Topic: The evolution of climate 1925 climatehistory

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The evolution of climate  1925 climatehistory


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  THE EVOLUTION OF CLIMATE
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PREFACE

Geologists very early in the history of their Science,
in fact as soon as fossils began to be examined, found
indisputable evidence of great variations in climate.
The vegetation which resulted in the coal measures
could have grown only in a sub-tropical climate, while
over these are vast remains of ice-worn boulders and
scratched rocks which obviously have been left by ice
existing under polar conditions. Such records were not
found only in one region, but cropped up in juxta-
position in many parts of the world. Remains of
sub-tropical vegetation were found in Spitzbergen, and
remains of an extensive ice-sheet moving at sea-level
from the south were clearly recognized in central and
northern India. At first it was simply noticed that the
older fossils generally indicated a warmer climate, and
it was considered that the early climate of a globe
cooling from the molten state would be warm and
moist, and so account for the observed conditions. It
was recognized that the ice remains were relatively
recent, and so far as a cause for the Ice Age was sought
it was considered that astronomical changes would be
sufficiënt.

It was only when geologists began to find records of
ice ages far anterior to the Carboniferous Age, and
astronomers proved by incontrovertible observations
and calculations that changes in the earth’s orbit, or
its inclination to that orbit, could not account for. the
ice ages, that the importance and inexplicability of the
geological evidence for changes of climate came to be
clearly recognized.
  VI

THE EVOLUTION OF CLIMATE

During the last few years much study has been given
to “ palseoclimatology,” but such a study is extremely
difficult. Only a very small fraction of the total surface
of the earth can be geologically examined, and of that
fraction a still smaller proportion has up to the present
been studied in detail. There has been a great tendency
to study intently a small region and then to generalize.
The method of study which has to be employed is
extremely dangerous. A geological horizon is deter-
mined by the fossils it contains. Wherever fossils of a
the strata are given the same

found in different parts of the world, and it is frequently
assumed not only that these rocks were laid down at
the same time, but that the conditions which they
indicate existed over the whole of the earth’s surface
simultaneously. Thus geologists teil us that the chmate
of the Carboniferous Age was warm and damp; of the
Devonian Age cool and dry; of the Eocene Age very
warm ; of the Ice Age very cold.

But has the geologist given sufficiënt attention to the
climatic zones during the various geological climates ?
It is true that the geologist has definitely expressed the
view that in certain ages climatic zones did not exist;
but from a meteorological point of view it is difficult
to see how the climate could have been even approxi-
mately the same in all parts of the world if solar radiation
determined in the past as in the present the temperature
of the surface of the earth.

The climatic zones of the various geological periods
will need much closer study in the future; the data
hardly exist at present, and the great area covered by
the ocean will always make the study difficult and the
conclusions doubtful. Admitting, for the saké of argu-
ment only, large changes in average conditions, but with
zonal variations of the same order of magnitude as those
existing to-day, the slow changes from period to period
will cause any given climatic state to travel slowly over

correlated by their fossils are
  PREFACE

vii

the surface of the earth, and this will so complicate the
problem as to make it doubtful whether any conclusions
can be reached so long as the same criteria are used to
determine both the geological epoch and the climatic
conditions.

These considerations apply more particularly to the
earlier records, while Mr. Brooks has confined his work
chiefly to the later records, beginning with those of the
Great Ice Age, in which climatic zones are clearly indi-
cated by the limits of the ice; but in this problem one
cannot confine one’s attention to a portion of the record,
for the test of any explanation must be its sufficiency
to explain all the past changes of climate. One will not
be satisfied with an explanation of the Great Ice Age
which does not explain at the same time the records of
earlier ice ages, of which there is indubitable evidence
in the Permo-Carboniferous and Pre-Cambrian periods,
and the records of widespread tropical or sub-tropical
conditions in the Carboniferous and Eocene Ages.
Whether Mr. Brooks’ theory for the cause of the recent
changes of climate satisfies this criterion must be left
to each reader to decide.

As Mr. Brooks says, the literature on this subject is
now immense, and it is most unsatisfactory literature to
digest and summarize. In the first place, many of the
original observations which can be used in the study of
past climates are hidden away in masses of purely geolo-
gical descriptions, and a great deal of mining has to be
done to extract the climatic ore. Then, again, most
of the writers who have made a special study of climatic
changes have had their own theoretical ideas and most
of their evidence has been ex parte. To take a single
example, for one paper discussing dispassionately the
evidence for changes in climate during the historical
period, there have been ten to prove either that the
climate has steadily improved, steadily deteriorated,
changed in cycles or remained unchanged. It is ex-
tremely difïicult to arrivé at the truth from such material,
  viiï THE EVOLUTION OF CLIMATE

and still more difficult to summarize the present state
of opinion on the subject.

It may be complained that Mr. Brooks has himself
adopted this same method and has written his book
around his own theory. But was there any alternative ?
There are so many theories and radically different
points of view that no writer could confine himself to
the observations and say what these indicate, for the
indications are so very different according to each theory
in turn. And new theories are always being propounded;
since Mr. Brooks commenced to write this book, Wegener
has put forward his revolutionary theory according to
which the polar axis has no stability, and the continents
are travelling over the face of the globe like debris on
a flood. Where is there solid ground from which to
discuss climatic changes if the continents themselves can
travel from the equator to the pole and back again in
the short period of one or two geological epochs ?

Mr. Brooks has studied deeply geology, anthropology,
and meteorology, and he has considerable mathematical
ability. By applying the latter to the results of his
studies he has developed a theory for the cause of climatic
ehanges based on changes of land and sea area, and on
changes of elevation of land surfaces, and naturally he
has made this theory the basis of his work.

That there will be some who are not able to agree
with him as to the sufficiency of the causes he invokes,
or who may even question whether he also has not
taken for granted what others dispute, goes without
saying ; but all will agree that he has presented a difficult
subject in a clear and condse way, and that meteorologists
(and may I add geologists ?) owe to him a deep debt of
gratitude.

G. C. Simpson
  CONTENTS

Prefaci ....... y

Introduction to thi Second Edition .   .   .4

I. Factors of Climate and the Causes of Cumatic Fluctuations i 5
II.   The Cumatic Record as a Whole .   .   .   32

III.   CONDITIONS BEFORE THE QüATERNARV IcE AgE .   .   4*

IV.   The Great Ice Age   .   .   .   .   •   47

V.   The Glacial History of Northern and Central Europe 55

VI.   The Mediterranean Regions ddring the Glacial Period 68

VII.   Asia ddring the Glacial Period .   .   .   .76

VIII.   The Glacial History of North America   .   .   .86

IX.   Central and South America .   .   .   .   97

X.   Africa ........ 103

XI.   Australia and New Zealand .   .   .   .109

XII.   The Glaciation of Antarctica .   .   .   . 114

XIII.   The Close of the Ice Age—The Continental Phase .   118

XIV.   The Post-Glacial Optimum of Climate   .   .   .127

XV.   The Forest Period of Western Edrope   .   .   .136

XVI.   The “ Classical ” Rainfall Maximum, 1800 b.c. to a.d. 500   140

XVII.   The Climatic Fluctuations since a.d. 500   .   .   149

XVIII.   Cumatic Fluctuations and the Evolution of Man .   .   159

XIX.   CUMATE AND HlSTORY   .   .   .   .   .102

Appendix—The Factors of Temperature .   .   .   i6fl

169


  INTRODUCTION TO THE
SECOND EDITION

On the whole, the first edition of “ The Evolution of
Climate ” met with a good reception. The meteoro-
logical interpretation of the succession of climatic
stages during the Quaternary Ice Age and subsequently
was especially welcomed, and it appears that with the
spread of our knowledge of the climatic conditions of
different parts of the world during the various geological
periods there will be increasing scope for work of this
kind. An important beginning has already been made
by F. Kerner-Marilaun (see later). The climatic
sequence should be a valuable guide to the complicated
stratigraphy of the Quaternary, and mainly on climatic
grounds it appeared to me most probable that the
Chellean industry, with its warm fauna, occupied the
Mindel-Riss interglacial. This conclusion was severely
criticized by several British archaeologists, on the ground
that work in France, especially by H. Obermaier, showed
that the Chellean industry probably feil in the Riss-
Wurm interglacial. The age of the Chellean is likely
to remain controversial for some time, but it may be
noted that the French archaeologist L. Mayet (i)1 places
the Chellean in the Mindel-Riss interglacial and at the
beginning of the Riss glaciation. A similar view is now
adopted by H. F. Osbom and C. A. Reeds (2) in a
valuable synthesis of the standards of Pleistocene classi-
fication; this is a reversal of the view which they
expressed in 1914. On the other hand, J. Reid Moir (3)
on the basis of his researches in East Anglia, and L.
Palmer (4) from work in south-east England, place the
1 These numbers refer to the Bibliography on page 12.

4
  INTRODUCTION   5

Chellean in the Gunz-Mindel interglacial. There are
thus three views to choose from, andfuture researches
alone can show which is correct. The question is of
climatic importance, because the greater part of the
Chellean is admitted to have been warm.

With regard to the climatic effect of volcanic dust,
Dr. W. J. Humphreys informs me that his suggestion
was that volcanic dust may act in conjunction with
mountain building and increased elevation of the
continents to produce glaciation. On page 18 the
figure for the maximum eccentricity should of course
have been 0.07775. H. Gams and R. Nordhagen have
made a number of helpful criticisms and suggestions.
Most of these are referred to in the summary of their
recent book (17); they will be introduced into the main
text when opportunity offers.

The past two or three years have seen great activity
in the study of past climates, and only a few of
these researches can be alluded to here. Ellsworth
Huntington and S. S. Visher (5) have published a new
hypothesis of the main cause of climatic variations.
According to their view the climate of the earth is
largely governed by changes in solar activity, acting on
the position and intensity of the storm beits. An
increase in solar activity, represented by an increase in
the relative sunspot numbers, is considered to result in
an increase of storminess, together with some displace-
ment of the storm tracks. When such a period of
increased solar activity occurs with extensive and high
continents, and perhaps with other favourable con-
ditions, such as a paucity of C02, a glaciation results.
This is considered to account for the Quaternary glacia-
tion and probably also for that of the Permo-Carbon-
iferous period, in which the storm tracks lay very far
south, and higher latitudes remained unglaciated because
they were occupied by deserts. Periods of slight solar
activity and few sunspots had slight storminess and
steady winds from the equator towards the poles, hence
  6 THE EVOLUTION OF CLIMATE

they were periods of mild and equable climate over the
whole earth. The variations of solar activity are con-
nected with changes in the distance of the nearest fixed
stars. The theory is attractive, but it presents several
very great difficulties. In particular the relationship,
if any, between sunspots and storminess at the present
day is still very obscure, and does not provide an adequate
basis for the enormous superstructure. In this country
at least it has not been well received.

A valuable summary of the palseoclimatological
evidence from the Antarctic has been presented by
C. S. Wright and R. E. Priestley (6). According to
this summary, the pre-Cambrian climate of Antarctica
was mainly warm temperate, with, however, indications
of frost action. In the Cambrian warm temperate to
tropical conditions prevailed ; in the Devonian possibly
temperate. In the Permo-Carboniferous period, during
the glaciation of the tropics, it appears that the high land
of Antarctica was an arid windswept desert, but in
sheltered lowlands a rich Glossopteris flora flourished.
There was a considerable seasonal range, but there is
no definite tracé of glacial conditions. In the Jurassic
a sub-tropical to warm temperate climate prevailed,
growing cooler through the Cretaceous, until in the
Eocene moraine-like deposits doubtfully suggest the
first Antarctic glaciation. In the Oligocene sub-
tropical to temperate conditions reappeared, followed
by the first undoubted glacial evidence. The Miocene
may have been a temperate interglacial period, but in
the Pliocene glacial conditions again appeared, and
persisted until the present, though with diminishing
intensity in recent times. This evidence must be taken
into account in future discussions of the causes of
climatic change.

F. Kerner-Marilaun (7) has studied the influence of
Permo-Carboniferous geography on the temperature
distribution, assuming a supply of solar energy similar
to that of to-day and the present position of the poles.
  INTRODUCTION   7

He finds that under these conditions a high Coastal
range of hills or plateau in northern India would pro-
bably be glaciated. His assumptions include a cold
Arctic ocean, and it is doubtful if this is vahd, but the
paper is a useful indication of the extent to which geo-
graphical changes might modify the present more or less
Zonal distribution of climates. The climatic conditions
of Permo-Carboniferous time are peculiar and now
appear to be well defined. There was a large expanse
of ocean in the northern hemisphere, with several large
islands or small continents, in the Coastal regions of
which the climate of the Coal Measures prevailed, moist
and probably rather warm. Isolated mountain areas
in the northern hemisphere, however, bore glaciers.
In the southem hemisphere, in which the equatorial
continent extended much farther south, the hardier
Glossopteris flora developed in high latitudes, and the
climate was probably equable but cool. Thus there was
a considerable temperature difference between the two
hemispheres, and this would lead to winds Crossing the
equatorial continent from south to north, similar to the
south-west monsoon of India. These winds would
deposit great quantities of moisture on the hills, which
at altitudes of about ten thousand feet would fall as
snow, originating the great ice-sheets of this period.
An investigation along these lines appears to present
the only possibility of accounting for the inversion of
zones in the Permo-Carboniferous period, apart from
displacements of the poles or Continental drift.

The theory of mild polar climates has also been
investigated by F. Kerner-Marilaun (. He found that
the land and sea distribution prevailing in the Upper
Jurassic and Middle Eocene periods would lead to
winter temperatures in the Arctic many degrees above
the present ones. He also found that the cooling effect
of the floating ice in the Arctic Ocean is so great that if
it could be cleared away the temperature over an open
ocean near the pole in January would be only a few
  8 THE EVOLUTION OF CLIMATE

degrees below freezing point. For some reason he did
not put these two results together, and apparently he
failed to realize that his researches showed that during
the two periods chosen the Arctic Ocean must have
been free of ice. A recalculation of his figures on this
basis (9) gave for the Upper Jurassic a January tempera-
ture in 750 N., approximatdy equal to that now found
in the Scilly Isles, while in the Middle Eocene it was
only a few degrees lower. The probable winter tem-
peratures calculated on climatological grounds thus fall
into very good agreement with those required by
palaeobotanists from the evidence of fossil floras.

The views of M. Depéret on the correlation of the
various Quaternary stages by means of changes of level
have attracted a great deal of attention. According
to Depéret the various changes of level which he
traced in the Mediterranean during the Quaternary
were due mainly to movements of the sea and only
locally to movements of the land, and he tracés the
Mediterranean raised beaches round the Atlantic coast
to the Baltic and also up the river valleys to the glaciated
regions, where they pass into glacial moraines. I
accepted Depéret’s system as applied to the Mediter-
ranean, but did not take seriously his extension of it
to the glaciated regions. Osborn and Reed (2), after
a careful examination, also find difficulty in accepting
Depéret’s correlation of the northern drifts. On the
other hand, it has been widely accepted in Europe as
a great advance. An objection to die scheme is that
each stage except the last includes both a glacial and an
interglacial phase; thus the Sicilian includes the Gun-
zian or Scanian glaciation and the Gunz-Mindel inter-
glacial, the Milazzian includes the Mindelian and the
Mindel-Riss, the Tyrrhenian includes the Rissian and
the Riss-Wurm, ^and the Monastirian includes the
Wurmian.

A. R. Dwerryhouse (10) has reinvestigated the glacia-
tion of north-eastern Ireland. He finds that this area
  INTRODUCTION   9

was covered first by Scottish ice from the Firth of Clyde,
and later by Irish ice from the hills of Donegal. The
two glaciations form part of a single maximum, and the
ice-sheets from the two centres were probably in contact
during part of the retreat of the Scottish ice. The
earlier work of Kilroe is mainly confirmed, with some
corrections of detail.

[Prometheus]

The late-glacial and post-glacial history of the Baltic
continues to be actively studied, and a number of papers
on the subject have appeared in the past two years.
E. Antevs (11) has contendëd that the Ancylus elevation
in the south-west Baltic region has been over-estimated.
He considers that during Ancylus time the Baltic was
never a true lake, but was an inland sea connected with
the Atlantic by a narrow channel, and kept fresh by the
enormous volume of water supplied by the melting
Scandinavian ice-sheet. This view is accepted by
G. de Geer, but is denied by H. Munthe. It is admitted
that the water was fresh, and if there was free com-
munication with the Atlantic it seems improbable that
the amount of thaw water during the cold dry winter
would be sufficiënt to keep out the sea water. From
the climatological point of view, however, the important
point is that die inflow of sea water at a higher tempera-
ture was interrupted, and it does not seem to matter
greatly which view is correct.

I. Hogbom (12) has reinvestigated “ fossil dunes ” of
northern Europe, and concludes that they were formed
by dry winds from west-north-west during Finiglacial
time (ca. 7000-6000 b.c.) and not to periglacial easterly
winds, as formerly supposed. The type of pressure
distribution reconstructed from the dunes and other
evidence resembles that prevailing during the cold
spell of spring.

G. de Geer (13) has been investigating the annual
clay-varves of the late-glacial period in North America.
It will be remembered that by an examination of similar
annual layers in Sweden he arrived at an absolute
  10 THE EV0LUTI0N OF CLIMATE

measure of the age of various stages of the retreat. He
considers that the succession of different thicknesses
in certain groups of annual layers in North America
bears so close a resemblance to parts of the Swedish
succession that they must refer to the same groups of
years, and on these grounds he has dated parts of the
final stages of the glacial period in North America.
The ice left the eastern end of Lake Champlain about
I,loo years before the end of the Ice Age in Sweden
(ca. 5000 b.c.). In Timiskaming (northern Ontario)
the recession was traced for over 600 years, the ice
leaving the district 297 years after the close of the Ice
Age in Sweden. This indicates that the melting of the
inland ice lasted somewhat longer in Canada than in
Sweden ; but de Geer considers that there can be no
more doubt as to the exact agreement between the
climatic conditions in the two regions. It is greatly
to be hoped that de Geer will publish a table showing
the relative thicknesses of each of his annual layers,
similar to that published by A. E. Douglass of the
width of annual tree-rings. Sir T. W. Edgeworth
David (14) has discovered similar banded clays associated
with the pre-Cambrian and Carboniferous tillites of
Australia, indicating a duration of 12,000 years in the
former case and about 4,000 years in the latter.

The fourfold division of the Quaternary Ice Age
adopted by Penck and Bruckner for the Alps is graduafly
being extended beyond the limits of Europe. Sir T. W.
Edgeworth David (15) accepts it for the glaciation of
Australia and Tasmania; he States that Tasmanian
man is now considered to date back probably to the
Rissian. The Australian type came later, but the
Talgai skull from near Warwick, Queensland, which is
placed in the Riss-Wurm interglacial, has Australian
affinities. As a result of Dainelli’s researches in the
Himalayan region, F. Loewe (16) has delineated a four-
fold glaciation of the western Himalayas. The second
ice-extension was the greatest, the positions of the
  INTRODUCTION   n

snow-line being: Glaciation I, unknown; II, 11,500
feet; III, 12,300 feet; IV, 12,550 feet. The fourth
glaciation was followed by retreat stadia as in the Alps.
No fossiliferous interglacial deposits are known, so that
the correlation with the Alpine stages is problematical.

Finally, I have to mention an important publication
by H. Gams and R. Nordhagen (17), deahng primarily
with the post-glacial climatic changes in central Europe,
but summarizing also the results of recent researches
in other parts of the Continent. Their summary
commences with the “ Great Interglacial ” (following
the Mindelian glaciation), in which they place the
Chellean industry. After this they intercalate a new
glacial stage, the Mühlbergian, followed by the short
Rabutz interglacial, in which they place the Acheulian.
This additional glaciation certainly clears up some
difficulties, and facilitates correlation with the (possibly)
five-fold American series (H. F. Osborn and C. A. Reeds
ignore the Iowan and so make the American series four-
fold); but much field-work will be required before
geologists will consent to such a modification of Penck
and Bruckner’s classic scheme. Gams and Nordhagen
consider the Rissian glaciation to have been the greatest,
instead of the Mindelian; it was followed by the
Rixdorf interglacial, also short. The Würrn glaciation
is divided into a number of stages—Schaffhauser Advance,
Laufen Oscillation, Mecklenburgian End Moraine,
Alleröd Oscillation, Fennoscandian End Moraine, fol-
lowed by the other familiar retreat stages. For the
post-glacial period the pioneer work of Axel Blytt is
regarded as thoroughly confirmed, and his terminology
is accepted. The temperature is considered to have
risen steadily through the dry Boreal Period (Continental
Phase, Azilian-Tardenoisian), the moist Atlantic Period
(Maritime Phase), and the dry sub-boreal Period (Later
Forest Phase), reaching a maximum 40 F. above the
present near 1000 b.c. It was at this period that the
hazel reached its greatest extension in northern Scan-
2
  12 THE EVOLUTION OF CLIMATE

dinavia, and not during the boreal period, as formerly
believed. About B.c. 850 occurred a sudden deteriora-
tion of climate, which in the Alps had almost the appear-
ance of a catastrophe. This begins the sub-Atlantic
Period (Later Peat-bog Phase) which in the opinion of
the authors corresponds with the Daun readvance of
the Alpine glaciers; after this the climate of Europe
passed by a series of oscillations to its present level.

If the results of all the remaining papers published in
the past two or three years were discussed, this preface
would grow to the size of another book. In the face of
such an outpouring of material one’s views require
constant adjustment, and the most urgent need at the
moment, as pointed out by Osbom and Reeds (2), is a
stable framework of classification for the Quaternary
period, which shall embody at once the glacial advances
and retreats, the river terraces and raised beaches, the
succession of faunas, both land and marine, and of
floras, the human industries and the waves of climate.
Unfortunately we seem now to be farther than ever
from such a framework. Let us hope that this is the
darkest hour which precedes the dawn, and that some
generally accepted framework will soon emerge.

C. C. P. B.

January, 1925.

BIBLIOGRAPHY

(1)   Mayet, Lucien. “ Corrélations géologiques et archéologiques des tempi

quaternaires.” Paris, C.-R, Ass. frattf. avanc. sci., 44 Session, Stras-
bourg, 1920, pp. 481-490.

(2)   Osbom, Henry Fairfield, and Chester A. Reeds. “ Old and new stan-

dards of Pleistocene diyision in relation to the prehistory of man in
Europe.” Buil. Geol. Soc. America, 33, 1922, pp. 411-490.

(3)   Moir, J. Reid. “ The Ice-age and Man.” Man, 1922, p. [52].
   . “ The antiquity of man in East Anglia.” Science

Progress, July, 1924, p. 129. See also The Times, August 22, 1924.
  BIBLÏOGRAPHY   13

(4)   Palm er, L. S. “The Ice-age and man in Hampshire.” Man, 1922,
p. 106.

-------------, and J. H. Cooke. “The Pleistocene deposits of the

Portsmouth district and their relation to man.” London, Proc.
Geol. Ass., 34, 1923, p. 253.

(3) Huntington, Ellsworth, and S. S. Visher. “ Climatic changes, their
nature and cause.” New Haven, 1922.

(6)   British (Terra Nova) Antarctic Expedition 1910-1913.   “ Glaciology,”

by C. S. Wright and R. E. Priestley. London (Harrison & Sons), 1922.

(7)   Kerner-Marilaun, F. “ Untersuchungen über die morphogene Klima-

komponente der permischen Eiszeit Indiens.” Wien, Sitzungsber.
Akai. Wiss., Matb.-nat. KL, Abt. x, 126 Bd., 1917, pp. 177-228.

(   ------------------.   “ Das akryogene Seeklima und seine Bedeutung

für die geologischen Probleme des Arktis.” Wien, Sitzungsber. Akai.
Wiss., 131, 1922, p. 133.

(9)   Brooks, C. E. P. “ The problem of mild polar climates.” London,

Q. J. R. Meteor. Soc., 31, 1923.

(10)   Dwerryhouse, A. R. “ The glaciation of north-eastern Ireland.”

London Q. J. G. S., 79, 1923, p. 332.

(11)   Antevs, Ernst. “On the late-glacial and post-glacial history of the

Baltic.” New York, N. Y., Geogr. Ren., 12, 1922, pp. 602-612.

(12)   Hogbom, I. “ Ancient inland dunes of northern and middle Europe.”

Stockholm, Geogr. Ann., 3, 1923, pp. 113-243.

(13)   Geer, G. de. “ Correlation of late-glacial annual day-varves in North

America with the Swedish time scale.” Stockholm, Geol. Foren.
Forb., 43, 1921, p. 70.

(14)   David, Sir T. W. Edgeworth. “The ' Varve Shales’ of Australia.”

Amer.J. Sd. (5), 3, 1922, p. 115.

(15)   -------------------------. “ Geological evidence of the antiquity

of man in the Commonwealth, with especial reference to the Tas-
manian aborigines.” Hobart, Papers and Proc. R. Soc. F asmania,
1923, pp. 109-150.

(16)   Loewe, F. “ Die Eiszeit in Kaschmir, Baltistan and Ladakh.” Berlin,

Zs. Ges. Erik., 1924, p. 42.

(17)   Gams, H., and R. Nordhagen. “ Postglaziale Klimaanderungen

und Erdkrustenb ewegungen in Mitteleuropa.” München, Geogr.
Geseüscb., Landesk. Forscbungen, H.25, 1923.
 
  THE EVOLUTION OF CLIMATE

CHAPTER I

FACTORS OF CLIMATE AND THE CAUSES OF CLIMATIC
FLU CTUATI ONS

T he climate of any point on the earth’s surf ace depends
on a complex of factors, some of them due to influences
arriving from outside the earth, and others purely
terrestrial. Since any variations of climate must be
due to a change in one or more of these, it is necessary,
before we can discuss changes of climate, to consider
briefly what the factors are.

The only important extra-terrestrial factor of climate
is the amount of radiant energy which reaches the
borders of the earth’s atmosphere from the heavenly
bodies—that is, from the sun, for the moon and stars
can be ignored in this connexion. The only other
conceivable factor is the arrival of meteorites, bringing
kinetic energy, which is converted into heat, and intro-
ducing cosmic dust into the atmosphere; but it is highly
improbable that this is of appreciable effect.

The amount of solar radiation1 which reaches the
earth depends in the first place on the total radiation
emitted by the sun, and in the second place on the
distance of the earth from the sun, both of which quanti-
ties are variable. It has been calculated that if other
factors remained unchanged an increase of ten per cent.
in the solar radiation would raise the mean temperature
of the earth’s surf ace by about 70 C., or between 12°

1 By this term we shall in future understand only that part of it which
is responsible for thermal effects.

15
  16   THE EVOLUTION OF CLIMATE

and 130 F., with, of course, a corresponding fall for a
decrease.

After the sun’s radiation reach.es the outer limits of
the earth’s atmosphere its nature and intensity are
modified by the composition of the air through which
it passes. In general the air itself is very transparent to
the small wave-lengths which make up the solar rays,
but the presence of fine dust, whether of volcanic or of
cosmic origin, has been shown by Humphreys to be a
distinct hindrance to their passage, so that volcanic
eruptions of an explosive nature, such as that of Krakatoa
in 1883, La Soufriére (St. Vincent) in 1902, or Katmai
(Alaska) in 1912, may result in a fall of temperature
over the world as a whole.

The temperature of the earth is determined by the
balance between the radiation received from the sun
and the terrestrial radiation to space, and a decrease in
the latter would be as effective in raising the mean
temperature as an increase in the former. The use of
glass for greenhouses depends on this principle; for glass
is transparent to heat rays of small wave-length, but is
largely opaque to the rays of greater wave-length which
make up terrestrial radiation. Certain constituents
of the atmosphere, especially water-vapour, carbon
dioxide and ozone, are effective in this way, and varia-
tions in the amount of these gases present may affect
the temperature.

The angle at which the sun’s rays strike the earth’s
Surface is a highly important factor. Within the Tropics
the sun at midday is nearly vertical throughout the
year, and the mean temperature in these regions is
correspondingly high; on the other hand, during the
long polar night the sun is not seen for half t;he year,
and very low temperatures prevail. There is thus a
seasonal variation of the heat received from the sun in
middle and high latitudes, the extent of which depends
on the “ obliquity of the ecliptic,” i.e. the inclination
of the earth’s axis to the plane of its orbit round the
  FACTORS OF CLIMATE   17

sun, and any changes in this factor must alter the seasonal
variation of climate.

Further, since the climate of any place depends so
closely on its latitude, it follows that if the latitude
changes the climate will change. A ship can change
its latitude at will, but we are accustomed to regard the
position of the “ firm ground beneath our feet ” relatively
to the poles as fixed within narrow limits. This stability
has, however, been questioned from time to time, mainly
on evidence derived from palaeoclimatology, and theories
of climatic change have been based on the wanderings
of continents and oceans. Finally, local climate is
intimately bound up with the distribution of land and
sea, and the marine and atmospheric currents resulting
therefrom, and on elevation above sea level, both of
which factors, as we shall see, have suffered very wide
variations in the geological past.

Nearly all the theories which have been put forward
to account for geological changes of climate, and espe-
cially the occurrence of the last or Quaternary Ice Age,
are based on the abnormal variation of one or other of
the above factors, and we may consider them briefly in
turn. Very few have ever been taken seriously. In
the first place, we can at once dismiss fluctuations in
the radiation emitted by the sun as a cause of great
changes of climate. It is true that many small fluctua-
tions are traceable directly to this cause, such as the
eleven-year periodicity of temperature and rainfall; but
these fluctuations are, and must be, greater at the
equator than at the poles, while the fall of temperature
during the Glacial period reached its maximum near
the poles and was least at the equator. Moreover, there
is not .the slightest direct evidence in support of such a
theory, and it can only be admitted when all other
hypotheses have failed.

The “ astronomical ” theory of the cause of climatic
fluctuations is associated chiefly with the name of James
Croll. Croll’s theory connects abnormal variations of
  18 THE EVOLUTION OF OLIMATE

climate with variations, firstly of the eccentricity of the
earth’s orbit, and secondly of the ecliptic. In periods
of high eccentricity the hemisphere with winter in
aphelion is cold because the long severe winter is far
from being balanced by the short hot summer; at the
same time the opposite hemisphere enjoys a mild equable
climate. This theory commanded instant respect, and
still finds a place in the text-books, but difficulties soon
began to appear. The evidence strongly suggests that
glacial periods did not alternate in the two hemispheres,
but were simultaneous over the whole earth; even on
the equator the snow-line was brought low down.
Moreover, on Mars the largest snow-cap appears on the
hemisphere with its winter in perihelion. Although
Croll’s reasoning was beautifully ingenious he gave very
few figures ; while the date which he gives for the
conclusion of the Ice Age, 80,000 years ago, has been
shown by recent research to be far too remote, 15,000
years being nearer the mark.

Croll’s theory has recently been revived in an altered
form by R. Spitaler, a Czecho-Slovakian meteorologist,
who calculated the probable alteration in the mean
temperature of each latitude under maximum eccen-
tricity 6^7775) and maximum obliquity (270 48'), the
distribution of land and water remaining unchanged.
The results are shown in the attached table, where —
means that the temperature was so much below the
present mean, and + that it was so much above.

   Aphelior   i December.   Aphelion June.   
   Winter. Summer. Year.      Winter.   Summer. Year.
   °F.   °F. °F.   °F.   "F. °F.
N. 600   - 9   + 15 -1   -s   -4 -1
30°   -13   + 13 -2   + 1   -8 -2
Equator   - 8   + 4 -2   + 1   -6 -2
S. 30°   - 6   + 1 -2   +3   -s -2
6o°   “ 2   - 1 -1   + 1   — 2 —1
  FACTORS OF CLIMATE   19

Spitaler claims that these differences are sufficiënt to
cause a glacial period in the hemisphere with winter in
aphelion, but from. this point his theory départs widely
from Croll’s. During the long severe winter great
volumes of sea water are brought to a low temperature,
and, owing to their greater weight, sink to the bottom
of the ocean, where they remain cold and accumulate
from year to year. But the water warmed during the
short hot summer remains on the surface, where its heat
is dissipated by evaporation and radiation. Thus,
throughout the cold period, lasting about 10,000 years,
the ocean in that hemisphere is steadily growing colder,
and this mass of cold water is sufficiënt to maintain a
low temperature through the whole of the following
period of 10,000 years with winter in perihelion, which
would otherwise be a genial interval. In this way a
period of great eccentricity becomes a glacial period over
the whole earth, but with crests of maximum intensity
altemating in the two hemispheres. Unfortunately the
numerical basis of this theory is not presented, and it
seems incredible that a deficiency of temperature could
be thus maintained through so long a period. Further,
the difficulty about chronology remains, and the work
brings the astronomical theory no nearer to being a
solution of the Ice Age problem than was Croll’s.

The theory which connects fluctuations of climate
on a geological scale with changes in the composition
of the earth’s atmosphere is due to Tyndall and Arrhenius,
and was elaborated by Chamberlin. The theory sup-
posed that the earth’s temperature is maintained by
the “ blanketing ” effect of the carbon dioxide in the
atmosphere. This acts like the glass of a greenhouse,
allowing the sun’s rays to enter unhindered, but absorbing
the heat radiated from the earth’s surface and returning
some of it to the earth instead of letting it pass through
to be lost in space. Consequently, any diminution in
the amount of carbon dioxide present would cause the
earth to radiate away its heat more freely, so reducing
  20 THE EV0LUTI0N OF CLIMATE

its temperature. But it is now known that the terrestrial
radiation which this gas is capable of absorbing is taken
up equally readily by water-vapour, of which there
is always sufficiënt present, and variations of carbon
dioxide cannot have any appreciable effect.

[Prometheus]

Brief mention may be made here of a theory put •
forward by Humphreys, who attributed gladation to
the presence of great quantities of volcanic dust in the
atmosphere. It would require an enormous output of
volcanic dust to reduce the temperature sufficiently;
but in any case the relation, if any, between vulcanicity
and temperature during the geological ages is rather
the reverse of that supposed by Humphreys, periods of
maximum volcanic action coinciding more frequently
with high temperatures than with low. Perhaps the
best comment on Humphreys’ theory is that in 1902
F. Frech produced its exact opposite, warm periods being
associated with an excess of vulcanicity and cold periods
with a diminution.

The theory which attributes climatic changes in
various countries to variations in the position of the
poles has been adduced in two mam forms. The first
is known as the Pendulation Theory, and supposes the
existence of two “ oscillation poles ” in Ecuador and
Sumatra. The latitude of these points remains un-
changed, and the geographical poles swing backwards
and forwards along die meridian of 10 E. midway
between them. Varying distances from the pole cause
changes of climate, and the movements of the ocean,
which adjusts itself to the change of pole more rapidly
than the land, causes the great transgressions and regres-
sions of the sea and the elevation and subsidence of
the land.

An alternative form put forward by P. Krdchgauer,
and recently brought up again by Wegener, explains
the apparent variations in the position of the pole, not
through a motion of the earth’s axis, but by the assump-
tion that the firm crust has moved over the earth’s core
  FACTORS OF CLIMATE   21

so that the axis, remaining firm in its position, passes
through different points of the earth’s crust. The cause
of these movements is the centrifugal force of the great
masses of the continents, which are distributed sym-
metricaUy .about the earth. Imagine a single large
continent resting on a sub-fluid magma in temperate
latitudes. Centrifugal force acting on this continent
tends to drive it towards the equator. There is thus a
tendency for the latitude of Europe to decrease. Similar
forces acting through geological ages have caused the
poles and equator to wander at large over the earth’s
surface, and also caused the continents to shift their
positions relatively to one another. According to
Wegener, in the Oligocene there was only a single
enormous continent, America being united to Europe
and Africa on the one hand, and through Antarctica to
Australia on the other; while the Deccan stretched
south-westwards nearly to Africa. The poles were in
Alaska and north of the Falkland Islands. The treat-
ment in Kreichgauer’s original book is speculative and
at times fanciful ] Wegener’s treatise appears to demand
more respectful attention, but is open to some vital
objections. In the first place, theories of this class
demand that the glaciation occurred in different regions
at widely different times, whereas we shall see in the
following pages that the evidence points very strongly
to a doublé glaciation during the Quaternary occurring
simultaneously over the whole earth. This objection,
which was fatal to Croll’s theory in its original form,
is equally fatal to theories of pole-wandering as an
explanation of the Quaternary Ice Age. Secondly, we
know that the last phase of this glaciation, known as the
Wisconsin stage in America and the Wurmian in Europe,
was highly developed only 20,000 years ago, and probably
reached its maximum not more than 30,000 years ago.
In the last 5000 years there has been no appreciable
change of latitude, at least in Eurasia; and it seems
impossible for the extensive alterations required in the
  22 THE EVOLUTION OF CLIMATE

geography of the world by Wegener’s theory to have
taken place in so short a time.

The great glaciation of the Permian period, referred
to in the next chapter, is a totally different matter.
During this time the ice-sheets appear to have reached
their maximum area, and to have extended to sea-level,
in countries which are at present close to the equator,
while lands now in high latitudes remained unglaciated.
It is true that at the present day glaciers exist at high
latitudes under the equator itself, and given a ridge
sufficiently steep and a snowfall suffidently heavy such
glaciers would possibly extend to sea-level; but even
these conditions would not give rise to the enormous
deposits of true boulder-clay which have been discovered,
and there seems no way of avoiding the supposition of
an enormous difference in the position of the pole
relatively to the continents at this time.

Wegener’s theory alone, however, requires that glacia-
tion should always have been proceeding in some part
of the globe (unless both poles were surrounded by wide
expanses of ocean), which is hard to reconcile with the
extremely definite and limited glaciations which geolo-
gical research has demonstrated. In these circumstances
we may tentatively explain the pre-Tertiary gladal
periods by combining Wegener’s theory of the move-
ments of continents and oceans as a whole with the
theory of changes of elevation and of land and sea
distribution which is outlined below. That is to say,
we may suppose that the positions of the continents
and oceans have changed, relatively both to each other
and to the poles, slowly but more or less continuously
throughout geological time; while at certain periods the
land and sea distribution became favourable for extensive
glaciation of the regions which at that time were in high
latitudes.

The geographical theory, which States that the Ice Age
was brought about by elevation in high latitudes, and
by changes in the land and sea distribution, though
  FACTORS OF CLIMATE

23

never seriously challenged, has sufïered until recently
from a lack of precision. The present author attempted
to remedy this by a close mathematical study of the
relation of temperature to land and sea distribution at
the present 'day. The method at attack was as follows:
from the best available isothermal charts of all countries
the mean temperature reduced to sea-level was read off
for each intersection of a ten-degree square of latitude
and longitude, for January and July, from 70° N. to 6o° S.
latitude; this gave 504 values of temperature for each
of these months. Round each point was next drawn
a circle with an angular radius of ten degrees, divided
into east and west semicircles. The area of each semi-
circle was taken as 100, and by means of squared paper
the percentage of land to the east and land to the west
were calculated; finally, in each month the percentage
of the whole circle occupied by land, ice, or frozen sea
was calculated, this figure naturally being greater in
winter than in summer. The projection used was that
of the “ octagonal globe,” published by the Meteoro-
logical Office, which shows the world in five sections,
the error nowhere exceeding six per cent.

These figures were then analysed mathematically,
and from them the effects on temperature of land to
the east, land to the west, and ice were calculated. The
detailed numerical results are set out in an Appendix;
it is sufficiënt here to give the following general con-
clusions :

(1)   In winter the effect of land to the west is always
to lower temperature.

(2)   In winter the effect of land to the east is almost
negligible, that is to say, the eastern shore of a continent
is almost as cold as the centre of the continent. The
only important exception to this rule is 70° N., which
may be considered as coming within a belt of polar
east winds.

(3)   In summer the general effect of land, whether to
the east or west, is to raise temperature, but the effect
  24 THE EVOLUTION OF CLIMATE

is nowhere anything like so marked as the opposite effect
in winter.

(4)   The effect of ice is always to lower temperature.

(5)   For every latitude a “ basal temperature ” can be
found. This is the temperature found near the centre of
an ocean in that latitude. This “ basal temperature ”
is a function of the amount of land in the belt of latitude.
Poleward of latitude 20° an increase of land in the belt
lowers the winter basal temperatures very rapidly and
raises the summer basal temperature to a less extent.
The “ basal temperature ” is important, since it is the
datum line from which we set out to calculate the
winter and summer temperatures of any point, by the
addition or subtraction of figures representing the local
effect of land in a neighbouring 10° circle.

As an illustration of the scale of the temperature
variations which may be due to geographical changes,
suppose that the belt between 50° and 70° N. is entirely
above the sea. Then we have the following theoretical
temperatures; for a point on 6o° N. at sea-level:
January — 30° F.; July 720 F.

Data for calculating the effect of ice are rather scanty,
but the following probable figures can be given, supposing
that the belt in question were entirely ice-covered :
January — 30° F. (as for land); July 230 F.

Supposing that the belt were entirely oceanic, the
mean temperature in 6o° N. would be :

January 290 F.; July 410 F.

These figures show how enormously effective the
land and sea distribution really is. From Appendix
it is easy to calculate the probable temperature distribu-
tion resulting from any arrangement of land and water
masses. Since the geography of the more recent geolo-
gical periods is now known in some detail, we have thus
a means of restoring past climates and discussing the
distribution of animals and plants in the light of this
knowledge. Of course it is not pretended that no other
possible causes of great climatic variation exist, but no
  FACTORS OF CLIMATE   25

others capable of seriously modifying temperature over
long periods are known to have been in operation. As
we shall see later, there are solar and other astronomical
causes capable of modifying climate slightly for a few
decades or even centuries, but these are insignificant
compared with the mighty fluctuations of geological
time.

In applying the results of this “ continentality ” study
to former geological periods the method adopted is that
of differences. The present climate is taken as a Standard,
and the temperatures of, for instance, the Glacial period
are calculated by adding to or subtracting from the
present temperatures amounts calculated from the change
in the land and sea distribution. This has the advantage
of conserving the present local peculiarities, such as those
due to the presence of the Gulf Drift, but such a pro-
cedure would be inapplicable for a totally different
land and sea distribution, such as prevailed during the
Carboniferous period. That it is applicable for the
Quaternary is perhaps best shown by the following
comparison of temperatures calculated from the distri-
bution of land, sea and ice with the actual temperatures
of the Ice Age as estimated by various authorities
(inferred fall) :

Locality.   Author.   Inferred Fall.   Calculated Fall.      
         Jan.   July.   Mean.
      «F.   °F.   •F.   •F.
Scandinaria   J. Geikie   More than 20   36   18   27
East Anglia   C. Reid   20   18   »3   IS
AIpi   Penck and Brückner   11   13   9   
Japan   Simotomai   7   9   5   7

It is seen that the agreement is quite good.

There is one other point to consider, the effect of
height. The existence of a great land-mass generally
implies that part of it at least has a considerable elevation,
  26   THE EVOLUTION OF CLIMATE

perhaps 10,000 or 20,000 feet, and these high lands
have a very different climate to the neighbouring low-
lands. Meteorologists have measured this difference
in the case of temperature and found that the average
fall with height is at the rate of i° F. in 300 feet. In
the lower levels the fall is usually greater in summer
than in winter, but at 3000 feet it is fairly uniform
throughout the year. Consequently, quite apart from
any change in climate due to the increased land area,
an elevation of 3000 feet would result in a fall of
temperature of 10° F., winter and summer alike. This
reinforces the effect of increased land area and aids in
the development of ice-sheets or glaciers.

The effect of geographical changes on the distribution
of rainfall are much more complicated. The open sea
is the great source of the water vapour in the atmosphere,
and since evaporation is very much greater in the hot
than in the cold parts of the globe, for considerable
precipitation over the world as a whole there must be
large water areas in the Tropics. In tempera te latitudes
the water vapour is carried over the land by onshore
winds, and some of it is precipitated where the air is
forced to rise along the slopes of hills or mountains.
Some rain falls in thunderstorms and similar local
showers, but the greater part of the rain in most tem-
perate countries is associated with the passage of “ depres-
sions.” These are our familiar wind- and rain-storms;
a depression consists essentially of winds blowing in an
anti-clockwise direction round an area of low pressure.

These centres of low pressure move about more or less
irregularly, but almost invariably from west to east in the
temperate regions. They are usually generated over
seas or oceans, and, since a supply of moist air is essential
for their continued existence, they tend to keep to the
neighbourhood of water masses or, failing that, of large
river valleys. In a large dry area depressions weaken or
disappear. Their tracks are also very largely governed
by the positions of areas of high pressure or anticyclones,
  FACTORS OF CLIMATE   27

which they tend to avoid, moving from west to east on
the polar side of a large anticyclone and from east to
west on the equatorial side. Since anticyclones are
developed over the great land areas in winter, this
further restricts the paths of depressions to the
neighbourhood of the oceans at that season.

For all these reasons the tracks of depressions, and
therefore the rainfall, are intimately connected with the
distribution of land and sea. In winter there is little rain-
fall in the interior of a great land mass, except where it is
penetrated by an arm of the sea like the Mediterranean ;
on the other hand, the coasts receive a great deal of rain
or snow. The interior receives its rain mostly in spring
or summer ; if the Coastal lands are of no great elevation
this will be plentiful, but if the coasts are mountainous
the interior will be arid, like the central basins of Asia.

The development of an ice-sheet is equivalent to
introducing perpetual winter in the area occupied by
the ice. The low temperature maintains high pressure,
and storm tracks are unable to cross the ice. At the
present day depressions rarely penetrate beyond the
outer fringe of the Antarctic continent, and only the
Southern extremity of Greenland is afïected by them.
Since the total energy in the atmosphere is increased by
the presence of an ice-sheet, which afïords a greater
contrast of temperature between cold pole and equator,
storms will increase in frequency and their tracks must
be crowded together on the equatorial side of the ice-
sheet. In the southem hemisphere we have great
storminess in the “ roaring forties ” ; south of Greenland
the Newfoundland banks are a region of great storminess.
Hence, when an ice-sheet covered northem and central
Europe the Mediterranean region must have had a
marked increase of storminess with probably rain in
summer as well as in winter.

But if snow-bearing depressions cannot penetrate an
ice-sheet, it may be asked how the ice-sheet can live.
The answer depends on the nature of the underlying
3
  28 THE EVOLUTION OF CLIMATE

country. A land of high relief such as Antarctica is,
and as Greenland probably is, rising to a maximum
elevation of many thousand feet near its centre, draws
its nourishment chiefly from the upper currents which
flow inward on all sides to replace the cooled air which
flows outwards near the surface. These upper currents
carry a certain amount of moisture, partly in the form
of vapour, but partly condensed as cirrus and even
cumulus cloud.

At low temperatures air is able to hold only a negligible
amount of water vapour, and this current, coming in
contact with the extremely cold surface of the ice, is
sucked dry, and its moisture added to the ice-sheet.
Probably there is little true snowfall, but the condensa-
tion takes place chiefly close to the surface, forming a
frozen mist resembling the “ ice-mist ” of Siberia.
Even if the central land is not high enough to reach
into the upper current at its normal level, the surface
outflow of cold air will draw the current down to the
level of the ice. This will warm it by compression, but
the ice-surface is so cold that such warming makes little
difference in the end. This process of condensation
ensures that after the ice reaches a certain thickness it
becomes independent of topography, and in fact the
centre of the Scandinavian ice-sheet lay not along the
mountain axis, but some distance to the east of it.

It is probably only on the edges of the ice-sheet, and
especially in areas of considerable local relief, that
snowfall of the ordinary type takes place, associated with
moist winds blowing in the front section of depressions
which skirt the ice-edge. But when conditions are
favourable this source of supply is sufficiënt to enable
these local ice-sheets to maintain an independent life,
merely fusing with the edges of the larger sheet where
they meet. Examples of such local centres in Europe
were the Irish and Scottish glaciers, and at a later stage
the Lofoten glaciers of the west of Norway, and in
America the Cordilleran glaciers of Columbia.
  FACTORS OF CLIMATE

29

Penck and Brückner have demonstrated that in the
Alps the increase of glaciation was due to a fall of tem-
perature and not to an increase of snowfall. The
argument is threefold:   firstly, the lowering of the

snow-line was uniform over the whole Alpine area,
instead of being irregular as it would be if it depended
on variations of snowfall; secondly, the area and depth
of the parent snow-fields which fed the glaciers remained
unchanged, hence the increased length of the glaciers
must have been due to decreased melting below the
snow-line, i.e. to lower temperatures; thirdly, the upper
limit of tree-growth in Europe sank by about the same
amount as the snow-line. The same conclusion holds
for the great Scandinavian and North American ice-
sheets, the extension of which was undoubtedly due to
a great fall of temperature. In the case of the Alps the
interesting point has come to light that the fall of
temperature, though due in part to increased elevation,
is mainly accounted for by the presence of the Scandi-
navian ice-sheet, which extended its influence for many
miles beyond the actual limits of glaciation, so that its
waxings and wanings are faithfully reproduced in those
of the Alpine glaciers, even to the details of the final
retreat after the last maximum.

[Prometheus]

It is only when we turn to tropical and sub-tropical
regions that we find variations of temperature unable
to account for increased glaciation. Not only were the
changes of land and sea distribution on a very much
smaller scale than further north, but the Appendix shows
that the temperature value of a corresponding change
of land area is also very much less. But the high inter-
tropical mountains—the Andes and Kenya and Kiliman-
jaro in central Africa—which to-day bear glaciers, in
Quaternary times carried much greater ones. We
cannot call in a fall of temperature, for the reason above
stated, and also because at lower levels there is no
evidence of colder conditions. In the Glacial period
the marine fauna was the same as to-day, and mountains
  30 THE EV0LUTI0N OF CLIMATE

which now fall short of the snow-line by a few hundred
feet were still unglaciated even then. The only alter-
native is increased snowfall on the higher mountains
Fortunately this fits in well with meteorological theory.
The rain and snowfall of tropical regions depends, first
of all, on the evaporation over the oceans. But evapora-
tion is profoundly influenced by the velocity of the
wind; and the wind, which in the Tropics represents the
strength of the atmospheric circulation, depends on the
thermal gradiënt between the equator and the poles;
since there is no evidence of any appreciable change
of temperature over the Tropics as a whole, while there
was a very great fall in cold-temperate and polar regions,
the thermal gradiënt, and therefore, ultimately, the
tropical and sub-tropical, rain and snowfall must have
been very greatly increased. Hence the increased
glaciation of high mountains near the equator, and hence
also the evidence of “ Pluvial periods ” in the sub-tropical
arid regions on either side of the equator.

Thus during Glacial periods we have :

(1)   Elevation in high latitudes caused a great increase
of land areas there.

(2)   Both elevation and increase of land area resulted
in a lowering of temperature, materially increased by
the gradual development of great ice-sheets.

(3)   These ice-sheets caused the development of sub-
sidiary ice-sheets on their Southern and western borders.

(4)   The lowering of temperature in high latitudes
increased the thermal gradiënt between equator and
poles, resulting in:

(a)   Increased snowfall, and hence increased
glaciation on high mountains near the equator.

(b)   Pluvial periods in the sub-tropical arid regions.

BIBLIOGRAPHY

Humphrey», W. J. “Physics of the air.” Phüadelphia, 1920. [Pt. 4,
pp. 556-629.]
  FACTORS OF CLIMATE

3i

Chamberlin, T. C. “ An attempt to frame a working hypothesis of the
cause of glacial periods on an atmospheric basis.” Journal of Geology
(American), Vol. 7, 1899, pp. 545-84» 667-85, 751-87. [Carbon
di-oxide theory.]

Croll, J. “ Climate and time in their geological relations.” London, 1875.
“ Discussions on climate and cosmology.” London, 1889. [Eccen-
tricity of earth’s orbit.]

Spitaler, R. “ Das Klima des Eiszeitalters.” Prag, 1921. Lithographed.
[Eccentricity of earth’s orbit. Reviewed in the Meteorological Maga-
zine, London, September, 1921.]

Simroth, H. “ Die Pendvdationstheorie.” Leipzig, 1908.

Kreichgauer, P. “ Die Aequatorfrage in der Geologie.” Steyr, 1902.

Wegener, A. “ Die Entstehung der Continente und Ozeane.” Die Wissen-
schaft, Bd. 66, Braunschweig, 1920.

Koppen, W. “ Ueber Aenderungen der geographischen Breiten und des
Klimas in geologischer Zeit.” Stockholm, Geografiska Annaler, 2,
1920, 1 p. 285-99.

Brooks, C. E. P. “ Continentality and temperature.” Quarterly Journal of
Royal Meteorological Society, Vol. 43, 1917, p. 169 ; and 44, 1918,
p. 253. [Influence of land and sea distribution.]

Enquist, F. “ Eine Theorie über die Ursache der Eiszeit und die geographis-
chen Konsequenzen derselbe.” Buil. Geol. Inst., Upsala, 13, 1915,
No. 2. [Influence of land and 'sea distribution.]

Hobbs, W. H. “ Characteristics of existing glaciers.” New York, 1911.
[Glacial anticyclone.]

TABLE OF GEOLOGICAL FORMATIONS

Quaternary

Recent

Pleistocene

Pliocene

Tertiary or
Cajnozoic

Mesozoic or
Secomdary

Permian

Carboniferou8

Palasozoic

Devonian

Silurian

Ordovician

Cambrian

Proterozoic

Pre-Cambrian
  CHAPTER II

THE CLIMATIC RECORD AS A WHOLE

It is a remarkable fact that one of the oldest known
sedimentary rocks is glacial in origin, and indïcated the

presence of an ice-sheet at a very early stage in the earth’s
history. This is a “ tilKte,” or boulder-clay, discovered
1   ""af. Coleman at the base of the Lower Huronian

Proterozoic) of Canada. It extends in an east

and west direction for 1000 miles across northern
Ontario, and northward from the northern shore of
Lake Huron for 750 miles. It rests on a scratched or
polished surface of various rocks, and the included
boulders are not always local, but some have been brought
from a considerable distance. AH these characters point
to a large ice-sheet.

Tracés of Proterozoic glaciations have been discovered
in various other parts of the world, and some of these
may be of the same age as the Canadian ice-sheet, but
they cannot yet be dated exactly. An interesting
example is western Scotland, which J. Geikie considered
to have been glaciated by ice from the north-west which
has since sunk into the North Atlantic. Other glacial
remains have no doubt been destroyed or deeply buried,
while some may stiH await discovery, and at present we
must be content to note the occurrence of a glacial
period at this time without attempting any description
of the distribution of climates over the globe. ‘

FoHowed a long period of milder climate indicated in
America by thick deposits of limestone with the remains
of reef-buüding organisms and other marine Hfe. This

32
  CLIMATIC RECORD AS A WHOLE 33

period may have been interrupted at least once by the
recurrence of gladal conditions, but the evidence for this
is doubtful. It must be remembered that the duration
of the Proterozoic was very great, at least as long as all
subsequent time, while the relics of it which are now
known to us are few and scattered, so that much which
happened during that time is a closed book. It is not
until the very close of the Proterozoic that we again
find indisputable evidence of widespread gladal
action.

This second great glaciation was placed originally in
the earliest Cambrian (see table of geological formations
at the end of Chapter I), but later evidence shows that
it is slightly older than the oldest deposit which can be
referred to this series, and it may be designated the Pre-
Cambrian glaciation. Tillites of this age have been
found in the middle Yangtse region of China and in South
Australia (where they extend from 20 miles south of
Adelaide to 440 miles north, with an east-west extension
of 200 miles). Gladal deposits which probably refer to
this period have been found also in India, both in the
Deccan and near Simla, over a wide area in South Africa,
and in the extreme north of Norway. This distribution
suggests the presence of two centres of gladation, one
between China, India and Australia, and the other north-
west of Europe. The south-eastem of these was the
most extensive, and probably indicates a ring of gladated
continents surrounding the pole, rather than a single
enormous ice-sheet.

During the Cambrian all evidence of gladal action
ceases, and we have, instead, evidence of a warm, fairly
uniform climate in the abundant marine life. This
continued during the Ordovidan and became accentuated
during the Silurian period, when reef corals lived in the
seas of all parts of the world. Terrestrial deposits are
curiously lacking in all this series, and this suggests that
in the absence of any great mountaïn building and
elevation shallow seas extended over almost the whole
  34 THE EVOLUTION OF CLIMATE

of the surface, accompaniè'd by mild oceanic chmates
extending to high latitudes.

At the close of the Silurian there was a period of
mountain building and the formation of continents.
The extinction of numerous species of marine organisms
and the rapid evolution of others point to the seas
becoming cooler and the stress of life more acute. In
the succeeding Devonian period there is evidence of
glacial conditions in South Africa in the form of a thick
series of quartzites with striated pebbles up to fifteen
inches long, but no typical boulder-clay has been dis-
covered. There are also some doubtful tracés from
England. The most noteworthy development of the
Devonian in the British Isles is, however, a thick deposit
of red sandstone (Old Red Sandstone) of the type that
is formed in shallow lagoons or inclosed basins, and
suggesting desert conditions, so that the rainfall of the
British Isles was probably slight.

These Continental conditions passed away towards the
close of the Devonian period, and once again extensive
warm oceans appear to have spread over a large part of
the globe, associated with the development of reef-
building corals. Climate continued warm and equable
throughout the greater part of the Carboniferous.
The important feature of this period is the great System
of coal-beds which extends through North America and
Europe to China, with northem and Southern limits
in 8o° N. (north-east Greenland and Spitzbergen) and
150 S. (Zambesi River). Wegener, summing up the
evidence, and considering especially the absence of
annual rings in the trees, concludes that the coal-beds are
the remains of the tropical rain-forest when the equator
lay across Europe some 30 degrees north of its present
position.

Towards the close of the Carboniferous period great
mountain-building set in, resulting in the formation of
the famous Gondwanaland, including south and central
Africa, Southern Asia, part of Australia and possibly
  CLIMATIC RECORD AS A WHOLE 35

Brazil. From a consideration of the glacial evidence,
however, it appears, as will be seen later, that this was
probably a ring of neighbouring and partly adjoining
land areas rather than a single enormous continent.
At the same time the climate became cooler, and a hardier
vegetation, known as the Glossopteris flora, developed in
the Southern hemisphere, including woody trees with
annual rings indicating seasons. The large insects of
the coal forests which did not undergo a metamorphosis
were replaced by smaller types which did pass through
such a stage ; this change of habit is considered to be
due to the winters having become severe, so that the
insects learnt to hibernate through them. In the early
Permian, Gondwanaland was occupied by great ice-
sheets, remains of which in the form of tiUites of great
thickness, ice-wom surfaces and striated boulders have
been found in South Africa, Belgian Congo, and Togo-
land, Tasmania and widely separated parts of Australia,
peninsular and north-westem India, and probably also
Afghanistan. In India the glacial striae show that the
ice-sheet was moving north, while in South Africa it was
moving south, i.e. away from the present equator in
both cases. Widespread glacial remains have been found
also in Brazil, northem Argentine and the Falkland
Islands, and there are probable tracés near Boston in
North America, in Armenia, the Urals and the Alps, and
possibly also in England.

Wegener’s reconstruction of the geography of this
period places the south pole a little to the south-east of
South Africa, surrounded by a great continent composed
of the junction of Africa, South America, Antarctica,
Australia, and an extended Deccan added to by smooth-
ing out the folds of the Himalayas. This great circum-
polar continent he considers to have been the site of an
immense ice-cap. The North Pole lay in the Pacific
Ocean, so that almost all the remaining land areas enjoyed
temperate or tropical climates.

It is admitted that this peculiar distribution of glacial
  36   THE EVOLUTION OF CLIMATE

remains apparently necessitates a position of the pole
somewhere near that described by Wegener, but the
theory of a single polar ice-cap extending beyond 50°
latitude on nearly all sides presents difficulties. From
the mechanism of the supply of snow to an ice-sheet
described in the precedlng chapter it follows that,
except close to the edges, all the moisture precipitated
must be brought in by upper currents. But even if we
take into account the increase in the strength of the
atmospheric circulation due to the introduction of an
ice-cap, there is a limit to the supply of moisture by this
process. All such mbisture has to cross the periphery,
and with increasing radius; the number of square miles
of area to each mile of periphery becomes greater, slowly
at first, then more and more rapidly. We shall see in
Chapter VIII that even the North American Quaternary
ice-sheet became unwieldy from this cause, and sufïered
several changes of centre.

Hence it seems that the rapprochement of the continents
in Permo-Carboniferous times need not have been so
complete as Wegener supposes, the glacial phenomena
being more readily explicable by a ring of continents
surrounding a polar sea, as in the case of the Quaternary
glaciation of the northern hemisphere. The local
Permian glaciations of Europe and North America, some
of which feil close to Wegener’s equator, are easily ex-
plicable as due to mountain glaciers similar to those of
Ruwenzori and other tropical mountains during the
Quaternary. There were interglacial periods in South
Africa, Brazil and New South Wales, which increase the
resemblance between the Permian and Quaternary Ice
Ages.

In Upper Permian times there was a widespread
development of arid climates, especially in the present
temperate parts of North America and Europe.
Wegener attributes this to the northerly position of the
equator bringing the sub-trppical desert belt (Sahara,
Arizona) to these regions. In the Trias these conditions
  CLIMATIC RECORD AS A WHOLE 37

gradually gave place to another period of widespread
warm shallow seas, with abundant marine life and corals
extending over a large part of the world, even to Arctic
Alaska. In the United States there are the remains of
the forests of this period, in which the tree-trunks show
very little evidence of annual rings, indicating that the
seasonal changes were slight, so that the elfmate had
again become mild and oceanic.

In the Lias (Lower Jurassic), there was crustal move-
ment and volcanic action accompanied by land-formation
and a gradual lowering of temperature. There yvas a
great reduction in the abundance and geographical range
of corals, and most of the species of insects are of dwarf
types. There is, however, no evidence of glacial action.

The Upper Jurassic period appears to have been
warmer than the Lias. Insects of a large size and corals
again attained a very wide distribution, but there is
enough difference in the marine faunas of different
regions to indicate a greater development of climatic
zones than in the extremely oceanic periods such as the
late Triassic. Schuchert points out that the plants of
Louis Philippe Land in 63° S. are the same, even to
species, as those of Yorkshire.

In the Cretaceous period the elfmate was at first
similar to that of the Jurassic, and trees grew in Alaska,
Greenland and Spitzbergen. These trees, however, show
marked annual rings, indicating a considerable differentia-
tion of seasons, while trees of this age found in Egypt are
devoid of rings. Towards the close of the Cretaceous
there were many crustal movements and great volcanic
outbursts, accompanied by a considerable reduction of
temperature, which led to the extinction of many forms
of life and the rapid evolution of others. There is no
evidence of glacial action during the Cretaceous, however,
though at the beginning of the Eocéne there was a local
glaciation of the San Juan Mountains of Colorado.
According to W. W. Atwood this glaciation was doublé,
the first stage being of the Alpine mountain glacier type,
  38 THE EVOLUTION OF CLIMATE

separated by an interglacial from the second stage, which
was of the Piedmont type (mountain glaciers spreading
out on the plain at the foot of the mountain). This
Eocene gladation has been found nowhere else, however,
and the climate of the Tertiary, which is discussed more
fully in the next chapter, was in general warm and
oceanic, becoming rigorous towards its close.

Summing up, we find that in the geological history of
the earth two main types of climate seem to have alter-
nated. Following on periods of great crustal movement,
and the formation of large land areas, the general climate
was cool, with a marked zonal distribution of temperature,
culminating during at least four periods in the develop-
ment of great sheets of inland ice. It is in such a period,
though, fortunately, not at its worst, that we are living
at present. During quiescent periods, on the other hand,
when these continents largely disappeared beneath the
sea, climate became mild and equable, and approached
uniformity over a great part of the world. At these
times, as soon as the surface water of the sea in high
latitudes began to cool, it sank to the bottom, and its
place was taken by warmer water from lower latitudes.
The oceanic circulation was very complete, but there
were practically no cold surface currents. Instead,
there was probably a general drift of the surface waters
from low to high latitudes (with an easterly trend owing
to rotation of the earth), and a return drift of cooled
water along the floor of the ocean. The formation of
sea-ice near the poles became impossible, while the wide-
spread distribution of marine life was facilitated.

The alternation of periods of crustal deformation with
periods of quiescence has frequently been noticed, and
has been termed the “ geological rhythm.” It may be
attributed to the gradual accumulation of small strains
during a quiescent period until the breaking point is
reached, when earth-movements take place until equi-
librium is restored, when the process is repeated.

The gradual erosion of the land by river and wave-
  CLIMATIC RECORD AS A WHOLE 39
.

[Prometheus]

action and the consequent shifting of the load provides
a certain amount of stress; but this is local, and calls for
local readjustments only. A more generally effective
agency may be the gradual slowing down of the earth’s
rotation under the influence of tidal friction. The
mechanism of this process was described by A. E. H.
Love (“ Nature,” 94, 1914, p. 254) : “ The surface of
the ocean, apart from waves and tides, is at any time a
figure of equilibrium answering to the speed of rotation
at the time, more oblate when the speed is greater, less
oblate when it is slower. Let us imagine that the litho-
sphere also is at some time a figure of equilibrium
answering to the speed of rotation at that time. If the
speed remained constant, the lithosphere would retain
this figure, and the matter within it would remain
always in the same configuration without having to
support any internal tangential stress. Now suppose
that the speed of rotation gradually diminishes. The
surface of the ocean will gradually become less and less
oblate. The lithosphere also will gradually become less
oblate, but not to such an extent as to make it a figure
of equilibrium answering to the diminished speed of
rotation, while the matter within it will get into a state
of gradually increasing internal tangential stress. The
effect on the distribution of land and water will be that
the depth of the ocean will gradually diminish in lower
latitudes and increase in higher latitudes, the latitudes
of no change being 350 ió' N. and S.

“ The internal tangential stress in the matter within the
lithosphere may increase so much that it can no longer
be supported. If this happens a series of local fractures
will take place, continuing until the lithosphere is again
adjusted much more nearly to a figure of equilibrium,
which will be less oblate than the original figure. The
effect on the distribution of land and water will be that
the depth of the ocean will increase rather rapidly and
spasmodically in lower latitudes and diminish in higher
latitudes.
  40

THE EV0LUTI0N OF CLIMATE

“ Accordingly, the kind of geological change which the
theory of tidal friction would lead us to expect is a sort
of rhythmic sequence, involving long periods of com-
parative quiescence, marked by what Suess calls ‘ positive
movements of the strand,’ in the higher latitudes, and
‘ negative movements ’ in the lower, alternating with
comparatively short periods of greater activity, marked
by rise of the land around the poles and subsidences in
the equatorial regions.”

The main periods of adjustment under this scheme
fall at the beginning and end of the Proterozoic, in the
Permo-Carboniferous and in the Quaternary. The two
latter at least were periods of great earth-movement,
while the two former were also Continental periods, since
the land masses were large and high enough to develop
ice-sheets.

The diflicult question raised by the low latitudes
in which the Pre-Cambrian and Permo-Carboniferous
glaciations were chiefly developed cannot yet be regarded
as solved, but the geological facts speak strongly in favour
of ice-sheets rather than mountain glaciers, and practi-
cally speaking it is meteorologically impossible for large
ice-sheets to extend to sea-level in the Tropics while the
rest of the world enjoys a temperate climate. The only
escape seems to be to assume a position of the South Pole
somewhere between Africa, India and Australia through-
out the whole of the Proterozoic and Palaeozoic periods.
On the other hand, from the Jurassic onwards, there is
no real support to the hypothesis that the positions of
the poles were other than they are now. Wegener’s
explanation of the Quaternary Ice Age we have seen to
be untenable. The period of transition appears to lie
in the later Permian and Triassic. The Proterozoic
and Permo-Carboniferous glacial periods were much less
defimte in the north than in the south-east; but such as
they were they appear to have been most severe in the
east of North America, where the ice was coming from
the north ; there are also some glacial tracés in Europe.
  CLIMATIC RECORD AS A WHOLE 41

This indicates that the position of the North Pole cannot
have been in the North Pacific Ocean, which is antipodal
to the South Indian Ocean. Hence it seems that what
we have to consider is not so much the wanderings of
the poles at large among the continents as the break-off
at the close of the Palaeozoic period of portions of the
Antarctic continent and their drift northwards towards
the equator. Without going into the mathematics of
the question, it seems just possible that the periodic
overloading of circumpolar continents by large ice-
masses could have this effect in the course of time,1 but
the suggestion is put forward tentatively for consideration
rather than as a definite hypothesis. We must be thank-
ful that in the next chapter we are on safer ground.

BIBLIOGRAPHY

Coleman, A. P. “ Climates and physical condition» of the early Pre-
Cambrian.” Geol. Mag. (6), Vol. t, 1914, p. 466.

Eckardt, W. R. “ Palaoklimatologie.” Sammlung Goschen, Leipzig, 1910.
Geikie, J. “ The Evolution of Climate.” Edinburgh, Scot. Geogr. Mag. (6),

1890, p. 57.

Grabau, A. W. “ Principles of Stratigraphy.” New York, 1913, pp. 74, et ieq.
Neumayr, M. “ Ueber klimatischen Zonen wahrend der Jura- und Kreide-
zeit.” Wien, Denkscbr. Ak. Sci., 47, 1883, p. 211.

Ramsay, W. “ Orogenesis und Klima.” Ofven af Fins ka Vetert! kaps. Soe.

Forb., 52, 1909-10, A, No. 11. Helsingfors, 1910.

Schuchert, C. “ Climates of Geologie Time, In: The climatic factor as
illustrated in arid America,” by Ellsworth Huntington. Washington,
1914, Pt. 2.

1 If the figure of the earth is adjusted to its speed of rotation before the
development of ice-sheets, the latter renders it too prolate, and there will
be a tendency for readjustment by the transference of mass towards the
equator.
  CHAPTER III

CONDITIONS BEFORE THE QUATERNARY ICE AGE

The third of the great periods into which the geological
record is divided is known as the Tertiary. Throughout
most of its length it appears to have been chafacterized
by remarkably mild and equable climatic condïtions
extending into comparatively high latitudes, so that the
west coast of Greenland, for instance, had a flora of
almost sub-tropical aspect. Since the plants in question
—chiefly palms and cycads—are not of identical species
with their present-day representatives, it is unsafe to
base numerical estimates of the temperature upon them,
but it is at least obvious that these regions were warmer
than they are at present.

Let us glance for'a moment at the geography of the
Tertiary period. The most noticeable point is a great
expanse of sea over south-eastern Europe, including the
Mediterranean countries, extending away over the
Black Sea and Caspian, and stretching in a great arm to
the Arctic Ocean, south of Novaya Zemlya. The
geology of the archipelago north of Canada is not yet
well known, but it seems probable that there was a con-
siderable area of Tertiary ocean there also. The sea
further encroached on the present boundaries of North
America, both east and west, and on the north-eastern
coast of Asia. Bearing in mind the principles set out in
the first chapter, we can infer from these changes a great
increase in the winter temperature of-the regions along
the Arctic circle. The increase reached a maximum
on the west of Greenland and in western Siberia, but
42
  BEFORE THE QUATERNARY ICE AGE 43

the west coast of Alaska also had a decidedly warm
climate in the late Miocene and Pliocene.

The basin of the Arctic Ocean, which already existed
at that stage, was raised to a temperature considerably
higher than the present by three great streams of warm
water flowing into it. If, as seems probable, the Bering
Strait was deeper, and the submarine ridge across the
North Atlantic less pronounced, the obstacles to the
outflow of cold water along the ocean floor were much
less than now. Finally, the winter temperatures of the
land masses to the south, and especially Siberia, being
already very much less severe owing to the sea over
Europe, the temperature of the water of the great rivers
flowing into the Arctic was not so low. For these reasons
the development of ice in the Arctic Ocean was very
much diminished, and possibly entirely absent, allowing
a great amelioration of the climate of Greenland, the
rigor of which is at present much enhanced by the ice
which flows down the Greenland Sea and round Cape
Farewell.

The cumulative effect of all these changes—greater
water area, greater inflow of warm surface water, less
inflow of cold river water, less ice-development—must
have been a mild equable rainy climate, entirely suitable
to a rich vegetation. The sub-tropical aspect of that
vegetation should not be stressed, for it was probably as
much an expression of the geological age of the period in
question as of its climate.

The objection may be raised that at the present time
the sub-antarctic islands in the great Southern Ocean
have the most maritime climate in the world, but are
not by any means places of opulent vegetation. The
difference is entirely accountea for by the presence of
the great ice-bearing Antarctic continent. lts effect
is twofold. Firstly, the glaciers shed into the Southern
Ocean an immense quantity of ice and ice-cold water
annually, which must have an appreciable effect on
temperature. Secondly, the presence of this ice-covered
4
  44 THE EVOLUTION OF CLIMATE

continent and the floating ice in its neighbourhood
ex ten ding as far as the sixtieth parallel, by forming a
marked contrast with the warmer waters further north,
greatly intensifies the strength of the atmospheric
circulation in these regions, resulting in the development
of a great succession of severe storms which sweep the
sub-antarctic islands. There are no great land-masses
to break the force of the wind, and these latitudes are
among the stormiest, windiest regions of the earth—gale
succeeding gale, winter and summer alike; and it is
largely to the extraordinary power of the wind that we
must attribute the desolate appearance of the islands.

The picture we have drawn of the high northern
latitudes in early Tertiary times is vastly different. A
great warm ocean occupied the Arctic regions, fed by
three ocean currents analogous to the Gulf Drift, and
the fall of temperature was gradual from the tropic to
the pole. The return colder currents were mainly along
the ocean floor and with little ice-formation the storms
were few and not severe. On the western shores of the
continents mild rain-bearing south-west winds prevailed,
and a quiet moist warm atmosphere existed which was
especially favourable to plant life. This favourable
state of affairs lasted until well on in the Miocene, and
then changes set in. The land and sea distribution
underwent essential modifications. The great Tertiary
continent or archipelago which is believed to have
existed in the western Pacific, and whose last remaining
summits now form the scattered islands of that ocean,
gradually subsided, and in its place elevation began in
higher latitudes. Bering Strait became narrow and
shallow, and was probably for a time entirely closed, while
the connexion between the Arctic and Indian Oceans
was closed permanently, leaving in its lowest areas a chain
of great inland seas and lakes, of which the Caspian and
Aral Seas are now the greatest representatives. The
Canadian Archipelago was probably raised above its
present level, and formed a great northern extension of
  BEFORE THE QUATERNARY ICE AGE 45

the American Continental area. The changes in the
Atlantic also were very extensive. The West Indies
were the site of a large and lofty Antillean continent;
further north a considerable land-mass existed east of
Newfoundland; Greenland was joined on the west to the
extended American continent, and considerably enlarged
to the south-east. Iceland, though it remained an
island, was elevated and probably neariy doubled in area,
and between Iceland and the north of Scotland was
developed a great submarine ridge, which may or may
not have risen above the sea in places. The British
Isles became a solid block of land, united with Continental
Europe across the English Channel and the great plain
which is now the North Sea. Scandinavia was elevated
by more than a thousand feet, and the elevation extended
at least as far as Spitzbergen. The Murman area had a
considerable extension. In eastern Asia the Sea of
Okhotsk was land and Japan was united to the mainland.

In the Southern hemisphere our knowledge is not
neariy so detailed. The presence of marine Middle-
Tertiary beds with temperate mollusca in Graham Land
and of plant-bearing beds in Seymour Island point to a
smaller Antarctic continent and very much warmer con-
ditions at this time in the South as well as in the North
Polar regions. For the close of the Tertiary, however,
we have strong grounds in the distribution of animals
and plants for assuming that the Antarctic continent was
greatly increased in size, with promontories uniting it to
Australia on the one hand and to South America on the
other. New Zealand was largely increased in area, and
South Africa probably extended further polewards.
The sub-antarctic islands attained a much greater area.
Conditions were ripe for the Ice Age in the Southern as
well as the northern hemisphere.

BIBLIO GRAPHY

Kerner von Marilaun, F. “ Synthese der morphogenen Winterklimate
Europas zur Tertiarzeit.” Wien, SitxBer, K. Akad. H tss, 12,2, 191 j,
pp. 233-98-
  46   THE EVOLUTION OF CLIMATE

Oibom, H. F. “ The age of mammals in Enrope, Aria and North America.”
8vo. New York, 1910.

Nathorst, A. G. “ On the fossil floras of the Arctic regions as eridence of
geological climates.” London, Bot. J. 2, 1913, pp. 197-202; and
Washington, Report Smithsonian Inst., 1911.

Dall, W. H. “ On climatic conditions at Nome, Alaska, during the Pliocene.”
Amer. J. Sci., ser. 4, Vol. 23, 1907, p. 457.

Nansen, F. “ The bathymetrical features of the North Polar seas, with a
discussion of the Continental shelves and previous oscillations of the
shore-line.” Norviegian N. Polar Exped., 1893-96. Scientific Results,
Vol. 4.

Spencer, J. W. “ Reconstruction of the Antillean continent.” Buil. Amer.
Geol. Soc., 6, 1895, pp. 103-40.

Wilckens, D. “ Die Mollusken der antarktischen Tertiarformation.” Wiss.

Ergebn. der Scbteed. Sudpolar Exped., 1901-3, Bd. 3, 1911.

Hedley, C. “ The palaeographical relations of Antarctica.” London, Proc.
Ltnneean Soc., 124, 1911-2, p. 80.
  CHAPTER IV

THE GREAT ICE AGE

As the land began to rise, the first effect was an increased
snowfall on the higher summits, and increased rainfall
on the rising coast lands. The rivers had an increasing
fall towards the sea, and rapidly carved out deep narrow
valleys, which were later developed by the ice into the
great fiords of Norway and other heavily glaciated
regions. But on the whole the first beginnings of the
Ice Age occurring towards the close of the Pliocene
period are obscure, and are likely always to remain so,
for the simple reason that the advancing and retreating
ice-sheets have wiped out most of the evidence of the
conditions which immediately preceded their advent.
The deteriorations of the climate had begun long before
the geographical changes outlined at the close of the
last chapter were complete, for mollusca of the Pliocene
beds in East Anglia indicate progressive refrigeration of
the North Sea at the same time as it became increasingly
shallow. At the close we have great shell-banks with
northern species which must have been piled up by
powerful easterly winds; these easterly winds show that
the storm tracks had been driven south of their present
course and suggest that the glacial anticyclone already
existed over Scandinavia. At the present day similar
shell-banks are forming on the coast of Holland under
the influence of the prevailing westerly winds. The
next series of deposits in this region are fresh-water
“ forest beds,” attributed to a greatly extended Rhine,
47
  48 THE EVOLUTION OF CLIMATE

and belong to the period when the North Sea had become
a plain.

It is no part of the plan of this work to go over the
geological ground of the Quaternary Ice Age, which has
already been so frequently and so efficiently covered.
All I can hope to do is to give a brief general account of
the succession of climatic changes involved, necessarily
incomplete since so much of the world is at present
insufficiently explored for glacial tracés. But a certain
amount of explanatory introduction is necessary.

In Europe and North America there are distinct
tracés of several separate glaciations with “ interglacial ”
periods when the climate approached or became even
warmer than the present. The time-relations of these
glaciations are not yet fully worked out, but there seems
Httle doubt that they were contemporaneous in the two
continents. The correlation is not perfect, however,
siricF the United States geologists recognized altogether
five glaciations. The explanation appears to belfhat the
equivalent of the Rissian glaciation in Americais doublé;
two stages, the'Illinoian md.Iowan, being recognizable,
separated by a retreatoFthe ice. The series is as follows :

Alp..   Northern Europe.   North America.   Date. B.C.
I Gunz   ?   Jerseyan or  Nebraskan   i
II Mindel   Lower Diluvium   Kansan   430,000-370,000
III Rits   Middle Diluvium   (Illinoian) jlowan )   130,000-100,000
IV Wurm   Upper Diluvium   Wi.consin   40,000- 18,000

The correlation is based on the amount of weathering
and erosion which the various deposits have undergone.
The time which has elapsed since the glaciers of the last
or Wurm stage were in active retreat has been estimated
by comparing the growth of peat-bogs, river-deltas, etc.,
  THE GREAT ICE AGE   49

during historical times with that since the last
retreat of the ice. But the most conclusive method is
due to the Swedish geologist Baron de Geer, who has
actually counted the years since the ice in its final retreat
left any particular point between Ragunda and the south
of Sweden. The work is based on the idea that the
lamination observed in certain marine and lacustrine
clays in Sweden is seasonal, the thick coarse layers being
due to the floods produced by the rapid melting of the
ice in summer, and the thinner and finer layers being
due to the partial cessation of melting in winter. By
correlating one section with another it is possible to date
any particular layer with great exactness, and further
to prove the existence of several great climatic fluctua-
tions during the retreat. The topmost limit of the
section is given by the surface of the old floor of Lake
Ragunda in Jemtland, which received its waters from
one of the permanent glaciers and was accidentally
drained in 1796. De Geer finds that the edge of the last
great^ce-sheets lay over Southern Scania about 12,000
years ago, and further estimates 8000 years for the retreat
across the Baltic. These results are in general agreement
with those obtained by other methods, and we may
accordingly, with some confidence, put the date when
the ice-sheet of the Wurm glaciation finally left the coast
of Germany at about 18000 b.c.

[Prometheus]

This period fixed, we have a datum ..for estimating the
duration of the interglacial periods.C_The moraines of
the Wurm glaciation present everywhere a very fresh
appearance, and the chemical change which the boulders
they contain have undergone is slight, while weathering
extends to a depth of scarcely a foot. The moraines of
the Riss glaciation are weathered somewhat more deeply,
and "those of the Mindel glaciation very much more.
Assuming that chemical weathering has proceeded
uniformly during the interglacial periods and ceased
during the glaciations, Penck and Brückner* who.. have
studied exhaustively the glaciation of the Alps, find that
  50 THE EV0LUTI0N OF CLIMATE

the Riss-Wurm interglacial lasted about three times as
long as the interval between the Wurm gladation and
the présent day, or 60,000 years, and the Mindel-Riss
interglacial about twelve times as long, or 240,000
years. No data are available for the Gunz-Mindel
interglacial, but it is provisionally made equal to the
Riss-Wurm, another 60,000 years.

No possibility of such direct measurement of the
duration of the glacial periods themselves has yet been
found. Penck and Brückner assume that the duration
equalled that of the Riss-Wurm interglacial, or 60,000
years in each case. This seems unnecessarily long.
The Yoldia Sea, the deepest part of which coincided
with the centre of the Scandinavian glaciation, appears
to have reached its greatest depth not more than 6000
years after the maximum of glaciation, indicating a lag
of this period. The subsidence of the land due to the
weight of the ice-sheet may have commenced some time
before the maximum of glaciation, but the duration of
the subsidence can hardly have been more than 10,000
years, and this is the limit for the second half of the Wurm
glacial period. Further, we know that during the growth
of the ice-sheets there was comparatively little melting,
for the rivers then had little power of carrying debris.
Recent measurements in Greenland give the rate of ice-
growth on the surface of the ice-sheet as 40 cm., or j.5
inches ,a_year; let us say a foot, and assume a marginal
loss equivalent to half this amount over the whole ice-
sheet. This gives us an average increase of six inches
a year, or 10,000 years for growth to a maximum thick-
ness of 5000 feet. On these grounds the estimated
duration of the Rissian glacial period has been reduced
to 30,000 years, and that of the Wurm period to 22,000
years. Only in the case of the long and complicated
Mindelian period, which, as will be seen later, was virtu-
ally a series of overlapping glaciations from various centres,
has the figure of 60,000 years been accepted. In the
present state of our knowledge no estimate of the
  THE GREAT ICE AGE   51

duration of the Gunz-Mindel interglacial can have any
value, and the dates are accordingly carried back only
to the Mindelian. In this way we obtain the time-scale
given on page 48.

The fourfold gladation has been recognized with
certainty only in Europe and North America, and even
in these countries there is considerable doubt whether
the northem ice-sheets shrank back as far as their present
narrow limits during the interglacial periods. The
long Mindel-Riss interglacial, which was probably the
Chellean stage of flint industry,1 was characterized by a
very warmth-loving fauna, and it is possible, even
probable, that during this period the glaciers melted
completely away, except on the very highest summits.
Of the climate of the Gunz-Mindel interglacial (termed
by J. Geikie the “ Norfolkian,” from the Cromer
Forest Bed), we have comparatively little evidence.
If the suggestion put forward in the following chapter
is correct, the Gunz-Mindel interglacial was merely a
local incident in the glaciation of the Alps, and not a
true interglacial at all. Even the Cromer Forest Bed
itself is not conclusive, since it is a river deposit largely
composed of material drifted from lower latitudes.
The Riss-Wurm interglacial (J. Geikie’s “ Neudeckian ”)
nowhere gives us evidence of a climate as warm as the
present, and as regards the Scandinavian and Canadian
ice-sheets may have been merely an extensive and pro-
longed oscillation of the ice-edge.

As regards the glaciation of Norway, the question has
been investigated recently by H. W. son Ahlmann, who
has published a long and detailed paper in English in
volumes 1 and 2 of the Swedish Geographiska Annalen.
He concludes that the morphology of Norway, without
reference to stratigraphical or biological data, gives
conclusive evidence of two glaciations. The first of
these was the greater, and between that time and the

1 This hai been the subject of much discussion recently. For a summary
see Science Progress, 17, 192a, October, p. 233.
  52 THE EVOLUTION OF CLIMATE

second smaller glaciations there occurred an interglacial
period of such considerable length that the greater
part of the present gorges was then formed by fluvial
erosion.

We may, accordingly, consider the Ice Age as fourfold
or doublé, according to the point of view from which we
regard it. In the Alps and other mountain ranges on
the borders of the great northern ice-sheets, which
respond very readily to small changes, it was fourfold.
In the peripheral regions of the northern ice-sheets
themselves it has an appearance of being threefold or
fourfold. In the more central regions of these great
ice-sheets, where response to climatic change is very
slow, there is no evidence of more than two glacia-
tions; but in these regions, where the destructive
effect of the ice reached its maximum, it is only by
the merest chance that evidence of interglacial periods
is preserved at all. And finally, in all other parts of
the world we have evidence of only two glaciations
at most.

. There is one deposit which is of considerable im-
pört'ance in the study of interglaciaTclimates, and that
is the loess. Loess is an exceedingly fine-grained hömo-
geneous deposit resulting from the gradual accumulation
of wind-blown dust on a surface sparsely covered with
vegetation. It is to be seen accumulating at the present
day in parts of south-east Russia and central Asia.
lts formation, except in closed basins, needs a climate
of the steppe character, with not much rainfall, and
especially with a long dry season. Now loess was very
extensively developed in Europe during the Qüaternary.
lts occurrence is peculiar, since it is found most widely
developed resting on the deposits of the Rissian glaciation,
and is never found resting on the moraines of the Wurm
glaciation. A little loess is found below the Riss
moraines, and it has also been found between the Riss and
Wurm moraines. In the pre-Rissian loess an implement
of Acheulian age was discovered in 1910 at Achenheim
  THE GREAT ICE AGE   53

(Alsace), by R. R. Schmidt and P. Wernert, indicating
that the deposit was formed towards the close of the
Chellean industry, when the climate was already cold and
dry. In the same section the younger loess seems to
fill completely the Riss-Wurm interglacial, since Mous-
terian implements were found at the base and Aurignacian
implements in the middle. The younger loess contains
remains of the jerboa and other rodents at present
inhabiting the Siberian steppes. It is therefore reason-
able to conclude that steppe conditions prevailed in
central Europe through practically the whole of the
Riss-Wurm interglacial, and the same probably applies
to the corresponding pre-Wisconsin interglacial in
America. But if a steppe climate prevailed in central
Germany there must have been very severe conditions
in Scandinavia, and probably the ice-sheet maintained
a quite considerable area there throughout the whole
period, though without encroaching on the Baltic basin.
In North America the loess was deposited by westerly
winds, indicating that the ice-development was not
sufficiënt to impose anticyclonic conditions in place of
the present prevailing westerly winds, and the same
appears to be true of Europe. Similar climatic con-
ditions were developed for a short time at the close of
the Wurm glaciation, but without any appreciable
development of loess. (See Chapter XIII.)

BIBLIO GRAPHY

Wright, W. B. “ The Quaternary Ice-age.” London, Macmillan,
I9H-

Brooks, C. E. P. “ The correlation of the Quaternary deposits of the British
Isles with those of the continent of Europe.” Ann. Rep. Smitbsonia*
Inst., 1917, pp. 277-375.

de Geer, G. “ A thermographical record of the late Quaternary climate.”
Ber. Internat. Gcologcnkongr., Stockholm, 1910. “ Die Veranderingen
des Klimas,” p. 303.

---------------. “ A geochronology of the last 12,000 years.” Ber

Internat. Geologenkongr., Stockholm, 1910, Vol. 1, p. 241.
  54 THE EVOLUTION OF CLIMATE

Penck, A., and Brückner, E. “ Die Alpen in Eiszeitalter.” Leipzig, 3 Vols.,-
1901-9.

Ahlmann, H. W., son. “ Geomorphological studies in Norway.” Stockholm,-
Geografiska Annaler, i, 1919, pp. 1-157, 193-252.

Richthofen, F. “On the mode of origin of the loess.” Geol. Mag.,
1882, p. 293.
  CHAPTER V

THE GLACIAL HISTORY OF NORTHERN AND CENTRAL EUROPE

The literature of the glacial period in Europe is stu-
pendous and is, further, of a highly contradictory nature.
Space does not permit of any summary of the great
conflict between the monoglacialists and the poly-
glacialists; it is sufficiënt to say that the latter often
went to extremes and so laid themselves open to defeat,
but the twofold nature of the glaciation is now widely
accepted. It must be understood, however, that the
following summary represents the views of a certain
section of geologists only, views which are not universaUy
held. In the British Isles especially, where the remains
of the maximum glaciation completely dominate those
of all the others, the theory of a single glaciation still
largely prevails.

When ice began to accumulate on the rising Scandi-
navian plateau it naturally formed at first on the
Norwegian mountains near the Atlantic, which was the
chief source of snowfall. These mountain glaciers
spread rapidly down the steep seaward slopes to the
west and more slowly down the gentler landward slopes
to the east. At this stage the centre of the ice-sheet,
and consequently the centre of the glacial anticyclone,
as soon as the latter developed a definite existence, lay
quite near the Norwegian coast. Under anticyclonic
conditions the circulation of the winds round the centre
is in the same direction as the motion of the hands of a
watch, combined with an outward inclination at an angle
of about thirty to forty-five degrees. Consequently,
while the centre lay in Norway, due north of the Alps,
35
  56   THE EVOLUTION OF CLIMATE

the prevailing winds in the latter must have been from
north-east, and therefore very cold. Accordingly, this
stage is probably contemporaneous with the Gunz
glaciation of the Alps. In the same way, over the
North Sea area the winds must have been easterly,
causing the currents which piled up the great sheÜ-
banks of the East Anglian coast, already referred to as
marking the end of the Tertiary and beginning of the
Quaternary period.

But the ice which reached the northern North Sea
broke up into icebergs not far from the coast, and floated
away, while that which moved east into the north of
Sweden could only be dissipated by melting and ablation,
processes which we have reason to believe went on very
slowly. Hence ice began to accumulate and spread over
a wide area east of the main Scandinavian mountain
chain. Fresh snow was deposited directly on this ice-
surface, until it gradually overtopped the mountains
which originally gave rise to it, and reversed the flow,
so that the ice actually moved uphill across the mountain
chain. As the centre of the ice-sheet moved eastward
the glacial anticyclone moved with it, and this new
position to the eastward caused an alteration in the
direction of the prevailing winds over the rest of Europe.
The Alps were now south-west of the anticyclonic
centre, and the winds in that district accordingly became
easterly instead of north-easterly. Of course, the
glacial anticyclone was now more intense, but in summer
in central Europe easterly winds are naturally so much
warmer than north-easterly winds that at first this in-
crease in intensity was not enough to counterbalance
the change in direction, and there was a slight improve-
ment in the Alpine climate. In the same way, over the
North Sea district the prevailing winds had now become
south-easterly instead of easterly, which would make
for a slight rise of temperature, as also would the
occasional depressions which would be able to make their
way in from the westward, bringing warm moist air from
  GLACIAL HISTORY OF EUROPE 57

the Atlantic and occasional rainfall. By this time the
process of elevation had converted the North Sea floor
into an extensive plain.

From Sweden and the Gulf of Bothnia the ice spread
out in all directions, extending in the east to the foot of
the Ural Mountains, which formed an independent
centre of glaciation; in the south-east over a large part
of European Russia, where it reached as far south as
latitude 40° in the Dnieper valley; in the south over
almost the whole of Germany as far as the Riesengebirge
and Harz Mountains; and in the south-east over the
whole of Holland and the North Sea basin. It should
be noted that Holland and Denmark were glaciated, not
by Norwegian ice, but by ice from the Baltic sheet which
had crossed Southern Sweden. The North Sea glacier
extended across East Anglia as far as Cambridge, while
a northem branch of it swept across Caithness and the
Orkney and Shetland Islands, but most of the British
Isles were glaciated from independent centres—the
Scottish Highlands, the Pennines, Cumberland, Wales
and northern Ireland.

With the growth of the glaciated area, and particularly
with its extension south-westward across the North Sea,
the Alpine climate again became very severe, and the
local glaciers and Piedmont ice-sheets of the Alps reached
their maximum development in the Mindelian. At
the same time the central plateau of France developed
a local plateau glacier of its own, and the Pyrenees
underwent their first and greatest glaciation, no tracés
of the Gunzian having been found in this range.

The British Isles show an interesting outward migration
of the local centres of maximum ice-development. The
Scandinavian glacier which invaded East Anglia extended
arctic anticyclonic conditions across the North Sea, and
induced a heavy snowfall over the high lands of Great
Britain. These, in consequence, developed independent
glaciers, which on their eastern sides fused with the
Scandinavian glacier and, partly by deflecting its flow,
  58 THE EVOLUTION OF CLIMATE

partly by intercepting some of its snowfall, pushed it
back into the North Sea plain. The Scottish glaciers
became strong enough to encroach on Ireland, partly in
the north-east, and partly by way of the Irish Sea and
St. George’s Channel (then a valley) on to the south-
east. This further extension of the cold area enabled
the Irish glaciers to develop, and these in turn pushed
back the Scottish glaciers until Ireland was solely
glaciated by Irish ice.

The Southern margin of the ice-sheet did not extend
beyond the Thames valley, but at some stage the English
Channel carried floating ice, which formed the deposits
of ice-borne boulders, of which that at Selsey is a well-
known example.

This great ice-sheet nowhere formed marked terminal
moraines, but its deposits fade away in thin beds of stiff
boulder-clay. This absence of moraines is probably
connected with the great thickness of .the ice-sheets,
which did not leave any appreciable nunataks or rocky
“ islands ” exposed in its path, so that there was nothing
to give rise to detritus on the surface of the ice. All the
transportation had to be carried on beneath the ice-
sheets, and these, penetrating into comparatively low
latitudes where the sun is powerful in summer, would
suffer gradual melting and ablation for some distance
from their margins. Near the actual ice-limit the
motion must have been slow and the thickness of the ice
small, so that conditions were against the accumulation
of thick beds of detritus.

On the borders of the ice-sheet the climate cannot
have been over-rigorous, for pre-Chellean man was able
to live almost up to the ice-edge. The air must have
been extremely cold, and there was a belt of high arctic
climate round the ice, but in the south and south-west
this appears to have been very narrow, and sub-arctic
conditions, no worse than those in which many races live
to-day, prevailed not very far from the ice. The con-
figuration of the ice-surface largely explains this. A
  GLACIAL HISTORY OF EUROPE 59

high steeply sloping wall of ice causes intensely violent
winds, carrying dense clouds of drift-snow—blizzards, in
fact, similar to those now experienced in parts of Antarc-
tica under similar circumstances, which sweep the land
bare of all life for a considerable distance. But a low
and gradually sloping surface, such as seems to have
existed near the borders of the maximum glaciation,
favours instead comparatively gentle winds without much
drift snow. It is only on the north-west ice-ridge,
where ice-cliffs fronted the sea and where severe storms
from the Atlantic were frequent in winter, that blizzards
occurred.

When the land in Scandinavia began to sink under the
ice-load more rapidly than the supply of snow could
build up the surface of the ice-sheet the force which
pushed out the ice in all directions from the centre
gradually died away, and the ice-masses over the North
Sea area—now probably again below sea-level—and
the low grounds of Europe were left derelict, with no
resources but the snowfall on their own surfaces. Under
these conditions they melted away more or less rapidly.
While these derelict ice-masses were still large, the
auxiliary peripheral centres in the Alps, Pyrenees and
British Isles maintained an independent existence for a
while, probably with fluctuations similar to those which
markecf the close of the last glaciation in the Alps, though
the evidence of these has now been wiped away. It is
even likely that the beginnings of the weakening of the
central source of supply helped the British ice to divert
the Scandinavian ice into the North Sea. Had there
been any powerful rivers bearing great masses of detritus
from.the south, as there are in Siberia, some of these
derelict ice-sheets might have been preserved for a time,
at least, as “ fossil ice,” but in western Europe conditions
were not favourable for this.

With the disappearance of the ice-sheets the general
climate of Europe must have passed through a series of
stages of amelioration, of which tracés can be found here
5

[Prometheus]

  6o THE EVOLUTION OF CLIMATE

and there, though the details are lost to us. Ültimately
temperate conditions again prevailed; and for a veiy long
time, approaching a quarter of a million years, Europe
cannot have differed greatly from present climatic
conditions. In Scandinavia the mammoth roamed in
forests of birch, pine and spruce ; further south the
mammoth is absent, and we find instead more Southern
forms—Elephas antiquus, resembling the Indian elephant,
Rhinoceros merckii, a Southern form, the sabre-toothed
tiger, cave-lion, cave-bear and cave-hyaena, wolf, beaver,
horse and various forms of deer, while the flora included
even such warmth-loving trees as the fig. Obviously,
during part of this interglacial period, the climate
must have been even warmer than the present.

Let us glance for a moment at the probable con-
ditions. One of the dominant features in the present
weather of Europe is the accumulation of floating ice
in the Arctic basin. This keeps the temperature low
and the pressure high—forms in fact during the spring
and summer months a temporary glacial anticyclone
similar in kind to, though of less intensity than, that
which has been described as covering the Scandinavian
ice-sheet. This anticyclone maintains on its Southern
edges a belt of easterly winds, and these winds enter into
the general circulation of the earth. Their effect is to
push southward the permanent storm-centres normally
situated near Iceland and the Aleutian Islands, and it
is these storm-centres which play a large part in causing
the rainy weather of northern and central Europe.
But occasionally—as in the remarkable spring and summer
of 1921—these conditions break down. The Arctic
Ocean becomes unusually ice-free and warm, the pressure
falls, and in consequence the storm-centres move
northward. Europe comes under the influence of the
permanent anticyclones of sub-tropical latitudes, rain-
bearing storms pass far to the northward, and we have a
dry warm summer of the Mediterranean type.

This is presumably what happened during the long
  GLACIAL HISTORY OF EUROPE 61

warm Mindel-Riss interglacial. For some reason, possibly
connected with a temporary widening and deepening
of the Bering Strait, the waters of the Arctic Ocean
became warmer and the amount of floating ice less.
Pressure became lower in the polar basin and therefore
higher over' the Atlantic and Europe, and fine warm
conditions prevailed in Europe as the normal climate
instead of only as an occasional event.

This warm interval was finally brought to a close by
the renewed elevation of Scandinavia, .and the ice-sheets
began to develop again, heralded by a period of dry
steppe climate. This time, however, the conditions
were different; the elevation was not so great, and was
more local. Hence the resulting glaciation was less
intense ; it filled the Baltic basin and extended some
distance on to the North German plain and into Holland.
It failed to reach the coast of Britain, but that it extended
some way across the North Sea plain is indicated by the
peculiar distribution of the Newer Drift of Britain, to
be referred to later. In the north of Norway the slope
of the ice towards the sea was very steep, so that many of
the Coastal hills extended above it as nunataks. The ice
extended into the channel between the mainland and the
Lofoten Islands (then a peninsula), but according to
Ahlmann these islands were an independent centre of
local glaciation, as the British Isles had been during the
preceding period, and the local ice met the main ice-
sheet in the fiords. On the coast of Nordland sufficiënt
land lay bare to harbour a small Arctic flora, and Vkero,
the southernmost island of Lofoten, had only small
hanging snow-banks.   1

The interpretation of the British glacial deposits is
still very much under discussion, but it seems probable
that the Scottish highlands formed a subsidiary centre
which glaciated the whole of Scotland and north-east
England, sending a stream south-eastward, which was
prevented from spreading across the North Sea plain by
the presence of Scandinavian ice to the east and impinged
  62 THE EVOLUTION OF CLIMATE

on the coast of Yorkshire and Lincolnshire, just reaching
the northern extremity of Norfolk. Many British
geologists regard this development as the concluding
phase of a single glaciation of Britain, but the differences
in the amount of weathering undergone are against such
an interpretation. At the same time there were local
glaciers in Cumberland, Wales and Ireland.

In England limits of this glaciation are characterized
by a well-marked series of end-moraines, which indicate
that the ice carried much surface detritus, and probably
ended in a steep clifiE. In Scandinavia, on the other hand,
the centre of glaciation again lay over the low ground
well to the east of the mountains, and the ice which
reached Germany and Denmark was still largely free of
surface detritus, and so did not form marked end-
moraines. There was a difference, however. On this
occasion, owing to the local nature of the elevation in
Scandinavia, the ice-sheet did not extend its borders so
far to the eastward, and the glaciation of Asia, as
described in Chapter VII, was slight. Europe came
more under the influence of cold north-easterly and
northerly winds, and life on the ice-borders was not so
easy as during the preceding glaciation. Man could still
live near the ice, but he took to makinghis home in caves,
and to clothing himself in skins for warmth.

After the ice had reached its Rissian maximum of
glaciation the climate improved somewhat. The ice-
edge retreated, leaving Denmark and the German coast,
and vacating the Baltic basin, but not disappearing
altogether from Scandinavia. At Rixdorf, near Berlin,
there is a bed of gravel deposited in this “ interglacial,”
containing numerous and well-preserved bones of the
mammoth, woolly rhinoceros, aurochs, bison, horse,
reindeer, red deer and other species of Cervus, musk ox
and wolf—a cold temperate to sub-arctic fauna. In
south Germany fresh water mollusca indicate that the
summers in that district were almost as |ivarm as at
present, but the winters were probably1 severe. As
  GLACIAL HISTORY OF EUROPE 63

described in the preceding chapter this “ interglacial ”
was the time of loess formation por excellence, with a
Continental climate and steppe conditions over much of
central Europe.

Investigations at Skserumhede in Denmark show that
this recession of the ice was accompanied by, and pre-
sumably due to, a fall in the level of the land relatively
to that of the sea, for at the beginning of the oscillation
the land lay about 350 feet above its present level, sink-
ing gradually to only 30 feet above present. Even at
its best during this interglacial the climate was almost
sub-arctic in Denmark. In northern Finland, on the
eastern edge of the ice-sheet, there was also an “ inter-
glacial,” with a slight improvement in the climate
accompanying a temporary submergence. But in
Scandinavia there are no tracés of any interglacial
deposits of this period, and considering the cold climates
which prevailed in Denmark and North Germany, it
seems probable that Scandinavia continued to be glaciated
during the whole period.

The mode of life among Mousterian men, who lived
during this “ interglacial,” also points to a severe
climate. For at this time man did not live in the open,
but in caves and rock-shelters, and the practice of wearing
the fur skins of animals as a protection against the cold,
begun in the preceding Rissian glacial period, was not
discontinued.

After the temporary subsidence had ceased, elevation
again set in, causing a readvance of the ice-sheet/s and
glaciers. The limits feil short of those of the precjeding
maximum, and the climate was not so severe, but;in its
general character it resembled that of the preceding
maximum, but was much stormier, and there were
probably frequent blizzards of the Antarctic type,
carrying drift-snow. The new ice-sheet carried more
surface detritus than its predecessors, presumably because
all the high ground was not covered, and it formed high
terminal moraines. The close association of cold ice
  64 THE EVOLUTION OF CLIMATE

and irregular masses of bare sand and stones, strongly
heated by the summer sun, set up a belt of powerful
conviction very favourable for the development of
blizzards; possibly there was something in the nature of
an ice-clifï down which the cold winds could blow wifh
great strength. At any rate, man found the near
neighbourhood of the ice unpleasant, and went, so that
there are no contemporaneous human implements near
the moraines. The lïmits of the Scandinavian ice-sheet
ran from the Norwegian coast across Denmark from north
to south, through North Germany and northern Russia,
and included Finland. The ice probably did not extend
far across the North Sea plain, and in the British Isles
there was no ice-sheet, but the high mountains of Scot-
land, Ireland, Wales and Cumberland bore small local
glaciers, which were long enough to reach the sea in the
Scottish highlands. The Alps bore considerable glaciers,
indicating a depression of the snow-line of about 3500
feet, corresponding to a temperature n° F. lower than
the present.

After this ice-development had reached its maximum
limits and remained there for perhaps a thousand years,
retreat set in, and the Scandinavian ice once more with-
drew from Germany and Denmark to the Baltic basin.
But its edge was never far from the German coast, and
occasionally readvanced across it, for numerous fos-
siliferous deposits are intercalated in boulder clay. The
fauna and flora, which are well known, point to an
arctic climate. At its best the mean temperature of
July rosé to about 50° F., and there was a vegetation
period of three or four months with an average
temperature of about 40° F., but these relatively mild
conditions lasted at most for a few decades or perhaps a
century at a time, and the winters were severe through-
out. The duration of the whole of this “ Baltic Inter-
stadial ” was from one to two thousand years.

Next followed the final readvance of the ice forming
the great “ Baltic ” moraines which fringe the Baltic
  GLACIAL HISTORY OF EUROPE 65

coast of Germany, turning northward in the west into
Denmark and in the east into Finland. There was a
correspondinj» redevelopment of glaciers in the Alps
(Bühl stage) and in the mountains of Ireland and Scot-
land, though these probably failed to reach the sea even
in Scotland. This period gave us a repetition of the
climate of the preceding maxima. In this case we have
definite evidence of the presence of a belt of easterly
winds on the Southern side of the ice-sheet, in a
series of “ barkans ” or fossil dunes in Holland,
Germany and Galicia. These dunes were formed
of fine ice-deposited material, and they are crescent-
shaped, with their convexities to the east, indicating
that they were built by strong easterly winds. A
moment’s consideration will show the truth of the
latter statement. Suppose there is an isolated round
hillock of sand exposed to strong easterly winds. The
sand grains will travel up the easterly windward slope
of the hillock and roll down the westerly leeward side.
In this way the whole hillock will advance very slowly
westwards. But in the centre, where the hillock reaches
its greatest height, the grains will take longer to reach
the highest point than near the edges, where they have
not to rise so high. At the edges a strong gust will
carry some of the heavier grains right over the dune,
while nearer the centre they will be left half-way, and
when the gust ceases will perhaps roll back to their
original position. In this way the margins of the /dune
will advance westward more rapidly than the c/entre,
producing the crescent shape with the convex side to
the east. At the time of their formation these dunes
must have had their steepest side to the westward, but
the westerly winds which have prevailed during the last
few thousand years have succeeded in modifying that
detail, without destroying the general shape of the
dunes, and the steepest slopes are now on the eastern
side. The preservation of the original shape, in spite of
the subsequent development of westerly winds, is due in
  66 THE EVOLUTION OF CLIMATE

part to the coating of vegetation, which protected the
dunes as soon as more favourable conditions occurred,
and probably in part to the lesser velocity of the wester-
lies. If the period of east winds and dune formation
had been long enough, we might have had another
deposit of loess, but it was short, and vegetation, which
is necessary to the genesis of true loess, had no time to
establish itself before the climate changed again with
the final retreat of the ice. The climate of this period
in Rumania has been ably described by G. Murgoci:
“ In general the prevailing climate of the time of the
formation of loessoid soils and blown sands must have
been that which is named by E. de Martonne the aralian
climate, a dry climate with some rain in spring to call
forth a poor and transient vegetation and to maintain
the flowing water in rivers and lakes. The temperature
with great extremes, in summer up to 1200 F. and in
winter below 20° F., was the characteristic of this
climate ; the atmosphere was very dry in the hot season,
but in the rest of the year there was some humidity in
the air and moisture in the soil, the water of the subsoil
being not very deep. The atmospheric precipitation
in this region was caused by the south-west wind just as
at present; but the dominant wind giving the character
of a dry Continental climate was the north-east wind
(Crivat) which has left its tracés in the fossil dunes of
the Baragan.”

A history of the changes of climate in Europe which
followed the maximum of the last readvance of the
ice-sheet must be left to later chapters.

BIBLIOGRAPHY

Brooks, C. E. P. “ The correlation of the Quaternary deposits of the Britiih
I»le» with those of the continent of Europe.” Ann. Rep. Smitbsonian
Jnst., 1917, pp. 277-375* [FuU list of references.]

Penck, A., and Briickner, E. “ Die Alpen in Eiszeitalter.” 3 Vols. Leipzig,
1901-9.
  GLACIAL HISTORY OF EUROPE 67

Gagel, C. “ Die Beweise für eine mehrfache Vereisung Norddeutschlands
in dikmaler Zeit.” Geol. Rundschau, 4, 1913, p. 39.

Wahmchaffe, F. “ Die Oberflachengestaltung des norddeutschen Flach-
landes.” Stuttgart, 1910.

Syastos, R. “ Le postglaciare dans 1’Europe centrale du nord et oriëntale.”
Ann. Sci. Univ. Jassy, 4, 1908, p. 48.

1

1
  CHAPTER VI

THE MEDITERRANEAN REGIONS DURING THE GLACIAL
PERIOD

Our knowledge of the histoiy of the Mediterranean
basin during the Glacial period is not nearly so com-
plete as is that of the more northem regions, chiefly for
the reason that during most of the period the land lay
above its present level, and except forlocal glaciers in
the mountain regions there was no ice to leave us a record
of the changing climates. Most of what we do know
relates to the relatively brief periods of submer-
gence.

At the beginning of the Glacial period the sea lay
some 500 feet above its present level, and we can tracé
the first appearance of a northem marine fauna. This
stage is known as the Calabrian; it is divided into two
horizons—a lower, in which northem forms are still rare,
and an upper, in which they are becoming abundant.
The most typical species are two mollusca whose present
habitat is the coast of Iceland—Chlamys (Pecteri) islandicus
and Cyprina islandica.

The Calabrian beach is not found on the coast of Spain
or at Gibraltar, and in Algeria it probably occurs at a
lower level. This suggests that the subsidence at this
period was local, and the western lands stood up as a
barrier against the Atlantic. There must have Deen a
channel of some sort, however, on the site of the present
Straits of Gibraltar, to provide an inlet for the im-
migrating northem mollusca. In the Maritime Alps,
68
  THE MEDITERRANEAN REGIONS 69

and again in the eastern Mediterranean, the Calabrian
beaches are at a much greater height owing to local
elevation.

After the formation of the Calabrian beach the whole
Mediterranean region was elevated above its present
level. This elevation must be contemporaneous with
the period of maximum elevation in north-west Europe
associated with the great Mindelian glaciation. It is
suggested that the “ sill ” of the outlet channel at
Gibraltar was raised above the level of the Atlantic,
and the Mediterranean became, first a closed salt lake,
and then a pair of lakes, the eastern fresh draining into
the western, which was salt, the two being separated by
a ridge of land between Italy and Tunis. This period
of elevation was long enough for a great deal of denuda-
tion to take place. Even in the Mediterranean this
was a time of severe climate. On the eastern side of
Gibraltar there are breccias, known as the “ Older
Limestone Agglomerate,” which reach a thickness of
100 feet in places, and are now much weathered. Similar
agglomerates are found in Malta. These resemble the
“ head ” of the south of England, and appear to be due
to frost action in a severe climate. In Corsica there are
tracés of four periods of mountain glaciation, and the
two oldest of these are provisionally correlated with the
Gunzian and Mindelian of the Alps. In the Balkan
highlands there are tracés of two distinct glaciations:
the older, which was the more general and reached the
greater intensity, probably corresponding to / the
Mindelian. In the Atlas Mountains there are great
boulder moraines which seem to belong to three disitinct
glaciations, the oldest extending to about 2000 feet
above sea-level, and the second terminating at about
4000 feet, while the third glaciation consisted of small
valley glaciers only.

Towards the close of the Mindelian glacial period the
land sank or the ocean rosé again, and the waters of the
Atlantic poured in, bringing with them a great number
  70 THE EV0LUTI0N OF CLIMATE

of high northern and Arctic mollusca. The theory has
been put forward that this influx was in the nature of a
debacle and carved out a deep gorge through the present
Straits of Gibraltar. The beaches deposited by this
sea lie at a height of 250 to 330 feet above the present
sea-level. The fauna resembles that inhabiting the
northemmost parts of Europe at the present day, and
the waters must have been several degrees colder than at
present. This stage is termed the Sicilian.

As the climate improved the land gradually rosé again,
and the next general raised beach lies at a height of
only about 100 feet in Southern Italy (except where
it has been elevated by local earth movements). Further
west it lies still nearer the present sea-level—twenty
. feet in the Balearic Islands and only seven feet on the
coast of Spain. On the coast of Algeria and Tunis this
beach is found at a height of about forty-five feet.

[Prometheus]

The beach contains no tracé of the northern fauna
found in the Sicilian stage; instead it is marked by an
assemblage of mollusca of a sub-tropical aspect, including
Strombus bubonius, Mytilus senegalensis and Cardita
senegalensis. The bones of large mammals are also
found, including the hippopotamus and Southern forms
of elephant (E. antiquus) and rhinoceros (Rh. merckiï).
This warm stage corresponds to the Chellean inter-
glacial fauna of northern Europe, though so far as I am
aware no Chellean implements have been found associated
with it.

About this time the Older Limestone Agglomerate of
Gibraltar had been worn into caves, in which are found
the bones of ibex, wild boar, leopard, spotted hyena,
Rhinocerus leptorhinus, Elephas meriaionalis, lion, Southern
lynx, bear, wolf, stag, horse, etc., so that the rock must
have been covered by a rich vegetation, and must haye
had a greater extent than now, and a connexion with
the continent of Africa. This is said to have been
followed by a submergence of about 700 feet with
numerous oscillations. This submergence, if it is really
  THE MEDITERRANEAN REGIONS 71

attributable to the interglacial, must have been extremely
local, and possibly it is much older.

After the warm CheUean period the Mediterranean
region rosé again, probably contemporaneously with the
rise which caused the Rissian glaciation of northern
Europe. But the climate was nothing like so severe as in
the Sicilian. We have no old beaches containing a mollus-
can fauna of this period, but at the Grotte au Prince near
Mentone, investigated by M. Boule, the Strombus beach
is overlain by a bed of cemented pebbles and “ hearths ”
containing Mousterian implements and bones of a
temperate fauna. The Newer Limestone Agglomerate
on the east of Gibraltar may have been formed during
this period. The Mediterranean lands remained above
their present level until the close of the Glacial
period.

Each glaciation of northern Europe must have been
a time of greater rainfall as well as of lower temperature
in the Mediterranean. The glacial anticyclone in the
north displaced the storms from the Atlantic, which
now mostly either skirt the north-west coast of Norway
or pass across Denmark into the Baltic. These storms
had to take a more southerly course, and entered the
Mediterranean basin either across the south of France
or in the neighbourhood of Gibraltar: tracks which are
still occasionally followed in winter. These storms
brought a rainfall much heavier than the present, and of
a different character. The Mediterranean is now a
“ winter rain region,” and the north of Africa is entirely
rainless for several months in the summer. But during
the “ Pluvial periods ” it is probable that rainj feil
throughout the year, though the winter still had more
than the summer. The winter rains were in the form
of steady falls of long duration, such as we experience
now in England, whüe the summer rains feil in short,
heavy showers, perhaps accompanied by thunder. The
Older Pluvial period, which corresponds to the Mindelian
glaciation, had these conditions in their greatest develop-
  72 THE EVOLUTION OF CLIMATE

ment. Depressions cannot live long without a supply
of moisture, either from the sea or from transpiring
vegetation, and at present such winter storms as enter
the Mediterranean are almost confined to its surface,
and on the African side rarely penetrate more than
one hundred miles inland. But at the period of greatest
elevation the shrunken Mediterranean offered no such
great attraction, and with a comparatively well-watered
Sahara the storms were able to pass much further south.
Consequently, northern Africa possessed a number of
large and permanent rivers which reached the sea. It
was along these rivers and their banks that the fauna
still inhabiting the Saharan oases made its way, to be
isolated there by the decrease of the rainfall, so that
crocodiles and many species of fish now live in isolated
pools and in rivers which lose themselves in the
sand.

In Egypt and Syria the first Pluvial period is doublé,
corresponding to the Gunz and Mindel glaciations,
with an intervening phase of feeble desert conditions,
during which, however, the rainfall remained greater
than the present. The second stage, corresponding to
the Mindelian, indicates very great activity; at this
time the Jordan Sea (Dead Sea) reached its greatest
area, extending to the northern end of the Sea of
Tiberias.

Conditions in Egypt at this time are very interesting.
South of Cairo the aïïuvial Nile muds are at most thirty
to thirty-five feet thick, and ten feet of this thickness
has been deposited since the time of Ramesis II. If
the rate of deposition has been uniform, this gives a
period of only 14,000 years for the deposition of the
whole thickness of the muds. The theory put forward
by Hume and Craig (British Association Report, 191^,
p. 382) is briefly as follows: The mud deposits of the
Nile valley are carried down with the flood waters of
the Blue Nile, Atbara, etc. These rivers rise in the
highlands of Abyssinia, where they are fed by the rains
  THE MEDITERRANEAN REGIONS 73

of the south-west monsoon. The incidence of the
monsoon is determined by anumber of factors, prominent
among which is the temperature of Southern Asia.
During the winter, at present, the low temperature of
the Himalayan and Tibetan region results in a great
outflow of cold air, which strikes the coast of Africa as
the cool dry north-east monsoon. During this time
there is very little rain in Abyssinia. It is only when
the Asiatic land-mass warms up in summer that the
south-west monsoon is established.

But during the Glacial period, as we shall see, there
was a great development of snow and ice on the Hima-
layas. The result was that winter conditions, i.e. the
north-east monsoon, prevailed more or less throughout
the year, and the rivers which feed the Nile contained
only a small volume of water. Hence they lost them-
selves in the desert before reaching Cairo, and the Nile
in its present form did not exist. On the other hand,
the westerly winds which at present bring a moderate
winter rainfall to the coast of Syria were greatly increased
in intensity and extended further south, replacing the
dry north and south winds now occupying the Nile
valley. The northerly winds prevailing in the Nile
valley in summer are associated with the low pressure
area over the neighbourhood of the Persian Gulf, which
in turn is due to the extremely high temperature expe-
rienced there. Even at the present day the highest hills
of Sinai penetrate above the north winds into a westerly
current, and a moderate fall of temperature over the
Persian Gulf would inhibit the north winds in the Nile
valley altogether and allow the westerly winds to reach
the surface. These strong westerly winds brought a
heavy rainfall to the hills, now almost rainless, between
the Nile and the Red Sea. Powerful streams descended
the western slopes of these hills, bringing great quantities
of debris, which formed delta-terraces forty or fifty feet
thick where the streams debouched on to the Egyptian
plain. These are especially well developed at Oina,
  74 THE EVOLUTION OF CLIMATE

the meeting place of several dry valleys from the hills,
and it is remarkable that they jtctually cross the present
site of the Nile valley and reach the desert on its western
side, additional evidence that the Nile was not then in
existence.

These gravel terraces contain numerous stone imple-
ments of early (pre-Chellean) types, showing that at
this time Egypt had sufficiënt rainfall of its own to
support human life.

The moist westerly winds carried the climate of the
Mediterranean coast far into the desert. For instance, in
the oasis of Khargeh, in latitude 250, grew the evergreen
oak and other 1   ' not now founa south of Corsica

The Mindelian Pluvial period was followed by a long
dry period corresponding to the Chellean, when desert
conditions supervened. The Nile as we know it first
appeared during this period. Terraces were formed on
the sides of the valley, probably during the submergence
which produced the Strombus beaches of the western
Mediterranean; these contain Chellean implements.
During the succeeding elevation the Nile cut its bed
below the present level.

The Rissian glaciation of northem Europe is repre-
sented in Egypt by a second rainy period, the Lesser
Pluvial period. Rain again feil on the Red Sea hills,
forming a newer set of gravel terraces, but these are
much smaller than the great Mindelian terraces. No
terraces are known representing the Wurmian period,
and the country does not seem to have been inhabited
at this time. Probably the climate was semi-desert,
with not enough rainfall of its own to support human
life, and yet without the fertilizing Nile floods to enable
human life to exist without rainfall. As has been said,
the present regime did not begin until the last glaciation
was nearly over, about 12,000 b.c.

and Southern
  THE MEDITERRANEAN REGIONS 75

BIBLIOGRAPHY

Gignoux, M. “ Le» formations marines pliocènes et quatemaires de 1’Italie
du Sud et de la Sicilië.” Ann. VUttiv. Lyon, n.s., Vol. i, fase. 36,
Paris, 1911, pp. 693.

Depéret, C. “ Les anciennes lignes de rivage de la cöte frangaise de la
Mediterranée.” Buil. Soc. Geol. de France, ser. 4, Vol. 6, pp. 207-30.

Douvillé, R. “ Espagne,” Hanibucb regional Geol., H. 7, 1911. (Includes
Gibraltar and Balearic Is.)

Hume, W. F., and Craig, J. I. “ The Glacial period and climatic change in
North-east Africa.” Rep. Brit. Assoc., 1911, p. 382.

6
  CHAPTER VII

ASIA DURING THE GLACIAL PERIOD

The great area of Asia is at present but little explored
for glacial tracés, but a certain amount of evidence has
been collected, and the data from the various mountain
districts are consistent enough to map out the general
trend of the history of the continent during the Ice Age.

The earth-movements which brought about the
present configuration of Asia were completed as regards
their major details by the close of the Tertiary period.
These movements left a number of great basins closed
in on all sides by enormous mountain walls; at first all
these basins contained lakes, and the subsequent geo-
graphical history has consisted largely in the gradual
silting up of the lakes and the development of more and
more arid conditions. The fluctuations of the Ice Age
were superposed on this secular desiccation, but except
in northern Siberia the part played by glaciation in the
history of the country has been relatively small.

Consider for a moment the relief of Asia. The
orographic centre may be taken as the great Pamir
plateau, the “ Roof of the World,” with an average
elevation exceeding the height of Mont Blanc, diversified
by ranges of mountains exceeding 25,000 feet in places.
East of this is the great plateau of Tibet, 10,000 to
17,000 feet, bounded on the south by the mighty Hima-
layas, and on the north by the mountains of Kuen Lun.
On the north the Pamir plateau 'is bounded by the
Alai range, passing north-east into the Tian-Shan
mountains, rising to 24,000 feet in Khan-tengri. Still
76
  ASIA DURING GLACIAL PERIOD 77

further north-east comes the Altai range, with an
elevation of 9000 feet. East of Lake Baikal lie a series of
ranges averaging 8000 feet in height, and passing into
the Stanovoi range of eastern Siberia and the mountains
of Kamchatka.

The Himalayas, owing to their heavy snowfall derived
from the south-west monsoon, bear numerous great
glaciers, but with the series of ranges extending from
the Pamirs to north-east Siberia the case is different.
These ranges all rise above the snow-line in places, but
owing to the scanty snowfall they bear at most a few
small glaciers on their northern sides, and none at all
on the slopes which face towards the deserts of western
China, and in all cases the glaciation is very slight in
comparison with their elevation.

This distribution was characteristic also of the Ice Age.
In the Pamirs there is evidence of two periods when the
glaciers had a greater extent; ..in the first they extended
to a level of 5000 feet, in the second to 7000 feet. The
present limit of the glaciers lies at about 10,000 feet.
The first glaciation was remote, for the moraines are
worn and weathered, but the second was much more
recent, for the moraines are fresh, and in some cases
there are still masses of “ dead ” ice buried beneath
great accumulations of debris and occasionally exposed
by slips.

In the Tian-Shan mountains there are remains of
two glaciations. The earlier was the greater, and the
glaciers descended well below 10,000 feet. This glacia-
tion was followed by a long interval, when the erosion
of the rivers converted the U-shaped glacial valleys into
V-shaped gorges. A second glaciation descended to
a level of 10,000 feet, and again developed U-valleys to
this level; the end moraines of these glaciers are young
and fresh-looking. In the Altai range there were also
two glacial periods. In the older and greater the snow-
line was depressed by 3000 feet. The glaciers attained
a length of twelve miles and descended to a level of
  78 THE EVOLUTION OF CLIMATE

only 3000 feet above the sea. The second glaciation
was less extensive.

So far we have been dealing with small mountain
glaciers only. But in north-eastern Siberia we find a
different state of affairs. The Stanovoi and Verkhoiansk
mountains were heavily glaciated, and during the first
glaciation were probably the centre of an actual ice-sheet
similar to that of Scandinavia. The ice descended the
valleys of the rivers Yana, Indijirka and Kolyma and
covered the New Siberian Islands, which were at that
time connected with the mainland. The upper valley
of the Lena was also heavily glaciated by an ice-sheet
moving southward, probably from the Patom highlands.
When this glaciation drew to a close the source of
supply among the mountains ceased, and the ice on the
lowlands and in the lower parts of the river valleys was
left stranded as “ dead ” ice. When the mountains
became free of ice, the re-born rivers carried great
quantities of moraine clay and other debris with them,
and flooding the ice-surface over wide areas deposited
their load above the ice. In course of time the remains
of the ice-sheet were deeply covered by a layer of earth
and stones, which prevented the ice from melting and
preserved it to the present day. This is the probable
origin of the well-known “ fossil ice ” of Siberia. Other
theories have been put forward, such as the freezing
of ground water during the winter, but none are satis-
factory, and that given here was generally adopted by
Russian geologists.

During the long warm interglacial which followed,
the surface of the thick earth-layer covering the ice bore
low-growing herbage in the same way as any other
earth-surface. (A parallel to this is found in Alaska,
where the glaciers terminate among the forests, which
actually grow over the moraines covering their snouts.)
The rivers cut their way down through the earth and ice,
exposing ice-cliffs, which were quickly buried by talus
from above. The mammoth and woolly rhinoceros
  ASIA DURING GLACIAL PERIOD 79

roamed the land, and their tusks remain in great numbers
as the “ fossil ivory ” of Siberia and the Arctic Ocean.
Still more remarkable is the fact that mammoths have
been found buried entire, and preserved by the frozen
ground to the present day. It is difficult to say how
the animals reached such a position, but most probably
they sank into swamps formed during the summer and
were quickly frozen.

In western Europe the mammoth and woolly rhino-
ceros are regarded as indications of severe climate, but
their presence in north-eastern Siberia in large numbers
is evidence of a climate probably somewhat warmer
than that of the present day, especially as regards the
length of the vegetation period. Probably the winter
snowfall also was less than now. It is difficult to see
how the fauna could have moved from, say, the New
Siberian islands into a warmer climate each winter,
for the winter climate becomes markedly more severe
as one penetrates south from the Arctic coast into the
interior. It is possible that the mammoth and woolly
rhinoceros hibernated during the winter.

After this interglacial there came a recrudescence of
glacial conditions. In this case, however, the Stanovoi
and Verkhoiansk mountains and the Patom highlands
were not buried in an ice-sheet, but became the centre
of great valley glaciers, which reproduced the well-
known glacial phenomena—corries, glacial terraces,
U-valleys, etc. The ice extended down the great river
valleys, leaving a typical moraine landscape on either
side, and again reached the New Siberian islands. In
course of time the climate ameliorated, again commencing
in the south, and again the ice of the glaciers was buried.
In the New Siberian islands the happenings are sum-
marized very expressively by a rock-section described by
Vollossovitsch. The bottom of the section is formed
by the older layer of “ fossil ice.” Above this is a sandy
clay with remains of meadow vegetation and shrubs,
followed by a fine clay with remains of alder and white
  80 THE EVOLUTION OF CLIMATE

birch, and the bones of mammoth and rhinoceros.
Above this comes another layer of “ fossil ice,” followed
by clay with the dwarf birch, Arctic willow, and bones
of musk ox, horse and later mammoth. After this the
Coastal regions sank beneath the sea for a time and
marine clays were formed in a climate somewhat warmer
than the present. When the land rosé again the con-
ditions resembled those now prevailing.

Though not part of Asia, reference may be made here
to the glaciation of Spitzbergen, which runs strictly
parallel with that of northern Siberia. The first glacia-
tion was of the “ ice-sheet ” type, originating in the
region north of Storfjiord, filling the whole of that
fiord and extending south of South Cape. Barentz
Land and Stans Foreland were at least partially ice-
covered. The ice-floor of Spitzbergen, which resembles
that of Siberia, may have originated during this glaciation.
This was followed by subsidence to 230 feet below
present level, and the ice retreated, giving place to an
“ interglacial,” during which frost was very active and
largely obliterated the tracés of the ice-sheet. This
“ interglacial ” was followed by a second extension of
the ice, which affected the valleys and fiords only, leaving
the plateaux free. This again was followed by subsidence
and a warm period.

In Southern Kamchatka there was a great develop-
ment of ice, but in the form of a network of glaciers
rather than of an inland ice-sheet. In the east the i,ce
reached the sea, but on the west it left a zone forty to
sixty miles broad, and up to a thousand feet high un-
glaciated, so that there was the same difference then as
now between the rainy east side and the drier west side
of the peninsula. The present snow-line in the centre
of Southern Kamchatka is about 5500 feet, and at the
maximum of the glaciation it must have been fully
3000 feet lower.

[Prometheus]

This glaciation was followed after an interval by a
second, which was confined to the mountains. The
  ASIA DURING GLACIAL PERIOD 81

moraines of this glaciation are much fresher than are
those of the earlier one.

In Japan the mountains were only just high enough
for glaciers to develop in the north. The moraines are
old and weathered, and their meaning has been disputed;
but recent work by Simotomai and Oseki seems to have
established their glacial origin. The depression of the
snow-line necessary to produce them—about 3000 feet—
fits in very well with that observed in adjoining parts
of the continent. The phenomena were confined to
small hanging glaciers in the Hida mountains which cut
out corries and descended to a level of about 8000 feet,
leaving small morainic ridges. This glaciation was
probably contemporaneous with the earlier and greater
glaciation of Siberia. To the succeeding interglacial
may be attributed the marine deposits found near
Tokio containing corals, at present living some distance
further south. No tracé of any subsequent glaciation
of Japan has yet been found.

J. S. Lee has recently called attention to the existence
of a glaciated area in northern China, the evidence for
which consists of moraines and striated slabs found in
Southern Chi-li, and a glaciated valley with travelled
boulders in the north of Shan-si. The glacial deposits
in Chi-li are closely associated with a layer of quartzite
pebbles which continues southward beneath the loess on
the eastern side of the Tai-hang range, and is attributed
to either torrential rain or the melting of glaciers.
J. Geikie had long ago stated that there once existed
ice-masses all over northern China, and considered that
the ice came from the Himalayas. This origin is impos-
sible, the probable source of the ice being the Yablonoi
mountains in Southern Mongolia.

In the Himalayas the glaciers formerly had a much
greater extension. The glaciers at present extend
downwards to 11-13,000 feet, but old moraines are
found at 7000 feet, and near Dalhousie on the Southern
slopes of the Dholadar range to 4740 feet.
  82 THE EVOLUTION OF CLIMATE

On the northern side of the Himalayas there was a
great development of ice over Tibet, but there was not
a real ice-sheet such as occurred further north. OJdham
records three separate periods of glaciation in Kashmir,
but it is not yet possible to discuss the glacial history
of the Himalayas in detail. The latter is likely to prove
complicated, since the range is still rising, and has prob-
ably been doing so either continually or intermittently
throughout the Quaternary.

The great development of ice in Tibet, which is now
semi-arid, owing to interception of the rain-bearing
winds by the Himalayan range, suggests a considerable
alteration in the present meteorological conditions.
The Tibetan snowfall was probably due to the Mediter-
ranean storms, which now give a small winter rainfall
in north-west India, and which during the Glacial period
greatly increased in strength and frequency and occurred
throughout the year (Chapters IV and VI), giving the
Pluvial period of North Africa. These storms would
pass across Persia and continue to the north of the
Himalayas, probably breaking up over the Tibetan
plateau.

It is evident that, taking northern Asia as a whole,
there have been two general glaciations, of which the
first was the more severe, separated by a long inter-
glacial, during which, in Japan at least, the climate
became appreciably warmer than the present. The first
glaciation is related to elevation in the Arctic basin,
which closed Bering Strait and united the New
Siberian islands to the mainland. It was almost cer-
tainly contemporaneous with the first glaciation (Gunz-
Mindel) of Europe. The ice began as glaciers on
the mountains as in Scandinavia, but, owing to the
scanty supply of snow, developed more slowly and only
reached the dignity of ice-sheets in north-east Siberia.
Then followed subsidence below the present level,
wider opening of the Bering Strait, warm ocean
currents and a long interglacial. After this there was
  ASIA DURING GLACIAL PERIOD 83

again elevation and a re-development of ice-sheets, but
apparently once only, and not twice as in Europe. This
glaciation probably corresponded in point of time
more or less with the Rissian, for the post-glacial dry
of central Asia appears to have been of enormous period
length.

There is one other phenomenon which must be con-
sidered in connexion with the glacial history of Asia,
and that is the loess. Loess has already been referred
to in connexion with the glaciation of Europe, but in
China its development is much greater. Richthofen,
who first studied this deposit attentively, and to whom
we owe the seolian theory of its origin, found that ït
was formerly deposited in China over a much greater
area than that over which it is accumulating at present,
and attributes this cessation of growth to the heavier
rainfall brought by the Glacial period, which enabled
the rivers to cut back their valleys and drain some of
the mountain basins, formerly enclosed. He considered
that loess can accumulate more rapidly in a closed
basin, where occasional floods leave behind them layers
of bare sand and mud, easily dried to dust, than in a
well-drained river valley where floods are rare.

In western Asia outside the limits of glaciation we
have further evidence of at least one Pluvial period in
the former far greater extent of all the enclosed lakes,
due partly to greater precipitation and partly to de-
creased evaporation. The Caspian Sea and Aral Sea
were extended to several times their present size and
united into a single sheet of inland water. Lake Lop-nor
was greatly increased in size, and many of the desert
basins, at present dry, were the sites of salt lakes. This
is especially the case in central Persia, where there were
large salt or brackish lakes.

These Pluvial conditions have not yet been correlated
with the glaciations of Asia, but, by analogy with the
conditions in America discussed in the next chapter,
there is little doubt that they were contemporaneous
  84 THE EVOLUTION OF CLIMATE

with one at least of the glaciations, and probably there
were two main Pluvial periods coinciding with the two
Glacial periods. At Baku, on the shores of the Caspian
river, Pumpelly has found old shore lines at heights
of 600, 500, and 300 feet above the present level of the
water. Still more interesting are the conditions found
by Sven Hedin in the Kavir basin of Persia. Here there
are lacustrine clays and silts referable to a Pluvial period
covered by beds of almost pure salt, suggesting a rapid
and complete drying up of the lake. Above this again
are further silts indicating a return of Pluvial conditions.
In addition to this the succession of silts and clays show
that there were several minor fluctuations superposed
on the main wet periods, giving ten moist phases
altogether.

BIBLIOGRAPHY

Many of the more important references are in Russian, and for theie

reference is made to summaries in other languages.

Sevastianov, D. P. “ On the glaciation of the extreme north-east of Siberia.”
J. 12 Congr. Russ. Nat., Moscow, 1910, No. 10, p. 491. (Russian, see
Geol. Centralblatt, 15, p. 205.)

Riesnitschanko, W. “ Ancient and modem glaciers of the south-western
Altai.” Mem. Russ. Geogr. Soc., 48, 1912, p. 357. (Russian, see
Geol. Centralblatt, 19, p. 131.)

Komarov, W. “ On the Quaternary glaciation of Kamchatka—Travels in
Kamchatka in 1908-9,” Vol. 1 (Russian, see N. J. Min., 1915, Pt. 2,
p. 117).

Merzbacher, G. “ Zur Eiszeitfrage in der nordwestlichen Mongolei.”
Peterm. Mitt., Gotha, 57, 1911, p. 18.

Prinz, Gyula. “ Die Vergletscherung des nördlichen Teiles des zentralen
Tien-8chan-Gebirges.” Wien, Mitt. K. K. geogr. Gesellscb, 52, 1909,
p. 10.

Obrutscher, W. A. “ Geological map of Lena gold-bearing region.” St.
Petersburg, 1907. [Text in Russian; see Geol. Centralblatt, 12,
PP-. 5°7-9-l

Simotomai, H. “ Die diluviale Eiszeit in Japan." Berlin, Zs. Ges, Erdkunde,
1914, p. 56.

Oseki, K. “ Some notes on the glacial phenomena in the North Japanese
Alps.” Edinburgh, Scot. Geogr. Mag., 31, 1915, p. 113.

Lee, J. S. “ Note on tracés of recent ice-action in North China," Geol,
Mag., 59, 1922, p. 14.

Burrard, S. G., and Hayden, H. H. “ A sketch of thé geography and geology
of the Himalaya Mountains and Tibet.” Calcutta, 1907-8.
  ASIA DURING GLACIAL PERIOD 85

Hogböm, G. “ Bidrag till Isfjordsomradets kvartargeologi.” Geol. Foren.

Stockholm Forh., ign. (Spitzbergen; résumé in German.)
Richthofen, F. Freih. von. “China.” 5 Vols., 1907-12. (Loess, see Vol. 1,
. P- 74 ff.)

Hedin, Sven. ** Some physico-geographical mdications of post-Pluvial
climatic changes in Persia.” Internat. Geol. Congr., Stockholm, 1911.
“Die Veranierung des Klim as."
  CHAPTER VIII

THE GLACIAL HISTORY OF NORTH AMERICA

The glaciation of North America was even greater and
more complicated than was that of Europe. It spread
from three main centres, the CordiUeran or Rocky
Mountain centre, the Keewatin centre west of Hudson
Bay, and the Labradorean centre. Vancouver Island
in the west and New Brunswick and Newfoundland in
the east, were also independent centres of glaciation,
and ice from the latter may have reached the coast of
the United States in places. The ice covered an area
of about 4,000,000 square miles, and the main ice-sheet
extended to 38° N., or twelve degrees further south than
the Scandinavian ice-sheet. Nine stages are recognized
by American geologists, though opinion is divided as
to whether all the stages of “ deglaciation ” represent
real interglacial periods. The sequence is as follows:

1.   Nebraskan, Jerseyan or pre-Kansan glaciation.

2.   Aftonian deglaciation.

3.   Kansan glaciation.

4.   Yarmouth deglaciation.

5.   Illinoian glaciation.

6.   Sangamon deglaciation.

7.   Iowan glaciation.

8.   Peorian deglaciation.

9.   Wisconsin glaciation.

On the other hand, in the northern part of the Rocky
Mountains there is evidence of only two Glacial periods,
separated by a single long interglacial, though perhaps
86
  THE GLACIAL HISTORY OF N. AMERICA 87

the second glaciation was doublé. Further south, out
of reach of the main ice-sheets, there are tracés of two
and in places three separate developments of valley
glaciers resembling those of the Alps.

As in the case of Europe, the literature of the subject
is extensive and conflicting, but the following summary
of the course of events represents the views of most
moderate American geologists.

The Quaternary period opened with extensive eleva-
tion of the whole North American continent, which
raised the Rocky Mountains several thousand feet above
their present level and extended the Continental area
over much of the northern archipelago. In the east
Newfoundland is considered to have been raised at
least 1000 feet, a movement which converted the
banks into dry land and interposed a large cold area in
the path of the moisture-bearing southerly winds. As
in northern Europe the high mountains of the west
were the first to develop large glaciers, which coalesced
into an ice-sheet, filling the valleys and rising up the
slopes of the mountains until it reached a thickness of
5000 feet. In Puget Sound the ice was 4000 feet
thick, but seawards the slope is very rapid and the ice
was unable to extend far from the shore. This ice-sheet
extended south-eastwards some distance into the United
States, forming the first ground moraine of that district.
Probably while this Cordilleran glaciation was still in
progress ice began to spread outwards also from the
Labradorean centre, forming the oldest drift of that
region. These oldest deposits are, however, not yet
well understood.

This oldest boulder-clay is separated from the moraines
of the main glaciation near its Southern limit by river
gravels containing the remains of mollusca and large
herbivorous mammals—extinct species of horse, the
hairy mammoth of the old world (Elephas primigenius),
and two other extinct species of elephant, and also the
true American mammoth. This is the Aftonian fauna
  88 THE EVOLUTION OF CLIMATE

which has been claimed as evidence of an Interglacial
period. That it evidences a retreat of the ice-edge in
that particular region is certain, but that the climate
became really temperate is very doubtful. More prob-
ably it corresponds to the Gunz-Mindel “ interglacial ”
of the Alps, and was formed when the Cordilleran
ice-sheet was retreating, but before the Keewatin sheet
had reached its maximum.

The Aftonian stage was followed by the Kansan
glaciation, when the ice-sheets reached their maximum
area over the greater part of North America. The
chief centre of glaciation at this stage was the Keewatin,
west of Hudson Bay. While it is certain that the
Keewatin centre reached its maximum later than the
Cordilleran, geological opinion in America is divided
as to whether or no the two ice-sheets ever coalesced,
but it is difficult to understand how an independent
ice-sheet could have grown up on the comparatively
low ground of the Keewatin centre. Most probably
the course of events here was an exact parallel of that
in the better-known Scandinavian region—the Cordil-
leran ice-sheet extended eastwards over the lower
ground until a glacial anticyclone developed east of the
Rockies. When this happened the supply of moisture
to the western part of the ice-sheet feil ofï somewhat,
and the eastern part took on an independent life, ulti-
mately becoming the main centre of glaciation. It was
while these changes were in progress that the Southern
limit of the ice retreated northwards and the “ Aftonian ”
deposits were formed.

The next stage (Kansan) occurred when the ice
from the Keewatin centre spread outwards in all direc-
tions, and in the south reached the maximum limits of
glaciation in America. In the west this sheet overlapped
on to the ground-moraine of the former Cordilleran ice,
but the Rocky Mountains were too far away and too
high for Keewatin ice to dominate them and overflow
them from east to west. Instead these mountains must
  THE GLACIAL HISTORY OF N. AMERICA 89

have maintained an extensive glaciation of their
own.

With the growth of the Keewatin centre the Labra-
doTean also decreased, but more slowly, and this change
was not associated with a retreat of the Southern ice-
edge, so that there was no corresponding “ interglacial ”
in the east of the United States. The moraines of these
older glaciations resemble those of the early ice-sheets
of Europe in presenting only featureless level surfaces
of boulder-clay without morainic ridges, lakes and the
other characteristics of ice-bearing surface detritus, and
there is no doubt that conditions at the Southern edge
were similar—the climate was severe in winter, but not
insupportable in summer. At the same time it was
decidedly more severe than the present, even as far
south as Florida} where there are colonies of northern
plants, which migrated southwards during the Ice Age,
still living on local cold slopes with a northerly aspect.
After the maximum of glaciation the disappearance of
the ice took place gradually and chiefly by ablation, for
there are none of the extensive river gravels and flood
terraces which we should find had the melting been
rapid. It is only in the valleys of the Rocky Mountains
that such deposits occur, testifying to conditions such
as obtained in the Alps.

The succeeding Yarmouth stage of deglaciation was
very long, corresponding in this respect to the Mindel-
Riss interglacial of Europe. The Kansan moraine was
weathered to a depth of ten or twenty feet, and four-
fifths of its surface was removed by the erosion of
streams and rivers. In the mountain districts the side
streams which had been left occupying “hanging
valleys” by the over-deepening of the heavily glaciated
main valleys, had time to cut out uniformly graded
broad V-shaped valleys descending to the level of the
main stream. In the Great Basin also, where the
periods of high water-level are considered to correspond
to the main glaciations, the interval of low water
  90 THE EV0LUTI0N OF CLIMATE

corresponding to the Yarmouth stage was very long.
A rough estimate of its length is about 200,000 years
—somewhat shorter than the Mindel-Riss. Actually,
though the Kansan and Mindelian glaciations were
approximately contemporaneous, the subsequent re-
currence of glaciation in America appears to have
preceded slightly that in Europe.

Of the climate of this stage we have unfortunately
httle evidence. Old land surfaces of this age are known,
containing deposits of peat and bones of the wood
rabbit and common skunk, but both of these animals
have a wide range. Perhaps the climate resembled the
present during most of the period ; there is no evidence
that it was ever warmer, and it appears quite likely that
ice-sheets maintained their existence in the far north
through the whole of this stage.

After this interglacial there set in a period of renewed
elevation in the Rocky Mountains and in the Labrador-
Newfoundland centres, which brought about a recur-
rence of the glaciation. In the Rocky Mountains the
ice was not so thick as in the preceding stage, but all
the valleys were occupied to a considerable depth and
the ice spread out to the eastward. The Labrador
ice-sheets also developed again, forming the Illinoian
glaciation, the moraines of which are found as far west
as Illinois, but no moraines are known of this age due
to the Keewatin ice-sheet. The latter developed later,
and is classed by some American geologists as a separate
glaciation, the Iowan, which is only certainly found in
northem Iowa, but may be represented further east
by a thin sheet of boulder-clay overlapping the Illinoian
moraine. The supposed interglacial between the Illi-
noian and Iowan, the “ Sangamon Stage,” is represented
only by land surfaces formed of the Illinoian ’ moraine
and covered by the loess or locally by the equivalent of
the Iowan moraine, and there is no evidence that the
ice-edge retreated far. Other American geologists,
including F. Leverett, do not recognize the existence of
  THE GLACIAL HISTORY OF N. AMERICA 91

[Prometheus]

a separate Iowan glaciation, and as the amount of
weathering and denudation undergone by the two
moraines differs very little, this seems the more natural
view. The natural explanation seems to be that this
was another case of “ glacial piracy,” the Keewatin ice-
sheet, owing to its lesser snowfall, developing more
gradually, and finally diverting the supply of moisture
from the Labradorean ice-sheet, until it reached a
maximum after the latter was already on the wane.
Both these sheets of drift present similar flat features
to the Kansan sheet, without morainic ridges.

Leverett’s interpretation of the succession is as
follows: The third (Illinoian-Iowan) glaciation was
followed by a period of moist climate, when peat-bogs
were formed on level poorly-drained surfaces, while
elsewhere coniferous forests developed. This was fol-
lowed by a period of dry steppe-like conditions with a
cold temperate climate, when the great American loess
sheet was deposited. This loess sheet extends northwards,
overlapping the Iowan moraine, and in places passing
under the Wisconsin drift. The material has come
from the west, and probably most largely from the dry
plains east of the Rocky Mountains, from which it
diminishes in thickness eastwards. But unlike Europe
this phase of steppe conditions was followed in America
by a definite interglacial, when the climate seems to
have become rather warmer than the present. In the
northem States an old land-surface formed on the loess,
and, termed the Peorian stage, is overlain by the Wisconsin
drift; but near Toronto, on the shores of Lake Ontario
and in the Don valley, the gap represented by this land-
surface is partly filled by a remarkable series of lacustrine
deposits known as the Toronto stage. The Lake
Ontario beds indicate a climate slightly colder than the
present, but the Don valley beds contain plants and
animals living in the central States, and refer to conditions
more favourable than those now found in the district.

The duration of this interglacial has been worked out

7
  92 THE EVOLUTION OF CLIMATE

in a remarkable way by A. P. Coleman, who on the
basis of wave-action estimated it as 62,000 years, which
agrees very closely with the 60,000 years found by
Penck and Brückner in the Alps. This period was not
long enough for streams in the “ hanging valleys ” to
cut out uniformly graded valleys down to the main
rivers, and was consequently much shorter than the
preceding interglacial.

The last glaciation of North America was the Wisconsin,
which closely resembles the Wurmian of Europe both
in its relations to the older glaciations and in the rough
topography and unworn character of its moraines. It
extended within the limits of the Kansan drift across
fully two-thirds of the continent, from Nantucket and
Cape Cod through Long Island, northern New Jersey,
Pennsylvania, Southern New York, Ohio, Indiana,
Illinois, Michigan, Wisconsin, Minnesota, Iowa and the
Dakotas, Manitoba, Saskatchewan and Alberta. At the
same time the Cordilleran centre probably bore increased
local valley glaciers.

Like the Wurm glaciation, the Wisconsin was doublé.
The older moraines are well-marked, and in places are
covered by a foot or two of loess, though thls deposit
reaches nothing like the thickness of that overlying the
moraines of the earlier glaciations. The moraine under
this loess is very little weathered, so that the time interval
was very short; possibly this loess is redistributed older
loess associated with glacial east winds. The ice of the
first glaciation melted very slowly and there is very little
gravel outwash to the moraines. But “ after the
Wisconsin ice-sheet had reached a position a little
outside the limits of the Great Lakes the retreat became
much more rapid, and large outwash aprons were formed
from which valley trains of gravel led far down the
drainage lines. From this position . . . the moraines
are practically free from loess-like silts.”1

From this point onwards the glacial history of America

1 Leverett, F. (see Bibliography).
  THE GLACIAL HISTORY OF N. AMERICA 93

is one of irregular retreat, with occasional halts or even
readvances resembling those of the Scandinavian ice.
Banded clays are found similar to those used so success-
fully by Baron de Geer in dating the retreat stages of
Scandinavia, and this geologist has recently been investi-
gating them, but until his results are worked out no
correlation with Europe can be attempted.

A natural clock of another type is provided by Niagara
Falls, which are cutting their Way back up the gorge
at a rate which has been definitely ascertained. Taking
into account the varying amounts of water which have
passed over the falls at different stages of post-glacial
geography, the duration since the region became free of
ice has been calculated at about 20,000 years, which
agrees closely with the time elapsed since the Scandi-
navian ice-sheet left the North German coast.

Before leaving North America it is necessary to give
a brief account of the phenomena outside the main
centres of glaciation, and especially of the history of
the Great Basin between the Siërra Nevada and Wasatch
Mountains. The lowest levels of this basin are at
present occupied by several salt lakes without outflow,
of which the largest is the Great Salt Lake, the level of
the water being determined by the balance between
inflow of the rivers and evaporation from the surface.
Twice in the past this balance has been decidedly more
favourable, and then the lakes grew to many times their
present size. The two greatest of these old lakes have
been fully described under the names of Lake Bonneville
(of which the Great Salt Lake is a vestige) and Lake
Lahontan, further to the west. The investigations have
shown that before the Glacial period, and extending
back into an unknown past, there was a period of great
aridity. To this succeeded a long period of high water,
during which, however, neither of the lakes overflowed.
This stage was followed by a very long period of great
aridity, during which the lakes dried up completely,
and all their soluble matter was deposited and buried
  94 THE EVOLUTION OF CLIMATE

by alluvial material. This period was followed by a
return of moist conditions, during which the water
reached a higher level than before, and in the case of
Lake BonneviUe actually overflowed into the Snake
river, cutting a deep gorge. This period, however,
was shorter than the preceding moist period. It was
followed by an irregular fall interspersed with occasional
slight rises, but ultimately both lakes descended below
their present level and probably again dried up com-
pletely. Both lakes suggest that this low level was
followed by a third rise to a height very slightly above
the present level, followed by a slow fall in recent years.

The relations of the periods of high water to the
glaciations are not clear in these large lakes, but in the
Mono Basin, a small basin further west, there is no
doubt that the two were almost contemporaneous, high
water accompanying the maxima of glaciation and
extending some way into the retreat phase. The very
long interval between the first and second period of high
water, several times that since the second period, agrees
with this correlation. We find then that south-west
of the main glaciated area there was a district of greater
precipitation or less evaporation, or more probably both.
This is confirmed by the vafley moraines of all this region
—Siërra Nevada, Uinta and Wasatch mountains,
Medicine Bow Range of northem Colorado, etc., all
of which indicate two glaciations, of which the first was
the greater, separated by a very long interval. In
several ranges the moraines of the second glaciation are
doublé, and some geologists consider that there were
three Glacial periods in these regions.

In the extremely arid region of Arizona, on the other
hand, which is considerably further south, the evidence
of the Gila conglomerates indicates that while frost was
very active, the increase of precipitation, though un-
doubtedly present, was comparatively slight. This
.shows that the climatic balance was not greatly disturbed,
the chief effect being an important lowering of tempera-
  THE GLACIAL HISTORY OF N. AMERICA 95

ture, probably due to cold northerly winds. The Gila
conglomerates are doublé, separated by a period repre-
senting present-day conditions.

Summing up the evidences of glacial climate in North
America, we find a striHng siimlarity to Europe. In
the north elevation and increased land area caused the
development of large ice-sheets, which appeared first
in the mountainous regions with a heavy snowfall, and
later spread over the drier plains and plateaux of the
interior. This first glaciation was long and complex.
Owing to the anticyclonic conditions which formed over
the ice, the rain- and snow-bearing depressions were
forced to pass further southward, causing greater snow-
fall on the mountains and high water-level in the lake
basins. This greater snowfall, together with the cold
conditions due to the existence of the ice-sheets to the
north, caused the development of mountain glaciers
south of the main glaciated region. In the east there
were cold northerly winds which carried a severe climate
as far south as Florida. This Glacial period was followed
by subsidence, and a long spell of dry, moderately warm
climate lasting perhaps 200,000 years, after which eleva-
tion and glacial conditions again set in. These conditions
were not so severe as the first, and their duration was
much less, while they were broken up by several intervals
of temporary recession of the ice, one of which, corres-
ponding to the Riss-Wurm period, lasted for 60,000
years, and perhaps should be considered as an “ inter-
glacial.” This period was marked in its early stages
by the deposition of the curious seolian deposit known
as “ loess,” indicating steppe conditions. After the last
glaciation there set in a stage of irregular retreat.

BIBLIO GRAPHY

Leverett, F. “ Comparison of North American and European glacial
deposit».” Zs.f. Gletsclerkünde 4, 1910, pp. 280, 323.

Wright, W. B. “ The Quaternary Ice Age.” London, 1914, Chs. 8-9.
  96   THE EVOLUTION OF CLIMATE

Attwood, W. W. “ The gladation of the TJinta Mountains.” J. Geol., 15,
,9°7> P- 79°*

Henderson, J. “Extinct glaciers of Colorado.” Col». U*iv. Studies, 3,
>905. P- 39-

Gilbert, G. K. “ Lake Bonne-nlle.” Washington, U.S. Geol. Survey Mono-
grapb I, 1890.

Russell, I. C. “ The geological history of Lake Lahontan.” Washington,
U.S. Geol. Survey Monograpb XI, 1885.

Coleman, A. P. “ An estimate of post-Glacial and interglacial time in
North America.” Rep. 12 Internat. Congr. Geol., 1913, p. 435.
  CHAPTER IX

CENTRAL AND SOUTH AMERICA

The scarcity of data which was bewailed in dealing
with Asia is still more marked in the case of South
America, and it will be necessary to present the glacial
history of that continent in the bar est outline only.
This is the more unfortunate as the chain of the Andes,
extending from north of the equator to high Southern
latitudes, is of enormous importance in glacial theory,
and especially in the question of simultaneity of glaciation
in the two hemispheres.

The beginnings of glaciation in South America are
obscure. The distribution of animals shows that towards
the close of the Tertiary the Falkland Islands were
greatly elevated and were United to Tierra del Fuego
and Patagonia, and this enlarged land area was connected
in some way with Australia and Tasmania, but the mode
of this latter connexion is not definitely known. This
question will be discussed more fully in Chapter XI;
it is sufficiënt to say here that the amount of elevation
may have reached 12,000 feet in Tierra del Fuego.
Equatorwards the elevation diminished, and near the
equator the land probably lay somewhat lower than
now.

In South Georgia the present glaciers greatly expanded,
until practically the whole island was buried in ice, and
the same is true of the Falkland Islands and Tierra del
Fuego, only the highest peaks remaining above the ice.
In the latter district there is so'me evidence of two
glaciations separated by an interglacial,jFthe earlier
97
  98 THE EVOLUTION OF CLIMATE

glaciation being due to a regional ice-sheet and the
later to smaller valley glaciers. The intricate coast-line
of the Falkland Islands and Tierra del Fuego points to
fiord er osion by ice which extended well beyond the
present limits of the land, and can only have occurred
during considerable elevation. As to the character of
the interglacial, little is known. In the Falklands there
is a bed of black vegetable soil full of tree-trunks, indi-
cating the existence of luxuriant forests and a temperate
climate. This deposit is overlain by boulder-clay, and
may be either interglacial or pre-glacial, but since it
was formed when the land stood at a comparatively low
level, while we have reason to believe (see Chapter XII)
that during the close of the Tertiary period these
islands were greatly elevated, it is probably an inter-
glacial formation, and indicates a great amelioration of
climate. In Gable Island, Tierra del Fuego, Halle
found beneath boulder-clay a Quaternary fauna of
barnacles and marine mollusca indicating a climate
slightly warmer than the present, and this probably
belongs to the same period. To the concluding stages
of the Glacial period in the Falklands belong the curious
“ stone rivers,” great streams of moss-grown boulders
which fiU the valleys, and under the influence of
temperature changes are probably still slowly ad-
vancing.

Passing further north to the Andes, between 390 and
440 south latitude, the glaciation was not so severe,
and its records are therefore clearer. The first result
of elevation was the cutting of deep canyons by the
rivers. This was followed, possibly without much
further elevation, by a fall of temperature, which in
this connexion may be attributed to the extension of
the Antarctic and Tierra del Fuego ice-sheets. Glaciers
now developed and spread down the canyons, leaving
moraines of great volume and height, associated with aü
the other criteria of glaciation. The snow-fields from
which these glaciers originated lay between 5000 and
  CENTRAL AND SOUTH AMERICA 99

6000 feet above the sea, and the snow-line lay at about
3000 feet instead of above 6000 as at present.

This glaciation was followed by a long interglacial,
during which the glaciers retreated to the highest
summits of the Andes. The length of this period is
indicated by the fact that the earlier moraines have
been eroded to such an extent that they no longer
present distinctly the typical features of glacial topo-
graphy, while the materials of which they are composed
are decayed to somewhat the same extent as the older
moraines of North America, the granite boulders espe-
cially being rotten and friable. This interglacial was
followed by a re-development of the glaciers, but to
nothing like the same extent as formerly ; their moraines
are smaller and fresh-looking, indicating that this
glaciation was comparatively recent.

Still further north, in latitude 2o°-25° S., we come
to a region of very slight snowfall, where the snow-line
lies higher than anywhere else on the face of the earth.
The glaciation here was comparatively unimportant,
the snow-line descending only 1600 to 2500 feet.
Here Keidel found moraines of three glacial advances,
and from his description it appears probable that the
earliest and greatest was separated by a considerable
interval from the two younger, the interglacial between
which was short and not characterized by a return to
present-day climatic conditions, since during this interval
there was very little weathering. Probably we have
here to do with two glaciations, of which the second
was doublé. In fact, some writers have described no
less than five glacial advances in the Argentine Andes,
but most of these are probably merely retreat stadia.

In Peru, W. Sievers reports the existence of two
glaciations separated by a considerable interval. The
present limit of the glaciers is about 15,200 feet; during
the first glaciation they descended to about 11,000 feet,
and during the second to 12,800 feet. The evidence is
very complete. In Ecuador, H. Meyer records a similar
  IOO THE EVOLUTION OF CLIMATE

bipartition. The oldest glaciation is represented by
trough-Kke valleys, enormous gravel terraces, and old
moraines much weathered ; the limits are far below the
present limits of glaciers, but have been much obscured
by subsequent erosion. This glaciation was followed
by a long period of steppe climate resembling the
present, during which the loess-like Cangagua formation
was deposited. This in turn was followed by a re-
advance of the glaciers to a level about 2700 feet below
the present limit. This glaciation is associated with
crescent-shaped moraines, corrie lakes, hanging valleys
and gravel terraces, covered with vegetation, but other-
wise fresh-looking. The snow-line lay about 1600 feet
below the present. Probably during the first glaciation
the Andes were invaded by numerous mountain plants
and animals related to North American forms—a valuable
piece of evidence which indicates that the glaciation
was contemporaneous with that in North America. In
Columbia and Venezuela there are tracés of Glacial
periods, but these have not yet been studied in detail.
The most northerly evidence of a Glacial period comes
from the Siërra Nevada de Santa Maria, near the north
coast of Venezuela in II0 N.

Except in Tierra del Fuego and Patagonia the ice
did not extend far from the mountains. But in the
eastern Argentine there is a great series of Quaternary
deposits known as the Pampean. This formation covers
200,000 square miles, and consists of at least ninety feet
of fine loam without a single pebble (except for a few
thin calcareous layers), but containing large numbers
of complete skeletons of mammals. It raises several
interesting problems. Apparently it represents the
whole course of the Glacial period. By some- geologists
it is considered to be a delta deposit of the combined
Parana and Paraguay rivers, but the absence of mollusca,
except in a marine intercalation near its summit, is
against this view, and Steinmann attributes it to seolian
agencies and compares it to the loess of Europe and
  CENTRAL AND SOUTH AMERICA ioi

[Prometheus]

North America. If this view is correct the Pampean
represents steppe conditions prevailing on the equatorial
side of the Patagonia-Falkland Islands ice-sheet. Appar-
ently before the incoming of the greatest cold the
Pampas were in part at least forest-clad, for in the older
beds are found peculiar forms of ground-sloths which
were adapted for forest life and have been found also
in cave-deposits of Brazil. At the maximum of glacial
conditions the Pampas probably had a steppe climate,
but the disappearance of the forests is to be attributed
rather to drought than to cold. Elevated glacier-
bearing Andes to the west and ice-sheets to the south
would render the Argentine extremely arid, and this
accounts for the gradual extinction of so many giant
forms whose remains are found in the Pampean deposits.
Conditions ultimately became too severe even for the
horse, which died out in South America. The marine
transgression which left its mark near the top of the
Pampean is probably post-glacial.

In Brazil, on the other hand, there is no evidence
that the climate has ever been drier than the present,
and in the semi-arid regions of the north-east it is even
probable that during the Glacial period the climate was
moister, presumably owing to the greater strength of
the rain-Tbearing east and north-east winds. Further
west in the Andes the existence of this wet period is
borne out by the former greater size of Lake Titicaca,
and there seems to be additional evidence to the same
effect in the Chilian deserts.

BIBLIOGRAPHY

Steinmann, J. “ Diluvium in Sfidamerika.” Zs, d. D, Geol. Gesellscb., 58,
1906, p. 215.

Meyer, H. “ In den Hochanden Ton Ekuador.” Berlin, 1907.

Sievers, W. “ Reise in Peru und Ecuador, ausgeführt 1909.” München
und Leipzig, 1914.
  io2 THE EV0LUTI0N OF CLIMATE

Keidel, H. “ Ueber den Anteil der Quartaren KJimaschwankungen an der
Gestaltung der Gebirgsoberflache in dem Trockengebiete der mittleren
und nördhchen Argentinischen Anden.” Congr. Geol. Internat., 12,
Canada, T913, p. 757.

Willis, Bailey. “ Physiography of the Cordillera de los Andes between
latitudes 390 and 440 S.” Congr. Geol. Internat., 12, Canada, 1913,
P- 733-

Halle, T. G. “ On Quaternaiy deposits and changes of level in Patagonia
and Tierra del Fuego.” Buü. Geol. Inst., Upsala, 9, 1908-9, p. 93.
  CHAPTER X

AFRICA

The Quaternary history of Africa can unfortunately be
dïsmissed in a very few words. The glaciation of the
Atlas Mountains has already been referred to in con-
nëxion with the Mediterranean region. Further south
we have no great mountain chain such as the Andes
extending above the snow-line over the whole extent
of the country, but merely a few isolated peaks. Three
of these, all close to the equator, are known to show
tracés of a greatly extended glaciation in the past:
Ruwenzori, just north of the equator, on the borders
of Uganda and the Congo, reaching an elevation of
16,794 feet, with the present snow-line at 15,000 feet,
and glaciers extending to 10,000 feet, formerly bore
glaciers extending down as far as 5200 feet; Kenya,
on the equator in Kenya Colony, height 17,040 feet,
present snow-line about 15,000 feet, past snow-line
12,000 feet, and old moraines at 10,000 feet; finally,
Kilimanjaro, 30 S., on the borders of Tanganyika terri-
tory, height 19,320 feet, present limit of glaciers 13,650
feet, past limit 4870 feet. Further south, the Draken-
berge Mountains, between Basutoland and Natal, were
glaciated on their higher summits. In none of these
cases have the remains of more than one glaciation been
described, but the mountains are still very little known
and this negative evidence is not conclusive. In the
neighbourhood of Ruwenzori there are several peaks,
which approach 12,000 feet, but these were not glaciated,
pointing-to a snow-line above this level. Unfortunateiy
103
  104 THE EV0LUTI0N OF CLIMATE

the latter piece of evidence is of doubtful validity since
these mountains are volcanic and possibly of post-glacial
age ; we may consider, however,' that the glaciation of
the central African mountains was characterized by a
great increase in the length of the glaciers with only a
slight depression of the snow-line, conditions showing
that the glaciation was due chiefly to an increase of snow-
fall, and only in a minor degree to a fall of temperature.
This conclusion is borne out by the low-level beds,
which nowhere show an appreciably lower temperature,
but abound in indications of a former greatly increased
rainfall. The first of these is the former greater size
of the African great lakes.

Abyssinia, as we have seen, was probably drier than at
present, but further south the rainfall must have been
considerably greater. Lake Kioga stood 600 feet above
its present level, and was connected with Lake Victoria.
Lake Victoria and the smaller lakes were twice theix
present size, and most of the broad valleys were filled
with water. Lake Magadi is the attenuated relic of a
vast sheet of water, and other great lakes have dis-
appeared entixely. One of these, in the Rift valley,
South of Lake Naivasha, has been mapped by Professor
Gregory and named after Professor Suess. Part of this
decrease of the lakes has undoubtedly taken place within
historie times, and part may be attributed to changes
in the drainage, but there remains enough evidence to
show that some time in the Ice Age the great lakes were
very much larger than the present.

Mr. E. J. Wayland, the Government Geologist_of
Uganda, informs me that in the old basin of Victoria
Nyanza there are masses of gravel which may be^ two
or three miles in brëadth, the surface of which forms two
terraces at different levels. Above the level of these
is an old peneplain with ancient beach gravels. Mr.
Wayland considers that this peneplain was formed
probably during the Pliocene by the first Victoria
Nyanza occupying a basin between folds. The initial
  AFRICA   105

high-level was due to the want of an outlet, but may
have been amplified by other causes. The level of the
lake then sank gradually to a considerably lower level,
after which it rosé again nearly to its old level and
remained there for a consideratie time. Dnring this
period the great gravel deposits were formed; they
contain flood deposits, espedally near their base. The
level of the lake then sank again and this part of the
basin was converted into a valley occupied by a river.
Subsequently the level rosé again sufficiently to carve
out the lower terrace in the gravels. Mr. Wayland
considers that the upper terrace may also represent a
.stage distinct from that in which the gravels were
actually deposited, but the upper terrace may be the
original surface of the gravels. Thus there is evidence
of two Pluvial periods in central Africa, of which the
first, probably córresponding with the great extension
of the mountain glaciers, was the greater. From the
archaeological evidence it appears to correspond with
the Mindelian glaciation of Europe.

A second line of evidence has been pointed out by
Q. W. Hobley. At the entrance to Kilindi Harbour,
Mombasa, there is a gap in the coral barrier through
which the fresh water from the river finds its way.
These gaps are always found opposite the mouths of
rivers, and are due to the inability of the coral polyp to
live in fresh or brackish water. In Pleistocene times the
land stood some seventy feet lower relatively to the sea,
and the old channel through the reef at this height is
almost doublé the width of the present channel, showing
that the river then had a greater volume, i.e. the rainfall
in its basin was greater.

But Africa is noteworthy chiefly for its deserts, and
the most important evidence of climatic change is
found in the deserts of Sahara and Kalahari. From
the time of the ancient Greeks it had been believed that
the Sahara was formerly the site of a great inland sea,
and the presence of this sea had even been suggested
  106   THE EVOLUTION OF CLIMATE

as the cause of the Ice Age in Europe, but recent investi-
gations have shown that this is not so ; the Sahara has
been land at least throughout the Tertiary period.
There is, however, abundant evidence that during the
Quaternary the rainfall was considerably greater than
the present. The presence of numerous animals closely
associated with water, such as the hippopotamus and
even the crocodile, in oases now entirely isolated, shows
that these oases were formerly connected with the big
rivers. The most definite evidence, however, comes
from Lake Tchad. This was formerly of much greater
area, but Chudeau and Freydenberg have made out a
whole series of changes from desert conditions in the
Tertiary through pluvial conditions in the Quaternary
back to desert conditions of the present. The sequence
is as follows:

1.   A regime of dunes.

2.   A slow transgression causing a long marshy period,

during which numerous plants whose remains
are found lived in the period.

3.   A slow regression.

4.   A rapid transgression (grey loam).

5.   A slow regression (clayey white loam with tracés

of roots).

6.   A transgression (white loam).

7.   Establishment of a new dune regime.

In the Chari basin east of Lake Tchad are the remains
of fish and shells, and also small pebbles of sandstone
and chalcedony, which are not local, but must have
been brought from the mountains of Tibesti by the
rivers Egnei and Toro when their current was much
stronger than at present. In Senegal, south of the
I5th parallel, the present dune sands are underlain by
an alluvial soil, indicating moister conditions preceding
the present climate. There is no means of dating the
moist periods indicated by these phenomena, but it is
reasonable to correlate them with the former extension
of the central African lakes.
  AFRICA   io 7

Passing south to the Kalahari, we find evidence of a
number of moist stages separated by drier intervals, but
they can apparently be grouped into two main Pluvial
periods, separated by a long interpluvial with steppe-like
conditions. One at least of these Pluvial periods must
be correlated with the former immense extension of
Lake Ngami and the Etosha Pan.

From Cape Colony there is some evidence of moister
conditions in the past, but the Quaternary variations
cannot be separated from those of historie times.

Before leaving Africa some reference must be made to
an interesting suggestion by C. W. Hobley, as to the
jnechanism_pf_climatic change in. tropical countries.
He notes that the north-east and south-we.st monsoons
extend to a height pf only. a few .thousand feet. Above
thèm are the very steady “ trade winds ” connected with
the general circulation of the atmosphere. In Kenya
Colony these blow from east or a little south of east.
“ Their effect is very marked on the high mountains of
the interior, such as Kenya, Kilimanjaro and Elgon;
in the early morning they are generally quite clear, but
about io a.m. the clouds sweep up from the S.S.E. and
collect on the mountains and blot them out from view for
the rest of the day. These are believed to be clouds bome
inland by the trade winds, and the moisture they carry
is precipitated mainly on the south and south-east sides
of the mountains.” Hobley suggests that there was
formerly a 'nearly continuous ridge of high land extending
north and south, and this caught the moisture from the
trade winds, so causing the Pluvial period, the evidence
for this ridge being the distribution of alpine plants on
the now isolated high mountains. An alternative ex-
planation is that the greater strength of the earth’s
circulation during glacial times caused the trade winds
to be much stronger and also to extend to a lower level
at the expense of the monsoons, just as the west winds
extended to a lower level in northern Egypt. This
would bring a great deal more moisture to be precipitated
  108 THE EVOLUTION OF CLIMATE

on the mountains, increasing the length of the glaciers
and also the volume of the rivers.

BIBLIOGRAPHY

Scott Elliott, G. F. “ The geology of Mount Ruwenzori and some adjoining
regions of tropical Africa.” Q.J.G.S., 51, 1895, p. 669.

Hobley, C. W. “ The alleged desiccation of East Africa.” Geogr. Journ.,
44, I9H, P- 467-

Freydenberg, H. “ Le Tchad et le Bassin du Chari.” Diss. Paris, 1908.
Passarge, H. “Die Kalahari.”
  CHAPTER XI

AUSTRALIA AND NEW ZEALAND

The continent of Australia has a relatively low relief,
only rising above the snow-line in Mount Kosciusko, and
glacial tracés have a relatively unimportant development.
The history of the region appears to be as follows:

In late Tertiary times the shore-line lay some distance
to the east towards New Zealand, this being a relic of a
much earlier connexion between the two lands. Towards
the close of the Tertiary earth movements set in, which
elevated the mountain belt of eastern Australia and
formed a land connexion with Tasmania and the Ant-
arctic continent. At the same time the land to the east
and the closed basins of central Australia were also
probably developed about this time. The climate was
then somewhat warmer than the present, at least on the
east coast, for the Australian barrier reef extended
further south. Probably at this time the Antarctic
ice-sheet did not reach the sea, and there was none of the
floating ice which is such an important factor in cooling
the Southern Ocean.

The next stage was the lowering of the snow-line on
Kosciusko to about 3000 feet below the present and the
development of extensive glaciers, which descended to
5500 feet above the sea, and attained an area of 80 to 100
square miles and a thickness of at least 1000 feet. Tas-
mania was also extensively ice-covered, probably by
glaciers which coalesced at low levels, forming what is
known as a “ piedmont ” ice-sheet, which possibly reached
the sea. The lowering of the snow-line in Tasmania is
109
  iio THE EVOLUTION OF CLIMATE

estimated as 6000 feet, corresponding to a faU in
temperature of 180 F. Probably a large part of this faU
is accounted for by the increased elevation, which may
have been several thonsand feet in Tasmania and more
than a thousand feet even in New South Wales. This
glaciation, which was probably dependent on the growth
of the Antarctic ice-sheet, was followed by a very long
interglacial, the duration of which has been estimated
by Professor David as 100,000 to 200,000 years. The
old moraines are much weathered and denuded, re-
sembling in this respect the older moraines of Europe.
No information is available as to the climate of this
interglacial period. Possibly some of the Quaternary
raised beaches with warmth-loving mollusca found in
unglaciated parts of Australia belong to this period,
and if so the climate was warmer than the present for
at least part of the time.

The interglacial was followed by uplift and a second
much less severe Glacial period, characterized by valley
glaciers on Kosciusko and in Tasmania, reaching the sea
in places on the latter island. It was at the close of this
'Glacial period that man reached Tasmania ; its con-
clusion is dated by Prof. David at about 10,000 years ago.
It was terminated by a period of depression below the
present level with a warm climate.

In the dry interior of Australia there is evidence that
at one time, probably during the maximum glaciation,
the rainfall was heavier than the present, and numerous
lakes were developed which have now been dry for a
very long time. It is possible that the artesian water
supply of Australia, which Gregory considers to be
“ fossil water ” accumulated under different conditions
from the present, is a vestige of the rainfall of this period.
Further north, in Java, the beds in which the famous
Pithecanthropus skeleton was found, believed to be lower
glacial, contain also plant remains similar to those now
found in the Khassian mountains of Assam, one of the
rainiest climates in the world. The climate of Java
  AUSTRALIA AND NEW ZEALAND in

during the maximum gladation was thus decidedly
rainier, and probably somewhat cooler than the present.

An extraordinaiy find which may be referred to here
is that of Professor Neuhauss, who discovered giant
erratics, scratched and poüshed, and moraines at sea-
level at the western end of Huon Gulf, New Guinea.
The region is very unstable, and is known to have stood
at a very much higher level, perhaps 10,000 feet or more,
in Quaternary times, and if the moraines indicate glaciers
terminating at 10,000 feet above the sea they are
explicable by a slight fall of temperature and increase
of snowfall.

Turning now to New Zealand, we find extensive
glacial remains on South Island, though not on North
Island. As in so many other countries, the Quaternary
opened with great elevation, which reached at least
1500 feet over the whole group. North and South
Islands were united with each other, with Stewart Island
and probably also the outlying islands, even including
the Chatham Islands, forming a great land-mass several
times the present area of New Zealand. On the Southern
part of this land-mass extensive glaciers were formed;
on the east these did not reach the present sea-level,
but on the snowy south-west they extended far below it,
so that the terminal moraines are now completely sub-
merged ; possibly they were never formed, but the debris
was floated away seaward on icebergs. Further north
moraines are found near the present shore line at many
places between Milford Sound and Hokitika, and morainic
mounds cover a large part of .the low ground. Still
further north they retreat inland, and in the Nelson
Province are not found lobew a level of 2000 feet at the
foot of Lake Rotoiti.

In the south-east a great moraine has been described
at the south end of Lake Wakatipu and others at the
north-east ends of Lakes Manapouri and Te Anau, but
none are found nearer the sea-coast. The glaciated
area of New Zealand was at least ten times the present
  112 THE EVOLUTION OF CLIMATE

[Prometheus]

ice-covered area, and the Tasman, the longest glacier in
New Zealand, was expanded from its present length
of 16 miles to at least 30 miles. Much of the apparent
fall of temperature shown by this glaciation was probably
due to the great elevation, but apart from this the ice
had a marked influence on climate. Outside the limits
of glaciation on the east is a thick deposit of typical loess,
which extends up to a level of 1000 feet on the flanks
of the hills. The occurrence of this loess points to a
steppe climate with dry, cold, southerly winds on the lee
side of the glaciated mountains, and is probably also
connected with the increase of land area. Further
north, north of Auckland in North Island, the present
treeless plains were covered by forests; for Kauri gum,
apparently very old, has been found. The sub-antarctic
islands—Campbell, Antipodes, etc.—were not covered by
either New Zealand or Antarctic ice, but were the
centres of local severe glaciations of their own.

The next stage was a great subsidence, during which
the glaciers retreated. The land sank below its present
level, raised beaches probably of interglacial age being
found at various heights ranging from 10 feet above the
sea at Manukau in the centre of North Island to 150 feet
at Taranaki, 200 feet at Cape Palliser, 400 feet on the
west coast of South Island, 500 feet at Amuri Blufï, and
even 800 feet in the entrances to the south-western
sounds. This great submergence was associated with
the deposition of extensive gravel deposits by the rivers.

The interglacial was followed by a second period of
elevation. It is not certain how far this went. Sub-
merged peat bogs have been found at a depth of nearly
600 feet below sea-level near Canterbury, but these may
belong to the early stages of the interglacial and not to
the post-Glacial period. On the other hand, the sub-
merged forests which are found at many points on the.
coast of New Zealand are evidently post-glacial and indi-
cate a slight rise above present level. At the same time
there was a renewal of the glacial conditions, but the ice
  !AUSTRALIA AND NEW ZEALAND 113

was confined to the valleys and had a much less extent
than in the first glaciation. This period seems to have
been followed by a slight submergence and a temporary
warm period.

BIBLIOGRAPHY

David, T. W. E. “ Australasie. Les conditions du cllmat auz époques
géologiques.” Rep. Congr. Geol. Internat., io, 1906, pp. 275-98.

Süssmilch, C. A. “ An introduction to the geology of New South Wales.”
Sydney, 1914.

Gagel, C. “ Beitrage zur Geologie von Kaiser-Wilhelms Land.” Beitr.
geol. Erforsch. deutscb. Scbutxgebieten, Berlin, 1913, H. 4.

Marshall, P. “ The glaciation of New Zealand.” Trans. New Zealand Inst.,
42, 1909, p. 334.

Salenka, M. L. et al. “ Die Pithecanthropusschichten auf Java.” Geol. und
Paltsol. Ergebnisse der Trinilexpedition (1907 und 1908). Leipzig, 1911.
  CHAPTER XII

THE GLACIATION OF ANTARCTICA

The great Antarctic continent offers a unique problem
to the glacial climatologist, for here we have a land area
with the theoretical snow-line already at sea-level, and
accordingly covered with a thick ice-sheet that leaves
only a few mountain ranges and nunataks exposed above
its surface, and yet in the past these ice-sheets and
glaciers have attained a thickness several thousand feet
greater, and have extended further north. Various
suggestions have been made to account for this former
extension, perhaps the most remarkable being that it
coincided with a milder and therefore snowier climate.
This, however, is untenable, for the Glacial period of
Graham Land and the South Orkneys is obviously a
southward extension of the Glacial period of Tierra del
Fuego, which was obviously due to a colder climate,
and ca'n be traced northward along the Andes into
tropical regions. A more fruitful suggestion is that as
one of the most potent factors in preventing the accumu-
lation of snow is at present the wind, it was a decrease
in the strength of the wind which enabled the ice to
reach a greater thickness. This is probably true in a
sense, the decrease of wind force being due to a great
increase in the area of the Antarctic continent during
the Quaternary.

We have seen that in the early Quaternary there was
great elevation in the south of South America and also
in Australia and New Zealand. The amount of this
elevation increased southward and was very great near
114
  THE GLACIATION OF ANTARCTICA 115

the polar circle. This is borne out by considerations
based on the distribution of living and fossil animals,
which point very definitely to a land connexion
between Australia and South America in Tertiary
and early Quaternary times, most probably by way of
Antarctica.

The first line of evidence is the distribution of the
marsupials, living and extinct. As is well known the
chief home of this type of mammal is now in Australia
and New Guinea, but in Tertiary deposits in Patagonia
remains of extinct forms known as Dasyurids have been
found, which are allied to Australian forms, and can only
have come from Australia, probably via Tasmania.
Secondly, there are two peculiar families of fresh-water
fishes, the Haflochitonia.ee and GalaxiicUe, the first
common to Australia and South America, while one
species of the second is found in New Zealand, Tasmania,
the Falkland Islands and Patagonia. Thirdly, Beddard
has found an intimate relation between the earthworms
of New Zealand, Eastern Australia and Patagonia.
Finally there is a curious similarity between the slugs of
Patagonia and those of Polynesia.

What is the explanation of these relationships ? As-
suming that there has been a land-connexion, it can have
been either by way of Antarctica or Polynesia. The
earthworms cannot endure a very severe climate, but on
the other hand there is a total absence of any tropical
forms common to Australia and South America, and the
general dissimilarity of the faunas shows that the con-
nexion cannot have been available for a very long period.
A study of the oceanic depths suggests that the Antarctic
connexion is the more probable. A comparatively
slight elevation would connect Patagonia and the Falk-
land Islands with the South Shetlands and Graham Land,
and an elevation of 12,000 feet would give a large land
connexion between Australia and the opposite coast of
Antarctica via Kerguelen. Forbes even postulates an
immense Tertiary Antarctica in which several forms of
  n6 THE EVOLUTION OF CLIMATE

animals and plants were able to evolve, but except
possibly in the case of the edentates this supposition is
not necessary.

The course of events may provisionally be taken as
follows : In late Tertiary times an elevation of at least
12,000 feet in the South Polar regions caused a great
increase in the area of Antarctica, which was united to
South America on the one hand and Australia on the
other. The northern shores of this continent were
far to the north of their present position, and though the
interior was very cold the coast lands had at first a
moderate temperature, and for a short time allowed
animals to migrate from Australia to South America or
vice-versa. But the high mountains of the interior were
already glaciated, and ice-sheets gradually crept down
their slopes. Owing to the small precipitation the
advance of the ice-sheets was slow, but ultimately,
probably in late Tertiary times, they approached the
coast, and the track along which migrations had taken
place was closed. The distribution of animals and
plants shows quite clearly that the land connexion was
maintained into the period of refrigeration. The shores
of the continent being further north, the pressure gradiënt
between the pole and the present coast was less, and
consequently the winds were lighter. This and the
diminished loss by calving into glaciers allowed the ice
to become thicker than it is now.

Hedley apparently considers that the migrations
referred to above took place in an interglacial period,
but the Patagonian beds in which the fossil marsupials
are found are Tertiary and not Quaternary. No direct
evidence of an interglacial period has been found in
Antarctica, nor, considering the intensity of the glaciation
which the country is even now undergoing, is any such to
be expected, and we can only infer from the bipartition
of the Glacial period in Australia, New Zealand and
South America—which, in New Zealand at least, was
associated with submergence—that there was probably a
  THE GLACIATION OF ANTARCTICA 117

similar bipartition in Antarctica. Nordenskjold States
that the submarine relief showing river erosion which,
in Tierra del Fuego, was developed partly at least in the
interglacial period, is also developed in West Antarctica.
It is improbable that the ice ever entirely vanished from
the continent. We shall see in Chapter XIV that even
the comparatively brief warm period known as the post-
glacial climatic optimum extended to the Antarctic
coast, and this is additional argument for extending the
much greater interglacial oscillation southward beyond
its known limits in Tierra del Fuego, Australia and New
Zealand, but here the matter must be left.

BIBLIOGRAPHY

Hedley, C. “ The palaeographical relations of Antarctica.” London, Proc.
Linneean Soc., 124, 1911-2, p. 80.

Lydekker, R. “ A geographical history of mammals.” Cambridge Univer-
•ity Press, 1896, pp. 125 ff.

David, T. W. E. “ Antarctica and some of its problems.” London, Geogr.J.
43, 19H, PP- 605-30.

Nordenskjold, O. “ Antarktis.” Handbucb Regional Geologie, Heft 15,1913.
  CHAPTER XIII

THE CLOSE OF THE ICE AGE—THE CONTINENTAL PHASE

In Chapter V we left the climatic history of northern
Europe at the point where the ice in its final readvance
had once more reached the German coast. But Scandi-
navia was now sinkin g, and the margin of the ice soon
began to retreat again. At the same time the Alpine
glaciers diminished in size, while the Irish and Scottish
glaciers disappeared. This is the critical period in the
change from glacial to temperate conditions, and, thanks
to the researches of the Swedish geologists, and especially
G. de Geer, H. Munthe and Gunnar Andersson, we
are very well acquainted with it. The change was not
uniform ; at first the recession was very slow, and there
were periods when for scores of years the ice-edge
remained stationary or even readvanced, but on the whole
the time was one of persistent amelioration. The
following description is based chiefly on W. B. Wright’s
summary of de Geer’s work.

After leaving the coast of Germany the ice-edge
appears to have remained in the western Baltic, retreat-
ing slowly for some 8000 years. About 10,000 b.c. it
lay along the Southern coast of Sweden, and during the
next 2000 years it withdrew to about 59° N. This
was the Gotiglacial stage. Here came a pause, when,
for 200 years, about 8000 b.c., owing presumably to
a change for the worse in the climate, the ice-edge
remained in one position, forming a great moraine.
Then came another period of very' rapid retreat, the
Finiglacial occupying nearly 3000 years, followed by a
118
  CLOSE OF THE ICE AGE   119

further halt of some duration near Ragunda, about
5000 b.c. After this the ice-sheet split into two portions,
and the Glacial period is regarded as over.

In the Alps there were similar periods of regression
and of halting or readvance. The first, known as the
Buhlstadium, corresponded to the Baltic readvance
(Chapter V). The second, the Gschnitz-stadium, with
a snow-line 2000 feet below the present (i.e. mean
temperature about 6° F. lower than now), has not been
dated, but probably occurred about 8000 b.c. This
was followed by a warmer period, probably as warm as
and drier than the present, after which the glaciers
readvanced about 5000 b.c. in the third or Daun-stadium,
when the snow-line was depressed 1000 feet (tempera-
ture 30 F. lower than now).

In the lower Nile valley the deposition of gravel
ceased, and that of mud began about 8000 b.c., indicating
that at this time the climate of north-east Africa reached
its present state of dryness.

It is at present difficult to give more than a tentative
explanation of these oscillations of climate during the
Retreat Phase. Northern Europe was at the time
passing through a complicated series of geographical
changes. As the ice left the Baltic basin the latter
became the site of a cold ice-lake, with narrow outlets to
the Atlantic by way of the Sound and the Beits. At
this time the recession was slow. Then the retreat of
the ice opened a connexion with the White Sea, and
elevation closed the outlet to the west. This probably
made the waters still colder, and the Fennoscandian
pause occurred. Elevation now closed the connexion
with the White Sea, and an entirely closed-in ice-lake
resulted. During this stage the retreat was slow, until
between 7000 and 6000 b.c., when the ice-sheet vacated
Scania, and direct communication between the Baltic
and the Atlantic was opened across Lakes Wener and
Wetter, and the climate, though still arctic at first,
became appreciably warmer by 6000 b.c.
  i20 THE EV0LUTI0N OF CLIMATE

For more than 10,000 years of the retreat, or until
6000 B.c., the ice-sheet was still sufficiently large and
powerful to maintain a border of Arctic anticyclonic
conditions on its southem edge. During the retreat the
mean annual temperature of southem Sweden increased
from 170 F. to 350 F., equivalent to a change from
North-east Greenland to South Greenland. The July
temperature rosé to about 430 F. On the North German
Plain still lived the reindeer and the fauna and flora of
the sub-Arctic tundras; the mean annual temperature
rosé to 450 F. by the close of the period. The land flora
in Sweden was entirely xerophilous, indicating a slight
rainfall. There is also geological evidence of a small
annual rainfall on the south-west coast of Norway.
This period covers the transition from Palaeolithic to
Neolithic culture.

It seems probable that the Continental character of
the climate of the final stages of the retreat phase was
slightly increased by astronomical causes, the obliquity of
the ecliptic being probably nearly one degree greater
about 7500 B.c. than it is now. In Germany and Sweden
this would have the effect of lowering the winter tempera-
ture and raising the summer temperature by rather more
than i° F.

While the land was still falling rapidly in the north of
Scandinavia and the Gulf of Bothnia, the coasts of
Germany and Denmark began to rise, and about 6000 B.c.
again closed1 the outlet of the Baltic, converting it into a
large fresh-water lake, the Ancylus lake. A similar lake was
formed farther east in central Finland. At this time the
south-west Baltic lands stood more than 100 feet higher
than at present. The land was probably still largely under
the influence of dry easterly winds, and the shutting out
of the Atlantic accentuated the Continental conditions,
and this stage in the climatic history of Europe is known
as the “ Continental Phase.” The winter climate was
severe; at first the summers were not especially warm

1 See reference to Antevs in this connexion.
  121

CLOSE OF THE ICE AGE

(July temperature about 540 F. in Southern Sweden).
This is probably the period of formation of the Ragunda
moraines, and of a readvance of the glaciers on the
Norwegian side of the divide, when the snow-line lay
200-300 metres lower than at present; it was also the
time of the Daun readvance in the Alps. But as the
land sank in the north and rosé in the south, the waters
of the Ancylus lake retreated farther and farther north,
and the summers became hot and dusty, with a mean
July temperature of about 6o° F. Everywhere in the
Baltic regions the older Ancylus beds show a monotonous
pine-wood, but in the upper Ancylus these are followed
closely by a number of plants and shrubs of Southern
type—black alder, curled birch, linden, etc. The
temperature continued to rise, and oak, Norway maple,
ash, and finally, in the southernmost parts of Sweden,
the common maple appeared. The last-named plant
has been found below the present level of the sea in
Ystad Harbour.

Under the influence of these conditions the remnant
of the Scandinavian ice-sheet again decreased in size,
until it split into two portions, the break occurring at
Ragunda, and this is considered by Scandinavian geolo-
gists to mark the end of the Ice Age in Europe. Gunnar
Andersson compares the climate of Southern Sweden at
this time to the Baraba Steppes in western Siberia, with
an annual rainfall of 12 to 16 inches, but this seems an
extreme estimate. The “ Karst ” flora of the lime-
stone areas of south-east Europe immigrated into
eastern Sweden during this period, and south-east
Europe probably gives a better idea of the climate of
Sweden during the Continental phase. Farther east,
in Finland, Kupffer describes the climate as resembling
that of central Russia. In central Germany the climate
was dry, with a mean temperature in the four summer
months of 63° F.; it resembled that of south-west
Russia. This period of warm summers began earlier
in Germany than in Sweden, and throughout this phase
  122 THE EVOLUTION OF CLIMATE

Scandinavia was occupied by a rich forest flora. The
hazel extended several degrees north of its present
position, and to higher levels, indicating a July tempera-
ture about J° F. higher than the present. In Southern
Norway the pine extended to much greater heights.
But the ivy and yew, whose limits depend on the winter
rather than on the summer temperature, showed no such
extension, indicating that the winters remained severe.
In Denmark there was a dry climate, fairly warm at the
close, with fir forests, though western Denmark is now
too wet for this tree. On the coast of Norway the seas
were still cold, so that there is a contrast between the
animal life of the sea and the plant life of the land. The
Alps also became warm and dry, and were occupied by
a xerophilous flora.

As the glacial anticyclone decreased in intensity,
depressions from the Atlantic began to take a more
northerly course, but were held up near the British Isles
and materially increased the rainfall. This is the first
peat-bog period of these islands, when the birch and pine
forests which had covered the non-glaciated lands during
the cold dry period gave way to extensive growths of
peat-bogs. Southern and eastern England, however,
largely escaped this damp period, sharing in the dry
climate of the Continent.

The absence of storms off the north-west coast of
Norway is shown by the forests which at this period
covered all the outermost islands of Norway as far as
Ingo Island, ofï North Cape. These islands are now
barren, and their afforestation indicates a drier and
especially a less stormy climate than the present, with a
decreased frequency of winds from the sea. These
conditions were well developed about 5000 B.c. This
is the Early Neolithic period. Owing to the great
development of forests, this period is sometimes called
the Early Forest period.

The late glacial history of North America was equally
complicated. Consider first the region of the St.
  CLOSE OF THE ICE AGE   123

[Prometheus]

123

Lawrence Estuary and the Great Lakes. As the Wis-
consin ice-sheet retreated across the present site of the
Lakes, the latter underwent a remarkable series of
fluctuations of area and outflow, which have been made
the subject of brilliant studies by several American
geologists. The opening stage began when the ice
abandoned the high ground south of the lakes, leaving
depressions bounded on the south by the hills and on
the north by the ice. The earliest of these in the basins
of Lakes Erie and Huron are known as the first and
second Lake Maumee. These gradually grew in size
and coalesced, forming several series of connected lakes,
to which various names have been given; thus Lake
Warren extended well outside the present limits of
Lake Erie and Southern Huron, and was held up by ice
over Lake Ontario and northern Huron. At a later
stage an enormous Lake Algonquin extended beyond
the combined limits of Lakes Superior, Michigan and
Huron, and communicated by broad channels with an
enlarged Lake Ontario known as Lake Iroquois, and with
Lake Erie. But even before this time the northern shores
of the lakes, relieved of the major portion of their ice-
load, had begun to rise rapidly, and ultimately reduced
the lakes to their present size.

These great areas of ice-cold water, bathing the Southern
edges of the ice-sheet, must have had an unfavourable
influence on the climate, keeping it cold and damp, and
preventing dry Continental conditions from becoming
established. They probably retarded the ice-retreat in
these regions quite considerably, so that a lobe of ice
was left here long after the edge had retreated north-
wards on either side. At the same time the climate
further south was dry, with seolian deposits; but as the
anticyclonic winds blew off the Atlantic the evidence
of drought is not so marked as in Europe.

After this slow retreat had been in progress for a con-
siderable time a submergence, known as the “ Champlain
stage ” set in, reaching a depth of at least 600 feet and
9
  I24 THE EVOLUTION OF CLIMATE

opening the St. Lawrence regions wide to the Atlantic,
which penetrated into Lake Ontario. The ice now
retreated rapidly under the influence of a maritime climate
little colder than the present. In phase this period corre-
sponds to the second Toldia Sea stage of Scandinavia ;
in point of time it was probably somewhat earlier. This
was followed by elevation, the first result of which was
to cut o ff the warm water am1   1 r 11

moraines, but a few thousand years earlier and probably
more marked. The continuance of elevation brought
on a long Continental period of extreme aridity, when
tree' grew on the peat-bogs of the eastern States, while
the lakes of the Great Basin further west were almost
or wholly dried up. At the maximum of the Continental
conditions the summers at least were warmer than at
present, as indicated by the northward extension of
various species of plants and fresh-water mollusca. The
winters were probably more severe. Possibly the great
aridity of this period was partly due to a sub-glacial
Continental anticyclone obstructing the path of depres-
sions across America from west to east. The drainage area
of the Great Basin received hardly any rainfall and was
a hopeless desert, but the Atlantic States were able to
grow trees on the old peat-bogs, probably with rainfall
derived from the Atlantic. By reference to the cutting
of Niagara gorge, we can infer that the warm dry period
began about 6ooo b.c., so that it corresponds exactly
with the Continental phase (Ancylus stage) of Europe.
This period of aridity was finally ended by a fresh sub-
mergence, the “ Micmac” which carried the land about
twenty feet below its present level.

In Yukon and Alaska, where the glaciation was not
nearly so severe as further to the south-east, the depres-
sion of the land by the ice-load and consequently the
subsequent rise on its removal were not great. There
were no complicated geographical changes, and corre-
spondingly there appear to have been nö fluctuations

temperature exactly analogous
  CLOSE OF THE ICE AGE   125

of climate, but only a gradual passage to present con-
ditions.

Even in Iceland there are indications of a dry period
following the last glacial maximum, for tree-trunks,
buried in the peat-bogs, show that the birch formerly
had a much greater extension. It is also quite possible
that there was an accentuation of desert conditions in
Asia during the retreat of the glaciers in Europe and
North America, which may have played a part in the
wave of Neolithic migration that appears to have over-
whelmed the artistic Palaeolithic races of western
Europe; but of this we have as yet no direct evidence.
The Neolithic invasion of Europe took place along two
main routes, the Nordics passing from the centre of Asia
north of the Caspian, across Russia to the Baltic shores,
where they became the Kitchen-midden people; and the
Alpine race passing from Transbaikalia, south of the Cas-
pian and Black Sea, into Southern Europe. The Nordics
drove before them an older race, characterized by the
transitional Maglemose culture, which passed from east
of Russia to the shores of the Baltic and ultimately to
England, where harpoons of Maglemose type have been
found beneath the peat of Holderness.

In the Southern hemisphere the Continental phase
does not appear to have been so well developed. The
uppermost part of the Pampean loess is possibly post-
glacial; more certainly so are the sand-dunes on the
coast near Buenos Aires, in which human remains have
been found in association with the bones of some extinct
animals. In New South Wales, after the retreat of
the glaciers, there was a period with land a little above
its present level, so ‘that the stools of Eucalyptus trees
are now found ten feet below sea-level; but there is no
evidence as to the climate óf this stage. In New Zea-
land we have no definite post-glacial beds of Continental
type. The occurrence of xerophilous plants, such as
Aciphylla, still living in a climate which is now decidedly
moist, may be a remnant of a Continental phase in New
  126 THE EVOLUTION OF CLIMATE

Zealand, or may date back to the steppe conditions of
the loess. As to Antarctica, we have, of course, no
evidence.

BIBLIOGRAPHY

Brooks, C. E. P. “ The evolution of climate in north-west Europe.”
London, Q. J. R. Meteor. Soc., 47, 1921, p. 173.

Wright, W. B. “ The Quaternary Ice Age.” London, 1914. (Ch. 17,
Late-gladal changes of level in North America.)

Bericht Internat. Geologenkongress, Stockholm, 1910. “ Die Veranderungen
des Klimas seit der Maximum des letzten Eiszdts.” Numerous papers,
dealing with Europe and North America.

Munthe, H. “ Studies in the Late-Quaternary history of Southern Sweden."

Stockholm, Geol. Foren. iForb., 22, 1910, pp. 1197-1292.

Antevs, E. On the late-gladal and post-gladal history of the Baltic. Geogr.
Re*., New York, 12, 1922, p. 521.
  CHAPTER XIV

THE POST-GLACIAL OPTIMUM OF CLIMATE

In most of the polar and temperate regions of the world
the Glacial period seems to have been separated from the
present by a short interval of slightly more maritime
climate. The existence of this phase was the chief point
brought out in the great collection of papers communi-
cated to the Stockholm meeting of the International
Geological Congress, which has frequently been referred
to in this volume. The pioneer work on the subject
has been done by the Scandinavian geologists, and we
may commence with a discussion of this period in the
countries bordering on the Baltic.

About 4000 b.c., at the conclusion of the Continental
phase referred to in the preceding chapter, a rapid
movement of submergence set in over the whole of the
Southern Baltic, and shortly afterwards the land-bar which
had formerly separated the fresh waters of the Ancylus
lake from the Atlantic gave place to a wide strait, through
which the waters of the ocean flowed into the Baltic
across Southern Sweden. Ultimately this channel be-
came wider than the present outlet between Sweden and
Denmark, and maritime influences penetrated to all
parts of the Baltic. In recognition of this influence the
period was termed by Blytt the “ Atlantic stage.”
The much greater freedom with which the waters of the
Atlantic were able to enter is shown by a comparison of
the “ isohalines ” of this period with those of the present
day. Isohalines indicate the degree of saltness of the
water; those of to-day can, of course, be measured
127
  128 THE EVOLUTION OF CLIMATE

directly, and show that in the Gulf of Bothnia the
water becomes continually less salt as we go northward,
for which reason many species of marine mollusca are
unable to live. By studying the distribution of the
mollusca in the Littorina Sea the isohalines of that period
have been reconstructed also, and show that the salt
content was much greater than at the present day,
indicating a greater influx of oceanic waters.

If we take a map showing a reconstruction of the
geography of Littorina time, and apply to it the formulae
given in the Appendix, comparing our results with the
inferences of Scandinavian and north German geologists
as to the temperature, we find that there is a remarkably
good agreement. Many of the palaeo-botanists comment
on the prolongation of the autumn into the present
winter, which is especially characteristic of a more
insular climate. The amounts of change in each case
are also in good agreement, except perhaps in the
Christiania region and in north Denmark, where the
geologists require a greater change than that calculated
from the land and sea distribution; this is probably
accounted for by a higher temperature in the waters
of the Atlantic. The maximum change as calculated
is shown in south-west Finland (winter 6° F. warmer,
summer 2° F. cooler). Finland is described as having
at that time the climate of western Europe, which
we may take as meaning winter 8° warmer, summer
3-40 cooler. There was thus a great change from the
extreme climate of the Continental phase with its hot
summers and severe winters and little rain, to an extremely
temperate climate with cool summers, mild winters and
a heavy rainfall. The warmth-loving plants which had
begun to immigrate during the later part of the Con-
tinental phase continued to spread, and probably the
highest average temperatures were reached at the time
of maximum submergence, but.now they were accom-
panied by plants for which a large rainfall is necessary,
and it seems that the average rainfall of Southern Sweden
  POST-GLACIAL OPTIMUM OF CLIMATE 129

must have been about 40 inches a year. The oak began
to dominate the forests in place of the hazel, and the
peat-bogs, which during the preceding dry period had
hardened into a firm surface on which birch and pine
were able to take root, again became moist, so that the
trees were choked by growths of bog-plants. On the
shores lived men of the Transition and Early Neolithic.
As the land rosé again and the Littorina Sea decreased in
area the climate again became drier and more rigorous.
In Denmark the forests of the Ancylus period gave place
to oak as the land sank, and there are also remains of two
water plants, the water-nut (Trapa natans), which is no
longer found in Denmark, and Najas marina, still living
in one isolated locality. Northern Denmark was broken
up into islands, among which marine deposits were
formed, containing the remains of Southern mollusca,
many of which are found in the kitchen-middings.
Most of the wood used by Neolithic man was oak ; there
is little fir and no beech.

In Norway the work of C. Brögger has made us familiar
with the Tapes beds, which correspond in point of time
to the Littorina stage of the Baltic. Tapes decussatus is
itself a Southern species of mollusc, and it is associated
with a very rich warmth-loving fauna. In Southern
Norway the geographical conditions were different from
those in Sweden, for the land reached its lowest level
relatively to the sea about the close of the Glacial period,
and has been rising throughout the post-glacial. The
seas show a progressive rise of temperature from 8° F.
below present at the close of the Glacial period to
40 F. above the present in the older Tapes beds. The
littoral climate at this stage resembled that prevailing
at present on the coast of northern England. After
this, as the land approached its present level, the tem-
perature feil again, and in the upper Tapes stage was
only 2° F. above the present.

The warm period represented by the Tapes beds is
found at intervals along the west coast of Norway, and
  130 THE EV0LUTI0N OF CLIMATE

we again find evidence of a submergence of the land
contemporary with the maximum temperature. These
conditions extend even as far north as Tromsö, within
the Arctic circle. In Spitzbergen there are raised
beaches 30 to 80 feet above the sea, containing remains
of molluscs and a species of Fucus, none of which are now
living so far north. On the land there are old peat-bogs
of great thickness, though peat mosses cannot now grow,
since the ground never thaws below a depth of 6-10
inches. It has been pointed out that a great number
of the plants now living in Spitzbergen are unable to
ripen their seeds under present climatic conditions,
though they must have done so in the past. Ripe seeds
of some species, in fact, have been found in the peat-
bogs, which are contemporaneous with the raised beach.
There is thus evidence of a very well-marked warm
period associated with submergence in Spitsbergen.

In Franz Josef Land, Nansen found raised beaches
with mussels 10 to 20 feet above the present level; this
shell does not now live so far north. In the White Sea
and on the Murman coast there are also raised beaches
with a Southern fauna. The warm period shown in the
beds of the New Siberian Islands has already been
referred to (p. 79).

Returning to the British Isles we find that in the
South the land was above its present level throughout
the whole of the post-glacial period. On the other
hand, a 25-foot beach is found in north-west England
(Formby and Leasowe marine beds), but without any
evidence as to climate ; the same applies to the 25-foot
beach of Scotland. It is only when we come to the
north-east of Ireland that we find evidence of conditions
appreciably warmer than the present, in the section of
the Alexandra Doek, B elf ast, where marine clays overlie
beds of grey sand and peat. The lower estuarine clay
is essentially a littoral clay, known as the Scrobicularia
zone. It is brownish-blue and sandy, and , contains in
abundance the roots and leaves of the grass-wrack
  POST-GLACIAL OPTIMUM OF CLIMATE 131

(Zostera marina), and a vast number of shells of a few
species which live between tide-marks, indicating that
die land stood 10 feet or so above its present level at
first, while the climate cannot have differed greatly from
that prevailing at the present day. It must have been
formed during a period of gradual depression, for
throughout its six feet or more of thickness it preserves
identical littoral characters. After a time this depression
became more rapid, and the upper estuarine clay began
to form—a light blue clay, very pure and unctuous, with
a very rich and well-preserved fauna, known as the
Thracia zone. The fauna has a decidedly Southern
aspect, and indicates that the coasts of north-east
Ireland had the present temperature of Bantry Bay—
an increase of at least 30 F. in the mean annual tempera-
ture. The Thracia zone is followed by a bed of yellow
shore sand, indicating re-elevation to about seven feet
above the present level.

Corresponding to the upper estuarine clay are raised
beaches at a height of 25 feet in north-east Ireland,
falling to 15 feet at Dublin, and to only 6 or 7 feet in
western Donegal and Sligo. The mollusca indicate a
somewhat higher temperature than the present. In the
beaches have been found flint scrapers and arrowheads
of early Neolithic type.

Looking further westwards, we find that Iceland,
which had undergone a slight elevation during the
Continental phase, so that peat was formed below
present sea-level, again subsided, falling to 10 or 12 feet
below its present level. During this subsidence the
temperature rosé, the greatest warmth coinciding with
the lowest level of the land. Species from the South-
west shores, where the temperature of the water is
directly influenced by the Gulf Drift, extended to the
cold northern coast. In some places the marine clays
of this period have been ploughed up by a subsequent
readvance of the glaciers.

From Greenland comes abundant evidence of a post-
  132 THE EVOLUTION OF CLIMATE

glacial warm period coincident with a subsidence of
about 8ofeet. Raised beaches all along the west coast
contain mollusca, some species of which are not now
living north of the St. Lawrence estuary. On the other
hand, some northern species which lived off the west
coast during the glacial maximum retreated northwards
during this period, and have not re-established themselves,
though the climate is now suitable. Further, K. Steen-
strup describes the occurrence of “ dead ice ” at several
places in North Greenland—masses of ice which have
become separated from their parent glaciers owing to
rapid recession, and are now buried in morainic matter.
Subsequently the ice again advanced, and in some cases
a new glacier has advanced over these masses of “ dead
ice.”

Passing to the mainland of North America, we find in
eastern Canada colonies of Southern mollusca, especially
oysters and quohogs, separated from their main area of
distribution south of Cape Cod by a wide area of cold
seas—the Gulf of Maine and Bay of Fundy. At the
beginning of the warm phase the land lay slightly below
its present level, but subsequently rosé above it. The
climate became still warmer, until its temperature
resembled that of the middle New England States. At
the same time the rainfall diminished and the peat-bogs
were replaced by forests of hardwood trees. In the
basin of the Great Lakes the warm period is represented
by gravel beds in the Niagara gorge, which from their
position must, according to the most recent determina-
tions, have been formed about 4000 to 3000 b.c. These
gravels contain shells of fresh-water mollusca, especially
species of Unio, which are not now living in the St.
Lawrence system, but are found in tributaries of the
Mississippi further south. Further south on the eastern
coast of the United States there are marine deposits
indicating a slight submergence, with a climate somewhat
warmer than the present.

Passing to South America, we find in Southern Pata-
  POST-GLACIAL OPTIMUM OF CLIMATE 133

gonia and Tierra del Fuego exactly similar evidence of a
post-glacial subsidence with a warmer climate than the
present. Raised beaches at a height of 50 feet contain
mollusca, some of which are now rare or extinct in that
locality, and in sheltered situations plants are found still
living whose nearest neighbours are some way to the
north.

[Prometheus]

In the same way, in Southern and eastern Australia
there are beaches a few feet above present level, con-
taining warmth-loving species of mollusca and indicating
a post-glacial warm period. Tliere is some evidence in
the distribution of plants and marine mollusca that this
warm period extended to New Zealand. Raised beaches
at a height of 50 to 180 feet are also known from many
places in Antarctica, and these contain mollusca, some of
which are not now living south of the sub-antarctic
islands. An interesting confirmation of'this has been
given by E. Philippi, from the results of an examination
of the sea-floor at four points in about 63° S., 75-950 E.,
all within the present limit of pack ice. The deposit
at present forming is poor in pelagic foraminifera, and
consequently contains little lime, but this deposit is
very thin, and beneath it is a much more calcareous clay
especially rich in Globigerina. The latter deposit is
still forming north of the limit of pack ice, and Philippi
concludes that at no very distant date the limits of ice
were further south, indicating warmer conditions. It
is interesting to note that a similar sequence has been
found in the Norwegian North Sea, the brown foramin-
iferous deposit (in this case containing Biloculina) being
known to be underlain as well as overlain by an
unfossiliferous grey clay attributed to the Glacial period.
Finally, with regard to Cape Colony, A. W. Rodgers
says : “ It is possible that the presence of marine mollusca
belonging to species that are only known in the living
state from the coast north of Pondoland, in the raised
beaches of Mossel and Algoa Bays, indicates that the sea
on the south coast was formerly warmer than now.”
  134 THE EVOLUTION OF CLIMATE

Thus we have evidence of a period of submergence
and climates warmer than the present from a large
number of places, including the Arctic Ocean and Green-
land, the temperate coasts of North America and Europe,
the Southern Ocean and Antarctica. The stage appears
to be missing on the temperate coasts of the Pacific, on
both the Asiatic and North American sides, and from
the whole of the Tropics. It is fairly certain that the
warm period occurred at the same time in eastern North
America and western Europe ; in the case of the Southern
hemisphere we have no direct proof of this, but in all
cases the deposits are comparatively recent, and since they
obviously refer to a similar state of affairs we may assume
that they are of the same date.

In the Baltic area we know that the great change of
level was due largely to a subsidence of the land and only
to a small extent to a rise of the sea. But in other parts
of the world the amount of submergence was remarkably
uniform at places in the same latitude, and decreased
steadily from the polar regions to about latitude 40-50°,
where it became zero. Now such a general change
suggests that it was the sea which rosé rather than the
land which sank, and points to some general cause which
piled up the waters of the oceans in the higher latitudes.
A possible cause of this nature has been adduced by
O. Pettersson, which he terms the “ tide-generating
force,” which reached one of its maxima in an 1800-year
cycle about 3500 b.c. This possibility will be dealt
with more fully in Chapter XVII.

BIBLIOGRAPHY

Bericht Internat. Geologenkongress, Stockholm, 1910. “ Die Veranderungen
des Klimas seit der Maximum des letzten Eiszeits.” Numerous
papers ranging trom the Arctic to the Antarctic.

Brooks, C. E. P. “ The evolution of climate in north-west Europe.”
London, Q-J- R. Meteor. Soc., 47, 1921, p. 173.

Praeger, R. LI. “ Report on the estuarine clays of the north-east of Ireland.”
Proc. R. Irtsb Acad., ser. 3, Vol. 2, 1892, pp. 212-89.
  POST-GLACIAL OPTIMUM OF CLIMATE 135

Goldthwait, J. W. “ The twenty-foot terrace and sea-cliff of the lower
St. Lawrence.” Amer. J. Science, ser. 4, Vol. 32, 1911, pp. 291-3x7.

Cowle>, H. C. “ A remarkable colony of northern plants along the Apala-
chicola River, Florida, and its significance.” Rep. 8 Internat. Geogr.
Congress, 1904, p. 599.

Shimer, H. W. “ Post-glacial history of Boston.” Amer. J. Science, ser. 4,
Vol. 40, 1915, pp. 437-42.

Halle, T. G. “ On Quatemary deposits and changes of lerel in Patagonia
and Tierra del Fuego.” Buil. Geol. Inst., Upsala, 9,1908-9, pp. 93-117.

Süssmilch, C. A. “ An introduction to the geology of New South Wales,”
Sydney, 1914.

Marshall, P. “ New Zealand.” Handbueb regional Geologie, Heft 5,1911.
  CHAPTER XV

THE FOREST PERIOD OF WESTERN EUROPE

Hitherto we have been dealing with climatic changes
which can be recognized with more or less certainty
over most of the polar and temperate regions of the world,
but we have now to describe a stage which appears to
have been peculiar to Europe and possibly Asia—the
Forest period. By 3000 b.c., or towards the close of
the Neolithic period, considerable elevation had again
taken place over the central latitudes of western Europe
(the northern parts of Norway and Sweden were still
several hundred feet below their present level). The
Southern part of the British Isles, which had remained
slightly elevated since the last Glacial period, had now
emerged to a height of nearly ninety feet above its present
level; the area of Ireland had increased appreciably and
part of the North Sea was land. The geographical
changes were not great, but they were sufficiënt to turn
the scale in the direction of a Continental climate in the
British Isles. The more or less complete closing of the
Straits of Dover, and the consequent bar to the free
circulation of the Gulf Drift, must have had an
appreciable effect on the climate in the direction of
continentality. At the same time the low level of
northern Norway, and possibly the persistence of warm
conditions in the Arctic basin, more and more attracted
depressions to the northernmost track, so that the
British Isles especially, and to a lesser extent Holland,
Germany, Southern Scandinavia and Russia, came more
persistently under the influence of anticyclonic con-
136
  FOREST PERIOD OF W. EUROPE 137

ditions. The rainfall of these countries diminished, and
the surface of the bogs dried sufficiently to enable forests
to grow in the western countries; in Germany heath-
plants took the place of bog-plants, while in Russia steppe
conditions supervened. The normal meteorological
conditions at this time in fact resembled those of the
memorable drought of 1921, which was characterized by
low pressure and stormy conditions in the Arctic Ocean
and a belt of high pressure and persistently fine weather
across central Europe.

During this phase the winters may have been severe,
but the summers were warmer than the present, for in
the peat-bogs of Ireland and Scotland are the remains
of trees larger than any now found in the neighbourhood.
The Irish bogs dried so completely that they were
extensively inhabited ; corded oak roads have been found
at this horizon, while in 1883 a two-story log house,
surrounded by an enclosure, was found in Drumkelin
Bog, Co. Donegal; it was twelve feet square and nine
feet in height, and a roadway led to it across the bog.
Both house and roadway were entirely constructed of
oak. With the hut were found a stone chisel and a
flint arrowhead. Beneath the floor were fourteen feet
of bog, and above the floor twenty-six feet. This time
was also one of relatively little wind movement, for stools
occur even in exposed positions on the mountain slopes
of western Ireland, where trees will only grow now in
"heltered positions near sea-level.

Further evidence of the very dry climate of this phase
is the frequent occurrence of trees apparently in situ
beneath the surface of fresh-water lakes, both in Ireland
and Scandinavia. I was able to examine one very good
example near Lough Toome in north-west Ireland.
An unusually dry spring had lowered the surface of the
water and a large number of tree-stools were exposed;
when these trees were growing the water-surface must
have been at least two feet below the level of the present
outlet. Most of the lakes in which these stools are
  i38 THE EVOLUTION OF CLIMATE

found are shallow upland basins with a small drainage
area, and if the present climate became drier they would
more or less completely disappear.

Mr. Fairgrieve has noted the action of blown sand on
the westward side of broken-off tree-stumps in a sub-
merged forest on the shore in south Wales, which,
though not conclusive, suggests dry conditions. Mr.
Fairgrieve also noted the direction of fall of twenty-one
trees, and found that in the great majority of cases they
were blown down by westerly winds.

The forest phase was short; according to the late
C. Reid the land again began to subside shortly after
3000 b.c., and by 1600 b.c., in Britain at least, had reached
its present level; this carries us to the beginning of the
Bronze Age. In connexion with Ellsworth Huntington’s
theory that the dampness of Ireland lowers the energy
of its inhabitants, it is interesting to note that this dry
period apparently corresponds to the legendary Heroic
Age, when the vigour of the Irish reached a level nev.er
lince attained. Civilization in Scandinavia also seems
to have benefited by the drier conditions, for Scandi-
navian technique advanced rapidly to a high level about
1800 B.c. But though there is evidence of a considerable
sea-borne commerce with Britain and Ireland, there
appears to have been comparatively little land traffic
between different parts of Scandinavia at this time.
In fact, to primitive man dense forest with thick under-
growth was almost impenetrable. But at the close of
the forest phase and the beginning of the peat-bog
phase the trees were weakening under conditions
becoming unfavourable. Such dying forests are marked
by the absence of undergrowth and young trees, and
afford safe and easy land communication. Accordingly
we find that by 1500 b.c. a considerable traffic had
developed across Scandinavia by land.

Although we have no direct evidence, the meteoro-
logical conditions suggest very strongly that the dry
belt extended across Russia into Siberia as a marked
  FOREST PERIOD OF W. EUROPE 139

period of desiccation, possibly worse than any droughts
of the historie period. At present Siberia receives its
rainfall mainly from depressions which cross Russia from
the Baltic or Black Seas, and follow a well-marked track
north of the central Asiatic mountains. But during
the forest period these tracks were abandoned, and the
majority of the depressions passed north-eastward ofï
the coast of Norway into the Arctic Ocean. The result
must have been a great diminution of rainfall over the
continent. We shall see later (Chapter XIX) that this
period of drought was of extraordinary importance in
human history. For during the moist maritime phase
central and eastern Europe, and probably also Asia, had
become extensively peopled by neolithic nomads of
Aryan and Semitic races, while the great river valleys of
the south were in the possession of dense agricultural
populations in a more advanced state of civilization.
As the climate became progressively drier and the pasture
diminished, the land was unable to support such a large
nomadic population, and there was a great outburst of
raiding and conquering expeditions directed southwards
and westwards, resulting in a succession of empires in
the rich Mesopotamian regions and neighbouring
countries, which form the beginnings of our history.
The beginnings of history in China also, about 2500 b.c.,
show that at this time the settled peoples of that country
were in trouble with the nomads of the interior.

BIBLIOGRAPHY

Reid, C. “ Submerged forests.” Cambridge University Press, 1913.
Lewis, F. J. “ The history of the Scottish peat-mosses and their relation
to the Glacial period.” Edinburgh, Scot. Geogr. Mag., 22,1906, p. 241.

10
  CHAPTER XVI

the“classical” rainfall maximum, 1800 b.c. to a.d. 500

About 1800 b.c., or the beginning of the Bronze Age
in Britain, the subsiding land finally attained approxi-
mately its present level. At the same time the climate
of western Europe deteriorated, becoming much more
humid and rainy, and there set in a period of intense
peat-formation in Ireland, Scotland and northern
England, Scandinavia and North Germany, known as
the Peat-Bog Period or Upper Turbarian. The peat-
beds choked and killed the forests which had developed
on the older peat-bogs, and grew up above the stools
and fallen trunks, so that we have two layers of
peat separated by an old forest. The forest level con-
tains neolithic articles, the peat contains gold collars,
bronze swords and pins, and other objects of the Bronze
Age. This growth also went on even over high ground,
which had not previousl; 1   11   r

near Enniskillen, and at o x __________ ______,______

Age caims and tumuli are found resting on rock and
covered by several feet of bog. Peat beds on the Frisian
dunes between two layers of blown s&nd are dated about
100 b.c., and some bogs in northern France were formed
during the Roman period. There is also some much-
disputed contemporary Latin evidence that at the time
of the Roman occupation the climate of Britain was damp
and boggy, while Gibbon (“ Decline and Fall of the
Roman Empire ”), referring to the climate of central

Professor Henry informs
  “ CLASSICAL ” RAINFALL MAXIMUM 141

Europe at the beginning 'of the Christian era, points to
some evidence that the climate was colder. This is,
that the Rhine and the Danube were frequently frozen
over, so that the natives crossed them with cavalry and
wagons without difficulty, although at the present time
this never happens. It is possible that this severe climate
is referred to in the Germanic legend of the “ Twilight
of the Gods,” when frost and snow ruled the world for
generations. The Norse sagas point to a similar cold
period in Scandinavia. This lapse of climate occurred
in the Early Iron Age, about 650 to 400 b.c., when there
was a rapid deterioration from the high Scandinavian
civilization of the Bronze Age. This deterioration of
culture was probably the direct result of the increased
severity of the climate.

This Pluvial period has been made the subject of
special studies by Ellsworth Huntington in several
important boots and papers; he finds evidence of a
distinctly Pluvial period in three regions—the Mediter-
ranean, central and south-western Asia, and an area
including the Southern United States and northern
Mexico. In the first of these, the Mediterranean,
Huntington considers that the Grseco-Roman civiliza-
tions grew up in a period of increased rainfall which
lasted from about 500 b.c. to a.d. 200. These States
were able to develop in comparative peace because during
this time there were no great invasions of nomadic
peoples from eastern Europe or central Asia, a fact
which points to good rainfall in these comparatively dry
regions, so that their inhabitants had no need to emigrate
in quest of a living. In the Mediterranean itself the
heavier rainfall allowed a solid agricultural basis which
produced a sturdy race of peasants who made good
soldiers. Owing to the greater cyclonic control of
climate and consequent changeable weather, these in-
habitants were more vigorous in mind and body, for
Huntington’s researches have demonstrated that long
spells of monotonous weather, either fine or rainy, are
  142 THE EVOLUTION OF CLIMATE

unfavourable for human energy. Finally the heavier
rainfall maintained a perennial flow in the rivers, giving
plentiful supplies of good drinking water. These con-
ditions broke down earlier in Greece than in Italy, as
the latter naturally has a heavier rainfall. Huntington
considers that the decline of Greece was largely due to
malarial poisoning, the decreasing rainfall causing the
river-flow to break down in summer, leaving isolated
pools forming a breeding ground for mosquitoes.

After a.d. 200 the climate of Italy also deteriorated.
The decrease of rainfall, combined with gradual
exhaustion of the soil, made wheat-growing more and
more difficult for the small agriculturalist, and the farms
came into the hands of large landowners, who worked
them by slave labour, and in place of wheat either grew
vines or olives or raised flocks and herds. The agricul-
tural population gravitated to Rome and a few other
large cities, and had to be fed by imported wheat. The
decline was probably aided by the introduction of
malaria, as in Greece.

In north Africa and Palestine the question is more
debatable. C. Negro, who has investigated the supposed
desiccation of Cyrenaica, concludes that there has been
no change of climate since Roman times, but a careful
study of his evidence suggests that his conclusions are
open to criticism. All that he has proved is that there
has been no marked Progressive decrease of rainfall since
about a.d. 200 ; he has ignored the possibility of great
fluctuations before and since that date. In north
Africa it seems difficult to believe that the great cities
of antiquity could have existed under present climatic
conditions, but when we turn to Palmyra in the Syrian
desert we have practically incontrovertible proof in the
great aqueducts, built to carry from the hill-springs to
the city large volumes of water which these springs no
longer deliver, so that even where they are intact the
aqueducts now carry only the merest trickle.

In Persia we find numerous ruins, which point to a much
  CLASSICAL ” RAINFALL MAXIMUM 143

greater population two thousand or more years ago.
This population lived by agriculture, and the remains
of their irrigation works are now found in regions where
running water never comes. Even the scanty population
of to-day can hardly live on the

population indicated by these ruined cities could have
existed without a very much greater supply of water.
The same condition is indicated by the ruined cities of
the great deserts of central Asia. These cities were
inhabited by agriculturalists, and the remains of tilled
fields, terraces and irrigation works abound in places
where the supply of brackish water would now be barely
sufficiënt for drinking purposes for such a large popula-
tion. Huntington has also made a careful study of the
water-level of the Caspian Sea in classical times, and finds
that there was a great period of high water extending
from unknown antiquity to about a.d. 400.

[Prometheus]

There is only one region in central Asia where the
population appears to have been less in classical times
than now, and that is the high basin of Kashmir. Hunt-
ington points out that this basin is at present near the
upward limit of agriculture, and any faH of temperature
and increase of snowfall would drive out the inhabitants.
But local legends point to such a period in the remote
past, corresponding to the period of increased habita-
bility of the central Asian deserts ; at its close there
were extensive migrations from Turkestan into
Kashmir.

Passing to America, we come to interesting evidence
of a very different class—I refer to the “ big trees ”
(Sequoia) of Califomia. Since these trees live in a semi-
arid climate, the amount of rainfall is the chief factor in
their growth, which finds an expression in the breadth
of the annual rings measured on the stump of the tree
when it is cut down. The method of utilizing the data
was due to A. E. Douglass. A careful comparison was
first made between the measurements of rings and the

country, and it is unbelievable
  144 THE EVOLUTION OF CLIMATE

rainfall measured at neighbouring stations, and a formula
was developed by which the rainfall of each year could
be reconstructed from the tree-growth with a high
degree of accuracy. In extrapolating to find the rainfall
for earlier years before rainfall measurements began,
various corrections had to be applied, for instance trees
grow more rapidly when young than when they are old,
while trees which are likely to live to a great age grow
more slowly at first than trees which die younger. These
methods were applied to nearly two thousand “ big
trees,” some of which were found to be four thousand
years old, but it is pointed out that the corrections
eliminate any progressive variation of cftmate which
may have occurred, so that the results show only “ cycles ”
of greater or lesser length. Summing up, Huntington
says: “ Judging from what we have seen of the rainfall
of to-day and its relation to the growth of the Sequoias,
high portions of their curve (of growth) seem to indicate
periods when the winters were longer than now, when
storms began earlier in the fall and lasted later into the
spring, and when mid-winter was characterized by the
great development of a cold Continental high-pressure
area, which pushed the storms of the prevailing zone of
westerly winds far down into sub-tropical regions and
thus caused sub-tropical conditions to invade what is
now the zone of equatorial rains.” Neglecting later
favourable periods, which are relatively short and unim-
portant, it is found that these conditions prevailed
very markedly between 1200 b.c. and a.d. 200, with
maxima about 1150 b.c., 700 b.c., and from 450 B.c. to
250 B.C.

Thus over the greater part of the temperate regions of
the northem hemisphere we have evidence of an im-
portant rainy period between the extreme limits of
1800 b.c. and a.d. 400 or 500. This period was best
developed from 1200 b.c. to a.d. 200, and reached its
maximum about 400 b.c. It constitutes a remarkable
wave of climatic variation, which is hitherto without
  “ CLASSICAL ” RAINFALL MAXIMUM 145

adequate explanation. A somewhat similar, though less
intense, wave which occurred about a.d. 1200-1300, and
which is described in the following chapter, was associated
by Wolf to a great outburst of sunspots which took place
about a.d. 1200. It is well known that sunspots are an
index of solar activity, the sun’s radiation being greater
at times of spot maximum than at times of spot minimum.
Greater solar radiation increases the evaporation over
the oceans, so that the air becomes more humid. This
moist air is carried by atmospheric currents over the
land, where the moisture is condensed into clouds and
greatly increases the rainfall. At the same time the 'cloud
canopy shuts off some of the direct heat of the sun, and
we have the curious paradox that at times of sunspot
maximum, or greatest solar radiation, the temperature
of the earth’s surface is lowest.

The connexion outlined above is, however, extremely
problematical for temperate regions. Since the absolute
sunspot maximum at A.D. 1200 is also very doubtful, it
will be realized that the evidence for the sunspot hypothe-
sis of the mediaeval rainfall maximum is extremely slender.
Furthermore, since we know nothing whatever about
the solar activity during the classical rainfall maximum,
we are still less in a position to extend the sunspot
hypothesis to that period also.

The interesting theory recently put forward by
O. Pettersson, already alluded to, provides a plausible
alternative explanation of the severe stormy climate of
the Peat-bog period, which reached a maximum near
400 b.c. Without going into details this theory is that
the strength of the tides depends on the relative positions
of the sun and moon, and the tides are greatest when
these act in conjunction, and also when they are nearest
to the earth. This fluctuation of strength passes through
various cyclic variations with periods of nine years,
about ninety years and about 1800 years, though the
lengths of the periods are not constant. The latter
cycle is most important to our purposes; according to
  i46 THE EVOLUTION OF CLIMATE

Pettersson’s calculations the fluctuations of the “ tide-
generating force ” were as follow :

Maxima 3500 B.c.   2100 b.c.   350 B.c. a.d. 1434

Minima 2800 B.c.   1200 b.c. a.d. 530.

Increased range of the tides means increased circula-
tion in the waters of the oceans, especially an increased
interchange between the warm North Atlantic and the
cold Arctic waters. It also means than an unusual
amount of ice is brought down from high into low
latitudes. Wide local variations of temperature of the
surface waters of the oceans cause increased cyclonic
activity, and hence we may expect a generally increased
storminess at times of maximum “ tide-generating
force,” and the reverse at times of minimum.

For the last maximum (a.d. 1434) Pettersson is able
to adduce a good deal of historical evidence of increased
storminess in north-west Europe and bad ice-conditions
near Iceland and Greenland, while Huntington has
found an increase of rainfall shown by the big trees of
California. The next preceding maximum, that of
360 b.c., marts the c 1   int of the Peat-bog

point to a severe climate about 650 B.c., which destroyed
an early civilization. This was the “ Twilight of the
Gods,” when frost and snow ruled the world for genera-
tions. The period was the Early Iron Age, when
civilization deteriorated greatly in north-west Europe.

Of the maximum of 2100 b.c. there is no tracé. It is
possible that the great Atlantic submergence of the
Maritime phase is connected with the tidal maximum
of 3500 B.c., but the phenomena were on a scale so much
greater than those of the more recent maxima that this
can hardly have been the sole cause.

The minima should have been characterized by
periods of relatively quiet stable climate with little ice
near Iceland and Greenland. That the last minimum,

phase. The Norse

Germanic myths
  “ CLASSICAL ” RAINFALL MAXIMUM 147

in a.d. 530, was such a period there is considerable
evidence in the high level reached by civilization at that
period in Scandinavia and by the revival in Ireland.
Again, about 1200 b.c., in the early part of the Peat-bog
phase, there is evidence of considerable traffic by sea
between Scandinavia and Ireland. The Irish Museum
has lately discovered a hoard of gold objects dated about
1000 b.c., in which the designs show a Scandinavian
origin. The minimum of 2800 b.c., which occurred
in the Forest phase, may have contributed to the dry
climate of that period, but otherwise has left no tracé.

Although at first sight the effect which Pettersson
sets out to explain seems out of all proportion to the
smallness of his cause, the coincidences after 2000 b.c.
are extremely interesting, and suggest that after the
land and sea distribution reached its present form the
astronomical cause adduced by Pettersson was possibly
effective, but before that date the astronomical cause,
if it existed, was masked by the much greater climatic
variations due to changes in the land and sea distribution.

The opinion has frequently been expressed that the
“ Classical ” and “ Mediasval ” rainfall maxima were
phenomena similar to the Glacial period, but less
intensive. This view is often carried to its logical
conclusion, that the thirty-five-year cycle, the eleven-
year, and still smaller cycles of climate, are also part
of the same series, and that the Glacial period and, let
us say, the three-year periodicity of rainfall are therefore
due to variations of the same agent, in this case the sun.
This logical extension of the theory is, however, com-
pletely untenable. The eleven-year periodicity is admit-
tedly connected with variations in the solar activity,
but there are other cycles which are completely indepen-
dent of such variations, such as, for instance, the annual
variation undergone by all meteorological elements,
which depends entirely on the inclination of the earth’s
axis. There is a weÜ-marked 4.8 year period in the
amount of ice off Iceland, the half-cycle of which is
  148 THE EVOLUTION OF CLIMATE

exactly equal to the distance travelled by the water
taking part in the North Atlantic circulation, divided
by the velocity with which it travels. There is, there-
fore, no a priori reason for assuming that the cause of
the Glacial period was identical with the cause of the
Classical and Mediaeval rainfall maxima. Further, in
the latter case, the chief phenomenon was the increase
of rainfall; the decrease of temperature was merely
incidental, but in the Glacial period the outstanding
feature was a great lowering of temperature in the polar
and temperate regions, and in this case it was the increase
of rainfall which was incidental.

BIBLIOGRAPHY

Lewis, F. J. “ The history of the Scottish peat-mosses and their relation to
the Glacial period.” Edinburgh, Scot. Geogr. Mag., 22, 1916, p. 241.

Brooks, C. E. P. “ The correlation of the Quaternary deposits of Great
Britain with those of the Continent of Europe.” Anti. Rep. Smitbsonian
Inst., 1917, p. 277.

Huntington, Ellsworth. “ The pulse of Asia.” Boston and New York,
1907.

--------------------.   “ The climatic factor as illustrated in arid America.”

Washington, Carnegie Institution, 1914.

—   “ World power and evolution.” New Haven, 1919,

pp. 186-207.

Pettersson, O. “ Climatic variations in historie and prehistorie time.”
Svenska Hydrogr.-Biol. Kotnm. Skrifter, Heft 5.
  CHAPTER XVII

THE CLIMAT1C FLUCTUAT10NS SINCE A.D. 500

The question of climatic changes during the historie
period has been the subject of much cüscussion, and
several great meteorologists and geographers have endeav-
oured to prove that at least since about 500 b.c. there
has been no appreciable variation. It is admitted that
there have been shiftings of the centres of population
and civilization, first from Egypt and Mesopotamia to
the Mediterranean regions, and later to northern and
western Europe, but these have been attributed chiefly
to political causes, and especially to the rise of Islam
and the rule of the “ accursed Turk.” Recently,
however, there has arisen a class of evidence which
cannot be explained away on political grounds, and
which appears to have decided the battle in favour of
the supporters of change ; I refer to the evidence of
the trees, explained in the preceding chapter. The
conclusions derived from the big trees of Califomia
have fallen admirably into line with archaeological work
in central America, in central Asia and other regions,
and have shown that the larger variations even of com-
paratively recent times have been very extensive, if not
world-wide, in their development.

Let us consider first the evidence of the trees. These
indicate that after the moist period ending about a.d. 400,
described in the preceding chapter, the rainfall was
generally light until about a.d. 1000, when it showed a
sharp rise, probably to the level attained in a.d. I.
(The correction for age renders an exact comparison
149
  ISO THE EVOLUTION OF CLIMATE

between periods a thousand years apart difficult.) This
period of abundant rainfall lasted some fifty years,
followed by a gradual decline to a brief minimum,
shortly before a.d. 1200. About 1300 occurred another
rapid rise, reaching a maximum before 1350 ; the period
of heavy rain continued a short white after 1400, when
a decline set in, reaching a minimum at 1500, after
which the rainfall recovered somewhat, and subsequently
maintained approximately its present level, with a slight
maximum about 1600 to 1645.

In the desert of Arizona, in regions at present too
dry for agriculture, there are abundant ruins, which are
attributed by Huntington to three periods :

(a)   Pueblo ruins, dating back to just before the
coming of the Spaniards (i.e. about a.d. 1600), and
indicating merely an increase of population at the
present centres.

(b)   Ruins of an older civilization, termed by Hunting-
ton the Pajaritan, during which numerous inhabitants
lived in places where at present no crops can be raised.
“ These people, as appears from their pottery, their
skulls and their methods of agriculture, belong to a
different civilization from that of the modern Pueblas
who inhabited Gran Quivera at the time of the coming
of the Spaniards. They had evidently disappeared long
before that date, as is evident from the present ruins of
their villages, and from the absence of any hint of their
existence in the early annals of the country ” (Geogr.
Journal, 40, 1912, p. 396).

The largest ruins of this type invariably lie near the
main lines of drainage. They consist of villages with
houses of several storeys. But digging down beneath
these ruins we find (c) tracés of an older occupation, and
ruins of a primitive type are also found on the plateaus
remote from any except small valleys. “ They are
usually small, and are greatly ruined, and seem to belong
to a time long anterior to the main large ruins.” Hun-
tington terms this type the Hohokam; unfortunately
  CLIMATIC FLUCTUATIONS SINCE A.D. 500 151

this and the Pajaritan occupations cannot be accurately
dated, bnt it is reasonable to connect them with the
rainfall maxima shown by the trees, about the time of
Christ, and in a.d. 1000 to a.d. 1300.

A similar snccession has been fonnd in the neighbour-
hood of Mexico City. The earliest tracé of occupation
is a crnde “ monntain pottery,” in ordinary river sand
and gravel. These deposits are succeeded by finer sand
with better pottery known as the “ San Juan ” type,
above which comes a culture layer with the remains of
houses. This is covered by a bed of “ tepetate,” a white
calcareous deposit frequently found in dry regions
where much water evaporates. The gravels suggest the
occasional heavy rains of arid countries. The San Juan
pottery extends throughout the “ tepetate,” which
probably corresponds to the dry period of a.d. 400-1000
in California.

Historical records in Mexico date back to the coming
of the Aztecs in a.d. 1325. They show that in 1325
and again in 1446 the level of the lake of Mexico was
high, but towards the end of the fifteenth century the
water was much lower. In 1520 it was high again;
in 1600 it was low, but high from 1629 to 1634. From
1675 to 1755 was a long dry period. On the whole the
climate from 300 to 600 years ago seems to have been
moister than that of to-day.

Still further south in the Peninsula of Yucatan recent
explorations have yielded results of extreme interest.
Yucatan lies within the tropical rain-belt, and is covered
by almost impenetrable forests. The climate is enervat-
ing and unhealthy, and the present inhabitants are
greatly lacking in vigour. In the forest, however, have
been found the ruins of ninety-two towns, some of them
of great size, and all remarkable for the beauty as well as
the solidity of their architecture.

These ruins belong to the great Mayan civilization.
Mayan history has been briefly summarized by Hunting-
ton as follows: “ First we have a long period of active
  152 THE EVOLUTION OF CLIMATE

development, during which the calendar was evolved
and the arts of architecture and sculpture were gradually
developed. . . . This time of marked growth must
have preceded the Christian era. Then comes . . . the
building of the great cities of Copan, Quirigua, Tikal
and others. These first great cities were in the Southern
part of the Maya area, on the borders of Honduras or
in eastern Guatemala. They lasted perhaps three or
four centuries; then quickly declined. So far as we
have any evidence, civilization never revived in this
Southern area, for the structures of the great period
have not been rebuilt by later inhabitants. Towards
the end of the period of greatness the centre of Mayan
culture moved northward. . . . The great period,
according to Bowditch, lasted from ioo b.c. to a.d. 350
. . then came a time of very low civilization, lasting
for centuries. ... A revival ensued about A.D. 900
or a.d. 1000, and architecture once more reached a high
pitch, but . . . only in northern Yucatan ; all the rest
of the country seems to have remained in darkness.
Moreover, this mediseval revival was relatively shortlived.
Since that time the condition of the Mayas has fluctuated
more or less, but on the whole there has been a decline.”
Now at the present day the densest and most Pro-
gressive population in Yucatan is found in the driest
part of the country, where the forest gives place to
jungle. If the line of separation between jungle and
forest were moved southward 300 miles, the former
would include all the districts where ruins are now
found. We see from the above summary that the
prosperous periods of Mayan history were just those
periods which in California were moist; in Yucatan
they must have been dry. Huntington’s explanation is
the theory of the “ shifting of climatic beits ” ; during
the rainy period in California the temperate storm-tracks
were shifted further southward. At the same time the
sub-tropical high-pressure belt, which at present lies
over the West Indies, was also shifted southwards and
  CLIMATIC FLUCTUATIONS SINCE A.D. 500 153

this brought a dry cool winter to Yucatan, with an
increased contrast of seasons, and consequently a more
invigorating climate.

In Asia, Huntington and other explorers have found
similar tracés of past variations of climate, a fascinating
account of which is given in “ The Pulse of Asia.” Space
wiïï not permit of a summary in detail, but the following
general conclusions may be quoted :1

“ If we omit the Volga and the European portions of
the Caspian drainage area, the limits (of the six basins
considered) lie over sixteen hundred miles apart from
north to south and over three thousand from east to
west. All this great area seems to have been subject
to the same great waves of climatic change.