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Post-glacial Crustal Movements And Climatic Changes

An interesting practical application of some of the preceding

generalizations is found in an attempt by C. E. P. Brooks[95] to

interpret post-glacial climatic changes almost entirely in terms of

crustal movement. We believe that he carries the matter much too far,

but his discussion is worthy of rather full recapitulation, not only

for its theoretical value but because it gives a good summary of

post-glacial changes. H
s climatic table for northwest Europe as

reprinted from the annual report of the Smithsonian Institution for

1917, p. 366, is as follows:

Phase Climate Date

1. The Last Great Arctic climate. 30,000-18,000 B. C.


2. The Retreat of the Severe continental 18,000-6000 B. C.

Glaciers. climate.

3. The Continental Phase. Continental climate. 6000-4000 B. C.

4. The Maritime Phase. Warm and moist. 4000-3000 B. C.

5. The Later Forest Phase. Warm and dry. 3000-1800 B. C.

6. The Peat-Bog Phase. Cooler and moister. 1800 B. C.-300 A. D.

7. The Recent Phase. Becoming drier. 300 A. D.-

Brooks bases his chronology largely on De Geer's measurements of the

annual layers of clay in lake bottoms but makes much use of other

evidence. According to Brooks the last glacial epoch lasted roughly from

30,000 to 18,000 B. C., but this includes a slight amelioration of

climate followed by a readvance of the ice, known as the Buhl stage.

During the time of maximum glaciation the British Isles stood twenty or

thirty feet higher than now and Scandinavia was "considerably" more

elevated. The author believes that this caused a fall of 1 deg.C. in the

temperature of the British Isles and of 2 deg.C. in Scandinavia. By an

ingenious though not wholly convincing method of calculation he

concludes that this lowering of temperature, aided by an increase in the

area of the lands, sufficed to start an ice sheet in Scandinavia. The

relatively small area of ice cooled the air and gave rise to an area of

high barometric pressure. This in turn is supposed to have caused

further expansion of the ice and to have led to full-fledged glaciation.

About 18,000 B. C. the retreat of the ice began in good earnest. Even

though no evidence has yet been found, Brooks believes there must have

been a change in the distribution of land and sea to account for the

diminution of the ice. The ensuing millenniums formed the Magdalenian

period in human history, the last stage of the Paleolithic, when man

lived in caves and reindeer were abundant in central Europe.[96] At

first the ice retreated very slowly and there were periods when for

scores of years the ice edge remained stationary or even readvanced.

About 10,000 B. C. the edge of the ice lay along the southern coast of

Sweden. During the next 2000 years it withdrew more rapidly to about

59 deg.N. Then came the Fennoscandian pause, or Gschnitz stage, when for

about 200 years the ice edge remained in one position, forming a great

moraine. Brooks suggests that this pause about 8000 B. C. was due to the

closing of the connection between the Atlantic Ocean and the Baltic Sea

and the synchronous opening of a connection between the Baltic and the

White Seas, whereby cold Arctic waters replaced the warmer Atlantic

waters. He notes, however, that about 7500 B. C. the obliquity of the

ecliptic was probably nearly 1 deg. greater than at present. This he

calculates to have caused the climate of Germany and Sweden to be 1 deg.F.

colder than at present in winter and 1 deg.F. warmer in summer.

The next climatic stage was marked by a rise of temperature till about

6000 B. C. During this period the ice at first retreated, presumably

because the climate was ameliorating, although no cause of such

amelioration is assigned. At length the ice lay far enough north to

allow a connection between the Baltic and the Atlantic by way of Lakes

Wener and Wetter in southern Sweden. This is supposed to have warmed the

Baltic Sea and to have caused the climate to become distinctly milder.

Next the land rose once more so that the Baltic was separated from the

Atlantic and was converted into the Ancylus lake of fresh water. The

southwest Baltic region then stood 400 feet higher than now. The result

was the Daun stage, about 5000 B. C., when the ice halted or perhaps

readvanced a little, its front being then near Ragunda in about latitude

63 deg.. Why such an elevation did not cause renewed glaciation instead of

merely the slight Daun pause, Brooks does not explain, although his

calculations as to the effect of a slight elevation of the land during

the main period of glaciation from 30,000 to 18,000 B. C. would seem to

demand a marked readvance.

After 5000 B. C. there ensued a period when the climate, although still

distinctly continental, was relatively mild. The winters, to be sure,

were still cold but the summers were increasingly warm. In Sweden, for

example, the types of vegetation indicate that the summer temperature

was 7 deg.F. higher than now. Storms, Brooks assumes, were comparatively

rare except on the outer fringe of Great Britain. There they were

sufficiently abundant so that in the Northwest they gave rise to the

first Peat-Bog period, during which swamps replaced forests of birch and

pine. Southern and eastern England, however, probably had a dry

continental climate. Even in northwest Norway storms were rare as is

indicated by remains of forests on islands now barren because of the

strong winds and fierce storms. Farther east most parts of central and

northern Europe were relatively dry. This was the early Neolithic period

when man advanced from the use of unpolished to polished stone


Not far from 4000 B. C. the period of continental climate was replaced

by a comparatively moist maritime climate. Brooks believes that this was

because submergence opened the mouth of the Baltic and caused the fresh

Ancylus lake to give place to the so-called Litorina sea. The

temperature in Sweden averaged about 3 deg.F. higher than at present and in

southwestern Norway 2 deg.. More important than this was the small annual

range of temperature due to the fact that the summers were cool while

the winters were mild. Because of the presence of a large expanse of

water in the Baltic region, storms, as our author states, then crossed

Great Britain and followed the Baltic depression, carrying the moisture

far inland. In spite of the additional moisture thus available the snow

line in southern Norway was higher than now.

At this point Brooks turns to other parts of the world. He states that

not far from 4000 B. C., a submergence of the lands, rarely amounting to

more than twenty-five feet, took place not only in the Baltic region but

in Ireland, Iceland, Spitzbergen, and other parts of the Arctic Ocean,

as well as in the White Sea, Greenland, and the eastern part of North

America. Evidences of a mild climate are found in all those places.

Similar evidence of a mild warm climate is found in East Africa, East

Australia, Tierra del Fuego, and Antarctica. The dates are not

established with certainty but they at least fall in the period

immediately preceding the present epoch. In explanation of these

conditions Brooks assumes a universal change of sea level. He suggests

with some hesitation that this may have been due to one of Pettersson's

periods of maximum "tide-generating force." According to Pettersson the

varying positions of the moon, earth, and sun cause the tides to vary in

cycles of about 9, 90, and 1800 years, though the length of the periods

is not constant. When tides are high there is great movement of ocean

waters and hence a great mixture of the water at different latitudes.

This is supposed to cause an amelioration of climate. The periods of

maximum and minimum tide-generating force are as follows:

Maxima 3500 B. C.--------2100 B. C.--------350 B. C.-------A. D. 1434

Minima ---------2800 B. C.--------1200 B. C.-------A. D. 530---------

Brooks thinks that the big trees in California and the Norse sagas and

Germanic myths indicate a rough agreement of climatic phenomena with

Pettersson's last three dates, while the mild climate of 4000 B. C. may

really belong to 3500 B. C. He gives no evidence confirming Pettersson's

view at the other three dates.

To return to Brooks' sketch of the relation of climatic pulsations to

the altitude of the lands, by 3000 B. C., that is, toward the close of

the Neolithic period, further elevation is supposed to have taken place

over the central latitudes of western Europe. Southern Britain, which

had remained constantly above its present level ever since 30,000 B. C.,

was perhaps ninety feet higher than now. Ireland was somewhat enlarged

by elevation, the Straits of Dover were almost closed, and parts of the

present North Sea were land. To these conditions Brooks ascribes the

prevalence of a dry continental climate. The storms shifted northward

once more, the winds were mild, as seems to be proved by remains of

trees in exposed places; and forests replaced fields of peat and heath

in Britain and Germany. The summers were perhaps warmer than now but the

winters were severe. The relatively dry climate prevailed as far west as

Ireland. For example, in Drumkelin Bog in Donegal County a corded oak

road and a two-story log cabin appear to belong to this time. Fourteen

feet of bog lie below the floor and twenty-six above. This period,

perhaps 3000-2000 B. C., was the legendary heroic age of Ireland when

"the vigour of the Irish reached a level not since attained." This, as

Brooks points out, may have been a result of the relatively dry climate,

for today the extreme moisture of Ireland seems to be a distinct

handicap. In Scandinavia, civilization, or at least the stage of

relative progress, was also high at this time.

By 1600 B. C. the land had assumed nearly its present level in the

British Isles and the southern Baltic region, while northern Scandinavia

still stood lower than now. The climate of Britain and Germany was so

humid that there was an extensive formation of peat even on high ground

not before covered. This moist stage seems to have lasted almost to the

time of Christ, and may have been the reason why the Romans described

Britain as peculiarly wet and damp. At this point Brooks again departs

from northwest Europe to a wider field:

It is possible that we have to attribute this damp period in

Northwest Europe to some more general cause, for Ellsworth

Huntington's curves of tree-growth in California and climate in

Western Asia both show moister conditions from about 1000 B. C. to

A. D. 200, and the same author believes that the Mediterranean lands

had a heavier rainfall about 500 B. C. to A. D. 200. It seems that

the phase was marked by a general increase of the storminess of the

temperate regions of the northern hemisphere at least, with a

maximum between Ireland and North Germany, indicating probably that

the Baltic again became the favourite track of depressions from the


Brooks ends his paper with a brief resume of glacial changes in North

America, but as the means of dating events are unreliable the degree of

synchronism with Europe is not clear. He sums up his conclusions as


On the whole it appears that though there is a general similarity in

the climatic history of the two sides of the North Atlantic, the

changes are not really contemporaneous, and such relationship as

appears is due mainly to the natural similarity in the geographical

history of two regions both recovering from an Ice Age, and only

very partially to world-wide pulsations of climate. Additional

evidence on this head will be available when Baron de Geer publishes

the results of his recent investigations of the seasonal glacial

clays of North America, especially if, as he hopes, he is able to

correlate the banding of these clays with the growth-rings of the

big trees.

When we turn to the northwest of North America, this is brought out

very markedly. For in Yukon and Alaska the Ice Age was a very mild

affair compared with its severity in eastern America and

Scandinavia. As the land had not a heavy ice-load to recover from,

there were no complicated geographical changes. Also, there were no

fluctuations of climate, but simply a gradual passage to present

conditions. The latter circumstance especially seems to show that

the emphasis laid on geographical rather than astronomical factors

of great climatic changes is not misplaced.

Brooks' painstaking discussion of post-glacial climatic changes is of

great value because of the large body of material which he has so

carefully wrought together. His strong belief in the importance of

changes in the level of the lands deserves serious consideration. It is

difficult, however, to accept his final conclusion that such changes are

the main factors in recent climatic changes. It is almost impossible,

for example, to believe that movements of the land could produce almost

the same series of climatic changes in Europe, Central Asia, the western

and eastern parts of North America, and the southern hemisphere. Yet

such changes appear to have occurred during and since the glacial

period. Again there is no evidence whatever that movements of the land

have anything to do with the historic cycles of climate or with the

cycles of weather in our own day, which seem to be the same as glacial

cycles on a small scale. Also, as Dr. Simpson points out in discussing

Brooks' paper, there appears "no solution along these lines of the

problem connected with rich vegetation in both polar circles and the

ice-age which produced the ice-sheet at sea-level in Northern India."

Nevertheless, we may well believe that Brooks is right in holding that

changes in the relative level and relative area of land and sea have had

important local effects. While they are only one of the factors involved

in climatic changes, they are certainly one that must constantly be kept

in mind.


[Footnote 95: C. E. P. Brooks: The Evolution of Climate in Northwest

Europe. Quart. Jour. Royal Meteorol. Soc., Vol. 47, 1921, pp. 173-194.]

[Footnote 96: H. F. Osborn: Men of the Old Stone Age, N. Y., 1915; J. M.

Tyler: The New Stone Age in Northwestern Europe, N. Y., 1920.]