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, 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 traces. But a certain
amount of explanatory introduction is necessary.
In Europe and North America there are distinct traces 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
little doubt that they were contemporaneous in the two continents.
The correlation is not perfect, however, since the United States
geologists recognized altogether five glaciations. The explanation
appears to be that the equivalent of the Rissian glaciation in
America is double; two stages, the Illinoian and Iowan, being
recognizable, separated by a retreat of the ice. The series is as
follows:
-----------+----------------+--------------------+----------------
Alps. |Northern Europe.| North America. | Date. B.C.
-----------+----------------+--------------------+----------------
I Gunz | ? | Jerseyan or | ?
| | Nebraskan |
II Mindel | Lower Diluvium | Kansan | 430,000-370,000
III Riss | Middle Diluvium| { Illinoian } | 130,000-100,000
| | { Iowan } |
IV Wurm | Upper Diluvium | Wisconsin | 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., 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 fluctuations 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 ice-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.
This period fixed, we have a datum for estimating the duration of the
interglacial periods. 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 the Riss-Wurm
interglacial lasted about three times as long as the interval between
the Wurm glaciation and the present 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 15 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 thickness 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 virtually 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 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 glaciation has been recognized with certainty only
in Europe and North America, and even in these countries there is
considerable doubt whether the northern 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,[3] 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 prolonged 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 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 double,
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 glaciations; 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 importance in the
study of interglacial climates, and that is the loess. Loess is an
exceedingly fine-grained homogeneous 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. Its 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 Quaternary. Its 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 (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 Mousterian 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 reasonable 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 sufficient to impose anticyclonic
conditions in place of the present prevailing westerly winds, and the
same appears to be true of Europe. Similar climatic conditions were
developed for a short time at the close of the Wurm glaciation, but
without any appreciable development of loess. (See Chapter XIII.)
BIBLIOGRAPHY
Wright, W. B. “The Quaternary Ice-age.” London, Macmillan, 1914.
Brooks, C. E. P. “The correlation of the Quaternary deposits of the
British Isles with those of the continent of Europe.” _Ann. Rep.
Smithsonian Inst._, 1917, pp. 277-375.
de Geer, G. “A thermographical record of the late Quaternary
climate.” _Ber. Internat. Geologenkongr._, Stockholm, 1910. “_Die
Veränderungen des Klimas_,” p. 303.
——. “A geochronology of the last 12,000 years.” _Ber. Internat.
Geologenkongr._, Stockholm, 1910, Vol. 1, p. 241.
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_, 1, 1919, pp. 1-157, 193-252.
Richthofen, F. “On the mode of origin of the loess.” _Geol. Mag._,
1882, p. 293.