GEOLOGICAL AND HISTORICAL ASPECTS OF CLIMATIC CHANGE 
aeozoic (Permo-Carboniferous or Permian) about 250 
million years ago; the Upper Proterozoic and earliest 
Cambrian about 500 million; and two or three in the 
early Proterozoic probably from 700 to 1000 million 
years ago. The Proterozoic Ice Ages are now represented 
only by seattered deposits, mostly in temperate regions. 
Early Cambrian or uppermost pre-Cambrian glacial 
deposits are found in small areas in the Lake Superior 
region and possibly in Utah, in the Yantze valley 
of China, near Simla (India), extensively in South 
Africa where they extend to within 29° of the equator, 
and in eastern and southern Australia. This glaciation 
was apparently similar to but more severe than that of 
the Quaternary. These early glaciations are chiefly of 
interest because they effectively dispose of the theories 
of a cooling earth. 
The Late Palaeozoic Ice Age, on the other hand, 
reached its greatest development in low latitudes on 
both sides of the equator, but mainly in the Southern 
Hemisphere. A great continent extending from South 
America across Africa and India to Australia carried 
true ice sheets which reached the sea in many places. 
There were large mountain glaciers in the eastern 
United States, but at the same time there was a rich 
valley vegetation in Europe, Asia, and North America 
resulting in the formation of thick beds of coal, even 
as far north as Spitsbergen. This peculiar distribution 
of climatic zones has been a great puzzle to geologists 
and climatologists, and was one of the chief bases of 
the theory of continental drift. A plausible explanation 
can be given in terms of the peculiar geography of the 
period, but this is deferred until the geographic factors 
of climate have been considered (see p. 1014). 
Between the major ice ages there is occasional evi- 
dence of local glaciation, but not all of it is accepted 
by geologists. The most important of these are (1) 
Jate Silurian and early Devonian in Alaska, eastern 
Canada, South Africa and, possibly southeast of Kash- 
mir, and (2) late Cretaceous and early Eocene in the 
Cordilleras of North America and in the Antarctic. 
These periods of glaciation occur about midway be- 
tween the major ice ages. 
Postglacial Climatic Changes. The changes of climate 
since the last maximum of the Wurm or Wisconsin 
glaciation are important for the theory of climatic 
change, because during that period the changes in 
land and sea distribution and in the elements of the 
earth’s orbit were small. Postglacial climatic changes 
in northern Europe are summarized in Table II and 
are based mainly on the study by Movius [24]. The 
dating is based on the work of de Geer on the glacial 
“varves” or annual layers of fluvio-glacial sediment. 
The retreat of the glaciers took place from the periph- 
ery inwards, mostly by melting and ablation. They 
did not decay at the centre, leaving large masses of 
“dead ice” at the margins to melt gradually, but 
continued active until they had nearly vanished. Their 
retreat was interrupted by three halts or slght re- 
advances, which differed from the main substages of the 
Wurm only in being superposed on a steady withdrawal. 
In the Alps, Britain, and Ireland there were similar 
1007 
stages which took the form of definite readvances or 
even new formation of mountain glaciers. In North 
America there were similar halts in the recession, but 
TaBLe II. Post@nactaL Succession IN Europe 
Date (B.c.) Climatic stage Climate Vegetation 
18,000-14,800) Pomeranian Arctic Tundra 
end moraine 
10, 000-8300 Allergd —_| Sub-Aretic— Pine, sedge, 
Oscillation temperate peat 
8300-7800 Fenno-Sean- | Arctic Dryas 
dian end 
moraine 
7800-6800 Pre-boreal Dry, cool Pine, hazel 
6800-5600 Boreal Dry, cold win- | Alder, oak, 
Ragunda sta- ters, warm elm 
dium 6800- summers 
6500 
5600-2500 Atlantic Warm, humid | Peat, oak, al- 
“Climatic der, lime, 
Optimum”’ elm 
2500-500 Sub-boreal Drier, becom- | Oak, giving 
ing cooler, place to 
variable pine 
500-0 Sub-Atlantie | Cool, wet Peat, beech 
these have not yet been definitely correlated with the 
European sequence. 
The Scandinavian geologists consider that the Ice 
Age ended about 6500 B.c., when the last remnants of 
the ice sheet split into two parts, but by this date 
the climate of most of Europe had become temperate. 
Near the periphery of the glaciated areas the Ice Age 
ended much earlier (about 20,000 B.c.), while Greenland 
and the Antarctic are still in the Ice Age, so that the 
choice of the date 6500 B.c. is arbitrary. 
The climate of the early postglacial period in Hurope 
was continental, with hot summers and cold winters. 
In the sixth millennium B.c. there was a change to a 
warm humid climate, with a mean temperature up to 
5F higher than the present mean and a heavy rainfall 
which caused a considerable growth of peat. This is 
known as the Climatic Optimum. In Scandinavia it was 
accentuated by subsidence of the land, which per- 
mitted a greater influx of warm Atlantic water into 
the enlarged Baltic known as the Litorina Sea, but the 
Climatic Optimum was so widespread—probably world- 
wide—that this cannot have been the only cause. Judg- 
ing by the flora of Spitsbergen, the Arctic Ocean was 
free of ice. : 
The sub-boreal climate was peculiar. On the whole 
there was a gradual decrease of temperature and rain- 
fall from the Climatic Optimum, but this was inter- 
rupted by long droughts in which the surface of the 
peat dried up, followed by returns to more rainy con- 
ditions. This alternation occurred several times, the 
main dry periods falling about 2200-1900, 1200-1000, 
and 700-500 s.c. The latter, which is termed the 
“Grenzhorizont,” was the best-developed and caused 
