GEOLOGICAL AND HISTORICAL ASPECTS OF CLIMATIC CHANGE 
ice from Scandinavia, for example, spread over eastern 
Britain and ice from Scotland occupied northern Ire- 
land. In North America the southern margin of the ice 
at its greatest extension was everywhere south of the 
Canadian border and in the Mississippi Valley it reached 
almost to 37°N. In Europe the Scandinavian ice ex- 
tended as far south as 50°N, and large piedmont glaciers 
spread out from the Alps. Mountain glaciers also 
reached far below their present limits in the Himalayas 
and the mountain ranges of the U.S.8.R., in the Rockies 
and coastal ranges of North America, the Andes, and 
New Zealand, and even on the equator in East Africa. 
Glaciers formed on many mountains now ice-free, in- 
cluding those in New Guinea and southeastern Aus- 
tralia. The latest estimates of the maximum area oc- 
cupied by land ice during glacial periods are about 13 
million square miles, including 5 million in the Antarc- 
tic, 414 million in North America, 114 million in Europe 
1005 
are sufficiently close to provide good evidence that all 
the major ice sheets were in existence at the same time. 
During glaciation, the snow line over the whole 
world was lowered by an average of about 2500 ft, 
more in the snowier and less in the drier regions. If 
this were due entirely to a fall of temperature, it would 
indicate a world temperature more than 7F below the 
present level. Probably the lowering of the snow line 
was due in part to heavier snowfall, but we can safely 
put the mean temperature at the peak of the Quaternary 
glaciation as at least 5F below the present. 
It is difficult to estimate the mean global temperature 
at the peak of a warm period. In the Arctic it was 
enormously higher than it is now, and was probably 
of the order of 50F, but in lower latitudes the rise was 
much smaller. A rough estimate of the increase for the 
world as a whole would be 10F, which is not excessive 
considering that we are still in an Ice Age, though in 
Tasie I. GuactaL SEQUENCE, QUATERNARY Ick AGE 
Al hi ds of North G . i Zh 
me Goreete: | Mayet North Ameria Raed 
Mankato 
Glacial Wurm (40-18) Weichsel Wisconsin CaRy Late glaciation le 5 
Tazewell 
Warthe Iowan ue 
€ I 115 
Interglacial Riss-Wurm Sangamon 
Glacial Riss (130-100) Saale Tllinoian Penultimate glaciation II 187 
I 230 
Interglacial Mindel-Riss Yarmouth 
Glacial Mindel (430-370) Elster Kansan Antepenultimate glaciation II 435 
I 476 
Interglacial Gunz-Mindel Aftonian 
Glacial Gunz (520-490) ? Nebraskan Early glaciation j II 550 
I 590 
and at least as much in Asia, and over 800,000 square 
miles in Greenland. The remainder is made up of a 
number of relatively small areas. The present ice- 
covered area is about 6 million square miles, almost 
entirely in the Antarctic and Greenland. In addition 
there was a great extension of floating ice in the oceans, 
especially the North Atlantic and probably also in the 
Antarctic. Altogether, nearly one-tenth of the earth’s 
surface must have been ice-covered. 
It is not likely that the ice everywhere reached its 
maximum extension and thickness simultaneously, since 
there was undoubtedly some migration of the centres 
of maximum accumulation. Flint [11] estimates that 
if the maxima were everywhere contemporaneous, sea 
level would have fallen by about 390 ft below the pres- 
ent level. This is a maximum figure, and is higher than 
previous estimates; it would have been considerably 
reduced by isostatic adjustment due to the weight of 
the ice. Shore deposits, coral reefs, etc., point to a lower- 
ing of sea level by more than 260 ft. The two figures 
an interglacial period. Thus we arrive at a difference 
of at least 15F as a measure of the contrast between 
a mild period and an Ice Age; less near the equator 
and much more near the poles. 
The Quaternary Ice Age was not a single uninter- 
rupted episode, but consisted of a number of advances 
and retreats of the glaciers and ice sheets. Penck and 
Briickner [27], in their classic researches into the glacia- 
tion of the Alps, recognised four main advances sep- 
arated by recessions to a climate not more unfavourable 
than the present. A similar arrangement has been 
found in North America, but in northern Europe the 
earliest glaciation has not been recognised, probably 
because its remains were swept away by subsequent 
more extensive ice sheets. The sequence is shown in 
Table I. In some areas the Mindel and in others the 
Riss glaciation was the most extensive; the Wurm was 
nearly everywhere much smaller. The extent of the 
Gunz is not well known. The Wurm glaciation included 
three or four main readvances and the Riss at least 
