CLIMATIC IMPLICATIONS OF GLACIER RESEARCH 
of the world’s present land area, as compared with a 
coverage of slightly more than 10 per cent by the 
glaciers of today [8, p. 451]. 
In North America, for example, the Wisconsin maxi- 
mum is evidenced by the outer limit of a sheet of rela- 
tively fresh young drift stretching across the continent 
from the Atlantic near New York to the Pacific near 
Seattle. This drift is composite, consisting of thin, ex- 
tensive layers of glacial deposits alternating in some 
regions with layers of loess and with beds of peat and 
other deposits of organic origin. Over wide sectors the 
individual layers of drift thicken to form series of sub- 
parallel, ridgelike moraines. 
Each drift layer is interpreted as the product of a 
considerable glacial expansion, while the moraines are 
believed to have been built by lesser pulses of increase 
in the ice. As both drift layers and moraines generally 
occur in off-lapping relationship, like the clapboards 
on a frame house, the implied history since the great 
thrust at the Wisconsin maximum is one of gradual 
shrinkage interrupted by renewed but ever-lessening 
re-expansions. 
The climatic implication of this sequence of events 
is, by analogy, a general increase of temperature punc- 
tuated by temporary, diminishing reversals of this 
trend. The implied climatic history is confirmed at 
various points by the ecologic relations of fossil organ- 
isms. No periodicity is apparent in the sequence. 
The areal extent of the ice at various times is clearly 
recorded by the successive moraines and layers of drift. 
Owing to the low relief of most of the glaciated country, 
former ice thicknesses are rarely indicated directly. 
However, evidence of progressive thinning, particularly 
in the later part of the time span represented, exists in 
this fact: Inward from the periphery of the Wisconsin 
drift, moraines become fewer, smaller, and less con- 
tinuous, and other features that indicate thin, slowly 
flowing or even stagnant ice become more abundant. 
A generally comparable sequence exists in northern 
Europe; however, definite time correlations between 
events in Kurope and those in North America have not 
yet been made. 
The so-called Climatic Optimum, a warm period be- 
lieved to have reached its climax some thousands of 
years before the beginning of the Christian era, is now 
widely recognized through the interpretation of organic 
evidence and through the history of fluctuations of 
lakes and sea level. The glacial record thus far includes 
very few indications of this warm time, doubtless be- 
cause glaciers then were reduced in area. Thus the 
critical evidence is apparently covered up by present- 
day glaciers which, despite their recent shrinkage, are 
still large when compared with their condition during 
the Climatic Optimum. Fossil wood incorporated in 
young moraines at two localities in western United 
States is believed to record forests that flourished dur- 
ing the Optimum and were later overwhelmed by 
glacier ice. Matthes [14, p. 520] believed that most of 
the glaciers in western United States ceased to exist 
during the Optimum and have since been reconstituted 
in what he called the “Little Ice Age.” 
1021 
Major Variations within the Pleistocene Epoch 
Glacial geology affords evidence of major climatic 
changes as far back as the beginning of the Pleistocene 
epoch, a million or more years ago. Because time and 
events have destroyed much of the record, it is far from 
complete, but the main outlines stand out with some 
distinctness [8]. 
In North America, and independently in Europe, four 
major glacial ages, of which the latest is the Wisconsin, 
are inferable. Geologists regard each of these ages as 
having been of the order of 100,000 years in duration, 
although it must be conceded that this figure is hardly 
more than a controlled guess, 
As far as their positions are now known, the outer 
limits of the four major drift sheets are subparallel. 
This fact implies that over wide regions, at least, the 
major relief features of the lands, the mountains and 
lowlands, remained much the same throughout this 
whole time, for if there had been any widespread change 
the patterns of the successive drifts would have differed 
more than they seem to do. Mountain uplifts known to 
have occurred during the Pleistocene epoch, such as 
coastal mountains in California and the Himalayas in 
southern Asia, would have exerted only a limited influ- 
ence on the world pattern of glaciers. 
The area covered by ice when at its maximum extent 
was of the order of 32 per cent of the land area of the 
world, compared with about 10 per cent covered today. 
That the climate was characterized by reduced tem- 
peratures and perhaps also greater precipitation in dis- 
tricts not covered by glaciers is established by two 
groups of phenomena. The first consists of large lakes 
and vigorous stream activity in western North America, 
central Asia, northern and eastern Africa, central Aus- 
tralia, and other regions now comparatively dry. The 
second consists of abundant evidence of former vigorous 
freezing and thawing of soil in districts where such 
activity is feeble or nonexistent today. 
Additional glacial evidence of major climatic varia- 
tion les, in mountain districts, in the discrepancy be- 
tween the altitudes of cirques occupied by valley gla- 
ciers today and the much lower altitudes of abandoned 
cirques. In some districts this discrepancy amounts to 
as much as 1200 meters. As there is an evident relation- 
ship between the general altitude of cirque floors in any 
district and the position of the regional snowline, the 
discrepancy between the two sets of cirques is a rough 
measure of the depression of the regional snowline dur- 
ing the glacial ages, as compared with its present 
position. 
The extent of glaciers during the times between the 
glacial ages is virtually unknown. That climates in the 
temperate zones were mild by comparison with the 
glacial climates in those zones is indicated by the char- 
acter of soils developed on glacial drift sheets and buried 
by the next succeeding drift sheets. The great thick- 
nesses and deep alteration of such soils constitute the 
basis for a general belief that the interglacial ages were 
much longer than the glacial ages, that they may have 
lasted 200,000 to 300,000 years each. Direct evidence 
of the character of interglacial climates comes from 
