PALEOGEOGRAPHIC MAPS OF NORTH AMERICA! 
BAILEY WILLIS 
U.S. Geological Survey 
QUATERNARY NORTH AMERICA 
North America during the Quaternary presents very unusual 
features. The land area is large. The margin of the continental 
plateau is now somewhat submerged, but probably has not been so 
throughout the period. Marine embayments are not extensive, 
except Hudson Bay, which is a fair example of the smaller epi- 
continental seas that have spread over various parts of the continent 
in the past. Mountain systems that are great in extent and height 
have grown from the places of the early Tertiary ranges of the Cordil- 
lera, which had been deeply eroded before the Pliocene. The 
Appalachian Mountains, which began to rise above the plains of 
eastern America possibly as early as the Eocene and which toward 
the close of the Miocene had ceased to grow at something less than 
half their present greatest height, have been raised to their existing 
altitudes during the Quaternary. 
These mountain features of North America are paralleled or 
exceeded in other continents and the period is thus characterized as 
one during which the forces that raise mountains have been decidedly 
active. 
In late Tertiary time great differences of climate developed. The 
equatorial, temperate, and polar zones became much more unlike 
than they ever had been, according to the geologic record. The 
Quaternary is distinguished by the development, the advances, and 
retreats of several ice sheets, whose combined areas are shown on the 
map. The expanse of ice was at no one time so great, but the entire 
area shown as ice was covered at one time or another, and some parts 
of it several times successively, by continental glaciers. 
« Published by permission of the Director of the U. S. Geological Survey. 
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