PALEOGEOGRAPHIC MAPS 



BAILEY WILLIS 

 U. S. Geological Survey 



15. 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 Avhich 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. 



The developments of topography and climate, including polar 

 refrigeration and corresponding modifications of oceanic conditions, 

 have greatly changed the environment of plants and animals, and 

 have resulted in special phenomena of competition and adaptation, 

 through which existing forms have been evolved. 



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