paleogeographic maps of north america

Paleogeographic Maps of North AmericaAuthor(s): Bailey WillisSource: The Journal of Geology, Vol. 17, No. 7 (Oct. - Nov., 1909), pp. 600-602Published by: The University of Chicago PressStable URL: http://www.jstor.org/stable/30058573 .

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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.

I Published by permission of the Director of the U. S. Geological Survey.

600



PALEOGEOGRAPHIC MAPS OF NORTH AMERICA

QUATERNARY NORTH AMERICA

LEGEND

OCEANIC BASINS

MARINE WATERS

CONTINENTAL DEPOSITS, SOMETIMES INCLUDING MARINE SEDIMENTS

LANDS ICE

POLAR MARINE CURRENTS

EQUATORIAL

60I



602 BAILEY WILLIS

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.



paleogeographic maps of north america

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