PALEOGEOGRAPHIC MAPS OF NORTH AMERICA? 
' BAILEY WILLIS 
U. S. Geological Survey 
I4. MIOCENE NORTH AMERICA 
In outline, North America during the Miocene resembled the 
continent during the Eocene. The surface was, however, less 
mountainous. ‘The sites of the Sierra Nevada and of the Coast 
Range of British Columbia were plains or low hilly lands. The 
Rocky Mountains of the United States were comparatively low. In 
British Columbia, and thence southward through Washington, 
Oregon, and Nevada occurred outflows of lava, which covered 
many thousand square miles, but which in general were not from 
volcanoes. Though probably subordinate in volume of lava 
erupted, volcanoes were numerous and they gave off quantities of 
volcanic ash, which formed deposits in lakes, particularly in western 
Montana and British Columbia. 
The elevation of the Rocky Mountains of western Montana and 
British Columbia by overthrust, and subsequently the development 
of longitudinal valleys and separate ranges by vertical displacements, 
probably began in the Miocene period and may have culminated dur- 
ing Pliocene or early Quaternary time. 
In the West Indian region the close of the Oligocene period was 
marked by a notable disturbance, which raised a folded mountain 
chain from Puerto Rico to Cuba and probably continuously to 
Yucatan. It may also have closed the Isthmus of Tehuantepec and 
possibly have temporarily connected Honduras with South America. 
Another possible line of connection is around the eastern end of the 
Caribbean through the Windward Islands. If, however, such a land 
link united North and South America it was but temporary. 
The effect of the Cuban elevation, or of some other geographic 
« Published by permission of the Director of the U. S. Geological Survey. 
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