PALEOGEOGRAPHIC MAPS OF NORTH AMERICA? 
BAILEY WILLIS 
U. S. Geological Survey 
13. EOCENE-OLIGOCENE NORTH AMERICA 
The Eocene-Oligocene aspect of North America differed from the 
Cretaceous and resembled the present. The east and west were 
united. The Cordillera had begun its development as a system of 
many mountain chains, most, if not all, of which are represented in 
existing ranges; yet few, if any, of which have had an uninterrupted 
growth. ‘They became high in the Eocene, but were greatly eroded 
in the Oligocene and Miocene. ‘The volcanic activity which marks 
the Cordillera was very notable during the Eocene. The eastern 
part of the continent remained low. 
By erosion of the mountains and by contributions from the 
volcanoes great thicknesses of sediment accumulated in interior 
basins of the Cordillera. ‘The deposits were in part fluviatile, in 
part eolian, in minor part lacustrine. On the map their distribution 
is shown by the ruling for continental deposits in the central west. 
In the Gulf region and also in Alaska extensive low lands and 
favorable climate produced extensive marshes which are now repre- 
sented by coal beds and are also indicated by the vertical ruling. 
The continental connections of North America during the Eocene 
and Oligocene appear to have been established and interrupted, as is 
shown by the relations of land animals. Osborn infers that there 
was intermigration with Europe during the Wasatch epoch,? and 
thenceforward separation from Europe until the Oligocene, when 
faunistic reunion took place. These inferences are suggested on the 
map by the temporary lands linking Alaska with Siberia and Green- 
land with England. 
« Published by permission of the Director of the U. S. Geological Survev. 
2 Osborn, H. F., ‘‘Cenozoic Mammal Horizons of Western North America,” 
U. S. Geological Survey Bull. 361, 1909. 
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