PALEOGEOGRAPHIC MAPS 

 TERTIARY 



BAILEY WILLIS 



U. S. Geological Survey 



13 AND 14. EOCENE-OLIGOCENE AND MIOCENE 



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. 



The region of the West Indies was the seat of an embayment of 



' Osborn, H. F., "Cenozoic Mammal Horizons of Western North America," 

 U. S. Geological Survey Bull. 361, 1909. 



222 



