THE CENOZOIC AGE. 209 



CHAPTER IV, 



THE CENOZOIC AGE IN NEBRASKA. EOCENE 

 TERTIARY EPOCH. 



Causes that Produced the Cenozoic Age. Tertiary Period. How Divided. 

 Eocene and Its Divisions in the West. Eocene Not Represented in Ne- 

 braska. Why Discussed Here. Its Peculiar Modern Vegetable and Animal 

 Life, and its Origin. Vermillion Group of the Eocene. Its Vegetable and 

 Animal Life. Green River Group, and Its Organic Remains. Fort Bridger 

 Group, and Peculiar Scenery. Its Animal Remains. Uintah Group. Close 

 of the Eocene. 



culmination of those physical changes that had been in pro- 

 JL gress during the whole of the latter portion of the Cretaceous 

 period inaugurated the Cenozoic Age. We have already seen that 

 successive portions of ihe old Cretaceous sea bottoms became dry 

 land. After the Dakota and Fort Benton groups, the first extended 

 land surface wrested from the Cretaceous sea was the Niobrara 

 Group. Portions of the Fort Pierre Group were next added. The 

 Fox Hills and Laramie are not exposed in Nebraska, but both may 

 and probably do exist in northwestern Nebraska beneath the super- 

 incumbent Tertiary. In fact, the Cretaceous period came to a close 

 by a very gradual uplift, not of single mountain masses or chains, 

 but by the elevation of the whole western portion or half of the 

 continent, along with mountain folding. Heretofore the highest 

 portion of the continent existed in the east with the Appalachian 

 chain as the central axis; now it came to be the western portion, 

 with the Rocky Mountains as the main axis. 



The emergence of the continent was most complete towards the 

 north. The great American Mediterranean Sea, which from the 

 middle Cretaceous period had extended from the Wasatch to the 

 meridian of eastern Nebraska and middle Kansas, had separated 

 the continent into two elongated bodies of American land. This 

 great sea had now become virtually extinct by the continued conti- 

 nental uprising, thus " giving free drainage to the sea, except along 

 a basin-like depression extending from the Wasatch Range east- 



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