THE EOCENE PERIOD 



799 



which contributed the Oligocene sediments were mostly low. Shale 

 is the dominant rock of the series, which is much thinner than 

 the Eocene. 



Considerable geographic changes occurred during the Oligocene, 

 or at its close, especially in the Gulf and Caribbean regions, 1 where 



Fig. 537. Oligocene Bad Lands of South Dakota. (Williston.) 



the Oligocene (early Oligocene) is commonly conformable on the 

 Eocene, and unconformable beneath the Miocene. 



FOREIGN 



Europe. The Oligocene is more distinctly differentiated from 

 he Eocene in Europe than in most parts of America. Toward the 

 close of the Eocene, the epicontinental sea of northern Europe was 

 greatly restricted, but considerable areas stood so near sea-level 

 hat slight changes served to greatly diminish or extend the epi- 

 continental waters. 



1 See references to the writings of Hill under Eocene. 



