THE EOCENE PERIOD. 



245 



The Oligocene is likewise represented among the terrestrial depos- 

 its of the western part of the continent. Following the Uinta stage 

 (p. 209) of the Eocene, physiographic and drainage relations were so 

 changed as to shift the sites of notable sedimentation. The next con- 

 siderable formation, the history of which is partially known, is the 

 White River formation, lying east of the northern Rockies. It occupies 

 an extensive area in northeastern Colorado, southeastern Wyoming, 

 western Nebraska (Brule and Chadron formations 1 ), and South Dakota, 

 and it may extend southward even to Kansas. 2 Clastic sediments pre- 



Fig. 435. — Bad Land erosion in the Brule clay, near Scolts Bluff, western Nebraska. 



(Darton, U. S. Geol. Surv.) 



dominate in the White River series, and clay predominates over coarser 

 material, but beds and lenses of sandstone and conglomerate (or sand 

 and gravel) occur at various places, and there are thin beds and lenses 

 of limestone and some volcanic ash. 



The origin of the White River beds has been the occasion of much 

 difference of opinion. They have usually been described as lacustrine, 

 but in recent years parts of them have been regarded as partly or 



1 Darton, Camp Clarke, Scotts Bluff, Edgemont, and Oelrichs folios, U. S. Geol. Surv 



2 Adams, Am. Geol., Vol. 29, p. 303. 



