296 



HISTORICAL GEOLOGY 



upper Miocene was a period of subsidence, with ideal conditions 

 for maximum deposition of sediments in local basins. During 

 Pliocene and early Pleistocene time there was a continuation of 

 many of the upper Miocene conditions, except that marine en- 

 vironment gave way to freshwater. ... A period of elevation 



Fig. 182 

 Toadstool Park": a view in the Bad Lands of western Nebraska. The 

 rocks are of Oligocene age. (After Darton, U. S. Geological Survey, Prof. 

 Paper 17.) 



and considerable local deformation in the early Pleistocene 

 inaugurated the present conditions on the Pacific Coast." x 



Development of Relief Features in the Eastern United 

 States. — The uplift of the great Cretaceous peneplain was an 

 event of prime importance for the eastern United States, because 

 it literally furnishes us with the beginning of the history of most of 

 the existing relief features of the Appalachian district as well as 



1 Ralph Arnold: Outlines of Geologic History, by Willis and Salisbury, 

 p. 248. 



