THE PLIOCENE PERIOD 839 



which cuts across strata (Ellensburg formation) of late Miocene 1 

 age. The planation must, therefore, have been later than that 

 part of the Miocene period represented by the Ellensburg formation. 

 At least the early part of the Pliocene period, if not most of it, 

 would seem to have been necessary for the accomplishment of this 

 great planation, so that the peneplain can hardly be thought to 

 antedate late Pliocene time. If this is correct, the main features 

 of the present topography of this rugged region are the result 

 primarily of Pleistocene erosion on the peneplain uplifted and de- 

 formed in, or subsequent to, late Pliocene time, and secondarily of 

 vulcanism, which has built up the great volcanic piles (Rainier) and 

 others) which affect the region. In British Columbia also, the 

 Pliocene is thought to have been primarily a time of erosion. 



Deformative movements of the erogenic type seem not to have 

 been common at the close of the Pliocene, but such movements 

 affected the Santa Cruz Mountains of California, where Miocene 

 (Monterey) and Pliocene (Merced) beds were deformed together. 2 



On the whole, the close of the Pliocene must be looked upon as 

 a time of great deformation, a critical period in the history of North 

 America. New lands were made by emergence from the sea, and 

 old lands were deformed and made higher; new mountains were 

 made, and old ones rejuvenated; streams were turned from their 

 courses in some places, and nearly everywhere started on careers 

 of increased activity. The fact that such notable changes, with 

 increased elevation of land, occurred during the epoch next pre- 

 ceding the glacial period, is one of the considerations which led to 

 the once wide-spread belief that the elevation was the cause of the 

 climate of the latter period. While there may be a connection 

 between the two things, it was probably not in the simple and 

 commonly accepted sense. 



The volcanic activity of preceding periods continued into the 

 Pliocene, and became somewhat pronounced near the end of the 

 period in different parts of the western Cordillera. Some of the 

 late igneous formations of the Sierras, and perhaps of northern 



1 Smith, Ellensburg, Wash., folio, U. S. Geol. Surv.; and Williatand Smith, 

 Professional Paper 19, U. S. Geol. Surv. 



2 Ashley, Jour. Geol., Vol. Ill, p. 434. 



