FORMATIONS AND PHYSICAL HISTORY 607 



tains seem to represent remnants of a deformed peneplain, which, 

 curried to the east and south, is continuous with an erosion plain 

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

 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 Pleisto- 

 cene erosion on the peneplain uplifted and deformed in Pliocene 

 time, or later, 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 pri- 

 marily 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 

 & 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 widespread belief that 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 com- 

 monly accepted sense. 



Volcanic Activity 



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 



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

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



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



