316 GEOLOGY. 



who have studied this region, broad valleys, which have subsequently 

 been elevated 2000 feet or more, were developed during the Pliocene. 

 Near the close of the period there was further elevation in this region, 

 and deep valleys were cut in the bottoms of the broad ones already 

 in existence. These valleys were continued out across the continental 

 shelf. Subsequent subsidence (and creep) has transformed part of 

 the valleys developed at this time into fiords. 1 The valley lakes of 

 this region occupy depressions which are thought to have been largely 

 excavated at this time, and subsequently transformed into basins by 

 warping, by glacial gouging, and by obstruction with glacial drift. 



It will be seen that the interpretations which have been put on 

 the phenomena in Washington and British Columbia are not altogether 

 consistent. They would be brought into harmony if the broad valleys 

 of the latter region, referred to the Pliocene, amounted to virtual penc- 

 planation of the region concerned. The amount of post-Pliocene 

 erosion in the Cascades, according to Smith and Willis, is much greater 

 than that in the Grand Canyon region, according to Dutton's inter- 

 pretation, but is more consistent with the later interpretations. 



Deformative movements of the orogenic 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 After the 

 deformation the range is thought to have been 1000 to 1200 feet higher 

 than now. 



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

 a time of great crustal movement, 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 Ozarkian epoch, the transition from the Ter- 

 tiary to the Pleistocene, was, so far as North America is concerned, 

 an epoch of great erosion. The fact that such notable changes, with 

 increased elevation of land, occurred during the epoch next preceding 

 the glacial period, led to a wide-spread belief that the elevation was 

 the cause of the climate of the latter period; and while there may 



1 Dawson, Science, Vol. XIII, 1901, p. 401 



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



