THE PLIOCENE PERIOD. 315 



to near the edge of the continental shelf. 1 There was notable fault- 

 ing in the Santa Cruz mountains of California at the end of the Plio- 

 cene, with uplift of the axis, while the flanks of the range remained 

 submerged. 2 The wide-spread unconformity between the Pliocene 

 and Pleistocene of the Pacific coast, is a further index of the great 

 changes of the time. 



There are submerged valleys 3 along the Pacific coast, as along 

 the Atlantic, but their excavation, instead of following the Ozarkian 

 uplift, is thought to have been the result of the post-Miocene move- 

 ment which folded up the Coast range, and shifted the coast line west 

 to the edge of the continental shelf. Some of them differ from the sub- 

 merged valleys of the Atlantic coast, in not being the continuations of 

 existing land valleys. The late Pliocene movements and lava flows, 

 the latter filling many of the valleys, so disturbed the drainage that 

 the streams no longer reached the sea at the same points as before. 

 . In Washington, present knowledge seems to point to the early 

 Pliocene as a time of prolonged erosion. The crests of the Cascade 

 mountains seem to represent remnants of a deformed peneplain which, 

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

 which cuts across strata (Ellensburg formation) of late Miocene 4 age. 

 The planation must, therefore, have been later than that part of the 

 Miocene period represented by the beds concerned. 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 

 (Ozarkian) time. If this view be correct, the main features of the 

 present topography of that most rugged region are the result primarily 

 of late Pliocene and Pleistocene erosion on the peneplain which was 

 uplifted and deformed in late Pliocene time, and secondarily of vul- 

 canism, 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. According to the interpretation of those 



1 Fairbanks, San Luis folio, U. S. Geol. Surv. 



2 Ashley, Journal Geol., Vol. III. 



3 LeConte, Bull. Geol. Soc. of Am., Vol. II, p. 325 



4 Smith, Ellensburg, Wash, folio, U. S. Geol. Surv.; also Willis and Smith, Pro- 

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



