838 GEOLOGY 



In the basin region, faulting and deformation l gave rise to de- 

 pressions between the Sierra Nevada and the Wasatch Mountains, 

 preparing the way for two great Pleistocene lakes (Bonneville and 

 Lahontan). It is probable that many other faults between the 

 Rockies and Sierras were developed at the same time, and in 

 many cases the movement seems to have been along fault planes 

 established earlier. 



In the Sierra region, the post-Tertiary (or late Tertiary?) up- 

 lift was still more marked. 2 Not only the deep canyons of these 

 mountains, but all the scenery of the high Sierras is post-Tertiary. 

 "Its bold, rugged, savage grandeur is due to its extreme recency. 

 The wildness of youth has not been tempered and mellowed by age." 3 

 The beginning of the re-elevation of the Sierras, after peneplanation, 

 is usually placed in late Miocene time. 



Still nearer the Pacific, notable changes marked the transition 

 to the Pleistocene. In some parts of southern California (Los 

 Angeles County) marine Pliocene beds are said to occur up to 

 altitudes of 6,000 feet, 4 and in others (San Luis Obispo), there was 

 folding (Fig. 545) and faulting, while the shore-line was pushed 

 out toward the edge of the continental shelf. 5 There are submerged 

 valleys 6 along the Pacific coast, as along the Atlantic, but their 

 excavation, instead of following the Ozarkian uplift, has been 

 referred to a somewhat earlier time. Some of these valleys differ 

 from those 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, are thought to have so dis- 

 turbed the drainage that the streams no longer reached the sea by 

 their former courses. 



In Washington, present knowledge points to the early Pliocene 

 as a time of prolonged erosion. The crests of the Cascade Moun- 

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

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



1 King, U. S. Geol. Expl. of the 40th Parallel, Vol. I, p. 542. 



2 LeConte, op. cit., and Diller, 14th Ann. Kept., U. S. Geol. Surv. 



3 LeConte, Jour. Geol., Vol. VII, pp. 529-530. 



4 Hershey, Am. Geol., Vol. XXIX, p. 364. 



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



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



