246 J. E. SPURR — ORIGIN AND STRUCTURE OF THE BASIN RANGES 



On the west the Sierra Nevada interposed between the Eocene lake 

 and the ocean a barrier, which, judging from the thickness of the lake 

 sediments, must have been high. If the east front of this range was 

 determined by faulting, the displacement must have originated before 

 this period. That the Tertiary movements which the Great basin un- 

 derwent did not in general affect the Sierra Nevada, however, is shown 

 by the circumstance that in this range the Tertiary strata lie nearly 

 horizontal. 



In the Colorado plateau Dutton* concludes that the region which had 

 been submerged during the Eocene was uplifted at its close and exposed 

 to denudation. 



MIOCENE MOVEMENTS 



King concluded that the entire western portion of the Great basin 

 sank at the close of the Eocene, forming Pah Ute lake, which covered 

 much of Nevada, Idaho, eastern Oregon, and part of California. Since 

 the Sierra Nevada was the lake's western boundar}^ he reasoned that 

 the eastern front of the range, which he believed a fault-scarp, originated 

 when the basin was formed. But it has just been shown that this de- 

 pression was made during the Eocene, and the movement permitting 

 Miocene sediments to extend, as King has described, may have been a 

 deepening of the earlier trough. The new sediments, so far as studied, 

 seem to have been laid down on the older beds without any striking 

 unconformit3\ 



After the Miocene closed, its sediments were u[)turned in folds attain- 

 ing a dip of as much as 25 degrees, and a new de[)ression was formed, 

 which received the Pliocene Shoshone lake.f 



In the Sierra Nevada there was little or no folding. The lofty range 

 which resulted from general uplift at the close of the Shasta-Chico Cre- 

 taceous'! had been actively eroded since then, and reached its stage of 

 least topographic relief in the Miocene. Late in the Tertiary, according 

 to Mr Diller,§ the elevation of the range was intensified by faulting. 

 Nevertheless this was a region of little disturbance as compared with 

 the Great basin. 



In the Colorado plateau the Miocene was a period of slight deforma- 

 tion, resulting in gentle swells of the strata. || 



PLIOCENE 



The Pliocene Shoshone Lake deposits in the Great basin are usuall}'' 



♦Second Ann. Repi't U. S. Geol. Survey, p. (;."). 



t King : Op. cit., p. 456. 



J J. S. Diller: Fourteenth Ann. Rept., part ii, p. 421. 



§ Op. cit., p. 4.33. 



II C. E. Dutton : Second Ann. Rept. U. S. Geol. Survey, p. 65. 



