EVOLUTION OF MODERN SURFACE FEATURES 43 



the Sierra Nevada block was now being uplifted, and was exerting a 

 climatic influence on the region to the east. Uplift continued into 

 early Pleistocene time, until the block had been raised 5,000 to 

 6,000 feet in the north and 7,500 to 9,000 feet farther south (Axel- 

 rod, 1957, p. 42). 



At the same time as the Sierra Nevada was being raised, block 

 faulting on an extensive scale disrupted the Great Basin, and was 

 largely responsible for shaping it into its present Basin and Range 

 topography (Van Houten, 1956, pp. 2821-2822). There was also a 

 gradual increase in altitude; basin floors which had stood at well 

 below 2,000 feet above sea level at the beginning of the Pliocene, 

 now stand at 3,000 to 5,000 feet. In part this was a result of sedi- 

 mentary filling of the basins, but to a much greater extent to 

 regional upwarp. 



The reader may note a discrepancy between the record as here set 

 forth for times of uplift in the Rocky Mountain region on the east 

 and the Great Basin and Sierra Nevada on the west. In the Rocky 

 Mountains, some geologists believe that the principal uplift was 

 before the middle of the Tertiary and diminished later, whereas the 

 Great Basin and Sierra Nevada seem to have been low up to middle 

 Tertiary time, and were greatly uplifted afterward. The inferred 

 history of the two areas is based on interpretations of necessarily 

 elusive evidence by various competent observers, but if the contrasts 

 are real, they had a significant influence on the geographic and cli- 

 matic evolution of the middle Cordillera. 



Pleistocene Environments. Events in the Great Basin and Sierra 

 Nevada during the Pleistocene are perhaps sufficiently familiar as 

 not to require detailed recital — the ice fields along the Sierra crest 

 and the valley glaciers below them, the smaller glaciers on higher 

 summits in the Great Basin, and the great lakes, such as Bonneville 

 and Lahontan, which flooded the lower country, in places to depths 

 of more than a thousand feet. Climatic fluctuations are recorded not 

 only by successive glacial moraines in the mountains, but by several 

 epochs of filling and dessication of the lakes. Existence of the lakes 

 indicates a much increased rainfall and implies, as well, that many 

 of the basins temporarily possessed exterior drainage (Hubbs and 

 Miller, 1948, pp. 21-29). 



The Great Basin has returned now to conditions of aridity ap- 

 proximately comparable to those at the end of Pliocene time but if. 



