266 GEOLOGICAL HISTORY OF LAKE LAHONTAK 



glaciers, while during the summer these conditions are reversed, seems 

 enough in itself to show that an extension of the winter conditions, from 

 whatever cause, for a greater portion of the year would favor the extension 

 of the present glaciers and the formation of new ones, as well as the increase 

 of the existing lakes and the flooding of valleys that are now arid through- 

 out the year. Prolonging the winter conditions in temperate latitudes would 

 therefore initiate a glacial epoch. This is the more evident as the climatic 

 change necessary to cause an extension of the existing glaciers of the 

 Sierra Nevada so as to approximate to their former magnitude, or the growth 

 of the existing lakes of the Great Basin until they equaled the extent of the 

 Quaternary water surface of the same region, need not be considered as a 

 climatic change of great intensity. 



There are no records of the former existence of glaciers within the basin 

 of Lake Lahontan, but the western border of its hydrographic basin was 

 once buried beneath a vast accumulation of snow and ice that covered all 

 the higher portions of the Sierra Nevada. The East Humboldt range, 

 which forms a portion of the eastern border of the same drainage area, 

 was also glacier-crowned. In the central portion of the basin, the Sho- 

 shone, Star Peak, and Granite ranges rise to an elevation of about 10,000 

 feet, and are reported by the geologists of the Fortieth Parallel Explora- 

 tion to bear evidence of foi-mer glaciation about their summits. 



. The former presence of extensive glaciers on the Sierra Nevada and 

 Wasatch mountains, and of ice fields of less extent on some of the inter- 

 mediate ranges, is sufficient to prove that during that time all the mountains 

 of the region must have been snow-covered for at least a large portion of 

 each year. This in itself — unless the temperature throughout the year was 

 below freezing — would necessitate the formation of lakes in the inclosed 

 basins between the ranges. In three instances in the Bonneville basin, and 

 at four localities near Mono Lake, the glacial and lake records of Quaternary 

 date overlap. The moraines at the western base of the Wasatch mount- 

 ains which descend below the level of the Bonneville beach have been de- 

 scribed by Mr. Gilbert ; *** in this instance, however, the relative age of the 

 moraines and lake terraces is indefinite. In the Mono basin a number of 



Mgecouil Ann. Keji. U. S. Gtol. Survey, p. 189. 



