Geol.— Vol. I.] TURNER— ORIGIN OF YOSEMITE VALLEY. 295 



of the western Cordilleran region was much more moist 

 during a former period than now. So far as can be judged 

 from the equal perfection of preservation of the lake de- 

 posits and from the similarity of the fossils contained in 

 them, they existed at the sam€ time and in fact resulted 

 from climatic conditions which could not have been local. 

 One of these conditions which favored the formation of 

 lakes would be greater precipitation of rain and snow. 

 This would likewise favor the formation of glaciers in the 

 cooler air of the mountains. Direct evidence, indeed, 

 exists of the coexistence of the glaciers and the early 

 Pleistocene lakes, as will be noted later. 



A large part of western Utah was formerly covered by a 

 body of water known as Lake Bonneville, the old shores of 

 which are recorded by the terraces which surround Great 

 Salt Lake Valley. These terraces are plainly visible from 

 the car window on the Central Pacific Railroad. Gilbert, 

 in his elaborate study of Lake Bonneville\ obtained evi- 

 dence of two periods of high-water, with an intervening 

 low-water epoch. During the first period the Yellow Clay 

 was deposited; in the low-water or inter-Bonneville epoch 

 alluvial deposits were formed; then came a second period 

 when the water rose hiorher than in the first high-water 

 epoch. During this second period the White Marl was de- 

 posited, and this rests unconformably upon the Yellow 

 Clay of the first high-water epoch. 



In western Nevada another Pleistocene lake, known as 

 Lake Lahontan, was studied by RusselP, who found three 

 sets of lake deposits separated by unconformities: (i) 

 lower lacustral clays representing the first high-water epoch ; 

 (2) medial gravels indicating shallow water; (3) upper 

 lacustral clays representing the second high-water epoch. 

 The history of these two lakes as read by Gilbert and Rus- 

 sell is thus the same. In his investigation^ of Mono Lake 



1 Monograph I, U. S. Geol. Surv., 1890. 



" Monograph XI, U. S. Geol. Surv., 1885. 



* ICighth Annual Report U. S. Geol. Surv., Part I, p. 305. 



