Little Studies in the Yosemite Valley. 



15 



one can find anywhere. Tenaya Canon, it appears, once pos- 

 sessed four glacial lakes, situated at successively higher levels. 

 All but the lowest, however, are now filled with sediment; 

 Mirror Lake alone survives as a remnant of the largest lake. 



After one has become familiar with all these lake basins in 

 the branch canons of the Yosemite Valley, and one has, more- 

 over, gained an insight into their mode of origin, one can scarce- 

 ly avoid reaching the conclusion that in the main valley, too, 

 there is a deeply eroded rock basin, now covered by the silts of 

 Lake Yosemite. The combined mass of the Tenaya and Merced 

 glaciers here must have eroded with particular vigor. The 

 very fact that each of these ice streams, by itself, was able to 

 excavate rock basins of considerable extent and depth, leaves 

 little doubt that united they achieved still larger erosional re- 

 sults. Besides, it has been noted that it is immediately below 

 the confluence of glaciers that the ice usually attains the greatest 

 power to excavate. 



The El Capitan moraine, then, is not to be given sole credit 

 for the creation of Lake Yosemite. That lake in all probability 

 lay in a rock basin eroded by the ice, and the only function of 

 the moraine dam was to raise the level of the waters, thus 

 increasing their depth and extent. 



In the meanwhile it should not be forgotten that the exis- 

 tence of the rock basin is purely inferential and is to be consid- 

 ered unproven until a series of borings along the whole length 

 of the valley shall afford the necessary facts. It is to be hoped 

 that some day such borings may be undertaken ; they would not 

 merely serve to solve a problem of great local interest, but would 

 contribute much-desired data regarding the still challenged 

 eroding efficiency of glaciers. 



That the Yosemite Valley has actually been occupied by gla- 

 cial ice no one now will venture to dispute ; were all other ice 

 signs in the valley rejected as untrustworthy, the El Capitan 

 moraine alone would afford evidence sufficient and irrefutable. 

 As to the extent to which the ancient glaciers have remodeled 

 and excavated the valley, nothing, perhaps would go further 

 towards settling this vexed question than a series of direct 

 measurements establishing beyond doubt the depth of former 

 Lake Yosemite. 



