48 GLACIAL AND SUEFACE GEOLOGY OF THE PACIFIC COAST. 



teorological conditions, tliey expose great rounded surfaces, of wliich the 

 smoothness is well calculated to deceive careless or inexperienced observers. 

 Similar surfaces may, however, almost invariably be found in the immediate 

 vicinity of such exposures, where the conditions are such that it is seen at 

 once that ice cannot have been the agent in bringing about the peculiar 

 smoothness and curved form of the rock. There are plenty of deeply 

 recessed portions of the Yosemite Valle}', where at the bottom of a cavity 

 entirely inaccessible to ice the same smoothness may be observed. Around 

 these are the overlapping edges of the successive plates, by the removal of 

 parts of which the cavity has been formed. These will all have sharp edges, 

 an occurrence entirely impossible if the removal of the wanting portion had 

 been effected by glacial erosion. 



The only thing in the Valley which might with some plausibilit}' be re- 

 /ferred to the agency of ice is the small moraine-like elevation at the head 

 ^ of the Bridal Veil Meadow, referred to in the Geology of California, Vol. I. 

 This, as well as the whole Vallej^, the writer has re-examined since that vol- 

 ume was written, and has become thoroughly convinced that it is not neces- 

 sarily of glacial origin. A moraine in the Yosemite Valley would without 

 doubt be composed of similar materials to those which constitute the present 

 talus at the bottom of the walls of the Valley. These are all made up chiefly 

 of coarse angular blocks of granite with a little finer granitic detritus mixed 

 with them, such material, in short, as would be formed by the cracking off 

 and falling down of portions of the walls of the Valley. The supposed mo- 

 raine, however, is made up of fine material ahnost entirel}', and it appears 

 very probable that it was formed at the time when the Valley was a lake, 

 by the crowding of the ice against the shore, at the time of the winter's 

 freezing. This mode of building up of walls and moraine-like accumulations 

 around the shores of lakes is a phenomenon of frequent occurrence. In tlio 

 case of the Yosemite the formation of such a pile of debris at its lower end 

 would be materially aided by the violence with which the wind frequently 

 blows up or down the Valley. This, even now, is occasionally almost sub- 

 merged at the time of the most rapid melting of the snows on the adjacent 

 heights, after winters of unconnnonl}' large precipitation. The Hetch-Hetchy, 

 on the other hand, according to Mr. Hofi^"mann, is transformed into a i-egular 

 lake at such times. 



It being, as the writer after a careful examination believes, a well-estab- 

 lished fact that the Yosemite was not occupied by ice during the time of the 



