Geol.— Vol. I.] TURNER^ORIGIM OF YOSEMITE VALLEY. 287 



although always where the ice mass appears to have been 

 very thick at one time, is such as not to suggest the prob- 

 ability of there having been sufficient weight of ice to 

 scoop out a basin in hard rock. Such is the basin occupied 

 by Lake Washburn (See Plate XXXIV), which is an expan- 

 sion of Merced River several miles up-stream from Yosemite 

 Valley. Even here, however, there is a steep bare granite 

 slope immediately west of the lake, but none up-stream, 

 from which direction the ice chiefly came. It is more than 

 likely that in many such cases the basin was originally pro- 

 duced by unequal disintegration of the granite in preglacial 

 time, this decomposed material being subsequently exca- 

 vated by the ice. 



These rock basins are sometimes more than a mile in 

 length. The blue color of the water of the lakes that 

 occupy them suggests some depth. Two of them were 

 sounded : the larger. Lake Tenaya, with a length of about one 

 mile, was found to have a maximum depth of about ninety 

 feet; the smaller, Johnson Lake, with a maximum diameter 

 of about one-fourth of a mile, showed a maximum depth of 

 about forty-five feet. As no rock bottom was found in either 

 of the basins, the depth to the solid rock must be in both 

 cases something more than is given above. In the illustra- 

 tion, the rock barrier forming the lower side of the basin of 

 Lake W^-shburn can be plainly seen, together with the out- 

 let of the lake. 



Helland ^ describes small rock basins lying in glacial 

 cirques, which may be explained by the sapping theory of 

 Johnson, referred to under " Glacial cirques." Helland 

 writes : — 



These lakes are not formed by moraines damming the water, but are truly 

 rock basins; and it does not seem likely that they were mainly scooped out 

 like the great lakes, along the sid.-s of which we often see groovings and 

 roches moutonnees; for in the little lakes one often sees sharp-edged blocks 

 covering the bottom. When the glaciers of the cirques filled these small 

 lakes so as to leave but little water, it seems probable that the water thus left 

 would freeze in winter, so that the whole tarn would thus be frozen to the 

 bottom and the rocks in that way broken loose. 



1 Quart. Jour. Geol. Soc. London, Vol. XXXIH, 1877, p. 165. 



