Studies in the Sierra 



tation of the material missing from between the mountains of the 

 summit was effected by glaciers, it yet remains to be considered 

 what agent or agents shaped the upper portions of these mountains, 

 which bear no traces of glacial action, and which probably were 

 always, as they now are, above the reach of glaciers. Even here we 

 find the glacier to be indirectly the most influential agent, constantly 

 eroding backward, thus undermining their bases, and enabling grav- 

 ity to drag down large masses, and giving greater effectiveness to the 

 winter avalanches that sweep and furrow their sides. All the summit 

 peaks present a crumbling, ruinous, unfinished aspect. Yet they 

 have suffered very little change since the close of the glacial period, 

 for if denudation had been extensively carried on, their separating 

 pits and gorges would be choked with debris; but, on the contrary, 

 we find only a mere sprinkling of post-glacial detritus, and that the 

 streams could not have carried much of this away is conclusively 

 shown by the fact that the small lake-bowls through which they flow 

 have not been filled up. 



In order that we may obtain clear conceptions concerning the 

 methods of glacial mountain-building, we will now take up the 

 formation of a few specially illustrative groups and peaks, without, 

 however, entering into the detail which the importance of the subject 

 deserves. 



The Lyell group lies due east from Yosemite Valley, at a distance 

 of about sixteen miles in a straight course. Large tributaries of the 

 Merced, Rush, Tuolumne, and San Joaquin rivers take their rise 

 amid its ice and snow. Its geographical importance is further aug- 

 mented by its having been a center of dispersal for some of the larg- 

 est and most influential of the ancient glaciers. The traveler who 

 undertakes the ascent of Mount Lyell, the dominating mountain of 

 the group, will readily perceive that, although its summit is 13,200 

 feet above the level of the sea, all that individually pertains to it is 

 a small residual fragment less than a thousand feet high, whose ex- 

 istence is owing to slight advantages of physical structure and posi- 

 tion with reference to the heads of ancient glaciers, which prevented 

 its being eroded and carried away as rapidly as the common moun- 

 tain mass circumjacent to it. 



Glacier wombs are rounded in a horizontal direction at the head, 

 for the same reason that they are at the bottom; this being the form 

 that offers greatest resistance to glacial erosion. The semicircular 



