Studies in the Sierra 



425 



per season, on the melting of the winter snow, when all weak- 

 ly constructed dams and drift-heaps are broken up and re- 

 formed. 



It is a fact of great geological interest that only that portion 

 of the general detritus of post-glacial denudation — that is, in 

 the form of mud, sand, fine gravel, and matter held in solution 

 — has ever at any time been carried entirely out of the range 

 into the plains or ocean. In the cafion of the Tuolumne River, 

 we find that the chain of lake basins which stretch along the 

 bottom from the base of Mount Lyell to the Hetch-Hetchy 

 Valley are filled with detritus, through the midst of which the 

 river flows ; but the washed boulders, which form a large por- 

 tion of this detritus, instead of being constantly pushed for- 

 ward from basin to basin, lie still for centuries at a time, as is 

 strikingly demonstrated by an undisturbed growth of immense 

 sugar-pines and firs inhabiting the river-banks. But the pres- 

 ence of these trees upon water-washed boulders only shows 

 that no displacement has been effected among them for a few 

 centuries. They still must have been swept forward and 

 outspread in some grand flood prior to the planting of these 

 trees. But even this grand old flood of glacial streams, whose 

 magnificent traces occur everywhere on both flanks of the 

 range, did not remove a single boulder from the higher to the 

 lower Sierra in that section of the range drained by the Tu- 

 olumne and Merced, much less into the ocean, because the low- 

 er portion of the Hetch-Hetchy basin, situated about half-way 

 down the western flank, is still in process of filling up, and as 

 yet contains only sand and mud to as great a depth as observa- 

 tion can reach in river sections. The river flows slowly 

 through this alluvial deposit and out of the basin over a lip of 

 solid bed-rock, showing that not a single high Sierra boulder 

 ever passed it since the close of the glacial period; and the 

 same evidence is still more strikingly exhibited in similarly 

 situated basins in the Merced Valley. 



Frost plays a very inferior part in Sierra degradation. The 

 lower half of the range is almost entirely exempt from its dis- 

 ruptive effects, while the upper half is warmly snow-mantled 

 throughout the winter months. At high elevations of from 

 ten to twelve thousand feet, sharp frosts occur in the months 



