Studies in the Sierra 



7S 



rents and low-voiced brooks is very much less than is vaguely 

 supposed. The brook which drains the south flank of the 

 Clouds' Rest ridge, above Yosemite Valley, in making its way 

 southward to join the Nevada Creek, is deflected to the west 

 by the right lateral moraine of the ancient Nevada glacier, and 

 compelled to creep and feel its way along the outside of the 

 moraine as far as to where it is caught between the moraine and 

 an escarpment which advances from the Clouds' Rest crest. 

 When halted here, it spread into a pool, and rose until it was 

 able to effect its escape over the lowest portion of the barrier. 

 Now, this stream, which in ordinary stages is about five feet 

 wide and a foot deep, seems to have flowed unfailingly in one 

 channel throughout all the long post-glacial centuries, but the 

 only erosion the moraine has suffered is the removal of sand, 

 mud, and some of the smaller boulders, while the large stones, 

 jammed into a kind of wall, are merely polished by the friction 

 of the stream, and bid fair to last tens of thousands of years. 

 The permanence of soils depends more upon their position and 

 mechanical structure than upon their composition. Coarse por- 

 ous moraine matter permits rains and melting snows to perco- 

 late unimpeded, while muddy and impermeable beds are washed 

 and wasted on the surface. 



Snow avalanches more resemble glaciers in their methods of 

 soil formation and distribution than any other of the post-gla- 

 cial agents. The century avalanche sweeps down all the trees 

 that chance to stand in its path, together with soils of every 

 kind, mixing all together without reference to the size of their 

 component fragments. Most of the uprooted trees are deposit- 

 ed in lateral windrows, heads downward, piled upon each other, 

 and tucked snugly in alongside the clearing; while a few are 

 carried down into the valley on the snout of the avalanche, and 

 deposited with stones, leaves, and burs, in a kind of terminal 

 moraine. 



The soil accumulations of annual avalanches are still more 

 moraine-like in form, and frequently attain a depth of from 

 forty to fifty feet. They are composed of mud, sand, coarse 

 granules, and rough angular blocks, avalanched from the moun- 

 tain side, and sometimes water-washed pebbles also, derived 

 from the channels of streams. 



