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Sierra Club Bulletin 



are rolled over and over, or swept forward partly suspended, 

 just as dust and sand-grains are by the wind. 



The transporting power of steeply inclined torrents is far 

 greater than is commonly supposed. Stones weighing several 

 tons are swept down steep canon gorges and spread in rugged 

 deltas at their mouths, as if they had been floated and stranded 

 like blocks of wood. The denudation of gorges by the friction 

 of the boulders thus urged gratingly along their channels is 

 often quite marked. 



Strong torrents also denude their channels by the removal 

 of blocks made separable from the solid bed-rock by the de- 

 velopment of cleavage planes. Instructive examples of this 

 species of denudation may be studied in the gorges between 

 the upper and lower Yosemite falls and the Tenaya Canon, 

 four miles above Mirror Lake. This is the most rapid mode 

 of torrent denudation I have yet observed, but its range is nar- 

 rowly restricted, and its general denuding effects inappreci- 

 able. 



Water-streams also denude mountains by dissolving them 

 and carrying them away in solution, but the infinite slowness 

 of this action on hard porphyritic granite is strikingly exempli- 

 fied by the fact that in the upper portion of the middle region 

 granite ice-planed pavements have been flowed upon incessant- 

 ly since they were laid bare on the breaking up of the glacial 

 winter without being either decomposed, dissolved, or mechan- 

 ically eroded to the depth of the one-hundredth part of an 

 inch. 



Wind-blown dust, mica flakes, sand, and crumbling chips 

 are being incessantly moved to lower levels wherever wind or 

 water flows. But even in the largest mountain rivers the 

 movement of large boulders is comparatively a rare occur- 

 rence. When one lies down on a river-bank opposite a boul- 

 der-spread incline and listens patiently for a day or two, a dull 

 thumping sound may occasionally be heard from the shifting 

 of a boulder, but in ordinary times few streams do much 

 boulder work; all the more easily moved blocks having been 

 adjusted and readjusted during freshets, when the current 

 was many times more powerful. All the channels of Sierra 

 streams are subjected to the test action of at least one freshet 



