426 



Sierra Club Bulletin 



of October and November, before much snow has fallen ; and 

 where shallow water-currents flow over rocks traversed by 

 open divisional joints, the freezing that ensues forces the 

 blocks apart and produces a ruinous appearance, without ef- 

 fecting much absolute displacement. The blocks thus loosened 

 are, of course, liable to be moved by flood-currents. This ac- 

 tion, however, is so limited in range, that the general average 

 result is inappreciable. 



Atmospheric weathering has, after all, done more to blur 

 and degrade the glacial features of the Sierra than all other 

 agents combined, because of the universaHty of its scope. No 

 mountain escapes its decomposing and mechanical effects. 

 The bases of mountains are mostly denuded by streams of 

 water, their summits by streams of air. The winds that sweep 

 the jagged peaks assume magnificent proportions, and effect 

 changes of considerable importance. The smaller particles of 

 disintegration are rolled or shoved to lower levels just as they 

 are by water currents, or they are caught up bodily in strong, 

 passionate gusts, and hurled against trees or higher portions 

 of the surface. The manner in which exposed tree-trunks are 

 thus wind-carved and boulders pohshed will give some concep- 

 tion of the force with which this agent moves. 



Where boulders of a form fitted to shed off snow and rain 

 have settled protectingly upon a polished and striated surface, 

 then the protected portion will, by the erosion and removal of 

 the unprotected surface around it, finally come to form a ped- 



quantity of post-glacial de- 

 nudation at that point. These boulder pedestals, furnishing so 

 admirable a means of gauging atmospheric erosion, occur 



Fig. 2. 



estal for the stone which 

 saved it. Figure 2 shows 

 where a boulder, B, has set- 

 tled upon and protected 

 from erosion a portion of 

 the original glaciated sur- 

 face until the pedestal, A, 

 has been formed, the height 

 of which is of course the 

 exact measure of the whole 



