■jOi- 



sapping action. In other wm-.K. tin; more pronounced the 

 constriction of the channel the more pronounced will be 

 the basin depths, produced by vertical corrasion. Similarly 

 the basin depth has a direct relation to the general depth 

 of the channel. The floor of the basin also becomes flatter 

 in such cases, because of the inability of the stream to 

 completely develop its lateral axis of corrasion 1 for the 

 same reason. Upon a declivity the stream velocity is 

 greatly increased and the structures of the channel base 

 are easily removed. This interesting case is discussed later. 

 Methods of Stream Coirasion.— (a) Relative strength 

 of streams 1 1 nil roc/,- structures. -The walls of the Yosemite 

 Valley or the allied San Joaquin Valley present the appear- 

 ance of a well-jointed rock mass, the strength of the 

 structure not being very great when opposed to the action 

 of a force moving along a declivity. Figure 8 represents 

 the Yosemitc struct nros appi >.\im,i'< iy. To remove a block 

 such as that represented by X Y Z in figure 8, very little 

 more than the power necessary to lift it is needed to dis- 

 lodge the block from the ledge. This relative weakness of 

 the granites of the Yosemite when acted upon by a down- 

 stream force is very pronounced. This feature will receive 

 fuller attention when the origin of roches moutonnees is 

 discussed. It will be seen however that a stream of 

 enormous volume such as the Yosemite Glacier of recent 

 time would be able to drag huge rock blocks from their 

 unstable anchorages on the channel declivities with the 

 greatest ease, little energy being required for the dislodge- 

 ment of the blocks beyond that necessary for their trans- 

 portation, the jointing of the rocks being so perfect. 



