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Sierra Club Bulletin. 



rise. On the evidence of these same characters it is 

 apparent no less that the process was rapid and its 

 period short. 



But head-wall sapping, with planation, is only one 

 process. Scour is another ; and scour acts vertically. The 

 greater basins, doubtless preglacial in their outlines on the 

 range flanks, have been expanded over the summit 

 region, and the wide-open, shallow troughs there are 

 essentially the products of glacial erosion, by head-wall 

 recession; and they are the measure of that erosion only 

 in part. The central canons, on the other hand, as 

 channel troughs of glaciation are doubtless preglacial as 

 to their courses. And in their central parts they may 

 have been deep. But in their upper parts, on the evi- 

 dence of their grade profiles, they are to be regarded, in 

 the main, as products of glacial erosion also. 



The sensibly level elements in these profiles, their 

 great length often, and the common occurrence of grade 

 reversal in the more exposed floors above the trunk- 

 canon head, seem to imply that the glacier advances 

 from the bottom, escaping from under its own weight; 

 that its lines of flow gradually rise, seeking to equalize 

 depth and load, because ablation planes away and thins 

 the forward part; and that, correspondingly, along the 

 bed, both pressure and velocity, and therefore erosive 

 vigor, diminish. 



