GLACIAL CANONS. 359 



Recapitulating, it appears that of the several elements 

 involved in glacial erosion, the first tends to deepen the gorge 

 and slightly to develop the U form, the second to develop the U 

 form, and perhaps very slightly to deepen the gorge, while the 

 third and predominant one tends strongly to widen the gorge 

 and protect its bottom, and less strongly to develop the U form. 

 It follows that the general tendency of glaciers must be to widen 

 rather than deepen the valleys they occupy, and to transform 

 V to U canons. Also, since the typical U gorge is just such as 

 would result from temporary occupancy of a V gorge by a 

 glacier, while the ordinary ratio of width to depth is less than 

 would obtain were the gorge eroded by glacial action exclusively, 

 it follows again that the characteristic glacial canons must be 

 only modified stream-canons. 



This conclusion explains, and is equally and directly corrob- 

 orated by, the first and sixth features of glacial canons. It also 

 fully warrants the assumption, in the following as in the forego- 

 ing discussion, of originally V shaped glacier-beds. 



III. 



As elsewhere shown, ^ corrasion of a stream is a function of 

 its volume, and, ceteris paribus, varies with, but less rapidly than 

 that element. In a region of rapid corrasion then, the main 

 stream must (unless the declivity be materially unlike) more 

 rapidly corrade its channel than does its minor tributary ; and the 

 tributary canon must accordingly enter its principal over a rapid 

 or at least a convex curve in longitudinal profile. 



If now the main canon become filled with ice and be trans- 

 formed from the V to the U type by its action, the distal 

 extremity of the tributary will be cut off and the original stream- 

 formed declivity replaced by the precipitous side-wall of the 

 normal glacier valley (fig. 3); and this result will follow whether 

 the tributary be filled with or free from ice, provided corrasion 



^ " The Formation of River Terraces " (recently published in Eleventh Annual 

 Report U. S. Geological Survey, 1891, pp. 259-272). 



