GLACIAL CANONS. 35 I 



of the subject in this paper is analytic, the work was primarily 

 synthetic and based directly on field observations and inferences 

 in the magnificent field of the southern Sierra. 



I. 



Glacial canons are characterized by several peculiar features : 



1. They are U shaped rather than V shaped in cross-profile; 



2. Small tributary gorges usually enter at levels considerably 

 above the cafion-bottoms ; 3. In longitudinal profile the canon- 

 bottoms are irregularly terraced — i. e., made up of a series of 

 rude steps of variable form and dimensions, — and some of the 

 terraces are so deeply excavated as to form rock-basins occupied 

 by lakelets ; 4. The canons are sometimes locally expanded into 

 amphitheatres ; 5. The cafion-bottom is not always obdurate rock, 

 but may consist of coarse fragmental debris in which individual 

 blocks are as deeply striated and as smoothly polished as are 

 the most solid ledges, though they may rest so insecurely in their 

 positions that a hand can overthrow them ; and 6. The volume 

 of glacial debris in moraine and valley deposits is but a small 

 fraction of the cubic content of the canon from which it was 

 derived. 



Of these features the first four suggest that glaciers are most 

 effective engines of erosion, while the last two indicate that 

 glacial erosion is inconsiderable. The source of the apparent 

 discrepance may be sought through analysis of the agencies 

 involved in the development of the four features first enumer- 

 ated. 



II. 



Whatever be the physical cause of ice-flow, the motion of a 

 glacier is unquestionably determined by ( i ) the weight of the 

 ice, (2) the declivity of the channel, (3) the share of potential 

 energy not expended in overcoming internal cohesion, and hence 

 available in producing mass motion, and (4) the friction against 

 bottom and sides of the channel ; of which factors the last two 

 (one of which is positive and the other negative) are indeter- 

 minate. The united effect of all — i. e., the total sum of potential 



