GLACIAL CANONS. " 363 



low declivity developed by varigradation ; and at such localities, 

 accordingly, glacial lakes, the submerged rock-basins characteris- 

 tic of fjords, and other evidences of energetic ice-action remain 

 after the melting of the ice.) 



It follows, then, that the third feature of glacial canons may 

 result simply from glacial occupation of water-cut canons ; and 

 since in the common mountain region from which the glaciers 

 have completely disappeared the irregularities of gradient pecu- 

 liar to such cafions are not greatly intensified, while glaciated 

 rock-basins are comparatively rare and of slight depth, it equally 

 follows that the occupation was only temporary, and the sum of 

 glacial erosion relatively inconsiderable. 



V. 



The immediate effect of the origin of a tributary cafion in a 

 developing drainage-system is the exposure of a greater length 

 of canon-wall to degradation ; from which effect in turn results 

 (under certain conditions of homogeneity of terrane and uni- 

 formity of altitude in the region, and hence of repeated bifur- 

 cation and wide dispersal of the branches of the nascent tributary) 

 the formation of an amphitheatre opening into the main canon. 

 Then, after the considerable development of the tributary, its 

 disposition will be, as shown by Warren,' to dam the main stream 

 and diminish the declivity above its confluence ; whereby lateral 

 corrasion will increase at the expense of vertical corrasion there. 

 Thus, by increased lateral corrasion the amphitheatre will ever 

 tend to expand within certain limits immaterial in this discussion. 

 Such amphitheatres, exhibiting the tortuous outlines character- 

 istic of fluvial erosion, have been well illustrated by Button,^ and 

 are common features in many mountain regions. 



If now a glacier enter and fill such an amphitheatre, its rate 

 of flow and similarly its rate of erosion on the given area will be 



^ " An Essay Concerning Important Physical Features Exhibited in the Valley of 

 the Minnesota River," 1874-7 ; and elsewhere. 



= " Tertiary History of the Grand Canon District," 1882, Chapter IX, and maps 

 accompanying. 



