260 W. D. WILCOX 



formation and their direction is not, like terminal or lateral 

 moraines, influenced by minor topographic features. 



From the foregoing it seems evident that these drift ridges 

 are a subglacial formation disposed under the ice along the 

 same lines as medial moraines would have had on the glacier 

 surface, and that they are a kind of crag and tail formation 

 resulting from the union of two glaciers. The fact that a rock 

 buttress is the initial point of these drift ridges, shows that 

 they were not the result of a short action at the close of the 

 ice invasion. The change of all the preglacial V-shaped valleys 

 to the present U-shaped form was accomplished by a great 

 amount of erosion and transportation of debris. The rock ridges 

 which commence and probably underlie the drift ridges are por- 

 tions of the old V-shaped valleys which by their position have 

 been preserved. They represent lines of protection from severe 

 erosive action, and it is therefore necessary that the rock should 

 be preserved along the same line in which the drift has been 

 deposited. These lake basins are therefore possibly in many 

 cases rock basins, but made much deeper by an* overlying drift 

 formation. 



It remains to inquire why the glaciers from the tributary 

 valleys did not cut out channels of even gradient, instead of 

 leaving these basins. Thus the bottom of Lake Louise is 230 

 feet below the very lowest part of its dam, and the lower sur- 

 face of its glacier must have ascended this slope upon entering 

 the Bow Valley. A study of existing glaciers shows that a tribu- 

 tary is always narrower after confluence with a larger glacier as 

 a result of the more rapid movement of the ice current. It is 

 probable that this contraction takes place in the vertical dimen- 

 sions as well as the horizontal, and thus causes the under sur- 

 face to ascend, while of course the upper maintains its level. 



Walter D. Wilcox. 



