CORRASION BY GRAVITY STREAMS. 307 



in a narrow canon a mile deep the immense glacier could 

 only have its mobility increased, and the basins were there- 

 fore deepened still more, while the channel cross-section 

 was beiug enlarged so as to allow of the harmonising of 

 velocities. Therefore, because a glacier must corrade 

 vertically above a point until it is unable to move its mass as 

 a whole along its bed over that point, the enormous depths 

 and flattish floors of Milford Sound basin were produced. 



3. The Finger Lake Region of New York.— Tarr 1 and 



Lincoln 2 both understood the relation existing between the 

 various lake basins of this district and the corrasive action 

 of the recent Ice Sheets. In these pages the Finger Lake 

 Region is selected, therefore, simply as an illustration of 

 the "Ice Flood Hypothesis." 



In the locality under consideration the general direction 

 followed by the ice sheet was from north to south. A range 

 of hills lay right athwart its course. The ice in its passage 

 swept over the range and left large morainic deposits upon 

 the crests in its decadent stage. The lake basins lie at 

 the feet of the hills and occur on the northerly aspect of 

 the range, that is, they lie in the deeper tracks used by 

 the basal ice in climbing the range. It may be mentioned 

 in passing that these lakes possess straight sides, hanging 

 valleys, and other characters common to Alpine lake basins. 



By some observers 3 the excavation of these lake basins 

 by ice action is denied, because of their peculiar situation 

 with respect to the general southerly movement of the ice 

 sheet. The important point to remember in this connection 

 is the fact that the ice sheet swept right over the range, 

 that is, it maintained itself as a stream over a channel 

 obstacle. Had it failed to scale the range it would here 



1 Tarr, see Literature. a Lincoln, see Literature. 

 3 Fairchild, Herman Le Roy — " Ice Erosion Theory a Fallacy " and 

 references. 



T— Nov. 3, 1909. 



