EXCAVATION BY GLACIERS 51 



not cut the first lines of a valley, but they constantly 

 deepen the valleys along which they move. As to how 

 much of this kind of excavation is due to the grinding 

 action of the vast weight of slowly moving ice and how 

 much is due to the huge and violent rushing torrent of 

 water which always underlies the ice and is hemmed in at 

 the sides of the valley by it, seems to be differently 

 estimated by American, Swiss, and English geologists. 



