198 



PHYSICAL GEOLOGY 



FIG. 204. End of the Alsek Glacier, Alaska. 



Commission.) 



(Netland, U.S. Boundary 



ground of a glacier the surface of the snow and ice is usually 

 concave, the movement is inwards toward the center, as well 

 as down valley. Farther down the valley, the surface is 

 commonly convex, in part because the marginal ice is melted 

 faster by heat reflected from the walls of the valley, and there 

 is accordingly movement toward the sides of the valley, as well 

 as along its axis. 



FIG. 205. The Zwillinge and Grenz Glaciers, Switzerland. Shows debris 

 on the ice, crevasses, etc. (R. T. Chamberlin.) 



