THE WORK OF SNOW AND ICE 133 



CHANGES PRODUCED BY THE CONTINENTAL GLACIERS 



The ice-sheets of North America produced many changes in the 

 surface which they covered. Some of these changes were brought 

 about by the erosion of the ice, and some by the deposition of the 

 drift. A brief statement of these changes will serve to review the 

 work of ice-sheets. It is important to remember, in this connection, 

 that the continental glaciers of North America developed on the 

 surface of a rather high plain, whose topography had been shaped, 

 in large measure, by river erosion. 



Changes Produced by Erosion 



1. On elevations. The ice was thick enough to cover the hills 

 and low mountains of the area shown in Fig. 136. As it passed 

 over them, it wore off their tops, and so tended to make the surface 

 smoother. Very small elevations were often worn away altogether, 

 but the ice had not strength enough to remove large hills or moun- 

 tains. It only changed their shapes a little, as shown by Figs. 123 

 and 124. 



2. In valleys. The ice deepened the valleys through which it 

 moved, and in many cases it deepened them more than it lowered 

 the hills. Where this was the case, the relief of the surface was in- 

 creased; but even where this was true, the roughness of the surface 

 was often diminished, for roughness depends on the frequency with 

 which elevations and depressions, such as hills and valleys, succeed 

 one another, and on the steepness of their slopes, quite as much 

 as on the amount of relief (compare Figs. 138 and 139). 



Fig. 138. Fig. 139. 



Figs. 138 and 139. These two figures show surfaces with the same amount 

 of relief, but the surface represented by Fig. 139 is the rougher. 



3. Rock basins. Another effect of ice erosion was to gouge out 

 hollows, or basins, in rock (Fig. 120), where the underlying rock 



