134 



PHYSIOGRAPHY 



was relatively weak. Such basins are less common in the area of 

 the continental ice-sheet than in the mountain valleys affected by 

 glaciers. 



Changes Produced by Deposition 



Sooner or later the ice deposited all of the material which it 

 gathered by erosion. Had the drift been equally thick everywhere, 

 its effect would have been to raise the surface without altering its 

 topography; but the drift is distributed with great inequality, and 

 therefore changes the surface greatly. 



Effect of drift on topography. The drift sometimes increases 

 the relief of the surface (Fig. 140) , but oftener decreases it (Fig. 141) . 



Fig. 140. Diagram to show how drift may be so disposed as to increase the 

 relief of the surface. The upper part of the Fig. represents drift, the 

 lower part the rock beneath. This should be compared with the fol- 

 lowing figure. 



Fig. 141. Diagram to illustrate how drift may decrease the relief of the 



surface. 



The drift was sometimes left in such a way as to make the surface 

 rougher than the surface of the rock below, even where the relief 

 was decreased. 



Both the erosion by the ice and the deposition of its drift pro- 

 duced topographic features very unlike those made by streams. 



Effect of drift deposits on drainage, a. Lakes. The drift filled 

 valleys at some points, but not at others. Where a valley is filled 

 at one point, water is likely to accumulate above the filling, as above 

 a dam, making a lake. Where a valley was filled in two places, as 



