218 



PHYSICAL GEOLOGY 



FIG. 230. Glaciated rock surface. The view shows also the relation of 

 drift to the bedrock beneath. Northern Ohio. (Stauffer.) 



times of considerable size, are dislodged. The freezing of 

 water in the openings of the rocks beneath the ice helps in 

 the process. 



General effect of erosion upon relief. Other things 

 equal, ice sheets erode most in regions where many slopes 

 oppose the advance of the ice. In flat regions the frozen 

 mantle rock has sometimes been overridden by thick ice, 

 and little disturbed. In rugged regions ice sheets tend to 



FIG. 231. Glaciated stones. 



