217 



The latter seems to have been the result over most of the 

 lake and prairie plains in northern United States. 



level surface may be replaced by a rough 

 one through the uneven deposition of 

 drift. 



TOPOGRAPHIC FEATURES DEVELOPED BY GLACIER EROSION 



How glaciers erode. Since it is much softer than rock, 

 pure ice accomplishes little or no wear upon smooth, firm 

 surfaces ; rather is it worn by the harder rock. As already 

 indicated, however, the 

 bottom ice is likely to be 

 charged with rock frag- 

 ments, and thus armed, 

 glaciers become efficient 

 agents of erosion. Their 

 rock tools are pressed 



with tremendous force FlG - 228 - Diagram showing how a nearly 



upon the surfaces over 

 and against which they 

 move, and each kind 

 does its appropriate work. 

 Clay particles tend to 

 smooth and polish, sand 

 grains and hard pebbles 

 to scratch (striate), and 

 bowlders to gouge and 

 groove the bedrock (Fig. 

 230). Meanwhile, the 

 tools are themselves worn. The weaker ones may be ground 

 into fine bits, even to rock flour. The stronger ones often are 

 marked typically ; their sides are worn flat, and, like the bed- 

 rock, are polished by clay and striated by sand (Fig. 231). 



Thick ice moving over much-jointed surfaces sometimes 

 quarries out blocks of rock by a process known as plucking. 

 The bottom ice is pressed by the great weight of that above 

 into the joints, bedding planes, and other openings of the 

 rocks, and as the glacier moves onward, fragments, some- 



FIG. 229. Diagram showing how glacial 

 drift may be so disposed as to replace 

 a hilly surface with a comparatively level 

 one. 



