268 



GEOLOGY 



less obvious characteristic is the constitution of the fine material, 

 for it is, as a rule, the product of rock grinding, not of rock decay. 

 Glaciated rock surfaces. Another distinctive mark which a 

 glacier leaves behind it is the character of the surface of the rock 

 on which the drift rests. This is generally smoothed by the severe 



Fig. 223. Section of drift showing its heterogeneity. 



abrasion to which it has been subjected, and the smoothed surfaces 

 (Figs. 203 and 224) are marked by grooves and st ria-. similar to those 

 on the stones of the drift (Fig. 205). Other distinctive features of 

 a glaciated area are the rounded bosses of rock (rochcs moidonnn'x. 

 Fig. 225), the rock basins, the lakes (Fig. 226), ponds, and marshes, 

 and the peculiar topographies resulting from the mutual erosion, 

 and the still more unequal deposition of the drift (Figs. 227 and 



