GLACIAL DEPOSITS 



161 



1 65. Section of drift showing its heterogeneity. 



Though notably worn, they are not rounded like the stones carried 

 by rivers. Many of them have sub-angular forms with planed and 

 brveled faces, the planes being striated and bruised (Fig. 147). 

 Ah-cnce of stratification, physical heterogeneity, and the striation 

 of at least a part of the stones are among the most distinctive char- 

 acteristics of glacial drift. A not less real though less obvious 

 characteristic is the constitution of the fine material, for it is, as a 

 rulr, 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 

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

 (Figs. 145 and 166) are marked by grooves and striae, similar to 



fose on the stones of the drift (Fig. 147). ' Other distinctive fea- 

 res of a glaciated area are rounded bosses of rock (roches mouton- 



