304 



GEOLOGY. 



Distinctive nature of glacial deposits. — The deposits made by glaciers 

 are distinctive. In the first place the ice does not assort its material, 

 and bowlders, cobbles, pebbles, sand, and clay are confusedly com- 

 mingled (Fig. 279). In this respect, the deposits of ice differ notably 

 from those of water. Furthermore, many stones of the drift show the 

 pecuHar type of wear which glaciers inflict. They are not rounded as 

 the stones carried by rivers, though they are notably worn. Many of 



Fig. 280. — Roche moutonn^e, Victoria Harbor, B. C. 



them have subangular forms with planed and beveled faces, the planes 

 being striated and bruised (Fig. 254). The absence of stratification, 

 the physical heterogeneity, and the striation of at least a part of the 

 stones are among the most distinctive characteristics of glacial drift. 

 A not less real though less obvious characteristic is the constitution of 

 the fine material, for it is in general not the product of rock decay, but 

 of rock grinding. The fine material handled by streams (except glacial 

 streams) on the other hand, is usually the product 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 are marked by grooves 

 and striae, similar to those on the stones of the drift (Figs. 255 and 256). 

 Other distinctive features of a glaciated area are the rounded bosses of 

 rock (roches moutonnees, Fig. 280; see also surface about the lakes. 



