THE WORK OF GLACIERS 



183 



which it carried along for a time and later dropped, either as hetero- 

 geneous, unstratified till, or as stratified sands, clays, and gravels. 

 The rock under- 

 lying the drift is 

 often smoothed and 

 striated (p. 157) 

 (Fig. 168), differ- 

 ing from that of 

 the non-glaciated 

 regions in this par- 

 ticular, as well as 

 in the fact that the 

 surface rock is usu- 

 ally fresh and does 

 not pass gradually 

 into soil, as the rot- 

 ten rock has been 

 removed by the 

 glaciers. The scratches and grooves (Fig. 



168. — A rock surface polished and striated by 

 glacial action. (Photo. L. E. Westgate.) 



169) on the surfaces of 

 glaciated rocks usually have a common direction (with some variation) 

 and show, as do the glacial bowlders or erratics (p. 156), the direction 



of the movement of 

 the ice. Harder por- 

 tions of the rock being 

 less easily smoothed 

 by the ice, project 

 slightly above the gen- 

 eral surface and also 

 show by the greater 

 abrasion on one side 

 (stoss) the direction 

 from which 

 came. The 



erosion on 



the ice 

 effect of 

 different 



rock 



is not 



169. — Rock grooved and polished by glaciers. 

 The excavation on the right is artificial. 



rock the more easily it will 

 actually be removed from 



kinds of 

 always in proportion 

 to their softness, al- 

 though the softer the 

 More material may 



be worn away, 

 a hard but much-jointed granite by the 



