856 



GEOLOGY 



distances. Within the glaciated area, one hill may be composed of 

 drift, while the next has only an interrupted veneer of it. The 

 drift may be thick on hills and thin in valleys, or, more commonly, 

 the reverse. No agent besides glaciers habitually leaves its deposits 

 so unequally distributed, and in such disregard of pre-existing 

 topography. 



7. Contact with underlying rock. The plane of contact be- 

 tween the drift and the rock beneath is generally, though not always, 



Fig. 568. Diagram to show how drift may be so disposed as to increase 

 the relief of the surface. This should be compared with the following 

 figure. 



Fig. 569. 



Diagram to illustrate how drift may decrease the relief of the 

 surface. 



sharply denned, and the surface of the rock is likely to be fresh and 

 firm (Fig. 203, p. 252). This relation is in contrast with that be- 

 tween the mantle rock and the underlying formations where there 

 is no drift (Fig. 210, p. 257). 



8. Striation and planation. 1 The rock surface beneatli the 

 drift, and especially beneath the unstratified drift, is frequently 

 polished, planed, striated (Fig. 203), and grooved. These feature 

 are wide-spread throughout the drift-covered area, and they occur 

 at all elevations where the drift occurs. The striae on the bed rock 

 beneath the drift are generally approximately parallel in any n;iven 



1 7th Arm. Kept., U. $. Geol. Surv., pp. l.V. 248. An rl.-il.nnin- 

 of this topic. 



