52 MAN AND THE GLACIAL PERIOD. 



ties in the compactness or hardness of the rock, the 

 natural action of running water is to hollow out the soft 

 parts, and leave the harder parts projecting. But, in the 

 phenomena which we are attributing to glacial action, 

 there has been a movement which has steadily planed 



- • ^r 



,ii«:« 



Fig. 19. — Bed-rock scored with glacial marks, near Amherst, Ohio. (From a pho- 

 tograph by Chamberlin.) 



down the surface of the underlying rock; polishing it, 

 indeed, but also grooving it and scratching it in a man- 

 ner which could be accomplished only by firmly held 

 graving-tools. 



This polishing and scratching can indeed be produced 



