234 THE ICE AGE IN NOBTH AMERICA. 



with their branches intact upon them, and their roots im- 

 bedded in the soil in which they grew. A stratum of this 

 soil even consists of moss and leaves and cones which origi- 

 nally formed a carpet over the forest floor. There can be no 

 doubt that, after the accumulation of sand burying the forest, 

 the glacier advanced for a great distance over it, attaining 

 a thickness at that point of two or three thousand feet. 



A little reflection will show that the advance of a glacier 

 upon a new field is analogous to that of the breakers in the 

 ocean over shallow bottoms, though the impressiveness of the 

 scene is disguised to the physical senses by the slowness of 

 the movement in the case of a glacier, and by the counteract- 

 ing effect of the heat which limits the advance of the ice-front, 

 Where the ice-movement is upon dry land, the front is ordi- 

 narily represented by a sloping field of ice covered with 

 debris deposited from the melting surface near its terminus. 

 For a long time investigators were puzzled by the fact that 

 many bowlders are found some miles south of the line mark- 

 ing the limit of glacial scratches upon the surfaces of the 

 rocks. But, in the light of the previous suggestions, it is 

 easy to see that for some distance back from the southern 

 margin there could have been no movement at all at the 

 bottom of the ice, so that bowlders upon the surface might 

 be transferred, as upon the summit of a breaker, from some 

 distance back of the front to some distance beyond the far- 

 thest limit attained by the lower strata of moving ice. This 

 narrow belt of glacial deposits bordering the limit indicated 

 by other glacial signs constitutes what Professor Lewis and 

 myself * agreed to call " the fringe " of the terminal moraine. 

 West of the Alleghanies this fringe is, as already shown, so 

 wide as to assume commanding importance, and everywhere 

 deserves more attention than we at first gave it. f 



This characteristic of the movement of glacial ice is illus- 

 trated by another phenomenon which has not been sufficiently 



* See " Second Geological Survey of Pennsylvania, Z," pp. 45, 206 et seq. 

 + Sec above, p. 149. 



