56 



AQUEOUS AGENCIES. 



jammed between the glacier and the confining rocks, partly fall into 

 crevasses and work their way to the bed, and partly are torn from the 

 rocky bed itself. But on the other hand, on account of their slow mo- 

 tion, glacier-erosion is by force of pressure, while that of water is by 

 force of impact. The effects of glacier-erosion differ entirely from 

 those of water : 1. Water, by virtue of its perfect fluidity, wears away 

 the softer spots of rock, and leaves the harder standing in relief ; while 

 a glacier, like an unyielding rubber, grinds both hard and soft to one 

 level. This, however, is not so absolutely true of glaciers as might be 

 supposed. Glaciers, for reasons to be discussed hereafter, conform to 

 large and gentle inequalities of their beds, though not to small ones, 

 acting thus like a very stiffly viscous body. Thus their beds are worn 

 into very remarkable and characteristic smooth and rounded depressions 

 and elevations called roclies moutonnees (Fig. 45). Sometimes large 



Fig. 45. — Koches Moutonnees of an Ancient Glacier, Colorado (after Hayden). 



and deep hollows are swept out by a glacier at some point where the 

 rock is softer or where the slope of the bed changes suddenly from a 

 greater to a less angle. If the glacier should subsequently retire, water 

 accumulates in these excavations and forms lakelets. Such lakelets are 

 common in old glacial beds. 



2. The lines produced by water- erosion, if detectible at all, are 

 always more or less irregular and meandering ; while those produced by 

 glaciers are straight %xA parallel (Fig. 46). 



Thus, smooth, gently-billowy surfaces, marked with straight, parallel 



