2l8 



THE GUIDE TO NATURE 



FIG. 



CUCIIEGAX ROCK, MOXTVILLE, COXXECTICUT. 



rial is carried on top of the ice. From 

 overhanging cliffs, frost and other 

 agencies continually detach blocks of 

 rock, large and small, which fall upon 

 the surface of a glacier. In the case of a 

 continental ice sheet, the amount of 

 material on top of the ice is relatively 

 small ; but here and there sharp, rocky 

 ridges protrude above the ice, and from 

 such ridges the blocks detached by 

 frost may fall upon its surface. 



A glacier or a continental ice sheet 

 carries thus at its bottom or on its sur- 



FIG. 



10. BOULDER XEAR COBALT, COXXEC- 

 TICUT 



face whatever loose material is fur- 

 nished to it. But it does more than 

 this. The ice with the material frozen 

 into its bottom abrades like a colossal 

 rasp the surface of bed rock over which 

 it passes. If the bed rock over which 

 it moves is intersected by cracks, the 

 ice penetrates into those cracks. It 

 freezes around the semi-detached 

 blocks between the cracks, and so 

 plucks them from their place and car- 

 ries them forward. If the ice moves 

 over a hummock in its path, it tears 

 off blocks of rock from the top of the 

 hummock, and the more rapid motion 

 of the superlicial ice carries these 

 blocks forward over the stagnant ice 

 in the lee of the hummock. Thus the 

 rock material transported by a glacier 

 is partly beneath the ice, partly on top 

 of the ice, and partly within the ice. 



When the ice finally melts, it drops 

 its load. Coarse material and tine are 

 left indiscriminately wherever they 

 happen to be when released from the 

 icy grip. Thus we see how it is that 

 the bed rocks of New England are gen- 

 erally covered, not with material result- 

 ing from the decomi^iosition and disin- 

 tegration of the underlying rock, but 

 with material of various origins trans- 

 ported by the glacier and dropped by 



