COLD AND ICE ON THE EARTH 



and pushes its rubbish along. It is not all stones. 

 Clay and earth mingle with it, often enclosing the stones ; 

 and the debris left by extinct glaciers of ages ago is some- 

 times called the boulder-clay. This is the deposit, earthy 

 and stony, that the glacier leaves on the floor of the 

 valley as it shrinks unless the river which usually 

 springs from the end of glaciers sweeps it away. Most 

 of the stones thus left are smoothed or polished and 

 covered with scratches or ruts, such as would be made 

 by rubbing against other hard pointed fragments of stone. 

 This is to be explained by the fact that these stones 

 as they were carried on by the glacier were rubbed on 

 the floor of rock over which the glacier was slipping. 

 If their journey was long enough, they stood a chance 

 of being rubbed away altogether and of finishing their 

 existence as sand or mud. What the valley did to the 

 glacier's stones, the stones did to the valley. They 

 scratched it and scored it. Every promontory of rock 

 which stood in the path of the ice had its angles and 

 corners ground away. The polish and the directions of 

 the scratches are especially remarkable, because, whether 

 the marks are mere lines or deep-worn ruts, they are 

 all on smooth surfaces, and they all run one way. That 

 way is the direction in which the glacier moved. How 

 high a degree of polish or how deep the markings may 

 be depends a good deal on the kind of rock over which 

 the glacier moved. Tough, close-grained rocks, such as 

 hard limestone, are sometimes polished to look like 

 marble. But there is a great deal of difference between 

 the smoothing effected by a river or a torrent and that 



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