LESSONS IN GEOLOGY. 363 



country during the glacial period; and may find some rational 

 explanation of the interesting and peculiar surface features which 

 were determined wholly, or in part, by glacial ice. 



The study of surface features is somewhat complicated by the 

 fact that coincident with the final retreat of the ice, the whole 

 region sank slowly down almost to sea level, and was cov- 

 ered with the broad, shallow lakes, or seas, of the Champlain 

 period, which greatly modified the surface left by the ice sheet. 

 Then the surface rose to about its present level, the lakes drained 

 away, still further modifying the old glacial surface, as the coun- 

 try gradually attained its present condition. 



Ice seems to be a brittle solid, but as it accumulates in the 

 mountains of Switzerland, it flows down those valleys like rivers, 

 slowly but surely following every change in direction, every vari- 

 ation in slope, and every irregularity of surface almost as per- 

 fectly as water does. 



As the ice moves down the valley loose fragments of rock are 

 pushed forward, abrading each other as they roll along; sharp 

 angles of rock are broken off, which, with rocks falling from cliffs 

 above, jostle along between the ice and the sides of the valley 

 abrading each other, till in time the whole mass of ice is filled 

 with sand, gravel, pebbles and fragments of rock. Those along 

 the bottom and sides, held firmly in the frozen grasp, make the ice 

 a gigantic rasp scratching, striating and grooving its rocky bed, 

 and smoothing out its sharper irregularities. Thus the ice river, 

 like the river of water, flows toward the sea with its burden of 

 mud, sand, pebbles, etc. At the end of the glacier the melting ice 

 forms a stream which carries away much of the fine dust from 

 the glacial mill ; but the pebbles, boulders and rocks, with much 

 of the finer materials accumulate, forming what is called a mo- 

 raine. A few hotter seasons cause the ice to retreat from the 

 moraine, covering the ground with clay, pebbles and boulders, 

 which otherwise would have been heaped on the moraine. A few 

 colder seasons and the ice advances again, pushing before it the 

 loose material it dropped on its retreat, until it again reaches 

 the moraine and perhaps crowds it forward. 



