Great Lakes, and the Valley of the JUississtjppi. 223 



leaving large bodies of cold fresh water * about their south- 

 ern margins, in which the mud produced by their grinding 

 action on the paleozoic rocks of the Lake District was first sus- 

 pended and then deposited. 



On the shores of Lake Erie these clays contain no boulders, 

 and very few pebbles, while farther North and West boulders 

 are more abundant. This is precisely what might be expected 

 from the known action of glacial masses on the surfaces over 

 which they pass. Their legitimate work is to grind to powder 

 the rock on which they rest: an effect largely due to the Band 

 which gathers under them, acting as emery on a lead wheel. 

 The water flowing from beneath glaciers is always milky and tur- 

 bid from this cause. Rocks and boulders are Bometimee frozen 

 into glaciers, and thus transported by them, but nearly all 

 the boulders carried along by a glacier are such as have fallen 

 from above; and a moraine can hardly be formed by a glacier 

 except when there are cliffs and pinnacles along its course. 



In a nearly level country, composed of sedimentary rocks 

 passed over by a glacier, we should have very little debris pro- 

 duced by it, except the mud flour which it grinds. 



The Erie clays would necessarily receive any gravel or 

 stones which had been frozen into the ice, either as scat:. 

 pebbles or stones, distributed to some distance from the glacial 

 mass by floating fragments of ice, or as masses of frozen gravel, 

 or larger and more numerous boulders near the glacier. In Borne 

 localities torrents would pour from the sides and from beneath 

 the glacier, so that here coarse material would alone resist the 

 rapid motion of the water, and the stratification of the sedi- 

 ments would be more or less confused. 



In regard to the cau8e of the gradual amelioration of the cli- 

 mate of the glacial epoch, by which the greal glaciers of the 



* Cold, because coming from the melting glacier, and depositing writb its ■■ ali- 

 ments no evidences of life ; frexh, because no marine sheila are found in it— only 

 drift-wood — while the equivalent " Champlain " clays on the coast are full of 

 marine Arctic shells. 



