366 PRACTICAL LESSONS IN SCIENCE. 



and Michigan nearly all the varieties of granitic rocks may be 

 found among the boulders of the hard pan. Large quantities of 

 drift have been transported from 300 to 500 miles, but for the 

 most part the transportation has seldom been more than 100 

 miles. But little is known about the thickness of the ice ; it seems 

 to have covered the mountains of New England, but was doubt- 

 less much thinner west and south, for in southwestern Wisconsin 

 and vicinity there is a large area over which no glacial ice ever 

 flowed; it is a "driftless area." The flow of ice over such low 

 gradients as seemed to prevail during the glacial period is not 

 well explained. It flows over similar gradients in Greenland, so 

 that it is not impossible, only difficult to understand. It is also 

 difficult to understand how the ice became so loaded through and 

 through with rocks, boulders, etc. 



The Laurentian tableland includes the oldest mountains, and 

 the great lakes occupy the oldest valleys on the continent. 

 Their great depth, some of them reaching two or three hun- 

 dred feet below the level of the sea, is not only evidence of 

 great erosion, but of the fact that the whole region for ages stood 

 at least from 1,000 to 1,500 feet higher than now ; for those old 

 channels must have had outlets to the sea. The glacier smoothed 

 up these old channels, perhaps changing their form somewhat, 

 but not materially; and when it retreated, its load of clay and 

 rocks filled many of them, and parts of many more, result- 

 ing in the formation of thousands of lakes, great and small, over 

 some parts of the glaciated area. 



The rocky bed of the Cuyahoga river, at Cleveland, is about 

 225 feet below the present level of Lake Erie. The beds of other 

 tributaries of Lake Erie are at about the same distance below its 

 level, which seems to indicate that the lake occupies an old valley, 

 which formerly had an outlet to the sea at a much lower level. 

 There is evidence of such a channel, now filled with glacial mate- 

 terial, extending from Lake Erie to the head of Lake Ontario. 

 The filling up of this old channel changed a portion of a river 

 valley into a lake, and the waters of this lake, finding an outlet 

 across the divide to Lake Ontario, formed the Niagara river, with 



