MICHIGAN ACADEMY OP SCIENCE. 23 



half of the peninsula, which now rises in i)laees 1,000 to 1,100 feet above 

 the surface of the lakes, seems to have no rock standing more than 250 

 to 300 feet above the lakes, there being 700 to 800 feet of drift on its 

 higher parts. It has been estimated by W. F. Cooper from plauimeter 

 measurements on maps showing present surface and bed rock surface, 

 issued bj^ the U. S. Geological Suiwej,^ that the entire southern penin- 

 sula of Michigan has an average of about' 300 feet of drift. The area 

 of thick drift covers also northwestern Ohio, northern Indiana, and 

 northeastern Illinois, and averages nearly 200 feet.- 



GLACIAL EROSIOX. 



The large amount of drift deposited at the ends and on the borders of 

 the ice lobes that occupied the basins of the Great Lakes calls for a 

 similar erosion of districts over which the ice jtassed. Considerable 

 material Avas gathered in Canada as we know from the occurrence of 

 Canadian bowlders and ijebbles in all parts of the Great Lakes region 

 and southward to the glacial boundary. But this appears to be a less 

 important constituent than the material derived from the softer rock 

 formations in and around the Great Lakes basins. The examination of 

 constituents of the drift made in Wisconsin, Illinois, Indiana, Ohio, 

 and Michigan, have shown from To to 80 ])er cent or more of material 

 derived from south of the Canadian boundary. 



WEIGHTIXG AND DEPRESSION BY ICE. 



That the region of the Great Lakes has experienced considerable in- 

 'crease in altitude on the withdrawal of the ice sheet is well shown by 

 the tilting of the shore lines of the glacial lakes that were the prede- 

 cessors of the present Great Lakes. This tilting is so closely connected 

 with the relief of the ice covered district from its burden of ice that it 

 seems highly probable that it came as a result of the relief from this 

 load. As will be shown later it amounts to several hundred feet in the 

 northern part of the Great Lakes legion, Avhere the load was heavy, and 

 dies out entirely in the southern part where the load Avas light. If the 

 unloading has given occasion for such a marked upward movement it 

 would seem a safe assumption that the accumulation of the ice had 

 caused a weighting down or de^jression from which this is a resilience. 

 The presence of a large amount of drift in the districts bordering the 

 Great Lakes, and especially that in the northern half of the southern 

 peninsula of Michigan, seems likely to have had a measurable effect 

 in preventing complete retur-n to preglacial altitudes. It is perhaps 

 to the great amount of drift east of the northern halt of the Lake ^Michi- 

 gan basin that the very slight rise of that region is due, a rise so slight 

 that it is scarcely measurable south of Manistee. 



It thus appears that the lake basins as we uoav find them are glacially 

 modified lowlands, which have been loaded in places by drift, and in 

 places eroded and weighted down by the ice. They are held up l)y rock 

 and drift banners to le\'els several hundred feet above their rock beds, 

 the lowest, Ontario, beiup- nearly 250 feet, and the highest, Superior, 000 

 feet above sea level, while the beds of all except Erie extend in places 

 l)elow sea level. 



'-Water Supply and Irrigation Taper 182, PI. II. 



