12 RURAL MirHKlAN 



lakes, is G02 feet above sea-level. A descent of 

 twenty-one feet brings its waters to Lake Huron. The 

 course through St. Clair Eiver and Lake and the 

 Detroit Eiver lowers the waterway 8.63 feet to the 

 level of Lake Erie. Then comes the stupendous drop 

 through the Niagara gorge to Lake Ontario at 246.19 

 feet elevation. Some 221 feet of the descent from 

 Lake Ontario must be overcome by canals or slack- 

 water navigation, before the Great Lakes can in any 

 proper sense be put in touch with the world's mari- 

 time trade. 



These vast "sweet Water seas," whose presence on 

 the borders of the State has so definitely influenced 

 the economic history of the commonwealth, have 

 themselves had an intricate, but interesting, geologic 

 history. The advance and recession of the glacial 

 ice, the elevation and subsidence of the surface of 

 the land, from time to time formed and reformed 

 lakes of varied shapes and sizes along the line of 

 the depressions which now contain their dwindled 

 remains. These prehistoric glacial lakes are known 

 by such names as Lake Saginaw, Lake Chicago, Lake 

 Algonquin, Lake Duluth, and Lake Ontonagon, while 

 the jSTipissing Great Lakes conformed on a some- 

 what larger scale to the Great Lakes of the present 

 era. Of these ancient bodies of water in the Michigan 

 area, the outlet was sometimes by way of the Georgian 

 Bay-Lake Simcoe route through Ontario; sometimes 

 via the Chicago-Illinois Eiver depression into the 

 Mississippi, or to the far northward over the line 

 of the low ground between the west end of Lake 



