<3ladal and ipregladal Xafees, 221 



Wright estimates that there is not less than 

 1,000,000 square miles of territory in North 

 America covered with glacial debris to an 

 average depth of 50 feet. Of course, the 

 depth varies in different places from a few 

 inches to several hundred feet. Of the carry- 

 ing power of these great glaciers we will speak 

 more fully in a future chapter. In preglacial 

 times the watershed of the Mississippi and of 

 the great rivers east of the Alleghany Moun- 

 tains, the Susquehanna and Hudson, extended 

 probably farther north than it does to-day. 

 The larger portion of the drainage area that 

 now finds an outlet through the River St. Law- 

 rence at one time undoubtedly drained off 

 through the Mississippi Valley into the Gulf 

 and the Valley of the Mohawk into that of the 

 Hudson. 



It is supposed by those who have made this 

 branch of geology a study that prior to the 

 glacial period a river flowed down through 

 Lake Superior, which connected with Lake 

 Michigan at a point near its present outlet at 

 Sault Ste. Marie, the channel of the river 

 passing down through what is now the bot- 

 tom of Lake Michigan, which had an outlet at 

 the head of the lake near Chicago and flowed 

 off into the Mississippi River. All of the lake 

 bottoms of this great chain, with the exception 

 of Lake Erie, are now below sea-level. The 

 reason for this exception will appear further 



