DRAINAGE OF THE GLACIAL PERIOD. 317 



advance and recession of the ice from the basin of the Great 

 Lakes. As the advancing ice closed up the outlets leading 

 into the St. Lawrence Valley the drainage was successively 

 turned through Lake Champlain into the Hudson. A little 

 later it was turned around the Adirondacks through the 

 Mohawk River Valley into the Hudson. Later still it was 

 turned over into the Susquehanna through the fissures now 

 occupied by the Finger Lakes of Central New York. A still 

 farther advance turned the vast current through the valley 

 occupied by the Grand, Mahoning and Beaver rivers into the 

 head-waters of the Ohio and helped in the formation of its 

 present tortuous channel. 



Meanwhile the advance of the ice farther west was produc- 

 ing numerous changes in the drainage of the upper lake region. 

 At first the drainage of Lakes Erie and Huron was turned 

 through the straits of Mackinaw into Lake Michigan and ran 

 off through the line of the present drainage canal into the 

 Mississippi through the Illinois River. Then, as soon as the 

 eastern end of Lake Superior was closed up, the drainage from 

 the western portion was through the St. Croix River at an 

 elevation of 466 feet above the lake into the Mississippi a 

 little below St. Paul. With the advance of the ice a little 

 further into the southern peninsula of Michigan the channel 

 through the Straits of Mackinaw was closed, forcing the water 

 over a low col leading from Saginaw Bay into the head- waters 

 of Grand River, and thence into Lake Michigan a short 

 distance below Grand Rapids, and so again over the line of 

 the Chicago drainage canal into the Mississippi basin. 



A still further advance closed up the entire passage into 

 Lake Michigan and turned the drainage southward from 

 Toledo, Ohio, through the pass at Fort Wayne, Indiana, into 

 the Wabash River and thence into the Ohio. 



Of course the direct evidence of these facts is not now 

 available, since the advancing ice has obliterated the beaches 

 and terraces which were built up before it. But on the retreat 



