DRAINAGE OF THE GLACIAL PERIOD. 319 



of the ice those lines were opened in reverse order and shore 

 lines and abandoned channels were left for inspection for all 

 time. These abandoned channels and the bordering high- 

 level gravel terraces are clearly marked through the line of 

 the St. Croix River in Wisconsin, through the line of the Chi- 

 cago drainage canal, through the pass at Fort Wayne and 

 along the whole course of the Wabash River in Indiana, and 

 to a greater or less extent through the other passes which 

 were occupied by the reversed glacial floods for a shorter 

 time further east. The shore lines of the temporary glacial 

 lakes whose levels were determined by the height of these 

 several passes are also clearly marked around the west end of 

 Lake Superior in the terraces above Duluth; and around 

 the southern shore of Lake Erie where at three levels already 

 mentioned they lead to Fort Wayne, at an elevation of 200 

 feet above the present level of the lake, and from Saginaw Bay 

 at levels of 150 and 100, respectively, to the upper and the 

 lower passes into Grand River, Michigan. 



Mr. Frank Leverett discovered a most interesting dis- 

 placement of a middle portion of the Mississippi River dur- 

 ing the farthest advance of the Illinoisan ice-sheet, which 

 pushed across the river for an average distance of from 

 thirty to forty miles between Clinton and Keokuk. This 

 forced the drainage along the border of the ice-sheet through a 

 channel still clearly marked through Jackson, Clinton, Scott, 

 Muscatine, Louisa, Des Moines, Henry, and Lee counties, 

 a distance of about 150 miles. But on the retreat of the ice, 

 the present channel was opened. 



Gravel terraces of great prominence also mark the course 

 of final glacial drainage through the Mohawk River in Cen- 

 tral New York, and a shore line around the southern side of 

 Lake Ontario is distinctly traceable from the mouth of the 

 Niagara gorge to the col at Rome, leading from the Ontario 

 basin into the Mohawk. 



