222 MAN AND THE GLACIAL PERIOD. 



Wayne outlet, two others at lower levels which are pretty 

 certainly marked by distinct beach ridges upon the south 

 side of Lake Erie. The first was opened when the ice had 

 melted back from the south peninsula of Michigan to the 

 water-shed across from the Shiawassee and Grand Rivers, 

 uncovering a pass which is now 729 feet above the sea. 

 This continued to be the outlet of Lake Erie-Ontario until 

 the ice had further retreated beyond the Strait of Mackinac, 

 when the water would fall to the level of the old outlet 

 from Lake Michigan into the Illinois River, which is a 

 little less than 600 feet, where it would remain until the 

 final opening of the Mohawk River in New York attracted 

 the water in that direction, and lowered the level to that 

 of the pass from Lake Ontario to the Mohawk at Rome.* 



A study of these lines of temporary drainage during 

 the Glacial period sheds much light upon the long lines 

 of gravel ridges running parallel with the shores of Lake 

 Erie and Lake Ontario. South of Lake Erie a series of 

 four ridges of different elevations can be traced. In Lo- 

 rain County, Ohio, the highest of these is 220 feet above 

 the lake; the next 160 feet; the next 118 feet; and the 

 lower one 100 feet, which would make them respectively 

 795, 755, 715, and 700 feet above tide. 



These gravel ridges are evidently old beach lines, and 

 indicate the different levels up to which the water was 

 held by ice-obstructions across the various outlets of the 

 drainage valley. The material in the ridges is water- worn 

 and well assorted, and in coarseness ranges from fine sand 

 up to pebbles several inches in diameter. The predomi- 

 nant material in them is of local origin. Where the rocks 

 over which they run are sandstone, the material is chiefly 

 sand, and where the outcropping rock is shale, the ridges 

 consist chiefly of the harder nodules of that formation 



* Mr. Warren Upham, in the Bulletin of the Geological Society 

 of America, vol. ii, p. 259. 



