xlvi FISHES OF ILLINOIS 



of preglacial drainage lines. Almost midwa3 T of its westward 

 course it crosses the Marseilles moraine. This, no doubt, for 

 a considerable period held a lake in the basin at the head of the 

 river, the Morris basin, but was eventual^ cut down to the bed 

 of this basin. From the Marseilles moraine, westward, the 

 channel found no prominent drift barriers to remove, but has 

 been compelled to cut down 50 to 75 feet into the rock in opening 

 an outlet from the Morris basin into the valley of the lower 

 Illinois (Leverett). 



The part of the " Chicago outlet" lying within the Morris 

 basin has an average width of 4 to 5 miles. A low bluff, formed 

 on the northern border of the basin, has a height of 15 to 20 

 feet, but on the southern border there is no bluff, that side 

 being heavily coated with deposits of sand. Below Morris the 

 width of the outlet averages only about one and a half miles. 

 The excavation is largely in soft St. Peter sandstone, there 

 being nearly continuous rock bluffs to a height of 60 to 75 feet 

 above the level of the bed of the outlet. In some places, as at 

 Starved Rock, the bluffs reach a height of 126 feet. Buffalo 

 Rock stands out in the valley, a big rocky island. 



In the 41 miles to the foot of the rapids near Utica the 

 stream falls 47 feet, or slightly more than 1 foot to the mile 

 (Leverett). This fall is far from regular, there being a series 

 of rock rapids separated by pools. 



In the Morris basin the shale bottom has been eroded in 

 places by the current and the hollows have been filled with 

 sand, but from the Morris basin to the bend of the river the 

 rock floor is swept clean. 



The old preglacial valley through . which the lower Illinois 

 flows, and where rock bed lies manj r feet below the bottom of the 

 present river, seems to have been so imperfectly filled by glacial 

 deposits that throughout nearly its entire length the stream is re- 

 established in its old course. The valley ranges in width from 

 two and a half to fully fifteen miles. Its greatest width is 

 reached just above the mouth of the Sangamon. The valley is 

 also very broad at the bend of the Illinois. The narrowest por- 

 tions are a short section near Peoria, where it passes through the 

 Shelbyville moraine, and a section embracing the lower 60 miles, 

 where it traverses the Mississipian and the Silurian limestones. 



The Illinois River bottom-lands are covered with patches of 

 timber, sand banks, mud-flats, and meadows. A good deal of 

 tjiis area is too low and marshy for cultivation, full of swamps, 



