Xlii FISHES OF ILLINOIS 



As a framework to this sketch of the Illinois River and the 

 waters of the Illinois basin generally, an outline of its geological 

 surroundings is essential. From its source to Peoria the river flows 

 through a district covered by the Wisconsin drift. From Peoria 

 to southern Pike county the outlines of its western border are cov- 

 ered by the Illinois drift capped by loess. From thence southward 

 they are nearly free from glacial drift, but are heavily coated with 

 loess, while those on the east have a moderate covering of Illinois 

 drift capped by loess. Within the Wisconsin drift the marshes, 

 bogs, and lakes are of small extent, but the drainage lines are, on the 

 whole, rather imperfectly developed. 



The portion of the basin lying in the Illinoisan is much better 

 drained. There are almost no marshes or swamps in it, except 

 those in the bottoms of the river itself, but there are numerous 

 shallow valleys which are poorly drained. 



The Illinois, as stated before, follows the old "Chicago outlet" 

 as far as its curve near Hennepin. In the 41 miles from the junction 

 of the Des Plaines and the Kankakee down to Utica, where appar- 

 ently a small preglacial tributary of the Illinois is entered, the 

 course of the present Illinois is independent of preglacial drainage 

 lines. Almost midway of its westward course it crosses the Mar- 

 seilles 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 

 eventually cut down to the bed of this basin. From the Mar- 

 seilles moraine, westward, the channel found no prominent drift 

 barriers to remove, but has been compelled to cut down SO 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. 



