THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS XXI 



Wabash and through it of the Ohio and Mississippi, whose general 

 direction is southeast. The drainage basins of these streams will 

 now be described in order. 



Rock River System 



The Rock Ri\'er system drains a part of southern Wisconsin and 

 most of the northwestern corner of Illinois. Its basin covers an area 

 of almost 11,000 square miles — 5,566 in Wisconsin and 5,419 in Illi- 

 nois (Leverett). This drainage basin is 40 to 50 miles wide in Wis- 

 consin, but near the state-line it reaches a width of about 80 miles. 

 It narrows again in Illinois to 40 miles, and then to 25 miles. Its 

 length is about 175 miles. The outline thus formed is comparable to 

 that of a pear, the stem toward Rock Island. The country in this 

 area is an undulating semi-prairie region. Large expanses of un- 

 broken prairie, groves and some more extensive bodies of timber, 

 swamps, and lakes, are all to be found within its limits. Almost all 

 of the basin lying within Wisconsin is covered with drift from the 

 Wisconsin glacier, but near Janesville Rock River breaks through 

 the "Kettle Moraine" of the Green Bay lobe of this glacier. South 

 of this the basin lies in drift of lowan and Illinoisan age. Although 

 the exact boundaries of these drift areas are not as yet definitely de- 

 termined, the western border of the lowan drift probably extends but 

 a few miles west of Rock River at any point, and for a short distance 

 below Rockford it follows nearly the course of the river. The sec- 

 tion of the basin lying in the Wisconsin drift is characterized by 

 extensive swamps and numerous small lakes, the drainage being 

 almost entirely independent of preglacial lines and consequently 

 imperfectly developed. The overflow from the swamps is gathered 

 into little meandering streams which have cut only small channels 

 in the soil. The rest of the basin is older country — undulating, well- 

 drained, and forming excellent farm-land except along Green River, 

 where there are many swamps and sand-hills. 



ROCK RIVER 



Rock River is, of course, the principal river in the system. It 

 rises in Dodge county, Wisconsin, in what was formerly Lake Hori- 

 con, but now is drained and has become an extensive marsh. The 

 lake, which existed until 1868, although a body of water formed by 

 an artificial dam, yet occupied the site of an ancient lake caused by 

 the body of drift which formed a natural barrier to the passage of 

 the water. Gradually this was eroded and the lake drained, prob- 



