INFLUENCE OF THE DRIFT UPON DRAINAGE. 483 



ROCK RIVER DRAINAGE BASIN. 



Rock River, which drains much of northwestern Illinois, has a length 

 of nearly 300 miles and a drainage area of about 11,000 square miles. 

 Nearly half its length and more than half its drainage area are in Wisconsin. 

 Its general course is southwest from southern "Wisconsin across northwestern 

 Illinois. From its source to Janesville, Wisconsin, it traverses a region 

 covered by drift of Wisconsin age, and its basin is characterized by extensive 

 swamps and numerous small lakes. The drainage appears to be largely 

 independent of preglacial lines. Just above Janesville the river crosses the 

 outer or "Kettle" moi-aine of the Green Bay lobe, and soon enters a broad 

 preglacial valley filled with a gravel deposit which heads in that moraine. 

 The bordering uplands are covered by drift of Iowan age, as well as by 

 earlier glacial deposits of Illinoian and possibly of still greater age. Below 

 Janesville, with the exception of a few miles near its mouth in Rock Island 

 County, Illinois, the present stream lies within the limits of the Iowan drift. 

 The western border of that drift, however, extends but a few miles west of 

 the stream at any point, and for a few miles below Rockford follows nearly 

 the course of the river. The portion of this drainage basin lying outside 

 the Wisconsin drift is generally undulating and well drained, but extensive 

 swamps occur along Green River, an eastern tributary. The western 

 tributaries lie mainly outside the limits of the Iowan, and the chief tribu- 

 tary, Pecatonica River, drains a small part of the Driftless Area. 



The preglacial valley entered by Rock River near Janesville, Wiscon- 

 sin, is followed southward a distance of 50 miles to the mouth of Kishwau- 

 kee River, a few miles below Rockford, Illinois. The river and valley 

 there part company, the valley continuing southward and apparently con- 

 necting with the Illinois at Hennepin, the river turning southwestward to 

 enter the Mississippi (see PI. XII). In this southwestward course is found 

 a series of small valleys separated by low divides which afforded the stream 

 a means of escape without producing a great amount of rock excavation, as 

 will appear from the detailed description given below. 



t 



THE PREOLACIAL DRAINAGE. 



The preglacial valley entered by Rock River near Janesville may be 

 traced without difficulty as far south as Rochelle, in southeastern Ogle 



