478 THE ILLINOIS GLACIAL LOBE. 



PLUM RIVER. 



Plum River lies partly within the Driftless Area, but its lower course 

 and two important eastern tributaries traverse glaciated districts on the 

 southeast border of the Driftless Area. By reference to the Savanna 

 topographic sheet it will be seen that the stream passes through a gorge in 

 southwestern Woodland Township, Carroll County, which apparently was 

 formerly the site of a divide between the Plum River drainage and a much 

 smaller stream entering the Mississippi at Savanna. The course of the 

 preglacial Plum River was probably southward through the glaciated dis- 

 trict, along a line a few miles to the east of its present lower course, but as 

 yet the precise location of the valley has not been ascertained. Carroll 

 Creek, one of the principal eastern tributaries of Plum River, apparently 

 discharged southward through Johnson Creek Valley, from a point irarae- 

 diatety east of Mount Carroll. It now passes westward through a series of 

 gorges in the vicinity of Mount Carroll and enters a small preglacial valley 

 about 2 miles west of that city. The interval between Carroll Creek and 

 Johnson Creek is completely filled with drift for a distance of about 4 miles. 

 The drift filling terminates abruptly near the point where the Chicago, 

 Burlington and Northern Railway crosses Johnson Creek. From that point 

 southward a preglacial valley fully a mile in width leads down to the 

 Mississippi. 



No stream of consequence enters the Mississippi on the east between 

 Plum River and Rock River. As the latter stream constitutes one of the 

 major tributaries, its discussion is taken up farther on. We pass, therefore, 

 to the district south of the mouth of Rock River. 



EDWARDS RIVER. 



Edwards River, a stream which has a length of about 60 miles, leads 

 westward from Henry County through Mercer County, Illinois, entering 

 the Mississippi nearly opposite the mouth of the Iowa River. Its head- 

 water portion consists of several streams which converge to form two forks 

 that unite in south-central Henry County. From the junction of these two 

 forks a remarkably direct westward course is taken, so that from points on 

 its bluffs views may be had for several miles up or down the stream. This 

 portion has only insignificant tributaries and drains a very narrow belt. 

 The directness of its course and the narrowness of its drainage basin are 



