ROCK RIVER DRAINAGE BASIN. 485 



reestablished along the preglacial line, though the lower half of its course 

 lies within the limits of the glacial district. The headwater portion of one 

 of the western tributaries of the Pecatonica — Yellow Creek— has been 

 diverted into Apple River, a tributary of the Mississippi. This stream also 

 makes slight deflections into the bordering bluff's in its lower course. Sugar 

 River, the principal northern tributary of the Pecatonica, occupies its pre- 

 glacial line, except, perhaps, at the headwaters. Many of the smaller trib- 

 utaries are also largely in preglacial lines. 



Leaf River, which now drains a portion of northern Ogle County 

 eastward into Rock River, is following a preglacial line which continued 

 eastward across the present course of Rock River, through an abandoned 

 channel known as "Stillman Valley," to the village of Stillman Valley, and 

 thence northeastward to the old Rock River Valley in southern Winnebago 

 County. Rock River follows the line of this preglacial valley for a few 

 miles in the vicinity of Byron, but in the reverse direction from the stream 

 which excavated the valley. Rock River also makes use of small tribu- 

 taries of the preglacial Leaf River. From the mouth of the present Leaf 

 River it passes up the valley of a small southern tributary for a few miles. 

 Before entering the old Leaf River Valley east of Byron it has utilized a 

 small northern tributary of that valley. Bluff Creek also, which enters 

 the present Rock River a few miles above Byron, has a preglacial course 

 southeastward across the present Rock River to a lower course of the pre- 

 glacial Leaf River. 



The Kishwaukee River, the first important eastern tributary of Rock 

 River south of the Wisconsin line, is in a new course for a few miles below 

 the junction of the north and south branches. It is not clear whether the 

 old mouth was a short distance north of the present mouth or whether the 

 stream passed southward up the south branch to the vicinitj^ of Fielding 

 and thence across to the old Rock River Valley near Esmond. The north 

 and south branches each occupy a preglacial valley for a few miles above 

 their junction, but the headAvater portions of each stream are in new val- 

 leys. The head of the north fork may not have been so far east as now, 

 for the effect of the ice sheet generally in northern Illinois has been to force 

 streams into the Rock-Illinois drainage basin from the east slope of the 

 limestone ridge which separated this basin from the Lake Michigan Basin. 



