482 THE ILLINOIS GLACIAL LOBE. 



and the Mississippi near Sutter. The valley is here well defined and nearly 

 as deep as at the "Big Meadow divide." From Sutter it leads down the 

 valley of a small stream known as Rock Run, passing- through a gap in 

 the marginal ridge of Illinoian drift just before entering the Mississippi 

 Valley. Throughout this entire distance of not less than 50 miles its 

 course is remarkably direct. 



The precise A'ariations in the elevation of its bottom have not been 

 determined. The eastern end near Bushnell, the "Big- Meadow divide," 

 and the divide at Sutter, all stand very nearly 650 feet above tide. With 

 the exception of the descent into the Mississippi Valley west from Sutter, 

 the slope of the valley bottom probably nowhere exceeds 5 feet to the 

 mile, and is usually scarcely half that amount. 



This singular valley appears to find its parallel in the valleys accom- 

 panying some of the eskers, an instance of which in Kane County has 

 already been discussed (p. 284). No esker, however, lies in the "Big 

 Meadow" Valley. It seems probable that the stream which excavated this 

 channel was confined beneath the ice sheet, and thus held to a direct course 

 across the low divides and shallow depressions covered by the ice. The 

 hypothesis of a piracy of the stream which formed this channel by the two 

 creeks which now drain it, Crooked Creek and Bear Creek, has been con- 

 sidered and seems untenable. The valle} T was evidently formed by a 

 stream having much larger volume than either of these creeks possesses 

 at the points where they depart from this old channel. Furthermore, the 

 course of the channel is such as can scarcely be supposed to have been 

 inaugurated without the confining influence of the ice sheet, for the present 

 courses of drainage are determined by the general slopes of the region and 

 are the natural lines of discharge. It seems, necessary, therefore, to refer 

 this abnormal drainage to a subglacial stream. 



In this connection it may be remarked that the system of parallel 

 streams in McDonough County, all bearing west-southwest, may have been 

 a result of slight channel development by subglacial streams along these 

 lines. Similarly the depressions occupied by Edwards River, Pope Creek, 

 and Copper Creek, in Mercer and Rock Island counties, may have been at 

 first avenues of discharge for subglacial waters. There seems, however, in 

 this latter district to have been a more decided development of ridges, 

 parallel with the depressions, than is found in the vicinity of the "Big 

 Meadow" channel. 





