520 THE ILLINOIS GLACIAL LOBE. 



The average width of the valley is nearly one-half mile, but its depth is 

 only about 20 feet. 



The Sangamon River also receives the drainage from a plain on its 

 southwest border south and west from Springfield. This plain shows a per- 

 ceptible descent toward the river, and the present divide between the tribu- 

 taries of the Sangamon and several streams which flow directly westward 

 to the Illinois apparently follows nearly the line of a preglacial rock divide. 

 With this exception the borders of the Sangamon watershed appear to be 

 determined bv accumulations of drift. 



CROOKED CREEK. 



This western tributary of the Illinoi. , which enters about 14 miles 

 below the mouth of the Sangamon, drains an area of nearly 1,400 square 

 miles. Its watershed lies immediately southwest of the Spoon River water- 

 shed. It extends on the northwest nearly to the bluff of the Mississippi, 

 there being one tributary in northern Hancock County, from which the 

 Mississippi bluff is distant less than 5 miles. 



The main stream has a southeastward course from eastern Hancock 

 County to its mouth, a distance of 60 miles. No important tributaries enter 

 from the west, but several creeks lead into it from the east, which have 

 lengths of 15 to 20 miles or more. These eastern tributaries present a 

 remarkable parallelism, and take a nearly uniform direction about S. 65° W. 

 As previously indicated, one of these tributaries, known as East Crooked 

 Creek, occupies a valley which continues beyond this watershed in direct 

 course to the Mississippi, and which is thought to have been formed by a 

 subglacial stream. (See discussion of Big Meadow channel, p. 481.) 

 Shallow channels may also have been opened by the same agency along 

 other eastern tributaries, and have occasioned their remarkably direct and 

 parallel courses. 



For a few miles near its mouth the coui'se of Crooked Creek has been 

 determined by a preglacial drainage line, but elsewhere the drainage 

 appeal's to be nearly independent of preglacial lines. A portion of the 

 divide between its watershed and that of Spoon River follows a low till 

 ridge. This ridge, however, is only a partial cause for the divide, since 

 the general altitude and slopes on its borders are such as to have located 

 the line of separation between the watersheds at about the present divide. 



