510 THE ILLINOIS GLACIAL LOBE. 



topographic sheet of this Survey, a portion of which is here reproduced 

 (see PI XIX). 



The principal tributaries of Fox River all lie on the west side, there 

 being - but a narrow strip of watershed on the east side of the river. Among 

 the tributaries may be mentioned Nippersink Creek, which enters about 6 

 miles south of the State line; Blackberry Creek, which enters opposite 

 Yorkville; Big and Little Rock creeks, which unite just before entering the 

 river, 5 miles below Yorkville; Somonauk Creek, which enters opposite 

 Sheridan, 12 miles farther down the river, and Big and Little Indian 

 creeks, which unite near their mouths and enter the river about 10 miles 

 above its junction with the Illinois. Nippersink Creek heads in the com- 

 posite morainic belt in northern McHenry County and takes a zigzag course 

 among its ridges through a series of marshes for a distance of nearly 20 

 miles before entering the river. Blackberry Creek, as previously indicated, 

 occupies an esker trough in its middle course, whose size is many times 

 greater than that of the remainder of the creek channel. Before entering 

 the esker trough the creek winds about among morainic, knolls near the 

 south end of the composite belt. In its lower course the creek does not 

 pass directly into Fox River, but follows down the valley about 8 miles, 

 draining a portion of the gravel plain which borders the river. The remain- 

 ing tributaries above mentioned all head in the Bloomington morainic 

 system or on the slope of its inner ridge, and take a somewhat direct south- 

 eastward course to Fox River Valley, following the slope of the plain. 

 They have narrow valleys and have sunk their beds but a few feet below 

 the level of the plain, except near their mouths, where they have cut down 

 to a level corresponding to that of Fox River. Some portions of the plain 

 west of Fox River are naturally very imperfectly chained, but by ditching 

 and tile chaining they have been brought to a high degree of fertility. 

 The slope of the plain averages usually several feet to the mile; conse- 

 quently drainage is easily developed. 



COVBL CKEEK. 



This small southern tributary of the Illinois drains an area of about 100 

 scpiare miles lying between the Marseilles moraine and Farm Ridge, the 

 inner ridge of the Bloomington morainic system. It enters the Illinois 



