518 THE ILLINOIS GLACIAL LOBE. 



depth, and this it maintains nearly to the mouth of the stream. The 

 depth, however, is increased nearly to 100 feet in portions of the course 

 between Springfield and Petersburg. The portion lying outside the Wis- 

 consin drift sheet, although generally shallow, is much wider than the 

 portion within the limits of that drift sheet, and bears evidence of having 

 been partially opened prior to the Wisconsin stage of glaciation. 



The amount of excavation accomplished prior to the Wisconsin stage 

 is especially well shown on tributaries of the Sangamon River, both north 

 and south of the main stream, which in some cases have been beheaded 

 because of the Wisconsin deposits. Thus Lake Fork, a small stream lead- 

 ing northwestward from the border of the Wisconsin drift in western Macon 

 County, has a valley about 20 feet in depth and fully one-half mile in 

 width, which was apparently formed entirely before the Wisconsin stage of 

 glaciation, for it now carries no stream adequate to erode a channel. The 

 Sangamon River and several of its tributaries are found to have similar 

 broad shallow valleys bordering narrower valleys of somewhat greater depth. 

 By affording this means for comparing the amount and kind of erosion 

 carried on before and since the Wisconsin invasion the Sang-amon watershed 

 becomes an important district for investigation. 



Several tributaries of the Sangamon have their sources within the 

 limits of the Wisconsin drift, among which may be mentioned North and 

 South Salt creeks, Kickapoo Creek, 1 and three headwater branches of Sugar 

 Creek. Kickapoo and Sugar creeks join Salt Creek a few miles above its 

 mouth, and therefore fall within the limits of its watershed. South Salt 

 Creek heads on the outer border of the Bloomington morainic system, in 

 southeastern McLean County, and flows southwestward across a gently 

 undulating plain to its junction with North Salt Creek, about 5 miles east 

 of Clinton. North Salt Creek has its source between the two large ridges 

 of the Bloomington morainic system, very near the source of the Sangamon 

 River. It passes southward through the outer ridge and across the undu- 

 lating plain south of it to its junction with the South Fork. Each of these 

 streams has a length of 25 or 30 miles above their junction. The united 

 stream passes westward through the Shelby ville moraine, entering the outer 

 border plain at Kenney, 8 miles southwest of Clinton. The general 



1 This Kickapoo Creek should lie distinguished from a stream of the same name entering the Illi- 

 nois at Peoria. 



