ILLINOIS RIVER DRAINAGE BASIN. 



501 



depression, found in the weight of the ice sheet, an additional cause is here 

 found in the great amount of drift deposited along the course of the old 

 valley. This valley from the vicinity of Pekin northward is filled, as well 

 as bordered, by heavy deposits of drift, which are rarely less than 200 feet 

 in depth and which reach a probable maximum of between 500 and 600 

 feet in the district north of Princeton, where the moraine occupies the old 

 valley. This weight of drift, unlike that of the ice sheet, still continues to 

 be an obstacle to the return of the valley floor to its former altitude, if not 

 a direct cause of depression. Possibly it even now is causing a depression 

 of that region, and the low gradient of this portion of the Illinois may 

 perhaps be explained in part by such a depression. 



In the following table the available data concerning the altitude of the 

 rock floor compared with the present Illinois are presented: 



Altitudes of rock floor and present lower Illinois River. 



Princeton (abandoned channel) 



Bureau Junction (Bureau Creek Valley) 



Hennepin 



Putnam 



Henry 



Peoria (Brigham's well) 



Pekin (city well) 



Beardstown 



Mouth of Illinois 



Bcllefontaine, Mo. (bridge) 



East St. Louis, 111. (bridge) 



Miles. 

 

 9 

 4 

 7 

 6 

 34 

 11 

 63 

 86 

 17 

 25 



1 438 

 438 

 438 

 429 

 427.3 

 418.6 

 403.6 

 402± 



Feet. 

 340 

 340 

 380 

 340 

 355 

 341 

 325 

 345 

 (?) 

 295 

 284 



a A dam located at Henry, Illii 

 at the natural level of the river. 



raises the river to 438.1 feet. The altitudes at points below Henry are given 

 THE UPPER ILLINOIS. 



In the 41 miles from the junction of the Des Plaines and Kankakee 

 down to Utica, where apparently a small preglacial tributary of the Illinois 

 is entered, the course of the present Illinois is independent of preglacial 

 drainage lines. About midway of its westward course it crosses the Mar- 

 seilles moraine. This no doubt for a considerable period held a lake in the 

 basin at the head of the river (the Morris Basin), but was eventually cut 

 down to the level of the low part of this basin. From the Marseilles 



