ILLINOIS EIVER DRAINAGE BASIN. 41)9 



statutes the southward continuation of the preglacial channel occupied by 

 Rock River in southern Wisconsin and northern Illinois. 



THE LOWER ILLINOIS. 



The lower Illinois Valley, as indicated above, seems to have been so 

 imperfectly filled by glacial deposits that throughout nearly its entire length 

 the stream is reestablished in the old course. The portion below the mouth 

 of the Sangamon River has been filled to a level less than 100 feet above 

 the present stream. The portion above the mouth of the Sangamon has 

 evidently been filled to irregular heights, as shown by terraces in the valley 

 and on its tributaries. The greatest filling seems to have been at Peoria, 

 where the Shelbyville moraine crosses. Terraces of tributary streams here 

 indicate a filling of not less than 170 feet above the present river level. 

 Portions of the valley above Peoria seem to have been filled to scarcely 

 100 feet above the stream, while the filling below Peoria declines rapidly 

 to a level 100 feet or less above the stream. The preglacial tributaries 

 leading into the lower Illinois are more completely filled than the main 

 valley. As far down as the mouth of the Sangamon it is impossible to trace 

 the courses of eastern tributaries, so complete has been the filling, and 

 below the mouth of that stream the tributary valleys are traceable for only 

 a few miles in the lower courses of the present streams. The western 

 tributaries, as shown below, may be traced in several instances some distance 

 back from the main valley. 



The valley of the lower Illinois ranges in width from 2i to fully 15 

 miles. Its greatest expansion is just above the mouth of the Sangamon, 

 and the full width of the preglacial valley at this point is not known. The 

 sandy bottoms have a -breadth of 12 to 15 miles, but the uplands to the 

 east are filled with drift which extends far below the level of the river bot- 

 toms, as shown by numerous well sections. The valley is also broad in the 

 vicinity of the bend of the Illinois.. The sandy and gravelly bottoms have 

 a width of 6 or 8 miles, and the preglacial valley has still greater width. 



The narrowest portions of the lower Illinois Vallev are a short section 

 at the city of Peoria, where it passes through the Shelbyville moraimc 

 system, and a section embracing the lower 60 miles, where it traverses the 

 Eocarboniferous and Silurian limestones. The reduced width near the 



