100 



lower courses of other tributaries in the southern basin The 

 altitude of the river at low water at Havana is given by Rolfe 

 ('94) as 429 feet, and data given by Cooley ('97, p. 60) indicate 

 an elevation of 422.96 feet above mean-tide level at the Gulf of 

 Mexico.* The fertile second bottoms, which are principally on 

 the eastern side of the river, lie from 30 to 75 feet above the first 

 bottoms, while the bluffs range in altitude from 450 to 800 feet, 

 the highest points being reached near Peoria, and near the mouth 

 in Calhoun county. The watersheds bounding the basin range in 

 height from 700 to 1 ,000 feet, but by far the greater part of the 

 area included has an elevation of 600 to 700 feet, being about the 

 average elevation for the state. The relief of the drainage 

 basin of the Illinois is thus quite insignificant. That part 

 which lies in southeastern Wisconsin is most diversified, while 

 that in the state proper, together with the Kankakee basin in 

 northwestern Indiana, is practically an unbroken plain. 



In common with the greater part of the state the basin of 

 the Illinois is covered by glacial drift. West and south of a 

 line drawn through Amboy, Peoria, Shelbyville, and Mattoon, 

 which marks the location of the Shelbyville moraine, this de- 

 posit is known as the "older drift," and is from 20 to 150 feet in 

 thickness. The drainage lines are here well developed, the 

 streams in many cases occupying preglacial channels ; but to 

 the north and east of this moraine the glacial deposit known as 

 the "newer drift" overlies the older, their combined thickness 

 ranging from 50 to 300 feet. Within this latter region the 

 drainage is not so well developed as it is in the older region to 

 the west and south of the moraine, and the streams, with the 

 exception of the Illinois, do not follow preglacial drainage 

 lines. Aside from a few minor streams between Havana and 

 Peoria, the whole basin of the Illinois above our plankton 

 station lies within the area of this newer drift. It consists of 

 extensive plains of glacial till, separated by glacial ridges or 



*"We seem to be reasonably certain that the elevation of the Illinois at its 

 mouth is 402.76 feet, and that at Beardstown it is 423 feet. We feel somewhat less 

 certain that at Peoria it is 436 feet, and at La Salle 440 feet. Beyond this we can only 

 approximate because the U. S. Engineers and Illinois Canal Commission differ so 

 widely." — Prof. C. W. Rolfe, in letter, Nov. 12, igoi. 



