140 



and apparently the entire area of Ma^on county lies within the 

 boundaries of the latter valley. The highest known point of 

 its rock surface is lower than that of any other county in Illi- 

 nois, and fifty feet below the present level of the river. Quite 

 probably this county lies in a forking of the preglacial valley, 

 an eastern fork being perhaps indicated by the drainage line 

 now oc<rupied by Sugar Creek and the lower ends of Salt Creek 

 and the Sangamon River, the two latter sections being in a pre- 

 glacial valley at least four miles wide. Records of wells and coal 

 shafts indicate great valleys beneath Bloomington and Cham- 

 paign, and another valley leading south fi-oni Lake Michigan 

 near the Indiana line, but at present it is impossible to trace 

 these old drainage lines. 



In northwestern Illinois the Illinoian invasion evidently 

 caused a radical readjustment of the river systems, turning 

 them westward across adjacent divides into other valleys be- 

 yond. ( Leverett. "99. PL XII. i The buried northward exten- 

 sion of the preglacial Illinois valley above Hennepin is appar- 

 ently continuous with that of Rock River above Rockford. At a 

 point just below this city the river now leaves its evident pregla- 

 cial channel, turning westward along side lines of its preglacial 

 tributaries, and is still cutting narrow passages across the rock 

 of the intervening divides. A considerable section of the Mis- 

 sissippi was temporarily crowded over some distance into Iowa, 

 where it has left a fairly well-marked channel that has greatly 

 modified the course of minor streams. It now leaves its great 

 preglacial valley not far below Fulton. 111., and runs south- 

 west at right angles across the still evident lines of preglacial 

 drainage, which appear to be directed eastwardly and to con- 

 verge in that direction. In the ^.-icinity of Rock Island the 

 flow of both the Mississippi and Rock rivers seems to be up- 

 stream with regard to preglacial lines, in order to cross into an- 

 other ancient valley at Muscatine. The probability that the pre- 

 glacial Mississippi channel swings eastward beneath the pres- 

 ent lower Rock River valley, and thence across to the bend 

 of the Illinois, is confirmed by the remarkably low sag, espe- 

 cially in rock levels, along this line. In that case, it must have 



