140 



and apparently the entire area of Mason 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 occupied 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 from 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, PI. XII.) 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 vicinity 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 



