14 THE ILLINOIS GLACIAL LOBE. 



smoothness of the rock floor is found in borings near Michigan City. Three 

 borings in a line leading from Michigan City westward 2 miles show a ridge 

 capped by Devonian shale at the middle boring, which stands 70 or 80 feet 

 above a rock floor of limestone at the other wells. 



Taking the basin as a whole, interesting contrasts appear. The 

 remarkable depth and the smoothness of the south-central portion of the 

 basin shown in the profile east from Racine seem to favor the view that 

 glacial erosion was there an agency of much consequence. The preserva- 

 tion of the shale at Michigan City (a few miles to the south) and the 

 apparent preservation of old escarpments in the midst of the basin a few 

 miles to the north, both being in more prominent and apparently better 

 exposed situations for effective erosion than the deep part between them, 

 seem to show comparatively little erosion. The evidence therefore as to 

 the amount of glacial erosion is somewhat confusing, and it will be found 

 difficult to eliminate this factor if an attempt to restore the preglacial 

 features of the basin be made. 



The reliefs of the region covered by the Illinois lobe are seldom of a 

 bold or conspicuous type. On the contrary, they are so gradual as to give 

 the impression that they are less than the instrumental determinations indi- 

 cate. There are, however, a few ridges with rock nuclei which are of 

 sufficient prominence to merit notice. 



The most prominent ridge of the region is found in the southern por- 

 tion of Illinois, at the southern border of the glaciated district. This ridge 

 crosses the State in a direction nearly due east and west from the bend in 

 the Ohio River just south of the mouth of the Wabash to the Mississippi 

 River near Grand Tower. Its crest ranges from about 700 to 1,047 feet 

 above tide (Rolfe), and its bi*eadth ranges from 5 or 6 to 10 or 12 miles. 

 It stands 300 to 600 feet above the lowlands on the north, their altitude 

 being but 400 to 550 feet above tide, and an even greater amount above the 

 lowest parts of the district on the south. This ridge seems to have limited 

 the extension of the ice sheet, for the drift was carried well up toward the 

 crest on the north slope, but no decisive evidence has been found that the 

 crest was overridden. 



From the western end of the ridge just noted, northwestward along 

 the Mississippi to St. Louis, an elevated limestone belt separates the river 

 valley from the Coal Measures district to the northeast. This belt is but 



