222 THE ILLINOIS GLACIAL LOBE. 



relationships are not fully determined. It may be clue largely to the cutting 

 down of gravelly portions of the Shelbyville sheet, such as that near 

 Mahomet just noted. It stands about 20 feet above the Sangamon River, 

 or about halfway from the level of the river up to the level of the till plain 

 on the west. In places the morainic knolls east of the river extend down to 

 the level of this gravelly plain, and seem to be merged with it so intimately 

 that there is a strong suggestion of similarity in age. However, it is not 

 fully established that even in such places the gravel is an outwash from the 

 moraine. As shown below, there is a light gravelly outwash near Mahomet 

 from the Champaign moraine over the till plain that borders it on the south. 

 It is possible that the gravel plain along the valley was built up in part at 

 that time, though the lightness of the deposit at Mahomet scarcely warrants 

 the reference of a large part of the filling to this source. 



INNER BORDER TRACT. 



Between the Cerro Gordo moraine and the outer ridge of the Champaign 

 morainic system there is a plain whose width nowhere exceeds 25 miles and 

 usually is about 12 or 15 miles. This plain has a very level surface, there 

 being few swells or knolls and but little variation in altitude. It is under- 

 lain by thick deposits of drift, except for a few miles near the State line. 

 Wells indicate that it may average nearly 200 feet, of which probably more 

 than one-half is older than the Shelbyville sheet. Since the majority of wells 

 obtain water without reaching the base of the Shelbyville sheet, few data 

 concerning the older drift are available from this region, but inferences 

 may be drawn to some extent from neighboring districts on either side, 

 where the data concerning thickness of the drift are more full. 



Wells in the vicinity of Areola are reported to have passed from the 

 soft till of the Shelbyville sheet into a harder till, presumably of the 

 older drift, at a depth of only 45 or 50 feet. In the vicinity of Tuscola 

 the wells usually pass from the soft till of the Shelbyville sheet to a harder 

 till, termed hardpan, at a depth of 35 to 40 feet. A prospect boring for 

 coal at Tuscola penetrated 186 feet of drift. It is probable that the thick- 

 ness of the Wisconsin drift is somewhat greater on the portion of the plain 

 to the west of these towns, as shown by records outside the Cerro Gordo 

 moraine already given. The thickness to the east of these villages is prob- 

 ably less rather than greater than in their vicinity, there being a general 

 decrease in thickness in that direction. 



