THE CHAMPAIGN MORAINIC SYSTEM. 231 



Creek. The slope of the country is such that the creek would have con- 

 tinued directly westward to the Wabash and reached that river in about 6 

 miles from the point where it encounters the drift ridge, but the deflection 

 occasioned by the drift ridge makes it necessary for it to flow not less than 

 25 miles before reaching the Wabash. At the northern end of the ridge, 

 near Rob Roy, knolls 20 to 30 feet high occur, and points in sec. 25, T. 

 21, R. 8 W., stand 50 or 60 feet above the bordering plain. To give added 

 morainic expression these prominent points are thickly strewn with bowlders. 



THICKNESS AND STRUCTURE OP THE DRIFT. 



The thickness of drift in this morainic system is to be measured by the 

 relief of its ridges rather than by the distance to rock, for beneath the level 

 of the base of the ridges older sheets of drift occur. The relief, as shown 

 above, nowhere reaches 100 feet and seldom exceeds 50 feet. The distance 

 to rock, on the other hand, rarely fall's below 50 feet^along the line of the 

 ridges, and in places is known to be 300 feet. In the Indiana portion and 

 in eastern Illinois, for some 20 miles west of the State line, rock is usually 

 entered at 100 feet or less, and there are numerous rock exposures along 

 the principal streams. Farther west the rock surface lies lower, and the 

 few borings which reach the rock indicate that the average thickness of the 

 drift in Champaign, Piatt, and McLean counties is not less than 200 feet, 

 while the maximum thickness is fully 300 feet. 



These morainic ridges are composed in the main of till. Gravel and 

 sand beds are occasionally found in the knolls and near the level of the 

 base of the ridges, but even in these situations they are of comparatively 

 limited extent. Along this morainic system in Indiana there is sufficient 

 gravel in the knolls to supply material for improving the roads in their 

 vicinity, but in Illinois road material is generally difficult to obtain. 

 Gravelly knolls were observed east of Ridge Farm village and on the 

 inner slope of the moraine near Champaign and Urbana. Along the sharply 

 ridged portion of the inner ridge in the vicinity of the State line, and also 

 in the sharp knolls near Fairmount, there is considerable gravel associated 

 with the till. It is possible that many knolls contain gravel which has not 

 yet been discovered. On the whole, water-bearing beds are more extensive 

 in the Indiana portion of these morainic ridges than in the Illinois portion. 

 In the latter district wells are often sunk to a level below the base of these 



