THE SHELBY VILLE MORAINIC SYSTEM. 213 



INNER-BORDER TRACT. 

 TOPOGRAPHY. 



The topography for some distance north and east from the border of 

 the Shelbyville drift is of a gently undulating type, not markedly different 

 from that of the thickened border. North and west from the Sangamon 

 River there is a triangular tract included between the Shelbyville and 

 Bloomington moraines in which swells 10 to 30 feet high are not uncom- 

 mon, but no well-defined morainic belt has been found. East from the 

 Sangamon River there are several small drift ridges lying between the Shel- 

 byville and Bloomington moraines, but the greater part of the surface is 

 plane. Aside from the ridges, the surface is less undulating than in the 

 tract west of the Sangamon, just mentioned. These ridges are discussed 

 individually and in some detail farther on. 



THICKNESS OF DRIFT. 



The Shelbyville till sheet covers this inner-border tract to a depth gen- 

 erally of 50 or 75 feet, and in places 100 feet or more. The depth seldom 

 falls below 50 feet. It is usually not difficult to ascertain the thickness of 

 the Shelbyville sheet, for well drillers report that it is much easier to pene- 

 trate than the underlying older drift. In the detailed discussion which 

 follows, these differences are set forth. 



STRUCTURE OF DRIFT. 



In the district west of the Illinois River the Shelbyville moraine is 

 separated from the Bloomington moraine by a narrow lowland, nowhere 

 more than 3 miles in width, and running to a point in northern Peoria 

 County and also at the border of the Illinois Valley. Apparently the drift 

 beneath this lowland is mainly till and has considerable depth, one well 

 having gone down 218 feet without entering rock. The well referred to is 

 located on the farm of John Miller, 4 miles north of Dunlap. The well 

 driller reports that the upper 69 feet was a soft till, mainly of blue color, 

 and probably to be referred to the Shelbyville drift sheet. The remainder 

 was a hard, brownish-gray till, apparently Illinoian. 



