THE SHELBYVILLE MORAINE. 197 



covered, while the north ridge was a prairie at the settlement of the 

 county. Each ridge is characterized by low knolls and shallow basins. 

 Occasionally the knolls reach a height of 30 feet, but they are usually only 

 10 or 15 feet in height. In the vicinity of Nevins a narrow outer ridge 

 appears for a few miles, which has sharp knolls 30 feet or more in height. 



Upon passing into Indiana the moraine loses the definite ridging which 

 it displays in Coles and Edgar counties, Illinois, and is represented in 

 northwestern Vigo County by a sheet of drift carrying only occasional 

 knolls. In a few cases the knolls reach a height of nearly 50 feet. The 

 portion of the moraine lying between the Wabash River and Big Raccoon 

 Creek is gently undulatory and more definitely ridged than that in north- 

 western Vigo County. From the Wabash Valley eastward to Greencastle 

 the moraine has generally a g-ently undulating surface, with few knolls 

 exceeding 20 feet in height. From Greencastle eastward the drift sheet 

 seems to be represented in places only by small knolls and ridges, standing 

 but 15 to 25 feet above the outer border. Among these knolls and ridges- 

 are plains of considerable extent in which the Shelbyville sheet seems to 

 be very attenuated. It becomes difficult to trace the border by the topog- 

 raph}*, and it has been found necessary to rely mainly upon the- structure 

 of the drift deposits. As indicated below, the removal of the loess and 

 the replacement by a bowlder}* drift gives the line at which this invasion 

 terminated sufficient definiteness to admit of mapping. 



STRUCTURE AND THICKNESS OF THE DRIFT. 



With the exception of the surface portion, which is variable in structure 

 because of local deposits of silt, sand, or gravel, the drift in this moraine 

 is, in the main, a typical till. For a depth of 8 to 12 feet, and occasionally 

 for 20 or 25 feet, from the surface it presents a brownish-yellow color, 

 which changes below to a grayish yellow and yellowish gray and finally to 

 a blue gray. The blue-gray till constitutes the main body of the drift 

 sheet. It as about as thick as the measure of the relief of the drift sheet 

 above the outer-border plain, which in Illinois, as above noted, ranges 

 from 60 to about 150 feet, and in Indiana seldom exceeds 40 feet. In 

 places a bed of sand or gravel is found at the junction of the yellow and 

 blue tills, suggesting that they may represent distinct deposits, but there 

 are many more places exposed to view where the yellow till grades down- 

 ward into the blue till. Furthermore, there are included within both the 



