THE SHELBYVILLE MORAINE. 199 



after the till which underlies it, for the latter is not leached except where 

 the surface leaching has extended below the base of the silt. As to the 

 origin of this silt, it maybe remarked that the calcareousness seems to indi- 

 cate derivation from the Shelbyville drift rather than from the Iowan loess. 

 The loess of the outlying districts is so thoroughly leached to a depth of 

 several feet that it can hardly be the source of supply. Exposures were 

 found near Princeville, in the district west of the Shelbyville moraine, 

 where a fresh calcareous silt such as caps the moraine overlies the Iowan 

 loess. Acid tests also indicate that the surface silt is more calcareous than 

 the underlying loess. 



Beneath this drift sheet, at about the level of the district outside the 

 moraine, the earlier or Illinoian drift is struck. The junction between the 

 earlier and later drift is often marked by a soil or other equally clear indi- 

 cation of an old land surface. The passage from one drift to the other is 

 readily recognized by well drillers because of the difference in hardness, 

 the earlier drift being partially cemented and much more difficult to pene- 

 trate than the overling later drift sheet. As noted below, exposures of 

 the earlier drift were found in the Illinois, Kaskaskia, and Embarras valleys 

 in Illinois. The majority of the streams in Illinois fail to reach the level 

 of the earlier drift in their passage through the moraine. Those of Indiana 

 more often have cut into the older drift. 



The thickness of the Shelbyville drift in the Illinois portion of the 

 moraine is much greater than that of the underlying' older drift, the aver- 

 age thickness of the former being nearlv, if not quite, 100 feet, while the 

 latter attains that thickness only in the valleys or lowland tracts which it 

 filled, and has in this region a general thickness scarcely half as great as 

 that of the Shelbyville drift, sheet. Our knowledge of the thickness of the 

 Shelbyville sheet is based mainly upon the relief of the moraine, and the 

 estimate of the thickness of the older drift is based upon its thickness in 

 districts outside the moraine, rather than upon borings within the limits of 

 the moraine, there being few wells which have reached its bottom. There 

 are, however, a sufficient number of records of deep wells along the 

 moraine, or in the districts north of it, to furnish a fair knowledge of this 

 earlier drift sheet. 



As may be inferred from previous statements, the Shelbyville drift 

 sheet is much thinner in the Indiana than in the Illinois portion of the 



