Pleistocene to Pre-pleistocene of Mississippi Basin. 363 



southern part of the drift-covered area was so far ameliorated 

 as to allow the growth of vegetation upon the drift of the 

 first episode. In southeastern Illinois and the adjacent parts 

 of Indiana, the ice advance of the second episode of the 

 first epoch seems to have been equal to, if it did not exceed, 

 that of the first episode. This seems not to have been the 

 case in some portions of southwestern Illinois, but in south- 

 eastern Illinois, and indeed in most of the southern part 

 of the state, the till immediately beneath the loess is re- 

 ferred to the second ice incursion of the first glacial epoch. 

 The loess here under discussion is therefore to be referred 

 to the close of the second glacial episode of the first glacial 

 epoch. But it may be traced across the limit of the drift 

 from north to south. The continuity is complete, and the 

 character of the formation is the same on both sides of the 

 line which marks the limit of ice advance. It is a continuous 

 mantle, overspreading alike the drift border on the north, and 

 the residuary earths which the ice did not disturb, on the south. 

 If, therefore, the age of the loess which covers the drift be 

 first glacial (first episode), the age of that which lies south of 

 the drift, in the area under discussion, is likewise first glacial. 



Between the relationship of the loess to the till north of the 

 limit of glaciation, and the relationship of the loess to the 

 residuary earths of the Paleozoic rocks immediately outside the 

 drift, there is one important difference to which reference has 

 already been made. The presence of a weathered and highly 

 oxidized zone, immediately subjacent to the loess, south of the 

 drift limit, is as conspicuous as is its absence to the north. 

 This oxidized zone is the upper surface of the residuary earths. 

 There is in places, a slight admixture of residuary material and 

 loess at the junction of the two. But the body of the residu- 

 ary earth is clearly separated from the body of the loess. The 

 fact of the existence of a long interval between the loess and 

 the residuary earths beneath, is as clearly indicated on the one 

 hand, as is the fact of the absence of an interval between the 

 loess and the underlying till on the other. The phenomena on 

 each side of the drift limit, strengthen the conclusion drawn 

 from the phenomena on the other. 



There is in the nature of the case no manifest reason why 

 the formation of loess should not have accompanied and fol- 

 lowed the ice action of the first episode of the first glacial 

 epoch, just as it accompanied and followed the second episode. 

 Assuming this to have been the case, there should be a two- 

 fold division of the loess south of the drift, theoretical, if not 

 observable. Such division has been sought for, but without 

 completely satisfactory results, throughout most of the area 

 under consideration. There are a few localities east of the 



