362 Chamberlin and Salisbury — Relationship) of 



must have been favorable for its preservation, bad it ever been 

 developed, seems to be conclusive against the hypothesis that 

 it ever existed. That the conditions were favorable for its pre- 

 servation is proved by its well-nigh universal presence under 

 the loess immediately south of the drift border, in the same 

 region. 



In many places, it may be clearly seen that the superficial 

 loess-mantle and the stony drift beneath, meet each other in a 

 thin zone of gradation. That is, the pebbles of the drift fre- 

 quently occur in the basal portion of the mantling loess, in 

 and just above the horizon where the imbedding matrix 

 changes from a gritty clay (till) to a gritless loam or loamy 

 clay (loess). In other places, there is a more or less marked 

 accumulation of drift pebbles'* immediately below the loess or 

 its clayey equivalent, marking its junction with the till. Both 

 these relationships find ready explanation in the hypothesis 

 that glacial waters covered the till, and spread the mantle of 

 loess upon it immediately after the ice retreated. And no 

 other hypothesis seems to meet the case. In the judgment of 

 the writers, therefore, the relationship between these two de- 

 posits, the till and the loess, as seen in innumerable sections, 

 in southeastern Illinois and southwestern Indiana, is such as to 

 admit of no second interpretation as to their sequence. The 

 loess, in the regions where such sections are found, was de- 

 posited immediately after the till, so far as not actually con- 

 temporaneous with it. We distinguish other sheets of loess, 

 contemporaneous with other stages of glaciation, and some of 

 these are separated from underlying till by old soils, but these 

 do not require special consideration here. The active agent 

 concerned in the production of the loess is believed to have 

 been water, and the material of the loess was in part derived 

 from the till beneath, and in part from the glacial silt carried 

 southward from the melting ice to the north. The evidence 

 that the loess is really a glacier-made silt, re-worked by water, 

 has been elsewhere discussed, f 



The age of the loess covering the drift along its southern 

 border, would seem to be clearly fixed relative to the under- 

 lying till. The till of the region belongs to the first glacial 

 epoch, as we have been accustomed to distinguish the glacial 

 epochs,^ and this loess belongs to the closing stages of the 

 same epoch, after the ice had retreated somewhat, but while 

 the region to the south of its edge was still overspread, in part 

 at least, by waters originating from it, 



The first glacial epoch embraces at least two episodes of 

 glaciation, separated by an interval when the climate of the 



* Steinsohle of the German geologists. 



f Sixth Annual Report, U. S. G. S., page 278 et seq. X Loc - cit -> P a S e 212 - 



