GENERAL ASPECTS OF ILLINOIAN DRIFT SHEET. 33 



cussed is not represented in this section, the question naturally arises whether 

 the silty material under the peat is not its equivalent. The question can 

 scarcely be decided from such fragmentary evidence as is now available. 

 This section appears to bring the loess into closer relation to the gummy 

 clay than had heretofore been supposed. If the blackened, humus-stained 

 surface of the gummy clay required but a few centuries for its development, 

 it would seem not unlikely that the deposition of this gummy clav and that 

 of the loess are to be referred to the same epoch of low altitude, an epoch 

 attended by more or less complete submergence, with interruptions or partial 

 emergence of the land. The reddened and leached' surface of the lllinoian 

 till apparently signifies a long exposure to atmospheric action. The balance 

 of evidence seems to favor a closer connection between the loess and the 

 gummy clay than between the latter and the underljdng lllinoian till. 



As the loess is discussed in some detail in connection with the Iowan 

 drift sheet, with which it is correlated, only a general statement concerning 

 it is made at this point. The entire surface of the lllinoian drift sheet 

 appears to have received a capping of loess or loess-like silt at about the 

 time of the Iowan ice invasion, the deposit being found midway between 

 the principal streams as well as along their borders, where it was first 

 recognized. It is much thicker on the borders of the Illinois and Missis- 

 sippi than on the divide between these streams or in the region east from 

 the Illinois. In much of southern Illinois the thickness is only 3 to 5 feet, 

 and the average thickness in districts east of the Illinois and Mississippi is 

 probably less than 10 feet. On the borders of these streams its thickness 

 is frequently 30 to 50 feet, though a portion of the valley border near the 

 corners of Iowa, Missouri, and Illinois is characterized by a thinner coating 

 of loess than is found to the north or south, the average thickness being 

 scarcely 10 feet. Aside from its thickening on the borders of the Illinois and 

 Mississippi, there is also a thickening on the borders of the Iowan drift sheet 

 in Carroll, Whiteside, Henry, and Bureau counties, as indicated in the 

 discussion of that drift sheet, 



sections of the iiiinoian drift. — For sections illustrating- the structure of the 

 lllinoian drift sheet, reference may be made to the portion of this report 

 devoted to the wells of Illinois (Chapter XIV). The well sections are taken 

 up by counties; but attention is called to the several drift sheets which are 



JION XXXVIII 3 



