MOKKIS M. LKIGHTON* 



INTERPRETATIONS 



Several important points are to be noted in the interpretation of this 

 section. 



1. There can be scarcely any doubt that the lower till is Illinoian in 

 age. The typical Illinoian till occurs but a few miles to the south and south- 

 west, with its surface at essentially the same elevation as it is here and with 

 similar relations to the overlying loess. 



South of the Wisconsin drift boundary, gumbotil has been found at the 

 top of the Illinoian, and it is impressive to find it here. As pointed out by 

 Kay and Pearce, 2 this peculiar tenacious, leached, and siliceous pebbly clay 

 appears to be the product of a long interval of chemical weathering of the 

 till before the deposition of the overlying deposit. 



2. The overlying loesslike silt, horizon 3 of the section, seems best 

 interpreted as a weathered loess. The color and weathered character of the 

 Illinoian gumbotil below, and the thickness and calcareous content of the 

 loesslike silt above, exclude the interpretation that the latter is the product 

 of extreme weathering of the gumbotil. The occurrence of rare pebbles in 

 the lower part is not unusual to the basal portion of any loess deposit which 

 rests on a pebbly formation from which pebbles could be introduced sec- 

 ondarily by organic agencies. The calcareousness of the lower part of this 

 deposit, although not notable, indicates that it was deposited subsequent to 

 the weathering of the gumbotil. The upper leached zone of o 1 /* feet, the 

 brownish color on the east side of the cut, and the overlying old soil shows 

 that this loesslike silt was subjected to subaerial agencies of weathering and 

 plant growth for a considerable period before the deposition of the overlying 

 fossiliferous loess. 



The presence of this soil zone and the loesslike clay beneath the fossil- 

 iferous loess has not been hitherto noted in this exposure. In the legend of 

 the photograph of the Farm Creek section in Monograph XXXVIII, Plate 

 XI, Leverett includes all of the deposits between the Illinoian till and the 

 Wisconsin till as "Iowan loess." 



In the railway cut, one-half mile to the east, Leverett reports and illus- 

 trates by photograph the occurrence of a peat bed 3-5 feet thick, below the 

 so-called "Iowan loess," and a 2- to ">-foot bed of silt between the peat bed 

 and the underlying Illinoian till.' Of the silt Leverett states that it "bears 

 some resemblance to the overlying Iowan loess in texture, but is not so cal- 

 careous and is of a deeper brown color. Whether it is similar in origin to 

 the loess can scarcely be decided." Parenthetically, it may be stated that 



: G. F. Kay and J. Newton Pearce, "The Origin of Gumbotil," Jour. Geol, Vol. 

 XXVIII (1920), pp. 89-125. 



3 Frank Leverett, "The Illinois Giacial Lobe," U. S. Geol. Surrey. Monograph 

 XXXVIII (1899), p. 128; PI. XI b. 



