186 THE ILLINOIS GLACIAL LOBE. 



underlain, in some cases at least, by Iowan drift. In central and eastern 

 Illinois the soil is in places underlain by a fossiliferous silt, referred with 

 some confidence to the Iowan loess. In eastern Illinois, as noted above, 

 the Iowan till may be present. This soil horizon, together with lower soil 

 horizons, was discussed by the writer in a paper presented at the Cleveland 

 meeting of the American Association for the Advancement of Science. 1 

 At that time the separation of the Iowan sheet from the Illinoian had not 

 been made and all the soils were referred to a single horizon. The later 

 developments have led the writer to separate the soils found at or slightly 

 below the base of the Wisconsin drift into two classes, one class being 

 thrown into the Sangamon stage, while the other is thrown into the stage 

 under discussion. It is not possible in all cases to decide to which class a 

 buried soil should be referred, for in some cases its existence is known only 

 through well records. Such separations as have been made may be found 

 in the discussion of the wells of Illinois, in the latter part of this report, 

 and in the discussion of the Iowan till above. 



In selecting a name for the horizon the ideal locality would be one in 

 which the earliest sheet of Wisconsin till overlies the Iowan till. In the 

 vicinity of Marengo, in McHenry County, a black muck has been found at 

 the base of the Wisconsin drift, and it apparently rests on Iowan till. This 

 might be taken as a type locality were it not that the Wisconsin drift at 

 that point may not include the Shelbyville or earliest Wisconsin sheet of 

 till. The same objection may be urged against buried soils found in Kane, 

 Dekalb, Lasalle, and Bureau counties, for in all these counties the outer 

 Wisconsin ridge appears to be the Bloomington moraine, and the limits of 

 the Shelbyville may be to the east of this ridge. It has seemed advisable, 

 therefore, in the selection of a type locality to pass to central Illinois, where 

 the Shelbyville sheet extends beyond the later sheets of Wisconsin drift. 

 This unfortunately carries us beyond the Iowan till, but the loess whose 

 deposition seems to be connected closely with the Iowan glaciation is there 

 well developed. The interval between the loess and Shelbyville till sheet 

 probably marks, as well, the time between the culmination of the Iowan 

 and Wisconsin glaciations, as if taken where the Shelbyville sheet overlies 

 Iowan till. 



1 For abBtract of paper, see Proc. Am. Assoc. Adv. Sci., Thirty-seventh Meeting, 1888, pp. 183, 184. 



