This study has confirmed the existence of a distinct mineralogical component in the upper part of the Yorkville Till Member of the Wedron Formation of Wisconsinan age in northeastern Illinois. This upper till unit occurs within the area of, but is not bounded by, the older Marseilles Morainic System and the younger, overriding Minooka Moraine. The analyses of samples from 79 locations that were used to identify, compare, and differentiate the two tills were: matrix grain size, clay mineral composition, carbonate content, and color.The upper till is a grayish brown, silty clay till with 76% illite and a dolomite-calcite ratio of 2:1. The lower till is an olive gray, silty clay till with 81% illite and a dolomite-calcite ratio of4:1. Results of this study indicate that (1) the two tills are separable and mappable units with the aid of laboratory data; (2) the lower till composes the bulk of the Marseilles Morainic System; (3) the upper till is thin between the moraines and constitutes the till of the Minooka Moraine; and (4) the upper till apparently represents a minor readvance and a concurrent change in source material.Ball State UniversityMuncie, IN 47306