A lacustrine silt layer has been identified within the Radnor Till Member of the Glasford Formation (Illinoian) in three outcrops and six boreholes in central Illinois. Two of three outcrops were sampled for study. The silt is up to 1.5 m thick, light gray in the lower part, and black with high organic content in the upper part. It contains fossil gastropods, osctracods, insect parts and, locally, pieces of wood. Heavy-mineral separations from the light gray part yielded a variety of minerals, including detrital pyrite. Separations from the organic-rich part yielded, almost exclusively, small masses (less than 3 mm) of authigenic pyrite. Binocular microscopic, ore-petrographic, and SEM examination indicated that the pyrite masses occur as a cement binding the silt grains, and in some cases binding crushed shell fragments. Some pyrite also occurs as replaced vegetal matter. These last two types of pyrite most likely formed diagenetically after a post-silt glacial advance. The cement formed under reducing and alkaline conditions and probably was precipitated by reaction of aqueous iron with hydrogen sulfide produced from the decay of the organic matter.