Stratigraphic investigations and detailed three-dimensional geologic mapping in the mid-1980s in Boone and Winnebago Counties, north-central Illinois, pro¬vided considerable new data allowing for revision of the stratigraphic frame¬work, which had been controversial and unresolved for almost 100 years. Significant findings were (1) the Win¬nebago Formation was not deposited during the early Wisconsin Episode as had previously been thought, but rather was deposited during the late Illinois Episode, and (2) the discovery of a lowermost coarse, sorted member of the Winnebago Formation, named the Beaver Creek, which was interpreted to have been deposited by proglacial melt¬water. An unresolved issue was the age of the Capron Member, the uppermost member of the Winnebago Formation. This report provides results from newly sampled locations of the Winnebago Formation using optically stimulated luminescence (OSL) dating of quartz grains from sand. Eight OSL ages for the Beaver Creek confirm its late Illinois Episode (late marine oxygen-isotope stage 6) deposition as well as that of overlying Winnebago Formation diamictons. The mean pooled age of the eight analyses is 136,610 ± 3,780 years BP. Dates range from 150,000 to 128,000 years ago, with five of the dates clustered between 132,000 and 139,000 years ago. A single acceptable OSL date of 106,000 years ago from a kame atop Capron Ridge helps clarify the age of the Capron Member, but not totally; the younger than expected age is probably due to bioturbation or some other mechanism of mixing. Although this OSL date is not definitive, it does suggest an Illinoian age for the underlying Capron diamicton.