Thick deposits of Roxana Silt are recognized only along the Illinois River (downstream from the Woodfordian terminal moraine) and are generally interpreted as being mainly loess, with the bulk accumulating from about 40,000 to 30,000 yr ago in association with an Altonian-age glacier in northeastern Illinois. Yet 11 14C dates indicate that southern Michigan was not ice-covered during that interval; thus, any proximate ice must have, at best, been restricted to Great Lakes basins, an interpretation supported by the absence of late Altonian till at critically located Michigan, and nearby, sites. Late mid-Wisconsinan ice did, however, obstruct eastern drainage of the ancestral Great Lakes. Such glacial blockage, the distribution of many Michigan organic deposits within pre-Woodfordian lacustrine sediments, and radiocarbon dates suggest that, more than once, late Altonian lakes associated with the Lake Michigan basin drained into the Illinois River. Erosion of lake and spillway bluffs along with repeated river fluctuations provided a source for the thick, geographically restricted Roxana Silt. Meanwhile, along other nearby rivers the supply was meager and the loess thin.