The Genesee Valley, a buried bedrock basin similar to the neighboring Finger Lakes, is filled with more than 200 m of glacial drift and Genesee River alluvium. Beneath a thin cover of late Wisconsin drift, glaciolacustrine sediments dated at 33,950 to 35,350 years BP gradationally enclose a deformation till and outwash containing reworked wood, peat fragments, and mammoth bones with ages ranging from 35,300 to 47,500 yrs BP. This till rests on an interstadial floodplain dated between 39,000 and 41,000 years BP. The 32 radiocarbon ages on organic remains document a Heinrich (H-4) time-equivalent advance into a proglacial lake 35 km south of Lake Ontario. The deformation till locally truncates interstadial point bar sands along a horizon containing plant roots in growth positions. Fluvial sand dates are from wood fragments, in-place rootlets, and small pelecypod shells with periostraca. The well preserved organics and the well documented stratigraphy provide a clear example of the potential limitations involved in using isolated dates on single samples from glacial drift to infer ages of discrete glacial events. The reworked nature of the wood, peat, and bone in the deformation till is obvious from the younger age of the in-situ organics in the winter layers of the rythmites and from the younger ages of the interstadial sediments below. The 4000-year age hiatus between the deformation till and the underlying fluvial sands is probably the result of reworking by interstadial meanders rather than ice erosion, as suggested by the chronostratigraphy of the modern Genesee River floodplain. A single finite age on one wood fragment, selected from the till or from the interstadial sediments, could have resulted in either a presumed early Wisconsin age or a late Wisconsin age for the deformation till, or for the entire section. Only the well-preserved stratigraphic relationships, the preserved sedimentary structures, and the well-preserved, abundant, organic remains reveal the true age and complexity of mid Wisconsin events at this site.