A previously undated late Wisconsin glacial advance in western New York State near the Bölling-Allerod/Younger Dryas transition (12,900 YBP) is verified from the Genesee Valley westward to Lake Erie. The Genesee Valley evidence includes 24 ¹⁴C ages ranging from 12,486 to 11,041 ¹⁴C years BP (calendar corrected 14,676 to 12,907 YBP) and indicates a readvance estimated as beginning between 13,000 and 13,300 YBP and incorporating trees that grew during the Bölling-Alerod warm episode (INTIMATE time scale age: 14,700 to 12,900 YBP). The Genesee Valley data, extending 43 km south of Lake Ontario, imply overriding of older glacial landforms without destroying preexisting moraines. A peccary skeleton preserved beneath the crest of the end moraine near Linwood, NY, preserves evidence of a proglacial quicksand setting at an active ice margin, which subsequently spread a thin till over the burial site. The peccary site details provide the best evidence for the young age (bone collagen; 13,307 YBP) of the advance, and are compatible with the youngest wood age (13,469 YBP) from till near Avon, NY. Discontinuous, clay-rich tills imply that a floating or partially grounded ice shelf is a plausible mechanism of emplacement. The age of the advance overlaps the most recent data for the limits of glacial Lake Iroquois in the Ontario basin. If a floating ice-shelf model is correct, Lake Iroquois must have persisted for a sufficient interval following the advance to explain the preservation of the apparently intact Lake Iroquois shorelines, which do not seem to record evidence of younger glacial deformation. Two borings for the Irondequoit Bay outlet bridge and older water supply wells also contain evidence of an ice advance out of the Ontario basin that compacted a thin interval of peat and/or gyttja (11,790 & 11,340 ¹⁴C BP or 13,621 & 13,194 YBP) at depths between 38 and 43 meters beneath the Irondequoit Bay barrier bar. Complementary evidence derived from the 4 major sites stretching westward into the Cattaraugus Creek basin near Springville, NY, is described in an online 2018 US Department of Energy/NY State Energy Research and Development Authority study at the West Valley Demonstration Project (WVDP). A comprehensive article focused on this regional ice advance is being prepared for publication with co-investigators from the WVDP studies.