Based primarily on lithologic differences, three Wisconsin drift sheets are recognized. The oldest is named the Olean; the middle, the Binghamton; and the youngest the Valley Heads. Beyond the Wisconsin drift border Illinoian drift is found, isolated into patches by erosion. Similar patches of Illinoian drift within the area covered by Wisconsin drift are postulated to explain the local lithologic vagaries of the latter. Olean ice, in overriding such patches of older drift, acquired material there-from to enrich locally the younger drift in the constituents common to the older. At Otto, New York, is seen in vertical section (1) weathered Illinoian drift, (2) peat, (3) Olean drift, (4) Binghamton drift.
The Wisconsin drift forming the east side of the re-entrant is older than that forming the west side.