LATE WISCONSIN GLACIAL ADVANCE AND RETREAT PATTERNS IN SOUTHWESTERN OHIO, USA

Despite the application of radiocarbon dating for more than three deca des along the southern margin of the Laurentide Ice Sheet, fundamental questions about the timing of glacial advances remain. For one of its sublobes, the Miami, we undertook areal mapping, detailed lithostrati graphic analysis, and radiocarbon dating to interpret four pulses of i ce advance. On top of the undated sediments deposited during the first advance is a major unconformity. The second advance occurred about 20 ,000 BP and marks the beginning of the late Wisconsin glaciation. A mi nor recession (more than 30 km) ensued, but plants did not reoccupy th e landscape. A third advance of the ice margin produced a stone-rich l odgement till to within 20 km of the late Wisconsin maximum. The final ice motion only occurred in the northern part of the study area and m ay be of local extent. Large accumulations of supraglacial gravity flo wtills and outwash mark the final ice-margin retreat. Of these, only t he second advance is well dated. This study implies that the number of advances of the ice margin is fewer than previously suggested. Conseq uently we argue that several of the sublobes across the southern margi n of the Laurentide Ice Sheet acted in unison for the interval of 22,0 00 to 18,000 BP implying ice-sheet external forcing.