Interpretation of seismic and glaciological data from the UpB camp on Ice Stream B, West Antarctica, suggests that the ice stream moves largely by deformation occurring within a meters thick, subglacial till layer resting unconformably on older sediments. The hypothesis that the bed of Ice Stream Β has been deforming at least since the post-Wisconsinan sea-level rise implies that a “till delta” probably tens of meters thick and tens of kilometers long has been deposited at the grounding line; recent gephysical measurements (Reference Shabtaie and BentleyShabtaie and Bentley, 1987; Reference Blankenship, Rooney, Alley and BentleyBlankenship and others, 1989) have provided evidence for the existence of this feature.

Significant sea-level fall would cause increased coupling between ice and till over the head of this delta, leading to erosion there, deposition at the grounding line, and conveyor-belt advance of the delta. The bathymetry of the Ross Sea suggests that this process would continue to the edge of the continental shelf for likely glacial maximum sea-level fall, leading to development of a low-profile, till-lubricated ice sheet. Subsequent sea-level rise would cause grounding-line retreat, leaving a meters thick, uniform till resting unconformably on older sediments. The modern Ross Sea is characterized by a regional angular unconformity, the Ross Sea unconformity, overlain by a meters thick, homogeneous diamicton that many investigators consider to be a till of Plio-Pleistocene age (e.g. Reference Anderson, Kurtz, Domack and BalshawAnderson and others, 1980), although both the age and depositional environment are debated. We hypothesize that the Ross Sea unconformity and overlying deposits record the extension of the UpB deforming till to the edge of the continental shelf during Wisconsinan (and earlier) sea-level low stands.

A paper reporting much of this work has been accepted for publication in Marine Geology (Reference Alley, Blankenship, Rooney and BentleyAlley and others, in press).