Most of the velocity of ice stream B near the Upstream B camp (UpB), West Antarttica, appears to arise from deformation of a seismically detected, subglacial till layer that averages 6 m thick. Available evidence indicates that the entire thickness of this till layer is deforming and is eroding subjacent bedrock into flutes parallel to ice flow and hundreds of meters across. The resulting till flux beneath UpB is equibalent to an average erosion rate of about 0.4 mm yr-1 in the catchment area and suggests that till deltas tens of kilometers long have been deposited at the grounding line during the Holocene. Such deltas should be charcterized by partial ice-till decoupling across a water fiem and by a small ice-air surface slope; they may have been discovered by recent geophysical work. ¿ American Geophysical Union 1987