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Most of the velocity of ice stream B near the Upstream B camp (UpB), West Antarctica, 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 equivalent 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 characterized by partial ice‐till decoupling across a water film and by a small ice‐air surface slope; they may have been discovered by recent geophysical work.

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Seismic experiments conducted on ice stream B, part of the marine ice sheet of West Antarctica, show a meters-thick layer immediately beneath the 1000-m-thick ice. A seismic experiment consisting of wide-angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer. Inversion of seismic travel times yields a compressional wave speed of less than 1700 m s -1 and a shear wave speed less than 160 m s -1 for the layer. These very low wavespeeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure.Based on wave speeds in other saturated, unconsolidated sediments, we believe that a porosity substantially greater than 0.32, probably around 0.4, and an excess of overburden pressure over pore pressure of only 50 kPa (0.5 bar) characterize the layer at this location.