Seismic experiments have shown that a meters thick layer of unconsolidated sediment exists beneath Ice Stream Β at a location that is several hundred kilometers up-stream from the grounding line. In previous work, we have proposed that a “coupling” line exists about 100 km upstream from the grounding line. Above the coupling line, we believe that the dynamic behavior of Ice Stream Β is dominated by deformation within the basal sediment layers and that erosion is taking place at the bottom of this layer. Below the coupling line, we believe that sliding on a lubricating water film becomes increasingly important and that the subglacial sediments are deposited as a delta. Preliminary results from a 30 km seismic profile near the coupling line of Ice Stream Β show a prominent horizontal reflector several meters below the base of the ice; this reflector is underlain by a sequence of reflectors, each with a down-stream dip of about 1%. We believe that the horizontal and dipping reflectors represent, respectively, the topsets and foresets of the hypothesized delta.