Discovery of Till Deposition at the Grounding Line of Whillans Ice Stream

We report on the discovery of a grounding-line sedimentary wedge (“till delta”) deposited by Whillans Ice Stream, West Antarctica. Our observation is that grounding-line deposition serves to thicken the ice and stabilize the position of the grounding line. The ice thickness at the grounding line is greater than that of floating ice in hydrostatic equilibrium. Thus, the grounding line will tend to remain in the same location despite changes in sea level (until sea level rises enough to overcome the excess thickness that is due to the wedge). Further, our observation demonstrates the occurrence of rapid subglacial erosion, sediment transport by distributed subglacial till deformation, and grounding-line sedimentation, which have important implications for ice dynamics, numerical modeling of ice flow, and interpretation of the sedimentation record. Subglacial sediment deformation and trans-port is an integral part of the glacial sys-tem, affecting or controlling both short-term (1) and long-term (2) glacier fluctuations through changes in the basal speed of the glaciers. Ex-tensive data sets on sediment transport are avail-able for many modern mountain glaciers (3), allowing assessment of the effects of sedimen