Ice streams drain much of the interior West Antarctic Ice Sheet and buffer the main ice reservoir from oceanic influences. The slow-flowing interior feeds the floating Ross Ice Shelf with ice via fast-flowing ice streams that are believed to modulate sea-level change through their control of inland ice storage. Understanding ice-stream behaviour, and predicting the response to climate change, requires a better knowledge of the subglacial geology. It is known that a thawed ice-bed and high-pressure basal water are necessary, but not sufficient, conditions to cause ice streaming. Moreover, it has been hypothesized that a soft sedimentary bed is also required, because of its intrinsic low frictional resistance to flow, and owing to its high erodibility so as to generate till that can deform and lubricate ice motion, or to bury rough features and smooth the bed for sliding. Here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill. The ice stream is within the basin and the ice outside the basin is slow-flowing. The basin fill presents an order-of-magnitude lower frictional resistance to ice flow than the subglacial material outside the basin. We conclude that the ice stream position is dependent on subglacial geology.
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