Several curvi-linear features are seen in the flat ice areas that border the downstream end of Ice Stream C, West Antarctica. Recently acquired radar data indicate that not all of these features are relict ice stream margins. This is an important distinction when considering the shutdown history of Ice Stream C (ISC). On the Duckfoot, only two of the four identified curvi-linear features are identified to be margins based on the presence of buried crevasses at two distinct depths. The remaining features likely do not represent past ice stream positions. In one case, the curvi-linear feature (which sidles alongside the edge of Siple Dome) is not associated with the presence of near-surface crevassing. The other feature (southernmost feature associated with the Duckfoot) is more complicated because it is associated with crevassing. It appears that crevasses here have "bulged" upwards (crevasses get shallower, from 18 m depth to 12 m) over a distance of 125 m. However, since crevasse depth returns back to its original position this feature does not appear to be a relict ISC margin.

Using accumulation rates that range from 0.06 to 0.1 m/a (ice equivalent) the oldest margin shutdown anywhere from 520-310 years ago. Similarly, the recent margin has a shutdown age that ranges from 280-170 years old (with an estimated error of 15%). Because there is no evidence of a more recent margin inboard of this one we believe this margin to correspond to the ~150 year old margin identified by Retzlaff and Bentley (1993) on the south side of ISC. We find no older margins of ISC in the flat ice terrain on the south side of ISC, near Ridge BC (nickname the Goosefoot). However, two relict margins of Whillans Ice Stream are identified here and dated at 350 years and 150 years (based on the fact that the most recent relict margin of WIS shut down at the same time as that of ISC because the two visible scar features in a Radarsat image connect downstream).

These margins indicate a rough sequence of events as follows. Approximately 500 years ago Siple Ice Stream (SIS) was an active distributary of ISC and ISC may have been up to 25 km wider near the grounding line (it is also possible that ISC was not wider but simply occupied a more northerly position at this time). 450 years ago (with an error of 70 years) SIS shut down (Smith, 2000). The northernmost margin of ISC may have shut down close to this time with the northernmost margin of WIS following closely. 250 years ago a major tributary of ISC (C0) shutdown. Following this is the entire shutdown of ISC and a shift in the position of the northern margin of WIS (at the grounding line) inwards. These changes indicate a rapidly-responding, dynamic coupling between adjacent ice streams.