Antarctica (Fig. 42.1) has been described as 'a continent for science'; it is certainly a continent of superlatives—the coldest, highest, driest, and certainly the least known and least understood. Just 30yr ago, vast tracts were unmapped and unexplored, and were represented by largely featureless maps showing only the routes of the handful of expeditions to have crossed the interior of the continent. The mapping of the continental ice sheet was, however, greatly improved in the 1970s as long-range airborne mechanism holds important implications for ice-stream mechanics, not least of which is that it suggests that they may be able to widen under steady state conditions and increase ice discharge (Clark et al., 2003b). This may partly explain why many palaeo-ice streams that operated during deglaciation are wider than their contemporary counterparts.

In places, the regular parallel pattern of ice-stream bedforms has been modified by the presence of ribbed moraines that clearly developed after the ice-stream bedforms were generated. Plate 41.2 shows an example of this superimposition in the ice-stream onset zone. Given that ribbed moraines are not associated with fast ice flow, we argue that they represent areas of high basal drag (sticky spots) that developed either during or in response to ice-stream shut-down. One possibility is that they resulted from basal freeze-on and are a manifestation of shearing and stacking of debris and/or fracturing of cold-based ice (see Hattestrand & Kleman, 1999), or some other unknown mechanism associated with a reduction in ice velocity.

The ice stream imprint is orientated approximately parallel to the overall deglacial direction and comparison with retreat patterns (isochrons) suggests that it operated for only a brief phase (<500yr?) during deglaciation, likely to be around 8.2kyr BP (Stokes & Clark, 2004). Its activity would have had a profound affect on the Keewatin Sector of the Laurentide Ice Sheet, considerably lowering its surface profile and accelerating its demise at the end of the last glacial cycle.

survey became practical, and as a result the first convincing maps of the interior of the continent were published (e.g. Drewry, 1983a). But despite the cooperative efforts of many nations, the coverage obtained from aircraft remained far from complete, and only with the advent of imaging satellites in the 1970s did we gain a truly continental perspective on the ice sheet. In the following two decades, the increasing supply of satellite data allowed highresolution mapping of the elevation and flow of the entire ice sheet. In the past half-decade, simple mapping has ceased to stretch our capabilities, and measurements of change in the ice

David G. Vaughan

British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK

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