Crescentic scours on palaeo-ice stream beds

Subglacial landforms include a range of depositional and erosional features formed by the interaction of ice or meltwater with a soft sedimentary or bedrock substrate. The genesis of many such landforms has received considerable scrutiny, but sets of half-moon- (crescent-) shaped scoured depressions that are known from a number of relict ice-sheet beds have received little attention. New data reveal that these enigmatic features, here termed crescentic scours, are more widespread than previously reported and may be important in understanding broader subglacial processes. Description Crescentic scours (CS) are imaged frequently on former ice-sheet beds (Fig. 1), but also on mid-latitude ice-sculpted terrains and beneath at least one modern Antarctic ice stream for which radar data are available (King et al. 2009). However, few CS have been described in detail. CS generally occur in three positions, each interpreted to have been previously occupied by fast-flowing ice streams or outlet glaciers: (1) they commonly wrap around the heads or seed points of drumlins or other streamlined, longitudinal bedforms (e.g. crag-and-tails) where sediments are thin to absent; (2) scours form linked chains against the up-ice face of bedrock protuberances or crags; and (3) on occasion, scours are found in isolation on flat or subdued beds with no apparent flow obstacle.

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