LateQuaternary glaciation in theHebrides sector of the continental shelf: wasStKilda overrun by theBritish‐IrishIceSheet?

Abstract

Until recently, theBritish‐IrishIceSheet (BIIS) was thought to have reached no farther than a mid‐continental shelf position in the Hebrides Sector,NWBritain, during the last glaciation (traditional model). However, recent discovery of widespread shelf‐edge moraines in this sector has led to a suggestion of much more extensive ice (AtlanticShelf model). The position of theStKilda archipelago, approximately mid‐way between theOuterHebrides and the continental shelf edge, makes it ideal as an onshore location to test which of the two competing models is more viable. To this end, we (i) reassessed the characteristics, stratigraphy and morphology of theQuaternary sediments exposed on the largest island (Hirta), and (ii) applied time‐dependent 2Dnumerical modelling of possible glacier formation onHirta. Instead of three glaciations (as previously suggested), we identified evidence of only two, including one of entirely local derivation. The numerical model supports the view that this glaciation was in the form of two short glaciers occupying the two valleys that dominateHirta. The good state of preservation of the glacial sediments and associated moraine of this local glaciation indicate relatively recent formation. In view of the low inferred equilibrium line altitude of the glacier associated with the best morphological evidence (∼120 m), considerable thickness of slope deposits outside the glacial limits and evidence of only one rather than two tills, aLateDevensian rather thanYoungerDryas age is preferred for this glaciation. Re‐examination of the submarine moraine pattern from available bathymetry suggests that the ice sheet was forced to flow aroundStKilda, implying that the ice was of insufficient thickness to overrun the islands. Accepting this leaves open the possibility that aStKilda nunatak supported local ice while the ice sheet extended to the continental shelf edge.