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Abstract

Hagafellsjökull Vestari, Iceland, surged in 1980 and advanced about 700 m. Subsequent retreat of the ice front has exposed a suite of streamlined bedforms and former subglacial sediments. Investigations were undertaken on the recently deglaciated forefield to explore the degree of ice–bed coupling during the surge. The morphometry of flutes at this site suggest that the ice was relatively well coupled to its bed. Widespread till wedges show that ploughing was a dominant process. These observations suggest that pervasive deformation of the sedimentary bed at high water pressures in surge was a strong possibility. However, evidence from the micromorphology of drumlin sediments shows laterally continuous clay bands preserved between till layers within a drumlin. Thus pervasive deformation of the whole sediment thickness did not occur. There is evidence only of a thin layer of deformation (<16 cm). We propose that at low effective pressures there was a reduction in the strength of the ice–bed interface rather than a reduction in sediment strength. Motion was concentrated at the sole with localized and shallow deformation where clasts ploughed through the substrate.

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