Evidence against pervasive bed deformation during the surge of an Icelandic glacier

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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.