The late-glacial ice-dammed lake Nedre Glomsjø in Mid-Norway: an open lake system succeeding an actively retreating ice sheet

The existence of large late-glacial ice-dammed lakes in Mid-Norway was recognised in the late 1800s. Mapping of the lakes’ distribution provided novel and valuable information regarding ice age palaeogeography and glacial isostatic adjustment, but the findings failed to convince the scientific community at the time. Divergent opinions concerning the inland lakes have lingered to the present, in particular as the purported glacial lake shorelines have been interpreted as subglacial or lateral meltwater features. In this study we revisit the largest palaeolake – glacial lake Nedre Glomsjø – and attempt to unravel the controversy using high-resolution airborne LiDAR data. Our mapping has revealed key and hitherto undescribed landforms indicative of an open lake system. De Geer moraines signal that the ice sheet retreated in a water-body corresponding to the elevation of the Nedre Glomsjø shorelines. The De Geer moraines also determine that the ice sheet in the region did not merely melt vertically down, but rather retreated actively southward, and we thus refute studies claiming that the purported shorelines formed along a closed ice-filled basin with stagnant ice. Based on the number of De Geer moraines, the ice-sheet margin is estimated to have retreated step-wise through our study area at a rate of 200–600 m yr-1. The study highlights the potential of mapping glacial lake remnants for assessing palaeoglaciology in an ambiguous sector of the Scandinavian Ice Sheet.

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