Abstract
ABSTRACTBasal records from nine lake basins, located within a small area inside the Younger Dryas Ra moraine in southern Norway, were analysed and radiocarbon dated. Six reworked shell fragments from the lowermost facies in five different basins date to the Bølling–Allerød chronozone and are interpreted to represent an ice‐free coastline before a Younger Dryas ice sheet readvance of at least 8 km. The widespread shell fragments within the Younger Dryas ice margin but pre‐dating the Younger Dryas are demonstrated using a systematic survey of the lowermost strata in a collection of lake basins. A further 13 dated samples, mostly in situ articulated bivalves found in clayey silt overlying the basal facies, yielded overlapping ages close to the Younger Dryas–Holocene boundary. The 13 shell dates, along with two additional similar ages obtained on terrestrial plant macrofossils from basal strata in two basins near the marine limit, provide a minimum‐limiting age of 11.6k cal a BP for ice sheet retreat from the Ra moraine. This suggests that the ice sheet in south‐east Norway retreated from its Younger Dryas maximum position nearly a thousand years later than previously assumed. Similar systematic approaches may lead to improved ice sheet reconstructions from other formerly glaciated coastlines.
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