Fennoscandia: Glacial landforms during the Bølling–Allerød Interstadial (14.6–12.9 ka)

In southern and eastern Fennoscandia, the deglacial landscapes of the Bølling–Allerød are well-known. Sequential ice sheet margin positions are marked by semicontinuous moraine ridges, glaciofluvial deltas and fans or sandar. These ice limits are commonly lobate in shape across a range of scales, fed by subglacial lineations and meltwater pathways with a radial distribution; the margin positions are commonly interpreted as products of minor readvances. Different lobes appear to behave differently in their magnitude of Bølling–Allerød retreat and in their propensity for oscillation or readvance. The nature of the terminal zone – land-terminating or subaqueous – is likely responsible, at least in part, although this suggestion is difficult to test rigorously given limited data available to constrain either the pattern or rates of ice margin retreat in the Baltic. The southern and eastern margins have a rich landform legacy of meltwater drainage, reflecting high summer temperatures. However, these conditions were established prior to the onset of the Bølling, and the Bølling–Allerød landscape is not distinct from earlier deglaciation either with respect to meltwater drainage as a dominant landscape characteristic or with respect to lobate and oscillating margins. In contrast, the western and northern ice sheet margins in a higher-relief topographic setting have preserved minimal landform evidence for Bølling–Allerød retreat, except at a few disparate sites where moraine ridges outboard of the well-mapped Younger Dryas limit have been dated to the intervening Older Dryas. The inboard extent of Allerød ice sheet retreat is poorly constrained all around the ice sheet; superimposed Younger Dryas advance landforms upon earlier retreat assemblages offer insight into the magnitude of Allerød retreat as well as changes in ice flow pathways, and are a potentially underexploited tool for reconstructing the geometry and dynamics of this pronounced retreat–readvance sequence.