Genesis of 'hummocky moraines' by thrusting in glacier ice: Evidence from Svalbard and Britain

Irregular mounds of glacial debris, commonly referred to as 'hummocky moraine', until recently were linked to ice stagnation during rapid climatic amelioration. However, recent work in Scotland has demonstrated that some hummocky moraine, dating from the Younger Dryas event (c. 10 000 years sp), was the product of deposition at active ice margins. Observations at modern high-arctic glacier margins in Svalbard (76-80 degrees N) indicate that some moraines of this type form under a dynamic glaciological regime, mainly by thrusting in active polythermal ice undergoing strong longitudinal compression. This situation is particularly common where there is a transition from warm-based ice in the interior of a glacier to cold-based ice at the margins or snout. From their morphological similarity (rectilinear slopes facing up-glacier and irregular downglacier faces), and apparent stacking of slabs of sediment of subglacial derivation, it is concluded that some assemblages of British moraines are also the product of thrusting. This process appears to be most dominant in polythermal glaciers, rather than in temperate glaciers. On this basis, it is tentatively suggested that the Younger Dryas in Britain was characterized by a climatic regime similar to that in Svalbard today (mean annual temperature - 5 degrees C; 400-1000 mm precipitation), and that the associated glaciers may have been of polythermal character.