Glaciology of the British Isles Ice Sheet during the last glacial cycle: form, flow, streams and lobes

The last ice sheet over the British Isles, together with other mid-latitude Pleistocene ice sheets, and in contrast to the modern ice sheets of Greenland and Antarctica, had a relatively low profile, low summit elevation and extensive, elongated lobes at its margin. A thermo-mechanically coupled numerical ice sheet model, driven by a proxy climate, has been used to explore the properties that would have permitted these characteristics to develop. The approach, the key to quantitative palaeoglaciology, is to determine the boundary conditions that permit the simulated ice sheet to mimic the evolution of the real ice sheet through the last glacial cycle. Simulations show how a British ice sheet may have been confluent with a Scandinavian ice sheet during some parts of its history and how unforced periodic and asynchronous oscillations could occur in different parts of its margins. Marginal lobes are a reflection of streaming within the ice sheet. Such streams can be ephemeral, dynamic streams located because of ice sheet properties, or fixed streams whose location is determined by bed properties. The simulations that best satisfy constraints of extent, elevation and relative sea levels are those with major fixed streams that strongly draw down the ice sheet surface. In these, the core upland areas of the ice sheet were cold based at the Last Glacial Maximum, basal streaming velocities were between 500 and 1000 ma⁻¹ compared with surface velocities of 10–50 ma⁻¹ in inter-stream zones, shear stresses were as low as 15–25 kPa in streams compared with 70–110 kPa in upland areas and 60–84% of the ice flux was delivered to the margin via streams.