LAUSR

During the Last Glacial Maximum (LGM), ice streams of the last British-Irish Ice Sheet (BIIS) controlled ice discharge from various dispersal centres. Deglaciation was characterised by shifts in ice-divide location and changes in internal ice-sheet dynamics, resulting in substantial flow switches and significant ice-stream reconfiguration, and hence modification of their landform signatures. We present new geomorphological mapping and 11 10Be cosmogenic nuclide ages from Northern England (Stainmore Gap, Eden Valley and Vale of York), that constrain regional dynamic ice-stream retreat following the LGM. We identify complex decoupling of competing ice lobes, characterised by early retreat of the North Sea Lobe and a minor re-advance of Stainmore ice prior to ∼20 ka. This was followed by rapid recession of the central Stainmore Gap, which was ice-free by 19.8 ± 0.7 to 18.0 ± 0.5 ka, contemporaneous with the recession of the Tyne Gap Ice Stream. In the southern Vale of Eden, Crossby Ravensworth Fell became exposed between 19.2 and 20.3 ka. The northwards ice-flow reversal in the Vale of Eden was associated with the development of ice domes across the northern Pennines, Howgill Fells and the Lake District. This shift in dispersal centres and ice divide migration likely triggered the rapid collapse of eastward ice stream corridors. The central sector of the BIIS rapidly collapsed back up into upland dispersal centres between 20 and 17.5 ka. This work highlights the role internal factors, such as topography, in driving ice-divide migration and flow switches during externally and climatically forced ice-sheet thinning.