Surface exposure dating (36Cl and 10Be) of post-Last Glacial Maximum valley moraines, Lake District, northwest England: some issues and implications

Surface exposure ages (36Cl and 10Be) have been obtained from 16 boulders at the margins of and within the limits of former valley glaciers at four sites in the Lake District, northwest England. The established view is that the Lake District functioned as an independent centre of ice dispersal during the Last Glacial Maximum (LGM; 2621 ka) of the Dimlington Stade (approximate to Greenland Stades 23) and that the glacial-depositional landforms at the sampled sites are products of stillstands or readvances of the ice margins following the LGM. However, several of the apparent 36Cl surface exposure ages pre-date rather than post-date the LGM. It is inferred that these boulders retain levels of 36Cl inherited from a previous period of exposure rather than indicating moraine formation prior to the LGM and their subsequent survival beneath LGM ice. An implication is that Lake District glaciers achieved limited erosion of bedrock during the build-up to and at the maximum of the LGM, and that glacial erosion rates increased at a late stage in that period. Single 36Cl and 10Be exposure ages from two sites suggest a stillstand or readvance of valley glaciers prior to the Loch Lomond (Younger Dryas) Stade (LLS; Greenland Stade 1) and Windermere (Lateglacial) Interstade (Greenland Interstade 1), but additional ages are required in order to confirm or refute these individual ages. An LLS age for glacial-depositional landforms at both these sites had previously been proposed. 36Cl surface exposure ages for boulders on moraines within the LLS limit do not fully support emplacement at that time; there is a possibility that these moraines are associated with a pre-LLS stillstand or readvance of the valley glacier. Copyright (c) 2013 John Wiley & Sons, Ltd.