Geomorphology and dynamics of the British-Irish Ice Sheet in western Scotland

Predicting the long-term behaviour of present-day ice sheets is hampered by the short timescales of our observations and restricted knowledge of the subglacial environment. Studying palaeoice sheets can help by revealing the nature and amplitude of past centennial- to millennial-scale ice sheet change. This thesis uses glacial sediments and landforms to examine the evolution of the partly marine-based British-Irish Ice Sheet (BIIS) and its bed, in western Scotland. Three zones of the former BIIS are considered: ranging from a mountain ice cap, to a core area of the ice sheet, to a peripheral marine-terminating sector. The topography of the subglacial landscape was an important in uence on the location of dynamic and stable components of the ice sheet. At an ice cap scale, zones of glacier inception and retreat were linked to catchment elevation and size. At the ice sheet scale, the migration of ice divides and thermal boundaries were focused through corridors of low relief subglacial topography. The main west-east ice divide of the BIIS in central Scotland migrated by 60 km, 10% of the ice sheet's width, through one such corridor during the glacial cycle. A major change in the ow regime of the BIIS in western Scotland accompanied the development of a marine-based sector on the Malin Shelf. As the BIIS advanced to the shelf edge, ice ow was drawn westwards { orthogonal to the earlier, geologically controlled, ow pattern. Retreat of the BIIS from the shelf edge occurred at an average rate of 10 m a-1, but was punctuated by at least one episode of accelerated retreat at 100 m a-1. In each zone of the BIIS examined, a rich palimpsest landscape is preserved and the role of earlier glaciations in conditioning or priming the landscape is highlighted. Western Scotland in particular is dominated by features relating to a 'restricted' mountain ice sheet, suggested to have been the prevailing ice sheet mode during the Early and Middle Quaternary. Where the last BIIS was underlain by soft sediments, glacier movement at the bed was facilitated by a combination of basal sliding and a localised mosaic of shallow deforming spots, allowing landform and sediment preservation. In places, till deposition was focused over permeable substrates acting to seal the bed, promote lower e ective pressures, and enhance motion by basal sliding. The modern land surface in western Scotland provides an approximation for the relief of the former glacier bed, and can be used for conceptual palaeoglaciological reconstructions. Areas of focused postglacial deposition have, however, obscured parts of the ice sheet bed, with demonstrable implications for quantitative palaeoglaciological analyses. Methods to improve the representation of former ice sheet bed in these areas are discussed and may be pertinent to future palaeo-ice sheet modelling exercises.