Growth and decay of a marine terminating sector of the last British-Irish Ice Sheet: a geomorphological reconstruction

The boundary conditions that govern ice sheet dynamics can change significantly with the development of marine margins. This paper uses the glacial landscape in western Scotland to reconstruct changes in the British-Irish Ice Sheet that accompanied the growth and decay of a marine sector over the Malin Shelf. Ice advanced from a restricted mountain ice sheet with tidewater margins after ∼35 ka BP, and reached the continental shelf in ∼7 ka (average rate of ∼30 m a−1). Early ice flow had been directed through north-south, geologically controlled, over-deepened fjords that were carved during previous 'restricted' glaciations. This flow regime was abandoned with development of the Malin Shelf ice sheet sector; ice flow direction switched by ∼90° and was drawn westwards towards the shelf edge. The marine ice sheet phase saw episodes of west-east ice divide migration by up to 60 km over west central Scotland, possibly linked to ice streaming and calving events at the ice sheet margin. However, permanent and stationary ice divides and zones of cold-based ice, associated with subglacial topographic highs, also characterised the marine glacial stage over western Scotland. The North Channel ice divide remained a constant, though migratory feature while the BIIS occupied the Malin Shelf; it finally collapsed at the end of the Killard Point Stadial when the Irish Ice Sheet began to rapidly decay ∼16.5 ka BP. This permitted the Scottish Ice Sheet to temporarily advance over north-east Ireland (previously identified as the East Antrim Coastal Readvance) before it too retreated, at rates in the order of 102 m a−1. Although the imprint of extensive shelf-edge ice sheet glaciation exists in the coastal landscape of western Scotland, the dominant landscape features relate to a restricted, marine-proximal mountain ice sheet with markedly different flow configurations. Similar first-order geomorphological features, relating to 'restricted' glacial conditions, are likely to be preserved in subglacial highlands under interior parts of modern ice sheets.