Submarine glacial landforms record

16 We use ~7,000 km2 of high-resolution swath bathymetry data to describe and map the submarine 17 glacial geomorphology, and reconstruct Late Pleistocene ice sheet flow configurations and retreat 18 dynamics within the Inner Hebrides, western Scotland. Frequently dominated by outcrops of 19 structurally complex bedrock, the seabed also comprises numerous assemblages of well-preserved 20 glacigenic landforms typical of grounded ice sheet flow and punctuated ice-margin retreat. The 21 occurrence and character of the glacially streamlined landforms is controlled in part by the shallow 22 geology and topography, however these factors alone cannot account for the location, orientation, 23 and configuration of the observed landforms. We attribute the distribution of these elongate 24 streamlined landforms to the onset zone of the former Hebrides Ice Stream (HIS) part of a major 25 ice stream system that drained 5-10% of the last British-Irish Ice Sheet (BIIS). We suggest this 26 geomorphic signature represents the transition from slow ‘sheet flow’ to ‘streaming flow’ as ice 27 accelerated out from an environment characterized by numerous bedrock obstacles (e.g. islands, 28 headlands), towards the smooth, sediment dominated shelf. The majority of streamlined landforms 29 associated with the HIS indicate ice sheet flow to the southwest, with regional-scale topography 30 clearly playing a major role in governing the configuration of flow. During maximal glacial conditions 31 (~27-23 ka) we infer that the HIS merged with the North Channel-Malin Shelf Ice Stream to form a 32 composite ice stream system that ultimately reached the continental shelf edge at the Barra33 Donegal Trough-Mouth Fan. Taken collectively however, the pattern of landforms now preserved at 34 seabed (e.g. convergent flow indicators, cross-cutting flow sets) is more indicative of a thinning ice 35 mass, undergoing reorganization during overall ice sheet retreat (during latter stages of Late 36 Weischselian glaciation). Suites of moraines overprinting the streamlined landforms suggest partial 37 stabilization of the HIS prior to the ice sheet retreating to more isolated, topographically confined 38 troughs and basins. Retreat from the shelf towards, and back into the Inner Hebrides may have been 39 rapid due the prevalence of overdeepened troughs. Within the near-shore fjord-like troughs and 40 deeps, basin-aligned streamlined landforms indicate the subsequent flow of thinner topographically 41 partitioned ice masses, and overprinted moraines record further ice margin retreat, potentially along 42 tide-water margins. This work provides the first geomorphological constraints for this large marine43 influenced sector of the former BIIS. We also shed new light on the glacial geomorphic record found 44 at the transition from terrestrial to marine continental-shelf settings, and examine the interplay 45 between substrate geology, bed topography/bathymetry, and grounding-line positions 46 relationships which are important for characterizing contemporary marine ice sheet margins. 47