Strongly divided opinion has led to competing, apparently contradictory, views on the timing, extent, flow configuration and decay mechanism of the last British Ice Sheet. We review the existing literature and reconcile some of these differences using remarkable new seabed imagery. This bathymetric data provides unprecedented empirical evidence of confluence and subsequent separation of the last British and Fennoscandian Ice Sheets. Critically, it also allows a viable pattern of ice-sheet disintegration to be proposed for the first time. Covering the continental shelf around the northern United Kingdom, extensive echosounder data reveals striking geomorphic evidence -in the form of tunnel valleys and moraines -relating to the former British and Fennoscandian Ice Sheets. The pattern of tunnel valleys in the northern North Sea Basin and the presence of large moraines on the West Shetland Shelf, coupled with stratigraphic evidence from the Witch Ground Basin, all suggest that at its maximum extent a grounded ice sheet flowed from SE to NW across the northern North Sea Basin, terminating at the continental shelf edge. The zone of confluence between the British and much larger Fennoscandian Ice Sheets was probably across the northern Orkney Islands, with fast-flowing ice in the Fair Isle Channel focusing sediment delivery to the Rona and Foula Wedges. This period of maximum confluent glaciation (c. 30-25 ka BP) was followed by a remarkable period of large-scale ice-sheet re-organisation. We present evidence suggesting that as sealevel rose, a large marine embayment opened in the northern North Sea Basin, as far south as the Witch Ground Basin, forcing the two ice sheets to decouple rapidly along a north-south axis east of Shetland. As a result, both ice-sheets rapidly adjusted to new quasi-stable margin positions forming a second distinct set of moraines (c. 24-18 ka BP). The lobate overprinted morphology of these moraines on the mid-shelf west of Orkney and Shetland indicates that the re-organisation of the British Ice Sheet was extremely dynamic -probably dominated by a series of internally forced readvances. Critically, much of the ice in the low-lying North Sea Basin may have disintegrated catastrophically as decoupling progressed in response to rising sea levels. Final-stage deglaciation was marked by near-shore ice streaming and increasing topographic control on ice-flow direction. Punctuated retreat of the British Ice Sheet continued until c. 16 ka BP when, following the North Atlantic iceberg-discharge event (Heinrich-1), ice was situated at the present-day coastline in NW Scotland.

Detailed geomorphological mapping of the Beinn Dearg massif in northern Scotland, was conducted to examine the maximum (Younger Dryas) extent, and earlier interstadial evolution, of an ice cap that existed during the Lateglacial period (14.7 -11.7 cal. ka BP). Landform evidence indicates a plateau ice cap configuration, with radial outlet glaciers, during the Younger Dryas. The interpreted age is supported by new cosmogenic exposure ages, and previously reported interstadial sediments beyond the ice cap margin. The reconstructed Younger Dryas Beinn Dearg ice cap covered 176 km 2 , with its summit positioned over the western side of the massif. Area-altitude balance ratio (AABR) equilibrium line altitudes (ELAs) of 570 -580 m were calculated for the ice cap as a whole. The empirically reconstructed ice cap is compared with recent numerical model simulations; both methods produce an ice cap with a similar configuration. However, differences are apparent in the extent of eastern and western outlets (±1-5 km), and in the spatial variation of ELAs. Results suggest that the numerical simulation over-estimates the extent of western ice cap sectors, and under-estimates the extent of eastern ice cap sectors. We attempt to quantify these differences in terms of ice cap mass balance and assess their possible causes. Geomorphological evidence for pre-Younger Dryas ice cap configuration indicates that the Beinn Dearg massif remained an important source during earlier deglaciation. In contrast, the neighbouring Fannich mountains acted as an 'unzipping' zone, and were ice free on their northern side by the Allerød (Greenland Interstadial 1c to 1a). Deglaciation continued over western parts of the Beinn Dearg plateau, with the possibility that glaciers remained in some central and eastern catchments, prior to (Younger Dryas) ice cap (re)growth.Andrew Finlayson (e-mail: afin@bgs.ac.uk), British Geological Survey, Edinburgh, UK and Institute of Geography, University of Edinburgh, UK; Nick Golledge (e-mail: nick.golledge@vuw.ac.nz), Antarctic Research Centre, Victoria University of Wellington, New Zealand; Tom Bradwell (e-mail: tbrad@bgs.ac.uk), British Geological Survey, Edinburgh, UK; Derek Fabel (e-mail: Derek.Fabel@ges.gla.ac.uk ), Department of Geographical and Earth Sciences, University of Glasgow, UK.Reconstructions of palaeo-, or formerly more extensive, ice masses in northwest Europe have enabled inference of past glacier mass balance and climate, and allowed the causes of ice mass fluctuations to be assessed (e.g. Ballantyne 1989;Dahl & Nesje 1992;Carr 2001;Rea & Evans 2007;Golledge et al. 2009;Nesje 2009). In the Scottish Highlands, the last decade has seen a renewed focus of research into the extent and behaviour of ice masses during the Lateglacial Younger Dryas (YD), or Loch Lomond Stadial (Greenland Stadial 1 (GS-1)) (12.9 -11.7 cal. ka BP (Lowe et al. 2008))(e.g. Ballantyne 2002Ballantyne , 2007aBenn & Ballantyne 2005;Finlayson 2006;Golledge 2007;Lukas & Bradwell 2010). Key outcomes of this research have been the...

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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 northsouth, 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 10 2 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.

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We use ∼7000km 2 of high-resolution swath bathymetry data to describe and map the submarine glacial geomorphology, and reconstruct Late Pleistocene ice sheet flow configurations and retreat dynamics within the Inner Hebrides, western Scotland. Frequently dominated by outcrops of structurally complex bedrock, the seabed also comprises numerous assemblages of well-preserved glacigenic landforms typical of grounded ice sheet flow and punctuated ice-margin retreat. The occurrence and character of the glacially streamlined landforms is controlled in part by the shallow geology and topography, however these factors alone cannot account for the location, orientation, and configuration of the observed landforms. We attribute the distribution of these elongate streamlined landforms to the onset zone of the former Hebrides Ice Stream (HIS) – part of a major ice stream system that drained 5–10% of the last British–Irish Ice Sheet (BIIS). We suggest this geomorphic signature represents the transition from slow ‘sheet flow’ to ‘streaming flow’ as ice accelerated out from an environment characterized by numerous bedrock obstacles (e.g. islands, headlands), towards the smooth, sediment dominated shelf. The majority of streamlined landforms associated with the HIS indicate ice sheet flow to the southwest, with regional-scale topography clearly playing a major role in governing the configuration of flow. During maximal glacial conditions (∼29–23ka) we infer that the HIS merged with the North Channel-Malin Shelf Ice Stream to form a composite ice stream system that ultimately reached the continental shelf edge at the Barra-Donegal Trough-Mouth Fan. Taken collectively however, the pattern of landforms now preserved at seabed (e.g. convergent flow indicators, cross-cutting flow sets) is more indicative of a thinning ice mass, undergoing reorganization during overall ice sheet retreat (during latter stages of Late Weischselian glaciation). Suites of moraines overprinting the streamlined landforms suggest partial stabilization of the HIS prior to the ice sheet retreating to more isolated, topographically confined troughs and basins. Retreat from the shelf towards, and back into the Inner Hebrides may have been rapid due the prevalence of overdeepened troughs. Within the near-shore fjord-like troughs and deeps, basin-aligned streamlined landforms indicate the subsequent flow of thinner topographically partitioned ice masses, and overprinted moraines record further ice margin retreat, potentially along tide-water margins. This work provides the first geomorphological constraints for this large marine-influenced sector of the former BIIS. We also shed new light on the glacial geomorphic record found at the transition from terrestrial to marine continental-shelf settings, and examine the interplay between substrate geology, bed topography/bathymetry, and grounding-line positions – relationships which are important for characterizing contemporary marine ice sheet margins

Glacigenic and fluvial deposits of variable lithological composition underlie many major cities in Europe and North America. Traditional geological mapping and 3D modelling techniques rarely capture this complexity as they use lithostratigraphic designations which are commonly based on genesis and age rather than lithological compositions.In urban areas, thousands of boreholes have been, and continue to be, drilled to facilitate the planning, design and construction of buildings and infrastructure. While these data may provide the basis for geological maps and 3D models based on lithological interpretation, they are too numerous for manual correlation to be undertaken efficiently. In this paper we explore the application of largely automated stochastic modelling techniques to develop predictive lithology models for glacial and fluvial deposits in the city of Glasgow, UK. These techniques are commonly used to assess facies variation in oilfield models and are applied here in an urban setting using over 4000 borehole records.Predictions derived from these methods have been evaluated by removing control data and rerunning the simulations. We demonstrate a moderate improvement in the prediction of lithology when using a lithologically-derived stochastic model compared with a conventionally interpolated lithostratigraphic model. It is possible to report uncertainty within the resulting models, either with probability maps or through a suite of plausible simulations of the lithologies across the study region. IntroductionThe growth and decay of high-and mid-latitude Pleistocene ice sheets has left 8% of the Earth's land surface, including one third of Europe and a quarter of North America, covered by glacigenic and fluvial deposits Gibbard, 2004a, 2004b). These deposits underlie many major cities and much of their associated infrastructure networks, and exert a significant influence on the groundwater system. Increasing urban development, and its demands (e.g. suitable foundation conditions, the need for waste storage, contaminant migration, drainage re-routing) requires that information about subsurface glacial deposits, which are often highly lithologically variable across short distances, is available for those involved in planning and construction (Campbell et al., 2010). A key challenge for the three-dimensional (3D) geological modelling community is therefore to represent these subsurface deposits in appropriate ways across large, city-wide areas (Culshaw, 2005;MacCormack et al., 2005;Kessler et al., 2009).In Glasgow, west central Scotland (Figure 1), the British Geological Survey (BGS), in partnership with Glasgow City Council and other local authorities, have used extensive borehole datasets to develop and successfully apply a suite of 3D Quaternary lithostratigraphic models (Merritt et al., 2007;Campbell et al., 2010) (Figure 2). A key strength of lithostratigraphic modelling is that it brings together the expertise of geologists and known geological relationships, enabling a geologically realistic r...

ABSTRACT. The nature and behaviour of sediment beneath glaciers influences how they flow and respond to changing environmental conditions. The difficulty of accessing the bed of current glaciers is a key constraint to studying the processes involved. This paper explores an alternative approach in which accessible sediments under the beds of former mid-latitude ice sheets are examined and related to changing ice behaviour during a glacial cycle. The paper focuses on the partly marine-based Pleistocene British Ice Sheet in the Clyde basin. A three-dimensional computation of subsurface glacial sediment distribution is derived from 1260 borehole logs. Sediment distribution is linked to an empirically-based reconstruction of ice sheet evolution, permitting identification of distinctive phases of sedimentation. Maximum sediment mobilisation and till deposition (<0.04 m a -1 ) occurred during ice advance into the basin from adjacent uplands. Subglacial processes were influenced locally by the relative stiffness of pre-existing sediments, the permeability of the sub-till lithology, and topography; the resulting mean till thickness is 7.7 m with a high standard deviation of 7.0 m. In places, focused till deposition sealed pre-existing permeable substrates, promoting lower effective pressures. Sediment remobilisation by meltwater was a key process as ice margins retreated back through the basin, upon deglaciation.