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2 3 1 Journal: BOREAS 4 5 6 2 Manuscript version: Major Revision 7 8 3 9 10 4 Title: Submarine sediment and landform record of a palaeo-ice stream within the British-Irish Ice 11 5 Sheet. 12 13 14 6 Short title: Submarine sediment and landform record of the Minch palaeo-ice stream 15 16 7 Article Type: Research paper 17 18 8 Keywords: Pleistocene, palaeoglaciology, continental shelf, marine mapping, glacial bedforms 19 20 9 For Review Only 21 22 10 23 24 11 25 26 27 12 28 29 13 30 31 14 32 33 34 15 35 36 16 37 38 17 39 40 18 41 42 43 19 44 45 20 46 47 21 48 49 50 22 51 52 23 53 54 24 55 56 57 25 58 59 26 60 27 Boreas Page 2 of 43

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2 3 4 1 Submarine sediment and landform record of a palaeo-ice stream within the 5 6 2 British-Irish Ice Sheet 7 8 9 3 10 11 12 4 TOM BRADWELL & MARTYN STOKER 13 14 5 15 16 17 6 Bradwell, T. & Stoker, M.S. 2014: Submarine sediment and landform record of a palaeo-ice stream within the 18 7 British-Irish Ice Sheet. Boreas, ...-... 19 20 For Review Only 21 8 22 23 9 This paper examines marine geophysical and geological data, and new multibeam bathymetry data to describe 24 25 10 the Pleistocene sediment and landform record of a large ice stream system that drained ca. 3% of the entire 26 11 British-Irish Ice Sheet at its maximum extent. Starting on the outer continental shelf NW of Scotland we 27 12 describe the ice-stream terminus environment and depocentre on the outer shelf and continental slope; 28 13 sediment architecture and subglacial landforms on the mid-shelf and in a large marine embayment (the 29 14 Minch); moraines and grounding line features on the inner shelf and in the fjordic zone. We identify new soft- 30 31 15 bed (sediment) and hard-bed (bedrock) subglacial landform assemblages in the central and inner parts of the 32 16 Minch that confirm the spatial distribution, coherence and trajectory of a grounded fast-flowing ice sheet 33 17 corridor. These include strongly streamlined bedrock forms and megagrooves indicating a high degree of ice- 34 18 bed coupling in a zone of flow convergence associated with ice stream onset; and a downstream bedform 35 19 evolution (short drumlins to km-scale glacial lineations) suggesting an ice-flow velocity transition associated 36 37 20 with a bed substrate and roughness change in the ice stream trunk. Chronology is still lacking for the timing of 38 21 ice stream demise; however, the seismic stratigraphy, absence of moraines or grounding-line features, and 39 22 presence of well-preserved subglacial bedfoms and iceberg scours, combined with the landward deepening 40 23 bathymetry, all suggest frontal retreat in the Minch was probably rapid, via widespread calving, before 41 24 stabilization in the nearshore zone. Large moraine complexes recording a coherent, apparently long-lived, ice- 42 43 25 sheet margin position only 5-15 km offshore strongly support this model. Reconstructed ice-discharge values -1 44 26 for the Minch ice stream (12-20 Gt yr ) are comparable to high mass-flux ice streams today, underlining it as 45 27 an excellent palaeo-analogue for recent rapid change at the margins of the Greenland and West Antarctic Ice 46 28 Sheets. 47 29 48

49 30 50 31 Tom Bradwell ([email protected]), British Geological Survey, West Mains Road, Edinburgh, EH9 3LA, UK; Martyn 51 32 S. Stoker ([email protected]) British Geological Survey, West Mains Road, Edinburgh, EH9 3LA, UK. 52 33 53 54 34 55 56 57 35 58 59 36 60 37 Page 3 of 43 Boreas

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2 3 1 Ice streams are fast-flowing, high mass-flux corridors that discharge the bulk of an ice sheet’s mass from 4 5 2 land to ocean. They can extend deep into the heart of ice sheets and often consist of a complex 6 7 3 branching network of tributaries; changes in ice stream flow variability greatly influence ice sheet mass 8 9 4 balance (e.g. Bentley 1987; Bamber et al. 2000; Truffer & Echelmeyer 2003; Rignot et al. 2011). Many 10 5 contemporary ice streams are grounded below sea level and all terminate in marine settings wherein 11 12 6 large volumes of ice are discharged into the ocean via calving. Marine sectors of ice sheets and ice 13 14 7 streams therefore play a pivotal role in the interconnected ocean-cryosphere system with changes in ice 15 16 8 stream flux, meltwater delivery and ice sheet retreat potentially impacting the ocean thermohaline 17 9 balance and raising global sea levels (e.g. Alley & MacAyeal 1994; Bard et al. 1996; Bigg et al. 2012; 18 19 10 Deschamps et al. 2012). In lieu of recent analogues for the current rapid rates of ice sheet change, 20 For Review Only 21 11 researchers are drawn to analogous settings from the Pleistocene when continental ice sheets existed 22 12 in the Northern Hemisphere with ice streams grounded below, or terminating at, sea level (e.g. Stokes 23 24 13 & Clark 2001; Sejrup et al. 2003; Dowdeswell et al. 2008; Andreassen et al. 2008). Better understanding 25 26 14 of palaeo-ice streams, particularly marine-based systems, and their role in large-scale ocean- 27 28 15 atmosphere-cryosphere events (e.g. Heinrich events) will greatly inform our understanding of present- 29 16 day ice sheet behaviour. Although it is widely accepted that Pleistocene-Holocene analogue studies can 30 31 17 provide valuable inferences about longer term ice-sheet behaviour, well constrained palaeo-ice stream 32 33 18 examples are still surprisingly rare (cf. Rinterknecht et al. 2014). 34 35 19 Knowledge of submarine sediments and landforms laid down by ice streams is a rapidly developing field 36 37 20 spurred on by the fact that new high-resolution data acquisition techniques have revealed much about 38 39 21 the geomorphology of glaciated continental shelves (O Cofaigh 2012). For instance, it is only in the past 40 41 22 10 years with the mainstream use of high-resolution multibeam echosounder (MBES) bathymetry that 42 23 large areas of formerly glaciated seafloor have been mapped and interpreted in detail. The combination 43 44 24 of detailed digital surface models (MBES data) and sub-bottom 2D and 3D acoustic profiles presents a 45 46 25 powerful tool for the remote (non-invasive) interpretation of seabed landforms and sediments. 47 48 26 Although geological evidence of ice streaming in the offshore record is well known from seismo- 49 50 27 stratigraphic studies (e.g. Vorren & Laberg 1997; Solheim et al. 1998; Evans et al. 2009), the submarine 51 52 28 geomorphological footprint of palaeo-ice streams has rarely been examined from source to sink – 53 29 notable recent exceptions being the Bjørnøyrenna, Andfjørden/Malangsdjupet and Uummannaq ice 54 55 30 stream systems (Andreassen et al. 2008; Rydningen et al. 2013; Dowdeswell et al. 2014). Several major 56 57 31 ice streams have now been identified within the former British-Irish Ice Sheet (Stoker & Bradwell 2005; 58 59 32 Golledge & Stoker 2006; Graham et al. 2007; Chiverrell et al. 2012; Howe et al. 2012; Hughes et al. 60 Boreas Page 4 of 43

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2 3 1 2014); all would have terminated on the continental shelf, on what is now seabed. To date, none has 4 5 2 had the various elements of its sediment and landform record described. 6 7 8 3 This paper summarises the submarine sediment and landform record relating to a large Pleistocene ice 9 4 stream system that drained the NW sector of the British-Irish Ice Sheet (BIIS). Overlaying valuable new 10 11 5 multibeam bathymetry data on older marine geophysical and geological data from the NW UK 12 13 6 continental shelf we describe the main elements of the system – its terminus and depocentre on the 14 15 7 outer continental shelf and slope; sedimentation styles on the mid-shelf and in a large marine 16 8 embayment (the Minch); and retreat and grounding-line features in the nearshore and fjordic zone of 17 18 9 NW Scotland. This work collates previously published research (e.g. Fyfe et al. 1993; Stoker et al. 1993, 19 20 10 1994, 2006; Stoker 1995,For 2013; Stoker Review & Bradwell 2005; Bradwell Only et al. 2007, 2008a,b,c) and presents 21 22 11 important new data highlighting the range of submarine glacial landforms preserved in this area and 23 12 their implications for understanding ice stream dynamics and the wider palaeoglaciology of this 24 25 13 important sector of the last BIIS. 26 27 28 14 29 30 15 Geographical and Quaternary geological setting 31 32 33 16 The geography of NW Scotland and the adjacent UK continental shelf has been shaped by successive 34 17 glaciations since at least ~0.44 Ma and probably since ~2.6 Ma BP (Gordon & Sutherland 1993; Thierens 35 36 18 et al. 2012). The result is over 1 km of vertical relief with strongly dissected mountainous terrain, u- 37 38 19 shaped valleys, overdeepened offshore rock basins and a deeply indented fjordic coastline (e.g. as seen 39 40 20 in Skye, Harris, Wester Ross, and NW Sutherland). The deeper waters of the Minch, a wide structurally 41 21 controlled (half-graben) bathymetric trough, extend onto the continental shelf as a broad, 30-40 km 42 43 22 wide, NW-trending channel, broadly defined by the 100 m isobath (Fig. 1). The continental shelf break is 44 45 23 defined in this region approximately by the 200 m isobath beyond which the continental slope descends 46 24 to over 1000 m at an angle of 2-4° on the upper slope, decreasing in angle downslope. The seabed 47 48 25 topography of the Hebrides Shelf and the Minch is highly variable, with numerous overdeepened 49 50 26 basins, shallow banks and steep-sided islands; hence, the overall shelf bathymetry does not simply 51 52 27 deepen with distance offshore. Of particular note are major nearshore bathymetric deeps on the inner 53 28 shelf, particularly east of Lewis and Harris, east of Skye, around the Summer Isles, and in the sea lochs 54 55 29 (fjords) where present-day water depths >200 m, and exceptionally >300 m, are encountered (Fig. 1). 56 57 58 30 The Cenozoic sediment sequence and architecture of the Hebrides Shelf and the Minch was the focus of 59 31 geophysical and marine geological investigations by the British Geological Survey (BGS) in the 1970s and 60 32 1980s, from which a Quaternary seismo-stratigraphic framework was established for the continental Page 5 of 43 Boreas

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2 3 1 margin offshore NW Scotland (Fyfe et al. 1993; Stoker et al. 1993). Numerous seismic reflection profiles 4 5 2 across the area reveal an extensive but irregular glacial (Pleistocene) unconformity with erosional 6 7 3 troughs cut into the Mesozoic and Cenozoic strata alongside isolated upstanding masses of resistant 8 9 4 basement or Tertiary volcanic rocks (such as the North Rona High, Shiant Islands, etc.) (Fig. 1). 10 5 Generally, the Pleistocene geology on the continental shelf takes the form of a landward stacking 11 12 6 succession of sediments younging from the shelf edge to the fjords (Fyfe et al. 1993; Stoker et al. 1993). 13 14 7 However, locally thicker glacigenic sediment sequences are seen on the shelf in overdeepened basins 15 16 8 where Pleistocene sediment thickness exceeds 100 m in places (Fig. 2). Key BGS boreholes on the 17 9 Hebrides Shelf penetrate stacked sequences of glacial diamictons (probably subglacial till), thick 18 19 10 glaciomarine deposits, and coarse-grained proglacial and morainic sediments (Fyfe et al. 1993; Stoker et 20 For Review Only 21 11 al. 1993). Using a range of marine geophysical data from around NW Scotland, Stoker & Bradwell (2005) 22 12 identified strongly parallel and highly elongate ridge-groove structures – interpreted as mega-scale 23 24 13 glacial lineations (MSGLS) – on the present-day seabed which, when taken together with the wider 25 26 14 geomorphological evidence, strongly suggested the presence of a palaeo-ice stream in the Minch at 27 28 15 times of extensive ice sheet glaciation. Reappraisal of seismic-profile data across the region also showed 29 16 MSGL surfaces to be preserved at various levels within the Quaternary succession (Fig. 2). Subsequently, 30 31 17 onshore digital surface models were used to better define the flow trajectory and catchment geometry 32 33 18 of this palaeo-ice stream (Bradwell et al. 2007). More recent terrestrial fieldwork work, supported by 34 35 19 Be-10 cosmogenic exposure analyses, has examined the surface profile and former thermal structure of 36 20 the ice sheet in NW Scotland and refined the location of certain ice stream tributaries (Bradwell et al. 37 38 21 2008b; Fabel et al. 2012; Bradwell 2013; Mathers 2014). 39 40 41 22 Well preserved, Late Pleistocene glacial sediment and landform assemblages in and around the Minch 42 23 and on the continental shelf offshore NW Scotland have been interpreted to represent the record of an 43 44 24 ice stream system within the former British-Irish Ice Sheet (Stoker & Bradwell 2005; Bradwell et al. 45 46 25 2007). At times of maximum Pleistocene glaciation, this ice stream flowed in a wide cross-shelf trough 47 26 depositing sediment in a large trough-mouth fan on the continental shelf slope – the Sula Sgeir Fan 48 49 27 (Stoker et al. 1993). At its maximum the trunk of the ice stream had a length of over 200 km and a width 50 51 28 of 40-50 km, and like most modern ice streams consisted of several (up to 10) convergent tributaries 52 53 29 merging into a single central flow corridor. An approximate ice-stream drainage area at times of 54 30 maximum configuration (since 0.5 Ma) is estimated to have been ca. 15,000-20,000 km2, equivalent to 55 56 31 ca. 3% of the total ice sheet area – although it is likely that this maximum configuration was atypical and 57 2 58 32 that drainage of a smaller area (<10,000 km ) was probably more typical (Stoker and Bradwell 2005; 59 60 33 Bradwell et al. 2007). Boreas Page 6 of 43

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2 3 1 It is thought that the last expansive ice sheet to glaciate the British Isles (MIS 2-3; Late Weichselian) 4 5 2 reached the continental shelf edge to the south of the Outer Hebrides, at the Barra Fan, and to the 6 7 3 north on the West Shetland Shelf (e.g. Stoker & Holmes 1991; Stoker et al. 1993; Peck et al. 2007; 8 9 4 Bradwell et al. 2008a). However, chronological constraint is weak and it is not currently known if (or 10 5 when) the Late Weichselian ice-sheet margin reached the continental shelf edge to the NW of Lewis. In 11 12 6 fact, until relatively recently, considerable debate centred on whether parts of northern Lewis had even 13 14 7 been glaciated during MIS 2-3 (e.g. Gordon & Sutherland 1993; Hall et al. 2003). As such, it cannot 15 16 8 presently be demonstrated that the Minch ice stream reached the shelf edge and directly fed the Sula 17 9 Sgeir Fan during the last glacial cycle (MIS 2-3) (Stoker & Bradwell 2005), although there is strong 18 19 10 evidence from cosmogenic 10Be analyses on glacially transported boulders that the ice sheet margin 20 For Review Only 21 11 extended at least as far as North Rona ~25-27 ka BP, ~40 km inboard of the shelfbreak (Everest et al. 22 12 2013). Seismic stratigraphy on the outer shelf and slope to the W of North Rona strongly suggest that 23 24 13 the ice sheet margin did extend to the shelfbreak earlier in the Mid to Late Pleistocene (prior to MIS 10) 25 26 14 (Stoker et al. 1993, 1994; Stoker & Bradwell 2005); nevertheless, the presence of floating ice shelves 27 28 15 during later more restricted glaciations (e.g. MIS2-3) cannot be ruled out. 29 30 16 31 32 33 17 Data and methods 34 35 18 A range of marine geological and geophysical datasets were used in this work. Chiefly, high-resolution 36 37 19 digital bathymetric multibeam echosounder survey data (MBES) in combination with 2D seismic profile 38 39 20 data were used to map the submarine glacial geomorphology and shallow sedimentary architecture of 40 41 21 selected areas of the continental shelf around NW Scotland. 42 43 22 The multibeam echosounder data from around NW Scotland was collected between 2006-2012 (Fig. 1) 44 45 23 by various survey vessels under contract to the Maritime & Coastguard Agency (MCA), and forms part of 46 47 24 an ongoing UK-wide MBES bathymetric survey programme conducted on behalf of the UK Hydrographic 48 25 Office (UKHO). In addition MBES data collected by the British Geological Survey (BGS) in 2005 was also 49 50 26 used. All MBES data are collected to UKHO survey standard using high-precision GNSS data with a 51 52 27 positional accuracy of <0.5 m in xyz and a resolution of <1 m. The final output data is made available at 53 28 8 m cell-size resolution or better. The raw bathymetric data were processed and gridded at the British 54 55 29 Geological Survey. Data manipulation and visualisation was conducted in Fledermaus software, allowing 56 57 30 full 3-D interrogation and enhancement of bathymetric surfaces. High resolution geotiffs of the surface 58 59 31 elevation data layers were imported into ArcGIS 10.1 where geomorphological features were digitising 60 32 manually using bathymetric hillshade and slope models to aid accuracy. Specific details relating to MBES Page 7 of 43 Boreas

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2 3 1 data acquisition are not detailed here but are available in the Reports of Survey, available on request 4 5 2 from MCA/UKHO (or BGS/NERC). 6 7 8 3 Single-channel seismic profile data collected by BGS between 1968 and 1985, part of a geological 9 4 mapping programme of the UK continental shelf, were used to map the shallow sub-surface geology. 10 11 5 Lower frequency sparker data (<1 kHz) is well suited for characterising shallow to intermediate depth 12 13 6 (Quaternary) submarine sediments and identifying the sediment/bedrock boundary. Seismic 14 15 7 penetration varies depending on substrate but is generally in excess of 50 m, with an optimum vertical 16 8 resolution of ~2-5m. High resolution tiffs of the seismic data, scanned from original records held at 17 18 9 BGS, were imported into image processing software for image enhancement; seismic interpretations 19 20 10 were manually digitisedFor on screen .Review This novel use of high Only-resolution bathymetric surface models 21 22 11 overlaid on seismic sub-bottom data allows characterization and interpretation of the seabed deposits 23 12 and the sub-seabed stratigraphic architecture, which in turn provides important insight on the style, 24 25 13 process history and chronology of depositional or erosional environment. 26 27 28 14 In addition to these main datasets, singlebeam echosounder data, from the global dataset, managed 29 15 and compiled by Olex AS (Trondheim), were used where multibeam data are lacking. The singlebeam 30 31 16 data generally have a positional accuracy of 10 m or less and vertical resolution of 0.5 to 1 m, but this 32 33 17 depends on sounding density which varies considerably across the study area (Fig. 1). Submarine 34 35 18 sediment and landform interpretations in this study also incorporate other published and unpublished 36 19 geological information, principally marine boreholes, shallow cores, and seabed grab samples from 37 38 20 around NW Scotland held by BGS-NERC, as well as published BGS Quaternary geological maps and 39 40 21 Offshore Regional Reports. 41 42 22 43 44 45 23 Results: Submarine glacial landforms and sediments 46 47 24 Starting at the continental slope and tracking back inshore, the following sections describe the main 48 49 25 submarine landform evidence and general sedimentary architecture relating to the large palaeo-ice 50 51 26 stream that periodically drained the NW sector of the Pleistocene BIIS. Descriptions and interpretations 52 53 27 are kept separate, with reference to previously published material where appropriate. 54 55 28 56 57 29 Continental slope – Ice stream terminus and depocentre 58 59 2 60 30 Description. – The Sula Sgeir Fan is a large 3750 km wedge-shaped sediment package on the 31 continental slope, at the termination of a wide bathymetrically deeper pathway or cross-shelf trough. Boreas Page 8 of 43

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2 3 1 The fan is steepest (3-4°) near the shelf break at 200-400 m water depth, and decreases in gradient with 4 5 2 distance downslope. The fan apron extends beyond the continental slope into water depths of 1200 m. 6 7 3 The upper 100-200 m of this large sediment accumulation is stratigraphically within the Upper Macleod 8 9 4 Formation (Stoker et al. 1993, 2011) and overlies a distinct regionally extensive unconformity that 10 5 mirrors the slope of the overlying sediment package (Fig.2). The Upper Macleod Fm comprises laterally 11 12 6 continuous slope-parallel sheet-like clinoforms with acoustically transparent or chaotic internal 13 14 7 reflection character – typical of diamictons and poorly sorted mass-flow sediments (Stoker et al. 1993; 15 16 8 Stoker 1995). A short core (BGS 59-08/42) from the upper slope section (within the Upper Macleod Fm) 17 9 recovered 2.5 m of very poorly sorted matrix-supported diamictic sediment with a highly variable clast 18 19 10 content in terms of its lithology, shape and size. Rare shell fragments as well as occasional shell and 20 For Review Only 21 11 sand layers were also noted within the diamict units (Stoker 1990). Although MBES data is lacking for 22 12 the Sula Sgeir Fan, good singlebeam echosounder bathymetry (Olex dataset) and seismic first-return 23 24 13 data show a network of large, subparallel, occasionally cross-cutting channels or gullies and overlapping 25 26 14 elongate lobes on the slope surface (Fig. 2). Unfortunately data coverage is poor on the steeper upper- 27 28 15 slope section, hence the full length of the gullies cannot be determined. However, numerous gullies 29 16 (n=16) can be traced descending the whole imaged slope from top to bottom, a distance of ca. 20 km. 30 31 17 These gullies are relatively wide compared to their depth but have sharply defined margins and flat- 32 33 18 bottomed box-shaped cross-sections. Typically gullies are spaced at 2-5 km intervals. Surface lobes are 34 35 19 coalescent or overlapping in plan morphology and increase in width downslope. Lobes generally start 36 20 on the mid slope within the well-imaged portion of the fan, and terminate at the slope foot where the 37 38 21 surface gradient falls appreciably. On the mid-slope, lobes are typically interspersed with the larger 39 40 22 gulleys. Lobe lengths range from 3 to 15 km; maximum widths at the slope foot range from 2 to 8 km 41 23 (Fig. 2). 42 43 44 24 Interpretation. – The location of this large discrete prograding sediment wedge at the mouth of a wide 45 46 25 cross-shelf trough marked by several overdeepened, glacial-sediment-filled, basins is entirely consistent 47 26 with its designation as an ice stream-fed trough-mouth fan (Vorren & Laberg 1997; Stoker 1995; Stoker 48 49 27 & Bradwell 2005). The Sula Sgeir Fan represents one of a number of Plio-Pleistocene prograding wedges 50 51 28 on the UK slope from the Barra Fan in the S to the Norwegian Channel an in the N (Sejrup et al. 2005). 52 53 29 Morphologically similar trough-mouth fans are found at the termini of well-established, formerly more 54 30 extensive, ice streams in contemporary settings (e.g. Greenland margin; Ó Cofaigh et al. 2003; 55 56 31 Dowdeswell et al. 2014). The slope-wide diamict-dominated packages are thought to be the product of 57 58 32 glacier-fed debris flows during the Mid-Late Pleistocene; and probably represent re-deposited 59 60 33 glaciomarine sediments from the outer shelf (Stoker 1990, 1995). Similarities between the lobe and 34 gully surface morphology of the Sula Sgeir Fan and other well-studied trough-mouth fans, such as those Page 9 of 43 Boreas

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2 3 1 on the Antarctic, Greenlandic and Norwegian shelf margins, indicate strong process linkages (e.g. 4 5 2 Dowdeswell et al. 2008; Gales et al. 2014). These glacially fed debris lobes and gullies represent 6 7 3 important pathways for sediment transfer beyond the continental shelf edge. 8 9 10 4 11 12 5 Outer- and mid-shelf moraines 13 14 15 6 Description. – The submarine landforms on the outer and mid-shelf, north of Lewis, are not currently 16 7 covered by MBES data, hence only a relatively low-resolution picture of submarine geomorphology 17 18 8 exists from singlebeam (Olex) echosounder data. On the outermost shelf at ca. 7°W, in water depths of 19 20 9 ~200 m, are a number ofFor long broad Reviewsediment ridges are up Only to 20-30 m high and 2 to 4.5 km wide with 21 22 10 low-angle slopes and wide poorly defined crestlines. BGS seismic profiles and echosounder bathymetry 23 11 show the longest ridge, close to the shelfbreak, to be in excess of 50 km (Stoker et al. 1993; Bradwell et 24 25 12 al. 2008a). Seismo-stratigraphically the broad ridges belong to the Mid to Late Pleistocene MacDonald 26 27 13 Formation (Table 1); although their precise age remains uncertain (Stoker & Holmes 1991; Stoker et al. 28 14 1993). A clear stratigraphical connection with the Sula Sgeir Fan has been established from seismic 29 30 15 profiles (Fig. 2), with the sediments on the distal flank of the outermost MacDonald Fm moraine 31 32 16 interdigitating with the uppermost mass-flow deposits (Upper Macleod Fm) on the trough-mouth fan 33 34 17 (Stoker 1990, 1995; Stoker et al. 1993). 35 36 18 Further inshore, on the mid-shelf ca. 6°W, two large broad arcuate ridges traverse the cross-shelf 37 38 19 trough north of Lewis in water depths of 100-120 m. Mapped by Bradwell & Stoker (in press), the outer 39 40 20 ridge (North Lewis Ridge) ranges in width from 2-5 km and has a maximum height of 20 m above the 41 21 surrounding sea floor. This discontinuous ridge can be traced for 40-50 km arcing round from offshore 42 43 22 northernmost Lewis to the Sula Sgeir-North Rona bedrock high in the north. Bathymetric cross profiles 44 45 23 show no preferred slope asymmetry. Seismostratigraphic correlations place the ridge within the Late 46 47 24 Pleistocene Elspeth Fm (Stoker et al., 1993, 2011) (Table 1). The inner broad ridge (North Minch Ridge) 48 25 is a well-imaged feature in the singlebeam data trending generally NE-SW between the North Lewis and 49 50 26 North Minch basins (Fig. 2). This continuous broad ridge is 20-25 km long, 10-20 m high and 3-6 km 51 52 27 wide, with linear and curvilinear sections in planform. Singlebeam echosounder data show that the 53 28 North Minch Ridge is asymmetrical in cross profile with a steeper NW-facing (distal) slope and a flattish 54 55 29 top (Bradwell & Stoker, in press). BGS seismic profiles show the ridge to be comprised of a ~30 m thick 56 57 30 acoustically structureless unit (Jean Fm), typical of diamicton, overlain by a strongly layered 58 59 31 conformable sediment package (Morag Fm) that drapes the underlying Quaternary topography (Fig. 2). 60 32 BGS borehole 77/08 on the flank of the North Minch Ridge recovered >20 m of stiff pebbly clay with Boreas Page 10 of 43

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2 3 1 arctic microfauna (Fyfe et al. 1993). Inshore of the North Minch Ridge, the Pleistocene geology on the 4 5 2 continental shelf thickens in the North Minch Basin where the Pre-Quaternary surface forms a broad 6 7 3 depression. BGS seismic profiles show a thick (<50 m) and extensive, generally acoustically transparent, 8 9 4 sediment unit at or close to seabed, across much of the central part of the Minch south of 58°30’N (Fyfe 10 5 et al. 1993). Defined as the Sheena Fm (Stoker et al. 2011) this unit has been proved in boreholes (e.g. 11 12 6 BGS BH 76/55) to be a dark grey soft clay with isolated dropstone clasts and a microfauna indicating 13 14 7 very cold, less than fully marine conditions (Fyfe et al. 1993). Although no absolute chronology currently 15 16 8 exists, the Sheena Fm can be seen in seismic profiles onlapping the full-glacial (MIS 2) Morag Fm, and 17 9 overlying the Lateglacial Annie Fm, dated in BGS Borehole 78/04 by radiocarbon assay (Graham et al. 18 19 10 1990). 20 For Review Only 21 22 11 23 24 12 Interpretation. – The large sediment ridges on the outermost shelf have been interpreted as moraines 25 26 13 (morainal banks) by Stoker & Holmes (1991) – and are primary evidence of ice-sheet glaciation on the 27 28 14 UK continental shelf NW of Scotland. Although the moraines are currently undated, seismostratigraphic 29 15 relationships between these morainic (ice-contact) diamictons and ice-proximal mass-flow sediments 30 31 16 on the upper part of the Sula Sgeir fan suggest coevality, and place them in the Mid to Late Pleistocene 32 33 17 (Stoker et al. 1993). Their position on the shelf edge indicates they were laid down during a maximal 34 35 18 glacial configuration, probably in a period of greatly lowered eustatic sea level (ca. 120 m below 36 19 present) (Lambeck 1993; Peltier & Fairbanks 2006). Based on their seismostratigraphical properties, 37 38 20 morphology and bathymetric setting (200 m below sea level, in a cross-shelf trough) we interpret the 39 40 21 ridges as large subaqueous moraines or grounding-zone features, similar to those seen on the West 41 22 Greenland Shelf (O Cofaigh et al. 2003, 2013; Dowdeswell et al. 2008, 2014), formed at the terminus of 42 43 23 an ice stream within a pre-Late Weichselian ice sheet (Bradwell & Stoker in press). 44 45 46 24 The large North Lewis and North Minch Ridges on the mid shelf are also interpreted as ice stream end 47 25 moraines or grounding-zone features based on their morphological and seismic affinities with ice- 48 49 26 marginal features mapped elsewhere (e.g. Sejrup et al. 2005, 2014; O Cofaigh et al. 2013; Dowdeswell 50 51 27 et al. 2014). Seismo-stratigraphic relationships (between Morag, Jean and Elspeth Fms (Fig. 2)) place 52 53 28 these moraines within the Late Weichselian – and considerably younger than those limits on the outer 54 29 shelf (Bradwell & Stoker in press). This chronology is supported by cosmogenic exposure-age analyses 55 56 30 from North Rona which suggest the last British-Irish Ice Sheet reached at least this far ca. 25 ka BP 57 58 31 (Everest et al. 2013). The location of these mid-shelf moraines in present-day water depths of 100-120 59 60 32 m suggest that they may have been laid down at or close to the contemporary sea level, with modelled 33 sea levels at 20-25 ka BP around 100 m below present on the mid to outer shelf (Lambeck 1993). The Page 11 of 43 Boreas

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2 3 1 implication is that the ice sheet margin at this time was probably grounded below sea level only in the 4 5 2 cross-shelf trough to the north of Lewis, and may have been grounded above sea level on low-lying 6 7 3 islands and exposed banks elsewhere on the mid-shelf (Bradwell & Stoker in press). Further inshore the 8 9 4 Late Weichselian sequence thickens in the North Minch Basin – an important mid-ice stream 10 5 depocentre. Seismic profiles show that much of the north Minch is draped by a 20-50 m thick 11 12 6 glaciomarine sediment package (Sheena Fm, Fig. 2) consistent with deposition in a large marine 13 14 7 embayment during ice sheet deglaciation, probably as relative sea level rose (Stoker et al. 1993). 15 16 8 Furthermore, the overall Pleistocene sediment architecture across the mid shelf and in the north Minch, 17 9 mapped from 2-D seismic records (Fyfe et al. 1993; Stoker et al. 1993), highlights a landward-stacking 18 19 10 succession of glacial sediments younging from the outer to inner shelf (Fig. 2) – typical of an ice stream 20 For Review Only 21 11 system confined to a cross-shelf trough (e.g. Sejrup et al. 2003; Andreassen et al. 2008; Rydningen et al. 22 12 2013; Dowdeswell et al. 2014). Interestingly, the stratigraphy and sedimentology of the uppermost 23 24 13 glacigenic units in the North Minch Basin are entirely consistent with ice sheet retreat from a stable 25 26 14 position (partly grounded above sea level), on a mid-shelf high near North Rona, into deepening water 27 28 15 where evidence of ice-sheet grounding is notably absent. 29 30 16 31 32 33 17 Inner shelf – exposed ice stream bed (transitional hard-to-soft bed assemblage) 34 35 18 Description. – Newly acquired MBES data from the central Minch (Fig. 3), covering 120 km2, shows a 36 37 19 diverse range of well-preserved Pleistocene glacial landforms in an area of discontinuous sediment 38 39 20 cover around a broad bedrock high (East Shiant Bank) in present-day water depths of 40-70 m. In the 40 41 21 southern half of the area numerous (>50) elongate streamlined mounds occur with equant, elliptical or 42 22 tapering planform outlines and broad poorly defined crestlines (Fig. 3). The mounds range in length 43 44 23 from 250 to 1300 m, and in width generally from 200 to 400 m. Elongation ratios (length/width) are 45 46 24 typically between 1.5 and 4. The seabed mounds are relatively low-elevation features ranging in height 47 25 from 5 to 15 m and can attain their maximum height at their any point along their length. In the 48 49 26 northern half of the area (north of 58°01’N) equant low-elongation seabed mounds are rare or absent. 50 51 27 Most of the sediment landforms in this area are linear narrow ridges with streamlined tapering 52 53 28 planforms and long profiles. These ridges range in length from 400 m to 1500 m, and in width generally 54 29 from 100 m to 300 m. Elongation ratios range from 4 to 14. Maximum ridge heights vary from 3 to 15 55 56 30 m; most ridges show a distinct long profile decrease in height. The seabed ridges in the extreme north 57 58 31 of the image appear to have bedrock at their highest point (southern ends), but are only partially 59 60 32 captured by the MBES data (Fig. 3). Other less pronounced ridge and groove features occur in the 33 northern part of the image. They have subtle seabed expression with typical amplitudes of only 3-5 m Boreas Page 12 of 43

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2 3 1 from crest to trough. These lineations are typically 1 to 2 km in length. Where Pleistocene/Holocene 4 5 2 sediment cover is patchy or absent, across ca. ~30% of the image, bedrock is observed at seabed – 6 7 3 probably Proterozoic sandstone, as proved in a nearby BGS borehole (Fyfe et al. 1993). Many bedrock 8 9 4 forms/outcrops have a streamlined geomorphological expression with an elongate teardrop-shaped 10 5 planform and tapering (N-S) height profile. 11 12 13 6 In the south, the long axis of these submarine landforms is generally orientated N-S, with a deviation of 14 15 7 10° either side of the mean; further north the long axes trend clearly swings to a NNE-SSW direction. 16 8 The surrounding streamlined bedrock also reflects this, with a swing from generally N-S orientated 17 18 9 forms in the south to NNE in the north of the image. BGS geophysical sub-bottom profiles across the 19 20 10 central area of landformsFor show the Review presence, absence and Only stratigraphic relationships of Quaternary 21 22 11 sediments (Fig. 4). Seismic line 30 traverses four elongate mounds (A-D) perpendicular to their long 23 12 axes and reveals a simple sediment stratigraphy with acoustically chaotic units, interpreted as glacial 24 25 13 diamicton, forming the bulk of the features. Across much of this area Quaternary sediment is thin (<20 26 27 14 m) and locally discontinuous; but where thickest, acoustically chaotic sediments overlie a strong 28 15 irregular reflector, mapped elsewhere as a glacially eroded bedrock surface (Fyfe et al. 1993). Marine 29 30 16 geophysical data from across the East Shiant Bank demonstrate that the seabed in this area consists of a 31 32 17 patchwork of wholly sediment, wholly bedrock and hybrid (bedrock-cored) landforms (Fig. 4). The use 33 34 18 of seismic data and high-resolution surface models together, in this way, ensures that the important 35 19 distinction between genetically different but morphologically similar landforms can be resolved. 36 37 38 20 Interpretation. – Collectively, the seabed landforms are interpreted as a single subglacial bedform 39 40 21 assemblage, similar to others in glaciated shelf settings (e.g. O Cofaigh et al. 2002; Shaw et al. 2006; 41 22 Rydningen et al. 2013). The MBES data shows smaller more equant drumlins in the south (elongation 42 43 23 ratios <2) becoming longer and narrower to the north, and grading into highly streamlined crag-and- 44 45 24 tails and glacial lineations (elongation ratios >8). We interpret the whole landform assemblage around 46 47 25 the East Shiant Bank to have formed beneath a grounded relatively fast-flowing ice sheet corridor and 48 26 therefore to be primary evidence of the palaeo-ice-flow trajectory at the bed of the Minch palaeo-ice 49 50 27 stream. Drumlins are thought to form by subglacial sediment accretion and/or sediment deformation 51 52 28 (Benn & Evans, 2010) and are primary evidence of flow instability near the ice/bed interface (Hindmarsh 53 29 1998; Fowler 2000; Clark 2010). Although the initiation and formation of drumlins is still largely 54 55 30 unknown, a marked change in physical properties of the bed substrate may be a governing factor 56 57 31 (Greenwood & Clark 2010; Phillips et al. 2010). We interpret the strong bedform attenuation across East 58 59 32 Shiant Bank – with increasing elongation ratios from <2 to >10, alongside the presence of parallel 60 33 streamlined glacial lineations – as the signature of a marked ice velocity increase from N to S, probably Page 13 of 43 Boreas

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2 3 1 relating to decreased basal drag and weaker ice-bed coupling as the ice stream transitioned from rough 4 5 2 (high-drag) bedrock to smooth (low-drag) predominantly soft-sediment (Figs 3, 4). We suggest this 6 7 3 landform assemblage is typical of transitional settings between glaciologically ‘hard-bed’ and ‘soft-bed’ 8 9 4 environments beneath ice streams. 10 11 5 12 13 14 6 Inner shelf – exposed ice stream bed (hard-bed assemblage) 15 16 7 Description. – Approximately 30 km to the SW of the East Shiant Bank, in the vicinity of the Shiant 17 18 8 Islands – the subaerial expression of a subsea Tertiary igneous intrusion – further submarine geological 19 20 9 evidence of ice sheet glaciationFor in theReview Minch is abundant. OnlyNewly acquired MBES data from SE of the 21 22 10 Shiant Islands at ca. 57°50’N (Fig. 1), shows an assemblage of streamlined crags and well-preserved 23 11 strongly directional landforms in an area of predominantly bedrock seabed ranging from 30-150 m 24 25 12 present-day water depths (Fig. 5). This region of seabed is characterised by its rugged relatively high- 26 27 13 relief bathymetry with numerous upstanding rock masses, some up to 40 m high, separating areas of 28 14 deeper smoother seabed with thin discontinuous sediment cover. Detailed mapping shows the 29 30 15 upstanding bedrock masses, clearly distinguishable on the MBES data, are generally elongate or 31 32 16 teardrop shaped in plan, with smooth flanks and occasionally grooved upper surfaces. Many of the 33 34 17 larger bedrock crags possess narrow tail-like ridges with streamlined tapering planforms and long 35 18 profiles (Fig. 5). These streamlined ‘tails’ range in length from 200 m to 1000 m, and are typically less 36 37 19 than 100 m wide; long axis orientations fall within 30 degrees east or west of N. Elongation ratios of 38 39 20 individual bedforms range from <2 to 8. In the central part of the area, in deeper waters to the north of 40 41 21 the main bedrock ridge (BP), the seabed displays a distinctly corrugated form with long straight, highly 42 22 parallel, ridges and grooves trending in a NNE to N direction and forming a convergent pattern (Fig. 5). 43 44 23 Crest-to-crest ridge spacings (equivalent to maximum groove widths) vary from 30 to 200 m, with 45 46 24 groove lengths (and ridge lengths) ranging from 300 to 1500 m. The ridge-groove forms are all 47 25 relatively low-relief subdued features typically <5 m in height (or depth). The ridges commonly attain 48 49 26 maximum height near their southern end; the grooves can attain maximum depth at any point along 50 51 27 their length. 52 53 28 BGS geophysical sub-bottom profiles across these features show the presence, absence and thickness of 54 55 29 Quaternary sediment cover in this area. Seismic line M13 shows a simple sediment stratigraphy in the 56 57 30 upper part of the seismic profile, with acoustically layered sediment discontinuously draping an 58 59 31 acoustically chaotic unit which rests unconformably on bedrock (Fig. 5). The thin upper unit is probably 60 32 Postglacial marine sediment. The lower sediment unit, probably glacial diamicton, thickens in the lee of Boreas Page 14 of 43

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2 3 1 a bedrock bump (large crag-and-tail) and thins or disappears in the area of grooved terrain. Analysis of 4 5 2 the seismic data coupled with the high-resolution digital surface (MBES) model allows areas of bedrock 6 7 3 at seabed to be spatially mapped with a moderate degree of confidence (Fig. 5). Interestingly, the main 8 9 4 area of ridge-groove terrain occurs in an area where Quaternary deposits are absent or very thin (below 10 5 the level of seismic resolution). Quaternary sediments are also notably absent from the pronounced 11 12 6 bedrock highs (StBH, BP; Fig. 5). Fields of discontinuous small transverse ridges, interpreted as 13 14 7 recessional moraines, occur sporadically across the multibeam image, typically in association with 15 16 8 pronounced highs (Fig. 5). (See ice-stream retreat features (below) for more interpretation.) 17 18 9 Interpretation. – The whole seabed geomorphology SE of the Shiant Islands is strongly reminiscent of 19 20 10 streamlined ‘hard-bed’ landscapesFor seenReview within palaeo-ice Only stream tracks in terrestrial settings (e.g. 21 22 11 Everest et al. 2005; Hughes et al. 2010; Eyles 2012). The strongly aligned submarine landforms are 23 12 interpreted as a single subglacial landform assemblage, similar to others seen in powerfully glaciated 24 25 13 submarine settings (e.g. O Cofaigh et al. 2002; Shaw et al. 2006; Dowdeswell et al. 2014). The 26 27 14 widespread presence of bedrock at seabed and relative absence of glacial deposits indicate that this is a 28 15 predominantly hard-bed landform assemblage. By definition therefore, the medium- and large-scale 29 30 16 streamlined forms are erosional bedforms; ranging from small- to large-scale crag-and-tails, streamlined 31 32 17 megagrooves, flutings, and streamlined submarine hills. Working around Antarctica, Wellner et al. 33 34 18 (2001) and O Cofaigh et al. (2002) related highly streamlined bedrock forms like these with the onset or 35 19 upper reaches of ice stream flow. In terrestrial settings, large-scale bedrock flutings and megagrooves 36 37 20 have been associated with the onset of ice streaming (Bradwell 2005, 2013; Bradwell et al. 2008b; Eyles 38 39 21 2012) and probably relate to a zone of increased sliding velocities in response to rapidly falling basal 40 41 22 shear stresses and weakened ice-bed coupling (Benn & Evans 2010). The absence of a continuous 42 23 sediment cover (i.e. a soft sedimentary bed) probably precludes full ice stream velocities in this part of 43 44 24 the inner shelf. However, the presence of highly elongate, strongly ice-flow-directional, bedrock forms 45 46 25 with convergent morphology strongly suggest flow confluence within the onset zone of an ice stream. 47 48 26 These observations of palaeo-ice-flow trajectory, in addition to the large broadly N-S flow-directed 49 50 27 bedforms around East Shiant Bank, support the earlier findings of Stoker & Bradwell (2005), and 51 52 28 confirm that a ~40-km-wide coherent flow corridor within the British-Irish Ice Sheet occupied the Minch 53 54 29 at certain intervals during the Late Pleistocene. 55 56 30 57 58 59 31 Inner shelf – ice-stream retreat features 60 32 Description. – Very few features relating to the style and rate of ice stream retreat have been reported Page 15 of 43 Boreas

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2 3 1 from the seabed in the northern Minch. All the available bathymetric and geophysical data show the 4 5 2 region of seabed from the North Minch ridge (58°36’N) to the East Shiant Bank (58°00’N) to be 6 7 3 unusually smooth and featureless. By contrast, conspicuous subaqueous moraines have been identified 8 9 4 near the present-day coastline of mainland NW Scotland, fronting the main fjords and deep bays in 10 5 present-day water depths of between 40 and 90 m (Stoker et al. 2006; Bradwell et al. 2008a; Bradwell & 11 12 6 Stoker in press). Singlebeam (Olex) bathymetry data show large arcuate ridges 10-20 km offshore the 13 14 7 Rubha Coigach and Stoer peninsulas and west of Cape Wrath (Bradwell et al. 2008c; Bradwell & Stoker 15 16 8 in press). The largest of these is the Eddrachillis Ridge which reaches 40 m in height, ~1000-2000 m in 17 9 width, and >40 km in length. The main ridge describes a broad arc through 90 degrees from near the N 18 19 10 tip of Stoer peninsula to make landfall in the vicinity of Sandwood Bay (Fig. 1). BGS seismic profile 20 For Review Only 21 11 85/04-03 crosses the Eddrachillis Ridge perpendicular to its crestline and shows a single thick generally 22 12 chaotic to acoustically transparent unit, typical of glacigenic diamicton, unconformably deposited on 23 24 13 underlying strata (Fig. 6). Five smaller inshore ridges, up to 10 m in height, comprised of the same 25 26 14 laterally continuous acoustic unit occur to the SE. The inner ridges have a very similar seismic expression 27 28 15 and morphology to submarine moraines in the Summer Isles region of NW Scotland (Stoker et al. 2006). 29 30 16 50 km to the south, new high-resolution MBES data show large ridge complexes at the mouths of Loch 31 32 17 Ewe, offshore Greenstone Point and around the Rubha Coigach headland, associated with, but 33 34 18 outboard of, seabed moraines identified in the Summer Isles region (Stoker et al. 2006). The main ridge 35 19 complex stretches from the mouth of Loch Ewe to near Greenstone Point and forms a long broad arc 36 37 20 open to the SE. New MBES data from the eastern Minch (MCA data), merged with existing MBES data 38 39 21 (Stoker et al. 2006), show the detailed geomorphology (Fig. 7). The largest ridges are comparable in size 40 41 22 to the Eddrachillis moraine complex – 1-2 km wide, up to 30 m high and at least 15 km long. BGS seismic 42 23 line 85/05-7 across the ridge complex at the mouth of Loch Ewe shows a single acoustically chaotic unit, 43 44 24 up to 40 m thick, unconformably overlying bedrock (Stoker et al. 2006). Superimposed on, and in places, 45 46 25 cross-cutting the main ridges, particularly around Greenstone Point and Rubha Coigach, are much 47 26 smaller delicate-looking discontinuous ridges 1-5 m high and <100 m wide (Fig. 7). In the shallows 48 49 27 around the Shiant Islands similar-sized small discontinuous transverse ridges are common between the 50 51 28 larger bedrock highs (Fig. 5). 52 53 54 29 55 56 30 Interpretation. – Based on their morphological and acoustic properties, we interpret all the larger 57 58 31 nearshore ridges as ice sheet end moraines – part of a large well-defined suite stretching almost 100 km 59 60 32 from the Eddrachillis Ridge via the Rubha Coigach moraines to the Loch Ewe-Greenstone Point Moraine 33 complex. Although MBES data of part of the intervening area is currently lacking, lower resolution Boreas Page 16 of 43

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2 3 1 echosounder data suggest a bathymetric connection between the Loch Ewe-Greenstone Point moraine 4 5 2 complex, the morphologically similar moraines around Rubha Coigach (Stoker et al. 2006) and those 6 7 3 further north. These large moraines in the eastern Minch are all substantial, well defined, morpho- 8 9 4 stratigraphically equivalent features in similar nearshore settings and water depths (Figs. 6, 7). The 10 5 moraine complexes indicate formation at the stable firmly grounded margin of marine-terminating ice- 11 12 6 sheet outlet lobes. The age of the Eddrachillis Ridge is not currently clear (Table 1); however this 13 14 7 substantial morainic deposit clearly overlies the Sheena Fm and may be laterally equivalent to the 15 16 8 Catriona Fm in the eastern Minch. Moreover, its thickness, well-preserved seabed expression and 17 9 superposed stratigraphic position strongly suggest a significant late-stage ice sheet advance or 18 19 10 prolonged stillstand. Morphological and stratigraphical evidence from the Loch Ewe–Greenstone-Point– 20 For Review Only 21 11 Rubha Coigach moraines also indicate a similar Late Weichselian ice-sheet terminus stillstand, prior to 22 12 the formation of the recessional moraine sequence in the Summer Isles, dated at ca. 15ka BP (Bradwell 23 24 13 et al., 2008c). 25 26 27 14 Superimposed on these large moraines are smaller ridges (Fig. 6), which we interpret as de Geer 28 15 moraines – laid down during retreat of a lightly grounded marine-terminating ice-sheet margin. Their 29 30 16 delicate and discontinuous nature is characteristic of a tidewater ice-front close to flotation, grounding 31 32 17 on bathymetric highs and floating in deeper water. Similar features have been described fronting 33 34 18 contemporary tidewater glaciers in West Greenland and Svalbard (Powell et al. 1996; Ottesen & 35 19 Dowdeswell, 2006). The superimposed relationship and cross-cutting geometry of the de Geer 36 37 20 moraines demonstrate that they are younger than the Loch Ewe–Greenstone Point moraine complex 38 39 21 (Stoker et al., 2009) and therefore probably relate to a separate later cycle of tidewater glacier 40 41 22 advance/retreat in the fjords of NW Scotland. 42 43 23 44 45 46 24 Inner shelf – Iceberg scours 47 48 25 Description. – In numerous places the seabed in the Minch is marked by fields of closely spaced, cross- 49 50 26 cutting, irregularly distributed furrows (Fig. 8). Most furrows are linear or curvilinear in plan form but 51 52 27 sinuous and vermicular forms are seen. Well-preserved furrows are particularly common on shallow 53 28 banks in the central part of the Minch. These occur within a narrow water-depth range; being abundant 54 55 29 between 70-90 m and absent in less than 50 m and more than 150 m water depth. Furrows generally 56 57 30 range in width from 50 to 200 m and are typically between 0.5 and 2 km long, although examples up to 58 59 31 4 km long are recorded (Fig. 8). Circular depressions or shallow pits (>2m) with similar width dimensions 60 32 are found interspersed with the furrows. Furrows are typically 1-2 m deep, with rare exceptions Page 17 of 43 Boreas

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2 3 1 exceeding 3 m in depth. Most have strongly v-shaped cross-profiles and many larger furrows have small 4 5 2 berms (<0.5 m high) (Fig. 8). Although their patterns can be irregular there is a general trend in furrow 6 7 3 orientation with most clustering between 000 and 045 (no directional trend implied). Furrows occur in 8 9 4 numerous places across the MBES data coverage area, but always in areas of soft-sediment at seabed. It 10 5 is worth noting that where furrows cut into soft-sediment banks they terminate abruptly at the edge of 11 12 6 the topographic feature. Similar curvilinear or sinuous, cross-cutting furrows and grooves have been 13 14 7 observed on sidescan sonar traces (Fig. 8) from the northern Minch and on the mid-shelf, especially in 15 16 8 the 70-100 m water depth range. 17 18 9 Interpretation. – The fields of irregular seabed furrows in the Minch are interpreted as iceberg scours 19 20 10 produced by the undersidesFor or keels Review of icebergs ploughing Only through soft sediment. Their morphology 21 22 11 and dimensions are consistent with iceberg ploughmarks seen at other mid- and high-latitude palaeo- 23 12 ice sheet margins (e.g. Evans et al. 2009; van Landeghem et al. 2009; Dowdeswell et al. 2014). Although 24 25 13 Pleistocene iceberg scours have been reported elsewhere on the continental shelf around the British 26 27 14 Isles (Beldersen & Wilson 1973; Stoker et al. 1993; van Landeghem et al. 2009) those in the central 28 15 Minch are particularly good examples because of their variation in size and shape, excellent 29 30 16 preservation state and the high quality of the seabed imagery (Figs. 3, 8, 9). The narrow water-depth 31 32 17 distribution (70-90 m) of the ploughmarks, and absence below 150 m, indicates relatively large but 33 34 18 uniform-sized icebergs, possibly relating to a local calving ice-margin. The strong orientation trend also 35 19 suggests a proximal source, rather than randomly drifting bergs from a number of disparate (far-field) 36 37 20 sources. Although the age of the iceberg scours has not been determined, their good state of 38 39 21 preservation – being not glacially or hydrodynamically modified – implies a relatively late (MIS 2) 40 41 22 formation date. 42 43 23 44 45 24 Discussion 46 47 48 25 Since the hallmarks of ice streaming were first identified in NW Scotland nearly 10 years ago (Stoker & 49 26 Bradwell 2005) a considerable amount of new topographic, bathymetric and geological data has 50 51 27 become available. The advent of high-resolution (5-m cell size) terrestrial digital surface models in 2005- 52 53 28 2006 (NEXTMap GB) allowed the identification of subglacial landform suites at a number of onshore 54 29 localities around the Minch. Ranging from large-scale crag and tails, several kms long, to narrow 55 56 30 drumlins ~1 km in length, these landform assemblages were used to map out the wider shape, flow 57 58 31 geometry and generalised onset zones of the Minch palaeo-ice stream (Bradwell et al. 2007) (Fig. 9). A 59 60 32 narrow strip of MBES data (25 x 2.5 km), acquired from the central part of the Minch in 2006 (M, Fig. 1), 33 showed subtle elongate seabed ridges and grooves interpreted as subglacial lineations trending roughly Boreas Page 18 of 43

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2 3 1 NNW-SSE (Bradwell et al. 2007). Unfortunately, the small size of the swath data coverage could not 4 5 2 definitely preclude ridge/groove formation by other geomorphic processes, such as iceberg scouring. 6 7 8 3 Around the same time, access to a global compilation of digital bathymetric soundings became available 9 4 – the Olex dataset. Interrogation of these bathymetric surface models, based on singlebeam echounder 10 11 5 data, has greatly enhanced our knowledge of submarine glacial landforms on continental shelves and 12 13 6 led to major revisions of ice-sheet reconstructions in NW Europe (Bradwell et al. 2008; Clark et al. 2012; 14 15 7 Sejrup et al. 2014; Bradwell & Stoker in press). However, these data are insufficiently detailed to resolve 16 8 features <500 m long, such as subglacial ice stream bedforms (Fig. 10). In the last 2 years, a large 17 18 9 amount of new high-resolution (<10 m cell size) MBES data has been acquired, chiefly by the MCA as 19 20 10 part of the UKHO’s majorFor programme Review to chart water depths Onlyaround the UK. A similar programme is also 21 22 11 being undertaken by the Irish government (INFOMAR, started 2005). This high-resolution bathymetry 23 12 data opens an important new chapter in the submarine geomorphology of NW Europe. Where 24 25 13 previously the seabed was apparently featureless or unresolvable, high quality data now exists (Figs. 1, 26 27 14 10). 28 29 15 Interrogation of new MBES datasets (in this study) has confirmed the former existence of a large 30 31 16 powerful ice stream within the bathymetric trough of the Minch. Seabed imagery presented here 32 33 17 highlights the abundance of streamlined elongate forms; their precise shape and form; and the overall 34 35 18 convergent, strongly directed flow set, generally orientated from S to N (Figs. 3, 4, 5). The presence of 36 19 elongate erosional bedforms, such as crag and tails, megagrooves and large streamlined bedrock forms, 37 38 20 on the seabed across the inner shelf, their progression downstream into highly elongate sedimentary 39 40 21 forms (long drumlins, MSGLs), and eventually into smooth sediment-dominated terrain, is typical of the 41 22 downstream evolution of ice stream beds in Pleistocene and contemporary settings (e.g. Wellner et al. 42 43 23 2001; O Cofaigh et al. 2002; Dowdeswell et al. 2008; Rydningen et al. 2013). Importantly, the pattern 44 45 24 and form of these submarine landforms is further enhanced by sub-bottom geophysical data. We 46 47 25 propose that the Pleistocene sediment architecture and glacial landform assemblages in and around the 48 26 waters of the Minch, NW Scotland, represent the most comprehensive expression of a palaeo-ice 49 50 27 stream system within the former British-Irish Ice Sheet (Fig. 9). As such it is an ideal place to conduct 51 52 28 further research into the geological and environmental legacy of rapid ice sheet and sea level change. 53 54 29 The remaining sections in this discussion summarise the main evidence for ice stream onset; palaeo- 55 56 30 flow geometry and flux; and discuss the probable mechanism of ice stream retreat set within a 57 58 31 framework chronology. 59 60 32 Page 19 of 43 Boreas

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2 3 1 Evidence of ice stream onset, flow convergence and topographic control 4 5 6 2 In high-relief terrain, topography plays the governing role in ice stream initiation (e.g. Sugden 1978; 7 8 3 Bentley 1987; Bamber et al. 2000; Winsborrow et al. 2010). This first-order topographic control on ice 9 4 streaming has been described in west Greenland where a downstream convergent configuration of 10 11 5 fjords in the Uummannaq region (70-72°N) controlled ice stream onset during successive glacial cycles 12 13 6 (Roberts et al. 2013; Lane et al. 2014). Furthermore, numerical models consistently show the co- 14 15 7 dependence between ice flow into topographic troughs and the initiation of localized ice streaming. It 16 8 is this process of ice funnelling through narrow ‘gates’ which increases ice flow and strain heating, that 17 18 9 in turn leads to increased meltwater production at the bed and, on impermeable beds, increased basal 19 20 10 sliding (Hindmarsh 2001;For Hall & Glaser Review 2003; Benn & Evans Only 2010). The configuration of fjords in NW 21 22 11 Scotland, although not the total relief, is similar to that in the Uummannaq region of W Greenland, with 23 12 an essentially dendritic network of major valleys and fjords converging in a central major trough (The 24 25 13 Minch). Support for ice-sheet flow convergence and ice stream onset in NW Scotland is seen in the 26 27 14 terrestrial and submarine landform record. Most notably in the MBES data from the western part of the 28 15 Minch, near the Shiant Islands (Figs 5, 9), where strongly streamlined fluted and grooved bedrock and 29 30 16 crag-and-tails indicate powerful ice flow to the NNE; and from the eastern part of the Minch at the 31 32 17 same latitude, in the Summer Isles region, where ice-carved land and seabed rock flutings and 33 34 18 megagrooves indicate strongly convergent ice flow from east to west (Bradwell et al. 2007, 2008b). 35 19 Where examined onshore, morphologically identical strongly streamlined bedrock outcrops show 36 37 20 abundant evidence of subglacial abrasion (e.g. long striae, whalebacks, p-forms and small-scale 38 39 21 erosional features), indicating a high degree of ice-bed contact and basal sliding by warm (soft) wet- 40 41 22 based ice (Roberts & Long 2005; Bradwell 2013; Roberts et al. 2013). Around Loch Laxford, a 5-km wide 42 23 zone of strongly streamlined bedrock terrain, with an abundance of wholly abraded landforms and ice 43 44 24 sculpted p-forms, has been used to map out a high-erosion corridor relating to an ice stream tributary in 45 46 25 the Loch Stack-Laxford trough (Bradwell 2013). This and other ice-stream ‘feeders’ from both sides of 47 26 the main Minch trough were proposed almost 10 years ago, based on landform patterns seen in high- 48 49 27 resolution digital surface models (Bradwell et al. 2007), but are only now being refined through detailed 50 51 28 terrestrial fieldwork and examination of previously hidden marine landforms. 52 53 54 29 55 56 30 Ice stream geometry and flux 57 58 59 31 Although the maximum thickness and extent of the BIIS ice sheet in the NW sector are still 60 32 unconstrained (cf. Ballantyne et al. 2008 with Fabel et al. 2012), the overall ice flow configuration is Boreas Page 20 of 43

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2 3 1 relatively well defined, at least during Minch ice stream operation. Newly identified submarine 4 5 2 landforms, presented here, confirm the strongly convergent geometry – from the Outer Hebrides, E or 6 7 3 NE; and from mainland Scotland, W or NW – towards a northerly direction in the central Minch. The 8 9 4 main flow geometry was probably directed by seafloor topography, with the fastest streaming flow in 10 5 the deepest parts of the Minch and slower or divergent flow over the broad bedrock highs, such as the 11 12 6 East Shiant Bank (Figs 1, 4). Fast-flow corridors in the fjords draining the high ground on either side of 13 14 7 the Minch would have converged in the central trough as a number of coalescent ice-stream tributaries, 15 16 8 as seen today in West Greenland and the Amundsea Sea sector of West Antarctica (e.g. Roberts et al. 17 9 2013; Joughin & Bamber 2005). Evidence of this is manifest in the 10-20 km long tapering bathymetric 18 19 10 extensions of submerged highs and headlands around the Minch – primary large-scale evidence of 20 For Review Only 21 11 streamlining by powerful northerly-directed ice sheet flow (Fig. 9). These mega-scale landforms are 22 12 absent north of 58°30’N suggesting either that they have been buried by later deglacial deposits, or that 23 24 13 the velocity (and power) differential between the coalescent tributaries was negligible within the trunk 25 26 14 of the ice stream north of this latitude. 27 28 15 Empirical ice-sheet reconstructions by McCarroll et al. (1995) and Ballantyne et al. (1998), using high- 29 30 16 level ‘trimlines’ to constrain ice sheet thickness in the region, have been significantly revised within the 31 32 17 past few years. Recent studies of the ice sheet’s vertical dimensions by Fabel et al. (2012), using 10Be 33 34 18 exposure-dating techniques, indicate that the Late Weichselian ice sheet overwhelmed the highest 35 19 mountains in NW Scotland preserving certain preglacial features beneath cold-based ice (cf. Ballantyne 36 37 20 et al. 1998; Stone & Ballantyne 2006). These 10Be exposure ages strongly suggest that the regional ice 38 39 21 surface had thinned to ca. 800 m above present-day sea level between Skye and the Summer Isles 40 41 22 region of NW Scotland by ~15 ka BP (Fabel et al. 2012). Although presently no empirical ice-sheet 42 23 thickness estimates exist for offshore sectors of the Minch ice stream, Ballantyne et al.’s (1998) 43 44 24 minimum ice height reconstruction indicates lower ice-sheet altitudes (typically ca. 450 m) around the 45 46 25 margins of the Minch than on the adjacent landmasses. Low-angle surface slopes associated with low 47 26 driving stresses, a soft sedimentary bed and streaming velocities would have maintained an ice sheet 48 49 27 elevation minimum in the central part of the Minch (ca. 58°N, 6°W), probably with an average ice 50 51 28 thickness of ca. 400 m (but with a +/- 100 m uncertainty). 52 53 54 29 Velocity estimates for present-day ice streams collected from satellite-borne radar data are wide 55 30 ranging and naturally depend on spatial setting and distance downstream – with typical flow values in 56 57 31 tributaries (~50-500 m yr-1) increasing rapidly in the trunk to around 1000-2000 m yr-1, and exceptionally 58 -1 59 32 >5,000 m yr (Joughin & Bamber 2005; Joughin et al. 2010; Rignot et al. 2011). Assuming a typical 60 33 velocity of 1000 m yr-1, the reconstructed cross-sectional area (40 km wide x 300-500 m thick) in the Page 21 of 43 Boreas

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2 3 -1 1 central part of the Minch ice stream trunk (ca. 58.25°N) would yield a discharge flux of 12-20 Gt yr , 4 5 2 depending on the ice thickness parameter used. These values are directly comparable to high mass-flux 6 7 3 ice streams draining the West Antarctic Ice Sheet today (e.g. Joughin & Bamber 2005; Benn & Evans 8 9 4 2010). By way of comparison, ice streams draining into the Weddell Sea and Ross Sea typically exhibit 10 5 discharge fluxes in the range 2-40 Gt yr-1; with the highest mass-flux ice streams in the Amundsen Sea 11 12 6 sector (Pine Island and Thwaites Glaciers) exhibiting discharges of ~50-70 Gt yr-1 (e.g. Vaughan et al. 13 14 7 2001; Joughin & Bamber 2005; Rignot et al. 2011). It is worth noting that although the reconstructed 15 16 8 discharge flux of the Minch palaeo-ice stream is similar to those in West Antarctica today, its drainage 17 9 basin area was considerably smaller: 104 km2 compared to 105 km2 typically in the Weddell and Ross Sea 18 19 10 sectors (Bentley 1987; Joughin & Bamber 2005). This size difference partly reflects the overall smaller 20 For Review Only 5 2 6 2 21 11 size of the BIIS – ca. 8 x 10 km compared to ~2 x 10 km for the West Antarctic Ice Sheet – and partly 22 12 reflects the first-order topographic controls on ice stream catchment in the British Isles. Generally 23 24 13 speaking, topographic ice stream systems have smaller, more dendritic drainage basins than pure ice 25 4 2 26 14 streams; Jakøbshavn Isbrae, in west Greenland, being a good example (area = ~10 km ) – equivalent in 27 28 15 size to the Minch palaeo-ice stream system. We suggest the comparable setting, geometry and flow 29 16 regime make the Minch ice stream an excellent palaeo-analogue for processes currently underway at 30 31 17 high flux ice sheet margins in West Antarctica and Greenland. 32 33 34 18 35 36 19 A framework chronology and probable mechanism of ice stream retreat 37 38 39 20 Constructing a chronology of ice stream retreat in the NW sector of the British-Irish ice Sheet is 40 41 21 currently the subject of a major renewed research effort (Clark et al. 2014). However, a simple 42 22 chronological framework can be constructed based on the existing sparse dataset. Geomorphological 43 44 23 (end moraines), geological (glacial diamicton) and seismostratigraphic (glacial erosion surfaces) 45 46 24 evidence taken together indicate it is very likely that the ice stream extended on to the mid-continental 47 25 shelf to the NW of Lewis during MIS 2-3. Indeed 8 new cosmogenic exposure ages from North Rona, ~40 48 49 26 km from the shelf edge, confirm that the last ice sheet overwhelmed this island at ~25 ka BP (= Late 50 51 27 Weichselian maximum extent), depositing boulders as it retreated (Everest et al. 2013). Another key 52 53 28 constraint relating to Minch ice stream decay is the biostratigraphic and radiocarbon chronology 54 29 derived from marine macro- and microfauna in BGS borehole 78/4, taken 5 km off eastern Lewis 55 56 30 (Graham et al. 1990). This detailed study showed open cold-water conditions were present in the Minch 57 58 31 at ca. 15 ka cal BP, with very cold, reduced salinity, waters persisting for some time before that. It is 59 60 32 worth noting that BGS Borehole 78/4 recovered 20.5 m of firm dark brown clay with pebbles below the 33 lowest bivalve sampled for radiocarbon analysis (Graham et al. 1990). The lowest 10 m of this unit, with Boreas Page 22 of 43

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2 3 1 its near absence of dinoflagellate cysts and macrofossils, is taken to represent a relatively shallow (<20 4 5 2 m) harsh glaciomarine environment, whilst the overlying 10 m with an increasing marine faunal 6 7 3 diversity is consistent with slightly milder glaciomarine conditions (Graham et al. 1990). The presence of 8 9 4 this single, seismo-stratigraphically undivided unit, with no internal erosional surfaces, indicates a 10 5 relatively long period of glaciomarine, and hence ice-sheet-free, conditions in this part of the western 11 12 6 Minch prior to 15 ka BP. 13 14 15 7 Given the currently weak chronology of MIS 2 deglaciation in the NW British Sector, being only 16 8 bracketed by two dated points (<25 and >15 ka BP), the rate and style of Minch ice stream retreat – fast 17 18 9 vs slow, collapse vs incremental – are still uncertain. It is important to note, however, that the absence 19 20 10 of moraines and groundingFor-zone featuresReview and the widespread Only presence of apparently unmodified 21 22 11 subglacial forms (e.g. drumlins, MSGLs, etc) across much of the central Minch (Fig. 9) indicate that the 23 12 mechanism of ice stream decay was not by incremental grounded retreat, at least in the main trunk of 24 25 13 the ice stream, and that retreat may have been rapid (Stoker & Bradwell 2005; Bradwell & Stoker in 26 27 14 press). In similar bathymetric settings, where the seabed deepens inshore, the most glaciologically 28 15 plausible style of ice stream retreat is a non-linear one triggered by instability of the marine margin (e.g. 29 30 16 Alley et al. 2005; Schoof 2012). This instability, brought about as grounded ice crosses the flotation 31 32 17 threshold, leads to rapid calving, increased drawdown, dynamic thinning and can trigger rapid ice 33 34 18 stream retreat or collapse (e.g. Alley et al. 2005; Pritchard et al. 2012; Schoof 2012). However, recent 35 19 numerical modelling studies have shown that this dynamic process is complicated by lateral drag 36 37 20 effects, particularly in relation to trough-width variations (Jamieson et al. 2012). When considering all 38 39 21 the evidence, we predict that large parts of the ice stream margin in the Minch were floating at key 40 41 22 times during deglaciation – bringing about rapid ice-front retreat by calving, in turn leading to a positive 42 23 feedback between increased drawdown, ice speed up and accelerated mass loss at the marine margin. 43 44 45 24 We suggest that the presence of the substantial Eddrachillis Bay, Rubha Coigach, Loch Ewe–Greenstone 46 47 25 Point Moraine complexes, in similar water depths, 5-15 km off the NW seaboard of Scotland are entirely 48 26 consistent with a single, stable, ice-sheet terminus grounding in coastal waters following rapid ice 49 50 27 stream retreat (Figs. 6, 7, 9). These major moraine complexes represent a significant, probably long- 51 52 28 lived, stillstand position of a grounded marine-terminating ice sheet margin stretching almost 100 km 53 29 from near Cape Wrath to the mouth of Loch Ewe; whilst a smaller independent ice cap probably 54 55 30 covered much of the Outer Hebrides. Although the exact timing of this major ice sheet stillstand in NW 56 57 31 Scotland is uncertain, it must have occurred after ~25ka BP when ice receded from North Rona on the 58 59 32 mid-shelf (Everest et al. 2013) and prior to ~15 ka BP, when the ice sheet margin had retreated inshore 60 33 of the Summer Isles (Bradwell et al. 2008c). A date between ~16-20 ka BP seems most probable. When Page 23 of 43 Boreas

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2 3 1 these moraine complexes formed the majority of the Minch would have been an open, but relatively 4 5 2 shallow, marine embayment. Well-preserved iceberg scours strongly suggest a proliferation of locally 6 7 3 sourced icebergs in the Minch at a late stage during deglaciation, possibly the product of widespread 8 9 4 calving events during the final stages of ice stream decay. 10 11 5 12 13 14 6 Conclusions 15 16 7 • Recently collected high quality, high-resolution, MBES data affords a view of the submarine 17 18 8 glacial landform record around NW Scotland in unprecedented detail. 19 20 9 • Newly identified,For well preserved, Review Late Pleistocene glacialOnly sediment and landform assemblages 21 22 10 in the Minch, supplementing those already described offshore NW Scotland, represent the most 23 11 complete record of an ice stream system within the former British-Irish Ice Sheet. 24 25 12 • Subglacial landform assemblages (both sediment and hard-bed) in the central and inner parts of 26 27 13 the Minch confirm the spatial distribution, coherence and trajectory of a grounded fast-flowing 28 29 14 ice sheet corridor. Notably these bedforms display a downstream evolution, from short 30 15 drumlins to km-scale glacial lineations, suggesting an ice-flow velocity transition in the trunk of 31 32 16 the ice stream. 33 34 17 • Elsewhere, in the inner parts of the Minch, strongly streamlined large-scale bedrock flutings and 35 36 18 megagrooves indicate a high degree of ice-bed coupling in a wide zone of flow convergence 37 19 characteristic of ice stream onset. 38 39 20 • Although currently the subject of further work, the available geomorphological evidence – 40 41 21 notably the lack of grounding-line features and presence of well-preserved subglacial bedforms 42 43 22 and abundant iceberg ploughmarks at seabed across much of the northern and central Minch – 44 23 combined with glaciological theory, suggest that the retreat of the marine portion of the Minch 45 46 24 palaeo-ice stream was probably rapid through a process of widespread calving and increased 47 48 25 drawdown, driven by grounding line dynamics and a landward deepening bed. 49 26 • Reconstructed ice-flux discharge values for the Minch ice stream (12-20 Gt yr-1) are comparable 50 51 27 to modern ice streams in West Antarctic and Greenland. We propose that the topographic 52 53 28 setting, flow regime and landform record make the Minch ice stream an excellent palaeo- 54 55 29 analogue for processes currently underway at high flux marine ice-sheet margins. 56 57 30 58 59 60 31 Boreas Page 24 of 43

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2 3 1 Acknowledgements 4 5 6 2 This research was funded by BGS-NERC (Energy & Marine Geology Directorate) and is a scientific 7 8 3 contribution to the MAREMAP programme. This work was largely undertaken prior to the start of 9 4 the NERC-funded (Consortium Grant) Britice-Chrono project, but has been partly supported by the 10 11 5 consortium and has benefitted subsequently from conversations with several consortium members 12 13 6 (most notably Chris Clark). Rhys Cooper (BGS) is thanked for processing MBES data. MCA (UKHO) 14 15 7 multibeam survey data is Crown Copyright and is provided by MCA (UKHO), who are gratefully 16 8 acknowledged. Singlebeam bathymetry data is from the Olex database, accessed under license by 17 18 9 BGS/NERC. The two anonymous reviewers are thanked for their comments. Published with the 19 20 10 permission of the ExecutiveFor Director, Review BGS (NERC). Only 21 22 11 23 24 25 12 26 27 13 28 29 14 Table 1: Relative setting of the stratigraphical units in NW Scotland; all belong to the Eilean 30 31 15 Siar Glacigenic Group (after Stoker et al. 2011). 32 33 16 34 35 Age (Ka) Hebrides Slope Hebrides Shelf The Minch Little Loch Broom 36 37 HOLOCENE (<10) Summer Isles Fm. 38 Catriona Fm. Annie Fm. LATE PLEISTOCENE Annat Bay Fm. 39 MacAulay Fm. 40 (10–25) Assynt Glacigenic Fm. 41 42 (including abundant recessional moraines) 43 44 45 ‘Unnamed diamicton Formation’ – Rubha Còigeach–Loch Ewe moraines 46 includes Eddrachillis Ridge moraine 47 Sheena Fm. Fiona Fm. Loch Broom Till Fm. 48 MID- TO LATE 49 PLEISTOCENE Morag Fm. 50 (25–450) MacIver Fm. Jean Fm. 51 52 Upper MacLeod Fm. MacDonald Fm. Aisla Fm. 53 (glacially-influenced Elspeth Fm. 54 slope-apron development throughout the mid- to Flora Fm. 55 late Pleistocene) 56 Shona Fm. 57 58 17 59 60 18 Page 25 of 43 Boreas

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2 3 1 List of Figures: 4 5 6 2 Figure 1: (A) Location and (B) physiography of study area. General bathymetry and 7 3 topography of NW Scotland and the surrounding continental shelf (1-km gridded DEM; compiled 8 4 from data held by BGS-NERC). Blue & magenta boxes indicate areas covered by MBES data (cut-off 9 10 5 date Dec 2013); blue areas acquired by MCA-UKHO; magenta areas acquired by BGS-NERC. All areas 11 6 of continental shelf within study area covered by singlebeam (Olex) dataset, except cross-hatched 12 7 areas where data density is poor. Numbered red boxes and lines indicate position of subsequent 13 14 8 figures (2-8). Key placenames marked. 15 9 16 17 10 Figure 2: Outer shelf and slope, glacial depocentres and stratigraphic architecture. (a) Main 18 11 Quaternary glacial depocentres on the NW UK continental shelf and slope. Isopachs derived from 19 20 12 grid of 2D seismic lines For(modified fromReview Stoker & Bradwell, 2005)Only. Line of geo-seismic transect (c) also 21 13 shown. (b) Hill-shaded singlebeam echosounder bathymetric image (Olex dataset) showing the 22 14 macrogeomorphology of the northern Hebrides Shelf. Note the well-defined cross-shelf trough and 23 24 15 the trough-mouth Sula Sgeir Fan. (c) Geoseismic transect across the northern Hebrides Shelf and 25 16 adjacent slope showing the glacial stratigraphic architecture of the middle to upper Pleistocene 26 17 succession and age of the underlying bedrock. Inset shows onlapping glacial diamicton architecture 27 28 18 and buried mega-scale glacial lineation surfaces (modified from Stoker & Bradwell, 2005). See Table 29 19 1 for key to lithostratigraphic formation names (in italics). 30 31 20 32 33 21 Figure 3: Ice stream bedforms in the Minch. (a) Hill-shaded multibeam bathymetric image and 34 35 22 (b) geomorphological map of submarine landforms at same scale. The drumlins, streamlined bedrock 36 23 forms and glacial lineations are interpreted as part of a single subglacial bedform assemblage on East 37 24 Shiant Bank, in the Minch. Note the strongly aligned long axis of forms and the general increase in 38 39 25 bedform elongation from south to north. StB – streamlined bedrock. MBES data collected by 40 26 Maritime & Coastguard Agency in 2010. Location of Figure 4 shown as white box. 41 42 27 43 28 44 45 29 Figure 4: Ice stream bedforms on East Shiant Bank in detail. Upper panel: Hill-shaded 46 30 greyscale multibeam image with geomorphological linework; blue lines – drumlinoid forms; red lines 47 31 – streamlined bedrock forms. (See Figure 3 for location). BGS seismic lines shown; drumlins labelled 48 49 32 A-E. Lower panel: Annotated BGS sparker profile across drumlins A-D showing glaciogenic sediment 50 33 thickness and localised postglacial drape overlying irregular bedrock surface. StB – Streamlined 51 34 bedrock at seabed surface; SBM – seabed multiple. MBES data collected by Maritime & Coastguard 52 53 35 Agency in 2010. 54 36 55 56 37 Figure 5: Ice stream bedforms near the Shiant Islands. Upper panel: Seabed geomorphology 57 58 38 and sediment distribution map showing medium- and large-scale streamlined glacial landforms. 59 39 Background DSM: hill-shaded grey-scale multibeam data (collected by Maritime & Coastguard 60 40 Agency in 2010). Location of seismic line M13 shown. Lower panel: annotated BGS sparker profile 41 across streamlined terrain. Note variable thickness of Pleistocene deposits (diamicton), and Boreas Page 26 of 43

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2 3 1 corrugated (ridge/groove) bedrock reflector. StBH – Streamlined bedrock high; LC+T – large crag and 4 5 2 tail; BMF – bedrock megaflute; BMG – bedrock megagroove; BP – bedrock plateau; SBM – seabed 6 3 multiple. 7 8 4 9 10 5 Figure 6: BGS sparker profile (upper panel) and seismostratigraphic interpretation (lower 11 12 6 panel) of the Eddrachillis Ridge, eastern Minch. This large 35-m high moraine and the smaller 13 7 recessional moraines to the SE have not been formally defined within the existing stratigraphic 14 8 framework, but represent a prominent relatively recent (?end-MIS2) ice-sheet margin position 15 16 9 offshore mainland NW Scotland. 17 10 18 11 19 20 12 Figure 7: (a) Hill-Forshaded mu Reviewltibeam bathymetric imageOnly and (b) geomorphological map of 21 13 seabed landforms, interpreted as a large moraine complex, offshore Greenstone Point, eastern 22 14 Minch. Merged MBES dataset: southern half collected by BGS in 2005; northern half collected by 23 24 15 MCA in 2010. (Lower panels) Bathymetric cross profiles of moraines along lines shown in (a). Note 25 16 the similarity in size and morphology between the large moraines here and those in Fig 6. 26 17 27 28 18 29 19 Figure 8: (a) Upper panel: Hill-shaded greyscale multibeam bathymetric data from the central 30 20 Minch showing area of dense cross-cutting iceberg scours in seabed sediment. Note the size 31 32 21 variation and general orientation (between 000-045) of scours; also note the abrupt edge of scour 33 22 marks at the margin of the broad sediment ridge (in water depths >100 m). Lower panel: 34 23 bathymetric profile across iceberg scours (X-X’) showing typical width:depth ratios and v-shaped 35 36 24 morphology. (b) BGS sidescan sonar profile from the eastern Minch showing seabed iceberg scours 37 25 in an area of morainic topography, 10 km NW of Rubha Coigach headland. MBES data collected by 38 26 Maritime & Coastguard Agency in 2010. 39 40 27 41 28 42 29 Figure 9: Submarine sediments and landforms associated with a palaeo-ice stream system 43 44 30 within the British-Irish Ice Sheet. Central panel: Map showing bathymetry, topography and main 45 31 palaeoglaciological features of the Minch ice stream system in NW Scotland. White lines show 46 32 proposed ice stream tributaries and flow lines (from Bradwell et al. 2007); thin black lines show 47 48 33 streamlined submarine extensions of headlands and submerged banks. Thick dashed lines – 49 34 reconstructed ice-stream terminus positions on continental shelf (from Bradwell & Stoker in press); 50 35 outermost is probably pre-MIS 2-3; inner limits are both MIS 2 (<25 Ka BP); thick solid grey line – 51 36 reconstructed ice-sheet margin position 5-15 km offshore NW Scotland, after demise of Minch ice 52 53 37 stream (currently undated). Diagonal hatching denotes area with absence of moraines or grounding- 54 38 line features, and preserved subglacial bedforms and iceberg scours at seabed. Red boxes indicate 55 39 locations of surrounding illustrative panels. (a) Image showing processed seismic first-return (seabed 56 57 40 surface) data from the shelf slope, highlighting the surface debris lobes and gulleys on the Sula Sgeir 58 41 Fan slope. (b) BGS sparker profile across the outer continental shelf and slope showing the seismic 59 42 architecture of the Sula Sgeir Fan (from Stoker & Bradwell, 2005). (c). Annotated BGS seismic profile 60 43 showing typical ice-stream sediment architecture on the mid-shelf, ~20 km N of Butt of Lewis. The 44 North Minch Ridge is the seabed expression of a thick morainic sediment wedge comprising several Page 27 of 43 Boreas

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2 3 1 stacked diamicton units. Note how the strongly layered Morag Fm, interpreted as distal glaciomarine 4 5 2 sediments, drapes and onlaps the underlying Aisla/Jean Fm. (d) BGS seismic profile showing exposed 6 3 and buried mega-scale glacial lineation (MSGL) surfaces associated with stacked diamicton 7 4 sequences, eastern Minch. Sidescan-sonar seabed image of the same location (~90 m water depth) 8 9 5 showing MSGLs interspersed with occasional iceberg scours in perspective view (from Stoker & 10 6 Bradwell 2005). (e) Dense field of iceberg ploughmarks in 70-90 m water depth, central Minch, 11 7 indicating abundant locally sourced icebergs at a late-stage of ice sheet retreat. (f) Annotated BGS 12 13 8 seismic profile across the Eddrachillis Ridge, eastern Minch - a large submarine push moraine 14 9 complex in 100 m water depth, part of a suite of conspicuous nearshore moraine complexes 15 10 stretching from near Cape Wrath to Loch Ewe. (g) Greyscale multibeam image of Greenstone Point 16 17 11 Moraine complex, eastern Minch, in 80 m water depth - part of the same suite of nearshore end 18 12 moraines as (f), indicating a stable (long-lived) grounded ice front position. Note the superimposed 19 13 discontinuous de Geer moraines suggesting retreat of a lightly grounded, partly floating, tidewater 20 For Review Only 21 14 ice sheet margin. (h) Greyscale multibeam image of drumlins on East Shiant Bank, central Minch, in 22 15 40 m water depth; evidence of subglacial sediment accretion and deformation at a hard-bed to soft- 23 16 bed transition within the trunk of the ice stream. (j) Greyscale multibeam image of erosional 24 25 17 streamlined bedforms, west of the Shiant Islands, western Minch – a good example of a hard-bed 26 18 subglacial landform assemblage in a submarine setting. Bedrock megagrooves and large-scale 27 19 flutings such as this have been the associated with the onset of ice streaming. All seismic sub-bottom 28 29 20 and sidescan sonar data in this figure collected by BGS-NERC; all MBES data collected by MCA. 30 21 31 22 32 33 23 Figure 10: Comparison of digital seabed surface models, highlighting the different resolution 34 24 and quality of bathymetric datasets now available at a single site in the Minch (, Long_). (a) GEBCO, 35 25 v3.0, grid cell size = 250 m; (b) Olex (version 2010), singlebeam echosounder data; typical grid cell 36 37 26 size = ca. 100 m; (c) MCA-UKHO MBES data; grid cell size = 8 m; (d) Close up of area in red box, 38 27 showing best resolution data. Same dataset as (c). 39 28 40 41 29 42 43 30 44 45 31 46 47 32 48 49 33 50 51 34 52 53 54 35 55 56 36 57 58 37 59 60 38 Boreas Page 28 of 43

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2 3 1 References 4 5 6 2 Alley, R.B., Clark, P.U., Huybrechts, P. & Joughin, I. 2005: Ice-sheet and sea-level changes. Science 310, 456- 7 3 460. 8 9 4 Alley, R.B. & MacAyeal, D.R. 1994: Ice-rafted debris associated with binge/purge oscillations of the Laurentide 10 5 Ice Sheet. Paleoceanography 9, 503-511. 11 6 12 7 Andreassen, K., Laberg, J. S. & Vorren, T. O. 2008: Seafloor geomorphology of the SW Barents Sea and its glaci- 13 8 dynamic implications. Geomorphology 97, 157–177. 14 9 15 10 Ballantyne, C.K., McCarroll, D., Nesje, A., Dahl, S.O. & Stone, J.O. 1998: The last ice sheet in NW Scotland: 16 11 reconstruction and implications. Quaternary Science Reviews 17, 1149–1184. 17 18 12 Bamber, J.L., Vaughan, D.G. & Joughin, I., 2000. Widespread complex flow in the interior of the Antarctic Ice 19 13 Sheet. Science 287, 1248-1250. 20 For Review Only 21 14 Bard, E., Hamelin, B., Arnold, M., Montaggioni, L., Cabioch, G., Faure, G. & Rougerie, F. 1996: Deglacial sea 22 15 level record from Tahiti corals and the timing of global meltwater discharge. Nature 382, 241-244. 23 24 16 Bledersen, R.H. & Wilson, J.B. 1973: Iceberg plough marks in the northeast Atlantic. Palaeogeography, 25 17 palaeoclimatology, Palaeoecology 13, 215-224. 26 18 27 19 Benn, D.I. & Evans, D.J.A. 2010: Glaciers and Glaciation. Second Edition. Arnold, London. 28 29 20 30 21 Bentley, C.R. 1987: Antarctic ice streams: a review. Journal of Geophysical Research 92(B9), 8843-8858. 31 22 32 23 Bigg, G. R., Clark, C. D., Greenwood, S. L., Haflidason, H., Hughes, A. L. C., Levine, R. C., Nygård, A. & Sejrup, H. 33 24 P. 2012: Sensitivity of the North Atlantic circulation to break-up of the marine sectors of the NW European ice 34 25 sheets during the last Glacial: a synthesis of modelling and palaeoceanography. Global and Planetary Change 35 26 98-99, 153–165. 36 27 37 28 Bradwell, T. 2005. Bedrock megagrooves in Assynt, NW Scotland. Geomorphology 65, 195-204. 38 39 29 Bradwell, T. 2013: Identifying palaeo-ice-stream tributaries on hard beds: mapping glacial bedforms and 40 30 erosion zones in NW Scotland. Geomorphology 201, 397-414. 41 31 42 32 Bradwell, T., Stoker, M. & Larter, R. 2007: Geomorphological signature and flow dynamics of the Minch palaeo- 43 33 ice stream, NW Scotland. Journal of Quaternary Science 22, 609-617. 44 34 45 35 Bradwell, T., Stoker, M.S., Golledge, N.R., Wilson, C.K., Merritt, J.W., Long, D., Everest, J.D., Hestvik, O., 46 36 Stevenson, A.G., Hubbard, A.L., Finlayson, A.G. & Mathers, H.E. 2008a: The northern sector of the last British 47 37 Ice Sheet: maximum extent and demise. Earth-Science Reviews 88, 207-226. 48 38 49 39 Bradwell, T., Stoker, M.S. & Krabbendam, M. 2008b: Megagrooves and streamlined bedrock in NW Scotland: 50 40 the role of ice streams in landscape evolution. Geomorphology 97, 135–156. 51 41 52 42 Bradwell, T., Fabel, D., Stoker, M.S., Mathers, H., McHargue, L. & Howe, J.A. 2008c. Ice caps existed throughout 53 43 the Lateglacial Interstadial in northern Scotland. Journal of Quaternary Science 23, 401–407. 54

55 44 56 45 Bradwell, T. & Stoker, M.S. In press: Detailed ice-sheet retreat pattern around Northern Scotland revealed by 57 46 marine geophysical surveys. Earth and Environmental Science, Transactions of the Royal Society of Edinburgh 104, 58 47 tbc. 59 60 48 Chiverrell, R.C., Thrasher, I., Thomas, G.S.P., Lang, A., Scourse, J., Van Landeghem, K.J.J., McCarroll, D., Cofaigh, 49 C., Evans, D. & Ballantyne, C.K. 2013: Bayesian modelling the retreat of the Irish Sea Ice Stream. Journal of 50 Quaternary Science 28, 200-209. Page 29 of 43 Boreas

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19 13 Earth-Science 20 14 Winsborrow, M., Clark, C.D.For & Stokes, C.R.Review 2010: What controls theOnly location of ice streams? 21 15 Reviews 103, 45–59. 22 23 16 24 25 17 26 27 18 28 29 19 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Boreas Page 34 of 43

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