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Quaternary geology of the Summer Isles region — a brief explanation of the geological map Summer Isles special sheet

M S Stoker and T Bradwell

Bibliographic reference: Stoker, M S, and Bradwell, T. 2011. Quaternary geology of the Summer Isles region – a brief explanation of the Summer Isles Special Sheet. Sheet Explanation of the British Geological Survey. 1:50 000 Special Sheet Summer Isles (Scotland).

Keyworth, Nottingham: British Geological Survey. © NERC 2011. All rights reserved.

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(Front cover) Looking west towards Gruinard Island from Mungasdale Beach [NG 9675 9340]. (Photographer: Tom Bradwell; (P731850).

(Rear cover)

Notes

The word 'region' refers to the area covered by the 1:50 000 Quaternary geology map. National Grid references are given in square brackets, and lie within the 100 km squares NB, NC, NG and NH. BGS offshore core and borehole sample stations are referenced by latitude and longitude (also noted in square brackets), and are registered in chronological order according to degree rectangles. Each degree rectangle covers 1° latitude by 1° longitude, and each rectangle is numbered by the geographical co-ordinates of its south-west corner. The rectangles included in this region are 57°N–06° W and 58°N–06° W. Cores collected by the Scottish Association for Marine Science (SAMS) are registered by a number that is prefixed by a notation that reflects the type of sample, i.e. GC represents gravity corer. The serial number given with the plate captions is the registration number in the National Archive of Geological Photographs, held at the BGS. Colour is described using Munsell® colour names and notations. All dates referred to in this account are expressed in calendar years before present (ka BP), and radiocarbon dates have been calibrated using Fairbanks et al. (2005).

Acknowledgements

The authors acknowledge contributions from John Howe and Kate McIntyre (Scottish Association for Marine Science, Oban) for their collaboration on the acquisition of geological cores, and provision of sedimentological information on the basinal infill in the North Annat Basin, respectively. The authors' thanks are due also to the masters and crew of the R V Calanus and the R V James Cook for their skill and assistance during the collection of the offshore geophysical and geological datasets. Jon Merritt and David Long (BGS) are thanked for their comments on an earlier version of the manuscript. The series editors are A G Stevenson and M A Woods. Figures were drawn by K Henderson and page setting was by A J Hill.

© Crown copyright. All rights reserved. Licence number 100017897/2011.

Quaternary geology of the Summer Isles region (summary from rear cover)

An explanation of the geological map Summer Isles special sheet (Scotland).

This Sheet Explanation contains a brief description of the Quaternary geology of the Summer Isles region, which covers the coastline of Wester Ross, the sea lochs of Loch Broom and Little Loch Broom, and the intervening belt of mountainous terrain that includes Ben Mór Coigach and An Teallach. As such, this region represents a classic fjord landscape with a highly indented, glacially sculpted coastline that is incised into the mountainous hinterland of north-west Scotland.

This survey provides the first comprehensive study of the distribution and composition of Quaternary deposits in this region, incorporating both onshore and offshore information. These were deposited mainly during the Lateglacial interval of the last (Devensian) ice age, between about 15 000 and 11 000 years ago, as the ice-sheet retreated landward from The Minch into the North-west Highlands. During deglaciation, this coastal region became an important sediment trap and has preserved a high-resolution record that reflects both local terrestrial and marine processes active during glacier retreat. An integrated onshore–offshore lithostratigraphy is presented that represents the sedimentary architecture of deglaciation in this region.

A distinctive glacial geomorphological imprint has also been recognised for the first time in the submarine landscape of the region. This includes: 1) bedrock megagrooves and streamlined till sheets that reflect the palaeo-ice flows of glaciers via the fjords towards The Minch, and which have been proved to be contiguous between the onshore and offshore landscapes; and, 2) a suite of 40 to 50 sea-floor moraine ridges that chart the oscillatory retreat of the ice-sheet from The Minch to topographically confined outlet glaciers in the fjords.

The submarine landscape also reveals a previously unknown history of slope instability on the sea floor. Major submarine landslides and slump deposits occur in Little Loch Broom, and locally elsewhere in the Summer Isles region. In addition, the sea floor has been locally disrupted by faulting and folding. A brief history of these movements and outline description of the resulting structures and deposits, including the terrestrial record, is presented. In addition, this Sheet Explanation includes information on the occurrence and effect on the sea floor of shallow gas as well as scouring by tidal currents. A knowledge and awareness of all of these processes is essential to understanding the natural environment of this region, and should be considered during the early stages of any planning and development.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the Quaternary strata (Figure 1) of the area covered by the Quaternary Geology 1:50 000 Special Sheet for the Summer Isles region, which is a joint onshore–offshore map published in 2011. The offshore Quaternary geology is largely based on new data acquired between 2005 and 2007. However, the eastern margin of The Minch succession is modified from the Sutherland and Little Minch 1:250 000 Sea Bed Sediments and Quaternary Geology map sheets. The onshore Quaternary geology is based on revision mapping between 2004 and 2009: the eastern portion from Loch Broom to Cnocan is a generalised compilation from BGS Scotland Sheet 101E Ullapool (Superficial & Bedrock) (in press); the central and southern portion is based on new mapping and supersedes the Quaternary geological linework on Scotland (Provisional Series) Sheets 101W Summer Isles, 91/100 Gairloch and 92 Inverbroom.

The Summer Isles region herein includes all the waters between the headlands of Greenstone Point in the south and Rubha Còigeach in the north; principally, Loch Broom, Little Loch Broom, Gruinard Bay, Annat Bay, and the waters around the Summer Isles (Figure 6)." data-name="images/P1004346.jpg">(Figure 3). " data-name="images/P1004345.jpg">(Figure 2). The eastern margin of The Minch forms the western limit of the area. Ben Mór Coigach forms a prominent mountain massif in the northern part of the region, whereas a number of rugged headlands backed by mountains, such as An Teallach, bound the southern part of the region. The Summer Isles comprise ten or so islands – Tanera Mór, Tanera Beg, Priest Island and Horse Island being the largest – plus a number of skerries (Figure 6)." data-name="images/P1004346.jpg">(Figure 3). Isle Martin and Gruinard Island are separate islands adjacent to the eastern and southern coastline, respectively. The sea lochs of Loch Broom and Little Loch Broom form major marine inlets; Loch Broom is 15 km long and 0.5 to 1.5 km wide, whereas Little Loch Broom is 12 km long and 0.5 to 2.0 km wide. As this landscape was carved and shaped predominantly by processes of glacial erosion, the term sea loch is analogous to fjord, which is a generic Nordic name for such a marine inlet. The adjacent coastal bathymetry reveals the juxtaposition of shallow-marine banks (<40 m below OD) and deeper incised troughs (up to 180 m below OD) (Figure 6)." data-name="images/P1004346.jpg">(Figure 3). The troughs represent the offshore continuation of the modern sea lochs of Loch Broom and Little Loch Broom, and form a number of discrete basins: the South-East, South and North Annat basins, and the Coigach, Tanera, Skerries and South Priest basins. The whole coastal region is separated from The Minch by a wide zone of predominantly bedrock that extends from Rubha Còigeach to Greenstone Point, including the Summer Isles, and is herein termed the Summer Isles sill (Figure 6)." data-name="images/P1004346.jpg">(Figure 3). " data-name="images/P1004345.jpg">(Figure 2).

The southern boundary of the North-West Highlands Geopark — a UNESCO 'European Geopark' (www.northwest-highlands-geopark.org.uk) — incorporates Rubha Còigeach, Tanera Beg, Tanera Mór, Horse Island and Ben Mór Coigach. Tourism borne out of the unique scenery of the region, together with the wide range of marine and coastal habitats and archaeological sites, is an important element of the local economy. Aquaculture (mostly salmon farming) and commercial fisheries (mostly prawns) are also significant components of the economy, employing a workforce that is nearly double the average of the Highlands as a whole.

The bedrock geology is dominated by coarse-grained, red, thick-bedded sandstone of Neoproterozoic age, belonging to the Torridon and Stoer groups. There are sporadic inliers of Archaean Lewisian orthogneiss, onto which the sandstone was deposited unconformably. Coarse-grained sandstones of the Applecross Formation form most of the surrounding mountains, for example, Ben Mór Coigach, Beinn Ghobhlach and An Teallach. The lower-lying peninsulas include breccias of the Diabaig Formation; red sandstone of the Aultbea Formation; sandstone and grey shale of the Cailleach Head Formation; and sporadic outliers of white Cambrian quartzite (Eriboll Formation). A north–south-trending strip of Cambro-Ordovician rocks, mainly quartzite with subordinate carbonate, crops out to the east of Ullapool which is, in turn, tectonically overlain by metasediments of the Neoproterozoic Moine Supergroup along the Moine Thrust Zone. The bedrock is cut by both north-east- and north-west-trending lineaments. The latter have exerted a control on the orientation of the sea lochs, whereas the intersection of lineaments in the outer part of the region has probably controlled the rectilinear pattern of the South Priest and Skerries basins. Further details of the bedrock geology may be found on the Maps listed in the Information sources section.

The present-day landscape of this region has been strongly shaped by Quaternary glaciation. Erosional landforms, such as troughs and corries, roches moutonnées, rock drumlins and meltwater channels dominate the coastal scenery, offshore islands and marine landscape. Bedrock megagrooves, glacial striae and far-travelled erratics indicate a south-east–north-west palaeo-ice flow of glaciers via fjords towards The Minch (Figure 6)." data-name="images/P1004346.jpg">(Figure 3). " data-name="images/P1004345.jpg">(Figure 2), where they coalesced to form a large, about 200 km long, palaeo-ice stream terminating at the edge of the continental shelf (Bradwell et al., 2007, 2008a, c; Stoker and Bradwell, 2005). Similar ice streams may have been active several times in the mid to late Pleistocene, most recently during the Late Devensian glaciation, between c. 28 ka BP and 15 ka BP. Glacial depositional features largely record the deglaciation of the region, and include till blankets, moraines, glaciofluvial outwash sheets, and blanket and fan deposits on the adjacent hills, peninsulas, and shallow marine banks. Glacimarine and marine deposits are preserved in the deeper-water basins. From a grounded margin amongst the Summer Isles, deglaciation was ongoing during the Lateglacial Interstadial, at about 14.5 ka BP, and by 13 ka BP ice caps had become topographically confined and restricted to the fjords. A late-stage re-advance of glaciers into the inner fjords occurred soon after 13 ka BP, during the Younger Dryas Stadial (c. 12.5–11.5 ka BP), before finally retreating into the valleys.

Tidal currents play a key role in the present-day hydrographic regime of the Summer Isles region. A strong spring–neap tidal variation has been recorded off north-west Scotland. This has been measured with a spring range of 4.5 m and a neap range of 1.6 m offshore Skye (Ellett and Edwards, 1983). The maximum speed of tidal streams during typical spring tides in this area is 1–2 ms−1 (Sager and Sammler, 1968).

Survey history

Prior to the latest offshore surveys (2005–2007) on which this sheet explanation is largely based, only the outer fringes of the coastal region, extending from the Summer Isles into The Minch, have been previously surveyed. The information gathered prior to 2005 was sparse, and was included on the published 1:250 000 scale Little Minch (including part of Great Glen) and Sutherland Sea Bed Sediments and Quaternary Geology offshore maps. On both of these maps, the Quaternary succession (where mapped) landward of the Summer Isles is undivided. In contrast, the Quaternary succession in The Minch has been surveyed in detail (Sutherland sheet). The eastern margin of this succession, between Rubha Còigeach and Greenstone Point, has been revised as part of this study.

Geophysical data were collected from the offshore area in 2005. The sea bed and submarine landforms were surveyed using swath bathymetry, whereas the sub-sea bed geology was imaged by surface-tow boomer seismic reflection profiling (Stoker et al., 2006). Lithological information was subsequently collected using a variety of corers, including: 1) in 2006, the SAMS 3 m gravity corer; and 2), in 2007, the BGS 5 m gravity corer, the 6 m vibrocorer, and the 15 m rockdrill. The rockdrill was used to test some of the shallow morainic banks, whereas the vibrocorer and gravity corer were both used to sample the basinal deposits. Although the length of the core, and hence depth of penetration, is limited by the length of the core barrel, an offset sampling strategy in areas of dipping strata allows an individual unit to be sampled at different stratigraphic levels.

The onshore geology of the region covered by this map was originally surveyed between 1885 and 1897, with some revisions between 1912 and 1915. The Geological Survey officers responsible for this work — B N Peach, J Horne, C T Clough, L W Hinxman, H M Cadell, and W Gunn — focused on recording the complexities of the bedrock geology related to the Moine Thrust Zone. Consequently, relatively little attention was paid to the superficial or 'drift' deposits in this area. For example, Sheet 101 Knockan was published showing only limited Quaternary deposits (alluvium and peat), and glacial deposits were not shown at all.

Between 2004 and 2010 a resurvey of the onshore Quaternary geology was undertaken by the BGS, primarily by Tom Bradwell. The resurvey combined the most up-to-date remote-sensing data and fieldwork. Colour monoscopic 1:10 000-scale vertical aerial photographs, existing large-scale geological maps, and the NEXTMap digital terrain model (Intermap Technologies) were viewed as semi-transparent layers within an ArcGIS environment. Boundaries of superficial deposits [superficial geological lines] were digitised on screen at a scale of 1:5000 and attributed using standard codes within the BGS Geological Spatial Database. Mapping was augmented in places using stereoscopic aerial photographs viewed in 3D using SocetSet. Field mapping was carried out by targeted walkover survey, during which geomorphological features were mapped and detailed logs of sedimentary exposures were recorded.

Onshore Quaternary geology linework overlaps with other BGS sheets currently in press or in production, including Sheet 101E (Ullapool, Superficial and Bedrock) (BGS, in press), Sheet 92E (Loch Fannich, Superficial and Bedrock), and Sheet 92W (Loch Maree). The Quaternary geological linework on provisional Sheets 91/100 (Gairloch) and 101W (Summer Isles) is superseded by the current work and the underlying digital database. Some geological and cartographic generalisation of the survey-scale data has been made in the production of this 1:50 000 scale product.

Chapter 2 Quaternary stratigraphy

The Quaternary stratigraphy depicted on the Summer Isles Quaternary Geology map represents a combined onshore–offshore lithostratigraphical scheme. The Quaternary succession comprises seven formations: 1) Fiona Formation; 2) Loch Broom Till Formation; 3) Assynt Glacigenic Formation; 4) Annat Bay Formation; 5) Ullapool Gravel Formation; 6) Annie Formation; and, 7) Summer Isles Formation (Figure 1). Of these, the Fiona, Annat Bay, Annie and Summer Isles formations are exclusively offshore units. The Assynt Glacigenic Formation includes four locally defined members within the map area: the Allt na h-Airbhe, Allt an t-Srathain, and Rhiroy members in Loch Broom, and the Rireavach Member in Little Loch Broom. For completeness, a fifth member — the Glen Douchary Member — is also included in this report; it occurs directly to the east of the area, and represents a suite of Younger Dryas morainic deposits. The Loch Broom Shell Bed together with a number of lithogenetic units, including late-stage debris-flow deposits, talus and scree, blown sand, fluvial and coastal deposits, and peat are locally mappable deposits, but cannot be attributed to specific formations.

In The Minch, much of the Quaternary succession comprises a palaeo-ice stream-related succession, up to 90 m thick (Stoker and Bradwell, 2005). Seaward of the Summer Isles sill, this succession is represented by the Fiona Formation. The former ice stream was fed by a number of ice-stream tributaries emanating from the coastal fjord region, including Loch Broom and Little Loch Broom (Bishop and Jones, 1979; Bradwell et al., 2007; Chesher et al., 1983) (Figure 6)." data-name="images/P1004346.jpg">(Figure 3). " data-name="images/P1004345.jpg">(Figure 2). In the Summer Isles area, the Loch Broom Till Formation was deposited from tributary glaciers during the most recent phase of ice-stream activity. The preservation of megascale glacial lineations at the sea bed in parts of The Minch attest to the rapid collapse of the ice stream subsequent to the Last Glacial Maximum, with The Minch essentially ice free by about 15 ka BP (Stoker and Bradwell, 2005).

Whereas the late-glacial to postglacial record in The Minch is a largely condensed sequence (the Annie Formation), the Summer Isles succession preserves a much thicker record of deglaciation, including evidence of ice survival into the Lateglacial Interstadial (Bradwell et al., 2008b; Stoker et al., 2009). The Assynt Glacigenic Formation is the most widespread unit, consisting of time-transgressive ice-contact and ice-proximal glacimarine and terrestrial glacigenic facies deposited across the whole region including headlands, valleys, shallow-marine banks and the deep floors of the fjord basins. A suite of 40 to 50 sea-floor moraine ridges, traced from the area between the headlands of Rubha Còigeach and Greenstone Point into the inner parts of Loch Broom and Little Loch Broom (Figure 6)." data-name="images/P1004346.jpg">(Figure 3). " data-name="images/P1004345.jpg">(Figure 2) and (Figure 6)." data-name="images/P1004346.jpg">(Figure 3), chart oscillatory retreat of the British Ice-Sheet from The Minch to topographically confined ice-cap outlet glaciers in the fjords (Bradwell et al., 2008b; Stoker et al., 2009). The Annat Bay Formation consists of ice-distal glacimarine deposits, laid down diachronously adjacent to the Assynt Glacigenic Formation. The Allt na h-Airbhe, Allt an t-Srathain and Rhiroy Members of the Assynt Glacigenic Formation represent late-stage re-advances of glaciers into Loch Broom; their association with the fan-delta deposits of the Ullapool Gravel Formation marks the oscillatory retreat of outlet glaciers from the fjords. The Summer Isles Formation is an exclusively Holocene marine unit.

Late Devensian

Fiona Formation (FONA)

The Fiona Formation is up to 90 m thick and is part of The Minch palaeo-ice stream succession. It rests unconformably on Permian and Mesozoic rocks. The top of the Fiona Formation is commonly exposed at sea bed across much of The Minch, between the Summer Isles and Lewis (Fyfe et al., 1993), where a mixture of gravel and mud has been recovered from the uppermost 0.5 m of the unit. Its seismic character (Figure 4) implies a stacked association of subglacial diamicton packages separated by megascale glacial lineation surfaces preserved at several levels within, and at the top of, the sequence (Bradwell et al., 2007; Stoker and Bradwell, 2005). The formation has been assigned a Late Devensian age (Fyfe et al., 1993); however, in the absence of deeper sample information, the stratigraphical range of the unit remains undetermined, and may be related to successive ice streams.

The eastern boundary of the Fiona Formation forms the western limit of the area described in this sheet explanation. The Summer Isles sill separates the Fiona Formation from the coastal region. Several sets of moraines are preserved at sea bed along the eastern edge of the Fiona Formation, informally termed the Rubha Còigeach–Loch Ewe moraine system (Stoker et al., 2009). These include a series of broad moraine ridges offshore Greenstone Point trending orthogonal to the coastline. They are possibly intermixed with bedrock, are up to 250 m wide and 20 m high, and located at the south-eastern end of the Greenstone Ridge, which is a major bathymetric high that extends north-north-west from Greenstone Point into The Minch (Figure 6)." data-name="images/P1004346.jpg">(Figure 3). " data-name="images/P1004345.jpg">(Figure 2). This probably represents a large, glacially streamlined till complex within the Fiona Formation (Bishop and Jones, 1979; Chesher et al., 1983) that formed between the ice-stream tributaries emanating from the Loch Broom region and Loch Ewe (Bradwell et al., 2007). Farther north, a series of broad, north–south-trending moraine ridges are preserved 3–4 km west of the Rubha Còigeach headland (Figure 6)." data-name="images/P1004346.jpg">(Figure 3). " data-name="images/P1004345.jpg">(Figure 2), trending subparallel or slightly oblique to the coastline and ranging from 50 to 250 m wide and 5 to 25 m high (Stoker et al., 2006). The largest individual moraine within this system occurs at the mouth of Loch Ewe, directly south-west of the map area, where it forms an arcuate ridge 1.5 to 2 km wide and 20 to 30 m high that essentially connects the headlands of Greenstone Point and Rubha Réidh; a distance of 6 to 7 km. This is the Loch Ewe Moraine (Stoker et al., 2006), which has a total thickness of 50 to 60 m. This moraine, and those offshore Rubha Còigeach, represent large late-stage recessional moraines of the Fiona Formation. This whole suite of moraines was probably deposited towards the end of a dynamic cycle of palaeo-ice stream collapse in The Minch, between about 20 and 15 ka BP (Stoker et al., 2009).

Loch Broom Till Formation (LBTI)

The Loch Broom Till Formation is up to 20 m thick, and forms a discontinuous, overconsolidated till sheet that rests unconformably on bedrock both onshore and offshore. The surface morphology of this till sheet is commonly streamlined, as observed on the flanks of outer Little Loch Broom and extending offshore beyond Stattic Point and Cailleach Head (Figure 6)." data-name="images/P1004346.jpg">(Figure 3), with lineations oriented parallel to surrounding streamlined bedrock features, such as roches moutonnées and megagrooves (Bradwell et al., 2008a). The formation comprises mainly till, but also includes gravel. The colour of the till varies from dull grey (5Y 6/1) to reddish brown (2.5Y 5/6) dependent upon the bedrock source of the sediment; generally either Moine psammite or Torridon Group sandstone. Although the Loch Broom Till Formation is separated from The Minch by the Summer Isles sill, it is most likely correlated with the upper part of the Fiona Formation (Stoker et al., 2009) (Figure 1)b.

The Loch Broom Till Formation principally comprises diamictons displaying a strong, pervasive, subhorizontal shear fabric. Onshore, at the type site at Allt an t-Srathain burn [NC 1085 9673], a 3.5 m-thick section of the Loch Broom Till Formation is exposed for about 100 m downstream (Stoker et al., 2009) (Plate 1). The grey, clay-rich, diamicton preserves a strong subhorizontal west-north-west-orientated clast fabric, with abundant bullet-shaped and faceted clasts. Clast lithologies typify the bedrock geology directly east of the area, with a predominance of Moine Supergroup psammite and lesser abundances of Cambrian Eriboll Formation sandstone (quartzite) and Torridon Group sandstones.

Offshore, the Loch Broom Till Formation is well preserved on shallow-marine banks, such as Martin Bank, as well as the submarine extensions of peninsula headlands (as noted above) (Figure 6)." data-name="images/P1004346.jpg">(Figure 3). On seismic profiles, the formation displays a variable seismic character (Figure 4) indicative of mixed lithologies and/or internal structure, as confirmed by core data. On Martin Bank, the Loch Broom Till Formation is commonly buried by up to 6 to 7 m of the Assynt Glacigenic Formation, though BGS cores 57-06/262 [57.9681° N, 05.3579° W] and 57-06/263 [57.9579° N, 05.3274° W], recovered by the 15 m rockdrill, proved up to 5 m of pebble to cobble grade gravel. The clasts comprised predominantly grey to dark grey psammite, semipelitic schist, and cream to pale grey quartzite, with subordinate Torridon Group arkosic sandstone. Due to the rotary action of the rockdrill, it is uncertain whether or not any original matrix material was washed away during the drilling operation. In contrast, the Loch Broom Till Formation occurs at or near to sea bed in Gruinard Bay, directly to the north-west of Gruinard Island, where BGS vibrocore 57-06/256 [57.9097° N, 05.5000° W] recovered a 1.52 m-long core section of diamicton. The diamicton is composed of abundant clasts of Torridon Group sandstone up to 8 cm long, and subordinate grey metamorphic clasts, set in a poorly sorted, weak red (10R 4/3) silty–sandy matrix (Plate 2). Many of the clasts are faceted and subangular to angular in shape, with fabrics varying from clast-to-matrix supported. In common with onshore exposures, clasts are preferentially orientated subhorizontally along their long axis. The red matrix is very stiff to hard and compact, and ranges from silt to medium-grained sand, but also includes sporadic, coarse- to very coarse-grained sand. On Martin Bank, discontinuous internal reflections on seismic profiles across the Loch Broom Till Formation may be attributable, in part, to this internal fabric.

On the basis of its sedimentology, morphological expression and internal shear fabric, the formation is interpreted as a subglacial lodgement till. Its streamlined form may have developed contemporaneously with the megascale glacially lineated surface that is currently exposed at sea bed atop the Fiona Formation in The Minch.

Assynt Glacigenic Formation (ASGL)

The Assynt Glacigenic Formation forms the most extensive deposit in the Summer Isles region, comprising a mix of lithofacies that range from a discontinuous onshore morainic veneer to thicker ice-contact, ice-proximal and glacimarine deposits (up to 100 m thick) in adjacent offshore basins. Although it mainly occurs landward of the Summer Isles sill and adjoining peninsulas, its offshore limit is interpreted to occur about 5 km west of the sill, marked by the outermost of a continuous sequence of 40 to 50 sea-floor moraines extending 30 km from The Minch to inner Loch Broom (Bradwell et al., 2008b; Stoker et al., 2009). This suite of pristine moraines is a major characteristic of the Assynt Glacigenic Formation, both onshore and offshore. West of the Summer Isles sill, the outer moraines form discrete, delicate features ranging from 10 to 30 m wide, and up to 5 m high, which are superimposed on the older Rubha Còigeach–Loch Ewe moraine system of the Fiona Formation. Farther east, the moraines are best preserved on Martin Bank and Cadail Bank (Figure 6)." data-name="images/P1004346.jpg">(Figure 3) where they are up to 20 m high and up to several hundred metres wide, and overlie either bedrock or the Loch Broom Till Formation. Some of these moraines have been traced into, and locally across, the adjacent basins, and several join with moraines onshore (Bradwell et al., 2008b). Boulders on moraine ridges in surface exposures near Achiltibuie and on the southern flank of Little Loch Broom have yielded ages of between 14.1 and 12.9 ka BP (Bradwell et al., 2008b). These are consistent with radiocarbon dates of marine shells recovered from contemporary glacimarine deposits in SAMS gravity core GC101 [57.9936° N, 05.3792° W] in the Tanera Basin, and BGS vibrocore 57-06/279 [57.9422° N, 05.2728° W] in the North Annat Basin, which gave ages of 13 973±73 ka BP and 14 111±98 ka BP, respectively (Stoker et al., 2009). Collectively, these data imply that the Assynt Glacigenic Formation in this region was deposited during the Lateglacial Interstadial.

Onshore, the Assynt Glacigenic Formation principally comprises heterogeneous clast-rich diamictons and interbedded gravels with diverse sedimentology. The stratotype site near Achduart in Coigach [NC 0470 0412] is characterised by boulder-strewn, moderately well-consolidated, sandy, clast-rich and clast-supported diamicton with thin interbeds of sand and silt. Clasts are typically angular to sub-rounded, with some faceting and only a low proportion of striated clasts. The lithology of the clasts largely represents the underlying bedrock, with the exception of sparse erratic boulders on the ground surface. At Achduart, the dominant (>95%) clast lithology is Torridon Group sandstone, in keeping with the local bedrock; but further inland, at other sites near Skiag Bridge beside Loch Assynt [NC 2303 2462], and within the Loanan Valley moraine system [NC 2430 1936], the Assynt Glacigenic Formation is comprised exclusively of angular to very angular Cambrian quartzite clasts. Gently dipping stratified gravel units and debris-flow related deformation structures are often present within exposures of the Assynt Glacigenic Formation. Soft-sediment deformation structures are locally present where differential loading has occurred during rapid sediment emplacement (Plate 3).

Offshore, the Assynt Glacigenic Formation is well preserved on shallow-marine banks as well as in the adjacent deeper-water basins. Its variable seismic character (Figure 4) reflects the interaction of both ice-contact and proglacial depositional processes during its formation (Stoker et al., 2009). The streamlined, structureless to chaotic sheet drape exposed on Martin Bank consists predominantly of diamicton as proved in numerous BGS cores, including a vibrocore at site 57-06/262 (57.9681° N, 05.3579° W) that recovered 0.64 m of massive, sandy diamicton with randomly orientated clasts of mainly Moine lithologies up to cobble grade (<8 cm) (Plate 4). In Little Loch Broom, lodged cobbles and boulders of Torridon Group conglomerate and sandstone are exposed at sea bed, atop a moraine ridge that caps the mid-loch sill (Plate 5).

Contemporary proglacial deposits, up to 10 m thick, lie ponded between the moraine ridges on Martin Bank, and comprise massive and colour-laminated clay and silty clay. Overlying the diamicton in BGS core 57-06/262, a 3.8 m-thick laminated section displays alternating grey and grey-brown laminae that range from 2 to 10 mm thick (Plate 4). This section, between 0.26 and 4.1 m depth in core 262, is taken as a reference section for the proglacial waterlain deposits. Stratigraphically equivalent proglacial deposits containing ice-rafted dropstones, as well as in situ shelly material and foraminifera, have also been proved in the deeper-water North Annat and Tanera basins, and in Loch Broom and Little Loch Broom, where they are locally interbedded with coarser-grained beds of reddish grey and olive-grey, very fine- to medium-grained sand with sporadic pebbles (Figure 5). In addition, a rare 3–4 m-thick section of sedimentologically similar interlaminated sand-silt-clay with sporadic pebbles is exposed in a coastal outcrop beneath Creag an Tairbh [NH 1346 9146], opposite Ullapool. This section shows clear evidence of internal deformation, including disrupted and convolute bedding, normal and reverse faulting, and loading and water-escape structures. Its isolated position, several tens of metres above the general level of comparable deposits within the Assynt Glacigenic Formation, suggests that this deposit may have been glaciotectonically rafted from the floor of Loch Broom during late-stage ice margin oscillation in this area (see below). The macro- and microfauna recovered from these proglacial deposits in the offshore cores are indicative of cold but not fully arctic conditions (Stoker et al., 2009).

The Assynt Glacigenic Formation includes five local members. The Allt na h-Airbhe, Allt an t-Srathain, Rhiroy and Glen Douchary members comprise diamicton deposited during late-stage glacier re-advances in Loch Broom, and the Rireavach Member in outer Little Loch Broom is an assemblage of mass-movement deposits. The Rhiroy Member overlies the Loch Broom Shell Bed, which has been dated at about 13 ka BP (see below).

The Allt na h-Airbhe Member (ANHA) represents the first of the late-stage readvances of outlet glacier lobes into Loch Broom. The stratotype for this member is the submarine landscape preserved between Ullapool [NH 128 938] and Corry Point [NH 142 923], where several domed and flat-topped curvilinear ridges, up to 20 m high and 500 m wide, trend oblique to the margins of the loch. Seismic reflection profiles reveal internal dipping reflectors that locally link into a décollement surface at the base of the ridges. This morphology and internal structure has been interpreted as an indicator of glacitectonic thrusting (Stoker et al., 2009). SAMS gravity cores GC125 [57.8881° N, 05.1679° W] and GC126 [57.8882° N, 05.1682° W] failed to penetrate more than 0.2 m into overcompacted sandy gravel on the flank of one of these ridges offshore Rubha Buidhe [NH 123 928]. The gravel comprised very poorly sorted, angular to subangular lithic clasts, including psammite, up to cobble grade, and a few shell fragments. The Allt na h-Airbhe Member may have a terrestrial expression in the deformed, laminated section at Creag an Tairbh (described above), but further work is needed to establish the exact stratigraphical relationships of this complex deposit. A series of moraines offshore Allt na h-Airbhe [NH 1152 9303] probably mark the limit of ice grounding in Loch Broom, which deepens into the outer loch beyond this point. However, seismic profiles indicate that a thin sequence of debris-flow deposits accumulated in the outer loch, most likely derived from the ice margin. The Allt na h-Airbhe Member predates the deposition of the Rhiroy Member (see below); it is also partly overlain by the prograding Ullapool fan-delta, which belongs to the Ullapool Gravel Formation.

The Allt an t-Srathain Member (ANSR) overlies the Loch Broom Till Formation onshore near Ullapool and, in the type section, is exposed in the banks of the Allt an t-Srathain channel [NH 1070 9678] where it reworks a gravel unit correlated with (or stratigraphically equivalent to) the Ullapool Gravel Formation (Plate 6). The Allt an t-Srathain Member at this location is a 1–3 m-thick, red-brown (2.5YR 4/4), well-consolidated, gravel-rich diamicton with a sand-dominated matrix. The diamicton has a weakly developed clast fabric and sporadic steeply dipping discontinuities, both recording an overall east–west directional sense. Numerous striated and lodged clasts can be observed and several have been fractured or sheared. The base of the Allt an t-Srathain diamicton is locally gradational with the underlying gravel unit, especially east of the burn. In the poorly accessible section west of the burn, the base of the Allt an t-Srathain Member is marked by a discontinuous unit of laminated clay and silts approximately 0.2 to 0.5 m thick. Rip-up clasts, 'till pebbles' and fold structures observed in exposures east and west of the burn attest to a degree of post- or synsedimentary deformation.

The Rhiroy Member (RHIR) is presently only defined in the offshore region of inner Loch Broom, where it is separated from the Allt na h-Airbhe Member by a buried moraine ridge within the loch between Rhiroy [NH 152 904] and Leckmelm [NH 163 909], which marks the limit of this late-stage re-advance deposit (Stoker et al., 2009). This moraine ridge is between 10 and 20 m high and, although covered by postglacial deposits in the loch, can be traced onshore as a number of boulder-strewn mounds between Rhiroy [NH 1525 8985] and Ardindrean [NH 1570 8875]. On the eastern side of Loch Broom, the geomorphological limit of the Rhiroy Member is unclear owing to dense forestry; a clay-rich diamicton overlying glaciotectonically disturbed laminated silts in Ardcharnich gorge [NH 1770 8890] probably represents its stratigraphical equivalent, although the diamicton may represent a subsequent glacier re-advance of slightly lesser extent. Seismic-profile data show that the base of the member is irregular and rests with erosional contact on older, basin infill deposits of the Assynt Glacigenic Formation, and, where proved by core data, overlies the Loch Broom Shell Bed. The top of the unit is unconformably overlain by the Summer Isles Formation. Its total extent within inner Loch Broom is obscured on seismic profiles by acoustic blanking due to the presence of shallow gas. The stratotype for the Rhiroy Member is provided by BGS vibrocore 57-06/267 (57.8655° N, 05.1063° W) (Figure 5), which recovered a 1.34 m-thick (2.06–3.4 m below sea bed), olive grey (5Y4/2), soft to firm, muddy and shelly diamicton with randomly orientated, matrix-supported clasts of quartzite, pelite and arkosic sandstone up to 10 cm long. The shelly material includes some well-preserved shells, including a single whole valve of the bivalve Mya truncate Linnaeus. Some of this material may have been incorporated from the underlying Loch Broom Shell Bed, which shows a crude stratification directly below the sharp contact with the diamicton. Radiocarbon dating of the Loch Broom Shell Bed in this core suggests that the Rhiroy Member is no older than 13 ka BP.

Although not yet formally defined, glacial deposits relating to the Younger Dryas chronozone are present within the Loch Broom catchment in the valleys dissecting the western portion of the Beinn Dearg massif. These hummocky morainic deposits, which were mapped by Sissons (1977) chiefly in Glen Douchary, Strath Mulzie, Coire Mathair Lathail and Coire Loch Tuath, all relate to a period of restricted mountain glaciation (or re-advance) during the Younger Dryas Stadial (12.9–11.5 ka BP). These glens were remapped during routine BGS surveys in 2006–2008, and for completeness, these morainic deposits are informally termed the Glen Douchary Member within the Assynt Glacigenic Formation.

In Little Loch Broom, the Rireavach Member (RIAV) comprises a discrete assemblage of mass-movement deposits of mixed lithology, up to 12 m thick, which occur in the outer part of the loch. These are derived mainly from a series of slide scars on the northern flank of the loch and the western side of the mid-loch sill, which collectively form the Little Loch Broom slide complex (see Geohazards; (Figure 6)). The base of the member is discordant with the underlying, undeformed sediments of the Assynt Glacigenic Formation, which are truncated at the contact with the mass-movement deposits; the top is generally covered by a veneer of postglacial deposits. The stratotype for the Rireavach Member is outer Little Loch Broom, between the outer- and mid-loch sills [NG 986 963] to [NH 020 940], within which the unit is confined. Its seismic character is consistent with a mass-transport origin (Figure 4), and three sediment cores have recovered sandy and muddy debris-flow deposits as well as intact slide blocks (Stoker et al., 2010). SAMS gravity core GC122 [57.9082° N, 05.3954° W] in combination with BGS vibrocore 57-06/286 [57.9079° N, 05.3957° W] penetrated 2.57 m into a debris-flow deposit off Corran Scoraig [NG 996 961] (Figure 6) and recovered massive, compact, reddish brown gravelly sand. In contrast, SAMS gravity core GC119 recovered a 0.2 m-thick sandy bed overlying 0.69 m of slightly sandy clay from the edge of the debris lobes derived from the Scoraig Slide. Discrete bodies of coarse lithic grains are scattered throughout the clay and possibly represent larger, matrix-supported intraclasts. Deposits of the Rireavach Slide were penetrated by SAMS gravity core GC087, which recovered 0.62 m of colour-laminated silty clay overlying a 0.14 m- thick bed of shelly, gravelly, sandy mud, in turn overlying homogeneous clay and silty clay. The contacts between all three beds are sharp. Although the laminated clay shows displacement of laminae along small faults within the bed, it retains coherency as an intact block and displays angular discordance with the underlying bed (Stoker et al., 2010).

Annat Bay Formation (ATBA)

The Annat Bay Formation occurs only in the glacially overdeepened Tanera, Coigach, North Annat, South Annat, Skerries and South Priest basins, as well as inner Little Loch Broom (Figure 6)." data-name="images/P1004346.jpg">(Figure 3). It comprises ice-distal glacimarine deposits up to 25 m thick, which are diachronous with the late-glacial Assynt Glacigenic Formation (Figure 1), and accumulated in the deep-water basins as they became increasingly distant from the receding fjord glacier grounding line(s) (Stoker et al., 2009). The base of the formation is marked by an angular unconformity with the Assynt Glacigenic Formation; its upper boundary is commonly an erosional surface overlain by the Summer Isles Formation. These unconformable relationships are especially enhanced on the margins of the basins.

BGS core data suggest that the Annat Bay Formation comprises predominantly silty mud with scattered shell fragments (Stoker et al., 2009). The stratotype for the Annat Bay Formation is provided by BGS vibrocore 57-06/277 [57.9471° N, 05.2696° W] (Figure 5), which penetrated 4.24 m into the upper part of the formation in the North Annat Basin (0.4–4.64 m below sea bed). The silty mud is homogeneous, dark greenish grey (GLEY 1 4/1) to dark grey (5Y 4/1), and poorly to very poorly sorted. It has a mean grain-size range of fine- to medium-grained silt, with a clay content that varies between 30 and 40%, increasing down the core. Foraminiferal tests and bivalve fragments are scattered throughout the core. The foraminifera from this and an adjacent vibrocore (57-06/278 [57.9421° N, 05.2717° W]), to indicate a mix of cold arctic and temperate species, providing some support for deposition during the Lateglacial Interstadial. At 4.29–4.31 m in core 57-06/277, there is a localised abundance of carbonate associated with very thin-bedded fine-grained sand, which probably represents a sporadic winnowed horizon. This implies current activity, and is consistent with the ponded to onlapping seismic character of the formation (Figure 4). Indeed, the infill geometry in some of the basins is asymmetric, with sedimentation focused on one side of the basin so that maximum sediment thickness does not necessarily follow the basin thalweg. This stratigraphical style is characteristic of modern-day glaciated fjords, where the Coriolis force influences sediment dispersal from plumes such that deposition is preferentially forced onto one side of the basin (Syvitski, 1989).

Loch Broom Shell Bed (LBSH)

The Loch Broom Shell Bed has been sampled in two BGS vibrocores from Loch Broom: 57-06/269 [57.9144° N, 05.2213° W] in the outer loch, and 57-06/267 [57.8655° N, 05.1063° W] in the inner loch, where beds 0.68 m (1.0–1.68 m below sea bed) and 0.64 m (3.4–4.04 m below sea bed) thick, respectively, have been proved (Stoker et al., 2009) (Figure 5). Core 57-06/269 is taken as the type section; it is a relatively undisturbed section, in contrast to the sequence in core 57-06/267, which was overridden during a late-stage glacial re-advance. The base of the outer loch shell bed is unconformable on the bioturbated top of the Assynt Glacigenic Formation; its top is unconformably overlain by the Summer Isles Formation. The shell bed in the inner loch also overlies bioturbated sediment of the Assynt Glacigenic Formation, but is overlain at the core site by the late-stage till of the Rhiroy Member (Figure 1).

In general, the shell bed is very poorly sorted and contains abundant shells and shell fragments dominated by bivalves, with some pebbles up to 4 cm set in a very soft, dark greyish brown (2.5Y 4/2), muddy and sandy matrix. However, in detail, the shell bed is divisible into three lithofacies units (Figure 5), of which the lower and middle units are comparable at the two sites where cored, but with different upper units. In both cores, the internal stratigraphy of the shell bed consists of: 1) a basal muddy sand/sandy mud (0.13–0.19 m thick) containing whole and fragmented shells, including Chlamys islandica (Müller) and Arctica islandica (Linnaeus); and, 2) a middle unit composed of a denser hash (0.3–0.36 m thick), with a predominance of large single valves of Chlamys islandica and subordinate Tridonta elliptica (Brown) that show evidence of boring, abrasion and encrustation. The shells are randomly orientated in both of these units. In core 57-06/269, the upper unit (0.19 m thick) of the outer loch shell bed contains abundant angular to subangular, granule to pebble-grade gravel clasts up to 4.5 cm in diameter, mixed with whole and fragmented shells, including Tridonta elliptica, set in a matrix of silty mud. The gravel clasts are probably ice-rafted material (Stoker et al., 2009). In core 57-06/267, the upper unit (0.15 m thick) of the inner loch shell bed consists of a crudely bedded accumulation of subhorizontally aligned large shells, dominated by Chlamys islandica, set in a sandy and muddy matrix. This bed is overlain by the late-stage glacial re-advance diamicton of the Rhiroy Member (Assynt Glacigenic Formation). At both core sites, the abundance of shell material contained within the shell bed contrasts markedly with the underlying deposits of the Assynt Glacigenic Formation, where shell material is either absent (57-06/269) or much reduced in abundance and lacking Chlamys islandica (57-06/267).

A radiocarbon date of 13 047±59 years BP was determined from Chlamys islandica taken from the inner loch shell bed (Stoker et al., 2009). This late-glacial age is supported by the presence of mixed assemblages of warm and cold water species of foraminifera in both cores, the superposition of a late-stage till on the inner loch shell bed, and the deposition of ice-rafted clasts in the upper part of the outer loch shell bed.

Ullapool Gravel Formation (ULGR)

The Ullapool Gravel Formation comprises sand and gravel deposits occurring in coastal locations, most notably around the margins of Loch Broom, Little Loch Broom and Loch Kanaird. Fan-deltas at Ullapool and Newton Loggie in Loch Broom form two of the most distinctive onshore occurrences of this unit, the limits of which have been extended offshore into the loch using swath bathymetric data (Stoker et al., 2009). The Ullapool fan-delta is a large sandy gravel accumulation, more than 30 m thick, that progrades, and extends more than halfway across the Loch Broom basin. Swath bathymetry and boomer profiles indicate that this fan-delta deposit oversteps, and therefore postdates, the ice-contact geomorphology associated with the Allt na h-Airbhe Member of the Assynt Glacigenic Formation. However, 2 km to the north-west, a similar fan-delta of the Ullapool Gravel Formation is locally overlain by the glacigenic Allt an t-Srathain Member (Plate 6). The geometry of the fan-delta deposits has been modified by post-depositional changes in relative sea level. The Ullapool fan-delta preserves former shorelines at approximately +15 m, +5 m, present-day (i.e. 0 m), and −10 m OD (Stoker et al., 2009). The raised sea-level evidence is also observed onshore at Rhue and Newton Loggie (Plate 7).

Natural exposures onshore, including the type section at Ullapool [NH 1284 9461], show that the Ullapool Gravel Formation consists of clast-supported, crudely bedded, glaciofluvial gravel with sporadic intercalated sand beds (Plate 8). Size sorting within individual beds is common, with cobble-grade material predominating. Clasts are typically rounded–subrounded and imbricated, and include lithologies from all local geological formations. The finer-grade gravel beds commonly display normal grading. Large-scale planar cross beds and foresets are exposed locally, with dips typically to the west. In contrast, eastward dips occur in the Newton Loggie fan-delta. Sandy units sporadically preserve ripple bedding, silt stringers and evidence of current reworking. SAMS core GC129 [57.8767° N, 05.1238° W] penetrated the top of the submerged fan-delta offshore Newton Loggie, and proved 0.29 m of gravel in a muddy and sandy matrix.

Late stage debris-flow deposits (DFD)

This time-transgressive lithogenetic unit occurs both offshore and onshore, and is characterised by deposits that reflect sidewall entry of material into the fjords and valleys as the outlet glaciers retreated. Offshore, this lithogenetic unit comprises a number of discrete, localised, basin-floor wedges, up to 8–9 m thick, which occur on the flanks of several basins, including outer Loch Broom, inner Little Loch Broom, Coigach Basin, and South and South-East Annat basins. The wedges generally occur sandwiched between the Annat Bay and Summer Isles formations, though where the former is absent they separate the Assynt Glacigenic and Summer Isles formations. Their seismic character and geometry (Figure 4) is a characteristic acoustic response of debris-flow deposits (Embley, 1980; Nardin et al., 1979). This is consistent with their lithofacies characteristics, which includes soft, pebbly, muddy diamicton recovered in SAMS gravity-core GC123 [57.9216° N, 05.2345° W] in Loch Broom, and folded beds of clay and sandy silt with intraclasts of muddy sand in SAMS gravity-core GC092 [57.8776° N, 05.3227° W] in Little Loch Broom (Stoker et al., 2010).

Onshore, localised large discrete sediment cones occur on the flanks of the main sea lochs and on steep valley sides. The cone-shaped geomorphology of these features indicates a point source; whereas their coarse-grained poorly sorted sedimentology implies close proximity to the sediment source. Good examples of these sediment cones occur on Creag an Tairbh [NH 1360 9123] opposite Ullapool, and on Beinn nam Ban [NH 1059 8945]. The deposit at Creag an Tairbh is a large cone of matrix-rich, poorly sorted, coarse-grained gravel and boulders, with sporadic fine-grained units (Plate 7). This glacially fed cone terminates at 20 m above sea level and partly overlies a glaciofluvial terrace within the Ullapool Gravel Formation, and therefore postdates the main period of late-glacial fjord glaciers. The debris cone is probably formed by sediment derived from a dwindling mass of ice that was located on the higher ground between Loch Broom and Little Loch Broom.

Late Devensian to Holocene

Annie Formation (ANNE)

The Annie Formation represents a late- to postglacial deposit that infills localised hollows on the sea bed, predominantly in The Minch region, but also west of Lewis (Fyfe et al., 1993; Stoker et al., 1993). It unconformably overlies bedrock and older glacial units, including the Fiona Formation. The seismic character of the Annie Formation indicates that the top of the unit is locally eroded at the present-day sea bed. The formation rarely exceeds 25 m in thickness, though it is exceptionally thick (up to 90 m) in a small basin offshore Stornoway on the western side of The Minch. BGS borehole 78/4 [58.1422° N, 06.2933° W] cored through the Annie Formation in this basin and proved a 52 m-thick sequence of dark grey to very dark grey (5Y 4/1–3/1), very soft to soft silty clay that becomes firmer towards the base of the core, accompanied by sporadic matrix-supported pebbles below 45.5 m depth. Biostratigraphical data have shown that the formation preserves a record of the transition from arctic to temperate environmental conditions (Graham et al., 1990), essentially representing the return of Gulf Stream waters to western Scotland concomitant with the withdrawal of the Late Devensian ice-sheet (Peacock and Harkness, 1990). Radiocarbon dating of mollusc shells indicates that this basin, and thus The Minch in general, was ice free by about 15 ka BP (14 891±192 ka BP; Graham et al., 1990; Stoker and Bradwell, 2005). The Devensian/Holocene boundary occurs at 16.02 m in the borehole. Radiocarbon dating of the uppermost unit, between 16 and 9 m depth, indicate an early Holocene age, [pre-9.2 ka] BP (9 211±196 ka BP; Graham et al., 1990) for this part of the sequence.

Holocene

Summer Isles Formation (SUMI)

The postglacial Summer Isles Formation is widespread throughout the offshore region. It is thickest in the basins where it predominantly comprises silty mud, but forms a coarser-grained, sandy and gravelly lag deposit on the shallow banks. In the basins, the formation is mostly no more than 5 m thick, though it is locally up to 12 m thick in outer Loch Broom where a mounded sediment drift is developed (Stoker et al., 2009). The base is commonly erosional, especially at the margins of the basins where the underlying Annat Bay Formation and older units have been locally scoured. The top of the formation is marked by the sea bed, which itself has been locally scoured and eroded, though it remains uncertain how active sedimentary processes are at the sea bed at the present time. Small-scale (a few centimetres) sediment ripples have been observed on backscatter imagery from the shallow banks. Localised mass-flow deposits have been observed on the basis of seismic character (Figure 4) and core data (Stoker et al., 2010) in Little Loch Broom, and the expulsion of shallow gas from the basins throughout the region has resulted in the top of the unit being locally disrupted by pockmarks (Stoker et al., 2006, 2009). These are described further in the Geohazardssection 3.

The stratotype section for the Summer Isles Formation is BGS vibrocore 57-06/267 [57.8655° N, 05.1063° W] (Figure 5). This borehole penetrated the feather-edge of the basin infill in inner Loch Broom, where it unconformably overlies the Rhiroy Member of the Assynt Glacigenic Formation. The type section is 2.06 m thick, and can be divided into three lithofacies units that define an overall fining-upward trend: 1) a basal, 0.16 m-thick unit of very coarse-grained, poorly sorted, carbonate gravel (shell hash) that includes several large valves of Arctica islandica, and very soft to soft rip-up clasts of mud; this is overlain by 2) an approximately 0.3 m-thick unit of medium- to coarse-grained, olive-grey (5Y 4/2), muddy, slightly gravelly and shelly sand; this grades upwards at about 1.6 m into 3) dark olive-grey (5Y 3/2) to very dark greyish brown (2.5Y 3/2), very soft to soft, silty mud, with sporadic shells including Turritella sp. This unit is bioturbated and very slightly sandy between about 1.6 and 0.9 m depth, but more generally homogeneous and massive from 0.9 m to sea bed.

Calibration of this core data with the seismic character of the Summer Isles Formation (Figure 4) suggests that the coarse-grained basal facies (unit 1) may correlate with an acoustically chaotic layer that is locally observed at the base of the formation. The overlying, increasingly finer-grained deposits correlate with the seismically well-layered texture that characterises most of the Summer Isles Formation. In combination, these data suggest that bottom-current activity persisted throughout the deposition of the Summer Isles Formation. Initial current activity was quite vigorous and erosional; however, finer-grained sediment accumulated as the current regime became more stable. The locally mounded geometry of the Summer Isles Formation, which is characteristic of sediment drifts, reflects differential erosion and deposition by bottom currents (Stoker and Bradwell, 2009; Stoker et al., 2009).

An early Holocene radiocarbon date of 8012 ± 53 years BP has been obtained from Arctica islandica from the basal bed in vibrocore 57-06/267, and foraminiferal analysis of the entire section revealed a warm-water assemblage (Stoker et al., 2009). A Holocene age has also been proved from the Tanera Basin (Figure 6)." data-name="images/P1004346.jpg">(Figure 3), where SAMS gravity core GC101 [57.9936° N, 05.3792° W] penetrated the 1.3 m-thick Summer Isles Formation, and derived radiocarbon dates of 7670 ± 44 years BP and 3853 ± 72 years BP from the bivalves Nucula sulcata (Bronn) (1.04 m) and Glossus humanus (Linnaeus) (0.54–0.59 m), respectively.

Unassigned lithogenetic units

Talus

Talus describes any accumulation of angular rock debris derived from a cliff or bedrock slope. Although no talus deposits are shown on the 1:50 000 map, principally owing to scale, numerous small areas of talus occur within the region. The most notable talus deposits occur at the foot of large rock slopes formed by Moine Group psammite, Torridon Group sandstone and Cambrian quartzite. These lithologies form tabular and blocky screes on steep slopes and beneath escarpments. Good examples are seen on the upper flanks of An Teallach, on Stac Pollaidh, on the western flanks of Cul Beag, and on Cnoc na Croiche [NC 1328 9444] near Ullapool, the latter is in quartzite. Talus thickness varies from 0.3 m to 3 m, and may be considerably thicker in places where periglacial processes have had more time to act.

Blown sand (BSA)

Wind-blown sand occurs as localised dunes and sheets on the landward side of several beaches in a number of bays. The sand is typically a mixture of fine-grained quartz grains and broken shell fragments of sand-grade (25–1000 μm). The proportion of shell fragments is more than 95% locally. More detailed study of the sedimentology of beaches in north-west Scotland was undertaken by Ritchie and Mather (1969). Within the Summer Isles region, the largest coastal accumulations of blown shell sand occur at Mellon Udrigle; flanking Gruinard Bay near Laide; near Little Gruinard; behind Mungasdale beach (see front cover), and backing Achnahaird Bay where individual dunes exceed 10 m in height. Smaller wind-blown accumulations exist on the Summer Isles themselves, for example on Isle Ristol, and on Tanera Mor. None of these aeolian deposits have been formally assigned to the lithostratigraphical scheme.

Where blown sand extends inland of coastal dunes and is stabilised by vegetation to form pasture it is known as machair. The machair deposits of Moravach [NC 0170 1330] at Achnahaird are quite considerable, extending over 0.7 km2 and reaching up to 20 m above present-day sea level. These large sand accumulations probably relate to periods of formerly higher sea level and possibly increased windiness. The machair deposits have stabilised over the latter part of the Holocene and may date from the period immediately after glaciation. Studies have shown (e.g. Ritchie and Mather, 1969) that some of the blown-sand deposits in north-west Scotland are in a state of sediment balance (i.e. sediment supply equals removal), whereas other dune complexes are currently undergoing erosion (i.e. net loss). The reasons for this imbalance are unclear.

Several high-elevation locations support discontinuous sheets or pockets of wind-blown sand. The best examples are on An Teallach, Ben Mor Coigach and Cul Beag. These aeolian deposits occur exclusively on Torridon Group sandstone above 600 m above sea level and have accumulated intermittently since the end of deglaciation at about 11 ka BP (Ballantyne and Morrocco, 2006). Their extent is localised (generally <0.1 km

) and they rarely exceed 2 m in vertical thickness, although they may have originally formed more extensive sheets. Studies show that many of these sand bodies have undergone erosion during the last few centuries (Morrocco et al., 2007).

Fluvial deposits

Most of the present-day rivers actively rework gravel deposits originally laid down as braidplains or outwash sheets by glacial rivers during ice-sheet retreat (about 15–12 ka BP). Although some postglacial sedimentation has occurred along several modern rivers courses, it is difficult in this part of Scotland to distinguish wholly modern alluvial deposits from reworked glaciofluvial deposits, therefore no alluvium (sensu stricto) is shown on the map. Several straths do, however, exhibit up to three river terraces. These are formed by rivers, both glacial and non-glacial, responding to changes in base level by incising and then redepositing sediment within the main channel. Several well-developed terraces are seen in Strath Canaird, Allt an t-Srathain, Strath Broom, and along the Dundonell and Gruinard Rivers where there has been local paraglacial and postglacial fluvial sedimentation. None of these fluvial deposits have been formally assigned to the lithostratigraphical scheme.

Coastal deposits

These can be broadly separated into storm beach, shoreface and tidal flat deposits. Although none are shown explicitly on the 1:50 000 map face, some coastal deposits are worthy of mention. Storm beaches consisting of berms or constructional ridges of cobble gravel and boulders flank numerous exposed bays and headlands. Good examples are found at Mellon Udrigle, Camas na Ruthaig, Ardmair, Camas Mor, Achiltibuie, Mungasdale beach (see front cover), Reiff, and Inverkirkaig. Most of these are probably relict features relating to formerly higher sea levels (+5 m OD) during the early to mid Holocene.

Shoreface processes typically rework pre-existing coarse-grained, unconsolidated sediments between low and high-water mark. At many locations, these sediments were laid down as glaciofluvial gravel sheets or fans during deglaciation. An excellent example of a 30 m wide cobble gravel beach formed of reworked pre-existing glacial sediments is found along the shoreline in Loch Broom, stretching from Ullapool Point to Allt an t-Srathain burn.

Tidal flats occur at the head of sea-lochs in the Summer Isles region. The largest expanses occur at the mouth of both the Dundonnell River and the River Broom (Plate 9) where, at low tide, almost 1 km2 of sandy, locally gravelly mud flats are revealed. Another large expanse of tidal flats occurs at the mouth of the River Canaird, around Keanchulish, where about 0.6 km2 of sandy mud flats are revealed at lowest tide. Holocene salt marshes are well developed at the head of Little Loch Broom, and also just landward of Achnahaird Beach. None of the coastal deposits mentioned are currently placed within the formal lithostratigraphical scheme.

Peat

Although not depicted on the map, peat covers much of the high ground and plateaux away from the coastal headlands. Its thickness is not considerable anywhere, and is typically less than 1 m thick. The Moine Group psammites north and east of Loch Broom are overlain by the most extensive peat deposits where they locally exceed 2 m in thickness; by contrast, the Lewisian terrain hosts only small relatively thin pockets of peat. All the peat in this part of north-west Scotland has accumulated in postglacial times (i.e. since 11 ka BP), and in places has been cut for fuel.

Chapter 3 Geohazards

Geological processes strongly influence the shape and nature of the landscape, including the sea bed. Assessing both terrestrial and submarine ground conditions is essential to understanding the natural environment, and should be considered at an early stage in the planning of future developments in order to mitigate against potential problems. In the Summer Isles region, glacial and paraglacial processes have left an indelible imprint on a landscape that varies in ground conditions, especially the submarine landscape. With a local economic emphasis on aquaculture and commercial fisheries, understanding the sea bed is important in terms of maintaining safe operations, particularly with regard to anything moored to, or located upon, the sea bed. Significant aspects of the submarine environment discussed here include shallow gas and pockmarks, sea-bed scouring, slope instability and neotectonic deformation.

Paraglacial slope instability and mass failure

Paraglacial processes include those non-glacial phenomena directly conditioned by former glaciation and deglaciation. For example, the withdrawal of ice-sheets and glaciers exposes sediments and rock slopes in an unstable state, which are prone to readjustment under gravity-driven processes. Many of the large-scale landslides and submarine slope failures in western Scotland may be regarded as paraglacial phenomena, formed in response to glaciation, but occurring long after deglaciation. Adjustment (or relaxation) of the landscape to nonglacial conditions has operated throughout the period since glaciation ended. Ballantyne (2002) has described the rate of sediment reworking in Scotland using a negative exponential exhaustion model, and suggests that many deglaciated Late Pleistocene landscapes are still in a state of readjustment.

The cliffed rocky coastline around the peninsula headlands, such as Cailleach Head and Rubha Coigeach, displays evidence of recent block failure into the sea. A particularly good example is seen on Eilean Dubh, where it can be best viewed from the Ullapool–Stornoway ferry. On the gentler sediment-mantled slopes, such as those flanking Loch Broom and Little Loch Broom, the glacial sediment cover has locally failed leaving distinct scarps on the hillsides (Plate 10). Although there are very few large rock-slope failures within the Summer Isles region, slope instability is widespread on a smaller scale. Many large glacially deposited boulders were left perched in precarious locations on slopes and summits, some of these have since moved downslope, either rapidly as rockfalls, or gradually as 'ploughing boulders'. Farther inland, scree and talus cones on mountains such as Stac Pollaidh, An Teallach and Ben Mór Coigach, mark the cumulative expression of displaced rock material over many centuries (see Quaternary stratigraphy). Many of these features are still active today, but were probably most active immediately after deglaciation.

Slope instability and mass failure is also widespread in the offshore area. The bounding walls of the sea lochs and the edges of the shallow-marine banks display an uneven, crenulated form, that is comparable to the adjacent coastline. Mass-flow deposits are commonly preserved downslope from the failure (Stoker et al., 2006; Stoker et al., 2010). The scale of these features ranges from several tens to several hundreds of metres in lateral extent, and include the 'Late-stage debris flows' (see Quaternary statigraphy).

In contrast, sediment slides up to 2 km long occur in Little Loch Broom and the North Annat Basin (Stoker et al., 2010)(Figure 6) and (Figure 7). The Little Loch Broom slide complex is a series of slides, including the Rireavach, Carnach and Scoraig slides, which have disrupted about 2.5 km2 of the sea bed in the outer part of the loch (Figure 6). The Rireavach Slide is the largest of the group and has three major scarp surfaces, 1–3 in (Figure 6), located between 55 and 75 m water depth. The scarp faces are between 7 and 15 m high and have slope angles of 7° to 15°. The displaced sediment, which has accumulated as mass-flow deposits on the loch floor, forms the Rireavach Member of the Assynt Glacigenic Formation from which it is derived. The Badcaul Slide occurs in the inner part of Little Loch Broom and is separated from the Little Loch Broom slide complex by the mid-loch sill (Figure 7). It is characterised by a number of terraces that step down into the inner loch via a series of scarps with slope angles up to 15°. The base of the present-day slope is at 110 m water depth, and is marked by a large debris lobe derived from the slide, which forms part of the late-stage debris-flow lithogenetic unit. The Holocene Summer Isles Formation overlies this unit.

In the North Annat Basin, the Cadail Slide can be traced for about 2.5 km around the landward end of the basin (Figure 7), where it disrupts sediment of the Assynt Glacigenic Formation. The main slide scarp is about 10 m high, with a slope angle of about 4°, and is exposed between about 45 and 55 m water depth; it may continue for about 5 m beneath the sea bed. A second scarp about 5 m high, with a slope angle of 5°, is exposed landward between 35 and 40 m water depth. Mass-flow deposits are preserved basinward of the main scarp, and are overlain by undisturbed sediments of the uppermost part of the Assynt Glacigenic Formation and overlying Annat Bay Formation, which onlap the slide scarp.

The stratigraphical evidence suggests that the large-scale slides were instigated about 14–13 ka BP, and are probably the paraglacial response of the landscape to deglaciation immediately following the removal of ice support during glacial retreat (Stoker et al., 2010). The more pervasive coastal failure may represent an ongoing response to paraglacial processes, as evidence of Holocene submarine mass failure has been found in Little Loch Broom (Stoker et al., 2010).

A separate style of mass failure is preserved in the basin-floor fjord deposits of the Assynt Glacigenic Formation in the outer part of Loch Broom, where slumping is facilitated by extensional faulting contained within the sediments. This failure is expressed at the sea bed as two distinct depressions at least 10 m deep and several hundred metres wide (Figure 7). The faults do not penetrate into the overlying units, which include the late-stage debris-flow lithogenetic unit and the Summer Isles Formation, and a late glacial age is inferred (Stoker and Bradwell, 2009). Neotectonic earthquake activity related to ice unloading is the most probable cause of this deformation. Holocene bottom-current activity has partially modified the shape of the depressions and influenced the type of sediment infill.

Shallow gas and pockmarks

Shallow gas derived from decaying organic matter is observed in the South Priest, Skerries, Coigach, North Annat and South Annat basins, and the inner basins within Loch Broom and Little Loch Broom (Figure 7). The presence of shallow gas is revealed on seismic-reflection profiles as a curtain of acoustic blanking that masks the seismic layering of the basinal sediments. In most basins, the gas originates from beneath a near- horizontal layer in sediments of the Annat Bay Formation that lies within 5–10 m of the sea bed, though in inner Loch Broom it occurs within 1–2 m of the sea bed in the Summer Isles Formation. The largest areas of shallow gas depicted on the seismic profiles range from 1.5–2 km2 and occur in the Skerries and Coigach basins, and in inner Loch Broom.

In most of the basins containing shallow gas, its presence is revealed by the occurrence of pockmarks; a gas-release feature expressed as a circular hollow or depression on the sea bed. These range from 1–10 m deep and 50–250 m wide, and occur as singular features, in clusters or as linear trails. The largest, singular pockmarks occur in the South Annat Basin and in inner Loch Broom, with sea-bed depressions up to 10 m deep, and 200 to 250 m wide. The densest cluster of pockmarks is in the Coigach Basin, where up to 19 per km2 are observed. Linear trails are well developed in the inner part of the Coigach Basin, and in inner Loch Broom and Little Loch Broom; these may reflect preferential fluid escape along pre-existing channels. No pockmarks are observed in the Skerries Basin, which has one of the largest areas of shallow gas.

The pockmarks are developed in the Holocene Summer Isles Formation, which suggests that these features may be active at the present-day, and may represent a hazard to sea-bed infrastructure.

Sea-bed scouring

Tidal currents have locally scoured the sea bed, and have formed areas of irregular sea-bed morphology with a relief of up to 20 m. The best examples of this occur in the North and South Annat basins, and in outer Loch Broom (Figure 7). Superficially, parts of this relief resemble pockmarks, though inspection of the seismic profiles reveals an erosional form, and occasionally a relief formed by differential erosion and deposition in a pre-existing hollow. The latter is evident in outer Loch Broom where a depression formed by neotectonic faulting and sea-bed collapse has been partially infilled by sediment deposited under the influence of bottom currents (Stoker and Bradwell, 2009). The depression may have focused the currents and created localised eddies that shaped the sediment fill. Although tidal currents have influenced the deposition of the Summer Isles Formation throughout the Holocene, the location and extent of present-day sea-bed erosion remains uncertain.

Information sources

Geological information held by the BGS relevant to the Summer Isles region is listed below. Enquiries concerning geological data for the district should be addressed to the National Geoscience Information Centre, BGS, Edinburgh. The current BGS Catalogue of Geological Maps and Books is available at the BGS website (www.bgs.ac.uk). BGS maps, memoirs, books and reports relevant to the region may be consulted at BGS and some other libraries. They may be purchased from the BGS Sales Desk, or via the bookshop on the BGS website.

Searches of indexes to some of the materials and documentary records collections can be made on the BGS website.

Geological enquiries, including requests for geological reports on specific sites, should be addressed to the BGS Enquiry Service at Keyworth.

Books

Maps

Documentary records collection

References

Most of the references listed below are held in the libraries of the British Geological Survey at Keyworth (Nottingham) and Edinburgh. Copies of the references can be purchased subject to the current copyright conditions. BGS library catalogue can be searched online at: envirolib.nerc.ac.uk.

Ballantyne, C K. 2002. Paraglacial geomorphology. Quaternary Science Reviews, Vol. 21, 1935–2017.

Ballantyne, C K, and Morrocco, S M. 2006. Scottish Landform Example 38: the windblown sands of An Teallach. Scottish Geographical Journal, Vol. 122, 149–159.

Bishop, P, and Jones, E J W. 1979. Patterns of glacial and post-glacial sedimentation in the Minches, north-west Scotland. 89–194 in The north-west European shelf seas: the sea bed and the sea in motion – 1. Geology and sedimentology.

Banner, F T, Collins, M B, and Massie, K S (editors). (Amsterdam: Elsevier.)

Bradwell, T, Stoker, M S, and Larter, R. 2007. Geomorphological signature and flow dynamics of The Minch palaeo-ice stream, north-west Scotland. Journal of Quaternary Science, Vol. 22, 609–617.

Bradwell, T, Stoker, M S, and Krabbendam, M. 2008a. Megagrooves and streamlined bedrock in NW Scotland: the role of ice streams in landscape evolution. Geomorphology, Vol. 97, 135–156.

Bradwell, T, Fabel, D, Stoker, M S, Mathers, H, McHargue, L, and Howe, J A. 2008b. Ice caps existed throughout the Lateglacial Interstadial in northern Scotland. Journal of Quaternary Science, Vol. 23, 401–407.

Bradwell, T, Stoker, M S, Golledge, N R, Wilson, C K, Merritt, J W, Long, D, Everest, J, Hestvik, O B, Stevenson, A G, Hubbard, A L, Finlayson, A G, and Mathers, H E. 2008c. The northern sector of the last British Ice Sheet: maximum extent and demise. Earth Science Reviews, Vol. 88, 207–226.

Chesher, J A, Smythe, D K, and Bishop, P. 1983. The geology of the Minches, Inner Sound and Sound of Raasay. Report of the Institute of Geological Sciences, No. 83/6.

Ellett, D J, and Edwards, A. 1983. Oceano-graphy and inshore hydrography of the Inner Hebrides. Proceedings of the Royal Society of Edinburgh, Vol. 83B, 143–160.

Embley, R W. 1980. The role of mass transport in the distribution and character of deep ocean sediments with special reference to the North Atlantic. Marine Geology, Vol. 38, 23–50.

Fairbanks, R G, Mortlock, R A, Chiu, T C, Kaplan, A, Guilderson, T P, Fairbanks, T W, and Bloom, A L. 2005. Marine radiocarbon calibration curve spanning 0 to 50 000 years B P based on paired 230U/234U/238U and 14C dates on pristine corals. Quaternary Science reviews, Vol. 24, 1781–1796.

Fyfe, J A, Long, D, and Evans, D. 1993. United Kingdom offshore regional report: the geology of the Malin–Hebrides sea area. (London: HMSO for the British Geological Survey.)

Graham, D K, Harland, R, Gregory, D M, Long, D, and Morton, A C. 1990. The biostratigraphy and chronostratigraphy of BGS borehole 78/4, North Minch. Scottish Journal of Geology, Vol. 26, 65–75.

Morrocco, S M, Ballantyne, C K, Spencer, J Q G, and Robinson, R A J. 2007. Age and significance of aeolian sediment reworking on high plateaux in the Scottish Highlands. The Holocene, Vol. 17, 349–360.

Nardin, T R, Hein, F J, Gorsline, D S, and Edwards, B D. 1979. A review of mass movement processes, sediment and acoustic characteristics, and contrasts in slope and base-of-slope systems versus canyon-fan basin-floor systems. Society of Economic Palaeontologists and Mineralogists Special Publication, No. 27, 61–73.

Peacock, J J, and Harkness, D D. 1979. Radiocarbon ages and the full-glacial to Holocene transition in seas adjacent to Scotland and southern Scandinavia: a review. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 81, 385–396.

Ritchie, W, and Mather, A S. 1969. The Beaches of Sutherland. (Perth: Countryside Commission for Scotland.)

Sager, G, and Sammler, R. 1968. Atlas der Gezeitenstrome für die Nordsee, den Kanal und die Irische See. (Rostock: Seehydrographischer Dienst der DDR.)

Sissons, J B. 1977. The Loch Lomond Readvance in the northern mainland of Scotland. 45–59 in Studies in the Scottish lateglacial environment. Gray J M, and Lowe J J (editors). (Oxford: Pergamon.)

Stoker, M S, and Bradwell, T. 2005. The Minch palaeo-ice stream, NW sector of the British–Irish Ice Sheet. Journal of the Geological Society of London, Vol. 163, 425–428.

Stoker, M S, and Bradwell, T. 2009. Neotectonic deformation in a Scottish fjord, Loch Broom, NW Scotland. Scottish Journal of Geology, Vol. 45, 107–116.

Stoker, M S, Hitchen, K, and Graham, C C. 1993. United Kingdom offshore regional report: the geology of the Hebrides and West Shetland shelves and adjacent deep-water areas. (London: HMSO for the British Geological Survey.)

Stoker, M S, Bradwell, T, Wilson, C K, Harper, C, Smith, D, and Brett, C. 2006. Pristine fjord land system preserved on the sea bed in the Summer Isles region, NW Scotland. Scottish Journal of Geology, Vol. 42, 89–99.

Stoker, M S, Bradwell, T, Howe, J A, Wilkinson, I P, and McIntyre, K. 2009. Late glacial ice-cap dynamics in NW Scotland: evidence from the fjords of the Summer Isles region. Quaternary Science Reviews, Vol. 28, 3161–3184.

Stoker, M S, Wilson, C R, Howe, J A, Bradwell, T, and Long, D. 2010. Paraglacial slope instability in Scottish fjords: an example from Little Loch Broom, NW Scotland. 227–244 in Fjordic depositional systems. Howe, J A, Austin, W E N, Forwick, M, and Paetzel, M (editors). Special Publication of the Geological Society of London, No. 344.

Syvitski, J P M. 1989. On the deposition of sediment within glacier-influenced fjords: oceanographic controls. Marine Geology, Vol. 85, 301–329.

Index map to the MAREMAP 1:50 000 Series

The map below shows the sheet boundaries and numbers of the 1:50 000 and 1:250 000 series geological maps.

(Index map)

British geological maps can be obtained from sales desks in the Survey’s principal offices. Through the BGS London Office at the Natural History Musuem, and from BGS-approved stockists and agents. Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.

Figures and plates

Figures

(Figure 1) Summary of the Quaternary succession (a) and relationship of the lithostratigraphical and lithogenetic units (b).

(Figure 6)." data-name="images/P1004346.jpg">(Figure 3). " data-name="images/P1004345.jpg">(Figure 2) Geography of the Summer Isles region showing extent of 1:50 000 scale map area (red box), location of key BGS offshore stratigraphical cores, and palaeo-ice flow reconstruction. Black box shows area covered in (Figure 6)." data-name="images/P1004346.jpg">(Figure 3).

(Figure 6)." data-name="images/P1004346.jpg">(Figure 3) Integrated swath bathymetric image and NextMap digital terrain model of the Summer Isles region. Inset box shows location of swath bathymetry enlarged in (Figure 6).

(Figure 4) Seismic characteristics of the Late Quaternary stratigraphical units (excluding the Loch Broom shell bed, which is unresolvable on seismic profiles).

(Figure 5) Graphic logs of key BGS stratigraphical cores showing predominant lithology, gross sedimentary structures and dated horizons along a transect from the North Annat Basin to inner Loch Broom. Cores located in (Figure 6)." data-name="images/P1004346.jpg">(Figure 3). " data-name="images/P1004345.jpg">(Figure 2).

(Figure 6) Swath bathymetric image of the Little Loch Broom slide complex, and the locations of BGS cores referred to in the text. Abbreviations: ER, eastern re-entrant; NR, northern re-entrant. 1–3, slide scars of the Rireavach Slide. See (Figure 6)." data-name="images/P1004346.jpg">(Figure 3) for location.

(Figure 7) Distribution of submarine geohazards.

Plates

(Front cover) Looking west towards Gruinard Island from Mungasdale Beach [NG 9675 9340]. (PhotographerTom Bradwell; (P731850).

(Rear cover)

(Index map) Index map to the MAREMAP 1:50 000 Series.

(Plate 1) Subglacial till of the Loch Broom Till Formation, Allt an t-Srathain [NH 1085 9673] near Ullapool. (Photographer Tom Bradwell; (P731851).

(Plate 2) Subglacial till of the Loch Broom Till Formation overlain by a coarse-grained marine lag deposit (Summer Isles Formation), BGS core 57-06/256 [57.9097° N, 05.5000° W], Gruinard Bay. (PhotographerMartyn Stoker; (P731852) & (P731853).

(Plate 3) Poorly sorted morainic diamicton of the Assynt Glacigenic Formation, near Achduart, Coigach [NC 0470 0412]. Note the well-developed soft-sediment loading structures. Pencil (for scale) is 15 cm long. (Photographer Tom Bradwell; (P731854).

(Plate 4) Diamicton overlain by colour-laminated glacimarine clay (both Assynt Glacigenic Formation), overlain by a coarse-grained marine lag deposit (Summer Isles Formation), BGS core 57-06/262 [57.9681° N, 05.3579° W], north-west end of Martin Bank. (Photographer Martyn Stoker; (P731855) & (P731856).

(Plate 5) Lodged boulders at sea bed on top of mid-loch moraine, Little Loch Broom [57.8942° N, 05.3313° W]. Squat lobster on left of image is about the size of an adult hand, excluding the claws. (Underwater image courtesy of Richard Shucksmith).

(Plate 6) Allt an t-Srathain Member (ANSR) – a late-stage re-advance member — at the type section [NH 1070 9678]: a) overlying Ullapool Gravel Formation (ULGR); and, b) close-up of poorly sorted diamicton. Height of hammer is 30 cm (Photographer Tom Bradwell; (P731858) & (P731864).

(Plate 7) View across Loch Broom towards Newton Loggie [NH 1430 9150]. A – late-stage glacigenic debris cone, which postdates B – Terrace of Ullapool Gravel Formation deposited when sea levels were 15 m higher than at present. C – mid-Holocene beach (+5 m OD). (Photographer Tom Bradwell; component images are (P731859) & (P731860); combined panorama (P731863).

(Plate 8) Ullapool Gravel Formation exposed within the large glacial fan-delta on which Ullapool is built [NH 1284 9461]. (Photographer Tom Bradwell; (P731861).

(Plate 9) Tidal flats exposed at low tide at the head of Loch Broom [NH 1750 8610]. (PhotographerTom Bradwell; (P731862).

(Plate 10) Paraglacial slope failure of till sheet (Assynt Glacigenic Formation on Loch Broom Till Formation), back of Badbea [NH 0275 9080], on southern flank of Little Loch Broom. (Photographer Martyn Stoker; (P731857).

Figures

(Figure 4) Seismic characteristics of the Late Quaternary stratigraphical units (excluding the Loch Broom shell bed, which is unresolvable on seismic profiles)

Formation or lithogenetic unit

Seismic character

Interpretation

SUMMER ISLES FORMATION

Weak, parallel laminated basin infill (locally mounded) that displays an onlapping to downlapping configuration. Basal chaotic layer locally present in Loch Broom. Reflections locally obscured by gas blanking. Seismically unresolvable lag deposit on shallow banks. Sporadic mounded deposits with chaotic internal signature in Little Loch Broom

Marine deposits strongly influenced by bottom currents. Localised mass failure

ANNIE FORMATION

Well-layered unit that infills localised hollows; variable onlapping to drape configuration. Reflectors truncated at sea bed, and locally obscured internally by gas blanking

Transitional glacimarine to marine deposits

ULLAPOOL GRAVEL FORMATION

Submerged parts of fan-deltas display large-scale (to 20 m) oblique-parallel sets of prograding strata (up to 10°)

Fluvioglacial outwash sheets and fan-deltas

LATE-STAGE DEBRIS FLOWS

Basin-floor wedges with chaotic internal reflection pattern; irregular (commonly rough) upper surface and variably planar to irregular (erosional?) basal surface

Discrete, localised debris-flow deposits

ANNAT BAY FORMATION

Layered basin infill with variable weak to strong internal reflections, which display a ponded to onlapping configuration, locally obscured by gas blanking

Distal glacimarine deposits, diachronous with Assynt Glacigenic Formation

ASSYNT GLACIGENIC
FORMATION

Acoustically structureless to chaotic morainal sheet drape on shallow banks contrasts with layered infill in basins, which displays a variable draped, onlapping and ponded reflection pattern. Basinal deposits locally disrupted by slumps and slides — forming mounded, acoustically chaotic, mass-movement deposits — and faulting and folding

Time-transgressive morainic, ice-contact and ice-proximal glacimarine deposits. Contemporaneous mass-failure and neotectonic deformation

LOCH BROOM TILL FORMATION

Patchy sheet-like unit that varies internally from homogeneous and chaotic to layered, with discontinuous, sub-parallel to wavy reflectors. Common streamlined upper surface. Locally indistinguishable from bedrock

Subglacial lodgement till

FIONA FORMATION

Largely dense, structureless to chaotic packages bounded by laterally continuous reflectors (including present-day sea bed) that depict a hummocky or corrugated form, with a relief of 5–15 m, ridge spacing of 100–500 m, and elongation ratios up to 70:1

Stacked diamicton (?) packages bounded by subglacially tectonised surfaces of megascale glacial lineations