3.1.1. The Minch and Hebrides Shelf

Along the continental shelf edge in present-day water depths of ∼180–200 m, and adjacent to the Sula Sgeir Fan, the bathymetry data shows a single, continuous, broad ridge gently arcuate to the SE, 3–5 km wide, 20–30 m high and up to 60 km long. This is the largest, outermost MacDonald Formation moraine mapped at the shelf break and previously described by Stoker (Reference Stoker, Dowdeswell and Scourse1990) and Stoker & Holmes (Reference Stoker and Holmes1991) (see section 1.2 above). Three morphologically similar, 10–15 km long, 2–5 km broad, arcuate ridges, are clearly visible 10–30 km to the E (inboard) of the outermost moraine. These ridges are mapped from bathymetry data in the same location, as fragmentary seabed moraines were recorded on seismic profiles by Stoker et al. (Reference Stoker, Hitchen and Graham1993, fig 77). As such, we interpret these outer shelf ridges collectively as broad subaqueous moraines (or grounding-line features) part of the Middle–Upper Pleistocene MacDonald Formation, with clear lateral extensions to the NE (Figs 4–6) (see section 3.1.2, below).

North of Lewis, on the mid shelf in water depths of 100–120 m, are two large broad ridges trending across the wide cross-shelf trough linking the Minch to the Sula Sgeir Fan. The outer ridge, which we call the North Lewis Ridge, ranges in width from 2 km to 5 km and has a maximum height of 20 m above the surrounding sea floor. This discontinuous ridge is 40–50 km long; however, the single-beam bathymetry data is sparse in the central part of the ridge system, which consequently remains undefined. The main ridge is curvilinear in form, arcing round from a northerly orientation offshore northernmost Lewis to a NE–SW orientation in its northern part; i.e., concave in plan form towards mainland Scotland. Bathymetric cross-profiles show no preferred slope asymmetry (Fig. 5). Seismic profiles show this region of seafloor is characterised by a featureless, in places acoustically transparent unit, which correlates with a 20 m-thick stiff foraminiferous clay proved in BGS borehole 77/08 (Fyfe et al. Reference Fyfe, Long and Evans1993). The inner broad ridge, which we here call the North Minch Ridge, is a well imaged feature in the single-beam data, trending generally NE–SW between the North Lewis and North Minch basins. This continuous ridge is 20–25 km long, 10–20 m high and 3–6 km wide, with linear and curvilinear sections in plan form. Single-beam echo sounder data show that the North Minch Ridge is asymmetrical in cross-profile in its central section, with a steeper NW-facing (distal) slope and a single broad crest line (Fig. 4). Bedrock does not outcrop in the vicinity of this ridge (BGS 1989, 1994). BGS seismic profiles show a thick Quaternary succession in and around the North Lewis Basin, and BGS borehole 77/08 on the flank of the North Minch Ridge recovered >20 m of stiff pebbly clay with arctic microfauna, indicating glaciomarine sedimentation (Fyfe et al. Reference Fyfe, Long and Evans1993). Approximately midway along the North Minch Ridge, a large broad flat-topped approximately rectangular mound occurs to the south, with a width of 5 km, a length of 8 km and a general N–S elongation. BGS seismic data show a considerable thickness of Quaternary sediment in this region (>50 m); however, the seismostratigraphic sequence is unclear, with the Pleistocene deposits left undifferentiated (Fyfe et al. Reference Fyfe, Long and Evans1993). Currently, no multibeam data exist for the outer or mid shelf NW of Lewis (Fig. 2); however, the eastern part of the North Lewis Ridge is captured by multibeam bathymetry data (Fig. 6). This high-resolution bathymetry reveals a broad ridge 2–3 km wide, with a generally smooth surface texture and no overall slope asymmetry, similar in form to large subaqueous moraines seen on the outer shelf (Stoker & Holmes Reference Stoker and Holmes1991) (Fig. 7). Of particular note is the irregular pitted and scoured surface of the ridge crest along a 5 km-long section (Fig. 7). In this relatively shallow shelf setting (<80 m present-day water depth), these are interpreted as keel marks of large icebergs, probably indicating former proximity to a calving ice-sheet margin.

Figure 7

(A) Hill-shaded multibeam bathymetry data, and (B) bathymetric cross-profiles of North Minch Ridge on the mid shelf, ∼40 km NW of Cape Wrath, in present-day water depths ca. 80 m. Note the irregular pitted surface, suggestive of iceberg keel marks and scours. Contains Maritime and Coastguard Agency MBES data.

Near the Flannan Isles, in the extreme west of the study area, are two large seabed ridges. The first is a broad low-elevation 15 km-long ridge trending NE–SW at around 100 m present-day water depth on the outer shelf (Figs 4, 5). This ridge is 2–5 km wide and typically 10–15 m high. BGS seismic profiles show a thin acoustically chaotic wedge of sediment draped on irregular basement bedrock in this area (Stoker et al. Reference Stoker, Hitchen and Graham1993). The second feature is a broad low-elevation ridge starting 20 km NW of Bernera, west Lewis, which we term the East Flannan Ridge. At its maximum, it is 4 km wide, 10 m high and 25 km long, and is weakly arcuate to the south where it is bounded by the Flannan Trough. This ridge may be partly bedrock controlled to the north, as seen in its irregular cross-profile, and terminates to the west on the submerged bedrock platform surrounding the Flannan Isles (Fig. 4). BGS seismic profiles show a thin poorly resolved sediment unit draped on bedrock, and seabed sediment maps show a broad belt of sandy gravel coincident with the East Flannan Ridge.

In the eastern Minch, large, arcuate seabed ridges occur in nearshore waters in present-day water depths of up to 100 m (Figs 5, 6). A number of these are clearly visible in the single-beam dataset, the most notable being a large ridge 1–2 km wide, up to 35 m high and nearly 20 km long, which extends N from the Stoer Penisula and arcs through almost 90 degrees to trend NW in the vicinity of Loch Laxford (Fig. 5). This large prominent sharp-crested ridge, here termed the Eddrachillis Ridge, has a simple asymmetric profile with a gentler eastern (proximal) flank and a steeper (distal) western flank – typical of large end moraines or grounding-zone features in submarine settings (e.g. Dowdeswell et al. Reference Dowdeswell, Ottesen, Evans, Ó Cofaigh and Anderson2008; Ó Cofaigh Reference Ó Cofaigh2012). Five similar but smaller sharp-crested ridges, 5–15 m high and 250–500 m wide, occur 1–2 km inshore of the Eddrachillis Ridge, all with similar asymmetric profiles – characteristic of recessional push moraines charting ice retreat from NW to SE.

A BGS seismic profile (sparker) across the Eddrachillis Ridge (Fig. 8) shows it is comprised of a single generally chaotic to acoustically transparent unit, with very few internal structures unconformably deposited on underlying strata, typical of glacigenic (morainic) diamicton laid down at the margin of a grounded ice sheet. The five smaller inshore ridges are comprised of the same laterally continuous acoustic unit. In the SE, this uppermost glacigenic unit directly overlies Precambrian bedrock; in the NW it is deposited on an offshore-thickening stacked sequence of Late Pleistocene sediments, interpreted elsewhere as subglacial and ice-proximal glaciomarine facies (Sheena Formation, Morag Formation) (Fyfe et al. Reference Fyfe, Long and Evans1993) (Fig. 8). The innermost ridge in the sequence has a very similar seismic expression and morphology to recessional moraines found on the seabed in the Summer Isles region to the south (see below) (Stoker et al. Reference Stoker, Bradwell, Howe, Wilkinson and Mcintyre2009). The stratigraphic position of the unnamed uppermost glacigenic unit (comprising the Eddrachillis Ridge and adjacent inshore ridges) within the existing regional seismostratigrphic framework is currently unclear. However, its superposed stratigraphic position and well-preserved seabed morphology demonstrate a late-stage glacial event, younger than the Sheena Formation (Upper Weichselian) but older than the Late-glacial climate reversal (Greenland Interstadial/Stadial 1), and therefore probably within the time period ∼20–15 ka BP.

Figure 8

BGS sparker profile (upper panel) and seismostratigraphic interpretation (lower panel) of the Eddrachillis Ridge, eastern part of the Minch. This large 30–40 m-high ridge and the smaller ridges inboard to the SE have not been formally defined within the existing Quaternary stratigraphic framework. The collective evidence suggests they are relatively late-stage (MIS2) subaqueous moraines recording grounded ice-sheet margin oscillations offshore mainland NW Scotland. For line of section, see Figure 1.

The Eddrachillis Ridge may represent a northern equivalent of a similarly prominent late-stage moraine system imaged at the mouth of Loch Ewe and around the Rubha Coigach headland (Stoker et al. Reference Stoker, Bradwell, Howe, Wilkinson and Mcintyre2009) (Figs 5, 6). The Loch Ewe–Greenstone Point moraine complex is 1–2 km wide, up to 30 m high and >15 km long and forms a long broad arc open to the SE. Low-resolution echo sounder data suggest a bathymetric connection between this moraine complex and the morphologically similar moraines around the Rubha Coigach headland ∼20 km to the NE, although multibeam data is lacking in this intervening area. Multibeam echo sounder data acquired by BGS (in 2005) and new baythmetry data from the eastern Minch acquired by MCA (in 2011) show the detailed geomorphology of this substantial moraine complex, in present-day water depths of 30–100 m. The larger moraines indicate firmly grounded retreat, whilst the superimposed de Geer moraines (1–5 m high; <100 m wide) suggest lightly grounded retreat of a tidewater glacier margin close to flotation (Stoker et al. Reference Stoker, Bradwell, Howe, Wilkinson and Mcintyre2009; Stoker & Bradwell Reference Stoker and Bradwell2011)

Morphologically similar seabed ridges occurring to the N of the Eddrachillis Ridge are well imaged on single-beam data (Fig. 5). They form a nested sequence of nearshore ridges, ca. 10–20 km to the W and NW of Loch Inchard, in present-day water depths of 40–65 m. They are referred to here as the NW Sutherland ridges (Fig. 5). These five or six ridges vary in size and shape, but all trend in the same direction (NE–SW); most of the ridges are generally arcuate in plan form, concave to the SE (Fig. 6). The outer ridges are broader and generally lower, whilst the inner ridges are higher and better defined; the innermost ridge is the most pronounced, being <1000 m wide and rising 20 m from the seabed. Most of the ridges have no overall slope asymmetry in cross-profile (Fig. 5). The inner ridges due west of the Sheigra headland, when projected along trend, would make landfall in the vicinity of Sandwood Bay; the outermost ridges would make landfall further N in the vicinity of Cape Wrath (Fig. 6). BGS seismic lines across this part of the eastern Minch show these sediment ridges to have a similar acoustic character to the Eddrachillis Ridge – with a single acoustically chaotic unit unconformably overlying an irregular bedrock reflector, characteristic of diamicton-dominated end moraines. Unfortunately, this group of well-developed ice-sheet moraines is outside the area covered by existing multibeam data (Fig. 2); hence more detailed morphological information is currently lacking.

All the nearshore moraines in the eastern Minch described in this section are relatively large, well defined, morpho-stratigraphically equivalent features in similar water depths. We therefore suggest they were broadly coeval in formation and informally assign these moraines to the Late Pleistocene deglacial succession, stratigraphically older than the Assynt Glacigenic Formation (<16 ka BP) further inshore and stratigraphically younger than the Fiona Formation–Loch Broom Till Formation (∼20–25 ka BP) (Stoker & Bradwell Reference Stoker and Bradwell2011).

In the south of the study area, from the Gairloch and Coigach headlands inshore to the heads of the fjords (Loch Ewe, Loch Broom and Little Loch Broom) are numerous transverse seabed ridges in water depths typically of 30–100 m (Fig. 9). This part of the eastern Minch, known as the Summer Isles region, was surveyed with multibeam echo sounder and sub-bottom acoustic profiling by BGS in 2005, and over 100 seabed cores were recovered in separate scientific cruises in 2006 and 2009. The results of these data acquisitions have been the focus of detailed geomorphological and geological studies (Stoker et al. Reference Stoker, Bradwell, Wilson, Harper, Smith and Brett2006, Reference Stoker, Bradwell, Howe, Wilkinson and Mcintyre2009; Stoker & Bradwell Reference Stoker and Bradwell2011). Over 50 seabed ridges have been mapped in the area trending generally SSW–NNE and varying in length from 0.5 km to 5 km; ridge spacings typically range from 500 m to 1000 m. There are two distinct sets: an older group of more substantial ridges (5–15 m high; 80–150 m wide), and a younger group of smaller more delicate ridges (2–5 m high; 30–50 m wide), superimposed on the earlier ridges (Fig. 9). Most ridges are simple in plan form, consisting of linear sections; however some are more intricate in plan form, describing intricate or zigzag patterns. Occasionally, ridges continue to water depths of 100 m, but in basins and bathymetric deeps the ridges are notably absent. The larger ridges are typically asymmetric in cross-profile, whilst the smaller more delicate ridges tend to be symmetrical. BGS seismic profiles show the ridges to be typically composed of a single acoustically chaotic or structureless unit draped unconformably on an irregular bedrock surface (Fig. 9). A core taken from the flank of a seabed ridge recovered a massive, poorly sorted, sandy to muddy diamicton, with numerous striated clasts of non-local lithology (Stoker et al. Reference Stoker, Bradwell, Howe, Wilkinson and Mcintyre2009). The whole suite of seabed ridges have been lithostratigraphically assigned to the Assynt Glacigenic Formation and are interpreted as subaqueous moraines (and/or de Geer moraines), formed at the grounded, or partially buoyant, tidewater margin of a retreating ice sheet (Stoker et al. Reference Stoker, Bradwell, Wilson, Harper, Smith and Brett2006, Reference Stoker, Bradwell, Howe, Wilkinson and Mcintyre2009). In places, around the mouths of Loch Broom and Little Loch Broom, these moraines have clear onshore counterparts which have been studied, and in places dated, with terrestrial cosmogenic nuclides (∼15 ka BP; Bradwell et al. Reference Bradwell, Fabel, Stoker, Mathers, Mchargue and Howe2008b).

Figure 9

Seabed moraines in the Summer Isles region, NW Scotland (mapped and studied previously by Stoker et al. Reference Stoker, Bradwell, Wilson, Harper, Smith and Brett2006, Reference Stoker, Bradwell, Howe, Wilkinson and Mcintyre2009; Bradwell et al. Reference Bradwell, Fabel, Stoker, Mathers, Mchargue and Howe2008b): (A) hill-shaded multibeam bathymetric image showing suite of transverse ridges (moraines) between Tanera Mor and Carn nan Sgeir. [Multibeam echo sounder data acquisition by BGS]; (B) hill-shaded multibeam bathymetric image of seabed moraines ∼10 km NW of Tanera Mor. Note the small delicate de Geer moraines (m) overprinting the larger (older) set of regional moraines (RM). Location of bathymetric profile (lower panel) shown by line; (C) BGS seismic reflection (boomer) profile of subaqueous moraines between Tanera Mor and Carn nan Sgeir. Note the asymmetric cross-profile. Rockhead/diamicton contact mapped where acoustically resolvable. Line of seismic profile shown in (A). Abbreviations: SBM=seabed multiple; BT=bottom tracking pulse. Modified from Bradwell et al. Reference Bradwell, Fabel, Stoker, Mathers, Mchargue and Howe2008b.

3.1.2. Central sector

The seabed in the central sector of the study area is generally shallower, more rugged and more topographically variable than the shelf areas to the SE and NW. The bathymetry is punctuated by a number of large structural bedrock highs and platforms on the mid shelf; these include the North Rona, Solan Bank, and Nun Rock–Sule Skerry Highs (Fig. 1). Extensive single-beam data coverage and good multibeam data coverage exists for this part of the shelf, affording an excellent view of the seabed geomorphology (Fig. 5). The seabed in this central sector is characterised by an abundance of seabed ridges, with over 100 mapped in this study.

Approximately 5–30 km from the shelf break, on the outer shelf, ∼30–50 km NE of North Rona, at least four large, broad ridges extend semi-continuously for ∼100 km in a NE–SW direction parallel to the continental shelf break. They are typically 3-4 km wide, increasing in width (to 4–5 km) to the NE, with smooth subdued cross-profiles (Fig. 5). The ridges are generally low-elevation features ranging in height from 10 m to 20 m. Overall, the ridges are broadly arcuate in plan form but with long linear sections (Fig. 6). The exact plan morphology of the ridges is not easy to establish, even with image enhancement, owing to their subdued form and the vertical resolution of the single-beam data. However, the new mapping is an improvement on that presented in Bradwell et al. (Reference Bradwell, Stoker, Golledge, Wilson, Merritt, Long, Everest, Hestvik, Stevenson, Hubbard, Finlayson and Mathers2008a) and gives a better impression of the lateral continuity and spatial interrelationship of these ridges; for example, it is clear that the outermost fragmentary ridge is cut out or overprinted, at least in part, by the adjacent inshore ridge (Fig. 6). BGS seismic profiles across this part of the outer shelf show acoustically structureless or chaotic packages in the upper part of the sequence (<40 m thick) which occasionally interdigitate with layered deposits near the shelf break. BGS seismostratigraphic mapping, supported by borehole evidence, places these NE–SW-trending broad ridges within the Murray or MacDonald Formations (Stoker et al. Reference Stoker, Hitchen and Graham1993; BGS 1994) – a Late Pleistocene suite of ice-proximal to glaciomarine sediments laid down prior to the Otter Bank Formation (Stoker & Holmes Reference Stoker and Holmes1991). Whereas, spatial relationships revealed in the new bathymetry data (this study) suggest that the inner three ridges can be traced onto the northernmost part of the Hebrides Shelf, seemingly offering a potential correlation with the MacDonald Formation moraines at seabed; their stratigraphic relationship (angular discordance) with other Pleistocene deposits must be taken into consideration in any proposed reconstruction (Fig. 4).

Further inshore, on the mid shelf to the NE of North Rona, another group of seabed ridges are well imaged on the single-beam bathymetry. For convenience, we term these collectively the N Rona Ridges (Fig. 5). Four large broad ridges, interspersed with several smaller ridges, occur in present day water depths of 80–130 m, in a region of highly variable bathymetry, including the North Rona Basin and western part of the Solan Bank High (Figs 5, 6). The larger ridges are 2–4 km wide and 10–20 m high; the smaller ridges are up to 2 km wide and of similar vertical relief (10–20 m). The North Rona ridges arc round from a NE–SW trend to an E–W trend in their eastern parts and extend for ∼50 km on the mid shelf, where they abut a larger feature (the Solan Bank Ridge) on the mid shelf. The inner suite of ridges appear to fan out from the North Rona High, suggestive of topographic pinning; whereas the outer suite of ridges are more shelf-edge parallel features (Fig. 6). Most of the ridges comprise discontinuous sections, with the longest continuous ridge extending for ca. 30 km. The spacing between successive ridges is relatively close near the North Rona High, with six main seabed ridges mapped in only 15 km of horizontal distance (Figs 5, 6).

Multibeam bathymetry covers only a small part of this area to the east of North Rona (Fig. 2). Here, the 3-D shape and smooth surface of the North Rona sediment ridges can be clearly seen, as well as their unconformable relationship with the underlying basement bedrock. These ridges have strong morphological similarities with subaqueous moraines mapped elsewhere in the study area (e.g., in the eastern Minch) and in other glaciated continental shelf settings (e.g., Shaw et al. Reference Shaw, Piper, Fader, King, Todd, Bell, Batterson and Liverman2006; Todd et al. Reference Todd, Valentine, Longva and Shaw2007; Dowdeswell et al. Reference Dowdeswell, Ottesen, Evans, Ó Cofaigh and Anderson2008). Unfortunately, this whole region is not well resolved in BGS seismic lines, as bedrock is near to seabed across much of the area and correlation of thin discontinuous Pleistocene sequences is difficult (Fig. 3). However, BGS Quaternary geology maps and offshore reports assign the Pleistocene deposits in this region to the Otter Bank Formation: a heterogeneous glaciogenic unit comprising ice-proximal diamictons, glaciofluvial/deltaic gravelly sands and glaciomarine pebbly muds (Stoker et al. Reference Stoker, Hitchen and Graham1993; BGS 1994). Interestingly, the narrower innermost E–W-trending North Rona moraine ridge seems to join, or be truncated by, a broader NNE–SSW-trending sediment ridge in shallow water, projecting from the northern edge of the Solan Bank High, but the exact morphological relationship remains unclear (Fig. 6).

A prominent NNE–SSW-trending bathymetric high crosses the mid shelf between the submerged bedrock high of the Solan Bank (60 m water depth) and the main Otter Bank Formation Moraines (100–120 m water depth). This prominent 10–20 km wide, 80 km long feature is a structural bedrock platform – referred to as the Solan Bank High. BGS investigations proved basement rocks of the Lewisian complex at seabed on the Solan Bank and the adjacent Sule Skerry-Nun Rock high (Ritchie et al. Reference Ritchie, Ziska, Johnson and Evans2011). BGS borehole 77/07 recovered weathered basement rocks at shallow depth beneath Tertiary and Neogene deposits on the spine of the Solan Bank High (Stoker et al. Reference Stoker, Hitchen and Graham1993). Although predominantly a bedrock feature, seismic records show that the eastern portion of the platform is capped by a thin Pleistocene sequence (Otter Bank Formation) which thickens to the northeast (Stoker et al. Reference Stoker, Hitchen and Graham1993). We map a low broad NNE–SSW-trending sediment ridge along the length of the eastern side of the Solan Bank High which we call the Solan Bank Ridge. It is 3–4 km wide and generally rises only 10–15 m above the surrounding seafloor, with low angle slopes and a very poorly defined crest line (Fig. 6). However, its geometry is consistent with other subtle ice sheet moraines seen on the mid and outer shelf. A morphologically similar broad, low ridge occurs on the northern part of the Solan Bank High where the glacigenic Otter Bank Formation thickens, forming more pronounced moraines (morainal banks) (Stoker et al. Reference Stoker, Hitchen and Graham1993).

Inshore of the Solan Bank High, the number and density of seabed ridges increase considerably (Fig. 10). This area of rugged seafloor, punctuated by numerous bedrock highs, is well imaged in both the single-beam and multibeam data. The density of seabed ridges is typified in the area S of Solan Bank around Nun Rock – a rugged basement high that reaches to within −30 m of present-day sea level (Figs 1, 4). Detailed mapping of ridges in this region reveals numerous sediment ridges with complex plan form morphology and variable 3D shape (Figs 10, 11). The ridges on this part of the shelf have notably more variable geomorphology than the broad simple plan form ridges seen on the outer and mid shelf (Figs 5, 6). Some show clear overprinting or superimposed relationships (Fig. 10). The seabed ridges inshore of the Solan Bank High and around Nun Rock generally fall into two types: broader, more substantial, ridges with widths ca. 200–1000 m and heights of 5–10 m; and narrow, more delicate, ridges with widths ca. 100 m and heights of only 1–3 m above the surrounding seafloor. The larger ridges have variable geomorphology, often with strongly arcuate sections and numerous inflection points (Fig. 11). Ridge crests are generally broad and rounded; along-ridge heights can vary; most are slightly asymmetric in cross-profile, steeper on their shoreward (proximal) side. They are also nearly always continuous features, with very few breaks or missing sections. By contrast, the smaller ridges tend to be simple in plan form, with linear or curvilinear sections, and rarely extend more than 2–3 km without a break. Ridge crests are sharper and better defined; however, slope asymmetry is often lacking.

Figure 10

Seabed moraines in the central sector, SW of Nun Rock High: (A) hill-shaded multibeam bathymetry data showing area of dense seabed ridges interpreted as ice-sheet moraines; (B) outline geomorphological map of seabed ridges. Note how the morphology of zigzag moraine (ZZM) overprints adjacent (older) ridges, suggesting a re-advance of the ice sheet margin; (C) hill-shaded greyscale bathymetric surface model, illuminated from the NW, used to map ridge morphology and crest lines in detail; (D) bathymetric cross-profiles of closely spaced, well developed seabed moraines SW of Nun Rock; profile lines shown in (A). For bathymetric colour ramp, see Figure 7. Contains Maritime and Coastguard Agency MBES data.

Figure 11

Seabed moraines in the central sector, S of Nun Rock High: (A) hill-shaded multibeam bathymetric image showing suites of transverse ridges between Nun Rock and Cape Wrath; (B) outline geomorphological map of ridges (large and small) interpreted as ice sheet moraines; stipple is bedrock at seabed; (C) bathymetric transects perpendicular to ridge crests showing cross-profile (2-D) morphology of selected ridges. Depths and distances in metres. Lines of transects shown in (A); (D) BGS seismic reflection (airgun) profile across seabed moraines S of Nun Rock. Line of profile shown in (A) and (B). Note the Pleistocene sediment package thickening in the vicinity of the Nun Rock moraine; sediment cover is thin or absent in places to the east. For bathymetric colour ramp, see Figure 7. Contains Maritime and Coastguard Agency MBES data. Abbreviations: NRM=Nun Rock Moraine; RM1, RM2=regional moraines; m1, m2=smaller recessional moraines; SBM=seabed multiple.

Immediately south of Nun Rock is an excellent example of one of the large continuous ridges found in the central part of the shelf off northern Scotland (Fig. 11). The ridge starts in the shallow water adjacent to Nun Rock and ends on the bedrock platform around Cape Wrath, traversing water depths from 55 m to 95 m below present-day sea level. Running almost due N–S and unbroken for 22 km, we term this the Nun Rock Ridge. The ridge ranges in width from ∼250 m to ∼1000 m and averages around 8–10 m in height. Bathymetric cross-profiles show that asymmetry is typical, with slope gradients of between 50:1 and 80:1 and a generally steeper E-facing slope (Fig. 11). The continuous, well-defined morphology of this feature on the seabed is striking. In particular, the arcuate, almost semi-circular, ridge sections concave to the E–SE; the numerous strong kinks or inflections where the ridge direction changes by up to 90 degrees; and the irregular, multi-lobed, overall plan form of the ridge (Fig. 11).

The BGS seismostratigraphic framework is poorly developed in the nearshore region around NW Scotland, with most deposits defined simply as “Quaternary undifferentiated” (Fig. 4). This is largely owing to the thin (<5–10 m) and patchy nature of the Pleistocene deposits on bathymetrically rough seabed (BGS 1989; Stoker et al. Reference Stoker, Hitchen and Graham1993). The Nun Rock Ridge is only poorly resolved on one or two BGS seismic profiles (airgun and sparker), owing to its small size, but is composed of a structureless, acoustically transparent unit, unconformably overlying highly reflective substrata interpreted as hard (?crystalline) bedrock (Fig. 11). BGS borehole 72/34, situated on the flank of the ridge, proved 8.5 m of grey silty, sandy clay with ice-worn pebbles and occasional sand lenses, typical of a glacial diamicton, overlying bedrock (Stoker et al. Reference Stoker, Hitchen and Graham1993). Maps made from seabed sediment samples show gravelly sands and gravels predominating on the highs (ridges), whereas sands predominate in the basins (BGS 1989). Taken collectively, the morphological and geological evidence, combined with the similarity to well-studied glacial features identified elsewhere on the seabed around NW Scotland (e.g. Stoker et al. Reference Stoker, Bradwell, Wilson, Harper, Smith and Brett2006, Reference Stoker, Bradwell, Howe, Wilkinson and Mcintyre2009), strongly favours the interpretation of the Nun Rock Ridge as a prominent end moraine, relating to a still-stand or oscillation of a grounded ice sheet margin.

Immediately adjacent to this seabed moraine, 1–2 km E, another less continuous but comparably-sized ridge mirrors its plan form shape with curves and inflection points in similar places along much of its length (Fig. 11). Approximately 10 km E of this, a further prominent seabed ridge occurs with similar dimensions and characteristic curved-and-kinked morphology. Like the Nun Rock Moraine, this broad-crested ridge is also asymmetric in cross-profile, with E- and W-facing slope gradients of 80:1 and 100:1 (Fig. 11). Based on their geomorphological similarity and spatial coherence with the Nun Rock Moraine, we also interpret these substantial ridges as end moraines, part of a suite of ‘regional moraines’ imaged in multibeam bathymetry data charting the retreat of a grounded ice sheet margin offshore northern mainland Scotland (Figs 6, 11).

The smaller, more delicate ridges, seen immediately W of the Nun Rock Moraine, form a different suite of four (or possibly five) ridges on the seabed. With wide spacings of 500–1000 m and heights of only 1–3 m, these ridges bear little similarity to marine (sediment transport) bedforms; their contextual setting adjacent to, and between, ice sheet moraines suggests a Pleistocene age and origin. Although geological and acoustic information about their composition is lacking, their morphological similarity to small subaqueous transverse moraines (de Geer moraines), identified elsewhere in formerly glaciated continental shelf settings, is strong (e.g. Ottesen & Dowdeswell Reference Ottesen and Dowdeswell2006; Todd et al. Reference Todd, Valentine, Longva and Shaw2007; Bradwell et al. Reference Bradwell, Fabel, Stoker, Mathers, Mchargue and Howe2008b). On this basis, we map these suites of smaller ridges in the vicinity of Nun Rock as de Geer-type moraines formed by incremental retreat of an ice-sheet grounding line over time (e.g. Lindén & Möller Reference Lindén and Möller2005). The stratigraphic and temporal relationship between these smaller moraines and the more substantial regional moraines is not clear, although there is a suggestion in the multibeam data that the former overprint or truncate the latter in places. It is very likely, judging by their good state of preservation, that all these moraines relate to the last (Late Weichselian) BIIS.

Multibeam echo sounder data also reveal an irregular hummocky area of seabed geomorphology, with faint linear N–S-orientated forms between the two large regional moraines (Fig. 11; RM1 and RM2). Sub-bottom geophysical profiles show thin and patchy Pleistocene sediment cover with bedrock at or close to seabed (Fig. 11); geological maps (BGS 1989, 1984) show undifferentiated Quaternary deposits in this area, with gravelly seabed sediments predominating. It is likely that some of the hummocks and discontinuous linear forms in the multibeam data may be glacigenic (morainic) in origin. It is notable that a further three small, delicate, morphologically distinct, but discontinuous ridges are seen with the same N–S orientation on the eastern margin of this hummocky area near RM2. We interpret these small discontinuous ridges as de Geer moraines, part of the Nun Rock moraines suite (described above).

The suite of regional moraines imaged between Cape Wrath and Nun Rock continue eastward at a high angle to, in places almost perpendicular to, the present-day north coast of Sutherland (Fig. 6). A notable deviation from this trend is offshore Faraidh Head, N of the mouth of Loch Eriboll in water depths of 50–70 m, where large, morphologically similar, but more pronounced, transverse ridges are seen to interrupt or overprint this regional pattern (Fig. 12). Interpreted as a subsequent suite of ice-marginal landforms, the Faraidh Head moraines form a group of up to ten discrete arcuate, but laterally discontinuous, nested ridges. Their geomorphological and acoustic similarity with the Nun Rock moraines, as well as their discontinuous concentric pattern, is strongly suggestive of a grounded or partly grounded ice margin retreating from N to S – evidence of a locally-sourced ice mass in the mountains of NW Scotland (Fig. 12). The superimposition of one set of moraines on another in this area suggests that a major local ice sheet/ice cap re-advance occurred in NW Sutherland and, furthermore, that final retreat of this ice mass probably occurred under marine conditions; hence the preservation of both sets of landforms.

Figure 12

Seabed moraines off the Sutherland coast due N of Faraidh head: (A) hill-shaded multibeam bathymetric image showing the area of complex multi-phase glacial geomorphology; (B) outline geomorphological map of transverse seabed ridges interpreted as ice-marginal landforms (moraines). Larger, older ridges (grey) are regional moraines relating to ice sheet retreat generally from west to east; smaller, younger ridges (black, FHM) relate to a later phase of advance and retreat of an ice mass sourced to the S, in NW Sutherland; (C) BGS seismic reflection (airgun) profile across seabed moraines off Faraidh Head. Line of survey shown in (A) & (B). Pleistocene sediment package is locally up to 20 m thick in ridges, although sequence is undivided. Exact stratigraphic relationship between regional moraines and Faraidh Head Moraines is uncertain, owing to low resolution of airgun profile; however, geomorphology shows the latter, more pronounced, ridges superimposed on the former, broader ridges, in places. Abbreviations: FHM=Faraidh Head Moraines; RM=Regional Moraines; SBM=Seabed multiple. For bathymetric colour ramp, see Figure 6. Contains Maritime and Coastguard Agency MBES data.

Further E in the central sector, the single-beam bathymetry data shows the suite of regional moraines continuing adjacent to theN coast of Sutherland and Caithness (Fig. 6). The spatial pattern of the ridges continues, with lobate or broadly arcuate plan forms open to the E or SE. Bathymetric cross-profiles also show similar height and widths to the moraines in the vicinity of Nun Rock, and generally steeper E-facing slopes. In several places, between Faraidh Head and Dunnet Head, these large moraines are truncated or overprinted by another set of equally substantial seabed ridges; implying a subsequent phase of glacial advance–retreat. One notable ridge complex highlights this relationship. A broad seabed sediment ridge can be seen extending offshore in an ENE direction to the east of Strathy Point (Fig. 6). If projected onshore, this ridge would make landfall in the vicinity of Melvich Bay; hence we call this the Melvich Ridge. Approximately 15 km N of Sandside (Reay) Bay, this broad subdued ridge is overprinted by another smaller, but more prominent seabed ridge with a well-developed crest line (Fig. 13). This sharp-crested ridge forms a wide arc stretching over 40 km from the Caithness coast, between Reay and Brims Ness, to 10 km W of Hoy in Orkney. Good quality single-beam bathymetry data shows that this ridge is 400–1000 m wide and typically 10–20 m high, with an unusual asymmetry, being generally steeper on its W-facing slopes. In places, the ridge crest line widens, bifurcates or splits into two well-defined ridges, with one ridge set ∼1 km inside the other (Fig. 13). At a point c. 10 km W of Rora Head on Hoy, the ridge sharply changes direction, and continues with the same distinctive sharp-crested morphology in a northerly direction in present-day water depths of 60–80 m. Unfortunately, this ridge is not well imaged in existing seismic sub-bottom survey lines; although an airgun profile clearly shows the twin-crested morphology of the ridge, its relatively high-angle slopes and asymmetry (steeper W-facing slopes) (Fig. 13). As elsewhere on the inner continental shelf, the 3-D morphology (e.g., height, width, shape, etc.) and geospatial similarity (e.g., spacing and trend) of the ridges between Faraidh Head and Dunnet Head to other seabed moraines imaged elsewhere on the UK continental shelf strongly suggest that they formed at the margin of a grounded ice sheet, probably during the Late Weichselian period. We suggest that the superimposed Reay Ridge represents a notable and spatially extensive moraine-forming event during overall ice-sheet retreat, when the ice sheet margin underwent a significant re-advance; in places, beyond the position of older moraines (Fig. 13).

Figure 13

Seabed moraines off the N Caithness coast: (A, B) hill-shaded single-beam echo sounder bathymetric images (Olex dataset); arrows highlight prominent arcuate sediment ridge (RR) abutting or overprinting older more subdued forms (MR): (A) illuminated from the NE; (B) illuminated from the NW. (C) BGS seismic reflection (airgun) profile across seabed offshore Caithness. Line of survey shown in (A), perpendicular to crest of ridges. Note the Pleistocene sediment package thickening to form a distinctive double crested ridge – the Reay Moraine. Steep-sided transverse sediment ridges to the E are similar to large Holocene sand waves seen elsewhere on the Continental Shelf; although seismic reflection data suggest some may be draped over Pleistocene (glacial) sediment cores. Abbreviations: MR=Melvich Ridge; RR=Reay Ridge; SBM=Seabed multiple.

Three to eight kilometres east of the Reay Ridge are numerous closely spaced, prominent, sharp-crested seabed ridges trending generally N–S, but with irregular spacings and occasionally branching plan form (Fig. 13). The ridges are 1–20 m high and up to 1000 m wide; some are symmetrical in cross-profile, whilst others have strong asymmetry (both E-and W-facing) with asymptotic slopes. They are currently only imaged in single-beam, not multibeam, data. Their setting at the western entrance to the Pentland Firth in a hydrographic region renowned for its strong tidal currents (Kenyon & Stride Reference Kenyon and Stride1970) suggests that they are probably large-scale, mobile (but long-lived), sediment transport features (i.e., sand waves). Similar features have been mapped as sand waves on the seabed between Orkney and the Scottish mainland (Stoker et al. Reference Stoker, Hitchen and Graham1993). Seismic (airgun and sparker) profiles across this field of ridges are equivocal, showing prismatic structureless sediment bodies unconformably overlying acoustically bedded hard substrate (probably bedrock) (Fig. 13). However, the possibility that the ridges are reworked, glacigenic features (end/de Geer moraines) or have a glacial sediment core, cannot be ruled out at this time.

3.1.3. West Shetland Shelf and Orkney–Shetland Platform

Considerable previous work has been done on characterising the seabed morphology, seismic architecture and Quaternary geology of the West Shetland Shelf. The following section draws on this literature, whilst summarising the range of glacial landforms seen in the bathymetry data on the continental shelf around Orkney and west of Shetland.

Adjacent to the shelf break, in water depths of 160–200 m, are a suite of ridges; two large broad ridges and two smaller intervening ridges (Figs 5, 6). The longest of these extends semi-continuously for >120 km, beyond the northern limit of the study area, in a NE–SW direction, parallel to the continental shelf edge. The broad ridges are the largest depositional features imaged on the continental shelf, being typically 3–5 km wide and ca. 20–30 m high; the two intervening ridges are only 1–2 km wide (Fig. 6). All the ridges have smooth low-aspect cross-profiles, with very broad almost imperceptible crests. NW-facing (distal) slopes tend to be slightly steeper than SE-facing (proximal) slopes, though the exact plan morphology of the ridges is not easy to establish, even with image enhancement, owing to their subdued form. The ridges are part of a wider suite of glacigenic deposits on the West Shetland Shelf, identified in seismic data by Stoker & Holmes (Reference Stoker and Holmes1991) and interpreted as ice-sheet end moraines. This whole suite of shelf-edge and mid-shelf moraines forms part of the Upper Pleistocene (Late Weichselian) Otter Bank Formation, as defined by Stoker et al. (Reference Stoker, Hitchen and Graham1993, Reference Stoker, Balson, Long and Tappin2011a). Further work on their acoustic character and internal architecture has been carried out by Davison (Reference Davison2005).

New mapping of these moraine ridges from echo sounder imagery (Fig. 6) shows their spatial relationships more clearly; with the inner broad ridge abutting, and partially overprinting, the outer ridge along the northernmost part of the shelf in the study area. The moraines appear to stop abruptly where the shelf edge (200 m water-depth contour) makes a re-entrant inshore – known as the Foula Bight (Figs 1, 6). BGS seismic profiles across this part of the outer shelf show relatively thin (<20 m thick) acoustically structureless or chaotic packages within the broad ridges (Otter Bank Formation) passing laterally into layered deposits, part of the Morrison Formation on the West Shetland Slope (Stoker et al. Reference Stoker, Hitchen and Graham1993; Stoker Reference Stoker, Scrutton, Stoker, Shimmield and Tudhope1995; Davison Reference Davison2005).

Approximately 20 km to the SW, on the mid shelf, is another suite of large broad ridges, (∼2–5 km wide; 20–50 m high) with concentric, arcuate or lobate morphology, some with zigzag or convoluted plan forms. The whole suite has been interpreted as a complex of Late Weichselian ice-sheet end moraines; the outermost ridges forming part of the Otter Bank Formation, the innermost forming part of the Stormy Bank Formation (Stoker et al. Reference Stoker, Hitchen and Graham1993). The geomorphology and seismic architecture of this sequence has been described previously in detail by Stoker & Holmes (Reference Stoker and Holmes1991), Stoker et al. (Reference Stoker, Hitchen and Graham1993) and Bradwell et al. (Reference Bradwell, Stoker, Golledge, Wilson, Merritt, Long, Everest, Hestvik, Stevenson, Hubbard, Finlayson and Mathers2008a). These seabed landforms are clear in the single-beam bathymetry and represent the densest clustering of large moraines on the continental shelf within the study area (Figs 5, 6). Our mapping merely refines earlier work, showing that there are nine or ten large nested ridges within 50 km horizontal distance; some of them showing clear overprinting relationships.

Correlation of the outermost Otter Bank Formation moraines in this group with the large moraines W of Shetland (Stoker & Holmes Reference Stoker and Holmes1991; Stoker et al. Reference Stoker, Hitchen and Graham1993) is possible, based on seismostratigraphical and geomorphological grounds. However, correlation of the inner moraines in this group (Stormy Bank Formation) with ridge fragments on the West Shetland Shelf to the N remains equivocal, as a large area of bedrock occurs at seabed in this area ∼60°N (Fig. 6).

To the SE of the relatively shallow Otter Bank moraines (80–120 m water depth), the seabed deepens inshore, reaching −180 m in the parts of the broad Westray Basin, before rising abruptly at the Orkney–Shetland Platform (Fig. 14). A conspicuous well-defined ridge with a distinctive lobate or looped plan form occurs in water depths of ca. 110–130 m on this adverse-sloping seabed, ∼10 km to the SE of the main Otter Bank–Stormy Bank moraine complex described above. This ridge forms the innermost (youngest) large regional moraine within the Otter Bank–Stormy Bank sequence; we call it the Westray Loop Moraine (Fig. 14). This sharp-crested ridge forms a 70 km-long loop, with a remarkable degree of morphological continuity. Good quality single-beam and multibeam bathymetry data show that this ridge is 400–1600 m wide and typically 10–20 m high with strong asymmetry, being notably steeper on its E-facing (proximal) slopes. In places, the ridge crest line widens or splits into two well-defined ridges. Occasionally, these ridge bifurcations enclose small intra-ridge basins; one such notable basin is ∼1 km² in area (Fig. 14). In other places, another small fragmentary ridge is set back 1–3 km inside the main ridge complex. The ridge has been previously interpreted by Bradwell et al. (Reference Bradwell, Stoker, Golledge, Wilson, Merritt, Long, Everest, Hestvik, Stevenson, Hubbard, Finlayson and Mathers2008a) as a lobate ice-sheet end moraine, possibly formed during a dynamic (?rapid) oscillation of the ice margin. BGS seismic data across the double-crested moraine would tend to support this interpretation, with strong parallel sub-surface reflectors within the ridge complex being truncated by a low-angle disruption plane at depth, interpreted here as a thrust (Fig. 14). Similar tectonic structures are often seen within contemporary thrust-block moraines, commonly associated with surging behaviour (Evans & Rea Reference Evans, Rea and Evans2003). Inboard of the Westray Loop Moraine, seismic reflection data show that the main part of the Westray Basin is covered in a relatively thick (>20 m), acoustically layered fill of glacial (?glaciomarine) and postglacial sediments overlying a strongly undulating, probably glacially moulded, erosion surface (Fig. 14).

Figure 14

Seabed moraines on the mid-shelf NW of Orkney: (A) hill-shaded echo sounder bathymetric surface model illuminated from the NW; dashed line shows join between single-beam and multibeam data; (B) hill-shaded multibeam bathymetry data showing the prominent Westray Loop Moraine (WLM) complex in detail (area of image shown in (A)). Lower panel: bathymetric profiles perpendicular to ridge complex showing different cross-profile (2-D) morphology of moraine at various points. Depths and distances in metres; (C) regional bathymetric transect showing depth profile of mid-shelf. Note the reverse (inshore-deepening) slope and location of main moraine complexes; (D) BGS seismic reflection (sparker) profile across Westray Loop Moraine and adjacent basin. Line of seismic profile shown in (A). Note the disrupted reflectors within the moraine ridge – possible evidence of glaciotectonic deformation. Contains Maritime and Coastguard Agency MBES data.

A large proportion of the seabed on the inner shelf around Orkney and Shetland consists of exposed bedrock, with structurally juxtaposed Lewisian, Moine, Devonian and Permo-Triassic rocks (Stoker et al. Reference Stoker, Hitchen and Graham1993; Ritchie et al. Reference Ritchie, Ziska, Johnson and Evans2011). This exposed bedrock is clearly imaged and can be mapped from the echo sounder bathymetry imagery, with a rugged highly irregular surface in multibeam data and a noisy chaotic seabed texture in single-beam data (Fig. 14). Across this predominantly bedrock seabed, numerous small sediment ridges are seen, particularly around northern Orkney and in the Orkney–Shetland Channel. These ridges generally occur in shallower water depths (<120 m below present-day sea level) and are well developed, continuous, linear features. Excellent examples are seen 5–20 km W of Westray on the gently sloping western flank of the Orkney–Shetland platform. Here, a suite of eight transverse ridges trends broadly NNE–SSW, in water depths of −120 m to −60 m, with linear, curvilinear or gently arcuate plan forms (Fig. 15). Most ridges can be traced semi-continuously for 20–30 km, although shorter ridge fragments <3–10 km in length also occur. Ridges are typically 100–300 m wide and 1–5 m in height. Ridge spacings are fairly regular and in the range of ∼1000–3000 m. Most ridges have simple crest lines, but one or two ridges bifurcate or display multiple crest lines. In cross-profile, ridge asymmetry is quite marked, with almost 60 % of moraine profiles (n=40) showing a steeper E-facing slope, although reversed asymmetry is noted locally (Fig. 15). Although no seismic sub-bottom profiles across these features were examined, the ridges are clearly draped directly on bedrock, corroborated by BGS seabed sediment maps, with little or no other Pleistocene deposits in the vicinity. The distinct morphological similarity of these transverse ridges with subaqueous moraines elsewhere strongly suggests that they are recessional moraines, probably de Geer moraines, deposited by a grounded ice front terminating in water (e.g., Lindén & Möller Reference Lindén and Möller2005; Ottesen & Dowdeswell Reference Ottesen and Dowdeswell2006). Large, isolated sandbanks, presumed to be Holocene, with distinctive slope morphology and E–W orientation are also seen in the shallows (<50 m water depth) close to the coast of Westray (Fig. 16).

Figure 15

Seabed moraines on the western flank of the Orkney–Shetland Platform: (A) hill-shaded multibeam bathymetry data showing suite of long transverse ridges W of Westray; (B) outline map of transverse seabed ridges interpreted as subaqueous recessional moraines. Red arrow indicates direction of ice-sheet retreat inferred from moraine morphology. Bedrock at seabed is stippled; (C) oblique view of seabed moraines, looking NW, showing line of bathymetric profile; generated in Fledermaus software. Lower panel: bathymetric profile (in metres) perpendicular to crest line of moraines along line X–X'; (D) morphometric analysis of moraines (from W to E; 40 cross-profiles). Grey profiles show end members: smallest (and most asymmetric) and largest (almost symmetrical) ridge. Red profile is more typical. [Note: other profiles removed to aid clarity]; (E) extract of seabed slope model (derived from bathymetric xyz data) for area shown in (A). Warm colours are steeper slopes. Note the generally higher slope angles on east-facing slopes, highlighting moraine asymmetry. Steep west-facing slopes occur locally. [Diagonal stripes are data and processing artefacts.] Contains Maritime and Coastguard Agency MBES data.

Figure 16

Seabed moraines on the Orkney–Shetland Platform: (A) hill-shaded multibeam bathymetry data showing suites of transverse ridges in the North Sound, offshore Papa Westray, Orkney. Onshore elevation model: hill-shaded NEXTMap Britain digital surface model; (B) summary map of transverse seabed ridges interpreted as subaqueous recessional (de Geer) moraines (black, grey lines). Red arrows indicate direction of ice-sheet retreat inferred from moraine morphology; large sandbanks are shown in blue; bedrock at seabed is stippled. Note how the moraines chart retreat in two different directions, suggesting separation of a palaeo-ice front around submerged bedrock highs (marked). Dashed line shows extent of multibeam data; (C) bathymetric profiles (in metres) perpendicular to crest line of moraines, showing typical slope asymmetry. Profile lines are shown in (A). Contains Maritime and Coastguard Agency MBES data.

Other equivalent transverse seabed ridges, with similar dimensions and ridge spacings, occur in the waters around the Orkney Islands, particularly in North Sound between Westray and Sanday in present-day water depths of 20–70 m (Fig. 16). Collectively, we assign a common glacigenic (morainic) origin to these suites of transverse ridges with similar 3-D morphology and dimensions. Although the slope asymmetry may vary (steeper distal vs proximal slope) as well as the absolute dimensions, the de Geer-type moraines mapped on the Orkney–Shetland platform are easily distinguished in multibeam bathymetry from more recently formed sandbanks, sand waves or linear bedrock structures (Fig. 16). Owing to the very thin and patchy distribution of Pleistocene deposits on this part of the inner shelf, the Quaternary sequence around northern Orkney is undivided. However, on the basis of the regional stratigraphic overview (Stoker et al. Reference Stoker, Hitchen and Graham1993) any glacigenic sediments in this region are expected to fall within the Stormy Bank Formation (Stoker et al. Reference Stoker, Balson, Long and Tappin2011a, Reference Stoker, McMillan and Watersb). The steep-sided, fresh-looking form and good state of preservation of the Westray and North Sound moraines is entirely consistent with a Late Weichselian age; although no absolute age constraints currently exist (Figs 15, 16).

Around 30–50 km to the E of the Pentland Firth, in relatively shallow water (60–80 m water depth) to the SE of Orkney, are a number of seabed ridges and banks of differing scale and morphology clearly imaged in both single-beam and multibeam data (Figs 5, 6, 17). The three largest (and oldest) features are a collection of NE–SW-trending low, broad, discontinous ridges and mounds. The main ridge belts are 1–4 km wide and elevated only 5–10 m above the surrounding seabed, with gentle slopes and wide subtle, sometimes multiple, crest lines (Fig. 6). These features continue to the NE, outside the area covered by this study. Quaternary deposits in the area to the E of Orkney and the outer Moray Firth are not well understood, owing to the thin and patchy nature of Pleistocene deposits; those that are defined in BGS maps and reports are left undifferentiated (BGS 2013). Furthermore, no formal correlation has yet been made between glacial sediment sequences and landforms to the E of Orkney with those to the W of Orkney (e.g. Otter Bank Formation, Stormy Bank Formation, etc) (Stoker et al. Reference Stoker, Hitchen and Graham1993; Andrews et al. Reference Andrews, Long, Richards, Thomson, Brown, Chesher and McCormac1990). Superimposed, in places, on top of these broad ridges is a second set of more delicate NE–SW-trending transverse seabed ridges. These form suites of narrow closely spaced curvilinear ridges with well-defined, sometimes bifurcating, crest lines. In one suite, approximately 40 km E of Duncansby Head, 15 well-developed but discontinuous ridges are seen in a horizontal distance of ∼10 km at the extreme eastern edge of the study area (Fig. 17). The longest ridges can be traced laterally for ∼20 km, although most are shorter and continuous over only 5–10 km or less. Most ridges occur within a well-defined elevation threshold 65–75 m below present-day sea level; several ridges terminate abruptly where the seabed deepens (>75 m water depth). Ridges are typically 100–200 m wide and 1–5 m high; ridge spacings are in the range of ∼300–1000 m. In cross-profile, ridge asymmetry is marked, with most ridges possessing a steeper W- or NW-facing slope, although symmetrical and reversed asymmetry is noted in one or two cases (Fig. 17). No high-resolution seismic lines were available for this area; hence the internal sediment architecture could not be examined. However, the co-spatial presence of (presumably Holocene) marine bedforms in the multibeam data, with markedly different morphologies and orientations to both generations of seabed ridges, suggests that the ridges are not related to present-day marine bed-forming processes (Fig. 17). This fact, and the strong morphological similarity of the smaller transverse ridges to subaqueous moraines elsewhere, strongly suggests that they are Pleistocene glacigenic landforms, probably de Geer moraines akin to those seen elsewhere around Orkney (cf. Figs 15, 16). The collective evidence suggests that they were deposited by a large ice mass terminating in shallow water and retreating in a northwesterly direction back towards Orkney. The larger more subdued ridges are probably older (?terrestrial) end moraine complexes deposited during an earlier stage of firmly grounded ice sheet retreat and subsequently overprinted by de Geer moraines with a similar orientation, recording retreat of a lightly grounded (partly floating) tidewater ice-sheet margin.

Figure 17

Seabed moraines SE of Orkney: (A) hill-shaded multibeam bathymetry data showing suite of transverse ridges ∼40 km E of Duncansby Head, Outer Moray Firth; (B) summary map of transverse seabed ridges interpreted as subaqueous recessional (de Geer) moraines (black lines). Red arrow indicates direction of ice-sheet retreat inferred from moraine morphology. Note the absence of moraines in deeper water areas (>80 m); (C) bathymetric profile (in metres) perpendicular to crest line of moraines showing typical slope asymmetry; profile line shown in (A); (D) oblique view of seabed ∼25 km ENE of moraine suite in (A), looking N; shows moraines and marine bedforms at strongly different, almost perpendicular, orientations. Lines of bathymetric profiles are also shown. Lower panel: bathymetric profiles (in metres) perpendicular to transverse features, showing the markedly different cross-profile of de Geer moraines (steeper slopes; greater relief) vs. marine bedforms. Note different scales on x and y axes. Contains Maritime and Coastguard Agency MBES data.