Rock avalanche scars in the geological record: an example from Little Loch Broom, NW Scotland

Proceedings of the Geologists’ Association 126 (2015) 698–711

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Rock avalanche scars in the geological record: an example from Little Loch Broom, NW Scotland Gareth Carter * British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, United Kingdom

A R T I C L E I N F O

A B S T R A C T

Article history: Received 16 March 2015 Received in revised form 15 September 2015 Accepted 16 September 2015 Available online 18 October 2015

An embayment in the southern rock slopes of Little Loch Broom, NW Scotland, was surveyed using a combination of morphological mapping and engineering geology techniques. The evidence points to a large-scale rock slope failure (rock avalanche) mechanism of formation, occurring during glacial downwasting and retreat, perhaps initiated by debuttressing and stress release. Periglacial activity, such as macrogelivation or large-scale frost-wedging, may have exacerbated the internal joint network by expanding the aperture dimensions of pre-existing joints leading to an increase in stresses and a reduction in friction across joint surfaces. The rockwall cavity was completely evacuated by the rock debris, leaving the embayment with a mantle of sediments originating from the glacial diamicton that drapes the plateau above the backwall. The absence of any rock avalanche debris deposit has led to the conclusion that the debris run-out extended onto the surface of the glacier and was supraglacially transported and dispersed, which has implications for glacial dynamics and local glacial history for Little Loch Broom. ß 2015 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.

Keywords: Rock avalanche Rock slope failure Little Loch Broom Deglaciation Last Ice Age

1. Introduction Whitehouse (1983) describes rock avalanches as being a ‘‘rapid flow-like movement of rock-rubble originating from the failure of all or part of a mountainside’’, and can incorporate a huge volume of bedrock material (>106 m3 in the case of ‘‘catastrophic’’ failure events) whilst travelling at speeds in excess of 200 km h 1 (Hewitt et al., 2008). Evidence of these large-scale failures is typically recorded in steep, mountainous settings that are experiencing (or have experienced during the Quaternary) periglacial or paraglacial conditions. It is therefore no surprise that the variety of causes and trigger mechanisms attributed to most Rock Slope Failure (RSF) events (of which rock avalanches fall into the ‘‘catastrophic’’ or ‘‘cataclysmic’’ failure category as described above) are mainly associated with bedrock structure (specifically the orientation of discontinuities such as bedding planes, joint/fracture sets, and tectonic cleavage planes) and stresses related to these environments. Debuttressing and oversteepening of rockwalls plays a major role in internal stress behaviour and the associated joint

* Tel.: +44 131 667 1000. E-mail address: [email protected]

characteristics within the bedrock. Oversteepening of the rockwall can lead to an increase in the self-weight shear stresses within the rock unit which, in turn, may be conducive to the generation of tensile stresses within joint sets along the base of the rock slope (Ballantyne, 2002; Deline, 2009). Debuttressing of the rockwall due to glacier retreat and downwasting can also lead to alterations in the structural joint regime. For example, stress-release in the rock mass may result in the propagation of pre-existing internal joint sets which can lead to instant or delayed failure of the bedrock unit (Ballantyne et al., 2014). External factors also play a part in RSF events; for instance earthquakes have been recorded as the primary cause of several rock avalanches during the 20th and 21st century, with the tragic Hattian Bala landslide a notable example (Dunning et al., 2007). In the periglacial/paraglacial environment, it is proposed that seismic activity associated with glacio-isostatic readjustment could be responsible for the failure of steep rock slopes (McColl, 2012). The climatic conditions associated with periglacial environments can also lead to additional stresses on bedrock, including large-scale frost-wedging (macrogelivation), whereby the apertures of pre-existing joint sets are widened through the freezing and subsequent expansion of water leading to a reduction in internal locking stress (Ballantyne, 1982). Today, the majority of documented rock avalanches are found in mountain belts across the globe, such as the Southern Alps of

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G. Carter / Proceedings of the Geologists’ Association 126 (2015) 698–711

New Zealand (e.g. Allen et al., 2011), the Mont Blanc Massif, Italy (Deline and Kirkbride, 2009), the eastern Alaska Range (Shugar et al., 2012) and the Karakoram Himalaya, Inner Asia (Hewitt, 2009). At lower altitudes, rock slope failures, including rock avalanches, have been observed along the fjord slopes of Norway (Braathen et al., 2004; Blikra et al., 2006; Hermanns et al., 2006; Osmundsen et al., 2009) and the west coast of Greenland (Kelly, 1980). Interestingly, present-day research is finding that rock avalanches are more frequent in glacially active mountain ranges than previously thought and increasing attention is being given to rock avalanches that flow onto the surface of glaciers (Fig. 1) (e.g. Deline, 2009; Hewitt, 2009; Shugar and Clague, 2011). Recent studies have indicated that glaciers can experience surges induced by rock avalanche cover and supraglacial transport of the associated debris can be significant (Hewitt, 2009; Shulmeister et al., 2009). Hewitt (2009) reports that rock avalanche debris

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deposited onto the Bualtar Glacier in the Karakoram Himalaya was transported 9 km from the bedrock source within a 20-year period, with about one third of that distance being achieved under surge conditions. During the initial surge following deposition of the rock debris, ice velocities increased from >0.8 m/day to a range of 7–11 m/day (Hewitt, 2009). In addition to increased ice velocities, it was noted that ice remained approximately 10–15 m higher in the areas of rock coverage when compared with the adjacent areas of the glacial surface; Hewitt (2009) concludes that this was due to a reduction in ablation. These changes in ice dynamics led to considerable alteration of the rock failure debris over the 20-year period, to the point where distinguishing between the rock avalanche deposit and other types of large supraglacial rock masses became extremely difficult (Hewitt, 2009). Although Scotland has remained free of ice since the termination of the Loch Lomond Stade (c. 12.9–11.7 ka), the landscape has

Fig. 1. Morsa´rjo¨kull Glacier, Vatnajo¨kull National Park, Iceland; an example of a rock avalanche deposited on top of a glacier (photo courtesy of Jeremy Everest & Emrys Phillips, The British Geological Survey).

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been heavily influenced by para- and peri-glacial activity (Ballantyne, 2012). This includes RSFs of various scales and sizes which have been recognised and mapped in the mountainous regions of Scotland for some considerable time now (e.g. Holmes and Jarvis, 1985; Ballantyne, 1986). Jarman (2006) compiles a database of RSF in the Scottish Highlands, based upon a minimum size threshold of 0.25 km2 which includes a total of 140 recorded events. The database is divided into categories based upon each RSF characteristic type; cataclysmic and sub-cataclysmic events, arrested (semi-intact) short-medium travel events, and slope deformations which includes sackungen failures (Jarman, 2006). Large-scale cataclysmic (also known as catastrophic) events are characterised by complete evacuation of the bedrock cavity and a low probability of reactivation due to the long run-out distance covered by the rock debris deposit which can typically travel beyond the slope foot (Jarman, 2006). The term sub-cataclysmic involves similar failure characteristics, however the rock debris deposit is typically backed up from the slope toe towards the rock slope cavity (Jarman, 2006). It is noteworthy that only two large rock avalanches have been identified in Scotland; the welldocumented Beinn Alligin rock avalanche (Ballantyne, 2003; Ballantyne and Stone, 2004) and the boulder accumulation in Strath Nethy (Jarman, 2006; Ballantyne et al., 2009). At present, all known RSF deposits across Scotland have been recorded in mountainous terrain. Given the similarities in bedrock geology and glacially derived landscapes (e.g. fjords) between northwest Scotland and the Norwegian coastline, it is perhaps surprising that while there have been several documented instances of RSF events within relatively low altitude fjord settings in Norway, none have been recorded within the sea lochs of Scotland. This paper aims to investigate an embayment feature set into the southern rockwall of the inner loch of Little Loch Broom, NW Scotland (Fig. 2), and (using a combined morphological and engineering geology approach) ascertain whether the origin of this feature could be the result of a rock avalanche event.

NW-trending faults are a common feature of the coastline of northwest Scotland, and it is one such fault, running through the bedrock below the seafloor sediments, that controls the orientation of Little Loch Broom (Stoker and Bradwell, 2009). NE-trending faults (such as the Coigach Fault located past the loch mouth and the famous Moine Thrust situated <5 km from the fjord head) intersect with the loch-base fault, which effectively splits the bedrock geology into a sequence of blocks (Stoker and Bradwell, 2009). Superficial deposits in the area generally reflect the glacial processes that have shaped the regional landscape. Glacial deposits that blanket the slopes of the sea loch include till, terrace and fan deposits, and glaciofluvial outwash sheets (Stoker et al., 2006). Glacial landforms such as moraines are well documented in the area (Ballantyne, 1993). Typical coastal and fluvial sediments are abundant along the foot of the slope, including alluvium, marine beach and raised beach deposits, along with river terrace deposits. Fig. 2 presents a simplified map of the geology and superficial deposits of the study area. The glacial history of Little Loch Broom and the surrounding area is still a debated topic, with various studies finding differing dates for regional deglaciation. Bradwell et al. (2007) suggest that during the Late Devensian an active ice stream existed across the Minch between mainland Scotland and the Outer Hebrides until ca. 17 k yr BP. Following ice stream disintegration, Bradwell et al. (2008) argue that large active glaciers existed in the area throughout the Lateglacial Interstadial (14.7–12.9 ka), with final deglaciation occurring around 11.7 ka BP in NW Scotland following the onset of the Holocene. However, recent work by Ballantyne and Stone (2012) suggests that the Wester Ross Readvance (WRR) around 14.7 ka represents the last period of glacial activity in low ground and fjords across NW Scotland, and they conclude that the entire Summer Isles region surrounding Little Loch Broom was completely deglaciated by ca. 14.0 ka at the latest. However, a general consensus has been achieved on a north westerly ice-flow direction across the area of interest (Fig. 2).

2. Bedrock and Quaternary geology of the study area

3. Methods

The bedrock geology of Little Loch Broom is dominated by the late Precambrian Torridonian sandstones which are subdivided into the Torridon Group (youngest), which unconformably overlies the Stoer Group (oldest) which in turn unconformably overlies the Lewisian Gneiss Complex (Beacom et al., 1999; Prave, 2002). The southern slopes of the inner loch are formed by rocks of the Applecross Formation. This is the thickest unit within the Torridon Group (3–3.5 km thick) and is comprised of sediments deposited in a braided fluvial and alluvial fan environment which prograded eastward through the basin, infilling a series of palaeovalleys (Owen, 1995; Strachan and Holdsworth, 2000). The deposits mainly consist of coarse, pebbly, red arkosic sandstones with occasional layers of conglomerate, which are part of a larger 5–6 km fining-upwards succession deposited by a major fluvial system (Ballantyne, 1981; Strachan and Holdsworth, 2000). Large bar structures (9 m thick) within the Applecross sandstones were deposited by a major river system, and abundant softsediment deformation structures signify turbulent palaeocurrent activity (Strachan and Holdsworth, 2000). The Applecross Formation has undergone very little post-depositional alteration, with beds typically dipping at 158 and retaining their original structure with little or no evidence of folding (Ballantyne, 1981). Quartzofeldspathic detritus within the Applecross Formation has been attributed to erosion of the Lewisian gneisses and palaeomagnetic data and Rb–Sr isotopic analysis have yielded an age of 977  39 Ma for this formation (Strachan and Holdsworth, 2000).

The site of interest is located on the southern slopes of the inner loch (Fig. 2). The coastal slope rises from present day sea level to a plateau at approximately 350 m OD, and has a NNE facing aspect. Prior to the commencement of fieldwork, a morphological interpretation of the slopes of Little Loch Broom was undertaken using stereoscopic colour and panchromatic aerial photographs. The photographs were taken during various Royal Air Force (RAF) sortie missions over NW Scotland, dating from 1946 to 1975 (1:10,000 scale photos). It was during this exercise that the embayment feature on the southern slopes of the inner loch was first identified, and a field investigation was deemed necessary. After conducting a field reconnaissance survey, morphological mapping was undertaken following published recommended methods (e.g. Savigear, 1965). Particular attention was given to slope breaks and gradient, recorded using an abney level, tape measure and sighting pole. In addition, the field morphological map recorded areas of substantial rock debris cover and bedrock outcrops. Subsequent interpretation of the resulting morphological map allowed for the development of a basic morphostratigraphic relative chronology to explain the evolution of the site (see Sections 6 and 7). Failures in rock slopes are often controlled by discontinuities (joints, bedding planes, faults, and shear planes) and the intersections of these fractures, which can result in wedges or blocks toppling or sliding out of the outcrop face (Hoek, 2007). Bedrock outcrops along the sidewall of the embayment allowed

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Fig. 2. Top: Study area, delineated by the red box. White arrows represent dominant ice-flow direction (based upon studies by Ballantyne et al. (1998) and Bradwell et al. (2008)). Bottom: Simplified geological map of Little Loch Broom (British Geological Survey, 2013a,b).

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Fig. 3. Field photographs showing examples of the Applecross Formation bedrock, associated bedding planes and discontinuity joint sets.

structural measurements to be taken as part of a detailed discontinuity survey and rock material descriptions were noted, in line with industry standards (British Standards Institute, 2003). As is standard practice with engineering geology, dip and dip direction of discontinuities were recorded using a compass clinometer (corrected for magnetic declination), applying the standard righthand rule. Field photographs of key rock mass features and discontinuities were taken as additional structural evidence of potential controls on rock slope stability in the area (Fig. 3). The structural data were plotted on equal-angle lower hemisphere stereographic projections using the commercially available plotting software package ‘‘Dips’’ (version 5.1), a registered trademark of Rocscience Inc. This software is specifically designed to assess planar, toppling and wedge failure based upon structural measurement datasets. 4. Bedrock lithology and fractures The Torridonian bedrock is well-exposed in a number of large outcrops from the southeast side of the embayment (trending roughly NE–SW), as well as in the backwall to the frontal edge of this landform. The embayment occurs within the sandstones of the Applecross Formation. In this area the formation comprises strong,

light pinkish red (weathered to grey), granular, medium to coarse grained, medium to thickly bedded, planar to trough cross-bedded, locally gravelly and pebbly sandstones. The distinct pinkish red colouring is due to the presence of hematite cement. Bed thickness varies from approximately 40 cm to 1.8 m. Laterally persistent bedding planes dip gently (158) to the NE and, in most instances, can be traced for the whole length of the outcrop (400 m). Apertures were generally noted to be relatively consistent across the vertical extent of the outcrop and there was no evidence of gap widening towards the top of the outcrop. These aperture gaps vary from tight (closed) to wide void spaces of 0–10 cm. Three joint sets were identified, defined by similarities in fracture orientation and dip angle which create three distinct clusters when plotted on the stereographic projection (Fig. 4). Based upon the BS 5930:1999 rock discontinuity description chart (British Standards Institute, 1999), the fractures can be described as being wide to very wide spacing (50 cm to 2.5 m) with high persistence and ‘X’ termination (outside exposure), and wavy, rough undulating surfaces. The apertures are generally moderately open to very open (1 mm to >1 cm) and are often damp with soil and lichen infill. Poles and planes can be seen plotted on a stereographic projection in Fig. 4 using Matheson’s method of graphically presenting discontinuity measurements in relation to

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Fig. 4. Stereographic projection of discontinuity joint sets using Matheson’s method (1983) of graphic display in relation to rock slope stability.

rock slope stability (Matheson, 1983). This plot demonstrates that, while wedge and toppling failures are less likely in this area, planar failure along steeply dipping NW orientated fractures is a possibility. At a regional scale, faults can be seen to trend roughly NE–SW, perpendicular to the sea loch slopes. As previously stated,

NW-trending faults are common along the west coast of Scotland and Little Loch Broom is no exception; Fig. 5 shows a series of these faults cross-cutting the NE–SW-trending structures. As a result, much of the bedrock along the coastal slope has been naturally divided into discrete blocks, bounded by cross-cutting NE and NWtrending faults. The embayment feature represents such a

Fig. 5. Regional fracture map of the inner loch, Little Loch Broom. Dashed red lines represent evidence of large-scale faulting and major joints. Black dashed line outlines the embayment feature.

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Fig. 6. Top: Geomorphological map showing areas of debris lobe cover, raised spur-like topographic feature and areas of abrupt step changes in slope associated with lateral bedrock outcrops. Bottom: Areas of the three key ‘Domains’ referred to throughout the document.

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structurally controlled rock slope block, with evidence of a NWtrending fault running along the backwall and NE-trending faults creating the sub-vertical SE limb and NW limb which is currently being exploited by the Allt Airdeasaidh stream. 5. Slope morphology A detailed morphological map of the embayment is shown in Fig. 6. The main embayment consists of a roughly horseshoe shaped indentation on the southern slope of the inner loch, measuring approximately 1 km along slope and 0.75 km from the top of the backwall to the toe. The 0.75 km2 area was covered, from the ‘‘toe’’ of the feature at sea-level to the glacial diamicton-covered plateau at approximately 350 m OD above the ‘‘backwall’’ of the NNE-facing embayment. The roughly ‘‘U’’ shaped cavity has a pronounced southeast limb and a more subtle northwest side (which may have undergone modification by fluvial actions of the Allt Airdeasaidh stream), and there is a prominent ‘‘toe’’ bulge along the coastline where the feature propagates out into the loch. Bedrock was noted to strike SE, parallel with the sea loch, with small (<1 m high) bedrock ‘‘steps’’ occurring at intervals along the lower slopes of the embayment (<100 m OD). The slopes above these steps have an undulating, hummocky topography with little evidence of bedrock until the exposure along the backwall at approximately 200 m OD. For ease of description, the study area is divided into three key domains: (1) backwall, (2) main terraced slope, and (3) elongated raised area on the SE margin of the embayment. 5.1. Backwall The NNE-facing backwall rises from approximately 200 m OD to 300 m OD and has a slope gradient of approximately 408. The diamicton blanketing the plateau above the backwall consists of poorly sorted subangular to subrounded cobbles and boulders set

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into a sandy gravelly matrix. Immediately above where the contact between bedrock and diamicton can be seen in the backwall (300 m OD) there are a series of elliptical-shaped depressions, the largest of which cuts 10 m down into the overlying glacial deposits. The slopes (288 gradient) of the largest hollow are sparsely vegetated and recently exposed glacial sediments can be observed, indicating relatively recent slope processes at work. The bases of these depressions dip towards the backwall and have very saturated basal sediments due to an abundance of active spring lines which feed incised gullies on the lower slopes. Below these scalloped depressions, extending 300 m from the foot of the backwall, there are a series of small overlapping debris cones which blanket the bedrock and terminate around 100 m OD elevation. As these debris cones appear to overlap and cross-cut one another, this would suggest several phases of transport and deposition. Lateral changes in the topographic profiles of the cones was observed; towards the NW side of the embayment the cones are less pronounced, appearing as an amalgamated blanket of debris with a lack of stream-cut gullies. In contrast, to the central and SE side of the slope, gullies clearly incise the debris cover indicating active gullying processes. Vegetation cover is wellestablished across the site, suggesting these are relic debris cones. However, the central to SE section of the site appears to still have an active system of sediment transport from above the backwall through stream erosion and gullying. 5.2. Main terraced slope Below 100 m OD, the slope undulates and is defined by several step-like changes in gradient. At several points across the slope, bedrock terraces can be seen to form steps which laterally traverse the embayment (Fig. 7). The undulating, sediment-covered surfaces of the terraces gently dip downslope (10–68 gradient) and these slope angles are relatively consistent laterally across the

Fig. 7. Field photograph highlighting areas of debris cover and gullying on the upper slopes and bedrock terraces on the slopes below.

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site (Fig. 6). The bedrock terraces are approximately 40–100 cm in height, which corresponds with bed thicknesses noted from outcrops observed in situ on slopes flanking the embayment. To the NW, these bedrock steps can be seen intersecting the Allt Airdeasaidh which exploits a pre-existing NE-trending fault. Towards the SE, the terraces are buried beneath the raised area of boulder-rich cover (see below). Small streams continue to incise downward into the sediments that drape the bedrock terraces, forming extensions of the deeper, more pronounced gullies at the foot of the backwall. 5.3. Raised area The third ‘‘domain’’ comprises a raised boulder-strewn area set into the SE section of the embayment. This arcuate spur-like landform trends SSE to NNW, and the undulating breaks in slope follow the natural curvature of this landform. Directly overlooking this area is the SE limb of the embayment formed from a prominent

outcrop of Applecross sandstone from which structural measurements were taken. It is likely that this outcrop is the source of numerous large (>1 m) blocks of sandstone that appear subangular to subrounded and weathered to light grey. Due to well-established lichen growth across these boulders, it is unlikely that they are the result of recent rockfall events. In addition, some boulders are embedded within the slope sediments to the point whereby the soil immediately upslope of the blocks has mounded up against the boulder and begun to submerge it (Fig. 8). There is an obvious change in vegetation type from the dense, rough heather and bracken covering the raised spur-like landform to the grasses and boggy, hydrophilic vegetation covering the rest of the embayment slopes, suggesting there is a change in underlying superficial geology which could affect drainage relating to changes in grain size and superficial sediment thicknesses. There are two main slope sections worth noting; a lower low-angled (4–88), gently undulating section and an upper steeper-angled (13–218) slope which would be consistent with a two-phase evolution of

Fig. 8. Raised spur-like landform dominating the SE margin of the embayment. Note the abundance of boulder cover and the obvious slope breaks. Also of interest are the soil and slope sediments into which the boulder debris is embedded.

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this landform. The lateral bedrock terraces of key domain no.2 terminate against the periphery of this feature, suggesting the raised area covers the terraces and is therefore of a relatively younger age. Finally, it is worth highlighting the presence of a large (>1 km long) NW–SE orientated drift ridge feature located on the plateau above the backwall of the embayment, which has been interpreted by Ballantyne (1993) as being a medial moraine formed when the ice from Little Loch Broom and that of the hanging valley above separated. The elliptical-shaped hollows described above encroach on the medial moraine, implying that the depressions are relatively younger. 6. Geomorphological interpretation The impact on the southern sidewall of Little Loch Broom is clearly consistent with this feature representing some form of slope failure which resulted in the loss of material from a substantial area of the slope. Domain 1 has been modified by gullying processes and the deposition of a debris mantle at the base of wall, and represents the backwall of the RSF. The embayment floor (domain 2) is characterised by the stepped bedrock terraces which can be traced laterally and appear to match with bedding outcrops in adjacent exposures, in particular the clearly visible outcrops along the course of the Allt Airdeasaidh stream. The heights of the stepped terraces are comparable to the bedding thicknesses noted in the Torridonian Sandstone, suggesting that these landforms represent in situ bedrock at or near slope surface. This domain has been interpreted as being the basal surface of the slope failure. Field evidence suggests that there is a relatively thin veneer of material mantling the floor of the embayment resulting in an undulating terrace surface. The slope angles of the terrace surfaces do not match with measured bedding dip direction, which suggests that the slope profile is influenced/controlled by the overlying sediment drape. The thickness of debris within the main embayment has implications for the overall mechanism for rockwall cavity development, as a series of gradual, retrogressively retreating rockfall or landslide events would be more likely to develop a thicker superficial deposit mantling the in situ, undisturbed bedrock due to the lower-energy nature of such small events (see Section 7). Large (>1 m) boulder debris is notably absent on the floor of the main embayment which might have been expected if the cavity in the southern wall was created by a series of small-scale RSFs or an extended period of rockfall activity. The only area that exhibits an abundance of large boulders and blocks is the raised spur-like feature to the SE margin of the embayment (domain 3). As this raised area truncates the lateral bedrock terraces, it is proposed that it postdates the main phase of embayment evolution. Based upon the morphological evidence outlined above, a relative chronology for the evolution of the embayment landform can be developed. The initial phase would involve a large-scale RSF (i.e. rock avalanche) of the southern rockwall of Little Loch Broom, which resulted in the formation of the main embayment. Following this, the exposed bedrock floor of the cavity (domain 2) would have been draped with a debris mantle potentially originating from the glacial diamicton that blankets the hanging valley above. This process may have been occurring in parallel with the formation of the talus slope (debris cones) noted at the foot of the backwall, potentially along with the initial gullying of the exposed cliff face and overlying rock and sediment debris. Another phase of evolution ensued with the development of the raised spur-like landform along the SE margin of the embayment; this is likely to be a colluvial creep lobe encompassing rock debris and boulders following a series of rockfall events originating from the overlooking Applecross Formation outcrop, which is orientated NE

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suggesting it is the result of a regional-scale NE–SW trending fault. The main breaks in slope (Fig. 8) imply a two phase development, resulting in the steeper upper slopes progressing into the gentler gradient, undulating lower slopes. Continuation or renewal of gullying along the eastern section of the backwall appears likely and this may have accompanied the formation of the scallopshaped hollow above the backwall of the main embayment. This gullying process continues today as several stream channels, originating from the scalloped hollows in the glacial diamict above the backwall, incise down into the talus slope and sediments that drape the floor of the embayment below. All the morphological features associated with various phases of evolution described above have substantial vegetation cover, implying a period of stability and are currently inactive. This has important implications for the relative age of the embayment creation. 7. Discussion: mechanism of rockwall embayment formation It is clear from the slope morphology of the embayment that a significant volume of bedrock has been removed from the southern wall of Little Loch Broom and a preliminary estimate of volume would be in the region of 93,750,000 m3. This volume was estimated based upon contours of the in situ bedrock flanking the embayment, extrapolated across the site to give an approximate volume of removed material. This method only gives a rough estimate as it does not account for any loss of slope material prior to the main rock slope failure event. 7.1. Cataclysmic, sub-cataclysmic, arrested or deformation event? The Little Loch Broom rockwall embayment was noted to have the basic profile (‘‘U’’ or horseshoe shaped) and some of the characteristics of a RSF feature (e.g. excavated rockwall cavity with a protruding toe at the slope base). The term RSF is described by Jarman and Ballantyne (2002) to indicate a ‘‘movement of large masses of rock under the influence of gravity’’ and can encompass many methods of failure; cataclysmic (or catastrophic) and subcataclysmic events whereby the failed rock material completely excavates the cavity and settles on the lower slopes or slope foot, non-complete (‘‘arrested’’) translational slides which typically result in a partially evacuated, yet still visually evident, cavity, and extensional/compressional rock slope deformation which is characteristically apparent through antiscarp arrays and creep features, and may not always create an obvious cavity in the rockwall (Jarman, 2009). Jarman (2009) provides an insight into the geometries of RSF scars incising into rockwalls, stating ‘‘cavities may be rectilinear (armchair), acute or obtuse wedges (often multiple), or planar slices’’. It is also noted that RSF can occur in various environments and are commonly recorded in fjord flanks at sea level (Jarman, 2009). The depiction of a rectilinear (armchair) shaped cavity, with a distinct backwall (domain 1) and cavity floor (domain 2; suggesting complete evacuation of failed bedrock debris), set into a steep-sided fjord rockwall corresponds well with the morphological evidence obtained during field mapping of the Little Loch Broom slopes. However, some key indicators of rock slope failure and deformation are notably absent from the Little Loch Broom rockwall cavity. Firstly, classic features of rock slope deformation are antiscarps (upslope-facing scarps) and tension furrows, for which there is no evidence within the embayment (Gutie´rrez-Santolalla et al., 2005; Jarman, 2006). Given the absence of these features, and the fact that no evidence of arrested failure blocks within the embayment was observed, rock slope deformation or slow-creep failure can be ruled out as a method of rockwall cavity formation. Additionally, a key missing component of RSF (particularly catastrophic rock avalanches) is a massive, long-runout debris

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Fig. 9. Multibeam bathymetry highlighting the main features of the seafloor immediately below the rockwall cavity. Dashed line shows the rough perimeter of the scar and the curved arrows show direction of movement.

deposit (Hewitt et al., 2008). While the slope toe can be observed to protrude out from the natural coastline to the extent that it influences the path of the A832 road (Fig. 6), no evidence of a substantial rock debris deposit was identified. Multibeam bathymetry data were acquired in 2005 during a British Geological Survey (BGS) and Scottish Association for Marine Science (SAMS) geophysical survey of the sea loch, and clearly show the seafloor area immediately offshore of the embayment slope toe (Stoker et al., 2006). Fig. 9 highlights some key features in the submarine setting, including a former meltwater channel and shallow gas pockmarks (Stoker et al., 2010). This could have implications for the timing of rock slope failure; if failure occurred immediately following deglaciation, perhaps due to effects of debuttressing, it would be expected that the debris deposit would fan out across the glacial trough floor and obscure any evidence of meltwater channels in the area. However, as the meltwater channel is still evident on the seafloor, it can only be concluded that no rock avalanche debris was deposited on the seabed directly below the rockwall cavity slopes. Finally, there is an absence of rock debris within the main embayment which suggests a series of smaller, retrogressive rockfall events gradually eroding the rockwall cavity over an extended period of time is unlikely. The above evidence would indicate several important points regarding rockwall cavity formation along the southern slope of Little Loch Broom; a large-scale rock slope failure occurred within the confines of regional-scale NE–SW and NW–SE trending faults which resulted in the development of an armchair cavity with a clear backwall (domain 1) and cavity floor (domain 2), however there is no rock debris deposit evident within the cavity itself. This would imply a cataclysmic (catastrophic) or sub-cataclysmic event whereby the rockwall material was completely evacuated from within the cavity, which would fit with a rock avalanche event being the mechanism of failure. In addition, the lack of evidence of a long-runout debris deposit on the seafloor situated directly below the embayment would indicate that some

form of sedimentary process has led to the removal of this rock material. 7.2. Rock avalanche model Any geological model clearly needs to explain the formation of the embayment in slope bedrock, the removal of a large volume of bedrock from the slope, and the absence of a toe or any rock debris within the floor of the embayment or glacial trough floor below. Consequently, the most likely model is that of a cataclysmic or subcataclysmic rock avalanche during glaciation of Little Loch Broom. The presence of the medial moraine (Fig. 10(A)) on the plateau surface above the embayment backwall (domain 1) indicates where ice filling Little Loch Broom separated from that occupying the hanging valley during the Last Glacial Maximum as the BritishIrish Ice Sheet (BIIS) began to retreat between 19 and 20 ka BP (Ballantyne, 1993; Clark et al., 2009, 2012). As no clear cut evidence for the embayment having been enlarged by ice filling Little Loch Broom was identified, it is suggested that the formation of this feature did not predate Devensian glaciation. The likelihood is, therefore, that the armchair embayment formed during deglaciation of Little Loch Broom. A plausible sequence of events would begin with the downwasting and retreat of ice within Little Loch Broom, exposing the bedrock trough walls (Fig. 10(B)). Glacial unloading and debuttressing of the rockwall would have resulted in a release of strain energy and subsequent joint extension and expansion of the internal joint network (Ballantyne, 2000). These factors led to destabilisation of the rockwall as regional-scale NW– SE and NE–SW trending faults extended and expanded, resulting in perpendicular intersections forming between these joint sets. It is also reasonable to suggest that as the Little Loch Broom ice experienced downwasting and the glacier within the hanging valley above the embayment backwall began to retreat, there would be a significant volume of meltwater percolating into the

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form of block failure could have occurred, leaving a deep scar incised into the trough wall. Indeed, the evidence fits well with descriptions by Braathen et al. (2004) of complex fields in Norway and more specifically that of planar fault geometry; numerous fault blocks leading to a domino-style block configuration which can result in total failure leaving a low gradient slope in the source area with adjacent unaffected moderately to steep gradient slopes. Given the evidence of a potential small-scale block-style failure observed in the field area (Fig. 3), it is possible to envisage a similar failure mechanism developing along steeply inclined discontinuity planes on a much larger scale creating a domino-style collapse within the confines of the large-scale, regional NE–SW and NW–SE trending faults. The sequence of events that could have produced this embayment are depicted in Fig. 10. 7.3. Supraglacial deposition of rock avalanche debris

Fig. 10. Schematic of proposed sequence of events leading to the formation of the rockwall embayment.

fractures and joint sets of the underlying bedrock. Given the north facing aspect of the slope, periglacial conditions would have been prolonged leading to large-scale frost-wedging, known as macrogelivation, resulting in a significant weakening of the pre-existing joints. This combination of paraglacial rock slope adjustment and periglacial weathering resulted in a cataclysmic rock avalanche (Fig. 10(C)). Results from the discontinuity survey suggest that planar failure is possible given the orientation and dip of bedding planes and joint sets (Fig. 4). As numerous subvertical joints intersect the gently dipping (<158) bedding planes, and regional fault mapping implies bedrock is segmented into blocks by roughly NE–SW and NW–SE trending faults, it would be reasonable to suggest that a

The final component of the geological model to address is the absence of boulder debris at the foot of the slope cavity. As previously stated, this is indicative of some physical process of sediment and rock debris removal and, as the slide scar does not appear to predate Devensian glaciation, it is therefore possible that the rock avalanche run-out occurred onto the surface of the retreating and downwasting ice occupying Little Loch Broom (Fig. 10(C)). Finer debris material would have been removed from the rock debris by wind, rain and snow melt action (Shugar and Clague, 2011). Some of the rock avalanche debris is likely to have been lost within crevasse openings and/or dispersed to the glacier margins; Hewitt (2009) reports on rock avalanche debris deposited on the surface of the Bualtar Glacier, Karakoram Himalaya, which was noted to reduce by more than half its original volume in just 19 years of monitoring due to these modes of sediment displacement. The same study highlighted significant sedimentological reworking of the deposit to the extent that, over an approximately 10-year period, the rock avalanche debris resembled typical heavy supraglacial debris (Hewitt, 2009). This evidence illustrates how rock avalanche debris can be dispersed over a relatively short timeframe and, as suggested here, how this could result in the absence of a debris lobe in the glacial trough below the rockwall cavity in Little Loch Broom. Instances of rock avalanche deposits on glacier surfaces are well documented (e.g. Deline, 2009; Hewitt, 2009; Shugar et al., 2012), and the effects of the resulting rock debris on glacier dynamics is an established field of research (Hewitt, 1988; Shugar and Clague, 2011; Sosio et al., 2012). As Deline (2009) highlights, rock avalanche debris acts as an effective insulator; 20 cm of debris cover has been shown to reduce daily ablation by up to six times when compare with ablation rates for clean ice (Deline, 2009). This can contribute significantly to a decrease in glacial retreat rates and indeed trigger glacial advances in some cases (e.g. Hewitt, 2009; Shugar et al., 2012). This has interesting implications for the glacial history of Little Loch Broom and the potential role played by supraglacial rock avalanche debris cover. In Fig. 9, an inner loch moraine is highlighted just at the foot of the rockwall cavity on the seafloor. While it is impossible to say for certain, it can be suggested that this moraine could represent a small readvance event driven by a combination of climatic elements and increased supraglacial debris cover originating from the proposed rock avalanche. As previously described, the geological model proposed relies upon a retreating and downwasting glacier to create the optimum conditions for tensile stress release and debuttressing. However, once the rock slope had failed, the glacier may have continued to retreat for a period until responding to the effects of reduced ablation combined with a climatic driver. If this was the case, then it suggests that further work needs to be undertaken to identify more moraines located within close proximity to a rock

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slope failure scar around Scotland, and review the glacial history for these areas in the context of rock avalanches and glacier dynamics. 8. Conclusions The following key points provide the conclusions from this study:  A large (93,750,000 m3) rock avalanche occurred during glacial retreat/downwasting, probably as a result of stress relief and debuttressing exacerbating regional-scale NE–SW and NW–SE trending faults.  The cavity surface has been draped with glacial diamicton originating from the plateau above the failure backwall. The sediments have been transported mainly by debris flow action, however glacial outwash may have played a role in the formation of the sediment mantle.  A colluvial creep lobe developed in the SE margin of the rockwall cavity, likely to have been formed by several phases of rockwall collapse from the outcrop above.  No evidence of the rock avalanche debris could be found onshore or offshore. Given the locations of the rockwall cavity (in a fjord wall), it is proposed that the rock debris flowed onto the surface of the retreating and downwasting glacier. This could have implications on the glacier dynamics in the area, and it is suggested that a moraine at the foot of the rockwall cavity is evidence of an advance which could possibly have been triggered by reduced ablation as a result of supraglacial debris cover.  This is the first documentation of such a large rock avalanche scar within a Scottish sea loch and requires more research in the surrounding fjords for evidence of similar events.

Acknowledgements This study was undertaken as part of an MSc thesis on slope behaviour which resulted in the successful completion of the Geological and Environmental Hazards course at the University of Portsmouth; thanks to lecturers Dr Malcolm Whitworth and Dr Andrew Gibson for guidance during this field study. Thanks to Dr Martyn Stoker of The British Geological Survey who allowed access to offshore datasets. Special thanks for internal British Geological Survey reviewers, in particular Emrys Phillips and Dave Long. Finally, thanks to external reviewers, Dr Danni Pearce and Dr Simon Carr, for extremely helpful and constructive suggestions and comments. This paper is published with permission of the executive director of the British Geological Survey, Natural Environmental Research Council. References Allen, S.K., Cox, S.C., Owens, I.F., 2011. Rock avalanches and other landslides in the central Southern Alps of New Zealand: a regional study considering possible climate change impacts. Landslides 8 (1), 33–48. Ballantyne, C.K., 1981. Periglacial landforms and environments on mountains in the northern Highlands of Scotland. (Unpublished doctoral thesis)University of Edinburgh, Edinburgh. Ballantyne, C.K., 1982. Depths of open joints and the limits of former glaciers. Scottish Journal of Geology 18 (2-3), 250–252. Ballantyne, C.K., 1986. Landslides and slope failures in Scotland: a review. The Scottish Geographical Magazine 102 (3), 134–150. Ballantyne, C.K., 1993. An Teallach. In: Gordon, J.E., Sutherland, D.E. (Eds.), Quaternary of Scotland. Springer Science and Business Media, Dordrecht, pp. 110–115. Ballantyne, C.K., McCarroll, D., Nesje, A., Dahl, S.O., Stone, J.O., 1998. The last ice sheet in north-west Scotland: reconstruction and implications. Quaternary Science Reviews 17 (12), 1149–1184. Ballantyne, C.K., 2000. Paraglacial adjustment of rock slopes: causes and consequences. Indian Journal of Geography and Environment 5, 1–22.

Ballantyne, C.K., 2002. Paraglacial geomorphology. Quaternary Science Reviews 21 (18), 1935–2017. Ballantyne, C.K., 2003. A Scottish Sturzstrom: the Beinn Alligin rock avalanche, Wester Ross. The Scottish Geographical Magazine 119 (2), 159–167. Ballantyne, C.K., Stone, J.O., 2004. The Beinn Alligin rock avalanche, NW Scotland: cosmogenic 10Be dating, interpretation and significance. The Holocene 14 (3), 448–453. Ballantyne, C.K., Schnabel, C., Xu, S., 2009. Exposure dating and reinterpretation of coarse debris accumulations (‘rock glaciers’) in the Cairngorm Mountains, Scotland. Journal of Quaternary Science 24 (1), 19–31. Ballantyne, C.K., Stone, J.O., 2012. Did large ice caps persist on low ground in northwest Scotland during the Lateglacial Interstade? Journal of Quaternary Science 27 (3), 297–306. Ballantyne, C.K., 2012. Chronology of glaciation and deglaciation during the Loch Lomond (Younger Dryas) Stade in the Scottish Highlands: implications of recalibrated 10Be exposure ages. Boreas 41 (4), 513–526. Ballantyne, C.K., Wilson, P., Gheorghiu, D., Rode´s, A`., 2014. Enhanced rock-slope failure following ice-sheet deglaciation: timing and causes. Earth Surface Processes and Landforms 39 (7), 900–913. Beacom, L.E., Anderson, T.B., Holdsworth, R.E., 1999. Using basement-hosted clastic dykes as syn-rifting palaeostress indicators: an example from the basal Stoer Group, northwest Scotland. Geology Magazine 136 (3), 301–310. Blikra, L.H., Longva, O., Braathen, A., Anda, E., Dehls, J.F., Stalsberg, K., 2006. Rock slope failures in Norwegian fjord areas: examples, spatial distribution and temporal pattern. In: Landslides from Massive Rock Slope FailureSpringer, The Netherlands, pp. 475–496. Braathen, A., Blikra, L.H., Berg, S.S., Karlsen, F., 2004. Rock-slope failures of Norway, type, geometry deformation mechanisms and stability. Norsk Geologisk Tidsskrift 84 (1), 67–88. Bradwell, T., Stoker, M., Larter, R., 2007. Geomorphological signature and flow dynamics of The Minch palaeo-ice stream, northwest Scotland. Journal of Quaternary Science 22 (6), 609–617. Bradwell, T., Fabel, D., Stoker, M., Mathers, H., McHargue, L., Howe, J., 2008. Ice caps existed throughout the Lateglacial Interstadial in northern Scotland. Journal of Quaternary Science 23 (5), 401–407. British Geological Survey, 2013a. DiGMapGB. 1,50,000 Bedrock Layer. ArcGIS Layer File. British Geological Survey, Keyworth, Nottingham. British Geological Survey, 2013b. DiGMapGB. 1,50,000 Superficial Layer. ArcGIS Layer File. British Geological Survey, Keyworth, Nottingham. British Standards Institute, 1999. BS 5930: Code of Practice for Site Investigations. British Standards Institute, London. Retrieved from: http://shop.bsigroup.com/ ProductDetail/?pid=000000000030190275. British Standards Institute, 2003. BS EN ISO 14689-1: Geotechnical Investigation and Testing. Identification and Classification of Rock. Identification and Description. British Standards Institute, London. Retrieved from: http://shop. bsigroup.com/ProductDetail/?.pid=000000000030162514. Clark, P.U., Dyke, A.S., Shakun, J.D., Carlson, A.E., Clark, J., Wohlfarth, B., Mitrovica, J.X., Hostetler, S.W., McCabe, A.M., 2009. The last glacial maximum. Science 325 (5941), 710–714. Clark, C.D., Hughes, A.L., Greenwood, S.L., Jordan, C., Sejrup, H.P., 2012. Pattern and timing of retreat of the last British-Irish Ice Sheet. Quaternary Science Reviews 44, 112–146. Deline, P., 2009. Interactions between rock avalanches and glaciers in the Mont Blanc massif during the late Holocene. Quaternary Science Reviews 28 (11), 1070–1083. Deline, P., Kirkbride, M.P., 2009. Rock avalanches on a glacier and morainic complex in Haut Val Ferret (Mont Blanc Massif, Italy). Geomorphology 103 (1), 80–92. Dunning, S.A., Mitchell, W.A., Rosser, N.J., Petley, D.N., 2007. The Hattian Bala rock avalanche and associated landslides triggered by the Kashmir Earthquake of 8 October 2005. Engineering Geology 93 (3), 130–144. Gutie´rrez-Santolalla, F., Acosta, E., Rı´os, S., Guerrero, J., Lucha, P., 2005. Geomorphology and geochronology of sackung features (uphill-facing scarps) in the Central Spanish Pyrenees. Geomorphology 69 (1), 298–314. Hermanns, R.L., Blikra, L.H., Naumann, M., Nilsen, B., Panthi, K.K., Stromeyer, D., ¨ tz Longva, O., 2006. Examples of multiple rock-slope collapses from Ko¨fels (O valley, Austria) and western Norway. Engineering Geology 83 (1), 94–108. Hewitt, K., 1988. Catastrophic landslide deposits in the Karakoram Himalaya. Science 242 (4875), 64–67. Hewitt, K., 2009. Rock avalanches that travel onto glaciers and related developments, Karakoram Himalaya, Inner Asia. Geomorphology 103 (1), 66–79. Hewitt, K., Clague, J.J., Orwin, J.F., 2008. Legacies of catastrophic rock slope failures in mountain landscapes. Earth-Science Reviews 87 (1), 1–38. Hoek, E., 2007. Practical Rock Engineering. Rocscience, Toronto. Available online at: http://www.rocscience.com. Holmes, G., Jarvis, J.J., 1985. Large-scale toppling within a sackung type deformation at Ben Attow, Scotland. Quarterly Journal of Engineering Geology and Hydrogeology 18 (3), 287–289. Jarman, D., Ballantyne, C.K., 2002. Beinn Fhada, Kintail: an example of large-scale paraglacial rock slope deformation. The Scottish Geographical Magazine 118 (1), 59–68. Jarman, D., 2006. Large rock slope failures in the Highlands of Scotland: characterisation, causes and spatial distribution. Engineering Geology 83 (1), 161–182. Jarman, D., 2009. Paraglacial rock slope failure as an agent of glacial trough widening. Geological Society, London, Special Publications 320 (1), 103–131.

G. Carter / Proceedings of the Geologists’ Association 126 (2015) 698–711 Kelly, M., 1980. A prehistoric catastrophic rock avalanche at Holsteinsborg, West Greenland. Geological Survey of Denmark Bulletin 28, 73–79. Matheson, G.D., 1983. Rock Stability Assessment in Preliminary Site Investigations. TRRL LR 1039. Department of the Environment, Department of Transport and Road Research Laboratory, Crowthorne. McColl, S.T., 2012. Paraglacial rock-slope stability. Geomorphology 153, 1–16. Osmundsen, P.T., Henderson, I., Lauknes, T.R., Larsen, Y., Redfield, T.F., Dehls, J., 2009. Active normal fault control on landscape and rock-slope failure in northern Norway. Geology 37 (2), 135–138. Owen, G., 1995. Soft-sediment deformation in upper Proterozoic Torridonian Sandstone s (Applecross Formation) at Torridon, northwest Scotland. Journal of Sedimentary Research 65 (3), 495–504. Prave, A.R., 2002. Life on land in the Proterozoic: evidence from the Torridonian rocks of northwest Scotland. Geology 30 (9), 811–814. Savigear, R.A.G., 1965. A technique of morphological mapping. Annals of the Association of American Geographers 55 (3), 514–538. Sosio, R., Crosta, G.B., Chen, J.H., Hungr, O., 2012. Modelling rock avalanche propagation onto glaciers. Quaternary Science Reviews 47, 23–40. Stoker, M.S., Bradwell, T., Wilson, C., Harper, C., Smith, D., Brett, C., 2006. Pristine fjord landsystem revealed on the sea bed in the Summer Isles region, NW Scotland. Scottish Journal of Geology 42 (2), 89–99.

711

Stoker, M.S., Bradwell, T., 2009. Neotectonic deformation in a Scottish fjord, Loch Broom, NW Scotland. Scottish Journal of Geology 45 (2), 107–116. Stoker, M.S., Wilson, C.R., Howe, J.A., Bradwell, T., Long, D., 2010. Paraglacial slope instability in Scottish fjords: examples from Little Loch Broom, NW Scotland. Geological Society, London, Special Publications 344 (1), 225–242. Strachan, R.A., Holdsworth, R.E., 2000. Proterozoic sedimentation, orogenesis and magmatism on the Laurentian Craton (2500–750 Ma). In: Woodcock, N., Strachan, R. (Eds.), Geological History of Britain and Ireland. Blackwell Science Ltd., Cambridge, pp. 52–72. Shugar, D.H., Clague, J.J., 2011. The sedimentology and geomorphology of rock avalanche deposits on glaciers. Sedimentology 58 (7), 1762–1783. Shugar, D.H., Rabus, B.T., Clague, J.J., Capps, D.M., 2012. The response of Black Rapids Glacier, Alaska, to the Denali earthquake rock avalanches. Journal of Geophysical Research: Earth Surface (2003–2012) 117 (F1), F01006. Shulmeister, J., Davies, T.R., Evans, D.J.A., Hyatt, O.M., Tovar, D.S., 2009. Catastrophic landslides, glacier behaviour and moraine formation – a view from an active plate margin. Quaternary Science Reviews 28 (11), 1085–1096. Whitehouse, I.E., 1983. Distribution of large rock avalanche deposits in the central Southern Alps, New Zealand. New Zealand Journal of Geology and Geophysics 26, 271–279.