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Rock slope failure in the

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The user has requested enhancement of the downloaded file. THE QUATERNARY OF THE LAKE DISTRICT Field Guide

Edited by Derek A. McDougall & David J.A. Evans

2015 Cover Photograph: Stony Cove Pike, looking towards Brothers Water and Ullswater (D. McDougall).

Produced to accompany the QRA Annual Field Meeting based at Field Studies Centre, 21-24 May 2015.

QRA contribution to The Geological Society’s Year of Mud.

© Quaternary Research Association, London, 2015. All rights reserved. No part of this book may be reprinted or utilized in any form or by any electronic, mechanical or other means, now known or hereafter invented, including photocopying and recording or in any information storage or retrieval system, without permission in writing from the publisher.

Printed by: Adlard Print & Reprographics Ltd., The Old School, The Green, Ruddington, Nottinghamshire, NG11 6HH.

Recommended reference: McDougall, D.A. and Evans, D.J.A. (eds) (2015) The Quaternary of the Lake District: Field Guide. Quaternary Research Association, London.

ISSN: 0261 3611 ISBN: 0 907 780 164

ii 5 Rock slope failure in the Lake District

P. Wilson and D. Jarman

Introduction Prior to 2004 rock slope failure (RSF) was a rather neglected aspect of Lake District Quaternary geomorphology. Before then, reports of RSFs had appeared sporadically, the earliest being that by Ward (1873b) describing prominent rock fissures on Helm Crag (NY 328093), Rosthwaite Cam (NY 254117) ), Kirk (NY 195104) and (NY 174075), all within rocks of the Borrowdale Volcanic Group (BVG). He considered the fissures might have been products of hillside slippage, but rejected this in favour of “earthquake shocks.” Later studies document individual failure sites as part of broader geological/ geomorphological investigations (Mitchell, 1931; Hay, 1942; Webb, 1990; Clark, 1992), and the maps and memoirs of British Geological Survey also indicate and describe several sites of RSF (British Geological Survey, 1997, 1999, 2001, 2004; Woodhall, 2000; Johnson et al., 2001). In addition to RSFs in the BVG some of these publications record failures on the (meta)sedimentary Group (SG) and on the Carrock Fell Igneous Complex. The Wainwright guide books to the Lakeland also illustrate and pass comment on some landslips and rockfalls, and on other areas of anomalous terrain since regarded as having RSF origins (Wainwright 1958, 1960, 1964, 1966). With publication of Wilson et al. (2004) and Clark and Wilson (2004), Lake District RSFs were brought to wider attention. A database of 48 sites, including several of large-scale rockfall, was provided by Wilson et al. (2004). Since then several more RSF descriptions with discussions of context, age and significance have been published (Wilson, 2005, 2011a, 2013; Wilson and Smith, 2006; Dykes et al., 2010; Davies et al., 2013; Wilson and Jarman, 2013; Jarman and Wilson, 2015), and the database has expanded to ~70 sites (Jarman and Wilson, unpublished). Also relevant here are the eight sites previously identified as relict rock glaciers, and reinterpreted by Jarman et al. (2013) variously as RSF (e.g. End;

83 Wilson, 2011a), RSF reworked as moraine, incremental debris accumulations, and bedrock forms (e.g. at Low Tarn and Nether Beck; Wilson, 2011b). Of these, Burtness Comb (NY 177147) has gone through three interpretations, with Clark and Wilson (2004) arguing that the tongue-shaped accumulation of coarse rock debris on the flank and floor of the cirque was the product of rock avalanching rather than being a Loch Lomond Stadial (LLS; 12.9-11.7 ka BP) moraine (Sissons, 1980) or a relict ice-cored (glacier-derived) rock glacier (Whalley, 1997).

Spatial distribution and areal extent Figure 5.1 shows the spatial distribution of Lake District RSFs according to size and in relation to bedrock geology. The authors have verified all sites in the field as either definite or probable RSFs, except where noted as ‘possible.’ Overall, the incidence of RSF is low compared with the densest Scottish Highland clusters but greater than peripheral Highland areas and Wales (Jarman, unpublished data).

Figure 5.1. Distribution and size of RSFs in the Lake District, and major rock groups. Selected sites are indicated and named (modified from Jarman and Wilson, 2015).

84 Although widely scattered within the Lake District, RSFs are sparse to absent in many areas, in particular the Scafell-Bowfell and Coniston ranges of the southern fells are almost devoid of RSFs. With a few notable exceptions, the - - range of the western fells, the northern sector of the range in the eastern fells, and the central fells are also virtually ‘empty quarters.’ Nevertheless two main ‘clusters’ of RSF are evident: (1) the High Street-southern Helvellyn range in the east, and (2) the Ennerdale-Buttermere area in the west/ northwest. Within these areas mini-clusters occur as in Grisedale (east) and in the - group (northwest); but superficially there appears to be no clear pattern to overall incidence of RSF. Curiously, RSF is almost absent south of the main W-E divide (Figure 5.1). The total area occupied by RSF is ~13 km2, average RSF size is ~0.18 km2 (cavity and deposit) and 14 sites exceed 0.25 km2, of which three (Dove Crags, NY 178208; Robinson, NY 204157; Clough Head, NY 328235) exceed 1 km2. These scale indicators compare closely with the Highlands.

Failure mode and bedrock geology The five principal modes of failure recognised in the Highlands (Jarman, 2006) are applicable in the Lake District (Figure 5.2): Cataclasmic and sub-cataclasmic: these RSFs have substantially evacuated their source cavities with the failed material extending to the slope foot or beyond. In the sub-cataclasmic mode the leading edge of debris may have reached the slope foot with other debris backed up above (Figures 5.2a and b). Rock avalanching is subsumed by these categories. Arrested slides: the failed materials remain as semi-intact or coherent masses that have travelled a relatively short distance downslope from their source cavity before ‘stabilising’ (Figure 5.2c). With greater distance of travel, slides are likely to undergo increasing amounts of disintegration and transform into cataclasmic/sub-cataclasmic failures. Slope deformations: encompass both extensional and compressional modes (Figures 5.2d and e). Those of the former category normally have distinct headscarps, and holes, fissures and grabens in their upper area, with an overall descending, forward-tilting morphology and areas of irregular gravitational sagging. In contrast, compressional deformations often lack a headscarp and are dominated by up-thrust ridges of ‘reverse-fault’ character that often divide slopes in a lattice manner. Furthermore these failures appear substantially coherent, sometimes being difficult to distinguish from adjacent slopes.

85 Figure 5.2. Representation of the five modes of failure recognised in the Lake District (after Jarman, 2006).

86 Figure 5.3. Bar chart indicating relationship between principal RSF modes and rock groups. BVG: Borrowdale Volcanic Group, SG: Skiddaw Group, Gr: Granitic intrusions; WG: Windermere Group; n=72.

Occurrences of each RSF mode in relation to bedrock geology are shown in Figure 5.3. RSFs of arrested slide and extensional deformation character dominate, with >20 cases of each. Failures recorded on the BVG are almost twice as common as those on the SG. Extensional deformation is favoured on the former, although arrested slides and compressional deformations are also well represented. The SG has proved particularly conducive to the arrested slide mode of failure; other modes register low counts on this rock group. However, rock type does not appear to be a prime control on overall RSF incidence (Figure 5.1).

Morpholocations The uneven spatial distribution of RSFs includes a marked absence from most of the main valley walls and passes, which may indicate their greater adjustment to and efficiency in the discharge of ice during later Quaternary stadials, particularly in the Devensian stage (~75–12 ka). Almost 20% of the RSFs are associated with breaches of glacial diffluence or transfluence. The Burnbank Fell RSF (NY 115216) and Beda Fell RSF (NY 427172) are within and downstream, respectively, of diffluent breaches, whereas the Robinson RSF and the Helm Crag RSF (NY 328093) are trough-wall failures downstream of transfluent breaches (Honister Hause and Dunmail Raise respectively). RSF at these and similar locations may reflect concentrated glacial erosion of bedrock and infer

87 Figure 5.4. The RSF on the west side of Caudale Moor and upstream of the transfluent breach. continuing trough enlargement due to recent augmentation of ice throughput. However, an obstacle to this hypothesis is the Kirkstone Pass breach (NY 401081) with its upstream RSFs on (NY 410119), Caudale Moor (NY 408097; Figure 5.4) and Middle Dodd (NY 396096). About 15% of RSFs are within tributary valleys with closed heads; some are single, isolated cases (e.g. Steel Fell, NY 316116) whereas others are clustered, as in the Hartsop-Martindale area of the High Street range. Recent concentrated glacial erosion of these troughs may again apply but why this should have been remains unclear. Very few RSFs (~8%) are in cirques but included here is the Burtness Comb rock avalanche (Clark and Wilson, 2004; Wilson and Jarman, 2013; Figure 5.5). Other cirque-RSFs comprise small rim failures as at Eller Peatpot (SD 141854) and Short Stile (NY 442118). The question of whether RSF has seeded cirques is addressed by Jarman and Wilson (2015) with respect to Dove Crags and considered below. A few cases are located on peripheral structural scarps possibly steepened by local ice streams: Carrock Fell (NY 352337 and 355329) and Souther Fell (NY 362296) adjacent to the Carrock End Fault on the eastern margin of the northern fells, and Cotley (SD 143 844; Figure 5.6) along the line of the Furness Fault in the southwest. 88 Figure 5.5. The upper (U) and lower (L) parts of the Burtness Comb rock avalanche debris tongue in relation to extent of proposed LLS glacier The upper part follows an oblique trajectory across the flank of the comb; was the source of the debris. The lower part is aligned along the comb floor. Also indicated are rock bastion (RB), distal spray fan, and rim failures (x). (From Wilson and Jarman, 2013).

Figure 5.6. The pre-LGM RSF at Cotley has created a cirque-like embayment on the southeastern flank of .

89 In some localities RSF is associated with concentrated fluvial erosion – possibly glacifluvial, as the cluster west of Knott (notably Meal Fell and Great Cockup, NY282337, NY277333) are close to the Trusmadoor overflow notch, while the Greta gorge incision (Latrigg, NY 285246 and 283242) cuts a moraine dam (see below).

Age No absolute dates have yet been obtained for any of the Lake District RSFs. Webb (1990) stated that the Robinson failure was of interglacial age, but gave no reason for that assertion and did not specify in which interglacial it had occurred. The Cotley failure was regarded by Johnson et al. (2001) as pre-dating the Last Glacial Maximum (LGM; ~27–21 ka BP) because it lacks a toe, suggesting either erosion by glacier ice or burial by till. For all of the RSFs documented by Clark and Wilson (2004), Wilson et al. (2004) Wilson (2005, 2011) and Wilson and Smith (2006) a post-LGM age was inferred because it was thought that none had been modified by the passage of glacier ice. The mismatch between cavity (~12–15 M m3) and slipmass (~3–6 M m3) volumes at Cotley was highlighted by Dykes et al. (2010), with Jarman and Wilson (2015) providing the approximate values, and can only reasonably be explained by glacial erosion of the slipmass. A pre-LGM origin for the large Dove Crags RSF has been proposed by Jarman and Wilson (2015). The slipmass is rather subdued with sparse indicators of RSF but the accumulated evidence for failure and the wider landscape anomalies presented by the valley of Gasgale Gill, in which the RSF sits, lend support to such an interpretation. Although of pre-LGM origin it was argued that the slipmass was pared back by a modest LGM valley glacier that also emplaced till on it. Following deglaciation the slipmass continued to move downslope narrowing the glaciated valley to give it its present illusory ‘fluvial’ profile. During the LLS a small glacier developed in the failure cavity (Sissons, 1980) and constructed moraine ridges across its floor. Dove Crags is one of the first substantial pre-LGM RSFs to be recognised in the British mountains, implying that others should be present. Clark and Wilson (2004), in discussion of the Burtness Comb rock avalanche, contended that there was no evidence for the cirque having held an LLS glacier and therefore the debris could have accumulated at any time since removal of ice associated with the LGM. On the basis of the trajectory taken by the rock avalanche, Wilson and Jarman (2013) argued for the presence of an LLS glacier

90 which acted to deflect debris from the Grey Crag source cliff along a trough between the ice and the outer slope of the cirque, the result being an oblique configuration to the upper part of the debris tongue (Figure 5.5). Although ascribing the upper part of the debris tongue to the LLS, the lower part does not require deflection by an LLS glacier to explain its position and alignment, and its source and age remain unclear. At present most other Lake District RSFs are probably best ascribed a post-LGM age, although Clough Head / Threlkeld Knotts presents some difficulties here (Davies et al., 2013; Jarman and Wilson, this volume), as may Robinson (Wilson and Jarman, this volume). Those sites that were covered by LLS glaciers may be of Holocene age but some post-LGM – pre-LLS failures could have survived LLS ice cover in situations where the ice was either cold-based and/or ineffectual in terms of its ability to erode. Some relatively small-scale RSFs have occurred within the last few hundred years at a few sites; brief details are given in Wilson et al. (2004). Although these events are trivial by comparison with the features listed in the database, they nevertheless indicate that many hillslopes remain susceptible to losing significant amounts of rock under certain circumstances. Examples include Deer Bield Crag (NY 306095), Far Easedale, from which the 60 m-high central buttress collapsed in 1997 (Figure 5.7; Craig, 2008). In 2011 concern was raised about the stability of part of Castle Rock (NY 321196), Vale of St. John, following reports that a 7 cm-wide crack had developed around the top of the north face of this celebrated rock climbing venue. Rock climbers were warned of the likely collapse of the face. The ‘semi’-detached mass, described as “the size of a small bungalow” underwent further movement over the winter of 2012/13. Another site of ‘recent’ RSF activity is Illgill Head (NY 156043). The shattered crags of the amphitheatre around the head of the scree shown in Figure 5.8 carry considerably less vegetation than the flanking buttresses, suggesting the crags are unstable and continue to lose material intermittently. On the ridge above the amphitheatre the terrain comprises a series of low grass-covered ridges with intervening depressions aligned parallel to the cliff top for ~200 m and extending ~60 m back from the edge (Figure 5.9). These features result from tensional spreading along the scarp and it is inferred that crag collapse has played a significant role in scree accumulation rather than intermittent freeze- thaw-generated rockfall alone (Wilson, 2005, 2013).

91 Figure 5.7. Debris of the 1997 collapse at Deer Bield Crag, Far Easdale.

Figure 5.8. The amphitheatre (top centre) and screes, Wast Water. The crags of the amphitheatre have a relatively sparse covering of vegetation in contrast to the flanking buttresses.

92 Figure 5.9. Grass-covered rock ridges and depressions resulting from tensional spreading on the cliff top above the amphitheatre shown in Figure 5.8.

Causes of failure In the absence of absolute ages the underlying causes of relict RSF are difficult to determine. Even when age has been established it is not always possible to identify a specific causal mechanism with certainty. Several mechanisms to account for RSF have previously been proposed, including glacial erosion and subsequent debuttressing, rock stress reorganisation, seismicity, tectonic activity, and climatic changes. In addition, RSF may reflect the crossing of stability thresholds in association with long-term deterioration in rock mass strength. Consequently ascertaining the trigger for many relict RSFs becomes a matter of conjecture. In formerly glaciated mountain regions RSFs are usually viewed as being a response to paraglacial (glacially conditioned) processes (Ballantyne, 2002; McColl, 2012). Glaciation and deglaciation have been regarded as factors that prepare slopes for subsequent failure by reducing rock mass stability through glacial erosion, and loading and unloading by glacier ice. These processes may trigger RSFs through stress release, but the role of in situ stresses also needs to be considered as, over long time-scales, they too may prepare rock slopes for failure as a result of progressive joint propagation through intact rock bridges (Stock et al., 2012). Three climatically-related factors have also been proposed as causes of paraglacial RSF, namely high cleft-water pressures, frost wedging,

93 and permafrost degradation. Thus the paraglacial tag encompasses a complex set of factors that are capable either singly or in some combination of generating RSFs. The Latrigg RSFs may be best regarded as of parafluvial or paraglacifluvial origin given their location above the gorge of the River Greta. The paraglacial tag has been attached to Lake District RSFs and this seems a reasonable approach to adopt because of the multiplicity and recency of glaciation. However, the lack of absolute ages is currently preventing the most likely cause(s) from being pinned down. Age estimates of 31 RSFs in Scotland and northwest Ireland obtained through cosmogenic isotope surface exposure dating enabled Ballantyne et al. (2014) to analyse the temporal pattern of failure in relation to the timing of deglaciation, rates of crustal uplift and periods of marked climate change. A similar dating exercise is required for Lake District RSFs.

Landshaping effects The overall impact of RSF on the mountain landscape is small, but locally it has truncated spurs (Burnbank Fell), widened troughs (Caudale Moor), sharpened crests and lowered cols (Fairfield, NY 357124), split ridges longitudinally (Helm Crag) and transversely (Crag Fell, NY 098143), dissected plateaus and their flanks (, NY 195104), enlarged cirques (Burtness Comb), created cirque-like embayments (Cotley), formed coarse boulder screes (Illgill Head, NY 156043), and has encroached substantially into preglacial relief (Dove Crags). RSFs are likely to have occurred widely over the multiple glacial—paraglacial cycles of the Quaternary, particularly in earlier cycles as fluvial valleys adapted to ice discharge and during the maximal mid-Quaternary ice sheet phases. It seems likely therefore that the cumulative effects of RSFs have been considerable. Pre- LGM RSF cavities will generally be much subdued and difficult to recognise, while failed masses and deposits will have been pared back or evacuated by succeeding ice advances (Jarman, 2009). The idea that RSF cavities have seeded some cirques goes back over 100 years (Clough, 1897; Bailey and Maufe, 1916) and continues to be discussed (Turnbull and Davies, 2006; Ballantyne, 2013b; Evans, 2013). The proposed pre-LGM Dove Crags RSF cavity (Jarman and Wilson, 2015) represents a rare case of cirque seeding in the Lake District with the cavity having been occupied and enlarged by ice during the LGM and again during the LLS. Most LLS glaciers in the Lake District fully occupied cirques (Sissons, 1980; McDougall, 2001, 2013) but some unusually large, angular cirques apparently nourished only small glaciers and

94 may be candidate sites for RSF origins. Such a site is Whelter Crag / Bottom (NY 465137), Haweswater, where an anomalously large cirque (rim 720 m OD; floor 400 m OD, and east-facing) has invaded peripheral open upland. The local LLS glacier is of contested extent (Sissons, 1980; McDougall, 2013). No trace of any RSF slipmass remains and the floor is conventionally stream-coursed. If the cirque is of RSF origins it would well predate the LGM. Two Lake District ‘cirqueforms’ already re-interpreted as RSF cavities may have good cirque-seeding potential. First, Cotley in spite of its peripheral escarpment location and floor elevation of 200-350 m OD has a good snowblow catchment and its east-facing headscarp element could nourish a small cirque glacier. But if the RSF is pre-LGM, as initially proposed by Johnson et al. (2001), there are few signs of LGM cirque-glacier activity. Second, Clough Head with its northwest aspect, high floor elevation of ~500 m OD, and some snowblow catchment “should readily be glacially occupied and thus seed a true cirque” (I.S. Evans, pers. comm., 2014). Davies et al. (2013) consider the RSF to be postglacial which may explain the absence of evidence for LLS glacier occupancy; Jarman and Wilson (this volume) discuss evidence for earlier origins, implying that the LLS may have been too brief for cirque initiation, as against mere re-occupation.

Future work In spite of recent advances in recognition, mapping and interpretation of Lake District RSFs greater understanding could be gained through several lines of enquiry. These include: ƒƒA programme of dating the RSFs with a view to establishing ages in relation to the timing of deglaciation and the pattern of post-LGM climatic changes. ƒƒDetermining which RSFs (or parts thereof) pre-date the LGM. ƒƒIdentifying sites that have failed on more than one occasion, irrespective of age. ƒƒRelating RSFs to local geological properties. ƒƒEstablishing the contribution that pre-LLS RSF has made to the volume of LLS moraines. ƒƒAppraising the spatial incidence of RSF in relation to glaciation history, in particular shifts in ice divides and dispersal routes, and more generally as an element in longer-term landscape evolution.

95 ISSN: 0261 3611 ISBN: 0 907 780 164

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