The Burtness Comb rock avalanche, English Lake District: a rare case of rock slope failure - glacier interaction.

Proceedings of the Geologists’ Association 124 (2013) 477–483

The Burtness Comb rock avalanche, English Lake District: a rare case of rockslope failure–glacier interaction

Peter Wilson a,*, David Jarman b

a Environmental Sciences Research Institute, School of Environmental Sciences, University of Ulster, Coleraine, Co. Londonderry BT52 1SA, Northern Ireland, United Kingdomb Mountain Landform Research, 16 Albert Place, Stirling, Scotland, United Kingdom

A R T I C L E I N F O

Article history:

Received 21 March 2012

Received in revised form 16 August 2012

Accepted 20 August 2012

Available online 22 October 2012

Keywords:

Rock avalanche debris tongue

Glacier ice

Loch Lomond Stadial

Lake District

A B S T R A C T

The alignment of the upper part of the Burtness Comb rock avalanche debris tongue, in the Lake District

of northwest England, indicates that glacier ice played a role in its emplacement. The debris initially

moved directly downslope but was deflected from this course on contact with the glacier and was

channelled along its margin. Debris deposition on an oblique alignment across a 25–308 slope was ice-

supported. With glacier wastage the debris settled, opening a sinuous longitudinal tension furrow. The

lower, older part of the debris tongue, aligned along the comb floor, may also have an association with

glacier ice and was likely sourced from a different area of the comb. Parts of both areas of the debris

tongue are underlain by fine-grained materials attributed to glacial deposition. This is one of very few

rock avalanche deposits in Great Britain for which a direct association with glacier ice can be

demonstrated, a sparsity at odds with what might be expected.

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1. Introduction

There has been much discussion and debate concerning theorigins and significance of accumulations of coarse rock debris inmountain areas of Great Britain but rather little attention has beendirected towards recognising the interaction between Lateglacialice masses and debris produced by rock slope failures (RSFs). This isa potentially important aspect of mountain geomorphologybecause local glacier reconstructions and derived palaeoclimaticparameters are dependent on accurate interpretation of deposi-tional landforms. Of the few such interactions that have beenrecognised none have yet been discussed in detail.

At least 600 and possibly over 850 RSFs exist in the older(Caledonian) mountain areas of Great Britain. They have beencategorised as: (1) near-in situ slope deformations displayingextensional or compressional morphology; (2) arrested (bothtranslational and rotational) rockslides with significant preserva-tion of rock structure, and (3) disintegrated forms, where thedebris has substantially evacuated its source and reached thelower slopes (sub-cataclasmic) or slopefoot/valley floor (cataclas-mic), the best developed being termed rock avalanches (Jarman,2006). Cataclasmic RSFs are rare in Great Britain, with only 33 goodcases of which only 16 exceed 0.05 km2 (cavity and deposit,

* Corresponding author.

E-mail address: [email protected] (P. Wilson).

0016-7878/$ – see front matter � 2012 The Geologists’ Association. Published by Else

http://dx.doi.org/10.1016/j.pgeola.2012.08.007

threshold area 0.01 km2). Graig Goch in the Cadair Idris range ofWales is by far the largest at 1.20 km2 area and 50 M m3 volume,and is also the only RSF damming a main valley (SH 710 088;Hutchinson and Millar, 2001). The largest in Scotland is on BeinnAlligin, at 0.47 km2 area and �3.5 M m3 volume (NG 867 604;Ballantyne and Stone, 2004).

Only five cataclasmic RSFs have been reported from the LakeDistrict of northwest England, three being minor crag collapsesin Borrowdale yielding inter alia the famous Bowder Stone (NY254 164; Wilson et al., 2004). A fourth case at Grasmoor End (NY160 206, 0.19 km2 area) in the Buttermere valley has recentlybeen reinterpreted from rock glacier or debris cone status(Wilson, 2011). The fifth cataclasmic case, also in the Buttermerevalley, is in Burtness Comb (NY 177 147; Fig. 1), which is thesouthernmost of three adjacent glacial cirques in the High Stilerange. A rock avalanche origin for the coarse rock debris in thecomb was proposed by Clark and Wilson (2004). Re-evaluationof the debris accumulation, while supporting the two-stage rockavalanche interpretation of Clark and Wilson (2004), favoursemplacement in part in association with a Loch Lomond Stadial(LLS; 12.9–11.7 ka) glacier.

2. The Burtness Comb rock avalanche complex

The RSF complex is an extended two-part tongue-shapedaccumulation of coarse rock debris extending to 0.14 km2 areaand 0.3 M m3 volume (Fig. 2A and B; Clark and Wilson, 2004).

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Fig. 1. Location of the Burtness Comb rock avalanche debris tongue. Also indicated are Loch Lomond Stadial glaciers (after Sissons, 1980; Hughes et al., 2012) and the

Grasmoor End rock slope failure (Wilson, 2011). Contours are in metres (� Crown copyright Ordnance Survey. All rights reserved).

P. Wilson, D. Jarman / Proceedings of the Geologists’ Association 124 (2013) 477–483478

On the basis of its location, morphology and composition, Clarkand Wilson (2004) argued that the debris tongue was morelikely to be a product of RSF, and in particular rock avalanching,rather than a lateral moraine (Sissons, 1980) or rock glacier(Whalley, 1997; Harrison et al., 2008) (the ‘protalus rampart’ ofOxford (1985) is an adjacent minor feature). Two phases ofavalanching were proposed based on morphological contrastsand Schmidt hammer R-values from the upper and lower partsof the debris tongue. A full description of the debris was given byClark and Wilson (2004), we summarise the main elementsbelow.

The upper part of the debris tongue (U on Figs. 2A, B and 3) is�300 m long and falls from �550 m OD to �440 m OD at anaverage gradient of 228 and in places is boulder strewn, withmany boulders exceeding 1 m in length. In cross-section thetongue is very asymmetric, the northern boundary ridgestanding �30 m above the crest of the southern flank. Thenorthern ridge stands 5 m above the depression that separates itfrom the adjacent hillside, and at its eastern end turns south in

the vicinity of a prominent rock bastion (RB on Fig. 2A and B)forming a high (>30 m), steep (�308), arcuate and, in longprofile, slightly concave frontal slope above the lower part of thedebris tongue. The outer face of the southern boundary ridgerises 10–14 m above the adjacent ground. The area between theboundary ridges is diversified by nested arcuate ridges, exposedconcentrations of large boulders, and vegetated areas.

The lower part of the debris tongue (L on Fig. 2A and B) extendsfor �400 m from �440 m OD to �310 m OD. It too has boulder-strewn areas and is asymmetric in cross-section, although less sothan the upper part, with a prominent northern boundary ridgethat at one place rises 9 m above the depression separating it fromthe adjacent hillside. The ridge is sinuous in plan. Exposures on theouter flank of the ridge show a 3–4 m thickness of angularboulders. The southern boundary ridge stands 3–5 m above theadjacent ground on its outer side. Between the marginal ridges is aseries of nested low arcuate ridges and mounds. Collapse pits indepressions between ridges, and resurgences of sub-surface floware also common.

Fig. 2. (A) Contour map showing configuration and location of the rock avalanche

debris tongue. RB is rock bastion and contours are in metres (� Crown copyright

Ordnance Survey. All rights reserved). (B) Satellite image showing the Burtness

Comb rock avalanche debris tongue with upper (U) and lower (L) parts and rock

bastion (RB) indicated.

P. Wilson, D. Jarman / Proceedings of the Geologists’ Association 124 (2013) 477–483 479

In focussing on the process reinterpretation, Clark and Wilson(2004) did not closely address the trajectory required to emplacethe upper deposit. That part of the tongue is neither aligneddirectly downslope towards the comb floor, nor is it aligned alongthe comb floor, rather it has an oblique configuration across a 25–308 slope (Fig. 3). Clark and Wilson (2004) took the rock avalanchesource to be on the broken crags immediately above the upperdeposit, from which the trajectory would be less anomalous. Wenow consider that the source was on Grey Crag, much higher andfurther back on the cirque flank rim, its continuing instabilityattested by furrow/antiscarp indications of incipient failure justbehind the rim (Wilson, 2005; Jarman, 2009), displaced andoutward dipping rock slabs at the slope crest, and fresh scarsfavoured by rock climbers (Figs. SI 2–5). A considerable deflectionfrom an unimpeded fall-line trajectory from the Grey Crag sourcearea has occurred, which now requires explanation.

3. The Burtness Comb LLS glacier

A LLS cirque glacier was proposed for Burtness Comb byManley (1959) and Pennington (1978), with Sissons (1980, Fig.5, glacier 11) showing the whole of the debris tongue as itsbounding moraine enclosing a cluster of hummocks – thismisinterpretation reflecting a classic case of landform mimicrythat has parallels with the substantially larger and considerablyyounger Triolet-Val Ferret boulder deposit of the Italian Alps(Orombelli and Porter, 1988). At maximum extent the glacierhad a surface area of 0.4 km2, a volume of 0.018 km3, and anequilibrium line altitude of 574 m (Sissons, 1980). Clark andWilson (2004) in rejecting a glacial origin for the debris alsofound no other depositional evidence for a glacier in the combduring the LLS, although they noted this was inconsistent withthe clear evidence for glaciers in the adjacent combs of similaraspect and elevation, as well as in less favourable settings. Wepropose that a LLS glacier did exist in Burtness Comb and canaccount for the trajectory taken by at least the upper, youngerpart of the rock avalanche debris.

Our reconstruction of the proposed LLS glacier at itsmaximum indicates it extended from high (�700 m OD) onthe cliffed backwall, beneath the High Crag–High Stile ridge, toan elevation of �350 m OD near the lip of the comb (Fig. 4). Theequilibrium line altitude of the glacier was �600 m, althoughprobably slightly lower beneath the northwest-facing slopesbelow High Crag than on the southeast-facing slopes beneathHigh Stile. The northern margin of the glacier was aligned acrossthe lower part of the talus below Grey Crag and from theretrended east across the area now occupied by some of the upperpart of the debris tongue and some of the lower part.

4. Deflection by glacier ice

The apparently anomalous alignment of the upper part of thedebris tongue can thus be explained by the presence of the glacier.Although the debris initially fell directly downslope from GreyCrag, on reaching the margin of the glacier it was deflected downthe narrow, shallow trough formed by the ice and the outer slope ofthe comb rather than travelling across the glacier surface. Ourreconstruction indicates that at the place of debris–ice contact theglacier presented a complex surface to the debris avalanche.Initially some debris may have been propelled across the ice butwould likely have fallen back into the trough due to gradientconstraints. The longitudinal gradient of the trough (�208) alongwhich the debris was deflected would have facilitated forwardmovement of the debris to create a ridge adhering to the outerslope and in the shallow trough, partially resting on the glaciermargin (Fig. 5A), with its toe wrapping around the southern flankof the rock bastion (RB in Figs. 2A, B and 4).

Four lines of evidence lend support to this proposal. First, thenorthern boundary ridge is split by an unusual sinuous longitudi-nal depression (Figs. 6 and SI 6). We interpret this as a tensionfurrow that opened as ice support was withdrawn and allowed thedebris ridge to settle downslope (Fig. 5B), in places developingelongate stone nets mimicking periglacial or even rock glacierbehaviour (Fig. SI 7). Second, the southern boundary ‘ridge’ is moreof a break of slope, which we suggest may represent the finalmargin of the glacier, along which the slumping debris tonguestabilised, akin to a retreat moraine (Figs. 5C and SI 8 – without thiscontainment, the slumping would have been more lobate); notethat although in views from above the upper deposit resembles aconventional rock avalanche tongue with a thinner interior, thetwo parallel ridges differ in character. Third, Clark and Wilson(2004) reported the presence of fine-grained material in shallowexposures on the steep frontal slope and on the outer face of the

Fig. 3. The upper part of the Burtness Comb rock avalanche debris tongue (U); the oblique trajectory is evident.

Fig. 4. Satellite image showing the relationship between the proposed Loch Lomond Stadial glacier and the rock avalanche deposit, with rock bastion (RB), Grey Crag source

area, distal spray fan, and rim-failures (X symbols) indicated.

P. Wilson, D. Jarman / Proceedings of the Geologists’ Association 124 (2013) 477–483480

Fig. 5. Schematic cross-sections demonstrating the mode of emplacement of the

upper part of the rock avalanche in association with glacier ice.

P. Wilson, D. Jarman / Proceedings of the Geologists’ Association 124 (2013) 477–483 481

southern boundary ridge, and boulder-lined furrows underlain byfine materials that evidently serve as drainage routes. The origin ofthese fine-grained materials is unclear but their locations suggestthat they are probably ice-marginal sediments that were buried byemplacement and/or settlement of the debris tongue, thusexplaining the apparent absence of glacial depositional landformsfrom this part of the comb. Springs emerge from the foot of thesouthern boundary ridge (Figs. 5C and SI 9), indicating thesubsurface presence of these materials are an impediment todrainage. Fourth, the slopes below the comb headwall crags arealmost devoid of fresh talus, which is abundant on both flanksimmediately beyond our proposed glacier limits.

Emplacement of the lower, apparently older part of the debristongue may also have been influenced by the presence of glacierice, although from its position this is less obvious. This part of the

debris tongue may not have come from Grey Crag; rather its sourcemay have been either the rim of the comb southeast of Eagle Cragwhere an incipient wedge failure extends for �100 m and/or theinnermost recess of the comb to the northwest of Eagle Crag whereridge crest furrows and shattered bedrock attest to former slopeinstability (Figs. 4 and SI 10). The debris is aligned along the floor ofthe comb (Figs. 2, 7 and SI 11–13) with a distinct bias to itsnorthern half and it laps onto the northern side slopes. Clark andWilson (2004) reported fine-grained material in shallow exposureson the frontal slope, and noted collapse pits, subsurface streamsand resurgences elsewhere. As with the upper part of the debristongue the implication is that the lower part is also underlain inplaces by in situ ice-marginal sediments, or the debris incorporatedand carried them forwards during emplacement. Rock avalanchetoes are usually well-contained and abrupt, indicating rapiddeceleration (Ballantyne, 2007a,b): observations suggest sprayfans generally result from free-fall impact. The toe here isunusually undefined, with associated blocky debris continuingto the intake wall at 300 m OD, having descended from a source at700 m OD and travelled �1 km. This implies a relatively rapid, longrunout debris flow, for which sturzstrom mechanisms are ofteninvoked but are not required if ice-assisted transit was available(Deline, 2009).

However, the general alignment of the older, lower deposit doesnot require deflection by an LLS glacier in the inner comb. Indeed, itis offset by �358 from the axis of the upper deposit, raising furtherdifficulties of trajectory.

5. Rock avalanche–glacier interactions

The Burtness Comb rock avalanche deposit is one of very fewin Great Britain for which clear evidence of direct interactionwith glacier ice can be demonstrated. Of the 33 recordedcataclasmic RSFs in Great Britain, only a handful have beensuspected of ice interaction, most famously and erroneously inthe case of Beinn Alligin (Torridon), which has been dated to themid-Holocene (Ballantyne and Stone, 2004). Indeed Holmes(1984) identified a mere four RSFs of any kind in the ScottishHighlands which suggested glacial disturbance, of which onlytwo are rock avalanches: at Maol Chean-dearg (south ofTorridon, NG 932 497, 0.25 km2) an extensive thin drape ofdebris has a neat broad lobate perimeter suggesting emplace-ment via a late glacier or neve bank (Robinson, 1977); a similarthin drape at Carn Ghluasaid (Cluanie, NH 140 119, 0.34 km2)appears to supply a swathe of erratic blocks extending 700 meast, suggesting that the rock avalanche toe extended onto thetrough glacier. A thick rock avalanche in Garbh Choire Mor(Fannaich, NH 254 673, 0.17 km2) is one of six cases within aglacial cirque, and ends in a thin toe drape, above morainiseddebris inferred to be from adjacent rim recesses.

Morainised rock avalanche debris might be expected to occurcommonly in cirques, where the RSF predates or has fallen onto theLLS glacier, but evidence is sparse. A supraglacial case wasidentified in Cwm Graianog, Snowdonia (SH 627 627, 0.16 km2),with a notably offset trajectory from the distinct cavity (Addison,1988; Fig. 8). It may likewise account for the smooth long-runoutlobe below Cwm-coch further up Nant Ffrancon (SH 635 613,0.16 km2), again with a bold cavity on Foel-goch.

Several deposits formerly regarded as rock glaciers have beenreinterpreted as rock avalanche–glacier interactions (Jarman et al.,in press), notably Cwm Bochlwyd (Snowdonia, SH 656 587,0.13 km2), Cyfrwy (Cadair Idris, SH 703 137, 0.08 km2), and LairigGhru (Cairngorms, NH 960 038, 0.05 km2), emplaced respectivelyat the foot of an LLS cirque glacier; after travelling across the toe ofone; and transported �750 m by a trough glacier. The remarkableabundant rockfalls filling the floor of Strath Nethy (Cairngorms, NJ

Fig. 6. View west along the crest of the upper deposit, looking into the comb. The white line indicates the floor of the sinuous longitudinal tension furrow, suggesting

sideslipping after withdrawal of ice support.

Fig. 7. View down Burtness Comb from Grey Crags with upper (U) and lower (L) parts of the rock avalanche outlined.

P. Wilson, D. Jarman / Proceedings of the Geologists’ Association 124 (2013) 477–483482

021 061) may also have been emplaced via a wasting stagnantglacier.

However, Burtness Comb appears unique in Britain in display-ing a rock avalanche–glacier interaction mimicking a lateralmoraine and produced by a trajectory down the ice margin (Figs. SI14–16; Cyfrwy is the most instructive comparator, with a similarsource high on a cirque flank (Jarman et al., in press)). The BurtnessComb rock avalanche debris tongue differs from the blocky lateralmoraine occasionally produced where a rockfall onto a glaciermargin becomes extended down valley as the glacier advances (e.g.

Coire nan Arr, Applecross, NG 810 420; Ladtjovagge, northern

Sweden (Hoppe, 1983)). The scarcity of evident rock avalanche-glacier interactions extends to the whole RSF population, althoughsome arrested translational slides have precariously steep toeswhich suggest ice support (e.g. Ben Hope northeast corrie, northernHighlands, NC 480 505). This scarcity is puzzling given that theparaglacial model (Ballantyne, 2002) suggests a tendency forslopes to fail as ice support is withdrawn (‘debuttressing’) andpermafrost degrades, implying a predominance of RSF activityaround deglaciation which should have yielded more numerousglacier interaction cases. Debuttressing theory has now beenmodified (McColl et al., 2010). It may be that the rebound stresses

Fig. 8. Cwm Graianog, Snowdonia, with probable most recent RSF cavity on rim (A)

and offset trajectory requiring cirque glacier to emplace as quasi-morainic debris

(B). Narrower tongue (C) down hanging lip may have travelled beyond end of

glacier.

P. Wilson, D. Jarman / Proceedings of the Geologists’ Association 124 (2013) 477–483 483

involved do not fully come into play until more completedeglaciation is achieved, and must then further await progressivefailure or appropriate triggering.

6. Conclusions

Re-assessment of the two-stage rock avalanche debris tongue inBurtness Comb favours emplacement of the upper part alongside abody of LLS glacier ice that deflected the debris from a directdownslope trajectory. The lower, older part of the debris tonguemay have been sourced from a different part of the comb, and alsointeracted with glacier ice but this is more difficult to establishwith certainty. Parts of both areas of the debris tongue areunderlain by fine-grained materials interpreted as glacial sedi-ments, and a tension furrow on the north boundary ridge of theupper part is thought to result from debris settlement as icesupport was withdrawn.

The Burtness Comb rock avalanche debris tongue is one ofvery few rock avalanche-glacier interactions to have beenreported from Great Britain and the only such feature to haveundergone detailed scrutiny. Other proposed examples requireverification in order to extend the emerging process-formrelationships. Burtness Comb is significant as one of the best-developed cataclasmic RSFs in Britain, and as the clearest case ofan interaction with LLS ice which appears to be extremely andpuzzlingly rare.

Acknowledgements

We thank Lisa Rodgers at the University of Ulster for preparingthe figures for publication. Comments by Dr Ian S. Evans(University of Durham) and an anonymous referee helped toimprove the original manuscript.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.pgeola.2012.08.007.

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