relict rock glaciers and protalus lobes in the british isles: implications for late pleistocene...

JOURNAL OF QUATERNARY SCIENCE (2008) 23(3) 287–304Copyright � 2007 John Wiley & Sons, Ltd.Published online 11 October 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/jqs.1148
Relict rock glaciers and protalus lobes in theBritish Isles: implications for Late Pleistocenemountain geomorphology and palaeoclimateSTEPHAN HARRISON,1 BRIAN WHALLEY2* and EDWARD ANDERSON3

1 School of Geography, Archaeology and Earth Resources, University of Exeter, Tremough Campus, Penryn, UK2 School of Geography, Archaeology and Palaeoecology, Queen’s University, Belfast, UK3 Bede College, Stockton on Tees, UK

Harrison, S., Whalley, B. and Anderson, E. 2007. Relict rock glaciers and protalus lobes in the British Isles: implications for Late Pleistocene mountaingeomorphology and palaeoclimate. J. Quaternary Sci., Vol. 23 pp. 287–304. ISSN 0267-8179.

Received 8 March 2007; Revised 12 June 2007; Accepted 4 July 2007

ABSTRACT: Many relict rock glaciers and protalus lobes have been described in mountainous areasof the British Isles. This paper reviews their distribution, chronology, supposed origin and develop-ment, and places the research within current investigations and knowledge. Rock glaciers and protaluslobes are located in a number of different topographic locations and settings. They developed at thebase of steep cliffs following the catastrophic failure of rock faces, at the base of scree slopes followingthe gradual accumulation of rock debris and in association with glaciers. Protalus lobes probablydeveloped in response to the permafrost creep of talus material while rock glaciers formed through the
deformation and sliding of large bodies of buried ice. Rock glaciers probably developed, or were lastactive, during the Younger Dryas, although the possibility exists that some of these landforms areDimlington Stadial in age. The development of protalus lobes during the Younger Dryas suggests thatprecipitation levels were low and permafrost was widespread during this time. The lack of rock glaciers(sensu stricto) in the British Isles compared with other mountain areas is believed to be a consequenceof the rock type and relative scarcity of weathered debris for their formation rather than a lack ofsuitable sites or appropriate environmental conditions. Copyright # 2007 John Wiley & Sons, Ltd.
KEYWORDS: rock glacier; protalus lobe; Younger Dryas; Dimlington Stadial; permafrost creep; glacier ice; British Isles.

Introduction and background

The international literature on rock glaciers and associatedfeatures has increased dramatically over the last 10 years. Thishas included papers on origin and structure as well asdescriptions and investigations from all continents. Althougha number of rock glaciers and related features have beendescribed from Great Britain (Ballantyne and Harris, 1994) andIreland (Wilson, 1990a,b, 1993) there has been no systematicattempt at a synthesis. Barsch (1992) has argued that theidentification of relict rock glaciers in upland areas should beregarded as a research priority but this has been largelyneglected in the British Isles. It is the purpose of this paper toreview the research carried out on rock glaciers and relatedfeatures in the British Isles, to describe and classify them, toinvestigate their potential as palaeoenvironmental indicatorsand to discuss the likely controls on their development.

Rock glaciers and protalus lobes are of interest for a number ofreasons. First, they represent a significant component of debris

* Correspondence to: B. Whalley, School of Geography, Archaeology andPalaeoecology, Queen’s University, Belfast BT7 1NN, UK.E-mail: [email protected]

movement from the base of steep slopes under a periglacialregime. Secondly, they are potentially important landforms for thereconstruction of palaeoenvironmental conditions, requiring:(1) the presence of permafrost or glaciers, (2) ‘relatively’ lowlevels of snowfall, and (3) a supply of rockfall debris for theirformation. In addition, it is interesting to speculate why relativelyfew rock glaciers have been found so far in the British Islescompared with their abundance in many other mountain regions.

Because there are no active rock glaciers or protalus lobes in theBritish Isles it is necessary to relate the relict features to modernanalogues. Despite the voluminous literature on rock glaciers, anumber of characteristics are disputed, including the mode offormation and environmental significance. Although we try toavoid such debates in this paper, it is necessary to place ourreview within the context of contemporary debates on active rockglacier and protalus lobe development. Thus, we start with a briefoverview of feature definitions and hypotheses of formation.

Definitions

Active rock glaciers have been described from many mountainenvironments in the world, including: the European Alps (e.g.

288 JOURNAL OF QUATERNARY SCIENCE

Barsch, 1978, 1988), North America (Ellis and Calkin, 1979);Asia (Schroder et al., 2005); South America (Dornbusch, 2000):Svalbard (Lindner and Marks, 1985; Isaksen et al., 2000); theFaeroe Islands (Humlum, 1998a), Greenland (Humlum,1982a), Antarctic Islands (Gordon and Birnie, 1986) andAntarctica (Lorrey, 2005). Active protalus lobes have been lesswell documented but examples in Svalbard (Humlum, 1982b;Sollid and Sørbel, 1992; Andre, 1994; Isaksen et al., 2000) areof particular importance. Findings from active present-dayfeatures will be considered in our interpretations of relict forms.

Both rock glaciers and protalus lobes are accumulations ofrock debris which have undergone slow movement by thecreep of an ice/debris mix or an ice core (Haeberli, 1985;Martin and Whalley, 1987; Barsch, 1988; Whalley andMartin, 1992; Humlum, 1998b). In their active state theyhave been regarded by some (Haeberli, 1985; Barsch, 1988)and in reviews by Martin and Whalley (1987) and Whalley andMartin (1992) as indicators of contemporary permafrost.However, other workers, e.g. Whalley and Azizi (1994) andPotter et al. (1998), argue that they may also be formed by theburial of glacier (sedimentary) ice by rock debris and aretherefore primarily of glacial origin. Barsch (1992) considersthat some rock glaciers may develop in till (and even protalusramparts) by the creep of permafrost-derived ice in thesediments. (A further complication has been noted whenauthors have referred to ‘periglacial’ when actually meaning‘permafrost’, the latter being a reference to thermal regimerather than location.)

In active rock glaciers, creep of the ice constituent mayproduce characteristic micro-relief such as ridges and furrows,while melting of ice cores induces differential thaw settlementleading to the development of collapse pits (‘thermokarst’) andlongitudinal furrows (Martin and Whalley, 1987). Rock glacierscommonly display steep, well-defined lateral margins and,when active, a steep snout. The latter is less pronounced in therelict state following melting of the ice content and postglacialmass movement. There is a marked absence of fine matrix,especially in the surface layers which tend to consist largely ofcoarse, angular rock debris (probably reflecting the importanceof macrogelifraction of bedrock in the supply of debris). Surfacelayers may also contain large bocks (>1 m long axis) which hasbeen used to suggest rockfall or cliff failure as a source of thedebris. For a discussion of topographic typologies see Martinand Whalley (1987) and Whalley and Martin (1992).

A number of attempts have been made to classify active rockglaciers using various criteria. These have included classifi-cations by shape and location (Wahrhaftig and Cox, 1959;Outcalt and Benedict, 1965) and by the nature of the icecontent, e.g. ‘glacier-cored rock glaciers’; see Whalley andMartin (1992) for a review. Barsch (1992) subdivides rockglaciers into two main types: ‘talus rock glaciers’, which havedeveloped below talus slopes and transported frost-shatteredbedrock fragments; and ‘debris rock glaciers’, which havedeveloped from the permafrost creep of morainic material. Thisclassification does not include rock glaciers which developedfollowing catastrophic rockfall events and can be differentiatedfrom rock avalanches (Johnson, 1984, 1987). The presentauthors argue that several of these relict rock glaciers arepresent in the British Isles and, as a result, Barsch’s (1992)genetic classification is not used in this paper. In an earlierreview of relict rock glaciers covering England and Scotland,Ballantyne and Harris (1994) classified them as protalus andmorainic rock glaciers. Again, this classification is consideredinappropriate since it does not account for the full range offeatures developed in the British uplands. The origin andformation of the debris giving rise to these features are by nomeans clear and all forms are inactive. In this paper we

Copyright � 2007 John Wiley & Sons, Ltd.

subdivide the range of features into generally distincttopographic forms: rock glaciers and protalus lobes.

Rock glaciers are markedly tongue-shaped (downslopelength is greater than across-slope width). Protalus lobes aregenerally located below cliffs, are wider than they are long andnot found below corrie headwalls where rock glaciers, sensustricto, are found (Hamilton and Whalley, 1995). Hamilton andWhalley (1995) considered that the term ‘rock glacier’ shouldonly be applied to valley-bottom features whose downslopelengths are greater than their widths (cf. Capps, 1910). For thoselandforms developed on valley sides (the ‘valley-wall rockglaciers’ of Outcalt and Benedict, 1965), Hamilton andWhalley (1995) used ‘protalus lobes’ following the terminologyof Richmond (1962). By adopting this scheme, a descriptiveframework can be established within which the differentprocesses responsible for feature formation can be discussed.Here we use the inclusive, and non genetic, term ‘discretedebris accumulations’ to incorporate protalus lobes, rockglaciers, rockfall/slide and landslide deposits. It also includesprotalus ramparts, although we touch on these only marginallyin this review. Ice is a component in those features currentlyactive so a subset would include ‘ice-debris landforms’.Furthermore, a scheme for the classification of relict rockglaciers must apply equally well to the classification of activeforms. Figure 1 shows a morphological typology of active‘ice-debris landforms’ (after Whalley, 2000) which includesprotalus lobes and rock glaciers.

Classification and identification of these ice-debris landformsin their relict state have also been problematic (Whalley andMartin, 1992; Hamilton and Whalley, 1995) as it is oftendifficult, if not impossible, to ascertain the nature of the formerice content. Further, the shape of a relict rock glacier doesnecessarily indicate the mode of formation or the age of thefeature. The most important problem is the misidentification inthe field caused by morphological and sedimentologicalaffinities between relict rock glaciers and protalus lobes andother discrete debris accumulation landforms such as protalusramparts, avalanche boulder tongues, moraine sequences androck slope failures of various types. Problems have also arisenbecause of the uncertain genesis of ice-debris landforms, evenwhen these features are actively moving. We thus usemorphological definitions because morphology has been usedto identify and map these features in the British Isles andIreland. However, a problem arising indirectly from the use of aclassification by form is that many authors have just referred toboth (i.e. ‘protalus lobes’ and ‘tongue-shaped’ or ‘valley floor’rock glaciers) just as ‘rock glaciers’ and thus potentiallyconfused the issue with respect to genesis, interpretation andhence environmental significance of the relict features.

Genesis

Active rock glaciers may contain an ice core, which can be ofburied glacial ice, or the void spaces may be filled with an ice‘cement’. It is possible that a combination of these occurs insome places. There is still some considerable debate about theorigin of this ice. Haeberli (1985) regards ice in rock glaciers asbeing solely of permafrost, rather than glacial, origin and rockglaciers are therefore seen as indicating the presence ofpermafrost, in at least a discontinuous form (see also thediscussion in Whalley and Martin, 1992). Despite thiscontention, the literature includes examples of rock glacierscontaining ice clearly derived from glaciers and existing in anenvironment where there is no thermal or landform evidence of

J. Quaternary Sci., Vol. 23(3) 287–304 (2008)DOI: 10.1002/jqs

Figure 1 Schematic of glacierised area showing a range of landforms associated with rock debris and ice masses of various types and sizes

ROCK GLACIERS IN THE BRITISH ISLES 289

permafrost (e.g. Whalley and Azizi, 2003). Under theseconditions the presence of a rock glacier cannot be used asconclusive evidence of permafrost. The snouts of glacierice-cored rock glaciers (which must necessarily have aninsulating cover of debris) can also reach lower altitudes than asmall glacier without a debris cover. As such, these features areambiguous and so is any palaeoclimatic interpretation. It couldbe argued that the presence of a rock glacier, but no moraines,in an area is suggestive that permafrost and not glacialconditions pertained (e.g. Blagborough and Farkas, 1968).However, a simple interpretation is that the rock glacier is asmall glacier with a debris cover in a glacially marginal area.Clearly, the debris supply is an important constituent in thebehaviour and preservation of small ice-debris systems.Figures 2 and 3 present schematic views of the permafrostand glacial origins of active rock glaciers, respectively.

With protalus lobes the origin of the debris is generally takento be weathered cliffs on valley walls. Such features can be seen

Figure 2 Permafrost origin of rock glaciers (protalus lobes) accordingto Barsch (1977)


commonly in Svalbard (e.g. Andre, 1994), although lessfrequently in the Alps. A general model is usually acceptedthat permafrost can allow meltwater to accumulate and freezeinterstitially. However, as we discuss below, creep of this porevolume ice is retarded by the debris (Whalley and Azizi, 1994),i.e. shear strength is greater than the strength without icepresent. Only where large lenses of ice occur can creep takeplace (Azizi and Whalley, 1995; Whalley and Azizi, 2003).Such analyses have an important role to play in theinterpretation of relict protalus lobes.

Distribution of relict rock glaciersand protalus lobes in the British Isles

In Great Britain and Ireland the majority of rock glaciers andprotalus lobes so far identified are located within Dimlington/Midlandian Stadial (26–13k yr BP) ice limits and thus post-datethis cold period. It is assumed that the only remaining periodcold enough for their formation is the Loch Lomond/Nahanagan Stadial (Younger Dryas; 11–10k yr BP) (Jonesand Keen, 1993). They are all relict features, rendered inactiveby climatic amelioration at the beginning of the Holocene.Most are located in close proximity to, but outside, YoungerDryas glacial limits. There are important exceptions to this andthese will be discussed below.

A total of 32 relict rock glaciers and protalus lobes have beenidentified in the British Isles, both from a review of earlierpublished work and from fieldwork undertaken by the authorsand colleagues (Tables 1–4). Although considerable debatesurrounds some of them, all have been included forcompleteness. Seven are described from England, eight fromScotland, four from Wales and ten from Ireland. We includesome ‘new’ examples from our observations as well as recentreinterpretations. The main features of these are describedbelow. Many of the features described show the difficulties ofunambiguously defining both typology and origin of mappableunits within the remit of discrete debris accumulations.


Figure 3 Glacier ice core model according to Johnson (1987), from Martin and Whalley (1992)


England

Seven of the relict landforms described here are from northernEngland. The feature at Grasmoor End (NY 162205) in thenorthwestern Lake District is described by Oxford (1985) as anaccumulation of talus with a series of indistinct flat-toppedbulges at its base. In the field the landform is poorly developedand described as ‘dubious’ by Ballantyne and Harris (1994, p.242). In addition, Oxford (1994) described a protalus rampartwith a lobate component on the downslope side below DeadCrags (NY 266317) in the Lake District and suggested that thiscan be seen as, ‘an incipient form of rock glacier’ (Oxford,1994: p. 162). This is further discussed in Huddart and Glasser(2005) although they do not suggest anything other than aprotalus rampart. Sissons (1980) has described several ‘fossilrock glaciers’ north of Wast Water at about 400–450 m asl.Although not very clear forms in the field they would beclassified as protalus lobes under our definition. Sissons alsomaps protalus ramparts and ‘gelifluction lobes’ and sheets,some with fronts up to 4 m high. It is possible that the GrasmoorEnd feature is a large gelifluction lobe, its terminus at about150 200 m asl. Sissons (1980, p. 20) also mapped ‘snow beddeposits’ (seemingly synonymous with ‘protalus ramparts’) andargues, noting the mapping and interpretational problem, that‘it is difficult to make a distinction between former snow bedsand very small glaciers’.

An elongate rock glacier occurs in Burtness Combe aboveButtermere (NY 175147, Whalley, 1997). The feature (Fig. 4(a))lies between 300 and 560 m asl and faces east. Its length is some700 m and it has a distinct left lateral furrow separating it fromthe hillside screes. The snout is distinct and is typical ofwell-formed relict rock glaciers. The debris supply for thisfeature is the cliff from Grey Crag (below High Stile). Morerecently, Clark and Wilson (2004) have argued that this featureis a rock avalanche deposit rather than a rock glacier.

At Cautley Crags in the western Pennines (Howgill Fells,Cumbria, SD 685976) Mitchell (1991) identified an arcuatedrift feature some 500 m in width developed at the base of thesouth-facing hillslopes of Yarlside (Fig. 4(b)). From the planformof the feature (with ridges parallel to the hillslope anddepressions between them) Mitchell (1991) argued that thisis a talus-foot or lobate rock glacier. Mitchell points out that thefeature had been interpreted as a terminal moraine, from aglacier in a combe to the south, by King (1976) and Harvey(1985). However, the presence of glacially striated and facetedclasts found in stream cut sections in the deposit suggest thatthis feature is at least partially of glacial origin and may be ofcomplex origin. Its low elevation (ca. 250 m) is more suggestiveof a large morainic or mass-movement feature rather than aprotalus lobe with permafrost ice.


Although not included in this review, large boulder lobes onDartmoor have also been attributed to permafrost creep(Harrison et al., 1996) and similar features occur on severalsummits in Snowdonia. With frontal risers up to 15 m in heightand with no evidence of a fine sediment infill, they are unlikelyto be the product of solifluction, and are much larger than theboulder lobes described from Scotland by Shaw (1977).

Scotland

All of the eight relict rock glaciers and protalus lobes found inScotland are in the Highlands. Five are in the Cairngorms orGrampians (although as many as eight may be found here;Ballantyne, pers. comm.): two in the Torridon massif and oneon the Isle of Jura. Those described in the Cairngorms by Sissons(1979), Chattopadhyay (1984) and Maclean (1991) can, in themain, be classified as protalus lobes (Fig. 5(a)). A number of‘snow-bed deposits’ (Sissons, 1979, see also above) have beenmisinterpreted as rock glaciers and vice versa (Ballantyne andHarris, 1994). The largest described in the area (some 2.4 km inwidth) is located on the western slopes above Strath Nethy(Maclean, 1991). This had been previously interpreted as‘snow-bed avalanche deposits’ (Sissons, 1979). Although muchof the material forming this relict feature appears to have comefrom large, discrete rockfalls, as shown by rockfall scars in thecliffs above it, the Strath Nethy rock glacier is classified here asa protalus complex lobe complex on the basis of the degradednature of these cliffs and the extensive screes which haveaccumulated below.

The northwest Highlands display three remarkable land-forms. On Ben Alligin in Torridon (NG 870600) a large rockfall(1.2 km long, 400–200 m wide and up to 15 m high, with avolume estimated as 1.5� 106 m3) from the cliffs of Sgurr Mor(Gordon, 1993; Fig. 5(b)) has provided the basis for an elongaterock accumulation. Much controversy surrounds this featureand the debate continues. Sissons (1975) argued that it was theresult of a rockfall reactivating a small glacier which occupiedthe southeast-facing cirque, the resulting landform representinga glacier-cored rock glacier. Whalley (1976), on the other hand,suggested that the feature had many of the characteristics oflarge catastrophic rock avalanches or ‘Bergsturz’. Ballantyne(1987) has joined the debate by arguing that the decayingglacier may have been larger than that envisaged by Sissons andtransported the debris downslope as a supraglacial layer.However, this does not easily explain the presence ofprecariously perched boulders which are found on the surfaceof the feature. Ballantyne and Stone (2004) have recentlyobtained a cosmogenic isotope date for this features which


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Copyright � 2007 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 23(3) 287–304 (2008DOI: 10.1002/jq


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Copyright � 2007 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 23(3) 287–304 (2008DOI: 10.1002/jq


)s

Figure 4 (a) Possible rock glacier below Grey Crag from Eagle Crag,Buttermere, Lake District, although this has also been interpreted as arock avalanche deposit (see text for discussion). (b) Protalus lobe, belowYarlside, Cautley Crags, Howgill Fells, Cumbria

Figure 5 (a) Protalus lobes in the Lairig Ghru (photo courtesy of JEGordon). (b) Ben Alligin, Torridon, rockslide/rock glacier deposit (photocourtesy of JE Gordon). (c) Beinn Shiantaidh, Jura, protalus lobes belowquartzite scree (photo courtesy of JE Gordon). (d) Possible rock glacierdeposit, associated with moraine sequence, west of Orval (NG 324992)Rum, Inner Hebrides


suggests a Holocene age for the deposit. On Baosbheinn (NG855676), also in Torridon, a substantial boulder deposit onthe northwest of this hill is another landform that has alsoengendered considerable debate. This feature has beenvariously described as a ‘protalus rampart complex’ (Sissons,1976) and a ‘lateral moraine with protalus ridges’ (Ballantyne,1986) associated with large rockfalls. The presence of collapsepits caused by the melting of buried ice led Ballantyne andHarris (1994) to argue further that at least part of the landformhad experienced rock glacier flow. Gordon (1993) suggests thatthe feature may be ‘regarded as a protalus or valley wall rockglacier and has affinities with Beinn Shiantaidh on Jura’.

Developed at the base of steep quartzite hills on Jura are aseries of boulder lobes and the biggest is the feature (Fig. 5(c))described by Dawson (1977) at Beinn Shiantaidh, Jura (NG521749) and classified as a ‘fossil lobate rock glacier’ but hereas a protalus lobe. Ballantyne (1999) describes these asrockslides and this re-evaluation is similar to that undertaken onrelated features (and on similar rock type) in Donegal by Wilson(2004), which is presented below. Also included in Fig. 5(d) is apossible moraine-rock glacier sequence seen on Rum. Theinterpretation of this, as no doubt with others still notrecognised in the field, shows the possible graduation in formsbetween various ‘discrete debris accumulations’.

Wales

Until the last two decades, relict rock glaciers had not beenidentified in the mountains of Wales. However, the elongate


debris accumulation at the back of Cwm Bochlwyd (Fig. 6(a))above Ogwen Valley in Snowdonia was interpreted as a relictrock glacier by Harrison (1992). Large rockfall debris streamshave developed below the northern flanks of Tryfan and theirdistal ends reach the A5 road. Although Harrison (1992)originally suggested that they were unmodified rockfalls, recentfieldwork has revealed the presence of transverse ridge and


Figure 6 (a) Rock glacier with rockfall debris, Cwm Bochlwyd (SH655589) Y Glyder, Gwynedd. (b) Possible moraine-rock glacier depos-its, east of Carnedd y Filiast (SH 625727) Glyder, Gwynedd. (c) Rockglacier area below cliffs on the north-facing slopes of Llechog (SH600536), Yr Wyddfa, Gwynedd. (d) Extensive hummocky area belowthe small Cwm du, Afon Ystwyth Dyfydd


furrow topography with depressions at the front of discretelobes on their surfaces. This means that they may also havebeen affected by deformation of ice and hence are relict rockglaciers (P. Wilson, pers. comm.). It is likely that other, similar,


landforms exist in the mountains of Wales. For example,discrete debris accumulations which lie on valley floors belowcliffs and could have been active rock glaciers occur below YGarn north of Tryfan in the Glyder range (Fig. 6(b)) and north ofthe Llechog ridge of Yr Wyddfa (Fig. 6(c)).

In the Nantlle valley well-developed debris lobes occur at thebase of extensive screes and the landform is interpreted as a relictrock glacier (Harrison and Anderson, 2001). This landform isdiscussed in more detail later. Similar landforms occur below thenorth-facing cliffs of Cader Idris, while a large debris accumu-lation has been described on Moelwyn Mawr (SH 658448) belowtalus slopes (Lowe and Rose, 1981; Campbell and Bowen, 1989).A feature, not dissimilar to that in Nantlle, has been attributed to alandslide (Watson, 1962) and is found at the southern end ofTal-y-llyn on the south of the Cader Idris massif.

Although this review is not primarily concerned with protalusramparts, their form and possible genetic relationships withother ‘discrete debris accumulations’ do bring them intodiscussion where they are large or related to cwms where icemay have accumulated. Some authors have attributed suchfeatures to ‘snow bed deposits’. Watson (1966) described largefeatures as drift accumulations and solifluction deposits withscarps in two north-facing cwms in the upper Yswyth valley(Fig. 6(d)). They have similarities with the features described byOxford (1985) at Dead Crags in the Lake District.

To our knowledge, and perhaps surprisingly, there are nodescriptions of discrete debris accumulations in Bannau Brychei-niog (Brecon Beacons), Mynydd Du and Fforest Fawr areas ofsouth Wales (Shakesby, 1992) other than the large protalusramparts under sandstone cliffs which may have developmentalrelationships to rock glaciers (e.g. Bannau Sir Gaer, Whalley andAzizi, 2003). An overview of these features is given by Campbelland Bowen (1989) and Shakesby (1992), who suggests that somecwms in the area lack moraines whereas others have complexforms which may be related to landsliding. Some ramparts andlobes are substantial enough to be considered as moraines andcan be likened to similarly large features that can be found in theLake District, e.g. Keskadale (High Hole) below Robinson (NY197177) – which has been mapped as both moraine and protalusrampart. The north face of Cader Idris has several protalus featuresincluding, in Cwm y Gadair, a feature now mapped by Sahlin(pers. comm., 2006) as a moraine but which had been interpretedas a rock glacier by Lowe (1983). These examples suggest that acontinuum of form and size exists between moraines, protalusramparts and rock glaciers which relates to the amount of debrisassociated with a creeping body of ice.

Ireland

In Ireland, most of the rock glaciers so far described in theliterature have been located in the mountains of Donegal andinvestigated by Wilson (1990a, 1990b, 1993). However, similarfeatures are also developed elsewhere. In the Nephin Beg ofwestern Mayo, O’Reilly (1989) has identified a possible rockglacier in a northeast-facing corrie of Corslive (Irish grid F903104). In Kerry, protalus lobes and rock glaciers have beenfound in Macgillycuddy’s Reeks developed on Old Red Sandstonelithologies (Anderson et al., 2001; Harrison and Anderson, 2002);a huge, elongate feature of complex origin occupies the floor of alarge cirque on Brandon Mountain, and relict rock glaciers havebeen reported from the northern slopes of the Dingle Peninsula.

Errigal Mountain (Fig. 7(a) and (b)) in Donegal displaysprobably the most impressive landforms attributed to permafrostcreep in the British Isles. These have all developed as protaluslobes and reflect considerable frost-shattering of the quartzitebedrock (Wilson, 1990a). Their combined volume is in excess of


Figure 7 (a) Errigal, Co. Donegal, showing protalus lobes below theactive screes; view from the southwest). (b) View from Errigal, Co.Donegal, showing small protalus lobes of quartzite. In the distance isAghla Mor (with a now disputed rock glacier). (c) Muckish (Co.Donegal) with protalus lobe in quartzite, and quarried snout area


11 million m3. Similar large features occur on Muckish Mountain(Fig. 7(c)) and on the Aghla Mountains (Wilson, 1990b, 1993).However, Wilson (2004) has recently revised his opinion onthese protalus lobe forms following new field evidence fromScotland (Sandeman and Ballantyne, 1996; Ballantyne andStone, 2004). Wilson’s discussion suggests that a rockslopefailure is more likely than ‘a relict rock glacier and protalusrampart origin for the Donegal debris landforms’. These formsthen become ‘rock glacier mimics’ (Whalley and Martin, 1992).We discuss the implications of this reassessment further below. Itis evident that the distinction between the various landforms andtheir topographical settings is important as this illustratesdifferences in form, the processes responsible for the features,their age and so their palaeoclimatic significance.

Examples of major types of discretedebris accumulations

From this overview we can thus identify three major types ofdiscrete debris accumulations:


1. ro
ck glaciers in corrie heads, perhaps associated with (cat-astrophic) rockfalls (including falls on to small glaciers);
2. ro
ck glaciers and protalus lobes associated with the perma-frost creep of talus;
3. ro
ck glacier and protalus lobe mimics (e.g. landslides withno glacial or permafrost association).
We now examine these main types and discuss specificexamples.

Rock glaciers associated withcatastrophic rockfalls

One of the best-developed rock glaciers developed below steepcliffs is found in the mountains of Snowdonia. The feature,located at the back of Cwm Bochlwyd above Ogwen Valley,forms an elongate boulder mass some 240 m in length and175 m wide and is situated within Loch Lomond Stadial glacierlimits identified by Gray (1982). The cwm is north-facing andsurrounded on its southern side by steep cliffs which rise to914 m OD. The boulder mass had been previously mapped asmoraines by Gray (1982) and as till by Addison et al. (1991).The surface of the feature is composed of large, angular blocksand, for the most part, is unvegetated and highly irregular.Three transverse ridges are present with clasts displaying higha-axis plunge angles separated by depressions showing lowangle plunges, possibly representing extending and compres-sive flow regimes. This configuration indicates movement of thedeposit at some time. In addition, circular depressions at therear of the feature are ‘tiled’ with platy clasts whose long axesare oriented towards the bottom of the depressions. These areinterpreted as ‘collapse pits’, suggesting that the surface of theboulder mass was let down slowly rather than in a catastrophicmanner. Slow deformation of the mass is also indicated by thepresence of a number of large blocks on the western flank of thelandform which are split, the pieces having moved a fewcentimetres apart relative to each other. Although a largewedge-shaped failure scar is set into the backwall above andbehind the feature, on these morphometric and sedimentolo-gical grounds Harrison (1992) argued that the feature was arelict rock glacier. However, a large component of the debrissupplied to the landform may have been contributed bycatastrophic rockfalls.

Rock glaciers and protalus lobes associatedwith the permafrost creep of talus

In contrast, the geomorphology of protalus lobes and relict rockglaciers found at the base of talus-covered slopes appears to beeasier to explain. A well-developed example occurs in theNantlle Valley in Snowdonia (Harrison and Anderson, 2001).The landform comprises a lobate-shaped mass of debris, 90 mlong and up to 270 m wide. It displays a steep frontal slope, upto 318, which rises to a maximum elevation of 22 m above thevalley floor. Elsewhere, the landform exhibits a low gradientsurface (2–88) and grades upslope into drift and talus deposits,although the contact between the lateral margins and adjacentslope debris is sharp. Towards the western end of the landformtwo small (<1.5 m high) bouldery ridges and a circular surfacedepression can be observed. Over much of the landformsurface there are irregular spreads of angular boulders, which inplaces appear to grade into talus deposits. Material recovered



from excavations in the landform demonstrated the presence ofhomogeneous sandy diamict containing subrounded clasts.

Being located outside Loch Lomond Stadial glacier limitsmeans that this landform may have formed at some stagefollowing the retreat of Dimlington Stadial ice sheets and beenreactivated during the periglacial climates of the Loch LomondStadial. This finding, however, lays open the suggestion that thefeature is in fact a large landslide and the similarities with theTal-y-llyn features described by Watson (1962) are striking.

Rock glaciers associated with the creep of morainic material

These are a subset of only a small number of morainic relictrock glaciers that have been found in the British Isles. Onepossible feature exists in the Owennfeana Valley on BrandonMountain; others may include those seen in Fig. 5(d) (Rum) andFig. 6(b). Ballantyne and Harris (1994) suggest that the Beinn anLochain and Beinn Alligin features in Scotland may be‘morainic rock glaciers’. However, the evidence here (seeGordon, 1993) is that such rockslides were just carried furtherby underlying ice rather than producing actual rock glaciers. Itis possible that the best-developed morainic feature in theBritish Isles is that described by Mitchell (1991) at Cautley Cragsin the western Pennines (Fig. 4(b)). However, here again thepossibilities of a landslide origin cannot be discounted.

Rock glacier and protalus lobe mimics

Well-developed protalus lobes exist in Macgillycuddy’s Reeksof southwest Ireland. The most prominent is named here ‘TheBone 1 rock glacier’ (Harrison and Anderson, 2002). Thislandform is located at 550 m asl on west-facing slopesoverlooking the An Gheadach Valley; it is lobate in shapeand 300 m in width across the slope and 100 m in length. Thesteeply inclined (up to 358) frontal riser is up to 24 m high. Fourtransverse ridges are observed on the surface of the feature.Wilson (1993) describes similar protalus lobes which haveformed on the Aghla Mountains in Donegal. Aghla Beg I is alobate debris accumulation extending 280 m downslope and530 m across-slope. Transverse ridges separated by hollowstestify to the flow of this feature at some stage. As the AghlaMountains were covered by Midlandian age glacial ice, Wilson(1993) attributes the formation of this landform to theNahanagan Stadial. However, Wilson’s (2004) re-evaluationof these features places them more firmly in the ‘mimic’category. In this case they are closely related to many otherpostglacial ‘Bergsturz’, commonly seen in Icelandic basalts(known as ‘berghlaup’; see Whalley et al., 1983) and whichhave been mistakenly identified as rock glaciers.

Discussion

Although there have been relatively few published studies it isclear that discrete debris accumulations, i.e. mappable units,which includes ‘rock glaciers’ and ‘protalus lobes’, are morenumerous in the British Isles than has previously beensupposed. What is less clear is the processes responsible fortheir formation, their palaeoenvironmental significance, theirage and the controls on their development. In addition, thedifferentiation between relict rock glaciers and protalus lobes


and other landforms is not always easily achieved. The fieldcharacteristics of these features, including locale and associ-ated geology, must be examined carefully before anypalaeoenvironmental conclusions can be made. We nowexamine the known controls on rock glacier and protalus lobeformation in relation to some of the identified featuresmentioned above.

Ground temperature, air temperature and precipitation maybe seen as the dominant climatic factors controlling thedevelopment of rock glaciers and protalus lobes, including theformation of ice and its creep. Other factors include aspect(partly controlling ice accumulation and preservation) andslope angle on which the deposit accumulates. Geology affectsthe nature of the weathered debris as well as the formation ofrockslides and slope failures. It would seem, however, that‘mixing and matching’ these variables may give rise to‘equifinal’ landforms.

Climatic controls

Temperature

Temperature, usually defined as mean annual air temperature(MAAT), determines whether permafrost is present or absentand hence whether a debris–ice mix can survive. In addition, asthe shear strength of ice increases with decreasing temperature(Patterson, 1994; Whalley and Azizi, 1994; Haeberli et al.,2006) this implies that rock glaciers and protalus lobes inwarmer conditions deform more easily and flow at highervelocities than those in colder environments. However,permafrost conditions are required to maintain an ice–rockmixture in the long term (Barsch, 1996). The lower thetemperature, the greater the shear strength of ice–rock mixtureswhich thus need to be at high slope angles and of considerablethickness for creep to occur. Some modelling of this has beendone by Azizi and Whalley (1996). Rock glaciers which arederived from thin glaciers under a protective blanket of debrismay continue to flow, even though the MAAT at that locationmay be above 08C. As active rock glaciers have been foundazonally, i.e. outside permafrost temperature limits (Whalleyand Martin, 1992), then rock glaciers cannot be used asindicators of a permafrost environment. This case also appliesto fossil forms.

Precipitation

Precipitation, in combination with temperature, controls theamount of snowfall and hence the likelihood of rock glacier andprotalus lobe formation. Low temperatures and high precipi-tation favour the growth of glaciers in locations which mightotherwise contain rock glaciers, as deep snow preventssufficient ground cooling and the development of permafrost.However, the time of year that snow falls also plays animportant role. If the ground freezes early and snow falls latethen permafrost is more likely to develop than with the reversesituation. Increasing development of permafrost and reducedprecipitation occur at high altitudes and in more continentallocations. Although it has been proposed (e.g. Haeberli, 1985;King, 1986) that rock glaciers follow continentality gradientsthere are examples of rock glaciers from maritime areas (e.g.Griffey and Whalley, 1979; Gordon and Birnie, 1986; Evans,1993; Whalley et al., 1995) that assert a contrary view. Theglacier model for rock glacier formation requires only that there


https://www.researchgate.net/publication/230259565_Description_and_origin_of_some_talus-foot_debris_accumulations_Aghla_Mountains_Co_Donegal_Ireland?el=1_x_8&enrichId=rgreq-ea7771b4-8c10-4cae-97a2-248ca0234851&enrichSource=Y292ZXJQYWdlOzIyOTQ0MjU4MztBUzoyNTg2NzczOTEyMjg5MjhAMTQzODY4NDkwNzI5Mw==


is glacier ice (protected by surface debris) in a suitable localityand that permafrost is not necessary.

Moisture for the creation of interstitial ice is required inprotalus lobes (Fig. 2) and this may be incorporated into thedebris mass by snowmelt processes or avalanches. While relictrock glaciers and protalus lobes at the foot of talus slopestherefore suggest the presence of low temperatures inassociation with low precipitation totals, the presence of rockglaciers associated with the instability of bedrock cliffs allowslittle to be said about the nature of the precipitation. This isbecause the features are controlled largely by other, mainlygeological, factors related to the size and flux of debrisparticles. Interstitial ice, even above ice saturation of pores in aboulder body, will not allow ice to creep (Whalley and Azizi,1994). Work by Azizi and Whalley (1996), however, shows thatfor any ‘reasonable’ amount of flow to take place the ice massneeds to be of considerable thickness so ice bodies severalmetres thick would have to be incorporated in order to producemeasurable flow rates (Fig. 8). Where such ice masses comefrom is, as yet, unclear. The easiest route could well besemi-permanent or relict (glacial) ice which is then covered bydebris. This may involve a two-stage process involving achange from ice body plus protalus rampart to protalus lobe by

Figure 8 (a) Finite element net with ice covered by ‘scree’ (from Whalley anmesh of Fig. 8(a). Finite element net with ice covered by ‘scree’ (from Wha


the addition of debris to the glacierette late in the develop-mental sequence. Again, environmental interpretation isdifficult as we do not have good analogues in present-dayenvironments.

Topographic controls

Relict rock glaciers and protalus lobes in the British Isles havedeveloped in topographically limited locations. Tables 1–4show the orientation and altitudes of rock glaciers in the BritishIsles and their concentrations on slopes of a northerly aspect.Most features occur at the bottom of slopes in mountainousregions which, through combinations of structure andlithology, provided enhanced debris supply. In addition, somefeatures require the presence of steep bedrock cliffs above themto provide the large rockfalls and the potential energy fordownslope travel. As a result, such rock glaciers arepreferentially found on corrie floors and in valley heads. Theremay be important structural controls as well. Evin (1987) hasdemonstrated that rock glaciers in the French and Italian Alpsare largely developed below cliffs with two or more structural

d Azizi, 2003). (b) Vertical and horizontal velocities for node 19 in thelley and Azizi, 2003)



trends which enhance the likelihood of rock failure. Althoughno work has been carried out in the British Isles to test thismodel, an assessment of the bedrock control is crucial to ourunderstanding of the locational factors in rock glacierdevelopment. This might be achieved through the study ofrock-mass strength indices in rockwalls upslope of relict rockglaciers. It must be said, however, that the number of suitablerock glaciers for this analysis is very limited, although this initself is a puzzle to be examined.

Landforms at the foot of scree slopes are apparently littlelimited in their locational requirements and have been foundon benches on mountain sides (e.g. the feature on Aghla Mor)and on valley sides (e.g. Strath Nethy fossil rock glacier), whilethe known examples of large boulder lobes are all locatedbelow rocky hilltops. Rock glaciers are located in valley orcorrie bottoms, below sites where glaciers accumulated and inthe vicinity of steep mountain sides which provided the debrisfor their formation.

Although relict rock glaciers and protalus lobes are found ona wide range of lithologies (see Tables 1–4), geology plays animportant role in their development. Protalus lobes and rockslope failures seem to be found where fracture of bedrockcreates blocky clasts (e.g. the quartzites of Jura and Donegal,Torridonian sandstone of northwest Scotland, granites of theCairngorms and the Old Red Sandstone of Kerry). Only rarely,as on the slates at Moelwyn Mawr for example, does bedrockwhich weathers to form platy clasts produce relict protaluslobes. The quartzites of Jura and Donegal are, however, alsoprone to mass movements and from these have developed thelandforms described by Wilson (1993, 2004).

Relict rock glaciers (type ii) situated below steep cliffs in theBritish Isles have shown no such geological control – ratherthey are developed where large-scale rockfall from steepglaciated cliffs has occurred. The reasons for this are not clear. Itmay be that rocks which weather mechanically to create blockyclasts form coherent bedrock masses and cliffs, while slates andother fissile bedrocks, which provide the continuous supply ofbedrock fragments, do not. Rock glaciers associated withmorainic systems require the input of rock material to theglacier system which may also have a similar geologicalcontrol. In any event, it is debris supply from specific rock typeswhich is a major influence on protalus lobe and rock glacierformation.

Debris supply

Protalus lobes below talus have probably developed inresponse to the gradual accumulation of frost-shattered androckfall debris and where movement has occurred through thedeformation of interstitial ice or an ice core. Relict examplessuch as the one at Moelwyn Mawr in north Wales, the featuresdescribed by Dawson (1977) on Jura, by Chattopadhyay (1984)in the Cairngorms and by Wilson (1990a, 1990b, 1993) innorthwest Ireland might fit into this category. The identifyingcharacteristics of these features include location outside LochLomond/Nahanagan Stadial glacier limits, low-angled ordegraded cliffs upslope of the debris accumulation and partiallyor completely vegetated surfaces.

In contrast, rock glaciers associated with catastrophicrockfalls are composite and polygenetic features where a verylarge proportion of the debris has accumulated in a single,catastrophic episode. The gross morphology of these featuresmay be related to this event with only slight subsequentmodification of the mass in a periglacial environment. Someactive rock glaciers are probably the result of both slow


incremental rockfall as well as large, ‘one off’ events. Anexample is the Arapaho Rock glacier in the Colorado Frontrange (Whalley, 1974). As cliff-foot features associated withcatastrophic rockfalls have been found at a number of sites inthe British Isles (e.g. on Cader Idris and in Cwm Bochlwyd inWales), it is clear that the ‘special circumstances’ (Sissons,1975, p. 85) of a large rockfall triggering a rock glacier responsemay not be particularly special after all. In areas ofcontemporary rock glacier development many of them havedeveloped as a consequence of the triggering of cliff instabilityand consequent high-magnitude low-frequency slope pro-cesses (Johnson, 1984).

Several of the rock glaciers in the British Isles may representthe deformation and flow of rock debris following burial ofglacier ice. Although Ballantyne and Harris (1994) suggest thatthe distinction between rock glaciers containing a glacial icecore and those with a permafrost ice cement may beterminological, examination of active examples shows thatthese exhibit distinct differences in rheology and formation(Whalley and Azizi, 1994). Form alone does not allow adistinction to be made between these possible origins.

Rockfalls may be associated with periods of cliff instabilityrelated to a number of possible factors. These include glacialunloading and seismic, neotectonic tremors associated withisostatic readjustment (e.g. Jarman, 2006), large-scale weak-ening of rock buttresses caused by intense periglacial weath-ering, permafrost melting or a combination of these factors. Thecharacteristics of these rock glaciers include location withinmapped Younger Dryas glacier limits, high and steeply angledcliffs upslope of the landform and largely unvegetated surfaces.The review of large ‘felssturz’ (rockfall events) in extraglacialareas of Austria by Meissl (1998) shows how significant suchevents may be even today. There is compelling evidence(Whalley et al., 1983; Whalley, 1984) that near-glacialconditions were, and are, important in the development ofrockfalls and slides. Permafrost warming post-Younger Dryasmay also have had a significant part to play, as has beensuggested for present-day rockfall production, and Whalleyet al. (1996) and Davies et al. (2003) have shown that largedebris accumulations are often associated with Little Ice Ageevents.

The proposed distinctions between relict rock glaciers andprotalus lobes may depend upon the mode of debrisaccumulation and represents a continuum between similarlandforms. However, Parsons (1987) has suggested that rockglacier types may be seen as a function of site conditions whichproduce distinct, mutually exclusive forms rather than acontinuous sequence. The formation of some relict rockglaciers may involve both catastrophic rockfalls and permafrostcreep of talus and are thus composite features which are noteasily classified. This would explain why glacier ice-cored rockglaciers might have snouts which are debris rich and perhapscontain interstitial ice or small lenses revealed by coring (seeWhalley and Martin, 1992, for review). The presence ofpermafrost thus allows such interstitial ice to be preserved onand around a glacier ice core (Whalley and Azizi, 2003). Such acomposite model is suggested by Avian et al. (2005) and can beextended to consider purely glacial deposits such as morainesas well as debris-covered glaciers and rock glaciers with aglacier ice core. This composite nature shows both thedifficulties of field recognition as well as providing a ‘unique’interpretation.

The relatively small number of rock glaciers presumed to beglacier ice-cored found in the British Isles is problematic. It maypossibly be related to the small amount of debris which fell onto the glacier surfaces during the latter part of the YoungerDryas as a consequence of the limited weathering occurring at


https://www.researchgate.net/publication/230259565_Description_and_origin_of_some_talus-foot_debris_accumulations_Aghla_Mountains_Co_Donegal_Ireland?el=1_x_8&enrichId=rgreq-ea7771b4-8c10-4cae-97a2-248ca0234851&enrichSource=Y292ZXJQYWdlOzIyOTQ0MjU4MztBUzoyNTg2NzczOTEyMjg5MjhAMTQzODY4NDkwNzI5Mw==


this time. In turn, this may be due to the limited size of mostcorrie-based glaciers and the relatively short period of timeavailable for debris accumulation and subsequent movement.The Llechog rock glacier (Yr Wyddfa/Snowdon, north Wales,Fig. 6(c)) might be a consequence of the ‘weak’, easilyweathered rock on that ridge compared with the much morestable cliff of Clogwyn d’ur Arddu just to the north, where thereis no rock glacier-like feature. Similar reasoning would alsoseem to apply to a permafrost model of rock glacier formation.

Taking this notion of debris-limited formation further, there isa similar paucity of extant rock glaciers in Norway, although nostatistical analysis has been performed. The reason may be thesame as in the British Isles, that the rock type is too resistant toproduce sufficient weathered rock debris that can accumulateon small glaciers. This contrasts with Iceland, where theglaciers are small but the fragmentation of basalt cliffs producessufficient debris to accumulate on glacier ice (see, for example,Whalley and Martin, 1994). Interestingly, Iceland also containsmany examples of discrete debris accumulations which havebeen attributed to ‘rock glaciers’ (protalus lobes) (Sigurðsson,1990) but are, in fact, the results of substantial rockslides andlandslides. In general, it would appear that we need to knowmuch more about the relative fluxes, as well as timings, ofdebris and ice inputs into systems which undergo flow. Asidefrom noting the influence of rockslides and landslides, rockfallsfrom valley sides or corrie heads contribute in complex ways tothe features developed below and studies such as that byGellatly and Parkinson (1994) in the Pyrenees may helpinterpretation using studies of present-day, marginally glacier-ized basins.

Flow and deformation processes

Despite controversy over the composition and origin of rockglaciers there is general agreement among supporters of bothgenetic models that rock glacier flow occurs as a result of slowinternal deformation in response to gravitational-inducedstress. The deformation is a result of thickness, surface slopeand the components of Glen’s (ice flow) law (Whalley andAzizi, 1994). According to the permafrost model, motion isachieved by the creep of interstitial pore ice (Wahrhaftig andCox, 1959; Barsch, 1977, 1978, 1992; Haeberli, 1985), orwithin interconnected lenses of segregation ice (Wayne, 1981).According to the glacial model, creep occurs within a body ofdebris-covered glacial ice (Whalley, 1974; Whalley andMartin, 1992, Whalley and Azizi, 1994, 2003).

Ice is often assumed to be a perfect plastic with a yieldstrength of 100 kPa, but since ice is a viscoplastic material it hasno definitive yield strength and strain can occur in response torelatively low stresses (Whalley and Azizi, 1994). For instance,in situ measurements of the internal deformation in glaciershave revealed that significant strain can occur when shear stresslevels are above 50 kPa at 08C (Patterson, 1994). However, theshear strength of a mixture of ice and rock debris (‘rockfill’) ismuch greater than ice alone, and empirical testing hasdemonstrated that the yield strength of rock debris saturatedwith pore ice is at least 300 kPa (Nickling and Bennett, 1984).Therefore significantly higher shear stresses are required toinduce deformation within an ice–rock debris mix. Addition-ally, the strain is very dependent upon the temperature of theice. The strain developed at 08C is some 50% greater than if theice is at�58C. This highlights several important implications forrock glacier studies (cf. Azizi and Whalley, 1995; Whalley andAzizi, 1994, 2003):


1. R
ock glaciers with a glacial core or a large body of laterallyextensive clean ice will flow in response to lower levels ofapplied shear stress than rock glaciers cemented by inter-stitial pore ice. The presence of massive ice acts as astructural weakness and thus helps to promote deformation.
2. U
nder similar stress and temperature conditions the strainrate within glacier ice-cored rock glaciers is greater andmore variable than within pore-ice rockfill.
3. If
permafrost is present, then the colder the conditions theless likely ice, especially interstitial ice, is to flow.
4. S
mall amounts of ice, if free to deform, will flow if the shearstress (thickness and surface slope) is sufficiently large andthe time period for creep to take place is sufficiently long.
5. In
general, however, if steep rockfill (protalus-lobe like)deposits are to creep then there needs to be a substantialvolume of ice within the deposit.
It has been suggested by Whalley and Azizi (1994) thatmodelling the dynamic properties of active rock glaciers canbe used as a test of their origin. The application of suchrheological models to relict rock glaciers is more problematicthough, since highly speculative assumptions of former thick-ness, density and ground temperature need to be made. How-ever, interpretation of shear stress calculated at the base of relictrock glaciers in relation to empirically derived values of theyield strength of ice and ice–rock debris mixtures may providesome intriguing clues as to the origin of relict features. Since theflow of a rock glacier is controlled by the yield strength of itsconstituent material (which in turn may reflect the origin of therock glacier) and the level of applied stress, it follows that if theformer stress regime of a relict rock glacier can be reconstructedthe level of calculated shear stress may be used to highlight themechanisms responsible for flow. Furthermore, if this infor-mation is considered along with the depositional characteristicsof the relict rock glacier it may provide evidence as to the originof these features.

Assuming that the sliding velocity at the base of rock glaciersis zero, basal shear stress which allowed internal deformationcan be calculated using the following equation:

tb ¼ r gh sina (1)

where:
tb ¼ basal shear stress (kPa); r ¼ specific gravity of rock (2100 kg m�3); g ¼ acceleration due to gravity (9.81 m s�2); a ¼ surface slope angle; h ¼ thickness (m).
The obvious problem with this approach is the uncertaintyassociated with estimating the unknown parameters of formerthickness of the landform and the ratio of ice to rock. However,progress can be made if realistic maximum and minimumvalues of shear stress are modelled using a range of appropriatevariables. For instance, the present topography of a relict rockglacier provides a minimum value for the slope gradient and ifthe landform is a protalus lobe (which many British Islesfeatures are) realistic levels of former shear stress can becalculated if a range of gradients up to the repose angle of thetalus slope are used.

As discussed above, the large debris accumulation withinCwm Bochlwyd is interpreted as a relict rock glacier whichformed due to the deformation of a glacial ice core. Thisconclusion is also supported by the evidence of estimatedformer basal shear stress. Calculation of maximum andminimum values of shear stress involved the followingassumptions:


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1. D

Cop

ensity ranged from 2250 kg m�3 (25:75 ratio of ice androck debris) to 1800 kg m�3 (50:50) ratio of ice and rockdebris).

2. T
hickness was 25 m (assuming there was a regular decline inslope beneath the maximum topographic height of thefeature).
3. S
lope gradient ranged from 108 to 158.
Values of basal shear stress are as follows: maximum142 kPa; minimum 77 kPa.

The calculated range of shear stresses at the base of the CwmBochlwyd rock glacier demonstrates that internal deformationcould not have occurred if the landform was formerlycomposed of rock debris cemented by interstitial pore ice.Instead, flow must have been aided by the existence of a largemass of laterally and longitudinally extensive ice. Thisconclusion is further supported by the occurrence of collapsepits on the surface of the rock glacier, which demonstratesdifferential thaw settlement due to the presence of large bodiesof internal ice. The origin of a mass of laterally extensive ice canbe explained by both the permafrost and glacial models of rockglacier formation (Whalley and Martin, 1992), but the positionof the Cwm Bochlwyd rock glacier within Younger Dryasglacial limits strongly suggests that the ice core was glacial inorigin. Application of this approach to the other rock glacierswithin Younger Dryas glacier limits leads to the sameconclusion.

Outside these limits, the majority of rock glaciers reported sofar can be classified as protalus lobes. There is generalagreement that these features result from the slow deformationof talus containing ice (Whalley and Martin, 1992) but there areseveral contrasting views on the nature and origin of the ice.Many researchers have advocated permafrost aggradation andsubsequent creep of interstitial pore ice to explain thesefeatures (White, 1976; Haeberli, 1985; Barsch, 1988).However, a number of authors have highlighted the need forbodies of laterally extensive ice to permit flow (Washburn,1979; Whalley and Martin, 1992), confirmed by the finiteelement modelling experiments of Azizi and Whalley (2003,Fig. 8). Lliboutry (1955) considers protalus lobes to be the resultof creep within basal ice following the burial of avalanche snowby rockfall debris as mentioned above and recent work byHumlum et al. (2007) provides examples of such rock glaciers.Alternatively, White (1981) argued that the presence of largeice bodies within talus can be attributed to the formation ofsegregation ice lenses, although it is not clear how this comesabout in freely draining rockfill on a slope. In addition, it hasbeen proposed that the formation of protalus lobes may berelated to the deformation of a core of glacial ice (Whalley,1974; Rudberg, 1986; Whalley and Martin, 1992).

The inferred presence of bodies of laterally extensive ice atthe base of relict protalus lobes is strongly supported bytheoretical consideration of the mechanics of rock glacier flow.According to Equation (1) shear stress at the point of failurewithin a rock glacier is partly a function of thickness and surfacegradient of the ice body. Although the surface gradient ofactive protalus lobes may approach the angle of repose of talus,the thickness of these features is not great enough to generatesufficient shear stress to exceed the yield strength of rockcemented by interstitial pore ice. Where estimated values of themaximum thickness of protalus lobes have been reported in theliterature, they are generally in the order of 10 m (Dawson,1977; Oxford, 1985, 1994; Wilson, 1993) and it is highlyunlikely that the thickness of any other exceeds 15 m. Byrearranging Equation (1) and assuming a density of2250 kg m�3, shear stress capable of promoting failure at thebase of a 358 talus containing 25% interstitial pore ice could

yright � 2007 John Wiley & Sons, Ltd.

only be generated if the material attained a thickness of at least23 m and at the pressure melting point. Azizi and Whalley’ssimulation suggests that very massive ice bodies must beincluded in the rock fill before flow can occur. Therefore theformation of protalus lobes in upland Britain must have beenaided by the presence of buried massive ice. If this is correctthen permafrost is not a necessary condition and debris fallscovering a relict snowbank (or small glacier ice body) may besufficient. However, it is as yet unclear what climaticconditions are required to preserve snowbank/ice bodies.

Palaeoenvironmental significance

Some research has suggested that rock glaciers are formedwhere temperatures and precipitation totals are relatively low(e.g. Haeberli, 1985; Barsch, 1988) and where precipitationincreases, ice glaciers develop. Haeberli’s (1985) modeldeveloped from rock glacier studies in the European Alps.King (1986) shows that these features form within constrainedcombinations of precipitation and temperature. Glacier iceforms outside these limits (e.g. when the mean annualtemperature falls below �28C and precipitation is higher than2500 mm per year). Problems are caused by the palaeoclimaticinterpretation of rock glaciers as indicators of permafrost andlow precipitation in northwest Scotland. Precipitation totalsduring the Younger Dryas are widely considered to have rangedfrom 3000 to 4000 mm per year (Lowe and Walker, 1984).Thus, suggestions that a relationship between low temperaturesand precipitation for rock glacier development exists, to theextent that they are useful as palaeoenvironmental indicators(Ballantyne and Harris, 1994), are untenable.

The palaeoclimatic significance of rock glaciers and protaluslobes (as well as perhaps protalus ramparts) is complicated by atleast three factors: temperature, available precipitation andavailable debris. With rock glaciers, for example:

1. T
he different genetic types of rock glaciers and protaluslobes probably reflect quite distinct environmental con-ditions but, at present, the climatic boundary conditionsassociated with each are unknown. For instance, rockglaciers formed by the burial of glacial ice by catastrophicrockfall debris may be related to climatic amelioration (andincreased cleft water pressure in cliffs), since this wouldtrigger glacier retreat, which in turn would enhance rockwall instability. Alternatively, if some rock glaciers arerelated to the creep of interstitial pore ice they reflectconditions which would encourage permafrost aggradationor the development of ice bodies sufficiently large to allowcreep. Palaeoclimatic interpretation is then complicatedsince it is very difficult (if not impossible) to determinethe origin of a relict rock glacier and at least some rockglaciers do not need permafrost to form.
2. It
is often unclear whether even active rock glaciers are inequilibrium with the prevailing climatic conditions.
3. It
is still unclear whether active rock glaciers have formedunder conditions of warming or cooling with the debris flux(where sufficiently great to accumulate) eventually limitingice melting.
Haeberli’s (1985) permafrost model outlined above may stillhave merit when applied to protalus lobes and its application toregional palaeoclimatic studies may still be valid. However, theuncertainties associated with the assessment of equilibriumconditions and the mechanical difficulties of interstitial ice



creep mean that interpretations need to be viewed with cau-tion.

With these caveats, Haberli’s suggestion of relationship ofthe distributions of protalus lobes when applied to the BritishIsles suggests that, for example, precipitation in southwestIreland during the Younger Dryas cannot have been greaterthan 2500 mm per year. Assuming this highest figure and alapse rate of 0.68C per 100 m, ‘The Bone’ protalus lobe inMacgillycuddy’s Reeks at 550 m asl implies that sea-level meantemperatures were about þ18C during the Stadial. In northWales, the Moelwyn Mawr rock glacier shows flow down toabout 400 m asl and those in the Ogwen Valley down to 350 m.Based on the aforementioned arguments, Gray’s (1982)assumption that the Loch Lomond Stadial in north Waleswas characterised by precipitation totals of about 3500 mm peryear may not be valid.

Reconciliation of these contrasting palaeoclimatic recon-structions might be possible, however, if rock glaciers andprotalus lobes developed preferentially in sites where snowfallwas easily stripped from the surface by high winds. There aresuggestions that the Loch Lomond Stadial climate in the BritishIsles may have included two distinct regimes (e.g. Bennett andBoulton, 1993): an early cool/wet phase and a later cold/dryphase. The implications of this are that high precipitation levelsduring the first half of the Loch Lomond Stadial were driven bycool and moist airstreams from the North Atlantic (Sissons,1979). This fuelled glacier development and prevented rockglacier formation since substantial winter snowfall andrelatively high summer temperatures prevented sufficientcooling of the ground. The later phase of the stadial wascharacterised by a more arid continental climate dominated bycold anticyclonic airstreams from Europe (Ballantyne andHarris, 1994). Under this extreme climatic regime intensewinter cooling of the ground aided by a thin snow covertriggered permafrost creep of debris in upland Britain andglaciers now starved of nourishment began to retreat (Bennettand Boulton, 1993).

This second phase provides a climatic mechanism for thedevelopment of relict rock glaciers in favourable locations atthe base of cliffs within Younger Dryas glacier limits (e.g. CwmBochlwyd). Their development can be outlined in a series oftheoretical stages:

1. G

Cop

laciers retreated due to precipitation starvation.
2. C atastrophic rockfalls occurred (due to glacier unloading,
permafrost melting and other processes).
3. S ubmergence of remnant glacier surfaces leading to the
development of glacier ice-cored rock glaciers (cf. Harrison,1992; Whalley et al., 1994).

While the majority of relict rock glaciers and protalus lobes inthe British Isles are believed to be Younger Dryas in age sincethey generally occur outside Younger Dryas glacial limitsand lie well within the area glaciated during the Dimlington/Midlandian Stadial (26–13k yr BP) (Ballantyne and Harris,1994), this morphostratigraphic argument may be flawedbecause there is abundant evidence to show that the progress-ive retreat of Dimlington/Midlandian Stadial ice sheetsoccurred under sustained cold climate conditions. Radiocar-bon dating of post-Dimlington Stadial lacustrine sedimentsdemonstrates that the ice sheet had vanished from Wales andnorthern England by 14.5k yr BP and had largely disap-peared from Scotland by 13k yr BP (Lowe and Walker, 1984;Jones and Keen, 1993). Deglaciation appears to have been inresponse to increased aridity since Coleoptera evidenceindicates that cool summers and intensely cold winters alsolasted until 13k yr BP (Atkinson et al., 1987).

yright � 2007 John Wiley & Sons, Ltd.

Ice core records show that periods of rapid cooling occurredthroughout the Late Pleistocene (Andrews, 1998; Alley et al.,2003) and the climatic conditions suitable for the formation ofrock glaciers and protalus lobes may have prevailed duringthese times and for a short while in some highland areasfollowing ice sheet retreat. Furthermore, the existence ofdecaying and stagnant ice masses at the base of steep slopes inprotected areas of accumulation (corries and cwms) coupledwith enhanced rates of debris supply, due to the effects ofglacial unloading and rockwall destabilisation, would encou-rage the formation of rock glaciers with a glacial ice core. Rockglacier and protalus lobe development may also have beeninitiated during cold periods of the Windermere Interstadial (ataround 12k yr BP). Conditions in southern Scandinavia at thistime were favourable for rock glacier development (Hammar-lund and Keen, 1994) and may have been so in the uplands ofBritain.

Much of the data on the climatic significance of rock glaciershas come from active features in the European Alps (e.g. Barsch,1992) and recently from North America (Potter et al., 1998;Steig et al., 1998). Most of these rock glaciers developed duringthe Little Ice Age and thus temperature/precipitation/availabledebris relate to these conditions. Although some protalus lobefeatures do appear to be of this age most pre-date the Little IseAge, although the precise age of these forms is unknown.Examples are known of Little Ice Age rock glaciers being formedand flowing through or across relict protalus lobes or acrossformer moraine sequences (e.g. Griffey and Whalley, 1979).This suggests that the formative, or at least regenerative,conditions for the former do not correspond to those of thelatter. In a situation such as the British Isles, this furthercomplicates matters, as does the apparent need for protaluslobes to have substantial ice cores in order to flow. It is possiblethat further investigation of active protalus lobes in Svalbardmight shed further light on the formation and structure of thesefeatures. Yet we still have problems in inferring former icecontent from features which are now ice free.

These findings and observations have important implicationsfor palaeoclimatic reconstruction and it is clear that moreresearch on the age of these landforms is required before anypotential for palaeoenvironmental reconstructions can berealised.

Conclusions

We use the non-genetic term ‘discrete debris accumulations’ toincorporate landforms such as protalus lobes, rock glaciers,rockfall/slide deposits that were, or might have been,associated with ice bodies during glacial and/or permafrostepisodes. Further, we distinguish between what have some-times been loosely called relict ‘rock glaciers’ and are heresubdivided into two distinct forms: rock glaciers and protaluslobes. This classification avoids the problems associated withapplying genetic classifications to relict features (such as‘ice-cored rock glaciers’) or those based on shape (which doesnot adequately distinguish between processes). The diverseideas of formation of active features are still rather confusingand contradictory. Taking what we do know of active forms torelict forms is therefore liable to misinterpretation.

Applying knowledge of active rock glacier and protalus lobesfrom the Alps and North America to inactive forms in the BritishIsles has further problems. The ice location is not always knownand either permafrost or glacially derived models may beapplicable. Furthermore, as most contemporary features



developed in response to Little Ice Age cooling the significanceof Late Holocene snow and ice versus debris fluxes combinedwith temperature regimes may not be appropriate analogues forYounger Dryas age landforms in the British Isles.

The location of rock glaciers and protalus lobes within lastglaciation ice limits suggests that these relict features developedduring the periglacial episodes of the Younger Dryas, althoughsome rock glacier development may have occurred during dry,cold periods at an earlier date. A number of rock glaciers arefound within Younger Dryas glacier limits and might beevidence for relatively low precipitation levels during at leastpart of this time. Meaningful palaeoenvironmental data fromthese landforms require that the timing of rock glacier andprotalus lobe initiation and activity be correctly dated.Unfortunately, such data are hard to come by, althoughcosmogenic isotope dating will be increasingly important.

A number of geomorphological features have previouslybeen misinterpreted as moraines, protalus ramparts androckfalls but may be relict rock glaciers or protalus lobes.Similarly, some of the landforms previously described asprotalus lobes (in the sense of this paper) may well be massiverockslope failure or landslides. Wilson’s (2004) recent changeof view of the Donegal protalus lobes/rock glaciers is a case inpoint. It is possible that there are more relict rock glaciers andprotalus lobes in the mountains of the British Isles waiting to bediscovered. Considering their prevalence in contemporaryperiglacial environments it is surprising that more have notbeen found. However, their overall paucity may be real anddue to the low weathering rates of most Caledonian rocks in thehigher hills. There may just not be sufficient debris to form thesefeatures from the mix of debris supply and snow or iceavailability. Where debris falls on to active glacier ice thenmoraines are formed; where ice flux is low or debris flux highthen rock glaciers with a glacier ice core are formed. Wheresufficient debris falls on thick snowbanks then protalus lobesmay be formed. Where there is some debris accumulation overa snowbank or incipient small glacier then a protalus rampartforms. Hence, changes in the mix of these components givesrise to different topographic forms. The complex nature of mostof the features discussed above is manifest in various ways:debris input and ice input types, rates and fluxes, localtopography aspect and altitude, maritime location as well astemperature all play important and varying roles. This widevariation in form and forcings does not appear, unfortunately,to be able to tell us a great deal about local precipitation andtemperature. For this reason, we cannot, currently, use relictdiscrete debris accumulations other than moraine sequencesand perhaps protalus ramparts, as palaeoenvironmentalindicators.

Acknowledgements The authors would like to thank the late DavidKeen for discussions on palaeoenvironments. The manuscript wasgreatly improved by the detailed reviews of Ole Humlum and PeterWilson. Vanessa Winchester, Dave Gove, Dave Passmore and TessaKingsley provided assistance in the field. We also thank John E. Gordonfor discussions and supply of photographs.

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relict rock glaciers and protalus lobes in the british isles: implications for late pleistocene...

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