Further glacier and snowbed sites of inferred Loch Lomond Stadial agein the northern Lake District, England
Peter Wilson* and Richard Clarkt
WILSON, P. & CLARK, R. 1999. Further glacier and snowbed sites of inferred Loch Lomond Stadialage in the northern Lake District, England. Proceedings of the Geologists' Association, 110, 321-331.Field mapping and evaluation of morphological and some sedimentological characteristics indicate thatthree debris accumulations in the hills centred on Knott in the northern Lake District result from glacialdeposition. Another debris accumulation is interpreted as a pronival (protalus) rampart that developed atthe downslope margin of a former snowbed. A Loch Lomond Stadial (LLS) age (11-10 k 14Cyears BP)
is inferred for all the landforms and extends distribution of LLS glaciation into a new area of the LakeDistrict. Reconstruction of the former glaciers reveals that two had larger surface areas than a substantialnumber of previously recognized LLS glaciers and estimated equilibrium line altitudes were lower thanthose for glaciers in the neighbouring Blencathra group hills. Glacier and snowbed development werefavoured by site aspects, which provided protection from direct solar radiation, and occurrence ofadjacent high ground, from which snow could be transferred by wind to contribute to glacier andsnowbed mass. It is possible that other LLS glacier and snowbed sites in the Lake District remain to bediscovered.
*School ofEnvironmental Studies, University of Ulster at Coleraine, Cromore Road, Coleraine, Co.Londonderry BT52 ISA, Northern Ireland. (email: [email protected])t Parcey House, Hartsop, Penrith, Cumbria CAll ONZ.
1. INTRODUCTION
The extent of 64 glaciers that existed in the Lake District ofnorthern England during the Loch Lomond Stadial (LLS)(11-10 k 14C years BP) was inferred by Sissons (1980) fromlocations of end (including lateral), hummocky and flutedmoraines, and boulder limits. In providing detailed maps ofthe field evidence used to delimit the glacier margins thiswork differed from earlier small-scale and schematicillustration by Manley (1959) and Pennington (1978) ofLLS glacier locations. Some sites of former glaciersproposed by Manley and/or Pennington were not acceptedby Sissons, who argued there was either an absence ofmorphological evidence or that the morainic materials therewere related to an earlier and more extensive ice cover.Sissons also identified glaciers at several locations notrecognized by Manley and/or Pennington as havingnourished LLS glaciers.
Recent studies indicate that the extent of some LLSglaciers may have differed from the limits mapped bySissons (1980). Former cirque and valley glaciers aroundthe High Raise-Ullscarf plateau have been re-interpreted byMcDougall (1995, 1997) as outlet glaciers of a plateauicefield and Evans (1997, following Manley (1959) andPennington (1978» has argued that the Grisedale glacierwas more extensive than Sissons considered it to be andalso occupied the Grisedale Tarn cirque. Moraine ridges offurther LLS glaciers have been recognized in Dovedale(Clark, 1992) and in part of the eastern Lake District(Wilson & Clark, 1998). Such findings imply that there may
Proceedings of the Geologists' Association, 110,321-331.
well be other, as yet unrecognized, LLS glacier sites andthat the full extent of LLS ice cover within the Lake Districtremains to be established.
This paper considers an additional four sites in thenorthern Lake District of inferred LLS glacial and periglacial debris accumulation. One, Nine Gills Comb, wasrecorded by Sissons (1980), but all have previously escapeddetailed study. All sites are among the hills centred on Knott[710 mOD, NY 296 330], which is separated from thenearby and higher hills of Skiddaw and Blencathra by theglaciated troughs of Dash Beck and the River Caldew (Fig.1).
The limits of three LLS glaciers and six debrisaccumulations attributed to deposition at the foot ofperennial snowbeds in the hills of the northern Lake Districtwere indicated by Sissons (1980). One snowbed deposit (inGlenderamackin Cove) was regarded by Evans (1994) asthe moraine of a LLS glacier. Three of the snowbed landforms are at Dead Crags, at the northern end of Skiddaw(Fig. 1). One of them forms an arcuate ridge at the base ofa steep slope and was thought by Sissons (1980), Ballantyne& Kirkbride (1986) and Oxford (1994) to be a protalusrampart. The other two landforms, which are terraces ratherthan ridges, were considered by Oxford (1994) to bemoraines related to activity of LLS niche-glaciers, whileWhalley (1997) suggested that one could be a relict rockglacier.
The three LLS glaciers identified by Sissons (1980) weresmall (0.13--0.45 km2) and occupied cirques on the easternand northern flanks of Blencathra group hills, their
establish its gross morphology. The frontal slope has alength of 88-108 m, a height of 25-35 m and maximumgradients of 30-32°. Beyond the crest of this slope onprofile A-B is a distinct but shallow depression 61 m wide(Fig. 3) containing peat at least 1 m thick. On profile C-Dthe depression is replaced by a gently sloping (2-4°) bench110 m wide; peat covers the bench and extends over thecrest of the frontal slope. Probings across the crest lineindicated a thickening of peat south of the crest, suggestingthe presence there of an infilled depression. A sheet ofdebris passes upslope from the depression and bench for145-190 m at a maximum gradient of 25°. These slopes areneither smooth nor rectilinear but are diversified by broadlongitudinal (downslope) ridges, between which areseepage zones or streams, and narrow transverse (acrossslope) benches. To the west of profile A-B the crest of thefrontal slope turns south to become the crest of a pronounced lateral ridge (Fig. 3) that rises from 415 m to480 m Ol) over a distance of 220 m at an average gradientof 17°. On its distal side the ridge stands 2 m above theadjacent ground.
The eastern part of the debris accumulation lacks the
Fig. 1. Glacialand periglacial debrisaccumulations in partsof theKnott and Skiddaw hills of the northern Lake District. NationalGrid co-ordinates [Grid Letters NY] are given at 2 Ian intervalsalongthe southern and western margins. Inset showsLakeDistrictlocation of researcharea.
equilibrium line altitudes (ELAs: the mean altitudes atwhich annual accumulation and ablation were equal whenthe glaciers were in equilibrium) were 502-704 m Ol), TheLLS glacier that occupied Glenderamackin Cove had anarea of 0.112 km2 (Evans, 1994) and an ELA of c. 500 mOf). Dimensions and ELAs for the proposed LLS glaciers atDead Crags have not been estimated (Oxford, 1994).
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Fig. 2. Debris accumulation in Nine Gills Comb, Little Calva.Explanation of symbols for this and other site diagrams: I, extentof debris accumulation (whether glacial or periglacial) showingsurveyed profilelines(A-B, etc),andridgecrestlinesandsummits.Solidlinedefining marginof debrisrepresents sharpbreakof slope,broken line indicates a less-clear margin. 2, crest of headwall. 3,low cliffs. 4, gully and alluvial fan. 5, watercourses. Contours inmetres.
1 kmj,Landforms mapped by
Sissons (1980) andOxford ( t 994)
Conlours (m)
~ Water courses
2. SITE AND LANDFORM DESCRIPTIONS
Nine Gills Comb
Nine Gills Comb [NY eastings 27/28 northings 32] issituated on the northern slopes of Little Calva (642 mOD)(Fig. 1). The extent of the debris accumulation is shown inFig. 2. Along its northern (downslope) and western marginsthe debris forms a sharp break of slope with the adjacentground, but its southern (upslope) and eastern margins areless clearly defined and were mapped using a combinationof gradient changes, vegetation changes, and presence!absence of bedrock outcrops. Debris area is estimated as c.0.17 km2 and has an elevation range from 385-530 mOD.Two profiles, locations shown in Fig. 2, were surveyedacross the western half of the debris accumulation to
GLACIER AND SNOWBED S ITES, LAKE DISTRICT 323
Fig. 3. The Nine Gills Comb debris accumulation seen from the east. The depressionlbench area is indicated by Ihe lower arrow and thelateral ridge definin g the western margin of the debris is indicated by the upper arrows. Foreground gullies are up to 10m in depth.
Fig. 4. Debris ridges north of Great Cockup. Symbols as for Fig. 2.
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distinct depression and bench morphology found to the westbut is differentiated by gradient changes into both concaveand convex slope facets. Debris extends to a higherelevation in this part of the cirque and there are numerousgullies cut by the principal stream and its tributaries (Figs 2& 3). Some gullies extend to the underlying Skiddaw Groupbedrock and show debris thicknesses of up to 10m. In theseexposures and another on the frontal slope of the bench,immediately west of profile C-D, the debris is poorly sortedand matrix-supported; most clasts are either angular orsubangular with marked edge-rounding and all are ofSkiddaw Group lithologies. Clasts with cross-cutting striaeare present at both locations. Grain-size analysis of the<2 mm fraction of two samples, one from the frontal slopeof the bench, the other from near the base of a gullyexposure in the eastern part of the debris accumulation,revealed a composition of 44-49% sand, 35-37% silt and16-19% clay.
Great Cockup
Great Cockup [526 mOD; NY 27 33] is an east-westtrending ridge c. 1 km north-northwest of Nine Gills Comb(Fig. 1). Two debris ridges occur at the foot of the steepnorth-facing slope of the ridge (Figs 4 & 5) are here referredto as the outer ridge (northernmost) and inner ridge(southernmost). Both possess a few shallow « 50 em)exposures in which all the clasts are of Skiddaw Grouplithologies and some are striated. At their upslope ends the
324 P. WILSON & R. CLARK
Fig. 5. Debris ridges north of Great Cockup seen from the southeast near Trusmadoor. Crest lines are indicated; inner ridge to left, outerridge to right.
ridges merge into the adjacent hillsides, the 'attachment'points being poorly defined.
The outer ridge projects downvalley for c. 200 m fromthe foot of the northwest slope of Meal Fell. It taperssharply and the crest line profile is stepped. With theexception of part of the proximal (southern) slope, fromwhich debris may have slumped, the ridge makes a sharpbreak of slope with the surrounding ground. From twoprofiles, locations shown in Fig. 4, surveyed across theridge the distal slope is 60-70 m in length, 28-32 m inheight, and has maximum gradients of 33-35°. Theproximal slope is 62-82 m in length, 15.5-18.5 m in height,and has maximum gradients of 20-26°. The ridge isasymmetrical in section; the distal slope is shorter, higherand steeper than the proximal slope. Ridge basal width is110-142 m.
The inner ridge is arcuate in plan and projects for c.370 m from the slopes of Great Cockup. Three summitsoccur along the undulating crest line. Basal breaks of slopeare sharp except along part of the proximal slope. Surveyedprofiles give a distal slope length of 60-64 m, a height of19-24 m, and maximum gradients are 28-31 0. Theproximal slope is 51-58 m in length, 7-17 m in height, andmaximum gradients are 12-22°. This ridge is clearly asymmetrical; the distal slope is longer, higher and steeper thanthe proximal slope. Ridge basal width is 109-112 m. Onprofile A-B the crest of the ridge is situated 110m from the
foot of the steep slope of Great Cockup, the correspondingdistance on profile C-D is 143 m. The inner ridge is longerthan the outer ridge and has generally shorter, lower and lesssteep slopes, and a narrower basal width.
South of the ridges relatively smooth vegetated slopesrise at average gradients of 27-28° towards the summit ofGreat Cockup. Bedrock outcrops are absent but severalprominent drainage lines are evident.
Dale Beck - Yard Steel
Yard Steel [NY 29 34] is a northern spur of Knottfalling into the valley of Dale Beck (Fig. 1). On the eastside of the spur a shallow embayment extends across-slopefor c. 300 m. The upper slopes of the embayment are steep(c. 30-40°) with a vegetated thin soil except where bedrockoutcrops occur. In contrast the lower slopes between 350 mand 420 m OD are mantled by an accumulation of debriswhich, at broadest, extends for c. 200 m both downslopeand across-slope and has an area of 0.029 km2 (Figs 6 & 7).The downslope margin of the debris has been eroded byDale Beck into a marked 7 m high scarp sloping at 34°.Above this scarp less steep (1-10°) slopes rise for 30 m to aridge crest with a 12 m long, 6° backslope that leads to apeat-filled depression 30 m in width. Maximum peatthickness is unknown but exceeds 1 m; the depth of thedepression below the ridge crest is at least 2.5 m. Upslope
from the depression a debris-covered slope with concaveand convex facets rises at gradients of 8-26° and terminateswhere the gradient increases to >30° and bedrock crops out.The lateral margins of the debris accumulation are definedby broad (60-70 m wide) ridges of which the southern ridgerises higher into the embayment than the northern ridge.Both ridges are asymmetrical: the northern has a 13° outerslope 6.5 m high and a 4.so inner slope 2.5 m high; thesouthern has a 4° outer slope 1-2 m high and an 8.5° innerslope 5 m high.
Although the northern extremity of the debris is indicatedon Fig. 6 as an integral part of the feature it is some 2-3 mlower than the continuation of the frontal ridge to the southand may consist of material of different derivation: there areno exposures. Scattered across and projecting from thelower part of the debris accumulation are BorrowdaleVolcanic Group boulders; the Yard Steel embayment iswithin the Borrowdale Volcanic Group outcrop. TheSkiddaw Group and Carrock Fell Complex crop out to thesouth but boulders of those lithologies have not beenrecorded on the Yard Steel debris accumulation.
Dale Beck - Clints Gill-Blea Gill
Clints Gill and Blea Gill [NY 30 34] are headwater tributaries of Dale Beck (Fig. 1) between which at 440-465 mOD a pronounced hillside accumulation of debris occurs(Fig. 8). The debris forms a northwest-facing cross-sloperidge, with an undulating crest 140 m in length, that risesfrom southwest to northeast. At its southwest end the ridgehas been modified by the dumping of spoil from mine
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326 P. WILSON & R. CLARK
Fig. 8. Debris accumulation between Clints Gill and Blea Gill, DaleBeck. Symbols as for Fig. 2.
workings; at its northeast end the ridge terminates in a 24°slope 7 m high (Fig. 9). Three profiles, locations shown inFig. 8, surveyed across the ridge show it to beasymmetrical: the distal slope has a length of 44-59 m, aheight of 18-22 m and maximum gradients in the range24-32°; the proximal slope is substantially shorter (7-9 m)and lower (1-2 m) with maximum gradients in the range
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7-18°. On profile C-D there is no proximal slope becauseinfill by alluvial fan debris from an upslope gully hascreated a bench; fan debris extends to the northeast andpartially infills the backing depression. The horizontaldistance between the ridge crest and the foot of the backingslope is 9.5-11 m.
3. DISCUSSION
Origin and age of the debris accumulations
For several independent reasons three of the debrisaccumulations (Nine Gills Comb, Great Cockup and YardSteel) are regarded as products of glacial deposition:
1. First, clasts with cross-cutting striae occur within thedebris at Nine Gills Comb and Great Cockup.
2. at those sites exposures show the debris to be poorlysorted and matrix-supported with angular and subangular edge-rounded clasts. These characteristics aregenerally taken as indicative of subglacial transport(Benn & Evans, 1998);
3. at Nine Gills Comb and Yard Steel there are distinctlateral ridges joining the frontal ridges of the debrisaccumulations; these are interpreted as lateral and endmoraine ridges respectively;
4. at each location straight-line profiles drawn from thecrest of the end moraine to the crest of the hillside abovethe debris indicate that the maximum thickness of snow/ice accumulation would have exceeded that normallyregarded as the threshold range (i.e. c. 20--30m; Gray,
Fig. 9. The Clints Gill-Blea Gill debris accumulation seen from the northeast. The cross-section asymmetry, rush-filled depression andproximity to the backing slope are evident.
GLACIER AND SNOWBED SITES, LAKE DISTRICT 327
1982; Shakesby, 1992) for the conversion of firn toglacier ice (see below);
5. there is no morphological evidence at any of the sites forthe former occurrence of large-scale landslide activitythe products of which have occasionally been mistakenfor glacial debris accumulations (cf. Rowell & Turner,1952; Mitchell, 1996; Shakesby & Matthews, 1996).
Taken in combination the available evidence suggests localglaciers caused debris emplacement.
Its morphology, location and close proximity (9.5-11 m)to the foot of the backing slope suggests the Clints GiIIBlea Gill debris ridge is a pronival (protalus) rampart thatdeveloped at the downslope margin of a former snowbed(firn field) (cf. Shakesby, 1997) rather than an accumulationof glacial (end or lateral moraine) or landslide debris. Evenallowing for alluvial fan construction and sedimentaccumulation by slopewash along the base of the backingslope subsequent to melting of the snowbed, the ridge-crestto backing-slope distance is unlikely to have exceeded c.20--25 m during rampart construction and the maximumthickness of snow is estimated to have been in the range10--15 m. Based on glaciological modelling of the thresholdconditions beyond which a snowbed undergoes significantmovement as a small incipient glacier (Ballantyne & Benn,1994) these values imply that the former snowbed did notundergo transformation to glacier ice but remainedstationary for a time after reaching its maximum dimensions. Therefore the ridge cannot be considered as an endmoraine. A lateral moraine origin is rejected because theridge is transverse to the likely surface slope of a valleyglacier occupying Dale Beck valley. The absence of a slopefailure scar on the backing slope and the lack ofirregularities on that slope and within the debris do notfavour interpretation of the ridge as a landslideaccumulation.
There is no direct evidence for moraine and rampart age.Development during the LLS is inferred because theirmorphology and position indicate debris transport awayfrom the backing slopes and such accumulations areunlikely to have been produced in association with theback/down-wasting Dimlington Stadial (DS) (26-13 k 14Cyears BP) ice sheet. Furthermore no evidence for an advanceof local glaciers between removal of DS ice and the start ofthe LLS in the Lake District has yet been presented. Aglacial advance and construction of associated moraines andperiglacial snowbed landforms during the LLS is regionallywell-documented through both geomorphological andbiostratigraphical evidence (Pennington, 1978; Sissons,1980; Oxford, 1985, 1994; Wilson & Clark, 1995; Mitchell ,1996) and therefore it is reasonable to infer that themoraines and rampart described here also developed at thattime.
Proposition of an LLS age for these landforms extendsdistribution of LLS glaciation into a new area of the LakeDistrict (cf. Manley, 1959; Pennington, 1978; Sissons,1980). Although the Nine Gills Comb debris accumulationwas indicated on Sisson's fig. 3 he categorized it as a
snowbed rather than a glacial landform . An LLS glacialorigin for the features has not previously been suggested .This may be due to the location of the hills centred onKnott, on the northern edge of the Lake District uplands, thealmost total lack of cirques, the relatively low elevation«710 m aD) and a presumption that the area receivedinsufficient stadial precipitation, such that it was consideredan unsuitable location for the generation of LLS glaciersand snowbeds. In addition, if these landforms had beenpreviously considered, their small areal extent may have ledto the belief that they could not be associated with LLSglaciers . However, reconstruction of the former glaciersdemonstrates that two of the ice masses had a greatersurface area than several of those reconstructed by Sissons(1980) (see below).
Freshness or clarity of form has also been used byprevious researchers to identify LLS moraine ridges in theLake District and it is possible that the landforms describedhere were seen but disregarded by Sissons (1980) becausethey failed to conform to his criteria defining 'freshness'. Inspite of all three glacial accumulations meeting Manley's(1959) 'freshness' definition he also failed to depict themon his map of LLS glaciers . Following work by Benn(1992) and McDougall (1997) demonstrating that LLSmoraine morphology results from a series of sedimentological and glaciological influences such as debris supplyvariations, glacier thermal regime, ice margin dynamics,and modifications resulting from meltwater erosion andparaglacial processes, operating during both deposition andice removal, Wilson & Clark (1998) drew attention to thesubjective way in which the term 'freshness' had beenapplied to Lake District moraines and suggested its pastapplication probably resulted in underestimates for theextent of LLS ice cover.
Reconstruction of the former glaciers
Determination of the boundaries of the three LLS glaciers attheir maximum extents was based, in part, on the positionsof end and/or lateral moraine ridges . Mapped lines definingthese ice margin positions and continued to the crest of theheadwalls delimit the outlines of the former glaciers (Fig.10). This method differs slightly from that used in previousreconstructions of LLS and earlier small glaciers in GreatBritain (e.g. Gray, 1982; Shakesby & Matthews, 1993;Harrison, Anderson & Passmore, 1998), for which theglacier upper margin was assumed to have been c. 30 mbelow the headwall crest. However, the approach isconsistent with the extent of some present-day smallglaciers in northern Sweden that descend from the headwallcrest (Rapp, Nyberg & Lindh, 1986) and was recognized bySissons (1980) as having probably occurred with respect tosome small LLS glaciers in the Lake District. Glacier areaswere estimated from these limits and, following Sissons(1974), the ELAs were determined applying the method ofarea-weighted mean altitude (AWMA), based on measurement of glacier surfaces between successive 20 m contours.The ELA was used to derive the accumulation-area ratio
328 P. WILSON & R . CLARK
Fig. 10. Reconstructions of LLS glaciers for: (a) Nine Gills Comb ,(b) Great Cockup , (c) Yard Steel. Contour interval is 20 m.
(AAR: the proportion of glacier area above the ELA) foreach former glacier. ELAs were also estimated based onAARs of 60±5% and, where present, the altitude of thehighest lateral moraine (cf. Evans, 1994; Mitchell , 1996).Selected characteristics of the former glaciers based on fieldsurveys and reconstructions are given in Table 1.
The largest (by area) and thickest of the former glacierswas nourished at the Great Cockup site followed by that inNine Gills Comb (Table 1). Respecti vely these glacierswere larger than 26 and 18 of the 64 LLS glaciersreconstructed by Sissons (1980). In contrast, the Yard Steelglacier was fractionally smaller than the smallest glacieridentified by Sissons but slightly larger than the smallestbody of LLS glacier ice subsequently recognized in theLake District by Wilson & Clark (1998) . Of the four LLSglaciers in the northern Lake District reconstructed bySissons (1980) and Evans (1994), only that in Bannerdalewas larger than the Great Cockup and Nine Gills Combglaciers (Table 1). Thus, these two former glaciers were notinsubstantial by Lake District standards.
Estimates of ELAs of LLS glaciers have been usedfrequently in Great Britain to draw palaeoclimaticinferences (e.g. Sissons, 1980; Corni sh, 1981; Gray, 1982;Ballantyne & Kirkbride , 1986; Ballantyne, 1989; Shakesby& Matthews, 1993; Wilson & Clark , 1995; Mitchell , 1996).The range of ELAs for the three former glaciers is 396492 m 00 (Table 1), within the range of values (284-785 m00) derived by Sissons (1980) for 64 LLS glaciers in theLake District. Therefore the regional palaeoclimaticparameters establi shed by Sissons do not require revision.However, the range of ELAs for the three former glacierslies entirely below that for the three former glaciers in theneighbouring Blencathra group hills (502- 704 mOD;Sissons, 1980) (Table 1). This suggests that topographyexercised a stronger influence on glacier development at thesites described here, which lie in an area of less precipitation and lower hills; the sites were probablyparticularly well placed to receive wind-blown snow fromadjacent areas. Glacier accumulation-area aspects rangefrom northwest to northeast (Figs 1 & 10): therefore thesites were protected to some degree from direct solarradiation during the ablation season. Furthermore eachaccumulation area is backed by gently-sloping high groundproviding ideal terrain across which the prevailing southerlywinds (Sissons , 1980) could have transferred additionalsnow to the sites, so making a significant contribution toglacier and snowbed mass. Of the three glacier sites thelargest extent of such high ground is above Yard Steel (Fig.1) but this site nourished the smallest glacier. A winddirection from slightly west of south would have been themost effective for snow transfer to that site. The smallestextent of high ground is above the Great Cockup glacier yetthat was the largest of the former glaciers . For snow transfer
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GLACIER AND SNOWBED SITES, LAKE DISTRICT 329
Table 1. Selected characteristics of LLS glaciers in the northern Lake District
Glacier Area Thickness Gradient AWMA-ELA AAR AAR(60±5%)- Highest lateral(km-) (m) CO) (mOD) (%) derived ELA moraine
Yard Steel 0.045 32 25 419 45 382-396 380Scales Tarn * 0.14 nr nr 704 nr nr None
Glenderamackin Cove* 0.112 40 26 c. 500 nr nr None
Bannerdale* 0.45 115 14-19 502 nr nr 520Bowscale Tarn* 0.13 95 17 538 nr nr None
AWMA Area-weighted mean altitudeELA Equilibrium line altitudeAAR Accumulation-area rationr not reported
data from Sissons (1980) and Evans (1994)
600 .,..--------------------,
Fig. 11. Area-altitude curves for the LLS glaciers: (a) Nine GillsComb, (b) Great Cockup, (c) Yard Steel.
100806040
Cumulative Area (%)
20
350
550
piedmont-glacier hypsometry, with steep but small uppersections and flatter, larger lower sections, for which anAAR of c. 50% or lower is more appropriate. Areaaltitude curves for the three former glaciers approximateto such hypsometry (Fig. 11).
500
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400
1. Alternative estimates of ELAs based on AARs of 60±5%and listed in Table 1 must be regarded with caution,particularly with respect to Nine Gills Comb, becausethere the range of values (46D-473 m aD) does notincorporate the highest altitude (480 m Off) of the lateralmoraine ridge. Such ridges are constructed only in theablation area (i.e. below the ELA) and their upper limitsthus provide an estimate of the ELA (Osmaston, 1975;Benn & Evans, 1998).
2. As noted by Evans (1994), an AAR of c. 50% may bemore appropriate for small glaciers which receive asubstantial part of their mass from wind-blown snow thataccumulates near the top of the glacier and serves tothicken the 'wedge of net accumulation' .
3. Leonard (1984) has shown that small glaciers may havean area-altitude distribution approaching that of
to both the Great Cockup and Nine Gills Comb glacierswind directions of south and southeast would have been themost effective and as these glaciers had surface areas anorder of magnitude greater than the Yard Steel glacier it isreasonable to assume that those winds were the dominantones. If that were so then snow was moved to the GreatCockup glacier not only from the ridge of Great Cockup butalso from the western ridges of Knott, and the col ofTrusmadoor probably functioned as an important passagefor snow movement. This indicates that snow transferred bywinds derives not only from ground sloping towards andconnecting with glacier surfaces but that upslope andacross-slope snow movement may be important (cf.Shakesby & Matthews, 1993; Wilson & Clark, 1995).
Although the AWMA-ELA derived AAR values(45-51 %) for the three former glaciers are lower than thosefrequently determined or assumed (c. 60±5%) for latePleistocene glaciers elsewhere (e.g. Meierding, 1982;Leonard, 1984; Murray & Locke, 1989; Dahl, Nesje &0vstedal, 1997) they are regarded as best estimates for threereasons.
330 P. WILSON & R. CLARK
The size and distinctiveness of the end and lateralmoraine ridges imply that the former glaciers had stablemargins for a considerable time when at their maximumextents. This is consistent with the findings of Sissons(1980) that some of the small LLS glaciers in the LakeDistrict produced some of the largest end moraines (e.g. thenearest such case to sites described here being the BowscaleTarn glacier and its moraine). The absence of recessionalmoraines at Nine Gills Comb and Yard Steel indicates thatice wastage was not interrupted by any significanttemporary halts or slight readvances. In contrast, at GreatCockup the inner ridge is interpreted to mark a phase ofice margin halt or readvance. The inner ridge is locatedc. 130-170 m behind the outer ridge, a distance that is nottoo dissimilar from the 100-150 m range noted by Sissons(1980) within which recessional moraines of Lake DistrictLLS glaciers are typically found. Why moraine morphologydiffers between the three sites is difficult to establish butwas probably controlled by a combination of thesedimentological and glaciological factors discussed above(Benn, 1992; McDougall, 1997).
4. CONCLUSIONS
Three of four debris accumulations mapped in the hills ofthe northern Lake District are considered to be the products
of glacial deposition during the LLS. The other debrisaccumulation is regarded as a pronival (protalus) rampartalso of LLS age. Reconstructions of the former glaciersdemonstrate that two of them were larger than a substantialnumber of the 64 LLS glaciers reconstructed by Sissons(1980) while the third was one of the smallest bodies of iceto have developed during the stadial. Topographic factors(site aspect and adjacent high ground) along with thecontribution of wind-blown snow adding to glacier masswere probably important controls in glacier nourishment.
The inferred LLS age for these landforms extendsdistribution of LLS glaciers and snowbeds into a new areaof the Lake District and raises the possibility that furthersites exist and remain to be discovered. As with the sitesdescribed above, northerly aspects of other hills on themargins of the area may have provided suitable locationsfor small ice and snow masses and warrant closeinvestigation.
ACKNOWLEDGEMENTS
Thanks are due to Kilian McDaid and Mark Millarfor diagram preparation and Nigel McDowell forphotographic processing. Comments by Colin Ballantyneand Stephan Harrison on an initial version of the paper areappreciated.
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Manuscript received 28 October 1998; revised typescript accepted 21 January 1999
