A small glacial cirque basin on Exmoor, Somerset

Stephan Harrison*, Ed Anderson" and David G. Passmore"

HARRISON, S., ANDERSON, E. & PASSMORE, D. G. 1998. A small glacial cirque basin on Exrnoor,Somerset. Proceedings oj the Geologists' Association, 109, 149-158. Geomorphological andsedimentological investigations at the Punchbowl on Exmoor, north Somerset, have demonstratedevidenceof cirque basin glaciation, some30 km south of the accepted southernlimit of Anglian ice coverin this area. The Punchbowl is a steep-sided, north-facing basin that has a subdued arcuate ridge at itsmouth. Analysis of rnacrofabrics, clast form and roundness and particle size on sediments forming theridge indicate it to consist of glacial diamict deposited as a terminal moraine. Further evidencesupportingthe presence of a formercirque basin glacier at this site is derivedfrom basin morphologyanda striated sandstone boulder. Palaeoglacierreconstruction gives a surface area of 0.38 km-, a maximumthickness of 46.5 m and a maximum basal shear stress value of 51 kPa. The ELA of the former glacierhas been calculated at 334 mOD. There is no evidence for multiple glaciations at this site, and, in theabsence of dating controls on the moraine, the glacier is provisionally assumed to have formed duringsevere climatic conditions associated with the southerly advance of ice sheets during the Anglian coldstage or during the Dimlington Stadial. Developmentof a glacier at this location appears to have been arare event during the Quaternary, and may have been facilitated by accumulation of windblown snowfrom the adjacent plateau.

'Centre jor Quaternary Science, Coventry University, Priory Street, Coventry CVI 5FB."School oj Geography, Middlesex University, Queensway, Enfield EN] 4SF.tDepartment oj Geography, Newcastle University, Tyneside NEI 7RU.

1. INTRODUCTION

At present, there is general agreement that the southernmostland-based glacial ice in the British Isles during theQuaternary was the Anglian glacial limit which runs fromthe northern coast of the Southwest Peninsula to thenorthern part of the Thames estuary (Bowen, Rose, McCabe& Sutherland, 1986; Jones & Keen, 1993). However, evid­ence is presented here of cirque basin glaciation in thePunchbowl on Exmoor, Somerset to the south of this limit.Straw (1995) has also highlighted the possibility that thissite may have contained a small glacier; he wrote that, 'if it(the basin) had reached a size sufficient for occupation by apermanent snow patch within which some internaldeformation was possible, it is, perhaps, Exmoor's soleinstance of an incipient glacial corrie' (p.22). However,Straw (1995) did not present any evidence to support thisidea. In this paper, detailed geomorphological andsedimentological evidence demonstrate the presence offormer glacial ice at this location. From this evidence it ispossible to reconstruct the dimensions of the former glacier,its basal shear stress and the Equilibrium Line Altitude(ELA).

2. DESCRIPTION

The Punchbowl is an impressive north-facing basin which iscarved into the broad plateau of Winsford Hill (426 mOD)(Figs 1,2 and 3). It is located at SS 883 345, some 2 km duewest of the village of Winsford. The area is underlain by

Proceedings of the Geologists' Association. 109, 149-158.

Upper Devonian Morte Slates and sandstones, and the silt­stones and slates of the Pickwell Down Beds; the junctionbetween these formations lies some 600 m north of thesouthern rim of the Punchbowl (Geological Survey ofEngland and Wales 1969, Sheet 294, Dulverton). The floorof the basin is at 300 m OD and the steep backwall rises to400 m 00 and is set at an angle up to 70°. Bedrock isexposed at several places on the backwall. The basin is upto 350 m wide and its downslope edge is marked by asubdued arcuate ridge up to II m high and 40 m wide whichhas been incised by a small stream (Fig. 4). This ridge is110 m in distance from the base of the backwall. The basinfloor is covered with thick vegetation and bog peat and thebasin sides are also vegetation covered.

3. METHODOLOGY

Geomorphic and sedimentological methods were used todetermine the origin of the ridge at the front of the basin andthe debris cover which mantles the adjacent plateau andvalley sides. Detailed geomorphological mapping of thePunchbowl (see Figs I and 4) was carried out usingenlarged base maps reproduced from the 1:10 000 OrdnanceSurvey map. The map was redrawn, scanned and digitized.Slope gradients were measured in the field using aclinometer and topographic profiles constructed from theI: 10000 map.

Five sites were selected to investigate the composition ofthe basin ridge and the plateau and valley-side debris

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Fig. L Geomorphological and location map of the Punchbowl. Somerset. The valleys 10 the west of the Punchbowl are also shown.

Fig. 2. The Punchbowl from the north.

GLAC IAL C IRQUE BASIN , EXMOOR

Fig. 3. The arcuate ridge from the back wall of the Punchbowl.

151

cove rs . At Site 1 (Fig. 4) a hand-du g pit meas uring 1.8wide x 1.5 long x 1.2 m in depth was excava ted into thesouthwe st-facing slope of the northwestern segment of thebasin ridge . The depth of the excavation was necessary toclear the wea thered profil e. Site 2 is a small naturalexposure on the western side of the southeastern segment ofthe basi n ridge some 1.5 m deep an d 1 m wide and islocated approximately 4 m below the ridge crest. Site 3 is astream-cu t section incised into the valley-side debris coverapproximately 250 m downstream of the frontal ridge and1.5 m above the present river level. It runs some 8 m alongthe valley floor and is up to 2.3 m deep. Site 4 is an exposedsect ion cut into the sediments on the plateau surface ofWinsford Hill , 100 m southwest of the top of the Punchbowlbackwall. It reaches a maximum of 1 m in depth. Site 5 is anatural ex posure cut into the ridge by the small river and is1.2 m deep and 2.5 m wide.

In order to assess the depo sitional origin of the sedimentexposed at each site the following standard techniques wereapplied: macrofabr ic analysis, clast form and roundness andpart icle size analys is. Macrofabric analysis was undertakenat Sites 1-3 on 50 clasts with a:b length ratios of 2:1 ormore. These sites were chosen to isolate potential fabricsformed by direct subglacial deposition and to be unaffectedby post-dep ositional modification . Therefore, Sites 1 and 2are situated on the stoss side of the basin ridge, as opposedto the lee slopes where sediments would have undergonerapid re-sedimentation by mass-wasting processes. For eachdata set three-dim ensional macrofabric diagrams wereco mputed using the procedure developed by Andrews &Shimizu (1966) and normalized eigenvalues (S t) were

calculated at a co nfidence int erval grea ter tha n 95 %following the technique of Mark (197 3).

Random samples of 50 clasts in the size range 25­100 mm were selected for clast form and rou ndnessanalyses at all five sites. Two samples were taken at Si te I(samples Ia and I b). Clas t shape was port rayed by plott ingc:a and b:a ratios on ternary diagrams and quantitative lyanalysed by the method outlined by Ballantyne (1982).Roundness values were calculated using Powers' ( l953)roundness charts. Particle size anal ysis of the finer than2 mm diameter fraction was conducted on samples from allfour sites using a combination of wet sieving (>63 urn) andsedi graph analysis « 63 urn). In addition , the sedimentprofile exposed at Site 3 was logged using standardlithostratigraphic techn iques.

4. RESULTS

Morph ometric evidence sugg esting significant erosion ofthe Pun chbowl includes the steep back wall and enclosedbasin shape of the hollow. To the wes t, and also incise d intoWinsford Hill are four other north ward-facing tributaryvalleys . Each of these are cut into the Morte Slates andPick well Down Beds; the latter rocks underlying theirheadwaters. These valleys are described below.

Riscombe Combe

Th is forms a semi-circular headed basin 300 m to the north ­wes t of the Punchbowl. Th e floor of the combe is smoothand rises gently from 270 m 00 to 300 m OO . Th e

152 S . H ARR IS O N ET AL .

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•Site 4

:?) moraine ridge ) rockwall .::1~ marsh

~ stream cutting Q trees -- ..10 - contours in metres

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headwalls of the combe are steep (average angle of 32°) androunded and rise to 370 m Ol) . The basin is up to 200 m inwidth and 350 m long. There is no stream occupying thefloor at the present time.

rises at a spring at 350 m Of) and the mouth of the combelies at about 280 m Ol) . The combe is about 300 m in lengthand 100 m wide at its widest part. Some 250 m further westis Ash Combe.

Unnamed combe

This combe is located 400 m due west of Riscombe Combe.Its morphology is less rounded than Riscombe Combe andthe floor is occupied by a small stream. The head of this

Ash Combe

This basin is occupied by a stream and the smoothed.rounded basin form displayed by both Riscombe and theunnamed combes is disrupted at Ash Combe by headward

Slabs

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Fig. 5. Clast form and roundness diagrams from the five sites.

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154 s . H AR RIS O N tcr AL.

0.0 -r----.,incision by the stream. As a result, the headwall is cut bytwo distinct stream channels. The headwall rises to 350 m00 and the combe is 400 m in length and up to 200 rn wide .Little Ash Combe lies 550 m to the west.

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0.2/ topsoil

Little Ash Combe

This basin , the most westerly of the four, also has the leastrounded headwall; this and the sides having been incised bystream erosion. The headwall rises to 350 m 00 and themouth of the combe is at 300 mOD, some 350 m to thenortheast. The combe is occupied by a small stream and isup to 100 m in width.

The geo morphology of the Punchbowl is shown in Figs Iand 4. The arcuate ridge completely encloses the basinmouth except where the stream has incised through it toform a v-shaped notch. There are no exposures of bedrockin the ridge . The sediments exposed at Site 2 near the baseof the ridge and Site I near the crest form a poorly-sorted,silty clay diamict with numerou s clasts. Elongate clastsdisplay a pronounced preferred orienta tion with their a-axespointing in a southwest (upslope) direction. Clasts fromthese sites have c:a ratios of 0.4 or less of 82% and 76%(Site I) and 78% (Site 2) and they are subrounded torounded (mea n roundness is 0.37, 0.38 and 0.38 respec­tively) (see Fig. 5). The clasts at Site 3 show a well­developed downslope macrofabric and are predominantly'slabby' in shape (98% have c:a ratios sOA ). The clasts aremainly angular or very angular (mean roundness of 0.2). Asectio n log from here (Fig. 6) shows 0.5 m of highlycontorted sediments containing angular clasts overlying asimilar, though uncontorted, unit some 1.2 m deep. Similarroundness and cla st form values are obtained from Site 4 (%c:a s O.4 is 98% and mean roundness of 0.2 1 (angular tosubangular) although it was not possible to obtainmacrofabrics from this site.

Differen ces between the sites are even more markedwhen using the percentage of c:a rat ios sO.3 as adiscrim inator. In this case, the samples from Sites I and 2on the arcuate ridge have c:a ratios of sO.3 of 50, 46 and46%, which compares with 98% for both of the samplestaken from sites above the Punchbowl (Site 4) anddownslope of the ridge (Site 3). Differences in roundness ofclasts between the ridge sites and those outside the ridge aresignificant at p=O.OOl. Particle size analysis of the finerthan 2 mrn frac tion does not distinguish between thesediments (Fig. 7); percentages of silt and clay lie between14 and 24%.

5. INTERPRETATION

The deposition al facies characteristics outlined aboveclea rly demonstrate distinct sedimentological contrastsbetween Sites I & 2 and Sites 3 & 4 which are probablyrelated to differences in origin. It is proposed here that thesediment exposed at Sites I and 2 is a glacial diamictdeposited at the margin of a small cirqu e glacier situated

involuted angulargrave ls and cobbles withsilty sandy matrix

angular gravels and cobb leswith silty sandy matrix

sa mple site

strea m channel

Fig. 6. Section log from Site 3.

within the Punchbowl basin. In contrast, the debris coverwhich mantles the adjacent plateau and valley sides isconsidered to be periglacial in origin. This interpretation isbased on the following two arguments .

\. The Punchbowl has a constructional arcuate ridge at itsmouth ; this is composed of matrix-supported diamictwith numerous subrounded and rounded clasts. Similarsedimentary constru ction al landfo rms are frequentfea tures at the ablation margins of glaciers in contem­porary settings, where the presence of rounded particlesmay be explained by the abrasion of clasts transportedalong subglacial pathways (Boulton, 1978). Thereforethis feature is interpreted as a term inal/l ateral moraineridge. In contrast, the adjacent slopes and plateau areasare mantled by a cover of clast-supported angular gravelsand cobbles with interstiti al fines. Significantly, theupper zone of this debris mantle is highly involuted (seeFig. 6). Consequently, this debris mantle is interpreted as

G LAC IA L C I R Q U E B A SIN . E X MOOR

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the product of prolonged perig lacial weathering ofregolith and ill situ cryoturbation stresses .

2. Macrofabric analyses from the ridge diamict demon­strate that the component clasts possess strong fabricsrelated to the highly orientated clustering of a:b planesparallel to the direction of flow (Fig. 4). Normalizedeigenvalues are compatible with subglacial till fabricsrecorded at contemporary glacial margins (Dowdeswell& Shar p, 1986; Hart, 1994; Benn, 1994, 1995) and areinterpreted as having been formed by subglacial deform­ation of diamict during the accretion of the moraineridge. The contrast in eigenvalu es between Sites 1 and 2may be related to differences in shear strength and strainrate of the diamict during the final stages of subglacialdeformation (cf. Dowdeswell & Sharp, 1986; Hart,1994; Benn, 1995). The sediment profile at Site 3 alsopossesses a strong clast fabric, but since the a:b planesare orientated downslope at dip angles parallel to theslope gradient, the macrofabric is interpreted to haveformed in response to solifluction activity. Furtherevidence supporting the development of glacial ice atthis site is demonstrated by the presence of a largesandstone boulder of local origin with a facetted andstriated face which is resting against the northern side ofthe wall about 5 m to the west of the stream (Fig. 8) onthe proxim al side of the ridge.

The depositional characteristics of the basin ridge demon­strate a glacial origin. However, its shape and positionwithin the basin could encourage speculation that it is aprotalus rampart. Indeed, Straw (1995) proposed this as apossible explanation for the basin ridge. Relict protalusramparts and moraine ridges can be notoriously difficult todifferentiate (Shakesby & Matthews, 1993). Ballantyne &Kirkbrid e (1986) proposed several criteria to facilitate theirdistinction. The points most relevant here are that protalusramp arts are always located at the base of a talus slope andthe rampart crest is usually within 30--40 m. of the talusfoot; they are normally composed of coarse openworkdebris with a variable amount of infill fines; rampart clastsare generally angular and slabby in form. Results of thecurrent study demonstrate that the topographic location andsedimentology of the basin ridge fails to match any of thesecriteria . For instance, the ridge crest is over 200 m from thebase of the steep northeast-facing slope which representsthe only realistic site for significant talus accumulation; theridge is composed of matrix-rich diamict and componentclasts are generally subrounded. Therefore, on the basis ofthis evidence the idea that the Punchbowl basin ridge is aprotalus rampart must be rejected .

Development of the other valleys to the west of thePunchbowl can be seen in the context of varying amounts ofsnowblow when winds were from an easterly direction and

156 S. H ARRISON ET AL.

Fig. 8. Boulder on the north side of the stream on the ridge showing facetted and striated surface.

blowing snow from the eastern and southeastern plateauareas of Winsford Hill. Their differing forms may reflectincreasing snow starvation to the west (owing to capture ofsnow by the Punchbowl) and hence increasing fluvialactivity at the expense of nival processes. The changingnature of the valleys cut into the north face of Winsford Hillmay therefore be taken to represent the transition fromvalley form dominated by glacial, nival and then fluvialprocesses with the Punchbowl probably having developedfrom a fluvially-incised valley to its present glacial form.

6. PALAEOGLACIER RECONSTRUCTION

The reconstruction of the surface area and morphology ofthe Punchbowl glacier was based on the evidence of thegeomorphic field mapping and followed the same procedureoutlined by several authors when reconstructing YoungerDryas cirque glaciers in the British upland s (e.g. Sissons,1974, 1980; Gray, 1982; Thorp, 1986; Ballantyne, 1989).Delimitation of the former ablation margin along the line ofthe terminal moraine ridge crest was straightforward;however, reconstru ction of the former limits in the headwallarea was more difficult because of the absence of trimlineevidence. Consequentl y. the former margin of the glacieraccumulation zone had to be extrapolated from the ablationarea. Assuming that the former accumulation zone extendedup to 30 m below the top of the backwall slope (cf. Gray,1982), the former surface area of the glacier was estimatedto be 0.38 km-'. The glacier contours were reconstru cted byanalogy to the characteristic contour pattern of contem-

porary glaciers. Former maximum glacial thickness wasestimated at 49.5 m by assuming that the present morph­ology of the basin floor has changed little since glaciationand superimposing elevation profiles of the reconstructedglacier and present land surface . Maximum basal shearstress was calculated to be 51 kPa (using an F factor of 0.8)employing the relationship outlined below:

1 =pgh sin a

where 1 is sheat stress, p is ice density, g is accelerationdue to gravity, h is ice thickness and a is slope gradient.

Although the value of basal shear stress must be regardedas approximate (because the shear stres s relationshipassumes zero basal slip or subglacial deformat ion), it fallswithin the range of 50-ISO kPa regarded as characteristic ofcontemporary glaciers (Paterson, 1994).

An estimate of the former ELA for the glacier wasestablished using the area-weighted mean method outlinedby Sissons (1974). This procedure is based on two assump­tions: (I ) during the glacial maximum the glacier was inequilibrium; (2) both the accumulation and ablationgradients have a linear relationship with altitude . Providingthat these assumptions can be satisfied then the position ofthe ELA can only be related to the distribution of altitude onthe glacier surface. Using this technique the ELA wascalculated to be 334 mOD. However, this approach may beflawed for two reasons. First, at present , there is no absoluteevidence that the glacier was entirely constrained by thebasin and it may have been an outlet of a small ice cappositioned over Winsford Hill (similar relationships are

G L ACIAL C I R QUE B ASIN, E X MOOR 157

observed by Gellatley, Whalley & Gordon, 1986). Second ,small glaciers do not necessarily have ELAs; in some yearsthey have positive mass balances and in other years theydisplay negative mass balances. Howe ver, by adopting asimple systems approach to glacier dynamics, the calcu­lation of an equilibrium line does prov ide a useful index ofthe relationships between a glacier, altitude and climate.

The reconstructed Punchbowl glacier is shown in Fig. 9.The development of glacial ice within the basin wasprobably partially controlled by the redistribution of plateausnow by wind. However, it is impossible to quantify theinfluence of snow-blow since the regional annualprecipitation at the time of glacier development is notknown.

7. AGE AND PALAEOENVIRONMENTALSIGNIFICANCE

At present , the age of the Punchbowl moraine ridge isunknown . The age of Pleistocene moraine ridges can bedetermin ed by dating material incorp orated within a diamictor associated glacigenic sediment, or by establishing local

morphostratigraphic relationsh ips. However, inspection ofexposed sections within the ridge revealed an absence ofmaterial suitable for dating, and its location south of theAnglian ice sheet limit undermines the application ofstandard morphostratigraphic methods. Furthermore , thesedimentary sequence beneath the basin floor rules outestablishing the minimum age of the infill by pollenstratigraphy (c. J. Caseldine , pers. comm .).

However, consideration of the geomorphic evidence doesshed some light on the likel y glacial history of ThePunchbowl. Two points are particul arly important.

I. At the mouth of the Punchbowl is a single arcuatemoraine ridge with a distinct crest and there are no otherrecessional or advance ridges within the basin. Beyondthe terminal moraine, the landscape reflects a periglaciallegacy and there appears to be no ev idence of formerglaciation here. Therefore, the moraine ridge appears tobe the result of a single glacial advance , though it may bean accretional feature related to more than one glacialphase. It is highly unlikely to be the product of multipleglaciations since there is no evidence that the basin wasoccupied by glacial ice on several occasio ns during theQuaternary.

2. The southerly location of the Punchbowl and the lowELA of the reconstructed glacier suggests that thedevelopment of glacial ice here was a rare event duringthe Quaternary and was probabl y initiated by the severeclimatic conditions which fuelled the southerly adva nceof ice sheets during the Anglian cold stage and theDimlington Stadia!. In comparison to the Dimlin gtonStadial ice sheet ELA calculated by Boulton, Smith,Jones & Newsome (1985), the reconstructed Punchbo wlELA appears anomalously low, implying that glacial icecould not have formed here during the DimlingtonStadia!. However, the presence of large plateau areasabove the site implies that the growth of glacial ice couldhave been considerably aided by the accumulation ofwindblown snow. Therefore the redistribution of snowinto a north-facing basin during the Dimlington Stadialmay have been sufficient to trigger the development ofglacial ice. If the moraine is of Anglian age, the absenceof thick mass-wasting deposits within the basin suggeststhat during later cold stages the Punchbowl was occupiedby a protective cover of nival ice which preventedinfilling of the depres sion.

8. CONCLUSIONS

x

-~)SO

Altit:mOD

Fig.9. Reconstruction of the Punchbowl glacier.

v Geomorphological and sedimentological investigations atthe Punchbowl have demo nstrated the presence of a formercirque basin glacier located 30 km to the south of theaccepted southern limit of Quaternary ice cover in this partof the British Isles. The absence of evidence for multipleglaciations in the Punchbowl, coupled with intensive peri­glacial modification of debris cover mantling the adjacentplateau and valley sides, suggests formation of glacial ice

158 S . HARRISON F:T A L .

her e was a localized and rare event. No d at ing co ntro ls areavaila ble fo r sedi ments fronting o r infi lli ng the basin;however, in view o f the southerly locat ion of the g lac ier. itis co ns ide re d m o st likely to have formed during the extremeclimatic conditio ns ass oc iated with the maximum advanc eof ei the r the A nglian or Dimlingt on Sta dial ice sheets .While rel ati vel y low ELA va lues for the recon structedPu nchb o wl g lacie r would seem to preclude a later dat e forthi s fea ture, th e gro w th of glacial ice during th e DimlingtonStadial m ay have been facilitated by accum ulatio n of wind ­blown snow from the adjacent plateau. The former pre senceof g lac ia l ice at rel at ively low altitudes on Exmoor sugges tsthat sim ila r ice masses should have devel op ed o n the higherand m ore westerly plateaus of Dartmoor and Bodmin Moor.

These find ing s suggest tha t othe r landforms on th e upl an dsof so uthwest En gl and may need re-evaluating in te rms of apot ential g lac ia l legac y in the la nd scap e .

ACKNOWLEDGEMENTS

The authors wis h to th an k Jan Foster and two anony mousreferee s who made useful com ments on an earl ier d raft ofthi s paper; the farmer at Withycombe Farm for access to thesi te and Te ssa Kin gsley wh o helped wi th fie ld work. Apreliminar y poster ve rsio n of thi s paper was presented at theBritish Geomo rphological Re se ar ch Group AnnualConferen ce at Dundee in September 1997.

REFERENCES

ANDR EWS, J. T. & SHIMIZU, K. 1966. Three-dim ensional vectortechnique for analyzing till fabris: discussion and FORTRANprogram. Canada Department of Mines and Technical SurveysBulletin,8,1 51-165.

BALLANTYNE, C. K. 1982. Aggregate clast form charac teristicsof deposits at the margins of four glaciers in the JotunheimenMassif, Norway. Norsk Geografisk Tidsskrift, 36, 103-11 3.

-- 1989. The Loch Lomond Readvance on the Isle of Skye.Scotland: glacier reconstruction and palaeocl imatic implications.Journal ofQuaternary Science, 4, 95- 108.

- - & KIRKBRIDE, M. P. 1986. The charac teristics andsignificance of some Lateglacial protalus ramparts in uplandBritain. Earth Surface Processes and Landf orms, 11,659-67 1.

BENN, D. I. 1994 . Fabri c shape and the interpretation ofsedimentary fabric data. Journal of Sedimentary Research. 64,9 10-9 15.

- - 1995. Fabric signature of subglac ial till deform ation ,Breidamerkurj (kull , Iceland. Sedimentology. 42 , 735-747.

BOULTON, G. S. 1978. Boulder shapes and grain size distributionsas indica tors of transport paths through a glac ier and till genesis .Sedimentology, 25, 773-799.

- , SMITH, G. D., JONES , A. S. & NEWS OME , 1. 1985.Glacial geo logy and glaciology of the last mid-latitude iceshee ts . Journal of the Geological Society, London, 142,447--474.

BOWEN, D. Q., ROSE, J., MCCABE , A. M. & SUTHERLAND,D. G. 1986. Correlation of Quaternary glacia tions in England,Ireland, Scotland and Wales. Quaternary Science Reviews. 5,299-340.

DOWDESWELL, J. A. & SHARP, M. 1986. Characterisation ofpebble fabrics in modem terrestri al glacigenic sed iments.Sedimentology. 33, 699- 7 10.

GELLATL EY, A. E, WHALLEY, W. B. & GORDON, J. E. 198fJ.Topographic control over recent glacier changes in southernLyngen Peninsula, North Norway . Norsk Geografisk Tidsskrift,40 (4) , 211-2 18.

GR AY, J. M. 1982. The last glaciers (Loch Lomond Advance) inSnowdonia. North Wales. Geological Journal. 17, 111- 133.

HART, J. K. 1994. Till fabrics associated with deformable beds.Earth Surface Processes and Landforms. 19, 15- 32.

JONES, R. L. & KEEN. D. H. 1993. Pleistocene Environments inthe British Isles. Chapman & Hall , London.

MARK , D. M. 1973. Analysis of axial orientation data includingtill fabris, Geological Society of America Bulletin. 84 ,1369- 1374.

PATERSON, W. S. B. 1994. The Physics of Glaciers . Pergamon,Oxford .

POWERS, M. C. 1953. A new roundn ess scale for sedimentaryparticles. Journal of Sedimentary Petrology. 23. I 17- 119.

SHAKESBY. R. A. & MAlTHEWS, J. A. 1993. Loch LomondStadial glacier at Fan Hir, Mynydd Du (Breco n Beacons). SouthWales: critical ev ide nce and palaeoclimat ic implications .Geological Journal , 28, 69-79.

SISSONS, 1. B. 1974. A lateglacial ice-cap in the CentralGramp ians, Transactions ofthe Institute ofBritish Geographers .62, 95-1 14.

-- 1980. Palaeoclimatic inferences from Loch Lomond Advanceglaciers. In (Lowe, 1. J., Gray, J. M. & Robinson, J. E.: eds)Studies in the Lateglacial of North-West Europe. Pergamon.Oxford, Pl'. 31--44.

STRAW, A. 1995. Aspects of the geomorphology of Exmoor. In(Binding, H.: ed.) The Changing Face of Exmoor. ExmoorBooks, Tiverton.

THORP, P. 1986. A mountain icefield of Loch Lomo nd Stadial age.western Gram pians, Scotland . Boreas. 15, 83-97.

Manuscript received 5 December 1997; typescript accepted 30 January 1998