Late Pleistocene deposits at Broughton Bay, Gower, South Wales:evidence for deposition at a non-marine Devensian ice margin

Stewart Campbell* and Richard A. Shakesbyr

CAMPBELL, S. & SHAKESBY, R. A. 1994. Late Pleistocene deposits at Broughton Bay,Gower, South Wales: evidence for deposition at a non-marine Devensian ice margin. Proceedingsof the Geologists' Association, 105, 167-185. The results of detailed sedimentological work onPleistocene deposits at Broughton Bay, west Gower, are presented. Ipswichian raised beach sedi­ments are overlain by a sequence of Devensian glacial and periglacial deposits and by Holocenedune sand. Particle size, clast fabric, lithological and SEM microtextural evidence show that threetypes of Pleistocene glacigenic deposit are present: (1) a lower, 'bedded' till series characterized byfine-grained, shelly diamictons; (2) an upper, stony till; and (3) till re-deposited by solifluction.Facies variations within in situ till were caused by the vertical transfer of materials within the basalice layers, as marine and estuarine sediments were entrained from the floor of Carmarthen Bayby southward-moving ice. These basal sediments, together with more far-travelled, englacially­transported material, were deposited contemporaneously on west Gower, at or near the maximumlimit of the Late Devensian Welsh ice sheet. The sedimentological and dating evidence is bestexplained by a conventional model of 'terrestrial' Late Devensian ice moving southwards ontoGower with a regionally stable landmass and eustatically-lowered sea-level. Reasons for rejectinga glaciomarine origin for the sediments are discussed. Soliflucted till, colluvial sediments and frostcracks at the site reflect periglacial activity towards the end of the Devensian Stage.

• Earth Science and Landscape Branch, Countryside Council for Wales, Plas Penrhos, FforddPenrhos, Bangor, Gwynedd, LL572LQ.t Department of Geography, University College of Swansea, Singleton Park, Swansea, WestGlamorgan, SA28PP.

1. INTRODUCTION

Gower is a key area for Late Pleistocene environ­mental reconstruction. This importance results from thelarge number of well-exposed sections in Pleistocenedeposits found around its rocky south and west shores.Of particular relevance are the sites which show a closejuxtaposition of what have been interpreted as in situglacial and periglacial deposits with richly fossiliferouscave and raised beach sediments. This wealth of evi­dence has a long history of investigation (e.g. Falconer,1860, 1868; Prestwich, 1892; Tiddeman, 1990; Strahan,1907a, b; Charlesworth, 1929; George, 1932, 1933;Wirtz, 1953; Mitchell, 1960, 1972; Bowen, 1970, 1971,1973a, b, 1974, 1977a, 1984), the main points of whichhave been summarized elsewhere (e.g. Bowen, 1981a;Bowen & Henry, 1984; Campbell & Bowen, 1989;Shakesby & Campbell, in press). During the last decadeor so, more detailed sedimentary analyses and the appli­cation of new analytical techniques, have begun toclarify the number, extents and timings of the variousglaciations which affected the peninsula (e.g. Bowen,1981b; Bowen, Sykes, Reeves, Miller, Andrews, Brew& Hare, 1985; Bowen & Sykes, 1988), as well as toestablish the ages of the widespread raised beachdeposits (e.g. Andrews, Bowen & Kidson, 1979;Bowen, 1981a; Campbell, Andrews & Shakesby, 1982;Bowen, Andrews, Davies & Henry, 1984; Bowen et al.,

Proceedingsofthe Geologists' Association, 105, 167-185.

1985; Campbell, 1984; Bowen & Sykes, 1988). Con­currently, detailed sedimentological and faunal studieshave provided a clearer insight into the palaeoenviron­ments of Gower during Mid- and Late Pleistocene times(e.g. Sutcliffe & Bowen, 1973; Stringer, 1975, 1977;Case, 1977, 1983, 1984; Campbell, 1984; Henry,1984a, b; Stringer, Currant, Schwarcz & Collcutt,1986).

Despite differences in detail, most workers have untilrecently agreed that a southward-moving, non-marineLate Devensian ice mass impinged directly on easternand northern Gower, while another ice mass movedwestwards along the Burry Estuary probably termin­ating at Whiteford Point (Fig. lB). Generally, icethicknesses have been regarded as insufficient to causeisostatic depression of the landmass. This traditional'terrestrial' model has been questioned recently with thesuggestion by Eyles & McCabe (1989, 1991) that manyLate Devensian deposits occurring around the low-lyingcoastal margins of western Britain are glaciomarine inorigin, formed while relative sea-level was raised, iso­statically, by as much as 140m above that of the present.However, this glaciomarine model has been rejected inaccounting for glacigenic deposits in North Wales(Harris, 1991; McCarroll & Harris, 1992), and else­where around the Irish Sea Basin, a 'terrestrial' model

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LATE PLEISTOCENE DEPOSITS AT BROUGHTON BAY. S . WALES 169

has been preferred (Thomas, Connaughton &Dackombe, 1985; Thomas & Kerr, 1987).

Up to 8 m of glacial and periglacial deposits occur atBroughton Bay, northwest Gower. Detailed accounts ofamino acid ratios and radiocarbon dates from the sitehave been given elsewhere (Campbell et al. , 1982;Campbell & Shakesby, 1985, 1986a, b) and wereinterpreted in terms of the non-marine model ofsedimentation . Amino acid ratios reported by Bowen,Reeves & Sykes (1986) from the site , however, suggestthat the glaciomarine model may be more appropriate.In this paper, previously unpublished data are presentedon the detailed stratigraphy, particle size charac­teristics , clast fabrics, lithological contents and quartzsand grain surface textures of the different deposits ,which are entirely consistent with a terrestrial model ofglacial deposition but which present a number of diffi­culties if a glaciomarine origin is proposed.

2. PHYSICAL BACKGROUND Al'l'D SETTING

Broughton Bay [SS 418 930], west Gower , lies on theeastern margin of Carmarthen Bay (Fig. 1A). The localsolid geology is relatively simple: strata underlying thePleistocene deposits form part of an extensive Carbon­iferous Limestone terrain that makes up much of southand west Gower (Fig. l.C), Llanmadoc Hill (186m 00),to the southeast of the bay, comprising Devonian sand­stones and conglomerates, forms a major topographicfeature, as does Rhosili Down (193 m) to the south. Thegenerally low-lying terrain between Twlc Point, BurryHolms, the north end of Rhosili Down and LlanmadocHill (i.e. Broughton Burrows and Llangennith Moors),is occupied by recent (Holocene) deposits, mainly dunesand (Fig. 1B).

3. STRATIGRAPHY AND SEDIMENTOLOGY

Pleistocene deposits at Broughton Bay crop out frombeneath Holocene sand in a 300 m long cliff sectionwhich runs east from Twlc Point (A-B in Fig. 1D). Thedetailed stratigraphy, as exposed in the early 1980s, isshown in Fig. 2. (Unfortunately, only small parts of thesection are now visible.) Five main Pleistocene bedshave been identified.

Bed I: Arenaceous/rudaceous sediments('raised beach deposits')

Near Twlc Point , there is a fragmentary raised beachdeposit cemented to a Carboniferous Limestone shoreplatform, close to the present-day High Water Mark.The deposit comprises rounded and occasionallyangular limestone clasts in a matrix of coarse sand andcomminuted marine shells. Whole shells (mainlyLittorina littoralis (L.» occur in the deposit (see section5: Biostratigraphy and dating) which interdigitates with

discontinuous coarse breccia of angular limestone clasts(head).

Bed 2: Shelly diamictons

Overlying the raised beach deposits are thinly beddeddiamictons up to 2.5 m in total thickness (Fig. 2). Theseare best developed near Twlc Point. These matrix­supported deposits vary in colour from dull yellowish­brown (Munsell Colour: lOYR 4/3) through greenish­yellow brown (10YR 4/2) to brownish-black (lOYR 312)and are highly consolidated. They contain abundantderived wood and marine shell fragments (see section 5:Biostratigraphy and dating). They are generally morefinely textured than the overlying diamicton (Fig. 3) andare interbedded with discontinuous bands of gravel,sand and silty-clay (usually less than 0.3 m in thickness;Fig. 3). Clasts in these beds rarely exceed pebble size(>64mm) but are commonly striated. In places, thelower , shelly diamictons are capped by a discontinuoussand and gravel layer up to c. a .7m thick . The latter,sub-horizontally bedded deposits are poorly sorted (Fig.3) and contain only occasional shell fragments. Thein situ lower diamictons and interbedded sedimentshave, on occasions, been exposed on the modern beach.

Bed 3: Stony diamicton

Overlying bed 2, often with an abrupt boundary, theupper stony diamicton is unfossiliferous, mostly un­stratified, dull yellowish-brown (lOYR 4/3), and up toc. 3 m thick . It is far stonier than bed 2, well consoli­dated, matrix-supported in places, but clast-supportedin others and many clasts (up to 2 m in size) are striated.Small impersistent layers of silt and clay are dispersedthroughout the bed, but overall it is texturally morehomogeneous than bed 2 (Fig . 3) .

Bed 4: Upper diamicton

The stony diamicton (bed 3) merges upwards at thewestern end of the section with a dull yellowish-brown(lOYR 4/3) or brown (lOYR 4/5) less well-consolidatedsediment up to c. 4 m thick with little evidence ofbedding. It differs little from bed 3 here and is onlydistinguishable by a coarser texture, a smaller pro­portion of silt and clay (Fig. 3) and by its fabric (seesection 6). Clasts from this bed are frequently striated.However, towards the eastern end of the sections, thisbed is sandier with only a few large clasts. It has thinhorizontal bedding and a sharp junction with the under­lying sediments (Figs 3 and 4).

Bed 5: Silty clay with pebbles

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LATE PLEISTOCENE DEPOSITS AT BROUGHTON BAY. S . WALES 171

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172 S . CAMPBELL AND R . A . SHAKESBY

large , bright brown (7.5YR 5/8) and dull yellowish­brown (lOYR 5/3) mottles. In places, however, thejunction with the underlying head (bed 4) is more clearlydefined. This bed ranges widely in thickness frome. 0.5 m to over 2 m and, in its upper part, containsoccasional , small subrounded pebbles. Near the lifeboatramp (Figs ID and 2), these deposits are overlain by alens of laminated brownish-black (lOYR 3/2) silty-clay(no longer exposed). All the silty-clay deposits arecapped by Holocene sand (Lees, 1982, 1983).

4. STRUCTURES

There are two main sets of structures. First, beds 2 and 3(in situ diamictons) have both been folded into gentleanticlines and synclines, examined both in cliff-sectionand in plan on the modem beach. They are clearestwhere highlighted by bedding, textural and colourdifferences in the lower shelly diamictons (bed 2) but areless pronounced in the overlying stony diamicton (bed 3)(Fig . 2).

The deformation structures form a series of 'domesand basins 'elongated in a north-south direction(Campbell & Shakesby, 1982; Campbell, 1984; Harris &Donnelly , 1991). A well-developed, 10m wide and 4mhigh dome occurs e. 125m along the exposure (Figs 2and 5) . At e. 165m (Fig. 2), a low-amplitude syncline inthe lower shelly diamictons sweeps upwards , forming anoverthrust at 172m (Fig. 2). Steeply-dipping beds seenin plan on the foreshore here demarcate a northwardcontinuation of the structure. Fault and thrust struc­tures are , however, generally uncommon in the sections,although near Twlc Point to the west, the intensity andamplitude of the deformation increase, and the shellydiamictons dip steeply westwards at e. 40°, presumablyresting against bedrock at a short distance inland(Fig . 2) .

Second, towards the eastern end of the section, up to3 m deep and 0.5 m wide, second generation crackspenetrate both beds 3 and 4, and in places affect thesurface of the lower, shelly diamictons (bed 2). They arelined with re-precipitated iron up to 1ern thick and filledwith silty-clay from bed 5 (Figs 2 and 4) . In plan , thecracks form a polygonal pattern.

S. BIOSTRATIGRAPHY AND DATING

Details of the known organic content of the beds and theresults of amino acid (Campbell et al., 1982; Davies ,1983; Bowen et al., 1986) and radiocarbon analyses(Campbell & Shakesby, 1985, 1986a, b; Worsley, 1986)have been presented elsewhere. Consequently, only abrief summary is given here .

The marine fauna

Identifiable marine shells from the raised beach (bed 1)consist largely of Littorina littoralis (L.) , although thoseof Patella vulgata L., Nueella lapillus (L. ) and Littorinalittorea (L.) are also present (Campbell et al. , 1982;Davies , 1983; Bowen et al., 1985).

The molluscan fauna from the shelly diamictons (bed2) is more diverse with 22 species identified (Table 1).All but two , Acanthoeardia tuberculata (L.) and Arealaetea (L.), live in the area today; only historical recordsof these two exist from the Bristol Channel (McMillan inCampbell et al., 1982). In addition to the listed species,one fragment from an echinoderm test (cf. Eehinus spp. )was found . No Arctic genera, such as Trophon orAstarte , were identified. The shell fauna thus constitutesa 'warm' assemblage, representing conditions verysimilar to the present day. The frequency of cocklefragments (Aeanthoeardia spp., Cerastoderma) and ofMacoma valves indicates a muddy sand environment;the presence of Area lactea and Aeanthoeardia tuber­eulata suggests conditions slightly warmer than now.

Wood fragments

Moderately well-preserved, dispersed , highly com­pressed (mostly <2cm thick) wood fragments have alsobeen found in the shelly diamictons (bed 2) . The largestfragments found were up to 15cm long and 4 em thick.Four large fragments were identified by Dr P. Rudall atthe Royal Botanic Gardens, Kew: three matched refer­ence material of the family Salicaceae (which includesSalix sp. [willow] and Populus sp. [poplar] - indistin­guishable from each other using wood anatomy alone) .The fourth , more seriously decomposed , was alsobelieved to be of a hardwood (cf. Salicaceae).

Amino acid and radiocarbon dating

Changes in the methods of amino acid analysis havetaken place since the original data were derived in theearly 1980s. An improved method has been used since1985 and it should be noted that the earlier data(Campbell et al. , 1982; Davies, 1983) are not directlycompatible with the latest results (e.g . Bowen et al.,1985; Bowen & Sykes , 1988). Early amino acid ratiosdetermined for specimens of Littorina littoralis from theBroughton Bay raised beach deposits were assigned anIpswichian Stage age (Campbell et al. , 1982) in view ofcomparable ratios obtained elsewhere in southernBritain (Andrews et al., 1979; Bowen, 1981a; Keen,Harmon & Andrews, 1981). In an extensive amino acidgeochronological study of Gower raised beach frag­ments, Davies (1983) provisionally ascribed theBroughton Bay raised beach to Oxygen Isotope Stage 5eand this was later confirmed by additional detailedamino acid measurements and Uranium Seriescalibration (Bowen et al. , 1985; Bowen & Sykes , 1988).

LATE PLEISTOCENE DEPOSITS AT BROUGHTON BAY, S. WALES 173

Fig. 4. Deep cracks filled with colluvial sediments penetrating soliflucted (a) and in situ (b) diamictons at eastern end of sections,

Fig. 5. Anticline developed in shelly diamictons, 125m from eastern end of exposure (see Fig. 2). Note camera case in centre forscale.

174 S. CAMPBELL AND R. A. SHAKESBY

Table 1. Mollusca from shelly diamictons at Broughton Bay

GastropodsPatella vulgata L.Gibbula magus (L.)Littorina saxatilis (Olivi)Littorina littoralis (L.)Turritella communis RissoNucella lapillus (L.)Ocenebra erinacea (L.)Nassarius reticulatus (L. )Buccinum undatum (L. )

BivalvesArca lactea L.MytilusedulisL.Ostrea edulis L.Chlamys varia (L.)Arctica islandicaAcanthocardia echinata (L.)Acanthocardia tuberculata (L.)Cerastoderma edule (L.)Venus striatula (da Costa)Venerupis rhomboides (Pennant)Venerupis pullastra (Montagu)Macoma balthica (L.)Corbula gibba (Olivi)

All the published amino acid ratios from the raisedbeach deposit at Broughton Bay seem to establish itsage as Ipswichian (= Oxygen Isotope Stage 5e). There isa difference of opinion, however, regarding the agerange of shells from the shelly diamicton, and this hasimportant implications concerning whether the ice sheetthat reached the site was 'terrestrial' or glaciomarine incharacter, as will be discussed later. The earliestpublished ratios for Littorina littoralis (Campbell et al.,1982; Table 2) indicated an age similar to that of theunderlying raised beach or slightly younger. Previouslyunpublished amino acid ratios, on other molluscs fromthe shelly diamictons (Table 2; Campbell, 1984), aresimilar to the published values. However, the two aminoacid ratios reported by Bowen et al. (1986) on Macomabalthica (L.) shells, presumed to be from the samedeposit, were interpreted as Middle and Late Devensianin age.

Radiocarbon assays on wood fragments from theshelly diamicton reported by Campbell & Shakesby(1985) yielded dates of >42000 BP (HAR-5443) [usingstandard method] and 68000 + 13000/- 5000 BP

(GrN-12508) [using isotopic enrichment method]).

6. CLAST FABRIC AND LITHOLOGICALMEASUREMENTS OF THE SEDIMENTS

The likely provenance of beds 2-4 at Broughton Baywas determined using clast fabric and lithological

1shell1 shell4 shells1 shellnumerous fragments2 almost intact shells1Y2 and one fragmentbodywhorl fragment1very young shell

2 valvesseveral fragmentsnumerous fragments4 fragments1fragmentfairly frequent fragments1fragmentfairly frequent fragmentsnumerous fragments1 beak and 2 other fragmentsa few fragmentsnumerous valves2 valves

analyses. Following the methology of Andrews (1971),the orientation (azimuth) and dip of 50 elongate clastswere measured for two sites in the shelly diamictons(bed 2), three in the stony diamicton (bed 3), and one inthe overlying, presumed soliflucted, material (bed 4).

Although stressing the need for the application of arange of sedimentological techniques in the interpret­ation of glacigenic deposits, Dowdeswell & Sharp (1986)demonstrated the contribution that eigenvalues canmake in discriminating between fabrics of differentglacial depositional environments. Eigenvalues haverecently been used as one of several techniques fordiscriminating between till and glaciomarine sediments(Harris, 1991;McCarroll & Harris, 1992). The approachhas also been applied to solifluction macrofabrics(Nelson, 1985). With one exception, the shelly and stonydiamictons have unimodal or bimodal fabrics with clearnorth-south or (for one shelly diamicton sample) south­north preferred orientations (Fig. 7). Clast dips are all< 45°, but those for the shelly diamicton are statisiticallysignificantly higher than those for the stony diamicton(p < 0.001; Kolmogorov-Smirnov two-sample test).The upper diamicton has a girdle-fabric (Woodcock,1977) and an approximate west-southwest to east­northeast preferred orientation which is similar to thelocal slope direction. All the fabrics have statisticallysignificant non-random fabrics (p < 0.05; Anderson &Stephens, 1972; Woodcock & Naylor, 1983), but theplot of S3 against S, eigenvalues shows that the shellyand stony diamicton samples have much stronger fabrics

LATE PLEISTOCENE DEPOSITS AT BROUGHTON BAY, S. WALES

Table 2. A summary of amino acid ratios from Broughton Bay

Sample Species Dalla: Liso· Author

Raised beach (bed 1)AAL-2453A Littorina littoralis 0.115 Campbell et al. (1982)AAL-2453B Littorina littoralis 0.107 Campbelletal. (1982)AAL-2453C Littorina littoralis 0.098 Campbelletal. (1982)AAL-2453D Littorina littoralis 0.115 Campbelletal. (1982)

Patella vulgata 0.150t Davies (1983)Nucella lapillus O. 125--{). 175t Davies (1983)

ABER-865A Littorina littorea 0.096 Bowenetal. (1985)ABER-865B Littorina littorea 0.106 Bowenetal. (1985)ABER-865C Littorina littorea 0.121 Bowenetal. (1985)ABER-865D Littorina littorea 0.091 Bowenetal. (1985)ABER-865E Littorina littorea 0.112 Bowenetal. (1985)ABER-866A Littorina littoralis 0.103 Bowenetal. (1985)ABER-866B Littorina littoralis 0.117 Bowenetal. (1985)ABER-866C Littorina littoralis 0.104 Bowenetal. (1985)ABER-866D Littorina littoralis 0.109 Bowenetal. (1985)ABER-866E Littorina littoralis 0.108 Bowen et al. (1985)

Till (bed 2)AAL-2442A Littorina littoralis 0.082 Campbell (1984)AAL-2442B Littorina littoralis 0.073 Campbell (1984)AAL-2442C Arctica islandica 0.200 Campbell (1984)AAL-2442D Arctica islandica 0.140 Campbell (1984)AAL-2442E Turritella communis 0.210 Campbell (1984)AAL-2454A Littorina littoralis 0.098 Campbell (1984)AAL-2454B Arctica islandica 0.250 Campbell (1984)AAL-2454C Arctica islandica 0.460 Campbell (1984)AAL-2454D Turritella communis 0.180 Campbell (1984)

ABER-867A Macoma balthica 0.070 Bowenetal. (1986)ABER-867B Macoma balthica 0.110 Bowen etal. (1986)

• Denotes combined or total amino acids.t Values estimated from graphs.

175

than the upper diamicton sample. These relative fabricstrengths were also confirmed by use of the An statisticaltest of Cornish (1979) and Ballantyne & Cornish (1979)(Campbell, 1984).

Clast lithological determinations were carried out onthe 8mm to 64mm size fraction (cf. Bridgland, 1986):10 samples (total number of clasts = 803) were analysedfrom the shelly diamictons (bed 2); 4 samples (252clasts)from the stony diamicton (bed 3); and 2 samples (143clasts from the overlying upper diamicton (bed 4). Anadditional sample was analysed from the better-sortedgravels found within bed 3. The results show minimalvariations between the samples (Fig. 6).

Provenance: interpretation

The striking similarity in the lithological content of thesamples suggests that all the material has been derived

from common sources. All samples are dominated byCarboniferous rocks (50-70%), principally sandstonesfrom the Pennant Measures. Smaller proportions ofCarboniferous Millstone Grit orthoquartzite andconglomerate (11%), limestone (2%), shale and mud­stone (12.5%), together with Devonian sandstone andconglomerate (9%) are present in the beds, indicatinga broad origin from the South Wales Coalfield to thenorth. The shelly and stony diamictons (beds 2 and 3)have similar lithological contents, although it is interest­ing to speculate that the higher limestone content of bed2 reflects the incorporation of material from localsources on (and off) northwest Gower where there is anextensive limestone outcrop. Although the measuredlithological differences between beds 2 and 3 are notstatistically significant, some support for this localsource comes from the evidence provided by quartzgrain surface texture analysis (see section 7).

176 S. CAMPBELL AND R. A. SHAKESBY

(a) lower, shelly diamictons (bed 2)

(c) soliflucted diamicton and head(bed 4)

Lithology

~ Devonian sandstone. siltstone and~ conglomerateII Car b oniferous 'Millstone Grit'L--J orthoquartzite and conglomerate

Itttt:j Carboniferous sandstone and siltstone

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(d) gravel from lens in stony diamicton(bed 3)

§ Carboniferous shale

g Carboniferous Limestone

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Fig. 6. Clast lithological composition of shelly, stony and soliflucted diamictons.

LATE PLEISTOCENE DEPOSITS AT BROUGHTON BAY, S. WALES 177

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0 0 OIP DIP

Fig. 7. Contour diagram of clast fabric data from the shelly, stony and upper diamictons plotted on Schmidt equal-areaprojections. The contour interval is two standard deviations (Kamb , 1959). Combined orientation data include the preferredorientation and dip of each sample (arrows with dip values). Eigenvalue ratios and clast dips are also shown.

In addition to characteristic Gower and South WalesCoalfield erratics, several clasts from much furtherafield were encountered in beds 2 and 3 including severalgranite and other igneous (but unidentified) clasts.Two clasts recovered closely match the St David's(Pembrokeshire) green granophyre; another resemblesthe Leinster Granite (T.R. Owen, pers. comm.). Inaddition, flint and chert fragments were recovered fromthe shelly diamictons (bed 2), and igneous rock typeshave been found in the beach shingle and were probablyderived from Pleistocene deposits in the cliff. Such far-

travelled clasts have normally been attributed to a pre­Devensian Irish Sea ice advance (e.g. George, 1933;Bowen , 1970,1974,1977b): flints were probably derivedfrom a Chalk outcrop in the Irish Sea Basin (Dobson,Evans & Whittington , 1973; Garrard & Dobson, 1974).These unusual erratics have probably been reworked bysouthward-moving Devensian Welsh ice from a suite ofpre-Devensian glacigenic deposits of mixed Irish Seaand Welsh provenance. This view is strongly sub­stantiated by the fabric evidence (see below).

The strong preferred orientations and comparatively

178 S . C A M P B E L L AND R . A . SHAKESBY

. . . . . . . . . . . . ICE ' .

(b ) After deformation of diamietons into "dome and basin" pattern

7. ANALYSIS OF QUARTZ GRAINSURFACE TEXTURES

Scanning electron microscopy (SEM) was used to deter­mine the quartz grain surface textures present on sandsamples (0.125-2.0 mm) from beds 2-4. Five sampleseach of 30 quartz sand grains were treated by standardtechniques (cf. Campbell & Thompson , 1991), coded bya colleague to avoid psychological bias (Culver , Bull ,Campbell , Shakesby & Whalley, 1983; Bull & Goudie,1987) and analysed . Two samples were from bed 2, twofrom bed 3 and one from bed 4. In all, 32 categories ofcommonly recognized surface texture were recorded.The percentages of grains in each sample exhibitinga given texture were calculated and are presented assurface feature variability plots (Fig. 9) (cf. Higgs, 1979;Walsh, Atkinson, Boulter & Shakesby, 1987). Surfacetextures for the shelly diamictons (bed 2) differ fromthose in beds 3 and 4. The samples from the shellydiamictons show an almost identical range of surfacetextures: the grains are dominantly subrounded torounded (categories 15 & 14; 73%) with only low tomedium relief (categories 30 & 31; 100%). Three mainsets of grain characteristics and surface textures arepresent in these samples. First, a few grains displayremnants of an euhedral crystal structure and evidenceof mechanical breakage, both of which show signs ofconsiderable subsequent abrasion. Second, most grainshave well-abraded, relatively smooth surfaces which aredensely covered with non-oriented V-shaped pits(category 9; 22%; Fig. 10) and crescentic gouges (% notrecorded) . Third , a few well-abraded grains show asecond generation of mechanical modification charac­terized by breakage blocks (categories 3 & 4; 50%),conchoidal fractures (categories 1 & 2; 35%) andparallel and arc-shaped steps (categories 5 & 8; 78% ;Fig. 11). Post-depositional chemical alteration , withsolution pits and hollows (category 26; 78%) andamorphous silica precipitation (category 24; 18%), iscommon on the grains but the effects only moderate.

Quartz grains from beds 3 and 4 (stony diamicton andupper diamicton, respectively) are similar but have abigger range of surface textures, dominated by large­scale mechanical features (Fig. 9). Both samples frombed 3 are dominated by grains with a subangular outline(category 16; 52%) and medium relief (category 31;73%) . Well-developed conchoidal fractures, breakageblocks , arc-shaped and parallel steps are common.Unlike the samples from the underlying bed, smoothly­abraded surfaces characterized by non-oriented V-pitsand crescentic gouges are relatively uncommon(category 9; 12%) . The number of grains showingchemical alteration and euhedral form are similar .Quartz grains from bed 4 have a somewhat more variedpattern of surface textures with more than 80% showingsigns of chemical alteration and some grains exhibitingsmooth, abraded surfaces covered with the smaller­scale , mechanically-formed V-pits and crescenticgouges.

predominant clast dip to north(nor th of dome)

- direction of ice movement N

- direction of ice movement N

':\.." ~ Ezlen l or ()u.I ler nar y aee:oon 8tUdled

loom.a..l..-~~~-,I

Broughton Bay

. . . . . . . . .. . . ICE ' .

predominant clast dip tosouth (south of dome)

S

S

(a) Before sediment deformation: clasts dipping consistently up-glacier

Fig. 8. Schematic interpretation of clast dip patterns atBroughton Bay.

PROFILES

.... . .

:·:;:::f&.1%1@+~±::;:: :·::;: : ::::;::::::;: :::: :;::4$¥~~1,::::::::

low dip angles for the stony diamicton samples areindicative of undeformed lodgement or melt-out till(Dowdeswell & Sharp, 1986). The higher clast dips ofthe shelly diamicton reflect the better development ofthe deformational structures in this bed compared withthe overlying one, clast dips tending to follow the formof the folds (Fig. 8). In support of this interpretation,the frequency distribution of dip angles in the shellydiamictons is similar to those of deformed lodgement tillaccording to Dowdeswell & Sharp (1986). If the upperdiamicton is regarded as glacigenic , then its fabric wouldsuggest possibly a highly deformed lodgement till or asediment-flow deposit, but in view of the downslopeorientation and the lithological content , a solifluctedorigin is favoured even though the fabric is atypicallyweak by comparison with fabrics reported by Nelson(1985).

1. Conchoidal fractures (large)2. Conchoidal fractures (small)3. Breakage blocks (large)4. Breakage blocks (small)5. Arc-shaped steps6. Random scratches and grooves7. Orientated scratches and grooves8. Parallel steps9. Non-orientated V-shaped pits

10. Meandering ridgesII. Dish-shaped concavities12. Upturned plates13. Micro -blocks14. Rounded grains15. Subrounded grains16. Subangular grains

~

~rnVl

"Ct"rn

~ornZtTletr1"CoVl

a~til:;lloc:o

~til>:<~

17. Angular grains18. Facet19. Cleavage flake20. Precipitation platelet21. Carapace22. Chemical, orientated V-pits23. Cleavage plane24. Silica precipitation (amorphous)25. Silica precipitation (euhedral)26. Solution pits and hollows27. Dulled solution surface28. Chattermarks29. Star-cracking30. Low grain relief31. Medium grain relief32. High grain relief

11 13 18 L7 L9 2 \ 23 28 27 29 31

SW'face feature

9 II 13 15 17 19 21 Z3 25 27 29 31

Surface feature

b. Lower, shelly diamictons (bed 2)100

90

90

70

60

50

40

30

20

100'''''', ,", ,"'''"" ,f. ,.", ."

d. Sand lens in stony diamicton (bed 3)100

90

80

70

60

50

40

30

20

10

:Ie

..!1r

.S

l'if.

t.S

l

9 II 13 15 17 19 21 23 25 27 29 31

Surface feature

9 II 13 15 17 19 21 23 25 27 29 31

Surface feature

9 11 13 15 17 19 21 23 25 27 29 31

Surface feature

c. Upper, stony diamicton (bed 3)100

90

eo70

80

50

40

a. Lower, shelly diamictons (bed 2)100

90

80

70

60

50

40

e. Soliflucted diamicton and head (bed 4)100

90

80

70

80 .

50

40

'if.

;f.

t.S

.~a.

is

!;f.

i.S

.~a.

Fig. 9. Scanning electron microscope surface feature variability plots. .....--.I\0

180 S . CAMPBELL AND R . A . SHAKESBY

./

35KU Xleee eee5 le~ UCS81Fig. 10. Typical, highly-abraded quartz grain from shelly diamictons (bed 2) , showing dense pattern of non-orientated V-shapedimpact pits and crescentic gouges , formed in a medium-high energy marine environment.

Fig. 11. Characteristic, glacially-formed , mechanical surface textures including arc-shaped steps (a), parallel steps (b) , conchoidalfractures (c) and breakage blocks (d).

LATE PLEISTOCENE DEPOSITS AT BROUGHTON BAY, S . WALES 181

Quartz grain surface textures: interpretation

Grains from the shelly diamictons show a range ofcharacteristics (rounded and subrounded grains; smoothsurfaces ' common V-pits and crescentic gouges) indica­tive of ~ subaqueous (presumed beach) origin (e.g.Krinsley & Doomkamp, 1973; Ying & Deonarine,1985). There appears to be some eviden~e of pr~vious

source rock/diagenetic environments (in particular,euhedral form reflecting a Pennant Sandstone source)and of mechanical breakages during transportation(glacial?) prior to substantial re-modelling of the grainsby marine processes (ct. Campbell & Thompson, 1991).A small proportion of grains shows evidence of latermechanical breakage (freshly-cleaved surfaces , break­age blocks, conchoidal fractures, arc-shaped andparallel steps) . Although grain fracturing in a highenergy beach environment cannot be ruled out, ~he

mechanical textures more likely indicate crushing dunngglacial transport (ct. Shakesby, 1989; Campbe.ll &Thompson, 1991). Grains from the sto~y diamictonshow a dominance of larger-scale, mechanically-formedsurface textures , usually associated with crushing i~

glacial environments (e.g. Krinsley & Takahashi,1962a, b; Krinsley & Doomkamp, 1973; Whalley &Krinsley, 1974; Whalley , 1979; Bull , 1981; Ito &Shimizu, 1989; Mahaney, 1990; Mahaney & And~es,

1991). The SEM analysis is consistent with the particlesize, clast lithological and fabric data in suggestmg thatthis bed is till. A few grains with subaqueous charac­teristics probably indicate derivation from the under­lying shelly diamictons which contain a higher. pro­portion of grains with subaqueous featu~es. Hlgh~r

variability of surface textures in the overlying bed 4 IS

consistent with the sediment being derived from beds 2and 3 beneath by solifluction. The fabric evidencesupports this view. The high percentage of grains in thissample that have been strongly affected by pos.t­depositional chemical activity probably reflect two m?mfactors . First, the susceptibility of the bed to weathenngis likely to have been increased by its location near tothe modem ground surface. Second, the effects ofweathering may have been enhanced by the less con­solidated nature and generally sandier texture of thematerial (Fig. 3).

8. DISCUSSION

The detailed sedimentological and stratigraphical datapresented here support the view, based mainly on datingevidence and expressed in earlier publications(Campbell et al., 1982; Campbell & Shakesby, 1985,1986a, b) , that the lower diamictons at B.rou~ht?n Bay(beds 2 and 3) were deposited by Devensian Ice m non­marine conditions.

The shelly diamictons (bed 2) are interpreted asprobable deformed lodge~ent or m~lt.out tills ~h!ch

contain sea-bed and estuanne matenals of Ipswichian

age (Campbell & Shakesby, 1~82 ; Campbell , .1984).SEM data show that these deposits are charactenzed mthe sand-size grade by a preponderance of beach sedi­ment, and that a small proportion ofthis marine materialhas been modified subsequently by glacial processes. Incontrast , the overlying bed (bed 3) is dominated bycharacteristically 'glacial' quartz grains. Since both bedspossess the same strongly north-south preferred orien­tation of clasts , it is probable that both were emplacedcontemporaneously beneath an ice stream that movedonto west Gower from the north. The low dip angles ofclasts in bed 3 suggest that it is an unaltered lodgementor melt-out till. The dominance of sea-floor sediments inthe lower beds (shell litter, beach sand, estuarine siltsand clays) reflects their incorporation from ~a~arthen

Bay as the ice moved south. The many thin units thatmake up the shelly diamictons (bed 2) therefore prob­ably reflect little-altered marine depositional structures.Shearing in the basal ice/sediment layers may have beensufficient to cause reorientation of constituent particlesaccounting for the strongly preferred clast orientationwhich closely mirrors that in the overlying stonydiamicton (bed 3). The short distance of transport m~y,

however have been insufficient to cause substantialremixing of the marine sediment and com~lete ~is­ruption of existing sedimentary structures, includingbedding planes. . .

In this model, beds 2 and 3 are Viewedas a contmuoussediment pile consisting, in the lower layers (bed 2), ofdominantly local material (shells , marine gravel, sa~d,

silt and clay from the floor of Carmarthen Bay) , Withincreasingly far-travelled material from Coalfieldsources (from major valley outlets includin¥ t~e

Loughor, Gwendraeth and Tywi valleys) occumng mthe overlying bed (ct. Boulton, 1972, 19?7). !he 'wa~'

shell fauna in the till represents a typical interglacialassemblage of Ipswichian age (supported by the ami.noacid ratios reported by Campbell et al., 1982) whichwould have been picked up from the dry floor. ofCarmarthen Bay by Late Devensian ice together Withestuarine sediments: certainly, the fauna is indicative ofwarm conditions, and some species reflect estuarineenvironments. The mechanisms involved in thedeformation of the sediment layers are unclear, but mayhave occurred in a variety of ways: the most likelymechanisms include the coalescence of ice sheets fromthe different valley outlets into the bay (ct. Hart, 1990);local loading structures caused by high pore waterpressures in the basal sediment layers (e .g. Hart &Boulton, 1991); or construction of basal sediment as icewas forced through the low col between L1anmadoc Hilland Twlc Point (Campbell , 1984).

Following wastage of the Late Devensian Welsh icesheet in this area , for which there is sedimentaryevidence in Rhosili Bay to the south in the form ofoutwash deposits, subaerial periglacial conditions pre­vailed . Glacigenic sediments in and around BroughtonBay were re-distributed downslope, where they accumu-

182 S. CAMPBELL AND R. A. SHAKESBY

lated as solifluction deposits (bed 4). Subsequently,cracks developed in the ground surface, presumablyduring the Devensian late-glacial, probably by frost­assisted processes rather than by desiccation or sub­sidence. This period probably also gave rise to theaccumulation of colluvial sediments (bed 5), washeddown .from surrounding sparsely-vegetated slopes ontothe earlier deposits and into the cracks on their surface.Elsewhere in coastal sections on Gower, there is someevidence to suggest that colluvial processes may havecontinued into the early Holocene (Bowen, 1970). Beds1-5 were buried beneath thick dune sand in theHolocene (Lees, 1982, 1983).

Although some aspects of the sedimentology andstatigraphy in the Pleistocene deposits at Broughton Bay(e.g. some unbroken marine shells, diamictons con­taining sands of a clear marine origin, muddy facies)might seem to lend support to a glaciomarine origin,there are major difficulties with applying this model.

(1) The most significant published evidence supportinga glaciomarine origin for the Broughton Baydeposits remains essentially a single amino acid ratioon a Macoma balthica shell indicating a LateDevensian age, presumably obtained from theshelly diamicton, although this is not stated (Bowenet aI., 1986; Table 2). Indeed, no details have beensupplied concerning the stratigraphical position orstate of preservation of the shell.

(2) The two amino acid ratios (one thought to be of LateDevensian, the other of Middle Devensian age)reported by Bowen etal. (1986) would imply that theshell assemblage in the shelly diamicton is part of amixed fauna of lpswichian, Middle Devensian andLate Devensian ages, but this conflicts with theentirely 'warm' marine shell assemblage of 22species painstakingly collected over many weeksfrom the whole range of fresh exposures in theglacigenic diamictons on the beach and in the cliff.It would be reasonable to expect to find at leastsome cold water-loving species if relative sea-levelhad been sufficiently high during the LateDevensian for marine shells to have lived at thattime in Carmarthen Bay. The range of sea tempera­tures favoured by Macoma balthica is broad so thatits presence alone does not lend weight to a glacio­marine origin.

(3) The amino acid ratio of supposed Middle Devensianage reported by Bowen et al. (1986) implies highrelative sea-levels at this time. This is at odds with:(a) the accepted views of small ice volumes inBritain at this time (Lowe & Walker, 1984); and(b) the likelihood that such small volumes of icewould have been insufficient to cause the isostaticdepression of the land surface in South Wales neces­sary for sea water to have occupied CarmarthenBay.

(4) An isostatically raised sea-level around Gowerduring Late Devensian times is difficult to reconcilewith the good preservation of easily eroded, pre­Late Devensian fossilferous deposits in cavesbeyond the ice limit and close to present sea-level(McCarroll,1993).

(5) The strong clast fabrics and comparatively low clastdips from beds 2 and 3 are difficult to reconcile withan ice-rafted origin for the sediments. Fabrics fromdiamictons formed by rain-out from such sedimentsare virtually random according to Domack &Lawson (1985). Furthermore, these authorsreported that high proportions (up to 32%) of theclasts in true glaciomarine sediments exhibited dipangles exceeding 45°. Significantly, no single clastdip greater than 45° was recorded in the BroughtonBay diamictons.

(6) The stratified nature and varied particle size of thedifferent units in bed 2 might be interpreted asthe product of re-sedimentation by slumping andmeltwater release in the proximal zone of a retreat­ing tidewater icc sheet (Eyles & McCabe, 1989,1991). However, this interpretation is at odds withthe well-preserved fold structures and consistentnorth-south clast fabrics in beds 2 and 3.

(7) There is no faulting of the diamictons, contrary towhat would be expected if major rebound hadoccurred following deglaciation (cf. Harris, 1991).

(8) If the shelly diamicton were representative ofre-sedimentation in the immediate vicinity of a tide­water ice-sheet margin, then the stony diamictonshould be composed of, or include, more distalsediments laid down as the ice retreated (d. Eyles &McCabe 1989; Scourse, Robinson & Evans, 1991).Thus a distal mud drape overlying the wholesequence would be expected. However, the stonydiamicton, rather than being finer, is coarser thanthe underlying shelly diamicton and contains nodiscrete unit of fines.

9. CONCLUSIONS

The detailed sedimentological data presented hereindicate that the glacigenic deposits at Broughton Bay,west Gower, are the product of a Late Devensian non­marine ice sheet close to its maximum limit.

The lithological content, clast fabrics, inclusion ofbeach sands as indicated by the quartz sand grain surfacetextures, abundant marine shells and shell fragments inthe glacigenic deposits are all consistent with 'non­marine' ice having moved southwards across theeustatically controlled dry floor of Carmarthen Bay,entraining estuarine and littoral sediments of IpswichianInterglacial age (= Oxygen Isotope Stage 5e). Deforma­tion structures in the glacigenic deposits (beds 2 and 3;shelly and stony diamictons, respectively) could be the

LATE PLEISTOCENE DEPOSITS AT BROUGHTON BAY. S . WALES 183

result of coalescence of ice sheets from different valleyoutlets into the bay , local loading structures or con­striction through a bedrock col. On the basis of its clastfabric , particle size and lithological content charac­teristics, bed 4 seems most likely to be a soliflucted till.Colluvial sediments (bed 5) were then laid down byslopewash processes probably also during periglacialconditions towards the end of the Devensian Stage. TheHolocene has been characterized by the accumulation ofthick deposits of dune sand .

Although there might seem to be some attractions inapplying the glaciomarine model to the Broughton Bayglacigenic deposits (the inclusion of marine sediments,some unbroken marine shells and an amino acid ratio ona Macoma balthica shell of reported Late Devensianage), these are far outweighed by the difficulties. Theyinclude an assemblage of entirely 'warm' marinemolluscs collected from the diamictons with nospecifically cold-loving types; the consistently strongN-S and S-N clast fabrics with comparatively low dipangles in the shelly and stony diamictons, which do notaccord with observed fabric patterns in glaciomarinedeposits; the well-preserved deformation structures inthe shelly and stony diamictons; no faulting in the

exposure indicative of a major isostatic rebound follow­ing ice retreat; and no evidence of a mud drape indica­tive of distal sedimentation following ice retreat.

ACKNOWLEDGEMENTS

In particular, we would like to thank Mr M. Bristow whoalerted us to the sections in Broughton Bay, and toMr A.R. Shefford who has generously allowed access tothem through his property ever since . We also grate­fully acknowledge Mr A. Cutliffe and Mr M. Williamsfor photographic and technical assistance, respectivelyand MrG. Lewis for drawing Fig. 7. S.C. wishes to thankNigel Peters, Jill Parsonson and Ely Ricketts for help inthe field, and the University of Wales for funding thisresearch. We would like to thank Mr N.D.W. Daveyand Drs J .E . Gordon, W.A. Wimbledon and P. Allenfor comments on earlier drafts of this paper. ProfessorN. Stephens kindly provided facilities at the Departmentof Geography, University College of Swansea andvaluable field and technical advice during the courseof this work . We are grateful to Dr C. Harris for helpwith the eigenvalue analyses carried out in this study.

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Received 28 October 1992; revised typescript accepted 24 January 1994.