The palaeoenvironmental history of Late Pleistocene deposits at Moel Tryfan, North Wales: evidence from Scanning Electron Microscopy (SEM) Stewart Campbell and Ian C. Thompson CAMPBELL, S. & I. C. THOMPSON, 1991. The palaeoenvironmental history of Late Pleistocene deposits at Moe! Tryfan, North Wales: evidence from Scanning Electron Microscopy (SEM). Proc. Geol. Ass., 102(2), 123-34. The transportational and depositional history of controversial sands from Moel Tryfan, North Wales, is reconstructed using Scanning Electron Microscopy to study the surface textures of quartz sand grains. Five major suites of sequentially superimposed surface textures indicate a complex history: i) Textures indicative of source rock and glacial environments are present; ii) Some of these textures show characteris- tics of major subaqueous (marine) modification; iii) A smaller proportion of grains shows evidence for a subsequent phase of glacial crushing and abrasion, and iv) then possibly a phase of fluvioglacial modification; v) Finally, many grains display slight post-depositional weather- ing. This limited chemical alteration may be consistent with the Late Devensian age suggested for the sediments. S. Campbell, Countryside Council for Wales, Plas Penrhos, Ffordd Penrhos, Bangor, Gwynedd, LL572LQ J. C. Thompson, Department of Geography, University College, Singleton Park. Swansea, West Glamorgan, SA2 BPP 1. INTRODUCTION Since their discovery by Joshua Trimmer in 1831, the high-level shelly drifts at Moel Tryfan (SH 520560; Fig. 1) have been a classic ground for glacial geologists. As Reade (1893) stated, the site became a 'battleground of contending theories' in the war between the 'Diluvialists and the Glacialists'. Although the concept of marine submergence to account for the drifts was finally abandoned in the early twentieth century, the origin and dating of the sediments have remained highly controversial. Over the years, quarrying has reduced the formerly extensive exposures of Pleistocene sediments, including the 'shelly drift', to a few small sections; even these are much degraded and partly obscured by slate spoil. In view of the poor current state and discontinuous nature of the exposures, an examination by SEM of the surface textures of quartz sand grains from two small samples of sands from within the enigmatic deposits, offers considerable scope for interpreting their sedimentary history without recourse to exten- sive excavations and, thereby, giving due regard to the outstanding value of the remaining beds (Campbell & Bowen, 1989). A brief abstract outlining the main results of this study has been given elsewhere (Campbell, 1990). 2. SETIlNG AND GEOLOGY Moel Tryfan rises to 427 m O.D. and is situated in the western foothills of Snowdonia, some 8.5 km east of Caernarfon Bay. The problematic, shell-bearing deposits are located mainly in the disused Alexandra Slate Quarry, which lies to the south-east of the Moel Tryfan summit (Fig. 1A). The continued existence of the exposures has always been precarious, particularly during the major expansion of the quarry (Greenly & Badger, 1899; Hicks et al., 1899; Greenly, 1900), and today the stratigraphic sequence is far from clear; although beds of till, shelly sand and gravel are still visible in separate parts of the quarry (Davies, 1988; Campbell & Bowen, 1989) where they overlie Cambrian grits and slates and, at one location, a deeply weathered and decomposed profile of the Cambrian bedrock. In piecing together the strat- igraphic evidence from the presently discontinuous exposures, it appears that the shelly sands (with interbedded silts) occur as an isolated remnant (up to c. 6 m thick) mainly along the south-east rim of the quarry; they are overlain by an apparently continuous bed of stony Welsh till (up to c. 3 m thick) which extends down the flanks of Moel Tryfan into the adjacent valley to the north-east (Fig. IC). Hicks et al. (1899) recorded a Pleistocene sequence up to 7.6 m thick overlying the slate bedrock (Fig. 1B). The shelly sands were described as yellow, with gravel pockets in which there were numerous and larger marine shell fragments, in addition to a number of characteristic Irish Sea erratics including granite clasts from the Lake District (Reade, 1893). The overlying till was a dark grey, unstratified deposit, with numerous clasts up to 1m across. These were largely subangular, many well-striated and mostly derived from North Wales; the riebeckite microgranite of Mynydd Mawr being especially abundant. In places, the sands and gravels interdigitated with till, being contorted into sharp folds at the junction of the beds. Despite former records to the contrary, Hicks et al. (1899) emphasised 123
Transcript
The palaeoenvironmental history of Late Pleistocene deposits at MoelTryfan, North Wales: evidence from Scanning Electron Microscopy
(SEM)
Stewart Campbell and Ian C. Thompson
CAMPBELL, S. & I. C. THOMPSON, 1991. The palaeoenvironmental history of LatePleistocene deposits at Moe! Tryfan, North Wales: evidence from Scanning ElectronMicroscopy (SEM). Proc. Geol. Ass., 102(2), 123-34. The transportational and depositionalhistory of controversial sands from Moel Tryfan, North Wales, is reconstructed using ScanningElectron Microscopy to study the surface textures of quartz sand grains. Five major suites ofsequentially superimposed surface textures indicate a complex history: i) Textures indicative ofsource rock and glacial environments are present; ii) Some of these textures show characteristics of major subaqueous (marine) modification; iii) A smaller proportion of grains showsevidence for a subsequent phase of glacial crushing and abrasion, and iv) then possibly a phaseof fluvioglacial modification; v) Finally, many grains display slight post-depositional weathering. This limited chemical alteration may be consistent with the Late Devensian age suggestedfor the sediments.
S. Campbell, Countryside Council for Wales, Plas Penrhos, Ffordd Penrhos, Bangor,Gwynedd, LL572LQ
J. C. Thompson, Department of Geography, University College, Singleton Park. Swansea, WestGlamorgan, SA2 BPP
1. INTRODUCTION
Since their discovery by Joshua Trimmer in 1831, thehigh-level shelly drifts at Moel Tryfan (SH 520560;Fig. 1) have been a classic ground for glacialgeologists. As Reade (1893) stated, the site became a'battleground of contending theories' in the warbetween the 'Diluvialists and the Glacialists'. Althoughthe concept of marine submergence to account for thedrifts was finally abandoned in the early twentiethcentury, the origin and dating of the sediments haveremained highly controversial. Over the years,quarrying has reduced the formerly extensiveexposures of Pleistocene sediments, including the'shelly drift', to a few small sections; even these aremuch degraded and partly obscured by slate spoil. Inview of the poor current state and discontinuousnature of the exposures, an examination by SEM ofthe surface textures of quartz sand grains from twosmall samples of sands from within the enigmaticdeposits, offers considerable scope for interpretingtheir sedimentary history without recourse to extensive excavations and, thereby, giving due regard to theoutstanding value of the remaining beds (Campbell &Bowen, 1989). A brief abstract outlining the mainresults of this study has been given elsewhere(Campbell, 1990).
2. SETIlNG AND GEOLOGY
Moel Tryfan rises to 427 m O.D. and is situated in thewestern foothills of Snowdonia, some 8.5 km east ofCaernarfon Bay. The problematic, shell-bearingdeposits are located mainly in the disused AlexandraSlate Quarry, which lies to the south-east of the Moel
Tryfan summit (Fig. 1A). The continued existence ofthe exposures has always been precarious, particularlyduring the major expansion of the quarry (Greenly &Badger, 1899; Hicks et al., 1899; Greenly, 1900), andtoday the stratigraphic sequence is far from clear;although beds of till, shelly sand and gravel are stillvisible in separate parts of the quarry (Davies, 1988;Campbell & Bowen, 1989) where they overlieCambrian grits and slates and, at one location, adeeply weathered and decomposed profile of theCambrian bedrock. In piecing together the stratigraphic evidence from the presently discontinuousexposures, it appears that the shelly sands (withinterbedded silts) occur as an isolated remnant (up toc. 6 m thick) mainly along the south-east rim of thequarry; they are overlain by an apparently continuousbed of stony Welsh till (up to c. 3 m thick) whichextends down the flanks of Moel Tryfan into theadjacent valley to the north-east (Fig. IC). Hicks et al.(1899) recorded a Pleistocene sequence up to 7.6 mthick overlying the slate bedrock (Fig. 1B). The shellysands were described as yellow, with gravel pockets inwhich there were numerous and larger marine shellfragments, in addition to a number of characteristicIrish Sea erratics including granite clasts from theLake District (Reade, 1893). The overlying till was adark grey, unstratified deposit, with numerous clastsup to 1 m across. These were largely subangular,many well-striated and mostly derived from NorthWales; the riebeckite microgranite of Mynydd Mawrbeing especially abundant. In places, the sands andgravels interdigitated with till, being contorted intosharp folds at the junction of the beds. Despite formerrecords to the contrary, Hicks et al. (1899) emphasised
123
124 STEW A RT C A M PB E LL AND IAN C . T H O M PS O N
Weathered bedrock in situ 0(Cambrian quartz-veined grits)
Stony Welsh till
Cambrian bedrock(grits, shales and slate)
Spoil heaps~~~
r:::::-::1G:::.:J
~
~
N
t
Fig. 1. Moel Tryfan site details. (A) location and setting, (B) detailed section adapted from Hicks et ai. (1899), (C) schematicrepresentation of present exposures .
that there was' ... no distinct evidence that the shellysand and gravel anywhere overlie the boulder clay'. Avaried marine shell fauna has been described fromthe sands and gravels at the site (Darbyshire, 1863;Jeffries, 1880; Davies, 1988). Species present includeMacoma balthica (L.), Cerastoderma edule (L.),Venur striatula (da Costa), Astarte borealis(Schumacher) and Turritella communis Risso (Davies ,1988).
3. PREVIOUS INTERPRETATIONS OF THEBEDS
Trimmer (1831) was the first to describe and interpretthe beds which he considered to be 'diluvial' orflood-formed. Many subsequent workers , includingBuckland (1842), Darwin (1842), Ramsay (1852),Lyell (1873), Mackintosh (1881, 1887) and Reade
(1892, 1893), similarly considered that marineagencies were , at least in part, responsible for thedeposits. Lyell (1873) noted that the marine shells inthe deposits ' .. . show that Snowdon and the highesthills which are in the neighbourhood of Moel Tryfanwere mere islands in the sea . . . '. Such was theentrenchment of belief that Trimmer (1831)considered that even glacial striae on slate bedrock inSnowdonia had been caused by 'diluvial currents'.Darbyshire (1863) recognised 56 different species ofmarine mollusc from the beds at Moel Tryfan and thislist was later updated by Jeffries (1880); these faunalanalyses further reinforcing the prejudice for a marineorigin.
Against this overwhelming tide of early opinion,Belt (1874) published a remarkably perceptive paperin which he refuted the marine origin of the shellybeds . He noted that ' . .. the shells are broken and
SEM ANALYSIS OF MOEL TRYFAN DEPOSITS 125
worn .. . they are just where they ought to be foundon the supposition that an immense body of icecoming down from Northern Ireland, Scotland andfrom Cumberland and Westmorland, filled the basinof the Irish Sea , scooped out the sand with the shellsthat had lived and died there, and thrust them far upamongst the Welsh hills that opposed its coursesouthward . .. '. He thus considered that the MoelTryfan shelly drifts were the product of a southwardmoving Irish Sea ice mass. Belt's interpretation wasalso adopted by Blake (1893) but, as late as 1910, theshelly beds were still regarded by Edward Hull,former Director of the Irish Geological Survey, asevidence for marine submergence (Davies, 1969).
4. DATING
Establishing the age of the beds at Moel Tryfan hasalso proved controversial. The restricted and highlevel occurrence of the deposits led Whittow & Ball(1970) to suggest that they were amongst the oldestPleistocene sediments in North and north-west Wales;perhaps being equivalent in age to the CricciethAdvance sediments in southern LlYn (Simpkins , 1968)and therefore of pre-Late Devensian age. Foster(1968, 1970) obtained a radiocarbon date of 33,740+2100/-1800 BP (1-2803) from a bulked shell samplefrom the beds. Since the shell material was derived,he considered that the glaciation responsible for itstransportation and deposition must post-date c. 34,000B.P. , indicating a Main Wurm (=Late Devensian) agefor the sediments. Although the reliability of such aprocedure has frequently been questioned (Shotton,1967, 1977; Boulton , 1968; Bowen, 1974), a LateDevenisan age for the Moe! Tryfan deposits has beenupheld on stratigraphic grounds (Bowen , 1977). Mostrecently, Davies (1988) applied amino acid datingtechniques to shells, notably Macoma, from the beds.The shells yielded a wide range of amino acid ratiossuggesting shell populations of extremely varied ages.The youngest shells, however, were believed to be ofMid and Late Devensian ages, thereby limiting theage of the glacial advance responsible for theirtransport and deposition to the Late Devensian(Davies, 1988).
5. SCANNING ELECTRON MICROSCOPY:RATIONALE
The hard surfaces of quartz sand grains preserve arange of markings that can reveal the differentenvironments through which grains may have passed(e.g. Krinsley & Doornkamp, 1973; Bull, 1981;Culver, Bull, Campbell, Shakesby & Whalley , 1983;Ying & Deonarine, 1985). In ideal conditions, notonly the primary or last transporting or depositionalmedium can be determined, but a series ofsequentially superimposed textures may be presentthat record a comprehensive palaeoenvironmental
history. The technique has become well established indistinguishing between various sedimentary environments in rock sequences of widely differing age, forexample , in sands of Triassic (Krinsley , Friend &Klimentidis , 1976), Permian (Krinsley & Smith,1981), Cretaceous and Palaeocene age (Higgs, 1979),and especially Pleistocene sediments (e .g. Krinsley &Takahashi, 1962a; Krinsley & Cavallero, 1970; Bull,1975, 1976, 1984; Campbell, 1984). Kuenen (1960)and Kuenen & Perdok (1962) performed experimentsto establish the effects of various mechanical actionson quartz grains, thus providing a sound empiricallink between process and depositional environment. Aplethora of works has since been published , the mostnotable being the 'Atlas of Quartz Grain SurfaceTextures' by Krinsley & Doornkamp (1973) whichillustrated quartz grains from a wide range ofsediments and environments including, glacial,subaqueous (e.g. marine and fluvial), aeolian, as wellas those showing diagenetic effects. More recently,Ying & Deonarine (1985) have provided an updatedatlas illustrating additional surface textures associatedwith deep-sea turbidites, alluvial deposits andlong-distance wind-blown sands . The increasingadoption of semi-quantitative techniques to analyseSEM data has yielded encouraging results as regardsthe accuracy of environmental discrimination based onsurface texture analysis (e.g. Margolis & Kellner,1969; Culver et al. , 1983). Although SEM analysis isbest used in conjunction with other analytical methods(e.g. Bull, 1981, 1984; Campbell, 1984), the techniquehas been used successfully alone, where othermethods have been either undiagnostic or inapplicable(e.g. Higgs, 1979; Krinsley and Wellendorf, 1980).SEM analysis of quartz grain surface textures is usedin the latter capacity in this study .
6. SAMPLING AND PREPARATION
Two small samples of the Moel Tryfan shelly sandwere collected in test tubes from a fresh face in theexposure (Fig. LC). These samples were gentlywet-sieved through 2 mm and 0.125 mm meshes. Thissize range of grains is commonly used in SEM studiessince it is the most likely to reflect a wide range ofwell preserved surface characteristics (Krinsley &Doornkamp, 1973; Bull, 1978; Higgs, 1979). Thesamples were cleaned with hydrochloric acid and thenstannous chloride solution to remove unwanteddebris , carbonate and iron-staining, which canotherwise mask the details of the grains' finer surfacecharacteristics (Krinsley & Doornkamp, 1973). Thegrains were finally washed in double-distilled waterand gently dried prior to sub-sampling . Ultrasoniccleaning techniques were not used since these canimpart surface textures to the grains (Porter, 1962).
Individual grains were randomly selected foranalysis using a binocular microscope and an incident
126 STEWART CAMPBELL AND IAN C. THOMPSON
light source. They were removed from the cleanedsample on the tip of a very fine, wetted paint brushand mounted individually on aluminium SEM stubscovered with double-sided adhesive tape. Thespecimens and stub were then sputter-coated with avery fine film of gold about 50- l(lO nm thick; thiseffectively provides an earth for the specimen, but issufficiently thin not to mask even the finest of thesurface textures. The grains were viewed with a JEOLJSM-35C Scanning Electron Microscope. Anoperating voltage of 30 kV and a working distancebetween the sample stub and the electron probe of15mm were used throughout this study to achieve thegreatest resolution of detail and, therefore, analyticalaccuracy.
7. OBSERVATION AND ANALYSIS
Thirty grains per sample were observed since such asample size has been shown to be satisfactory foraccurate environmental discrimination in other recentstudies (Tovey, Eyles & Turner, 1978; Culver et al.,1983; Campbell, 1984; Ying & Deonarine, 1985).Each grain was examined by energy dispersive X-rayanalysis to ensure that it was of quartz and thenthoroughly scanned at magnifications generally between about 200 and 5000 times. Morphological andmicrotextural analysis of quartz sand grains requirescareful comparison and recording of diagnostic surfacetextures. In this study, a range of 33 commonlyrecognised quartz grain surface textures (modifiedfrom Krinsley & Doornkamp, 1973; Culver et al.,1983; Walsh, Atkinson, Boulter & Shakesby, 1987)was used (Table 1); the presence of any given texture
on more than 10% of a grains' surface being recordedon tally sheets. These features fall into three maincategories; general grain characteristics such asroundness and relief (often inherited from sourcerock); mechanical features formed after removal fromsource rock and by transport dynamics; and thoseformed by post-depositional chemical activity.
In addition to recording the general graincharacteristics and the presence or absence of thevarious surface markings (Fig. 2), great care wastaken to record the precise relationships of thedifferent surface textures. This is vital in order todemonstrate which particular microtextural suiteformed first. A set of notes was therefore compiledfor each grain, carefully recording the sequences ofsurface texture superimposition. This is an indispensable aid for interpreting numerical surface texturedata (Campbell, 1984).
8. RESULTS
The percentages of grains showing particular texturesare presented graphically (Fig. 2). The surface featurevariability plots for both samples of sand grains (Fig.2A & B) are remarkably similar, showing littlesignificant variability between samples and a reassuring consistency in the analytical methods (the authorsanalysed, independently, one sample each of thegrains). The salient features of the surface textureassemblages for both samples are so similar that thedata sets have been combined (Fig. 2C); the followingremarks refer to the combined variability plot.Photomicrographs of key surface features andmicrotextural assemblages are illustrated in Figs. 3
TABLE 1. Surface texture categories used in SEM analysis of quartz sand grainsfrom the shelly sand beds at Moel Tryfan.
1. Conchoidal fractures (large)2. Conchoidal fractures (small)3. Breakage blocks (large)4. Breakage blocks (small)5. Arc-shaped steps6. Random scratches and grooves7. Oriented scratches and grooves8. Parallel steps9. Non-oriented V-shaped pits
18. Facet19. Cleavage flake20. Precipitation platelet21. Carapace22. Chemical, oriented 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 relief33. Crescentic gouges or scars
The numbers which precede each of the surface features in this table, are used todenote the same categories in the surface feature variability plots (Fig. 2).
Fig. 2. Scanning Electron Microscope surface featur e variability plots. (A) Moel Tryfan Sample A, (B) Moel Tryfan SampleB, and (C) Moel Tryfan combined variability plots for Samples A & B.
A B
c
E
bb
ps
cf
D
F
vp
vp
cg
cg
Fig. 3. Photomicrographs of quartz sand grains from shelly sand at Moel Tryfan. (A) Much-abraded interlocking euhedralquartz crystals (secondary quartz overgrowths) from a diagenetic environment (source rock). The abraded surfaces showevidence of pitting effected in a marine environment. (B) Possible glacially formed conchoidal fractures (cf). These textureshave been marine abraded in the same manner as the euhedral growths in Fig. 3A, resulting in a largely rounded and pittedsand grain. (C) Typical well rounded marine sand grain with low surface relief. (D) Detail of surface of marine sand grainshowing dense pattern of non-oriented V-shaped impact pits (vp) and crescentic gouges or scars (cg) caused by grain to graincollisions in water. E. Remnant surface of well abraded marine grain (rs) with superimposed textures, including largeconchoidal fractures (cf), breakage blocks (bb), arc-shaped steps (as) and parallel steps (ps), possibly caused by glacialcrushing. (F) Characteristic glacially formed surface textures superimposed on a marine grain that now has a subangularoutline and medium to high surface relief.
SEM ANALYSIS OF MOEL TRYFAN DEPOSITS 129
A
30 KIJ X430 1100 _.-~ 1 oi10U UCS86
c
B
~-"'::~ -,",-30KU X7S00 . 1212 -. 1 . 0U~ U C S S 6
DFig. 4. Pholomicrographs of quartz sand grains from shelly sand al Moel Tryfan . (A) Glacial textures including classicexamples of arc-shaped steps (as) and parallel steps (ps). (B) Slight abrasion and pitting along the edges of otherwi se sharpand fresh glacially formed textures; such edge abrasion may have occurred during meltwater transportation . (C) An excellentexample of a pitted and etched surface caused by the solution of silica. This pronounced etching is, however, atypical of theMoel Tryfan samples as a whole and may therefore be diagenetic (source rock) rather than post-depositional in origin . (D)High magnification detail of amorphous silica precipitation on an othe rwise very well rounded and pitted marine-type sandgrain.
and 4. The information recorded along the bottomedge of each micrograph refers, in turn , to theoperating voltage of the microscope ; the magnification; the micrograph number; the scale in microns ;and the date the photomicrograph was taken . Sincethe magnification factor varies with the degree ofphotographic enlargement, the micron bar must bereferred to for scale.
Both samples were dominated by rounded andsubrounded grains (81.66%) (categories 14 & 15,Table 1), with only a very small percentage of grains(1.66%) possessing angular outlines (17). The vast
majority of grains was characterised by surfaces withmedium to low relief (> 98% )(30, 31). Non-ori entedV-shaped impact pits (9) were the most frequ entmechanically formed surface texture on the grains(>78%) , although crescentic gouges (33) were alsocommon (>41 %) . A smaller proportion of grainsshowed other mechanically formed features such aslarge and small conchoidal fractures (>48%)(1 ,2),large and small breakage blocks (>21%)(3,4) andparallel and arc-shaped steps (>34%)(8, 5). Featuresassociated with chemical modification of grain surfaces(for example , solution pits and hollows , 26) were
130 STEWART CAMPBELL AND IAN C. THOMPSON
common, although the degree of modification wasusually slight, and rarely completely masked any ofthe underlying mechanically formed textures.
9. INTERPRETATION OF THE DATA
At the outset it should be stated that one method ofsedimentary analysis, in this case Scanning ElectronMicroscopy, can never be entirely conclusive; othermethods of investigation might well have yieldeddifferent evidence. Nevertheless, we believe that theSEM data presented here give valuable newinformation about an important and controversialdeposit-one that cannot be easily studied by othermethods without extensive disruption to anddestruction of the remaining beds.
Initial assessment of the numerical surface texturedata indicates the presence of two main suites ofmicrotextures. First, there are mechanically formedtextures including conchoidal fractures, breakageblocks and steps (Figs. 3F & 4A) which are usuallyassociated with high energy, particularly glacial,environments (e.g. Krinsley & Takahaski, 1962a;Krinsley & Doornkamp, 1973; Whalley & Krinsley.1974; Whalley, 1979); comparable textures have alsobeen produced experimentally by crushing (Krinsley& Takahashi, 1%2b). Although such textures havefrequently been associated with glacial environments,they are by no means exclusively produced by glacialprocesses; quartz grains from grus and material freshlyliberated from crystalline sources are also characterised by abundant conchoidal fractures and generically related arc-shaped steps and parallel steps (e.g.Higgs, 1979; Culver et al., 1983). A second set oftextures, comprising non-oriented V-shaped impactpits and crescentic scars or gouges (Fig. 3D),occurring on largely rounded/subrounded grains withlow to medium relief (Fig. 3C), are associated withrelatively high energy subaqueous environments, forexample, beaches (Krinsley, Silberman & Newman,1964; Krinsley & Funnell, 1%5; Hey, Krinsley &Hyde 1971; Steiglitz & Rothwell, 1972).
A closer inspection of the sequence of surfacetexture formation, reveals a more complex history;five major suites of sequentially superimposedtextures can be recognised. First, the oldest surfacefeatures present are diagenetic textures inherited fromthe source rock and also some textures characteristicof glacial environments (Fig. 3A & B). The formerinclude remnants of formerly interlocking secondaryquartz crystals (quartz overgrowths) clearly identifiedby their euhedral form; such characteristic diageneticfeatures are well documented in the geologicalliterature (e.g. Waugh, 1970; Pittman, 1972; Bull,1976; Wilson, Bateman & Catt, 1981; Campbell,1984). The glacial textures that can be recognisedfrom this early part of the sand grains' history aredominated by conchoidal fractures (Fig. 3B). Second,
all of these diagenetic and glacial textures have beenconsiderably worn (Fig. 3A & B) and altered bysubaqueous (probably marine) activity, accountingfor the widespread occurrence of impact pits andgouges (Fig. 3D) on generally well abraded,rounded/subrounded grains (Fig. 3C). Indeed, subaqueous modification characteristics are dominant inthe samples to the extent that some grains (Fig. 3C)show no evidence of other processes, having beencompletely smoothed and abraded to a well roundedform with virtually no surface relief. The strongdevelopment of these characteristics and, in particular, the high density of non-oriented V-shaped impactpits and crescentic gouges, leaves little doubt that thissubaqueous modification occurred in a marineenvironment (Krinsley & Doornkamp, 1973; Ying &Deonarine, 1985). Third, a smaller proportion ofgrains, that has clearly first been thoroughly abradedin a marine setting, exhibits subsequent fresh breakages and textures frequently associated with glacialcrushing (Fig. 3E & F). Although these freshmechanical breakages are common on grains takenfrom glacial sediments, they can in fact be produced inother high energy environments. It cannot thereforebe ruled out that these breakages did not originate ina high energy beach environment. However, the widerange of mechanical textures associated with thisphase of grain modification in the Moel Tryfansamples, rather than the occurrence of simple cleanbreakages, leads us to believe that this phase ofmechanical modification occurred during glacialtransportation. Fourth, some of these freshly brokengrains also show signs of subsequent, but slight,abrasion, particularly to the fractured edges of thegrains (Fig. 4B). In comparable studies (e.g.Shakesby & Campbell, 1980; Ying & Deonarine,1985) such edge abrasion has been attributed to grainto grain collisions in water; in this case, the limitedalteration could be explained by brief transport inglacial meltwater. Finally, all grains show a smallamount of post-depositional chemical, subaerialweathering and modification in the form of silicadeposition (Fig. 40) and silica solution and etching(Fig.4C).
10. DISCUSSION
The transport of materials from the floor of the IrishSea to the summit of Moel Tryfan has traditionallybeen regarded as showing a substantial verticaldisplacement of a generally southward-moving IrishSea ice-sheet, amounting to some 400 m in a distanceof only about 10 km (e.g. Bowen, 1977). It has largelybeen assumed that the deposits were dredged from thefloor of the Irish Sea Basin (e.g. Whittow & Ball,1970) and subsequently redeposited at or near thebase of that ice-sheet (Davies, 1988). The precisemechanisms of entrainment, transportation and
SEM ANALYSIS OF MOEL TRYI'AN DEPOSITS 131
deposition, however, have never been adequatelyexplained.
The present stratigraphic observations confirm thebroad relationships of the beds given by earlierworkers (e.g. Greenly & Badger, 1899; Hicks et al.,1899); namely, that a grey, stony Welsh till,widespread on the eastern flanks of Moel Tryfan,overlies a more localised sequence of shelly sands,silts and gravels (Fig. l C). The contortions recordedat the junction of the till and sand beds (Hicks et al.,1899), and still visible locally, seem to suggest that theIrish Sea sediments derived from the north weresubsequently overridden by Welsh ice . An origin asload structures, however, cannot be ruled out. Alikely path for such Welsh ice would be thenorth-westerly running valley between the CwmDwythwch massif and Mynydd Mawr; this mayexplain the preponderance in the till of clasts of thedistinctive Mynydd Mawr microgranite (Hicks et al.,1899). The lack of sedimentary structures and thesedimentological1y uniform nature of the sand bedspresently exposed at the site (Davies , 1988) inhibitpalaeoenvironmetnal interpretation of this part of thesequence. The SEM surface texture data presentedhere and the condition of the derived marine fauna,however, do offer further clues as to the origin ofthese beds .
From the common occurrence of whole mol1uscshel1s in the sand and gravel beds Davies (1988)suggested transportation in a large frozen block,implying limited reorganisation of the sediment andminimal abrasion of the contained fauna (ct.Thompson & Worsley, 1966; Eyles & McCabe, 1989).Such a mechanism does not accord with the SEM datawhich show that a substantial proportion of themarine sand grains has been crushed and brokenduring glacial transportation. These glacial1y modifiedgrains also show evidence for a subsequent phase ofsubaqueous activity, here propounded to haveoccurred in a meltwater (fluvioglacial) environment.Furthermore, Davies' suggestion does not accord withthe fact that where the shelly beds are still visible,most of the shel1s are highly comminuted. Indeed, thefragmented and polished condition of many of theMoel Tryfan shel1s and the recognition of mixedhabitat assemblages (Darbyshire, 1863; Jeffries, 1880)have been used as strong evidence for glacial grindingand mixing (Charlesworth, 1957). Whereas the wholeshells appear largely to have come from gravel lenses(e .g. Hicks et al. , 1899), the comminuted fragmentsare dispersed throughout the sands (Foster, 1968). Itis therefore plausible that the sea-bed sediments wereentrained and carried by the Irish Sea ice , and thenredeposited by meltwater; the selective distribution ofthe whole and comminuted shells being the result ofnatural subaqueous sorting. The SEM data from thesampled sands we have taken as showing that theentrained Irish Sea marine sediment was substantially
modified and mixed by glacigenic processes. Fluvioglacial re-sorting of the deposits negates the need toinvoke complex and selective mechanisms of sedimentfreezing onto the glacier sole which are otherwisenecessary to explain the distribution of the whole andbroken shells .
If the shelly silts , sands and gravels were final1ydeposited in a fluvioglacial environment , as suggestedhere , then the surface of the Irish Sea ice-sheet neednot have been locally much higher than the summit ofMoel Tryfan itself. It is quite feasible that glaciallytransported and modified marine debris was carriedupwards through the ice profile from the basal ice andsediment layers, for example, by shearing near an icemargin. Without detailed sedimentological data it isimpossible to be precise about the specific processesand environments which led to the final deposition ofthis englacial debris (e.g. kame, esker, kameterrace?). The shelly sediments may wel1 have beendeposited in a subaerial ice-marginal situation, priorto being overridden by a Welsh ice-stream. Such adepositional environment is consistent with thesedimentary model recently proposed for the Irish SeaBasin, including Llyn and the coastal margins ofnorth-west Wales, by Eyles & McCabe (1989). Theseworkers envisaged that during wastage of the LateDevensian Irish Sea ice-sheet , regional1y highsea-levels occurred as the result of glacio-isostaticdepression of the land surface. This created a glacialtidewater environment which resulted in rapid calvingof the ice margin, local glaciomarine sedimentationand catastrophic decay and disintegration of theice-sheet. During this general process of ice wastage,the margin of the Irish Sea ice-sheet may temporarilyhave been located along the northern coast of Llynbeing excluded by hills from the rest of the peninsula.Cols between these hills, however, may have acted asdrainage channels carrying debris-charged meltwaterto mid and southern Llyn where, it is proposed, thesediments were deposited in a series of Gilbert-typedeltas built out into the sea (Eyles & McCabe, 1989).Such subaerial meltwater could have deposited theMoel Tryfan sands and gravels.
Since Moe) Tryfan lies very close to thisreconstructed recessive ice margin, the valleys andtopographic depressions adjacent to the site may alsohave funct ioned in a similar manner, carryingmeltwater and sediment to the south. The sortedshelly deposits on the flanks of Moel Tryfan may haveaccumulated , therefore, either subglacial1y or subaerially , at significantly higher altitudes than theproposed delta complexes at Bryncir and Bodfuan(Cors Geirch) to the south and west. While thedeposits may be of the same age, the results presentedhere cannot be used to support the claim thatsouthern and mid Llyn lay beneath the sea at thistime.
Past records of till beneath the shelly sand and
132 STEWART CAMPBELL AND IAN C. TIIOMPSON
gravels have never been confirmed (Hicks et al., 1899;Greenly, 1900; Foster, 1968). However, they leave theintriguing possibility that the sequence is even morecomplex than presently thought . They further ensurethat a satisfactory resolution of the stratigraphic,sedimentological and chronological problems, thathave already led to over 150 years of debateregarding the site, will have to await furtherdiscoveries and the application of hitherto unused andnewly developed techniques.
11. CONCLUSIONS
Despite problems of interpreting evidence from asingle method of sedimentary analysis , the results ofthis study indicate a complicated history for the sandgrains in the Moel Tryfan shelly sands. It appears thatgrains have passed from their original rock source(s)through glacial, marine, glacial again and perhapsfinally, meltwater environments to reach their presentposition. The simplest hypothesis to explain thestratigraphic sequence at the site is to regard theshelly silts, sands and gravels as deposits entrained byice from the floor of the Irish Sea Basin , substantiallymodified during transport by glacial grinding andcrushing and, finally , redeposited by meltwater. Thesereworked Irish Sea sediments subsequently appearto have been overridden by a Snowdonian ice-stream
which deposited a stony till. At present , there is nostratigraphic or dated evidence which suggests otherthan a Late Devensian age for both of these separateglacial events; the limited chemical weathering of theconstituent sand grains is consistent with this mostrecent ice-sheet phase. The results presented here,although based on a limited set of data, furtherdemonstrate the effectiveness of the SEM techniquefor retrieving detailed palaeoenvironmental information where other standard techniques cannot beapplied or are undesirable.
ACKNOWLEDGEMENTS
The research for this paper was undertaken when bothauthors were members of the Department ofGeography, University College of Swansea. Inparticular, therefore, we would like to thank ProfessorNicholas Stephens for facilities in the GeographyDepartment and Mr Alan Cutliffe for help with theproduction of the SEM micrographs. The technicalassistance of Mr Malcolm Williams (Department ofMetallurgy) with the operation of the SEM is alsomuch appreciated. The authors would like to thankMr N. D . W. Davey and Drs P. Allen , J . E. Gordonand W. A . Wimbledon for critical comments on themanuscript. Ian Thompson acknowledges the supportof a NERC studentship.
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