A Late Devensian rock glacier in the Nantlle ValleyWord文档格式.docx
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A Late Devensian rock glacier in the Nantlle ValleyWord文档格式.docx
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Sciences,
CoventryUniversity,
PrioryStreet,
Coventry,CV15FB,UK.
s.harrison@coventry.ac.uk
Anderson,E.
DepartmentofGeography,
BedeCollege,
HaleRoad,
Billingham
TableofContents
Introduction
Rockglaciers
Methodology
Landformdescription
Interpretation
Discussion
Debrissupply
Formationofburiedice
Palaeoclimaticimplicationsandage
Conclusions
Acknowledgements
References
Figures
Abstract
Inthispaperwedescribearelictrockglacieronthesouth-facingflankoftheNantlleValley,NorthWales.Thelandformiscomposedofdebrisofbothglacialandtalusorigin.Macrofabricanalysesundertakenondebrisabovethesteepfrontalslopedemonstrateclastfabricsformedbythecompressiveshearingofdebrisandtheseareconsideredtoreflectthethrustingofmaterialovertheslowermovingfrontalmargin.Theabsenceofsurfacecollapsestructuressuggeststhattherockglacierdidnotcontainalargebodyofice,whichfurtherimpliesthattheicewhichfacilitatedmovementwasperiglacialinorigin.Atpresent,theabsoluteageoftherockglacierisnotknown,butitisconsideredmorelikelythatitformedduringthefinalstagesoftheDimlingtonStadialoftheDevensianglaciation(LateGlacialMaximum),thanduringtheYoungerDryas.
Keywords:
RockGlacierNorthWalesperiglacialLateGlacialMaximum
AtpresentrelativelyfewrelictrockglaciershavebeenidentifiedinuplandareasoftheBritishIsles.Areviewoftheliteraturerevealsthatfewerthanthirtyhavebeenreportedandthevalidityofanumberofthesehasbeenquestioned(cf.BallantyneandHarris1994).ThelargestconcentrationislocatedintheCairngorms(Sissons1979,BallantyneandHarris1994),whereuptoeighthavebeendocumented.InIreland,severaloccurintheDonegalmountains(Wilson1990a,b,1993)andintheMacgillycuddy'
sReeks(Andersonetal.1998)
andothershavebeenreportedintheEnglishLakeDistrict(Sissons1979,Oxford1985)andinSnowdonia(Harrison1992).AllareconsideredtobeLateglacialinagesincetheyoccuroutsideYoungerDryas(11-10kaBP)glaciallimits(BallantyneandHarris1994).ThescarcityofrelictrockglaciersinuplandBritainandIrelandmayreflectthefactthatclimaticconditionsordebrissupplyratesduringtheLateglacialwerenotconducivefortheirwidespreadformation.Nevertheless,thelimitednumberofreportedrelictrockglaciersmayalso,inpart,reflecttheproblemsassociatedwiththeircorrectidentification(BallantyneandHarris1994).Relictrockglacierscanresembleprotalusramparts,rockavalanchedepositsorlargebodiesofmorainicdebris,anddifferentiationisfurthercompoundedsincealltheselandformscanoccupysimilartopographiclocations(BallantyneandKirkbride1986,WhalleyandMartin1992,Shakesby1997).
AnexampleoftheproblemsofcorrectinterpretationishighlightedbythedebatesurroundingthehugebouldermassbelowSgurrMoronBeinnAlligininnorthwestScotland.
Sissons(1975)arguedthatitwastheresultofarockfallwhichreactivatedasmalldecayingglacieroccupyingthesoutheast-facingcirque;
theresultinglandformrepresentingaglacierice-coredrockglacier.
Whalley(1976),ontheotherhand,suggestedthatthefeaturehadmanyofthecharacteristicsoflargecatastrophicrockavalanchesorsturzstroms.
Ballantyne(1987)hasjoinedthedebatebyarguingthatthedecayingglaciermayhavebeenlargerthanenvisagedbySissonsandtransportedthedebrisdownslopeasasupraglaciallayer.Inviewoftheproblemsofidentificationandclassification,therefore,webelievethatanumberofrelictrockglaciersmayhavebeenmis-identifiedandhaveyettobedescribed.
IntheNantlleValley,NorthWales,Gray(1982)mappedafeaturewhichheinterpretedasaprotalusrampart.Inthispaperwesuggestthatthislandformisinfactarockglacier.Ourproposalisbasedonmorphologicalsimilaritieswithcontemporaryrockglaciersandconsiderationofclastmacrofabricdataderivedfrompitsexcavatedintothelandform.Thefeatureislocatedat160mODatthebaseofthesteepsouthfacingslopeofMynyddMawr695mOD(Fig.1).ThisareaisunderlainbyOrdovicianlow-grademetamorphiclithologiesslate,phylliteandquartzite)andwaslastglaciatedduringtheDimlingtonStadial26-13kaBP;
(Rose1985)whentheentirelandscapewasprobablysubmergedbeneaththickicesheets(Boultonetal.1977,1985).
Rockglaciersarelarge-scalebodies(20-100mthick)ofice-richdebriswhichflowdownslopeatlowvelocities(0.1-1.0ma-1)inresponsetotheshearingstressesgeneratedbytheirunsupportedweight.Theyaretypicallytongueorlobateshaped,anddevelopintospatulateformswheretheyflowontolow-reliefareas.Activerockglaciersarecharacterisedbyasteepfrontalslopewhichisatorneartheangleofrepose(35-40°
).Theuppersurfacesofrockglaciersareusuallycoveredbylargeclasts(0.1-5.0m)andoftendisplayacurvingridgeandfurrowtopographywhichreflectstheflowregimeofthefeature.Inaddition,enclosedsurfacehollowscansometimesbeobservedandareconsideredtobecausedbythepartialorcompletemeltingofinternalicebodies.Activerockglaciersarecommoninmanyhighreliefalpineandpolarenvironments,whilerelictexampleshavebeennotedinmanymountainousareascurrentlyexperiencingmoretemperateclimaticconditions.Activerockglaciersoccurbelowthesnowlineandabovethelowerlimitofdiscontinuouspermafrost,andhenceareoftenviewedasbeingcharacteristicofcontinentalclimateregimesHaeberli1985,Barsch1992).However,anumberhavebeenreportedfrommaritimeenvironments(Eyles1978,Whalley1974,BirnieandThom1982,Humlum1982,MartinandWhalley1987,André
1994).
Theoriginofrockglaciersisasubjectofintensedebate.Manyauthorsconsiderthemtobepurelyperiglacialfeaturesandattributeflowtotheinternaldeformationoficerichlenseswithinpermafrost(WahrhaftigandCox1959,Haeberli1985,Barsch1977,1988,1992).Otherauthorshaveproposedthatsomerockglaciershaveformedthroughtheburialanddeformationofglacialice(OutcaltandBenedict1965,Potter1972,Whalley1974,Johnson1980,Humlum1982,1996;
WhalleyandMartin1992,Evans1993,Whalleyetal.1994,HamiltonandWhalley1995).Athirdmodelconsidersthatcertainrockglaciersareformedbyrockslides(Johnson1978,1984,1987).Thisbodyofworksuggeststhatrockglacierformationmayreflectgeomorphicequifinality.Indeed,severalauthorshaveproposedthatsomerockglaciersarepolygeneticinoriginandaretheresultofacombinationofperiglacial,glacialandcatastrophicrockslideprocesses(e.g.WhalleyandMartin1992).
Inordertoestablishtheoriginofthestudylandformanditsgeomorphicrelationshipwiththeadjacentslopedepositsanumberofstandardmethodswereused.Detailedgeomorphologicalmappingoftheareawascarriedoutusingenlargedbasemapsreproducedfromthe1:
10000OrdnanceSurveymap.TopographicprofileswereconstructedusingaLeicatotalstation.Atfoursiteshandexcavatedpitsmeasuring2.0mwide,1.8mlongandatleast1.4mdeep,weredugtoinvestigatethecompositionofthefeatureandadjacentslopematerials.Pits1-3werelocatedontherelativelyflat,uppersurfaceofthelandform,whereasPit4wassituatedonthedebrisslopetotheeast.Inallfourpitssedimentcharacteristicswereassessedusingmacrofabricanalysis,clastformandroundnessanalysis.Macrofabricanalysisinvolvedmeasuringthea:
baxisazimuthof50clastswitha:
blengthratiosof2:
1ormore.Foreachdatasetthree-dimensionalmacrofabricdiagramsandnormalisedeigenvalues(Mark1973)(ataconfidenceintervalgreaterthan95%)werecomputedusingStereoplotsoftware.Randomsamplesof50clastsinthesizerange25-100mmwereselectedforclastformandroundnessanalysis.Clastshapewasportrayedbyplottingc:
aandb:
aratiosonternarydiagramsandquantifiedusingtheprocedureoutlinedbyBallantyne(1982).RoundnessvalueswerecalculatedusingaPowersroundnesschart.
Thelandformcomprisesalobe-shapedmassofdebris,90mlongandupto270mwide(Figs1and2).Itdisplaysasteepfrontalslope,upto31°
Figs3and4),whichrisestoamaximumelevationof22mabovethevalleyfloor.Elsewherethelandformexhibitsalowgradientsurface-2°
to8°
;
(Fig.5)andgradesupslopeintodriftandtalusdeposits(Figs1-3),althoughthecontactbetweenthelateralmarginsandadjacentslopedebrisissharp.Towardsthewesternendofthelandformtwosmall(<
1.5mhigh)boulderyridgesandacircularsurfacedepressioncanbeobserved(Fig.1).Overmuchofthelandformsurfacethereareirregularspreadsofangularboulders,whichinplacesappeartogradeintotalusdeposits.MaterialrecoveredfromPits1to3(Figs4and6),whichwereexcavatedintoareaswhichlackedthissurfacerubble,demonstratedthepresenceofhomogenoussandydiamict.Componentclastsofthisdiamictareangulartosubroundedandhavemeanc:
aratiosof0.36-0.42.Inaddition,macrofabricanalysisofdebrisatthebaseofthepitsrevealedapronouncedpreferredorientationwiththea-axesofelongateclastsdippingsteeplyintothelandforminanortherlydirection(Fig.6).ThematerialfromPit4,excavatedintotheadjacentdriftslope,isalsoahomogeneoussandydiamictandcomponentclastsdisplaysimilarroundnessandformcharacteristicstothoseexposedinPits1to3(Fig.6).However,thewelldev
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