Travel

A small glacial cirque basin on Exmoor, Somerset

Description
A small glacial cirque basin on Exmoor, Somerset
Categories
Published
of 10
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Share
Transcript
  AsmallglacialcirquebasinonExmoor,Somerset StephanHarrison ,Ed Anderson andDavidG. Passmore HARRISON,S.,ANDERSON,E.   PASSMORE D.G. 1998. Asmallglacialcirquebasinon Exrnoor Somerset. Proceedings oj theGeologists'Association, 109, 149-158. GeomorphologicalandsedimentologicalinvestigationsatthePunchbowlonExmoor,northSomerset,havedemonstratedevidenceofcirquebasinglaciation,some30kmsouthoftheacceptedsouthernlimitofAnglianicecoverinthisarea.ThePunchbowlisasteep-sided,north-facingbasinthathasasubduedarcuateridgeatitsmouth.Analysisofrnacrofabrics,clastformandroundnessandparticlesizeonsedimentsformingtheridgeindicateittoconsistofglacialdiamictdepositedasaterminalmoraine.Furtherevidencesupportingthepresenceofaformercirquebasinglacieratthissiteisderivedfrombasinmorphologyandastriatedsandstoneboulder.Palaeoglacierreconstructiongivesasurfaceareaof 0.38km-, amaximumthicknessof 46.5 mandamaximumbasalshearstressvalueof 51 kPa.TheELAoftheformerglacierhasbeencalculatedat334mOD.Thereisnoevidenceformultipleglaciationsatthissite,and,intheabsenceofdatingcontrolsonthemoraine,theglacierisprovisionallyassumedtohaveformedduringsevereclimaticconditionsassociatedwiththesoutherlyadvanceoficesheetsduringtheAngliancoldstageorduringtheDimlingtonStadial.DevelopmentofaglacieratthislocationappearstohavebeenarareeventduringtheQuaternary,andmayhavebeenfacilitatedbyaccumulationofwindblownsnowfromtheadjacentplateau. 'CentrejorQuaternaryScience,CoventryUniversity,PrioryStreet,CoventryCVI5FB. School oj Geography,MiddlesexUniversity,Queensway,EnfieldEN]4SF.tDepartment oj Geography,NewcastleUniversity,TynesideNEI7RU. 1.INTRODUCTION Atpresent,thereisgeneralagreementthatthesouthernmost land-based glacialiceintheBritishIslesduringtheQuaternarywastheAnglianglaciallimitwhichrunsfromthenortherncoast of the Southwest Peninsulatothenorthernpart of theThamesestuary(Bowen,Rose, McCabe   Sutherland,1986;Jones   Keen,1993).However,evidenceispresentedhereofcirquebasinglaciationinthe Punchbowl onExmoor,Somersettothesouth of thislimit.Straw(1995)hasalsohighlightedthepossibilitythatthissite may havecontainedasmallglacier;hewrotethat,  if it(thebasin)hadreachedasizesufficientforoccupationbya permanent snowpatchwithin which someinternal deformationwaspossible,itis,perhaps, Exmoor s soleinstance of anincipientglacialcorrie'(p.22).However,Straw(1995)didnotpresentanyevidencetosupportthisidea.In this paper, detailedgeomorphological and sedimentologicalevidence demonstrate thepresence of formerglacialiceatthislocation. From thisevidenceitispossibletoreconstructthedimensions of theformerglacier,itsbasalshearstressandtheEquilibriumLineAltitude(ELA). 2.DESCRIPTION ThePunchbowlisanimpressivenorth-facingbasinwhichiscarvedintothebroadplateau of WinsfordHill(426 mOD) (Figs 1,2 and3).ItislocatedatSS883345,some2kmduewest of thevillage of Winsford. The areaisunderlainby Proceedings of theGeologists'Association. 109,149-158. Upper Devonian Morte Slatesandsandstones,andthesiltstonesandslates of thePickwell Down Beds;the junctionbetween theseformationsliessome 600 mnorth of thesouthernrimofthe Punchbowl (GeologicalSurvey of EnglandandWales1969, Sheet 294,Dulverton). The floor of thebasinisat 300 m OD andthesteepbackwallrisesto 400 m   andissetatanangleupto70°.Bedrockis exposed atseveralplacesonthebackwall.Thebasinisupto 350 mwideanditsdownslope edge ismarkedbyasubduedarcuateridgeuptoIImhighand40mwidewhichhasbeenincisedbyasmall stream (Fig.4).Thisridgeis110mindistancefromthe base of thebackwall. The basinflooriscoveredwiththickvegetationandbogpeatandthe basin sidesarealsovegetationcovered. 3. METHODOLOGY Geomorphic andsedimentologicalmethodswereusedtodeterminetheorigin of theridgeatthefront of thebasinandthedebris cover whichmantlestheadjacentplateauandvalleysides.Detailedgeomorphologicalmapping of thePunchbowl(seeFigsIand4)wascarriedoutusingenlargedbasemapsreproducedfromthe1:10000 Ordnance Surveymap.Themapwasredrawn,scannedanddigitized. Slope gradientswere measured inthefield using aclinometerandtopographicprofilesconstructedfromtheI: 10000 map. Five siteswereselectedtoinvestigatethecomposition of thebasinridgeandtheplateauandvalley-sidedebris 0016-7878/98$10·00©1998 Geologists'Association  ISO .>0  '0   . . -   セ ::::::::::::   . .. セ   s.   RR SON tcr  v z.. , »<: Fig.LGeomorphologicalandlocationmapofthePunchbowl.Somerset.Thevalleys 10 thewestofthePunchbowlarealsoshown.Fig.2.ThePunchbowlfromthenorth.  GLA C IAL CI RQUEBASIN , EXMOOR Fig.3.ThearcuateridgefromthebackwallofthePunchbowl. 151 covers.AtSite1(Fig.4)ahand-dugpitmeasuring1.8wide x 1.5long x 1.2mindepthwasexcavatedintothesouthwest-facingslopeofthenorthwesternsegmentofthebasinridge.Thedepthoftheexcavationwasnecessarytocleartheweatheredprofile.Site2isasmallnaturalexposureonthewesternside of thesoutheasternsegmentofthebasinridgesome1.5mdeepand1mwideandislocatedapproximately 4 mbelowtheridgecrest.Site3isastream-cutsectionincisedintothevalley-sidedebriscoverapproximately250mdownstreamofthefrontalridgeand1.5mabovethepresentriverlevel.Itrunssome 8 malongthevalleyfloorandisupto2.3mdeep.Site 4 isanexposedsectioncutintothesedimentsontheplateausurfaceofWinsfordHill,100msouthwestofthetopofthePunchbowlbackwall.Itreachesamaximumof 1 mindepth.Site5isanaturalexposurecutintotheridgebythesmallriverandis1.2mdeepand2.5mwide.Inordertoassessthedepositionaloriginofthesedimentexposedateachsitethefollowingstandardtechniqueswereapplied:macrofabricanalysis,clastformandroundnessandparticlesizeanalysis.MacrofabricanalysiswasundertakenatSites 1- 3on50clastswitha:blengthratios of 2:1ormore. The sesiteswerechosentoisolatepotentialfabricsformedbydirectsubglacialdepositionandtobeunaffectedbypost-depositionalmodification.Therefore,Sites1and2aresituatedonthestossside of thebasinridge,asopposedtotheleeslopeswheresedimentswouldhaveundergonerapidre-sedimentationbymass-wastingprocesses.Foreachdatasetthree-dimensionalmacrofabricdiagramswerecomputedusingtheproceduredevelopedbyAndrews   Shimizu(1966)andnormalizedeigenvalues(S t) werecalculatedataconfidenceintervalgreaterthan95%followingthetechniqueof Mark (1973).Randomsamplesof50 cla stsinthesizerange 25 100mmwereselectedforclastformandroundnessanalysesatallfivesites.TwosamplesweretakenatSite I (samples I aand I b).Clastshapewasportrayedbyplottingc:aandb:aratiosonternarydiagramsandquantitativelyanalysedbythemethodoutlinedbyBallantyne(1982).RoundnessvalueswerecalculatedusingPowers'(l953)roundnesscharts.Particlesizeanalysis of thefinerthan2mmdiameterfractionwas condu ctedonsamplesfromallfoursitesusingacombinationofwetsieving(>63 urn) andsedigraphanalysis«63 urn). Inaddition,thesedimentprofileexposedatSite3wasloggedusingstandardlithostratigraphictechniques. 4. RESULTS MorphometricevidencesuggestingsignificanterosionofthePunchbowlincludesthesteepbackwallandenclosedbasinshapeofthehollow.Tothewest,andalsoincisedintoWinsfordHillarefourothernorthward-facingtributaryvalleys.EachofthesearecutintotheMorteSlatesandPickwellDownBeds;thelatterrocksunderlyingth eir headwaters.Thesevalleysaredescribedbelow. RiscombeCombe Thisformsasemi-circularheadedbasin300mtothenorthwestofthePunchbowl.Thefloorofthecombeissmoothandrisesgentlyfrom270m   to300mOO.The  152 S.H ARR ISON ETAL . o 2  l } m II  _ .   セ   __ I I R\ \I • ite4  ? moraineridge ro kw ll  :: 1 marsh   streamcutting Q trees   .. 10 - contoursinmetres Fig.4.MacrofabricsfromthearcuateridgeandSite3. headwallsofthecombearesteep(averageangleof32°)androundedandriseto370mOl).Thebasinisupto200m in widthand350mlong.Thereisnostreamoccupyingtheflooratthepresenttime.risesataspringat350mOf)andthemouthofthecombeliesatabout280mOl).Thecombeisabout300m in lengthand100mwideatitswidestpart.Some250mfurtherwestisAshCombe. Unnamedcombe Thiscombeislocated400mduewestofRiscombeCombe.ItsmorphologyislessroundedthanRiscombeCombeandthefloorisoccupiedbyasmallstream.Theheadofthis AshCombe Thisbasinisoccupiedbyastreamandthesmoothed.roundedbasinformdisplayedbybothRiscombeandtheunnamedcombesisdisruptedatAshCombebyheadward  Slabs Sile 1. Site lb Site 2 lOlh  セ   ィ 0 r 80-1 X .Mean   セ   » o roundness セセ Q セ •••• セ セ :.: = 98 | エ Z   セ   » % 60 oj 5.0• Standard = 0.17 ,0\ セ r deviation 0.08.':'. = 0.07'...-; o . 40 セ ?J o セ l A'SA'SA' R wA' II C Site 3 Site 0 SiteS tTl n .50   » Vl Z m I >< X =037 1 X .038 1 X .038 1 X .02 l X  21 l X .0.33 セ 5 5 5 5 5.0.0.065.0   .04 5 . 003 50   7 00   Key Site1a Site lb Site2Site3SileoSiteS Fig.5.Clastformandroundnessdiagramsfromthefivesites. lJ> W
Search
Similar documents
View more...
Related Search
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks