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UrbanGeologyinLondonNo.30

©RuthSiddall;UCL,January2015;revisedJuly2015.

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AnUrbanGeologist’sGuidetotheFossilsofthePortlandStoneThePortlandseaswerewarm,fullofnutrientsandthereforefulloflife,andplentyofevidenceforthiscanbeseenonthewallsofLondon’sbuildings.GoodreviewsanddiscussionofthegeologyofthePortlandStonearetobefoundintheexcellentwebpagescreatedbyIanWest(West,2013,2014)andinCope(2012).AnexcellenttreatiseonPortlandStoneasabuildingmaterialhasrecentlybeenpublishedbyHackman(2014).TheunitofuppermostJurassicage,theTithonianwhichisknownlocallyasthePortlandian(Cope,2012).Thereforetheserocksare~150millionyearsold.Stratigraphically,thebuildingstonesformtheupperpartofthePortlandLimestoneFormationandarecalledtheFreestoneMember.

Marinereptilessuchaspliosaursandichthyosaurswouldhaveswamtheseseas,thoughtheirremainsarerarelyencounteredinthestone.Varietiesofbivalvesaremostfrequentlyobserved,alongwithcoquinasofmarinesnail(gastropod)shells.Thewaterswererelativelyshallow,buttidal,theywerelagoon-typeenvironments.Thisenvironmentbringsabouttheformationofparticlesofcarbonatesandknownasooidsorooliths.Ooidsarespheroidalgrainswhichformaroundkernalsoffinesandgrainsorshellfragments.Inthewarm,tropicallagoons,tidalcurrentsrollthegrainsbackwardsandforwards,andthereforethecarbonateisprecipitatedconcentricallytoformooids.Thesehaveamaximumsizeofaround1mmdiameter,andareoftenvisiblewiththeaidofahandlens,andsometimesthenakedeye.ThereforethePortlandFreestoneisanooliticlimestone,withvariableshellcontent.TheBaseBedisrelativelyfreeoffossils.TheWhitbedcontainsvariableamountsoffossils,rangingfromafewscatteredoysterstocross-bedsectionsrichinshelldetritusandevenpatchreefs,upto3-4mdiametercomposedofoystersorthecorallinealgaSolenopora.TheRoachrepresentsashellybeachdominatedbyshellfossils,acoquina.

Above;ablockofRoachplacedontopofcross-beddedWhitbedat23KingStreet,W1.

AmajorfeatureofsomesectionsofthePortlandStoneistheleachingofaragoniticshellmaterial.Weneedtolearnsomecarbonategeochemistryheretotrytounderstandthisprocess.Calciumcarbonateisthemainconstituentofmostlimestonesandalsoshell,coralsandexoskeletonsofothermarineinvertebrates.Calciumcarbonatecanexistintwoformsor‘polymorphs’whichhavethesamechemistrybutdifferentcrystalstructures.Thesetwoformsarecalledaragoniteandcalcite.Mostshell-forminganimalssecretearagoniteastheirexoskeleton.Aragoniteisgeologicallyunstableandusualchangestothegeologicallystablepolymorphcalciteduringfossilisation.InthecaseofthePortlandStone,whichwasrapidlyupliftedfollowingitsdeposition,muchofthearagonitehasdisappearedcompletely.ThisisbecausethisformationrapidlylithifiedandthearagonitewasdissolvedawaybymeteoricwatersinthefreshwaterenvironmentoftheoverlyingPurbeckBeds.

ThelinedrawingsusedbelowhavebeenadaptedfromEricRobinson’shandout‘JurassicandCretaceousFossils’,whichinturnwereadaptedfromillustrationspresentedinArkell(1947).Howeveritisalwaysworthremindingourselvesthatthefacesofstonesweseeonbuildingsareartificialandthereforemany



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sectionsthroughthefossilsareobservedanditisrarethatperfectexampleswillbeseen.Palaeontologistsbecomeadeptatrecognisingallpossiblesectionsthroughashell.

TheFossils

Gastropods-Aptyxiellaportlandica,the‘PortlandScrew’.ThisisthemosticonicfossilofthePortlandStone,butitisonlyencounteredinthecoquinafaciescalledtheRoach.

Above:AptyxiellaportlandicaonthenewwingofBBCBroadcastingHouseonGreatPortlandStreet.

ThePortlandScrewisaturretedgastropodwhichischaracteristicofandprettymuchrestrictedtotheRoachandso,asistypicalinthisfacies,thearagoniteshellhasbeenleachedaway.Thisleaves5-6cmlong(aboutthelengthsofyourfingers),arrow-shaped,externalmouldsoftheshellsandthespiral‘screws’whichrepresenttheinternalcastsoftheshells.AptyxiellaportlandicaisnotfoundinmainlandoutcropsofthePortlandLimestoneFormationandsoitcanbeconsideredasatrulydiagnosticPortlandbeast,andcharacteristicoftheRoachcoquinas.

Above:Screw-shapedinternalcastsofAptyxiellaportlandica.Thesearenotthebodypartsofthesnail,butthecarbonatemudwhichhasfilledintheshells.TheseexamplesarefromBBCBroadcastingHouseonGreatPortlandStreet.

Bivalves



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ThecommonestbivalveencounteredinthePortlandstoneistheoysterLiostreaexpansa.Oysterssecretecalciteshellsandtheyarethereforewellpreserved.Anyonewhohaseverseenanoystershellonabeachoradinnerplatewillknowthattheyappeartobeformedofthinlayersofcalcite,butanyonewhohasevertriedtocrackoneopenwillalsoknowtheyareextremelyrobust.

Aboveleft;diagramofL.expansaafterArkell(1947).Right;asectionthroughanoyster,probablycuttingthroughclosetothehingeoftheshell,inWhitbedonBBCBroadcastingHouse.Thefinelylayeredstructureoftheshellcanbeclearlyseen.

OysterfragmentsareverycommonlyseenthroughtheWhitbedandRoach,andindeedreeflikemassesofoystersdevelopedintheWhitbed.ThebrokenshellsarealsoconcentratedinlagsinsomevarietiesoftheWhitbedasseenbelowatPaternosterSquare.

Above:oystershelllaginPortlandWhitbed.Thefieldofviewis~20cm.Below:anoyster-bedinweatheredWhitbedusedforbenchesattheEconomistPlaza,StJames’s.Thisstoneiscutparalleltobedding,FOV~50cm



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Plicatuladamoni,avarietyofthespinyshellsspondylus,areverysimilartotheoysters.Theyhadthinridgesalignedconcentricallyalongtheirshellswhichenabledthemtoanchorthemselvesinthemud.Theseshellsaredifficulttodistinguishfromoystersandpectens(seebelow)unlessyoucanseethesespinelikeridgesincrosssectionasintheexamplebelow,whichshowsbothvalvesoftheshell.

Moreoftenthannot,P.damoniformsinternalcastswhicharerathernondescriptandthey,unsurprisingly,resemblethefleshypartsofoystersinshape.

Trigoniashellsarenowlargelyextinct(thereisonesurvivingvarietythatlivesinoneplaceinAustralia).ThespeciespredominantlyfoundinthePortlandStoneisLaevitrigoniagibbosa.Trigoniidbivalvesarethick-walledshells,whichmeansthattheywereabletowithstandhighenergyenvironments.Theshellsarealsotypicallyhighlyornamentedwithribsandknobs,thelattertechnicallyreferredtoascostae(singular:costa).ThecommonestvarietyfoundinthePortlandFreestoneisLaevitrigoniagibbosa,whichhastwoformsonewithribsandtheotherwithcostae.

Above:varietiesofL.gibbosaafterArkell(1947).

TypicaloftheRoach,thearagoniticshellsofL.gibbosahavebeendissolvedaway,leavingexternalmouldswhichshowtheshellsurfaceornamentation.Alsopresentaretheinternalcastswhichreplicatetheformoftheinterioroftheshell.ThesearegiventheirownLatinnameMyorphorellaincurva.Itisnothardtoimaginehowthesecastscanbecomedetached.Ontheirowntheseoddlookingfossilsareknownas‘steinkern’,literallystonehearts.SimilartrigoniidcaststoM.incurvaweretermedhippocephaloidesby17thCenturynaturalhistorianRobertPlottbecausetheyresembledrathermournfulhorsesheads.HowevertothePortlandquarrymentheybecameknownas‘osses’‘eadsinthelocalvernacular.ImagesoftheseandPlott’sdrawingcanbeseenontheNaturalHistoryMuseum’swebsite1.

1http://www.nhm.ac.uk/nature-online/earth/fossils/fossil-folklore/fossil_types/bivalves02.htm



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Above:MouldsofL.gibbosainRoachatBBCBroadcastingHouse.Theseexamplesshowgooddetailoftheornamentedshellsurfaces.Eachshellisabout5cmacross.Below:mouldsandaninternalcastofL.gibbosaclearlyshowingthecostaeontheshellsurfaces.

BelowisadrawingofaperfectM.incurva(afterArkell,1947).Theimageontheright(FOV~15cm)showsmoretypicalcasts(aswellasaTrigoniashellmould).



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Left:TwosectionsthroughoncearticulatedvalvesofL.gibbosa,nowleachedaway.Bychance,themasonhascutthroughalongthelongaxesoftheshellsgivingtheeffectof‘angelwings’.Theribsandcostaecanbeseenontheupperexampleofthetwo.Onceagain,thisisfromtheRoachusedonthenewwingofBBCBroadcastingHouse.

Camptonecteslamellosusisavarietyofpecten.Pectenarescallopshellsandcharacteristicallyhavewelldeveloped‘wings’alongtheirhinge.ThevarietyareextremelycommonalongsomebedsofthePortlandStone.Theyarefrequentlywellpreservedasthey,likeoysters,precipitatecalciteandnotaragonite.Theshellsarearoundthesizeofmodernediblescallops.

Aboveleft;diagramtoshowtheappearanceofthewholeofCamptonecteslamellosusshell(afterArkell,1947).Right;halfapectenwithoneofthe‘wings’preservedattheendoppositemyfinger.ThisexampleisonthefrontstepsoftheSladeSchoolatUniversityCollegeLondon.

PairsofarticulatedvalvesofC.lamellosusareoftenseenintheWhitbed.Theycanbedistinguishedfromthespondylusdescribedabovebythelackofspinesandtheflatteningofthehingezone.Unlikemodernediblescallops,whichhaveoneflatandoneconvexvalve,C.lamellosushastwoconvexvalves.



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Above:acrosssectionofCamptonecteslamellosusinWhitbedonBBCBroadcastingHouse.FOV~12cm.

AlsopresentonWhitbedandtheRoacharethemusselsMytilussuprajurensis,tellinshellsPleuromyatellina,therock-boringbivalveLithophagaandvarietiesofcocklesincludingProtocardiadissimilis.Thesearelesseasytodistinguishastheyareoftenfragmentary.Theoyster-like,butaragoniteprecipitatingIsognomonlisteriisalsoknownfromthereef-formingfacies,predominantlyasinternalmoulds(West,2014,Townson,1975).

Left:Pleuromyatellina.Theseshellsare~2cmacross.

Ammonites

Ammonites,spiralshellsoccupiedbysquid-likecreaturesareclassicMesozoicfossils.Thegiantammonite,Titanitesanguiformes,istheclassicfossilofthePortlandStone,thoughtheyarerarelyobservedinbuildingstones,andiftheydotheyaregenerallyfragmentary.Wholespecimensareuptoametreacross.Anotherspecies,T.giganteus,ofsimilardimensionsalsooccursbutislessabundantthanT.anguiformes(Wimbledon&Cope,1978).

Above:imagesofTitanitesanguiformes(left)andT.giganteus(right).Bothspecimensare~1mdiameter(adaptedfromWimbledon&Cope,1978).



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VisitorstotheIsleofPortlandcanobserveagreatnumberofthesefossilssetintogardenwallsandactingasthesignsforquarries.IndeedatheoryastowhyT.anguiformesisrarelyencounteredinbuildingstones,isthatwhentheyarefound,theyarekeptasprizesbythequarrymen.GoodphotosofgoodspecimenscanbeseenonIanWest’swebsite2.TomyknowledgetheonlyexampleonaLondonbuildingstoneisonthefrontstepsoftheSladeSchoolofFineArtatUniversityCollegeLondon.Herethepavingslabstakeseveralslicesthroughthesamefragmentofawhorl(below).

Alga

TheredalgaeSolenoporaportlandicaformedsmallpatchreefsinthePortlandStoneseas(Fürsichetal.,1994).Theyarenotparticularlycommon,butwhenencounteredbyquarrymen,theyarequarriedasacoherentstoneforuseinasingleproject.AgoodexampleofsuchauseisatCaxtonHouse,TothillStreetSW1.ThesereefsaredominatedbySolenopora.However,whitefragmentsofSolenoporaarecommonthroughoutthemoreshell-richvarietiesofPortlandStoneandthoseformedinmoredynamicenvironments.Theyresembleinsizeandshapecauliflowerfloretsandshowdistinctivegrowthrings,whichsometimesshowpreferentialweatheringofalternatebands.Thesearebelievedtorepresentseasonalphasesofgrowth(seeWright,1985).

Above:Solenopora‘floret’withseasonalgrowthringsinPortlandWhitbedobservedinPaternosterSquare. 2http://www.southampton.ac.uk/~imw/jpg-Portland/11PT-Albion-Ammonite-Roach.jpg



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Above:SolenoporaweatheringoutontheparapetofWaterlooBridge.FOV5cm.

Above:SolenoporareeffaciesatCaxtonHouse,TothillStreet,SW1.FOV~20cm.

West(2014)hasalsoidentifiedthebryozoanHypororporaportlandicaencrustingshellsinthesereefs.



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Tracefossils

Tracefossilsarethepreservedmarksmadebyfeedingormovingorganismsonsediment.ThesearecommoninthePortlandStone,butareoftensubtleanddifficulttosee.Themostfrequentlyencounteredareburrowswhichcanbeformedbyanumberoforganismsbutprimarilycrustaceans,ThesearecalledThalassinoides.Theseburrowsvaryinsizefrom2-3cmindiametertomuchlarger(10cm)burrowsandtheymaybeinfilledwithshelldebrisandcarbonatemud.TypicallyThalassinoidesareabranchingnetworkofburrowsandstructuresresemblingadouble-endedYarecommonthroughoutthePortlandStoneFormation(seeTownson,1975).Theseburrowsareorientatedsub-paralleltobedding.

Above:ThalassinoidesburrowsinPortlandStoneWhitbed.ThestepsoftheKathleenLonsdaleBuildingatUniversityCollegeLondon.

Othertracesoforganismscanbefoundfromtheircircularorovoidboringsintoshellsandsediment.Thoseinoystershellsorreeffaciesaremostobviousandwereformedbyworms(i.e.Glomerulasp.,Townson,1975),spongesandtherock-boringbivalveLithophaga(alsoknownasdatemussels–modernvarietiesdoresembledatefruits).Thisshellcharacteristicallylivesinshallowwaterorthelittoralzone.

Above:Left;pelletofoysterreefboredbythebivalveLithophagasp.,formingtheovoidstructureintheuppercentre.Right:bringsinoystershellsbyspongesorworms.BothexamplesareonBBCBroadcastingHouse.



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VertebrateFossils

BonesarerarelyseeninPortlandbuildingstone,butDelair&Wimbledon(1993)recordseveralvarietiesofLateJurassicreptilesfromthebuildingstonememberandequivalentstrata,includingcrocodiles,turtles,Ichthyosaurs,PleisiosaursandsaurischiandinosaursincludingJurassicCoastfavouritesMegalosaurusandIguanodon.IamalsoreliablyinformedbyMarkGoddenofAlbionStonethatbonesandteethareoccasionallyfoundinthequarries(Goddenpers.comm.&alsoseeGodden,2012).ExamplescanbeseeninthePortlandMuseum.

TheonlyexampleIhaveencounteredinLondon’sbuildingstoneisonStMargaret’sWestminster,whereanerodedbonecobbleisseenontheSEfaçadeoftheChurch(below).

ReadmoreaboutPortlandStoneanditsfossils…

Arkell,W.J.,1947,TheGeologyoftheCountryaroundWeymouth,Swanage,CorfeandLulworth.MemoiroftheGeologicalSurvey,HMSO,386pp.

Cope,J.C.W.,2012,GeologyoftheDorsetCoast;Geologists’AssociationGuideNo.22.,Chapter5,SouthDorset;IsleofPortland,94-103.

Delair,J.B.&Wimbledon,W.A.,1993,ReptiliafromthePortlandStone(UpperJurassic)ofEngland:Apreliminarysurveyofthematerialandliterature.,ModernGeology,18,331-348.

Fürsich,F.T.,Palmer,T.J.&Goodyear,K.L.,1994,GrowthanddisintegrationofbivalvedominatedpatchreefsintheUpperJurassicofSouthernEngland.,Palaeontology,37(1),131-171.

Godden,M.,2012,Portland’squarriesanditsstones.http://www.dorsetgeologistsassociation.com/Portland-Stone/Portland_Stone_Document_-_7_June_12.pdf

Hackman,G.,2014,StonetobuildLondon:Portland’slegacy.,FollyBooksLtd.,MonktonFarleigh.,311pp.

NHMFossilFolklore:Bivalves;http://www.nhm.ac.uk/nature-online/earth/fossils/fossil-folklore/fossil_types/bivalves02.htm

PortlandMuseum:http://www.portlandmuseum.co.uk

Siddall,R.,2015,PortlandBone:http://wp.me/p53QQu-57



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Townson,W.G.1975.LithostratigraphyanddepositionofthetypePortlandian.JournaloftheGeologicalSociety,London,619-638.

West,I.M.2013.FossilsofthePortlandGroup,UpperJurassic:GeologyoftheWessexCoast(includingtheDorsetandEastDevonUNESCOWorldHeritageCoast-JurassicCoast).Internetfieldguide.Version:26thAugust2014.;http://www.southampton.ac.uk/~imw/portfoss.htm

West,I.M.2014.TheIsleofPortland:General.GeologyoftheWessexCoast(partofJurassicCoast,DorsetandEastDevonWorldHeritageSite).Internetfieldguide.Version:24thAugust2014.http://www.southampton.ac.uk/~imw/Portland-Isle-Geological-Introduction.htm

Wimbledon,W.A.&Cope,J.C.W.1978.TheammonitefaunasoftheEnglishPortlandBedsandthezonesofthePortlandianStage.JournaloftheGeologicalSociety,London135,183-190.

Wright,V.P.1985.SeasonalbandinginthealgaSolenoporajurassicafromtheJurassicofGloucestershire,England.JournalofPalaeontology,59,721-732.

LondonBuildingswithgoodandvariedexamplesofPortlandStonePortlandStone,invariousstagesofweathering,canofcoursebeseenalloverLondon.Theexamplesbelowarechosentobestshowthefossils.CaxtonHouse,TothillStreetSW1–ThebuildingiscladinstonefromaSolenoporapatchreef.

BBCBroadcastingHouse,GreatPortlandStreetW1–Onthenewextensionsofthisbuilding,excellent‘exposures’ofRoachandWhitbedcanbeseen.

EconomistPlaza,StJames’sStreetSW1–ThebuildingwhichintroducedPortlandRoachascladding.

GreenParkTubeStation,PiccadillySW1–mostvarietiesofPortlandStoneareusedhere.Thestationalsodisplay’sJohnMain’swork‘SeaStrata’whichillustratesgiantsizePortlandFossils.

NewStockExchange,PaternosterSquareEC4–thisandotherbuildingsinPaternosterSquareshowgodexamplesofWhitbedvarieties.

StMargaret’sWestminster–avertebrateboneontheSEcornerofthechurch.

UCLFrontQuad,GowerStreetWC1–ThisisagoodanaccessibleplacetoseeweatheredWhitbedandtheammoniteonthefrontstepsoftheSladeSchool(northendofthequad).

WaterlooBridgeSE1–Thebridgeisbuiltfromacross-beddedvarietyofWhitbed,richinshellfragmentsandSolenoporadebris.

MeHotelStrandWC2–thebuildingiscladwithfossil-packedCoombefieldWhitbed.

Howtocitethisarticle:Siddall,R,2015,AnUrbanGeologist’sGuidetotheFossilsofthePortlandStone.,UrbanGeologyinLondonNo.30,http://www.ucl.ac.uk/~ucfbrxs/Homepage/walks/PortlandFossils.pdf

©DrRuthSiddall,UniversityCollegeLondon,GowerStreet,LondonWC1E6BT,UK:[email protected]://www.ucl.ac.uk/~ucfbrxs/Homepage/UrbanGeology.htm;Facebook|Cultural&UrbanGeology|Twitter:@R_Siddall|#UrbanGeology