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Urban Geology in London No. 30 ©Ruth Siddall; UCL, January 2015; revised July 2015. 1 An Urban Geologist’s Guide to the Fossils of the Portland Stone The Portland seas were warm, full of nutrients and therefore full of life, and plenty of evidence for this can be seen on the walls of London’s buildings. Good reviews and discussion of the geology of the Portland Stone are to be found in the excellent web pages created by Ian West (West, 2013, 2014) and in Cope (2012). An excellent treatise on Portland Stone as a building material has recently been published by Hackman (2014). The unit of uppermost Jurassic age, the Tithonian which is known locally as the Portlandian (Cope, 2012). Therefore these rocks are ~ 150 million years old. Stratigraphically, the building stones form the upper part of the Portland Limestone Formation and are called the Freestone Member. Marine reptiles such as pliosaurs and ichthyosaurs would have swam these seas, though their remains are rarely encountered in the stone. Varieties of bivalves are most frequently observed, along with coquinas of marine snail (gastropod) shells. The waters were relatively shallow, but tidal, they were lagoon-type environments. This environment brings about the formation of particles of carbonate sand known as ooids or ooliths. Ooids are spheroidal grains which form around kernals of fine sand grains or shell fragments. In the warm, tropical lagoons, tidal currents roll the grains backwards and forwards, and therefore the carbonate is precipitated concentrically to form ooids. These have a maximum size of around 1 mm diameter, and are often visible with the aid of a hand lens, and sometimes the naked eye. Therefore the Portland Freestone is an oolitic limestone, with variable shell content. The Base Bed is relatively free of fossils. The Whitbed contains variable amounts of fossils, ranging from a few scattered oysters to cross-bed sections rich in shell detritus and even patch reefs, up to 3-4 m diameter composed of oysters or the coralline alga Solenopora. The Roach represents a shelly beach dominated by shell fossils, a coquina. Above; a block of Roach placed on top of cross-bedded Whitbed at 23 King Street, W1. A major feature of some sections of the Portland Stone is the leaching of aragonitic shell material. We need to learn some carbonate geochemistry here to try to understand this process. Calcium carbonate is the main constituent of most limestones and also shell, corals and exoskeletons of other marine invertebrates. Calcium carbonate can exist in two forms or ‘polymorphs’ which have the same chemistry but different crystal structures. These two forms are called aragonite and calcite. Most shell-forming animals secrete aragonite as their exoskeleton. Aragonite is geologically unstable and usual changes to the geologically stable polymorph calcite during fossilisation. In the case of the Portland Stone, which was rapidly uplifted following its deposition, much of the aragonite has disappeared completely. This is because this formation rapidly lithified and the aragonite was dissolved away by meteoric waters in the freshwater environment of the overlying Purbeck Beds. The line drawings used below have been adapted from Eric Robinson’s handout ‘Jurassic and Cretaceous Fossils’, which in turn were adapted from illustrations presented in Arkell (1947). However it is always worth reminding ourselves that the faces of stones we see on buildings are artificial and therefore many

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Page 1: An Urban Geologist’s Guide to the Fossils of the Portland ...ucfbrxs/Homepage/walks/PortlandFossils.pdf · An Urban Geologist’s Guide to the Fossils of the Portland Stone

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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


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Fossils 2 types of fossils 2 types of fossils 1. body fossils: actual remains of living things (bones). 2. trace fossils: impression left by once living