UrbanGeologyinLondonNo.30
AnUrbanGeologist’sGuidetotheFossilsofthePortlandStone
ThePortlandseaswerewarm,fullofnutrientsandthereforefulloflife,andplentyofevidenceforthiscan
beseenonthewallsofLondon’sbuildings.GoodreviewsanddiscussionofthegeologyofthePortland
StonearetobefoundintheexcellentwebpagescreatedbyIanWest(West,2013,2014)andinCope
(2012).AnexcellenttreatiseonPortlandStoneasabuildingmaterialhasrecentlybeenpublishedby
Hackman(2014).TheunitofuppermostJurassicage,theTithonianwhichisknownlocallyasthe
Portlandian(Cope,2012).Thereforetheserocksare~150millionyearsold.Stratigraphically,thebuilding
stonesformtheupperpartofthePortlandLimestoneFormationandarecalledtheFreestoneMember.
Marinereptilessuchaspliosaursandichthyosaurswouldhaveswamtheseseas,thoughtheirremainsare
rarelyencounteredinthestone.Varietiesofbivalvesaremostfrequentlyobserved,alongwithcoquinasof
marinesnail(gastropod)shells.Thewaterswererelativelyshallow,buttidal,theywerelagoon-type
environments.Thisenvironmentbringsabouttheformationofparticlesofcarbonatesandknownasooids
orooliths.Ooidsarespheroidalgrainswhichformaroundkernalsoffinesandgrainsorshellfragments.In
thewarm,tropicallagoons,tidalcurrentsrollthegrainsbackwardsandforwards,andthereforethe
carbonateisprecipitatedconcentricallytoformooids.Thesehaveamaximumsizeofaround1mm
diameter,andareoftenvisiblewiththeaidofahandlens,andsometimesthenakedeye.Thereforethe
PortlandFreestoneisanooliticlimestone,withvariableshellcontent.TheBaseBedisrelativelyfreeof
fossils.TheWhitbedcontainsvariableamountsoffossils,rangingfromafewscatteredoysterstocross-bed
sectionsrichinshelldetritusandevenpatchreefs,upto3-4mdiametercomposedofoystersorthe
corallinealgaSolenopora.TheRoachrepresentsashellybeachdominatedbyshellfossils,acoquina.
Above;ablockofRoachplacedontopofcross-beddedWhitbedat23KingStreet,W1.
AmajorfeatureofsomesectionsofthePortlandStoneistheleachingofaragoniticshellmaterial.Weneed
tolearnsomecarbonategeochemistryheretotrytounderstandthisprocess.Calciumcarbonateisthe
mainconstituentofmostlimestonesandalsoshell,coralsandexoskeletonsofothermarineinvertebrates.
Calciumcarbonatecanexistintwoformsor‘polymorphs’whichhavethesamechemistrybutdifferent
crystalstructures.Thesetwoformsarecalledaragoniteandcalcite.Mostshell-forminganimalssecrete
aragoniteastheirexoskeleton.Aragoniteisgeologicallyunstableandusualchangestothegeologically
stablepolymorphcalciteduringfossilisation.InthecaseofthePortlandStone,whichwasrapidlyuplifted
followingitsdeposition,muchofthearagonitehasdisappearedcompletely.Thisisbecausethisformation
rapidlylithifiedandthearagonitewasdissolvedawaybymeteoricwatersinthefreshwaterenvironmentof
theoverlyingPurbeckBeds.
ThelinedrawingsusedbelowhavebeenadaptedfromEricRobinson’shandout‘JurassicandCretaceous
Fossils’,whichinturnwereadaptedfromillustrationspresentedinArkell(1947).Howeveritisalways
worthremindingourselvesthatthefacesofstonesweseeonbuildingsareartificialandthereforemany
©RuthSiddall;UCL,January2015;revisedJuly2015.
1
UrbanGeologyinLondonNo.30
sectionsthroughthefossilsareobservedanditisrarethatperfectexampleswillbeseen.Palaeontologists
becomeadeptatrecognisingallpossiblesectionsthroughashell.
TheFossils
Gastropods-Aptyxiellaportlandica,the‘PortlandScrew’.ThisisthemosticonicfossilofthePortland
Stone,butitisonlyencounteredinthecoquinafaciescalledtheRoach.
Above:AptyxiellaportlandicaonthenewwingofBBCBroadcastingHouseonGreatPortlandStreet.
ThePortlandScrewisaturretedgastropodwhichischaracteristicofandprettymuchrestrictedtothe
Roachandso,asistypicalinthisfacies,thearagoniteshellhasbeenleachedaway.Thisleaves5-6cmlong
(aboutthelengthsofyourfingers),arrow-shaped,externalmouldsoftheshellsandthespiral‘screws’
whichrepresenttheinternalcastsoftheshells.Aptyxiellaportlandicaisnotfoundinmainlandoutcropsof
thePortlandLimestoneFormationandsoitcanbeconsideredasatrulydiagnosticPortlandbeast,and
characteristicoftheRoachcoquinas.
Above:Screw-shapedinternalcastsofAptyxiellaportlandica.Thesearenotthebodypartsofthesnail,butthe
carbonatemudwhichhasfilledintheshells.TheseexamplesarefromBBCBroadcastingHouseonGreat
PortlandStreet.
Bivalves
©RuthSiddall;UCL,January2015;revisedJuly2015.
2
UrbanGeologyinLondonNo.30
ThecommonestbivalveencounteredinthePortlandstoneistheoysterLiostreaexpansa.Oysterssecrete
calciteshellsandtheyarethereforewellpreserved.Anyonewhohaseverseenanoystershellonabeach
oradinnerplatewillknowthattheyappeartobeformedofthinlayersofcalcite,butanyonewhohasever
triedtocrackoneopenwillalsoknowtheyareextremelyrobust.
Aboveleft;diagramofL.expansaafterArkell(1947).Right;asectionthroughanoyster,probablycuttingthrough
closetothehingeoftheshell,inWhitbedonBBCBroadcastingHouse.Thefinelylayeredstructureoftheshell
canbeclearlyseen.
OysterfragmentsareverycommonlyseenthroughtheWhitbedandRoach,andindeedreeflikemassesof
oystersdevelopedintheWhitbed.Thebrokenshellsarealsoconcentratedinlagsinsomevarietiesofthe
WhitbedasseenbelowatPaternosterSquare.
Above:oystershelllaginPortlandWhitbed.Thefieldofviewis~20cm.Below:anoyster-bedinweathered
WhitbedusedforbenchesattheEconomistPlaza,StJames’s.Thisstoneiscutparalleltobedding,FOV~50cm
©RuthSiddall;UCL,January2015;revisedJuly2015.
3
UrbanGeologyinLondonNo.30
Plicatuladamoni,avarietyifthespinyshellsspondylus,areverysimilartotheoysters.Theyhadthinridges
alignedconcentricallyalongtheirshellswhichenabledthemtoanchorthemselvesinthemud.Theseshells
aredifficulttodistinguishfromoystersandpectens(seebelow)unlessyoucanseethesespinelikeridges
incrosssectionasintheexamplebelow,whichshowsbothvalvesoftheshell.
Moreoftenthannot,P.damoniformsinternalcastswhicharerathernondescriptandthey,unsurprisingly,
resemblethefleshypartsofoystersinshape.
Trigoniashellsarenowlargelyextinct(thereisonesurvivingvarietythatlivesinoneplaceinAustralia).The
speciespredominantlyfoundinthePortlandStoneisLaevitrigoniagibbosa.Trigoniidbivalvesarethickwalledshells,whichmeansthattheywereabletowithstandhighenergyenvironments.Theshellsarealso
typicallyhighlyornamentedwithribsandknobs,thelattertechnicallyreferredtoascostae(singular:
costa).ThecommonestvarietyfoundinthePortlandFreestoneisLaevitrigoniagibbosa,whichhastwo
formsonewithribsandtheotherwithcostae.
Above:varietiesofL.gibbosaafterArkell(1947).
TypicaloftheRoach,thearagoniticshellsofL.gibbosahavebeendissolvedaway,leavingexternalmoulds
whichshowtheshellsurfaceornamentation.Alsopresentaretheinternalcastswhichreplicatetheformof
theinterioroftheshell.ThesearegiventheirownLatinnameMyorphorellaincurva.Itisnothardto
imaginehowthesecastscanbecomedetached.Ontheirowntheseoddlookingfossilsareknownas
‘steinkern’,literallystonehearts.SimilartrigoniidcaststoM.incurvaweretermedhippocephaloidesby
17thCenturynaturalhistorianRobertPlottbecausetheyresembledrathermournfulhorsesheads.However
tothePortlandquarrymentheybecameknownas‘osses’‘eadsinthelocalvernacular.Imagesoftheseand
Plott’sdrawingcanbeseenontheNaturalHistoryMuseum’swebsite1.
1http://www.nhm.ac.uk/nature-online/earth/fossils/fossil-folklore/fossil_types/bivalves02.htm
©RuthSiddall;UCL,January2015;revisedJuly2015.
4
UrbanGeologyinLondonNo.30
Above:MouldsofL.gibbosainRoachatBBCBroadcastingHouse.Theseexamplesshowgooddetailofthe
ornamentedshellsurfaces.Eachshellisabout5cmacross.Below:mouldsandaninternalcastofL.gibbosa
clearlyshowingthecostaeontheshellsurfaces.
BelowisadrawingofaperfectM.incurva(afterArkell,1947).Theimageontheright(FOV~15cm)shows
moretypicalcasts(aswellasaTrigoniashellmould).
©RuthSiddall;UCL,January2015;revisedJuly2015.
5
UrbanGeologyinLondonNo.30
Left:Twosectionsthroughoncearticulatedvalves
ofL.gibbosa,nowleachedaway.Bychance,the
masonhascutthroughalongthelongaxesofthe
shellsgivingtheeffectof‘angelwings’.Theribsand
costaecanbeseenontheupperexampleofthetwo.
Onceagain,thisisfromtheRoachusedonthenew
wingofBBCBroadcastingHouse.
Camptonecteslamellosusisavarietyofpecten.Pectenarescallopshellsandcharacteristicallyhavewell
developed‘wings’alongtheirhinge.ThevarietyareextremelycommonalongsomebedsofthePortland
Stone.Theyarefrequentlywellpreservedasthey,likeoysters,precipitatecalciteandnotaragonite.The
shellsarearoundthesizeofmodernediblescallops.
Aboveleft;diagramtoshowtheappearanceofthewholeofCamptonecteslamellosusshell(afterArkell,1947).
Right;halfapectenwithoneofthe‘wings’preservedattheendoppositemyfinger.Thisexampleisonthefront
stepsoftheSladeSchoolatUniversityCollegeLondon.
PairsofarticulatedvalvesofC.lamellosusareoftenseenintheWhitbed.Theycanbedistinguishedfrom
thespondylusdescribedabovebythelackofspinesandtheflatteningofthehingezone.Unlikemodern
ediblescallops,whichhaveoneflatandoneconvexvalve,C.lamellosushastwoconvexvalves.
©RuthSiddall;UCL,January2015;revisedJuly2015.
6
UrbanGeologyinLondonNo.30
Above:acrosssectionofCamptonecteslamellosusinWhitbedonBBCBroadcastingHouse.FOV~12cm.
AlsopresentonWhitbedandtheRoacharethemusselsMytilussuprajurensis,tellinshellsPleuromya
tellina,therock-boringbivalveLithophagaandvarietiesofcocklesincludingProtocardiadissimilis.These
arelesseasytodistinguishastheyareoftenfragmentary.Theoyster-like,butaragoniteprecipitating
Isognomonlisteriisalsoknownfromthereef-formingfacies,predominantlyasinternalmoulds(West,
2014,Townson,1975).
Left:Pleuromyatellina.Theseshellsare~2cmacross.
Ammonites
Ammonites,spiralshellsoccupiedbysquid-likecreaturesareclassicMesozoicfossils.Thegiantammonite,
Titanitesanguiformes,istheclassicfossilofthePortlandStone,thoughtheyarerarelyobservedinbuilding
stones,andiftheydotheyaregenerallyfragmentary.Wholespecimensareuptoametreacross.Another
species,T.giganteus,ofsimilardimensionsalsooccursbutislessabundantthanT.anguiformes
(Wimbledon&Cope,1978).
Above:imagesofTitanitesanguiformes(left)andT.giganteus(right).Bothspecimensare~1mdiameter
(adaptedfromWimbledon&Cope,1978).
©RuthSiddall;UCL,January2015;revisedJuly2015.
7
UrbanGeologyinLondonNo.30
VisitorstotheIsleofPortlandcanobserveagreatnumberofthesefossilssetintogardenwallsandacting
asthesignsforquarries.IndeedatheoryastowhyT.anguiformesisrarelyencounteredinbuildingstones,
isthatwhentheyarefound,theyarekeptasprizesbythequarrymen.Goodphotosofgoodspecimenscan
beseenonIanWest’swebsite2.TomyknowledgetheonlyexampleonaLondonbuildingstoneisonthe
frontstepsoftheSladeSchoolofFineArtatUniversityCollegeLondon.Herethepavingslabstakeseveral
slicesthroughthesamefragmentofawhorl(below).
Alga
TheredalgaeSolenoporaportlandicaformedsmallpatchreefsinthePortlandStoneseas(Fürsichetal.,
1994).Theyarenotparticularlycommon,butwhenencounteredbyquarrymen,theyarequarriedasa
coherentstoneforuseinasingleproject.AgoodexampleofsuchauseisatCaxtonHouse,TothillStreet
SW1.ThesereefsaredominatedbySolenopora.However,whitefragmentsofSolenoporaarecommon
throughoutthemoreshell-richvarietiesofPortlandStoneandthoseformedinmoredynamic
environments.Theyresembleinsizeandshapecauliflowerfloretsandshowdistinctivegrowthrings,which
sometimesshowpreferentialweatheringofalternatebands.Thesearebelievedtorepresentseasonal
phasesofgrowth(seeWright,1985).
Above:Solenopora‘floret’withseasonalgrowthringsinPortlandWhitbedobservedinPaternosterSquare.
2http://www.southampton.ac.uk/~imw/jpg-Portland/11PT-Albion-Ammonite-Roach.jpg
©RuthSiddall;UCL,January2015;revisedJuly2015.
8
UrbanGeologyinLondonNo.30
Above:SolenoporaweatheringoutontheparapetofWaterlooBridge.FOV5cm.
Above:SolenoporareeffaciesatCaxtonHouse,TothillStreet,SW1.FOV~20cm.
West(2014)hasalsoidentifiedthebryozoanHypororporaportlandicaencrustingshellsinthesereefs.
©RuthSiddall;UCL,January2015;revisedJuly2015.
9
UrbanGeologyinLondonNo.30
Tracefossils
Tracefossilsarethepreservedmarksmadebyfeedingormovingorganismsonsediment.Theseare
commoninthePortlandStone,butareoftensubtleanddifficulttosee.Themostfrequentlyencountered
areburrowswhichcanbeformedbyanumberoforganismsbutprimarilycrustaceans,Thesearecalled
Thalassinoides.Theseburrowsvaryinsizefrom2-3cmindiametertomuchlarger(10cm)burrowsand
theymaybeinfilledwithshelldebrisandcarbonatemud.TypicallyThalassinoidesareabranchingnetwork
ofburrowsandstructuresresemblingadouble-endedYarecommonthroughoutthePortlandStone
Formation(seeTownson,1975).Theseburrowsareorientatedsub-paralleltobedding.
Above:ThalassinoidesburrowsinPortlandStoneWhitbed.ThestepsoftheKathleenLonsdaleBuildingat
UniversityCollegeLondon.
Othertracesoforganismscanbefoundfromtheircircularorovoidboringsintoshellsandsediment.Those
inoystershellsorreeffaciesaremostobviousandwereformedbyworms(i.e.Glomerulasp.,Townson,
1975),spongesandtherock-boringbivalveLithophaga(alsoknownasdatemussels–modernvarietiesdo
resembledatefruits).Thisshellcharacteristicallylivesinshallowwaterorthelittoralzone.
Above:Left;pelletofoysterreefboredbythebivalveLithophagasp.,formingtheovoidstructureintheupper
centre.Right:bringsinoystershellsbyspongesorworms.BothexamplesareonBBCBroadcastingHouse.
©RuthSiddall;UCL,January2015;revisedJuly2015.
10
UrbanGeologyinLondonNo.30
VertebrateFossils
BonesarerarelyseeninPortlandbuildingstone,butDelair&Wimbledon(1993)recordseveralvarietiesof
LateJurassicreptilesfromthebuildingstonememberandequivalentstrata,includingcrocodiles,turtles,
Ichthyosaurs,PleisiosaursandsaurischiandinosaursincludingJurassicCoastfavouritesMegalosaurusand
Iguanodon.IamalsoreliablyinformedbyMarkGoddenofAlbionStonethatbonesandteethare
occasionallyfoundinthequarries(Goddenpers.comm.&alsoseeGodden,2012).Examplescanbeseenin
thePortlandMuseum.
TheonlyexampleIhaveencounteredinLondon’sbuildingstoneisonStMargaret’sWestminster,wherean
erodedbonecobbleisseenontheSEfaçadeoftheChurch(below).
ReadmoreaboutPortlandStoneanditsfossils…
Arkell,W.J.,1947,TheGeologyoftheCountryaroundWeymouth,Swanage,CorfeandLulworth.Memoirofthe
GeologicalSurvey,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:A
preliminarysurveyofthematerialandliterature.,ModernGeology,18,331-348.
Fürsich,F.T.,Palmer,T.J.&Goodyear,K.L.,1994,Growthanddisintegrationofbivalvedominatedpatchreefsin
theUpperJurassicofSouthernEngland.,Palaeontology,37(1),131-171.
Godden,M.,2012,Portland’squarriesanditsstones.http://www.dorsetgeologistsassociation.com/PortlandStone/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/fossilfolklore/fossil_types/bivalves02.htm
PortlandMuseum:http://www.portlandmuseum.co.uk
Siddall,R.,2015,PortlandBone:http://wp.me/p53QQu-57
©RuthSiddall;UCL,January2015;revisedJuly2015.
11
UrbanGeologyinLondonNo.30
Townson,W.G.1975.LithostratigraphyanddepositionofthetypePortlandian.JournaloftheGeologicalSociety,
London,619-638.
West,I.M.2013.FossilsofthePortlandGroup,UpperJurassic:GeologyoftheWessexCoast(includingtheDorset
andEastDevonUNESCOWorldHeritageCoast-JurassicCoast).Internetfieldguide.Version:26thAugust2014.;
http://www.southampton.ac.uk/~imw/portfoss.htm
West,I.M.2014.TheIsleofPortland:General.GeologyoftheWessexCoast(partofJurassicCoast,Dorsetand
EastDevonWorldHeritageSite).Internetfieldguide.Version:24thAugust2014.
http://www.southampton.ac.uk/~imw/Portland-Isle-Geological-Introduction.htm
Wimbledon,W.A.&Cope,J.C.W.1978.TheammonitefaunasoftheEnglishPortlandBedsandthezonesofthe
PortlandianStage.JournaloftheGeologicalSociety,London135,183-190.
Wright,V.P.1985.SeasonalbandinginthealgaSolenoporajurassicafromtheJurassicofGloucestershire,
England.JournalofPalaeontology,59,721-732.
LondonBuildingswithgoodandvariedexamplesofPortlandStone
PortlandStone,invariousstagesofweathering,canofcoursebeseenalloverLondon.Theexamplesbelow
arechosentobestshowthefossils.
CaxtonHouse,TothillStreetSW1–ThebuildingiscladinstonefromaSolenoporapatchreef.
BBCBroadcastingHouse,GreatPortlandStreetW1–Onthenewextensionsofthisbuilding,excellent
‘exposures’ofRoachandWhitbedcanbeseen.
EconomistPlaza,StJames’sStreetSW1–ThebuildingwhichintroducedPortlandRoachascladding.
GreenParkTubeStation,PiccadillySW1–mostvarietiesofPortlandStoneareusedhere.Thestationalso
display’sJohnMain’swork‘SeaStrata’whichillustratesgiantsizePortlandFossils.
NewStockExchange,PaternosterSquareEC4–thisandotherbuildingsinPaternosterSquareshowgod
examplesofWhitbedvarieties.
StMargaret’sWestminster–avertebrateboneontheSEcornerofthechurch.
UCLFrontQuad,GowerStreetWC1–ThisisagoodanaccessibleplacetoseeweatheredWhitbedandthe
ammoniteonthefrontstepsoftheSladeSchool(northendofthequad).
WaterlooBridgeSE1–Thebridgeisbuiltfromacross-beddedvarietyofWhitbed,richinshellfragments
andSolenoporadebris.
Howtocitethisarticle:
Siddall,R,2015,AnUrbanGeologist’sGuidetotheFossilsofthePortlandStone.,UrbanGeologyinLondon
No.30,http://www.ucl.ac.uk/~ucfbrxs/Homepage/walks/PortlandFossils.pdf
©DrRuthSiddall,UniversityCollegeLondon,GowerStreet,LondonWC1E6BT,UK:r.siddall@ucl.ac.uk
Downloadsfromhttp://www.ucl.ac.uk/~ucfbrxs/Homepage/UrbanGeology.htm;
Facebook|Cultural&UrbanGeology|Twitter:@R_Siddall|#UrbanGeology
©RuthSiddall;UCL,January2015;revisedJuly2015.
12