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UrbanGeologyinLondonNo.36

©RuthSiddall;UCL,April2017

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Afire-spittingvolcanoinDeptford?UrbanGeologyinSoutheastLondon

RuthSiddall

No,therearenovolcanoesinDeptford,andtherehaveprobablyneverbeenanythroughoutatleastthelast1000millionyears.ThetitleofthisurbangeologicalwalkcomesfromanobservationofanastonishedandbewilderedresidentoftheGleichburginGermanywhomay(ormaynot)havewitnessedavolcaniceruptionthereinthe18thCentury(seeGrattanetal.,2000).InFebruary2017,SmashFestUKcelebratedvolcanoesandallthingsvolcanicintheircommunitysciencefestivalbasedinDeptford,southeastLondon.Partofthiseventincludedageologicaltourofthearea’s(mainly)igneousrocks,usedaspaving,streetfurnitureandbuildingmaterials.Thisisawriteupofthattourwiththeadditionsofafewotherlocalbuildingsworthyofgeologicalinterest.ThewalkstartsattheAlbanyTheatreonDouglasWay,SE84AG.FromthemainentranceofTheAlbany,turnleftandthenleftagaincrossingthemarketplaceintoDeptfordMarketYard.DeptfordMarketYardTheredevelopmentofDeptford’sMarketYardandtheimpressiveVictorianCarriageRampwasundertakenbyFarrerHuxleyAssociatesin2015.TheRampitselfwasdesignedandbuiltfrombrickbyColonelGeorgeThomasLandmann(1780-1854)oftheRoyalEngineersin1835.TheLondon&GreenwichRailwaywasLondon’sfirstrailwayline,openingin1836andpassengerswerecarriedonanelevatedline,hencetherequirementforanaccessramp.ThelineranfromGreenwichtoLondonBridge,andDeptfordwasthefirststop.The2015redevelopmenthasplacedshopsandcafésintheRamp’sarchesandhaspavedtheYardtomakeaverypleasantplaceforsocialisingonthewaytoandfromthestation.TheMarketYardhasbeenpavedwitharangeofChinesegranitesandrelatedrockswhichareverypopularforcitypavingatpresent;mainlybecausetheyareavailableinlargequantitiesandarerelativelycheap,whilstbeinghard-wearingandattractive.Granitesareigneousrocks;thosethathavecrystallisedfromthesolidificationofameltand



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canformcanforminmanyenvironmentswithintheEarth’scrust.Theyare‘Plutonic’rocksratherthan‘Volcanic’.NamedaftertheGreekgodoftheunderworld,Pluto,granitescrystallisefroma‘magma’underground,usuallyatdepthsof5ormorekilometres.Somegranitescanoccuratmuchshallowerdepthsandthesecanbethemagmachambersthatfeedvolcanoesandarethesourceof‘lava’whichisliquidontheEarth’ssurfacebeforeitfreezes.Asarocktype,a‘granite’isdescribedasbeingaplutonicigneousrockcomposedofthemineralsquartz,micaandfeldspar.Allgranitesbydefinitionhavethisbasicrecipe,butlikeice-creams,theycancomeinallkindsofdifferentflavours.Tracemineralsandelementscanalterthecolourofthemajormineralsandgrainsizeandshapecanvary.AlthoughastonemasonwouldcallallthestonesusedhereinDeptfordMarketYard‘granites’,geologicallywehaveherethreegranitesandarockcalledadiorite.Luckilytheyarealleasytodistinguishtoo,beingdifferentcolours.

LuoyuanGranite(topleft)haslargepinkandwhitefeldsparcrystalsaswellasquartzandablackvarietyofmica(biotite).ItisfromFujianProvinceinSEChina.Ithasanoverallpinkappearanceandisaverystrikingstone.ShandongRustGraniteisayellow-browngranitewhichgetsitsnamefromrustyspots,stripesandpatchesthroughoutthisrockswhichhavegivenanotherwisewhitegraniteayellowtorusty-browncolour.Therustisduetoironwhichhaspercolatedthroughthegraniteasithasbecomeweathered.LaizhouGranite(belowleft)isaratherunusuallookinggranite,withamedium-grained,greygroundmassmadeupofthethreemaingraniteminerals,quartz,micaandfeldspar.Howeveritisscatteredwithlargercrystals,2-3cmlong,ofwhiteandpink,well-shaped,feldsparcrystals.Someofthepinkcrystalshavewhiterims.Thecolourchangesrepresentsubtlechangesinchemistryasthemeltwascrystallising.Itiseasytomistaketheselargercrystalsforchewinggumonthepavement,butthenamefortheselargecrystalsinafinegrainedgroundmassisphenocrysts.ZhangpuGreenGraniteistheoddoneouthere,itisadiorite,anigneousrockcomposedoffeldspar,whichisusuallywhiteorgreyandadarkgreenmineralcalledhornblende,andthisiswhythisstoneasagrey-greencolour.Ithascrystallisedfromameltofadifferentchemistryfromthatofgranites.However,tostonecontractors,prettymuchalligneousrocksare‘granites’.

ZhangpuGreencomesfromFujianProvinceonthecoastofsouthernChina.Luoyuan,LaizhouandShandongGranitescomefromShandongProvinceonthecoastofsoutheastChina.AllwereintrudedduringtheMesozoicera,partofthegeologicaltimescalethatcomprisestheTriassic,JurassicandCretaceousPeriods,thetimeofthedinosaurs!ThiswasthetimethatthelandmassthatisnowsouthandeastChinawasbeingformedbyplatetectonicprocesses.TheSouthChinaSeawasbeingforcedundertheAsiancontinentalcrustbuildingmountainsalongthenewcoastlines.DuringtheMesozoic,thispartofSEAsiawouldhavebeenliketheAndeanMountainsofSouthAmericatoday–highmountainsandlotsofvolcanoes.Thegranitesanddioriteswehaveseenherewouldhavebeentheplumbing,feedingmagmatothevolcanoeswhereitcouldbeeruptedaslavasandvolcanicash.



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OnleavingDeptfordMarketYard,turnleftalongDeptfordHighStreet.AsmallpieceofevidenceofEnglishmagmatismcanbefoundonthecanbespottedonthekerb.FfinchStreetAVictoriankerbstonemadeofCornishGranitesurvivesamongsttheconcreteonthecornerofFfinchStreetandDeptfordHighStreet(below).TheCornishgranitesdidnotfeedvolcanoes,buttheydidproducespectacularrockswhichwerewidelyusedasbuildingstonesthroughoutthe19thCentury.

Thewhite,brick-shapedcrystalsarethemineralfeldspar,oneoftheessentialcomponentsofthegraniterecipe.Thecrystalshereshowsomealignment.Peopleoncebelievedthatthisalignmentwasduetothesolidcrystalsflowinginaliquidmelt.Howevermoltengraniteisextremelyviscous,somuchsothatwewouldnotthinkofitasaliquid.Ithasaconsistencymorelikepeanutbutter.Wenowbelievethatthesephenocrystsformedlaterinthecrystallisationhistoryofthegraniteandanyalignmentthatweseeisprobablymorelikelytoberelatedtodeformationoftherockratherthanflow.Ifyouwanttoseesomefossils,thentakeashortdiversiondowntoStPaul’sChurch.StPaul’sChurchStPaul’sisafineBaroquechurch,designedbyarchitectThomasArcherandcompletedin1730(seeCherry&Pevsner,2002).Ithasasemi-circularportico,supportedbycolumnsatthefrontentranceandporchesaccessedbystaircasesonthenorthandsouthsides.Geologicallywemustnowtakeadetourawayfromourigneousrocksandtheirassociationswithvolcanoes,forthechurchisbuiltfromLondon’smostfamousbuildingstone,PortlandStone.The17thcenturyarchitectInigoJoneswasinfluentialinbringingPortlandStonetoLondon.HeusedittobuildtheRoyalBanquetingHouseontheMallin1620.AftertheGreatFireofLondonin1666,useofPortlandStonetookoffanditwasusedbyChristopherWrentorebuildStPaul’sCathedral.FromthenonitbecameLondon’smainbuildingstoneanditwasoftenusedforcivicbuildingsandchurches.PortlandStoneisapale-greylimestoneoflatestJurassicage,deposited145millionyearsagoinwarm,shallow,tropicalseas,fulloflife.Evidenceofthiscanbeseeninthefossilshellsthatareeasilyspottedinthisstone.Giventhisbuildingisnearly300yearsold,thePortlandStonehasweathered,andtheharder



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fossilsnowstandproudofthesurface.Theseareoystershellsandthelaminatedstructureoftheirshellshasbeenwellpreserved.ThepresenceofthesefossilstellsusthatthisisthevarietyofPortlandStoneknownasWhitbed.

FossilsinthestonesofStPaul’sChurch.Topleftandright,PortlandWhitbedwithfossiloystershellfragmentsweatheringproudofthesurface.Bottomleftandright,varietiesofPurbeckStonewithvariousfossilisedfreshwatershellsandalgae.PurbeckStoneisEngland’s‘marble’.Itisnotatruemarbleinthegeologicalsensebutitisanarchitecturalmarbleinthesensethatitisalimestonethattakesagoodpolish.Itwasverypopularinecclesiasticaldecorations.IntheexteriorofStPauls,weseeitaspavingonthenorthandsouthsideporches,whichareaccessedbysymmetricalstaircases.Theseareasarepavedwithared,sandylimestonecalledMansfieldRed,fromNottinghamshire,PortlandStoneandthevarietiesofPurbeckstonewhichareillustratedabove.GeologicallythePurbeckBedsoverlythePortlandStone,markingnotonlyamajorchangefrommarinetofreshwaterenvironments,butalsothechangefromtheJurassictotheCretaceousera.Thefossilsindicatethischangeinenvironmentwithfreshwatersnailsandmusselstakingtheplaceoftheseashellfossilsfound



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inthePortlandStone.PurbeckStonehasalsorevealedrarefossilsofreptileboneandinsectsaswellasoccasionaldinosaurfootprints.PurbeckhasbeenquarriedsincetheRomanperiodfromquarriesaroundthevillageofLangtonMatraversontheIsleofPurbeckinDorset.Thebedsarethinandthereisalotofvariabilityintheappearanceofthevariousstonesand‘marbles’extractedfromthesestrata.LeavethechurchyardandturnleftbackontoDeptfordHighStreet.RetraceyoustepsbacktothestationandTomi’sKitchenoppositethestationendofDeptfordMarketYard.Tomi’sKitchenTomi’sKitchenRestaurantat126,DeptfordHighStreetusedtobeapub,TheMechanicsArms1,andithasverytypicalstoneworkforaLondonPub(seeSiddall,2014).Pubswereintendedtolooksmartandclassy,butnottooclassy!NofancymarblesinLondon’spubs.ThestoneofchoicewasacolourfulgraniteusuallyfromScotlandorScandinavia.HereatTomi’sKitchenwehavered,ScandinaviangranitesandinfactbothhereareoriginallyfromtheeastcoastofSweden.LiketheoneswehaveseenaboveintheMarketYard,theyhavethestandardquartz+feldspar+micarecipe.ThetwovarietiesareSwedishBalmoralGranite(belowleft)andGraversforsGranite(belowright).

Bothgranitescontainbrick-redfeldsparsandtheblackmica,biotite.ThewhitemineralinSwedishBalmoralGraniteisquartz,andthisgivesthegraniteared-and-whiteappearancethatcanbeseeninthephotographabove.GraversforsGraniteontherightismoreofapurplecolouroverall,andthisisdowntothequartzbeingalmostvioletincolour,partlyduetothefactithasbeencrushedandalsobecausethereisalotofironoxide(themineralhematite)inthistoo.Theseareextremelyancientrocks,almosttwobillionyearsold(1800millionyearsold).ThesearecertainlytheoldestthingsonDeptfordHighStreet!ContinuedowntheHighStreettoGiffinStreet.GiffinStreetHereintheplazaonGiffinStreet,wegettoseesolidifiedlavaflowsinDeptford.Thisstoneisusedforthecubeseatingblocksintheplaza.ThisisanotherChinesestonecalledBlackPearlBasalt.Abasaltisavolcanicigneousrock.Basaltsarecomposedofthedarkcolouredmineralsolivine,pyroxeneandavarietyoffeldsparandareconsequentlyblackincolour.Thesebasaltswereeruptedasenormouslavaflows,fiftymetresthick,duringtheCretaceous,around140millionyearsago.Theseeruptionswouldhavebeen

1 TheLostPubsProject:http://www.closedpubs.co.uk/london/se8_deptford_mechanicsarms.html



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unimaginablyhuge,nothinglikethemhasbeenseeninhumanhistory.ThegraniteswehaveseeninDeptfordMarketYardandtheirrelatives,wouldhavebeenthemagmasfeedingtheseenormousvolcanoeswhicharetruesupervolcanoes.Therecraterswouldhavebeen50milesacrossandmostoftheeruptiveproductswouldhavebeenvolcanicashandpyroclasticflows.Basaltswereonlyasmallproportionmaterialerupted.Howeverthesolidifiedflowsarestillenormous,formingcolumns(whicharetypicalofthickbasaltflows)whichcanbeseeninthequarriesinBailininFujianProvince.Thesecolumnsformasthelavacoolsandsolidifies,andareupto2-3metresdiameter(seebelow,right).Asaresult,largeblockswithnocrackswithinthemcanbeproduced.

Aboveleft;theporphyrybasaltatGiffinStreet.Right;TheBlackPearlquarriesinFujianProvince,China(photofromStoneContacthttp://www.stonecontact.com/quarries-2209/china-black-basalt-quarry).Lookingcloselyatthepolishedsurfacesoftheblocksandyoucanseethattheycontainblackspots.Thesearephenocrystsofthemineralpyroxene.Ifyoucanseeanygreencrystals,thesearethemineralolivine,sonamedbecauseitisolivegreen.Youcanalsoseethatthegroundmassisveryfine-grained.Thisvarietyoftextureinrocksiscalleda‘porphyry’.ContinuesouthalongDeptfordHighStreet.DeptfordHighStreetThegreenpavingcobblesaroundthejunctionwithGiffinStreetandDouglasWayareZhangpuGreenGranite.ThepatternofdifferentcolouredgranitesseeninDeptfordMarketYardisonceagainusedontheHighStreet;pinkisLuoyuanGranite,greyisLaizhouGraniteandthebrownisShandongRustGranite.



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ThelaststoponDeptfordHighStreetonthecornerofNewCrossRoadistheBroadwayRestaurant,ariotofgranites.BroadwayRestaurantOnthecornerofDeptfordHighStreetandDeptfordBroadway,TheBroadwayRestaurantalsohadaformerlifeasapub,TheCenturion2.Itisalsocladinavarietyofgranites,threeScottishandoneFinnish.PeterheadGraniteisthepalepinkgraniteusedformostofthesquaresectionpillarsbetweenthewindows.AsthenamesuggestitisfromPeterhead,afishingtownnorthofAberdeenontheeastcoastofScotland.IthassalmonpinkfeldsparsandsmokygreyquartzandlooksabitlikecoarseArdennespaté.Althoughnolongerquarriedtoday,thisstonewasverymuchinfashionduringtheVictorianandEdwardianperiodsandisusedtocladagreatmanypubsandusedasplinthsforstatuesetc.Itwasintrudedjustover400millionyearsagoduringtheformationofScotland’smountains.ThegreygranitepanelsbetweenthewindowsareCairngallGranitewhichalsocomesfromPeterhead,andthiswasquarriedclosetothepinkgranitequarriesatFlushing.Thequarriesarelongsinceclosedandarenowariflerange.CairngallGraniteismucholderthanPeterhead,at475millionyears.Justnextdoortothemainrestaurantat14,DeptfordHighStreetthebuildingschemecontinues.Atsometimeintherecenthistoryofthebuilding,therehasbeenneedtoreplacesomeofthePeterheadGranite,presumablybecauseitwasdamaged.Asitisnolongerworked,ithasbeenreplacedusingBalmoralGranitefromFinland.Thisgranitehascrystalsformingamosaicofbrick-redfeldsparsanddark-grey,almostblack,quartzisone-and-a-halfbillionyearsold.Itcomesfromthesouth-westcoastofFinland.

ThesmallpillarsoneachsideofthedooraremadefromDalbeattieGranitefromsouth-westScotlandalsoaround400millionyearsold.Thishaswhiteapalepinkfeldspars,blackbiotitemicaandgreyquartz.Thepillarnthelefthandsidehasadark-grey,raggedpatchinit.Thisisafeaturethatgeologistscalla‘xenolith’whichliterallymeansa‘strangerock’.Itisnotpartofthegranite,butpartofthepre-existingcountryrocksthatthemeltwasintrudedinto.Thereforeitisarockthatisolderthanthegranitethathasnotbeenincorporatedintoitandnotdissolvedintothegranitemagma(left).

ThisistheendofthemaintouroftheurbangeologyoftheareaaroundDeptfordHighStreet.HoweverifyouareheadingtowardsNewCross,thereareafewbuildingsworthyofgeologicalnotealongNewCrossRoad.InLondon’ssuburbs,mostbuildingisinbrick,butpubsandcivicbuildingsaregoodplacestospotsomegeology.TheNewCrossInnandNewCrossHouseareoppositeeachother,atthejunctiononNewCrossRoadandLewishamWay.NewCrossInnAnotherearly20thCenturypub,theNewCrossInnhasitsgroundfloorcladwithaScandinavianigneousrock.ThistimeitisfromthetownofLarvikinsouthernNorwayanditisnamedlarvikite.Larvikiteisnotagranitebecauseitdoesnotcontainanyquartz.Itisavarietyofrockcalledamonzonite.ItwasintrudeintotheOsloriftvalley(nowtheOsloFjord)around290MaduringthePermian.Larvikitehasbeenquarriedinacommercialscalesincethe1880sandasbecomeverypopularworldwideasastrong,hard-wearingandveryattractivestone.Itisfamousforitsiridescentfeldsparswhichshimmerandchangecolourasyou 2TheLostPubsProject:http://www.closedpubs.co.uk/london/se8_deptford_centurion.html



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moveyourhead;sometimesappearinggreyandsometimeselectricblue.Anumberofvarietiesofthisstonerangingincolourfromalmostblack,throughbluetogreyarequarried.ThisisthegreyvarietywhichgoesbythenameMarinaPearlLarvikite.Theimagebelowshowsthegrey,iridescentfeldspars.Theblackmineralsarebiotitemica(whichwehaveseeninsomeofthegranites)andpyroxene(whichwesawinthebasaltinGiffinPlace).

LarvikiteontheNewCrossInn.

WeknowthatthelarvikitemagmaschambersoftheOsloriftweredefinitelyfeedingvolcanoes.Weseetheproductsofthesevolcanoeswhichstillsurviveassolidifiedlavaflows,withverydistinctivediamond-shapedorrhombicfeldspars.Theserocksareknownasthe‘rhombporphyries’andtheyareavarietyoflavaswhichgeologistscalllatites.TheyareuniquetothePermianmagmatismoftheOslorift.Duringtheiceages,thesevolcanicandplutonicrockswereheavilyeroded(andthisprocesshelpedtoexposethelarvikiteintrusions).Chunksofrhombporphyrywerecarriedsouthbyglaciersandicesheetsanddumpedinmorainesandglacialsedimentswhentheicesheetsmelted.TheycannowbeenfoundintheglacialgravelsofSEEngland,andbecauseoftheiruniqueness,weknowexactlywheretheycamefrom.Thishasgivenuslotsofinformationabouthowfar,howfastandinwhatdirectiontheicesheetsmoved.

ApolishedslabofrhombporphyryfrozenlavaintheNationalMuseumofScotland.Thelargestcrystalsare1cmlong.PhotobyRuthSiddall.



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TheNewCrossHouseAcrosstheroadfromtheNewCrossInnistheNewCrossHouse.ThisiscladinBritishgranites;MerrivaleGranitefromDartmoorandCairngallGranitefromPeterheadinAberdeenshire,Scotland.WehaveseenCairngallearlieronTheBroadwayRestaurant.MerrivalecomesfromtheotherendoftheBritishIslesandisoneofthesuiteofgranitesintrudedintoDevonandCornwallaround300millionyearsago.Ithaslargewhitefeldsparphenocrystsandslightlybrownishquartz.Cornishgranitestypicallyhavetwotypesofmica;theblackbiotiteandasilveryvarietycalledmuscovite.Thiscanbeseensparklingintherock,especiallyifthesunisshining.MerrivaleGranitewasquarriedbytheprisonersincarceratedinDartmoorPrison.

StonesontheNewCrossHouse.Left;CairngallGraniteandright;MerrivaleGranite.

DeptfordTownHallLikeStPaul’sChurch,theoldDeptfordTownHallbuildingisconstructedofPortlandStone.TheintricatecarvingabovethemaindoorisenabledbyPortland’spropertiesasafreestone,thatis,astonewhichhasnopreferredweaknessandthereforecanbecarvedinanydirection.Asaconsequence,elaborateandintricatecarvings,asobservedinthisfishycoatofarms,representingDeptford’smaritimeassociations(below).ThebuildingwasdesignedbyarchitecturefirmLanchesterStewartPickardsandbuiltin1905.



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References

Cherry,B.&Pevsner,N.,2002,TheBuildingsofEngland:LondonNo.2,South.,YaleUniversityPress,Newhaven&London.,808pp.

Grattan,J.P.,Gilbertson,D.D.&Dill,A.,2000,AfirespittingvolcanoinourdearGermany?DocumentaryevidenceforalowintensityvolcaniceruptionoftheGleichbergin1783,in:McGuireW.G.,Griffiths,D.R.,Hancock,P.L.&Stewart,I.S.(Eds.)TheArchaeologyofGeologicalCatastrophes,SpecialPublications,GeologicalSocietyofLondon,171,307-315.

Siddall,R,2014,London’spubgeology:aspotter’sguide.,UrbanGeologyinLondonNo.20,18pp.,http://www.ucl.ac.uk/~ucfbrxs/Homepage/walks/PubGeology.pdf

Glossary

Basalt–avolcanicrockcomposedofthemineralsfeldspar,pyroxeneandolivine.

Diorite–aplutonicigneousrockcomposedfromthemineralsfeldsparandhornblende.

Freestone–arockwithnoplanesofweakness,meaningitisverygoodforthree-dimensionalcarving.

Granite–aplutonicigneousrockthatiscomposedoftherockformingmineralsquartz,feldsparandmica.

Igneous–atypeofrockthathasformedbythecrystallisation(freezing)ofasilicatemelt.

Lava–asilicatemeltontheEarth’ssurface,eruptedfromavolcano.

Magma–asilicatemeltwithintheEarth.

Mineral–asolidmaterialwithadefinedchemicalformula.

Phenocryst–alargecrystalinafinergrainedgroundmassinanigneousrock.

Plutonic–referringtoanigneousrockwhichhasformedwithintheEarth’scrust.

Porphyry–anigneousrockwithphenocrysts,whichthereforehasaspeckledappearance.

Pyroclasticflow–atorrentofrockfragmentsandashproducedbyaveryexplosivevolcaniceruption.

Rock–asolidmaterialmadeupofminerals.

Stonecontractor–thebusinessthatliaiseswithaquarrytosupplystoneattherequestofarchitects.

Volcanic–referringtoanigneousrockwhichhasbeeneruptedontotheEarth’ssurface.

IndexofStones

BalmoralGranite–BroadwayRestaurant(right)

BlackPearlBasalt–Giffinstreetsquare

CairngallGranite–BroadwayRestaurant,TheNewCrossHouse

CornishGranite–FfinchStreet

DalbeattieGranite–BroadwayRestaurant

GraversforsGranite–Tomi’sKitchen

LaizhouGranite–DeptfordMarketYard,HighStreet.

LincolnshireLimestone–StPaul’sHouse

LuoyuanGranite–DeptfordMarketYard,HighStreet.

MansfieldRed–StPaul’sChurch

MarinaPearlLarvikite–NewCrossInn



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MerrivaleGranite–TheNewCrossHouse

PeterheadGranite–BroadwayRestaurant

PortlandStone–StPaul’sChurch,DeptfordTownHall

PurbeckStone–StPaul’sChurch

ShandongRustGranite–DeptfordMarketYard,HighStreet.

SwedishBalmoralGranite–Tomi’sKitchen

ZhangpuGreenGranite–DeptfordMarketYard,HighStreet.

DeptfordMarketYardandCarriageRamp.

Howtocitethisarticle:Siddall,R,2017,Afire-spittingvolcanoinDeptford?UrbanGeologyinSoutheastLondon.,UrbanGeologyinLondonNo.36,11pp.,http://www.ucl.ac.uk/~ucfbrxs/Homepage/walks/Deptford.pdf

LocationsofthebuildingsdescribedaboveandbriefdetailsofthestonesusedcanalsobefoundintheLondonPavementGeologywebsite:http://londonpavementgeology.co.uk©DrRuthSiddall,UniversityCollegeLondon,GowerStreet,LondonWC1E6BT,UK:[email protected]

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