Glaciers, ice patches and archaeology

Ice masses can switch between being ice patches and glaciers over time. During a colder climate, like “the Little Ice Age”, ice patches could increase in size ... Glaciers,icepatchesandarchaeology Howcanarchaeologicalfindssurviveinsideorbeneathmassiveglaciers?Well,theycan’t!Theconstantmovementoftheicecrushestheartefactsandeventuallydumpsthesadremainsatthe mouthoftheglacier.Whenyouhearaboutexcitingdiscoveriesfromglaciers,thesefindstendtoberelativelyrecent,likemissingairplanes or WW1soldiers. ApropellerfromaC-53SkytrooperDakota,thatcrash-landedontheGauliglacierinSwitzerlandin1946.Partsoftheairplanemeltedoutagainin2012.Source:http://www.thehistoryblog.com/archives/date/2012/08/11 Luckilyforglacierarchaeologistsnotalliceinthehighmountainsmoves.Wewouldhavemademuchfewerfinds ifitdid!Non-movingfieldsoficemaybeattachedtoglaciers,andaregoodplacestolookforartefacts.Icepatches,isolatednon-movingaccumulationsofice,alsoexistandareexcellentcontextsforthepreservationofartefacts,oncelostinthesnow. Twotypesoficeinthemountains:Icewithcrevassesareglaciersthatmoveanddestroyarchaeologicalfinds.Icewithoutcrevasses,butwithdownhillmeltwaterchannels,areicepatcheswhichdonotmoveandcanpreservearchaeologicalfinds.ThesummittotheleftisGaldhøpiggen(2469m.o.h.),thehighestmountaininNorthernEurope.Photo:LarsPilø,OpplandCountyCouncil. Glacieroricepatch? Thebasicdifferencebetweenaglacierandanicepatchisthataglaciermoves,whileanicepatchdoesnotmovemuch(butreadbelow).Icemassescanswitchbetweenbeingicepatchesandglaciersovertime.Duringacolderclimate,like“theLittleIceAge”,icepatchescouldincreaseinsizetoapointwheretheybecamesufficientlythick(25-30m)tostartmovingandturnintoglaciers.Essentiallyallglaciershavestartedasicepatches. Veryfinelylaminatediceisvisibleinthisicepatch.Photo:LarsPilø,OpplandCountyCouncil. Theiceinthelargericepatchescandeformduetothepressurefromtheoverlyingice.Thisprocessisveryslow,comparedtothefastmovementsofglaciers.Thelowericelayersarecompressedandstretched,whichcanleadtoveryfinelylaminatedice.Ifthetopographybelowtheicepatchisflat,asinthephotoabove,thelaminationwillbequiteeven.Ontheotherhand,ifthetopographyisuneven,thestratigraphywillbemoreundulating,sometimesleadingtostrangeandbeautifullayeringintheice(seephotobelow). Oldicewithastrangeandcurvedstratigraphyisexposedontherightsideofthisicepatch.Theweird-lookingicestratigraphyiscausedbythecompressedice,incombinationwiththeuneventopographybelowtheiceandthemelting.Photo:ReidarMarstein. Icepatchesasarchaeologicalsites Evenwheniceisnotmovingordeformingonlyveryslowly,asafindspotitprovidesspecialchallengesforarchaeologists.Icepatchesreactquicklytochangesinclimate,mainlywinterprecipitation,summertemperatureandwind,andprevailingwinddirectionandstrengthduringthewinterseason.Evenifobjectswereoriginallylostinthesnow,mostofthemhavemeltedoutoftheiceatsometimeortimesinthepast,andbeenrecoveredbysnowandice.Thisisthereasonwhymostartefactsarefoundonthegroundandnotonthesurfaceoftheice.Objectsareonlyfoundontheicewhenthemeltingreachesicelayerspreviouslyuntouchedbymelting. Afourthousandyearsoldwoodenarrowshaft,foundontheicesurfaceinJotunheimenin2014,whenmeltingreachedoldice.Photo:ØysteinRønningAndersen,OpplandCountyCouncil. Findingartefactsmeltingoutoftheicetheyoriginallywereencapsulatedinhappensonlywhenoldicemelts.Suchfindsarethereforequiterare. Manyfindsaredisplacedbymeltwaterand/orwind,andpartsofthesameobjectmaybefoundseveralhundredmetersapart.Thedistributionoffindsonthesitesisthusdifficulttointerpret.Aretheartefactsfoundwheretheywereoriginallylostoraretheydisplaced? Findspots(redcircle)forpartsofthesameBronzeAgeskiatLendbreen.Thedistancebetweentheuppermostandlowermostfindis250m.Photoandillustration:LarsPilø,OpplandCountyCouncil. Amajorfactorforthepreservationofanartefactfoundatanicepatchisthelengthofexposure.Thisfactorisoftencorrelatedwiththedistanceofthefindspotfrompresentdayice–thefurtherawayfromtheice,thepoorerthepreservation.Thismayappearself-evident,butsometimeslocalconditionsaroundthefindspotmaybebeneficialtoartefactsurvival.Thiscould,forexample,bealargeboulderthathascollectedextrasnow,andprovidedshadow. EllingUtvikWammerholdinga1500yearsoldarrow,foundduringsurveyinJotunheimenin2011.Photo:JulianMartinsen,OpplandCountyCouncil. Differenttypesofartefactsalsopreservedifferently,dependingonthematerialtheyaremadeof.Arrowheadsmadeofstoneormetalpreservewell,butarehardtospotamongthestones,ifthereisnotanassociatedarrowshaft.Wood,birchbarkandbonepreservebestoftheorganicmaterials,whilesinew,hideandtextiledeterioratemorerapidly. Thelandscapesurroundingicepatchesalsovariesalot.Someicepatchesaresituatedinalandscapewithactiveprocesses,forinstancesteepslopeswhererocksmove.Othersareinflatterareaswithactivepermafrost.Suchactivegeomorphologicalprocessesofferpoorpreservationconditionsforartefacts. Alotmoreresearchisneededtogainadeeperunderstandingofwhatthefindsrecoveredfromtheglacialicecantellusaboutpasthumanactivitiesonthesites.