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Siccar Point - Den Mest Berømte Geologiske Lokalitet?

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Siccar Point - Den Mest Berømte Geologiske Lokalitet? Siccar Point - den mest berømte geologiske lokalitet? Berwick-kysten med Siccar Point set fra vest. Kystklinten er 30-40 m høj. (Foto: Forfatteren) Af Ole Humlum, Institutt for Geofag, Oslo tydning en dannet person med bred interesse som en usædvanlig dygtig landmand, men Universitet for også mange andre aspekter af tilværel- han havde også i videnskabelige kredse sen. etabletet sig som en tilsvarende højt anset Vi er i året 1788, dvs. et år inden Blandt hans øvrige interesser ved siden geolog. At være en anset geolog i Skotland stormen på Bastillen i Paris. Det af medicin kan nævnes så forskellige emner stillede i 1788 store krav. Det er nemligt globale klima er forholdsvis køligt, som landbrug, kemi, meteorologi, fi losofi og tidsmæssigt sammenfaldende med sidste dels fordi man fortsat lever under kinesisk sprog. James Huttons første og dy- del af den såkaldte “Scottish Enlightement”, beste interesse var dog landbrug. Han arve- der begynder omkring 1730 og varer ved Den lille Istid, dels fordi det kun er de familiens gård nær Duns i Berwickshire, indtil omkring 1790. I disse 60 år fremstod få år efter det gigantiske Laki-vul- øst for Edinburgh, og udviklede med tiden Skotland som Europas ubestridte, kultu- kanudbrud (1783-84) på Island. På denne til et mønsterlandbrug. Hans ideer og relle ledernation. Filosoffen David Humes, det tidspunkt anses vor planets alder erfaringer vedrørende landbrug blev senere økonomen Adam Smith, kemikeren Joseph for at være 6.000 år, i det mindste i samlet i en manual “The Elements of Agri- Black, ingeniøren James Watt, poeten culture”, på ikke mindre end 1.100 sider. Robert Burns er nogle af samtidens store den europæiske kulturkreds. Gården var beliggende nær de 400-500 m kulturelle skikkelser i Skotland. Fra at være høje Lammermuir Hills, hvor særligt kolde en nation i strid med sig selv og sine naboer Dette tal var bestemt på grundlag af et intervaller af Den lille Istid fl ere gange hav- ændres Scotland til at være et samfund i po- detaljeret studium af Biblen foretaget af de resulteret i fejlslagen høst, hungersnød James Ussher (1581-1656), den anglikanske og frafl ytning specielt i tidsrummet 1685- Ærkebiskop af Armagh, Irland. Med tilsva- 1705. Også i Huttons egen tid, mellem 1740 rende sikkerhed mente Ussher iøvrigt også og 1770, var forholdene vanskelige som at kunne bestemme, at Jorden ville gå under følge af kort vækstsæson. i oktober 2004. Sir Isaac Newton (1643- Derfor forsøgte Hutton, i lighed med 1727) havde tidligere foretaget en mere mange andre bønder, at effektivisere udnyt- videnskabelig beregning af alderen, baseret telsen af de lavere liggende landbrugsom- på en antagelse, om at Jorden oprindelig råder, hvor det fortsat var muligt at modne var helt smeltet. Newton konkluderer, at korn, trods tidernes generelle klimatiske afkølingen måtte have taget omkring 50.000 ugunst. Et af midlerne til effektivisering år. Dette resultat var dog blevet ignoreret af var at etablere bedre dræning af markerne. samtiden, da det jo ikke var foreneligt med Under gravningen af drænkanaler fandt den offi cielle alder på kun 6.000 år. Hutton ved fl ere lejligheder sten og fast fjeld indeholdende fossiler af fortidige dyr Hutton – en nuanceret videnskabsmand og planter herunder arter, der ikke længere Samme år, dvs i 1788, stiger den skotske levede i Scotland. Herved var der langsomt landmand og geolog James Hutton (1726- vokset en stærk geologisk interesse frem, og 1797) med to venner ombord i en lille båd han blev i kraft af sine observationer efter- ved Dunglass Burn og sejler på tur langs hånden overbevist om, at planeten var langt kysten øst for Edinburgh. Hutton var var i mere dynamisk og udsat for store miljøskift, øvrigt født i Edinburgh 3. juni 1726. Han end man kunne formode på grundlag af ska- studerede medicin ved universiteterne i belsesberetningen. Paris og Leyden og var i ordets bedste be- I 1788 var han derfor ikke kun anset 10 GeologiskNyt 5/09 Inkonformiteten på Siccar Point set fra syd. I forgrunden ses den stejltstillede gråvakke fra Silur overlejret af svagt hældende sandsten fra De- von. Den synlige del af de devone afl ejringer er ca. 1 m mægtig. (Foto: Forfatteren) sitiv udvikling med hovedvægt på kultur og venner, John Playfair og Sir James Hall. øvre bjergarts nederste lag. Skarpkantede videnskab. The Royal Society of Edunburgh Mellem byerne Dunbar og Eyemouth gik de fragmenter af den nedre bjergart var fl ere oprettes i denne gyldne periode, og tidens i land på en lille halvø kaldet Siccar Point. steder synlige i det nederste lag af den øvre fremstående forskere og kulturelle personer Stedet var ikke tilfældigt valgt. Hutton bjergart. mødtes regelmæssigt her eller andre steder i havde jordbesiddelser ved det nærliggende Hutton og hans venner indså øjeblikkelig Edinburgh for at udveksle og diskutere nye Reston og havde diskuteret de lokale geolo- betydningen af dette: Hver af lagserierne ideer. Ifølge en samtidig beretning udmær- giske forhold med en anden landmand, Sir måtte have krævet lang tid at afsætte, og kede disse sammenkomster sig ved at være James, der også var en interesseret geolog. undervejs måtte der også have været tid til hyggelige, uformelle, ofte videnskabelige, Sir James viste Hutton en skitsetegning, føst at introducere en foldning af den ældste og altid uden at deltagerne udviste forud- som han havde gjort ved Siccar Point, vi- bjergart, således at den kom til at stå om- intagne holdninger til emner under debat sende eksistensen af to forskellige lagserier, trent lodret. Herefter måtte der have været uanset tema. liggende over hverandre, adskildt af en ure- tid til ved erosion at fjerne en del af denne, Det var på denne frugtbare kulturelle gelmæssig grænsefl ade. Det syntes at være og desuden måtte der være tid til at afl ejre baggrund at James Hutton udviklede sine en yderst lovende lokalitet, med henblik på den øvre, lagdelte bjergart, samt dennes geologiske ideer. Hans omhyggelige geolo- hvad Hutton var på udkig efter. efterfølgende hærdning. Endelig måtte det giske iagttagelser i Skotland og andre steder Derfor gik Hutton og hans venner i land nuværende landskab naturligvis også have gjort over en periode på ca. 30 år havde ef- på Siccar Point, hvor de stod overfor en tid til at udvikles. Dette krævede blandt terhånden gjort ham stærkt skeptisk over for spetakulær og besynderlig geologisk forma- andet, at stedets karakter sideløbende måtte den offi cielle alder på Jorden; 6.000 år. Han tion. To serier af tydeligt lagdelte bjergarter have undergået ændring fra hav til land og mente, at den virkelige alder måtte være ligger over hinanden, men ikke på simpel sandsynligvis tilbage til hav igen. Selv om langt højere. Ellers var der ikke mulighed parallel måde. Nederst ligger med næsten de to lagserier således synligt var i direkte for at forklare alle de miljøændringer, som lodret lagstilling en grå lagdelt bjergart kontakt med hverandre, måtte de med hen- geologien på forskellig vis oplyste om. Så (greywackes), og derover ligger med kun syn til deres dannelse derfor have været ad- han var på udkig efter en eller fl ere geologi- svag hældning en anden lagdelt bjergart skilt af et betydeligt tidsrum, i løbet af hvil- ske lokaliteter, hvor hypotesen om Jordens med rødlig farve (Old Red Sandstone and ket mange store naturbegivenheder havde alder på kun 6.000 år med stor sikkerhed conglomerates). Lagserierne står således udspillet sig. Planeten havde bestemt ikke kunne falsifi ceres. mere eller mindre vinkelret på hinanden. henligget uden nævneværdige ændringer Grænsefl aden imellem de to bjergarter er siden sin tilblivelse. Termen “unconformity” Spektakulær opdagelse på Siccar Point uregelmæssig, og fl ere steder ses lag fra den blev knyttet til beskrivelse af dette fæno- Med i båden havde Hutton i 1788 to geolog- underliggende bjergart at stikke lidt op i den men, hvor to bjergarter med uens lagstilling GeologiskNyt 5/09 11 grænser op til hverandre langs en fl ade skabt ved erosion. John Playfair publicerede senere en be- skrivelse af besøget ved Siccar Point, som med tiden er blevet en klassiker i den viden- skabelige litteratur (se boks nedenfor). Jordens alder til revision Observationen ved Siccar Point overbeviste på den ene side Hutton og hans venner om at vor planet er langt mere dynamisk end tidligere formodet, ikke mindst det forhold at landoverfl adens retning åbenbart kunne drejes 90 grader i forhold til op-ned. På den anden side indså de samtidig, at Siccar Point på overbevisende måde demonstrerede, at skabelsesberetningens alder for Jorden på 6.000 år var alt for lav. Med deres kendskab til geomorfologiske processer virke, var det langt mere sandsynligt, at planetens alder skulle tælles i hundrede af millioner af år, Kontakten mellem de silure og devone (øverst) afl ejringer. Den synlige del af profi let måler eller måske endda milliarder. Begrebet “an ca. 1 m fra højre til venstre. Grænsen mellem de to bjergarter ses at være uregelmæssig, og abyss of time” blev introduceret. angulare fragmenter af den silure gråvakke er indlejret i den devone sandstens nederste del. I dag ved vi, at de to lagserier synlige (Foto: Forfatteren) ved Siccar Point er fra henholdsvis Silur og Devon, og deres dannelse indbyrdes er ad- skildt af 55 millioner år i tid. Den videnska- belige effekt af Huttons tolkning skulle vise hypotesen langsomt ophøjet til teori i løbet fysiske principper. Godt nok havde James sig at blive enorm. Mange geologer anser af de næste 50-70 år. Senere forskere har Hutton og andre geologer i de forudgående derfor i dag Siccar Point som Jordens mest naturligvis i høj grad bygget på de logiske 50-70 år argumenteret for en meget højere berømte geologiske lokalitet. konsekvenser af Huttons analyse fra Siccar alder end den tidligere formodede alder på Huttons hypotese om Jordens høje alder Point.
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