Chlorine-36 dating and the bluestones of Stonehenge.

Chlorine-36 dating and the bluestones of Stonehenge. Chlorine-36 dating has important potential for archaeology, butrecent Chlorine-36 dates on `bluestones' of Stonehenge have beenmisinterpreted. Professor D.Q. Bowen of the University of Wales Affiliated institutionsCardiff University Cardiff was once a full member of the University but has now left (though it retains some ties). When Cardiff left, it merged with the University of Wales College of Medicine (which was also a former member). , Cardiffand colleagues have dated a fragment of igneous rock igneous rock:see rock. igneous rockAny of various crystalline or glassy, noncrystalline rocks formed by the cooling and solidification of molten earth material (magma). reported as havingbeen found at Stonehenge (exact type unknown, but not a spotteddolerite dol��er��ite?n. Chiefly BritishA dark, fine-grained igneous rock; diabase.[French dol��rite, from Greek doleros, deceitful (from its easily being mistaken for diorite) ) at 14,000[+ or -]1900 years, and surfaces of outcrops at CarnMenyn in Preseli at 5400[+ or -]400 and 4900[+ or -]400 years (Bowen etal. 1994; Bowen 1994: 211). This information has been interpreted asindicating that the bluestones of Stonehenge could not have beentransported to the site of Stonehenge by ice, because the ice sheetswere extensive enough only at c. 400,000 years or earlier, when thefragment and outcrop were apparently still buried or covered, and notexposed for ice transport (Bowen 1994: 211; Hawkes 1994; BritishArchaeology News 1995). Chlorine-36 dating gives an estimate of the length of time that arock surface has been exposed to the atmosphere, by measuring the amountof Chlorine-36 produced by exposure of the rock to cosmic radiation. Ifthe rock or surface has been covered or buried, the date obtained willreflect the reduced time of exposure to air. Thus a Chlorine-36 date mayreflect either recent exposure of a surface due to processes such asfrost shattering, or an original exposure date. This difficulty ofinterpretation is why Chlorine-36 dating is normally done on boulders orlava surfaces whose erosional history is known (e.g. Phillips et al.1991). Professor Bowen and colleagues have obtained a date of c. 14,000years exposure time for the fragment from Stonehenge. However, it is notpossible to tell if this is an original exposure date, or if thefragment was brought to Salisbury Plain by ice 400,000 years ago orearlier, and was subsequently buried within superficial deposits onSalisbury Plain for part of its history. Or it could have been brokenoff a larger erratic lying on Salisbury Plain, by natural processes suchas frost shattering. The rock type of this fragment is unknown, and thesample now completely destroyed (Professor D.Q. Bowen, in discussion atthe meeting of the Lithic lith��ic?1?adj.Consisting of or relating to stone or rock.Adj. 1. lithic - of or containing lithium2. lithic - relating to or composed of stone; "lithic sandstone" Studies Group, Cardiff, 28 January 1995), soit may be nothing to do with the bluestone bluestone,common name for the blue, crystalline heptahydrate of cupric sulfate called chalcanthite, a minor ore of copper. It also refers to a fine-grained, light to dark colored blue-gray sandstone. monoliths. Carn Menyn loses material from outcrop surfaces every year throughfrost shattering. This will reduce the date obtained on an outcrop. Adate of 5000 years could represent a preserved quarried surface (inwhich case it might be expected to show quarry marks), or it could be afrost-shattered surface. Even if it is a quarried surface, its relevanceto Stonehenge is questionable, because the bluestones were erected thereabout 4000 years ago. Thus, even if it is considered that the bluestones were moved byhuman transport, the Chlorine-36 dates do not tell us at what era thatmovement might have taken place. The article in British Archaeology suggests that dating a monolithsurface of a bluestone at Stonehenge will resolve the problem.Unfortunately, it will not. At least some of the bluestones were dressedand the argument continues about which, and how many were altered inthis way. Some have also been damaged by tourists. A dressed or damagedbluestone will give a Chlorine-36 date reflecting total exposure time -for example, about 4000 years if it is dressed but not subsequentlydamaged. A variety of older dates could be obtained, depending onwhether the bluestone was removed from Wales by a glacier as an erratic,buried for part or all of its glacial transport, or broken up byerosional processes in post-glacial times. We welcome further evidence in the Stonehenge debate, but areconcerned about misinterpretations. Chlorine-36 dating might help thedebate by obtaining large numbers of dates on Preseli outcrops (to givea mean date of oldest exposed surfaces). Dates on monoliths atStonehenge could be useful if obtained for undressed and undamagedsurfaces, but even these would be subject to interpretation. In themeantime Adv. 1. in the meantime - during the intervening time; "meanwhile I will not think about the problem"; "meantime he was attentive to his other interests"; "in the meantime the police were notified"meantime, meanwhile , Chlorine-36 dates give no evidence either for or againstglacial transport of the bluestones of Stonehenge. ReferencesBowen D.Q. 1994. Late Cenozoic Wales and south-west England,Proceedings of the Ussher Society 8: 209-13. Bowen, D.Q., F.M. Phillips& D. Elmore 1994. Chlorine-36 dating British ice-sheets, Abstractsof the American Geophysics Union, 1994 Fall Meeting: 226. BritishArchaeology. 1995. Dating gives clue to Stonehenge riddle, BritishArchaeology 1 (February). Hawkes, N. 1994. Stonehenge dating dispelsicesheet theory, The Times: 5 December 1994. Phillips, F.M., M.G. Zreda,S.S. Smith, D. Elmore, P.W. Kubik, R.I. DORN & D.J. Roddy. 1991. Ageand geomorphic ge��o��mor��phic?adj.Of or resembling the earth or its shape or surface configuration. history of Meteor Crater, Arizona, from cosmogenic cos��mo��gen��ic?adj.Produced by cosmic rays.[cosm(ic ray) + -genic.]Adj. 1. [Cl.sup.36] and [C.sup.14] in rock varnish, Geochimica et CosmochimicaActa 55: 2695-8.