Those famous Chlorine 36 dates

I've been looking at the film called "Stonehenge -- Secrets of the Stones."  Thanks to Pete for drawing attention to it.  I'll put up another post with a YouTube link on it, in case anybody is interested.

One of the interesting sequences in the film (produced in 1998) is the bit featuring Prof David Bowen, who wanders about in the monument patting sarsens and apparently thinking that they are bluestones, and then produces two lumps of stone which he wants to date, using the chlorine 36 technique.  He says that the stones have come from the 1958 dig -- so presumably they have come from the collections made in the Richard Atkinson excavations of that year.  God only knows where they came from on the site, and what their history might be.  That would be quite enough to put most people off, but not Prof DB.

Look at the stones in these pics.  The smaller one is obviously a piece of spotted dolerite -- but what is the other one?  Looks like a bit of Chilmark stone to me --  please correct me if I'm wrong.  Could it be a piece of volcanic ash?  Or a piece of rhyolite?  Anyway, we don't hear anything more about the big lump on the TV programme -- but the smaller one ended up being crushed to a powder in the USA (at Purdue University) and subjected to Chlorine 36 dating.  The date apparently came out as showing that the stone was first exposed to the atmosphere around 40,000 years ago.  On that basis the good Professor says -- on the film -- that "there is no way that this rock could have been transported by glacier ice from Preseli to Stonehenge."  He says that the last ice sheet that might have affected this area was in existence about 650,000 years ago -- goodness knows where he got that date from........

This finding was widely purported in the media to demonstrate conclusively that glaciation could not have moved the bluestones to Stonehenge.  of course, it does nothing of the sort.  It does not even support the human transport theory.

Prof Bowen's dating exercise -- also involving other dates, with two from Carn Meini,  has been heavily criticised by other geologists. It is in my view entirely useless -- we do not know where the sample came from either at Stonehenge or at the place of its origin.   fUnless you know precisely where a sample came has come from, ie what its exposure to cosmic radiation and weathering might have been over many thousands of years, you can say NOTHING reliable or scientific about the age of the stone surface you are purportedly dating. And to say anything sensible, you need at least two different dating techniques, based on the characteristics of the rock.

This was pointed out by Olwen Williams-Thorpe and others some years ago.  I'm a bit confused, because there is another date (for the big sample in the picture?) that was published in 1994 --- around 14,000 years BP.    Other dates for rock surfaces at Carn Meini were 5,400 BP and 4,900 BP.  Which rock surfaces?   Chosen because they looked fresh, or because they looked old?  facing east or west?  On top of a cliff or in the shadow of a cliff? We have no idea................

PLEASE will somebody do some sensible cosmogenic dating one day, to help us to understand what has gone on at Stonehenge? 

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Antiquity, Volume: 69  Number: 266  Page: 1019–1020
Chlorine-36 dating and the bluestones of Stonehenge
Olwen Williams-Thorpe, D. Graham Jenkins, Judith Jenkins and John S. Watson

http://findarticles.com/p/articles/mi_hb3284/is_n266_v69/ai_n28663881/

Chlorine-36 dating and the bluestones of Stonehenge

by Olwen Williams-Thorpe, D. Graham Jenkins, Judith Jenkins, John S. Watson

Chlorine-36 dating has important potential for archaeology, but recent Chlorine-36 dates on `bluestones' of Stonehenge have been misinterpreted. Professor D.Q. Bowen of the University of Wales, Cardiff and colleagues have dated a fragment of igneous rock reported as having been found at Stonehenge (exact type unknown, but not a spotted dolerite) at 14,000[ or -]1900 years, and surfaces of outcrops at Carn Menyn in Preseli at 5400[ or -]400 and 4900[ or -]400 years (Bowen et al. 1994; Bowen 1994: 211). This information has been interpreted as indicating that the bluestones of Stonehenge could not have been transported to the site of Stonehenge by ice, because the ice sheets were extensive enough only at c. 400,000 years or earlier, when the fragment and outcrop were apparently still buried or covered, and not exposed for ice transport (Bowen 1994: 211; Hawkes 1994; British Archaeology News 1995).
Chlorine-36 dating gives an estimate of the length of time that a rock surface has been exposed to the atmosphere, by measuring the amount of Chlorine-36 produced by exposure of the rock to cosmic radiation. If the rock or surface has been covered or buried, the date obtained will reflect the reduced time of exposure to air. Thus a Chlorine-36 date may reflect either recent exposure of a surface due to processes such as frost shattering, or an original exposure date. This difficulty of interpretation is why Chlorine-36 dating is normally done on boulders or lava surfaces whose erosional history is known (e.g. Phillips et al. 1991).
Professor Bowen and colleagues have obtained a date of c. 14,000 years exposure time for the fragment from Stonehenge. However, it is not possible to tell if this is an original exposure date, or if the fragment was brought to Salisbury Plain by ice 400,000 years ago or earlier, and was subsequently buried within superficial deposits on Salisbury Plain for part of its history. Or it could have been broken off a larger erratic lying on Salisbury Plain, by natural processes such as frost shattering. The rock type of this fragment is unknown, and the sample now completely destroyed (Professor D.Q. Bowen, in discussion at the meeting of the Lithic Studies Group, Cardiff, 28 January 1995), so it may be nothing to do with the bluestone monoliths.
Carn Menyn loses material from outcrop surfaces every year through frost shattering. This will reduce the date obtained on an outcrop. A date of 5000 years could represent a preserved quarried surface (in which case it might be expected to show quarry marks), or it could be a frost-shattered surface. Even if it is a quarried surface, its relevance to Stonehenge is questionable, because the bluestones were erected there about 4000 years ago.
Thus, even if it is considered that the bluestones were moved by human transport, the Chlorine-36 dates do not tell us at what era that movement might have taken place.
The article in British Archaeology suggests that dating a monolith surface of a bluestone at Stonehenge will resolve the problem. Unfortunately, it will not. At least some of the bluestones were dressed and the argument continues about which, and how many were altered in this way. Some have also been damaged by tourists. A dressed or damaged bluestone will give a Chlorine-36 date reflecting total exposure time - for example, about 4000 years if it is dressed but not subsequently damaged. A variety of older dates could be obtained, depending on whether the bluestone was removed from Wales by a glacier as an erratic, buried for part or all of its glacial transport, or broken up by erosional processes in post-glacial times.
We welcome further evidence in the Stonehenge debate, but are concerned about misinterpretations. Chlorine-36 dating might help the debate by obtaining large numbers of dates on Preseli outcrops (to give a mean date of oldest exposed surfaces). Dates on monoliths at Stonehenge could be useful if obtained for undressed and undamaged surfaces, but even these would be subject to interpretation. In the meantime, Chlorine-36 dates give no evidence either for or against glacial transport of the bluestones of Stonehenge.

References

Bowen 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, Abstracts of the American Geophysics Union, 1994 Fall Meeting: 226. British Archaeology. 1995. 
Dating gives clue to Stonehenge riddle, British Archaeology 1 (February). 
Hawkes, N. 1994. Stonehenge dating dispels icesheet 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. Age and geomorphic history of Meteor Crater, Arizona, from cosmogenic [Cl.sup.36] and [C.sup.14] in rock varnish, Geochimica et Cosmochimica Acta 55: 2695-8.