New Bevins / Ixer paper on certain rhyolites

Carn Alw, the crag on the north side of Preseli which was thought by Thomas to have been the source for the Stonehenge rhyolites

A new paper by Richard Bevins and Rob Ixer has just been published online -- thanks to Rob for drawing my attention to it.  This one is not about the Stonehenge rhyolite debitage and the link with Craig Rhosyfelin -- but about HH Thomas and his conviction that the source of the Stonehenge rhyolites (or at least some of them) was the crag called Carn Alw, on the north side of the Preseli ridge.  The details of the paper, and the Abstract, are here:

"Carn Alw as a source of the rhyolitic component of the Stonehenge bluestones: a critical reappraisal of the petrographical account of H.H. Thomas"
Richard E. Bevins, Rob A. Ixer
Journal of Archaeological Science, In Press, Accepted Manuscript, Apr 2013
doi:10.1016/j.jas.2013.03.017
http://www.sciencedirect.com/science/article/pii/S0305440313001076

Abstract
The source of the Stonehenge bluestones was first determined in the early 1920’s by
H.H. Thomas who was an officer with the Geological Survey of England and Wales. He
determined that the so-called ‘spotted dolerites’ could be petrographically matched to a
small number of outcrops in the Mynydd Preseli district in south-west Wales. The
bluestones, however, comprise a number of additional lithologies, including rhyolite and
‘calcareous ash’, as well as various sandstones. Thomas was convinced that the volcanic
lithologies in the bluestone assemblage were all sourced from a small area at the
eastern end of the Mynydd Preseli, with the rhyolites originating from the prominent
outcrop known as Carn Alw. Recently, the provenancing of these rhyolites to Carn Alw
has been questioned on the evidence of whole-rock geochemistry. This raised concerns
over the original petrographical attribution. Accordingly a re-investigation was
undertaken of the rhyolite petrography by re-examining the original specimens used by
Thomas. Three of the original four thin sections studied by Thomas were re-examined,
along with a newly made thin section from the fourth of Thomas’ samples as the original
thin section could not be located. The new petrographical evidence demonstrates
convincingly that the two pairs of samples from the Preseli and Stonehenge as examined
by Thomas do not match despite his contention and confirms that Carn Alw is not the
source of the Stonehenge rhyolites which Thomas described. This reinforces the
geochemical evidence presented recently and supports the contention that Craig Rhos-y-felin, to the north of Mynydd Preseli, is the principal source of rhyolitic debris in the Stonehenge Landscape.

So HH Thomas is under scrutiny here -- which is fine by me, since geologists have, by and large, been very reluctant to put his work under close scrutiny, even though I have done so in THE BLUESTONE ENIGMA and on this blog:

http://brian-mountainman.blogspot.co.uk/2013/03/is-bluestone-myth-based-on-scientific.html
http://brian-mountainman.blogspot.co.uk/2010/08/hh-thomas-geologist-seduced-by-dreams.html
http://brian-mountainman.blogspot.co.uk/2011/03/more-on-hh-thomas.html
http://brian-mountainman.blogspot.co.uk/2010/02/did-hh-thomas-cook-books.html
http://brian-mountainman.blogspot.co.uk/2010/03/two-great-hoaxes-piltdown-skull-and.html

On the contents of the new paper, the authors home in on the four rhyolite samples studied by Thomas and used as the basis for the "Carn Alw connection."  They point out that the matches between the Stonehenge samples and the Carn Alw samples are not at all as convincing as Thomas pretended, and that the problem of accurately matching thin section slides one against another is exacerbated in this case because Thomas did not make it clear whether he was looking at his slides through plain polarised light or through crossed polarised light.  This is a somewhat esoteric matter, and the illustrations used in the paper are very revealing in this respect!

At any rate, this is an interesting study, and we must be grateful to the authors for yet another fascinating piece of detective work.  They are perfectly justified in giving close scrutiny to Thomas's confident assertion that  "…conclusive evidence is furnished by the fact that the remaining four igneous masses of Stonehenge, the rhyolites, are identical in colour, mode of weathering, and all structural and mineralogical details with the rhyolites that occur at Carn Alw."  The authors point out that the four rhyolitic (and dacitic) Stonehenge orthostats are ash-flow tuffs, with pumice, lithic
and crystal fragments and in one case well-preserved glass shards. In contrast, they say that the Carn
Alw samples described by Thomas (and now reexamined) are recrystallized, typically spherulitic rhyolitic lavas which might have originally been part of a thick lava flow or dome.  One of Thomas's thin sections seems to match the foliated rhyolite from Craig Rhosyfelin, but the others are from unknown sources.  However, there is a suggestion that a possible source area might be to the west of Tycanol Wood, where air-fall tuffs were examined by Lowman and Bloxham in 1981.  (If that source were to be confirmed, that would be interesting indeed, since I have speculated many times on Tycanol and Carnedd Meibion Owen as being a possible area for the glacial entrainment of erratics...........)

In conclusion, Bevins and Ixer say:  "not a single rhyolite fragment from the Stonehenge orthostats or debitage from anywhere in the Stonehenge Landscape can be attributed to Carn Alw."  That is a pretty definitive statement, which may well be correct.  However, I would have liked a little more hard information on the actual samples examined from Carn Alw -- it is a big rock outcrop with considerable geological variation within it.  Where were the samples taken?  Have the authors eliminated the possibility that Thomas's samples were not taken from a different part of the outcrop from that sampled recently?

Finally, I am rather concerned by the manner in which Rhosyfelin has been pulled into this paper.  Quote:  "Bevins et al. (2012) presented geochemical evidence to suggest that the majority of the Stonehenge rhyolitic rocks were derived not from Carn Alw but from Craig Rhos-y-felin....."  As mentioned before, I am not at all convinced by that statement -- it is far too sweeping, and the authors should have said, much more cautiously,  "... the majority of the Stonhenge rhyolitic debitage from sampled locations" came from Rhosyfelin.

Quote:  The present study ".......serves to test the proposal by Bevins et al. (2012) and Ixer and Bevins (2012) that Craig Rhos-y-felin rather than Carn Alw is the main source of rhyolitic debris in the Stonehenge Landscape."  Again, that is far too grand an objective, given the very small number of samples looked at here and the very limited knowledge which we still have of "the Stonehenge landscape" -- most of which has of course not been investigated by anybody at any time.

Quote from the abstract and the conclusion:  "This reinforces the geochemical evidence presented recently and supports the contention that Craig Rhos-y-felin, to the north of Mynydd Preseli, is the principal source of rhyolitic debris in the Stonehenge Landscape."

With due respect to the authors, what their new paper does is demonstrate that Carn Alw was not the source for the rhyolite orthostats or for the fragments collected by Gowland. As they say, they still have no idea where the source area for these fragments (or for the four rhyolite orthostats at Stonehenge) might be.  The new work does nothing to reinforce the Rhosyfelin connection.  If anything, it weakens it, since the authors are now confirming that other sources exist for at least some of the material at Stonehenge.........

Finally, what are we to make of the work of HH Thomas all those years ago?  Was he a charlatan who was actually involved in scientific fraud in pursuit of his great goal of demonstrating that the bluestones were carried from Preseli to Stonehenge by human agency?  I have long suspected that when he made his famous lecture and wrote his famous paper, the evidence he presented was partial and carefully selected, with "inconvenient" information simply left out.  He was a darling of the archaeological establishment, and he knew perfectly well what an impact his thesis would have.  In  the event his evidence and his conclusions were subjected to hardly any scrutiny -- a point which has not been made often enough.  So was he involved in a scientific fraud on a par with that of the Piltdown Skull?  It's possible.  Bevins and Ixer are far too polite to suggest such a thing in their paper -- but they do suggest that his work was less competent than it might have been, even given that he was working on his samples almost a century ago.  Was he careless?  Disorganized?  Or maybe just so obsessed with his thesis that he was blind to the subtleties of interpretation which Bevins and Ixer have now applied, when looking at exactly the same thin sections?