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Sunday, 7 April 2013

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

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:

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?


Anonymous said...

I recently helped the Sky at Night team filming their latest episode which was shown tonight. It will be repeated on Thursday, BBC4 at 7:30. The Lake House meteorite is featured and they mention glaciation on Salisbury plain,

Anonymous said...

"Was he a charlatan who was actually involved in scientific fraud"

He was a Harkness Scholar and was awarded a 1st class BA degree in Natural Sciences.

He won the Sedgwick prize in 1903 and was also assistant to Professor Sollas at Oxford, earning B.A. and B.Sc. at Oxford. From 1901 to 1911, he was geologist to the Geological Survey of Great Britain and was a petrographer from 1911 to 1935 working for the Geological Survey Department.

He was a leading paleobiologist and carried out some work on carboniferous palaeobotany. At Cambridge was awarded Sc.D. in 1914.

Just a simple academic whose 'qualifications' have been proven once again to be meaningless.

"Or maybe just so obsessed with his thesis that he was blind to the subtleties of interpretation"

A distant relative?

BRIAN JOHN said...

Your point being that all experts are idiots?

Anonymous said...

Yet another pay to view!

Cue another comment about reading the primary sources.

Aren't these two incompatible?

BRIAN JOHN said...

I know -- it's a real bugger, and effectively bars people who are not working in institutions from reading the primary sources. But I find that if you send a nice Email message to the lead author, citing the paper you would like, they are normally very willing to send you a PDF.

GCU:intwominds said...

That is EXACTLY what I do.
I pay for all my independant research from my OAP fund- well The Constantine Palaeologos Research Fund does.
I read the primary lit by buying it- if I cannot scrounge it.
I have a list of those slimy b... who do not send Pdfs.
They are few but KNOWN. I know where they work.

BRIAN JOHN said...

Myris -- you sound like a character in my novels -- one Joseph Harries the Wizard. His classic curse is to send somebody a note which simply says: "I know who you are, and I know what you have done." That, of course, normally does the trick......

GCUintwominds said...

Perhaps I am, my nickname at uni was Idris.

BRIAN JOHN said...

Idris the Wizard? I like it. Didn't know you had Welsh roots.....

Constantinos Ragazas said...

So it's back to Rhosyfelin. Deja vu all over again! But still no RC-date data from MPP! Dating some shells taken from fissures atop the Craig may resolve this. More rocks probably wont as decisively!


BRIAN JOHN said...

Shells on top of Craig Rhosyfelin?!! Do you know something the rest of us don't know, Kostas?

Constantinos Ragazas said...


If fresh water shells are found on top of Craig Rhosyfelin that RC-date to say 3000BP wont that indicate the Craig was under water at the time it was a “bluestone quarry”? And wont this decisively settle the issue? Of course, no fresh water shells have been found. Because no one looked for them! Any bets if MPP will look for them?


BRIAN JOHN said...

Kostas -- no whalebones have been found on top of Mynydd Preseli either, presumably because nobody has ever bothered to look for them.

Constantinos Ragazas said...


Very funny! And very sophistic!

I found a walesbone once some forty years ago on the beach of Provincetown in Cape Cod. And Barbara my friend with me at the time told me I will live forever! I liked her tail better!

So you think finding fresh water shells on top of the Craig is as improbable as finding walesbones. But is it as insignificant? Especially if they carbon-date to 3000BP?


Anonymous said...

"Your point being that all experts are idiots?"

A clear contradiction.

My point is that most transformational experts are not academics. Darwin, Einstein and Newton were all experts but were not institutionalised as they all were undistinguished in their educational careers as they found it totally irrelevant.

Those who seek higher academic qualification need to encompass the accepted 'belief system' if they are to be successful. Hence your comment on H.H. Thomas being a 'charlatan' is inaccurate as he was just regurgitating accepted institutional beliefs, which now a hundred years on can be scientifically disproven.

BRIAN JOHN said...

I accept that some academics can be "institutionalised" and can allow themselves to go with accepted belief systems if they perceive that that will be good for their careers. In all walks of life there are people who follow "the easy route" -- not just in science and academia. But HHT was not just regurgitating accepted beliefs --- he was intent upon making a "scientific coup" which would give him maximum publicity and prestige. In some ways that is what drives the modern generation of senior academics -- the need to tell a spectacular "new" story which may or may not stand up to serious scientific scrutiny. Scientific method should be at the heart of everything -- but sadly it often isn't.....

GCU:Intwominds said...

Newton was Lucian Prof of Mathmatics at Cambridge for over 30years. Is there a bigger chair?
If this is "undistinguished in an educational career" I would like the same please.

Einstein lived much of his life in working (sheltered) in Princeton.

There are good and readable biographies of all three.

TonyH said...

Very interesting you mention that west of Tycanol Wood could be a source area for rhyolites as you say you've mentioned it as a possible location for the entrainment of erratics.

Anonymous said...


Alan Sugar holds two Doctorates of Science, awarded in 1988 by City University and in 2005 by Brunel University. Alan is an 'expert' at making money, but he is no academic.

Universities are very good at 'after the event' qualifications.

BRIAN JOHN said...

I don't have any geological evidence in support of that speculation -- but the geomorphology and glaciology both point to it being quite possible.

BRIAN JOHN said...

Anon -- honorary degrees are what they say they are --- nobody expects that the award of such a degree involves any great understanding of science! On the other hand, examined masters degrees and doctorates should be worth quite a lot, if the supervision, refereeing and oral cross-examinations are properly conducted. What are you trying to say?