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Tuesday, 30 August 2016

Rhosyfelin spot provenancing and the "few square metres" myth

 An illustration of the manner in which the "Locality 8" foliation layer and fracture plane can be exposed at multiple locations across the Rhosyfelin rock face, not to mention at other multiple locations in the neighbourhood.

 Quote from Ixer and Bevins 2011:
"This is the first time that any lithics from Stonehenge have been unequivocally assigned to an area of a few square metres, namely to within a very small single outcrop or couple of outcrops......"
Rob Ixer and Richard Bevins, Archaeology in Wales 50, 2011, pp 21-31
How many times have we heard the claim that geologists Richard Bevins and Rob Ixer have identified the source of certain foliated rhyolites in the Stonehenge debitage to "within a few square metres" on the Rhosyfelin rock face?  Too many -- and as we have pointed out very often, the claim was unreliable when it was first made, and it remains unreliable today since no new evidence has been published to support it.  Other geomorphologists who have visited Rhosyfelin are just as sceptical as the three of us who wrote the papers questioning the reliability of the "bluestone quarry" hypothesis:

Brian John, Dyfed Elis-Gruffydd and John Downes. 2015. OBSERVATIONS ON THE SUPPOSED “NEOLITHIC BLUESTONE QUARRY” AT CRAIG RHOSYFELIN, PEMBROKESHIRE". Archaeology in Wales 54, pp 139-148. (Publication 14th December 2015)

Brian John, Dyfed Elis-Gruffydd and John Downes (2015a).  "Quaternary Events at Craig Rhosyfelin, Pembrokeshire."  Quaternary Newsletter, October 2015 (No 137), pp 16-32.

There have been a number of posts on this blog on this subject, including the following:

The trouble is that the claim, having been prematurely and incautiously made, has been accepted as the truth by archaeologists who cannot realistically be expected to know any better -- and so the myth, having been manufactured, continues to roll.  One of the most absurd byproducts  of it is the claim by Prof MPP that he now has the precise location of an "extraction point" from which one of the Stonehenge bluestone monoliths was taken.  He says that he knows this because the geologists have told him it is true....... it's always a good trick to avoid responsibility for a statement by pretending that you have it on the good authority of somebody who is more expert than you are.......

So why is the "few square metres" claim unreliable?

Firstly, none of the thin sections from the foliated rhyolite debitage at Stonehenge precisely matches the published thin section from locality 8 near the tip of the Rhosyfelin spur.  There is quite a lot of variation in the "jovian fabric" of the samples from Stonehenge and in the samples from the Rhosyfelin - Pont Saeson area. It's all very well for the geologists to say "We are the experts.  trust us. We know what we are talking about."  In that case, show us the colour of your evidence, and we might believe you.

Secondly, we have received no answer to this point:  if the jovian fabric shown in the Location 8 thin section is typical of one particular foliation layer on one particular fracture plane, surely that "signature" will also exist wherever else that foliation / fracture plane relationship occurs?  By my reckoning, similar if not identical samples might also have been taken from approximately 40 sq metres of the currently exposed rock face, and also from any number of outcrops within and outside the Brynberian river valley, and also from parts of the crag subsequently removed by glacial erosion and other processes.  The geologists cannot prove that the "similar" or "related" samples found in the Stonehenge rhyolitic debitage have not come from hundreds of metres or even kilometres away.  It is no argument to say that all of the other samples taken in the Pont Season / Rhosyfelin area are different from that taken from Location 8.  Of course they are different, since they came from different positions in the rhyolitic sequence, some from above the "special" foliation plane and some from below it.  You would expect them to be different -- some very different indeed, and others just slightly so............

Thirdly, we must question the claim by Ixer and Bevins that in excess of 99.9% of of the Stonehenge rhyolitic ‘debitage’ can be petrographically matched to the rhyolitic rocks found around Craig Rhosyfelin and Pont Saeson.  It is largely because of that statement that the media have become obsessed with the idea that the source of at least some of the bluestones has now been found.  However, as I have pointed out before on this blog, the 99.9% figure is meaningless, since we have no idea how many rhyolite  fragments have been examined, how they were distributed in the Stonehenge Layer,  how close together the collection points were, and what degree of selection was employed in the collection of samples during the respective digs.  Percentage figures like this should never be used without a full presentation of the numerical data from which they are supposedly derived.   In any case, we have no idea which rhyolites occur in the Stonehenge Layer in those large segments of Stonehenge that have not been excavated, and in the soils of the area around Stonehenge, and what their frequency may be.

Abundant over-egging of the pudding going on here.  I'm not questioning for a moment the competence of the geologists concerned.  Just their tendency to go off-topic.  They have a lot to answer for, since they are the ones who set this whole quarrying wild goose chase going when they would have been far wiser to simply present their evidence without seeking to over-emphasise its archaeological importance.

Will the geologists now please retract the "few square metres" claim, and allow common sense to prevail?  


TonyH said...

Isn't it a pity that your detailed explanation of why the "few square metres" claim is unreliable will not be presented to Mr & Mrs Joe Punter, readers of the tabloid press such as the Daily Mail and Express, or even The Guardian, Times etc!

TonyH said...

......The reason being, of course, that it is beyond the attention span of most casual readers and, perhaps more importantly, newspaper editors, and so is deemed not newsworthy.

BRIAN JOHN said...

I am intrigued by the manner in which the geologists have -- metaphorically speaking, of course -- dug a deep hole for themselves, for reasons that are still unclear. In the quote below, from 2012, Rob is actually talking about location 8, and not location 9, but he seems convinced that the spot provenancing allowed him and Richard Bevins to claim an accuracy of 10 sq m -- that means a surface exposure of just over 3m x 3m. So in 2011, the geologists told MPP where to dig, and rejoiced with him when that big chunk of rock was uncovered. So from 2011 onwards they have been completely wedded to the quarrying hypothesis, and that is apparent in much of the subsequent geological literature relating to the Preseli igneous outcrops. Has their objectivity as scientists been compromised as a result? Opinions on a post card please.........

".......... matching the distinctive ‘rhyolite with fabric’ debitage (first seen in Stone’s stones) from Stonehenge to very detailed sampling along the Welsh outcrop showed that rocks from the extreme north-east of Craig Rhos-y-felin (‘site 9’) were identical to Stonehenge rhyolites showing the 'Jovian’ texture. This suggests an almost impossible provenance of ten square metres. The archaeologists were told where to dig. In September 2011, Professor Mike Parker-Pearson of Sheffield University and his team cleared the vegetation from the northern end of Craig Rhos-y-felin and excavated. They found, just a few metres from site 9, a large proto-orthostat, a large joint block set for its journey to Salisbury Plain......."

Rob Ixer, Digging into Stonehenge’s past. Mineral Planning, issue 143 / October 2012, p 13

BRIAN JOHN said...

The characteristics and exposures of a particular foliation in a rhyolite sequence are a bit difficult to understand. Imagine a novel with 200 pages. The book has 20 chapters. In the middle of Chapter 10 is one page (numbered 100) that is black. Instead of lying the book flat, lay it on end, with the spine downwards and the long edges of the pages exposed upwards. Now carve a landscape out of the book, while keeping it tightly shut. You will be able to see the edge of that infamous black page along the ridge, sometimes on high points in the landscape and sometimes in low ones. If you then take samples from that thin black line you will always find the characteristics (or "signature") of that particular page. If you take samples from pages close to it they may be closely related in that they are still parts of Chapter 10, but the further you go away, to one side or another, the more "different" they will become. At the far edges of the book (or the geological outcrop, if you want to call it that) you will have samples from Chapter 1 and Chapter 20, which will be very different in some respects but similar in others, in that they are still parts of the same novel.

This is what the geologists have done in the Rhosyfelin - Pont Saeson area, claiming that the sample taken from Locality 8 so unique that it cannot be replicated from anywhere else. Their other samples have all come from other pages of the book -- or from other foliations, related or different to a greater or lesser degree. They have forgotten that the Locality 8 sample can be replicated wherever the black edge of page 100 pops up in the landscape.

In any case, as I have pointed out often, there is NOT an identical match between any one sample from Rhosyfelin and any one sample from the Stonehenge debitage. Maybe the debitage samples have come from page 85, or 98, or 107 or 125? Similar and related, but not the same. And those actual fragments of rock could have come from multiple locations in the landscape -- thousands of them -- where the edges of those particular page edges or foliations have outcropped in the past.

There now. That is probably all as clear as mud.

BRIAN JOHN said...

I have been looking again at the Ixer / Bevins paper on Rhosyfelin and the rhyolites with the Jovian fabric / planar foliations. Because the foliations are metamorphic in origin, and planar or flat, as distinct from wavy and distorted, we can assume that they are very extensive -- as suggested in my comments above. Individual foliations might stretch across country for kilometres -- I can see no evidence that supports the contention by the authors that ....."these very distinctive rhyolitic rocks can be traced for no more than 150 m from the northeasternmost end of Craig Rhos-y-felin." (p 22). Elsewhere they talk of these special rocks being restricted to an area about 300m across, with the peculiar Jovian fabric rocks being much more localised. What they mean is that they have no samples from further afield, from other parts of the same planar foliations, because there are no convenient outcrops. Neither can I see any reason why the tip of the crag near sampling locality 8 has been selected as the likely source for some of the Stonehenge debitage, other than the fact that it is easily accessible and that it looks -- to the geologists -- as if it might have been a very handy quarry! Hmm -- here comes that ruling hypothesis again........