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Wednesday, 3 February 2016

That "Jovian" fabric -- how accurate can "spot provenancing" be?






One of the interesting points that came up in discussions after my seminar last week in Swansea was the claim by geologists Rob Ixer and Richard Bevins that they have identified the source of some chips of "foliated rhyolite with Jovian fabric" at Stonehenge to within a few square metres of sampling point 8 at the tip of the Rhosyfelin spur.  I have dealt with this claim before, more than once, and have expressed my scepticism:

http://brian-mountainman.blogspot.co.uk/2014/04/on-significance-of-rhosyfelin-locality-8.htmhttp://brian-mountainman.blogspot.co.uk/2015/09/rhosyfelin-and-spot-provenancing.html
http://brian-mountainman.blogspot.co.uk/2014/04/rhosyfelin-foliation-plane.html

Although the geologists have been rather cautious about their interpretation, it has been seized upon with relish by the archaeologists -- and has led to all that wild speculating about the "monolith extraction point" which has been much photographed, mostly with MPP indicating exactly where a Stonehenge bluestone is supposed to have actually come from. Of course, he cites the geologists as his authority.   Has MPP been chastised by the geologists?  It does not appear so.......

Following my presentation, without any prompting from me, some of those present were mystified about the degree of accuracy claimed, on a craggy outcrop from which rock has been removed down through the millennia by a variety of different processes.  So herewith are my attempts to articulate the concerns:

As I understand it, the foliated rhyolites from across this area have subtle variations on the "Jovian" texture theme -- no two samples look precisely the same in thin section.  Presumably that means that each foliation layer has its own "signature" which is different from the foliation layers above and beneath it.  It does not appear that the geologists yet have enough detail -- at least not in the published articles thus far -- to say whether the "signature" of each layer remains consistent laterally, right across a foliation surface or plane.  Several other geomorphologists have pointed out to me that if a particular foliation plane has its own signature, then any point at which that plane is exposed (maybe miles away from Rhosyfelin) could be a source for the fragments found at Stonehenge.  We can see on the photo above that the foliation plane exposed in the yellow strip closest to the camera (the supposed "monolith extraction point") is also exposed at multiple other points across the face.  That face is not flat, and it is not a single fracture plane -- the face is made up of multiple fracture surfaces, some set more than 50 cms "deeper" than others.  So the celebrated fragments at Stonehenge could have come from any one -- or several -- of those exposures -- or more likely from exposures of that same foliation layer from parts of the crag long since destroyed.  Or -- and this is perfectly feasible -- from localities many miles away from Rhosyfelin.

Which brings me to the point that we still haven't seen a thin section slide from Rhosyfelin that precisely matches up with a thin section slide from the Stonehenge rhyolite debitage.  Rob and Richard, if you are reading this, and if you have such a "matching pair",  please send them along and I'll happily reproduce them.  Alternatively, you might wish to publish the slides in this strange thing called the "primary literature", in which case we look forward to reading the forthcoming article.

Sad as I am to bring this up again, I have to repeat that we seem to have a case of geological "over-interpretation" here -- which has, as we all know,  led to the committing of a multitude of sins by the archaeologists.


This is the figure published by Ixer and Bevins which shows the sampling points used in the collection of rhyolite samples.  As we can see, point 8 is near the tip of the spur and all the other points are on the SE flank of the ridge. It appears that no samples were taken in the original research programme from the NW face of the ridge, which has attracted so much attention........  No doubt other samples will have been collected from the rock face and analysed by now, and we look forward to reading about them in due course.


 

5 comments:

chris johnson said...

Ah, the elephant in the room.

It always struck me as strange that the basis for this endeavour in the Presceli is not examined critically. It speak volumes for the reputations of Ixer/Bevins that their findings go unchallenged, although when you read carefully what they say and what they don't say then they are more careful than the popular press would have us believe.

In any other field, a technique which is mastered by so few people in the world would set alarm bells ringing - but apparently not in British Archaeology. Myris used to remark that Dr Ixer's science was a dying art and perhaps this is why so few people seem capable to comment on it.

Helen said...

Hi Brian

Submitting this as a comment as I don't know how to contact you directly, so please feel free to delete/not to publish.

Just to pass on this link to a YouTube video in which, quote, "Eminent archaeologist Mike Parker Pearson gives the Council for British Archaeology's 37th annual Beatrice de Cardi lecture on the Stonehenge landscape."

https://www.youtube.com/watch?v=BZ2lR5_N0vc

I don't know when the talk occurred but the video's date of posting is given as 2 February 2016. You may already have seen it, of course, in which case I apologise for duplication.

There appears to be nothing new in what MPP has to say but it's always interesting to hear it from the proverbial horse's mouth.

Regards
Helen

BRIAN JOHN said...

Thanks Helen == must watch this right through one day when I have time on my hands.....

BRIAN JOHN said...

I'm more than a little intrigued that this post -- which in effect says that the so-called spot provenancing of Stonehenge foliated rhyolites to within a few square metres at Rhosyfelin cannot be deemed to be reliable -- has elicited no response from the geologists. So we'll take that as agreement, shall we?

chris johnson said...

The section on Carn Goedog is most enthralling. MPP waxes lyrical about this oh so obvious Neolithic quarry, complete with work floors, roadways, and gaps where monoliths have been extracted. A marvellous flight of fancy.