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Saturday, 7 May 2016

Rhosyfelin -- more on the "monolith extraction point"

 Prof Mike Parker Pearson shows visitors where the "monolith extraction point" is, near the outer end of the Rhosyfelin spur.  Photo: The Duke.

We have had a lot of discussion on this site already about the accuracy -- or otherwise -- of the "spot provenancing" by Rob Ixer and Richard Bevins by which they claim to have fixed to within a few square metres the origin of certain foliated rhyolite fragments found at Stonehenge. 

This is just one of my previous posts:

MPP refers to "the removal of the rhyolite pillar from its recess" and thinks that he knows exactly when that happened, because carbonized hazelnut shells dated to 4590 BP and c 4667 BP, were found in a small occupation layer just 1.5 m away from it. That is about as logical, as I have pointed out, as dating a sea cave in the cliffs to six years ago because a piece of flotsam with a date of 2010 on it is found on the beach nearby.   In the Antiquity paper by MPP et al, there is also a claim that signs of wedges and levers have also been found in the recess:

Mike Parker Pearson, Richard Bevins, Rob Ixer, Joshua Pollard, Colin Richards, Kate Welham, Ben Chan, Kevan Edinborough, Derek Hamilton, Richard Macphail, Duncan Schlee, Jean-Luc Schwenninger, Ellen Simmons and Martin Smith (2015). Craig Rhos-y-felin: a Welsh bluestone megalith quarry for Stonehenge.   Antiquity, 89 (348) (Dec 2015), pp 1331-1352. 

As Dyfed Elis-Gruffydd, John Downes and I have argued in print, the "recess" (which isn't really a recess at all) is an extremely unlikely place from which a monolith might have been taken,  and it is also unlikely that the rhyolite fragments from Stonehenge came from here anyway.  Both the geologists and the archaeologists have been swept along on a wave of euphoria, and have forgotten to look critically at the evidence on the ground.  I took another look at Rhosyfelin yesterday, and this is what I observed.

1.  Petrography.  Bevins and Ixer's sampling point density is inadequate for the degree of precision claimed.  A close reading of their texts, and a close examination of their published thin sections, suggests that some of the Stonehenge foliated rhyolite fragments might have come from other outcrops in the Pont Saeson area.  Furthermore, the geologists do not appear to have considered the possibility that the fragments at Stonehenge have come from parts of the crag which have been entirely removed by glacial erosion.  The foliations in the local rhyolite rock are for the most part very thin indeed -- some are less than 1mm thick, and in places wafer-thin flat flakes have broken off the parent rock along foliation planes:

Wafer-thin flakes of foliated rhyolite found in areas where the rock has been weathered.  These are approx 1 mm thick.  Does every foliated layer (and there are many thousands of them) have its own petrographic signature?

The geologists have not told us, in spite of being asked several times, whether each foliation or thin layer has a characteristic signature which is different in some measurable way from the layers above and below it.  If each layer does have a "signature" that can be picked up through petrography and petrology, presumably it can be followed laterally in all directions along the plane.  That means you should be able to pick it up wherever that foliation layer outcrops -- metres or even kilometres distant from the point at which it was first recognized.  Or do these characteristics change laterally along the plane?  If so, by how much do they change?  None of the published thin sections from the rhyolite fragments at Stonehenge matches precisely the sample taken from point 8 near the tip of the crag, as we have pointed out before.

The other thing which I noticed on yesterday's visit is that the foliations exposed at the surface  in MPP's 2m "recess"are approximately the same ones that are exposed about 8 m away along the rock face, on the other side of a large slab which projects out by about 25 cms.  There is an area of exposed rhyolite approx 8 m long and 4 m high (32 sq m) from which the Stonehenge fragments might have come.  Even further along the face, the same fracture plane is exposed, adding at least another 10 sq m to the "potential source area."  Finally, we have to get rid of the myth that the Rhosyfelin rock face coincides with a single fracture plane.  There are partial exposures of foliated rhyolites on sections of multiple fracture planes, making this a wonderful "field laboratory" for rhyolite enthusiasts.

This section of the face exposes multiple fracture planes which are close to the foliations exposed in the "recess" off photo to the left.  The foliated rhyolite fragments at Stonehenge are just as likely to have come from this section of the face, if they did not come from exposures of the same layers hundreds of metres if not kilometres away, or from exposures long since destroyed.

Anyway, I am more convinced than ever that the Stonehenge rhyolite fragments cannot be provenanced to "within a few square metres" at Rhosyfelin, and that the geologists have been carried away by their work -- which is quite impressive enough and which really did not need to be over-sold.

2.  Rock mechanics.  Prof MPP clearly thinks that a coherent pillar or monolith could have been levered off the rock face at the position of his famous "recess" and then carted off to some proto-Stonehenge in the neighbourhood before making its way to Stonehenge.  A brief examination of the rock face in the vicinity is enough to dispose of this idea once and for all.  There have been many posts on this blog about the Rhosyfelin fracture patterns, including this one:

I have also made the point that even the larger blocks and slabs of rhyolite lying around in and on the rockfall debris at the site are riddled with cracks which would make successful human transport away from the site extremely unlikely.  The "picnic table" is a case in point -- I suggested about a year ago that any attempt to move it would result in it falling to pieces, and that it was the most useless bluestone ever.

Well, I have looked at that "recess" again and have discovered that the rock face displays at least 13 crossing fractures.  Some are quite substantial, and others are hairline cracks.  They can all be traced across to adjacent rock faces, and they run in all directions.  One big fracture plane has been exploited by natural processes to create a "back slope" at the base of the "recess"  -- this is not unique, but it has caused MPP to refer to a "notch" which he thinks was used to lever a monolith away from the face.  Any slabs or blocks that have come away from the face hereabouts will have been riddled with the same fractures, and my guess is that they have fallen away in bits and pieces, and have ended up as rubble.

Close-up of part of the "monolith extraction recess" at Rhosyfelin, showing at least ten intersecting fracture planes.  Anything falling from this location would have been similarly weakened, and the likelihood of a coherent monolith being taken away from here is effectively zero.


TonyH said...

A poseur, n'est pas?

BRIAN JOHN said...

No objection to somebody doing a bit of modelling -- one has to do what one can, these days, to keep body and soul together. What I really need here is some informed comment from geologists, whether they know this site or not......

BRIAN JOHN said...

Where are the pet rock boys on this issue? Thunderous silence -- I still want to know how the "petrographic signature" of a particular foliation layer varies across that layer laterally, and how it varies from those layers above and below it in the sequence. This is absolutely crucial if we are to assess the reliability of the "within a few square metres" claim.