Rhosyfelin - some geological questions

Jovian texture shown in thin section from a sample from site 8 near the tip of the Rhosyfelin spur (photo: Ixer and Bevins)

 Prof Mike Parker Pearson pointing to the exact site from which a rhyolite orthostat was taken away and transported to Stonehenge (photo: The Duke).  According to the photographer, "Even evidence of cutouts for wedges to be driven in to split the stone away from the face are visible. Awesome site."  So there we are then.

 In recent weeks I have visited Rhosyfelin in the company of more than a dozen glacial geomorphologists and have had some rather interesting discussions.  Most of them have raised one question which is worth sharing:  "What is the actual evidence which enables the geologists to claim that they have provenanced much of the rhyolite debitage found at Stonehenge to the very place where Prof MPP is standing in the above photo?"  That question relates in particular to the fact that there is not a particularly dense cover of sampling points across the landscape, and that great tracts of the Stonehenge landscape debitage haven't been sampled either.   Some of the visitors have read the literature, and they are intrigued by the sheer bravado of statements like these: 

"The overwhelming majority of the Stonehenge rhyolitic ‘debitage’, namely that belonging to Groups A-C, can be sourced from the Pont Saeson area and perhaps entirely from Craig Rhos-y-felin, but from more than one site on the crags."

"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; it may not be the last."

CRAIG RHOS-Y-FELIN, PONT SAESON IS THE DOMINANT SOURCE OF THE STONEHENGE RHYOLITIC ‘DEBITAGE’
Rob Ixer and Richard Bevins, Archaeology in Wales 50, pp 21-31

"The ‘rhyolite with fabric’ (almost all is Craig Rhos-y-felin rhyolitic tuff) and the ‘volcanics with sub-planar texture’ debitage are universally distributed within the Stonehenge Landscape." (from "Chips off the old block" by Ixer and Bevins)

".......... 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 squares 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......." (As pointed out before, that should be "site 8".....)


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

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As I have indicated before on this blog, I too have some doubts about the  confidence with which the geologists can cite the Rhosyfelin crag as "the dominant source" for rhyolite fragments found in the Stonehenge area.  So some more questions:

1.  Of the 1200 or so rhyolite fragments found in the Stonehenge landscape and looked at by the geologists, how many have been used for thin section analysis?

2.  What assurance do we have that the thin sections published are actually typical of the whole assemblage?

3.  How much of the Stonehenge landscape has been excavated (5% at most?), and how much of the Stonehenge monument itself has been excavated (50% at most?).

4.  How many rhyolite sites in the Pont Saeson -- Rhosyfelin -- Brynberian area have now been sampled, and how many of those have been used for the preparation of thin sections?

5.  Are there not other localities in the Pont Saeson area which have provided thin sections with "Jovian fabric" which are virtually identical with the one from locality 8 on the crag itself?

6.  Is there any evidence that suggests that all or most of the rhyolite debitage at Stonehenge comes from one or two destroyed orthostats?

7.  Have you considered the possibility that the "rhyolite with fabric" fragments found in some parts of the Stonehenge landscape (I use those words advisedly) have come from parts of the crags in the Rhosyfelin - Pont Saeson area that no longer exist, having been entrained by overriding glacier ice?

8.  How much variation is there in the fabric of the rhyolites across the 50 m or so of the exposed rock face?

9.  How much variation is there in the fabric as you go deeper into the cliff face and examine some of the the foliated rhyolites exposed side-on where cliff face sections have sheared away? 

10.  Have you sampled the "proto-orthostat", and can you identify exactly where on the crag it has come from?

None of this detracts from the very smart provenancing work that has been done.  It's just that several of us share concerns about some of the statements made and conclusions drawn.........  Apologies if some of these questions are already answered in some of the papers published!  Can't keep track of everything -- but repeat answers will be appreciated.

To find further discussions on this issue,  just put "Jovian fabric" into the search box.