The Carn Goedog bluestone provenancing is unsupported by the evidence

The geology of the north flank of Mynydd Preseli.  Source:  BGS mapping.

Some quotes from Bevins, Ixer and Pearce, 2014

https://www.cambridge.org/core/journals/antiquity/article/megalith-quarries-for-stonehenges-bluestones/AAF715CC586231FFFCC18ACB871C9F5E

Quote:

The principal conclusion presented here is that at least 55% of the Stonehenge monoliths and fragments analysed to date can be sourced to Carn Goedog. The PCA plots support the association of the Group 1 Stonehenge dolerites with Carn Goedog but also suggest that Group 3 dolerites might come from Carn Goedog.....

Quote:
Group 3 samples might be from an as yet un-sampled part of the Carn Goedog outcrop, bearing in mind that Jones et al. (2005), on the basis of PXRF investigations, identified that a number of the eastern Preseli outcrops were geochemically heterogeneous. Further sampling of the Carn Goedog intrusion would serve to clarify if this is the case or not.

Bevins, Ixer and Pearce — Carn Goedog paper, 2014.
Journal of Archaeological Science 42 (2014) 179e193

As we know, these rather circumspect conclusions have been solidified, over the course of five years, on the basis of no additional evidence, info firm statements from Parker Pearson et al (2019) that "At least five bluestone pillars (stones 33, 37, 49, 65, 67) were taken from Carn Goedog, and probably many more."  Elsewhere they claim that a further 4 bluestone megaliths at Stonehenge have also probably come from Carngoedog -- making 9 in all:

Recent geochemical analysis has revealed two main groups of Stonehenge spotted dolerite, the larger of which (Stones 33, 37, 49, 65 & 67) can be matched most closely with Carn Goedog (Bevins et al. 2014). The second group (Stones 34, 42, 43 & 61) has not been provenanced to a specific Preseli outcrop but may derive from Carn Goedog or from nearby outcrops such as Carn Breseb or Carn Gyfrwy. 

I have already scrutinized this claim, and have pointed out that all 9 of these so-called quarried pillars are not freshly quarried pillars at all, but battered erratic boulders and slabs.

https://brian-mountainman.blogspot.com/2019/03/the-other-carngoedog-bluestone.html
https://brian-mountainman.blogspot.com/2019/02/yet-another-quarry-hunters-hypothesis.html

Dear Reader,

Do these look to you like freshly quarried pillars?  If they do, you might need to get your eyes checked........

 Some of the boulders and slabs claimed by Parker Pearson to have been quarried at Carngoedog.  Pics -- courtesy Simon Banton

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To summarise:

The existence of the Carngoedog bluestone quarry is completely dependent upon the accurate provenancing of fragments and samples of spotted dolerite from Stonehenge being traced back unequivocally to the Carn Goedog tor.  As we know, Bevins, Ixer and Pearce (2014) claim to have done that, on the basis of just three samples from the tor, three from Carn Breseb and two from Carn Sian -- although we have no information at all as to where on the tors the sampling points were.  I have raised serious concerns about this provenancing, since none of the Stonehenge samples match precisely with the samples from Carngoedog.  

Now I have followed up some references, and have discovered the paper summarised below.  It's a paper about a dolerite sill from Tal y Fan in Snowdonia, around 110m thick and emplaced in rather similar circumstances to the sills of the Fishguard Volcanic Series in NE Preseli.  What the paper shows is that within sills of this scale, there is great differentiation in the chemical composition and petrological characteristics of a sill from its edges in towards the centre.  There are also differences attributable to post-emplacement metamorphism or alteration.  

Quote from Bevins, Ixer and Pearce 2014:

For example, in a study of the altered, 110 m thick dolerite sill of Tal y Fan (an intrusive sheet of igneous rock) Merriman et al. (1986) were able to model flow differentiation processes during magma emplacement involving the segregation of olivine towards the centre of the intrusion on the basis of concentrations of MgO and Ni, elements which are preferentially partitioned into olivine.

One of the interesting things about this 1986 paper is that Richard Bevins is one of the authors!  So a sample taken from near the edge of the sill is not typical of the centre of the sill, nor of other parts affected by metamorphism.  We have no idea whether the three samples from Carn Goedog came from the centre of the sill, or near one of the edges.  

There must have been similar differentiation in all of the other extensive sills on the north flank of Preseli -- the Carn Bica sill, the Carn Sian sill,  the Carn Ddy Fach sill, the Craig Talfynydd sill and all the others.  So how can the authors of the 2014 paper know that the characteristics of the fragments taken from Stonehenge do not have better matches in unsampled parts of one or more of the other sills in the area?  Their sampling density in the field is nowhere near being adequate to rule out that possibility.

In other words, there is no way that they can state with any degree of confidence that they have actually identified Carn Goedog as a source for certain Stonehenge bluestones.  They should go back to Preseli, take maybe 50 more samples across the full width of the sills already mapped, and then tell us what the analytical results are.  Until they have done that, we can dismiss their "provenancing work" as useless.

If that sounds all very technical, apologies.  It's actually a matter of very simple logic.  Elementary, my dear Doctor Watson.

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Petrological and Geochemical Variations within the Tal y Fan Intrusion: a Study of Element Mobility During Low-Grade Metamorphism with Implications for Petrotectonic Modelling
R. J. MERRIMAN R. E. BEVINS T. K. BALL
Journal of Petrology, Volume 27, Issue 6, 1 December 1986, Pages 1409–1436,
https://doi.org/10.1093/petrology/27.6.1409
Published: 01 December 1986

Abstract

The Tal y Fan Intrusion is an altered olivine dolerite sheet emplaced into a coeval sequence of subaqueous volcanic rocks of Caradoc (Ordovician) age in NE Snowdonia, Wales. Primary mineral and chemical variations across the 110 m thick sheet suggest that the magma was drawn from a zoned magma chamber, although the intrusion consolidated predominantly as a single cooling unit. An horizon of ferrodolerite resulted from in situ fractionation. Secondary mineral assemblages are indicative of the prehnite-pumpellyite and prehnite-actinolite fades, suggesting metamorphic alteration conditions of approximately 310°C and 1-85 kb. Major elemental variation largely reflects primary mineral variations across the intrusion, although Ca, Al, and Na show limited mobility in the outermost 4-5 m, related to breakdown of plagioclase feldspar during metamorphism. The LIL elements Rb, Sr, K, and Ba were highly mobile, particularly in the marginal zones, whereas Th, in addition to the incompatible elements Zr, Y, Ti, P, Nb, Ta, Hf, and the REE, was immobile even in the marginal zones. Accordingly petrotectonic modelling based on discriminant diagrams using these immobile elements is considered most reliable. The Tal y Fan Intrusion has characteristics transitional between N-type and E-type MORB, similar to tholeiitic within plate basalts. In contrast with other Ordovician volcanic sequences of the Welsh Basin, no subduction component is identified in the Tal y Fan magma, the LIL element enrichment observed being related to alteration.

Merriman, R.J., Bevins, R.E., Ball, T.K., 1986. Geochemical variations within the Tal y Fan intrusion: implications for element mobility during low-grade meta- morphism. J. Petrol. 27, 1409-1436.

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POSTSCRIPT -- CRAIG RHOSYFELIN

From a previous post, in 2015.
A gentle reminder of why the "spot provenancing" of foliated rhyolites to within a few sq metres at Rhosyfelin is also nonsense:

http://brian-mountainman.blogspot.co.uk/2015/09/rhosyfelin-and-spot-provenancing.html

This is an extract from our new paper, to be published on Monday:
Parker Pearson has pointed out to many visitors the “exact location” from which an orthostat was taken from the rock face and hauled or carried off to Stonehenge. That assertion appears to be based on the statement from Ixer and Bevins (2011, 2014) that they had provenanced certain rhyolite flakes at Stonehenge to “within a few square metres” of their sampling point 8, near the tip of the Rhosyfelin spur. However, they have not adequately demonstrated that level of precision, either through published thin sections from Stonehenge and Rhosyfelin samples, or through analysis of a very dense pattern of sampling points. The Stonehenge rhyolite flakes could even have come from a section of the spur which has been removed by the processes of glacial entrainment. Also, the “shelf” from which the monolith is supposed to have been removed is heavily abraded by either meltwater or ice action, indicating that it cannot have been quarried during the Neolithic.

The thin section showing the petrography (the Jovian fabric) of the foliated rhyolite at Locality 8 as discussed by Bevins and Ixer in their papers. Thanks to them for the photo.

The geologists have never demonstrated in print that there is anything in the Stonehenge rhyolitic debitage that is identical to the rock exposed at Richard Bevins's original sampling point 8........  If they have thin sections that prove a match, here is an offer to publish them on this blog......

In any case, as far as I can see, the petrography of the foliated rhyolite debitage at Stonehenge is quite variable, when seen in thin sections, as is the petrography of the samples taken from the Pont Saeson - Rhosyfelin area. A confounding factor is that the rock face at Rhosyfelin coincides with a series of closely-spaced fracture planes, extending for about 50 south-westwards from the tip of the spur. Bits and pieces of the same fracture plane are exposed along the whole face, which means that widely separated samples will have an identical, or almost identical, petrography. I am sure that samples will have been taken, and that this will have been confirmed by the geologists. I would like to see that in print. So whatever the nature of the rock at point 8 (location 11) may be, the same rock will occur in many other locations along the rock face and in other locations along the valley side where rock exposures can no longer be seen.

In summary, the level of accuracy in the geological "spot provenancing" for this locality is another fantasy, cited repeatedly and deliberately by the archaeologists as part of their campaign to "prove" that this is a Neolithic quarry. No matter what they may claim, this is NOT scientific evidence.