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Tuesday 9 September 2014

More thoughts on Carn Goedog

On mulling it over, I need more convincing if I am to accept that ANY of the Stonehenge bluestones have actually come from this outcrop........

I have been thinking about Carn Goedog lately, given the huge amount of publicity that was given, almost a year ago, to the idea that "Carn Goedog was the source of the spotted dolerites found at Stonehenge."  The paper which gave rise to all the excitement was this one, by Bevins, Ixer and Pearce:

"Carn Goedog is the likely major source of Stonehenge doleritic bluestones: evidence based on compatible element geochemistry and Principal Component Analysis"
Journal of Archaeological Science
Richard E. Bevins, Rob A. Ixer, Nick J.G. Pearce
Journal of Archaeological Science, 19 November 2013

If you put in "Carn Goedog" into the search box you will see several entries and much discussion!  But this is what I said in November:

1.  There is no new fieldwork here.  The authors have not been doing any new sampling at Stonehenge, and nor have they been furiously collecting new samples on Mynydd Preseli.

2.  The authors have re-analysed samples already collected over past decades by Richard Bevins and others, and by Olwen Williams-Thorpe and her colleagues prior to the publication of this big report:  Thorpe, R.S., Williams-Thorpe, O., Jenkins, D.G., Watson, J.S., Ixer, R.A., Thomas, R.G.,
1991. The geological sources and transport of the bluestones of Stonehenge, Wiltshire, UK. Proc. Prehist. Soc. 57, 103e157.

3.  A total of 68 samples have been re-analysed using very sophisticated techniques:  22 samples from Stonehenge (from monoliths and debitage) and 46 samples of Preseli dolerite -- spotted and unspotted.  It's worth noting that only 3 of these samples have come from Carn Goedog (which is a very big hillside tor stretching across several hundred metres of terrain.)

4.  It is concluded that 11 (and possibly 12) of the 22 Stonehenge dolerite samples have probably come from the Carn Goedog outcrops. These are called Group 1 samples.

5.  Three of the Stonehenge samples have possibly come from the Cerrigmarchogion area, on the crest of the Preseli ridge and some way to the south-west of Carn Goedog. These are called Group 2 samples.

6.  Five of the Stonehenge samples and the Boles Barrow sample have come from outcrops in the Carn Gyfrwy-Carn Breseb-Carn Alw area (including the un-named outcrop west of Carn Ddafad-las).  These are called Group 3 samples.

7.  One sample from Stonehenge (numbered SH44) has not been assigned to any of the three groups mentioned above.

8.   There is a possibility that Group 1 and Group 3 dolerites might be closely related -- and may ALL have come from the Carn Goedog outcrop.  By the same token, some of them or most of them might have come from some other as yet unsampled part of the same intrusion, which stretches cross-country for about 3 km before being truncated by faults at both ends.

9.  Carn Meini -- for long assumed to be the location of THE bluestone quarry -- does not figure at all in the new provenancing work, and we can see from the detailed mineralogy plots that the Carn Meini samples (around 18 of them?) seem to fall outside the ranges of the three Groups of Stonehenge samples analysed by the authors. 

10.  The dolerite sills in this part of eastern Preseli are highly variable or "geochemically heterogenous."  Also, they have still not been adequately mapped, and whilst there seems to have been quite intensive sampling of the Carn Meini outcrops in the past,  there is still a shortage of samples from the Carn Goedog outcrops.   Expect more surprises when more fieldwork is undertaken!

11.  So has the Carn Meini Quarry now been replaced in the minds of archaeologists by the Carn Goedog Quarry?  If it has, they should think again.  There is a lot of variation in the Stonehenge samples, so even the eleven Group 1 samples have NOT all come from the same precise location.  Overall, we are looking at Carn Goedog rocks that have possibly come from an area several hundred metres across -- and quite possibly some of them have come from outcrops or source areas as yet unsampled.

12.   The 22 Stonehenge samples analysed in this paper have probably come from 22 different locations.  Some of the samples show strong similarities, but each one is unique.  What the geologists have done here is to have narrowed down the provenancing of the sampled Stonehenge dolerites to three or four areas within this landscape of dolerite sills in eastern Preseli -- but there is no way we can talk about point locations or quarries.

13.  When this is all put together with the work that the same authors, and others, have done on the rhyolites and sandstones found in the Stonehenge bluestone and debitage sample assemblage, it still seems to me that we are looking at an assemblage of glacial erratics that have come from many different places -- but preferentially from eastern Preseli -- on the route of the Anglian Irish Sea Glacier.

14.  Remember that there are 43 known bluestones at Stonehenge.  There is also a vast area of debitage at Stonehenge that has not been excavated or sampled.  The authors of this paper look forward to the day when EH will allow ALL of the bluestones to be sampled and analyzed -- and will allow the "unexplored" parts of the Stonehenge regolith also to be sampled.  Expect further surprises -- but maybe not in our lifetimes.........

15.  Many thanks to Rob Ixer for sharing this paper and making it possible for me to conduct this review of the main findings.   He and his fellow authors will no doubt correct me if I have got anything wrong!!


 Intrusions and Fishguard Volcanic Group rocks in eastern Preseli 
(Source:  Bevins, Ixer amd Pearce.)

I'm now having serious misgivings about the degree of certainty expressed by the authors in this case. 

First, I'm concerned about the emphasis on the actual tor called Carn Goedog, which happens to be prominent -- but because it is a prominent landscape feature that does not mean it is necessarily the place from which Stonehenge bluestones have actually come.  That line of reasoning is very much an anthropocentric one -- based on the idea that the bluestones were carried by human beings, and that they are more likely to have taken them from somewhere prominent rather than from somewhere insignificant.  That is not very scientific -- and in fact it verges on the careless.  Glaciers are not as impressed by upstanding lumps of rock as human beings apparently are.......

Second, when one looks at the map of dolerite sills and other outcrops on the flank of Preseli, one sees that the "Carn Goedog" sill of spotted dolerite is a very extensive one, stretching from near Carn Alw through Carn Goedog itself and thence westwards towards the mountainside between Mynydd Bach and Hafod Tydfil.  To the east of Carn Goedog, the outcrop stretches across about 2 km of mountainside before running up towards the crest of the ridge at Carn Breseb and thence to Carn Ddafad-las.  The western half of this sill outcrop has only been sampled in two places, both (so far as we can tell) close together at Carn Goedog tor itself.  So over the rest of this extensive outcrop we have no idea how similar -- or how different -- the spotted dolerites may be.  That means that the geologists are NOT in a position to state unequivocally that the bulk of the Stonehenge spotted dolerites have come from Carn Goedog itself.  Before they can do that, they need to show us a string of samples from along the length of the sill outcrop, and to show us that all the others are different!

The authors of the paper admit that there are difficulties:

The bivariate plots suggest that the three Stonehenge dolerite
geochemical groups are derived from three principal geographical
sources, namely Carn Goedog (Group 1), Cerrigmarchogion or
possibly Craig Talfynydd (Group 2) and a set of outcrops including
Carn Breseb, Carn Gyfrwy, those in the vicinity of Carn Alw and the
un-named outcrop immediately west of Carn Ddafad-las (Group 3)
(but note that sample SH44 remains unassigned in this study).
Whilst this agrees in part with the suggestions of Thorpe et al.
(1991) on the basis of whole rock geochemistry and Ixer (1996,
1997) on the evidence of reflected light microscopy 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, rather than the outcrops
listed above. What seems likely, and is hinted at in some of the
bivariate plots (e.g. Figs.10 and 11), is that the Groups 1 and Group 3
magmas were derived from the same magma batch and that the
difference between Group 1 and Group 3 dolerites relates to
varying mineral concentrations (resulting from crystal fractionation
and/or crystal accumulation) in the samples and that the
compatible element plots highlight this feature. 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

What this says is that the Group 3 samples -- from the eastern part of the same sill of spotted dolerite -- are in some ways very similar from those taken from Carn Goedog.  That's not surprising, since a single sill -- if indeed it is just one sill -- should consist of magma from a single source, intruded and solidified at the same time.  It's interesting that the authors suggest that the "Group 3" dolerites in the Stonehenge collection might, like the "Group 1" dolerites, have come from Carn Goedog.  I don't quite follow the logic in that; one might equally claim that because of the "geochemically heterogenous" nature of the dolerites in this sill, NONE of them have actually come from the famous outcrop.

I don't deny that the geological work reported in this paper is very impressive.  Having thought long and hard about it, I'm just not convinced -- on the basis of the evidence presented -- that ANY of the Stonehenge bluestones have actually come from Carn Goedog.

 The terrain in the Carn Goedog - Carn Alw area.  Carn Goedog is bottom left.  Carn Alw is top right.  Carn Breseb is bottom right.  On this mountainside there are many localities from which erratic boulders might have been entrained from the same spotted dolerite sill by 
overriding glacier ice.

One of the reasons for my scepticism is that when one looks in detail at the landscape between Carn Goedog and Carn Alw, over a distance of about one km, one sees many outcrops of spotted dolerite (presumably all parts of the same sill) and sound evidence of glacial erosion and glacial entrainment in the form of ice-smoothed slabs, littered erratic boulders, and "scooped-out" hollows where ice has at some stage been moving uphill towards the crest of the Preseli ridge.  More of which anon.......


PeteG said...

heads up!

PeteG said...

look out for a new Stonehenge story after tonight 1AM embargo

Myris of Alexandria said...

Some detail on this would be welcome, this is the sort of thing that should be in the real literature.
I think that more sampling of CD and others is planned or may have happened.

BRIAN JOHN said...

Pete -- an article that's more than ten years old? I suppose the Boscombe Bowmen are about ton ride forth again? Await news with bated breath.....

BRIAN JOHN said...

Myris -- there is not much here that is publishable -- certainly nothing new. What I'm doing is applying some careful scrutiny to the published paper and pointing out that the designation of Carn Goedog as the place where the Stonehenge spotted dolerites probably came from seems to be based more upon the fact that it is a PROMINENT HILLOCK than upon incontrovertible geological provenancing.......

BRIAN JOHN said...

To explain a bit further. Re the Carn Goedog spotted dolerite sill -- we can see on the map that it is truncated by a fault that runs more of less N-S along the cwm to the west of the tor. The fault line is assumed to run right across the Preseli ridge, giving rise to the col which is the lowest crossing of the ridge -- this is why the route was used by the drovers and their animals. I have assumed that the sills which extend to the west of this fault, on the northern slopes of Mynydd-bach, are part of the Carn Goedog sill. True or false? There appears to have been no sampling which will help us to answer this question....... But we do know that the sill as shown on the geological map is inaccurate. There are certainly several outcrops of dolerite on the mountainside where there are supposed (according to the map) to be Ordovician sediments.....