In the light of all the current interest in the Boles Barrow bluestone, let's forget for the moment the arguments about whether it was originally stolen from Stonehenge or was genuinely found within the core of a particular long barrow. (Parker Pearson currently seems to believe that the stone did NOT come from Stonehenge, and that it was put into the long barrow at least 5,500 years ago.) Let's concentrate instead on what its geological provenance might be.
Parker Pearson et al (2015) refer to it as a "broken but un-dressed bluestone pillar fragment" -- so the assumption is that somewhere or other, the rest of this mysterious pillar remains to be found.
It is generally assumed that it came -- like most of the other spotted dolerite bluestones -- from the Preseli area of North Pembrokeshire. Ixer and Bevins have stated that it is a "group 3" spotted dolerite, with characteristics different from the Carn Goedog spotted dolerites. They have speculated that it most closely matches the spotted dolerites outcropping in Carn Breseb, Carn Gyfrwy, Carn Alw and an unnamed outcrop west of Carn Ddafad-las. These outcrops are associated with doleritic intrusions up to 2km east of Carn Goedog, whose precise details are still not known, although the geologists think that they may all be components of the same elongated dolerite sill.
I have examined the evidence here:
Then it gets a bit confusing, because the geologists suggest in their paper that although the "group 3" spotted dolerite samples are different from those at Carn Goedog, they might have come from there after all, because the dolerites contain a lot of lateral internal variation. Confused? So am I............. and that is probably because the density of sampling points on the Preseli outcrops is far from adequate for really sound conclusions to be drawn.
So might the supposed "Boles Barrow pillar" which has fallen into bits have come from Carn Goedog after all? That seems to be where the opinion (but not necessarily the evidence) is taking certain people who have a quarrying hypothesis to uphold.......
We can shortly expect a new paper on the Boles Barrow bluestone. Various little birds have tweeted that it was taken from its perch in Salisbury Museum out into the fresh air in May, so that a detailed laser scan could be completed. So it has by now been subjected to the full 3D treatment, which will be revealed in all its glory. That's great -- I look forward to seeing the results. While they were about it, did the investigators take some samples from the surface for cosmogenic dating? Is the "old" part of the boulder (or pillar fragment, if you want to call it that) demonstrably older and more weathered than the "broken" or fractured face? If so, what are the dates?
One of the little tweeting birds tells me that the close examination of the underside of the stone supposedly provides evidence of how it was extracted from its bedrock position. So the latest hypothesis is that the Boles Barrow stone is not just a piece of a lost pillar, but that the pillar itself can be shown to have been taken from a particular location on a particular outcrop. Rumour has it that Prof MPP is on the trail, and that he is excavating a specific quarry site. That has to be Carn Goedog, since he cannot possibly have hunted across several square kilometres of terrain looking for a space from which an imaginary spotted dolerite pillar of unknown dimensions might have come from! He has already laid out the trail by referring (in his CA article) to precise spots on the Carn Goedog tumbledown tor from which monoliths have been extracted. I couldn't see them, and I'm not sure anybody else could either, but he has the eye of faith.......
The last question is this. If the Boles Barrow boulder (or pillar remnant) belongs to spotted dolerite group 3, could one of the other known group 3 stones at Stonehenge actually be the larger part of the same stone? The candidates are bluestones 34, 42, 43 and 61. All of these are spotted dolerite, and all are sampled.
34 -- well rounded small boulder or squat pillar, placed on end. Spotted dolerite with pinkish spots -- in the bluestone circle.
42 -- recumbent wedge-shaped stone with heavy wear on edges. Densely spotted dolerite? Projecting through the turf, in the bluestone circle.
43 -- recumbent slightly flattened boulder with heavy wear on edges. Densely spotted dolerite? Projecting through the turf, in the bluestone circle.
61 -- a slab-like small pillar in the bluestone horseshoe.
The only one of these that looks remotely like a pillar is SH61. Could that be the rest of the Boles Barrow stone? Watch this space........
Postscript: I have checked the details for whole-rock analysis in table 1a of the Carngoedog paper by Bevins, Ixer and Pearce. The geochemical makeup of SH 61 is NOT the same as that of the Boles Barrow boulder. In fact the sample from the boulder does not match up precisely with any of the other samples -- if anything, the closest match appears to be with stone SH42. But I can see why the Group 3 spotted dolerites are grouped together, and that the assumption is that they have all come from roughly the same area. But spot provenancing on the basis of the current evidence? Frankly, impossible. Maybe Myris will give us an opinion on this?
Carn Goedog is the likely major source of Stonehenge doleritic bluestones: evidence based on compatible element geochemistry and Principal Component Analysis (2013),
Richard E. Bevins, Rob A. Ixer, Nick J.G. Pearce