Is it possible for geologists to provenance rock fragments found in one place to a locality a great distance away, with an accuracy of "a few square metres"?
As readers of this blog will know, Prof MPP claims to know exactly where at least one Stonehenge bluestone has come from, and he has pointed the spot out to many visitors to the Rhosyfelin dig site. OK, you might say, he's just repeating what the geologists have told him.....
But hang on a minute. If you read the papers by Ixer and Bevins rather carefully they are for the most part rather circumspect, and they are very cautious about making claims that might be difficult to sustain under close scrutiny. So who's responsible for the hype here? This is important, because the story of this "precise provenancing" has been picked up by the media and by all sorts of august institutions like the Pembs Coast National Park and the Dyfed Archaeological Trust -- so the truth does become rather important. So what are the facts?
Well, we have (according to a mysterious source) about 40-60 thin sections of rock samples from the Rhosyfelin - Pont Saeson area, and we can probably assume that there are an approx equal number of thin sections from the "foliated rhyolite" debitage in the Stonehenge area. So do the matches match the hype? Here we have a problem, because very few of these "Jovian fabric" slides have been published. No problem with that, since no researchers ever publish ALL their data, and as Myris has reminded us, they try to publish slides that are "typical" or "representative" of the group as a whole.
The slides I have been able to find are reproduced below, with full acknowledgement to Richard Bevins and Rob Ixer and their colleagues.
The top thin section -- actually representing the rocks at the tip of the spur at Rhosyfelin -- is the crucial one here, and what we need is something from Stonehenge that matches it almost exactly.
The sample from the Heel Stone fragment is not all that good a match, because the black blobs look rather like tadpoles on that one (excuse the scientific language.......). The closest match is that of Sample STH08-738, but even that is not all that close, because the black blobs are less frequent and there is a sort of melted or fluidized look to the slide which is absent in the one from Rhosyfelin. And the larger phenocrysts (right word?) are more prominent and texturally distinct at Rhosyfelin than they are in the Stonehenge sample.
Could the Heel Stone sample have come from another part of the Rhosyfelin crags, or is it matched to something further away? Could OU18 have come from Pont Saeson PS10? They do look rather similar.......
What we don't know at present is the extent of lateral variation in texture along a foliation plane, or the extent to which samples taken from the rock face at Rhosyfelin are representative of those rocks a metre or two "behind" the rock face, or higher up or lower down. Maybe that info will be forthcoming in a future paper?
So on the evidence currently before us, M'Lud, I suggest that the case is not proven. I do not see anything in the Stonehenge samples to convince me of an identical match. Neither do we know how big a variation there is in the Stonehenge material. Could the samples all have come from one or two stones that have been broken up, or could they have come from a much wider range of material in the Stonehenge landscape and transported by ice from the Rhosyfelin - Pont Saeson neighbourhood? Now there may well be other samples hidden away that we have not yet been shown. But I suspect that if there had been "better" Stonehenge samples, we would have seen them by now. On the principle of "Show, don't tell" may we have a look at any other thin sections that might help to convince us that there really is a match to "within a few square metres" between something at Stonehenge and something at Rhosyfelin?