The Altar Stone may NOT have come from Wales

ORS outcrops and (marked by triangles) the locations of ORS beds sampled and used for comparative purposes with the Altar Stone surfaces and with associated debitage fragments at Stonehenge.

A friend attended a talk by Richard Bevins the other day, at which he highlighted this latest piece of research on the Altar Stone:

"Linking derived debitage to the Stonehenge Altar Stone using portable X-ray fluorescence analysis."
Richard E. Bevins, Nick J.G. Pearce, Rob A. Ixer, Stephen Hillier, Duncan Pirrie and Peter Turner
Mineralogical Magazine (2022), 1–13
doi:10.1180/mgm.2022.22
Published online by Cambridge University Press: 22 March 2022


https://www.cambridge.org/core/journals/mineralogical-magazine/article/linking-derived-debitage-to-the-stonehenge-altar-stone-using-portable-xray-fluorescence-analysis/23E9A00C0C2B9FAC0BC74AC11AAE5B2D

Abstract

The Altar Stone at Stonehenge in Wiltshire, UK, is enigmatic in that it differs markedly from the other bluestones. It is a grey–green, micaceous sandstone and has been considered to be derived from the Old Red Sandstone sequences of South Wales. Previous studies, however, have been based on presumed derived fragments (debitage) that have been identified visually as coming from the Altar Stone. Portable X-ray fluorescence (pXRF) analyses were conducted on these fragments (ex situ) as well as on the Altar Stone (in situ). Light elements (Z<37) in the Altar Stone analyses, performed after a night of heavy rain, were affected by surface and pore water that attenuate low energy X-rays, however the dry analyses of debitage fragments produced data for a full suite of elements. High Z elements, including Zr, Nb, Sr, Pb, Th and U, all occupy the same compositional space in the Altar Stone and debitage fragments, and are statistically indistinguishable, indicating the fragments are derived from the Altar Stone.  Barium compares very closely between the debitage and Altar Stone, with differences being related to variable baryte distribution in the Altar Stone, limited accessibility of its surface for analysis, and probably to surface weathering.

A notable feature of the Altar Stone sandstone is the presence of baryte (up to 0.8 modal%), manifest as relatively high Ba in both the debitage and the Altar Stone. These high Ba contents are in marked contrast with those in a small set of Old Red Sandstone field samples, analysed alongside the Altar Stone and debitage fragments, raising the possibility that the Altar Stone may not have been sourced from the Old Red Sandstone sequences of Wales. This high Ba ‘fingerprint’, related to the presence of baryte, may provide a rapid test using pXRF in the search for the source of the Stonehenge Altar Stone.

Conclusions

We tested the potential link between debitage fragments from excavations at Stonehenge identified visually as being derived from the Stonehenge Altar Stone by pXRF to ascertain if the vis-ual link was valid. The Altar Stone was analysed in situ on a wet morning following a night of rain, contrasting with the ex situ, dry setting for the debitage fragments. Accordingly, the surface of the Altar Stone at the time of analysis was wet. This caused attenu- ation of the signal for light elements (below Z=37). However, strong correlations for elements above Z = 37 leads us to conclude that the fragments were indeed derived from the Altar Stone. This shows that even though there are widely publicised concerns over the pXRF technique, in this case it did provide a credible geochemical comparison between the derived fragments and source monolith and highlights the value of the technique in this particular archaeological context. In addition, being a non-destructive technique, it does not compromise the integrity of the ancient monument.

Having matched the derived fragments to the Altar Stone we are now in a position to interpret with confidence the data obtained to date on the derived fragments in the search for the source of the Altar Stone. The pXRF results showed the Altar Stone to have high Ba contents which is in agreement with the high modal % baryte identified previously using automated SEM-EDS analysis. This rather unusual mineralogy will be a key element in our search for the origin of the Altar Stone.

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We have known about this research for some time -- and indeed it has been obvious for years that pXRF analyses had to be done on the Altar Stone exposed surfaces.  So it's good to see the research published at last.  Thankfully, this is a straight geology paper unsullied by any archaeological speculation about stone transport mechanisms or haulage routes.  

I need to read the paper more carefully, but for me the stand-out features are as follows:

1.  It now seems probable that the fragments at Stonehenge assumed to have come from the Altar Stone did indeed come from it -- but as far as the specialists are concerned, I suspect that the jury is still out.  The researchers had a problem in that their sampling work on the exposed surfaces of the Altar Stone was done in extremely wet conditions, whereas the fragments were studied dry.  To the untrained eye (such as mine) there seem to be substantial differences between the mineralogy of the fragments and the stone surfaces (Figure 3) -- and I was not all that convinced by the convoluted arguments in the text which put the differences down largely down to wet / dry sampling errors.

2.  Having promoted very heavily the idea that the Altar Stone came from the Senni Beds somewhere in mid Wales or the Welsh Borders, Bevins, Ixer and their colleagues now seem to have abandoned that idea altogether. That's quite a lesson for them and for everybody else as well -- the provenencing of erratic boulders (or Stonehenge bluestone monoliths) is only as good as your sampling density allows it to be.  As I have agued over and again, there is no way that Bevins and Ixer have provenanced spotted dolerite monoliths to Carn Goedog or foiliated rhyolite fragments to Rhosyfelin, because they still do not know where all the outcrops are.

3.   All this throws the provenencing of the Altar Stone up into the air again, with the intriguing possibility that it might actually be rather local -- having come, quite possibly, from no further away than Frome............