Some of the ideas discussed in this blog are published in my new book called "The Stonehenge Bluestones" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click

Thursday 5 December 2019

Stonehenge - more on the Devonian (?) sandstones

The petrographic differences between the four studies samples.  Note that the old "type section 277" has just been examined visually, and that the other three -- including the new type section -- have been subjected to QEMSCAN analysis.  That may have resulted in analytical error -- but the authors are confident that the differences are significant and that they will withstand scrutiny.  

I have been looking again at the 2019 paper by Ixer et al on the carbonate-cemented micaceous sandstone samples collected over the years at Stonehenge:

Alternative Altar Stones? Carbonate-cemented micaceous sandstones from the Stonehenge Landscape
by Rob Ixer, Richard Bevins, Peter Turner, Matthew Power and Duncan Pirrie
Wiltshire Archaeological & Natural History Magazine, vol. 112 (2019), pp. 1–13

The six-tonne recumbent Altar Stone is perhaps the most enigmatic of all the Stonehenge bluestones, differing markedly from the others in size, tonnage, lithology and origin. It has therefore had more than its fair share of speculation on all of these aspects and many questions remain: was it always recumbent, was it a singleton or half a twin, where did it come from? Clearly it is not from the Preseli Hills hence the debate as to its geographical origins for over a century. However, any provenancing of the Altar Stone must rely on a detailed and accurate lithological and petrographical description. New descriptions of material labelled ‘Altar Stone’ held in museum collections and a re-evaluation of suggested Altar Stone debitage using automated scanning electron microscopy and linked energy dispersive analysis using QEMSCAN technology suggests that modiļ¬cation of the published petrographical descriptions is needed. A new ‘typical Altar Stone’ description is provided including the presence of early cementing barite and a better characterisation of the clay content. These new data should continue to narrow the search for the geographical origin of the Altar Stone, one that is expected to be at the eastern end of the Senni Formation outcrop, an outcrop that reaches as far east as Abergavenny in the Welsh Marches.

The real significance of this paper is that having studies assorted thin sections (only 4 were suitable for comparative analysis) the authors now feel that the famous thin section 277 may NOT have come from the Altar Stone at all, and that thin section SH18-196 should now be taken as the "type section" for it.  The reason is that the petrography of sample 277 is out on a limb, with various striking differences with samples FN593, HM13-3y/1 and SH18-196.  Those three have close similarities -- close enough in the view of the authors to have come from the same stone.  

What this means is that the carbonate-cemented micaceous sandstones have come from at least two different locations -- probably within the Devonian outcrops of south and west Wales.

There are a further 8 samples of similar sandstones which have not been analysed in this study, and no matter what the geologists may say, there is always a possibility that at least some of these have come from additional sites.  

Note that these are all different from the Lower Palaeozoic sandstones which have just been studied for this paper:

‘No provenance is better than wrong provenance’: Milford Haven and the Stonehenge sandstones,  by Rob A. Ixer, Richard E. Bevins, Duncan Pirrie, Peter Turner and Matthew Power.
Wiltshire Archaeological and Natural History Magazine 113, 20220, pp 1-15.

See also:

No comments: