The Stonehenge Sandstone Mystery (1) -- The Altar Stone
Since my previous posts about the Heddle Collection of Stonehenge slides at the Hunterian Museum in Glasgow, I can report that there is no skulduggery going on! Dr John Faithfull has been in touch, and he tells me that the "missing" slides to which I referred are not missing at all, but are in the possession of the Open University and / or the National Museum of Wales, where geologists are presumably examining them carefully as we speak.
He also referred me to this Open University site, where more information may be found:
Leaving the matter of the strange "Altar Stone" sample that appears to be made of igneous rather than sedimentary material, let's home in on the sandstone samples. We are lucky to have a keynote paper to guide us here -- "A detailed re-examination of the petrography of the Altar Stone and other non-sarsen sandstones from Stonehenge as a guide to their provenance," by R.A. Ixer and P. Turner, published in Wiltshire Archaeological and Natural History Magazine, 99 (2006), pp 1-9. The authors describe the Altar Stone as "a purplish-green micaceous sandstone" which appears to have originated on the ORS Senni Beds of South Wales. (They argue very convincingly that the place of origin was NOT the Cosheston Beds on the shores of Milford Haven, as assumed by archaeologists for many years.) Here is a part of their description of the rock examined (from a slide held in Salisbury Museum): " Microscopically the rock is a very fine- to fine-grained, well-sorted sandstone with a mean grain size of 0.13mm and a maximum grain size of approximately 0.32mm. The clastic grains are angular to subrounded and the modal group is subangular..............The rock has a homogeneous fabric but bedding is observed by thin, less than 0.5mm wide, opaque mineral-dominated, heavy mineral laminae.......There is very little detrital, interstitial material. Authigenic kaolinite is locally present but the main matrix is a pervasive carbonate cement........" There is much more detail in the paper.