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 Rob A. Ixer. Dept. of Geology, University of Leicester, University Road, Leicester, LE1 7RH Email r.ixer@btinternet.com Peter Turner. School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT The Altar Stone at Stonehenge has a disputed origin and sandstones from both the Devonian (Old Red Sandstone) Cosheston Group and the Senni Beds in South Wales have been proposed as possible parent lithologies. A thin section of the Altar Stone together with those of four fragments of non-sarsen sandstone excavated at Stonehenge and attributed to the Cosheston Group sandstones have been re-examined. These detailed petrographical descriptions are compared/contrasted with earlier published accounts and with new descriptions of typical lithologies from the Senni Beds of South Wales. The Altar Stone is petrographically very similar to fine- to medium-grained, calcareous sandstones found in the Senni Beds and this is consistent with the suggestion they are the best candidates for being the parent rock. The Altar Stone is, however, very different lithologically from the four fragments; comparing their petrography with that published confirms that that they are probably not Cosheston Group sediments and that their degree of deformation may indicate that they are older than the Devonian. As their origin remains unknown it is still not possible to provenance them. None of the five samples originated from the Old Red Sandstone rocks around Milford Haven. Introduction A lithologically unremarkable, grey-green, micaceous sandstone is perhaps the most famous Welsh lithic export in the world. Stone 80 (numbering after Atkinson, 1979), namely the fallen ‘Altar Stone’ from Stonehenge, at nearly 5m long, is the largest bluestone (following the convention that uses the term to denote all non-sarsen lithologies) at the site, weighs over six tonnes and is the only sedimentary rock amongst the otherwise igneous bluestones (Figure 1). The geographical, as opposed to mythical, origin of this sandstone (alongside all the other stones at Stonehenge) has been a matter of speculation since the mid 18th century and is succinctly reviewed in Thorpe et.al. (1991, 119-124). William Stukeley (1740) thought the Altar Stone to be a Derbyshire marble, Phillips in Maskelyne (1878, 151) noted it was ‘a grey sandstone composed of quartz sand, silvery mica and dark grains (possibly hornblende)’ and suggested it was Devonian (Old Red Sandstone) or Cambrian in age and Davies on the same page in Maskelyne noted the nearest similar looking sandstones cropped out at Frome in Somerset. H. H. Thomas (1923, 244-245) too, believed the Altar Stone to be Devonian in age and his is the most complete, published, macroscopic and microscopic description of the Altar Stone to date. He suggested that the Altar Stone compared well with certain green, micaceous and calcareous Old Red Sandstones in