Don't get me wrong. I just love the idea that the Altar Stone at Stonehenge has come from the ORS Senni Beds -- but since this is a scientific blog, every now and then we need to ask some inconvenient questions.
The current orthodoxy is that the Altar Stone is from the Senni Beds -- and not from the Cosheston Beds on the shore of Milford Haven. The foundation of all of that is the paper by Ixer and Turner-- much cited -- dating from 2006. That's quite a while ago, in geological terms.
Reference: Ixer, R.A. and Turner, P. 2006. A detailed re-examination of the petrography of the Altar Stone and other non-sarsen sandstones form Stonehenge as a guide to their provenance. Wiltshire Archaeological & Natural History Magazine 99, 1–9.
I have been looking at the paper again, and am forcefully reminded that there is actually a huge amount of doubt surrounding this identification. The centrepiece of the work by Ixer and Turner was a thin section from the Salisbury Museum collection, labelled "277 Altar Stone Stonehenge." Who took the sample, and when? Is sample 277 one of the Cunnington samples? Quote: "Cunnington (1884) identified five fragments of the Altar Stone, that he assumed resulted from dressing of the stone, amongst his loose finds, although they are now missing."
Quote: "Therefore, the thin section labelled ‘277 Altar Stone Stonehenge’ in the Salisbury Museum Collection is likely to remain, for the foreseeable future, the only piece of the monolith available for investigation. It is imperative then that it should be described as fully as possible and that this description becomes widely available."
In Discussion (Quote):
This, paper represents the first detailed description of the Altar Stone for over eighty years and is in broad agreement with H.H. Thomas other than his identification of abundant garnet and glauconite. Glauconite is a green, chlorite-like mineral and so, if present, has been subsumed under chlorite in the present description. The disparity over the amount of garnet is more significant and puzzling. Thomas noted significant amounts of garnet in his ‘heavy residues’ (Thomas, 1923, 244) but did not report garnet in his thin section description of the Altar Stone. Although trace amounts of garnet can be overlooked/underestimated in thin section the present study could not confirm significant amounts of garnet microscopically. The presence and amount of garnet is important as Thomas was struck by the coincidence between the garnet-rich nature of his Altar Stone ‘heavy residues’ and the unusually garnetiferous nature of the Cosheston Beds and it was the presence of these unusual amounts of garnet in both, that led him to suggest the Cosheston Group might have been the origin of the Altar Stone. Without further sampling (this would require many grammes of Altar Stone to crush before separating the heavy minerals) the garnet problem must remain unresolved.