At https://www.sciencedirect.com/science/article/pii/S2352409X23003905 … there is an article published in the Journal of Archaeological Science, that clims the Altar Stone at Stonehenge has a more exotic origin than it was previously thought. It is the largest of the so called blue stones, rocks that do not have an origin on Salisbury Plain. Most of them have been traced back to western Wales, and the latest theory is that a stone circle in the Preseli region was brought stone by stone from there and re-erected in Wiltshire, some considerable distance. Not all the blue stones come from Preseli but most have been traced to somewhere in Pembrokeshire or the Preseli region. As there is a large basin of sandstone in Wales, overlapping into what is now England, it has been assumed the altar stone came from an outcrop of that formation. It now seems it does not originate from there – and the search is now on from where it might have come from. Optical petrography, portable XRF and automated SEM-EDS, as well as Raman Spectroscopic techniques, have shown it could not have come from the same area as the other blue stone group. Its origin is now been sought in northern Britian – possibly as far north as the Orkneys. However, this latest discovery might further suggest a quite different origin to the mainstream archaeological mantra. The blue stones may have been erratics from the last Ice Age that were washed up on Salisbury Plain, or somewhere close to that location. Stones can be transported by the snout of glaciers – and also by meltwaters as the ice melted. Heavy run off perhaps. Or were other factors involved?
I can see why they will look for similar sandstones in the Orkneys as recent archaeology there has thrown up some interesting information on the ability of ancient people to erect megalithic structures. Was there some connection with locations in the south of Britain?
For further information see https://www.livescience.com/archaeology/100-year-old-origin-theory-of-stonehenges-iconic-altar-stone-could-be-wrong-scientists-say …. and
https://www.sarsen.org/2023/09/the-altar-stone-not-welsh-so-where-is.html … and www.stonesofstonehenge.org.uk … for an image of every stone at Stonehenge