Orcadian Basin Rock at Stonehenge!

I expect you all have heard that the Altar Stone at Stonehenge has been proved to be from the Old Red Sandstone (ORS) of North East Scotland. The original paper is in Nature and can be read HERE. 

The Altar Stone, seen here underneath two bigger Sarsen stones, which new research suggests is actually Scottish not Welsh © Professor Nick Pearce/PA Wire

Nature has also provided a video which is well worth viewing.

The origen has been established using extremely sophisticated analysis techniques on individual crystals in a couple of thin sections. These proved that the zircon, rutile and apatite crystals present (and there were not many of them!) could only come from the Orcadian Basin. The Nature paper goes into excruciating detail on this, but it is summed up in the following figure.

How the stone got to Stonehenge will, no doubt, lead to furious debate. Tha Nature article posits that it was "anthropogenically transported"! What I find more interesting is why the builders of Stonehenge thought that a stone from so far away was necessary for the site.