Comment by dboreham

The rock formation identified is at the surface throughout the Orkney Islands. The River Avon seems to be at least close to navigable by a barge carrying a 5m long stone, requiring less than a mile of transport across land to Stonehenge. So it doesn't seem to require extremely implausible means to get it there. As to why -- perhaps the Stonehenge civilization came from Orkney originally, bringing their sacred stone with them? Or perhaps Orkney was a sort of Rivendell back then -- it's easier to defend as an isolated island group. You might go get a big stone from the monks there to establish the authenticity of your satellite colony.