Stonehenge is in Wiltshire. In reality, Stonehenge is thought to date from around 3300 BC. Geology seems to suggest that at least some of the stones were quarried in the Prescelly Mountains in southwest Wales and floated up the Bristol channel on rafts, so that Geoffrey's account of seaborne stones from the west could preserve a factual tradition. But clearly it is not possible to tie Stonehenge to the Arthurian period.
The stones weigh up to 50 tons each, the tallest of which rises 24 feet. The whole complex, including the banks and ditches, is about 450 feet in diameter. The most famous center stones are in the middle and the arrangement has a diameter of approximately 55 feet.
According to Geoffrey of Monmouth, the Saxon's had, through trickery, slaughtered the British at a meeting of nobles. King Ambrosius, brother of Uther Pendragon, wanted a memorial to those slain by the devious Saxons. Merlin suggested transporting the stones from the Giant's Ring at the top of Mount Killaraus in Ireland back to Britain. Ambrosius though this would be a fitting memorial, and an army of Britons went to Ireland to get the stones. Many tried and failed, and only Merlin was able to transport them back to Britain, and make Stonehenge. The story then goes on to say that Stonehenge served as a grave for Ambrosius, as well as Constantine, Arthur's successor.
"Aurelius ordered Merlin to erect round the burial-place the stones which he had brought from Ireland. Merlin obeyed the King's orders and put the stones up in a circle round the sepulchre, in exactly the same way as they had been arranged on Mount Killaraus in Ireland, thus proving that his artistry was worth more than any brute strength. (Monmouth, History, 198)"
Southern England Arthurian Sites