The Longstone of Minchinhampton

I spent yesterday evenings beautiful sunset by the stone, just made it in time weaving through hte maze of roads which all seem to bring you out at the same place. Note the stone of identical texture built into the wall. Could this be as antiquated? I know that someone once speculated that this was the entrance to a barrow.
Tis a pretty place indeed but returned with a dreadful headache. Not a stone for brittle heads!