This reasonably petite stone (3ft high according to the Somerset Historic Environment Record) stands on top of Whitstone Hill. It was a Hundred Stone - the meeting place of the High Sheriff's court every October, when wine would be poured over it at the meeting's start.
I'm kind of loathe to add this - is it really prehistoric? It's certainly in what must be a prominent spot on high ground. Why would you have a tradition of pouring wine over a stone if it was only a boundary marker to you? The hill and Hundred are named after it (white stone) - would you name your Hundred after a stone you put there yourself??