Thanks 4W - I thought at the time I took that picture that it might be the remains of a kerb but wasn't confident enough to say so. The barow just looks too man-made to be natural. One thing which did occur to me was that it might have been carved out of a morane mound, and shaped to fit the purpose befor the stones were placed. I don't suppose we'll ever know, but it is a spectacular wee place.