A query below had me trying to stand upright in a gale taking a photo of a wall. The query was about flint when I'd said pebbles. The above is not the same wall, which was unavailable, but one nearby. One assumes that this is a case of using local materials, in this case beach pebbles. There's not loads around the area, but there's enough in a few of the older back streets to make it a small local feature.