Water walk

Went for a short walk at a rather chilly and dank Cotswold Water Park. Apart from the moss, this stone caught my eye because it doesn't seem to be an integral part of the dry-stone wall and the upper portion appears to be circular. Unfortunately I didn't have time to stop and examine it closely but I wonder whether it once served some purpose other than being just the bookend of a wall. It does appear to have some kind of inscribed markings.