Just south of the pond there is a little site that is probably not very old. There are rock piles, a few short stretches of stone wall, lots of stone under foot, and a small quarry. This is one of the things I found at this site. First, a photo of the odd stone, with debris on top:
Second, with debris cleaned away and a half meter stick to show the size:Third and fourth, the whole picture:
It seems this could be entirely accidental. The split block appears to be granite. A geologist friend of mine said it could have been used for a headstone and this piece was discarded. Certainly, the tree seems to have pushed it up a bit, and those small stones under the boulder look like they might have just flaked off the lower edge of the boulder. So, what looks like ceremony, maybe in this case, is not.