About a mile and a bit north east from the souterrain Cnoc A' Chaisteal, is a big stone, a large Clach, and seeing as it's on route to the ferry, I have to stop and look.
The stone is massive, it's not set into the ground but rests somewhat precariously on top of it, surely it didn't come to rest in that position, it just has to have been stood up.
Canmore says this...
Clach an t-Sagairt is a large stone block set on edge, facing SE, and measuring 8ft long(I think they mean tall), 11ft broad, and 4 1/2ft thick, with a Latin cross incised on it towards the sinister top corner of its face.
This cross is also known as Clach na h'Ulaidh and Crois Aona'ain.
It may have marked a sanctuary limit of St Columba's Chapel......
None of that negates the idea that it was up and had meaning to ancient man, so I've added it as a natural feature, like a mountain, cave, tree or mole hill.