Like Postie I was intrigued by Clach An T-Sagairt and wondered its purpose. It is set just to the north of Dun Rosail.
The 'sinister' cross is still there and that could mark the edge of various Saints influences or a meeting place.
However the stone has been standing there for thousands upon thousands of years, no reason why the prehistoric peoples didn't help it stay up.
After a good look round I had look at the ground on the west side. To me there appears to chocks helping keep the stone erect so possibly it isn't a natural setting. It certainly is an impressive stone that looks far into the west and the Atlantic.
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.
Among the various names that have been recorded for the boulder are 'Crois Aona'ain' and 'An'adhan', suggesting a traditional association with St Adomnan which would be appropriate in an area with dedications to Columba (iii). The name 'Clach an t-Sagairt' ('the priest's stone') is often associated with meeting-places for recusant worship (iv), but this seems unlikely on North Uist. Martin about 1700 described a stone 'which the natives call a cross', and in 1878 it was believed to be 'the site of a general meeting place of the Picts for worship'