Finally found a photo of the cliffside wall facing this stack. Though the sitepage has gone from here anyone still interested can find this image here :-http://tech.ph.groups.yahoo.com/group/circlesettler/photos/view/89e2?b=3&m=f&o=0
This doesn't resemble 20thC work at places such as the Brough of Birsay and the Brough of Deerness in any way. If late Viking or earlier the question now is whether this feature is co-eval with the landbridge or post-dates its collapse.