Visited 6.6.12
This stone is right next to the road and acts as a fence post.
There are many stones like this all over Orkney acting as fence posts.
I am no expert but I don’t know why this one has been listed as prehistoric when clearly most/all of the others are ‘modern’.
Although I am sure Wideord knows more about these things than me (which wouldn’t be hard!)
this was the nearest category I could find for the humungous water-worn? stone lying across the base, anywhere else it would be the Giant's Linen Smoother or somesuch