Aye, I recall similar on top of Rough Tor (http://www.themodernantiquarian.com/post/20499), though I didn't get to look at the top of the Logan stone. I wonder if the top of the Cheesewring has similar 'Drudical Basons'.

Borlase's sketches seem to involve quite a bit of artistic license, as do his interpretations ;)

I'm still quite taken by the idea that even if they are natural features, they my have held some significance in the minds of prehistoric folk. They remind me of things that get labelled as 'artificially enhanced natural features' when they turn up in association with bona fide rock art.