I'm assuming the stones at Coldrum are mostly/all sarcen?
'Cos this:http://www.themodernantiquarian.com/post/6070 reminds me of the hours of endless fun I've had staring at the natural patterning caused by fossilised mangrove roots in Wiltshire stones, then concocting half-baked thories about how the signifiance of natural holes in stone are related to the meaning of cup-marks.

Have vast amounts of fun Mr S :)

God knows if I'll be able to walk by then, I'm housebound with no milk, no bread and no bloody cigarettes!!

If not, I just spotted these in Castleton, one of my fave places anywhere http://www.themodernantiquarian.com/site/648 so the next time I'm Oop North I'll drag boy wonder out to find 'em :)