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 :)