I read that very recently - certainly within the last couple of months - in an article which had an image but I can't remember where - it must have been an internet or newspaper article rather than a book. i think the piece was about rock art becasue it mentioned the stonehenge stone in passing
actually thinking about it, it may have been in the information in the Pre-History gallery at the British Museum which we recently visited because I don't normally read things about stonehenge. I will look next time I'm there