Have to say that, from the pic, it doesn't looked chalked to me (oh, an EXPERT now, are we?)
But... It looks like it's not a first: http://www.themodernantiquarian.com/post/22540
I know it's a different location, but it's not an isolated problem...
"Dear English Heritage/Historic Scotland
Please could you put signs, or add to your literature, something to tell people not to do this?
It'll be dead cheap and really easy.
Thank you"
G x