Eyup WNH,

I can't for the life of me see why someone would need to chalk that spiral to see it.
It's not exactly as if it's faint is it?

You sure it's chalk?

I can't see anything resembling chalk in your photo. Looks just like natural lichen and moss to me.
Long Meg's rings have a dark smooth look which I assume is finger oil from people tracing the patterns.