British Rock Art Blog [Click here]
Interesting especially if you are interested in man-modified rocks.

For example this in a comment:
"Hi George
Quartz hammered into fissures sounds very intriguing, will have to look that site up.
It would be interesting to know the thinking behind that. Another glimpse of thier ritual practices?
At the scandanavian sites the cracks were packed with small stones to “seal” the flints and bits of pottery etc inside. Apparently they also brought ‘ancestral flints’ to one outcrop, ie mesolithic flints that were already ancient when carvings were in use."

As I read this blog, I see the writer is pre-occupied with the same sort of questions about rock art as I am here about rock piles.