Sorry, but it's abrasion that's the problem. If everyone rubbed chalk / soil/ peat / sheep shit / anything with particles onto a rock, then in time it will get eroded. Badly. Very badly, come to think of it.

TE's equation of rock art erosion: E=mc2
Where E is erosion (over a predescribed period of time)
m is vigourousness of rubbing, (i.e. mass imposed on the particulate as force F, where F=ma)
c (a constant) is the speed of light. Note well, that if the user exceeds this speed of rubbing, he will have completely eroded the rock carving before he has begun applying the abrasive material.

Yours cosmologically,

TE