Petroglyphs are subject to weathering, damage from wandering livestock, erosion from people who can't keep their hands to themselves, and alterations from misguided folks who try to make the pictures more visible by carving the lines a bit deeper. Some of these carved rocks have been taken away and are currently housed in museums around the world, but taking them out of their context at sacred sites also erodes their significance. Not sure what the right balance will be.