<<Also, what about grafitti (ancient Egypt, Pompeii,
20th c. American urban, etc)? >>
If you have not already, check out the "fabulous ruins of Detroit" website I posted a couple of months ago; it has a part called "Lascoux Detroit" that compares graffiti on abandoned buildings in Detroit to cave art.
www.detroityes.com/home/htm
-Lydia de Toledo