"Pornographic" petroglyphs from western China

Slate.com article on Chinese petroglyphs
Don't be taken in by the lurid title on this article; it is a layman's introduction to some carvings found in western China. There is a link to an article by a UPenn professor, Viktor Mair, who has excavated and studied a site related to the petroglyphs, a site containing mummies. The latter article is scholarly and long, but worth reading. The Slate article, however, is the one with at least one picture of the petroglyphs.
The date of the petroglyphs is uncertain. Some people put them at 1000 BCE, while others put them earlier. Much earlier.
If I find a link to more pictures of the petroglyphs, I'll add it to this post.