The Valley of Fire State Park near Las Vegas, Nevada hosts a significant collection of petroglyphs. The rock art was created by a culture known as the Basket makers (2000-1450 years ago) and the Anasazi/Ancient Pueblo group (1500-850 years ago). The Anasazi lived throughout the Southwest and were responsible for creating the impressive cliff dwellings at Mesa Verde National Park. They mysteriously disappeared around 1300, a fact which still has archeologists searching for answers.
The arid climate of the Southwest combined with the fact that petroglyphs are carved in stone has allowed for their preservation. Only modern man with his graffiti poses a threat.
Please don't ever touch the petroglyphs, and be careful of the vegetation when you are viewing the rock art!