Petroglyphs are rock carvings or engravings often associated with prehistoric people to represent simple scenes and symbols. This is one of the oldest forms of art on the planet, and its evidence can be found on rocks and boulders throughout the world.
The above photo is a sample of a petroglyph. It is a carving of an elk on a rock in Arizona. Typically, petroglyphs are simply made by scratching the surface of a rock with a stone or another hand held object. They often last through the ages.