The human mark on nature is perhaps as old as Homo sapiens. The mark so often is indelible, as manifested in species extinctions and global climate change. In its benign form, the mark of man is mere artwork and graffiti, slowly fading from rocky pages. For millennia, numerous nomadic and pueblo peoples have occupied the American southwest and left as their trace a myriad of petroglyphs and pictographs. Petroglyphs are made by scratching, pecking, or engraving, often most distinctly on the desert varnish of rock walls. Pictographs are formed by application of mud-based paints, using spattering or finger painting.