A petroglyph is a mark made into a stone surface by humans to represent some object. This is contrasted to what is often referred to as ‘rock painting’, which is a design or image painted or drawn on to the surface of the rock. Those are called pictographs. Petroglyphs images are pecked, scratched or ground into the surface of the rock. In some areas the authors used a hammerstone and a pebble as a chisel. 

In Southwestern deserts, petroglyphs are found on canyon walls, rocks, on cliff sides where time and weather and the unique chemistry of the rock, adds a color to the rock surface. This coating usually consists of iron and manganese dioxides mixed with other things such as the by products lichens. This is called a 'desert varnish' or patina. With a patina or varnish the rock surfaces often appear shiny or wet.

Sometimes images are also cut into a surface that is not discolored. 

Some images on are on high, flat canyon walls or steep rock faces sometimes as far from the ground as a dozens of meters.