How are petroglyphs made?

Petroglyphs from The Narrows.

Because different kinds of rocks have different hardnesses, petroglyphs can be made by using a harder rock to peck away on an exposed softer rock surface. Other examples were engraved by "cutting" or scratching into the rock, again with the use of a harder implement. At The Narrows, a rock art site in northwest Arkansas, Jerry Hilliard of the Arkansas Archeological Survey unearthed fragments of fire-hardened sandstone that were used to carve some of the petroglyphs found there.

Some surfaces on which rock art appears also show evidence of preliminary preparation. People used hammerstones to chip away protrusions. They used heavier stone hammers, perhaps combined with sand for "grit," as abraders to provide a smoother working
surface.

4. How are pictographs made?