Petroglyphs, one of the most familiar forms of rock art, are created by scratching the dark surface of a rock so that the lighter color of stone beneath shows through. Humans carve symbols into the rock to communicate and illustrate. Sometimes percussion has been used as the method of abrasion.
The result of humans painting symbols on rocks is called pictographs. Minerals naturally occurring in the landscape can be ground and combined with organic materials (water, egg or oil, for instance) to make cohesive paint. Colors seen frequently in pictographs include red, yellow, black, white, blue and green.
Symbols and images produced by one or more round, cup-shaped depressions ground into rock are known as cupules. They are found all over the world and exist in almost every time period of human history. Cupules can occur on horizontal or vertical rock surfaces and are often the size of a quarter.
Creations of symbols or designs on the earth’s surface are called geoglyphs. Geoglyphs are made by intaglios or rock alignments. Intaglios involve removing dark pebbles to expose the light-colored soil underneath. Rock alignments use rocks or earth mounds lined up in meaningful ways to form a design. Geoglyphs are fairly large in scale — generally more than four yards.