At a general level, any artificially created mark that is cut, engraved, incised, etched, gouged, ground or pecked into, or applied with paint, wax, or other substance (organic or mineral) onto, a rock surface. Within this broad field, the term petroglyph is applied to marks made by carving, incising, engraving, pecking, or grinding the rock surface, while the term pictograph refers to marks made by painting organic and mineral pigments onto the surface.
The individual marks in rock art are often referred to as motifs; groups of motifs in close juxtaposition as panels; and places where one or more panels have been identified as sites. It is recognized, however, that in making these identifications and using such terminology a structure is being imposed that may not have been known or relevant to the people who made the rock art in the first place. Indeed, use of the very term ‘art’ carries with it a series of assumptions from modern western culture about the nature of the various marks that can be observed.
Rock art is extremely widely spread around the world and appears to have been made since Lower Palaeolithic times for a variety of purposes, such as during religious rites, depicting historical or mythological events and narratives, as decoration, or to mark territories and routeways. Dating rock art is often extremely difficult because of its general open and exposed positions and lack of associations. Interpreting the motifs used is also difficult. The most widespread motif is the simple cup or hollow, either singly or in groups. Some rock art contains motifs that are symbols, shapes and lines; in other cases people, animals, objects, events, and structures are depicted.
Many different kinds of rock surface have been used in the creation of rock art, but a number of key situations are widely recognized: parietal panels are those on the walls of a natural structure such as a cave or rock‐shelter; open‐air panels are those on natural earthfast boulders and rock outcrops that lie unprotected in essentially open countryside; monument‐based panels are those found on the faces of stones incorporated into the fabric or structure of deliberately constructed monuments—some of these pieces may have been open‐air rock art before being lifted and used in monument building; and mobiliary rock art is where panels occur on the surface of stones that have been relocated from their source and may have been moved several times in the past—essentially portable pieces of rock art.
http://www.rockart.org The Rock Art Foundation, for promoting the conservation and study of Native American rock art in the lower Pecos region of southwestern Texas.
http://www.cupstones.f9.co.uk/links.htm Index of rock‐art websites across the world.