or, they are created using a combination of the two techniques. Parietal art is found very widely throughout the world, and in many places new examples are being discovered. The defining characteristic of rock art is that it is placed on natural rock surfaces; in this way, it is distinct from artworks placed on constructed walls or free-standing sculpture. As such, rock art is a form of landscape art, and includes designs that have been placed on boulder and cliff faces, cave walls, and ceilings, and on the ground surface. Rock art is a global phenomenon, being found in many different regions of the world. There are various forms of rock art. Some archaeologists also consider pits and grooves in the rock known as cupules, or cups or rings, as a form of rock art. Although there are exceptions, the majority of rock art whose creation was recorded by ethnographers had been produced during rituals. As such, the study of rock art is a component of the archaeology of religion. Rock art serves multiple purposes in the contemporary world. In several regions, it remains spiritually important to indigenous peoples, who view it as a significant component of their cultural heritage. It also serves as an important source of cultural tourism, and hence as economic revenue in certain parts of the world. As such, images taken from cave art have appeared on memorabilia and other artefacts sold as a part of the tourist industry. Types Paintings In most climates, only paintings in sheltered sites, in particular caves, have survived for any length of time. Therefore, these are usually called "cave paintings", although many do survive in "rock-shelters" or cliff-faces under an overhang. In prehistoric times, these were often popular places for various human purposes, providing some shelter from the weather, as well as light. There may have been many more paintings in more exposed sites, that are now lost. Pictographs are paintings or drawings that have been placed onto the rock face. Such artworks have typically been made with mineral earths and other natural compounds found across much of the world. The predominantly used colours are red, black and white. Red paint is usually attained through the use of ground ochre, while black paint is typically composed of charcoal, or sometimes from minerals such as manganese. White paint is usually created from natural chalk, kaolinite clay or diatomaceous earth. Once the