For thousands of years Indian people left evidence of their presence on the land with rock art: pictographs and petroglyphs. Pictographs are created by painting on rock surfaces with natural pigments while petroglyphs are pecked, carved, or abraded into the surface of the rock.
Pictographs are usually found under protective ledges or in caves where they have been protected from the weather. In producing pictographs, Indians used natural pigments such as iron oxides (hematite or limonite), white or yellow clays and soft rock, charcoal, and copper minerals. These natural pigments were mixed to produce a palette of yellow, white, red, green, black, and. In mixing the powdered mineral pigments into paint, organic binders were used. This included a combination of fluids such as plant juices, eggs, animal fat, saliva, blood, urine, and water. This pigment, when freshly applied, actually stains the rock surface. As natural weathering evaporates the water or organic binder with which the pigment was mixed, it actually seeps into microscopic pores of the rock by capillary action. Thus the pigment becomes part of the rock. The pigments were generally applied by finger painting.
Shown above is Pictograph Cave near Billings, Montana.
Petroglyphs are usually found on sandstone or basalt. These stones have a dark surface or patina which has formed slowly as a result of weathering and microbial/chemical alterations. By pecking or carving the stone, this patina is removed and the lighter-colored unweathered stone is exposed. This creates a visual contrast.
In the Pacific Northwest, pecking was the most common method of creating petroglyphs: the rock surface was repeatedly struck with a sharp piece of harder stone to produce a shallow pit that was then gradually enlarged to form the design. Hammer and chisel stones would be used to gain greater control.
Over time, petroglyphs will darken, or repatinate, as the weathering process continues on the rock surface. However it can take hundreds to thousands of years for the process to completely repatinate the petroglyphs, depending on microclimatic factors at specific sites.
One of the problems with regard to rock art is attempting to determine what these symbols mean. Symbols are an important part of culture and when they are taken out of the context of the culture in which they were created, it is difficult, if not impossible, to understand what they meant to the people who created them.
Some rock art sites appear to have been associated with the vision quest. Following a vision quest, the supplicants would paint a pictograph to commemorate the experience. Sites used to commemorate the vision quest experience are usually in relatively inaccessible, isolated areas like those chosen for vision quests. The symbolism at these sites often shows humans, animals, sun symbols, dots, crosses, and geometric abstracts.
Some rock art sites may have been associated with hunting magic and ceremonies. Among most of the North American tribes, it was felt that animals had spirits controlling their behavior. Therefore, certain rituals would be carried out prior to the hunt in which the animal spirits would be asked to allow that some animals be taken for the good of the human group.
It is not always possible to determine the age of rock art. However, archaeologists are able to assign rough time periods to many sites. In some instances this can be done stylistically: for example, in what is now the U.S., the portrayal of horses on a rock art panel means that it was made after the Indians acquired horses.