Dated Portable Art
Many of the ancient Paleolithic paintings and carvings in the famous caves of France and Spain can be fairly securely dated because they closely resemble engraved designs on animal bone that have been found in dated archaeological deposits. In these cases, the portable art and the rock art are sufficiently similar in style and technique to infer that they are contemporaneous. Unfortunately, on the Columbia Plateau portable art is not often found in well-dated contexts, but a few examples are known. Probably the most definitive are near The Dalles, where a number of Tsagiglalal carvings have been found in cremation burials that date between A.D. 1700 and A.D. 1800, and almost identical Tsagiglalal petroglyphs occur on nearby cliffs. Certainly, given the stylistic complexity of this figure, both the portable and rock art examples must be of the same age.
Elsewhere at sites along the middle Columbia, archaeologists have recovered a few pieces of stone and bone sculptured in human and animal form. Some of the human forms show distinctly bared teeth; some humans and animals have ribs clearly shown. All examples with teeth or ribs date within the last twelve hundred years. In the same region are also occasional rock art portrayals of human and animal figures with teeth and ribs. McClure (1984) argues convincingly that these rock art depictions date to the same time period as the carved portable objects. Finally, at an occupation site in southern British Columbia, a small, cylindrical stone painted with red dots and lines was recovered from a context more than two thousand years old (Copp 1980). The designs would fit in any of the area’s pictographs, indicating that simple geometric paintings are of considerable age in the region.
Portrayal of Datable Objects
Rock art from the northern Great Plains region of Alberta, Canada, Montana, and South Dakota shows thousands of historic items, including horses, guns, wagons, European Americans, and buildings. Since the dates at which these objects were brought to the area are known, the rock art is dated to the same time period. Likewise, rock art on the north Pacific coast shows sailing ships that can be reliably dated to historic times.
On the Columbia Plateau, approximately thirty sites show examples of horses or mounted humans (Boreson 1976; McClure 1979a). The drawings include both pictographs and petroglyphs, and occur in all areas of the region. We can reliably date these depictions after about A.D. 1720, when horses were first introduced onto the Columbia Plateau by Indians who had obtained them from Spanish settlements in New Mexico (Haines 1938). A few sites also contain drawings of a gun, a European American, and brands that date to the historic period.
In addition to historic items shown in rock art, depicted atlatls (throwing sticks used to propel stone-tipped darts) and bows and arrows are reliable time markers. The bow and arrow was first introduced onto the Columbia Plateau between two and three thousand years ago; before that, hunting weaponry was the spear or atlatl and dart. Atlatls are shown occasionally in rock art throughout the western United States. On the Columbia Plateau, probable atlatl depictions occur at petroglyph sites near The Dalles and on the Snake River south of Lewiston, Idaho. These panels were probably carved before the Indians acquired the bow and arrow. On the other hand, bows and arrows are relatively common, both as petroglyphs and pictographs, throughout the Columbia Plateau, occurring at sites in British Columbia, Washington, Oregon, and Idaho. These designs must date within the last two thousand years.
Superimpositions
Occasionally one design will be carved or painted over another; thus superimposed, the overlapping figure must have been made more recently than the original. A series of superimpositions, involving stylistically distinct examples, can be useful in developing a generalized chronology for a region’s rock art, since the relative ages of each style can be shown. In some areas of the world (e.g., Valcamonica, Italy; Dinwoody, Wyoming; Coso Range, California), frequent superimpositions characterize rock art, and relative chronologies have been developed from detailed analysis of many instances of superimpositioning.
In Columbia Plateau rock art superimpositions are rare, although they occur occasionally in some parts of the region, most frequently along the middle and lower Columbia River. However, despite some tantalizing clues, we have not yet recognized a clear pattern of any specific design or style overlapping another. Instead, it appears that in this area a few superimpositions were done during approximately the past thousand years in a seemingly random pattern. Possibly further study will yield information that is of more value for turning these clues into a relative chronology.
Patination
Many petroglyphs along the middle Columbia River and the lower Snake River were made by pecking through the dark patina on the basalt bedrock characteristic of these areas. This patina, sometimes called desert varnish, is a brown to black stain that colors exposed rock surfaces. It occurs most often on stones in hot, arid portions of the world. Some scientists suggest that this patina forms due to chemical weathering and leaching of iron and manganese oxides from the stone, while others hypothesize that airborne microorganisms oxidize these minerals and concentrate them on the rock surface. In either case, the process is a slow one and desert varnish takes considerable time to develop.
When a petroglyph is pecked or carved through the patina on a rock surface, it exposes the lighter colored interior stone and creates a negative image, with the paler petroglyph showing on an otherwise dark background. If conditions for patina development still exist after the petroglyph is made, the newly exposed surfaces gradually begin to acquire the desert varnish. After a long period the design will be repatinated; it will have essentially the same patina as the unaltered rock face. Although repatination of petroglyph designs does not provide an absolute age (since exposure, temperature, humidity, and other factors influence the rate of patina formation), petroglyphs repatinating differently on the same surface are useful for creating a relative chronology. In such cases, those that appear fresher are younger than those that have repatinated to an appearance closer to the original surface of the stone.
Differential patination has been used on petroglyphs near The Dalles and at Buffalo Eddy on the Snake River south of Lewiston, Idaho, to suggest relative chronologies. At The Dalles, petroglyphs in the pit and groove style invariably are heavily patinated and weathered. At two sites, these contrast with much fresher appearing designs pecked at a later time. The heavy repatination of all of these pit and groove petroglyphs is consistent with that noted in the Great Basin area of Utah, Nevada, and California. There the appearance of this art style has been dated between five and seven thousand years ago. While we cannot automatically assign an equally ancient age to the pit and groove petroglyphs at The Dalles, certainly they are the oldest in this locale.
At Buffalo Eddy, Nesbitt described two rock art styles (1968). A naturalistic style shows primarily mountain sheep, deer, and humans wearing horned headdresses, while a “graphic” style is composed of triangles, circles, dots, and lines arranged in geometric patterns. At Buffalo Eddy, the naturalistic petroglyphs are usually repatinated, some very heavily. In contrast, the graphic designs are reported as fresher looking and cut through the patina on the rock surface. In one instance graphic style designs may actually be superimposed on repatinated naturalistic designs. In this case, the naturalistic drawings of men and mountain sheep clearly seem older than the graphic geometric designs.
Weathering
Variations in the weathering of different designs are often used in conjunction with patination studies, but such variations can also be applied to pictographs, which are not affected by patination. Many Columbia Plateau pictographs show weathering differences: some designs at a site will be very “fresh” looking while others will appear somewhat faded or will be partially covered by mineral deposits. Since different artists likely used paints of slightly different composition and color, differential weathering of pictographs is not as reliable an indicator of the passage of time as is differential patination of petroglyphs, but it does serve to indicate that paintings were made at different times.
The lack of evidence of extensive pictograph weathering during the last
