When Was Rock Art Made?
Petroglyphs are made by removing the outer dark surface of rocks, called rock varnish or patina, to reveal the lighter rock underneath. Stone is not directly datable in the time frame of human history. But patina takes a long time to form. It is created by bacteria living on the rock and attaching clay particles to the rock. In the process they incorporate the element manganese, which gives a dark color. As soon as patina is removed, bacteria start to create patina again on the new surface. Roughly, we can say that darker petroglyphs are older, because they have been exposed longer to formation of patina. But this works only roughly, because surfaces exposed in different directions and to different weather will form patina at different rates.
Pictographs are made by painting on rock surfaces. Though they most often use mineral pigments, they may use an organic “binder” to attach the pigment to the rock surface so it will stay there for a long time. If either pigments or binder contain carbon, they may possibly give a basis for radiocarbon dating. But this also has limits, because of organic contamination, and because we do not wish to destroy painted rock art to date it.
There are other chemical processes affecting rock surfaces which are being explored and which hold out promise for the future.
Rock Art As Part of Archaeology
Sometimes the production of images on a rock leaves a record in the ground just below that rock. Stones used for pecking petroglyphs, brushes used for applying paint, mortars used to grind pigments and small fireplaces used to change minerals into pigments, these and other objects can link rock art with more typical archaeological materials found in the ground. In that same ground there may be charcoal from a hearth or other organic materials permitting radiocarbon dating. And there are other ways of treating rock art as simply one more kind of evidence in archaeology.
When none of these kinds of “abolute” dating is available, relative dating may still be possible. The relative darkness of petroglyphs on a single rock surface may give their relative dates. What is called “seriation” of the frequencies of design motifs may show that one motif A was very early, increased gradually, then motif B began to occur more frequently as motif A was fading from use. Which motifs tend to occur often together, and which never do?
Who Made Rock Art?
Some rock art can be identified with particular “archaeological” cultures, that is, cultures whose typical signatures can be recognized by archaeologists, even if they do not know whether the people who carried that culture have any reasonably direct descendants today. Sometimes this is because of where the rock art occurs, as at habitation sites of a particular culture. Sometimes it is because objects are pictured in the rock art which have been discovered in archaeological excavations, or which are known from ethnography (the study of living cultures, often early records of contact between Europeans and peoples living in some area)
In certain very fortunate situations, we know who the people were who created rock art, we even know their names as individuals. Or we may know the symbols they conventionally used, because they also used these symbols and pictorial conventions when writing their biographies in ledger books — for example Indian scouts working in the US Army in the 1800’s. Such a ledger book may be signed. Or we may not know the individual who made particular rock art, but we may know that the individual belonged to a cultural group whose rock art consistently used those symbols in particular ways, similar to that in the ledger books.
What Did Rock Art Mean to Its Creators and Ancient Audiences?
So-called “biographic” rock art of the Northern Plains is probably the rock art where we can be most certain of parts of its meaning, because of the very close relation between the ledger art just referred to and rock art using the same symbols. Both ledger books and the related rock art often recorded biographies of individuals, their accomplishments, victories in battle, and other events. Or they may record partial histories of entire villages or peoples. Such records are partly like the buffalo skins known as “Winter Counts” which could continue for 80 years, one important event each year. So there can be something close to history here. Quite a large vocabulary of signs can now be interpreted in this style of rock art, not because of any single “Rosetta Stone”, but because of the combination of details preserved in many places.
For other kinds of rock art, we may not be so lucky. We may have to use a very wide range of techniques of analysis, to gain partial clues from each, until many clues point in the same direction. Which symbols occur together often, which rarely, which never do? Which occur in particular kinds of locations (residential vs. ceremonial)? What is the geographic distribution of particular rock art traditions, and did it change through time? Do the symbols used in one tradition of rock art resemble those used in another tradition?
Designs on shields, both real shields and rock-art images of shields, are very likely to be symbolic, whether of powers which a warrior relied on for help in battle, or of other cultural ideas. Particular types of design may have been favored by particular cultures. Rock art, whether shields, large human-like figures, or even mere handprints, may have been used mark territory, homes, food storage, or other things. Small shields in less public places might have merely meant that the bearer(s) of that shield design were there, perhaps as part of a larger alliance. Clan symbols might be used in similar ways.
Other explanations which may be valid for some rock art include these: Girls’ puberty ceremonies; Vision quests; Prayers for rain; Hunting magic (hoping to ensure a good hunt); Pictorial representations of hunts showing where nets were placed, how game was driven into nets; astronomical indicators of the seasons; elements of rituals and ceremonies; echoes (voices from within the rocks); patterns often “seen” after consuming psychoactive plants; patterned phenomena of the natural world. The list goes on and on.
But the understanding or “interpretation” of rock art symbols, alone or in combination, remains very difficult. Simply because a symbol looks like something to us, it may not have looked at all like that for the people who created the rock art using it. Two symbols which we judge the “same” may have been very different symbols for some culture. Evidence will often be indirect, fragmentary, and even seemingly contradictory. To be on a sure footing in interpretation, we have to use every clue available from every branch of science which studies ancient and modern cultures. And even then, there are many things we will just never know. We need to be very modest when we think we do know, and keep gathering new kinds of information we had not earlier realized could be relevant. Even Plains Sign Language for example may hold some clues.
For additional information on petroglyph’s check out