Proceedings of the National Academy of Sciences
National Academy of Sciences
Rock art compels interest from both researchers and a broader public, inspiring many hypotheses about its cultural origin and meaning, but it is notoriously difficult to date numerically. Barrier Canyon-style (BCS) pictographs of the Colorado Plateau are among the most debated examples; hypotheses about its age span the entire Holocene epoch and previous attempts at direct radiocarbon dating have failed. We provide multiple age constraints through the use of cross-cutting relations and new and broadly applicable approaches in optically stimulated luminescence dating at the Great Gallery panel, the type section of BCS art in Canyonlands National Park, southeastern Utah. Alluvial chronostratigraphy constrains the burial and exhumation of the alcove containing the panel, and limits are also set by our related research dating both a rockfall that removed some figures and the rock’s exposure duration before that time. Results provide a maximum possible age, a minimum age, and an exposure time window for the creation of the Great Gallery panel, respectively. The only prior hypothesis not disproven is a late Archaic origin for BCS rock art, although our age result of A.D. ∼1–1100 coincides better with the transition to and rise of the subsequent Fremont culture. This chronology is for the type locality only, and variability in the age of other sites is likely. Nevertheless, results suggest that BCS rock art represents an artistic tradition that spanned cultures and the transition from foraging to farming in the region. Archaeology is focused upon material records, contextualized in time. Rock art is a record with the potential to provide unique insight into the dynamics and evolution of culture, but it generally lacks stratigraphic or chronologic context. Interpretation of the origin and meaning of rock art is indirect at best, or simply speculative. In the case of some pictographs, pigments may include or have enough accessory carbon for accelerator mass spectrometry (AMS) radiocarbon dating (1⇓⇓–4). In other special situations, such as caves, minimum age constraints have been obtained by various techniques of dating material that overlies or entombs rock art (5⇓–7). However, most rock art remains undatable and researchers rely upon stylistic comparison and indirect associations with artifacts at nearby sites (8, 9). The case in point for this study is arguably the most compelling and debated rock art in the United States—the Barrier Canyon style (BCS) of the Colorado Plateau. Previous attempts to derive an absolute chronology have failed and its age remains unknown, with widely ranging hypotheses that have remained untested until now. The continued development of dating techniques offers new possibilities for hypothesis testing. The optically stimulated luminescence (OSL) signals from mineral grains make it possible to date the deposition of most sediment that is exposed to a few seconds of full sunlight before burial, and its use in the earth and cultural sciences has greatly increased (10, 11). Among the latest applications of OSL are techniques dating the outer surfaces of rock clasts that have become shielded from light, including those with archaeological context (12⇓⇓–15). Recent work has furthermore used the “bleaching” profile of decreasing luminescence signal toward the surface of rock to estimate exposure time to sunlight (16, 17). Using these dating tools, we can constrain the age of rock art and gain new insight into past cultures and landscapes. Here, we synthesize results from three approaches to dating the type section of BCS art, the Great Gallery in Canyonlands National Park of southeastern Utah. Through dating the full alluvial stratigraphy and a rockfall event that both have incontrovertible cross-cutting relations with the rock art, and then by determining the exposure duration of a painted rock surface, we greatly narrow the window of time when the rock art was created. These approaches do not require direct sampling of rock art and have strong potential for application to other archaeological and surface processes research. Although our results are only for the type section of BCS art, and chronological variability should be expected for the style across the region, they suggest that BCS art coincides with the transition to agriculture in the northern Colorado Plateau and may not have been limited to a specific archaeological culture.
Pederson, J. L., Chapot, M. S., Simms, S. R., Sohbati, R., Rittenour, T. M., Murray, A. S., Cox, G. 2014. Age of Barrier Canyon-style rock art constrained by cross-cutting relations and luminescence dating techniques. PNAS 111, 12986-12991. https://doi.org/10.1073/pnas.1405402111