AMS Radiocarbon and Cation-Ratio Dating of Rock Art in the Bighorn Basin of Wyoming and Montana

Abstract

Samples of organic matter and rock varnish from seven rock-art sites in the Bighorn Basin of Wyoming and Montana were collected for dating purposes. Petroglyphs sampled include Dinwoody-style figures, shield-bearing warriors, and other well-known Plains rock-art motifs. Accelerator mass spectrometry (AMS) dating of 10 petroglyphs yielded dates from the Early Archaic to the Protohistoric periods. A strong numerical relation between varnish leaching and time was found for petroglyphs older than 1,000 years, permitting the derivation of a cation-leaching curve (CLC) and calibrated cation-ratio (CR) ages for 15 different petroglyphs. No clear numerical relation between varnish leaching and time was found for petroglyphs less than 1,000 years old, possibly due to historical damage or past environmental conditions. As a result, calibrated CR ages could not be derived for six petroglyphs, and they are considered to be only younger than 1,000 years. Although further research is needed to establish whether one CLC can be used for all petroglyphs in the region, these studies constitute the first numerical chronology for rock art in the Bighorn area. Results indicate the occurrence of spatially discrete, but temporally concurrent styles in the Bighorn Basin during the last 800-900 years.