In 2002, eight pigment samples were collected from three rock art sites in the Big Belt Mountains of west-central Montana. Samples from Hellgate Gulch (24BW9), Avalanche Mouth (24BW19), and the Gates of the Mountains (24LC27) were dated using plasma-chemical extraction and accelerator mass spectrometry. The dates were statistically indistinguishable with ages of 1170 ± 45, 1225 ± 50, and 1280 ± 50 B.P. When calibrated, these ages range from 650 to 990 cal A.D. This corresponds to the early Late Prehistoric period on the Northwestern Plains. An oxalate accretion sample overlying a painted area at another site, Big Log Gulch (24LC1707), provided a minimum age of 1440 ± 45 B.P. for the rock art present at this site. The dated images at the four sites fit within the Foothills Abstract and Eastern Columbia Plateau rock art traditions.
Chippindale and Taçon 1998). Based on a typical constellation of clues observed at more than two dozen sites across the northern Plains, this multiplicity of evidence includes relative weathering, superimposition, association with dated archaeological deposits, reuse of images, the portrayal of subject matter, seriation of different pigments, and direct radiocarbon dating (Greer 1995; Keyser and Klassen 2001, 163-165; Keyser et al. 2012, 201-203; Scott et al. 2005). From this research, rock art scholars propose that Foothill’s Abstract traditional imagery was first painted sometime in the middle-late Archaic period.