AMS Dates from Four Late Prehistoric Period Rock Art Sites in West Central Montana

Article excerpt

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.

Keywords: rock art, plasma-chemical extraction, radiocarbon dating, Foothills Abstract tradition, Eastern Columbia Plateau tradition

Since 1997, the Helena National Forest has maintained an active rock art research and conservation program in the Big Belt Mountains of west central Montana (Greer and Greer 1997, 2001; Loubser 2001a, 2002,2004; Scott and Davis 2004; Scott et al. 2000). Natural deterioration, vandalism and increased site visitation necessitated the development of conservation plans and accurate baseline data for monitoring and law enforcement purposes. Two of the largest Big Belts rock art sites in Hellgate Gulch (24BW9) and the Gates of the Mountains (24LC27) are now recorded in detail, and graffiti has been removed from the Hellgate Gulch site to discourage future vandalism (Loubser 2001a; Scott et al. 2000). A large forest fire in the north Big Belts in 2000 precipitated post-fire erosion control at the Hellgate Gulch pictographs (Davis 2001; Scott et al. 2000), rock art survey in the burn area (Greer and Greer 2001), and site condition assessments (Loubser 2001a). The accelerator mass spectrometry (AMS) dating project with Texas A&M University and Lawrence Livermore National Laboratory described in this paper is a continuation of this rock art investigation program.

Only a few attempts have been made to date central Montana rock art using AMS radiocarbon dating (Greer 1995: 48-50; Loendorf 1992). Paint pigment seriation has therefore been used to establish a relative chronology for central Montana rock art, which spans from 3000 B.C. to A.D. 1000 (Greer 1995). The notable absence of Ceremonial tradition and Biographic tradition rock art images also indicates that many central Montana rock art sites pre-date A.D. 1000. Although these are useful temporal reference points, radiocarbon dates are necessary to build an accurate rock art chronology.

Rock art condition assessments completed by Loubser (2001a, 2002) identified images at four sites that were in deteriorated condition and flaking from the rock face. In 2002, small flakes of paint pigment exfoliating from the panels at these select sites were carefully collected for AMS radiocarbon dating. An oxalate accretion sample overlying a red pigment smear was also collected from the Big Log Gulch Pictograph site (24LC1707). In an effort to be sensitive to potential tribal and rock art conservation concerns, samples were not removed from intact (non-exfoliating) images, although such sampling may have provided finer temporal and stylistic resolution to this study.


The Big Belt Mountains lie just east of the Continental Divide on what is roughly the boundary between the Eastern Columbia Plateau and the Northwestern Plains. The majority of pictograph sites in the Big Belts (and surrounding mountain ranges) are characteristic of the Foothills Abstract rock art tradition (Keyser and Klassen 2001:151-175). This tradition, which may have originated in the Middle Plains Archaic period, is characterized by extensive red pigment washes with superimposed images composed of elongated (stick-like) human figures, zoomorphs, handprints, smears, fmgerlines, maze and mask-like figures, and geometric designs. …