ARCHAEOLOGICAL GEOLOGY OF THE SHEEP ROCK SPRING SITE, LATE PLEISTOCENE TO HOLOCENE, MISSOURI RIVER HEADWATERS REGION, SOUTHWEST MONTANA

The Sheep Rock Spring site (24JF292) is located on the south flank of Bull Mountain NE of Whitehall, MT, in the Missouri River Headwaters region, in a small valley between the resistant mass of Sheep Rock and a smaller bedrock tor (“Camel’s Hump”) to the west. Excavations revealed a four-fold sequence of late Quaternary deposits: (1) a basal boulder diamict marking a rock avalanche of angular clasts from the tor; (2) diamicts and stratified sediments from down-valley debris flows; (3) late Pleistocene to early Holocene channel and overbank alluvium with a paleosol ~10.0 ¹⁴C ka; and (4) a Holocene cross-valley alluvial/colluvial fan, reflecting intermittent deposition and lowered effective vegetative cover. The sequence reflects local events superimposed upon a regional signal of early Holocene warming and drying, a time-transgressive trend as demonstrated by Albanese and Frison in 1995 for Wyoming and Montana, and by Wilson in 1983 for the Canadian plains and foothills. Twenty-seven radiocarbon measurements on bone collagen, charcoal, wood, and organic sediment from alluvium and colluvium range back to >10.2 ¹⁴C ka and cultural associations reach in excess of 9.0 ¹⁴C ka. Bones of extinct megafauna are in interstitial sediments among the landslide boulders but a date of ~20.7 ¹⁴C ka on residual bone organics is problematic because of an unusual ¹³C/¹²C ratio. This Late Pleistocene fauna includes large mountain sheep, camel, horse, and cheetah. Regional evidence suggests ice-free conditions after ~14.5 ¹⁴C ka. Archaeological evidence suggests intermittent, very short-term camping by humans, with associated tool-production activities. Given the depositional setting, some artifacts probably have been fluvially or colluvially redeposited, a concern also for some of the radiocarbon samples. Projectile points in dated contexts add to growing evidence for introduction and use of relatively small corner, side, and corner/side-notched types from terminal Paleoindian times forward to the late Holocene.