The late Pleistocene–early Holocene archaeological record of the interior Pacific Northwest is dominated by what has been regionally referred to as the Western Stemmed Tradition (WST). While various efforts have attempted to clarify the chronology of this tradition, these have largely focused on data from the Great Basin and have been disproportionately preoccupied with establishing the beginning of the tradition due to its temporal overlap with Clovis materials. Specifically focusing on the Columbia Plateau, we apply a series of Bayesian chronological models to create concise estimates of the most likely beginning, end, and span of the WST. We then further explore its chronology by modeling its temporal span under various parameters and criteria so as to better identify places in the chronology that need further work and those that are robust regardless of data iteration. Our analysis revealed four major findings: (1) WST conservatively dates between 13,000 and 11,000 cal BP, likely extending to ~13,500 cal BP; (2) the most problematic period for WST is its termination; (3) the WST is incredibly long-lived compared to roughly contemporary Paleoindian traditions; and (4) the WST was seemingly unaffected by the onset of the Younger Dryas.

The Marmes Rockshelter archaeological site in southeastern Washington state contains a > 11 kyr stratigraphic record that was excavated in the 1960s but only recently analyzed in detail. We present the results of physical, chemical, and isotopic analyses of archived Marmes sediments from rockshelter, hillslope, and floodplain locations. Multiple lines of evidence including éboulis production, soil chemistry, and δ13C and δ18O signatures in soil organic matter and calcium carbonate suggest that relatively cool, moist conditions 10,600 to 9700 14C yr BP were followed by relatively warm and dry conditions as early as 9000 14C yr BP. Warm and dry conditions extended to the late Holocene, followed by a return to cooler and moister climate. The limited range of δ13C and δ18O values in Marmes paleosols suggests that the magnitude of moisture and temperature shifts was locally buffered in the lower Snake River Canyon but adequate to generate significant changes in sedimentation and soil formation, possibly due to nonlinear geological and pedological processes. These buffered canyon environments were well suited for establishing residential bases associated with foraging and logistical collecting strategies and may have minimized the influence of climate changes in food resource abundance.