Assessing Paleoenvironmental and Geomorphic Variability in Relationship to Paleoindian Site Burial; Centennial Valley, Montana

Wave action along the shores of Lima Reservoir in Centennial Valley, Montana is actively eroding the southern margins of three neighboring Paleoindian sites. Despite ostensible similarity among the sites, major site formation differences are apparent in exposed sediments. Shoreline cutbank exposures one-to-five meters high connect the sites and reveal a complicated geomorphic history. Although each site contains artifact evidence of terminal Pleistocene-early Holocene occupations, Paleoindian components at these three localities occur in very different contexts: one is buried, while the other two are apparent surface scatters. This raise the question of why sites of the same age are in both buried and exposed contexts. Moreover, buried sites are more likely to have preserved spatial layout and sites with buried components are more likely to be considered significant under National Register of Historic Places criteria. These factors therefore prompt the management question of where might other buried sites be located in the valley? In order to answer these questions, I used a multi-pronged approach including optically stimulated luminescence dating, sediment grain size analysis, stratigraphic profiling and sediment facies analysis. I accomplished two nested objectives with this research. First, I reconstructed the last 60,000 years of geomorphic events for the area surrounding the three sites in order to determine what conditions resulted in site burial. Second, I used those findings to outline criteria for differentiating occupation-age and pre-occupation-age stratigraphic layers in Centennial Valley. I determined, in part, that cultural-age deposits are present at both high and low elevations and that they may be marked by a specific soil sequence. The oldest packages, far pre-dating potential human occupation, are deep lake and high energy stream sediments that may be recognized by soil color alteration and thick gypsum horizons