Cooper’s Ferry/Nipéhe, stratigraphic integrity, and confirming early inland dispersal: Evaluating peopling of the Americas modeling

Statistical models shape debates over the timing and geography of the initial peopling of the Americas, but their value depends on testing their assumptions against archaeological evidence. We examine the Apparent Stratigraphic Integrity Index (ASI), used to infer postdepositional mixing at Late Pleistocene sites and support a relatively late occupation of temperate North America. ASI describes vertical discontinuity in binned artifact-frequency profiles; it does not directly measure artifact movement or stratigraphic integrity. At Cooper’s Ferry/Nipéhe, Idaho, the 5-centimeter absolute-elevation bins used to calculate ASI do not reflect the actual stratigraphic positions of artifacts across the irregular LU2 surface on which LU3 loess accumulated. The resulting ASI value cannot support claims of large-scale vertical mixing. Independent chronological, stratigraphic, pedogenic, archaeological, and excavation evidence supports the integrity of the ∼16,000–calibrated-years-before-the-present occupation. Cooper’s Ferry/Nipéhe provides early inland evidence expected under coastal-migration models, showing that models should be evaluated against archaeological evidence rather than used to override it.