GEOMORPHOLOGY, CHRONOSTRATIGRAPHY AND GEOARCHAEOLOGY OF THE LOWER BIG FORK RIVER VALLEY, NORTHERN MINNESOTA

Geomorphic and chronostratigraphic evidence document the alluvial events and the sedimentological processes related to prehistoric archaeological occurrences along the Big Fork River, in northern Minnesota. The river has eroded into sediments of glacial lake Agassiz since 9,500 radiocarbon years before present (RCYBP) and formed three Holocene (post-Glacial) terraces. The middle and lower terraces consist of distinct relict meanders and a swale-and-ridge topography; these mark past channel positions resulting from lateral migration. Based on stratigraphic excavations, 19 drill-cores, and using 33 radiocarbon dates from the lower terrace, the sedimentological and site formational processes associated with late Holocene ("Woodland") artifacts can be inferred. The multi-component, stratified Hannaford site is situated in the active floodplain of a meandering river that has produced two sets of strata. The lower series consists of coarse-grained strata that are primarily the result of point bar deposition. The upper fine-grained sediment series represents overbank flooding events. Most of the Woodland artifacts are within the upper series and represent primary archaeological contexts. Some artifacts are located within point bar deposits and are in secondary position. Radiocarbon dates indicate the lower terrace was formed by processes of alluvial aggradation that have been dominant since at least 3,000 RCYBP. The stratigraphic cross-sections document a progressive expansion of land associated with prehistoric human presence from about 1,300-650 RCYBP. This is an example of 1) geomorphic evolution in a deglaciated area that can be compared to wider patterns of environmental change, and 2) the role alluvial processes play in the interpretation of the prehistoric archaeological record.