EVALUATION AND APPLICATION OF GEOPHYSICAL METHODS IN IDENTIFYING POTENTIAL NATIVE AMERICAN EARTHEN MOUNDS IN THE UPPER MISSISSIPPI BASIN, USA

Subtle mound-like landforms can be genetically ambiguous features within a landscape. A variety of geomorphological and anthropological processes can result in these equifinal forms being difficult to interpret. Being able to reliably and noninvasively differentiate them is important for legal as well as cultural and spiritual reasons. Here, we utilize and test a suite of non-invasive geophysical methods to elucidate characteristics observed within mound-like landforms in order to identify diagnostic indicators of anthropogenic origin. We conducted ground penetrating radar (GPR), electrical resistivity, and magnetometry surveys on mounds at the Kiwanis site in western Wisconsin. Genesis of these mounds is ambiguous given the presence of aeolian landforms in immediate proximity. As a control, we applied these same methods to previously identified anthropogenic mounds at the nearby Belle Creek site in eastern Minnesota. Data from both locations indicate increases in electrical resistivity and range in magnetic gradient within or near the mounds, suggesting an anthropogenic origin. 500 MHz GPR data show strong, semi-continuous horizontal reflections at depth within each mound. We interpret these to be the stratigraphic contact between the mound and underlying sediments. This reflection becomes discontinuous directly beneath the mound apex, indicating that it may be a prepared surface. Inclined reflections dip away from the apex of each mound in all directions. Since this is inconsistent with predominant southerly winds responsible for aeolian deposition at Kiwanis, we interpret these to represent grainflow during construction of the mound or during post-construction hillslope diffusion. A rectangular reflection measuring 2 x 4 x 0.5 m is visible in the center of the Kiwanis mound and cannot be explained via aeolian processes. We hypothesize this to be remnants of a mortuary feature due to its shape and orientation. We conclude that the Belle Creek and Kiwanis site mounds are similar in genesis, and internal anomalies at Kiwanis further support an anthropogenic origin. The methods applied here have proven effective as a non-invasive approach to identifying anthropogenic mounds and should be considered in future studies of ambiguous mound-like forms.