INTERPRETATION OF LATE WOODLAND EFFIGY MOUND CONSTRUCTION TECHNIQUES FROM THREE GEOELECTRICAL TECHNIQUES

Three geoelectrical techniques were employed over two specific effigy mounds in Indian Mounds County Park in Jefferson County, Wisconsin to better define the subsurface under and around these mound structures. The entire site (40,000 m²) was first surveyed using a Geonics EM31 conductivity meter which provided a low resolution view of the subsurface to a depth of approximately 6 meters. The ground conductivity varies from 0.1 to 2.0 mS/m and has a predominant east to west gradient with the higher conductivity to the east. We interpret this as variations in the glacial deposits, with little signature from the mounds.

Electrical resistivity profiles (ERT) were conducted longitudinally and transversely on two of the mounds (61 and 56) using an ARES electrical profiling system with twenty four electrodes spaced two meters apart. The lines coincided with the ground penetrating radar (GPR) lines. The electrical resistivity profiles provide subsurface information to a depth of approximately 1 to 5 meters over the length of the line. Mound 61 (lizard mound) shows significant resistivity variations. We interpret some of the longitudinal changes as indications of robber pits, where as the lateral variations are most likely from pre-mound construction activity. Mound 61 has greater resistivity variations than Mound 58 (bird mound). We interpret this as differences in mound preparation.

GPR surveys using 225 and 450 MHZ antennae were conducted along the same lines as the ERT. The 450 MHZ survey provided better resolution of the soil lenses/layers and gives more details of the construction techniques.

The combined results of the three techniques give an internally consistent non-invasive view of the area which adds to the archeological interpretation of the site.