ELECTRICAL RESISTIVITY GROUND IMAGING (ERGI) INVESTIGATION OF TOPOGRAPHIC DEPRESSIONS NEAR THE SERPENT MOUND NATIVE AMERICAN EFFIGY, ADAMS COUNTY, OHIO, USA

Serpent Mound, a prehistoric Native American effigy mound in the form of a snake, is the largest serpent effigy in the United States. The effigy is ~424 m long, 0.5-1.5 m high, and 2.6-6.2 m wide. Three circular topographic depressions are located ~180 m south-southwest of the mound. Some researchers suggest that the depressions are areas where material was excavated for use in constructing the mound (i.e., borrow pits). Other investigators contend that the depressions are sinkholes formed by dissolution and collapse of the underlying carbonate bedrock. Electrical resistivity ground imaging (ERGI) surveys were conducted to provide subsurface information regarding the origin of the depressions.

The three depressions are sequentially aligned along a SW-NE, roughly linear trend. Three ERGI surveys were conducted. One survey spanned the southwest and middle depressions, and the other two surveys traversed the middle depression and were oriented approximately perpendicular to each other. The surveyed depressions are ~30 m in diameter with relief on the order of 0.7-1.6 m. The study area is situated on a small plateau adjacent to Brush Creek, a tributary of the Ohio River. Bedrock is the Upper Silurian Pebbles Dolomite which is overlain by unconsolidated sediment. The area is located in the unglaciated portion of Ohio and lies on the southwestern part of a fractured and faulted, late Paleozoic meteorite impact structure.

ERGI profiles show a ~1-4 m thick layer of low-resistivity (50-400 Ohm-m) surficial sediment that is readily discernible from the higher resistivity (500-2000 Ohm-m) carbonate bedrock. Surface topography generally parallels bedrock topography beneath the depressions, though exceptions are apparent. A prominent high resistivity (3000-6000 Ohm-m) feature is present beneath the middle depression and likely represents a collapse breccia. Beneath the other depression, a potential silt-filled void (~2 m high, ~3-4 m wide; resistivity of 60-160 Ohm-m) is evident, possibly overlain by collapse breccia. Collectively, the ERGI results suggest that the depressions are sinkholes rather than borrow pits. The depressions and the possible silt-filled void are comparable in scale to solution voids evident in outcrop and also to sinkholes documented elsewhere in the area.