GEOPHYSICAL SUBSURFACE INVESTIGATION OF THE SERPENT MOUND AREA (OHIO, USA) USING ELECTRICAL RESISTIVITY GROUND IMAGING (ERGI): EVALUATION OF BEDROCK CONTROLS ON SURFACE FEATURES

Serpent Mound, a prehistoric Native American effigy mound in the form of a snake, is the largest serpent effigy in the United States (~424 m long). This study used electrical resistivity ground imaging (ERGI) to evaluate whether surface features associated with the Mound and surrounding area were influenced by the subsurface geology. The study focused on two areas: one south of the mound where three topographic depressions are evident, the other within the effigy. The study area is situated on a small plateau adjacent to a tributary of the Ohio River. Bedrock is Upper Silurian dolostone overlain by unconsolidated sediment.

The three depressions (~30 m in diameter, relief of 0.7-1.6 m) are thought to be either areas where material was excavated for use in mound construction (i.e., borrow pits) or sinkholes formed by dissolution and collapse of the underlying carbonate bedrock. ERGI results show a layer of low-resistivity surficial sediment that is readily discernible from the higher resistivity bedrock. Surface topography generally parallels bedrock topography. A high resistivity feature beneath one depression likely represents a collapse breccia. Beneath another depression, a potential silt-filled void is evident, possibly overlain by a collapse breccia. Collectively, the ERGI results suggest that the depressions are sinkholes.

Two ERGI surveys were conducted within the effigy. Some researchers contend that a portion of one Mound convolution may have undergone erosion or damage (and was later repaired) in association with sinkhole subsidence or sub-Mound drainage associated with a closed depression, citing the convolution’s location at the head of a gully. An ERGI survey across this convolution indicates an underlying high resistivity feature that is possibly a collapse breccia associated with sinkhole formation, a small void, or subsurface weathered bedrock. Regardless, the feature is consistent with the aforementioned scenario. A second ERGI survey was conducted in the head-oval area to investigate whether a large cave or cavern system exists in the subsurface, as purported by some people. ERGI results in this area indicate no such features. The likely karst features evident on all ERGI profiles are comparable to solution voids and weathering features evident in bedrock outcrops.