ARE RIDGES WITHIN CHANNELS ESKERS, SENECA COUNTY, OHIO?

Across the karst landscape of Seneca County, discontinuous meltwater channels 3–6 m deep and parallel to ice flow direction contain ridges resembling eskers. The ridges with 1–2 m relief are usually found in groups, sometimes parallel with each other, straight, but also curvilinear where the channel bends. Regional mapping indicates 3–4 m of till at the surface overlying Columbus Limestone bedrock. Sinkholes within the channels contribute to multibasinal channel bottoms. It is hypothesized that the ridges are eskers because of their location within a meltwater channel and eskers are observed nearby outside channels. Because eskers usually consist of highly resistive sequences of sand and gravel with horizontal or cross-bedding structures, a combination of ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) data were collected across and within the channels, and for 200 m along a ridge to determine ridge composition. GPR data were collected with 100 MHz antennae attached to a cart and set for continuous sampling. The ERT data were collected with 1 m electrode spacing in a dipole-dipole array. The ERT result consists of two distinct resistivity units with an undulating contact between them. The lower unit with higher resistivity (>1000 Ωm) is interpreted as limestone bedrock and has an undulating upper surface. The upper unit with lower resistivity (<500 Ωm) is interpreted as till, consistent with a geological map of the area and soil auger samples to ~2 m depth. The GPR data consists of mostly faint reflections parallel to the ground surface with shallow penetration depths. The ridges are composed of low resistivity till instead of the anticipated high resistive sand and gravel characteristic of eskers, hence, the hypothesis is rejected. An alternative interpretation for the ridges is elongated low-relief bedrock ridges resulting from unequal subglacial meltwater erosion subsequently draped with till.

Keywords: ERT. GPR. Eskers. Meltwater Channels