THE SURFICIAL GEOLOGY AND BEDROCK TOPOGRAPHY OF LAWRENCE COUNTY, OHIO

The Ohio Geological Survey is completing a 5-year plan to finish the statewide surficial geology map of Ohio. The last area that required new mapping is Lawrence County, which previously had no detailed mapping of unconsolidated material. Lawrence County is the southernmost county in Ohio and borders both Kentucky and West Virginia. The county is located on the Allegheny Plateau, lies south of the glacial margin, and features high topographic relief. Pennsylvanian-aged bedrock in the county is a source of coal, clay, and iron that has supported industrial development mainly along the Ohio River. Surficial geology “stack” mapping was used to delineate the extents and thicknesses of different unconsolidated lithologic units. These pseudo–3-D mapping products use stacked labels and line symbology to indicate the variability of unconsolidated units in the subsurface. To evaluate the thickness of unconsolidated deposits, a detailed bedrock topography map was interpolated from contours of bedrock surface elevation at a 50-foot contour interval. This surficial geology and bedrock topography mapping indicate that unconsolidated deposits generally are thin throughout the county, apart from those found in the Ohio River Valley. The primary unconsolidated units within the county include colluvium/residuum, alluvium, lake silts/clays, and sand/gravel. By area, the most dominant unconsolidated unit is colluvium/residuum derived from weathered and eroded Pennsylvanian-aged bedrock. Alluvial deposits generally are thin at the headwaters of tributaries and thicken towards the Ohio River. Lake silts/clays are preserved on terraces above modern floodplains and buried beneath alluvial deposits in some partially buried valleys. Sand-and-gravel deposits are concentrated within the Ohio River Valley and are the thickest unconsolidated materials, reaching about 100 feet thick within well-preserved terraces. Interpretations of unconsolidated materials and geomorphology of the county provide evidence of at least one major drainage reversal during the Quaternary Period. Barbed tributaries, cross-sectional valley widths, and lacustrine deposits point towards stream piracy via a slackwater lake downcutting a paleo-drainage divide in the modern Symmes Creek watershed.