The new Bedrock Geologic Map of Ohio

The Ohio Division of Geological Survey achieved a milestone in September 2006, when 5,000 copies of the new Bedrock Geologic Map of Ohio were printed. The publication of this map marks the culmination of a 25-year effort by Survey staff to remap the bedrock geology of Ohio. The 2006 Bedrock Geologic Map of Ohio is a work of artistry and science. It contains an intricate mosaic of colors and patterns that depict the distribution of near-surface bedrock geology across the state. Essentially, the map displays the bedrock units at ground surface, or the fi rst bedrock unit encountered beneath surfi cial deposits that cover the bedrock. The map shows the distribution of 45 bedrock units across the state on a modern base map. Sections of the new bedrock map, such as portions of southeastern Ohio, strongly resemble the 1920 map and testify to the excellent geologic mapping conducted between 1890 and 1920. Of course, economically important Pennsylvanian-age coal-bearing rocks dominate southeast Ohio, a fact that focused the Survey’s work through the early and mid twentieth century. The detailed mapping of those rocks is refl ected on the 1920 state geologic map, and in many respects is carried over to the 2006 version. However, the new map’s depiction of western Ohio contains an immense amount of added detail in stark contrast to the 1920 map. This is largely because buried-valley systems were only roughly understood prior to 1920. The best example of such a major update is the 450 feet-deep, ancestral Teays Valley and its tributaries that deeply slice through several Silurian-age bedrock formations from London, Ohio to Grand Lake St. Mary’s and beyond to reveal older Ordovician bedrock beneath. Also included with the map is a cross section. The section traces along a line from the Ohio River in Washington County, to Bellefontaine in Logan County, and then to Williams County in the northwest corner of the state. The cross section at the base of the map is a vertiThe 2006 Bedrock Geologic Map of Ohio is the fi rst major update of the bedrock map of the state since 1920. This map is the result of over 20 years of intensive data gathering, map drawing, and digital compilation by Division staff. The map is at a scale of 1:500,000 (one inch equals 8 miles) and the map dimensions are 42 inches in height by 50 inches in width.