DISTRIBUTION OF NATURAL ARCHES IN KENTUCKY

Natural arches can be found throughout the eastern United States, but Kentucky is home to more than 2,900—more than any other state in the region. However, the geology of Kentucky’s arches is understudied. This work examined the rock units, landscape positions, and joint orientations of 332 natural arches across the state.

In Kentucky, natural arches are found atop ridges, along sides of cliffs and ridges, along streams, and in sinkholes. Most of the arches occur in the Eastern and Western Kentucky Coal Fields and form in thick, cliff-forming, jointed sandstones. The Lower Pennsylvanian Corbin Sandstone Member of the Grundy Formation, located along the western margin of the Eastern Kentucky Coal Field’s Red River Gorge Geological Area in east-central Kentucky, features the largest concentration of natural arches in the state. The longest span of any arch occurs in western Kentucky, in the Lower Pennsylvanian Caseyville Sandstone in the faulted, Fluorspar District of the Mississippian Plateaus physiographic region. However, the largest clearances and widths of arches in the state occur in Mississippian limestones of the Slade Formation in the Eastern Kentucky Coal Field.

Natural arches are often controlled by jointing. Span and joint orientations were measured for 262 natural arches. Lintels, the areas of rock above the opening, varied in orientations, but approximately a third were parallel or sub-parallel to local jointing. Approximately, a fourth of the lintels occurred at right or oblique angles to joint orientations. General trends in joint and arch orientations related to major faults or fault systems were observed in some regions.