Jeptha Knob - Clay Village, KY - Impact Craters on Waymarking.com

Three sites in Kentucky bear the scars of ancient impacts by meteorites: the Jeptha Knob in Shelby County, a site near Versailles in Woodford County, and a site near Middlesboro in Bell County. A meteorite impact usually forms a roughly circular crater, called an astrobleme, and can crack the Earth's crust in a characteristic circular pattern. Astroblemes may show a "rebound structure" where a central core of rock has been brought up from deeper underground by the impact. The three Kentucky astroblemes represent the highly eroded cores of the astroblemes that were situated under the original craters; the crater walls eroded long ago. Each of these structures is characterized by a circular belt of arc-shaped faults cross cut by faults radiating outward from the central core of intensely broken rock. In the past, these structures were referred to as "cryptoexplosive" because their origin was uncertain.

A good example of an astrobleme is Jeptha Knob. Jeptha Knob is located in Shelby County in north-central Kentucky and can be observed on the Shelbyville and Waddy geologic quadrangle maps (1:24,000 scale). This nearly 3-mile-diameter group of hills is visible just north of Interstate Highway 64. It contrasts sharply with the surrounding rolling farm land. A paper by Cressman (1981) describes the geology of the astrobleme, shown on the map above. The colors on the map represent different rock units. The black lines (many dashed) are faults. The rock units shown in pink in the center of the ring of faults have been uplifted relative to the rocks outside of the ring.

The cross section across the Jeptha Knob astrobleme is a representation of what the rocks beneath the surface of Jeptha Knob look like. It dramatically illustrates how the unit of rock shown in pink in the center of the astrobleme has been pushed upward. This is the geologic remnant of the uplifted rebound structure in the center of the crater. Geologists think the structure was formed around 425 million years ago, during the Ordovician Period.