Jeptha Knob, Shelby County’s buried crater

Underneath is a circular structure nearly three miles across: a core of uplifted, shattered Ordovician limestone and shale ringed by concentric and radial faults, later buried under flat-lying Silurian rock. William Linney of the Kentucky Geological Survey first described it in 1887, and Walter Bucher mapped it in 1925 and called it "cryptovolcanic," the word still on its roadside marker.

Most geologists now suspect a meteorite or comet strike around the end of the Ordovician, roughly 440 million years ago, and unusually high iridium in its broken rock points that way, but it isn't on the official list of impact craters because no shatter cones or coesite have been found. The knob is on private land. The years on this story are the human ones, from Linney’s 1887 report to Bucher’s 1925 map.

The map pin marks the Kentucky Historical Society’s Jeptha’s Knob marker at Clay Village; the knob itself is private.