. . "Limestone." . . . . . . . . . . . . . . . . "Contributions to stratigraphy"@en . . . . . . "The Lexington limestone (Middle Ordovician) of central Kentucky"@en . . . . . . . . . . "Geology, Stratigraphic." . . "Limestone Kentucky." . . "Geology, Stratigraphic Ordovician." . . "Kentucky." . . "Ordovician Geologic Period." . . "Kentucky Geological Survey," . . "From 425 to 500 million years ago" . . "Geology." . . "Geological Survey (U.S.)," . . "Geology Kentucky." . .