The Cynthia Clay Pit

The Cynthia clay pit, in Hinds County about a mile and a half northwest of milepost 94 of the Natchez Trace Parkway, was for years the best outcrop of the Yazoo Clay in Mississippi. The Miss-Lite division of Jackson Ready Mix mined the late Eocene marine clay there to make lightweight aggregate at the state's only such plant, which opened on January 22, 1958 and closed in 1992 after 34 years. By then the pit walls exposed more than 130 feet of the formation, and David Dockery of the Mississippi Office of Geology predicted that its lower levels would fill with water once the pumping stopped.

The pit was best known for whales. Mississippi's state fossils are two archaeocete whales of the late Eocene sea, and both have been found there: vertebrae and other remains of Basilosaurus cetoides, about sixty feet long, turned up often, along with the sixteen-foot Zygorhiza kochii and vertebrae of an eighteen-foot sea snake, Pterosphenus. A partial skeleton from the pit, with skull, jaws, teeth, vertebrae and forelimb bones, became the type specimen of a new genus and species, Cynthiacetus maxwelli, described by Mark Uhen in 2005 and held by the Mississippi Museum of Natural Science. Shark teeth nearly three inches high have also been found.

The clay records a sea in transition. A 1984 survey listed 56 mollusk species from the pit, 14 of them unnamed and known only there, and found five Paleocene and Eocene mollusk genera making their last North American appearance in this horizon. In 1985 a bulldozer uncovered pods of tar in the clay, the residue of escaping hydrocarbons.