The Clays Ferry Formation is a newly recognized major rock-stratigraphic unit in south-central Kentucky. It includes strata that were previously divided on the basis of the fauna into (in ascending order) the Cynthiana Formation of A. F. Foerste, the Fulton Shale, and the Million Shale of J. M. Nickles as used by A. F. Foerste. The Clays Ferry consists of interstratified thin-bedded shale, limestone, and siltstone. It ranges from about 120 to 220 feet in thickness, and is of late Middle and early Late Ordovician age. The Clays Ferry Formation overlies and intertongues with thick-bedded limestone of the Lexington Limestone ; it grades into the overlying Garrard Siltstone. INTRODUCTION The name Clays Ferry Formation is here given to a mappable unit of interbedded shale, limestone, and siltstone that crops out in southcentral Kentucky. The Clays Ferry Formation overlies thick-bedded granular limestone of the Lexington Limestone as defined by Black and others (1965). It is overlain by the Garrard Siltstone. The type locality (figs. 1 and 2) is on the south side of the Kentucky River in Madison County across the river from the settlement of Clays Ferry, Fayette County, Ky. The type section (p. B14) is exposed in a gully a few tens of feet northeast of the eastern span of the Interstate Highway 75 bridge over the Kentucky River at Clays Ferry and in roadcuts on the northeast side of the highway in the southern part of the Ford quadrangle, Kentucky. This report is based in large part on the geological mapping of Kentucky being conducted by the U.S. Geological Survey in cooperation with the Kentucky Geological Survey. Areas not currently being mapped were studied chiefly in the spring and summer of 1963. The writers are indebted to D. F. B. Black, E. R. Cressman, and G. C. Simmons for information concerning the stratigraphy of the Clays Ferry Formation. R. J. Ross, Jr., helped collect fossils at the type locality. Bl B2 CONTRIBUTIONS TO STRATIGRAPHY LEXINGTON Mount Sterling o Winchester E 0 Richmond VALLEY VIEW QUADRANGLE Harrodsburg