The area under discussion is practically coincident with that underlain by Chester and Lower Mississippian south and east of the Western Kentucky coal basin, and includes Breckinridge, Meade, Hardin, Grayson, Larue, Hart, Warren, Barren, Allen, Logan, and Todd counties. Numerous wells have been drilled in these counties to test the so-called "Corniferous," or limestone beneath the Chattanooga shale.

The study of such samples as are available demonstrates the enormity of the erosional unconformity between the Beechwood (Hamilton) and the directly underlying formation, which may be Silver Creek (Hamilton), Jeffersonville (Onondaga), a member of the Silurian or Upper Ordovician. This thin limestone rests on older and older strata as the crest of the Cincinnati arch is approached on the east and has been entirely removed in many places close to the present outcrop.

The Jeffersonville with its basal sand also overlaps from Clear Creek onto the Louisville (Silurian) limestone. At the south end of the basin the pre-Jeffersonville unconformity is less striking than on the east, but the Chattanooga shale rests on strata ranging in age from Hamilton through Jeffersonville and Clear Creek to the Bailey (Helderberg), and in northern Tennessee the shale directly overlies the Upper Silurian.

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