Rivers across unglaciated portions of the Appalachian Plateaus of Tennessee and Kentucky are deeply entrenched, almost without exception. Widespread gravel deposits on upland surfaces, combined with broad straths and terraces inset beneath the highlands indicate a history of base-level stability punctuated by periods of river incision. Determining the exact timing of episodic incision historically has been difficult due to a combination of unsuitable dating methods and poorly preserved surface materials. Recently, advances in analytical techniques have allowed researchers to constrain the incision history by utilizing the hydrologic link between multilevel cave systems and regional rivers. In this study, we date clastic sediments deposited in caves associated with the Cumberland River using cosmogenic 26 Al and 10 Be, and show that they correspond to: (1) deposition of upland (Lafayette-type) gravels between ca. 5.6 Ma and ca. 3.5 Ma; (2) initial incision of regional rivers into the Highland Rim after ca. 3.5 Ma; (3) development of the Parker strath during the interval between ca. 3.5 Ma and ca. 2 Ma; (4) incision of the Parker strath at ca. 2 Ma; (5) shorter cycles of incision after ca. 1.3 Ma associated with terraces above the modern floodplain; and (6) regional aggradation at ca. 0.8 Ma. Burial ages of cave sediments record more than 5 m.y. of incision history within the unglaciated Appalachian plateaus and constrain the time needed to develop multilevel cave systems on plateau margins.

The Old Kentucky River system was a major contributor to the Teays River, draining southwestern Ohio and much of eastern Kentucky. The trunk river flowed northward from southeastern Kentucky throughout Frankfort and Carrollton, and then past Cincinnati and Dayton, joining the Teays River near Springfield, Ohio. North of the glacial boundary, which lies along the modern Ohio River, the course of the Old Kentucky River has been modified, and is today largely buried by drift. Although dissection is extensive to the south, there are many remnants of this entrenched and broadly meandering Teays-age valley system and of its sub-upland predecessors. These valleys contain areas of upward-fining, deeply weathered gravel, composed mainly of rounded quartz, chert, and silicified limestone pebbles derived from the headwaters of the system. Modern rivers have been entrenched 30 to 100 m below the Old Kentucky River valley and its main tributaries, the Old Licking and South Fork. The Old Kentucky River system was severed from the Teays when glaciation dammed its downstream reaches, forcing a reversal in flow direction between its junction with the Teays in west-central Ohio and Carrollton, Kentucky, and causing westward overflow into the Old Ohio River system. Piracy by the Old Ohio may also have contributed to the integration of the Old Kentucky and Old Ohio River basins. Ponded sediment is present in some of the now-abandoned valley remnants east of Cincinnati. As a result of glacial damming, the headwaters of the Teays River in southeastern Ohio and West Virginia overflowed westward across the Manchester divide into the Old Kentucky River drainage basin. All of these events led to establishment of the modern Ohio River system.