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.

Multiple cosmogenic nuclides with different decay rates can be used to date exposure and burial of rocks over the timescales of radioactive decay. This paper reviews the development of such dating methods over the past ∼50 years, beginning with a historical perspective on early meteorite studies, and later focusing on recent examples in the terrestrial field using the 26 Al- 10 Be pair in quartz. Two classes of terrestrial applications are discussed in detail. The first involves the use of 26 Al and 10 Be in rock or sediment that has experienced a complex history of repeated exposure and burial. In these cases, the cosmogenic nuclides can only provide a minimum near-surface age. Examples include sediment from beneath desert sand dunes, and rocks from beneath cold-based glaciers. The second class of application uses 26 Al and 10 Be to date discrete burial events, in cases where sediment has experienced a simple history of exposure followed by rapid burial. Examples include cave sediments, alluvial deposits, and sediment buried beneath glacial till. Finally, the half-lives of 26 Al and 10 Be are discussed, with special attention given to discrepant estimates of the 10 Be half-life. It is shown that geologic data are consistent with either half-life estimate of 1.51 m.y. or 1.34 m.y., but more closely conform to the shorter half-life.