Pliocene-Pleistocene incision of the Green River, Kentucky, determined from radioactive decay of cosmogenic Al-26 and Be-10 in Mammoth Cave sediments

Cosmogenic Al-26 and Be-10 in sediments washed into Mammoth Cave, Kentucky, record the history of 3.5 m.y. of water-table position, governed by incisi on and aggradation of the Green River, a tributary of the Ohio River. Upper levels of the cave formed during a period of slow river incision and were later filled with sediment due to river aggradation at 2.3-2.4 Ma, A brief surge of river incision ca, 2 Ma was followed by river stability and cave-p assage formation at a lower level. Rapid incision through 15 m of bedrock c a, 1.5 Ma was prompted by repositioning of the Ohio River to its present co urse along an ice-sheet margin. Renewed incision ca, 1.2 Ma and aggradation at 0.7-0.8 Ma correlate with major ire advances in the Ohio River basin. M easurements of Al-26 and Be-10 also indicate that sandstone-capped uplands have maintained slow erosion rates of 2-7 m/m.y. for the past 3.5 m.y., des pite accelerated Pleistocene river incision rates of similar to 30 m/m.y.