The Ohio River Story - Part V (GM-27)
The Falls of the Ohio, like many topographic features along the Ohio River, have their origin in glacial events. Sand and gravel deposited during the latest period of partial valley filling date from the most recent glacial advance, which reached almost to Cincinnati. The fill covered many low hills within the valley. About 15,000 years ago the ice retreated from the Ohio River basin, and the river, no longer loaded with glacial debris, began to cut downward. At Clarksville its course then lay across a small knob of fossiliferous limestone. Here the falls were formed as the stream cut downward. This is the only place along the Ohio River where the riverbed is cut into bedrock from shore to shore.

Our Hoosier State Beneath Us: Geomorphology