Clifton Gorge is a classic example of a canyon cut into dolomite and shale bedrock dating to the Silurian Period 400 million years ago. Meltwater released from a retreating continental glacier about 15,000 years ago created the gorge, whose cool, moist conditions have preserved a diversity of native plants, including the red baneberry bush, that are now rare elsewhere in Ohio. During the nineteenth century, the Little Miami River powered grist and cotton mills, paper factories, and breweries in the gorge. In 1973, the upper gorge became a state nature preserve.