Geologically, the canyon was cut into dolomite and shale bedrock dating to the Silurian Period some 400 million years ago.[3] It was created by meltwater from glaciers about 15,000 years ago.[3] The Little Miami River rushing through the narrow canyon created a water power source in the nineteenth century for grist mills, cotton mills, paper factories, and breweries.[3] But by the late 1800s, most of the industrial activity ended when water power ceased to be an economical source of energy.[4] Eight acres of the area was declared a National Natural Landmark in 1967,[5] and the upper gorge became a state nature preserve in 1973.[6]
Facilities
The preserve has a Nature Center, and three miles of trails for hiking and biking.[7] It also adjoins the 752-acre John Bryan State Park, which offers additional trails as well as boating, fishing, camping, and other amenities.[4][8] That park, in turn, adjoins the 1,125 acre Glen Helen Nature Preserve, which has its own 15-mile network of footpaths, and which includes the famous Yellow Spring that gave the nearby town its name.[9][10][11]
