The sedimentary rocks of the canyon walls are believed to be the remains of the ancestral Appalachian Mountains, which were eroded into inland seas during the Devonian Period 350 million years ago. Along the course of the Genesee River, on its three-day journey from Pennsylvania to Lake Ontario, over 100 million years of deposition are revealed in the rock layers like pages of geologic time. Alternating strata of sandstones and shales create a variety of soft-colored hues in the vertical cliff, with some dark shales formed from highly organic clays that have a fuel-oil odor from decayed plants and animals, and other rusty-brown bands containing oxidized minerals such as iron. Midway down the 350-foot cliff, a year-round spring is creating a huge stalagmite of tufa weighing several tons, a centuries-old rock formed from precipitated lime mixed with algae and moss.