Ausable Chasm is a sandstone gorge near Keeseville in northeastern New York, carved by the Ausable River over about two miles to depths of roughly 150 feet. Formed after the last glacial period as retreating waters cut into Potsdam Sandstone, it preserves striking evidence of ancient tidal flats as well as dramatic features such as Rainbow Falls and several dry former channels. People were drawn to the gorge long before organized tourism began in 1873, and the site later developed bridges, walkways, industry powered by the falls, and visitor facilities. It remains a major Adirondack attraction valued for both its scenery and its clear geologic history.