Wissahickon Gorge (National Natural Landmark)

The Wissahickon Valley within Philadelphia's Fairmount Park was designated a National Natural Landmark in 1967 for its steep-sided gorge carved into Wissahickon schist. In places the gorge reaches depths on the order of a hundred feet, creating a pronounced urban canyon through which Wissahickon Creek flows. The valley contains historic mills, a long network of trails, and the 19th‑century Valley Green Inn adjacent to the creek.