Next below these is the late Devonian and early Mississippian Huntley Mountain Formation, which is made of relatively soft grayish-red shale and olive-gray sandstone. This is relatively hard rock and forms many of the ridges. Below this is the red shale and siltstone of the Catskill Formation, about thick and some 375 million years old. This layer is relatively soft and easily eroded, which helped to form the Pine Creek Gorge. Cliffs formed by the Huntley Mountain and Catskill formations are visible north of the park at Barbour Rock. The lowest and oldest layer is the Lock Haven Formation, which is gray to green-brown siltstone and shale over 400 million years old. It forms the base of the gorge, contains marine fossils, and is up to thick.