 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 760 feet ( 230 m ) 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 600 feet ( 180 m ) thick . 
