 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.The Allegheny Plateau has a continental climate, with occasional severe low temperatures in winter and average daily temperature ranges of 20 °F (11 °C) in winter and 26 °F (14 °C) in summer.