The national park sits astride the Iapetus suture, the line along which the former Iapetus Ocean closed during the Silurian period as the former micro-continent of Avalonia to the south collided with the continent of Laurentia to the north. The suture and the great slab of subducted oceanic crust from beneath the former ocean are concealed from view by a thickness of sedimentary rocks which subsequently accumulated over the area. During the Devonian period a granite pluton was emplaced in the north (under what is now the Cheviot Hills) and volcanic activity led to the accumulation of various volcanic rocks whilst to the south (under what is now the North Pennines, a granite batholith was emplaced.