 Exmoor is a dissected plateau of Devonian sedimentary rock , rising to 517 metres ( 1 @,@ 696 ft ) at Dunkery Beacon . It extends into Devon but the majority of the area is in Somerset . Much of the area is a National Park . The landscape is one of rounded hills , with hogs @-@ back cliffs at the coast due to geological movements . Because of high rainfall there are boggy areas and the part by the Chains is a Geological Conservation Review site recognised as being nationally important for its south @-@ western lowland heath communities and for transitions from ancient semi @-@ natural woodland through upland heath to blanket mire . The Chains provides palynological record of a mid to late Flandrian vegetation history on Exmoor . The pollen sequence in the peat is calibrated by radiocarbon dating . The <unk> area demonstrates the Trentishoe Formation of the Hangman Sandstone Group . The Hangman Sandstone represents the Middle Devonian sequence of North Devon and Somerset . These unusual freshwater deposits in the Hangman Grits , were mainly formed in desert conditions . 
