 Exmoor is loosely defined as an area of hilly open moorland in west Somerset and north Devon in South West England . It is named after the River Exe , the source of which is situated in the centre of the area , two miles north @-@ west of Simonsbath . Exmoor is more precisely defined as the area of the former ancient royal hunting forest , also called Exmoor , which was officially surveyed 1815 – 1818 as 18 @,@ 810 acres ( 7 @,@ 610 ha ) in extent . The moor has given its name to a National Park , which includes the Brendon Hills , the East Lyn Valley , the Vale of Porlock and 55 kilometres ( 34 mi ) of the Bristol Channel coast . The total area of the Exmoor National Park is 692 @.@ 8 square kilometres ( 267 @.@ 5 sq mi ) , of which 71 % is in Somerset and 29 % in Devon . 
