 Exmoor has 55 kilometres ( 34 mi ) of coastline . The highest sea cliff on mainland Britain ( if a cliff is defined as having a slope greater than 60 degrees ) is Great Hangman near Combe Martin at 318 metres ( 1 @,@ 043 ft ) high , with a cliff face of 250 metres ( 820 ft ) . Its sister cliff is the 250 metres ( 820 ft ) Little Hangman , which marks the edge of Exmoor . The coastal hills reach a maximum height of 314 metres ( 1 @,@ 030 ft ) at Culbone Hill . 
