 The Somerset Levels run from the coast up to 30 kilometres ( 19 mi ) inland . These wetlands cover 600 square kilometres ( 232 sq mi ) , most of which is no higher than 8 metres ( 26 ft ) above sea level . There are coastal marine clay deposits , and further inland there are many peaty areas . Dotted within this wetland landscape are slightly raised inter @-@ glacial " islands " called burtles . These have been settled from the Mesolithic onward , with wooden causeways linking them to higher ground . There are also rocky outcrops , such as Brent Knoll and Glastonbury Tor which have also housed ancient settlements . 
