 Exmoor is an upland area formed almost exclusively from sedimentary rocks dating from the Devonian and early Carboniferous periods . The name of the geological period and system , ' Devonian ' , comes from Devon , as rocks of that age were first studied and described here . With the exception of a suite of Triassic and Jurassic age rocks forming the lower ground between Porlock and Timberscombe and from Minehead to Yarde ( within Exmoor National Park but peripheral to the moor itself ) , all of the solid rocks of Exmoor are assigned to the Exmoor Group , which comprises a mix of gritstones , sandstones , slates , shales , limestone , siltstones and mudstones . Quartz and iron mineralisation can be detected in outcrops and subsoil . The Glenthorne area demonstrates the Trentishoe Member ( formerly ' Formation ' ) of the Hangman Sandstone Formation ( formerly ' 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 . As this area of Britain was not subject to glaciation , the plateau remains as a remarkably old landform . The bedrock and more recent superficial deposits are covered in part by moorland which is supported by wet , acid soil . 
