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.