The following pages are concerned with the Earth Science of Exmoor. The pages explain the underlying structure of sedimentary rocks and explains how and when this developed. Click the link below for further information

Geology & the Landscape

Building stone

Geological Features

Click one of the links below to read further information about individual features on Exmoor. Underneath the Links is some more information about Exmoor's features.

Structures

Fossils

Minerals

Drift

Environment

Geological Glossary

Rocks

The moors of Devon and Cornwall are mostly of granite and Exmoor is unusual in the West Country in being of an upland of sedimentary rocks. These are classified as grits, sandstones, siltstones and mudstones according to particle size. Most show a slaty cleavage - lines of fracture caused by pressures of earth movements. They are largely of Devonian age. A special feature is that they show an uninterrupted sequence through the Devonian and into the succeeding Carboniferous period.

Earth movements have created a variety of structures within the rocks such as folds and faults. The Exmoor coast is a good place to study such features because of their variety within a small area. The movements also concentrated minerals within the rocks and caused volcanic activity that introduced other minerals to the area. Few of the minerals are unusual but their exposure along the coast provides opportunity for their study.

The sequence of rocks shows changing environments within the Devonian period. Throughout much of that time the area where Exmoor’s rocks were forming was on the southern edge of a mountainous continent in tropical latitudes. The rocks were laid down in a variety of conditions that changed with coastal and land erosion, continental drift and earth movements. Conditions included deep and shallow seas, rivers, estuaries, deltas, temporary lakes and deserts. Such conditions are indicated by the size and shape of the grains in the rocks, the way they are bedded, structures such as ripple and sole marks and the types of fossils they contain. Fossils on Exmoor are not generally well preserved because of the crushing and movement of the rocks after they were formed but the fossils that can be found are important for dating and comparing the rocks with other areas as well as providing information about past environments.

In more recent times Exmoor was close to the edge of ice sheets, producing what is known as a periglacial climate. In this climate the ground was frozen for long periods and ice activity produced weathering and erosion of rocks resulting in loose materials known as drift. Within this drift are structures that show much about the activity of ice and the changing conditions within the Ice Age.

Landforms

Exmoor National Park consists of Exmoor, the Brendon Hills and the Vale of Porlock. Within the National Park Exmoor can be defined as a plateau ranging from about 250m to 519m in altitude, but the plateau extends beyond the park boundary to the south and to the Brendon Hills in the east. Exmoor is unusual in Britain in being an un-glaciated upland. As a result the plateau remains as a remarkably old landform and may rank amongst the oldest features on the Earth’s surface. The southern part of the Vale of Porlock was eroded at least as long ago and is also a remarkably ancient feature.

Unusually smooth and convexly rounded hillsides are a special feature of Exmoor. The hog’s back cliffs, typical of West Country coasts, are a related feature, owing much of their shape to weathering and erosion in the Ice Age.

Many special features can be related to changes in sea level. These include the coastal waterfalls, The Valley of Rocks, the Lyn delta, the cobble ridge and submarine forest at Porlock. Inland there are unusual knolls within valleys, such as at Cow Castle, Flexbarrow and Alse Burrow, which appear to be fragments of river terraces, also related to sea level changes.

Important sites

The most important geological sites in Britain were listed in a survey known as the Geological Conservation Review. On Exmoor all such sites have been incorporated into Sites of Special Scientific Interest or became Geological Sites of Special Scientific Interest. There are four of the latter sites within the National Park. At Glenthorne (13ha, 33 acres) there are unusual freshwater deposits in the Hangman Grits, which were mainly formed in desert conditions. At Dean Steep (2ha, 5 acres) the uninterrupted transition between Lower and Middle Devonian rocks is well illustrated and at the River Lyn (17 ha, 42 acres) the valley of a tributary of the West Lyn and the Glen Lyn Gorge show clear erosional and depositional features relating to the flood of 1952. At Combe Martin Bay, part of a SSSI stretching out of the National Park to the west, rocks are of national importance for showing changing environments within the Middle Devonian period from dry land through shallow and deep sea deposits and clearly illustrating a variety of folds of different sizes. Its limestones also provide valuable marker horizons to correlate with other areas where exposure is poor or sequence interrupted. Thus Devonian rocks on Exmoor can be used to date rocks of similar age throughout the world.