Introduction

This guide explains the geology and Earth science of Exmoor. It describes the underlying sedimentary rocks, how they were formed, and how they have influenced the landscape. It includes information on geological history, rock types, fossils, mining, and a glossary of geological terms.

Geology and the Landscape

Exmoor is known for its extensive views across moorland and pasture. The hills are generally smooth, level-topped and often form long ridges running from south-east to north-west. These ridges reflect the orientation of the underlying rock layers.

Rock exposures on the uplands are limited. Former quarries, mines and excavations used for building stone, lime production and ore extraction are often overgrown or filled in.

The coastline provides some of the best geological exposures. Between Minehead and Baggy Point, cliffs reveal a sequence of rock layers that allow geologists to study much of Exmoor's geological history. Differences in rock hardness influence the shape of the coast. Harder rocks form prominent features such as Hurlstone Point and Culbone Cliffs, while softer rocks are more easily eroded.

Coastal Safety

Visitors should take care when accessing beaches:

• Tides rise quickly and the tidal range can be large.
• Always ensure there is a safe route off the beach.
• Cliff faces may be unstable and dangerous.
• Stay on recognised paths.
• Seek local advice about routes and tide conditions.

Geological History

Most of Exmoor is underlain by Devonian rocks. The Devonian Period lasted approximately from 410 to 360 million years ago. These rocks were formed mainly from sediments deposited in shallow seas, lakes and river deltas, although some were deposited in desert environments.

Over time, layers of mud and sand hardened into rock. Later, two crustal plates collided, compressing and folding the rock layers. This created major arches (anticlines) and troughs (synclines) across the region. Many smaller folds were formed during the same process.

The oldest rocks are exposed around the Valley of Rocks on Exmoor's northern side. Progressively younger rocks are found towards the south.

Later Geological Development

After the Devonian Period:

• Devonian rocks were eroded.
• New Red Sandstone sediments were deposited under arid, desert-like conditions.
• During the Jurassic Period, the sea returned and marine sediments containing abundant fossils accumulated.
• More recent deposits include peat, river alluvium, blown sand and glacial deposits.

Evidence of the Ice Age

During the last Ice Age, ice sheets probably reached the coasts of North Devon and West Somerset. Freeze-thaw processes shattered rock and caused debris to move downslope, forming deposits known as "Head".

Rock Sequence and Geological Structure

Normally, younger rocks are deposited above older rocks. Determining the sequence on Exmoor is complicated because many rock layers have been folded and overturned.

The folds become less severe towards the north, suggesting the main compressive force came from the south. Geological evidence indicates that folding occurred around 300 million years ago.

Many shales were transformed into slates by pressure during folding. These slates developed cleavage planes along which they split easily. Sandstones and limestones were less affected by deformation.

Depositional Environments

The characteristics of sediments and fossils help geologists understand ancient environments.

General principles include:

• Coarse sediments are usually deposited nearer to land.
• Fine sediments settle farther offshore.
• Fossils are more commonly preserved in water-laid sediments.
• Coral fossils indicate deposition in clear water away from major river sediment sources.

Geological Map and Cross-Sections

The original publication contains a geological map of the Exmoor region and cross-sections showing the arrangement and folding of rock layers across the landscape. These illustrations demonstrate how sedimentary rocks were deposited, folded by tectonic forces and later exposed through erosion.

Accessibility note: Alternative text descriptions should be added if the map and diagrams are reproduced online.

Rocks of Exmoor

Jurassic and Triassic Rocks

These include:

• Lower Lias
• Watchet Beds
• Langport Beds
• Cotham Beds
• Westbury Beds
• Sully Beds
• Tea Green and Grey Marls
• Keuper Marl
• Upper Sandstone and Pebble Beds

These rocks contain abundant fossils, particularly ammonites, and were deposited in shallow seas, lagoons, rivers and scree environments.

Carboniferous Rocks

Culm Measures

Mainly purple sandstones containing:

• Plant fossils
• Fish fossils
• Thin coal seams

These formed in river delta and coastal lagoon settings.

Devonian Rocks

Pilton Beds

Sandstones and shales containing fossil shells and trilobites. Deposited in shallow marine conditions.

Baggy and Marwood Beds

These rocks show evidence of wave action and burrowing organisms. Plant fossils occur at some inland sites. They are thought to represent river delta and lagoon environments.

Pickwell Down Beds

Purple, brown and green sandstones. A volcanic ash layer has yielded fragments of armoured fish. Deposited near river mouths.

Morte Slates

Smooth grey and purple slates with occasional sandstone layers. Deposited in shallow seas. Fossils are uncommon.

Ilfracombe Beds

Slates, limestones and sandstones containing fossil corals and crinoids. Deposited in shallow marine conditions.

Hangman Grits

Sandstones showing ripple marks and channels. Occasional shell and plant fossils indicate deltaic and desert environments.

Lynton Beds

Dark blue-grey slates and sandstone layers. Deposited in shallow seas that became progressively shallower over time.

Mining on Exmoor

Mining was once important across Exmoor and the Brendon Hills, although no mining currently occurs within the National Park.

Major minerals extracted included:

• Iron
• Copper
• Lead
• Silver
• Zinc
• Antimony
• Manganese
• Gold

Iron mining reached a peak production of 46,000 tons annually from the Brendon Hill Mines in 1877.

Silver mining at Combe Martin dates back to at least the thirteenth century. Silver from the area was worked extensively during the reign of Elizabeth I and was later made into decorative items purchased by Queen Victoria.

Most mining ceased by the early twentieth century because operations became uneconomic. Since Exmoor became a National Park in 1954, proposals for new mining have generally been opposed because of concerns about impacts on landscape, traffic, noise and pollution.

Mineral exploration has continued in some areas, and uranium has been discovered near Simonsbath.

Glossary

Alluvium
Loose material deposited by rivers.

Basal
At the base or beginning of a rock sequence.

Bioturbation
Disturbance of soft sediment by burrowing organisms.

Crustal Plates
Large sections of the Earth's crust that move slowly over time.

Dip
The angle between a rock layer and the horizontal.

Haematite
An iron ore mineral (iron oxide).

Lenticular
Lens-shaped or flattened-oval in form.

Lode
A sheet-like body of minerals, particularly metallic ores.

Matrix
Fine-grained material in which larger objects are embedded.

Mould
An impression left by an organism or fossil.

Period
A major subdivision of geological time.

Strata
Successive layers of rock.

System
A group of related rock layers from a particular period.

Trace Fossil
Evidence of an organism's activity, such as tracks or burrows, rather than the organism itself.

Tuff
Rock formed from volcanic ash.

Unconsolidated
Not yet hardened into rock.