Geology

Haytor Quarries and Granite Railway SX 766 771 A classic symbol of Devon, Haytor is one of the most visited locations on Dartmoor and it is probably the most easily accessible of the large Dartmoor tors.

HaytorA short walk up the grassy slopes from car parks and bus stops alongside the road leads to the rocks. The panoramic views from around the rocks are well worth the walk and at the same time you can look at the 300 million years old granite.

Boulders of the granite can also be seen in walls and loose blocks near to the road. At a height of 457 metres and situated right on the eastern side of the moor, it provides excellent views of the coastline, the Teign estuary and the rolling countryside between, with the ridge of Haldon behind.

There are some old Haytor quarries to the north of the tor where the granite was worked for building stone. The unique Granite Tramway was used in the mid-1800’s to transport the stone from the moor to Teignmouth and sent for use in London’s buildings and elsewhere.

Blackingstone Rock SX 786 856 is a large tor situated in the eastern part of Dartmoor National Park. It exhibits many of the typical features of the Dartmoor Granite. Of particular note is that the coarse grained granite contains very large crystals of feldspar. The term ‘feldspar’ encompasses a group of pale coloured rock-forming minerals; Blackingstone Rock has examples up to several centimetres long. The tor also displays characteristic jointing.

The Granite Way SX 517 852 - SX 589 944 is an 11 mile cycle and walkway running between Okehampton and Lydford along the north western edge of Dartmoor. It is mostly traffic-free, largely following the course of the former Southern Region railway line. A journey along the Granite Way offers good views of the granite landscape of Dartmoor, as well as a number of specific sites of geological interest.

The impressive Lydford Gorge SX 503 839 has a depth of 35metres, is almost 2 km long and is of considerable importance for interpreting the geology of the local area. Within the gorge it is possible to see extensive exposures of mudstones, sandstones, limestones and cherts ranging in age from Upper. Devonian (c370 million years) to Lower Carboniferous age (350 million years). Some of these rocks contain important fossil remains that have proved crucial in dating the geology. However, perhaps the most impressive aspect of the site is the structure of the gorge itself. This provides a classic example of river gorge formation followed by ‘river capture’ and has many features associated with this process. These include the spectacular 27 metre high Whitelady waterfall and the exciting Devil’s Cauldron whirlpool, along with the imprints of potholes now many metres above the present river level.

Brent Tor SX 471 804 is one of the most impressive rock outcrops in Dartmoor. With St Michael’s Church at its top, it makes a distinctive and famous silhouette on the Dartmoor skyline.

BrentorThe Tor is unusual as it is one of the few on Dartmoor not to be made of granite. It is formed from basaltic lava which flowed some 350 million years ago into a shallow sea that covered the area during the Lower Carboniferous and Devonian periods. As the lavas flowed out into the sea some solidified into globular masses known as pillow lavas. Others were broken up by explosive contact with the sea water.

This lava formed a mound on the sea floor which was then eroded by sea currents with the resulting debris being washed down the slopes of the mound. Debris of this nature can still be seen loose on the southern slopes of the Tor.

Merrivale SX 540 760 located on the west side of Dartmoor, this site is home to a number of impressive landforms that are defined by the underlying geology and demonstrate the effects of weathering during the Ice Age. Dartmoor was never glaciated but still suffered the effects of the cold conditions, known as periglacial activity. Solid ice sheets came as far south as the coast of North Devon. The area has a number of tors, including Roos Tor, Cox Tor and the Staple Tors. These Tors were exposed when the overlying material was eroded by periglacial activity. They are surrounded by areas of clitter (rock debris) and blockfields (flat or only gently sloping areas of frost-shattered rock), and boulder runs are also present.

The slopes around Cox Tor display a number of unusual earth hummocks. These vary considerably in size and shape but can be up to two metres in diameter and about 0.5 metres high. They are generally composed of a fine silty loam that again resulted from weathering during the Ice Age.

Bellever Tor & Higher Cherrybrook Bridge Quarry SX 645 764 & SX 635 771 Situated in the centre of Dartmoor National Park, the area around Bellever Tor is easily accessible and is a popular site for walkers. It provides a good example of a hill-crest granite tor. Features seen here and at the nearby Cherrybrook Bridge Quarry illustrate the effects of weathering on granite and give a good indication of how the Dartmoor tors were formed.

From the adjoining Forestry Commission plantation, there is a pleasant and relatively gentle walk up the moorland slopes towards the summit of Bellever Tor. The tor shows well developed and flat-lying tabular jointing. Weathering has penetrated these joints causing the disintegration of the granite into large slabs which now form the debris, known as clitter that surrounds the tor. The tor is also cut by widely-spaced vertical joints which have weathered into broad gullies.

The effects of weathering on granite are also well demonstrated at the nearby Higher Cherrybrook Quarry, located by the roadside some 2km to the northwest of Bellever Tor. Here, the degree of loosening and weakening of the granite can be related to the proximity to the joints which, over millions of years, have allowed the rock to be attacked through the chemical and physical forces of water. In addition to its geological significance, the site offers excellent views over South Dartmoor. The area also exhibits many remains of prehistoric settlement, including tombs and stone rows and circles.

Upper and Lower Burrator Quarries SX 549 677 Located on the south-western edge of Dartmoor close to the impressive Burrator Reservoir are the disused quarries.

The Upper Burrator Quarry offers an opportunity to view a rare exposure of the contact between the Dartmoor granite and Devonian rocks. Indeed, veins of pink granite can be seen penetrating these rocks which were once slates resulting from the deep burial and intense deformation of mudstone originally laid down in marine conditions. However, the high temperatures that resulted from the intrusion of the hot granite transformed them into re-crystallised rocks known as hornfels. Minerals such as black tourmaline have been formed in the original slate.

The Lower Burrator Quarry exposes granite intersected by a series of joints. Joints are fractures in the granite resulting from stresses caused by cooling, by pressures from earth movements or by the removal of a load when the rocks once above the granite are eroded away.

Tavistock & Tavistock Railway Cutting SX 484 740 Tavistock, originally founded in 974 AD with the building of the Benedictine Abbey, has been greatly influenced by the local geology. The surrounding area once supported a thriving mining industry. Indeed, the extraction of minerals such as tin, copper and arsenic is documented as early as 1305.

Tavistock is now a part of the Cornwall and West Devon Mining Landscape World Heritage Site in recognition of the considerable legacy of this mining activity. During the nineteenth century the town was completely remodelled by the 7th Duke of Bedford, Francis Russell, and his steward John Benson using the profits gained from mining. In the process a number of impressive public buildings were built, as was model housing for workers - virtually unheard of at the time.

The buildings include the Cornmarket (1835) and the Guildhall (1863). Many were created using a variety of local materials including the green volcanic Hurdwick Stone, quarried just a few miles away, and granite from Pew Tor on Dartmoor. The workers cottages were made from brick and local rubble stone.

The rocks underlying Tavistock can be seen in several places in the town, such as Tavistock Railway Cutting (SX 4722 7413 – SX 4788 7448). Greenish grey Devonian slates occur in the southwestern part of the cutting (SX 4722 7413) and towards the road bridge. Beyond this bridge black slates, also Devonian, can be observed. At the north eastern end of the section is evidence of volcanic activity with lavas containing cavities caused by the expansion of trapped gasses and finely grained deposits of volcanic ash.

Morwellham Quay SX 445 697 Much of the Tamar Valley in west Devon was once home to a thriving mining industry. This industry needed a transport system and a number of small quays were built along the River Tamar to ship out the mined material. One such example is the restored Morwellham Quay, now an open-air museum and visitor centre.

Copper ore taken from the nearby George and Charlotte mine, first worked in the early 1700s, was shipped from here. This small mine has many features characteristic of the other mines found throughout the Tamar Valley but here you can actually journey underground and experience something of the working conditions of the miners during the 19th century.

Morwellham Quay is part of the Cornwall and West Devon Mining Landscape World Heritage Site.

Finding out more.

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