GEOLOGY AND THE LANDSCAPE
There are wonderful panoramic views from the high moorland and pastures of Exmoor. The hills are smooth, level-topped and often elongated to form ridges.
There is little rock exposure in the uplands. Quarries, scrapes and mines, originally dug to obtain local stone for building, lime or ore, are now overgrown or filled in. In contrast the dramatic beauty of the coast owes much to the variety of exposed rock types. There are magnificent cliff exposures between Minehead and Baggy Point that make it possible for a cross-section of all the strata to be sampled.
The strata are divided into named rock groups according to their main rock type and fossil content. Each name is made up of the rock and a place where it is exposed, e.g. Morte Slates. Variations in the hardness of rock are picked out by the sea. The prominent headland of Hurlstone Point and Culbone cliffs are good examples. The headland and cliffs made up of harder rock than Porlock Bay in between.
Geological history
Sedimentary rocks of the Devonian system lie under most of Exmoor. The name 'Devonian' refers to the fact that the rocks are common and accessible in Devon. It is also used to name rocks found elsewhere but formed at the same time i.e. between about 410 and 360 million years ago, the Devonian period.
Most of the rocks were deposited as layer on layer of mud or sand in the shallow waters of seas, lakes or river deltas. These layers gradually hardened into rocks and were later squeezed between two colliding crustal plates, one to the north and the other to the south. The intense pressure folded the rocks across the West Country into an east-west arch-like fold (anticline) and trough (syncline) within which were lots of smaller folds. The crest of the arch lies close to the coast of Exmoor but has been eroded to the west where the Bristol Channel now lies. The oldest rocks can be seen in the Valley of Rocks in the eroded core of the anticline along the northern side of the moor. Progressively younger layers become exposed towards the southern edge.
Small patches of other rocks, once more extensive, have survived erosion. In the Vale of Porlock and down the eastern edge of the exposed Devonian rocks there are some red, stony, sandy and silty sediments with no fossils. The larger fragments in these New Red Sandstone rocks are broken down pieces of Devonian rock. This shows that the New Red Sandstone sequence was formed after, and from the erosion of, the rocks of Devonian times. Their general nature suggests that the erosion took place on land in desert-like conditions.
After this period when the area was dry land, about 290 to 210 million years ago, the sea once again invaded at least part of the area, for rocks of Jurassic age (210 to 145 million years ago), with many marine fossils, are preserved around Selworthy and Minehead
Unconsolidated, surface deposits are widespread and of much more recent date. Some of these relate to the Ice Age when an ice-sheet probably pressed against the West Somerset and North Devon coasts.
Usually the ground would have been frozen but during times of thaw at least a surface layer melted and large quantities of frost-shattered rock fragments slid down-slope in a muddy matrix to form unsorted deposits on the lower valley sides. These deposits are known as 'Head'. River-laid alluvium covers the floors of some valley bottoms while blown sand forms Braunton Burrows and peat is found on some of the high moors.
Environment
The original nature of the sediments and their fossil content help us to work out what the conditions were like when the sediments were laid down. In water, coarser sediments settle out before finer ones. In the sea, they are, therefore, found beneath or closer to the rivers which brought them there than the finer ones. Fossils are most likely to occur and be preserved in sediments formed in water rather than on land. Because corals live in clear water their presence in local limestone suggests that they grew far from rivers.
Sequence
As might be expected, newer rock layers normally lie on top of older, and the rock key is arranged on this basic principle. However working this out was complicated towards the southern margin of the area because the strata are folded quite severely. The northwards overturning of the folds and their decreasing severity in the same direction shows that the dominant folding push came from the south. As rocks of Carboniferous age are also included in the folding but not the New Red Sandstone (Permo-Triassic) strata, the movements are dated at about 300 million years ago. The collision of the two crustal plates already referred to is thought to be responsible and explains why the older rocks lie in east-west bands across the area. Recognised changes in the sequence of fossilised life-forms also help to order the rock layers by age. There are some difficulties locally because fossils are generally infrequent and often distorted by the pressure of and shearing caused by earth movements.. The sandstones and limestones were changed little by the the earth movements but shales were hardened into slates with planes of weakness, known as cleavage, along which they tend to split.
Mining
No mining is carried out in the National Park area at the moment but remains of old workings can be seen in many places.
'Investigations have proved the existence of a series of parallel lodes running approximately E - W. The dip of the lodes varies from 40º to 60° South, and various experts' estimates have computed the volumes of ore at many millions of tons. Shafts have proved the existence of continuous lenticular lodes of high quality haematite ore.
South, and various experts' estimates have computed the volumes of ore at many millions of tons. Shafts have proved the existence of continuous lenticular lodes of high quality haematite ore.
This 1909 'Description of the Brendon Hills Iron Ore Mines' given to the South Wales Institute of Engineers indicates the general arrangement of the main iron-bearing lodes of the Exmoor area which clearly follow the geological structure. They extend from the Brendon Hills to south west of Simonsbath. Unfortunately the iron ore was not in continuous seams but in lens-shaped masses connected by clay or quartz-filled fissures. The main period of their exploitation was in the second half of the 19th century, with a maximum annual output of 46,000 tons from the Brendon Hill Mines in 1877. Some remaining structures together with old photographs give an idea of what the mines and the mineral railway built to serve them were like.
Mines near North Molton yielded copper as well as iron ores, with smaller amounts of the ores of lead, zinc, antimony, manganese - and gold! These minerals entered the Exmoor rocks about 300 million years ago following an upthrust of molten rock caused by the collision of the crustal plates. They crystallised from hot vapours or solutions pushed out of the main mass of molten rock which cooled to form the granites of Dartmoor and Cornwall.
Silver-lead workings at Combe Martin are recorded from the 13th century when Edward I's daughter, Eleanor, received a dowry of 270 lb. of Combe Martin silver. In the reign of Elizabeth I a new, highly profitable, silver lode was worked and in the 19th century several brooches of Combe Martin silver were bought by Queen Victoria.
Although most of the mining activity finished towards the end of the century as workings became uneconomical, occasional operations on the Brendons didn't end until 1910. Renewed investigations during the First and Second World Wars proved to be of no long term economic value.
In 1954 Exmoor became a National Park. Any new attempts at mining would be resisted by the National Park Authority because of possible ill-effects on the beauty of the scenery and on noise, traffic and pollution levels. However exploration for minerals has continued, particularly in the North Molton and Combe Martin areas, and uranium has been discovered near Simonsbath. There has been concern about the possibility of pollution from potential oil exploration in the Bristol Channel.