GEOLOGICAL FEATURES

Structures

Folding and Mineralisation

At the end of the Carboniferous period, about 290 millions years ago a great continent moved up from the south moved up to collide with the continent on the edge of which Exmoor's rocks were forming. The collision forced the edge of the southern continent under the northern continent. At depth the light surface rocks melted and welled up under the surface to cool slowly as the granites of Dartmoor and Cornwall. At the same time hot solutions of minerals were forced through the Exmoor rocks, sometimes working themselves into the cracks caused by the folding. Other minerals appear to be older as they form beds within the rocks, suggesting that they were laid down at the same time or separated out because of the chemical conditions at particular depths in the sediments. An effect of folding has been to concentrate these minerals in the apexes of the folds to produce what are known as saddle reefs. This has happened with the silver minerals on the coast at Combe Martin.

Bedding and Cleavage

Bedding and Cleavage Crapstone

Exmoor's rocks were laid down in layers known as beds. Often there are slight differences between the beds. For instance most of the sediments laid down on the sea bed came from rivers wearing away adjacent mountain ranges. At times of flood quantities of coarse sediment would be brought down and at other times the rivers would carry finer sediment. These built up as distinctive horizontal layers of different sediments. Later earth movements would tilt and buckle the beds. The same earth movements and the depth of rock itself caused great pressures in the rocks. In places the pressures transformed the structure and chemistry of the minerals within the rocks. The clay minerals in the finer sediments such as mudstones and siltstones were often changed into minerals with a platy structure, producing layers at roughly right angles to the direction of pressure. The rocks tend to break along these layers, which are known as 'cleavage'. In this way shales are transformed into slates and many Exmoor rocks have a 'slaty cleavage'. Such rocks are known as 'metamorphic' or changed rocks. The cleavage is often mistaken for bedding and is sometimes referred to as 'false bedding'. It usually cuts across the bedding, especially in folds and the bedding is best distinguished by changes in colour and texture, as in the photograph.

Minor Thrust Folds

Minor Thrusts Wildersmouth

Most of Exmoor's rocks were laid down as layers or beds one on top of the other. At the end of the Carboniferous period, about 290 million years ago, there was a great period of earth movements and mountain building known as the Caledonian orogeny, because it particularly affected Scotland. Exmoor's rocks were forming in a sea between two continents and the continents collided, forcing the rocks out of the sea and cracking and buckling them in the process. Close to the collision point to the south and west of Exmoor the effects were very dramatic and the rocks were squashed like a concertina. On Exmoor itself they tended to be more gently folded. However, different types of rock reacted in different ways. The finer, more thinly bedded rocks tended to shear whereas the coarser, more thickly bedded rocks tended to form broad folds. Within the Ilfracombe beds are many alternating thin layers of coarse and fine rocks and limestones resulting in tiny folds. The great pressures during the folding have caused changes in the minerals in the rocks so that some have become slaty and others, the limestone in particular, have become distorted and moved like plastic.

Fossils

Devonian Corals

Rillage Point Devonian Corals

The continents are formed of relatively light rocks which behave as if they were floating on the heavier rocks underneath. Thermal currents moving deep inside the Earth cause very slow movements of these surface rocks, resulting in what is known as continental drift. When most of Exmoor's rocks were forming 350-400 million years ago, the area was lying in the tropics well south of the equator and has since drifted northwards. Many of the rocks were laid as sediments in shallow seas and, at times when there was little sediment being brought to the sea from the neighbouring land, coral reefs grew in the clear, warm waters. The reefs now appear as thin bands of limestone, particularly within the Ilfracombe Beds. It is in these bands that most local fossils are found. The one in the picture is a solitary coral, Pachypora. Others formed in reef-building colonies although each coral is in itself a colony of small animals.

Chondrites

Rillage Point Tentaculites

Most of Exmoor's rocks belong to the Palaeozoic era, the time when it was originally thought that life began on Earth. As a result, the creatures around at the time tended to be small and simple. Many had no hard parts which could later be preserved as fossils. There was little life on land and some Exmoor rocks were formed in lifeless deserts. However, fossils, including some of the earliest fish fossils, can be found in Exmoor rocks. Many are misshapen or fragmented because of the great pressures and shearing of the rocks during earth movements. Of the few that survive, those known as Chondrites are probably the most common. These are known as trace fossils because they are the traces of creatures rather than the creatures themselves which have been preserved. As a result it is difficult to guess what sort of creatures left these marks in the rocks. They used to be called tentaculites, then fucoids because people thought they were fossilised tentacles or seaweeds. Some think they are the droppings of fish but most now think that they are the burrows of worms or crustaceans.

Minerals

Silver/Lead Vein, Camel's Head, Combe Martin

Camels Head Silver Lead Vein

Combe Martin is famous for its silver mines but silver is only a very small proportion of the other minerals found there. It is usually found in galena, a lead mineral similar in colour to silver. The two minerals were difficult to separate, particularly at the height of the mining between the 14^th and 16^th centuries, when the ore was sent to south Devon for processing. These minerals tend to be concentrated in folds in the shales of the Ilfracombe Beds and are accompanied by strings of quartz, which miners followed to locate the pockets of valuable minerals.

The Camel's Head is a rock on Combe Martin beach so named from its appearance from a distance. A mineral vein runs through the rock and adjoining headland. It has been quarried where it outcrops and an adit was cut into the headland at right angles to the vein but no silver was found there.

Drift

Raised Beach and Head, Lee Bay

Lee Bay Raised Beach and Head

During the Ice Age the sea level dropped during the cold, glacial stages when ice caps formed and rose again when the ice melted in the warm, interglacial stages. During the last interglacial period, about 100,000 years ago, the sea level rose to about 25 feet/ 8 metres higher than present sea level. At Lee this resulted in a beach forming in the bay at the mouth of the Lee Stream. Beaches of this height and age are sometimes known as Patella beaches after the limpet shells they contain. During the following glacial period the sea level dropped and the beach was left high and dry. The continual freezing and thawing of the ground in the cold climate caused soil slippage known as solifluction. A mixture of soil and scree known as head ran down the valley sides to cover the beach. The head has partly protected the beach from erosion but landslipping is frequent in the loose material.

Submerged Forest, Porlock Weir

Porlock Weir Submerged Forest

From the end of the last glacial period, about 10,000 years ago, the melting of ice caps has caused the sea level in the Bristol Channel to rise about 40 metres. It has been at roughly its present level for the last 2000 years but is still rising very slowly.

Between 7000 and 8000 years ago the area that is now Porlock Beach was more than five miles inland. It was a flat, low lying area and the climate was warm and wet. The area was thickly wooded and Mesolithic people lived by hunting and fishing. They probably hunted wild cattle, the bones of which have been found here. The stumps of trees were preserved in the marshy conditions in which they grew and have today been revealed as the sea has risen to erode them. At low tide can be seen tree trunks, a thin layer of peaty soil and a large amount of grey clay soil which is now inhabited by sea shells known as piddocks.

Terminal Curvature, Hangman Grits, Scob Hill

Hangman Grits Scob Hill Terminal Curvature

At times during the Ice Age Exmoor experienced what is described as a periglacial climate. This means that Exmoor was not covered by ice but was sufficiently close to ice sheets to have an extremely cold climate. In this climate the soil was permanently frozen at depth but the surface would thaw out on warm summer days. With little vegetation to hold it together, the topsoil on slopes slipped down over the frozen layer as a mudslide. At the same time any rock near the surface would be broken by the continual expansion and contraction of the water in it as it froze and thawed. The broken rock was carried downhill by the mudslides to fill valleys with a loose material known as head. The slipping caused the layers of rock nearest the surface to be bent over in the direction of the slope, forming what is known as terminal curvature.

Environment

Unconformity, Little Hangman

Little Hangman Unconformity

A break in the sequence of rocks is known as an unconformity. Usually it is where layers of rock of different ages lie on top of one another and there appears to be a sequence missing in between. Exmoor's Devonian and Carboniferous rocks are remarkable in their lack of unconformity and show an unusually intact sequence for their age and the length of time they represent. However, they do contain some minor unconformities. The red Hangman Grits were largely formed on dry land as desert sands. However, some were formed at the mouth of a river and others in shallow seas. In a few places they were also formed in inland lakes and rivers. All were clearly formed in or near to the sea at a time when the coastline was changing and at times there was and at times there was erosion and at other times deposition. Here at Combe Martin red desert sandstones lie uncomfortably on top of grey slate sandstones formed in shallow waters.

Ripple Marks, Rugged Jack, Valley of Rocks

Rugged Jack Ripple Marks

Sedimentary rocks tell us much about the past environments in which they were formed. Exmoor's Devonian rocks each have different facies or characters which are due to the changing environment of the time. Many were formed in shallow seas just offshore but the coastline kept changing with erosion and deposition and as sea levels changed so that a particular area may be at one time in shallow water, at another in deep water and another again on dry land or under a river delta. These rocks pictured in the Valley of Rocks have ripple marks similar to those that may be found on a beach today, showing that they were laid down in shallow water. Other marks, known as sole marks, show currents from deeper water or shelving parts of the sea bed.

Geological Glossary