Exmoor is underlain by sedimentary rocks such as sandstones and slates. Originally, these were mostly laid down in the shallow waters of seas, lakes and river deltas during the Devonian (417 — 354 million years ago) and Carboniferous (354 — 290 m.y.a.) periods.
So why is Exmoor now such a dramatic upland landscape? The explanation can be found in the ancient movement of the Earth's crust. A period of tectonic plate collision caused intense pressure resulting in the rocks being compressed, folded like a concertina and faulted. Exmoor is an upfaulted block of harder Devonian rocks which contrasts with the softer rocks of the Carboniferous age to the south. During the subsequent eons there has been a considerable degree of erosion which has sliced through layers of rocks of different ages, meaning that the geology of a variety of times is now exposed in the cliffs and steep valley sides.
Exmoor is a disected plateau of Devonian sedimentary
rock, rising to 517 m at Dunkery Beacon. It extends into Devon but the
majority of the area is in Somerset. Much of the area is a National
Park.
The landscape is one of rounded hills, with hogs-back cliffs at the coast
due to geological movements. Because of high rainfall there are boggy
areas and the part by the Chains is the wildest in Somerset. The action
of streams has cut combes through the hills down to the sea, which are
now wooded. Much of Exmoor is open heathland. There is an outlier of
Exmoor at North Hill near Minehead. Iron working was formerly carried
out, probably from the Roman period onward.
The Exmoor 'plateau' is an area of unglaciated
upland (250-500m) approximately 200 million years old - some 180 million
years older than the Alps in Europe. It is thought to rank among the
oldest features on the Earths surface
Exmoor is made up of sedimentary rocks (grits, siltstones,
sandstones, mudstones) which show a slaty cleavage; the rock has been
altered under pressure so that it is split into flat plates. They are
mainly of Devonian Age. They are special because they show an uninterrupted
sequence through the Devonian into the succeeding Carboniferous period
(385-355 million years ago). Devon and Exmoor was the first area where
rocks of this age were studied - hence the name.
Unusually smooth and convexly rounded hillsides are a special
Exmoor feature. The Hogs-back cliffs owe much of their shape to weathering
and erosion during the Ice Age.
Changes in sea-level are responsible for other landform
features such as the Valley of Rocks (Lynton),
the coastal waterfalls, the Lyn Delta, the cobble or shingle ridge and
submarine forest at Porlock. There are
unusual knolls within inland valleys such as Cow Castle (beside the River
Barle below Simonsbath), Flexbarrow
and Alse Barrow which appear to be fragments of river terraces.
Most of Exmoor is underlain by Devonian sandstones, slates and shales, with minor amounts of limestone. The oldest rocks, the Lynton Slates, occur in the core of an anticline extending eastwards from Lynton. Successively younger Devonian rocks comprise the Hangman sandstone, Ilfracombe Slates (with thin limestones), Morte Slates, Pickwell Down Sandstones, Upcott Slates, Baggy Sandstones and Piton Shales. They were extensively and complexly folded and faulted during the Variscan earth movements about 300 million years ago. On the eastern side of the moor the Permo-Triassic rocks of the Vale of Porlock contrast with the surrounding hills of Devonian sandstone. They fill a fault-bounded basin of red Mercia Mudstone and Luccombe Breccia. At Selworthy, there are Jurassic Blue Lias shales and limestones. At the western extremity of the area, Braunton Marsh and Braunton Burrows are underlain by extensive areas of Quaternary deposits, including blown sand, alluvium and river terrace deposits. In Porlock Bay, a fine shingle ridge is present.
The Exmoor plateau has been deeply incised by short rivers flowing down its steep northern side and in places cascading over the high coastal cliffs as waterfalls. To the south, where Devonian rocks abut the Carboniferous shales and grits of the Culm Measures, larger deep, very steep-sided valleys form the headwaters of the Barle, Quarm and Exe.
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 running approximately south east - north west. The ridges follow the alignment of the rocks underneath.
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. The names of the main groups are made up of the rock and a place where they can be studied, 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 are made up of harder rock than Porlock Bay.
xmoor creates a diamond on the geographic landscape straddling North Devon and Somerset with approximately 2/3rds of the area in the latter county. Its area stretches from Elworthy to the East and Combe Martin in the West, from Lynmouth in the North to Exebridge in the South. Much of its Northerly backbone rises to over 1,300ft that peaks at 1,705ft at Dunkery Beacon. Cheek by jowl with the sea, yet its remote rolling moors transport you to peaceful isolation. Steep sided combes and goyles carve through the landscape, sometimes with rocky outcrops with sparse vegetation, others densely wooded; trees desperately clinging to the sides. Bright sun with stark skies frequently gives rise to 'Exmoor moods'. Most of the rocks found on Exmoor are considerably old dating back to the Devonian period of the Palaeozoic era some 400 million years ago, and even the more recent rocks are from the Mesozoic Era the Triassic and Jurassic just a mere 250 million years old. A fuller detail of Exmoor's Geology can be found in a concise introductory book of that title by R.A.Edwards.
See National Park Leaflet |
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