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| Cornwall (inc Isles of Scilly) |
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| Overview |
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The geology of Cornwall is dominated by rocks that belong to the Devonian and Carboniferous Periods of geological time. In Cornwall the Devonian rocks comprise mudstones and volcanic lavas and ashes (tuffs) that were originally deposited in a deep water basin which was bordered to the north and east by shallow shelf seas and the Devonian Old Red Sandstone continent.
During the Carboniferous marine conditions persisted and thick deposits of mudstones, sandstones and some limestones were deposited. During the latest part of the Carboniferous, movements in the Earth’s crust led to the intrusion of a large body of granite into the Devonian and Carboniferous rocks, which were also folded and faulted under the intense heat and pressure. The outcrops of the granite give rise to acidic soils which support moorland, peat bogs and acid grassland. The geological activity of the Late Carboniferous was related to the closure of the ocean in which the marine Carboniferous sediments were deposited. There is direct evidence for the closure of the ocean in the form of the rocks of the Lizard Peninsula, which represent the remnants of a former piece of ocean crust.
Apart from rocks of the Lizard Peninsula and the Devonian and Carboniferous slates and mudstones, there is a small outcrop of marine sands and clays of Tertiary age at St. Erth. The last two million years of time, covering the so-called Ice Age, is represented in the County by raised beach deposits and shattered rock debris (known as head) in valley bottoms and on slopes formed under tundra-like conditions. |
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| Pre-Cambrian (?) |
The rocks of the Lizard Peninsula form one of the most interesting suite of rocks in England. A Pre-Cambrian age (older than 600 million years) has been assigned to them, although their exact date is difficult to pin down. The southern half of the Peninsula comprises a complex array of metamorphic and igneous rocks including serpentinite, schist, gabbro and granite, which are faulted up against Devonian slates along a fault running roughly between Mullion Island and Nare Point. The serpentinite rock represents the remnants of part of an ancient ocean floor into which the other igneous rocks were intruded. This whole complex was, due to movements of the plates of rock covering the Earth’s surface, thrust up against continental rocks during the middle Devonian, resulting in the “welding” of the ocean floor rocks onto the continental landmass.
The unique geology of the Lizard peninsula, particularly serpentinite, gives rise to a similarly unique range of heathland and wetland habitats that support many rare plants and invertebrates. |
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| Devonian |
During the Devonian (417-354 million years ago) Britain was part of a large continental mass that included Europe and North America, which lay approximately 10o south of the Equator. What is now Cornwall was at this time located on the margin of an ocean; termed the Rheic Ocean. The coastline stretched approximately from the Bristol Channel to the Thames estuary (or at least where they are now). In these marine waters a thick sequence of muds and limestones were deposited. The marine Devonian succession shows a progressive deepening. The Lower Devonian Dartmouth Slates (slates and siltstones) are rich in fossil fish, trilobites, crinoids and sponges all of which are more typically shallow water. In the Middle Devonian deeper water conditions have produced mudstones, thin limestones and slates which rarely yield fossils.
Upper Devonian rocks in Cornwall vary in their nature across the County. In the south, around the Lizard, slates, limestones and conglomerates occur. These rocks include fragments of older rocks derived from a land mass just to the south. Further to the north (Port Isaac-Padstow area), deeper water mudstones and limestones were deposited. In the Padstow area, the Upper Devonian was marked by large outpourings of underwater lava (known as pillow lavas), which can be seen exposed in the cliff section at Pentire Point. |
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| Carboniferous |
In Cornwall, Lower Carboniferous rocks extend in a narrow belt from the coast south of Boscastle to Launceston. On the coast these comprise thin black shales, sandstones and lavas and volcanic ash (tuff) which were slowly deposited in the marine basin. These rocks are intensely folded and faulted. Volcanic activity was widespread during Lower Carboniferous times and extensive sheets of lava and tuff occur in north Cornwall and today masses of this lava give rise to prominent land features in the Launceston area.
In north-east Cornwall, between Tintagel and Widemouth, the spectacular cliff section exposes the contorted dark grey shales and thin sandstones of the Crackington Formation. This thick sequence of rocks, which forms part of the so-called Culm Measures of North Cornwall and North Devon, were deposited in a relatively deep sea basin. The muds that now form the slates were deposited on the floor of the basin, which was occasionally disturbed by currents of muddy water carrying sands and even pebbles that swept down the flanks of the basin. These turbidity currents and the deposits they produced may represent the product of earthquakes which disturbed unstable sediments on the submarine slopes flanking the basin. The Crackington Formation is overlain by the more massive sandstones with siltstones and shales of the Bude Formation, which forms the cliff section north of Widemouth and inland to the border with Devon. Where the Culm Measures outcrop, they give rise to heavy acidic soils, making farming difficult. Reflecting this and the oceanic climate, the predominant land use is grass production for livestock, but where this is not intensive then areas of species-rich habitat known locally as Culm Grassland still occurs.
The entire Carboniferous (and Devonian) succession within Cornwall were subjected to intense compressive forces and heat during the mountain building period know as the Variscan Orogeny (late Carboniferous to early Permian) which has resulted in the spectacular folding and faulting of the Carboniferous strata. The culmination or late stages of the Variscan Orogeny was marked by the intrusion of the five large granite bodies or “plutons” that now form the Isles of Scilly, Land's End, Carnmenellis, St Austell (or Hensbarrow) and Bodmin Moor. These granite bodies are, in fact, the upper surface of a much larger and single parent body, the Cornubian Batholith, which extends from the Isles of Scilly to Dartmoor. This body is often interpreted as the product of the melting of part of the Earth’s crust caused by continent-continent collision. Fluids escaping along fractures from the cooling granite bodies were responsible for depositing a wide range of minerals including metal ores, including those rich in tin, copper, lead and zinc. There is a close geographical connection between the granites of Cornwall and the location of important mining districts. These areas of granite provide a strong geological contrast with the surrounding Devonian and Carboniferous shales and mudstones. This contrast is reflected by the heathland vegetation, granite landforms and upland acid moorland that these granites support as opposed to the more fertile grasslands found on the surrounding slates. |
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| Tertiary (comprising Neogene
and Palaeogene)
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| At St Erth in west Cornwall and Crousa Down on the Lizard, there are small outcrops of sand and clay of possible Tertiary age. The St Erth Beds comprise sands and clays which have yielded marine molluscs of Pliocene, or early Pleistocene age (1.5-2 million years old) age. These sediments were deposited in a shallow sea that covered this part of south-west Britain and which cut an erosional platform into the surrounding landmass. Subsequent uplift of the land has led to this platform being located at a present-day height of about 130m. |
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| Quaternary |
Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets have been instrumental in forming the landscape we see today.
Ice sheets did not reach as far south as Cornwall. However, the area was subject to arctic-like conditions during the glacial periods and the repeated changes in sea-level have left their mark around the coastline. During the glacial periods, large amounts of water were locked up in the ice sheets to the north and as a consequence sea-level was up to 100 m lower than its present level. During the warmer interglacial periods, water was released from the ice and sea-level was higher than today. Coastal features such as old sea cliffs and raised beaches, produced by these sea level fluctuations, are found along the Cornish coast and in the Isles of Scilly. The arctic conditions also led to the formation of large amounts of shattered rock, known as head, which now occupies valley bottoms and slopes. Areas of fine silt, known as loess, occur in some locations, notably the Lizard Peninsula. This material was wind transported and derived from the extensive vegetation-free plains during the colder glacials. |
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| Geological Highlights: |
- The granitic rocks of Cornwall are enriched in natural
radioactive isotopes, including those of thorium and potassium
and, particularly, of uranium. The energy from the decay
of these radioactive elements is dissipated as heat which
has led to the exploration of the South West granites as
a source of geothermal energy.
- The circulation of hot aqueous fluids associated with
the emplacement of the Cornish granites has led to the formation
of mineral deposits. Veins within and close to the granites
mostly carry tin, in places with tungsten and arsenic. In
the rocks immediately around the granites copper and arsenic
may be found at some distance from the granite outcrops.
In addition to the metals and minerals listed above, Cornwall
has yielded iron, manganese and antimony as well as small
amounts of uranium, bismuth, nickel, cobalt, molybdenum
and gold.
- Apart from the development of mineral veins, fluid circulation
in the granites has, in a number of places, resulted in
the breakdown of the feldspar minerals in the granite and
their replacement by the clay mineral kaolinite. This alteration
is very patchy, but where complete, the rock becomes soft
and friable and the clay can be selectively removed by hydraulic
mining. Kaolin alteration is a feature of many areas of
the granites, but is particularly extensive in the western
part of the St Austell Granite which is an important centre
for the china clay (kaolin) industry. Britain is one of
the world’s largest producers and exporters of china
clay.
- There has been a long history of mining and quarrying
in Cornwall, particularly for tin, copper, arsenic and lead.
The earliest tin ore production was probably from river
gravels and other surface deposits, while in more recent
times, deep mining of veins has been of greater importance.
The last remaining deep tin mine to operate was South Crofty,
which closed in 1998. Copper is the second most important
metal worked from the Cornish veins. Systematic mining of
the metal does not appear to have commenced before the 16th
Century. In the first half of the 19th century production
increased considerably and more than 40% of the world’s
output was obtained from Cornwall and Devon. The production
of arsenic from the mines of Cornwall has left a considerable
legacy of pollution stemming from waste dumps and from the
effects of former refining operations.
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| Local sites |
| The following localities represent, in part, the geology of this county. Each locality has a grid reference, a brief description of how to get there and a short summary of the geology you are likely to find. All the localities listed are openly accessible. |
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Kynance Cove (part of the West Lizard SSSI)
Boscastle-Widemouth (SSSI) |
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