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Holsworthy Group
The Holsworthy Group is a late Carboniferous lithostratigraphic group (a sequence of rock strata) in north and east Cornwall and Devon in southwest England. The name is derived from the Devon town of Holsworthy. The Group comprises (in ascending order i.e. oldest first) the Crackington, Bideford and Bude formations. In the Launceston area the group is represented by the Bealsmill Formation. It was formerly known as the Upper Culm Group. The ''Crackington Formation'' is over 1km thick in the Exeter area but amounts to only 250m around Bideford. It consists of mudstones with lesser sandstones and siltstones displaying rhythmic bedding, typical of turbidite deposits. The overlying ''Bideford Formation'' is over 1200m thick in places and consists of interbedded sandstones, mudstones and siltstones with a range of different characters including turbidites, channel sandstones, cross-bedded and slumped sandstones and locally contains burrows and rootlets. The ''Bude Formation'' is cal ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chevron (geology)
Chevron folds are a structural feature characterized by repeated well behaved folded beds with straight limbs and sharp hinges. Well developed, these folds develop repeated set of v-shaped beds. They develop in response to regional or local compressive stress. Inter-limb angles are generally 60 degrees or less. Chevron folding preferentially occurs when the bedding regularly alternates between contrasting competences. Turbidites, characterized by alternating high-competence sandstones and low-competence shales, provide the typical geological setting for chevron folds to occur. Perpetuation of the fold structure is not geometrically limited. Given a proper stratigraphy, chevrons can persist almost indefinitely. Fold process In response to compressional stress, geological beds fold in order to minimize dissipation of energy. Given an unconstrained bed, folding does so by correspondingly minimizing bending and thus develops a sinusoidal geometry. In a stratigraphic sequence, b ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Lithostratigraphy
Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are primarily igneous or sedimentary relating to how the rock was formed. Sedimentary layers are laid down by deposition of sediment associated with weathering processes, decaying organic matter (biogenic) or through chemical precipitation. These layers are often distinguishable as having many fossils and are important for the study of biostratigraphy. Igneous layers occur as stacks of lava flows, layers of lava fragments (called tephra) both erupted onto the Earth's surface by volcanoes, and in layered intrusions formed deep underground. Igneous layers are generally devoid of fossils and represent magmatic or volcanic activity that occurred during the geologic history of an area. There are a number of principles that are used to explain the appear ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Geology Of Cornwall
The geology of Cornwall, England, is dominated by its granite backbone, part of the Cornubian batholith, formed during the Variscan orogeny. Around this is an extensive metamorphic aureole (known locally as killas) formed in the mainly Devonian slates that make up most of the rest of the county. There is an area of sandstone and shale of Carboniferous age in the north east, and the Lizard peninsula is formed of a rare section of uplifted oceanic crust. Coasts Cornwall forms the tip of the south-west peninsula of the island Great Britain, and is therefore exposed to the full force of the prevailing winds that blow in from the Atlantic Ocean. The coastline is composed mainly of resistant rocks that give rise in many places to impressive cliffs. The north and south coasts have different characteristics. The north coast is more exposed and therefore has a wilder nature. The prosaically-named ''High Cliff'', between Boscastle and St Gennys, is the highest sheer-drop cliff in Corn ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Carboniferous System Of Europe
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago (Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferous'' means "coal-bearing", from the Latin '' carbō'' ("coal") and '' ferō'' ("bear, carry"), and refers to the many coal beds formed globally during that time. The first of the modern 'system' names, it was coined by geologists William Conybeare and William Phillips in 1822, based on a study of the British rock succession. The Carboniferous is often treated in North America as two geological periods, the earlier Mississippian and the later Pennsylvanian. Terrestrial animal life was well established by the Carboniferous Period. Tetrapods (four limbed vertebrates), which had originated from lobe-finned fish during the preceding Devonian, became pentadactylous in and diversified during the Carboniferous, including early amphibian lineages ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Clast
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks by physical weathering.Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-5 Geologists use the term clastic to refer to sedimentary rocks and particles in sediment transport, whether in suspension or as bed load, and in sediment deposits. Sedimentary clastic rocks Clastic sedimentary rocks are rocks composed predominantly of broken pieces or ''clasts'' of older weathered and eroded rocks. Clastic sediments or sedimentary rocks are classified based on grain size, clast and cementing material (matrix) composition, and texture. The classification factors are often useful in determining a sample's environment of deposition. An example of clastic environment would be a river system in which the full range of grains being transported by ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life. About 20% to 25% of sedimentary rock is carbonate rock, and most of this is limestone. The remaining carbonate rock is mostly dolomite, a closely related rock, which contains a high percentage of the mineral dolomite, . ''Magnesian limestone'' is an obsolete and poorly-defined term used variously for dolomite, for lime ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Lava
Lava is molten or partially molten rock ( magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or underwater, usually at temperatures from . The volcanic rock resulting from subsequent cooling is also often called ''lava''. A lava flow is an outpouring of lava during an effusive eruption. (An explosive eruption, by contrast, produces a mixture of volcanic ash and other fragments called tephra, not lava flows.) The viscosity of most lava is about that of ketchup, roughly 10,000 to 100,000 times that of water. Even so, lava can flow great distances before cooling causes it to solidify, because lava exposed to air quickly develops a solid crust that insulates the remaining liquid lava, helping to keep it hot and inviscid enough to continue flowing. The word ''lava'' comes from Italian and is probably derived from the Latin word ''l ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chert
Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood. Chert is typically composed of the petrified remains of siliceous ooze, the biogenic sediment that covers large areas of the deep ocean floor, and which contains the silicon skeletal remains of diatoms, silicoflagellates, and radiolarians. Precambrian cherts are notable for the presence of fossil cyanobacteria. In addition to microfossils, chert occasionally contains macrofossils. However, some chert is devoid of any fossils. Chert varies greatly in color (from white to black), but most often manifests as gray, brown, grayish brown and light green to rusty redW.L. Roberts, T.J. Campbell, G.R. Rapp Jr., "Encyclopedia of Mineralogy, Second Edition", 1990. R.S. M ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Turbidite
A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing Turbidites were first properly described by Arnold H. Bouma (1962), who studied deepwater sediments and recognized particular "fining-up intervals" within deep water, fine-grained shales, which were anomalous because they started at pebble conglomerates and terminated in shales. This was anomalous because within the deep ocean it had historically been assumed that there was no mechanism by which tractional flow could carry and deposit coarse-grained sediments into the abyssal depths. Bouma cycles begin with an erosional contact of a coarse lower bed of pebble to granule conglomerate in a sandy matrix, and grade up through coarse then medium plane parallel sandstone; through cross-bedded sandstone; rippled cross-bedded sand/silty sand, and finally la ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Bideford
Bideford ( ) is a historic port town on the estuary of the River Torridge in north Devon, south-west England. It is the main town of the Torridge local government district. Toponymy In ancient records Bideford is recorded as ''Bedeford'', ''Byddyfrod'', ''Bedyford'', ''Bydeford'', ''Bytheford'' and ''Biddeford''. The etymology of the name means "by the ford", and records show that before there was a bridge there was a ford at Bideford where River Torridge is estuarine, and at low tide, it is possible, but not advisable, to cross the river by wading on foot. The Welsh means "this is the way" or "this is the road" owing to the Celtic legacy of the Dumnonians and their common ancestry with the Welsh. History Early history Hubba the Dane was said to have attacked Devon in the area around Bideford near Northam or near Kenwith Castle, and was repelled by either Alfred the Great (849–899) or by the Saxon Earl of Devon. The manor of Bideford was recorded in the Domesday B ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Exeter
Exeter () is a city in Devon, South West England. It is situated on the River Exe, approximately northeast of Plymouth and southwest of Bristol. In Roman Britain, Exeter was established as the base of Legio II Augusta under the personal command of Vespasian. Exeter became a religious centre in the Middle Ages. Exeter Cathedral, founded in the mid 11th century, became Anglican in the 16th-century English Reformation. Exeter became an affluent centre for the wool trade, although by the First World War the city was in decline. After the Second World War, much of the city centre was rebuilt and is now a centre for education, business and tourism in Devon and Cornwall. It is home to two of the constituent campuses of the University of Exeter: Streatham and St Luke's. The administrative area of Exeter has the status of a non-metropolitan district under the administration of the County Council. It is the county town of Devon and home to the headquarters of Devon County C ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Launceston, Cornwall
Launceston ( or , locally or , kw, Lannstevan; rarely spelled Lanson as a local abbreviation) is a town, ancient borough, and civil parish in Cornwall, England, United Kingdom. It is west of the middle stage of the River Tamar, which constitutes almost the entire border between Cornwall and Devon. The landscape of the town is generally steep particularly at a sharp south-western knoll topped by Launceston Castle. These gradients fall down to the River Kensey and smaller tributaries. The town centre itself is bypassed and is no longer physically a main thoroughfare. The A388 still runs through the town close to the centre. The town remains figuratively the "gateway to Cornwall", due to having the A30, one of the two dual carriageways into the county, pass directly next to the town. The other dual carriageway and alternative main point of entry is the A38 at Saltash over the Tamar Bridge and was completed in 1962. There are smaller points of entry to Cornwall on mino ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
