99-572: Carboniferous Limestone is a collective term for the succession of limestones occurring widely throughout Great Britain and Ireland that were deposited during the Dinantian Epoch of the Carboniferous Period . These rocks formed between 363 and 325 million years ago. Within England and Wales, the entire limestone succession, which includes subordinate mudstones and some thin sandstones,
198-455: A Mohs hardness of 2 to 4, dense limestone can have a crushing strength of up to 180 MPa . For comparison, concrete typically has a crushing strength of about 40 MPa. Although limestones show little variability in mineral composition, they show great diversity in texture. However, most limestone consists of sand-sized grains in a carbonate mud matrix. Because limestones are often of biological origin and are usually composed of sediment that
297-457: A bloom of cyanobacteria or microalgae . However, stable isotope ratios in modern carbonate mud appear to be inconsistent with either of these mechanisms, and abrasion of carbonate grains in high-energy environments has been put forward as a third possibility. Formation of limestone has likely been dominated by biological processes throughout the Phanerozoic , the last 540 million years of
396-434: A carbonate rock outcrop can be estimated in the field by etching the surface with dilute hydrochloric acid. This etches away the calcite and aragonite, leaving behind any silica or dolomite grains. The latter can be identified by their rhombohedral shape. Crystals of calcite, quartz , dolomite or barite may line small cavities ( vugs ) in the rock. Vugs are a form of secondary porosity, formed in existing limestone by
495-612: A central quartz grain or carbonate mineral fragment. These likely form by direct precipitation of calcium carbonate onto the ooid. Pisoliths are similar to ooids, but they are larger than 2 mm in diameter and tend to be more irregular in shape. Limestone composed mostly of ooids is called an oolite or sometimes an oolitic limestone . Ooids form in high-energy environments, such as the Bahama platform, and oolites typically show crossbedding and other features associated with deposition in strong currents. Oncoliths resemble ooids but show
594-449: A change in environment that increases the solubility of calcite. Dense, massive limestone is sometimes described as "marble". For example, the famous Portoro "marble" of Italy is actually a dense black limestone. True marble is produced by recrystallization of limestone during regional metamorphism that accompanies the mountain building process ( orogeny ). It is distinguished from dense limestone by its coarse crystalline texture and
693-949: A composition reflecting the organisms that produced them and the environment in which they were produced. Low-magnesium calcite skeletal grains are typical of articulate brachiopods , planktonic (free-floating) foraminifera, and coccoliths . High-magnesium calcite skeletal grains are typical of benthic (bottom-dwelling) foraminifera, echinoderms , and coralline algae . Aragonite skeletal grains are typical of molluscs , calcareous green algae , stromatoporoids , corals , and tube worms . The skeletal grains also reflect specific geological periods and environments. For example, coral grains are more common in high-energy environments (characterized by strong currents and turbulence) while bryozoan grains are more common in low-energy environments (characterized by quiet water). Ooids (sometimes called ooliths) are sand-sized grains (less than 2mm in diameter) consisting of one or more layers of calcite or aragonite around
792-412: A considerable fraction of the limestone bed. At depths greater than 1 km (0.62 miles), burial cementation completes the lithification process. Burial cementation does not produce stylolites. When overlying beds are eroded, bringing limestone closer to the surface, the final stage of diagenesis takes place. This produces secondary porosity as some of the cement is dissolved by rainwater infiltrating
891-465: A distinct South Wales-Mendip province which extends from Pembrokeshire in the west through southern Carmarthenshire , Glamorgan and south Powys to Monmouthshire , Gloucestershire and north Somerset . These rocks continue eastwards at depth beneath Oxfordshire . The Carboniferous Limestone sequence of South Wales and the Bristol area is currently (2012) subdivided thus: The limestone found north of
990-483: A drop of dilute hydrochloric acid is dropped on it. Dolomite is also soft but reacts only feebly with dilute hydrochloric acid, and it usually weathers to a characteristic dull yellow-brown color due to the presence of ferrous iron. This is released and oxidized as the dolomite weathers. Impurities (such as clay , sand, organic remains, iron oxide , and other materials) will cause limestones to exhibit different colors, especially with weathered surfaces. The makeup of
1089-621: A few million years, as this is the most stable form of calcium carbonate. Ancient carbonate formations of the Precambrian and Paleozoic contain abundant dolomite, but limestone dominates the carbonate beds of the Mesozoic and Cenozoic . Modern dolomite is quite rare. There is evidence that, while the modern ocean favors precipitation of aragonite, the oceans of the Paleozoic and middle to late Cenozoic favored precipitation of calcite. This may indicate
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#17328440627811188-455: A few thousand years. As rainwater mixes with groundwater, aragonite and high-magnesium calcite are converted to low-calcium calcite. Cementing of thick carbonate deposits by rainwater may commence even before the retreat of the sea, as rainwater can infiltrate over 100 km (60 miles) into sediments beneath the continental shelf. As carbonate sediments are increasingly deeply buried under younger sediments, chemical and mechanical compaction of
1287-460: A greater distance across the limestone as underground channels and chambers fill up. Large sinkholes are called 'swallowholes' or 'potholes'. Gaping Gill , Alum Pot and the Buttertubs are well-known examples. Dry valleys are valleys without streams. Watlowes Valley is an excellent example. It was formed originally by a subglacial meltwater stream which existed during the last major Ice Age . After
1386-490: A high percentage of the mineral dolomite , CaMg(CO 3 ) 2 . Magnesian limestone is an obsolete and poorly-defined term used variously for dolomite, for limestone containing significant dolomite ( dolomitic limestone ), or for any other limestone containing a significant percentage of magnesium . Most limestone was formed in shallow marine environments, such as continental shelves or platforms , though smaller amounts were formed in many other environments. Much dolomite
1485-435: A limestone sample except in thin section and are less common in ancient limestones, possibly because compaction of carbonate sediments disrupts them. Limeclasts are fragments of existing limestone or partially lithified carbonate sediments. Intraclasts are limeclasts that originate close to where they are deposited in limestone, while extraclasts come from outside the depositional area. Intraclasts include grapestone , which
1584-466: A lower Mg/Ca ratio in the ocean water of those times. This magnesium depletion may be a consequence of more rapid sea floor spreading , which removes magnesium from ocean water. The modern ocean and the ocean of the Mesozoic have been described as "aragonite seas". Most limestone was formed in shallow marine environments, such as continental shelves or platforms . Such environments form only about 5% of
1683-525: A lower diversity of organisms and a greater fraction of silica and clay minerals characteristic of marls . The Green River Formation is an example of a prominent freshwater sedimentary formation containing numerous limestone beds. Freshwater limestone is typically micritic. Fossils of charophyte (stonewort), a form of freshwater green algae, are characteristic of these environments, where the charophytes produce and trap carbonates. Limestones may also form in evaporite depositional environments . Calcite
1782-523: A mechanism for dolomitization, with one 2004 review paper describing it bluntly as "a myth". Ordinary seawater is capable of converting calcite to dolomite, if the seawater is regularly flushed through the rock, as by the ebb and flow of tides (tidal pumping). Once dolomitization begins, it proceeds rapidly, so that there is very little carbonate rock containing mixed calcite and dolomite. Carbonate rock tends to be either almost all calcite/aragonite or almost all dolomite. About 20% to 25% of sedimentary rock
1881-448: A plausible source of mud. Another possibility is direct precipitation from the water. A phenomenon known as whitings occurs in shallow waters, in which white streaks containing dispersed micrite appear on the surface of the water. It is uncertain whether this is freshly precipitated aragonite or simply material stirred up from the bottom, but there is some evidence that whitings are caused by biological precipitation of aragonite as part of
1980-449: A radial rather than layered internal structure, indicating that they were formed by algae in a normal marine environment. Peloids are structureless grains of microcrystalline carbonate likely produced by a variety of processes. Many are thought to be fecal pellets produced by marine organisms. Others may be produced by endolithic (boring) algae or other microorganisms or through breakdown of mollusc shells. They are difficult to see in
2079-413: A regiment of Roman auxiliary troops. Its remains are located beneath the town of Loughor . Stone defences were added to the earthen ditch and rampart by AD 110 and the fort was occupied until the middle or end of that century. However, it was later abandoned for a time and in the early 3rd century the ditch naturally silted up. It appears to have been brought back into use during the reign of Carausius who
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#17328440627812178-535: A rock drawing of a red deer from the same period. This may be the oldest cave art found in Great Britain . Gower is also home to menhirs or standing stones from the Bronze Age . Of the nine stones , eight remain today. One of the most notable of the stones is Arthur's stone near Cefn Bryn . Its 25-ton capstone was most likely a glacial erratic (a piece of rock/conglomerate carried by glacial ice some distance from
2277-645: A scraper; flint flakes; a bone spatula; a needle & bead; and animal bones – the remains of domesticated animals, cat and dog. Archaeologists Alasdair Whittle and Michael Wysocki note that this period of occupation may be "significant", with respect to Parc Cwm long cairn, as it is "broadly contemporary with the secondary use of the tomb". In their article published in The Proceedings of the Prehistoric Society (vol.64 (1998), pp. 139–82) Whittle and Wysocki suggest corpses may have been placed in caves near
2376-462: Is The Beacon at Rhossili Down at 193 metres (633 ft) overlooking Rhossili Bay. Pwll Du and the Bishopton Valley form a statutory Local Nature Reserve . The southern coast consists of a series of small, rocky or sandy bays, such as Langland and Three Cliffs , and larger beaches such as Port Eynon , Rhossili and Oxwich Bay . The north of the peninsula has fewer beaches, and is home to
2475-422: Is a hard sedimentary rock made largely of calcium carbonate. It is generally light-grey in colour. It was formed in warm, shallow tropical seas teeming with life. The rock is made up of the shells and hard parts of millions of sea creatures, some up to 30 cm in length, encased in carbonate mud. Fossil corals, brachiopods and crinoids are frequently in evidence as components of Carboniferous Limestone; indeed
2574-399: Is a partly restored Neolithic chambered tomb . The megalithic burial chamber, or " cromlech ", was built around 6,000 BP. In the 1950s, members of Cambridge University excavating in a cave on the peninsula found 300–400 pieces of flint related to toolmaking, and dated it to between 14,000 and 12,000 BC. In 2010, an instructor from Bristol University exploring Cathole Cave discovered
2673-560: Is also favored on the seaward margin of shelves and platforms, where there is upwelling deep ocean water rich in nutrients that increase organic productivity. Reefs are common here, but when lacking, ooid shoals are found instead. Finer sediments are deposited close to shore. The lack of deep sea limestones is due in part to rapid subduction of oceanic crust, but is more a result of dissolution of calcium carbonate at depth. The solubility of calcium carbonate increases with pressure and even more with higher concentrations of carbon dioxide, which
2772-428: Is an area of almost bare, flat rock and is arguably the most fascinating feature of any area of Carboniferous limestone. They develop after the rock has been exposed by the scouring action of an ice sheet or glacier . Existing joints are subsequently exploited by the action of chemical weathering carbonation to form deep grykes and rounded blocks called clints . Grykes have a habitat of their own, which encourages
2871-482: Is an uncommon mineral in limestone, and siderite or other carbonate minerals are rare. However, the calcite in limestone often contains a few percent of magnesium . Calcite in limestone is divided into low-magnesium and high-magnesium calcite, with the dividing line placed at a composition of 4% magnesium. High-magnesium calcite retains the calcite mineral structure, which is distinct from dolomite. Aragonite does not usually contain significant magnesium. Most limestone
2970-402: Is carbonate rock, and most of this is limestone. Limestone is found in sedimentary sequences as old as 2.7 billion years. However, the compositions of carbonate rocks show an uneven distribution in time in the geologic record. About 95% of modern carbonates are composed of high-magnesium calcite and aragonite. The aragonite needles in carbonate mud are converted to low-magnesium calcite within
3069-474: Is clusters of peloids cemented together by organic material or mineral cement. Extraclasts are uncommon, are usually accompanied by other clastic sediments, and indicate deposition in a tectonically active area or as part of a turbidity current . The grains of most limestones are embedded in a matrix of carbonate mud. This is typically the largest fraction of an ancient carbonate rock. Mud consisting of individual crystals less than 5 μm (0.20 mils) in length
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3168-416: Is commonly white to gray in color. Limestone that is unusually rich in organic matter can be almost black in color, while traces of iron or manganese can give limestone an off-white to yellow to red color. The density of limestone depends on its porosity, which varies from 0.1% for the densest limestone to 40% for chalk. The density correspondingly ranges from 1.5 to 2.7 g/cm . Although relatively soft, with
3267-513: Is completely dry, and a great attraction to rock climbers. A gorge is a steep-sided valley , often formed in a limestone area as the result of the collapse of a roof above a cave system. Gordale Scar is an excellent example. Caves are common subsurface features in limestone landscapes. In the Yorkshire Dales , there are numerous caves, three of which – Ingleborough Cave , White Scar Caves and Stump Cross Caverns – are now show caves for
3366-420: Is controlled largely by the amount of dissolved carbon dioxide ( CO 2 ) in the water. This is summarized in the reaction: Increases in temperature or decreases in pressure tend to reduce the amount of dissolved CO 2 and precipitate CaCO 3 . Reduction in salinity also reduces the solubility of CaCO 3 , by several orders of magnitude for fresh water versus seawater. Near-surface water of
3465-545: Is converted to low-magnesium calcite. Diagenesis is the likely origin of pisoliths , concentrically layered particles ranging from 1 to 10 mm (0.039 to 0.394 inches) in diameter found in some limestones. Pisoliths superficially resemble ooids but have no nucleus of foreign matter, fit together tightly, and show other signs that they formed after the original deposition of the sediments. Silicification occurs early in diagenesis, at low pH and temperature, and contributes to fossil preservation. Silicification takes place through
3564-503: Is deposited close to where it formed, classification of limestone is usually based on its grain type and mud content. Most grains in limestone are skeletal fragments of marine organisms such as coral or foraminifera . These organisms secrete structures made of aragonite or calcite, and leave these structures behind when they die. Other carbonate grains composing limestones are ooids , peloids , and limeclasts ( intraclasts and extraclasts [ ca ] ). Skeletal grains have
3663-460: Is described as coquinite . Chalk is a soft, earthy, fine-textured limestone composed of the tests of planktonic microorganisms such as foraminifera, while marl is an earthy mixture of carbonates and silicate sediments. Limestone forms when calcite or aragonite precipitate out of water containing dissolved calcium, which can take place through both biological and nonbiological processes. The solubility of calcium carbonate ( CaCO 3 )
3762-617: Is described as micrite . In fresh carbonate mud, micrite is mostly small aragonite needles, which may precipitate directly from seawater, be secreted by algae, or be produced by abrasion of carbonate grains in a high-energy environment. This is converted to calcite within a few million years of deposition. Further recrystallization of micrite produces microspar , with grains from 5 to 15 μm (0.20 to 0.59 mils) in diameter. Limestone often contains larger crystals of calcite, ranging in size from 0.02 to 0.1 mm (0.79 to 3.94 mils), that are described as sparry calcite or sparite . Sparite
3861-462: Is distinguished from micrite by a grain size of over 20 μm (0.79 mils) and because sparite stands out under a hand lens or in thin section as white or transparent crystals. Sparite is distinguished from carbonate grains by its lack of internal structure and its characteristic crystal shapes. Geologists are careful to distinguish between sparite deposited as cement and sparite formed by recrystallization of micrite or carbonate grains. Sparite cement
3960-830: Is known as the Carboniferous Limestone Supergroup . Within Great Britain the suite of rocks known traditionally as the Carboniferous Limestone Series was deposited as marine sediments in three distinct ‘provinces’ separated by contemporary landmasses. One of these landmasses was the Wales-London-Brabant Massif , an east–west aligned belt of land stretching through central Wales and the English Midlands to East Anglia and on into Belgium . The limestones deposited to its south form
4059-579: Is known for its coastline , popular with walkers and outdoor enthusiasts, especially surfers. Gower has many caves, including Paviland Cave and Minchin Hole Cave. The peninsula is bounded by the Loughor Estuary to the north and Swansea Bay to the east. Gower Area of Outstanding Natural Beauty covers 188 km , including most of the peninsula west of Crofty , Three Crosses , Upper Killay , Blackpill and Bishopston . The highest point of Gower
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4158-587: Is named for the limestone which characterises the heart of the Peak District and through which deep gorges have been cut by rivers such as the Wye, Dove and Manifold . The limestone is concealed beneath younger rocks to the east and west and to the north through the South Pennines . To the north the limestone is exposed once again in east Lancashire and in the Yorkshire Dales . There are numerous limestone hills in
4257-560: Is one of the first minerals to precipitate in marine evaporites. Most limestone is formed by the activities of living organisms near reefs, but the organisms responsible for reef formation have changed over geologic time. For example, stromatolites are mound-shaped structures in ancient limestones, interpreted as colonies of cyanobacteria that accumulated carbonate sediments, but stromatolites are rare in younger limestones. Organisms precipitate limestone both directly as part of their skeletons, and indirectly by removing carbon dioxide from
4356-473: Is otherwise chemically fairly pure, with clastic sediments (mainly fine-grained quartz and clay minerals ) making up less than 5% to 10% of the composition. Organic matter typically makes up around 0.2% of a limestone and rarely exceeds 1%. Limestone often contains variable amounts of silica in the form of chert or siliceous skeletal fragments (such as sponge spicules, diatoms , or radiolarians ). Fossils are also common in limestone. Limestone
4455-472: Is produced by decaying organic matter settling into the deep ocean that is not removed by photosynthesis in the dark depths. As a result, there is a fairly sharp transition from water saturated with calcium carbonate to water unsaturated with calcium carbonate, the lysocline , which occurs at the calcite compensation depth of 4,000 to 7,000 m (13,000 to 23,000 feet). Below this depth, foraminifera tests and other skeletal particles rapidly dissolve, and
4554-514: Is secondary dolomite, formed by chemical alteration of limestone. Limestone is exposed over large regions of the Earth's surface, and because limestone is slightly soluble in rainwater, these exposures often are eroded to become karst landscapes. Most cave systems are found in limestone bedrock. Limestone has numerous uses: as a chemical feedstock for the production of lime used for cement (an essential component of concrete ), as aggregate for
4653-579: Is the most abundantly available rock. However it is extensively quarried for other purposes: Limestone Limestone ( calcium carbonate CaCO 3 ) 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 CaCO 3 . 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
4752-588: The Act of Union resulted in the Lordship of Gower becoming part of the historic county of Glamorgan with the southwest part becoming the hundred of Swansea . Agriculture remains important to the area with tourism playing an ever-increasing role in the local economy. The peninsula has a Championship status golf course at Fairwood Park just off Fairwood Common, which twice hosted the Welsh PGA Championships in
4851-566: The Arnside and Silverdale AONB and in the southern Lake District e.g. Whitbarrow Scar with coastal exposures around the northern margins of Morecambe Bay such as Humphrey Head . An outcrop extends from Kirkby Stephen along the western side of the Vale of Eden and wraps around the northern margin of the Lake District as far as Cleator Moor . North again, it is a major landscape forming feature in
4950-524: The Black Mountain to Cribarth above the upper Swansea Valley . It is here referred to as the ‘north crop’ as distinct from a sub-parallel outcrop, the ‘south crop’ which defines the southern rim of the South Wales Coalfield . The outcrop continues through Ystradfellte to Pontneddfechan , Penderyn and Pontsticill . It then runs near the southern margin of the national park via Trefil and
5049-702: The Braose family until the death of William de Braose, 2nd Baron Braose in 1326, when it passed from the family to the husband of one of his two daughters and co-heiresses, Aline and Joan. In 1215 a local lord, Rhys Gryg of Deheubarth , claimed control of the peninsula, but in 1220 he ceded control to the Anglo-Norman lords, perhaps on the orders of his overlord, Llywelyn ap Iorwerth . As an Anglo-Norman peninsula isolated from its Welsh hinterland but with coastal links to other parts of south Wales and southwest England, it developed its own Gower dialect of English. In 1535,
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#17328440627815148-634: The Gower Peninsula ( Penrhyn Gŵyr ) is in South West Wales and is the most westerly part of the historic county of Glamorgan , Wales . It projects towards the Bristol Channel . In 1956, the majority of Gower became the first area in the United Kingdom to be designated an Area of Outstanding Natural Beauty . Until 1974, Gower was administered as a rural district . It was then merged with
5247-661: The Great Orme at Llandudno , the neighbouring Little Orme and a zone of country in inland Denbighshire running through Denbigh and Ruthin . A broader belt forms high ground immediately east of the Clwydian Hills extending south to form the impressive west-facing Eglwyseg escarpment north of Llangollen and continuing as a broken outcrop southwards beyond Oswestry . There are a few outcrops in Shropshire such as Titterstone Clee Hill and at Little Wenlock . The White Peak
5346-565: The Green Coast Award 2005 for "natural, unspoiled environment": Rhossili Bay , Mewslade Bay , Tor Bay , Pwll Du Bay , and Limeslade Bay . Other beaches: The Llethryd Tooth Cave, or Tooth Hole cave, is a Bronze Age ossuary site in a limestone cave, about 1,500 yards (1.4 km) north north west of the Parc Cwm long cairn cromlech, on private land along the Parc Cwm valley, near
5445-452: The Llangattock escarpment to Blorenge where it turns southwards. A very narrow ‘east crop’ and ‘south crop’ run by Cwmbran and north of Cardiff. It turns west again to meet Swansea Bay at Porthcawl . West of the bay, the rock forms the renowned southern coast of Gower between Mumbles Head and Worms Head . There are further occurrences in the Vale of Glamorgan , both inland and on
5544-1136: The North Pennines and thence through Northumberland to the Northumberland Coast where it extends to the Scottish border at Berwick-upon-Tweed . There are scattered outcrops along the north coast of the Solway Firth . Limestones occur in southern Ayrshire and in a very broken band running northeastwards through the Pentland Hills towards Edinburgh . There are limited outcrops on the coasts of East Lothian and Berwickshire , isolated outcrops in Fife and Stirlingshire and further occurrences around Greenock and Dumbarton . Carboniferous Limestone occurs most famously around The Burren in County Clare , western Ireland where it produces one of western Europe's most important karst landscapes. Carboniferous Limestone
5643-732: The Swansea Urban Area . Wales is known to have been inhabited since at least the Upper Paleolithic period, and the Gower Peninsula has been the scene of several important archaeological discoveries. In 1823, archaeologists discovered a fairly complete Upper Paleolithic human male skeleton in Paviland Cave. They named their find the Red Lady of Paviland because the skeleton is dyed in red ochre , though later investigators determined it
5742-410: The cockle -beds of Penclawdd . The northern coast is mainly salt marsh , and is used for raising Gower salt marsh lamb which was registered as a Protected Designation of Origin in 2021 under UK law and in 2023 under EU law. The interior is mainly farmland and common land . The population mainly resides in small villages and communities with some suburban development in eastern Gower; part of
5841-623: The county borough of Swansea . From 1974 to 1996, it formed the Swansea district . Since 1996, Gower has been administered as part of the unitary authority of the City and County of Swansea . Since its establishment in 1999, the Gower Senedd constituency has only elected Labour members. The Gower constituency in Westminster had previously also elected only Labour Members of Parliament (MPs) since 1908;
5940-938: The 1990s. Meanwhile, the Gower Golf Club at Three Crosses hosts the West Wales Open, a two-day tournament on Wales' professional golf tour , the Dragon Tour. Gower is part of the Swansea travel to work area . There are six castles on the Gower Peninsula: Landimore Castle—also known as Bovehill Castle— Oystermouth Castle , Oxwich Castle , Pennard Castle, Penrice Castle , Weobley Castle and numerous cairns and standing stones. Four beaches have Blue Flag beach and Seaside (2006) awards for their high standards: Bracelet Bay , Caswell Bay , Langland Bay and Port Eynon Bay . Five other beaches have been given
6039-496: The Earth's history. Limestone may have been deposited by microorganisms in the Precambrian , prior to 540 million years ago, but inorganic processes were probably more important and likely took place in an ocean more highly oversaturated in calcium carbonate than the modern ocean. Diagenesis is the process in which sediments are compacted and turned into solid rock . During diagenesis of carbonate sediments, significant chemical and textural changes take place. For example, aragonite
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#17328440627816138-628: The Wales-London-Brabant Massif and south of the emergent Southern Uplands block is identified as a separate northern province. It is characterised by the presence of numerous ‘blocks’ and ‘basins’ each with its own particular depositional style. To the north of the Southern Uplands are the limestones of the Scottish Midland Valley stretching from Ayrshire and Arran in the west to Fife , Lothian and Berwickshire in
6237-519: The Yorkshire Dales and Brecon Beacons. Typically from 1–20 m deep and 1–60 m across, they form as a result of the subsurface collapse of limestone or through the more gradual dropping of surface material into caves. Streams flowing from higher impermeable slopes sink into the ground when they reach permeable limestone. During dry spells all water sinks very quickly on reaching the limestone, through sinkholes . In wetter conditions water flows
6336-430: 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
6435-434: The base of roads, as white pigment or filler in products such as toothpaste or paint, as a soil conditioner , and as a popular decorative addition to rock gardens . Limestone formations contain about 30% of the world's petroleum reservoirs . Limestone is composed mostly of the minerals calcite and aragonite , which are different crystal forms of calcium carbonate ( CaCO 3 ). Dolomite , CaMg(CO 3 ) 2 ,
6534-657: The beds. This may include the formation of vugs , which are crystal-lined cavities within the limestone. Diagenesis may include conversion of limestone to dolomite by magnesium-rich fluids. There is considerable evidence of replacement of limestone by dolomite, including sharp replacement boundaries that cut across bedding. The process of dolomitization remains an area of active research, but possible mechanisms include exposure to concentrated brines in hot environments ( evaporative reflux ) or exposure to diluted seawater in delta or estuary environments ( Dorag dolomitization ). However, Dorag dolomitization has fallen into disfavor as
6633-477: The bottom with the concave face downwards. This traps a void space that can later be filled by sparite. Geologists use geopetal structures to determine which direction was up at the time of deposition, which is not always obvious with highly deformed limestone formations. The cyanobacterium Hyella balani can bore through limestone; as can the green alga Eugamantia sacculata and the fungus Ostracolaba implexa . Gower Gower ( Welsh : Gŵyr ) or
6732-399: The ceiling as the water continues to drip. When the water drips on to the floor of the cavern some evaporation occurs here also leaving a trace of limestone. Again over thousands of years a stalagmite is formed. A stone pillar is formed when a stalagmite and stalactite meet. Because it is brittle, the use of Carboniferous Limestone for building stone tends to be limited to those areas where it
6831-665: The coast. An important outlier is that of the Forest of Dean basin which forms the cliffs of the Wye Valley , straddling the England/Wales border and extends southwestwards through Chepstow to Undy . The larger part of the Mendip Hills are formed from Carboniferous Limestone, showing notable geomorphological features, including Cheddar Gorge , Burrington Combe and the show cave of Wookey Hole . The Avon Gorge west of Bristol and
6930-681: The coastal cliffs at Clevedon and Weston-super-Mare are cut in this rock. The limestone islands of Flat Holm and Steep Holm are prominent in views across the mouth of the Severn Estuary . There are limited outcrops on the Isle of Man and more extensive ones in Anglesey notably along the Menai Strait , around Benllech and towards Puffin Island . The Carboniferous Limestone belt extends eastwards to form
7029-462: The depositional fabric of carbonate rocks. Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles, based on criteria such as whether the grains were originally in mutual contact, and therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of
7128-469: The deposits are highly porous, so that they have a spongelike texture, they are typically described as tufa . Secondary calcite deposited by supersaturated meteoric waters ( groundwater ) in caves is also sometimes described as travertine. This produces speleothems , such as stalagmites and stalactites . Coquina is a poorly consolidated limestone composed of abraded pieces of coral , shells , or other fossil debris. When better consolidated, it
7227-406: The earth's oceans are oversaturated with CaCO 3 by a factor of more than six. The failure of CaCO 3 to rapidly precipitate out of these waters is likely due to interference by dissolved magnesium ions with nucleation of calcite crystals, the necessary first step in precipitation. Precipitation of aragonite may be suppressed by the presence of naturally occurring organic phosphates in
7326-615: The east. Though of Carboniferous age, the limestones of this Scottish province are not assigned to the Carboniferous Limestone Supergroup. The Carboniferous Limestone is widespread throughout Ireland. The Carboniferous Limestone is a significant landscape-forming rock unit in each of the depositional provinces of Great Britain within which it is found. Within Pembrokeshire the Carboniferous Limestone forms
7425-421: The fact that the rocks are now above sea-level and no longer horizontal. The 'classic limestone walk' is a circular 10 km route from the field centre on the north side of Malham Tarn to the village of Malham, UK via Watlowes Valley and back again via Gordale Scar . Surface depressions, typically funnel-shaped and variously known as shakeholes, sinkholes , solution hollows and dolines are very common in
7524-399: The first refers to the grains and the second to the cement. For example, a limestone consisting mainly of ooids, with a crystalline matrix, would be termed an oosparite. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample. Robert J. Dunham published his system for limestone in 1962. It focuses on
7623-572: The formation of distinctive minerals from the silica and clay present in the original limestone. Two major classification schemes, the Folk and Dunham, are used for identifying the types of carbonate rocks collectively known as limestone. Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks . Based on composition, there are three main components: allochems (grains), matrix (mostly micrite), and cement (sparite). The Folk system uses two-part names;
7722-429: The geologic record are called bioherms . Many are rich in fossils, but most lack any connected organic framework like that seen in modern reefs. The fossil remains are present as separate fragments embedded in ample mud matrix. Much of the sedimentation shows indications of occurring in the intertidal or supratidal zones, suggesting sediments rapidly fill available accommodation space in the shelf or platform. Deposition
7821-467: The growth of shade-loving ferns such as hart's tongue and dog's mercury . During the last Ice Age, a stream is thought to have poured over Malham Cove - the most spectacular feature in the Yorkshire Dales. At the end of the Ice Age the limestone, which had been frozen solid, once again became permeable, allowing the water to disappear through its joints. Now Malham Cove is a high cliff (83 m high) – it
7920-424: The ice retreated, the valley was further developed by a meltwater stream flowing across the limestone while it was frozen solid. Watlowes Valley is a particularly good example of a dry valley because it has a textbook profile - the south-facing side is less steep than the north-facing side. This results from the weathering and mass movement processes that have operated in the post-glacial period. A limestone pavement
8019-422: The limestone is pervious. As this happens the limestone is dissolved and removed in solution. Caverns are often found below the surface in the limestone and as the lime-rich water finds its way underground it begins to drip from the roof of the cavern. It is cold underground so there is little evaporation but some does take place leaving a trace of limestone on the roof. Over thousands of years a stalactite forms from
8118-510: The longest run (with Normanton and Makerfield ) of any UK constituency. This ended in 2015 when the Conservatives took the seat. In 2017, it returned to Labour. The area of both constituencies covers the peninsula and the outer Gower areas of Clydach , Gowerton , Gorseinon , Felindre , Garnswllt and encompasses the area of the historic Lordship of Gower apart from the city of Swansea. About 70 square miles (180 km ) in area, Gower
8217-410: The ocean basins, but limestone is rarely preserved in continental slope and deep sea environments. The best environments for deposition are warm waters, which have both a high organic productivity and increased saturation of calcium carbonate due to lower concentrations of dissolved carbon dioxide. Modern limestone deposits are almost always in areas with very little silica-rich sedimentation, reflected in
8316-402: The public. In Ireland there is a large number of show caves open to visitors - Crag Cave , Ailwee Cave and Marble Arch Caves . The stalagmite and stalactite are the two main subsurface features in a Carboniferous Limestone area. These are formed when rainwater - a weak carbonic acid capable of dissolving limestone - percolates through it via the grykes and joints underground. This means
8415-426: The reaction: Fossils are often preserved in exquisite detail as chert. Cementing takes place rapidly in carbonate sediments, typically within less than a million years of deposition. Some cementing occurs while the sediments are still under water, forming hardgrounds . Cementing accelerates after the retreat of the sea from the depositional environment, as rainwater infiltrates the sediment beds, often within just
8514-482: The relative purity of most limestones. Reef organisms are destroyed by muddy, brackish river water, and carbonate grains are ground down by much harder silicate grains. Unlike clastic sedimentary rock, limestone is produced almost entirely from sediments originating at or near the place of deposition. Limestone formations tend to show abrupt changes in thickness. Large moundlike features in a limestone formation are interpreted as ancient reefs , which when they appear in
8613-641: The rock is full of fossils. Carboniferous Limestone has horizontal layers (beds) with bedding planes, and vertical joints. These joints are weaknesses in the rock, which are exploited by agents of both denudation and weathering . They also lead to an important characteristic of Carboniferous Limestone – its permeability . Water seeps through the joints in the limestone. This creates a landscape geologists call karst , which lacks surface drainage but which has all manner of characteristic surface and subsurface features. The Carboniferous Limestone has been folded and faulted by massive Earth movements which can be seen by
8712-659: The rock outcrop from which it came): the builders dug under it and supported it with upright stones to create a burial chamber. The remains of Sweyne Howes on Rhossili Down, Penmaen Burrows Tomb (Pen-y-Crug) and Nicholaston Long Cairn are three other well-known Neolithic chambered tombs. During the Bronze Age, people continued to use local caves for shelter and for burying their dead. Bronze Age evidence, such as funeral urns, pottery and human remains, has been found in Tooth Cave at Llethryd, Culver Hole (Port Eynon) and Cathole Cave . With
8811-460: The rock. The Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample. A revised classification was proposed by Wright (1992). It adds some diagenetic patterns to the classification scheme. Travertine is a term applied to calcium carbonate deposits formed in freshwater environments, particularly waterfalls , cascades and hot springs . Such deposits are typically massive, dense, and banded. When
8910-479: The sediments increases. Chemical compaction takes place by pressure solution of the sediments. This process dissolves minerals from points of contact between grains and redeposits it in pore space, reducing the porosity of the limestone from an initial high value of 40% to 80% to less than 10%. Pressure solution produces distinctive stylolites , irregular surfaces within the limestone at which silica-rich sediments accumulate. These may reflect dissolution and loss of
9009-662: The sediments of the ocean floor abruptly transition from carbonate ooze rich in foraminifera and coccolith remains ( Globigerina ooze) to silicic mud lacking carbonates. In rare cases, turbidites or other silica-rich sediments bury and preserve benthic (deep ocean) carbonate deposits. Ancient benthic limestones are microcrystalline and are identified by their tectonic setting. Fossils typically are foraminifera and coccoliths. No pre-Jurassic benthic limestones are known, probably because carbonate-shelled plankton had not yet evolved. Limestones also form in freshwater environments. These limestones are not unlike marine limestone, but have
9108-594: The spectacular coastal cliffs at St Govan's Head along from which are features such as Huntsman's Leap and the Green Bridge of Wales , a natural arch. It forms prominent headlands such as those of Stackpole Head and Lydstep Point and the cliffs at Tenby . A narrow, intensely quarried outcrop runs inland from Carmarthen Bay through Carmarthenshire from Kidwelly , entering the Brecon Beacons National Park at Llandyfan and extending westwards through
9207-537: The transition into the Iron Age, hill forts (timber fortifications on hill tops and coastal promontories) and earthworks began to appear. The largest example of this type of Iron Age settlement in the Gower Peninsula is Cilifor Top near Llanrhidian. Roman occupation brought new settlement. The Romans built Leucarum , a rectangular or trapezoidal fort at the mouth of the River Loughor , in the late 1st century AD to house
9306-576: The village of Llethryd. In 1961 the cave was rediscovered by cavers , who found human bones. An excavation was carried out by D.P. Webley & J. Harvey in 1962 revealing the disarticulated remains (i.e. not complete skeletons) of six adults and two children, dated to the Early Bronze Age or Beaker culture . Other finds are now held at the National Museum of Wales , Cardiff : Early Bronze Age, or Beaker, collared urn pottery; flaked knives;
9405-533: The water by photosynthesis and thereby decreasing the solubility of calcium carbonate. Limestone shows the same range of sedimentary structures found in other sedimentary rocks. However, finer structures, such as lamination , are often destroyed by the burrowing activities of organisms ( bioturbation ). Fine lamination is characteristic of limestone formed in playa lakes , which lack the burrowing organisms. Limestones also show distinctive features such as geopetal structures , which form when curved shells settle to
9504-553: The water. Although ooids likely form through purely inorganic processes, the bulk of CaCO 3 precipitation in the oceans is the result of biological activity. Much of this takes place on carbonate platforms . The origin of carbonate mud, and the processes by which it is converted to micrite, continue to be a subject of research. Modern carbonate mud is composed mostly of aragonite needles around 5 μm (0.20 mils) in length. Needles of this shape and composition are produced by calcareous algae such as Penicillus , making this
9603-516: Was actually a male. This was the first human fossil to have been found anywhere in the world, and is still the oldest ceremonial burial anywhere in Western Europe . The most recent re-calibrated radiocarbon dating in 2009 indicates that the skeleton can be dated to around 33,000 Before Present (BP). In 1937 the Parc Cwm long cairn was identified as a Severn-Cotswold type of chambered long barrow . Also known as Parc le Breos burial chamber, it
9702-416: Was likely deposited in pore space between grains, suggesting a high-energy depositional environment that removed carbonate mud. Recrystallized sparite is not diagnostic of depositional environment. Limestone outcrops are recognized in the field by their softness (calcite and aragonite both have a Mohs hardness of less than 4, well below common silicate minerals) and because limestone bubbles vigorously when
9801-496: Was worried about Irish raids, but was abandoned again before the 4th century. A Norman castle was later built on the site. Following the Norman invasion of Wales the commote of Gŵyr passed into the hands of English-speaking barons, and its southern part soon became Anglicised . In 1203 King John (1199–1216) granted the Lordship of Gower to William III de Braose (died 1211) for the service of one knight's fee . It remained with
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