Misplaced Pages

#443556

45-966: The Woolhope Beds consist mainly of shales which are generally calcareous and pass frequently into irregular nodular and lenticular limestone. In the Malvern Hills there is much shale at the base, and in places the limestone may be absent. These beds are best developed in Herefordshire ; they appear also at May Hill in Gloucestershire and in Radnorshire . Common fossils are Phacops caudatus , Encrinurus tuberculatus , Orthis calligramma , Atrypa reticularis , and Orthoceras annulatum . The Wenlock Shales are pale or dark-grey shales which extend through Coalbrookdale in Shropshire , through Radnorshire into Carmarthenshire . They appear again southward in

90-492: A grandson of Cunedda Wledig , who was granted the lordship of the area. Merionethshire was a maritime county, bounded to the north by Caernarfonshire , to the east by Denbighshire , to the south by Montgomeryshire and Cardiganshire , and to the west by Cardigan Bay . With a total area of 1,731 km (668 sq miles), it was one of the more sparsely populated counties in Great Britain . The Merioneth area remains one of

135-414: A hard, fissile, metamorphic rock known as slate . With continued increase in metamorphic grade the sequence is phyllite , then schist and finally gneiss . Shale is the most common source rock for hydrocarbons ( natural gas and petroleum ). The lack of coarse sediments in most shale beds reflects the absence of strong currents in the waters of the depositional basin. These might have oxygenated

180-575: A result, became a mere Cantref of Gwynedd. Dunoding is naturally divided in the middle, by Tremadog Bay and the gorges and marshland of the Glaslyn river ; Ardudwy is the portion south of that divide. In the early 13th century, Llywelyn Fawr , Owain Gwynedd's grandson, established a distinct territorial unit comprising Ardudwy and Meirionydd (which is immediately south of Ardudwy), and gave it to his own son, Gruffydd , as an appanage . In 1221, however, Gruffydd

225-785: A series of thin limestones within shales or as thick massive beds; it is sometimes hard and crystalline and sometimes soft, earthy or concretionary . It is typically developed at Wenlock Edge , where it forms a striking feature for some 20 mi. It appears very well exposed in a sharp anticline at Dudley , whence it is sometimes called the Dudley Limestone; it occurs also at Aymestrey , Ludlow , Woolhope , May Hill, Usk and Malvern. The fossils include corals in great variety ( Halysites catenularis , Favosites aspera , Heliolites interstinctus ), crinoids ( Crotalocrinus , Marsupiocrinus , Periechocrinus ), often very beautiful specimens, and trilobites ( Calymene blumenbachii ,

270-429: Is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g., kaolin , Al 2 Si 2 O 5 ( OH ) 4 ) and tiny fragments ( silt -sized particles) of other minerals, especially quartz and calcite . Shale is characterized by its tendency to split into thin layers ( laminae ) less than one centimeter in thickness. This property

315-1201: Is accompanied by telogenesis , the third and final stage of diagenesis. As erosion reduces the depth of burial, renewed exposure to meteoric water produces additional changes to the shale, such as dissolution of some of the cement to produce secondary porosity . Pyrite may be oxidized to produce gypsum . Black shales are dark, as a result of being especially rich in unoxidized carbon . Common in some Paleozoic and Mesozoic strata , black shales were deposited in anoxic , reducing environments, such as in stagnant water columns. Some black shales contain abundant heavy metals such as molybdenum , uranium , vanadium , and zinc . The enriched values are of controversial origin, having been alternatively attributed to input from hydrothermal fluids during or after sedimentation or to slow accumulation from sea water over long periods of sedimentation. Fossils , animal tracks or burrows and even raindrop impressions are sometimes preserved on shale bedding surfaces. Shales may also contain concretions consisting of pyrite, apatite , or various carbonate minerals. Shales that are subject to heat and pressure of metamorphism alter into

360-754: Is also the southernmost mountain in Great Britain to exceed an altitude of 900 metres. However, the mountain of Cadair Idris 893 m (2,929 ft) to the south of Dolgellau was better known and hugely popular with hillwalkers. Other mountains include Arenig Fawr and the Rhinogydd . The chief rivers were the Dwyryd , the Mawddach , the Dyfi and the Dee , while in the south the Dulas formed

405-422: Is called fissility . Shale is the most common sedimentary rock. The term shale is sometimes applied more broadly, as essentially a synonym for mudrock , rather than in the narrower sense of clay-rich fissile mudrock. Shale typically exhibits varying degrees of fissility. Because of the parallel orientation of clay mineral flakes in shale, it breaks into thin layers, often splintery and usually parallel to

450-638: Is composed of about 58% clay minerals, 28% quartz, 6% feldspar , 5% carbonate minerals, and 2% iron oxides . Most of the quartz is detrital (part of the original sediments that formed the shale) rather than authigenic (crystallized within the shale after deposition). Shales and other mudrocks contain roughly 95 percent of the organic matter in all sedimentary rocks. However, this amounts to less than one percent by mass in an average shale. Black shales, which form in anoxic conditions, contain reduced free carbon along with ferrous iron (Fe ) and sulfur (S ). Amorphous iron sulfide , along with carbon, produce

495-430: Is evidence that shale acts as a semipermeable medium, allowing water to pass through while retaining dissolved salts. The fine particles that compose shale can remain suspended in water long after the larger particles of sand have been deposited. As a result, shales are typically deposited in very slow moving water and are often found in lakes and lagoonal deposits, in river deltas , on floodplains and offshore below

SECTION 10

#1732855200444

540-455: Is more likely to form nonfissile mudstone than shale. On the other hand, black shales often have very pronounced fissility ( paper shales ) due to binding of hydrocarbon molecules to the faces of the clay particles, which weakens the binding between particles. Lithification follows closely on compaction, as increased temperatures at depth hasten deposition of cement that binds the grains together. Pressure solution contributes to cementing, as

585-452: Is reduced. In addition to this physical compaction, chemical compaction may take place via pressure solution . Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact. It is during compaction that shale develops its fissility, likely through mechanical compaction of

630-766: The Girvan area are the Blair and Straiton Beds. In Ireland , there is more rock of Wenlock age. In the Dingle Peninsula one finds the Ferriters Cove Beds , a thick series of shales, slates, and sandstones with lavas and tuffs . In County Tipperary and County Mayo are the Mweelrea Beds and others. Lime and flagstones are the most important economic products of the British Wenlock rocks. Shale Shale

675-764: The Lake District , the Wenlock rocks are represented by the Brathay Flags , the lower part of the Coniston Flags series. In southern Scotland , their place is taken by the variable Riccarton Beds of Kirkcudbright shore, Dumfriesshire , Riccarton , and the Cheviots . In Lanarkshire , they are represented by sandstone in the form of greywackes and by shales. In the Pentland Hills , one finds mudstones, shales, and grits. In

720-775: The Meirionnydd district of Gwynedd , with a small area in the north east, Edeirnion Rural District , becoming part of the Glyndŵr district of Clwyd . Until 1974, Merionethshire was divided into civil parishes for the purpose of local government; these in large part equated to ecclesiastical parishes (see the table below), most of which still exist as part of the Church in Wales. Most of these parishes ended up in Gwynedd, but those marked D are in Denbighshire. Chapelries are listed in italics. As

765-399: The U.S. Gulf Coast . As sediments continue to accumulate, the older, more deeply buried sediments begin to undergo diagenesis . This mostly consists of compaction and lithification of the clay and silt particles. Early stages of diagenesis, described as eogenesis , take place at shallow depths (a few tens of meters) and are characterized by bioturbation and mineralogical changes in

810-460: The north-west of Wales . 'Merioneth' is an anglicisation of the Welsh placename Meirionnydd (for the geographical area) or Sir Feirionnydd (for the county), with a 'double' ⟨nn⟩ , but the variant with a single ⟨n⟩ is sometimes found in older works The name is derived from that of the earlier cantref of Meirionnydd . This supposedly took its name from Meirion,

855-651: The wave base . Thick deposits of shale are found near ancient continental margins and foreland basins . Some of the most widespread shale formations were deposited by epicontinental seas . Black shales are common in Cretaceous strata on the margins of the Atlantic Ocean , where they were deposited in fault -bounded silled basins associated with the opening of the Atlantic during the breakup of Pangaea . These basins were anoxic, in part because of restricted circulation in

900-560: The Dudley locust, Phacops caudatus and Illaenus ( Bumastus ) barriensis ). Brachiopods are abundant ( Atrypa reticularis , Spirifer plicatilis , Rhynchonella cuneata , Orthis , Leptaena , Pentamerus ). Lamellibranchs include the genera Avicula , Cardiola and Grammysia whilst Murchisonia , Bellerophon and Omphalotrochus are common gastropod genera. Common cephalopod genera include Orthoceras , Phragmoceras and Trochoceras . The greater part of

945-532: The Silurian patches in Gloucestershire, Herefordshire and Monmouthshire . They thicken from the south northward. The fossils are on the whole closely similar to those in the limestones above with the natural difference that corals are comparatively rare in the shales, while graptolites are abundant. Six graptolite zones were recognized by Gertrude Elles in this formation. The Wenlock Limestone occurs either as

SECTION 20

#1732855200444

990-643: The Wenlock-age rocks change their character and become more slaty or arenaceous . They are represented in this area by the Moel Ferna Slates , the Pen-y-glog Grit , and Pen-y-glog Slates . All of those layers belong to the lower part of the Denbighshire Grits , a great series of slates and grits 3,000 feet (910 m) thick. Similar deposits of similar age occur on this horizon still farther north. In

1035-421: The black coloration. Because amorphous iron sulfide gradually converts to pyrite , which is not an important pigment, young shales may be quite dark from their iron sulfide content, in spite of a modest carbon content (less than 1%), while a black color in an ancient shale indicates a high carbon content. Most shales are marine in origin, and the groundwater in shale formations is often highly saline . There

1080-506: The clumps of clay particles produced by flocculation vary in size from a few tens of microns to over 700 microns in diameter. The floccules start out water-rich, but much of the water is expelled from the floccules as the clay minerals bind more tightly together over time (a process called syneresis ). Clay pelletization by organisms that filter feed is important where flocculation is inhibited. Filter feeders produce an estimated 12 metric tons of clay pellets per square kilometer per year along

1125-408: The color of the rock. Red, brown and green colors are indicative of ferric oxide ( hematite – reds), iron hydroxide ( goethite – browns and limonite – yellow), or micaceous minerals ( chlorite , biotite and illite – greens). The color shifts from reddish to greenish as iron in the oxidized ( ferric ) state is converted to iron in the reduced ( ferrous ) state. Black shale results from

1170-412: The context of underground coal mining , shale was frequently referred to as slate well into the 20th century. Black shale associated with coal seams is called black metal. [REDACTED] Media related to Shale at Wikimedia Commons Merionethshire Merionethshire , or Merioneth ( Welsh : Meirionnydd or Sir Feirionnydd ), was one of the thirteen historic counties of Wales , in

1215-571: The county boundary. Waterfalls such as Pistyll Cain and small lakes were numerous, the largest being Bala Lake (4 miles (6.4 km) long and 1 mile (1.6 km) broad). The region which became Merionethshire previously constituted the Cantrefs of Meirionydd and Penllyn , and the Commote of Ardudwy . Prior to the 10th century, Ardudwy formed part of the principality of Dunoding , while Meirionydd and Penllyn were part of Powys . Welsh records from

1260-456: The death of Madog ap Maredudd , the powerful ruler of Powys, and the death of his immediate heir, Madog's remaining sons divided Powys between them. Penllyn was the portion which went to Owain Brogyntyn . Owain was too weak, compared with his father, to resist Gwynedd's aggressive behaviour, and was forced to become a vassal of Owain Gwynedd , the son of Gruffudd who now ruled Gwynedd; Penllyn, as

1305-504: The death of Llywelyn in 1282. Two years later, in 1284, King Edward issued the Statute of Rhuddlan , terminating Gwynedd 's existence as a state. The former appanage of Ardudwy-Merionydd, together with Penllyn, which had been part of Gwynedd for less than 150 years, were converted into Merionethshire (taking the name from Meirionydd). Merioneth was an important part of the Welsh slate industry in

1350-612: The end of 1098 (with Norwegian assistance). Gruffudd ap Cynan , the heir to the principality of Gwynedd , came to an accommodation with the Normans, who restored him to power in Gwynedd, excepting the Perfeddwlad . Once Hugh died (in 1101), Gruffudd made further representations to King Henry I , who in response granted Dunodin to Gruffudd, as well. Gruffudd's sons engaged in expansionist attacks on surrounding territory, taking Meirionydd from Powys in 1123, and annexing it to Gwynedd. Following

1395-461: The end of this period, and later, treat Dunoding as a vassal of Gwynedd , ruled by an ancient cadet branch of the same family. Nevertheless, according to John Edward Lloyd , Dunoding had been independent of Gwynedd, at the time of Cadfan ap Iago (in the early 7th century), and before. The Norman presence in England, after 1066, was the most significant factor which disrupted this pattern. In 1067,

Wenlock Group - Misplaced Pages Continue

1440-490: The known Silurian fauna of Britain comes from Wenlock rocks; J. Davidson and G. Maw obtained no fewer than 25,000 specimens of brachiopods from seven tons of the shale. Not only are there many different genera and species but individually certain forms are very numerous. The three principal zonal graptolites are, from above downwards: Monograptus testis , Cyrtograptus linnarssoni , and Cyrtogra murchisoni . When traced northward into Denbighshire and Merionethshire ,

1485-592: The mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. The clay minerals may be altered as well. For example, smectite is altered to illite at temperatures of about 55 to 200 °C (130 to 390 °F), releasing water in the process. Other alteration reactions include the alteration of smectite to chlorite and of kaolinite to illite at temperatures between 120 and 150 °C (250 and 300 °F). Because of these reactions, illite composes 80% of Precambrian shales, versus about 25% of young shales. Unroofing of buried shale

1530-433: The narrow Atlantic, and in part because the very warm Cretaceous seas lacked the circulation of cold bottom water that oxygenates the deep oceans today. Most clay must be deposited as aggregates and floccules, since the settling rate of individual clay particles is extremely slow. Flocculation is very rapid once the clay encounters highly saline sea water. Whereas individual clay particles are less than 4 microns in size,

1575-567: The nineteenth and twentieth centuries, with major quarrying centres at Blaenau Ffestiniog in the north of the county and Corris in the south, with other large quarries at Abergynolwyn , Aberllefenni , Arthog , and the Cwm Ebol quarry at Pennal . In 1947, ahead of his marriage to Princess Elizabeth , Lieutenant Philip Mountbatten was created Earl of Merioneth , as well as Duke of Edinburgh and Baron Greenwich , by his father-in-law, King George VI . An administrative county of Merioneth

1620-415: The original open framework of clay particles. The particles become strongly oriented into parallel layers that give the shale its distinctive fabric. Fissility likely develops early in the compaction process, at relatively shallow depth, since fissility does not seem to vary with depth in thick formations. Kaolinite flakes have less tendency to align in parallel layers than other clays, so kaolinite-rich clay

1665-505: The otherwise indistinguishable bedding planes . Non-fissile rocks of similar composition and particle size (less than 0.0625 mm) are described as mudstones (1/3 to 2/3 silt particles) or claystones (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are siltstones . Shales are typically gray in color and are composed of clay minerals and quartz grains. The addition of variable amounts of minor constituents alters

1710-817: The presence of greater than one percent carbonaceous material and indicates a reducing environment. Pale blue to blue-green shales typically are rich in carbonate minerals . Clays are the major constituent of shales and other mudrocks. The clay minerals represented are largely kaolinite , montmorillonite and illite. Clay minerals of Late Tertiary mudstones are expandable smectites , whereas in older rocks (especially in mid-to early Paleozoic shales) illites predominate. The transformation of smectite to illite produces silica , sodium , calcium , magnesium , iron and water. These released elements form authigenic quartz , chert , calcite , dolomite , ankerite , hematite and albite , all trace to minor (except quartz) minerals found in shales and other mudrocks. A typical shale

1755-444: The richest source rocks may contain as much as 40% organic matter. The organic matter in shale is converted over time from the original proteins, polysaccharides , lipids , and other organic molecules to kerogen , which at the higher temperatures found at greater depths of burial is further converted to graphite and petroleum. Before the mid-19th century, the terms slate , shale and schist were not sharply distinguished. In

1800-546: The rulers of Gwynedd and Powys invaded England, in support of Eadric the Wild , a leader of continued Saxon resistance against the Normans. When Northern England revolted against in 1080, the Normans responded by preemptively attacking, and then occupying Wales, to prevent any further Welsh assistance to the English. In 1094, the Welsh decided to revolt, but Hugh of Chester , the nearest Norman magnate, successfully re-captured North Wales by

1845-433: The sediments, with only slight compaction. Pyrite may be formed in anoxic mud at this stage of diagenesis. Deeper burial is accompanied by mesogenesis , during which most of the compaction and lithification takes place. As the sediments come under increasing pressure from overlying sediments, sediment grains move into more compact arrangements, ductile grains (such as clay mineral grains) are deformed, and pore space

Wenlock Group - Misplaced Pages Continue

1890-507: The strongest Welsh-speaking parts of Wales , although places like Barmouth and Tywyn are very Anglicised. The coastline consisted alternately of cliffs and stretches of sand and the area generally was the most mountainous in Wales; a large part of the Snowdonia National Park had been within its boundaries. The highest point ( county top ) was Aran Fawddwy near the village of Dinas Mawddwy at 905 m (2,970 ft), which

1935-414: The waters and destroyed organic matter before it could accumulate. The absence of carbonate rock in shale beds reflects the absence of organisms that might have secreted carbonate skeletons, also likely due to an anoxic environment. As a result, about 95% of organic matter in sedimentary rocks is found in shales and other mudrocks. Individual shale beds typically have an organic matter content of about 1%, but

1980-623: Was created under the Local Government Act 1888 on 1 April 1889. The first election to the new authority was held in January 1889. The county council was originally based at County Hall in Smithfield Street in Dolgellau before moving to modern facilities at Cae Penarlag in Dolgellau in 1953. The county was abolished under the Local Government Act 1972 on 1 April 1974. The bulk formed

2025-454: Was stripped of these lands for ruling them too oppressively. In 1245, Gruffydd's half-brother, Dafydd , launched an attack against his uncle - King Henry III - eventually resulting in the loss of the Perfeddwlad . When Gruffydd's son, Llywelyn , allied with the enemies of Edward I (Henry's son) and tried to recover the Perfeddwlad, Edward launched a huge invasion of Gwynedd, resulting in

#443556