In geology and related fields, a stratum is a layer of sedimentary rock or soil, or igneous rock that were formed at the Earth's surface, with internally consistent characteristics that distinguish it from other layers. The "stratum" is the fundamental unit in a stratigraphic column and forms the basis of the study of stratigraphy; each layer is one of a number of parallel layers that lie one upon another, laid down by natural processes. They may extend over hundreds of thousands of square kilometers of the Earth's surface. Strata are seen as bands of different colored or differently structured material exposed in cliffs, road cuts and river banks. Individual bands may vary in thickness from a few millimeters to a kilometer or more. A band may represent a specific mode of deposition: river silt, beach sand, coal swamp, sand dune, lava bed, etc. Geologists categorize them by the material of beds; each distinct layer is assigned to the name of sheet based on a town, mountain, or region where the formation is exposed and available for study.
For example, the Burgess Shale is a thick exposure of dark fossiliferous, shale exposed high in the Canadian Rockies near Burgess Pass. Slight distinctions in material in a formation may be described as "members". Formations are collected into "groups" while groups may be collected into "supergroups". Archaeological horizon Geologic formation Geologic map Geologic unit Law of superposition Bed GeoWhen Database
Poitiers is a city on the Clain river in west-central France. It is a commune and the capital of the Vienne department and of the Poitou. Poitiers is a major university centre; the centre of town is picturesque and its streets include predominantly historical architecture religious architecture and from the Romanesque period. Two major battles took place near the city: in 732, the Battle of Poitiers, in which the Franks commanded by Charles Martel halted the expansion of the Umayyad Caliphate, in 1356, the Battle of Poitiers, a key victory for the English forces during the Hundred Years' War; this battle's consequences provoked the Jacquerie. The city of Poitiers is strategically situated on the Seuil du Poitou, a shallow gap between the Armorican and the Central Massif; the Seuil du Poitou connects the Aquitaine Basin to the South to the Paris Basin to the North. This area is an important geographic crossroads in Western Europe. Poitiers's primary site sits on a vast promontory between the valleys of the Clain.
The old town occupies the slopes and the summit of a plateau which rises 130 feet above the streams which surround it on three sides. Thus Poitiers benefits from a strong tactical situation; this was an important factor before and throughout the Middle Ages. Inhabitants of Poitiers are referred to as Poitevins or Poitevines, although this denomination can be used for anyone from the Poitou province. One out of three people in Poitiers is under the age of 30 and one out of four residents in Poitiers is a student; the climate in the Poitiers area is mild with mild temperature amplitudes, adequate rainfall throughout the year although with a drying tendency during summer. The Köppen Climate Classification subtype for this type of climate is "Cfb". Poitiers was founded by the Celtic tribe of the Pictones and was known as the oppidum Lemonum before Roman influence; the name is said to have come from the Celtic word for Lemo. After Roman influence took over, the town became known as Pictavium, or "Pictavis", after the original Pictones inhabitants themselves.
There is a rich history of archeological finds from the Roman era in Poitiers. In fact until 1857 Poitiers hosted the ruins of a vast Roman amphitheatre, larger than that of Nîmes. Remains of Roman baths, built in the 1st century and demolished in the 3rd century, were uncovered in 1877. In 1879 a burial-place and tombs of a number of Christian martyrs were discovered on the heights to the south-east of the town; the names of some of the Christians had been preserved in inscriptions. Not far from these tombs is a huge dolmen, 6.7 metres long, 4.9 metres broad and 2.1 metres high, around which used to be held the great fair of Saint Luke. The Romans built at least three aqueducts; this extensive ensemble of Roman constructions suggests Poitiers was a town of first importance even the capital of the Roman province of Gallia Aquitania during the 2nd century. As Christianity was made official and introduced across the Roman Empire during the 3rd and 4th centuries, the first bishop of Poitiers from 350 to 367, Hilary of Poitiers or Saint Hilarius, proceeded to evangelize the town.
Exiled by Constantius II, he risked death to return to Poitiers as Bishop. The first foundations of the Baptistère Saint-Jean can be traced to that era of open Christian evangelization, he was named "Doctor of The Church" by Pope Pius IX. In the 4th century, a thick wall 6m wide and 10m high was built around the town, it was 2.5 km long and stood lower on the defended east side and at the top of the promontory. Around this time, the town began to be known as Poitiers. Fifty years Poitiers fell into the hands of the Arian Visigoths, became one of the principal residences of their kings. Visigoth King Alaric II was defeated by Clovis I at Vouillé, not far from Poitiers, in 507, the town thus came under Frankish dominion. During most of the Early Middle Ages, the town of Poitiers took advantage of its defensive tactical site and of its location, far from the centre of Frankish power; as the seat of an évêché since the 4th century, the town was a centre of some importance and the capital of the Poitou county.
At the height of their power, the Counts of Poitiers governed a large domain, including both Nouvelle-Aquitaine and Poitou. The town was referred to as Poictiers, a name commemorated in warships of the Royal Navy, after the battle of Poitiers; the first decisive victory of a Western European Christian army over a Muslim power, the Battle of Tours, was fought by Charles Martel's men in the vicinity of Poitiers on 10 October 732. For many historians, it was one of the world's pivotal moments. Eleanor of Aquitaine resided in the town, which she embellished and fortified, in 1199 entrusted with communal rights. In 1152 she married the future King Henry II of England in Poitiers Cathedral. During the Hundred Years' War, the Battle of Poitiers, an English victory, was fought near the town of Poitiers on 19 September 1356. In the war In 1418, under duress, the royal parliament moved from Paris to Poitiers, where it remained in exile until the Plantagenets withdrew from the capital in 1436. During this interval, in 1429 Poitiers was the site of Joan of Arc's formal inquest.
The University of Poitiers was founded in 1431. During and after the Reformation, John Calvin had numerous converts in Poitiers and the town had its share of the violent proceedings which underlined the Wars of Religion throughout France. In 1569 Poitiers was defended by Gui de Daillon, comte du Lude, against G
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particules and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes; the particles that form a sedimentary rock are called sediment, may be composed of geological detritus or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, transported to the place of deposition by water, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and piling up on the floor of water bodies. Sedimentation may occur as dissolved minerals precipitate from water solution; the sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 8% of the total volume of the crust.
Sedimentary rocks are only a thin veneer over a crust consisting of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata; the study of sedimentary rocks and rock strata provides information about the subsurface, useful for civil engineering, for example in the construction of roads, tunnels, canals or other structures. Sedimentary rocks are important sources of natural resources like coal, fossil fuels, drinking water or ores; the study of the sequence of sedimentary rock strata is the main source for an understanding of the Earth's history, including palaeogeography and the history of life. The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology. Sedimentology is part of both geology and physical geography and overlaps with other disciplines in the Earth sciences, such as pedology, geomorphology and structural geology. Sedimentary rocks have been found on Mars. Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: clastic sedimentary rocks, biochemical sedimentary rocks, chemical sedimentary rocks, a fourth category for "other" sedimentary rocks formed by impacts and other minor processes.
Clastic sedimentary rocks are composed of other rock fragments that were cemented by silicate minerals. Clastic rocks are composed of quartz, rock fragments, clay minerals, mica. Clastic sedimentary rocks, are subdivided according to the dominant particle size. Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel and mud; the classification of clastic sedimentary rocks parallels this scheme. This tripartite subdivision is mirrored by the broad categories of rudites and lutites in older literature; the subdivision of these three broad categories is based on differences in clast shape, grain size or texture. Conglomerates are dominantly composed of rounded gravel, while breccias are composed of dominantly angular gravel. Sandstone classification schemes vary but most geologists have adopted the Dott scheme, which uses the relative abundance of quartz and lithic framework grains and the abundance of a muddy matrix between the larger grains.
Composition of framework grains The relative abundance of sand-sized framework grains determines the first word in a sandstone name. Naming depends on the dominance of the three most abundant components quartz, feldspar, or the lithic fragments that originated from other rocks. All other minerals are considered accessories and not used in the naming of the rock, regardless of abundance. Quartz sandstones have >90% quartz grains Feldspathic sandstones have <90% quartz grains and more feldspar grains than lithic grains Lithic sandstones have <90% quartz grains and more lithic grains than feldspar grainsAbundance of muddy matrix material between sand grains When sand-sized particles are deposited, the space between the grains either remains open or is filled with mud. "Clean" sandstones with open pore space are called arenites. Muddy sandstones with abundant muddy matrix are called wackes. Six sandstone names are possible using the descriptors for grain composition and the amount of matrix. For example, a quartz arenite would be composed of quartz grains and have little or no clayey matrix between the grains, a lithic wacke would have abundant lithic grains and abundant muddy matrix, etc.
Although the Dott classification scheme is used by sedimentologists, common names like greywacke and quartz sandstone are still used by non-specialists and in popular literature. Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles; these fine-grained particles are transported by turbulent flow in water or air, deposited as the flow calms and the particles settle out of suspension. Most authors presently
The Morvan is a mountainous massif lying just to the west of the Côte d'Or escarpment in Bourgogne-Franche-Comté, France. It is of Variscan age, it is composed of granites and basalts and formed a promontory extending northwards into the Jurassic sea. The Morvan has a strong musical tradition, it combines them to make its own. At its heart nowadays is the protected area of the Parc naturel régional du Morvan, its main town is Château-Chinon on the D978 between Autun. Several of its valleys have been dammed to form reservoirs; this is a unknown place internationally, so most information is in French. Lormes.net, a place in the Morvan Description of The Morvan Parc du morvan and pollution sources Le Morvan
The Armorican Massif is a geologic massif that covers a large area in the northwest of France, including Brittany, the western part of Normandy and the Pays de la Loire. It is important because it is connected to Dover on the British side of the English Channel and there has been tilting back and forth that has controlled the geography on both sides, its name comes from a Gaul area between the Loire and the Seine rivers. The massif is composed of metamorphic and magmatic rocks that were metamorphosed and/or deformed during the Hercynian or Variscan orogeny and the earlier Cadomian orogeny; the region was uplifted. The Cantabrian Mountains and the Armorican Massif were rift shoulders of the Bay of Biscay; the competent old rocks of the Armorican Massif have been eroded to a plateaulike peneplain. The highest summit, the Mont des Avaloirs, is just 417 m above sea level; the western part of the Armorican Massif are the Monts d'Arrée. During the Neoproterozoic, older parts of the Armorican Massif formed the northern margin of the paleocontinent Gondwana.
During the Paleozoic era, the Armorican Massif was part of the microcontinent Armorica, which also included terranes found in the Vosges, Black Forest and Bohemian Massif further east. Armorica rifted off the northern margin of Gondwana somewhere during the Ordovician or Silurian periods to move northward and collide with Laurussia during the Hercynian orogeny; the oldest rocks of the massif are Neoproterozoic sediments of the Brioverian Supergroup which were deformed and metamorphosed during the Cadomian orogeny. These are overlain by lower Paleozoic sediments; the whole sequence was deformed and intruded by felsic magmas during the Hercynian orogeny. The massif is cut in three by two major late Hercynian southeast-northwest striking shear zones; the divisions are called the North and South Armorican Zones. The north was less deformed during the Hercynian orogeny than the south; the South Armorican Zone is considered part of the core of the Hercynian orogeny, comparable to the Moldanubian Zone of southern Germany and central Europe.
Late Hercynian granitoid. The northern parts of the Armorican Massif have less intrusive rocks, although a small zone in the northwest of Brittany forms an exception
Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, the processes by which they change over time. Geology can include the study of the solid features of any terrestrial planet or natural satellite such as Mars or the Moon. Modern geology overlaps all other earth sciences, including hydrology and the atmospheric sciences, so is treated as one major aspect of integrated earth system science and planetary science. Geology describes the structure of the Earth on and beneath its surface, the processes that have shaped that structure, it provides tools to determine the relative and absolute ages of rocks found in a given location, to describe the histories of those rocks. By combining these tools, geologists are able to chronicle the geological history of the Earth as a whole, to demonstrate the age of the Earth. Geology provides the primary evidence for plate tectonics, the evolutionary history of life, the Earth's past climates. Geologists use a wide variety of methods to understand the Earth's structure and evolution, including field work, rock description, geophysical techniques, chemical analysis, physical experiments, numerical modelling.
In practical terms, geology is important for mineral and hydrocarbon exploration and exploitation, evaluating water resources, understanding of natural hazards, the remediation of environmental problems, providing insights into past climate change. Geology is a major academic discipline, it plays an important role in geotechnical engineering; the majority of geological data comes from research on solid Earth materials. These fall into one of two categories: rock and unlithified material; the majority of research in geology is associated with the study of rock, as rock provides the primary record of the majority of the geologic history of the Earth. There are three major types of rock: igneous and metamorphic; the rock cycle illustrates the relationships among them. When a rock solidifies or crystallizes from melt, it is an igneous rock; this rock can be weathered and eroded redeposited and lithified into a sedimentary rock. It can be turned into a metamorphic rock by heat and pressure that change its mineral content, resulting in a characteristic fabric.
All three types may melt again, when this happens, new magma is formed, from which an igneous rock may once more solidify. To study all three types of rock, geologists evaluate the minerals; each mineral has distinct physical properties, there are many tests to determine each of them. The specimens can be tested for: Luster: Measurement of the amount of light reflected from the surface. Luster is broken into nonmetallic. Color: Minerals are grouped by their color. Diagnostic but impurities can change a mineral’s color. Streak: Performed by scratching the sample on a porcelain plate; the color of the streak can help name the mineral. Hardness: The resistance of a mineral to scratch. Breakage pattern: A mineral can either show fracture or cleavage, the former being breakage of uneven surfaces and the latter a breakage along spaced parallel planes. Specific gravity: the weight of a specific volume of a mineral. Effervescence: Involves dripping hydrochloric acid on the mineral to test for fizzing. Magnetism: Involves using a magnet to test for magnetism.
Taste: Minerals can have a distinctive taste, like halite. Smell: Minerals can have a distinctive odor. For example, sulfur smells like rotten eggs. Geologists study unlithified materials, which come from more recent deposits; these materials are superficial deposits. This study is known as Quaternary geology, after the Quaternary period of geologic history. However, unlithified material does not only include sediments. Magmas and lavas are the original unlithified source of all igneous rocks; the active flow of molten rock is studied in volcanology, igneous petrology aims to determine the history of igneous rocks from their final crystallization to their original molten source. In the 1960s, it was discovered that the Earth's lithosphere, which includes the crust and rigid uppermost portion of the upper mantle, is separated into tectonic plates that move across the plastically deforming, upper mantle, called the asthenosphere; this theory is supported by several types of observations, including seafloor spreading and the global distribution of mountain terrain and seismicity.
There is an intimate coupling between the movement of the plates on the surface and the convection of the mantle. Thus, oceanic plates and the adjoining mantle convection currents always move in the same direction – because the oceanic lithosphere is the rigid upper thermal boundary layer of the convecting mantle; this coupling between rigid plates moving on the surface of the Earth and the convecting mantle is called plate tectonics. The development of plate tectonics has provided a physical basis for many observations of the solid Earth. Long linear regions of geologic features are explained as plate boundaries. For example: Mid-ocean ridges, high regions on the seafloor where hydrothermal vents and volcanoes exist, are seen as divergent boundaries, where two plates move apart. Arcs of volcanoes and earthquakes are theorized as convergent boundaries, where one plate subducts, or moves, under another. Transform boundaries, such as the San Andreas Fault system, resulted in widespread powerful earthquakes.
Plate tectonics has provided a mechan