In structural geology, geological fold occurs when one or a stack of flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds, they occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales. Folds form under varied conditions of stress, hydrostatic pressure, pore pressure, temperature gradient, as evidenced by their presence in soft sediments, the full spectrum of metamorphic rocks, as primary flow structures in some igneous rocks. A set of folds distributed on a regional scale constitutes a fold belt, a common feature of orogenic zones. Folds are formed by shortening of existing layers, but may be formed as a result of displacement on a non-planar fault, at the tip of a propagating fault, by differential compaction or due to the effects of a high-level igneous intrusion e.g. above a laccolith.
Folds are classified by their size, fold shape and dip of the axial plane. A fold surface seen in profile can be divided into limb portions; the limbs are the flanks of the fold and the hinge is where the flanks join together. The hinge point is the point of minimum radius of curvature for a fold; the crest of the fold is the highest point of the fold surface, the trough is the lowest point. The inflection point of a fold is the point on a limb; the hinge points along an entire folded surface form a hinge line, which can be either a crest line or a trough line. The trend and plunge of a linear hinge line gives you information about the orientation of the fold. To more describe the orientation of a fold, one must describe the axial surface; the axial surface is the surface defined by connecting all the hinge lines of stacked folding surfaces. If the axial surface is a planar surface it is called the axial plane and can be described by the strike and dip of the plane. An axial trace is the line of intersection of the axial surface with any other surface.
Folds can have, but don't have a fold axis. A fold axis, “is the closest approximation to a straight line that when moved parallel to itself, generates the form of the fold.”. A fold that can be generated by a fold axis is called a cylindrical fold; this term has been broadened to include near-cylindrical folds. The fold axis is the same as the hinge line. A fold can be shaped as a chevron, with planar limbs meeting at an angular axis, as cuspate with curved limbs, as circular with a curved axis, or as elliptical with unequal wavelength. Fold tightness is defined by the size of the angle between the fold's limbs, called the interlimb angle. Gentle folds have an interlimb angle of between 180° and 120°, open folds range from 120° to 70°, close folds from 70° to 30°, tight folds from 30° to 0°. Isoclines, or isoclinal folds, have an interlimb angle of between 10° and zero, with parallel limbs. Not all folds are equal on both sides of the axis of the fold; those with limbs of equal length are termed symmetrical, those with unequal limbs are asymmetrical.
Asymmetrical folds have an axis at an angle to the original unfolded surface they formed on. Folds that maintain uniform layer thickness are classed as concentric folds; those that do not are called similar folds. Similar folds tend to display thickening of the hinge zone. Concentric folds are caused by warping from active buckling of the layers, whereas similar folds form by some form of shear flow where the layers are not mechanically active. Ramsay has proposed a classification scheme for folds, used to describe folds in profile based upon curvature of the inner and outer lines of a fold, the behavior of dip isogons. that is, lines connecting points of equal dip on adjacent folded surfaces: Anticline: linear, strata dip away from axial center, oldest strata in center irrespective of orientation. Syncline: linear, strata dip toward axial center, youngest strata in center irrespective of orientation. Antiform: linear, strata dip away from axial center, age unknown, or inverted. Synform: linear, strata dip toward axial center, age unknown, or inverted.
Dome: nonlinear, strata dip away from center in all directions, oldest strata in center. Basin: nonlinear, strata dip toward center in all directions, youngest strata in center. Monocline: linear, strata dip in one direction between horizontal layers on each side. Chevron: angular fold with straight limbs and small hinges Recumbent: linear, fold axial plane oriented at low angle resulting in overturned strata in one limb of the fold. Slump: monoclinal, result of differential compaction or dissolution during sedimentation and lithification. Ptygmatic: Folds are chaotic and disconnected. Typical of sedimentary slump folding and decollement detachment zones. Parasitic: short wavelength folds formed within a larger wavelength fold structure - associated with differences in bed thickness Disharmonic: Folds in adjacent layers with different wavelengths and shapes Folds appear on all scales, in all rock types, at all levels in the crust, they arise from a variety of causes. When a sequence of l
Shale is a fine-grained, clastic sedimentary rock composed of mud, a mix of flakes of clay minerals and tiny fragments of other minerals quartz and calcite. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility, it is the most common sedimentary rock. Shale exhibits varying degrees of fissility, breaking into thin layers splintery and parallel to the otherwise indistinguishable bedding plane because of the parallel orientation of clay mineral flakes. Non-fissile rocks of similar composition but made of particles smaller than 0.06 mm are described as mudstones or claystones. Rocks with similar particle sizes but with less clay and therefore grittier are siltstones. Shales are composed of clay minerals and quartz grain, are grey. Addition of variable amounts of minor constituents alters the color of the rock. Black shale results from the presence of greater than one percent carbonaceous material and indicates a reducing environment.
Black shale can be referred to as black metal. Red and green colors are indicative of ferric oxide, iron hydroxide, or micaceous minerals. Clays are the major constituent of other mudrocks; the clay minerals represented are kaolinite and illite. Clay minerals of Late Tertiary mudstones are expandable smectites whereas in older rocks in mid- to early Paleozoic shales illites predominate; the transformation of smectite to illite produces silica, calcium, magnesium and water. These released elements form authigenic quartz, calcite, ankerite and albite, all trace to minor minerals found in shales and other mudrocks. Shales and mudrocks contain 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 and sulfur. Pyrite and amorphous iron sulfide along with carbon produce the black coloration; the process in the rock cycle which forms shale is called compaction.
The fine particles that compose shale can remain suspended in water long after the larger particles of sand have deposited. Shales are deposited in slow moving water and are found in lakes and lagoonal deposits, in river deltas, on floodplains and offshore from beach sands, they can be deposited in sedimentary basins and on the continental shelf, in deep, quiet water.'Black shales' are dark, as a result of being 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 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/burrows and raindrop impact craters are sometimes preserved on shale bedding surfaces.
Shales may contain concretions consisting of pyrite, apatite, or various carbonate minerals. Shales that are subject to heat and pressure of metamorphism alter into a hard, metamorphic rock known as slate. With continued increase in metamorphic grade the sequence is phyllite schist and gneiss. Before the mid-19th century, the terms slate and schist were not distinguished. In the context of underground coal mining, shale was referred to as slate well into the 20th century. Bakken Formation Barnett Shale Bearpaw Formation Burgess Shale Marcellus Formation Mazon Creek fossil beds Oil shale – Organic-rich fine-grained sedimentary rock containing kerogen Shale gas Shale gas in the United States Wheeler Shale Wianamatta Shale Media related to Shale at Wikimedia Commons
Ammonoids are an extinct group of marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs referred to as ammonites, are more related to living coleoids than they are to shelled nautiloids such as the living Nautilus species; the earliest ammonites appear during the Devonian, the last species died out in the Cretaceous–Paleogene extinction event. Ammonites are excellent index fossils, it is possible to link the rock layer in which a particular species or genus is found to specific geologic time periods, their fossil shells take the form of planispirals, although there were some helically spiraled and nonspiraled forms. The name "ammonite", from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble coiled rams' horns. Pliny the Elder called fossils of these animals ammonis cornua because the Egyptian god Ammon was depicted wearing ram's horns; the name of an ammonite genus ends in -ceras, Greek for "horn".
Ammonites can be distinguished by their septa, the dividing walls that separate the chambers in the phragmocone, by the nature of their sutures where the septa joint the outer shell wall, in general by their siphuncles. Ammonoid septa characteristically have bulges and indentations and are to varying degrees convex from the front, distinguishing them from nautiloid septa which are simple concave dish-shaped structures; the topology of the septa around the rim, results in the various suture patterns found. Three major types of suture patterns are found in the Ammonoidea: Goniatitic - numerous undivided lobes and saddles; this pattern is characteristic of the Paleozoic ammonoids. Ceratitic - lobes have subdivided tips, giving them a saw-toothed appearance, rounded undivided saddles; this suture pattern is characteristic of Triassic ammonoids and appears again in the Cretaceous "pseudoceratites". Ammonitic - lobes and saddles are much subdivided. Ammonoids of this type are the most important species from a biostratigraphical point of view.
This suture type is characteristic of Jurassic and Cretaceous ammonoids, but extends back all the way to the Permian. The siphuncle in most ammonoids is a narrow tubular structure that runs along the shell's outer rim, known as the venter, connecting the chambers of the phragmocone to the body or living chamber; this distinguishes them from living nautiloides and typical Nautilida, in which the siphuncle runs through the center of each chamber. However the earliest nautiloids from the Late Cambrian and Ordovician had ventral siphuncles like ammonites, although proportionally larger and more internally structured; the word "siphuncle" comes from the New Latin siphunculus, meaning "little siphon". Originating from within the bactritoid nautiloids, the ammonoid cephalopods first appeared in the Devonian and became extinct at the close of the Cretaceous along with the dinosaurs; the classification of ammonoids is based in part on the ornamentation and structure of the septa comprising their shells' gas chambers.
While nearly all nautiloids show curving sutures, the ammonoid suture line is variably folded, forming saddles and lobes. The Ammonoidea can be divided into six orders, listed here starting with the most primitive and going to the more derived: Agoniatitida, Lower Devonian - Middle Devonian Clymeniida, Upper Devonian Goniatitida, Middle Devonian - Upper Permian Prolecanitida, Upper Devonian - Upper Triassic Ceratitida, Upper Permian - Upper Triassic Ammonitida, Lower Jurassic - Upper CretaceousIn some classifications, these are left as suborders, included in only three orders: Goniatitida and Ammonitida; the Treatise on Invertebrate Paleontology divides the Ammonoidea, regarded as an order, into eight suborders, the Anarcestina, Clymeniina and Prolecanitina from the Paleozoic. In subsequent taxonomies, these are sometimes regarded as orders within the subclass Ammonoidea; because ammonites and their close relatives are extinct, little is known about their way of life. Their soft body parts are rarely preserved in any detail.
Nonetheless, much has been worked out by examining ammonoid shells and by using models of these shells in water tanks. Many ammonoids lived in the open water of ancient seas, rather than at the sea bottom, because their fossils are found in rocks laid down under conditions where no bottom-dwelling life is found. Many of them are thought to have been good swimmers, with flattened, discus-shaped, streamlined shells, although some ammonoids were less effective swimmers and were to have been slow-swimming bottom-dwellers. Synchrotron analysis of an aptychophoran ammonite revealed remains of isopod and mollusc larvae in its buccal cavity, indicating at least this kind of ammonite fed on plankton, they may have avoided predation by squirting ink, much like modern cephalopods. The soft body of the creature occupied the largest segments of the shell at the end of the coil; the smaller earlier segments were walled off and the animal could maintain its buoyancy by filling them with gas. Thus, the smaller sections of the coil would have floated ab
Sea urchins or urchins are spiny, globular animals, echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal to 5,000 metres, their tests are round and spiny from 3 to 10 cm across. Sea urchins move crawling with their tube feet, sometimes pushing themselves with their spines, they feed on algae but eat slow-moving or sessile animals. Their predators include sea otters, wolf eels, triggerfish. Like other echinoderms, urchins have fivefold symmetry as adults, but their pluteus larvae have bilateral symmetry, indicating that they belong to the Bilateria, the large group of animal phyla that includes chordates, arthropods and molluscs, they are distributed across all the oceans, all climates from tropical to polar, inhabit marine benthic habitats from rocky shores to hadal zone depths. Echinoids have a rich fossil record dating back to the Ordovician, some 450 million years ago, their closest relatives among the echinoderms are the sea cucumbers.
The animals have been studied since the 19th century as model organisms in developmental biology, as their embryos were easy to observe. Species such as the slate pencil urchin are popular in aquariums, where they are useful for controlling algae. Fossil urchins have been used as protective amulets. Sea urchins are members of the phylum Echinodermata, which includes sea stars, sea cucumbers, brittle stars, crinoids. Like other echinoderms, they have five-fold symmetry and move by means of hundreds of tiny, adhesive "tube feet"; the symmetry is not obvious in the living animal, but is visible in the dried test. The term "sea urchin" refers to the "regular echinoids", which are symmetrical and globular, includes several different taxonomic groups, with two subclasses: Euechinoidea and Cidaroidea or "slate-pencil urchins", which have thick, blunt spines, with algae and sponges growing on them; the "irregular" sea urchins are an infra-class inside the Euechinoidea, called Irregularia, include Atelostomata and Neognathostomata.
Irregular echinoids include: flattened sand dollars, sea biscuits, heart urchins. Together with sea cucumbers, they make up the subphylum Echinozoa, characterized by a globoid shape without arms or projecting rays. Sea cucumbers and the irregular echinoids have secondarily evolved diverse shapes. Although many sea cucumbers have branched tentacles surrounding their oral openings, these have originated from modified tube feet and are not homologous to the arms of the crinoids, sea stars, brittle stars. Urchins range in size from 3 to 10 cm, although the largest species can reach up to 36 cm, they have a rigid spherical body bearing moveable spines, which gives the class the name Echinoidea. The name "urchin" is an old word for hedgehog; the name is derived from Latin ericius, hedgehog. Like other echinoderms, sea urchin early larvae have bilateral symmetry, but they develop five-fold symmetry as they mature; this is most apparent in the "regular" sea urchins, which have spherical bodies with five sized parts radiating out from their central axes.
The mouth is at the anus at the top. Several sea urchins, including the sand dollars, are oval in shape, with distinct front and rear ends, giving them a degree of bilateral symmetry. In these urchins, the upper surface of the body is domed, but the underside is flat, while the sides are devoid of tube feet; this "irregular" body form has evolved to allow the animals to burrow through sand or other soft materials. Sea urchins may appear to be incapable of moving. Sometimes the most visible sign of life is the spines, which are attached to ball-and-socket joints and can point in any direction. Sea urchins have no visible eyes, legs, or means of propulsion, but can move but over hard surfaces using adhesive tube feet, working in conjunction with the spines; the internal organs are enclosed in a hard shell or test composed of fused plates of calcium carbonate covered by a thin dermis and epidermis. The test is rigid, divides into five ambulacral grooves separated by five interambulacral areas; each of these areas consists of two rows of plates, so the sea urchin test includes 20 rows of plates in total.
The plates are covered in rounded tubercles which contain the sockets to which the spines are attached by ball and socket joints. The inner surface of the test is lined by peritoneum. Sea urchins convert aqueous carbon dioxide using a catalytic process involving nickel into the calcium carbonate portion of the test. Most species have two series of spines and secondary, distributed over the surface of the body, with the shortest at the poles and the longest at the equator; the spines are hollow and cylindrical. Contraction of the muscular sheath that covers the test causes the spines to lean in one direction or another, while an inner sheath of collagen fibres can reversibly change from soft to rigid which can lock the spine i
Tunnels of Gibraltar
The tunnels of Gibraltar were constructed over the course of nearly 200 years, principally by the British Army. Within a land area of only 2.6 square miles, Gibraltar has around 34 miles of tunnels, nearly twice the length of its entire road network. The first tunnels, excavated in the late 18th century, served as communication passages between artillery positions and housed guns within embrasures cut into the North Face of the Rock. More tunnels were constructed in the 19th century to allow easier access to remote areas of Gibraltar and accommodate stores and reservoirs to deliver the water supply of Gibraltar; the 20th century saw by far the greatest extent of tunnelling when the Rock was turned into a huge underground fortress capable of accommodating 16,000 men along with all the supplies and equipment needed to withstand a prolonged siege. The tunnelling ceased in 1968 when the British Army's last specialist tunnelling unit was disbanded. Since the tunnels have progressively been turned over to the civilian Government of Gibraltar, although a number are still owned by the Ministry of Defence and some have been sealed off as they are now too dangerous to enter.
The Rock of Gibraltar is a klippe of Jurassic dolomitic limestone with series of'shale' formations underlying and in part overlaying the limestone. It consists of a series of carbonate deposits some 400 to 600 metres thick, varying from dark grey bituminous dolomite at the base through to a thick homogeneous sequence of light or medium grey fine-grained limestones; the formation appears to have been laid down in a tropical environment somewhat similar to the Bahamas today, on the basis of fossil evidence an Early Jurassic age has been proposed for the Gibraltar Limestone, though in appearance it has a strong resemblance to the Carboniferous Limestone that underlies large parts of England and Wales. The formation's thick bedding, extensive cementation and dolomitisation have made it a stable engineering material. Tunnelling has been possible throughout the Rock, with the exception of one region where it has been precluded due to high ground water pressures. In some areas, zones of weak rock or intrusions of the underlying or overlying'shales' have caused tunnel instability.
The tunnels of Gibraltar were constructed in five distinct phases. After the capture of Gibraltar in 1704, the British began constructing fortified defensive lines on the north-west flank of the Rock of Gibraltar – the King's Lines, Queen's Lines and Prince's Lines; these consisted of trenches cut into the solid rock or natural ledges which were made more defensible by building masonry walls and scarping the cliffs below. The Lines were constructed in stages, building on earlier defences constructed by the Moors and Spanish; the first man-made tunnels in Gibraltar were created during the Great Siege when members of the British Army's Soldier Artificer Company dug out a tunnel behind the North Face of the Rock. It was intended to reach a position called the Notch, inaccessible by any other route, where the British garrison sought to install a new artillery battery to cover a blind angle; the tunnel was the initiative of the Company's Sergeant-Major, Henry Ince, who began the task on 25 May 1782.
As the works progressed, the tunnellers decided to create an opening in the cliff face to provide them with ventilation. It was realised that this would offer an excellent firing position. By the end of the siege, the newly created Upper Gallery housed four guns, mounted on specially developed "depressing carriages" to allow them to fire downwards into Spanish positions on the isthmus to the north; the Notch was not reached. The tunnelling continued after the siege to build a series of connecting galleries and communication tunnels to link them together with the Lines. By the end of the 18th century, nearly 4,000 feet of tunnels had been dug; these comprised: Windsor Gallery – 659 feet Queen's Union Gallery – 304 feet Upper Union Gallery – 437 feet Lower Union Gallery – 232 feet Prince's Gallery – 220 feet King's Gallery – 710 feet Queen's Gallery – 1,079 feet The second phase of tunnelling was driven by a number of disparate requirements towards the end of the 19th century as Gibraltar's naval base became a key strategic asset for the United Kingdom.
Two short tunnels were constructed in Camp Bay on the southwest end of Gibraltar in 1880 to provide access to a quarry. In 1895, the Army dug out two natural caves under Windmill Hill to create an underground ammunition store, the Beefsteak Magazine. Major new construction works in the harbour at the end of the century prompted the Admiralty to construct the Dockyard Tunnel between 1898–99, driving through the entire width of the Rock in an east-west direction to reach Sandy Bay on the east side, where quarries were being worked to provide stone for the construction of the dry docks. In 1901, the Admiralty followed up by extending the natural Ragged Staff Cave to form a tunnelled underground magazine adjoining the harbour. Gibraltar's long-running water supply problems were tackled between 1898 and 1900 with the construction of four underground reservoirs, they were supplied with rainwater gathered from catchment areas on the Great Sand Dune on the east side of the Rock and conveyed to the
Barbary macaques in Gibraltar
From the Atlas Mountains and the Rif Mountains of Morocco, the Barbary macaque population in Gibraltar is the only wild monkey population on the European continent. Although most populations in Africa are experiencing declining populations due to hunting and deforestation, the population of Barbary monkeys in Gibraltar is increasing; some 300 animals in five troops occupy the Upper Rock area of the Gibraltar Nature Reserve, though they make occasional forays into the town. As they are a tailless species, they are known locally as Barbary apes or rock apes, despite being monkeys; the local people refer to them as monos when conversing in Spanish or Llanito. The name Barbary refers to the Berber People of Morocco who since the beginning of history had ties with the animals surrounding their region, as the Barbary macaques; the macaque population had been present on the Rock of Gibraltar long before Gibraltar was captured by the British in 1704 and according to records, since prior to reconquest of Gibraltar from the Muslims.
It was during the Islamic period. In his work Historia de la Muy Noble y Más Leal Ciudad de Gibraltar, written between 1605 and 1610, Alonso Hernández del Portillo, the first chronicler of Gibraltar, wrote: "But now let us speak of other and living producers which in spite of the asperity of the rock still maintain themselves in the mountain, there are monkeys, who may be called the true owners, with possession from time immemorial, always tenacious of the dominion, living for the most part on the eastern side in high and inaccessible chasms." In his History of Gibraltar, Ignacio López de Ayala, a Spanish historian like Portillo, wrote of the monkeys: "Neither the incursions of Moor, the Spaniards nor the English, nor cannon nor bomb of either have been able to dislodge them." Repeated introduction of animals and the lack of reliable data concerning founders of the Gibraltar macaque population has obscured their origin. The fact that all extant Gibraltarian mtDNA haplotypes were found in North Africa, combined with the lack of fossil evidence of M. sylvanus in Gibraltar at the end of the last glaciation diminishes the possibility that the Gibraltar macaques represent or include any remnant of the original European population, a possibility which can not be excluded.
Indeed, it had been earlier suggested that the original Gibraltar macaques were a remnant of populations that had spread throughout Southern Europe during the Pliocene, up to 5.5 million years ago. The Macaca sylvanus species is declining. About 75% of the total population is found in the Middle Atlas Mountains. During the Pleistocene, this species inhabited the Mediterranean coasts and Europe, reaching as far north as Germany and the British Isles; the species decreased with the arrival of the Ice Age, to extinction in the Iberian Peninsula 30,000 years ago. The Gibraltar Barbary macaques are considered by many to be the top tourist attraction in Gibraltar; the most popular troop is that of Queen's Gate at the Ape's Den, where people can get close to the monkeys. They will approach and sometimes climb onto people, as they are used to human interaction, they are still wild animals and will bite if frightened or annoyed. The macaques' contact with large numbers of tourists was causing the integrity of their social groups to break down, as they began to become dependent on humans.
This induced the monkeys to forage in the town, resulting in damage to buildings and vehicles. Close contact with humans has led to the macaques learning how to open pockets and unzip handbags and rucksacks in order to steal food from humans. For these reasons, deliberately feeding the macaques in Gibraltar is now an offence punishable by law. Anyone caught feeding the monkeys is liable to be fined up to £4,000. Gibraltar's Barbary macaque population was under the care of the British Army and the Gibraltar Regiment from 1915 to 1991, who controlled a population that consisted of a single troop. The'Keeper of the Apes' would keep the official records, maintaining an up-to-date register for each ape, listing their births and names and supervising their diet, which they drew every week; the food allowance of fruit and nuts was included in the budget, set by the War Office at £4 a month in 1944. They would humorously announce births in the'Gibraltar Chronicle':— "Rock Apes. Births: To Phyllis, wife of Tony, at the Upper Rock, on 30th June 1942— a child.
Both doing well." Much to the delight of readers. They were named after governors and high-ranking officers. Any ill or injured monkey needing surgery or any other form of medical attention was taken to Royal Naval Hospital Gibraltar and received the same treatment as would an enlisted service man; when UK-based infantry units were withdrawn and garrison duty was left to the Gibraltar Regiment, the Government of Gibraltar took over responsibility for the monkeys. Lt Bill Parker of the Royal Artillery Major W O Skelton of the Royal Artillery Gunner Wilfred Portlock of the Royal Artillery Regiment Sgt Alfred Holmes of the Gibraltar Regiment Cpl. Ernest Asquez of the Gibraltar Regiment On 11th May 1954, Queen Elizabeth II and the Duke of Edinburgh visited the ape packs while on a visit to Gibraltar. A photograph captured the Queen feeding a Barbary ape while the Duke of Edinburgh stood next to battle-dressed ape-keeper Gunner Wilfred Portlock; the monkeys are managed by the Gibraltar Orni
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