For the Skiddaw group of hills, see Skiddaw Group
28-716: The Skiddaw Group is a group of sedimentary rock formations named after the mountain Skiddaw in the English Lake District . The rocks are almost wholly Ordovician in age ( Tremadoc through Arenig to Llanvirn epochs ) though the lowermost beds are possibly of Cambrian age. This rock sequence has previously been known as the Skiddaw Slates , the Skiddaw Slates Group and the Skiddavian Series. Its base
56-409: A body of water or beneath ice. Unconsolidated surficial materials may also be given a lithology. This is defined by grain size and composition and is often attached to an interpretation of how the unit formed. Surficial lithologies can be given to lacustrine , coastal, fluvial , aeolian , glacial , and recent volcanic deposits, among others. Examples of surficial lithology classifications used by
84-411: A continually-increasing extent of metamorphism. Metamorphic facies are defined by the pressure-temperature fields in which particular minerals form. Additional metamorphic rock names exist, such as greenschist (metamorphosed basalt and other extrusive igneous rock) or quartzite (metamorphosed quartz sand). In igneous and metamorphic rocks, grain size is a measure of the sizes of the crystals in
112-407: A group must not be defined by fossil taxonomy. Lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop , in hand or core samples , or with low magnification microscopy. Physical characteristics include colour, texture, grain size , and composition. Lithology may refer to either a detailed description of these characteristics, or
140-459: A group. Formations are the fundamental unit of stratigraphy. Groups may sometimes be combined into supergroups . Groups are useful for showing relationships between formations, and they are also useful for small-scale mapping or for studying the stratigraphy of large regions. Geologists exploring a new area have sometimes defined groups when they believe the strata within the groups can be divided into formations during subsequent investigations of
168-408: A hand lens, the visible mineralogy is included as part of the description. In the case of sequences possibly including carbonates , calcite - cemented rocks or those with possible calcite veins, it is normal to test for the presence of calcite (or other forms of calcium carbonate ) using dilute hydrochloric acid and looking for effervescence . The mineralogical composition of a rock is one of
196-461: A particular depositional environment and may provide information on paleocurrent directions. In metamorphic rocks associated with the deeper levels of fault zones , small scale structures such as asymmetric boudins and microfolds are used to determine the sense of displacement across the zone. In igneous rocks, small-scale structures are mostly observed in lavas such as pahoehoe versus ʻAʻā basaltic flows, and pillows showing eruption within
224-569: A rock describes the relationship between the individual grains or clasts that make up the rock. Sedimentary textures include the degree of sorting , grading , shape and roundness of the clasts. Metamorphic textures include those referring to the timing of growth of large metamorphic minerals relative to a phase of deformation—before deformation porphyroclast —after deformation porphyroblast . Igneous textures include such properties as grain shape, which varies from crystals with ideal crystal shapes ( euhedral ) to irregular crystals (anhedral), whether
252-612: A sample. The colour of a rock or its component parts is a distinctive characteristic of some rocks and is always recorded, sometimes against standard colour charts, such as that produced by the Rock-Color Chart Committee of the Geological Society of America based on the Munsell color system . The fabric of a rock describes the spatial and geometric configuration of all the elements that make it up. In sedimentary rocks
280-432: A summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone , slate , basalt , or limestone . Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. In certain applications, such as site investigations , lithology is described using a standard terminology such as in
308-772: A wacke sandstone which is the rough equivalent of the Watch Hill Formation, itself a wacke sandstone as is the Loweswater Formation. Within the Central Fells are the Buttermere Formation and the overlying Tarn Moor Formation. These are matched by the Murton Formation (grey slates and thin sandstones) and the Kirkland Formation (mudstones with tuffs and lavas) at Cross Fell. The Buttermere Formation
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#1732844875864336-537: Is impractical, they may be classified chemically using the TAS classification . This is based on the total content of silica and alkali metal oxides and other chemical criteria. Sedimentary rocks are further classified by whether they are siliciclastic or carbonate . Siliciclastic sedimentary rocks are then subcategorized based on their grain size distribution and the relative proportions of quartz, feldspar, and lithic (rock) fragments. Carbonate rocks are classified with
364-684: Is interpreted as an olistostrome . The Tarn Moor and Kirkland Formations contain some volcaniclastic rocks. The inlier to the south at Black Combe contains the wackes of the Knott Hill Formation. The group underlies the Borrowdale Volcanic Group in the southern and central Lake District and the Eycott Volcanic Group in the northern part of the district. The Kirkland Formation has provided fossils of Dichograptus octobrachiatus and Heminectere rushtoni . The sequence
392-482: Is not exposed but in its main outcrop area, it is considered to be in excess of 5,000 metres (16,000 ft) thick though less elsewhere. It consists largely of mudstones and siltstones with subordinate wacke -type sandstones . Their main occurrence is within the northern and central fells of the Lake District, either side of the major ENE-WSW aligned Causey Pike Fault , but inliers are found at Black Combe in
420-477: Is the diameter of the grains and/or clasts that constitute the rock. These are used to determine which rock naming system to use (e.g., a conglomerate , sandstone , or mudstone ). In the case of sandstones and conglomerates, which cover a wide range of grain sizes, a word describing the grain size range is added to the rock name. Examples are " pebble conglomerate" and "fine quartz arenite ". In rocks in which mineral grains are large enough to be identified using
448-471: The Dunham or Folk classification schemes according to the constituents of the carbonate rock. Metamorphic rock naming can be based on protolith , mineral composition, texture, or metamorphic facies . Naming based on texture and a pelite (e.g., shale , mudrock ) protolith can be used to define slate and phyllite . Texture-based names are schist and gneiss . These textures, from slate to gneiss, define
476-460: The QAPF classification , which is based on the relative content of quartz , alkali feldspar , plagioclase , and feldspathoid . Special classifications exist for igneous rock of unusual compositions, such as ultramafic rock or carbonatites . Where possible, extrusive igneous rocks are also classified by mineral content using the extrusive QAPF classification, but when determining the mineral composition
504-922: The European geotechnical standard Eurocode 7 . The naming of a lithology is based on the rock type . The three major rock types are igneous , sedimentary , and metamorphic . Igneous rocks are formed directly from magma , which is a mixture of molten rock, dissolved gases, and solid crystals. Sedimentary rock is formed from mineral or organic particles that collect at the Earth's surface and become lithified . Metamorphic rock forms by recrystallization of existing solid rock under conditions of great heat or pressure. Igneous rocks are further broken into three broad categories. Igneous rock composed of broken rock fragments created directly by volcanic processes ( tephra ) are classified as pyroclastic rock . Pyroclastic rocks are further classified by average fragment ( clast ) size and whether
532-695: The area. It is possible for only some of the strata making up a group to be divided into formations. An example of a group is the Glen Canyon Group , which includes (in ascending order) the Wingate Sandstone , the Moenave Formation , the Kayenta Formation , and the Navajo Sandstone . Each of the formations can be distinguished from its neighbor by its lithology , but all were deposited in
560-440: The fragments are mostly individual mineral crystals , particles of volcanic glass , or rock fragments. Further classifications, such as by chemical composition , may also be applied. Igneous rocks that have visible mineral grains ( phaneritic rocks) are classified as intrusive , while those that are glassy or very fine-grained ( aphanitic ) are classified as extrusive rock . Intrusive igneous rocks are usually classified using
588-454: The main visible fabric is normally bedding , and the scale and degree of development of the bedding is normally recorded as part of the description. Metamorphic rocks (apart from those created by contact metamorphism ), are characterised by well-developed planar and linear fabrics. Igneous rocks may also have fabrics as a result of flow or the settling out of particular mineral phases during crystallisation, forming cumulates . The texture of
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#1732844875864616-483: The major ways in which it is classified. Igneous rocks are classified by their mineral content whenever practical, using the QAPF classification or special ultramafic or carbonatite classifications. Likewise metamorphic facies, which show the degree to which a rock has been exposed to heat and pressure and are therefore important in classifying metamorphic rocks, are determined by observing the mineral phases that are present in
644-531: The past also been used as units for chronostratigraphy and geochronology . These are the Rotliegend and Zechstein (both of Permian age); Buntsandstein , Muschelkalk , and Keuper ( Triassic in age); Lias , Dogger , and Malm ( Jurassic in age) groups. Because of the confusion this causes, the official geologic timescale of the ICS does not contain any of these names. As with other lithostratigraphic ranks,
672-401: The rock shows highly nonuniform crystal sizes (is porphyritic ), or whether grains are aligned (which is described as trachytic texture). Rocks often contain small-scale structures (smaller than the scale of an individual outcrop). In sedimentary rocks this may include sole markings , ripple marks , mudcracks and cross-bedding . These are recorded as they are generally characteristic of
700-478: The rock. In igneous rock, this is used to determine the rate at which the material cooled: large crystals typically indicate intrusive igneous rock, while small crystals indicate that the rock was extrusive. Metamorphism of rock composed of mostly a single mineral, such as quartzite or marble , may increase grain size ( grain growth ), while metamorphism of sheared rock may decrease grain size (syntectonic recrystallization ). In clastic sedimentary rocks, grain size
728-612: The same vast erg . Not all these formations are present in all areas where the Glen Canyon Group is present. Another example of a group is the Vadito Group of northern New Mexico . Although many of its strata have been divided into formations, such as the Glenwoody Formation , other strata (particularly in the lower part of the group) remain undivided into formations. Some well known groups of northwestern Europe have in
756-767: The south of the Lake District and at Cross Fell in the North Pennines . In the Northern Fells of the Lake District, the Skiddaw Group comprises five formations of which the earliest/lowest is the Bitter Beck Formation. This is succeeded by the Watch Hill Formation, then the Hope Beck, Loweswater and Kirk Stile Formations in ascending order. The inlier at Cross Fell comprises just the Catterpallot Formation,
784-498: Was affected by low-grade regional metamorphism and deformation associated with the Acadian Orogeny , causing the dominant fine-grained parts of the sequence to become slates . The resulting slaty cleavage is parallel to the axial plane of regional folds . Group (stratigraphy) In geology, a group is a lithostratigraphic unit consisting of a series of related formations that have been classified together to form
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