Alluvial fan
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A fan-shaped deposit of material deposited at the mouth of a gorge or valley where it emerges onto less steeply sloping terrain.
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Anticline
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A type of fold, where the rocks are folded upwards into an arch.
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Bed
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A layer of sedimentary rock. Beds can vary in thickness from a few millimetres to many metres.
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Breccia
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A sedimentary rock made up of angular fragments of rock in a finer- grained matrix. Similar to cemented scree.
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Calcite
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A mineral made of calcium, carbon and oxygen — CaCO3; the main (calcium carbonate) component of limestone and marble.
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Conglomerate
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A sedimentary rock made up of rounded cobbles and boulders of rock in a finer-grained matrix.
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Crinoid
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A fossil sea lily made up of small ossicles or plates of calcium carbonate. These plates are the major constituent of crinoidal limestone.
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Cross-bedding
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A feature of sedimentary rocks deposited by flowing water or strong tidal currents, where the sediment is lain down in sloping laminae.
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Cryoturbation
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Disturbance to the soil caused by repeated freezing and thawing, typically under periglacial conditions.
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Diagenesis
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A term used to describe the process of how sediment is transformed into rock.
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Dolomite
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A mineral made of calcium, magnesium, carbon and oxygen — CaMg(CO3)2; also the name given to a rock made up of dolomite.
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Galena
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Lead sulphide — PbS.A heavy, grey shiny mineral with cubic cleavage. Principal lead ore mined on Mendip.
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Gruffy ground
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A local term used to describe the uneven ground produced by mining, possibly derived from ‘groovy ground’.
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Inlier
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A term used to describe an area of older rocks surrounded by younger rocks.
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Karst
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A term used to describe a limestone landscape. Karst features include caves, sinking streams, closed depressions and dry valleys.
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Limestone
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A sedimentary rock consisting mainly of calcium carbonate grains such as ooliths, shell and coral fragments and lime mud. Often fossiliferous.
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Loess
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A fine-grained, wind-blown silty soil, often derived from rock flour produced by glaciers.
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Oolith
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A grain of calcium carbonate formed by precipitation of calcium carbonate around a nucleus, much like a pearl. A limestone made of ooliths is known as an oolite.
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Outlier
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A term used to describe an area of younger rocks surrounded by older rocks.
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Pericline
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A type of fold, similar to anticline where the rocks are folded upwards, but plunging at each end, forming an elongate structure like an upturned boat.
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Periglacial
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A term used to describe very cold conditions. Often used to denote the climate or environment close to the edge of an ice sheet.
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Phreatic
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A term used to describe caves which were formed below the water table.
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Rake
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A miner’s term for a mineral vein.
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Scree
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A deposit of angular rock fragments at the base of a cliff, often formed by frost shattering during periglacial climates.
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Smithsonite
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Zinc ore. Zinc carbonate — ZnCO3, also known as calamine.
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Sump
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Water-filled cave passage, sometimes passable by cave divers.
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Swallet
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A local term for a stream sink, commonly developed at the contact between the Avon Group and the Black Rock Limestone.
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Syncline
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A type of fold, where the rocks are folded downwards into a U-shape.
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Thrust fault
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A fault that is near horizontal or gently dipping, and along which older rocks have been forced over younger rocks.
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Unconformity
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A term used to describe the surface between two sets of rocks where there is a time gap or a layer of rocks missing. Where the younger rocks are folded at a different angle to those above, this is termed an angular unconformity.
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Vadose
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A term used to describe caves which were formed above the water table.
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Wadi
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An Arabic term used to describe an ephemeral water course, only active after storms or during the rainy season.
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