Jersey: description of 1:25 000 Channel Islands Sheet 2
|From: Bishop. A. C. and Bisson. G.1989. Jersey: description of 1:25 000 Channel Islands Sheet 2. Classical areas of British geology, London: HMSO for British Geological Survey. © Crown copyright 1989.|
Jersey - description of 1:25 000 Channel Islands Sheet 2
Authors and contributors
- A. C. Bishop. BSc. PHD Department of Mineralogy, British Museum (Natural History), Cromwell Road, London SW7.
- G. Bisson, BSc, ARSM Little Newcombes, Newton St Cyres, Exeter, Devon EX5 5AW
- J. C. Brides, MA, PhD Natural Environment Research Council Polaris House, North Star Avenue, Swindon, Wilts SN2 1 ET
- R. A. Clark, BSc, PhD Department of Earth Sciences, University of Leeds, Leeds LS2 417
- D. G. Helm, BSc, PhD Department of Earth Sciences, Goldsmiths' College, University of London, Rachel McMillan Building, Creek Road, London SE8 .3BU
- = D. H. Keen, BSc, PhD Department of Geography, Coventry (Lanchester) Polytechnic, Priory Street, Coventry CV1 5FB =
- G. J. Lees, BSc Department of Geology, University of Keele, Keele, Staffordshire ST5 5BG
- A. E. Mourant, MA, DPhil, DM, FRCP, FRS The Dower House, Maison de Haut, Longueville, St Saviour, Jersey, Channel Islands
- J. T. Renouf, Bsc., PhD Maison Petit Port, St Brelade, ,Jersey, Channel Islands
- C. M. Thomas, BSc, PhD Esso Norge, PO Box 60, N4033 Forus, Norway
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|Adamellite||Granite in which alkali feldspar and plagioclase occur in about equal amounts.|
|Agglomerate||A volcanic rock formed of pyroclastic blocks or fragments generally more than 64 mm in diameter.|
|Air-fall tuff||A tuff formed by consolidation of fine-grained pyroclastic debris (ash) which was laid down on land from the air.|
|Amphibolite||A metamorphic rock consisting mainly of amphibole and plagioclase.|
|Amygdale||A gas bubble or cavity in an igneous rock which has been filled with secondary minerals.|
|Andesite||A lava of intermediate composition consisting of plagioclase feldspar (usually andesine) and one or more ferromagnesian minerals.|
|Andinotype||An orogeny characterised by sedimentation in fault-margined furrows, andesites, I-type tonalites, burial metamorphism, cauldron batholiths that feed volcanoes, and vertical movement with minimal shortening.|
|Anticline||A fold, generally convex upward (an arch), the core of which contains the older rocks.|
|Aphyric||Adjective applied usually to a fine-grained igneous rock which lacks phenocrysts.|
|Aplite||A light-coloured, fine- to medium-grained acid igneous rock with an equigranular texture and consisting mainly of quartz and feldspar.|
|Aplogranite||A light-coloured, even-grained plutonic acid igneous rock consisting mainly of quartz and alkali feldspar.|
|Apophysis||An offshoot from an igneous intrusion.|
|Appinite||A dark-coloured plutonic igneous rock rich in hornblende which commonly occurs as elongate prismatic crystals.|
|Ash||Unconsolidated fine-grained pyroclastic debris.|
|Assimilation||Incorporation of foreign material into a magma.|
|Aureole||The rocks adjoining an igneous intrusion that have suffered contact metamorphism.|
|Authigenesis||The process whereby minerals are formed in place within a sedimentary rock during or after deposition.|
|Autobreccia||A rock composed of angular fragments, formed by a process that is penecontemporaneous with the deposition or consolidation of the rock.|
|Axial plane||The surface that passes through successive hinge lines within a fold.|
|Axial trace||The line of intersection of the axial plane or axial surface of a fold with the Earth's surface.|
|Axiolite||A spherulitic aggregate elongated along a central axis.|
|Basalt||A fine-grained lava or minor intrusion composed mainly of calcic plagioclase and pyroxene with or without olivine.|
|Base-surge||A cloud of gas and solid debris that moves rapidly outward from the base of a volcanic explosion column.|
|Bouguer anomaly||A gravity anomaly left after corrections have been made for the attraction effect of topography.|
|Braid||To branch and rejoin repeatedly, forming a network.|
|Breccio-conglomerate||A sedimentary rock consisting of angular and rounded sedimentary clasts.|
|Camptonite||A lamprophyre consisting mainly of plagioclase (usually labradorite) and brown hornblende, the hornblende forming elongated prismatic crystals.|
|Cataclasite||A rock formed by shattering less severe than would produce a mylonite.|
|China-stone||Any form of granitic rock used in the manufacture of china and usually, but not necessarily, kaolinised.|
|Clast||A grain or fragment in a sedimentary rock.|
|Cleavage||Aligned and closely spaced tectonic surfaces along which a rock tends to split.|
|Columnar jointing||Prismatic fractures in lavas, sills or dykes that have resulted from cooling.|
|= Concretion =
||= A nodular or irregular mass formed by the secondary precipitation of minerals about a nucleus or centre in a sedimentary rock. =
|Conglomerate||A sedimentary rock consisting of cemented rounded pebbles or clasts.|
|Contact metamorphism||Metamorphism resulting from the emplacement of a body of magma.|
|Cross-cut||A mine tunnel driven through barren rock, commonly to intersect a mineral deposit.|
|Crystal fractionation||The separation of a cooling magma into parts of differing composition by the successive crystallisation and settling of different minerals at progressively lower temperatures.|
|Devitrification||The replacement of glassy texture by crystalline texture in a volcanic rock during or after cooling.|
|Dextral fault||A fault in which the rock on the far side appears to have been moved horizontally to the right.|
|Diagenesis||The sum of the processes involved in changing a sediment into a sedimentary rock.|
|Diorite||A plutonic igneous rock of intermediate composition consisting essentially of plagioclase and hornblende, with or without biotite and pyroxene.|
|Disconformity||A break in the stratigraphic sequence without major structural discordance.|
|Dolerite||A medium-grained igneous rock consisting mainly of calcic plagioclase and pyroxene, commonly with an ophitic texture, with or without olivine.|
|Dolmen||A prehistoric burial chamber.|
|Drag fold||A fold produced by differential movement between beds on the limb of a large fold or by shearing in a fault zone.|
|Dune||A ripple-like bedform greater than 10cm in height and 1 m in wavelength.|
|Enclave||An inclusion or fragment enclosed in an igneous rock.|
|Epiclastic||An adjective applied to sedimentary rock formed of fragments derived by weathering and erosion of older rocks.|
|Epidiorite||A metamorphosed basic igneous rock, consisting principally of amphibole and plagioclase.|
|Eugeosynclinal||A major elongate structural and sedimentological unit of the Earth's crust, with a thick sequence of deep-water sediments and characteristic igneous rocks.|
|Euhedral||An adjective applied to mineral grains in igneous rocks that are bounded by their natural crystal faces.|
|Eutaxitic texture||The texture in tuffs where shards and pumice are flattened to give a banded or streaky appearance.|
|Facies||The lithology, structure, fauna, etc., of a rock unit.|
|Felsite||A fine-grained igneous rock composed mainly of quartz and feldspar. Fiamme Collapsed pumice fragments in ignimbrite, commonly with ragged terminations.|
|Flow banding||A structure characterised by alternating layers of slightly .different composition and texture owing to the movement of magma, most common in silicic lava flows.|
|Fluxion banding||Flow banding.|
|Flysch||A marine sedimentary facies comprising a thick sequence of sandstones, shales and mudstones, typically found on the borders of the Alps.|
|Fold axis or hinge-line||The line at which the two sides or limbs of a fold meet.|
|Gabbro||A coarse-grained intrusive igneous rock composed essentially of basic plagioclase and pyroxene with or without olivine.|
|Gneiss||A foliated metamorphic rock in which layers of coarsely granular minerals alternate with layers or lenticles of platy minerals.|
|Graded bed||A sedimentary unit in which the grains normally show a progression from coarse below to fine above. Some beds display inverse grading.|
|Granite||In common usage, a coarse-grained acid igneous rock consisting essentially of quartz, feldspar and mica. More precisely, the feldspar is predominantly (greater than 2/3) alkali feldspar.|
|Granoblastic||A textural term used of regionally metamorphosed rocks having mineral grains of the same general size.|
|Granophyre||A granitic rock in which the bulk consists of micrographic intergrowths of quartz and potassic feldspar, giving a texture called granophyric.|
|Gravity gradient||A measure of the change in the value of gravity relative to horizontal distance.|
|Greenschist facies||Regionally metamorphosed rocks produced under conditions of low temperature and low to medium pressure.|
|Greywacke||A poorly sorted sandstone with more than 15 per cent interstitial matrix and angular to subangular grains of quartz, feldspar and lithic fragments.|
|Heavy mineral||A mineral with specific gravity greater than 2.9; a mineral that will sink in bromoform.|
|Hornfels||A rock consisting of fine equidimensional grains without preferred orientation, produced by contact metamorphism.|
|Humic||Derived from plants.|
|Hydrothermal||Relating to hot solutions emanating from a magma.|
|Ignimbrite||A subaerial pyroclastic rock formed by the deposition and consolidation of ash flows and glowing avalanches (nuees ardentes).|
|Imbrication||A sloping and overlapping arrangement.|
|Intraclast||A sedimentary rock fragment derived penecontemporaneously from within the sedimentary basin, e.g. pebbles in an intraformational conglomerate.|
|Isochron||A straight line constructed most commonly by plotting the ratio 87Rb/86Sr against the ratio 87Sr/86Sr, or 207Pb/204Pb or 208Pb/204Pb against 206Pb/204Pb, for different rocks or minerals from the same geological body. The slope of the isochron is a function of the age of the body.|
|Isocline||A fold with parallel limbs. Isotopic dating The determination of the age of a rock by methods based on the nuclear decay of certain natural chemical elements contained within it.|
|Kaersutite||A titanium-bearing amphibole.|
|Keratophyre||A soda-rich acid to intermediate lava or minor intrusion.|
|Kersantite||A lamprophyre consisting mainly of biotite and plagioclase, usually accompanied by augite and/or hornblende.|
|Lacustrine||Relating to a lake.|
|Lahar||A mudflow composed of volcaniclastic material.|
|Lamprophyre||A general term for dark, porphyritic intrusive igneous rocks composed of phenocrysts of one or more of dark mica, pyroxene, amphibole or olivine in a groundmass of alkali feldspar or plagioclase. Some lamprophyres are feldspar-free.|
|Lapilli||Fragments in the range of 5 to 50 mm ejected by volcanic eruption.|
|Liquefaction||Quicksand effect in soft sediments owing to sudden increase in pore fluid pressure and loss of cohesion.|
|Lithic||Made of rock.|
|Machair||Low-lying sandy environment.|
|Mafic.||Rich in ferromagnesian minerals|
|Magnetic anomaly||A departure from the normal magnetic field of the Earth.|
|Megacryst||A crystal significantly larger than the grains in the surrounding groundmass of an igneous rock. Metasediment Metamorphosed sedimentary rock.|
|Metasomatism||Metamorphism involving a change in the bulk composition of the affected rock.|
|Microlites, microliths||Microscopic needle-like crystals generally found in volcanic rocks and having some determinable optical properties.|
|Minette||A lamprophyre in which biotite forms phenocrysts and orthoclase is the main feldspar.|
|Molasse||A sequence of sedimentary rocks laid down in intermontane basins.|
|Monchiquite||A lamprophyre composed essentially of phenocrysts of pyroxene, olivine and titanium-bearing amphibole in an isotropic groundmass consisting largely of analcime, commonly highly altered.|
|Monzonite||A coarse-grained igneous rock intermediate in composition between syenite and diorite, containing approximately equal amounts of potassic feldspar and plagioclase.|
|Mortar structure||A structure produced by the mechanical fracture of rocks, especially granites and gneisses, such that the comminuted grains of quartz and feldspar surround larger grains of the same minerals.|
|Mylonite||A compact fine-grained streaked and flinty rock formed by severe granulation and shearing of rocks during dynamic metamorphism.|
|Neomagmatic||An adjective referring to magma formed by partial or complete refusion of pre-existing rocks.|
|Orthogneiss||A coarse-grained rock produced by the regional metamorphism of igneous rocks.|
|Palaeocurrent||A current that flowed in the geological past.|
|Palaeomagnetism||The study of natural remanent magnetisation in order to determine the intensity and direction of the Earth's magnetic field in the geological past.|
|Palaeoslope||A slope that existed in the geological past.|
|Parasitic fold||A relatively small fold on the limb or in the hinge of a larger congruous fold of the same generation.|
|Parataxitic texture||An extreme variation of eutaxitic texture in tuffs, in which the shards and pumice lumps are flattened and appear to have been drawn out.|
|Pegmatite||An exceptionally coarse-grained igneous rock. Most pegmatites are granitic and form irregular dykes, lenses or veins, especially near margins of intrusions.|
|Pericline (doubly plunging fold)||A fold in which the beds dip outwards from the centre (dome) or towards the centre (basin).|
|Perlitic texture||Small-scale arcuate cracks caused by cooling in volcanic glass.|
|Petrographical||Pertaining to the descriptive aspects of the study of rocks (petrology).|
|Phenocryst||A crystal in an igneous rock that is conspicuously larger than those of the matrix in which it is set.|
|Plunge||The inclination of a fold axis. Plutonic Relating to igneous rocks formed at great depth in the Earth. P|
|neumatolysis||Alteration of a rock, or crystallisation of minerals, by gases emanating from a magma.|
|Poikilitic||A texture in which smaller crystals of one mineral are enclosed within a larger crystal of another.|
|Poikiloblastic||A texture in a metamorphic rock formed where a recrystallised mineral surrounds relicts of earlier minerals.|
|Polygenetic||Originating in more than one way.|
|Porphyritic||A texture in which larger crystals in an igneous rock are set in a finer-grained groundmass, as in a porphyry.|
|Porphyroblast||A large, usually well shaped crystal, that has grown in a finer-grained matrix during metamorphism.|
|Pressure solution||The mass transfer of material by fluid diffusion from one part of a rock to another as a result of grainto-grain contact during tectonic deformation.|
|Propylite||A hydrothermally altered andesite or related rock containing secondary minerals such as chlorite, zoisite and calcite.|
|Provenance||Source areas from which fragments in sedimentary rocks have been derived.|
|Proximal||Close to the source of supply of sedimentary material.|
|Pseudomorph||A mineral whose outward crystal form is that of another, which it has replaced.|
|Pumice||A highly vesiculated glassy lava, usually of rhyolite and light enough to float in water.|
|Pyroclastic||Adjective describing a elastic rock formed by explosion or eruption from a volcanic vent.|
|Quartz-wacke||A sedimentary rock with 15 per cent or more matrix in which framework grains are mainly quartz.|
|Radiocarbon date||The age of a deposit determined by measuring the content of carbon-14 ("C) in organic material.|
|Raised beach||A beach deposit left at a level above that of the modern beach, following a lowering of sea level.|
|Rhyolite||A fine-grained acid extrusive igneous rock, commonly porphyritic and flow-banded.|
|Ripple mark||A small sand ridge formed by the movement of water or sediment over unconsolidated sediment. Ripples usually have a cross-laminated internal structure useful in determining the palaeocurrent direction.|
|Schist||A strongly foliated metamorphic rock in which the lamellar or elongate minerals show parallel orientation.|
|Scree||An accumulation of angular rock fragments, usually at the foot of and derived from a cliff or hill.|
|Shard||A glass fragment typically found in pyroclastic rocks, having distinctive cuspate margins.|
|Shear||A fracture caused by shearing due to compressive stress.|
|Sinistral fault||A fault in which the rock on the far side appears to have been moved horizontally to the left.|
|Sole mark||A sedimentary structure at the base of a bed produced by currents and used to determine the direction of palaeocurrent flow.|
|Spessartite||A lamprophyre consisting mainly of phenocrysts of hornblende set in a groundmass of sodic plagioclase.|
|Spherulite||A mass of radiating crystals, usually spherical in shape.|
|Stalagmite||A column of calcareous material deposited on the floor of a cave by water dripping from the roof.|
|Strike slip||The component of the movement on a fault that is parallel to the strike of the fault.|
|Subhedral crystal||A mineral within an igneous rock only partly bounded by its natural crystal faces.|
|Sub-volcanic||Relating to the region below the Earth's surface where dykes and sills are intruded.|
|Supracrustal||Descriptive of rock that overlies the basement.|
|Syenite||A plutonic rock consisting essentially of alkali-feldspars and amphibole or dark mica.|
|Syncline||A fold, generally convex downward, the core of which contains the younger rocks.|
|Synclinorium||A composite regional syncline composed of lesser folds.|
|Synplutonic||A term used to describe a dyke intruded into a granite rock before the granite had solidified.|
|Tear fault||A fault in which the movement has been substantially in a horizontal sense.|
|Tectonic||Relating to the forces involved in the large-scale structural evolution of the upper part of the Earth's crust.|
|Tectonic pitting||The indentation of one sedimentary grain by another.|
|Trace fossil||A sedimentary structure, such as a track or boring, left by an animal in the geological past.|
|Tuff||A lithified deposit of volcanic ash.|
|Tuffite||An admixture of pyroclastic (>25 per cent) and epiclastic (>25 per cent) material.|
|Turbidity current||A turbulent current laden with suspended sediment at the base of a column of water that flowed down a slope under the influence of gravity.|
|Unconformity||A break in the stratigraphical sequence marked by a structural discordance.|
|Vergence||The direction in which a fold is inclined or overturned.|
|Volcaniclastic||Composed mainly of volcanic rock fragments.|
|Wrench fault||A tear fault.|
|Xenocryst||A crystal in an igneous rock to which it is not genetically related.|
|Xenolith||An inclusion in an igneous rock to which it is not genetically related.|