Glossary
This glossary aims to provide simple explanations of some of the geological terms used in this publication. The explanations are not intended to be comprehensive definitions, but concentrate instead on the way in which the terms are used here.
A |
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| Algae | Large group of simple photosynthetic plants with unicellular organs of reproduction |
| Alluvial | Pertaining to the action of rivers and floods |
| Ammonoids | A subclass of molluscs, typically characterised by a coiled, chambered shell, with distinctive lines (sutures) between the chamber walls and the outer walls of the shell |
| Amygdale | A gas bubble cavity in an igneous rock that has been infilled later with minerals |
| Andesite | A fine-grained igneous rock of intermediate composition, typically containing phenocrysts of feldspar and pyroxene |
| Anticline | A convex upwards fold with the oldest rocks in the centre |
| Armour-stone | Large stone block used in coastal defence and other engineering works |
| Aureole | Sonesurrounding an igneous intrusion in which the rocks have been affected by heat from the intrusion, typically metamorphosing the rock to hornfels |
B |
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| Basalt | A fine-grained, dark igneous rock composed of calcic plagioclase and pyroxene, may also contain olivine |
| Bedding | Layering in sedimentary rocks parallel to the original surface of deposition (true bedding) or inclined to it (false, current or cross bedding) |
| Bivalve | A class of molluscs with paired shell valves (eg mussels) |
| Brachiopods | Solitary marine shelled invertebrates, the shell is made of two unequal valves |
| Breccia | A rock composed of angular fragments greater than 2 mm in diameter |
| Bryozoans | Small aquatic colonial animals |
C |
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| Calcareous | Containing calcium carbonate |
| Cementstone | A name used to describe a limestone, usually containing clays, that is, or was, used to make cement |
| Chronostratigraphy | The standard hierachical definition of geological time units |
| Clast | A fragment in a pyroclastic or sedimentary rock |
| Cleavage | Splitting, or the tendancy to split, along parallel closely positioned planes in a rock |
| Conodonts | An extinct group of marine animals whose most commonly preserved parts are microscopic elements, made of mainly calcium phosphate, some of which superficially resemble small fish teeth and worm jaws |
| Crinoids | Marine animals (sea lillies) composed of calcareous plates |
| Cross-bedding | Layers in a sedimentary rock inclined to bedding and related to the original direction of current flow |
D |
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| Drumlin | A low, rounded hill of glacial till, which was moulded into a streamlined shape by glacier ice passing over it |
| Dyke | A sheet of igneous rock emplaced along a steep, generally vertical fracture, normally discordant to the structure of host rocks |
E |
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| Era | The largest division of geological time, divided into Periods |
| Extrusive | Describes igneous rocks that have been extruded onto the Earth's surface, rather than being intruded beneath the surface (intrusive) |
F |
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| Fault | A fracture in rocks along which some displacement has taken place |
| Feldspar | A group of rock-forming minerals consisting of silicates of aluminium, sodium, potassium and calcium |
| Freestone | Any fine-grained sandstone or limestone that can be sawn easily |
G |
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| Gastropods | Molluscs belonging to the class Gastropoda, usually with coiled shells (eg snails) |
| Geomorphology | The study of landforms and the processes that form them |
| Goniatites | An extinct group of ammonoids useful in determing time zones within sequences of rocks |
| Graptolite | A group of extinct colonial marine organisms. They consist of one or more branches or stipes in which individuals in the colony occur in rows |
| Greywacke | A sandstone containing a high proportion of silt, clay and rock fragments in addition to quartz grains |
H |
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| Hornfels | A hard fine-grained metamorphic rock adjacent to an igneous intrusion, that has been partly or completely recrystallised by the heat from the intrusion |
| Hydrothermal | Processes involving the reaction of hot groundwaters with pre-existing rocks, resulting in changes in the mineralogy and chemistry of the rocks, the formation of mineral veins and replacive 'flat' deposits |
I |
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| Igneous rock | A rock that has formed from the cooling of magma (molten rock) |
| Inlier | An outcrop of older rock surrounded by rocks of younger age. |
J |
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| Joint | A fracture, or potential fracture, in a rock along which there has been no displacement |
L |
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| Lapilli-tuff | A pyroclastic rock in which 25-75% of the clasts are netween 2 mm and 64 mm in diameter |
| Limestone | Sedimentary rock composed mostly of calcium carbonate |
| Lithology | The character of a rock expressed in terms of its mineral composition, structure, grain size and arrangement of its constituents |
| Lithostratigraphical | The determination of the stratigraphical relationship of rocks based on their lithology |
M |
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| Magnetic anomaly | The value of the local magnetic field remaining after the subtraction of the dipole portion of the Earth’s field |
| Metamorphism | The process of change in the mineralogy and structure of a rock as a result of the effects of heat and/or pressure |
| Monocline | A steep flexure of rocks on either side of which the strata are horizontal or dip at only low angles |
| Monograptid | A graptolite with a single stipe |
O |
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| Orogeny | An episode in Earth history that produced crustal thickening following the collision of tectonic plates and resulting from magmatism, folding thrusting and accretion, leading to regional uplift and mountain building |
| Ostracod | A group small arthropods with a twin shell |
| Outlier | A remnant of a younger rock surrounded by older strata |
p |
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| Palaeomagnetic | Describing the remnant magnetic characteristics frozen in a rock during its formation and reflecting the direction of the Earth’s magnetic field at that time |
| Pegmatitic | Textural description of an area within an igneous rock that is notably more coarsely crystalline than the surrounding rock |
| Periglacial | A zone or environment peripheral to glaciers, so that it is very cold but not covered by ice-sheets Phenocryst A crystal in an igneous rock, usually of near perfect shape, that is larger than that in the groundmass |
| Plagioclase | A very common group of rock-forming feldspar minerals |
| Porphyritic | Describing igneous rocks in which larger crystals (phenocrysts), are set in a finer grained or glassy groundmass |
| Pseudomorph | A mineral or aggregate of minerals that have replaced a pre-existing mineral, and having retained the original mineral's shape |
| Pyroclastic | Describes rocks that form directly by explosive ejection from a volcano |
| Pyroxene | A group of magnesium, iron and calcium silicate minerals |
R |
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| Ripple marks | Small scale ridges and troughs formed by the flow of water or wind over unconsolidated sandy or silty sediment. The fossilised equivalent of ripples found today on beachs and river sands |
S |
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| Saprolite | A soft, decomposed rock, formed in place by chemical weathering. It is characterised by its retention of some of the structures that were present in the rock from which it was derived |
| Seatearth | The fossil soil underlying a coal seam |
| Sedimentary | Describes rocks formed by the accumulation of fragments from the wasting of previous rocks or organic materials, deposited as layers of sediment |
| Siliceous | Rich in silica (SiO2) |
| Stratigraphy | The definition and description of the stratified rocks of the Earth's crust |
| Syncline | A concave-upwards fold with the youngest rocks in the centre |
T |
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| Throw | The amount of vertical movement on a fault |
| Tor | A mass of rock rising above the surrounding landscape with free faces on all sides; originally used in south-west England for distinctive residual masses of rock, mostly granitic, capping hills |
| Trachyte | A fine-grained igneous rock of intermediate composition, typically containing phenocrysts of feldspar and mica |
| Tufa | A porous or cellular deposit of calcium carbonate deposited from lime-rich springs |
| Tuff | A pyroclastic rock composed of clasts with an average grain size less than 2 mm |
| Turbidity current | A dense, turbulent sub-marine flow of mixed water and sediment, capable of very rapid movement |
U |
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| Unconformity | A substantial break in the succession of rocks following a period of erosion or non deposition |
V |
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| Volcaniclastic | Describes a rock containing clasts derived from volcanic activity |
