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A Palaeontological Glossary

Alluvial deposits, alluvium:  Material that has been deposited by a river on its flood plain, usually composed of sands and gravels.

Aragonite:  Chemical composition: CaCO3. A mineral found in the shells of some ammonites, bivalves etc. It is converted to calcite with heat and pressure.

Arenaceous rocks:  Sedimentary rocks containing particles with grain sizes of between 1/16 mm and 2mm. Generally sandstones.

Argillaceous rocks:  Sedimentary rocks containing particles with grain sizes below 1/16mm. Examples include shales, mudstones, siltstones and clays.

Banded structure:  A rock containing definite bands, e.g. banded flint.

Bed:  A layer within a sedimentary rock, different to those above and below, and characterised by a certain lithology, fossil assemblage, colour etc.

Bedding, bedding plane:  A definite change in the character of a rock, which is parallel to the surface of deposition. In many cases it is possible to split a rock along its bedding planes.

Bedrock:  Unweathered rock found below soil or sediments.

Benthos:  Those organisms that live on the surface or in the top few centimetres of the sea floor.

Bioclastic:  Applicable to sediments composed of broken fragments of organic skeletal matter e.g. bioclastic limestones.

Biocoenosis:  A fossil assemblage that has been buried during conditions of low energy deposition i.e. in an environment with very weak currents. These assemblages contain fossils that were closely associated in life. E.g. the Wenlock Limestone.

Biofacies:  A unit of rock that contains a fossil assemblage indicative of one particular environment.

Biozone:  Rocks deposited during the life-span of one particular species.

Bone bed:  A rock that has a relatively high quantity of bone pieces, teeth, scales etc. in its composition.

Bone breccia:  A mass of bones, teeth etc, usually encountered in terrestrial caves. The organic material is cemented by calcium carbonate and does not contain bedding.

Boulder:  A rock that is over 256mm in size.

Boulder clay:  Material left behind by glacial and fluvio-glacial conditions. It has a clayey matrix which contains rocks varying in size from the sub-millimetre to boulder size.

Box-stones:  Hollow concretions.

Breccia:  A rock composed of varyingly sized, angular fragments, which have been cemented together.

Brickearth:  Loess material that has been reworked by fluvial action.

Calcite:  Chemical composition: CaCO3. A mineral that is regularly found in the composition of many different types of fossils. Aragonite (found in ammonite shells etc.) is converted to calcite under conditions of heat and pressure.

Carbonates:  Those rocks and minerals which have CO3 within their composition. Examples include limestone (CaCO3), Malachite (CuCO3CU(OH)2) and dolomite (CaMg(CO3)2).

Carbonisation:  A form of fossilisation where an organism’s organic content is reduced to a thin carbon film. Commonly encountered in plant fossils.

Cast:  A three dimensional, fossilised representation of the original organism, part of an organism or traces left by an organism. The counterpart to a mould.

Cement:  That material which binds together particles within a rock.

Chalk:  A very pure limestone from the Upper Cretaceous.

Chert:  Rocks composed of silica, found in layers, sheets and nodules.

Clastic rocks:  Rocks that have been formed from eroded or weathered particles of other rocks, e.g. sandstones, breccias etc.

Clay:  A rock composed of particles smaller than 1/256mm. It can be distorted easily (it is plastic) when wet.

Coal:  A rock composed of plant material that has undergone compaction. There are numerous types of coal, with the younger, brown coloured lignite being regarded as low quality, while older, black anthracite is regarded as being of high quality. Deposits of coal tend to be found among sandstones and shales.

Cobble:  A rock with a size of between 64mm and 256mm.

Coccoliths:  Tiny circular plates produced by plankton and composed of CaCO3. Chalk is largely composed of coccoliths.

Compaction:  The process whereby, during diagenesis, a sediment’s grains are packed together and pore spaces and water are largely eliminated.

Conchoidal:  Denoting a rock fracture that is curved and has concentric ripples radiating from the point of impact.

Concretions:  Masses formed, usually around a nucleus, during diagenesis. Two examples are flint nodules within chalk, and mudstone nodules within shale. Concretions are often fossiliferous, with the fossil providing the nucleus for growth.

Cone-in-cone structure:  A structure, often formed of calcite, that is sometimes mistakenly identified as a fossil. Its appearance is that of a suite of cones stacked within each other.

Conformable:  Denoting a sequence of deposits that has accumulated without a break in deposition.

Conglomerates:  Rocks composed of rounded pebbles cemented together.

Consolidation:  Another word for ‘diagenesis’ – the process during which sediments are compacted and/or cemented, to become rocks.

Contact metamorphism:  A change in the character of surrounding rocks when subjected to intense heat from magma intrusions.

Cross/current-bedding:  Bedding planes that are inclined and often cross and terminate each other. Generally formed in river sediments.

Deformation:  When a rock layer’s structure is altered by tectonic forces. Examples of deformation include folding and faulting.

Derived fossil:  A fossil incorporated into younger sediments after it has been weathered or eroded out of its original matrix.

Desiccation cracks:  Cracks formed in muds and clays etc. due to rapid dehydration of their surfaces. These can be found in fossil form and evidence a terrestrial depositional environment.

Diagenesis:  The process during which sediments are compacted and/or cemented, to become rocks.

Differential erosion/weathering:  Caused by differences in the resistance of rocks and particles within rocks. This can be applied on a small scale e.g. a fossil weathering out of its surrounding matrix, a large scale e.g. valleys naturally cutting through less resistant rocks, and any other scale in-between.

Dimorphism:  Where one species is found in two distinctly different forms, for instance when the male of an ammonite species is smaller than the female.

Dip slope:  A slope that runs roughly parallel to the gradient of the rock bedding below.

Dogger:  A large, calcareous nodule or concretion. Spherical or sub-spherical in shape.

Dolomitisation:  The process whereby a calcium carbonate rock is converted to double calcium magnesium carbonate – ‘dolomite’. Fossils are usually destroyed during this change.

Drift:  Material left behind by glacial and fluvio-glacial conditions.

Epifauna:  Those organisms that live attached to other, larger organisms. Examples include the corals, bryzoa, worms and bivalves that are found attached to echinoids of the chalk.

Erratic:  Rocks of pebble size or above that have been transported from their original source and often end up out of context with the geology of the area that they were transported to. In Britain, erratics are usually the legacy of glaciers.

Evaporite:  A rock formed by the evaporation of saline water, e.g. rock salt.

Exfoliation:  A form of weathering where, due to expansion and contraction during varying thermal conditions, thin layers split away from rocks.

Exposure:  An exposed area of in situ rock.

Fault:  A fracture in rock along which there has been movement. There are numerous types, including normal faults, reverse faults and pivot faults.

Faunizone/assemblage zone:  Strata that contain a certain faunal assemblage.

Flint:  A form of chert. Found in the Cretaceous Chalk as nodules and thin layers. It has a conchoidal fracture and often contains fossils.

Fold, folding:  A deformation of rock strata, usually caused by tectonic forces.

Fracture: How a homogenous rock will break, e.g. flint has a conchoidal fracture.

Gastrolith:  A stone that was swallowed by a prehistoric animal such as a dinosaur or marine reptile to aid in the break-down of food in the stomach. These are generally smooth in appearance and may be found as erratics.

Geode:  A rock containing a crystal lined hollow.

Granular texture:  Describing the texture of a rock containing roughly equally sized grains.

Gravel:  Strictly speaking, rocks with sizes of between 2mm and 4mm.

Holotype:  One specimen of a species that is used as a standard to which others thought to be of the same species can be compared. The specimen may be the holotype because it was the first of it species to be found/described or because it shows the various features of the organism most clearly.

Horizon:  A rock unit that is recognisable due to a distinctive lithology or fossil assemblage.

Inclusion:  One substance enclosed by another, e.g. an insect in amber.

Index fossil:  A fossil species that characterises a certain horizon by its abundance, but is not solely restricted to that horizon.

Induration:  Another word for ‘diagenesis’ - the process whereby sediment is converted to rock.

Inferior:  Meaning ‘lower’, e.g. ‘Inferior Oolite’ translates to ‘Lower Oolite’.

Inlier:  An ‘island’ of older rocks completely surrounded by younger rocks.

In situ:  Rocks or fossils that are within their original strata and not loose.

Interbedded:  Between two beds, e.g. a layer of coal may be interbedded between two layers of sandstone.

Jointing:  Breaks within rock layers, across which there has been no perceivable movement. In sedimentary rocks jointing is usually produced by tectonic activity.

Karst scenery:  A limestone landscape typified by rock structures that have been modified by the slow dissolution of the rock. This chemical weathering accentuates the joints and fractures within the limestone and creates gullies, caves and underground rivers etc. 

Laminations:  Suites of thin strata.

Lias:  The Lower Jurassic

Lignite:  Young, brown coloured coal. Considered to be of a low quality when compared to older, black anthracite.

Limestone:  A rock composed of calcite or dolomite. Often fossiliferous, although dolomitic limestones are less so than calcitic. 

Lithifaction:  Another word for ‘diagenesis’ - the process whereby sediment is converted to rock.

Lithofacies:  A rock type that is characteristic of a certain environment.

Lithology:  Referring to the physical character of a rock or sediment.

Loess:  Wind blown sand, deposited (in the case of the UK) under periglacial conditions.

Marine band:  A stratum containing marine fossils that is interbedded between two non-marine strata.

Marker bed/horizon:  An easily recognisable stratum that can be used to correlate rock sections that were deposited contemporaneously (at the same time), in different locations.

Marl:  A calcareous mudstone.

Mineral:  A naturally formed homogenous solid that has a definite chemical composition. Often crystalline.

Mould:  An impression of the original item. The counterpart of a cast.

Nodule:  A round or sub-round concretion of pebble size.

Oolite:  An old name for the Upper Jurrasic of Britain and Europe.

Oolith:  A spherical or sub-spherical rock particle. These contain a nucleus that has had a mineral (usually calcite) built-up around it.

Orogeny:  A period during which mountains are formed, due to the collision of crustal plates. Orogenies can cause extensive folding of rock layers, an example of this being the folding produced in northern Pembrokeshire by the Caledonian Orogeny. 

Outcrop:  The area over which a certain rock unit is found, either exposed at the surface or covered by soil etc. The area showing a certain rock unit on a geological map.

Outlier:  An ‘island’ of younger rocks, completely surrounded by older rocks.

Overburden:  Loose material that rests upon solid rock. Also used in quarrying to refer to any un-usable rock layers that are found above a layer of economic importance.

Palaeoclimatology:  The study of previous climates by drawing climatological inferences from sediments and the fossil types that they contain.

Palaeocurrent:  A representation of a fossil current within a rock, inferred from sedimentological structures such as ripple marks and/or cross bedding etc.

Palaeoecology:  The ecology of fossil assemblages, e.g. how fossil organisms interacted with each other, how sediment types lead do different fossil assemblages etc.

Palaeogeography:  A reconstruction of a previous geography, e.g. where the sea was in relation to the land surface or where the continents were in relation to each other.

Palynology:  The study of fossil pollens and spores.

Pebble:  A rock with a size of between 4mm and 64mm.

Pelagic:  Describing marine animals that swim freely or float within the water.

Permeability:  Denoting the ease with which water can flow through a rock.

Petrifaction:  The process whereby an organism’s structure is converted to rock.

Phanerozoic:  That period of time during which obvious and abundant life has existed – from the Cambrian to the present day.

Placer deposits:  Deposits of heavy and valuable minerals that have been concentrated by the action of water.

Planktonic:  A more commonly used synonym for ‘pelagic’ - describing marine animals that swim freely or float within the water.

Porosity:  The ratio between open spaces within a rock that can hold water, and solid material that cannot.

Preferred orientation:  Describing rock particles that have a certain common orientation, usually due to currents.

Provinance:  The area from which material making up sediments (and therefore, rocks) has come.

Pudding stone:  A synonym for ‘conglomerate’.

Raised beach:  A wave cut platform, sometimes covered by beach deposits, that is now above the current sea level. This is due to either a fall in sea level, or a rise in the land surface relative to sea level.

Reef:  A mass made up of in situ, organic skeletal material (originating from organisms such as corals, bryzoa, brachiopods etc.), organic debris transported to the site, and also a small amount of chemical precipitate.

Remanié fossils:  Hard parts of organisms that have accumulated over time, before eventual burial. Usually the concentration of organic material has occurred due to a lack of sedimentation. The fossils are often rolled and abraded.

Residual deposit:  That part of a rock left behind after chemical weathering.

Rock mechanics:  The study of the mechanical properties of rocks e.g. porosity, sheer strength and crushing strength etc.

Rotten stone:  The siliceous residue left behind after the weathering of certain types of limestones.

Roundness:  A measure of the curvature of the edges and general shape of a rock or rock particle. These can be said to be angular, sub-angular, sub-rounded, rounded or well-rounded.

Sand:  Rock particles with a size of between 1/16mm and 2mm.

Scree:  An accumulation of loose rock pieces, often found on slopes below outcrops of in situ rock.

Sessile:  Denoting a non-mobile organism.

Shelf facies:  Those sediments and their floral and faunal contents that accumulate on the shallower, near land, ‘shelf’ areas of an ocean.

Silicification:  Whereby silica in solution is introduced into a non siliceous rock e.g. the formation of flint nodules within chalk.

Silt:  Rock particles with a size of between 1/256mm and 1/16mm.

Slickenslides:  When rock surfaces slide over each other under pressure, e.g. during a rotational land slip. The rocks will be left with a characteristic polish as well as grooves and striations in the direction of movement.

Slump:  Mass movement of unconsolidated material down a slope, producing a pile of debris at the bottom.

Stratigraphic nomenclature:  A set of words that divide up geological time, e.g. ‘era’, ‘period’ and ‘stage’. Please see the ‘Stratigraphic Nomenclature’ guide on the guides page for a more comprehensive explanation.

Stratigraphy:  The study of layered (stratified) rocks, including the description of their physical characteristics and the correlation of strata between locations.

Stratum:  Another name for a bed or a layer. Plural is ‘strata’.

Streak:  The colour a mineral produces when scratched across a non-glazed porcelain surface. Often the streak colour is different to the colour of the mineral.

Striations/striae:  Small grooves and scratches on rock surfaces, often caused by the movement of glaciers over in situ material.

Terrigenous sediments:  Sediments formed on land, and also sediments that have come from the land surface but are deposited in the sea.

Test:  The proper word to use when referring to various creatures’ (e.g. echinoids and forams) shells or skeletons.

Till:  Another name used for ‘drift’ or ‘boulder clay’.

Trace fossils:  Fossils of impressions, track-ways etc. and not of actual animals.

Type locality:  A location that has been chosen to be a standard to which other locations with the same rock units can be compared. Usually type localities best display the rock units in question.

Unconformity:  Put most simply, an unconformity usually presents itself as a break in deposition. This is most easily seen when there has been some folding of the older rocks, before new sediments are deposited – leading to horizontal bedding sitting on top of folded bedding.

Undercutting:  Where the base of a cliff or river bank etc. is eroded at a faster rate than the material above. At a certain point in time, a section of the structure will collapse due to a lack of support from below.

Uniformitarinaism:  The concept that the present is the key to the past. For instance if current bedded sediments can be observed at the present to have been deposited by rivers then it stands to reason that current bedding in rocks many millions of years old will also be due to sediment deposition in rivers.

Varve:  A layer or suite of layers deposited over a period of a year. These may be encountered when investigating lake sediments for instance. Larger particles will be washed into lakes in the winter when there is more rain and smaller particles in the summer – producing a pronounced banding effect in the accumulated material.

Vein:  An accumulation of minerals along a fault or a joint. The minerals are usually igneous in origin. 

Weathering:  The process whereby rocks are broken down by such forces as wind, rain, temperature changes, bacteria, chemical attack and plants etc. Weathering affects rocks in situ.

Wind erosion:  Erosion caused by wind-bourn particles. Usually encountered in arid locations.

Zone fossil:  A fossil species that characterises a certain zone and is not found outside of it.

Definitions adapted from A Dictionary of Geology by D. G. A Whitten and J. R. V. Brooks. Published by Penguin Books Ltd, 1972. 


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