Glossary of terms

  • Accretionary prismThe sediment scraped from a descending tectonic plate as it is subducted. Accretionary prisms show complex patterns of deformation, as in the Tertiary rocks of the Scotland District.
  • Acicular crystalsCrystals that grow to form long thin needles are said to have an acicular or needle-like habit.
  • Acidic tuffsA volcanic tuff of rhyolitic composition, such as an unwelded or a welded tuff (see below). Any acidic igneous rock has more than 60% SiO2.
  • AgglomeratesA rock formed from large, irregular, pyroclastic fragments in a finer matrix or groundmass. This term is used mainly in connection with volcanic agglomerates.
  • AllochthonousAn allochthonous sedimentary succession has been moved by tectonic forces after it was deposited, for example, a thrust-faulted nappe.
  • AlluviumMaterial that has been deposited by a river on its flood plain, usually composed of sands and gravels.
  • Amorphous silica aggregatesConsidered to be a polymerisation of Si(OH)4 molecules to form clumps of SiO2 (silica) along with water that is both present from the initial solution and from the condensation reaction that transforms the Si(OH)4 to SiO2.
  • AndesiteA fine-grained intermediate igneous (volcanic) rock containing the feldspars (see below) oligoclase or andesine.
  • AragoniteChemical composition: CaCO3. A mineral found in the shells of some ammonites, bivalves etc. It is converted to calcite with heat and pressure.
  • ArcuateCurved in an arc.
  • Arenaceous rocksSedimentary rocks containing particles with grain sizes of between 1/16 mm and 2mm. Generally sandstones.
  • Argillaceous rocksSedimentary rocks containing particles with grain sizes below 1/16mm. Examples include shales, mudstones, siltstones and clays.
  • Assemblage zoneStrata that contain a certain faunal assemblage.
  • AutochthonousAn autochthonous sedimentary succession has not been moved by tectonic forces since it was deposited. The Pleistocene coral cap of Barbados is an autochthonous succession, resting on the allochthonous Tertiary deposits of the island.
  • Band planeWhen agates are cut, they often have successive bands that follow the agate cavity’s general shape or may be a series of successive curves that resemble a man-made fortification. These frontal views are complimented by a slanted view, above or below the narrow band, when it can be seen as a cross-section of a planar layer. We call that view the ‘band plane view’. Think of a slice of bread – viewed edge on, it’s a slice or band of bread and, viewed from above or below, it’s a flat plane of bread!
  • Banded fortification sheathsThe successive fortification bands that are seen surrounding some needle-like inclusions in agates.
  • Banded structureA rock containing definite bands, e.g. banded flint.
  • Basal complexAn outmoded idea that islands in the Caribbean and elsewhere were formed on top of foundered continents.
  • BedA layer within a sedimentary rock, different to those above and below, and characterised by a certain lithology, fossil assemblage, colour etc.
  • BeddingA 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.
  • BedrockUnweathered rock found below soil or sediments.
  • BenthicThe benthic zone is the ecological region at the lowest level of a body of water such as an ocean or a lake, including the sediment surface and some sub-surface layers.
  • BenthosThose organisms that live on the surface or in the top few centimetres of the sea floor.
  • BioclasticApplicable to sediments composed of broken fragments of organic skeletal matter e.g. bioclastic limestones.
  • BiocoenosisA 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.
  • BiofaciesA unit of rock that contains a fossil assemblage indicative of one particular environment.
  • BiozoneRocks deposited during the life-span of one particular species.
  • Bone bedA rock that has a relatively high quantity of bone pieces, teeth, scales etc. in its composition.
  • Bone brecciaA mass of bones, teeth etc, usually encountered in terrestrial caves. The organic material is cemented by calcium carbonate and does not contain bedding.
  • BotryoidalRefers to the cauliflower shaped, three-dimensional texture seen in some agates.
  • BoulderA rock that is over 256mm in size.
  • Boulder clayMaterial 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 crabThe box crabs, also known as shame-faced crabs (family Calappidae), have a box-shaped carapace and conceal their anterior (‘face’) behind their chelae, hence their two common names.
  • Box-stonesHollow concretions.
  • Brazil Law Twins quartzSuch quartz crystals form when right and a left hand quartz structures are combined in a single crystal. Think of a stack of sub-microscopic quartz crystals that stack so that their “c” or optic axis is perpendicular to the direction of stacking and each successive sub-microscopic crystal is twisted a bit from its predecessor. The overall twist of these quartz crystal fragments can be either a twist in a counter-clockwise direction, which is a ‘right-handed twist’ or it can twist in a clockwise direction, which is a ‘left-handed twist’.
  • Brazil Twinning ExperimentsIdentification of quartz Brazil Law Twins is difficult and is done visually by identifying crystal growth and etching patterns on crystal faces. Also, the beautiful iris effect (a rainbow display of colour) in agates is due to Brazil Law Twinning of the chalcedony in the fortification bands that disperse white light into a rainbow of colour. Experiments with crossed-Nichol prisms (use of polarizing effects) show a distinctive zigzag pattern in thin-sections of Brazil Law Twin quartz. This pattern is called “Runtzel Banderung”, after the German physicist who discovered it.
  • BrecciaA rock composed of varyingly sized, angular fragments, which have been cemented together.
  • BrickearthLoess material that has been reworked by fluvial action.
  • CalciteChemical 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.
  • Caledonian OrogenyThe great mid-Palaeozoic episode of mountain building in northern Europe, which built the Caledonian mountains of Scotland.
  • CallianassidThe mud shrimps (family Callianassidae) are burrowers whose principal calcification in the exoskeleton is in the chelae. They are rarely preserved complete, but their chelae may be locally abundant.
  • Cap rockAn impermeable sedimentary rock, such as a mudrock or evaporite, through which fluids cannot migrate. Cap rocks overlie traps formed by reservoir rocks and in a position that prevents further migration of petroleum.
  • CarapaceThe carapace is the ‘shell’ (=exoskeleton) of a crab, protecting the dorsal and lateral parts of the ventral surface. In most families, it is well calcified and easily fossilised.
  • Carbonaceous layerA bed rich in carbon, probably containing ample fossilised plants or plant debris.
  • CarbonatesThose rocks and minerals which have CO3 within their composition. Examples include limestone (CaCO3), Malachite (CuCO3CU(OH)2) and dolomite (CaMg(CO3)2).
  • CarbonisationA form of fossilisation where an organism’s organic content is reduced to a thin carbon film. Commonly encountered in plant fossils.
  • Caribbean plateThe crustal plate on which most of the Caribbean islands are situated.
  • CastA three dimensional, fossilised representation of the original organism, part of an organism or traces left by an organism. The counterpart to a mould.
  • CementThat material which binds together particles within a rock.
  • ChalcedonyA variety of quartz that is microcrystalline. Some chalcedony in agate is fibrous. This is caused when the tiny quartz fragments are stacked up, which gives them a thread-like appearance.
  • Chalcedony amygduleThe name given to a cavity in hardened lava, which has been filled with the variety of quartz called chalcedony.
  • ChalkA very pure limestone from the Upper Cretaceous.
  • Channelized volcaniclasticTuffaceous extrusive rocks preserved in a channel. The channel most probably formed by the erosive action of tuffs at the time of eruption, although they could also infill a dry river valley.
  • Chelae(sing., chelae): The enlarged anterior-most pair of legs of a decapod crustacean, modified as pinching claws.
  • ChelipedA cheliped is the a joint of the anterior limbs that directly supports a chela.
  • ChertA siliceous sedimentary rock. It commonly forms by precipitation of silica from solution (see silicification below) in a pre-existing sedimentary rock, such as a limestone.
  • ChloriteA disparate group of layer-lattice minerals with a distinctive green colouration.
  • ChloritizationThe introduction of, production of, replacement by, or conversion into chlorite
  • Clastic rocksRocks that have been formed from eroded or weathered particles of other rocks, e.g. sandstones, breccias etc.
  • ClayA rock composed of particles smaller than 1/256mm. It can be distorted easily (it is plastic) when wet.
  • CoalA 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.
  • CobbleA rock with a size of between 64mm and 256mm.
  • CoccolithsTiny circular plates produced by plankton and composed of CaCO3. Chalk is largely composed of coccoliths.
  • Cold seepAn area of the sea floor where geochemical energy sources, such as hydrogen sulphide, other reduced sulphur compounds and/or methane, are being released onto the sea floor. Cold seep communities rely on geochemical rather than photosynthetic energy; the primary producers are chemosynthetic bacteria.
  • CompactionThe process whereby, during diagenesis, a sediment’s grains are packed together and pore spaces and water are largely eliminated.
  • ConchoidalDenoting a rock fracture that is curved and has concentric ripples radiating from the point of impact.
  • ConcretionsMasses 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.
  • Condensation reactionA chemical reaction in which molecular water is produced, often by splitting off hydroxyl groups from the parent molecule.
  • Cone-in-cone structureA 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.
  • ConformableDenoting a sequence of deposits that has accumulated without a break in deposition.
  • ConglomeratesRocks composed of rounded pebbles cemented together.
  • Connate watersWater trapped in the pores of a sediment at the time of deposition.
  • ConsolidationAnother word for ‘diagenesis’ – the process during which sediments are compacted and/or cemented, to become rocks.
  • Contact metamorphismA change in the character of surrounding rocks when subjected to intense heat from magma intrusions.
  • Cross-beddingBedding planes that are inclined and often cross and terminate each other. Generally formed in river sediments.
  • DaciteAlso known as rhyolite, this is a fine-grained to glassy volcanic rock.
  • Dactylus(pl., dactyli): The first segment of a limb of the thorax, that is, its termination.
  • DeformationWhen a rock layer’s structure is altered by tectonic forces. Examples of deformation include folding and faulting.
  • Derived fossilA fossil incorporated into younger sediments after it has been weathered or eroded out of its original matrix.
  • Desiccation cracksCracks 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.
  • DiagenesisThe process during which sediments are compacted and/or cemented, to become rocks.
  • Diapiric melangeAn intrusive body, either igneous or sedimentary (muds, evaporites) in origin, which pierces and deforms the overlying beds into domed structures. Mud diapirs of the diapiric melange in the Scotland District deformed the basal complex, but, in turn, originated within the lower parts of the complex.
  • DiapirismAn intrusive body, either igneous or sedimentary (muds, evaporites) in origin, which pierces and deforms the overlying beds into domed structures. Mud diapirs of the diapiric melange in the Scotland District deformed the basal complex, but, in turn, originated within the lower parts of the complex.
  • Differential erosionCaused 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.
  • DiffractionLight is deviated from its straight ahead/line path when it passes through a tiny slit, tiny hole, or very near a sharp edge. The process by which this takes place is called diffraction. ‘Tiny’ means a slit or hole that is about equal to the wavelength of the light. Following its passage through such a small slit or hole, or near a sharp edge, white light is dispersed into a rainbow of colours. That dispersion of light is called the diffraction pattern.
  • DiffusionThe process whereby molecules of a substance move in a random path from a region of high concentration to a region of lower concentration. Think about walking across a street, which is filled by people jostling about – you don’t walk a straight line to get across the street and you are also jostled about.
  • DimorphismWhere one species is found in two distinctly different forms, for instance when the male of an ammonite species is smaller than the female.
  • Dip slopeA slope that runs roughly parallel to the gradient of the rock bedding below.
  • DoggerA large, calcareous nodule or concretion. Spherical or sub-spherical in shape.
  • DolomitisationThe process whereby a calcium carbonate rock is converted to double calcium magnesium carbonate – ‘dolomite’. Fossils are usually destroyed during this change.
  • DriftMaterial left behind by glacial and fluvio-glacial conditions.
  • DripstoneCalcitic deposits produced by precipitation from dripping water, rich in dissolved calcium carbonate; usually formed in caves within limestones.
  • DynomenidThe family Dynomenidae a group of crabs that preferentially inhabit corals.
  • Echelon diffraction gratingThis is a device that is a microscopic stairway, where the width and height of each step is about equal to the wavelength of light, that is, around 500 nanometres.
  • Electric dipole natureThe strength of the electric dipole and the geometric shape of the electric field it produces. An ideal electric dipole is a composed of a positive charge (+q) and an equal size negative charge (–q) separated by some distance ‘d’. The electric field caused by such a combination of electric charge has a particular geometric pattern, which is the same as that seen around a bar magnet. It can also be described mathematically.
  • EpifaunaThose 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.
  • ErraticRocks 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.
  • Eutaxitic texture“Said of banded structure in certain extrusive rocks, resulting from the parallel arrangement and [alternations] of layers of different textures, mineral composition, or colour. Commonly applied to banded structures … in welded tuff” (Neuendorf et al., 2005, p. 220).
  • EvaporiteA rock formed by the evaporation of saline water, e.g. rock salt.
  • ExfoliationA form of weathering where, due to expansion and contraction during varying thermal conditions, thin layers split away from rocks.
  • ExposureAn exposed area of in situ rock.
  • FaultA fracture in rock along which there has been movement. There are numerous types, including normal faults, reverse faults and pivot faults.
  • Faunizone zoneStrata that contain a certain faunal assemblage.
  • FeldsparsThese are the most significant single group of silicate minerals that form rocks.
  • Fixed fingerThe lower part of the pinching claw, rigidly attached to the chela.
  • FlintA form of chert. Found in the Cretaceous Chalk as nodules and thin layers. It has a conchoidal fracture and often contains fossils.
  • Flow bandingFlow structures commonly seen in rhyolitic rocks
  • FoldA deformation of rock strata, usually caused by tectonic forces.
  • FoldingA deformation of rock strata, usually caused by tectonic forces.
  • ForaminiferaProtistan (= unicellular) microfossils, with a skeleton composed of calcite, agglutinated particles or secreted organic matter (tectin). Foraminifera include both planktic (for example, Globigerina) and benthic taxa (for example, Amphistegina).
  • Fore reefThe seaward side of a reef, sloping into deeper water from the reef crest.
  • Forearc basinThe basin on the oceanward side of a volcanic arc, that is, on the side where subduction is occurring.
  • Fortification bandThe common pattern in agates, which consists of concentric bands of varying colour and width that reminds people of crenulations in a fortress wall. Such a band has three structurally different parts. The first layer formed consists of tiny quartz fragments or crystallites (nanometre to tens of nanometres in size) where the crystal axes are essentially indistinguishable. The second layer is the chalcedony layer composed of stacks of quartz crystal fragments, from hundreds of nanometres to micrometer in size, where the crystal axes are distinguishable. The final layer of a fortification band is the coarse or euhedral quartz, which is tens of micrometers to centimetres in size and have the “c-crystal” axis roughly perpendicular to the fortification band.
  • Fortification segmentsA bird’s-eye view of a fortress shows different straight lines or circular segments, which often meet at guard towers. In an agate, the birds-eye view provided by a cutting a slice from the agate often reveals a series of curved segments, straight segments or circular segments, which join together. (In agate, there is often a very thin line where the segments meet.) Such a pattern of curves, straight lines and circles reminded early agate collectors of the bird’s-eye view of a fortress and called the resulting pattern ‘fortification banding’.
  • Fracture How a homogenous rock will break, e.g. flint has a conchoidal fracture.
  • GastrolithA 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 errati
  • GeochronologyThe subdivision of geological time into intervals of known duration.
  • GeodeA rock containing a crystal lined hollow.
  • Geothermal gradientThe sequential difference in temperature of the Earth’s crust with increasing depth.
  • Globigerina(Foraminifera): Protistan (= unicellular) microfossils, with a skeleton composed of calcite, agglutinated particles or secreted organic matter (tectin). Foraminifera include both planktic (for example, Globigerina) and benthic taxa (for example, Amphistegina).
  • GrainstoneA grain-supported, clastic, mud-free limestone.
  • Granular textureDescribing the texture of a rock containing roughly equally sized grains.
  • GravelStrictly speaking, rocks with sizes of between 2mm and 4mm.
  • Gravity anomalyThe difference between measured gravity and that expected based on theoretical determinations. A negative gravity anomaly, as occurs in Barbados, is a lower measurement than would otherwise be predicted. This can be explained by the island resting on a thick pile of ‘light’ Cenozoic sediments and sedimentary rocks.
  • GreywackeA ‘dirty’, unsorted sandstone, deposited probably no great distance from the source rock, and including a wealth of grains of contrasting mineralogy, size and degrees of roundness.
  • GroundmassA finer-grained material making up the main body of a rock, in which larger units appear.
  • HemipelagicHemipelagic sedimentary sequences are deposited on the continental shelf and continental slope.
  • HermatypicA reef-building organism, such as certain colonial species of scleractinian corals.
  • Hermit crabThe hermit crabs (family Paguridae) are a group with a poorly mineralised carapace that live attached to the insides of discarded gastropod shells. Their fossil record is mainly based on the well-mineralised chelae.
  • HolotypeOne 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.
  • HorizonA rock unit that is recognisable due to a distinctive lithology or fossil assemblage.
  • IgnimbriteCommonly extrusive, pyroclastic igneous rocks, such as sillars and welded tuffs (see below). Such eruptions are invariably explosive and form from nuée ardentes or glowing cloud eruptions, such as the one that destroyed St Pierre in Martinique in 1902, killing 27,000 people in just a few minutes.
  • In situRocks or fossils that are within their original strata and not loose.
  • InclusionOne substance enclosed by another, e.g. an insect in amber.
  • Inclusions in an agateAggregates of a non-silica compound. They include non-silica crystals that are usually needle-like or miniature fern-like tendrils, or resemble moss that is twisted, looped and thread-like.
  • Index fossilA fossil species that characterises a certain horizon by its abundance, but is not solely restricted to that horizon.
  • Index of refractionThe ratio ‘n’ of the speed of light in a vacuum ‘c’, or interstellar space, to the speed of light in a material ‘v’ is called the material’s index of refraction (n = c/v). The higher a material’s index of refraction, the greater the deviation of light from its original direction when it enters the material at an angle.
  • IndurationAnother word for ‘diagenesis’ – the process whereby sediment is converted to rock.
  • InferiorMeaning ‘lower’, e.g. ‘Inferior Oolite’ translates to ‘Lower Oolite’.
  • InlierAn ‘island’ of older rocks completely surrounded by younger rocks.
  • InterbeddedBetween two beds, e.g. a layer of coal may be interbedded between two layers of sandstone.
  • IsochronousOccurring or formed at the same time.
  • JointingBreaks within rock layers, across which there has been no perceivable movement. In sedimentary rocks jointing is usually produced by tectonic activity.
  • KarstTopography associated with limestone (or other soluble rock such as evaporites), generally in regions of at least moderate rainfall and produced by solution, which occurs mainly in the subsurface.
  • Karst sceneryA 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.
  • LaminationsSuites of thin strata.
  • LapilliFragments of igneous rocks of between 4 and 32mm in size.
  • Latite(or trachy-andesite): is an intermediate igneous rock, considered the extrusive equivalent of monzonite (syeno-diorite). Normally it is porphyritic – with phenocrysts usually of plagioclase feldspar, and a groundmass of augite and orthoclase
  • Length-fast chalcedonyThis is a variety of fibrous stacked crystallites of quartz with their c-axis perpendicular to the direction of stacking (or increasing length of the fibrous quartz).
  • LiasThe Lower Jurassic
  • Liesegang bandingThe regular zones or bands of precipitated chemical materials, when a mineral such as red iron oxide (which precipitates from an aqueous solution as a catalytic agent) moves through into undisturbed solution to reinitiate the process. This process again results in precipitation of iron oxide into a narrow band, freeing the catalytic agent to continue its movement through the solution or silica gel. This banding is called ‘Liesegang banding’ and forms in response to diffusion controlled movement of iron and oxygen in the solution or rock.
  • LigniteYoung, brown coloured coal. Considered to be of a low quality when compared to older, black anthracite.
  • LimestoneA rock composed of calcite or dolomite. Often fossiliferous, although dolomitic limestones are less so than calcitic.
  • LithifactionAnother word for ‘diagenesis’ – the process whereby sediment is converted to rock.
  • LithofaciesA rock type that is characteristic of a certain environment.
  • LithologyReferring to the physical character of a rock or sediment.
  • LoessWind blown sand, deposited (in the case of the UK) under periglacial conditions.
  • Marine bandA stratum containing marine fossils that is interbedded between two non-marine strata.
  • Marker bedAn easily recognisable stratum that can be used to correlate rock sections that were deposited contemporaneously (at the same time), in different locations.
  • MarlA calcareous mudstone.https://depositsmag.com/wp-admin/admin.php?page=pms-basic-info-page
  • MineralA naturally formed homogenous solid that has a definite chemical composition. Often crystalline.
  • MouldAn impression of the original item. The counterpart of a cast.
  • Mountain Uplift TheoryCT Trechmann’s hypothesis that the uplift of mountains was driven by the gravitational pull of the moon.
  • Mud diapirsAn intrusive body, either igneous or sedimentary (muds, evaporites) in origin, which pierces and deforms the overlying beds into domed structures. Mud diapirs of the diapiric melange in the Scotland District deformed the basal complex, but, in turn, originated within the lower parts of the complex.
  • Mud shrimpThe mud shrimps (family Callianassidae) are burrowers whose principal calcification in the exoskeleton is in the chelae. They are rarely preserved complete, but their chelae may be locally abundant.
  • Mud volcanoesA mud diapir that has breached the ground surface and is extruding mud to form a structure of positive relief.
  • Nano-aggregateA collection of tiny, nanometre-size particles, loosely clumped together.
  • Nano-particleA particle, such as a molecule or group of molecules, which is about one nanometer in the longest dimension.
  • NappeA large-scale, allochthonous rock unit formed by thrust faulting, for example, the Oceanics of Barbados.
  • Net charge separationThe condition where electrical charges appear or, as physicists say, ‘are induced’ on an uncharged object, because of a nearby charged object. The net charge on the object is still zero. A rod-like aggregate with net charge separation arising through induction acts like an electric dipole.
  • NoduleA round or sub-round concretion of pebble size.
  • Non-equilibrium processesIn the science of thermodynamics (TD), a system that is not in TD equilibrium has driving forces: thermal, electrical, mechanical, temperature, chemical potential, concentration gradients, and so on, which tend to be spontaneously driven toward TD equilibrium. Many natural processes are non equilibrium processes involving the transfer or use of released energy to drive processes such as chemical reactions.
  • Non-equilibrium, small-scale chemical microenvironmentshen the non-equilibrium TD processes occur in a micro-volume of space or solutions, rather than in the large volumes common to everyday experience, we call them processes in non-equilibrium, small-scale chemical microenvironments. They are natural processes that drive the system, small though it may be, towards TD equilibrium. In these microenvironments, the thermodynamic laws are not well understood.
  • NucleationThe process where a foreign particle or structure acts as a scaffold for a crystal to grow on. It eliminates need to create a new surface for crystal growth and the associated surface energy requirements.
  • Nucleation, particularly heterogeneous nucleationThe process where a foreign particle or structure acts as a scaffold for a crystal to grow on. It eliminates need to create a new surface for crystal growth and the associated surface energy requirements.
  • OlistostromeAn allochthonous sedimentary mélange formed by chaotic submarine slides or debris flows of blocks into deeper water.
  • Olistostromic blocksAn allochthonous sedimentary mélange formed by chaotic submarine slides or debris flows of blocks into deeper water.
  • OoliteAn old name for the Upper Jurrasic of Britain and Europe.
  • OolithA spherical or sub-spherical rock particle. These contain a nucleus that has had a mineral (usually calcite) built-up around it.
  • OphiolitesAre pieces of oceanic plate thrust onto the edge of continental plates (obducted). The term applies to an assemblage of igneous rocks that match the sequence found at mid-ocean ridges: basaltic pillow lava above, gabbro and peridotite below.
  • Orogenic activityMountain-building processes.
  • OrogenyA 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.
  • OutcropThe 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.
  • OutlierAn ‘island’ of younger rocks, completely surrounded by older rocks.
  • OverburdenLoose 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.
  • PacketA fault-bounded tectonostratigraphic unit consisting of more or less deformed rocks, but with a definable internal stratigraphy.
  • PackstoneA grain-supported, clastic limestone with some interstitial lime mud.
  • PalaeobathymetryThe depth of deposition of an ancient, commonly sedimentary rock unit, determined using uniformitarian principals and preferably using multiple criteria, such as the data made available from sedimentology, body fossils and trace fossils.
  • PalaeoclimatologyThe study of previous climates by drawing climatological inferences from sediments and the fossil types that they contain.
  • PalaeocurrentA representation of a fossil current within a rock, inferred from sedimentological structures such as ripple marks and/or cross bedding etc.
  • PalaeoecologyThe ecology of fossil assemblages, e.g. how fossil organisms interacted with each other, how sediment types lead do different fossil assemblages etc.
  • PalaeogeographyA 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.
  • PalynologyPalynology: The study of fossil pollens and spores.
  • Patterned chalcedonyChalcedony that forms in repeating bands, arc, swirls, and lines of various colours in agates. While the colours are understood to result from different non-silica molecules such as iron oxide, the process for the formation of repeating bands, arc, swirls and lines is not understood.
  • PebbleA rock with a size of between 4mm and 64mm.
  • PelagicDescribing marine animals that swim freely or float within the water.
  • PermeabilityDenoting the ease with which water can flow through a rock.
  • PetrifactionThe process whereby an organism’s structure is converted to rock.
  • PhanerozoicThat period of time during which obvious and abundant life has existed – from the Cambrian to the present day.
  • Pheno-fibre quartz bandThis is interpreted as an agate band where quartz crystal, growth resembles a fibre, but actually is an assemblage of long, thin and perhaps twisted crystals.
  • PhenocrystLarge crystals found in igneous rocks.
  • Placer depositsDeposits of heavy and valuable minerals that have been concentrated by the action of water.
  • PlagioclaseMinerals within the feldspar family (see above), made up of sodium and calcium.
  • PlanktonicA more commonly used synonym for ‘pelagic’ – describing marine animals that swim freely or float within the water.
  • Plumes (agates)Refer to the tree-like or fern-like growths of branched non-silica minerals surrounded by the chalcedony of the agate.
  • PorosityThe ratio between open spaces within a rock that can hold water, and solid material that cannot.
  • PortunidThe family Portunidae includes the swimming crabs, which have the posterior-most pair of legs modified as flattened paddles.
  • Preferred orientationDescribing rock particles that have a certain common orientation, usually due to currents.
  • Propodus(pl., propodi): The second segment of a limb of the thorax, that is, the segment that supports the dactylus.
  • ProvinanceThe area from which material making up sediments (and therefore, rocks) has come.
  • Pudding stoneA synonym for ‘conglomerate’.
  • Pyroclastic`Fragmentary rocks produced by explosive volcanism, such as an acidic tuff
  • RadiolariansSiliceous protistan (= unicellular) microfossils, commonly with complex and ornate skeletons.
  • Raised beachA 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.
  • RaninidThe family Raninidae are called frog-crabs because of their appearance.
  • ReefA 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 fossilsHard 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.
  • Reservoir rockAn incompressible, porous, permeable sedimentary rock in which oil and/or gas can accumulate following migration from a source rock. A reservoir rock of suitable stratigraphic or structural geometry, for example, an anticline capped by an impermeable mudrock, forms a hydrocarbon trap in which petroleum may accumulate.
  • Residual depositThat part of a rock left behind after chemical weathering.
  • Residual magmatic fluidsRefers to that portion of the solution and gas in an agate-forming cavity that is a leftover or a residue from the original exposure of the magma to earth surface or ocean bottom conditions.
  • RhyoliteA fine-grained, extrusive igneous rock of acidic composition, similar to that of a granite.
  • Rock mechanicsThe study of the mechanical properties of rocks e.g. porosity, sheer strength and crushing strength etc.
  • Rotten stoneThe siliceous residue left behind after the weathering of certain types of limestones.
  • RoundnessA 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.
  • RutileTitanium dioxide, or TiO2, which is a naturally occurring mineral. In agates, it is often found as needle-like crystals.
  • Sagenitic spraysRefer to the needle-like or acicular inclusions in agate, which are often in a radiating pattern.
  • SandRock particles with a size of between 1/16mm and 2mm.
  • ScreeAn accumulation of loose rock pieces, often found on slopes below outcrops of in situ rock.
  • SessileDenoting a non-mobile organism.
  • Shelf faciesThose sediments and their floral and faunal contents that accumulate on the shallower, near land, ‘shelf’ areas of an ocean.
  • Silica monomersThe tetra-hedral molecules described by the equation Si(OH)4.
  • Silicic eruptionsLarge-scale explosive volcanic eruptions (rather than basaltic eruptions, which are mostly effusive).
  • SiliciclasticA sedimentary rock formed from mechanically-derived fragments of pre-existing, silica-rich rocks. Examples of siliciclastic rocks include mudrocks, siltstones, sandstones and conglomerates.
  • SilicificationWhereby silica in solution is introduced into a non siliceous rock e.g. the formation of flint nodules within chalk.
  • SilicificationThe process by which silica is introduced into non-siliceous rocks, either by the filling of spaces between pores or by replacement (for example, of calcite).
  • SiliclasticSedimentary rocks consisting of fragments of silicic components (mostly quartz, feldspars and heavy minerals).
  • SillarAn ignimbrite that lithified “… after deposition by recrystallisation due to the activities of escaping hot gases and fluids” (Wyatt, 1986, p. 161).
  • SiltRock particles with a size of between 1/256mm and 1/16mm.
  • SlickensidesStriated surfaces on a fault plane, gouged out as fault moves. Therefore, a slickenside indicates the direction in which a fault has moved.
  • SlickenslidesWhen 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.
  • SlumpMass movement of unconsolidated material down a slope, producing a pile of debris at the bottom.
  • Source rockSedimentary rocks in which hydrocarbons are generated. Source rocks are invariably fine-grained sedimentary rocks (=mudstones), with low permeability and porosity, that were deposited under low energy conditions and which contain a large proportion of organic matter.
  • South American plateThe crustal plate on which the continent of South America and adjacent floor of the western Atlantic Ocean are situated.
  • Step-like, crypto crystal-facetsThink of the steps on a very steep, nearly vertical stairway. Then, picture the steps (hence, ‘step like’) as being made of very tiny (hence ‘crypto”) quartz crystals stacked up, one on top the other, so that two of the six, long crystal ‘facets’ on each one forms the riser and tread of each crystalline stair-step.
  • Stratigraphic nomenclatureA 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.
  • StratigraphyThe study of layered (stratified) rocks, including the description of their physical characteristics and the correlation of strata between locations.
  • StratumAnother name for a bed or a layer. Plural is ‘strata’.
  • StreakThe colour a mineral produces when scratched across a non-glazed porcelain surface. Often the streak colour is different to the colour of the mineral.
  • StriaeSmall grooves and scratches on rock surfaces, often caused by the movement of glaciers over in situ material.
  • StriationsSmall grooves and scratches on rock surfaces, often caused by the movement of glaciers over in situ material.
  • Subduction zoneThe elongate and arcuate region where one tectonic plate is sliding under the edge of a second plate, forming an oceanic trench.
  • SyngenicTwo geological processes or events occurring at the same time.
  • TectonostratigraphyThe stratigraphy of tectonic units (packets) in each of which may be a definable sedimentary succession.
  • Tension fissure“A fracture that is the result of tensional stress in a rock” (Neuendorf et al., 2005, p. 661). They are commonly associated with the displacement of structures like joints (Fig. 2A). Also known as tension joints or tension fractures.
  • TerraceA flat-lying area of land in a region of stepped topography, with a steep descending slope on its more seawards margin and a steep ascending slope on its more landwards margin (these steep slopes may be cliffs or ancient cliff lines).
  • Terrigenous sedimentsSediments formed on land, and also sediments that have come from the land surface but are deposited in the sea.
  • TestThe proper word to use when referring to various creatures’ (e.g. echinoids and forams) shells or skeletons.
  • TillAnother name used for ‘drift’ or ‘boulder clay’.
  • Trace fossilsFossils of impressions, track-ways etc. and not of actual animals.
  • TurbiditesSedimentary deposits formed by settling of a suspended sediment load derived by rapid, downslope transport, usually formed in deep water.
  • Type localityA 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.
  • UnconformityPut 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.
  • UndercuttingWhere 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.
  • UniformitarinaismThe 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.
  • Unwelded tuff(=sillar): a pyroclastic rock (=tuff), in which the grains have not been welded together by its included hot gases and the weight of overburden. Contrast with welded tuff.
  • VarveA 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.
  • VeinAn accumulation of minerals along a fault or a joint. The minerals are usually igneous in origin.
  • Volcanogenic strataSedimentary strata derived from eruption of particulate volcanic rocks, such as ash and tuff.
  • Wall-banded agateRefers to agate in which the fortification bands repeatedly mimic the shape of the cavity in which the agate forms. If the cavity is spherical, the wall-banding fortifications in three-dimensions are a series of concentric shells of chalcedony, or in cross-section, a series of concentric circles.
  • WeatheringThe 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.
  • Welded tuff“A glass-rich pyroclastic rock that has been indurated by the welding together of its glass shards under the combined action of the heat retained by particles, the weight of the overlying material, and hot gasses” (Neuendorf et al., 2005, p. 719).
  • Wind erosionErosion caused by wind-bourn particles. Usually encountered in arid locations.
  • Zone fossilA 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., together with terms used throughout our Deposits Magazine. printed issues. New terms added with every issue.