Acid rock An igneous rock with 10% or more free quartz. Granite is an example of an acid igneous rock.
Aeolian deposits Sediments, usually sand, deposited after transport by the wind.
Alkali feldspar Potassium-bearing members of the feldspar group of silicate minerals.
Alkaline A term for an igneous rock which is rich in sodium and potassium.
Alluvial Processes or landforms involving deposition of sediment; river channels occupying river sediment rather than bedrock.
Amphibole The name for a group of silicate minerals, with a double chain structure, often found in metamorphic rocks.
Amphibolite A metamorphic rock composed mainly of amphibole.
Andesite A fine-grained intermediate volcanic igneous rock characterised by the presence of oligoclase or andesine feldspar.
Avalonia An ancient continent that consisted approximately of present day England, Wales and northern France.
Basalt A black fine-grained, basic igneous rock, commonly forming lava flows and consisting of iron oxide and silicate minerals including feldspar and pyroxene.
Basement The oldest rocks recognised in a given area, usually a complex of metamorphic and igneous rocks underlying all the sedimentary formations.
Bedding planes The junctions between rock layers marking the original surfaces onto which sediment was deposited (e.g. in sandstone).
Bedload Sediment transported along the bed of a river by rolling, sliding or saltating.
Boulder clay Glacial deposit consisting of boulders (grains varying in size) in a clay-dominated matrix. Boulder clay is laid down beneath a glacier or ice sheet and is typically unsorted, i.e. the grains/boulders are not in an arrangement of large at the base and small at the top of the sequence.
BP Years before present.
Brachiopod A group of bivalved marine animals varying in size from 5 mm to 20 cm and having either a calcareous or chitinous shell. They superficially resemble mollusc shells.
Breccia A sedimentary rock containing angular rock fragments which are greater than 2mm across.
Caldera A very large volcanic crater which may arise by the collapse of surface rocks into an underground magma chamber.
Caledonian Orogeny The mountain-building event that occurred when England and Scotland collided around 430 million years ago. Prior to the orogeny, Scotland formed part of the continent of Laurentia and England formed part of the continents of Avalonia and Baltica. Between these continents lay an ocean called the Iapetus. Plate tectonics caused the continents to move towards each other, closing the ocean. During this time, the ocean floor was subducted beneath Laurentia. On the final closure of the Iapetus Ocean, the continents collided and a period of mountain building took place.
Cambrian A period of geological time, dating from about 545 to 490 million years ago.
Carboniferous A period of geological time, dating from about 350 to 290 million years ago.
Cenozoic (Cainozoic) An era of geological time, dating from about 65 to present day (comprising the Tertiary and Quaternary periods).
Clasts Mineral (rather than organic) particles of sediment, ranging from sand grains to boulders.
Cleavage plane The plane of mechanical fracture in a rock. Cleavage planes are normally sufficiently closely spaced to break the rock into parallel-sided slices.
Coarse-grained A descriptive term for a rock with large constituent grains or crystals (a few mm or more across).
Columnar jointing A joint pattern, related to cooling of large bodies of volcanic rock. Thick lava flows, especially basalts, develop vertical cooling joints which neatly divide the rock into many polygonal columns. The columns arise owing to contraction during the cooling of the molten rock and are usually hexagonal but sometimes are pentagonal or heptagonal.
Continental drift Movement of the continents over the Earth’s surface.
Coprolite Fossilised excrement of fish, reptiles, birds or mammals.
Cretaceous A period of geological time, dating from about 140 to 65 million years ago
Crust The Earth’s outermost layer. The continental crust is about 40km thick and consists mostly of granite; the oceanic crust is about 10km thick and consists mostly of basalt.
Dalradian Supergroup A group of metamorphosed sediments laid down in Scotland and Ireland between 800 and 600 million years ago. The term has both geographical and chronological significance and may be used as an adjective.
Deformation A term used to describe changes in the shape, attitude or volume of a layer of rock after its formation.
Deglaciation Disappearance by melting of ice covering an area. Deglaciation can vary greatly in speed, and ranges in nature from the retreat of a melting ice margin to the downward melting and fragmentation of stagnant ice.
Devensian A geological time period (between 115,000 and 10,000 years ago). This period includes the most recent ice sheet glaciation of northern Britain.
Diorite A coarse-grained plutonic igneous rock of intermediate composition, consisting essentially of intermediate plagioclase feldspar.
Dolomite A calcium-magnesium carbonate mineral, CaMg(CO3)2.
Duplex A stack of thrust-bounded rock slices, bounded by a roof thrust and a floor thrust, formed through continued thrusting along a floor thrust with successive collapse of thrust ramps.
Erratic Rock transported by a glacier and deposited on the Earth’s surface a distance from its origin. Often, the rock type is different from the rock on which it overlies.
Esker A sinuous ridge of sand and gravel deposited by meltwater in a subglacial channel.
Eurypterid Arthropods allied to the scorpions, varying in size from 10 cm to 2m in length. Originated in the marine environment but shifted to predominantly brackish-fresh water during their geological history.
Feldspar The general term for a group of important aluminosilicate (rich in Al and Si) minerals. Includes plagioclase feldspar, which contains Na and/or Ca; and alkali feldspar, which contains K. Feldspars are the most common minerals in the Earths’ crust.
Fine-grained A descriptive term for a rock in which the constituent grains or crystals are not large enough to be seen with the naked eye.
Fold A flexure in a rock, with a change in the amount and direction of dip of the structural planes.
Foreland An area of rock which is undeformed during an episode of mountain-building.
Formation A rock unit, being a subdivision of a group or subgroup.
Fossil The remains of a once-living organism that is typically older than 10,000 years. The remains are preserved in sedimentary rock, where the hard body parts, e.g. skeleton, teeth, or scales (and occasionally the soft body parts) are preserved in a number of ways through chemical interaction with the surrounding rock and groundwater. Trace fossils are fossils of the tracks, impressions and burrows made by organisms.
Gabbro A coarse-grained igneous rock consisting of feldspar and magnesium- and iron-rich silicate minerals, including pyroxene and sometimes olivine.
Geological Conservation Review (GCR) A comprehensive assessment of all key sites of geological and geomorphological importance within the UK. All sites selected by the GCR are of national or international importance for geology or geomorphology.
Glaciofluvial sediments Sediments originally deposited by meltwater associated with glacial ice. The term glaciofluvial is used interchangeably with fluvioglacial.
Gneiss A coarse-grained metamorphic rock, with compositional layering and alignment of minerals, produced under conditions of regional high pressure.
Granite A coarse-grained igneous rock consisting of quartz, feldspar and very commonly mica.
Graptolite An extinct group of marine colonial organisms consisting of one or more branches (stipes). The individuals of the colony (zooids) were situated in cups arranged along the stipe.
Hornblende A member of the amphibole group of minerals
Hornblendite A rock composed almost entirely of hornblende.
Iapetus Ocean An ancient ocean that no longer exists and is only recognisable by the sediments that were laid down in it. It was formed by splitting of a continent called Laurentia about 600 million years ago and disappeared when other continents named Avalonia and Baltica collided with Laurentia between 500 and 400 million years ago.
Igneous rocks Rocks formed by the solidification of molten rock or magma.
Ignimbrite A type of rock consisting of fragmental volcanic material produced through explosive volcanic activity, transported and deposited by high temperature pyroclastic flows.
Interglacial Prolonged period of warm global climate when ice retreats to high mountains and the polar regions only.
Intrusion (noun) A body of igneous rock which has forced itself into pre-existing rocks, either along some definite structural feature, or by deformation and cross-cutting of the invaded rocks. (verb) The action of magma forcing itself into pre-existing rocks.
Imbrication The formation of a sequence of thrust-bounded rock slices, through continued thrusting along a floor thrust with successive collapse of thrust ramps. The stack of thrust slices produced in this way is known as an imbricate stack (also termed a duplex) and has an appearance rather like a stack of roof tiles.
Jurassic A period of geological time, dating from about 205 to 140 million years ago.
Kettle hole A hollow in sand and gravel where an underlying body of ice has been melted out.
Lateglacial Interstadial A relatively warm period between the end of the full glacial conditions (c.13,000 years ago) and the onset of the Loch Lomond Stadial (c. 11,000 years ago).
Laurentia An ancient continent that formed about 600 million years ago by the fragmentation of the super continent called Rodinia. The metamorphic rocks of Laurentia form the lower part of the crust of much of Scotland and crop out at the surface as the gneisses of the Lewisian Foreland;.
Lewisian Foreland See entry on Laurentia.
Lewisian Gneiss Gneissic rocks, around 3000 million years old and with a complex history, which form the basement for much of Scotland.
Limestone A sedimentary rock composed mainly of calcium carbonate (CaCO3), usually as the mineral calcite.
Loch Lomond Readvance An advance of glaciers in many parts of Scotland, in response to the cold period of the Loch Lomond Stadial.
Loch Lomond Stadial A brief reversion to cold conditions after the Late Devensian ice sheets melted. This episode probably occurred between 11,000 and 10,000 years ago.
Magma Molten rock that forms igneous rocks upon cooling.
Mantle The main bulk of the Earth, between the crust and the core, ranging from depths of about 40-2900km. It is composed of dense mafic silicates and divided into concentric layers by phase changes that are caused by the increase in pressure with depth.
Marble A metamorphosed limestone produced by recrystallisation mainly under conditions of thermal metamorphism.
Mass movement Various down-slope movements of materials (including rock) under the influence of gravity, often assisted by water but not in flowing water.
Matrix The finer-grained material constituting the main part of a rock, in which larger clasts or phenocrysts may be set.
Meltwater channels Channels formed by glacial meltwater, often located away from the course of normal rivers. Can form under the ice, or in front or alongside it.
Metamorphic rocks Rocks of the Earth’s crust which have been altered by the effects of heat, pressure or chemically active fluids.
Metasediment Metamorphosed sedimentary rock.
Mesozoic An era of geological time, dating from about 250 to 65 million years ago (comprising the Triassic, Jurassic and Cretaceous periods).
Mineral Naturally occurring, inorganic substance that typically has a crystalline structure. Each mineral has characteristic properties such as hardness, colour, lustre, cleavage and relative density and each has a characteristic chemical composition, e.g. the mineral quartz is silicon dioxide (SiO2). Minerals are the components of rocks.
Mudstone A sedimentary rock, composed mostly argillaceous or clay-bearing grains.
Old Red Sandstone describes the terrestrial sedimentary and igneous rocks approximately equivalent in age to the Devonian marine rocks found in southwest England and Europe. Devised by Sir Roderick Murchison in 1859, the ORS was split into three divisions: Lower, Middle and Upper. Exact correlation of the ORS with stages of the Devonian is still under scrutiny. The base of the Lower ORS has been dated to the topmost Silurian (Wenlock-Llandovery) and the top of the Upper ORS is through to be in the lowermost Carboniferous at around 360Ma.
Ophiolite / obducted ophiolite Forms during subduction. Sequence of rock that represents a part of the oceanic crust (and in some cases upper mantle) that has been forced up onto a continental landmass during subduction (i.e. instead of being subducted with the rest of the oceanic plate, the ophiolite section becomes stuck on the continental landmass).
Ordovician A period of geological time, dating from about 490 to 440 million years ago.
Outlier A limited area of younger rocks completely surrounded by older rocks.
Oxygen isotope analysis A method of broadly determining past climate. The ratio of oxygen isotopes found fixed in shells in ocean sediments, or in old layers of ice, is largely determined by the relative size of the Earth’s ice sheets in the past. A plot of the changing ratio shows a proxy record of changing climate, in which cold or warm periods have been numbered for reference as isotope stages.
Palaeozoic An era of geological time, dating from about 550 to 250 million years ago (comprising the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods).
Periglacial Cold climates (with average annual temperatures of -15 to -1oC and seasonally snow-free ground), or surface features created under those climates.
Permian A period of geological time, dating from about 290 to 250 million years ago.
Permineralised A form of fossilisation, whereby minerals (from the surrounding groundwater) are deposited within the porous structures of the organism. This can preserve details of the organism down to the cellular level.
Phenocryst A crystal in an igneous rock larger than the other crystals of the matrix, and formed during the early stages of crystal development.
Phyllite A cleaved metamorphic rock having affinities with both slates and schists. The term is rather loosely used for rocks which are coarser-grained and less perfectly cleaved than slates, but which are finer-grained and better cleaved than mica schists.
Picrite A class of ultrabasic rocks consisting of 90% or more iron and/or magnesium- containing minerals.
Pillow lavas Pods of cooled lava that have the rounded appearance of pillows. Formed when lava enters the sea, whereupon the outer surface cools quickly, and the inner lava remains fluid for longer. Provides evidence for submarine volcanic eruptions.
Plate The Earth’s crust and part of the underlying mantle are divided into several rigid plates which glide over the underlying, viscous mantle. The movement of the plates (plate tectonics) is due to convection currents in the mantle.
Pollen Microscopic spores generated by higher flowering plants
Porphyritic A textural term describing igneous rocks containing relatively large crystals set in a finer grained groundmass.
Porphyry An igneous rock in which phenocrysts constitute 25% or more of the volume, the groundmass should be fine grained. The term may be used in conjunction with a mineral name, for example, quartz porphyry for a rock containing phenocrysts of quartz.
Precambrian A period of geological time, dating from the consolidation of the Earth’s crust around 4600 million years ago to about 550 million years ago.
Pyroxene The name for a group of silicate minerals, with a chain structure. Pyroxenes are typically found in basic rocks.
Pyroxenite A rock composed almost entirely of pyroxene.
Quartzite A rock composed almost entirely of quartz.
Quaternary The last 2 million years of the earth’s history, corresponding approximately to the era during which the planet has been extensively glaciated. Prior to the Quaternary, glaciations were rare.
Radiocarbon dating Technique for finding the age of organic remains (i.e. formerly living organisms) formed in the recent past. The technique relies on the measurement of the decay of carbon-14 atoms (contained in all organisms). The number of atoms decaying in a given time interval will decrease at a known rate, and so by measuring the number of decaying atoms, the age of the sample can be found. Another method relies on directly measuring the ratio of carbon-14 to other carbon atoms. The number of decaying carbon-14 atoms in samples older than c.40,000 years is so small as to be virtually immeasurable, and other dating techniques have to be used to find the age of these samples.
Radio-isotope dating Dating of rocks by measuring the different isotopes of elements within the rocks, and comparing them to known rates of decay of radioactive isotopes.
Ramp A high-angled part of a thrust fault which cuts up through rock layers.
Rhyolite Fine-grained to glassy acid volcanic rock mineralogically similar to granite.
Roof thrust The upper bounding thrust of a duplex.
Sedimentary rocks rocks formed through the process of deposition of particles consisting of either minerals or rock fragments.
Schist A metamorphic rock characterised by a parallel arrangement of most of its minerals.
Shale Fine-grained sedimentary rock formed from the accumulation of clay or mud particles, with a strong fissility along one plane.
Shearing Deformation of a body by translation of one part of it relative to another.
Sill A sheet-like body of igneous rock which is conformable with (parallel to) the bedding or structural planes of the host rock.
Sill A sheet-like body of igneous rock intruded along the bedding or structural planes of earlier host rocks.
Siltstone A sedimentary rock consisting of a predominance of silt-grade material.
Silurian A period of geological time, dating from about 440 to 420 million years ago.
Slate A low-grade metamorphosed fine-grained rock which have developed a well marked cleavage but have experienced little recrystallisation.
Stadial A cold period, interrupting warmer conditions, that is too short-lived for an ice sheet to fully form.
Stratigraphy The study of stratified rocks especially their sequence in time, the character of the rocks and the correlation of beds in different localities.
Subaerial Geomorphic processes that involve exposure to the atmosphere, i.e. not in rivers or the sea.
Suspended load Sediment carried within river flow, but not dissolved in it.
Tectonic Structural, related to movements of the Earth’s crust.
Tertiary A period of geological time, dating from about 65 to 2 million years ago (comprising both Palaeogene and Neogene periods).
Thin section Petrologists (those who study rock) use thin sections to study the mineralogy of rocks. A thin section is a microscopic glass slide to which a flat slice of rock is attached and ground to 30 microns thick (three hundredths of a millimetre). At this thickness, light can pass through the rock, allowing the properties of the minerals to be studied using polarised and the plane polarised light.
Thrust/Thrust plane A low-angle fault (or fracture) over which rocks are transported, with the direction of transportation up the dip of the fault. Thrusts form when the Earth’s crust is placed under compression, such as in zones of continental collision.
Till Sediment laid down directly from ice, either underneath or at the margins of ice bodies. Till usually consists of a wide range of sediment from boulders to mud.
Torridonian A rock unit formed in the Precambrian in Scotland, characterised by red sandstone, with the type exposures being in Torridon.
Triassic A period of geological time, dating from about 250 to 205 million years ago.
Trilobite A group of extinct marine arthropods.
Troctolite A type of Gabbro.
Tuff The general name for rocks formed from fragmental volcanic material that has been blown into the atmosphere by explosive activity.
Volcaniclastic Where volcanic ash and other debris has been transported and reworked through mechanical action, such as by wind or water, the resulting deposits (and subsequently rocks) are termed ‘volcaniclastic’.
Volcano A vent or fissure in the Earth’s crust, through which molten rock and hot gases escpe to the surface of the land, or the bottom of the sea.
Volcanic vent The vent through which volcanic materials are erupted onto the Earth’s surface. After volcanism has ceased, volcanic vents are often infilled with breccias composed of fragments of the volcanic rock and the rocks which surround the vent.
Zone fossils If a particular species of fossil is restricted to a narrow unit of time and is also abundant, it can be used to identify the age of a stratigraphic unit, i.e. if the fossil is found in a rock, the age of the rock is then known. Also known as index fossils.