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A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z


Alluvium: detrital material deposited along the course of a river in (usually) temporary accumulations.

Anticline: an upfold in the rocks, usually in the form of an arch, the beds dipping away on either side. Anticlines preserve old rocks in their cores, with progressively younger strata on the margins.

Aragonite: one of the forms of calcium carbonate, CaCO3. It is only stable in seawater and is increasingly rare in older rocks. This is because it tends to revert to calcite, a polymorph which is stable in fresh water.


Bed: see 'stratum'

Bedding plane: the surface between two adjacent strata.

Bivalve: a mollusc with a shell composed of two valves. Each valve is unsymmetrical in most cases, so making bivalves relatively easy to distinguish from brachiopods. Cockles, mussels and oysters are bivalves.

Brachiopod: a marine invertebrate with a shell composed of two, hinged valves. Unlike bivalved molluscs, each valve is symmetrical, and the two tend to be of unequal size.

Braided stream: a stream consisting of interwoven, anastomosing channels. Characteristic of streams with a high sediment load and high rate of discharge.

Breccia: a rock-type consisting of angular fragments embedded in a finer matrix. Breccia differs from conglomerate in the angularity of its component fragments.


Calcite: stable polymorph of calcium carbonate (CaCO3). It is the major component of most limestones, and while soluble in water, is mineralogically stable out of the sea.

Caledonian Orogeny: a period of mountain-building which took place at the end of the Silurian.

Calcrete: soil cemented by calcium carbonate

Coal: a deposit of carbonised plant material formed in swamps with dense tropical forest cover. The most important coals in Britain are Carboniferous in age.

Coal Measures: that part of the Carboniferous System which is coal-bearing. There are three divisions: the Upper, Middle and Lower Coal Measures.

Conglomerate: rock-type composed of rounded pebbles in a finer matrix.

Coral: animals related to jellyfish and sea-anemones, these creatures secrete skeletons of calcium carbonate. They live affixed to the sea-floor, and can form large accumulations called 'reefs'. Many more live separately. Some corals comprise a single individual, while others are colonial. In the Carboniferous, two varieties of coral existed. These were the Rugose Corals (solitary, horn-shaped) and the colonial Tabulate Corals. Both are now extinct.

Crinoid: sea-lily, or stalked echinoderm; related to the starfish and the sea-urchin. Animals, not plants, the crinoids live attached to the sea-floor in dense groves, and when they die their skeletons disintegrate into very many component parts (plates and ossicles). These are the major component of crinoidal limestones.

Cross-bedding: a sedimentary structure consisting of minor, concave-upward bedding planes set at an angle to the true bedding. Can form in deserts, in seas and in rivers, but the scale and form of the cross-beds may be used to determine the environment of deposition very closely.


Detritus: granular material produced by the erosion of preexisting rocks and transported to depositional environments where it goes to form new sedimentary rocks.

Dip: the maximum angle of inclination of folded strata. It is measured in degrees from the horizontal and is at right-angles to 'strike'.

Disturbance: a term used mostly in South Wales to describe a major zone of complex faulting.

Dolomite: calcium magnesium carbonate (CaMg (C03)2).

Downthrow: the vertical distance travelled by the downwardly displaced block along a normal fault.

Dyke: a sheet of igneous rock, emplaced in molten state, which cuts across bedding in a sedimentary sequence.


Erratic: a piece of rock, transported away from its place of origin and dumped in an area where the rocks are quite different.


Facies: This word has many meanings in geology, and takes on different attributes in different contexts. In the sense most commonly used in this book, however, it refers to the sum total of all a sedimentary rock's characteristics; its grain-composition, size, shape, sedimentary structures, bedding and matrix. By patiently establishing a rock's facies, geologists can link it with the depositional environment in which it formed.

Fault: a fracture in the rocks along which adjacent blocks have moved. There are many types, classified on the style of this movement (See Fig.6).

Fold: where strata have been buckled.

Formation: a unit of rocks which is. consistently recognizable over wide areas and which is therefore useful in mapping.

Fossil: the physical remains of a once-living organism, preserved in the rocks. Can also be used to describe inanimate objects, eg., a 'fossil' soil, or a 'fossil' volcano.
freshwater phreatic: that part of the phreatic zone where the interstitial water is fresh.
geology: the study of the earth.


Geomorphology: the study of landforms and the processes which shape them.

Glacial till: very poorly sorted deposit composed of large blocks, cobbles, boulders, pebbles, sand and very fine rock flour, dumped by melting ice-sheets. Ancient examples, hardened into rock, are called tillite.

Glaciation: a period during which the polar ice-caps extend towards the equator, covering large areas of the Earth.

Groundwater: water which circulates in the phreatic zone (ie., beneath the water-table).


Head: chaotic breccia formed by solifluxion at the end of glacial periods. Found in large amounts along the south coast of Gower.

Hercynian Orogeny: period of mountain-building which took place at the end of the Carboniferous Period. Responsible for the folding and faulting seen in Gower.

Hiatus: in the geological sense, a period of cessation of deposition. A gap in the stratigraphic record where time has passed unrepresented by any sediment.

Hinge: the axis about which the folding of strata has taken place.


In situ: occurring in original position.

Incompetence: exhibited by a bed or a formation which possesses insufficient mechanical rigidity to bear the loads imposed by folding and to transmit them evenly.

Induration: the hardening process which turns a sediment into a rock.

Interglacial: the warmer period between times of extended polar ice and very low temperatures.


Joint: a crack in a rock along which no movement has taken place. May reflect stresses due either to overburden or compression by Orogenic movement.

Joint-set: a system of joints trending in similar direction through a formation.


Karst: very rugged erosion pattern found in limestone areas where the predominant erosional process is solution of calcium carbonate by rainwater.


Limb: one half of a fold.

Lime mud: very fine mud formed of microscopic crystals of calcium carbonate.

Limestone: a rock-type composed primarily of calcium carbonate.


Marine phreatic: that portion of the phreatic zone in which the groundwater is salt.

Marine transgression: when sea-level rises (or land sinks) and large areas of land become inundated. Withdrawal of sea, leading to increased land area, is called a 'regression'. The geological record is punctuated by major transgressions and regressions. A major transgression, for example, marks the base of the Carboniferous System, when Devonian desert environments gave way to shallow, tropical seas and limestone deposition.

Marker-horizon: a distinctive bed or formation which can be used by a geological map-maker to locate himself in the stratigraphic sequence. Particularly useful in between very thick, monotonous units .

Matrix: in a rock, the material which was deposited between the constituent grains. Not to be confused with cement, which is precipitated between grains after deposition. Matrix is particulate matter, of the same age as the larger particles embedded within it.

Micrite: microscopic particles of calcium carbonate. See 'lime mud'.

Millstone Grit: this is a rock- name for the sediments laid down during the Namurian.
mineralisation: the introduction of mineral matter into a rock; as, for example along faults, or close to certain deep igneous bodies.

Misfit stream: a stream running in a valley far too deep to have been caused by present agencies.

Mixed zone: (also 'Mixed phreatic zone' or 'mixing zone') portion of the phreatic zone where freshwater meets salt.


Namurian: period of time spanning the transition from limestone deposition to coal-measure conditions in Britain.

Nose: of a fold - where the fold dies away.


Old Red Sandstone (ORS): sediments of the Devonian.

Oolith: a spherical bead of calcium carbonate, about the size of a pin-head. It has an onion-skin construction and forms(in massive quantities)in certain limestone-depositing environments. Each oolith has a nucleus (sand-grain, or a fragment of shell) around which precipitation first starts. A rock made of ooliths is an oolite, and are described as being 'oolitic'.

Orogeny: a period of mountain-building, during which rocks are deformed (folded, faulted).

Outcrop: where rocks come to the surface and are exposed, or "crop out".


Palaeontology: the study of fossil organisms.

Pericline: a small, local fold which dies out quickly in both directions. It is a term more usually applied to anticlines than synclines, though it can be used in both contexts.

Period: the time-span over which a geological system was laid down. The time-unit corresponding to a known body of rock defined by stratigraphic palaeontology.

Phreatic: the zone beneath the water-table, where the pores are full of groundwater. May be subdivided on whether that water is fresh, salt or brackish into 'freshwater' imarinel or 'mixed' phreatic zones. The chemistry of the groundwater in these zones has great consequences for the changes which take place in rocks immediately after deposition (their ‘diagenesis').

Piedmont plain: a broad stretch of alluvial material at the foot of a mountain range, formed by coalescing alluvial fans and consisting of material eroded from the mountain chain.

Plunge: the dip of a fold axis.

Polymorph: some substances (eg., calcium carbonate) have many crystal forms, each with different degrees of stability. Each of these forms is known as a polymorph.

Pyrites: iron sulphide (FeS2); 'Fool’s Gold'.


Raised beach: a trace of an ancient marine erosion level cut into the cliff, representing a period when sea-level was higher, relative to the land, than it is today.

Regression: a general retreat of the sea, exposing large areas of the globe to subaerial weathering.


Sandstone: rock-type composed of sand-grade grains (usually of quartz).

Sediment: a deposit of particulate material. If indurated (hardened), can become a sedimentary rock.

Shale: a rock-type made from muddy sediment characterized by pervasive fissility which causes them to break up readily in the hand. Rocks of similar grain-size, but without this fissility, are called mudstones.

Shelf sea: shallow sea covering the margins of the continents. These are very narrow at the moment, but in the geological past, sea-levels have been generally much higher, and much greater areas of the continents have been under water.

Sink-hole: hole in the ground through which a stream disappears.

Slickensides: scratches on fault-planes caused by the grinding of one block past another. They can be used to determine the sense of movement along a fault.

Solifluction: 'soil-flow';a form of soil creep on slopes which are subject to freeze-thaw action. Common in environments around the edges of ice sheets.

Spit: stretch of sand, attached to the land at one end, and extending out into the sea. Commonly formed by the drift of sediment along the coast, and often extending from headlands across bays.

Stratigraphy: the study of the ordering of rock units.

Stratum (pl. strata): unit of sedimentary rock, bounded by bedding planes and having reasonable lateral persistence. A layer of rock.

Strike: the direction of a horizontal line drawn upon an inclined plane. It lies at rightangles to the direction of dip.

Swallet: a crater-like depression in limestone country formed by collapse of the bedrock through dissolution.

Syncline: a downfold of rock. Synclines preserve young rocks in their cores, with older rocks towards the outside.

System: the rocks laid down during a Period (Carboniferous, Jurassic, etc.) The Cretaceous System was created during the Cretaceous Period. Defined by stratigraphic palaeontology.


Thrust: a low-angle reverse fault. The Gower 'thrusts' are perhaps more properly described as reverse faults, since they have a fairly high inclination.
topography: the shape of the land.

Trilobite: an extinct arthropod with a distinctively three-lobed form.


Unconformity: plane separating a lower series of rocks which have been folded and faulted and eroded, from an upper series deposited on top of them at a much later date. It is the plane which represents all the 'lost' time in between, the time taken to deform and wear down the older sequence.

Uniformitarianism: a basic tenet of geological science. It states that the ancient world operated according to the same laws, with the same processes that we observe around us in the modern world. Therefore, to interpret the sedimentary record correctly, we must study the present so as to make accurate analogies. Hence the expression 'the present is the key to the past'.


Vadose zone: that portion of the subsurface beneath the land, lying above the water table. The pores are therefore only partially filled with water and are regularly flushed with rainfall.

Valves: the component parts of a hinged shell, such as might be seen on a bivalved mollusc or a brachiopod.

Vein: a sheet-like body of mineral material emplaced along a fault-line or joint, or any other kind of fissure.


Water-table: the surface of the phreatic zone which divides it from the vadose zone. That level beneath the land at which the pores of rock or soil become completely full of groundwater.

Wave-cut platform: a plane, cut by marine erosion, when sea-level was higher than it is today for a prolonged period.