Download GLOSSARY

Home Page
GLOSSARY
Aeolian
Caused by or related to wind.
Alluvium
Material (usually sand, gravel and mud) transported and
deposited by a river.
Altiplanation terraces
Terraces formed on hill-slopes by the concentration of frost
shattering along sub-horizontal bands, for instance springlines.
Amplitudes
Half the distance measured at right angles between the
crest and trough of a fold.
Anastomosing
Of streams, branching and rejoining irregularly to produce a
net-like pattern.
Anticline
An arch shaped fold.
Antiformal folds
A fold closing upwards for which no information is available
on the direction in which the succession becomes younger.
Aragonite
An orthorhombic carbonate mineral, CaCO3, found in
sedimentary rocks.
Argillaceous
Descriptive of a detrital sedimentary rock with particles
<4mm.
Atrypids
Type of brachiopods.
Attitude
The relation between some directional feature in a rock and
a horizontal plane defined by strike and dip in the case of
planar features e.g. joints, bedding, and strike and plunge in
the case of linear features e.g. fold axis.
Axial plane/axial planar
cleavage
A set of cleavage planes generally parallel to the axial
planes (planes containing two crystallographic axes) of a
fold and related to the formation of the fold.
Basalt
A fine grained dark basic igneous rock.
Basaltic
Composed of basalt, a volcanic rock, rich in iron-magnesium
minerals and without quartz.
Basic lava
see basic rock.
Basic rock
Igneous rock with relatively low silica content.
Bedding (beds)
Layers or stratum of sedimentary rock bounded above and
below by bedding planes.
Bedding Plane
A distinct surface separating two beds which marks a break
Educational Register of Geological Sites
in the continuity of sedimentation caused, for example, by a
period of erosion or cessation of sediment supply.
Bedrock
Unweathered rock beneath unconsolidated material.
Berms
A triangular feature orthogonal to the shore with a
subhorizontal top and a more steeply dipping seaward
surface found on certain beaches.
Bioclast/bioclastic
Grains consisting of the skeletal material of organisms.
(Bioclastic limestone is a limestone in which the
predominant grains are bioclasts).
Biostratigraphy
The use of fossils to estimate the relative age of rocks.
Biotite
see mica.
Bivalve
(lit. 'two halves') A name for the class of Mollusca better
known as Lamellibranchiata or Pelecypoda.
Bryozoa
A group of colonial animals, usually marine, typically within
a coral-like structure.
Block
An angular fragment of rock with a diameter >256mm.
Blockfield
An accumulation of coarse detritus on a level or gentlysloping surface in a mountainous area, comprising local
rocks broken up by frost action.
Blockstreams
Blocks of rock on a hill-slope, alligned with a superficial
stream-like appearance, and formed by periglacial action.
Bluff
A steep projecting bank.
Boudinage
Pull apart structures, found in greatly deformed sedimentary
and metamorphic rocks, which resemble a chain of
sausages.
Boulder
A rounded rock fragment with a diameter >256mm.
Boulder clay (till)
Glacial debris deposited directly from ice, comprising a wide
variety of grain sizes.
Braciopods (Brachiopoda)
A group of bivalved marine animals varying in size from
5mm to 20 cm and having either a calcareous or a chitinous
shell.
Breccia
A rock generally formed of more or less angular fragments
of pre-existing rock.
Breccio-conglomerate
Any sedimentary rock composed of coarse fragments
between breccia and conglomerate, i.e. with approx. equal
numbers of angular and rounded clasts.
Bryozoans
(polyzoa) Phylum consisting of aquatic colonial organisms.
The skeletal material may be of chitin or calcium carbonate,
but only the calcareous forms are found as fossils.
Buff
A dull light yellow colour
Calcarenite
Limestones of grain-size 1/16mm.-2mm built up of
fragments of pre-existing rocks which have been produced
by the processes of weathering and erosion, and in general
transported to a point of deposition.
Calcrete
A phenomenon of desert soils, where evaporation causes
concretions of limestone to form in the sub-soil.
Calcite
The most common carbonate mineral, the principal
component of limestone.
Carbonaceous
Pertaining to or containing carbon.
Carbonate
1. A mineral type containing the carbonate radical (CO3)2-.
Carboniferous
A Geological period of time (see figure 2.1).
Cemented
The result of the process of cementation by which clastic
sediments (i.e. composed chiefly of fragments derived from
pre-existing rocks and minerals) are converted into
sedimentary rock by precipitation of a mineral cement
between the sediment grains, forming an integral part of the
rock.
Cenomanian
A stratigraphic stage name for the base of the basal
European Upper Cretaceous.
Chalcedonic
see silica
Chert (or flint)
Bands, layers or nodules of silica found formed within
sedimentary rocks.
Chrondrites
Fossil burrows with a branching shape, possibly formed by
marine worms.
Clast
A particle of rock or single crystal which has been derived
by weathering and erosion.
Cleavage
The planes along which rocks split as a result of the
recrystallisation and reorientation of component minerals in
response to directed pressure and metamorphism during
folding. The term cleavage is also applied to the ability of
some minerals to split along planar surfaces defined by their
internal structure. The feldspars, micas, chlorite, fluorite
and barite show well developed cleavage.
Clitter
A type of blockfield made up of a scatter of granite boulders.
Calcite, aragonite and dolomite represent 3 groups of
carbonate minerals.
2. A sediment composed of calcium, magnesium, and/or
iron e.g. limestone, dolomite.
Cobble
A rock fragment between 64 and 256 mm in diameter. It is
larger than a pebble and smaller than a boulder.
Conglomerate
A sedimentary rock formed of rounded pebbles and cobbles
derived from earlier rocks and set in a fine textured matrix of
sand or silt.
Conodonts
Microscopic phosphatic toothlike structures which range in
time from Cambrian to Jurassic. They are used in the
stratigraphic division of the Devonian and Carboniferous.
Continental
Formed or generated on land, as opposed to in the ocean or
seas.
Coralliterous
Containing corals.
Country rock
The rock into which magma or mineralisation is intruded or
emplaced.
Crenulation
Small-scale folding superimposed on folding of a larger
scale.
Cretaceous
A geological period of time (see figure 2.1).
Crinoid/crinoidal
A class of echinoderms (animal kingdom) commonly called
sea lily since they resemble plants often attached to the sea
floor. They are composed of calcite parts that form the main
constituent of some limestones.
Cross bedding
A series of inclined bedding planes having some
relationship with either the direction of flow, angle of rest, or
rate of sediment supply.
Cryoturbated
‘Patterned ground’ formed in response to the mixing
activities of ice.
Cupola
A small, dome-like proturberance on a larger igneous rock
intrusion.
A term applied indiscriminately to the 'repeat' unit of either
cyclic sedimentation or rhythmic sedimentation.
Cyclothems
Decolment (Decollement)
Literally an unglueing or unsticking. The plane of dislocation
caused by an upper series of rocks folding and in the
process sliding over a lower.
Deformation
A geological process in which the application of force
causes a change in geometry, such as the production of a
fold or fault.
Detrital
Rocks derived from material originating from pre-existing
rocks.
Devonian
A geological period of time (see figure 2.1).
Dextral
A term applied to a tear.
Dip
The angle measured from horizontal of an inclined bedding
plane.
Dicordant
A term used to describe an igneous rock showing a crosscutting relationship to bedding and foliation.
Distal turidite
The fine silty or sandy deposits formed when a submarine
sediment flow (or "avalanche") begins to dissipate, at some
distance from its source.
Dolerite
A medium-grained basic igneous rock.
Dolomite
A carbonite mineral found in magnesium limestone, formed
by dolomitisation (replacement of the calcium of a calcium
carbonate precursor by magnesium).
Downfaulted
Rocks on the downthrown side of a fault are described as
downfaulted. Downthrown is the side of a fault appearing to
have moved downward relative to the other side.
Dyke
A steeply inclined, or near vertical wall-like intrusion of
igneous rock.
Echinoderm
A group of entirely marine animals having a skeleton made
up of calcareous plates or spicules.
Elvan
A Cornish mining term for a dyke cutting granite.
Eocene
An epoch (see figure 2.1).
Epoch
A third order geological time unit.
Era
A first order geological time unit.
Erratics
Stones transported by a glacier and deposited far from its
point of origin.
Euhedral
A term applied to grains displaying fully developed crystal
form.
Exsolution
The development of two or more compositionally different
crystal phases from a solid solution, usually as cooling takes
place.
Fan
A slope of detritus increasing in width down the slope.
Fault (faulting)
A fracture in rocks along which movement has taken place
as a result of stresses in the earth’s crust. A thrust fault or
thrust is a low angle or flat-lying fracture on which the
overlying rocks have been pushed over the underlying
rocks. A normal fault is a high angle fault in which the
hanging wall (overlying side) has moved downwards relative
to the footwall (rock beneath).
Fault gouge
A fine-grained clay-like substance formed by the grinding of
rock material as a fault develops.
Fault plane
The plane along which a fault acts.
Feldspar
A group of calcium sodium potassium aluminium silicates
that are major components of igneous rocks. Orthoclase is
a potassium feldspar. Plagioclase is a sodium-calcium
feldspar with a range of composition from a sodium rich
variety to a calcium rich variety. Orthoclase and sodium rich
plagioclase are found in igneous acid rock. More calcium
rich plagioclases are found in basic igneous rocks.
Fenestrae
Generally refers to small elongated cavities (often filled with
mineral or sediment) in a limestone.
Ferruginous
Containing iron.
Fissile/fissility
The property of a fine-grained rock which has surfaces of
weakness along which it splits easily.
Fissure
A cleft; a longitudinal opening.
Flexure (hinge)
The locus of points (usually a line) of maximum bending or
curvature along a folded surface.
Flow
The permanent deformation that has a continuous strain
distribution.
Flow banding
Layering in an igneous rock or metamorphic rock produced
by flow.
Flowstone
A type of speleothem (chemical precipitates found in caves)
deposited by water flowing over the walls or floor of a cave.
Fluvial/fluviatile
Material deposited by rivers, usually in the form of sand,
gravel and silt.
Fold/Folding
The crumpling deformation of layered rocks by directed
stress during a mountain building (orogenic) episode.
Folding may occur in several successive phases during an
orogeny (period of mountain building) with the alignment of
the folds being related to the direction of the stress. The
shape and nature of the folds can vary according to the
depth at which the folds formed and the intensity of the
deformation so that the folds can be upright, asymmetrically
overturned, or collapsed into a recumbent position.
Foliation
A continuous or discontinuous layer structure in
metamorphic rocks formed by the segregation of different
minerals in streakes or lenticles, or by the alternation of
bands of different textures.
Formation
A fundamental geological unit used in the local classification
of strata and rocks.
Gastropods
Mollusca, a group of marine, freshwater, or terrestrial
molluscs ranging in size from 1mm to 10 cm. The shell is
usually coiled, the coiling being typically spirals, although
plane spirals do occur.
Geology
The study of the solid earth and its history.
Geomorphology
The study of the form of the ground and the processes
which shape it.
Glacial
1. Adjective referring to a glacier or ice sheet.
2. A period during an Ice Age.
Glaciation
The covering and alteration of the Earth’s surface by
glaciers and ice sheets.
Glauconite
A green mineral, essentially a hydrous potassium iron
silicate, closely related to the micas.
Goniatite
Mollusca (Cephalopoda) a group of entirely marine molluscs
in which the foot is modified to a ring of tentacles around the
mouth.
Graben
A downthrown block between two parallel faults.
Granite
A coarse-grained, acid igneous rock in which quartz is
associated with plagioclase and orthoclase feldspars as
major constituents. Biotite and/or muscovite are also
important constituents.
Greensand
A sand or sandstone with a green colour because of the
presence of dark green grains of glauconite.
Greisen
A deposit formed by hydrothermal processes.
Hades
The angle made with the vertical by a fault plane.
Head deposits
Superficial deposits consisting of rock rubble and
disaggregated rock moved and redepositied by the slow
downslope movement of water-logged earth during the later
Pleistocene.
Hornblende
A dark green or black lustrous silicate mineral, commonly
found in igneous or metamorphic rock.
Hornfels
A general term for any rock recrystallised by thermal
metamorphism.
Hyaloclastites
A type of volcanic rock, formed from ash deposits or flows.
Hydrothermal
The condition associated with igneous activity involving the
migration of heated or superheated water.
Igneous rock
Rock formed by crystallisation of the component minerals
from a molten state.
Intruded (Intrusive rock)
The process whereby molten rock has been forced into
cracks and fissures in other rocks.
Imbrication
State of being imbricate i.e. bent and hollowed like a roof
tile; lapping over each other, like tiles.
Indurated
Hardened.
Inter-
Between.
Isoclinal fold
A fold with parallel or near parallel limbs.
Joint/Jointing
Spaced fractures in all types of rocks resulting from the
release of contained stress or the effects of laterally applied
stress.
Jurrassic
A geological period of time (see figure 2.1)
Kaolinised/kaolinisation
The alteration of feldspars in granitic rocks to the white clay
mineral, kaolin by the circulation of heated water through the
rock.
Karst
A type of topography characterised by caves, caverns, dry
valleys, underground drainage and dolines (a bowl, cone or
well-shaped depression in limestone areas).
Keuper Marl
A red marl (a friable mixture of subequal amounts of micrite
and clay minerals) of the European Upper Triassic period.
Kink-band
A fold with straight limbs and angular hinges where the
limbs are markedly asymmetrical.
Lacustrine
Formed in lakes and lagoons.
Laminae/lamination
A fine, discrete layer of rock 0.005 - 1.00 mm thick.
Lamprophyres
Medium grained, commonly igneous rocks containing
relatively large crystals set in a finer-grained groundmass,
intermediate rocks, which contain a predominance of
ferromagnesian minerals.
Lava
Molten rock material at the surface.
Lenses/lenticular
An ore or rock body that is thick in the middle and thin on
the edges.
Lignite
A soft, low rank, earthy, brown-black coal.
Limestone
A rock comprising almost entirely of calcium carbonate.
Limonite
Brown iron ore.
Lineation
A general term for any rock feature showing a linear
structure.
Lithology
The description of the characteristics of rocks, as seen in
the hand-specimens and outcrops on the basis of colour,
grain size and composition.
Load casts
Rounded lobes of sandy material that protrude into an
underlying finer layer, most commonly occurring in
interbedded sandstones and mudstones.
Lustre/lustrous
A mineral property caused by the interference of light with
the mineral surface e.g. pearly or dull are terms to describe
lustre.
Magma
A molten fluid, formed within the crust or upper mantle of the
Earth, which may consolidate to form an igneous rock.
A large elevated feature, usually in a mountainous belt,
differing topographically and structurally from the lower
adjacent terrain.
Massif
Massive
1. (of homogenous rock bodies) having great bulk.
2. (of rocks) having homogenous structure or texture.
Matrix
The fine grained material of a rock in which larger bodies
may be set.
Megacrysts
A crystal in an igneous rock which is large compared to
those in the matrix.
Metasedimentary
Metamorphosed sedimentary rocks.
Metamorphic rock
Any of a class of rocks that are the result of partial or
complete recrystallisation in the solid state of pre-existing
rocks under conditions of temperature and pressure that are
significantly different from those obtaining at the surface of
the Earth.
Metamorphism
The processes which produce structural and mineralogical
changes in any type of rock in response to physical and
chemical conditions differing from those under which the
rocks originally formed.
Metamorphosed
Changed into a different shape; transformed.
Metavolcanic
Metamorphosed volcanic rocks.
Mica
A group of hydrous silicate minerals with a layered structure
which gives them a very perfect cleavage with a smooth
shining surface. Muscovite, a potassium alluminium mica, is
colourless and transparent. Biotite, a potassium iron
aluminium mica, is brown and can appear nearly black.
Both are common constituents of granite.
Micaceous
Containing or resembling mica.
Micrite/micritic
An abbreviation of microcrystalline calcite; very fine grained
carbonate making up the matrix in limestones.
Microgranite/
microgranitic
A medium grained, microcrystalline igneous rock with the
composition and texture of granite.
Mollusca
A group of invertebrate animals which may be terrestrial,
fresh water, or marine in habitat, commonly having a shell
secreted by the mantle.
Moraine
A depositional landform generated directly by a glacier.
Mudstone
A sedimentary rock of mud grade i.e. particles <62 µm.
Muscovite
see mica.
Mylonite
A hard, fine-grained, chert-like rock with banded or streaky
structure formed by the extreme granulation of rocks that
have been pulverised during faulting or intense dynamic
metamorphism.
Nappes
A fold in which the axial plane is horizontal or subhorizontal.
Normal fault
see fault.
Obelisk
An upright four-sided pillar that gradually tapers towards the
top.
Orogeny
A major phase of deformation, folding and metamorphism of
the earth’s crust associated with the formation of mountains,
volcanic activity and granite intrusion.
Orthoclase
see feldspar.
Orthoquartzite
An unmetamorphosed sedimentary quartzite.
Ossicle
A small bone.
Ostracods
Arthropoda, a phylum of segmented animals which have an
external shell consisting of chitin, strengthened in some
cases by CaCO3 and which have jointed limbs.
Outcrop
The total area over which a particular rock unit occurs at the
surface.
Outlier
An area of younger rocks surrounded by older rocks.
Outwash
Gravel and sand deposited by meltwater streams (resulting
from melting ice and snow) on land.
Palaeo-
Ancient.
Pebble
A rounded rock fragment of 4 - 64 mm in diameter.
Pegmatitic (Pegmatite)
A very coarse-grained igneous rock having a grain size of
3cm or larger, with crystals occasionally reaching a metre or
so in length.
Penecontemporaneous
Almost at the same time.
Periglacial
Period
(of areas) having locations, conditions, processes and
topographical features, adjacent to the borders of a glacier.
A second order geological time unit.
Permian
A geological time period (see figure 2.1).
Petrography
(Petrographically)
The systematic description of rocks in a specimen and thin
section.
Phosphatised
Preserved in or replaced by calcium phosphate.
Pillow lava
A volcanic rock of basaltic (see basalt) composition
comprising rounded, sack-like bodies, 0.2-2m in diameter,
separated from each other by rinds. Forms when lava flows
come into contact with water.
Pleistocene
A geological time period (the last Ice Age) (see figure 2.1).
Porphyry/porphyritic
Description of an igneous rock containing large crystals in a
groundmass of smaller crystals or glass. Often interpreted
in terms of a two stage cooling procedure.
Prod and Groove Casts
Prod marks/casts are found on the surface of fine-grained
sediment in the form of an asymmetrical mark with one end
better defined than the other: this is the downstream end
where the object carried by the current was buried more
deeply. A groove cast is a long, straight, sometimes ridgelike structure on the underside of a sandstone bed produced
by the filling of a groove on the surface of an underlying
mudstone.
Quarternary
Most recent geological period (see figure 2.1).
Quartz
A silicon dioxide translucent or white mineral commonly
forming hexagonal crystals and an essential constituent of
granite. It is also a major component of most sandstones
and siltstones. Quartz often occurs as veins in folded or
mineralised rocks.
Quartzite/quartzitic
A metamorphic rock consisting primarily of quartz grains
usually formed by the metamorphism of a sandstone.
Raised beach
A beach deposit stranded at altitude marking the position of
a former shoreline above the present sea level.
Rejuvenation
Rejuvenation takes place with the relative uplift with respect
to sea level of a region which has developed a mature
drainage system. Streams whose base level has fallen, cut
down very rapidly and become incised into the land in an
attempt to re-create the previous long-profile of equilibrium.
Relict
Rhombic
An original texture or structure visible through a later texture
or structure.
Having the figure of a rhombus.
Ripple marks
Small scale ridges and troughs formed by the flow of wind or
water over loose sand-grade sediment.
Rugose
Wrinkled.
Saccaroidal
Having a granular texture resembling that of sugar.
Schist
A finely layered metamorphic rock.
Scree
A heap of rock debris produced by weathering at the base of
a cliff.
Sandstone
A sedimentary rock composed of sand-sized grains in a
matrix of clay or silt, and bound together by a cement that
may be carbonate.
Sedimentary Rock
A rock formed by the consolidation of sediment.
Sedimentology
1. The scientific study of sedimentary rocks and the
Schist
Metamorphosed fine grained rocks, often shiny.
Shale
Fine grained sedimentary rock formed by consolidation of
clay, silt or sand that accumulates in deltas and on lake and
ocean bottoms. Shales may be black, red, grey or brown.
Shear(ed)
Deformation in which the angular relationship between
material lines in a body change i.e. a rotational stress or
strain.
Siderite
A carbonate mineral of iron, sometimes of importance as an
ore.
Silica
Silicon dioxide occurring in many rocks and minerals.
Chalcedony is a particular variety of silica often found as a
deposit lining or filling cavities in rocks.
Silicate
Any of numerous, often complex, compounds that contain
silicon and oxygen. This is the largest class of minerals.
Silt
Fine, detrital material between sand and clays particle size.
Slate
A fine-grained metamorphic rock derived mostly from shale.
It is characterised by slaty cleavage, i.e. the ability to be split
into large, thin, flat sheets. Such cleavage is due to the
alignment of platy minerals into parallel planes, which
processes responsible for their formation.
2. the origin, description, and classification of sediments.
Slickenside
occurs in metamorphism.
A smooth or polished fault surface.
Slip-off slope
A low gradient slope on the inside of a meander bend.
Solifluction
The downhill movement of soil or scree cover as a result of
the alternate freezing and thawing of the contained water.
Solution
1. An important process of chemical weathering in which a
Sorted (graded)
(of a sediment) composed of particles of uniform size.
Spicules
A small spine or needle.
Staddon Grit
A well known local rock type (N.B. grit is a hard, coarse
grained sandstone).
Start Complex
An ancient and very important series of structurally and
chemically altered rock occurring in South Devon
Strata (pl. stratum)
Beds of sedimentary rocks.
Stratigraphy
(stratigraphic)
The study of the stratified rocks especially their sequence in
time, the character of the rocks and the correlation of beds
in different localities.
Strike
1. n. The direction taken by a structural surface such as a
Stromatoporoids
A group of extinct organisms with a mesh-like skeleton
forming sheet-like domes or discoidal or dendroid masses,
often forming reefs with corals.
Structural feature
A feature produced by the displacement or deformation of
rocks, such as a fault or fold.
Syncline
A generally U-shaped fold with the younger rocks in its core.
Tabular
Descriptive of a crystal or feature (e.g. jointing) with a broad,
flat, commonly rectangular form.
Talus
A synonym of scree.
Tension-gash
Small fractures, often filled with quartz or other minerals,
formed during deformation of brittle rocks.
Terrace
A nearly flat landform with a steep edge formed by a variety
of processes.
Land-formed sediments. The term is applied (a) to
sediments formed and deposited on land and (b) to material
Terriginous
(Terrigenous Sediments)
mineral in contact with a solvent is dissociated into its
component ions.
2. A fluid containing ions.
fault or bedding plane as it intersects the horizontal; it is
the compass of the horizontal line in an inclined plane.
2. v. To be aligned in a direction at right angles to the
direction of dip.
derived from the land when mixed-in with purely marine
material.
Tertiary
A geological period of time (see figure 2.1)
Tholeiites
An important type of basalt.
Thrust
see fault.
Tourmaline
A group of magnesium, iron, aluminium, boron, silicate
minerals commonly found in veins in igneous and
metamorphic rocks. Tourmaline can in some cases replace
many of the other minerals in the rock (tourminalisation).
Transcurrent Fault
A fault where the principal displacement is parallel to the
strike of the fault plane which is vertical or nearly so.
Triassic
A geological period of time (see figure 2.1)
Tuff
Rock composed mainly of volcanic ash.
Turbidite
Slurries of sediments and water behave as a discrete fluid
phase when poured into fresh or sea water. The
movements of these masses of slurry are termed turbidity
currents and the sediment deposited as a result of such a
current is termed a turbidite.
Type Locality/Site
A locality/site selected as a standard for a stratigraphic unit
(a stratum or body of strata recognisable as a unit that may
be used for mapping, description, or correlation).
Unconformable
(unconformity)
Not following the underlying sequence of rock in structure or
age.
Vein
A sheet-like or tabular body of one or more minerals formed
by complete or partial infilling of a fracture within a rock. A
group of veins with a regular orientation is a vein system.
Ventifacts
A pebble faceted by the abrasive effects of wind-blown
sand. The facets develop along the length of the pebble in
a 'down-wind' direction.
Vesicular (Vesicles)
Small spherical or ellipoidal cavities found in volcanic lavas,
which are produced by bubbles of gas trapped during the
solidification of the rock.
Volcanic bomb
Volcanism
A mass of liquid lava thrown through the air during a
volcanic eruption which rotates and takes on a characteristic
shape and structure.
Volcanic activity.
Xenolith
A foreign crystal inclusion in an igneous rock.