Glossary

Acidic

Relating to igneous rocks containing over 63 per cent silica

Altiplanation terrace

Hillside bench cut in solid rock and formed in periglacial conditions by processes involving solifluction and mass movement

Argillaceous

Relating to sediment composed of clay sized particles

Autoclastic breccia

A rock formed by mechanical crushing in situ

Basalt

Fine-grained, dark coloured igneous rock, usually extrusive, composed mainly of plagioclase in the labradorite to bytownite range and pyroxene

Base metal

Common chemically active metal; for example, lead, copper, zinc

Basic

Relating to igneous rocks with less than 52 per cent of silica

Benthonic

Relating to bottom-dwelling marine life

Bornite

Sulphide of copper and iron (Cu₅FeS₄) with iridescent tarnish (Peacock Ore)

Boulder

Clay Glacial deposit consisting of unsorted boulders and pebbles in clay matrix. Synonymous with till

Brachiopod

Solitary marine invertebrate characterised by having two symmetrical but dissimilar shells or valves

Breccia

A coarse-grained elastic rock composed of angular rock fragments

Breccia pipe

Roughly cylindrical body of intrusive breccia

Caledonian orogeny

Name used for the orogenic belt (the Caledonides) stretching from Ireland through Britain to Scandinavia. Dated as end-Silurian but also used to include earlier pulses

Chalcopyrite

Bright brass-yellow ore mineral of copper (CuFeS₂)

Chronostratigraphy

Branch of stratigraphy which relates strata to time

Cleavage

Fissile structure in a rock produced by deformation or metamorphism which enables it to split into thin laminae along secondary aligned fractures

Comagmatic

Related to igneous rocks interpreted as having been derived from a common parent-magma

Contourite

Sedimentary rock deposited from a contour-following bottom current. Usually recognised as a layer of relatively coarse-grained sediment in marine muds or silts

Dolerite

Medium-grained, igneous intrusive rock of similar composition to basalt

Drumlin

Low, oval hill or ridge of glacial till built under the margin of the ice and shaped by its flow so that the longer axis is parallel with the direction of movement of the ice

Epiclastic

Related to a rock formed by the breakdown and consolidation of preexisting rocks

Fluvioglacial

Related to the deposits produced by meltwater streams flowing from a glacier

Fold

Bend of a planar structure in rocks, for example, bedding planes or cleavage

Fold axis

Line on a map which traces the crest or trough of a fold

Galena

Bluish grey ore mineral of lead (PbS)

Geosyncline

Large-scale downwarp in the surface of the earth in which thousands of metres of sedimentary and volcanic rocks accumulate

Graptolites

Extinct group of colonial marine animals with a corneous skeleton. In Dictyonema the colony formed a conical net

Greenschist facies

Low grade regional metamorphism corresponding to a temperature range of 300 to 500°C

Greywacke

An impure sandstone with more than 15 per cent interstitial matrix consisting of mica, chlorite and quartz. Grains include quartz, feldspar and lithic rock fragments

Head

Deposit consisting of locally derived unsorted material formed by solifluction usually under periglacial conditions

Hiatus

Break in continuity of the stratigraphic record either by erosion or non-deposition, and the time-value associated with this period

Hyaloclastite

A deposit formed by the rapid chilling of a lava or magma where it flows into water or saturated sediment causing it to shatter into small angular fragments

Hydrothermal alteration

The alteration of rocks or minerals by the action of hot water circulating underground

Hyolithid

Extinct group of molluscs with a narrowly conical lidded shell

Intermediate

Relating to igneous rocks containing 52 to 63 per cent silica

Intrusive breccia

Heterogeneous mixture of angular fragments which has been mobilised and intruded into its present position

Lapilli

Fragments between 2 and 64 mm in diameter ejected by a volcanic eruption

Lava breccia

Autoclastic breccia produced by the fragmentation of the upper and lower crusts of a lava during now

Lithostratigraphy

That part of stratigraphy which deals with the nature and composition of strata

Load cast

A sole mark or depression on the base of a bed caused by unequal settling and compaction of the overlying material

Magnetite

Ore mineral of iron (Fe304)

Mass-flow deposits (mud flow)

Deposit formed by mass-movement of soil and rock debris down a slope. May be triggered by earthquake or by oversaturation

Mass wasting

Dislodgement and downslope movement of soil and rock material under gravity. Includes processes such as solifluction, rock slides, soil creep and mass-flow

Microdiorite

Medium-grained intermediate igneous rock consisting of plagioclase in the oligoclase to andesine range. Other minerals may include augite, hornblende, biotite, hypersthene. Primary quartz is normally less than 5 per cent

Microgranite

Medium-grained acid igneous rock containing quartz and feldspar with some dark minerals which may include hornblende and biotite

Microtonalite

Medium-grained intermediate igneous rock of similar composition to microdiorite but containing more than 10 per cent primary quartz

Molybdenite

Lead-grey flaky molybdenum mineral (MoS₂)

Moraine

Debris eroded and redeposited by a glacier consisting of unsorted unstratified till. Lateral and terminal moraines are the deposits at the side and end of a glacier respectively

Oolith

Round or oval accretionary particle in a sedimentary rock (0.25 to 2 mm in diameter)

Orogeny

Process of formation of mountains

Pericline

A fold in the form of a dome or basin

Periglacial

Relating to the conditions and processes existing around the margins of a glacier or ice-sheet

Phenocryst

Large conspicuous crystal in a porphyritic rock

Porphyry

An igneous rock of any composition containing phenocrysts in a fine-grained groundmass

Porphyry copper

An ore deposit composed of a large body of rock containing small quantities of disseminated chalcopyrite and other sulphide mineral

Pyrite

Brass-yellow ore of iron known as 'fools gold' (FeS₂)

Pyrrhotite

Silver-yellow magnetic form of iron sulphide

Quartz-microdiorite

A form of microdiorite with between 5 and 10 per cent quartz

Quartz wacke

Impure sandstone containing more than 15 per cent detrital matrix (sericite and chlorite). Grains are mainly of quartz with less than 10 per cent feldspar and less than 10 per cent of rock fragments

Rhodochrosite

Pink or reddish ore of manganese (MnCO₃)

Solifluction

Downslope flow of waterlogged soil and other unsorted material

Spessartine

Red-brown to yellowish manganese aluminium silicate (variety of garnet)

Sphalerite

Yellowish brown to dark brown zinc ore (ZnS)

Subarkose

Sandstone with little detrital matrix (less than about 15 per cent) with grains predominantly of feldspar with less abundant lithic clasts and quartz

Subgreywacke

Sandstone with little detrital matrix (as in subarkose). Grains consist of quartz, rock fragments and feldspar. The proportion of rock fragments exceeds that of feldspar

Subduction

Process whereby one part of the earth's crust descends beneath another

Subvolcanic

Relating to intrusions and other phenomena in the basement beneath a volcano

Till

Synonymous with boulder clay

Trilobite

An extinct group of marine anthropods (class Trilobita) in which the exoskeleton was divided longitudinally into three lobes (Figure 20)

Tuff

A rock formed by the consolidation of volcanic ash

Tuffite

A mixed rock consisting of >25 per cent pyroclastic and >25 per cent epiclastic or detrital material

Turbidite

The consolidated deposit of a turbidity current. These rocks are characterised by graded bedding, moderate sorting and well developed sequence of bedding structures (Bouma cycle)

Xenolith

A foreign inclusion in an igneous rock