Geological glossary

Alluvium Loose deposits of clay, silt sand and gravel laid down on floodplains by the action of rivers in the recent past. Andesite A fine-grained volcanic or intrusive rock, intermediate between basalt and dacite in composition (53 to 63 per cent silica).

Angular A descriptive term for rock fragments or sand grains with sharp corners.

Bed An individual layer of sediment or a stratum of sedimentary rock.

Bioclastic limestone A limestone with common fragments of calcareous organisms.

Breccia A rock made up of large, angular fragments in a finer matrix that may be of a similar or a different material (see also, Volcanic breccia).

Crinoid Fossilised remains of shelly marine creatures, related to sea urchins, usually attached by a stem composed of discs (ossicles).

Cross-bedding A feature of sedimentary rocks formed by the movement of sand grains in currents to produce layering oblique to the margins of the beds.

Dacite A fine grained volcanic or intrusive rock containing 63 to 70 per cent silica; it commonly has quartz phenocrysts.

Deformation Any natural process that bends, twists or fractures rocks.

Diorite A medium- to coarse-grained intrusive igneous rock composed principally of the minerals plagioclase feldspar, hornblende, and/or pyroxene; the equivalent of andesite.

Dolomitic, dolomitised Descriptive terms for a limestone that has had some of its calcium carbonate replaced by magnesium carbonate.

Erosional unconformity (see ‘unconformity’)

Feldspar A commonly occurring aluminium silicate mineral of potassium, sodium and calcium (see plagioclase feldspar).

Floodplain The flattish floor of a valley composed of alluvium and prone to flooding.

Fragmental Descriptive term, usually reserved for rocks made up of volcanic fragments ranging from ash-size to blocks (see volcanic breccia).

Granodiorite A coarse-grained intrusive igneous rock similar to granite, but with more plagioclase feldspar than potassium feldspar.

Igneous rock Rock formed when molten magma cools and solidifies. It includes extrusive rocks erupted from volcanoes (e.g. andesite) and intrusive rocks that cool beneath the Earth’s surface (e.g. diorite).

Intrusion A body of igneous rock formed from molten magma that has been introduced into pre-existing rock. Laminae, Lamination The narrowest type of layering in sedimentary rocks, less than 1cm in thickness.

Magma Molten rock from the Earth’s interior, which cools and solidifies to form igneous rocks.

Mudstone A fine-grained sedimentary rock originally composed of clay or mud.

Plagioclase feldspar A very common rock-forming aluminium silicate mineral with varying contents of calcium and sodium; usually grey in colour.

Phenocryst A relatively large and usually conspicuous crystal, surrounded by smaller crystals and formed in the mass of a porphyritic igneous rock.

Proximal In Charnwood Forest, a term describing rock types with particular features showing that they formed close to a volcano.

Pyroclastic flow A rapid avalanching of ash and rock fragments down the flanks of an erupting volcano.

Quartz The crystalline form of silica (silicon dioxide, SiO2 ).

Quartz diorite An intrusive igneous rock with the same minerals as diorite but with between 5 and 20 per cent quartz.

Sandstone A sedimentary rock composed of sand-sized grains (i.e. generally visible to the eye, but less than 2mm in size). Volcaniclastic sandstone is where all of the grains are of volcanic origin.

Sedimentary rock A rock that is commonly formed by the binding together (lithification) of sediment particles (e.g. sandstone, siltstone, mudstone).

Scree An accumulation of broken rocks around the slopes and bases of crags.

Siltstone A sedimentary rock composed of silt-sized grains (i.e. only just visible to the eye).

Subrounded A term referring to the appearance of rock fragments or grains, the corners of which are not quite angular and which therefore have been somewhat worn by abrasion. Rounded or well-rounded fragments are called pebbles.

Superficial deposits Material laid down in geologically very young times as a loose (unconsolidated) mantle across the bedrock.

Tectonic Rock-deforming processes and resulting structures that affect large sections of the Earth’s crust

Tuff A collective term for consolidated pyroclastic rocks (i.e. rocks formed directly from volcanic eruptions) with fragments less than 64mm in mean diameter. Lapilli tuff is a rock composed of pyroclastic fragments of any shape with a mean diameter of 2 to 64 mm.

Turbidite A rock formed from sediment that settled out of turbid water carrying particles of widely varying grain size. Characteristically displays graded bedding.

Turbidity current A current on the sea floor caused by the movement of a body of dense, turbid or sediment-laden water.

Unconformity A surface of contact between two rock units, which represents a hiatus in the geological record, usually due to a combination of erosion (erosional unconformity), tectonic activity and a cessation of sedimentation.

Volcanic block An angular volcanic fragment, in excess of about 6.5cm size, formed by the disintegration of a congealed lava flow or volcanic dome.

Volcanic breccia A pyroclastic rock composed of volcanic blocks.

Volcanic dome A mass of viscous lava that cools and hardens within the crater. When a volcanic dome forms, pressure from the volcano’s gases can build up and eventually the dome disintegrates into avalanches of hot volcanic rock

Volcaniclastic A general term for rocks composed wholly or in part of volcanic fragments i.e. fragments originating from volcanic eruptions (pyroclastic material) or from the erosion of volcanoes (epiclastic material).