Alluvial fan

A fan-shaped deposit of material deposited at the mouth of a gorge or valley where it emerges onto less steeply sloping terrain.

Anticline

A type of fold, where the rocks are folded upwards into an arch.

Bed

A layer of sedimentary rock. Beds can vary in thickness from a few millimetres to many metres.

Breccia

A sedimentary rock made up of angular fragments of rock in a finer- grained matrix. Similar to cemented scree.

Calcite

A mineral made of calcium, carbon and oxygen — CaCO₃; the main  (calcium carbonate) component of limestone and marble.

Conglomerate

A sedimentary rock made up of rounded cobbles and boulders of rock in a finer-grained matrix.

Crinoid

A fossil sea lily made up of small ossicles or plates of calcium carbonate. These plates are the major constituent of crinoidal limestone.

Cross-bedding

A feature of sedimentary rocks deposited by flowing water or strong tidal currents, where the sediment is lain down in sloping laminae.

Cryoturbation

Disturbance to the soil caused by repeated freezing and thawing, typically under periglacial conditions.

Diagenesis

A term used to describe the process of how sediment is transformed into rock.

Dolomite

A mineral made of calcium, magnesium, carbon and oxygen — CaMg(CO₃)₂; also the name given to a rock made up of dolomite.

Galena

Lead sulphide — PbS.A heavy, grey shiny mineral with cubic cleavage. Principal lead ore mined on Mendip.

Gruffy ground

A local term used to describe the uneven ground produced by mining, possibly derived from ‘groovy ground’.

Inlier

A term used to describe an area of older rocks surrounded by younger rocks.

Karst

A term used to describe a limestone landscape. Karst features include caves, sinking streams, closed depressions and dry valleys.

Limestone

A sedimentary rock consisting mainly of calcium carbonate grains such as ooliths, shell and coral fragments and lime mud. Often fossiliferous.

Loess

A fine-grained, wind-blown silty soil, often derived from rock flour produced by glaciers.

Oolith

A grain of calcium carbonate formed by precipitation of calcium carbonate around a nucleus, much like a pearl. A limestone made of ooliths is known as an oolite.

Outlier

A term used to describe an area of younger rocks surrounded by older rocks.

Pericline

A type of fold, similar to anticline where the rocks are folded upwards, but plunging at each end, forming an elongate structure like an upturned boat.

Periglacial

A term used to describe very cold conditions. Often used to denote the climate or environment close to the edge of an ice sheet.

Phreatic

A term used to describe caves which were formed below the water table.

Rake

A miner’s term for a mineral vein.

Scree

A deposit of angular rock fragments at the base of a cliff, often formed by frost shattering during periglacial climates.

Smithsonite

Zinc ore. Zinc carbonate — ZnCO₃, also known as calamine.

Sump

Water-filled cave passage, sometimes passable by cave divers.

Swallet

A local term for a stream sink, commonly developed at the contact between the Avon Group and the Black Rock Limestone.

Syncline

A type of fold, where the rocks are folded downwards into a U-shape.

Thrust fault

A fault that is near horizontal or gently dipping, and along which older rocks have been forced over younger rocks.

Unconformity

A term used to describe the surface between two sets of rocks where there is a time gap or a layer of rocks missing. Where the younger rocks are folded at a different angle to those above, this is termed an angular unconformity.

Vadose

A term used to describe caves which were formed above the water table.

Wadi

An Arabic term used to describe an ephemeral water course, only active after storms or during the rainy season.