This page of the glossary covers A to C. You can navigate to the next section from the links at the bottom of this page.
Ablation Occurs where more glacial ice is lost by melting and evaporation each year than is replaced by the build up of snow.
Accretion The build-up of sediment solely by the action of the forces of nature, e.g. through deposition by water or wind.
Aeolian Deposits Sediments made up of windblown sand or dust
Alluvial fan A fan-shaped deposit of sediment left by a fast flowing river or stream that has lost velocity upon entering a broad, relatively flat valley.
Ammonite A coiled, chambered fossil shellfish dominant in the Jurassic and Cretaceous seas and distantly related to the modern squid and octopus
Anticline A convex fold in rock, the central part of which contains the oldest rocks.
Asthenosphere The uppermost layer of the mantle, located below the lithosphere. This zone of soft, easily deformed rock exists at depths of 100 kilometres to as deep as 700 kilometres.
Basalt A fine-grained igneous, extrusive rock dominated by dark-coloured minerals.
Bedding Parallel layers in a sedimentary rock.
Bedrock A general term for the solid rock that underlies soil and other unconsolidated surface material.
Brachiopod A type of shelled animal with two valves that was common in ancient seas, but is much rarer today. Sometimes known as a ‘lamp shell’.
Boulder clay Glacial deposit consisting of clay containing boulders (of varying size). Boulder clay is laid down beneath a glacier or ice sheet. Boulder clay is also know as till.
Braided river A network of converging and diverging streams separated from each other by narrow strips of sand and gravel.
Breccia A sedimentary rock containing angular rock fragments which are greater than 2mm across.
Caledonian Orogeny The mountain-building event that occurred when England and Scotland collided around 430 million years ago.
Cambrian A Period of geological time, dating from about 545 to 495 million years ago.
Carboniferous A Period of geological time, dating from about 354 to 290 million years ago.
Cenozoic (Cainozoic) An Era of geological time, dating from about 65 million years ago to present day (comprising the Tertiary and Quaternary Periods).
Chert A member of a group of sedimentary rocks that consist primarily of microscopic silica crystals.
Clay A very fine grained sediment that is made up of particles less than 0.004mm in diameter. Clay typically contains a lot of water and deforms easily when squeezed. On heating it becomes hardened and is widely used in the brick making and ceramic industries.
Coal Typically black in colour coal is a member of a group of easily combustible, organic sedimentary rocks composed mostly of plant remains and containing a high proportion of carbon.
Conglomerate A sedimentary rock containing rounded rock fragments greater than 2 mm in diameter.
Continental drift Movement of the continents over the Earth’s surface.
Cretaceous A Period of geological time, dating from about 142 to 65 million years ago.
Crinoid A marine invertebrate, usually possessing a cup-shaped body and five or more feathery tentacles (often also known as sea-lilies). A few hundred species presently exist, but thousands of extinct species have been found as fossils, particularly from the Palaeozoic Era.
Cross bedding A sedimentary rock texture characterised by overlapping and cross-cutting bedding at an angle to the main layers of bedding. Cross bedding is typical of aeolian sands, beach and deltaic deposits.
Crust The Earth's outermost layer. The continental crust is about 40km thick and consists mostly of granite; the oceanic crust is about 10km thick and consists mostly of basalt.