Dictionary of Geology

Confused about that strange geological term? Then our quick and easy guide to geology can explain that technical geobabble.

Arthropod – An animal with an external skeleton and jointed legs arranged in pairs.

Ammonite – An extinct group of marine Cephalopods, similar to modern squid living in coiled shells. They are excellent time markers and can be used to age specific rock layers.

Bivalve – A type of mollusc inside two shells with a bilateral symmetry, for example a muscle.

Brachiopod – A type of marine animal that lives inside two shells. They are much rarer today than in the geological past.

Bryozoans – A colony to tiny marine animals which filter feed from the water column with fine tentacles. They can also use the tentacles to move across the seafloor.

Cephalon - The head section of a trilobite.

Crinoid – They are also known as sea-lilies, they have feathery arms to filter the passing water currents and most attached to the sea floor with a stem formed of ossicles. Their fossil history extends back to Cambrian times.

Cystoid - These are extinct echinoderms and most lived attached to the sea floor by stalks, and are distinguished from other echinoderms by triangular pore openings.

Dendrites – A characteristic tree-like structure usually formed by fine mineral crystals.

Fossil – The remains of a plant or animal that lived and died in a past geological age.

Fossiliferous – The description for a rock rich in fossils.

Gastropod – A mollusc living in a single coiled shell and one muscle. Slugs and snails are gastropods.

Genal Spines – The spikes that protrude from certain species of trilobites heads. They may have been for defence.

Geology – a science dedicated to the history of the Earth as recorded in the rocks, natural hazards and economic resources.

Graptolite – Colonial animals ranging from the Cambrian to the Lower Carboniferous. They can have a branching shape or a long thin elongated shape.

Hypostome - A shield-shaped structure from the underside of a trilobite, and thought to be a mouth part.

Igneous Rock– A rock which has formed from magma after it has cooled and solidified.

Matrix – A mass of rock in which fossils, crystals or gems are embedded.

Metamorphic Rock– A rock that has formed when the original sedimentary or igneous rock (protolith) is altered by heat and/or pressure to recrystallise, for example in a mountain building event.

Micrite - A classification of limestone which is composed of lime mud with grains less than 4µm.

Mineral – A naturally forming solid through geological processes that have a specific chemical composition, ordered atomic structure and physical properties.

Pygidium - The tail section of a trilobite.

Rock – A naturally occurring collection of minerals to form a consolidated mass.

Rugose Coral - These are an order of extinct corals that have simple or compound skeletons with internal skeletal structures consisting mainly of three elements, the septa, tabulae, and dissepiments.

Sedimentary Rock– A rock that is made up of the weathered remains of other rocks, biological processes or chemical precipitation for example salts.

Sp. – This indicates that the fossil is from a known genus but is unidentified down to species level.

Sparite - A classification of limestone which is composed of grains larger than 4µm and is crystalline.

Sutura Pattern – The wavy lines that connect sections of the ammonite shell together. They became more irregular and complex as the ammonites evolved.

Tabulate Coral - These are a form of coral, which are almost always colonial, forming colonies of individual hexagonal cells known as corallites defined by a skeleton of calcite, similar in appearance to a honeycomb.

Thorax – The body section of a trilobite. This part is composed of ribs and split into three lobes. This allowed the trilobites to roll into defensive balls.

Trilobite – An extinct group of marine arthropods with an exoskeleton made up of three parts, vertically and horozontally.

Whorl – A single coil of a gastropods shell.

If there are any other terms you would like explained please let us know by leaving a comment below.

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