Geological Time Periods

And a Glossary of Terms
EraSedimentary Rocks of the British IslesEpoch
Barton, Bracklesham & Bagshot BedsEocene
London Clay, Reading & Thanet Beds
Upper Greensand & Gault
Lower Greensand & Speeton Clay
Wealden Clay
Hastings Beds
Purbeck & Portland Beds
Kimmeridge & Oxford Clays
Upper & Lower Oolite
Liassic & Rhaetic
Keuper Marl & SandstoneTriassic
Bunter Sandstone

Permian MarlPermian
Magnesian Limestone
Coal MeasuresCarboniferous
Millstone Grit & Culm Measures
Carboniferous Limestone
Upper & Middle Old Red SandstoneDevonian
Lower Old Red Sandstone

 Precambrian, Torridonian, Charnian & Longmyndian 
Metamorphic Rocks
 Schists, Gneisses & Quartzite 
Igneous Rocks

Geological Glossary

Cambrian the first epoch of the Palaeozioc era which lasted 100 million years, during which marine invertebrates, such as trilobites, first appeared; from Medieval Latin Cambria 'Wales', where outcrops of these rocks occur.
Carboniferous the fifth and penultimate epoch of the Palaeozioc era, lasting for 80 million years during which time the coal measures were formed; from Latin carbo 'charcoal', modern adjective meaning literally 'yielding carbon (or coal)'.
Cenozoic the most recent geological era, beginning about 70 million years ago and characterized by the development and increase of mammals; from Greek kainos 'new, recent' and zoikos 'animal, life'.
Clay a very fine-grained material consisting of quartz, hydrated aluminium silicate and fragments of organic matter, occurring in soils and sedimentary deposits. It has remarkable plasticity when wet, becoming hard upon heating, qualities which have seen its use in the manufacture of bricks, tiles, ceramics, cement, etc; from Old English claeg, related to Old German klia, Latin glus, Greek gloios 'glue, sticky oil'.
Cretaceous third and final epoch of the Mesozoic era, lasting for 65 million years during which chalk deposits were formed and the first flowering plants appeared; from Latin cretaceus from creta, literally 'Cretan earth' or chalk.
Culm a formation of shales and sandstones occurring between productive coal measures, and thus deposited during the Carboniferous epoch; the word possibly stems from Old English col 'coal', meaning however, coal-mining waste.
Devonian the fourth epoch of the Palaeozioc era, the first in the Upper Paleozoic, lasting for 50 million years during which amphibians first appeared; named after the English county of Devon, where outcrops of these rocks occur.
Eocene second epoch of the Tertiary period, lasting for 20 million years during which hooved mammals first appeared; from Greek eos 'dawn, early' + kainos 'recent'.
Epoch a point in time which marks the beginning of a distinct period in history, in geology, a unit of geological time during which a series of sedimentary rocks are deposited; from New Latin epocha from Greek epokhe 'cessation'.
Era a period of time considered to possess a certain distinctive character; there are four distinct eras of geological time, the Palaeozoic, Mesozoic, Tertiary and Pleistocene, each of which are divided into a number of epochs; from Latin aera 'counters'.
Extrusive igneous rocks formed from magma issuing from volcanoes on the Earth's surface and along fissures and cracks in the ocean beds; from Medieval Latin extrusio, from Latin extrudere 'to thrust out'.
Gault a stiff, compact clay formation found in rocks of the lower Cretaceous epoch in eastern England; of unknown meaning and origin.
Gneiss a coarse-grained metamorphic rock, often banded or foliated, which represents the last stage in the metamorphism of rocks prior to melting; from Old German ganeist 'spark', related to Old Norse gneista 'to give off sparks'.
Holocene the second and most recent epoch of the Quaternary era, which began 10,000 years ago, and in which we are now living; from Greek holo 'wholly' + kainos 'recent'.
Igneous rocks which have been formed from molten magma either at or beneath the Earth's surface; from Latin igneus 'fiery', from ignis 'fire'.
Intrusive igneous rocks formed from magma squeezed between spaces within the strata of the Earth's crust, often deep underground; from Latin intrudere 'to thrust in'.
Jurassic second epoch of the MesoZoic era, lasting for 45 million years during which dinosaurs and ammonites flourished; from French jurassique from the Jura mountain range in eastern France.
Marl a fine-grained sedimentary rock consisting of clay, minerals and silt, used as a fertilizer; from Latin marga 'marl, fertilizer'.
Mesozoic era or period of geological time or sedimentary rock formation that began 225 million years ago at the start of the Triassic period and ended some 155 million years later at the end of the cretacious period, some 65 million years before present; the second or middle period of geological time, hence the name, from Greek misos 'middle' + zoe 'life' + -ic suffix denoting relationship.
Metamorphic rocks which have been considerably altered from their original composition and structure by exposure to both pressure and heat, deep within the Earth's crust; from Latin via Greek meta-morphe 'form-changing'.
Miocene fourth epoch of the Tertiary period, which lasted 14 million years; from Greek meion 'less' + kainos 'recent'.
Oligocene third epoch of the Tertiary period, lasting 15 million years; from Greek oligos 'few' + kainos 'recent'.
Oolite a type of sedimentary rock consisting of small spherical grains within a fine matrix; from New Latin oolites literally 'egg stone'.
Ordovician the second epoch of the Palaeozioc era lasting 60 million years during which marine invertebrates flourished; named after the native British Ordovices tribe which inhabited the area in North wales where outcrops of these rocks occur.
Palaeocene first epoch of the Tertiary period lasting for 10 million years; from Greek palaios 'old, ancient' + kainos 'recent', denoting its association among the newest geological epochs.
Palaeozoic the first era of geological time which began about 600 million years ago with the cambrian period and lasting about 375 million yeras until the end of the permian period; from Greek palaios 'old, ancient' + zoe 'life' + modern adjectival suffix -ic which denotes relationship.
Permian the sixth and final epoch of the Palaeozioc era which lasted 45 million years; named after Perm on the Kama River in Central Russia, where outcrops of these rocks were first identified.
Pleistocene the first epoch of the Quaternary era, lasting about 990 thousand years and characterized by extensive glaciation of the northern hemisphere and the evolutionary development of Man; from Greek pleistos 'most' + kainos 'recent'.
Pliocene fifth and final epoch of the Tertiary period, lasting for 10 million years during which many modern mammals appeared; from Greek pleion 'more' + kainos 'recent'.
Quaternary the most recent geological era, consisting of the Pleistocene and Holocene epochs; from Latin quaternarius 'containing four', denoting its status as the fourth era of geological time.
Schist a metamorphic rock consisting of parallel bands of micaceous minerals which can be split into thin layers along these planes; from French schiste, from Latin lapis schistos 'splittable stone', from Greek skhizein 'to split'.
Sedimentary rocks formed by the accumulation and consolidation of mineral and organic fragments that have been eroded and deposited by the action of water, wind or ice; from Latin sedimentum 'settling', from sedere 'to sit'.
Shale a fine-grained, dark, laminated sedimentary rock formed by the compression of successive layers of clay; from Old English scealu 'shell'.
Silurian the third epoch of the Palaeozioc era, the last of the Lower Paleozoic, lasting for 40 million years during which fish firrst appeared in the Earth's oceans; named after the native British Silures tribe from South Wales, in whose former territories outcrops of these rocks appear.
Slate a compact, fine-grained, metamorphic rock formed by the action of heat and pressure on shale, it is easily split along natural cleavage planes into thin sheets, commonly used as a roofing material; from Old French esclat 'to fragment'.
Tertiary the third era of sedimentary rock formation, divided into Palaeocene, Eocene, Oligocene, Miocene and Pliocene epochs or series. From Latin tertius 'third', it being the third period of geological time.
Triassic first epoch of the Mesozoic period, lasting for 45 million years during which time the reptiles flourished; from Latin trias or 'triad', with reference to its three divisions.