BGS Lexicon of Named Rock Units

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The BGS Lexicon of Named Rock Units provides BGS’s definitions of the lithostratigraphic, lithodemic, and litho-morpho-genetic geological units of the United Kingdom and its continental shelf.

The Lexicon focuses mainly on geological units of Member, Formation and Group rank (and equivalent) but it also includes information on some units of lesser rank, notably economically important coal seams and laterally extensive marine bands. It includes superficial and bedrock units. It includes synonyms and other names not currently recognised by BGS or regarded as obsolete.

A full Lexicon entry includes geological unit name, a persistent unique identifier (the Lexicon ‘computer code’), preferred map code, currency of the name, rank, parent unit, geological age, lithology, definitions of boundaries, thickness, previous and alternative names, geographical extent, type localities, and a reference list.

You can search on full or partial names of geological units.


The Lexicon is a dynamic database where new entries are added and existing entries are revised, upgraded and reclassified.

The information held is a current BGS interpretation. Free access is provided on the basis that the data are ‘as seen’ and are subject to revision without notice.

Search the BGS Lexicon of named rock units


Bedrock deposits: rocks, older than Quaternary age, generally lithified, that underlie the superficial deposits.

Litho-morpho-genetic units: geological units that are defined by their shape and mode of origin. They are generally superficial and unnamed, for example unnamed raised beach deposits, glacial moraines, and river terrace deposits.

Lithodemic units: geological units that do not obey the Law of Superposition, for example igneous intrusions and ancient highly deformed and metamorphosed terranes.

Lithostratigraphic units: geological units that obey the Law of Superpositioni.e. that are deposited in layers with the oldest at the base and the youngest at the top, for example sedimentary rocks and lava flows.

Superficial deposits: deposits of Quaternary age, generally unlithified, that overlie the bedrock.


Please cite the Lexicon as follows:

British Geological Survey. 2020. The BGS Lexicon of Named Rock Units [online]. Keyworth, Nottingham. Available from

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