The Lexicon of Named Rock Units provides definitions of lithostratigraphic, lithodemic, and litho-morpho-genetic geological units of the United Kingdom and its associated continental shelf. The Lexicon focuses mainly on units of Member, Formation, Group and higher rank (and equivalents) 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 the BGS or regarded as obsolete. Full Lexicon entries include geological unit name, a persistent unique identifier, map code, currency, rank, parent unit and rank, age, lithology, definitions of boundaries, thickness, previous and alternative names, geographical extent, type localities, and bibliographical references.