The BGS Lexicon of Named Rock Units

Rock collection The Lexicon of Named Rock Units database provides BGS definitions of terms that appear on our maps and in our publications.

You can search for full or part-names of rock units.

Lexicon fields

Full Lexicon entries include:

  • details of rock unit name
  • rank, parent unit and parent unit rank
  • age and lithology
  • lower and upper boundaries
  • thickness
  • geographical extent
  • stratotypes

The database holds definitive online information about lithostratigraphical and lithodemic units and other, less formal, units.


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

The Lexicon information 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.

All entries carry a code describing the level of information held including:

  • Formal and Informal [F and I] entries — fully defined
  • Partial [P] entries that lack some elements of a full definition
  • Code Pending [CP] entries cover units that are not yet defined
  • Code Only [C] entries refer to units not scheduled for definition.

Formal and informal definitions

Only about 25% of Lexicon entries currently include full definitions — formal or informal. The detail level in each category is broadly comparable, but the informal definitions do not meet all the criteria of modern stratigraphical procedures. Informal definitions may become formal as mapping progresses and knowledge of the rocks improves. Older full definitions were flagged simply as formal or informal, but it is now recognised that some definitions might be valid only locally, whereas others are of national, or even international, applicability. In the future these distinctions will be recorded. Meanwhile, the full formal and informal definitions can be considered applicable in the areas of the stratotypes described, and potentially across wider geographical extents.


The Lexicon took over some functions of a BGS database/dictionary (BGS.Dic_Strat) that was built to hold unique codes for all rock units shown on BGS maps and referred to in publications up to and after its creation. Some entries were neither allostratigraphical, lithostratigraphical nor lithodemic. Thus the Lexicon has inherited entries related to rock units that cannot be defined in terms of a modern formal stratigraphy. Such quasi-stratigraphical entries include those for some superficial deposits (e.g. alluvium and head) that form expedient units for mapping purposes, and for un-named bedrock bodies (including un-named igneous intrusions) that are labelled with a specific lithology.

For further information on the Lexicon of Named Rock Units, please contact Dr Tim McCormick.