Formal Codes used in the BGS Lexicon of Named Rock Units

Types of entry identified by the following 'formal codes' are held within the Lexicon:


These carry only a Computer Code and 'RESERVED' in the 'Name of rock unit' field. They are transient entries, intended to avoid the need for deletion of named database entries if units are renamed before they are formalised, a process that sterilises the computer code. RESERVED CODE entries are upgraded to INFORMAL or FORMAL FULL entries when appropriate definitions are produced.


These relics of the pre-existing BGS.Dic_Strat database have been merged into the Lexicon with no significant modification or enhancement. In some cases more detailed information exists within the original paper data than appears in the digital database. Many non-mnemonic codes for 'Precambrian' units were derived according to an earlier coding system. Entries for divisions (including named veins and minor intrusive rocks) that are non-stratigraphical were allocated computer codes for other reasons. Most will be re-designated CODE ONLY.


Originally Dic_Strat entries, these units have computer codes that are no longer appropriate. Some earlier codes were based on a mixed litho- and chronostratigraphical nomenclature that is no longer valid and, as the initial code character indicated the chronostratigraphical component, the mnemonics are no longer helpful. To avoid later confusion the codes are not deleted but are maintained as 'obsolete', and the user is referred to an entry with a new code.


Also derived from Dic_Strat entries, the name has been confirmed as no longer appropriate for use by BGS and its associated code unsuitable for use with a replacement or revised name. Name and code are flagged 'obsolete' and the name is recorded as 'non-current'.

These OBSOLETE status levels are added to appropriate data entries as they are encountered during the day-to-day operation of the Lexicon, and some DIC_STRAT entries will be regraded to this status.


The named unit is currently accepted and used elsewhere, but it has not been adopted for use by the BGS. Potentially, the details provided may be at any level between CODE ONLY and FULL.


CODE ONLY entries are of four broad types:

  • 6.1 Most apply to units at bed (or equivalent) level, such as named coal seams and marine bands, that do not require FULL Lexicon entries. The lack of a current BGS requirement for FULL Lexicon data does not preclude its provision where it is desirable; hence these entries may include limited additional data and can be upgraded to FULL Lexicon status.
  • 6.2 Fewer entries relate to nomenclature that falls outside formal rock naming conventions, such as alluvium and un-named till or terrace deposits (here termed 'quasi-stratigraphical'). Also included are various named (but undefined) and un-named igneous intrusions that have appeared on geological maps and need a unique code for use in computer operations.
  • 6.3 Still fewer entries apply to expedient informal combinations of names that are more fully defined elsewhere (eg Stewartby and Weymouth members, undivided), or terms used for convenience, where specific named units cannot be identified (eg Chalk, undivided or Carboniferous rocks, undivided). Such nomenclature is used commonly on small scale maps and in borehole coding.
  • 6.4 A few entries represent data at an interim stage, following initial application for a computer code, and will eventually be upgraded to FULL entry status.

CODE ONLY status is added to appropriate data entries as they are encountered during the day-to-day operation of the Lexicon, and some DIC_STRAT entries will be regraded to this status.


Some original DIC_STRAT entries, mainly at group level, have been defined more fully as a temporary measure. The definitions were compiled by geologists with no direct experience of the relevant rocks, on the basis of a literature search, and the accuracy of the data is unverified. The details must be treated with caution. As survey progresses, new or modified entries will be produced for named units that remain current. Other terms will be flagged as obsolete as new divisions are identified and defined.


These entries are reliable, but do not include the full suite of data required by the modern Lexicon. The partial data are derived from archived application forms compiled by BGS geologists when justifying and defining named rock units to an earlier, less detailed, database specification.


These entries are for rock units, not mapped by the BGS, that are included on compiled 'provisional' 1:50 000 scale geological maps. The entries include only a sub-set of the data provided by FULL entries and the terms may become regarded as obsolete if new survey work is carried out.


FULL entries are those that meet all the standards of the modern Lexicon of Named Rock Units. They include units that are:

  • 10.1 F - FORMAL, with definitions meeting all the requirements of the North American Stratigraphic Code.
  • 10.2 I - INFORMAL, relating to units that either fail to meet all the requirements of the Stratigraphic Code or are used 'informally' as a descriptive expedient (eg the Upper and Lower Lincolnshire Limestone members are informal divisions of the Lincolnshire Limestone Formation). Some INFORMAL entries may eventually be made FORMAL.
  • 10.3 As Lexicon development proceeds the FULL entries will be sub-divided:
  • FL or IL - FULL FORMAL or INFORMAL definition of LOCAL applicability;
  • FN or IN - FULL FORMAL or INFORMAL definition of NATIONAL applicability;
  • FW or IW - FULL FORMAL or INFORMAL definition of (potentially) WIDER applicability.

The data associated with FORMAL and INFORMAL FULL Lexicon entries are verified according to internal BGS standards, presenting the current BGS definition of the specified unit. It is emphasised that each entry reflects a BGS interpretation. Definitions may or may not include elements of definitions originating elsewhere, and it is possible that some definitions may not be accepted outside BGS. Likewise, rock unit names considered obsolete by BGS geologists may still be in use outside BGS.