1:250,000 Scale Offshore Geology Data

Introduction

The data comprises primarily polygons (or areas) and lines. Each polygon is attributed with information based on the name (often lithostratigraphical) of the unit and its lithology or composition. These polygons are arranged into a single theme: bedrock geology ('solid'). The lines include faults and the coastline. Coverage of point data, e.g. dips and strikes, is limited and is not yet available.

The data is available in latitude / longitude decimal degrees WGS84 (World Geodetic System 1984). The data are seamless, as far as possible, with polygons and coding consistent across the original map (or digital tile) boundaries. The geological codes and descriptions are included as integral components of the data. This 1:250 000 data is the only scale that provides cover of the offshore area. It has been merged into a single file for each theme, Sea-Bed Sediments and Bedrock

Onshore, a lithostratigraphical nomenclature is used with rocks routinely mapped at formation, member and bed level. Offshore, this is often impossible and rocks are commonly referred to by age. Where it is possible, mapping is usually only to group or formation level at best, and even these may not be differentiated into individual units, but shown only as part of a package of units.

The sea-bed sediments layer is only mapped offshore, where it commonly forms a veneer on the sea bed. The map is based on sea-bed grab samples of the top 0.1m, combined with cores and dredge samples as available. A standard Folk triangle classification has been used based on the gravel percentage and the sand to mud ratio.

Back to top

Polygon layers

The digital geological maps typically show the polygon theme as described below.

Theme Comment Age
Bedrock (or 'solid') geology Mostly consolidated natural rocks Pre-Quaternary age, older than about 1.8 million years
Seabed Sediments

The top layer of sediment covering the sea bed, classified by % gravel and the sand to mud ratio. (After Folk, 1954)

Polygon Attribute Table Description

The Attribute Level is an incremental number used to identify the types of additional information supplied in Geology Digimap datasets.

Attribute level: 14 [see note 1 below]

Data Field Explanation of data field Comment
SHAPE Necessary for the ESRI Shapefile format indicating polygon, polyline or point data.
LEX_ROCK A two-part code, LEX & ROCK, used to label each polygon of Geology Digimap data and for creating map keys or legends. See note 2
LEX Lexicon (or LEX) code. First part of the LEX_ROCK label. Up to 5 characters (mostly letters). An abbreviation of the rock unit or deposit as listed in the BGS Lexicon of Named Rock Units: e.g. GOG. See note 3
LEX_D Description of the Lexicon code above giving the name of the unit: e.g. GREAT OOLITE GROUP is the full name of the unit coded as GOG.
ROCK Rock Code. Second part of the LEX_ROCK label. A code abbreviation, up to six letters, for the type of rock or lithology e.g. LMST.
ROCK_D Description of the rock code above: e.g. LIMESTONE is the lithology of the rock unit coded as LMST.
RCS RCS code. An alternative code abbreviation (or a string of such codes joined by + signs with square brackets used for subordinate types), each up to 6 characters, for the type of rock or lithology as based on the hierarchical BGS Rock Classification Scheme (RCS): e.g. MDST + [CONG]. See note 4
RCS_D Description of the RCS code(s) above giving the lithology of the unit: e.g. MUDSTONE (UNDIFFERENTIATED) with CONGLOMERATE is the description of the rock coded as MDST + [CONG].
RANK Rank of the unit in the lithostratigraphic or lithodemic hierarchy: e.g. GROUP or SUITE. See note 5
BED_EQ Bed equivalent. Lexicon code for the unit at bed or equivalent level where applicable. See note 6
BED_EQ_D Description of BED_EQ above; name of unit at Bed level.
MB_EQ Member equivalent. Lexicon code for the unit at member or equivalent level where applicable.
MB_EQ_D Description of MB_EQ above; name at Member level.
FM_EQ Formation equivalent. Lexicon code for the unit at formation or equivalent level where applicable.
FM_EQ_D Description of FM_EQ above; name at formation level.
SUBGP_EQ Subgroup equivalent. Lexicon code for the unit at subgroup or equivalent level where applicable.
SUBGP_EQ_D Description of SUBGP_EQ above; name at subgroup level.
GP_EQ Group equivalent. Lexicon code for the unit at group or equivalent level where applicable.
GP_EQ_D Description of GP_EQ above; name at group level.
SUPGP_EQ Supergroup equivalent. Lexicon code for the unit at supergroup or equivalent level where applicable.
SUPGP_EQ_D Description of SUPGP_EQ above; name at supergroup level.
MAX_TIME_D Maximum age of the unit, to the most accurate time (or geochronological) division possible: e.g. ASBIAN. Same if unit spans only one time division
MIN_TIME_D Minimum age of unit, to the most accurate time (or geochronological) division possible: e.g. ALPORTIAN.
MAX_TIME_Y Maximum age, in years, of the oldest time division during which the geological unit was formed: e.g. 333800000. See note 7
MIN_TIME_Y Minimum age, in years, of the youngest time division during which the geological unit was formed: e.g. 320710000.
MAX_INDEX Maximum index. A number representing the maximum age (earliest time) of the unit: MAX_TIME_D field. Used for GIS querying and legend building: e.g. 1322120. See note 8
MIN_INDEX Minimum index. A number representing the minimum age (latest time) of the unit: MIN_TIME_D field. Used for GIS querying and legend building: e.g. 1321340.
MAX_AGE Maximum Age. Name of the Age of maximum geochronological time applicable: e.g. ASBIAN. Same if unit spans only one Age. See note 9
MIN_AGE Minimum Age. Name of the Age of minimum geochronological time applicable: e.g. ALPORTIAN.
MAX_EPOCH Maximum Epoch. Name of the Epoch of maximum geochronological time applicable: e.g. VISEAN. Same if unit spans only one Epoch
MIN_ EPOCH Minimum Epoch. Name of the Epoch of minimum geochronological time applicable: e.g. NAMURIAN.
MAX_SUBPER Maximum Sub-period. Name of the Sub-period of maximum geochronological time applicable: e.g. DINANTIAN. Same if unit spans only one Sub-period
MIN_SUBPER Minimum Sub-period. Name of the Sub-period of minimum geochronological time applicable: e.g. SILESIAN.
MAX_ PERIOD Maximum Period. Name of the Period of maximum geochronological time applicable: e.g. CARBONIFEROUS. Same if unit spans only one Period
MIN_ PERIOD Minimum Period. Name of the Period of minimum geochronological time applicable: e.g. PERMIAN.
MAX_ERA Maximum Era. Name of the Era of maximum geochronological time applicable: e.g. PALAEOZOIC. Same if unit spans only one Era
MIN_ERA Minimum Era. Name of the Era of minimum geochronological time applicable: e.g. MESOZOIC.
MAX_EON Maximum Eon. Name of the Eon of maximum geochronological time applicable: e.g. PROTEROZOIC. Same if unit spans only one Eon
MIN_EON Minimum Eon. Name of the Eon of minimum geochronological time applicable: e.g. PHANEROZOIC.
PREV_NAME Previous name(s) for the unit as listed in the BGS Lexicon of Named Rock Units. New at level_14, See note 10
BGS_TYPE The Geology Digimap theme e.g. bedrock, superficial, mass movement or artificial. New at level_14, See note 11
BGSREF BGS Reference colour for the polygon based on the LEX_ROCK code pair. The default printing colour defined as a 3-digit number: e.g. 434 (for Claygate Member_mudstone and sandstone). Used for legend building to give a similar appearance to the published map.
BGSREF_LEX Alternative BGS reference colour at the Lexicon code level, LEX, as defined above: e.g. 434 (no alternative needed as no clashes so same as above).
BGSREF_FM Alternative BGS reference colour at the formation level, FM_EQ, as defined above: e.g. 323 for London Clay Formation which includes Claygate Member.
BGSREF_GP Alternative BGS reference colour at the group level, GP_EQ, as defined above: e.g. 424 for Thames Group which includes London Clay Formation.
BGSREF_RK Alternative BGS reference colour for the lithology ROCK code, as defined above: e.g. 822 for mudstone and sandstone lithology of Claygate Member.
SHEET Geological map sheet number, map sheet name and the tile version that the polygon appears on: e.g. EW069_BRADFORD_V3; SC032E_EDINBURGH_V3. See note 12
VERSION Version number and attribute level of the digital data: e.g. V3_14 is version 3, with attribute level 14. The version number is changed when a new dataset is released following major changes or periodic update. Data with the same attribute level have the same structure. As fields are added, renamed or removed so the attribute level is changed.
RELEASED Date the Geology Digimap data were converted into release format.
NOM_SCALE Nominal scale of the published (or compiled) information used to prepare the digital data: e.g. 10000 for 1:10 000 [including 1:10 560], 25000 for 1:25 000, 50000 for 1:50 000 [including 1:63 360] and 100000 for 1:100 000 maps. See note 13
NOM_OS_YR The latest year date of Ordnance Survey information contained in the topographic base used for the original printed geological map (or the base used for Geology Digimap compilations). Fuller details are available if required.
NOM_BGS_YR The latest year date of the principal BGS geological information contained in the digital tile. This is usually the year of publication of the most up-to-date map sheet. Where no published map was available it is the year of compilation for Geology Digimap. Fuller details are available if required.
MSLINK Used for BGS QA purposes.

Additional notes

Note 1 The attribute level is an incremental number used to identify the types of additional information supplied in Geology Digimap datasets; a new number is used whenever the information fields attached to the data are changed. Programming code used to query one dataset will probably operate on another at the same attribute level; users are advised to check the functionality of their GIS applications when the attribute level is changed.
Note 2 In the preparation of Geology Digimap data each LEX_ROCK pair is given a unique number which is then used to link to other BGS databases and thereby provide the information used to populate the other information fields.
Note 3 The Lexicon is a database of named rock units and definitions which can be viewed at Digimap Lexicon The majority of stratified rock units are given a lithostratigraphical name whilst non-stratified units, such as igneous intrusions and some metamorphic bodies, have a lithodemic name. As these are mutually exclusive Geology Digimap uses the same field names for both types.
Note 4 The BGS Rock Classification Scheme (RCS) is available in 4 Volumes which can be downloaded free at: http://www.bgs.ac.uk/bgsrcs/home.html
Note 5 Further explanatory notes are available on lithostratigraphic units and lithodemic units at: glossary
Note 6 The parentage of each rock unit is provided, in so far as it is available. Thus a named unit of bed rank may be part of a named member, which is itself part of a formation. Several formations may make up a group and several groups may form a supergroup. A formation is the prime mapping-unit and need not be divided up into named members or beds; nor does a formation have to belong to a group or supergroup.
Note 7 These are ages, in years as shown on the BGS Geological Time chart where they are expressed as 'million years'. Some of these values are interpolations; the +/- error ranges are not provided here. The age range given is that for the time period ascribed to each geological unit in the BGS Lexicon. They do not give absolute age measurements made on the individual geological units.
Note 8 The index number is a hierarchical 7-digit number, based on the geochronological time, that allows rock units to be ordered approximately for legend building, or selected or queried by their age.
Note 9 Sub-divisions of Age called Chrons are also shown in this field as a hyphenated name, for example Streffordian-Actonian is used for the Actonian Chron which is the early part of the Streffordian Age. New fields for maximum and minimum chrons have not been created as they are only used in the Caradoc at present.
Note 10 Possible previous name(s) for the unit are given as listed in the BGS Lexicon of Named Rock Units. These names may have been used for all or part of particular polygons in particular areas as shown in Geology Digimap data e.g. Keuper Marl and Keuper Series are two of the possible previous names used for the Mercia Mudstone Group. The field length is limited and long lists are truncated to 250 characters due to the Shapefile format.
Note 11 There are four types of mapped unit in Geology Digimap: bedrock (formerly solid), superficial (formerly drift), mass movement or artificial and these are held as separate layers of geological data for use in GIS.
Note 12 The version suffix will be particularly relevant to some users of 1:50 000 scale data. For the upgrade from Version 2 to Version 3, the accuracy of every specified point (or node) was reduced from 1 mm to 1 m on the ground. In practice this has meant that the shape of every polygon has been revised. The inclined margins of each digital tile were also redefined as a series of 1 m steps. The shape of each tile has thus been 'permanently' fixed and they now fit together 'jigsaw like' with their neighbours. This minimises the creation of minor gaps and overlaps appearing in the data when it undergoes GIS processing as happened previously in the 1:50 000 dataset with mm resolution.
Note 13 Digital data should normally only be used at scales similar to the source data; for example 1:50 000 data are not suitable for use at 1:10 000 scale without great caution. See guidance on use of geological map data.

Back to top

Linear Feature Layers

The linear features shown include fold axes (anticline or syncline) and faults (normal, thrust, reverse) and are organised into a number of themes. The faults show crossmarks on their downthrown side.

Back to top

Important Notes

 

Do not over-enlarge the data; for example do not use 1:250 000 nominal scale data at 1:100 000 or 1:50 000 working scale. The scale of the original information is indicated by the nominal scale attribute (NOM_SCALE) embedded in the data.

 

Back to top