BGS Geology - Linear theme

Linear example

Linear features at the ground surface or bedrock surface (beneath superficial deposits) are digitised and geologically attributed. Their availability in the digital data therefore depends on the detail shown on the printed map. They are organised into six main categories:

  • ROCK e.g. coal seam, gypsum or ironstone bed; marine band
  • FAULT e.g. normal, thrust, reverse
  • FOLD AXIS e.g. anticline, syncline
  • MINERAL_VEIN e.g. mineral vein
  • ALTERATION_AREA e.g. limit of dolomitisation
  • LANDFORM e.g. buried channel margin, glacial drainage channel margin

These relate primarily to the bedrock theme, being either an intrinsic part of it (ROCK) or MINERAL_VEIN) or affecting it (FAULT, FOLD AXIS or ALTERATION_AREA). The LANDFORM theme may relate to either the bedrock or superficial theme. Most of these linear features are identified as either observed or inferred.

Many of the linear features are attributed generically, for example a ROCK line may be identified as a coal seam or gypsum bed; a marine band or mussel bed fossil horizon. However, these rock units can also be identified with LEX_RCS codes in the same way as polygons and carry the same associated information fields. Thus particular coal seams or marine bands may be identified with specific LEX codes such as:

  • YCL (Yard Coal, Leicestershire),
  • AGMB (Aegiranum Marine Band).

Similarly other linear features such as faults and mineral veins may also be attributed with more specific information, for example the name of the fault or the composition of the mineral.

Printed paper maps may also show other 'concealed' linear features such as faults or coal seams on the 'sub-Triassic' surface, i.e. the assumed outcrop of these features (probably identified in coal mines or boreholes) projected on the unconformity below Triassic strata. These underground features are not included in the BGS Geology linear theme in order to minimise the potential confusion they could cause to users if interpreted wrongly as being in overlying bedrock strata at the surface or just beneath superficial deposits.

Hutton field: well correlation diagram.