Henry Thomas de la Beche (1796–1855) was perhaps the first person to geologically survey an entire Ordnance Survey one-inch topographical map sheet. His first completed sheet (Old Series 22) was undertaken at his own expense in 1830–31 and covered south-east Devon and part of Dorset, including Lyme Regis where he was then living.
In May 1832, de la Beche succeeded in obtaining financial support from the Board of Ordnance to undertake a geological survey of Devonshire. One stipulation was that he should consult with the Geological Society to come up with an agreed scheme of colours. The resulting table of 16 formations soon became inadequate as more rock units were identified during the course of mapping. Today BGS recognises some 2500 formations — see the BGS Lexicon of Named Rock Units.
Founding of the Geological Survey
With the support of the Geological Society, de la Beche succeeded in establishing the Geological Survey on a formal basis, on 11 July 1835, as a department within the Ordnance Survey. Mapping progressed into Cornwall and by 1837 the whole of south-west England had been geologically surveyed.
Preservation of field maps and fossils
The ‘Devonian Controversy’ of 1834–39 involved a dispute over the application of fossil evidence to stratigraphic interpretation. The dispute threatened to undermine the authority of the Geological Survey, but it led to an appreciation of the need to establish a scientifically based collection of fossils in support of the mapping programme.
De la Beche preserved his field maps (cut up into slips) as a record of his actual observations. The BGS now holds more than three million fossil specimens, about half a million rock samples (excluding borehole material) and 68 512 registered field slips (excluding Old Series one-inch slips, which are currently unregistered).
Application of colour to geological maps
The colours chosen to depict rock formations usually attempted to match the overall colour of the rocks themselves, as exemplified by William Smith’s celebrated geological map of England and Wales (1815) and the Geological Society’s own version compiled by G B Greenough (1819), the latter being the starting point for de la Beche’s first colour scheme. As more formations were recognised, it became necessary to employ a wider range of colours or to combine them with ornament.
The colours themselves were based on watercolour pigments available from art dealers, but in due course a need arose for the survey to negotiate the production of specially formulated pigments.
Why the constant need for revision?
Given the rapid pace at which the Government of the day expected the Geological Survey to proceed and the cartographic limitations imposed by the one-inch scale, de la Beche soon came to recognise the provisional nature of the geological mapping being conducted at that time. In addition, the significance of superficial deposits was not recognised until the survey had reached the northern Welsh Borders and East Anglia, where glacial deposits thickly blanket the bedrock geology.
The introduction of topographical maps at the six-inch scale in the mid-nineteenth century created a need, and fulfilled a desire, for more detailed geological information.
This process of adding greater detail and refinement has continued up to recent times, with each re-survey adding greater understanding to what is essentially an interpretation based on the best available evidence.
Mapping in the 21st century
Since 2007, the BGS has dispensed with the use of paper field slips and now gathers field data utilising a tablet PC.
The BGS offers free access to geological map data via BGS Geology Viewer, a free resource that is designed to work on most modern browser-enabled devices.