For much of the Geological Survey's existence, the County Series of maps were the standard large-scale maps on which geological mapping was undertaken. These maps are based on the Ordnance Survey County (or six-inch to the mile) series of maps. These maps were cut up to be used in the field to record geological observations, and on return to the office, the geology was transferred to a complete County Series map, which after approval was known as a 'standard' (England / Wales) or 'clean copy' (Scotland). This dataset contains the 'standard' or 'clean copy' County Series maps held by BGS. Geological maps represent a geologist's compiled interpretation of the geology of an area. A geologist will consider the data available at the time, including measurements and observations collected during field campaigns, as well as their knowledge of geological processes and the geological context to create a model of the geology of an area. This model is then fitted to a topographic basemap and drawn up at the appropriate scale, with generalization if necessary, to create a geological map, which is a representation of the geological model. Explanatory notes and vertical and horizontal cross sections may be published with the map. Geological maps may be created to show various aspects of the geology, or themes. The most common map themes held by BGS are solid (later referred to as bedrock) and drift (later referred to as superficial). These maps are hard-copy paper records stored in the National Geoscience Data Centre (NGDC) and are delivered as digital scans through the BGS website.