The BGS collection of 1:10 560 / 1:10 000 National Grid Series geological maps. These maps are based on the Ordnance Survey National Grid series of maps, which are defined by the 10 km intervals of the larger 100 km square identified by a specific two-letter code. Each map is thus denoted by a unique reference, e.g. SP 29 NW. SP=100 km square; 29=10 km square; NW=5 km square. Since field mapping is generally undertaken at the scale of 1:10 000 (or equivalent), these maps are the largest-scale main series of geological maps that BGS holds. A small number of remote areas were mapped at 1:25 000 scale, the subsequent maps are also at 1:25 000 scale and are included in this series. The equivalent to the National Grid Series prior to the 1960s is the County Series (at 1:10 560 scale). In the 1960s, this series started to be replaced by 6 inches to 1 mile (1:10 560 scale) National Grid sheets based on the four quadrants (NW, NE, SW, SE) of a 10 km Ordnance Survey National Grid square. Areal coverage provided by the National Grid series of large-scale maps is limited in extent and the preceding County series of six-inch maps can still be the most up to date map available for some areas. 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, for the most part, hard-copy paper records stored in the National Geoscience Data Centre (NGDC) and are delivered as digital scans through the BGS website.