These maps are based on the Ordnance Survey quarter-inch to the mile series of maps, for England / Wales and Scotland. Most maps in this series show solid geology only, but there are a few drift maps within the New Series maps of England / Wales. There are three distinct series of quarter-inch maps: - Geological map of England and Wales. Quarter-inch series 1:253 440: Old Series (1889 - 1906). This is a set of hand-coloured maps which were published between 1889 and 1895 with later revisions. They were engraved onto copper. The series was issued as 15 sheets, where sheet 3 was an index to colours. - Geological map of England and Wales. Quarter-inch series 1:253 440: New Series (1906-1977). Following the popularity of the Old Series 'Quarter-inch' map, a New Series of colour-printed maps was issued. This was a long-lived series, with sheets still being published in the late 1970s. Maps were published between 1906–1977. The series was issued as 15 sheets, where sheet 3 was an index to colours. - Geological Survey of Scotland. Quarter-inch series 1:253 440 (1904-1977). These Scottish maps were published in parallel with the English / Welsh New Series, and was issued as 17 sheets. The quarter-inch mapping was superseded in the 1970s - 1980s by the Universal Transverse Mercator (UTM) Series geological maps of the UK and Continental Shelf. 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.