These maps are based on the Ordnance Survey 'Old Series' One-Inch maps of England and Wales. They were the first 1:63 360 geological map series produced by the Geological Survey in England and Wales. Sheets were initially published as full sheets, and later as quarter sheets (NW, NE, SW, SE). Earlier maps are hand-coloured, later maps are colour-printed. The maps show both solid and drift geology. The OS 'Old Series' maps extended as far north as a line drawn between Preston and the Humber. To the north of this, the maps use the same sheetlines as the 1:63 360 New Series, and carry two sheet numbers, which refer to the Old Series (91-110) and New Series (1-73) numbering. These maps are common to both series. A sheet that covers the Isle of Man is also included in the series. 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. 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.