Between November 1917 and June 1918, 12 maps at 1:10 000 scale of western Belgium (part of the British-occupied sector of the Western Front) were printed for the British army. These classified the ground according to its suitability for the excavation of dug-outs. Compilation was largely by the Welsh-born Australian T. W. Edgeworth David, using pre-war Belgian geological maps and data from c. 1000 British and Belgian boreholes. Lithostratigraphical units were coloured primarily in shades of red to indicate relatively ‘good’ (dry) strata, contrasted with ‘bad’ (wet) units coloured in shades of blue. The maps constitute the first engineering–environmental geology series to be published for British use, and arguably the first published large-scale engineering geology map series per se. Such little-known innovations in World War I, by British, US, German and Austro-Hungarian military geologists, were further developed in World War II. Wartime expertise consequently provided part of the foundation for the development of engineering geology postwar.