German hydrogeological maps prepared for Operation ‘Sealion’: the proposed invasion of England in 1940

During the Second World War, German militarygeologists assisted preparation for an invasion of Britain planned for September 1940. A military geology group deployed in support of the German 16th Army generated ten 1:50 000-scale water-supply maps plus accompanying explanatory texts to cover southeastern England. Two additional military geology units generated at least nine water maps for the adjacent south-central areas assigned to the 9th Army, maps different in their presentational style and in adopting 1:100 000 and 1:250 000 as well as 1:50 000 scales. Initially, water was to be obtained by both armies from untapped springs and drive wells: partly-perforated steel tubes hammered <7 m into suitable ground. Topographical base maps were derived from Ordnance Survey maps of the most similar scale. These were annotated by the geologists to show positions of springs of different types, key engineering works, regions differing in well-driving potential, and regions of contaminated or brackish groundwater, with more detail shown on 16th than 9th Army maps. Preparation essentially within the late July to mid-September 1940 time frame was made possible by extensive use of Ordnance and British Geological Survey publications, information readily available, rather than data from other sources. The maps were important aids to operational planning since substantial quantities of potable water were required for both men and horses of the two invading armies. From hindsight and more detailed local knowledge, it seems that the maps would have been useful but imperfect guides to obtaining adequate supplies of drinking water.