The existing geological maps show many lithological as well as stratigraphical inaccuracies. A new reconnaissance survey has been made. This survey included a study of the thickness and the lithology of the Quaternary sediments and a lithological and stratigraphical study of the upper part of the Tertiary substratum.The study has been realized mostly by geo-electric probings with direct current and the electrodes in Wenner arrangement. About 500 probings were executed over the whole territory of the West-Flanders. They are grouped together in areas where terrain conditions allowed the application of this method. The data comprise at first the values of the apparent resistivity, which have been measured in each point by electrode separations varying between 1 and 100 m. The values of the apparent resistivity, measured by the electrode separations of 10, 20 and 40 metres are represented by resistivity maps. The specific resistivity Q o for the upper layer and Q b for the substratum are reported on two other maps. On the other hand by ihterpretation of the data, an isopach map of the Quaternary sediments and an isohyps map of the geo-electric contact level have been established.The resistivities measured for sediments saturated with water are mentioned. The resistivity maps made it possible to distinguish different electrically homogeneaus regions, which correspond also to hydrologic unities.The Dune Belt forms a region of high resistivity, where the depth of the Ypresian clay has been determined between De Panne and Nieuwpoort. In the vicinity of Knokke the depth of the Bartonian clay has been measured. Between Nieuwpoort and Heist the gea-electric method could not be used since either the Tertiary sediments have the same composition as the Quaternary overburden, or salt water is present in the subsoil. In the Polders the rapid horizontal variations in resistivity are connected with the degree of the salinity of the phreatic water. In the Transition Region between the Polders and the Inner Flanders two zones can ...