In the delta of the rivers Fluvia and Muga, the Quaternary is hydrogeologically the most important deposit. It is underlain by Pliocene marls and clays. From geo-electrical soundings and bore-hole data a map with depth contours of the Quaternary-Pliocene boundary plane was constructed. Near the coast the Quaternary consists of two aquifers. One of the restrictions of the resistivity method is that the second aquifer is too thin in respect to its depth. Therefore only the formation resistivities of the complete Quaternary were calculated. By comparing the map of formation resistivities with maps of the clay percentage and the water resistivities in the Quaternary, prospective areas for waterwinning can be delineated. To the north of the delta, Silurian granites and schists crop out. A gravimetric survey over the contact shows a gently sloping boundary of the granite and schist with the Eocene-Pliocene of ca. 15°. From geo-electrical data a comparable inclination of 10° was found. The pre-Eocene topography here is thought to have been formed under tectonic control but the inclination of the Eocene-Pliocene to granite/schist would seem to be too shallow to correspond to a normal fault plane.