AbstractHydro‐pedological investigations were carried out in a small forested watershed on the Lower Emsian formations of the Lower Devonian in the Luxembourg Ardennes. These rocks consist of weakly metamorphosed shales and quartzites, of which only the shales are undergoing appreciable weathering, and are composed of quartz, sericite, albite and chlorite, with minor amounts of haematite (goethite) and traces of pyrite, rutile and apatite. The alumino‐silicates react with CO2charged water, forming kaolinite, with vermiculite and smectite as intermediate phases and releasing cations and silica for solution. From a hydrological study and a detailed programme of soil, spring and river water sampling the year could be subdivided into three characteristic hydrological periods. The runoff during these periods could be separated into various components, some of which have a characteristic water chemistry. The amount of material removed in solution was 7,448 kg for the years 1973/1974 and 1974/1975 (220 kg/ha/year). The output of material in solution is probably much greater than the output of material in suspension and as bedload. The chemical erosion rate could be calculated with these output data, after making allowance for atmospherically supplied materials and biomass recycling; it amounts to 4,435 kg (131 kg/ha/year), which is 59.5 per cent of the total solutional out