Abstract A sulfurous spring emerges at an outcrop of Bowland Shale Formation on Crimpton Brook, Lancashire. The spring is marked by the precipitation of white and yellow jarosite. Comparable light sulfur isotope compositions suggest that the jarosite was derived from surface oxidation of pyritic shale. The shale contains anomalous concentrations of selenium and other trace elements, in common with other exposures of the Bowland Shale Formation in the north of England. The selenium is resident in the pyrite, and was released during oxidation and incorporated into the jarosite in Crimpton Brook. The small scale of the water flow at the site and the remote situation pose no environmental problem, but the data exemplify the potential danger of pyrite weathering into groundwater on a larger scale.