AbstractStream sediments were sampled systematically in nine areas totalling 1100 sq. milesThroughout this paper 1 sq. mile = 2.589 km2. underlain in part by marine black shales ranging in age from Ordovician to Cretaceous. Regional patterns of molybdenum in the sediment were related to the underlying bedrock. Soils developed from the black shale parent material together with associated pasture herbage in eight of the areas contained above normal concentrations of molybdenum (>3 parts/million).Detailed studies in four of the areas confirmed the high molybdenum content of stream sediment, rock, soil and pasture. Examples are shown of the importance of both glacial drift and of specific soil factors such as pH in the interpretation of geochemical reconnaissance data for agriculture. Clinical hypocuprosis in cattle is already recognised over parts of two of the molybdenum‐anomalous areas. In the other two, copper deficiency disorders are not recognised but may well occur at a sub‐clinical level. It is suggested that appreciable acreages of the United Kingdom underlain by marine black shales may be enriched in molybdenum and be potentially limiting to livestock performance.