Thick mudstone formations crop out over large areas of Britain and it is commonly necessary to subdivide them and make correlations within them for civil engineering and other economic purposes. Although at first sight many of these formations appear to consist of monotonously uniform lithologies that are commonly deeply weathered and rarely exposed, they can yield sufficient field data for detailed stratigraphical and structural interpretations to be made. Such data includes lithological variations and macrofunal changes used in combination with field mapping and geophysical borehole logging. In almost all cases the geological subdivisions so recognized are more detailed than the variations revealed by indiscriminate engineering soil-testing. Visual inspection of mudstones in the field, combined with selective testing, can provide a better guide to their behavioural characteristics.