The Passage beds, which are commonly exposed in Mendip caves, comprise mechanically weak calcareous shale beds and tough coarse grained limestones (biosparrudites), it is the biosparrudites which have shown preferential erosion. The biosparrudites have an uneven surface with the fossils standing proud of the matrix. There are thus two scales of differential erosion, firstly between the shale and the coarse grained limestone beds and secondly within the coarse limestone bed, between fossils and matrix. Chemical and microscopic work show that the former is attributed to differences in composition and the latter is primarily controlled by the texture of the limestone. For biosparrudites it is invalid to relate the dissolution rate of the components to the rate of erosion of the whole rock.