Increases in soil bulk density beyond the optimum reduce land productivity and, where soil is affected, may be difficult to remedy. Elucidating the mechanisms causing compaction is a prerequisite to sustainable management of fragile soils. We examined a dense grey soil in Western Australia in which the dominant physical feature of the subsoil was coarse prismatic structure. The prisms were approximately hexagonal in horizontal section with an average side length of 0.66 m. The top of the prisms reached to within approximately 0.07 m of the soil surface, their sides becoming indistinguishable below approximately 0.9 m. The vertical faces of the prisms were coated by material similar in composition to the topsoil and separated from it by a transition material of intermediate composition. Soil within the prisms had a bulk density at maximum swelling which reached a maximum of 1.86 g cm(-3) in the upper subsoil. We investigate the hypothesis that such a high bulk density could have developed as a result of a simple three-stage process: (i) soil shrinkage as the profile dries over summer leading to widening of cracks between prismatic peds, (ii) infilling of cracks by detached topsoil which adds to coating thickness, and (iii) swelling during the winter, now partially restricted by coating material, leading to compression of the prismatic peds. We present a model which accounts quantitatively for this process and explain how soil physical characteristics might facilitate it. The dense upper subsoil (7-60 cm) limits root penetration and prolongs the period of transient waterlogging of the topsoil during winter, adversely affecting subsequent crop performance. Our work suggests that stabilizing surface soil to minimize soil detachment could be a relevant management objective on these structurally unstable soils in order to prevent subsoil compaction.