Abstract A system of natural, vertical and mutual perpendicular joints is described in the clay pits of the Rupelian Boom Clay in the Antwerp area of North Belgium. Joints are the dominant discontinuity surfaces in the clay outcrops. Neither the stress evolution of the clay during burial and uplift nor the regional tectonic history can explain the tensional jointing. It is suggested that the negative horizontal stresses required for the joint formation in clays are caused by shrinkage of the clay when the formation was located near the surface. The origin of the loss of pore fluid in a several tens of metres thick clay layers remains unknown. The joints in the Boom Clay are a scarce field example of the possibility of lateral contraction of a clay layer, without involving tectonics or burial/uplift as an origin. When such a jointed clay layer is buried again, the presence of the vertical joints might offer pathways for fluid migration through a relatively impermeable layer. A general relationship between a maximal depth beneath which no tensional joints can occur and the cohesion of the clay has been derived. In the case of the Boom Clay this limiting depth is around 40–50 m. The time of the joint formation in the outcrop area is most probably late Oligocene/early Miocene. The burial history of the clay at a particular location can be used as a predictive tool for the presence or absence of tensional joints.