The stratotype of the Dormaal Sand Member, representing the lower part of the Tienen Formation (formerly Upper Landenian, Upper Landen Formation or Sparnacian auct.), exhibits a cyclic sedimentation pattern of fluviatile origin. This palaeoriver system was inhabited by a large variety of freshwater biota and bordered by dense warm-temperate to subtropical forests, occasionally set afire by lightning. Abundant contemporaneous land plants and terrestrial vertebrate remains accumulated in this depocentre, together with large quantities of reworked Campanian chalk pebbles and late Palaeocene fish teeth, resulting from local and upstream erosion. The Dormaal Sands represent the infill of a large palaeovalley, cut during a eustatically induced sea-level lowstand, which was presumably related to plate-tectonic convergence. For the moment it is justified to assume that the burial of the Dormaal mammal fauna post-dated this major sea-level fall, but preceded the events which led to the well-known kaolinite influx and the subsequent, globally detectable negative carbon isotope excursion, which are useful criteria for the definition of the Palaeocene/Eocene boundary. However, as long as no consensus has been reached on this boundary definition, the exact chronostratigraphic position of the Dormaal Sand Member and its mammal fauna remains unresolved.