Abstract The Tertiary deposits in Belgium are marine shelf to coastal deposits formed in the southern part of the North Sea Basin. Lithologically they vary from calcareous deposits at the beginning of the Paleogene, almost indistinguishable from the underlying Maastrichtian chalks, to marls, clays and sands towards the top. In northern Belgium, these deposits reach thicknesses of several hundreds of meters. Stratigraphically they cover almost the whole Tertiary, albeit with many important hiatal intervals. The stratigraphy in the region was established quite firmly from outcrops, already in the former century. International stage names such as Ypresian and Rupelian are derived from well studied outcrop areas in this region, and several more regionally used stage names were defined in Belgium such as Montian, Landenian, Bruxellian, Ledian, Wemmelian and Tongrian. As these chronostratigraphic names have become obsolete, the names of their type localities are now reserved for lithostratigraphic units: groups, formations and members (Maréchal, 1991). In fact this is even more appropriate as these stratigraphic units were originally individualized on the basis of lithological characteristics and much less on grounds of stratigraphically significant fossils.