The precise stratigraphic position of the sediments in the Heist-op-den-Berg and Beerzel hills has been under debate for a long time. According to the geological map of Flanders, these deposits are included in the Diest Formation. Reference data for the Diest and Kasterlee Formations are defined and compared to new sediment-petrological data from samples collected in the Heist-op-den-Berg and Beerzel area. This comparative study indicates that the sediments are similar to the Kasterlee Formation. Additionally, a gravel bed observed at the base of the deposits at the Heist-op-den-Berg and Beerzel hills is similar to the basal gravel of the Kasterlee Formation at Olen. The Kasterlee Formation at Heist-op-den-Berg and Beerzel is subdivided into three members. The lithofacies of the lowermost member is comparable to the Diest Formation and is explained through significant reworking. The occurrence of the Kasterlee Formation in the study area implies the formation of a bay or estuary in the late Miocene sea extending towards the south. The curvature in the present base of the Kasterlee Formation and the considerable thinning of the Diest Formation to the south indicates ongoing subsidence of the Campine basin and uplift of the Caledonian Brabant Massif during the late Miocene and Pliocene.