After the catastrophic surge in the delta region of SW Netherlands in February 1953 an ambitious scheme of waterworks was erected, the deltaworks. The geological research preceding many of the new large infrastructural works resulted in a massive bore campaign. These so-called ‘Deltadienst boreholes’ cover SW Netherlands, and represent mainly Pliocene and Quaternary deposits, rich in fossil shells.Ongoing research into the faunal successions provides a picture of molluscan diversity in the southern North Sea Basin during the Pliocene and Quaternary. In general a decline in species numbers is seen, with termophile and endemic groups most affected. However, immigration events, especially of Arctic–Pacific species, from the Middle Pliocene onwards, have greatly influenced the faunal development of the North Sea.The deltadienst successions, together with new successions described from the Antwerp (Belgium) Pliocene, will lead to an improved molluscan biozonation for the southern margin of the North Sea Basin.