J. H. van Voorthuysen, K. Toering, and W. H. Zagwijn. “The Plio-Pleistocene Boundary in the North Sea Basin Revision of Its Position in the Marine Beds”. Netherlands Journal of Geosciences (January 1, 1972): 627-639. Accessed May 8, 2026. https://njgjournal.nl/index.php/njg/article/view/14567.