G.F.W. Herngreen, C. Hartkopf-Froder, and G.H.J. Ruegg. 1994. “Age and Depositional Environment of the Kuhfeld Beds (Lower Cretaceous) in the Alstatte Embayment (W Germany, E Netherlands)”. Netherlands Journal of Geosciences 72 (January):375-91. https://njgjournal.nl/index.php/njg/article/view/12522.