N. Keutgen, J.W.M. Jagt, P.J. Felder and E.A. Jagt-Yazykova 2010. Stratigraphy of the upper Vijlen Member (Gulpen Formation; Maastrichtian) in northeast Belgium, the southeast Netherlands and the Aachen area (Germany), with special reference to belemnitellid cephalopods. Netherlands Journal of Geosciences. 89, (Sep. 2010), 109 - 136. DOI:https://doi.org/10.1017/S0016774600000731.