Todt, W., and H. Lippolt. “K-Ar Age Determinations on Tertiary Volcanic Rocks: 5. Siebengebirge, Siebengebirge-Graben”. Journal of Geophysics, Vol. 48, no. 1, Mar. 1980, pp. 18-27, https://journal.geophysicsjournal.com/JofG/article/view/259.