At the Stöffel hill, trachytic and basaltic ash layers overlay
older clay and silicified sand of the Arenberg Formation
(Fig. 2) (Ahrens 1936; Pirrung 1997; Hilder 2003). The
volcanic ash erupted from two to three diatremes that were
oriented parallel to the Para Rhine Graben (NNW to SSE).
The middle (northern) diatrem was a small trachytic caldera
or a Bims volcano (Pirrung 1998; Pirrung et al. 2001). After
the end of the eruption (dated by a lava flow within
posteruptive breccias to have an isotopic age of 24.79±
0.05 Ma; Mertz et al. 2007),