Figure 4. The Dedenborn outcrop

The Dedenborn outcrop. (a) The classic view of the outcrop, showing the characteristic cuspate-lobate structures at bedding contacts. (b) Overview of the outcrop, looking at high angle to bedding. (c) NW-SE profile of the fold structure in the area, showing bedding, cleavage and the location of mullions. Modifed after Pilger and Schmidt (1957). (d) Photograph of the outcrop in 1955, showing lateral variations in mullion morphology (arrows), which is not visible in (b) (from Pilger and Schmidt (1957)). (e) Overview of the outcrop, emphasizing the angle between mullion axis and delta lineation. (f) Side view of two mullions, showing the quartz vein terminating at the cusps (yellow, arrows) and the trace of cleavage. (g) Detailed drawing of several mullions in profile (from Pilger and Schmidt (1957)). (Click for enlargement)

A small roadside outcrop near the village of Dedenborn in the N-Eifel found its way into many structural geology textbooks as the type example of mullions (Fig. 4).

The outcrop is situated in the steep to overturned limb of a regional series of upright folds with well-developed axial planar cleavage. The rocks consist of alternating psammite-pelite layers. The classic view of the outcrop (Fig. 4a) shows the bottom of a sandstone layer, which broke along the contact to the adjacent shale. This surface forms a series of regular, highly cylindrical, asymmetrical cuspate-lobate structures (Fig. 4f,g) with the cusps pointing to the sandstone. In the sandstone layer cleavage is only weakly developed. The sandstone layer slowly grades upwards into a shale, and the backside of the outcrop consists of shale with a well-developed slaty cleavage. A very detailed description of this outcrop is given in (Pilger and Schmidt, 1957a; Pilger and Schmidt, 1957b; Pilger and Schmidt, 1957c; Pilger and Schmidt, 1957d).

Two observations not usually mentioned in textbooks (but documented in the literature) are: (1) The sandstone layer contains a series of spindle-shaped quartz veins which terminate in the cusps between the mullion lobes (Fig. 4f), and (2) the intersection lineation between cleavage and bedding (the delta lineation), best seen on another bedding plane which has not developed mullions) makes an angle of about 30 degrees with the long axes of the mullions (Fig. 4e).