Mullion and boudin structures in the Lower Devonian of the Ardennes and Eifel have been discussed in the literature in three languages since 1907. Two outcrops in Dedenborn, Germany, and Bastogne, Belgium served as examples in several textbooks for mullion and boudin type localities, respectively. Both structures are characterized by cylindrical cuspate-lobate structures on pelite-psammite contacts, with the regional slaty cleavage at a high angle to bedding. We review the confusing evolution of nomenclature, demonstrate the common origin of both structures and propose that all such structures in this area with a large angle between bedding and cleavage are from now on called mullions.

We review the most important observations of mullion (formerly boudin) structures and the models proposed for their evolution, and present data from a database of 150 outcrops containing mullion structures. Although most authors agree that these long, strikingly regular, cuspate-lobate structures formed during layer-parallel shortening, there is less agreement about the origin of the arrays of quartz veins, which are always present between individual mullions. An important observation is the angle between the mullion axis and the delta-lineation. This angle is generally non-zero but never more than 40¾, strongly suggesting that the mullions did not form as buckling instabilities at the pelite-psammite interface.

We propose a two-phase development of these structures. First, at the late stages of burial to over 8 km depth in a passive continental margin, and after the development of close to lithostatic pore pressures, a regionally extensive series of closely spaced sub-parallel veins were formed in mode I fractures in the psammite layers at high angle to bedding. Then, during the early phases of sub-horizontal Variscan shortening, which happened to take place at high angles to the pre-existing veins, the cuspate-lobate structures were initiated at the vein tips due to the competence contrast (veins > psammite > pelite).

The shape of the lobes between mullions, which can be accurately measured, is a potential indicator of the power law exponent n in sand-shale sequences deforming under very low grade metamorphic conditions.