The term 'boudin' is generally acknowledged in the geological terminology to describe structures that are the result of layer-parallel extension, ie 'boudinage'. The structures in the Ardenne-Eifel area, however, turn out not to comply with the latter definition, which lead to a controversy on the true nature of these particular structures, ie the 'boudinage question'. Eventually a renewed interest in the veins allowed to give a conclusive answer to this controversy. The key to the solution has to be found in a multidisciplinary approach, integrating structural, mineralogical, geochemical and microthermometrical analyses. This integrated study demonstrates that the structures in the Ardenne-Eifel area are a composite of two different deformation events. First, the veins formed during the deepest stages of the burial history, in close association with the regional low-grade burial metamorphism. Second, veining was followed by layer-parallel shortening at the onset of the Variscan orogeny, resulting in a cuspate-lobate morphology of the interface between psammite and pelite, ie 'mullions'.
This mullion development occurred at both upper and lower interfaces of the psammitic layers pinnen by the pre-existing veins, giving rise to the particular 'sausage'-like structures, that gave initially rise to the term 'boudin'. The current study irrefutable proves that the term 'boudin' should be abandoned to describe the particular 'double-sided' mullions in the Ardenne-Eifel area.