Photograph of the month

Abstract

Roofing slate quarrying of global importance is carried out in the Upper Ordovician rocks of the Westasturian-Leonese zone of the Iberian Variscan belt. Several orders of folds with an associated cleavage developed during the first regional deformation phase. The photograph (view looking SE) shows an anticline-syncline pair developed in a multilayer made up dominantly of slates with thin intercalations of siltstones and fine grained sandstones deformed by parasitic folds with wavelengths of 10e20 cm. The larger folds have a near perfect similar geometry (Ramsay class 2) in the slate layers. The classical controversy about the origin of similar folds is posed at this outcrop. The well-developed axial planar slaty cleavage as well as the centimetric buckling folds indicates an important shortening perpendicular to the axial plane. The centimetric competent layers show folding with regular wavelength and amplitude on the limbs and hinges indicating that these parasitic folds formed early during the initial layer parallel shortening of the multilayer. The progress of folding gave rise to the asymmetry observed in the limbs of the metric folds. Another section (40 m2 in size) of this amazing structure is permanently exhibited on a geological wall in the Science Museum CosmoCaixa (Barcelona, Spain). Location: Pizarras del Carmen Quarry, La Baña, Spain. N 42º16.310´, W 6º43.990'