The detritic deposits of the Bierzo may be devided into two groups : the first one, in the lower part of the sedimentation, is fine with gravel-beds ; the other, in the upper part, is mixed with conglomeratic-beds. On the basis of comparison with thoses of Castilla (marls with gypsum called marls of the Paramo) the grey fades at the top of the fine deposits is thought to be Upper Vindobonian. The alluvial piedmont fades of Las Medulas was deposited during the tectonic period of the Upper Miocene ; but the sedimentation of the latest conglomeratic fades, which presents kaolinit in the clay minerals, is assumed to have occured during the warm and weter Pliocene. Morphologically, there are various kinds of fault-line scarps, some with slope of the fine sediments toward the basin center ; some with slope of the upper conglomeratic sediments. An erosion nivel was been formed before the conglomeratic sedimentation, an other one, most recent, took place after.