Precambrian clastic rocks, deposited under unstable conditions, were folded before in a relatively stable environment shallow marine sedimentation spread out over the whole area. Silico-clastic sediments were deposited from Cambrian to Devonian, except from the Lower-Middle Cambrian when carbonate deposition dominated. With a hiatus in sedimentation during the Llanvirn to Llandovery the influence of a rising block, NNE of the present area, started. During the Silurian this rise resulted in development of clastic sequences trending to thin towards the N. From the Devonian to Upper Carboniferous sedimentation circumstances became less stable. As a result an alternation of clastic and carbonate rocks developed. Towards the end of the Devonian epeirogenetic uplift and tilting of the northern part of the area resulted in strong erosion and consequently the uppermost transgressive Devonian sandstone rests on a variety of older deposits. The Sabero-Gordón line separates the uplifted area in the north from the area where continued subsidence and sedimentation took place during the Upper Devonian. During the Lower Carboniferous differences in sedimentation circumstances were strongly reduced resulting in the deposition of the Alba Formation all over the area. During the Namurian the Sabero-Gordón Une renewed its function as a facies boundary between a northern and a southern area. Together with the development of the progress of the maximal Carboniferous sedimentation towards the north the initial folding of the Hercynian orogenesis started south of it. After the orogenesis oblong coal basins developed during the Stephanian B along normal faults approximately parallel to the strike of the folding. After folding of these coal basins a long period of non-deposition followed which ended in the Upper Cretaceous when sedimentation took place along the southern border of the folded Palaeozoic. The Tertiary morphogenetic uplift of the Cantabrian Mountains is accompanied by continental deposits forming the border of the Duero basin. During the Hercynian orogenesis major deformation took place in the Leoides (Fig. 3). The Sabero-Gordón line separates the Leónides in a strongly folded area in the south and an area with thrust sheets north of it. The shape of folds and thrusts is mainly determined by the lithological properties of the Palaeozoic rocks. Table 2 shows the rocks units which are supposed to have their own tectonic-style. In the southern area (Alba synclinorium) minor folding is an important feature. Based on a symmetry-concept most of these folds are parasitic folds. In some places minor folds in the folded area as well as in the thrust area show that deformation took place by gravity-stress. The León line separating the Leonides from the Asturides seems to have no significance as a fundamental structural line in this area.