The structures in the SW part of the Cantabrian Mountains have much in common with those of the Foothills Belt of the Rocky Mountains, the Alps and the Central European Hercynian orogene, and their origin can be explained in the same way as that of the structures in these orogenes. The greywacke sedimentation and the folding both migrated from the internal to the external part of the original basin during the Upper Carboniferous. The folds and thrust faults run parallel to the axis of the original basin. The basement has been broken into large blocks in the shape of parallelograms, along the boundary faults of which local deviations of the regional directions occurred.