This individuality naturally has its basis, or at least its correspondences, in the structure and geological constitution of the continental mass. Although modern investigations have shown the events through which its morphology was modeled more and more complicated, there is no doubt that the Iberian relief derives its originality essentially from the fact that the Alpine orogeny not only has not canceled, but it has, so to speak, adapted to a design which, in its general lines, dates back to the Hercynian corrugation, if not even to even more ancient (Caledonian) phases. The existence, from the Paleozoic onwards, of a rigid central nucleus – the meseta- along the edges of which, without being able to deform it, the orogenetic efforts manifested during the Tertiary sector have had the effect of forming, in the peripheral areas, a crown of reliefs and depressions which that nucleus ended up with isolate from the original marine contact. It is probable that the meseta once extended much more towards the west: on this side, in fact, the current margin corresponds to a line of faults (from the mouth of the Duero to Cape Spain Vincenzo, through Abrantes and Alcocer), whose geological significance it is made more evident by the instability (accentuated seismicity) of southern Portugal and by the nephelite python of the Serra de Monchique. With all this, the delimitation of the lower basin of the three great rivers of the plateauit is marked precisely by the contact of this with the various tectonic units from which neighboring Portugal results.
According to cachedhealth.com, the long period of quiescent emergence of the meseta, barely disturbed by partial transgressions of the Mesozoic seas, contrasts with the relative brevity (in a geological sense) of the Cenozoic paroxysms, to which the current architecture of the Iberian relief, more or less slightly retouched, is essentially due from postpliocene epyrogenic movements. Hence the opposition, not exclusive of Spanish territory, of young or rejuvenated forms to old or even decrepit forms. Contrast, however, does not always have its stratigraphic and geo-lithological equivalent. The penepians of the plateau, as well as with the demolition of the very ancient Paleozoic relief, are evidently related to the leveling action of the fluvial-lacustrine deposits, almost everywhere of great power, which hide their ancient basement on a large surface: where these are missing, as in the section southwest of New Castile, the lines of the ancient Hercynian relief are still evident, if only for the direction imposed on the surface waters (Extremadura, elbow of the Guadiana). These Neogenic and Quaternary continental deposits are indeed typical of the mesetanorth, of which they cover the largest part and where they continue in the finitima depression of the Ebro and, beyond the Sierra de Guadarrama, in La Mancha, while the contemporary sedimentations of the Andalusian trench have marine origin, and only after the recent Pliocene do they cloak of alluvial material. In contrast to the widespread diffusion of the Cenozoic, the crystalline and paleozoic formations in the W and SW mainly stand out in Spanish territory, the Mesozoic above all in the E. and N. The former alone constitute the western edge of the meseta from Galicia to the Tagus, emerging again over wide spaces in the sierrasmedians; the latter continue both in Asturias and the Cantabrians, as well as between the Tagus and Guadalquivir, where from Extremadura they go as far as La Mancha, forming the same raised edge of the Sierra Morena. Much more varied and complicated is the geological constitution of the eastern edge of the plateau: almost everywhere, however, the Paleozoic basement is masked by powerful piles of Mesozoic depositions, among which the stratigraphically rather varied ones of the Cretaceous period prevail. it continues, through the Basque depression, on the southern flank of the Pyrenean mountain range, as well as, beyond the Júcar, in the mountainous alignments of the Betic system.
However, the greater complexity of the peripheral reliefs compared to the meseta should not lead us to believe that the morphology of the latter is ideally simple. Its relative rigidity did not prevent postocene orogenetic thrusts from making their echo reach it, albeit by tracing and renewing more ancient tectonic accidents. The emergence of the median anticlines (Sierra de Gata, de Gredos and de Guadarrama; M. di Toledo), the surcretion of the margins (Cantabrian rim, M. Universali, Sierra Morena) and the accentuation of the internal depressions (pits of Toledo and del Guadiana) introduce these varieties into the natural landscape, which passes within a short space of the arid and flat páramosCastilian to the rugged slopes of Peñalara (2406 m.), on whose summit traces of the Pleistocene glaciation are clearly visible. But even without appealing to these contrasts, the central mountain range, which traditionally divides the Old from the New Castile (the latter about 300 m lower than the first), is in reality the limit of two morphologically stale zones. The typical plains of the northern campos are followed, in Spain delle Sierras, by a hilly landscape with soft undulations, accentuating as one proceeds towards the Spain and especially towards the SW, where the relief takes on an ever more decisive importance. Only the flat expanse of the steppes of La Mancha recalls the páramos due to its perfect horizontalityof León, but here, more than the common geological history, the presence, in the superficial layers of the soil, of the chalky clays of the Sarmatian and of the bibulous limestones of the upper Miocene comes into play as a determining factor.
