The mouths of the Minho, Douro and Tagus form estuaries which provide good harbours for shipping but all rapidly become unnavigable toward the interior. In fact only the Guadalquivir can really be classed as navigable for any appreciable distance from the sea, ships being able to ascend it as far as Seville. The others flow in deep rocky valleys which they have cut across the Meseta. Their water supply is scanty, their deeply cut valleys are difficult to cross, and the river beds so far below the level as to make them useless for irrigation. The Ebro makes its way to the Mediterranean by way of a tortuous gorge and so affords no means of river transportation to the sea. Its great importance lies in its plentiful waters for irrigation purposes derived from both the Pyrenees and the Cantabrian mountains. Lakes of any consider able size are few, the only important ones being two coastal la goons in the Mediterranean—the Albufera de Valencia, the Mar Menor in Murcia—and the Laguna de la Janda in Cadiz behind Cape Trafalgar. There are many small alpine lakes, and small salt lakes are to be found in every steppe region.
Geologically the Iberian peninsula consists of a great massif (the Meseta) which has been composed by Archaean, Palaeozoic, and eruptive rocks partly concealed by a covering of Tertiary strata but characterized by the absence, excepting in its margins, of any marine deposits of Mesozoic age, and bordered on the north, east and south by zones of folding in which the Mesozoic and early Tertiary beds are involved. The Meseta is a fragment only of the great Hercynian mountain system which was formed across Europe at the close of the Carboniferous period. The earth-block which forms the Meseta was individualized at the close of the Palaeozoic era. During the Mesozoic era the Hercy nian system of which it is a part was shattered and large portions of it sank beneath the sea and were covered by Mesozoic and Tertiary strata. But other portions remained above the sea and of these the Meseta was one. Around it the deposits of the Jurassic and Cretaceous seas were laid down. During the Tertiary era these deposits together with the earlier Tertiary beds were crushed against the old massif, thus forming the folded zones of the Baetic ranges on the south, the hills of southern Aragon in the east. and the Pyrenees on the north.
There are clear evidences of earlier (Huronian and Caledonian) movements on the Meseta, but worn folds of Carboniferous formations embedded in the Hercynian structure may represent in places an older discordant structure dependent on earlier move ments.
Tertiary movements soldered to the Meseta the formations by the north-east, where the folding action lay within the early (Palaeogene) Tertiary period, and of the south-east, where the folding was most intense, in the later Neogene period before the Tortonian epoch. The Tertiary pressures also caused many im portant movements in the Hercynian massif itself. Ancient frac tures were rejuvenated, large interior basins were formed, and the strata were folded and crushed with great violence in the borders of the massif. During the Pliocene period the Meseta was given
a general tilt westward. Later differential Tertiary movements modified the Central Cordillera system and accentuated the sepa ration of the northern from the southern submeseta, leaving them at very different altitudes from each other and from the marginal depressions and lowlands.
The areas of regional metamorphism on the Meseta are formed of plutonic rocks, broadly granites, which pass upwards into an Archaean, or strata crystalline series in Galicia, Portugal, and the Central Cordillera system. They appear elsewhere aligned with the Palaeozoic series, principally along a line from Alcantara to Andu jar. This series shows these levels—the lowest, Augen-gneiss; the intermediate, micaceous gneiss with schists and crystalline lime stones intercalated; and the uppermost, lustrous schists and phyllites. These materials appear now at the surface among gra nitic rocks—the massif of Galicia and North Portugal,—the cen tral sierras,—in the Montes de Toledo, and in South Portugal on the plateau of Evora and Beja which extends eastward by the Sierra de Aracena to the Sierra Morena point north of Seville. Palaeozoic.—The oldest Palaeozoic strata are referred, from their included fossils to the Cambrian, Ordovician and Silurian systems. They range through a vast region of Andalusia, Estre madura, Castile, Salamanca, Leon and Asturias, and along the peaks of the Pyrenees and the Cantabrian mountains. In eastern Galicia and Asturias, and in western Leon, the Cambrian and Silurian trendlines swing round a quarter-circle following the out line of Galicia and northern Portugal. The Silurian of the sierras (Ocej6n, Alto Rey) lying to the north-east of the Sierra de Guadarrama trends south–south-west, south, and, finally, south-west. Round the gneissic vestiges of the Toledan massif the Cambrian and Silurian, partly metamorphosed, in the neighbourhood of the granites, trend west-ward and the north west-ward in the Montes de Toledo and the Sierra de Altamira. From the Sierra Morena front the trendlines, running at first parallel to the line of front, swing to the west and west–north-west round the strata-crystalline of southern Portugal. Farther east they run freely to the north-west in the great belt of Palaeozoic which stretches from La Mancha to beyond the Portuguese fron tier. The older Palaeozoic of the north culminates in the Teleno (7,181 ft.) belonging to the Montanos de Leon. In the south the Sierra Morena is the scarped southern edge of the main mass of the Palaeozoic, cut by a series of faults partly of Permian and partly of Pliocene date. The Palaeozoic also formed the two pla teaux to the south of the Sierra Morena and a low plateau in the east notable for the complex lead-silver zone of Linares and La Carolina; and a broad plateau in the west extending from the Andevalo district south of the Sierra de Cuacena to the western shores of Portugal.