Geology and geomorphology
Sil Canyons (Ourense/Lugo)
The basin is located at northwest corner of the Iberian Peninsula y distributed betweenGalician-Leonese Massif and the southwestern part of the Cantabrian Mountains. The highest altitudes are found in the eastern mountains, beingPeña Orníz (2,191 m), Pico Cornón (2,188 m)YPeña de los Años (2,158 m) the highest points in the basin. All three are located at the headwaters of the Sil River, between Somiedo and La Babia, and are border peaks of the basin. On the other hand, the peakCabeza de la Yegua, in the Aquilano mountains, with 2,142 m, is the highest point that is entirely in the basin.The average altitude is 714 meters above sea level.
The orography is varied and complex, defined by the variety and geological activity and by the erosive action of the rivers on the substrates, which are generally very resistant. In the upper zone of the Miño the orography is smooth (Terra Chá and Meseta de Lugo) but the Sil basin is characterized by having a much more abrupt relief, with deep valleys and steep slopes (for example , the Sil Canyons, of tectonic origin).
Lithologically, quartzite, slate, sandstone and limestone predominate, followed by granites with two micas and other granites.
Terraza labrada en la roca. Proximidades desembocadura rio Mouro (Monção)
Fracturación cauce rio Mouro en (Barbeita-Monção)
Terraza labrada en la roca. Proximidades desembocadura rio Mouro (Monção)
geological setting
From a geological point of view, the territory ofthe Miño basin is located in the oldest area of the Iberian Peninsula, in the so-called Iberian Massif, which covers approximately the western half of the peninsula. The Iberian Massif is the largest extant fragment of a great chain of mountains, which extended over thousands of kilometres, the Variscan or Hercynian mountain range, originating some 300 million years ago por the collision of two continents.
The construction of this mountain range involved the folding of the rocks and the transformation of the sediments that had been deposited between the continents into metamorphic rocks (due to the increase in temperature and pressure).In the deepest areas of the mountain range, the rocks melted and large amounts of granitic magmas originated., which, taking advantage of the fractures produced in the collision, rose rapidly and settled in more superficial areas of the crust, cooled and solidified.
Whatthe suture zone of the continents that collided would be close to what is now Galicia, the degree of metamorphism of the rocks and the intrusions of granitic rocks increase from East to West, because toward the East the thermal and mechanical energy received is less.
The fractures and folds caused by the formation of the Variscan mountain range, are structural directions that continue to influence the relief of the Iberian Peninsula.
After,early jurassic, about 200 million years ago, the Variscan mountain range began to fragment. The opening of the Atlantic Ocean and the Bay of Biscay gave rise to the individualization of the Peninsula, which became a separate island from Europe.
in the Cenozoic(66 to 2.58 million years ago) Africa begins to move north, while the opening of the Atlantic Ocean and the Bay of Biscay continues, which rotates the Iberian Peninsula so that it "embeds" in Europe, raising the Pyrenean Mountains. Compression during this orogeny reactivates ancient Hercynian fracture systems.
The genesis of the Pyrenean Mountain Range, has determined in all this region a very irregular relief, characterized by a complex succession of flat areas located at different altitudes that are cut by embedded valleys. We must add the existence of tectonic depressions filled with sediments. This creates a mosaic of raised surfaces and sunken areas, which is called piano key relief.
The location of these morphostructural units is sometimes controlled by the discontinuities of the Hercynian base, but their formation and evolution are linked to the clash of the Iberian Peninsula against France that originated the Pyrenees, they are a continuation to the west of the Pyrenean mountain range._cc781905- 5cde-3194-bb3b-136bad5cf58d_
geological landmarks
Geological points of interest, called LIG (Places of Geological Interest) in Spain and Geossites in Portugal, are geological formations and structures, landforms, rocks or other geological manifestations that have a unique and/or representative character at a didactic and scientific level and that allow knowing, studying and interpreting the origin and evolution of the earth, its modeling processes, climates and landscapes of the past and the present.
geomorphology
The Miño river is born in O Pedregal de Irima, a scree of quartzite blocks that descends from the slopes of the Sierra de Meira. The waters of small springs, which arise thanks to a fault, flow under the stones, until emerging to the surface in the plain. Six kilometers downstream, in theFuenteminana lagoon, its flow increases, since there is an important upwelling of water there.
From there it heads south,crossing the “Terra Cha” region, which is a flattened surface, surrounded by reliefs. Since the land is flat,the inaccuracy of the hydrography is pronounced: lagoons, gándaras, canals, multiple watercourses abound on this land.
From the surroundings of Rábade the convergence of the waters is accentuated, andthe river Miño definitively takes a course towards the south, creating a much clearer valley, especially downstream from Lugo. It gradually sinks against the slope on the surface of Vilalba-Chantada (the bottom of the Portomarín depression dominates the Miño from a hundred meters and that of the Chantada depression from more than 250 meters ) avoiding the depressions of Sarria and Monforte.
Its most important tributary,the Sil,It is born in the Cantabrian mountain range, in the Leonese region of La Babia.Its course successively follows two large fractures N60º, that of Barco-A Rúa and then that of Quiroga. It flows at the bottom of gorges that only widen in the basins, such as El Bierzo, A Rúa and Quiroga.Its tributaries also follow fracture lines and will develop very embedded valleysseparating raised surfaces.
The Sil and the Miño come together in Os Peares, where there is an important embedding, which abruptly disappears before reaching Ourense. Until its mouth, the Miño is exploiting a series of NE-SW and NS fractures that alternately lead it towards the ocean.
Near Ribadavia, 25 km before reaching the international course, the Miño runs through a fairly wide valley, which narrows as it approaches the northernmost point of Portugal, without the nature of the rock changing. Where the international stretch begins, the river cuts through a chain of mountains, which reaches 1,150 meters on the right bank and 1,357 on the left bank, in the Castro-Laboreiro mountain range.
From Cortegada-Fugueira to Arbo-Melgaço no accumulation terraces have been found, there are only terraces carved into the rock at various levels. A little above one of them is the tower of Melgaco.The area in which the terrace deposits are found uninterruptedly begins near As Neves and Barbeita.
Aboutfrom the lateral valley of the Mouro river, the Miño valley begins to widen.Close to Monção-Salvaterra, on one hand, and Valença-Tui on the other,the Miño flows through tectonic valleysthat have North-South direction (Tea, Troporiz, Louro). At each intersection of valleys, the Miño valley widens, forming a basin. That of Valença-Tui extends to the south, in a wide inlet, between O Fao and Sao-Paio.
near the mouth, the river widens up to 1,300 meters and includes alluvial islands, such as Canosa and Boega. The mouth, which is not very deep, is reduced to a width of 400 meters to the north, by the granite projection of Ponta de Santa Tecla, and to the south by the dunes of Caminha-Moledo.
Like the other Portuguese river valleys, toothe Miño valley would have been eroded to a greater depth than it is today and later filled in by materials from the lower terrace. Based on the data from the surveys carried out for the construction of the international bridge between Valença and Tui, it is known that in times of greatest marine regression, the Miño River excavated its valley below its current level. C Teixeira assumes that the rock must be located at least 44 meters deep, since the pilasters of the international bridge, 22 meters below the low-water level, rest on pebble deposits.
Sources:
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Lautensach, H. (1945). Formation of two interglacial terraces in Portugal and their relationship with the problems of the ice age. Publicações, Geological Society of Portugal, I, 39 p.
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Nonn, H. (1966). Les regions cotieres de la Galicia: étude geomorphologique. Th. Doc. Fac. Lettres, Univ. Strasbourg, 591 pp.
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Vergnolle, C. (1990). Morphogenese des reliefs cotiers associes a la marge continentale Nord-Espagnole. The example of the NE of Galicia. Galician State Seminar, Nova Terra Series, 1, 315 pp.
