Portugal constitutes lands of the most diverse geological ages, and this diversity corresponds to a remarkable variety of the landscape from the morphological and anthropogeographical point of view. Portugal is part of the western edge of the Meseta, which is made up of granite soils, but these lands reach the sea only in the northern part of the country; in the southern one the Meseta is limited by a series of dislocations oriented from north to south, which leave between the Meseta and the Atlantic a vast selvedge of tertiary lands crossed by the Tagus and the Sado. In the Algarve, on the other hand, Palaeozoic formations appear which reconnect with those of the Sierra Morena.
According to 800zipcodes, turning now to examine more particularly the geological structure of the country we observe that a block of granite soils occupies almost all of northern Portugal, that is the region called Entre Douro and Minho (between the Duero and the Minho) and the Beira Alta; in the Lower Beira, the Cambrian soils (conglomerates, quartzites and limestones) prevail. The Traz os Montes, which includes the NE part. of Portugal, is on the contrary made up of schist soils. The soils of the Mesozoic (shell and lithographic limestones of the Jurassic period, white clay and rudist limestones of the Cretaceous period) constitute a large part of northern Extremadura; only at N. of the mouth of the Tagus do granite rocks reappear in the Sintra massif. In S. del Tago the more recent soils of the Paleozoic (lower Carbonic) and the Tertiary, Miocene and Pliocene periods predominate. The soils of the Pliocene, with marls, clays and marine sands, prevail in the area between the mouth of the Tagus and Santarem; those of the Miocene (limestone and sandstone), in the area located upstream of Santarem up to Abrantes, and cover vast plains where the Tagus and the Sado flow. To S. and E. of the aforementioned plains, in the central part of the Alemtejo, there are crystalline schist soils, with outcrops of porphyry and granite, and soils of the Upper Silurico (conglomerates and limestones). Finally, a vast area of Carbonic soils (schists and sandstones) extends into the Algarve; only the Serra de Monchique is made up of granite soils.
Of the great plateau of the Iberian Peninsula, Portugal occupies the westernmost part which gradually descends towards the Atlantic Ocean; in this part the plateau presents reliefs and valleys deeply engraved by the action of the rivers.
From the mrfological point of view, a clear distinction can be made between the part of Portugal that lies north of the Tagus and that located south of this river. In fact, while the northern part is mountainous, to the south, on the other hand, plateaus engraved by shallow valleys prevail; at N. del Tago the heights of less than 400 m. they represent 52%, in S. 97% of the territory.
In the northern part there are numerous greenhouses (Serra de Soajo, 1415 m.; do Gerez, 1361 m.; de Larouco, 1338 m.; de Marão, 1415 m.) Which detach from those of south-eastern Galicia and branch off fan-shaped with general direction NE.-SO. gradually degrading towards the Duero. The reliefs (Serra or Gerez, de Larouco, etc.) rarely have the jagged appearance of the high mountains, with sharp peaks, steep and steep walls; rather rounded shapes predominate on which the comparative action of atmospheric agents is evident. In S. del Duero, in Beira Alta, the heights drop significantly with the greenhouses Montemuro (1382 m.), Gralheira (1116), de Caramulo (1071), da Lapa (986), Marofa (974). Here too the rather strong erosion, due to the abundance of rains and the melting of snow, he acted on the mountainous areas, transforming them into rounded ridges and plateaus deeply engraved by the waters that go to the Duero, the Vouga, the Mondego. Between the course of the Mondego and that of the Zézere stands the highest chain of Portugal, the Serra da Estrella which has a NE.-SO direction. and culminates at 1991 m. in the Malhão da Estrella, the highest point in the Portuguese territory.
Towards the NE. the Serra lowers rapidly, while in the SW. continue with the Serra de Lousã. The great granite massif of the Serra da Estrella, engraved by a series of transversal valleys, has a very rough and wild appearance on the Zézere side; this is the so-called “Serra Brava” which is difficult to access, while on the opposite side, from which the Mondego and the various tributaries of the Duero originate, the slopes are less steep; it is the “Serra Mansa”. On the highest peaks of the Serra, which has rather domed shapes, low and bushy vegetation grows; the sides, on the other hand, are covered with wonderful oak and chestnut woods. The snow dwells on the sides of the Serra from November to May, feeding the Mondego and other less important watercourses at the time of thaw. To S. della Serra da Estrella, separated by the course of the Zézere rise the Serra do Moradal (1080 m.), the Serra Guardunha (1923 m.) and the Serra de las Mesas (1366 m.). At the Serra da Estrella, the small mountain range that forms the backbone of the peninsula located north of Lisbon and between the lower reaches of the Tagus and the ocean. Extremadura extends to S. del Tago, a region formed by the alluvial plains of the Tagus and Sado rivers. The low valley of the Tagus, an alluvial plain known as the “Ribatejo”, is one of the most fertile regions of Portugal, while the low valley of the Sado is marshy and therefore not very favorable to human settlement. The Alemtejo is very uniform from a morphological point of view, in which the last offshoots of the Serra Morena die. Vast plateaus extend as far as the eye can see and give the landscape a sad and monotonous aspect: they are the “Charnecas”, often completely uncultivated and uninhabited. Only the granite greenhouse of S. Mamede (1025 m.) And further to S. la Serra Ossa (640 m.) And the Serra Monfurado interrupt the monotony of the landscape. The mountains of the Algarve, which are formed by a series of greenhouses arranged as an amphitheater and completely oriented towards the south (Serra do Malhão, 579 m; Serra de Monchique, 902 m., The highest), also represent an extension of the folds of the Serra Morena.
In Portugal earthquakes are frequent and the areas of maximum seismicity are located in the lower valley of the Tagus (Ribatejo), and in the coastal area of the Algarve. These regions have been tried by numerous disasters; famous is that of November 1, 1755, known as the Lisbon earthquake, which was accompanied by a strong tsunami, and that of April 23, 1909 in the Ribatejo in the NE. of Lisbon.
