Mexico appears as the continuation of the area of highlands which, starting from Alaska, occupy the entire western side of North America and are collectively known as the Rocky Mountains. The bundle of high lands, which in the United States extends between the Pacific Ocean and the Prairie area, is gradually narrowing south of Oaxaca, where North America joins the Central one at the Isthmus of Tehuantepec. But politically, Mexico also embraces the territories of the states of Chiapas and Tabasco and the entire Yucatán Peninsula, these lands belonging to Isthmian America. It follows the natural distinction between the highly developed continental part (1,700,000 sq km) and the much smaller southern or rather south-eastern part (250,000 sq km).
The largest continental section is made up, as is well known, by a large central plateau closed to the east and west by two series of reliefs, indicated together with the names of Sierra Madre Occidental and Sierra Madre Oriental; at the outer foot of the latter, along the Gulf of Mexico, there is a low coastal area which continues southwards the plain of Texas and which is still 300 km wide. in northern Tamaulipas, it then narrows to just about twenty kilometers in correspondence with Veracruz. Also on the Pacific side we find a remarkably large flat area in the parched and rugged territory of Sonora and Sinaloa, but reduced to short stretches of coastal selvedge in the rest of the coast up to Soconusco. L’mesas, north and central, from the mountains of Oaxaca descending rapidly at noon on the isthmus of Tehuantepec. The boundary between the two parts can be considered the line marked by the deep and recessed valley of the Río Balsas, to which the deep incision of the Río Papaloapán corresponds towards the east.
The Sierra Madre Occidental is formed by a bundle of roughly parallel chains that run from north-west to south-east, separated from each other by long longitudinal furrows and by short and deep transverse valleys, comparable to the famous cañonesof Colorado; this beam, more than 300 km wide. to the north, towards noon, it almost narrows into a single chain, the Sierra de Nayarit, which stretches as far as the Río Grande de Santiago and beyond to the 20th parallel, where the imposing group of Nevado and Volcán rises de Colima. It is a corrugated region already in the Eocene, whose folds, formed by archaic and primary soils, were covered at the beginning of the Tertiary and in subsequent periods by masses of volcanic rocks. The height of the mountainous beam increases from N. to S. reaching 1790 m. in Cerro Álamos (Sonora), 3450 m. in the Cerro de Pimal (Sierra de Nayarit) and 4335 m. in the Nevado de Colima. For Mexico 2002, please check commit4fitness.com.
At the latitude of the latter peak, the Sierra Madre bends to the east, limiting the southern and highest section of the plateau to the south, the so-called Mesa de Anahuac, while along the Pacific coast runs another series of reliefs that starting from Cape Corrientes extends through the territories of the states of Colima, Guerrero and Oaxaca up to the Isthmus of Tehuantepec. This last series, which seems to be connected through the Tre Marie islands to the relief of Baja California, is indicated in Mexico with the overall name of Sierra Madre del Sur, distinguished from the Mesa di Anahuac by the valleys of Balsas and Papaloapán, as mentioned above.. The Sierra Madre del Sur, made up largely of crystalline archaic rocks, schists and gneisses, is deeply eroded by watercourses which, rejuvenated by a recent uplift, have been able to affect deep valleys penetrating right into the heart of the raised region, enough to be mistaken in some cases with fracture lines or pits of tectonic origin, as happened for the Río Balsas valley. The territory of Oaxaca, so tormented, clearly shows the signs of this long travail. The Sierra del Sur culminates in the Zempoaltepec at 3140m.
The Sierra Madre Oriental, overall less wide and less elevated than the Occidental, is formed by a series of largely discontinuous folds, between which the valleys of the rivers that descend to the Gulf of Mexico penetrate. Here too the various sections take different local names (Sierra del Carmen, San Marco, Paila, San Martín, etc.) and the reliefs rise towards the south reaching 3664 m. in the Peña Nevada. At the latitude of Tampico the beam narrows and rises, complicated by volcanic activity, which has poured out large lava fields and erected huge volcanic cones, such as the Cofre de Perote (4090 m.) And the Pico de Orizaba (5700 meters), the highest peak in all of Mexico. It is here that the great series of volcanic cones which, starting from the mentioned Volcán de Colima,
Volcanism played a large part in the formation of the Mexican territory starting from the Eocene, and as is evident from the fact that effusive rocks (andesites, rhyolites, basalts) are found throughout Mexico, but in more recent periods, from the Pliocene to the Quaternary, and in the historical epoch volcanic activity was particularly concentrated in this southern section of the plateau, building an imposing series of gigantic cones, aligned along the so-called Mexican volcanic axis. The volcanic systems rise in the very rugged area that seems to close the central plateau at noon and to which Mexican geographers give various overall names (Gran Cordillera volcánica, Arista Meridional de la Mesa, Sistema tarasco – nahua). The series begins in the west with the Volcán de Colima (3860), followed by the Tacintaro (3845),nahuatl Citlaltepetl (Cima della Stella) recalls the vision of the sparkling peak of eternal snow, which can be seen silhouetted against the blue sky looking from the torrid beach of Veracruz. Further north there are other systems, such as the Ceboruco (2200) on the left of the Río Santiago, while the isolated cone of the Volcán de Tuxtla (1764) rises on the coast of Veracruz, and in the high lands of Chiapas the very active Tacaná.
