Sierra Madre

Sierra Madre, The Sierra Madre.Encyclopædia Britannica, Inc.mountain system of Mexico. It consists of the Sierra Madre Occidental (to the west), the Sierra Madre Oriental (to the east), and the Sierra Madre del Sur (to the south). These ranges enclose the great central Mexican Plateau, which itself is a part of the system—although the northern portion of the plateau also is considered to be part of the Basin and Range Province in the United States—and is broken by blocks of mountain ranges and large ephemeral drainage basins called bolsones (“pockets”).

Physical features


The Mexican Plateau is composed largely of uplifted and folded strata of the Mesozoic Era (about 250 to 65 million years ago). Formations from the second half of the Mesozoic predominate among exposed rocks, but older sediments are widespread. The region’s elevation, folding, and faulting were contemporaneous with the uplift of the Rocky Mountains. Early in the Cenozoic Era (about the past 65 million years), great outpourings of lava terminated a long interval of weathering and erosion. Subsequently the large plateau block was uplifted, the displacement being greater to the south than to the north. Its margins have been dissected into the deeply gouged angular landscape of the Sierra Madre Occidental and the more rounded but rugged terrain of the Sierra Madre Oriental.


The Sierra Madre Oriental, composed largely of folded sedimentary rocks of the Cretaceous Period (about 145 to 65 million years ago), owes its present relief to uplift, faulting, and erosion since about 23 million years ago. Igneous intrusion forms are numerous.

Marking the southern edge of the Mexican Plateau and spanning Mexico from coast to coast is a zone where volcanism developed during two episodes. The first episode occurred during the early and middle parts of the Cenozoic Era, when immense quantities of lava were poured over the land. The second volcanic episode, which began in the Pliocene Epoch (about 5.3 to 2.6 million years ago) and has continued to the present, is associated with the development of such spectacular volcanic peaks as Pico de Orizaba (18,406 feet [5,610 metres]), Popocatépetl (17,930 feet [5,465 metres]), Iztaccíhuatl (17,159 feet [5,230 metres]), Nevado de Toluca (14,977 feet [4,565 metres]), and Colima (13,451 feet [4,100 metres]). There were major changes in drainage patterns following the formation of the Cordillera Neo-Volcánica during the second episode. In 1759 and 1943, respectively, the smaller volcanoes of El Jorullo and Paricutín were formed. Additional peaks, cinder cones, and other evidence of volcanism occur throughout the region.

To the west of the Isthmus of Tehuantepec in southern Mexico, the great escarpment of the Southern Highlands rises several thousand feet. This deeply dissected mountain mass reveals east-to-west folds in its base rocks of metamorphosed Cretaceous sediments and intrusives. The narrower western section of the highlands is known as the Sierra Madre del Sur. It is breached by the Balsas River.


Cobre Canyon in the Sierra Madre Occidental, in Chihuahua, with (centre right) an abandoned miner’s house.© Kerrick JamesThe Sierra Madre Occidental extends approximately 700 miles (1,100 km) from northwest to southeast and is about 100 to 300 miles (160 to 480 km) in width. Summits mostly exceed elevations of 6,000 feet (1,800 metres), and some peaks rise above 10,000 feet (3,000 metres). From the east the mountains present relatively slight but broken relief; from the west they appear as formidable escarpments whose benched flanks drop into deep canyons called barrancas. The great mountain ranges and canyons trend northwest to southeast, which is generally the trend of the folds in the Mesozoic and older basement strata. The local relief may exceed 5,000 feet (1,500 metres), and the majestic dimensions of some of these barrancas—notably Barranca del Cobre (“Copper Canyon”)—compare to the Grand Canyon of the United States.

Extending southeastward from the Big Bend of the Rio Grande (called Río Bravo del Norte in Mexico) are a series of low mountains that are composed of folded sedimentary strata. South of Monterrey these mountains become a striking range, the Sierra Madre Oriental. Summit elevations are about 7,000 feet (2,100 metres), but some peaks rise above 10,000 feet, such as the Cerro Peña Nevada. Steep-walled narrow valleys, a number of which are north to south in alignment, lie in the sierra, and there are several gateways from the Gulf of Mexico lowlands to the plateau.

The core of Sierra Madre del Sur is outlined by a narrow and discontinuous coastal plain to the southwest and by the structural depression of the Balsas River to the north. To the east of this depression is the Sierra Mixteca, which forms a highland bridge between the Cordillera Neo-Volcánica and the Southern Highlands. A labyrinth of narrow ridges and steep-flanked valleys forms the Southern Highlands. Much of this broken country crests at elevations of about 6,500 feet (2,000 metres), and only a few peaks exceed 10,000 feet. Whereas the Mexican Plateau’s volcanic mantle is relatively free of dissection, the Southern Highlands have been stripped of much of their volcanic cover. The older basement rocks are exposed in what are probably the most highly dissected landscapes of Mexico. There is very little flatland in the region. Abruptly terminating the region on the east is a magnificent escarpment that overlooks the Isthmus of Tehuantepec.



The tierra templada, which includes elevations from 3,000 to 6,000 feet (900 to 1,800 metres), has mean temperatures ranging from the mid-60s to the mid-70s F (about 18 to 24 °C). Much of highland Mexico is in this thermal zone. It is the most pleasant of the zones for human habitation. During winter, frost may occur. In the cooler tierra fria the mean annual temperatures are between the mid-50s and mid-60s F (about 13 and 18 °C). Above 10,000 to 11,000 feet (3,000 to 3,300 metres) the mean annual temperature is less than 50 °F (10 °C), and above 14,000 to 15,000 feet (4,300 to 4,600 metres) it is less than 32 °F (0 °C).


Plant and animal life




Tarahumara Indians in the Sierra Madre Occidental, Mexico.Art Resource, New YorkThe remoteness, relative inaccessibility, and many habitable valleys of the Sierra Madre made it a refuge for a number of indigenous peoples who succeeded in retaining much of their aboriginal culture for two or three centuries after their first contact with the Spaniards. These groups include the Huichol and Cora in the southern states of Nayarit and Jalisco, the Tarahumara (Rarámuri) in the north (Chihuahua), and a number of other groups, especially in the Sierra Madre del Sur. The gradual acculturation of these peoples to the Mexican mestizo way of life was unavoidable, however, and in many areas the use of traditional languages, clothing patterns, and house types has been replaced; many young adults have abandoned the ways of their ancestral societies. Technological progress furthered this process in the second half of the 20th century and the beginning of the 21st. Trucks and buses and even small airplanes capable of operating with short landing strips increasingly have been used to transport goods and people, replacing the traditional horses, mules, and burros.





Study and exploration
