Sierra Madre Occidental

The Sierra Madre Occidental ('Western Sierra Madre') is a major mountain range system which runs northwest–southeast through northwestern and western Mexico, and along the Gulf of California. The Sierra Madre is part of the American Cordillera, an almost continuous chain of mountain ranges that forms the western "backbone" of the Americas.

Etymology

The Spanish name sierra madre means "mother mountain range" in English, and occidental means "western", these thus being the "Western mother mountain range". To the east, from the Spanish oriental meaning "eastern" in English, the Sierra Madre Oriental range or "Eastern mother mountain range" runs generally parallel to the Sierra Madre Occidental along eastern Mexico and the Gulf of Mexico.

The range extends from northern Sonora, a state near the Mexico–U.S. border at Arizona, southeast towards to the Trans-Mexican Volcanic Belt and Sierra Madre del Sur ranges. The high plateau that is formed by the range is cut by deep river valleys. This plateau is formed from volcanic rock overlying a basement of metamorphic rock.

This uplift has caused changes in weather patterns; increased rainfall occurring in the mountains has provided areas where ecosystems can form in wetter areas than surrounding land. This water source forms watersheds that provide the arid surroundings with water that makes it possible to irrigate and farm crops. The wet ecosystems are islands of biodiversity, differing significantly from what would otherwise be a desert landscape. Oak forests are the predominant plant life, and extend into the lowland deserts.

This forest and canyon land provided a place for a variety of indigenous people to live, until Spanish settlers with associated mestizos came into the area to found towns for the silver mines in the area. The major industries in the area now are agriculture and forestry, which have become contentious because of land degradation and the native population's opposition to these practices.

Geography

thumb|left|Copper Canyon in Chihuahua, Mexico

thumb|[[Basaseachic Falls]]

The range trends from the north to southeast. Canyons cut by the rivers of the wet western slopes exist in addition to those of the northeast slopes, notably the Copper Canyon.

Location

The range runs parallel to the Pacific coast of Mexico, from just south of the Arizona-Sonora border southeast through eastern Sonora, western Chihuahua, Sinaloa, Durango, Zacatecas, Nayarit, Jalisco, Aguascalientes to Guanajuato, where it joins with the Sierra Madre del Sur and the Eje Volcánico Transversal (Transverse Volcanic Axis) of central Mexico after crossing . The mountains range from from the Gulf of California in the north, but begin to approach within of the Pacific in the south. These mountains are generally considered to be part of the much larger American cordillera, the mountains extending from Alaska down to these across western North America. This is a dramatic landscape of steep mountains formed by a high plateau that has been cut through with canyons including Copper Canyon, larger and, in places, deeper than the Grand Canyon. This plateau has an average elevation of with most of the more eroded canyons on the western slope, due to the higher moisture content. The exact elevations of the highest peaks are not known within accurate enough ranges to determine their relative elevation. The highest point is probably Cerro Mohinora, located at . Estimates for the height of the mountain start around and go up to . However, Cerro Barajas, at , may be as high as In this area, the Sierra Madre begins to give way to the Basin and Range Province. Subranges of this area include the Sierra de Alica and the Sierra Pajaritos, both in Nayarit, The Yaqui drains into the Gulf of California, as do the Fuerte River in Sinaloa and the Humaya River further south. The Río Grande de Santiago drains from the southern slopes of the Sierra Madre. Along the more arid eastern slopes of the mountains, the Nazas River and Aguanaval River drain the mountains into a closed basin. These mountains supply 90% of the water used for irrigation within the watershed. and the El Cajón Dam upstream from it. To provide irrigation water for farming the dry basin, the Nazas river was dammed in the 1930s and 1940s. This has led to the former drainage lakes in the area drying up, and soil depletion from the elimination of the river as a source of soil. Due to the continued logging in the area, erosion on the slopes of hillsides has increased.

Human

Numerous small populations live in Copper Canyon, some of them around old missions. Many settlements consist of single families or small family groups near agricultural fields. There are also a few towns of note, including Creel, Chihuahua, with 5,000 inhabitants; it serves as a stop on the train line. South of Copper Canyon is the town of El Salto, Durango, situated along highway 40. To the west is Copala, Sinaloa.

To the south live the indigenous Cora people in the towns of Jesús María and La Mesa del Nayar. In addition, 1,000 people live in the village of Huajimic in Nayarit. The Bolaños River valley of Jalisco is populated with mining communities, such as San Martín de Bolaños, which has a population of around 3,000.

Few paved roads cross the mountains, meaning that travel times can be long, even between cities that are relatively close. In the north, Mexico Federal Highway 16 connects Hermosillo, Sonora and La Junta, Chihuahua. In the mountains, the La Entrada al Pacifico Corridor is an incomplete road intended to connect Los Mochis, Sinaloa and Chihuahua. Durango and Mazatlán are connected by highway 40, The lower of these groups is a series of volcanic rocks formed by lava flows. The surface of the plateau is made up of a second series of volcanic ignimbrites, covered mostly by clay rich organic soils. Some intrusions in the area have deposited ore veins that are commercially viable.

The landscape was shaped after rock placement. Rivers cut valleys into the plateau, following folds in the rocks. Rocks have broken off of the slopes, forming rocky sides. The bedrock is covered by the clay rich soils, except in eroded areas, where water carries away deposits leaving a stony surface.

Geological history

The basement is thought to be Older North American basement, especially in the North, The northern extent of the Sierra Madre is the Cortés Terrane, although the northern extreme of this contains a small portion of the Carborca Terrane. Some of this area has a series of Paleozoic sedimentary sequences from a deep marine environment underlying it, considered by some to be part of the Cotes Terrane and by others to be transported. In some areas, distinct sedimentary rocks can be identified by shallow-water Silurian and Mississippian sedimentary rocks underlying deep-water Pennsylvanian and Permian, which are divided by Mississippian rhyolite. The Permian clastics contain some detrital rock that sit alongside low-grade metamorphosed sedimentary and volcanic rocks with serpentinite. The southern part of the range is made up of the Guerrero Composite Terrane, a large body, which is actually a complex of five different subterranes, not all of them within the main body of the Sierra Madre. The main subterranes of the Guerrero Composite Terrane that are within the Sierra Madre are the Tahue and Zihuatanejo terranes. a marine basin which separated the island arc that came to form the Guerrero terrane from the accretions that came to form the Sierra Madre terrane. It contains a lower formation made up of pillow basalts with pillow breccias, tuffs, and shales underlying a group of pelagic limestones, oozes, and turbidites. Three of these calderas are in Copper Canyon. The lava has formed a series of mafic rocks that comprise the Southern Cordilleran Basaltic Andesite Suite. The tuffs are above a thick formation of lava rock. Around five or six units have been identified, mostly around the Copper Canyon area. These tuffs have allowed wide-ranging correlation with formations in other geographical areas, for example Death Valley.

During the Miocene, three periods of volcanism marked separate events in the Sierras. Each period is marked by different volcanic processes. The first was a continuation of the Oligocene flare-up, which lasted through the early Miocene. This may be a distinct second occurrence of the same process, placing silica-rich lava above Oligocene rocks. Episodes of volcanism continued into the Quaternary. The surface of the plateau is almost exclusively from the second series of flows causing the rock that is most visible to be ignimbrites with lava flow layers. The faulting decreases in the center of the range where the faults are mostly small normal faults, but increases once again on the western slope. The southern boundary of the Sierra Madre Occidental with the Jalisco block is a series of folds, thrust, and left lateral faults date from the middle Miocene. This may be evidence for a left transpressional shear zone.

Mineral deposits

On a smaller scale, mineral deposits are distributed in formations of the Sierra Madre, although the events that placed these deposits vary throughout the mountains. The Guerrero composite terrane contains many of the economic deposits, often found in ore bodies younger than the Cretaceous. These are believed to be caused by later intrusions into the early flows. Fissure-vein deposits have been divided into two groups of fissure vein deposits, which are divided geographically into an eastern and western series. Some also occur as epithermal deposits. Also, some gold occurs along with copper in Cenozoic iron-oxide skarn and replacement deposits along the Pacific. small deposits Soils are commonly phaeozems that are up to . Due to the high clay content, which blocks water from being absorbed into the soil, larger rock particles in the soil help reduce runoff and soil losses from erosion. Areas that are covered in stone are known as lithosols, and make up most of the remaining surface in the mountains, Acidic soils may have lower calcium content, leading to lower plant soil quality. Soils along the eastern slopes have increased organic content and a clay heavy layer that is developed more than those of the more arid eastern lands. Other problems include overgrazing, which has led to terracing from cattle paths and the formation of a soil crust, and soil acidification, which poses a risk to some grasslands as of 2002. The high pressure begins to break down and move south during September or October; however, the breakdown is usually at a slower pace than the movement north. This high pressure belt is associated with the mechanics that form the Bermuda High.

The climate in the mountains is mild. Summer temperatures maintain a constant level. Maximum temperatures are typically in July at around 31 C Below-freezing temperatures can occur in the higher mountains, with precipitation occurring as snowfall certain times of year.

During the summer monsoon, wind patterns undergo large-scale changes. As the high pressure moves north, air is pulled off the Gulf of Mexico, bringing easterly winds aloft to the mountains. The mountains cause convection during the following monsoon season, Monsoon rains come to the Sierra Madre in June as the high pressure area moves north, leading to wet summer seasons. This causes east winds bringing moisture from the Gulf of Mexico. On the western side of the mountains, which are affected by the largest change in precipitation between the dry season and monsoon season, Seasonal variation in humidity is also present with average May and June relative humidity around 50%, with 25% possible during the day. but the forests in the mountains vary by elevation and place in the range. This ecoregion is noted for its high biodiversity and large number of endemic species, and for allowing some species to extend their range.

Flora

The dominant plants vary across the range, and the habitat that they present varies from scattered woodland to both deciduous and coniferous forest.

The elevation and latitude in the range are the major determining factors in the dominant plants. Oaks dominate the lower reaches of the mountains, where stands grow down to about . Some oak species even continue into the surrounding ecosystems. In the western slopes of the range, cloud forest also occurs, mostly on mid-elevation slopes with little exposure to wind or sun in the southern end of the range. At higher elevations pines begin to grow alongside the oaks, and pines begin to dominate the oaks at higher elevations. At high elevations and in the north, mixed conifer forests become the dominant ecosystem. These forests are composed primarily of pines and firs, which begin to grow at these elevations. There are also grasslands on some mountain tops, surrounded by the forest that occur in those areas.

Fauna

The mountains are often a corridor for species allowing more diversity in the area, by combining species from different regions. Animals typically associated with arid and mountainous habitats share the mountains with species associated with habitats further south.

There are many carnivorous and omnivorous mammals native to the area, although several are extirpated or are part of threatened populations. The Mexican wolf's range in the mountains is limited to the foothills of Durango or the animal may have been extirpated; the black bear population is small, is confirmed extinct. Jaguars and ocelots are also now rare. On the eastern slopes there are American badger, coyote and gray fox that live across the mountains. cliff chipmunk, rock squirrel, Mexican fox squirrel, and various species of Cricetidae. Raccoons, hog-nosed skunk, and hooded skunk live in the mountains. The white-nosed coati lives on the western slopes, and the ring-tailed cat lives throughout. Many species of bats live in the mountains also.

Birds frequent the mountains, some as a migratory path and others as native species. The migratory birds often use the area to the west as an overwintering area, and migrate north along the western slopes. In the northern canyons birds may be divided into different habitats, or more specifically, the different types of forest, leading to differences in range by elevation. Some of this difference by elevation may even extend the range of some species, enabling tropical species to use the mountain valleys for migration. Bird species endemic to the Sierra Madre Occidental include tufted jay, Invasive fish compete with the native species which has been a cause of concern among researchers, The Chihuahuan Desert is found to the northeast and east, the Meseta Central matorral and Central Mexican matorral to the southeast, and the Sinaloan dry forests to the west and southwest. These lower elevations are usually graded into by a stand of tropical deciduous forest, which has a large portion of flora made up of flowering plants and legumes. The ecotone, made up of the tropical deciduous forest, is commonly sparsely populated, sometimes only by small stands of trees. It may be referred to as savanna. Other groups, including Conservation International, work to protect animal species in the area. Basaseachic Falls National Park is a national park in the mountains containing the highest waterfalls in the country. Additionally, there are reserves such as La Michilía in the southern Sierra Madre, or the Tutuaca reserve in western Chihuahua, which was established in 2000 under a new system of management. Other protected areas in the Sierra include the Bavispe, Campo Verde, Cerro Mohinora, Papigochic, and Sierra de Álamos–Río Cuchujaqui flora and fauna protection areas. The Cuenca Alimentadora del Distrito Nacional de Riego 043 Estado de Nayarit, established in 1949 to protect forests and watersheds, is now a natural resources protection area which conserves much of the southern Sierra.

People and history

thumb|upright|An early picture of Tarahumara

The mountains are home to several indigenous nations speaking Uto-Aztecan languages, including the Tarahumara in the central portion of the range, the Huichol in the southern part of the range, and the Tepehuanes in the eastern slopes.

Prehistory

Archaic cultures are known to have inhabited the Sierra Madre along with the rest of the surrounding lands, but little is known of the culture, although it does vary between the northern and southern extremes of the range. The agrarian Chalchihuites Culture began as farming communities around the first century AD and remained small communities until 300 AD when new agricultural techniques caused a large growth in population until around 500 AD. This culture began to decline in the ninth century. The Mogollon culture has been described in the northern foothills. Sites date the emergence of the culture to around 900 AD, and its abandonment to around 1000–1100 AD. During the period estimated for Mogollon abandonment, the Rio Sonora culture began agricultural developments along the Rio Sonora and rivers flowing into it. This culture produced pottery and lived in pit houses, until 1150 to 1200 when they began to construct surface dwellings. Construction of these houses ended around AD1550. He would explore the western coast and cross the Sierra Madre in 1564. Early Spanish influence in the area came from the new silver mines, the first of which was established in 1567 at Santa Barbara. As sites opened for mining, missionaries came to the areas and had a strong presence by 1595. The first meeting with the Tarahumara came in 1607, when a missionary went up the valley to meet them. and lasted for two years. A revolt arose in 1648, primarily among the Tarahumara. In 1767, Jesuits were expelled from New Spain, ending the mission program. After the Apache faced heightened resistance to raiding from the Spanish in Chihuahuan, they moved down to the Sierra Madre in 1794. By 1800, raiding had essentially ended. From there they raided Sonora and the surrounding land until General George Crook entered the mountains in 1883 to discuss Geronimo's return. Geronimo eventually surrendered in 1886. Since most of the railroads were American, the foreign influence led to unrest among the people living in the mountains, causing the Tomochic rebellion in 1892. In addition to the railroads, canals were constructed in the 1890s in the Yaqui and Mayo river valleys. These canal projects failed, however, owing to the resistance that the natives of the valleys put up against the projects. Loggers began arriving in the mountains in the late 1800s. In response to the new industry, the Tarahumara moved further into the canyons. Silver mines were forced to make payments to the rebels, but were allowed to continue operating. The raids subsided by 1919, and Villa returned peacefully to the foothills.

In order to construct irrigation projects in the lowlands below the valleys, irrigation projects were never completed in the Sonoran hills; instead, the Yaqui River was dammed in the 1930s.

World War II and opium cultivation for morphine

During World War II, the United States experienced shortages of medical morphine after opium supplies from Asia were disrupted by the Pacific War. In response, Mexican authorities, in cooperation with U.S. officials, expanded regulated opium poppy cultivation in northwestern Mexico, including rural areas surrounding Culiacán in the state of Sinaloa. Farmers in the mountainous regions of Sierra Madre Occidental, near the city of Culiacán, produced opium that was processed into legal morphine for wartime medical use by Allied forces. Although the program ended after 1945, the agricultural knowledge, smuggling routes, and local intermediary networks developed during the wartime period persisted. Historians and criminologists have identified these postwar networks as an early foundation for later illicit drug trafficking organizations in Sinaloa.

References

External links

A list of the birds present in the region