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Sierra Madre Occidental
Mountain Range (cordillera)
Geographical view to Mexico and its mountain ranges Sierra Madre Oriental, Sierra Madre Occidental and Sierra Madre del Sur.
Country	Mexico
State	Sonora, Chihuahua, Sinaloa, Durango, Zacatecas, Nayarit, Jalisco, Aguascalientes, Guanajuato
Region	Northern Mexico
Borders on	Sea of Cortez–Pacific-W Altiplanicie Mexicana-E Trans-Mexican Volcanic Belt-S (Cordillera Neovolcanica)
Highest point	Cerro Mohinora
- elevation	10,863 ft (3,311 m)
- coordinates	25°57′22″N 107°2′52″W / 25.95611°N 107.04778°W / 25.95611; -107.04778
Length	932 mi (1,500 km), NW x SE
Width	150 mi (241 km), W x E
File:Geographic Map of Mexico.jpg

The Sierra Madre Occidental is a mountain range in western Mexico. Sierra Madre means "Mother Range". The range extends from near the Arizona border down to the Sierra Madre del Sur. The high plateau that is formed by the range is cut by deep river valleys. This plateau is formed from igneous rocks overlying a basement of metamorphic rock.

This uplift has caused changes in weather patterns, with rainfall occurring in the mountains providing islands of available land for wetter ecosystems to form than in the surrounding land. This water 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, in what would otherwise be a desert landscape. Oak forests are the predominant plant life and extend into the low land deserts.

This forest and canyon land provided a place for a variety of indigenous people to live, until Spanish settlers came into the area to found towns for the silver mines in the area. The major industry in the area now are agriculture and forestry, which have become contentious because of perceived land degradation and the native population's fight against this.

Geography[link]

Copper Canyon in Chihuahua, Mexico

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.

Location[link]

The range runs north to south, from just south of the Sonora–Arizona 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 1,250 km (780 mi).^[1] The mountains range from 300 km (190 mi) from the Gulf of California in the north, but begin to approach within 50 km (31 mi) of the Pacific in the South.^[2] 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.^[3]

Features[link]

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.^[4] This plateau has an average elevation of 2,250 m (7,380 ft) with most of the more eroded canyons on the western slope, due to the higher moisture content.^[1]

The highest elevations occur in the Tarahumara range.^[5] The exact elevations of the highest peaks are not known within accurate enough ranges to determine the relative elevation of peaks. The highest point is probably Cerro Mohinora, located at 25°57′N 107°03′W / 25.95°N 107.05°W / 25.95; -107.05. The height of the mountain ranges from estimates of 3,040 m (9,970 ft)^[6] up to 3,300 m (10,800 ft).^[7] However, Cerro Barajas, at 26°24′N 106°5′W / 26.4°N 106.083°W / 26.4; -106.083, may be as high as 3,300 m (10,800 ft)^[8] although other sources give 3,170 m (10,400 ft) as the elevation.^[9] Cerro Gordo, at 23°12′N 104°57′W / 23.2°N 104.95°W / 23.2; -104.95, may have an elevation between 3,350 m (10,990 ft)^[10] and 3,340 m (10,960 ft).^[11]

Geology[link]

The Sierra Madre Occidental is a high plateau of volcanic rock that is eroded in areas to reveal a basement composed of plutonic and sedimentary rocks underlying the two groups of extrusive volacanics.^[12] 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 below 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. Rock has been broken off of the slopes, forming rocky sides. This is covered by the clay rich soils, except in eroded areas, where water carries away deposits leaving a stony surface.

Geological History[link]

The basement rock in the area is composed of metamorphised formations that show no signs of the volcanism that would later come to characterize the area. This formation is primarily composed of granite, gneiss, and schist in the North, with a slightly younger metamorphosed clastic sedimentary and volcanic schist in the South.^[13] After the metamorphosed basement, marine sedimentary rocks form most of the rock. Local exporsure of Argillite and Limestone is present in localized deposits.^[14] Commercially viable deposits of a range of minerals exist throughout the range.

The Durango Volcanic field

Towards the end of the Cretaceous, magmatism in the area became more active, forming the first major igneous series in the area. The igneous series are made up of formations of plutonic and volcanic rock, which would later be exposed. Interbedded with these rocks are sedimentary deposit rocks. In the center of the range, some of these rock have been deformed by tectonic action that occurred at the same time. The southern part of the range contains none of the volcanism that is apparent in the northern range. These formations ended in the Paleocene.^[13] Eocene volcanism formed a series of andesitic and rhyolitic formations in the area, with spatial and temporal variations throughout. Most of the gold and silver deposits are also in these rocks.^[15]

An example of the ignimbrite units

In the Oligocene ash flows became the predominate formation of the area, with interbedded lava flows between. These ash flows began the second series of high magmatism formations. Units are a series of ignimbrite formations layered one atop another, that are sometimes broken by lava flows.^[16] This ignimbrite formations in this area cover the largest area of any known series, with ten calderas identified in the province.^[17] Three of these calderas are placed in Copper Canyon. The lava has formed a series of mafic rock that form the Southern Cordilleran Basaltic Andesite Suite.^[18] 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 actually allowed wide ranging correlation with formations in other geographical areas, for example Death Valley.^[19]

Structures[link]

The mountains are made up at the surface of large scale ignimbrite sheet which have been incised by rivers flowing from rainfall in the mountains.^[20][21] 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.^[15] The region has a general strike from just west of north to just south of east.^[21] The large faults along the eastern slope^[15] have caused the slope to be made up of large escarpments that face into the interior basins.^[21] 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.^[15]

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. It was mentioned that gold and silver deposits occur in the formations caused by early magmatism.^[15] These are believed to be caused by later intrusions into the early flows. These intrusions have been divided into two groups of fissure vein deposits, which are divided geographically into an eastern and western series.^[22] In the east, the gold and silver are accompanied by lead and zinc. Other minerals were deposited in more recent formations with veins and contact deposits more common than the early porphyry deposits. In addition to precious metals, tin, iron, and uranium occur in the mountains due to later deposits.^[22] Methods of iron deposition vary throughout the range with lava flows, ash flows, and replacement occurring in different parts of the range.^[15] Uranium is present mostly in rhyolitic rocks.^[22] Small tin deposits occur in the rhyolitic ignimbrites throughout.^[15] These are the main deposits providing a variety of minerals occurring in different formations throughout the entire range.

Most soil in the mountains is covered by an organic rich layer over layers rich in clay.^[23] Due to the high clay contents, which block 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.^[23] The last major soil type are vertisols which are thin mineral poor soils. Soils at higher elevations experience more leaching, due to increased rainfall.^[24] 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.^[21] The thickness of soil layers also increases.

Climate[link]

The climate varies considerably between the northern and southern extents of this long mountain range. Seasonal variations also occur due to the presence of large bodies of warm water delivering warm damp air from each side of the range. This region undergoes a seasonal variety with two wet seasons, including a summer monsoon, and two dry seasons each year.

Temperature, Pressure, and Wind[link]

Air pressure changes in Northern Mexico and the Southwest United States are the main causes of seasonal variation in the Sierra Madre. A high pressure area that rests over the mountains in the winter begins to move north of the mountains into New Mexico during the month of June.^[25] The high pressure begins to breakdown 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.^[26]

The climate in the mountains is mild. Summer temperatures maintain a constant level. Maximum temperatures are typically in July at around 31 C^[26] with average temperatures on the order of 16 C during this same period.^[27] Below freezing temperatures can occur in the higher mountains, with precipitation occurring as snowfall certain times of year.^[28]

During the summer monsoon, wind patterns undergo large scale changes.^[25] In May there is very little large scale wind current through the mountains, most circulates around the mountains,^[25] although onshore flow is present.^[29] As the high pressure moves north, air is pulled off the Gulf of Mexico, bringing easterly winds aloft to the mountains.^[25] On the east slope of the mountain daytime upslope winds flow to the crest, where the flow joins westerly winds.^[30] The mountains cause convection during the following monsoon season,^[29] during which nocturnal winds at low levels in the atmosphere move moisture overnight.^[30] After the high pressure breaks down, wind patterns return to those prior to the monsoon season.

Precipitation[link]

Precipitation varies on both annual and seasonal scales in the Sierra Madre. Annual differences in regional climate are associated with the El Niño-Southern Oscillation with El Niño years leading to a wetter climate.^[27] 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. Rainfall from the monsoon is especially focused on the mountainous regions causing 10 to 15 inches of rain.^[25] The southern region receives more of the rainfall than the northern. On the western side of the mountains, which are affected by the largest change in precipitation between the dry season and monsoon season,^[29] rainfall can exceed 30 cm (12 in) in a single month.^[25] Much of this precipitation occurs as tropical storms. As the high pressure over New Mexico breaks down, rainfall in the Sierra Madre end. Because of the monsoon, the summer accounts for the majority of rainfall in the area.^[27] The spring and fall dry season separate out a weaker wet season in the winter. In addition to increasing in the southern ranges of the mountains, rainfall increases in the higher elevations of the Sierra Madre Occidental.^[27]

Humidity[link]

Humidity in the mountains varies at wide levels as does rainfall. In the early morning before dawn, the air is almost saturated (often over 90% relative humidity) near the mountain tops with low clouds over the range. Lower in the range, air is not as near saturated in the morning.^[31] Seasonal variation in humidity is also present with average May and June relative humidity around 50%, with 25% possible during the day.^[26] Also during the day, relative humidity drops and cloud levels rise as temperatures rise.^[31] The humidity of the range is also influenced by the same seasonal changes as rainfall. In the dry season dew points are low, around -22 deg C in the south of the range and -24 to -26 in the north in May. In June, dew points begin to increase from the May levels up to -20 in the North and -16 in the south. By July these dew points are at -14 throughout the range. The range allows higher dew points throughout all these periods than the surrounding low lands. This rise in humidity accompanies the monsoon.^[29]

Water[link]

The Rio Santiago

The mountains act as a source of water, in an otherwise arid environment from the increased precipitation from the mountain range. This has led many of the rivers that runoff of the mountains to become a source of water for irrigation in the surrounding lands. From the need of water to irrigate the mountains, dams were built, which has caused several environmental concerns in the area.

Major Rivers[link]

As the northern end of the mountains are more arid, the Yaqui River, which drains that area is an important source of irrigation in the state of Sonora.^[32] The Yaqui drains into the Gulf of California along with the Humaya River in Sinaloa and the more southern Fuerte River.^[33] The Río Grande de Santiago drains 100000 km^2 from the Southern Slopes of the Sierra Madre.^[34] Along the more arid eastern slopes of the mountains, the Nazas River and Aguanaval River drain the mountains in a closed basin.^[35] North of this system is the Conchos River which drains in the Rio Grande.^[33]

Human Use and Effects[link]

Along many rivers the arid conditions have caused many courses to be dammed to provide water for irrigation. These dams have caused some of the concerns along with those caused by the other activities. The Yaqui has been dammed with three large reservoirs along its course.^[33] The Río Grande de Santiago has also been dammed including the Aguamilpa dam begun in 1991^[36] and the El Cajón Dam (Mexico) upstream from it.^[37] In order to provide irrigation water for farming the dry basin, the Nazas river was dammed in the 1930s and 40s.^[38] 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.^[39]

Ecology[link]

Sierra Madre Occidental pine-oak forests

The Sierra Madre Occidental pine-oak forests are found at elevations of 1,500–3,000 m (4,900–9,800 ft) throughout the range.^[40] This ecoregion is noted for its high biodiversity and large number of endemic species.^[41]

Flora[link]

Oaks dominate the lower reaches of the mountains, with stands down to about 1,000 m (3,300 ft). Some oak species even continue into the surrounding ecosystems.^[42] In the Western slopes of the range, cloud forest also occurs, mostly on mid-elevation slopes with little exposure to wind or sun^[43] in the southern end of the range.^[44] At higher elevations pines begin to grow alongside the oaks, with pines dominating the oaks at higher elevations.^[45] At high elevations and in the north Mixed Conifer Forests become the dominate ecosystem. These forests are composed primarily of pines and firs, which begin to grow at these elevations.^[46] The high mountain tops may also be grasslands surrounded by the forest that occur in those areas.^[45]

Plants in the area typically have large responses to the change in climate with each season. Monsoon season is the largest of these effects, with leaf buds following only a few months after the rains begin. Flowering season occurs in the months preceding the summer monsoon.^[42] This is only the peak flowering season, with plants flowering throughout the year. Due to the early flowering, fruit is ripe and seeds are dispersed at the beginning of the rains.^[46] There are also areas where plants have little response to the seasons, particularly those areas that experience moist conditions, caused by the elevations. These plants may maintain leaves through times when other areas are in the dry season, during which most deciduous trees have no leaves.^[47]

Surroundings[link]

The Sonoran Desert is found in the northwestern foothills at 1,000–2,000 m (3,300–6,600 ft).^[48] 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.^[46] These flowering plants are mostly from the genus Bursera.^[49]

Preservation[link]

Preservation efforts in the modern day, include those from Edwin Bustillos.^[50] Other groups work to protect animal species in the area including Conservation International.^[51] Basaseachic Falls National Park is a national park in the mountains containing the highest waterfalls in the country.^[5]

People and History[link]

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, and the Huichol in the southern part of the range. In addition to the Tarahumara and Huichol, the Tepehuano also inhabited the mountains.^[52]

Prehistory[link]

Archaic cultures are known to have inhabited the Sierra Madre along with the rest of the surrounding lands, but little of known of the culture, although it does vary between the northern and southern extremes of the range.^[53] 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.^[54] 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 abandonment to around 1000-1100 AD.^[55] 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.^[56] Construction of these houses ended around AD1550.^[53]

Spanish Sierra Madre[link]

In 1554 Francisco de Ibarra came to Durango and began to settle the area.^[57] He would explore the western coast and cross the Sierra Madre in 1564.^[58] Early Spanish influence in the area came from the new silver mines, the first of which was established in 1567 at Santa Barbara.^[59] As sites opened for mining, missionaries came to the areas and had a strong presence by 1595.^[60] The first meeting with the Tarahumara came in 1607, when a missionary went up the valley to meet them.^[60] Opposition to the missionaries built up, leading to the Tepehuán Revolt. This revolt began in 1616 in the town of Santa Catalina in Durango^[61] and lasted for two years.^[60] The discovery of silver at Parral in 1631 led to the Northern limit of settlement moving into Tarahumara lands.^[60]

The Jesuits renewed missionary activity in the 1630s, with a new program that increased in scope until around 1650. Tarahumara moved further into the mountains in order to escape the conditions.^[60] A smallpox epidemic caused a revolt in 1645.^[62] In 1648 a revolt arose, primarily among the Tarahumara.^[60] A drought caused a pair of revolutions in 1650 and 1652.^[62] The last major revolt began in March of 1690 among the Tarahumara and ended in 1698.^[63] Demands from labor drafts increased in the 1720s, placing a new pressure on missions.^[62] Criticism of the Jesuits rose in the 1730s with complaints of Jesuit Commerce, which was established by mission stores.^[64] In 1767, Jesuits were expelled from New Spain, ending the mission program.^[60]

A new colonial official José de Gálvez came to New Spain and established new laws that caused a boom in silver mining activity in the 1770s.^[65] After the Apache faced heightened resistance to raiding from the Spanish in Chihuahuan, they moved down to the Sierra Madre in 1794.^[66] By 1800, raiding had essentially ended.^[60]

Mexican Sierra Madre[link]

After leaving San Carlos in Fall of 1881 Geronimo and other Apache moved to the mountains.^[66] From here they would raid Sonora and the surrounding land until general George Crook entered the mountains in 1883 to discuss Geronimo's return. After 1886, these mountains served as refuge for the remaining Chiricahua Apache known as the Nameless Ones (Nnee Nnaahí) who refused to surrender with Geronimo to U.S. forces.^{[citation needed]} Pancho Villa fled to the mountains in 1894, becoming a bandit.^[67]

Pancho Villa

Railroads came to the area starting in the 1880s and 1890s, which allowed many old mines in the region to reopen at a profit.^[68] 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.^[69] In addition to the railroads, canals were constructed in the 1890s in the Yaqui and Mayo river valleys.^[70] However, these canal projects failed due to the resistance that the natives of the valleys put up against the projects.^[71] Loggers began arriving in the mountains in the late 1800s. In response to the new industry, the Tarahumara moved further into the canyons.^[68]

The 1910 revolution began in Chihuahua and spread over the mountains throughout the winter of 1910-1911.^[72] Silver mines were forced to make payments to the rebels, but were allowed to continue operating.^[68] General John Pershing began to follow Villa in 1916 and entered the mountains; however, he received no information from the local population.^[73] By 1919 the raids subsided 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.^[70] Dams continued to be constructed in the 1940s and 1950s on the Yaqui River and its tributaries with the largest reservoir, behind the Oviachic dam, completed in 1952.^[71] Dams were also constructed on the Mayo river. Because of the dams, soil conditions fell, and more works were required to improve soil quality.^[71] The mountain range was crossed by the Ferrocarril Chihuahua al Pacifico railroad completed 1962.^[39] The railroad went up for sale in 1996, but no transaction was ever completed.^[74]

References[link]

Wikimedia Commons has media related to: Sierra Madre Occidental

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Sierra Madre (Spanish for "mother mountain range") may refer to one of several mountain ranges:

Mexico's three Sierra Madre mountain ranges

• In Mexico:
  • Sierra Madre Occidental, a mountain range in northwestern Mexico and southern Arizona
  • Sierra Madre Oriental, a mountain range in northeastern Mexico
  • Sierra Madre de Oaxaca, a mountain range in south-central Mexico
  • Sierra Madre del Sur, a mountain range in southern Mexico
  • Sierra Madre de Chiapas, a mountain range which extends from southeastern Mexico
• In Central America:
  • Sierra Madre de Chiapas, the continuation of this mountain range crossing southern Guatemala, northern El Salvador, and western Honduras
• In the Philippines:
  • Sierra Madre (Philippines), a mountain range on Luzon island
• In the United States:
  • Sierra Madre Mountains (California), a mountain range in the state of California
  • Sierra Madre, California, a town in Los Angeles County located at the foot of the San Gabriel Mountains
  • Sierra Madre Range (Wyoming), a mountain range in the south central portion of the state

Entertainment[link]

• The Treasure of the Sierra Madre, a 1927 novel
• The Treasure of the Sierra Madre (film), 1948 film made from the novel

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