Sierra Madre Occidental

The Sierra Madre Occidental is a major mountain range system that 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".<ref name = "Phillips2010">Template:Cite web</ref> 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

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, 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 Template:Convert.<ref name="Centres of Plant Diversity">Template:Cite web</ref> The mountains range from Template:Convert from the Gulf of California in the north, but begin to approach within Template:Cvt of the Pacific in the south.<ref name = "Topography and Drainage">Template:Cite web</ref> 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.<ref name="Ferrari">Template:Cite book</ref><ref name="HildebrandKTOro"/>

Regions

Sierra Tarahumara or Tarahumara is the name for the region of the Sierra Madre beginning at the Durango border and extending north. This name comes from the Tarahumara natives.<ref name = "Gajdusek1953">Template:Cite journal</ref> 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.<ref name = "MaricopaGrandCanyon">Template:Cite web</ref> This plateau has an average elevation of Template:Convert with most of the more eroded canyons on the western slope, due to the higher moisture content.<ref name = "Centres of Plant Diversity"/>

The highest elevations occur in the Tarahumara range.<ref name = "Standish2009" >Template:Cite book</ref> 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 Template:Coord. Estimates for the height of the mountain start around Template:Convert<ref name = "Mathiasen2010">Template:Cite journal</ref> and go up to Template:Convert.<ref name="Valenzuela">Template:Cite web</ref> However, Cerro Barajas, at Template:Coord, may be as high as Template:Convert<ref name="PBBarajas"/> although other sources give Template:Convert as the elevation.<ref name = "Jurado2005"/> Cerro Gordo, at Template:Coord, may have an elevation between Template:Convert<ref name = "PBGordo"/> and Template:Convert.<ref name="Relieve"/>

The southern end of the mountains may be referred to as the Sierra Huichola.<ref name="Stephens">Template:Cite journal</ref> In this area, the Sierra Madre begins to give way to the Basin and Range Province.<ref name = Henry2000>Template:Cite journal</ref> Subranges of this area include the Sierra de Alica and the Sierra Pajaritos, both in Nayarit,<ref name = Canseco2007/> and the Sierra los Huicholes, Sierra de Morones, Sierra Nochistlán, and Sierra Fría extending eastwards into Jalisco, Zacatecas, and Aguascalientes.

Water

The Rio Santiago

The mountains act as a source of water, in an otherwise arid environment from the increased precipitation from the mountain range. As such, rivers that have headwaters in the mountains provide water for irrigation in the surrounding lands. The need for water to irrigate prompted the construction of dams, which has been the source of several environmental concerns in the area.

The northern end of the range is more arid; hence, the Yaqui River, which drains that area, is an important source of irrigation in the state of Sonora.<ref name = "Adams2005">Template:Cite journal</ref> The Yaqui drains into the Gulf of California, as do the Fuerte River in Sinaloa and the Humaya River further south.<ref name = "Benke2005">Template:Cite book</ref> The Río Grande de Santiago drains Template:Cvt from the southern slopes of the Sierra Madre.<ref name="Cisneros2011">Template:Cite thesis</ref> Along the more arid eastern slopes of the mountains, the Nazas River and Aguanaval River drain the mountains into a closed basin.<ref name = "Gutierrez2005">Template:Cite journal</ref> These mountains supply 90% of the water used for irrigation within the watershed.<ref name = "Gonzales-Barrios2002" /> North of this system is the Conchos River which drains into the Rio Grande.<ref name = "Benke2005"/>

Along many rivers the arid conditions have caused courses to be dammed to provide water for irrigation. These dams have caused concerns along with those caused by other activities. The Yaqui has been dammed with three large reservoirs along its course.<ref name = "Benke2005"/> The Río Grande de Santiago has also been dammed, including the Aguamilpa Dam begun in 1991<ref name = "Fuentes2006">Template:Cite book</ref> and the El Cajón Dam upstream from it.<ref name = "Velasquez2006">Template:Cite book</ref> To provide irrigation water for farming the dry basin, the Nazas river was dammed in the 1930s and 1940s.<ref name = "Wolfe2010">Template:Cite web</ref> 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.<ref name="MexDefAU1996">Template:Cite web</ref>

Human

Numerous small populations live in Copper Canyon, some of them around old missions.<ref name = Smith2008>Template:Cite web</ref> Many settlements consist of single families or small family groups near agricultural fields.<ref>Template:Cite web</ref> There are also a few towns of note, including Creel, Chihuahua, with 5,000 inhabitants; it serves as a stop on the train line.<ref>Template:Cite book</ref> South of Copper Canyon is the town of El Salto, Durango, situated along highway 40.<ref>Template:Cite web</ref> To the west is Copala, Sinaloa.<ref>Template:Cite web</ref>

To the south live the indigenous Cora people in the towns of Jesús María and La Mesa del Nayar.<ref name = "Warner">Template:Cite conference</ref> In addition, 1,000 people live in the village of Huajimic in Nayarit.<ref name="Huajimic">Template:Cite web</ref> 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.<ref>Template:Cite web</ref>

Few paved roads cross the mountains, meaning that travel times can be long, even between cities that are relatively close.<ref name = "McDowell2007">Template:Cite news</ref> In the north, Mexico Federal Highway 16 connects Hermosillo, Sonora and La Junta, Chihuahua.<ref name="Roalson2002">Template:Cite journal</ref> In the mountains, the La Entrada al Pacifico Corridor is an incomplete road intended to connect Los Mochis, Sinaloa and Chihuahua.<ref>Template:Cite web</ref> Durango and Mazatlán are connected by highway 40,<ref name = "Swanson1978"/> around 24 degrees north latitude.<ref name = "McDowell2007"/>

Geology

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 volcanics.<ref name="BroncoGold">Template:Cite web</ref> 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,<ref name = "Swanson2006"/> and a number of different accreted terranes, which are mostly covered by Cenozoic volcanism.<ref name = "Centeno2008">Template:Cite web</ref> 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.<ref name = "Centeno2008"/> Some of the northern end may also be underlain by a Precambrian basement placed around the same time as the Grenville Orogeny.<ref name = "Albrecht2000">Template:Cite journal</ref> 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.<ref name = "Ferrari2007">Template:Cite web</ref> 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.<ref name="Davis1999"/>

South of the Cortes and Carborca Terranes is the San José de García Terrane, which is a combination of Cretaceous arc volcanics and volcaniclastics, which may be thrusting over the Cortes terrane.<ref name = "Centeneo2003">Template:Cite web</ref> 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.<ref name = "Centeno2008"/> However, the terrane is mostly covered by volcanics and sedimentary deposits, so it is only visible where erosion has revealed it.<ref name = "Potra2011">Template:Cite thesis</ref> The main subterranes of the Guerrero Composite Terrane that are within the Sierra Madre are the Tahue and Zihuatanejo terranes.<ref name = "Centeno2008"/> Dividing the Guerrero Terrane from the rest of the Sierra Madre terranes is a boundary that is thought to represent the Early Cretaceous Arperos Basin,<ref name="Keppie">Template:Cite journal</ref> 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.<ref name = "Freydier2000">Template:Cite journal</ref> 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.<ref name = "Freydier2000"/>

Covering the southern basement are sedimentary and metamorphosed sedimentary rocks containing argillite, limestone,<ref name = "Ferrari1999"/> volcanic schist, slate, and greywacke which were exposed in the canyon of the Santiago River.<ref name="Ferrari2007"/> Near these sediments late Oligocene to early Miocene granite and granodiorite intrusive bodies occur. A possible cause of this is that these bodies were roof pendants which were uplifted by plutons.<ref name = "Ferrari1999"/> Mesozoic limestone also occurs in northern portion of the mountains, mostly from the Cretaceous.<ref name = "McDowell2007"/>

File:Durango volcanic field.jpg

The Durango Volcanic field

Towards the end of the Cretaceous, the Laramide orogeny increased the activity of magmatism in the area, forming the first major igneous series in the area.<ref name = "Cosca2003"/> 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 rocks have been deformed by tectonic forcing 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.<ref name = "Ferrari2007"/> 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.<ref name = "McDowell1981"/>

File:Sierra.madre.occidental.volcanics.JPG

An example of the ignimbrite units

In the Oligocene, ash flows became the predominant deposit of the area, with interbedded lava flows between. These ash flows began the second series of high magmatism formations. The Mid-Tertiary ignimbrite flare-up formed a series of ignimbrite formations, layered one atop another,<ref name = "Cosca2003"/> that are sometimes broken by lava flows.<ref name = "Swanson2006"/> The ignimbrite formations in this area cover the largest area of any known series, with ten calderas identified in the province.<ref name = "Aguirra2002" >Template:Cite web</ref> 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.<ref name = "Swanson2002">Template:Cite web</ref> 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.<ref name = "Knott2005">Template:Cite web</ref>

During the Miocene, three periods of volcanism marked separate events in the Sierras. Each period is marked by different volcanic processes.<ref name = NietoSam1999>Template:Cite journal</ref> 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.<ref name = "Ferrari2007"/> Also in the early Miocene more intermediate basaltic andesites occur along faults and in grabens.<ref name = NietoSam1999/> Beginning in the early Miocene and continuing into the middle, an arc of andesite was placed during the spreading of the Gulf of California.<ref name = "Ferrari1999"/> The basin and range-style faulting of the middle-late Miocene took place at the same time as the placement of alkali basalts,<ref name = "McDowell1981"/> In the westernmost slopes mafic dikes formed. These events have also been linked to the subduction of the Farallon Plate.<ref name = "Ferrari2002">Template:Cite journal</ref> Episodes of volcanism continued into the Quaternary.<ref name = "McDowell1981"/>

Structures

The Sierras are believed to be a thick core covered by volcanics and eroded by numerous rivers. Some have suggested that the basement of the mountains have numerous intrusions made by mafic magma.<ref name = "Ferrari2007"/> At their surface, the mountains are made up of large-scale ignimbrite sheet that has been incised by rivers flowing from rainfall in the mountains.<ref name = "Montgomery2001">Template:Cite journal</ref><ref name = "Hawley1969">Template:Cite conference</ref> 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.<ref name = "McDowell1981"/> The region has a general strike from just west of north to just south of east.<ref name = "Hawley1969"/>

Faults in the area tend to be younger than the upper volcanics, excluding those associated with calderas,<ref name = "McDowell1981"/> but do occur in distinct periods.<ref name = "Swanson2006"/> The large faults along the eastern slope<ref name = "McDowell1981"/> have caused the slope to be made up of large escarpments that face into the interior basins.<ref name = "Hawley1969"/> These faults are commonly covered by alluvium and basalt, making them difficult to discern, but can be found in certain fault blocks.<ref name = "McDowell1981"/>

Grabens have formed in the north, and some of these are filled with volcanic and sedimentary floors,<ref name = "Ferrari1999"/> and long depressions have formed in the southern Sierra Madre.<ref name = "Aguirre2003">Template:Cite journal</ref> 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.<ref name = "McDowell1981"/> In fact, the major normal faults tend to diverge around the center of the core and join at either end.<ref name = "Swanson2006"/> Some of these faults may have allowed ignimbrites to erupt from the surface, and so there may be some association between the ignimbrite occurrence and faults;<ref name = "Aguirre2003"/> this can be used to find out information about different kinds of basement rock present.<ref name = "Ferrari2007"/>

There are two faults dividing the Sierra Madre from the central Mexican Plateau. The more southern one is the north–south Aguascalientes fault extending Template:Convert from Altos de Jalisco to near Zacatecas, where it joins the more northern northwest–southeast trending San Luis–Tepehuanes fault extending from Guanajuato to Durango.<ref>Template:Cite web</ref> 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.<ref name = "Ferrari2007"/> The area between this boundary and the Aguascalientes fault is filled with numerous grabens and half-grabens, as well an east–west left shear zone, the El Roble shear zone. At the west end of these grabens and half-grabens are the Pochotitan and the more northerly San Pedro fault systems, which form the western boundary of the faults in the southern mountains.<ref name = "Ferrari2002"/> In the north there is an unknown boundary between the Cortes Terrane and the Guerrero Composite Terrane. Some theorize that this is a section of the Mojave–Sonora Megashear, but this theory is not universally accepted; however, the Megashear is important, and, in fact, is necessary for reconstructions of the opening of the Gulf of California, as of 2003.<ref>Template:Cite journal</ref>

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.<ref name="Potra2011" />

Gold and silver deposits most commonly occur in the altered andesites of the early volcanics,<ref name = "McDowell1981"/> typically these deposits are between 30 and 50 million years old.<ref name = "Clark1982">Template:Cite journal</ref> 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.<ref name = "Clark1982"/> In the east, the gold and silver are accompanied by lead and zinc. In the south, the Bolaños mine district is contained with the Bolaños Graben.<ref>Template:Cite journal</ref> Some also occur as epithermal deposits. Also, some gold occurs along with copper in Cenozoic iron-oxide skarn and replacement deposits along the Pacific.<ref name="Potra2011"/>

In addition to gold and silver and their related deposits, tin, iron, and mercury occur, sometimes in later units,<ref name = "Clark1982"/> mostly in later volcanics, and Molybdenum occurs in the mountains, often along with older copper deposits<ref name = "McDowell1981"/> or in Oligocene deposits in the east.<ref name = "Clark1982"/> The belt of tin deposits lies mostly within the eastern slopes from Durango to Guanajuato.<ref name = "Clark1982"/> In these areas, tin occurs in rhyolitic ignimbrites in many, possibly over 1000,<ref name = "Huspeni1984">Template:Cite journal</ref> small deposits<ref name = "McDowell1981"/> of cassiterite. Typically these deposits are divided from overlying ignimbrites by breccia, which may also contain ore. Many of these are thought to be hydrothermal deposits<ref name = "Huspeni1984"/> To the east of these units is a belt of more mercury deposits, followed by another belt of manganese deposits, although these deposits are mostly limited to Chihuahua and Hidalgo.<ref name = "Clark1982"/> Iron occurs in three different regions placed by different mechanisms. Along the Pacific there the numerous skarn and replacement deposits mentioned above with relation to gold.<ref name = "Clark1982"/> Iron occurs in Durango in the form of magnetite lava flows, which are often surrounded by smaller hematite deposits, thought to be ash flows.<ref name = "McDowell1981"/> There is also a belt of iron deposits further east.<ref name = "Clark1982"/>

Soil

Most soil in the mountains is covered by an organic-rich layer over layers rich in clay.<ref name = "Descroix2000">Template:Cite journal</ref> Soils are commonly phaeozems that are up to Template:Convert.<ref name = "Descroix2000"/> Some soils are cambisols that are less than Template:Convert deep and well-drained.<ref name = "Gonzales-Barrios2002">Template:Cite conference</ref> 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,<ref name = "Descroix2000"/> especially on upper slopes.<ref name = "Gonzales-Barrios2002" /> The last major soil type are vertisols, which are thin mineral-poor soils. Soils at higher elevations experience more leaching, due to increased rainfall.<ref name = "Goldberg1982">Template:Cite journal</ref> 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.<ref name = "Hawley1969"/> The thickness of soil layers also increases.

Problems with soil degradation have been aggravated by the cultivation of drug crops, leading to deforestation of many areas.<ref name = "Pajares2011">Template:Cite web</ref> 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.<ref name = "Gonzales-Barrios2002" />

Climate

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

Air pressure changes in northwestern 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.<ref name = "NOAANAM">Template:Cite web</ref> 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.<ref name="Garatuza2001">Template:Cite webTemplate:Dead link</ref>

The climate in the mountains is mild. Summer temperatures maintain a constant level. Maximum temperatures are typically in July at around 31 C<ref name = "Garatuza2001"/> with average temperatures of the order of 16 C during this same period.<ref name = "Heyerdahl2003"/> Mean annual temperatures are between Template:Convert above Template:Convert.<ref name = USPO1992>Template:Cite web</ref> Below-freezing temperatures can occur in the higher mountains, with precipitation occurring as snowfall certain times of year.<ref name = "eoearth">Template:Cite web</ref>

During the summer monsoon, wind patterns undergo large-scale changes.<ref name = "NOAANAM"/> In May there is very little large-scale wind current through the mountains, most circulates around the mountains,<ref name = "NOAANAM"/> although onshore flow is present.<ref name = "Douglas1992">Template:Cite journal</ref> As the high pressure moves north, air is pulled off the Gulf of Mexico, bringing easterly winds aloft to the mountains.<ref name = "NOAANAM"/> On the east slope of the mountain, daytime upslope winds flow to the crest, where the flow joins westerly winds.<ref name = "Fawcett2002">Template:Cite journal</ref> The mountains cause convection during the following monsoon season,<ref name = "Douglas1992"/> during which nocturnal winds at low levels in the atmosphere move moisture overnight.<ref name = "Fawcett2002"/> Extensive lightning activity occurs during this season. After the high pressure breaks down, wind patterns return to those prior to the monsoon season.

Precipitation and humidity

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.<ref name = "Heyerdahl2003">Template:Cite web</ref> 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,<ref name = "Douglas1992"/> in the monsoon season rainfall can exceed Template:Convert in a single month.<ref name = "NOAANAM"/> The southern region receives more of the rainfall than the northern.<ref name = "Douglas1992"/> Much of this precipitation occurs as tropical storms. As the high pressure over New Mexico breaks down, rainfall in the Sierra Madre ends. Because of the monsoon, the summer accounts for the majority of rainfall in the area.<ref name = "Heyerdahl2003"/> 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.<ref name = "Heyerdahl2003"/>

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.<ref name = "Nesbitt2007">Template:Cite journal</ref> Seasonal variation in humidity is also present with average May and June relative humidity around 50%, with 25% possible during the day.<ref name = "Garatuza2001"/> Also during the day, relative humidity drops and cloud levels rise as temperatures rise.<ref name="Nesbitt2007"/> The humidity of the range is also influenced by the same seasonal changes as rainfall. In the dry season dew points are low at high elevations (500 hPa), around Template:Convert in the south of the range, and Template:Convert in the north in May. In June, dew points begin to increase from the May levels up to Template:Convert in the north and Template:Convert in the south. By July these dew points are at Template:Convert throughout the range. The range allows higher dew points throughout these periods than the surrounding low lands. This rise in humidity accompanies the monsoon.<ref name = "Douglas1992"/>

Ecology

File:Sierra Madre Occidental....jpg

Sierra Madre Occidental pine-oak forests

The mountains create a diverse setting for plants and animals by creating a different set of conditions from the surroundings. The Sierra Madre Occidental pine-oak forests are found at elevations of Template:Convert throughout the range,<ref name="POF">Template:WWF ecoregion</ref> 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,<ref name = "Bye1994">Template:Cite journal</ref> 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 Template:Convert. Some oak species even continue into the surrounding ecosystems.<ref name = "CSDS2006">Template:Cite web</ref> In the western slopes of the range, cloud forest also occurs, mostly on mid-elevation slopes with little exposure to wind or sun<ref>Template:Cite journal</ref> in the southern end of the range.<ref>Template:Cite journal</ref> At higher elevations pines begin to grow alongside the oaks, and pines begin to dominate the oaks at higher elevations.<ref name="Biopark">Template:Cite web</ref> 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.<ref name = "Felger1994">Template:Cite journal</ref> There are also grasslands on some mountain tops, surrounded by the forest that occur in those areas.<ref name = "Biopark"/>

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.<ref name = "CSDS2006"/> 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.<ref name = "Felger1994"/> 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.<ref>Template:Cite journal</ref>

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.<ref name = "Felger1994Out">Template:Cite web</ref>

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<ref name="Gingrich1993">Template:Cite thesis</ref> or the animal may have been extirpated;<ref name = LopezGon2012>Template:Cite journal</ref> the black bear population is small,<ref name="Gingrich1993"/> and the Mexican grizzly bear that once resided here<ref name="Wright 1913">Template:Cite book</ref> is confirmed extinct.<ref>Template:Cite journal</ref> Jaguars and ocelots are also now rare.<ref name = "Gingrich1993"/> On the other hand, mountain lion and bobcat are more common.<ref name = LopezGon>Template:Cite web</ref> On the eastern slopes there are American badger, coyote and gray fox that live across the mountains.<ref name=LopezGon2012/> Native ungulates include the white tailed deer,<ref name="Gingrich1993"/> mule deer, and collared peccary.<ref name=LopezGon2012/> Rodents and lagomorphs include the eastern cottontail, which lives in the forests of the western slopes,<ref>Template:Cite book</ref> cliff chipmunk, rock squirrel, Mexican fox squirrel, and various species of Cricetidae.<ref name = "Hafner2012">Template:Cite journal</ref> 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.<ref name =LopezGon2012/>

A variety of lizards live in the area and surroundings. The Tarahumara frog lives in the oak and pine stands preferring moving water to pools. The southern extent of the Sonoran mountain kingsnake's range is the Chihuahuan desert into the mountains. Other snakes and frogs live in the mountains, and many species live in the western slopes.<ref>Template:Cite book</ref>

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.<ref name = "Melcher2005">Template:Cite web</ref> 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.<ref name = "Stager1954">Template:Cite web</ref> 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,<ref name = "Gingrich1993"/> the "gray-eared" subspecies of red warbler, eared quetzal, thick-billed parrot, and formerly included the imperial woodpecker, which now may be extinct.<ref name = "Gingrich1993"/>

The rivers and streams of the range contain some of the southernmost trout populations in North America. These populations may be native, although this is not universally accepted.<ref name = Hendrickson>Template:Cite web</ref> Invasive fish compete with the native species which has been a cause of concern among researchers,<ref name = "Felger1994Out"/> along with concerns about the effects of logging on these species.<ref name = "Gingrich1993"/>

Surroundings

The Sonoran Desert is found in the northwestern foothills at Template:Convert.<ref name = "WWF">Template:WWF ecoregion</ref> 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.<ref name = "Felger1994"/> These flowering plants are mostly from the genus Bursera.<ref>Template:Cite journal</ref> 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.<ref name = "Felger1994"/>

Preservation

Preservation efforts in the modern day include those of Edwin Bustillos.<ref name="goldman">Template:Cite web</ref> Other groups, including Conservation International, work to protect animal species in the area.<ref name = "CI2012">Template:Cite web</ref> Basaseachic Falls National Park is a national park in the mountains containing the highest waterfalls in the country.<ref name = "Standish2009"/> Cumbres de Majalca National Park was designated in 1939.<ref name = "Jurado2003">Template:Cite journal</ref> Additionally, there are reserves such as La Michilía in the southern Sierra Madre,<ref>Template:Cite web</ref> or the Tutuaca reserve in western Chihuahua, which was established in 2000 under a new system of management.<ref name = "Fule2004">Template:Cite web</ref> 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

File:Tarahumaras1.jpg

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.<ref name = "Stacy2003">Template:Cite book</ref>

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.<ref name = "Phillips1989">Template:Cite journal</ref> 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.<ref name = "Austin2001">Template:Cite book</ref> 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.<ref name = "Lister1958">Template:Cite book</ref> 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.<ref name = "Phillips2011">Template:Cite web</ref> Construction of these houses ended around AD1550.<ref name = "Phillips1989"/>

Spanish Sierra Madre

Francisco de Ibarra came to Durango in 1554, and began to settle the area.<ref name = "History2012">Template:Cite web</ref> He would explore the western coast and cross the Sierra Madre in 1564.<ref>Template:Cite web</ref> Early Spanish influence in the area came from the new silver mines, the first of which was established in 1567 at Santa Barbara.<ref name = "Schmal2004">Template:Cite web</ref> As sites opened for mining, missionaries came to the areas and had a strong presence by 1595.<ref name = "Spicer1962">Template:Cite book</ref> The first meeting with the Tarahumara came in 1607, when a missionary went up the valley to meet them.<ref name = "Spicer1962"/> Opposition to the missionaries built up, leading to the Tepehuán Revolt. This revolt began in 1616 in the town of Santa Catarina de Tepehuanes in Durango,<ref>Template:Cite conference</ref><ref>Template:Cite web</ref> and lasted for two years.<ref name = "Spicer1962"/> The discovery of silver at Parral in 1631 led to the northern limit of settlement moving into Tarahumara lands.<ref name = "Spicer1962"/>

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.<ref name = "Spicer1962"/> A smallpox epidemic caused a revolt in 1645.<ref name = "Deeds1989">Template:Cite journal</ref> A revolt arose in 1648, primarily among the Tarahumara.<ref name = "Spicer1962"/> A drought caused a pair of revolutions in 1650 and 1652.<ref name = "Deeds1989"/> The last major revolt began in March 1690 among the Tarahumara and ended in 1698.<ref name="Schroeder1998"/> Demands from labor drafts increased in the 1720s, placing a new pressure on missions.<ref name = "Deeds1989"/> Criticism of the Jesuits rose in the 1730s with complaints of Jesuit Commerce, which was established by mission stores.<ref>Template:Cite book</ref> In 1767, Jesuits were expelled from New Spain, ending the mission program.<ref name = "Spicer1962"/>

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.<ref>Template:Cite journal</ref> After the Apache faced heightened resistance to raiding from the Spanish in Chihuahuan, they moved down to the Sierra Madre in 1794.<ref name="Rushforth2010">Template:Cite web</ref> By 1800, raiding had essentially ended.<ref name = "Spicer1962"/>

Mexican Sierra Madre

File:Villa close up.jpg

Pancho Villa

After leaving San Carlos in the fall of 1881, Geronimo and other Apache moved to the mountains,<ref name = "Rushforth2010"/> and became known as the Nednai, sometimes referred to as the "Pinery Apache" or "Bronco Apache".<ref name = "Arreola2012">Template:Cite journal</ref> 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.<ref name = "Arreola2012"/> Pancho Villa fled to the mountains in 1894, becoming a bandit.<ref name = "Hickman"/>

19th century

Railroads came to the area starting in the 1880s and 1890s, which allowed many old mines in the region to reopen at a profit.<ref name = "Knight1986">Template:Cite book</ref> 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.<ref>Template:Cite book</ref> In addition to the railroads, canals were constructed in the 1890s in the Yaqui and Mayo river valleys.<ref name = "Sanderson1981">Template:Cite book</ref> These canal projects failed, however, owing to the resistance that the natives of the valleys put up against the projects.<ref name = "West1993">Template:Cite book</ref> Loggers began arriving in the mountains in the late 1800s. In response to the new industry, the Tarahumara moved further into the canyons.<ref name = "Knight1986"/>

20th century

The 1910 revolution began in Chihuahua and spread over the mountains throughout the winter of 1910–1911.<ref name = "Camin1993">Template:Cite book</ref> Silver mines were forced to make payments to the rebels, but were allowed to continue operating.<ref name = "Knight1986"/> General John J. Pershing began to follow Pancho Villa in 1916 and entered the mountains; however, he received no information from the local population.<ref name = "Gonzales2002">Template:Cite book</ref> 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.<ref name = "Sanderson1981"/> Dams continued to be constructed in the 1940s and 1950s on the Yaqui River and its tributaries with the largest reservoir, behind the Álvaro Obregón Dam completed in 1952.<ref name = "West1993"/> Dams were also constructed on the Mayo river. Because of the dams, soil condition deteriorated, and more work was required to improve soil quality.<ref name = "West1993"/> The mountain range was crossed by the Ferrocarril Chihuahua al Pacifico railroad, completed in 1961.<ref name = "MexDefAU1996"/> The railroad went up for sale in 1996, but no transaction was ever completed.<ref name="Campos2000">Template:Cite web</ref>

See also

• List of birds of the Sierra Madre Occidental
• Sierra Madre
• Sierra Madre Occidental pine-oak forests
• Sierra Madre Oriental

External links

• Template:Commons category-inline