Geologic Formations

Mountains in the monument are the remnants of ancient volcanoes. The large formations that make up the mountains are old rock, with little opportunity for plant life to take hold. Only after this rock is broken down through the process of erosion, are plants able to reap the benefits of fertile soil.

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The flat desert basin seemingly abruptly meets with mountain ranges.

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Basin and Range

Organ Pipe Cactus National Monument makes up only a small part of the massive feature called the Basin and Range Provence. This region stretches from its eastern boundary in West Texas to its western boundary at the foothills of the Sierra Nevada in California. It stretches from deep in western Mexico in the south to the Columbia Plateau of Oregon and Idaho in the north. The Basin and Range Provence crosses almost every ecosystem imaginable, from wetland to desert, from forest to tundra, and from beaches to mountains, but it is unified by its unique geology.

At least 30 million years ago, one of Earth’s tectonic plates (the moving puzzle pieces on which the continents and oceans sit) collided with the western edge of North America. Geologists call it the Farallon Plate. This began a process called subduction, whereby the Farallon Plate was drawn underneath North America.

As the Farallon Plate was drawn under, it kept moving to the east, creating a massive amount of pressure that compressed the western part of North America. In addition, the Farallon Plate was subducted at an unusually shallow angle, further lifting this compressed portion of the continent until a large bubble of continental crust swelled to create a massive area of western highlands across the continent.

Eventually, the Farallon Plate was completely swallowed by the Earth, drifting under North America. With no more compression coming from the west and no more force coming from below, the crust of North America began to relax and spread under its own weight and compression, collapsing the western highlands. Faults formed where the compressed rock split, creating a repeating pattern of mountain-valley-mountain-valley that stretched across the western half of the continent. Imagine an accordion at its peak of compression being pulled apart, with the ridges of the accordion being the mountains and the spaces between being the valleys.

The valleys, of course, became the “basins,” and the mountains became the “ranges,” giving this area of North America its name. Because of this varied topography, the climate and environment at the top of these mountain ranges often differ radically from that at the bottom of the valleys. This creates an archipelago of “sky islands” that stretches across the Basin and Range Province, each isolated from the others by the differing climate of the basins between.

Another way this type of terrain isolates ecosystems from each other is by separation of rivers and streams. Because the basins between the ranges are also isolated from each other, rivers, streams, and lakes often have no outlet or connection to each other. This means that rivers often empty into dry salt flats, sand dunes, or simply disappear beneath the surface under layer upon layer of loose sediment. The Quitobaquito pupfish and Sonoyta mud turtle are species that were stranded by this form of isolation and left to adapt to their local surroundings.

 

Because of the cooler temperatures at higher elevations, plants like mountain agave thrive in the high mountains, while it would struggle on the lower desert basin.

NPS Photo/Cate Blanch

Sky Islands

Thanks to the basin and range uplift, hundreds of roughly north-south mountain ranges dot the desert southwest, each separated from the others. These are the Sky Islands of the Basin and Range Provence, and each tells a slightly different story. They are all, however, deeply influenced by the climate of the past: specifically, the Last Ice Age.

The Wisconsin Glaciation of the late Pleistocene lasted from around 100,000 years ago to around 11,000. During this time, global temperatures were significantly lower, causing glaciers to advance from the poles deep into what is now the continental United States. This glacial period is named for the evidence of glaciers found in Wisconsin, but continental glaciers extended as far south as New York and Pennsylvania.

In addition to the better-known continental glaciers, alpine glaciers extended far south along the spine of the Cascade and Sierra Nevada mountain ranges on the west coast of North America. A combination of these two types of glaciers forced wind patterns farther south, and the Jet Stream (which today crosses the northern and central United States) was forced into what is now the Desert Southwest. Consequently, cool, moist air was diverted here, creating a climate drastically different from that of the present. In fact, because colder temperatures often bring drier climates, the Southwest is one of the only regions on the planet that was wetter during the Last Ice Age than today.

Where there are deserts today, the Pleistocene Southwest was mostly covered by prairies and subtropical savannas. The higher elevations were home to unique forests, some of which joined with the forest ecosystems of the Rocky Mountains to the north or the Sierra Madre to the south, allowing plants and animals to colonize them from those larger, taller ranges.

As the climate began to warm at the end of the Wisconsin Glaciation, the Sierra and Cascade glaciers mostly melted off, allowing the Jet Stream to return to its current position. The Southwest became hotter and drier, creating deserts where once there were grasslands. This new habitat created a fantastic opportunity for organisms from the south to colonize. In fact, many of the classic plants associated with this region, such as the saguaro and organ pipe cacti, are relatively recent arrivals, only colonizing after the end of the Wisconsin Glaciation.

For the resident plants and animals that had adapted to the cooler, wetter conditions of the Last Ice Age, however, life became increasingly difficult. There were essentially three options: adapt, move, or die out. Many species adapted to the spreading desert, with selective pressures forcing them to find innovative ways to conserve water and survive the long, hot summers. Many more died out completely. For some, however, the cooler mountain ranges of the Basin and Range Provence provided a refuge that let them persist.

In the Ajo Range, you can find oaks, junipers, and agaves growing among grasses and leafy shrubs: remnants of the flora that covered much of the area over 11,000 years ago. Because these species were isolated from other populations, they began to adapt to the unique pressures of their local environments. They share a common ancestor with other members of their groups that followed a slightly different evolutionary path. They are some of the last members of their respective groups that managed to hang on in this region. Learn more about the fire ecology of sky islands across Arizona.

 

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Canyons

Canyons are deep valleys with steep sides, and they are easy to find throughout the monument. The geologic forces that created this area were at play when some larger canyons formed in the monument, but many canyons are still being shaped every year. Water is an incredibly powerful force in the monument, shaping the plant and animal communities, as well as the physical landscape.

Water, especially when moving fast, can carve away stone. During the summer monsoons, huge volumes of rain will fall very quickly, and naturally flow to lower elevations. Sometimes, as water pools and flows down the mountain, it will encounter a tight spot or bottleneck, where it picks up speed and force. The sheer amount of water and speed of flow will erode the face of a mountain and will create a path that water will likely take again during the next storm, repeating the process.

 

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Tinaja

Similar to a the formation of a canyon, water may cut away stone, but then just pool there if it is low enough. When heavy amounts of water flow through the rocky landscape of a mountain, the amount and force of the runoff may carve divots in relatively flat rocks. As a pocket forms, more fast-flowing water enters the divot and gains momentum, further carving out the divot into a natural basin or tinaja.

Tinajas hold water, sometimes year-round, in shade or high elevation. These reliable pools of water attract insects and other animals, and historically provided water to native people, then miners and ranchers, and their cattle.

 

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Arroyo/Washes

A common feature of dry places with seasonal rain are the riverbeds that spring to life during wet weather. As the powerful summer monsoons shower the landscape in water, the crust that formed on the ground resists soaking up the water. The runoff must flow into low points in the landscape, like an arroyo, also known as a wash.

During particularly large storms, a typically dry arroyo can very quickly play highway for a raging river of water up to 10 feet deep (3 m). These sudden and violent runoff events actively carve the landscape, and can transport boulders, trees, cacti, and unsuspecting people away.

Arroyos are unique, even when dry! These dry beds are lower in elevation than the surrounding area, with walls sometimes reaching high above you. Because cold air sinks, this low point on the land allows cool air to collect, and even flow like water from higher elevations. This network of airflow makes the arroyo act like a sort of air conditioning duct to the monument. This incredible feature hosts plants, animals, and fungi that seek out relief from the often sweltering world above.

Arroyos carve their way throughout the monument. Along the biologically diverse bajadas, an arroyo may add the perfect element for a unique arrangement of plants and animals, like the rare acuña cactus, or deer and mountain lions. Learn about wash monitoring in the Sonoran Desert.

 

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Alluvial Fans

Although the monument is nestled in the heart of a desert, it experiences seasons of rain. In the winter, it will experience persistent soaking rain, and in the summer, the monsoon storms soak the Earth in often violent washes. When the water gets moving along an arroyo, or cascades down from a mountain, it will pick up loose clay, sand, and even boulders and transport them. The debris that is picked up and deposited elsewhere is called alluvium.

An alluvial fan forms when flowing water that is carrying debris is allowed to spread out. When water spreads out, there is less force pulling the debris along, so it settles. The pattern that forms when all of the debris is settled, and the water has soaked into the ground is fan like in shape most obvious way to see an alluvial fan is by looking at a bajada.

 

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Bajadas

While exploring Organ Pipe Cactus National Monument, you will often see a gentle slope connecting the base of a mountain range with the valley below. These features are called bajadas, derived from the Spanish term for a downward slope. Bajadas create a unique geophysical environment and habitat for Sonoran Desert life.

While bajadas seem to blend seamlessly with both the valleys and the mountains, they are a distinct feature with aspects of both. Bajadas are made up of debris, from tiny sand grains up to large boulders, washed down from the mountain ranges. During the monsoon, flash floods can move large amounts of sediment from the tops of the mountains. Over time, this sediment piles in slopes at the base.

Bajadas hold some of the most diverse plant communities in the monument. Down the bajada’s slope, soil temperature and salt levels increase, and soil moisture and grain size decrease. Even though the landform can appear rather uniform, plant communities can change radically over the downward slope.

Palo verde, organ pipe cactus, and saguaro are much more common at the tops of bajadas, whereas creosote, salt bush, and other scrubby plants are much more common at the bottom. Toward the middle of the bajada, communities merge to form what is known as a transition zone or “ecotone”. This middle ground is home to diverse communities that contain representatives of both higher and lower plant communities, as well as a few species that need both, and a variety of animals.

 

 

Manyy beautiful geologic formations make up the Sonoran Desert ecosystem and each play a unique role in the landscape. Explore below to find out more about formations you'll experience at Organ Pipe Cactus, and visit the Arid and Semi-arid Region page on the NPS Geology website to learn more about geology in the Sonoran Desert.