Basin and Range (Transierra) Region Physical Geography

Joel Michaelsen

I. Introduction

The Basin and Range region in California covers a roughly triangular area bounded on the west by the Sierra Nevada crest, on the east by the Nevada border, and on the south by an indistinct boundary with the Mojave Desert (Figure 1). This region is the western portion of a much larger province that extends from the Sierra Nevada eastward across all of Nevada into Utah, where the Hurricane Cliffs and Wasatch Front constitute its eastern boundary. Called "Basin and Range" due to its characteristic sequences of high mountain ranges and deep valleys (all oriented roughly north-south), the same general area is also referred to as the "Great Basin" because the entire vast area has no external drainage outlets.

California's portion of this province has exceptional large-scale topographic variability: the highest and lowest points in the contiguous 48 states are only 85 miles apart. Mt. Whitney in the high Sierra crest is the highest point with an elevation of 4,416m (14,480') (Figure 2), while Badwater in Death Valley is the lowest point with an elevation of -86m (-280') (Figure 3). Generally, elevations of both mountain crests and valley floors decrease moving eastward from the Sierra Nevada. For example, the elevations of the valleys drop from 6,700' in Mono Basin (Figure 4) to 4,000' in the Owens Valley (Figure 5) to about 1,500' in the Panamint and Saline Valleys (Figure 6) to below sea level in Death Valley (Figure 7).

II. Geologic History

Most of the oldest rocks in California are found in the Basin and Range region and farther south in the Mojave Desert. Some of the most prominent exposures are in the mountains that form the east face of Death Valley (Figure 8). These rocks, which have been exposed by extensive faulting, appear to match the rocks in the inner gorge of the Grand Canyon, which have been exposed by the down cutting of the Colorado River. It is likely that similar rocks underlay much of southeastern California and the wider southwestern United States. The oldest are estimated to be around 1.4-1.7 billion years old, and their composition indicates they were accreted onto the western margin of the ancient North American continental core. By around 1.2 billion years ago, the region was part of an ancient supercontinent that geologists have named Rodinia (Figure 8). According to current understanding of paleogeography, the area that is now southeastern California was adjacent to continental material than eventually became Australia and Antarctica.

About 700 or 800 million years ago the Rodinia supercontinent began to split apart (Figure 9). As it split, rifts formed and faulting eventually dropped the area low enough so that it became the floor of a coastal ocean off the western margin of the remaining North American continent. Starting about 600 million years ago, and persisting for the next 350 million years, most of southeastern California was part of a stable, broad, gently sloping continental shelf environment. It was on the passive, western margin of the North American continent that was moving east, colliding with Eurasia, and producing the Appalachians and other mountains in the eastern U.S. The marine environment in the west was probably similar to what is currently found along the Gulf Coast and Florida. Deep layers of sediment accumulated over the long, quiescent period and formed the widespread sedimentary rocks that are exposed throughout the Basin and Range region. Often the same rock strata can be traced over large distances, reinforcing the picture of a broad, shallow shelf area. The most common rock types are carbonates, such as limestone, that are composed of the remains of marine organisms (Figure 10). Also present are sandstone layers with sand that probably originated on beaches or dunes. The rocks are generally light colored and show little or no evidence of terrestrial sediments, indicating that the adjacent coast must have been low-lying and lacking large rivers.

Starting about 250 million years ago, North America changed direction and started to drift west as it split away from Eurasia. Western North America became an active continental margin colliding with and accumulating various different pieces of crustal material. As the oceanic plate to the west was subducted beneath North America, a major mountain building episode, known as the Nevadan Orogeny, created the ancestral Nevadan range in the general location of the current Sierra Nevada. Molten material from the subducted oceanic plate rose through the North American crust but cooled below the surface to form the intrusive igneous rocks of the Sierra Nevada granites. While the largest concentrations of these rocks are west of the Basin and Range region, there are a number of individual intrusions, or batholiths, in the area. One of the most striking is in Papoose Flat in the Inyo Mountains (Figure 11) where the contacts are very distinct between older distorted and metamorphosed sedimentary rocks and the granitic rocks that intruded into the then existing rocks. These granitic rocks have eroded into picturesque outcrops that look quite different than the surrounding sedimentary outcrops. Another granitic batholith is found in the Cottonwood Basin area of the White Mountains, where it forms a clear contrast with the very light-colored carbonate rocks (dolomites) that cover most of the range (Figure 12).

Around 25 million years ago a large-scale change occurred in the interaction of tectonic plates along the west coast of North America. The oceanic plate that had been converging against the North American plate for a long period was completely subducted beneath the North American plate, along with a spreading zone between it and the Pacific plate. The Pacific plate's motion was (and is) northwestward, tangential to the North American plate. This change had major impacts on coastal regions as the San Andreas Fault system developed, but its impact was also profound in the Basin and Range region where it generated stretching and expansion of the crust. Geologists have estimated that, overall, the stretching of the crust has increased the east-west extent of the greater Basin and Range province by about 250km. One of the areas where the stretching has been most intense, Death Valley, has roughly doubled in width as rocks in the Panamint Range on the west side of the Valley have moved 75km to the northwest, splitting away from rocks on the east side that were formerly adjacent.

The major consequence of the crustal stretching has been the breaking of the crust into large blocks that have tilted and shifted relative to each other along roughly parallel north-south trending fault zones (Figure 14). This tilting and faulting produced the distinctive alternation of high fault-block mountain ranges and deep down-dropped valleys that characterizes the whole Basin and Range province. It is important to understand that rocks exposed in the Basin and Range region are usually hundreds of millions of years old, but the geologic processes that produced the modern landforms and exposed the rocks are actually quite recent, in geologic terms. The extension of the crust is continuing at an estimated rate of about 1mm/yr while the uplift of mountain blocks relative to adjacent valleys is also ongoing. An historic example of this activity was the major earthquake in 1872 in the Owens Valley that nearly leveled the small town of Lone Pine and killed more than 20 residents. The earthquake, with an estimated magnitude of 7.8 on the Richter Scale, produced about 5' of vertical motion and around 20' of horizontal motion. The vertical motion elevated the Sierra Nevada slightly higher above the Owens Valley.

The extension, and associated thinning, of the crust has also spawned a considerable amount of volcanic activity starting about 15 million years ago and continuing to the present. Volcanic landforms such as cinder cones, craters, and lava flows of recent origin are common throughout the region, particularly in the valleys. One especially impressive feature is the Long Valley Caldera, a 15km by 30km depression north of Bishop (Figure 15). It was created by a massive explosive eruption a little more than 700,000 years ago that spread ash across a large portion of the western U.S. After the eruption, the land surface collapsed to create the caldera depression. Various lines of evidence indicate that a shallow magma chamber is still present beneath the area. This evidence includes unusual earthquake activity, a rising dome in the center of the caldera, and releases of carbon dioxide that have produced substantial areas of die-off in the forests in the Mammoth Lakes area. Mammoth Mountain itself is a much younger volcanic peak near the western rim of the caldera (Figure 16).

III. Landforms

As noted above, the dominant characteristic of the Basin and Range region are the series of nearly parallel high mountain ranges and deep valleys that were created by large-scale block faulting associated with the crustal extension (Figure 17). The steep east face of the Sierra Nevada forms a clear and dramatic western boundary for the region (Figure 18). The Sierra Nevada is an asymmetrical tilted fault block range with a gentle western slope and steep eastern slope. The high Sierra crest is above 14,000' while the floor of Owens Valley is around 4,000', so the elevation changes by nearly two vertical miles in about 12 horizontal miles (producing the impressive average slope of 17%). Many landform features on the floor of the Owens Valley, including the scarp associated with the 1872 Lone Pine quake, provide evidence of the ongoing faulting activity that produced this elevation difference. Another distinctive and picturesque feature in the Owens Valley is the Alabama Hills (Figure 19). The Alabama Hills are composed of the same granitic rock that covers most of the high Sierra, but they are in a block that was left behind when the Sierran uplift began. The gnarled appearance of the Alabama Hills is much different than the high Sierra because of the dissimilar environmental conditions under which the rock has been weathering. In warm, semi-arid environments like the floor of the Owens Valley, granite is subject to chemical weathering that causes the rock to disintegrate around joints and cracks, creating rounded boulder piles and rock gardens. In the cold, high Sierra environment, dominant weathering processes involve mechanical scraping and splitting by glacial ice and freezing and thawing of water. The rock remains solid and supports the sort of steep cliffs and jagged ridges that characterize the east face of the Sierra.

The eastern wall of the Owens Valley is formed by the White and Inyo Mountains, which are structurally part of the same uplifted block (Figure 20). While not quite as precipitous as the eastern face of the Sierra, this range is still quite impressive, rising to over 14,000' at White Mountain Peak. There is ample evidence of faulting in this range, as well. For example, one distinctive feature along the base of the White Mountains is a series of truncated triangular slope facets created by an abrupt break in slope at the fault zone. The rapid uplift of the range created steep slopes and canyons leading up to elevated flats that have not been eroded or dissected yet. They range in elevation from 7,000' to 9,000' in the Inyos (Figure 21) and up to 12,000' in the White Mountains (Figure 22).

East of the Whites and Inyos are another string of deep valleys, most notably the Saline and Panamint Valleys, which drop to elevations of about 1,000'. The crest of the Inyos rises to over 11,000' only eight miles west of the floor of the Saline Valley, creating an average slope of 24% — quite a bit steeper than the eastern face of the Sierra. The east slope of the Inyos is exceptionally steep and rugged and is only partially dissected by several deep, steep-walled canyons (Figure 23). The Saline Valley is a deep structural depression with a southern entrance above 6,000' and a northern one above 7,000'. If the climate was wet enough, the basin would fill with water to a depth of about 3,000' before it spilled into Death Valley, which would make it the deepest lake in the Western Hemisphere. The Saline Valley, now part of Death Valley National Park, is one of the more remote places in California (Figure 24). It is accessed by a single 80-mile dirt road and a couple of rough jeep trails and has no services of any kind (including cell phone reception).

The Panamint Mountains, east of the Panamint Valley, rise to 11,000' at Telescope Peak, and form the western wall of Death Valley. This face forms yet another two mile vertical change over 12 miles of horizontal distance producing an average slope of 17% (Figure 25). Death Valley is a deep hole, several thousand feet below all of the access points except for the Amargosa River bed at the southern end. The are many distinctive landforms that bear witness to the rapid down-dropping of the valley and uplifting of the adjacent ranges. Impressively large alluvial fan deposits with elevations of around 3,000' and horizontal extents of 5-6 miles have been built from the mouths of the Panamint Range canyons to the floor of the valley (Figure 26). As the valley continues to subside, these fans continue to grow. On the eastern side of the valley, the mountains are lower but still extremely rugged, rising abruptly from the floor of the valley. The slopes below Dante's View (5,500' on the eastern side of the valley) are so steep that it is not possible to see their bottoms (Figure 27).

IV. Climate

The Basin and Range region has been identified as "The Land of Little Rain" in a collection of essays by author and early 20th-century Owens Valley resident, Mary Austin. The region's primary source of precipitation is midlatitude storms moving into California from the Pacific Ocean, however the nearly unbroken wall of the Sierra Nevada squeezes out most of the moisture as the air masses are forced to rise, creating an arid "rain shadow" in the lands to the east. Here valley floors are dry enough to qualify as true deserts. Death Valley, of course, is famous for its aridity and has an annual average precipitation of less than 6cm (Figure 28). (Generally, places with annual totals less than about 25cm are considered deserts.) The Owens Valley is wetter than Death Valley, but still dry enough to easily meet the desert criterion, with totals ranging from about 13cm to 18cm (Figure 29). Precipitation totals in the valleys north of Bishop do rise some with increasing elevations, but remain in the desert or near-desert range (20cm-35cm). The high mountains catch enough precipitation to exceed the desert limit, at least above 8,000' or so. In the White Mountains, for example, a UC research station at 10,000' averaged 33cm (Figure 30), while one at 12,500' averaged 48cm. For purposes of comparison, the Sierra Nevada at this latitude and similar elevations probably averages more than 150cm.

The majority of the region's precipitation comes from midlatitude storms during the colder part of the year. Throughout the region, about 70% of the annual total typically falls between November and April. Occasional summer thunderstorms can be very intense, however, and the flash flooding and debris flows they produce play important roles in shaping the landscape. Generally, summer precipitation is closely tied to topography, so it plays a more important role in the high mountains. Also, because the source of the summer moisture is the Arizona monsoon (a moist unstable flow that originates in Mexico), areas in the southern and eastern portions of the region see more summer thunderstorm activity. The Panamint Mountains probably experience the most thunderstorms, whose violent flash floods have helped to produce their many narrow, steep-sided canyons.

True to the continental location of the region, temperatures vary widely in time and space. At any given location there are large temperature variations on both daily and seasonal time scales. At Bishop (Figure 31), for example, average daily temperature ranges are 20oC (36oF) or more. Monthly average temperatures at Bishop range from about 3oC (37oF) in December to nearly 25oC (77oF) in July. Taking into account these large daily ranges, the difference between winter minimum and summer maximum temperatures is quite extreme — December average daily minimum temperature of -6oC (22oF) vs July average daily maximum temperature of 36oC (98oF). The primary factor determining spatial variations in temperature at any time of day or year is elevation. Typically, temperatures decline by more than 8oC/km (4.5oF/1,000'). Elevation differences of over 3,000m between valley floors and ridge tops, therefore, translates into temperature differences of around 25oC (45oF). As a result, valley floors will be reasonably comfortable in winter when mountains are extremely cold, while the mountains will be comfortable when valley floors are blazing hot in summer. For example, in January the average daily maxima and minima in Death Valley are 19oC (66oF) and 4oC (39oF), respectively, while at 10,000' in the White Mountains (Figure 32) they are 0oC (32oF) and -13oC (8oF). Similarly, in July the Death Valley maxima and minima (Figure 33) are 46oC (116oF) and 30oC (87oF), while in the White Mountains they are 19oC (66oF) and 3oC (37oF). Thus, at any time of year it is possible to be reasonably comfortable in some places and very uncomfortable in others.

While aridity is the dominant characteristic of the current climate, conditions were quite different in the relatively recent past. During the last glacial period the climate of the Basin and Range was cooler and damper. Increased moisture and reduced evaporation promoted the formation of large lakes in most of the deep basins. This so-called pluvial period peaked around 15,000 years ago as the glaciers in the Sierra Nevada began to melt, pouring significant amounts of runoff into the region. A connected drainage system was created starting in the Mono Basin and Owens Valley and eventually ending up in Death Valley, where the floor of the valley filled with prehistoric Lake Manly to a depth of as much 600'. Along the way this drainage collected runoff from Owens Lake Basin, China Lake, Searles Lake, and the Panamint Valley, all of which are interior drainages today. The pluvial lakes and the rivers connecting them created many landforms which still exist as relicts of these past climatic conditions. Old shoreline features are present in most of the basins, as are the dry drainage channels of the former rivers. An example of the latter is Fossil Falls (Figure 34) in the southern Owens Valley, where a lava flow deposit blocked the river draining south out of Owens Lake, creating a waterfall. In the Searles Lake basin relict shorelines are clearly visible both on the west side near the inflow channel through aptly named Poison Canyon (Figure 35) and on the east side where the outflow channel heads toward Panamint Valley and Death Valley (Figure 36). The Trona Pinnacles in the Searles Lake basin are a particularly unique and striking set of relict landforms (Figure 37). They are tufa towers composed of calcium carbonate that precipitated out of the water of pluvial Searles Lake (Figure 38). Another distinctive pluvial landscape is the badlands area around Zabriskie Point in Death Valley (Figure 39). The soft clay sediments that make up the badlands were deposited on the floor of pluvial Lake Manly and then uplifted and exposed to erosional forces by rapid faulting since the end of the pluvial period.

V. Vegetation

Vegetation species adapted to arid and semi-arid conditions dominate throughout the Basin and Range region, however as climate here is closely tied to elevation, it is not surprising that vegetation tends to show strong vertical zonation. The lowest elevations — the floors of valleys such as Death Valley and Saline Valley (Figure 40) — are dominated by the creosote scrub vegetation community similar to that which is widespread in the Mojave Desert (Figure 41). Joshua Tree woodland, another vegetation community mainly identified with the Mojave, extends north into the Basin and Range region in an elevation zone between about 1,000m and 1,500m (Figure 42). The most common vegetation community is Great Basin sage scrub, which replaces creosote at elevations of around 1,000m and above (Figure 43). Its dominant plant species, Great Basin sagebrush (Figure 44), is very widely distributed throughout the west. Not only does it form the dominant species in sage scrub, but it also forms the understory in pinyon-juniper woodland, which is found in the 1,500m - 3,000m range and is also widespread in semi-arid regions of the west and southwest (Figure 45). Montane coniferous forests, common in the Sierra Nevada, do not occur in the mountains of the Basin and Range region. The higher elevations above the range of pinyon-juniper woodland do have scattered stands of limber pine and the exceptionally long-lived bristlecone pine. Bristlecones occur in isolated populations in the Basin and Range mountains, most notably in California's White Mountains and often grow in harsh environments where there is little other vegetation (Figure 46). Tree ring studies have shown that individuals can live for several thousand years, making them the oldest living trees on the planet. By combining chronologies from old living individuals with data from dead trees that are been preserved by the lack of decay in the harsh environment, scientists have been able to assemble records over 8,000 years long that provide information on past climates.