The spatial context of the Basin and Range Physiographic Province within the United States is shown in Figure 15.1-1. The province occupies about 300,000 square miles in much of the western and southwestern part of the United States. The region comprises almost all of Nevada, the western half of Utah, southeastern California, and the southern part of Arizona and extends into northwestern Mexico. From Figure 15.1-1 it is seen that eight physiographic provinces and the Mexican border adjoin the Basin and Range Province. These are: the Columbia Plateaus to the north; The Cascade-Sierra Mountains along most of the western border with shorter segments of the Pacific Border and Lower California Provinces; the Mexican border is shown in the illustration as the province border, however the province extends into Mexico; the southern portion of the eastern boundary coincides with the southwestern border of the Great Plains Province; the southern extent of the Southern Rocky Mountains Province and then the southern, southwestern and northwestern boundary of the Colorado Plateaus; and a portion of the western boundary of the Middle Rocky Mountains.

The Basin and Range is a semi desert with an extremely complex geologic history. The province has more than 400 mountain ranges if the small mountain ranges are included that are the remains of crustal rocks that were upraised by faulting along north-south lines. Eroded materials from the ranges move downslope into the basins.

A very small-scale shaded relief image of the United States in Figure 15.1-2 shows the rugged terrain in the western third of the United States. The general boundaries of the Basin and Range Province, while not readily apparent everywhere, can be generally seen by contrasts of the north-south trending mountain ridges with the bordering terrains. Note that this image is available from USGS in various formats from postcard to wall map.

 The province has a varied and remarkable topography consisting mainly of numerous roughly parallel fault block mountain ranges tending north-south and separated by nearly flat desert basins. The up-thrown side of these faults forms mountains that rise abruptly and steeply, and the down-dropped side creates low valleys. Basins occupy much more of the area than ranges; in fact, in the distant future the ranges will be largely buried beneath their own waste material, especially in the northern part - the Great Basin - where there is only interior drainage.

Among the mountains of the province certain characteristics are prevalent. Usually the range is one of many more or less parallel ranges. Lengths of 50 to 70 miles are common; there are smaller and larger ones and the maximum altitude of the mountain ranges can be more than 13,000 feet but most are less than 10,000 feet in altitude. Within its length there is no great or sudden variation in height and breadth though the crest may be very jagged with bold faces and cliffs essentially devoid of vegetation. The range is not deeply notched and segmented; the bulk is fairly continuous; a general straightness is more common than not. The generalized slope from the base to the summit is not excessive but the appearance of steepness is enhanced by the fact that the slope does not flatten out near the base, The abrupt meeting of the valley floor and mountain side, and the uniform slopes of the latter, are among the striking features of the province.

The rocks of the Basin and Range vary considerably, but in this dry climate the limestones are as resistant to weathering and erosion as many of the sandstones and granites.

The mountains rise abruptly 3,000 to 5,000 feet above the intermountain desert basin. The altitudes of the basins vary from below sea level (at Death Valley and Salton Sea) to about 5,000 ft above sea level. The mountains have irregular crestlines with worn peaks and flanks where gullies and waterworn slopes are plentiful.

The valleys or basins begin downslope from the base of the rock outcrops. The weathered and transported materials become finer and the slopes decrease as the centers of the basins are approached. Many of the valleys are closed and contain playas (i.e., dry lake beds).

With few exceptions the landscapes are typical of dry regions. The dominant landforms, in addition to the ranges themselves, are alluvial fans, bajadas, pediments, bolsons, and playas. Vegetation is sparse and wind erosion is active and produces large sand dune areas in several locations.

Only a few of the highest areas in the province were glaciated and almost all of these are in the northern section. However, Pleistocene lakes occupied the interior basins. Some of these are very large along the edges of the province in the Great Basin Section. On the eastern edge was Lake Bonneville that was much larger than its Great Salt Lake descendant. On the western edge was Lake Lahontan with the present-day Carson sink as its deepest point. Many others occupied interior basins during the cooler climate intervals of the Pleistocene.

National Parks and Monuments in the Basin and Range that illustrate its physiographic features include Death Valley and Joshua Tree National Monuments, California; Lake Mead Recreational Area, Nevada and Arizona; Saruaro Cactus, Organpipe Cactus, and Chiricahua National Monuments, Arizona; Big Bend National Park, Texas; and Lehman Cave National Monument, Nevada. Others could be added as well: Great Salt Lake and parts of the salt desert around it; Carson Sink and parts of the mountains bordering it; and some of the other lakes, basins, and ranges near the foot of the Sierra Nevada.

Outstanding features of the Basin and Range include:

• Complex geology and recent faulting.
• Structural rather than erosional valleys.
• Large numbers and large size of Pleistocene lakes.
• Three quite different kinds of ground in every valley - gravel alluvial fans rising from the valleys to the base of bordering mountains, dry lake beds or floodplains in the central parts of the valleys, and rocky bordering mountains.
• Deposits of various kinds of salts in the playas.

The Basin and Range Province has five sections of unequal size, but it is commonly subdivided into northern and southern halves. The northern half would include the Great Basin Section that is located north of about latitude 36 degrees. It is the largest section and mountains take about half of this area. Only minor marginal tracts drain to the sea. South of the Great Basin Section in California and southwestern Arizona is the Sonoran Desert Section. This section is much lower in altitude, has smaller mountain ranges, and is only about one-fifth the size of the Great Basin. Also large areas are without concave basins of internal drainage. The Salton Trough Section has a center that is several hundred feet below sea level. Its southward extension into Mexico embraces the Gulf of California. Between the Sonoran Desert Section and the Colorado Plateau Physiographic Province and stretching east beyond the Rio Grande, is the northward continuation of the Mexican Highland Section. This section is similar to the Great Basin Section in its major features. Finally, on the eastern margin of the province, mainly in New Mexico, there is a north-south strip that combines some features of this province with others of the Colorado Plateau Province. This is the Sacramento Section and it has plateaus tilted like the mountains in the northern part of the Great Basin.

The Great Basin makes up about half of the Basin and Range Province and the total area of the section is 190,000 square miles that are about evenly divided between mountains and basins. It centers in Nevada but extends into the adjoining states. It is bordered by the Sierra Nevada range on the west, the Wasatch Mountains on the east, the Columbia Plateau on the north, and the Mojave Desert on the south. The Great Basin consists typically of the north-south mountain ranges that vary in length of 50 to 75 miles and have widths of 6 to 15 miles. Perhaps the most common altitudes are 3,000 to 5,000 ft above their bases and 7,000 to 10,000 ft above sea level. The western face of the Sierra Nevada blocks rain-bearing winds off the Pacific Ocean from reaching the Great Basin forming a rain shadow over the entire region. The annual rainfall of 6 to 12 inches in the basin supports little more than sparse desert or semi-desert vegetation. The Great Basin is particularly noted for its internal drainage, in which precipitation falling on the surface leads eventually to closed valleys and does not reach the sea.

Five subdivisions of the Great Basin may be distinguished on the basis of their structure, topography, hydrography, and kind of ground. These are: A, The Central Area elevated basins and ranges; B, The Bonneville Basin east of the central area; C, The Lahontan Basin west of the central area; D, The Lava and Lake Area at the northwest corner of the section; and E, The Southern Area. Figure 5.1.6-1 is a small-scale shaded relief image of the states of Nevada and Utah with the five subdivisions labeled. Note that at this scale the image of mountains in the vicinity of "A" and "C" has been characterized as a group of caterpillars, all crawling irregularly northward.

The Central Area, in the vicinity of "A" in Figure 22.3, is characterized by valleys that are mainly 5,000 feet in altitude. Some are closed, but none contain perennial lakes. Dry lakebeds and alluvial flats make up about 10% of the Central area. The remaining part is about equally divided between the mountains and the gravel fans sloping from them. A large part of the Central Area drains to the Lahontan Basin by way of the Humbolt River. The mountain ranges in the eastern and northern parts of the Central Area are linear ridges of complexly deformed rocks that are mainly limestone. To the west the rocks are mainly sandstone, siltstone, and shale derived from volcanic rocks. Block faulting of these folded and faulted blocks produced the basins and ranges. At about the same time that this block faulting started there were extensive eruptions of lavas and tuffs in the southwest part of the Central Area, and these blockfaulted, volcanic rocks form the ranges there. The downfaulted blocks throughout the Central Area are buried under debris washed into the valleys by erosion of the uplifted blocks. The block faulting has continued into historic time, and in many valleys the gravel fans and other valley fill are faulting along with the bedrock. This recent faulting and tilting of the block mountains has contributed to some of the drainage anomalies to be described later.

The Bonneville Basin, in the vicinity of "B" in Figure 5.1.6-1 is the eastern subdivision of the Great Basin and covers most of western Utah. It is structurally similar to the Central Area but lower in elevation. In most of this subsection the basins are below 5,000 feet in elevation. They are slightly higher in southwestern Utah. In the Great Salt Lake, the lowest part of the subsection, the elevation is at 4,200 feet. Playas and alluvial fans make up about 40% of the basin and the mountains cover about another 25% of the basin. The remaining 35% of the surface cover is gravel fans. Thus the proportions are different from those of the Central Area. The structure and composition of most of the mountain ranges in the Bonneville Basin are like those of the northeastern part of the Central Area. The ranges are mostly complexly folded and faulted rocks that were later divided into structural blocks by block faulting. To the south, volcanoes form some of the mountain ranges; in the north the volcanic rocks are young and occur mostly in the basins. The Bonneville Basin lacks exterior drainage and has three major lakes: the Great Salt Lake, Utah Lake, and Sevier Lake. Sevier Lake is dry much of the time, but would be perennial if it received the water that is consumed for irrigation.

The Lahontan Basin, in the vicinity of "C" in Figure 5.1.6-1, is located between the Central Area and the Sierra Nevada. It is structurally similar to the Bonneville Basin. The greater part of its area is alluvial flat and playa, and it too contains some large lakes - Pyramid Lake, Lake Winnemucca, and the playa at Carson Sink at the mouth of the Humbolt River. The mountains are fault blocks in much the same manner as the folded and faulted mountains in the Central Area and the Bonneville Basin.

The Lava and Lake Area northwest of the Lahontan Basin, in the vicinity of "D" in Figure 5.1.6-1, is topographically higher than the Lahontan Basin. It is a block faulted lava plateau with numerous high volcanic cones. Several lakes - Honey Lake, Eagle Lake, and others in the Klamath, Oregon, are crowded against the foot of the Cascade Mountains at the edge of the Great Basin. The topographic grain of the Lava and Lake Area has much less linearity than the other parts of the Great Basin because the thick, extensive lava flows and volcanic cones are young and they mask the effects of the block faulting.

The Southern Area in the vicinity of "E" in Figure 5.1.6-1 bears some structural resemblances to the Central Area but is lower, both structurally and topographically. Toward the south, the mountain ranges trend northward, but they are separated from the ranges in the Central Area by a northwest-trending belt of mountains and hills, most of which are small and have very irregular outlines. This belt, that parallels the diagonal southwest border of Nevada, extends from the vicinity of Lake Mead to the vicinity of Walker Lake. It coincides with the Las Vegas shear zone, along which there seems to have been some miles of lateral movement; the blocks on the southwest side of the shear zone have moved northwestward relative to the blocks on the northeast side. The displacement seems to decrease northwestward. Along the western boundary of the Southern Area, at the foot of the Sierra Nevada, are Mono Lake and Owens Lake. Death Valley, that covers more than 200 square miles and is more than 250 feet below sea level, collects the runoff from almost 9,000 square miles of surrounding area, but is rarely flooded. Most of the time it is a dry, salt-encrusted playa. The southeastern part of the Southern Area drains to the Colorado River.

Figure 5.1.6-2 is a small-scale geology map of Nevada that occupies the majority of the Great Basin. At this scale the general distribution of the ridges are well defined. The ridges in the center of the state (i.e., the Central Area) are mainly igneous and sedimentary rocks. The white areas surrounding the ridges are alluvial valley fill and playas. The large orange areas in the northwest corner (i.e., the Lava and Lava Lake Area) and the middle of the northern border are volcanic masses. The yellow areas are tuffaceous sedimentary rocks (i.e., reworked and consolidated volcanic tuff).

Figure 5.1.6-2. Nevada geology map. (source: http://geology.about.com)

Figure 5.1.6-3 is a mosaic of four shaded relief images prepared from four Nevada 1:250,000-scale USGS DEMS. At this scale the general boundary between the rocks of the mountain masses and the downslope weathered materials is rather well defined. The rock areas are heavily textured. Sharp ridges are seen on some mountains but not on all. Contouring in the weathered materials is related to artifacts associated with low surface gradients. The white square outline indicates the area shown in the larger scale Figure 5.1.6-5, and 5.1.6-4.

Figure 5.1.6-4 shows four different means of portraying DEM data is presented. The shaded relief image has been discussed. The contour image with 100 m contours reveals less information. It is difficult to delineate rock areas from weather materials. Filled contours or contour labels would improve information content. The image map shows the configuration of the ridge crests, but no information in the low areas. A better gray scale range would add more information. The wireframe image was prepared by filtering the DEM data for only every fifteenth value in the x and y dimensions and thus shows only generalized terrain configuration. but nothing conclusive. The basin in which "B" is placed shows details of erosional drainage channels. For example, the "B" is placed in a main channel draining the mountain to the north. The topography in the area of "C", on the west side of the north-south mountain range, is even more diverse. While there is evidence to suggest older and more highly eroded alluvial slopes, there are also indications of low inselbergs or maybe even pediments with bedrock near the surface. The ridge at "D", while slightly more rounded at the crest than the ridges to the north-east and south-west, has similar features on either side and could, as a reasonable first guess, be considered as portions of the same geology but faulted to the present positions. The higher elevations in the vicinity of "E" are spatially more irregular and round at the tops as are the higher elevation features to the west of the ridge at "D".

Figure 5.1.6-5 is a shaded relief image mosaic of 1:24,000-scale USGS DEMs from the area indicated in Figure 5.1.6-3. There is additional terrain information seen at this scale in the rock as well as the weathered materials area. At "A" however the basin is without much detail. There are hints in the image of alluvial fans that must be there.

Figure 5.1.6-6 is a mosaic of image maps of the same DEM quadrants as Figure 5.1.6-5. This illustration shows more clearly the nature and elevations of the crests and the drainage at the higher elevations, at the expense of any detail in areas of lower elevation.

The Great Salt Lake and the Bonneville Salt Flats, with other associated playas and lakes in northwestern Utah, represent a significant physiographic area of the United States. However, images related to terrain configuration show little if any information in these generally flat areas because local relief at the scale of our present USGS DEM data is lacking, except for isolated ridges. One might look for old strandlines and terraces associated with former lake levels, but that will be left as an exercise for the reader.

The Payette and Snake River Sections of the Columbia Plateau border this area on the east by the Middle Rocky Mountains and on the north.

Similar to Lake Bonneville in the Pleistocene glacial epoch, the Lahontan Basin was a vast lake similar to the Bonneville Basin on the eastern side of the Great Basin with fault block mountain ridges projecting above the water. Today the greater part of the area is alluvial flats and playas with some large lakes and isolated ridges.

This area is the eastern portion of a large volcanic area in the north-western portion of the Great Basin that extends into south-central Oregon, northeastern corner of Nevada, and north-western corner of California. Figure 5.1.6-7 is a small scale shaded relief image in the western portion of the Lava and Lake Area and annotated with the physiographic province and section codes. Two distinctly different terrains in this area will be described: The volcanic area in the vicinity of the Lava Beds National Monument and a portion of the Modoc Plateau in the northwestern corner of Nevada.

The obvious features in the center of this image are the large volcanoes. The largest, Medicine Lake shield volcano, is seen in the lower center. It has the blue lake in its crater. The small lake from which Medicine Lake volcano derives its name lies within the 7 by 12 km central caldera. The Medicine Lake volcano is located about 50 km east-northeast of Mount Shasta, that is the largest of the Cascade stratovolcanoes. The Medicine Lake volcano dominates the local geology. Located on the north flank of the Medicine Lake shield volcano, Lava Beds National Monument consists of many lava flows, 17 cinder cones, 300 known lava tubes, and other volcanic features, several of which were formed during the past hundred years.

The Lava Beds National Monument is part of the lava-covered Modoc Plateau desert of northeastern California and is included in the Basin and Range Province because typical fault-block structures extend into the area from the Tule and Klamath Falls, Oregon basins immediately north. The monument is part of the much larger Medicine Lake shield volcano that dominates the local geology. The area also shares geologic character with the nearby Cascade Mountain volcanic province to the west and the extensive volcanism associated with the Columbia Intermontane Province to the northwest.

Figure 5.1.6-8 shows two adjacent shaded relief images and their related contour images with 100 meter contour interval that were prepared from 1:250,000-scale USGS DEMs (i.e., Alturas-E and Alturas-W). These images are in the extreme northeast corner of California, as shown by the left two inscribed white rectangles in Figure 5.1.6-7. The area at "A" in Figure 5.1.6-8 is on the northwestern slope of Medicine Lake volcano and the Lava Beds National Monument lies on the northern side of this volcano. Note the number of small cinder cones on the volcano slopes that show in the shaded relief image and contour map. Gillem's Bluff, a prominent Basin and Range fault scarp, is located to the east of "B" that shows as a ridge on the contour map having a bluff on the east side that extends towards the center of the Medicine Lake volcano. The surface of the shaded relief in the vicinity of "C" appears relatively smooth with large isolated cinder cones near the Medicine Lake volcano. At "D" the surface continues to be generally smooth with the surface dropping abruptly to the east and south in what looks like a cliff, but only shows as one contour line in the contour map. The very smooth area to the northeast of "D" is the basin of Goose Lake that has water while the identically smooth feature directly east of "D" on the east side of the ridge is a dry valley of a former lake. The ridge at "E" is similar to the Great Basin ridges seen in previous illustrations.

Figure 5.1.6-9 is a 3x3 mosaic of 1:24,000-scale shaded relief images within the area shown in Figure 5.1.6-8. What is immediately apparent is that considerable information not seen on the smaller 1:250,000-scale images becomes apparent at the larger scales. For example, the lake in the caldera of the Medicine Lake volcano is seen at "A." The Callahan flow at "B" has lobes that are distinguishable. At "C" the lobes appear to have a rougher surface. The mafic basalt flows are typically very fluid, producing smooth pahoehoe surface morphologies. Extensive flows on relatively flat terrain (such as that in the northern half of the monument) developed pressure plateaus near the ends of the flows. With increasing silica content, the morphology of the flows become more rugged and form rough-surfaced aa or have very rugged, blocky surfaces commonly described as block lava flows. At "D" the crater rim appears to be breached and a flow has spilled down the side of the volcano. The large conical hill at "E", on the side of the Medicine Lake volcano, has no caldera. At "F" what appeared to be a single escarpment at Gillern Bluff is seen to be three escarpments. The feature at "G" would seem to be a portion of a volcano side. The surface in the vicinity of "H" appeared smooth at the smaller scale but shows some structure at this scale. And at "I" the Tule Lake surface still appears to be very smooth.

Figure 5.1.6-9. Mosaic of nine 1:24,000-scale California shadedrelief images in the vicinity of Lava Beds national Monument.

Figure 5.1.6-10 is a collage of three images prepared from the Caldwell, California, 1:24,000-scale DEM. The shaded relief image shows significantly more detail in this image than in the reduced scale image produced from the same data. While this should be a truism (i.e., larger image scales always reveal more information), The viewer might not have the proper context, afforded by the more general and smaller scale images, to correctly interpret the larger scale information. When viewing Figure 5.1.6-10 refer to Figures 5.1.6-8 and 5.1.6-9 for context.

The lava flow patterns on the northeastern flank of Medicine Lake volcano in the vicinity of "A" in Figure 5.4.1-10 suggests that the large cinder cone volcano posed an obstacle and diverted flows to either side. At "B" note the smooth surface of the older flows. At "C" and "D" flows are seen associated with two smaller volcanoes. The volcano at "C" has a breached caldera and a small lava flow on its western side. The volcano at "D" appears to have a well-formed caldera rim with the lava only on the western side. At "E" the crater is large but the caldera is not very deep. And there is a small feature adjacent to it's base on the northwestern side which is thought to be a very small cinder cone. The remains of a small eroding cinder cone are seen at "F" with its associated flows of limited extent. A large cinder cone at "G" has no distinguishable lava flow patterns of it's own, but it, like the cone at "E" has a much smaller cone on it's north edge. The regional grade is downward to the northeast away from the Medicine Lake volcano and the gentler slopes have a surface pattern suggestive of old flows with subdued ash-covered surfaces.

The perspective view in Figure 5.1.6-10 was also produced from the Caldwell, California 1:24,000-scale DEM and is oriented looking from the lower northeastern corner to the higher southwestern corner on the slopes of Medicine Lake volcano.

A unique feature of the southern area of the Great Basin is Death Valley National Park that straddles the California-Nevada border. It is located just east of the Sierra Nevada range. Death Valley National Park is the largest national park outside of Alaska (3.3 million acres) and one of the newest, created in 1994. Death Valley is an enclosed basin 130 miles long and from 6 to 14 miles wide. Death Valley is the hottest, driest, and lowest of the national parks and the lowest land elevation in North and South America. The maximum temperature ever recorded in North America was at Death Valley in 1913 when 134 degrees F was measured in the shade. The average annual precipitation is low, only 1.7 inches, and some years have no recorded rainfall. The cause is that Death Valley lies in a triple rain shadow effect in that the Sierra Nevada, Argus, and Panamint ranges all help drain most of the moisture from Pacific air masses moving inland. Local storms of high intensity in the surrounding mountains, however, cause floods that in a few hours remodel the landscape significantly.

The topographic relief within the National Park is notable. Telescope Peak, near the southern border, is 11,049 feet high; less than 2 miles to the east is Badwater, the lowest point in the western hemisphere, 282 feet (about 86 meters) below sea level. Death Valley is sandwiched between the Panamint and Amargosa mountain ranges in a down-dropped fault block valley. Death Valley is not a true valley. Streams form true valleys and sea level is the base level of erosion. Death Valley is a structural basin formed by down faulting of a large block - the "valley" or graben - and the up faulting of the mountain ranges surrounding it. Death Valley is a structural basin formed by down faulting of the mountain ranges surrounding it. The process continues today with the mountains on either side rising and the valley dropping. However, the sinking valley is partially offset by erosion and deposition of debris. Geologists have determined that beneath Badwater, the lowest point in the valley, a 9,000-foot thickness of debris rests on the bedrock floor of the Death Valley basin.

Figure 5.1.6-11 is a mosaic of two 1:250,000-scale shaded relief images in the Death Valley National Park vicinity. Note how the ridges resemble those seen in Nevada, except here the valleys may be narrower between the ridges. In this mosaic Death Valley is in the center of the images (the southern end of Death Valley has been annotated with white squares) representing areas of higher resolution to follow in Figure 5.1.6-12.

Figure 5.1.6-12 shows eight 1:24,000-scale USGS DEMs in the lowest portion of Death Valley with only the lower portions of the ridges to either side. The individual DEMs and their associated maximum and minimum elevations in meters follows.

Note that the official minimum elevation of minus 86 meters in Death Valley is in the vicinity of Badwater, however the DEMs registered a few meters less than that (presumably measurement error). However, the uniform tones in the shaded relief images are due to the lack of surface elevation structure. Death Valley contains one of the world's largest salt pans, consisting of a variety of salts, but they will not be visible from the DEM data due to their level nature and lack of structure.

During the Great Ice Age when the climate was colder and wetter, glaciers occupied mountains within an expanded Death Valley drainage system. Lakes formed and disappeared repeatedly in Death Valley over a period possibly exceeding a million years. At its height, Death Valley held a lake about 600 ft deep and close to 100 miles long. Lakes formed and disappeared repeatedly since then. Their phases are recorded in strand lines on the valley sides. These strand lines will not be discernable from the DEM resolution involved.

Death Valley is renowned for its alluvial fans or debris aprons at the edges of the valley. These fans are the result of uplift by earthquakes of the mountains and down dropping of the valleys. Sediments are transported from higher areas during storms, making beautiful fan-shaped accumulations of rocks and soils and filling the edges of the valley. The map in Figure 5.1.6-13 shows contour line extending below the mountain ridges into the valley. These contours represent areas of alluvial fans. It is easily seen that the areas of alluvial fans are much larger on the west side of the valley than those on the east side. Literature provides two reasons for this difference. One reason is that the region is tilting to the east and this will raise the western side to provide a greater runout for the alluvial materials. The second reason provided is that the drainage areas to the west that contribute to these fans are from higher terrain that receive much more moisture than the terrain on the east side of the valley.

Figure 5.1.6-13 is a larger shaded relief image produced from the Hanaupah Canyon DEM data with a 25-meter contour overlay. Note the crenulations in the contours is related to the drainage structure on the alluvial fans.