The geology, physiography, and ecology of the western United States is quite varied. Look at this shaded relief map that shows most of the region. One feature should catch your eye almost at once: the many stand-alone elongated mountain systems trending north-south that cover a wide area centered on Nevada. This is the Basin and Range Province, which owes its character to differential up-down movements of the Earth's crust along normal faults as the region has been uplifting in response to subduction brought about by the westward movement of North America:
Compare the above map with the color-shaded relief map of most of Nevada.
The bulk of the Basin and Range Province is within Nevada and small parts of California and Colorado. But as this subdivisions map shows, the characteristic block faulting extends into Arizona and New Mexico, and a bit into Mexico itself:
This Landsat image mosaic shows much of the Basin and Range province in most of Nevada but includes Death Valley, the southern Sierra Nevada block, and a bit of the Great Valley (San Joaquin) near Bakersfield, are in California; see map below the mosaic.
Major C.E. Dutton, an early explorer of the American West, described these mountains as they appeared on a map as resembling "an army of caterpillars crawling northward out of Mexico". That is re-enforced by the next image, a shaded topographic depiction of the region made from DEM data (method described on page 11-5; another example is on page 7-2). This rendition emphasizes the notable flatness of the valley floors in the basins.
This aerial oblique photo brings out the strong parallelism among neighboring mountain ranges in Nevada.
Such characteristic topography in the Basin and Range results from a complex structural history dominated by block faulting (somewhat like the faulting we described in Kenya on page 3-2). The region, as it underwent tensional stresses during uplift of the crust while the Sierra Nevada mountain block to the west and the Southern Rocky Mountains to the east were forming, responded by fracturing. The fractures trend mostly north-south, along which segments dropped down (faulted), leaving adjacent range blocks higher. The present elevations can be greater than 2,700 m (8,900 ft), producing relief of 900 to 1,520 m (3000 to 5000 ft) relative to the basins. Among the major ranges in this scene are the Shoshone, Toiyabe, Toquima, Monitor, Ruby, and Hot Creek Mountains; higher slopes are forested, as suggested by the reds in this September scene.
Although most of these mountains are not high (moderate relief), they can appear imposing as they are approached. Here is the Toiyabe Range as seen from a basin to its east:
The intermontane basins between ranges are back-filled with great amounts of rock debris descending downslope, so that the valley floors move upwards as the ranges wear down. The central (lowest) surfaces of some of the basins contain playas (deposits of fine sediment left after intermittent lakes evaporate following the rainy season) that are light-toned in the image. Between playas and ranges, along the transitional zone known as a piedmont (literally, "foot of the mountain"), are deposits of coarser sediment (up to cobbles and boulders in size), mixed with clays that make up alluvial fans (dark gray in the image).
The ranges can be quite distinctive, standing out between valleys as bare rock, since many are almost devoid of arboreal vegetation. This scene is in Nevada:
Nevada has experienced relatively recent volcanism. Parts are covered by thick beds of volcanic ash. Basaltic lavas also extrude from vents. This Landsat image shows the Timber Mountain caldera (lower right), another caldera atop a mountain (upper right), and several lava flows:
Most of Nevada is "owned" by the Federal government. That was one reason why a large tract covering hundreds of thousands of acres was selected to develop the Nevada Test Site where first surface and then underground nuclear testing was carried out for several decades (usually in volcanic ash deposits). This "secret" base was entered from Camp Mercury about 75 miles north of Las Vegas (in a recent relaxation of the off-limits policy, since underground tests are no longer carried out, the public can now tour part of this facility). The writer (NMS), when he was part of the Atomic Energy Commission's Plowshare program, visited the NTS many times over five years. The principal areas within NTS, where most of the nuclear explosions took place, are Yucca and Frenchman Flats. This Landsat-4 TM image shows that part of the basin complex, the Rainier Mesa on the left, and limestone mountains to its right. The large white patch in the upper right is Groom Lake, a playa near which is the "infamous" Site 51, supposedly where super secret military tests were carried out (I never visited this area but could hear a rocket engine being tested there). Below the image is an enlargement of the Yucca Flats section of the scene in which are a number of craters. Several, such as Sedan (see Section 18) were explosion craters but the majority were collapse craters caused when an underground nuclear explosion produced a cavity which then failed allowing the alluvium above to sink into it, leaving a circular depression at the surface.
Just to the northeast of Yucca Flats around Groom Lake is the famous (more properly, infamous) Area 51 which is probably the most supersecret stretch of land in the United States. Here are a Russian KVR-1000 photograph and an IKONOS image of facilities within this area, which lies about 110 km (75 miles) northeast of Las Vegas:
The mystery of the nature of government activities at Area 51 remains. Many think it is where either the U.S. military builds radical new airplanes or keeps captured UFOs. The U.S. Air Force does have a presence there. One confirmed activity is the testing of rocket engines.
Here is a space image of part of southern Nevada that contains Las Vegas (green patch), which we examined earlier on page 4-1. East of L.V. is the Hoover (Boulder) Dam which has created Lake Mead on the Colorado River.
Nearby Lake Mead is the largest manmade freshwater lake in the U.S. This is a close-up satellite view:
This IKONOS high resolution image shows the Hoover dam and the gorge in which it was built in the 1930s:
Now, back to the confirmable. Another typical block fault range occurs north of Tucson, AZ and is capped by a pine forest. At the foot of the Santa Catalina Mountains, shown below, is a thick, deeply gullied alluvial fan. You can check the setting of this area in the images of the Tucson, AZ area on page 4-1.
The dry desert air in the Tucson region means that materials are less susceptible to material erosion or degradation. Near the Davis-Monthan Air Force Base is a stretch of open land where hundreds of surplus (retired) military aircraft are kept parked - mainly as a reservoir of spare parts for other planes. Here is a Digital Globe space view of some of these:
One of the classic areas within the Basin and Range, near its southwest margin, contains Death Valley and other block fault ranges. We show it below along with a map indicating the names of the principal topographic features.
Use this map to locate mountain ranges in this MODIS image of much the same area between the southern tip of the Sierra Nevada and the Spring Mountains near Las Vegas. The Owens Mountains are the long slender range east of the Sierras. Look particularly for the Panamint Mountains, a range whose tallest peak is over 11000 ft, as shown in the photo below the image:
Below the Death Valley area, the Basin and Range ends against the Mojave Desert. That region is located within the structural salient north of the southern California Transverse Ranges (see page 7-2).. Small ranges within the Mojave closely resemble the barren block fault mountains of the Basin and Range types. In west central Mojave Desert one of the larger towns in this desert terrain (where summer temperatures can reach 120° F) is Barstow. It is near several small mountain ranges and is just north of Apple Valley and Victorville at the edge of the San Bernadino Mountains of the Tranverse ranges. Note the alluvial fan (blue) made from an ephermal stream coming from those mountains. Here is a color radar image of an area near Barstow, California, made from SIR-C C- and X-band imagery (see page 8-7).
The Basin and Range type of geology is widespread over western North America (see map above). From Oregon-Idaho through Nevada and Utah, it spreads east into southern Arizona and New Mexico into northern Mexico itself. One area of classic ranges and valleys is around Phoenix, along with Las Vegas the two fastest growing large cities (each over 1,000,000 people) in the southwest U.S. The next image was an experiment done in the early days of Landsat-1 to use MSS Band 7 (IR) to render vegetation green. The fertile (irrigated from reservoir lakes) land in the Salt River Valley within which Phoenix was built shows up as actively growing in this February, 1973 image:
Part of the sprawl of fast-growing Phoenix appears in this aerial oblique photograph:
Around and south of Phoenix the mountain ranges are small and isolated. North are the high, forested Mazatzal Mountains - a much larger block that includes also the Sierra Ancha and Pinal Mts. In the latter, note a pinkish red patch, which marks mine waste from the Globe-Miami-Superior copper mining district. In these mountains is the dammed Roosevelt Lake: it is a wonder on weekends to find 1000s of power boats being hauled from Phoenix to cruise those waters.
The desert lands described above have a surprising paucity of sand dunes. One dune field that lies near the borders between California and Nevada, and the Mexican state of Sonora was photographed from the International Space Station:
Water from the Colorado River is diverted into the All-American Canal (visible in the image above). This 129 km (80 mile) long canal is the longest of its kind in the world. Part is shown in this ASTER image, which encompasses the uninhabited desert. The photo below shows the canal as it enters the Imperial Valley of California:
By now, you have gained experience with picking out features in the landscape at the medium scales presented in Landsat imagery. Before finishing our trip, we challenge you to apply your experience by locating or identifying geographic and geologic landmarks in a space image somewhat to the south of our main flight line. This image is much smaller in scale, covering about 1,100 km (684 miles) on a side. Make this a game of finding the places listed below. Look over the black and white Day-Vis HCMM image that shows most of the southwestern U.S. (part of southern California and Nevada, most of Arizona, a bit of Utah, and small segments of the Mexican states of Baja California and Sonora), including some of the Basin and Range.
6-11: Use a U.S or World Atlas to aid in correlating landmarks in the scene with mapped features. To help you get oriented, the red numbers 1, 2, and 3 are the Salton Sea, Lake Mead, and Lake Powell, respectively. Pin these down on an atlas to give you a feel for the scale. Then, relying on the maps, identify what is at or around the numbers 4 through 10. (The answers are below). Finally, without the aid of number guides, try to find the Gulf of California, San Diego, the Imperial Valley, the Sierra Nevada range, Las Vegas, Phoenix, and Tucson. Also, what is the name of the great geologic fault that marks (as a straight tonal boundary) the southern edge of the Mojave Valley? ANSWER
We now swing northward to the interior U.S. West north of the Basin and Range. Several geomorphic provinces come together in northern Utah, northwestern Wyoming, Idaho and Montana. This MODIS wide-angle image shows these states and the landforms within:
The MODIS image points to a westward deflection of the Rocky Mountain belt of folded rock ranges as it enters Montana. This next Landsat scene shows the style of expression of the mountaneous topography - one of individual blocks separated by broad valleys:
In the image are the towns of Helena, Bozeman, and Livingston. The ranges include Madison, Gallatin, Tobacco Root, and Crazy. The light blue patch just in from the center left margin demarks the tailings pile from the Butte and Anaconda copper mine district.
In northern Montana is the popular Glacier National Park, seen here as a space image and a ground view. The geology is dominated by sedimentary limestones, exposed in rugged peaks now steepened by glaciation. The nearby Lake Louise, in Banff National Park, Alberta, Canada shows the layering of limestones even better:
Within the Northern Rocky Mountains, individual ranges are given their own names. Here are a Landsat image and then a perspective view made from SIR-C data of the valley containing Missoula, MT (the state's largest town) as it is bounded to the east by the Mission Range.
The Idaho segment of the Northern Rocky Mountains begins with several NW-trending individual ranges, separated by valleys and a more continuous mountain mass to the west, as seen on the left side of this Landsat-1 image:
Idaho' state capital, Boise, is typical of a small western mountain city (but big enough for Boise State University's football team to have been in the top 25). We see it first in a ground panorama and then from space:
On previous pages in this Section note was given to sand dunes - a hallmark of deserts and desert like physiographies - in various states including Nebraska and Colorado. Idaho, although more north and in a climate that has a semi-temporate feel, also has sand dunes. The field shown below lies between Idaho Springs and Yellowstone Park. It was produced during the last ice age less than 10000 years ago.
The northern end of the Basin and Range abuts against the Snake River Plains in Idaho, seen here in this Landsat-1 full scene:
The Idaho Rocky Mountains are seen near the top. The Plains themselves are a series of bluish-gray lava flows (mostly basalt) that began some 10 million years ago. More recent is the large Craters of the Moon National Monument - the conspicuous black blotches - seen in two Landsat views. Water from the Snake River is used to irrigate the farms that produce Idaho potatoes, sugar beets, barley, wheat.
This colorized SIR-C radar image takes a closer look at the Craters of the Moon.
In 1963, when the writer (NMS) was working in the AEC's Plowshare program (at Livermore, CA) for engineering uses of nuclear explosives, I was given the responsibility of finding a site in the plains basalts for Project Schooner - a proposed 100 kiloton nuclear explosion in basalt designed to make a crater (this was a prototype experiment to determine feasibility of using nuclear devices to dig a second Panama Canal in Central America). The most promising area was Bruneau Canyon just west of the left edge of the above false color Landsat image. After more than a month on site, and several expensive drill holes, I recommended abandoning this once promising area (the county where the detonation would have taken place has 25000 cows and less than 5000 people - logistically favorable) when unexpected lake beds were found buried by younger flows near the surface - these would have compromised the diagnostics of the cratering event by creating multiple echoes. (The beds consisted of loose, fine-powdered dust; the dust clogged local roads and got into auto engines; one day 4 of 5 field vehicles were out of commission requiring us to leave the area clinging to a drilling water truck). Here is a space view of the Bruneau Canyon area and the Canyon itself.
The Snake River Plains are the eastern extension of the Columbia Plateau. That physiographic unit is dominated by a series of flat-lying stacked units of basalt that were extruded over a wide area in Tertiary and Quaternary times. Here is a map of the Province.
The best exposure of basaltic lava flows in the Columbia Plateau is Hells Canyon, a gorge cut by the Snake River.
Typical Plateau terrain, consisting of relatively gentle rolling plains, is shown in this astronaut photo that displays the junction of the Snake River (right) with the Columbia River just south of Richland, Washington. The Hanford nuclear energy research facility lies to its north.
Both the Basin and Range and Columbia Plateaus conjoin with Pacific Coast physiographic units, examined on page 6-9.