Southwestern U. S. Geology Gallery

IMPORTANT NOTICE: All photos copyrighted by Raymond Wiggers. If you are an educator or student and would to like to use any of these images, e-mail me and let me know how the images will be used. Please also credit me as the photographer. I ask that all companies, organizations, and government agencies contact me about my fees for the use of my photos, and about obtaining higher-quality versions on CD-ROM. Thanks for your understanding and compliance with the law.

To go directly to a particular state represented in this gallery, click on its name below:

- Arizona

- California

- Colorado

- New Mexico

- Trans-Pecos (Westernmost) Texas

4. At the Kelso Dunes site, looking east toward the Providence Mountains. This range, like so many others in the American Southwest, is a manifestation of the extensional tectonics of the Basin and Range Province. In this vast region, about 300,000 square miles in extent, the Earth's crust is being stretched apart. As a result, large blocks -- the basins -- have been dropped downward along normal faults. They are bordered by upthrust blocks -- ranges -- such as this one.

6. Above the tree line and facing the Nokhu Crags, a snaggle-toothed ridge at the northern end of the Never Summer Range. This type of ridge, originally carved by small valley glaciers, is known as an arete. Note that the fans of rock debris or talus come in two distinct colors: battleship gray and plum purple. This reflects the fact that they're derived from different types of bedrock exposed above them.

5. The Monument, located near the city of Grand Junction, is most famous for its scenic overlooks. The stratigraphic section ranges from Precambrian rocks in the canyon bottoms to Cretaceous strata at the top. This is Colorado Plateau country: a land of essentially flat-lying strata that has been raised to great altitude for reasons that are still not abundantly clear. Beyond the mesa in the foreground lies the valley of the Colorado River.

7. The view northward and a little to the right of the previous shot. In center foreground is Lake Agnes, a tarn or perched, glacially derived pond. To its right, the bicolored talus from the Nokhu Crags; in the center distance, the Medicine Bow Range. As majestic and seemingly eternal as they appear, individual mountains are, as the great nineteenth-century geologist John Wesley Powell put it, ephemeral landforms. They are continually besieged by the forces of weathering and erosion, as the huge amount of talus in these photos testifies, and in relatively short order they are brought low. However, they are often replaced by new high ground formed from their roots, through a process of rebound geologists call isostasy.

8. At a lower altitude, on the shore of Lake Agnes. Note that the bank is still snowy in July -- as it should be, given its range's name. Note the bulging, lobate mass of rock debris looming just above the trees and the tarn. In all likelihood, this is a rock glacier, which has some of the same properties of form and movement as an ice glacier. It may even contain the latter, a survivor from colder times that is now hidden by bouldery camouflage.

10. Rabbit Ears Peak, east of Steamboat Springs. This distinctive landmark is another demonstration of the formidable powers of erosion. It was formed when the softer rock of a volcano's exterior was sculpted away to reveal the harder material of the neck, or lava-plugged vent, that once brought ash and molten rock to the surface. And notice the radial dikes, also composed of more resistant rock, that extend downhill and into the foreground.

45. This picture, taken in the Franklin Mountains along Route 375 north of downtown El Paso, contains about a semester's worth of geology. For one thing, this range contains some of the most ancient rocks in the Lone Star State. They date to the Middle Proterozoic Eon, about 1.4 billion years ago. Above them, and quite visible even in the thumbnail, are the reddish Cambrian strata of the Bliss Formation, capped by the lighter and still younger Ordovician El Paso and Montoya Formations. The west-ward dip of these upper layers, so obvious here, is due to the fact that the Franklins are a block of the Earth's crust that was tilted backward as it was thrust up relative to the flat-floored basin just to the east. (Also see the discussion of Basin and Range landforms accompanying Photos 4, 19, and 31, above.

46. Another view from Route 375, also known as Trans-Mountain Road. The roadcut at center, on the other side of the highway, exposes the Proterozoic Castner Formation, consisting of such metamorphic rocks as slate and marble. At right, in the distance, stretches the floor of the down-dropped basin to the east of the Franklin Mountains.

2. Detached chunks of Supai sandstone and conglomerate, at the same locale. Note the diagonal crossbedding pattern formed by more or less parallel laminae of sand. Such crossbedding is the planar variety, formed by deposition on a dune slipface, in a stream channel or delta, and in other environments, too.

3. A view of the Kelso Dunes, Mojave National Preserve, in San Bernardino County. This great dune field is situated in a basin bordered on the south by the Granite Mountains and on the east by the Providence Mountains. This is a superb place to see both active dunes that are still migrating -- some are over 600 feet high -- and those that are fixed by vegetation. As you walk up a windy slope, small lizards scamper ahead of you and dive into the sand, like dolphins playing at the prow of a ship. Also, as many visitors have noted, the windblown sand moving down the slipface often generates a booming sound that can be rather unnerving.

47. A closeup of a Route 375 roadcut showing the very steeply dipping slate beds of the Castner Formation. Note the light-colored dike of Red Bluff Formation granite that cuts diagonally across the slate layers. Dikes are formed when magma -- molten rock -- finds its way into fissures of preexisting rock.

49. It's only about half as old as the Castner Formation, but the age of this Ordovician El Paso Formation limestone, at the southern end of the Franklins and along Scenic Drive, is still almost unimaginable by human standards. The rock hammer provides scale.

50. This jumble of loose rock debris, along the Route 375 shoulder, represents a much more recent chapter in geologic history. Termed colluvium, this rubble has been brought down its mountainside by the forces of gravity and water in sheet flow, rather than by streams in distinct channels.

51. Yes, it really is that white, and no, it isn't snow. In the heart of the Salt Flat bolson, along Route 180, with the Guadalupe Mountains displaying the high, dark mass of the Capitan Reef in the background. As noted above, bolson is a regional term for a down-dropped basin between fault-block ranges. Here, evaporite minerals, especially gyp-sum and halite (rock salt), have crystallized after being deposited by an ephemeral lake that will reappear whenever there is sufficient runoff from the surrounding hills. This lowpoint in a bolson is called a playa.

52. Looking like the ramming prow of an advancing battleship, the southern end of the Guadalupe Mountains provides one of the Southwest's quintessential vistas. Worthy of of federally protected status even if they weren't a geologic marvel, the Guadalupes are in fact an exhumed, 260-million-year-old tropical reef and its underpinnings. In Permian times, much if not all of this scene would have been under water. The upper portions of the mountains at right were the reef proper; El Capitan, the resistant peak at center, was originally part of the forereef, where debris, ripped by surf and storms, came to rest. Below and in front of the mountains are the strata formed from deposits that collected in the deep-water basin nearby.

55. Another fascinating feature at the Brushy Creek exposure, just a few meters from the channel-fill site. The bowed strata shown here do not indicate regional compression of the crust after the sediments turned to rock. Rather, they point to localized deformation of water-logged sediments long before the sand was cemented together.

53. One of the Delaware Basin roadcuts most frequented by geologists is this exposure of the Permian Brushy Creek Formation, along Route 62/180 just south of the Guadalupe Mountains. This and other exposures in the area illustrate the dynamics of deposition in deep water close to a continental shelf. At this spot there is an excellent example of channel fill, where a body of sand, borne by a submarine current surging down a slope, scoured out and then filled a channel (represented by the light-colored rock at rght) through preexisting sediments (the darker layers at lower left).

54. A closeup of the lefthand side of the channel cut by the underwater torrent of sand. That sand, now a massive, buff-colored sandstone, has many joints. These fractures were caused when, over a long span of time, the heavy layers of rock originally sitting atop this formation were eroded away. Relieved of the confining weight, the sandstone was free to expand and crack.

56. From the Permian Reef Trail, in the McKittrick Canyon section of the national park. The projecting crest of carbonate rocks, the upper reaches of the Guadalupe Mountains, is the reef; El Capitan is out of the picture to the south (left).

11. This cliff face near the northeastern terminus of the park's Walnut Canyon Scenic Drive shows massive back- reef deposits of limestone and other sedimentary strata laid down in the Permian, during the formation of the world- famous Capitan Reef also exposed at Texas' Guadalupe Mountains National Park (see below) . This area boasts one of the most extensive cavern complexes on the planet. If you click on the thumbnail photo to see the magnified view, you'll notice, if you look carefully, a sharply upward-bent section of rock layers above the cave mouth. This is one of the park's enigmatic tepee structures.

12. A closeup of another tepee structure in Walnut Canyon. There are several theories that seek to describe the origin of these unsual features. One states that seawater seeped into the sediments that now form these strata. When the water evaporated, carbonate minerals precipitated and pushed the layers upward.

13. And the most photographed and scrutinized tepee structure of them all, shown here in postcrepuscular light. This most accessible example is on the edge of the park's Visitor Center parking lot. It is part of the Permian Tansill Formation, though some of the others in the park are in the underlying Yates Formation.

57. A wash near the park's McKittrick Canyon visitor center. This is the cobble-plated bed of an intermittent stream -- one that does not flow, at least at the surface, for much of the year. Desert washes can, however, be bone- dry one moment and inundated by a flash flood the next.

39. If you drive between Marfa and Alpine on U.S. 90, you have your choice of two marvels -- one enshrined in mass culture and completely bogus, and the other relatively subtle to the untrained eye but certified genuine. The first is the designated viewing area for the Marfa Lights, described by the eternally credulous as UFOs or other occult phenomena .(In fact, they are atmospherically distorted headlights of vehicles on Route 67 south of Marfa.) The second is that you traverse, in the vicinity of Paisano Pass, the inner workings of an ancient volcano. Though thirty-five million years of erosion have leveled much of its bulk, splendid roadcuts near the turnoff to the Paisano Encampment reveal the interior of its caldera -- the zone of catastrophic implosion that was formed when the volcano's supply of magma was depleted and the chamber that once held it collapsed. Here, the maroon breccia that bears proof of the collapse is transected by a dike of light-tinted rhyolite.

40. At the same roadcut. A closeup of the collapse breccia -- sedimentary rock formed from angular chunks of other materials originally part of more homogeneous rock units. Among the rock fragments present here are white rhyolite, a form of solidified lava rich in quartz -- it's the extrusive equivalent of granite -- and black, quartz- poor basalt, another type of hardened lava.

41. A few meters from the previous shot. This is an exam-ple of slickensides -- a scratched and polished surface that indicates a.fault, where there has been relative displacement between two adjoining masses of rock. This fault may be the result of the main episode of caldera collapse or of subsequent movement.

42. South of the town of Marathon, just to the west of Route 385, stand a string of flatirons. These features, the light-colored landforms in front of the distant hill, are formed from steeply dipping and resistant strata that have been shaped by streams that cut through them as the preexisting rock surface was eroded. The flatirons are composed of a white chert -- a very hard rock composed of chemically precipitated quartz -- named the Caballos Novaculite. This formation is Silurian to Devonian in age.

43. Farther south from the previous site, just off Route 385 and close to the Persimmon Gap entrance of Big Bend National Park, is the Stillwell Ranch. Here collectors hunt for agates and other precious minerals for a reasonable fee, after checking in at the ranch store. This hillside in the Ranch demonstrates that most desert floors are not covered with blowing sand, as many folks from wetter climes suppose, but with the stony surface geologists call desert pavement.

44. Overlooked by most mineral collectors but at least as significant as any sought-after agate is a type of pale gray sedimentary rock found in abundance on the Stillwell Ranch. Of Cretaceous age, it's calcarenite (a quartz-rich limestone) . This particular specimen shows two inter-esting features that give clues to both the modern climate and the ancient environment in which the rock was formed. The beautifully preserved pattern of polygonal cracks, a sort of nonbiologic fossil, show that the sandy lime mud dried out in the open air and sun before it was buried by younger sediments and turned to stone. (See the DeKalb County section of my Illinois Geology Gallery for a modern equivalent.) In contrast, the much deeper fissure at right is a relatively recent occurence, and a textbook example of a type of physical weathering very common in deserts. Rocks are poor conductors of heat; when they finally warm up in the searing daytime heat, they retain that heat long after sundown, while the temperature of the surrounding air plummets. It's similar to plunging a heated glass vial into icewater: the thermal stress suddenly fractures the rigid stone, often with an audible crack or pop.

14. Heading south from the park's Persimmon Gap entrance. The gash in the hillside, glowing in the low- angle light of late afternoon, is a rockslide scar. Slides like this are one form of mass wasting -- the downhill movement of material on steep slopes. Notice the line of white dots in the sky, which are visible when you double-clik the thumbnail. As much as I'd like to suggest they're a squadron of UFOs (or maybe the Marfa Lights taking a daytrip -- see the text for Photo 39, below) they are actually markers on a power or telephone line. Imagine the number of hits this website would get if I did let my quest for truthfulness lapse a little.

16. Nothing speaks more eloquently of the mysteries of time and timelessess than a canyon in the West. The park has three such great gorges cut by the Rio Grande: Boquillas, Mariscal, and Santa Elena. A walk down the Boquillas Trail, near the park's eastern tip, inspires reverence, or should. The stream that now marks the U.S.-Mexican border has sliced its way through the imposing Sierra del Carmen. Exposed on the wall faces are, from bottom to top, the Del Carmen Limestone, the Sue Peaks Formation, and the Santa Elena Formation, all of Cretaceous age.

15. Geology's closest approximation to the Mr. T Look, on the road to Boquillas Canyon. It looks as though this hummock of shale of the Cretaceous Pen Formation is sporting a Mohawk haircut, but in fact the narrow spine that surmounts it is a dike formed from lava that filled a fissure in the host rock and then was partially exposed by erosion. I pity the fool who visits the park and doesn't take the time to invesigate this intriguing feature.

17. The Mexican side of the Boquillas Canyon. The great cliff-forming formation of the upper canyon walls is the Santa Elena, which here reveals a magnificent fault (the diagonal trace running from upper right to bottom center) . The cliff section on the left side has moved down relative to the right side -- as gravity would seem to dictate, given the angle of the trace. This type of rupture in the Earth's crust is therefore sometimes called a gravity fault, though it is more commonly referred to as a normal fault.

20. The park's centerpiece and a haven for rare plant and animal species, the Chisos Mountains loom above the Chihuahuan-Desert floor. The eroded remains of extrusive and intrusive igneous rocks, the Chisos highland provides ample evidence of the Big Bend region's violent volcanic phase, which lasted from about 38 million to 20 million years ago. This was followed by the establishment of the modern Basin and Range topography, where the stretching of the crust created a series of titled, fault- block mountain ranges and flat bolsons. (The term bolson is a synonym for fault basins or grabens, and means purse in Spanish.)

22. The mouth of the 1,400-foot-deep Santa Elena Canyon. The human visitors on the trail help provide scale. The ascending sequence of formations exposed in the walls is the much same as it is in Boquillas Canyon, but at the base and by the footpath the Telephone Canyon Formation is also present. Note how the massive, vertical-cliff- forming limestones of the Del Carmen and Santa Elena Formations are stained with black managanese oxide -- chemical weathering on an especially grand scale.

24. Humankind as a geologic force -- sort of. This chunk of limestone fallen from the cliff face has acquired a high-grade polish on its upper surface that makes the rock look as glossy as artist's marble. Apparently, the polish is the work of generations of human posteriors, because this perch, with its breathtaking view up the canyon, is irresistible to resting hikers.

25. A small faulted wedge of limestone at the same locale. One can discern at least several other, bigger faults in the canyon's walls, but even they do not compare in scale to the great Terlingua Fault that separates the highlands containing this canyon from the low, downdropped block to the north of the canyon's mouth. The vertical displacement between the two sides is approximately 3,000 feet, or almost 1 kilometer.

21. In the Chisos Mountain's picturesque Basin, facing west on the Window View Trail. At an altitude of approximately 1,500 feet (460 meters) above the surrounding desert floor, this locale has a cooler microclimate that comes as a most welcome relief in the summer months.

26. Tuff Canyon, carved by Blue Creek, with the Chisos Mountains in the background. This is one of the best places in the park to examine the region's turbulent volcanic past. Here basalt lava flows are overlain by layered deposits of what geologists call lapilli-tuff. This rock type is formed by eruptions that produces hot clouds of gas and debris that roll down a slope. If the descending cloud hugs the ground, it is a pyroclastic flow; if it billows upward more, it's a pyroclastic surge. In places in the canyon one can also spot debris flows, where viscous mixtures of water, mud, and large rock fragments have been deposited instead.

27. On the floor of Tuff Canyon. In this section of the wall, the erosive power of Blue Creek has revealed both the record of volcanic activity and a striking structural feature as well. A normal fault transects the middle of the rock, and you can see how the relatively horizontal beds have been displaced downward on the right side of the gash.

28. In many places, large chunks of rock, termed ballistic blocks, are embedded in the pumice-lapilli. This and the other big basalt fragments were part of the material carried downhill by pyroclastic surges -- a testament to the surges' ability to transport big objects some distance before they fell to the ground. Note the depression under the block when the latter slammed into the preexisting deposit. The pen placed beside the ballistic block provides scale.

29. Just down the Ross Maxwell Scenic Drive from the Tuff Canyon pulloff. Cretaceous beds in and near the park bear impressive examples of petrified wood, and it seems reasonable to suppose that this is a tree trunk that was preserved in its original vertical position. But Big Bend is one of those places where reasonableness is not always the order of the day. Actually, this stumplike feature is is a small volcanic neck, where lava congealed in a vent and, millions of years later, was exposed when the softer surrounding rock was eroded away.

19. North of Boquillas Canyon stands the rampart of the Sierra del Carmen. This great limestone cliff, so beautiful in the rose and golden tints of sunset, stands near the southeastern tip of America's great Basin and Range Province. At the foot of the cliff runs a fault where the sierra, a horst or fault-block range, meets the graben or downdropped basin.

30. Do you do hoodoos? This roadside attraction, which looks as though it were designed by the great Catalan architect Gaudi, stands along the state park's Route 170. This paved two-laner features some of the most magnificent scenery and brain-tickling geology in the whole solar system. A hoodoo is a grotesque erosional landform, which in this case has been carved out of a deposit of tuff, or somewhat-hardened volcanic ash.

31. Farther west along Route 170. The Rio Grande mean-ders close to the road; beyond it is Mexico. While most bolsons in the Southwest are closed basins with no exterior drainage, this one and others aligned with it share the unusual distinction of having been breached by this major stream. How the river accomplished this is still a matter of debate.

32. To explore the heart of the state park one must be willing to leave the paved highway far behind and take gravel roads that lead into the heart of what was, until recently, the private holdings of Big Bend Ranch. Along the way to the park headquarters, in the Cuevas Amarillas locale, are low yellow cliffs of Rawls Formation tuff, which is not particularly tough. As seen in Photo 30, it often erodes quite readily into curious shapes. Note the slump blocks behind the ocotillo shrubs. The blocks have fallen away from the cliff. Some have been cracked appart, presumably by thermal stress.

33. A noteworthy form of biological weathering in a Cuevas Amarillas rock shelter. Here the transforming agent was Homo sapiens -- American Indians who bored pits into the tuff for the grinding of the legumes and grains that probably were a significant part of their diet.

34. For the geologist, the Big Bend region's Holiest of Holies is the evocatively named Solitario, a great oval dome in the Earth's crust some 9 by 8 miles (14 by 13 kilometers) . This is its outer rim, seen from the northwest. The structure began to form some 36 million years ago, when a great mass of magma pushed preexisting strata upward. Some of the magma actually reached the surface and caused what were no doubt spectacular volcanic eruptions. Special permission is required to enter the Solitario; the rocky and rutted state-park road into it is challenging in places. To see an excellent satellite image of the whole uplift, go to this NASA site.

35. The northeastern rim of the Solitario, from the inside of the dome. On the cliff the Caballos Novaculite (see also Photo 42, below) has been contorted into compressional folds. This deformation occurred long before the creation of the dome. In fact, it's part of the southwesternmost exposure of the eroded Ouachita Mountains. This range, like the ancient Alleghenian Mountains of the Eastern states, owed its existence to the the construction of the supercontinent Pangea, some 300 to 250 million years ago. The folds here reflect the collision of ancestral North and South America.

36. In the Solitario, 1.1 miles (1.8 kilometers) north of Tres Papalotes. At a water gap steeply dipping dark chert of the Ordovician to Silurian Maravillas Formation crops out. Like the Caballos Novaculite, ths chert is a very resistant rock formed of quartz chemically precipitated at the bottom of a long-vanished sea.

37. Another Solitario outcrop, about 0.9 mile (1.6 kilometers) north of Tres Papalotes. This is the Tesnus Formation. (The name Tesnus, incidentally, was coined by spelling Sunset backwards.) Younger than the Maravillas but still venerable by Big Bend standards, these strata are Mississippan to Pennsylvanian in age. They too were tilted by the Ouachita mountain-building event. Note the top of the exposure, where the beds have been bent, much more recently, to the right, i.e., downslope. This is a good demonstra- tion of the mass-wasting process called soil creep (for more evidence of this process in a very different environment, see Photo 26 of my Illinois Geology Gallery.

58. From a hilltop on the Woodward Ranch, off Route 118 south of Alpine. This is another favorite venue for agate collectors. In the foregrond, the heavily grazed surface could be mistaken for a Viking landing site on Mars, were it not for the cholla cactus and the other vegetation in the distance. This is a land of basalt lava flows that poured forth some 35 to 20 million years ago.The peak that dominates the skyline is Cathedral Mountain.

59. A roadcut along Route 118, not far south of the preceding locale. Here a darker basalt flow lies atop an older, reddish flow. In between, just below the the slight overhang, is a paleosol (ancient soil) that formed before the younger eruption buried it. For more on the significance of this site's paleosol, see Photo 37 in my Soils Gallery.

1. One of the most dramatic physiographic boundaries in the United States is a high scarp that runs from west to east through the central section of Arizona. Known as the Mogollon Rim, it divides the high Colorado Plateau from the Basin and Range region to the south. Here at an old uranium-mine site at Promontory Butte, just east of Kohls Ranch, three great Southwestern rock units, also on display in the Grand Canyon, are visible. In the foreground, the gray Supai Group is overlain by the reddish Hermit Shale Formation; high in the background is the cliff-forming Coconino Sandstone Formation. All are of Permian age, with the exception of the lower part of the Supai, which is Pennsylvanian.