Strangers in a Strange Land: Just how flat is flat?

We've looked at two playa surfaces now in the series Strangers in a Strange Land, our tour of Death Valley and the Mojave Desert of eastern California. The first was the salt pan at Badwater, the lowest point in North America at minus 282 feet, and Bonnie Claire Playa, where rocks seem to have a mind of their own.

Playas have to be counted as pretty much the flattest places on Earth (abyssal plains on the deep ocean floor are pretty flat, too, but aren't exactly accessible). And they are flat. When rare flash floods bring clay-rich water onto the playa surface, the water spreads out, and the clay stays in suspension long enough to be spread evenly across the lake.

I was thinking about flatness today when I was running errands at the department. Topographic maps are the tool of choice for geologists for mapping and plotting. They are also used by anyone who really wants to know where they are when backpacking, hiking, camping, hunting, or whatever. I generally blame topo maps for dragging me into geology. I was a scout, and I was fascinated by the maps we used while hiking. I collected them, I practiced my orienteering/compass skills constantly, and eventually became the "go-to" guy when we were out in the wilds. They called me Captain Eagle-Eye Wrong-Way, because it was just about impossible to get me lost if I could see any two mountains and had a compass and map (even today students ask "How far?" and when I answer, they say "Garry-miles or real miles"?).

A couple of years back I was going through the department map collections, and I found a map that I realized would defeat my efforts at orienteering. I saluted the map-makers, and posted the map on the wall in the department to be a shrine to the pratfalls of topographical arrogance. It is pictured below, and people familiar with contour lines will notice a near complete lack of them. The red lines form a grid made of square miles, so this map is about 6 1/2 by 8 miles, or about 52 square miles. The contour interval is 5 feet, which means that elevations across this entire quadrangle vary by only a few feet, primarily because of some 8 foot high dunes on the lower left-hand corner.

The map is part of the Carson Sink, a place in western Nevada where a number of rivers come to die. This was part of the ghastly last 40 miles where settlers in the 1840-50s came to grief as they tried to reach the California gold fields. Even today, a notation on the map mentions "no roads or trails". Well played, topographers, well played!

Strangers in a Strange Land: Even the stones have minds of their own...

Death Valley is the very epitome of (easily visited) remoteness and loneliness. There are outposts of civilization at Furnace Creek, Stovepipe Wells, and Scotty's Castle, but the rest of the park is a vast wilderness with only a few roads, paved or otherwise. I've called this blog series "Strangers in a Strange Land" because most humans are strangers here, and it seems like almost everything about this landscape is truly strange. One of those strange landscapes are playa surfaces. I previously talked about the Death Valley salt pan, one aspect of an interior drainage desert, but there are many other "lakes" in the park which never collected much in the way of salt. They have smaller drainage basins, and no river systems fed into them, so the most common mineral on the floor of these playas is fine-grained clay. They will occasionally have water following flash flood events, but most of the time they are dry, and the clay breaks up into millions of small, mostly hexagonal mudcracks.

Pure clay makes for poor soils, and plants do not grow on the playas. So...no plants, few animals or insects, no particularly interesting minerals. Why are these students so fascinated with the surface of the playa? Well, there is one thing on this playa that seems "alive"...it's the rocks. They've been moving around...

It's strange for large pebbles and small boulders to be on the playa in the first place if you think about it. Playas are the tail end of a flash flood sequence. There might be a mudflow careening down a desert drainage, tearing at the cliffs and carrying large rock fragments. But then the flood reaches the alluvial fans on the flanks of the mountains, and with the gentler slopes, the flood slows and the larger fragments drop out. The only sediment that usually reaches the playa surface is a brown slush containing only suspended clay particles. Sometimes there are rock outcrops found within the playa or along the margins, so the rocks can be easily explained. But how are they leaving behind trails?

By now, readers familiar with Death Valley are thinking that we are visiting Racetrack Playa in Death Valley National Park, but we aren't. Although the Racetrack is justly famous for the number and size of its moving stones, it turns out that moving stones can be found in at least eight other places across the Basin and Range Province in Nevada and California. I haven't been to the Racetrack yet (it is at the end of a 25 mile long gravel road), but I have seen the phenomenon at two other places: along Highway 50 on the Carson Sink in Nevada, and on this playa that we visited in February, Bonnie Claire, just outside of Death Valley National Park.

So, how is it that they move? If you are looking for a definitive answer, I am sorry, I don't actually know. No one truly does, because no one has ever seen it happen. I think we can safely dismiss stories of aliens playing jokes, elves, and pixies, and of strange magnetic forces. The stones lie on exceedingly flat surfaces, so gravity sliding is probably not involved. It seems that only one force exists in the Death Valley region that can account for the movement of these rocks: the wind.

Wind seems a real stretch. Even first-time geology students know that wind normally can only move sand and dust particles. It would take extraordinary conditions for wind to be able to push rocks that weigh many pounds. Yet it happens. Extremely high winds are not unusual in the Death Valley region, especially during and after winter storms. Studies are ongoing as to whether wet saturated clay is slick enough to allow for the movement of stones in such winds.

There may be another important factor. The playas where the rocks move are at higher elevations where freezing temperatures are common. If enough rain has fallen, water may cover the playas and freeze around stones sitting on the playa surface. It is suggested that broken sheets of ice around the stones can act as sails, helping push the rocks along. This idea is supported by rock trails that move in tandem, as if the rocks were physically connected. This explanation works for me, but additional testing of some of the rocks shows that ice cannot completely explain the movement (researchers put stakes around some stones, and they moved out, passing through spaces barely bigger than the stones themselves).

So the moving stones are still a bit of a mystery. One of these times, someone is going to brave a horrific windstorm and will be standing there with a camera when the stones are moving about. At that time we will be able to see the elves themselves pushing the rocks along!

Ron Schott has allowed me to post some of his photographs taken at Racetrack Playa in Death Valley National Park. The first is a gigapan 360 degree view of the playa (click on the image below to see how gigapans work; it's pretty amazing). More gigapans of Racetrack and other parts of Death Valley can be found here: http://www.gigapan.com/gigapans?gallery_id=7940&order=most_popular&page=2&per_page=10

More of his shots can be found on his Flicker page at http://www.flickr.com/photos/rschott/tags/racetrackvalley/

A final note: if you visit Racetrack or any of the other moving stone sites, please leave the stones alone. They are mysterious, but not mystical. They are normal run-of-the-mill rocks with no unusual properties other than being on a playa, and the position of each stone is recorded with GPS. Stealing the rocks (and that is what you would be doing) will screw up years of research. You will also take from the enjoyment from other visitors.

Strangers in a Strange Land: Even the volcanoes are a bit weird

Death Valley is a Strange Land, and we were strangers within it. Even the volcanoes are kind of odd, at least if you think volcanoes are big cones that put out lava flows and clouds of seething ash. The presence of the volcanoes is not strange in itself; it is a natural consequence of crustal extension. As normal and detachment faults broke up the crust, decompression allowed partial melting of the underlying mantle, producing basaltic magmas. Sometimes the magma rose into the crust, and melted the granitic rock into rhyolite lava that produced explosive caldera eruptions, including the one we observed in this earlier post. There was also some cinder cone activity at the south end of Death Valley, including a cone that was offset by a right lateral fault. But it is the northern end of Death Valley that has one of the stranger "volcanoes" that you might want to investigate if you ever visit.

Ubehebe Crater is a half-mile wide hole in the ground, around 600-700 feet deep. It's a bit unlike your stereotypical volcano, in that there are no lava flows or ash deposits. It is a big hole, but the walls of the crater are composed of orange and yellow lake sediment, and a nicely exposed fault (above). There are just little fragments of basaltic rock spread over several square miles, and a dozen or so craters. Ubehebe is the biggest, and Little Hebe (below) is one of the youngest, at least in appearance.

The other oddity is the bizarre layering along the margins of the craters (below). They are composed of bits and fragments and sedimentary rock and basalt that have been blown out of the crater onto the surrounding terrain. But no lava flows in or around the craters at all. So what was exploding?

Basalt is not as explosive as other types of lava, and other cinder cones in the region along the margins of the valley formed normal cinder cones and lava flows, which can be seen in the picture below. The Ubehebe Craters are more in the middle of the valley, but in a spot where a fault was present which allowed magma to rise through water-saturated lake sediment. The water was heated far past the normal boiling point because of the higher pressure at depth. Eventually the temperature rose so high that the water flashed to steam, and the ensuing explosions formed the craters and layering along the rim. Such volcanic features are called maars.

The National Park Service has recently completed a renovation of the parking area, and has posted new interpretative signs, including the aerial view of the site (below). It allows us to see the overlapping nature of the craters, showing their relative ages. The Ubehebe Craters have always been known to be young features, less than 10,000 years, but some recent research indicates they could be as little 800-2,100 years old. Media reports kind of played up the "recent" angle a bit much, suggesting they could erupt any moment (sillies, those things won't happen until December when the Mayan Calendar ends). I guess that is what the media needs to do to sell things, but at least people learn that there are things in Death Valley other than reptiles and sand.

Walking down into Ubehebe is a really breath-taking experience (I mean that literally, especially when one is trying to climb back out). I was struck by the youthful appearance of the slopes. One has to allow for the arid climate, but I have seen quarries and failed housing developments that look more gullied, and more recent.

Check out the miniature wineglass canyon below, with an alluvial cone forming the base, and the valley in the shadow above forming the cup. Sort of a Death Valley mountain range in miniature.

Ubehebe Craters are a fascinating place to visit, but are pretty isolated unless you are headed to Scotty's Castle. On the other hand, the Craters are on the road to one of Death Valley's strangest sights of all, which we discuss in the next post, even though I've never been there. But I have pictures of what makes it strange!

Strangers in a Strange Land: Salt and the end of all things

In February and March I made a series of trips to the Mojave Desert and Death Valley, and since then I have been conducting a blog journey through the basic principles of geology as exposed in this extraordinary desert environment (Strangers in a Strange Land). In the last post we were looking up at the Black Mountains and the strange Turtleback faults, but today we are looking down, down as far as we can look on the North American continent. We have reached Badwater and the salt pan of Death Valley.

The end of all things? Strange topic I suppose, but I have been watching Frozen Planet on the Discovery Channel over the last few weeks, and it has had an emotional impact. Not so much the cute animals and the little dramas of survival against their predators and so on. That gets to be manipulative after a while and loses impact with me. It was more watching the Emperor Penguins, and their struggle to survive as the last year-round non-flying vertebrate life form on the continent of Antarctica (Oh, except for this...).

Antarctica was once a lush forested continent. It once teemed with life, with dinosaurs that wandered through the trees and swamps, along with birds, amphibians, and later on, marsupial mammals. Antarctica was in fact the bridge by which ancient marsupials reached Australia for the first time, where they thrived.

But life in Antarctica was doomed. As long as the continent was still connected to the other southern continents, warm ocean currents kept the landmass temperate enough to hold back the formation of glaciers. But sometime in the early Cenozoic era, around 50 million years or so, the last connections to the other continents were severed. Antarctica was isolated, not just by water, but by cold ocean currents that surrounded the continent. The glaciers grew larger and larger, turning into ice sheets thousands of feet thick. As more and more land disappeared, animal and plant species disappeared, unable to adapt to the new frigid conditions.The thread of life frayed to the point of snapping.


Source: http://en.wikipedia.org/wiki/File:Kaiserpinguine_mit_Jungen.jpg

Except for those penguins. Somehow, they have continued to survive, adapting their reproductive habits to the seasonal patterns, standing their ground through the harsh winters, standing for months on end with an egg, and later a chick between their legs. The parents don't eat for months at a time, trading parental duties while their mates chase fish many miles away. They stand, literally, at the edge of survival in the harshest land imaginable on this planet.

What do these musings have to do with Death Valley? The "end of all things" can be interpreted any number of ways, and here, the extremes run the opposite direction. Death Valley is the product of the disruption of the Earth's crust, a disruption so complete that no river drainages in the province will ever reach the sea. Death Valley is the sump, the low place to which all waters flow, on the surface and underground. It is the end of rivers. And Death Valley is, like Antarctica, a place where animal and plant life face insurmountable challenges. Out on the salt flats life ends.

Death Valley was a pleasant environment once upon a time. During the Ice Ages, which dominated much of the last two million years (equaling about half the time that the Death Valley graben has existed), the valley collected glacial meltwater, and thus was a freshwater lake. The surrounding mountains were still semi-arid, but forests of juniper and other trees extended to the shores of the lakes, along with extensive grasslands. A diverse ecosystem thrived; mammoths, mastodons, horses, camels, antelope grazed here, and were hunted by carnivores like sabertooth cats, American lions, and wolves. At some point in time, a connection was made with the Colorado River drainage or some other river system now lost to us. So there were fish in the lakes.

No one could mistake Mono Lake for a verdant paradise, but forests come almost to the shoreline.

But things changed. Around 11,000 or 12,000 years ago the latest stage of the ice ages, the Tioga, ended. The rivers shrank to a trickle and gave out. The vast lake in Death Valley, over 100 miles long, and 600 feet deep, evaporated bit by bit, and with each dry season the salt content climbed. Finally, the last of the water left, and a vast salt flat was created. The desert baked in the summer sun.

Some animals migrated elsewhere, and but most went extinct. Bighorn sheep and coyotes are the largest animals still found anywhere in the region, but you won't find them here on the salt pan. It is one of the most sterile environments on the planet (the biologists will have to let me know if bacteria can thrive in a pure salt environment like this). And what of the fish?

Nope, no fish here anymore...

As pointed out before, Death Valley was the sump where all the regional water collected. It still is. The surface water is wildly inconsistent, but springs tap into underground reservoirs, and they flow on a year-round basis, and have been doing so since the last of the freshwater lakes dried up. A precious few species of fish took refuge in these springs and survive to the present day. In Death Valley the survivors are the pupfish (Cyprinodon). Like their penguin counterparts in the southern hemisphere, they survive in some of the harshest climates in the world. Some of the Death Valley species persist in water that reaches 100 degrees, and salinity that is three times that of seawater.

They're also hard to photograph...

The most endangered species is the Devils Hole Pupfish. The entire population occupies a single cavern opening in Ash Meadows just east of Death Valley. The population has hardly ever exceeded 300 individuals, but it sometimes dips to only two or three dozen. As if the 92 degree water and the precarious food supply (a ledge of algae covered rock) isn't enough, they sometimes have to put up with the bizarre effects of distant earthquakes (check out this video). But through it all the pupfish have found a way to adapt survive.

That's the entire population of Devils Hole Pupfish down there....

Death Valley is the hottest and one of the driest places on the planet. And the climate is warming. What changes are in store? It is hard to know. But I have a feeling that the rare years when the Amargosa River flows all the way to the valley floor will become all the rarer.

It will not be so often that we can see a six-inch deep lake on the floor of Death Valley, and imagine how it once was...

Strangers in a Strange Land: I'm feeling detached...is that normal? Looking at Death Valley's faults

Strangers in a strange land is my latest blog series, dealing with the geology of one of California's most unique environments, the Basin and Range Province and the Mojave Desert. I visited both in the last few months, and I have been inviting the readers of this blog to learn the basic principles of geology through the eyes of my community college students. The last few posts have concerned the nature of faulting. Sixty years ago, as far as anyone was concerned, there were essentially four kinds of faults, left and right lateral, normal and reverse. There was a variation of a reverse fault, one with a low angle plane of movement called a thrust. Thrusts result from compressional forces, and are commonly seen the Appalachians, the Alps and the Himalayas, all of which are produced at convergent plate boundaries.

Thrust faults generally push older rocks over younger rocks, and a number of Mesozoic-aged thrust faults are found in the mountains of Death Valley. Levi Noble, one of the first geologists to map Death Valley, found many of them. As noted in the last post, he also found a series of unusual "thrusts" that were distinguished by having younger rock on top of older sequences. A nearly horizontal young rock-on-old rock fault can be seen crossing the middle of the photo above.

Another geologist, H. D. Curry, who worked the region in the late 1930s noticed some very strange structures in the Black Mountains on the east side of Death Valley. He called the huge dome-like mountains turtleback faults. That's the Copper Canyon turtleback in the picture at the top of this post. The turtlebacks (there are three obvious ones in the Black Mountains, and a number of somewhat cryptic ones scattered through other mountain ranges across the region) are made of ancient Proterozoic-aged metamorphic rock, and their surfaces are fault planes. When rocks are found above the fault planes, they are invariably younger. The enigmatic young-on-old 'thrusts' and turtlebacks seemed to be closely related. 

Other geologists started to question the idea that these faults were compressional. Even though Noble was initially sure that the faults were thrusts, over the years he began to accept that his first perceptions might possibly be invalid. The 'thrusts" were associated with numerous normal faults, and it was known by then that the Basin and Range province was produced by extensional forces in middle and late Cenozoic time. Finding the odd exception to the rule is not unusual, but by the 1970s, geologists were finding these enigmatic fault systems all across the Basin and Range province. As they realized the extensional nature of these faults, they knew they had discovered a counterpart to the compressional thrust faults. These low-angle normal faults were soon being called detachment faults, and the dome-like systems of ancient metamorphic rocks were termed metamorphic core complexes.

The picture above shows the exposure of the Badwater Turtleback where it plunges into the subsurface in the canyon above Natural Bridge in the central part of Death Valley. It is an astounding feeling to lay one's hands on the surface between young volcanic rocks and the ancient rock from deep in the continental crust. Recognition that these structures were an entirely new kind of fault zone did not provide answers to all of the questions raised by their discovery. The nature of detachments and metamorphic core complexes is still a field of ongoing research and controversy.

The exact nature of the turtlebacks in Death Valley is not a settled issue. Are the turtlebacks like the other core complexes, or are they a more unique local phenomena involving steeper normal faults? You can check out this report by Miller and Pavlis for a review of the issues involved, or pick up the excellent guide to the geology of Death Valley by Miller and Wright.

Diagram of a 'typical' detachment fault from Spencer and Reynolds, 1989, Middle Tertiary Tectonics of Arizona
and Adjacent Areas, Arizona Geological Society Digest 17, p. 539-574.

When one stands on the top of the Black Mountains and looks across the incredible Death Valley graben to the Panamint Mountains on the far side, one can barely comprehend the implication of the discovery of detachment faulting and metamorphic core complexes. The Panamints were once on top of the Black Mountains, or nearly so. And only a short time ago in the geologic sense, just a few million years before the present.

Strangers in a Strange Land: Not finding what you're not looking for...

Perception is a funny thing. Our preconceived biases exert a powerful influence on our interpretation of our observations. We understand what we know, and we can't really conceive of that which we don't know. Maybe Donald Rumsfeld was onto something when he famously said "There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know." Maybe he was expressing a deep philosophical truth. Or maybe he was a prevaricating politician. But there is a truth revealed here that applies to science, and geology in particular. We are very good at not finding what we are not looking for.

Charles Walcott, a respected paleontologist, discovered the incredible Burgess Shale fossil assemblage in 1909. Although he collected tens of thousands of specimens of exquisitely preserved creatures from the Cambrian period, he didn't completely realize their full potential as a window into the evolution of the earliest complex life forms. He tended to shoehorn odd specimens into known phyla and classes without exploring the possibility that these animals could be entirely different organisms from phyla that are long extinct.

Often scientists will see something new, and despite their insight, they don't fully follow through. Alfred Wegener was a geological visionary who collected extensive evidence that the continents of the world had once been collected together in a huge supercontinent that he called "Urkontinent" (German word meaning "origin of the continents"). The story is told in all geology classes about how his ideas were not at first accepted by the geological community (there were some good reasons for the geologists to be skeptical, and but other objections were rather unfair). What strikes me is a rarely noted factoid: he thought of Pangea (Urkontinent) as the starting point, that continents began as a single landmass. I may be wrong about this (and I am open to correction), but he seems to never have realized that Pangea may have been constructed from the collision of earlier continents. Yet he used mountain ranges that seem to cross from one continent to another (such as the Appalachian Mountains in the U.S. and similar mountains in Scotland and Scandinavia) as his evidence for the existence of Pangea. Mountains like these were the suture zones where older continental fragments collided and melded together. There may have been as many as four "urkontinents" in the last two billion years! Would a fuller understanding of these earlier landmasses changed Wegener's approach to dealing with his hypothesis? It's impossible to say. He died in 1930 at the age of only 50. 

Which brings us to Death Valley and our recent trip. Our "strangers in a strange land" had just learned at an earlier stop that there are four kinds of faults: normal, reverse, right lateral and left lateral. Low angle reverse faults, thrusts, are well-known from pioneering studies in the Alps and Appalachian Mountains. This was pretty much the status of fault classification in the 1940s when the first pioneering studies were being done in the Death Valley region.

Levi Noble was one of the first geologists to map the geology in Death Valley, and the project was daunting. He was up to the task, and his maps have stood the test of time. But some of his interpretations were colored by his expectation of finding the kinds of faults he had seen before. He knew that low angle faults were thrusts, because no one had seen anything different. And thrusts are caused by compression, which generally causes older rock to be pushed up and over younger rocks. Noble found plenty of thrust faults exposed in the mountains around Death Valley.

When he mapped in the Black Mountains on the east side of the valley, he found some real oddities. In some places, faults seemed to be everywhere, to the extent that the crust simply seemed to be chopped up into small bits that were only a few hundred feet across. Layers that were hundreds of feet thick elsewhere where only a few tens of feet in these outcrops, including the roadcut in the picture above, which has been called Exclamation Rock or Exclamation Point.  Noble called these unique sequences, somewhat logically, chaos. And underneath the chaos he mapped a low angle fault. It probably looked much like the fault seen in the picture at the top of this post. He called it, quite reasonably the Amargosa Thrust. But there was something odd about it.

The person in the picture above is walking in a canyon that has been carved through one of the low angle faults like the one mapped by Noble. Above the fault plane is a sequence of Neogene volcanic rocks (a few million years old). The rocks in the gorge are Proterozoic gneiss and schist...more than a billion years old.

What's wrong with this picture??

Strangers in a Strange Land: Confront your faults, it's good for your sole

The Strangers in a Strange Land continued their journey through Death Valley National Park last month. We had spent some time observing and interpreting a unique outcrop east of Shoshone, and after a short break in the urban nightmare of Shoshone itself (one gas station, one coffee shop, an RV park or two) we headed through the Black Mountains over Jubilee Pass into the south end of Death Valley itself. Although the road is paved, the south end sees few of the tourist buses and casual visitors who spend most of their time at Badwater and the resort at Furnace Creek. But the geology is wonderful. Especially if you are interested in faults...

Source: National Park Service

As can be seen in the diagram above, there are four basic faults type, normal (caused by extensional force), reverse (compressional forces), and strike-slip (the fault above is a left-lateral strike-slip; the blocks would move the opposite direction if they were along a right lateral fault). Strike slip faults are caused by shearing motion.

Normal and reverse faults can be distinguished by observing the relative motion of the headwall and footwall (as shown above). Extension causes the headwall move down relative to the footwall, making a normal fault. Compression forces the headwall upward relative to the footwall, forming a reverse fault.

Our students had just learned about these four basic fault types at the Charlie Brown outcrop, but they were looking at fault planes in a roadcut. As we entered Death Valley, we started seeing the effects of recently active faults on the landscape. We stopped along the road near a couple of odd features that don't really make sense on a valley floor where deposition should be the dominant process. It was a particularly instructive spot, as we could see evidence of movement along two kinds of faults from one viewpoint.

First is the terrace at the top of the post (also seen in the Google Earth image above). The gravelly sediments in the photo are from an alluvial fan along the base of the Black Mountains. The surface was once a smooth gentle slope, but fault motions lifted the rocks into the terrace, forming a fault scarp. The black rock is intriguing...it is basalt, which apparently rose through the crust along the weakened rock in the fault zone.

The second fault is less obvious from the valley floor where we were standing, but if you look carefully you can see that the eroded cinder cone has been split in two, and the portion of the cone on the far side of the fault has moved to the observer's right. It is a right lateral strike-slip fault crossing the valley floor. The offset is clearer when seen from above, as in the Google Earth image below.

This juxtaposition of two kinds of faults raises questions. Two different forces are clearly at work here, shearing and extension. Are they both presently active, or has the stress regime changed in recent time from extension to shearing or vice versa?

At this point we are below sea level on the floor of Death Valley, a 100+ mile-long fault trough. Mountains rise high on both sides of the valley, with a total relief of more than 11,000 feet (few places on the continent can claim such extreme elevation changes over so short a distance). Such fault valleys are termed grabens (the German word for grave or trench), while the mountains are termed horsts (German for eagle's nest or aerie).

Although we could not see an example from where we were standing, the Death Valley region also has examples of reverse or thrust faults (thrusts have a fault plane angle of less than 45 degrees). They have a tendency to push older rocks over younger, as can be seen below along the Keystone Thrust west of Las Vegas. The gray layered rocks are Paleozoic limestone formations (400-500 million years old) which have been pushed over the bright yellow and orange rocks of Mesozoic sandstone formation (around 200 million years or so). Check out Georney's on the ground visit of the Aztec Sandstone at Red Rock Canyon here. These faults are not currently active.

 Our students were treated to examples of most of the fault types within the course of a day. A nice simple explanation for the existence of Death Valley. The land stretched and grabens developed. Oh that it could be so easy. We rounded a corner and had our first view of one of Death Valley's turtleback faults. The story was about to get complicated...

Strangers in a Strange Land: Tuff luck, it's all your fault, so don't be an ash about it.

So, what do you do first? You sketch it. Drawing forces you to recognize patterns that you might not otherwise see. Then you stick your nose on it. How many times have I dragged reluctant students on a field trip only to have them sit on the far side of the highway trying to text someone when there was no cell service? It wasn't the case with these students. This group was a bunch of go-getters.You do what you can to identify the rocks in the roadcut, at whatever skill levels you've reached. This trip to a Strange Land has brought together many strangers who have had no classes in geology and some who have had many.

We have to establish some possibilities. We sit down and talk it over. What in the world could that black stuff be? The whole outcrop is layered. Doesn't that make it a sedimentary exposure? Tilted, you think? An example of original horizontality? What if it were volcanic? That makes it basalt in the middle, but what is the red and brown stuff? Is it a flow or an intrusion? A book suggests that this is an intrusive sill. It that possible?

We start to organize our thoughts and questions. Look at the picture below...it was taken in an abandoned pit at the Black Mesa Coal Mine on the Navajo Reservation in Arizona. It is composed of light colored sedimentary layers. Although not visible in this image, there are coal seams between these layers. Could the mysterious black layer be a coal seam?

Or another serious possibility. At the other end of Death Valley I encountered an intrusive sill, a place where molten basalt forced its way between the layers of light-colored limestone. The basalt has weathered deeply to a brownish-red color, but a fresh exposure would be black. Could the Charlie Brown outcrop be a sill? How would the sill affect the color of the surrounding layers? Would it oxidize the iron in the sediments, turning the rocks reddish?

What about a buried lava flow? Maybe sediments were laid down, then a thin lava flow covered them. Then new layers of sediment buried the basalt flow. How would that look?

Armed with new information, the students take a second look. And they come back. "It's not coal, the rocks aren't right, and an intrusion shouldn't have holes and cavities in it. And the rocks on either side of the dark stuff don't look like normal sedimentary layers. We don't think any of your explanations work."

Their closer inspection of the rock reveals that the "layers" aren't really layers at all; all the contacts are gradational, one color slowly merging into another, getting progressively darker and darker until it turns black and shiny. It's obsidian! Or more properly, vitrophyre, a glass-rich volcanic rock. This outcrop is showing us something else entirely. The entire outcrop is a single volcanic deposit that formed in a single vigorous eruption.

A few million years ago a small rhyolite magma chamber broke through the crust and erupted violently, producing a small caldera and coating the surrounding landscape with seething hot ash. The first ash to hit the ground cooled quickly. But the interior of the ash deposit was still so hot that the portion about 10 feet above the base fused into the volcanic glass of the vitrophyre. The uppermost ash layers cooled quickly and did not darken like the rock in the interior. This exposure is an excellent example of a welded tuff.

I really love this outcrop...it encapsulates very well the concept of the scientific method. We see a phenomena that raises questions. We do a preliminary investigation that results in a number of possible explanations (hypotheses). We test each one, assuming that one of the hypotheses will be supported by the evidence, and that the other hypotheses will be shown to be wrong. Like many times in science, all the proposed explanations turn out to be incorrect, and we go back to square one, not yet at an answer, but far more knowledgeable about our mystery. In the end, if we are diligent, and sometimes lucky, we arrive at an answer that fits all the evidence.

At this outcrop we have the added benefit of being able to learn about the various kinds of faults, and use that knowledge to identify the faults found in the same roadcut. The left side of the fault (the headwall) is down relative to the right side (the footwall), making this a fine example of a normal fault, which is generated by extensional forces. The entire region, the basin and range province, has been stretched and broken, so the faults in this one outcrop are a microcosm of the faulting found throughout this strange landscape.

This is the kind of outcrop that shows that diagrams on a chalkboard can never be as powerful a learning tool as standing on the ground staring at and manipulating the rocks.

It was time for a bathroom and a cold drink...we headed down to the village of Shoshone and got ready to see the heart of Death Valley. In a coming post....