Strangers in a Strange Land; or more like Strangers in their own land

Strangers in a Strange Land is an exploration of the geology of Death Valley National Park and the Mojave National Preserve, as it might be seen (and possibly even understood) by a new student of geology. My trip turns out to have been in two parts, a five day journey over the President's holiday weekend with 30 students, and a second trip (just completed) to the Mojave National Preserve (with the National Association of Geoscience Teachers). Mrs. Geotripper and I made use of a spare day with a quick swing through Death Valley, exploring some of the places we've never seen with students.

I've been known to complain about being upstaged while lecturing by furry or scaly animals on my field trips. Really, would someone go to a park like Yosemite just to see deer or chipmunks??? Then again, something like this happens....

We were headed up a rather rough dirt track into a lesser known part of the park called Hole in the Wall (which will have a separate post of its own later on). We were surprised to see a herd of Desert Bighorn Sheep (Ovis canadensis nelsoni), six ewes and three lambs, grazing in the canyon along the road. I've been coming to Death Valley for more than twenty years, and this was the first time I had seen any here. That is partly a function of my short visits, but is also a consequence of big changes in the population of the sheep in the park.

Twenty years ago, one rarely saw bighorn sheep, but almost always saw feral burros, descendants of animals let loose by miners a century ago. The burros competed for forage and water sources, so the sheep declined in population (they also declined because of diseases introduced by domestic sheep). The park service instituted an effort to remove something like 3,000 burros, leaving a population of around 100. The bighorns have rebounded nicely.

The sheep were cautious about our presence. They were grazing on both sides of the road we were following, and they moved away from us deliberately, but not in a panic. We drove slowly so as to not scare them. Eventually they moved up the hill a bit and waited until we left the area (having taken 30-40 photos of course). We went up the canyon for the next hour, and when we came back down, they were along the road again.

We stopped and decided to have lunch but stopped upstream of the herd, so as to bother them as little as possible. I saw one or two peeking around the corner down the canyon a few times.

Eventually we had to leave so we got in the car and drove down the canyon. They watched us from above the road, but by now were not retreating. They just waited and looked down on us. It was a transforming moment for me.

Bighorn sheep are an integral part of the desert landscape of the western United States. They appear in tens of thousands of petroglyphs, far more than other animals, and thus were an imprtant part of Native American culture. With the arrival of Europeans, both the Native Americans and the sheep became strangers in their own land. I was so glad to see that the bighorns were reclaiming at least a corner of their ancestral home.

Strangers in a Strange Land; or maybe the opposite

Which of the two pictures below is more strange? Which has creatures that are more out of place?

I know that my series on Strangers in a Strange Land has been about Death Valley and the Mojave National Preserve, but the title seems appropriate here as well. We were chasing a storm eastward into the Sierra Nevada foothills to see whether the flowers were making a spring appearance. There weren't too many just yet. The storms this week dropped nearly three inches of rain in our area, which may not sound like much, but it is a quarter of our yearly average, and yet it only gets us to about 60% of normal. It's been a pretty bad drought, in other words.

We were driving past a ranch in the foothills, and the grazing animals looked out of place. We stopped for a closer look and realized that there were llamas out in the field. Llamas! What could be a stranger sight in the grassy foothills of the Sierra Nevada?

When we think of grazing animals in this region, we mainly think of cows. But from a geological point of view, the llamas make a great deal more sense. Why? Because they are camelids. That doesn't seem to make any more sense, since most people know that camels are only found in the Middle East and Africa. But the llamas and the camels should feel perfectly at home in California and the rest of the North American continent. Their evolutionary heritage is right here.

The first camels evolved here around 45 million years ago, and dozens of species evolved in the wide grasslands across North America. Some groups moved into Asia a few million years ago over the Bering Land Strait. Two or three million years ago other groups crossed over the newly formed Isthmus of Panama when North and South America became connected. Their story parallels the evolutionary history of the horses, who also traveled both directions out of North America.
They continued to thrive in North America until just 11,000 or 12,000 years ago when the camelids and the horses and a host of other large species went extinct. The reasons may be related to climate changes or to overhunting by recently arrived humans, or some other unknown agent, but they disappeared.

The cows that graze our grasslands today arrived much more recently with European settlers, but they evolved in Europe and Asia. They are the true strangers in our strange land.

The llamas are grazing over the graveyards of their distant ancestors.

Postscript: If you want to know more about the camels that are being excavated in the Central Valley, check out the Fossil Discovery Center in Madera County

Strangers in a Strange Land: A River Runs Through It, or at least used to

"After three days in the desert fun
I was looking at a river bed
And the story it told of a river that flowed
Made me sad to think it was dead"

from A Horse with No Name by Dewey Bunnell

Oh, the songs of our youth. A Horse With No Name by America came out in the olden days of 1972, and was one of those songs that imprinted itself into my junior high psyche in such a way that it can never leave. I think they call it an earworm, known to drive people nuts, but it is a pleasant one for me. It was a bit of an anthem for us backpackers and young desert rats. The lyrics from the song popped into my head as we pulled up to the next stop on our Strangers in Strange Land tour of Death Valley and the surrounding desert region of eastern California.

For first-time visitors, pretty much everything about the desert is strange. For first year geology students, the mystery is deepened by that little bit of extra knowledge they possess that has them observing the rocks and surrounding mountains, trying to understand how this landscape might have developed.

So what was going on here? We had arrived at the eastern foot of the southern Sierra Nevada and parked in the midst of a lava flow from a cinder cone in the Coso Volcanic Field called Red Hill. The age is not precisely known, but it probably is less than 130,000 years old. The lava flow is covered by tan colored eolian (windblown) dust. Up to this point nothing seems particularly strange, but in the picture above, one can see a gap at the edge of the lava flow, a little bit to the right. Why is it there?

Having walked a quarter mile over rough lava surfaces, it was a bit of a surprise to come across this sage covered flat area that seemed to be composed only of sand and silt, with only a few scattered boulders of the basalt. Something was a bit off about this part of the lava flow.

A look ahead confirmed the odd nature of this stop (below). The lava flow had turned into an intricate labyrinth of deep holes and channels. Even if you barely know anything about lava flows, it is clear that lava doesn't do this sort of thing. This lava has clearly been scoured by a pretty large river, the deep rounded pits being potholes formed by swirling masses of pebbles and gravel. The name of our stop, Fossil Falls, more or less supports the contention that a river once flowed here.

But this is a desert. No rivers are found nearby, and the few streams that flow off the nearby slopes of the Sierra Nevada sink quickly into the gravel-rich alluvial fans long before reaching this site. Could the channel be the result of flash floods? It doesn't seem likely, because the channel upstream has not been cleansed of the easily eroded silt and sand for a very long time. The growth of brush in the old channel suggests stability.

There is a story here...until the 1920s, a large natural lake existed just north of Fossil Falls. It was called Owens Lake, and it was the natural sump for most of the streams and small rivers flowing off the eastern flank of the Sierra Nevada. Although the lake covered an area 8 miles by 12 miles, it was usually no more than 30 feet deep, and in the intense desert heat the water left only by evaporating. There could be no outlet unless the water reached a depth of 200 feet or so. If it spilled over the water would be directed downstream right towards Fossil Falls. The lake is mostly dry today because Los Angeles has diverted the water than once flowed into the basin.

Where did all the water come from? Clearly at some time in the past the climate was cooler and wetter than it is today. The glacial ice ages that affected the region during the last two million years provide a reasonable explanation. Glacial meltwater flowed through Mono Lake and the Owens River, filling up Owens Lake which in turn spilled over into other lake basins farther to the south. These bodies of water are called pluvial lakes.

The potholes are still active today, but in a different way. The rare precipitation events will occasionally leave water standing for weeks in some of them. The silt in the bottom contains the desiccated eggs of small delicate fairy shrimp. When they get wet, the shrimp hatch and grow quickly to maturity in a race to produce eggs before the pothole is dry again. I found a few of them warming up in a small pool that still contained water despite the dry year.

Still, seeing the ghosts of long gone and forgotten rivers is sort of sad. There is no crashing of water cascades, just the quiet breezes (or gales, if a storm is blowing through).  But there was a previous trip...2005 was a very wet year, and we were following on the heels of a major storm. When we reached the falls, I heard something I had never heard before. Water flowing in the ancient gorge! There was no mistaking these chocolate colored trickles with the big ice age rivers, but it was impressive to see anyway.

This is the joy of teaching geology! From small inconsistencies in a landscape come grand stories of climate change, glaciers, vast freshwater lakes, and fertile ground for the curiosity and imagination of our students.

paleomagnetism in rocks that provided unexpected proof of plate tectonics. James Hutton staring at a Roman fortification in Scotland that led to the recognition of geologic time. Nicolas Steno studying "devils tongues" and discovering the biologic origin of fossils.  Geology isn't just an academic discipline; it is an adventure pretty much like no other.

Fossil Falls can be found a few miles south of the Coso Junction Rest Area on Highway 395. Turn east on Cinder Road (just south of Red Hill, the prominent cinder cone next to the highway), and follow the signs on the gravel road about a mile to the recently improved parking area and trailhead (vault toilet and picnic tables available, and a small primitive campsite).

Strangers in a Strange Land: The long journey of a sea lily

The larva emerged from the egg and began drifting in the gentle current. It was a bilaterally symmetrical body that recalled a small chordate, but it wasn't. The ciliated body allowed it to swim to some extent but within a few hours it settled to the sea floor and a remarkable change occurred. It began to develop an odd five-fold symmetry. That placed the organism in the lineage of the phylum echinodermata.

Attribution: Pacific Ring of Fire 2004 Expedition. NOAA Office of Ocean Exploration; Dr. Bob Embley, NOAA PMEL, Chief Scientist: http://www.flickr.com/photos/noaaphotolib/5114735036/in/photostream/

Over time the organism developed a stalk with a root-like holdfast, and a flower-like "head" on top. As much as it looked like a plant, it was indeed an animal. The "flower" was a food collecting organ with feather-like appendages that captured small bits of food drifting by in the currents. The edible particles were transported to the mouth at the center of the "flower" and consumed. The stalk gave the organism an advantage over bottom dwelling filter feeders by extending above the other benthic species. These crinoids, also known as sea lilies, were hugely successful in their time, with thousands of them anchored to some parts of the ocean floor, waving like a field of wheat at the bottom of a warm tropical sea.

This particular individual lived for several years, but during a tropical storm the turbulent water broke the stalk off, and the calyx sank to the seafloor and the crinoid died. The calyx broke into numerous calcite plates, and the organism was added to the remains of thousands of crinoids that had died before. In time the bits of stalk and little platelets were buried under the constantly accumulated debris from crinoids that were born much later. Over time the water was mostly squeezed out and the crinoid debris began to lithify, that is, it turned to solid stone. More sediment covered the limestone, providing additional pressure, and soon the remains of the small crinoid were pretty much removed from the effects of surficial processes. Then came the vast void of time. For more than 100 million years, the crinoid stalk simply existed, pretty much without change, other than a very gradual buildup of pressure as the rock was buried ever deeper under thick sequences of sand, silt and limestone.

Time ebbs and flows. Sometimes things happen very quickly, but in other situations time seems endless. The crinoid lived for only a few years, but it lay buried for millions. In this kind of immortality, sentience would be a curse. Entombed in total darkness and silence for endless millennia, insanity would be a blessing. Time wore on, and the only indications of activity were the occasional earthquake and slow change of orientation and temperature. It was also getting hotter, for big things were happening above.

The ocean had disappeared, and the rocks that had lain at the bottom of the sea were now pushed up into a mountain range. Out to the west, a subduction zone had developed, and the resulting compressional forces had buckled the rocks into a series of folds and thrust faults. A few tens of miles away, vast bodies of magma were intruding into the overlying rock, although the fragments of the crinoid were not directly effected by the igneous activity. They remained entombed in the darkness, and over time the mountains above were smoothed by erosion to a fairly muted landscape of low hills.

More millions of years passed, but the pace of change increased. There were more earthquakes, and there was an imperceptibly slow change in the amount of pressure. One day there was a crack as the stressed rock relaxed by fracturing. And then again. And then, for the first time in millions of years, water started seeping through the small fractures. The water was not salty; it was groundwater coming from rainstorms far above. Sound became more perceptible; thunder and the shattering of stones rolling down a hill.

One day, light appeared as a rock fall exposed the bit of limestone encased in what turned out to be a high cliff. A short time later, the limestone bearing the fragments of the crinoid fell out of the cliff onto the talus slope below. After the insanely slow pace of change over millions of years, events were now dazzlingly fast. One afternoon, a particularly intense thunderstorm produced a mudflow that tore away the base of the talus slope, and the chunk of rock containing the crinoid was lifted and carried in a slurry of rock and fine-grained sediment. This one storm only carried the boulder a few hundred yards, but the next storm and the one after carried the rock for several miles out onto an alluvial fan.

The rock sat, exposed to the occasional rainstorm, the endless heat of the blazing sun, and the growth of small shrubs and perennials. The surface of the rock became etched, and the structure of the original crinoid stem became obvious, enough that a semi-sentient being perceived that the one stone was different than the others nearby. He picked it up, considered the nature of the once living organism, snapped a picture, and laid the stone on the ground again. He wandered off.

300 million years after the small crinoid lived and died, the last vestiges of its existence would soon dissolve away, and its constituent atoms would be distributed through the soil. It will soon be gone.

And that is the story of the crinoid I found last month.

I've always thought about rocks this way, but in presenting the narrative in this style, I was no doubt influenced by The Planet in a Pebble: A Journey into Earth’s Deep History by Jan Zalasiewicz, which I reviewed a while back. Strangers in a Strange Land is my series on the geology revealed during a series of trips to Death Valley and the Mojave National Preserve in February and March.

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: 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: 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....

Strangers in a Strange Land: Discerning the Story in the Rocks at Red Rocks

In the last post, we were introduced to the basic rules of stratigraphy as they stood exposed in the spectacular cliff faces of Red Rock Canyon State Park in California's Mojave Desert. A classroom chalk board can only provide a cartoon of these concepts, so there is nothing quite like being there and laying your hands on these records of the Earth's past. We were on the road with 30 community college students, most of whom were seeing this landscape for the first time. After an introduction to the stratigraphic principles of superposition, original horizontality, lateral continuity, and cross-cutting relationships, I sent the students forth to test their newly acquired knowledge against the cliffs above. We weren't quite to the point where we could do geologic mapping, but I provided them with an outlined photo of the scene (below) and asked them to describe the rocks, and identify any structures.

This is one of the finest moments a teacher gets to have. I wasn't teaching at all; my students were teaching themselves. They were crawling up and down the slopes, taking notes, and discussing what they were seeing. OK, it's true there was no cell phone service, so what else were they gonna do?

The layers were readily labeled on the basis of color alone, and most of the students developed some version of the picture below (click to enlarge):

I climbed up the slopes to have a closer look at some the exposures. What happened here 8-12 million years ago? The red and brown layers (A, C, and D) proved to be arkosic sandstones, the kind of thing one might find in river channels and alluvial fans in a region of relatively high relief (lots of rapid erosion). Some of the finer-grained brown sediments formed in river floodplains. These rocks had clearly formed on land.

The white layers (B) with puffy deposits that kind of looked like ground hamburger were ash deposits, evidence of regional volcanic activity. The ancient environment here was beginning to look a little bit dangerous!

The upper cliffs and slopes (layer F), proved to be evidence of the greatest geological violence possible, a rhyolitic tuff breccia. The rock formed when hot ash rolled over the landscape, picking up gravel and debris and incorporating the material in the nearly molten tuff deposit. This was no place to be hanging about...but plenty of animals did. The layers of the Dove Springs Formation are a treasure trove of Miocene fossils, one of richest beds found in California (or pretty much anywhere, really).

The animals found in the sedimentary sequences here include include extinct elephants, rhinos, three-toed horses, giraffe-like camels, saber-toothed cats, and bone-crushing dogs as well as smaller animals like ancestral skunks, martens, alligator lizards, rodents, and shrews. (follow the links to descriptions of each type of animal on the Los Angeles Natural History Museum website).

Faults in a cartoon drawing on a chalkboard are a lot easier to see than most faults in the real world. A number of the students noticed how the layers ended and identified the fault that caused them to be offset (see the arrows in the diagram above). I headed up to take a closer look at the fault surface...because I think everyone should know their faults...

We only had a few precious days to see as much as possible on this trip, so we couldn't spend as much time at Red Rock Canyon as we would have liked. A fair number of geology programs do mapping exercises here, spending several days on site. It sounds like fun...if it isn't too hot. It was time for us to move on, so we loaded up the vans and drove north on Highway 14 towards our next destination at the south end of the Owens Valley. That will be part of the next post.

If postings seem sparse in the coming week, it would be because we are about to embark on part two of our Strangers in a Strange Land journey: an exploration of the Mojave Scenic Preserve in the desert south of Death Valley. We're attending the spring meeting of the Far Western Section of the National Association of Geoscience Teachers at the Desert Research Center at Zzyzx. Hope to see a few of you there!

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...