Cartoons Come to Life: The Value of Field Studies at Red Rock Canyon

I spend a lot of time in classrooms drawing cartoons. Lots of them, diagrammatically representing folds, faults and stratigraphy, trying to communicate to my students how these structures tell the story of the Earth. The problem for many of my students is that these are just cartoons. Cartoons on Saturday mornings (do ANY of you remember when that was the only time one got to watch cartoons?) do represent life in a way, but only if you have a real life to compare them to. And that's the problem for many students these days. They have no real-life experiences in the outdoors with which to compare these drawings.

And thus, the value of a field studies course. There is nothing quite like having the privilege of standing beneath a cliff, enjoying and appreciating the scenery, yes, but also being able to understand the story it tells. Our trip to Death Valley a few weeks ago included a fossil hunt at the first stop, but our second was a site where basic principles of geology stand out in stark relief, without a need for a chalkboard cartoon (I forgot the chalkboard this trip anyway). We had arrived at Red Rock Canyon State Park in the Mojave Desert. Just stepping out of the vehicles presented us a cliff face that was a physical representation of the diagram at the top of the post, a series of brightly colored layers of sand, siltstone and volcanic tuff transected by a fault.

The order of the layering provides a fine example of "superposition", the principle that layered rocks are stacked oldest to youngest, unless they have been overturned. This is one of the earliest principles of geology, described originally by Nicolas Steno in the 1600s. Looking east along the cliff we could also see the physical manifestation of another Steno principle, that of original horizontality. Most sedimentary environments produce horizontal strata (think floodplains or lakes, or shallow seas). If layers are tilted, some kind of force has acted on them, and being here in southern California, faults might be at fault.

A discussion of these basic principles was followed by a short "mapping" project. We weren't quite to the proficiency of working with maps, but the students set out to propose how they would organize the layers into formations and members that could be used to tell a logical story as to how these rocks could have come to have the structure and appearance they have today.

Red Rock Canyon does have a fascinating story as it turns out. The red and brown layers, called the Dove Springs Formation, record deposition of silt and sand in river floodplains and ephemeral lakes between 12.5 to 7.5 million years ago. The semiarid savanna environment supported a diverse ecosystem that included extinct elephants (gomphotherium), ancestral rhinos, three-toed horses, giraffe-like camels, saber-toothed cats, and bone-crushing ancestors to the bears and dogs. There are also numerous smaller fossils like ancestral skunks, alligator lizards, and shrews, with a total plant and animal species count of more than one hundred. The ecosystem suffered the occasional catastrophe as it was buried by volcanic ash. Basaltic lava flowed across the region.

One of the student's discoveries was a somewhat less visible fault that shifted the layers. It was a great introduction into the mind of field geologists, seeing the world the way earth scientists do. It was an interesting spot, but there were many more to come on the road ahead.

Speaking of roads and cartoons come to life...here's the Roadrunner (pic is actually from Joshua Tree National Park)....

...and Wiley E. Coyote. Despite living in the most desolate driest place in North America, it looks like this one has actually caught a few roadrunners! Seriously, this coyote lives only a few miles from Badwater, the hottest and lowest part of Death Valley.

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...