Out in America's Never Never: Getting There

Photo by Mrs. Geotripper

A journey into the Never Never of America has to have a beginning in the green lands. From our base in the fertile Central Valley, we traveled through 200 miles of irrigated farmlands. They would be part of the desert were it not for a massive water project drawing water from northern California. We crossed the Sierra Nevada at Tehachapi Pass and continued into the Mojave, a desert that exists because of the rain shadow effect. Pacific storms are wrung dry by the imposing walls of the Coast Ranges, Sierra Nevada and Transverse Ranges.

Photo by Mrs. Geotripper

The western Mojave could hardly be mistaken for wilderness outback. Los Angeles has spilled over the mountains, with hundreds of thousands of people living in towns like Palmdale and Lancaster. Power lines and railroads crisscross the desert, as well as numerous paved highways.

We made a short stop at the massive open pit mine at Boron, one of the world's largest sources of borate minerals. The mine, operating since the 1920s, is now 500 feet deep, a mile long, and half a mile wide. A visitor center perched on the edge provides a marvelous view of the operation.

As we continued eastward, the trappings of civilization began to fall away. Barstow is a crossroads town of just over 20,000 people. At the north end of town, a road provided access to Fort Irwin (eerily accompanied by a lot of white crosses from traffic fatalities), but our route took us on a side road to Rainbow Basin.

19 to 13 million years ago, the Barstow region was not a desert. It was a savanna, with wandering streams and ephemeral lakes. The grasslands included groves of palm trees, and numerous grazing animals that were the ancestors to the elephants, antelope, horses, and camels that are familiar to us today. The grazers were preyed upon by a collection of primitive canines and felines, as well as amphicyon, a large carnivore that can visualized as an ancestor to the bears and dogs. The sediments that collected in the isolated basins accumulated to a depth of more than 3,000 feet, with a brightly colored assemblage of freshwater limestone, sandstone, siltstone, conglomerate, along with a series of volcanic ash layers that made it possible to date the formation.

The layers were later caught up in the deformation that began as the San Andreas and other faults warped and twisted the crust. In the center of Rainbow Basin, the Barstow Formation was deformed into a textbook example of a syncline, a downward pointing fold in the rocks. This is the syncline I wrote about a few weeks ago. I used the outcrop as an introduction to basic stratigraphic principles, and the students sketched the relationships, and outlined a sequence of events to account for the scene ahead of us. Here is the sketch as I would have done it...

The principle of superposition tells us that the oldest rocks will be found at the base of the sequence, so the Barstow Formation is the oldest rock visible. The principle of original horizontality tells us that most sediments are laid down in flat layers, so we can state that the Barstow Formation has since been twisted from its original position. The principle of lateral continuity shows us that the red and green layers of the Barstow have been eroded, because they end against an overlying layer instead of the edge of the original basin. The erosion surface is termed an unconformity. The principle of cross-cutting relationships tells us that the faulting took place after the deposition of the Barstow Formation, but before the erosion that produced the unconformity. And after all of this, erosion exposed the rocks for us to see on a warm Saturday in June at the start of our long journey.

We headed east from Barstow on Interstate 40, and all evidence of civilization disappeared, with the exception of the narrow strip of pavement we were following. The desert spread out for miles in every direction. As the sun disappeared beneath the horizon, we made the turn north to Hole in the Wall within the Mojave National Preserve, a national park for all intents and purposes except for the right of a few to shoot wild "game". When the vans were parked, the silence was striking. We had reached the land I'm calling Never Never.

For the first time in many months, I just stared at the sky. Early on, the sky was dominated by the quarter moon, but later, when the moon had set, the sky filled with stars. I did a time exposure over Woods Mountains...

It was a beautiful and peaceful night, the first for me in a long time. It was dreamtime.

The Other California: Prairies Past and Some Nice Folding

Visitors at Calico Ghost Town in the Mojave Desert geomorphic province are treated to an authentic old mining town complete with bars, melodrama theatres, staged gunfights, and portraits in oldtime clothing. They may even learn a bit of genuine history of the old silver mining district from the occasional sign. Calico is California's Official Silver Rush Ghost Town (discussed in a previous post), and is definitely worth a stop, but not necessarily for the reason envisioned by those who run the park. The parking lot sits in the gulch below the town, and exposes some of the finest folds to be seen anywhere. They have appeared in many geology textbooks ("textbook examples" of anticlines, the upward pointing folds, and synclines, the downward pointing folds).

The origin of these folds has been the subject of much discussion, but their proximity to an active transverse fault (the Calico fault) suggests they developed as a result of transpressional stress. Place a sheet of paper flat on a table, and put your palms on each end. Moving one hand towards you and the other away from you will cause crumpling across the middle. Those crumples are transpressional folds.

A bit farther to the west, the Barstow Formation is exposed at Rainbow Basin Natural Area, administered by the Bureau of Land Management. An upside-down rainbow, the Barstow Syncline, is one of the most striking features. I talked about the area in a previous post.

In a marked contrast to the volcanic rocks that make up the core of the Calico Mountains, the Barstow layers are sedimentary rocks. Sedimentary structures such as mudcracks (below) and ripplemarks indicate that the sediments accumulated in a faulted basin in a moderately tropical and occasionally arid environment, essentially similar to the African Savannah of today's world. It was a world of grasslands and palm trees that existed between 13 and 19 million years ago, based on radiometric dates of volcanic ash layers that bracket the sedimentary strata.

An African savannah. Visions of elephants, giraffes, zebras, and rhinos. But not in America, right? Well, time for a new vision. The most stunning aspect of the Barstow Formation is the diversity of fossil species found in its layers. Numerous species of ancestral horses and camels, a rhino ancestor (Aphelops), and an early elephant called a gomphotherium (similar to those big elephants in the Lord of the Rings movie, only much smaller) are among the discoveries. Feeding on these planteaters were one of the earliest cats (Pseudaelurus), an ancestor to the coyotes (Tomarctus), and a probable forebear to the bears and dogs called an Amphicyon (pictured below).

Photograph by Ghedoghedo

The extreme diversity of the fossil assemblage, and the fact that the formation is well-dated led to the establishment of a portion of the geologic time scale called the Barstovian stage of the Miocene epoch, referring to the time period from 16.3 to 13.6 million years ago.

One further aspect of the fossil record in the Barstow Formation are calcareous nodules that contain nicely fossilized insects. Insects are rarely preserved as fossils, but the bugs in the lake sediments were replaced by silica and were supremely preserved, to the extent that even leg hairs are visible.

Learn more here:

A technical article from the GSA Bulletin on the Barstow Formation (abstract; full article requires fee or GSA membership)

A nice slideshow about the Barstow Formation and the fossils it contains, from the San Bernardino County Museum.

This has been an entry in the series on the Other California.

Faults and Folds Weekend Fotos!

Following the example of Silver Fox and Lost Geologist, I decided to throw a fault and fold foto into the geomix this weekend. This is also still on the theme of my impending Colorado Plateau journey, not because it is there on the plateau, but because this is often my first stop on our trip. It has it all for a first day geology student: faults, folds, stratigraphy, unconformities, paleontology, economic geology, and nearby archaeology. Geologists from southern California will recognize it right away as the Barstow Syncline, located within the Barstow Formation, and exposed in the appropriately named Rainbow Basin Natural Area. I did my very first field mapping here, back in 1979 or so, with Pomona College.

The syncline lies near exposures of the Calico fault, an active right-lateral strike slip fault. The Landers quake in 1991 ruptured part of the fault zone. A minor offset cuts the left limb of the syncline and other faults slice through the basin, making it an excellent place for mapping.

The layers exposed in the fold are terrestrial and fresh water sediments of the Miocene Barstow Formation, a critically important stratigraphic and paleontological element of southern California geology. The vertebrate fossils found here include early species of elephants, camels, antelope, three-toed horses, cats, and many others. In some nearby areas, silicified insects have been discovered as well. Because the layers of the formation are bracketed by dateable volcanic ash layers, they form the basis of the Barstovian Land Mammal Stage of the geological time scale. When similar species of animals are found in other parts of the country, they can be dated to around 15 million years by comparison to the Barstovian fauna.

Looking for a good example of an angular unconformity? Check out the layer of alluvium across the top of the fold! The region eroded to a relatively flat surface in Pleistocene time and was covered by alluvium, but more recently the land was warped upward and is presently being rapidly eroded.

The Barstow Formation was the host rock for barite and silver ores, as well as borate minerals, and a number of mining ghost towns dot the region. Calico is the most famous, and has been developed as a regional county park (read: tourist trap), but it is a nice enough place to visit. Kenny Rogers and the First Edition once actually produced a two disk record album full of songs about the ghost town. Farther to the east, the Calico Early Man Site offers evidence for the presence of humans in the western hemisphere far earlier than is accepted by most archaeologists.

Want to see Rainbow Basin? The Bureau of Land Management site offers the directions and information on camping and touring the region.

Update: It didn't take long to find out that others really love the place, too (and beat me to the blog post!). Ron Schott has a blog entry here (although I am having trouble following the link), and Dave Schumaker has one here. Thanks for the info!