The Other California: Yokohl Valley Drive in the Southern Sierra Nevada...and a "Blank Spot" on the Map

To be clear, the "blank spot" on the map is MY blank spot, not the cartographer's. For all the geological explorations I've been documenting here at Geotripper for the last five years, there is a frustratingly large area that I haven't set foot in for thirty years or more. It's the southernmost Sierra Nevada.

I imagine that many people miss that the Sierra Nevada continues for many miles south of Sequioa National Park, but it is a fascinating region containing a national monument, four wilderness areas, a state demonstration forest, and a county park preserving a sequoia grove. I spent some of the finest weeks of my boyhood in this region, at a scout camp (Circle B Scout Ranch; now disbanded and sold off), and on a fifty mile week-long hike through the Little Kern River Valley and Maggie Mountain. But my boy scout days were a very long time ago, and I have not managed to get back to the region since.

And that is a true shame, because the southern Sierra Nevada is one of the most beautiful parts of California. A few weeks ago, I made a brief foray into the mountains of my youth, taking an excursion along Yokohl Valley Drive, which led us from Springville to Highway 198 near Three Rivers.

The road doesn't exactly go anywhere. It's mostly there to provide access to a number of ranches, but along the way there are some gorgeous vistas, and springtime provided us with some beautiful flower displays. At our first stop, a Red-tailed Hawk watched us carefully...

The road crosses a divide at 2,700 feet, and a look east gave us a tantalizing view of some of the high country. The high peaks include Dennison and Moses Mountains, and hidden in the folds below the peaks are Balch Park (Tulare County), and Mountain Home State Demonstration Forest. The rest of the mountains are preserved as Giant Sequoia National Monument, established by President Clinton in 2000. Nearly half of the remaining groves of the Giant Sequoia trees were preserved by the designation.

Much of Yokohl Valley Drive cuts through granitic rocks of the Sierra Nevada Batholith. The hilltops show some nice examples of spheroidally weathered granite boulders.

The wildflower displays were a bit scattered in this dry year, but several sections were dappled with blue, purple and gold. The tallest plants are bush lupine, with yellow fiddlenecks filling the lower areas.

In a few scattered spots, the golden poppies dominated. It won't be but a few weeks before the flowers are gone, and the dry heat of the summer settles in.

At the bottom of the long hill I was surprised to see some mima mounds! These enigmatic low hummocks (sometimes also called "hogwallow" mounds) are found in widely scattered locations along the eastern Central Valley and western Sierra Nevada foothills. They were once thought to be burial mounds, but there is no evidence of this. Other hypotheses include preferential tunneling by gophers and ground squirrels, ancient stabilized sand dunes, heaving effects dating from the ice ages, and wave effects from earthquake activity. And aliens, of course.

I don't know which hypothesis fits best with the phenomenon, but rodents make the most sense to me. But that isn't nearly as fun as weird earthquake wave-forms.

Google Earth provides a satellite perspective on the strange mounds...

The Other California is my long on-again, off-again series exploring the fascinating geological localities that don't often appear on the tourist's postcards.

Postscript: Of course, such a beautiful place would have to be slated for development: http://www.saveyokohlvalley.org/ . I hope that since the site hasn't been updated for a few years, and there were no resorts in Yokohl Valley that I could see, maybe it has been tabled or cancelled...

A Soggy Day in Joshua Tree National Park: On the road with the NAGT

I spent the last weekend geotripping in Joshua Tree National Park with fellow members of the Far Western Section of the National Association of Geoscience Teachers. We hold these meetings twice a year at locations throughout California, Nevada and Hawaii. They are a great way to share teaching ideas and to collect samples and photos of some of the most spectacular scenery on the planet.

Part of our trip was an exploration of the Geology Road Tour in the middle of the park. The road is an unpaved track that is recommended for four-wheel drive vehicles. We would try to see if that was true on a rainy day! 

It was only sprinkling at first, as we crossed the Queen Valley pediment. Here and there inselbergs rose from the flat valley floor. These 'islands' of rocks that are remnants of mountain ridges that eroded away long ago.

The process of erosion continues into the present day. Can you see the horizontal scars across the boulders in the pictures above and below? These are areas of intense chemical weathering that formed when the soil levels were about 6-7 feet higher than today. The soils have eroded away within the last 20,000 years or so.

Another interesting form of erosion on these granite boulders is the development of tafoni, the hollows and honeycomb pits that occur on some boulders. The origin of the pits is somewhat unclear, but is no doubt a phenomenon of chemical weathering, possibly when the rock was still buried in the soil.

The hollow above was big enough to sit in. Others have had the idea of using the hole for shelter, as there was a grinding hole in the floor of the shallow cave. I also noticed a lot of charcoal in the soil at the site, meaning that the site was also utilized as a campsite by the local Native Americans.

We passed Malapai Hill, a 400 foot high basaltic intrusion. The volcanism resulted from decompression and partial melting of mantle material during a "reorganization" of the crust of southern California as the San Andreas fault system was taking shape. We didn't have time to check it out, but a climb of the hill reveals some nice examples of columnar jointing and fragments of mantle materials in the basalt.

Malapai Hill, photo by Mrs. Geotripper

We dropped down into Pleasant Valley, although with the building storm it was looking less and less pleasant by the moment. The abrupt valley wall marks the trace of the Blue Cut fault, a left lateral strike-slip fault that trends eastward through the park.

We stopped for a look at the Gold Coin Mine (not much equipment left), and searched for a few petroglyphs on the dark metamorphic rocks at the mine site.

At the contact with the granite, I saw a very nice example of a xenolith ('alien stone'), a piece of older rock that was incorporated into the granite when it was still molten. The xenolith was more resistant to erosion, and now stands out.

The weather was getting nastier by the second. The road crossed a playa surface in the center of Pleasant Valley. After the first eight cars passed through, the surface had become 'unstable', for lack of a better term. For the first time in five years of driving my Subaru, I went fishtailing on a road. My car has been stable on all manner of icy and snowy surfaces, but this was...fun. When we left two hours later, other cars were driving down the one way road. I wonder how they fared.

At the other end of the valley we found some of the most interesting rocks, a series of aplite dikes cutting across the Proterozoic Pinto Gneiss. These rocks are part of a complex that includes the oldest rocks in California (and part of an extensive belt of mountains that once extended across the southern United States).

 The rocks have been intricately folded and intruded by small granite veins. The complex chemical interactions have resulted in mineralization, leading many miners to seek out valuable ores in these rocks, mostly without success.

We continued around the one-way loop, on not too unreasonable roads while the rain continued to pour down on us. Luckily the playa surface did not extend to the other end of the loop, and we had no problems crossing the valley.

The view across the valley was interesting...our road out could be seen meandering up the alluvial slope. The inselbergs from the first part of the road tour can be seen on the skyline through the falling rain.

We made it back to the paved highway without incident, but the trip was cut a bit short because our last destination was Key's View. On a normal desert day, the view is incredible, looking across the Coachella Valley to Salton Sea, the San Andreas fault, and the San Jacinto Mountains. But not on this particular afternoon. The 30% chance of showers had turned into a full-fledged storm, so we headed back to town.

A trip spoiled by rain? Absolutely not. There is nothing quite like the sights and smells of a rainstorm in the desert. It was a great day!

Water in a Dry Land: Joshua Tree National Park, and Getting Tanked

A lake in the desert? That can't be right...

Joshua Tree National Park has a bit of a schizophrenic aspect. Geologicially, the park is divided into vast tracts of metamorphic rock separated from large areas of granitic rock. Topographically, the park is divided into the higher Mojave Desert and the lower Colorado Desert (the High Desert and Low Desert of SoCal weather reports). The elevation and rock differences result in large contrasts in water availability, and thus vegetation; the iconic Joshua Tree is not found in the lower parts of the park, for instance.

The higher parts of the park exceed 3,500 feet, and the climate is cooler and wetter ("wetter" being a relative term), and this higher elevation desert even receives the occasional snow flurry in winter. My vivid memory of January camping at Joshua Tree as a Boy Scout was that of coldness, absolute extreme cold (probably merely close to freezing, but I was a southern Californian; we panic when the temperature drops below 50 degrees).

There's more water in the high desert, enough that the landscape could almost be classed as savanna. These higher elevations supported enough grass that a few ranchers scratched out a living in the region. Cattle need water, but the water doesn't flow at the surface much of anywhere in the park. The ranchers built a few small dams in scattered drainages to catch the runoff from rare rainstorms. These so-called tanks were of varying quality, and some have fallen into such disrepair that they no longer hold water, but others survive. One of the nicer (and popular) short hikes in the park visits Barker Dam in the Wonderland of Rocks area. We had a pleasant stroll there last week, on a sparkling clear day.

With a bit of revegetation effort and pathway control, one can see how grasses could almost support a cow, although the measure would be acres per cow rather than cows per acre. The grazing no doubt put severe pressure on the local population of bighorn sheep, who are occasionally seen in the vicinity of the reservoir.

The trail winds past some unique desert oak trees and a variety of yucca called Parry Nolina, which blooms into the showy stalks seen in the picture above. The bedrock in the area is entirely composed of a variety of granite called monzogranite, which formed deep underground as magma, and cooled slowly to produce the crystal-rich rock. As the granitic rock was exposed at the surface, expansion and stresses caused the rock to fracture along joints which became the focal point of weathering and erosion. The boulders weather more readily at corners and edges, giving them a rounded aspect. The process is called spheroidal weathering.

Closer to the dam, the rock is less jointed, and breaks mostly parallel to the surface, forming a dome-like structure, which is a relatively rare sight at Joshua Tree, and more familiar in places like the Sierra Nevada. This is the process of exfoliation (below).

The little reservoir captures the rare runoff from thunderstorms, and the resulting lake attracts all manner of birds, insects, mammals and other creatures, including tourists.

A look at the dam reveals what probably amounts to a lack of sophisticated dam engineering, but it has lasted for more than sixty years.

The area downstream of the dam is a beautiful landscape of rock and vegetation. A number of petroglyphs and pictographs (some unfortunately recent) can be found hidden in these rocks. The ranchers had probably built on top of a natural reservoir in the rock, and the area has been a source of water for a long time.

So what about these Joshua Trees? Is there anything geological about them? That will be next time...