Saturday, December 21, 2013

Accretionary Wedge #63 AND Berry Go Round, Two Blog Carnivals and One Post on Serpentine

I don't usually post two blogs in one day, but the latest Accretionary Wedge deadline is approaching, and I'll be on the road again, doing Christmassy things with my family, so I might not have a chance later on. Our host of the double carnival is month is Hollis at In the Company of Plants and Rocks. The topic for both carnivals is as follows: Plants and Rocks (or Rocks and Plants). 

Reading the rather wide-open topic immediately brought to mind a subject dear to my heart, the state rock of California and a group of beautiful minerals: serpentine. There was a bit of political controversy a few years back about serpentine and its past use as a source for asbestos, and I did quite a few blogs on the subject. But even earlier I had written a blog post for my Other California series that explored the little known but geologically interesting areas of California, the ones that rarely showed up on tourist postcards. These are the kinds of places you seek out when you've seen the Golden Gate Bridge and Yosemite Valley. I am rejuvenating a post from the series, one I wrote about the Red Hills in the Sierra Foothills near Jamestown and Knights Ferry. It first appeared on March 27, 2010, and is mostly reproduced here with minor changes. It was titled "There's an Endemic in those Hills!" Oh, that's right, it's epidemics we're supposed to worry about. An endemic refers to plant species found in specific limited locations. There are a number of these in the Red Hills "Area of Critical Environmental Concern", a rather high-falutin' name for an area that less than two decades ago was barely more than an open garbage dump scarred by numerous off-road vehicle trails. The rare and endemic species are there for a very geologic reason, the subject of this post.

The Other California is my continuing blog series on those places in California that people don't generally find on the postcards at all our tourist traps. I've been following a regional theme, traveling through the northernmost provinces, but the Other California has a temporal pattern as well, and late March is the perfect time to talk about the Red Hills, located in the Sierra Nevada Mother Lode near the Gold Rush town of Chinese Camp (I talked about the area around La Grange a few days ago for the same reason).

Much of lowland California is currently covered with a green carpet of grass (mostly of exotic and invasive origin) along with the occasional oak tree, but as you can see in the pictures above, there are a few places where the grass and oak trees are missing, and a profusion of flowers and scattered pines thrive instead. Why are the oaks and grass missing?

The Mother Lode is famous as the source of the ores during the Gold Rush in 1848-53, and many people know of the association of quartz veins with the gold. What is perhaps less known is that the Mother Lode consists mostly of metamorphic rocks like slate, greenstone, and marble, not the granite that is found in the higher parts of the Sierra Nevada. These metamorphic rocks are the twisted and baked remains of sea floor muds and silts, lime from tropical reefs and shelves, and volcanic rock from the oceanic crust. These collections of crustal rocks (called "exotic terranes") were transported across the Pacific Ocean and slammed (in the geologic sense; they moved at maybe 2 inches a year) into the western edge of the North American continent, mostly in the late Paleozoic and Mesozoic eras (the Mesozoic, from around 251 to 65 million years ago, is best known as the "age of the dinosaurs"). The different terranes are separated from one another by major fault systems.

At times the crustal terranes also include rocks from beneath the crust. This rock hails from the underworld of the earth's mantle, and includes dunite and peridotite, composed primarily of the mineral olivine (known to most people as the gemstone peridot). The rock readily alters to serpentine, California's state rock. These rocks are also collectively called ultramafic rocks, for their high content of magnesium and iron (fe, the 'fi' part). They also contain small, but significant amounts of nickel and chrome.

When ultramafic rocks are brought to the surface, they are far out of chemical equilibrium with the ambient conditions, which means they are easily attacked by oxygen, water and organic acids. Clay is a common product of this process, as well as red or yellow iron oxides (from which the Red Hills take their name). The surface layer resulting from this weathering process is of course soil. We tend to think of soil as a rich surface layer that supports plant life, but some soils lack the necessary nutrients for most kinds of plant growth. This is definitely the case for soils developed on ultramafic rocks, which lack nitrates, phosphorus, and potassium. To make things worse, chrome and nickel are actually toxins. Hence, only specialized species can thrive on these rocks.

The shrubby Ceanothus, or Buckbrush (above) and Gray Pine (below) are two plants that are more or less indifferent to the odd soil conditions. They grow elsewhere, but compete very well in ultramafic soils. A large number of flower species are also indifferent to the soils, but the only grasses found in the region are native species. The European and Asian grass species that have overwhelmed most of the prairies in the Central Valley, Coast Ranges and Sierra foothills cannot grow on the serpentine soils.

There are a number of endemic species that grow on these soils, and at least one is found nowhere else in the world (California verbena, Verbena californica). Other rare endemics include Rawhide Hill onion (Allium tuolumnense), Layne's butterweed (Senecio layneae), Congdon's lomatium (Lomatium congdonii) and the Red Hills soaproot (Chlorogalum grandiflorum). A fairly common serpentine endemic is the Milkwort Jewelflower (Streptanthus polygaloides). Alas, I arrived very late in the afternoon and had no time to search them out (and to be truthful, I am better at identifying rocks and minerals).

Though closely associated with the rocks of the Mother Lode, the serpentine and dunite were remarkably free of gold, and so the Red Hills were mostly ignored by the miners. Farmers couldn't grow much in the soils, and grazing conditions were not favorable, so the when the federal government came into possession of these lands in 1848, they couldn't even give them away! So this swath of land, about 7,000 acres worth, was administered, somewhat indifferently, by the Bureau of Land Management. The landscape suffered the abuses of modern civilization, with trash heaps, motorcycle trails, and unrestrained target shooting. The recognition that the area was a unique geologic and biologic treasure led to the restriction of shooting and off-road vehicle use in 1991. Private groups assisted in cleaning up the trash heaps and a trail network was established, so today the Red Hills are a delightful place to visit, especially in the spring when the wildflowers are at their stunning best. And I could be wrong, but I don't think I've seen any postcards with pictures of the area.

If you want to learn more, or pay a visit, information about the Red Hills can be found on this BLM website , and the nature trail brochure PDF can be found here.