Posted by: James Wapotich | April 23, 2018

Trail Quest: Serpentine landscapes of the Figueroa Mountain area

It’s hard not to fall in love with the serpentine rocks found in our mountains. Sometimes smooth and shiny, sometimes brittle and crumbly, the blue-green rock is found in several locations in Santa Barbara County with some of the most accessible outcrops found along Figueroa Mountain Road.

Serpentine is the state rock of California and takes its name from its mottled pattern, which is sometimes reminiscent of snakeskin.

Recently, Santa Ynez Valley Natural History Society hosted a field trip into the mountains with geologist Susie Bartz and naturalist Liz Gaspar. We visited four sites along Figueroa Mountain Road and learned about serpentine rocks and soils, and the plants that grow on them.

Serpentinite Serpentine ultra mafic roch Figueroa Mountain Los Padres National Forest blue point

Serpentinite

Our first stop is a large outcrop of serpentine rock along the road between Sedgwick Reserve and the Midland School property. Here, we’re afforded exceptional views of Zaca Ridge, including Grass Mountain and Zaca Peak.

With this backdrop, Ms. Bartz explains how these rocks were formed, but not without first acknowledging the work of Thomas Dibblee; “Tom was a geologist who mapped the entire San Andreas Fault, out to 25 miles on its eastern side and to the coast on the western side. He mapped about 40,000 square miles on foot over his 75-year career.”

A friend of Mr. Dibblee, Ms. Bartz later helped bring his maps to publication. The maps, which are overlaid onto USGS topographic maps, provide a detailed overview of the geologic formations found in a particular area and are a great resource for understanding the geology of our local area. Dibblee maps can be purchased at Santa Barbara Museum of Natural History, as well as viewed online at https://ngmdb.usgs.gov/mapview.

Serpentine rocks are part of the Franciscan assemblage of rocks that can be found in a 35-mile long swath across our local mountains from Blue Canyon to Figueroa Mountain. The formation is bordered primarily on the north by Camuesa Fault and on the south by Little Pine Fault.

Franciscan rocks were formed 150-60 million of years ago as a result of the movement of the Earth’s tectonic plates. There are two types of plates, oceanic and continental, and as they move around and interact they meet at either a convergent boundary, where one plate dives under another, or at a transform boundary, where one plate slides past another along a fault such as San Andreas Fault.

Oceanic plates are formed at mid-ocean spreading centers that open as one plate dives under another. Because oceanic plates are thinner and denser they’re often subducted under less dense continental plates, slowly disappearing like a conveyer belt.

“What happens is a lot of bulldozing and scraping of all the stuff on the surface of the sea floor, which accumulates little by little on the underbelly of the edge of the continent,” Ms. Bartz explained.

This accumulation of material along the plate boundary is called an accretionary wedge and is a mix of rocks, including both sedimentary and metamorphic rocks. Metamorphic rocks are rocks that started out as one thing and have been chemically altered by heat and pressure.

susie bartz ranger peak serpentine santa ynez valley natural history society los padres national forest camuesa fault

Geologist Susie Bartz points out Camuesa Fault on the landscape near Ranger Peak

Ms. Gaspar in turn highlighted the work of Arthur Kruckeberg, who was among the early researchers to focus on the flora and ecology of serpentine soils in California. She then led us on a short walk where we could see the disparity between plants growing on serpentine soils versus non-serpentine soils.

“Everything is turned upside down with regard to what plants need,” Ms. Gaspar pointed out. “Plants need a high ratio of calcium to magnesium, but on serpentine soils it’s the opposite, magnesium is high and calcium is low. Serpentine is also low in phosphorous, nitrogen, and potassium, which plants need.”

Plants growing on serpentine soils not only have to tolerate these nutritional deficiencies but also heavy metals such as nickel, iron, and magnesium. And yet, nature has found a way to use serpentine soils, with plants evolving over time to tolerate and even thrive on them.

Serpentine formations cover just 1.5 percent of California’s total land area, but account for 11 percent of California’s endemic species, that is, plants that are only found in California.

Our next stop is along an old road cut, about a mile before Figueroa Mountain Ranger Station. The unpaved access road may have been used for prospecting or mining and leads past a debris slide composed of serpentine rock, where Ms. Bartz explains how serpentinite is formed.

Surprisingly, one of the key ingredients in the formation of serpentinite is water, along with heat, pressure, and time. The parent rock for serpentinite is peridotite, which is formed deep within the Earth’s crust near the mantle. It’s formed at mid-ocean ridges where sea water is able to find its way down into the Earth’s crust, slowly soaking the peridotite over millions of years.

The addition of water makes the rock lighter which brings it up towards the surface. As it’s driven past other rocks it’s not only exposed to additional pressure, but its surface is sometime pressed and scraped creating slickensides, which is what can give serpentinite its shiny appearance.

The addition of water also changes the lattice structure of peridotite as it alters to serpentinite. Whereas, peridotite is hard to break, serpentinite weathers more easily, forming soil that some plants are able to grow in.

One evolutionary pathway is that some plants have random mutations in their gene pool that make them “pre-adapted” to tolerating serpentine soils. As the plants on serpentine soils further adapt and diverge physically they can’t successfully reproduce with their original counterparts, becoming their own species or variety.

Another way plants can become restricted to serpentine soils is if their non-serpentine counterparts are out-competed by other species, leaving just the ones growing on serpentine soils. In Santa Barbara County we have four serpentine endemic species.

liz gaspar serpentine plants ranger peak trail santa ynez valley natural history society los padres national forest

Naturalist Liz Gaspar points out chaparral plants growing on serpentine soil along Ranger Peak Trail

Our third stop along Figueroa Mountain Road is at a grassy hillside across from an outcrop of serpentine rocks near the top of Davy Brown Trail. Here, Ms. Gaspar points out where Devil’s onion might be found. We haven’t had enough rain to see any, but the plant, with its pink-tinted white flowers typically blooms from April to June.

Another serpentine endemic is Santa Barbara jewelflower, which is found only in the San Rafael Mountains. An annual plant, it blooms from May to July. Its nearest relative is found further east in the Transverse Ranges where it mostly grows on granitic soil. The rareness of the plant highlights both the unique habitat provided by serpentine soil and the importance of protecting these areas.

The third endemic growing in our area is leather oak, which we’ll see at our next stop. All three of these are known as strict endemics, meaning 95 percent or more of them grow on serpentine soil.

The fourth endemic in our area is Sargent cypress. It is a broad endemic, meaning 85 to 94 percent of them grow on serpentine soils. Similar in appearance to juniper, the plant was first recorded in our area by local ranger Edgar Davison. Good examples of it can be found along Old Catway Jeep Road, which starts near Davy Brown Campground, and along Cuesta Ridge in San Luis Obispo County.

Plant distribution maps can be found online at Jepson Herbarium eFlora, http://ucjeps.berkeley.edu/eflora, and Calfora, www.calflora.org.

Our last stop is near Ranger Peak. From the top of Ranger Peak Trail we make our way down to the small saddle where the trail branches. From this intersection, we’re treated to a great view eastward out across the San Rafael Mountains and Santa Ynez Valley.

Visible on the landscape is a unique situation where a long break in the plants reflects the transition from Monterey shale to Franciscan rocks, which are separated by Camuesa Fault. North of the fault is chaparral and to the south are mostly grasslands.

From the saddle we hike down along the trail to where a mix of chaparral plants is growing on serpentine soil. Here, Ms. Gaspar points out scrub oak and has us study its leaves and overall appearance. She then points out the serpentine endemic leather oak and we repeat the exercise. Both oaks have stellate hairs on the underside of their leaves, however leather oak also has the small hairs on the topside and its leaf edges curve under.

It is interesting to consider that serpentinite was formed in the Earth’s crust, under the ocean, and is now near the top of the San Rafael Mountains; and not only were some plants able to adapt to serpentine soil, but in some cases are now found almost nowhere else.

Ms. Bartz will be leading a geology field trip in December to the Red Rock area and leads programs for NatureTrack, which connects kids with nature.

Ms. Gaspar worked as the park naturalist for 20 years at Cachuma Lake and is the co-author of Wildflowers and Other Plants of the Cachuma Lake Region. She leads field trips and is on the board of Santa Ynez Valley Natural History Society.

Founded in 2000, the non-profit, member supported organization provides natural history and environmental education through public lectures and field trips in the Santa Ynez Valley region. For a list of upcoming events and programs go to, www.syvnature.org.

This article originally appeared in section A of the April 23rd, 2018 edition of Santa Barbara News-Press.


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