Songs of the Wilderness

Oso Canyon is one of many canyons associated with the San Rafael Mountains. A loop hike to Hidden Potrero is one of the ways to take in the rich scenery and geology that the canyon has to offer.

The hike to Hidden Potrero, along Buckhorn-Camuesa Road, is about 4.5 miles, and provides great views out across the canyon. An alternate route back, also 4.5 miles, can be made through Oso Canyon, and can include a quarter-mile side trip down to the site of an old mercury mine.

To get to the trailhead from Santa Barbara, take State Route 154 over San Marcos Pass to Paradise Road. Turn right onto Paradise Road and continue past the river crossing to Lower Oso Day Use Area. At Lower Oso, turn left and continue towards Upper Oso, along what’s referred to as Romero-Camuesa Road.

The meadow at Hidden Potrero Camp

A mile later, the road arrives at the turnoff for Upper Oso Campground. Stay to the right, and continue a short way to the locked forest service gate, which represents the trailhead. From here, Romero-Camuesa Road continues, unpaved, as a forest service access road, and is more commonly referred to as Buckhorn-Camuesa Road. Parking is found near the gate. An adventure pass is required park within the Lower Santa Ynez River Recreation Area.

From the trailhead, continue along Buckhorn-Camuesa Road. The road is popular with OHV (off highway vehicle) and motorcycle enthusiasts, and so you will need to be alert to this traffic while hiking along the road.

Buckhorn-Camuesa Road follows Oso Creek for the first .75 miles, and passes through what could be described as Oso Narrows. Here, the watercourse has cut through the exposed Matilija sandstone.

At the .75-mile mark, the road arrives at the official beginning of Santa Cruz Trail. From here, Santa Cruz Trail continues up Oso Canyon towards Nineteen Oaks; this is the return trail for the loop part of the hike.

From the Santa Cruz Trail junction, continue on Buckhorn-Camuesa Road as it starts to climb away from Oso Creek. The road makes a winding set of turns as it gains elevation, before starting to level out and parallel Oso Canyon. Here, one is treated to views out across the canyon towards Alexander Saddle and Little Pine Mountain.

It’s also here, looking down into the canyon, that one can more easily spot where the rock type transitions from Matilija sandstone to Espada Formation and into Franciscan rocks. The transition line also marks the location of Little Pine Fault, which uplifted the older Espada and Franciscan rocks. Tracing the canyon west to east, one can see where beige sandstone transitions to olive-gray Espada rocks, and then past that into the more varied geology of greenish-brown Franciscan and blue-green serpentine outcroppings.

Serpentine rock

Both Franciscan assemblage and serpentine rocks are metaphoric, i.e. rocks that have been changed physically and chemically from exposure to high heat and pressure beneath the earth’s surface. Franciscan rocks were once sedimentary and igneous rocks on the floor of the Pacific Ocean, on what was then the Farallon Plate. Those rocks were metamorphosed through the process of subduction. Serpentine was formed beneath the earth’s crust and was changed as it rose to the surface.

As the eastern moving Farallon Plate subducted under the North American Plate, the Franciscan material was scraped off the Farallon Plate and added to the western edge of North America. As more and more of the Farallon Plate subducted under the North American Plate, it brought the Pacific Plate behind it into contact with the North American Plate.

However, unlike the Farallon Plate, the Pacific Plate was, and still is, moving northward. Instead of sliding under the North American Plate and forming a subduction zone at the plate boundary, the Pacific Plate is sliding north against it, creating a transform fault, i.e. the San Andreas Fault, where the two plates meet.

As the Pacific Plate moved north against the North American Plate, it picked up material from the North American Plate, which it dragged along with it. One of the blocks of land it picked up was the Transverse Mountains, which used to be down by San Diego. Over the course of time they were dragged northward and turned 90 degrees, arriving where they are now, just south of the San Rafael Mountains.

An interpretive guide for the geology of Oso Canyon can be found at, http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsm9_034048.pdf. Included in the guide is a Dibblee-style geologic map of the area that can be helpful in identifying rock types and their transitions. Similar USGS geologic maps can be viewed online at http://ngmdb.usgs.gov/maps/mapview/.

Hidden Potrero Camp is seen from a nearby hill-size outcropping of Franciscan Rock

At about the 2.75-mile mark, the road arrives at Camuesa Connector Trail, which leads down to the Santa Ynez River. Here, the Franciscan rocks become more apparent on the landscape. And as the road continues, it passes a large, exposed outcropping of serpentine rock.

About a half mile past Camuesa Connector Trail, look for a trail turnoff on your left. This is the beginning of the trail that leads down to Nineteen Oaks for the return part of the loop hike. The trail follows an old road cut that leads down towards the mine site. The trail is unmarked, but is recognizable by the metal sign that reads “No Motor Vehicles” and the fence across the trail, both of which are there to remind motorcyclists not to ride on non-OHV designated trails.

From here it’s roughly another 1.25 miles along Buckhorn-Camuesa Road to Hidden Potrero Camp. As you near the camp, you’ll see on your left a large outcropping Franciscan rock that forms a good size hill. Just past the outcropping is a side trail that leads through a small meadow and arrives at the campsite.

Hidden Potrero Camp was built in the late 1920s, and was used as a base camp when the road was being constructed. The word potrero is Spanish for pasture. The camp has two picnic tables and a metal fire ring.

Just a short way past Hidden Potrero Camp, Buckhorn-Camuesa Road branches. Camuesa Road continues eastward over towards Mono Campground, and Buckhorn Road continues north over the San Rafael Mountains.

Little Pine Mountain is seen from the old mine road that leads from the Buckhorn-Camuesa Road down towards Nineteen Oaks

For the hike back, return along Buckhorn-Camuesa Road to the beginning of the old mine road. The trail down to Nineteen Oaks follows the old road cut as it descends down towards Oso Creek. The trail is in generally good shape and offers views out across the canyon towards Little Pine Mountain.

About 1.25 miles from Buckhorn-Camuesa Road, the trail makes a 90 degree turn and continues towards Nineteen Oaks. At this turn, the old road cut continues straight, descending down towards Oso Creek and the mine site.

The trail down to the mine site is more overgrown and eroded, but is still easy to follow down to the creek. At the creek the trail disappears; however, at this point you’re almost at the mine site, which is across the creek. There is little to find at the site, and it’s a bit of a scramble to the top of the tailings leftover from the mine. At the top, one can find orange-colored metamorphic rocks associated with cinnabar deposits, similar to those found at the quicksilver mines along the Santa Ynez River.

From the turnoff to the mine, the main trail continues another quarter mile down canyon and arrives at Nineteen Oaks. The camp has two sites under the oaks, each with a picnic table and fire ring. Past the camp, the trail arrives at the intersection with Santa Cruz Trail.

Turn left onto Santa Cruz Trail and continue downstream. Here, the trail passes through more intriguing rock features, such as red-banded chert, serpentine and blue schist, before transitioning back into Matilija sandstone. The trail then arrives back at Buckhorn-Camuesa Road, which leads back to Upper Oso.

This article originally appeared in section A of the December 1st, 2014 edition of Santa Barbara News-Press.