Bay Nature magazineApril-June 2008


The Fremont Peak Experience

April 1, 2008

Eleven miles south of San Juan Bautista and up a narrow, winding road is one of Northern California’s hidden treasures—Fremont Peak State Park, straddling the San Benito and Monterey county line. The park is an extraordinary place, where you can look back in time to our state’s early history, or even further back into geologic time when the peak’s rocks were formed. Or further than that into astronomic time as you look into the deep sky of our Milky Way galaxy through the Fremont Peak Observatory’s fine telescope. Stretch your mind and time travel or stay firmly rooted to our planet on the park’s hiking trails, which take you through some of the Bay Area’s least common rocks and give you spectacular views from the peak’s 3,169-foot summit.

Captain John C. Fremont—explorer, scientist, and surveyor—camped on the peak in 1846 during a survey of the Salinas area, then still part of Mexico. He took refuge on Gavilan Peak, as it was then called, after an altercation with General Jose Castro, commandant at the Monterey Presidio, who had ordered Fremont to leave California. In defiance, Fremont built a log stockade and raised a U.S. flag at the top of the peak that now bears his name. Shortly thereafter, however, he and his troops departed in the middle of the night to avoid a confrontation with Mexican troops.

Fremont Peak is now a small state park that happens to be the best place in the greater Bay Area to see ancient and far-traveled metamorphic rocks that occur only west of the San Andreas Fault. Elsewhere in Northern California they occur mainly in small outcrops or on private land. Fremont Peak is at the northern end of the Gavilan (or Gabilan) Range, between the Santa Lucia Range to the west and the Diablo Range to the east and north. The San Andreas Fault lies just east of the Gavilan Range, and that is the key to the park’s extraordinary geology. The Pacific Plate, west of the fault, is sliding past the North American Plate at an average rate of about two inches per year on its way to Alaska. Fremont Peak, on the eastern edge of the Pacific Plate, is heading north with it.

The Gavilan Range is composed of granitic and metamorphic rocks found only west of the San Andreas. Geologic evidence indicates these rocks were rafted to their present location from several hundred miles to the south by movement along the fault. The granitic rocks were likely formed as a southern extension of the Sierra Nevada about 80 to 100 million years ago when an ancestral Pacific Plate (the Farallon Plate) was colliding with the North American Plate. The denser oceanic Farallon Plate sank beneath the continental plate in a process called subduction. Like the granitic rocks of Half Dome in the Sierra Nevada, those of the Gavilan Range were formed deep below the surface as the sinking Farallon Plate began to melt. The resulting molten rock, or magma, intruded the rocks of the North American Plate, metamorphosing them. The magma solidified into granitic rocks like those at Fremont Peak, which occur in only a few places in the Bay Area—Montara Mountain on the Peninsula, the Farallones, Point Reyes, and Bodega Head.

The story of the even rarer metamorphic rocks is a remarkable one of travel through time and space. They probably formed from sediments deposited in a warm shallow sea about 300 to 500 million years ago, when the landmass that became the North American Plate was thousands of miles to the south near the equator. These sediments hardened over time into rock, forming mudstone and limestone, which in turn were metamorphosed by tremendous heat and pressure into schist and marble, respectively, as the magma intruded them. After the plate collision ended, the overlying rocks were eroded away, exposing the granitic rocks and small remnants of the metamorphic rocks.

At Fremont Peak, unlike in most of the Gavilan Range, marble is the dominant metamorphic rock. It forms the peak itself. Much of the forested area of the park is underlain by schist, which glitters with white mica. The lower ridge on which the observatory is located is composed of quartzite, which, in our climate, is less resistant to weathering than the marble of the peak.

The vegetation at Fremont Peak clearly reflects the underlying rock. Grassland and forest grow on the schist and marble; chaparral on the granitic bedrock. A short walk down the fire road past the gate at the end of the Oak Point Campground loop takes you through the oak forest, in spring strewn with large clusters of yellow false lupine. Abruptly the forest gives way to chaparral, characterized here by brittle-leaf manzanita and chamise. Look underfoot to see why. The dark soil formed on the metamorphic rocks underlying the forest gives way to light sandy soil on granitic rock. This soil is very porous and does not retain enough moisture for trees. Only chaparral vegetation can grow on it.

Wildflowers are abundant here after a rainy winter. Bright California poppies color the grasslands. Purple lupine thrives on the south-facing slopes of the ridge near the observatory. Look for fremontia, a bush with showy yellow blossoms, along the road up to the peak.

The Fremont Peak Trail circles the peak and is the best trail for rocks, flowers, and expansive views to the west. Stop to read the Fremont Historical Plaque in the upper parking lot, then start up the hill on the gated service road. The trail branches off to the right.

The marble-derived soil along the trail is host to wildflowers, including shooting stars, linanthus, and woodland stars, while bright orange lichen grows on the rock itself. Close up, the rocks exhibit interesting crystalline textures. At many outcrops, the marble displays a beautiful mosaic of coarse crystals. The marble is metamorphosed limestone and dolomite, both of which are carbonate rocks consisting of the mineral calcite. You can recognize the dolomite by its cream color and the unusual —elephant-skin— surface that forms as it weathers. Limestone is fluted and pitted by rainwater, which is slightly acidic, running down the rocks and dissolving the calcite. Carefully run your hand over the limestone rocks to see how sharp the edges can be. Fieldwork in such rocks is very hard on the hands, knees, and clothes.


California Fuchsia
In late summer and early fall, California fuchsia addswelcome color to a tawny landscape. Beneath the flower is some of thepark’s coarse crystalline marble. Photo by Michael Kran.

One of the most interesting deposits you will see along this trail is the mineral barite. Look for a heavy snowy white mineral—it’s much heavier than the rocks with which it is associated. The Fremont Peak barite is found as veins in the marble. This barite is especially high grade and was mined between 1916 and 1920, mainly for use in flat wall paint. In those years the peak accounted for about 25 percent of California’s barite production. A barite pit and a tailings pile lie along the trail on the peak’s southwest side, and tailings of other mines can be seen along Rocky Ridge to the west.

As the trail comes out of the woods into rolling grassland, views open up to the west. The Salinas River meanders through its wide valley, the entire curve of Monterey Bay lies before you, and if the air is clear, you can see northwest to Santa Cruz.

A moderate climb takes you up to the saddle between the peak and the transmission towers. The rock here is schist, which is less erosion-resistant than the marble. From this saddle, nimble and careful hikers can scramble up the marble blocks to the summit for a spectacular 360-degree view. To the north and east are the volcanic peaks of the Quien Sabe Volcanics in the Diablo Range. The San Andreas Fault runs down the San Benito River valley in the foreground. To the south, the Gavilan Range stretches into the distance. Across the Salinas Valley to the west lie the Santa Lucia Range and to the northwest the Santa Cruz Mountains, both underlain by granitic rocks. To the north, the flat valley in which San Juan Bautista lies is the former lakebed of Lake San Juan, which probably formed when movement along the San Andreas Fault dammed stream drainage flowing to the west. In the distance is the Santa Clara Valley.

On a warm day the Valley View Trail provides relief from the sun; the path starts from the upper parking lot and takes you down and along the hillside through shady oak woodlands. Where it crosses the fire trail, turn north to see the abrupt change from forest to chaparral where metamorphic bedrock meets granitic bedrock.

It is not only geology and history that make Fremont Peak special. On weekends without much moonlight, you can peer into deep space thanks to the Fremont Peak Observatory telescope. The peak is just high enough that most nights it rises above the fog that so often envelops the Salinas-Monterey area, blocking out the lights of Salinas, Santa Cruz, and Monterey. With those areas in fog and the lights of San Jose far away, the dark skies are ideal for star-gazing, and Bay Area amateur astronomers have been holding star parties on the peak for almost 50 years.

One of the first groups to hold a star party here, in the late 1960s, was the San Mateo Astronomical Society. It turned out to be such a great site that other groups soon joined them. It was not unusual to have 200 people and 100 telescopes in the park on a summer weekend. Soon amateur astronomers began thinking about an enclosed observatory at Fremont Peak, but it took more than a decade to bring the idea to fruition.

From the beginning, state park rangers welcomed the groundbreaking notion of volunteers operating a public facility in the park, and the Fremont Peak Observatory Association (FPOA) was organized to make it happen. The observatory was built with donations of equipment, money, time, and labor, and in 1986 visitors got to look at the deep sky through a large 30-inch telescope built by amateur astronomers Denni and Kevin Medlock—who ground the mirror and built the scope in their garage. Today public star parties are held on weekends near a new moon. After a program in the observatory’s meeting room, the roof rolls back, the telescope is pointed at the sky, and you are transported from Earth to outer space as planets, nebulae, star clusters, double stars, and other deep sky objects appear.

So choose a weekend near a new moon to visit the park. Enjoy the rocks, flowers, and views during the day, grab a little nap and supper, then marvel at the skies at night. The amateur astronomers on hand bring their own telescopes and are happy to share both their knowledge and their scopes. This spring star parties begin on Saturday, April 5. See the Fremont Peak Observatory Association website for the schedule.

On April 27 this year, an old fashioned picnic will celebrate the 100th anniversary of Fremont Peak Days. This annual event, sponsored by the Veterans of Foreign Wars and the Native Daughters of the Golden West, started in 1908 to commemorate Captain John C. Fremont’s U.S. flag raising.

Whenever you go to Fremont Peak, consider stopping in San Juan Bautista to get a close-up look at traces of the San Andreas Fault, which runs through San Juan Bautista State Historic Park. Along the northeast side of the mission, you can see breaks in a wall and a small escarpment parallel to the stone bleachers.

Getting There

Take Highway 101 south to Highway 156. Go east toward Hollister to the stoplight at San Juan Bautista. Turn right onto San Juan Canyon Road. At the Y intersection at Mission Vineyard Road, jog left and continue up San Juan Canyon Road about 11 miles to the park.

About the Author

Doris Sloan's articles on the geology of the Bay Area have been appearing in Bay Nature since our first issue. Her book Geology of the San Francisco Bay Region was published by UC Press in 2006.

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