Geology Cafe

Field Trip to the Fremont Peak State Park,
San Benito County, California

Geology Field Trips & Guidebooks
Field-trip overview: A field trip to Fremont Peak State Park includes spectacular views of the Central Coast region of California near Monterey Bay. Fremont Peak is the highest peak at the north end of the Gavilan Range. The drive from San Juan Bautista to Fremont Peak State Park is an 11-mile drive up San Juan Canyon Road. Features along the way include the trailhead to the Juan Bautista de Anza National Historic Trail (which follows the Old State Road). Rocks exposed along San Juan Canyon Road include sandstone, weathered granite (saprolite) and marble and schist (in Fremont Peak State Park). A stop along San Juan Canyon Road provides a sweeping view of the San Andreas Fault in the Hollister Hills State Recreational Vehicle Park. A half-mile loop trail leads from a parking area to the mountaintop at Fremont Peak. Camping is available at the park.

Directions: From US Highway 101 take CA Highway 156 East (via the ramp to San Juan Bautista/Hollister.

Follow CA Highway 156 east for 3.4 miles to the first stoplight at the Alameda (the Windmill Market is near the intersection with the Alameda in San Juan Bautista).

Turn right at the light on The Alameda.

Continue south on The Alameda to a four way intersection follow the sign to Fremont Peak State Park via San Juan Canyon Road. (See Stops A-C below. Stops are illustrated on the geologic map.)

Be cautious driving up the narrow and winding road; some of the turns have poor visibility. Stop D is an about six miles up the canyon at a pull off on the right that provides as sweeping view of the Hollister Hills State Recreational Vehicle Park.

Follow San Juan Canyon Road for 11 miles to the mountaintop destination: Fremont Peak State Park (Stop E).
Click on images for a larger view.
Regional map Geologic Map of Fremont Peak
Regional map showing Fremont Peak State Park Geologic Map of the Fremont Peak region1
Sign says 11 miles to Fremont Peak Last rails in San Juan
4-way intersection of Mission Vineyard Road, San Juan Canyon Road, Old State Road (Anza Trail) and the Salinas Road near San Juan
Last rails to Cement Plant cross San Juan Canyon Rd.
Anza Trail San Juan Mine Dump
Old Stage Road trailhead Old mine dump tailings pile
Road-trip to Fremont Peak State Park

The park headquarters for Fremont Peak are located on Second Street near the Plaza at San Juan Bautista State Historic Park near downtown San Juan Bautista. From San Juan Bautista, plan a few hours minimum to drive to Fremont Peak, do a mile hike around the mountain peak, and plan to have a picnic is the campground area before you return to San Juan Bautista.

Stop A—Intersection of San Juan Canyon Road: Drive south on the The Alameda from downtown San Juan Bautista across CA Highway 156 and proceed 0.3 miles to Stop A at the intersection of several roads: Mission Vineyard Road (to the left), Fremont Peak Road (middle road to left) and Salinas Road (straight and to right). Note that the trailhead access road to the Juan Bautista de Anza National Historic Trail—a hiking/horseback riding trail that follows the unpaved historic Old Stage Road between Salinas and San Juan Bautista. Also note the section of railroad tracks in the intersection of the four roads. The San Juan Pacific Railway began operation in 1907 and provided rail service from the San Juan Portland Cement Plant to the Southern Pacific at Chittenden. The San Juan Pacific Railway operated a seven mile line, the Mission Route, starting near to San Juan Bautista.

Stop B—Cement Plant mine tailings: Follow San Juan Canyon Road. Be cautious when stopping on the road to examine views. Near the mouth of San Juan Canyon is a large mine tailings pile that was associated with a large cement plant that operated in the lower San Juan Canyon from around 1903 to closure near the end of World War II.

The Flint Ranch deposit, a principal source of supply, lies along the tops of the steep hills above San Juan Canyon. In the Gavilan Peak deposits, the limestone shows all degrees of silicification. At several places near the top of the peak, deposits of white, high-grade barite have been found in the limestone and proved large enough to repay mining. The Gavilan limestone is older than the granitic rock and has been changed to highly crystalline marble. The color ranges from dark blue to nearly pure white. The Flint Ranch deposit (abandoned quarry) is located up a side canyon the Pioneer Park (owned by the local Church of Jesus Christ of Latter Day Saints).

Stop C—Vaqueros Sandstone outcrops: A brief stop might be possible if no traffic is visible along San Juan Canyon Road. Large, scenic outcrops of Vaqueros Sandstone (or Vaqueros Formation) are visible on the left (north) side of San Juan Canyon about a half mile north of the intersection (Stop A). The Vaqueros Sandstone is Upper Oligocene and Lower Miocene in age. The massive sandstone units of Vaqueros Formation are cliff- and ledge-forming units throughout the Coast Ranges of Central California. The rock formation consists mostly of medium-grained sandstone with zones of shale and conglomerate. Locally abundant marine shell fossils suggest the sediment was deposited in a relatively shallow shelf environment. Vaqueros Sandstone crops out in the Santa Cruz Mountains at Castle Rock State Park and near Gilroy at Chitactac-Adams County Park. It crops out in many locations all the way down the coast to the Santa Barbara region. In the San Juan Bautista region, Vaqueros Sandstone crops out along the Salinas Road, and the form outcrops along Rocks Road and Highway 101 south of the large eucalyptus grove south of the intersection of CA Highway 156 (made famous by the 1957 Alfred Hitchcock movie "Vertigo").
Vaqueros  Sandstone
Outcrops of dipping strata of the Vaqueros Sandstone (Oligocene-Miocene) along San Juan Canyon Road
No Trespassing sign
A friendly reminder
San Jan Canyon Road
San Juan Canyon Road on granite ridge
Hollister Valley
Hollister Valley and Quien Sabe Range in distance
Stop D—Hollister Hills overlook: San Juan Canyon Road follows a linear drainage until it rises to a ridge line. Along the lower part of the canyon Vaqueros sandstone crops out in road cuts. Near the ridge line the character of the bedrock changes (along with the character of the soil and the vegetation it supports. The road crosses the Vergeles Fault, a splay fault of the San Andreas Fault that cuts through the western foothills of the Gavilan Range. On the north side of the fault is Vaqueros Sandstone; on the north side is Cretaceous-age granitic rocks. As the road rises out of the canyon the vegetation changes to chaparral community of plants that thrive on the well-drained sandy soil and weathered bedrock. Notice the deeply weathered character of the granite on the right side of the road. In many places the uniformly coarse-grained granite that locally displays quartz- and feldspar-rich veins and pegmatite intrusions. Continue about a mile up the canyon and pull off on the right at a prominent pull off (large enough for a dozen cars). Examine scenic vistas on either side of the road.

To the east side of the road is the Hollister Hills State Recreational Vehicle Park. The motorcycle trails in the canyon east of the road cut through granite terrane throughout the hillslopes covered with chaparral. In the distance lower in the canyon the vegetation changes into grassland with scattered oak forest. The change in vegetation marks the location of the San Andreas Fault where it cuts through the foothills (Hollister Hills) of the Gavilan Range. The view extends across Hollister Valley to the Quien Sabe Range southward into the San Benito River valley south of Hollister.

To the west side of the road is a view that extends northward over a ridge that bears the scar of an abandoned quarry where marble and silicified rock was mined for the manufacture of cement at the former cement plant near San Juan Bautista. Loma Prieta Peak in the southern Santa Cruz Mountains is in the distance.



Stop E—Fremont Peak State Park:
As the San Juan Canyon Road continues it ascent to the mountain peak it enters upland forest with abundant oak and bay laurel trees. In the spring the Fremont Peak upland region usually provides a spectacular wildflower display. Fremont Peak is the highest peak at the north end of the Gavilan Range (gavilan means hawk in Spanish).

Near the entrance to Fremont Peak State Park the road crosses a geologic boundary between the Cretaceous granitic rocks to older bedrock consisting of marble, gneiss and schist of undermined age (due to a lack of identifiable fossils). Despite having been heavily metamorphosed, some of the rocks do preserve bedding structuresthinly laminated layers in the rock resembles rocks of late Proterozoic to Cambrian age in the Death Valley and Mojave region of southern California.

Fremont Peak State Park encompasses a portion of the high mountain peak and includes a campground, group area, and an astronomical observatory. On moonless nights astronomy groups frequently gather on the mountaintop park for star gazing. In a parking area near the observatory there are often dozens of telescopes are set up and shared for public viewing.

A kiosk to pay for a $6 day-use pass is located in the parking lot near the campground entrance. A trailhead parking area is located a short distance farther up the mountain. Both a paved road (maintenance vehicles only) and a hiking trail lead to the mountain top. It is recommended to follow the hiking trail up and return down the steeper paved road. The combined trail provides a counterclockwise loop around the mountain top.
saprolite dikes and intrustions
Igneous veins in weathered granite along road
Intrusion in weathered granite
Pegmatite dike in weathered granite along road
Abandoned Cement Mine
Old cement quarry mine in San Juan Canyon and Santa Cruz Mountains in distance
Hollister Hills SRVP
Change in vegetation shows location of San Andreas Fault in Hollister Hills SVRP

Loop Trail Walk to Fremont Peak

The hike to Fremont Peak is about a mile and involves about a 400 foot climb in elevation. Be prepared for windy conditions and heat on clear sunny days. The peak is frequently enshrouded in fog or clouds in the mornings or during inclement winter. It is recommended to check weather forecasts before traveling to the park. However, the weather is typically perfect for a day trip or camping trip throughout the year. The closest services available are in San Juan Bautista. Be aware of poison oak and uneven footing along the trail. Rattlesnakes, although rare, may be present, so watch where you step or place your hands when examining or climbing on outcrops.

Near the trailhead are displays describing the events associated with the arrival of US Army explorer, John C. Fremont, and his group of "surveyors" sent west to explore the west coast. The arrival of the crew in the central coast region in 1847 came unannounced and they were threatened with arrest if captured by troops under authority of General Commandante General of the Mexican Army, José Antonio Castro. Historical accounts describe Fremont defied orders to leave, instead Fremont's crew climbed "Gavilan Peak" (former name of Fremont Peak) and planted a United States flag and set up crude fortification. The Mexican Army troops stationed in San Juan Bautista responded, but a preliminary skirmish only involved shouting and profanities from both sides. Legend is that the wind blew down Fremont's flag, and they decided to flee instead of facing the odds of surviving a military encounter with an overwhelming force of Mexican troop. (The fact that there no water on the mountain top may have influenced their decision.) Fremont and his crew fled before the Mexican troops returned and they continued their journey to Sutter's Fort and Sacramento where they helped organize a revolt against the Mexican authority and declared the region as the Bear Flag Republic. The California Gold Rush of 1849 began a flood of non-Spanish speaking people into the region. With the overwhelming surge of immigrants, the republic became the state of California on September 9th, 1850.

Along the hiking trail examine outcrops of crystalline marble and partially metamorphosed dolomitic limestone. Blocks and small outcrops of schist, gneiss, and slate are visible along the trail. The trail passes through a col (or gap) along a ridge line along the crest of the mountain. On the north side of the ridge the mountainside is forested whereas on the south side the mountainside is grass covered and open with spectacular views encompassing the Salinas Valley and Monterey Bay region. Several small mines with mine tailings are located along the trail and are visible on the private ranch land along the ridge outside of the park. The small mines were dug to gather barite and travertine ore that was processed for paint (whitewash) for the Mexican communities in the region.

The trail loops around the mountaintop, climbs through a couple switchbacks ,and joins the paved road in the vicinity of a radio tower complex on a peak adjacent to the higher Fremont Peak. A short but rugged trail, more of a climb, is required to get to the high point near the flag pole on the peak. Use extreme caution climbing over the rugged rocks, but the sweeping 360° view at the top is worth the effort.

Following the horizon in a counterclockwise view: To the south is a view of the crest of the Gavilan Range and the rift valley of the San Andreas Fault extending south to San Benito Mountain at the south end of San Benito County. To the east is the Gavilan Range which includes the volcanic peaks of the Quien Sabe Range on the east side of Hollister Valley. To the north is the Santa Clara Valley extending northward to San Francisco Bay. To the northwest is the Santa Cruz Mountains with the high peak of Loma Prieta and Ben Lomond Mountain near Santa Cruz. To the west is a scenic view of the marble ridge (of which Fremont Peak is a part). Beyond the ridge is the coastal plain around Monterey Bay extending from Santa Cruz, Watsonville, Moss Landing (indicated by the tall power plant towers), Salinas. To the west is the Monterey Peninsula and the Santa Lucia Range on the opposite side of the Salinas Valley. The highest peaks in the coast ranges are part of the Ventana Wilderness at the south end of the Santa Lucia Range.

The core of the Gavilan Range is mostly granitic rocks with large "roof pendants" (masses of older metamorphic rock) that escaped destructive melting when the granitic magma intruded into crust in Cretaceous time (between about 80 to 90 million years ago). A large block of crust that would become the Gavilan Range was ripped off of southern California batholith (crust beneath an ancient volcanic arc) and carried northward by plate tectonic forces associated with the San Andreas Fault and older fault systems that proceeded it. The Gavilan Range is bounded on the east by the San Andreas Fault, and its south and eastern flank is blanketed by younger Tertiary-age sedimentary rocks.

The structure of the pendant block that forms the ridge line at Fremont Peak is revealed by the steeply northward dipping bands of layers of marble exposed on the south side of the ridge. Close examination of some of the marble outcrops show that in many places bedding plains are preserved, having escaped complete destruction in metamorphic phase in the formation of the marble and schist. Some of the marble outcrops display stromatolite-like texture (stromatolites are flat-layered, lumpy or mound-shaped accumulations of calcareous sediment associated with the growth of lime-secreting cyanobacteria. The possible occurrence of stromatolites and the apparent lack of shell fossils suggest that the original sediments may be latest Precambrian (Proterozoic) to Cambrian age before shell fossils became abundant in the fossil record.

In many areas the marble has been completely recrystallized into massive granular crystalline rock: marble and dolomite marble. Pods of dense, white, crystalline barite (barium sulfate) were mined around the mountain top. Fragments of barite still can be found in the historic mine tailings. Travertine (banded freshwater limestone) occur along small cavern-like fissures throughout the mountain top and can be seen near the flagpole on the peak

Selected References

California Department of Parks and Recreation, [2012], Fremont Peak State Park, official park website: http://www.parks.ca.gov/?page_id=564

Dibblee, Thomas, W, 1979, Preliminary Geologic Map of the San Juan Bautista Quadrangle: U.S. Geological Survey Open-File Report 79-375, scale 1:24,000. See a revised online version at: http://geologycafe.com/maps/SJBgeologicmap.html

Perazzo, P.B. and Perazzo, G., 2012, San Benito County List of Stone Quarries, Etc. Stone Quarries and Beyond website: available online at: http://quarriesandbeyond.org/states/ca/quarry_photo/ca-san_benito_photos.html

1Wagner, D.L., Green, H.G., Saucedo, G.J., and Pridmore, C.L., 2002, Geologic map of the Monterey 30'x60' Quadrangle and adjacent areas, California. California Geological Survey, Regional Geologic Map No. 1, 1:100000 scale: http://www.quake.ca.gov/gmaps/RGM/monterey/monterey.html

Fremont Peak flag pole and marble outcrops
Fremont Peak with outcrops of marble
Gavilan Range crest
Crest of Gavilan Range south of Fremont Peak
View of Fremont Peak campground
Looking northeast over Fremont Peak Campground
Loma Prieta
Looking north toward Loma Prieta Peak from the peak
Marble Ridge at Fremont Peak
Marble forms a resistant ridge west of Fremont Peak
Salinas Valley
Salinas Valley south west of Fremont Peak
Barite mine at Fremont Peak
Old barite mine on Fremont Peak
Stromatolite
Possible stromatolites preserved in marble bedrock
http://geologycafe.com/fieldtrips/fremont_peak.html
9/11/2012