National Association of Geoscience Teachers, Far West Section

Fall 1998 Meeting, Modesto, California


San Andreas Fault Near Hollister Field Guide


Jeffrey W. Tolhurst, Ph.D.
Columbia College





            This field guide is a general overview of the geologic setting along the San Andreas fault system near the towns of Hollister, San Juan Bautista, and Aromas in San Benito County, California.  Since this is a one-day trip, discussion of the seismotectonic and stratigraphic details will be limited in scope.  These details are discussed in more depth in a number of publications, including Allen (1946), Rogers (1969, 1980), Dibblee (1980), Vedder, Howell, and McLean (1983), Ernst (1987), Bucknam and Haller (1989), Hill, Eaton, Ellsworth, Cockerham, Lester, and Corbett (1991), Sims (1993), and Matthews (1997).


Summary of the Geologic Setting along the San Andreas Fault near Hollister


            The San Andreas transform fault extends for approximately 1,200 km (740 mi) (Norris and Webb, 1990) displacing two active spreading centers – one north of Cape Mendocino and the other buried underneath the southern end of the Salton Trough (see Figure 1).  It is the longest fault in California.  Seismic data indicates the fault is probably between 7 km (4.3 mi) and 20 km (12 mi) deep, averaging about 14 km (8.5 mi) (Norris and Webb, 1990).  The fault occupies a zone of active displacement that seldom is greater than 0.8 km (0.5 mi) wide and is often much less and consists of many closely spaced braided or anastomosing fractures.  These active zones tend to lie within a belt of parallel features that range up to 18 km (11 mi) in length and are 150 to 760 m (500 to 2,500 ft) wide.  Many of these slip surfaces have had histories of movement different from today’s. 

Movement first began along the fault in southern California, when the plate boundary separating the Pacific and Farallon plates, the East Pacific Rise (see Figure 2), reached the continental border about 29 million years ago during the Oligocene (Howard, 1979).   Movement along the fault in the central California region probably did not occur until the Miocene, approximately 21 million years ago, following passage of the Mendocino Triple Junction. 

A. C. Lawson apparently first named the fault in 1895 after San Andreas Lake, which lies in the fault zone south of San Francisco.  It was first recognized as a continuous structural feature following the 1906 San Francisco earthquake.  The fault has produced a number of other significant damaging earthquakes including the 1906 event (see Table 1).  Large scale offset of approximately 305 km (189 mi) along the fault during the Cenozoic was first reported by Hill and Dibblee (1953) as they mapped two juxtaposed terranes: 1) the Salinian block on the southwest; and 2) the Franciscan complex on the northeast.




Figure 1: The San Andreas fault system extends from the Mendocino Triple Junction to just south of the Salton Sea (modified from Norris and Webb, 1990).





Figure 2: Evolution of the western margin of North America over the past 30 million years.






June 1838

San Francisco


January 9, 1857

Fort Tejon


April 18, 1906

San Francisco


October 17, 1989

Santa Cruz


June 28, 1992




Table 1: Historical earthquakes with magnitude > 7.0 on the San Andreas fault.



            The Salinian block, or Salinian composite terrane (SCT), is composed of granitic and high-grade metamorphic rocks sandwiched between two fault systems – the San Andreas on the northeast and the Sur-Nacimiento on the southwest.  Mesozoic Franciscan assemblage rocks, comprised of well bedded distal to midfan turbidites, shaly melanges, and serpentinized peridotites, bound the elongate SCT on both sides (Ernst, 1987). 

            The San Andreas and Calaveras faults are the major structural elements encountered near Hollister and San Juan Bautista and divide the area into three major structural blocks (Rogers, 1980).  The spatial relationships between the Gabilan, San Justo, and Diablo blocks will be shown in the field at Stop 1.  The Gabilan block, west of the San Andreas, consists of SCT rocks.  The San Justo block is comprised of Franciscan assemblage rocks overlain by Pliocene, Plio-Pleistocene, and Quaternary clastic sediments.  The Diablo block, northeast of the Calaveras fault zone, is composed of Cretaceous age sedimentary rocks of the Great Valley Sequence of Bailey, Irwin, and Jones (1964), and is underlain by the Franciscan complex.  Geodetic measurements by Savage and Prescott (1974) suggest east-west extension in this block, which contrasts to northwest right-lateral shear taking place west of the Calaveras fault zone (Rogers, 1980). 

            Recent studies of the chronology of displacement on the San Andreas fault in central California suggest that three phases of evolution of the fault system have occurred (Sims, 1993).  The first stage began with the eruption of the early Miocene volcanic rocks about 24 Ma.  The Neenach-Pinnacles volcanics, produced by the passage of the Mendocino Triple Junction, were soon cut by the growing San Andreas Transform system (Matthews, 1976). The Salinian composite terrane, a magmatic arc of early to late Cretaceous granitic rocks, which bound the fault along its southwestern edge near Hollister, was detached from the Mojave and Sierran blocks during this phase (Mattinson, 1990; Seiders and Cox, 1992).  The rate of slip during the first phase probably averaged about 10 mm/yr (0.39 in/yr) over a period of 8 m.y. (Sims, 1993).

            During the second phase the San Andreas stepped eastward and slip was transferred to the new trace of the fault.  The rate of slip probably averaged about 8 mm/yr (0.31 in/yr) over a period of about 7 m.y. during this phase.

During the third phase the San Andreas experienced deformation, which led to bending of the fault to the point that it could no longer conduct slip around the bend.  A new, stable, straight segment was formed.  Slip rates along this segment were estimated by Sims to be about 33 mm/yr (1.3 in/yr) over a period of about 5 m.y.





Field Trip to Hollister/Cienega Road/San Juan Bautista/Aromas
Jeffrey W. Tolhurst, Ph.D.
Columbia College

Michael Whittier, M.S. Candidate
California State University, Stanislaus


Objectives: By the end of this trip participants will: 1) visit and observe several classic localities along the San Andreas and Calaveras faults near the town of Hollister (see Figure 3) that demonstrate different scales of offsets that have occurred over the past few years to 21 million years; and 2) visit an active quarry operated by the Graniterock Corporation, near the town of Aromas, to observe mining of crushed granite that outcrops along the San Andreas fault.


Road Log:


Cum.       Interval 


0.0       0.0       Begin trip – depart from Modesto Junior College.


0.8       0.8       Union Pacific Railroad


            Railroads played an integral part of the history and settlement of the Great Valley. Early pioneers created towns and supply hubs for the gold fields in the foothills (Miller, 1983). These early towns in the Great Valley were located along the rivers, as were the railroads. The steam locomotives had a steam generating capacity of about 30 miles, so the older towns up and down the valley have a similar spacing. In many Central Valley cities where the railroad had a dominant influence, street grids were aligned with the tracks, rather than to the U.S. Township and Range System. The rather bewildering downtown area of Modesto is oriented to the Union Pacific railway. On of the city’s more unique landmarks is the railroad that traverses the middle lane of Ninth Street. Visitors are often surprised to hear train whistles behind them in the left turn lane!

            Prior to December 7, 1870 the town of Modesto was named Paradise City, and was located on the banks of the Tuolumne River southwest of the present downtown area (Brotherton, 1975).  When the railroad was completed, the town was re-established adjacent to the tracks. The town became the county seat in 1871 and was incorporated in August 1, 1884. The townspeople wanted to name the town after one of the railroad owners, a Mr. Ralston, but he was too ‘modest’ to accept the honor, and so ‘Modesto’ was adopted instead.


1.2       0.4       Turn right onto the Highway 99 south on-ramp. Drive 2.5 miles to the Crows Landing off-ramp.


3.7       2.5       Take the Crows Landing off-ramp and turn right (south).


11.0     7.3       Land Use Patterns

As we leave the city, agricultural land use patterns along Crows Landing Road become readily apparent. These include row cropping and orchard farming.  The row crops are mainly dedicated to feeding local livestock populations and include corn, alfalfa and hay. The crops are chopped, cut or bailed and fed to cattle for milk and meat production.

            The orchards consist primarily of walnuts and almonds, but also include cherries, peaches and grapes. For more details of orchard farming in the Great Valley, see appendix B, “Geographic notes on the Great Valley: Orchard Farming”.

Soil types and their specific mineral concentrations control the distribution of the crops grown. At first we see deep-rooted plants such as trees because sandy loams located here are the most productive soils for these plants.  As Crows Landing Road approaches the San Joaquin River, the types of plants and land use activities change.  Corn and alfalfa, which have shallow root systems, occupy the large fields on either side of the road.  Dairy farms began to dot the area. These choices in land use are influenced by the underlying geology.  For many thousands of years, channels have drained stream runoff, floods, and more recently, irrigation waters to the same sites near the San Joaquin River.  Increased salinity and alkalinity levels in soils close to the river (see Figure 4) has led to programs developed to mitigate these effects in hopes of increasing crop yield in these areas.

            The soils on the opposite side of the river do not have the high saline-alkaline chemistry (see Figure 5).  From east to west, soils grades from finer materials, such as clays, into the coarser sandy loams. We will first see row crops of alfalfa, beans, or corn near the river, then mostly walnut orchards as we approach Interstate 5.


25.3     14.3     Ogden-Martin's Stanislaus Resource Recovery Facility


The plant to the right of Interstate 5 (I-5), at the Crows Landing interchange, incinerates municipal solid waste.  It burns a total of 800 tons of garbage per day, in two furnaces generating 18.5 megawatts of electricity, which is sold to the Pacific Gas and Electric Company (PG&E). For details, see appendix B, “Geographic notes on the Great Valley”.


26.0     0.7       Citrus Groves


            The dark green, heavily foliated trees to the left of I-5 are citrus groves.  Visible here are orange, grapefruit, and lemon trees. They are easily distinguished from the walnut trees in the area, which are taller, broader, and more expansive.  Almond trees are shorter and fuller looking.  The walnut and almond orchards are steadily being relocated to the flanks of the Valley away from prime agricultural soils due to urban sprawl.  The citrus trees have been planted here because they grow best on these hills and slopes due to geological and meteorological conditions found here (see appendix B, “Geographic notes on the Great Valley: Citrus Trees”).


29.6     3.6       California Aqueduct (C.A.)


The large canal crossing under I-5 winds southward toward O'Neill Forebay, near the town of Santa Nella.  The sources for the aqueduct water include Lake Shasta and Lake Oroville.  The California Aqueduct is part of the State Water Project (see appendix B, “Geographic notes on the Great Valley: Federal and State Water Projects”).


30.5     0.9       Orestimba Creek


Creeks and streams on the western side of the San Joaquin Valley, flow east toward the San Joaquin River, providing diverse riparian habitats that stand in stark contrast to the surrounding dry grasslands. Currently the creeks are dry but during the wetter winter months, they may have flows averaging 50 to 200 cubic feet per second (cfs) or more.  These ephemeral streams occupy canyons that established their courses when the range was uplifted in latest Cenozoic time (Page et al., 1998).  One of the largest and deepest of these is Del Puerto Canyon, the subject of Sunday's field trip.

            Just past the rise out of the Orestimba Creek flood plain, I-5 briefly passes exposures of the Valley Springs Formation.  Its type section lies about 140 miles to the north and west in the foothills of the Sierra Nevada Mountains near New Hogan Reservoir.  This formation is lower Miocene to upper Oligocene in age.  The outcrop here shows a yellowish-gray to tan clay-rich sandstone and sandy claystone that represent a near shore depositional environment.  In the type area, the unit is composed of primarily of rhyolitic tuff and stream deposits. 


30.9     0.4       Open Range Lands and European Grasses


In stark contrast to the lush green hills and mountains of late winter and early spring, dry annual grasses offer a dry and barren backdrop to the San Joaquin Valley.  This landscape is not representative of what existed 150 years ago.  The hills and mountains were light green with expansive populations of endemic bunchgrasses (family Gramineae), (Robbins and others, 1920).  This has changed radically as European settlers brought exotic grass species that soon overwhelmed the native bunchgrasses (for more details, see appendix B, “Geographic notes on the Great Valley: Natural Vegetation”).


46.5     15.6     Take exit for Santa Nella and Highway 33.  Stay to the right and follow the road through town south toward Highway 152.


48.3     1.8       Delta Mendota Canal (D.M.C.)


The Delta Mendota Canal is part of the California Valley Project (CVP).  This canal, like the California Aqueduct, carries water from wetter northern regions of the state to the drier southern areas (see appendix B, “Geographic notes on the Great Valley: Federal and State Water Projects”).


49.0     0.7       O'Neill Forebay


            At the top of the rise outside of Santa Nella traveling south on Highway 33 just past the wildlife refuge, the Delta Mendota Canal again crosses under the road followed in turn by the California Aqueduct.  These canals are transporting water from the O’Neil Forebay to points south.  (See appendix B, “Geographic notes on the Great Valley Federal and State Water Projects”).        


50.2     1.2       Turn right, just before the overpass, and head west on Highway 152.


52.4     2.2       Basalt Area (toll area)


            The Basalt Area road leads to reservoir access for camping, fishing and boating.  It is named for Basalt Hill lying just south of the terminus of this road, which is an erosional remnant of a Miocene basalt flow of the Quien Sabe volcanics (Bennison and others, 1991).  It now acts as a cap rock for the more easily eroded Great Valley Sequence.  Most of the fill used to create the San Luis Dam was quarried from Basalt Hill (Trefzger, 1963).  The Quien Sabe volcanics are similar in age to the rhyolite at Pinnacles National Monument but they were erupted nearly 200 miles apart (Norris and Webb, 1990).  They are adjacent to one another today because of the offset along the San Andreas Fault.


53.4     1.0       William R. Gianelli Pumping-Generating Plant


            The silver and red facility located at the base of San Luis Dam, to the left (south) of Highway 152, is the William R. Gianelli Pumping-Generating facility (for more details, see appendix B, “Geographic notes on the Great Valley: Federal and State Water Projects”).


            Rocks of the Upper Cretaceous Panoche Formation underlie the plant and dam.  At the dam, prior to construction, Payne (1962) mapped the layer, which contains conglomerates and thick sandstones interbedded with thin lenses of shale, as the Ciervo Formation.  The foundation was built on weathered conglomerates, and the pump-release tunnels were carved through both rock strata.  During construction, the depth and size of the tunnels were increased to reach more consolidated material and to allow for thicker cement linings to be poured for reinforcement near small faults found near the dam site.


55.0     1.6       Romero Visitor Center


 (Optional stop)  Turn left and proceed up the short road leading to the Romero Visitor Center, which is run by the Department of Water Resources. A dirt parking area is located between the visitor center and Highway 152.  Park and walk down toward the reservoir.  Just below the high stand benchmark are excellent exposures of the Upper Cretaceous Panoche Formation of the Great Valley sequence (GVS) which at this location is mostly composed of sandstones and interbedded conglomerates.  The unit represents deposition within an inner-fan channel or submarine-canyon fill (Bennison et al., 1991).


57.7     2.7       Ortigalita Fault - Great Valley Sequence and the Franciscan Complex


The road crosses the Ortigalita fault, indicated by the linear embayment along the Reservoir (see Figure 3). The high angle fault marks the boundary between the tilted Great Valley Sequence (GVS) and the Franciscan Complex.  It can be traced for approximately 60 miles (96km) from Panoche Valley northwest to near Livermore.  The nature of motion on the fault is controversial, but is primarily vertical (Bartow, 1985; Bennison et al., 1991).

The Franciscan Complex in this area is composed primarily of graywacke sandstone and shale deposited in the accretionary prism of the subduction zone that was active on the western margin of North America in middle and late Mesozoic time.  The rocks have been severely disrupted by the churning action of subduction and metamorphosed by deep burial within the crust.  The metamorphic minerals found in the rocks indicate high pressure, but relatively low temperatures (the blueschist facies: some of the characteristic minerals give the rocks a distinct bluish cast).


57.8     0.1       Oak Grassland & Woodland


Having now passed the Ortigalita Fault, we see a major change in the macro-flora.  Oak trees are commonly seen on either side of the highway from here to Casa De Fruita on the other side of Pacheco Pass (for more details, see appendix B, “Geographic notes on the Great Valley: Native Vegetation”).


63.5     5.7       Pacheco Pass - Elevation 1,368 feet


This pass divides the headwaters of Pacheco Creek, which drains west into the Pajaro River, from San Luis Creek which drains eastward into San Luis Reservoir.  The mountains here are a typical orographic barrier.  Summer thermal low conditions in the Central Valley pulls cooler marine air in from Monterey Bay, creating ideal windsurfing conditions on the O’Neill Forebay, with typically foggy mornings experienced from the pass to Monterey Bay.  In the winter months, the moisture-laden marine air cools as it rises over the mountains, condenses, and falls as rain.  Thus the west side tends to be wetter, while the east side experiences a rain shadow effect and is dryer.

            The pass was named for ranch owner Don Francisco Perez Pacheco.  He came to the region in 1819 as an artillery maintenance man.  In 1824 he was given a citation for bravery while helping to put down a revolt by the indigenous peoples and was promoted to comandante of the guard at the Customs House in Monterey.  A short time afterward he was again advanced to the Presidio of Monterey as comandante.  By 1833 he accepted a land grant and settled near Mission San Juan Bautista.  He continued to be deeded additional parcels of land, and by 1852 his San Justo Rancho was one of the largest landholdings in the region (Renach, 1933).  His rancho extended from Hollister to Los Banos, and included the Pacheco Pass area.

            A historical adobe structure dating from the rancho days was located in the valley now covered by San Luis Reservoir.  An effort was made to move the building to safer ground, but in transit the house collapsed and was destroyed beyond repair.


63.7     0.2       Dinosaur Point


The road leads to an area where Fresno State University and the University of California, Berkley completed a joint paleontological dig.  Geologists excavated a thirty-foot long mosasaur fossil.  This road was the old highway over Pacheco Pass prior to the construction of the San Luis Reservoir.


65.7     2.0       Serpentinite Outcrop


            Serpentine is exposed on the north side of the highway.  Bodies of serpentine form from the alteration of peridotites and other ultramafic rocks in the Franciscan Complex and Coast Range Ophiolite.  The serpentine masses are often in the form of diapirs, which forced their way up through the crust and along fault zones (Coleman, 1996).  As a consequence, whenever serpentine is exposed, large tectonic displacements can be inferred (Page et. al., 1998).

            Serpentinite is a rock composed of serpentine minerals such as antigorite, lizardite and or chrysotile, (Brooks, 1987).  The term serpentine was derived from the Latin word serpentinus meaning serpent rock by Agricola in 1546 (O'Hanley, 1996).  It alludes to the dark green splotched scaly appearance.  Serpentinite contains very high concentrations of magnesium (Mg) and iron (Fe) along with other elements such as nickel, chromium, or selenium.  Serpentine is relatively stable at the surface, although it eventually breaks down in humid climates, leaving iron-rich smectites and talc residues.  In arid regions it will persist, forming mountains and resistant outcrops.

            Serpentine soils often support a population of unique, endemic, and rare plants.  They normally have toxic levels of trace elements restricting the type of species that can establish themselves, but certain plants, known as hyperaccumulators, thrive in this environment.  Most  plants can store minute amounts of toxic materials, but plants adapted to serpentine soils have evolved the ability to concentrate nickel, cobalt, zinc, and other minerals in their foliage, in amounts up to 1% of their dried weight (Reeves, 1983).  These plants tend to act like tiny mining operations, concentrating mineral resources that can be harvested and refined in useful quantities.  It has been suggested that areas with serpentine soils could produce economic benefits not otherwise available through farming or ranching.


66.8     1.1       Meta-igneous Rocks


The protruding outcrops on each side of the road are a metagabbro unit within the Franciscan Complex.  They are texturally related to other prominent peaks and hills in the area such as Ortigalita Peak, located 20 miles (32km) to the southeast.  Mafic magma, dated at around 100 Ma by Mattinson and Echeverria (1980), intruded the Franciscan Complex under high pressure prior to metamorphism and recrystallization.

The isolated nature of these bodies suggests an extrusive component (i.e. volcanic necks).  As the more resistant metavolcanics were uplifted, they were differentially weathered, creating the knobs, hills and peaks on either side of the road.


74.8     8.0       Highway 152 passes the offramp to Casa De Fruita, the very epitome of the roadside tourist trap.  Don’t miss the petting zoo, the cup flipper, and the train rides for the little kids!


76.7     1.9       Turn left onto highway 156 toward the town of Hollister


81.1     4.4       At stop light turn left onto San Felipe Road, (still Highway 156).


82.3     1.2       Hollister city limits.


Hollister was named after W. W. Hollister, a Civil War colonel who was instrumental in the creation of the town.  Around 1855 Hollister and the Flint-Bixby Company purchased the 35,619-acre San Justo Rancho land grant from Don Pacheco, mentioned above.  They paid $370,000 for the property to establish better trade routes to and from the quicksilver mines of New Idria (Myler, 1970).  By November of 1868 Hollister had proposed the layout and location of a new town and had sold some of his holdings in that area.  A group of fifty local farmers, called the San Justo Homestead Association, named the town in honor of its benefactor.  On February 12, 1874, San Benito County was founded with Hollister as the county seat.


84.9     2.6       Turn right onto Hill Street and park in the parking lot at the top of the hill.


Stop 1: Walk to the west side of the softball field to a bluff overlooking the Hollister area to the south and the San Benito Valley to the north.  The physiography of this region is shown in Figure 4.  There are two triangular concrete blocks in the grass at the top of the bluff, the site of two USGS benchmarks.  These blocks were at one time used by the USGS as part of a crustal deformation monitoring program, using laser-ranging equipment.  A laser distance meter was set up at the site, then used in conjunction with reflecting prisms stationed on nearby peaks, to accurately measure the distances and directions across the San Andreas fault system.  The geodetic measurements gave information on creep rates as well as strain accumulation across the San Andreas and Calaveras faults.

Park Hill is a horst that has been displaced vertically along the Calaveras fault (see Figure 5).  The main trace of the fault trends along the northwest side of the hill.  If you follow the fault trace to the northwest, another small hill is visible in the distance.  This is another horst along the Calaveras fault.  The fault cuts along the northwest side of this hill, too.  Matthews (1997) reports the rate of creep along the Calaveras fault in Hollister has varied through time.  Town records of sidewalk construction indicate that there was no movement along this trace between 1910 and 1929.  Then creep averaged 8 mm/yr (0.31 in/yr) from 1929 to 1961.  Between 1961 and 1967 the rate was 15 mm/yr (0.59 in/yr).  Since 1979, slip at one site in Hollister averaged 6.6 mm/yr (0.26 in/yr) and 12 mm/yr (0.47 in/yr) at another (Sims, 1989).

            If you turn toward the southwest, you will face the main trace of the San Andreas fault.  The fault runs northwest to southeast below the Gabilan range in the distance, behind the hills in the foreground.  The town of San Juan Bautista (a stop later in the day) is approximately 12.9 km (8 mi) west of Hollister. 

As you face south and look down into the town of Hollister from the bluff, you will be looking out over where the Calaveras fault travels underneath many of the homes built unknowingly on top of it.  Our next stop will examine some of the cultural features disrupted by faulting.  Return to the vehicle.


85.1     0.2       Drive back down Hill Street to San Felipe Road (Highway 156) and turn right.


85.5     0.4       Turn right onto 4th Street.


85.7     0.2       Turn right onto Locust Avenue and park the vehicle on the right hand side of the street. 


Stop 2: [Note: Please practice geo-courtesy on this walk by staying out of people’s yards and keeping on the public sidewalks.]  Walk to the corner of Locust and Fourth.  The sidewalk on the south side of the street is dated 1910 and was offset 22.9 cm (9 in) by 1967 (Rogers and Nason, 1967; Matthews, 1997).  The fault trace travels directly under the house.  The curb, sidewalk, and stone wall have all been offset here with a right-lateral sense of motion.



Figure 3: Location map of the Hollister area showing USGS 7.5’ quadrangles covering field trip area (modified from Rogers, 1980).




Figure 4: Physiography of the San Andreas fault near Hollister and San Juan Bautista.  Major streams are indicated as follows: SR, Salinas River; SBR, San Benito River; DPV, drainage from Priest Valley (modified from Dibblee, 1980).




Figure 5: Map of the Calaveras fault zone through Hollister (modified from Rogers, 1969).



            Walk back along Locust (north) to the alley on the north side of the garage next to the house you just visited.  The north wall of the garage has been displaced right-laterally at least 25 cm (1 ft).

            Continue north along Locust Street to the next house.  The concrete walkway to the front porch has been offset in discreet units, each sliding past the next in a relative, right lateral sense.  The fault is now under the front yard of this house and continues across the sidewalk and across the street, toward the northwest.  En echelon cracks are visible in the

street here.  The curb in front of the house shows evidence of crustal shortening.  The slabs have been buckled upward.  Within a few tens of feet in this area there is evidence of compression, tension, and shearing.  Look carefully and you will see all three where the fault obliquely crosses the sidewalk and street (see Figure 6).

            Cross the street and continue north toward the base of Park Hill, the horst from Stop 1.  As you turn the corner on Locust Street, look westward along the curb and notice the offset there.  The house just south of the curb lies directly on the fault.  It is being sheared by the fault’s action.  Return to the vehicle.


85.8     0.1       Drive to the end of the block and turn right onto Virginia Drive.


85.9     0.1       Turn right onto West Street.


86.0     0.1       Cross 4th Street and continue straight ahead to 6th Street.


86.1     0.1       Turn right onto 6th Street and park about half way down the block on the right side of the street.


Stop 3: The change in topography indicates a small scarp produced by the Calaveras fault.  Curbs on both sides of the street are right-laterally displaced.  The curb on the north side had a 11.4 cm (4.5 in) offset at a construction joint in 1967 (Roger and Nason, 1967; Matthews, 1997).  The concrete retaining wall on the north side shows world-class right lateral displacement (see Figure 7).  Photos of this wall are commonly included in physical geology text books illustrating active fault creep and resulting deformation.  Return to the vehicle.


86.2     0.1       Turn right onto Powell Street.


86.3     0.1       Turn right onto 5th Street.


86.6     0.3       Turn right onto San Benito Street.


88.1     1.8       Turn right onto Union Road.


88.4     0.3       Turn left onto Cienega Road.


89.8     1.5       Intersection with Hospital Road.  This road crosses the San Benito River approximately 1 km (0.5 mi) north of the intersection and was destroyed by flooding during the “El Nino” winter of 1997-8 (see Figure 8).  The Graniterock Co. owns a sand and gravel operation on the San Benito River near the crossing.






Figure 6:  Photo showing different manifes-tations of strain where the Calaver-as fault obliquely crosses a curb and street at the northwest corner of Virginia and Locust Streets in Hollister.  (All photos taken by J. Tolhurst unless otherwise noted).



92.1     2.3       Note the landslide headscarp at the watershed divide here.  The drainage basin on your right (west) lies along the San Andreas fault and consists of loosely consolidated (low shear strength) Miocene sediments that have felt the effects of repeated seismic activity and heavy rainfall, as well as land use and vegetation changes due to grazing.  The combination of these factors has resulted in the many shallow soil slips, slumps, and earthflows visible.


93.5      1.4       Hollister Hills State Vehicular Recreation Area entrance.  This motorcycle and offroad vehicle (ORV) park was created around 1980 after Howard Harris, a local geologist, rancher, and farmer, sold the property to the State of California.  As a side note, Howard graduated from the U.C. Berkeley geology department in 1930 and, in 1940, sold a patent on a process he had perfected for extracting magnesium from dolomite to his friend W. Kaiser (Kaiser Refractories).  Kaiser sold the magnesium to companies that manufactured incendiary devices used in World War II.  Howard used the money (c. $30,000) to buy up the ranches surrounding his property, expanding from about 500 to 5,000 acres in size.   Sediment retention structures within the park help to mitigate sediment production from ORV use.


95.5     2.0       Calera Winery (formerly Almaden Winery).  Park along the right side of the road and walk to the drainage ditch across the access road on the south side of the main warehouse.


Stop 4:  (Note: The owner of Calera Winery, Pat DeRose, appreciates a phone call, (408) 636-9143, before the arrival of curious geo-visitors).  The drainage ditch (see Figure 9) was constructed in 1948, when the present building housing the winery was rebuilt.  The San Andreas fault runs directly under the winery and crosscuts the ditch, offsetting it right-laterally.  The creeping nature of the fault was discovered accidentally after the previous winery facility was destroyed by the fault’s motion.  Geologists were brought in to help the owners understand what was happening to the building.  It was determined that the fault was actively creeping aseismically at the rate of about 12 mm/yr (0.47 in/yr) (Steinbrugge, et al, 1960).  In the past 50 years approximately 61 cm (24 in) of offset has been measured.  Steinbrugge also documented that 92% of the creep activity during a one-year period occurred in four distinct “spasms” totaling 34 days (Steinbrugge, et al, 1960; Matthews, 1997).  Long term creep rates in the central California portion of the San Andreas fault are 22 mm/yr (0.87 in/yr) near the Pinnacles and reach a maximum of approximately 30 mm/yr (1.2 in/yr) 26 km (16 mi) further to the southeast, near Bitterwater (Matthews, 1997).  The fault appears to be partially locked at the winery since the creep rate is on the order of 12 to 13 mm/yr (0.47 to 0.51 in/yr).  It is completely locked northwest of San Juan Bautista.  Surface rupture during the 1906 San Francisco did not occur at the winery, though significant damage was noted.  At Hollister, aseismic creep accounts for approximately 13 mm/yr (0.51 in/yr) of movement.  Geodetic measurements across the San Andreas, Calaveras, and Hayward faults give 35 to 41 mm/yr (1.4 to 1.6 in/yr); the amount of strain released by aseismic creep on the San Andreas is 0 mm/yr (0 in/yr), 6 mm/yr (0.24 in/yr) on the Hayward fault, and 3 mm/yr (0.12 in/yr) on the Calaveras fault in the South Bay area (Matthews, 1997).  Ward (1990) reports that the Hayward and Calaveras faults are siphoning off approximately 20 mm/yr (0.79 in/yr) of the Pacific/North




Figure 7: View east along Sixth Street sidewalk and concrete wall offset right laterally by creep along the Calaveras fault.







Figure 8:  Photo shows flood damages incurred during winter of 1997-8 along Hospital Road, near Hollister.



American plate motion from the San Andreas fault.  Return to the vehicle, turn around and drive north, back along Cienega Road.


102.6   7.1       Intersection of Cienega and Union Roads.  Turn left onto Union Road.


105.0   2.4       Entrance to San Justo Reservoir.  Water is piped from the San Luis Reservoir over Pacheco Pass into this reservoir for farming/irrigation in the San Benito Valley.


106.2   1.2       Intersection with Highway 156.  Turn left onto 156.


109.2   3.0       Turn left onto Mission Vineyard Road.


109.7   0.5       San Andreas fault.  Park on the right side of the road.


Stop 5: The San Andreas fault crosses the road at this point.  There is a slight depression in the road that marks the location.  The road is aligned north/south and the fault crosses at an angle from the southeast to the northwest.  En echelon cracks are clearly visible in the pavement.  The USGS has a seismographic station directly on the fault on the east side of the road behind a barbed wire fence.  Radio transmitting equipment lies behind the row of trees.  An abandoned creep metering system is visible here.  Two poles cemented into the ground on opposite sides of the fault were used to measure movement across this segment of the fault.  As you look toward the northwest you will notice the ground level is topographically depressed by the fault, though there is no actual sag pond.  This segment of the fault creeps aseismically with the rate of creep increasing toward the south.


109.8   0.1       90 degree right bend in road; follow this bend.


110.3   0.5       Turn right at Salinas Road.


110.7   0.4       Cross Highway 156 and continue onto The Alameda.


110.9   0.2       Turn right onto Franklin Street.


111.0   0.1       Turn left onto 2nd Street.


111.1   0.1       Park along the right side of the street and walk back to the edge of the bluff next to the grassy area in front of Mission San Juan Bautista.


Stop 6: The town of San Juan Bautista is built on a northeast-facing scarp of the San Andreas fault.  The bluff represents this feature.  The mission was severely damaged during the 1906 San Francisco earthquake, which exhibited approximately 430 km (270 mi) of surface rupture from near where you are standing to near Cape Mendocino, just south of Ferndale, in Humboldt County.  The San Andreas fault is locked along the segment that ruptured in 1906 and creeps aseismically from just north of here south to Parkfield.  Aseismic creep decrease to




Figure 9: Drainage ditch offset by creep along the San Andreas fault at the Calera Winery along Cienega Road.  Photo taken January 1998.


zero within 19 km (12 mi) to the northwest and increases to a maximum of 30 mm/yr (1.2 in/yr) 67 km (42 mi) to the southeast (Matthews, 1997). 

The Mission is now a popular tourist attraction. The USGS once had a seismographic station mounted on the small adobe foundation.  The housing and equipment have been removed.  Return to the vehicle.


111.3   0.2       Continue along 2nd Street then turn right onto Monterey Street.


111.4   0.1       Turn left onto 1st Street, which becomes the San Juan Highway once it leaves town.  Stay on the San Juan Highway.


111.9   0.5       Slow down and look to your left along a fence line perpendicular to the road.  The fence was built across the San Andreas fault and is noticeably offset here at Nyland Ranch due to creeping of the San Andreas fault.  The Flint Hills are on your right and represent a horst, which is bounded on the far side by the Calaveras fault.


113.5   1.6       Turn left onto Anzar Road.  This road follows the San Andreas fault for several miles and passes a classic sag pond.  It was the intended field trip route to the A.R. Wilson quarry, owned and operated by the Graniterock Company.  The route has been changed due to a landslide that occurred on April 22, 1998, which has closed the road (see Figure 10).  The landslide snapped two main gas pipelines to Santa Cruz County and left over 60,000 people without gas for water heating, cooking, heat, and other needs.  It took 750 Pacific Gas & Electric and Southern California Edison employees going door to door to restore service.  The landslide also impounds water that normally drains to the south along the fault and has caused concern among homeowners living along Anzar road.  It destroyed one home; the homeowners first noticed their driveway was cracking; then the windows in their house were jammed; then the doors wouldn’t open; finally a 3-foot crack – the eventual headscarp - opened up just uphill of the home.  They were able to quickly remove their belongings and move to safety before the whole house was destroyed (Jerry Allen, Graniterock Company, pers. comm., 1998).  This area can be accessed from the town of Aromas, to the north and is worth a visit.  Due to time constraints we will not visit this segment of the fault.


114.4   0.9       Turn right onto Searle Road.


115.1   0.7       Turn left onto Highway 129 from Searle Road.


117.5   2.4       Cross the Pajaro River near Chittenden Pass.  Homes in this area experienced serious flooding this past winter.  Downstream the Pajaro River overflowed its banks and spilled onto the floodplain near Watsonville as its levies burst.


118.5   1.0       To your right you will see an earth dam constructed by Graniterock Company, which increases the capacity of Soda Lake, an old meander cutoff of the Pajaro River (Wills and Manson, 1990).  Granitrock pumps a slurry of fine grained material from the quarry into Soda Lake.  The lake experienced liquefaction of the fine grained material during both the October 1989 Loma Prieta and the April 1990 Chittenden earthquake swarm (Wills and Manson, 1990).  We will view Soda Lake from the quarry.


119.6   1.1       Visible in the embankment along the right side of the road are horizontal groundwater wells installed by Caltrans to drain the hillslope of excess water.  The rate of flow increased dramatically following the 1989 Loma Prieta earthquake (Jerry Allen, Graniterock Company, pers. comm., 1998).


120.3   0.7       Turn left onto Rogge Lane.


121.4   1.1       Turn left onto Quarry Road.


121.8   0.4       Entrance to Graniterock’s A. R. Wilson Quarry.  Follow the main road up the hill to the offices on the left side (follow signs).


Stop 7:  Graniterock Company’s Arthur R. Wilson quarry, located in the westernmost tip of San Benito County, along the Pajaro River, near the town of Aromas (see Figure 11), produces crushed aggregate used in the construction industry.  The quarry trends southeast for approximately 2 km (1.24 mi) and is up to 610 m (2,000 ft) wide. 

The following excerpt from the company’s promotional pamphlet gives a brief and simplified geologic history of the deposit:


“[The quarry’s] story began more than 200 million years ago, when a great mass of molten granite began to push up from the depths of the earth through limestone, sandstone and clay on the bed of an ancient ocean.  The granite cooled, contracted and cracked, and was folded, broken, crushed and uplifted as the Pacific Plate slowly drifted by the continent of North America.  The fortuitous location of the granite directly upon the San Andreas Fault would ease future mining of this pre-fractured rock.”


Allen (1946) initially classified the rock as quartz diorite; Stinson and others (1983) later reclassified it as hornblende gabbro of Cretaceous age.  The Pliocene Purisima Formation and Pleistocene Aromas Formation, which thicken and dip to the southwest (Higgins, 1989), overlie the deposit and are considered overburden.  The gabbro is bounded on the northeast by the San Andreas fault zone, which has crushed the rock, making it easily quarried by bulldozers.  Blasting only occurs once or twice a month on small, less-fractured masses.  Mechanical crushing is greatly reduced due to the natural crushing done by the fault, thus reducing the overall cost of excavation and processing.  Mining at the site would probably not occur were it not for the geologic processes that have acted here (Higgins, 1989). 



Figure 10: A Pacific Gas & Electric Company drilling foreman checks out a landslide along Anzar Road, near Aromas, along the San Andreas fault.  The landslide severed gas service to Santa Cruz and northern Monterey Counties in late April, 1998 following heavy El Nino rains (photo by Lewis Stewart).


            In 1871 civil engineers for the Southern Pacific railroad, forging the main coastal line through Chittenden Pass, discovered the plutonic outcrop.  It was perfect for use as ballast to form railroad beds for track laid throughout the state.  Warren Porter and Arthur Wilson purchased the quarry and the company was incorporated on Valentine’s Day, 1900.  The 1906 San Francisco earthquake damaged equipment at the site, but the devastation also caused a demand for construction and the company rebounded.  Street paving became increasingly important as the automobile replaced the horse and buggy and Graniterock’s concrete was used for much of the local area.  The company continued to expand and change over the years producing concrete, asphalt, and base aggregate.  Other products include drain rock, sand for fill, railroad ballast, and riprap.  In the 1980’s the company modernized, spending over $16 million, and acquired the largest mobile primary rock crusher in the world as well as a state-of-the-art, computer-controlled automated truck and rail car loading system, which has reduced the entrace-exit loading time for trucks from 25 to 10 minutes (Higgins, 1989).

Before the rock can be mined the overburden must be removed at the quarry site.  A 3.2 km (2 mi) long conveyor system (see Figure 12) transports the waste rock to a site where it is placed.  Reclamation creates a buffer zone between the mine and surrounding populated areas as the stockpiles are recontoured and revegetated (Higgins, 1989).  The underlying gabbro is quarried and loaded into a self-propelled, mobile rock crusher, which is the largest of its type in the world (Jerry Allen, Graniterock, pers. comm. 1998).  The crusher takes up to 106 cm (42 in) diameter rocks and breaks them into 25.4 cm (10 in) and smaller.  Then a 1.6 km (1 mi) long conveyor system transports the rock to a secondary and tertiary plant where it is screened, washed, and sorted by size (see Figure 13).  Water is used in the process and the plant has its own water treatment and recycling facility.  Unused fines from the filtration process are piped to Soda Lake, across Highway 129, to settle out.  Liquefaction occurred in this lake following the 1989 Loma Prieta earthquake and 1990 Chittenden earthquake swarm (Wills and Manson, 1990).  Once the rock is sorted by size, it is then stockpiled over an underground tunnel and conveyor system.  A computer system controls the size of material distributed to the company’s clients.  It is then loaded onto trucks or trains to be transported to where it is needed.  Rail transport keeps hundreds of trucks off the roads daily, reducing noise, traffic congestion, and air pollution (Higgins, 1989).

The quarry is capable of producing up to 3,000 tons of rock per hour and peaks during the summer.  On a typical day there may by 100 railroad cars and 400 trucks transporting aggregate from the processing plant.  There are about 70 employees during the summer and about 50 during the winter months working at the quarry.  Haul trucks cost approximately $450,000; the rock crusher initially cost $3,000,000 and today would run about 3 to 4 times that amount.  The newest loader (purchased this year) cost $1,100,000 and typically burns about 180 gallons of diesel fuel per day.  The company is planning to upgrade the automated truck and rail car loading system in the near future.  The contact person for tours of the facility is Jerry Allen (831) 768-2000.

Return to Modesto via Highway 129 east to Highway 101 south to Highway 156 east to Hollister, then Interstate 5, retracing your route back to Modesto Junior College.







Figure 11: Location map showing Graniterock company’s Arthur R. Wilson quarry, San Benito County, California.  (Figure modified from Higgins, 1989). 






Figure 12: Overburden conveyor system used by Graniterock Company to transport tailings 3.2 km (2 mi) from the quarry to the dump site.  During reclamation, the non-native eucalyptus trees visible in this picture are removed and native vegetation (including oak trees, bunch grasses, poison oak, etc.) is planted.




Figure 13: Screening plant at the Arthur R Wilson quarry.  The building in the upper left of the photo is the computerized truck and rail loadout structure.  The main trace of the San Andreas fault trends along the outcrop visible in the upper right part of the photo.  (Photo digitized from Higgins, 1989).






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