Geology at Point Reyes National Seashore and Vicinity …

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Geology at Point Reyes National Seashore and Vicinity, California:
A Guide to San Andreas Fault Zone and the Point Reyes Peninsula

Trip highlights: San Andreas Fault, San Gregorio Fault, Point Reyes, Olema Valley, Tomales Bay, Bolinas
Lagoon, Drakes Bay, Salinian granitic rocks, Franciscan Complex, Tertiary sedimentary rocks, headlands, sea
cliffs, beaches, coastal dunes, Kehoe Beach, Duxbury Reef, coastal prairie and maritime scrublands

Point Reyes National Seashore is an ideal destination for field trips to examine the geology and natural history of the

San Andreas Fault Zone and the North Coast of California. The San Andreas Fault Zone crosses the Point Reyes
Peninsula between Bolinas Lagoon in the south and Tomales Bay in the north. The map below shows 13 selected field
trip destinations where the bedrock, geologic structures, and landscape features can be examined. Geologic stops
highlight the significance of the San Andreas and San Gregorio faults in the geologic history of the Point Reyes
Peninsula. Historical information about the peninsula is also presented, including descriptions of the aftermath of the
Great San Francisco Earthquake of 1906.

Figure 9-1. Map of the Point Reyes National
Seashore area. Numbered stops include:
1) Visitor Center and Earthquake Trail,
2) Tomales Bay Trail,
3) Point Reyes Lighthouse,
4) Chimney Rock area,
5) Drakes Beach,
6) Tomales Bay State Park,
7) Kehoe Beach,
8) McClures Beach,
9) Mount Vision on Inverness Ridge,
10) Limantour Beach,
11) Olema Valley,
12) Palomarin Beach,
13) Duxbury Reef
14) Bolinas Lagoon/Stinson Beach area.

Features include: Point Reyes (PR), Tomales Bay
(TB), Drakes Estero (DE), Bolinas Lagoon (BL),
Point Reyes Station (PRS), San Rafael (SR), and
San Francisco (SF), Lucas Valley Road (LVR),
and Sir Francis Drake Boulevard (SFDB).

Planning Your Trip

The Bear Valley Visitor Center (Park Headquarters) near Olema is about an hours’ drive north of the Golden Gate
Bridge. Point Reyes is located approximately 35 miles north of San Francisco on Highway 1. The park is accessible from
San Rafael via Sir Francis Drake Boulevard or from Lucas Valley Road (the latter is discussed in the road log presented
below). Although a trip to Point Reyes from anywhere in the Bay Area can be accomplished on a long day, fieldtrip
planners should consider spending a night camping or utilizing overnight accommodations either inside or near the park
to have more time to enjoy the experience. However, there is no car camping in the park. Call the Bear Valley Visitor
Center well in advance to inquire about group overnight accommodations. It is advisable to check weather forecasts and
tide conditions before traveling to Point Reyes. The weather can be very windy and cold along the coast, particularly near
the lighthouse on the Point Reyes headlands. More information can be found on the Point Reyes National Seashore
website at: http://www.nps.gov/pore/.

The park is open daily from sunrise to sunset throughout the year. Visitor Center hours are as follows: Bear Valley

Visitor Center, weekdays, 9:00 a.m. to 5:00 p.m., weekends and holidays 8:00 a.m. to 5:00 p.m.; Lighthouse Visitor
Center, Thursday through Monday, 10:00 a.m. to 4:30 p.m., Lighthouse stairs and exhibits (weather permitting), 10:00
a.m. to 4:30 p.m., Lens Room open as weather & staffing permit, all Lighthouse facilities are closed Tuesdays and
Wednesdays; Ken Patrick Visitor Center, weekends and holidays, 10:00 a.m. to 5:00 p.m. On weekends during whale
watching season (December to April) the road to Point Reyes is closed to private vehicles, but shuttle bus transport is
provided.

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Geology of Point Reyes—An Overview

Point Reyes National Seashore is an exceptional geologic observatory for many reasons, but it is perhaps most
notable for its association with the San Andreas Fault Zone and Great San Francisco Earthquake of 1906. The San
Andreas Fault Zone is a dividing line between rocks of disparate origin and represents the classic boundary between the
North American and Pacific plates—with the Point Reyes Peninsula residing on the Pacific Plate, and the rest of Marin
County being part of the North American Plate. Olema Valley and the submerged valleys flooded by Tomales Bay and
Bolinas Lagoon are part of the San Andreas Rift Valley (fig. 9-2).

Rocks on the east side of the fault consist of Franciscan Complex, a mix of oceanic crustal rocks that formed in late

Mesozoic time (Jurassic and Cretaceous) and were gradually accreted onto the North American continental margin by
plate tectonic motion. Bolinas Ridge along the east side of the rift valley consists mostly of sandstone and metasandstone
of Cretaceous age. Pillow basalts are exposed along the highway near Nicasio Reservoir. The pillow basalts formed on
submarine volcanoes associated with ancient spreading centers in the Pacific Ocean basin. Basalt that has been altered to
greenstone crops out along Highway 1 south of Bolinas. Other rocks in the Franciscan Complex exposed along Highway
1 include chert, shale, and argillite; they represent rocks that formed from sediments that accumulated in mid-ocean to
outer continental margin environments. Serpentinite occurs in scattered outcrops along the highway and throughout the
Mt. Tamalpais area. Serpentinite is a mineralogically complex rock of ultramafic composition (rich in magnesium and
iron). Serpentinite is derived from rocks that originally crystallized deep in the ocean crust or mantle before undergoing
physical and chemical alteration during migration to the surface. A sliver of Franciscan rocks that contain limestone (the
Calera Limestone of the Permanente Terrane) occurs within the San Andreas Fault Zone in Olema Valley. The limestone
accumulated as limey sediments on the crest of a submarine volcano or plateau.

Point Reyes Peninsula is an elevated block of ancient crystalline basement with a sedimentary cover of Tertiary
sedimentary rocks and some Quaternary marine terrace, alluvial, and dune deposits (fig 9-2). The crystalline basement
consists of Cretaceous granitic rocks and some ancient metamorphic rocks (schist, gneiss, and marble). These rocks are
called Salinian Complex, named for an extensive belt of granitic and crystalline metamorphic rocks exposed in the Salinas
Valley region. However, the name Sur Series also applies because the basement rocks of Point Reyes are closely
associated with equivalent crystalline basement rocks exposed along the Big Sur coast south of Monterey. Salinian
crystalline rocks are exposed along the crest of Inverness Ridge and in the Point Reyes headlands. In the Point Reyes
headlands, a thick sequence of conglomerate and sandstone of Paleocene age rest nonconformably on the Salinian granitic
basement. The granitic basement and conglomerate are in turn are unconformably overlain by sedimentary formations of
middle Miocene age. Along Inverness Ridge, middle Miocene marine sedimentary rocks (Laird Sandstone and Monterey
Formation) rest directly on crystalline basement (the Point Reyes Conglomerate is not present). A sequence of late
Miocene and Pliocene formations (Santa Margarita Sandstone, Santa Cruz Mudstone, and Purisima Formation) rests
unconformable on top of the Monterey Formation. The Miocene and Pliocene sedimentary rock formations are folded
into a broad syncline with a northwest-trending axis and runs between Inverness Ridge and Point Reyes.

Three separate faults merge near Bolinas Lagoon and in the southern Olema Valley to form the San Andreas Fault
Zone at Point Reyes: the Golden Gate Fault, San Andreas Fault, and San Gregorio Fault (Bruns and others, 2002) (see fig.
9-2). These faults extend south through the Gulf of the Farralones. The Golden Gate Fault runs along the eastern shore of
Bolinas Lagoon and crosses the Gulf west of the Golden Gate before running onshore in the vicinity of Lake Merced.
The North Coast segment of the San Andreas Fault is the trace that ruptured in the 1906 earthquake. It comes onshore
near the east end of Stinson Beach. The San Andreas extends southward beneath the Gulf and comes onshore in the
vicinity of Mussel Rock in Daly City. The northern extension of the San Gregorio Fault extends onshore in between the
town of Bolinas and Duxbury Point on the southern end of the Point Reyes Peninsula. To the south, the San Gregorio runs
along the San Mateo Coast and extends across Monterey Bay. These three faults merge into a narrow (kilometer-wide)
fault zone that extends northward through Olema Valley and under Tomales Bay. A large thrust fault, the Point Reyes
Fault, runs offshore near the Point Reyes Headlands and is probably responsible for ongoing uplift of the headlands
region. All of these faults show signs of tectonic activity extending from late Miocene time to the present.

The composition and characteristics of the Salinian basement and Paleocene conglomerate suggests that the Point

Reyes Peninsula actually migrated northward along the San Gregorio Fault System to its present location. Salinian
granitic rocks and Paleocene conglomerate on the Monterey Peninsula at Point Lobos are nearly identical to the rocks on
Point Reyes. In addition, the late Miocene to Pliocene age sedimentary sequence (Santa Margarita Sandstone, Santa Cruz
Mudstone, and Purisima Formation) are essentially identical to the same stratigraphic sequence on the east side of the San
Gregorio Fault in Santa Cruz County.

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Elevated marine terraces along with ancient alluvial and coastal dune deposits suggest that the Point Reyes Peninsula

has been rising throughout the Quaternary Period. However, ongoing sea-level rise associated with the melting of the
extensive continental glaciers is responsible for the flooding and Holocene sediment filling of ancient valleys beneath
Tomales Bay, Drakes Estero, and Bolinas Lagoon. Coastal processes associated with the modern marine transgression are
responsible for the erosional development of the sea cliffs and headlands, and the sediment accumulation on ocean
beaches, dunes, and the back filling of baylands and lagoons throughout the peninsula.

Figure 9-2. Generalized geologic map of Point Reyes National Seashore after Clark and Brabb (1997), Blake and others
2000), and Bruns and others (2002).

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Cultural History of the Point Reyes Region

[Summarized after Gilliam (1962), Caughman and Ginsberg (1987), and from National Park Service sources]

We don’t know when humans first appeared on the Point Reyes Peninsula. Archeological studies report human

activity in California extending back to near the end of the Pleistocene Epoch, nearly 10,000 years ago. The record of
early human history in the region has probably been lost to the nearly 120 meter rise in sea level since the final maximum
of the Wisconsin glaciation about 18,000 years ago. At that time, the shoreline was about 50 kilometers west of its present
location, and the Point Reyes Peninsula was part of a system of low inland hills. At least for the last several thousand
years, people utilized the peninsula as recorded by archeological evidence. The Coast Miwok Indians lived in villages
along the bays; they hunted, fished, and gathered food and living supplies from the region’s abundant wildlife and natural
resources. The Miwok people were probably descendents of earlier cultures in the region.

Historians still debate whether Sir Francis Drake stopped at Point Reyes. Drake was an English pirate and explorer

serving Queen Elizableth I. Drake had a ship, the Golden Hind, loaded with stolen Spanish treasure taken from
settlements in the New World on his voyage north from Cape Horn. Facing the prospects of revenge by Spanish galleons,
he choose not to spend more time seeking a fabled Northwest Passage, but rather sail west to return to England. Before
heading west, Drake returned south along the California coast seeking a location to repair and resupply his ship before the
long journey home. Journals from Drake’s adventurous voyage (1577 to 1580) described the landing at a location with
white cliffs, similar to the White Cliffs of Dover. In 1579, Drake’s crew careened the ship to repair damage and remove
barnacles possibly somewhere on one of the sheltered shores of Drakes Estero, inland of Drakes Bay. However, no
authenticated artifacts from Drake’s voyage have been recovered in the Point Reyes area.

The earliest mention of Point Reyes is in historic records of the 1595 shipwreck of the Spanish galleon, the San
Agustin. The ship, captained by Sebastián Rodrígues Cermeño was on her return journey from the Philippines. Despite a
damaged ship and a mutinous tone from the crew, Cermeño chose to spend three weeks exploring the coast in the vicinity
of Point Reyes. However, the ship was probably anchored near the mouth of Drakes Estero when, on a November day, an
unexpected storm drove the ship into the coast. Several of the crew members were lost. The remains of the ship and cargo
were abandoned. Still following Cermeño’s command, the crew modified and crowded into the San Agustin’s small
launch vessel, and heroically paddled nearly 1500 miles south to Spanish outposts in Mexico. In his exploration, he
named the coastal embayment between Point Reyes and Point San Pedro, “La Baya de San Francisco,” but never actually
sailed through the Golden Gate or visited the land that now bears the name. Chinese Ming pottery recovered from the
shipwrecked cargo of the San Agustin was recovered from Miwok Indian village sites. The loss of the San Agustin was
perhaps the first of many shipwrecks that have occurred in the treacherous waters around Point Reyes. Point Reyes
extends nearly 15 kilometers seaward, perpendicular to the coast. During major coastal storms, many ships driven by
strong winds and currents were unable get far enough west to get around Point Reyes ended up trapped and shipwrecked
in Drakes Bay.

On a return journey to Spain from Cape Mendicino, in 1603 Spanish explorer Sebastián Vizcaíno mistook what is

now the entrance to Tomales Bay for the mouth of a great river. He named it Rio Grande de San Sebastián. On the
Roman Catholic holiday of the Three Kings (January 6, 1603), Vizcaíno sighted the headlands and gave the name, La
Punta de Los Tres Reyes, and hence the name, Point Reyes. Not until 1793 when another Spaniard, Juan Matute, explored
the area, did Vizcaíno’s river prove to be a bay. Matute named his new bay Puerto Nuevo.

The bay and peninsula were home to the Miwok Indians who had settlement along the shore. Both Drake and
Vizcaíno reported friendly encounters with the Miwok people who provided supplies for the ships. Unfortuantely, like
most native California cultures, the Miwok population was decimated by disease, subjugation, and slavery during the era
of Mexican colonization. A mission was built in 1817 in what is now San Rafael. During Spanish rule, the Point Reyes
Peninsula was divided into land grant ranchos. However, after the Mexican Revolution of 1821, the San Rafael Mission
was secularized, and the few remaining Miwok that escaped or were released from subjugation had lost their land.

After California was ceded to the United States after the Mexican American War of 1848, the Point Reyes Peninsula

became the possession of a San Francisco law firm. The peninsula was then subdivided into dozens of dairy and cattle
farms. For nearly 75 years, dairy products and cargo from the ranches were loaded onto schooners that docked in Drakes
Estero and Tamales Bay and shipped to San Francisco. The Gold Rush brought a wave of immigrant pioneers to the area.
Bolinas was a thriving logging town by 1850. Lumber and fire wood were loaded at Bolinas Lagoon for shipment to San
Francisco. Over time, Bolinas and nearby Stinson Beach grew with support from tourism and inhabitants who chose to
live in the coastal communities and commute to the city. The town of Olema was established in 1859 and for a short

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9.0

15.5
21.5
25.2

26.3
26.5
29.3
30.5
33.4

33.7
36.9

37.4
37.6
38.3
38.7
38.8

40.0

period became a commercial center for the peninsula. In the early 1870s, the North Pacific Coast Railroad was built to
link the port of Sausalito near the Golden Gate to parts of Marin and Sonoma counties. From San Rafael, the rail line
followed Lagunitas Creek valley to the south end of Tomales Bay. By 1875, the rail line extended northward along the
east side of the bay to the town of Tomales. By 1877, the railroad was extended to serve lumber mills in the Russian
River area. In 1883, the town of Point Reyes Station was developed. The new train stop two miles north of Olema caused
the older established town to fade in significance. While in operation, the railroad transported lumber, farm produce,
commuters and tourists until the rail line ceased operation in 1933.

Point Reyes National Seashore was authorized for addition to the National Park System on September 13, 1962. The

park now encompasses 71,000 acres, with additional extensive land holdings in the region managed in cooperation with
Golden Gate National Recreation Area and California State Parks. The southeastern end of the peninsula is set aside as
part of the Philip Burton Wilderness Area and Research Natural Area. The Tule Elk Reserve encompasses the north end
of the peninsula including Tomales Point. Much of the western end of the peninsula consists of pastoral lands associated
with dairy and cattle ranches that are maintained in cooperation with the National Park Service. The U.S. Coast Guard
maintains small stations near Abbotts Lagoon and at Palomarin Beach, and private land holdings encircle the small
communities of Bolinas, Olema, and Inverness Park.

Road Logs From Golden Gate Bridge To Point Reyes National Seashore’s Park Headquarters

Golden Gate Bridge to Bear Valley Visitor Center (Park Headquarters) via Lucas Valley Road
Mileage
0

Description
Golden Gate Bridge Vista Area on Highway 101 (northbound lane at north end of bridge).
Restrooms are available at the Vista Area.
Pass exit to Highway 1 to Stinson Beach (this is recommended as a return route – it is 31 miles to
the Bear Valley Visitor Center/Park Headquarters along Highway 1).
Pass Sir Francis Drake Boulevard (this is an optional park access route, but has many more stop
lights than Lucas Valley Road).
Take the Lucas Valley Road exit from Highway 101. Turn left (west) on Lucas Valley Road.
Large outcrops of Franciscan Complex rocks occur along road.
The route passes through groves of coast redwoods (Sequoia sempervirens) in Samuel P. Taylor
State Park.
Turn right (north) on Nicasio Valley Road. Nicasio Reservoir is on the left.
The road passes the small town of Nicasio.
Nicasio Reservoir is on the left. Many Franciscan Complex “knockers” outcrops are in this area.
Turn left on Point Reyes-Petaluma Road toward Point Reyes Station.
Pillow basalts are exposed in roadside outcrops on the left near a sign for Golden Gate National
Recreation Area. These rocks are part of the Nicasio Terrane of the Franciscan Complex
Bear right across a bridge to continue on Point Reyes-Petaluma Road to Point Reyes Station
Turn left on Highway 1 (Shoreline) at Point Reyes Station. The town of Point Reyes Station offers
restaurants, gas, and other travel services. Highway 1 (Shoreline) makes a few turns through town.
See the discussion and historic photographs of the Point Reyes Station area below.

NOTE: Optional Stop 2 – Tomales Bay Trail is located along Highway 1 at 1.7 miles north of
Point Reyes Station on the east side of the bay. See the stop 2 discussion below.
The historic Point Reyes Station (a brick building) is on the right (see fig. 9-3 below).
Turn right on Sir Francis Drake Boulevard just past the bridge over Lagunas Creek.
Turn left on Bear Valley Road.
Pass Limontour Road on right (see Stop 10 – Limantour Road description below).
A small dirt road to the left leads to a parking area along the San Andreas Fault. An undeveloped
path that starts at the parking area leads to the top of a pressure ridge developed along the fault. The
top of the grassy knoll is a good place to view the sag ponds, marshlands, and fault scarps at the
south end of Tomales Bay (see fig. 9-7).

For the next mile Bear Valley Road follows the trace of the San Andreas Fault that ruptured in the
1906 earthquake. The road follows and crosses a fault scarp; sag ponds and marsh along Olema
Creek are on the right.
Turn right into the Bear Valley Visitor/Park Headquarters area near the red barn.

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Stop 1 – Bear Valley Visitor Center and Earthquake Trail (see description below).

40.2

Point Reyes Station and the 1906 Earthquake

Shortly after the 1906 earthquake and fire that destroyed much of San Francisco, reports started arriving about

surface ruptures and damage in the Olema Valley and the Tomales Bay-side community of Point Reyes Station. Reporters
and earthquake investigators made their way north to document the damage associated with rupture along a fault line that
would eventually be known as the San Andreas Fault. Perhaps the most famous pictures from those early investigations
include a picture of a toppled train at Point Reyes Station taken with a borrowed camera by one of train’s engineers, and
pictures of the fault rupture taken by USGS geologist Karl Grove Gilbert (1907).

Figure 9-3. A famous historic photograph taken by an engineer
of a derailed train that tipped over at the Point Reyes Station
during the 1906 earthquake (image from Jordan, 1907). The
narrow gauge train was stopped at the station on a section of
track that ran roughly parallel to the fault several hundred
meters east of the rupture. Based on the engineer’s description
of the rocking motion that fell the locomotive, Anooshehpoor
and others (1999) calculated the earthquake motion created
ground acceleration that was in the range of 0.7 to 1.1g.

Figure 9-4. Photograph by Gilbert (1907) of surface rupture
along the fault near Point Reyes Station. The view is looking
northwest toward marshlands at the south end of Tomales Bay.
The photograph illustrates a classic example of a sidehill bench.
Note the extent of timber removal on Inverness Ridge in the
distance.

Figure 9-5. View looking southeast along the fault near Point
Reyes Station. Note the woman for scale near the top of the hill
to the left of the fault scarp.

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Figure 9-6. This view of an offset road near Point Reyes Station
shows 20 feet of horizontal slip cause by the 1906 earthquake.
Photograph by Gilbert (1907).

Figure 9-7. This view is looking north from the top of a shutter
ridge located along Bear Valley Road about 1.5 miles north of
the Visitor Center, Inverness Ridge to the left (west). The
wetlands of southern Tomales Bay are near the center of the
image, and Point Reyes Station is on an elevated terrace to the
right (east). A scarp along the main trace of the San Andreas
Fault is visible as vegetation change along a break in slope
located just above the parked vehicle on the left.

Stop 1 – Bear Valley Visitor Center (Park Headquarters) and Earthquake Trail

Stop highlights: San Andreas Fault scarp, 1906 earthquake rupture, park geology and ecology exhibits

The Point Reyes National Seashore, Bear Valley Visitor Center (Park Headquarters) located near the town of Olema

is the best place to start a field trip in the park. The Visitor Center provides brochures, maps, book sales, and exhibits
about the land, wildlife, and the Coast Miwok Indians that inhabited the peninsula before European contact. It is
recommended that you check the Point Reyes National Seashore website and call the Visitor Center before planning a trip
to the park.

The Visitor Center is located near the historic 1906 rupture of the San Andreas Fault. Fault rupture and ground failure

features in the area around Point Reyes Station and Olema were described and photographed by USGS geologist, Karl
Grove Gilbert (1907), and by others. Although little physical evidence remains from the great earthquake, the National
Park Service maintains a trail to the 1906 fault rupture zone (figs 9-8 and 9-9). The Earthquake Trail starts in the Visitor
Center parking lot. Earthquake Trail guides are available at the Visitor Center. Signs and displays along this half mile
walk provide basic information about plate tectonics and the geologic setting and history of the San Andreas Fault. A
fault scarp is clearly visible along the trail. The fault scarp represents the cumulative effects of slip from many
earthquakes over many thousands of years that have occurred along this active strand of the San Andreas Fault; only a
small fraction of the vertical relief along the fault scarp is from the 1906 rupture. More features related to earthquake
faults can be seen along the Rift Zone Trail. This trail roughly follows the trace of the 1906 rupture from the Visitor
Center southward for four miles to the trailhead at Five Brooks along Highway 1. Paleoseismic studies based on trench
excavations by Neimi and Hall (1992) indicate that the slip rate of the 1906 trace (North Coast segment of the San
Andreas Fault) averaged about 24±3 mm per year for the past 2,000 years, and that the recurrence interval for large
earthquakes is in the range of 221±40 years.

According to K.G. Gilbert, who recorded the 1906 earthquake’s impacts, the only recorded casualty in the Point
Reyes region was a cow that was supposedly crushed and killed upon falling into an open fissure during the earthquake. It
was later revealed that the cow story was really a hoax perpetrated by a farmer who had dumped a dead cow into an open
fissure. The sensational story was widely distributed before it was revealed to be a joke on the reporters.

108

Figure 9-8. A reconstructed historic fence offset by the San
Andreas Fault in 1906. This exhibit is along the Earthquake
Trail near the Bear Valley Visitor Center. The original fence at
this location was offset about 18 feet. As much as 26 feet of
horizontal surface slip was reported on the fault in the Olema
area (Gilbert, 1907).

Figure 9-9. Blue posts mark the trace of surface rupture by the
1906 earthquake. The Shaffer Ranch barn was built on the trace
of the fault and was damaged by the earthquake. This view is
looking north from the offset fence exhibit area along the
Earthquake Trail.

Stop 2 – Tomales Bay Trail (Golden Gate National Recreation Area)

Stop highlights: San Andreas Rift Valley, Tomales Bay, Franciscan Complex, blueschist knockers, marshland and
coastal prairie habitats

The Tomales Bay Trail is a good optional destination to examine unusual outcrops of Franciscan Complex bedrock in
a scenic setting on the east side of Tomales Bay. The Tomales Bay Trail is located 1.7 miles north of Point Reyes Station
along Highway 1 (Shoreline), and is 4.3 miles north of the Bear Valley Visitor Center (Stop 1) near Olema.

The Tomales Bay Trail area is located at the north end of a marine terrace that extends southward through the Point
Reyes Station area. From the parking area, the trail crosses a low incised plateau-like area (part of the old terrace) before
dropping down to the salt marsh at the south end of Tomales Bay. Of geologic interest are outcrops of metamorphosed
sandstone which rise above the coastal prairie near the parking area (fig. 9-10). The features are called “blueschist
knockers”—blueschist being the grade of metamorphic change that the rock has experienced, imparted to the rock by
crystallization of the mineral glaucophane under moderate crustal depth pressure and at relatively low temperature. The
word “knocker” is a regional geologic term used to describe lone outcrops of bedrock that rise above the surrounding
landscape in areas typically underlain by Franciscan Complex. Over time as erosion has worn down the landscape,
resistant blocks of metamorphic sandstone and other rock are left behind as softer, more fractured and weathered material
erodes away. Knockers typically occur in areas where bedrock has been heavily sheared and mixed through the ongoing
tectonic development of the region, beginning with tectonic forces associated with plate-tectonic transport and
subduction, and later uplift and shearing along fault zones. The French word mélange (meaning mix) is applied to areas
of sheared rock in the Franciscan Complex where rock masses are often too small and discordant to be mapped as
individual geologic units. Rocks of the Franciscan underlie the countryside east of Tomales Bay.

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