Geology Of Salt Point State Park
Sue Ellen Hirschfeld, Ph. D. Professor Emerita
Department of Geological Sciences California State University, East Bay
Salt Point State Park provides spectacular vistas of the ocean, with rugged
offshore rocks and steep sea cliffs that take the full impact of the waves. The
rocks are sculpted into an infinite variety of forms and shapes. Extending
underwater, the rocks offer a range of habitats to a wide variety of marine
plants and animals. Divers can enjoy the rich underwater world. Uphill from the
coast, the park continues to the top of the coastal ridge. Habitats change from
coastal grassland to forests of Bishop pine, madrone, tanoak, and redwoods. A
pygmy forest of stunted cypress, pine, and even redwoods, as well as a large
open “prairie,” provide unique surprises for the hiker. What makes Salt Point
State Park so special? What has created this unique landscape? There are many
more questions than we can easily answer, but we can begin to unravel the
mysteries of its origin and formation. We can look beneath the surface at the
dramatic geologic processes that create this magnificent landscape. The terrain
of the park has been formed and modified over tens of millions of years. The
processes involved in its formation include those processes that move continents
and create oceans, build mountains and generate destructive earthquakes. To
fully appreciate the geologic history of Salt Point State Park, it is helpful to
understand how the rocks of the park formed and what dynamic processes were
involved in the creation of the coastal mountains of California.
ROCKS
There are three types of rocks: igneous, sedimentary, and metamorphic, defined
by how they are formed. Igneous rocks were molten at some time in their history.
The melt is called magma when it is found beneath the earth’s surface or lava
when it is erupted onto the surface. When the melt cools, it forms a rock made
of intergrown, interlocking crystals composed of several different minerals.
When the melt cools slowly, the crystals have time to grow large,
producing an igneous rock such as granite. If the melt cools quickly, the
crystals that form are very small, often too small to be seen with the unaided
eye. Basalt is an example of an igneous rock which is formed from lava that
cooled quickly.
Sedimentary rocks are formed on the earth’s surface by the action of surface
processes, such as weathering, erosion, deposition, and cementation. When any
type of rock (igneous, metamorphic, or sedimentary) is exposed at the earth’s
surface, it comes in contact with the atmosphere. Oxygen in the atmosphere and
weak acids in rainwater react with the rocks. The exposed rocks are chemically
altered and mechanically broken apart into smaller and smaller pieces, a process
called weathering. The rock fragments, known as sediment, are then transported
by wind, rivers, ocean currents, or glaciers. This transportation process is
termed erosion. Eventually the rock particles are deposited in some low place,
such as on the bottom of a lake or on the floor of the ocean, and they
accumulate layer upon layer. With the passage of time, the weight of the
overlying sediment and the precipitation of minerals in between the rock
particles cements the grains together and transform the loose sediment into
solid sedimentary rock. Sedimentary rocks are classified based on the size of
the particles making up the rock. Large, rounded pebbles cemented together form
a conglomerate. Sand-sized particles form a sandstone, while mud and clay-sized
particles form mudstone and shale. Each of these types of sedimentary rock can
be seen at Salt Point State Park and will be described in greater detail later
in this guide. If sedimentary rock or crystallized igneous rock is deeply buried
and subjected to high temperatures and pressures, it will be altered to a new
rock called metamorphic rock. Metamorphic rocks are often made of crystals like
igneous rock, but the crystals are arranged in layers (called foliation),
reflecting the modifying heat and pressure. Examples of metamorphic rocks are
quartzite, marble, slate, schist, and gneiss (pronounced “nice”). Igneous and
metamorphic rocks, originally formed at depth, can be uplifted by
mountain-building processes and become sedimentary rocks. These sedimentary
rocks, in turn, may be buried and converted into metamorphic rock or melted to
become igneous rock, thus completing the rock cycle. All of the rocks along the
coastline in the park are sedimentary sandstones, conglomerates and mudstones.
Metamorphic and igneous rocks can only be found as large, rounded pebbles within
the conglomerates. Conglomeratic
pebbles vary in the color of their crystals. Pebbles composed of large white and
black crystals are igneous granite. Volcanic pebbles are usually dark with a
scattering of tiny light-colored crystals. All-white pebbles are usually white
quartz. These types of igneous rock are fairly abundant. Pebbles made of
metamorphic rock are often dark in color and may show alternating dark and light
layers of small crystals. Identifying these different rock types may be
especially difficult when the pebble has been rounded and polished and the
sample is small. Even experts may have difficulty, so don’t get discouraged.
READING THE STORY IN THE ROCKS
Rocks contain a record of their geologic history: how, when, and where they
formed. Geologists are able to read the story contained within the rocks, and
they can interpret and recreate the history of the California coast through
geologic
time. It doesn’t take a professional to do this. With a little background, you
can
begin to look beneath the surface and take a voyage back through time.
THE ROCK CYCLE
MOVEMENT OF PLATES, BUILDING OF MOUNTAINS
The story begins over 100 million years ago with the formation and movement of
large blocks of the earth’s crust and upper mantle called plates. The outer
portion
of the earth is divided into about a dozen rigid plates that are “floating” on a
plastic-like portion of the upper mantle (the layer of the earth beneath the
crust).
These plates are in motion; some move apart and some move toward each other.
Where plates move apart, molten magma comes to the surface in the rift and
cools to form new oceanic crust.
A sea-floor spreading ridge to the west generates the Pacific Ocean plate which
is subducted under the
North American plate. Heat generated where the plates collide causes melting of
the crust. The molten magma rises to form volcanoes at the surface. Large bodies
of molten rock cool at depth, forming the granite of the ancestral Sierra Nevada
mountains. Ocean floor sediments and portions of the oceanic crust are scraped
off the subducting plate against the North American plate and pile up,
eventually forming the rocks of the Coast Range mountains. These “sea-floor
scrapings” are called the Franciscan Assemblage. When this process occurs under
the ocean, the process is called sea-floor spreading. As spreading occurs and
new crust is formed, the plates move away from each other. As the plates
separate, they move toward other plates. Where plates collide, one plate moves
down under the other, a process called subduction. Collision and subduction of
plates are the processes that created most of the rocks of California. Millions
of years ago, the Pacific Ocean plate moved eastward away from a spreading ridge
and collided with the North American plate. As the two plates collided, the
North American plate acted like a gigantic snowplow and scraped off a thin
portion of the Pacific Ocean plate as it was being consumed. Over millions of
years, these “sea floor scrapings” piled up at
the margin of the North American plate and today make up much of the rock of the
northern coastal mountains.
For hundreds of millions of years, the West Coast of North America had been a
collision-type plate margin. There was a sea-floor spreading ridge to the west,
generating the Pacific Ocean plate. The Pacific Ocean plate, formed at the
spreading ridge, moved toward the western margin of the North American plate,
collided with it and was subducted beneath the North American plate. Farther to
the east, molten rock, generated by the friction developed as the two plates
collided, rose to form volcanoes at the surface and, at depth, cooled to form
the granite of the ancient Sierra Nevada mountains.
SAN ANDREAS FAULT
About 25 million years ago, the California coastline went through a dramatic
change. A new type of plate margin formed. Instead of colliding, the Pacific and
North American plates move past each other along the San Andreas fault. The
cause of this change was the North American plate overtaking and overriding the
eastern portion of the Pacific plate and the spreading ridge. More and more of
the Pacific plate has been overridden, and the San Andreas fault has become
longer.
Today the San Andreas fault, the boundary between these two huge plates,
traverses the State of California from the head of the Gulf of California in the
south to Point Arena in the north. The San Andreas fault crosses through the
eastern part of Salt Point State Park. This segment of the fault ruptured in the
great San Francisco earthquake in 1906. Reconstruction of interactions between
the North American plate and the Pacific Ocean plate over the last 40 million
years. 40 million years ago, a spreading ridge generated the oceanic plate
which was subducted under the North American plate. 20 million years ago, North
America moved westward and overrode a corner of the spreading ridge, the oceanic
plate and the subduction zone, thus forming the beginning of the San Andreas
fault boundary where the two plates slide past each other. Today, the San
Andreas fault has lengthened as more of the Pacific Ocean plate is overridden.
Sea-floor spreading and subduction
still occur to the north and to the south. The Pacific Ocean plate is now moving
northwest along the San Andreas fault.
All of the California continental crust to the west of the San Andreas fault is
attached to the Pacific Ocean plate and is moving northwest with the Pacific
Ocean plate. This sliver of continental crust is called the Salinian block. The
Salinian block has moved hundreds of miles to the north along the San Andreas
fault system along with all the younger rocks which formed on top of it,
including those at Salt Point State Park (Figure 4B). Because of this northward
migration of the Salinian block, the rocks of Salt Point State Park on the east
of the San Andreas fault are very different in composition and in age from the
rocks on the west side of the fault.
The rocks in the park to the east of the San Andreas fault are called the
Franciscan Assemblage and make up much of the Coast Range mountains. Franciscan
rocks are made of the deep ocean sediments and portions of oceanic crust scraped
off the descending Pacific Ocean plate as it was subducted about 100-150 million
years ago. Franciscan rocks are difficult to see in the park because the portion
of the park east of the San Andreas fault is covered with dense forest and
soil, and the Franciscan rocks are poorly exposed. However, excellent examples
of Franciscan rocks can be seen in road cuts along the coast south of the park
between Fort Ross and Bodega Bay. As a result of the mountain building processes
that have raised portions of the California coast and the movement along the San
Andreas fault, the rocks of Salt
Point State Park have been folded and, in some places, faulted. These folds and
faults can be seen in the rocks along the coast.
ROCKS ALONG THE COAST AT SALT POINT
GERMAN RANCHO FORMATION
The rocks along the beautifully rugged coastline are tilted sedimentary rocks,
mostly sandstones with interbeds of conglomerates and mudstones, part of the
German Rancho Formation. A formation is a group of rocks having similar
composition. This one is named for a local geographic landmark. In 1846, Rancho
German was a large land grant that extended north from Fort Ross. Rocks of the
German Rancho Formation can be found from Fort Ross to Point Arena.
North of Fort Ross the sequence is thought to be as much as 18,000 feet thick!
The sedimentary rocks of the German Rancho Formation were formed in a submarine
basin on the then submerged Salinian block 40-60 million years ago(during the
Paleocene to Eocene epochs of the geologic time scale). The marine basin and
Salinian block were at that time situated 200-260 or more miles to the south of
where Salt Point State Park is located today (Figure 4B). These rocks have been
moved that distance along the San Andreas fault in the last 20 million years!
Geologists use the composition of the pebbles and sand to locate the original
mountainous source area for the sediments now distantly separated
along the fault. In this way they are able to reconstruct the past geography and
environment at the time these rocks were forming.
TURBIDITY CURRENTS, TURBIDITES & DEEP-SEA FANS
The thick layers of sandstone alternating with layers of conglomerate and
mudstone which make up the German Rancho Formation are interpreted to have been
deposited by flows of dense, turbulent, sediment-laden water flowing down
a submarine canyon. These high density flows are called turbidity currents. The
rocks formed from the deposition of sediments from turbidity currents are termed
turbidites. Rapid sedimentation on slopes of the submarine basin may result in
instability. Intense storm activity or an earthquake may trigger a submarine
slide that starts the turbidity current moving. The flow moves downslope, down a
submarine canyon and then out onto the ocean floor where the sediment is
deposited on a deep-sea fan. A modern example of these processes and deposits
can be found at Monterey
Bay. Today, rocks in the high Sierra Nevada mountain range are weathered, and
the sediment is carried by streams and rivers into the Sacramento and San
Joaquin rivers, through the Delta to San Francisco Bay and out the Golden Gate
to the
ocean. There, longshore currents (currents flowing parallel to the coast),
driven by prevailing winds, carry the sediments southward along the coast
forming the beaches from San Francisco to Monterey. In Monterey Bay, the
sediments are funneled down the Monterey submarine canyon where they flow in
underwater channels and finally come to rest on
the Monterey deep-sea fan. The fan has numerous channels at the top which carry
the coarsest sediment, such as conglomerates. The sediment gets finer farther
down the fan and between the channels where the sediment may have
overflowed the banks of the channels. The sandstones and
conglomerates at Salt Point are thought to have formed in the
channels, and the thinner beds of sandstone and mudstone are thought to have
formed between the channels on a large, submarine deep-sea fan millions of years
ago. These channel and interchannel deposits can be seen in many places along
the sea cliffs How did these sedimentary rocks from a deep-sea fan,
deposited thousands of feet beneath the ocean, get raised to their present
position above sea level? These same processes that created the Coast Range
mountains can be seen in action today. On October 17, 1989, a 7.1 magnitude
earthquake rocked the San Francisco Bay region. The epicenter was in the Santa
Cruz mountains. The San Andreas fault ruptured along 25 miles, at a depth of
between 2 and 11 miles beneath the surface. The fault did not break the surface
as it did in 1906. After the earthquake, surveys
of the surrounding peaks indicated that the Santa Cruz mountains had moved about
4 feet upward and 6 feet to the north. There is evidence at Salt Point State
Park of similar kinds of uplift and northward migration. The sandstones, shales,
and conglomerates of the German Rancho Formation were originally deposited in
horizontal layers, like the layers of a cake. The layers are now tilted, or
tipped, along the entire coastline of Salt Point State Park. This tilting
exposes rocks of different ages. In a sequence of sedimentary rocks, the strata
at the bottom is the oldest (first deposited) and the strata at the top is the
youngest (last deposited). If the sequence of strata remains horizontal and
flat, the only way to see what is below the surface is to cut a slice into it,
as has occurred where the Colorado River has cut the Grand Canyon through a mile
of rock, exposing older and older rock as you descend to the bottom of the
gorge. Alternatively, when rock is tilted, and erosion carves away the edges,
older and older rock is exposed at the surface. At Salt Point State Park, the
oldest rocks are at the southern park boundary, and you come upon younger and
younger rock layers as you walk northward up the coast. About a third of a mile
south of Horseshoe Point, the rocks are tilted in the opposite direction, and
the layers get progressively older to the north. This change in tilt is because
the rocks are folded into a huge downfold called the Horseshoe Point syncline.
The youngest layers are at the center of the syncline, while the layers get
progressively older away from the center. The tilting not only exposes rocks of
different ages, it exposes rocks of different hardness and different resistance
to weathering and erosion. The waves wear away the weaker rock layers from the
harder ones, forming coves among the more resistant points and headlands.
MARINE TERRACES
As you drive or walk from Highway 1 to the coastal edge at Salt Point State
Park, you will notice the broad, flat surface above present sea level where the
waves break along the cliffs. These flat surfaces are called marine terraces,
and they are
old, uplifted ocean floor. To visualize how these terraces formed, imagine what
would happen if the water were suddenly drained from the ocean. You would find a
gently sloping surface running from the beach offshore to the west. Imagine what
would happen if the land were suddenly raised 20 feet (and the ocean returned to
its previous level). You would have a broad gently sloping surface like the one
you drive across to reach the coast. In fact, the rocks that rise above the
terrace level are ancient sea stacks, similar to the resistant rocks off the
coast today that take the initial impact of the waves before they reach the sea
cliff.
WATER, WIND AND SALT
The sandstone sea cliffs look as if a sculptor shaped and carved the rocks into
all manner of imaginative forms. In fact, the sculptor is the wind, waves, and
sea spray. Look beyond their amazing shapes and forms into their origin and
history. If you look carefully at the sandstones along the coast, you can see
the layering in the sandstone and also see that the layers are tilted at an
angle, exposing them to the elements. Some of the sandstones are harder because
they are better cemented than adjacent sandstone layers. Layers that contain
more clay may be softer. The waves and wind are able to etch and remove the
softer, exposed layers, leaving the harder layers standing as ridges and ribs.
The massive sandstones and conglomerates form the points and headlands; the
coves form where the rocks contain more mudstone or the rock has been fractured.
FRACTURES AND FAULTS
The extent to which rocks are broken is another important factor in how easily
rocks are eroded. A fault is a break along which movement occurs; a fracture or
joint is just a crack in the rock. The close proximity of the San Andreas fault
and
the tilting of the rocks indicate that the rocks have been subjected to stress.
In some cases, the rocks respond by simply cracking; in other cases, the rocks
on the two sides of the break move, one side relative to the other. As you walk
along
the headlands, notice that the massive sandstones are highly fractured.
Differences in color of the rocks due to weathering often accentuate the
fractures. Faults also break up the rock. The fault plane where rocks have been
broken can
be recognized by polished surfaces called slickensides. These can be seen along
the road down to Gerstle Cove.
Faults can also be recognized by seeing the strata on one side of the fault
displaced from the strata on the other side, or by seeing rocks juxtaposed which
may be entirely different in composition. Faults are also recognized where
layers of rock
are tilted at a different angle from the rocks across the fault. These fault
features are very apparent at Gerstle Cove and in the cove to the north of Salt
Point. Gerstle Cove is a cove because the shattered rocks in the fault zone have
been
more easily removed by the waves.
WEATHERING
One of the most unusual and beautiful features of the sandstone along the sea
cliffs is the development of a honeycomb-like network called tafoni. The exact
process of formation of tafoni is not entirely understood. The waves and salt
spray leave salt crystals on the sandstones. Salt and water interact with the
cement between the sand grains and within minute fractures in the rock.
Alternately, some portions are hardened while others are loosened. This creates
the lacy, box-like pattern. Large, rounded rocks, some even standing on
pedestals, are found on some of the points of rock along the coast. These
rounded rocks are called concretions. The concretions represent areas within the
sandstone layers where the sandstone is better cemented than in the surrounding
sandstone and, therefore, are more resistant to weathering and erosion.
WAVE EROSION
One of the most awesome sights is a view of winter storm waves battering the
coast. Waves strike the rocks with tremendous force. Water is massive material;
a cubic yard of water weighs about a ton! Seismographs used to detect
earthquakes can actually register the minute tremors caused by the sudden impact
of tons of
water striking solid rock at the coast. Storm waves are even more destructive
than the force of the water alone because waves pick up and hurl sand and
boulders against the shoreline. As the waves break, water pressure forces sea
water into every tiny crack, enhancing chemical weathering of the rock as the
water evaporates. Much of the wave energy is focused on the headlands that
project into the ocean. The waves are bent (refracted) around the headland so
the force of the wave is directed against the sides of the headland as well as
at the point. This causes erosion along the sides, leading to the formation of
sea arches. If erosion isolates the point of a headland, sea stacks form.
FOSSILS
Fossils are traces or remains of once living organisms now preserved in rock. At
Salt Point State Park, fossils can be found in the sandstone and mudstone
exposed in the sea cliffs. These are trace fossils, or ichnofossils (“ichno”
meaning footprint or track), which are the tracks, trails, burrows, or borings
made by organisms in the sediment in which they lived. Unlike body fossils, such
as shells or bone that are the actual hard-part remains of the organism, trace
fossils are indications of the organism’s behavioral activity, such as feeding
traces, locomotion tracks, or of its home-dwelling burrow. These fossils may not
be obvious at first glance. They appear as a series of straight, curved or
branched tubes, about ¼” to 1” in diameter, within the sedimentary layers. In
cross-section, they appear as small circles. Traces are produced by a variety of
animals, such as crabs, clams, and worms. It is not always possible to identify
the organism that produced the track, trail or burrow. However, it has been
determined that certain types of trace fossils found together represent a
particular depositional environment. Some traces represent shallow water, others
near-shore environments, and still others progressively deeper offshore waters.
The Salt Point trace fossils are thought to be from a deep-water environment.
GEOLOGIC PROCESSES AT
SALT POINT STATE PARK
Salt Point State Park provides an opportunity to view geologic processes in
operation today. Take a trip through time and space to explore the millions of
years of Earth history recorded in the rocks. See the evidence of great plates
colliding and passing. Touch rocks that formed deep on the ocean floors,
composed of materials eroded from mountains long since vanished. See these
geologic wonders for yourself by following the Field Guide to Salt Point State
Park.
Selected Bibliography
Farmer, J. D., and M. F. Miller. “A Deep Water Trace Fossil Assemblage from the
German Rancho Formation, Stump Beach, Salt Point State Park”, Modern and Ancient
Biogenic Structures, Bodega Bay, California and Vicinity. Annual Meeting Pacific
Section, Society of Economic Paleontologists and Mineralogists, Field Trip 3,
pp. 3-13. 1981.
Graham, S. A., and K. D. Berry. “Early Eocene Paleogeography of the central San
Joaquin Valley, Origin of the Cantua Sandstone”: Armentrout, J. M., et al.,
Cenozoic Paleogeography of the Western United States, Symposium 3, Pacific
Section, Society of Economic Paleontologists and Mineralogists. 1979.
Pestrong, R. Tafoni, Pacific Discovery. Volume 43, No. 2, (1990), pp. 15-21.
Porter, B. S. History of Salt Point 1845-1890. Unpublished manuscript. Cultural
Resource Management Unit, Resource Protection Division, California Dept. of
Parks
and Recreation. 1982.
Wentworth, C. M. “Upper Cretaceous and Lower Tertiary Strata Near Gualala,
California, and Inferred Large Right Slip on the San Andreas Fault,” Dickenson,
W. R., and A. Grantz (eds.), Proceedings of Conference on Geologic Problems of
the San Andreas Fault System. Stanford University Publication of Geologic
Science,
Volume 11, pp. 130-143. 1968
21
22
Suggested Readings
Alt, D. D., and D. W. Hyndman. Roadside Geology of Northern California.
Missoula, Montana: Mountain Press Publishing Company, 1975.
Bascom, W. Waves and Beaches, the Dynamics of the Ocean Surface. New York:
Doubleday and Company, 1964.
Iacopi, R. Earthquake Country. Menlo Park, California: Lane Books, 1969.
Pestrong, R. Tafoni, Pacific Discovery. Volume 43, No. 2, (1990), pp. 15-21.
Press, F., and R. Stever. Earth. San Francisco: W. H. Freeman and Co., 1982.
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