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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