The Geology of Salt Point State Park

Salt Point State Park provides the visitor 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. There is also a pygmy forest of stunted cypress, pine and even redwoods, and a large open "prairie".

What makes Salt Point State Park so special? What has created this unique and unusual landscape? There are many more questions than we can easily answer, but we can begin to unravel the mysteries of the park's 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 the Salt Point State Park, it is helpful to understand how the rocks of the park formed and the dynamic processes involved in the creation of the coastal mountains of California.

ROCKS AT SALT POINT

There are three types of rocks: igneous, sedimentary, and metamorphic defined on how they are formed. Igneous rocks were molten at some time in their history. The melt is called magma when it's 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 a lava that cooled quickly.

Sedimentary rocks are formed on the earth's surface by surface processes, such as weathering, erosion, deposition, and cememtation. When any type of rock (igneous, metamorphic, or sedimentary) is exposed at the earth's surface, it comes in contact with the atmosphere which is very corrosive. The rocks are mechanically broken apart and react chemically with oxygen in the atmosphere and weak acids in rain water . These weathering processes breaks the rock into smaller particles that are then transported by wind, running water, ocean currents, or glaciers; they are eroded. 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 by layer. The weight of the overlying sediment and the precipitation of minerals in between the rock particles turns 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 rocks 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.

All of the rocks along the coastline in the park are sedimentary sandstones, conglomerates and mudstones. The only metamorphic and igneous rocks are found in the large, rounded pebbles in the conglomerates. Granite pebbles are made of white and black crystals. Volcanic pebbles are usually dark with a scattering of tiny light-colored crystals. All-white pebbles are usually white quartz (or quartzite). These types 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 backround, you can begin to look beneath the surface and take a voyage back through time.

The story begins over a hundred million years ago, and involves the formation and movement of large blocks of crust, 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. 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, North America acted like a gigantic snowplow and scraped off a thin portion of the Pacific 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 year, the West Coast of North America has been a collision-type plate margin. There has been a sea-floor spreading ridge to the west, generating the Pacific Ocean plates. The oceanic 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. In the process the rocks of the Coast Ranges were "scraped off" the descending plate and uplifted. Farther east, molten rock, generated by the friction developed as the two plates collided, rose to form granite of the ancient Sierra Nevada mountains.

THE 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 moved past each other along a fault: 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.  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.

The rocks of Salt Point State Park on the west of the San Andreas fault are very different in composition and age from the rocks on the east side. The reason for these differences are because the Park straddles the San Andreas fault. All of the California continental crust to the west of the San Andreas fault is attached to the Pacific oceanic plate and is moving northwest with the Pacific plate. That portion of Salt Point State Park situated west of the San Andreas fault is part of this sliver of continental crust called the Salinian block. These rocks in the park were formed about 40 - 60 million years ago in a marine basin on the Salinian block. The basin in which the rocks were deposited was then situated 200-260 or more miles to the south of where Salt Point State Park is located today. These rocks have been moved that distance along the San Andreas fault in the last 20 million years!

The rocks in the park to the east of the San Andreas fault are very different in composition and age. They are called Franciscan rocks (the name applied to a group of rocks making up much of the Coast Ranges) and are made of the deep-ocean sediments and portions of oceanic crust scraped off the descending Pacific 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 heavily covered with forest and soil, and Franciscan rocks are just poorly exposed. However, excellent examples of Franciscan rocks can be seen 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 STATE PARK

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 a similar composition. They are named for a local geographic landmark. Rancho German, in 1846, was a large land grant that extended north from Fort Ross. Rocks of the German Rancho Formation can be found exposed 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 40-60 million years ago during the Paleocene to Eocene Series of the geologic time scale. These rocks were originally deposited in horizontal layers, like the layers of a cake. Along the entire coastline of Salt Point State Park, the layers are now tilted or tipped. 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. On the other hand, when rock is tilted, and erosion carves the edges, older and older rock is exposed at the surface. At Salt Point State Park, the oldest rocks are at the southern boundary and they get younger as you progress northward up the coast. About a third of a mile south of Horseshoe Point the rocks are tilted in the opposite direction and get progressivley older to the north. This is because the rocks are folded into a downfold called the Horseshoe Point syncline. The youngest rocks are at the center of a syncline, and they get older away from the center.

The tilting not only exposes rocks of different ages, it exposes rocks of different hardness and 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.

40 - 60 million years ago, when the rocks of the German Rancho Formation formed , sediments worn from the surrounding mountains were carried by rivers down to the ocean. The mud, sand, and pebbles were transported out onto the floor of a deep submarine basin that was located far to the south of where Salt Point is located today. The composition of the pebbles is used by geologists to reconstruct the geography and environment at the time these rocks were forming.

TURBIDITY CURRENTS AND DEEP-SEA FANS

The thick layers of sandstone alternating with layers of conglomerate and mudstone in a particular succession, forms a pattern that is 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. Rapid sedimentation on slopes of the 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 the 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 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 funnelled off 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 in 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 mudstones are thought to have formed in 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? The processes that created the coast range mountains can be seen in action today.  On October 17th 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 11 and 3 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 6 feet upward and 4 feet to the north. There is evidence at Salt Point State Park of similar kinds of uplift and northward migration.

MARINE TERRACES

As you hike or drive along the coast, you will notice broad, flat surfaces above sea level.  These are called marine terraces, part of the old, uplifted ocean floor. 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. You would have a 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 seacliff.
 
 
 

WATER, WIND, AND SALT

The sandstone seacliffs 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 their layers 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 mudstones or the rock has been fractured.

Another important factor in how easily rocks are eroded is how broken they are. 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 indicates 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 eccentuates 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. Another way to recognize faults is to see the strata on one side of the fault displaced from the strata on the other side, or the rocks may be entirely different in composition or be tilted at a different angle from rocks across the fault. These features are well displayed 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 sandstones along the seacliffs 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 leaves salt crystals on the sandstones. Salt and water interacting with the cement between the sand grains, and in minute fractures in the rock alternately hardens portions and loosens others, creating the lacy, box-like pattern.
 
 
 
 
 
 
 
 
 
 
 

On some of the points of rock along the coast are large rounded rocks, some are even standing on pedestals. 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 is more resistant to weathering and erosion.
 
 
 
 
 
 
 
 
 
 
 
 

WAVE EROSION

One of the most awesome sights is to see winter storm waves battering the coast. Waves strike the rocks with tremendous force. Water is massive stuff; 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 which project out 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 leads to erosion along the sides leading to the formation of sea arches. If the erosion isolates the point of a headland, sea stacks form.
 

TRACE 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 sandstones and mudstones exposed in the sea cliffs. These are trace fossils or ichnofossils (ichno = 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), which are the actual hard-part remains of the organism, trace fossils are indications of the organisms behavioral activity such as feeding traces, locomotion tracks, or of its home-dwelling burrow.
 
 
 
 
 
 
 
 

Trace fossils may not be obvious at first glance. They appear as a series of straight, curved or branched tubes, about 1/4 inch to an inch 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 associations of trace fossils characterize particular depositional environments. In other words, there is an assemblage of traces representing shallow water, near-shore environments, and different assemblages in progressively deeper offshore waters. The Salt Point trace fossils represent a deep-water association.
 
 
 

GEOLOGIC PROCESSES AT SALT POINT STATE PARK

Salt Point State Park provides the visitor an opportunity to view geologic processes in operation today and to take a trip through time and space, to explore the millions of years of earth history recorded in the rocks, to see the evidence of great plates colliding and passing, and to see rocks that formed deep on the ocean floors, composed of materials eroded from mountains long sinced vanished.  A geologic field guide to the coast between Gerstle Cove and Stump Beach is available for purchase at the Visitor's Center at the park.

Written and illustrated by Dr. Sue E. Hirschfeld, Emerita Professor of Geology, Department of Geological Sciences, California State University, Hayward, February, 2001.

 

Copyright Dr. Sue Ellen Hirschfeld

Site by Table Mountain Web Design