The geology of Point Reyes National Seashore

Although not as widely known as Yellowstone and Yosemite, Point Reyes National Seashore is particularly loved by residents of the San Francisco Bay Area. Just an hour’s drive from downtown San Francisco, Point Reyes feels a world away and provides a welcome refuge from urban life. Most also know that the San Andreas fault (SAF) extends through the park and that this part of the fault moved during the Great San Francisco Earthquake in 1906.

Relief map of the San Francisco Bay Area showing major faults (red lines: SAF=San Andreas fault; SGF=San Gregorio fault; HF=Hayward fault) and the epicenter of the 1906 San Francisco earthquake (blue star). Click on the image, and any others in the blog, to get a larger size.

The epicenter (place on Earth’s surface directly above an earthquake) was offshore, just west of the entrance to San Francisco Bay, and of course that part of the fault could not be directly observed. But a famous geologist—G.K. Gilbert—walked nearly the entire length of the on-land part of the fault after the earthquake, taking photographs and drawing diagrams to illustrate his observations. The fault ruptured for 477 km (296 miles), from Shelter Cover, located about a 4.5-hour drive north of San Francisco to San Juan Batista, located about a 2-hour drive south of San Francisco. The greatest amount of offset along the fault was at Olema, near the Point Reyes National Seashore Visitor’s Center, a worthwhile stop.

At the Visitor’s Center, you can walk the “earthquake trail” that crosses the SAF I am emulating with outstretched hands. Although this is a “Hollywood” fence, it was constructed to demonstrate the amount of offset (~7 m / 20 ft) that occurred here during the earthquake. The SAF is a transform fault that is the boundary between the Pacific and North American plates that are sliding past each other. This view is to the east, with the Pacific plate in the direction I am looking and the North American plate behind me. The SAF is a right-lateral type of transform fault. You can tell this from the photo—looking across the fault, the plate on the other side has moved to the right. This works no matter which side of the fault you’re standing on.

Modern seismographic instrumentation was not available in 1906, but geophysicists have since used various methods to estimate a magnitude of 7.9 for the earthquake, which was a breakthrough event for science. Prior to 1906, the cause of earthquakes was not understood, but as a result of this large and well documented event, geoscientists figured out that earthquakes occurred because of fault movement—forces associated with plate motions cause pressure to build up on fault surfaces until the amount of force exceeds the strength of the fault (i.e., overcomes friction), and the fault moves, sending out seismic waves that reverberate throughout the earth and shake the ground surface.

At the entrance to the Visitor’s Center, there is a red barn next to the SAF. This photo, taken in 1906, shows J.C. Branner, Stanford geology professor, turning his back on an unsightly mess. It seems that cow manure shoveled out the barn window had created a pile along the exterior wall. During the earthquake, the manure pile was moved many meters to the NW, along with the Pacific plate! The photo is from the Branner Collection at Stanford University.
This is another photo of the SAF, looking southeastward, soon after the earthquake occurred. This location is between the barn and the offset fence (two photos above) at the Visitor’s Center. Today, posts mark this location, where there is a slight indentation, but the slope has been mostly smoothed out by erosion over the past 100+ years. Photo by G.K. Gilbert.
The red star on this geologic map of the Point Reyes Peninsula is the approximate location of the Visitor’s Center. The map shows the contrast in rock type across the SAF that is a result of millions of years of right-lateral motion. The KJf rock on the east side of the fault is the Franciscan complex (named after San Francisco!)—rock that was scraped off the down-going Farallon (oceanic) plate and accreted to the continent during the Mesozoic Era when the Sierra Nevada was an active chain of volcanoes and underlying plutons (see figures in my last post on October 30).

The Kg rock on the west side of the fault is granite that was formed as plutons between the southern Sierra Nevada and the Peninsula Range, but that during the past ~7 million years has been transported northward from southern California by movement on the SAF. The granite has moved northward ~450 km (270 miles), since the Pacific plate encountered the North American plate and captured the western edge of California! The brown- to orange-colored units are marine sedimentary rocks that also formed far to the south and have been transported northward with the Pacific plate. Geology adapted from Clark and Brabb, 1997. Note the Point Reyes thrust fault (PRF)—we found out that the fault, rather than curving around to the east, continues southward (dashed black line).
This view is looking south along the SAF, which has created a valley occupied at the north end by Tomales Bay (this photo) and at the south end by Bolinas Bay and Lagoon (see geology map above and last photo in this blog). Arrows show the relative direction of the plates—North American on the left (east) side and Pacific on the right (west) side. Practice “standing” on one side of the fault and looking across to the other side—you will see that the plate on the other side is moving to the right, because it is a right-lateral transform fault.
This map shows the current plate configuration along the west coast of the U.S. Since the Pacific plate first encountered the North American plate in southern California about 27 million years ago, the continental edge has evolved from a subduction zone to a suite of transform faults (in this simplified map, only the SAF is shown). The extent of the SAF has continued to lengthen northward—only our small Cascadia Subduction zone remains from what extended along the entire west coast prior to 27 million years ago. The Mendocino fraction zone is named after the coastal town of Mendocino, where the SAF ends and the subduction zone to the north begins. You may wish to view the map of California geology and the plate reconstruction 100 Ma in my previous post, on October 30.

While a professor at San Francisco State University, I and my students completed many studies of the Quaternary sediments (yellow areas marked Q in the geology map) to better understand how the fault zone has evolved during the past 200,000 years (recent history for the planet!). While working in the fault valley, I kept looking at the peninsula to the west and wondering why it was elevated—the transform SAF was causing the peninsula to slide northward, but it was not responsible for uplifting the ridges.

This is a view to the NW across the Point Reyes Peninsula. The dashed line is the hinge line for a large-scale synclinal fold (to emulate a syncline, cup your hand to create low area in the middle and high areas on each side). Sedimentary layers on the east (right) side are tilting westward toward the hinge and creating the high topography of Inverness Ridge; layers on the west (left) side are tilting eastward toward the hinge and creating the topography of Point Reyes, the westernmost extent of the peninsula. The fold structure implies that compressive forces have acted across this region.

Our investigations involved features on land and under the ocean. On land we discovered that a series of flat surfaces indent the western slope of Inverness Ridge. These features, called marine terraces, can be used to figure out how fast the uplift is happening.

This is what marine terraces look like and how they form. At sea level, wave action erodes a wave-cut platform into the bedrock. Because Earth has been experiencing alternating glacial and interglacial cycles during the past ~3 million years, sea level has been regularly rising and falling. Today, since we are in an interglacial period, sea level is high. But 18,000 years ago, during the last glaciation, sea level was about 140 m (450 ft) lower than it is today. The terraces tell us that the land was being uplifted while sea level was falling and so when it rose again, the land was at a higher elevation and the waves then cut a new platform. Since the old platform was elevated above sea level, it became an inactive terraced surface. We measure the elevation of the terrace’s inner edge, and try to get an age for the overlying sediments—with time and distance we can then calculate a rate.
This view to the south shows the flight of terraces that indent the western slope of Inverness Ridge. The area labeled “ls” is a part of the peninsula where extensive landsliding has disrupted the terraced surfaces, which are visible again at the south end of the peninsula near Bolinas (see geologic map). The landslide zone is an area called “The Lakes”, where back-tilted landslide blocks create low areas that collect water.
We used a high-precision GPS (Global Positioning System) unit to measure terrace elevations (shown are 3 of my students with GPS device) and we used a luminescence dating technique to get ages of the sediments overlying the terraced surfaces. With these data, we were able to determine that Inverness Ridge has been rising at a rate of 0.4–1.0 mm/yr, much less than the side-by-side rate of the SAF at ~24 mm/yr, but enough to create the topography that makes Point Reyes such a special place.
This diagram shows how terrace surfaces correlate to times of high sea level (ka=thousands of years ago). The odd numbers (marine isotope stages) indicate interglacial periods with high sea level; in between are the glacial periods (even numbers not shown) with low sea level. The Glen transect is a terrace flight south of Sculptured Beach (see geologic map); we found higher rates of 1.0 mm/yr (same as 1 m/ka) at the south end of the peninsula near Bolinas. Diagram from Scherer, A., 2004, MS thesis at San Francisco State University. Keep in mind that average rates on faults are long-term rates. In the short term, faults sit quietly for many years, and then move abruptly, with large offset, during earthquakes.
This diagram demonstrates how the Point Reyes Peninsula has been lifted up above the ocean during the past ~330 ka (=330,000 years ago). Diagram from Bidgoli, T., 2002, BS thesis at San Francisco State University.

But what faults have been responsible for the uplift? To find these faults, we needed to examine data from the seafloor west of the peninsula. In the 1970s and 80s, oil companies conducted seismic surveys that involve sending sound pulses from a ship that are strong enough to penetrate the seafloor and bounce off of rock layers below. Companies also drilled holes into the seafloor to collect samples of the rock types. Fortunately, they didn’t find much oil and gave up the pursuit, but the data are now in the public domain and we were able to access them through the U.S.G.S. office in Menlo Park, California (thanks Holly Ryan!).

This is a seismic profile across the Point Reyes thrust fault (PRF—red line on profile) that is offshore from Point Reyes (see black line on geologic map above). View is toward the NW, with land to the right (NE). Now we know why the western point is high—because it has been lifted upward by movements on the PRF. Thrust faults are a result of compressive forces; notice that the older Kg (Cretaceous granite) rock has been lifted upward (east side of fault) to the same level as younger sedimentary rocks (Tp, Tsc, Tm, on the west side of fault). The sedimentary rocks have been folded (no longer flat lying) as a result of the compression, just as observed on land in the Point Reyes syncline (see geologic map and photo across the syncline above).

The vertical axis of the seismic profile is depth in TWTT (two-way travel time), the amount of time it takes for the sound pulse to travel down through the rocks and back up again to the recorders. With the time, and knowing the velocity of sound through rock types, it is possible to calculate the depth of penetration, which in this case is about 3 km (1.8 miles). The length of the profile (Shot points at 33 m/shot) is about 23 km (14 miles). V.E. (vertical exaggeration) = 3x. Figure from Stozek, B., 2012, MS thesis at San Francisco State University.
Because it’s under water, it’s not possible to see the Point Reyes thrust fault. But it is possible to see a well-exposed thrust fault if you visit Kehoe Beach on the north end of the peninsula (see geology map). This view is to the east, from the beach. The yellow arrows indicate the location of 2 thrust faults that lifted up older rocks next to younger rocks. Kg=Cretaceous granite (plutonic igneous); Tl=Tertiary Laird Formation (sedimentary). See geologic map above for location. The yellow box is Jay for scale. We aren’t certain when these faults were active—the youngest rocks involved are about 20 million years old, so the faults are certainly younger than that and probably much younger than that.
Another interesting place to visit is Sculptured Beach (see geologic map), which has a magnificent example of an angular unconformity, the surface that I’m standing on (red line). An angular unconformity occurs where sedimentary layers are tilted below (Tm), but flat-lying above (Qt). The Tm rock (Monterey Formation) is a marine sedimentary rock that is about 20 million years old. It was formed in southern California and became folded (tilted) during its journey northward with the Pacific plate. Once in this coastal location, it was eroded by waves to create a wave-cut platform; then sediments (Qt) flowed down from the hills and out to the beach a mere 50,000 years ago. They have not been tilted.
This view is looking north to Bolinas Lagoon and beyond along the valley formed by the SAF (dashed red line). To the left (west) is the Point Reyes Peninsula and to the right (east) is mainland Marin County with Bolinas Ridge. The faults that are uplifting the southern end of the peninsula are a southern extension of the Point Reyes thrust fault and other thrust faults located west of the SAF. Note the broad flat terrace, called “The Mesa”, west of the lagoon.

Notice the sand spit across the south end of Bolinas Lagoon, which is covered by homes. This is Stinson Beach—located on a pile of loose sand, on an active fault that is a plate boundary—this is not a good place to buy your dream home! But it’s fine for a week’s vacation—if you rent only.

Park History. Point Reyes is one of the younger parks in the national park system. A few small county parks had been established, but in the mid 1950s, there were plans for a large development at Limantour Beach (see geology map above). In 1958, the National Park Service proposed establishing a national seashore at Point Reyes, and in 1959, Congressman Clem Miller began his first term in U.S. House of Representatives and made establishing the Point Reyes National Seashore his greatest priority. He was successful when the park was established in 1962 by President John F. Kennedy. Unfortunately, Clem died less than a month after this, but I encountered his gravestone one day while hiking on the west slope of Inverness Ridge—his marker appropriately looks out to sea from the beautiful peninsula that he was instrumental in saving.

Earlier on, coastal Miwoks had occupied the region for at least 10,000 years, before European explorers landed here. The Englishman Sir Frances Drake, for whom Drakes Bay and Beach are named, arrived in the last 16th century; at the beginning of the 17th century, the Spaniard Vizcaíno arrived and named the location “Punta de los Reyes”. Unfortunately, within a few centuries, most of the native peoples were gone.

References

Clark, J., Brabb, E., 1997, Geology of Point Reyes National Seashore and Vicinity, California: U.S.G.S. Open-File Report 97–456 (https://pubs.usgs.gov/of/1997/of97-456/).

National Park Service site about the geology of Point Reyes: https://www.nps.gov/pore/learn/nature/geologicactivity.htm.

U.S.G.S. site about the Great 1906 San Francisco Earthquake: https://earthquake.usgs.gov/earthquakes/events/1906calif/18april/.

Grove, K., and Niemi, T., 2005, Late Quaternary deformation and slip rates in the northern San Andreas fault zone at Olema Valley, Marin County, California: Tectonophysics journal.

Grove, K., et al., 2010, Accelerating and spatially-varying crustal uplift and its geomorphic expression, San Andreas fault zone north of San Francisco, California: Tectonophysics journal.

Field trip guide to Point Reyes National Seashore and vicinity: https://pubs.usgs.gov/of/2005/1127/chapter9.pdf.

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