Geology of Point Reyes National Seashore
Geology of Point Reyes National Seashore
Point Reyes National Seashore was formed nearly 29 million years ago, but tectonic activity has created a dynamic landscape that is still on the move today. While the structures of the seashore are relatively young, the rocks that form it are much older. The rocks that we see today were formed in the Late Jurassic as two tectonic plates were colliding (159-144 million years ago). The oceanic Farallon plate was subducting, or sinking, under the North American continental plate. Once the Farallon plate was completely subducted, cracks in the Earth, called faults, began to form. Here, a transform fault, or a strike-slip fault was formed, which occurs when rocks slide past one another. This area is now on the Pacific Plate, and Point Reyes lies in an area called the Salinian Block. It is between two sets of strike-slip faults and is 40-70 km wide and over 500 km in length. It is also close to the famous San Andreas Fault. East of the San Andreas, Point Reyes consists of the Franciscan Complex. The Franciscan Complex is composed of very deformed and highly metamorphosed (a change in rock type due to changes in heat and pressure) graywacke, mudstone, volcanic materials, chert, and limestone. The area to the west of the San Andreas Fault consists of vast areas of Cretaceous-age granite with recent sedimentary rocks overlain. A geologic map of Point Reyes can be viewed here: https://pubs.usgs.gov/of/1997/of97-456/.
Point Reyes is of particular interest to many geologists because it sits on the San Andreas Fault. In the picture below you can see just where the fault zone lies- it is also the meeting point of the oceanic Pacific Plate and the continental North American Plate. However, the fault itself cannot be seen. The San Andreas Fault is built up of many large and small additional faults to create one big system. It travels southwest out of Tomales Bay towards Bolinas lagoon (Figure 2). These scattered faults run parallel to one another, and the resulting mountainous ridges (and on either side of the red line below) are called ‘shutter ridges’. On the west of Tomales Bay is the granite rock, and on the east side of Tomales Bay is a mix of the sedimentary rock types. Because the park sits within this fault system, the park is on the move. Movement along the San Andreas fault is up to 5 cm a year.
The California coast is always eroding due to constant wave movement from the Pacific Ocean, but the rocks can also move vertically. Uplift is a phenomenon where the Earth’s crust is pushed upward due to tectonic activity. Features called ‘marine terraces’ can be used to tell us how fast rocks are being uplifted compared to sea level.
At sea level, the waves from the ocean erode and cut away at the cliff’s edge, creating flat areas called terraces. As time passes, land is pushed upward while sea level also changes, forming new terraces . Sea level is affected by Earths glaciation cycle: when the Earth freezes over, there is less water in the oceans (since its all frozen), and sea level falls. When the Earth is not frozen, there is less frozen water and more available as liquid in the ocean, so sea level rises. At Point Reyes, many marine terraces can be seen by looking out at the coast . The rate of marine terrace formation can be determined by measuring how far the rocks were uplifted and how long that took.
The ocean can be a dangerous force in the park as well. Events called landslides can block roads, tumble rocks into the sea, and potentially cause harm. A landslide is a large sliding of rocks and other earth materials down a slope; it can be down a mountain, cliff, or even into roads and trails. Because Point Reyes is on the coast, and the ocean is constantly eroding the seashore, it can sometimes take away too much material from underneath a cliff and the entire cliff collapses. When this happens, the event is called a ‘coastal landslide’ or ‘cliff retreat’. Over time, as water pounds into the rock, bits and pieces are chipped away until there is no support, and the slope is over-steepened leading to a cliff collapse. Also, high and strong waves can crash into the cliff and cause movement within the cliff, causing a landslide.
USGS REFERENCES FOR ADDITIONAL INFORMATION: