Is Tomales Point Moving Northwest Faster Than it is Being Eroded?

On my birthday, as I hiked to Tomales Point, I had one simple question. Is Tomales Point moving northwest faster than it is being eroded by the ocean?

The implications of this question are huge. If it is moving northwest faster than it is being eroded, it is actually moving northwest. If not, it is either stationary or moving southeast. Most of us know the Pacific Plate side of the San Andreas Fault is moving northwest. But the features riding on top of it may not be.


This is not an entirely esoteric question. There are implications for the sediment budget of Bodega Bay and the north side of the Point Reyes peninsula. If Tomales Point is a conveyor belt of granite being slowly fed into a rock-crusher (the ocean waves), the crushed rock in the form of sand ends up in Bodega Bay and on the Great Beach of Point Reyes.

Tomales Point jutting out into foggy Bodega Bay, with the
Sonoma County coastline on the horizon.

Question 1: Northward Movement
This is a two-part question. The first part asks how fast the crustal plates are sliding past each other. The Pacific Plate is moving northwest past the North American Plate at a rate of about 2.5 inches per year. 1.5 inches of that is occurring on the San Andreas Fault. The rest of the movement is on other faults, notably the ones in the Eastern Sierra.

This movement is generally not continuous creep, although there are places along the San Andreas Fault that do creep. The movement occurs in periodic bursts that we call earthquakes. The 8.3 magnitude 1906 earthquake that devastated San Francisco broke the ground surface for 300 miles and moved it 25 feet in the Point Reyes and Shelter Cove areas. The similar-in-magnitude 1857 Fort Tejon earthquake moved the ground 30 feet at Tejon Pass. Since 1769, there have been 13 earthquakes as large or larger than the 6.9 magnitude Loma Prieta earthquake that interrupted the World Series in 1989--the largest earthquake so far to jiggle the modern Bay Area.

At the other extreme, the Point Reyes peninsula sat docked next to the Monterey peninsula from 60 million years ago to 15 million years ago, before lurching northward again over those last 15 million years. If you imagine 15 million years of wave action eroding the peninsula, complicated by rises and falls in sea level and other tectonic activity, it makes you wonder how much bigger the landmass would have been back then.

Question 2: Southward Erosion

The Point, with Bodega Bay in the background.

Which brings us to our second question. How fast are the waves eroding Tomales Point? One helpful piece of information is that the cliffs at Duxbury Point and Bolinas Point are receding at a rate of 1-3 feet per year (Clague, 1969). But these points face south instead of north, and are not made out of granite like Tomales Point, but a softer sedimentary rock. And 1-3 feet is quite a range!

But despite those hanging questions, if we compare the 1.5 inches per year of northward movement to the low-end of the 1-3 feet of erosion per year, it seems clear that the erosion is winning--by a factor of about 10 to 1! So if we assume Tomales Point is eroding ten times faster than it is moving north, it means that it used to extend out into Bodega Bay much farther, and at one point probably enclosed Bodega and Tomales Bays as a single body of water opening to the north much like Tomales Bay does today! As it erodes south, Tomales Bay is getting shorter and Bodega Bay is getting more open to the ocean.

The northernmost yellow lupine in Point
Reyes National Seashore.

How fast is this happening? At a rate of about 1 foot per year, it means that 5,000 years ago Tomales Point was a mile farther out to the north than today--and five miles north 25,000 years ago! That means that 25,000 years ago Bodega Bay and Tomales Bay were one closed system opening up to the north of Bodega Head.

In light of this fascinating result, the sediment budget seems the least of the practical questions--salmon biogeography now rises to the top of the questions in my mind. If this were true, 25,000 years ago the Salmon swimming up Lagunitas Creek left the open ocean at the point where Salmon Creek enters the ocean north of Bodega Bay, and 50,000 years ago they left the open ocean at the mouth of the Russian River!

Wow. Talk about Watershed Stories!

Getting Real (data on bluff retreat)
So according to Griggs, 1994/2005, rate of cliff retreat in granitic rock is "a few centimeters per year" and Griggs 2004 says sedimentary bluffs retreat 15-30 cm per year. So 1-3 feet appears to be an alarmist overestimate or a very local condition at Bolinas Point. Given the large size of Duxbury Reef it is probably a local condition.

For those of you who didn't memorize the basic English-Metric conversions in school, 1) I'm sorry for your difficulties living in an English system country and a Metric system world; and 2) An inch is 2.54 centimeters. If you do the math, "a few centimeters" of granite bluff erosion is about the same annual rate as the movement north on the San Andreas Fault. So already our conclusions on erosion rates in the above section are huge overestimates and now Tomales Point appears to be a stationary feature over recent geologic time!

I say "already" because I'm not done yet changing my previous conclusions. At the end of the last ice age 21,000 years ago, sea level was 120 meters lower and the entire continental shelf was exposed. So the ocean wasn't close enough to erode Tomales Point, and you have to stop the clock on erosion. On the other hand, my conclusions above are correct about Tomales Point and Bodega Head being connected--much more recently that 25,000 years ago, more like 11,000 years ago! However we don't know if Lagunitas Creek flowed to the north or south of Bodega Head back then. Probably south, I'm guessing.

(Figure 6.10 from the Sea-Level Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future (2012) report released in June. All the figures in the report are blurry when enlarged. The entire continental shelf was exposed 21000 yrs ago when coastlines began  retreating globally as sea level began rising. Tomales Point is at the top of the figure.

So my question has been answered. Tomales Point is eroding at about the same rate that it is moving north, and this feature remains stationary, with the sediment produced from its erosion filling Bodega Bay and the local beaches around the mouth of Tomales Bay.

The prettiest thistle grows in the carpets of poppies near the point.

On my birthday hike I had hoped to run into a ranger that might know the answer to my question. I'm glad I didn't because it was a lot more fun to figure out this way!

REFERENCES
Primary sources listed above were cited in:
1. Evens, Jules G., Natural History of the Point Reyes Peninsula, UC Press, 2008
2. Sea-Level Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future (2012)
3. Lecture: Mighty Mammoth; Geological Insights into Eastern California's Most Majestic Volcano by Dr. Brandon Browne