Exploring California’s Marine Protected Areas: Duxbury Reef State Marine Conservation Area

From ancient trees to undersea kelp forests, fog-drenched coastlines to ice-carved granite peaks, the natural landscapes of California offer a diverse range of outdoor adventures. But have you ever wondered how these iconic surroundings were formed?

Close your eyes and imagine stepping into a time machine programmed to take you back millions of years into California’s geologic past. On arrival, you may find yourself staring down a volcanic chain that will become the backbone of the Sierra. The grinding of subducting continental plates where the ocean meets the land is a geological process that swallows and uplifts the ancient seafloor. When the mighty San Andreas Fault ruptured about 30 million years ago, it twisted, uplifted, and transformed massive portions of California, constructing the topography of much of the coastline and surrounding landscapes that we know today.

If you don’t have a time machine readily available to you, a trip out to the coast can provide enough evidence to guide you through part of the diverse geological story of California. Many of these ecologically and geologically unique coastal places are safeguarded by a network of 124 marine protected areas (MPAs), managed by the California Department of Fish and Wildlife.

Duxbury Reef State Marine Conservation Area (SMCA) is an excellent MPA to explore rock formations in the San Andreas Fault zone and the surrounding Point Reyes National Seashore. Duxbury Reef is the largest shale reef in California and one of the largest in North America. The Monterey shale that composes the reef was formed during the Miocene Epoch (spanning about 5 to 23 million years ago), when a mixture of volcanic ash and small plankton called diatoms sank to the seafloor, creating layers of fine-grained, silica-rich sheets of rock over time.

shale rock
Monterey shale is a type of sedimentary rock that forms when sediments are deposited on a surface and cemented and compacted together over time.
CDFW photo by S. Worden

Duxbury Reef formed at the southern coastal portion of modern-day San Francisco and has been at the mercy of the San Andreas Fault ever since. Locations to the west of the fault’s fissure are creeping northward about one inch per year as the fault line slips. This slow northward creep has pushed Duxbury Reef to its current location on the Marin County coast near the small community of Bolinas, about 30 miles north of where the reef was originally formed.

surfgrass
Surfgrass exposed at low tide.
photo by C. King, NOAA MBNMS

Protecting nearly three miles of rocky shore, Duxbury Reef SMCA safeguards substantial habitat where a diverse array of marine species live. Researchers who study the rocky intertidal zone at Duxbury Reef as part of the state’s MPA Monitoring Program have identified over 100 different species of invertebrates, seaweeds, and marine plants that cling to the reef as the tide ebbs and flows. One of these marine plants, the emerald green surfgrass, is abundant at Duxbury Reef; the flat sheets of shale provide the perfect environment for it to thrive. Surfgrass (and its close relatives) is the only flowering plant that lives in the ocean and is considered a foundation species, which means it provides many ecological benefits to areas where it grows. Surfgrass meadows help to bolster the biodiversity of species at Duxbury Reef by providing a safe place for organisms to live among its roots and long, vine-like blades. Dense beds of surfgrass dampen the force of the waves washing over the rocky shore and improve water quality by trapping sediments and converting carbon dioxide to oxygen, which can help to counteract the effects of ocean acidification.

The gentle slope of Duxbury Reef also makes exploring the tidepools on foot accessible to almost everyone. It is a popular destination for people seeking a glimpse of curious crabs, jewel-toned sea stars, and feathery seaweeds. The Point Reyes National Seashore Association provides information on tidepool etiquette, tide tables and weather conditions, and the natural wonders you may discover on your tidepool adventure. Watch this short video about life in the splash zone, which was partly filmed at Duxbury Reef SMCA, to prepare for your visit!

Don’t forget to check the MPA regulations  for Duxbury Reef SMCA and the Marin County COVID 19 safety measures before you leave! You could plan a relaxing day at the beach fishing from the shore for finfish, but no other take is allowed. Even the rocks that formed millions of years ago are protected at Duxbury Reef SMCA and the other MPAs in the coastal network. Although a fossil embedded in Monterey shale might make a nice addition to your rock collection, leave it behind to help write the next chapter in Duxbury Reef’s natural history.

Duxbury Reef State Marine Conservation Area is one of the 124 MPAs in California’s statewide MPA Network. Please visit CDFW’s MPA website for more information, and sign up to receive updates about the MPA Management Program.


MPA logo

post by Sara Worden, CDFW Environmental Scientist

Learn more about MPAs by diving into the Exploring California’s Marine Protected Areas series!