Southern California’s sandy beaches are more than just popular spots for surfing and sunbathing—they’re the product of a dramatic geologic story that’s been unfolding for millions of years. With their sweeping ocean views and turquoise waters, these iconic coastlines attract millions every year. But few people stop to think about how these beaches actually came to be.

To get the full picture, you have to go way back—about 200 million years, to the Mesozoic era. Back then, the land we now know as Southern California was underwater, part of a vast oceanic plate. As the North American continent drifted westward, it collided with and began to override the Pacific plate. This slow-motion crash, called subduction, set the stage for the coast we see today.

This subduction zone generated intense heat and pressure, melting portions of the oceanic crust and upper mantle. The resulting magma rose to the surface, forming a chain of volcanic islands and large underground magma chambers. Over time, these chambers cooled and solidified into granite, forming what’s now known as the Southern California batholith—an enormous mass of igneous rock that underlies much of the region. This tectonic activity also helped uplift and shape many of the mountain ranges we see today, including the Santa Monica and San Gabriel Mountains.

Beach sand, particularly in Southern California, is primarily composed of quartz and feldspar mixed with silvery mica and milky quartz. These minerals originally existed in the granite of the local mountains, miles from the shoreline. Studies have shown that much of the sand on Southern California beaches actually comes from the San Gabriel mountain range. 

“Sediment that’s derived from granite-type watersheds is generally comprised of a lot of quartz,” says UCLA geography professor Tony Orme. “It tends to be light in color.”

The San Gabriel Mountains are part of the Transverse Ranges, are known for their rugged terrain, diverse ecosystems, and recreational opportunities, stretching approximately 68 miles from Los Angeles County to San Bernardino County.

It may be surprising to learn that the San Gabriel Mountains, towering over Los Angeles, play a critical role in forming the region’s stunning beaches. They are, in fact, the primary source of much of Southern California’s beach sand, particularly around Los Angeles. But how does this granitic mountain material end up miles away on the beach?

The answer lies in the forces of erosion and weathering. The mountains’ granite is gradually worn down over time by rain, wind, and cycles of freezing and thawing. This erosion process, which can take millions of years, results in smaller and smaller particles. Rainfall and streams transport these eroded particles down the mountain slopes and into the regions rivers.

These rivers, such as the Los Angeles and San Gabriel Rivers, act as conveyor belts, carrying the eroded material – the future sand of our beaches – toward the Pacific Ocean. Renowned geomorphologist Douglas Sherman of the University of Alabama has extensively studied these sediment transport processes, highlighting their importance in coastal formation.

Sand continuously migrates from land to sea. As rivers met the ocean, they deposited their sediment load, forming deltas. Coastal currents then took over, redistributing these sediments along the shoreline, a process known as longshore drift. Waves, powered by the coastal winds, continually pushes this sediment onto the shore, gradually creating the wide, sandy beaches we enjoy today.

This ongoing transfer is accompanied by watershed run-off and the erosion of bluffs and hillsides, which carry sand toward the beach. Grains of sand then embark on a southward journey along the coast, while the smaller sediment particles are swept further offshore and deposited deep on the ocean floor.

While there is still widespread belief among geologists that most of California’s sand originates in the mountains, two relatively recent studies conducted by researchers at the University of California, San Diego have suggested that another key source of erosion might be the grand sea cliffs of the region.

“Much to our surprise,” expressed Scott Ashford, formerly a professor of engineering at UCSD, and now at Oregon State, who employed a mobile laser imaging system to examine coastal formations for one of the studies. “It’s revealing that our comprehension of the beach system isn’t as thorough as we’ve presumed.”

His research analyzed six years’ worth of imaging data from the 50-mile (80-kilometer) coastline stretching from Dana Point to La Jolla. Previously, geologists had conjectured that up to 90% of the beach sand in this sector originated from deposits transported by coastal rivers, but Ashford’s research indicated that the sea cliff erosion accounts for some 67% of Southern California’s beach sand. However, since Ashford’s study was focused on such a small area of the coast, many geologists are wary of embracing his conclusions.

The coastal journey of the sand concludes either when it is blown inland to form dunes or more frequently, when it descends into a submarine canyon, such as Monterey Canyon in Northern California. The deep underwater chasm of a canyon signifies the endpoint of a littoral cell. A littoral cell is a unique coastal region where sand embarks on a journey from land into the ocean, traverses down the coast, and then exits the system. The volume of sand accessible to beaches equals the quantity entering the littoral cell minus the quantity exiting. Changes to this sand budget can result in the contraction or even complete vanishing of beaches.

The formation of Southern California’s beaches is not a completed process but an ongoing one. Waves and currents continue to shape the coastline, sometimes depositing sand to widen the beach, and at other times eroding the shoreline. Los Angeles has paved most of its major rivers, reducing the amount of sand that comes from the mountains onto the beaches. In fact, it is not uncommon for Southern California beaches to be missing close to 50% of their historical sand supply.

California has added sand to its beaches for decades through projects called “nourishment”. These projects are often used to restore eroded beaches and protect against sea level rise. Sand is typically dredged offshore and pumped onto the shore, where trucks spread it around. The goal is to widen the beach so that wave energy breaks sooner and dissipates towards the bluff face.

Rosanna Xia’s book, California Against the Sea: Visions for Our Vanishing Coastline (2023) is an excellent source of information on California beach erosion and the threats posed by the loss of significant portions of the coast. The book explores how human activities like coastal development, urbanization, and dam construction have intensified natural erosion processes. She provides a historical context for these developments and their long-term impacts, while also exploring innovative adaptation strategies and community-led efforts to protect the coastline. Balancing a sense of urgency with cautious optimism, Xia presents a vision for a resilient future where informed policies and sustainable practices can help safeguard California’s coastal treasures for generations to come.

Understanding the geological history of Southern California’s beaches not only adds depth to our appreciation of these natural wonders but also highlights the need for careful stewardship. By minimizing our environmental impact, reducing development and mitigating the effects of climate change, we can ensure that these incredible landscapes continue to evolve and endure for generations to come.