We’re all beach people. We’re attracted to the coast for the beauty, serenity and the waves. There are many variations to coastlines around the globe and these changes are the factors that create the great variety of waves. In this multiple-part article, we’ll analyze coastal geology to better understand how differences in a coast’s dynamics dictate how waves break in a given area.
Geology is the Science of the rocks that create landforms and the processes that change those rocks.The coastal environment has many dynamic factors. Plate tectonics, wave energy, weather, rivers, and humans all have an impact on coastal geology. The coast is always changing. Some changes happen daily, like the movement of sand. While others, like the uplifting of land from plate movement, take place over the course of hundreds, thousands and millions of years.
If you observe any stretch of coast, you’ll see that waves break differently on varying parts of the beach. This is because the Ocean bottom varies. Bathymetry is the underwater contours of the seafloor. It is often overlooked as a surf variable. We cannot directly see it and much of it is fixed for our lifetime. But a closer investigation reveals the dynamic and vital story of our coastlines.
Swell approaching a coastline will always refract and bend toward shallow water. Reefs, sandbars, points and submarine canyons cause swell energy to focus and shoal. Because there is a variety of depths along a coast, waves break differently everywhere. Sandbars frequently shift and move as the sand is carried by longshore currents. California reefs are uplifted remnants of the coastal bluffs and marine terraces. Points are created by the uneven shape of a dynamic coast. Submarine canyons form when fresh water runs off the land and erodes a chasm in the continental shelf.
California’s coast primarily consists of sedimentary rocks in identifiable layers. All of these layers are associated with ancestral rock formations and their subsequent weathering, erosion and deposition. According to a sign at San Elijo State Beach, “The coastal bluffs were formed by the accumulation of mineral and organic sediments. In more recent times, the Ocean level has receded, leaving the sedimentary deposits exposed in elevated marine terraces. Once exposed, these terraces were eroded along the seaward margins, leaving the steep coastal bluffs present in the park today.”
Three prominent layers include:
The Del Mar Formation– the bottom and oldest layer of the San Diego coast sedimentary rocks. It is often greenish or gray mudstone, containing many fossils, laid down in muddy lagoons 45-50 million years ago. The Del Mar Formation now forms many of the reef wave setups we have in San Diego.
Torrey Sandstone– a large layer of light colored sandstone that is the main constituent in San Diego’s coastal bluffs. It is roughly the same age as the Del Mar Formation and was laid down as a sandbar and beach deposit.
Monterey Formation– an oil rich layer that is responsible for the tar on beaches and the offshore oil rigs towards Santa Barbara. It is 6-16 million years old and comprises the remains of billions of microorganisms that once swam in a shallow sea. When they died, they sank to the bottom and were covered by sand and silt. With pressure, heat and time, the organisms became hydrocarbons: the source of our much beloved oil. The Monterey Formation is seen throughout the Central Coast and into the Coastal Range Mountains. It makes up many of Central California’s reef breaks.
Mark Bordelon- Irvine Valley College
PBS Coastal Geological Processes