I recently led a field trip at the Mendocino Coast Botanical Gardens to observe, enjoy, and ponder the grandeur of Mendocino’s coastal landforms. In the process, I pecked at the tip of the iceberg of our understanding of the immense and dynamic forces that are actively shaping California’s evolving coastline.
The landscape before us is continually being transformed by the elements, by the ocean and wave action, and by tectonics. All of these mechanisms act as erosional as well as depositional forces on the landscape — perpetually building it up and then taking it away.
The landforms emblematic of the Mendocino coast, such as sea stacks, rock arches, headlands, and reefs appear stalwart, but for those of us who have been around for at least a few decades, we know that in reality these oceanfront features can be fleeting.
The collapsed roofs of sea caves are now “sinkholes” precariously pockmarking the landscape of the North Coast bluffs, collapsed sea arches become sea stacks overnight, and rock failure severs the tips of peninsulas, isolating plant and small mammal communities on brand-new near-shore islets.
One such coastal landform the greater public may or may not recognize, but residents of the immediate coast call home, are marine terraces — the flat, raised coastal plateaus also known as mesas, fringing much of California’s coastline. Marine terraces give us somewhat of a safe place above the waves to live our lives and conduct our business, but these landforms are actually dynamic tectonic landscapes, and home to fragile and vanishing ecosystems along California’s coastline.
Marine terraces begin their existence beneath near-shore waters as platforms of bedrock being carved flat by wave action, and then raised above sea level by the steady tectonic uplift of the San Andreas Fault system. Over the past two million years as the sea levels cyclically rose and fell throughout the ice ages of the Quaternary period, marine terraces were repeatedly carved flat beneath the waves during high sea stands (transgressions), and then raised above the waves when sea level dropped during regressions.
This repeating cycle of rising and falling sea levels, in concert with steady tectonic uplift, has created stair-stepped flights of marine terraces, ascending in elevation and age as they are uplifted inland from the coast over hundreds of thousands of years.
While the marine terraces along the majority of the California coast from Pacifica south to the Mexican border are commonly composed of softer, younger and more erodible sedimentary sandstones and mudstones of the Pacific Plate, the marine terraces of the North Coast, and the Mendocino coast in particular, are constructed primarily of a harder and older Greywacke bedrock of the North American Plate, which retains its shape as it is uplifted.
Greywacke is a bluish-gray partially metamorphosed sandstone, formed from sediments washed down from ancestral mountain ranges into deep sea subduction zone environments between 30 to 200 million years ago. The greywacke terraces of the Mendocino coast are usually capped by younger golden-colored marine deposits of loosely formed sandstones, clay, gravel and silts.
One of California’s endemic coastal habitats adapted to marine terraces is the Coastal Terrace Prairie. Famous for vibrant spring wildflower displays along the Mendocino coast, such as along the bluffs of MacKerricher State Park, California’s coastal terrace prairies are long-lived, low profile, species-rich herbaceous ecosystems adapted to wind, salt spray and the moisture from marine fog.
California’s coastal prairies are one of earth’s top carbon-sequestering ecosystems, storing up to 50% soil carbon in the deep rich coastal prairie soils through the very long roots of the prairie’s keystone perennial bunch grass species. California’s coastal prairies are the most endangered and the most species diverse of all grassland and prairie ecosystems in the U.S. And in California, this ecosystem is critically endangered with most of its historic range on California’s marine terraces lost to human development and invasive plant species.
Rowena Forest is a physical geographer and a science writer at Cal Geographic.com