The Point Arena peninsula is a tectonic traveler defined by the San Andreas Fault. It’s the northernmost outpost of emergent Pacific Plate landmass on the California coast, and the tiny tip of the spearhead of an uplifted and migrating sedimentary complex west of the plate boundary that constructs the majority of the California coast from the top of Manchester Beach at Alder Creek south to Mexico.
The San Andreas Fault and fracture zone cuts a sharp linear path incising the immediate coastline north of the Golden Gate, appearing to shave off thin strips of land along its path before “exiting” back under the ocean via the Alder Creek fault valley on the central Mendocino Coast.
The three coastal landmasses along this northern path west of the San Andreas Fault are the Point Reyes Peninsula, Bodega Head, and the Point Arena block extending from Fort Ross up to Alder Creek. They are defined as tectonostratigraphic terranes (not terrains), which are distinct geologic units bound by tectonic faults, and are transiting to the northwest with the motion of the Pacific Plate as the boundary between the Pacific and North American plates continues to evolve and lengthen, forming the California coast.
The greater Point Arena Terrane is composed of sedimentary geologic formations including sandstones, mudstones, and shale deposited and lithified within ancient coastal marine basins of the Miocene epoch approximately 10-18 million years ago, as the Pacific Plate came into contact with North America and the San Andreas tectonic boundary evolved. The Point Arena Terrane is underlain by very old Salinian Block basement rock of the Mesozoic era, also an identifying component of these migrating tectonostratigraphic terranes.
Combine the striking quality of the golden-hued layers of Point Arena’s sedimentary strata with the highly active tectonics of the underlying San Andreas fracture zone, and you get the spectacular sedimentary landforms that Point Arena and the California Coastal National Monument are famous for.
These include immense steeply tilted strata and bedding planes observable along the coastal cliff faces and the towering uplifted walls of Arena Cove; very large pillow-shaped and spherical concretions (“bowling balls”); geometric fold belts and hogback ridge outcrops; linear and geometric emergent reef structures; waveform laminations as patterns in the rock faces, and many more.
The geologic structures of the Point Arena peninsula and region are tectonically uplifted by the compressional forces of the San Andreas fault system to form a series of marine terraces, which are raised wave-cut platforms that compose Mendocino’s flat coastal plateaus.
A network of fragile and rare coastal ecosystems unique to California that evolved to the sedimentary landscapes of the Mendocino Coast span the marine terraces, slopes, gulleys, lowlands, cliff faces and higher elevations of the Point Arena region. Coastal prairies, Northern coastal scrub, Maritime chaparral, and seasonal as well as permanent palustrine wetlands are all found within this habitat mosaic.
These are rare and imperiled native coastal landscapes that are refuges to threatened and endangered plant and animal species, including red-legged frogs that rely on Point Arena’s tiny patches of permanent freshwater pools that are adversely impacted by the wetland foraging habits of grazing cattle, as well as by coastal development.
For further exploration and photos of Point Arena’s striking geology, tectonics, and biogeography, go here.
Rowena Forest is a physical geographer and science writer at Cal Geographic.