16.2: California Coastline Anatomy

California Coastline Anatomy

The shoreline anatomy of California follows patterns found elsewhere around the world, wherever tectonic uplift exceeds erosion and sea level rise.

Headlands, as mentioned in the previous section, are rock promontories that stick out into the ocean relative to the rest of the coast (see Figure \(\PageIndex{1}\)). The dynamics of wave refraction cause incoming waves to warp, or bend, around headland, concentrating the erosive power of the crashing waves on the sides of headlands. The headland flanks erode quicker than the point of the headland, and much quicker than the embayments further back.

This phenomenon of uneven erosion is known as differential erosion, and this accounts for much of the shape of California’s coastline.

Figure \(\PageIndex{1}\): Typical coastal features resulting from differential erosion, including headlands, sea caves, sea arches, and sea stacks. "Coastal features" by Steven Newton, is licensed under CC BY-NC 4.0. Access a detailed description.

Because of the “pincer” focus of refracting waves, headlands inevitably erode along their sides first, forming sea caves (Figure \(\PageIndex{2}\)). Sea cave indentations may grow until they break through to the other side and meet in the middle, forming a thoroughgoing sea arch (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{2}\): A well-developed sea cave along the coast north of Santa Cruz, CA. "Sea cave" by Steven Newton, is licensed under CC BY-NC 4.0. Access a detailed description.

Figure \(\PageIndex{3}\): A sea arch south of Jenner, CA. "Sea arch" by Steven Newton, is licensed under CC BY-NC 4.0. Access a detailed description.

Arches are vulnerable to collapse, and in geologic time, we should think of these dramatic structures as temporary. Indeed, changes are visible even in human time frames; one may go to Santa Cruz and visit the Natural Bridges Park and wonder why there seems to be only one “bridge,” only one remaining arch; the other two fell down, in 1905 and 1980.

When an eroded headland, undercut by sea caves which merge to become a sea arch, reaches its end stage, the last arch collapses and leaves the point of the headland as an island out to sea. This island is then called a sea stack (see Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{4}\): Distant sea stacks, remnants of a former headland, at the confluence of the Russian River and the Pacific Ocean, Jenner, CA. "Sea stacks" by Steven Newton, is licensed under CC BY-NC 4.0. Access a detailed description.

Sometimes sea stacks then act as a barrier that creates an area of low wave energy in the longshore drift, which allows sand to build up between the sea stack and the coast. This is called a tombolo (see Figure \(\PageIndex{5}\)).

Figure \(\PageIndex{5}\): A tombolo created from a former sea stack, which has accumulated enough sand behind it to reattach to shore. Near Jenner, CA. "Tombolo" by Steven Newton, is licensed under CC BY-NC 4.0. Access a detailed description.

Recognizing sea stacks, and mentally tracing them back to the existing shore, can give one a breathtaking view of how the coast used to look before wave refraction sculpted the complex features we see today.

Wave refraction is not the only erosive work of waves. As waves crash ashore, several times per minute, 24/7, they carry with them sand, pebbles and even small boulders during particularly energetic waves. This incessant “sand blasting” of the rocks under the waves creates flattened, wave-cut platforms. Most of the time, you don’t see wave-cut platforms because they are under water. However, in a tectonically active emergent coastline, these wave-cut platforms get raised up out of the water by successive quakes.

When these wave-cut platforms are above sea level, they stand out as flat zones along the coast, punctuated by narrow steep cliffs, with further plateaus inland. This pattern is called marine terraces. Many developed places along the California coast are built on marine terraces, because of the desirability of flat land. The city of San Diego, in particular, is built on staggered marine terraces (Figure \(\PageIndex{6}\)).

Figure \(\PageIndex{6}\): A diagram showing three marine terraces, with the oldest to the left and the youngest to the right, and a newly forming terrace under the water. "Marine terraces" by Steven Newton, is licensed under CC BY-NC 4.0. Access a detailed description.

If the wave-cut platforms have hosted headlands which eroded into sea stacks, then the marine terraces may also preserve “fossil” sea stacks, now completely on dry land, as in Figure \(\PageIndex{7}\).

Figure \(\PageIndex{7}\): A marine terrace with “fossil” sea stacks, rubbed smooth by now-extinct Columbian mammoths (Mammuthus columbi). Note Aspen the dog for scale. Goat Rock, Jenner, CA. "Mammoth rocks" by Steven Newton, is licensed under CC BY-NC 4.0. Access a detailed description.

References

1. Abbott, P. L. (1999). The Rise and Fall of San Diego. Sunbelt Publications.