Coastal Processes: Cabrillo Beach, CA

Natural Beach

This beach is typical of most of the beaches along the Palos Verdes Peninsula coastline. Strong waves strip off most of the sand and leave behind the heavier pebbles, cobbles and boulders. The sediment here is termed bimodal, meaning that there are two distinct grain sizes: 1) pebbles, cobbles and boulders weathered out of the beds that comprise the cliffs backing this beach and 2) coarse sand, comprised of quartz, rock fragments, garnets = occasionally) and various biological components such as sea urchin spines and shells.

Photo looking NNW towards the outer beach

Photo looking NNW towards the outer beach showing the bimodal nature of the sediments.

The cliffs behind the beach are a part of the Monterey Formation, Altamira Shale Member, and include siltstones, diatomite, sandstones and conglomerates. These rocks were once deep marine sediments – muds, clays and sands located on the sea floor in deep ocean basins 35 million years ago. Anoxic conditions formed in these basins due to their great depths. The clayey nature and the thin bedding of the Monterey Formation rocks make them prone to landslides and erosion. Houses located at the tops of these cliffs are constantly threated by erosion, especially during the winter rainy season.

Offset beds indicate where the Cabrillo Fault exits out of the cliff and heads into the ocean.

Exposed during low tide is a wave-cut terrace. These terraces form by waves moving back and forth over the bedrock below. Tide pools are common in these terraces. Organisms found in these tide pools include sea anemone, hermit crabs, crabs, purple sea urchins, whelks, sea hares, chiton, and sea stars. Recently it has become difficult to see sea stars in the tide pools of Palos Verdes due to a wasting disease affecting the area.

Photo looking west towards Santa Catalina Island = in backgroud).