Cabrillo Beach Coastal Processes Trip

Natural Beach

Sediment Analysis

(click image for larger view)
(click image for larger view)
Sample collected on Saturday, October 24, 2020. Yellow grid is for scale. The sediment is comprised of subangular to angular rock fragments (dark to tan), shell fragments (white to cream), sea urchin spines (purple), and minor quartz (clear grains), plagioclase (white), orthoclase (pinkish-tan), and hornblende (black grains).            Sample collected on Monday, Juy 20, 2020. Yellow grid/quadrat is for scale. The sediment at this station is bimodal - that is, it has two predominate grain sizes. The smaller grains are a coarse sand (see left). The larger ones are comprised of siltstone, sandstone, and some granitic clasts.


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 that form a steep beach slope. 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 taken on 3/16/19 looking east/southeast.

Photo taken on 7/20/20 looking north. Note how the surface of each of the clasts is flat and parallel to the slope of the beach. This is called imbrication and indicates that waves flow over this area, plaining the clast surfaces flat, frequently.

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 south towards Santa Catalina Island (in background).


Coastal Erosion at Cabrillio Beach. Photo looking north. Waves, storms, and humans play a major role in the erosion of sea cliffs. Structures atop the the cliffs are in danger of being destroyed went the cliff collapses.

Storm drains like the ones shown below allow water to exit from the streets and over the cliffside in a controled manner, reducing erosion.

Drainage pipe, looking south.

Another drainage pipe, looking west-southwest.


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