Station 4 - "Natural" Beach
Sediment Samples
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. The images below show both modes.
Smaller Clasts
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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 amounts of quartz (clear grains), plagioclase (white), orthoclase (pinkish-tan), and hornblende (black grains).
The amount of broken shells and sea urchin spines vary considerably, ranging from 5 - 50%. Thus, sometimes the sediment can be classified as lithogenous (<30% biologic material) to biogenous/biogenic (>30% biologic material)
The grains in this sample range in size from 1 mm (very coarse sand) to 40 mm (medium pebble), but the average size is closer to 4 mm.
Larger Clasts
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Sample collected on Monday, Juy 20, 2020. Yellow grid/quadrat is for scale. The larger ones are comprised of siltstone, sandstone, and some granitic clasts.
Clast size varies wildly, ranging from 30 mm (pebbles) to >264 mm (boulders).
Beach Structure
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.
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 is looking NE.
Coastal Erosion and Erosion Control Methods
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.
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.
Sometimes, erosion control measures aren't enough. From January through April of 2024, the Southern California region recieved an abnormaly large amount of rain, with between 16-23" recorded in the Palos Verdes Peninnsula where Cabrillo Beach is located. Many instences of mass wasting occurred throughout the state, including this one on the cliffs just behind the beach at Station 4. Photo taken on 4/10/24 and is looking to the north west.