Cabrillo Beach Coastal Processes Trip

White's Point

Sediment Analysis

Wave energy: Moderate to Strong
  Small Clasts Large Clasts
Slope steepness: Moderate
 Steep
Grain size: 0.5 to 2.5 mm
4 to 44 mm
Biologic Compenents: ~ 10% shells ~ 5% shells
Lithologic Compenents: Quartz, Plagioclase, Orthoclase, Hornblende, Magnetite during periods of heavy wave activity Siltstone and sandstone clasts, some sea glass
Angularity of Clasts: Subangular to subrounded Subrounded, some imbricated

 

The sediment at Whites Point varies, but is generally in the "coarse" category. To the left is a snapshot of what the beach is comprised of in the tide pool area - mostly subrounded clasts derived from the cliffs backing the beach. Some sea glass can also be found. Note that the clasts show imbrication - that is, they have flat surfaces that parallel the ground. Imbrication forms due to a constant flow of water over the surface of the clast; in this case, waves.

 

 About 15 million years ago, at the same time the Altamira shale was deposited, submarine volcanoes were active. Basalt intrusions can be found along the south and west flanks of the hill, and pillow lava can even be found at the top of the peninsula. At the bottom of the ramp is a sequence of light gray and cream colored rocks that have been compressed and folded into a “C” shape (called a recumbent fold). The Miocene-aged rocks in the Southern California region are known as an important source of petroleum. Tar seeps are a common feature - the La Brea Tar Pits is an excellent example. Petroleum has stained some of the siltstones shown here a light grey.

Tectonic stresses from the nearby Cabrillo and Palos Verdes faults have compressed and contorted many of the rocks along the Palos Verdes Peninsula. The "C" shaped fold in these siltstones of the Altamira shale formed as a result of these forces

The beaches here are covered with not sand by coarser clasts ranging in size from pebbles to boulders. Unlike at Malaga Cove, where the large clasts have weathered out of the cliffs backing the beach, the source of the pebbles and cobbles is the ocean itself. It takes energy to move sediments. Since the Palos Verdes Peninsula is a headland, it receives the full force of the incoming waves' energy. Thus the waves are strong enough here to move smaller clasts such as sand and gravels. This leaves behind the larger clasts on the beach face.

The constant back-and-forth motion of the waves planes the bedrock along the coast flat, forming a wave-cut terrace. In some locations along the coast the waves are strong enough to strip the sediments off of the beach and exposes the wave-cut terrace. These terraces make for excellent tide pools. Due to the strong waves, tidepooling in this location should only be done during low tide.
View of the waves creating a wave-cut terrace from the top of the cliffs. Photo looking west.