Cabrillo Beach Coastal Processes Trip

Bluff Cove

First image is a satelite view of Bluff Cove, the second is a geology map of area

 

Sediment Analysis

Wave energy: Moderate
Slope steepness: Moderate to Steep
Grain size: 8 to >64 mm
Biologic Compenents: less than 1% shells
Lithologic Compenents: Siltstone and sandstone clast
Angularity of Clasts: Subrounded, some imbricated

 

Palos Verdes marine terrace #4 creates a nice, flat area for the dirt parking lot that leads to the Bluff Cove trail. The trail is good, although a bit steep in areas, and gives you a fabulous view of the Pacific Ocean and the wave-cut terrance forming beneath the waves at the base of the cliff.

What are terraces? The constant back and forth motion of the waves planes the bedrock flat, creating a wave-cut terrace. Tectonic activity on nearby faults will uplift the land - including the terrace, which is now quite literally high and dry. Begining around 3 million years ago, uplift on nearby faults began to move the Palos Verdes Peninsula up out of the ocean. Movement on the fault wasn't constant, however, and during pauses in uplift a terrace would form. At first, the land was an island, then the southern side began to move up more than the the the rest of the peninsula. How do we know this? There are a total of 13 terraces on the peninsula. The upper two completely encircle the top of the hill, indicating that it had been an island. The remaining are all along the coastline, with the most recent terrace forming beneath the waves.

Not too far down the trail is a fork; the right hand path leads to Flat Rock Point, with Flat Rock and Bit Rock peaking out of the water.


If you were to take the left-hand fork, the trail will continue down to the beach. Exposed here in the cliffs are the cream-colored diatomaceous-rich shales, cherty shale, porcelanite, diatomite, tuff, sandstone and minor amounts of limestones comprising the Altamira Shale member of the Monterey Formation. A series of folds can be seen as well - an anticline (rocks arch downward), a syncline (rocks arch upwards), and another anticline. The axes of these folds are dipping towards the ocean, and this has caused the rocks in the synclide to fail, forming a landslide that obscures the folds.

The map below is from Donald Kupfer' 1940 Bachelor's thesis (Geology of Bluff Cove). On the right side there are three dashed lines, the outer ones with arrows pointing towards the line, the middle one has arrows pointing away from the line. These represent the anticlines and syncline of Bluff Cove.

 

Heading south along the beach you will pass two more landslides. The rock type has changed to the San Onofre Breccia, comprised of subangular clasts of blueschist, some greenschist, and rarely garnet eclogite. The mix of colors in the clasts, combined with the greenish-grey matrix, makes these rocks stand out in the cliffs.