At the top of the cliffs that back Bluff Cove is a nice, flat area which people use to park their cars before heading down to the beach via Bluff Cove trail. The reason this area is so flat is because it is a marine terrace – an area that once was a shoreline but has been uplifted due to tectonic activity. To discover why we have to go back 30 million years in time.
Thirty million years ago, during the Oligocene, our plate boundary looked very different than it does today. Just off shore there is a subduction zone which was busy subducting the Farallon Plate beneath the North American Plate. Just west of the trench is a ridge which is spreading the Farallon Plate away from the Pacific Plate. Somewhere around 20 – 25 million years ago at the Oligocene/Miocene boundary, the subduction zone began to subduct the spreading center. This caused a new transform fault to form – one we now call the San Andreas Fault Zone. As time went on this transform grew and separated the trench into two trenches (the Middle America and Cascadia) and the spreading center into two ridges (the Rivera Rise and the Gorda/Juan de Fuca Ridge), each now more than a thousand miles apart.
Then, about 5 million years ago, the plate boundary moved to its present location. The San Andreas Fault makes a left-stepping bend just north of Los Angeles County and put the entire region under compressional as well as shear tectonic stresses. This is, in part, why the geology of the Los Angeles Region is so complex. Rotation of the Pacific Plate 3 million years ago put further stress on the area, in addition to the development of the Palos Verdes Fault and numerous other faults.
Thus, the Palos Verdes Peninsula is a broad anticline, with the south-western limb dipping towards the Pacific Ocean.
Three million years ago Palos Verdes began to rise up out of the Pacific Ocean. First, as an island, then, as uplift continued, as the peninsula we see today. The area would be uplifted, pause, and then be uplifted again. During each of these tectonic pauses a coastline would form. This coastline would then up uplifted along with the rest of the area, and then a new coastline would form. This happened at least 13 times over the past 3 million years. How do we know this? Geologists have identified 13 marine terraces on the Palos Verdes Peninsula – the top two completely encircle the top of the peninsula, which is how we know that, initially, the area was once an island. Bluff Cove is located on the 4th terrace.
Now carefully make your way down Bluff Cove Trail. You will have a beautiful view of the coastline and the development of terrace #1 located just beneath the waves. Under your feet is a sill of basalt which intruded into the Altamira Shale. How do we know it is a sill and not a flow? Because the shale above and below the basalt are metamorphosed. The basalt sill continues down the cliff to form Flat Rock Point and, out in the ocean, Flat Rock and Bit Rock. The basalt transitions into the Altamira Shale as you continue down the trail. Here, the Altamira Shale is comprised of cream-colored diatomaceous-rich shales, cherty shale, porcelanite, diatomite, tuff, sandstone and minor amounts of limestone.
The trail ends on Bluff Cove Beach. Turn and look at the cliffs. Before you is a large landslide within the Altamira Shale. The slide appears to be sliding along the axis of a syncline – a special type of fold that develops as beds are folded downward. On either side of the syncline are two anticlines – folds with beds that arch upward. You can’t see the folds due to the landslide.
Walking south along the beach you will exit the landslide area and find two more landslides. In this area you will find the Miocene-aged Green Breccia. In the breccia are clasts of quartzite, green gneiss, and gluacophane schist.
Carefully exit the parking lot and make a left (north) on Paseo Del Mar. Paseo Del Mar end at the Malaga Cove Trail parking lot.