During the Miocene, erosion of rocks in the area we now call Southern California were being deposited: Silts and diatomaceous oozes in deep ocean basins, sand along rivers, coastal areas and shallow marine environments; and pebbles, cobbles and sand along rivers. Volcanic activity was also common during this time, depositing layers of ash over the entire region. Over time, these sediments lithified into rocks.
Tectonic activity along the many faults in the Southern California region uplifted these rocks (called the Monterey formation in this area) above sea level. Weathering began to erode some rocks, forming the cobble beaches common along the Palos Verdes Peninsula, and altering others, such as the ash beds. Ash beds will often alter to form a clay called bentonite. Bentonite is very slippery when wet! Uplift of the Palos Verdes region along the Palos Verdes Fault tilted theses rocks further, causing them to tilt towards the Pacific Ocean. The term geologists use to describe when bedrock tilts in the same direction of the slope is called adverse bedding and is a major cause for mass wasting events such as landslides.
Image from: "The Slippery Slope of Litigating Geologic Hazards: California's Portuguese Bend Landslide " by David L. Ozsvath of the Department of Geography/Geology at the University of Wisconsin-Stevens Point
The Portuguese Bend Landslide is a combination of an earthflow, rotational and translational slide. The modern slide is a reactivation of a larger, ancient slide. What caused this area to fail again? The Portuguese Bend landslide began its modern movement in August 1956. Approximately 260 homes and associated roads had been built in the area. The added weight of the houses, the addition of water from lawn irrigation, and the excess weight placed at the top of the slide from the building of Crenshaw Blvd. reactivated the slide. Six weeks later, the entire eastern edge of the landmass was; by the summer of 1957, it was moving at a rate of up to 5 inches/day. By 1961 over 150 homes had been severely damaged or destroyed.
Over time, corrective measures such as dewatering wells, surface drainage improvements, and lack of homes have slowed the rate of movement to less than 1 cm per day. Palos Verdes Drive South constantly needs to be repaved at a cost of $150,000/year.
So, what caused this area to fail again?
- adverse bedding of the Monterey formation
- bentonitic clay layer (forms the slide plane along which the slide is moving)
- erosion of toe by ocean waves (undermines the stability of the slide)
- Residential development of area in 1950s (added extra weight and water to the slide)
Now we head to our next stop, Whites Point public beach. Drive back down to Palos Verdes Drive, turn left and continue southeast. Palos Verdes Drive turns into 25th Street. Continue on 25th Street to Western. Turn right on Western and veer left on Paseo del Mar. Whites Point entrance (Royal Palms State Beach) is on the right. Park in the parking lot at the top of the hill and walk down to the bottom of the hill.