Sunken City

In the early 1920s, a devloper named George H. Peck had great plans for the Point Fermin area of San Pedro, California. The desire to build houses in the beautiful Palos Verdes Peninsula was rampant, and developers planned out and built communities with little thought to the geology of the area. The first hint of a problem showed up on January 2, 1929, with first a broken water main, then a broken gas pipe at the Ocean View Inn hotel on Paseo Del Mar. Residents began to move out and shortly thereafter the cliff failed, taking with it several buildings.

What happened? Three million years ago the Peninsula began to rise out of the Pacific Ocean. As it did, the land began to buckle and fold into a series of anticlines and syclines. The result is that the beds on the coastal side of the Peninsula are all dipping in the same direction as the slope (aka a dip slope) aproximately 10-15 degrees to the south. This is also called adverse bedding and is a leading cause of slope failure wherever the condition is found. It is especially problematic in coastal areas, where the crash of the waves on the cliffs undermines the area, making an already bad problem worse.

Another problem is the bedrock itself. The Miocene aged Altamira Shale member of the Monterey Formation is comprised of thin layers of siliceous and phosphatic shale - which is very slick when wet. Most landslides in the Southern California region are within shales and siltstones found in Miocene aged formations.

Initally, the landslide scarp was 8-10 ft wide and at least 30 ft deep. The lowering of the water table in the area and a small earthquake on July 8, 1929, increased the rate of movement, and, with time, has slowed. Erosion continues, however. A 2007 study by the USGS found that the cliffs in the Point Fermin area have retreated 64 ft in the past 70 years.

Animation showing how dip slope failures can occur.	The Point Fermin Landslide (aka "Sunken City" is on the other side of these cliffs. Note how the beds are dipping towards the ocean.
Annotated image of the Point Fermin Landslide, after Miller 1956
Map of the slide area showing the lots that were lost during the 1929 slide event. Red lines added.
Close up view of the Sunken City/Point Fermin Landslide