The Sunken City Geology
Overview
The Sunken City, as we know it today, came to be when a huge portion of Point Fermin, San Pedro, collapsed into the ocean, as a consequence of a massive landslide. This event, which occurred in 1929, left in its wake a trail of destruction. In essence, there are many kinds of landslides experienced in Southern California. These are identified by the U.S. Geological Survey (2017) as “shallow, rapid debris flows triggered by heavy rainfall, rock falls from steep slopes as a result of earthquakes, and sower moving slumps and earth flows on slopes that can fail under many different conditions.” It was the latter that was to blame in the case of the Sunken City.
History
In addition to being geologically complex, the area south of California has also been known to be tectonically active (the U.S. Geological Survey (2017). According to the Survey, it is this very complexity that controls not only where, but also when landslides occur, as was the case in San Pedro, Los Angeles. In 1929, a landslide that would end up being devastating on the public utilities front and with regard to reconfiguration of the landmass started on one of California’s prominent beach fronts – in the area of San Pedro. With the gradual slip of the land into the sea, emergency services managed to evacuate a significant number of people and save property, save for a few houses that were eventually swallowed up by the ocean. At the time, the landslide’s unexpected onset baffled geologists, and as a matter of fact, most were largely caught flatfooted. It was not until the landslide was underway that geological experts started investigating the area (Hyndman and Hyndman, 2016). However, as it has been observed elsewhere in this text, there may have been massive failure on the part of public officials...
The development of the site was done by George Peck, whose objective, according to Das and Sobhan (2016), was to establish an exclusive residential area offering its residents amazing views of the Pacific Ocean.
Today, the area is totally different from the way it looked pre-1929 – with the landscape having been totally reconfigured by the landslide. In recent times, the area has experiences several landslides, with the last one having been experienced 8 years ago (Bramlett, 2011).
Bentonite and Tectonic Activity
Sunken City’s bedrock comprises of sedimentary rock (layered), with one key feature of the layers, for purposes of this discussion, being bentonite. Bentonite weakens when it gets wet – with the weakness increasing with the level of wetness (Mutton, 2009). According to a Feasibility Report on Rancho Polos Verdes, Los Angeles County, CA, “bentonites are rich in the clay material montmorillonite, and become very weak when saturated by water” (U.S. Army Corps of Engineers, 2000, p. 3-1). As the report further point out, the landslide movement is in this case triggered by high levels of ground water which, in essence, “impact on the bentonite layers, as well as causing the saturated soils to become heavier and less stable, particularly along steep bluff areas” (U.S. Army Corps of Engineers, 2000, p. 3-1). In the case of the sunken city, the waves eroded the cliff’s underbelly, causing the layer to sip in water. Over time, it could no longer hold the weight and it at this point that the slide into the ocean begun. The slip into the ocean was, at first, not drastic – with approximately one foot of the…
