The Sunken City
Posted by: TerryDad2
N 33° 42.385 W 118° 17.269
11S E 380661 N 3730349
Quick Description: This fenced area was once a beautiful neighborhood with an ocean view. A rotational landslide condemned the entire area.
Location: California, United States
Date Posted: 1/11/2010 9:40:09 PM
Waymark Code: WM81ZC
A small parking lot is adjacent to the fenced area. No legal access is available to go out on the slide area. You will view the slide from the parking area through the fence.
The bedrock of this area is the layered sedimentary rock of the Monterey Formation. The layers in the rock gently slope toward the sea. Some of these layers are a soft clayey material called bentonite. Bentonite swells and weakens when it gets wet.
Initial movement of this slide in 1929 appears to have been translational. Evidence for this comes from two wells (a water and oil well) that were sheared off near sea level. In a translational slide, material above a relatively flat plane of weakness slides downhill. Picture a pile of tilted books. At some point the books will begin to slide off each other. In this case, the plane of weakness was likely a bentonite layer that was saturated by the ocean.
Later, as the ocean eroded the base of the cliff, undercutting the foundation of the cliff, the plane of the slide curved up to the surface. The character of the slide became rotational. Now the head of the slide (the area furthest uphill) began to sink down faster than toe (the downhill section) changing the angle the beds dip. (see Cal State Long Beach Virtual Field Trip for a graphic.)
Sections of the slide continue to move especially after heavy rains.
The above information was compiled from the following sources:
Foster, John, The Point Fermin Landslide in A Day on the Palos Verdes Peninsula, Arthur Brown and John Cooper Editors, Pacific Section SEPM, October 7, 2006
POINT FERMIN, CSULB Geology's (visit link