POINT FERMIN
While there are several interesting geologic features at Point
Fermin, the most striking is a landslide that destroyed several homes. CSULB field geology classes often
stop at Point Fermin and the "Sunken City" to learn about
mass movements (or wasting).
There are several types of mass movements. Several are listed in the table below:
Type of Movement |
Description |
Speed of Material Movement |
Specific kinds of mass wasting within this type |
Falls |
The vertical distance traveled by the mass is greater then the horizontal distance. |
Extremely fast (more then 10 feet/second). |
Rock, debris or earth falls, topples. |
Slides |
The movement of rock is along a plane. If the plane is flat, it is called translational. If the plane is curved, it is called rotational. |
Moderately fast (5 feet/month) to extremely slow (less then 1 foot every 5 years). |
If the slide is rotational and has several parallel curved planes of movement, it is called a slump. |
Flows |
The moving mass becomes broken-up, and looses any internal structure. The mass resembles a viscous fluid. |
Rapid to extremely fast (more then 1 foot/minute), except creep, which is extremely slow (less then1 foot every 5 years). |
Mudflows and earthflows. creep occurs when the movement is only soil, and is very slow. |
Point Fermin has classic examples of rotational slides. There are several causes
to the landslide activity at Pt. Fermin and other nearby locations. The underlying geology
has bedding that dips slightly toward the ocean.
The dominant factor is wave action that eroded the base of the seacliff. Wave erosion
oversteepened the slope.
Eventually, a plane of weakness developed. Unlike the Portuguese Bend Landslide, the plane
of weakness at this locality is not flat, but curved and cylindrical.
Eventually, gravitational force overcame frictional force, and the block of rocks rotated. Notice
that the rock layers that once dipped toward the ocean now are dipping toward land. This is
called a back-rotated block.
As it rotated, it also was eroded. In the end, the center of mass of the block is now lower then
where it once was.
Here is how the sequence occurred:
Evidence of a rotational slide is still seen
today.
The sequence described above is oversimplified. In actuality, there were 6-8 planes
of weaknesses that developed, creating 6-8 landslide blocks all moving at thier own individual
speeds.
Movement began in the early 1940's when several broken water pipes were noticed. Movement abated
in the early 1960's, but damage to homes and streets had already occurred. Some concrete
from the foundations of the houses and the street can still be seen today at Point Fermin.
A CSULB student inspects the ruins of an area now called "The Sunken City."
Also visit CSULB Geology's Landslide
Talk Story for more information on landslides and mass wasting.
Choose one of the Palos Verdes sites
GO BACK TO THE MAIN MAP
Evidence of a rotational slide is still seen today.
The sequence described above is oversimplified. In actuality, there were 6-8 planes of weaknesses that developed, creating 6-8 landslide blocks all moving at thier own individual speeds.
Movement began in the early 1940's when several broken water pipes were noticed. Movement abated in the early 1960's, but damage to homes and streets had already occurred. Some concrete from the foundations of the houses and the street can still be seen today at Point Fermin.