The Portuguese Bend Landslide


*
A labeled overview of the lower section of the Portuguese Bend Landslide looking towards the east.

The Portuguese Landslide is perhaps one of the largest and most famous landslides in California, covering about 260 acres. It is visited by several of the CSULB geology classes as an example of mass wasting. Mass wasting is the movement of rock material downslope due to Earth's gravity.

There are several types of mass movements:

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.
Table modified from Varnes, 1978, Fig. 2.1.

The Portuguese Landslide is a translational slide. In contrast, the Point Fermin landslide is a rotational slide.

The Portuguese Bend Landslide actually occured in two pulses. The first began about 37,000 years ago. At this time, a cross section of Palos Verdes looked something like the diagram below:
*
Notice that bedding is slightly steeper then the profile of the land.

A plane of weakness developed. A diagrammatic sketch is shown below:
*
Notice that the plane of weakness is parallel to bedding.

The land above this plane slid towards the ocean. The actual cause of this weakness that occured 37,000 years ago is not well understood. Total displacement is also not known. This movement, called the "Ancient Landslide," is no longer active. The result looked like the diagram below:
*

After this movement ceased, a new, more shallow plane of weakness formed. An ancient volcanic eruption produced the geologic layer that developed the recent plane of weakness. The thick (50 ft) volcanic ash and subsequent thinner layers altered to a clay called bentonite. Bentonite absorbs much water and can lose its cohesiveness, forming slip planes along which the land can slide. It is believed that this layer may have played a role in the Point Fermin Landslide activity. A diagrammatic sketch of this is shown below:
*

Here is how the landslide appears today:
*
The initial movement is evidenced by Long Valley, which runs parallel to Crenshaw Blvd. in the city of Rolling Hills. This area is called the Crenshaw Extension.

*

The recent slide has been moving sporadically over the last several thousand years. However, renewed activity was noticed in 1956, when a portion of the southeast of the slide showed signs of movement. People living in the city of Rolling Hills, which lies just north of the slide, introduced large amounts of water into the ground through garden watering and leaky septic tanks. This reactivated the slide that would have otherwise remained dormant. The active movement affects roughly 80 acres of land. The most active part of the slide is in the southeast corner, and has moved over 600 feet since 1956.

The effects of the landslide activity are readily apparent throughout the area. The picture below was taken along Palos Verdes Drive South. These are parallel fractures caused by tension (or forces pulling in opposite directions away from each other).
*

Some other indications of active sliding are more obvious, such as the constant roadwork needed to maintain Palos Verdes Drive South.
*

Below is a picture taken looking west from the easternmost extent of the landslide. Land in the foreground is stable, while land in the background is within the landslide. The road's curb has been offset by movement. This, however, is not indicative of the total movement of the slide in that area, only movement since the curb was straight, which was about two years ago.
*

The following two pictures were taken just to the right of the previous picture, facing towards the north. They show where exactly the land becomes unstable along Palos Verdes Drive South. (Note that the gate shown on the far left of the picture is the one described in the directions to the pillow basalt.)
*
The boundary between stable and unstable land is not easily seen by this picture. But if it is labeled, the division between the two should be easily seen.
*

Also visit CSULB Geology's Landslide Talk Story for more information on landslides and mass wasting.

*****
***

VISIT ONE OF THESE VIRTUAL FIELD TRIP LOCATIONS

* * * * * * * * *

*
GO BACK TO THE MAIN MAP
*