Portuguese Bend
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The Portuguese Bend region is the largest area of natural vegetation remaining on the Palos Verdes Peninsula. Though once slated for development (and in fact the projected route of Crenshaw Boulevard through the area still shows on many maps), the area is geologically unstable and is unsuitable for building.
The geographical location and geological history of the peninsula make the remaining habitat extremely valuable for ecological and other scientific reasons. The peninsula, which was an island in recent geological time, has close floral and faunal similarities to the Channel Islands. This feature makes the Portuguese Bend Landslide area a natural research laboratory for the study of island biogeography and evolutionary ecology.
The vegetation found in the area is coastal sage scrub. This plant community supports a surprising number and variety of species. There are at least three races of birds resident on the peninsula that are found nowhere else except the Channel Islands. These are the insular forms of the Orange-crowned Warbler, Pacific-slope Flycatcher, and Allen's Hummingbird. The same phenomena has been documented for plant species. A species of live-forever, Dudleya virens, which is endemic to the Channel Islands and the Palos Verdes Peninsula, is found near Point Vicente.
The area also serves as habitat to many migrating birds moving through the region in fall and spring. The Peninsula is a headland that juts into the Pacific Ocean several miles further than the surrounding coastline. Migrating terrestrial and shore birds, flying over the open ocean on their north-south migration along the Pacific Flyway, spot this headland and stop to rest and feed. Many of these birds will stay and spend the winter in the area. Thus, the geographic position makes this habitat much more important than might otherwise be expected.
In general, the area has been lightly disturbed, and much natural vegetation remains. Intense disturbances, in the form of heavy off-road vehicle and pedestrian use, have been limited. Grazing has also taken place at one time.
Information Source(s): Survey/Interview, Literature, ERC/UCLA, CNACC/CNPS.
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[edit] Portuguese Bend Landslide
[edit] General Information
The Portuguese Bend Landslide is a slow moving landslide which is located on the Palos Verdes Hills. These hills form an elongated topographic dome that rises from sea level to altitudes of more than 430 meters. The main Portuguese Bend Slide spans, at its greatest, 2700 feet wide and 3900 long. The regional climate is characterized by mild winters and hot summers, with an annual precipitation of 25 to 100 centimeters. Precipitation occurs primarily during the winter months.
During the late Pleistocene, the Palos Verdes hills were an offshore island. The island later became a peninsula, when the region between the island and the mainland filled with alluvial deposits from the mountain ranges near the Los Angeles basin. The Palos Verdes Hills are part of an uplifted block, with a northwest trend, bounded on the northeast by the Palos Verdes fault zone. Most of the movement along this fault is dip-slip, resulting in an uplift of about 1 km of the Palos Verdes Hills relative to the Los Angeles basin.
[edit] Surficial Deposits
Surficial deposits of the Palos Verdes Hills consists of stratigraphic layers, in order from oldest to youngest:
- Late Pleistocene stream terrace gravel
- Late Pleistocene marine terrace deposits
- Late Quaternary dune sand
- Late Quaternary soil
- Late Quaternary talus
- Late Quaternary non-marine terrace cover
- Late Quaternary slope wash and creep deposits
- Holocene beach deposits
- Holocene alluvium.
The ground surface in the central and southern parts of the district is low and hummocky, reflecting the location of numerous late Quaternary landslides.
[edit] Historical Sliding
The Portuguese Bend Landslide has been conductive to ground failure for approximately 250,000 years. The landslide spans 260 acres with an average thickness of 135 feet. The ground failure occurs on an overall smooth surface approximately 100 feet below the surface. The ground failure over the years has been due to seaward-dipping strata, rock weakness and continual coastal erosion. Prehistoric landslides are believed to be so extensive that they destroyed the formation of higher wave-cut benches. The active slide consists of landslide rubble such as bedded blocks, which are rare among most landslides. The bedded blocks measures ten feet in diameter and they appear in landslide rubble. This shows ground disturbance which could eventually cause land the land to slide.
[edit] Modern Sliding
[edit] Cause
The onset of sliding has been attributed to the housing development that started in the 1950s. Homeowners tried to say it was the county's fault for constructing a road near the headscarp but they didn't win this case. Geologists all associate the onset of slip to irrigation, installation of pools and septic tanks that increased ground water levels. These homeowners had built houses on top of the land prior to the slide. Some homes were damaged, but others remain intact. These homes which remain occupied now have water and sewage lines available to them. The new lines were constructed above ground, so the slide can move freely below them.
[edit] Description and Characterization
The first movement indication occurred on Friday, August 17th 1956. Cracking occurred in the foundation of a recently built structure. The cracks were repaired, but new cracking occurred just days later. Ten days after the first indication of movement, cracking propagated to Palos Verdes Drive South. By September 4th, 10 cm offset was observed on the Palos Verdes Drive South. By mid-September, more pronounced cracking was observed. Distortion became noticeable on the Portuguese Bend pier on October 4th of that year. A survey station was installed to monitor ground movement. The station recorded movement of 7-10 cm per day. By the end of October, movement slowed to 2 cm per day. Drilling projects over the next several months determined the location of the slip surface, and the location of standing groundwater.
The major part of the landslide consists on severely broken rubble. Individual rock fragments appear to be from the middle and upper units of the Altamira Shale, consisting of tuffaceous and silty shale. Fractured blocks of diatomaceous shale found on the south slope of Blakesley HIll. Individual fragments of chert and cherty shale are found throughout the slide, though they are more abundant went of Portuguese Canyon. Found at the toe of the slide, between Inspiration Point and Portuguese Point, are weathered boulders of basalt.
Drainage within the Portuguese Bend district is characterized by a prevailing southward flow. Several canyons cross the distract and act as the primary drainage features, including the Altamira and Portuguese Canyons. These canyons trend southward through the slide complex. Drainage is controlled locally by landslide-related features, such as resistant landslide blocks or channels cut in softer zones.
[edit] References
1. Vonder Linden, Karl. "The Portuguese Bend Landslide." Engineering Geology, December 1989, Vol. 27, Issue 1-4, pp. 301-373
