Portuguese Bend slides

What is moving

The Palos Verdes Peninsula is built on the Monterey Formation, over 2,000 feet thick in this part of the coast. Within the Monterey is the Altamira Shale — a thin-bedded sedimentary rock with successive clay layers. Within the Altamira is the Portuguese Tuff — a 50-to-75-foot volcanic ash bed altered through geologic time to bentonite clay. The 2018 Zone 2 EIR puts the mechanism plainly: "In the presence of water, bentonite becomes very slippery and has been a major contributing factor for landslides in Rancho Palos Verdes." Initial slide movement on this formation began 120,000 to 500,000 years ago. The modern slides are reactivations of ancient ones.

The Ancient Portuguese Bend Landslide (APBL) is roughly 900 acres. The modern Portuguese Bend Landslide (PBL) — the one you can see today — is approximately 260 acres within the APBL footprint. The Abalone Cove Landslide (ACL) — the immediate neighbor to Abalone Cove Shoreline Park and to the two parcels named in the position paper — is approximately 85 acres. Zone 2, the City's moratorium zone addressed by the 2011 LGC Valley study, covers 112 acres and 47 undeveloped lots; it abuts the ACL on the south.

1956 and the Crenshaw extension

The prehistoric slide plane at Portuguese Bend was documented in a federal geological report in 1946. The Albers opinion a generation later noted that the 1946 report "was known to both the county and the developers." Ten years after the report, during the Crenshaw Boulevard extension project in 1956, Los Angeles County placed approximately 175,000 cubic yards of fill within a road easement and adjacent lands. The fill overloaded the ancient slide plane. The Portuguese Bend Landslide reactivated.

Approximately 134 of 160 affected homes were destroyed. The slide moves approximately three feet per year on average, with peak movement of 1.5 inches per day after heavy rainfall. In 1965 the California Supreme Court, in Albers v. County of Los Angeles, affirmed aggregate damages of $5.36 million against Los Angeles County under Article I §14 of the California Constitution — "private property shall not be taken or damaged for public use without just compensation." Albers was an inverse condemnation case; it did not stop the slide.

Groundwater is the controlling factor

The single most important finding in the geological record of this landslide, as far as the position paper at /position/ is concerned, is from Robert Stone & Associates in 1979:

"evaluation of the driving force produced by the groundwater head indicates it is the controlling factor causing the slide movement." (Stone & Associates 1979, cited at Zone 2 EIR §4.5-6)

The 2018 Zone 2 EIR, drawing on the 2011 LGC Valley, Inc. study, elaborates:

"Early in the development of the Portuguese Bend area, septic systems, leach lines and cesspools installed as part of residential development on the APBL contributed high volumes of water directly into the landslide." (Zone 2 EIR §4.5-5)

"Groundwater was concluded to be the most likely agent responsible for the slide movement of the 80-acre ACL." (Zone 2 EIR §4.5-5)

Stone & Associates recommended against further development in Zone 2 until slide movement was stopped within the ACL, the water table was lowered, and surface drainage was improved. That was 1979. In 1980 the Abalone Cove Landslide Abatement District (ACLAD) began drilling dewatering wells. By 1985 the ACL had nearly stopped moving. ACLAD currently operates approximately twenty-two dewatering wells. The City of Rancho Palos Verdes contributes roughly $1 million per year to road repairs in the complex.

Now

As of March 2026, the City of Rancho Palos Verdes land-movement updates record the Abalone Cove Landslide moving at 2.14 inches per week — three orders of magnitude faster than the 2007-2017 average recorded in the Zone 2 EIR. Wayfarers Chapel, built on the same formation, was dismantled in 2024 after recording 2 feet per month of movement.

The landslide is not a historical event. It is the present tense.