Beginning as far back as the late 1800s there are
records of landslides in the La Conchita area. In essays written by a
resident that are nearly a century old, there is reference to debris flows
covering cultivated lands and damage to structures caused by mass wasting
events.
On January 10, 2005, La Conchita, California experienced a
landslide and debris flow, the second in less than ten years (the first was
in the spring of 1995.) 10 people were killed in this tragic event and homes
lost in the 156-home community. Suffering the loss of their family members
were Jimmie Wallets and his daughter Jasmine. Read
more...
La Conchita was once a thriving residential town on the
east side of Highway 101 across from La Conchita-Seacliff Beach and Mussel
Shoals. Located north of Ventura with its nearest city, Carpinteria just a five or ten minute drive away (to the north), the
hamlet consists of several streets and a gas station/ general store. A railroad
track separates residents from the highway and aside from the regularly
scheduled views of passing trains, they enjoy the blue Pacific out the
windows and doors of their homes.
By Jeffrey J. Hemphill Remote Sensing Research Unit Department
of Geography University of California Santa Barbara
Beginning as far back as the late 1800s there are records of
landslides in the La Conchita area. In essays written by a resident that are
nearly a century old, there is reference to debris flows covering cultivated
lands and damage to structures caused by mass wasting events. The following
material summarizes important historical landslide events researched as part of
the legal investigation by one Kristing Coddington and documented in a
chronology spanning 1865 to 1958 (Coddington, 1998). In 1865 there was a wagon
trail through this section of coastline that connected Santa Barbara and Ventura
which a surveyor described as follows: “the character of this road was so
changeable in consequence of the falling down of masses of earth from the
cliffs, which in some places were 400 feet high, and from the washing of the
earth by the waves, that the road for the transportation of goods was nearly
worthless.” Southern Pacific Railroad laid tracks through La Conchita in 1887;
two years later in 1889 sections of the tracks were buried by the first of two
landslides. In December 1889 the Ventura Free Press reports that, “West of
Ventura somewhere on the cliffs there is a dead engine stalled in the
landslides…” The January 29, 1909 edition of the Santa Barbara Independent
reported on the Punta Gorda slide, and, as a consequence of this slide, changes
to the rail and the section of road running through La Conchita were made. The
author of this article described the stability of the area surrounding La
Conchita by saying: “The character of the soil of the mountain, which rises
almost abruptly from the sea, is such that there can be no security from slides,
such as the avalanche of dirt and rocks that last Saturday swept down on the
road and buried a work train.” As a result of this slide, Southern Pacific
Railroad had to excavate several railroad cars, and major plans for construction
of a causeway and the reinforcement of the railroad began. In an effort to
reduce the hazard imposed by the steep cliff face, Southern Pacific Railroad
bulldozed flat the area adjacent to the railroad which facilitated the
development of the area and the eventual construction of houses. In 1924 the La
Conchita del Mar subdivision was established. It consisted of approximately 330
lots and another 47 lots in a row up against the base of the cliff (SMC, 1996).
A zoning map drawn in the early 1930’s shows that most of the land in the town
of La Conchita was agricultural and, according to the legend on the drawing, the
lowest value parcels were actually composed of mudflow debris.
Geologic History
Geologic evidence suggests that the 1995 landslide, and other landslide events
along this section of coastline, are a relatively frequent occurrence. The
distinguished geologist, William C. Putnam, described the geology of this area
in detail in a report authored for the Geologic Society of America in 1942. The
report points out numerous signs that there have been significant changes to the
topography of this area in geologically recent times. For example, the exposed
strata visible on the cliff face and in areas on top of the bluffs contain
shells of species that established themselves in warmer postglacial times
(Putnam, 1942). Marine sediments, mollusk shells, and sandstone boulders with
bore holes and barnacles can be found in various locations in this area (Putnam,
1942; Sylvester, 1988; Harden, 1986). There are eight or nine distinct scarps
where the different uplifted wave cut platforms are visible near the surface
(Sylvester, 88; Harden, 1987). The Punta Gorda marine terrace is about 1300 feet
above sea level in the middle of the La Conchita ranch, and it is estimated to
have been at or near sea level between 40,000 and 60,000 year ago. Located
nearby is a fluvial terrace that was dated using charcoal deposits and
determined to be as young as 2000 years old (Harden, 1987). Based on a detailed
analysis of soil samples, it is estimated that uplifting rates are about 4.2 – 5
meters per 1000 years. In terms of tectonic uplifting, this is one of the
fastest in the world (Sylvester, 1988).
Two drainages border the La Conchita ranch, the Padre San Juan and Javon
Canyons, and both are deeply incised and have steep eroding walls. Putnam (1942)
noted the young, but abnormally deep, well-developed drainage pattern in this
area as being a result of tectonic uplifting. He also noted the presence of
gravel benches in the canyon walls that developed during episodes of canyon
cutting followed by filling in the recent geologic past. Putnam attributed the
deep canyon cutting and filling episodes to two possible causes, climatic
fluctuations that caused changes in the amount of winter runoff and tectonic
uplifting. As far as landslides are concerned, Putnam (1942) states that,
“Nearly every square foot of surface on the hill slopes underlain by upper Pico
clay shale is in motion down slope or has moved in the very recent geologic
past.” Given the geologic composition and the tectonic origins of cliffs above
La Conchita, it is difficult to understand how development was allowed in the
area at the base of the cliffs, given the high risk of landslides.
Geologic stability of subsurface soil layers and the slope of the hillside are
important considerations for landslide risk assessment. Also important are the
hydrologic environment and proximity to active faults. One influential feature
in this area is the Red Mountain Thrust Fault that runs from the Ventura River
to the east, around the Red Mountain Dome above the La Conchita Ranch, and off
shore near Rincon Point up the coast to the west of La Conchita. This fault has
exerted significant influence on the drainage pattern visible around the study
area; differential erosion rates of resistant Miocene age strata over weak clay
and shale has caused the canyons bordering the ranch above La Conchita to become
deeply incised in some areas and restricted in others (Putnam 1942). The canyons
are confined where they cross the resistant rock layers upturned by the Red
Mountain Fault, and the canyons are deep where they traverse weaker clay and
shale layers. -End
