Geology Cafe

Landslide Hazards—An Awareness Guide

Table of Contents

Image Gallery—Landslide Hazards and Features in California

Below is a collection of selected images illustrating landslides, debris flow deposits, and related features with exampes from California. Click on any image for a larger view.
California's landslide hazards map   Map of California's landslide hazards showing general incidence (or occurrence) and susceptibility (or likelihood of occurring). The Coast Ranges of central and northern California and the coastal Transverse and Peninsular Ranges of southern California have moderate to high levels of landslide incidence because these areas receive significant amounts of rain, typically as winter storms. Also, much of the bedrock in these mountain areas are, geologic-speaking, relatively young and and consist of poorly consolidated sedimentary rocks of marine origin that were uplifted into mountain ranges within the past few million years (and are continuing to rise as demonstrated by the frequency and abundance of earthquakes in the region). The volcanic deposits of the southern Cascade Range of northern California are also relatively very young and poorly consolidated and are highly prone to landsliding. In contrast, even though the high high mountains of Sierra Nevada Range receive abundant rainfall, the bedrock in the region consists mostly of hard, dense granitic and metamorphic rocks and have thin soils, and are therefore generally not as susceptible to landsliding. The mountains of the southern Basin and Range Province and Mojave desert region also general have hard bedrock, but they also don't receive abundant precipitation except during infrequent, intense storms.
Rockfall in Arroyo Seco Canyon   A small rockfall along a road in Arroyo Seco Canyon (Monterey County). Many similar small rockfalls occurred along roads and trails during and after a short, but intense, rainstorm in the Ventana Wilderness region of the coastal Santa Lucia Range. Some rockfalls are initiated when plant roots expand and dislodge rocks during winter rains come after a long, dry summer season. Also, flowing water, change the balance and stability of loose rock and soil by the addition of the weight of rainwater, and possibly other factors induce rockfalls and landslides after storms.
Talus slope in Arroyo Seco Canyon   A talus slope at the base of a cliff of sedimentary rocks (Monterey Formation) along Arroyo Seco Canyon (Monterey County). The talus slope formed from the ongoing breakdown of the sedimentary rock exposed in the cliff. Rockfalls occur frequently enough that plans don't have the opportunity to become established on the talus slope below the cliff.
Devils Postpile   Devils Postpile National Monument (Inyo County) is appropriately named for the large, blocky talus slope below cliff of columnar basalt. The rock columns formed from a lava flow that flowed down an ancient valley in the eastern Sierra Nevada Range. The cliff slope formed when a glacier and stream erosion later scoured out the valley. Rockfalls in the high mountains of the west are mostly initiated by water freezing and expanding in cracks, and the collapse of steep slopes in over-steepened valleys carved by glaciers in the recent Ice Ages of the Pleistocene Epoch (roughly between 11,000 and 2 million years ago).
Talus slope on Mission Peak   A talus slope on Mission Peak near Fremont (Alemeda County). The blocky slope consists of materials that accumulated from rockfalls at the base of a steeply dipping hogback of sedimentary rocks (Briones Formation, Late Miocene, about 12 to 8 million years old). The abundance of grass amongst the boulders suggests that slope is now relatively stable compared to perhaps an earlier period.
Worthan Creek Canyon   Marine shales of Late Cretaceous-age (mostly Moreno Formation, about 80 to 65 million years old) along Worthan Creek Canyon-southwest of Coalinga in Fresno County. Shale is highly susceptible to landsliding because of its soft physical character and because it weathers into clay (the material of which it originally formed). Note that little plant cover becomes established on the talus slopes below the high cliffs. Note that shale slopes are not blocky because shale typically weathers into small flakes and mud. This is in part to the arid summer climate and lack of water-retaining soil on the slope, and the high degree of erosion that takes place during infrequent, but often intense, storms.
Kings Canyon   This airliner view of the high peaks and glaciated valleys of the Sierra Nevada in the Kings Canyon National Park (southeastern Fresno County). Whereas the valleys were carved by alpine glaciers in the recent Ice Ages of the Pleistocene Epoch (about 11,000 to 2 million years ago), a fresh winter snow cover highlight how the over-steepened slopes are susceptible to avalanches. Avalanche chutes extend from high slopes, cutting through forests on the lower slopes to the valley floor where snow, rock, and other material accumulate. In many ways, the ancient alpine glaciers that once filled these valleys behave like great slump-like landslides. In fact, glacial deposits share many similar landscape features associated with large landslides.
Mount Shasta   Mount Shasta is the largest volcano in the Cascade Range—located in Siskiyou County in northern California). This view shows a large debris chute on the western flank of the volcano. Mount Shasta is actually a complex volcano that has had multiple eruption vents on the summit and sides of the volcano that have built up the great composite cone over the past few million years, with large eruption having occurred after the end of the last Ice Age (about 11,000 years ago. The volcano (like other Cascades Range volcanoes) has produced massive landslides and lahars in prehistoric times, some of which were sizably greater than the 1980 eruption of Mount St. Helens.
La Conchita Landslide   Aerial view of the La Conchita Landslide in California (Ventura County). This landslide formed on the hillslopes above the residential community built on a narrow coastal plain along the Pacific Ocean between Santa Barbara and Ventura. The landslide formed in soft (poorly consolidated) sediments on the western slope of Rincon Mountain. On January 10, 2005, an earthflow (landslide) at La Conchita destroyed or seriously damaged 36 homes and killed 10 people. Another landslide occurred in the same vicinity in 1995 (without catastrophic results) and the hillslopes in the area display abundant evidence of landslide activity in the prehistoric past. Note the bowl-shaped character of the upper escarpment area and the lobe-shaped geometry of the landslide deposits at the base (or toe) of the landslide. (Photograph by R.L. Schuster, U.S. Geological Survey, 1995.)
Landslide near Pacheco Pass   A slump in Pacheco State Park in the Diablo Range in western Merced County. This slump formed in deeply weather bedrock consisting of serpentinite (the State rock of California). A small stream (located near the trees in this image) had undercut the slope, helping to initiate the ground failure. Note the arc-shape scarp at the top of the slump and the lumpy and fractured surface of the slump itself.
Landslide near Paicines   A slump along Highway 25 in valley of the San Benito River south of Paicines in San Benito County. This region has numerous landslides that occur in relatively young and poorly consolidated sediments of both marine and continental origin (Late Miocene to Pliocene age; roughly about 14 to 4 million years old). This rugged valley is a rapidly evolving tectonic setting —it basically follows the trace of the San Andreas Fault Zone.
Landslide near Paicines   Another view of the slump along Highway 25 south of Paicines in San Benito County. This view shows the rugged, lobe-shaped character of the landslide deposits at the toe of the slump. The wind mill tower in the image is about 40 feet tall for scale.
Limekiln Trail Landslide   This slump is along the Limekiln Trail near Lexington Reservoir in Santa Clara County (south of Los Gatos). The slump formed in deeply-weathered serpentinite bedrock and soil on the steep mountainsides along the valley Limekiln Creek on the flank of Sierra Azul Ridge. Note the leaning and dead manzanita shrubs on the toe of this active landslide.
Slump fissure in Forest of Nisene Marks  

One of many fissures along the Big Slide Trail located in the Forest of Nisene Marks State Park in Santa Cruz County. This was one of several large fissures that opened at the headwall escarpment area of a large landslide complex as a result of groundshaking near the epicenter of the 1989 Loma Prieta earthquake. The magnitude 6.9 Loma Prieta earthquake of October 17, 1989 occurred just after 5:00 pm and lasted between 10 and 15 seconds. The earthquake initiated or activated dormant landslides throughout the southern Santa Cruz Mountains.

Deranged forest in Forest of Nisene Marks   This view shows a "drunken"or "deranged" forest in association with the large landslide complex in the Forest of Nisene Mark in Santa Cruz County. Many trees fell during the 1989 Loma Prieta earthquake, and whole groves of trees have bent trunks from having adjusted themselves back to a vertical direction after the ground surface rotated in areas of deep-seated slumps.
Slump pond in Forest of Nisene Marks   This pond formed at the head of a great slump along the Big Slump Trail in the Forest of Nisene Marks in Santa Cruz County. The disruption of the landscape by landslides creates locations where water can pond. This stagnant pond formed at the head of a large slump block. Fractures and fissures on landslides, particularly in the toe area, are commonly the location of springs and seeps.
Highland Way Landslide  

The Highland Way Landslide of January, 1997 in Santa Cruz County. The landslide took out a section of road and force the abandonment of a home. Additional landsliding occurred here in 1998. The road has since been repaired, but the fresh landslide escarpments are still visible (even after construction repairs have smoothed out the typically chaotic landscape of a slide area). The steep, forested landscape throughout this area displays abundant evidence of landslide activity—both active slides and other more ancient slides that are currently dormant. The slide area is along the southern flank of Loma Prieta Peak in the rift valley of the San Andreas Fault. The bedrock exposed in the slide area is marine shale and sandstone of early Tertiary age (Eocene Mount Madonna Sandstone; deposited between 34 to 56 million years ago).

Mission Peak Landslide  

The Mission Peak Landslide near Fremont (Alameda County) occurred in April 1998 occurred during an El Nino season. This view is from the Mission Peak Region Preserve. The landslide is about a mile long and was considered one of the largest landslides in the Bay Area. The landslide occurred on undeveloped land but threatened homes at the toe. This view shows the barren escarpment on the upper flank of Mission Peak and the rugged, tongue like flow that extends down slope to the lower left (covered with chaparral-type vegetation). Some of the older landslide deposits on the lower slope were already covered with chaparral before the modern landslide occurred. Residential portions of the city of Fremont are to the left. The landslide is part of a larger and more ancient landslide complex that extends for many miles along Mission Ridge.

Mission Peak Landslide   Headwall escarpment of the Mission Peak Landslide (Alameda County). Large fissures in the slope above the landslide ominously suggest that additional landslides will likely happen here in the future. The Mission Peak Landslide of March 22, 1998 represents a partial reactivation of an enormous bedrock landslide complex. The massive landslide complex has developed beneath Mission Ridge over a period of perhaps tens of thousands of years. 
Santa Rosa Islands landslides   Small landslides are scattered across the grassy slopes of Santa Rosa Island (Channel Islands National Park). Many small landslides occurred throughout the Channel Islands and nearby mainland during a series of winter storms early in 1999. The landslides occurred in a region where the surface soil is underlain by soft sedimentary rocks (Monterey Formation).
Panoche Hills landslide   A cross-sectional profile of an old landslide is exposed along the side of Dosados Canyon in the Panoche Hills in western Fresno County. The original landslide developed on a slope consisting of steeply dipping shale (Moreno Formation of Late Cretaceous age, about 70 million years old). Stained layers are highlighted by salts and iron minerals precipitated by groundwater along fractures and glide planes along the base of the landslide deposits. Note the arc-shape profile of parts of the slump and overlapping tongues sediment within the landslide deposits. The small landslide eventually stabilize, possibly when the stream channel migrated to a new position before carving the modern canyon.
Landslide at Palomarin Beach  

A large slump is an impressive landscape feature at Palomarin Beach in southern Point Reyes National Seashore (Marin County). A great landslide complex extends along the coast in this area for several miles. Near the center of the image is a great slump block that is gradually slipping seaward. The landslide is forming in soft marine mudrocks of the Santa Cruz Mudstone of late Miocene-age (about 8 million years old). The large slump of Santa Cruz Mudstone is capped by younger marine terrace deposits. These marine terrace deposit were originally horizon but are now tilted and offset by rotation of the large slump block. At low tide, a small plunging syncline is exposed and may represent a structure associated with the development of the of the slump in the past, but coastal erosion has carved the sea cliff landward to its present position. Note the Marin Headlands in the distance (to the northwest).

Landslide at Mussel Rock Park   A massive landslide complex at Mussel Rock Park masks the main trace of the San Andreas Fault. Note the high, barren slump escarpment in the background. Ongoing coastal erosion at the toe of this slide area promotes the landward migration of the high wall of the landslide escarpment. Numerous homes have been destroyed, damaged, or condemned along this coastal landslide complex in Daly City. Part of the landslide complex was used as a landfill in the past.
Thornton State Beach landslides   View looking northward from Mussel Rock Park along the sea cliffs at Thornton Beach State Park (San Mateo County). The rolling slope in the foreground is part of the toe area of a large landslide complex at Mussel Rock Park. The sea cliffs are exposures of the Merced Formation. The Merced Formation consists of poorly consolidated marine and coastal sedimentary deposits that accumulated roughly between 3 and 0.5 million years ago. The Ocean Shore Railroad built along this section of coast in 1905 to 1906 was severely damaged by the 1906 earthquake. Because of ongoing troubles due to landsliding, the rail line was abandoned in 1920. In the 1930s, the rail line was widened to accommodate Highway 1. However, shortly after major landsliding occurred in association with a magnitude 5.3 earthquake on March 22, 1957 the route was once again abandoned. Today only a trace of the highway cut can still be seen about midway up the sea cliff.
The Devils Slide   Devil's Slide is a notorious landslide area along Coast Highway 1 in San Mateo County. The landslide has been occurring where steeply dipping, faulted and folded sedimentary rocks are slipping above a steeply inclined surface of underlying weathered granite bedrock of Montara Mountain. Several landslide chutes and failure zones are present in the area. The landslide extends from 900 feet (275 m) high on the mountain down to sea level and the glide planes of the slump blocks extend as much as 150 feet (46 m) below the surface. The landslide complex at Devil's Slide has a long and expensive history. Landslide failures disrupted travel along the first road built across the slide area in the late 1890s, and the road was eventually abandoned. Starting in 1905, the Ocean Shore Railroad attempted to operate a rail line across the area below the present road level, but it was abandoned in the 1920s because of the chronic troubles with landsliding at Devil's Slide and elsewhere along this coastal route. The State Department of Highways completed the first version of the coastal highway along the abandoned rail line in 1936 and this is partly the route of Highway 1. However, landsliding and road closures have constantly plagued the route, and millions of dollars have been expended in endless repairs (Williams, 2001).
Goleta Submarine Landslide Complex   The Goleta Submarine Landslide Complex is located in the Santa Barbara Channel near Coal Oil Point near the towns of Goleta and Santa Barbara. This image was generated from is a multibeam-bathametric data. It shows large tongues of sediment created by undersea landslides that extend in some places more than 6 miles (10 kilometers) onto the seafloor in the Santa Barbara Channel from their source of failure along the margin near the break in slope of the continental shelf edge. Research shows that a record submarine landslide failures in the area began about 200,000 years ago, with two of the large landslide lobe having formed about 10,000 and 8,000 years ago. Scientists are concerned that such offshore landslides could cause tsunamis that affect the Santa Barbara coast with little advanced warning (Fisher and others, 2005; Image by Gary Greene).
Bathymentry and relief map of Monterey Bay region   Topography and Bathymetry of the Monterey Bay and San Francisco Bay region. The image shows that submarine erosion is occurring offshore along the outer margin of the continental shelf and continental slope. Submarine landslides move blocks of materials and sediments churned up in density currents (also called turbidity currents) down slope into the abyssal plain that extends seaward from the base of the continental slope. A great submarine channel extends seaward from the base of Monterey Canyon.
Underwater landslide deposits at Mono Lake  

Underwater landslide deposits exposed in the Mono Lake Basin (Mono County). The landslide deposit is sandwiched between normal flat-lying lake sediments. The deposits formed when the surface of Mono Lake was perhaps several hundred feet higher during one of the ice ages of the Pleistocene Epoch. The underwater landslide may have been initiated by strong earthquake.

Point Reyes Headlands   Point Reyes Headlands at Point Reyes National Seashore (Marin County). Wave erosion is carving away away at the base of uplifted granite headlands causing over-steepening of the slope. This results in landslides that drop material into the ocean or beaches in small coves. The headlands at Chimney Rock on the east end of the headlands are capped by marine terrace deposits and a soil horizon that are perhaps only about 150,000 years old. They are now several hundred feet above sea level. The occurrence of these deposits illustrate how relatively the coastline is changing from the combine effects of sea-level change, tectonic uplifts, and coastal erosion (including landsliding).
Kehoe Beach landslide chute   Debris chute and deposits on Kehoe Beach in Point Reyes National Seashore (Marin County). Episodic storms and landslides fall to the beach along the chute (causing further erosion). The material accumulates as a cone of alluvial debris at the base of the slope. High waves during high tides and coastal storms strip away the alluvial debris, mostly moving it offshore or reworking it into beach deposits along the coast.
Telescop Peak and Hanupah Canyon alluvial fan   Hanupah Canyon alluvial fan with Telescope Peak (elevation 11049 feet; 3368 m) in the Panamint Range rises above the Badwater Basin playa (below sea level) in Death Valley National Park (Inyo County). Light-colored channels indicated the location of stream channels that only drain to the valley floor during episodic storms. Some of these stream discharge events are in association with debris flows or debris floods that discharge materials onto the alluvial fan. This view is looking west from Dantes Peak in the Black Mountains on the east side of Death Valley.
Alluvial fans in Death Valley near Badwater   Aerial view of alluvial fans located at the mouths of canyons along the western flank of the Black Mountains in Death Valley National Park. Badwater, the lowest elevation in North America (-282 feet;-86 m). The alluvial fans form from sediments carried by episodic debris floods and debris flows draining from canyons in the Funeral Range. The sediment overload in the flood is dropped on the alluvial fan where the speed of the flood slows at the change in slope. Sediments fill broad channels on the alluvial causing the drainage to constantly shift across the surface, gradually building up the relatively even cone- or fan-shaped alluvial deposit. Coarser materials (boulders) generally accumulate at the top of the fan, whereas finer materials (sand, silt, and clay) are transported down slope to the basin.
Copper Canyon alluvial fan   Copper Canyon Fan in Death Valley National Park (Inyo County) is often illustrated as a "classic alluvial fan"in textbooks. Note the radial pattern of stream channels starting at the mouth of the Copper Canyon. Darker areas on the fan are areas where desert plants (mostly Creosote Bush) are established on the fan whereas the lighter colored areas are active stream channel (void of vegetation and filled mostly with gravel). The base of the fan merges with the dry lake bed (mud flats and salt flats) in southern Death Valley. The lake bed and flats are occasionally flooded with a thin veneer of water during infrequent wet periods, usually during or following abnormally wet winter seasons.
Mormon Point alluvial fan  

Alluvial fan at Mormon Point in Death Valley National Park (Inyo County). The small size of the alluvial fan relative to the large canyons in the Black Mountains is because the east side of southern Death Valley is sinking relative to the west side of the valley. In addition, glacial Lake Manly filled Death Valley during the Ice Ages of the Pleistocene Epoch. Waves and currents in Lake Manly redistributed sediments throughout the lake basin area. At its maximum extent, the lake was about 800 feet deep. Shoreline terraces associated with the lake's shoreline are visible near the center-right in this image. The lake finally dried up about 9,000 years ago, and the modern alluvial fans on the east side of Death Valley have accumulated since that time.

Note the narrow character of the lower canyon near its mouth but that the valley broadens upstream. This landscape characteristic is called an "hour-glass canyon". This landscape morphology forms as the headwater region of the drainage expands over time, yet the lower stream channel is confined to an ever-deepening canyon. As a result, when a rainstorm occurs across the headwaters region, the runoff of this larger upland region is concentrated in the canyon "bottleneck"near the canyon mouth. This is where the most hazardous conditions exist during and following a desert storms and is the site where debris flow or debris floods have their greatest impact.

Furnace Creek alluvial fan  

Alluvial Fan at Furnace Creek in Death Valley National Park (Inyo County). This view is an aerial photograph taken in 1948. Note that in 1948 the development of the Furnace Creek Ranch area was limited to only a small portion of the fan. Dark streaks are vegetation growing along stream channels on the lower part of the alluvial fan. Today, park development including campgrounds, lodges, housing, a gulf course, date palm orchards, and a settlement of the Timbisha Shoshone Tribe occupy portions of the alluvial fan area. The image shows Furnace Creek Wash drains from the Amargosa Ranges into Death Valley where if forms a massive alluvial fan. On August 15, 2005, a flood from 1-2 inches of rain in the Amargosa Ranges came down Furnace Creek. The event killed two people, destroyed cars, and damaged roads and buildings. Large quantities of rock debris was deposited on the alluvial fan at Furnace Creek and elsewhere in the region. Other major floods occurred along Furnace Creek Wash in 1939, 1941, and 1985.

Panamint Valley alluvial fan   Debris flow deposits on an alluvial fan in Panamint Valley at the base of Panamint Butte in the Cottonwood Mountains in Death Valley National Park (Inyo County). Note how the stream channel is incised near the top of the fan, but the debris flow deposit is spread out on the lower fan surface.
Boulder in Marble Canyon   A large granite boulder blocks the stream channel in Marble Canyon in the Cottonwood Mountains in Death Valley National Park (Inyo County). The boulder is roughly 10 feet (3 meters) in diameter. The closest bedrock occurrence of similar granite is nearly 8 miles (12 km) away in the central Cottonwood Mountains. The boulder moved to its present location possibly entrained in a massive debris flood or debris flow in prehistoric times.
Mosaic Canyon   Alluvial deposits and large boulders of bedrock in Mosaic Canyon in Death Valley National Park (Inyo County). Episodic storms produce debris floods and debris flows in all canyons in the Death Valley region. Great storms that happen perhaps only once or twice a century will wipe out tell-tale traces of smaller events and will move sediments trapped in the upper canyon onto the alluvial fans in the valley below.
Bear Gulch   This view is of a debris cave in eastern Pinnacles National Park (San Benito County). Large boulders from rock falls and debris flows cover or fill narrow gorges associated with stream valleys carved in volcanic rock. The rock exposed in the Pinnacles area consists of massive volcanic (pyroclastic) debris and welded tuff that accumulated on the flanks of an ancient volcano that formed along the San Andreas Fault Zone about 23 million years ago. Tectonic forces along the fault zone ripped the volcano apart and moved the Pinnacles portion to its current location. Over time, tectonic uplift and erosion have carved the old volcanic deposits to their current landscape configuration. Seasonal climate (wet winters, dry summers) are conducive to erosion in the form of debris floods or flows in the region during the winter months.
Chalone Creek debris flow deposits   This view shows the dry stream bed of Chalone Creek in eastern side of Pinnacles National Park (San Benito County). The image shows a plant community partially established on stream bed after a massive debris flow the previous year. In 1998 heavy rains resulted in debris flows that destroyed the bridge and spread gravels throughout the valley. The debris flows possibly contributed to the death of the trees in the image. The high Pinnacles that are the prominent feature of the park are just to the left of the center of the image.
Stevens Creek debris flow deposits   Debris flow deposits partly cover a road in Stevens Creek Canyon (Santa Clara County). The debris flow occurred during a storm in late January 2003. Most of the major streams draining the Santa Cruz Mountains were dammed for flood and water resource management; their reservoirs serve as catchment basins for sediments that would otherwise be carried downstream to the alluvial fans that are today completely developed into the South Bay's urban communities.
Fanglomerate in Split Mountain, Anza Borrego State Park   Ancient debris flood and debris flow deposits exposed in the canyon walls in Fish Creek Wash exposed Split Mountain in Anza Borrego State Park (eastern San Diego County). The "fanglomerate"beds, consisting of sandstone and conglomerate beds formed from sediments deposited on an alluvial fan on the margin of the Salton Trough in the early stages of its formation in Late Miocene time (between 15 and 5 million years ago). Each of the conglomerate beds represents a debris flow or flood event. The entire sequence shown in this sequence probably represents several dozen major storm events that occurred within a period of several thousands of years. Minor storm events are probably poorly represented or not preserved in the sequence.