Where do Devil's Slide landslides occur?

Devil's Slide Trail is a paved hiking and bicycling trail through a coastal erosion landscape south of the Pedro Point Headlands in San Mateo County, California. Its scenery is amazing. Its ground and surroundings are unstable and slippery. Along the trail, travelers enjoy the spectacular views of the Pacific Ocean. With awe, visitors also scan the walls and slopes of crumbling and sliding sedimentary rocks.

Powerful forces are at work at Devil's Slide. The above clip from an interpretive panel along Devil's Slide Trail illustrates the slide area with its cliffs and steep slopes. The panel labels the ongoing activity as “The Big Squeeze” and describes this phenomenom in detail:

Devil's Slide sits between San Pedro Ridge, a rock ridge squeezed upward by the earth's tectonic forces, and Montara Mountain. The slide is about half a mile wide and extends 900 feet from the ridge top to the ocean. Millions of years of upward pressure has broken and weakened the rock of these cliffs. Water trapped underground causes the weakened rock to move. At the same time, the pounding surf washes away the bottom of the slide. Devil's Slide continues to move into the Pacific Ocean, part of the natural process that shapes our ever changing coastline.

Devil's Slide Trail mostly traverses areas of sedimentary rock. A short trail section at the south leads over granitic rock. The geology-focused panel asks readers to notice how different the cliffs at the south end are compared to those at the north end. It continues:

The weathered rock face to the south is the granitic rock of Montara Mountain [about miles southeast from Devil's Slide], the same rock found in the Sierra Nevada Range. In contrast, the rough layers of sedimentary rock at the north end were once ocean floor. Not quite as old as the Montara Mountain rock, these layers of shale and sandstone have been thrust up and folded, over millions of years, by forces deep within the earth.

The panel also answers the questions asked in the title of this post:

The landslides occur where the sedimentary rock has been thrust over the granitic rock, causing broken, weakened ground.

San Pedro Point and sedimentary rock slopes between Devil's Slide Trail and the ocean

Keywords: geology, tectonics, landslides, sedimentary rocks, granitic rocks, Big Squeeze, San Pedro Ridge, Montara Mountain.

Explore the Geology of Devil's Slide:
Devil's Slide (California) [en.wikipedia.org/wiki/Devil's_Slide_%28California%29].
The Rocks of Devil's Slide [ww2.kqed.org/quest/2013/03/28/the-rocks-of-devils-slide].
Devil's Slide Tunnels Project [www.dot.ca.gov/dist4/dslide/images/devils%20slide_1updated.pdf].

A short paved segment of the California Coastal Trail between Oregon and Mexico: Devil's Slide Trail

A paved path through a landscape of coastal erosion: Devil's Slide Trail

After the opening of the Tom Lantos Tunnels, also called Devil's Slide Tunnels—two road tunnels now bypasses a two-lane segment of State Route 1 between Pacifica and Montara in San Mateo County, California—the 1.3-mile-long coastal highway stretch was updated and beautified to become a paved multi-track hiking and bicycling trail with spectacular ocean overlook platforms. It is named Devil's Slide Trail due to the history of rockfalls and landslides that frequently happen along this part of the Californian coast with the San Andreas Fault meeting the Pacific Ocean a few miles further north. A Devil's Slide information panel describes this trail within California's varied and ever-changing landscapes as follows:

The 1.3-mile-long Devil's Slide Trail is built on what was once a treacherous stretch of Highway 1 and the site of frequent landslides, accidents, and closures. When the Tom Lantos Tunnels opened in 2013, the San Mateo County Parks Department began converting this segment of the old highway to this public multi-use, non-motorized trail.
The trail opened in 2014, and offers spectacular ocean views and unparalleled opportunities for viewing birds and marine mammals.
The Devil's Slide Trail is a section of the California Coastal Trail, which will extend 1,200 miles from Oregon to Mexico.

A Devil's Slide vista device with San Pedro Point in the background

Occasional detourings may be required, since Devil's Slide continues to slide into the Pacific Ocean, and the rising ocean keeps attacking the Devil's bottom, accelerating the erosion bottom-up.
 

A bird's-eye view of birds at the Devil's bottom

While strolling or recreating along the easily accessible, well-maintained Devil's Slide Trail between the Pedro Point Headlands and the west-side slopes of San Pedro Mountain, your escape strategies during a strong earthquake are very limited. High walls and slippery slopes on one side; steep, unsafe cliffs on the other, dropping down to a beach or into the pounding ocean surf. If this would be a mine site or a construction area, hard hats would certainly be required. But the Devil's Slide coast, today, is a preserve-like open space celebrating successful restoration efforts to regrow Common Murre colonies. The natural uniqueness, refreshing sea breezes and amazing views lets one forget potential natural hazards.

North Devil's Slide trailhead

Along the trail you will find many interpreted panels informing about Devil's Slide history, the big squeeze, land protection, environmentalist Ollie Mayer, seabird colonies and much more about the local natural history.


Getting to the Devil's Slide trailheads

Devil's Slide Trail is a San Mateo County Park. There are trailheads at either end of the trail, which can be accessed before entering or after leaving a Tom Lantos Tunnel. Parking lots are limited.

The south trailhead is on the west side of Highway 1, about two miles north of the Montara State Beach parking area.

The north trailhead is to your right, after driving southbound on Highway 1 for about one mile from Pacifica. A trail that will connect the north trailhead with Pacifica is in planning.

Beachside geology: the Kehoe cliffs

Contorted and faulted white dikes cutting through Granite of Inverness Ridge

Most people visit a beach to play and recreate. Kehoe Beach invites visitors to additionally experience cliffs-on geoscience and to explore the natural history of the San Andreas Fault Zone. Kehoe Beach is an excellent, easily accessible field-trip destination in western Marin County north of San Francisco. Doris Sloan featured the geology of the Kehoe region in a recent Bay Nature article [1].

The generalized geologic map of Point Reyes National Seashore after Clark and Brabb (Figure 9-2 in [2]) shows that the area south of the Kehoe Beach Trail consists of late-Pleistocene-to-Recent features: alluvial depositions, beaches and sand dunes; obvious to today's visitor. The units north of Kehoe Trail include Laird Sandstone from the middle Miocene and Salinian granitic rocks from the Cretaceous period; the latter forming the Inverness Ridge.

The Laird Sandstone you can see at the Kehoe trailhead is the oldest of the sedimentary rocks in the area [3]. It originated by deposition when the sea rose or the land sank between 16 to 11 million years ago. On top of it, Monterey Formation sandstone was deposited eight to six million years ago. This formation is distributed from south to north over the Point Reyes Peninsula and exhibits a strong similarity with deposits found in Monterey further south. Therefore, it is assumed that Point Reyes, riding north on the eastern edge of the Pacific plate, was located in the Monterey area up until Miocene times [4]. Layers of uplifted Monterey Formation are exposed north of where the Kehoe Beach Trail traverses a sand-dune ridge to meet the shoreline.

Monterey Formation exposure next to Kehoe Beach Trail

Walking north on Kehoe Beach, you soon will find yourself in front of Laird Sandstone cliffs. Continue north and you continue walking back in time, geologically speaking. The gray granitic rocks to the north have been uplifted relative to those of the sandstone cliffs on the south. A small landslide marks the contact zone, a vertical fault between the sandstone cliffs and the Salinian granitic formation. The latter is also called the Granite of Inverness Ridge, which is plutonic igneous rock that formed about 85 million years ago (within the Cretaceous period between 145 to 66 million years ago) deep in the crust from magma produced during long-ago plate collisions [1].

A striking feature of the lower granitics are the bold white dikes cutting through them in puzzling, strangely twisted patterns (top picture). Doris Sloan helps us to solve the puzzle, or, at least, to make sense of it in plate-tectonic terms [1]:

If Yosemite comes to mind when you think of granite, this outcrop of granitics north of the landslide may puzzle you. No high cliffs of light-colored rock; no straight white dikes—bands of lighter, younger rock—cutting through the granitic rocks for hundreds of feet. Here you see a jumble of multicolored rocks, cut by contorted and faulted white dikes. The rocks are so mixed up and fractured, so different, that you would think there is no connection to Yosemite. However, these granitic rocks once formed the southernmost end of the Sierra Nevada, about 350 miles south of their present location; they ended up on the other side of the San Andreas Fault system and got hijacked by the Pacific plate on its way north.

Vertical fault between formations

Point Reyes is a merger of different points in time and space. You can be sure that this peacefully appearing location has gone through lots of sinking, lifting and shifting—and still does. Rising sea levels and predicted earthquakes will have their future impact on this preserve.

Keywords: geology of Point Reyes, shifting ground, deposition, sedimentation, earth science, plate tectonics.

References and more to explore
[1] Doris Sloan: A Trip Through Time On the Pacific Plate. Bay Nature July-September 2015, pp. 12-16.
[2] Geology at Point Reyes National Seashore and Vicinity, California [pubs.usgs.gov/of/2005/1127/chapter9.pdf].
[3] The Laird Sandstone [www.marin.edu/~jim/ring/ptreyes/pr_tl.html].
[4] Monterey Formation [www.marin.edu/~jim/ring/ptreyes/pr_tm.html].

Kehoe Beach Trail

Kehoe Beach

Kehoe Beach Trail is a short, level, 0.6-mile-long path connecting the Kehoe trailhead on Pierce Point Road with the sandy beaches and dramatic cliffs of the northern section of Point Reyes National Seashore, a California coast preserve. The trail and beach is named after the nearby Kehoe Ranch. The hiking trail follows a broad and low valley with views of grassy hills, coastal scrub and sandstone rocks. Not very far from the trailhead, you will approach patches of marshland. A freshwater creek is coming into view. Depending on the seasonal weather and tide conditions, the creek ends at Kehoe Beach, as in the picture above, or may be mixed with saltwater during a storm. The sand dunes next to the creek are part of a sensitive wildlife area, in which western snowy plovers nest and hatch between March and September.

Freshwater creek parallel to Kehoe Beach Trail

As the trail approaches the beach and the sound of the surf becomes louder, you will get to a bench in the sand. The beach and ocean is only a few steps away. The loose-sand trail traverses a low sand-dune ridge with a right-side exposure of Monterey Formation sandstone. You'll find more interesting outcrops, formations and crumbling cliffs while strolling north to where the beach terminates into a rock-strewn, pounding-surf zone after about half a mile. When the fog does not blow in, you may be able to locate—across a fifteen-mile-stretch of sand beaches with treacherous surf—the rock peninsula with the Point Reyes Light in the distant south.

Getting to Kehoe Beach Trailhead
Head north from the Bear Valley Visitor Center. After 0.2 miles turn left and go north on Bear Valley Road towards Inverness. Continue northbound alongside Tomales Bay on Sir Francis Drake Boulevard. After one mile from Inverness the road bends left, ascending Inverness Ridge. At the junction in the mountains, take right-side Pierce Point Road, leading north and around Tomales Bay State Park to the junction with the road to Hearts Desire Beach. Continue on Pierce Point Road, pass a couple of historic farms and also pass the Abbotts Lagoon Trailhead. Kehoe Beach Trailhead is coming up on your left about two miles north of the latter. Kehoe Beach Trail is open to pets-on-leash walking and bicycling—mud and sand bicycling it is over some sections of the mostly single-track trail.

More to explore:
[1] San Francisco Bay Area Hiker: Kehoe Beach, Point Reyes National Seashore, National Park Service, Marin County [www.bahiker.com/northbayhikes/kehoe.html].
[2] Nature Dogs: Kehoe Beach, Point Reyes National Seashore [www.naturedogs.com/hike.php?id=286].
[3] Geology at Point Reyes National Seashore and Vicinity, California [pubs.usgs.gov/of/2005/1127/chapter9.pdf].

A short trail to learn about the long history of the Coast Miwok people: the Kule Loklo Trail near Olema

Kule Loklo Trail near the Bear Valley Visitor Center

The 0.3-mile-long Kule Loklo Trail west of Olema leads from the Bear Valley Visitor Center to a rebuilt Coast Miwok Indian Village—maintained entirely by volunteers—in what is now the Point Reyes National Seashore, a park preserve in Marin Country north of San Francisco, California. Kule Loklo means Bear Valley in the Coast Miwok language, which is no longer spoken. But the culture and spirit of the People of the Coast is still alive and of interest to everyone looking for a balanced life style within natural richness and restraint.

The trail begins by underpassing some low-leaning trees. It continues between a left-side meadow and trees to its right. An interpreted panel along the trail informs:

Kule Loklo acorn granary

The abundance of plant life in the nearby forests nourished the Coast Miwok for generations. Each plant offered a wealth of uses. Oak trees dropped their bounty of acorns in the fall and were an important food source as the earth rested during the winter. Tule grass from surrounding marshes was woven into mats or bundled together for canoes or kotcas (houses).

Today, this is not an area to experience typical California oak woodland anymore. Before arriving at the village, you will walk in front and then pass through a stand of tall eucalyptus trees (blue gum trees)—native to Australia, but alien to pre-Columbian Native Americans. In the village at the trail end, there is a reconstructed granary, illustrating how the Coast Miwok stored acorns (umpa), gathered in fall, for future use and protected from insect pests and other animals. During spring and summer months, they relocated from their inland villages to the estuaries and the coast to catch salmon and to gather seaweed, clams, abalone and other seafood during low tides. Beads and ceremonial regalia, which they made from shells, are still found in the area.

Kotcas at Kule Loklo

The reconstructed village further consists of shelters and gathering places. There are several conical-shaped kotcas (also spelled kotchas) you or your kids may want to check out by stepping or crawling into. Coast Miwok families of five or more individuals are said to have lived in one such structure. These living structures, according to another panel, were either made of tule grass or redwood bark. While the tule homes lasted for a couple of years, the redwood bark buildings lasted longer. 

Getting to the Kule Loklo trailhead

From Stinson Beach, drive north on Highway 1. At the northern tip of the Bolinas Lagoon, Highway 1 continues in northwest direction through Olema Valley between the Bolinas Ridge and Point Reyes mountain ranges. Just north of Olema, turn left on Bear Valley Road. After about one mile, turn left at the signed junction to get to the Park Headquarters and the visitor center. The Kule Loklo trailhead is to your right, next to the first parking lots you are getting to. 

From Point Reyes Station, drive south on Highway 1. After overpassing a creek, turn right towards Sir Francis Drake Boulevard. After about one mile turn left on Bear Valley Road. Go southbound for about three miles to get to the Park Headquarters junction.

References and more to explore
[1] California Indians: Miwok, Coast & Lake [factcards.califa.org/cai/miwokcoastandlake.html].
[2] John Littleton: Tracing Forgotton Footsteps. MAPOM Blog, July 13, 2015 [blog.mapom.org/category/coast_miwok]

Sandy Meadow Trail in the Sierra Nevada

Fluted volcanic cliff structures south of Sandy Meadow Trail

The Sandy Meadow Trail is located north of Highway 4 between Ebbetts Pass and the Calaveras Big Trees State Park, Sierra Nevada, California. Despite its name, this single-path hiking and horseback riding trail winds through coniferous forest; only short sections traverse meadows. The trail connects the corrals on Highway 4 with Wheeler Lake further west.

Forest-framed sandy meadow

From its trailhead at Highway 4, Sandy Meadow Trail ascends northbound and soon enters the Mokelumne Wilderness. At the Stanislaus National Forest sign the trail bends west and stays fairly level for about 1.5 miles. Without challenges you should reach the first sandy meadow. The trail crosses Sandy Meadow Creek and then enters forest again. Some rock outcrops next to the trail offer nice views of the Sierra including the mountains surrounding the Blue Lakes area further north.

The trail continues westwards, up and down through forest; boulder-strewn in places. My favorite rock along the trail is a granite pyramid pointing skyward in an opening of the at intervals dense pine and fir forest. 

A natural pyramid, I assume

Frequently, you will encounter fallen trees. The trail is cleared of some trees and occasionally a path has been cut through a trunk. Other trunks you have to climb over or find your way around. At several locations, the trail crosses a creek or a meadow. It can be difficult to recognize the path in areas, where the trail leads over the sandy soil of a meadow or traverses wet and boggy patches. You want to memorize the location of the trail point at the forest opening to avoid track searching on your return trip. Sandy Meadow Trail is marked with little discs showing a silver-white backpacker with a hiking stick on blue ground (see picture above). These markers are fixed to trees. They guide hikers in both directions of the trail, but are too small to be seen across larger forest openings.

Hiking towards the upper end of Jackson Canyon, you are going to traverse meadows with stunning views to the south. My topographic hiking map—the National Geographic “Carson-Iceberg, Emigrant and Mokelumne Wilderness Areas” trail map—shows the location names Cliff Meadow and Avalanche Meadow. From east to west, the south is “blocked” by a steep, continuous cliff wall: a slightly curved cirque of fluted volcanic cliffs, a fraction of which is shown above. Darkening clouds moving over the cliffs from the southwest may easily cause a haunted feel in a psychically sensitive visitor. The cliffs show overhangs and caves. I am sure bird watchers will get their thrill exploring the avian life of these cliffs. Up to now, I didn't succeed in resourcing any ornithological information concerning this amazing cliff structure. It looks like a paradise for eagles and vultures. North-facing however, the livelihood of this vertical otherworld may be limited by enduring snow and ice curtains after a long, precipitation-rich winter season.

Chances are you or your party will be on their own in this cliff backyard, since Sandy Meadow Trail is not as frequently traveled as many other trails in the Sierra.

Getting to the Sandy Meadow Trailhead

From Minden, Nevada, take Highway 88 south to Woodfords. Turn left on Highway 89 to Markleeville. Continue alongside the East Fork Carson River and manage all the switchbacks uphill and over the Pacific Crest. From Ebbetts Pass, continue westbound past the Hermit Valley Historical Landmark. Find the Sandy Meadow Trailhead sign on the left side of the narrow road, just a short distance west from the Mosquito Lakes.  

From Arnold, California, go eastbound on Highway 4 past Calaveras Big Trees State Park and Lake Alpine. Find the Sandy Meadow Trailhead sign on the right side of the narrow road, about three miles east from the Wood Chuck Basin Trailhead.  

From Carson Pass via PCT to upper Forestdale Creek and Blue Lakes

Forestdale Creek pond with Elephants Back's south side

The hiking distance along the Pacific Crest Trail (PCT) across the Mokelumne Wilderness between Carson_Pass and Ebbetts Pass is about twenty five miles; one way. For non-backpackers, an out-and-back trip to Blue Lake viewpoints is a great day-hike option. This trip rewards hikers with scenic views of the ancient Elephants Back lava dome and various shallow ponds within an subalpine landscape east of the Sierran crest.

PCT traversing slopes east of Elephants Back

The trip begins at the Carson_Pass_Information_Center by following the Tahoe to Yosemite Trail (TYT) and the PCT with ascending switchbacks southbound through dense forest that is thinning while approaching shallow Frog Lake. In a short distance from the junction with the short side-path to Frog Lake you will arrive at the next junction where the TYT continues on toward Winnemucca Lake, Round Top Lake and Fourth of July Lake and where the PCT veers left and south around the steep east side of Elephants Back. Soon, the mountains west of Markleeville and ranges of the Carson-Iceberg Wilderness are coming into view.

Forestdale Creek and Forestdale Divide Road

From the crest next to Elephants Back, the PCT steeply drops into the basin of Forestdale Creek. The PCT crosses at least three tributary streams of Forestdale Creek—interspersed with rock outcrops—before winding uphill towards a beautiful, lightly forested area with meadows and ponds, backdropped by granite walls and talus slopes. The top picture shows one of the Forestdale ponds and the southeast-facing walls of Elephants Back. After about five miles from Carson Pass, you will reach a trail crossing. A few steps to the left is an unmarked trailhead on Forestdale Divide Road. To the right, a trail leads down into Summit City Canyon.

The trail marker at the crossing gives a distance of eighteen miles to Ebbetts Pass. Continuing southbound toward that direction, the PCT soon intersects Forest Divide Road, from where it traverses open slopes with sweeping canyon views. Then, the PCT enters the conifer forest northeast of the Upper Blue Lake. After about a mile, the lake is shining through the forest trees.  

Upper and Lower Blue Lake