Saturday, October 3, 2015

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].

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