Devil's Slide, positioned between San Pedro Ridge and Montara Mountain, is about half a mile wide and extends 900 feet from the ridge top to the ocean below. Millions of years of upward pressure have 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.
In the past, falling rock from the slide had repeatedly damaged this road, causing Highway 1 to be closed dozens of times since it opened in 1937. This destructive cycle continued until it finally closed on March 25, 2013, the day the Tom Lantos Tunnels opened to provide motorists with a safer route through the mountains. Landslides occur where the sedimentary rock has been thrust over the granitic rock, causing broken, weakened ground.
Walk the trail for a demonstration of coastal erosion geology. Notice how different the cliffs at the south end look compared to those at the north end. The weathered rock face to the south is the granitic rock of Montara Mountain, the same rock found in the Sierra Nevada Range. In contrast, the rough layers of sedimentary rock at this northern end were once the 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 result of this effect can be seen in the cliffs along the trail near the posted coordinates. You can see another fine example of this effect if you look out across the water toward San Pedro Rock!
To log this Earthcache, simply email me your answers to the following questions:
Please do not post the answers in your log entry.
Question 1. Look over the cement barrier on either side of the trail (no need to physically cross it) at any fallen parts from the sedimentary rock. Describe the way the rock has fallen off. Has it crumbled away in clumps like dirt or broken off cleanly?
Are there more large chunks of rock or are there more small stones or finely ground pebbles?
Question 2. What colors do you see among the sedimentary rock?
Question 3. Look at the sedimentary rock along the East side of the trail. Over millions of years forces deep within the earth have given the rock a striped/ribbon effect. Which direction are these stripes/ribbons running? Vertical, horizontal, diagonal?
Congratulations to deadley on FTF!