There is parking in either the State Beach parking lot or on the dirt shoulder along PCH. Follow the old roadbed out to the coordinates. The coordinates take you up to the fence but do not take you past it.
Years ago, Pacific Coast Highway (PCH) followed this abandoned stretch of road around Mugu Rock on the seaward side. However, now PCH has been realigned to cut through the mountain. This realignment was required due to the repeated washing out of the highway around the point. The high erosion rate at the point is due to a combination of ocean wave refraction and geology.
Ocean wave refraction concentrates wave energy on the rocks that stick furthest out in the ocean. Waves hitting the point of Mugu Rock have the most energy because these waves have not slowed down much by the ocean bottom. Then as the waves move past the rock that sticks out in the water, the part of the wave in contact with the rock slows while the wave further away continues at its original speed. The results in the wave angling in toward the rock. Thus the cliff upon which the road was built was subject to the highest amount of erosion and repeatedly was washed out.
Mugu Rock is made up of a Formation that has characteristics that make it erodible. The rock in this area is the Topanga Formation. This formation is composed of layers of sandstone separated by siltstone, shale, or mudstone deposited in the early Miocene. Each layer has a different hardness and erode at different speeds. The sandstone makes up the hard lighter colored layers while the darker layers that break up into small rocks are made up of the siltstone, shale, and mudstone.
The contact between the two types of rocks forms a bedding plane. It is a weak point in the rock. As the material down-slope gets eroded away, the weight of the up-slope rock eventually exceeds the friction and cohesion along a bedding plane, and the rock slides down-slope along the bedding plane. This particular form of slide can be referred to as a rock slide or transitional slide. It is one where material slides along a relatively planar surface with little or no rotational movement.
Logging requirements:
Send me a note with :
- The text "GC4NWQG PCH the Ocean Wins" on the first line.
- The number of people in your group (put in the log as well).
- Examine the rock exposure and determine which type of rock likely was eroded out as the waves hit it?
- Based on 3 which bedding plane are rock slides most likely to occur on?
- Watch the waves as they hit both the rock jutting out into the ocean and as they move onto the beach. Do you agree that the most energetic waves hit the rock furthest out to sea? Do you see the wave refraction?
The following sources were used to generate this cache:
- http://www.indiana.edu/~g105lab/1425chap12.htm
- http://www.ga.gov.au/hazards.html
- http://geology.com/usgs/landslides/
- United States Geological Survey Fact Sheet 2004-3072