Sequit Point is part of Leo Carrillo State Park. If you use the State Park parking lots there is an entrance fee. If you can find parking along PCH be sure to follow the parking restrictions and use extreme caution crossing the road. Use appropriate trails going down to the caves. Be sure to evaluate the tides and waves to be sure it is safe to go down to the caves.
Sequit Point is an outcropping of the Upper Topanga Formation. This formation is a light gray sandstone that was deposited in the Middle Miocene (17-13 million years ago). These sediments were deposited in a shallow marine environment. This sandstone formation is hard and relatively resistant to erosion.
At this location, the sandstone has been eroded into a little depressions that look a bit like honeycombs. These honeycombs erode out through a physical erosional process. A physical erosional process is one where particles are removed from the rock without changing their chemical makeup. This is as compared to chemical erosion where the rock’s chemical makup is changed during the removal of particles from the main body of the rock.
The physical erosion on these rocks starts with the repeated sea spray. Since these droplets of water have been sprayed on the rocks instead of evaporating, they still contain dissolved salt. The spray wets the surface of the sandstone and some percolates into the rock in between the grains. When the rock drys out, the water evaporates, but the salt is left behind. As the salt crystals grow, they expand putting pressure on the grains of sand around them. At some point, the pressure is enough to pry out the individual sand grain.
This is a self perpetuating process that feeds on itself. The removal of grains creates a depression where more sea mist collects relative to flat areas. This concentrates the growth of crystals in the depression where additional grains are pried out creating a larger depression. And the growth continues. Sometimes a white salt crystal growth is seen in and around the depressions.
For those who would like a more advanced description here is a quote from www.tafoini.com: “Salt crystals preferentially press against confining walls of rock capillaries because they are not able to expand into smaller, unfilled, capillaries due to their atomic nature. The larger capillaries are preferentially filled because less chemical free energy is needed to fill larger spaces than smaller ones. After the larger capillaries become completely filled with salt crystals, the crystals are less likely to expand into unfilled smaller capillaries and instead exert force onto the surface of the confining walls. The pressure in the pore space caused by the precipitation of salt crystals increases as the crystals grow to the threshold of bond rupture (which varies by mineral type) or to the point that the chemical potential is raised high enough to promote salt crystal growth into smaller capillaries.“
The result are the multitude of small cavities in the rock face. This grouping of small cavities is called tafoni or honeycomb weathering. At other locations where tafoni form, these cavities can merge together to form caves.
Logging requirements:
Send me a note with:
- The text "GC6A7BM Sequit Point Tafoni" on the first line
- The number of people in your group (put in the log as well).
- Feel the inside of some of the tafoni or caves. What are the size of the eroded grains?
- Compare the size of the eroded grains to the size of the grains in the sandstone.How deep and wide are the tafoni?
- In a 1 square foot area, about how many tafoni are there?
- Why do you think there aren’t more tafoni or they haven’t grown bigger?
The following sources were used to generate this cache:
- Dibblee, T.W., and Ehrenspeck, H.E., 1990, Geologic map of the Point Mugu and Triunfo Pass quadrangles, Ventura and Los Angeles Counties, California: Dibblee Geological Foundation, Dibblee Foundation Map DF-29, scale 1:24,000
- County of Los Angeles Department of Regional Planning 2014, Santa Monica Mountains Local Coastal Program Technical Appendices, January 2014
- Hall, Clarence A. Jr., Introduction to the Geology of Southern California and Its Native Plants. University of California Press. Copyright.