The hike up to the base of the cliff with the caves is short, but steep at times. The easiest trail to the cliff base is almost directly south of the coordinates (34.48259444, -120.23799167), but you will still have to duck under some overhanging limbs and climb the rock a bit to get a good look at the erosional features. While the coordinates take you up on the side of the cliff where some close up views of the tafoni can be found, I will accept answers from anywhere on the cliff face (i.e. you don't have to climb the cliff face if you aren't able to)
These cliffs are made up of the Vaqueros Formation. The Vaqueros Formation was deposited in a near-shore marine environment during the Oligocene (about 26 to 24 million years ago). This formation is almost entirely sandstone with a few areas with well-rounded pebble size clasts. Recent movement along the nearby Santa Ynez Fault have tilted these layers into an anticline to form the Santa Ynez Mountains. In this area, the beds tilt 40 to 60 degrees down into the ground.
Sea spray from the nearby ocean is regularly blown up to these cliffs. Since these droplets of water have been blown here 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 type of physical weathering.
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 cliff face. This grouping of small cavities is called tafoni or honeycomb weathering. The larger cavities, or caves likely formed as the small cavities merged together and grew deeper.
Logging requirements:
Send me a note with:
- The text "GC58RBG Gaviota Wind Caves and 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.
- During your visit, was their any salt crystal growth.
- On the way back down the trail, what are the approximate coordinates of another large outcrop that has tafoni?
The following sources were used to generate this cache:
- Dibblee. 1950. Geology of Southwestern Santa Barbara County California. State of California Division of Mines. Bulletin 150. June.
- Gregory A. Miles and Catherine A. Rigsby. "Lithostratigraphy and Depositional Environments of the Vaqueros and Upper Sespe/Alegria Formations, Hondo Field, Santa Barbara Channel, California." SEPM Core Workshop No. 14. San Francisco, June 3, 1990.
- http://www.tafoni.com/Salt-Weathering.html