Santa Rosa Plateau Vernal Pools EarthCache
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Santa Rosa Plateau Vernal Pools
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The vernal pools of the Santa Rosa Plateau are the only known examples of Southern Basalt Flow Vernal pools. The basalt on the Santa Rosa Plateau is the key to the large number of vernal pools there, since intact basalt is nearly impervious to penetration by water. Only 14 vernal pools survive in Riverside County; 13 of them are protected on the Santa Rosa Plateau. Of the 13 pools at the Santa Rosa Plateau, eight are on Mesa de Burro, four are on Mesa de Colorado and one is on Mesa de la Punta.
The basalt came from a spreading center that opened up when a part of the North American Plate was torn away from the Oceanside / Camp Pendleton / San Onofre Area by the movement of the Pacific Plate about ten million years ago. That part of the North American Plate was carried north and rotated to form the western part of the Transverse Ranges, including the Channel Islands / Santa Barbara area. That basalt flooded southern Orange County and southwest Riverside County, and eventually covered the nearly-flat landscape, producing an almost absolutely-flat landscape.
The surface of the basalt had many small (~inch length) irregularities. Rain lingered in the small depressions, and the acid produced by dissolved carbon dioxide from the air weathered those depressions more than the surrounding small projections. Water runoff carried away most of the weathered basalt, deepening the depression.
The depressions grew with every rainfall, merging with neighboring small depressions over and over again over millions of years. Eventually, the surface was covered with large depressions ~0.5 mile wide, with relief of typically 100 feet elevation difference between the center of the depression and the rims. The Main Pool on the Vernal pool Trail is the best place to see a depression with a nearly-intact rim. The small pools near the Vernal pool Trailhead have had their north and south rims removed by erosion; only their west and east rims are still intact.
When most of the basalt area was intact, perhaps in the period from two to ten million years ago, it is possible that the depressions contained permanent lakes, not vernal pools, and the Santa Rosa Plateau area was a Land of a Thousand Lakes, and not a vernal pool area. However, no one knows for sure what the fluvial regime was prior to the erosion of most of the basalt.
Once most of the basalt covering the entire area was removed by erosion, leaving the remnant basalt in mesas with steep sides, each of the remaining depressions was eventually deeply cut by stream erosion, initially in a single location. This is the first step of the same process that removed the surrounding basalt. This drastically lowered the water level in each depression, and resulted in the creation of vernal pools. At the Santa Rosa Plateau, the pool depths are measured in inches, not feet.
That water has only two ways out: evaporation or percolation. Which is most important depends on the rate of those two processes.
Data shows that percolation is the most important process. Without further rainfall, the pool depth declines by one inch per week, independent of the evaporation rate. The decline of one inch per week does not depend on season, the air or water temperature, or how windy it is.
This write up is condensed from the works of Tom Chester.
You do not have to enter the Santa Rosa Plateau Ecological Reserve to view these two Vernal Pools. Both are viewable from the highway, Via Volcano.
The first vernal pool can be seen by parking at 33 30.507N, 117 17.646W.
This is a public highway, so use caution and pull all the way off into the pullout.
Looking towards the East and just past the fence, you will see the outline of the pool. Depending on the time of year, this can be either dry, or wet.
Parking for the second vernal pool is located less than .2 miles south at 33 30.326N, 117 17.596. Like the first pool, you will look towards the East and just beyond the fence.
NOTE -- Physical caches are not permitted in the Santa Rosa Plateau.
To claim this cache – log the cache, and send us an email answering the following:
1. What the water level was at the time you saw the pools? Estimate
2. What color were the flowers/plants inside the pool
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