Siesta Valley
Map SatelliteA readily observable syncline in this valley makes it of particular value for educational use. A trough-shaped valley with relatively steep slopes, 40 - 65 degrees, the synclinal axis follows the valley floor. The non-marine sedimentaries of the Siesta formation, claystones, siltstone and fine-grained sandstones dating to the Pliocene, lie conformably on the lower Pliocene Moraga formation, primarily andesite with interbedded clastic rocks. The Siesta rocks were deposited in the fresh-water lakes formed by the preceding Moraga lava flows. These beds, with a dip angle often more than 80 degrees, have been tightly folded into a U-shaped trough. Some invertebrate fossils have been found in the Siesta formation.
The Siesta formation has poor slope stability and there have been numerous landslides in the valley, often obscuring the bed outcrops. Soils on the upper slopes are Los Osos clay loam; on the lower slopes, Diablo clay and alluvium deposits.
Vegetation is predominantly grassland, with some northern coastal scrub and a mixed evergreen forest. Eucalyptus are present.
Integrity: Roads, powerlines, etc., are found in the valley. A freeway crosses the syncline.
Use: Portion private. Educational, research, observational.
Ref: Case, J. E. 1963. Geology of a Portion of the Berkeley and San Leandro Hills, California. Ph.D. Dissertation, Unpub., Univ. of Calif., Berkeley.
March 1976
Inventory of California Natural Areas
Revision © 2009 Steven Louis Hartman