Lithology and Stratigraphy
This subsection is predominantly folded, faulted, and generally metamorphosed sedimentary and volcanic rocks of the Franciscan Complex that are largely covered by Pleistocene sediments, dune sand, and, on the bay side of the peninsula, recent alluvium.
This subsection is just northeast of the San Andreas fault. San Bruno Mountain, San Miguel Hills, Twin Peaks, and a few low hills of Franciscan rock are surrounded by a plain that is Quaternary marine and dune sand deposits. There is some recent alluvium on the bay side of the peninsula. The subsection elevation range is from sea-level to a little over 1000 feet on San Bruno Mountain. Fluvial erosion is the main geomorphic processes. Fluvial deposition is an important process on recent alluvial plains in and above the tidal zone on the bay side of the peninsula. Wind has been an active geomorphic agent and still is on the west side of the peninsula.
The soils are mostly Lithic Haplustolls and Pachic Argiustolls on bedrock, Ustic Dystropepts on marine sediments, and Psamments on stabilized dune deposits. Xerorthents and Pachic Argixerolls predominate on bedrock and marine terraces on the bay side of the peninsula. Soils in alluvium adjacent to the bay are Fluvaquents, and Hydraquents. Much land has been created in the last century by the addition of fill to the bay. Most, but not all, of the soils are leached free of carbonates. Soluble salts accumulate in some soils that are in or near the tidal zone. The soils are mostly well drained, except some in alluvium adjacent to the bay. The soil temperature regimes are isomesic, and possibly some thermic on the bay side of the peninsula. Soil moisture regimes are mostly ustic, but xeric on the bay side of the peninsula.
The predominant natural plant communities are California oatgrass series and Coast live oak series on hills, with California sagebrush series on shallow soils, and California oatgrass series on alluvial plains. The dunes support a succession of plant communities, from bare dune through herbaceous communities and Coyote brush series. There is some Pickleweed series in estuaries along San Francisco Bay.
Characteristic series by lifeform include:
- Dune vegetation: Dune lupine - goldenbrush series, Native dunegrass series, Sand-verbena - beach bursage series, Yellow bush lupine series.
Saltmarsh vegetation: Cordgrass series, Ditchgrass series, Pickleweed series, Saltgrass series, Sedge series.
Grasslands: California annual grassland series, California oatgrass series, Pacific reedgrass series.
Shrublands: Blue blossom series, California sagebrush series, Coyote brush series.
Forests and woodlands: California bay series, Coast live oak series.
The mean annual precipitation is about 20 to 25 inches. It is practically all rain. Summer fog is common. Mean annual temperature is about 56° to 58° F. The mean freeze-free period is about 250 to 300 days.
Runoff is rapid from the hills and slow across the alluvial plains. Streams are dry through most of the summer. There is little overland flow from the area of stabilized dunes. Natural lakes are absent, other than ponding behind sand dunes.