Lithology and Stratigraphy
Late Quaternary alluvium predominates in this subsection. There are a few small areas of Quaternary marine sediments, and there are a few small hills of Franciscan formation rocks - some along the Hayward fault and others surrounded by alluvium.
The alluvial plain is mostly gently sloping to nearly level alluvial fans. Hills projecting above the fans are steep to moderately steep. The subsection elevation range is from sea-level up to about 600 feet on hills along the Hayward Fault. Fluvial erosion is the main geomorphic process. Fluvial deposition is an important process on recent floodplain and alluvial fans, but most of the stream sediments are washed across the alluvial plain to estuaries of the San Francisco-San Pablo Bay system.
The main soils are Typic Xerorthents, Entic Haploxerolls, Pachic Argixerolls, Mollic Palexeralfs, Chromic and Typic Pelloxererts, Aquic Natrixeralfs, and Aeric Haplaquepts on the alluvial plain. On hills they are predominantly Lithic Xerochrepts and Typic Argixerolls. Most, but not all, of the soils are leached free of carbonates. Soluble salts accumulate in poorly to somewhat poorly drained soils just above the tide zone. The soil temperature regimes are thermic (nearly mesic). Soil moisture regimes are mostly xeric (nearly ustic), and some aquic.
The predominant natural plant communities are Coast live oak series on hills and California oatgrass series or Needlegrass grasslands on the alluvial plain.
Characteristic series by lifeform include:
- Grasslands: California annual grassland series, California oatgrass series, Purple needlegrass series.
Forests and woodlands: Coast live oak series.
The mean annual precipitation is about 20 to 30 inches. It is practically all rain. Mean annual temperature is about 52° to 56° F. The mean freeze-free period is about 250 to 275 days.
Runoff is rapid from hills, but slow across alluvial plains. All but the larger streams are dry through most of the summer. Natural lakes are absent.