Lithology and Stratigraphy
Late Quaternary alluvium is predominant. There are small areas of Pliocene and Pleistocene nonmarine sediments.
The alluvial plain is mostly gently sloping to nearly level floodplain and alluvial fans. There is a small area of low, moderately steep hills between the Santa Clara and San Benito Valleys, and a small area of Quaternary stream terraces along the San Benito River. The subsection elevation range is from sea-level up to about 250 feet on the alluvial plains and up to about 1000 feet on the hills west of Hollister. Fluvial erosion and deposition are the main geomorphic processes.
The soils are mostly Typic Xerorthents, Typic Calciaquolls, Typic and Mollic Haploxeralfs, Fluvaquentic Haploxerolls, Typic Pelloxererts, and Typic Palexeralfs. On hills they are mostly Chromic Pelloxererts, Typic Argixerolls, and Typic Palexeralfs. Most, but not all, of the soils are leached free of carbonates. The soils are mostly well drained, but some are somewhat poorly to moderately well drained. Soluble salts accumulate in poorly to somewhat poorly drained soils just above the tide zone, and there are small areas of Natrixeralfs not far above the tide zone. The soil temperature regimes are thermic (nearly mesic). Soil moisture regimes are mostly xeric (nearly ustic).
The predominant natural plant communities are Valley oak series and California oatgrass series or Needlegrass grasslands. Coast live oak series, Needlegrass grasslands, and Blue oak series occur on hills and terraces.
Characteristic series by lifeform include:
- Grasslands: California annual grassland series, California oatgrass series, Purple needlegrass series.
Forests and woodlands: Blue oak series, Coast live oak series, Valley oak series.
The mean annual precipitation is about 12 to 20 inches. It is practically all rain. Mean annual temperature is about 56° to 60° 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.