Lithology and Stratigraphy
This subsection contains predominantly Pliocene nonmarine sediments that are only slightly consolidated. There are smaller areas of Quaternary terraces and recent alluvium.
This subsection is on very gently sloping terraces that are dissected and eroded to form gently sloping to moderately steep slopes. There are small areas of recent floodplain and terraces along streams that cross from mountains of the northern California Coast Ranges to reach the Sacramento River. The subsection elevation range is from about 100 to about 200 feet. Fluvial erosion and deposition are the main geomorphic processes.
The soils are mostly Typic Palexeralfs, Typic Haploxeralfs, Entic and Typic Chromoxererts, and Typic Argixerolls. Soils on recent alluvium are Typic Xerorthents, Typic Xerochrepts, Aeric Haplaquepts, and Haploxeralfs. Most of the soils are well drained, but some in recent alluvium are somewhat poorly drained. Bicarbonate weathering and leaching and accumulation of clay in subsoils are the main pedogenic processes in the terrace soils. Soil temperature regimes are thermic. Soil moisture regimes are mostly xeric, with some aquic on floodplains.
The predominant natural plant communities are Needlegrass grasslands and Blue oak series. Fremont cottonwood series occurs along streams.
Characteristic series by lifeform include:
- Grasslands: California annual grassland series, Needlegrass series.
Vernal pools: Northern hardpan vernal pools.
Forests and woodlands: Blue oak series, Fremont cottonwood series.
The mean annual precipitation is about 20 to 25 inches. It is practically all rain. Mean annual temperature is about 59° to 62° F. The mean freeze-free period is about 250 to 275 days.
Streams in this subsection drain to the Sacramento River. All but the larger streams are generally dry during the summer. There are no lakes, but there is temporary ponding in vernal pools on Pleistocene terraces.