Lithology and Stratigraphy
This subsection contains predominantly Eocene, Miocene, and Pliocene nonmarine sedimentary rocks. There are smaller areas of Plio-Pleistocene terraces and recent alluvium.
This subsection is on gently sloping to moderately steep hills and dissected terraces. There are small areas of floodplain and recent terraces along streams that cross from mountains of the central Sierra Nevada to reach the Sacramento and San Joaquin Rivers. The subsection elevation range is mostly about 200 to 600 feet, but up to 1211 feet on Valley Springs Peak. Fluvial erosion and deposition are the main geomorphic processes.
The soils are mostly shallow Typic and Ultic Haploxerolls, shallow Typic Xerochrepts, Lithic Mollic Haploxeralfs, and Ultic Palexeralfs. On Plio-Pleistocene terraces there are Haploxeralfs, Palexeralfs, and Durixeralfs. Fluventic Haploxerolls and Aquic Xerofluvents are common on floodplains. 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, plus accumulation of silica in the subsoils of terrace soils. Soil temperature regimes are thermic. Soil moisture regimes are xeric, with some aquic on floodplains.
The predominant natural plant communities are Needlegrass grasslands. There is some Blue oak series, and Northern claypan vernal pools are common. Fremont cottonwood series occurs along streams.
Characteristic series by lifeform include:
- Grasslands: California annual grassland series, Purple needlegrass series.
Vernal pools: Northern claypan vernal pools.
Forests and woodlands: Fremont cottonwood series, Mixed willow series.
The mean annual precipitation is about 20 to 25 inches. It is practically all rain. Mean annual temperature is about 58° to 62° F. The mean freeze-free period is about 250 to 275 days.
Streams in this subsection drain to the Sacramento River. The Cosumnes, Mokelumne, Stanislaus, and Tuolumne Rivers cross the subsection. 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.