Subsection 263Al
Marin Hills and Valleys

Lithology and Stratigraphy.  This subsection is dominated by Franciscan sedimentary, minor volcanic, and metamorphic rocks of the Central, Coastal, and Eastern Belts.  Most of them are intensely deformed.  The Franciscan strata are poorly dated, but they range from Jurassic through Cretaceous, and in the Coastal Belt, into the early Tertiary.  Ultramafic rocks are scattered through the Franciscan Complex. There are small areas of Pliocene andesite on the northeast, opposite the Sonoma volcanics, and recent alluvium.

Geomorphology.   This is a subsection of mountains and hills with rounded ridges, steep and moderately steep sides, and narrow canyons.  Most of the mountains are elongated in north-northwest to northwest directions, although Mt. Tamalpais is oriented southwest-northeast.  The elevation ranges from sea-level up to 2604 feet on Mt. Tamalpais.  Mass wasting and fluvial erosion are the main geomorphic processes.

Soils.  The soils are mostly Typic Haploxerolls, Entic Haploxerolls, and Ultic Argixerolls, and, in the southwestern part of the subsection, Lithic Haplustolls and Typic and Pachic Argiustolls.  The soil temperature regimes are mostly mesic and thermic, and in the southwestern part of the subsection, isomesic.  Soil moisture regimes are mostly xeric and, in the southwestern part of the subsection, ustic.  Udic soil moisture regimes are common on the lower slopes in steep canyons.

Vegetation.   The natural plant communities, from isomesic and udic to thermic and xeric soil temperature and moisture regimes, are mainly Redwood series, Douglas-fir - tanoak series, and Coast live oak series.  Grasslands are predominantly California oatgrass series or Pacific reedgrass series in the southwestern part and Needlegrass grasslands in the remainder of the subsection.  Chamise series is present on shallow soils.

Characteristic series by lifeform include:
Grasslands: California annual grassland series, Foothill needlegrass series,  Introduced perennial grassland series, Pacific reedgrass series, Purple needlegrass series.
Shrublands: Chamise series, Leather oak series.
Forests and woodlands: Bishop pine series, Black oak series, California bay series, Coast live oak series, Douglas-fir - tanoak series, Mixed oak series,  Redwood series, Sergeant cypress series, Tanoak series, Valley oak series.

Surface Water.  Water runs off rapidly, and most of the smaller streams are dry by the end of the summer, except in the southwestern part of the subsection.  There are no natural ponds or lakes in the subsection, but several reservoirs.