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This
subsection is the central part of the northern California Coast Ranges
that is influenced somewhat by marine air, but lacks summer fog.
It has a temperate and humid climate that is cooler during summer and wetter
than the Eastern Franciscan subsection. MLRAs 5c, 5d, 14c,
14d, 15c, and 15d.
Lithology
and Stratigraphy. This subsection is dominated by Jurassic and
Cretaceous Franciscan graywacke, greenstone, chert, melange, and serpentinite
of the Central and other Belts, and, in the southwestern part of the subsection,
Cretaceous marine sedimentary rocks other than those of the Franciscan
Complex. They are intensely folded and faulted. Nonmarine Pliocene
sedimentary rocks are present in some low areas and Quaternary alluvium
is present in the valleys.
Geomorphology.
This is a subsection of mountains with rounded ridges, steep and moderately
steep sides, and narrow canyons, but with several broad valleys (for example,
Round Valley and Potter Valley). Most of the mountains
are elongated in north-northwest to northwest directions and have subequal
summits with increasing elevation toward the interior. The
elevation range is from about 300 feet up to 6175 feet on Big Signal Peak.
Mass wasting and fluvial erosion are the main geomorphic processes.
No other part of California has a greater density (area/area) of landslides.
Soils.
The soils are mostly Dystric Xerochrepts and lesser amounts of Ultic Haploxeralfs,
Xerumbrepts, and Lithic Xerorthents in northern part; and predominantly
Ultic Haploxerolls and lesser amounts of Haploxeralfs, Argixerolls, and
Dystric Lithic Xerochrepts in the southern part of the subsection.
Most of the soils are leached free of carbonates, but the subsoil is calcareous
in some Xerolls. Few surfaces are old enough, because of active erosion,
to have Palexeralfs, Palexerults, or Palehumults. The soil temperature
regimes are predominantly mesic, but includes thermic in the southern part
of the subsection. Soil moisture regimes are xeric (nearly udic or
ustic).
Vegetation.
The predominant natural plant communities are Douglas-fir - tanoak series
with Needlegrass grasslands and Oregon white oak series in the northern
part; and a mosaic of Mixed conifer series, Needlegrass grasslands, Blue
oak series, and Chamise series in the southern part of the subsection.
The mosaic is controlled by slope aspect, lithology, and soils. Blue
oak series is most common on south-facing slopes and at lower elevations.
Needlegrass grasslands are present in areas of Franciscan melange, which
are quite susceptible to mass wasting by slide and flow. Chamise
series prevails on south-facing slopes with shallow soils.
Climate.
The mean annual precipitation is about 35 to 110 inches. Most of
the precipitation is snow at higher and rainfall at lower elevations.
Mean annual temperature is about 40° to 58° F. The mean freeze-free
period is from 125 days at higher elevations to 250 days at lower elevations.
Surface Water. Runoff is rapid and all but the larger streams are dry by the end of the summer. Natural lakes are absent, but there are a few reservoirs in the area.
The area is widely known for extremely high sediment delivery by the
streams (for example, the Eel River).
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