Subsection 263Ab
Northern Franciscan

Lithology and Stratigraphy.  This subsection is dominated by Jurassic and Cretaceous Franciscan sedimentary, minor volcanic, and metamorphic rocks of the Eastern Belt.  They are intensely folded and faulted.  Plio-Pleistocene marine and nonmarine sediments are the predominant strata between the Lost Man fault and the ocean, south of the mouth of the Klamath River.  There are small areas of recent alluvium along the Klamath and Smith Rivers and Pleistocene gravels on mountain ridges between the Klamath River and the Lost Man fault.

Geomorphology.  This is a subsection of mountains with rounded ridges, steep and moderately steep sides, and narrow canyons.  Most of the mountains are elongated in north-northwest to northwest directions and have subequal summits with increasing elevation toward the interior.   Plio-Pleistocene sediments southwest of the Lost Man fault have been elevated nearly 1000 feet above sea-level and highly dissected. The subsection elevation range is from sea-level up to 3092 feet on School House Peak.  Mass wasting and fluvial erosion are the main geomorphic processes.  The density (area/area) of landslides is very high.

Soils.  The soils are mostly Typic Humitropepts and Mollic Hapludalfs.  Typic Tropohumults and Ultic Hapludalfs are the common soils on Plio-Pleistocene sediments.  Typic Tropofluvents and Cumulic Humaquepts occur in alluvium along the major streams.  Most of the soils are leached free of carbonates and the older ones are strongly acid.  The soil temperature regimes are predominantly isomesic.  Soil moisture regimes are mostly udic with some aquic.

Vegetation.   The predominant natural plant community is Redwood series.  Douglas-fir - tanoak series occurs on upper slopes and ridges.  Sitka spruce occurs along the coast.  There are patches of Black oak series, Oregon white oak series, and Pacific reedgrass series on south-facing slopes and ridges.  Red alder series is common in riparian areas.

Characteristic series by lifeform include:
Grasslands: Introduced perennial grassland series, Pacific reedgrass series.
Saltmarsh vegetation: Pickleweed series, Saltgrass series.
Shrublands: Salal - California huckleberry series.
Forests and woodlands: Black oak series, California bay series, Douglas-fir - tanoak series, Grand fir series, Red alder series, Redwood series, Sitka spruce series, Tanoak series, Western hemlock series.

Surface Water.  Runoff is rapid and many of the smaller streams are dry by the end of the summer.   The Klamath and Smith Rivers, which drain from the Klamath Mountains, cross this subsection to reach the ocean.  Natural lakes are absent, other than minor and temporary ponding by landslides.