Subsection M261Ar
Trinity Mountain - Hayfork

Lithology and Stratigraphy.  This subsection is dominated by Mesozoic granitic rocks and Paleozoic to Jurassic gabbro and metavolcanic and metasedimentary rocks, including serpentinized peridotite, of the Sawyers Bar and Western Hayfork terranes.  It is between Siskiyou fault on the east and the Salt Creek fault on the west and southwest.  The granitic rocks are generally quartz diorite to slightly more silicic rocks.  There are nonmarine sedimentary rocks of the Weaverville Formation in a graben occupied by Hayfork Valley.  Quaternary alluvium occurs in Hayfork Valley and along the Trinity River and its tributaries.

Geomorphology.  This is a subsection of mountains with rounded summits, steep sides, and narrow canyons.  The mountains are generally aligned toward the north, although the Hayfork graben and Hayfork Divide are aligned toward the northeast.  Slopes in the Hayfork graben are nearly level on floodplains to moderately steep on the Weaverville Formation.  There are floodplains and terraces in Hayfork Valley and along the Trinity River and its tributaries.  The elevation range is from about 1800 feet up to 6273 feet on Hayfork Bally.  Mass wasting and fluvial erosion are the main geomorphic processes.

Soils.  The soils are mostly Dystric and Dystric Lithic Xerochrepts and Ultic Haploxeralfs, plus shallow Dystric Lithic Xerochrepts on granitic rocks and Mollic Haploxeralfs on serpentinized peridotite.  Palexeralfs and Mollic Haploxeralfs predominate on the Weaverville Formation.  Soils at higher elevations are mostly Typic and Lithic Xerumbrepts.  Lithic Xerochrepts, Lithic Argixerolls, and Mollic Haploxeralfs occur at lower elevations, adjacent to the Great Valley.  Soils on alluvium in Hayfork Valley are mostly Argixerolls and Aquic Xerofluvents.  The soils are well drained, except somewhat poorly drained soils in Hayfork Valley.  Soil temperature regimes are mostly mesic, with some frigid at higher elevations and thermic at lower elevations.  Soil moisture regimes are xeric.

Vegetation.   The predominant natural plant communities are Douglas-fir series, Mixed conifer series, Douglas fir - ponderosa pine series, and Ponderosa pine series.  White fir series is predominant at higher elevations.  Jeffrey pine series occurs on serpentinized peridotite.  Oregon white series is predominant in Hayfork Valley.  Chamise series and Mixed chaparral communities occur at lower elevations.

Characteristic series by lifeform include:
Grasslands: California annual grassland series, Montane meadow habitat.
Shrublands: Brewer oak series, Bush chinquapin series, Chamise series, Greenleaf manzanita series, Holodiscus series, Huckleberry oak series, Tobacco brush series, Wedgeleaf ceanothus series, Whiteleaf manzanita series.
Forests and woodlands: Black oak series, Birchleaf mountain-mahogany series, Canyon live oak series, Douglas-fir series, Douglas-fir - ponderosa pine series, Foothill pine series, Interior live oak series, Jeffrey pine series, Knobcone pine series, Mixed conifer series, Oregon white oak series, Ponderosa pine series, White alder series, White fir series.

Surface Water.  Runoff is rapid.  Most of it drains to the Trinity River and its  tributaries.  Runoff from the southeast end of the subsection drains to Cottonwood Creek, a tributary of the Sacramento River.   The streams are mostly perennial on the west, and many of the smaller ones are ephemeral on the east.  There are no lakes in the subsection.