Subsection M261At
Trinity Alps

Lithology and Stratigraphy.  This subsection is dominated by Paleozoic metavolcanic rocks of the Salmon Hornblende Schist and metasedimentary and metavolcanic rocks of the  Abrams Mica Schist, or Grouse Ridge Formation, that are between the Trinity fault on the east and the Siskiyou fault on the west.  The age of metamorphism is Devonian, which is when the Trinity fault was active.  These older rocks were intruded by Mesozoic granitic rocks, generally quartz diorite to slightly more silicic rocks.  Quaternary glacial till and outwash occur on high plateaus and occupy many valleys.

Geomorphology.  This is a subsection of mountains with rounded ridges, steep sides,  and narrow canyons, except at higher elevation where glaciers have carved cirques, aretes, horns, and U-shaped valleys.  Glacial moraines are extensive and outwash terraces occur from moraines down-valley into adjacent subsections.  The elevation range is from about 4000 feet up to 9002 feet on Thompson Peak.  Mass wasting and fluvial erosion are the main geomorphic processes.  Glacial processes have been prominent during the Pleistocene.

Soils.  The soils are mostly Dystric Xerochrepts and Lithic, Entic, Typic, and Pachic  Xerumbrepts at lower elevations.  Soils at higher elevations are mostly Lithic and Typic Cryorthents and Entic and Typic Cryumbrepts.  Bare rock is common at higher elevations, particularly on granitic rocks.  The soils are well drained.  Soil temperature regimes are frigid and cryic.  Soil moisture regimes are xeric.

Vegetation.   The predominant natural plant communities, from lower to higher elevations, are White fir series, Red fir series, and Mountain hemlock series.   Jeffrey pine series, Foxtail pine series,  and Mixed subalpine forest series occur on serpentinized peridotite.  Vascular plants are absent where bedrock is exposed, except along joints that are prominent in granitic rocks.

Characteristic series by lifeform include:
Grasslands: Green fescue series, Montane meadow habitat, Nebraska sedge series, Rocky Mountain sedge series, Shorthair reedgrass series, Subalpine meadow habitat.
Seeps: Darlingtonia series.
Shrublands: Brewer oak series, Bush chinquapin series, Greenleaf manzanita series, Holodiscus series, Huckleberry oak series, Low sagebrush series, Montane wetland shrub habitat, Mountain alder series, Mountain heather - bilberry series, Sitka alder series, Subalpine upland shrub habitat, Subalpine wetland shrub habitat, Tobacco brush series, Wedgeleaf ceanothus series.
Forests and woodlands: Curlleaf mountain-mahogany series, Douglas-fir series, Douglas-fir - ponderosa pine series, Foxtail pine series, Incense-cedar series, Jeffrey pine series, Lodgepole pine series, Mixed conifer series, Mixed conifer subalpine series, Mountain hemlock series, Red fir series, Subalpine fir series, Western white pine series, White fir series, Whitebark pine series.

Surface Water.  Runoff is rapid.  It drains to the Trinity River and its tributaries, except for a small area in the northwest part of the subsection which is in the South Fork Salmon watershed.  Streams are mostly perennial.  There are many  small lakes, or ponds, in cirque and paternoster basins.