Subsection M261Ad
Siskiyou Mountains

Lithology and Stratigraphy. This subsection is dominated by serpentinized peridotite, gabbro, diabase, and Paleozoic to Jurassic metavolcanic and metasedimentary rocks of Rattlesnake Creek terrane and by Mesozoic gabbro and granitic rocks.  The granitic rocks are generally quartz diorite to slightly more silicic rocks.  Quaternary glacial till and outwash occur in many small areas.

Geomorphology.  This is a subsection of mountains with rounded ridges, steep sides, and narrow canyons.   Preston Peak is an individual mountain that stands about 1000 feet or more above subequal summits of other mountains and ridges in the subsection.  Cirques and moraines occur near the crest of the Siskiyou Mountains and on Preston Peak and the north side of Bear Peak.   Both large and small landslides are common.  The elevation range is from about 2000 feet up to 7309 feet on Preston Peak.  Mass wasting and fluvial erosion are the main geomorphic processes.  Glacial processes have been much less extensive.

Soils.  The soils are mostly Dystric and Dystric Lithic Xerochrepts and Typic Haploxerults on metamorphic rocks; Ultic Haploxeralfs and Dystric Xerochrepts on gabbro and diabase; Dystric and shallow Dystric Xerochrepts and Typic Haploxerults on granitic rocks; and Lithic Ruptic-Xerorthentic, Typic Xerochrepts and Typic Haploxeralfs on serpentinized peridotite.  Soils at the higher elevations are mostly rocky Lithic Xerumbrepts and Dystric Xerochrepts, plus Lithic, Entic, and Typic Xerumbrepts on granitic rocks and Xerumbrepts on glacial till.  The soils are well drained, except in small glacial basins.  Soil temperature regimes are mesic at lower elevations and frigid, with some cryic, at higher elevations.  Soil moisture regimes are xeric.

Vegetation.   At lower elevations, the predominant natural plant communities are Douglas-fir - tanoak series and Douglas-fir series, and, on ultramafic rocks, Jeffrey pine series.   Canyon live oak series is common on very steep rocky slopes with stony soils.  The higher elevations are mostly White fir series, with some Red fir series and Mixed subalpine forest series.  Port Orford-cedar series occurs on some wet mountain sideslopes and along streams.  There are also some unique plant communities in the Siskiyou Mountains called “Enriched stands in the Klamath Mountains” (Sawyer and Keeler-Wolf, 1995).

Characteristic series by lifeform include:
Grasslands: Idaho fescue series, Montane meadow habitat, Rocky Mountains sedge series.
Seeps: Darlingtonia series, Fen habitat.
Shrublands: Brewer oak series, Bush chinquapin series, Greenleaf manzanita series, Holodiscus series, Huckleberry oak series, Sadler oak series, Sitka alder series, Subalpine upland shrub habitat, Tobacco brush series, Wedgeleaf ceanothus series.
Forests and woodlands: Alaska yellow-cedar stands, Douglas-fir series, Enriched stands in the Klamath Mountains, Incense-cedar series, Jeffrey pine series, Knobcone pine series, Lodgepole pine series, Mixed subalpine forest series, Mountain hemlock series, Port Orford-cedar series, Red fir series, Western white pine series, White fir series.

Surface Water.  Runoff is rapid.  It drains to the Smith River on the west and to the Klamath River and its tributaries on the east.   Larger tributaries are perennial and smaller streams are mostly perennial.   There are some small lakes, or ponds, in cirque and paternoster basins. Temporary ponding occurs on some landslides.