Subsection M261Aa
Western Jurassic

Lithology and Stratigraphy.  This subsection is dominated by clastic Jurassic marine sedimentary rocks and greenstone of the Galice and Rogue formations.  The sedimentary rocks have been slightly metamorphosed to form metagraywacke and slate.  In the northern part of the subsection there are some serpentinized peridotite, gabbro, diabase, and volcanic rocks.  The rocks are moderately to intensely folded and faulted.

Geomorphology.  This is a subsection of mountains with rounded ridges, steep sides,  and narrow canyons.  There are narrow  floodplains and high terraces along the Smith, Klamath, and Trinity  Rivers and their tributaries.  Both large and small landslides are common.  The elevation range is about 250 to 4000 feet.  Mass wasting and fluvial erosion are the main geomorphic processes.

Soils.  The soils are mostly Dystric and Dystric Lithic Xerochrepts, and, from east to west, Ultic Haploxeralfs and Typic Haploxerults.  Xeric Haplohumults occur on high terraces.  The soils are well drained.  Soil temperature regimes are predominantly mesic.   Soil moisture regimes are xeric, bordering udic along the western edge of the subsection at the northern end.

Vegetation.   The predominant natural plant communities are Douglas-fir - tanoak series and Douglas-fir series.   Canyon live oak series is common on very steep rocky slopes with stony soils.  Port Orford cedar series occurs on wet mountain sideslopes and along streams.   Arroyo willow series and Mixed willow series occur in riparian areas along streams.  Jeffrey pine series occur on soils formed from serpentinized peridotite.

Characteristic series by lifeform include:
Grasslands: Rocky Mountains sedge series.
Shrublands: Brewer oak series, Greenleaf manzanita series, Holodiscus series, Montane wetland shrub habitat, Mountain alder series, Sadler oak series, Tobacco brush series, Wedgeleaf ceanothus series.
Forests and woodlands: Black oak series, Canyon live oak series, Douglas-fir series, Douglas-fir - tanoak series, Incense-cedar series, Jeffrey pine series, Knobcone pine series, Oregon white oak series, Montane wetland shrub habitat, Red fir series, Western hemlock series, White alder series, White fir series.

Surface Water.  Runoff is rapid.  The Smith, Klamath, and Trinity Rivers cross the subsection.  Larger tributaries are perennial and smaller streams are mostly perennial.  There are no permanent lakes, although temporary and some permanent ponding occurs on and behind some landslides.