Subsection M261Ai
Eastern Klamath Mountains

Lithology and Stratigraphy.  This subsection contains a variety of Devonian through  Jurassic metavolcanic and metasedimentary rocks, including metamorphosed andesite, rhyolite, pyroclastic rock, graywacke, shale, minor chert, and limestone.  From oldest to youngest, they are designated Copley greenstone, Kennett Formation, Bragdon Formation, Baird Formation, McCloud Limestone, Bollibokka Group, Pit Formation, Modin Formation, Arvison Formation, Bagley Andesite, and Potem Formation.   Non marine sedimentary rocks of the Weaverville Formation occur in a north-northeast aligned graben that is parallel to the Trinity River in the Musser Hill area.

Geomorphology.  This is a subsection of mountains with rounded summits, steep sides, and narrow canyons.  It is hilly with moderately steep slopes, rather than mountainous with steep slopes, in the trough that is occupied by the Weaverville Formation.  The elevation range is from about 700 adjacent to the Great Valley up to 6252 feet on Grizzly Peak.  Mass wasting and fluvial erosion are the main geomorphic processes.

Soils.  The soils are mostly Dystric, Dystric Lithic, and Lithic Xerochrepts, Ultic Haploxeralfs, and Typic Haploxerults.  Xeric Haplohumults are common on older land surfaces.  Soils on the Weaverville Formation are mostly Mollic Haploxeralfs.  The soils are well drained.  Soil temperature regimes are predominantly mesic, with some frigid at higher elevations and thermic adjacent to the Great Valley.  Soil moisture regimes are xeric.

Vegetation.   The predominant natural plant communities are Mixed conifer series, Douglas-fir - ponderosa pine series, and Ponderosa pine series.  Blue oak series and Mixed chaparral communities occur on south-facing slopes at lower elevations.  Canyon live oak series is common on very steep rocky slopes with stony soils.  White fir series occurs at higher elevations.

Characteristic series by lifeform include:
Grasslands: California annual grassland series.
Shrublands: Brewer oak series, Chamise series, Chamise-wedgeleaf ceanothus series, Greenleaf manzanita series, Tobacco brush series, Wedgeleaf ceanothus series, Whiteleaf manzanita series.
Forests and woodlands: Birchleaf mountain-mahogany series, Black oak series, Blue oak series,  Canyon live oak series, Douglas-fir series, Douglas-fir - ponderosa pine series, Engelmann spruce series, Foothill pine series, Interior live oak series, Knobcone pine series, Jeffrey pine series, Port Orford-cedar series, McNab cypress series, Mixed conifer series, Oregon white oak series, Ponderosa pine series, Valley oak series, White alder series.

Surface Water.  Runoff is rapid.  It drains to the Trinity River on the west and to the Sacramento River and its tributaries in the eastern part of the subsection.   All but the larger streams are dry through much or most of summer.  There are no natural lakes, but there are some reservoirs in the subsection.