Section M331B Bighorn Mountains
Geomorphology. There are high mountains with sharp crests, rolling uplands, and dissected hills, with alpine glaciation dominating the upper third of the area. The rugged hills and mountains are cut by many narrow valleys with steep gradients. Elevation ranges from 4,000 to 13,000 ft (1,220 to 3,962 m). This Section is within the Middle Rocky Mountains physiographic province.
Lithology and Stratigraphy. The central part of the Section is Precambrian quartz monzonite to quartz diorite in the north and Precambrian gneiss in the south. The periphery of the Section is Paleozoic carbonates and shales. A small area in the extreme northeast of the Section is Cretaceous sandstones, siltstones, and shales.
Soil Taxa. Soils include cryic Borolls, Ochrepts, and Boralfs. These soils are generally shallow to moderately deep, but some deep soils occur in alluvial and colluvial basins. Textures are generally loamy or sandy, with large amounts of rock fragments.
Potential Natural Vegetation. Kuchler mapped potential vegetation as Douglas-fir forest and western spruce--fir forest (50 percent) and wheatgrass--needlegrass--shrubsteppe (50 percent). Common tree species include lodgepole pine, Douglas-fir, subalpine fir, and Engelmann spruce. Idaho fescue, bluebunch wheatgrass, and mountain big sagebrush are common grass and shrub species.
Fauna. Birds are typical of the Rocky Mountains. Species include ferruginous and Swainson's hawks, golden eagle, blue grouse, sage grouse, mountain plover, Steller's and gray jay, Clark's nutcracker, Townsend's solitaire, green-tailed towhee, and western tanager. Species nearing the edge of their ranges are calliope hummingbird, indigo bunting, and clay-colored sparrow. Typical herbivores and carnivores include white-tailed deer, mule deer, elk, moose, pronghorn, black bear, bobcat, and cougar. Smaller common herbivores include the snowshoe hare, yellow-bellied marmot, and the northern flying squirrel. Bison are historically associated with this Section. Herpetofauna typical of this Section are the spotted frog, rubber boa, boreal toad, bloched tiger salamander, and, at lower elevations, the prairie rattlesnake.
Climate. Precipitation ranges from 15 to 40 in (380 to 1,020 mm), with much occurring as spring and fall rains. Climate is cold continental with dry, cold winters. Temperature averages 36 to 43 F (2 to 6 C). The growing season lasts 45 to 90 days.
Surface Water Characteristics. This area has medium to fine density dendritic patterns with moderate gradients. Streams are deeply entrenched as they leave the mountains. Lakes occur in glaciated terrain, as well as in high elevation cirques and basins. Major streams include the Tongue, Shell, and Tensleep.
Disturbance Regimes. Fire, insects, and disease are the dominant natural sources of disturbance. Fire has historically been fairly frequent, low intensity, and patchy; however, fire suppression has caused this pattern to change to less frequent, more intense, larger fires.
Land Use. The land is used for timber harvest, livestock grazing, wildlife habitat, watershed, and recreation.
