Geoscience Reference
In-Depth Information
resistant sedimentary strata. the vegetation is predomi-
nantly forests and woodlands of ponderosa pine, with
grasslands at lower elevations. the hills north of new-
in the western part of the Black Hills, the sedimentary
strata form a comparatively flat area known as the Lime-
stone Plateau, with an elevation of 6,200 to 7,200 feet.
the underlying Madison Limestone (see chapter 2) is
permeable to water, which has enabled the formation of
numerous caverns, including Wind cave and Jewel cave.
Some creeks disappear into the bedrock before emerging
again, such as Spearfish and Boulder creeks in the north.
Spearfish creek has cut through 400 feet of Paleozoic
sandstones, forming the deepest and most spectacular
canyon in the Hills. the vegetation of the plateau is pri-
marily ponderosa pine forests and savannas, commonly
intermingled with bur oak woodlands in the north and
groves of white spruce in canyons or on north slopes.
Also found are grasslands and foothill shrublands. A few
of the grasslands still have native species, but most are
now dominated by smooth brome, timothy, and other
introduced plants.
12
the fifth geomorphic region is the igneous, granitic
core of the Hills, which has been exposed through the
erosion of sedimentary rocks. Known as the central
Area, it has many pinnacles and steep slopes (fig. 16.4).
vegetation is predominantly ponderosa pine forest, with
occasional groves of white spruce and aspen in wetter
habitats. Several lakes occur in the Hills, such as Sylvan
Lake, but dams on creeks have created all of them.
the Bear Lodge Mountains, located to the northwest
of the Black Hills, occupy a smaller area and have less-
well-defined geomorphic regions. the highest point is
Warren Peak at 6,655 feet, about 600 feet lower than
Harney Peak. except for the absence of white spruce and
a few other species, the vegetation of the Bear Lodge
Mountains is similar to that of the Black Hills.
104°
35
Belle Fourche
55
50
Spearfish
40
Sundance
Lead
40
60
45
50
55
45
40
50
Rapid
City
35
44°
Newcastle
48
Custer
45
50
45
N
Hot
Springs
10
0
10 mi
15
0
15 km
Ardmore
Fig. 16.3. Mean annual precipitation patterns (shown in cen-
timeters) in the Black Hills and Bear Lodge Mountains. Divide
centimeters by 2.54 to obtain inches. Adapted from Boldt and
Van Deusen (1974).
on the interior side of the Hogback Rim is the Red
Valley—a name that stems from the red shales of the
Spearfish Formation that give a characteristic red color
track” because of its oval shape, the valley was formed
by the erosion of softer shales, siltstones, and sand-
stones underlying the more resistant rocks of the Hog-
back Rim. Grasslands predominated in the valley when
european immigrants arrived, as they do today where
the land has not been cultivated. the towns of Sun-
dance, Spearfish, Hot Springs, and the western part of
Rapid city are found in the Red Valley, which ranges in
elevation from about 3,000 to 3,600 feet.
Further to the interior, above the Red Valley, are
the Minnelusa Foothills, composed of harder, erosion-
Vegetation Mosaic
nowhere in Wyoming and adjacent states are ponder-
osa pine forests so extensive fig. 16.5; see fig. 1.5). they
occur on nearly all soil types and exposures, occupy
more than 90 percent of the Black Hills and Bear Lodge
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