Beyond the limits reached by the ice of the continental glaciers, the
Unglaciated Missouri Plateau displays the greatest variety of landforms
of any section of the Great Plains. In western Montana, many small
mountain masses rise above the general level of the plateau, including
the Highwood, Bearpaw, and Little Rocky Mountains near the margin of the
glaciated area, and the Judith, Big Snowy, Big Belt, Little Belt,
Castle, and Crazy Mountains farther south (fig. 22). Many of these, such
as the Crazy, Castle, Judith, and Big Snowy Mountains, are areas
uplifted by large, deeply rooted, intrusive igneous bodies called
stocks, which have been exposed by subsequent erosion of the arched
overlying sedimentary rock layers. Some, such as the Highwood and
Bearpaw Mountains, are predominantly piles of lava flows, although in
the Bearpaws the related intrusive bodies of igneous rock form a part of
the mountains. The Big and Little Belt Mountains were formed by
mushroom-shaped intrusive igneous bodies called laccoliths, which have
spread out and domed between layers of sedimentary rocks. A number of
igneous bodies also intrude the rocks of the Missouri Plateau around the
periphery of the Black Hills. Devils Tower, the first feature to be
designated a National Monument, is the best known of these igneous rock
features (fig. 23).
[Illustration: _Figure 22.—The Highwood Mountains seen from the Little
Belt Mountains, Mont. Photograph by I. J. Witkind, U. S. Geological
Survey._]
The uplift and volcanism that formed these mountains took place before
the streams began to cut downward and segment the Great Plains. The
mountains had been greatly dissected before the advent of the Great Ice
Age, when alpine glaciers formed on the Castle and the Crazy Mountains
and flowed down some of the stream-cut valleys. Alpine glacial features
such as cirques, in the high parts of the mountains, and glacially
modified U-shaped valleys (fig. 24) are impressive evidence of this
glaciation.
[Illustration: _Figure 23.—Devils Tower National Monument, Wyo. An
igneous intrusive body exposed by erosion. Photograph by F. W.
Osterwald, U. S. Geological Survey._]
The Missouri River and its tributaries—the Sun, Smith, Judith,
Musselshell, and Yellowstone Rivers in Montana and the Little Missouri
River in North Dakota—have cut down into the Missouri Plateau, cut broad
upland surfaces at many levels, and established confined valleys with
valley floors flanked by terrace remnants of older floodplains. Locally,
high buttes that are remnants of former interstream divides rise above
the uplands. Large lakes also were formed in most of these tributary
valleys because of damming by the continental ice sheets.
[Illustration: _Figure 24.—U-shaped, glaciated valley of Big Timber
Creek, Crazy Mountains, Mont. Photograph by W. C. Alden, 1921, U. S.
Geological Survey._]
West of the Black Hills, in Wyoming, the Tongue River and the Powder
River have excavated the Powder River Basin and produced similar
features (fig. 25). The east-flowing tributaries of the Missouri
River—the Knife, Heart, and Cannonball Rivers in North Dakota and the
Grand, Moreau, Belle Fourche, Cheyenne, Bad, and White Rivers in South
Dakota—similarly have shaped the landscape.
Most of these rivers flow in broad, old valleys, established more than 2
million years ago, before the first advance of the continental ice
sheets. Some of these valleys have been widened by recession of the
valley walls by badland development. Badlands are formed by the cutting
action of rivulets and rills flowing down over a steeply sloping face of
soft, fine-grained material composed mainly of clay and silt. The
intricate carving by thousands of small streams of water produces the
distinctive rounded and gullied terrain we call badlands. Badlands
National Monument in South Dakota (fig. 26) has been established in the
remarkable badlands terrain cut into the White River Group along the
north valley wall of the White River, and the South Unit of Theodore
Roosevelt National Memorial Park is in the colorful badlands of the
Little Missouri River, formed on the Fort Union Formation (fig. 27).
The White River also has cut a steep scarp along its southern wall that
is called the Pine Ridge escarpment. This escarpment defines the
boundary between the Missouri Plateau and the High Plains here.
[Illustration: _Figure 25.—View northeast across the Deckers coal mine
and the Tongue River in the Powder River Basin, southeastern Montana.
Typical terrain of unglaciated Missouri Plateau. Small mesas with
cliffed escarpments on capping layer of resistant sandstone, such as
those in the foreground, are common. Coal mine is about 1 mile across.
Photograph by R. B. Taylor, U. S. Geological Survey._]
The landscape of the Unglaciated Missouri Plateau has been determined
largely by the action of streams, but in some areas igneous intrusions
and volcanoes have produced small mountain masses that interrupt the
plain, and valley glaciers have modified the valleys in some of these
mountains.
[Illustration: _Figure 26.—Badlands in Badlands National Monument, S.
Dak. Photograph by W. H. Raymond, III, U. S. Geological Survey._]
[Illustration: _Figure 27.—Badlands of the Little Missouri River in
South Unit of Theodore Roosevelt National Memorial Park, N. Dak. View
looking northwest from Painted Canyon Overlook along Interstate Highway
94, west of Belfield._]