The Geologic Story of the Great Plains / A nontechnical description of the origin and evolution of the landscape of the Great Plains

by Donald E. Trimble

MISSOURI PLATEAU

Beginning about 5 million years ago, regional uplift of the western part of the continent forced streams, which for 30 million years had been depositing sediment nearly continuously on the Great Plains, to change their behavior and begin to cut into the layers of sediment they so long had been depositing. The predecessor of the Missouri River ate headward into the northern Great Plains and developed a tributary system that excavated deeply into the accumulated deposits near the mountain front and carried away huge volumes of sediment from the Great Plains to Hudson Bay. By 2 million years ago, the streams had cut downward to within a few hundred feet of their present level. This region that has been so thoroughly dissected by the Missouri River and its tributaries is called the Missouri Plateau.

About 2 million years ago, after much downcutting had already taken place and river channels had been firmly established, great ice sheets advanced southward from Canada into the United States. (See [figure 2].) These continental glaciers formed, advanced, and retreated several times during the last 2 million years. At the north and east margins of the Missouri Plateau they lapped onto a high area, leaving a mantle of glacial deposits covering the bedrock surface and forcing streams to adopt new courses along the margin of ice. The part of the Missouri Plateau covered by the continental glaciers now is referred to as the Glaciated Missouri Plateau. South of the part once covered by ice is the Unglaciated Missouri Plateau.

Preglacial Drainage

Before the initial advance of the continental ice sheets, the Missouri River flowed northeastward into Canada and to Hudson Bay. Its major tributaries, the Yellowstone and the Little Missouri joined the Missouri in northwestern North Dakota. The east-flowing Knife, Heart, and Cannonball Rivers in North Dakota also joined a stream that flowed northward to Hudson Bay.

Glaciated Missouri Plateau

When the continental ice sheets spread southward into northern Montana and the Dakotas, a few isolated areas in Montana stood above the surrounding plain. These are mostly areas that were uplifted by the intrusion of igneous bodies long before the streams began downcutting and carving the land. The northernmost of these isolated mountains, the Sweetgrass Hills, were surrounded by ice and became nunataks, or islands of land, in the sea of advancing ice, which pushed southward up against the Highwood Mountains, near Great Falls, the Bearpaws south of Havre, and the Little Rockies to the east.

Much of the northern part of Montana is a plain of little relief that is the surface of a nearly continuous cover of glacial deposits, generally less than 50 feet thick. This plain has been incised by the east-flowing postglacial Teton, Marias, and Milk Rivers.

In North Dakota, a high area on the east side of the Williston basin acted as a barrier to the advance of the ice, most of which was diverted southeastward. The margin of the ice sheet, however, lapped onto the bedrock high, where it stagnated. Earlier advances moved farthest south; the later advances stopped north of the present course of the Missouri River—their maximum position marked by ridges of unsorted, glacially transported rock debris (till) called terminal moraines. North of the terminal moraines is a distinctive landscape characterized by a rolling, hummocky, or hilly surface with thousands of closed depressions between the hills and hummocks, most of them occupied by lakes. This is the deposit left by the stagnant or dead ice, and it is called dead-ice moraine. The rolling upland in North Dakota that is covered by dead-ice moraine and ridges of terminal moraines from the last glacial advances is called the Coteau du Missouri ([fig. 20]). A gently sloping scarp, several hundred feet high and mostly covered by glacial deposits (referred to collectively as drift), separates the Coteau du Missouri from the lower, nearly flat, drift-covered plains of the Central Lowland to the east. This escarpment, which is called the Missouri escarpment, is virtually continuous across the State of North Dakota southward into South Dakota. The base of the Missouri escarpment is the eastern boundary of the Great Plains in these northern states.

Figure 20.—Ground moraine on the Coteau du Missouri, northwestern North Dakota. Photograph by R. M. Lindvall, U. S. Geological Survey.

The advancing ice front blocked one after another of the northward-flowing streams of the region, diverting them eastward along the ice front. Shonkin Sag, north of the Highwood Mountains near Great Falls, Mont., is an abandoned diversion channel of the Missouri River, occupied when the ice front stood close to the north slopes of the Highwoods. Much of the present course of the Missouri River from Great Falls, Mont., to Kansas City, Mo., was established as an ice-marginal channel, and the east-flowing part of the Little Missouri River in North Dakota was formed in the same way. These valleys were cut during the last 2 million years.

The north-flowing part of the Little Missouri River and the east-flowing courses of the Knife, Heart, and Cannonball Rivers in North Dakota are for the most part older, preglacial courses. The Little Missouri was dammed by the ice, and its waters impounded to form a huge lake during the maximum stand of the ice, but the deposits of this glacial lake are few and make no imprint on the landscape.

The valley of the east-flowing, glacially diverted part of the Little Missouri River, however, is markedly different from that of the north-flowing preglacial river. It is much narrower and has steeper walls than the old valley. Because it is younger, it is little modified, except by huge landslides that have affected both walls of the valley. Tremendous rotated landslide blocks in the North Unit of Theodore Roosevelt National Memorial Park are some of the best examples of the slump type of landslide to be seen anywhere ([fig. 21]).

Melting ice at the front of the glaciers provided large volumes of meltwater that flowed across the till-mantled surface in front of the glacier as it melted back toward Canada. This meltwater took many courses to join the glacially diverted Missouri River, and these sinuous meltwater channels wind across the dead-ice moraine and the older, less hummocky ground moraine between the Coteau du Missouri and the Missouri River. Locally the sediment carried by the meltwater streams was banked against a wall of ice to form a small hill of stratified drift that is called a kame. Streams flowing in tunnels beneath the ice formed sinuous, ridgelike deposits called eskers, and in places the meltwater deposits form broad flat areas called outwash plains.

Figure 21.—Rotated slump blocks in huge landslide, North Unit of Theodore Roosevelt National Memorial Park, N. Dak. Note that layering of Fort Union Formation in cliffs on skyline, where landslide originated, is horizontal.

This rather limited variety of landforms, then, characterizes the landscape of the Glaciated Missouri Plateau. The landforms themselves are testimony to their glacial origin and to the great advances of the continental ice sheets. This is a stream-carved terrain that has been modified by continental glaciers and almost completely covered by a thick blanket of glacially transported and deposited rock debris, locally hundreds of feet thick. Subsequent stream action has not altered the landscape greatly.

Unglaciated Missouri Plateau

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]).

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.

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.

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.

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.

Figure 26.—Badlands in Badlands National Monument, S. Dak. Photograph by W. H. Raymond, III, U. S. Geological Survey.

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.