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.