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.