geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...

a sag, or trough, is a depressed, persistent, low area; the opposite of an arch, or ridge, a raised, persistent, high area. The terms sag and arch were used historically to describe very large features, for example, characterizing North America as two arches with a sag between them. Also, a sag is a former river bed which has been partially filled with debris from

glaciation A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate be ...

or other natural processes but which is still visible in the surface terrain. Sags formed by the former river beds of large rivers often become the valleys of smaller streams after a change of course by the main river. Examples of sags include the former continuations of the

Grand Grand may refer to: People with the name * Grand (surname) * Grand L. Bush (born 1955), American actor * Grand Mixer DXT, American turntablist * Grand Puba (born 1966), American rapper Places * Grand, Oklahoma * Grand, Vosges, village and c ...

, Moreau, and

White White is the lightest color and is achromatic (having no hue). It is the color of objects such as snow, chalk, and milk, and is the opposite of black. White objects fully reflect and scatter all the visible wavelengths of light. White o ...

rivers in

South Dakota South Dakota (; Sioux: , ) is a U.S. state in the North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Lakota and Dakota Sioux Native American tribes, who comprise a large porti ...

in the United States. Before the last ice age, these rivers continued eastward past their current confluences with the present course of Missouri River. The sags are prominently visible on the

plateau In geology and physical geography, a plateau (; ; ), also called a high plain or a tableland, is an area of a highland consisting of flat terrain that is raised sharply above the surrounding area on at least one side. Often one or more sides ...

of the

Missouri Coteau The Missouri Coteau, or Missouri Plateau, (french: Coteau du Missouri) is a large plateau that stretches along the eastern side of the valley of the Missouri River in central North Dakota and north-central South Dakota in the United States. In t ...

, allowing small streams to drain into the Missouri from its eastern side.

Notes

Geomorphology Glaciology {{Geomorph-stub