| Summary: | Figure 2. Map showing major physiographic divisions in North Dakota ince, it also has physiographic characteristics similar to the Central Lowlands Province. It is probably most accurate to think of the Missouri Coteau region as the transitional zone between the two provinces, as mentioned above. The Missouri Plateau (or Missouri Slope Uplands) The broad valleys, hills, and buttes of the Missouri Plateau, southwest of the Missouri River, are largely the result of erosion of flat-lying beds of sandstone, siltstone, claystone, and lignite. Most of the exposed sediments belong to the Paleocene-age Fort Union Group and were deposited by ancient rivers flowing away from the rising Rocky Mountains between about 65 and 55 million years ago. From about 10 to five million years ago, streams began eroding the sediments that had so long ago been deposited, dissecting the plateau with a series of rivers flowing northeast to Hudson Bay. The modern landscape over most of southwestern North Dakota thus formed over an exceptionally long period of time, unlike the much more recent topography of the glaciated portion of the state. Part of the Missouri Plateau south and west of the Missouri River was glaciated during the Pleistocene, but in most places the only visible evidence of glaciation is an occasional erratic boulder or thin patch of glacial sediment. The glaciations that affected these areas were early ones that occurred long before the glaciations whose deposits and landforms are so evident in the eastern and northern parts of the state. 1999-2001 North Dakota Blue Book 465
|
|---|