| Summary: | Some important erosional features, such as the James and Sheyenne River valleys, which were carved by large rivers of glacial meltwater, are found in glaciated areas, but most of the topography in northern and eastern North Dakota was formed when the glaciers rearranged the materials over which they were flowing. A thick layer of reworked sediment, some of it transported great distances by the glaciers, but most of it derived locally, completely changed the shape of the land during Pleistocene time. Glacier sediment contains a broad mix of minerals, providing the basis for rich soils. Soils developed on shale and sandstone tend to be less fertile than those formed from glacier sediment. Regional Historical Geology The accompanying 4-part diagram (fig. 2) shows how the glaciers affected North Dakota and nearby areas during the Pleistocene Epoch, or "Ice Age." The first map (fig. 2-a) shows conditions at the end of Tertiary time, just prior to the advance of the earliest glaciers. The oldest rocks, which were found at the edge of the Canadian Shield on the North Dakota-Minnesota border and to the north in Manitoba, consisted of igneous and metamorphic rocks of Archean age, some of which may have been as much as 2.5 billion years old. To the west, carbonates of Ordovician, Silurian, and Devonian age lapped onto the Canadian Shield. These were overlain in northeastern North Dakota and southern Manitoba by rocks of Jurassic age. Over much of the central and northern part of the area, the land was covered by Cretaceous shales. Lignite-bearing sandstones, shales, and siltstones of Tertiary age were found in the Williston Basin, in western North Dakota and eastern Montana. Before the Ice Age began, the area was a gently rolling plain that sloped to the north, toward Hudson Bay. Western North Dakota and eastern Montana were drained by north and northeast-flowing rivers that included the Yellowstone, Little Missouri, and Missouri Rivers. Most of central North Dakota was drained by an extensive north-flowing river system that entered Manitoba east of the Turtle Mountains. This river drained areas that are today drained by the Knife, Cannonball, Heart and other rivers. Much of eastern North Dakota, parts of northwestern Minnesota, and northern South Dakota, were drained northward by an early Red River of the North. It's likely that all of these rivers joined somewhere to the north of North Dakota and that all of the runoff made its way to Hudson Bay. The area was glaciated several times during the two-million-year duration of the Pleistocene Epoch, but our knowledge of early glaciations is extremely limited. fig. 2-b shows the hypothetical extent of one of the early glaciers, one that advanced perhaps 600,000 years ago. This early glacier blocked the northerly route of the Little Missouri River, forcing it to flow to the east and south, along the margin of the ice sheet. As a result of its new, steeper route, the Little Missouri River began eroding the badlands, a process that continues today. Each time 1995 North Dakota Blue Book 513
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