| Summary: | Figure 7, Paleoenvironmental reconstruction of Theodore Roosevelt National Park during part of the Paleocene. This reconstruction by Bruce R. Erickson, Science Museum of" Minnesota, St, Paul, is based on fossils found in the Bullion Creek Formation at the Wannagan Creek fossil site, just west of the South Unit. The Missouri River4 Before North America was first glaciated during the Ice Age about two million years ago, all of the rivers in North and South Dakota and eastern Montana drained northeastward into Canada to Hudson Bay. There was no Missouri River carrying drainage from the northern mid-continent to the Gulf of Mexico, Why is the modern situation so different than two million years ago? When the glaciers advanced southward over North Dakota, they cut off the natural northeastward flow of these rivers, forcing them to find new paths and carve new valleys. By studying the modem and ancient river valleys, geologists have learned a lot about the history of the Missouri River. The modern Missouri River valley in North Dakota consists of a number of "segments" of valley that are quite different from one another. Some of these valley segments are broad, six to 10 miles wide from edge to edge, with gentle slopes from the adjacent upland to the valley floor. Others are narrow, less than two miles wide, with 1999-2001 North Dakota Blue Book 477
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