| Summary: | drainage in North Dakota is about 10,000 years old; some of the early routes of the "Missouri River" may have lasted much longer than that. Even though the glaciers that covered North Dakota at various times during the Pleistocene Epoch flowed in various directions, evidence to document them is scarce. In parts of southwestern North Dakota, which were not reached by the most recent glaciers, the only evidence of the early glaciations is an occasional erratic, the remaining glacial sediment having been removed by erosion long ago. Only the most recent glaciation, the Wisconsinan glaciation, is well documented; all of the landforms and near-surface sediment in the northern and eastern part of the state were deposited by Wisconsinan glaciers. Fig. 2-c shows conditions when the ice was melting from North Dakota for the last time, at the end of the Wisconsinan glaciation. By 13,000 years ago, active glacial ice had melted from the central and northwestern parts of the state. Because the melting glacier by this time was much thinner than it had been several thousand years earlier, it could no longer flow over high areas such as the Turtle Mountains, Prairie Coteau, and Missouri Coteau (stippled areas on the diagram). Consequently, the glacier became more lobate and flowed around these higher areas. Large portions of the glacier that had been flowing over the high areas became covered with debris, stagnated, and took much longer to melt than did the cleaner ice on the nearby lowlands. Areas where the ice stagnated remain today as rugged, hummocky moraine. As the glacier continued to melt, large meltwater lakes became dammed along its southern edge (fig. 2-d). The largest of these was Lake Agassiz, but other meltwater lakes, such as Lakes Souris, Regina, and Dakota also formed in areas south of the receding glacier. When these lakes drained, broad, flat areas of silt and clay were left on the former lake bottoms. Late Pleistocene Glaciation The Pleistocene Epoch is often referred to as the Ice Age since continental ice sheets spread across Canada and the northern United States during this time. Evidence from North Dakota and elsewhere suggests several major periods of glaciation during the two million years of the Pleistocene Epoch, each lasting for thousands of years. The approximate maximum extent of the glaciers that covered North America during the Pleistocene is shown on fig. 2-b. The glaciers that advanced into North Dakota profoundly influenced the topography we see here today. During the Wisconsinan, which started about 75,000 years ago, the glaciers didn't advance quite so far as some of the earlier ones. The glaciers fluctuated back and forth between northerly and southerly regions before they finally retreated from North Dakota about 9,000 years ago. Though the climate began to warm 1995 North Dakota Blue Book 515
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