| Summary: | The Turtle Mountains are underlain by rocks of the Cretaceous Fox Hills and Hell Creek Formations and the Paleocene Cannonball Formation, all covered by a thick layer of glacial sediment. In early Pliocene or late Miocene time (maybe 5 or 6 million years ago), the area that is now the Turtle Mountains was apparently part of a broad plain that sloped to the northeast. Rivers and streams flowed over the plain from the west and southwest, making their way to Hudson Bay. Then, in Pliocene time, maybe 5 million years ago, erosion increased markedly and large amounts of material were removed as deep valleys dissected the plain. It is not really known why this cycle of erosion began. Perhaps the area was uplifted by geologic forces so that streams began to cut into the sediments they had been flowing over, or perhaps the climate changed. As sediment was eroded away, new hills and valleys were shaped and, gradually, as the sediments surrounding the Turtle Mountains were carried away to Hudson Bay, a large mesa was left standing where the Turtle Mountains are today. The reason the outlier developed where it did is not entirely clear. The uppermost bedrock unit on the Turtle Mountains (beneath the covering of glacial sediment) is the Tertiary Cannonball Formation, which is not notably resistant to erosion. It is possible, of course, that some kind of resistant layer was present throughout much of the erosion cycle; perhaps a part of the lower Bullion Creek Formation. It is also possible that additional drilling in the area will eventually penetrate a remnant of some resistant material not yet found. If any resistant layer exists, it is deeply buried beneath glacial sediments. About two million years ago, at the beginning of the Pleistocene Epoch, the climate turned colder and, as snow built up to great depths near Hudson Bay, glaciers formed and flowed southward, out of Canada into North Dakota. As the climate fluctuated, glaciers advanced and receded, flowing over and around the Turtle Mountains several times. Finally, about 25,000 years ago, the Late Wisconsinan glacier flowed southward over the Turtle Mountains. During Late Wisconsinan time, the most recent major glaciation, the Turtle Mountains were continuously buried under the actively moving glacial ice for about 10,000 years. The movement of the glacial ice over the obstruction formed by the Turtle Mountains caused the ice to become compressed and resulted in shearing within the glacier. The shearing of the ice at the edge of the Turtle Mountains caused large volumes of rock and sediment to be incorporated into the ice. As the climate moderated between about 15,000 and 13,000 years ago, the glacier thinned and its margin receded northward. Since the Turtle Mountains rise 600 to 800 feet above the surrounding area, and since ice 200 or 300 feet thick will flow under its own weight, glacial flow continued for awhile on either side of the Turtle Mountains. At the same time, the glacier on top of the Turtle Mountains stagnated, leaving several hundred feet of debris-covered ice covering the surface. In areas surrounding the Turtle Mountains, where shearing of material into the glacier had not been as intense, the ice was cleaner, and it simply melted away, although as the glacier continued to move over these lower areas around the edge of the Turtle Mountains, it did deposit a few feet of sediment in places. However, as the debris-covered, 540 2001-2003 North Dakota Blue Book
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