Page 529

• Language: unknown
• Subjects: Canada; Hudson; Hudson Bay; The Sentinel; glacier*
• Online Access: http://cdm16921.contentdm.oclc.org/cdm/ref/collection/ndbb/id/13286

layers that can be seen today in the badlands areas. Bentonite is composed principally of the clay mineral montmorillonite, which if it contains sodium as an exchangeable ion, can swell conspicuously when wet; calcium bentonites are only slightly swelling or non-swelling. The Sentinel Butte bentonite is an iron- and sodium-rich montmorillonite, one of three major Paleocene-age bentonites in North Dakota shown to have been derived from volcanic ash. The bentonites in the badlands can absorb up to several times their weight in water and they are extremely slick and mobile when wet. They form a characteristic popcorn-like surface when dry. Formation of the Badlands Although the layers of sedimentary rock exposed in the Little Missouri Badlands range from Cretaceous through Eocene age, the badlands themselves - the hills and valleys - are not nearly that old. The Little Missouri River began to carve the badlands about 600,000 years ago, when a glacier diverted it from its northward course. Prior to glaciation, the river had flowed northward to a confluence with the early Yellowstone River and thence northeastward into Canada and on to Hudson Bay. After the glacier diverted the Little Missouri River eastward, it flowed over a shorter, steeper route than before. As a result, a vigorous cycle of erosion and downcutting began, initiating badlands development. Thus, the badlands are ultimately a product of glacier processes. Without glaciation, the Little Missouri Badlands would not have formed. About the only conspicuous evidence of glaciation remaining in the Little Missouri Badlands is the occurrence of glacial erratics in the northern part of the area. Patches of sand and gravel of glacial origin are also found on some buttes, overlying Sentinel Butte Formation beds. In addition to the areas of sand and gravel, a thin, widespread mantle of loess is present in upland areas, adjacent to eroded, badlands slopes and, in a few places, thin deposits of glacial till are found. Erosion Rates A great amount of sediment has been eroded and carried away by the Little Missouri River since the Little Missouri Badlands began to form. In the southern part of the area, near the river's headwaters in northeastern Wyoming, the river has cut down about 25 meters below the level at which it was flowing when the glaciers diverted it. Near Medora, the valley floor is 80 meters lower than the pre-diversion level. Still farther north, near the confluence of the Missouri and Little Missouri Rivers, the east-trending portion of the river flows in a valley that has been incised to a depth of more than 200 meters since glaciers diverted the river-It can be calculated that the average rates of erosion are (approximately): Missouri-Little Missouri confluence area, 3 centimeters/100 years; Medora area, 1 centimeters/100 years; Headwaters area, 4 millimeters/100 years. Although these rates of Chapter Ten - Natural History 529