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• Language: unknown
• Subjects: Browns; Canada; Glacial Lake; Hudson; Hudson Bay; Thunder Bay; glacier*; Ice Sheet
• Online Access: http://cdm16921.contentdm.oclc.org/cdm/ref/collection/ndbb/id/10628

In North Dakota, these include the glacial Lake Souris plain in McHenry and Bottineau Counties, the glacial Lake Dakota plain in Dickey and Sargent Counties, the glacial Lake McKenzie plain in Burleigh County, and many smaller Jake plains. By far the largest of these was glacial Lake Agassiz, once the largest proglacial lake in North America. Like its namesake Louis Agassiz (1807-1873), the father of glacial geology, glacial Lake Agassiz's influence was felt far and wide. Evidence of glacial Lake Agassiz occurs over an area of roughly 365,000 square miles, an area five times the size of the state of North Dakota, although at no single time did the lake ever cover this entire area. Ice margin positions and lowering of outlets by erosion combined to limit the size of the lake at any given time. Glacial Lake Agassiz was the last in a series of proglacial lakes that must have formed in the Red River Valley many times during the Ice Age, each time north-draining rivers were impounded by ice sheets spreading south out of Canada, and again as these glaciers receded. Evidence of glacial Lake Agassiz's history is found principally in: 1) its stratigraphy, or layers of sediment, and 2) its border of shoreline features. The stratigraphy of the sediments of the Red River Valley implies that the Valley was repeatedly occupied by glaciers, lakes, and rivers, each leaving behind distinct deposits. Several different parallel beach ridges and erosional scarps also show that the lake stabilized at different levels over its history. The history of glacial Lake Agassiz is therefore one of fluctuation, punctuated by periods of relatively stable lake-levels. Fluctuations were caused by the alternate advance and retreat of glaciers, use of different outlets, catastrophic inflows from meltwater floods, and postglacial rebound. The history of Lake Agassiz in North Dakota covers approximately 2,700 years, from 11,700 years ago until 9,000 years ago. The lake began to form in the Red River Valley when the glacier retreated north of the drainage divide between the Hudson Bay and Mississippi River drainage basins, near Browns Valley, Minnesota. North-draining rivers were blocked on the north by the ice sheet. The lake that formed ahead of the retreating glacier was small at first and overflowed southward into the Minnesota River Valley. As the glacier margin receded northward, the lake expanded, flooding the Red River Valley. By about 11,500 years ago, the North Dakota shoreline was lined by an open forest of spruce and deciduous trees mixed with sagebrush-dominated openings. Wooly mammoths roamed the shore. Between about 11,200 and 9,900 years ago, after the glacier margin had receded north of Thunder Bay, Ontario, the lake level dropped in a series of still stands as successively lower outlets to Lake Superior were opened. During this time, the exposed parts of the previously flooded lake floor were covered by spruce and marsh. The lake probably completely drained from North Dakota by 9,900 years ago, but between 9,900 and 9,500 years ago, the glacier readvanced in Canada, blocking the eastern outlets to Lake Superior. Lake Agassiz flooded the Red River Valley again, draining southward into the Minnesota River. By 9,000 years ago, after the glacier had again receded, Lake Agassiz retreated from North Dakota. 488 Chapter Ten - Natural History