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Share Map. At no time did a single lake flood the entire area shown on this map. Rather, during the time the southern part of the area in North Dakota and Minnesota was flooded, beginning about 11, years ago, the northern regions were still covered by glacial ice.
By the time the northern areas in northern Manitoba were flooded, as recently as 7, years ago, much of the southern portion of the lake plain had already drained. Even so, the entire indicated area was at some time submerged beneath the water of Lake Agassiz. Throughout most of the area, little lake sediment was deposited the thickest lake deposits are found along the North Dakota-Minnesota border , but rather, the lake plain is mainly a wave-washed plain.
The map also shows the locations of several of the smaller glacial lakes that existed at various times. These lakes were not all contemporaneous with one another, or with Lake Agassiz. Many of them date to the time prior to the formation of Lake Agassiz, earlier than 12, years ago.
Banded Lake Agassiz sediments near Mayville. Black layers were deposited during winters, thicker, light-colored layers during the summer photo by John Bluemle. Evidence for glacial Lake Agassiz is found principally in: 1 its stratigraphy, or layers of sediment, 2 its border of shoreline features; and 3 its flat topography. The stratigraphy of the sediments of the Red River Valley imply that the Valley was repeatedly occupied by glaciers, lakes, and rivers.
The history of glacial Lake Agassiz is one of fluctuating levels, punctuated by periods of relative lake-level stability. Fluctuations were caused by the alternate advance and retreat of glaciers, use of different outlets, catastrophic inflows from meltwater floods, and postglacial rebound.
I'll give the briefest of possible summaries of the history of glacial Lake Agassiz: The history of Lake Agassiz in North Dakota covers approximately 2, years, from 11, years ago until 9, 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; this is 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, 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. Nomadic bands of humans hunted the mammoths and other game - perhaps they were able to catch fish from the lake. Between about 11, and 9, 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, years ago, but between 9, and 9, years ago, the glacier readvanced in Canada, blocking the eastern outlets to Lake Superior. By 9, years ago, after the glacier had again receded, Lake Agassiz retreated from North Dakota for the last time. The Red River Valley takes on a whole new dimension when viewed from the air.
It is exceptionally flat, of course, but it is also marked by oxbows, grooves, spring pits, differential compaction ridges, deltas, and other features. When viewed from a low-flying airplane, the central part of the Red River Valley appears terribly scratched. These scratches, or grooves, are up to six miles long, generally 3 to 10 feet deep, and up to feet wide. The grooves were made by icebergs as they were blown over the surface of the lake during spring breakup. The bottoms of the icebergs dragged in the soft lake sediment, forming the grooves.
The grooves are almost imperceptible from the ground, but are reflected in "beaded shelterbelts," which consist of alternating tall and short trees whose growth characteristics are controlled by soil fertility differences between the grooves and the higher areas between the grooves.
In parts of Walsh and Grand Forks counties, irregular depressions up to a mile wide and 15 feet deep can be seen. These are spring pits, where salty water is seeping to the surface and slowly removing, or sapping, surface sediments as it flows away. The salty water comes from buried sandstone formations. Photo by J. The array of ice-drag markings in this area is striking and seems to indicate plowing by ice in several different directions.
Groundwater had moved eastward through the preglacial sandstone beds and seeped to the surface in eastern North Dakota for millions of years prior to the Ice Age, but when the surface was sealed by glacial ice, the water could not escape to the surface. As a result, considerable pressure built up in the water beneath the glacial ice.
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