A reason to visit North Dakota.
In 1823, William A. Keating, an English-born American geologist, accompanied Major Stephen H. Long on his expedition
to the upper Mississippi River and the Red River of the North. Keating noticed that the Red River's valley was
extraordinarily flat, sixty miles (100 km) wide in some places. Today, we know it falls no more than 230 feet (70 m) in the 545 miles (877 km) between Lake Traverse and Lake Winnipeg, most of it in the upper reaches near Lake Traverse.
Over the succeeding years, later explorers, and various geographers and geologists noticed some other peculiar features of the area:
- The Red River Valley is just the southernmost tip of a large flat surface extending into most of southern Manitoba and
- Just past the southern tip of Lake Traverse is the northern end of Big Stone Lake, which drains into the Minnesota River. Between the two, the valley walls make a continuous gorge.
- Long ridges of sand and gravel stretch for long distances above the river, at constant elevations. Sand and gravel deposits appear where certain rivers cross these ridges. Ridges of this sort also snake through southern Manitoba.
- Beneath this exceedingly flat surface are many alternating light and dark layers of sediment, known as "varved sediments".
- The terminal moraines in certain parts of Manitoba end abruptly where they meet the varved sediment plain.
Eventually it was recognized that towards the end of the last (Wisconsin) ice age, a vast lake covered most of southern Manitoba and western Ontario, with extensions into Saskatchewan, Minnesota, and North Dakota. The region's natural topography trapped glacial meltwater between high ground on the southern side, and the retreating edge of the continental ice sheet on the northern side. The sandy ridges were fossil beaches of the lake, and the large sand deposits were river deltas. The varved sediments were precisely what would form at the bottom of a glacial lake. The lake must have once drained southward into the Minnesota River valley, and carved the gorge between the lakes. In 1879, the lake was named after Louis Agassiz, the man who propounded the theory of Ice Ages (specifically, that the till that covered most of Northern Europe and North America was brought by glaciers that once blanketed the land).
It would be misleading to suggest that Lake Agassiz covered this entire area at one time: As the glacier retreated, advanced, and retreated again, the lake advanced and retreated with it. Geographers now recognize five phases of the
- Cass Phase, 11700 years ago. The lake was confined to the southern end of what is now the Red River Valley. It drained southward through the Warren River, into what woukld become the Minnesota River. The Sheyenne River emptied into the lake, eventually forming a large delta.
- Lockhart Phase 11600-11200 years ago. The lake expanded into what is now Canada; the Pembina and Assiniboine Rivers drained into the lake. The lake contracted and rose as the ice advanced again.
- Moorhead Phase, 11000-9900 years ago. The ice retreated and unblocked a passage that is now the Rainy River in northern Minnesota. The lake dropped in level so that its southern tip was around Fargo, where the Sheyenne formed the Moorhead Delta. Eventually, the lake dropped enough so that the entire US part of the Red River valley was exposed.
- Emerson Phase, 9900-9500 years ago. The ice sheet advanced, plugging up the outlet, and the lake rose to drain through the southern tip again. The lake reached its largest extent at this time, about 350,000 square kilometers (135000 square miles). The Minnesota River outlet was carved below the level of the Rainy River outlet.
- Nipigon Phase, 9500-8500 years ago. The lake overflowed one ice dam in western Ontario after another, causing the lake to drop over and over. Numerous fossil beaches snake through the Prairie Provinces. A dramatic gorge near Lake Nipigon indicates where the lake (intersected?) drained into Lake Superior for a time.
By 7500 years ago, the ice had completely melted and Lake Agassiz drained away for good. The Nelson River now drains most of the area once covered by the lake. The largest remnants of Lake Agassiz are Lake Winnipeg, Lake Winnipegosis, and Lake Manitoba in Manitoba. Other important remnants include the Lake of the Woods and many of the lakes of northwestern Minnesota such as Red Lake.
Warren Upham, The Glacial Lake Aggasiz, United States Geological Survey 1894
North Dakota State University Libraries,
United States Government Documents Resources for North Dakota Libraries
Explore maps of Minnesota! See the gorge separating the two lakes at
"Lake Agassiz: Child of Ice", Don McCollor, University of North Dakota
Earthscapes: The Red River Valley