The Minnesota River Valley is a trench that originates on the western border of Minnesota, slices southeastward to Mankato, and then bends sharply to the northeast, where it joins the Mississippi River Valley at St. Paul near Fort Snelling.

This capacious valley, up to 60 meters (m) deep and in places almost 5 kilometers (km) wide, was excavated about 12,000 years ago when the rising waters of a large, newly formed glacial lake, Lake Agassiz, fed by melting glacial ice, overtopped a natural earth dam near what is now Browns Valley.

The ensuing flood not only dropped the level of the lake significantly but scarred irreparably the land surface across which it raced. Successive floods, each as energetic as the first, pulsed down the watercourse established by the initial burst, forming the River Warren. The turbulent waters eroded deeply into the soil and under-lying bedrock before the glacial lake was finally drained dry. (Remnants of the southern shores of Lake Agassiz can still be seen in the northwest corner of Stevens County.)

Eventually, the Minnesota River established itself as the successor to the mighty outlet of that glacial lake. Although an important event in its own right as part of the geologic history of Minnesota, the excavation of that great trench across the midsection of the state also brought into view through a window 400 km long a geologic past spanning more than 3,500 million years.

Low outcrops of dark red rock on the valley floor near Montevideo, blasted through to make way for U.S. Hwy. 212, sparkle to the passing motorist as the sun is reflected back from the crystal faces of minerals that are among the oldest materials ever to be identified in the crust of the earth. From quarries in the area huge blocks of rock equally as old were raised by cranes to be loaded and shipped throughout the United States as architectural stone. The importance of these rocks to an understanding of the early history of the earth is so great that geologists from all over the world come to western Minnesota to study them in their natural surroundings.

The minerals and structures in these primitive rocks reflect an environment of formation much different from that in which they now lie as outcrops. They indicate recurring melting, crystallization, and deformation at temperatures and pressures known to exist only tens of kilometers beneath the earth's surface. Elevation of the rocks to their present position in the crust and their exposure to today's environment occurred as the result of a remarkable series of geologic events, from mountain building to the erosion of great volumes of overlying bedrock.

These oldest rocks, so solid and fresh looking in outcrops that can be seen in Granite Falls, Ortonville, Montevideo, in Big Stone National Wildlife Refuge, and along the shores of the lower end of Lac qui Parle Lake, contrast areas along the sides of the river valley where the rock has decomposed into soft, clay-like substance called kaolin, a result of prolonged exposure to subtropical weather conditions more than 70 million years ago

Away from the valley, the surrounding uplands are generally devoid of bedrock exposures. Instead, the land surface is underlain by a thick blanket of stony sediments that represents still another drastic change in the environment, a great ice age that furnished a cover of glacial till. That period of cold climate and glacial activity ended just 12,000 years ago. Minnesota sits astride two of North America's largest topographical provinces, the Laurentian Upland and the Interior Lowland. The meeting of these two provinces forms the Continental Divide. This divide can be crossed in Browns Valley and at locations in Stevens County. The variety of geological features found on the prairies of west central Minnesota provide area visitors with a number of options on where and how to explore the area.

Information is from: Minnesota's Geology by Richard W. Ojakanga and Charles L. Matsch, University of Minnesota Press, Minneapolis, 1982.

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