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Minnesota’s Geology

Minnesota’s Geology

Richard W. Ojakangas
Charles L. Matsch
Drawings, charts, and graphs by Dan Beedy
Copyright Date: 1982
Edition: NED - New edition
Pages: 268
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  • Book Info
    Minnesota’s Geology
    Book Description:

    Minnesota’s Geology provides a history of the past 3.5 billion years in the area’s development. In accessible language, Minnesota-based geologists Richard W. Ojakangas and Charles L. Matsch tell the story of the state’s past and offer a guide for those who want to read geological history firsthand from the rocks and landscapes of today. "Minnesota’s Geology sets a standard of excellence for books about the geology of a region. . . an unusually well-written and well-balanced book." Science Books and Films

    eISBN: 978-0-8166-8167-9
    Subjects: Geology

Table of Contents

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    • 1 The Briefing
      (pp. 3-13)

      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 lake, 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...

    • 2 Minnesota’s Place in Geologic History
      (pp. 15-20)

      Minnesota is situated near the low-relief center of the North American continent, about equidistant from North America’s two big mountain belts, the Rockies and the Appalachians. One might think it unfortunate that Minnesota has no mountains. But Minnesota had its day. Twice—2,700 million years ago and again about 1,800 million years ago—sizable mountains were part of the Minnesota and Lake Superior scene. Spectacular volcanism was commonplace at those times and 1,100 million years ago as well. But 2,700 million years is a tremendously long expanse of time for erosion to have been at work. The 225-million-year-old Appalachians and...


    • 3 Early Precambrian Time (4,500 to 2,500 Million Years Ago)
      (pp. 23-33)

      The rocks of Early Precambrian age are exciting, if for no other reason than their great antiquity. These rocks contain clues to the conditions and events of a distant part of the Earth’s past. Interpreting these old clues is indeed exciting. For example, the study of many rocks from the northern part of the state indicates a history of explosive volcanism. Although the fires are now quiet, the “brimstone” remains for close scrutiny by the earth scientist.

      Lower Precambrian rocks underlie much of Minnesota, but they are mostly covered with glacial sediment. Figure 3-1 shows diagrammatically their relations to younger...

    • 4 Middle Precambrian Time (2,500 to 1,600 Million Years Ago)
      (pp. 35-45)

      History is forever in the making. Events of one era may well influence events of a distant, later era. This fact can be illustrated no better than here in Minnesota. Iron-bearing sediment deposited in a Middle Precambrian sea about 2,000 million years ago, probably in part by the action of seemingly insignificant microscopic bacteria, has had a most profound effect on the socioeconomic framework of today’s Minnesota. For that matter, Minnesota’s iron played a very important part in shaping the United States into an industrial giant.

      Middle Precambrian time could well be called the age of iron, in a geological...

    • 5 Late Precambrian Time (1,600 to 600 Million Years Ago)
      (pp. 47-61)

      Late Precambrian time extends from about 1,600 million years ago to the beginning of the Paleozoic (“old life”) Era, an age of abundant fossils about 600 million years ago. Thus, like the Middle Precambrian, the Late Precambrian spans a billion years. A lot can happen in a billion years, and a lot did!

      Upper Precambrian rocks in Minnesota can be divided into three main groups. The oldest consists of quartz sandstones, the next oldest consists of 1,100-million-year-old lava flows and gabbroic intrusions, and the youngest consists of postvolcanic sandstones and other sediments. These are shown diagrammatically on the composite generalized...

    • 6 Post-Precambrian Time (600 to 2 Million Years Ago)
      (pp. 63-95)

      While rifting, volcanism, and sedimentation were occurring in the mid-continental region, western, eastern, and northern North America were experiencing the early stages in the development of major downwarped zones calledgeosynclines. These zones, in the oceans and peripheral to the Precambrian Canadian Shield and the rest of the central stable continent, orcraton, measured thousands of kilometers long and a few hundred kilometers wide. The Cordilleran Geosyncline was developing in the west; the Appalachian Geosyncline, in the east; and the poorly studied Franklin Geosyncline, in the far north (Figure 6-1). The first two are of great importance, for they eventually...

    • 7 The Quaternary Period (2 Million Years Ago to the Present)
      (pp. 97-122)

      A few kilometers northwest of Montevideo, on the partially wooded floodplain of the Chippewa River, lies a 20-ton block of fine-grained, dark green-gray rock. Many years ago, local observers recognized the fact that this giant boulder contrasted sharply in color and texture with the nearby bedrock, which is a pink, coarse-grained granitic gneiss. They concluded that the dark stranger had traveled to its resting place from a distant source in outer space, and the rock became locally known as the Montevideo Meteorite (Figure 7-1). Centuries earlier, Indians had also been impressed with some of the boulders littering the landscape of...


    • 8 Metallic Minerals
      (pp. 125-149)

      Iron ore in the Lake Superior region was first discovered near Negaunee, Michigan, in 1844 by a surveying party when great variations were noted in the behavior of compass needles. The canal and locks at Sault Ste. Marie at the east end of Lake Superior were completed in 1855, and that same year the first iron ore from Negaunee, on the Marquette Range, moved through the locks, destined for Cleveland. The Menominee Range began producing in the 1870s and the Gogebic Range was opened up in the 1880s (see Figure 4-7).

      Meanwhile, what was going on in Minnesota? In 1855,...

    • 9 Nonmetals, Fuels, and Water
      (pp. 151-158)

      Metals are the glamour elements when it comes to mineral exploration and exploitation, and the iron deposits of Minnesota deserve premier status in an inventory of the state’s mineral wealth. Or do they? If mineral resources include water, then a price must be set on lakes, streams, swamps, and groundwater reserves. But how does one determine in dollars and cents the value of rivers and lakes as effluent disposal systems, transportation routes, and water supplies? It is difficult, and even impossible when biological, recreational, and aesthetic uses are added to the picture. Consider also the exploitation of the soil in...


    • 10 Northeastern Minnesota
      (pp. 161-197)

      Belying its character as part of an old peneplain surface, northeastern Minnesota holds both the highest and lowest elevations in the state and therefore has the most rugged relief. From a high of 701 m above sea level on Eagle Mountain in central Cook County, the land drops to 183 m at the shoreline of Lake Superior. And were the lake waters to disappear, the land would bottom out about 400 m lower. Much of the landscape owes its character to differential erosion of the bedrock, both by running water and by glaciers. It is surprising to realize that, although...

    • 11 Northwestern Minnesota
      (pp. 199-209)

      The Red River of the North, which forms the northern half of the western border of Minnesota, flows northward along the axis of a broad topographic basin covered by the silts and clays of Glacial Lake Agassiz. Upper Red Lake, Lower Red Lake, and Lake of the Woods are remnants of that large water body, which drained away sometime before 8,000 years ago. The exposed lakebed, which stretches eastward all the way to International Falls, accounts for some of the flattest terrain to be seen anywhere, interrupted only by the incised valleys of tributary streams and by hundreds of kilometers...

    • 12 Central Minnesota
      (pp. 211-221)

      Rugged relief, poor drainage that is reflected in the great number of lakes, and sparse outcrops of bedrock all indicate the importance of glaciation in creating the landscape of central Minnesota. “Moraine terrain” is an apt general name for the area. The upper Mississippi River is the major watercourse in the central part of the state, and the St. Croix River forms the eastern boundary of this segment of the state. Mille Lacs Lake is one of the larger lakes in Minnesota. At an elevation of 380 m, its surface is almost 200 m higher than that of Lake Superior....

    • 13 Southwestern Minnesota
      (pp. 223-231)

      A flatiron-shaped plateau, called the Coteau des Prairies by early French explorers, dominates the regional topography of eastern South Dakota and southwestern Minnesota (Figure 13-1). This distinctive “highland of the prairies” rises dramatically to elevations of more than 640 m in South Dakota. The eastern flank of the plateau descends in a series of broad steps to merge with the Minnesota River Lowland, a southeastern extension of the topographic basin occupied by the Red River of the North. The entire landscape owes its major form to a similar arrangement of the bedrock surface, even though important details of local topography...

    • 14 Southeastern Minnesota
      (pp. 233-244)

      Southeastern Minnesota contains two sharply contrasting landscapes directly related to the glacial history of the area. Bordering the Mississippi River Valley and extending westward to the Bemis-Altamont moraine system is a lake-free terrain with a well-integrated and, in places, deeply entrenched stream system. Inside and to the west of the moraines, the land surface is dotted with lakes, and the streams are not so well developed, a direct reflection of the youthfulness of the surface, ice-free only since the demise of the Des Moines Lobe about 13,000 years ago. The entire region rises topographically from the Minnesota River and Minneapolis...