Great Plains Geology (Discover the Great Plains) - Softcover

Diffendal, R. F.

Softcover

ISBN 10: 0803249519 ISBN 13: 9780803249516

Verlag: UNIV OF NEBRASKA PR, 2017

Inhaltsangabe

Great Plains Geology concisely guides readers through the geological development of the Great Plains region. It describes the distinct features of fifty-seven geologic sites, including fascinating places such as Raton Pass in Colorado and New Mexico, the Missouri Breaks of Montana, and the Ashfall Fossil Beds in Nebraska. This guide addresses the tricky question of what constitutes the Great Plains, showing that the region is defined in part through its unique geologic features.

 

Über diesen Titel

Über die Autorin bzw. den Autor

R. F. Diffendal Jr. is professor emeritus in the School of Natural Resources at the University of Nebraska–Lincoln. He is the author of Lewis and Clark and the Geology of the Great Plains.

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Great Plains Geology

By R. F. Diffendal Jr.

UNIVERSITY OF NEBRASKA PRESS

Copyright © 2017 Board of Regents of the University of Nebraska
All rights reserved.

ISBN: 978-0-8032-4951-6

Contents

List of Illustrations, xi, 
Preface, xv, 
Acknowledgments, xvii, 
Introduction, xix, 
1. What Is the Great Plains?, 1, 
2. Geologic History of the Great Plains, 13, 
3. Visiting the Great Plains, 33, 
SITES IN CANADA, 
MONTANA, 
NORTH DAKOTA, 
SOUTH DAKOTA, 
WYOMING, 
NEBRASKA, 
KANSAS, 
COLORADO, 
NEW MEXICO, 
TEXAS, 
Afterword, 175, 
Appendix 1. Geologic Subdivisions of the Great Plains, 177, 
Appendix 2. Chronology of the Development of Some Geological Concepts, 183, 
Appendix 3. Cautions for Travelers on the Great Plains, 185, 
Glossary, 187, 
Bibliography, 195, 
Index, 201, 

CHAPTER 1

What Is the Great Plains?

Drawing the Boundaries

With the exception of the western edge, bordered mostly by the Rocky Mountains, the Great Plains is a region (also called a physiographic province) with few obvious boundaries. In the years since John Wesley Powell first mapped, described, and named it, few authors have agreed where the Great Plains begins or ends. In contrast, most of the other physiographic provinces of the United States, Canada, and Mexico are rather sharply delineated. Fifty published maps of the region show many different versions of its supposed boundaries (fig. 2). Some include land as far north as the Canadian Northwest Territories, as far south as Mexico, as far east as Illinois, or as far west as Utah. Some more recent ones have even more unusual boundaries, including one that puts Indiana, Michigan, and Ohio in the Great Plains but leaves out any states south and southwest of Kansas. Almost all have boundaries drawn as solid lines, indicating a certainty that is clearly not so. In fact, all are approximations based on either the understanding or the misunderstanding of the individual map maker.

To me, the extremes are not parts of the Great Plains. Most of the western boundary lines shown in figure 2 cluster along the break between the plains and the eastern slopes of the Rocky Mountains. The northern boundaries lie mostly in the southern parts of the Canadian Prairie Provinces, and the southern boundaries lie mostly in western Texas. The eastern boundaries are more problematic. However, two clusters of boundary lines are recognizable. One of these clusters mostly follows the upper Mississippi Valley; the other lies to the west of that some hundreds of miles.

Drawing a boundary for the Great Plains is no easy task for someone who wants to be precise. To make matters worse, the region is classified by some as a "physiographic province," an area similar in geologic structure, and by others as a "natural region," an area with similar climate, vegetation, and physical features like elevation (fig. 3).

According to the geologist Charles B. Hunt the basic differences between physiographic provinces are structural, referring to uplift, earthquake faulting, bending and folding of rock layers, volcanism, or combinations of these processes. The variances in these processes and the degree to which they have shaped a given area create physically distinct regions and sections therein. Anyone approaching the Rocky Mountains from the plains can see an abrupt change and would generally agree that two distinct regions meet at this physical boundary.

The transition from the gently tilted rocks of the plains to the more steeply tilted rock layers of the foothills marks the general boundary between the western Great Plains and the adjacent Rocky Mountains to the west. This same kind of distinct visual break can also be seen at the southwestern margin of the Great Plains where that physiographic province meets the faulted rocks of the Basin and Range Province in western Texas and southeasternmost New Mexico and along the Balcones Fault Zone in south central Texas where the southeastern Great Plains border adjoins the western margin of the Coastal Plain Province.

Defining the eastern, northern, and southern boundaries of the Great Plains is much more difficult. Except for the boundary in south central Texas, there is no sharp structural break between the Great Plains and the plains regions continuing to the east, north, and south of it. The Great Plains slopes gently eastward from its western boundary to the plains areas to its east. Sediments and sedimentary rock layers beneath the land surface usually appear to be only gently tilted.

In the face of these uncertainties, where do I draw the boundary of the Great Plains? Because this is a geologic work, I will base my boundary on geologic features if at all possible. Those can change over long geologic periods, but such changes require time spans of a magnitude greater than do those related to culture and dependent upon climate. My geologic boundary for the Great Plains and the boundaries of its sections are shown in figure 4 and can also be seen on figure 5. The boundaries suggested previously by most geologists are very close to those in my figure.

The western boundary begins at the break between the Great Plains and the structurally complex Rocky Mountains, running from Alberta, Canada, south to north central New Mexico. From there the line continues south and then southeast along the break between the Great Plains and the easternmost parts of the Basin and Range Province to just east of the Big Bend area of Texas.

The northern border, in Alberta, runs along the south side of the Athabasca River valley, then turns eastward into western Saskatchewan south of the Christina River. From there I have drawn the line at the topographic break of the Missouri Coteau, a low escarpment extending from western Saskatchewan south across North and South Dakota. In Nebraska, recent geologic events left a sometimes thick mantle of sedimentary deposits that mask where the Missouri Coteau might have been so that the line cannot be extended there with any certainty. For this reason I have drawn the line in Nebraska and northeastern Kansas along the approximate western boundary of the Pleistocene continental glaciers. This line is marked by the end of glacial till deposits or erratic boulders either at the land surface or found in drill holes.

The line then continues southwest across Kansas, western Oklahoma, and western Texas following the eastern edge of sedimentary rocks of Cretaceous age and then the eastern edge of the Miocene Ogallala Group. I have then drawn the line along the approximate northern boundary of the Central Texas Uplift, then south and west along the Balcones Fault Zone (finally another structural break) to the Rio Grande in the vicinity of Del Rio, Texas.

Most of the earlier maps showing a Great Plains boundary line have stopped the line at the Rio Grande. I have continued the line across the border into a small part of northeastern Mexico, as some Mexican mappers have done. This makes sense to me because the relevant geologic formations and structures do not stop at the river in Texas but continue on into Mexico. This continuation can easily be demonstrated either by reviewing geologic and topographic maps of the area or by standing on the Texas side of the Rio Grande and looking across the river at the rocks exposed along the valley sides to the south in Mexico.

To further complicate the picture, different geologists have divided the Great Plains Province into ten subdivisions called sections (fig. 4). The boundaries between sections are...

Details zur Ausgabe

• Verlag: UNIV OF NEBRASKA PR
• Erscheinungsdatum: 2017
• Sprache: Englisch
• ISBN 10: 0803249519
• ISBN 13: 9780803249516
• Einband: Taschenbuch
• Anzahl der Seiten: 240
• Kontakt zum Hersteller: Nicht verfügbar
• Verantwortliche Person: preigu; mail@preigu.de; ; Lengericher Landstr. 19; Osnabrück; 49078; Deutschland