Great Plains Geology (Discover the Great Plains) - Softcover

Diffendal Jr., R.F.

Softcover

ISBN 10: 0803249519 ISBN 13: 9780803249516

Publisher: Bison Books, 2017

Synopsis

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.

 

About this title

About the Author

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.

Excerpt. © Reprinted by permission. All rights reserved.

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 sometimes fairly sharply defined by a structural element like a major fault or fold, but most have boundaries that are more arbitrary. Please see appendix 1 for more information on these subdivisions.

How Did the Great Plains Come to Be?

Now that I have established a boundary for the Great Plains, you might ask how this physiographic province came to be. The major characteristic of the region is a fairly flat land surface. Here and there, however, the land surface changes. How did that happen? What explains the appearance of the Great Plains today? The results of geologic change over millions of years provide some answers to these and other questions, such as:

Why does the Great Plains tilt downward toward the east? Collisions of tectonic plates along the western edge of North America produced folds and earthquake faults (fig. 6). The Rocky Mountains were pushed up along some of these faults. As the mountains rose, so also did the plains to the east, where they rose highest nearest to the mountain front. The result is a land surface that tilts downward to the east (fig. 7). After the faulting occurred, increases in the volume of mineral crystals in the rocks buried deeply beneath the mountains and the nearby plains further uplifted the entire region to the altitudes of today.

Why do the Black Hills stick up in the middle of flat lands? The rocks beneath the Black Hills of South Dakota were pushed up by compression forces coming mostly from the west to form a huge elongated, blister-shaped fold that is generally steeper on the eastern side (fig. 8). After the folding, the rocks decayed by exposure to the atmosphere and living organisms and winds and rivers eroded parts of the uplifted rocks to produce the rugged, mountainous landscape that we see today.

Why are there so many volcanic cones, rocks, and hardened lava flows on the Great Plains in southeastern Colorado and northeastern New Mexico? A line of weakness or lineament in the Earth's crust runs from east central Arizona to the Great Plains of northeastern New Mexico and southeasternmost Colorado. From time to time the rocks beneath the surface here have broken, and molten rock from deep beneath the Earth has welled out onto the land to form the cones and flows (fig. 9). Some of these volcanic rocks formed less than 20,000 years ago and appear fresh and new.

Why is the High Plains Aquifer thickest and most widespread in Nebraska and thinner to the north and south? The water in the High Plains aquifer is contained in pore spaces in river deposits filling many ancient valleys. The rivers in these valleys drained much of the Rocky Mountains and adjacent plains. In Nebraska, wide and deep valleys sloped eastward and northeastward across the state, the general trend of the thickest and most widespread deposits of the aquifer. Ancient rivers to the north in the Dakotas and to the south in the Great Plains of Kansas, southern Colorado, western Oklahoma, western Texas, and eastern New Mexico did not carve out such deep valleys and generally did not leave behind deposits as thick as those in Nebraska.

Where did the sand in the Nebraska Sand Hills and other sand dune areas of the Great Plains Come from and why? Ancient rivers that crossed the plains in the past carried gravel and sand grains and deposited them in their valleys. Dunes in the Nebraska Sand Hills and other parts of the Great Plains were formed when exposed sand, left behind when the courses of the rivers changed, was picked up by winds and deposited in the form of dunes during prolonged major droughts (fig. 10).

What caused the "prairie potholes" of Southern Alberta, Saskatchewan, and Manitoba and eastern North and South Dakota? When the ice sheets covering this part of the Great Plains melted, debris carried by the ice was left behind as a blanket-like deposit covering much of the bedrock. During the melting, pieces of ice sometimes broke off the sheet and were buried in the glacial deposits. When this buried ice melted, it left behind basin-shaped depressions on the land surface that filled with precipitation or groundwater. These wetland areas are often called "prairie potholes" or "kettles" (fig. 11).

Is there any evidence on the Great Plains related to the end of the Age of Dinosaurs? Most geologists agree that tectonic and climatic effects of large meteorite or comet impacts and major volcanic activity at the end of the Cretaceous Period (see fig. 1) marked the end of the Age of Dinosaurs. At this time the Great Plains area was covered by a sea. The impacts produced tsunamis that left distinctive kinds of sediments in southeastern South Dakota and northeastern Nebraska. The youngest layers of Late Cretaceous sediments in the Great Plains are enriched with the element iridium and also contain minerals indicating meteorite impacts.

CHAPTER 2

Geologic History of the Great Plains

A brief geological history of the province can help you appreciate the many features that you can see while traveling on the Great Plains. But first you must understand some basic geologic concepts.

Some Geologic Concepts

Today most geologists view the Earth as a single system with the atmosphere, hydrosphere, lithosphere, and biosphere all interconnected to produce the world that we inhabit.

A few geological discoveries and general geologic principles (see appendix 2) will be useful for you to know. In 1669 Nicolas Steno, a Dane, wrote a small book outlining three key principles still used in the study of sediments and sedimentary rocks (fig.12 A-C; bottom layer, labeled 1, is oldest). Sediment, carried into water, moves to the floor of the body of water (anything from a mud puddle to an ocean basin), where it accumulates in nearly horizontal layers (the principle of original horizontality). The layers accumulate one at a time, with the first deposited at the bottom and others deposited above it (principle of superposition). The layers are continuous across the body of water (principle of lateral continuity).

James Hutton, a Scot, published his Theory of the Earth with Proofs and Illustrations in 1795. In this and later publications, Hutton and others explained that the Earth's rocks were formed in cycles. Mountains form and partially erode away and sea levels rise and fall repeatedly through geologic time. Hutton observed that any geologic feature, such as a fault, that cuts across another must be the geologically younger feature, an observation that later became known as the law of crosscutting relationships (fig. 12 D).

Then, in 1815, William Smith, an Englishman, prepared the first geologic map of Great Britain. Smith established the principle or law of floral and faunal succession (fig. 12 A–D). That is, fossil species occur in layers deposited in sequence one at a time, with the oldest at the bottom and the youngest at the top. Species succeed one another in a definite and recognizable order, and each geologic formation can have flora and fauna different from the formations above and below. This understanding has allowed geologists to match up layers of rock of roughly equivalent geologic age locally and worldwide and to establish a worldwide geologic rock column and relative timescale.

The development of other fundamental concepts continued. While correlating layers of sediment and sedimentary rock using the fossils they contained, geologists discovered that there was often an abrupt change in fossil types below and above major surfaces of erosion or nondeposition in the rock column. These surfaces, which James Hutton noted in 1795, signal gaps of time between the sediment deposits. These gaps are called unconformities. Geologists have designated the rock sequences between major unconformities as "systems."

Starting in 1877, Grove Karl Gilbert wrote on the development of river valleys. He noted that, in most cases, rivers have, through time, eroded the valleys in which they flow.

By 1913, scientific technology had reached a point of development where isotopes of elements could be detected and their proportions in minerals and rocks could be measured. In that year, Arthur Holmes of Great Britain used ratios of uranium and lead isotopes in minerals found in some rocks in the geologic column to make the first absolute geologic timescale. Since then, improved technology and new dating techniques have allowed for further refinement of the timescale (fig. 1). Figure 1 is not Holmes's original timescale but the most recent iteration. Note that there are parallel terms in the table for the rocks deposited in certain time spans (erathem, system, series) and for the actual spans of time (era, period, epoch). Geologists separate these so that they know if they are talking about rocks or about the time when they formed.

Events in Earth's History

I pointed out in 1991 that it is impossible to separate the geologic development of the Great Plains from the development of the Earth as a whole. The principal drivers of geological changes on Earth through time are the sun's radiant energy and the Earth's internal heat. After life-forms appeared, they became a third driver. The amount of solar energy that arrives on Earth has generally increased through time over billions of years, but it also varies on human timescales.

The Earth, once fully molten, began to cool about 4.6 billion years ago to its current state. Throughout Earth's history, convection currents have circulated hotter materials from the mantle, the part between the Earth's core and its outer layer or crust, toward the surface and have moved cooler materials back down. As the Earth's crust developed by the concentration of elements with lighter atomic weights at the surface and the oceans and atmosphere developed through the process of outgassing, the ocean basins began to form.

Early in Earth's history, around 3.8 billion years ago or earlier, continents began to form. The minerals and rocks of these lands were subject to the weathering and erosional processes prevailing at those times. Some of these processes may have differed from today's because then the atmosphere contained far less free oxygen and nitrogen and the oceans far less salt.

Also, the oceans may not have been as deep as today because today's deepest ocean basins may be due, in part, to overall cooling and contraction of rocks beneath the sea floor. Continents continued to build up and break up through time as convection currents in the mantle moved oceanic crust and land masses around the surface and recycled oceanic crust.

It is clear that the climate has never been static. In the 1950s, when "continental drift" was being discussed as a theory, geologists studying rocks in the southern hemisphere discovered erosional features and deposits of sediments in Precambrian rocks in Africa, South America, Antarctica, Australia, and India that were formed or deposited by ice sheets. Today, ancient glacial striations (parallel lines or grooves) on those older rocks extend in different directions on these continents. When, however, scientists fitted the continents together like pieces of a jigsaw puzzle, they found that the striations all radiated out from a central area on the reconstructed supercontinent they called Gondwanaland.

(Continues...)

Excerpted from Great Plains Geology by R. F. Diffendal Jr.. Copyright © 2017 Board of Regents of the University of Nebraska. Excerpted by permission of UNIVERSITY OF NEBRASKA PRESS.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Edition details

• Publisher: Bison Books
• Publication date: 2017
• Language: English
• ISBN 10: 0803249519
• ISBN 13: 9780803249516
• Binding: Paperback
• Number of pages: 240