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Title: The Geologic Story of the Great Plains
Author: Trimble, Donald E.
Language: English
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[Illustration: DENVER, COLORADO]
_But from these immense prairies may arise one great advantage to the
United States, viz., the restriction of our population to some certain
limits, and thereby a continuation of the union. Our citizens being so
prone to rambling, and extending themselves on the frontiers, will,
through necessity, be constrained to limit their extent on the west to
the borders of the Missouri and the Mississippi, while they leave the
prairies, incapable of cultivation, to the wandering and uncivilized
Aborigines of the country.
Zebulon Pike_
Exploratory Travels Through The Western Territories of North America
comprising a voyage from St. Louis, on the Mississippi, to the source
of that river, and a journey through the interior of Louisiana and the
north-eastern provinces of New Spain. Performed in the years 1805,
1806, and 1807, by order of the Government of the United States. By
Zebulon Montgomery Pike. Published by Paternoster-Row, London, 1811:
W. H. Lawrence and Company, Denver, 1889. Quotation from pages
230-231, 1889 edition.
The GEOLOGIC STORY of
The GREAT PLAINS
By DONALD E. TRIMBLE
_A nontechnical description of the origin and evolution of the landscape
of the Great Plains_
GEOLOGICAL SURVEY BULLETIN 1493
UNITED STATES DEPARTMENT OF THE INTERIOR
CECIL D. ANDRUS, _Secretary_
GEOLOGICAL SURVEY
H. William Menard, _Director_
[Illustration: U. S. DEPARTMENT OF THE INTERIOR · March 3, 1849]
UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1980
Library of Congress Cataloging in Publication Data
Trimble, Donald E.
The geologic story of the Great Plains.
(U.S. Geological Survey Bulletin 1493)
Bibliography: p. 50
Includes index.
Supt. of Docs. no.: I 19.3: 1493
I. Geology—Great Plains. I. Title.
II. Series: United States Geological Survey Bulletin 1493.
QE75.B9 no. 1493 [QE71] 557.3s [557.8] 80-607022
For sale by the Superintendent of Documents, U.S. Government Printing
Office
Washington, D.C. 20402
CONTENTS
Introduction 1
What is the Great Plains? 5
The Great Plains—its parts 7
Early history 10
Warping and stream deposition 11
Sculpturing the land 18
Landforms of today—The surface features of the Great Plains 19
Black Hills 20
Central Texas Uplift 22
Raton Section 23
High Plains 25
Missouri Plateau 32
Preglacial Drainage 33
Glaciated Missouri Plateau 33
Unglaciated Missouri Plateau 36
The Colorado Piedmont 42
Pecos Valley 45
Edwards Plateau 46
Plains Border Section 48
Epilogue 49
Acknowledgments 49
Some source references 50
FIGURES
FRONTISPIECE. Aerial photograph of Denver.
1. Index map 3
2-3. Maps showing:
2. Physical divisions of the United States and maximum extent
of the continental ice sheets 6
3. The Great Plains province and its sections 8
4. Photograph of Mescalero escarpment and southern High Plains 9
5. Geologic time chart 12
6-8. Maps showing:
6. Paleogeography of U.S. in Late Cretaceous 14
7. Structural setting of the Great Plains 14
8. Progressive southward expansion of areas of deposition 17
9. Photograph of Big Horn strip mine at Acme, Wyo. 18
10. Black Hills diagram 21
11-16. Photographs showing:
11. Weathering of granite at Sylvan Lake in the Black Hills 22
12. Capulin Mountain National Monument, N. Mex. 24
13. Mesa de Maya, Colo. 25
14. Spanish Peaks, Colo. 26
15. “The Gangplank,” Wyo. 27
16. Scotts Bluff National Monument, Nebr. 28
17. Aerial photograph of the Nebraska Sand Hills 30
18. Map of the Nebraska Sand Hills 31
19-30. Photographs showing:
19. Loess plain in Nebraska 32
20. Ground moraine on the Coteau du Missouri in North Dakota 34
21. Slump blocks in North Unit of Theodore Roosevelt National
Memorial Park, N. Dak. 36
22. Highwood Mountains, Mont. 37
23. Devils Tower National Monument, Wyo. 38
24. Glaciated valley in Crazy Mountains, Mont. 39
25. Powder River Basin in vicinity of Tongue River 40
26. Badlands National Monument, S. Dak. 41
27. Badlands of Little Missouri River in South Unit of
Theodore Roosevelt National Memorial Park, N. Dak. 42
28. Hogback ridges along the Front Range, Colo. 43
29. Pawnee Buttes, Colo. 44
30. Edwards Plateau, Tex. 47
TABLE
1. Generalized chart of rocks of the Great Plains 15
The GEOLOGIC STORY of
The GREAT PLAINS
By Donald E. Trimble
INTRODUCTION
The Great Plains! The words alone create a sense of space and a feeling
of destiny—a challenge. But what exactly is this special part of Western
America that contains so much of our history? How did it come to be? Why
is it different?
Geographically, the Great Plains is an immense sweep of country; it
reaches from Mexico far north into Canada and spreads out east of the
Rocky Mountains like a huge welcome mat. So often maligned as a drab,
featureless area, the Great Plains is in fact a land of marked contrasts
and limitless variety: canyons carved into solid rock of an arid land by
the waters of the Pecos and the Rio Grande; the seemingly endless
grainfields of Kansas; the desolation of the Badlands; the beauty of the
Black Hills.
Before it was broken by the plow, most of the Great Plains from the
Texas panhandle northward was treeless grassland. Trees grew only along
the floodplains of streams and on the few mountain masses of the
northern Great Plains. These lush prairies once were the grazing ground
for immense herds of bison, and the land provided a bountiful life for
those Indians who followed the herds. South of the grasslands, in Texas,
shrubs mixed with the grasses: creosote bush along the valley of the
Pecos River; mesquite, oak, and juniper to the east.
The general lack of trees suggests that this is a land of little
moisture, as indeed it is. Nearly all of the Great Plains receives less
than 24 inches of rainfall a year, and most of it receives less than 16
inches. This dryness and the strength of sunshine in this area, which
lies mostly between 2,000 and 6,000 feet above sea level, create the
semiarid environment that typifies the Great Plains. But it was not
always so. When the last continental glacier stood near its maximum
extent, some 12,000-14,000 years ago, spruce forest reached southward as
far as Kansas, and the Great Plains farther south was covered by
deciduous forest. The trees retreated northward as the ice front
receded, and the Great Plains has been a treeless grassland for the last
8,000-10,000 years.
For more than half a century after Lewis and Clark crossed the country
in 1805-6, the Great Plains was the testing ground of frontier
America—here America grew to maturity (fig. 1). In 1805-7, explorer
Zebulon Pike crossed the south-central Great Plains, following the
Arkansas River from near Great Bend, Kans., to the Rocky Mountains. In
later years, Santa Fe traders, lured by the wealth of New Mexican trade,
followed Pike’s path as far as Bents Fort, Colo., where they turned
southwestward away from the river route. Those pioneers who later
crossed the plains on the Oregon Trail reached the Platte River near the
place that would become Kearney, Nebr., by a nearly direct route from
Independence, Mo., and followed the Platte across the central part of
the Great Plains.
Although these routes may have seemed long and tedious to those dusty
travelers, they provided relatively easy access to the Rocky Mountains
and had a continuous supply of fresh water, an absolute necessity in
these plains. The minds of those frontiersmen surely were occupied with
the dangers and demands of the moment—and with dreams—but the time
afforded by the slow pace of travel also gave them ample opportunity for
thought about the origins of their surroundings.
Today’s traveler, who has less time for contemplation, races past a
changing kaleidoscope of landscape. The increased awareness created by
this rapidity of change perhaps is even more likely to stimulate
questions about the origin of this landscape.
[Illustration: _Figure 1.—Index map of the Great Plains showing route of
Lewis and Clark and the Santa Fe and Oregon Trails._]
For instance, the westbound traveler on Interstate Highway 70 traverses
nearly a thousand miles of low, rounded hills after leaving the
Appalachians; the rolling landscape is broken only by a few flat areas
where glacial ice or small lakes once stood. Suddenly, near Salina,
Kans., the observant traveler senses a difference in the landscape.
Instead of rounded hills, widely or closely spaced, he sees on the
skyline flat surfaces, or remnants of flat surfaces. As he climbs gently
westward these broken horizontal lines stand etched against the sky.
About 35 miles west of Salina he finds himself on a broad, flat plateau,
where seemingly he can see forever. True, in places he descends into
stream valleys, but only briefly, for he soon climbs back onto the flat
surface.
This plateau surface continues for 300 miles to the west—to within 100
miles of the abrupt front of the Rocky Mountains. East-flowing streams,
such as the Smoky Hill, the Saline, the Solomon, and the Republican
Rivers and their tributary branches, have cut their valleys into this
surface, but these valleys become increasingly shallow and disappear
entirely near the western rim of the plateau in eastern Colorado.
The distant peaks of the Rockies are seen for the first time as the
traveler approaches the escarpment that forms the western edge of this
great plateau. After crossing the escarpment near Limon, Colo., he
begins the long gentle descent to Denver, on the South Platte River near
the foot of the mountains that loom so awesomely ahead. He has crossed
the Great Plains. The distances have been great, but the contrasts have
been marked.
Had our traveler selected a different route, either to the north or
south, he would have found even greater contrasts, for the Great Plains
has many parts, each with its own distinctive aspect. Why should such
diverse landscapes be considered parts of the Great Plains? What are
their unifying features? And what created this landscape? Has it always
been this way? If not, when was it formed? How was it formed?
We will look here at some of the answers to those questions. The history
of events that produced the landscape of the Great Plains is interpreted
both from the materials that compose the landforms and from the
landforms themselves. As we will see, all landforms are the result of
geologic processes in action. These processes determine not only the
size and shape of the landforms, but also the materials of which they
are made. These geologic processes, which form and shape our Earth’s
surface, are simply the inevitable actions of the restless interior of
the Earth and of the air, water, and carbon dioxide of the atmosphere,
aided by gravity and solar heating (or lack of it). They all have helped
sculpture the fascinating diversity of the part of our land we call the
Great Plains.
WHAT IS THE GREAT PLAINS?
The United States has been subdivided into physiographic regions that,
although they have great diversity within themselves, are distinctly
different from each other (fig. 2).
From the Rocky Mountains on the west to the Appalachians on the east,
the interior of our country is a vast lowland (see cover) known as the
Interior Plains. These plains are bounded on the south by a region of
Interior Highlands, consisting of the Ozark Plateaus and the Ouachita
province, and by the Coastal Plain. In the Great Lakes region, the
Interior Plains laps onto the most ancient part of the continent, the
Superior Upland. West of the Great Lakes it extends far to the north
into Canada. Certainly the Rocky Mountains are distinctly different from
the region to the east, which is the Great Plains. The Great Plains,
then, is the western part of the great Interior Plains. The Rocky
Mountains form its western margin. But what determines its eastern
margin?
During the Pleistocene Epoch or Great Ice Age, huge glaciers formed in
Canada and advanced southward into the great, central, low-lying
Interior Plains of the United States. (See figure 2.) These glaciers and
their deposits modified the surface of the land they covered, mostly
between the Missouri and the Ohio Rivers; they smoothed the contours and
gave the land a more subdued aspect than it had before they came. This
glacially smoothed and modified land is called the Central Lowland.
Although the ice sheets lapped onto the northern part, the Great Plains
is the largely unglaciated region that extends from the Gulf Coastal
Plain in Texas northward into Canada between the Central Lowland and the
foot of the Rocky Mountains. Its eastern margin in Texas and Oklahoma is
marked by a prominent escarpment, the Caprock escarpment. Its southern
margin, where it abuts the Coastal Plain in Texas, is at another abrupt
rise or scarp along the Balcones fault zone.
[Illustration: _Figure 2.—Physical divisions of the United States and
maximum extent of the continental ice sheets during the Great Ice Age._]
THE GREAT PLAINS—ITS PARTS
Within the Great Plains are many large areas that differ greatly from
adjoining areas (fig. 3). The Black Hills stands out distinctively from
the surrounding lower land, and its dark, forested prominence can be
seen for scores of miles from any direction. At the southern end of the
Great Plains is another, less imposing, forested prominence—the Central
Texas Uplift. Most impressive, perhaps, is the huge, nearly flat plateau
known as the High Plains, which extends southward from the northern
border of Nebraska through the Panhandle of Texas, and which forms the
central part of the Great Plains. The east and west rims of the southern
High Plains are at high, cliffed, erosional escarpments—the Caprock
escarpment on the east and the Mescalero escarpment on the west. The
north edge of the High Plains is defined by another escarpment, the Pine
Ridge escarpment, which separates the High Plains from a region that has
been greatly dissected by the Missouri River and its tributaries. There,
several levels of rolling upland are surmounted by small mountainous
masses and flat-topped buttes and are entrenched by streams. This region
is the Missouri Plateau. The continental glacier lapped onto the
northeastern part of the Missouri Plateau and altered its surface.
The South Platte and Arkansas Rivers and their tributaries have
similarly dissected an area along the mountain front that is called the
Colorado Piedmont, and the Pecos River has excavated a broad valley
trending southward from the Sangre de Cristo Mountains in New Mexico
into Texas. The Mescalero escarpment separates the Pecos Valley from the
southern High Plains (fig. 4). South and east of the Pecos Valley,
extending to the Rio Grande and the Coastal Plain, is a broad plateau of
bare, stripped, flat-lying limestone layers bearing little but cactus
that is called the Edwards Plateau. Green, crop-filled valleys with
gently sloping valley walls and rounded stream divides trend eastward
from the High Plains of western Kansas and characterize a Plains Border
section. And finally, between the Colorado Piedmont on the north and the
Pecos Valley on the south, volcanic vents, cinder cones, and lava fields
form another distinctive terrain in the part of the Great Plains called
the Raton section.
[Illustration: _Figure 3.—The Great Plains province and its sections._]
[Illustration: _Figure 4.—Mescalero escarpment and the southern High
Plains (Llano Estacado) south of Tucumcari, N. Mex., Photograph by C. D.
Miller, U. S. Geological Survey._]
Can such diverse parts of our land have a sufficiently common origin to
justify their being considered part of one unified whole—the Great
Plains? Probably so, but to understand why, we must examine some of the
earlier geologic history of the Great Plains as well as subsequent
events revealed in the present landforms. We will find that all parts of
this region we call the Great Plains have a similar early history, and
that the differences we see are the results of local dominance of
certain processes in the ultimate shaping of the landscape, mostly
during the last few million years. The distinctive character of the
landscape in each section is determined in part by both the early events
and the later shaping processes.
EARLY HISTORY
The Interior Plains, of which the Great Plains is the western, mostly
unglaciated part (fig. 2), is the least complicated part of our
continent geologically except for the Coastal Plain. For most of the
half billion years from 570 million (fig. 5) until about 70 million
years ago, shallow seas lay across the interior of our continent (fig. 6
). A thick sequence of layered sediments, mostly between 5,000 and
10,000 feet thick, but more in places, was deposited onto the subsiding
floor of the interior ocean (table 1). These sediments, now consolidated
into rock, rest on a floor of very old rocks that are much like the
ancient rocks of the Superior Upland.
About 70 million years ago the seas were displaced from the continental
interior by slow uplift of the continent, and the landscape that
appeared was simply the extensive, nearly flat floor of the former sea.
WARPING AND STREAM DEPOSITION
Most of these rocks of marine origin lie at considerable depth beneath
the land surface, concealed by an overlying thick, layered sequence of
rocks laid down by streams, wind, and glaciers. Nevertheless, their
geologic character, position, and form are exceptionally well known from
information gained from thousands of wells that have been drilled for
oil. The initial, nearly horizontal position of the layers of rock
beneath the Interior Plains has been little disturbed except where
mountains like the Black Hills were uplifted about 70 million years ago.
At those places, which are all in the northern and southern parts of the
Great Plains, the sedimentary layers have been warped up and locally
broken by the rise of hot molten rock from depth. Elsewhere in the
Interior Plains, however, earth forces of about the same period caused
only a reemphasis of gentle undulations in the Earth’s crust.
These undulations affected both the older basement rocks and the
overlying sedimentary rocks, and they take the form of gentle basins and
arches that in some places span several States. (See sketch map, figure
7.) A series of narrow basins lies along the mountain front on the west
side of the Great Plains. A broad, discontinuous arch extends southwest
from the Superior Upland to the Rocky Mountain front to form a buried
divide that separates the large Williston basin on the north from the
Anadarko basin to the south.
While the flat-lying layers of the Interior Plains were being only
gently warped, vastly different earth movements were taking place
farther west, in the area of the present Rocky Mountains. Along a
relatively narrow north-trending belt, extending from Mexico to Alaska,
the land was being uplifted at a great rate. The layers of sedimentary
rock deposited in the inland sea were stripped from the crest of the
rising mountainous belt by erosion and transported to its flanks as the
gravel, sand, and mud of streams and rivers. This transported sediment
was deposited on the plains to form the rocks of the Cretaceous Hell
Creek, Lance, Laramie, Vermejo, and Raton Formations. Vegetation thrived
on this alluvial plain, and thick accumulations of woody debris were
buried to ultimately become coal. This lush vegetation provided ample
food for the hordes of three-horned dinosaurs (_Triceratops_) that
roamed these plains. Their fossilized remains are found from Canada to
New Mexico.
[Illustration: _Figure 5.—Geologic time chart and the progression of
life forms. Note Cretaceous_ Triceratops, _Oligocene_ Titanotheres, _and
Miocene_ Moropus.]
GEOLOGIC TIME
The Age of the Earth
The Earth is very old—4.5 billion years or more according to recent
estimates. Most of the evidence for an ancient Earth is contained in
the rocks that form the Earth’s crust. The rock layers themselves—like
pages in a long and complicated history—record the surface-shaping
events of the past, and buried within them are traces of life—the
plants and animals that evolved from organic structures that existed
perhaps 3 billion years ago.
Also contained in rocks once molten are radioactive elements whose
isotopes provide Earth scientists with an atomic clock. Within these
rocks, “parent” isotopes decay at a predictable rate to form
“daughter” isotopes. By determining the relative amounts of parent and
daughter isotopes, the age of these rocks can be calculated.
Thus, the results of studies of rock layers (stratigraphy), and of
fossils (paleontology), coupled with the ages of certain rocks as
measured by atomic clocks (geochronology), attest to a very old Earth!
[Illustration: _Figure 6.—Generalized paleogeographic map of the United
States in Late Cretaceous time (65 to 80 million years ago), when most
of the Great Plains was beneath the sea._]
[Illustration: _Figure 7.—Structural setting of the Great Plains.
Williston basin and Anadarko basin are separated by a midcontinental
arch._]
[Illustration: Table 1.—Generalized chart of rocks of the Great Plains]
Geologic age Missouri High Plains—Plains Pecos
Millions of Plateau—Black Border—Colorado Valley—Edwards
years ago Hills Piedmont Plateau—Central
Texas
Quaternary
Pleistocene Glacial deposits, Alluvium, sand Piedmont, terrace,
alluvium, and dunes, and loess and bolson
terrace deposits deposits
2 erosional surface
Tertiary
Pliocene EROSION
5 Flaxville Gravel Ogallala formation
and Ogallala
Formation
Miocene Arikaree Formation Arikaree Formation
22-24 erosional surface
Oligocene White River Group White River Group Mostly missing
because of
erosion or
nondeposition
37-38 erosional surface
Eocene Wasatch and Golden
Valley Formations
53-54 Dawson Arkose
Paleocene Fort Union Denver, Poison
Formation Canyon, and Raton
Formations
65
Cretaceous Hell Creek and Vermejo and Laramie
Lance Formations Formations
Fox Hills Sandstone Trinidad and Fox
Hills Sandstones
Shales, sandstones, and limestones
deposited in Late Cretaceous sea
Dakota Sandstone Dakota Sandstone
and Lakota
Formation
Glen Rose and
Edwards Limestones
136
Jurassic Sundance Morrison Formation Jurassic rocks not
Formation, Ellis present
Group, and
Unkpapa Sandstone
190-195
Triassic Dominantly red rocks
225
PALEOZOIC Paleozoic rocks, undivided
570
PRECAMBRIAN Precambrian rocks, undivided
As the mountains continued to rise, the eroding streams cut into the old
core rocks of the mountains, and that debris too was carried to the
flanks and onto the adjoining plains. The mountainous belt continued to
rise intermittently, and volcanoes began to appear about 50 million
years ago. Together, the mountains and volcanoes provided huge
quantities of sediment, which the streams transported to the plains and
deposited. The areas nearest the mountains were covered by sediments of
Late Cretaceous and Paleocene age (table 1)—the Poison Canyon Formation
to the south, the Dawson and Denver Formations in the Denver area, and
the Fort Union Formation to the north (fig. 8). Vegetation continued to
flourish, especially in the northern part of the Great Plains, and was
buried to form the thick lignite and subbituminous coal beds of the Fort
Union Formation (fig. 9). The earliest mammals, most of whose remains
come from the Paleocene Fort Union Formation, have few modern survivors.
Beginning about 45 million years ago, in Eocene time, there was a long
period of stability lasting perhaps 10 million years, when there was
little uplift of the mountains and, therefore, little deposition on the
plains. A widespread and strongly developed soil formed over much of the
Great Plains during this period of stability. With renewed uplift and
volcanism in the mountains at the end of this period, great quantities
of sediment again were carried to the plains by streams and spread over
the northern Great Plains and southeastward to the arch or divide
separating the Williston and Anadarko basins (fig. 8). Those sediments
form the White River Group, in which the South Dakota Badlands are
carved. In addition to the _Titanotheres_, huge beasts with large, long
horns on their snouts who lived only during the Oligocene (37 to 22
million years ago), vast herds of camels, rhinoceroses, horses, and
tapirs—animals now found native only on other continents—grazed those
Oligocene semiarid grassland plains.
[Illustration: _Figure 8.—Progressive southeastward expansion of areas
covered by Paleocene, Oligocene, and Miocene-Pliocene sedimentary
deposits._]
Powder River basin
Denver basin
Raton basin
PLAINS
Margin of Oligocene deposition
Margin of Miocene-Pliocene deposition
[Illustration: _Figure 9.—Big Horn coal strip mine in Fort Union
Formation at Acme, Wyo. Photograph by F. W. Osterwald, U.S. Geological
Survey._]
Sometime between 20 and 30 million years ago the streams began
depositing sand and gravel beyond the divide, and, for another 10
million years or more, stream sediments of the Arikaree and Ogallala
Formations spread over the entire Great Plains from Canada to Texas,
except where mountainous areas such as the Black Hills stood above the
plains. Between 5 and 10 million years ago, then, the entire Great
Plains was an eastward-sloping depositional plain surmounted only by a
few mountain masses. Horses, camels, rhinoceroses, and a strange
horselike creature with clawed feet (called _Moropus_) lived on this
plain.
SCULPTURING THE LAND
Sometime between 5 and 10 million years ago, however, a great change
took place, apparently as a result of regional uplift of the entire
western part of the continent. While before, the streams had been
depositing sediment on the plains for more than 60 million years,
building up a huge thickness of sedimentary rock layers, now the streams
were forced to cut down into and excavate the sediments they had
formerly deposited. As uplift continued—and it may still be
continuing—the streams cut deeper and deeper into the layered stack and
developed tributary systems that excavated broad areas. High divides
were left between streams in some places, and broad plateaus were formed
and remain in other places. The great central area was essentially
untouched by erosion and remained standing above the dissected areas
surrounding it as the escarpment-rimmed plateau that is the High Plains.
This downcutting and excavation by streams, then, which began between 5
and 10 million years ago, roughed out the landscape of the Great Plains
and created the sections we call the Missouri Plateau, the Colorado
Piedmont, the Pecos Valley, the Edwards Plateau, and the Plains Border
Section. Nearly all the individual landforms that now attract the eye
have been created by geologic processes during the last 2 million years.
It truly is a young landscape.
LANDFORMS OF TODAY—The surface features of the Great Plains
The mountainous sections of the Great Plains were formed long before the
remaining areas were outlined by erosion. Uplift of the Black Hills and
the Central Texas Uplift began as the continental interior was raised
and the last Cretaceous sea was displaced, 65 to 70 million years ago.
They stood well above the surrounding plains long before any sediments
from the distant Rocky Mountains began to accumulate at their bases. In
southern Colorado and northern New Mexico, molten rock invaded the
sedimentary layers between 22 and 26 million years ago. The Spanish
Peaks were formed at this time from hot magma that domed up the surface
layers but did not break through; the magma has since cooled and
solidified and has been exposed by erosion. Elsewhere the magma reached
the surface, forming volcanoes, fissures, and basalt flows. A great
thickness of basalt flows accumulated at Raton Mesa and Mesa de Maya
between 8 and 2 million years ago. Volcanism has continued
intermittently, and the huge cinder cone of Capulin Mountain was created
by explosive eruption only 10,000 to 4,000 years ago. Most of these
volcanic masses were formed before major downcutting by the streams
began. Other igneous intrusions and volcanic areas in the northern Great
Plains similarly were formed before the streams were incised.
To examine the origin of the present landscape and of the landforms
typical of the various sections of the Great Plains, it is convenient to
begin with the Black Hills, the Central Texas Uplift, and the Raton
section simply because they were formed first—they existed before the
other sections were outlined.
BLACK HILLS
The Black Hills is a huge, elliptically domed area in northwestern South
Dakota and northeastern Wyoming, about 125 miles long and 65 miles wide
(fig. 10). Rapid City, S. Dak., is on the Missouri Plateau at the east
edge of the Black Hills. Uplift caused erosion to remove the overlying
cover of marine sedimentary rocks and expose the granite and metamorphic
rocks that form the core of the dome. The peaks of the central part of
the Black Hills presently are 3,000 to 4,000 feet above the surrounding
plains. Harney Peak, with an altitude of 7,242 feet, is the highest
point in South Dakota. These central spires and peaks all are carved
from granite and other igneous and metamorphic rocks that form the core
of the uplift. The heads of four of our great Presidents are sculpted
from this granite at Mount Rushmore National Memorial. Joints in the
rocks have controlled weathering processes that influenced the final
shaping of many of these landforms. Closely spaced joints have produced
the spires of the Needles area, and widely spaced joints have produced
the rounded forms of granite that are seen near Sylvan Lake (fig. 11).
Marine sedimentary rocks surrounding the old core rocks form
well-defined belts. Lying against the old core rocks and completely
surrounding them are Paleozoic limestones that form the Limestone
Plateau (fig. 10). These tilted layers have steep erosional scarps
facing the central part of the Black Hills. Wind Cave and Jewel Cave
were produced by ground water dissolving these limestones along joints.
These caves are sufficiently impressive to be designated as a national
park and a national monument, respectively. Encircling the Limestone
Plateau is a continuous valley cut in soft Triassic shale. This valley
has been called “the Racetrack,” because of its continuity, and the Red
Valley, because of its color. Surrounding the Red Valley is an outer
hogback ridge formed by the tilted layers of the Dakota Sandstone, which
are quite hard and resistant to erosion. Streams that flow from the
central part of the Black Hills pass through the Dakota hogback in
narrow gaps.
[Illustration: _Figure 10.—Diagram of the Black Hills uplift by A. N.
Strahler (Strahler and Strahler, 1978). Used by permission._]
Dakota Sandstone hogback
Limestone plateau
Belle Fourche River
Spearfish
Bear Butte
Sundance
Red Valley
Rapid City
Red Valley
Hot Springs
Cheyenne River
Edgemont
Mt. Rushmore National Monument
Jewel Cave National Monument
Wind Cave National Park
[Illustration: _Figure 11.—Jointed granite rounded by weathering at
Sylvan Lake, in the central part of the Black Hills, S. Dak._]
The Black Hills, then, is an uplifted area that has been carved deeply
but differentially by streams to produce its major outlines. Those
outlines have been modified mainly by weathering of the ancient core
rocks and solution of the limestone of the Limestone Plateau.
CENTRAL TEXAS UPLIFT
The domed rocks of the Central Texas Uplift form a topography different
from that of the Black Hills. Erosion of a broad, uplifted dome here has
exposed a core of old granites, gneisses, and schists, as in the Black
Hills, but in the Central Texas Uplift, erosion has produced a
topographic basin, rather than high peaks and spires, on the old rocks
of the central area. A low plateau surface dissected into rounded ridges
and narrow valleys slopes gently eastward from the edge of the central
area to an escarpment at the Balcones fault zone, which determines the
eastern edge of the Great Plains here. Northwest of the central basin
the Colorado River flows in a broad lowland about 100 miles long, but
the northern edge of the uplift, forming a divide between the Brazos and
the Colorado Rivers, is a series of mesas formed of more resistant
sandstone and limestone.
The cutting action of streams, modified or controlled in part by
differences in hardness of the rock layers, has been responsible for the
landforms of the Central Texas Uplift. Weathering of the old core rocks
has softened them sufficiently to permit deeper erosion of the central
area, and solution of limestone by ground water has formed such features
as Longhorn Caverns, 11 miles southwest of Burnet, Tex.
RATON SECTION
Volcanism characterizes the Raton section. The volcanic rocks, which
form peaks, mesas, and cones, have armored the older sedimentary rocks
and protected them from the erosion that has cut deeply into the
adjoining Colorado Piedmont to the north and Pecos Valley to the south.
The south edge of the Raton section is marked by a spectacular
south-facing escarpment cut on the nearly flat-lying Dakota Sandstone.
This escarpment is the Canadian escarpment, north of the Canadian River.
Northward for about 100 miles, the landscape is that of a nearly flat
plateau cut on Cretaceous rock surmounted here and there by young
volcanic vents, cones, and lava fields. Capulin Mountain is a cinder
cone only 10,000 to 4,000 years old (fig. 12). Near the New
Mexico-Colorado border, huge piles of lava were erupted 8 to 2 million
years ago onto an older, higher surface on top of either the Ogallala
Formation of Miocene age or the Poison Canyon Formation of Paleocene
age. (See table 1.) These lava flows formed a resistant cap, which
protected the underlying rock from erosion while all the surrounding
rock washed away. The result is the high, flat-topped mesas, such as
Raton Mesa and Mesa de Maya (fig. 13), that now form the divide between
the Arkansas and Canadian Rivers. At Fishers Peak, on the west end of
Raton Mesa, about 800 feet of basalt flows rest on the Poison Canyon
Formation at about 8,800 feet in altitude. Farther east, on Mesa de
Maya, about 400 feet of basalt flows overlie the Ogallala Formation at
altitudes ranging from about 6,500 feet at the west end to about 5,200
feet at the east end, some 35 miles to the east. The Ogallala here rests
on Cretaceous Dakota Sandstone and Purgatoire Formation, for the Poison
Canyon Formation was removed by erosion along the crest of a local
uplift before the Ogallala was deposited.
[Illustration: _Figure 12.—Capulin Mountain National Monument in
northeastern New Mexico. This huge cinder cone, which erupted between
4,000 and 10,000 years ago, rises more than 1,000 feet above its base.
Photograph by R. D. Miller, U.S. Geological Survey._]
East of the belt of upturned sedimentary layers that form the hogback
ridges at the front of the Rocky Mountains, the layered rocks in the
Raton Basin have been intruded in many places by igneous bodies, the two
largest of which form the Spanish Peaks (fig. 14), southwest of
Walsenburg, Colo. These two peaks are formed by igneous bodies that were
intruded 26 to 22 million years ago and have since been exposed by
removal of the overlying sedimentary rock layers by erosion. Radiating
from the Spanish Peaks are hundreds of dikes, nearly vertical slabs of
igneous rock that filled fractures radiating from the centers of
intrusion. Erosion of the sedimentary layers has left many of these
dikes as conspicuous vertical walls of igneous rock that project high
above the surrounding land surface. Some of these dikes north of
Trinidad, Colo. extend eastward for about 25 miles, almost to the
Purgatoire River.
[Illustration: _Figure 13.—Lava-capped Mesa de Maya, east of Trinidad,
Colo. Spanish Peaks in left distance. Mesa rises about 1,000 feet above
surrounding area. Photograph by R. B. Taylor, U.S. Geological Survey._]
The northern boundary of the Raton section is placed somewhat
indefinitely at the northern limit of the area injected by igneous
dikes. The eastern boundary of the Raton section is at the eastern
margin of the lavas of Mesa de Maya and adjoining mesas, where
lava-capped outliers of Ogallala Formation are separated from the
Ogallala of the High Plains only by the canyon of Carrizo Creek.
HIGH PLAINS
At the end of Ogallala deposition, some 5 million years ago, the Great
Plains, with the exception of the uplifted and the volcanic areas, was a
vast, gently sloping plain that extended from the mountain front
eastward to beyond the present Missouri River in some places. Regional
uplift of the western part of the continent forced the streams to cut
downward; land near the mountains was stripped away by the Missouri, the
Platte, the Arkansas, and the Pecos Rivers, and the eastern border of
the plains was gnawed away by lesser streams. A large central area of
the plain is preserved, however, essentially untouched and unaffected by
the streams, as a little-modified remnant of the depositional surface of
5 million years ago. This Ogallala-capped preserved remnant of that
upraised surface is the High Plains. In only one place does that old
surface still extend to the mountains—at the so-called “Gangplank” west
of Cheyenne, Wyo. (fig. 15). In places, as at Scotts Bluff National
Monument, Nebr. (fig. 16), small fragments of this surface have been
isolated from the High Plains by erosion and now stand above the
surrounding area as buttes.
[Illustration: _Figure 14.—Spanish Peaks, southwest of Walsenburg, Colo.
Igneous rocks and many radiating dikes exposed by erosion. Photograph by
R. B. Taylor, U.S. Geological Survey._]
[Illustration: _Figure 15.—Looking east toward Cheyenne at “the
Gangplank.” Interstate Highway 80 and the Union Pacific Railroad follow
the Gangplank from the High Plains in the distance onto the Precambrian
rocks (older than 570 m.y.) of the Laramie Mountains in the foreground.
Photograph by R. D. Miller, U.S. Geological Survey._]
[Illustration: _Figure 16.—Aerial view of Scotts Bluff National
Monument, Nebr. Buttes on the south side of the valley of the North
Platte River isolated by erosion from High Plains in the background.
Highest butte stands about 800 feet above valley floor._]
The High Plains extends southward from the Pine Ridge escarpment, near
the South Dakota-Nebraska border (fig. 3), to the Edwards Plateau in
Texas. The Platte, the Arkansas, and the Canadian Rivers have cut
through the High Plains. That part of the High Plains south of the
Canadian River is called the Southern High Plains, or the Llano Estacado
(staked plain). The origin of this name is uncertain, but it has been
suggested that the term Llano Estacado was applied by early travelers
because this part of the High Plains is so nearly flat and devoid of
landmarks that it was necessary for those pioneers to set lines of
stakes to permit them to retrace their routes.
The Llano Estacado is bounded on the west by the Mescalero escarpment
(fig. 4) and on the east by the Caprock escarpment. The southern margin
with the Edwards Plateau is less well defined, but King Mountain, north
of McCamey, Tex., is a scarp-bounded southern promontory of the High
Plains. The remarkably flat surface of the Llano Estacado is abundantly
pitted by sinks and depressions in the surface of the Ogallala
Formation; these were formed by solution of the limestone by rainwater
and blowing away or deflation by wind of the remaining insoluble
particles. Many of these solution-deflation depressions are aligned like
strings of beads, suggesting that their location is controlled by some
kind of underlying structure, such as intersections of joints in the
Ogallala Formation.
The solution-deflation depressions are less abundant north of the
Canadian River, but occur on the High Plains surface northward to the
Arkansas River and along the eastern part of the High Plains north of
the Arkansas to the South Fork of the Republican River.
Covering much of the northern High Plains, however, are sand dunes and
windblown silt deposits (loess) that mantle the Ogallala Formation and
conceal any solution-deflation depressions that might have formed. The
Nebraska Sand Hills (fig. 17), the largest area of sand dunes in the
western hemisphere, is a huge area of stabilized sand dunes that extends
from the White River in South Dakota southward beyond the Platte River
almost to the Republican River in western Nebraska but only to the Loup
River in the northeast part of the High Plains (fig. 18). Loess covers
the western High Plains southward from the sand dunes almost to the
Arkansas River, and to the South Fork of the Republican in the eastern
part. This extensive cover of loess has created a fertile land that
makes it an important part of America’s wheatlands (fig. 19).
[Illustration: _Figure 17.—Aerial view, looking northwest, of the
Nebraska Sand Hills west of Ashby, Nebr._]
Other, smaller areas of sand dunes lie south of the Arkansas River
valley. The only large areas of sand dunes on the Llano Estacado, or
Southern High Plains, are along the southwestern margin near Monahans,
southwest of Odessa, Tex.
Oil and gas are present in the Paleozoic rocks that underlie the High
Plains at depth. Gas fields are ubiquitous in much of the eastern part
of the High Plains between the Arkansas and Canadian Rivers. Just south
of the Canadian River, at the northeast corner of the Southern High
Plains, a huge oil and gas field has been developed near Pampa, Tex. Oil
and gas fields also are abundant in the southwestern part of the
Southern High Plains, south of Littlefield, Tex.
[Illustration: _Figure 18.—The Sand Hills region of Nebraska. Arrows
show inferred direction of dune-forming winds. Map from Wright (1970),
used by permission._]
WYOMING
Badlands National Monument
Missouri River Valley
JAMES RIVER LOBE
MINNESOTA
IOWA
SOUTH DAKOTA
NEBRASKA
Rosebud
Valentine
DES MOINES LOBE
NEBRASKA
Ashby
SANDHILLS
Platte River Valley
IOWA
MISSOURI
NEBRASKA
KANSAS
COLORADO
Muscotah
TOPEKA
EXPLANATION
Transverse dunes
Longitudinal dunes
Wind-blown sand
Loess thickness (in feet)
[Illustration: _Figure 19.—Little-modified loess plain in southeastern
Nebraska. Photograph by Judy Miller._]
The surface of the High Plains, then, has been little modified by
streams since the end of Ogallala deposition. It has been raised by
regional uplift and pitted by solution and deflation, and large parts of
it have been covered by wind-blown sand and silt. It has been drilled
for oil and gas and extensively farmed, but it is still a geological
rarity—a preserved land surface that is 5 million years old.
MISSOURI PLATEAU
Beginning about 5 million years ago, regional uplift of the western part
of the continent forced streams, which for 30 million years had been
depositing sediment nearly continuously on the Great Plains, to change
their behavior and begin to cut into the layers of sediment they so long
had been depositing. The predecessor of the Missouri River ate headward
into the northern Great Plains and developed a tributary system that
excavated deeply into the accumulated deposits near the mountain front
and carried away huge volumes of sediment from the Great Plains to
Hudson Bay. By 2 million years ago, the streams had cut downward to
within a few hundred feet of their present level. This region that has
been so thoroughly dissected by the Missouri River and its tributaries
is called the Missouri Plateau.
About 2 million years ago, after much downcutting had already taken
place and river channels had been firmly established, great ice sheets
advanced southward from Canada into the United States. (See figure 2.)
These continental glaciers formed, advanced, and retreated several times
during the last 2 million years. At the north and east margins of the
Missouri Plateau they lapped onto a high area, leaving a mantle of
glacial deposits covering the bedrock surface and forcing streams to
adopt new courses along the margin of ice. The part of the Missouri
Plateau covered by the continental glaciers now is referred to as the
Glaciated Missouri Plateau. South of the part once covered by ice is the
Unglaciated Missouri Plateau.
Preglacial Drainage
Before the initial advance of the continental ice sheets, the Missouri
River flowed northeastward into Canada and to Hudson Bay. Its major
tributaries, the Yellowstone and the Little Missouri joined the Missouri
in northwestern North Dakota. The east-flowing Knife, Heart, and
Cannonball Rivers in North Dakota also joined a stream that flowed
northward to Hudson Bay.
Glaciated Missouri Plateau
When the continental ice sheets spread southward into northern Montana
and the Dakotas, a few isolated areas in Montana stood above the
surrounding plain. These are mostly areas that were uplifted by the
intrusion of igneous bodies long before the streams began downcutting
and carving the land. The northernmost of these isolated mountains, the
Sweetgrass Hills, were surrounded by ice and became nunataks, or islands
of land, in the sea of advancing ice, which pushed southward up against
the Highwood Mountains, near Great Falls, the Bearpaws south of Havre,
and the Little Rockies to the east.
Much of the northern part of Montana is a plain of little relief that is
the surface of a nearly continuous cover of glacial deposits, generally
less than 50 feet thick. This plain has been incised by the east-flowing
postglacial Teton, Marias, and Milk Rivers.
In North Dakota, a high area on the east side of the Williston basin
acted as a barrier to the advance of the ice, most of which was diverted
southeastward. The margin of the ice sheet, however, lapped onto the
bedrock high, where it stagnated. Earlier advances moved farthest south;
the later advances stopped north of the present course of the Missouri
River—their maximum position marked by ridges of unsorted, glacially
transported rock debris (till) called terminal moraines. North of the
terminal moraines is a distinctive landscape characterized by a rolling,
hummocky, or hilly surface with thousands of closed depressions between
the hills and hummocks, most of them occupied by lakes. This is the
deposit left by the stagnant or dead ice, and it is called dead-ice
moraine. The rolling upland in North Dakota that is covered by dead-ice
moraine and ridges of terminal moraines from the last glacial advances
is called the Coteau du Missouri (fig. 20). A gently sloping scarp,
several hundred feet high and mostly covered by glacial deposits
(referred to collectively as drift), separates the Coteau du Missouri
from the lower, nearly flat, drift-covered plains of the Central Lowland
to the east. This escarpment, which is called the Missouri escarpment,
is virtually continuous across the State of North Dakota southward into
South Dakota. The base of the Missouri escarpment is the eastern
boundary of the Great Plains in these northern states.
[Illustration: _Figure 20.—Ground moraine on the Coteau du Missouri,
northwestern North Dakota. Photograph by R. M. Lindvall, U. S.
Geological Survey._]
The advancing ice front blocked one after another of the
northward-flowing streams of the region, diverting them eastward along
the ice front. Shonkin Sag, north of the Highwood Mountains near Great
Falls, Mont., is an abandoned diversion channel of the Missouri River,
occupied when the ice front stood close to the north slopes of the
Highwoods. Much of the present course of the Missouri River from Great
Falls, Mont., to Kansas City, Mo., was established as an ice-marginal
channel, and the east-flowing part of the Little Missouri River in North
Dakota was formed in the same way. These valleys were cut during the
last 2 million years.
The north-flowing part of the Little Missouri River and the east-flowing
courses of the Knife, Heart, and Cannonball Rivers in North Dakota are
for the most part older, preglacial courses. The Little Missouri was
dammed by the ice, and its waters impounded to form a huge lake during
the maximum stand of the ice, but the deposits of this glacial lake are
few and make no imprint on the landscape.
The valley of the east-flowing, glacially diverted part of the Little
Missouri River, however, is markedly different from that of the
north-flowing preglacial river. It is much narrower and has steeper
walls than the old valley. Because it is younger, it is little modified,
except by huge landslides that have affected both walls of the valley.
Tremendous rotated landslide blocks in the North Unit of Theodore
Roosevelt National Memorial Park are some of the best examples of the
slump type of landslide to be seen anywhere (fig. 21).
Melting ice at the front of the glaciers provided large volumes of
meltwater that flowed across the till-mantled surface in front of the
glacier as it melted back toward Canada. This meltwater took many
courses to join the glacially diverted Missouri River, and these sinuous
meltwater channels wind across the dead-ice moraine and the older, less
hummocky ground moraine between the Coteau du Missouri and the Missouri
River. Locally the sediment carried by the meltwater streams was banked
against a wall of ice to form a small hill of stratified drift that is
called a kame. Streams flowing in tunnels beneath the ice formed
sinuous, ridgelike deposits called eskers, and in places the meltwater
deposits form broad flat areas called outwash plains.
[Illustration: _Figure 21.—Rotated slump blocks in huge landslide, North
Unit of Theodore Roosevelt National Memorial Park, N. Dak. Note that
layering of Fort Union Formation in cliffs on skyline, where landslide
originated, is horizontal._]
This rather limited variety of landforms, then, characterizes the
landscape of the Glaciated Missouri Plateau. The landforms themselves
are testimony to their glacial origin and to the great advances of the
continental ice sheets. This is a stream-carved terrain that has been
modified by continental glaciers and almost completely covered by a
thick blanket of glacially transported and deposited rock debris,
locally hundreds of feet thick. Subsequent stream action has not altered
the landscape greatly.
Unglaciated Missouri Plateau
Beyond the limits reached by the ice of the continental glaciers, the
Unglaciated Missouri Plateau displays the greatest variety of landforms
of any section of the Great Plains. In western Montana, many small
mountain masses rise above the general level of the plateau, including
the Highwood, Bearpaw, and Little Rocky Mountains near the margin of the
glaciated area, and the Judith, Big Snowy, Big Belt, Little Belt,
Castle, and Crazy Mountains farther south (fig. 22). Many of these, such
as the Crazy, Castle, Judith, and Big Snowy Mountains, are areas
uplifted by large, deeply rooted, intrusive igneous bodies called
stocks, which have been exposed by subsequent erosion of the arched
overlying sedimentary rock layers. Some, such as the Highwood and
Bearpaw Mountains, are predominantly piles of lava flows, although in
the Bearpaws the related intrusive bodies of igneous rock form a part of
the mountains. The Big and Little Belt Mountains were formed by
mushroom-shaped intrusive igneous bodies called laccoliths, which have
spread out and domed between layers of sedimentary rocks. A number of
igneous bodies also intrude the rocks of the Missouri Plateau around the
periphery of the Black Hills. Devils Tower, the first feature to be
designated a National Monument, is the best known of these igneous rock
features (fig. 23).
[Illustration: _Figure 22.—The Highwood Mountains seen from the Little
Belt Mountains, Mont. Photograph by I. J. Witkind, U. S. Geological
Survey._]
The uplift and volcanism that formed these mountains took place before
the streams began to cut downward and segment the Great Plains. The
mountains had been greatly dissected before the advent of the Great Ice
Age, when alpine glaciers formed on the Castle and the Crazy Mountains
and flowed down some of the stream-cut valleys. Alpine glacial features
such as cirques, in the high parts of the mountains, and glacially
modified U-shaped valleys (fig. 24) are impressive evidence of this
glaciation.
[Illustration: _Figure 23.—Devils Tower National Monument, Wyo. An
igneous intrusive body exposed by erosion. Photograph by F. W.
Osterwald, U. S. Geological Survey._]
The Missouri River and its tributaries—the Sun, Smith, Judith,
Musselshell, and Yellowstone Rivers in Montana and the Little Missouri
River in North Dakota—have cut down into the Missouri Plateau, cut broad
upland surfaces at many levels, and established confined valleys with
valley floors flanked by terrace remnants of older floodplains. Locally,
high buttes that are remnants of former interstream divides rise above
the uplands. Large lakes also were formed in most of these tributary
valleys because of damming by the continental ice sheets.
[Illustration: _Figure 24.—U-shaped, glaciated valley of Big Timber
Creek, Crazy Mountains, Mont. Photograph by W. C. Alden, 1921, U. S.
Geological Survey._]
West of the Black Hills, in Wyoming, the Tongue River and the Powder
River have excavated the Powder River Basin and produced similar
features (fig. 25). The east-flowing tributaries of the Missouri
River—the Knife, Heart, and Cannonball Rivers in North Dakota and the
Grand, Moreau, Belle Fourche, Cheyenne, Bad, and White Rivers in South
Dakota—similarly have shaped the landscape.
Most of these rivers flow in broad, old valleys, established more than 2
million years ago, before the first advance of the continental ice
sheets. Some of these valleys have been widened by recession of the
valley walls by badland development. Badlands are formed by the cutting
action of rivulets and rills flowing down over a steeply sloping face of
soft, fine-grained material composed mainly of clay and silt. The
intricate carving by thousands of small streams of water produces the
distinctive rounded and gullied terrain we call badlands. Badlands
National Monument in South Dakota (fig. 26) has been established in the
remarkable badlands terrain cut into the White River Group along the
north valley wall of the White River, and the South Unit of Theodore
Roosevelt National Memorial Park is in the colorful badlands of the
Little Missouri River, formed on the Fort Union Formation (fig. 27).
The White River also has cut a steep scarp along its southern wall that
is called the Pine Ridge escarpment. This escarpment defines the
boundary between the Missouri Plateau and the High Plains here.
[Illustration: _Figure 25.—View northeast across the Deckers coal mine
and the Tongue River in the Powder River Basin, southeastern Montana.
Typical terrain of unglaciated Missouri Plateau. Small mesas with
cliffed escarpments on capping layer of resistant sandstone, such as
those in the foreground, are common. Coal mine is about 1 mile across.
Photograph by R. B. Taylor, U. S. Geological Survey._]
The landscape of the Unglaciated Missouri Plateau has been determined
largely by the action of streams, but in some areas igneous intrusions
and volcanoes have produced small mountain masses that interrupt the
plain, and valley glaciers have modified the valleys in some of these
mountains.
[Illustration: _Figure 26.—Badlands in Badlands National Monument, S.
Dak. Photograph by W. H. Raymond, III, U. S. Geological Survey._]
[Illustration: _Figure 27.—Badlands of the Little Missouri River in
South Unit of Theodore Roosevelt National Memorial Park, N. Dak. View
looking northwest from Painted Canyon Overlook along Interstate Highway
94, west of Belfield._]
THE COLORADO PIEDMONT
The Colorado Piedmont lies at the eastern foot of the Rockies, (fig. 1)
largely between the South Platte River and the Arkansas River. The South
Platte on the north and the Arkansas River on the south, after leaving
the mountains, have excavated deeply into the Tertiary (65- to
2-million-year-old) sedimentary rock layers of the Great Plains in
Colorado and removed great volumes of sediment. At Denver, the South
Platte River has cut downward 1,500 to 2,000 feet to its present level.
Three well-formed terrace levels flank the river’s floodplain, and
remnants of a number of well-formed higher land surfaces are preserved
between the river and the mountains. Along the western margin of the
Colorado Piedmont, the layers of older sedimentary rock have been
sharply upturned by the rise of the mountains. The eroded edges of these
upturned layers have been eroded differentially, so that the hard
sandstone and limestone layers form conspicuous and continuous hogback
ridges (fig. 28). North of the South Platte River, near the Wyoming
border, a scarp that has been cut on the rocks of the High Plains marks
the northern boundary of the Colorado Piedmont. Pawnee Buttes (fig. 29)
are two of many butte outliers of the High Plains rocks near that scarp,
separated from the High Plains by erosion as is Scotts Bluff, farther
north in Nebraska. To the east, about 10 miles northwest of Limon,
Colo., Cedar Point forms a west-jutting prow of the High Plains.
The Arkansas River similarly has excavated much of the Tertiary piedmont
deposits and cut deeply into the older Cretaceous marine rocks between
Canon City and the Kansas border. The upturned layers along the mountain
front, marked by hogback ridges and intervening valleys, continue nearly
uninterrupted around the south end of the Front Range into the embayment
in the mountains at Canon City. Skyline Drive, a scenic drive at Canon
City, follows the crest of the Dakota hogback for a short distance and
provides a fine panorama of the Canon City embayment.
[Illustration: _Figure 28.—Hogback ridges along the Front Range west of
Denver, Colo. South Platte River emerges from the mountains and cuts
through hogbacks in middle distance. Photograph courtesy of Eugene
Shearer, Intrasearch, Inc._]
Extending eastward from the mountain front at Palmer Lake, a high divide
separates the drainage of the South Platte River from that of the
Arkansas River. The crest of the divide north of Colorado Springs is
generally between 7,400 and 7,600 feet in altitude, but Interstate
Highway 25 crosses it at about 7,350 feet, nearly 1,500 feet higher than
Colorado Springs and more than 2,000 feet higher than Denver. From the
crest of the divide to north of Castle Rock, resistant Oligocene Castle
Rock Conglomerate (which is equivalent to part of the White River Group
of the High Plains) is preserved in many places and forms a protective
caprock on mesas and buttes. This picturesque part of the Colorado
Piedmont looks quite different from the excavated valleys of the South
Platte and Arkansas Rivers.
Much of the terrain in the two river valleys has been smoothed by a
nearly continuous mantle of windblown sand and silt. Northwesterly
winds, which frequently blow with near-hurricane velocities, have
whipped fine material from the floodplains of the streams and spread it
eastward and southeastward over much of the Colorado Piedmont.
Well-formed dunes are not common, but alined gentle ridges of sand and
silt and abundant shallow blowout depressions inform us of the windblown
origin of this cover.
[Illustration: _Figure 29.—Pawnee Buttes in northeastern Colorado.
Buttes isolated by erosion from High Plains in the background. Ogallala
Formation caps top of Buttes. White River Group forms lower part. The
top of the highest butte is about 240 feet above the saddle between the
two buttes. Photograph by R. D. Miller, U. S. Geological Survey._]
In the Colorado Piedmont, then, the erosional effects of streams are the
most conspicuous features of the landscape, but these are enhanced by
the steep tilting of the layered rocks along the western margin as a
result of earth movement and modified by the nearly ubiquitous products
of wind action, which have softened the landscape with a widespread
cover of windblown sand and silt.
PECOS VALLEY
South of the land of volcanic rocks that is the Raton section, the Pecos
River has cut a broad valley from the Sangre de Cristo Mountains, in New
Mexico, southward to the Rio Grande, and has removed the piedmont cover
of Ogallala Formation and cut deeply into the underlying rocks. The
Ogallala Formation capping the High Plains to the east forms a rimrock
at the top of the sharp Mescalero escarpment, which is the eastern
boundary of the Pecos Valley. (See figure 4.) The western boundary of
the Pecos Valley is the eastern base of discontinuous mountain ranges.
The great thickness of Tertiary deposits that formed on the northern
Great Plains did not accumulate here, and the Pecos River has cut its
valley into the older marine sedimentary rocks. The rocks underlying the
surface of much of the Pecos Valley are upper Paleozoic limestones.
The soluble nature of limestone is responsible for some of the most
spectacular features of the landscape in the Pecos Valley. For about 10
miles north and 50 miles south of Vaughn, N. Mex., collapsed solution
caverns in upper Paleozoic limestones have produced an unusual type of
topography called karst. Karst topography is typified by numerous
closely spaced sinks or closed depressions, some of which are very deep
holes, caused by the collapse of the roof of a cave or solution cavity
into the underground void, leaving hills, spines, or hummocks at the top
of the intervening walls or ribs separating the depressions.
Although the karst in the vicinity of Vaughn is perhaps the most
conspicuous solution phenomenon, sinks and caves are common throughout
the Pecos Valley. At Bottomless Lakes State Park east of Roswell, N.
Mex., seven lakes occupy large sinkholes caused by the solution of salt
and gypsum in underlying rocks.
The most spectacular example of solution of limestone by ground water is
Carlsbad Caverns, N. Mex., one of the most beautiful caves in the world.
This celebrated solution cavity is preserved in a national park.
The Pecos River along much of its present course flows in a
vertical-walled canyon with limestone rims. The Canadian River, flowing
eastward from the Sangre de Cristo Mountains, has cut a deep canyon
along the northern part of the Pecos Valley section. The sharp rims of
the Dakota Sandstone at the Canadian escarpment, north of the Canadian
River, form the northern boundary of the Pecos Valley section.
The sharp, northeast-trending broken flexure called the Border Hills
that is crossed by U. S. Highway 70-380 about 20 miles west of Roswell
is a unique landform of the Pecos Valley. This markedly linear upfolded
(anticlinal) structure forms a ridge more than 30 miles long and about
200 feet high.
As in the Colorado Plateau, windblown sand and silt mantle the landscape
in many places, but the greatest accumulations are along the base of the
Mescalero escarpment at the northeast and southeast corners of the Pecos
Valley section.
East of the Pecos River, in the southeast part of the Pecos Valley, the
underlying rocks have yielded much oil and potash. Oil fields are common
east of Artesia and Carlsbad, and potash is mined east of Carlsbad.
The Pecos and Canadian Rivers and their tributaries have created the
general outline of the landscape of the Pecos Valley, but underground
solution of limestone by ground water and the collapse of roofs of these
cavities have contributed much detail to the surface that characterizes
the Pecos Valley today.
EDWARDS PLATEAU
South of the Pecos Valley section, the Pecos River continues its journey
to the Rio Grande in a steep-walled canyon cut 400 to 500 feet below the
level of a plateau surface of Cretaceous limestone from which little has
been stripped except a thin Tertiary cover of Ogallala Formation (fig.
30). To the east, the plateau has been similarly incised by the Devils
River and the West Nueces and Nueces Rivers. East of the Nueces to the
escarpment formed by the Balcones fault zone, the southern part of the
Edwards Plateau has been intricately dissected by the Frio, Sabinal,
Medina, Guadalupe, and Pedernales Rivers and their tributary systems.
San Antonio and Austin, Tex., are located on the Coastal Plain at the
edge of the Balcones fault zone.
[Illustration: _Figure 30.—Rio Grande and the flat-lying limestone
layers of the Edwards Plateau downstream from the mouth of the Pecos
River. Mexico on the left side of picture. Photograph by V. L. Freeman,
U. S. Geological Survey._]
The Pecos River, and to a lesser extent the Devils and Nueces Rivers,
particularly in their lower courses, have entrenched themselves deeply
in the plateau in remarkable meandering courses of a type that is
usually found only in broad, low-lying floodplains. These stream courses
reflect the stream environment prior to regional uplift.
Sinkholes pit the relatively undissected limestone plateau surface in
the northeast part of the Edwards Plateau, and some underground solution
cavities in the limestone are well-known caves, such as the Caverns of
Sonora, southwest of Sonora, Tex.
Oil and gas fields are widely developed in the northern part of the
Edwards Plateau, but only cattle ranches are found in the bare southern
part.
Ancient oceans deposited the limestones that now cap the Edwards
Plateau; streams planed off the surface of the flat-lying limestone
layers and entrenched themselves in steep-walled valleys; and ground
water dissolved the limestone and created the solution cavities that are
the caves and sinks of the Edwards Plateau. Water has created this
landscape.
PLAINS BORDER SECTION
The Missouri Plateau, the Colorado Piedmont, the Pecos Valley, and the
Edwards Plateau all were outlined by streams that flowed from the
mountains. On the eastern border of the Great Plains, however, headward
cutting by streams that have their source areas in the High Plains has
dissected a large area, mainly in Kansas. This Plains Border Section
comprises a number of east-trending river valleys—of the Republican,
Solomon, Saline, Smoky Hill, Arkansas, Medicine Lodge, Cimarron, and
North Canadian Rivers—and interstream divides, most of which are
intricately dissected.
North of the Arkansas River, the east-flowing Republican, Solomon,
Saline, and Smoky Hill Rivers have incised themselves a few hundred feet
below the Tertiary High Plains surface and have developed systems of
closely spaced tributary draws. The interstream divides are narrow, and
the tributary heads nearly meet at the divides. This intricately
dissected part of the Plains Border section is called the Smoky Hills.
Some isolated buttes of Cretaceous rocks left in the upper valley of the
Smoky Hill River are called the Monument Rocks. A large area of rounded
boulders exposed by erosion south of the Solomon River, southwest of
Minneapolis, Kans., is called “Rock City.” These boulders originated as
resistant nodules (concretions) within the Cretaceous rocks that
contained them.
South of the Arkansas River is a broad, nearly flat upland sometimes
referred to as the Great Bend Plains. The Medicine Lodge River has cut
headward into the southeastern part of the Great Bend Plains and created
a thoroughly dissected topography in Triassic red rocks that is locally
called the Red Hills. In a few places, badlands have formed in the Red
Hills.
Some large sinks or collapse depressions have formed because of solution
of salt and gypsum at depth by ground water. Big and Little Basins, in
Clark County in south-central Kansas, were formed in this way.
Sand dunes have accumulated in places, especially near stream valleys.
Dunes are common, for example, along the north side of the North
Canadian River.
Oil and gas fields are widely developed in the southeast part of the
Plains Border section—in the Smoky Hills, the Great Bend Plains, and the
Red Hills.
The Plains Border section, like the Missouri Plateau, the Colorado
Piedmont, and the Pecos Valley, is primarily a product of stream
dissection. The differences in the outstanding landforms of the section
are mainly the result of differences in the hardness of the eroded
rocks.
EPILOGUE
The Great Plains, as we have seen, is many things. It contains thick
layers of rock that formed in oceans, and younger layers of rocks
deposited by streams. These rocks have been affected by earth movements
and injected by hot molten rock, some of which reached the surface as
volcanic rock. The rocks have been carved by streams, dissolved by
ground water, partly covered by glaciers, and blown by winds. All of
these agents have played important roles in determining the landscape
and the landforms of the Great Plains. But the streams were the master
agent. They formed the great depositional plain that was to become the
Great Plains, and then began to destroy it—leaving only the High Plains
to remind us of what it was. Those long miles we travel across the High
Plains are a journey through history—geologic history.
ACKNOWLEDGMENTS
This narrative history of geologic and biologic events in the Great
Plains had its origin in a study intended to identify potential National
Natural Landmarks in the Great Plains, commissioned by the National Park
Service. William A. Cobban, G. Edward Lewis, and Reuben J. Ross of the
U. S. Geological Survey were collaborators in that study, and some of
their contributions to the history of life on the Great Plains have been
incorporated into this narrative, which was undertaken at the urging of
Wallace R. Hansen.
The photographic illustrations, other than those obtained from the film
library of the U. S. Geological Survey, were provided by the interest
and effort of my friends and colleagues of the Geological
Survey—including C. R. Dunrud, V. L. Freeman, C. D. Miller, R. D.
Miller, F. W. Osterwald, R. L. Parker, W. H. Raymond, III, Kenneth
Shaver, and R. B. Taylor—and by Eugene Shearer, Intrasearch, Inc.,
Denver, Colo. Without their help this publication would not have been
possible.
SOME SOURCE REFERENCES
Alden, W. C., 1932, Physiography and glacial geology of eastern Montana
and adjacent areas: U. S. Geological Survey Professional Paper 174,
133 p.
Bluemle, J. P., 1977, The face of North Dakota—the geologic story: North
Dakota Geological Survey Education Series 11, 73 p.
Colton, R. B., Lemke, R. W., and Lindvall, R. M., 1961, Glacial map of
Montana east of the Rocky Mountains: U. S. Geological Survey
Miscellaneous Geologic Investigations Map I-327.
Colton, R. B., Lemke, R. W., and Lindvall, R. M., 1963, Preliminary
glacial map of North Dakota: U. S. Geological Survey Miscellaneous
Geologic Investigations Map I-331.
Curtis, B. F., ed., 1975, Cenozoic history of the southern Rocky
Mountains—Papers deriving from a symposium presented at the Rocky
Mountain Section meeting of the Geological Society of America,
Boulder, Colorado, 1973: Geological Society of America Memoir 144,
279 p.
Darton, N. H., 1905, Preliminary report on the geology and underground
water resources of the central Great Plains: U. S. Geological Survey
Professional Paper 32, 433 p.
Flint, R. F., 1955, Pleistocene geology of eastern South Dakota: U. S.
Geological Survey Professional Paper 262, 173 p.
Frye, J. C., and Leonard, A. B., 1965, Quaternary of the southern Great
Plains, _in_ Wright, H. E., Jr., and Frey, D. G., eds., The
Quaternary of the United States—A review volume for the 7th Congress
of the International Association for Quaternary Research: Princeton
University Press, p. 203-216.
Howard, A. D., 1958, Drainage evolution in northeastern Montana and
northwestern North Dakota: Geological Society of America Bulletin,
v. 69, no. 5, p. 575-588.
Johnson, R. B., 1961, Patterns and origin of radial dike swarms
associated with West Spanish Peak and Dike Mountain, south-central
Colorado: Geological Society of America Bulletin, v. 72, no. 4, p.
579-590.
Judson, S. S., Jr., 1950, Depressions of the northern portion of the
southern High Plains of eastern New Mexico: Geological Society of
America Bulletin, v. 61, no. 3, p. 253-274.
Keech, C. F., and Bentall, Ray, 1971, Dunes on the plains—The Sand Hills
region of Nebraska: Nebraska University Conservation and Survey
Division Resources Report 4, 18 p.
Lemke, R. W., Laird, W. M., Tipton, M. J., and Lindvall, R. M., 1965,
Quaternary geology of northern Great Plains, _in_ Wright, H. E.,
Jr., and Frey, D. G., eds., The Quaternary of the United States—A
review volume for the 7th Congress of the International Association
for Quaternary Research: Princeton University Press, p. 15-27.
Mansfield, G. R., 1907, Glaciation in the Crazy Mountains of Montana:
Geological Society of America Bulletin, v. 19, p. 558-567.
Pettyjohn, W. A., 1966, Eocene paleosol in the northern Great Plains,
_in_ Geological Survey research 1966: U. S. Geological Survey
Professional Paper 550-C, p. C61-C65.
Robinson, C. S., 1956, Geology of Devils Tower National Monument,
Wyoming: U. S. Geological Survey Bulletin 1021-I, p. 289-302.
Smith, H. T. U., 1965, Dune morphology and chronology in central and
western Nebraska: Journal of Geology, v. 73, no. 4, p. 557-578.
Stormer, J. C., Jr., 1972, Ages and nature of volcanic activity on the
southern High Plains, New Mexico and Colorado: Geological Society of
America Bulletin, v. 83, no. 8, p. 2443-2448.
Strahler, A. N., and Strahler, A. H., 1978, Modern physical geography:
New York, John Wiley & Sons, 502 p.
Thornbury, W. D., 1965, Regional geomorphology of the United States: New
York, John Wiley, 609 p.
Wright, H. E., Jr., 1970, Vegetational history of the Central Plains,
_in_ Pleistocene and recent environments of the central Great
Plains: Kansas University Department of Geology Special Publication
3, p. 157-172.
INDEX
[Italic page numbers indicate major references]
A
Page
Acknowledgments 49
Agriculture 30
Alaska 11
Anadarko basin 11, 16
Arikaree Formation 18
Arkansas River 2, 7, 23, 25, 29, 30, 42, 43, 44, 48
Artesia, N. Mex. 46
Austin, Tex. 47
B
Bad River 39
Badland development 39
Badlands National Monument 39
Balcones fault zone 23, 46
Basalt flows 20, 24
Bearpaw Mountains 33, 36
Belle Fourche River 39
Bents Fort, Colo. 2
Big Basin, Kans. 48
Big Belt Mountains 36
Big Snowy Mountains 36
Bison 1
Black Hills 7, 11, 18, 19, 20, 37
Border Hills 46
Bottomless Lakes, N. Mex. 45
Brazos River 23
Burnet, Tex. 23
C
Camels 16, 18
Canada 1, 5, 33
Canadian escarpment 23, 46
Canadian River 23, 29, 30, 46
Cannonball River 33, 35, 39
Canon City, Colo. 43
Caprock escarpment 7, 29
Capulin Mountain 20, 23
Carlsbad, N. Mex. 46
Carlsbad Caverns, N. Mex. 45
Carrizo Creek 25
Castle Mountains 37
Castle Rock, Colo. 43
Castle Rock Conglomerate 43
Caverns of Sonora 47
Cedar Point 43
Central Lowland 5, 34
Central Texas Uplift 7, 19, 20, 22
Cheyenne, Wyo. 27
Cheyenne River 39
Cimarron River 48
Cirques 38
Clark County, Kans. 48
Climate 2
Coal 16
Coastal Plain 5, 7, 10, 47
Colorado 19
Colorado Piedmont 7, 10, 19, 23, 42, 48, 49
Colorado Plateau 46
Colorado River 23
Colorado Springs, Colo. 43
Coteau du Missouri 34, 35
Crazy Mountains 37
Creosote 1
Cretaceous Period 11, 16, 19, 24, 43, 46, 48
D
Dakota hogback 43
Dakota Sandstone 22, 23, 24, 46
Dawson Formation 16
Dead-ice moraines 34
Definition 1
Deformation 11
Denver, Colo. 42
Denver Formation 16
Deposition 10, 11, 32, 44
Devils River 46
Devils Tower, Wyo. 37
Differential erosion 23, 25, 42
Dikes 25
Dinosaurs 16
Drift 34
E
Edwards Plateau 10, 19, 29, 46, 48
Eocene Epoch 16
Epilogue 49
Erosion 18
Escarpments 4, 7, 23, 34
Eskers 35
F
Farming 30
Fishers Peak 23
Fissures 20
Forests 1, 2, 7
Fort Union Formation 16, 40
Fossils 16
Frio River 46
Front Range 43
G
Gangplank 27
Gas 30, 47, 49
Glaciation 2, 5, 11, 33
Grand River 39
Great Bend, Kans. 2
Great Bend Plains 48, 49
Great Falls, Mont. 33, 35
Great Ice Age 5
Great Lakes 5
Guadalupe River 46
Gulf Coastal Plain 7
H
Harney Peak 20
Havre, Mont. 33
Heart River 33, 35, 39
Hell Creek Formation 11
High Plains 7, 10, 25, 45, 48
Highwood Mountains 33, 35, 36
Horses 16, 18
Hudson Bay 32, 33
I
Ice Age 5
Independence, Mo. 2
Interior Highlands 5
Interior Plains 5, 11
Interstate Highway 25 43
Interstate Highway 70 4
Introduction 1
J
Jewel Cave 21
Joints 20
Judith Mountains 36
Judith River 38
Juniper 1
K
Kames 35
Kansas 10, 48
Kansas City, Mo. 35
Karst topography 45
Kearney, Nebr. 2
King Mountain 29
Knife River 33, 35, 39
L
Laccoliths 37
Lake development 34, 39
Lance Formation 11
Laramie Formation 16
Lava flows 37
Lewis and Clark expedition 2
Limestone Plateau 21, 22
Limon, Colo. 4, 43
Little Basin, Kans. 48
Little Belt Mountains 36
Little Missouri River 33, 35, 38, 40
Little Rocky Mountains 33, 36
Littlefield, Tex. 30
Llano Estacado 29, 30
Loess 29
Longhorn Caverns 23
M
Marias River 33
McCamey, Tex. 29
Medicine Lodge River 48
Medina River 46
Mesa de Maya 20, 24, 25
Mescalero escarpment 7, 29, 45, 46
Mesquite 1
Mexico 1, 11
Milk River 33
Minneapolis, Kans. 48
Miocene Epoch 23
Missouri escarpment 34
Missouri Plateau 7, 19, 20, 32, 48, 49
Missouri River 5, 7, 25, 32, 33, 35, 38
Montana 33
Monument Rocks 48
Moraines 34
Moreau River 39
_Moropus_ 18
Mount Rushmore 20
Musselshell River 38
N
Nebraska 7, 29
Nebraska Sand Hills 29
Needles area, Black Hills 20
New Mexico 7, 19, 45
North Canadian River 48, 49
North Dakota 33, 34, 35
Nueces River 46
Nunataks 33
O
Oak trees 1
Odessa, Tex. 30
Ogallala Formation 18, 23, 24, 25, 27, 29, 45, 46
Ohio River 5
Oil 30, 46, 47, 49
Oklahoma 7
Oligocene Epoch 16, 43
Oregon Trail 2
Ouachita province 5
Outwash plains 35
Ozark Plateaus 5
P
Paleocene Epoch 16, 23
Paleozoic Era 21, 30, 45
Palmer Lake 43
Pampa, Tex. 30
Pawnee Buttes 42
Pecos River 7, 25, 45, 46
Pecos Valley 7, 10, 19, 23, 45, 48, 49
Pedernales River 47
Pike, Zebulon iii, 2
Pine Ridge escarpment 7, 29, 40
Pioneers 2
Plains Border Section 19, 48
Platte River 2, 25, 29
Pleistocene Epoch 5
Poison Canyon Formation 16, 23, 24
Powder River 39
Powder River Basin 39
Purgatoire Formation 24
Purgatoire River 25
R
Racetrack, The 22
Rainfall 2
Rapid City, S. Dak. 20
Raton Basin 24
Raton Formation 16
Raton Mesa 20, 23
Raton section 10, 20, 23, 45
Red Hills 48, 49
Red Valley 22
Republican River 4, 29, 48
Rhinoceroses 16, 18
Rio Grande 7, 45
Rocky Mountains 5, 19
Roswell, N. Mex. 45
S
Sabinal River 46
Salina, Kans. 4
Saline River 4, 48
San Antonio, Tex. 47
Sand dunes 44, 49
Sand Hills, Nebr. 29
Sangre de Cristo Mountains 7, 45, 46
Scotts Bluff National Monument 27, 42
Sedimentation 10, 11, 32
Shonkin Sag 35
Sinkholes 47, 48
Skyline Drive, Canon City, Colo. 43
Smith River 38
Smoky Hill River 4, 48
Smoky Hills 48, 49
Soil development 16
Solomon River 4, 48
Solution cavities 45, 47, 48
Sonora, Tex. 47
South Dakota 20, 29, 33, 34
South Dakota Badlands 16
South Platte River 4, 7, 42, 43, 44
Spanish Peaks 19, 24
Spruce trees 2
Stream deposition 11, 32
Summary 49
Sun River 38
Superior Upland 5, 10
Sweetgrass Hills 33
Sylvan Lake 20
T
Tapirs 16
Tertiary Period 42, 43, 45, 46, 48
Teton River 33
Texas 7
Theodore Roosevelt National Memorial Park 35, 40
Till 34
_Titanotheres_ 16
Tongue River 39
Trails 2
Trees 1, 2, 7
Triassic Period 21
Triceratops 16
Trinidad, Colo. 25
U
Uplift 11, 16, 19, 32, 37
V
Valley development 39
Vaughn, N. Mex. 45
Vegetation 1, 2, 7, 10, 16
Vermejo Formation 16
Volcanoes 16, 20, 40
W
Walsenburg, Colo. 24
Warping 11
Well-drilling 11
West Nueces River 46
White River 29, 39, 40
White River Group 16, 40, 44
Williston basin 11, 16, 33
Wind Cave 21
Wind deposition 44
Wyoming 20, 39
Y
Yellowstone River 33, 38
[Illustration: U. S. DEPARTMENT OF THE INTERIOR • March 3, 1849]
Transcriber’s Notes
--Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.
--In the text versions only, text in italics is delimited by
_underscores_.
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