The GEOLOGIC STORY 

of the

GREAT PLAINS

QE75 

B9 

no.1493

RECEIVED

JUNO 21981

O.SJUI, LIBRARIES

GEOLOGICAL SURVEY BULLETIN 1493
A
The GEOLOGIC STORY of

The GREAT PLAINS
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.

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

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 deMaya, Colo. .................... 25

14. Spanish Peaks, Colo. ..................... 26

15. "The Gangplank," Wyo. .................. 27

16. Scotts Bluff National Monument, Nebr. ....... 28
FIGURES

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 southcentral 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.
ower .* BlackVCheyS>

___*JilL___J___.-

McCameu San 

Angelo

/

...... j __ !- -..J^rss- <*~v J

y Figure 1. Index map of the Great 

Plains showing route of Lewis and 

* Clark and the Santa Fe and Oregon Trails. \^ Corpus CM
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 30,0 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
250

0 250 500 750 KILOMETERS

Figure 2. Physical divisions of the United States and maxim

sheets during the Great Ice Age
"^ 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.

THE GREAT PLAINS- ITS PARTS

M Within the Great Plains are many large areas that differ

j. 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 squth-

> ward 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

r another escarpment, the Pine Ridge escarpment, which sepab? rates the High Plains from a region that has been greatly dissected by the Missouri River and its tributaries. There,

r several levels of rolling upland are surmounted by small 

Y mountainous masses and flat-topped buttes and are env trenched 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 

x* Valley, extending to the Rio Grande and the Coastal Plain, is
EDWARDS ] UPL|FT 

PLATEAU l

Figure 3. The Great Plains province and its sections.

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.

8

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.

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.

10
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,

Figure 5 (overleaf) Geologic time chart and the progression of life 

forms. Note Cretaceous Triceratops, Oligocene Titanotheres, 

and Miocene Moropus.

11
PERIOD 1 MISSISSIPPIAN

0 l PERIOD

GEOLOGIC TIME

The Age of the Earth

The Earth is very old 4.5 billion years or more according to

otes. Most of the evidence for on ancient Eai 

lined in the rocks that form the Earth's crust. The rock layers 

..lemselves 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 elemen 

'hose isotopes provide Earth scientists with an atomic clock. Within 

mese rocks, "parent" isotopes decay at a predictaLI - ""- " *- 

"daughter" isotopes. By determining the relative a...»».,. u , h 

and daughter isotopes, the age of these rocks can be calculate

Thus, the results of studies of rock layers (stratigraphy), and of 

fusils (paleontology), coupled with the ages of certain rocks as mi

d by atomic clocks (geochronology), attest to a very old Ear

\Volca

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.

SUPERIOR UPLAND 

(Precambrian shield)

0 250 500 750 KILOMETERS Arches

Figure 7. Structural setting of the Great Plains. Williston basin 

and Anadarko basin are separated by a midcontinental arch.

14
 t t

Table 1. Generalized chart of rocks of the G

*- £"

3 s

Tertiary

Geologic age

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Cretaceous

Jurassic

Triassic

PALEOZOIC

PRECAMBRIAN

Millions 

of years 

ago

i-y -»o

 hS 

 225 

 570 

Missouri PlateauBlack Hills

Glacial deposits, alluvium, and 

terrace deposits

erosio 

ER

Flaxville Gravel and Ogallala 

Formation 

Arikaree Formation

erosio

White River Group 

erosio

Wasatch and Golden Valley 

Formations

Fort Union Formation

Hell Creek and Lance Formations 

Fox Hills Sandstone

High Plains-Plains Bord

Colorado Piedmont

Alluvium, sand dunes, a

lal surface 

OSION

Ogallala Formation 

Arikaree Formation 

lal surface

White River Croup 

nal surface

Dawson Arkose

Denver. Poison Canyon

Raton Formations

Vermejo and Laramie F

Trinidad and Fox Hills S

Shales, sandstones, and limestones deposited in Late Creta

Dakota Sandstone and Lakota 

Formation

Sundance Formation, E His Group, 

and Unkpapa Sandstone

Dakota Sandstone

Morrison Formation

Dominantly red 

Paleozoic rocks, u

Precambrian rocks, 
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 threehorned dinosaurs (Triceratops) that roamed these plains. 

Their fossilized remains are found from Canada to New 

Mexico.

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

16
Figure 8. Progressive southeastward expansion of areas covered 

by Paleocene, Oligocene, and Miocene-Pliocene sedimentary 

deposits.

17
Figure 9. Big Horn coal strip mine in Fort Union Formation at 

Acme, Wyo. Photograph by F. W. Osterwald, U.S. Geological 

Survey.

found native only on other continents grazed those Oligocene semiarid grassland plains.

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

18
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 thatis 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

19
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 Qakota 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

20
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.

Figure 10. Diagram of the Black Hills uplift by A. N. Stmhler 

(Strahler and Strahler, 1978). Used by permission.

21
Figure 11. Jointed granite rounded by weathering at Sylvan Lake, 

in the central part of the Black Hills, S. Dak.

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.

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

22
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

Volcariism 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 southfacing 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

23
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.

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

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.
Figure 13. Lava-capped Mesa de Mm/a, 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.

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.

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,

25
Figure 14. Spanish Peaks, southwest of Walsenburg, Colo. Igneous intrusive 

erosion. Photograph by R. B. Taylor, U.S. Geol

r it » 
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.

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..

27
Figure 16. Aerial view of Scoffs 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.

r (
The High Plains extends southward from the Pine Ridge

y 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 Cana-

^ dian 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 Mescai lero 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

A 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 

a- 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

j northward to the Arkansas River and along the eastern part

N 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

 v part of the High Plains (fig. 18). Loess covers the western

29
Figure 17. Aerial view, looking northwest, of the Nebraska Sand 

Hills west of Ashby, Nebr.

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).

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.

30
104° 102° 100°

40°

39° - EXPLANATION Loess thickness (in feet)

Figure 18. The Sand Hills region of Nebraska. Arrows show inferred direction of dune-forming winds. Map from

Wright (1970), used by permission.
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 windblown 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 estab32
lished, 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

33
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.

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 icemarginal 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.

35
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

36
Figure 22. The Highwood Mountains seen from the Little Belt 

Mountains, Mont. Photograph by I. J. Witkind, U.S. Geological 

Survey.

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).

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

37
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.

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

Figure 23. Devils Tower National Monument, Wyo. An igneous 

intrusive body exposed by erosion. Photograph by F. W. Osterwald, U.S. Geological Survey.

38
Figure 24. U-shaped, glaciated valley of Big Timber Creek, Crazy 

Mountains, Mont. Photograph by W. C. Alden, 1921, U.S. Geological Survey.

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.

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 eastflowing 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

39
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.

(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.

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.

Figure 26. Badlands in Badlands National Monument, 5. Dak. 

Photograph by W. H. Raymond, III, U.S. Geological Survey.

40
"f^r- ' 
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-millionyear-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

42
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.

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

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.
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. Wellformed 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.

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 moveFigure 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.
ment 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.

45
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

46
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.

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.

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 wellknown 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 flat47
lying limestone layers and entrenched themselves in steepwalled 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 southcentral Kansas, were formed in this way.

48
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 landf orms 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 landf orms 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.

49
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, Cblo. Without their help this publication would hot 

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 1-327.

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 1-331.

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.

50
* Judson, S. S., Jr., 1950, Depressions of the northern portion of the south- 

>^ ern 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.

I 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: 

~j Geological Society of America Bulletin, v. 19, p. 558-567. 

? Pettyjohn, W. A., 1966, Eocene paleosol in the northern Great Plains, in 

t 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 1021-1, 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: 

h 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.

51

INDEX

(Italic page numbers i

Page 

A

35 a .

Artesia,N.Me,~ ________

B

Badlands National Monument 

Balcones fault zone -

earpp w j°u~v'ns

ig nowy oun ams

Black Hills _____ - _ - _ ___ 

Border Mills ______ _ - -

C 

Camels

Canadian escarpment -

aproc escarpment

CarlcKa^ M MAY

Carlsbad Caverns, N. Mex. -

Castle Mountains 

as e oc ong omera e

Central Lowland 

30

11 1 A

, 7, 23, 25, 29, 30,

42, 43, 44, 48 

 46

____ 29

39

36

19, 20, 37 

46

 y\

, __ _ _ 23 46

 - 23, 29, 30, 46

____ 45

____ 27

 -.- . A\

47

43

______ ^ 34

. _-7. 19. 20. 22

ndicate major references]

eyenne iver

imarron iver

Colorado Piedmont - - 7, 10, 19, 23, 42,

C° °ra I M- mSS ' ° °

au u iss un

Cretaceous Period 11, 16, 19, 24, 43, 

D

e mi ion

eposi ion , ,

evis iver

evi s ower, yo.

Qfjf ( _____ ______ . . __ _____ _________

E, F

ocene poc

pi ogue

rosion

scarpments , ,

a mg

is ers ea

Mssures

°r" ,S .

ort nion ormation

F°SS1R'

Page

27 

39 

48 

38 

48 

2 

16 

10,47 

19 

48,49 

46 

23 

43 

34, 35 

37 

1 

46,48

43 

24,46 

16 

34 

1 

11 

42 

16 

32,44 

46 

37 

25,42 

25 

16 

34

46,48 

16 

49 

18 

23,34 

35

30 

23 

20 

1, 2, 7 

16, 40 

16 

46 

*n

53
Page M Page

Gangplank 27 

Gas 30, 47, 49

Glaciation 2, 5, 11, 33

.................. 39

________ 2

 48, 49

 33, 35

 5 

.................. 5

.................. 46

________ 7

Grand River 

Great Bend, Kans. - 

Great Bend Plains ----- 

Great Falls, Mont. - 

Great Ice Age 

Great Lakes 

Guadalupe River 

Gulf Coastal Plain -

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, J, K

Ice Age -

Independence, Mo. -

Interior Highlands -

Interior Plains 

Interstate Highway 25 -

Interstate Highway 70 

Introduction -

Joints 

Judith Mountains 

Judith River -

Juniper 

Kames -- - - 

Kansas 

Kansas City, Mo. 

Karst topography 

Kearney, Nebr. 

King Mountain 

Knife River 

.. 5

.... ... ... 2

..... 5

.... .... ... 5, 11

. ...... 43

4

.... ..... ... i

................. 20 

.. .. . 36

.... . 38

..... i

.... 35

..... 10, 48

. ... .... ... 35

. 45

.... ... 2

. ..... 29

... . 33,. 35, 39

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

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

Odessa, Tex. 

Ogallala Formation 

 30

....... ... ig, 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

Laccoliths 

Lake development 

Lance Formation 

Laramie Formation 

Lava flows 

Lewis and Clark expedition 

Limestone Plateau 

Limon, Colo. 

Little Basin, Kans. 

Little Belt Mountains 

.. ... 37

. 34, 39 

.... 11

. 16

........... 37

....... 2

..... 21, 22

----- 4, 43

.... 48

........... 36

Little Missouri River - - 33, 35, 38, 40

Little Rocky Mountains - 

Littlefield, Tex. 

Llano Estacado 

Loess - 

Longhorn Caverns 

33, 36

30

29, 30

29

23

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

54
Page Page

Purgatoire Formation 

R

Rainfall

Rapid Citv S Dak _ _

Raton Basin 

Raton section 10, 20, 

Red Hills __ ___ __ -

Red Valley

S

Salina, Kans. - - 

a" "tomo' ex '

Sand Hills Nebr

Shonkin Sag

Skyline Drive, Canon City, Colo. 

Smith River

Smokv Hill River

 ° Y '

ion cavi IBS

South Dakota Badlands 

South Platte River 4 7 42

pams' hP ' ' '

39 

24 

25

22 

2 

20 

24 

16 

20, 23 

23,45 

48, 49 

22 

29,48 

16, 18 

7,45 

5, 19 

45

46 

4 

4, 48 

47 

44, 49 

29 

45,46 

27, 42 

11, 32 

35 

47, 48 

43 

38 

4, 48 

48, 49 

16 

4, 48 

47,48 

47 

33,34 

16 

43,44 

19, 24 

2

Sun River --------------

upenor pan

weetgrass i s

T

Tertiary Period - : 42, 43, 45

Theodore Roosevelt National Memorial

 pin . _ _ ___ _

itanot ercs

rees

riassic erio

U, V

ermejo orma ion

W, Y

a sen urg, o o.

"' "ecesRlver

'te 'VCr

Wind deposition 

49 

38 

5, 10 

33 

20

16 

, 46, 48 

33 

7

35,40 

34 

16 

39 

2 

1, 2, 7 

21 

16 

25

. 32, 37

39 

45 

, 10, 16 

16 

, 20, 40

24 

11 

11 

46 

39, 40 

40,44 

, 16, 33 

21 

44 

20, 39

« IR

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