The

Geologic TheGeologic The

 S Story

Geologic

 Story

Geologic

of

U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1699

Prepared in cooperation with the U.S. Bureau of Land Management

��������

�����

Gunnison Gorge

National Conservation Area,

Colorado
COVER: Gunnison River canyon looking north toward Grand Mesa in the distance. More information 

about this image is shown in figure 18. 
By Karl S. Kellogg

Prepared in cooperation with the U.S. Bureau of Land Management

Professional Paper 1699

U.S. Department of the Interior

U.S. Geological Survey

TheGeologic Story of Gunnison Gorge

National Conservation Area,

Colorado
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

This publication is also available on line at: http://cpg.cr.usgs.gov/pub/profpapers.html

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.
Contents

Introduction ................................................................................................................................................... 1

A Brief Sense of Geologic Time......................................................................................................... 3

Geologic Evolution of Gunnison Gorge .................................................................................................... 4

The Basement Rocks of the Gorge ................................................................................................... 5

Ancestral Uncompahgre Highlands ................................................................................................. 8

The Sedimentary Rocks of the Gorge............................................................................................... 8

Jurassic Rocks............................................................................................................................ 9

Entrada Sandstone ............................................................................................................ 9

Wanakah Formation ........................................................................................................ 11

Morrison Formation......................................................................................................... 11

Cretaceous Rocks..................................................................................................................... 16 

Burro Canyon Formation................................................................................................. 16

Dakota Formation............................................................................................................. 16

Mancos Shale .................................................................................................................. 17

Surficial Deposits: A Record of Late Tertiary and Quaternary Erosion, Deposition,

and Climate Change............................................................................................................. 18

The Laramide Orogeny: Building the Rocky Mountains.............................................................. 20

The Ute Indian Fault: A Prominent Laramide Feature.................................................................. 20

The Gunnison River Paradox ........................................................................................................... 22

Factors that Control the Gunnison River’s Ability to Downcut ................................................... 27

How Steep is the Gradient Through the Gunnison River Gorge? .............................................. 29

Why Are the Canyon Walls so Steep?............................................................................................ 29

Environmental Issues in the Gunnison Gorge National Conservation Area ................................... 32

Selenium and Salt Contamination from the Mancos Shale........................................................ 32

Effects of Damming on the River’s Profile...................................................................................... 32

Effects of the Upriver Dams on River Ecology .............................................................................. 32

Erosion Due to Off-Road Vehicle Use............................................................................................. 33

Acknowledgments ..................................................................................................................................... 33

References Cited and Suggested Reading ............................................................................................ 35

Glossary of Selected Terms...................................................................................................................... 36

Figures

1. Index map showing the location of Gunnison Gorge National Conservation Area.......... 1

2. LANDSAT 7 image of the Gunnison Gorge National Conservation Area............................ 2

3. Geologic time chart..................................................................................................................... 3

4. View from 7 km (4.5 mi) north of Montrose looking northeastward at the

Gunnison uplift ............................................................................................................................. 5

5. Photograph of amphibolite (dark rock) intruded by coarse-grained,

speckled black and white Pitts Meadow Granodiorite, which is, in turn,

intruded by medium-grained white aplite................................................................................ 6

6. Speckled Pitts Meadow Granodiorite enclosing a dark, fine-grained inclusion.............. 7

iii
7. Light-colored pegmatite dikes cutting the Pitts Meadow Granodiorite............................. 8

8. Diagram showing the ancestral (late Paleozoic) Rocky Mountain uplifts

and adjacent basins of Colorado .............................................................................................. 9

9. Generalized stratigraphic column of the sedimentary and basement

rock units exposed in the Gunnison Gorge National Conservation Area......................... 10

10. Entrada Sandstone along the Chukar trail............................................................................. 11

11. View of the north slope of Smith Fork canyon ...................................................................... 12

12. View looking west from the top of Chukar trail of the south end of the

Ute Indian fault........................................................................................................................... 12

13. Iron-oxide-stained ooliths stand out in weathered relief in sandy limestone

in upper part of the Wanakah Formation............................................................................... 13

14. Chert-pebble conglomerate, typical of Burro Canyon Formation...................................... 16

15. A 45-cm- (18-in.-) thick coal bed in lower part of Dakota Sandstone............................... 16

16. Typical exposure of Mancos Shale near the Eagle Valley trail and Chukar Road.......... 18

17. Regional map and cross section of the lower Gunnison River area, showing the 

Laramide uplifts and major faults............................................................................................ 21

18. View looking north along Gunnison Gorge showing the dramatic offset

of Proterozoic and overlying Mesozoic rock near the north end of

the Ute Indian fault.................................................................................................................... 22

19. Close-up view of the north terminus of the Ute Indian fault............................................... 23

20. View looking northwest at a north-trending splay (branch) of the Ute Indian fault....... 26

21. Schematic diagrams showing the relationship of the Colorado and

Gunnison Rivers to the Uncompahgre uplift over the past few million years ................. 28

iv
v

Simplified Geologic

 Map of Gunnison Gorge

National Conservation Area,

Colorado
���

���

���

��

����

���

���

���

���

��

����

���

���

��

����

���

��

��

���

�� ������ �������� ����������

����� ����������

��������� ������� ����������

��������� ��������� ��� ������������

������� ������ ��������� ��� ������������

������� ������ ������������ ��� ���� ���������

������ ����� ������ �����������

������ ��������� ��� ����� ������ ���������

������ ��� ����� �����������

�������� ���������� ����� ���� ������ ���������

������� ������ �� �������� ��������� ���

������� ��������� ������ ���

������ ���������

������� ��������� ������� ���������

������� ���� ����������� �� ���������

��������� ������� �� ����� ������������

������ ���� ������ ��������� ���� ������� ������

������� ������������

����� ������ ������������ ������ ������������

����� ������ ������������� ������������ �������

����������� ������ ������������

������� ���������� ������ ������ ������������

���� ������ ������ ������������

��������� ������ ������������

������� ������� �������� ��� ������������

����� ������������� �������

������������ ����� ������������� ��������

������ ����� ��������� �� ��������� ��������

����������� �������� �� �������� �����

�������� ������������ ����

LIST OF MAP UNITS

vi

(Above and Facing page) Simplified geologic map of Gunnison Gorge National 

Conservation Area and vicinity. Geology compiled and modified from Hansen (1968,

1971) and Ellis and others (1987). Shaded-relief base created by D. Paco VanSistine.

See Kellogg and others (2004) for a large, detailed version of this map.

�

�

�

�� �

�� �

��

�

��
vii

��������

�����

�� �� �����������������

�

�

�� ��������

������������

����������������

��������������

��������

�����

���������

������������

���� UNCOMPAHGRE PLATEAU

�������� �����

�����

��������

�����

����

������

�����

���������

�������

���������

�����

�����������

���

��

��

���

���

����

������

���������

������

��������

�����

��������

������������������

������������

��������

Area of figure 1

���°���

��°���

Hidden north of an escarpment of barren shale badlands and arid sandstone outcroppings near Montrose, 

Colorado, is one of the West’s most beautiful and unspoiled 

canyons—the lower reaches of the Black Canyon of the 

Gunnison. In an effort to preserve this unique yet highly 

accessible resource for future generations, the 234 km2

(57,725-acre) Gunnison Gorge National Conservation Area 

(hereafter referred to as the NCA) was created by Congress 

in 1999 (figs. 1 and 2). An additional 29.7 km2 (7,344 acres) 

were added by Congress in 2003. The outstanding scenery, 

Introduction

magnificently exposed geological formations, and recreational opportunities within the NCA attract over 10,000 

visitors a year. Kayaking, rafting, fishing, hunting, hiking, 

mountain biking, horseback riding, and camping are a few 

of the activities that the NCA has to offer. The spectacular 

geologic formations within the gorge document a geologic 

record that encompasses more than a third of the age of the 

Earth. Unlike the almost inaccessible depths of the Black 

Canyon of the Gunnison National Park, several good hiking 

trails descend into the NCA (fig. 2).

By Karl S. Kellogg

Figure 1. Index map showing the location of Gunnison Gorge National Conservation Area and Black 

Canyon of the Gunnison National Park.

TheGeologic Story of Gunnison Gorge

National Conservation Area,

Colorado
������

������

������

����

������������

�����

����������

�

�����

������

�����

���

�����

GUNNISON GORGE

NATIONAL 

CONSERVATION AREA

����� ����

��� ������

���

�����

������������

�����

������

�������

��� ������

�

�����

������

�����

����� ����

��������

��� � �

����� ��������

����� ������

����� ��������

����� ������

GUNNISON GORGE

NATIONAL 

CONSERVATION AREA

��������

����

92

������

����

�����

������ ������

��� ������

 to Montrose

�

� � � ������������

� �������

������

������

�������

This guide is intended to introduce the visitor to the remarkable 

geological features of the NCA and 

to explain the processes that created 

them. The geologic story involves 

ancient seas that covered the land, 

uplift of mountains, extensive erosion, 

faulting, volcanism, and a geologically recent history of deep canyon 

incision. This guide describes these 

features in non-technical terms; geological concepts and terms used as the 

story unfolds are defined at the end of 

the guide.

The NCA shares the same general 

geologic history as the adjacent Black 

Canyon of the Gunnison National Park. 

For the reader who desires an interesting and in depth (but easy to understand by the non-specialist) explanation of the regional geology and 

geologic history of the Black Canyon 

of the Gunnison, I refer you to Wallace Hansen’s excellent book “Black 

Canyon of the Gunnison—in depth” 

(Hansen, 1987). It provides many 

additional geological details relevant 

to the NCA. In addition, three geologic maps covering most of the NCA 

(Hansen, 1968, 1971; Ellis and others, 

1987) show in detail the distribution 

of the various geologic formations 

and structural features. An excellent 

waterproof topographic map, published 

by National Geographic Society and 

compiled from U.S. Geological Survey 

1:24,000-scale quadrangle maps, covers all except the southernmost part of 

the NCA; the map shows access to all 

roads and trails in the NCA. 

Figure 2. LANDSAT 7 image of the Gunnison Gorge National Conservation Area (yellow 

dashed line), showing paved roads (solid double lines), dirt roads (dashed double lines) 

and trails (dashed red lines).

2 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado
��� ��� ������ �����

��������

�����������

��������

�������

���������

������

���������

���� ����������

����� ���������� �����������

�����������

��������

��������

���������

����������������

������������������

������������������

����������

��������

����������

��������

��������

�������

�������������

�������������

��������

��������

����������

��������

��������������

������������

�������

����������������������

���

���

����

����

����

����

����

���

���

���

���

���

���

���

���

���

���

���

���

���

���

�������������������

�������

�����

�������

�����

�

�

�

�

�

�

A Brief Sense of Geologic Time

The period of time during which rocks exposed in the Gunnison River canyon were formed is immense: more than one-third 

of the Earth’s 4.5-billion-year history. Yet, the carving of the gorge itself occurred just “yesterday,” geologically speaking—only 

over a couple of million years. A simple analogy is sometimes useful to gain a better grasp of geologic time. If all of geologic 

time were condensed into one 24-hour day starting at midnight, the hard “basement” rocks exposed in the inner walls of the gorge 

formed at about 3:30 p.m. during a period of deep burial, recrystallization (metamorphism), and intrusion by granitic magma 

(molten rock). In real time, this event took place about 1.7 billion years ago in the early Proterozoic era (point A on the geologic 

time scale shown in fig. 3). The first hard-bodied animals (those that left abundant fossil remains) appeared in the Earth’s oceans 

at 9:05 p.m. (543 million years ago; point B in fig. 3). However, the oldest sedimentary rocks in the canyon, which directly overlie 

the eroded crystalline basement rocks, were not deposited until 11:09 p.m. (about 160 million years ago; point D in fig. 3); this is 

long after the first dinosaurs appeared at 10:46 p.m. (about 230 million years ago; point C in fig. 3). The last dinosaur took its final 

breath at 11:39 p.m. (65 million years ago; point E in fig. 3), and the first true humans walked the Earth at 11:59:22 p.m. (point F in 

fig. 3), which is also the approximate time (about 2 million years ago) that downcutting of the Gunnison River gorge began.

Figure 3. Geologic time chart. The letters (A–F) refer to events mentioned in the text.

Introduction 3
4 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado

The Gunnison River canyon is a fantastic geologic laboratory, in which we can examine close at hand both the rocks and the 

geologic structures present in the gorge, as well as learn about various processes that shaped the surrounding land. We will begin by 

looking at the ancient basement rocks of the inner canyon, which 

formed many kilometers deep within the Earth’s crust. These 

rocks have risen to the Earth’s surface during several periods of 

uplift and mountain building, and they eventually eroded to form 

a widespread erosional surface. We will then look at the sequence 

of sedimentary rocks that are on the eroded basement surface and 

explain the environments in which these rocks were deposited. 

The next part of the story involves a period of regional mountain 

building, called the Laramide orogeny, during which many large 

crustal blocks (including the Gunnison uplift) rose. Uplifts of 

the crustal blocks marked the initial growth of the present Rocky 

Mountains, although they were subsequently modified over time 

by considerable additional uplift, volcanism, and erosion. Finally, 

we’ll look at how the canyon itself was paradoxically carved right 

through the heart of the Gunnison uplift. Why didn’t the river, 

instead, flow around this uplifted region? 

The Gunnison uplift is a broad, topographically high 

region that extends from just south of the Gunnison River 

northward to the West Elk Mountains. The uplifted region can 

be seen when looking north from near Montrose (fig. 4). In 

the uplift, the bedding south of the Gunnison River is tilted (or 

dips) generally to the southwest, in some places quite steeply. 

Farther northeast, the bedding dips gently in the opposite 

direction (to the northeast), forming a broad arch with the crest 

or highest point of the arch located just northeast of the Gunnison River where it flows through the canyon. 

T

Geologic Evolution

of Gunnison Gorge

Geologic Evolution

Hikers sitting on Pitts Meadow Granodiorite high above the Gunnison 

River. View is to the north (downriver). Photograph by Jeff Karson.
Figure 4. View from 

7 km (4.5 mi) north of 

Montrose looking northeastward at the Gunnison uplift. The Dakota 

Formation and Burro 

Canyon Formation cap 

the highest ridges, and 

the light-colored badlands below are formed 

on the Mancos Shale. 

The hayfield is on an old 

(Pleistocene) terrace of 

the Uncompahgre River, 

which is now about 6 km 

(4 mi) to the west.

 The Basement Rocks of the Gorge

The oldest rocks in the gorge—the mostly steep, darkcolored rock walls—formed during the Proterozoic Eon (fig. 3). 

These rocks are sometimes referred to as Precambrian rocks, the 

basement complex, or simply basement rocks. The oldest basement rocks are metamorphic, meaning that the original sedimentary or igneous rocks were recrystallized due to elevated 

heat and pressure during burial many kilometers deep in the 

Earth’s crust. Heat from a large igneous intrusion (such as a 

granite) may also recrystallize or metamorphose rocks adjacent 

to it. The metamorphic event that formed the metamorphic 

rocks of the canyon took place about 1.74 billion years ago.

Certain sandstones rich in feldspar (the most common 

rock-forming silicate mineral in the Earth’s crust), lava flows, 

or tuff (rock formed by welding or fusion of volcanic ash) can 

recrystallize into a foliated metamorphic rock called gneiss; 

shale can metamorphose into a mica-rich metamorphic rock 

called schist; and quartz-rich sandstone can recrystallize into 

the metamorphic rock quartzite or quartzitic gneiss.

Excellent outcrops of layered quartzitic gneiss are 

exposed along the Chukar trail, which follows Chukar Canyon 

down to the Gunnison River. The rock is composed of as 

much as 85 percent quartz, variable amounts of feldspar, and 

a silvery, transparent mica called muscovite. The original rock 

was impure quartz sandstone, probably deposited in an ancient 

riverbed. In places, where the original quartz sandstone graded 

into more shaly beds, the rock metamorphosed into schist. 

Commonly, the mica flakes in the schist are aligned parallel 

to the foliation (the layering in the rock); the foliation is not 

always parallel to the original bedding. 

Mica schist, containing muscovite, is exposed on the canyon walls at several places just downstream from the mouth 

of Chukar Canyon. The schist also commonly contains small, 

brown, tabular crystals of staurolite (an iron-bearing aluminous silicate mineral), fine-grained, fibrous, white sillimanite 

(a pure aluminous silicate mineral), and small, well-formed, 

equant, red crystals of garnet (an iron-bearing aluminous 

silicate mineral), all formed at high temperatures and pressures 

during metamorphism. 

At a few places in the canyon, a dark metamorphic rock 

called amphibolite is interlayered with a lighter colored igneous rock (the Pitts Meadow Granodiorite, described next). 

The amphibolite is generally black and composed mostly of 

hornblende (a common, rock-forming, black, tabular, silicate 

mineral) and variable amounts of biotite (a black mica) and 

plagioclase feldspar. Amphibolite probably formed from the 

metamorphism of basalt lava or a sedimentary rock derived 

from the breakdown of basalt lava. Amphibolite is exposed 

along the river about a mile upriver from Smith Fork (fig. 5) 

and can also be seen at several places along the Ute trail. It is 

also prominently exposed several miles up Red Canyon. 

The Basement Rocks of the Gorge

Geologic Evolution of Gunnison Gorge 5
���

���

���

���

���

6 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado

About 1.0 km (0.6 mi) downriver from Chukar Canyon, 

the rock type changes dramatically to an igneous rock that 

formed deep in the Earth’s crust as a magma that intruded the 

older metamorphic rocks. This igneous rock is called the Pitts 

Meadow Granodiorite. Most of the basement rocks exposed 

in the canyon in the NCA are composed of the Pitts Meadow 

Granodiorite. The rock is dark gray and composed of uniform, 

coarse crystals of quartz and feldspar (the quartz is slightly 

darker and has a more glassy luster than the opaque, white 

feldspar, which is mostly plagioclase), black, platy crystals 

of biotite mica, and black, tabular hornblende. The contrast 

between the white and black minerals gives the rock a speckled 

appearance (fig. 6). The composition is quite variable and the 

rock locally shows flow-induced layering (foliation) caused by 

the alignment of the mineral grains. A particularly dark variety 

of the granodiorite is exposed along the canyon walls for the 

first kilometer (0.6 mi) downriver from where the river first 

encounters the Pitts Meadow Granodiorite below Chukar Canyon (Hansen, 1968).

Numerous dark inclusions that are finer grained and 

contain a slightly higher percentage of black minerals than the 

granodiorite itself are present throughout the Pitts Meadow 

Granodiorite (fig. 6). These inclusions, generally less than 

about 10 cm (4 in.) long, but locally much larger, formed by 

solidification along the margins of the magma chamber as 

the rock slowly cooled; they were subsequently ripped off the 

chamber wall by moving, viscous magma and incorporated 

into the melt. The Pitts Meadow Granodiorite intrudes the 

metamorphic rocks and has been dated by the radiometric 

method at 1.71 billion years (Jessup and others, 2002).

At some places in the canyon are exposures of a distinctive layered rock, called migmatite, composed of alternate 

layers of gray quartz-feldspar-mica gneiss and lighter colored 

granitic rock that was injected as magma generally parallel to 

the layering or foliation in the gneiss. The source for the lightcolored layers, which are generally less than about 5 cm (2 in.) 

thick in the canyon, was probably the Pitts Meadow Granodiorite. Migmatite is much more common upriver in the Black 

Canyon of the Gunnison but is exposed at several localities in 

the NCA near the mouths of Red Canyon (called Crystal Creek 

on older maps) and Smith Fork. 

Figure 5. Photograph 

of amphibolite (Xa) 

intruded by coarsegrained, speckled, 

black and white Pitts 

Meadow Granodiorite 

(Ypm), which is, in turn, 

intruded by light-colored, coarse-grained 

pegmatite and white 

aplite (YXp), composed 

almost entirely of quartz 

and feldspar. Aplite 

locally grades into 

coarse-grained pegmatite. Pocket knife is 9 cm 

(3.5 in.) long.
Throughout the canyon are very noticeable, abundant, 

irregular-shaped or tabular, light-colored rock bodies that cut 

the canyon walls. The tabular variety is called a dike, and 

these rock bodies are composed of a rock called pegmatite. 

Pegmatite is a very coarse grained rock composed mostly of 

crystals of alkali feldspar, quartz, and mica (mostly the transparent muscovite variety). In places, the individual feldspar 

crystals are as much as one meter (3 ft) or more in length. An 

example of pegmatite that is well known to river runners is the 

“T dyke” (the spelling of “dyke” is the British version), which 

is about 2 km (1.2 mi) downriver from Red Canyon (fig. 7). 

The “T-dyke” is actually two prominent pegmatite dikes that 

intersect to form a “T.” Pegmatite bodies and dikes are present 

almost everywhere basement rock is exposed along the river. 

Pegmatite forms from hot, water-rich magma commonly left 

over after a large granitic (including granodiorite) magma 

cools and partly solidifies. The water dissolved in the magma 

tends to lower the melting temperature, allowing the melt to 

stay liquid after the main granitic mass solidifies, contracts, 

and cracks. Then, the last-remaining melt is injected into the 

irregularly oriented cracks, in both the cooling granite body 

and in older, surrounding rocks, to form pegmatite dikes. The 

water-rich nature of the melt also permits the growth of very 

large crystals, because water makes the melt less viscous 

(more “runny”), which allows atoms to move readily and be 

easily ordered into crystal lattices.

Repeatedly during Middle and Late Proterozoic time, 

deep-crustal rocks were uplifted many kilometers during complicated and poorly understood periods of mountain building. 

By the close of the Proterozoic Eon (about 540 million years 

ago), the mountains were eroded to form a vast erosional surface, or peneplain, which stretched over a large part of what is 

now North America, including Colorado. This surface is sometimes referred to as the “Great Unconformity” (an unconformity is a gap in the geologic record that generally represents a 

long period of erosion or non-deposition of sediment).

Beginning late in the Cambrian Period (about 500 million years ago), the land began to slowly sink, and sandstone 

and limestone were deposited over the peneplain; most of the 

limestone was deposited in a shallow sea. Cambrian sedimentary rocks are the oldest rocks that contain abundant fossils, 

because before Cambrian time most organisms did not have 

hard body parts that could be preserved. Although deposition 

of sedimentary rocks continued intermittently throughout the 

Paleozoic Era in Colorado and much of North America, none 

of these Paleozoic rocks is preserved in the NCA region, as 

they are elsewhere in most of Colorado. The reason for the 

removal of the Paleozoic rocks from this region brings us to 

the next chapter of the geologic story. 

Figure 6. Speckled 

Pitts Meadow Granodiorite enclosing a dark, 

fine-grained inclusion.

Geologic Evolution of Gunnison Gorge 7
8 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado

Ancestral Uncompahgre Highlands

Beginning during the Pennsylvanian Period and lasting into the Permian Period (refer to fig. 3), various regions 

of western North America were uplifted to form a chain of 

mountains generally referred to as the “ancestral Rocky Mountains.” One of the uplifted areas was the ancestral Uncompahgre highlands (fig. 8), which extended across southwestern 

Colorado. Another large uplift, the ancestral Front Range 

highlands, lay to the east. As both highlands rose, the overlying sedimentary rocks were stripped away, and the underlying 

crystalline basement rocks were deeply eroded. Tremendous 

quantities of sediment were shed into basins surrounding these 

mountain uplifts; some basins, such as the “central Colorado trough” between the ancestral Uncompahgre and Front 

Range highlands, contained shallow, enclosed seas in which 

thick deposits of salt and gypsum were deposited due to rapid 

evaporation of seawater in the hot, arid climate.

In the NCA, it took millions of years, until the middle of 

the Jurassic Period, for erosion to reduce the crystalline rock 

core of the ancestral Uncompahgre highlands to a flat peneplain. After the erosion cycle, sedimentation across the area 

resumed, and a new chapter of the geologic story began. 

The Sedimentary Rocks of the Gorge

A sequence of sandstone, shale, limestone, and gypsum 

that overlies the eroded basement surface of the ancestral 

Uncompahgre highlands tells a story of ancient rivers, lakes, 

swamps, and vast oceans. Dinosaurs roamed the earth during 

the middle Jurassic Period when the oldest sedimentary rocks 

of the region were deposited, and their bones are locally found 

in some of these rocks. To the northwest, such as in Colorado 

National Monument near Grand Junction, a thick sequence of 

older sedimentary Triassic rocks were deposited on the flanks 

of the highlands, but no Triassic sediments were deposited in 

the NCA area.

The sedimentary rocks are grouped into sequences called 

formations that are generally of distinct age and rock type; 

subdivisions of formations are called members. The complete 

sequence of formations (and their various members) in the 

NCA is shown in figure 9. The following descriptions start 

with the oldest and stratigraphically lowest rocks.

Figure 7. Light-colored pegmatite dikes cutting the Pitts 

Meadow Granodiorite, 2 km (1.2 miles) downstream from Red 

Canyon. River runners call the intersecting pattern the “T dyke,” 

although it is actually two intersecting dikes. 
������

�����

�������

�����

Future

Denver

AZ

UTAH

WYOMING

NEBRASKA

NEW MEXICO OKLAHOMA

���������������

������

��������

������������

KANSAS

COLORADO ��������� ����� �����

�������

��

��������� ����������� ���������

�������

��������

������

Figure 8. Diagram showing the ancestral (late 

Paleozoic) Rocky Mountain uplifts and adjacent 

basins in Colorado. Note the location of the Proterozoic rocks, now exposed in the NCA and Black 

Canyon of the Gunnison National Park, within the 

ancestral Uncompahgre highlands.

Jurassic rocks

 Entrada Sandstone (Middle Jurassic) 

This conspicuous, brightly colored formation, deposited 

directly on basement rocks, is exposed extensively in the canyon. For example, the trail down Chukar Canyon to the river 

(Chukar trail) crosses the yellow and pink Entrada Sandstone 

(fig. 10), and the lower part of the Ute trail follows the Entrada 

along much of its way. The formation consists of fine-grained, 

thick-bedded, eolian (deposited by wind) sandstone containing 

mostly indistinct cross-beds that represent the frontal faces of 

ancient sand dunes. In most places, the sandstone is quite soft, 

although it commonly forms cliffs and weathers into large, 

rounded forms. Near the mouth of Smith Fork, a pale graygreen, chromium-bearing layer at the top of the formation is 

about a meter (several feet) thick and has been traced over a 

large region of southwestern Colorado. Elsewhere, this layer 

is locally associated with vanadium deposits. The Entrada 

Sandstone ranges in thickness from 15 to 30 m (50 to100 ft), 

generally thins eastward, and pinches out against low, eroded 

Proterozoic remnants of the ancestral Uncompahgre highlands. 

The basal few feet of the Entrada Sandstone consist of 

conglomerate containing pebbles of granite, gneiss, pegmatite, 

and quartz that are derived from the underlying Proterozoic 

basement. A good place to see the basal conglomerate and a 

well-developed, lithified (turned to rock) paleosol (ancient 

soil) that formed on the basement just before the Entrada was 

deposited is along the Ute trail, about 2.3 km (1.4 mi) north of 

its junction with the river.

The Entrada Sandstone was deposited as sand dunes in 

an ancient lowland desert that probably was near a seaway, 

because to the west, in Utah, the Entrada contains marine 

mudstone. At the close of deposition of the formation, the 

sea began to encroach on the area, and the upper few feet of 

sandstone contain water ripple marks, which signify a change 

to the shallow-marine deposits of the Wanakah Formation.

Geologic Evolution of Gunnison Gorge 9
ENTRADA

SANDSTONE

MANCOS SHALE

DAKOTA SANDSTONE

BURRO CANYON

FORMATION

BRUSHY

BASIN 

MEMBER

SALT WASH

MEMBER

WANAKAH

 FORMATION

MORRISON FORMATION

PONY EXPRESS

LIMESTONE MBR.

100

200

300

400

500

600

700

800

900

1,000

1,100

0 0

100

200

300

Sandstone, crossbedded

Sandstone

Conglomerate

Shale

Shaly sandstone

Gypsum bed; gypsiferous

 sediments

Limestone bed

Coal bed

UPPER

 CRETACEOUS

LOWER 

CRETACEOUS

UPPER

 JURASSIC

PROTEROZOIC

BASEMENT ROCKS

TIDWELL

MEMBER " sandstone

bed of

Smith Fork"

METERS FEET

MIDDLE

 JURASSIC

Pegmatite, granodiorite,

 and granite

Metamorphic rocks

EXPLANATION

10 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado

Figure 9. Generalized stratigraphic 

column of the sedimentary and basement rock units exposed in the Gunnison Gorge National Conservation Area. 

Thicknesses of some units are variable 

and are generalized in this figure. 
Jw

Je

Jmt

�b

Jmt

Figure 10. Entrada 

Sandstone (Je) along the 

Chukar trail. Some people 

have remarked on the 

resemblance of the outcrop 

to the Starship Enterprise. 

Dark outcrops of Proterozoic basement rocks (b) 

are below the Entrada, and 

the light-colored slopes 

above are underlain by the 

Wanakah Formation (Jw) 

and the Tidwell Member of 

the Morrison (Jmt). Contact 

line is dashed where 

approximately located.

 Wanakah Formation (Middle Jurassic)

This relatively thin formation consists of white, gray, and 

tan shale, mudstone, gypsum, and friable (crumbly) sandstone. 

It is well exposed in Chukar Canyon and in the lower slopes 

of Smith Fork canyon. From a distance, the formation appears 

as drab, gray, almost featureless slopes (figs. 11 and 12), but 

on closer inspection individual layers and rock types become 

evident. The base of the Wanakah Formation is marked at 

most localities by a 0.3–1.2-m- (1–4-ft-) thick, locally brecciated (composed of broken, angular pieces), fetid (oily smell 

on freshly broken surfaces), gray limestone, called the Pony 

Express Limestone Member; it is particularly well exposed 

in Chukar Canyon, just above the Entrada Sandstone. The 

limestone locally contains small (less than 2 mm [0.1 in.] in 

diameter) spherical bodies, which resemble fish roe and are 

called ooliths (pronounced oo-oo-liths). Ooliths are thought to 

form by growth around a nucleus, such as a sand grain, as they 

roll back and forth on the sea floor. 

Gypsum (hydrated calcium sulfate) crops out prominently in the Wanakah Formation as beds as thick as one 

meter (several feet) that are interlayered with soft, pale-pink, 

gypsiferous mudstone and calcareous (has calcium carbonate 

cement) sandstone. The gypsum is a snow-white, sugary-textured variety called alabaster, which is mined in other areas 

for the manufacture of sheetrock. The gypsum is evidence that 

the shallow Wanakah seaway was closed to circulation and 

that the climate was hot, so that evaporation was high. These 

conditions caused the dissolved minerals in the sea water to 

concentrate and precipitate on the sea floor. The total thickness 

of the Wanakah Formation is only about 15 m (50 ft).

The similarity of the Wanakah to the overlying Tidwell 

Member of the Morrison Formation caused previous geologists (for example, Hansen, 1987) to include the beds of the 

Tidwell with the Wanakah Formation. However, detailed 

studies, particularly those of Robert O’Sullivan (1992), have 

shown that there is a considerable gap (unconformity) in the 

geologic record between the top of the Wanakah Formation 

and the base of the Morrison Formation.

 Morrison Formation (Upper Jurassic)

The Wanakah seaway gradually dried up and was mostly 

replaced by broad, meandering rivers and shallow lakes in 

which the Morrison Formation was deposited. However, 

the shallow sea was still present during much of early Morrison deposition. Although dinosaurs roamed the land when 

the Entrada and Wanakah Formations were being deposited, 

the Morrison Formation is particularly famous for the spectacular and diverse dinosaur bones that have been recovered 

at such places as Morrison, Colorado (the locality for which 

the formation is named), Canon City, and Dinosaur National 

Monument in northwestern Colorado. An excellent and easily 

accessible exposure of the Morrison Formation is on the north 

side of Smith Fork, although much of the formation can also 

be observed along the upper part of the Ute trail. The upper 

Morrison is easily observed along the Eagle Valley trail in the 

southwest part of the NCA (fig. 2). The formation is divided 

into three members: the lower Tidwell Member, the middle 

Salt Wash Member, and the upper Brushy Basin Member.

Geologic Evolution of Gunnison Gorge 11
��

��

���

���

���

��

Ute

Indian fault

��

��

�� ���

���

���

��

��

�� ��

Figure 12. View looking west from the top of Chukar trail of the south end of the Ute Indian fault 

in upper Chukar Canyon. The fault (at the left margin of the image) passes into a monocline in the 

Wanakah Formation (Jw). Other units are: Proterozoic basement rocks (b); Entrada Sandstone 

(Je); Tidwell (Jmt), Salt Wash (Jms), and Brushy Basin (Jmb) Members of the Morrison Formation; and Burro Canyon Formation (Kb). Contact line is dashed where approximately located; 

fault is dotted where concealed.

Figure 11. View of the north slope of Smith Fork canyon, about 1 km (0.6 mile) upstream from 

the Gunnison River. (Je, Entrada Sandstone; Jw, Wanakah Formation; Jmt, Tidwell Member of 

the Morrison Formation; Jms, Salt Wash Member of the Morrison Formation; Jmb, Brushy Basin 

Member of the Morrison Formation; Kb, Burro Canyon Formation). Contact line is dashed where 

approximately located.

12 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado
The Tidwell Member, which resembles the Wanakah 

Formation in the NCA region and is easily misidentified with 

it, is about 60 m (200 ft) thick. It consists of gray and yellow 

gypsum-bearing shale and sandy shale that form a continuous 

slope, which extends down to a similar slope on the Wanakah. 

A 3-m- (10-ft-) thick, yellow, coarse-grained sandstone marks 

the base of the member. The upper part of the Tidwell Member contains gray, cherty, algal limestone. Native Americans 

commonly used chert in the manufacture of arrow and spear 

points. The limestone formed from calcareous algae, as shown 

by the preserved, wavy algal structures in the rock.

Some sandy limestone beds near the top of the formation also contain small, iron-stained (“rusty”), limy, spherical 

ooliths (fig. 13). In the Smith Fork area, but not in areas to the 

south, is a fine-grained, friable, crossbedded, light-yellowishgray, eolian sandstone, as much as about 25 m (80 ft) thick. 

Hansen (1968) named this unit the “Junction Creek Sandstone Member of the Wanakah Formation;” now it is simply 

called the “sandstone bed of Smith Fork” and is included in 

the Tidwell Member of the Morrison Formation (O’Sullivan, 

1992). This sandstone was deposited as dune sand on the 

temporarily dried-up floor of the ancient seaway that existed 

during early deposition of the Morrison Formation.

The Salt Wash Member is composed chiefly of light-gray, 

massive to crossbedded, fine-grained, cliff-forming sandstone 

and interbedded red, gray, and green shale. The sandstone is 

stained red from the overlying red shale. Uranium deposits 

elsewhere in Colorado and Utah are commonly hosted by the 

Salt Wash Member. This member is about 35–52 m (120–170 

ft) thick in the NCA. 

The Brushy Basin Member consists of thin-bedded, red, 

yellow, gray, and green shale and light-yellow sandstone. In 

places, a distinctive pebble conglomerate bed, containing 

red and green pebbles (Hansen, 1968, calls it the “Christmas tree conglomerate”), crops out near the middle of the 

member. Dinosaur bones, such as the famous bone beds of 

Dinosaur National Monument, are locally abundant in the 

Brushy Basin Member. In the NCA, the member is 96–99 m 

(315–325 ft) thick.

Figure 13. Iron-oxide-stained 

ooliths stand out in weathered relief 

in sandy limestone in upper part of 

the Wanakah Formation. 

Geologic Evolution of Gunnison Gorge 13
14 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado
Geologic Evolution of Gunnison Gorge 15

Prominent cliff of light-tan, locally cross-bedded, quartz sandstone 

in the Salt Wash Member of the Morrison Formation in the Smith 

Fork canyon. Black staining is desert varnish, an iron-manganese 

oxide common in arid climates, although some of the reddish 

staining is probably from overlying red sediments of the Brushy 

Basin Member of the Morrison.
Figure 14. Chert-pebble conglomerate, typical of the Burro Canyon Formation. Pocket knife is 9 cm (3.5 in.) long.

16 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado

Cretaceous rocks

 Burro Canyon Formation (Lower Cretaceous)

Sandstone and conglomerate that characterize the Burro 

Canyon Formation are prominently displayed in cliffs along 

much of the canyon rim. The formation consists of light-yellow, gray, and tan, thick-bedded, quartz-rich sandstone that 

commonly contains well-rounded, white, gray, and black chert 

pebbles, especially near the top of the formation (fig. 14). 

Cross-beds are conspicuous in most of the sandstone and conglomerate beds. In most places, the lower part of the formation 

is interbedded shale and crossbedded, coarse-grained sandstone. The pebbles within the Burro Canyon Formation formed 

in streams that had enough power to transport the pebbles 

from as far away as Utah or Nevada (Hansen, 1987). The base 

of the formation marks an unconformity above the Morrison 

Formation. A good place to examine the Burro Canyon Formation is along the Eagle Valley trail in the southwest part of the 

NCA (fig. 2). The thickness of the Burro Canyon Formation is 

variable, from about 8 to 35 m (25 to 115 ft). 

 Dakota Formation (Upper Cretaceous)

The Dakota Formation 

is widespread throughout 

the Rocky Mountain region. 

Tilted, resistant sandstone 

of the formation commonly 

forms linear ridges called 

hogbacks. In the NCA, the 

Dakota consists mostly of 

fine-grained, thin, platy, 

quartz-rich sandstone and 

dark-gray to black, carbonaceous (containing black, 

organic material) shale. Chert 

pebbles are present in some 

of the sandstone. Coal beds 

less than 2 ft thick are locally 

present near the base. Ripple 

marks, worm tracks, and 

leaf and twig impressions 

are common. An excellent, 

easily accessible exposure of 

the entire Dakota Formation, 

including a 0.5-m-thick (1.5-

ft-thick) coal bed, can be seen 

along the Eagle Valley trail 

(fig. 15).

Most of the Dakota 

was deposited by slow-moving streams and in swamps; 

however, the upper part was deposited on coastal tidal flats 

adjacent to an encroaching seaway and interfingers with the 

overlying marine Mancos Shale. The thickness of the Dakota 

Formation ranges from about 20 to 34 m (65 to 110 ft).

Figure 15. A 45-cm- (18-in.-) thick coal bed, partly mud coated, 

in lower part of the Dakota Formation. The coal bed contains a 

thin sandstone bed in the upper part. This outcrop is exposed 

along the Eagle Valley trail several hundred meters (about 1,000 ft) 

northeast of the Chukar Road. Pen is 13 cm (5 in.) long.
 Mancos Shale (Upper Cretaceous)

This widespread formation forms drab, gray badlands 

over much of the Uncompahgre River valley, which forms 

the west side of the NCA. The rocks of the Mancos consist 

mostly of dark-gray, silty, clay shale or mudstone. The 

shale is fissile (contains platy layers); in contrast, the mudstone is more massive and blocky. A few thin, fine-grained, 

silty sandstone layers are near the base. The Mancos Shale 

is very soft, easily eroded, and underlies mostly broad valleys in the region. This poorly drained formation forms a 

barren, “lunar” landscape of light-gray rounded hills and 

gullies (figs. 4 and 16), and it becomes a sticky gumbo 

in wet weather. This Mancos landscape is referred to as 

“adobe lands,” a very suitable name. The Mancos Shale 

was deposited in a shallow inland sea, and careful searching reveals fossils of clams, ammonites (extinct cephalopods related to the modern chambered nautilus), and shark 

teeth. The Mancos basin slowly sank as clay from rivers 

feeding the inland sea was deposited. Deposition probably 

kept up with sinking, so that the water depth remained 

nearly constant. The Mancos Shale is very thick, more 

than 1,500 m (5,000 ft), although the upper part has been 

eroded away in the NCA.

Geologic Evolution of Gunnison Gorge 17

View to the southwest at Pleasure Park, at the confluence of 

North Fork (left) and the Gunnison River (behind trees at right). 

The layered rocks at the top of the ridge are Dakota Formation. 

The prominent light-tan cliff is the Burro Canyon Formation, and 

the reddish slopes below the Burro Canyon are underlain by the 

Morrison Formation. Old (middle? Pleistocene) terrace gravel 

forms the flat lower surfaces on both sides of the Gunnison River.
18 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado

Surficial deposits are relatively thin deposits of erosional 

materials that locally cover the land surfaces, particularly in 

stream valleys. They reveal important information about the 

more recent (last couple of million years or so) geologic history of the region.

At a few places in the NCA (such as near the highest 

point of Chukar Road; fig. 2), deposits of boulder gravel contain large, rounded, black boulders composed of volcanic rock. 

This boulder gravel is alluvium deposited by the ancestral 

Gunnison River, probably before or during the early stages 

of major canyon downcutting about 2 million years ago. The 

boulders were eroded from extensive beds of Oligocene and 

Miocene volcanic breccia (rock composed of angular igneous 

rock fragments) and ash-flow tuff (consolidated or fused ash 

from a volcanic eruption). Such beds are now exposed to the 

east of the NCA, but they probably covered parts of the NCA 

at one time. (See “The Gunnison River Paradox” section for 

discussion of these volcanic rocks.) The boulder gravel deposits are so old and eroded that no terrace surfaces are preserved.

Terrace gravel represents older alluvium in river- or streamdeposited beds that are now above present stream channels due 

to the downward erosion of the streams. The deposits underlie 

flat, terrace-like surfaces; generally, the higher a terrace surface 

is above a stream channel, the older is its underlying terracegravel deposit. Terrace gravel accumulated during periods when 

deposition in stream channels predominated over channel cutting. In this region, terraces formed when glaciers were actively 

eroding the high mountain valleys at the headwaters of the Gunnison River and were dumping vast quantities of sediment into 

the river system. The last major period of glaciation (called the 

Pinedale glaciation in the Rocky Mountains) is dated at about 25 

to 14 thousand years ago, and in many places (mostly outside the 

NCA, such as along the Uncompahgre River and lower Gunnison River) a prominent terrace was produced at that time. Higher 

terraces locally record several older glacial periods. If the ages of 

terrace deposits at different heights above a stream can be determined, the rates of downcutting can be calculated. So, terraces 

are important recorders of the erosional history of the Gunnison 

River and its tributaries.

Figure 16. Typical exposure of Mancos 

Shale near the Eagle Valley trail and Chukar 

Road. Scarring is caused by indiscriminant 

off-road vehicle use.

 Surficial Deposits:

 A Record of Late Tertiary and Quaternary Erosion, Deposition, and Climate Change
Landslides are common in the canyon, especially 

on steep slopes underlain by relatively weak rocks of the 

Wanakah and Morrison Formations. Recent landslides have a 

hummocky surface and a steep breakaway zone at the upper 

margin, but most landslides in the NCA have been inactive for 

a long time, so they are deeply eroded. These old landslides 

formed during periods of higher precipitation than currently 

exists, such as during the last (Pinedale) glacial interval. 

Talus deposits are sloping aprons of angular rock fragments as much as 3 meters (10 ft) long, generally beneath 

cliffs where rock fragments accumulate due to rockfall. Talus 

accumulates more rapidly during episodes of heavy precipitation such as in glacial periods. In the more arid times we live 

in, very recent rockfall composed of freshly broken rocks 

without lichens is found at several places in the canyon.

The youngest and most widespread surficial deposit in 

the NCA is valley alluvium, which is composed of stratified 

sand, gravel, and boulders in river and stream channels. Valley alluvium is actively transported and deposited by flowing 

water, especially during periods of flooding. Sand, gravel, 

and boulders in the alluvium are generally rounded by stream 

action; some boulders are as large as cars.

Geologic Evolution of Gunnison Gorge 19

What appears to be a fault relationship in this north-facing view is 

instead a large, almost unbroken landslide block of Brushy Basin 

Member of the Morrison Formation (reddish, layered rock) that 

has slid toward the viewer and is now overlying gray Mancos 

Shale. This outcrop is about 1 km (0.6 mi) north of Chukar Road 

and about 2 km (1.2 mi) from Peach Valley Road.
Figure 17 (facing page). Regional 

map and cross section of the lower 

Gunnison River area, showing the 

Laramide uplifts (areas underlain by 

Lower Cretaceous and older rocks) 

and major faults. UF, Ute Indian fault; 

RRF, Red Rocks fault; CF, Cimarron 

fault. Cross section A–A’ is drawn at 

twice the scale of the map

Prominent monocline in the Morrison 

Formation, just north of Chukar Road. 

This fold is probably caused by draping 

of beds over a buried north-trending 

basement fault. This is a different fold 

from the larger monocline overlying 

the buried, northwest-trending Red 

Rocks fault. The gray mudstone in the 

foreground is Mancos Shale.

20 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado

The Laramide Orogeny: 

Building the Rocky Mountains

The present Gunnison uplift rose during the Late Cretaceous and early Tertiary Periods, concurrent with an episode 

of great mountain building called the Laramide orogeny. This 

was a time of crustal shortening driven by continental-scale 

forces (commonly called plate tectonic forces, generated 

by movement of various plates of the Earth’s outer shell or 

lithosphere). These huge forces caused large crustal blocks to 

rise in a somewhat irregular pattern across the western interior 

of North America to form various ranges of the present Rocky 

Mountains. However, later crustal extension, erosion, volcanism, and additional late Tertiary uplift significantly modified 

the original uplifts. The Laramide uplifts are usually bounded 

on one or more sides by thrust faults, inclined at low angles 

(less than 45o), that were the breakage planes along which 

older rocks (commonly including basement rocks) were 

shoved over younger rocks. The movement of a block up an 

inclined thrust fault tends to tilt the thrusted block, causing 

formerly flat-lying sedimentary rocks to become folded and 

locally broken.

The Gunnison uplift is a relatively small uplift compared with many other uplifts of Laramide age. It is a simple 

northeast-tilted block of basement rocks that is bounded on 

the southwest by two major reverse faults, the Red Rocks 

and Cimarron faults (fig. 17). These reverse faults are similar 

to thrust faults except that they are steeper than 45o. Only the 

west end of the exposed Red Rocks fault is exposed in the 

NCA. However, its buried trace farther to the west within the 

basement rocks is evident by the draping and folding of the 

overlying sedimentary rocks over the fault to form a structure called a monocline. 

The Ute Indian Fault:

A Prominent Laramide Feature

The NCA has one of the most spectacularly exposed 

Laramide faults in Colorado—the Ute Indian fault—which is 

clearly visible along much of the gorge (cover photograph and 

figs. 12, 18, 19, and 20). At most places, the fault dips about 

40o to the west and its trace is parallel to the west structural 

margin of the Gunnison uplift. The Ute Indian fault is a thrust 

fault along which Proterozoic basement rocks were shoved up 

and to the east as much as 350 m (1,150 ft) vertically above 

Jurassic sedimentary rocks. Additional reverse faults in the 

basement rocks west of the Ute Indian fault are concealed 

beneath sedimentary rocks, which have been tilted to the 

west. These hidden faults dip to the east and have movement 

opposite to that of the Ute Indian fault. Basement rocks were 

shoved up and to the west on these hidden faults, and the 

faults mark the west margin of the Gunnison uplift. 

The southern end of the Ute Indian fault is prominently 

exposed in Chukar Canyon (fig. 12) and generally follows the 

Gunnison River canyon northward, crossing the river at several 

places. At both its south and north ends, the fault undergoes a 

remarkable decrease in displacement over a short distance. At 

both ends, the vertical displacement across the fault decreases to 

zero, and the fault disappears from view into the basal sedimentary and basement rocks. At both the north and south ends, the 

overlying beds are not broken by the fault but are folded over it, 

becoming locally steepened into a monocline (fig. 19). The Ute 

Indian fault is also well exposed along the Ute trail, about 0.5 

km (0.8 mi) from where the trail meets the river. Where the fault 

is exposed, it is a 1–2-m-wide (3–6-ft-wide) breccia zone that 

separates the Pitts Meadow Granodiorite on the west from the 

Entrada Sandstone on the east. 
� �� �� �������������

� �� ��������

� � �������������

� � ��������

�������� ������

��������������

������

���������������

� ��

���������������

�������

���������

����������������

������������������������������������

��������������������

�����������

��������������������

�����������������������������������������������������������

�������������������������������

�������������������������������������

��������������������������������������������

��������������

������������������������U is upthrown side; D is downthrown side; 

dashed where location is uncertain

��������������������������

�����������

�

�

��

�

���

��

����

���

���

��������

������

��

��

���

�

�

�

�

� �

��������

�����

�����������

�����

�����

����

�

�

�

�

�����

��������

�����

������

���������

������

��������

��������

��������

�����

�������

������

����������

�����������

������

��������

�����

���������

������������

����

���°���

��°��

Geologic Evolution of Gunnison Gorge 21
Kdb

Jmb

Jms

Jmt

Je

Jw

Kdb

Jmb

Jms

Jmt

�b

�b

Je

Jw

����� �����

�����

������

�����

������

Je

Jms

Jmb

Jmt

�����������

Je

JeJe �������������������

�������������������

Jms

Jmb

Jmt

JwJw

�����������

The Gunnison River Paradox

Rivers tend to seek the easiest path for their channels and, 

therefore, usually follow zones of soft, easily eroded rock. For 

example, the Uncompahgre River has cut a wide valley mostly 

in the soft Mancos Shale west of the Gunnison uplift. The 

Gunnison River, in contrast, has cut its channel through the 

hard basement rocks of the Gunnison uplift rather than taking 

an easier path through softer rocks. 

A number of factors contributed to this paradox. Following the Laramide orogeny, erosion began to degrade the 

Gunnison uplift until by late Eocene time a vast erosional 

surface had formed. This surface consisted of rounded hills 

and relatively flat areas, and it formed on both the crystalline 

basement and the overlying sedimentary rocks. Slow-moving, 

meandering rivers flowed northwestward across the region 

into a large lake in what is called the Uinta-Piceance Creek 

basin, northwest of the Gunnison uplift. The thick lake deposits of the basin include the famous oil shale of the Eocene 

Green River Formation. 

During the Oligocene Epoch, tremendous outpourings 

of volcanic rock from two sources covered large parts of the 

region, including most, if not all, of the NCA. About 30 million 

years ago (early Oligocene time), lava flows and voluminous 

mudflows from a large volcano in the West Elk Mountains, 

just to the east of the NCA, spread over much of the region 

and solidified into a formation called the West Elk Breccia. 

The original volume of the West Elk Breccia is estimated at 

more than 400 km3 (90 mi3) (Hansen, 1987). About 2–3 million 

years after the West Elk Breccia was deposited, tremendous 

caldera eruptions, at least 30 km (20 mi) to the southeast in the 

San Juan Mountains, sent sheets of hot ash across the region; 

these caldera eruptions lasted less than a million years (about 

28–27.5 million years ago). Calderas are enormous volcanoes, 

with craters commonly wider than 10 km (6 mi), which erupt 

as much as thousands of cubic kilometers (hundreds of cubic 

miles) of hot ash that is so hot it welds itself into a rock called 

ash-flow tuff. Some of the hot ash clouds flowed as far to the 

north as the south slopes of the West Elk Mountains, covering 

older volcanic rocks. None of these volcanic rocks is preserved 

in the NCA because they have been eroded away, although 

large rounded boulders (composed mostly of black or darkgray tuff and volcanic breccia) are preserved in boulder gravels 

deposited about two million years ago by the Gunnison River 

before it began cutting the canyon. These gravel deposits are 

now found high on hillsides above the canyon.

Figure 18. View looking north along Gunnison River canyon showing 

the dramatic offset of Proterozoic and overlying Mesozoic rocks near 

the north end of the Ute Indian fault. Units are: Proterozoic basement 

rocks (b); Entrada Sandstone (Je); Wanakah Formation (Jw); Tidwell 

(Jmt), Salt Wash (Jms), and Brushy Basin (Jmb) Members of the Morrison Formation; and Dakota Formation and Burro Canyon Formation 

(Kdb). Contact lines are dashed where approximately located; fault is 

dotted where concealed.

22 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado
Ute Indian

fault

Old river gravel

Ute Indian

fault

Old river gravel

���

���

��

��

��

���

���

��

��

Figure 19. Close-up view of the north terminus of the Ute Indian fault. Note how the ductile sedimentary rocks are more easily folded 

than the brittle Proterozoic basement rocks (b), and therefore “drape” over the fault. Other exposed units are: Entrada Sandstone (Je); 

Wanakah Formation (Jw); and Tidwell (Jmt), Salt Wash (Jms), and Brushy Basin (Jmb) Members of the Morrison Formation. Contact 

lines are dashed where approximately located; fault is dotted where concealed.

The ancestral Gunnison River was frequently diverted 

by outpourings of volcanic rock. It was ultimately confined 

between piles of volcanic rocks from volcanic centers in the 

West Elk Mountains and San Juan Mountains. Volcanic rocks 

upriver from the NCA are unusually thick, an indication that 

they are preserved in a slight crustal downwarp (a syncline). 

This structure is along and parallel to the river, so it served to 

further confine the river’s course (Hansen, 1987). Once the 

Gunnison River was entrenched between two piles of volcanic 

rocks, it could not seek a new channel, even when it encountered resistant rocks, such as Proterozoic basement rocks. 

After the Gunnison River eroded through the relatively 

soft volcanic and sedimentary rocks overlying the Gunnison 

uplift, it began to cut its channel into the harder basement 

rocks in a process called superimposition. This is a process 

whereby a drainage pattern from a formerly existing geologic 

terrane is impressed onto a different geologic terrane during 

erosion and downcutting. If it were not for the confinement of 

the Gunnison River between thick piles of volcanic rock, the 

river might have been positioned a few kilometers to the south 

in an area of easily eroded sedimentary rocks rather than in 

the hard, crystalline basement rocks of the Gunnison uplift. A 

broad river valley, similar to the Uncompahgre River valley, 

would have formed, and the gorge of the NCA and Black Canyon of the Gunnison would not have been carved. 

Geologic Evolution of Gunnison Gorge 23
24 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado
Geologic Evolution of Gunnison Gorge 25

Kayakers exploring upriver from Duncan trail. The Canyon walls 

here are Pitts Meadow Granodiorite.
Figure 20. View looking northwest at a north-trending splay (branch) of the Ute Indian fault in the wall of the Gunnison River gorge 

about 0.8 km (0.5 mi) downriver from Chukar Canyon. Proterozoic rock (b) was thrust eastward over Entrada Sandstone (Je). On the 

west (left) side of the fault, the Proterozoic rock is layered quartzitic gneiss; east of the fault it is mica schist. This splay of the Ute Indian 

fault dies out in the overlying, easily deformed, gypsum-rich beds of the Wanakah Formation (Jw). Landslide deposits (Ql) cover parts of 

the rock outcrops. The near outcrop at the left, at river level, is a large Proterozoic pegmatite. Lines are dashed to indicate where they 

are approximately located.

pegmatite

Ute

Indian

fault

pegmatite

Ute

Indian

fault

��

��

��

��

��

��

��

��

��

��

26 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado

The Gunnison River began to cut its gorge about 2.5 

million years ago, a remarkably short time ago, in terms of 

geologic time. How do we know this? The evidence, cited by 

Hansen (1987), is based on the isotopic dating of a volcanic 

ash bed in Bostwick Park, immediately east of the eastern 

boundary of the NCA. The ash is 1.2 million years old and 

is present about 365 m (1,200 ft) below the rim of Black 

Canyon of the Gunnison and about an equal distance above 

the bottom of the present canyon. If the ash was deposited 

at the then bottom of the canyon 1.2 million years ago, the 

average rate of downcutting at Bostwick Park since that time 

has been about 0.3 m (1 ft) per 1,000 years. Although the rate 

of downcutting undoubtedly was not constant, this rate yields 

an approximate total time for downcutting of the canyon of 

about 2.5 million years.
��������

�����

Factors that Control the

Gunnison River’s Ability to Downcut

Downcutting of a river’s channel can only occur when 

a river has a distant outflow region that is significantly lower 

in altitude than the rocks into which it is eroding. This level, 

called the stream’s base level, is the ultimate level to which a 

stream or river can erode. The base level concept is extremely 

important when considering how the Gunnison River canyon formed. Somehow, the base level of the Gunnison River 

was lowered dramatically about 2.5 million years ago, thus 

allowing the river to cut deeply into the Gunnison uplift. A 

large canyon that crosses the Uncompahgre uplift south of 

Grand Junction, called Unaweep Canyon (fig. 17), presents 

an intriguing clue as to how this might have occurred. A large 

river clearly carved Unaweep Canyon, but, curiously, it now 

contains only small streams that are incapable of carving such 

a canyon. Stanley Lohman (1961) suggested that the ancestral 

Colorado River once flowed through Unaweep Canyon (fig. 

21A). Sometime during the Pliocene Epoch, the flow of the 

Colorado River, upstream from its ancestral junction with 

the Gunnison River, was “captured” by headward erosion of 

a downstream tributary of the Colorado River near present 

Grand Junction (fig. 21B). The Colorado was then diverted 

into the valley of the tributary stream and flowed around 

the northwest end of the Uncompahgre Plateau. The erosive 

power of the Colorado River added to the tributary and caused 

the river channel to cut down mostly through the soft Mancos 

Shale, thereby significantly lowering the river’s base level. 

This river capture left only the flow of the Gunnison River 

through Unaweep Canyon. Evidence recently found by Robert 

Scott and his coworkers (2002) shows that the Uncompahgre Plateau was being uplifted at this time, so that the flow 

through Unaweep Canyon, deprived of the flow of the Colorado River, was unable to downcut as rapidly as uplift, causing 

the Gunnison River to be dammed and creating a lake (fig. 

21B). Later, a north-flowing tributary of the Colorado River 

captured the lake’s outflow, and the Gunnison River then 

regained its connection with the Colorado River near present 

Grand Junction (fig. 21C). This reconnection of the Gunnison 

with the Colorado River greatly increased the river’s erosive 

power because its base level was suddenly lowered to that of 

the Colorado River, creating a steeper gradient. Initially, the 

increased gradient was across a region underlain by Mancos 

Shale, which was easily eroded by the increased cutting power 

of the river. Concurrently, tributary streams cut into the soft 

Mancos, and the valley was widened. Gradually, the steepened 

gradient moved upstream until it encountered the hard, crystalline basement rocks of the Gunnison uplift, causing rapid 

downcutting into the basement rocks.

A major debate currently among geologists is whether 

the Gunnison uplift (as well as other uplifts in Colorado) 

underwent renewed uplift in the late Tertiary while the Gunnison River was cutting its gorge. Some geologists argue that 

regional erosion of the surrounding soft sedimentary rocks 

and exhumation of the Gunnison uplift was induced mostly by 

climatic change to wetter conditions and that renewed uplift 

either did not occur or was of minor significance. Other geologists point to old (mostly late Eocene), tilted, warped, and 

possibly faulted erosional surfaces as evidence for significant 

regional late Tertiary uplift. Both factors, climatic change and 

renewed uplift, may have played a role, but what is clear is 

that the river began its journey into the hard, crystalline heart 

of the uplift only about 2.5 million years ago. 

Geologic Evolution of Gunnison Gorge 27
�������

������

�������

������

�������

������

����

������

�����

��������

��������� ��������

�����

�����

�

��������

������

��������

����

��������

����

��������

����

������

�����

�

������

�����

�

������

�����

������

�����

������

�����

������

�����

��������������

��������

�����

������

�����������

�����������������������

������

���������������������������������������

�����������������������������������������

��������������������

��������������������

�����

����

���� ����� �������� �� ���

��������� ������ �� �����

������ �� �� ���

�������� ���� �

�������� ���� �

10 KILOMETERS 

0

0 5

5 10 MILES

Figure 21. Schematic diagrams showing 

the relationship of the Colorado and Gunnison Rivers to the Uncompahgre uplift over 

the past few million years (after Lohman, 

1961, as modified by Scott and others, 

2002). A, Early in the Pliocene Epoch (3–5 

million years ago); B, Later in the Pliocene; 

C, Late Pliocene to present. In all diagrams, 

yellow and red areas indicate highlands; 

red areas indicate a high Miocene basalt 

plateau, the remnants of which form Grand 

Mesa. Refer to the text for an explanation 

of each diagram. 

28 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado
How Steep is the Gradient Through the 

Gunnison River Gorge? 

From Chukar Canyon to North Fork, a distance of 20.4 

km (12.7 mi), the average gradient is 3.8 m/km (20 ft/mi), 

although the gradient over some reaches of the gorge is twice 

this amount. The average gradient across the Gunnison uplift 

(from Blue Mesa Reservoir, 50 km [31 mi] upriver from the 

NCA, to North Fork) is 8.1 m/km (43 ft/mi), although Hansen 

(1987) points out that there are cataracts within the Black Canyon where the gradient is as high as 68 m/km (360 ft/mi).

Geologic Evolution of Gunnison Gorge 29

Why Are the Canyon Walls so Steep?

Two factors mostly control the steepness of the canyon 

walls. First, the Proterozoic basement rocks are very resistant 

to erosion, so where the river gradient is steepest, most of the 

river’s energy is spent in downward cutting instead of lateral 

cutting. Second, because the Gunnison River flows through 

the highest part of the exhumed Gunnison uplift, much of the 

regional drainage is diverted away from the river. Most of the 

tributaries in the Gunnison River canyon are small, so they do 

not significantly erode the canyon walls.
30 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado

The Boulder Garden rapids. Rooster Barnhart (BLM River Ranger) 

is standing on a boulder of Pitts Meadow Granodiorite.
Geologic Evolution of Gunnison Gorge 31
32 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado

Selenium and Salt Contamination

from the Mancos Shale

Selenium and various dissolved salts (mostly sulfates) 

naturally exist in small concentrations in marine sedimentary 

rocks. These substances are leached by groundwater and form 

a small but measurable component in the water from springs 

and seeps that emanate from the Mancos Shale. Increases in 

irrigation in the Uncompahgre and Gunnison River valleys 

(as well as in the Grand Valley near Grand Junction) have 

dramatically increased the load of selenium and dissolved salts 

into the surface-water systems. Consequently, there is rising 

concern that this increase may have significant health effects 

on humans, fish in the rivers, and livestock. The problem is 

serious enough that the U.S. Geological Survey is undertaking 

a study to determine the geologic factors governing the concentration of selenium and other soluble material in the Mancos Shale, the environmental impacts of these substances due 

to increased irrigation, and the possible health risks involved.

Effects of Damming on the River’s Profile

Downcutting by the river would normally be greatest 

during periods of highest runoff, which is usually in the spring 

during rapid snowmelt in the higher parts of the Gunnison 

River basin. However, three dams upriver from the NCA that 

were built in the early 1960s—Blue Mesa, Morrow Point, and 

Crystal Dams—have drastically changed flow patterns. Peak 

discharge before the dams were built was commonly more 

than 15,000 cubic feet per second (cfs); the historic record of 

19,000 cfs was in 1921. The controlled release from the dams 

now rarely exceeds 5,000 cfs, and the scouring power of the 

river has consequently been severely diminished. 

The decrease in discharge will inevitably change the 

profile of the river over time. As flash floods sweep piles 

of bouldery debris down steep tributaries, the river will be 

unable to remove this debris as effectively as it once did. 

Also, talus below steep cliffs will also build up. These factors will gradually lead to damming of the river at various 

points, especially at the mouths of side canyons, and will 

result in steeper rapids at the ensuing boulder fields. Perhaps 

river runners wishing for enhanced whitewater challenges 

may not object to these changes!

Effects of the Upriver Dams on River Ecology

The decreased sediment load and the decreased ability of 

the river to transport sediment due to the upstream dams have 

had a profound effect on the ecology of the river. Fine-grained 

sediment, now derived only from tributary streams, is deposited along the riverbanks rather than being scoured by spring 

floods. The result is a proliferation of riverbank vegetation 

(Elliott and Parker, 1997). Grasses, scouring rushes, poison 

ivy, tamarisk, and other water-loving plants now crowd the 

riverbanks. Cottonwood, box elder, and willow trees are also 

making inroads along the river corridor. BLM is now attempting to remove all tamarisk, an introduced invasive species, 

from the NCA. 

Before the dams were built, spring floods brought large 

quantities of sand and gravel down the river, which were 

deposited in eddies and reaches of slower moving water. The 

floods also invaded lowlands adjacent to the main channel and 

deposited sand and gravel bars that were spawning grounds 

for a variety of fish, such as the razorback sucker, Colorado 

pikeminnow, humpback chubb, and bonytail. These habitats 

are now mostly, if not entirely, gone from the Gunnison River. 

The gravel bars of former floods remain above river level and 

are mostly overgrown with vegetation. In addition, the reduced 

flow has resulted in the reduction of fine sand and gravel in 

the river channel, leaving mostly large cobbles and boulders. 

These changes in the channel character have severely threatened the ability of native fish to reproduce.

Perhaps even a greater threat to the endangered, warmwater fish species is the drastically lower water temperature 

caused by the dams. During the spring and summer, the water 

emerging from the bottoms of the reservoirs is considerably 

colder than the water in the river before dams were built. In 

addition, introduced cold-water species, such as northern pike 

and trout, compete with the diminishing numbers of native 

fish. This has raised the issue of whether the advantages of 

having a blue-ribbon trout-fishing stream outweigh the loss of 

native species. 

Environmental Issues in the

Gunnison Gorge National Conservation Area

Environmental Issues in the
Erosion Due to Off-Road Vehicle Use

One of the recreational activities enjoyed by visitors to 

the NCA is traversing the multitude of challenging, scenic 

trails and roads using mountain bikes, motorcycles, all-terrain 

vehicles (ATVs), and 4-wheel-drive vehicles. Numerous trails 

and roads have been established for this purpose, and most 

visitors are happy to obey the signs imploring people to stay 

on the established routes. However, an increasing number of 

people are ignoring these rules, and numerous areas, particularly in the Mancos badlands, are beginning to show the 

effects of indiscriminant off-road travel (fig. 16). Tracks from 

both 2- and 4-wheeled vehicles cause gullying, increased wind 

erosion, and are increasingly visible in many places. 

Many people made this geological guide possible 

and deserve heartfelt thanks from the author. Karen Tucker, 

Manager of Gunnison Gorge National Conservation Area for 

the U.S. Bureau of Land Management, initiated this study and 

led the author on his initial tour of the area. Bruce “Rooster” 

Barnhart, Gunnison Gorge River Ranger, organized a 

geological traverse of the river, pointed out many places of 

interest, and shared his knowledge of canyon lore and history. 

Richard Grauch (U.S. Geological Survey) provided information regarding the environmental setting of the Mancos Shale. 

John Elliott (U.S. Geological Survey), who has studied the 

geomorphology of the Black Canyon of the Gunnison and 

Gunnison Gorge National Conservation Area for many years, 

offered insights into the recent alluvial history of the canyon. 

Robert Scott, Bruce Bryant, Ralph Shroba, William Johnson, and Craig Brunstein (all of the U.S. Geological Survey) 

made numerous suggestions to improve the geological story. I 

thank Carol Quesenberry for her original artwork and for the 

wonderful design and layout of this book. Pioneering work 

by Wallace Hansen (U.S. Geological Survey, retired) on the 

geology of the region provided the foundation for this guide. 

In addition, several conversations with Hansen clarified some 

puzzling aspects of the geology.

Acknowledgments

Acknowledgments 33
34 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado
Elliott, J.S., and Parker, R.S., 1997, Altered streamflow 

and sediment entrainment in the Gunnison Gorge: Water 

Resources Bulletin of the American Water Resources Association, v. 33, no. 5, p. 1041–1054.

Ellis, M.S., Gaskill, D.L., and Dunrud, C.R., 1987, Geologic 

map of the Paonia and Gunnison area, Delta and Gunnison 

Counties, Colorado: U.S. Geological Survey Coal Investigations Map C–109, scale 1:100,000.

Hansen, W.R., 1968, Geologic map of the Black Ridge 

quadrangle, Delta and Montrose Counties, Colorado: U.S. 

Geological Survey Geologic Quadrangle Map GQ–747, 

scale 1:24,000.

Hansen, W.R., 1971, Geologic map of the Black Canyon of the 

Gunnison River and vicinity, western Colorado: U.S. Geological Survey Miscellaneous Investigations Map I–584, 

scale 1:31,680.

Hansen, W.R., 1987, The Black Canyon of the Gunnison—in 

depth: Tucson, Arizona, Southwest Parks and Monuments 

Association, 58 p.

Jessup, Mikah, Karlstrom, Karl, Connelly, James, and Willaims, M.L., 2002, Tectonic evolution of the Black Canyon 

of the Gunnison, Colorado: new U/Pb geochronology and 

evidence for both Paleoproterozoic and Mesoproterozoic 

deformation and metamorphism: Geological Society of 

America Abstracts with Programs, v. 34, no. 6, p. 181.

Kellogg, K.S., Hansen, W.R., Tucker, Karen, and VanSistine, 

D.P., 2004, Geologic map of the Gunnison Gorge National 

Conservation Area, Delta and Montrose Counties, Colorado: U.S. Geological Survey Scientific Investigations Map 

I–2825, scale 1:45,000.

Lohman, S.W., 1961, Abandonment of Unaweep Canyon, 

Mesa County, Colorado, by capture of the Colorado and 

Gunnison Rivers: U.S. Geological Survey Professional 

Paper 424–B, p. B144–B146.

National Geographic Society, undated, The Black Canyon of 

the Gunnison National Park, Colorado, USA: Trails Illustrated Map 245, scale 1:28,000.

References Cited and 

Suggested Reading

References Cited and Suggested Reading 35

Edited by F. Craig Brunstein, Central Publications Group

Publication design and digital layout by Carol Quesenberry, Central 

Publications Group

Original drawings by Carol Quesenberry, except for Rocky Mountain 

Juniper branch, which is from G.B. Sudworth’s Forest Trees of 

the Pacific Slope (1908), and drawings in the Geologic Time Chart, 

which are by various USGS graphic artists

Except as noted, photographs by Karl Kellogg (U.S. Geological 

Survey) and personnel of the U.S. Bureau of Land Management; 

cactus photograph by Craig Brunstein

(Facing page) A Water Ouzel (American Dipper) nest perched on 

a ledge on a vertical cliff of Pitts Meadow Granodiorite.

(Facing page insert) Water Ouzel (American Dipper). 

Courtesy of the National Park Service and U.S. Fish and 

Wildlife Service.

O’Sullivan, R.B., 1992, The Jurassic Wanakah and Morrison 

Formations in the Telluride-Ouray-western Black Canyon 

area of southwestern Colorado: U.S. Geological Survey 

Bulletin 1927, 24 p. 

Scott, R.B., Steven, T.A., Betton, C.W., Cole, R.D., Aslan, 

Andres, and Hood, W.C., 2002, Evidence for late Cenozoic 

uplift of the Uncompahgre Plateau, northeastern Colorado 

Plateau: Geological Society of America Abstracts with Programs, v. 34, no. 6, p. 323.
A

Alkali feldspar See feldspar.

Alluvium Material deposited by a stream or 

running water. Alluvium is generally stratified 

and composed of grains ranging in size from 

clay to boulders.

Amphibolite Black or dark-gray metamorphic rock composed almost entirely of hornblende, lesser amounts of plagioclase, and 

little or no quartz. 

Anticline Generally a convex-upward 

(“arch-shaped”) fold in layered rocks; opposite of a syncline.

Aplite Fine- to medium-grained intrusive 

igneous rock composed almost entirely of 

equal-size crystals of feldspar and quartz; 

commonly forms dikes. 

Ash As applied to volcanic rocks, refers to 

the fine, airborne particles from a volcanic 

eruption. If hot enough, may weld into a tuff.

B

Base level The level below which a stream 

cannot erode its bed; for example, the base 

level of the Mississippi River is mean sea level.

Biotite A black mica; an iron-potassiumaluminum silicate mineral forming bendable flakes.

Breccia Sedimentary or volcanic rock composed of angular fragments. In sedimentary 

breccia, the angularity of the fragments indicates that they have not traveled far from their 

source. Also, a rock composed of cemented, 

angular fragments that originated by movement along a fault. 

C

Caldera More or less circular depression, 

commonly larger than 10 km in diameter, 

formed by the collapse of an area following 

an enormous volcanic eruption that evacuates 

an underlying magma chamber. The caldera 

is subsequently filled by a great thickness of 

ash-flow tuff from the eruption.

Chert An extremely hard rock containing 

microcrystalline, interlocking quartz crystals 

or amorphous silica (SiO2). Chert generally 

has a waxy texture, glass-like fracture, comes 

in a variety of colors (due to impurities), and 

often forms nodules in limestone and limy 

sandstone. Bedded chert may also result from 

deposition of siliceous skeletons of one-celled 

organisms called radiolaria.

Cross bed A sandstone bed that formed on 

the downstream or downwind side of a moving dune-shaped mass of sand; it is inclined to 

the plane of the overall bedding.

D

Dike A tabular igneous intrusion that cuts 

across the bedding or foliation of the enclosing rock.

E

Eolian Pertaining to the deposition of material by wind, such as in sand dunes.

F

Feldspar Along with quartz, feldspar minerals are among the most common rock-forming 

minerals. The two primary types of feldspar 

are plagioclase, a sodium-calcium aluminum 

silicate, and alkali feldspar, a potassium aluminum silicate.

Foliation A planar fabric in metamorphic 

rock formed by the alignment of mineral grains. Causes a banded or “streaky” 

appearance. 

36 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado

Glossary of Selected Terms
G

Gneiss A foliated metamorphic rock commonly composed mostly of feldspar, quartz, 

and lesser amounts of mica.

Granite A coarse-grained, intrusive igneous 

rock containing approximately equal amounts 

of plagioclase and alkali feldspar and greater 

than 20 percent quartz. Also contains minor 

amounts of other minerals such as biotite.

Granodiorite Similar to granite, but the 

percentage of plagioclase to total feldspar 

(alkali feldspar plus plagioclase) is greater 

than 65 and less than 90; also generally contains a higher percentage of black minerals 

(mostly biotite and amphibole), so is darker 

than most granites.

H

Hornblende A black, prismatic silicate mineral containing calcium, magnesium, iron, and 

aluminum; hornblende is the primary mineral 

in amphibolite and is common in many dark 

igneous and metamorphic rocks. 

I

Igneous rock A rock that solidified from 

molten or partially molten material. Granite, 

granodiorite, pegmatite, ash-flow tuff, and 

lava are examples.

L

Limestone Sedimentary rock composed 

chiefly of calcium carbonate.

Lithosphere The upper, approximately 100-

km-thick, relatively strong shell of the Earth. 

Includes the crust and upper part of the mantle 

and forms the various tectonic plates that 

move relative to each other (the crust, mantle, 

and core are the three layers that make up the 

Earth); also see plate tectonics.

M

Magma Hot, molten material generated 

within the Earth that can intrude other rocks 

beneath the Earth’s surface or erupt at the surface. May contain suspended rock fragments 

or crystals.

Metamorphic rock Rock derived by mineralogical and (or) chemical changes in pre-existing rock due to heat, pressure, and (or) shearing. These changes generally occur either 

adjacent to igneous intrusions or at depth (at 

least 10 km) in the Earth’s crust.

Mica Common rock-forming mineral characterized by thin, flexible flakes and glassy 

(vitreous) surfaces; see biotite and muscovite. 

Migmatite Layered crystalline rock composed of well-foliated gneiss containing 

igneous (or igneous appearing) layers parallel 

to foliation that were either injected from a 

nearby magma source or formed by partial 

melting of adjacent rock. Migmatite layers are 

commonly very contorted. 

Monocline A fold in which otherwise uniformly and gently dipping (tilted) sedimentary 

beds are locally steepened. Vertical movement 

on a buried fault in the underlying basement 

rock commonly causes such steepening.

Muscovite A mica; a potassium-aluminum 

silicate mineral consisting of clear, bendable flakes. 

O

Oolith A small (commonly less than 2 mm), 

round body, resembling fish roe, in a sedimentary rock, generally composed of calcium 

carbonate. Formed by back and forth rolling 

and gradual build-up of concentric layers 

around a nucleus such as a sand grain. Forms 

oolitic limestone or sandstone and is found 

locally in the Wanakah Formation.

Orogeny The process of mountain building, 

generally associated with folding, faulting, 

and the intrusion of magma.

Glossary of Selected Terms 37
P

Pegmatite A very coarse grained igneous 

rock composed mostly of feldspar, quartz, 

and subordinate mica; the rock is close to the 

composition of granite. Typically forms dikes.

Peneplain A low, nearly featureless, flat land 

surface produced by erosion nearly to base level.

Plagioclase See feldspar.

Plate tectonics A theory, generally accepted, 

whereby the lithosphere of the Earth is 

divided into a number of plates that move 

relative to one another and interact at their 

boundaries in various ways to produce earthquakes, mountains, ocean trenches, ocean 

ridges, and great faults (such as the famous 

San Andreas Fault in California). 

Q

Quartz Next to feldspar, the most common 

mineral in the Earth’s crust. Generally a clear 

or white (impurities may create almost any 

color), very hard mineral composed of silicon 

dioxide (SiO2).

Quartzite A metamorphic rock composed 

chiefly of quartz and formed by recrystallization of quartz sandstone or chert.

R

Radiometric method The measurement of the 

relative amounts of an unstable “parent” isotope of an element and a “daughter” isotope 

that forms by the radioactive decay of the parent. Once the rate of decay is determined, the 

age of the mineral or rock sample in which 

the measurement was taken can then be calculated. For example, the uranium-lead method 

generally uses tiny zircon (zirconium silicate) 

grains extracted from a rock sample. Zircon 

contains relatively high amounts of uranium 

(“parent”) in its crystal lattice, and the amount 

of lead (“daughter”) derived from the decay 

of uranium is measured on an instrument 

called a mass spectrometer.

Reverse fault A high-angle (more than 45°) 

fault on which the upper block (the body of 

rock above the fault) has moved upward relative to the lower block.

S

Sandstone A sedimentary rock composed 

mostly of angular to rounded, cemented 

grains of quartz, feldspar, other minerals, 

and rock fragments; may contain a matrix 

of silt or clay. 

Schist A strongly foliated metamorphic 

rock (can be split into slabs parallel to the 

aligned minerals in the rock) that generally 

contains abundant mica. Schist forms from 

the metamorphism of shaly (mud-rich) 

sedimentary rocks. 

Sedimentary rock A rock formed at the 

Earth’s surface. Sedimentary rocks are formed 

from consolidation and cementation of loose 

sediment (examples are shale and sandstone), 

from chemical precipitation from solution 

(examples are rock salt and gypsum), and 

cementation of organic remains or secretions 

(examples are coal and most limestone).

Shale A finely laminated sedimentary rock 

formed by the consolidation of mud (clay and 

(or) silt).

Silicate mineral A mineral whose crystal 

structure is composed of molecules composed 

of one silicon atom and four oxygen atoms 

(SiO4 tetrahedra) grouped either individually (as in quartz), or in various geometrical 

arrangements with metallic atoms. Most minerals in the Earth are silicate minerals.

Surficial deposit Generally uncemented 

sediment or rock fragments overlying bedrock 

derived by generally recent geological processes; includes alluvium, landslide deposits, 

talus, and glacial deposits.

Syncline Generally a concave-upward 

(“trough-shaped”) fold in layered rocks; 

opposite of an anticline.

38 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado
T

Talus Angular rock fragments that accumulate due to rockfall, commonly beneath a cliff.

Terrace A bench-shaped surface along a 

valley margin representing a period when the 

stream was at the level of the bench, before 

it cut to its present level. Higher terraces are 

therefore older than lower terraces. Terraces 

are generally covered with terrace gravel 

deposited by the earlier stream. 

Thrust fault A low-angle (less than 45o) fault 

on which the upper block (the body of rock 

above the fault) has moved upward relative to 

the lower block.

Tuff Welded ash from a volcanic eruption. 

Ash may settle from a hot, eruptive cloud. If 

hot enough, the ash may fuse into a hard rock 

called ash-flow tuff or welded tuff. 

Glossary of Selected Terms 39

U

Unconformity A generally significant break 

or gap in the geologic record wherein one 

rock in a stratigraphic sequence is overlain 

by a significantly younger rock. Implies 

a period of time during which erosion 

occurred, with loss of a previously formed 

geologic record, or a period during which 

no deposition occurred. 

View downstream (north) from near the Ute Trail. The base of the Entrada Sandstone is on basement rocks 

(mostly amphibolite and migmatite here), and is visible at the upper left and right sides of the photograph.
40 The Geologic Story of Gunnison Gorge National Conservation Area, Colorado

View downstream (north) from just above the confluence of 

Smith Fork with the Gunnsion River. Basement rocks here 

are mostly migmatite.