Mountains, Glaciers, and Mines

U.S. Department of the Interior

U.S. Geological Survey

The Geological Story of the Blue River Valley, Colorado, 

and Its Surrounding Mountains

Circular 1400

Mountains, Glaciers, and Mines

The Geological Story of the Blue River Valley, Colorado, 

and Its Surrounding Mountains

By Karl S. Kellogg, Bruce Bryant, and Ralph R. Shroba 

Circular 1400

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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reproduce any copyrighted material contained within this report.

Suggested citation:

Kellogg, K.S., Bryant, Bruce, and Shroba, R.R., 2016, Mountains, glaciers, and mines—The geological story of the 

Blue River valley, Colorado, and its surrounding mountains: U.S. Geological Survey Circular 1400, 46 p., 

http://dx.doi.org/10.3133/cir1400.

Library of Congress Cataloging-in-Publication Data

Kellogg, Karl S.

 Mountains, glaciers, and mines : the geological story of the Blue River Valley, colorado, and its surrounding 

mountains / by Karl S. Kellogg, Bruce Bryant, and Ralph R. Shroba.

 pages cm. -- (Circular ; 1400)

 Includes bibliographical references.

 ISBN 978-1-4113-3966-8 (pbk.)

 1. Geology--Colorado--Blue River Valley. 2. Mountains--Colorado. I. Bryant, Bruce, 1930- II. Shroba, R. R. III. Title.

 QE92.B58K45 2016

 557.88’65--dc23

 2015034498

ISSN 1067-084X (print)

ISSN 2330-5703 (online)

ISBN 978-1-4113-3966-8
Overview of This Report

This report describes, in a nontechnical style, the geologic history and mining 

activity in the Blue River region of Colorado, which includes all of Summit County. 

The geologic story begins with the formation of ancient basement rocks, as old as 

about 1700 million years, and continues with the deposition of sedimentary rocks 

on a vast erosional surface beginning in the Cambrian Period (about 530 million 

years ago). This deposition was interrupted by uplift of the Ancestral Rocky Mountains 

during the late Paleozoic Era (about 300 million years ago). The present Rocky 

Mountains began to rise at the close of the Mesozoic Era (about 65 million years 

ago). A few tens of millions years ago, rifting began to form the Blue River valley; 

a major fault along the east side of the Gore Range dropped the east side down, 

forming the present valley. The valley once was filled by sediments and volcanic 

rocks that are now largely eroded. During the last few hundred-thousand years, 

at least two periods of glaciation sculpted the mountains bordering the valley 

and glaciers extended down the Blue River valley as far south as present Dillon 

Reservoir. Discovery of deposits of gold, silver, copper, and zinc in the late 1800s, 

particularly in the Breckenridge region, brought an influx of early settlers. The 

world-class molybdenum deposit at Climax, mined since the First World War, 

reopened in 2012 after a period of closure. 

The report includes a glossary to explain geologic terms used in the text, and 

numerous photos, maps, and diagrams illustrate the geologic principles discussed. 

References for further reading are also included. 
iv 

Contents 

Overview of This Report...............................................................................................................................iii

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

Geological Story of the Blue River Region .......................................................................................4

Laying the Foundation—Proterozoic Basement Rocks .................................................................5

Ancient Seas and Long-Vanished Mountains................................................................................11

The Rise of the Rocky Mountains .............................................................................................................14

Igneous Activity and the Laramide Orogeny..................................................................................17

The Gore Fault Zone and Faults Cutting Proterozoic Rocks of the Gore Range.......................18

The Continent Pulled Apart—Development of the Northern Rio Grande Rift ...................................19

The Upper Oligocene to Upper Miocene Troublesome Formation—

Was It All That Troubling?....................................................................................................23

What Happened to All the Sediment That Once Filled the Blue River Valley? .........................23

The Ice Ages.................................................................................................................................................24

Landslides and Spreading Mountains......................................................................................................29

Landslides ............................................................................................................................................29

What Are Sackungen and How Do They Form? ............................................................................29

Rich Ores of the Blue River Valley Region—Their Geology and Mining History ..............................31

Hard-Rock Mining in the Blue River Valley ....................................................................................32

Climax Molybdenum Deposit ............................................................................................................34

Acknowledgments.......................................................................................................................................35

Suggested Reading......................................................................................................................................36

Glossary.........................................................................................................................................................39

Information Boxes

1. Dating Rocks by Using Zircons...................................................................................................6

2. Dating Igneous Rocks by Using Radioactive Decay of Potassium to Argon....................20

3. Earthquakes and Age of Recent Faulting ...............................................................................22

4. What Apatite Fission Tracks Tell Us about Uplift and Cooling History...............................23

5. Railroads and Mining .................................................................................................................34
 v

Figures

1. Photograph of cirque and tarn at the head of Boulder Creek, Gore Range.......................vi

2. Geographic map, Blue River valley region ...............................................................................2

3. Photograph across Blue River valley ........................................................................................3

4. Diagram showing divisions of geologic time ...........................................................................4

5. Photograph of migmatitic biotite gneiss, Williams Fork Mountains.....................................5

6. Photograph of amphibolite intruded by thin sills of granite and pegmatite dikes,

Tenmile Canyon.............................................................................................................................7

7. Simplified geologic map of the Blue River valley region........................................................8

8. Stratigraphic column of sedimentary and igneous rocks, Blue River valley region .......10

9. Diagram of late Paleozoic Ancestral Rocky Mountain uplifts and adjacent basins 

in Colorado...................................................................................................................................11

10. Schematic cross sections through the Blue River valley region .......................................12

11. Photograph of Dakota Sandstone in road cut, Interstate Highway 70,

 two miles west of Williams Range thrust ..............................................................................13

12. Photograph of black shale, lower part of Pierre Shale, 

below the Williams Range thrust.............................................................................................13

13. Photograph of Williams Range thrust fault near Keystone .................................................15

14. Photograph of late Oligocene dike of hornblende latite porphyry, Haystack Butte........16

15. Photograph of Minturn Formation and Paleoproterozoic granite juxtaposed by

Gore fault, Booth Creek .............................................................................................................18

16. Diagrammatic map of Rio Grande rift......................................................................................19

17. Photograph of moderately welded upper Oligocene tuff, South Brush Creek.................20

18. Photograph of early morning view of the Gore Range from Ute Pass...............................21

19. Schematic east-west cross section showing proposed relationship of

down-to-the-east normal faults, Blue River valley ...............................................................22

20. Map showing extent of Pleistocene glaciers, Blue River valley region............................25

21. Photograph of unconsolidated till of Bull Lake glaciation, Dillon Reservoir.....................26

22. Photograph of unconsolidated till of Pinedale glaciation, Dillon Reservoir .....................26

23. Photograph of active rock glacier, South Rock Creek, Gore Range ..................................27

24. Photograph of gravel, Mesa Cortina ......................................................................................28

25. Photograph of sackungen near crest of southern Williams Fork Mountains...................30

26. Schematic cross section showing development of sackungen, 

Williams Fork Mountains...........................................................................................................30

27. Schematic map of Colorado Mineral Belt ..............................................................................31

28. Photograph of placer mining on Swan River .........................................................................32

29. Photograph of Wellington mine on French Gulch .................................................................33
Figure 1. View to the northwest of a cirque and glacial lake (tarn) at the head of Boulder Creek in the Gore Range. The bedrock in this 

area is 1700-million-year-old Cross Creek Granite.
Introduction

Perhaps no other place in Colorado contains such a variety 

of rocks and geological features as the Blue River region, and 

this guide offers a wonderful opportunity to explore the geology 

of this beautiful mountain area. Some of the oldest rocks in the 

central Rocky Mountains make up the high peaks of the Gore 

Range (fig. 1), Tenmile Range, and western Front Range, and 

a variety of younger rocks and geological deposits record a 

geological story that is beautifully displayed at many places in 

the Blue River region. This guide will help you turn the pages 

of this story. 

An early influx of settlers in the latter half of the 

19th century is closely tied to the geology of the region. 

Placer and hard-rock mining was the first major economic 

draw in several areas, such as around the towns of Breckenridge, 

Montezuma, and Kokomo (the latter town is now buried under 

the mine tailings of the Climax molybdenum mine). More 

recently, a multibillion-dollar ski industry, as well as many 

other outdoor activities, attracted a new wave of settlers who 

fueled rapid population growth. Several world-class ski resorts 

(such as Keystone, Copper Mountain, Breckenridge, and 

Arapahoe Basin) are situated in the Blue River drainage basin. 

Dillon Reservoir and Green Mountain Reservoir, behind dams 

constructed on the Blue River in the early 1960s, provide 

boating and fishing in the summer. The old Dillon town site 

lies under the water of Dillon Reservoir; the town was moved 

to its present location during dam construction (fig. 2).

From its headwaters near Hoosier Pass, the Blue River 

flows approximately 50 miles north through or near the towns 

of Breckenridge, Frisco, and Silverthorne to join the Colorado 

River at the town of Kremmling (fig. 2). Tenmile Creek, with 

headwaters near Fremont Pass, is a major tributary of the 

Blue River. The Blue River valley is flanked on the west by 

the rugged Gore Range and on the east by the Williams Fork 

Mountains, which is actually the large western ridge of the 

more extensive Colorado Front Range; the impressive Tenmile 

Range lies between Tenmile Creek and the Blue River. 

Summit County, whose county seat is Breckenridge, lies 

entirely within the drainage of the Blue River, and there is a 

good reason that this county has such a lofty name. The lowest 

point in the county, along the lower Blue River, is more than 

7,500 feet above sea level, and a large expanse of the county 

lies in the alpine zone above tree line (approximately 11,500 feet 

elevation). Two of Colorado’s “fourteeners” (peaks more than 

14,000 feet high) lie along the Continental Divide on the Summit 

County–Park County line (fig. 2): Torreys Peak (14,267 feet) 

and Grays Peak (14,278 feet). These are the only two fourteeners, 

out of about 53 total in Colorado, that lie along the Continental 

Divide. In fact, Grays Peak is the highest point on the Continental 

Divide in North America. From many places in the Blue River 

valley, the distinctive double summits of Grays and Torreys Peaks 

are visible. The county contains one other fourteener, Quandary 

Peak (14,265 feet), in the Tenmile Range. Many old mining 

roads cross the high country and are open to four-wheel-drive 

vehicles in the summer and make good routes for ski touring 

in the winter. A gravel road (not plowed in winter) follows 

an abandoned railroad grade over the Continental Divide at 

Boreas Pass. 

The Gore Range forms one of Colorado’s most impressive 

and rugged alpine environments, with steep, rocky peaks, 

glacially carved cirques, and numerous high mountain lakes 

(fig. 1). Mt. Powell (13,448 feet), the highest peak in the 

range, gives way to the north to a gentler and lower meadowy 

ridge (Elliott Ridge). The high Gore Range is part of the 

Eagles Nest Wilderness, and most of the high country is 

accessible by hiking trails. The jagged Gore Range contrasts 

sharply with the gentle, rolling alpine meadows of the 

Williams Fork Mountains, despite similar rocks underlying 

both ranges. As we will see, there is a good geological 

explanation for this puzzling contrast in topography. 

Mountains, Glaciers, and Mines

The Geological Story of the Blue River Valley, Colorado, 

and Its Surrounding Mountains

By Karl S. Kellogg, Bruce Bryant, and Ralph R. Shroba
 Creek

91

9

70

70

40

40

6

Frisco

Dillon

Vail

Breckenridge

Silverthorne

Kremmling

Hot Sulphur

Springs

Montezuma

Copper

Mountain

Heeney

Parshall

40°00’

106°30’

FRONT RANGE

GORE RANGE

Williams 

 Fork

Blue River

Blue River

Blue River

Creek

Colorado

River

Swan River

Tenmile

Creek

Gore

Boreas

Pass

Continental

Continental

Divide

Divide

Snake River

 GREEN

 MOUNTAIN

 RESERVOIR

WILLIAMS

FORK

RESERVOIR

 DILLON

 RESERVOIR

GRAND

Booth

Creek

CLIMAX

MINE

106°00’ 

Loveland

Pass

French Gulch

GIBSON HILL

WELLINGTON

 MINE

BOSS 

MINE

SAINTS JOHN

MINE

Straight Creek

Slate Creek

Creek

Creek

Black

Boulder Creek

Piney River

Creek

North Willow Creek

N Rock

Hoosier

Pass

Sheephorn

Creek

Muddy

Creek

WOLFORD MTN

RESERVOIR

Troublesome Creek

Cataract

Creek

Georgia

Pass

Corral

Creek

Beaver

Creek

EAGLE

HAYSTACK

BUTTE

S Brush 

WILLIAMS FORK MOUNTAINS

MESA

CORTINA

GOLD RUN

ELLIOTT RIDGE

Otter

Acorn Creek

Little

Muddy

Creek

Piney

Ridge

GREEN 

MTN

Fremont

Pass

EisenhowerJohnson

Tunnels

TENMILE 

RANGE

SAWATCH RANGE

TORREYS

PEAK

GRAYS

PEAK

Beaver Creek

Spruce

Creek

 MT

POWELL

QUANDARY

PEAK

 

39°30’

SUMMIT

PARK

River

Topogra p hic divide

GRAND

SUMMIT

GRAND

PARK

SUMMIT

EAGLE

SUMMIT

SUMMIT

) (

) (

) (

) (

) (

Brush

Creek

S Rock Creek

Denver

COLORADO

Map area

Lake, reservoir

Topographic divide

County line

Continental Divide

Road

0 5 10 MILES

0 5 10 KILOMETERS

Mine or mined area

EXPLANATION

2 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Figure 2. Geography of the Summit County region. In the southeast portion of the map, county boundaries follow 

the Continental Divide.

Base modified from U.S. Geological Survey

1:100,000-scale digital data, 2011
BLUE RIVER NORMAL FAULT

WILLIAMS RANGE THRUST FAULT

Early Proterozoic basement rock 

Mesozoic sedimentary rock and Quaternary surficial deposits

Early Proterozoic basement rock 

Early Proterozoic basement rock 

Figure 3. View to the north of the Blue River valley, taken from the north end of the Tenmile Range. The Williams Range thrust (mostly 

covered by landslide debris) on the west flank of the Williams Fork Range (western part of the more extensive Front Range) is shown, as 

is the Blue River normal fault at the eastern edge of the Gore Range. Interstate 70 is visible in the right part of the photo. The edge of the 

town of Frisco is seen in front of an arm of Dillon Reservoir.

Introduction 3

This booklet provides an overview of the geology of the 

Summit County region, which includes the entire Blue River 

drainage basin (fig. 2). Many studies have contributed to our 

understanding of this geologic story. In particular, the reader is 

referred to Kellogg and others (2008, 2011) and Ruleman and 

others (2011) for a more thorough discussion of the geology, 

supported by detailed geological maps and cross sections. 

The geologic map and cross sections used in this report were 

simplified from these sources. Because understanding the 

geological story requires a familiarity with some geologic 

terms and concepts, a glossary at the end of this booklet 

defines geological terms. References to more detailed 

descriptions of the geology of the region are listed at the 

end of this booklet.
Eon Era Period Epoch

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Late Cretaceous

Early Cretaceous Phanerozoic

Proterozoic

Cenozoic

Mesozoic

Paleozoic

 Neoproterozoic

 Mesoproterozoic

 Paleoproterozoic 

Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

Age

(million years ago) 

 0.012

2.6

5.3

23.0

33.9

55.8

65.5

99.6

145.5

Archean

Hadean

 200

 251

299

318

359

416

444

488

542

1000

1600

2500

4000

4600

4 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Geological Story of the Blue River Region

The geological story of the Blue River region stretches for 1780 million years, almost 40 percent of the age of the Earth. 

This lengthy history is divided into six major chapters: 

1. Proterozoic history, mostly represented by 

an early Proterozoic (Paleoproterozoic ) 

period of thick sedimentation and widespread 

volcanism, followed by voluminous intrusions 

of mostly granitic magma, and recrystallization 

(during metamorphism) and deformation of 

the deeply buried sedimentary and igneous 

rocks. Later in the Proterozoic Era (during 

the Neoproterozoic ), mostly granitic 

magma locally intruded the earlier igneous 

and metamorphic rocks.

2. Following a poorly understood late 

Proterozoic (Neoproterozoic ) period of 

uplift and deep erosion of the Proterozoic 

metamorphic and intrusive rocks to a vast 

surface of low relief, a long period of 

sedimentation throughout the entire region 

transpired from Late Cambrian to Late 

Cretaceous time (fig. 4). Much of this 

sedimentation was on the floors of ancient 

seas, although this part of the story was 

interrupted by a late Paleozoic period 

of mountain building, producing the longsince-eroded “Ancestral Rocky Mountains.” 

3. Late Cretaceous to early Tertiary mountain 

building, called the “Laramide orogeny,” and 

thick accumulation of debris shed off the rising 

mountains, during a period of crustal contraction 

(shortening). It was during the Laramide orogeny 

that the early roots of the present Rocky 

Mountains were formed, although they were 

subsequently much modified.

4. Middle to late Tertiary basin development 

(including development of the rift underlying 

the Blue River valley) during pulling apart of 

the Earth’s crust, followed by renewed uplift 

of the southern Rocky Mountain region.

5. The ice ages, times of global cooling, extensive 

glaciation, and deep erosion. Included are 

other factors that changed the landscape during 

the Quaternary Period , such as widespread 

landsliding and gravitational spreading of 

mountains. 

6. Lastly, a section is devoted to the fascinating 

mining history of the region and the geology 

of the ores that fed the mines. 

Figure 4. Divisions of geologic time. Ages from U.S. Geological Survey Fact Sheet 

2010–3059 (U.S. Geological Survey Geologic Names Committee, 2010).
Introduction 5

During part of the Paleoproterozoic Era (between about 

1780 and 1740 million years ago), thick marine sediments 

and volcanic rocks of various composition accumulated in 

deep basins in the northern Colorado region. Keep in mind 

that the Earth is about 4600 million years old, so these rocks 

were deposited when the Earth was 60 percent as old as it is 

now. We know almost nothing about the history of the area 

before this time because no older rocks are preserved. During 

an episode of rock deformation (that is to say, an orogeny) 

that lasted about 110 million years, these layered rocks were 

pervasively folded, stretched, and intruded by large molten 

bodies of magma, most of which solidified into granite. These 

processes occurred many miles deep in the Earth’s crust, where 

elevated heat and pressure cause most minerals to form into 

larger grains, or even to react chemically to form new minerals. 

At these extreme conditions, rocks deform in a ductile manner, 

much like taffy. The combination of recrystallization and 

intense deformation produced metamorphic rocks and 

destroyed most evidence of the types of rocks from which 

they were derived. 

Figure 5 (below). Migmatitic biotite gneiss on east side of crest 

of Williams Fork Mountains. The light-colored, pink igneous phase 

grades into coarse-grained pegmatite clots in the upper part of 

the image. This pink phase is interpreted to have formed by partial 

melting of the darker biotite gneiss. Pencil is about 7 inches long 

for scale. 

The metamorphic rocks include quartz-feldspar gneiss 

(possibly metamorphosed volcanic tuff), biotite gneiss and 

schist (metamorphosed sandstone and shale), amphibolite and 

hornblende-plagioclase gneiss (probably metamorphosed basalt 

and andesite, and sediments derived from these volcanic rocks), 

minor quartzite, and “calc-silicate gneiss” (metamorphosed 

limey or calcareous sediments, including limestone). In most 

places, the metasedimentary and igneous rocks are now 

complexly interlayered, mixed, and deformed, as is shown in 

figure 5. 

Laying the Foundation—Proterozoic Basement Rocks
6 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

 Many of the more quartz- and feldspar-rich metasedimentary 

rocks, such as biotite gneiss, partially melted and then formed 

a rock called migmatite (fig. 5), which in most cases displays 

alternating light and dark layers. The light layers are a melt 

phase, produced from partial melting of the darker layers (the 

remaining original rock) and have a composition resembling 

granite. In some migmatites, the light-colored melt phase that 

was injected between the layers of metamorphic rocks came 

from a more distant source (fig. 6). The degree of migmatite 

formation decreases toward the northwest, such that migmatites 

are rare among the metamorphic rocks north of Green Mountain 

Reservoir (fig. 7). Zircon dates from several localities in the 

region indicate that metamorphism peaked and migmatites 

formed about 1697 million years ago (refer to the Denver 

West map by Kellogg and others, 2008, for a more detailed 

discussion of migmatites and their age).

Contemporaneous with or closely following the 

approximately 1700-million-year-old metamorphic and 

deformational event, extensive batholiths and smaller bodies 

of mostly granodiorite and granite (called stocks) intruded 

the older metamorphic rocks. For example, the approximately 

1700 million-year-old Paleoproterozoic Cross Creek Granite 

(figs. 7 and 8) underlies large areas of the Gore Range, and 

other stocks and batholiths ranging in composition from 

gabbro to granite intruded between 1725 and 1710 million 

years ago over a widespread region in Colorado, including 

the Front Range and Sawatch Range. 

The Paleoproterozoic basement of the Front Range region 

was extensively modified during a second period of deformation,

heating, and intrusion of mostly granitic rocks about 1400 million 

years ago. Plutons that intruded during this orogeny are 

widespread in the Front Range region; several are mapped in 

the southern Tenmile Range near the Climax mine and near 

the Eisenhower-Johnson Tunnels on Interstate 70.

Almost all basement rocks are cut by irregular-shaped 

dikes of light-colored, very coarse grained intrusive rocks 

called pegmatites. These dikes, composed almost entirely of 

feldspar, quartz, and minor amounts of biotite (black mica) 

and in some places muscovite (clear mica), solidified from 

granitic melts where water and other volatile phases concentrated 

the last liquid left during cooling and solidification. Small 

pegmatite bodies and dikes are shown in figures 5 and 6. 

The Proterozoic rocks of the Front Range and Gore Range 

are transected by northeast- to east-trending, discontinuous zones

of intense deformation that formed during both the approximately 

1700-million-year-old and 1400-million-year-old episodes of 

deformation and granite intrusion. These zones, called shear 

zones, are analogous to faults because differential movement 

of rocks takes place across them. However, along a fault, 

movement takes place along a sharp break; in a shear zone, 

movement between rocks on either side is distributed over 

a wide zone. Shear zones generally form at depth, where 

temperature and pressure are high enough that rocks are no 

longer brittle but act in a ductile fashion, like soft putty. The 

mineral grains in these zones were typically reduced in size 

and sheared into parallel layers. In extreme cases of shearing 

and grain-size reduction, a hard, very fine grained rock called 

mylonite resulted. The Tupps Creek shear zone in the northern 

Gore Range (fig. 7) is a particularly well exposed shear zone. 

Information Box 1.

Dating Rocks by Using Zircons

Tiny zircon grains are found in very small quantities in virtually all rocks of granitic composition, including metamorphic rocks. 

These zircons can be used to date the time that the igneous and metamorphic rocks formed, using the radioactive decay of 

isotopes of uranium to isotopes of lead. This decay occurs at a precisely known rate, so that if we know the ratios of the various 

isotopes of lead and uranium in the zircons (measured on an instrument called a mass spectrometer), we can calculate the age 

of the rock. Zircons in igneous rocks record the time at which the rock cooled from a magma, when the zircons first crystallized. 

However, the zircon grains in most metamorphosed sedimentary rocks were washed in from some distant source, so their age 

(1785–1740 million years old in the Blue River region) is the age of the rock that was eroded and provided the zircon-bearing 

sediment. This age, therefore, represents only the maximum age of deposition of the sediment (later to become a metamorphic 

rock) in which the zircons are found. Zircons are tough little minerals, so metamorphism does not change them very much. 
Introduction 7

Figure 6. Amphibolite intruded by numerous thin sills of granite, in turn cut by small pegmatite dikes. Nearby foliated granite, correlated 

with the granite sills, was dated at about 1755 million years old (Kellogg and others, 2011). Outcrop in Tenmile Canyon (Officers Gulch). 

Knife is about 3 inches long.
0 5 10 MILES

0 5 10 KILOMETERS

Denver

COLORADO

Map area

Springs

 

106°30’

GORE 

A

B

B’

106°00’

 

A’

40°00’

39°30’

Topograp hic divide

Frisco

Dillon

Vail

Breckenridge

Silverthorne

Kremmling

Hot Sulphur

MOSQUITO

GORE FAULT

BLUE RIVER 

NORMAL 

FAULT

FAULT

FRONT

WILLIAMS

RANGE

THRUST

Williams 

Fork

Blue River

Blue River

Blue River

Creek

Colorado River

Swan Tenmile

Creek

River

ZONE

Gore

TUPPS

CREEK

SHEAR

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Continental

Continental

Divide

Divide

Snake

River

GREEN

 MOUNTAIN

 RESERVOIR

WILLIAMS

FORK

RESERVOIR

 DILLON

 RESERVOIR

MONTEZUMA

STOCK

Blue Ridge

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BROSS

Booth

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Piney

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8 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Base modified from U.S. Geological Survey

1:100,000-scale digital data, 2011
Middle Park Formation (Paleocene

 and Upper? Cretaceous)

Mesozoic sedimentary rocks

 Permian and Pennsylvanian

 sedimentary rocks

Mississippian to Cambrian

 sedimentary rocks

Mesoproterozoic granite

Paleoproterozoic plutonic and

 metamorphic rocks

Surficial deposits (Quaternary) 

 extensive deposits only

Old landslide deposits (Quaternary

 and Pliocene) west side

 Williams Fork Mountains

Basin-fill deposits (Miocene and 

 Oligocene) including

 Troublesome Formation

Mafic volcanic rocks (early

 Miocene and Oligocene)

Intrusive rocks (early Oligocene

 to Late Cretaceous)

EXPLANATION

 

 

 

 

 

 

 

Contact

Fault—Dotted

 where concealed

Normal fault—Bar and

 ball on downthrown side;

 dotted where concealed

Thrust or reverse fault—

 Teeth on upper block;

 dotted where concealed

Anticline

Syncline

Overturned syncline

Shear zone

Continental Divide 

Mzu

Xu

Pzu

Tb

Qu

Ti

Tv Yg

Pzl

QTl

Introduction 9

Figure 7 (pages 8 and 9). Simplified geology of the Blue River valley region, based mostly on detailed maps of 

Kellogg and others (2008, 2011) and Ruleman and others (2011), and references therein. 

Geology of the Blue River Valley Region
Maroon

Formation

Minturn

Formation

Entrada Sandstone

Chinle Formation

State Bridge Formation

Morrison Formation

Leadville Limestone

Belden Formation

TERTIARY

Late Cretaceous

JURASSIC CRETACEOUS

Early

 Cretaceous

Dakota 

Formation

Benton 

Formation

Niobrara 

Formation

Pierre Shale

Middle Park

Formation 0– >2,000 ft

8,500 ft

100–360 ft

540 ft

310 ft

165 ft 

50–160 ft

0–330 ft

0–6,250 ft

0–200 ft 

Chaffee Group

Fremont Dolomite

Harding Quartzite

Manitou Formation

Sawatch Quartzite 

0–160 ft

0–500 ft

Proterozoic

rocks

0–3,950 ft

Late Jur.

Early Jur.

Early Miss.

Late Tri. 

Tri. &

Permian Early Permian to Middle Pennsylvanian 

TRIASSIC PENNSYLVANIAN

Middle Pennsylvanian

Early Miss.

to

Late

MISS. TO 

CAMBRIAN

Cambrian

MISS. PRO- TEROZOIC

Proterozoic Paleocene Miocene and Oligocene

 

Troublesome Formation is light-brown and gray-brown siltstone, arkosic

sandstone, and conglomerate; locally contains tuff and tuffaceous sandstone 

and scattered outcrops in Blue River valley. More than 25 basalt and

andesite flows best exposed on Piney Ridge.

Multicolored siltstone, mudstone, arkosic sandstone, and conglomerate.

Windy Gap volcanic member (basaltic breccia) at base. Thickness not to scale.

Brownish-gray to black platy shale and mudstone with interbedded

fine-grained sandstone. Marine. Thickness not to scale.

Upper Smoky Hill Shale Member is platy-weathering, gray, calcareous shale;

lower Fort Hays Limestone Member is blocky, gray, very fine grained limestone. 

Marine. 

Upper crystalline limestone (6 feet thick) overlies rusty siltstone, shale, and 

worm-burrowed sandstone that overlies dark-brown to black platy shale. Marine. 

Hard quartz sandstone beds interlayered with dark-gray to black coaly shale. 

Deposited by rivers and in swamps. 

Gray to greenish-gray and maroon claystone in upper part; lower part light-yellow

fine-grained sandstone, shale, and gray limestone. Deposited by rivers and in swamps. 

Light-yellow, well-sorted, fine- to medium-grained, commonly cross-bedded,

carbonate-cememted sandstone. Quartz grains frosted. Eolian.

Thin-bedded, dark-red, pink, and greenish-gray silt, shale, and fine-grained

sandststone. Fluvial. In places, basal Gartra member (0–20 feet thick) is white,

pale-orange, cross-bedded sandstone, and pebble conglomerate. 

Thin-bedded, micaceous, brick-red shale, siltstone and fine-grained sandstone. 

One 16-foot-thick bed of gray, arkosic conglomerate in middle of formation. Fluvial. 

Grayish-red, reddish-brown, and orange-red, medium- to coarse-grained, 

micaceous sandstone, and conglomerate. Contains minor beds of 

red shale and shaly limestone. Fluvial. Thickness not to scale.

Orange-red to maroon siltstone, shale, arkosic sandstone, and conglomerate, and

stratigraphically important beds of limestone and dolomite. Fluvial and nearshore.

Deposited on flanks of ancestral Front Range uplift. Thickness not to scale. 

Dark-gray to black, fossiliferous shale and subordinate limestone, dolomite, and

sandstone. Marine.

Massive, gray, fossiliferous limestone and dolomite; karst surface locally contains thin, 

yellowish clay and silt with chert fragments (Molas Formation).

Pre-Leadville sequence is thin- to medium-bedded dolomite, shale, and sandstone. 

Basal Sawatch Quartzite is hard, white to pale pink quartz sandstone, locally

containing worm burrows. Sequence entirely marine. Sparsely and discontinuously

preserved in southern part of Blue River basin and Vail region. Sequence entirely

eroded from area of ancestral Front Range uplift during late Paleozoic.

Mostly metamorphic and granitic rocks.

Troublesome

Formation 

0–750 ft 

0–130 ft

Troublesome

 Formation, 

other basin-fill

deposits, and

lava flows 

Lava flows

0–750 ft

PERMIAN

10 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Figure 8. Stratigraphic column of sedimentary and igneous rocks in the Blue River valley region. Cretaceous and Tertiary intrusive rocks, 

and rocks and deposits younger than the Troublesome Formation, are not shown. The Middle Park, Pierre Shale, Maroon, and Minturn 

Formations may be thicker than represented in the stratigraphic column.
100 MILES

100 KILOMETERS

0

0

ANCESTRAL

FRONT

RANGE

HIGHLANDS

Denver

Basin

Paradox

Basin

C E N T R A L

C O L O R A D O

T R O U G H

UTAH

WYOMING NEBRASKA

OKLAHOMA

APISHAPA UPLIFT

ANCESTRAL UNCOMPAHGRE

HIGHLANDS

Map area

Denver

COLORADO

ARIZONA NEW MEXICO

KANSAS

Figure 9. Late Paleozoic Ancestral Rocky Mountain 

uplifts and adjacent basins in Colorado. Area of 

figure 7 outlined in red.

Introduction 11

Ancient Seas and Long-Vanished Mountains 

Sometime during the latest Proterozoic Era, a widespread 

and poorly understood period of uplift and erosion exposed 

deep levels of the basement rocks across a nearly flat surface 

that extended over much of what is now North America. 

Shallow seas covered this surface during most of the time 

between deposition of the Upper Cambrian Sawatch Quartzite 

and Middle Pennsylvanian Belden Formation (fig. 8), which 

are well exposed near Minturn, just west of the map area. The 

surface at the base of these sedimentary rocks, just above the 

underlying, much-older basement rocks, is referred to as the 

Great Unconformity.

Life forms proliferated in the Cambrian Period. Although 

some soft-bodied organisms lived before the Cambrian Period, 

it was not until the Paleozoic Era that organisms generally 

developed hard body parts that were well preserved as fossils. 

Uplift of the Ancestral Front Range, one of many late 

Paleozoic uplifts in the western and central United States 

(collectively called the Ancestral Rocky Mountains), began 

during the Middle Pennsylvanian Epoch (fig. 9). As the rocks 

rose, nearly all pre-Pennsylvanian sedimentary rocks from the 

upper Blue River valley area were eroded, with the exception 

of limited areas of Ordovician to Mississippian rocks south of 

Breckenridge (fig. 7, unit Pzl), which were just off the western 

flank of the Ancestral Front Range (fig. 9). Rocks as old 

as the Cambrian Sawatch Quartzite are beautifully exposed 

in Eagle County near Minturn, just southwest of Vail. In the 

northern Gore Range, which lies near the central part 

of the Ancestral Front Range, all rocks older than the Upper 

Jurassic Morrison Formation were completely eroded. Coarse 

sediment (sand, cobbles and boulders) was shed southwestward 

into the flanking Central Colorado trough (fig. 9), forming 

coarse alluvial fans and fluvial (river) deposits that later 

solidified to form the Pennsylvanian Minturn Formation and 

overlying Pennsylvanian and Permian Maroon Formation 

(fig. 7, unit Pzu; fig. 8). These two thick formations are 

exposed throughout large areas in southwestern Summit County 

west of the Gore fault zone. Deposition of the coarse sediment 

of the Minturn Formation was interrupted periodically by 

incursions of the sea, as demonstrated by marine limestone 

interbedded with the sandstones. Some of the limestone 

subsequently altered to dolomite by the addition of magnesium 

derived from hot groundwater, probably during the Laramide 

orogeny (which we will discuss in the next section). The Maroon 

Formation contains almost no limestone. The grain size of 

the clastic deposits (siltstone, sandstone, and conglomerate) 

decreases toward the axis of the Central Colorado trough (fig. 9), 

in which a saline sea with limited circulation deposited thousands 

of feet of evaporite (halite or rock salt, and gypsum) at the same 

time that the Minturn Formation was deposited. 

Also at the same time, sediments similar to those of the 

Minturn and Maroon Formations were shed eastward off 

the Ancestral Front Range uplift. Those rocks are now exposed 

along the eastern margin of the Front Range, forming the 

Fountain Formation, which makes up the prominent Flatirons 

near Boulder, Red Rocks Amphitheatre west of Denver, the 

spectacular rock formations of Roxborough State Park 

south of Denver, and the spires of the Garden of the Gods 

near Colorado Springs. 

The Gore fault zone, which lies along the west side of the 

Gore Range (figs. 7 and 10), is actually a complicated array of 

intersecting steep faults that defines the approximate eastern 

structural boundary of the Central Colorado trough. The Gore 

fault zone has had a protracted tectonic history, beginning 

at least as early as the early Paleozoic, and perhaps even the 

Precambrian, although most deformation along this zone is 

Laramide in age.
u u

u

u Ti Ti

Xu Xu

Xu Xu Xu

Xu

Xu

Xu

u u

u

l

l

l

GORE FAULT

ZONE

DILLON

RESERVOIR

WILLIAMS

RANGE THRUST

Keystone

window

MONTEZUMA STOCK SWAN MOUNTAIN

 STOCK

GORE RANGE FRONT RANGE

BLUE RIVER

NORMAL 

FAULT

B B’

Xu Xu

Xu Xu

Xu

Xu

Xu Xu Xu

Xu

Tb Qu

WILLIAMS

RANGE

THRUST

BLUE RIVER

NORMAL

FAULT

GORE FAULT

ZONE

GORE RANGE

FRONT RANGE

0

5,000

10,000

Gore

Creek

Williams

Fork

River

2X VERTICAL EXAGGERATION

FEET

A A’

0

5,000

10,000

FEET

0

5,000

10,000

FEET

0

5,000

10,000

FEET

15,000 15,000

15,000 15,000

Qu

0 5 10 KILOMETERS

0 5 10 MILES

EXPLANATION

Qu

Tb

Ti Xu

Quaternary surficial deposit

Tertiary basin fill

Tertiary intrusive granite

Mesozoic rock undifferentiated

Paleocene and Upper Cretaceous

Middle Park Formation

Fault—Dotted where uncertain;

arrows show relative motion

Upper Paleozoic rock

Lower Paleozoic rock

Early Proterozoic basement rock

12 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

The land of the Ancestral Front Range was above sea 

level, and ongoing erosion exposed basement rock until the 

Jurassic Period, when the region once again was eroded to a 

near-sea-level planar surface and the near-shore deposits of the 

Upper Jurassic Morrison Formation, deposited by rivers and in 

Formation lies directly on Proterozoic rocks in the northern 

Gore Range. The Morrison Formation is famous for the many 

dinosaur fossils that have been discovered in it, most famously 

in Dinosaur National Monument in northwestern Colorado 

and northeastern Utah. The quartz sandstones and interbedded 

carbon-rich shales of the Lower Cretaceous Dakota Sandstone 

deposits along beaches and slow-moving rivers, and in swamps. 

The Dakota Sandstone, which forms “hogbacks” (sharp, 

rocky ridges) west of Denver, in many places is a valuable 

petroleum-reservoir rock. Gradual sea-level rise (accompanied 

in some places by tectonic subsidence beginning in the late 

Early Cretaceous Epoch) caused inundation of the entire 

region by the vast Late Cretaceous Western Interior Seaway 

that covered much of the central part of North America. About 

9,000 feet of marine strata, mostly shale and minor limestone 

and sandstone (Benton Group, Niobrara Formation, and the 

Figure 10. Cross sections (slices through the Earth’s upper crust) running generally southwest to northeast through 

the Blue River valley region; see figure 7 for lines of section A-A’ and B-B’. Vertical exaggeration is 2, which increases 

angle of dip on structures. Cross sections are approximately twice as long as the line of section shown in figure 7. The 

Mesozoic rocks are as old as the Late Triassic Chinle Formation and as young as the Upper Cretaceous Pierre Shale. 

The upper Paleozoic (an informal term) here refers to Mississippian through Permian rocks; lower Paleozoic (also an 

informal term) here refers to Cambrian through Devonian rocks.
W I L L I A M S

R A N G E

T H R U S T (C O V E R E D) 

Introduction 13

Figure 12. Black shale of the lower Pierre Shale below the Williams Range thrust. Prominent cliff just below the 

thrust is Hygiene Sandstone Member of the Pierre Shale. Due to weathering, the sandstone forms pale yellowish 

talus which in many places slides downhill to cover underlying shale. The hill in the foreground also contains thin 

sandstone beds near the top. The trace of the Williams Range thrust is shown by the dashed line with teeth; rocks 

above the thrust have moved towards the observer. View is east across Green Mountain Reservoir; the vehicle on 

road in lower extreme right provides scale.

Figure 11. Dakota Sandstone exposed in road cut along Interstate Highway 70, two miles west of the Williams 

Range thrust. The small west-directed thrust fault (marked by white line showing relative motion along it) is similar 

to the much larger Williams Range thrust to the east. Note the small fold (steepened beds marked by arrow) above 

the thrust plane. View is to the northwest. Distance across road cut in photo is about 60 feet. 
14 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

The Rise of the Rocky Mountains 

The Laramide orogeny, which began about 70 million 

years ago and ended about 50 million years ago, marked the 

first episode in the building of the present Rocky Mountains. 

This time interval straddled the end of Late Cretaceous time 

(when the dinosaurs disappeared) and the beginning of Tertiary 

time (fig. 4). The Laramide orogeny was a time of crustal 

contraction (shortening), uplift, faulting, igneous activity, 

erosion, and sedimentation in sinking marginal basins. The 

Laramide orogeny formed major mountains and basins in the 

same places where they now exist, although these early Rockies 

would have looked vastly different from the Rockies we see 

today. Some of the peaks may have been higher than present 

ones, and extensive erosion during the past 50 million years 

and deposition of thick accumulations of sediment in marginal 

basins resulting from this erosion have greatly modified the 

shape of the land. 

Early stirrings of the Laramide orogeny were marked by 

the withdrawal of the Western Interior Seaway from the region 

after 69 million years ago—the age of the youngest ammonite 

fossils in the Pierre Shale. (The Pierre Shale is a thick marine 

shale and sandstone sequence that is exposed extensively in 

the Blue River valley, and ammonites were ocean-dwelling 

invertebrates, an extinct group of cephalopods related to squids, 

octopuses, and chambered nautili.) 

A sequence of upward-coarsening rocks as young as 

64 million years old documents this initial, slowly accelerating 

Laramide mountain uplift. This sequence is clearly exposed 

near Golden on the eastern side of the Front Range, but the 

only rocks in the northern Blue River region area that record 

Laramide uplift are the fine- to very coarse-grained sandstones 

and conglomerates of the thick Paleocene-age Middle Park 

Formation. This rock unit formed from sediments that were 

shed mostly off the small Laramide uplifts that were formed 

during movement of the Williams Range thrust fault and, near 

Hot Sulphur Springs, by movement of the Mount Bross thrust 

fault, another thrust fault (fig. 7). This relationship suggests 

that uplift and erosion may have started in the Blue River valley 

region about 65 million years ago—the boundary between 

Cretaceous and Tertiary Periods. The earliest intrusive rocks, 

granitic sills in the southern part of the Blue River region, 

were emplaced about this same time. Boulders and cobbles 

in the upper part of the Middle Park Formation are mainly 

derived from Proterozoic basement rocks, indicating that the 

basement was exposed during deposition of the Middle Park 

sediment. The basement was brought to the surface from deeper 

levels, largely by moving up and over the younger rocks along 

the thrust faults (such as the Mount Bross fault), thereby 

causing the highlands or uplifts and effectively locally thickening 

the Earth’s crust. 

The Laramide-age Williams Range thrust fault defines the 

western structural boundary of the Front Range (by convention, 

it is not called the Williams Fork thrust fault, after the Williams 

Fork Mountains, which would make more sense). Figure 11 

shows a very small thrust in Dakota Sandstone in a roadcut 

near Dillon that provides a miniature model of the Williams 

Range thrust fault; like this small thrust fault, the Williams 

Range thrust fault dips gently or is nearly horizontal. East of 

Dillon, rocks above the fault have been carried at least 6 miles 

westward relative to rocks beneath the fault. This movement 

is demonstrated by a small area just west of the Montezuma 

stock (fig. 7), where erosion has locally cut through the basement 

rock above the fault, creating a “window” through which we 

can see the fault and the Cretaceous sedimentary rocks 

beneath it (fig. 13). This window is almost 5 miles east of the 

main exposure of the fault. The age of the thrust is suggested 

by its suspected connection to the Elkhorn thrust to the south, 

although the possible connection is obscured by Tertiary 

intrusive rocks and overlying surficial deposits. The Elkhorn 

thrust is as young as late Paleocene (about 56 million years old). 
Brecciated

and oxidized

gneiss

Main fault

Sheared and

silicified

Pierre Shale

“Baked”

Pierre Shale

(hornfels)

Migmatitic

biotite

gneiss

Figure 13. Structural zones of the Williams Range thrust fault near Keystone, exposed in a “window” 

through the thrust near the west end of the Montezuma stock. Heat from magma that formed the 

stock baked the Pierre Shale, which then transformed into a hard rock called hornfels. Hat near main 

fault shows scale. View is to the northwest.

The Rise of the Rocky Mountains 15
16 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Figure 14. Dike emplaced 26–25 million years ago (late 

Oligocene) of hornblende latite porphyry forms Haystack Butte. 

View shows northwest end of dike, which is about 25 feet thick. 

View to the southeast.
The Rise of the Rocky Mountains 17

Igneous Activity and the Laramide Orogeny

The Laramide orogeny was also marked by voluminous 

intrusions of irregular stocks, sills, and dikes ranging from 

gabbro to granite (or basalt to rhyolite in finer-grained rocks), 

mostly in the southern part of the Blue River Basin. Some of 

these intrusive rocks may have breached the surface, forming 

volcanoes that have long since eroded. 

The oldest of these rocks, such as sills and stocks of 

the Pando Porphyry (much more prominently exposed near 

Leadville, south of the Blue River valley), are about 

72 million years old, which is about the time at which the 

Laramide orogeny began. The Pando Porphyry is a fine-grained 

granite with larger grains (called phenocrysts) of quartz. 

However, the youngest intrusive rocks, associated with the 

Climax granite porphyry intrusive complex near Fremont 

Pass (fig. 2), are about 24 million years old (late Oligocene); 

so intrusive activity continued after the close of the Laramide 

orogeny. The largest intrusive bodies, which have granitic 

compositions, intruded about 40 million years ago. They 

include the large granite porphyry stock at Swan Mountain 

south of Dillon (fig. 7), the stock just east of Boreas Pass, and 

the Montezuma stock (including numerous nearby dikes and 

small intrusive bodies) (fig. 7). At Green Mountain (fig. 2), 

just north of Green Mountain Reservoir, sills, dikes, and small 

stocks of fine-grained gray porphyry (called latite and trachyte 

porphyry) are about 31.5 million years old, although a prominent 

dike of similar composition north of Green Mountain at 

Haystack Butte (figs. 2 and 14) is about 25 million years old. 
GORE 

FAULT

18 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Let us now turn our attention to the Gore Range and 

the enigmatic Gore fault zone that marks the west side of the 

Gore Range. Sorting out the history of movement along the 

Gore fault zone (figs. 7 and 10) is complicated because the 

zone was recurrently active at least as long ago as the late 

Paleozoic, and perhaps as long ago as the Neoproterozoic. 

Most movement along the Gore fault zone, however, took 

place during the Laramide orogeny; so most faults of the 

zone are thrust faults or reverse faults that formed during 

contraction of the Earth’s crust. The Gore fault zone also 

contains some faults that moved during crustal extension after 

the Laramide orogeny, discussed in the next section. At the 

surface, beds of the Minturn Formation are nearly vertical 

or are overturned immediately southwest of the fault zone. 

A particularly well-exposed fault contact between steeply 

dipping Minturn Formation and fractured Paleoproterozoic 

granitic rock (Cross Creek Granite) can be seen near Booth 

Creek, just north of Vail (fig. 15). All lower Paleozoic rocks 

along and northeast of the fault zone were eroded during 

late Paleozoic uplift, except for thin remnants of Cambrian 

Sawatch Quartzite and Ordovician dolomite that are locally 

preserved in fault-bounded blocks. 

The north-northwest-trending faults that cut the Proterozoic 

basement of the Gore Range have also had a very long history 

that may record at least four periods of movement, here listed 

from oldest to youngest: 

1. Movement related to late Paleozoic uplift of the Ancestral 

Front Range; 

2. Uplift related to the Laramide orogeny; 

3. Strike-slip movement (both sides of the fault moved 

horizontally) related to a poorly understood mid-Tertiary 

interval of deformation; and 

4. Crustal extension related to formation of the Rio Grande rift 

(discussed in the next section), which produced late 

Oligocene and younger normal faulting, crustal heating, 

and uplift parallel to and along the Blue River normal fault. 

Another set of east-northeast-striking faults cuts the 

Proterozoic basement of the Gore Range and is probably 

related to differential uplift of the range during the Tertiary 

Period. A particularly large down-to-the-northwest fault system 

just north of Mt. Powell (fig. 2) separates the high, rugged 

terrane south of the fault from the lower and gentler topography 

of Elliott Ridge. 

Figure 15. The Gore fault (white line; dotted where concealed by Quaternary deposits) near Vail. Steeply west-sloping beds of the 

Minturn Formation left of the fault are in contact with fractured Paleoproterozoic granite (Cross Creek Granite) on the right. Arrows 

show relative movement above and below the fault. View is west across Booth Creek.

The Gore Fault Zone and Faults Cutting Proterozoic Rocks of the Gore Range
Map area Denver

Leadville

Blue River valley

Alamosa

Santa Fe

Taos

Albuquerque

Socorro

Las Cruces

MEXICO

BASINS

THE

RIO GRANDE

RIFT

NEW MEXICO

COLORADO

OF 

The Continent Pulled Apart—Development of the Northern Rio Grande Rift 19

Following the Laramide orogeny, which persisted until 

the early Eocene (about 50 million years ago), a period of 

relative quiescence and erosion preceded a renewed episode 

of crustal unrest, beginning in the Oligocene, about 30 million 

years ago. Crustal extension and increased volcanism dominated 

the geologic events of the region during this time and formed 

north-trending grabens (down-dropped crustal blocks bounded 

by faults) of the Rio Grande rift (fig. 16). The rift extends 

southward from the Blue River valley, steps westward into 

the upper Arkansas River valley, extends southward into the 

river valley of the Rio Grande of southern Colorado and New 

Mexico, and continues into northern Mexico. Normal faults 

associated with formation of the Rio Grande rift also extend 

north into Wyoming, although the Blue River valley is the 

northernmost topographical expression of the rift.

Alkali basalt (which contains relatively high amounts 

of potassium and sodium as compared to other basalts) and 

andesite lava flows cap ridges west of Green Mountain Reservoir, 

particularly on and near Piney Ridge (fig. 2), where more than 

25 flows are stacked. These rocks are part of a large, eroded 

volcanic field that extended from the Blue River westward at 

least to State Bridge along the Colorado River, about 18 miles 

southwest of Kremmling. These types of basalts are typical 

of those that erupted during periods of crustal extension and 

rifting. Ages of basalt flows near State Bridge were calculated 

from the decay of isotopes of potassium to argon. These ages 

are all between 24.0 and 22.5 million years old, similar to 

three ages of about 23.6 million years from andesite lava flows 

in the northern Blue River Basin near Otter Creek. 

The graben underlying the Blue River valley is called a 

“half-graben” because only one major fault, the Blue River 

normal fault (figs. 7 and 10), bounds the graben on the west 

side; there is no large normal fault on the east side of the 

graben. As a result, bedrock strata beneath the valley floor 

generally tilt down to the west. Unlike other parts of the 

Rio Grande rift to the south, the Blue River half-graben is 

relatively narrow, and Tertiary basin-fill deposits that once 

filled the graben have been mostly eroded. Only a few 

hundred feet of basin-fill deposits (sandstone, gravel, and 

volcanic rocks) remain in several places, and those are mostly 

mantled by thick late Tertiary and Quaternary gravels and 

Quaternary glacial deposits. Elsewhere in the graben, 

Cretaceous rocks (mostly shale and sandstone of the Pierre 

Shale are exposed. The oldest dated volcanic rocks interbedded 

with the basin-fill sedimentary rocks are about 27 million years

old (late Oligocene), which marks the approximate time of initial 

rifting. These dated rocks include ash-flow tuffs (fig. 17) which 

flowed into the early-formed valley as hot, incandescent clouds 

of ash from an exceptionally large caldera (“supervolcano”) 

eruption or eruptions to the south in Colorado, possibly from 

the San Juan Mountains in southwestern Colorado. When 

these hot clouds of ash settled, they were hot enough to weld 

into a relatively hard rock.

Figure 16. The Rio Grande rift. Tan areas represent basin-fill 

deposits that once filled rift valleys (grabens); associated faults 

are not shown. Basin-fill deposits of the Blue River valley have 

largely been eroded.

The Continent Pulled Apart—

Development of the Northern Rio Grande Rift
20 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Information Box 2.

Dating Igneous Rocks by Using Radioactive Decay 

of Potassium to Argon

How are these potassium-argon ages calculated? The method is similar to that used to measure the age of very old rocks in 

which isotopes of uranium decay to isotopes of lead in tiny zircon crystals (see information box 1. Dating Rocks by Using Zircon). 

The potassium-argon and argon-argon methods use the decay properties of a radioactive isotope of potassium ( 40K), which 

decays at a precisely known rate to various isotopes of argon. The relative amounts of each of these isotopes in an igneous rock 

can be measured by a mass spectrometer, which, in turn, can be used to calculate the age of an igneous rock since the time it 

cooled from a magma. The isotopes are generally measured in grains of the minerals sanidine, biotite, or hornblende.

Figure 17. Outcrop of moderately welded upper Oligocene tuff near South Brush Creek. About 60 percent of the rock is very fine 

grained matrix; the balance is made up of crystals of quartz, biotite, plagioclase, and sanidine (high-temperature potassium feldspar), 

and small fragments of obsidian (volcanic glass), light-colored volcanic rocks, and Proterozoic gneiss. This tuff is about 27 million years 

old, probably came from a volcanic eruption many miles to the south, and is one of the oldest rocks infilling the very early valley of the 

ancestral Blue River. Hammer head 8 inches long. 
The Continent Pulled Apart—Development of the Northern Rio Grande Rift 21

The Blue River normal fault along the east side of the 

Gore Range offsets the top of the basement rocks vertically by 

at least 3,600 feet (fig. 18). In addition, many of the Mesozoic 

rocks of the Blue River valley, dominated by the more than 

5,000-feet-thick Pierre Shale, are cut by several normal faults 

that are all down on their east sides. The Pierre Shale was 

originally more than 8,000 feet thick in the area, but the upper 

3,000 feet have been removed by erosion. The consistent

down-to-the-east displacement of faults in the Blue River valley 

suggests that they may curve downward into a nearly flat plane 

of weakness, possibly the reactivated Laramide-age Gore fault, 

which also curves down to the east (fig. 19). Movement along 

the Blue River normal fault may be as young as Holocene, but the 

timing of youngest movement, undoubtedly accompanied by 

intense earthquake activity, is controversial (refer to information 

box 3, Earthquakes and Age of Recent Faulting). 

The controversy about the age of faulting continues. 

For example, low (less than about 6 feet high), subtle fault 

scarps west of Interstate Highway 70, just northwest of Frisco, 

cut gravel deposits of Pliocene(?) to middle Pleistocene age. 

Figure 18. Early morning view of the Gore Range from Ute Pass. View is west across the Blue River valley toward the valley of Slate 

Creek. The Blue River normal fault lies along the eastern base of the range, behind the large, rounded, forested hill in foreground. This 

hill is underlain mostly by bouldery material (till) that was deposited by glacial processes. Rusty-red color of lodgepole-pine forest due 

to widespread infestation by pine-bark beetles. 

Subtle scarp-like features that cut glacial till as young as late 

Pleistocene (deposited during the Pinedale glaciation) have 

also been identified along the Blue River normal fault northwest 

of Frisco and Silverthorne, suggesting possible Holocene 

movement, but further study is needed to verify the origin of 

these features. 

The flanks of the Blue River valley, along the Gore Range 

and western Front Range, uplifted faster than areas more distant 

from the valley. We know this from studies of apatite fission-track 

ages (see information box 4. What Apatite Fission Tracks Tell 

Us about Uplift and Cooling History). Oligocene to late Miocene 

fission-track ages (as young as 6 million years old) from rocks 

adjacent to the graben in the Gore Range and western Front 

Range are considerably younger (by several tens of millions of 

years) than fission-track ages recorded farther from the graben. 

This relationship indicates that uplift along the flanks of the 

Blue River graben is relatively young as compared with uplift 

in areas more distant from the graben. 
2 10

10 2

3 MILES

3 KILOMETERS Sense of Laramide movement 

(Late Cretaceous and early Tertiary)

Sense of late Tertiary movement

Sense of early Laramide and younger 

movement reversed during Rio Grande(?)

age movement

Cretaceous undifferentiated

Triassic-Jurassic undifferentiated

Paleozoic undifferentiated

Proterozoic basement rock,

undifferentiated

EXPLANATION

GORE

FAULT

CENTRAL Gore Range

COLORADO

TROUGH

Blue River

Valley

Williams Fork

Mountains

WEST EAST

SEA

LEVEL

WILLIAMS RANGE

THRUST

YXu

YXu

YXu

Ku Ku

YXu

BLUE RIVER

NORMAL FAULT

3 MILES

-3 MILES

Ku

YXu

|u

|u

^-Ju

^-Ju

^-Ju

22 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Information Box 3.

Earthquakes and Age of Recent Faulting

Although Colorado is not generally regarded as an area with substantial earthquake risk, large earthquakes have occurred 

in the Front Range region. An earthquake of inferred Richter magnitude 6.5 caused considerable damage in the northern 

Front Range in 1882, and scattered earthquakes of smaller magnitude periodically shake the region. Because the Blue River 

valley lies near the northern terminus of the Rio Grande rift—an active zone of crustal extension, elevated heat flow, and 

late Tertiary uplift along its flanks—one might expect a relatively high seismic hazard potential. 

Paradoxically, the faults in the Blue River valley have not been historically active, and this part of the Rio Grande rift is 

seismically quiet (although the timing of youngest movement along normal faults in the valley, such as the Blue River 

normal fault, is controversial). The well-known Colorado geologist Ogden Tweto suggested that the prominent scarp that 

defines the Blue River normal fault along the east side of the Gore Range indicated Holocene movement. One indication of 

Holocene, and perhaps currently active, movement are “gouge boils.” These features are mounds as high as 36 feet found in 

treeless areas above the trace of the Blue River normal fault. They are composed of fine-grained “fault gouge,” interpreted 

by Tweto as material that extruded upward along the fault zone and shouldered aside bouldery material. Other geologists 

have challenged Tweto’s assessment of Holocene faulting and have, in fact, suggested that movement on the Blue River 

normal fault was probably no younger than the middle of the Pleistocene Epoch. 

Figure 19. Cartoon east-west cross section across the Blue River valley, approximately between lines of section A-A’ and B-B’, 

showing proposed relationship of down-to-the-east normal faults, including the Blue River normal fault, which suggests that these 

Rio Grande-age (Oligocene or younger) faults merge into a detachment surface at depth, possibly the older (Laramide-age) Gore fault 

(relationship suggested by Kellogg, 1999). 
The Continent Pulled Apart—Development of the Northern Rio Grande Rift 23

The Upper Oligocene to Upper Miocene 

Troublesome Formation—

Was It All That Troubling? 

Northeast of the Blue River half-graben lies a separate 

downwarp, the Middle Park Basin (fig. 7). This basin underlies 

a generally north-northwest-trending, sagebrush- and grasscovered broad valley of relatively low topographic relief. The 

basin is bounded in places by normal faults, so it is a broad 

rift basin that formed at approximately the same time as other 

basins of the Rio Grande rift. The Middle Park Basin is filled 

with several hundred feet of brown to yellowish-tan and 

relatively soft rocks of the Troublesome Formation (fig. 7, 

part of unit Tb)—siltstone, sandstone, conglomerate, and 

minor volcanic ash deposits. The sediments composing these 

rocks were deposited during late Oligocene to late Miocene 

time (about 28 to 11 million years ago) in part by streams in 

and near the headwaters of the ancestral Colorado River. 

Volcanic ash deposited from distant volcanic eruptions can be

used to date these deposits. The soft, easily eroded Troublesome 

Formation locally forms a sparsely vegetated, badlands 

landscape in Middle Park. The rocks of the formation are not 

really troublesome to understand; the formation is named 

after Troublesome Creek near Kremmling, whose wet or 

muddy siltstone was probably difficult for early wheeled 

vehicles to cross. 

What Happened to All the Sediment 

That Once Filled the Blue River Valley?

The Blue River valley undoubtedly contained a very 

thick sequence of sedimentary rocks by the beginning of the 

Pliocene Epoch (about 5 million years ago). Because most of 

these rocks have been removed by erosion, it is not clear how 

thick this sequence was. These rocks were equivalent to those 

of the Troublesome Formation in the Middle Park Basin, but 

they also included younger sediment that was shed off the Front 

Range and into flanking valleys, including the Blue River 

valley, during a poorly understood period of late Miocene and 

early Pliocene erosion. The thick Miocene to perhaps early 

Pliocene Ogallala Formation, which composes a large aquifer 

of the eastern plains of Colorado and adjacent states, was also 

deposited at this time from the debris eroded from the Front 

Range and adjacent ranges north and south of the Front Range. 

Geologists continue to debate whether the erosion was caused 

by renewed uplift, causing streams to incise into the mountains 

owing to steeper gradients, or whether it was caused by climate 

change, in which increased precipitation gave streams additional 

erosive power. Both these factors may have been at work. 

Deposition ceased by about 5 million years ago. Rivers 

began eroding the thick sequence of upper Tertiary sedimentary 

rock and, eventually, cut into the underlying older rocks. 

The Blue River was no exception. It eroded virtually all the 

Tertiary sedimentary rocks in the Blue River valley, so that 

now only a few vestiges of these rocks remain. Much of the 

underlying Cretaceous rock has also been eroded and washed 

into the Colorado River, which carried most of the sediment 

into the Gulf of California.

Information Box 4.

What Apatite Fission Tracks Tell Us about Uplift and Cooling History

Apatite (calcium phosphate, CaPO4), is found as scattered tiny crystals in virtually every rock. Fission tracks in apatite 

crystals are produced by high-speed particles released by the spontaneous decay of 238U (which occurs in very small 

amounts in apatite). The tracks are retained generally below about 100 °C (they anneal, or disappear, above this temperature). 

The density of these fission tracks, measured on a polished surface of a rock under a microscope, is proportional to the 

time since the rock cooled below about 100 °C; so they can be used to calculate the time at which a rock cooled through 

this temperature. A rock at depth is hot owing to the geothermal gradient in the Earth’s crust, and it cools as an area uplifts 

and overlying rock is removed by erosion. Ages calculated from fission tracks, then, provide a good way to determine both 

the cooling and uplift history of the rocks
24 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Glacial erosion and deposition, primarily during the 

Pleistocene Epoch, were the last major events to modify the 

high mountain landscape of Summit County. Glaciers slowly 

transformed V-shaped river valleys into straighter U-shaped 

valleys, and they deposited large masses of glacially transported 

rock debris, called till, when the ice melted. The straight or 

curved ridges composed of till are called moraines. Large 

streams issuing from the melting ice redeposited some till 

in broad plains as glacial outwash. These periods of glacial 

erosion and deposition resulted from periodic major climate 

changes which drove several glacial-interglacial cycles. Global 

climatic cooling began about 2.6 million years ago at the 

beginning of the Quaternary Period and intensified after 

about 900 thousand years ago, although it was interrupted 

periodically by limited periods of warming. Time will tell 

whether the present period of global warming marks an abrupt 

end to this cyclic trend. 

During each of these glacial events, the landscape looked 

considerably different from how it looks now. Long rivers of 

ice partly filled the upper parts of high mountain valleys, and 

perennial snowfields probably mantled some of the adjacent 

high terrain. Glaciers in the Rocky Mountains advanced and 

retreated at least 12 times during the Pleistocene, but because 

each advance tended to overrun or conceal evidence of previous 

advances or glaciations, only the last two major glaciations left 

widespread evidence of their extent and imprint on the modern 

landscape. The Pinedale glaciation, the last major glacial event, 

took place between about 30 and 12 thousand years ago; the 

Bull Lake glaciation, the next older glacial event, was active 

between about 190 and 120 thousand years ago. Both were 

named for glacial deposits on the western and eastern flanks of

the Wind River Range in Wyoming, respectively. The approximate 

extent of glaciers of Pinedale and Bull Lake age in the Blue 

River region is shown in figure 20. 

Till and locally minor amounts of glacial outwash of the 

Pinedale glaciation form well-preserved, hummocky moraines 

that are widespread in larger valleys along the east flank of 

the Gore Range and in the Blue River valley and its major 

tributaries. Moraines of the Bull Lake glaciation are much 

more smoothed and rounded by erosion than those of Pinedale 

age. Bull Lake till is much more weathered, so that boulders 

are more disintegrated and the upper 6 to 9 feet of the till is 

oxidized (light-yellow or brown color) and turned partially to 

clay (fig. 21). Pinedale-age till, in comparison, is much fresher 

and commonly is grayer, with little soil or clay development 

(fig. 22). Typically, till lacks visible layering and contains 

particles ranging in size from silt to boulders. 

The youngest glacial deposits in cirques of the Gore 

Range form sharp-crested moraines; they may correlate with 

those in the Front Range that are about 12 thousand years 

old and younger. These tills lie above present tree line near 

cirque headwalls, typically above an elevation of 11,000 feet. 

They were deposited during minor glacial advances after the 

Pinedale glaciation. These deposits are within areas that were 

covered by glaciers during the Pinedale glaciation (fig. 20).

Small glaciers still exist in some of the highest cirques 

of the Gore and Tenmile Ranges. These glaciers are concealed 

by a layer of boulders and finer debris, which effectively 

insulates the underlying ice, which generally is not visible. 

Consequently, these glaciers are called “rock glaciers” (fig. 23). 

Active rock glaciers, those that have a substantial ice core, 

move slowly down valley and have a steep, bouldery front 

face. Inactive rock glaciers, in which the ice core has mostly 

or completely melted, have a more gentle front face, and dense 

lichens cover most exposed rock surfaces. 

The Ice Ages

Glacier—Photograph by Dirk Beyer—Wikimedia Commons; Creative Commons Attribution–Share Alike 3.0 Unported
b

b

b

b

b

b

b

p 

p 

p

p

b

Extent of

 Pinedale

 glaciation

Extent of Bull Lake

 glaciation beyond

 Pinedale glaciation

0 5 10 MILES

0 5 10 KILOMETERS

Frisco

Dillon

Vail

Breckenridge

Silverthorne

Kremmling

Hot Sulphur

Springs

Denver

COLORADO

 Williams 

 Fork

Blue River

Blue River

Blue River

Creek

Colorado

River

Swan

River

Tenmile

Creek

Gore

Continental

Continental

Divide

Divide

Snake River

 GREEN

 MOUNTAIN

 RESERVOIR

River

WILLIAMS

FORK

RESERVOIR

 DILLON

 RESERVOIR

106°30’ 106°00’

EXPLANATION

40°00’

39°30’

 

FRONT

GORE

GRAND

EAGLE

SUMMIT

PARK

LAKE

Slate 

Creek

Creek

Black

RANGE

RANGE

Figure 20. Blue River valley region showing extent of Pleistocene glaciers. Bull Lake glaciers (active about 190 to 

120 thousand years ago) extend in most places a mile or two beyond the extent of Pinedale glaciers (active about 30 

to 12 thousand years ago), although extent of Bull Lake glaciers in some valleys is not known. In the southeast part of 

the map, county boundaries follow the Continental Divide.

The Ice Ages 25

Base modified from U.S. Geological Survey

1:100,000-scale digital data, 2011
Figure 21. Unconsolidated till of Bull Lake glaciation in a 10-foot-high, wave-cut exposure along Dillon Reservoir, characterized by 

strong oxidation (brown color) in the lower part.

26 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Figure 22. Unconsolidated till of Pinedale glaciation in a 6-foot-high, wave-cut exposure along shore of Dillon Reservoir. Soil is minimally 

developed and the till’s general gray color indicates that the material is only slightly oxidized. 
The Ice Ages 27

Here are a few particular facts regarding Bull Lake and 

Pinedale glaciers that occupied the upper Blue River valley 

and its tributaries (fig. 20):

• The longest glacier in the Blue River drainage system, 

determined by the extent of glacial till, flowed down 

the valley of Tenmile Creek as far north as the dam at 

Dillon Reservoir. It was 18 miles long during the Bull 

Lake glaciation and only one mile shorter during the 

Pinedale glaciation; extensive glacial outwash and 

till deposited by the Pinedale glacier underlie the 

town of Frisco. 

• The glacier that occupied the upper Blue River valley 

south of Breckenridge was about 8 miles long; 

terminal moraines formed during Bull Lake and 

Pinedale glaciations are just south of Breckenridge. 

• The glacier in the Snake River valley was 11 miles long.

• The Williams Fork valley glacier was 16 miles long 

during the Bull Lake glaciation and was only 0.3 miles 

shorter during the Pinedale glaciation. 

• Glaciers flowing out of the eastern part of the Gore 

Range were as long as 10.5 miles, as in the valley of 

Slate Creek. 

• The glacier that occupied the valley of Gore Creek 

on the west side of the Gore Range was about 15 miles 

long during the Bull Lake glaciation. 

• The lowest elevation reached by any glacier in the 

Blue River drainage system (about 8,250 feet), was 

at the terminus of the 9-mile-long glacier in the valley 

of Black Creek, now beneath the level of Green 

Mountain Reservoir. 

The oldest pre-Bull Lake glacial deposits, deposited before 

900 thousand years ago (early Pleistocene), are now mostly or 

completely buried by younger glacial deposits. Cold (glacial) 

and warm (interglacial) climatic cycles before 900 thousand 

years ago produced smaller global ice volume and were of 

much shorter duration than those after 900 thousand years ago.

Exposures of old, coarse, unstratified, deeply weathered, 

gravelly deposits that resemble till have been identified in several 

places. These gravels typically lack morainal form and are 

preserved a short distance down-valley from exposures of 

Pinedale or Bull Lake till. However, it is not always obvious 

if some deposits that resemble till are, in fact, till of pre-Bull 

Lake age. To be cautious, such deposits are called “diamictons,” 

which implies that the genetic origin is unknown. Such diamictons 

are exposed at Mesa Cortina (fig. 24) and Gold Run; they may 

be large debris-flow deposits (flowed as bouldery mud) or 

alluvium rather than glacial till. These gravels are particularly 

interesting not only for the enormous size of the boulders 

(some are at least 25 feet long), but also for their gold content. 

These deposits were locally mined for placer gold, mostly in 

the 1860s and 1870s. 

Figure 23. View, facing west, of the head of South Rock Creek in 

the Gore Range showing a large, active rock glacier. Although no 

ice is showing, the hummocky piles of rocks are likely cored by 

ice which is slowly moving down the valley. We know it is active 

because the steep front of the rock glacier is unstable and at the 

angle of repose; rocks tumble down the steep front almost daily. 

The front of the upper part of the rock glacier is particularly high 

and steep.

Progressively older glacial deposits (or diamictons, deposits 

that resemble till but are of uncertain origin) on the east side

of the Gore Range contain progressively higher percentages 

of Dakota Sandstone clasts. This relationship suggests that 

Dakota Sandstone bedrock was much more extensively 

exposed during the early Pleistocene and was increasingly 

covered by the debris of succeeding glacial episodes. 
28 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Streams draining from glaciers produced broad, gravelly 

glacial outwash deposits, such as those underlying Frisco. 

Particularly during deglaciation—times of substantially 

greater stream flow and sedimentation—debris embedded in 

glacial ice was released and inundated downstream areas. 

Temperatures during glacial episodes were certainly much 

colder than they are now. In many periglacial environments, 

nonglaciated areas such as along ridges beyond the limit of 

glacial ice, vigorous freeze-thaw action and increased 

precipitation would have promoted slope instability and 

intensified mass movement in the Front and Gore Ranges. 

These periglacial processes formed such features as block 

fields, block streams or stripes, and patterned ground. 

Patterned ground refers to the polygonal shapes outlined by 

rocks that were forced to the surface and moved laterally 

during long episodes of freeze-thaw action. 
Landslides and Spreading Mountains 29

Figure 24 (facing page). Gravel at Mesa Cortina, showing 

crude stratification and strong oxidation. The deposit is deeply 

weathered and may be as old as Pliocene. It is interpreted either 

as a set of large debris flows or alluvium rather than glacial 

till. The extent of these deposits beneath younger Quaternary 

deposits is unknown, as is the thickness. The face was exposed 

by late nineteenth century hydraulic mining for placer gold. 

Outcrop about 1,000 feet northwest of I-70, west of Silverthorne.

Landslides and Spreading Mountains

Landslides

Landslide deposits of varying sizes are a common feature 

along slopes of the Blue River valley. Most large landslides 

probably formed or were reactivated under periglacial conditions 

during the Pleistocene, although some of these slides were 

recurrently mobilized during the Holocene Epoch (the most 

recent 12,000 years or so). Some large landslide deposits in 

glaciated valleys along the east side of the Gore Range formed 

after glaciers retreated and glacial ice no longer provided lateral 

support to weakly consolidated material on steep, unstable 

slopes. A particularly large, old, and eroded landslide complex 

(fig. 7, unit QTl), involving only Proterozoic gneiss and 

granitic rocks, underlies the west slope of the Williams Fork 

Mountains (which form the western ridge of the more extensive 

Front Range). This deposit owes its origin largely to the highly 

fractured and weak bedrock underlying the ridge. 

Conditions that contribute to landslides commonly include 

the following: (1) oversteepening of slopes by fluvial or glacial 

erosion at the base of slopes, or human undercutting of slopes 

(or both); (2) bedding oriented approximately parallel to slope; 

(3) deforestation by logging, mining, other human activity, 

or fires; (4) high water content in consolidated rock units or 

unconsolidated deposits (by intense or prolonged rainfall, 

or rapid or protracted snowmelt); and (5) vertical contrast in 

density or plasticity of geologic materials. 

In the high country of the Gore Range, cliffs in 

Proterozoic rocks frequently produce rockfalls, especially 

in the spring when water from daytime melting of snow 

freezes and expands at night in cracks in rocks. Repeated 

freeze-thaw cycles tend to dislodge unstable rocks from cliffs. 

Talus deposits are the products of rockfall from high, steep, 

rocky outcrops.

What Are Sackungen and How Do They Form? 

The crest of the Williams Fork Mountains is relatively 

smooth and rounded, and it maintains a nearly constant altitude 

of about 12,000 feet. The underlying Proterozoic bedrock is

densely fractured and relatively weak, a condition that has induced 

gravitational spreading of the entire range, somewhat like a 

large pile of bread dough. Peculiar features produced by this 

process are called “sackungen.” Sackungen is a German word 

(singular form is sackung) for deep fractures with uphill-facing 

scarps that are induced by the gravitational spreading and are 

common along and near the crest of the range (fig. 25). The 

subdued topography of the Williams Fork Mountains is in 

stark contrast to that along the rugged crest of the Gore Range 

(fig. 18), where Proterozoic rocks underlying the range are 

relatively unfractured and strong, although sackungen do 

occur in several strongly fractured local areas near the Blue 

River normal fault. 

Laramide-age movement along the Williams Range 

thrust fault probably is responsible for the dense fracturing of 

the rocks underlying the Williams Fork Mountains, which 

weakened the entire range. As erosion deepened the bounding 

valleys, these highly fractured mountains became gravitationally 

unstable and spread sideways slightly, forming cracks or 

sackungen (fig. 26). The gravitational instability also caused 

widespread landslides on the west side of the Williams Fork 

Mountains (fig. 7, unit QT1). These enormous landslide 

deposits are composed almost entirely of fragments and blocks 

of Proterozoic rock as long as about 50 feet derived from the 

strongly fractured upper plate of the Williams Range thrust 

fault (that is, rocks above the thrust plane). The landslide 

deposits moved downslope and concealed most of the trace

of the thrust fault and much of the Cretaceous shale and 

sandstone (mostly Pierre Shale) on the west side of the range. 

Most of this landslide complex is deeply eroded and no longer 

retains a hummocky landslide morphology, suggesting that at 

least some of the landslides may be as old as Pliocene. 
Figure 25. View to the south of sackungen (three of which are marked by arrows) near crest of southern Williams Fork Mountains. 

The sackungen, or deep-seated extension fractures, are marked by uphill-facing scarps caused by gravitational spreading of the entire 

ridge. Tenmile Range in background.

30 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Figure 26. Schematic cross section through Williams Fork 

Mountains showing the development of sackungen and landslides 

as a result of pervasive fracturing. 

TT

 Antislope

scarps

(sackungen)

Fractured

crystalline

rocks Landslide

deposits

WILLIAMS

RANGE

THRUST

Crystalline 

basement rocks

Mesozoic 

sedimentary

rock

Syn-thrust

 surface

Figure 27 (page 31). The Colorado Mineral 

Belt extends northeastward from the San Juan 

Mountains in the southwest part of Colorado to 

the Front Range near Boulder. The belt includes 

most of the major mining districts in the state 

(red dots); the Cripple Creek gold mine is a 

major exception. The Colorado Mineral Belt also 

contains many of the state’s “fourteeners”—

mountain peaks more than 14,000 feet high.
40°

38°

108° 104°

SUMMIT

COUNTY

Gold Hill

Breckenridge

Climax

Leadville

Aspen

Tincup

Telluride

Ouray

Cripple Creek

Rico

HENDERSON

 MINE

Gilman

Monarch

La Plata

Caribou

Denver

Lake City

Creede

Silverton

Central

 City

Boulder

Summitville

Crested Butte

COLORADO

Rich Ores of the Blue River Valley Region—Their Geology and Mining History 31

If you look at a map of all major mining districts in 

Colorado, you will notice that most of them lie in a 

northeast-trending belt that extends from the southwestern 

corner of the state to Boulder. This belt is referred to as the 

Colorado Mineral Belt (fig. 27). It includes almost all of the 

major metal mining districts in the state—such as Aspen, 

Climax, Leadville, Silverton, and Telluride. The large gold 

mine at Cripple Creek, Colorado, is a major exception; it 

lies considerably east of the belt. All of the rich mines in 

Summit County lie along the Colorado Mineral Belt. It has been 

suggested that a huge, northeast-trending body of deeply buried 

Laramide and younger granite and related rocks provided the 

heat and mineralizing fluids that deposited the ores.

Historically, the richest gold deposits in Summit County 

were near Breckenridge, in gravel placer deposits along the 

upper Blue River, the Swan River (fig. 28), French Gulch,

and Tenmile Creek (fig. 2). These gravels were dredged for 

100 years, from 1859 to 1959, although most of the gold was 

produced between 1906 and 1924. Total value of gold from 

these placers was more than $15.5 million at then-current 

prices of $21/ounce to $35/ounce (approximately $1.0 billion 

at 2014 gold prices). A 29-oz gold nugget was reportedly 

found in gravel along the lower Swan River. 

Hydraulic mining for gold used powerful streams of water 

shot from hoses aimed onto gravels; the resultant slurry was 

run through sluice boxes. At Mesa Cortina (figs. 2 and 24) 

near Silverthorne and at Gold Run near Breckenridge, hydraulic 

mining began in 1860, and the gravelly deposits were worked 

intermittently until 1934; early miners called the Mesa Cortina 

gravels “buffalo placers.” These deeply weathered gravels (fig. 24) 

may be as old as Pliocene and are probably debris-flow deposits 

or alluvium rather than pre-Bull Lake glacial deposits.

Rich Ores of the Blue River Valley Region—

Their Geology and Mining History 
32 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Hard-Rock Mining in the Blue River Valley

Hard-rock mining in the Breckenridge district commenced 

in 1869, and the beautiful gold wire, foil, and nuggets mined 

from the district, especially from Farncomb Hill about a 

mile north of French Gulch, are well-known items in many 

museum mineral collections. Gold was the principal commodity 

produced until the railroad to Leadville (and continuing on to 

Denver) was completed in 1880, which gave access to smelters 

that could process ores containing lead, zinc, and copper. The 

Wellington mine, on French Gulch about 1.5 miles east of 

Breckenridge (fig. 29), was by far the largest gold mine in the 

district. In this mine, northeast-striking, southeast-dipping veins 

as wide as about 10 feet of galena (lead sulfide), sphalerite 

(zinc sulfide), copper-bearing minerals, and pyrite (iron sulfide) 

produced considerable amounts of gold, silver, lead, zinc, and 

copper. The mine operated until 1958 when it was permanently 

closed. During its last 10 years of operation alone, the Wellington 

mine produced about 2,320 ounces of gold, 174,000 ounces of 

silver, 116 million pounds of lead, 16.8 million pounds of zinc, 

and 464,000 pounds of copper. 

Extensively mined deposits in the Gibson Hill area, just 

northeast of Breckenridge, are confined to a pervasively altered, 

44-million-year-old granite porphyry (quartz monzonite porphyry, 

to be specific) laccolith, radial dikes surrounding the laccolith, 

and peripheral sills, all of which intrude Cretaceous shale, 

limestone, and sandstone. A laccolith is a flat-bottomed, domeshaped body that intruded between sedimentary beds. Intrusion 

breccia is common. Mineralization is essentially restricted to 

the porphyry, which contains a stockwork of gold-bearing, 

iron oxide–quartz veinlets; it is richest in the upper oxidized 

(“epigene”) zone, which extends as much as 80 feet below the 

surface. Accompanying the quartz are, in decreasing order

of relative abundance, massive, earthy, yellow and orange iron 

oxides and sulfate, fine white mica and, rarely, visible native 

gold. Deeper, unoxidized (“hypogene”) ore includes, in decreasing 

order of abundance, pyrite, sphalerite, and galena. The presence 

of galena was thought to be a particularly favorable indication 

of nearby gold. A cluster of mines on the heavily worked north 

side of Gibson Hill produced at least 70,000 ounces and possibly 

as much as 170,000 ounces of gold.

Figure 28. Gold dredge (red arrow) working up Swan River; view is to the north (downstream). The piles of gravel (placers) are the work of 

this dredge, which had worked down to the Blue River. The small, dilapidated machine (yellow arrow) in front of the larger craft is an old 

steam dredge. The dredges are long gone and the placer deposits now have been mostly smoothed for development. This photograph 

was taken in 1909 by the famous geologist Frederick L. Ransome, who extensively studied the Breckenridge mining district.
Rich Ores of the Blue River Valley Region—Their Geology and Mining History 33

Figure 29. Wellington mine on French Gulch, Breckenridge mining district. Most of the mine dumps shown are from the Oro mine workings; 

the vein underlying the Oro workings lies diagonally up the hill to the right, nearly along the line of dumps. The Wellington mine is located 

near the contact between Dakota Sandstone and Morrison Formation, and the vein extends into adjacent Tertiary porphyritic granite. This 

photo was taken in 1909 by Frederick L. Ransome.

Mines in the Montezuma district produced mainly silver 

and lead from pyrite-galena-sphalerite veins. The Saints John 

mine, discovered in 1865, is one of the oldest silver mines in the

state and is known for the high-grade ruby silver ore mined there. 

Deep-red ruby silver is also called pyrargyrite (Ag3SbS2).

Mines in the Kokomo district on Tenmile Creek exploited 

massive blanket-shaped deposits (“mantos”) of lead, zinc, and 

silver sulfide that replaced limestone beds in the Minturn 

Formation (fig. 10). Minor amounts of gold were also recovered. 

Mining ceased in 1951 and the towns of Kokomo, Recon, 

and several other small settlements are now largely buried 

under the tailings and wasterock of the Climax mine in upper 

Tenmile Creek. 

Several small mines in the Tenmile Range mostly operated 

in the latter part of the 19th and early 20th centuries. They 

produced gold, silver, lead, zinc, and copper from mixed-sulfide 

quartz veins cutting Proterozoic rock, although it has been said 

that in most cases, more money was put into the ground than 

was taken out. The ruins of one of these mines, the Boston 

mine, lie in Mayflower Gulch.

In the Gore Range, prospects or small mines containing 

lead, copper, zinc, and silver in quartz and quartz-carbonate 

veins were exploited during the latter part of the 19th and 

early 20th centuries. The largest of these, the Boss mine on

the eastern side of the Gore Range, was mined for silver-bearing 

galena and sphalerite from east-dipping quartz veins in

Proterozoic gneiss just west of the Blue River normal fault. 

A small but rich deposit of sphalerite and silver-bearing galena 

was mined between 1937 and 1950 at the Four Star mine, 

about 600 feet east of Green Mountain dam, from fracture 

fillings in sandstone of the Dakota Sandstone (fig. 10). This 

mineralization probably accompanied emplacement of the 

early Oligocene intrusive porphyry at Green Mountain.

Economically marginal fluorite or fluorspar (calcium 

fluoride) in a matrix of Dakota Sandstone fragments, called 

the Hammer fluorspar deposits, was mined from brecciated 

sandstone blocks in a landslide deposit about 3 miles northwest 

of Silverthorne. This fluorspar-bearing deposit was probably 

transported a significant distance to the east by landsliding 

from a now-buried, faulted and mineralized sandstone of the 

Dakota Sandstone. 
34 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Climax Molybdenum Deposit

The Climax mine near Fremont Pass, currently operated 

by Freeport-McMoRan Copper and Gold, is reported to be the 

largest, highest-grade, and lowest-cost molybdenum ore body 

in the world. The mine is in Lake County, just south of the 

Summit County line (fig. 2), although extensive tailings ponds 

from the mine lie along the headwaters of Tenmile Creek in 

Summit County. Molybdenum’s many uses include alloys 

with steels to make hardened automotive and jet-turbine parts, 

construction equipment, stainless steel, tool steel, lubricants, 

automobile safety airbags, and an agent used for removing 

sulfur from crude oil. 

The deposit was discovered in 1879 by Charles Senter 

during the Leadville silver boom, but he did not know what 

the silvery-gray, greasy mineral (molybdenite, MoS2

) was 

until a chemist analyzed it in 1895. At that time, molybdenum 

had no commercial value, and it was not until it was discovered 

to harden steel that an industry was born. Commercial production 

began in 1915, and by the close of World War I, Climax was 

producing 90 percent of the world’s molybdenum. The price 

of molybdenum crashed after the war, and the mine shut down 

until 1924, when peace-time uses for molybdenum-hardened 

steel again made mining profitable. By 1959, Climax produced 

35,000 tons/day of ore and was the world’s largest underground 

mine. These underground workings have mostly collapsed 

(purposefully “block-caved” by removing supporting rock) 

into a gigantic “glory hole,” testifying to the extent of these 

underground workings. Starting in about 1972, large-scale 

open-pit mining began, and the benches of that phase can 

be seen above the glory hole on the steep mountainside. For 

obvious reasons, mining at this time was restricted to areas not 

already caved into the glory hole.

Between 1915 and 1987, the Climax mine produced 

421.5 million tons of ore, yielding 1,885 million pounds 

(almost a million tons) of molybdenum. This massive production 

created an enormous problem, however: where was all the mine 

waste to be placed? The solution was to use almost the entire 

upper reach of Tenmile Valley for storing tailings, thereby 

burying almost the entire Kokomo mining district, including the 

small towns of Recon, Kokomo, Robinson, and Carbonateville, 

although most of them were ghost towns by then.

The high price of molybdenum caused many porphyry 

copper mines (in which mineralization was caused by a 

porphyritic granite) to begin producing the metal as a byproduct 

of copper production, which flooded the market and caused 

the price to drop. The Climax mine could not compete, and 

the mine was placed on an inactive status in 1995. However, 

as the price of molybdenum once again increased sufficiently, 

the mine was put back into production in 2012. As of 2014, 

recoverable reserves are estimated at 145 million tons of ore, 

averaging 0.23 percent molybdenum, which amounts to about 

335 million pounds of molybdenum. It is interesting to consider 

that before erosion removed most of the richest ore, the deposit 

may have contained as much as 4 million tons of minable 

molybdenum, making it, as far as is known, the world’s greatest 

deposit of molybdenite.

Information Box 5.

Railroads and Mining

The railroad history of Summit County and its environs is closely tied to the mining history. Transportation into and out 

of Summit County from 1860 to 1880 was by horse and wagon on primitive roads over the high passes, but transport of 

goods and mine products clearly got a huge boost with the advent of railroad service in the early 1880s. Railroads made it 

possible to supply substantial quantities of coal to power the smelters and mills and to ship the ore concentrates from the 

mills to the smelters. The ore was generally concentrated by a mill at or near the mine, but the nearest smelters were in 

Leadville, about 10 miles south of Summit County over Fremont Pass.

Two railroads competed to serve Summit County. The Denver, South Park and Pacific Railroad came from the east over 

Boreas Pass from South Park to Breckenridge. Its rail line went on to Frisco and then up Tenmile Creek to access the 

Kokomo mining district (now buried under mine tailings from the Climax mine in upper Tenmile Creek), and continued to 

Fremont Pass and eventually to Leadville. The other railroad, Denver and Rio Grande Railroad, was built from Leadville 

over Fremont Pass and down Tenmile Creek to the Blue River at Dillon (the original townsite is now under Dillon 

Reservoir), and then east to Keystone; the planned continuation to Montezuma was never completed. 

As mining declined and automobiles and trucks and the roads on which they traveled improved, the railroads were 

abandoned. The Denver and Rio Grande Railroad from Leadville to Dillon was abandoned in 1924, and its roadbed, except 

a stretch above the Climax tailings ponds, is followed by the present highway up Tenmile Creek. Looking across Tenmile 

Creek from the highway, one can see parts of the roadbed of the Denver, South Park, and Pacific Railroad that came from 

Breckenridge. This line was abandoned in 1938 and the section over Boreas Pass is now a gravel road that is closed 

during the winter.
Acknowledgments 35

The deposit is composed of a stockwork of molybdenite 

in small veinlets associated with multiple high-silica granitic 

porphyry stocks and rhyolite dikes containing elevated amounts 

of fluorine, molybdenum, tin, tungsten, uranium, thorium, and 

niobium. The ore occurs in three separate dome-shaped shells, 

each of which is associated with a separate granitic intrusion, 

which collectively form an intrusive complex, emplaced during 

a 9-million-year period from 33 to 24 million years ago. The 

molybdenite stockwork is associated with pervasive deposition 

of potassium feldspar and is just above a zone of massive 

“flooding” of silica-rich fluids that produced much secondary 

quartz. A western, unmined, part of the ore body is dropped 

down several thousand feet below the elevation of the main ore 

zone along the down-on-the-west Mosquito fault (fig. 7), which 

moved mostly during late Oligocene and Miocene crustal 

extension associated with formation of the Rio Grande rift.

Beside molybdenum, the mine has also yielded tin, tungsten, 

copper, rare-earth elements, and pyrite (used in the production 

of sulfuric acid) as byproducts. Sulfides are separated by 

the froth-flotation process, and waste from this process now 

underlies the large tailings ponds along Tenmile Creek. Water 

that flows from the ponds is treated to remove heavy metals 

and reduce acidity before being released into Tenmile Creek. 

A dense network of diversion canals intercepts water from 

all streams draining into the ponds and diverts it around the 

tailings ponds, thus minimizing contamination of downstream 

areas. Efforts are currently underway to reclaim some of the 

tailings by spreading top soil and planting trees. 

Acknowledgments

This booklet was improved tremendously 

by reviews and conversations with Jack 

Reed, Jr., and Ric Page of the U.S. Geological 

Survey. Mary-Margaret Coates further 

improved the booklet with careful editing, 

and Carol Quesenberry brought it to life 

with her artistic layout. The authors wish to 

thank them for their valuable input.

Railway—Photograph by Walter Siegmund—Wikimedia Commons; Creative Commons Attribution–Share Alike 3.0 Unported
36 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Suggested Reading

Age of the Rocks and Uplift History

Bryant, Bruce, Marvin, R.F., Naeser, C.W., and Mehnert, H.H., 1981, Ages of igneous rocks in 

the South Park–Breckenridge region, Colorado, and their relation to the tectonic history of 

the Front Range uplift, Chapter C, in Shorter contributions to isotope research in the western 

United States, 1980: U.S. Geological Survey Professional Paper 1199, p. 15–26.

Kellogg, K.S., Shroba, R.R., Premo, W.R., and Bryant, Bruce, 2011, Geologic map of the eastern 

half of the Vail 30 ́ x 60 ́ quadrangle, Eagle, Summit, and Grand Counties, Colorado:

U.S. Geological Survey Scientific Investigations Map 3170, scale 1:100,000, with 49 p. pamphlet. 

Naeser, C.W., Bryant, B., Kunk, M.J., Kellogg, K.S., Donelick, R.A., and Perry, W.J., Jr., 2002, 

Tertiary cooling and tectonic history of the White River uplift, Gore Range, and western 

Front Range, central Colorado—Evidence from fission-track and 40Ar/39Ar ages, in Kirkham, 

R.M., Scott, R.B., and Judkins, T.W., eds., Late Cenozoic evaporite tectonism and volcanism 

in west-central Colorado: Geological Society of America Special Paper 366, p. 31–53.

U.S. Geological Survey Geologic Names Committee, 2010, Divisions of geologic time—Major 

chronostratigraphic and geochronologic units: U.S. Geological Survey Fact Sheet 2010–3059, 2 p.

The Laramide Orogeny

Tweto, Ogden, 1975, Laramide (Late Cretaceous–early Tertiary) orogeny in the Southern 

Rocky Mountains, in Curtis, Bruce, ed., Cenozoic history of the Southern Rocky Mountains: 

Geological Society of America Memoir 144, p. 1–44. 

Ore Deposits

Bookstrom, A.A., 1990, Igneous rocks and carbonate-hosted ore deposits of the central Colorado 

Mineral Belt, in Beatty, D.W., Landis, G.P., and Thompson, T.B., eds., Carbonate-hosted sulfide 

deposits of the central Colorado Mineral Belt: Economic Geology Monograph 7, p. 45–65.

Lovering, T.S., 1934, Geology and ore deposits of the Breckenridge mining district, Colorado: 

U.S. Geological Survey Professional Paper 176, 64 p.

Lovering, T.S., 1935, Geology and ore deposits of the Montezuma quadrangle, Colorado: 

U.S. Geological Survey Professional Paper 178, 119 p.

Parker, B.H., Jr., 1974, Gold placers of Colorado (Book 1): Quarterly of the Colorado School 

of Mines, v. 69, no. 3, 268 p.

Ransome, F.L., 1911, Geology and ore deposits of the Breckenridge district, Colorado: 

U.S. Geological Survey Professional Paper 75, 187 p.

Singewald, Q.D., 1951, Geology and ore deposits of the upper Blue River area, Summit County, 

Colorado: U.S. Geological Survey Bulletin 970, 74 p.

Wallace, S.R., and Snow, G.G., 1980, The porphyry molybdenite deposit at Climax, Colorado, 

in Bryant, Bruce, and Beaty, D.W., eds., Mineral deposits and geology of central Colorado: 

American Geophysical Union field trip guidebook T129, p. 38–44. 
Suggested Reading 37

Precambrian History

Reed, J.C., Jr., Bickford, M.E., and Tweto, Ogden, 1993, Proterozoic accretionary terranes 

of Colorado and southern Wyoming, in Van Schmus, W.R., and Bickford, M.E., eds., 

Transcontinental Proterozoic provinces, in Reed, J.C., Jr., and six others, eds., Precambrian 

conterminous U.S.: Geological Society of America, The geology of North America, v. C-2, 

p. 211–228.

Tweto, Ogden, 1987, Rock units of the Precambrian basement in Colorado: U.S. Geological 

Survey Professional Paper 1321–A, 54 p.

Quaternary History

Chadwick, O.A., Hall, R.D., and Phillips, F.M., 1997, Chronology of Pleistocene glacial advances 

in the central Rocky Mountains: Geological Society of America Bulletin, v. 109, p. 1443–1452. 

Kellogg, K.S., 2001, Tectonic controls on a large landslide complex—Williams Fork Mountains 

near Dillon, Colorado: Geomorphology, v. 41, p. 355–368.

Varnes, D.J., Radbruch-Hall, D.H., and Savage, W.Z., 1989, Topographic and structural conditions 

in areas of gravitational spreading of ridges in the western United States: U.S. Geological 

Survey Professional Paper 1496, 28 p.

West, M.V., 1978, Quaternary geology and reported surface faulting along east flank of Gore Range, 

Summit County, Colorado: Quarterly of the Colorado School of Mines, v. 73, no. 2, 66 p.

Railroad History

Ormes, Robert, 1975, Tracking ghost railroads in Colorado: Colorado Springs, Colorado, 

Century One Press, 148 p.

Regional Geology 

(including maps; some references also include discussion of ore deposits) 

Bergendahl, M.H., 1963, Geology of the northern part of the Tenmile Range, Summit County, 

Colorado: U.S. Geological Survey Bulletin 1162–D, scale 1:24,000, 19 p.

Chronic, Halka, and Williams, Felicie, 2002, Roadside geology of Colorado: Missoula, Montana, 

Mountain Press Publishing Company, 399 p.

Kellogg, K.S., Bartos, P.J., and Williams, C.L., 2002, Geologic map of the Frisco quadrangle, 

Summit County, Colorado: U.S. Geological Survey Miscellaneous Field Studies Map 

MF–2340, scale 1:24,000.

Kellogg, K.S., Shroba, R.R., Bryant, Bruce, and Premo, W.R., 2008, Geologic map of the 

Denver West 30′ × 60′ quadrangle, Colorado: U.S. Geological Survey Scientific Investigations 

Map 3000, scale 1:100,000.

Kellogg, K.S., Shroba, R.R., Premo, W.R., and Bryant, Bruce, 2011, Geologic map of the eastern 

half of the Vail 30′ × 60′ quadrangle, Eagle, Summit, and Grand Counties, Colorado: 

U.S. Geological Survey Scientific Investigations Map 3170, scale 1:100,000.

Matthews, Vince, Landon, Susan, and KellerLynn, Katie, eds., 2003, Messages in stone—

Colorado’s colorful geology: Colorado Geological Survey, 157 p.
38 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

Tweto, Ogden, Bryant, Bruce, and Williams, F.E., 1970, Mineral resources of the Gore Range–

Eagles Nest Primitive Area and vicinity, Summit and Eagle Counties, Colorado: 

U.S. Geological Survey Bulletin 1319–C, 127 p. 

Tweto, Ogden, Moench, R.H., and Reed, J.C., Jr., 1978, Geologic map of the Leadville 1° × 2° 

quadrangle, northeastern Colorado: U.S. Geological Survey Miscellaneous Investigations 

Series Map I–999, scale 1:250,000.

Wallace, C.A., Keller, J.W., McCalpin, J.P., Bartos, P.J., Route, E.E., Jones, N.N., Gutierrez, 

Francisco, Williams, C.L., and Morgan, M.L., 2003, Geologic map of the Breckenridge 

quadrangle, Summit and Park Counties, Colorado: Colorado Geological Survey Open-File 

Report 02–7, scale 1:24,000. (Contains a particularly good overview of mineral deposits in 

the area.)

Widmann, B.L., Bartos, P.J., McCalpin, and Jackson, Jeffrey, 2004, Geologic map of the 

Copper Mountain quadrangle, Summit, Eagle, Lake, and Park Counties, Colorado: Colorado 

Geological Survey Open-File Report 03–20, scale 1:24,000.

Rio Grande Rift

Chapin, C.E., and Cather, S.M., 1994, Tectonic setting of the axial basins of the northern and 

central Rio Grande rift, in Keller, G.R., and Cather, S.M., eds., Basins of the Rio Grande 

rift—Structure, stratigraphy, and tectonic setting: Geological Society of America Special 

Paper 291, p. 5–25.

Kellogg, K.S., 1999, Neogene basins of the northern Rio Grande rift—Partitioning and asymmetry 

inherited from Laramide and older uplifts: Tectonophysics, v. 305, p. 141–152.

Tweto, Ogden, 1979, The Rio Grande rift system in Colorado, in Riecker, R.E., ed., Rio Grande 

rift—Tectonics and magmatism: Washington, D.C., American Geophysical Union, p. 33–56.

Seismic History

Spence, William, Langer, C.J., and Choy, G.L., 1996, Rare, large earthquakes at the Laramide 

deformation front, Colorado (1882) and Wyoming (1984): Bulletin of the Seismological 

Society of America, v. 86, p. 1804–1819.
Glossary 39

Glossary

A

alluvial fan A cone- or fan-shaped deposit composed of sediments deposited by debris flows 

and streams where a smaller (tributary) valley adjoins a more level surface in a larger valley.

aluminosilicate A mineral in which aluminum substitutes for silica in part of the crystal structure.

ammonite A member of an extinct group of cephalopods, related to squids, octopuses, and 

chambered nautili that have a thick, intricately ornamented, segmented shell. Ammonites died 

out at the end of the Paleozoic Era and their fossil remains are valuable for identifying the age 

of the rocks in which they occur.

amphibole Group of generally dark (commonly black) aluminosilicate minerals that has a 

prismatic, elongate form; various proportions of iron and magnesium produce the dark color. 

amphibolite A black or dark-gray metamorphic rock composed almost entirely of amphibole 

(variety hornblende) and plagioclase; amphibole is greater than 50 percent of rock; commonly 

interpreted as metamorphosed basalt or andesite.

anticline A convex-upward (arch-shaped) fold in layered rocks, such that the oldest rocks are 

in the center of the fold.

apatite fission-track age Age determined from counting density of fission tracks in a highly 

polished slide containing the mineral apatite (CaPO4). Fission tracks are produced by high-speed 

particles released from the spontaneous decay of 238U. The tracks generally are retained below 

about 100 °C (and are annealed, or disappear, above this temperature), so the calculated age 

gives the time at which the rock containing the apatite cooled through 100 °C. This calculated 

age is commonly used to reconstruct the rock unit’s uplift history, because rock at depth, which 

is hot owing to the geothermal gradient in the Earth’s crust, cools as an area is uplifted and 

overlying rock is removed by erosion. 

andesite Fine-grained, dark-colored extrusive rock, commonly containing phenocrysts of sodic 

plagioclase; slightly more silicic than a basalt and less silicic than a dacite. 

ash-flow tuff A generally hard, fine-grained, rock formed from an incandescent cloud erupted 

from a volcano; sometimes called an ignimbrite.

B

basalt Black, fine-grained volcanic rock, commonly containing phenocrysts of hornblende, 

pyroxene, and olivine, but little or no quartz.

basement The complex of igneous and metamorphic rocks that underlies sedimentary rocks 

and deposits or is exposed at the surface.

batholith An igneous intrusion that has at least 40 square miles of surface exposure and no 

known floor.

biotite Platy black aluminosilicate mineral, a type of mica, common in igneous and metamorphic 

rocks; contains potassium and various proportions of iron and magnesium; common mineral in 

biotite gneiss.

breccia A rock composed of angular fragments usually bound together by a fine-grained 

matrix; may be formed in a fault zone or a debris flow.
40 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

C

calcareous Said of a rock or deposit that contains calcium carbonate.

calc-silicate gneiss A metamorphic rock consisting mainly of calcium-bearing silicate minerals 

and formed by metamorphism of impure limestone or dolomite.

cephalopod A marine mollusk belonging to the class Cephalopoda; includes octopuses, 

squids, chambered nautili, and extinct ammonites.

clastic Pertaining to a rock or sediment composed principally of broken or eroded fragments 

(clasts) that are derived from preexisting rocks that have been transported some distance from 

their place of origin; sandstone and conglomerate are examples.

cross section Diagram of a slice through the Earth that shows subsurface geology.

D

dacite Generally gray, fine-grained, extrusive or intrusive igneous rock, commonly containing 

phenocrysts of plagioclase and quartz; fine-grained equivalent of granodiorite.

dike A tabular igneous intrusion that cuts across surrounding rock.

dip The angle that a sloping surface makes with the horizontal; measured perpendicular to the 

strike of the surface. 

dolomite A carbonate sedimentary rock resembling limestone containing the mineral dolomite 

(CaMg(CO3)2); the rock commonly forms by addition of magnesium to existing limestone. 

F

feldspar Common rock-forming aluminosilicate mineral with wide compositional range; two 

major groups are plagioclase [sodium-calcium feldspar (Na,Ca)Al(Si,Al)Si2O8)] and potassium 

feldspar (KAlSi3O8).

felsic Refers to an igneous rock containing an abundance of light-colored minerals; an adjective 

derived from shortened forms of “feldspar” plus “silica.”

fluorite Calcium fluoride (CaF2), a translucent mineral commonly found in veins associated 

with ores of lead, tin, and zinc; usually purple or blue.

fluvial An adjective pertaining to a river or stream.

foliation A general term for a planar arrangement of textural or structural features, such as 

mineral grains, in a rock; usually refers to the layering in a metamorphic or igneous rock.

froth-flotation process A method of separating sulfide minerals, such as galena, sphalerite, 

molybdenite, and pyrite from waste rock. Ground ore is placed in a vat with 1 percent pine oil 

or turpentine and water. Forced air forms an oily froth which adheres to the sulfides (but not 

the silicate minerals) when the mixture is agitated. The sulfide-rich froth is skimmed off, the 

sulfides are separated, and the pine oil or turpentine is recycled.
Glossary 41

G

gabbro Dark colored, medium- to coarse-grained intrusive rock containing calcic plagioclase; 

black, iron-magnesium minerals; and little or no quartz; coarse-grained equivalent of basalt. 

glory hole A large open pit from which ore is being or has been extracted; some glory holes 

form by purposeful collapse (block caving) of underground mine workings, such as at Climax.

gneiss A foliated metamorphic rock in which minerals are aligned in response to stress during 

regional metamorphism. 

graben Elongate, down-dropped block bounded by normal faults, produced during crustal 

extension; a rift valley. A half-graben is bounded only on one side by a fault or faults, so it tilts 

into the bounding fault (or faults), such as in the Blue River half-graben.

granite A medium- to coarse-grained intrusive or plutonic rock containing approximately 

equal amounts of plagioclase and potassium feldspar and greater than 20 percent quartz. In 

most cases in the text, “granite” refers to a rock that contains a plagioclase/potassium-feldspar 

ratio of 35 to 65 percent (a monzogranite).

gypsum Hydrated calcium sulfate (CaSO4·2H2O); an evaporite mineral.

H

halite Common salt (NaCl); an evaporite mineral.

hornblende A black amphibole mineral, common in dark igneous and metamorphic rocks 

such as basalt, gabbro, or amphibolite. 

I

igneous rock A rock formed by the solidification of a magma, such as basalt or granite.

intrusion A body of igneous rock (batholith, stock, or dike) that penetrates Earth’s crust.

intrusion breccia A mixture of angular fragments of surrounding rocks that has been forcibly 

injected into a preexisting zone of weakness; may include an igneous component in the matrix 

and is generally very altered to clays and other minerals.

isotope One of two or more species of the same chemical element, differing only in the number 

of neutrons in the nucleus; isotopes of the same element have the same number of protons, 

but have slightly different physical and chemical qualities owing to their mass differences, by 

which they can be separated.

L 

laccolith A dome-shaped igneous body, generally with a flat floor that intrudes parallel 

(concordant) to preexisting layers or beds; less than 5 miles in diameter.

Laramide A period of mountain building and igneous activity in western North America from 

the Late Cretaceous to the early Eocene (about 70–50 million years ago); the Laramide orogeny.

latite A porphyritic extrusive rock containing roughly equal amounts of plagioclase and 

potassium feldspar, little or no quartz, and a fine-grained to glassy matrix.

limestone A sedimentary rock composed mostly or entirely of the fine-grained mineral 

calcite (CaCO3).
42 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

M

magma Naturally occurring molten rock material, generated within the Earth, capable of 

extrusion (such as a lava) or of intrusion. It solidifies into an igneous rock that may or may not 

contain crystals and rock fragments.

manto A flat-lying, bedded ore deposit, generally following a sedimentary bed.

metamorphic rock A rock that has recrystallized owing to a variable combination of heat and 

pressure (commonly induced by deep burial or proximity to an igneous intrusion). For example, 

shale may metamorphose into a schist or gneiss. 

metasedimentary A metamorphic rock derived from metamorphism of a sedimentary rock.

migmatite A rock containing interlayered metamorphic and igneous phases; the igneous phase 

is commonly granitic in texture and composition and may be produced by in-place partial melting 

of the metamorphic phase or by injection from elsewhere.

moraine A mound, ridge, or other distinct accumulation of glacial debris (till) left after melting 

of a glacier.

mylonite A very fine grained, hard, banded or streaky rock produced by intense shearing.

N

normal fault An inclined fault produced by extension in the Earth’s crust, such that the rocks 

above the fault plane move down the fault, resulting in younger rocks overlying older rocks 

across the fault. The Blue River normal fault is an example.

O

orogeny A period of crustal deformation; a mountain-building event.

P

pegmatite A very coarse-grained intrusive igneous rock usually found in dikes and pods in or 

near the margins of batholiths (large bodies of granitic rock); generally has the composition of 

a granite (roughly equal amounts of plagioclase, potassium feldspar, and quartz, with abundant 

mica); some feldspar or quartz crystals may be longer than one foot.

periglacial Processes or conditions that are active near margins of glaciers or in regions subject 

to intense freeze-thaw processes, such as alpine environments. Periglacial deposits commonly 

contain angular blocks derived from underlying bedrock and include block fields and bouldery 

stone stripes on hillsides formed by downslope creep. 

plagioclase A white to gray, common rock-forming mineral; sodium-calcium aluminosilicate 

[(Na,Ca)Al(Si,Al)Si2O8].

phenocryst A visible crystal in an igneous rock, surrounded by a finer-grained matrix. 

pluton A body of igneous rock that formed beneath the Earth’s surface from consolidation of 

a magma.

porphyry An igneous rock containing phenocrysts in a finer-grained matrix.

potassium feldspar A light-pink to white, common rock-forming mineral; potassium 

aluminosilicate (KAlSi3O8); a major constituent of granitic rocks. 

pyroxene A black, equant, iron-magnesium aluminosilicate mineral, common in dark igneous 

rocks such as basalt and gabbro.
Glossary 43

Q

quartz Silicon dioxide (SiO2); one of the most common rock-forming minerals.

quartzite A granular metamorphic rock composed entirely of quartz grains; informally, also 

refers to a hard quartz sandstone in which quartz grains are cemented by silica (SiO2).

quartz monzonite A medium- to coarse-grained intrusive rock containing approximately equal 

amounts of plagioclase and potassium feldspar and less than 20 percent quartz; informally, a 

relatively low-quartz granite.

R

relief Refers to the physical shape or unevenness of the Earth’s surface. A low-rolling or flat 

region is a surface of low relief; a mountainous or deeply incised region is a surface of high relief.

reverse fault A fault with a plane that slopes (dips) greater than 45° toward the block that has 

been relatively raised; indicative of crustal shortening.

rhyolite A fine-grained, generally porphyritic igneous rock, commonly containing phenocrysts 

of quartz and potassium feldspar (generally a variety called sanidine). May originate either 

from volcanic activity or in shallow intrusive bodies.

S

sackung A deep-seated, trench-like fissure produced by gravitational spreading of a mountain 

range or ridge. Forms uphill-facing scarps and small ridge-top grabens.

sanidine A variety of high-temperature, clear potassium feldspar commonly found in volcanic 

rocks such as trachyte and rhyolite. 

scarp A steep slope or zone produced by erosion, sliding, or faulting; the head of a younger 

landslide is commonly marked by a scarp.

schist A strongly foliated, commonly layered, crystalline rock, formed by regional dynamic 

metamorphism (metamorphism accompanied by shearing); rich in mica, so that it can be split 

into thin flakes or slabs.

sedimentary rock A rock, such as sandstone, shale, or limestone, that has resulted from the 

consolidation or cementation of loose sediment.

sill An igneous intrusion injected parallel to the layers of existing rock, such as sedimentary 

beds, producing an approximate tabular form.

stock An igneous intrusion that is less than about 60 square miles in area, and generally 

crosscuts (is discordant to) the surrounding rock. Smaller than a batholith, which is greater than 

60 square miles in area.

stockwork A mineral deposit composed of a three-dimensional network of closely spaced 

veins that are dense enough that the entire mass can be mined.

strike The direction or trend of a horizontal line contained in a sloping surface; perpendicular 

to the dip of the surface.

strike-slip fault A steep fault in which the two sides move horizontally with respect to each 

other. 

surficial deposit Unconsolidated material, mostly Quaternary in age, at or near the Earth’s 

surface, such as alluvium, glacial till, and landslide deposits. 

syncline A concave-upward (trough-shaped) fold in layered rocks, such that the youngest 

rocks are in the center of the fold.
44 Mountains, Glaciers, and Mines: The Geological Story of the Blue River Valley, Colorado

T

thrust fault A fault with a plane that slopes (dips) less than 45°; produced during crustal 

shortening such that older rocks are generally shoved above younger rocks. The Williams 

Range thrust fault is an example.

till Unstratified, unsorted (heterogeneous fragments of many sizes), typically bouldery deposit; 

composed of fragments at one time entrained in glacial ice, but now commonly composed of 

fragments preserved in moraines after the glacier melted. 

trachyte A fine-grained, generally porphyritic igneous rock containing potassium feldspar 

(sanidine), minor mafic minerals (biotite, hornblende, or pyroxene), and little or no quartz.

tuff A general term for all consolidated pyroclastic rocks (formed from an incandescent cloud 

of volcanic material). May show postconsolidation flow bands and contain commonly flattened 

fragments of pumice (frothy volcanic rock).

Summit County—Photograph by Aaron Raufman—Wikimedia Commons; 

Creative Commons Attribution–Share Alike 3.0 Unported


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Edited by Mary-Margaret Coates 

Illustrations by Mary Berger

Layout and graphic design by Carol A. Quesenberry

For more information concerning this publication, contact:

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Box 25046, Mail Stop 980

Denver, CO 80225

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Or visit the Geosciences and Environmental Change Science Center Web site at:

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This publication is available online at:

http://dx.doi.org/10.3133/cir1400
Kellogg and others—Mountains, Glaciers, and Mines—The Geological Story of the Blue River Valley, Colorado, and Its Surrounding Mountains—Circular 1400

ISSN 1067-084X (print)

ISSN 2330-5703 (online)

http://dx.doi.org/10.3133/cir1400