The Geologic Story of Colorado’s Sangre de Cristo Range

U.S. Department of the Interior

U.S. Geological Survey

Circular 1349
Cover shows a landscape carved by glaciers. Front cover, Crestone Peak on left and the three summits of Kit Carson Mountain on right. 

Back cover, Humboldt Peak on left and Crestone Needle on right. Photograph by the author looking south from Mt. Adams.
The Geologic Story of Colorado’s

Sangre de Cristo Range

By David A. Lindsey

A description of the rocks and landscapes of the Sangre de Cristo Range

and the forces that formed them.

Circular 1349

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

This and other USGS information products are available at http://store.usgs.gov/

U.S. Geological Survey

Box 25286, Denver Federal Center

Denver, CO 80225

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1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Lindsey, D.A., 2010, The geologic story of Colorado’s Sangre de Cristo Range: U.S. Geological Survey Circular 1349, 

14 p.
iii

Contents

The Oldest Rocks ...........................................................................................................................................1

Uplifts and Unconformities...........................................................................................................................2

A Map of the Range.......................................................................................................................................4

The Ancestral Rockies..................................................................................................................................6

The Laramide Orogeny..................................................................................................................................8

The Rio Grande Rift......................................................................................................................................10

The Ice Ages.................................................................................................................................................13

References....................................................................................................................................................14

Acknowledgments.......................................................................................................................................14

Figures

1. Precambrian rocks south of Crestone: (A) layered gneiss; (B) pink granite; 

(C) dark igneous rock (gabbro) with granitic veins.................................................................1

2. Uplifts and unconformities. Rocks of early Paleozoic age lap onto an old highland 

to the south and were partly dissolved and eroded before rocks of late Paleozoic 

age were deposited......................................................................................................................3

3. Geologic map of Colorado’s Sangre de Cristo Range.............................................................4

4. The Ancestral Rockies. Map and photographs of Crestone Conglomerate.......................6

5. At Marble Mountain, fossil mounds called bioherms in limestone of the Minturn

Formation have been thrust over the younger Sangre de Cristo Formation along the 

Marble Mountain thrust fault ........................................................................................................7

6. A redbed under the microscope, showing iron oxide ............................................................8

7. The Laramide orogeny. Photographs of folded and faulted rocks of Pennsylvanian 

and Permian age and section showing structure...................................................................9

8. The Rio Grande rift. Map and photographs of the Sangre de Cristo fault in the San

Luis Valley ....................................................................................................................................10

9. The ice ages. Aspen cover a lateral moraine in Black Canyon, north of Valley View 

Hot Springs ..................................................................................................................................13
iv
By David A. Lindsey

The Oldest Rocks

The oldest rocks are like those in other Colorado mountains—Precambrian gneiss from 1.8–1.7 billion years ago. 

Gneiss (layered metamorphic rock formed by heat and pressure—fig. 1A) makes up much of the range east of the Great 

Sand Dunes. Once the gneiss was sedimentary or igneous rock, 

but it rarely retains the original texture of these rocks. Chemical and isotopic composition is the only guide to the origin 

of gneiss. Much of the light-colored gneiss on Blanca Peak is 

thought to have formed by metamorphism of a large igneous 

intrusion (called a “pluton”). Plutons form when large balloonshaped bodies of magma rise into the middle and upper levels 

of the Earth’s crust and cool. About 1.7–1.4 billion years ago, 

after the first episodes of metamorphism and igneous intrusion had ended, four more plutons invaded the crust; they are 

exposed along the west side of the range north and south of 

Crestone, and in the headwaters of Sand Creek west of Marble 

Mountain. Although these intrusions all look like granite (fig. 

1B), their mineral composition actually varies; geologists 

assign specific names to them, names like monzogranite and 

quartz monzonite. They determine these names by identifying 

and estimating the amount of each mineral in the rock. Besides 

the large granite plutons, dikes of gabbro (a heavy, dark igneous rock composed of iron and magnesium minerals—fig. 1C) 

and pegmatite (a coarse-grained version of granite) intrude both 

gneiss and younger plutons.

The forces of metamorphism and intrusion come from 

plate tectonics, the process by which continent-sized slabs 

of the Earth’s crust break apart by rifting and regroup by 

plate collision. During Precambrian time the old core of 

the continent lay to the north, in Wyoming. Sea-floor plates 

from the south—some with volcanic island chains—collided 

*The Sangre de Cristo Mountains extend from near Santa Fe, New Mexico, to Salida, Colorado. In Colorado and northernmost New Mexico, the name 

“Culebra Range” refers to the Sangre de Cristo Mountains south of La Veta Pass, and the name “Sangre de Cristo Range” refers to the mountains north of the 

pass. This document discusses the Sangre de Cristo Range north of the pass.

Figure 1. Precambrian rocks south of Crestone: (A) layered gneiss; 

(B) pink granite; (C). dark igneous rock (gabbro) with granitic veins. 

Picks (12 inches in length) and pen (6 inches) show scale.

There is no record of the beginning of time 

in the Sangre de Cristo Range*. Almost 3 billion 

years of Earth history are missing, but the rest is 

on spectacular display in this rugged mountain 

landscape. This is the geologic story of the Sangre 

de Cristo Range.

The Geologic Story of Colorado’s Sangre de Cristo Range 

A

B

C
2 The Geologic Story of Colorado’s Sangre de Cristo Range

with this old continent. Sea-floor plates usually are heavier 

than continents and are swept beneath them (a process called 

“subduction”). However, some plate fragments are torn away 

and plastered against the continent—this is how continents 

grow. Rocks in the subducted plate melt and plumes of molten 

rock rise through the newly enlarged continent, solidifying 

below the surface as plutons or erupting as volcanoes. Today 

the results of this process of ancient continental growth are 

exposed as gneiss and granite in mountain uplifts like the 

Sangre de Cristo Range.

Uplifts and Unconformities

At many places in the world, including the Sangre de 

Cristo Range, a great gap in time separates younger rocks from 

those of Precambrian age. We recognize time gaps where rock 

layers that should be present—because they are seen elsewhere—are missing. Geologists call these horizons of missing rocks “unconformities.” Unconformities are important to 

understanding Earth history; they are strong evidence for uplift 

and erosion (see fig. 2).

If you look up the valleys on the west side of the range 

from Crestone north, you will see large cliffs of gray Mississippian age limestone—this is the Leadville Limestone, deposited 

from about 360 to 326 million years ago. The Leadville Limestone is underlain by less obvious, thinner formations of dolomite and sandstone of Ordovician and Devonian age, deposited 

between 490 and 360 million years ago. At the bottom of this 

sequence are the Ordovician formations, oldest to youngest: the 

Manitou Dolomite, Harding Sandstone, and Fremont Dolomite, 

and the Devonian Chaffee Formation (dolomite and sandstone). 

At different places, both the Manitou and Harding lie unconformably on Precambrian rocks. This unconformity represents 1 

billion years of Earth history missing from the Sangre de Cristo 

Range; all of the evidence for this history is gone. Whatever 

rocks formed in this place, whether igneous or sedimentary, 

were removed by erosion before the Ordovician seas covered it. 

By Ordovician time, a broad highland stood above the Ordovician sea over what is now southern Colorado. The Ordovician 

sea slowly encroached on the uplift, producing an onlapping 

sequence of successively younger formations. By Mississippian time, when the Leadville Limestone was deposited, the sea 

probably covered the entire highland. In fact, we find marine 

limestone of this age throughout the Rocky Mountains.

Then sea level fell, and a second unconformity began 

to form. Land displaced shallow seas; rocks were exposed 

to weathering and erosion. In the warm, humid climate, 

limestone dissolved; caves and caverns formed. The Leadville Limestone collapsed to form a jumble of sharp rock 

fragments we call “karst,” a German name for a region in 

Slovenia, where water has dissolved much of the landscape. 

Weathering and erosion stripped Paleozoic rocks from a 

wide area of southern Colorado. No rocks of Ordovician to 

Mississippian age are found in the Sangre de Cristo Mountains south of the Great Sand Dunes. On the highland, the 

two unconformities merge into one, indivisible break in the 

rock record. To the south (for example, where highway 160 

crosses the mountains at La Veta Pass), rocks of Pennsylvanian age, deposited about 320–300 million years ago, lie 

directly on Precambrian rocks.
Uplifts and Unconformities 3

Leadville Limestone

Late Paleozoic (Pennsylvanian) rocks

Precambrian rocks

Harding Sandstone

Fremont Dolomite

Manitou Dolomite

ORDOVICIAN UPLIFT

ORDOVICIAN BASIN

Hayden Pass Great Sand Dunes

100 MILES

160 KILOMETERS

Crestone La Veta Pass CO-NM Boundary

North South

Chaffee Formation

karst breccia

karst breccia

Pre-Ordovician unconformity

End-Mississippian unconformity

METERS

300

FEET

1,000

0

0

0

Figure 2. Uplifts and unconformities. Rocks of early Paleozoic age lap onto an old highland to the south and were partly 

dissolved and eroded before rocks of late Paleozoic age were deposited.
4 The Geologic Story of Colorado’s Sangre de Cristo Range

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 Peaks

RATON

BASIN

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WESTCLIFFE FAULT

SANGRE DE CRISTO FAULT

SAN LUIS

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37°30' La Veta

Westcliffe

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MOUNTAINS

SANGRE

DE

CRISTO

RANGE

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Map compiled by T. Klein, E. DeWitt, and C. San Juan, U.S. Geological Survey. 

0

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20 30 KILOMETERS

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A Map of the Range

Understanding geology begins with a map. The geology shown in 

figure 3 is based on detailed maps made over many years, then combined in digital form to make an up-to-date geological database. 

Geologic features were selected from the database to produce the 

simplified map shown here. The map shows the distribution of 

rocks, faults, and folds. Rocks have been grouped together by age 

and origin; for example, rocks of late Paleozoic age, deposited 

when the Ancestral Rockies were uplifted and eroded 320–250 

million years ago, are grouped together.

Faults, where rocks have broken and moved past one 

another, are shown by heavy lines. Faults formed by 

compression of the Earth’s crust are called thrust faults; 

on figure 3, they are marked by a “T” on the upthrown 

side. Faults formed by extension are called normal 

faults; they are marked by a ball and bar on the 

downthrown side. Fold trends are shown by thin 

black lines; anticlines, where rocks are bowed up 

like an upside-down “U,” are marked by opposing arrows across the trendline; synclines, 

where rocks are bowed down like a “U,” are 

marked by converging arrows. An arrow on 

the trendline shows the plunge direction 

of the fold.
A Map of the Range 5

Oldest rocks: Island arcs collide with 

 ancient continent to north, rocks meta-

 morphosed to gneiss; granite plutons 

 invade the crust

Uplifts and unconformities: Paleozoic 

 seas cover the land after erosion 

 removes older rocks; Paleozoic rocks 

 exposed to weathering and erosion

Ancestral Rockies: Gondwana collides 

 with Laurentia; Central Colorado trough 

 subsides, fills with sediment; redbeds form

Laramide orogeny: Plate collision 

 uplifts and compresses the crust; Raton 

 basin subsides, fills with sediment

Rio Grande rift: Sangre de Cristo 

 Mountains rise; Wet Mountain and 

 San Luis Valleys subside

Volcanic eruptions fill paleovalleys

Permian

Pennsylvanian

Mississippian

Devonian

Ordovician

Cambrian

Silurian

MESOZOIC

LATE

EARLY

PALEOZOIC

PRECAMBRIAN

CENOZOIC Tertiary

Quaternary

Jurassic

Cretaceous

Triassic

Ice ages: glaciers carve the present mountains

CORRELATION OF MAP UNITS AND GEOLOGIC HISTORY

0

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MILLIONS OF YEARS

Figure 3 (pages 4 and 5). Geologic map of Colorado’s Sangre de Cristo Range. BP, Blanca Peak; CP, Cottonwood Peak; CN, Crestone Needle; 

EM, Eureka Mountain; GP, Gibson Peak; HP, Hayden Pass; LVP, La Veta Pass; M, Mt. Marcy; MA, Mt. Adams; MM, Marble Mountain; MP, Mosca 

Pass; TP, Twin Peaks; VS, Valley View Hot Springs.

: Normal fault, ball and bar

 on downthrown side

F Anticline, arrow shows

 direction of plunge

Thrust or reverse fault,

 “T” on upper plate,

 dotted where concealed

T

Late Precambrian and early Paleozoic

Intrusives in Wet Mountains

Precambrian

Gneiss

Granite

Syncline, arrow shows

 direction of plunge

EXPLANATION

Quaternary

Alluvium

Alluvial fans

Windblown sand

Glacial deposits

Tertiary

Intrusives

Sediments

Volcanics

Mesozoic

Sedimentary rocks

Paleozoic sedimentary rocks

Late Paleozoic

Early Paleozoic
6 The Geologic Story of Colorado’s Sangre de Cristo Range

Figure 4 (pages 6 and 7). The Ancestral Rockies. (A) A map 

of the Western United States, showing the Ancestral Rocky 

Mountains about 300 million years ago. From Lindsey and others, 

1986. Colorado was near the equator at that time; however, 

it is shown at its present position. Deposits in the Crestone 

Conglomerate, formed by erosion of the Ancestral Rockies: (B) 

stream-rounded pebbles and structures formed by flowing water 

in sandstone (pick 12 inches in length for scale); (C) cobbles and 

boulders in poorly sorted debris flows; and (D) mudflows (top half 

of picture) and stream deposits (bottom half) in cliffs southwest of 

Eureka Mountain.

Today’s location

Sangre de Cristo Range

110° 100° 90°

40°

30°

EPICONTINENTAL SEA

Apishipa

highland

Front Range

highland

PALEOZOIC MOUNTAINS

10° S

EQUATOR

0

0

KILOMETERS 500

MILES 300

EXPLANATION

Highlands and uplifts

Deep basins

Coarse sand

Shallow seas

Salt basins

Uncompahgre highland Central Colorado Trough

Wichita uplift

A

The Ancestral Rockies

The first Rocky Mountains, called the “Ancestral Rockies,” began to rise about 320 million years ago during Pennsylvanian time (fig. 4A). Like the present Rocky Mountains, 

deep fault-bounded basins separated individual ranges of the 

Ancestral Rockies but, unlike the present Rockies, these basins 

were filled with shallow seas. Geologists continue to speculate 

about how the Ancestral Rockies formed, whether by compression and thrusting of the crust or by strike-slip faulting, or some 

combination of these, perhaps when two large ancient continents, Laurentia (North America and Europe combined) and 

Gondwana (South America and Africa combined) collided.

However they formed, the Ancestral Rockies were largely 

worn down by erosion by the end of Permian time, about 

250 million years ago. West of the present Sangre de Cristo 

Range, an uplift called the Uncompahgre highland shed silt, 

sand, and coarse gravel (figs. 4B, C, D) into a basin called the 

central Colorado trough. The sedimentary fill of the trough is 

exposed throughout central Colorado today. In the Sangre de 

Cristo Range, north of the Great Sand Dunes, the sedimentary 

fill reached more than 2 miles thick. Geologists divide this fill 

into the Pennsylvanian Minturn and the Pennsylvanian and 

Permian Sangre de Cristo Formations; today it makes up most 

of the backbone of the range. The Crestone Conglomerate 

Member of the Sangre de Cristo Formation, which makes up 

Crestone Needle and many peaks in the central part of the 

range, was deposited nearest the uplift. It is so coarse that it 

contains boulders as much as 6 feet in diameter.

B

C
The Ancestral Rockies 7

Figure 5. At Marble Mountain, 

fossil mounds called bioherms 

(B) in limestone of the Minturn 

Formation (hm) have been thrust 

over the younger Sangre de Cristo 

Formation (Phs) along the Marble 

Mountain thrust fault (MMT).

B B B

MMT

hm

Phs

MMT

The Minturn Formation contains cyclic sequences of 

sedimentary rocks that record the rise and fall of sea level 

along a coastline dominated by river deltas. These cycles are 

thought to reflect the growth and retreat of glaciers on the 

great southern continent, Gondwana, during Pennsylvanian 

time. When the glaciers of Gondwana melted, sea level rose 

and limestone with marine fossils was deposited in warm, 

shallow seas. When the glaciers grew, sea level fell and rivers 

covered the former sea bottom with coarse sand. Land plants, 

such as ferns and even small trees, grew in swamps along the 

coast. The best place to see these sea-level cycles and their 

fossils is in valleys on the east side of the range.

At a few places on the shallow seafloor, invertebrate 

marine animals, such as crinoids, brachiopods, and coral—and 

also algae—were so numerous that their remains form mounds 

called bioherms hundreds of feet thick. Fragmental textures 

and coarse calcite in cavities show that the bioherms were 

partly dissolved and re-cemented while they grew, probably as 

they were alternately exposed and submerged at sea level. The 

best place to see the bioherms is Marble Mountain, on the east 

side of the range (fig. 5).

When the Uncompahgre highland rose quickly, coarse 

sand, pebbles and even boulders overwhelmed the shallow 

seas around it. Near the highland, floods and debris flows 

swept out of steep canyons and spread coarse gravel onto 

alluvial plains. Sediment accumulated rapidly, and the basins 

around the Ancestral Rockies sank miles deep.

D
8 The Geologic Story of Colorado’s Sangre de Cristo Range

Figure 6. A redbed under the microscope, showing iron 

oxide (black) in pores between sand-size grains of feldspar (K, 

stained orange for identification) and quartz (Q, clear); scale is 

500 micrometers (1 micrometer=1/1,000 millimeter), lower right. 

Although iron oxide imparts a red color to the rock, it appears 

black under the microscope because it is opaque in light shined 

through a thin section from below.

All around the flanks of the Ancestral Rockies, 

groundwater entered the newly deposited basin sediments. 

These sediments were not composed of pure quartz, but of 

unaltered, chemically unstable grains of feldspar and iron and 

magnesium minerals, such as hornblende and biotite. As soon 

as oxidizing groundwater began to react with these unstable 

minerals, iron began to move out of the minerals and into 

adjacent pore spaces, where it formed an insoluble gel of iron 

oxide (fig. 6). This gel gradually crystallized to the red iron 

oxide mineral, hematite. Today, this hematite gives the rocks 

their red color that imparts a red-pink glow to the mountains 

(especially at sunrise and sunset). Perhaps it was the red 

hematite that inspired 18th century Spanish explorers to name 

the mountains “Sangre de Cristo” (Blood of Christ).* 

*Here are two versions. At the time of his death in the 1700s, while looking up at the red mountains at sunset, the priest Francisco Torres is said to have whispered the words “Sangre de Cristo.” A second story: in 1719, after seeing the snowy red mountains at sunrise, the explorer Antonio Valverde y Cosio named the 

mountains “Sangre de Cristo.”

The Laramide Orogeny

“Laramide orogeny” is the name given to the processes 

that formed the Rocky Mountains about 80 to 40 million years 

ago, during Late Cretaceous and early Tertiary time. Faults 

and folds (figs. 7A, B) of the Laramide orogeny—some are 

large enough to show on the map in figure 3—can be seen 

throughout the Sangre de Cristo Range. They were formed by 

compression and uplift when tectonic plates beneath the North 

Pacific Ocean were forced (subducted) under the lighter continental crust of the western North American plate. Where the 

continental crust was thin, west of a line that passes through 

western Montana, Wyoming, and Utah, the eastward motion of 

subducting crust produced a broad north-south belt of lowangle faults (overthrusts). East of the overthrust belt, in the 

Rocky Mountain interior of North America, where the crust 

was thick, large fault-bounded uplifts and basins formed.

In Colorado, the old Uncompahgre and Front Range 

highlands of the Ancestral Rockies were uplifted again. 

However, the present site of the Sangre de Cristo Range was 

not one of these highlands. At the beginning of the Laramide 

orogeny, a shallow sea extended west over the area of the 

present Sangre de Cristo Range. A now-vanished uplift, 

sometimes called the Laramide San Luis highland, rose to the 

west of the present San Luis Valley. The San Luis highland 

may have connected to the Laramide Uncompahgre highland 

to the northwest, forming a single large uplift across southwestern Colorado.

Commonly, deep-rooted, high-angle reverse faults surround such interior uplifts on one or more sides. Less often, 

compression of highland against basin fill produces a stack 

of low-angle thrust faults. A belt of thrusts and high-angle 

faults extends nearly the entire length of the Sangre de Cristo 

Mountains, from a few miles south of Salida almost to Santa Fe 

on the south. This thrust belt formed on the east side of the San 

Luis highland. Some of the thrusts are magnificently exposed 

in Pennsylvanian and Permian rocks in the northern half of the 

range. Immediately north of the Great Sand Dunes, thrusts have 

carried Precambrian gneiss over Paleozoic limestone and sandstone. These thrusts are exposed in “windows” where streams 

have cut down to expose younger rocks below. In the window 

north of the Sand Dunes, shown in the cross section (fig. 

7C), as much as 2 miles of Pennsylvanian sedimentary rocks 

have been removed by thrust faults, and the thrust itself has 

been folded and faulted as the upper plate continued to move. 

A second window is exposed along the trail to Mosca Pass 

immediately east of the Sand Dunes (see map, fig. 3), and a 

third small window is exposed high in the range east of Blanca 

Peak. Rocks, which in Pennsylvanian time lapped onto the late 

Paleozoic Uncompahgre uplift, lie on rocks that were deposited 

many miles to the east in the central Colorado trough.

500μm

Q

Q

Q

K

K

K
The Laramide Orogeny 9

As thrust plates piled up on the east side of the San Luis 

highland, their weight depressed the west side of the Raton 

Basin. The basin began to sink and fill with sediment brought 

by rivers from the highland. Erosion and thrusting proceeded 

together. Along the west side of the Raton Basin, newly 

deposited sediment—exposed now as sandstone in Huerfano 

Park—was uplifted and eroded. Streams inundated the basin 

with coarse sand and gravel, and coal swamps formed along 

the shoreline of the great inland sea that lay to the east. These 

coal deposits were mined for steelmaking at Pueblo, for generating electricity, and for heating thousands of homes. Today 

they are the source of natural gas for home heating.

Figure 7. The Laramide orogeny. Photographs of folded and 

faulted rocks of Pennsylvanian and Permian age: (A) folded and 

overturned Minturn Formation (hm) overlies upturned beds of 

Sangre de Cristo Formation (Phs) along the Spread Eagle Peak 

thrust fault (SEPT) on Mt. Marcy; teeth on upthrown side of 

thrusts; photograph by S. Soulliere Johnson; (B) the Gibson Peak 

syncline, a large fold in the Sangre de Cristo Formation northeast 

of Crestone. (C) Section showing thrust-faulted structure of the 

Sangre de Cristo Range north of the Great Sand Dunes (see 

line “A–B” on fig. 3), based on interpretation of surface geology 

by Lindsey, Johnson, and others (1986). Erosion has exposed a 

window into rocks beneath a thrust fault that underlies most of 

the range; gneiss and granite west of Marble Mountain have 

been thrust over Paleozoic rocks; more faults may underlie the 

main thrust, but queries (?) emphasize that the deep structure 

cannot be known without drilling or geophysical surveys.

+5

–5

0

+3

–3

pegn

Pegr

pegn

Phs

hm

hm

Phs

Phs

hm

pegn

QT

pegn

pegn

QT pegr

Window exposes

hidden thrust fault

Sangre de Cristo Fault

Marble Mountain

pegn Phs

pegn

MDO

? MDO

?

?

MDO

?

?

A B

0

MILES FROM SEA LEVEL

KILOMETERS FROM SEA LEVEL

0

0

5 KILOMETERS

2.5 MILES

EXPLANATION

Late Paleozoic Sedimentary Rocks

Quaternary and Tertiary sediments

Sangre de Cristo Formation

Minturn Formation

Mississippian, Devonian, and Ordovician Rocks

Early Paleozoic Rocks

Precambrian rocks

Granite

Gneiss

QT

Phs

hm

pegr

pegn

MDO

C

A

B
10 The Geologic Story of Colorado’s Sangre de Cristo Range

The Rio Grande Rift

The Rio Grande rift fundamentally changed the geology of southern Colorado. About 35–27 million years ago, 

before the rift (a type of fault valley) formed, volcanoes in the 

San Juan Mountains and in the Bonanza area west of Villa 

Grove erupted volcanic ash over the entire landscape. Much 

of the ash has been eroded, but it is still found in old valleys 

dating from that time. One of these ancient valleys (called 

“paleovalleys”) has been mapped on the east side of the range 

northwest of Westcliffe, where a stream crossed the area of 

the present Sangre de Cristo Range more than 30 million 

years ago. Ash formations also have been found in boreholes, 

drilled to explore for oil in the San Luis Valley. These ash 

beds were deposited before rifting.

About 26 million years ago, in what is now New Mexico 

and Colorado, the Earth’s crust began to pull apart. Why rifting happens is not fully understood, but during rifting heat and 

molten rock rise from the mantle, uplifting and stretching the 

Earth’s crust. Rifting is always accompanied by block-faulting 

and volcanic eruptions. The Sangre de Cristo Range contains 

many small intrusions of igneous rocks dating from the time of 

rifting or slightly before. The nearby Spanish Peaks is famous 

for igneous dikes, many of which radiate from central intrusions of igneous rock.

The San Luis and Wet Mountain Valleys began to drop 

down as the Earth’s crust arched upward, stretched, and collapsed. Today the Sangre de Cristo Range stands as a remnant 

of this uplift. The Wet Mountain Valley has faults on both 

sides, but the San Luis Valley has only one faulted boundary 

on the east, the Sangre de Cristo fault (figs. 8A, B). At most 

places, the fault actually consists of several branches. One 

branch follows the range front and dips west into the valley at 

about 60 degrees. At about 10 miles deep, the fault zone probably flattens; at this depth, rocks that are brittle at the surface 

begin to flow like putty. Geophysical surveys reveal that more 

than 3 miles of sediment fills the valley on the downthrown 

side of the fault beneath the Great Sand Dunes. Down-faulting 

has tilted the lowest fill toward the range, but the upper layers 

are still horizontal, indicating that the fill was deposited during—not before—rifting.

Fission-track studies of apatite (calcium phosphate—our 

bones and teeth are made of a variant of this mineral), a trace 

mineral found in most rocks, show that rifting was especially 

rapid about 20–12 million years ago. During that time uplift 

and erosion removed much of the rock cover from the range; 

the remaining rocks cooled, fission tracks began to form, and 

the radiometric clock began ticking. Fission tracks are defects 

left in crystals when uranium atoms split apart. Fission tracks 

form in proportion to the amount of uranium originally present 

and, for apatite, the time since the mineral cooled below about 

100°C. They have been used extensively to study the history of 

uplift and rifting along the Rio Grande Valley.

The range rose again during the last million years or so. 

Alluvial fans and fault scarps that encircle the southwest sides 

of Blanca Peak and Twin Peaks record rapid uplift there 

38°00'

38°30'

37°30'

38°15'

105°45' 105°30' 105°15' 105°00'

37°45'

0 10

0 10 

20 

20

30

KILOMETERS 

MILES 

VS 

TP 

SANGRE DE CRISTO RANGE 

WET MOUNTAINS 

WET MOUNTAIN VALLEY 

Sangre de Cristo fault 

Sangre de Cristo fault 

 GREAT 

SAND DUNES SAN LUIS VALLEY 

Westcliffe fault 

HP 

CP

BP

A

(fig. 8A, C). If you look closely at the mountain front south of 

the Great Sand Dunes, you will see fault offsets in alluvium 

that formed only a few thousand years ago. North of the Great 

Sand Dunes, many more scarps are apparent along the range 

front. Large flatiron-shaped features called “faceted spurs” cut 

off the ridgelines along the Sangre de Cristo fault (fig. 8D). 

The best time for viewing fault scarps is in the morning or 

evening, when the low sun angle casts long shadows.

Rift faulting often forms closed basins with no throughgoing drainage, and the Rio Grande Valley was no exception. 

Eventually the Rio Grande cut through bedrock to connect these 

basins into a single drainage. The San Luis Valley was still 

closed about a half million years ago and, during rainy periods, 

a lake occupied the valley north and east of Alamosa. Sediments 

and shoreline features from Lake Alamosa are still preserved 

in a few places southeast of the city. Then the Rio Grande cut 

through volcanic rock that separated the lake from basins to 

the south, and the lake drained. Perhaps at this time and later, 

during the ice ages, strong winds swept over the dry lake basin, 

picking up the dust and sand that formed the Great Sand Dunes.

Figure 8 (pages 10 and 11). The Rio Grande rift. (A) Map showing 

major faults and topographic expression of the Rio Grande rift in 

the vicinity of the Sangre de Cristo Range. BP, Blanca Peak; CP, 

Cottonwood Peak; HP, Hayden Pass; TP, Twin Peaks; and VS, Valley 

View Hot Springs. Photographs showing the Sangre de Cristo fault 

in the San Luis Valley: (B) Looking north at fault scarps (f) and 

alluvial fans near Valley View Hot Springs (VS). 
The Rio Grande Rift 11

VS

f

f

B
TP

12 The Geologic Story of Colorado’s Sangre de Cristo Range

CP

s s s BC PC SC

C

D

Figure 8 (continued). (C) Alluvial fan south of the Great Sand Dunes, formed during uplift of mountains around Twin Peaks (TP). The Sangre 

de Cristo fault runs along the foot of the range. (D) Faceted spurs (S, triangular slopes at ends of ridges) and alluvial fans along the fault 

between Hayden Pass and Valley View Hot Springs. Piney Creek (PC), Steele Canyon (SC), Black Canyon (BC), and Cottonwood Peak (CP).
The Ice Ages 13

The Ice Ages

The rugged scenery of the Sangre de Cristo Range owes 

much to erosion by glaciers. Jagged glacier-carved ridges 

divide the range. Large rock basins called “cirques” occupy the 

heads of glaciated valleys. Glaciers sculpted the sharp hornshaped summits of Blanca and the Crestones, and scoured 

rock basin lakes in the valleys below. Along the range front, 

meltwater streams deposited fans of gravel and sand. Around 

the southern and western sides of Blanca Peak, fan surfaces are 

steep and fan gravel accumulated to great thicknesses as faulting continued to down-drop the San Luis Valley (fig. 8C).

Glacial valleys have a characteristic U-shaped cross 

section. The bedrock in glacial valleys displays grooves and 

scratch-marks where the ice has dragged embedded stones 

over it. Such valleys always contain scattered deposits of 

poorly sorted boulders and clay (called “glacial till”). Glacial 

landforms composed of till are called “moraines.” At their 

downstream ends, valley glaciers deposit natural dams of till 

called “terminal moraines.” Upstream, wherever the ice front 

pauses, they deposit still more till dams, called “recessional 

moraines.” On valley walls, steps where ice has rubbed against 

the bedrock and till has accumulated (“lateral moraines”) 

mark the former height of glacial ice (fig. 9). Meanwhile, back 

downstream at the ice front, meltwater streams emerge from 

the glacier and rework the moraine, forming a gravel deposit 

called an “outwash fan.” Almost every glaciated valley in the 

Sangre de Cristo Range has such an outwash fan.

Valley glaciers formed repeatedly in the Rocky Mountains during the last 1.8 million years; this time is known 

as the Pleistocene Epoch of the Quaternary Period. Valley 

glaciers advanced at least three times in the Sangre de Cristo 

Range. The oldest widespread deposits consist of weathered 

till and outwash gravel. These deposits, called “Bull Lake” 

after the type locality in Wyoming, are probably between 

190,000 and 130,000 years old. The surface of Bull Lake till 

is smooth and deposits are deeply weathered to a brown color. 

Even older (pre-Bull Lake) glacial deposits are known from 

one place on the east side of the range. About 30,000–12,000 

years ago, the last glacial advance scoured the valleys and 

left fresh deposits of till and outwash. These deposits, called 

“Pinedale” after another Wyoming locality, form hummocky 

surfaces and are only weakly weathered. Except for a cover of 

trees, they look as if they formed yesterday.

Alternating periods of global cooling and warming 

cause valley glaciers to advance and retreat. During long cool 

periods—typically hundreds to thousands of years long—snow 

accumulates year-after-year in high valleys. Accumulation 

exceeds summer melting and, eventually, the snow becomes 

ice and flows down the valley under its own weight. As long 

as the supply of snow and ice exceeds the loss from melting, 

the front of the glacier advances down the valley, often reaching the foot of the mountains.

Before the industrial age, periods of global cooling 

and warming were driven by cycles in the Earth’s orbit and 

rotational axis. The Earth’s path around the sun varies from 

nearly circular to elliptical over cycles of about 100,000 and 

413,000 years. The tilt of the Earth’s axis of rotation changes 

from 22.1° to 24.5° and back every 41,000 years. Finally, 

the Earth’s axis of rotation moves around a pole of the plane 

defined by the Earth’s orbit around the sun every 26,000 years. 

This rotation, together with the gravitational effects of other 

planets on the Earth’s orbit, produces a 19,000- to 23,000-year 

cycle called the “precession of the equinoxes.” This cycle is 

so-called because it causes the Earth’s equinox to move backward (precess) a few minutes every year. The net result of all 

these cycles is a complex cooling and warming pattern—called 

“Milankovitch cycles” after their discoverer—that currently 

brings a glacial advance about every 100,000 years.

Figure 9. The ice ages. Aspen 

cover a lateral moraine in Black 

Canyon, north of Valley View Hot 

Springs.
14 The Geologic Story of Colorado’s Sangre de Cristo Range

References

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of Mines and Mineral Resources Bulletin 160, 317 p.

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Miscellaneous Field Studies Map MF-1878, scale 1:24,000.

McCalpin, J.P., 1982, Quaternary geology and neotectonics of 

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Pierce, K.L., 2003, Pleistocene glaciations of the Rocky 

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Scott, G.R., and Taylor, R.B., 1975, Post-Paleocene Tertiary 

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Acknowledgments

Photograph of Mt. Marcy by S. Soulliere Johnson; 

geologic map by T. Klein, E. DeWitt, and C. San Juan; 

reviewed by D.H. Knepper, Jr., and T. Klein; edited 

by Jon Raese; graphic design and layout by Mari L. 

Kauffmann (Contractor, ATA Services). Work supported 

by Bradley Scholar Program grant to D.A. Lindsey.


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Lindsey—Geologic Story of Colorado’s Sangre de Cristo Range—Circular 1349