U.S. Department of the Interior

U.S. Geological Survey

Circular 1374

Prepared in cooperation with the National Park Service

Geology Along Mosca Pass Trail, Great Sand Dunes 

National Park and Preserve, Colorado
!.

!.

!.

!.

!.

!.

!.

Blanca

 Peak

Marble

Mountain

Crestone

 Peak

Spanish

 Peaks

Hayden

 Pass

Mosca

 Pass

Salida

La Veta

 Pass

 Twin

Peaks

Carbonate

Mountain

California Peak

Medano

 Pass

Horn

 Peak

Arkansas

River

Mt. Zwischen

Blanca

 Peak

Marble

Mountain

Crestone

 Peak

Valley View

Hot Springs

Villa

 Grove

Crestone

Alamosa

Blanca

Gardner

La Veta

Westcliffe

Cañon

 City

Cuchara

River

U.S. HIGHWAY 50

Valley View

Hot Springs

Villa

 Grove

Crestone

Alamosa

Blanca

GREAT SAND DUNES 

NATIONAL PARK

AND PRESERVE

HUERFANO

PARK

Spanish

 Peaks

Gardner

Hayden

 Pass

Mosca

 Pass











105°45’ 105°30’ 105°15’ 105°00’

38°30’

38°15’

38°00’

37°45’

37°30’

La Veta

Westcliffe

Salida

Cañon

 City

WET

MOUNTAINS

SANGRE

DE

CRISTO

RANGE

La Veta

 Pass

 Twin

Peaks

Carbonate

Mountain

California Peak



Medano

 Pass



Horn

 Peak

Arkansas

River

Huerfano River

Cuchara

River

Arkansas

River

SAN

LUIS

VALLEY

WET

MOUNTAIN

VALLEY

HUERFANO

PARK

U.S. HIGHWAY 160

COLORADO HIGHWAY 17

COLORADO HIGHWAY 69

U.S. HIGHWAY 50

SIERRA

BLANCA

Mt. Zwischen

Index map.
Geology Along Mosca Pass Trail, 

Great Sand Dunes National Park 

and Preserve, Colorado

By David A. Lindsey, Terry L. Klein, Andrew Valdez, and Robert J. Webster

A Trail Guide and Introduction to the Geology of the Sangre de Cristo 

Range East of Great Sand Dunes 

Prepared in cooperation with the National Park Service

Circular 1374 

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: 2012

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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., Klein, T.L., Valdez, Andrew, and Webster, R.J., 2012, Geology along Mosca Pass Trail, Great Sand Dunes 

National Park and Preserve, Colorado: U.S. Geological Survey Circular 1374, 18 p.
iii

Contents

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

A Trail into the Earth .............................................................................................................................1

And a Trail into the Valley....................................................................................................................1

About Technical Terms ........................................................................................................................1

Geologic Setting.............................................................................................................................................2

Range-Boundary Faults, Alluvial Fans, and Pediments..................................................................2

The Mosca Creek Window..................................................................................................................6

The Trail Guide................................................................................................................................................8

Locating Yourself on the Trail..............................................................................................................8

Rocks Along Montville Nature Trail ...................................................................................................9

Rocks Along Mosca Pass Trail .........................................................................................................10

Magnetic Anomalies ...................................................................................................................................17

More Information.........................................................................................................................................18

Acknowledgments.......................................................................................................................................18

References Cited..........................................................................................................................................18

Figures

1. Composite photograph of the Sangre de Cristo Range, looking north from the 

ridgeline..........................................................................................................................................2

2. Geologic map of the region around the Mosca Pass Trail ....................................................3

3. Geologic cross sections through the Sangre de Cristo Range at Mosca Pass 

showing (A) The present-day range and (B) The big picture 70–60 million 

years ago........................................................................................................................................4

4. Photograph showing young fault boundary on the southwest side of the 

Sierra Blanca.................................................................................................................................5

5. Photograph showing old fault boundary on the east side of the range between 

Mosca Pass and Medano Pass..................................................................................................5

6. Topographic map showing location of points of geologic interest along the 

Montville Nature Trail and the Mosca Pass Trail ....................................................................8

7–15. Photographs at points on the Mosca Pass Trail:

7. At point 1: Montville Nature Trail: (A) Light-colored igneous dike and brown 

thrust breccia, (B) Red sandstone east of dike, and (C) Slickensides in 

light-colored igneous dike...........................................................................................................9

8. At point 2: (A) Looking east along trail, (B) Slickensides of greenish chlorite 

and white quartz on sandstone block, and (C) Looking west at sandstone cliffs 

above fault ...................................................................................................................................10

9. At point 3: (A) Looking west at trail in notch cut through sandstone, (B) Broad 

fold in Minturn Formation sandstone bed along the trail, and (C) Small fault with 

slickensides .................................................................................................................................11

10. At point 4: (A) Looking east along trail, (B) Near-horizontal slickensides in outcrop 

showing movement parallel to Laramide compression direction, and (C) Sandstone 

beds with white quartz granules of lower Minturn Formation............................................12
iv

11. At point 5: (A) Looking west along trail at a fault breccia and (B) Close-up 

showing fractures and rotated blocks ....................................................................................13

12. At point 6: (A) Looking west at reverse fault and (B) Deformation shown by 

closely spaced fractures filled with quartz............................................................................14

13. At point 7: Looking east along trail at exposures of steeply dipping sandstone 

beds...............................................................................................................................................15

14. North of point 8: Mosca Creek thrust marked by gneiss breccia above sandstone 

of Minturn Formation..................................................................................................................15

15. At point 9: (A) Breccia zone in gneiss above the Mosca Creek thrust and 

(B) Gneiss of Twin Peaks pluton immediately above breccia zone....................................16

16. Photograph of Mosca Pass from the east trailhead.............................................................17
Introduction

A Trail into the Earth

Mosca Pass Trail takes the hiker on a journey into the 

Earth’s crust. Here you can see the results of tremendous 

tectonic forces that bend and tear rocks apart and raise 

mountain ranges. 

The trail begins near the Sangre de Cristo fault, which 

separates the Sangre de Cristo Range from the San Luis 

Valley. The valley is part of the Rio Grande rift, a series 

of fault basins extending from southern New Mexico to 

central Colorado, wherein the Earth’s crust has been pulled 

apart during the last 30 million years. Thousands of feet of 

sediment, brought by streams mostly from the Sangre de 

Cristo Range, fill the San Luis Valley beneath the Great Sand 

Dunes. 

The trail ends at Mosca Pass overlooking Huerfano 

Park. The park is part of the larger Raton Basin, formed by 

compression of the Earth’s crust during the Laramide orogeny, 

which occurred 70–40 million years ago. Massive highlands, 

the remnants of which are preserved in the Sangre de Cristo 

Range, were uplifted and pushed over the western side of 

the Raton Basin. Streams eroded the highland as it rose and 

filled the Raton Basin with sediment. After the sediment 

was compacted and cemented to form sedimentary rock, 

the Huerfano River and other streams began to excavate the 

basin. Over an unknown but long timespan that probably 

lasted millions of years, relatively soft sedimentary rocks were 

removed by the river to form the valley we call “Huerfano 

Park.”

Between the ends of the trail, the hiker walks through 

an erosional “window,” or opening, into red sedimentary 

rocks overridden by gneiss, a metamorphic rock, during 

the Laramide orogeny, which is described in more detail in 

following sections. This window gives the hiker a glimpse 

into the Laramide highland of 70-40 million years ago that 

preceded the present-day Sangre de Cristo Range. The window 

is the focus of this trail guide. At the east end of the trail, 

near Mosca Pass, another trail follows the ridgeline south to 

Carbonate Mountain (Winger and Winger, 2003). Immediately 

after reaching the first summit above tree line, this trail crosses 

a narrow valley where red rocks mark an extension of the 

window across the range. Stunning vistas of the Sangre de 

Cristo Range extend north to the horizon (fig. 1). The uplifted 

range stands in sharp contrast to the San Luis Valley on the 

west and Huerfano Park on the east.

And a Trail into the Valley

Geology affects geography, and geography affects 

history. At Mosca Pass the range-boundary faults are close 

together. Streams have eroded the range to allow passage 

through low passes and short valleys. 

Mosca Pass has long been a gateway to the San Luis 

Valley because it is one of the shortest routes across the Sangre 

de Cristo Range (National Park Service, 2011; Harlan, 1976). 

Indians and Spanish explorers probably crossed the range 

through Mosca Pass. Here the fur trader Antoine Robidoux 

crossed from the Great Plains into the Rocky Mountains in the 

early 19th century. The expeditions of Zebulon Pike in 1807 

and John C. Fremont in 1848 crossed the Sangre de Cristo 

Range in the winter, but they probably went through nearby 

Medano Pass. After 1870 a toll road crossed the range at 

Mosca Pass giving access by horse and wagon for commerce 

and mail into the San Luis Valley. The road washed out during 

heavy rains in the early 1900s and never fully reopened. Today 

travelers and commerce from the east use the highway across 

La Veta Pass to the south. The Mosca Pass Trail is maintained 

for hikers and horses.

About Technical Terms

Like any scientific field, geology has its share of 

technical terms. We attempt to describe and illustrate names of 

rocks and other features as we follow the trail; look for these 

descriptions and definitions under “Geologic Setting” and in 

“The Trail Guide.” Inevitably, however, you will come across 

some terms that we have not described adequately. Rather 

Geology Along Mosca Pass Trail, Great Sand Dunes 

National Park and Preserve, Colorado

By David A. Lindsey,1 Terry L. Klein,1 Andrew Valdez,2 and Robert J. Webster 3

1

 U.S. Geological Survey, MS 973, Box 25046, Federal Center, Lakewood, 

CO 80225.

2

 Great Sand Dunes National Park and Preserve, 11500 Highway 150, 

Mosca, CO 81146.

3 OXY USA Inc., 5 Greenway Plaza, Suite 110, Houston, TX 77046.
2 

than provide yet another glossary at the end of this guide, we 

encourage you to look for descriptions online or to consult 

guidebooks with glossaries of geologic terms. The pocket 

guide “Rocks Above the Clouds” (Reed and Ellis, 2009) 

has descriptions and pictures of rocks, structures, and other 

geologic features common to the mountains of Colorado. A 

geologic time scale (U.S. Geological Survey Geologic Names 

Committee, 2007) is available online.

Geologic Setting

Range-Boundary Faults, Alluvial Fans, and 

Pediments

Before ascending the trail, study the geologic map (fig. 2) 

and cross sections (fig. 3). Notice the orientation of the Sangre 

de Cristo fault in the first cross section (fig. 3A). It dips west 

at a high angle, arbitrarily drawn at 60 degrees, although we 

cannot measure the dip accurately. The west side (hanging 

wall of fault) has moved down against the east side (footwall 

of fault). We call such high-angle faults with down-dropped 

hanging walls “normal faults.” In contrast to the Sangre de 

Cristo fault, the west side (hanging wall) of the boundary fault 

on the east side of the range has moved up, instead of down 

like a normal fault. The angle of the fault is arbitrarily shown 

dipping west 45 degrees near the surface but is interpreted to 

flatten beneath the Sangre de Cristo Range to the west (fig. 3A). 

Where it is exposed a few miles to the north, the east-side 

boundary fault dips west less than 20 degrees. We call such 

low-dipping faults “thrusts.” Thrust faults dip at low angles, 

typically less than 45 degrees, and usually bring older rocks 

over younger rocks. If the dip is greater than 45 degrees, the 

same fault is called a “reverse fault.” Another line of evidence 

for westward flattening of the east-side boundary fault beneath 

the Sangre de Cristo Range is from magnetic anomalies, to be 

discussed later.

The cross section in figure 3A represents the Sangre de 

Cristo Range as it appears at Mosca Creek now. Figure 3B

shows a big-picture cross section through the range at about 

70–60 million years ago. The Sangre de Cristo normal fault 

has been removed from the cross section because it formed 

later, after the time represented by the cross section in figure 

3B. However, at 70–60 million years ago, compression of 

the Earth’s crust was forming the east-side boundary fault. 

The boundary fault separated a highland (labeled “Laramide 

highland” in fig. 3B) and an adjacent basin (labeled “Raton 

Basin”) to the east. As the highland rose, it was eroded 

by streams that filled the basin with sediment. Later, after 

about 55 million years ago, another highland centered over 

the present Wet Mountains (see index map) began to rise, 

shedding additional sediment into the northwestern part of the 

Raton Basin from the northeast.

The topography of the west side of the Sangre de Cristo 

Range is more rugged than the east side of the range. This 

difference in topography reflects the different ages of the 

faults on the west and east sides of the range. On the west side, 

scarps of the Sangre de Cristo fault cut alluvium close to the 

range front and large flatiron-shaped surfaces (called “faceted 

spurs”) cut off the ends of ridges (fig. 4). Segments of the 

range front are straight and abrupt. In contrast, the range front 

on the east side lacks fault scarps in alluvium, does not have 

faceted spurs, and is irregular and subdued (fig. 5). The Sangre 

de Cristo fault on the west side of the range formed during the 

time of Rio Grande rifting (from 30 million years ago to the 

present); the east-side boundary fault is older and assigned to 

the Laramide orogeny (70–40 million years ago). 

Figure 1. Composite photograph of the Sangre de Cristo Range looking north from the ridgeline summit at tree line—elevation 10,268 feet, 

about 1.5 miles north of Carbonate Mountain (see index map). The range is delineated by the Sangre de Cristo normal fault on the left 

and an eroded thrust fault of Laramide orogeny age, on the right; the Great Sand Dunes are on the left and Huerfano Park is on the right. 

Mosca Creek and Mosca Pass are located in the forested lowlands just beyond the treeless foreground.
Geologic Setting 3

Figure 2. Geologic map of the region around the Mosca Pass Trail, compiled by Terry L. Klein, Ed DeWitt and Carma San Juan from 

mapping by Aughenbaugh (1958), Burford (1960), Bruce and Johnson (1991), Webster (2000), and D.A. Lindsey (unpub. mapping, 1985).

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: : :

105°32' 105°30' 105°28' 105°26'

0 1 2 3 KILOMETERS

0 1 2 MILES

A

A'

North Arrastre Creek

Carbonate

Mountain

South Arrastre

Creek

Mosca Creek

Morris

Gulch

Mosca Pass

Campground

Garden Creek

∆

COLORADO HIGHWAY 150

Pass Trail

Mosca

COUNTY ROAD 583

GREAT SAND DUNES

NATIONAL PARK 

VISITOR CENTER

∆

 Peak 

11,268

MOSCA

CREEK

THRUST

SANGRE

DE

CRISTO

FAULT

EAST

SIDE

BOUNDARY

FAULT

MOSCA

CREEK

THRUST

*mq

Xvm

Xvm

Xitp

Qf

Ti

Xi

Qal

Qe

Yim

Tbf

Ta

Ti

Qal

Qe

Ti

Ti

Yim

Xi

Xi

Xitp

Xi

Ta

Ta

Ta

Tbf Ta

Tbf

Ti

*mq

Qal

Qf

Ti

Ti

Xitp

Xi Ta

Ta

Xvm

Ta

*mu

Qe

Xi

*mq

6

1 2 3 4 5 7 8 9

37°42'

37°44'

*

mu

Tbf

*mu

Xvm

Ta

 

 

*mq

 

Xitp

 Metamorphosed mafic intrusive rocks

 Metamorphosed granite at Twin Peaks 

 Metamorphosed intermediate-composition

 intrusive rocks

 Metamorphosed volcanic rocks

Depositional or intrusive contact—Dotted where inferred

Normal fault—Dashed, approximate location;

 dotted, concealed; ball and bar on downthrown side

Thrust or reverse fault—Dashed, approximate location;

 dotted, concealed; teeth on upthrown side

Roads

Trails

Points described in text

Qf

Quaternary

 Alluvium

 Eolian sand

 Alluvial fan deposits

Tertiary

 Basin fill deposits

 Andesite flows

 Intrusions (dikes and plugs)

Pennsylvanian

 Minturn Formation, undivided

 Minturn Formation, lower part

 (quartz-pebble redbeds)

Ti

 

 

Xi

Qal

Qe

 

EXPLANATION

1

Yim

Proterozoic gneisses
4 Geology Along Mosca Pass Trail, Great Sand Dunes National Park and Preserve, Colorado

Figure 3. Geologic cross sections through the Sangre de Cristo Range at Mosca Pass. (A) Cross section 

through the present-day range along the ridge side immediately north of the Mosca Pass Trail. (B) Cross 

section showing “the big picture” of thrusting that occurred in the early part of the Laramide orogeny 70–60 

million years ago. Numbers beside faults in 3B indicate interpreted order of fault movement: 1, Mosca Creek 

thrust; 2, east-side boundary fault; and 3, younger thrust breaking into Raton Basin.

Tbf

Xi?

Xitp

Tbf

mq

Xi Ta

MDO?

SANGRE

DE CRISTO

FAULT

MOSCA

CREEK

THRUST

Mosca

Creek 

window

mu

Pop-up

mq

Xi?

?

EAST-SIDE 

BOUNDARY FAULT

Qf

mq?

MDO?

 WEST

3 

0

2

1

MILES ABOVE SEA LEVEL

EAST

0

2

1

3

4

KILOMETERS ABOVE SEA LEVEL

0

0 3 KILOMETERS

2 MILES

NO VERTICAL EXAGGERATION

Bulge

Structure

concealed

?

Structure

concealed

mq

NO VERTICAL EXAGGERATION

 WEST EAST

?

5

56 7 9 3 2 4

1

Trail

level

8

MDO? + mq

MDO? + mq Paleozoic and Mesozoic rocks

Precambrian rocks

Proterozoic, Paleozoic, and Mesozoic rocks

Raton Basin

Laramide highland

Erosion by streams

Basin fill

Compressional force

NO SCALE

Area of A

Basin subsidence

1

3

2

Proterozoic rocks

1

MDO? + mq

?

A

B

Tbf

mu

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mq

 

Xitp Metamorphosed Twin Peaks granite

 Metamorphosed intermediate-composition

 intrusive rocks

Depositional or intrusive contact—Dotted where inferred

Bedding in sedimentary rock

Fault—Dashed, approximate location

Arrows show sense of movement along faults

Points described in text

Qf

Quaternary

 Alluvial fan deposits

Tertiary

 Basin fill deposits

 Andesite flows

Pennsylvanian

 Minturn Formation, undivided

 Minturn Formation, lower part

 (quartz-pebble redbeds)

Mississippian, Devonian, and 

 Ordovician sedimentary rocks

 

 

Xi

 

EXPLANATION

1

MDO

Proterozoic gneisses

A A'
Geologic Setting 5

Figure 4. The young fault 

boundary on the southwest 

side of Sierra Blanca (see 

index map) is revealed 

by faceted spurs on the 

mountainside and scarp lines 

at the heads of alluvial fans. 

Photograph was taken from 

U.S. highway 160, 2 miles east 

of junction with Colorado 

highway 150 (5 miles south of 

the range front).

Figure 5. The old fault 

boundary on the east side of 

the Sangre de Cristo Range 

between Mosca Pass (left) 

and Medano Pass (right) has 

embayed valleys and rounded 

slopes. The distant grassy 

slopes are underlain by alluvial 

gravel shed from the range 

during uplift millions of years 

ago. The fault boundary is 

exposed as a Laramide age 

thrust south of Medano Pass 

but is concealed beneath 

pediment gravel (grassy slope) 

near Mosca Pass. Photograph 

was taken from Colorado 

highway 69, 2 miles east of 

Gardiner, Colorado (16 miles 

east of range front).
6 Geology Along Mosca Pass Trail, Great Sand Dunes National Park and Preserve, Colorado

The Sangre de Cristo fault passes through the parking lot 

at the west end of the Mosca Pass Trail. The fault is marked 

by scarps in alluvium and bedrock. A good example of a scarp 

in alluvium extends north from the trees on the north side of 

the lot (best viewed from the paved road north of here). A 

bedrock-alluvium scarp is located immediately south of the 

parking lot. If you hike southeast from park headquarters past 

the water tank and up the trail to Morris Gulch, you will walk 

alongside another fault scarp in alluvium. These scarps in 

alluvium represent surface ruptures above the bedrock fault 

when the fault last moved, a few thousand years ago. Farther 

south, extensive scarps cut the alluvium around the foot of 

Sierra Blanca (the Spanish name for the mountains around 

Blanca Peak; see index map). The ridgelines extending from 

the mountains are truncated by faceted spurs (fig. 4). This side 

of the Sangre de Cristo Range is geologically young, formed 

by normal faulting during the last few million years.

Where streams emerge from the mountains, they deposit 

alluvial fans in response to decreasing channel slope. During 

the ice ages, streams swollen with glacial meltwater deposited 

large fans of coarse gravel hundreds of feet thick around the 

southwestern side of Sierra Blanca (fig. 4) (Peterson, 1971). 

Valleys of Sierra Blanca have the signature U-shaped cross 

section of valleys carved by glaciers. When the climate 

warmed after the last ice age, deposition stopped and streams 

began cutting canyons into the fans. In contrast to the valleys 

of the Sierra Blanca, the valleys around Mosca Pass (see index 

map) were never occupied by glaciers. The valley of Mosca 

Creek and those to the north have V-shaped cross sections 

carved by streams. Streams in these unglaciated valleys 

deposited small alluvial fans like the one beneath the trailhead 

parking lot. Such alluvial fans are much smaller than their iceage counterparts around Sierra Blanca.

The boundary fault east of Mosca Pass is concealed by 

younger rocks (figs. 2 and 3). Huerfano County road 583 

ascends Mosca Pass from the east through the canyon of 

May Creek, a tributary of the Huerfano River. The canyon is 

cut through a thick sequence of sandstone belonging to the 

Minturn Formation of Pennsylvanian age. After leaving the 

canyon, the landscape opens onto grasslands composed of 

rounded ridgelines and valleys that slope gently away from the 

Sangre de Cristo Range (fig. 5). The surfaces of the grasslands 

are underlain by loose, coarse gravel visible in road cuts and a 

prominent borrow pit along the road. This gravel is composed 

almost entirely of gneiss eroded from the mountains to the 

west. At places the grassy surfaces underlain by gravel extend 

into the mountains giving the range front a slightly embayed 

appearance, although the overall impression of the front 

from a distance is linear. No fault scarps cut alluvium on the 

grasslands, and no truncated spurs mark the ends of ridgelines. 

As the road ascends into the range by switchbacks, it cuts 

through fractured gneiss, a metamorphic rock. No major fault 

is exposed in the gneiss. 

The boundary fault east of Mosca Pass must be concealed 

under the gravel-mantled grasslands. Along Huerfano County 

road 583 only about 2 miles separate exposures of Proterozoic 

gneiss on the west from sandstone of Pennsylvanian age on 

the east. Just a few miles north of the road, the boundary fault 

is exposed. There, Proterozoic gneiss overlies sandstone of 

Pennsylvanian age along a fault that dips west beneath the 

range. To the north, the topographic front of the range is nearly 

coincident with the boundary fault, reflecting the contrast 

in erosional resistance between gneiss (more resistant) and 

sandstone. At the road and to the south, however, the mountain 

front has retreated west about 1 mile, indicating that the 

range front has been eroded for a long time. The presence of 

cemented gravel and volcanic rock beneath the gravel points 

to earlier episodes of erosion.

The gravel beneath grassy surfaces was eroded from 

the mountains by streams millions of years ago. As the 

Huerfano River excavated the topographic basin now called 

“Huerfano Park,” it paused during times when erosion 

balanced deposition, leaving behind stable surfaces underlain 

by gravel. Where these surfaces flank streams in the park, they 

are called “terraces.” In uplands drained by streams emerging 

from the mountain front, they are called “pediments.” The 

streams flowing from the mountains today are the same ones 

that formed the pediments, but they have gradually cut into 

the pediments, so that today the pediments stand above stream 

level. Gravel deposits beneath the pediments are much thinner 

than the alluvial-fan deposits on the southwest side of Sierra 

Blanca. No meltwater rushed down the streams between 

Mosca Pass and Medano Pass, and no glaciers filled their 

valleys, probably because this part of the range was never high 

enough to support year-round accumulation of snow and ice.

The Mosca Creek Window

Between the Sangre de Cristo fault and the east-side 

boundary fault, erosion has exposed a structural window 

beneath another fault, called the “Mosca Creek thrust” (figs. 2 

and 3A). This thrust is correlated with other thrusts south of 

Carbonate Mountain and at California Peak (see index map) 

and north of the head of Garden Creek. Its topographic and 

structural position high in the range indicates that it overlies 

the east-side boundary fault. The west side of the Mosca 

Creek thrust has been cut off by the Sangre de Cristo fault, 

confirming that the thrust was there first. Likewise, the east 

side of the Mosca Creek thrust is interpreted to have been cut 

by the east-side range-boundary fault, but that interpretation is 

less certain because the boundary fault is obscured by younger 

rocks. These relations only specify relative age; they do not 

distinguish between great gaps in time (such as between 

Laramide thrusting and Rio Grande rifting) and smaller gaps 

(such as the movement of individual Laramide age faults). 
Geologic Setting 7

The Mosca Creek thrust and overlying rocks rode on 

other rocks above the east-side boundary fault, bending and 

breaking as the latter advanced over the Raton Basin (fig. 3B). 

A younger thrust fault is shown breaking east in the footwall 

of Paleozoic and Mesozoic rocks, beneath the western side of 

the Raton Basin. Such thrusts can actually be observed at the 

surface in Huerfano Park. Possibly other thrusts overlay the 

Mosca Creek thrust; if so, they have been removed by erosion. 

The east-side boundary fault is shown as flattening west, 

beneath the Laramide highland, but whether the fault actually 

flattens to horizontal beneath the highland or continues down 

into deeper levels of the Earth’s crust is unknown. Such 

uncertainties are indicated by question marks and notes on the 

cross sections.

Four Proterozoic gneiss units overlie the Mosca Creek 

thrust—three gneiss units about 1.7 billion years old (units 

Xvm, Xi, and Xitp) and one gneiss about 1.4 billion years 

old (unit Yim) (see descriptions of gneiss units in Explanation 

for figs. 2 and 3). The three 1.7-billion-year-old gneiss units 

consist of (1) dark gneiss (Xvm, metamorphosed volcanic 

rocks), found within mixed gneiss; (2) mixed light- and 

dark-layered gneiss (Xi, metamorphosed intermediate 

composition intrusive rocks); and (3) light-colored gneiss 

(Xitp, metamorphosed granite of Twin Peaks) around Mosca 

Pass. The 1.4-billion-year-old gneiss consists of small areas 

of dark (mafic) metamorphosed intrusive rocks within older 

gneiss.

The window exposes red sandstone, siltstone, and shale 

of the lower member of the Pennsylvanian Minturn Formation, 

which is about 300 million years old. In addition to its red 

color, the lower member contains distinctive quartz pebbles 

and granules that set it apart from the rest of the Minturn 

Formation. It is never more than about 1,000 feet thick, a key 

measurement used in constructing the cross section of figure 

3A. The cross section (fig. 3A) also shows sedimentary rocks 

of early Paleozoic age and gneisses of Proterozoic age beneath 

the Minturn Formation. Nowhere are these rocks exposed 

in the window, but their presence is predicted by exposures 

elsewhere in the range. For example, a thrust fault south 

of Carbonate Mountain, thought to be an extension of the 

Mosca Creek thrust, overlies a thin layer of lower Paleozoic 

sedimentary rocks on gneiss.

The Minturn Formation in the window was deformed 

and metamorphosed by heat and pressure. Small faults and 

fractures traverse the Minturn, and in places beds have been 

tilted and folded. The original red sedimentary rock of the 

Minturn Formation has been recrystallized to form hard 

metamorphic rock with abundant white quartz and green 

chlorite concentrated in fractures, indicating temperatures 

reached several hundred degrees Celsius. Spectacular striated 

fracture surfaces called “slickensides” are everywhere. The 

thrust itself is marked by broken, angular fragments (breccia) 

of gneiss overlying red Minturn Formation. The thrust breccia 

is as much as 100 feet thick in some places.

Several large reverse faults cut through the window 

of Minturn Formation (Webster, 2000) (figs. 2 and 3). The 

reverse faults are visible along the trail and were spectacularly 

illustrated in a series of annotated photographs by R.J. Webster 

(2000) of the valley wall and ridge north of Mosca Creek. 

Movement on most of the reverse faults was from west to east, 

but one fault moved rocks in the opposite direction. Together, 

these faults formed a bulge of deformed Minturn Formation 

beneath the thrust, accounting for the change in dip of the 

thrust from west to east. The thrust warped as the gneiss above 

it moved over bulges in the underlying Minturn Formation and 

later when the entire package of rocks moved on the east-side 

boundary fault. Thus, the resulting surface of the Mosca Creek 

thrust is very irregular; it rises from 8,300 feet at the trailhead 

to 11,200 feet at one point along the range crest south of 

Mosca Pass. The reverse faults below the thrust are interpreted 

as originating in Proterozoic rocks, but some faults may well 

have their origins in the Minturn Formation.

Rocks in the window are also cut by east-trending tear 

faults (Webster, 2000). All of the tear faults mapped in the 

window have small displacements. Tear faults are commonly 

vertical and strike across the trend of other structures; they 

reflect differences in the amount of horizontal movement 

or transport during thrusting. Tear faults also indicate the 

presence of another thrust below the window, evidence for the 

projection of the east-side boundary fault beneath the range 

(fig. 3). Tear faults in the window form zones of weak, easily 

erodible rock. Thus stream valleys may follow tear faults. One 

tear fault appears to follow Mosca Creek; it is not shown on 

figures 2 and 6, because it follows the creek and the trail so 

closely that including it would obscure the trail shown in the 

figures. South of Mosca Creek, a prominent limestone bed 

with marine fossils has been offset by another tear fault. The 

tear fault on Mosca Creek may have weakened the rocks in the 

window, promoting downcutting and influencing the location 

of the creek channel.

Mosca Creek, as well as Morris Gulch to the south and 

Garden Creek to the north, have cut steep, narrow canyons 

most of the way through the range. Unlike the other canyons 

in the Sangre de Cristo Range, which become steep near their 

heads, the upper part of Mosca Creek flows through a broad 

meadow. The gradient of Mosca Creek changes from about 

550 feet/mile in the sandstone window (first 2 miles) to about 

300 feet/mile in the gneiss east of the window (last mile), 

suggesting that rock type has influenced stream gradient. 

Morris Gulch and Garden Creek, both entirely in gneiss, have 

steep gradients averaging about 1,000 feet/mile. Most canyon 

cutting happens during floods like the ones that washed out the 

wagon road in Mosca Creek in the early 1900s. Steep canyon 

walls provide abundant sediment to Mosca Creek by means of 

both gravity-driven sliding and slope wash.

Two trails, the Montville Nature Trail and the Mosca Pass 

Trail, leave from the parking lot at the mouth of Mosca Creek, 

0.25 mile northeast of the Park Visitor Center. We recommend 

that you walk the Nature Trail first, stopping at point 1, where 

you can see gneiss and fault breccia, the Mosca Creek thrust 

fault, a dike, slickensides, and red sandstone of the Minturn 

Formation—all in one place.
8 Geology Along Mosca Pass Trail, Great Sand Dunes National Park and Preserve, Colorado

Locating Yourself on the Trail

You can use the topographic map (fig. 6) to locate 

approximately each point of interest. In our descriptions we 

provide some approximate distances between points and 

reference a prominent notch cut in rock along the trail about 

0.5 mile east of the point where the Mosca Pass Trail leaves 

the Montville Nature Trail. If you have a global positioning 

system (GPS) unit, use the coordinates given in the captions 

for figures 7 to 15 to locate each point of interest along the 

trail. These coordinates are formatted in degrees and decimals 

using the 1983 North American Datum (NAD83). NAD83 

coordinates should also be close to those of the 1984 World 

Geodetic System (WGS84), which is the default datum for 

most GPS units and is used by Google Earth, so you can 

probably leave your unit set on the default. Comparisons 

with locations in Google Earth indicate that the coordinates 

should be accurate within 50 feet and, in most cases, 10–20 

feet. Be aware that paper versions of the 1:24,000-scale U.S. 

Geological Survey (USGS) topographic maps of Mosca Pass 

(published in 1967) and Zapata Ranch (published in 1965) 

are based on an older standard, the 1927 North American 

Datum, which may not agree well with coordinates in a newer 

datum. If your GPS provides elevations, they can also serve 

as a general guide to confirm your location. Comparison of 

GPS elevations indicate that they should be within one contour 

interval (40 feet) of the elevation indicated on the topographic 

map (fig. 6). Finally, photographs of the trail help to locate 

most of the points.

Figure 6. Topographic map showing location of points of geologic interest along the Montville Nature Trail and the Mosca Pass Trail. 

Faults shown on figure 2 are shown here for reference. Contour interval is 40 feet.

(

(

(

(

(

(

(

(

(

(

(

(

(

(

(

(

(

(

(

MOSCA

PASS

TRAIL

MONTVILLE

NATURE TRAIL

0 0.5 1 

0 0.5 1 MILE

1.5 KILOMETERS

105°30'30” 105°28'30”

37°44'

6

1

2 3

4 5

8

9 7

Thrust or reverse fault—Dashed, approximate location;

 dotted, concealed; teeth on upthrown side

Trails

Points described in text

EXPLANATION

1

The Trail Guide
The Trail Guide 9

Rocks Along Montville Nature Trail

POINT 1: The short self-guided Montville Nature Trail 

follows a loop north of the entrance to the Mosca Pass Trail. 

From the Mosca Pass trailhead parking lot east of the Visitor 

Center, enter the trail through a grove of cottonwood and 

aspen trees and take the Montville Nature Trail immediately 

to your left. Cross Mosca Creek, and walk uphill through 

switchbacks cut in gravelly soil of decomposed gneiss. As you 

reach the summit of the loop, you will come to a large lightcolored outcrop that forms a small point on the ridgeline (point 

1, figs. 6 and 7A). This outcrop is an igneous dike formed 

when molten magma intruded the Mosca Creek thrust about 

30 million years ago. The dike extends up the ridge, crossing 

an old irrigation ditch and the trail leading north to the 

campground. Immediately west of the dike on the nature trail, 

brown rock composed of gneiss fragments is thrust breccia. 

Immediately east of the dike are tilted beds of red sandstone 

and shale of the Minturn Formation (fig. 7B). You are entering 

the window beneath the thrust.

Before proceeding east beyond the dike, take a look 

at the nearly vertical, northwest-trending fractures in the 

dike. The fractures have brown, striated surfaces called 

“slickensides” (fig. 7C). The striae (technically, these are 

called “slickenlines”) mark the direction of movement 

along surfaces. Slickensides commonly display small 

curved fractures called Reidel shears that reveal the sense 

of movement (Petit, 1987). Slickenside striae in the dike are 

steeply inclined and show down-to-the-west movement, like 

the fault scarp in alluvium north of the parking lot. These 

slickensides were formed during Rio Grande rifting and are 

younger than the gently dipping slickensides exposed along 

the trail in gneiss west of the dike and in sandstone east of the 

dike. The gently dipping slickensides probably formed during 

the Laramide orogeny. 

Slickensides in sandstone of the Mosca Creek 

window have been studied several times with conflicting 

interpretations. In the most recent study, Webster (2000) 

concluded that most of the slickensides were formed during 

the Laramide orogeny, and that the striae show movement 

along a line 70 degrees east of north. That direction is typical 

of Laramide fault movement in this part of Colorado and is 

consistent with the slickensides in gneiss and sandstone along 

the trail near point 1. 

Follow the nature trail east, down to the bridge that crosses 

Mosca Creek. On the south side of the creek, take a look at the 

boulder with a plaque on it; the green striated surfaces on the 

boulder are slickensides. Now cross back to the north side of 

the creek and proceed east up the Mosca Pass Trail. 

Figure 7. Photographs at point 1, Montville Nature Trail (lat 

37.73443°N., long 105.50714°W., elev. 8,320 feet): A, Looking 

southeast up the trail at a light-colored igneous dike. Brown 

rock in foreground is thrust breccia gneiss; valley of Mosca 

Creek in background. B, Looking east from igneous dike at 

red sandstone of the Minturn Formation in the Mosca Creek 

window. C, Slickensides (above pen) in light-colored igneous dike 

showing down-to-west movement of Rio Grande rift faulting. Pen 

approximately 6 inches long shows scale. 

A

B

C
10 Geology Along Mosca Pass Trail, Great Sand Dunes National Park and Preserve, Colorado

Rocks Along Mosca Pass Trail

POINT 2: As you walk up the trail through red rocks of 

the Minturn Formation, about 0.4 mile east of the plaque at 

the trail entrance, you will encounter numerous examples of 

slickensides in both outcrops and loose blocks (point 2, figs. 6 

and 8A). Many of these examples show Reidel shears (fig. 8B). 

Slickensides in the Mosca Creek window are nearly always 

composed of green chlorite and white quartz.

Just west of point 2, a prominent gulch at the foot of cliffs 

on the north side of the trail (fig. 8C) marks the westernmost 

of five reverse faults mapped in the Minturn Formation (see 

cross section, fig. 3A, unit *mq). Rocks on the west side of 

the fault moved up and east. Although small parallel faults are 

exposed in the cliffs west of the fault, the cliffs are composed 

of erosion-resistant sandstone, in contrast to easily eroded 

crushed rock near the fault. Such cliff-and-gulch topography 

results from variable resistance of rocks to erosion along fault 

zones. 

Figure 8. Photographs at point 2 (lat 37.73200°N., long 

105.49837°W., elev. 8,610 feet): A, Looking east along the trail: 

slickensides, left foreground (S). B, Slickensides of greenish 

chlorite and white quartz on this sandstone block record 

fault movement in the Mosca Creek window during Laramide 

thrusting. Slickensides are composed of surfaces called Reidel 

shears that are inclined to the actual plane of movement. 

Photograph located in C. Blue pen approximately 6 inches long 

shows scale. C, Looking west at sandstone cliffs below skyline. 

Blocks with slickensides (S and 8B) along trail. A reverse 

fault runs down the gulch behind the spruce tree in center of 

photograph.

A

B

C
The Trail Guide 11

POINT 3: The trail passes by the wooden sign labeled 

“Sangre de Cristo Wilderness” and up through a prominent 

notch cut through rock (point 3, figs. 6 and 9A), located about 

600 feet farther up the trail from point 2. About 75 feet east 

of the notch is a broad anticline (upfold), defined by a single 

sandstone bed (fig. 9B). The bed represents the channel of an 

ancient stream during Pennsylvanian time; the shale above 

the sandstone bed represents overbank sediment deposited on 

an ancient floodplain as the channel meandered across it. The 

anticline was formed by compression and buckling of beds 

as the Mosca Creek thrust moved across the area during the 

Laramide orogeny. Small faults cut off the upfolded sandstone 

bed on both sides. On the west side of the anticline, two small 

faults are indicated by sheared rock along the trail about 50 

feet east of the notch. Where the trail descends into a small 

gully about 140 feet east of the notch, another small fault cuts 

the anticline (fig. 9C). Slickensides on the fault show that the 

east side has moved up and over the anticline. Faults like these 

form on the sides of folds where beds have been crowded 

together; commonly, they bend and disappear into shale beds. 

These faults are in the footwall of the second reverse fault 

(counting from the west in the cross section of figure 3A. The 

second reverse fault crosses the trail about 250 feet west of the 

notch near the wilderness sign but is concealed by talus and 

vegetation.

Figure 9. Photographs at point 3 (lat 37.73245°N., long 105.49616°W., elev. 8,730 feet): A, Looking west at trail in notch cut 

through sandstone. B, Broad fold in sandstone bed of Minturn Formation along the trail 75 feet east of notch. The fold was 

formed during the Laramide orogeny. The bed is about 5 feet thick. A small fault with slickensides is exposed above the trail, 

behind tree. C, Looking up at the small fault (between arrows) with slickensides (beside top of walking stick), east side of 

anticline. Photograph located in B, behind tree. In A and C, walking stick is approximately 5 feet long showing scale.

A B

C
12 Geology Along Mosca Pass Trail, Great Sand Dunes National Park and Preserve, Colorado

POINT 4: About 650 feet east of the anticline, you 

can find good examples of many sedimentary and structural 

features (point 4, figs. 6 and 10A). Slickenside surfaces and 

close-spaced fractures of varying orientation abound. Nearly 

horizontal slickensides display striae that are parallel to the 

direction of Laramide compression (fig. 10B); other, steeply 

inclined slickensides with striae of varying orientations record 

other components of strain. Beds of white quartz granules 

and red sandstone reveal the gentle dip here (fig. 10C). In 

the Sangre de Cristo Range, granules and pebbles of white 

quartz in red sandstone are diagnostic of the lower member 

of the Minturn Formation. Quartz is one of the most resistant 

minerals to weathering and erosion. These quartz granules 

survived a long period of weathering over 300 million years 

ago, after which they were deposited by streams in the lower 

member of the Minturn Formation.

Red sandstone containing quartz granules and pebbles 

has been found beneath thrust faults at other localities north 

and south of Mosca Creek (Rhodes, 1964). This relation 

suggests that the Mosca Creek thrust underlies a very large 

area, from south of California Peak to a point several miles 

north of the head of Garden Creek.

Figure 10. Photographs at point 4 (lat 37.73155°N., long 105.49443°W., elev. 8,820 feet): A, Looking east along the trail. B, Nearhorizontal slickensides showing movement parallel to the compression direction of the Laramide orogeny. Note closely spaced striated 

surfaces of green chlorite and white quartz near blue pen; these are Reidel shears. Photograph is located at arrow in A. Blue pen is 

perpendicular to striae. C, Sandstone beds with white quartz granules are characteristic of the lower member of the Minturn Formation. 

Orientation of beds is approximately horizontal. Inclined veins (V) of white quartz and near-vertical slickensides (S) of green chlorite cut 

across beds. Veins are located at arrow in A. Blue pen approximately 6 inches long shows scale.

A

B

C
The Trail Guide 13

POINT 5: The trail crosses fault breccia about 600 

feet east of point 4 (point 5, figs. 6 and 11). Fractures with 

prominent light-brown altered margins separate blocks of red 

sandstone and shale. Altered margins probably formed by 

oxidizing waters that either ascended during hydrothermal 

activity or descended from the surface. The original bedded 

structure of the rocks is visible in individual breccia blocks; 

the orientation of bedding in the individual blocks is highly 

variable. Fault breccia is exposed for about 120 feet along the 

trail. The breccia marks the west strand of an east-directed 

fault and the western boundary of highly deformed Minturn 

Formation in the bulge shown on the cross section of fig. 3A. 

Loose blocks of rock and soil on steep slopes south of Mosca 

Creek indicate the probable continuation of the fault.

Figure 11. Photographs at point 5 (lat 37.73160°N., long 105.49261°W., elev. 8,860 

feet): A, Looking west along trail at fault breccia with rotated blocks and altered 

zones along fractures. B, Close-up showing light-brown altered fracture margins and 

rotated blocks in fault breccia. Walking stick approximately 5 feet long shows scale.

A

B
14 Geology Along Mosca Pass Trail, Great Sand Dunes National Park and Preserve, Colorado

POINT 6: About 700 feet farther up the trail from point 

5, look up at a cliff overhanging a prominent fracture (point 

6, figs. 6 and 12A). The fracture is a strand of an east-directed 

reverse fault (located on fig. 6). Closely spaced fractures 

filled with quartz attest to the high degree of deformation of 

the sandstone in the footwall of the fault (fig. 12B). These 

fracture fillings are themselves sheared and folded revealing 

a complicated history of deformation as the overlying fault 

moved. The original sedimentary layers within these beds 

have been completely obliterated, yet bedding surfaces remain 

sufficiently intact to determine that they dip about 20 degrees 

north. Rocks in the bulge of faulted Minturn Formation 

between points 5 and 7 were compressed between two faults 

that moved in opposite directions.

Figure 12. Photographs at point 6 (lat 37.73165°N., long 

105.49118°W., elev. 8,990 feet): A, Looking west at east strand 

of reverse fault (dashed) at point 6 (see fig. 6); fault is marked 

by a prominent left-dipping fracture in rock below the cliff. 

B, Deformation in footwall of fault along trail: closely spaced 

fractures filled with quartz (arrows point to examples) cut across 

red siltstone beds, located behind bush in A. Bedding trends 

from upper left to the middle right side of picture and dips about 

20 degrees away from the observer. Pick approximately 12 

inches long shows scale.

A

B
The Trail Guide 15

POINT 7: Steeply dipping sandstone beds of the 

Minturn Formation are exposed along the trail at point 7, 

located about 900 ft east of point 6 (figs. 6 and 13). Steeply 

inclined slickensides parallel to bedding indicate east-side-up 

movement. An exposure in the trees about 200 feet to the 

east reveals beds dipping about 70 degrees west, and 300 feet 

farther east beds dip gently north about 10 degrees. A westdirected reverse fault separates steeply dipping beds on the 

west from gently dipping beds on the east (see location of fault 

at point 7 on fig. 6). The steeply dipping beds were upturned 

by the west-directed fault. This is the eastern boundary of the 

bulge in the Minturn Formation below the Mosca Creek thrust 

fault.

POINT 8: The trail crosses a small alluvial fan about 

0.55 mile east of point 7, where a gully enters Mosca Creek 

from the north. This is point 8 (fig. 6) on the trail. Walk 

north of the trail about 100 feet into a clearing on the fan and 

look across the gully at a contact between reddish-brown 

rocks below light-brown rocks on the hillside (fig. 14). The 

contact is the Mosca Creek thrust fault. The reddish-brown 

rocks below the thrust contact are gently dipping beds of 

sandstone in the Minturn Formation. The light-brown rocks 

above the contact are brecciated gneiss. The thrust contact is 

easily located within a few feet between bedrock exposures; 

however, typically you cannot touch the contact because it is 

covered by soil and talus.

Figure 13. Photograph at point 7 (lat 37.73087°N., long 105.48865°W., 

elev. 9,020 feet), looking east along trail at exposures of steeply 

dipping sandstone beds. Beds strike across the trail and dip steeply 

to the west, toward the observer. Additional exposures along trail 

are located in the trees. A west-directed reverse fault lies beyond 

the trees in the background.

Figure 14. Photograph north of point 8 (lat 37.73148°N., long 

105.47889°W., elev. 9,270 feet), looking west across gulch from 

100 feet north of the trail, showing the Mosca Creek thrust fault. 

The thrust (dashed line) lies at the contact between light-colored 

gneiss breccia (above) and dark reddish-brown sandstone of the 

lower member of the Minturn Formation. 
16 Geology Along Mosca Pass Trail, Great Sand Dunes National Park and Preserve, Colorado

POINT 9: The Mosca Creek thrust fault crosses the trail 

about 0.3 mile east of point 8 (fig. 6). Between points 8 and 9, 

the thrust breccia is exposed on the hillside all along the north 

side of Mosca Creek. The breccia is as much as 100 feet thick 

here and is developed entirely within gneiss above the thrust. 

It can be easily examined on the rocky hillside at point 9 

where the trail leaves the trees along Mosca Creek and enters 

a small grassy area with scattered aspen (point 9, fig. 15A). 

Fractured and brecciated blocks of light- and dark-colored 

gneiss, pegmatite (a coarse-grained variant of granite), and 

dark igneous rocks have been mixed together by the intense 

grinding action of the thrust movement. The cliffs to the west 

are composed entirely of breccia. You have walked through 

the Mosca Creek window. 

At point 9 you can examine the light-colored gneiss 

(fig. 15B) above the thrust breccia. This unit is exposed along 

the trail from here to the ridgeline at Mosca Pass. The lightcolored gneiss at Mosca Pass consists of mostly feldspar and 

quartz; it is metamorphosed granite. The gneiss is included 

with several bodies of rock referred to as the Twin Peaks 

pluton, a group of metamorphosed igneous intrusions named 

for Twin Peaks, south of Mosca Creek (see index map). The 

gneiss was formed by metamorphism of the granitic rocks of 

the pluton. Proceed another mile up the trail through lightcolored gneiss to the east entrance at Mosca Pass (fig. 16).

Two other types of gneiss also crop out around the 

Mosca Creek window. Gneiss resulting from metamorphism 

of intermediate-composition intrusions (feldspar-rich rocks 

Figure 15. Photographs 

at point 9 (lat 37.73073°N., 

long 105.47317°W., elev. 

9,410 feet): A, Breccia 

zone in gneiss above the 

Mosca Creek thrust fault 

is exposed on this rocky 

hillside and in the cliffs 

beyond. The breccia here 

is as much as 100 feet 

thick. Person standing 

on the slope shows 

scale. B, Gneiss of Twin 

Peaks pluton immediately 

above breccia zone. Pick 

approximately 12 inches 

long shows scale.

A

B
The Trail Guide 17

with less than 20 percent quartz, such as quartz monzonite) is 

exposed north and south of the Mosca Creek window along the 

west side of the range, and gneiss representing metamorphism 

of volcanic rocks crops out south of the window (fig. 2). Small 

dikes of pegmatite (coarse-grained igneous rock composed of 

quartz and feldspar) and amphibolite (a dark metamorphosed 

igneous rock composed mainly of amphibole minerals) cut 

through the gneiss. 

Magnetic Anomalies

Most of the gneiss in the Sangre de Cristo Range contains 

small amounts of iron-bearing minerals that make it slightly 

magnetic. However, the light-colored gneiss at Mosca Pass 

may not be magnetic because it contains very few iron-bearing 

minerals. Sedimentary rocks (sandstone, siltstone, shale, and 

limestone) are also nonmagnetic. The Minturn Formation, a 

sedimentary rock, is exposed in the window beneath the Mosca 

Creek thrust and in Huerfano Park east of Mosca Pass, and 

the younger, overlying Sangre de Cristo Formation is exposed 

north of Mosca Pass. A folded section of the mile-thick 

Minturn Formation could produce a large negative magnetic 

anomaly under the range near Mosca Pass. The light-colored 

gneiss above the Mosca Creek thrust and the 1,000-footthick lower member of the Minturn Formation beneath the 

thrust in the Mosca Creek window (fig. 3A) are probably not 

thick enough to produce negative magnetic anomalies by 

themselves.

Magnetic intensity of different types of rocks in the 

range has been mapped by airborne surveys, which can also 

detect the magnetic intensity from rocks well below ground 

surface. These surveys show a positive magnetic anomaly 

centered between the mouths of Mosca and Morris Creeks on 

the west side of the range and a negative anomaly beneath the 

eastern half of the range and extending beneath the pediment 

to the east. The positive anomaly indicates that magnetic rock 

(possibly iron-bearing gneiss) underlies the western part of 

the range at Mosca Creek. The negative anomaly indicates 

that nonmagnetic rocks (light-colored, nonmagnetic gneiss 

or sedimentary rocks) underlie the eastern part of the range 

at Mosca Pass. This observation is explained if the east-side 

boundary fault dips west beneath the range and overrides a 

thick section of Minturn Formation as shown in the cross 

sections (fig. 3).

Figure 16. Mosca Pass from the east trailhead. Grassy areas are underlain by gneiss of the Twin Peaks pluton.
18 Geology Along Mosca Pass Trail, Great Sand Dunes National Park and Preserve, Colorado

More Information

The references cited below provide detailed information 

about the geology of the area around Mosca Creek; some of 

these may be accessible only through university libraries. You 

can find more information about the geologic history of the 

Sangre de Cristo Range in U.S. Geological Survey Circular 

1349 (Lindsey, 2010). Two references (Reed and Ellis, 2009 

and U.S. Geological Survey Geologic Names Committee, 

2007) provide descriptions and definitions of geologic terms 

and geologic time. 

Acknowledgments

We thank C. San Juan for help in preparing the geologic 

map and V.J.S. Grauch for sharing her knowledge of magnetic 

anomalies and their implications for cross sections in the 

Mosca Pass region. J.S. Caine and S.A. Minor provided 

helpful insights about the boundary fault east of the range. All 

are employees of the U.S. Geological Survey. We thank Fred 

Bunch, National Park Service, for a copy of the thesis by N.B. 

Aughenbaugh and for his assistance throughout this project. 

C.J. Fridrich, U.S. Geological Survey, and Danielle York, 

National Park Service, provided technical reviews. This work 

was supported by the U.S. Geological Survey Bradley Scholar 

program and the National Park Service.

References Cited

Aughenbaugh, N.B., 1958, Geology of the Sangre de Cristo 

Mountains at Mosca and Morris Creeks: Ann Arbor, Mich., 

University of Michigan, M.S. thesis, 40 p. 

Bruce, R.M., and Johnson, B.R., 1991, Reconnaissance 

geologic map of parts of the Zapata Ranch and Mosca Pass 

quadrangles, Alamosa and Huerfano Counties, Colorado: 

U.S. Geological Survey Miscellaneous Field Studies Map 

MF–2168, scale 1:24,000.

Burford, A.E., 1960, Geology of the Medano Peak area, 

Sangre de Cristo Mountains, Colorado: Ann Arbor, Mich., 

University of Michigan, Ph.D. dissertation, 201 p.

Harlan, George, 1976, Postmarks and places: Denver, Colo., 

Golden Bell Press, 190 p.

Lindsey, D.A., 2010, The geologic story of Colorado’s Sangre 

de Cristo Range: U.S. Geological Survey Circular 1349, 

14 p. (Also available at http://pubs.usgs.gov/circ/1349/.)

National Park Service, 2011, Great Sand Dunes—

History & culture: National Park Service Website, accessed 

11/08/2011, at http://www.nps.gov/grsa/historyculture/

index.htm.

Peterson, R.C., 1971, Glaciation in the Sangre de Cristo 

Range, Colorado, in James, H.L., ed., Guidebook to the San 

Luis Basin, Colorado, 22nd Field Conference, Sept. 30–

Oct. 1, 1971: Socorro, N. Mex., New Mexico Geological 

Society, p. 165–167.

Petit, J.P., 1987, Criteria for the sense of movement on fault 

surfaces in brittle rocks: Journal of Structural Geology, v. 9, 

no. 5/6, p. 597–608.

Reed, Jack, and Ellis, Gene, 2009, Rocks above the clouds: 

Golden, Colo., Colorado Mountain Club Press, 240 p.

Rhodes, J.A., 1964, Stratigraphy and origin of Pennsylvanian–

Permian rocks of the Huerfano Park quadrangle, Colorado: 

Ann Arbor, Mich., University of Michigan, Ph.D. 

dissertation, 176 p.

U.S. Geological Survey Geologic Names Committee, 2007, 

Divisions of geologic time—Major chronostratigraphic 

and geochronologic units: U.S. Geological Survey Fact 

Sheet 2007–3015, 2 p., available at http://pubs.usgs.gov/

fs/2007/3015/.

Webster, R.J., 2000, Kinematic analysis and structural style of 

the Mosca Creek window, west side of the Sangre de Cristo 

Mountains, Alamosa County, Colorado: Wichita, Kans., 

Wichita State University, M.S. thesis, 164 p. 

Winger, Charlie, and Winger, Diane, 2003, The essential guide 

to Great Sand Dunes National Park & Preserve: Golden, 

Colo., Colorado Mountain Club Press, 239 p.

Publishing support provided by: 

Denver Publishing Service Center

For more information concerning this publication, contact:

Center Director, USGS Central Mineral and Environmental Resources 

Science Center

Box 25046, Mail Stop 973

Denver, CO 80225

(303) 236-1562

Or visit the Central Mineral and Environmental Resources Science

Center Web site at: http://minerals.cr.usgs.gov
Front Cover. Mosca Creek window in foreground; Great Sand Dunes and Sangre de Cristo Range in 

 background. Photograph from ridgeline summit, elevation 10,268 ft, by D.A. Lindsey.

Back Cover. Looking down Mosca Creek at Great Sand Dunes, by D.A. Lindsey.
Lindsey and others— Geology Along Mosca Pass Trail, Great Sand Dunes National Park and Preserve, Colorado—U.S. Geological Survey Circular 1374