Great Sand Dunes National Park & Preserve 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2006/011 

National Park Service 

U.S. Department of the Interior 

Natural Resource Program Center 

Great Sand Dunes National Park and Preserve 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2006/011 

Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, Colorado 80225 

March 2006 

U.S. Department of the Interior 

Washington, D.C.
The Natural Resource Publication series addresses natural resource topics that are of interest 

and applicability to a broad readership in the National Park Service and to others in the 

management of natural resources, including the scientific community, the public, and the NPS 

conservation and environmental constituencies. Manuscripts are peer- reviewed to ensure that 

the information is scientifically credible, technically accurate, appropriately written for the 

intended audience, and is designed and published in a professional manner. 

Natural Resource Reports are the designated medium for disseminating high priority, current 

natural resource management information with managerial application. The series targets a 

general, diverse audience, and may contain NPS policy considerations or address sensitive 

issues of management applicability. Examples of the diverse array of reports published in this 

series include vital signs monitoring plans; "how to" resource management papers; 

proceedings of resource management workshops or conferences; annual reports of resource 

programs or divisions of the Natural Resource Program Center; resource action plans; fact 

sheets; and regularly- published newsletters. 

Views and conclusions in this report are those of the authors and do not necessarily reflect 

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not constitute endorsement or recommendation for use by the National Park Service. 

Printed copies of reports in these series may be produced in a limited quantity and they are 

only available as long as the supply lasts. This report is also available from the Geologic 

Resource Evaluation Program website (http://www2.nature.nps.gov/geology/inventory/

gre_publications) on the internet, or by sending a request to the address on the back cover. 

Please cite this publication as: 

Graham, J. 2006. Great Sand Dunes National Park and Preserve Geologic Resource Evaluation 

Report. Natural Resource Report NPS/NRPC/GRD/NRR—2006/011 National Park Service, 

Denver, Colorado. 

NPS D- 71, March 2006 
 GRSA Geologic Resource Evaluation Report iii

Table of Contents 

List of Figures .............................................................................................................. iv

Executive Summary ...................................................................................................... 1

Introduction ................................................................................................................... 3

Purpose of the Geologic Resource Evaluation Program ............................................................................................3

Geologic Setting .........................................................................................................................................................3

Park History................................................................................................................................................................4

Geologic Issues............................................................................................................. 9

Water Issues...............................................................................................................................................................9

Earthquake Potential ................................................................................................................................................11

Oil and Gas Development ........................................................................................................................................11

Mining Issues............................................................................................................................................................12

Potential Research Projects .....................................................................................................................................13

Geologic Features and Processes............................................................................. 15

Dune Features and Processes .................................................................................................................................15

Wetlands ..................................................................................................................................................................16

Features in the Sangre de Cristo Range ..................................................................................................................17

Map Unit Properties .................................................................................................... 22

Map Unit Properties Table........................................................................................................................................27

Geologic History.......................................................................................................... 27

Precambrian .............................................................................................................................................................27

Paleozoic Era ...........................................................................................................................................................27

Mesozoic Era............................................................................................................................................................30

Cenozoic Era............................................................................................................................................................31

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

References................................................................................................................... 41

Appendix A: Geologic Map Graphic .......................................................................... 45

Appendix B: Scoping Summary................................................................................. 47

Attachment 1: Geologic Resource Evaluation Products CD 
 NPS Geologic Resources Division iv 

List of Figures 

Figure 1. Map of Great Sand Dunes National Park and Preserve..................................................................................6

Figure 2. Great Sand Dunes and Sangre de Cristo Mountains ......................................................................................7

Figure 3. West to east geologic cross-section across the San Luis Valley.....................................................................7

Figure 4. Geographic setting of Great Sand Dunes National Park and Preserve...........................................................8

Figure 5. Physiographic map of the dune field, Great Sand Dunes National Park and Preserve.................................14

Figure 6. Eolian sand deposits in Great Sand Dunes National Park and Preserve......................................................18

Figure 7. The three major eolian sand deposits at Great Sand Dunes National Park and Preserve............................19

Figure 8. Common dune types.....................................................................................................................................20

Figure 9. Sand ripples on sand dunes..........................................................................................................................20

Figure 10. Great Sand Dunes wetland.........................................................................................................................21

Figure 11. Music Pass, Sangre de Cristo Mountains ...................................................................................................21

Figure 12. Geologic time scale.....................................................................................................................................33

Figure 13. Middle Proterozoic paleogeography of the southwestern United States .....................................................34

Figure 14. Ordovician, Mississippian, Pennsylvanian, and Permian paleogeography of the southwestern US ...........36

Figure 15. Late Jurassic and Late Cretaceous paleogeography of the southwestern United States ...........................37

Figure 16. Early Tertiary (Eocene) paleogeography of the southwestern United States..............................................38
 GRSA Geologic Resource Evaluation Report 1

Executive Summary 

This report has been developed to accompany the digital geologic map produced by Geologic 

Resource Evaluation staff for Great Sand Dunes National Park and Preserve in Colorado. 

It contains information relevant to resource management and scientific research. 

Great Sand Dunes National Park and Preserve is home to 

the tallest sand dunes in North America with some dunes 

rising 750 feet (230 m) above the San Luis Valley floor. 

Unique to the Rocky Mountain physiographic province, 

the high- altitude desert dunes of the park and preserve 

represent one of the most complex dune systems in the 

world. The 30 square miles (78 sq km) of Great Sand 

Dunes are part of a larger dune field that includes a 

relatively flat sand sheet and sabkha and covers 150 

square miles (389 sq km). High velocity, bi- directional 

winds shape the dunes, which offer the visitor an 

exceptional experience of dynamic eolian processes and 

sand dune morphology. 

The dunes of Great Sand Dunes lie on the eastern edge 

of the San Luis Valley at the base of the Sangre de Cristo 

Mountains. The dune field is part of the 84,000 total 

acres of the park and preserve that also protect alpine 

lakes, tundra, six peaks over 13,000 feet (3,962 m), 

ancient spruce and pine forests, large stands of aspen and 

cottonwood, grasslands, and wetlands. Rocks 

representing all of the geologic eras are exposed in the 

Sangre de Cristo Mountains and Great Sand Dunes 

National Park and Preserve. 

Precipitation in the Sangre de Cristo Mountains provides 

surface runoff throughout the year and maintains the 

Medano and Sand Creeks which define the borders of 

the sand dunes. Water flowing in these shallow, braided 

streams seeps through the porous sand feeding 

underground aquifers that provide water for both 

agriculture and domestic use in the arid San Luis Valley, 

the largest intermontane basin in Colorado. 

Archeological evidence suggests that the San Luis Valley 

has been inhabited for thousands of years. 

The large dunes are the reason that Great Sand Dunes 

was established as a national monument in 1932. Geologic 

issues that affect the resource management of the dunes, 

as well as other features in the park and preserve, fall into 

four broad categories: 

• Water issues that affect dune preservation 

• Earthquake potential along the Sangre de Cristo 

mountain front 

• Oil and gas development in the Baca grant, north and 

west of Great Sand Dunes 

• Past mining activities 

Located in an arid climate, Great Sand Dunes is 

dependent on limited water resources. Both surface 

water and groundwater are critical for the preservation 

of the dunes and dune ecosystem. The delicate balance 

among precipitation, surface flow, and groundwater 

sustains the dune morphology, surge flow and surface 

flow conditions in Medano and Sand Creeks, and 

interdunal ponds and wetlands. If the groundwater table 

is lowered, surface water is lost to the subsurface, sand 

isn’t recycled, and the dunes are negatively impacted. 

Surface water and groundwater also are precious 

resources for agriculture and potable water supplies. As 

population within Colorado and in neighboring states 

continues to grow, demands on the water resources in 

San Luis Valley will continue to generate management 

issues for Great Sand Dunes. 

The Sangre de Cristo fault zone passes beneath the park 

and defines the east boundary of the San Luis basin. Only 

one seismic event has been recorded in the last 120 years, 

however, paleoearthquakes mark the Sangre de Cristo as 

one of Colorado’s few active fault zones. Surface shaking 

from earthquakes may impact water tanks, pipelines, and 

most of the other facilities at Great Sand Dunes that lie in 

the vicinity of faults. The Great Sand Dunes Visitor’s 

Center was built on a fault scarp and the campground is 

located on a landslide and/or debris flow that may be 

reactivated during a seismic event. 

The approximately 1,800- acre Baca land grant lies north 

and west of the park and preserve. Exploration for gold 

in the area led to the accidental discovery of oil on the 

Baca grant. However, economic deposits of gold or oil 

have not been found, and no new exploratory wells have 

been drilled since the mid- 1990s. Nevertheless, previous 

exploration targets may become more economically 

attractive in the future. Increased exploration on the 

Baca grant may lead to an increase in groundwater 

withdrawal and negative impacts on ground water and 

surface water quality and the dune ecosystem at Great 

Sand Dunes. 

Past mining operations in the Sangre de Cristo watershed 

may affect water quality in the park. While some 

prospect excavations in the main Cold Creek drainage 

channel can generate acids when exposed to water, many 

of the past mining sites are small, without concentrated 

mineralization, and are located some distance from 

stream channels so that they pose little threat to surface 

water quality. Past placer gold mining activity in the 

dunes does not present a water quality issue for Great 

Sand Dunes. 

Research projects proposed in a workshop, held at the 

park in 1998, may help define these geologic issues for the 

resource manager. Potential areas of research are 

identified in the Geologic Issues section of this report. 
 NPS Geologic Resources Division 2 

The dunes of Great Sand Dunes form one large dune 

feature called a draa that is constantly being shaped and 

reshaped by a complex wind regime. Bimodal and 

complex wind patterns form reversing and star dunes on 

the draa. Unimodal winds form parabolic, barchan, and 

transverse dune forms. Surge flow in Medano Creek and 

Sand Creek is a rare hydrologic process and plays a 

significant role in recycling sand back onto the dunes. 

Features on the sand sheet and sabkha are more subdued 

than on the sand dunes, yet both environments are 

critical for the maintenance of dune features. 

Both riverine and palustrine wetlands are recognized in 

Great Sand Dunes. Riverine wetlands form along creek 

channels while palustrine wetlands are associated with 

interdunal ponds and depressions that nearly intersect 

the water table. Important habitat for a variety of species, 

palustrine wetlands have decreased markedly in recent 

years. 

With the passage of the Great Sand Dunes National Park 

and Preserve Act in 2000, thousands of acres of diverse 

mountain and valley landscapes were added to the park. 

Alpine glacial features are common in the Sangre de 

Cristo Mountains and structural features, generated 

during the Laramide Orogeny, are also found in the 

strata exposed in the mountain range. Rock units of 

Precambrian, Paleozoic, and Mesozoic age are exposed 

in the Sangre de Cristo Range. A wide variety of marine 

and terrestrial depositional environments are 

represented by these strata. Each mappable rock unit has 

characteristics which may be important to the resource 

manager regarding resistance to erosion, fossil contents, 

mineral and cultural resources, and suitability for 

development. Great Sand Dunes National Park and 

Preserve now preserves and protects not only the tallest 

dunes in North America, but also pieces of the geologic 

record that extend back in time approximately 1,700 

million years in the Great Sand Dunes region.

 
 GRSA Geologic Resource Evaluation Report 3

Introduction 

The following section briefly describes the regional geologic setting and the National 

Park Service Geologic Resource Evaluation Program. 

Purpose of the Geologic Resource Evaluation Program 

Geologic resources serve as the foundation of park 

ecosystems and yield important information for park 

decision making. The National Park Service Natural 

Resource Challenge, an action plan to advance the 

management and protection of park resources, has 

focused efforts to inventory the natural resources of 

parks. The geologic component is carried out by the 

Geologic Resource Evaluation (GRE) Program 

administered by the NPS Geologic Resources Division. 

The goal of the GRE Program is to provide each of the 

identified 270 “Natural Area” parks with a digital 

geologic map, a geologic report, and a geologic 

bibliography. Each product is a tool to support the 

stewardship of park resources and each is designed to be 

user friendly to non- geoscientists. In preparing products 

the GRE team works closely with park staff and partners 

(e.g., USGS, state geologic surveys, and academics). 

The GRE teams hold scoping meetings at parks to review 

available data on the geology of a particular park and to 

discuss the geologic issues in the park. Park staff are 

afforded the opportunity to meet with the experts on the 

geology of their park. Scoping meetings are usually held 

in each park individually, although, to expedite the 

process, some scoping meetings are multipark meetings 

held for an entire Vital Signs Monitoring Network. 

For additional information regarding the content of this 

report, refer to the Geologic Resources Division of the 

National Park service, located in Denver, Colorado, with 

up to date contact information at the following Website 

http://www.nature.nps.gov/geology. 

Geologic Setting 

Great Sand Dunes National Park and Preserve is located 

in the San Luis Valley of Colorado, where the Sangre de 

Cristo Range bends to form a topographic pocket, or 

reentrant (figure 1). Near Blanca Peak and partially 

encompassing the great Sangre de Cristo Mountains that 

rise 6,000 fee1 (825 m) above the San Luis Valley floor, 

Great Sand Dunes protects 84,000 acres of sand dunes, 

alpine lakes and tundra, six peaks over 13,000 feet (3,962 

m), ancient spruce and pine forests, large stands of aspen 

and cottonwood, grasslands, and wetlands. Much of this 

diverse landscape was added to Great Sand Dunes 

National Monument in 2000 with the passage of the 

Great Sand Dunes National Park and Preserve Act. The 

original monument that was designated in 1932 was 

created in order to preserve the 19,200 acres (7,700 

hectares of dunes) (figure 2). 

The high- altitude desert dunes of Great Sand Dunes are 

unique in the Rocky Mountain physiographic province 

and represent one of the most complex dune systems in 

the world. The dune field, of which Great Sand Dunes is 

a part, covers 150 sq miles (389 sq km). Eolian (wind) 

processes constantly shape and reshape the dunes, 

forming dune features that appear and disappear on a 

regular basis. Classic examples of reversing and star dune 

types populate the dune field. High velocity, 

bidirectional winds that shape the dunes are funneled in 

and out of the valley through three relatively low passes 

in the Sangre de Cristo Range – Mosca, Medano, and 

Music passes. Medano Pass also is the source area for 

Medano Creek. 

Precambrian igneous and metamorphic rocks, Paleozoic 

and Mesozoic sedimentary rocks, Tertiary igneous and 

sedimentary rocks, and unconsolidated Quaternary 

sediments are exposed in the Sangre de Cristo 

Mountains and Great Sand Dunes National Park and 

Preserve (Map Unit Properties Table) (Bruce and 

Johnson 1991; Johnson et al. 1989; Lindsey et al. 1986). 

Gaps in the geologic record are represented by 

unconformities, which reflect periods of either erosion 

or nondeposition. The Precambrian rocks span the Early 

Proterozoic from 1.6 to 2.5 billion years ago and consist 

of quartz monzonite, diorite, and gneiss. 

The Paleozoic and Mesozoic exposures are primarily 

sedimentary rocks of the Pennsylvanian Minturn 

Formation (285 to 320 million years ago (Ma)) and the 

Pennsylvanian- Permian Sangre de Cristo Formation 

(250 to 300 Ma). Both formations consist mostly of shale, 

sandstone, and conglomerate. Mississippian, Devonian, 

and Ordovician strata are mapped as one unit, but 

include the Lower Mississippian Leadville Limestone, 

the Mississippian(?) to Lower Devonian Chaffee Group, 

the Upper Ordovician Fremont Dolomite, the Middle 

Ordovician Harding Sandstone, and the Lower 

Ordovician Manitou Limestone (Johnson et al. 1989). 

Rocks of Silurian age are missing in the region while only 

sporadic exposures of the Cambrian Sawatch Quartzite 

are present. In most of the region, the Ordovician 

Manitou Limestone rests on eroded Precambrian 

basement. In the Sangre de Cristo Mountains, the 

Middle Jurassic Entrada Sandstone and Upper Jurassic 

Morrison Formation represent the Mesozoic Era. 

Tertiary rocks in the Sangre de Cristo Mountains consist 

of Miocene and Oligocene igneous rocks and stratified 

sediments of the Santa Fe Formation that were deposited 

in the Pliocene and Miocene epochs. A variety of 

unconsolidated Quaternary- age deposits has been 

mapped in the Sangre de Cristo Mountains and the 

eastern part of Great Sand Dunes (Lindsey et al. 1986). 
 NPS Geologic Resources Division 4 

The San Luis Valley is the result of extensional, tectonic 

forces rather than erosional processes. Crustal extension 

began about 30 Ma and initiated the extensive volcanic 

activity that built the San Juan Mountains which border 

the San Luis Valley to the west (figure 3). Since extension 

began, displacement along normal faults in the basinbounding fault zone adjacent to the Sangre de Cristo 

Mountains has amounted to over 30,000 vertical feet 

(9,200 m) (Kluth and Schaftenaar 1994). Great Sand 

Dunes lies within this fault zone. The dip of this fault 

zone ranges from 45 degrees to 60 degrees (Kluth and 

Schaftenaar 1994). 

As much as 21,000 feet (6,400 m) of Tertiary and 

Quaternary age sediments underlie the asymmetric San 

Luis Valley as a result of fault movement (Kluth and 

Schaftenaar 1994). Aquifers in these sediments are critical 

to the survival of the dunes and agriculture in the valley. 

The northern part of the San Luis Valley, north of the 

San Luis Hills, has been termed the Alamosa Basin 

(Brister and Gries 1994). The Alamosa Basin, in turn, has 

been subdivided into two smaller basins: a western 

graben (the Monte Vista graben) and a deeper, narrower, 

eastern graben (the Baca graben) (figure 3) (Brister and 

Gries 1994). Separating the two sub- basins is a basement 

high called the Alamosa horst. The Baca graben has a 

greater degree of tilting than the Monte Vista graben 

(Brister and Gries 1994). 

The Baca graben also contains a thicker sequence of 

sediments than the Monte Vista graben. Up to 9,930 feet 

(3,027 m) of post- Eocene sediments were deposited in 

the Monte Vista graben compared to almost 16,000 feet 

(4,900 m) of sediments in the Baca graben. The basin fill 

thins to 5,400 feet (1650 m) on the Alamosa horst 

(McCalpin 1994). The degree of tilting and amount of 

sedimentary fill is due to the Baca graben’s proximity to 

the Sangre de Cristo fault zone (Brister and Gries 1994). 

The San Luis Valley is the largest intermontane basin in 

Colorado, stretching almost 135 miles (217 km) from 

north to south and 56 miles (90 km) from east to west 

(figure 4). At the latitude of the dunes, the valley is 40 

miles (64 km) wide. The northern terminus of the San 

Luis Valley is Poncha Pass, which separates the Rio 

Grande drainage from the Arkansas River drainage 

system (Johnson 1967). Except for the San Luis Hills, the 

surface of the San Luis Valley is remarkably flat. 

Elevations range from 7,546 feet (2,300 m) in the center 

of the valley to 7,874 feet (2,400 m) on the alluvial fans 

along the mountain front (Wurster and Cooper 2000). 

The San Luis Valley also can be subdivided into a 2,900 

square mile (7,600 sq km) closed basin to the north and 

an open basin through which the Rio Grande flows to 

the south (figure 4). Streams flowing into the closed 

basin form a system of wet meadows and ephemeral 

lakes (San Luis Lakes on figure 4). Evapotranspiration is 

the only outlet for water in the closed basin (Wurster and 

Cooper 2000). 

The headwaters of the Rio Grande River drain part of the 

glacially carved peaks of the San Juan Mountains. The 

Rio Grande is the major drainage in the valley and its 

watershed covers 4,633 sq miles (12,000 sq km) (Wurster 

and Cooper 2000). Eroded debris is carried into the San 

Luis Valley where, as the river’s gradient decreases at the 

foot of the mountains, sediment is deposited in an 

alluvial fan. The Rio Grande then meanders slowly in a 

southeasterly direction past Alamosa, Colorado and 

across a broad, flat flood plain (Johnson 1967). 

The San Luis Valley is an arid environment with an 

average annual precipitation of only 8 inches per year 

(200 mm/yr) (Wurster and Cooper 2000). The rain 

shadow effect created by the San Juan Mountains that 

form the western border of the valley contributes to this 

arid climate by creating a barrier to moisture swept into 

the area from the southwest and west. Located on the 

eastern border of the San Luis Valley, Great Sand Dunes 

receives only a bit more precipitation than the center of 

the valley. Thunderstorms between June and September 

contribute 60 percent of the annual precipitation. 

Snowfall in the mountains during the winter months is 

typically 12 times greater than in the center of the valley. 

Only a few days pass each year during which the sun 

does not shine in the San Luis Valley. High temperatures 

range from 50 degrees Fahrenheit (10 degrees 

Centigrade) during January to 90 degrees Fahrenheit (32 

degrees Centigrade) in July (Wurster and Cooper 2000). 

When the clouds are pushed against the Sangre de Cristo 

Mountains, thunderheads build and may drop over an 

inch (2.5 cm) of water in less than an hour on the dry, 

rocky foothills above the dunes. The rocky slopes are 

unable to absorb such intensive downpours, and the 

water runs swiftly downhill along any trail, arroyo, tire 

track, or other available path, forming gullies several feet 

deep in only a few minutes. These flash floods typically 

last only a few minutes before fading to a trickle. 

In the northwest corner of Great Sand Dunes, a wetland 

complex has developed on part of the sand sheet that 

surrounds 4,000 acres (10,000 hectares) of the tallest 

dunes in the park. In this area, dunes seldom exceed 33 

feet (10 m) in height and most of the dunes have been 

stabilized by vegetation (Wurster and Cooper 2000). 

Here, interdunal wetlands occur in depressions between 

the dunes. Most wetlands are less than 0.2 acres (0.40 

hectare) in size. Sand Creek, drains 15 sq miles (39 square 

km) of the Sangre de Cristo Mountains, and flows 

northwest of the interdunal wetland area, this is the 

largest stream entering Great Sand Dunes. Flow in the 

stream is seasonal, occurring between April and October. 

Park History 

Humans have inhabited the San Luis Valley for 

thousands of years. (Trimble 1972; Kiver and Harris 

1999). Projectile points made 15,000 years B.C. have been 

found in the dunes and other locations in the valley 

(Trimble 1972). Hunters belonging to the Folsom and 

Clovis cultures hunted mammoth, bison, giant sloth, and 

other game in the valley about 10,000- 11,000 years ago 
 GRSA Geologic Resource Evaluation Report 5

(Kiver and Harris 1999). Bones of a now- extinct bison 

along with the fluted spear points that killed the animal 

were found west of the park near remains of a campfire 

used to roast the kill 10,000 years ago. 

Pueblo Indians were the only hunters in the valley for 

centuries until 1300 A.D. when Apaches and Navajos 

arrived from the north and the Utes entered Colorado 

from the west. The Utes soon established their 

dominance in the mountains, and they were the most 

recent Native Americans to hunt in the grasslands 

adjacent to the dunes. They were in the valley when Don 

Diego de Vargas and his exploration party, the first 

Europeans, arrived in 1694 and also when Zebulon Pike, 

of the United States Army, stumbled down Medano Pass 

with ten hungry, freezing men in 1807, looking for the 

source of the Red River. Upon seeing the dunes, Pike 

recorded “sandy hills...[whose] appearance was exactly 

that of the sea in a storm, except as to color, not the least 

sign of vegetation existing on them.” (Trimble 1972). 

Pike’s journal gave the world the first written description 

of the dunes. 

Eventually, through a series of broken treaties beginning 

in 1855, the Utes were pushed from the San Luis Valley to 

a small reservation in the dry, southwestern corner of 

Colorado. Farmers and ranchers moved into the valley 

beginning in the 1850s, although the dry climate and 

harsh winters limited development. Miners scoured the 

Sangre de Cristo Mountains looking for gold and other 

precious minerals and by the 1890s, a “fair- sized” camp 

sprang up in North Arrastre Canyon, southeast of Great 

Sand Dunes (Trimble 1972). The canyon’s name derives 

from the arrastra, or ore- grinding stone, that the 

prospectors found and which was probably left by the 

Spanish in the 1600’s. No significant ore deposits were 

discovered and the camp was abandoned about as 

quickly as it had been built. 

A richer strike was discovered north of the dunes in 1876. 

Duncan, Colorado, became the new boomtown until 

1900, when the owners of the Baca land grant, upon 

which the town sprawled, decided to enforce their 

property rights and evicted the entire town. Residents 

moved across the fence, taking their town with them, and 

renamed it Liberty. The ore eventually ran out and the 

miners vanished. Today, only a few disintegrating 

buildings remain to mark the boom times at Liberty. 

While prospectors’ picks rang in every nook and cranny 

of the mountains, Ulysses Herard began raising cattle up 

the Medano Pass trail in 1878. To many, Herard is the 

official character of Great Sand Dunes (Trimble 1972). 

Stories of Herard feeding full- curl bighorn rams lumps 

of salt by hand and killing bears with a hatchet are as 

colorful as the land itself. Herard single- handedly raised 

6,000 head of cattle, was quite well read, and lived in his 

meadow until the 1930’s. The mountain above Medano 

Pass is still referred to by some as Herard Peak. 

All of this human activity surrounded the sand dunes but 

rarely penetrated them. Sand won’t support grain 

production or cattle and makes a poor foundation for 

cabins. The dune sand does contain a trace of gold, 

however, and the fever briefly lured miners into the area 

in the 1930s. The Volcanic Mining Company once built a 

mill along Medano Creek near today’s picnic area, but 

the operation was not profitable and the buildings were 

torn down. 

In order to preserve the dunes, some visionary women of 

the valley got together in 1930, led by Mrs. Elizabeth 

Spencer of Monte Vista and Mrs. Millicent Velhagen of 

Alamosa, and encouraged the creation of a Great Sand 

Dunes National Monument (Trimble 1972). Supported 

by valley residents, the state of Colorado, and 

Washington, D.C., their dreams became reality in 1932 

when President Herbert Hoover proclaimed the area a 

National Monument. 

National designation, however, does not necessarily 

discourage modern developers. Developers, spurred by 

the abundant groundwater beneath the monument, 

designed 13,000 houses to be built along or near the 

monument’s edge (Kiver and Harris 1999). Because of 

concern that a lowering of the water table would 

threaten the preservation of the dunes as well as park 

plants and wildlife, the Great Sand Dunes National Park 

and Preserve Act was passed in 2000, derailing these 

developments. Groundwater in the San Luis Valley, 

however, remains a precious commodity especially for 

thirsty Front Range cities, primarily metropolitan 

Denver. 
 NPS Geologic Resources Division 6 

Figure 1. Map of Great Sand Dunes National Park and Preserve. Map courtesy of the National Park Service. 
 GRSA Geologic Resource Evaluation Report 7

Figure 2. Great Sand Dunes and Sangre de Cristo Mountains. Photo courtesy of the National Park Service. 

Figure 3. West to east geologic cross-section across the San Luis Valley. The structural valley is deeper on the eastern edge where 

movement is on the Sangre de Cristo Mountain-front fault. From NPS, http://www.nps.gov/grsa/resources/wind_regime.htm (access 

December 10, 2005). 
 NPS Geologic Resources Division 8 

Figure 4. Geographic setting of Great Sand Dunes National Park and Preserve in southern Colorado. Black arrows indicate wind 

direction. Blue lines indicate fluvial systems. Brown areas indicate mountain ranges. Modified from Kiver and Harris, 1999. 
 GRSA Geologic Resource Evaluation Report 9

Geologic Issues 

A Geologic Resource Evaluation scoping session was held for Great Sand Dunes 

National Park and Preserve on August 21, 1998, to discuss geologic resources, to address 

the status of geologic mapping, and to assess resource management issues and needs. The 

following section synthesizes the scoping results, in particular, those issues that may 

require attention from resource managers. 

The sand dunes are the key to the dune field ecosystem 

of Great Sand Dunes National Park and Preserve 

(Resource Management Strategy 1994). They are the 

common thread binding the physical, biologic, and 

cultural subsystems in the Great Sand Dunes area. In 

order to understand and manage this ecosystem as a 

whole, resource managers must understand the eolian 

processes that shaped the dunes, the present status of the 

dunes, and their range of dynamic variations (Resource 

Management Strategy 1994). These dynamic variations, 

or fluctuations, not only need to be identified but also 

the causes of these fluctuations need to be determined. 

This section addresses the following geologic issues 

important to Great Sand Dunes: 

• Water issues and dune preservation 

• Earthquake potential 

• Oil and gas development 

• Mining issues 

The section also lists potential research projects 

discussed during the 1998 scoping workshop 

(Appendix B). 

Water Issues 

Water is the lifeblood of the Great Sand Dunes. Without 

water, creeks would no longer flow across the sand, the 

dunes and the sand deposits around them would change, 

and wetlands and ponds that support a rich diversity of 

life would dry up. Because of this integral relationship 

between water and sand, Congress passed the Great 

Sand Dunes National Park and Preserve Act, which 

authorized the creation of a national park (the former 

Great Sand Dunes National Monument) and a national 

preserve that includes the entire hydrological system of 

the dunes, from tundra to wetlands (figure 1). 

Preservation of the sand sheet that lies west of the dune 

field, a major supplier of sand, is critical to the 

preservation of the dune field (figure 1). Groundwater 

withdrawal may adversely impact the sand sheet. In 

addition, water withdrawal may impact the health of the 

environment in Great Sand Dunes. The amount of water 

available for ecosystem management and drinking water 

supplies, is a major concern for the park and preserve. 

All of the dune features, including the unique 

morphology of the dunes, surge flow in Medano and 

Sand Creeks, and interdunal ponds and wetlands are 

closely tied to the delicate balance among precipitation, 

surface flow, and groundwater. Slightly more 

precipitation falls at Great Sand Dunes than in the center 

of the valley due to its location at the base of the 

mountains (Chatman et al. 1997). The monsoons of July, 

August, and September bring in the greatest amount of 

precipitation. Although, the month- by- month 

precipitation is less during the winter than summer, 

snowfall is sufficient to produce a substantial snow pack 

in the mountains. 

Water in the San Luis Valley is a precious resource. In 

1991 an application by American Water Development 

Incorporated to pump 200,000 acre- ft of water per year 

from San Luis Valley aquifers and to transport it via 

pipeline to the Denver vicinity was denied. The proposed 

project would have lowered the water table 150 feet (46 

m) along the park’s boundary. Water resources of the 

San Luis Valley will continue to be sought by entities 

outside the valley – both within Colorado and from other 

states (Emery 2005). Management and monitoring of 

surface water and groundwater in the San Luis Valley 

and at Great Sand Dunes are critical to dune 

maintenance and sustainability. 

Surface Hydrogeology: Most of the streams flowing out of 

the Sangre de Cristo Mountains are small and end within 

a few miles of the mountain front (Andrew Valdez, NPS 

geologist, personal communication, November, 2005). 

Two streams flow along segments of the dune field 

perimeter (figure 5). Sand Creek, the larger of the two, 

flows along the northwestern side and Medano Creek 

flows along the east and southeastern sides of the dune 

field. While Sand Creek occasionally reaches some playa 

lakes located 10 miles (16 km) from the mountain front, 

both streams infiltrate the groundwater system. 

Since 1992, discharge has been measured on each stream 

with Parshall flumes placed near the location where the 

streams enter park service land. Peak flow on Sand Creek 

ranges from 54 to 225 cubic feet per second (cfs) and 

occurs in May and June. Base flow for Sand Creek varies 

from 0 to 1 cfs. Peak flow for Medano Creek fluctuates 

from 9 to 65 cfs with a base flow of 2- 3 cfs (Valdez 1996). 

These two creeks erode sand from some parts of the 

dune field and deposit it in others. Both streams and 

their tributaries begin in the high, glaciated valleys of the 

Sangre de Cristo Range as typical mountain streams with 

steep gradients and rocky channels. However, once their 

gradient lessens, they become braided streams choked 
 NPS Geologic Resources Division 10 

with a large sand load. Surge flows occur in the braided 

streams at regular intervals and are critical for the 

maintenance and sustainability of the dunes. A surge 

flow is a pulse of water that can flush more sand down 

the stream than steady flow. Surge flow is dependent on 

stream discharge levels, water depths, and flow 

velocities. A decrease in surface flow would lead to a 

lower mean velocity in the creeks resulting in a lower 

sediment transport rate (Wiegand 1977). 

Two of three distinct regions of dune development are 

influenced by the action of Sand and Medano Creeks 

(figure 5). The thick sand deposits and closely spaced 

reversing dunes of Medano Creek ridge are a direct 

response to the availability of sand supplied by Medano 

Creek (Valdez 1996). Sand availability and a complex 

wind regime result in the Sand Creek star dunes. 

Prevailing winds from the southwest blow sand back into 

the dunes. The great thickness of dunes in the park 

comes from vertical dune growth that is facilitated by 

excess sand and multiple wind directions (Valdez 1996). 

If surge flow is eliminated, steady flow will become the 

dominant process and less sand will be transported 

downstream becoming available for recycling into the 

dunes. 

Groundwater Hydrogeology: Except for water lost to 

evapotranspiration, all the water in Sand and Medano 

Creeks normally infiltrates a shallow unconfined aquifer 

(Chatman et al. 1997; Rupert and Plummer 2004). The 

existence and natural maintenance of the dune field are 

dependent on maintaining historic groundwater levels in 

this aquifer. The unconfined aquifer and a deeper 

confined aquifer are the two principal aquifers at Great 

Sand Dunes. 

The unconfined aquifer is located in the Alamosa 

Formation, which consists of unconsolidated gravels, 

sands, clays, and conglomerates. In most locations, the 

water table is within 10 feet (3 m) of the surface and is 

generally less than 100 feet (30 m) thick. A clay layer 

about 8 feet (2.4 m) thick and about 300 feet (91 m) deep 

separates the unconfined aquifer from the confined 

aquifer, also located within the Alamosa Formation. The 

eastward extent of the clay layer extends toward the base 

of the Sangre de Cristo Mountains is not known (Rupert 

and Plummer 2004). 

The confined aquifer ranges from 500 feet (152 m) to 

5,000 feet (1,524 m) thick. On the east side of the valley, 

an unconformity marks the base of the active confined 

aquifer while on the west side, the base of the aquifer lies 

above ash- flow tuffs. Artesian water from this confined 

aquifer was discovered in about 1887 and within four 

years, about 2,000 flowing wells had been developed 

(Emery 2005). The confined aquifer continues to be a 

primary water source in the valley. 

Radiocarbon dating of groundwater in the confined 

aquifer indicates it is about 30,000 years old (plus or 

minus 3,000 years) (Rupert and Plummer 2004). For 

comparison, the last major ice advance (Wisconsin) 

during the ice age occurred about 20,000 years before 

present. Studies have shown that fluctuations in the 

interdunal ponds are not tied to the confined aquifer. 

A third aquifer, called the “passive confined aquifer,” has 

also been identified in the San Luis Valley (Chatman et al. 

1997). This is the deepest aquifer. Water within the 

aquifer is “passive” and does not move. The passive 

confined aquifer is based in the Precambrian basement 

rocks, the San Juan volcanic units, and the underlying 

Blanco Basin/Vallejo Formations. Little is known about 

this aquifer but hydrologists believe the aquifer has low 

hydraulic conductivity and poor water quality. 

Although less than 10 inches (25 cm) of precipitation falls 

in the area each year, a large aquifer system exists in the 

subsurface. Estimates of over 2 billion acre- feet of water 

stored in aquifers above 6,000 feet (1,829 m) have been 

made. The San Juan Mountains provide much more 

recharge to the aquifers than the Sangre de Cristo 

Mountains, yet the Sangre de Cristo Range is locally 

important to the aquifers in Great Sand Dunes. 

Aquifers in the vicinity of Great Sand Dunes National 

Park and Preserve are important for four reasons 

(Chatman and others 1997): 

• they are sources of agricultural and potable water 

• they influence streamflow 

• they create interdunal ponds where the dune surface 

intersects groundwater 

• they influence wetlands 

The proposal by American Water Development 

Incorporated to extract 200,000 acre- feet each year 

from the San Luis Valley led the NPS and other agencies 

to conduct research into the hydrologic system of the 

park and preserve. Studies using 21 groundwater 

monitoring wells, the Parshall flumes on Medano and 

Sand Creeks, and Schlumberger soundings and resistivity 

testing to map the water table near the creeks showed 

seasonal fluctuations in the water table. These studies 

showed fluctuations of up to 10 feet (3 m) in shallow 

wells (20 ft; 6 m) near the creeks and fluctuations of less 

than 1 foot (0.3 m) in deeper creek wells (100 ft; 30 m) and 

wells away from the streams (Valdez 1996). Results from 

the research predicted that any significant lowering of 

the water table would increase the gradient between the 

streams and groundwater and decrease the extent and 

volume of flow, the ability for surge flow to develop, and 

the ability of these streams to transport sand. If the 

creeks cannot transport sand back to the upwind side of 

the dune field, the long- term viability of the dune field 

will be jeopardized (Rupert and Plummer 2004). 

Wetlands associated with interdunal ponds and 

depressions along the western edge of the dune field are 

also affected by changes in the shallow unconfined 

aquifer. The number of ponds decreased from 36 in 1936 

to zero in 1966, and then gradually increased to 9 in 1995 

(Chatman et al. 1997; Ruppert and Plummer 2004). 
 GRSA Geologic Resource Evaluation Report 11

Interdunal ponds intersect the water table of the 

unconfined aquifer. Thus, their occurrence is directly 

affected by fluctuations in the water table of the 

unconfined aquifer. Lowering the water table of the 

shallow unconfined aquifer, results in an immediate 

decrease in the water levels of the interdunal ponds 

(Rupert and Plummer 2004). The water table may be 

lowered due to periods of drought, during periods of 

over pumping of the unconfined aquifer, or through 

lowering of local base level by downcutting of Big or 

Little Spring Creeks, both of which originate in the park 

and flow to the southwest (figure 1). 

To better understand the hydrologic cycle at Great Sand 

Dunes, NPS staff are studying how snow pack, storm 

runoff, and the position of the water table relate to 

stream flow rates and duration (Valdez 1996). While the 

cycle is relatively simple – snow melts, flows down hill in 

streams, and soaks into the ground – predicting what 

changes in any part of the cycle would do to the other 

parts of the cycle is more complex. Data regarding the 

monitoring system, rates of discharge, water quality, 

groundwater flow direction, and gauge data are available 

in Chatman and others, 1997, and Rupert and Plummer, 

2004. 

The predominant feature in Great Sand Dunes is the 39 

square- mile (100 sq km) dune field. Some of the dunes 

are as much as 750 feet (230 m) high. If the water table 

lowers, the process of sand recycling will be significantly 

reduced. Over time, if the dune field were unbounded by 

Sand Creek and Medano Creeks scientists expect that 

the dunes would spread out over a larger area at a lower 

height. 

Earthquake Potential 

Earthquakes present a potential problem due to surface 

shaking that might affect water tanks, pipelines, and most 

of the other facilities that lie in the vicinity of fault lines. 

The faults associated with Great Sand Dunes may act as 

barriers to groundwater flow thus causing additional 

pressure buildup along the faults that may lead to 

eventual failure. The Visitor’s Center is currently located 

on a fault scarp and the campground is located on 

unstable ground (landslide or debris flow) that may be 

reactivated during a seismic event. 

The Sangre de Cristo Range was rapidly uplifted about 19 

Ma in the Miocene and uplift has continued into recent 

time. The approximately 1.2 miles (2 km) of relief on the 

range occurred primarily in the Late Miocene (Lindsey 

et al. 1986). Fault scarps, from multiple earthquakes, 

offset Quaternary deposits and landforms are evidence 

of continued uplift. The average vertical displacement on 

recent faults is 2.9 to 9.5 feet (1.2- 2.9 m) (McCalpin 1994). 

Magnitude 7 and greater earthquakes that accompany 

displacement have a return interval of 10- 47 thousand 

years. The last two earthquakes along the Sangre de 

Cristo mountain front fault with a magnitude greater 

than 7 occurred about 10- 13 thousand years ago and 7.6 

thousand years ago, respectively. Holocene displacement 

of the Sangre de Cristo fault marks it as one of 

Colorado’s few active faults (McCalpin 1994). 

The Holocene and Quaternary faults in the San Luis 

Valley are not associated with any historical seismic 

event. In the last 120 years only one earthquake has been 

recorded, in 1952 (Stover 1988; McCalpin 1994). 

Oil and Gas Development 

Hydrocarbon exploration and development may 

negatively impact groundwater quality, scenic views, 

dark night sky, ambient sound levels, and plant and 

animal communities. 

Interest in oil and gas development in the vicinity of 

Great Sand Dunes began in 1992 when Challenger Gold 

Inc. accidentally discovered oil while they were 

exploring for gold on the Baca land grant. The oil was 

discovered in 28 shallow exploratory drill holes. The 

discovery prompted Challenger (now doing business as 

Lexam Explorations) to redirect its gold exploration in 

favor of concentrating on developing the petroleum 

potential on the Baca lands. In 1995 two exploratory oil 

and gas wells (Baca #1 and Baca #2) were drilled by 

Lexam along the eastern edge of the valley near 

Crestone. The oil and gas operator stated that unrefined 

oil and gas was found in shallow cores in these wells, but 

the data are not available to the public. Neither of the 

wells was completed as a producing well (Lisa Norby, 

Geologic Resources Division, 2/06). Two additional 

exploratory drilling permits were obtained to test the 

hydrocarbon potential on the Baca lands but the permits 

expired in 2003 and the wells were never drilled. 

The oil and gas drilling target in the park called the 

Crestone Prospect is a structural anticline in the 

Crestone sub- basin of the Baca Graben. The Baca 

Graben is part of the Tertiary aged rift system extending 

from south- central Colorado to Mexico. An additional 

prospect called the Pole Creek Prospect is a shallow oil 

target in a fault block near the margin of the basin 

(http://www.lexamexplorations.com). Both drilling 

targets were identified on seismic and gravity data. The 

geologic plays are interpreted by Lexam to contain 

Mesozoic- aged source (Mancos shale) and reservoir 

rocks (Dakota sandstone), but there are no publicly 

available geologic data to confirm or deny the presence 

of the Mesozoic strata in the basin. The most commonly 

held geologic interpretation is that the Mesozoic oil and 

gas source and reservoir rocks were eroded and are no 

longer present in the eastern portion of the San Luis 

Valley. However, recently Bob Kirkham of the Colorado 

Geological Survey and Andrew Valdez of Great Sand 

Dunes National Park located some outcrops that appear 

to be Dakota sandstone and Morrison Formation in a 

part of the Baca Ranch that is now managed by the US 

Forest Service (written communication Andrew Valdez, 

Great Sand Dunes National Park and Preserve 3/2/2006). 

In 2004 Petro Hunt, an oil and gas company based in 

Dallas Texas leased all of oil and gas interests on the Baca 

Ranch from Lexam Exploration and Conoco/Phillips. 
 NPS Geologic Resources Division 12 

They conducted a 60 mile seismic survey to confirm the 

subsurface Crestone oil and gas prospect. The results of 

the seismic survey were discouraging and Petro Hunt 

elected to not drill the Crestone Prospect and released 

the leases to Lexam (pers. communication Steve Bressler, 

Petro Hunt 2/24/2006). Based on the Geologic Resources 

Division analysis of the limited available data, the lack of 

oil and gas production in the San Luis Basin, and minimal 

interest by oil and gas companies in drilling the Crestone 

Prospect, the division has concluded that the possibility 

of a commercial oil and gas discovery in the park is 

unlikely and highly speculative. Any future oil and gas 

development in the park would be regulated by the 

National Park Service’s Nonfederal Oil and Gas Rights 

regulations art 36 CFR Part 9 Subpart B regulations. 

Mining Issues 

Lexam Explorations, Inc., originally prospected the Baca 

grant for gold in 1990. They patterned their program 

after the Battle Mountain Gold Company’s San Luis 

Mine, located near the town of San Luis, CO. The San 

Luis Mine hosts disseminated gold of hydrothermal 

origin in a 24 to 27 million- year- old low- angle, 

detachment- style fault zone associated with the Rio 

Grande rift. Upward- migrating gold- bearing 

hydrothermal fluids at the San Luis Mine were trapped 

by a clay layer similar to the one that acts as the 

petroleum trap on the Baca grant oil shows. The 

hydrothermal fluids tended to pool below the clay zone. 

Minor quantities of sphalerite and galena are also present 

in the San Luis Mine which was prospected for lead and 

zinc decades ago. Copper is also present at the San Luis 

Mine. (http://www.nature.nps.gov/geology 

/parks/grsa/geology, access December 16, 2005). 

Lexam drilled 42 exploratory drill holes around 

Deadman Creek in and near the foothills on the Baca 

grant, but a reserve estimate could not be substantiated. 

While gold exploration has currently been suspended, an 

increase in gold prices could issue a return of the 

exploration program. The structural zone that is thought 

to host the gold may extend beneath the park and 

preserve (http://www.nature.nps.gov/geology/parks 

/grsa/geology, access December 16, 2005). Although 

private companies do not have to submit exploration 

reports to the state, the Colorado Geological Survey 

knows of no new gold exploration on the Baca grant 

(James Cappa, Chief, Mineral and Mineral Fuels Section, 

Colorado Geological Survey, personal communication, 

December 21, 2005). Any company seeking to develop 

hard rock minerals in the park and preserve would need 

to obtain a National Park Service special use permit prior 

to undertaking any activity. 

Lode Mining: A lode is a mineral deposit in consolidated 

rocks such as a vein, as opposed to a placer deposit found 

in loose gravels. Small mining operations and prospects 

were active in Great Sand Dunes in the past prior to the 

inclusion of the area in the National Park System. The 

park and preserve is closed to the location of new mining 

claims under the Mining Law of 1872. Numerous 

abandoned mine and prospect excavations exist 

upstream of and in the watershed that drains into the 

park. Mineral- development activities potentially could 

affect water quality of the park by introducing heavy 

metals and increasing acidity. 

Most of the past lode mining activity within Great Sand 

Dunes occurred in the Cold Creek watershed on a small, 

unnamed tributary of Cold Creek in the Sangre de 

Christo Mountains in the northern part of the Preserve 

(figure 1). There is a 2,300 cubic yard dump at the main 

site, but little metallized or sulfidized rock is present, 

therefore acid mine drainage and heavy metal loading 

from this site are unlikely to impact water quality 

(http://www.nature.nps.gov/geology/parks/grsa/geology, 

access December 16, 2005). 

Three other groups of prospect excavations are located 

north of the main Cold Creek drainage channel and all 

are thought to be sources of at least some of the ironoxide cementation that occasionally occurs in Sand 

Creek. If iron and heavy metal sulfides are present, the 

cations can dissolve in the water creating sulfuric acid 

commonly know as acid mine drainage. These sites may 

require further investigation (http://www.nature.nps.gov 

/geology/parks/grsa/geology, access December 16, 2005). 

Other excavations of lode deposits in Great Sand Dunes 

are not thought to be detrimental to water quality 

because the sites are small, lack concentrated 

mineralization and occur some distance from drainages. 

This is confirmed by water quality measurements. These 

sites include excavations in Sawmill Canyon, on the 

Wellington tract, and at Denton Spring. A detailed 

description of these sites was prepared by Chatman in 

1995 and should be on file at the park headquarters 

(Chatman 1995; http://www.nature.nps.gov/geology 

/parks/grsa/geology, access December 16, 2005). 

The Myrtle K Mine and mill site are upstream of the 

dunes on Sand Creek on a narrow fault zone that 

contains iron and copper sulfide mineralization. There is 

a 2,100 cubic yard dump, a boiler, and a five- stamp mill, 

all immediately adjacent to the creek. Slightly elevated 

levels of arsenic, boron, copper, manganese, and zinc 

have been found in samples taken from the fault and a 

shear zone in the site. 

Workings on Medano Creek, Buck Creek, Morris Gulch, 

Evans Gulch, and South Arrastre Creek in the Sangre de 

Christo Mountains, and a working in the Sand Creek 

watershed about 0.5 miles (0.8 km) downstream from the 

Myrtle K Mine have very little impact on water quality. 

This is because the workings are small, they lack sulfide 

mineralization, the bedrock tends to neutralize any acid 

produced, and they are not close to any drainage 

channels, or some combination of these factors 

(http://www.nature.nps.gov/geology/parks/grsa/geology, 

accessed December 16, 2005). Historical information 

suggests that some copper may be present in Mosca 

Creek, but limited water quality testing has been negative 

for copper. If a decline in the water quality is detected in 
 GRSA Geologic Resource Evaluation Report 13

the future, the source of mineralization should be 

investigated. 

Placer Mining: An unsuccessful gold rush was sparked in 

1928 when gold was found to be associated with 

magnetite grains in dune field sand. Claims were located 

on lands in the dune field as well as on Medano Creek. 

Mining ended by 1938 due to low gold concentrations, 

although some prospecting continued to 1941. In 1980 

another attempt to recover placer gold occurred at some 

unknown site on Medano Creek, probably inside the Rio 

Grande National Forest. The operation was terminated 

due to lack of access routes (http://www2.nature.nps. 

gov/geology/parks/grsa/geology, access December 16, 

2005). 

The abandoned placer claims do not pose a water quality 

issue for Great Sand Dunes. All equipment and materials 

have been removed. No mercury anomalies have been 

found in the water quality testing done since 1990. 

Potential Research Projects 

Coupled with these geologic issues are potential research 

projects that might help to better define them for the 

resource manager. At the workshop (Appendix B), the 

following projects were proposed: 

• Date the dunes in order to get a better idea of dune 

history. 

• Map the existing fault lines. 

• Map groundwater flow patterns. 

• Differentiate paleo- fluvial channels within the park 

and link this research with global warming studies, 

human history, and hydrologic history. 

• Find the depth to bedrock, especially in the basin 

located west of the major dune field. Interest was 

expressed at the workshop in drilling below the 

confining layers. Projected drilling depth was 

estimated to be about 300 feet (91 m). 

• Study the impact of groundwater withdrawal on 

sabkha environments. 

• Study the rate of infiltration of moisture into the soil. 

Questions were raised concerning meteoric water, 

groundwater, and the origin of the vapor phase. 

• Define the relationship between soil moisture and 

vegetation on dunes. 

• Research why light colored ripples are coarser grained 

than dark colored ripples on the dunes, the different 

mineralogy of the dunes, and the provenance of the 

sands. Workshop participants were interested in why 

the light colored and dark colored sands didn’t mix 

and questioned if they were the products of different 

wind regimes or some other factor. 

• Study the impact of livestock grazing of vegetation on 

the sand sheet and its impact on the dunes. Dune 

preservation, vegetation, groundwater, and grazing are 

all interrelated, and a better understanding of this 

interrelationship might be beneficial to the resource 

manager. 

The dune system at Great Sand Dunes National Park and 

Preserve is one of the most complex in the world. Great 

Sand Dunes offers scientists an unparalled opportunity 

to study high- altitude, cold climate dune ecosystems in 

detail. 
 NPS Geologic Resources Division 14 

Figure 5. Physiographic map of the dune field, Great Sand Dunes National Park and Preserve. From http://www.nps.gov/grsa/resources 

/maps/dune_field_relief.jpg. 
 GRSA Geologic Resource Evaluation Report 15

Geologic Features and Processes 

This section provides descriptions of the most prominent and distinctive geologic features 

and processes in Great Sand Dunes National Park and Preserve. 

Dune Features and Processes 

Great Sand Dunes National Park and Preserve contains 

the tallest dunes in North America. Standing 750 feet 

(230 m) above the San Luis Valley floor, these dunes are 

internationally significant as a high- altitude, seasonally 

cold climate eolian system. The Great Sand Dunes are 

actually one giant dune with many smaller dunes 

superimposed on its surface. This type of large dune 

feature is called a draa, or giant dune field (Andrew 

Valdez, http://sangres.com/np/sanddunes/ 

faq.htm, access December 12, 2005). 

Main dune field: Roughly 39 square miles (100 sq km) of 

dunes between Sand Creek and Medano Creek are 

constantly being shaped and reshaped by wind (figure 6). 

Prevailing southwesterly winds carry sand that has been 

deposited on the floor of the San Luis Valley to the 

Sangre de Cristo Mountain front. Medano Pass, at about 

9700 feet (2960 m) elevation, is the lowest of the three 

passes east of Great Sand Dunes. Although far removed 

from the preserve, the pass plays a major role in dune 

formation by funneling northeasterly winds across the 

dune field (Valdez 1996; Rupert and Plummer 2004). The 

resulting bimodal wind regime changes dune behavior 

from migratory to vertically growing. 

Sand dunes grow to enormous heights not only because 

of a complex wind regime, but also due to a sand 

recycling process. Medano and Sand Creeks seasonally 

carry sand from the east and north side of the dune field 

and redeposit it where southwesterly winds can 

transport it back to the dune field. 

Surge flow in the braided channel sections of the two 

streams plays a significant role in transporting sand 

beyond the western edge of the dune field so that it can 

be recycled onto the dunes. Surge flow is a rare 

hydrologic phenomena in nature because the process 

requires a high flow regime and a smooth, mobile 

channel (Chatman et al. 1997). Each surge is a pulse, or 

wave, of water released at regular intervals from 

bedforms called antidunes. Antidunes are relatively large, 

sinuous ripples that form in the fast flowing, smooth 

channel. When the unstable antidunes collapse, they 

release their stored water. Because the channel is sandy 

and creates little turbulence, the pulse of water continues 

downstream in a discrete packet and picks up more 

water from other antidunes (Schumm et al. 1982; Valdez 

1996). 

The magnitude of the surge wave depends on water 

depth. Surging usually occurs at 15 to 20- second intervals 

in depths of only a few inches. During spring runoff, 

surge waves may reach 8 to 10 inches (20 to 25 cm) high 

with 40 to 60- second cycles. As antidunes break, they 

carry a large sand load downstream. The sediment is 

deposited westward of the main dunes and 

southwesterly winds blow it back into the dune field. 

Sand Creek and Medano Creek also contribute to two of 

three distinct regions of dune development shown on 

figure 6 (Valdez 1996). First, sand supplied by Medano 

Creek forms the thick sand deposits and closely spaced 

reversing dunes in the area known as the Medano Creek 

ridge. The north trending dunes cannot hold all of the 

sand supplied to them so sand fills their troughs, forming 

a second northeast trending ridge that becomes the 

horizon of the ridge. Secondly, the Sand Creek star 

dunes are the result of a complex wind regime that 

buffets sand from Sand Creek. The third area, the central 

dune field, isn’t affected by the streams and so displays 

the simplest dune formation. Found in the central dune 

field are large north- trending reversing dunes, with an 

occasional star dune, that are separated by vegetated 

troughs. 

Individual sand grains move in three ways: saltation, 

creep, and suspension. Saltation results when grains 

bounce slightly above the surface when blown by the 

wind. Wind velocities must be greater than 13 mph (21 

km/h) to transport sand. Bouncing sand grains reach 

heights of less than 2 inches (5 cm) and move 

horizontally 2 to 4 inches (5 to 10 cm). When a sand grain 

collides with other grains, it “creeps” by rolling or 

making small jumps. When blown high in the air, a sand 

grain moves by suspension. Saltation accounts for 95 

percent of sand grain movement; creep accounts for 

about 4 percent; and suspension accounts for 1 percent of 

sand grain movement. 

The active main dune field extends as much as 5 miles 

(8 km) westward from the Sangre de Cristo Mountain 

front. The sand dunes contain approximately 170 billion 

cubic feet (4.8 billion cu m) of sand. Still, the main dune 

field covers the least area of three sand deposits in the 

Great Sand Dunes system (figure 6). 

Sand sheet: Extending as much as 20 miles (32 km) 

westward from the Sangre de Cristo Mountain front is 

the more aerially extensive sand sheet deposit (figure 6). 

The sand sheet covers 180 square miles (466 sq km) and 

primarily is composed of low- relief, vegetationstabilized sand dunes (figure 7B) (Rupert and Plummer, 

2004). The thickness of the eolian sand sheet is about 140 

feet (43 m). 

Vegetation in the sand sheet helps to stabilize the sand 

and prevent it from moving. However, active parabolic 
 NPS Geologic Resources Division 16 

and barchan dunes are located on the south and west 

areas, respectively, of the sand sheet. Over a 63- year time 

span, total net movement of the parabolic dunes was 0.2 

to 0.4 miles (0.4 to 0.6 km). The barchan dunes moved 

less, averaging net movement of 0.1 to 0.2 miles (0.2 to 0.3 

km) (Marin and Forman 2004). The dunes move farther 

during drought years when vegetation and surface water 

resources are reduced. During the most recent drought 

in the late 20th to early 21st centuries, there was an 8 

percent increase in the rate of parabolic dune movement 

between 1998 and 1999 compared to prior wet years 

(Marin and Forman 2004). 

Sabkha: Like the sand sheet, the sabkha is a subtle feature 

at Great Sand Dunes (figure 7C). The third major sand 

deposit, the Sabkha lies farther west in the valley, near 

the San Luis Lakes (figure 6). The sabkha covers 120 

square miles (311 sq km) and is characterized by sand 

grains held together by evaporite minerals (salts). 

Because the sand in the sabkha is hardened by minerals, 

the wind cannot shape it into dunes. Seasonal ponds 

form on the sabkha and these ponds, along with other 

wetlands, are an important part of the greater Great Sand 

Dunes ecosystem, supporting rare shorebirds, mammals, 

amphibians, and the endangered Slender spiderflower. 

The sabkha would slowly disintegrate without regular 

saturation and evaporation. 

Dune types: Bimodal and complex wind patterns form 

reversing and star dunes in the dune field, while areas 

with unimodal winds, such as the sand sheet and 

mountain flanks, develop parabolic, barchan, and 

transverse dunes (figure 8). Special dune features such as 

climbing dunes, coppice dunes, large blowouts, and sand 

ramps are also present. 

Parabolic dunes and barchan dunes are crescent- shaped 

dunes (figure 8). Where the tails of parabolic dunes point 

upwind, however, the tails of barchan dunes point 

downwind. Parabolic dunes have a relatively limited sand 

supply compared to transverse dunes, and vegetation 

often anchors the tails of the dunes. Consequently, winds 

remove, or blow out, the sand from the center of the 

dune and form a nose pointing downwind. Parabolic 

dunes, therefore, are also called blowout dunes. 

At Great Sand Dunes, parabolic dunes have been divided 

into parabolic dunes of accumulation and parabolic 

dunes of deflation (Johnson 1967). Parabolic dunes of 

deflation cover a large area southwest of all other dune 

types. Low mounds of loose sand, covered by grass and 

shrubs, form the parabolic dunes of deflation. Blowouts 

measure from a few feet to several thousands of feet 

across, and may have been the reason for the lakes and 

playas that now characterize this area of the dunes. 

Parabolic dunes of accumulation are both active and 

fixed, but both form downwind of the parabolic dunes of 

deflation (Johnson 1967). The fixed parabolic dunes of 

accumulation are upwind from the active parabolic 

dunes of accumulation. The fixed dunes are only a few 

feet higher than the parabolic dunes of deflation, but the 

active dunes rise for 100 to 200 feet (30- 60 m) on a 

relatively steep slope from the fixed dunes to the 

transverse dunes. Sparse vegetation covers both 

parabolic types. Only small amounts of sand collect on 

the leeward side of the fixed parabolic dunes of 

accumulation. The sand from the blowouts seems to 

bypass the fixed parabolic dunes of accumulation and 

settle onto the active parabolic dunes of accumulation 

(Johnson 1967). 

In contrast to parabolic dunes, the tails of the crescentshaped barchan dune point downwind because there is 

little vegetation available to stabilize the limited sand 

supply (figure 8). Barchan dunes are not as aerially 

extensive as are reversing dunes (Johnson 1967). A grove 

of ponderosa pine is presently being inundated by 

advancing barchan dunes. In addition to the sand supply 

from the southwest, sand is being added to the barchan 

dunes from sand sheet material already in place. 

Reverse dunes and star dunes are the result of highly 

variable wind directions and may constitute the majority 

of dune forms at Great Sand Dunes (Kiver and Harris 

1999). Although the prevailing wind direction is from the 

southwest, especially during the spring, variable winds 

occur throughout the rest of the year. With variable 

winds, the sand tends to blow back westward and stack 

itself into higher levels rather than migrating 

horizontally. Star patterns form when the wind changes 

to several directions (figure 8). Great Sand Dunes has 

some of the world’s most impressive reversing dunes. 

Sand that accumulates on the far edges of the dune field 

piles against the steep flanks of the Sangre de Cristo 

mountains in climbing dunes and sand ramps. The 

climbing dunes at Great Sand Dunes are between 

Medano and Cold Creeks. The dunes rise from about 

8,800 to 9,500 feet (2,680 to 2,896 m) in about one mile 

(1.6 km) (Johnson 1967). The climbing dunes have no 

regular dune like surface features. They seem to form in 

irregular mounds and swales and are practically devoid 

of vegetation. 

Sedimentary structures that form on the dunes include 

animal tracks, avalanching foreset beds, and eolian ripple 

marks (figure 9). Mudcracks may also form between the 

dunes. 

Wetlands 

As part of the National Wetland Inventory, the U.S. Fish 

and Wildlife Service mapped and classified the wetlands 

in the park and preserve in 1992 (figure 10). These 

wetlands are defined in Chatman and others (1997). 

Briefly, seven different types of riverine wetland systems 

and five palustrine wetlands are the responsibility of 

Great Sand Dunes resource management. 

Riverine wetlands occur along the channels of Medano 

and Sand Creeks and their tributaries. A palustrine 

wetland is one comprised of perennial and intermittent 

ponds and depressions that nearly intersect the water 

table and that support hydrophytes. In Great Sand 
 GRSA Geologic Resource Evaluation Report 17

Dunes, they occur along the banks of creeks, in 

abandoned stream channels, in the sand sheet, and the 

interdunal areas of the active sand dune field. 

Of particular interest to Great Sand Dunes resource 

managers, are the palustrine wetlands associated with 

interdunal ponds and depressions that nearly intersect 

the water table near the western edge of the dune field 

(Chatman et al. 1997; Wurster and Cooper 2000; Rupert 

and Plummer 2004). The interdunal ponds and 

depressions provide important habitat for a variety of 

species and have decreased markedly in recent years. 

Lowering of the water table may lead to continued 

disappearance of the interdunal ponds and wetlands. 

Features in the Sangre de Cristo Range 

Glacial features: The Sangre de Cristo Mountains contain 

alpine glacial features common to many mountain ranges 

in the Rocky Mountains of Colorado (figure 11). Cirques, 

arêtes, horns, and U- shaped valleys are among the more 

common glacial features. Tiejeras Peak, the highest point 

in Great Sand Dunes at 13,604 feet (4,146 m), is a glacial 

horn. 

Structural features: The glacially sculpted Sangre de Cristo 

Mountains are fault- block mountains that form a lofty 

backdrop to the sand dunes at Great Sand Dunes. Major 

west- dipping reverse faults exposed in the Sangre de 

Cristo Range include (from west to east) the Crestone, 

Sand Creek, Deadman, and Spread Eagle thrusts. These 

faults date from the Late Cretaceous- middle Eocene 

Laramide Orogeny (about 65- 55 Ma) (McCalpin 1994). 

In Great Sand Dunes the Crestone Thrust is a complex 

fault zone at least 2,600 feet (800 m) wide. It contains 

numerous fault slivers of pre- Pennsylvanian 

sedimentary rocks. Mylonite, slatey cleavage, and 

chloritoid phyllite mark the footwall of the thrust. Like 

the Crestone Thrust, the Sand Creek Thrust moved 

Precambrian basement rocks up and eastward over late 

Paleozoic rocks, such as the Sangre de Cristo Formation. 
 NPS Geologic Resources Division 18 

Figure 6. Eolian sand deposits in Great Sand Dunes National Park and Preserve. From http://www.nps.gov/grsa/resources/ 

maps/GRSA_Sand_Deposits.jpg. 
 GRSA Geologic Resource Evaluation Report 19

7 A. Active dune field 

7 B. Sand sheet 

7 C. Sabkha 

Figure 7. The three major eolian sand deposits at Great Sand Dunes National Park and Preserve. A) the active dune field; B) the sand sheet, 

and C) the sabkha (aerial view). From Andrew Valdez, http://www.nps.gov/grsa/resources/wind_regime.htm (access December 10, 2005.) 
 NPS Geologic Resources Division 20 

Figure 8. Common dune types. Arrows indicate wind direction. 

Figure 9. Sand ripples on sand dunes caused by wind, Great Sand Dunes National Park and Preserve. Photo courtesy of 

http://www.nps.gov/grsa/resources/photos_dunes.htm (access December, 2005). 
 GRSA Geologic Resource Evaluation Report 21

Figure 10. Great Sand Dunes wetland. Photograph courtesy of the National Park Service. 

Figure 11. Music Pass, Sangre de Cristo Mountains. Glacial features in this photograph include a horn, an arête, a cirque basin, and a Ushaped valley. Photograph courtesy of the National Park Service. 
 NPS Geologic Resources Division 22 

Map Unit Properties 

This section provides a description for and identifies many characteristics of the map 

units that appear on the digital geologic map of Great Sand Dunes National Park and 

Preserve. The table is highly generalized and is provided for informational purposes 

only. Ground disturbing activities should not be permitted or denied on the basis of 

information contained in this table. More detailed unit descriptions can be found in the 

help files that accompany the digital geologic map or by contacting the NPS Geologic 

Resources Division. 

The following Map Unit Properties Table (MUPT) 

serves as a stratigraphic column for Great Sand Dunes 

National Park and Preserve and identifies specific 

properties of the different formations on the digital 

geologic map. The darker black horizontal lines in the 

MUPT represent major regional unconformities in the 

stratigraphic record. Geologic features and processes 

often occur in or can be restricted to a particular 

stratigraphic unit (group, formation, or member). This 

section ties together the geologic features with the 

formation properties, and also with the accompanying 

digitized geologic map. 

The MUPT includes several properties specific to each 

unit in the stratigraphic column including a description 

of the lithology, the map symbol, erosion resistance, 

paleontologic and cultural resources, hazards, mineral 

resources, suitability for development, and a 

miscellaneous “other” column. 
Map Unit Properties Table 

Age Map Unit (Symbol) Unit Description Erosion Resistance Paleontological Resources 

Alluvium 

(Qal) 

Unconsolidated sand, gravel and clay deposited by streams; includes Holocene stream deposits, Pleistocene glacial outwash, & locally, colluvium; up to several tens of feet thick. 

Low None 

Alluvial fan deposits 

(Qaf) Poorly sorted, coarse sand & gravel deposited by distributary stream systems Low None 

Eolian sand 

(Qes) 

Surficial deposits of well- sorted sand covered by sparse vegetation; thin and discontinuous, interspersed with alluvial sand & clay N of Deadman Creek. Low None 

Dune field 

 (Qdf) Wind- blown sand piled in various dune types. Low None 

Sandsheet (Qssh) Unconsolidated sand Low None 

Sabhka (Qsbk) Clay and salt Low None 

Landslide deposits 

 (Ql & Qls) 

Angular rock debris of all sizes; forms hummocky topography; up to 100 ft (30 m) thick. 

Low if disturbed by seismic activity or by construction 

None 

Rockfall deposits (Qrf) Accumulations of talus below large outcrops. High None 

QUATERNARY 

(Holocene)

Rock- glacier deposits 

(Qrg) 

Angular blocks at base of steep slopes & valley floors; lobate & tongue- shaped; 32-100 ft (10- 30 m) thick. High None 

Pinedale glacial deposits, undiff. 

(Qpu) 

Gray stratified alluvium of boulders, cobbles, pebbles & sand; forms mostly undissected & coalescing fans; locally covered by thin deposits of eolian sand; thickness unknown, may be as much as 330 ft (100 m) thick. 

High 

Pinedale alluvial fan deposits 

(Qpf) Boulders, cobbles, pebbles, sand; some boulders as much as 10 ft (3 m) in diameter. High 

Pinedale lacustrine clay 

(Qpl) Glacial lake clay above terminal moraine on Willow Creek; thickness unknown. Low if disturbed 

Pinedale outwash deposits 

(Qpo) 

Gray, well- washed, stratified alluvium of boulders, cobbles, pebbles, sand; below terminal moraines on South Colony Creek; about 26 ft (8 m) thick. 

Sand will erode more quickly than boulders 

Pinedale till 

(Qpt) 

Gray, unsorted, unstratified deposit of boulders, cobbles, pebbles, sand & clay; 50-

100 ft (15- 30 m) thick. 

Sand & clay will erode more quickly than boulders 

Pinedale & Bull Lake deposits, 

undiff. (Qpbu) Boulders, cobbles, pebbles, sand Variable 

Bull Lake alluvial fan deposits 

(Qbf) Sorted, stratified alluvium of boulders, cobbles, pebbles, sand; forms dissected fans. High except at times of intense runoff 

Bull Lake outwash deposits 

(Qbo) 

Gray, sorted, stratified alluvium of boulders, cobbles, pebbles, sand; located in valleys below terminal moraines; thickness about 32 ft (10 m). 

High except at times of intense runoff 

QUATERNARY 

(Pleistocene) 

Bull Lake till 

(Qbt) 

Yellow- gray, unsorted, unstratified compact deposit of boulders, cobbles, pebbles, sand & clay; 160 ft (50 m) thick on South Colony and Willow Creeks. High 

Mammoth femur & bison phalange (Pinedale/Bull Lake?) 

Unconformity 

Santa Fe Formation 

 (Ts) 

Tan, pink, buff, orange, brick red, light olive, and light gray to black stratified sand & gravel mostly from Precambrian and upper Paleozoic rocks; 1,300 ft (400 m) thick in subsurface. 

High None documented in GRSA 

Granite & granodiorite 

(Tgr) 

Small irregularly shaped intrusions; tan to light- gray; leucocratic, medium- to coarse- grained; seriate; nonfoliated; plagioclase, microcline, & quartz in subequal 

amounts with 1%- 5% biotite; accessory minerals include magnetite & apatite. 

High None 

Andesite 

(Ta) Andesitic lahar, biotite latite, & dense hornblende- pyroxene andesite. High None 

Mafic volcanic rocks 

(Tmv) 

Black, vesicular, porphyritic olivine basalt; composed of several flows; rests unconformably on Sangre de Cristo Formation; 250 ft (75 m) thick High None 

Felsic dikes 

(Tf) 

Light- gray to white, fine- grained, felsic dike as much as 10 ft (3 m) wide; composed of microphenocrysts of quartz, feldspar & sparse biotite in groundmass of same minerals. 

Phenocrysts will weather more quickly than groundmass 

None 

TERTIARY 

(Oligocene/ Eocene) 

Mafic dikes 

(Tm) 

Dark gray- green to black, aphanitic to medium- grained; generally massive, locally 

porphyritic; nonfoliated; as much as 10 ft (3 m) wide. High None 

Unconformity 
Age Map Unit (Symbol) Unit Description Erosion Resistance Paleontological Resources 

UPPER 

JURASSIC Morrison Formation (Jm) Interbedded gray to brown shale, gray to brown sandstone, & minor gray, fine- grained limestone. 250 ft (75 m) thick. Shale has low resistance None documented in GRSA 

MIDDLE 

JURASSIC Entrada Sandstone (Je) Thin- bedded, white to tan, friable quartz sandstone, fine- to medium- grained, well- sorted, well- rounded & frosted. 39 ft (12 m) thick. Friable surface grains, otherwise high None documented in GRSA 

Unconformity 

Sangre de Cristo Formation 

Undivided 

(PPNsu) 

Red arkosic sandstone, conglomeratic sandstone, siltstone, shale, minor limestone; cross- bedding in sandstone; ripple marks, cross- laminations, mudcracks, raindrop 

impressions, burrows in siltstone & shale; thin limestone is unfossiliferous, marine limestone contains bryozoan fossils; max exposed thickness of about 4,600 ft (1,400 m) at Horn Peak. 

Feldspars and limestone will erode more quickly than quartz sandstone 

Bryozoan fossils in marilimestone beds on MusiCreek 

Sangre de Cristo Formation 

Crestone Member 

(PPNsc) 

Red conglomerate, conglomeratic sandstone, & minor siltstone & shale; boulders & cobbles of Early Proterozoic gneiss, syenite & quartz. monzonite; sandstone is horizontally laminated & cross- bedded; exposed between Sand Creek Thrust and Little Sand Creek Thrust; 6,500 ft (2,000 m) thick. 

Variable 

Crinoid columns, brachiopods, sponge spicules, bryozoans; Eomarginifera sp & 

fusulinids found in biohermal units; trunks Calamites in clastic beds 

UPPER 

PENNSYLVANIAN/PERMIAN Sangre de Cristo Formation 

Lower Member 

(PPNsl) 

Red arkosic sandstone, conglomeratic sandstone, siltstone, & shale; sandstone 

cross- bedded; siltstone & shale contain ripple marks, cross- lamination & sparse mudcracks; conformably overlies Minturn Fm.; interfingers with overlying Crestone Conglomerate Member; thickens northward; thickness about 2,000 ft 

(600 m) west of Rito Alto Peak. 

High None documented 

MIDDLE PENNSYLVANIAN Minturn Formation (PNm) 

Gray & red arkosic sandstone, conglomerate, siltstone, shale, and minor marine limestone; occurs in thrust plates west of Mosca Pass; max thickness about 6,500 ft (2,000 m) at Middle fork of Taylor Creek. PNmu: upper part; PNmumls: marker limestone PNmcls: crinoidal silty limestone, about 755 ft (230 m) below top of unit; 39- 52 ft (12- 16 m) thick. PNmush: shale & siltstone member PNmols: oolite limestone PNmt: main turbidite member; upward- fining units of sandstone, siltstone, & shalePNmbls: biohermal limestone unit PNml: lower part PNmq: quartzose red- beds; arkosic & quartzose sandstone & siltstone 

High 

Abundant crinoid columnals, brachiopodssponge spicules, bryozoans 

Unconformity 

MISSISSIPPIAN, 

DEVONIAN, 

ORDOVICIAN Sedimentary rocks, undifferentiated (MDOr) 

Leadville Limestone (Lower Mississippian): Dark gray cherty massive dolomitic 

limestone; locally contains limestone breccia; 230 ft (70 m) thick. Chafee Group, Dyer Dolomite (Mississippian- Devonian): Yellow- gray cherty 

dolomite; 100 ft (30 m) thick. Chafee Group, Parting Quartzite (Mississippian- Devonian): Gray, massive 

quartzite and dolomite; 50 ft (15 m) thick. Fremont Dolomite (Upper Ordovician): Dark- gray, coarsely crystalline dolomite; 

230 ft (70 m) thick. 

High None documented 
Age Map Unit (Symbol) Unit Description Erosion Resistance Paleontological Resources 

MIDDLE 

ORDOVICIAN Harding Sandstone (Oh) Pale- red, thin- bedded quartzite; 100 ft (30 m) thick. High None documented 

LOWER 

ORDOVICIAN Manitou Limestone (Om) Dark- gray, fine- grained, cherty dolomitic limestone; 200 ft (60 m) thick. High None documented 

Unconformity 

MIDDLE 

PROTEROZOIC Quartz Monzonite (Yqm) Stock of gray to pink, medium- grained, equigranular, leucocratic, quartz monzonite, N of Willow Creek; 35% plagioclase, 30% microcline, 30 % quartz, 2%- 5% biotite & magnetite; locally small pegmatite dikes; extensively altered to chlorite, epidote, & muscovite; located along the Sangre de Cristo fault near Crestone; 1.45 billion years old. Vein quartz (Yqmqz) High None 

Unconformity 

Pegmatite 

(Xp) Coarse- grained igneous rock; interlocking crystals; composition of granite High None 

Tonalite gneiss 

(Xto) 

White to light- gray- green, buff- weathering; 60% plagioclase, 30% quartz, 8% mafic minerals, 2 % or less potassium feldspar; porphyroblastic with aggregates of mafic minerals as much as 0.6 inches (1.5 cm) across; groundmass grain size 

generally 3- 4 mm; weak foliation; original mafic minerals thoroughly altered to epidote, chlorite, & muscovite; K- feldspar increases near major faults; contains numerous mafic dikes as much as several tens of feet across; dikes most abundant near contact with metadiorite (Xdi); contact with metadiorite is interlayered & metadiorite xenoliths occur in tonalite gneiss near contact; located near southern end of Sangre de Cristo wilderness area. 

Plagioclase will erode faster than quartz None 

Metagabbro (Xg) Metamorphosed gabbro High None 

Quartz diorite 

(Xqd) 

Plagioclase, quartz, & hornblende or biotite; accessory apatite, zircon, & opaque oxides. High None 

Diorite 

(Xdi) 

Dark- green to dark- gray, fine- to medium- grained, seriate to equigranular, locally foliated hornblende diorite; 50- 705 euhedral to subhedral hornblende, 30%- 50% plagioclase, up to 5 % quartz as inclusions in plagioclase; quartz averages about 1% of rock; accessory minerals (about 2% of rock) are magnetite, sphene, & apatite; extensively altered to sericite, chlorite, epidote; locally injected by numerous interconnecting veins of leucocratic biotite granodiorite to tonalite. 

Alteration of dark minerals None 

Mafic intrusive rocks 

(Xm) 

Dark gray- green to black, aphanitic to coarse- grained, basaltic & gabbroic dikes & 

sills; generally massive, locally porphyritic; subequal amounts of plagioclase & hornblende with 5%- 10% opaque minerals; pyroxene- & olivine- bearing varieties occur locally; nonfoliated; sharp contacts with host rocks; map units may include mafic intrusive rocks of Tertiary age. 

Mafic igneous rocks will erode faster than felsic igneous rocks 

None 

LOWER 

PROTEROZOIC 

Quartz Monzonite 

(of Music Pass) 

(Xqm) 

Gray to pink, coarse- grained, foliated, quartz monzonite porphyry; white to pink, subhedral, microcline phenocrysts 25%- 45%; groundmass is about 60% plagioclase, 20% quartz, 20% biotite, up to 1% sphene and magnetite; foliation locally well developed; boudinage of microcline phenocrysts accompanies increased foliation intensity, particularly near contacts of unit; faulted contacts with Paleozoic sedimentary rocks; intrudes Xgn, Xlgn, & Xq; mafic pods & large mafic xenoliths common locally. 

Biotite & feldspars will erode faster than quartz 

None 
Age Map Unit (Symbol) Unit Description Erosion Resistance Paleo Resources 

Quartzite 

(Xq) 

Black & gray to purple & white, layered quartzite breccia; blocks of layered quartzite range from mm to tens of meters across; matrix is crushed quartzite with minor iron oxide & muscovite; numerous fractures crisscross blocks; 90%- 95% quartz, 3%- 8% opaque minerals, minor muscovite & biotite; opaque minerals include magnetite, specular hematite, earthy hematite; opaque minerals define layers in blocks probably reflecting original bedding; occurs as a narrow, faultbounded 328- ft (100 m) wide layer between PPNsc & Xqm & as scattered, poorly exposed xenoliths within quartz monzonite. 

High None 

Syenite & Monzonite 

(Xsm) 

Reddish- tan, medium- to coarse- grained porphyry stock; nonfoliated to weakly 

foliated; subhedral microcline phenocrysts; 50%- 80% microcline, up to 35% plagioclase, 1%- 5% quartz, & up to 10% biotite with minor muscovite, zircon, & sphene; moderate alteration of plagioclase & biotite; fault contact with Paleozoic sedimentary rocks; interlayered with Xgn. 

High None 

Leucocratic gneiss 

(Xlgn) 

80% of unit white to medium gray, light- pink to light- gray weathering; 30% quartz, 30% microcline; 35 % plagioclase, 4% biotite; variably massive, laminated 

and layered; foliated; locally migmatitic; locally has porphyroblasts of feldspar up to 2 inches (5 cm); 20% of unit is gray gneiss with composition of granodiorite; 40% plagioclase, 25% quartz, 20% microcline, 15% mafic minerals; original mafic minerals commonly altered to chlorite; complexly interlayered with light- colored gneiss; quartz veins & pods & small pegmatites common. 

Biotite & feldspars will erode faster than quartz 

None 

Hornblende gneiss 

(Xhgn) 

Schistose to massive, fine- to medium- grained quartz- plagioclase- hornblende gneiss; dominantly a well- foliated, laminated gneiss having sharply bounded, continuous, dark- and light- colored layers 1- 5 mm thick. 

Hornblende & plagioclase erodes faster than quartz 

None 

LOWER 

PROTEROZOIC 

Mixed gneiss 

(Xgn) 

Mafic & felsic gneiss & micaceous schist; pink, gray & black; layered, laminated, & foliated; layers discontinuous, rarely traceable more than 0.3 miles (0.5 km); contains leucocratic, biotite- quartz- plagioclase- microcline gneiss & hornblendebearing varieties of leucocratic gneiss interlayered with mafic quartz- plagioclasehornblende gneiss; locally includes fine- to coarse- grained, weakly foliated to nonfoliated alaskitic gneiss; all varieties in this unit are extensively altered to chlorite, epidote, & sericite; dikes & sills ranging in composition from aplite & pegmatite to basalt cut the unit; exposed along west side of Sangre de Cristo Range for about 60 miles (100 km). 

Dark minerals will erode faster than lighter minerals 

None 
 GRSA Geologic Resource Evaluation Report 27

Geologic History 

This section highlights the map units (i.e., rocks and unconsolidated deposits) that occur 

in Great Sand Dunes National Park and Preserve and puts them in a geologic context in 

terms of the environment in which they were deposited and the timing of geologic events 

that created the present landscape. 

Precambrian 

During the Early Proterozoic (about 1,800 – 2,000 Ma) 

(figure 12), erupting volcanoes formed a chain of islands 

in an ocean south of the edge of the North American 

craton in Wyoming (Reed et al. 1987; Scott et al. 2001). 

Like today’s volcanic islands in the Pacific Ocean, these 

Proterozoic islands formed along the margins of 

converging lithospheric plates. Volcanic rocks, with 

compositions indicative of continental margin 

magmatism, and sedimentary rocks are widespread over 

a 300 mile- wide (500- km) belt in Colorado. 

About 1,740 Ma, during a period of compressive 

mountain building, the rocks were folded into tight, 

chevron- like folds and metamorphosed to high- grade, 

metamorphic gneisses in a regional orogeny (mountainbuilding episode) that culminated with the suturing of 

Colorado onto the Wyoming province to the north 

(figure 13) (Hutchinson, 1976; Tweto 1980; Gregson 1992; 

Scott et al. 2001). This Precambrian orogeny converted 

the graywacke, arkose, and shale into the migmatitic 

metasedimentary rocks that are now exposed in the 

Sangre de Cristo Mountains. 

After the end of this first period of compressional 

tectonics and mountain building, Middle Proterozoic 

granites (1470- 1390 Ma) intruded into the metamorphic 

rocks (Hutchinson 1976; Tweto 1980; Gregson 1992). 

Major faulting and shearing accompanied intrusion and 

another generation of folds was generated near the shear 

zones. 

Rocks representing the next 1,170 million years were 

removed by erosion. Erosion cut deeply into the Early 

Proterozoic rocks, removing miles of the overlying, less 

metamorphosed rocks, and exposing the highly 

metamorphosed cores of ancient mountains. 

The Pike’s Peak Plutonic Event was the last major 

Precambrian intrusive event in Colorado (Hutchinson 

1976; Gregson 1992). During this time (1040- 980 Ma), the 

Pike’s Peak Granite intruded into considerably shallower 

depth than the 1,750 year- old granites. Restricted to the 

Pike’s Peak area, the Pikes Peak granite may have even 

breached the surface to form volcanic rocks in places. 

Rocks representing about 400 million years of 

Precambrian time following emplacement of the Pikes 

Peak granite are missing from Colorado (Tweto 1980). 

Beginning about 600- to 800 Ma, the tectonic regime in 

Colorado changed from one of compression to one of 

extension (Stone 1986; Hoffman 1989; Burchfiel et al. 

1992). About 800 Ma, the west coast of North America 

began to rift apart. With rifting, a new western edge of 

the continent formed. The Late Proterozoic coastline 

trended south from what would eventually become 

northeastern Washington through western Idaho and 

eastern Oregon into the area occupied by the Sierra 

Nevada of California today. 

Whenever continents begin to break apart, some of the 

basalt that rises in the rift produces swarms of diabase 

dikes and sills in the continental crust. Diabase, with the 

same composition as basalt, is often indicative of 

continental rifting. The diabase dikes and sills in the 

Sangre de Cristo Mountains attest to this period of 

tectonism in Colorado. 

Paleozoic Era 

Cambrian and Ordovician Periods (542-444 Ma): The 

Cambrian and Early Ordovician Periods in the western 

United States were a time of tectonic quiescence (figure 

12) (Stewart 1980; Poole et al. 1992). A passive margin 

developed on the western margin of North America that 

prevailed until the advent of the Antler Orogeny in the 

Middle Devonian (figure 14A). Cambrian rocks are not 

exposed in the park and preserve, and Ordovician strata 

are mapped together with the Mississippian and 

Devonian rocks in the digital geologic map on the 

attached CD. 

The dark- gray, fine- grained, cherty, dolomitic marine 

limestone of the Lower Ordovician Manitou Limestone, 

like the other Ordovician, Devonian, and Mississippian 

strata, is exposed in fault slices in the Sangre de Cristo 

Mountains. An unconformity separates the Manitou 

Limestone from the underlying Proterozoic rocks. 

The Middle Ordovician Harding Sandstone that was 

metamorphosed to quartzite also is unconformable with 

underlying Proterozoic gneiss. The metamorphosed 

marine sandstones represent the initiation of widespread 

marine submersion in North America (Ross and Tweto 

1980; Witzke 1980). 

Marine carbonate deposition returned in the Upper 

Ordovician and is represented by the Fremont Dolomite 

(Johnson et al. 1989). Following deposition of the 

Fremont Dolomite, the shoreline along the western 

margin of the Transcontinental Arch moved westward as 

a major regressive episode affected Colorado. Regression 

would continue throughout the Silurian and into the 

Early Devonian. 
 NPS Geologic Resources Division 28 

The end of the Ordovician Period (444 Ma) is marked by 

one of the five most extensive mass extinctions recorded 

in the fossil record (figure 12). Meteor impacts, 

glaciation, sea level rise, sea level fall, and global warming 

have all been offered as mechanisms leading to various 

extinction episodes in the geologic record, but for the 

Ordovician Period, no definitive explanation has been 

accepted. 

Devonian Period (416-359 Ma): About 100 million years of 

geologic record, from the Silurian to Upper Devonian, 

are missing in Great Sand Dunes. By the beginning of the 

Devonian, the seas that had covered most of the North 

American continent had receded, or regressed, and the 

shoreline formed a southwest- northeast trend through 

west- central Utah and southeastern Nevada. Offshore, a 

shallow carbonate ramp dipped westward while onshore, 

extensive tidal flats spread for miles from western 

Colorado to central Utah. Crustal plate collision along 

the western margin initiated the Antler Orogeny and the 

next major transgression in western North America. 

Transgressions onto the continent were episodic and 

rapid. A gradual regression occurred in the late Middle 

Devonian followed by another transgression in the 

Upper Devonian (Johnson et al. 1991). The Dyer 

Dolomite and the Parting Formation in the Sangre de 

Cristo Mountains represent these episodic 

transgressions and regressions (Ross and Tweto 1980; 

Johnson et al. 1989). 

Mississippian Period (359-318 Ma): The Lower 

Mississippian Leadville Limestone in southwestern 

Colorado is a dark- gray, massive limestone that locally 

contains limestone breccia (Johnson et al. 1989; Lindsey 

et al. 1986). In central Colorado in the vicinity of Great 

Sand Dunes, the formation increases in clastic material. 

Although mapped together with Devonian and 

Ordovician sedimentary rocks, elsewhere the Leadville 

Limestone is subdivided into three members, in 

ascending order: Gilman Member, Redcliff Member, and 

the Castle Butte Member. The Gilman Member, at the 

base of the Mississippian, is comprised of fine- tocoarse- grained, well rounded, quartz sandstones, sandy 

dolomite breccias, and limestone- pebble conglomerates. 

Strata of the Redcliff Member rest unconformably on the 

Gilman Member with basal limestone- pebble 

conglomerates and sandstone lenses deposited locally in 

channels scoured into the underlying Gilman Member at 

the unconformity (De Voto 1980). Thinly bedded, 

stromatolitic dolomite mudstones, intraclast 

wackestone- packstones, and dolomite breccias 

characterize the Redcliff Member. A solution- erosion 

unconformity separates the Redcliff Member of the 

Leadville Limestone from the overlying Castle Butte 

Member. The unconformity is marked by a breccia soilrubble zone and by collapse breccia- filled sinkholes and 

caves indicative of karst topography. 

Resting unconformably above the Redcliff, the Castle 

Butte Member consists dominantly of pelletal, oolitic, 

and mixed- skeletal grainstones and packstones. The 

lime mud content in the grain- supported rocks generally 

decreases upward in the section. With uplift and 

subaerial exposure of these carbonate rocks during the 

Middle and Upper Mississippian, solution erosion 

developed a karst topography on the top of the Castle 

Butte Member strata similar to the paleo- karst 

topography marking the top of the Redcliff Member (De 

Voto 1980). 

The Lower Mississippian Leadville Limestone represents 

the final transgressive pulse of the Antler Orogeny (figure 

14B). As the highlands to the west were thrust above sea 

level at the beginning of the Mississippian, warm marine 

water spread over eastern Utah giving rise to an 

extensive carbonate platform. Transgression continued 

in the Lower Mississippian, and the shoreline 

encroached into western Colorado, forming intertidal 

and restricted subtidal environments. Stromatolitic 

dolomite and carbonate mudstones, pelletal packstones, 

and wackestones in the lower member of the Leadville 

Limestone in southwestern Colorado represent these 

shallow marine environments (De Voto 1980). In the 

Great Sand Dunes region, the sandstones in the Gilman 

Member may have been derived from erosion of the 

subaerially exposed, Devonian Chaffee Formation. The 

carbonate mudstones and dolomite of the Redcliff 

Member indicate shallow, restricted- circulation, 

subtidal environments as the transgression spread into 

central Colorado (De Voto 1980). 

The sea transgressed into central Colorado one more 

time in the Middle Mississippian (Castle Butte Member 

of the Leadville Limestone). High- energy, shoal- water, 

oolite banks developed on topographic- structurally high 

areas while open- circulation, subtidal environments 

were created as the sea moved across central Colorado. 

When the thrust faulting ceased and the orogeny came to 

an end in the Middle Mississippian, the shoreline 

migrated back westward. As the sea regressed, and the 

ocean basin between the Antler Highlands and Colorado 

filled with clastics deposited in deltaic systems, fan- delta 

systems, alluvial fans and eventually, fluvial systems. 

Pennsylvanian Period (318-299 Ma): In south- central 

Colorado, the Middle Pennsylvanian Minturn Formation 

can be subdivided into three distinct lithologic facies: a 

turbidite- bearing facies, a limestone- bearing facies, and 

a redbed facies (Lindsey et al. 1986). Turbidite deposits 

characteristically contain many turbidite cycles (density 

current), which consist of upward- fining subcycles of 

sandstone, siltstone, and shale. The contact is sharp 

between the shale of one turbidite cycle and the 

overlying sandstone or conglomerate of the next cycle. 

The turbidite- bearing facies is interpreted to be the 

result of fluvial- dominated fan delta deposition that 

migrated seaward onto the sea bottom below the wave 

base. (Lindsey et al. 1986). A fan delta is an alluvial fan 

that builds into standing bodies of water. At the time the 

Great Sand Dunes region was located on the western 

margin of the central Colorado trough, a depression that 
 GRSA Geologic Resource Evaluation Report 29

developed along the eastern border of the uplifted late 

Paleozoic Uncompahgre Highland (part of the Ancestral 

Rocky Mountains). The turbidite- bearing facies is 

gradational into the overlying limestone- bearing facies. 

The limestone- bearing facies consists of shallow marine 

limestone, shale, siltstone, as well as arkosic sandstone, 

conglomerate and conglomeratic sandstone. Upwardfining subcycles of arkosic sandstone, siltstone, and shale 

also are characteristic of this facies (Lindsey et al. 1986). 

Limestone beds contain marine fossils, oolites, skeletal 

fragments, and stromatolites along with sedimentary 

structures such as planar and wavy lamination, ripple 

cross- lamination, and trough cross- bedding. About 

3,200 feet (1,000 m) below the top of the formation is a 

980- foot (300 m) thick zone of lenticular biohermal 

limestone units that extend 4,900 feet (1,500 m) along the 

strike of the unit. The fossils include crinoid columnals 

as much as 1 foot (0.3 m) long, brachiopods, bryozoans, 

phylloid algae, gastropods, pelecypods, rugose corals, 

fusulinids, and conodonts. Associated with the limestone 

beds are burrowed sandstone and siltstone units. 

Interfingering with these deposits are upward- fining 

cycles of lenticular- bedded, channeled, and crossbedded conglomerate and sandstone capped by thin 

intervals of siltstone and shale. Trunks of Calamites

(horse- tail tree) were found in growth position in some 

of these clastic deposits. 

The limestone- bearing facies of the Minturn Formation 

is also the result of fan deltas prograding over a sea 

bottom in the area of the Central Colorado Trough, but 

in contrast to the turbidite- bearing facies, the 

limestone- bearing facies represents deposition above 

wave base (Lindsey et al. 1986). Shallow marine 

limestone, shale, siltstone, and arkosic sandstone were 

deposited in the prodelta and interdeltaic regions. 

The wide variety of textures and fossil debris in the 

limestone are the result of shallow marine environments 

impacted by energy regimes that range from low energy 

to strongly agitated waves. Conglomerate, conglomeratic 

sandstone, and sandstone mark the delta front. Sandy 

foreset beds are the most distinctive feature in delta front 

sandstone units and formed as small deltas prograded 

into shallow standing water. Arkosic sandstone, siltstone 

and shale in upward- fining subcycles represent braided 

streams. The trunks of Calamites in growth position are 

rooted in shale interpreted as a rare coastal swamp 

deposit and are engulfed by pebbly sand deposited by 

braided streams (Lindsey et al. 1986). 

Upward- fining cycles of quartzose and arkosic 

sandstone and siltstone constitute the redbed facies. The 

assignment of the redbeds to the Minturn Formation is 

speculative and these deposits have not been studied in 

detail (Lindsey et al. 1986). The fining- upward cycles in 

the redbed facies have been interpreted as alluvial 

deposits, but this hypothesis needs more testing (Lindsey 

et al. 1986). 

Upper Pennsylvanian and Permian Periods (333-251 Ma): 

The Pennsylvanian and Permian Periods were times of 

great tectonic upheaval around the globe. All the major 

landmasses were coming together to form one 

supercontinent, Pangaea. The paleo- Gulf closed as 

South America was sutured to the Gulf Coast, Africa and 

Europe collided with the eastern seaboard to form the 

Appalachian Mountains, and on the western margin, the 

Pacific lithospheric plate was subducting beneath the 

North American continent. 

Ripple effects of the South America and North America 

suturing resulted in the uplift of the Ancestral Rocky 

Mountains (figure 14C). The Ancestral Rocky 

Mountains, less extensive than today’s Rocky 

Mountains, are characterized by the Uncompahgre and 

Front Range Uplifts. In addition to the area of today’s 

Uncompahgre Plateau, the Uncompahgre uplift 

encompassed the resurgent San Luis uplift to the southsoutheast of the Uncompahgre highland and extended 

into north- central Mexico. Erosion of the Uncompahgre 

highland and episodic transgressive and regressive 

episodes are reflected in the Sangre de Cristo Formation. 

North of Great Sand Dunes, the undivided Sangre de 

Cristo Formation is caught up in thrust slices and 

consists of red arkosic sandstone, conglomeratic 

sandstone, siltstone, shale, and minor limestone. The 

strata are arranged in upward- fining alluvial cycles 

estimated to be 3 to 100 feet (1- 30 m) thick (Lindsey et al. 

1986). The sandstone and conglomeratic sandstone are 

cross- bedded. Ripple marks, cross- laminations, 

mudcracks, possible raindrop impressions, and burrows 

formed on the siltstone and shale and support a fluvial 

mode of deposition. The thin, (3 ft; 1 m) limestone beds 

are unfossiliferous, fine- grained, nodular to laminated 

and may be paleosol deposits. Marine limestone beds on 

Music Creek contain abundant bryozoan fossils typical 

of Permian and Pennsylvanian strata. An unconformity 

separates the Sangre de Cristo Formation from the 

overlying Entrada Sandstone at Loco Hill. 

In some areas, the Sangre de Cristo Formation can be 

divided into two members: 1) a Crestone Conglomerate 

Member, and 2) a Lower Member. The red 

conglomerate, conglomeratic sandstone, and minor 

siltstone and shale of the Crestone Conglomerate 

Member represents debris flow, mudflow, streamflow, 

and sheetflow deposits on proximal parts of alluvial fans 

and are exposed in thrust sheets east of Great Sand 

Dunes (Johnson et al. 1989). The coarse conglomerate 

contains boulders, cobbles, and pebbles of Precambrian 

metamorphic and igneous rocks in a matrix of 

feldspathic sand and clay. The sandstone is horizontally 

laminated and cross- bedded. The Crestone 

Conglomerate Member unconformably overlies Early 

Proterozoic gneiss near Medano Creek but in other 

areas, it unconformably overlies the Minturn Formation. 

The Lower Member is a red arkosic sandstone, 

conglomeratic sandstone, siltstone, and shale arranged in 

fining- upward cycles wherein the coarser conglomeratic 
 NPS Geologic Resources Division 30 

sandstones grade upward to finer grained siltstones and 

shales. Sedimentary structures in the sandstone, 

siltstone, and shale include cross- bedding, ripple marks, 

cross- lamination, and mudcracks compatible with distal 

alluvial fan deposits (Lindsey et al. 1986). The Lower 

Member is conformable with the underlying Minturn 

Formation, and in the Horn Peak area, this unit 

interfingers with the Crestone Conglomerate Member. 

The close of the Permian Period brought the third, and 

most severe, major mass extinction of geologic time 

(figure 12). Although not as well known as the extinction 

event that exterminated the dinosaurs at the end of the 

Mesozoic Era, the Permian extinction was much more 

extensive. Thousands of species of insects, reptiles, and 

amphibians died on land. In the oceans, coral formations 

vanished, as well as many species of snails, urchins, sea 

lilies, fish, and the once- prolific trilobites. 

Mesozoic Era 

Triassic Period (251-199 Ma): No Triassic rocks are 

exposed in the Great Sand Dunes area. During the 

Triassic, the supercontinent Pangaea reached its greatest 

size. All the continents had come together to form a 

single landmass that was located symmetrically about the 

equator (Dubiel 1994). Most of Colorado was above sea 

level and experienced nondeposition (Maughan 1980). 

The extensive epicontential seas that flooded the craton 

in the Paleozoic were gone. At the time, the 

Uncompahgre Highlands were still contributing 

sediment to western Colorado while erosion had created 

a nearly level plain in southwestern Colorado (Stewart et 

al. 1972A). Where exposed, continental rocks of the 

Western Interior form a complex assemblage of alluvial, 

marsh, lacustrine, playa, and eolian deposits (Stewart et 

al. 1972B). 

Pangaea began to break apart in the Late Triassic and 

Early Jurassic (about 195 to 216 Ma), and the monsoon 

climate that characterized the Early Triassic changed as 

the western North American interior slowly rotated into 

a position farther north of the Triassic equator. In the 

Late Triassic, the Colorado Plateau became a desert. 

Jurassic Period (199-145 Ma): During the Jurassic, 

extensive ergs (eolian sand seas) similar to the Sahara and 

Sahel regions today covered a region that was located 

about 18 degrees north latitude at the beginning of the 

Jurassic and about 30- 35 degrees north latitude at the 

end of the Jurassic (figure 15A) (Kocurek and Dott 1983; 

Peterson 1994). This is the latitude of the northeast trade 

wind belt. Most modern hot deserts of the world occur 

within the trade wind belt and during the Jurassic Period, 

the climate of the Colorado Plateau appears similar to the 

modern Western Sahara of Africa. 

In the Sahara, the world’s largest desert, only 10 percent 

of the surface is sand- covered. The Arabian Desert, 

Earth’s sandiest desert, is only 30 percent sand- covered. 

The Jurassic deserts that inundated the Colorado Plateau 

for roughly 40 million years (not counting the time 

represented by erosion) contained sand dunes that may 

be the largest recorded in the history of Earth (Kocurek 

and Dott 1983). These were ergs that formed on a coastal 

and inland dune field affecting the present areas of 

southern Montana, eastern Utah, westernmost 

Colorado, southwest Colorado, northeastern Arizona, 

and northwestern New Mexico (Kocurek and Dott 1983; 

Peterson 1994). 

No Early Jurassic strata are exposed in Great Sand 

Dunes, but the Middle Jurassic Entrada Sandstone is the 

most widespread of the preserved late Paleozoic and 

Mesozoic eolianites (wind deposited sandstone) in the 

western US. Unlike the dunes at Great Sand Dunes, 

however, the cross- stratified Entrada Sandstone was 

deposited in an extensive dune field in a back- beach area 

(Doelling 2000). The Entrada is a thinly- bedded, friable, 

white to tan quartz sandstone. Typical of eolian deposits, 

the fine- to medium- grained sandstone is well- sorted 

and well- rounded. The grains appear translucent or 

frosted as a result of its eolian origin (Lindsey et al. 1986). 

Three marine transgressive- regressive cycles pushed the 

shoreline back and forth across the northern and 

northwestern parts of the Western Interior Basin during 

the Lower and Middle Jurassic Period. As the sea 

regressed to the north in the Upper Jurassic, the 

extensive Morrison Formation was deposited across the 

continental Western United States (figure 15A). Worldrenowned for its dinosaur fossils, the Morrison is 

exposed north of Great Sand Dunes in an overturned 

syncline at Loco Hill (Lindsey et al. 1986). In this area, 

the Morrison is an interbedded gray and brown shale, 

gray to brown sandstone, and minor gray, fine- grained 

limestone that ranges about 164- 230 feet (50- 70 m) thick. 

It was probably deposited in alluvial and near- shore 

marine environments. 

Cretaceous Period (145-65 Ma): Cretaceous rocks have 

been eroded from the Great Sand Dunes region. 

Accelerated lithospheric plate collision in the Cretaceous 

caused mountains to build along the western margin of 

North America as the Sevier Orogeny formed a roughly 

north- south trending thrust belt that is well defined in 

present- day southern Nevada, central Utah, and western 

Montana. As the mountains rose in the west, the Gulf of 

Mexico separating North and South America continued 

to rift open in the south, and seawater began to move 

northward into the expanding Western Interior Basin. 

Marine water also began to transgress from the Arctic 

region. The Western Interior Seaway became an elongate 

basin that extended from today’s Gulf of Mexico to the 

Arctic Ocean, a distance of about 3,000 miles (4,827 km) 

(figure 15B) (Kauffman 1977). 

West- dipping thrust faults, high- angle reverse faults, 

and folds exposed in the Sangre de Cristo Mountains 

date from the Laramide Orogeny (Late Cretaceous to 

Mid- Tertiary, about 75- 35 Ma) (figure 16) (Lindsey et al. 

1986; Kluth 1986). The Creston, Sand Creek, Deadman, 

and Spread Eagle thrusts displaced Precambrian 

basement rocks eastward over Paleozoic rocks (typically 

Pennsylvanian Sangre de Cristo Formation). The 
 GRSA Geologic Resource Evaluation Report 31

Crestone Thrust is a complex fault zone at least 2,600 

feet (800 m) wide. Within the fault zone are slivers of 

pre- Pennsylvanian sedimentary rocks. 

Cenozoic Era 

Tertiary Period: A hiatus of approximately 86 million 

years exists between the Jurassic Morrison Formation 

and the Eocene (55.8- 33.9 Ma) Blanco Basin Formation. 

The Blanco Basin Formation (0- 2,280 feet; 0- 696 m 

thick) is restricted, in the Great Sand Dunes region, to 

the Monte Vista graben, west of the Alamosa horst 

(Brister and Gries 1994). The reddish shales, siltstones, 

and sandstones of the Blanco Basin Formation represent 

alluvial sediments deposited in the Blanco Basin, a 

north- northwest trending basin formed sometime in the 

Paleocene to Eocene due to strike- slip faulting during 

late Laramide Orogeny (figure 16). 

The andesitic lava flows and volcaniclastic rocks of the 

Conejos Formation (0- 7,550 feet, 0- 2,300 m thick) also 

are restricted to the western portion of the San Luis 

Valley. The volcanoclastic rocks and lava flows 

originated from the active volcanoes in the San Juan 

volcanic field from 35 to 30 Ma (Brister and Gries 1994). 

Ash- flow tuffs (1,250- 1,900 feet, 380- 580 m thick) were 

deposited over the Conejos Formation in the Monte 

Vista graben and directly upon Precambrian basement 

rocks in what is now the Baca graben, east of the Alamosa 

horst. The ash- flow tuffs mark a tectonic regime change 

that occurred about 29 Ma. The region of south- central 

Colorado that had been deformed by compression 

during the Laramide Orogeny now began to be extended 

as the area was pulled apart by tectonic forces (Chapin 

and Cather 1994). 

With extension came volcanism and the formation of the 

San Juan volcanic field. Regional ash- flow sheets were 

emplaced prior to the rift- related half grabens that 

would soon develop and fill with sediments. Volcanic 

rocks may include dark- brown (mafic) olivine basalt 

flows and flow breccias that are more than 246 feet (75 

m) thick and light- gray to white (felsic) sills and dikes of 

ranging from 3- 26 feet (1- 8 m) to 164 feet (50 m) thick 

that erupted approximately 26.5 Ma (Lindsey et al. 1986; 

Bruce and Johnson 1991). The sills and dikes have 

intruded along thrust and reverse faults in both Paleozoic 

and Precambrian rocks (Lindsey et al. 1986; Bruce and 

Johnson 1991). 

After the emplacement of the uppermost Oligocene ashflow tuff, the Rio Grande rift extended northward and 

the northern part of the San Luis Basin began to split 

apart (Brister and Gries 1994; Chapin and Cather 1994). 

In the early stages of rifting, the eastern half of the San 

Luis Basin (Baca graben) was progressively tilted 

eastward along the Sangre de Cristo Range. 

As the basin opened, streams carried sediments from the 

San Juan volcanic field and deposited them in alluvial 

fans much like the alluvial fan that has developed where 

the Rio Grande River enters the San Luis Valley today. 

Additional material was supplied to the basin from the 

Sangre de Cristo Range that was rapidly uplifted about 19 

Ma in the Miocene. Shorter, steeper alluvial fans were 

constructed along the eastern margin of the Baca basin 

due to episodic movement along the basin- bounding 

fault. These sediments became the Santa Fe Group, 

which consists of the Santa Fe Formation and the 

overlying Alamosa Formation in the Great Sand Dunes 

area. Santa Fe Group strata are up to 3.5 miles (5.6 km) 

thick. 

The Santa Fe Formation is found throughout the basin in 

the subsurface and locally, along the basin margin. The 

stratified volcanic and sedimentary clastics of the Santa 

Fe Formation came from two primary sources; 1) 

volcanic debris from the western San Juan volcanic field, 

and 2) Precambrian granitic and metamorphic rocks and 

Paleozoic limestones and clastic rocks from the rising 

Sangre de Cristo Range (Brister and Gries 1994). 

Sedimentary structures in the formation are indicative of 

fluvial processes. 

The Alamosa Formation overlies the Santa Fe Formation 

in the subsurface but is not exposed in the Great Sand 

Dunes region. In the subsurface, the gray, black, and 

green claystones of the Alamosa Formation are 

conformable with the underlying beds of the Santa Fe 

Formation (Brister and Gries 1994). Wells drilled into the 

Alamosa Formation have uncovered fossil debris 

including ostracodes, bones, peat, wood fragments, and 

mollusk shell fragments. 

Quaternary Period: Poorly cemented sandstone horizons 

that can be correlated over broad areas rest at the top of 

the Alamosa Formation, and these sandstone layers are 

the primary source of groundwater for irrigation in the 

San Luis Valley. The formation may be as much as 1,800 

feet (550 m) thick. Pleistocene (0.6- 0.9 Ma) fossils of 

fish, birds, and mammals, along with volcanic ash and 

fresh- water mollusk shells, have been found at Hanson’s 

Bluff near the town of Alamosa (Brister and Gries 1994). 

The discovery of methane in the Alamosa Formation 

helped stimulate intermittent oil and gas drilling in the 

San Luis Valley (Brister and Gries 1994). 

A new round of flood basalts about 4.5 Ma barred the 

surface drainage from the northern to the southern part 

of the San Luis Valley. Environments favorable to lake 

and fluvial sedimentation were created by subsequent 

internal drainage. The basalts that solidified north of the 

San Luis Hills horst acted as a hydrologic divide until 

middle to late Pleistocene (somewhere between 0.69 to 

0.3 Ma) when the Rio Grande River eroded a gorge 

through the basalt (Brister and Gries 1994). 

Mapped in the Great Sand Dunes area are two 

Pleistocene (1.8 to 0.01 Ma) alpine glacial deposits: the 

Bull Lake glacial deposits and the Pinedale glacial 

deposits. The rounded crests of the Bull Lake moraines 

and the weathered surface of Bull Lake boulders suggest 

that the Bull Lake alluvium, outwash, and till are older 

than the Pinedale deposits (Meierding and Birkeland 
 NPS Geologic Resources Division 32 

1980). Bull Lake glaciation probably correlates with a 

major climatic event recorded in deep- sea cores about 

130,000 to 150,000 years before present (BP) (Meierding 

and Birkeland 1980). 

In contrast to the Bull Lake glacial deposits, the Pinedale 

glacial deposits retain many original depositional 

features (Meierding and Birkeland 1980). Pinedale 

moraines have steep sides (up to 35 degrees), sharp 

crests, large aerial extent, and a high frequency of surface 

boulders. Boulders in the sagebrush zone also appear to 

be relatively fresher than the Bull Lake boulders. 

Radiocarbon dates of organic matter between Pinedale 

tills indicate that the ice margin reached its maximum 

extent once before 30,000 years BP and three times 

between 30,000 and 12,000 years BP. Dated lake 

sediments in the eastern Front Range suggest a maximum 

stand of Pinedale ice between 23,000 and 21,000 years 

BP. Pinedale glaciers began receding from their 

outermost position in the Front Range about 14,000 

years ago and retreated to the cirques by at least 10,000 

years BP (Meierding and Birkeland 1980). 

Glacial erosion formed a large debris fan where the Rio 

Grande River flowed out of the San Juan Mountains into 

the San Louis Valley. The Rio Grande flowed eastward 

into a large lake in the valley, the remnant of which 

remains as San Luis Lake. San Luis Lake still occupies the 

enclosed basin in the northern part of the valley. In the 

area of San Luis Lakes, the Rio Grande took a sharp 90 

degree turn to the south. Since the Pleistocene, the river 

has gradually migrated to the southwest forming its 

present, gently curved channel. 

About 12,000 years ago, the large valley glaciers in the 

mountains began to recede and the cooler, wetter climate 

that had prevailed in the San Luis Valley gradually 

became the arid climate of the valley today. The 

abundant vegetation disappeared and the prevailing 

southwest winds, blowing from 15 mph (24 km/hr) to 60 

mph (97 km/hr), began moving sand across the valley 

floor to the eastern edge of the valley where the Sangre 

de Cristo Mountains formed a sand trap for the high 

altitude eolian deposits (Johnson 1967; Kiver and Harris 

1999). 

Alluvium, alluvial fan deposits, landslides, and rockfalls 

contribute to other Holocene deposits adjacent to the 

dune field and in the Sangre de Cristo Mountains. Sand, 

gravel, and clay deposited by streams compose the 

alluvium deposits and include both Holocene and 

Pleistocene stream deposits. Spread along the west side 

of the Sangre de Cristo Mountains, these deposits form 

alluvial fans. Landslide deposits form hummocky 

topography underlain by angular rock debris. These 

deposits may be as much as 100 feet (30 m) thick and 

differ from rockfall deposits in that rockfall deposits 

consist of the talus that accumulates at the base of large 

outcrops and cliffs (Lindsey et al. 1986; Johnson et al. 

1989; Bruce and Johnson 1991).
 GRSA Geologic Resource Evaluation Report 33

Figure 12. Geologic time scale; adapted from the U.S. Geological Survey and International Commission on Stratigraphy. Red lines indicate 

major unconformities between eras. Included are major events in life history and tectonic events occurring on the North American continent. 

Absolute ages shown are in millions of years. 
 NPS Geologic Resources Division 34 

Figure 13. Speculative Paleogeographic map of the southwestern United States during the Middle Proterozoic, about 1700 Ma. Map courtesy 

of Dr. Ron Blakey, Northern Arizona University, http://jan.ucc.nau.edu/~rcb7/pcpaleo.html. 
 GRSA Geologic Resource Evaluation Report 35

14 A. Middle Ordovician, about 470 Ma. Western North America was a passive tectonic margin. 

14 B. Early Mississippian, about 340 Ma. Western North America was an active tectonic margin (Antler Orogeny). 
 NPS Geologic Resources Division 36 

14 C. Early Pennsylvanian, about 300 Ma. Tectonic collisions between North and South America 

results in the Colorado Orogeny and the rise of the Ancestral Rocky Mountains in Colorado. 

14 D. Early Permian, about 270 Ma. The Ancestral Rocky Mountains still supply sediment to the Great Sand Dunes vicinity. 

Figure 14. Some time slices of Paleozoic paleogeography of the southwestern United States. A) Middle Ordovician. The Cambrian 

depositional and tectonic patterns continue. An oceanic arc approaches from the west. The ocean off western North America subducts 

below the arc, and the arc closes in on the North America coast. B) Early Mississippian. The colliding Antler arc emplaces strata deposited 

in deep water over the continental shelf to form the Antler Orogeny. The weight of the thickened crust causes eastern Nevada and western 

Utah to subside. Farther east a broad carbonate shelf blankets much of the Western Interior C) Early Pennsylvanian. The Ancestral Rockies 

rise across the southern Western Interior. The Antler Mountains are already being worn down by erosion and even begin subsiding in some 

areas. A marine basin covers most of western Utah and eastern Nevada. D) Early Permian. Parts of the Ancestral Rockies remain high and 

shed sediments into adjacent basins. Maps courtesy of Dr. Ron Blakey, Northern Arizona University, 

http://jan.ucc.nau.edu/~rcb7/pcpaleo.html. 
 GRSA Geologic Resource Evaluation Report 37

15 A. Late Jurassic, about 150 Ma, paleogeographic map of the southwestern United States. 

15 B. Late Cretaceous, about 75 Ma, paleogeographic map of the southwestern United States. 

Figure 15. Some time slices of Mesozoic paleogeography of the southwestern United States. A) Late Jurassic paleogeography. The Morrison 

fluvial system expands eastward across the Western Interior. The Nevada Orogeny begins in the west. B) Late Cretaceous paleogeography. 

The Western Interior seaway slowly retreated to the northeast. Some of the World's greatest dinosaur remains are found in these deposits. 

The Sevier Orogeny along the western margin of North America was at its climax. Maps courtesy of Dr. Ron Blakey, Northern Arizona 

University, http://jan.ucc.nau.edu/~rcb7/pcpaleo.html. 
 NPS Geologic Resources Division 38 

Figure 16. Early Tertiary (Eocene), about 50 Ma at the end of the Laramide Orogeny. As the Rocky Mountains were uplifted, basins formed 

and subsided between major uplifts. Huge lakes filled the basins during the Eocene. Although the lakes were in basins, paleobotanical data 

suggests that their absolute elevation was near the current elevation of the deposits. This fact suggests that much of the Rocky Mountain 

region and the Colorado Plateau were uplifted in the Eocene. Mountainous terrain existed in Nevada, western Utah, and central and southern 

Arizona. Streams drained these uplands onto the Colorado Plateau and probably into the lakes. Map courtesy of Dr. Ron Blakey, Northern 

Arizona University, http://jan.ucc.nau.edu/~rcb7/pcpaleo.html. 
 GRSA Geologic Resource Evaluation Report 39

Glossary 

This glossary contains brief definitions of technical geologic terms used in this report. Not all 

geologic terms used are referenced. For more detailed definitions or to find terms not listed 

here please visit http://wrgis.wr.usgs.gov/docs/parks/misc/glossarya.html. 

active margin. A continental margin where significant 

volcanic and earthquake activity occurs; commonly a 

convergent plate margin. 

alluvial fan. A fan- shaped deposit of sediment that 

accumulates where a high gradient stream flows out of 

a mountain front into an area of lesser gradient such as 

a valley. 

barchan dune. A crescent- shaped dune with arms or 

horns of the crescent pointing downwind. The 

crescent or barchan type is most characteristic of the 

inland desert regions. 

baseflow. Stream flow supported by groundwater flow 

from adjacent rock, sediment, or soil. 

baselevel. The lowest level to which a stream can erode 

its channel. The ultimate base level for the land surface 

is sea level, but temporary base levels may exist locally. 

basement. The undifferentiated rocks, commonly 

igneous and metamorphic, that underlie the rocks of 

interest. 

basin (structural). A doubly- plunging syncline in which 

rocks dip inward from all sides (also see dome). 

basin (sedimentary). Any depression, from continental 

to local scales, into which sediments are deposited. 

batholith. A massive, discordant pluton, greater than 100 

km2

, (39.6 mi2) often formed from multiple intrusions. 

breccia. A coarse- grained, generally unsorted, 

sedimentary rock consisting of cemented angular 

clasts. 

clastic. Rock or sediment made of fragments or preexisting rocks. 

chlotitoid. A greenish mica- like mineral 

climbing dune. A dune formed by the wind piling sand 

against a mountain slope. 

craton. The relatively old and geologically stable interior 

of a continent (also see continental shield). 

cross-bedding. Uniform to highly- varied sets of inclined 

sedimentary beds deposited by wind or water that 

indicate distinctive flow conditions. 

debris flow. A rapid and often sudden flow or slide of 

rock and soil material involving a wide range of types 

and sizes. 

delta. A sediment wedge deposited at a stream’s mouth 

where it flows into a lake or sea. 

dike. A tabular, discordant igneous intrusion. 

dune. A low mound or ridge of sediment, usually sand, 

deposited by wind. Common dune types include the 

barchan dune, longitudinal dune, parabolic dune, and 

transverse dune (see respective listings). 

eolian. Formed, eroded, or deposited by or related to the 

action of the wind. 

evaporite. Chemically precipitated mineral(s) formed by 

the evaporation of solute- rich water under restricted 

conditions. 

fan delta. An alluvial fan that builds into a standing body 

of water. The landform differs from a delta in that a 

fan- delta is next to a highland and typically forms at 

an active margin. 

graben. A down- dropped structural block bounded by 

steeply- dipping, normal faults (also see horst). 

horst. An uplifted structural block bounded by highangle normal faults. 

island arc. A line or arc of volcanic islands formed over 

and parallel to a subduction zone. 

lacustrine. Pertaining to, produced by, or inhabiting a 

lake or lakes. 

landslide. Any process or landform resulting from rapid 

mass movement under relatively dry conditions. 

longitudinal dunes. Dunes elongated parallel to the 

direction of wind flow. 

migmatite. Literally, “mixed rock” with both igneous and 

metamorphic characteristics due to partial melting 

during metamorphism. 

mylonite. A fine- grained rock form in shear zones 

composed of pulverized rock. 

orogeny. A mountain- building event, particularly a wellrecognized event in the geological past (e.g. the 

Laramide orogeny). 

packstone. A sedimentary carbonate rock composed of 

self- supporting granular material (wackestone). 

Pangaea. A theoretical, single supercontinent that 

existed during the Permian and Triassic Periods (also 

see Laurasia and Gondwana). 

parabolic dunes. Crescent- shaped dunes with horns or 

arms that point upwind. 

passive margin. A tectonically quiet continental margin 

indicated by little volcanic or seismic activity. 

pluvial lakes. Lakes formed during earlier times of more 

abundant precipitation. 

prodelta. The part of a delta below the level of wave 

erosion. 

progradation. The seaward building of land area due to 

sedimentary deposition. 

red beds. Sedimentary strata composed largely of 

sandstone, siltstone, and shale that are predominantly 

red due to the presence of ferric oxide (hematite) 

coating individual grains. 

regression. A long- term seaward retreat of the shoreline 

or relative fall of sea level. 

ripple marks. The undulating, subparallel, usually smallscale, ridge pattern formed on sediment by the flow of 

wind or water. 

reversing dune. A dune form by wind from variable 

(often opposite) directions. 
 NPS Geologic Resources Division 40 

sabkha. A coastal environment in an arid climate where 

evaporation rates are high. The term is also applied to 

interior flat areas where, from either deflation or 

evaporation, saline minerals are present (salt flat). 

suture. The linear zone where two continental 

landmasses become joined due to subduction. 

tectonic. Relating to large- scale movement and 

deformation of Earth’s crust. 

transverse dunes. Dunes elongated perpendicular to the 

prevailing wind direction. The leeward slope stands at 

or near the angle of repose of sand whereas the 

windward slope is comparatively gentle. 

turbidite. A sedimentary rock fromed by a turbidity 

current, 

turbidity current. A swift current laden with suspended 

sediment. 

unconformity. A surface within sedimentary strata that 

marks a prolonged period of nondeposition or 

erosion. 

wackestone. A mud- supported sedimentary carbonate 

rock (packstone). 
 GRSA Geologic Resource Evaluation Report 41

References 

This section provides a listing of references cited in this report. A more complete geologic 

bibliography is available and can be obtained through the NPS Geologic Resources 

Division. 

Brister, B.S., and Robie R. Gries. 1994. Tertiary 

stratigraphy and tectonic development of the Alamosa 

basin (northern San Luis Basin), Rio Grande rift, 

south- central Colorado. In Basins of the Rio Grande 

Rift: Structure, Stratigraphy, and Tectonic Setting, 

edited by G.R. Keller and S.M. Cather. Boulder: 

Geological Society of America Special Paper 291, 

39- 58. 

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

geologic map of parts of the Zapata Ranch and Mosca 

Pass Quadrangles, Alamosa and Huerfano Counties, 

Colorado. U.S.G.S. Miscellaneous Field Map, MF2168, scale: 1:24,000. 

Burchfiel, B.C., D.S. Cowan, and G.A. Davis. 1992. 

Tectonic overview of the Cordilleran orogen in the 

western United States. In The Cordilleran Orogen: 

Conterminous U.S., edited by B.C. Burchfiel, P.W. 

Lipman, and M.L. Zoback. Boulder: Geological 

Society of America, The Geology of North America, 

Vol. G- 3, 407- 480. 

Chapin, C.E. and S. M. Cather. 1994. Tectonic setting of 

the axial basins of the northern and central Rio 

Grande rift. In Basins of the Rio Grande Rift: Structure, 

Stratigraphy, and Tectonic Setting, edited by G.R. 

Keller and S.M. Cather. Boulder: Geological Society of 

America Special Paper 291, 5- 25. 

Chatman, M.L. 1995. A preliminary Literature Search, 

Inventory, and Assessment of Mines and Prospects in 

and near the National Monument with Emphasis on 

Potential Water Quality Degradation: Great Sand 

Dunes National Monument, Colorado. Unpublished 

report in Great Sand Dunes National Park and 

Preserve files. 17 pages, 1 appendix. 

Chatman, M., Dave Sharrow, and Andrew Valdez. 1997. 

Water resources management plan Great Sand Dunes 

National Monument, Colorado. NPS Water Resources 

Division, 197 pages. 

De Voto, R. H. 1980. Mississippian stratigraphy and 

history of Colorado. In Colorado Geology, edited by 

H.C. Kent and K.W. Porter. Denver: Rocky Mountain 

Association of Geologists: 57- 70. 

Doelling, Helmut H. 2000. Geology of Arches National 

Park, Grand County, Utah. In Geology of Utah’s Parks 

and Monuments, edited by D.A. Sprinkel, T.C. Chidsey, 

Jr., and P.B. Anderson. Salt Lake City: Utah Geological 

Association Publication 28: 11- 36. 

Dubiel, R. F. 1994. Triassic deposystems, 

paleogeography, and paleoclimate of the Western 

Interior. In Mesozoic Systems of the Rocky Mountain 

Region, USA, edited by Mario V. Caputo, James A. 

Peterson, and Karen J. Franczyk. Denver: Rocky 

Mountain Section, SEPM (Society for Sedimentary 

Geology): 133- 168. 

Emery, Philip A. 2005. Hydrogeology of the San Luis 

Valley, Colorado, An Overview – and a look at the 

Future, http://www.nps.gov/grsa/resources 

/wind_regime.htm (accessed December 12, 2005). 

Gregson, Joe D. 1992. Geology and tectonics of the 

Ancestral Uncompahgre Uplift and the Colorado 

Orogeny. In Uncompahgria Journal, edited by Joe D. 

Gregson. Grand Junction: Mesa State Geology 

Department: 19- 46. 

Hoffman, P.F. 1989. Precambrian geology and tectonic 

history of North America. In The Geology of North 

America; An Overview, edited by A.W. Bally and A.R. 

Palmer. Boulder: Geological Society of America, The 

Geology of North America, Vol. A, 447- 512. 

Hutchinson, R. M. 1976. Precambrian geochronology of 

western and central Colorado and southern Wyoming. 

In Studies in Colorado Field Geology: Professional 

Contributions, edited by R. C. Epis and Robert J. 

Weimer. Golden: Colorado School of Mines: 73- 77. 

Johnson, Jess G., Charles A. Sandberg, and Forrest G. 

Poole. 1991. Devonian lithofacies of western United 

States. In Paleozoic Paleogeography of the Western 

United States – II, edited by John D. Cooper and Calvin 

H. Stevens. Los Angeles: Pacific Section, SEPM 

(Society for Sedimentary Geology), Vol. 1, 83- 106. 

Johnson, R.B. 1967. The Great Sand Dunes of Colorado. 

USGS Professional Paper 575- C: 177- 183. 

Johnson, B.R., R. M. Bruce, and D.A. Lindsey. 1989. 

Reconnaissance geologic map of the Medano Pass 

Quadrangle and part of the Liberty Quadrangle, 

Alamosa, Huerfano, and Saguache Counties, 

Colorado. U.S.G.S. Miscellaneous Field Map, MF2089, scale: 1:24,000. 

Kauffman, E. G. 1977. Geological and biological 

overview: Western Interior Cretaceous Basin. 

Mountain Geologist, Vol. 14, 75- 99. 
 NPS Geologic Resources Division 42 

Kiver, Eugene P., and David V. Harris. 1999, Geology of 

U.S. Parklands. New York: John Wiley & Sons, Inc., 

644- 652. 

Kluth, C. F. 1986. Plate tectonics of the ancestral Rocky 

Mountains. In Paleotectonics and Sedimentation in the 

Rocky Mountain Region, edited by J. A. Peterson. 

American Association of Petroleum Geologists, 

Memoir 41, 353- 369. 

Kluth, C.F. and C.H. Schaftenaar. 1994. Depth and 

geometry of the northern Rio Grande rift in the San 

Luis basin, south- central Colorado, In Basins of the Rio 

Grande Rift: Structure, Stratigraphy, and Tectonic 

Setting, edited by G.R. Keller and S.M. Cather. 

Boulder: Geological Society of America Special Paper 

291, 27- 37. 

Kocurek, G. and Robert H. Dott, Jr. 1983. Jurassic 

paleogeography and paleoclimate of the central and 

southern Rocky Mountain region. In Mesozoic 

Paleogeography of the West- Central United States, 

edited by Mitchell W. Reynolds and Edward D. Dolly. 

Denver: Rocky Mountain Section, SEPM (Society for 

Sedimentary Geology), 101- 118. 

Lindsey, D.A., B.R. Johnson, S.J. Soulliere, R.M. Bruce, 

and K. Hafner, K. 1986. Geologic map of the Beck 

Mountain, Crestone Peak, and Crestone Quadrangles, 

Custer, Huerfano, and Saguache Counties, Colorado. 

U.S.G.S. Miscellaneous Field Map, MF- 1878, scale 

1:24,000. 

Marin, Liliana, and Steven L. Forman. 2004. 20th century 

dune movements at the Great Sand Dunes National 

Park and Preserve, Colorado and relation to drought 

variability. Boulder: Geological Society of America, 

Abstracts with Programs, Vol. 36, 229. 

Maughan, E.K. 1980. Permian and Lower Triassic 

Geology of Colorado. In Colorado Geology, edited by 

H.C. Kent and K.W. Porter. Denver: Rocky Mountain 

Association of Geologists, 103- 110. 

McCalpin, James P. 1994. General Geology of the 

Northern San Luis Valley, Colorado. In Basins of the 

Rio Grande Rift: Structure, Stratigraphy, and Tectonic 

Setting, edited by G.R. Keller and S.M. Cather. 

Boulder: Geological Society of America Special Paper 

291. Also available online at http://www.nps.gov/grsa/ 

resources/docs/Trip2021.pdf (access December 12, 

2005). 

Meierding, T.C., and P.W. Birkeland. 1980. Quaternary 

Glaciation of Colorado, In Colorado Geology, edited by 

H.C. Kent and K.W. Porter. Denver: Rocky Mountain 

Association of Geologists, 165- 174. 

National Park Service. 1994. Resource Management 

Strategy: Great Sand Dunes National Monument, 1994. 

Department of Interior, NPS document. 

Peterson, Fred. 1994. Sand dunes, sabkhas, stream, and 

shallow seas: Jurassic paleogeography in the southern 

part of the Western Interior Basin. In Mesozoic Systems 

of the Rocky Mountain Region, USA, edited by Mario V. 

Caputo, James A. Peterson, and Karen J. Franczyk. 

Denver: Rocky Mountain Section, SEPM (Society for 

Sedimentary Geology), 233- 272. 

Poole, Forrest G., John H. Stewart, A.R. Palmer, Charles 

A. Sandberg, R. J. Madrid, R.J. Ross, Jr., L.F. Hintze, 

M.M. Miller, and C.T. Wrucke. 1992. Latest 

Precambrian to latest Devonian time; Development of 

a continental margin. In The Cordilleran Orogen: 

Conterminous U.S., edited by B.C. Burchfiel, P.W. 

Lipman, and M.L. Zoback. Boulder: Geological 

Society of America, The Geology of North America, 

Vol. G- 3, 9- 56. 

Reed, J. C., Jr., M.E. Bickford, W.R. Premo, J.N. 

Aleinikoff, and J.S. Pallister. 1987. Evolution of the 

Early Proterozoic Colorado province: Constraints 

from U- Pb geochronology. Geology, Vol. 15, 861- 865. 

Ross, R. J., Jr. and Ogden Tweto. 1980. Lower Paleozoic 

sediments and tectonics in Colorado. In Colorado 

Geology, edited by H. C. Kent and K. W. Porter. 

Denver: Rocky Mountain Association of Geologists, 

47- 56. 

Rupert, Michael G. and L. Niel Plummer. 2004. Groundwater flow direction, water quality, recharge sources, 

and age, Great Sand Dunes National Monument, 

south- Central Colorado, 2000- 2001. U.S.G.S. 

Scientific Investigations Report 2004- 5027, 28 pages. 

Also available online at http://pubs.usgs.gov/sir/ 

2004/5027 (access December 12, 2005). 

Schumm, Stanley A., D.W. Bean, and Mike D. Harvey. 

1982. Bed- form Dependant Flow in Medano Creek, 

Southern Colorado. Earth Surface Processes and 

Landforms, Vol. 7, pp. 17- 28. 

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Livaccari, J.B. Johnson, R.R. Shroba, and R.P. 

Dickerson. 2001. Geologic map of Colorado National 

Monument and adjacent areas, Mesa County, 

Colorado. U.S.G.S. Geologic Investigations Series I2740, 1:24,000 scale. 

Stewart, John H. 1980. Geology of Nevada. Carson City: 

Nevada Bureau of Mines and Geology, Special 

Publication 4, 136 pages. 

Stewart, John H., Forest G. Poole, and R.F. Wilson. 

1972A. Stratigraphy and Origin of the Triassic 

Moenkopi Formation and related Triassic strata in the 

Colorado Plateau region. U.S.G.S. Professional Paper 

691, 195 pages. 
 GRSA Geologic Resource Evaluation Report 43

Stewart, John H., Forest G. Poole, and R.F. Wilson. 

1972B. Stratigraphy and Origin of the Chinle 

Formation and related Triassic strata in the Colorado 

Plateau region. U.S.G.S. Professional Paper 690, 336 

pages. 

Stone, Don S. 1986. Seismic and borehole evidence for 

important pre- Laramide faulting along the axial arch 

in northwest Colorado. In New Interpretations of 

Northwest Colorado Geology, edited by Don S. Stone. 

Denver: Rocky Mountain Association of Geologists, 

19- 36. 

Stover, C.W., B.G. Reagor, and S.T. Algermissen. 1988. 

Seismicity map of the State of Colorado. USGS Map 

MF- 2036, scale 1:1,000,000. 

Trimble, S.A. 1972. Great Sand Dunes. Globe, Arizona: 

Southwest parks and Monuments Association, 33 

pages. 

Tweto, Ogden. 1980. Precambrian geology of Colorado. 

In Colorado Geology, edited by H. C. Kent and K. W. 

Porter. Denver: Rocky Mountain Association of 

Geologists, 37- 46. 

Valdez, Andrew D. 1996. The Role of Streams in the 

Development of the Great Sand Dunes and Their 

Connection with the Hydrologic Cycle. In Geologic 

Excursions to the Rocky Mountains and Beyond, edited 

by R.A. Thompson, M.R. Hudson, and C.L. Pillmore. 

Denver: Colorado Geological Survey. Also available 

online at http://nps.gov/grsa/resouces 

/wind_regime.htm (accessed December 12, 2005). 

Wiegand, J.P. 1977. Dune Morphology and 

Sedimentology at Great Sand Dunes National 

Monument. Unpublished Master’s thesis, Colorado 

State University. 

Witzke, Brian J. 1980. Middle and Upper Ordovician 

paleogeography of the region bordering the 

Transcontinental Arch. In Paleozoic Paleogeography of 

the West- Central United States, edited by T. D. Fouch 

and E. R. Magathan. Denver: Rocky Mountain Section, 

SEPM (Society for Sedimentary Geology), 1- 18. 

Wurster, F.C., and D.J. Cooper. 2000. Analysis of 

interdunal wetland disappearance at Great Sand 

Dunes National Monument, Colorado. NPS 

document, 103 pages. 
 NPS Geologic Resources Division 44 
 GRSA Geologic Resource Evaluation Report 45

Appendix A: Geologic Map Graphic 

The following page provides a preview or “snapshot” of the geologic map for Great Sand 

Dunes National Park and Preserve. For a poster size PDF of this map or for digital 

geologic map data, please see the included CD or visit the GRE publications webpage: 

http://www2.nature.nps.gov/geology/inventory/gre_publications.cfm 

 GRSA Geologic Resource Evaluation Report 47

Appendix B: Scoping Summary 

The following excerpts are from the GRE scoping summary for Great Sand Dunes 

National Park and Preserve. The scoping meeting occurred on August 21, 1998, 

therefore, the contact information and Web addresses referred to herein may be 

outdated. Please contact the Geologic Resources Division for current information. 

Notes from the field: Geologic Resources Inventory, 

Great Sand Dunes National Monument, CO 

Date: August 21, 1998 

Geology Workshop Report 

Bruce Heise 

Geologic Resources Division 

Summary 

Field Trip: Park resource specialist Andrew Valdez led 

NRPC staff on an afternoon field trip explaining the 

park’s geology and identifying resource concerns and 

ongoing research. The sand that makes up the dunes is 

the primary resource of the park. The sand is constantly 

recycled as it is caught up in a complex cycle of winds 

alternately crossing the San Luis Valley to the west and 

the Sangre de Cristo Range to the east. Maintaining that 

cycle is essential to the preservation of the dunes. There 

are three geologic components to the park and dune 

system; a stable sand sheet surrounding the dunes; the 

active dune fields, and a sabka system (internally drained 

evaporative basin) to the west of the park. There are 

some xxx square miles of dunes in the field, of which xx 

square miles are in the park. Additionally, there are 

metamorphic crystalline rocks exposed in the northeast 

corner of the park, which, while having no impact on the 

dunes, none the less represent a significant geologic 

feature. We observed sites where the sand lapped onto 

the bedrock as well as onto alluvial fans extending from 

the mountain front. 

Research: Sand traps identify the provenance of the sand 

and the effect varying levels of vegetation have on dune 

stabilization and motion. NRPP money funding for 

study? 

Re- establishing abandoned USGS gauging stations at 

several of the creeks leading down off the mountain. 

Early 90s research revealed that the sand sheet west of 

the park provided a critical element in the sand cycle. 

Maintaining that sand sheet west of the park was critical 

to dune preservation. 

There is ongoing research in the region related to an 

anticipated proposal to mine the ground water of the San 

Luis valley 

Resource Concerns 

In the early 1990s a proposal was put forth to mine the 

extensive ground water resources of the San Luis Valley. 

The park was concerned about the effect this would have 

on the dunes and the sand cycle. The local agriculture 

community was equally concerned about the impact on 

their irrigation source. Although that proposal was 

defeated, a similar one is anticipated in the near future. 

There will be significant state involvement in any future 

proposal. 

RMP statements 

Acquire MF maps from USGS multiple copies for park, 

I&M, GRD. 

Talk to NRCS about soils mapping in Great Sand Dunes 

(and other CO parks) for status, including digital. 

FAQ 

• How old, 

• How thick, 

• How tall, 

• Where do they come from, 

• Are they changing? 

Cannot answer age, have a minimum thickness of 710 + 

130 well depth, height is 710 max, some changing/motion, 

Coop sales data 

• Geol Map of Colorado Hwy map – 127 

• So. Rockies Hwy Map 60 

Jim McCalpin: 

Our Index Map indicates MF 2089, question about 

mapping extent, maps done in support of SdC wilderness 

Two new references: add to list. 

Digital map use: 

• General public – cobble together existing maps and 

pull out a compilation 

• Best situation would be to dedicate a specific map a la 

ROMO 

• State mapping projects probably of limited interest – 

no hazards in large population areas 
 NPS Geologic Resources Division 48 

Hazards 

• VC on fault scarps, 

• Campground location possible on landslide/debris 

flow, 

• No interest shown by USGS on McCalpin proposal to 

investigate SdC fault for potential active status 

Adams State: senior projects possible for mapping, break 

out Precambrian units. 

CSM seismic work report available. 

CGS study by Kirkham (sp) on seismic hazards for area? 

Georef doesn’t appear to be capturing PhD dissertations. 

I-1594 displays fault lines taken from McCalpin map.

Recommended compilation pieces: 

• Bedrock from two MF 2089, 2168 

• Surficial from I 1594 

• Dunes from 575- C 

• Units would be the three sand units: active dunes, sand 

sheet, sabka 

• There is flexibility to allow a GIS theme for the 

different units, 

• General belief is that a popular publication, including a 

map showing informal units, would be popular and 

worthwhile for interp 

Issues 

1. Date the dunes: USGS lab closed, need funding, 

samples remain with USGS, cost about $600/sample, 

Scientific investigations Water Resource Research 

Report 

1. Surface shaking, earthquake issues, water tanks, 

pipelines – most facilities in the vicinity of fault lines, 

tie down to mapping 

2. Ground water relation to faulting – 1976 thesis 

showing faults acting a barrier to ground water 

passage, another reason to study faults 

3. May require another more complete map of faulting 

4. Differentiate paleo fluvial channels within park – link 

to global warming studies, there is existing climatic 

history of dunes (also human history, hydrology 

history) 

5. Depth to bedrock – major basin fault located west of 

major dune field, seismic in 1995? paper on Rio 

Grande rift, drilling for depth this next fiscal year, to 

confining layer then just below, maybe to 300 feet 

6. Fryeberger issues – preservation of sand sheet critical 

to dune preservation – grazing of veg on sand sheet 

and its impacts on dunes 

7. Water withdrawal – impacts on sabka 

8. Main concern is with water QUANTITIES, wind and 

shape of mountain front cannot be managed, 

emphasis 

9. Research into sands, light colored ripples are coarser 

grained, different mineralogy, is provenance 

different? Why don’t they mix? Different wind 

regimes? 

10. Infiltration of moisture into soil rates: Meteoric 

water? Ground water? What is vapor phase coming 

from? 

11. Relationship of vegetation on dunes to soil moisture 

Interpretive Needs 

• Products 

• Xbedding post card 

• Geologic publications 

• Other postcards – xbeds, geol map 

• Exhibits 

• Sand peel 

• Rotating 

• In house training document geologic piece of the story. 

Can we get a copy of this 

• Publish proceedings 

• Electronic Documents – online 

• GSA proceedings from 1996 on San Luis Valley area 

• Talk to Bob (Higgins) on including inland dune parks 

in coastal processes symposium. Bill Wellman, Jane 

Beyner? Possible initiators. Including academic 

community 

• Report on the park – Andrew would edit/coordinate 

Possible Mary Mickelsen as author? Using 

Cooperating org, I&M, GRD project monies 

• Additional data coming in – deep well 

• Is there Lexam (Tom Watkins geologist) data available 

Unique Geologic Features 

Surge flows 

Disturbed Lands – Steensen 

• Some prospect pits and shafts 

Hazards and Issues 

• Water – from WRD 

• Hazards – Jim McCalpin write up from proposal 

History of Geologic Exploration in Area 

• Human history – Mary 

• Possible funding by I&M year end money
The Department of the Interior protects and manages the nation’s natural resources and cultural heritage; provides scientific and other 

information about those resources; and honors its special responsibilities to American Indians, Alaska Natives, and affiliated Island 

Communities. 

Great Sand Dunes National Park and Preserve 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2006/011 

NPS D-71, March 2006 

National Park Service 

Director • Fran P. Mainella 

Natural Resource Stewardship and Science 

Associate Director • Michael A. Soukup 

Natural Resource Program Center 

The Natural Resource Program Center (NRPC) is the core of the NPS Natural Resource Stewardship and 

Science Directorate. The Center Director is located in Fort Collins, with staff located principally in 

Lakewood and Fort Collins, Colorado and in Washington, D.C. The NRPC has five divisions: Air 

Resources Division, Biological Resource Management Division, Environmental Quality Division, Geologic 

Resources Division, and Water Resources Division. NRPC also includes three offices: The Office of 

Education and Outreach, the Office of Inventory, Monitoring and Evaluation, and the Office of Natural 

Resource Information Systems. In addition, Natural Resource Web Management and Partnership 

Coordination are cross- cutting disciplines under the Center Director. The multidisciplinary staff of NRPC 

is dedicated to resolving park resource management challenges originating in and outside units of the 

national park system. 

Geologic Resources Division 

Chief • David B. Shaver 

Planning Evaluation and Permits Branch Chief • Carol McCoy 

Credits 

Author • Dr. John Graham

Editing • Sid Covington 

Digital Map Production • Stephanie O’Meara, Eileen Ernenwein, Victor DeWolfe, Giorgia deWolfe, and 

Andrew Valdez 

Map Layout Design • Melanie Ransmeier
Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, CO 80225 

http://www.nature.nps.gov/geology/inventory/ 

(303) 969-2090 

EXPERIENCE YOUR AMERICATM 

National Park Service 

U.S. Department of the Interior