National Park Service

U.S. Department of the Interior

Natural Resource Stewardship and Science

Pecos National Historical Park

Geologic Resources Inventory Report

Natural Resource Report NPS/NRSS/GRD/NRR—2015/951 
ON THE COVER

The last Spanish mission church at Pecos National Historical Park was completed by 1717 (view 

looking south). National Park Service photograph by Chris Dietrich (Resource Information 

Services Division). 

THIS PAGE

Limestone of the Alamitos Formation crops out along the Pecos River. National Park Service 

photograph by Mario Armijo (Pecos National Historical Park).
Pecos National Historical Park

Geologic Resources Inventory Report

Natural Resource Report NPS/NRSS/GRD/NRR—2015/951 

Rebecca Port

National Park Service Geologic Resources Division

Geologic Resources Inventory

PO Box 25287 

Denver, CO 80225 

April 2015

US Department of the Interior

National Park Service

Natural Resource Stewardship and Science

Fort Collins, Colorado
ii NPS Geologic Resources Division

The National Park Service, Natural Resource Stewardship and Science office in Fort 

Collins, Colorado, publishes a range of reports that address natural resource topics 

These reports are of interest and applicability to a broad audience in the National Park 

Service and others in natural resource management, including scientists, conservation 

and environmental constituencies, and the public. 

The Natural Resource Report Series is used to disseminate comprehensive information 

and analysis about natural resources and related topics concerning lands managed by 

the National Park Service. The series supports the advancement of science, informed 

decision-making, and the achievement of the National Park Service mission. The series 

also provides a forum for presenting more lengthy results that may not be accepted by 

publications with page limitations. 

All manuscripts in the series receive the appropriate level of peer review to ensure that 

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

the intended audience, and designed and published in a professional manner. This 

report received informal peer review by subject-matter experts who were not directly 

involved in the collection, analysis, or reporting of the data.

Views, statements, findings, conclusions, recommendations, and data in this report do 

not necessarily reflect views and policies of the National Park Service, U.S. Department 

of the Interior. Mention of trade names or commercial products does not constitute 

endorsement or recommendation for use by the U.S. Government.

Printed copies of this report are produced in a limited quantity and they are only 

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

Inventory website (http://www.nature.nps.gov/geology/inventory/

gre_publications.cfm) and the Natural Resource Publications Management website 

(http://www.nature.nps.gov/publications/nrpm/). To receive this report in a format 

optimized for screen readers, please email irma@nps.gov. 

Please cite this publication as: 

Port, R. 2015. Pecos National Historical Park: Geologic resources inventory report. 

Natural Resource Report NPS/NRSS/GRD/NRR—2015/951. National Park Service, 

Fort Collins, Colorado. 

NPS 430/128395, April 2015
PECO Geologic Resources Inventory Report iii

Contents

Page

List of Figures................................................................................................................. v

List of Tables .................................................................................................................. v

Executive Summary ..................................................................................................... vii

Products and Acknowledgments.................................................................................. x

GRI Products................................................................................................................................................................ x

Acknowledgments....................................................................................................................................................... x

Geologic Setting and Significance................................................................................ 1

Geologic Setting.......................................................................................................................................................... 1

Geologic Significance and Connections to Park Stories................................................................................................. 1

Geologic Features and Processes.................................................................................. 7

Geologic Foundation ................................................................................................................................................... 7

Sangre de Cristo Mountains......................................................................................................................................... 9

Glorieta Pass..............................................................................................................................................................11

Glorieta Mesa ............................................................................................................................................................11

Fluvial Features..........................................................................................................................................................12

Folds .........................................................................................................................................................................12

Faults.........................................................................................................................................................................13

Slope Movements......................................................................................................................................................15

Paleontological Resources..........................................................................................................................................16

Caves and Karst Resources.........................................................................................................................................18

Earthquakes...............................................................................................................................................................19

Geologic Resources in a Cultural Context ...................................................................................................................20

Geologic Resource Management Issues ..................................................................... 27

Fluvial Erosion............................................................................................................................................................27

Water Quantity and Quality .......................................................................................................................................31

Disturbed Land Restoration........................................................................................................................................31

Abandoned Mineral Lands Hazards............................................................................................................................32

Slope Movement Hazards and Risks...........................................................................................................................33

Paleontological Resource Inventory, Monitoring, and Protection .................................................................................34

Caves and Associated Landscape Management ..........................................................................................................34

Land Subsidence........................................................................................................................................................35

Earthquake Hazards and Risks....................................................................................................................................35

External Energy or Mineral Development....................................................................................................................35

Interpretation and Education......................................................................................................................................35

Geologic History .......................................................................................................... 37

2,500 million–542 million years ago (Proterozoic Eon): Development of the Structural Foundation..............................37

542 million–252 million years ago (Paleozoic Era): Rise and Fall of the Ancestral Rocky Mountains..............................38

252 million–66 million years ago (Mesozoic Era): Sedimentary Basins..........................................................................40

The past 66 million years (Cenozoic Era): Mountain Building, Erosion, and Emergence of the Modern Landscape........40

Geologic Map Data...................................................................................................... 43

Geologic Maps ..........................................................................................................................................................43

Source Maps..............................................................................................................................................................43

GRI GIS Data..............................................................................................................................................................43

Geologic Map Poster .................................................................................................................................................44

Map Unit Properties Table..........................................................................................................................................44

Use Constraints..........................................................................................................................................................44

Glossary........................................................................................................................ 45

Literature Cited............................................................................................................ 51
iv NPS Geologic Resources Division

Contents (continued)

Page

Additional References ................................................................................................. 57

Geology of National Park Service Areas......................................................................................................................57

NPS Resource Management Guidance and Documents...............................................................................................57

Climate Change Resources.........................................................................................................................................57

Geological Surveys and Societies................................................................................................................................57

US Geological Survey Reference Tools........................................................................................................................57

Appendix A: Scoping Participants .............................................................................. 58

2006 Scoping Meeting Participants............................................................................................................................58

2013 Conference Call Participants..............................................................................................................................58

Appendix B: Geologic Resource Laws, Regulations, and Policies............................. 59

GRI Products CD ................................................................................................ attached

Geologic Map Poster ........................................................................................in pocket

Map Unit Properties Table ...............................................................................in pocket
PECO Geologic Resources Inventory Report v 

List of Figures

Figure 1. Physiographic province map of New Mexico. ................................................................................................. 1

Figure 2. Pecos National Historical Park map................................................................................................................. 2

Figure 3. General stratigraphic column for Pecos National Historical Park...................................................................... 3

Figure 4. Geologic time scale. ...................................................................................................................................... 4

Figure 5. Douglas-fir microclimate................................................................................................................................ 5

Figure 6. Paleozoic arkosic sandstone........................................................................................................................... 8

Figure 7. Large scale physiographic map.....................................................................................................................10

Figure 8. Glorieta Mesa..............................................................................................................................................11

Figure 9. Terraces and alluvial gravel deposits.............................................................................................................12

Figure 10. Limestone cliffs confine the Pecos River. ....................................................................................................12

Figure 11. Block diagrams of two types of folds .........................................................................................................13

Figure 12. Map of folds in Pecos National Historical Park. ...........................................................................................13

Figure 13. Schematic illustrations of fault types. .........................................................................................................14

Figure 14. Schematic illustrations of selected slope movements. .................................................................................15

Figure 15. Steep banks along the Pecos River. ............................................................................................................16

Figure 16. Mudstone and sandstone cliffs. .................................................................................................................16

Figure 17. Fossils of the Alamitos Formation...............................................................................................................17

Figure 18. Fossils of the Sangre de Cristo Formation...................................................................................................17

Figure 19. Limestone caves ........................................................................................................................................ 18

Figure 20. Earthquake probability map.......................................................................................................................19

Figure 21. Petroglyphs ...............................................................................................................................................20

Figure 22. Ceramic jar................................................................................................................................................20

Figure 23. Prehistoric tools.........................................................................................................................................21

Figure 24. Obsidian and chert sources in the Jemez Mountain area ............................................................................22

Figure 25. Pecos Pueblo stone walls...........................................................................................................................23

Figure 26. Freshly made adobe bricks at Pecos mission church....................................................................................24

Figure 27. Fluvial erosion............................................................................................................................................27

Figure 28. Bedrock outcrop on the Pecos River...........................................................................................................28

Figure 29. Potential bedrock outcrops along Glorieta Creek and the Pecos River .........................................................28

Figure 30. Artificial levee............................................................................................................................................29

Figure 31. Aerial photograph of artificial levee ...........................................................................................................29

Figure 32. Restored wetlands of Glorieta Creek..........................................................................................................29

Figure 33. Erosion along arroyos ................................................................................................................................30

Figure 34. Glorieta Creek after a rain event in September 2013..................................................................................30

Figure 35. Beaver dams on Glorieta Creek..................................................................................................................31

Figure 36. Lower Glorieta Creek, pre-restoration ........................................................................................................32

Figure 37. Lower Glorieta Creek, during restoration ...................................................................................................32

Figure 38. Rock slide into the Pecos River...................................................................................................................33

Figure 39. Undercut banks on the eastern side of the Pecos River...............................................................................33

Figure 40. Old Colonias Road.....................................................................................................................................34

Figure 41. Mississippian Period paleogeographic maps...............................................................................................37

Figure 42. Pennsylvanian Period paleogeographic map...............................................................................................38

Figure 43. Permian Period paleogeographic map........................................................................................................39

Figure 44. Cretaceous Period paleogeographic map ...................................................................................................40

Figure 45. Cenozoic Era paleogeographic map ...........................................................................................................41

List of Tables 

Table 1. Clastic sedimentary rock classification and characteristics. ............................................................................... 7

Table 2. Simplified classification of igneous rocks. ........................................................................................................ 9

Table 3. Geology data layers in the Pecos National Historical Park GRI GIS data...........................................................44
PECO Geologic Resources Inventory Report vii

Executive Summary

The Geologic Resources Inventory (GRI) is one of 12 inventories funded by the National 

Park Service (NPS) Inventory and Monitoring Program. The Geologic Resources Division 

of the NPS Natural Resource Stewardship and Science Directorate administers the GRI. 

This report synthesizes discussions from a scoping meeting for Pecos National Historical 

Park (New Mexico) on 28 March 2006 and a follow-up conference call on 11 April 2013, 

which were held by the Geologic Resources Division to determine geologic resources, the 

status of geologic mapping, and geologic resource management issues and needs. It is a 

companion document to previously completed GRI digital geologic map data. 

This Geologic Resources Inventory report was written 

for resource managers to support science-informed 

decision making, but it may also be useful for 

interpretation. The report was prepared using available 

geologic information. The NPS Geologic Resources 

Division did not conduct any new fieldwork in 

association with this report. Sections of the report 

discuss distinctive geologic features and processes within 

Pecos National Historical Park, highlight geologic issues 

facing resource managers, describe the geologic history 

leading to the present-day landscape, and provide 

information about the GRI geologic map data. A geologic 

map poster (in pocket) illustrates these data. The Map 

Unit Properties Table (in pocket) summarizes report 

content for each geologic map unit. 

Pecos National Historical Park preserves more than 

12,000 years of human history in north-central New 

Mexico, approximately 40 km (25 mi) southeast of Santa 

Fe. Within the park’s roughly 2,700 ha (6,700 ac) are the 

remains of the Pecos Pueblo and other American Indian 

structures, Spanish colonial missions, a section of the 

Santa Fe Trail, dozens of circa-1700 Hispanic homestead 

sites, and the Glorieta Battlefield of the Civil War. 

Today, the park consists of two units: the Pecos Unit and 

the Glorieta Unit. The Pecos Unit was created when the 

State of New Mexico set aside the ruins of the pueblo 

and mission as a state monument in 1935, and in 1965 

President Lyndon Johnson signed a bill establishing the 

site as Pecos National Monument. For decades the 

monument occupied less than 160 ha (400 ac). In 1990 

the park was renamed Pecos National Historical Park 

and expanded to include Forked Lightning Ranch, which 

completely surrounded the old monument. Later that 

same year, the Glorieta Unit, which includes the 

Cañoncito and Pigeon’s Ranch subunits, was added to 

preserve the Civil War battlefield of Glorieta Pass. 

Although primarily recognized for its cultural resources, 

a strong connection exists between the cultural and 

geologic landscapes of the park. The area now preserved 

as Pecos National Historical Park is on a high mountain 

pass (Glorieta Pass) that facilitated travel and trade 

between the high plains to the east and the Rio Grande 

valley to the west. The park is situated along the pass at 

an elevation of about 2,100 m (7,000 ft). To the north of 

the pass, near Taos for example, the Sangre de Cristo 

Mountains rise to 4,000 m (13,000 ft) above sea level. To 

the south Glorieta Mesa looms over the pass, rising to 

2,304 m (7,559 ft) above sea level at its highest point. The 

geologic processes that formed Glorieta Pass paved the 

way for a rich and complex human history.

Geologic features and processes of particular 

significance for resource management at Pecos National 

Historical Park were identified during a 2006 GRI 

scoping meeting and a 2013 follow-up conference call. 

They include the following:

• Geologic Foundation. The GIS data show four major 

rock/deposit types: (1) Quaternary deposits (surficial 

units which includes artificial fill) less than 2 million 

years old, (2) Tertiary deposits (gravels and the 

Tesuque Formation) perhaps up to 10 million years 

old, (3) Mississippian through Triassic sedimentary 

rocks approximately 360 to 200 million years old, and 

(4) Precambrian igneous and metamorphic rocks more 

than 1.4 billion years old. Geologists are able to 

reconstruct the history of the land based on the 

structure and composition of these rocks and 

unconsolidated deposits. 

• Sangre de Cristo Mountains. The Sangre de Cristo 

Mountains are a north–south-trending range that 

terminates in foothills at Pecos National Historical 

Park. The mountains tell a long history culminating in

the most recent major geologic event of the area—the 

Laramide Orogeny—which raised the modern Rocky 

Mountains, including the Sangre de Cristo Mountains, 

between about 70 and 40 million years ago. 

• Glorieta Pass. Glorieta Pass is a natural, high mountain 

pass carved through uplifted sedimentary rocks in the 

former location of the Pecos River. Today, tributaries 

of the Pecos River dissect the pass. The pass functions 

as a gateway between the Great Plains and the Rio 

Grande valley. It was critical to the cultural events that 

occurred in and near Pecos National Historical Park.

• Glorieta Mesa. Glorieta Mesa is an elevated, broad, 

flat-topped area of land on the south side of Glorieta 

Pass. It is a dominant feature of the park’s viewshed. It 

showcases layer upon layer of sedimentary rocks. The 

steep, high escarpment of the mesa marks the 

boundaries of the Rocky Mountain, Great Plains, and 

Basin and Range provinces.
viii NPS Geologic Resources Division

• Fluvial Features. Three streams flow through the units 

of the park: the Pecos River, Glorieta Creek, and 

Galisteo Creek. The topography of the area is largely a 

result of fluvial processes downcutting through 

uplifted land for the last 10 million years, since the late 

Miocene Epoch. 

• Folds. Folds are curves or bends in originally flat rock 

strata. A series of folds occurs throughout the foothills 

of the Sangre de Cristo Mountains in Pecos National 

Historical Park.

• Faults. A fault is a fracture in rock along which 

movement has taken place. Four fault zones, three 

named faults, and nearly 400 smaller fault features 

were mapped in the vicinity of Pecos National 

Historical Park. These features are included in the GRI 

GIS data set. Two of the fault zones intersect the 

Cañoncito Subunit. Episodes of motion along the 

faults occurred more than a billion years ago

(Proterozoic Era) and as recently as 5 million years ago 

(Neogene Period). 

• Slope Movements. Slope movements are the 

downslope transfer of soil, regolith, and/or rock under 

the influence of gravity. Most slope movements in 

Pecos National Historical Park occur in relation to 

fluvial processes where stream erosion has created 

steep banks or has exposed bedrock along the Pecos 

River and Glorieta Creek. Slope movements may also 

occur on areas of disturbed land or as landslides. 

• Paleontological Resources. Fossils are evidence of 

ancient life preserved in a geologic context. Petrified 

wood is the only fossil that has been documented in 

Pecos National Historical Park. However, discovery of 

more fossils is a strong possibility if field-based 

inventories were conducted because rock units 

mapped in the park are known to contain fossils in 

other locations.

• Caves and Karst Resources. Two caves, Baca Cave and 

“small cave,” are in Pecos National Historical Park. 

Both caves are closed to the public. Most of the 

geologic units mapped in the park contain some 

amount of karst-forming carbonates such as limestone 

or dolomite. However, no karst topography is exposed 

in the park.

• Earthquakes. The area around Pecos National 

Historical Park has experienced some earthquakes in 

recent history. Since 1962—when earthquake 

monitoring began in the area—no earthquakes greater 

than a magnitude 3.5 have been recorded. Nearly 100 

smaller earthquakes have been recorded.

• Geologic Resources in a Cultural Context. Geologic 

processes created the landscape on which cultural 

events played out in and near Pecos National 

Historical Park. Additionally, the inhabitants of Pecos 

Pueblo used geologic resources to produce ceramic 

pottery, bricks, and stone tools, and for building stone 

and glass.

Geologic issues of particular significance for resource 

management at Pecos National Historical Park were 

identified during a 2006 GRI scoping meeting and a 2013 

follow-up conference call. They include the following:

• Fluvial Erosion. Accelerated fluvial erosion along the 

Pecos River and Glorieta Creek has led to narrowing 

of some stream channels, loss of riparian habitat, 

increased sediments in waterways, and threats to 

cultural resources.

• Water Quantity and Quality. Water quantity and 

quality issues in Pecos National Historical Park are 

related to geologic processes. In 2011, a section of 

Pecos River had temperature and turbidity (suspended 

sediment) levels that exceeded federal standards. This 

may be partially the result of disturbed lands with 

exposed surfaces from which excess sediments are 

eroded and transported into the river, and a lack native 

vegetation to provide shade to the waterways. Drought 

has reduced surface water quantity.

• Disturbed Land Restoration. Prior to establishment of 

Pecos National Historical Park, decades of livestock 

grazing, clearing of vegetation, and development by 

settlers created areas of disturbed land. These areas are 

especially susceptible to erosion because the naturally 

developed drainage has been altered and stabilizing

vegetation has been removed. The National Park 

Service has already completed several restoration 

projects within the park. At the time of this report, 

park managers were seeking assistance to address the 

remaining disturbed lands.

• Abandoned Mineral Lands Hazards. Abandoned 

mineral lands present a variety of resource 

management issues such as visitor and staff safety and 

environmental quality of air, water, and soil. The Baca 

Mine, which was excavated into a natural cave—Baca 

Cave, is closed to the public to prevent injury to park 

visitors and impacts to cave resources. Remains of the 

Pecos Mine operation, located adjacent to the Pecos 

River and 26 km (16 mi) upstream from the park, may 

be adversely affecting water quality in the park by 

contributing heavy metals leached from waste piles 

into the Pecos River. 

• Slope Movement Hazards and Risks. At the time of 

this report, slope movements (gravity-driven 

processes) were not affecting infrastructure in Pecos 

National Historical Park. Slumping does occur along 

river corridors and may be impacting backcountry 

areas and cultural resources.

• Paleontological Resource Inventory, Monitoring, and 

Protection. All paleontological resources are nonrenewable and subject to science-informed inventory, 

monitoring, protection, and interpretation as outlined 

by the 2009 Paleontological Resources Preservation 

Act. A field-based survey could determine the source 

of petrified wood found in Pecos National Historical 

Park and the presence of additional fossils.

• Caves and Associated Landscape Management. The 

Federal Cave Resources Protection Act requires the 

identification of “significant caves” in NPS areas, the 

regulation or restriction of use as needed to protect 

cave resources, and inclusion of significant caves in 

land management planning. Park staff monitors Baca 

Cave but a detailed cave management plan was not 

available at the time of this report.
PECO Geologic Resources Inventory Report ix

• Land Subsidence. Subsidence has not been 

documented in Pecos National Historical Park but has 

the potential to occur. Geologic units that are mapped 

in the park have caused land subsidence in locations

outside the park, where it occurred following the 

dissolution of limestone and gypsum rock.

• Earthquakes Hazards and Risks. Earthquakes have 

historically occurred in the area. However, the

potential for damaging earthquakes is low.

• External Energy and Mineral Development. In the 

past, mining has taken place in and near Pecos 

National Historical Park, but current and future 

development is of limited concern to park resource 

managers. Based on regional seismic and exploration 

activities, the rock formations in the park are not 

believed to have commercially exploitable mineral 

deposits, and the various strata are not associated with 

oil and gas producing beds. 

• Interpretation and Education. Interpreters at the park 

often include geology information in their talks and 

tours. Park staff should continue to work with local 

geologists and could request 

interpretation/education/outreach program assistance 

from the NPS Geologic Resources Division.
x NPS Geologic Resources Division

Products and Acknowledgments

The NPS Geologic Resources Division partners with institutions such as Colorado State 

University, the US Geological Survey, state geological surveys, local museums, and 

universities to develop Geologic Resources Inventory products. This section describes those 

products and acknowledges contributors to this report. 

GRI Products

The objective of the Geologic Resources Inventory is to 

provide geologic map data and pertinent geologic 

information to support resource management and 

science-informed decision making in more than 270 

natural resource parks throughout the National Park 

System. To realize this objective, the GRI team 

undertakes three tasks for each natural resource park: (1) 

conduct a scoping meeting and provide a summary

document, (2) provide digital geologic map data in a 

geographic information system (GIS) format, and (3) 

provide a GRI report (this document). These products 

are designed and written for nongeoscientists. 

Scoping meetings bring together park staff and geologic 

experts to review and assess available geologic maps, 

develop a geologic mapping plan, and discuss geologic 

features, processes, and resource management issues that 

should be addressed in the GRI report. Following the 

scoping meeting, the GRI map team converts the 

geologic maps identified in the mapping plan to digital 

geologic map data in accordance with the GRI data 

model. After the map is completed, the GRI report team 

uses these data, as well as the scoping summary and 

additional research, to prepare the GRI report. 

The compilation and use of natural resource information 

by park managers is called for in the 1998 National Parks 

Omnibus Management Act (section 204), 2006 National 

Park Service Management Policies, and the Natural 

Resources Inventory and Monitoring Guideline (NPS75). The “Additional References” section and Appendix 

B provide links to these and other resource management 

documents and information.

Additional information regarding the GRI, including 

contact information, is available at 

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

17 February 2015). The current status and projected 

completion dates of products are at

http://www.nature.nps.gov/geology/GRI_DB/Scoping/Q

uick_Status.aspx (accessed 17 February 2015). 

Acknowledgments

Additional thanks to Paul Varela (Pecos National 

Historical Park) and Mario Armijo (Pecos National 

Historical Park) for providing photographs, Vincent

Santucci (NPS Geologic Resources Division), Justin 

Tweet (NPS Geologic Resources Division) and Erica 

Clites (University of California Museum of 

Paleontology) for providing paleontological resource 

information, and Katie KellerLynn (Colorado State 

University) for providing the scoping summary. Joel 

Wagner (NPS Water Resources Division) provided 

photographs and information regarding Glorieta Creek 

restoration. Trista Thornberry-Ehlrich (Colorado State 

University) drafted some graphics used in this report.

Author

Rebecca Port (NPS Geologic Resources Division) 

Review

Cheri Dorshak (NPS Pecos National Historical Park) 

Jeremy Moss (NPS Pecos National Historical Park) 

J. Michael Timmons (New Mexico Bureau of Geology 

and Mineral Resources) 

Jason Kenworthy (NPS Geologic Resources Division)

Editing

Katie KellerLynn (Colorado State University) 

Report Formatting and Distribution

Jason Kenworthy (NPS Geologic Resources Division)

Rebecca Port (NPS Geologic Resources Division)

Source Maps

New Mexico Bureau of Geology and Mineral Resources

GRI Digital Geologic Data Production

Georgia Hybels (Colorado State University)

Andrea Croskrey (NPS Geologic Resources Division) 

GRI Geologic Map Poster Design

Rachel Yoder (Colorado State University)

Georgia Hybels (Colorado State University) 

GRI Geologic Map Poster Review

Georgia Hybels (Colorado State University) 

Rebecca Port (NPS Geologic Resources Division)

Jason Kenworthy (NPS Geologic Resources Division)
PECO Geologic Resources Inventory Report 1 

Geologic Setting and Significance 

This section describes the regional geologic setting of Pecos National Historical Park, as well 

as summarizes connections between geologic resources and other park resources and 

stories. 

Geologic Setting

Pecos National Historical Park is located at the junction

of three major, and very different, physiographic 

provinces (fig. 1). Diverse geologic features occur in 

every direction. Most of the park is technically in the 

extreme southern part of the Rocky Mountain province, 

which extends into northern New Mexico with the 

Sangre de Cristo Mountains (fig. 1). They are a 

continuation of the Front Range, a major section of the 

Rocky Mountain system (Anderson 1956). The Great 

Plains province is to the east of the park (fig. 1). The 

portion of the plains adjacent to New Mexico is a 

remnant Tertiary landform occasionally interrupted by 

erosional and volcanic features (Trimble 1980). The 

Basin and Range province is to the south and west (fig. 

1). It is a series of north–south-trending mountains 

separated by wide valleys. Cutting through the Basin and 

Range is the Rio Grande rift (fig. 1), a natural low area 

that native peoples and later settlers utilized (KellerLynn 

2006). Spanning 1,000 km (600 mi)—from Chihuahua in 

northern Mexico through central New Mexico to 

Leadville in central Colorado —the Rio Grande rift is a 

Figure 1. Physiographic province map of New Mexico. Pecos 

National Historical Park is at the junction of three major 

provinces, the Southern Rocky Mountain, Basin and Range 

(includes the Rio Grande Rift), and the Great Plains 

provinces. National Park Service graphic by Rebecca Port 

(NPS Geologic Resources Division).

major Basin-and-Range feature of crustal extension. The 

southern part of rift began to pull apart about 36 million 

year ago (Eocene Epoch; see fig. 4). In the north, rifting 

began in about 22 million years ago (Miocene Epoch) 

(Price 2010).

The Pecos Unit and Pigeon’s Ranch Subunit are both 

completely within the Rocky Mountain province. The 

Pecos Unit is bisected by the Pecos River (fig. 2). On the 

east side of the river, the Sangre de Cristo Mountains

terminate in foothills. The west side of the river is 

broader and flatter. The Pigeon’s Ranch Subunit is also 

in the foothills. It is bisected by ephemeral Glorieta 

Creek (fig. 2). The Cañoncito Subunit covers the very 

southern edge of the foothills and extends south over a 

small portion of Glorieta Mesa, which is in the Basin and 

Range province. The Cañoncito Subunit is bisected by 

ephemeral Galisteo Creek (fig. 2).

The general stratigraphy in the park is Paleozoic Era

strata hundreds of millions of years old overlain by much 

younger alluvium (stream deposited sediment). The 

Paleozoic rocks of the Pecos River valley floor and 

Sangre de Cristo foothills consist of 320 million–270 

million-year-old soft mudstones, sandstones, siltstones, 

limestones, and conglomerates of the Alamitos (PNal) and 

Sangre de Cristo formations (PPNsc) of the Pennsylvanian 

and early Permian (see figs. 3 and 4; and poster, in 

pocket). In many places the Sangre de Cristo Formation 

is buried up to 8 m (25 ft) beneath alluvium (Qa, Qc, Qvf, 

Qt, QTg; fig. 3). Most of the valley is covered by this 

Quaternary alluvial fill (see poster, in pocket), and a 

mantle of thick soil derived from weathering and 

decomposition. The pueblo, mission ruins, and visitor

center are located on alluvial fill dating from when the 

site was occupied by the Pecos River whose channel has

now migrated to the east. The underlying Pennsylvanian 

and Permian strata of the Alamitos and Sangre de Cristo 

formations crop out along the rivers, as do ancient

igneous and metamorphic rocks, billions of years old

(Precambrian time and/or Proterozoic Era; fig. 4). 

To the southwest of the Pecos Unit and across Interstate 

25 is the broad, elevated Glorieta Mesa. The mesa 

showcases 300 million–200 million-year-old sedimentary 

rocks (Permian to Triassic periods; figs. 3 and 4, and 

poster, in pocket). The Cañoncito Subunit is the only 

park unit whose boundary extends into part of the mesa.

Geologic Significance and Connections to Park Stories

Pecos Pueblo was built on a broad lowland area between 

the southernmost point of the Sangre de Cristo 

Mountains and the northern edge of the Glorieta Mesa. 

That this site was used by so many people and selected as 
2 NPS Geologic Resources Division

Figure 2. Pecos National Historical Park map. Pecos National Historical Park consists of two units, the Pecos Unit and the 

Glorieta Unit, which includes the Cañoncito and Pigeon’s Ranch subunits. The Pecos Unit is bisected by the Pecos River, the 

Pigeon’s Ranch Unit is bisected by Glorieta Creek, and The Cañoncito Unit is bisected by Galisteo Creek. National Park 

Service map, available at http://www.nps.gov/peco/index.htm (accessed 5 August 2014).

the location for the pueblo is due in large part to its 

physical setting, which resulted largely from geologic 

processes. The low area surrounding the pueblo served 

as a natural passage, or “gateway,” between the Rio 

Grande valley and the high plains. In addition, this 

location receives solar heating from its southern 

exposure, is out of the way of cold air which commonly 

exits Pecos Canyon, has access to water, and has 

unobstructed views of the surrounding landscape. All of 

these qualities made the area ideal for settlement. Today, 

Interstate 25 traverses the pass within view of the historic

pueblo and church. Together, these features “span 

centuries of southwest history as a testament to the role 

of geology and geographical location in human 

experience” (Bezy 1988, p. 18).

The Battle of Glorieta Pass, fought on 26–28 March 1862, 

has ties to the geologic landscape. It was a decisive battle 

of the Civil War, ultimately ending the war in the west. 

The purpose of the Glorieta Unit is to preserve and 

interpret this battle. In the valley of the Pigeon’s Ranch 

Subunit battle lines were established, artillery was set up 

and skirmishing took place. On the north side of the 

valley, “Sharpshooter’s Ridge” was utilized as a lookout 

(National Park Service 1993). During a skirmish, Union 

forces used the landscape to their advantage by 

traversing Glorieta Mesa and descending undetected 

into the Cañoncito Subunit, where they destroyed the 

Confederate supply camp (National Park Service 1993).

Geologic features and processes in the park are also 

linked to natural resources. For example, the sliding of 

sandstone slabs atop weathered mudstone of the Sangre 

de Cristo Formation creates crevices with a microclimate 

ideal for the growth of Douglas-fir (Pseudotsuga 

menziesii); fig. 5) (Mario Armijo, staff, Pecos National 

Historical Park, written communication, 24 September 

2014). Also, owls have been documented near the cool, 

quiet, and protected cavities formed in the limestone 

rock of the Alamitos Formation (see “Caves and Karst 

Resources” section) (Mario Armijo, written 

communication, 24 September 2014). 
PECO Geologic Resources Inventory Report 3 

Era Period Epoch MYA* Rock/Sediment Unit Description Cenozoic

Quaternary

Holocene 0.01–

present Alluvium (Qa) Unconsolidated clay, silt, sand, and gravel filling valleys. 

Pleistocene 2.6–0.01

Colluvium, valley fill, and 

terrace deposits (Qc, Qvf, 

and Qt) 

Talus, fine grained deposits in low-relief 

areas, and gravel-capped deposits 

associated with modern streams. 

Gravel (QTg) 

Rounded Paleozoic and Proterozoic 

clasts deposited prior to the 

establishment of the modern Pecos 

River and Glorieta Creek channels. 

“Tertiary” 

Neogene Pliocene

66.0–2.6

Units of this age interval are not mapped within Pecos National Historical 

Park. 

Miocene

Paleogene

Oligocene

Eocene

Paleocene

Mesozoic

Cretaceous Upper 100–66.0

Lower 145–100

Jurassic

Upper 164–145

Middle 174–164

Lower 201–174

Triassic 

Upper 237–201 

Chinle Group (TRc)**

Light gray to grayish orange or tan, 

conglomeratic sandstone, and reddish

brown mudstone. Mapped in the 

Glorieta Unit (Cañoncito). 

Santa Rosa Formation (TRs, 

TRsu, TRsm, TRsl) 

Sandstone, mudstone, and siltstone 

with limestone and chert conglomerate. 

Some crossbedding present. Fossil 

wood is common in the lower member. 

Mapped in the Glorieta Unit 

(Cañoncito)

Middle 247–237

Moenkopi Formation (TRm) 

Grayish red to purplish red sandstones, 

conglomerates, mudstones, and 

Lower 252–247 siltstones. 

Paleozoic 

Permian

Lopingian 

(Upper) 260–252

Artesia Formation (Pa) Reddish brown sandstones, siltstones, 

and gypsum. 

Guadalupian 

(Middle) 272–260 San Andres Formation (Psa) Light gray, thin-bedded, silty limestone 

and calcareous sandstone. 

Glorieta Sandstone (Pg) Yellow to buff, thick-bedded quartz 

sandstones. 

Cisuralian 

(Lower) 299–272 Yeso Formation (Py) 

Red thick-bedded mudstone; finegrained sandstone; and buff, coarsegrained arkosic sandstone. 

Sangre de Cristo Formation 

(PPNsc) 

Brownish red to purple mudstone and 

buff to dark brown, arkosic 

conglomeratic sandstone. 

Pennsylvanian Upper 307–299

Alamitos Formation (PNal) 

Gray, fossiliferous, marine limestone;

coarse-grained arkosic sandstone; and 

dark gray to reddish shales. 

*Millions of years ago.

**Nomenclature of the “Chinle Group” has changed over time. This report follows the “Chinle Group” nomenclature as per the GRI source 

maps published in 1997 and 2002 (see “Geologic Map Data” section). More recently, the US Geological Survey and some state surveys prefer 

the “Chinle Formation” nomenclature. Refer to the US Geological Survey GEOLEX for additional information 

(http://ngmdb.usgs.gov/Geolex/Units/Chinle_4997.html, accessed 9 April 2015).

Figure 3. General stratigraphic column for Pecos National Historical Park. All of the units in the column above are mapped 

within the park’s boundaries. Colors are standard colors approved by the US Geological Survey to indicate different time 

periods on geologic maps; they also correspond to the colors on the Map Unit Properties Table (in pocket). Age is in millions 

of years before present and indicates the time spanned by associated epoch or period. Rock/sediment units associated with 

those epochs or periods may not encompass the entire age range.
4 NPS Geologic Resources Division

Figure 4. Geologic time scale. The divisions of the geologic time scale are organized stratigraphically, with the oldest 

divisions at the bottom and the youngest at the top. GRI map abbreviations for each time division are in parentheses. 

Compass directions in parentheses indicate the regional locations of events. Boundary ages are millions of years ago (MYA). 

National Park Service graphic using dates from the International Commission on Stratigraphy 

(http://www.stratigraphy.org/index.php/ics-chart-timescale; accessed 9 April 2015).
PECO Geologic Resources Inventory Report 5 

Figure 5. Douglas-fir microclimate. Sandstone slabs that have slid atop weathered mudstone of the Sangre de Cristo 

Formation (PPNsc) create cool, shaded crevices ideal for the growth of Douglas-fir. National Park Service photograph by

Mario Armijo (Pecos National Historical Park).
PECO Geologic Resources Inventory Report 7 

Geologic Features and Processes 

This section describes noteworthy geologic features and processes in Pecos National 

Historical Park. 

The geologic landscape is the stage upon which episodes 

of natural and cultural history take place. It is important 

to recognize and interpret the geologic features and 

processes in Pecos National Historical Park because of 

their intrinsic value and the potential they have to impact 

other park resources and visitors. 

During the 2006 scoping meeting (see KellerLynn 2006) 

and 2013 conference call, participants (see Appendix A) 

identified the following geologic features and processes:

• Geologic Foundation

• Sangre de Cristo Mountains

• Glorieta Pass

• Glorieta Mesa

• Fluvial Features

• Folds

• Faults

• Slope Movements

• Paleontological Resources

• Caves and Karst Resources

• Earthquakes

• Geologic Resources in a Cultural Context

Geologic Foundation

The GIS data shows four major rock/deposit types: (1) 

Quaternary deposits (surficial units, including artificial 

fill), (2) Tertiary deposits (gravels and the Tesuque 

Formation), (3) Mississippian through Triassic 

sedimentary rocks, and (4) Precambrian igneous and 

metamorphic rocks. Three of the four groups are 

composed of sedimentary rocks and deposits.

Sedimentary Rocks and Deposits

The three main types of sedimentary rocks are clastic, 

chemical, and organic. Clastic sedimentary rocks are the 

products of weathering, erosion, transportation, and 

deposition of rock fragments called “clasts.” The clastic 

rocks and deposits in the GIS data were originally 

deposited by water in streams, seas, or lakes (Johnson 

1969) or by hillslope processes.

Clastic sedimentary rocks are named after the size of 

clasts (table 1). Higher-energy depositional 

environments, such as fast-moving streams, deposit 

larger (heavier) clasts while transporting smaller (lighter) 

clasts. Where water moves slowly or is stagnant, such as 

in lakes, the water cannot transport even the smallest 

clasts and they are deposited. Wind also transports and 

deposits sand-sized or smaller clasts. Many clastic 

sedimentary rocks appear red, which is due to oxidation 

(rusting) of iron-bearing minerals in the sediments that 

make up the rock.

Clast size, types of fossils and minerals, and depositional 

structures within the sedimentary layers indicate the 

original depositional environment. Geologists can then

determine the history of the land. The geologic history of 

the Pecos National Historical Park area is described in 

the “Geologic History” section of this report. 

Chemical sedimentary rocks form when ions 

(microscopic particles of rock dissolved during chemical 

weathering) precipitate out of water. For example, 

carbonate rocks, such as limestone (calcium) or dolomite 

(calcium and magnesium) have a carbonate (CO3

2-) ion.

Table 1. Clastic sedimentary rock classification and characteristics.

Rock Name Clast Size Depositional 

Environment

Example from GRI GIS data

(unit symbol)

Conglomerate

(rounded clasts, “gravel”)

or Breccia (angular clasts)

>2 mm

(0.08 in) [larger] Higher Energy Gravel deposits (Qtg)

Sandstone 1/16–2 mm

(0.0025–0.08 in) Sangre de Cristo Formation (PPNsc)

Siltstone 1/256–1/16 mm 

(0.00015–0.0025 in) Alluvium (Qa) and Valley Fill (Qvf) 

contain silt and clay, as well as other 

clast sizes. Claystone <1/256 mm

(0.00015 in) [smaller] Lower Energy

Note: Claystones and siltstones can also be called “mudstone,” or if they break into thin layers, “shale.”
8 NPS Geologic Resources Division

Organic sedimentary rocks are composed of organic 

remains (e.g., coal) or were produced by the 

physiological activities of an organism (e.g., 

accumulation of calcium carbonate shells of marine 

organisms to form limestone).

Quaternary Deposits

Quaternary deposits are the youngest units in the GRI 

GIS data set. They blanket the surface in a relatively thin 

layer of sediments that have accumulated mainly in 

valleys and on terraces. This category also includes 

artificial fill (Qaf), though no artificial fill is mapped in 

the park.

Quaternary alluvium, terrace deposits, and valley fill (Qa, 

Qt, and Qvf, respectively) line active perennial and 

ephemeral stream channels of the Pecos River, Glorieta 

Creek, and Galisteo Creek. These deposits were made by 

streams on river beds, flood plains, and on alluvial fans. 

They consist of unconsolidated clay, silt, sand, and 

gravel. Terrace deposits are typically capped by gravel. 

Tertiary Deposits

Tertiary deposits in the GIS data include three units: the 

Galisteo Formation (Tg), gravel (Tgv) and the Tesuque 

Formation (Tt). These units are not mapped within the 

park.

The Galisteo Formation (Tg) tops hills north of Glorieta 

Pass and caps the Glorieta Mesa south of the park. It is a 

cross bedded sandstone interbedded with siltstone and 

mudstone layers. 

The Tertiary gravel deposits (Tgv) form highs in the 

landscape, up to 460 m (1,500 ft) above the modern 

Pecos River (Read and Rawling 2002). The gravels top a 

ridge north of the Pigeon’s Ranch Subunit and cap parts 

of the Glorieta Mesa, including Rowe Peak. Their 

colluvium mantles hillsides. The gravels consist of 

sedimentary, igneous, and metamorphic clasts derived 

from the variety of much older Precambrian and 

Paleozoic rocks exposed throughout the area.

The Tesuque Formation (Tt) is mapped in a small area 

southwest of the Cañoncito Subunit. It is arkosic 

sandstone, meaning that it is rich in feldspar (at least 25% 

of the rock). Arkosic sandstone forms from the 

weathering of feldspar-rich rocks such as granite. The 

weathered clasts must have been deposited rapidly under 

dry or cold conditions in order for the feldspar not to 

chemically weather into other minerals.

Mississippian through Triassic Sedimentary Rocks

The GRI GIS data set contains several types of 

Mississippian through Triassic sedimentary rocks 

representing a variety of depositional environments. The 

majority of the units that are mapped within Pecos 

National Historical Park belong to this group.

Mississippian sedimentary rocks are not mapped within 

the park. The Mississippian Espiritu Santo (Mes) and 

Tererro (Mt) formations are restricted to a small area in 

the southwest corner of the map area (see poster, in 

pocket). These units are limestones and dolomites, which 

Figure 6. Paleozoic arkosic sandstone. Arkosic sandstone of 

the Alamitos (PNal) and Sangre de Cristo (PPNsc) formations 

is exposed in Pecos National Historical Park. These rocks 

contain abundant feldspar and probably were derived from 

the weathering of Precambrian granite nearby. Note the red 

color indicating oxidation (rusting) of iron in the rock. 

National Park Service photograph by Mario Armijo (Pecos 

National Historical Park).

were deposited in deep areas of prehistoric inland seas 

that once occupied the area.

Two Pennsylvanian units are mapped within the park, 

the Alamitos (PNal) (inside cover) and Sangre de Cristo 

(PPNsc) formations. These units comprise the foothills of 

the Sangre de Cristo Mountains inside the park (see

“Sangre de Cristo Mountains” section). These 

formations are made up of interbedded layers of 

limestone, mudstone, shale, and arkosic sandstone (fig. 

6). 

The three other Pennsylvanian units in the GRI GIS data 

are not mapped within the park. The La Posada 

Formation (PNlp) is limestone with banded chert along 

both sides of the Pecos River just north of the Lisboa 

Springs fault to the north of the Pecos Unit. The Sandia 

Formation (PNs) is arkosic sandstone with shale in the 

southwest portion of the map near the Mississippian 

units. It was deposited at the same time as the lower part 

of the La Posada Formation. Finally, the Madera 

Formation (PNm) is limestone mapped to the northwest 

of the park and was deposited at the same time as the 

Alamitos and upper part of the La Posada formations.

Permian through Triassic units comprise the layers of 

Glorieta Mesa. Refer to the “Glorieta Mesa” section of 

this chapter for more information about this landform

and the geologic units.

Precambrian Igneous and Metamorphic Rocks

Precambrian igneous and metamorphic rocks are 

mapped mainly at higher elevations north and northwest 

of the park where they make up the core of the Sangre de 

Cristo Mountains (see poster, in pocket). The core of the 

mountains consists of igneous rocks such as granites, 

quartz monzonites, granodiorites, and metamorphic 
PECO Geologic Resources Inventory Report 9 

Table 2. Simplified classification of igneous rocks.

Volcanic rock 

(extrusive, cooled on 

Earth’s surface) 

Relative % 

silica (Si/O) 

Relative % 

mafic minerals 

(Fe/Mg)

Plutonic equivalent

(intrusive, cooled 

beneath Earth’s surface) 

Example from GRI 

GIS data

(unit symbol)

Rhyolite Higher Lower Granite Granite to Granitic Gneiss 

(Xgg)

Dacite Granodiorite Granodiorite (Xgr)

Andesite Diorite Diorite (Xd)

Basalt Lower Higher Gabbro Not present on map

“Higher” and “Lower” indicate relative percentages silica (Si/O) or mafic minerals containing iron (Fe) and magnesium (Mg).

rocks (Wilks 2005) which are approximately 1.7 billion to 

1.4 billion years old (J. Michael Timmons, New Mexico 

Bureau of Geology and Mineral Resources, associate 

director for mapping programs, written communication, 

24 November 2014). These core rocks are either granitic 

intrusions or the result of metamorphism of pre-existing 

sedimentary and igneous rocks (Clark 1966).

Within the park, Precambrian rocks are mapped only in 

the Cañoncito Subunit. Granite to granitic gneiss (Xgg) is 

the most extensively mapped of all the igneous and 

metamorphic rocks in the GRI GIS data (see poster, in 

pocket). Granite is an intrusive (“plutonic”) igneous 

rock, which means that it formed from the cooling of 

molten material beneath Earth’s surface. Volcanic rocks 

are formed when molten material cools at Earth’s 

surface. Igneous rocks are classified by texture (grain 

size, shape, orientation), as well as the percentage of

major minerals (quartz, alkali feldspar, and plagioclase) 

present in the rock (table 2). The granite in Xgg is 

medium to coarse grained and consists predominantly of 

the silicate minerals plagioclase, potassium feldspar, and 

quartz.

The gneiss in Xgg is metamorphosed granite. Rocks can 

be metamorphosed—altered—by high temperature 

and/or pressure to form metamorphic rocks. 

Metamorphism occurs mainly in two settings: contact or 

regional. Contact metamorphism is associated with the 

intrusion of molten material where rocks adjacent to the 

intrusion are “baked” by the high temperatures. Regional 

metamorphism is associated with large-scale tectonic 

events, such as mountain building orogenies. Gneiss 

forms due to regional metamorphism. The two major 

subdivisions of metamorphic rocks are foliated (minerals 

are aligned in “stripes”) and non-foliated (minerals are 

oriented randomly). The gneiss in Xgg is foliated.

Sangre de Cristo Mountains

The Sangre de Cristo Mountains are the portion of the 

Rocky Mountains that terminates in Pecos National 

Historical Park. The foothills of the Sangre de Cristo 

Mountains cover the eastern half of the Pecos Unit, from 

the Pecos River east. The mountains tell the story of one 

of the last major geologic events to occur near the park—

the Laramide Orogeny—which raised the Rocky 

Mountains (see “Geologic History” section).

The Sangre de Cristo Mountains are a north–southtrending range that extends from south-central Colorado 

to northern New Mexico. In New Mexico, the range is

subdivided into the Taos Range on the west and the 

Cimarron Range on the east. The Taos and Cimarron 

ranges are separated by the high—approximately 2,510 m 

(8,240 ft) in elevation—and relatively flat Moreno Valley 

(fig. 7; Clark 1966). The park is at the southern end of the 

Taos Range.

The Cimarron Range is not as long as the Taos Range, 

extending less than 80 km (50 mi) south from the New 

Mexico-Colorado border. South of the Cimarron Range 

is the Ocate Plateau, a 23 million–3 million-year-old lava 

flow of the Miocene and Pliocene epochs (fig. 4; Wilks 

2005).

The Taos Range continues beyond the length of the 

Cimarron Range and splits into two ridges separated by 

the Pecos River valley. The main ridge swings westward 

and southward to form the Santa Fe Range. The lesser, 

eastern leg of the split continues southward and is 

known as the East Range (other common names include 

the Las Vegas Range and Elk Mountain-Mora Range). 

Both ranges terminate in foothills just north of Glorieta 

Mesa and Interstate 25. The foothills of the East Range 

extend into the eastern portion of the Pecos Unit of the 

park and the foothills of the Santa Fe Range cross into 

the Pigeon’s Ranch Subunit (fig. 7). 

The foothills are mapped as the Pennsylvanian and 

Permian sedimentary Alamitos (Pnal) and Sangre de 

Cristo (PPNsc) formations (see poster, in pocket). They

border the much higher igneous and metamorphic core 

of the mountain range. The layers of sedimentary rocks 

in the foothills dip away, to the south, from the higher 

mountains. Their slope is steepest north of the park, 

close to the core of the high mountains (Johnson 1969), 

and decreases to nearly horizontal as they approach the 

base of Glorieta Mesa.
10 NPS Geologic Resources Division

Figure 7. Large scale physiographic map. Pecos National Historical Park is along Glorieta Pass, a naturally formed high 

mountain pass (white dashed line). The Pecos Unit of the park is in the Pecos River Valley. To the south of the park is the 

broad, elevated Glorieta Mesa and to the north of the park are the Sangre de Cristo Mountains. Note the low areas on 

either side of the pass—the Rio Grande valley and the high plains. For thousands of years humans traveling between these 

areas have passed through and inhabited the area now preserved by the park. National Park Service graphic by Rebecca 

Port (NPS Geologic Resources Division), using imagery by Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX, 

Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community.
PECO Geologic Resources Inventory Report 11

Glorieta Pass

Glorieta Pass is a natural, 50 km (30 mi) long, high 

mountain pass which parallels Interstate 25 from the 

Cañoncito Subunit to Starvation Peak (fig. 7). The pass 

separates the Sangre de Cristo Mountains from Glorieta 

Mesa. The elevation of the pass is at least 1,800 m (6,000 

ft) in all places and reaches its highest point in Glorieta at 

2,265 m (7,432 ft) above sea level (Johnson 1969). This 

high point marks the drainage divide between Galisteo 

Creek, which flows westward to join the Rio Grande at 

Santo Domingo Pueblo, and Glorieta Creek, which flows 

eastward to the Pecos River a few kilometers south of the 

town of Pecos (Bauer et al. 1995).

The Pecos River is responsible for carving most of 

Glorieta Pass by eroding relatively soft sedimentary rock 

between the more resistant rocks of the Sangre de Cristo 

Mountains to the north and Glorieta Mesa to the south 

(Johnson 1969; Bezy 1988). 

The pass is the lowest passable route across the 

mountains and mesa and was critical to the cultural 

events that occurred in and near Pecos National 

Historical Park. It was a “gateway” for a great variety of 

people travelling between the Great Plains in the east and 

the Rio Grande valley in the west. Additionally, it was the 

site of the Civil War Battle of Glorieta Pass (see

“Geologic Resources in a Cultural Context” section). 

Glorieta Mesa

Glorieta Mesa is an elevated, broad, flat-topped, area of 

land south of Pecos National Historical Park (fig. 7). It is 

separated from Glorieta Pass by a steep escarpment (see 

“Glorieta Pass” section). Though only mapped within the 

Cañoncito Subunit, Glorieta Mesa is a dominant feature 

of the Pecos Unit’s viewshed. It showcases layer upon 

layer of sedimentary rock formations (fig. 8). The steep, 

high escarpment of the mesa marks the boundary 

between the Rocky Mountain and the Basin and Range

provinces. 

The rocks of Glorieta Mesa (PPNsc, Py, Pg, Psa, Pa, TRm, 

TRsl, TRsm, Trsu, TRs) were originally deposited in 

horizontal layers between 300 million and 200 million 

years ago (Permian and Triassic periods; fig. 4). During 

the Laramide Orogeny (mountain-building event), the 

layers were tilted slightly—almost undiscernibly—to the 

south (Johnson 1969). 

The uppermost (youngest) rock layer on the mesa is 

yellow sandstone—the Santa Rosa Formation (TRs, Trsu, 

TRsm, TRsl; see poster, in pocket). It caps the very top of 

the mesa including Cerro de Escobas, the highest point at 

2,503 m (8,212 ft) above sea level (Johnson 1969). 

Although it is as much as 140 m (450 ft) thick elsewhere 

in New Mexico, the Santa Rosa Formation was eroded to 

about 15 m (50 ft) on top of the mesa (Johnson 1969). 

Beneath the Santa Rosa Formation is the grayish red 

Moenkopi Formation (TRm, sandstone), which is 

underlain by the orange Artesia Formation (Pa, siltstone). 

The contact between the units is a disconformity—a 

Figure 8. Glorieta Mesa. The top photograph shows the rock 

sequence on Glorieta Mesa. This photograph was taken 

looking southwest from Pecos National Historical Park 

toward the second highest point on the mesa, Rowe Peak. 

The bottom photograph shows the view of the mesa taken 

from Old Denver Highway looking toward the highest point 

on the mesa, Cerro de Escobas. National Park Service 

photographs by Mario Armijo (Pecos National Historical 

Park).

break in deposition where the bedding planes above and 

below the contact are essentially parallel—which makes 

the units difficult to distinguish. Additionally, both of 

these formations are mostly covered by vegetation and 

talus. Thus, the map displays the units as one (TRm+Pa) in 

some locations.

The Glorieta Sandstone (Pg) is extremely hard and 

resistant to erosion and makes up the yellow sandstone 

bench that covers the main escarpment of the mesa. The 

Glorieta Sandstone is easily visible due to its distinctive 

light-yellow to grayish white color (fig. 8). It takes its 

name from the mesa due to its prominence.

The Yeso Formation (Py) is a reddish brown sandstone 

and siltstone beneath the Glorieta Sandstone. 

The lowermost (oldest) unit of the mesa is the grayish 

red and gray sandstone of the Sangre de Cristo 

Formation (PPNsc) (fig.8). The formation is much thicker 

than it appears at Glorieta Mesa. Only the top fifth of this 

unit is visible; the remaining 4/5ths of the unit extends 

beneath the mesa and valley floor. The Sangre de Cristo

formation is mapped in all units of the park.
12 NPS Geologic Resources Division

Fluvial Features

Pecos National Historical Park is in the upper Pecos 

River watershed. Three streams flow through the units of 

the park. The perennial Pecos River flows south through 

the Pecos Unit. Glorieta Creek flows east through the 

Pigeon’s Ranch Subunit and into the Pecos Unit of the 

park where it joins the Pecos River. Galisteo Creek flows 

west through the Cañoncito Subunit (fig. 2). Glorieta 

Pass is the drainage divide between Glorieta and Galisteo 

creeks. 

Pecos River

The drainage basin for the upper Pecos River is between 

the Santa Fe Range and East Range of the southernmost 

Sangre de Cristo Mountains (fig. 7; Anderson 1956). The 

Pecos River flows south from the mountains and through 

Pecos National Historical Park. After exiting the park, 

the river follows the Glorieta Mesa escarpment for 

approximately 24 km (15 mi) before breaching the mesa 

at Glorieta Pass near San Jose (Johnson 1969).

The topography of the area today is largely a result of the 

Pecos River downcutting through uplifted land in the last

Figure 9. Terraces and alluvial gravel deposits. Terraces and 

gravel deposits mark the former path of the Pecos River. In 

the upper photograph, the Pecos River is not visible because 

it is deeply incised, but a terrace marking its former 

elevation can be seen on the gently sloping area between 

the two steep cliffs. In the lower photograph, rounded 

gravel and cobbles next to the river are evidence of the 

location of the former river bed. National Park Service 

photographs by Paul Varela (Pecos National Historical Park).

Figure 10. Limestone cliffs confine the Pecos River. The 

Pecos River has downcut through limestone of the Alamitos 

Formation (PNal) in some locations, exposing the rock as 

cliffs. The erosion resistant limestone confines the river to a 

narrow channel and prevents meandering. National Park 

Service photograph by Mario Armijo (Pecos National 

Historical Park).

5 million years, since the Miocene Epoch (fig. 4). Deep 

alluvial gravel deposits and a series of terraces mark the 

former course of the river (fig. 9; Bezy 1988; Perkins et al. 

2005; Johnson et al. 2011). Lower, younger terraces were 

often sites for villages or agricultural fields (Bezy 1988). 

The river migrated eastward to its present location.

Stream erosion along the Pecos River as well as Glorieta 

Creek is ongoing, exposing bedrock in some locations 

(fig. 10; Bezy 1988; see “Fluvial Erosion” section). 

Folds

Folds are curves or bends in originally flat structures, 

such as rock strata, bedding planes, or foliation. The two 

primary types of folds are anticlines which are “Ashaped” or convex and synclines which are “U-shaped” 

or concave (fig. 11). Both types of folds can be 

overturned—tilted past vertical—by continued or future 

tectonic forces. Folds frequently “plunge” meaning the 

fold axis tilts. As bedrock is compressed, anticlines and 

synclines form adjacent to each other. A monocline is a 

step-like fold where dip steepens markedly over an 

otherwise gently dipping area. 

The southeastern part of the Sangre de Cristo Mountains 

is characterized by large asymmetrical folds with the 

steepness of the fold increasing to the north, suggesting 

that the forces deforming the rocks increased to the 

north (Baltz and Bachman 1956). The majority of the 

folds developed during the Laramide Orogeny between 

about 70 million and 40 million years ago (see “Geologic 

History” section).

Seven named folds—two anticlines, two synclines, and 

three monoclines—are included in the GRI GIS data 

along with several unnamed smaller folds. They are all in 

the eastern half of the GIS data (see poster, in pocket). 

Anticlines and synclines alternate across the landscape 

with the occasional monocline between them. From 

north to south the named folds are the La Cueva 
PECO Geologic Resources Inventory Report 13

Figure 11. Block diagrams of two types of folds. The 

southeastern part of the Sangre de Cristo Mountains is 

characterized by large anticline (A) and syncline (B) folds. As 

bedrock is compressed, anticlines and synclines form 

adjacent to each other (C). Folds in the vicinity of Pecos 

National Historical Park frequently “plunge” (D, meaning 

the fold axis tilts) to the south or southwest.

Figure 12. Map of folds in Pecos National Historical Park. 

Axes of the anticlines tend to be aligned with ridges 

whereas axes of synclines are aligned with valleys. This is 

especially apparent on the axis of the Pecos syncline where 

the contact between the Alamitos (PNal) and Sangre de 

Cristo (PPNsc) formations (black dashed line) in the valley is 

arched opposite the direction of plunge. For a description of 

units labeled on the map see the Map Unit Properties Table. 

National Park Service graphic by Rebecca Port (NPS Geologic 

Resources Division) using GRI GIS data. The base map was 

compiled from a variety of best available sources from 

several data providers, including the U.S. Geological Survey, 

National Park Service, DeLorme, NOAA, and ESRI. 

anticline, Monastery monocline, Glorieta Mesa syncline, 

Pecos syncline, Los Piñones monocline, Pecos Pueblo 

anticline, and finally in the far southeast corner of the 

Pecos Unit, the Manzanita monocline. Several of these 

folds cross into the Pecos Unit, the only park unit 

containing mapped folds (fig. 12). The axis of the Pecos 

syncline crosses Glorieta Creek inside the park and goes 

directly through the original park boundary which 

includes the ruins.

The axes of the folds are generally oriented to the north 

or northwest, closely lining up with the ridges and valleys 

of the Sangre de Cristo foothills. Where the folds plunge 

(tilt away from horizontal), they do so to the south or 

southwest. Both the Alamitos (PNal) and Sangre de Cristo 

(PPNsc) formations are folded. The folds have some 

structural control over the location of valleys and ridges 

in the foothills. Axes of the anticlines tend to be aligned 

with ridges whereas axes of synclines are aligned with 

valleys. This is especially apparent on the axis of the 

Pecos syncline where the contact between the Alamitos 

(PNal) and Sangre de Cristo (PPNsc) formations in the 

valley is arched opposite the direction of plunge (fig. 12). 

Faults

A fault is a fracture in rock along which movement has 

taken place. Faults are classified based on relative motion 

of rocks on either side of the fault plane. The three 

primary types of faults are normal, reverse, and strikeslip (lateral) (fig. 13).

All three types are mapped in the vicinity of Pecos 

National Historical Park, including four named fault 

zones (Agua Sarca, Borrego, Glorieta Mesa, and PicurisPecos), three named faults (Apache Canyon, Escobas, 

and Lisboa Springs), and nearly 400 unnamed fault 

features. A fault zone is a fault that is expressed as 

numerous small fractures and may be hundreds of 

meters wide. The Glorieta Mesa and Picuris-Pecos fault 

zones cut through the Cañoncito Subunit and intersect 

with the Borrego fault zone northwest of the subunit

(Bauer et al. 1995). Faults are not mapped in the other 

two park units. As many as six or seven episodes of fault 

activation (and reactivation), ranging in age from 

Proterozoic (more than 541 million years ago) to 

Neogene (less than 23 million years ago), have been 

suggested (Bauer et al. 1995). 

Agua Sarca Fault Zone 

The Agua Sarca fault zone is a north–south-trending fault 

zone in the northwest corner of the extent of the GRI 

GIS data. It runs through granite and granitic gneisses 

(Xgg). Fault type and amount of displacement is not 

known.

Borrego Fault Zone

The Borrego fault zone parallels the Agua Sarca fault 

zone to the east and extends farther south crossing 

Interstate 25. The fault zone is primarily in granite and 

granitic gneiss (Xgg) although it bounds blocks of the 

Sangre de Cristo Formation (PPNsc) in the south section 

and blocks of the Madera Formation (PNm) in the north 
14 NPS Geologic Resources Division

Figure 13. Schematic illustrations of fault types. Movement 

occurs along a fault plane. Footwalls are below the fault 

plane and hanging walls are above. In a normal fault, 

crustal extension (pulling apart) moves the hanging wall 

down relative to the footwall. In a reverse fault, crustal 

compression moves the hanging wall up relative to the 

footwall. A thrust fault is similar to a reverse fault, but has 

a dip angle of less than 45°. In a strike-slip fault, the relative 

direction of movement of the opposing plate is lateral. 

When movement across the fault is to the right, it is a rightlateral (dextral) fault, as illustrated above. When movement 

is to the left, it is a left-lateral (sinistral) fault. A strike-slip 

fault between two plate boundaries is called a transform 

fault. Graphic by Trista Thornberry-Ehrlich (Colorado State 

University).

section. Fault type and amount of displacement is not 

known.

Glorieta Mesa Fault Zone

The Glorieta Mesa fault zone is a north–south-trending

zone where the fault type and amount of displacement is 

not known. It runs through the east side of the 

Cañoncito Subunit along the border of Glorieta Mesa

where the fault’s west side has moved down relative to its 

east side. The faults cut through all the layers of Glorieta 

Mesa (see “Glorieta Mesa” section).

The Picuris-Pecos Fault Zone

The Picuris-Pecos fault zone encompasses many north 

striking, high-angle faults that cutt rocks ranging in age 

from Proterozoic to Paleogene (Cather et al. 2011). The 

Picuris-Pecos fault is the main fault. The zone of faults

has been traced for more than 84 km (52 mi), from the 

northern Picuris Mountains south of Taos to 24 km (15 

mi) south of the Cañoncito Subunit. It may extend the 

entire length of the state if probable linkages to other 

fault zones are included (Cather et al. 2011).

The Picuris-Pecos fault zone consists of faults that 

exhibit both lateral (strik-slip motion) and vertical 

(normal motion) displacement. The Picuris-Pecos strikeslip fault exhibits 37 km (23 mi) of net right lateral 

(dextral) movement (Bauer and Ralser 1995; Cather et al. 

2011). That is the largest displacement of any fault in the 

central and Southern Rocky Mountains (Cather et al. 

2011) and is arguably the largest-displacement, strike-slip 

fault exposed in the state (Bauer and Ralser 1995).

Vertical displacement is shown in the GRI GIS data 

north of the Cañoncito Subunit where granite and 

granitic gneiss (Xgg) has been displaced upward on the

west side of the fault zone relative to the Madera 

Formation (PNm) to the east.

The timing of motion along the fault is unclear with 

considerable geologic evidence indicating the fault was 

repeatedly reactivated (Miller et al. 1963) over hundreds 

of millions of years from the Proterozoic Eon through 

the Neogene Period (fig. 4). Some movement most likely 

occurred during the time between the emplacement of 

the youngest granite about 1.4 billion years ago and the 

Pennsylvanian Period about 323 million years ago (Bauer 

and Ralser 1995; Chapin and Cather 1981). However, 

most of the movement occurred during the early 

Ancestral Rocky Mountain orogeny (beginning about 

320 million years ago) and the Laramide Orogeny 

(between about 80 million and 40 million years ago) 

(Chapin and Cather 1981; Bauer and Ralser 1995; Kelley 

1995; Erslev et al. 2004; Fankhauser 2005; Cather et al. 

2008). Based on the age difference of rocks on opposing 

sides of the Picuris-Pecos fault it appears the fault was

also active in the Cenozoic Era (66 million years ago to 

present). Later (23 million years ago to present), rift 

related faulting was concentrated near the Picuris-Pecos 

fault zone (Bauer et al. 1995). These periods of 

movement along the fault were determined using 

sedimentological evidence such as carbonate mud that 

filled fissures about 359 million–323 million years ago 

during the Mississippian Period (fig. 4; Bauer and Ralser 

1995; Erslev et al. 2004; Fankhauser 2005; Cather et al. 

2008).

Apache Canyon Fault

The Apache Canyon fault is an east–west striking normal 

fault to the north of the Cañoncito Subunit. The fault 

separates granite and granitic gneiss on the northern 

(upthrown) side of the fault from the younger Sangre de 

Cristo Formation (PPNsc) on the south side. The fault 

connects the Borrego and Picuris-Pecos fault zones.

Escobas Fault

The Escobas fault is a north-trending normal fault that 

runs through the east side of Glorieta Mesa and to the

west of the Pecos Unit. It crosses Cerro de Escobas, the 
PECO Geologic Resources Inventory Report 15

highest point on the mesa. The GIS data do not include 

strike and dip of the fault but it can be inferred from the 

map that the east side has moved up relative to the west.

The Escobas fault cuts through all the layers of Glorieta 

Mesa (see “Glorieta Mesa” section).

Lisboa Springs Fault

The Lisboa Springs fault is an east–west-striking fault. 

Fault type and amount of displacement is not known. 

The fault runs through the Alamitos Formation (PNal). 

The north side has moved up relative to the south side. 

North of the Pecos Unit, the fault crosses the Pecos 

River. The west end of the Lisboa Springs fault lies 

between the El Molino Mills site and the tailings ponds 

to the south.

Slope Movements

Slope movements are the downslope transfer of soil, 

regolith, and/or rock under the influence of gravity. Soil 

creep, rockfalls, debris flows, and rock avalanches are 

common types of slope movements (fig. 14). These 

processes and the resultant deposits are also known as 

“mass wasting” and commonly grouped as “landslides.” 

Slope movements occur on time scales ranging from 

seconds to years. Slope movements create geologic 

hazards and associated risk in many National Park 

System units, including Pecos National Historical Park 

(see “Slope Movement Hazards and Risks” section).

Large-scale slope movements are not common in Pecos 

National Historical Park (KellerLynn 2006). Most slope 

movements in the park occur in relation to fluvial 

processes where stream erosion has created steep banks 

(fig. 15) or has exposed bedrock (fig. 16) along the Pecos 

River and Glorieta Creek. These movements are seldom 

hazardous. Slope movements may also occur on areas of 

disturbed land (see “Disturbed Lands Restoration” 

section).

Landslide deposits (Ql) are included on the GIS data and 

are predominantly composed of large sandstone blocks 

that moved downslope from cliffs during rockfalls. The 

only landslide mapped in the park extends slightly onto 

the north edge of the Cañoncito Subunit. The remaining 

landslide deposits are on Glorieta Mesa and associated 

with rockfalls originating from the Santa Rosa Formation 

(TRs).

Colluvium deposits (Qc and Qca) are also included on the 

GIS data. Colluvium refers to soil and rock fragments 

that were deposited at the base of gentle slopes by 

rainwash, sheetwash, or slow downhill movement. This

Figure 14. Schematic illustrations of selected slope movements. Different categories of slope movement are defined by 

material type, nature of the movement, rate of movement, and moisture content. Types of slope movements shaded in 

green have the potential to occur in Pecos National Historical Park. Graphic by Trista Thornberry-Ehrlich (Colorado State 

University) after Varnes (1978).
16 NPS Geologic Resources Division

Figure 15. Steep banks along the Pecos River. Most slope 

movements in the park occur in relation to fluvial processes. 

A flood in September 2013 created these steep and unstable 

banks along the Pecos River. Slope movements such as 

slumps, soil fall, and debris topple occur here. National Park 

Service photograph by Paul Varela (Pecos National Historical 

Park).

Figure 16. Mudstone and sandstone cliffs. Most slope 

movements in the park occur in relation to fluvial processes. 

Where stream erosion has exposed bedrock, the rocks are 

susceptible to rockfalls and rockslides. In the above photos, 

underlying mudstone is eroding faster than overlying 

resistant, hard arkosic sandstone, further increasing 

susceptibility to slope movements. National Park Service 

photographs by Mario Armijo (Pecos National Historical 

Park).

differs from fluvial deposits which are deposited by 

continuously flowing water in channels like streams and 

rivers. Colluvium surrounds the base of Glorieta Mesa 

extending into both of the Glorieta Units. It also blankets 

some of the benches on the mesa itself. The GRI GIS data 

include one mapped deposit of colluvium at the southern 

end of the Pecos Unit on the east side of the Pecos River. 

This deposit consists of Tertiary gravels (QTg) eroded 

from upslope. 

Paleontological Resources

Paleontological resources (fossils) are any evidence of 

life preserved in a geologic context (Santucci et al. 2009). 

Body fossils are any remains of the actual organism such 

as bones, teeth, shells, or leaves. Trace fossils are 

evidence of biological activity; examples include 

burrows, tracks, or coprolites (fossil dung). Fossils in 

NPS areas occur in rocks or unconsolidated deposits, 

museum collections, and cultural contexts such as 

building stones or archeological resources. As of April

2015, 258 parks had documented paleontological 

resources in at least one of these contexts. The NPS 

Geologic Resources Division Paleontology website, 

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

cfm (accessed 17 February 2015), provides more 

information. 

Although fossils have been documented in only the 

Sangre de Cristo (PPNsc) and Alamitos (PNal) formations 

within Pecos National Historical Park, all of the geologic 

units mapped in the park, except for Xgg, have the 

potential to contain fossil resources (Vince Santucci, 

senior geologist, and Justin Tweet, guest scientist, NPS 

Geologic Resources Division, email communications, 22 

January 2015). Two Pennsylvanian age geologic units 

mapped in the park, the Sangre de Cristo Formation 

(PPNsc) and the Madera Formation (PNm), are known to 

contain abundant fossils in locations outside of the park 

(Koch and Santucci 2003). Pennsylvanian strata in New 

Mexico include a great diversity of depositional 

environments and more than 1,250 reported fossil 

species (Kues 1982). 

Staff members from Pecos National Historical Park have 

photographic documentation of marine invertebrate 

fossils from the Alamitos Formation (fig. 17). The fossils 

include marine bivalves, gastropods, crinoids, and 

ammonites. 

Petrified wood was reported in the north and east 

sections of the Pecos Unit (Reed et al. 1999; Dand Jacobs, 

personal communication, cited in Johnson et al. 2011, p. 

46). The geologic source of the wood was not mentioned

in those publications, but it may have come from the 

lower member of the Triassic Santa Rosa Formation 

(TRsl) where Read and Rawling (2002) documented fossil 

wood. The Santa Rosa Formation is only mapped in the 

Cañoncito Subunit of the park. It is mapped on Glorieta 

Mesa where the wood may have eroded out of the rock 

and been brought down to the Pecos Unit by natural 

processes. 
PECO Geologic Resources Inventory Report 17

Figure 17. Fossils of the Alamitos Formation. Marine invertebrates have been documented in the Alamitos Formation (PNal) 

within the park. Clockwise from the top left the identifiable fossils are a gastropod, an ammonite, bivalves, and crinoids. 

National Park Service photographs by Mario Armijo (Pecos National Historical Park).

The Sangre de Cristo Formation (PPNsc) is fossiliferous 

within Pecos National Historical Park. During the 1930s, 

researchers visited the area and collected a small 

assemblage of vertebrate material from between 0.8 and 

1.2 km (0.5 and 0.75 mi) north of the mission ruins, 

within the modern boundaries of the Pecos Unit. Fossils 

brought back to University of California Museum of 

Paleontology include two bone fragments attributed to 

the amphibian Eryops (fig. 18) (Erica Clites, museum 

scientist, University of California Museum of 

Paleontology, personal communication to Justin Tweet, 

January 2015). Romer (1960) reported that a shark tooth 

and bones of a diadectid (a “reptile-like amphibian”) and 

Sphenacodon (a “mammal-like reptile” similar to 

Dimetrodon but with no “sail” on its back) were found 

within 1.2 km (0.75 miles) of the Pecos ruins. The finds 

are sometimes attributed to the Abo Formation (Romer 

1960) or Cutler Formation, but are now known to belong 

to the Sangre de Cristo Formation.

Though not documented in the park, the Madera 

Formation contains marine inveretebrate fossils in 

several locations throughout New Mexico and in 

Colorado (Koch and Santucci 2003). Kues et al. (1997) 

reported them in Sandoval County. Kues (1987) 

Figure 18. Fossils of the Sangre de Cristo Formation. Bone 

fragments attributed to Eryops were discovered in the 

1930s near the ruins in the Pecos Unit of the park. 

Photograph by Erica Clites (University of California Museum 

of Paleontology).

described a well-preserved specimen of the large 

gastropod Pharkidonotus megalius in south-central New 

Mexico. Schram and Schram (1979) discovered shrimp 

in the Manzanita Mountains. Kues and Kietzke (1981) 

report a large assemblage of the eurypterid 

Adelophthalmus luceroensisi from Valencia County, in 

central New Mexico. A sequence of fusulinids from 

Huerfano Park, Colorado, helped to deduce a marine 

transgressional depositional environment (Tischler 

1963). Conchostracans (“clam shrimp”) and brachiopods 

were reported in the Manzano Mountains by Huber et al.
18 NPS Geologic Resources Division

(1989). Other invertebrates reported in the Madera 

Formation include fossil plants and insects in the 

Manzano Mountains in Bernalillo County (Huber et al. 

1989).

The Madera Formation also contains vertebrate fossils. A 

diverse vertebrate assemblage from New Mexico was 

presented by Rowland et al. (1997). In addition, Berman 

(1973) described a new genus and species of amphibian 

(Lafonius lehmani) from the Manzano Mountains, in 

north-central New Mexico. Disarticulated fish were also 

reported from the Manzano Mountains (Huber et al. 

1989). Cook and Lucas (1998) reported a locality in 

central New Mexico that includes various fragmentary 

fossils, reptiles, an amphibian, and a shark (Koch and 

Santucci 2003). 

According to the GRI GIS data, the Madera Formation 

(PNm) is not mapped in the park. Some authors 

considered the Alamitos Formation (PNal)—which is 

mapped in the Pecos Unit of the park—to be equivalent 

to the Madera Formation (Baltz and Myers 1984, 1999). 

However, the Alamitos exposures in the vicinity of the 

park are unlike the typical Madera Formation (Kues 

2001). This reduces the possibility that Madera 

Formation-type fossils may be found within the Alamitos 

Formation rocks of the park.

Caves and Karst Resources

Caves are naturally occurring underground voids. Karst 

is a landscape that forms through the dissolution of 

soluble rock, most commonly carbonates such as 

limestone or dolomite (Toomey 2009). Excluding 

Quaternary surficial units, 98.55% of the area of Pecos 

National Historical Park is mapped as geologic units that 

contain some amount of karst-forming carbonate rock 

such as limestone or dolomite (PNal, PPNsc, Py, Pg, Psa, Pa, 

TRm, TRs; see Map Unit Properties Table, in pocket; Land 

et al. 2013). However, no karstic features, for example 

sinkholes, “losing streams,” springs, or internal drainage, 

occur in the park. As of April 2015, cave or karst 

resources are documented in at least 140 parks. The NPS 

Geologic Resources Division Cave and Karst Resources 

website, http://www.nature.nps.gov/geology/caves/

index.cfm (accessed 17 February 2015), provides more 

information. 

In some locations the Alamitos Formation (PNal) crops 

out as cliffs along the Pecos River. In places, the 

limestone of the formation is dissolving and forming 

small dissolution caves (fig. 19). Owls have been sighted 

in the vicinity of these small caves and may be inhabiting 

the caves (Mario Armijo, Pecos National Historical Park, 

staff, written communication, 24 September 2014).

A karst cave located outside the park, near the Pecos 

mine, was used for ceremonies by Pueblo Indians 

(Anderson 1956). The cave extends northwest more than 

1.6 km (1 mi) into a hillside composed of Magdalena 

limestone (Anderson 1956). Evidence of fires, 

arrowheads, and broken arrow shafts, suggest the cave 

was used by hunting parties during the 19th and early 

20th centuries and may have provided shelter to local 

Figure 19. Limestone caves. Small caves are forming in 

outcrops of the Alamitos Formation (PNal, limestone). 

National Park Service photograph by of Mario Armijo (Pecos 

National Historical Park).

Indians into the 1940s (Anderson 1956). This cave is 

beyond the extent of the GRI GIS data set.

Non-karst Caves

Two caves, Baca Cave and “small cave,” are documented 

in the park. Both are closed to the public. The Baca Cave

is a natural opening in sandstone that prospectors 

expanded to create a mine (Burger and Allison 2008; see 

“Abandoned Mineral Lands Hazards” section). The Baca 

Cave is not a karst feature. It formed by the erosion of 

sandstone rather than through dissolution of limestone. 

The cave developed between a sandstone layer and a 

limestone layer within the Sangre de Cristo Formation. 

The upper layer is grainy sandstone approximately 9 m 

(30 ft) thick (Burger and Allison 2008). The lower unit is 

a gray, limestone bed a few feet below the floor of the 

cave entrance and is about 8 m (26 ft) thick (Burger and 

Allison 2008). The east-west cave passage developed 

along a joint between these layers (Johnson et al. 2011). 

Joints are fractures in rock and provide avenues for 

water erosion. The joint along which the cave developed 

formed in response to the stress associated with normal 

faulting in the area (Johnson et al. 2011). The joint

developed perpendicular to the normal fault plane. 

The cave has a surveyed length of 73.7 m (241.8 ft), a 

surveyed depth of 9.14 m (30.0 ft), and a volume of 

approximately 283 m3

 (9,994 ft3) (Burger and Allison 

2008). The NPS Geologic Resource Division assessed the 

bat habitat of Baca Cave (KellerLynn 2006).

Baca Cave is currently closed to the public for safety 

reasons and to protect cultural resources (Johnson et al. 

2011). However, Burger and Allison (2008) reported 

graffiti and garbage in the cave, indicating some human 

access (see “Caves and Associated Landscapes 

Management” section). 

“Small cave” is another non-karst cave within the park. It 

is located at the base of a cliff approximately 20 m (60 ft) 

southeast of Baca Cave and about 5 m (15 ft) higher in
PECO Geologic Resources Inventory Report 19

Figure 20. Earthquake probability map. The star marks the location of Pecos National Historical Park. A magnitude 5.5 

earthquake has a 0.25–0.30 probability (25%–30% “chance”) of a occurring within 30 mi (50 km) of the park in the next 100 

years. Five earthquakes greater than magnitude 1 have occurred within 80 km (50 mi) of the Pecos Unit of the park since 

1962. The area of increased probability to the south is the northern extent of the Socorro Seismic Anomaly. US Geological 

Survey map created at http://geohazards.usgs.gov/eqprob/2009/index.php (accessed 3 June 2014).

elevation. It appears to have formed along a joint 

associated with the same north–south normal fault as the 

Baca Cave (Burger and Allison 2008). Three passages in 

“small cave” can be followed for about 12 m (39 ft) 

before becoming too tight to traverse; it is possible that 

this cave is connected to Baca Cave (Burger and Allison 

2008).

Earthquakes

Earthquakes are ground vibrations—shaking—that occur 

when rocks suddenly move along a fault, releasing 

accumulated energy (Braile 2009). Earthquake intensity 

ranges from imperceptible by humans to total 

destruction of developed areas and alteration of the 

landscape. The magnitude is a measure of the energy 

released by an earthquake. Earthquakes can directly 

damage park infrastructure, or trigger other hazards such 

as slope movements that may impact park resources, 

infrastructure, or visitor safety. Braile (2009), the NPS 

Geologic Resources Division Seismic Monitoring 

website (http://nature.nps.gov/geology/monitoring/

seismic.cfm; accessed 17 February 2015), and the US 

Geological Survey Earthquakes Hazards website 

(http://earthquake.usgs.gov/; accessed 17 February 2015) 

provide more information. Also see the “Earthquake 

Hazards and Risk” section in this report.

According to the US Geological Survey Earthquake

Probability map (fig. 20; http://geohazards.usgs.gov/

eqprob/2009/index.php; accessed 3 June 2014), a 

magnitude 5.5 earthquake has a 0.25–0.30 probability 

(25%–30% “chance”) of occurring near Pecos National 

Historical Park in the next 100 years. A 5.5 magnitude 

earthquake is considered a “moderate” earthquake. 

New Mexico earthquakes have been instrumentally 

located since 1962 by the New Mexico Institute of 

Mining and Technology. During this time period, even 

“moderate” earthquakes have been uncommon in New 

Mexico, and the Rio Grande rift, a region west of the 

park which was seismically active during its formation 

roughly 30 million years ago, has remained quiet. An area 

of concentrated, seismic activity surrounding Socorro, 

referred to as the “Socorro Seismic Anomaly,” is

approximately 240 km (150 mi) southwest of the park. 

This area produces a disproportionate share of the state’s 

earthquakes and has generated the strongest historical 

earthquakes (Sanford et al. 2006). Earthquake activity in 

this region is attributed to crustal extension (pulling 

apart) as molten material (magma) moves beneath the 

surface (Fialko and Simons 2001).

Since 1962 Pecos National Historical Park has not 

experienced any significant earthquakes—earthquakes 

greater than 3.5 magnitude. However, between 1962 and 
20 NPS Geologic Resources Division

2010, nearly 100 earthquakes with a magnitude of 3.5 or 

less were recorded within 80 km (50 mi) of the park, 

indicating that earthquakes do occur in the area, 

although the vast majority are minor and not felt by 

humans (Sanford et al. 2006; Pursley et al. 2013). The five 

strongest of these recorded earthquakes ranged in 

magnitude from 2.7 to 3.5. 

Geologic Resources in a Cultural Context

The geologic history and geologic landscape of Pecos 

National Historical Park played a role in the history and 

lives of the people that have passed through and 

inhabited the area. The inhabitants of Pecos Pueblo used 

geologic resources for producing ceramic pottery, bricks, 

and stone tools (“lithics”), and as building stone and 

glass. Petroglyphs are visible on some of the gritty arkosic 

sandstone outcrops of the Sangre de Cristo Formation 

(PPNsc) (fig. 21).

Figure 21. Petroglyphs. Prehistoric inhabitants of the Pecos 

area carved petroglyphs into the arkosic sandstone of the 

Sangre de Cristo Formation (PPNsc). National Park Service

photograph by Mario Armijo (Pecos National Historical 

Park).

Travel Route and Battlefield

For more than 800 years Glorieta Pass has provided 

travelers with a connection between the Pecos River 

valley in the east and the Rio Grande valley in the west. 

The pueblo and mission were both situated along this 

“gateway” corridor. In 1541, Francisco Vasquez de 

Coronado traversed the pass in route to explore the 

plains; Spanish friars followed in the 1660s, attempting to 

convert Plains Indian tribes; Apaches and Comanches 

entered the Pueblo area from the east (Bauer et al. 1995). 

The Santa Fe Trail crossed the pass from the 1820s to the 

1880s, followed by the railroad and Interstate 25 today. 

Glorieta Pass and neighboring Glorieta Mesa played 

significant roles in the outcome of the Battle of Glorieta 

Pass. This battle, fought on 26–28 March 1862, was a 

decisive, ending the Civil War in the far west (National 

Park Service 1993). The Glorieta Unit of the park 

interprets these stories. While the skirmishing occurred 

in the valley near the Pigeon’s Ranch Subunit and 

unbeknownst to the Confederates, a detachment of 

Union soldiers made their way west over the top of 

Glorieta Mesa, out of sight from the battleground, and 

into the Confederate supply camp at Cañoncito, which 

they attacked and destroyed (James 1979).

Ceramics

Ceramic pottery is made out of clay. Most of the ceramic 

pottery recovered by archeologists at Pecos National 

Historical Park has been broken over time into small 

fragments—sherds (fig. 22; Powell 2002). Locally 

excavated minerals (see “Mining” section) may have 

been the materials used to make glaze paints for some 

Pecos glaze ware ceramics (Jeremy Moss, Pecos National 

Historical Park, chief of cultural resources, written 

communication, 5 August 2014). 

Figure 22. Ceramic jar. This ceramic jar was reconstructed 

from 36 micaceous-clay pottery sherds recovered from Pecos 

National Historical Park. National Park Service photograph, 

and additional information, available at 

http://go.nps.gov/peco_jar (accessed 9 April 2015).

Lithics

Lithics, or stone tools, used by Pueblo Indians in and 

around the Pecos Valley include projectile points, 

manos, and metates (fig. 23; Kidder 1932; Kilby and 

Cunningham 2002). One Clovis-type projectile point has 

been reported from the upper Pecos Valley (Nordby 

1981). In addition, the mid-section of a Folsom point was 

found in the Tecolate Range in the north east section of 

the Pecos Unit (Kilby and Cunningham 2002).

Stone was also used to craft hammer stones, pot 

polishers, arrow shaft straighteners, smoking pipes, and 

other ceremonial objects (Kidder 1932). Some lithic 

materials were locally available from within a 30 km (20 

mi) radius of the park, such as chert and chalcedony 

from the Madera Formation (PNm) and sandstones and 

quartzite of the Dakota Formation (Kilby and 

Cunningham 2002). The Dakota Formation was not 

mapped in the extent of the GRI GIS data. More 

abundant and higher-quality lithic material such as 

petrified wood, lignite, flint, turquoise, fibrolite ax heads,
PECO Geologic Resources Inventory Report 21

Figure 23. Prehistoric tools from Pecos National Historical Park. Investigators found a variety of lithics during a cultural 

resource inventory at the park in the mid-1990s. National Park Service photographs.

salt, obsidian, and shell was acquired from more distant 

regions through trade (Bezy 1988; Kilby and 

Cunningham 2002). Lithics derived from source rocks 

found in the Rocky Mountains, Basin and Range, and 

Great Plains have all been discovered in Pecos National 

Historical Park.

Rocky Mountain Lithic Materials

Some Rocky Mountain lithic materials were sourced 

locally. Bezy (1988) reported schists and quartzites (Xf) 

from nearby terraces that were used as hammer stones, 

pot polishers, arrow shaft straighteners, smoking pipes, 

and other ceremonial objects.

Obsidian and chert were obtained from the Jemez 

Mountains which surround the Valles caldera northwest 

of Santa Fe. The Jemez Mountains comprise a volcanic 

field that started erupting episodically about 14 million 

years ago (Miocene Epoch; fig. 4) and culminated during 

two caldera-forming events 1.62 million and 1.25 million 

years ago (Pleistocene Epoch; see GRI report about 

Bandelier National Monument by KellerLynn in review). 

Obsidian (“volcanic glass”) produced during these 

eruptions was prized throughout the Southwest and 

Southern Plains as exceptional source rock for stone 

tools. The obsidian sources of the Jemez Mountains are 

located 80–100 km (50–60 mi) northwest of Pecos 

National Historical Park. Three locations appear to have 

been the most important for lithic raw material: El 

Rechuelos, Cerro Toledo, and Valle Grande (fig. 24), 

each of which has a distinctive chemical signature and 

can be differentiated by x-ray florescence or neutron 

activation (Baugh and Nelson 1987; Glascock and Neff 

1993). The Rio Grande and its tributaries that drain the 

Jemez Mountains, such as the Chama and Jemez river 

systems, are potential secondary sources of the same 

obsidian. 

Chert (flint) was acquired from Cerro Pedernal, or 

locally just “Pedernal,” a mesa in northern New Mexico

on the northern flank of the Jemez Mountains 

approximately 100 km (60 mi) from Pecos National 

Historical Park (fig. 24; Kilby and Cunningham 2002) . 

The name is Spanish for “flint hill.” The chert was 

obtained from Miocene sedimentary deposits of the 

Tesuque Formation (Tt) (Wilks 2005). 

Basin and Range Lithic Materials

Relative to adjacent physiographic provinces, the Basin 

and Range contains few important sources of lithic raw 

materials, although there are chert deposits in the 

Madera and San Andres formations similar to those 

found in the same formations in the Rocky Mountain 

province (Kilby and Cunningham 2002). The Rocky 

Mountain sources were more likely utilized for chert in 

the Pecos Valley because they were closer. 

However, alluvial deposits of the Rio Grande provided a 

closer and important secondary source of Jemez-derived

obsidian (Kilby and Cunningham 2002). Evidence of 

utilization of Rio Grande deposits has been document as 

far north as Bandelier National Monument and as far
22 NPS Geologic Resources Division

Figure 24. Obsidian and chert sources in the Jemez Mountain area. Three locations (El Rechuelos, Cerro Toledo, and Valle 

Grande; red dots) appear to have been the most important for obsidian used by Pueblo Indians. Cerro Pedernal (blue dot) 

was a source of chert. Graphic by Rebecca Port (NPS Geologic Resources Division) after information from Kilby and 

Cunningham (2002). The shaded relief base map was compiled from a variety of best available sources from several data 

providers, including the U.S. Geological Survey, NOAA, and ESRI. 
PECO Geologic Resources Inventory Report 23

south as Tularosa Basin (Head 1999). In addition, 

obsidian derived from Mount Taylor about 180 km (112 

mi) west of Pecos was discovered in alluvial deposits 

within 110 km (68 mi) of the park (Kilby and 

Cunningham 2002). The prehistoric significance of the 

Mount Taylor derived obsidian is not well understood.

Great Plains Lithic Materials

Despite relatively great distances, chert or flint derived 

from the Great Plains is fairly common in the cultural 

resources of Pecos National Historical Park. Alibates 

flint from Alibates Flint Quarries National Monument 

(Texas) has been found within the park, although the 

quarries are more than 350 km (220 mi) east of Pecos

(Kilby and Cunningham 2002). Chert from the Edwards 

Limestone in Texas, about 480 km (300 mi) from Pecos, 

has also been discovered in the park (Kilby and 

Cunningham 2002).

Pecos Pueblo Stone Walls

Pecos Pueblo was enclosed by a wall built with stone

rather than adobe (fig. 25; Winship 1904; Nordby 1981). 

The inhabitants of the Pecos Valley constructed pueblo 

walls with igneous and metamorphic cobbles carried 

from nearby terraces and Glorieta Creek, as well as

conglomerate and sandstone blocks of the Sangre de 

Cristo Formation (PPNsc), which underlies most of the 

area in the park (Johnson 1969; Bezy 1988) (see poster, in 

pocket). These rocks split naturally into sizes and shapes

ideal for building stones (Johnson 1969).

Mission Churches

Flagstone was quarried locally from the Sangre de Cristo 

Formation (PPNsc) for walkways, steps, and shelves in 

Spanish mission churches (Johnson 1969). Missionaries 

made adobe bricks with the deep red soil from the Pecos 

River and Glorieta Creek valleys (Johnson 1969). Adobe 

bricks are made in a similar manner at the park today as 

part of preservation efforts (Jeremy Moss, written 

communication, 5 August 2014) (fig. 26). 

Selenite (crystalline variety of gypsum) and white mica 

(muscovite) were used for window panes because they 

split easily into thin transparent sheets (a characteristic

known as “perfect basal cleavage”) (Johnson 1969; Bezy 

1988). The source of the selenite was probably southwest 

of the Cañoncito Subunit in a large gypsum deposit in the 

either the San Andres (Psa) or Artesia (Pa) formations 

(Johnson 1969). The gypsum deposit was also used as a 

source of plaster of Paris to cover interior walls (Johnson 

1969). The mica came from large crystals in the 

Precambrian muscovite schist (Xms) and quartz 

muscovite schist (Xqms) mountain rocks north of the 

park (Johnson 1969).

Mining

Though no mining of large economic value has occurred 

within Pecos National Historical Park (KellerLynn 

2006), mines near the park have been economically 

viable in the past. The mining history of the park and 

surrounding area is primarily important from a cultural 

perspective and in relation to lasting environmental 

impacts. This section presents a brief history of mineral 

exploration and mines in and near the park (see also 

“Water Quantity and Quality” and “Abandoned Mineral 

Lands Hazards” sections).

Pueblo Indians were the first prospectors and miners in 

the area. More than 500 years ago (beginning sometime 

between 1100s and 1500s), they extracted minerals to be

used for artifacts and pigments, such as lead ore to make 

glaze for pottery (Warren and Weber 1979). 

Archeological remains associated with their mining 

activities include turquoise quarries and tunnels, lead 

mines, cobbing and sorting workshops, campsites, 

hearths, and sherd and tool scatters (Warren and Weber 

1979). Prehistoric mines are not documented in the park; 

however, mineral artifacts belonging to Pueblo Indians

Figure 25. Pecos Pueblo stone walls. The stone walls surrounding Pecos Pueblo were constructed with igneous and 

metamorphic cobbles carried from nearby terraces and Glorieta Creek, as well as conglomerate and sandstone blocks of the 

Sangre de Cristo Formation. These rocks split naturally into sizes and shapes ideal for building stones. National Park Service 

photograph by Paul Varela (Pecos National Historical Park).
24 NPS Geologic Resources Division

Figure 26. Freshly made adobe bricks at Pecos mission church. Each summer, selected areas of the mission church are 

restored using adobe bricks. The bricks are made in much the same manner as they were hundreds of years ago. The new 

adobe bricks act as a shelter or protective layer that encases the original material. National Park Service photograph.

have been discovered (Kilby and Cunningham 2002). 

The nearest evidence of prehistoric mining is in the 

Cerillos mining district, 32 km (20 mi) west of the park 

(Warren and Weber 1979). Warren and Weber (1979) 

asserted that the role and importance of prehistoric 

mining to Southwestern history is not well understood

and deserves further study.

In 1581, Spanish explorers unsuccessfully began 

prospecting for silver in the land of the Pueblos 

(National Park Service 2014). Later, during the late 19th

and early 20th centuries, European settlers began largescale exploration for industrial and precious minerals 

(Bezy 1988). The GRI GIS data includes prospects and

ditches located roughly 4 km (2 mi) northwest of 

Glorieta Pass. Several mines were subsequently opened 

in the area. 

One mine and several small aggregate quarries existed 

inside the park. The nearest significant mine to operate 

near the park was the Pecos mine. The remains of some 

smaller mines have also been documented in the area

(Anderson 1956). 

The Old Bradley mine, 6.4 km (4.0 mi) north of the 

Pigeon’s Ranch Subunit, briefly produced gold-silver ore 

from the late 1890s to 1905 (Anderson 1956). According 

to Anderson (1956), the gold was found between the 

contact of a diabase and granite body. The geologic unit 

that was mined was most likely Precambrian granite 

(Xgg) (Wilks 2005). The location of this mine is beyond 

the extent of the GRI GIS data. 

Also north of the park, was a copper mine, the Fairview 

lode mine. The GRI GIS data includes the location of the 

Fairview lode mine. The mine occurs in the Sangre de 

Cristo Formation (PPNsc). 

A World War I–era iron mine was located near the rim of 

Glorieta Mesa, south of the Pecos Unit (Kelley 1950; 

Anderson 1956). None of the geologic units mapped in 

this area (PPNsc, Py, Pg, Psa) on the GRI GIS data contain 

iron ore minerals. It is unclear what unit was mined at 

this location. The GRI GIS data includes the location of a 

mine feature identified as a “shaft” on the rim of Glorieta 

Mesa which may be the feature discussed by Kelley 

(1950) and Anderson (1956). The shaft is located near the 

contact of the Glorieta sandstone (Pg) and the Artesia 

Formation (Pa).

Baca Mine

According to the NPS database of Abandoned Mineral 

Lands, only one underground mine, the Baca mine,

existed inside Pecos National Historical Park (see also 

Burghardt et al. 2014). The Baca mine is also a cave (see 

“Cave and Karst Resources” section). The cave was a 

natural opening that prospectors expanded into an adit 

and a shaft (KellerLynn 2006). Several passages appear to 

have been enlarged, and juniper logs used to support the 

roofs of the enlarged passages are still present. Local 

legend maintains that the cave was enlarged by treasure 

hunters. 

The Baca mine may have been opened in search of 

uranium or copper; however, the cave contains no 

obvious valuable minerals. This finding is supported by 

the geology of the surrounding area which is primarily 

mapped as the Sangre de Cristo Formation (PPNsc). 

Lindgren et al. (1910) discussed copper deposits in the 

Sangre de Cristo Formation which may be a reference to 

the Baca mine (Virgil Leuth, New Mexico Bureau of 

Geology and Mineral Resources, personal 

communication, cited in KellerLynn 2006, p. 6).

Anderson (1956) reported numerous copper showings 

(small particles) in sandstone on the west side of the 

Pecos River. Making a comparison to the GRI GIS data, 

Anderson (1956) may also be referring to the Sangre de 

Cristo Formation (PPNsc). To date, no ore bodies have 

been found in the Sangre de Cristo Formation and it has 

never been developed for copper mining (Anderson 

1956).
PECO Geologic Resources Inventory Report 25

Gravel Pits

More recently, probably between 1965 and 1975, gravel 

was extracted from pits in an area that is now within the 

park’s boundary. The gravel was used as aggregate

during construction of Interstate 25 (Dan Jacobs, 

personal communication, cited in Johnson et al. 2011, p. 

46). Most of the pits were reclaimed prior to the 

expansion of the park; however, park staff reclaimed one 

pit in the late 1990s (KellerLynn 2006) and one gravel pit 

remains in the south part of the park alongside the Old 

Colonias Road, just before it dips into the Pecos River 

Valley (Dan Jacobs, personal communication, cited in 

Johnson et al. 2011, p. 46). The GRI GIS data set includes 

the location of the remaining gravel pit (see “Disturbed 

Lands Restoration” section). 

Pecos Mine

The Pecos mine, also known as the Terrero, Willow 

Creek, Hamilton, or Cowles mine, is 23 km (14 mi) north 

of Pecos National Historical Park along the Pecos River 

(Anderson 1956; Bezy 1988; McLemore 1995). It was the 

only productive mine ever developed in the Pecos River 

basin (Anderson 1956). From 1927 to 1939, the Pecos 

mine was the largest producer of zinc, lead, gold, and 

silver in New Mexico (Anderson 1956; Johnson and 

Deeds 1995; McLemore 1995). These minerals were 

exported and the tailings—waste—were left near the 

village of Pecos (Bezy 1988).

Massive ore deposits of the sulfide minerals galena, 

sphalerite, and chalcopyrite contained within highly 

sheared Proterozoic metamorphic rocks produced the 

zinc, lead, copper, silver, and gold (Anderson 1956; 

Johnson and Deeds 1995; McLemore 1995). In the 

vicinity of the mine, these rocks are only exposed at the 

surface in a small area where the Pecos River and its 

tributaries have cut through the overlying limestone and 

shale of the Magdalena group (PNm and PNs; Anderson 

1956).

Crushed ore was transported via aerial tramway 19 km 

(12 mi) south to the El Molino Mill (or Alamitos Canyon 

Mill) in Alamitos Canyon, 0.5 km (0.3 mi) west of the 

town of Pecos (McLemore 1995). The mill tailings were 

conveyed to the tailings ponds in Alamitos Canyon, 

downstream from the mill and immediately north of the 

current park boundary. The GRI GIS data include the 

location of the El Molino Mill and associated tailings 

ponds.

Pecos mine closed in 1939; the commercial ore was 

exhausted and mine conditions made it uneconomic to 

continue (Anderson 1956; McLemore 1995). Minor 

reprocessing of mine dumps occurred in 1943–1944 

(McLemore 1995). In 1950, the surface was purchased by 

the State of New Mexico to become part of the Bert 

Clancey Fish and Wildlife Area, administered by the 

New Mexico Game and Fish Department. The mineral 

rights were transferred to a trust and periodically leased 

to interested mining companies (McLemore 1995). State 

and federal agencies used materials from the mine waste 

pile at the Pecos mine for road construction and as fill in 

various campgrounds from the 1930s through the 1970s 

(McLemore 1995). Since the late 1980s, the mineralized 

zones at the Pecos mine have been considered too deep 

and too limited to be mined economically (McLemore 

1995).
PECO Geologic Resources Inventory Report 27 

Geologic Resource Management Issues 

This section describes geologic features, processes, or human activities that may require 

management for visitor safety, protection of infrastructure, and preservation of natural 

and cultural resources in Pecos National Historical Park. The NPS Geologic Resources 

Division provides technical and policy assistance for these issues. 

During the 2006 scoping meeting (see KellerLynn 2006) 

and 2013 conference call, participants (see Appendix A) 

identified the following potential geologic resource 

management issues: 

• Fluvial Erosion

• Water Quantity and Quality

• Disturbed Lands Restoration

• Abandoned Mineral Lands Hazards

• Slope Movement Hazards and Risks

• Paleontological Resource Inventory, Monitoring, and 

Protection

• Caves and Associated Landscape Management

• Land Subsidence

• Earthquakes Hazards and Risks

• External Energy and Mineral Development

• Interpretation and Education

Resource managers may find Geological Monitoring

(Young and Norby 2009) useful for addressing some of

these geologic resource management issues. This 

information is available at 

http://go.nps.gov/geomonitoring (accessed 17 February 

2015). The manual provides guidance for monitoring 

vital signs—measurable parameters of the overall 

condition of natural resources. Each chapter covers a 

different geologic resource and includes detailed 

recommendations for resource managers and suggested 

methods of monitoring.

Fluvial Erosion

Fluvial erosion—erosion caused by rivers and streams—

has changed the landscape of Pecos National Historical 

Park (fig. 27; KellerLynn 2006). Although some amount 

of erosion and change to channel morphology is within 

the realm of natural stream evolution, downcutting and 

bank erosion are proceeding at accelerated rates along 

some sections of the Pecos River and Glorieta Creek

within the park. In addition, excessive erosion along 

arroyos—flat-bottomed channels where flow is 

intermittent—has removed soil and exposed bedrock in 

places. 

Rapid downcutting has produced narrow and incised 

stream channels. This channel morphology may result in 

loss of riparian habitat, threats to cultural resources, and 

risk of slope failure (see “Slope Movement Hazards and 

Risks” section). In locations where the erosion-resistant 

Figure 27. Fluvial erosion. Severe erosion is a concern at 

Pecos National Historical Park. Photographs were taken near 

the far northwest boundary of the Pecos Unit. Note person 

for scale (red circle) in top photograph. During flash floods 

and seasonal storms, ephemeral arroyos flood and 

dramatically erode their banks. The steep banks produced 

are at risk of slope movements. National Park Service 

photographs by Mario Armijo (top) and Paul Varela 

(bottom; Pecos National Historical Park).

Alamitos Formation (PNal) crops out along the 

waterways, it restricts lateral channel migration—

meandering—and forces rapid downcutting (figs. 10, 28, 

and 29). In locations where arkosic sandstone and 

mudstone have been exposed, slabs of the hard and 

resistant sandstone is at risk of sliding or falling as the 

mudstone weathers rapidly beneath it (fig. 16). 

Bank erosion can lead to increased sediment in 

waterways, undercut banks (see “Slope Movement 

Hazards and Risks” section), loss of riparian habitat, and 

threats to cultural resources. Staff from the NPS Water 

Resources Division conducted a riparian condition 

assessment on the Pecos River and lower Glorieta Creek 

in July 2010 (see Wagner and Martin 2011). A riparian 

area was considered in “proper functioning condition” if 
28 NPS Geologic Resources Division

Figure 28. Bedrock outcrop on the Pecos River. The Alamitos 

Formation (PNal) crops out along several meanders (white 

arrow) along the Pecos River in the park. Bedrock outcrops 

restrict lateral channel migration of the Pecos River south of 

its confluence with Glorieta Creek. Esri, DigitalGlobe, 

GeoEye, i-cubed, USDA, USGS, AEX, Getmapping, Aerogrid, 

IGN, IGP, swisstopo, and the GIS User Community imagery, 

annotated by Rebeca Port (NPS Geologic Resources 

Division).

streamflow forces and channel processes were in 

dynamic equilibrium. Excessive erosion and sediment 

loading indicated a “non-functional” stream system 

condition.

Pecos River

Three reaches along the Pecos River in the park were 

rated in “proper functioning condition” (the highest 

possible rating) by Wagner and Martin (2011). However, 

an earlier study of the entire Upper Pecos River 

Watershed by La Calandria Associates, Inc. (2007) 

concluded that stream bank erosion of the Upper Pecos 

River was contributing excessive sediments along the 

segment that stretches from south of the village of Pecos 

to the confluence with Cow Creek. According to the 

2007 study, half of the banks of the 16.2 km (10.1 mi) 

stretch of the Pecos River from Alamitos Canyon to the 

confluence with Cow Creek were eroding, with an 

estimated 709 kg (1,563 lbs) or 182 m3

/yr (238 yd3 or 

6,416 ft3

/yr) of soil being lost each day (La Calandria 

Associates, Inc. 2007). This stretch includes the portion 

of the Pecos River that runs through the park.

Recommended remedial actions included constructing

upland erosion control measures (head cut repair, check 

dams), repairing roads to reduce erosion (improved 

culverts, crossings, ditches, and road alignments), 

evaluating streams for possible alterations to reduce 

width (and facilitate formation of meanders and pools), 

and revegetating stream banks (La Calandria Associates, 

Inc. 2007). Restoring native vegetation on the banks of 

the Pecos River would provide shade, cooling the stream 

to more natural temperatures, as well as reduce bank 

erosion and sediment load (La Calandria Associates, Inc. 

2007).

Figure 29. Potential bedrock outcrops along Glorieta Creek 

and the Pecos River. White arrows show the location of 

potential outcrops of the Alamitos Formation (PNal) that 

may be inhibiting natural channel migration or are a 

potential source of slope movements. The red line indicates 

the park boundary. The top photograph shows an 

expanded view of the the area outlined in white on the 

lower photograph. Esri, DigitalGlobe, GeoEye, i-cubed, 

USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, 

swisstopo, and the GIS User Community imagery, annotated 

by Rebeca Port (NPS Geologic Resources Division).

At the time of this report, remedial actions have not been 

taken because the 5-km- (3-mi-) segment of the Pecos 

River that flows through the park is not representative of 

the excessive sediments and erosion along the full stretch 

of the river as reported by La Calandria Associates, Inc. 

(Cheri Dorshak, park ranger, Pecos National Historical 

Park, written communication, 12 September 2014). The 

best available data for the park is therefore the report by
PECO Geologic Resources Inventory Report 29

Figure 30. Artificial levee. The photograph shows a levee that impaired Glorieta creek by restricting the creek to an 

artificially narrow corridor. This limited the potential size, structural complexity, and habitat value of the riparian system. 

The National Park Service removed the levee in 2013 and the area is currently being revegetated. National Park Service 

photograph taken in 2011 by Joel Wagner (NPS Water Resources Division).

Wagner and Martin (2011). However, the data provided 

by La Calandria Associates, Inc. (2007) may be useful for 

understanding overall conditions of the Upper Pecos 

River Watershed.

Glorieta Creek

Wagner and Martin (2011) rated one reach along the 

lower Glorieta Creek as “functional-at risk,” which 

indicates that the riparian area was functioning at the 

time but that soil, water, vegetation, or related attributes 

had made it susceptible to degradation. This rating was 

largely due to an artificial levee that was impairing the 

creek’s natural processes (figs. 30 and 31). This was a 

remaining levee left in place following restoration 

activities along Glorieta Creek in 2000 (see “Disturbed 

Lands Restoration” section). The levee, constructed 

adjacent to the lower Glorieta Creek, constrained the 

channel to an artificially narrow corridor, which 

prevented natural stream erosion and limited the 

potential size, structural complexity, and habitat value of 

the riparian system (Wagner and Martin 2011). The 

artificial levee was susceptible to erosion by floods which

could break through and deposit excessive sediment into 

the restored wetlands to the east and north of the levee, 

and into downstream aquatic habitats. The remaining 

levee was removed in November 2013 and the area is 

currently being restored with native vegetation (fig. 32; 

Cheri Dorshak, Pecos National Historical Park, park 

ranger, written communication, 12 September 2014).

In addition to the issues associated with the levee, 

accelerated erosion of Glorieta Creek has destroyed 

areas of bordering land that were once used for 

agriculture and livestock grazing by Hispanic and 

European settlers (Bezy 1988). Grazing coupled with 

timbering, which removed stabilizing vegetation, may

have triggered the accelerated erosion (Bezy 1988). The 

former grazing areas, now devoid of soil and vegetation, 

consist of exposed rocks of the sedimentary Alamitos 

Formation (PNal; Johnson et al. 2011). 

Figure 31. Aerial photograph of artificial levee. The levee 

(red dashed lines), constructed adjacent to the lower 

Glorieta Creek, constrained the channel to an artificially 

narrow corridor, which prevented natural stream erosion. 

Note the nearly right angle turn in the creek. The NPS Water 

Resources Division recommended removal of the levee in 

2011. It was removed in 2013. National Agriculture Imagery 

Program (NAIP) photograph from 2009, annotations by 

Rebecca Port (NPS Geologic Resources Division).

Figure 32. Restored wetlands of Glorieta Creek. The 

National Park Service removed an artificial levee and 

planted native vegetation in an area adjacent to Glorieta 

Creek. This photograph shows the area in its restored 

condition. National Park Service photograph by Paul Varela 

(Pecos National Historical Park).
30 NPS Geologic Resources Division

Arroyos

Arroyos are typically dry stream beds, but may flow and 

even flash flood on a seasonal basis or temporarily, 

following thunderstorms. Arroyos, therefore, erode 

periodically and sometimes severely washing away soil 

and vegetation and exposing bedrock (fig. 33). Riparian 

habitat was lost along Glorieta Creek as it entrenched its 

channel through the former valley floor, forming a steepwalled, flat-bottomed arroyo in the upper reach of the 

stream where flow is intermittent (Bezy 1988). In the 

Cañoncito Subunit, arroyo cutting is an issue on steep 

slopes. Water funneled out of the interstate culverts is 

increasing downcutting (Staff, Pecos National Historical 

Park, conference call, 11 April 2013).

Figure 33. Erosion along arroyos. Arroyos that drain into the 

Pecos River have become so eroded that bedrock is exposed 

along their beds. National Park Service photographs by 

Mario Armijo (top) and Paul Varela (bottom; Pecos National 

Historical Park).

Threats to Cultural Resources

Even under natural conditions, erosion along rivers may 

be a concern for the preservation of cultural resources 

(KellerLynn 2006). As of 2011, the cultural site nearest 

the Pecos River was 23 m (75 ft) from the river bank with 

all others more than 25 m (82 ft) away (Johnson et al. 

2011). This is far enough away that fluvial erosion is not 

likely to be a threat. These sites would only be at risk 

during floods (fig. 34). However, there is no evidence 

Figure 34. Glorieta Creek after a rain event in September 

2013. Even under natural conditions, erosion along rivers 

may be a concern for the preservation of cultural resources 

These sites would most likely only be at risk during floods. 

National Park Service photograph by Gary Zbel available 

online http://www.nps.gov/peco/planyourvisit/fishing.htm

(accessed 10 April 2015).

suggests that overbank flooding occurs nearby, and these 

cultural sites have persisted for decades to centuries. 

Thus, flooding is likely not an imminent threat to these 

resources (Johnson et al. 2011). 

The cut banks of Glorieta Creek periodically erode, most 

notably during times of high flow. Cultural resources are 

at risk of loss during these periods (staff, Pecos National 

Historical Park, conference call, 11 April 2013). Park staff 

does not regularly monitor the banks. No bank armoring 

or other mitigation strategies are in place (staff, Pecos 

National Historical Park, conference call, 11 April 2013). 

Major storms, which would exacerbate the problem, 

have not occurred since at least 2010; however, the banks 

are currently undercut to such a degree that slumping 

could occur without a storm (see “Slope Movement 

Hazards and Risks” section”). 

Recommended Actions

Park-wide, restoration or remedial actions that aim to 

diminish or bring under control erosion that results in 

unacceptable degradation of natural and cultural is 

outlined as a desired condition in the park’s resource 

stewardship strategy (National Park Service and 

Colorado State University 2011). However, predicting

the effects of any particular bank restoration or other 

remedial action on erosion is difficult. Nevertheless, 

remediation techniques need to be designed to function 

during periods of high flow because the vast majority of 

erosion and downstream sediment transportation 

happens during periods of high flow, such as spring 

runoff or summer thunderstorms. 

In the Geological Monitoring chapter about fluvial 

geomorphology, Lord et al. (2009) described methods 

for inventorying and monitoring geomorphology-related 

vital signs, including (1) watershed landscape 

(vegetation, land use, surficial geology, slopes, and 

hydrology), (2) hydrology (frequency, magnitude, and 
PECO Geologic Resources Inventory Report 31

duration of stream flow rates), (3) sediment transport 

(rates, modes, sources, and types of sediment), (4) 

channel cross section, (5) channel planform, and (6) 

channel longitudinal profile.

Water Quantity and Quality

In 2011, the section of Pecos River that flows through the 

park was classified as “impaired” due to temperature and 

turbidity (suspended sediment) levels that exceeded 

federal standards (Johnson et al. 2011). Disturbed lands 

adjacent to the river may be partially responsible for the 

river’s excessive temperature and turbidity because the 

lands lack native vegetation that provides shade, and they 

are also eroding large amounts of sediments into the 

river valley (see “Disturbed Lands Restoration” section).

The drought of the 2010s has reduced surface water 

quantity, which in turn exacerbates turbidity issues 

(Johnson et al. 2011). 

Abandoned mineral lands upstream from the park may 

be contributing metals and other contaminants to the 

park’s water supply and affecting water quality (see 

“Abandoned Mineral Lands Hazards” section).

Water quality issues not related to geologic processes are 

beyond the scope of this report. The NPS Water 

Resources Division can assist park managers with 

concerns about such issues (see 

http://nature.nps.gov/water/; accessed 19 February 

2015). 

Disturbed Land Restoration

Where natural processes and conditions have been 

impacted by human activities such as development or 

agriculture, disturbed land restoration (DLR) may be 

needed. Lands disturbed by human activity often cause 

unwanted and long-lasting problems that affect other 

resources. Many of these disturbances obliterate soil 

profiles, exacerbate the invasion of exotic plants, result 

in contamination of water and soil, and cause erosion 

and sedimentation. These damages, in turn, frequently 

impair the quality of habitats, disrupt ecosystem 

functions, and cause problems for managing areas as 

wildlands. 

Prior to the park’s establishment, decades of livestock 

grazing, clearing of vegetation, and development by 

settlers created areas of disturbed lands (KellerLynn 

2006). These activities began in the early 1900s. By the 

1940s livestock were excluded from the 26- ha (64-ac) 

core of the park, and in June 1967 grazing ceased. 

Erosion was noticed shortly thereafter. The disturbed 

lands are especially susceptible to erosion because the 

naturally developed drainage has been altered and 

stabilizing vegetation has been removed.

The National Park Service has completed several DLR 

projects in Pecos National Historical Park. In 1978, small 

ravines throughout the park were planted with 

piñon/juniper brush in an attempt to halt erosion 

(National Park Service 1988). Disturbed lands along the 

lower Glorieta Creek were restored in 2000 (fig. 32; see 

“Lower Glorieta Creek Restoration Project” section). At 

the time of this report, park managers were seeking 

technical assistance from the Natural Resource 

Stewardship and Science (NRSS) Directorate to address 

the remaining disturbed lands and associated erosion 

issues in the park. 

A more recently developed area of disturbed lands 

involves wetlands along Glorieta Creek. Until 2011, flow 

in the creek was increased (albeit artificially) by well 

water discharge from the Glorieta Conference Center 

upstream from the Pecos Unit. After closure of the 

conference center, flow in Glorieta Creek was 

significantly reduced. Wetlands may be lost if the creek 

flow remains at the currently low level. Additionally, a

beaver dam approximately three m (9 ft) high at the 

northern part of the Creek is currently impeding the flow 

of water downstream (fig. 35) (Cheri Dorshak, written 

communication, 12 September 2014). 

Park resource managers are trying to determine what, if 

any, actions should be taken to restore the wetlands 

created by the previous “unnatural” flow levels in the 

creek (Pecos National Historical Park staff, conference

call, 11 April 2013). Historic photographs do not indicate 

Figure 35. Beaver dams on Glorieta Creek. A beaver dam at 

the northern part of the Glorieta Creek is currently 

impeding the downstream flow of water. National Park 

Service photographs by Mario Armijo (Pecos National 

Historical Park).
32 NPS Geologic Resources Division

that wetlands are the natural condition (Pecos National 

Historical Park staff, conference call, 11 April 2013). 

Flow may resume to the area without any action taken by 

the National Park Service; in 2014, the conference center 

was purchased by new owners and it is yet to be 

determined if discharge has increased (Cheri Dorshak, 

written communication, 12 September 2014). High 

rainfall events during 2014 have caused the creek to flow 

above and over the beaver dam and all the way into the 

Pecos River (Cheri Dorshak, written communication, 12 

September 2014). 

Lower Glorieta Creek Restoration Project

In the 1980s, landowners at the time converted 

abandoned sand and gravel quarries into two reservoirs 

on the north side of Glorieta Creek approximately 0.4 

km (0.25 mi) upstream from its confluence with the 

Pecos River (La Calandria Associates, Inc. 2007). The 

landowners constructed levees to hold water in the 

reservoirs, altering stream flow and sediment 

transportation in the creek (fig. 36). The NPS Water 

Resources Division completed a restoration project in 

2000 by removing levees, re-contouring the land, and 

planting native vegetation to create 2 ha (5 ac) of 

wetland-riparian habitat (fig. 37; La Calandria Associates, 

Inc. 2007). The restored riparian wetlands and 

woodlands have clearly benefited marsh-favoring species 

such as red-winged blackbird (Agelius phoenicus) and 

common yellowthroat (Geothlypys trichas) (fig. 32; 

Johnson et al. 2003). 

During the 2000 restoration project, a portion of the

levee surrounding the “upper reservoir” was left in place 

as a temporary measure against erosion (Wagner and 

Martin 2011). In the stretch of lower Glorieta Creek that 

was adjacent to this remnant levee, the channel was 

forced to stay within an artificially narrow meander 

between the levee and a high terrace to the south (fig. 31; 

Wagner and Martin 2011). The reduced sinuosity

increased the stream’s gradient in that area. However, 

water and sediment were able to flow through the 

channel and onto the floodplain without excessive 

erosion or deposition. Furthermore, the creek appeared

to function properly in large storms (Wagner and Martin 

2011). 

Staff at the NPS Water Resources Division and Colorado 

State University’s Public Lands History Center strongly 

recommended removal of the remaining levee because a

large flood could cause the creek to erode a new channel 

through the levee and then flow through the restored 

wetlands (Wagner and Martin 2011; National Park 

Service and Colorado State University 2011). Removal of 

the levee would better connect the restored riparianwetland area to the stream corridor, allow the riparian 

system to reach its full potential for habitat size, diversity, 

and quality, and eliminate the threat of excessive 

sediment deposition (Wagner and Martin 2011). Because 

of this recommendation, NPS staff removed the last 

portion of the levee in December 2013, and the area is 

undergoing revegetation (Cheri Dorshak, written 

communication, 12 September 2014). 

Figure 36. Lower Glorieta Creek, pre-restoration. Former 

landowners converted abandoned sand and gravel quarries 

into two reservoirs (red dashed lined) on the north side of 

Glorieta Creek (yellow dashed line) approximately 0.4 km 

(0.25 mi) upstream from its confluence with the Pecos River. 

Aerial image provided by Joel Wagner (NPS Water 

Resources Division).

Figure 37. Lower Glorieta Creek, during restoration. The NPS 

Water Resources Division completed a restoration project in 

2000 by removing levees, re-contouring the land, and 

planting native vegetation to create 2 ha (5 ac) of wetlandriparian habitat. National Park Service photograph taken in 

1999 by Joel Wagner (NPS Water Resources Division).

Abandoned Mineral Lands Hazards

Abandoned mineral lands (AML) are among the many 

types of disturbed lands in the National Park System. 

AML sites may be (1) underground and surface mines, 

(2) placer and dredge sites, (3) oil, gas, and geothermal 

wells, or (4) associated facilities.

AML features present a variety of resource management 

issues such as visitor and staff safety and environmental 

quality of air, water, and soil. AML features can also

provide habitat for bats and other animals. Resource 

management of AML features requires an accurate 

inventory and reporting. All AML features should be 

recorded in the AML database (the NPS Geologic 

Resources Division may be able to provide assistance). 

An accurate inventory can identify human safety hazards, 

and facilitate closures, reclamation, and restoration of 

AML features. When appropriate for resource 

management and visitor safety, AML features can also 

present opportunities for interpretation as cultural 

resources. 
PECO Geologic Resources Inventory Report 33

Baca Mine

According to the NPS AML database (accessed 5 June 

2014) and Burghardt et al. (2014), Pecos National 

Historical Park contains one AML site, the Baca mine. 

The Baca mine is also a cave (see “Caves and Associated 

Landscape Management” section). The Baca mine 

features an adit and a shaft, and was possibly a uranium 

or copper play (KellerLynn 2006). Park administrators

closed the area surrounding the Baca mine with fencing 

and signs to address public safety concerns (KellerLynn 

2006). 

Pecos Mine

The remains of the Pecos mine operations, though not 

within the park’s boundaries, still have the potential to 

affect water quality inside the park. Waste rock piles 

generated by the Pecos mine cover approximately 8 ha 

(19 ac) and are within 150 m (500 ft) of the east bank of 

the Pecos River, 26 km (16 mi) north of the park. In some 

places the piles are up to 14 m (45 ft) thick (Johnson and 

Deeds 1995). Waste rock piles are potential sources of 

acid mine drainage. The underlying clay and limestone 

bedrock may provide some buffer capacity for any acid

drainage and may also limit the transport of dissolved 

metals in groundwater (Johnson and Deeds 1995).

Wetlands may also be effective in buffering low pH and 

limiting the transport of dissolved metals. Nevertheless, 

low pH and elevated levels of cadmium, copper, lead, 

and zine have been measured in seeps downslope of the 

waste rock piles (Johnson and Deeds 1995). Elevated 

metal levels and low pH have also been measured in 

Willow Creek, which flows across a portion of the waste 

rock piles. The seeps and Willow Creek flow into the 

Pecos River and may be affecting its water quality. For 

example, fish die-offs at the Lisboa Springs State Fish 

Hatchery, 18 km (11 mi) downstream from the Pecos 

mine, may be related to reduced water quality in the 

Pecos River, the source of water for the hatchery (US 

Fish and Wildlife Service 1993).

Prior to 2002, some surface runoff was diverted away 

from the waste rock piles (Johnnie Green, Cyprus Amax 

Minerals Company, and Stephen Wust, New Mexico 

Environment Department, personal communication, 

cited in National Park Service 2005). The Pecos mine site 

was remediated in 2002 (Johnson et al. 2011). Although 

heavy metal concentrations have decreased postremediation, levels are still sufficiently high to warrant 

limits on fish consumption (Johnson et al. 2011). 

Mercury is the greatest concern; its concentration has 

not decreased appreciably since the remediation 

(Johnson et al. 2011). The Upper Pecos Watershed 

Association may be a helpful forum for communication 

among the National Park Service, the Pecos valley 

community, and the New Mexico Environment 

Department about monitoring and other aspects of the 

Pecos mine cleanup (La Calandria Associates, Inc. 2007). 

Slope Movement Hazards and Risks

Slope movements are a common type of geologic 

hazard—a natural or human-caused condition that may 

impact park resources, infrastructure, or visitor safety. 

Risk is the probability of occurrence combined with the 

expected degree of damage or loss that may result from 

exposure to a hazard, or the likelihood of a hazard 

causing losses (see Holmes et al. 2013). 

Scoping meeting and conference call participants 

suggested that maintained infrastructure in the park was 

not affected by slope movements (KellerLynn 2006; 

Pecos National Historical Park staff, conference call, 11 

April 2013). However, slumping, sliding, and rockfalls do 

occur along the river corridors and may be impacting 

backcountry areas and cultural resources (fig. 38; see

“Fluvial Erosion” section). Undercut banks are especially 

susceptible to slope failure (fig. 39). Some abandoned 

roads, such as Old Colonias Road (fig. 40), are sloughing 

away. However, these roads are not maintained and their 

loss is not a concern of resource managers (KellerLynn 

2006).

Figure 38. Rock slide into the Pecos River. Where limestone 

beds of the Alamitos Formation (PNal) dip towards the river, 

they are at risk of sliding or falling into the water as the 

river carves deeper through the strata. National Park Service

photograph by Paul Varela (Pecos National Historical Park).

Figure 39. Undercut banks on the eastern side of the Pecos 

River. Undercut banks are especially susceptible to slope 

failure. National Park Service photograph by Mario Armijo.
34 NPS Geologic Resources Division

Figure 40. Old Colonias Road. Old Colonias road sits atop 

limestone bedrock (Alamitos Formation). The road has 

sloughed away to the extent that it is barely recognizable 

as a roadway today. National Park Service photograph by 

Paul Varela (Pecos National Historical Park).

In the Geological Monitoring chapter about slope 

movements, Wieczorek and Snyder (2009) described five 

vital signs for understanding and monitoring slope 

movements: (1) types of landslide, (2) landslide causes 

and triggers, (3) geologic materials in landslides, (4) 

measurement of landslide movement, and (5) assessment 

of landslide hazards and risks. In addition, the following 

publication and websites provide detailed information 

about slope movements, monitoring, and mitigation 

options: Highland and Bobrowsky (2008), 

http://landslides.usgs.gov/ (accessed 17 February 2015), 

http://www.nature.nps.gov/geology/hazards/index.cfm

(accessed 17 February 2015), and 

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

monitoring/slopes.cfm (accessed 17 February 2015). 

Paleontological Resource Inventory, Monitoring, and 

Protection

All paleontological resources are non-renewable and 

subject to science-informed inventory, monitoring, 

protection, and interpretation as outlined by the 2009 

Paleontological Resources Preservation Act (see 

Appendix B). As of April 2015, regulations associated 

with the act were being developed. As described in the 

“Paleontological Resources” section, petrified wood is 

the only type of fossil currently documented within 

Pecos National Historical Park, although Sangre de 

Cristo and Madera formations have potential for fossil 

discoveries. 

A field-based paleontological resource survey can 

provide detailed, site-specific descriptions and resource 

management recommendations that are beyond the 

scope of this report. Although a park-specific survey has 

not yet been completed for Pecos National Historical 

Park, a brief summary of paleontological resources was 

completed by Koch and Santucci (2003). An updated 

summary report is currently being prepared (Vince 

Santucci, senior geologist, NPS Geologic Resources 

Division, email communication, 22 January 2015). A 

field-based survey would help determine the source of 

the petrified wood, as well as identify the variety of 

species and abundance of marine invertebrates in the 

Alamitos Formation and the presence of additional 

fossils in the remaining geologic units. The NPS Geologic 

Resources Division can provide assistance in conducting 

such a survey. 

In the Geological Monitoring chapter about 

paleontological resources, Santucci et al. (2009) 

described five methods and vital signs for monitoring in 

situ paleontological resources: (1) erosion (geologic 

factors), (2) erosion (climatic factors), (3) catastrophic 

geohazards, (4) hydrology/bathymetry, and (5) human 

access/public use.

Caves and Associated Landscape Management

Cave features are non-renewable resources. The Federal 

Cave Resources Protection Act of 1988 requires the 

identification of “significant caves” in NPS areas, the 

regulation or restriction of use as needed to protect cave 

resources, and inclusion of significant caves in land 

management planning. The act also imposes penalties for 

harming a cave or cave resources and exempts park 

managers from releasing specific location information 

for significant caves in response to a FOIA request (see 

Appendix B). A park-specific cave management plan has 

not yet been completed for Pecos National Historical 

Park. Such plans include a comprehensive evaluation of 

current and potential visitor use and activities, as well as 

a plan to study known and discover new caves. The NPS 

Geologic Resources Division can facilitate the 

development of such a plan.

Two caves, Baca Cave and the informally named, “small 

cave,” are documented in the park. Both are closed to 

protect visitors and resources. Archeological, 

paleontological, and biological studies related to the cave 

and karst resources at Pecos National Historical Park 

have not been conducted, although Soto (2013) 

summarized the caves and associated resources in the 

park. 

Park staff currently monitors the cave (Johnson et al. 

2011). Burger and Allison (2008) noted several potential 

hazards, including a rockfall hazard at the southern 

fissure in the cave and falling hazards on climbs into a 

small dome and a 9-m- (30-ft-) deep shaft. Burger and 

Allison (2008) recommended maintaining the closure 

and monitoring it annually for signs of unauthorized use. 

They also recommended that graffiti be removed and the 

cave surveyed for biological and archeological resources. 

Installing a gate may not be practical as described in the 

“Abandoned Mineral Lands” section. Development of a 

cave management plan is recommended.

In the Geological Monitoring chapter about caves and 

associated landscapes, Toomey (2009) described 

methods for inventorying and monitoring cave-related 

vital signs, including the following: (1) cave meteorology, 

such as microclimate and air composition; (2) airborne 

sedimentation, including dust and lint; (3) direct visitor 

impacts, such as breakage of cave formations, trail use in 

caves, graffiti, and artificial cave lighting; (4) permanent 

or seasonal ice; (5) cave drip and pool water, including 

drip locations, rate, volume, and water chemistry, pool 

microbiology, and temperature; (6) cave microbiology; 

(7) stability issues associated with breakdown, rockfall, 
PECO Geologic Resources Inventory Report 35

and partings; (8) mineral growth of speleothems, such as 

stalagmites and stalactites; (9) surface expressions and 

processes that link the surface and the cave environment, 

including springs, sinkholes, and cracks; (10) regional 

groundwater levels and quantity; and (11) fluvial 

processes, including underground streams and rivers.

Land Subsidence

Scoping meeting and conference call participants (see 

Appendix A) did not identify land subsidence as a 

geologic resource management issue. However, literature 

on geologic hazards in New Mexico, including Haneberg 

(1992), describes land subsidence as a potential hazard

within and near Pecos National Historical Park. 

Near Santa Rosa, approximately 145 km (90 mi) 

southeast of the park, Sweeting (1972) described 

subsidence relief attributed to the dissolution of 

limestone and gypsum in the Permian San Andres 

Formation (Psa) and the collapse of sandstone in the 

overlying Santa Rosa Formation (TRs, TRsu, TRsm, TRsl). 

Both of these formations are present at or near the 

surface in the Cañoncito Subunit of the park.

Another cause of land subsidence in the Pecos River 

valley in the last 5 million years (Pliocene Epoch; fig. 4) 

has been the dissolution of Permian halite deposits and 

collapse of overlying strata (Bachman 1974, 1984; 

Gustavson and Finley 1985; Osterkamp et al. 1987). 

However, the geologic descriptions of the Permian units 

in the GRI GIS data do not mention the presence of 

halite. 

Earthquake Hazards and Risks

Scoping meeting (see KellerLynn 2006) and conference

call participants did not identify any recent earthquakes 

in the area of the park. The potential for damaging 

earthquakes in the area is low (Sanford et al. 2006, 

Pursley et al. 2013). According to the US Geological 

Survey earthquake probability map (fig. 20; 

http://geohazards.usgs.gov/eqprob/2009/index.php;

accessed 3 June 2014), a magnitude 5.5 earthquake has a 

probability of 0.25–0.30 (25%–30% “chance”) of 

occurring near Pecos National Historical Park in the 

next 100 years.

In the Geological Monitoring chapter about earthquakes 

and seismic activity, Braile (2009) described the 

following methods and vital signs for understanding 

earthquakes and monitoring seismic activity: (1) 

monitoring earthquakes, (2) analysis and statistics of 

earthquake activity, (3) analysis of historical and 

prehistoric earthquake activity, (4) earthquake risk 

estimation, (5) geodetic monitoring and ground 

deformation, and (6) geomorphic and geologic 

indications of active tectonics.

External Energy or Mineral Development

Although mining has taken place in and near Pecos 

National Historical Park at both the Baca and Pecos 

mines (see “Abandoned Mineral Lands Hazards” 

section), current and future development is of limited 

concern to park resource managers. Based on regional 

seismic and exploration activities, the rock formations in 

the park do not appear to have commercially exploitable 

mineral deposits, and the various strata are not 

associated with oil and gas producing beds (National 

Park Service 1995). The federal government owns all of 

the mineral rights for lands within the park boundary 

that are under the jurisdiction of the National Park 

Service (Reed et al. 1999).

The NPS Geologic Resources Division is available to 

provide the park with policy and technical assistance 

regarding minerals and energy issues. Recommendations 

include remaining aware of public and private mineral 

ownership and speculation, exploration, or drilling 

activity on lands in the park’s vicinity. The NPS Geologic 

Resources Division Energy and Minerals website, 

http://nature.nps.gov/geology/minerals/index.cfm

(accessed 17 February 2015), provides additional 

information. 

Interpretation and Education

One of the interpretive themes in the park’s foundation 

statement (National Park Service 2009) relates the park 

story to the local geology: The natural features of the 

landscape, including the Pecos River and its tributaries,

established the backdrop against which people (past and 

present) adapt their survival strategies. Interpreters at the 

park often include geology information in their talks, 

such as during tours of the ruins.

During the 2006 scoping meeting, park staff discussed 

the possibility of a local geologist training interpreters 

and conducting field trips for visitors (KellerLynn 2006).

Since the scoping meeting several local geologists have 

visited the park and worked with the staff (Christine 

Beekman, Pecos National Historical Park, chief of 

interpretation, personal communication, cited in 

Johnson et al. 2011, p. 47). Virgil Lueth of New Mexico 

Bureau of Geology and Mineral Resources has led one 

geology tour for his staff. Jennifer Lindline, geology 

professor at New Mexico Highlands University, 

presented a poster on park geology at the 2007 Pecos 

Conference. She has also led several field trips in the

park that included park staff. A few years ago the park 

offered a “Geology and Human Environment” tour. 

Since the 2006 scoping meeting, the New Mexico Bureau 

of Geology and Mineral Resources published The 

Geology of Northern New Mexico’s Parks, Monuments, 

and Public Lands (Price 2010), which may be a useful 

reference for park staff. Park staff should continue to 

work with local geologists and could request 

interpretation/education/outreach program assistance 

from the NPS Geologic Resources Division. For 

example, a Geoscientists-In-the-Parks (GIP) intern could 

assist with development of geology themed 

interpretation, education, and outreach products. The 

GIP website, http://nature.nps.gov/geology/

gip/index.cfm (accessed 10 April 2015), provides 

additional information 
PECO Geologic Resources Inventory Report 37

Geologic History

This section describes the chronology of geologic events that formed the present landscape of 

Pecos National Historical Park. 

The landscape at Pecos National Historical Park is an 

expression of hundreds of millions of years of geologic 

history and processes that continue to shape the land 

today. The rocks mapped in the park and surrounding 

area can be compiled into four groups, from oldest to 

youngest: (1) Precambrian igneous and metamorphic 

rocks more than 1.6 billion years old, (2) Paleozoic and 

early Mesozoic sedimentary rocks between about 350 

million and 200 million years old, (3) Tertiary gravels 

about 5 million years old, and (4) surficial units less than 

2 million years old. The geologic history of these rock 

units includes development of ancient basement rock 

and structure foundation, the rise and fall of the 

Ancestral Rocky Mountains, the assembly of the 

supercontinent Pangaea, subsequent uplift, erosion, and 

the emergence of the modern landscape. 

2,500 million–542 million years ago (Proterozoic Eon):

Development of the Structural Foundation

The oldest rocks in Pecos National Historical Park and 

surrounding area are more than 1.6 billion years old. 

These Proterozoic igneous and metamorphic rocks form 

the “basement” under all younger rocks. These ancient 

basement rocks are exposed at the surface where 

movement along faults brought them up and millions of 

years of erosion removed overlying rocks. Proterozoic 

rocks are mapped at the surface primarily in the 

northwest corner of the map area, which not 

coincidentally, is cut by many faults. A small band of 

granite and granitic gneiss (Xgg) is mapped in the 

Cañoncito Subunit (see poster, in pocket). 

The older rocks are primarily metamorphic granitic 

gneiss (Xgg), schist and phyllite (Xqms, Xf, Xbp, and Xms),

Figure 41. Mississippian Period paleogeographic maps. Western North America was inundated by a shallow sea 340 million 

years ago (left graphic). Dolomites and limestones were deposited in the area around Pecos National Historical Park 

(location marked by a star). Toward the end of the Mississippian Period (325 million years ago, right graphic) sea level fell, 

exposing the marine sedimentary rocks. Erosion ensued, which left only a thin sequence of limestone and dolomite behind. 

Throughout this time, the supercontinent Pangaea was coming together as the future South American landmass collided 

with the southern margin of North America (converging landmasses shown with white arrows). Base paleogeographic map

by Ron Blakey (Colorado Plateau Geosystems, Inc.), available at http://cpgeosystems.com/index.html (accessed 14 July 

2013), annotated by Rebecca Port (NPS Geologic Resources Division). 
38 NPS Geologic Resources Division

and amphibolite (Xa). These rocks originated as granite, 

shale, volcanic rocks, and amphibole that were changed 

under great heat and pressure, and in the case of 

amphibole, also recrystallized. As is common with 

basement rocks, their total thickness is unknown but 

may be more than 3,000 m (10,000 ft) (Baltz and 

Bachman 1956). Younger Proterozoic granite (Xg and 

Xmg), granodiorite (Xgr), and diorite (Xd) intruded into 

the older metamorphic rocks to form sills, dikes, and 

larger “bodies” of igneous rock (Baltz and Bachman 

1956).

The ancient rocks were originally deposited in a 

sedimentary basin where moderately deep seas created 

thick accumulations of clay (shale) (Clark 1966). Igneous 

rocks (granite) interlayered with the shale are indicative 

of periods of tectonic activity (Baltz and Bachman 1956; 

Clark 1966). Later in Proterozoic Eon, compressive 

forces metamorphosed and tightly folded the shale and 

granite (Baltz and Bachman 1956; Clark 1966). 

542 million–252 million years ago (Paleozoic Era): Rise 

and Fall of the Ancestral Rocky Mountains

Little is known about the early Paleozoic geologic history

of north-central New Mexico. Early Paleozoic rocks 

(Devonian and older; fig. 4) are not present in the region 

(Baltz and Bachman 1956). During much of this time the 

area was elevated and deposition did not occur. Of the 

few rocks that were deposited, weathering and erosion 

later stripped them from the land creating an 

unconformity in the stratigraphic record. An 

unconformity is a horizon in the rock record separating 

strata of different ages. Unconformities are gaps in the 

rock record and strong evidence for uplift and erosion. 

Because of this period of erosion, late Paleozoic rocks 

(Missippian and younger) lie directly on Proterozoic

rocks in most areas of the Sangre de Cristo Mountains

(Lindsey 2010).

359 million–323 million years ago (Mississippian Period) 

Approximately 340 million years ago, the landscape was 

subdued and seas inundated the entire southwestern half 

of North America (fig. 41). The supercontinent Pangaea 

was just beginning to form as ancestral North America 

moved east toward Europe and Africa. During this time 

period limestone and dolomites were deposited in a sea 

that covered much of New Mexico, including the area of 

Pecos National Historical Park.

Toward the end of the Mississippian Period, an interval 

of lowered sea level exposed the marine sedimentary 

rocks (fig. 41). Erosion left only a thin sequence of 

limestone and dolomite behind. Mississippian rocks (Mt

and Mes) are mapped at the surface in the southwest 

corner of the extent of the GRI GIS data, which is 

outside of the park. Younger rocks cover Mississippian 

strata in most places on the map.

323 million–299 million years ago (Pennsylvanian Period) 

About 320 million years ago at the beginning of the 

Pennsylvanian Period, the Ancestral Rocky Mountains 

Figure 42. Pennsylvanian Period paleogeographic map. The 

Ancestral Rocky Mountains (Frontrangia and Uncompahgria) 

rose during the Pennsylvanian Period. At this time, the area 

of Pecos National Historical Park (location marked by a star) 

was situated on the southern edge of the Rowe-Mora basin 

(red dashed line). Both offshore-derived (calcareous ooze) 

and land-derived (clastic) sediments accumulated in the 

basin. Some of these sediments formed the limestone, 

shale, and sandstone of the Alamitos Formation (PNal) 

which underlies the eastern portion of the Pecos Unit of the 

park. Base paleogeographic map by Ron Blakey (Colorado 

Plateau Geosystems, Inc.), available at 

http://cpgeosystems.com/index.html (accessed 14 July 

2013), annotated by Rebecca Port (NPS Geologic Resources 

Division). 

began to rise as a result of the coming together of the 

supercontinent Pangaea. The ancestral Rockies consisted 

largely of Precambrian metamorphic rock, forced 

upward through layers of Mississippian limestone and 

dolomites. The uplift created two large ranges—

Frontrangia and Uncompahgria—located roughly in the 

current locations of the Front Range and the San Juan 

Mountains (fig. 42). 

The area now occupied by Pecos National Historical 

Park was near the southern edge of a depression—the 

Rowe-Mora basin or Taos trough—which occupied the 

valley between the southern ends of the neighboring 

ancestral ranges (fig. 42). A basin is a structural 

depression that accommodates the accumulation of 

sediment. The Rowe-Mora basin developed as a result of 

east–west compression (Baltz and Bachman 1956) and 

was covered in a shallow sea throughout most of 

Pennsylvanian Period (fig. 42; Kues 2001). Interbedded 

layers of offshore–derived calcareous ooze and 

mountain–derived clastic sediments built up in the basin

and formed thick deposits of sedimentary rock (Clark 

1966). Occasional retreat of the sea produced thin layers 

of deltaic shales and terrigenous (land-derived) 

mudstones and sandstones (Kues 1984).
PECO Geologic Resources Inventory Report 39

Figure 43. Permian Period paleogeographic map. By the end 

of the Permian Period, the Ancestral Rocky Mountains were 

largely word down by erosion. Land-derived sand and mud 

were deposited in the area of the park (location marked by 

a star). These sediments formed the sandstone and 

mudstone of the Sangre de Cristo (PPNsc), Yeso (Py), 

Glorieta (Pg), and Artesia (Pa) formations. During an interval 

of relatively higher sea level calcareous sediments were 

deposited to form the limestone of the San Andres 

Formation (Psa).. Base paleogeographic map by Ron Blakey 

(Colorado Plateau Geosystems, Inc.), which is available at 

http://cpgeosystems.com/index.html (accessed 14 July 

2013), annotated by Rebecca Port (NPS Geologic Resources 

Division).

The Alamitos Formation (PNal) is the only unit entirely 

from the Pennsylvanian Period within the park. It covers 

the eastern half of the Pecos Unit (see poster, in pocket). 

The formation consists of primarily calcareous ooze, and 

lesser amounts of mud and sand deposited in a shallow

sea, which compacted and cemented into limestone, 

shale, and sandstone, respectively. The limestone is

known to contain fossils in exposures outside of the park

(Koch and Santucci 2003). 

Toward the end of the Pennsylvanian Period the sea 

began a gradual regression. Sediments deposited in the 

basin shifted from largely offshore-derived to landderived. This transition is illustrated in the sediments of 

the Sangre de Cristo Formation (PPnsc). 

299 million–252 million years ago (Permian Period) 

The beginning of the Permian Period marks the 

completion of the transition from shallow marine to 

terrestrial deposition. The sea receded and the Ancestral 

Rocky Mountains continued to erode, filling the RoweMora basin with sediments (fig. 43; Clark 1966). During 

its lifetime, the Rowe-Mora basin accumulated between 

about 300 and 900 m (1,000 and 3,000 ft) of Paleozoic 

sediments in the area of Pecos National Historical Park

(Baltz and Bachman 1956). 

Sediments of the Sangre de Cristo Formation which 

began accumulating in the Pennsylvanian continue to 

accumulate into early Permian time. The rocks formed 

are predominantly non-marine mudstone and sandstone 

with the occasional thin layer of limestone. The 

mudstone and sandstone are the result of a migrating, 

meandering stream system; the red clay represents fluvial 

deposition, and the white sandstone represents deltas 

and beaches (KellerLynn 2006).

Today, the Sangre de Cristo Formation underlies the 

western half of the Pecos Unit and the entire Pigeon’s 

Ranch Subunit (see poster, in pocket). The pueblo and 

mission were built on a low ridge of red, maroon, and 

purple mudstones and tan-to-red sandstones of this 

formation (Johnson et al. 2011).

Throughout the rest of the Permian Period, sediments 

accumulated forming, from oldest (topographically 

lowest) to youngest (topographically highest), the Yeso 

Formation (Py), the Glorieta sandstone (Pg), the San 

Andres Formation (Psa), and the Artesia Formation (Pa). 

The units comprise much of Glorieta Mesa and are 

exposed along the base and sides of the mesa. Within the 

park these units are only mapped in the Cañoncito 

Subunit. They are primarily sandstone, with some 

mudstone, siltstone, and thin limestone layers, which 

represent periodic transgressions of the sea.

The Yeso Formation consists of mud and sand that

accumulated in an intertidal environment, perhaps a 

coastal plain (Johnson 1969; Baars 1974). During a period 

of rising sea level, waves eroded some of the Yeso 

Formation producing quartz sand that was redeposited 

just offshore to become the Glorieta Sandstone (Pg;

Johnson 1969). The high concentration of resilient 

quartz and lack of less stable minerals in the Glorieta 

Sandstone indicates that the sediments were recycled 

multiple times. The high concentration of quartz made

the Glorieta Sandstone resistant to erosion and it 

therefore forms ledges on the mesa.

Following the deposition of the Glorieta Sandstone, 

Permian seas continued to advance and the marine 

limestone of the San Andres Formation (Psa) 

accumulated in slightly deeper water. A final regression 

of the sea occurred toward the end of the Permian 

Period resulting in the deposition of the Artesia 

Formation (Pa)—a quartz sandstone similar in 

composition to the Glorieta Sandstone. The Artesia 

Formation contains some gypsum, a mineral precipitated 

when seawater evaporates. Ripple marks are also present, 

indicating deposition in a coastal environment. Like the 

Glorieta Sandstone, the high quartz concentraiton of the 

Artesia Formation makes it resistant to erosion, and 

today it is a cliff former on Glorieta Mesa. 

By the end of the Permian Period, the Ancestral Rockies 

were largely worn down by erosion and neighboring 

basins had filled with sediments shed from the eroding 

highlands (Lawton 1994; Lindsey 2010).
40 NPS Geologic Resources Division

252 million–66 million years ago (Mesozoic Era):

Sedimentary Basins

During the Mesozoic in the Rocky Mountain region, 

subsidence events triggered by tectonic plate collisions 

farther to the west created a succession of sedimentary 

basins (Baltz and Bachman 1956; Lawton 1994). 

Throughout the era, 1,200 to 1,500 m (4,000 to 5,000 ft) 

of sediments accumulated in these basins (Baltz and 

Bachman 1956). Most of these sediments were deposited 

during the Late Cretaceous Period. They are not 

preserved near the park today (Baltz and Bachman 1956). 

The stratigraphic record shows unconformities—periods 

of non-deposition and/or erosion—separating overlying 

basinal deposits (Lawton 1994).

Mesozoic strata in the extent of the digital geologic data 

are all from the Triassic Period (252 million–201 million 

years ago). They are found capping Glorieta Mesa. 

Within the park, Mesozoic rocks are only mapped in the 

Cañoncito Subunit (TRm, TRsl, TRsm, TRsu, TRs, TRc).

During the Early Triassic, the red sandstone of the 

Moenkopi Formation (TRm) was deposited by rivers and 

in lakes. This unit is separated from the underlying 

Artesia Formation (Pa) by an unconformity. Rocks from 

the Middle Triassic Period are missing on Glorieta Mesa. 

Johnson (1969) deduced that sedimentation did not 

occur during Early Triassic time in northern New 

Mexico and older rocks were slightly eroded.

Later in the Triassic Period, seas advanced over the land 

in the area around northern New Mexico and Pecos 

National Historical Park. Layers of sand derived from 

the weathering of older rocks (TRm) accumulated just 

offshore in shallow water and would eventually form the 

rocks of the Santa Rosa Formation (TRsl, TRsm, TRsu, TRs; 

Clark 1966; Johnson 1969).

Continental deposits accumulated in the Rocky 

Mountain region throughout much of the Jurassic Period 

(201 million–145 million years ago), though rock units of 

this age are not found in the vicinity of the park (Johnson 

1969). Baltz and Bachman (1956) report evidence of 

some gentle folding in the Late Jurassic Period. 

During the Cretaceous Period (145 million–66 million 

years ago), the active basin deepened considerably as 

Pangaea broke apart. Sea water inundated North 

America, creating the Western Interior Seaway that

connected the Arctic Ocean to the newly formed Gulf of 

Mexico (fig. 44). Thick shales and limestones were 

deposited in New Mexico; however, these layers were 

removed by erosion in the area around the park. 

Withdrawal of the Western Interior Seaway at the end of 

the Cretaceous Period gave way to the most recent major 

mountain building event in the region—the Laramide 

Orogeny (Clark 1966).

The past 66 million years (Cenozoic Era): Mountain 

Building, Erosion, and Emergence of the Modern 

Landscape

“Laramide Orogeny” is the name given to the processes 

that formed the Rocky Mountains, starting about 70 

Figure 44. Cretaceous Period paleogeographic map. Toward 

the end of the Mesozoic Era, Pangaea had separated into 

the landmasses recognizable as modern-day continents. The 

Atlantic Ocean basin had opened considerably (diverging 

plates shown with white arrows), allowing a sea to bisect

North America. The Western Interior Seaway connected the 

Arctic Ocean to the Gulf of Mexico. Thick shales and 

limestones were deposited near the park (location marked 

by a star). These layers were later removed by erosion. 

Graphic annotated by Rebecca Port (NPS Geologic Resources 

Division). Base paleogeographic map created by Ron Blakey 

(Colorado Plateau Geosystems, Inc.), which is available at 

http://cpgeosystems.com/index.html (accessed 14 July 

2013).

million years ago and ending roughly 40 million years ago 

(Late Cretaceous and early Cenozoic time; fig. 4) . The 

Sangre de Cristo Mountains were uplifted as part of the 

Rocky Mountains during this time. Flat slab 

subduction—a shallowly subducting oceanic plate—

roughly 1,100 km (700 mi) to the west caused the rise of 

the region much farther inland than would normally be 

expected for a mountain range associated with a 

subduction zone (fig. 45).

Laramide mountain building raised Precambrian 

crystalline rocks from great depths that now make up the 

core of the Sangre de Cristo Mountains. In the process, 

overlying sedimentary Paleozoic strata (PNal, PPNsc, Py, 

Pg, Psa, Pa) was also raised and, in the area of the park, 

tilted toward today’s location of Glorieta Mesa. In the 

Pecos Unit strata are inclined to the southwest; in the 

Pigeon’s Ranch Subunit strata dip to south; in the 

Canoncito Subunit strata dip primarily to the southeast 

except where they have been disrupted by faults. The tilt 

in all locations of the Paleozoic strata is steepest near the 

high mountains to the north and gradually decreases 

toward Glorieta Mesa where it becomes almost 

unnoticeable (Johnson 1969). These young sedimentary 

rocks have long been eroded away from the crest of the 

mountain range, but they are still present in the foothills 

(PNal and PPNsc).
PECO Geologic Resources Inventory Report 41

Figure 45. Cenozoic Era paleogeographic map. By 50 million 

years ago, the Laramide Orogeny had raised the modern 

Rocky Mountains, including the Sangre de Cristo Mountains. 

Today, Pecos National Historical Park (location marked by a 

star) is on the southern foothills of the range. Base 

paleogeographic map by Ron Blakey (Colorado Plateau 

Geosystems, Inc.), available at http://cpgeosystems.com/

index.html (accessed 14 July 2013), annotated by Rebecca 

Port (NPS Geologic Resources Division).

Faults and folds of the Laramide Orogeny can be seen 

throughout the Sangre de Cristo Mountains (Lindsey 

2010). The faults formed by compression and uplift 

associated with the converging tectonic plates to the 

west. Folding and complex thrusting in an east–west or 

northeast–southwest direction affected the rocks 

mapped in the park (Baltz and Bachman 1956). The 

basement complex in the Sangre de Cristo Mountains 

was mildly deformed, creating folds (Baltz and Bachman 

1956), which are included in the GRI GIS data. Folding 

and thrusting to the east continued until about 35 

million–30 million years ago into the late Eocene and 

early Oligocene epochs (fig. 4; Clark 1966).

Lava flows and intrusive igneous features that 

accompanied Laramide mountain building are common

in the Sangre de Cristo Mountains but are absent in the 

area around the park (Johnson 1969).

After mountain building commenced, nearly 40 million 

years of erosion shaped the land into currently 

recognizable features such as the Pecos River valley and 

Glorieta Mesa. Gravels (Tgv and QTg) derived from the 

Precambrian core of the Sangre de Cristo Mountains 

were deposited south of the mountains from about 23 

million to 2 million years ago, that is, throughout the 

Neogene and early Quaternary periods. The Pecos River 

and Glorieta Creek established their valleys during this 

time. Underlying strata controlled the paths the rivers 

took. Softer rocks (PNal and PPNsc) were preferentially 

eroded becoming valleys, while harder rocks (Pg and Pa) 

resisted erosion and now stand out as cliffs and ledges.
PECO Geologic Resources Inventory Report 43

Geologic Map Data 

This section summarizes the geologic map data available for Pecos National Historical 

Park. The Geologic Map Poster (in pocket) displays the map data draped over imagery of 

the park and surrounding area. The Map Unit Properties Table (in pocket) summarizes this 

report’s content for each geologic map unit. Complete GIS data are included on the 

accompanying CD and are also available at the GRI publications website: 

http://go.nps.gov/gripubs (accessed 17 February 2015). 

Geologic Maps

Geologic maps facilitate an understanding of an area’s 

geologic framework and the evolution of its present 

landscape. Using designated colors and symbols, these

maps portray the spatial distribution and temporal 

relationships of rocks and unconsolidated deposits. 

Geologic maps can be divided into two primary types: 

surficial and bedrock. Surficial geologic maps typically 

encompass deposits that are unconsolidated and formed 

during the past 2.6 million years (Quaternary). Surficial 

map units are differentiated by geologic process or 

depositional environment. Bedrock geologic maps 

encompass older, typically more consolidated 

sedimentary, metamorphic, and/or igneous rocks. 

Bedrock map units are differentiated based on age 

and/or rock type.

Geologic maps often depict geomorphic features, 

structural interpretations (such as faults or folds), and 

locations of past geologic hazards that may be 

susceptible to future activity. Anthropogenic features 

such as mines or quarries, as well as observation or 

collection locations, may be indicated on geologic maps. 

The American Geosciences Institute website, 

http://www.americangeosciences.org/environment/publi

cations/mapping (accessed 17 February 2015), provides 

more information about geologic maps and their uses. 

Source Maps

The GRI team digitizes paper maps and converts digital 

data to conform to the GRI GIS data model. The GRI 

digital geologic map product includes essential elements 

of the source maps such as map unit descriptions, a 

correlation chart of units, a map legend, map notes, 

references, and figures. The GRI team used the following 

sources to produce the digital geologic data set for Pecos 

National Historical Park. These sources also provided 

information for this report.

Ilg, B., P. W. Bauer, S. Ralser, J. Rogers, and S. Kelley. 

1997. Preliminary geologic map of the Glorieta 7.5-min 

quadrangle, Santa Fe County, New Mexico (scale 

1:12,000). Open-File Geologic Map OF-GM-11. New 

Mexico Bureau of Geology and Mineral Resources, 

Socorro, New Mexico. 

Read, A., and G. Rawling. 2002. Preliminary geologic 

map of the Pecos 7.5-min quadrangle, San Miguel and 

Santa Fe counties, New Mexico (scale 1:24,000). 

Open-File Geologic Map OF-GM-52. New Mexico 

Bureau of Geology and Mineral Resources, Socorro, 

New Mexico.

GRI GIS Data

The GRI team implements a GIS data model that 

standardizes map deliverables. The data model is 

included on the enclosed CD and is also available at 

http://science.nature.nps.gov/im/inventory/geology/Geo

logyGISDataModel.cfm (accessed 17 February 2015). 

This data model dictates GIS data structure, including 

layer architecture, feature attribution, and relationships 

within ESRI ArcGIS software. The GRI team digitized 

the data for Pecos National Historical Park using data 

model version 2.1. The GRI Geologic Maps website, 

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

.cfm (accessed 17 February 2015), provides more 

information about GRI map products. 

GRI digital geologic data are included on the attached 

CD and are available through the NPS Integrated 

Resource Management Applications (IRMA) portal 

(https://irma.nps.gov/App/Reference/Search?SearchTyp

e=Q; accessed 17 February 2015). Enter “GRI” as the 

search text and select a park from the unit list.

The following components are part of the data set:

• A GIS readme file (peco_gis_readme.pdf) that 

describes the GRI data formats, naming conventions, 

extraction instructions, use constraints, and contact 

information

• Data in ESRI geodatabase GIS format

• Layer files with feature symbology (table 3) 

• Federal Geographic Data Committee (FGDC)–

compliant metadata

• An ancillary map information document 

(peco_geology.pdf) that contains information 

captured from source maps such as map unit 

descriptions, geologic unit correlation tables, legends, 

cross-sections, and figures. 

• An ESRI map document (peco_geology.mxd) that 

displays the digital geologic data
44 NPS Geologic Resources Division

Table 3. Geology data layers in the Pecos National Historical Park GRI GIS data.

Data Layer On Poster? 

Geologic Cross Section Lines No

Geologic Attitude and Observation Localities No

Alteration and Metamorphic Feature Points No

Geologic Point Features (springs) No

Mine Point Features No

Mine Feature Lines No

Geologic Line Features (arkose beds and calcite veins) Yes

Map Symbology Yes

Folds Yes

Faults Yes

Mine Area Feature Boundaries No

Mine Area Features No

Geologic Contacts Yes

Geologic Units Yes

Geologic Map Poster

The Geologic Map Poster displays the GRI digital 

geologic data draped over an aerial photograph of the 

park and surrounding area. Not all GIS feature classes 

are included on the poster (table 3). Geographic 

information and selected park features have been added 

to the graphic. Digital elevation data and added 

geographic information are not included in the GRI GIS 

data set, but are available online from a variety of 

sources. Contact GRI for assistance locating these data.

Map Unit Properties Table

The Map Unit Properties Table lists the geologic time 

division, symbol, and a simplified description for each of 

the geologic map units in the GRI GIS data. Following 

the structure of the report, the table summarizes the 

geologic features, processes, resource management 

issues, and history associated with each map unit. 

Use Constraints

Graphic and written information provided in this report

is not a substitute for site-specific investigations. 

Ground-disturbing activities should neither be permitted 

nor denied based upon the information provided here.

Please contact GRI with any questions.

Minor inaccuracies may exist regarding the locations of 

geologic features relative to other geologic or geographic 

features on the Geologic Map Poster. Based on the 

source maps scales and US National Map Accuracy 

Standards, geologic features represented in the geologic 

map data are horizontally within 6 m (20 ft) on the 

1:12,000-scale source map (Ilg et al. 1997) or 12 m (40 ft) 

on the 1:24,000-scale source map (Read and Rawling

2002) of their true locations. 
PECO Geologic Resources Inventory Report 45

Glossary 

This section contains brief definitions of selected geologic terms relevant to this report. 

Definitions are based on those in the American Geosciences Institute Glossary of Geology

(5th edition; 2005). Additional terms are defined at

http://geomaps.wr.usgs.gov/parks/misc/glossarya.html (accessed 17 February 2015).

active margin. A tectonically active plate boundary

where lithospheric plates are converging, diverging, or 

sliding past one another. Compare with “passive 

margin.”

adit. A horizontal passage from the surface into a mine.

alluvium. Stream-deposited sediment.

amphibolite. A rock consisting mainly of amphibole and 

plagioclase minerals with little to no quartz.

anticline. A fold, generally convex upward (“A”-shaped) 

whose core contains the stratigraphically older rocks. 

Compare with “syncline.”

arkose. A commonly coarse-grained, pink or reddish 

sandstone consisting of abundant feldspar minerals.

arroyo. A small, deep, flat-floored channel or gully of an 

ephemeral or intermittent stream in the arid and 

semiarid regions of the southwestern United States.

axis. A straight-line approximation of the trend of a fold

along the boundary between its two limbs. “Hinge 

line” is a preferred term.

basement. The undifferentiated rocks, commonly 

igneous and metamorphic, that underlie rocks 

exposed at the surface, commonly sedimentary. In 

many regions the basement is of Precambrian age, but 

it may be much younger. Also, Earth’s crust below 

sedimentary deposits that extends down to the 

Mohorovicic discontinuity.

basin (sedimentary). Any depression, from continental to 

local scale, into which sediments are deposited.

bedding. Depositional layering or stratification of 

sediments.

bedrock. Solid rock that underlies unconsolidated, 

superficial material and soil.

biotite. A generally dark colored rock-forming mineral of 

the mica group.

braided stream. A sediment-clogged stream that forms 

multiple channels that divide and rejoin.

calcareous. Describes a substance that contains calcium 

carbonate. When applied to a rock name it implies that 

as much as 50% of the rock is calcium carbonate.

calcite. A carbonate (carbon + oxygen) mineral of 

calcium, CaCO3; calcium carbonate. It is the most 

abundant cave mineral.

carbonate. A mineral group composed of carbon and 

oxygen plus an element or elements; for example 

calcite, CaCO3; and dolomite, CaMg(CO3)2. 

cement (sedimentary). Mineral material, usually 

chemically precipitated, that occurs in the spaces 

among the individual grains of sedimentary rocks, thus

binding the grains together.

chalcedony. A cryptocrystalline variety of quartz.

chert. An extremely hard sedimentary rock with 

conchoidal fracturing, consisting mostly of 

interlocking crystals of quartz. 

chronology. The arrangement of events in their proper 

sequence in time.

clast. An individual constituent, grain, or fragment of a 

rock or unconsolidated deposit, produced by the 

mechanical or chemical disintegration of a larger rock 

mass.

clastic. Describes rocks or sediments made of fragments 

of preexisting rocks. 

coastal plain. Any lowland area bordering a sea or ocean, 

extending inland to the nearest elevated land, and 

sloping very gently seaward; may result from the 

accumulation of material along a coast.

colluvium. A loose, heterogeneous, and incoherent mass 

of rock fragments and soil material deposited via 

surface runoff or slow continuous downslope creep; 

usually collects at the base of a slope or hillside, but 

includes loose material covering hillsides.

compaction. The process whereby fine-grained sediment 

is converted to consolidated rock.

compression. A type of deformation where Earth’s crust 

is pulled apart.

conglomerate. A coarse-grained, generally unsorted, 

sedimentary rock consisting of cemented, rounded 

clasts larger than 2 mm (0.08 in) in diameter.

contact. The surface between two types or ages of rocks. 

convergent plate boundary. A boundary between two 

plates that are moving toward each other. Essentially 

synonymous with “subduction zone” but used in 

different contexts. 

cross-bed. A single bed, inclined at an angle to the main 

planes of stratification; the term is commonly 

restricted to a bed that is more than 1 cm (0.4 in) thick.

cross-bedding. Uniform to highly varied sets of inclined 

beds deposited by wind or water that indicate flow 

conditions such as direction and depth. 

cross section. A graphic interpretation of geology, 

structure, or stratigraphy based on mapped and 

measured geologic extents and attitudes, depicted in a 

vertical plane (i.e., a cut or profile view). 

cuesta. A hill or ridge with a gentle slope on one side and 

a steep slope on the other.

cutbank. A steep, bare slope formed by lateral erosion of 

a stream.

deformation. The process of folding, faulting, shearing, 

or fabric development in rocks as a result of Earth 

stresses.

delta. The low, nearly flat, alluvial tract of land at or near 

the mouth of a river, commonly forming a triangular or 

fan-shaped plain of considerable area; resulting from 
46 NPS Geologic Resources Division

the accumulation of sediment supplied by the river in 

such quantities that it is not removed by tides, waves, 

and currents.

diabase. An intrusive igneous rock consisting primarily 

of the minerals labradorite and pyroxene.

differential erosion. Erosion that occurs at irregular or 

varying rates due to differences in the resistance and 

hardness of surface material: softer and weaker rocks 

are rapidly worn away, whereas harder and more 

resistant rocks remain to form ridges, hills, or 

mountains.

dike. A narrow igneous intrusion that cuts across 

bedding planes or other geologic structures.

diorite. A coarse-grained, intrusive (plutonic) igneous 

rock characteristically containing plagioclase, as well 

as dark-colored amphibole (especially hornblende), 

pyroxene, and sometimes a small amount of quartz; 

diorite grades into monzodiorite with the addition of 

alkali feldspar. 

dip. The angle between a bed or other geologic surface 

and the horizontal plane. 

dip-slip fault. A fault with measurable offset where the 

relative movement is parallel to the dip of the fault. 

discharge. The rate of flow of surface water or 

groundwater at a given moment, expressed as volume 

per unit of time.

disconformity. An unconformity in which the bedding 

planes above and below are parallel and usually 

separated by an uneven erosion surface indicating an 

interruption in the orderly sequence of sedimentary 

strata.

displacement. The relative movement of the two sides of 

a fault; also, the specific amount of such movement. 

divergent plate boundary. A boundary between two 

plates that are moving apart, characterized by midocean ridges at which sea-floor spreading occurs. 

dolomite (rock). A carbonate sedimentary rock 

containing more than 50% of the mineral dolomite 

(calcium-magnesium carbonate).

downcutting. Stream erosion in which cutting is directed 

primarily downward, as opposed to laterally. 

downwarping. Subsidence of Earth’s crust on a regional 

scale as a result of crustal loading by ice, water, 

sediments, or lava flows. Compare to “upwarping.”

drainage. The manner in which the waters of an area 

flow off in surface streams or subsurface conduits; 

also, the processes of surface drainage of water from 

an area by streamflow and sheet flow, and the removal 

of excess water from soil by downward flow. 

drainage basin. The total area from which a stream 

system receives or drains precipitation runoff.

emergence. A change in the levels of water and land such 

that the land is relatively higher and areas formerly 

under water are exposed; results from either an uplift 

of land or fall of water level. Compare to 

“submergence.” 

erosion. The general process or group of processes that 

loosen, dissolve, wear away, and simultaneously move 

from one place to another, the materials of Earth’s 

crust; includes weathering, solution, abrasive actions, 

and transportation, but usually excludes slope 

movements. 

escarpment. A steep cliff or topographic step resulting 

from vertical displacement on a fault or as a result of 

slope movement or erosion. Synonymous with

“scarp.”

eustatic. Describes a worldwide rise or fall in sea level.

evaporite. A sedimentary rock composed primarily of 

minerals produced from a saline solution as a result of 

extensive or total evaporation of the solvent (usually 

water).

extension. A type of deformation where Earth’s crust is 

pulled apart. 

extrusive. Describes igneous rock that has been erupted 

onto the surface of the Earth. 

fault. A break in rock characterized by displacement of 

one side relative to the other. 

felsic. Derived from feldspar +silica to describe an 

igneous rock having abundant light-colored minerals 

such as quartz, feldspars, or muscovite; also, describes 

those minerals. 

fissile. Capable of being easily split along closely spaced 

planes. 

flagstone. A hard, fine grained, often micaceous 

sandstone that occurs in extensive thin beds and splits 

uniformly along bedding planes for use in terrace 

floors, retaining walls, and the like.

flat slab subduction. Refers to the subduction of one 

tectonic plate beneath another at a relatively shallow 

angle. 

flint. The homogeneous, dark-gray or black variety of 

chert.

floodplain. The surface or strip of relatively smooth land 

composed of alluvium and adjacent to a river channel, 

constructed by the present river in its existing regimen 

and covered with water when the river overflows its 

banks. A river has one floodplain and may have one or 

more terraces representing abandoned floodplains.

fluvial. Of or pertaining to a river or rivers. 

fold. A curve or bend in an originally flat structure, such 

as a rock stratum, bedding plane, or foliation; usually a 

product of deformation.

foliation. A preferred arrangement of crystal planes in 

minerals. Primary foliation develops during the 

formation of a rock and includes bedding in 

sedimentary rocks and flow layering in igneous rocks. 

In metamorphic rocks, the term commonly refers to a 

parallel orientation of planar minerals such as micas. 

Secondary foliation develops during deformation 

and/or metamorphism and includes cleavage, 

schistosity, and gneissic banding.

formation. Fundamental rock-stratigraphic unit that is 

mappable, lithologically distinct from adjoining strata, 

and has definable upper and lower contacts.

fossil. A remain, trace, or imprint of a plant or animal that 

has been preserved in the Earth’s crust since some past 

geologic time; loosely, any evidence of past life.

geomorphology. The study of the general configuration 

of surface landforms and their relationships to 

underlying structures, and of the history of geologic 

changes as recorded by these surface features.

gneiss. A foliated metamorphic rock with alternating 

bands of dark and light minerals. Varieties are 

distinguished by texture (e.g., augen gneiss), 
PECO Geologic Resources Inventory Report 47

characteristic minerals (e.g., hornblende gneiss), or 

general composition (e.g., granite gneiss).

Gondwana. The late Paleozoic continent of the Southern 

Hemisphere and counterpart of Laurasia of the 

Northern Hemisphere; both were derived from the 

supercontinent Pangaea.

gradient. A degree of inclination (steepness of slope), or 

a rate of ascent or descent, of an inclined part of 

Earth’s surface with respect to the horizontal; 

expressed as a ratio (vertical to horizontal), a fraction 

(such as m/km or ft/mi), a percentage (of horizontal 

distance), or an angle (in degrees).

granite. A coarse-grained, intrusive igneous (plutonic) 

rock in which quartz constitutes 10%–50% percent of 

the felsic (“light-colored”) components and the alkali 

feldspar/total feldspar ratio is generally restricted to 

the range of 65% to 90%; perhaps the best known of 

all igneous rocks.

granitic. Pertaining to or composed of granite.

granitoid. A general term for all coarse-grained igneous 

rocks dominated by quartz and feldspar minerals.

granodiorite. A coarse-grained intrusive (plutonic) 

igneous rock intermediate in composition between

quartz diorite and quartz monzonite, containing 

quartz, plagioclase, and potassium feldspar as the felsic 

(“light-colored”) components, with biotite, 

hornblende, or, more rarely, pyroxene, as the mafic 

(“dark-colored”) components. 

gravel. An unconsolidated, natural accumulation of 

typically rounded rock fragments resulting from 

erosion; consists predominantly of particles larger 

than sand, greater than 2 mm (1/12 in) across.

graywacke. A dark gray, firmly indurated, coarse-grained 

sandstone that consists of poorly sorted angular to 

subangular grains of quartz and feldspar, with a variety 

of dark rock and mineral fragments embedded in a 

compact clayey matrix.

gypsum. A sulfate (sulfur + oxygen) mineral of calcium 

and water, CaSO4 • 2H2O.

igneous. Describes a rock or mineral that solidified from 

molten or partly molten material; also, describes 

processes leading to, related to, or resulting from the 

formation of such rocks.

intertidal. Pertaining to the benthic ocean environment 

or depth zone between high water and low water; also, 

pertaining to the organisms of that environment. 

Synonymous with “littoral.”

intrusion. The process of emplacement of magma into 

preexisting rock. Also, the igneous rock mass formed. 

intrusive. Pertaining to intrusion, both the process and 

the rock body.

joint. A break in rock without relative movement of 

rocks on either side of the fracture surface.

karst topography. Topography characterized by 

abundant sinkholes and caverns formed by the 

dissolution of calcareous rocks.

landslide. A collective term covering a wide variety of 

slope-movement landforms and processes that involve 

the downslope transport of soil and rock material en 

masse under the influence of gravity.

Laurasia. The late Paleozoic continent of the Northern 

Hemisphere and counterpart of Gonwana of the 

Southern Hemisphere; both were derived from the 

supercontinent Pangaea.

lava. Molten or solidified magma that has been extruded 

though a vent onto Earth’s surface.

law of original horizontality. A general law of geology: 

Water-laid sediments are deposited in strata that are

horizontal or nearly horizontal, and parallel or nearly 

parallel to the Earth’s surface. The law was first clearly 

stated by Steno in 1669.

law of superposition. A general law upon which all 

geologic chronology is based: In any sequence of 

layered rocks—sedimentary or extrusive (volcanic) 

igneous—that has not been overturned, the youngest 

stratum is at the top and the oldest at the base; that is, 

each bed is younger that the bed beneath, but older 

than the bed above it. The law was first clearly stated 

by Steno in 1669.

left-lateral fault. A strike-slip fault on which the side 

opposite the observer has been displaced to the left. 

Synonymous with “sinistral fault.” 

limb. One side of a structural fold.

limestone. A carbonate sedimentary rock consisting of

more than 95% calcite and less than 5% dolomite. 

lithic reduction. The process of detaching lithic flakes 

from a lump of stone, often using a hammerstone, 

wood, or bone.

lithification. The conversion of sediment into solid rock.

lithology. The physical description or classification of a 

rock or rock unit based on characteristics such as 

color, mineral composition, and grain size.

magma. Molten rock beneath Earth’s surface capable of 

intrusion and extrusion. 

mass wasting. Dislodgement and downslope transport of 

a mass of rock and/or unconsolidated material under 

the direct influence of gravity. In contrast to “erosion,” 

the debris removed is not carried within, on, or under 

another medium. Synonymous with “slope 

movement.” 

matrix. The fine-grained material between coarse grains 

in an igneous or sedimentary; also refers to rock or 

sediment in which a fossil is embedded.

meander. One of a series of sinuous curves, bends, loops, 

turns, or windings in the course of a stream, produced 

by a mature stream swinging from side to side as it 

flows across its floodplain or shifts its course laterally 

toward the convex side of an original curve.

megacryst. Any crystal or grain in an igneous or 

metamorphic rock that is significantly larger than the 

surrounding groundmass or matrix.

megacrystic. A rock which contains megacrysts.

member. A lithostratigraphic unit with definable 

contacts; a subdivision of a formation.

mesa. A broad, flat-topped erosional hill or mountain 

bounded by steeply sloping sides or cliffs.

metamorphic rock. Any rock derived from preexisting 

rocks that was altered in response to marked changes 

in temperature, pressure, shearing stress, and chemical 

environment. One of the three main classes of rock—

igneous, metamorphic, and sedimentary.

metamorphism. The mineralogical, chemical, and 

structural changes of solid rocks, generally imposed at 
48 NPS Geologic Resources Division

depth below the surface zones of weathering and 

cementation.

mica. A group of abundant silicate (silicon + oxygen) 

minerals characterized by perfect cleavage, readily 

splitting into thin sheets. Examples include “biotite” 

and “muscovite.”

mineral. A naturally occurring inorganic crystalline solid 

with a definite chemical composition or compositional 

range.

monocline. A one-limbed fold in strata that are otherwise 

flat-lying. 

monzonite. An intrusive (plutonic) igneous rock, 

intermediate in composition between syenite and 

diorite, containing approximately equal amounts of 

alkali feldspar and plagioclase and very little quartz. 

Monzonite contains less quartz and more plagioclase 

than granite.

muscovite. A light-colored silicate (silicon + oxygen) 

mineral of the mica group, KAl3Si3O10(OH)2, 

characterized by perfect cleavage in one direction and 

the ability to split into thin, clear sheets.

nonconformity. A type of unconformity developed 

between sedimentary rocks and older rocks (plutonic 

igneous or massive metamorphic rocks) that had been 

exposed to erosion before the overlying sediments 

covered them.

normal fault. A fault in which the hanging wall appears to 

have moved downward relative to the footwall; the 

angle of dip is usually 45°–90°. 

obsidian. An extrusive (volcanic) igneous rock, 

specifically a black or dark-colored volcanic glass, 

usually composed of rhyolite, and characterized by

conchoidal fracture.

ooze. A pelagic sediment consisting of at least 30% 

skeletal remains of pelagic organisms, the rest being 

clay minerals.

orogeny. A mountain-building event.

outcrop. Any part of a rock mass or formation that is 

exposed or “crops out” at Earth’s surface. 

oxidation. The process of combining with oxygen.

paleogeography. The study, description, and 

reconstruction of the physical landscape in past 

geologic periods.

paleontology. The study of the life and chronology of 

Earth’s geologic past based on the fossil record.

Pangea. A supercontinent that existed from about 300 to 

about 200 million years ago and included most of the 

continental crust of the Earth, from which the present 

continents were derived by fragmentation and

continental drift. During an intermediate stage of the 

fragmentation—between the existence of Pangea and 

that of the present continents—Pangea split into two 

large fragments, Laurasia on the north and Gondwana

on the south.

parent material. The unconsolidated organic and 

mineral material from which soil forms.

parent rock. Rock from which soil, sediment, or other 

rock is derived. 

parting. A plane or surface along which a rock readily 

separates.

passive margin. A continental plate boundary where no 

plate-scale tectonism is taking place; plates are not 

converging, diverging, or sliding past one another.

pebble. A small rounded rock, especially a waterworn 

stone, between 4 and 64 mm (0.16 and 2.5 in) across. 

pegmatite. An intrusive igneous rock consisting of 

exceptionally coarse-grained, interlocking crystals, 

generally granitic in composition and commonly in 

irregular dikes, lenses, and veins, especially at the 

margins of batholiths.

pelagic. Said of marine organisms whose environment is 

the open ocean.

pelitic. A sedimentary rock composed of clay, 

representing a consolidated volcanic ash deposit of 

clay-sized particles.

phyllite. A metamorphic rock, intermediate between 

slate and mica schist, with minute crystals of graphite, 

sericite, or chlorite that impart a silky sheen 

(“schistosity”).

plate boundary. A zone of seismic and tectonic activity 

along the edges of lithospheric plates, resulting from 

the relative motion between plates. 

plate tectonics. A theory of global tectonics in which the 

lithosphere is divided into about 20 rigid plates that 

interact with one another at their boundaries, causing 

seismic and tectonic activity along these boundaries.

point bar. A low ridge of sand and gravel deposited in a 

stream channel on the inside of a meander, where flow 

velocity slows.

Precambrian. A commonly used term to designate all 

rocks older than the Cambrian Period of the Standard 

Global Chronostratigraphic Scale. It includes the

Archean and Proterozoic eons and represents 90% of 

geologic time.

principle of uniformity. The assumption of uniformity of 

causes or processes throughout time and space; the 

uniformity of natural laws. Not synonymous with 

“uniformitarianism.”

quartz. The only silicate (silicon + oxygen) mineral 

consisting entirely of silicon and oxygen, SiO2; silicon 

dioxide. Synonymous with “crystalline silica.”

quartz monzonite. An intrusive (plutonic) igneous rock 

of granitic composition but with about as much 

plagioclase as alkali feldspar.

regolith. The layer of unconsolidated rock material that 

forms the surface of the land and overlies or covers 

bedrock; includes rock debris of all kinds, volcanic 

ash, glacial drift, alluvium, loess, and aeolian deposits, 

vegetal accumulations, and soil. Etymology: Greek 

“rhegos” (blanket) + “lithos” (stone).

regression. Long-term seaward retreat of the shoreline 

or relative fall of sea level.

reverse fault. A contractional high-angle (greater than 

45°) dip-slip fault in which the hanging wall moves up 

relative to the footwall. 

rift. A region of Earth’s crust where extension results in 

formation of many related normal faults, commonly

associated with volcanic activity.

right-lateral fault. A strike-slip fault on which the side 

opposite the observer has been displaced to the right. 
PECO Geologic Resources Inventory Report 49

ripple marks. The undulating, approximately parallel and 

usually small-scale pattern of ridges formed in

sediment by the flow of wind or water.

rock. An aggregate of one or more minerals (e.g., granite), 

a body of undifferentiated mineral matter (e.g., 

obsidian), or a body of solid organic material (e.g., 

coal).

rockfall. The most rapid type of slope movement in 

which a newly detached fragment of bedrock of any 

size falls from a cliff or other very steep slope, traveling 

straight down or in a series of leaps and bounds down 

a slope. 

sand. A clastic particle smaller than a granule and larger 

than a silt grain, with a diameter ranging from 1/16 to 2 

mm (0.0025 to 0.08 in).

sandstone. Clastic sedimentary rock composed of 

predominantly sand-sized grains.

scarp. A steep cliff or topographic step resulting from 

displacement on a fault or as a result of slope

movement or erosion. Synonymous with

“escarpment.”

schist. A medium- to coarse-grained, strongly foliated, 

metamorphic rock with eminently visible mineral 

grains, particularly mica, which are arranged parallel, 

imparting a distinctive sheen, or “schistosity,” to the 

rock.

sediment. An eroded and deposited, unconsolidated 

accumulation of rock and mineral fragments.

sedimentary rock. A rock resulting from the 

consolidation of loose sediment that has accumulated 

in layers; it may be “clastic,” consisting of mechanically 

formed fragments of older rock; “chemical,” formed 

by precipitation from solution; or “organic,” consisting 

of the remains of plants and animals. One of the three 

main classes of rock—igneous, metamorphic, and 

sedimentary. 

sedimentation. The process of forming or accumulating 

sediment into layers, including the separation of rock 

particles from parent rock, the transportation of these 

particles to the site of deposition, the actual deposition 

or settling of the particles, the chemical and other 

changes occurring in the sediment, and the ultimate 

consolidation of the sediment into solid rock. 

sequence. A rock-stratigraphic unit that is traceable over 

large areas and defined by sediment associated with a 

major sea level transgression–regression. 

shale. A clastic sedimentary rock made of clay-sized 

particles and characterized by fissility. 

shear. Deformation resulting from stresses that cause 

contiguous parts of a body to slide relatively to each 

other in a direction parallel to their plane of contact. 

sheetwash. A sheetflood occurring in a humid region.

Also, the material transported and deposited by the 

water of a sheetwash. Used as a synonym of “sheet 

flow” (a movement) and “sheet erosion” (a process).

siltstone. A clastic sedimentary rock composed of siltsized grains.

slope movement. The gradual or rapid downslope 

movement of soil or rock under gravitational stress. 

Synonymous with “mass wasting.”

slump. A generally large, coherent slope movement with 

a concave failure surface and subsequent backward 

rotation relative to the slope.

soil. The unconsolidated portion of the Earth’s crust 

modified through physical, chemical, and biotic 

processes into a medium capable of supporting plant 

growth.

sorting. The dynamic process by which sedimentary 

particles having some particular characteristic (such as 

similarity of size, shape, or specific gravity) are 

naturally selected and separated from associated but 

dissimilar particles by the agents of transportation.

strata. Tabular or sheetlike layers of sedimentary rock; 

layers are visually distinctive from other layers above 

and below. The singular form of the term is stratum, 

but is less commonly used.

stratigraphy. The geologic study of the origin, 

occurrence, distribution, classification, correlation, 

and age of rock layers, especially sedimentary rocks.

stream. Any body of water moving under gravity flow in 

a clearly confined channel.

stream channel. A long, narrow depression shaped by the 

concentrated flow of a stream and covered 

continuously or periodically by water.

stream terrace. A planar surface along the sides of a 

stream valley representing the remnants of an 

abandoned floodplain, stream bed, or valley floor 

produced during a former stage of erosion or 

deposition.

strike. The compass direction of the line of intersection 

of an inclined surface with a horizontal plane.

strike-slip fault. A fault with measurable offset where the 

relative movement is parallel to the strike of the fault. 

Described as left-lateral (sinistral) when relative 

motion of the block opposite the observer is to the left, 

and right-lateral (dextral) when relative motion is to 

the right. 

structure. The attitudes and relative positions of the rock 

masses of an area resulting from such processes as 

faulting, folding, and igneous intrusion.

subduction. The process of one lithospheric plate 

descending beneath another. 

subduction zone. A long, narrow belt in which

subduction takes place.

submergence. A rise of water level in relation to land, so 

that areas of formerly dry land become inundated; 

results from either a sinking of the land or rise of water 

level. Compare to “emergence.”

subsidence. The sudden sinking or gradual downward 

settling of part of Earth’s surface.

syncline. A generally concave upward fold of which the 

core contains the stratigraphically younger rocks. 

Compare with “anticline.”

tailings. The portions of milled ore that are regarded as 

too poor to be treated further.

talus. Rock fragments, usually coarse and angular, lying 

at the base of a cliff or steep slope from which they 

have fallen. 

tectonic. Describes a feature or process related to largescale movement and deformation of Earth’s crust.

terrace. Any long, narrow, relatively level or gently 

inclined surface (i.e., a bench or steplike ledge) that is 
50 NPS Geologic Resources Division

bounded along one edge by a steeper descending slope 

and along the other edge by a steeper ascending slope, 

thus breaking the continuity of the slope; commonly 

occurs along the margin and above the level of a body 

of water, marking a former water level. 

terrestrial. Describes a feature, process, or organism 

related to land, Earth, or its inhabitants.

terrigenous. Describes material or a feature derived from 

the land or a continent.

topography. The general morphology of Earth’s surface, 

including relief and locations of natural and humanmade features.

transform fault. A strike-slip fault that links two other 

faults or plate boundaries such as two segments of a 

mid-ocean ridge. 

transform plate boundary. A type of plate boundary at 

which lithosphere is neither created nor destroyed, 

and plates slide past each other on a strike-slip fault.

transgression. Landward migration of the sea as a result 

of a relative rise in sea level.

unconformable. Describes strata that do not succeed the 

underlying rocks in immediate order of age or in 

parallel position, especially younger strata that do not 

have the same dip and strike as the underlying rocks. 

Also, describes the contact between unconformable 

rocks.

unconformity. A substantial break or gap in the geologic 

record where a rock unit is overlain by another that is 

not next in stratigraphic succession, resulting from 

either a change that caused deposition to cease for a 

considerable span of time or erosion with loss of the 

previously formed record.

uniformitarianism. The assumption that “the geological 

forces of the past differ neither in kind nor in energy 

from those now in operation,” which was the basis 

advocated by Charles Lyell for interpreting past 

phenomena by analogy with modern ones. Thus the 

cliché, “the present is the key to the past.”

uplift. A structurally high area in Earth’s crust produced 

by movement that raises the rocks.

volcanic. Pertaining to the activities, structures, or rock 

types of a volcano.

wash. A broad, gravelly, dry stream bed, generally in the 

bottom of a canyon that is periodically swept by a 

torrent of water. The term is used especially in the 

southwestern United States. 

weathering. The physical, chemical, and biological 

processes by which rock is broken down, particularly

at the surface.
PECO Geologic Resources Inventory Report 51

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PECO Geologic Resources Inventory Report 53

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PECO Geologic Resources Inventory Report 55

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PECO Geologic Resources Inventory Report 57

Additional References 

This section lists additional references, resources, and websites that may be of use to 

resource managers. Web addresses are valid as of April 2015. Refer to Appendix B for laws, 

regulations, and policies that apply to NPS geologic resources. 

Geology of National Park Service Areas

NPS Geologic Resources Division (Lakewood, 

Colorado)

Energy and Minerals; Active Processes and Hazards;

Geologic Heritage: 

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

NPS Geologic Resources Inventory: 

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

fm. 

NPS Geoscientist-In-the-Parks (GIP) internship and 

guest scientist program: 

http://www.nature.nps.gov/geology/gip/index.cfm

NPS Views program (geology-themed modules are 

available for Geologic Time, Paleontology, Glaciers, 

Caves and Karst, Coastal Geology, Volcanoes, and a 

variety of geologic parks): 

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

NPS Resource Management Guidance and Documents

1998 National parks omnibus management act: 

http://www.gpo.gov/fdsys/pkg/PLAW105publ391/pdf/PLAW-105publ391.pdf

Management Policies 2006 (Chapter 4: Natural resource 

management): 

http://www.nps.gov/policy/mp/policies.html

NPS-75: Natural resource inventory and monitoring 

guideline: 

http://www.nature.nps.gov/nps75/nps75.pdf

NPS Natural resource management reference manual 

#77: http://www.nature.nps.gov/Rm77/

Geologic monitoring manual (Young, R., and L. Norby, 

editors. 2009. Geological monitoring. Geological 

Society of America, Boulder, Colorado): 

http://nature.nps.gov/geology/monitoring/index.cfm

NPS Technical Information Center (TIC) (Denver, 

Colorado; repository for technical documents): 

http://www.nps.gov/dsc/technicalinfocenter.htm

Climate Change Resources

NPS Climate Change Response Program Resources: 

http://www.nps.gov/subjects/climatechange/resources

.htm

US Global Change Research Program: 

http://globalchange.gov/home

Intergovernmental Panel on Climate Change: 

http://www.ipcc.ch/

Geological Surveys and Societies

New Mexico Bureau of Geology and Mineral Resources: 

http://geoinfo.nmt.edu/

US Geological Survey: http://www.usgs.gov/

Geological Society of America: 

http://www.geosociety.org/

American Geophysical Union: http://sites.agu.org/

American Geosciences Institute: 

http://www.americangeosciences.org/

Association of American State Geologists: 

http://www.stategeologists.org/

US Geological Survey Reference Tools

National geologic map database (NGMDB): 

http://ngmdb.usgs.gov/

Geologic names lexicon (GEOLEX; geologic unit 

nomenclature and summary): 

http://ngmdb.usgs.gov/Geolex/geolex_home.html

Geographic names information system (GNIS; official 

listing of place names and geographic features): 

http://gnis.usgs.gov/

GeoPDFs (download searchable PDFs of any 

topographic map in the United States): 

http://store.usgs.gov (click on “Map Locator”)

Publications warehouse (many publications available 

online): http://pubs.er.usgs.gov

Tapestry of time and terrain (descriptions of 

physiographic provinces): 

http://tapestry.usgs.gov/Default.html
58 NPS Geologic Resources Division

Appendix A: Scoping Participants 

The following people attended the GRI scoping meeting for Pecos National Historical Park, 

held on 28 March 2006, or the follow-up report writing conference call, held on 11 April 

2013. Discussions during these meetings supplied a foundation for this GRI report. The 

scoping summary document is available on the GRI publications website: 

http://go.nps.gov/gripubs (accessed 23 February 2015). 

2006 Scoping Meeting Participants

Name Affiliation Position

Doug Bland New Mexico Bureau of Geology and 

Mineral Resources Geologist

Tim Connors NPS Geologic Resources Division Geologist

Dennis Ditmanson Fort Union National Monument and Pecos 

National Historical Park Superintendent

Bruce Heise NPS Geologic Resources Division Geologist

Dan Jacobs Pecos National Historical Park Chief Ranger

Katie KellerLynn Colorado State University Research Associate

Ron Kerbo NPS Geologic Resources Division Cave Specialist

Virgil Leuth New Mexico Bureau of Geology and 

Mineral Resources Geologist

2013 Conference Call Participants

Name Affiliation Position

Jason Kenworthy NPS Geologic Resources Division Geologist, GRI Reports Coordinator

Rebecca Port NPS Geologic Resources Division Geologist, Technical Writer

Dennis Carruth Pecos National Historical Park Superintendent

Greer Price New Mexico Bureau of Geology and 

Mineral Resources Director and State Geologist

Michael Timmons New Mexico Bureau of Geology and 

Mineral Resources Associate Director for Mapping Programs

Regina Carrico Pecos National Historical Park Chief Ranger

Sue Eininger Pecos National Historical Park Archeologist
PECO Geologic Resources Inventory Report 59

Appendix B: Geologic Resource Laws, Regulations, and 

Policies 

The NPS Geologic Resources Division developed this table to summarize laws, regulations, 

and policies that specifically apply to National Park Service minerals and geologic 

resources. The table does not include laws of general application (e.g., Endangered Species 

Act, Clean Water Act, Wilderness Act, National Environmental Policy Act, or National 

Historic Preservation Act). The table does include the NPS Organic Act when it serves as the 

main authority for protection of a particular resource or when other, more specific laws are 

not available. Information is current as of April 2015. Contact the NPS Geologic Resources 

Division for detailed guidance.

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Caves and Karst Systems

Federal Cave Resources Protection 

Act of 1988, 16 USC. §§ 4301 – 

4309 requires Interior/Agriculture to 

identify “significant caves” on 

Federal lands, regulate/restrict use of 

those caves as appropriate, and 

include significant caves in land 

management planning efforts. 

Imposes civil and criminal penalties 

for harming a cave or cave resources. 

Authorizes Secretaries to withhold 

information about specific location of 

a significant cave from a FOIA 

requester.

National Parks Omnibus 

Management Act of 1998, 16 USC. 

§ 5937 protects the confidentiality of 

the nature and specific location of 

cave and karst resources.

Lechuguilla Cave Protection Act 

of 1993, Public Law 103-169

created a cave protection zone (CPZ) 

around Lechuguilla Cave in Carlsbad 

Caverns National Park. Within the 

CPZ, access and the removal of cave 

resources may be limited or 

prohibited; existing leases may be 

cancelled with appropriate 

compensation; and lands are 

withdrawn from mineral entry. 

36 C.F.R. § 2.1 prohibits possessing/ 

destroying/disturbing...cave 

resources...in park units.

43 C.F.R Part 37 states that all NPS 

caves are “significant” and set forth 

procedures for determining/releasing 

confidential information about specific 

cave locations to a FOIA requester.

Section 4.8.1.2 requires NPS to 

maintain karst integrity, minimize 

impacts.

Section 4.8.2 requires NPS to protect 

geologic features from adverse effects 

of human activity.

Section 4.8.2.2 requires NPS to 

protect caves, allow new development 

in or on caves if it will not impact cave 

environment, and to remove existing 

developments if they impair caves.

Section 6.3.11.2 explains how to 

manage caves in/adjacent to 

wilderness.
60 NPS Geologic Resources Division

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Paleontology

National Parks Omnibus 

Management Act of 1998, 16 USC. 

§ 5937 protects the confidentiality of 

the nature and specific location of 

paleontological resources and 

objects.

Paleontological Resources 

Preservation Act of 2009, 16 USC. 

§ 470aaa et seq. provides for the 

management and protection of 

paleontological resources on federal 

lands.

36 C.F.R. § 2.1(a)(1)(iii) prohibits 

destroying, injuring, defacing, 

removing, digging or disturbing 

paleontological specimens or parts 

thereof.

36 C.F.R. § 13.35 prohibition applies 

even in Alaska parks where the surface 

collection of other geologic resources 

is permitted.

Regulations in association with 2009 

PRPA are still being finalized (April

2015).

Section 4.8.2 requires NPS to protect 

geologic features from adverse effects 

of human activity.

Section 4.8.2.1 emphasizes Inventory 

and Monitoring, encourages scientific 

research, directs parks to maintain 

confidentiality of paleontological 

information, and allows parks to buy 

fossils only in accordance with certain 

criteria.

Rocks and Minerals

NPS Organic Act, 16 USC. § 1 et 

seq. directs the NPS to conserve all 

resources in parks (which includes 

rock and mineral resources) unless 

otherwise authorized by law.

Exception: 16 USC. § 445c (c) – 

Pipestone National Monument 

enabling statute. Authorizes 

American Indian collection of catlinite 

(red pipestone).

36 C.F.R. § 2.1 prohibits possessing, 

destroying, disturbing mineral 

resources...in park units.

Exception: 36 C.F.R. § 7.91 allows 

limited gold panning in Whiskeytown.

Exception: 36 C.F.R. § 13.35 allows 

some surface collection of rocks and 

minerals in some Alaska parks (not 

Klondike Gold Rush, Sitka, Denali, 

Glacier Bay, and Katmai) by nondisturbing methods (e.g., no pickaxes), 

which can be stopped by 

superintendent if collection causes 

significant adverse effects on park 

resources and visitor enjoyment.

Section 4.8.2 requires NPS to protect 

geologic features from adverse effects 

of human activity.

Park Use of Sand and Gravel

Materials Act of 1947, 30 USC. § 

601 does not authorize the NPS to 

dispose of mineral materials outside 

of park units.

Exception: 16 USC. §90c 1(b) the 

non-wilderness portion of Lake 

Chelan National Recreation Area, 

where sand, rock and gravel may be 

made available for sale to the 

residents of Stehekin for local use as 

long as such sale and disposal does 

not have significant adverse effects 

on the administration of the National 

Recreation Area. 

None applicable.

Section 9.1.3.3 clarifies that only the 

NPS or its agent can extract parkowned common variety minerals (e.g., 

sand and gravel), and

-Only for park administrative uses.

-After compliance with NEPA and 

other federal, state, and local laws, 

and a finding of non-impairment.

-After finding the use is park’s most 

reasonable alternative based on 

environment and economics.

-Parks should use existing pits and 

create new pits only in accordance 

with park-wide borrow management

plan.

-Spoil areas must comply with Part 6 

standards

-NPS must evaluate use of external 

quarries.

Any deviations from this policy require 

written waiver from the Secretary, 

Assistant Secretary, or Director.
PECO Geologic Resources Inventory Report 61

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Upland and Fluvial Processes

Rivers and Harbors Appropriation 

Act of 1899, 33 USC. § 403

prohibits the construction of any 

obstruction, on the waters of the 

united states, not authorized by 

congress or approved by the USACE.

Clean Water Act 33USC. § 1342

requires a permit from the USACE 

prior to any discharge of dredged or 

fill material into navigable waters 

(waters of the US (including 

streams)).

Executive Order 11988 requires 

federal agencies to avoid adverse 

impacts to floodplains. (see also D.O. 

77-2) 

Executive Order 11990 requires 

plans for potentially affected 

wetlands (including riparian 

wetlands). (see also D.O. 77-1)

None Applicable.

Section 4.1 requires NPS to manage 

natural resources to preserve 

fundamental physical and biological 

processes, as well as individual species, 

features, and plant and animal 

communities; maintain all components 

and processes of naturally evolving 

park ecosystems.

Section 4.1.5 directs the NPS to reestablish natural functions and 

processes in human-disturbed 

components of natural systems in 

parks unless directed otherwise by 

Congress.

Section 4.4.2.4 directs the NPS to 

allow natural recovery of landscapes 

disturbed by natural phenomena, 

unless manipulation of the landscape 

is necessary to protect park 

development or human safety.

Section 4.6.4 directs the NPS to (1) 

manage for the preservation of 

floodplain values; [and] (2) minimize 

potentially hazardous conditions 

associated with flooding

Section 4.6.6 directs the NPS to 

manage watersheds as complete 

hydrologic systems and minimize 

human-caused disturbance to the 

natural upland processes that deliver 

water, sediment, and woody debris to 

streams

Section 4.8.1 directs the NPS to allow 

natural geologic processes to proceed 

unimpeded. Geologic 

processes...include...erosion and 

sedimentation...processes.

Section 4.8.2 directs the NPS to 

protect geologic features from the 

unacceptable impacts of human 

activity while allowing natural 

processes to continue.
62 NPS Geologic Resources Division

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Soils

Soil and Water Resources 

Conservation Act, 16 USC. §§ 2011 

– 2009 provides for the collection 

and analysis of soil and related 

resource data and the appraisal of 

the status, condition, and trends for 

these resources.

Farmland Protection Policy Act, 7 

USC. § 4201 et. seq. requires NPS to 

identify and take into account the 

adverse effects of Federal programs 

on the preservation of farmland; 

consider alternative actions, and 

assure that such Federal programs 

are compatible with State, unit of 

local government, and private 

programs and policies to protect 

farmland. NPS actions are subject to 

the FPPA if they may irreversibly 

convert farmland (directly or 

indirectly) to nonagricultural use and 

are completed by a Federal agency or 

with assistance from a Federal 

agency. Applicable projects require 

coordination with the Department of 

Agriculture’s Natural Resources 

Conservation Service (NRCS).

7 C.F.R. Parts 610 and 611 are the 

US Department of Agriculture 

regulations for the Natural Resources 

Conservation Service. Part 610 

governs the NRCS technical assistance 

program, soil erosion predictions, and 

the conservation of private grazing 

land. Part 611 governs soil surveys and 

cartographic operations. The NRCS 

works with the NPS through 

cooperative arrangements.

Section 4.8.2.4 requires NPS to

-Prevent unnatural erosion, removal, 

and contamination.

-Conduct soil surveys.

-Minimize unavoidable excavation.

-Develop/follow written prescriptions 

(instructions).
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.

NPS 430/128395, April 2015
National Park Service

U.S. Department of the Interior

Natural Resource Stewardship and Science

1201 Oakridge Drive, Suite 150

Fort Collins, CO 80525

www.nature.nps.gov

EXPERIENCE YOUR AMERICATM

PECO Map Unit Properties Table, Page 1 of 6 

Map Unit Properties Table: Pecos National Historical Park 

Colored map units indicate those mapped within Pecos National Historical Park. Bold text corresponds to headings in the Geologic Features and Processes, Geologic Resource Management Issues, and Geologic History sections of the report.

Age Map Unit

(Symbol) Geologic Description Geologic Features and Processes Geologic Resource Management Issues Geologic History

QUATERNARY 

(Holocene)

Artificial fill

(Qaf) 

Anthropogenic deposits of rock, soil, gravel or other material 

to fill depressions, open space, or construct dams.

Mapped at El Molino Mills Site and dams.

Geologic Resources in a Cultural Context—El Molino 

Mills Site processed ore from Pecos Mine. None documented.

The past 66 million years: Mountain Building, 

Erosion, and Emergence of the Modern Landscape—

artificial fill was placed over the El Molino Mills Site and 

used to create dams in the tailings ponds.

Alluvium

(Qa) 

Unconsolidated valley fill sediments in active perennial and 

ephemeral stream channels and on floodplains. Composed 

of clay, silt, sand, and gravel. Grades into minor colluvial and 

alluvial fans at valley margins. Thickness 08 m (0–25 ft).

Mapped in all units of the park.

Fluvial Features—Qa represents modern stream deposits 

in channels and on floodplains. 

Fluvial Erosion—accelerated erosion in the Pecos River 

and Glorieta Creek affects these deposits.

Slope Movement Hazards and Risks—slumping occurs 

along river corridors.

The past 66 million years: Mountain Building, 

Erosion, and Emergence of the Modern Landscape—

the Laramide orogeny raised the Rocky Mountains, 

including the Sangre de Cristo Mountains. Weathering and 

erosion carried silt, sand, and gravel down from the high 

mountains to the Pecos valley. Qa represents recent 

deposits along the banks and in the floodplains of the 

Pecos River and Glorieta Creek.

QUATERNARY 

Colluvium

(Qc) 

Locally derived talus from nearby cliffs. Commonly occurs as 

a veneer of variable thickness, 0–9 m (0–30 ft), on slopeforming bedrock.

Mapped in all units of the park.

Slope Movements—Qc consists of talus deposits. Slope Movement Hazards and Risks—slope movements 

may occur.

The past 66 million years: Mountain Building, 

Erosion, and Emergence of the Modern Landscape—

the Laramide orogeny raised the Rocky Mountains, 

including the Sangre de Cristo Mountains. Weathering and 

erosion carried silt, sand, and gravel down from the high 

mountains to the Pecos Valley.

Colluvium and 

alluvium

(Qca) 

Intermixed, unconsolidated colluvial and alluvial deposits. 

Locally contains well-developed soils. Thickness 0–6 m (0–20 

ft). 

Fluvial Features—Qca represents modern stream 

deposits mixed with colluvium. 

Slope Movements—Qca formed as a result of hillslope 

processes. 

Slope Movement Hazards and Risks—slumping occurs 

along river corridors.

Valley fill

(Qvf) 

Unconsolidated to consolidated, fine-grained valley fills

composed of clay, silt, and sand along the margins of active 

stream channels and in areas of low relief or where runoff is 

not channelized. Qvf may form terraces when incised by 

modern arroyos. Thickness 0–12 m (0–40 ft).

Mapped in the Pecos Unit and the Pigeon’s Ranch Subunit. 

Fluvial Features—Qvf represents modern stream 

deposits.

Fluvial Erosion—accelerated erosion in the Pecos River 

and Glorieta Creek affects these deposits.

Landslide deposits

(Ql) Large sandstone blocks. 

Slope Movements—Ql consists of predominantly rockfall 

deposits that are downslope from sandstone cliffs of the 

Santa Rosa Formation.

None reported.

The past 66 million years: Mountain Building, 

Erosion, and Emergence of the Modern Landscape—

weathering and erosion of the cliff-forming Santa Rosa 

Formation produced these rockfall deposits. 

Terrace deposits 

(Qt) 

Undivided, gravel-capped terrace deposits along the Pecos 

River and Glorieta Creek. Thickness 0–80 m (0–260 ft). 

Mapped in all units of the park.

Fluvial Features—Qt marks the position of past stream 

channels.

Geologic Resources in a Cultural Context—Pueblo 

Indians used much older (Paleozoic Era) cobbles from this 

unit to build the stone walls of Pecos Pueblo.

Fluvial Erosion—accelerated erosion in the Pecos River 

and Glorieta Creek affects these deposits.

Slope Movement Hazards and Risks—slumping occurs 

along river corridors.

The past 66 million years: Mountain Building, 

Erosion, and Emergence of the Modern Landscape—

the Laramide orogeny raised the Rocky Mountains, 

including the Sangre de Cristo Mountains. Weathering and 

erosion carried silt, sand, and gravel down from the high 

mountains to the Pecos Valley. These are recent deposits 

along terraces of the Pecos River and Glorieta Creek.
Colored map units indicate those mapped within Pecos National Historical Park. Bold text corresponds to headings in the Geologic Features and Processes, Geologic Issues, and Geologic History sections of the report.

PECO Map Unit Properties Table, Page 2 of 6 

Age Map Unit

(Symbol) Geologic Description Geologic Features and Processes Geologic Resource Management Issues Geologic History

QUATERNARY-TERTIARY 

Gravel deposits, 

younger

(QTg) 

Gravel and cobble deposits composed of rounded Paleozoic 

and Proterozoic clasts. Thickness 0–9 m (0–30 ft). 

Mapped in Pecos Unit. 

Geologic Resources in a Cultural Context—Pueblo 

Indians used Paleozoic cobbles from this unit to build the 

stone walls of Pecos Pueblo. Around 1965–1975, gravel 

was extracted from this unit for use as aggregate in 

interstate construction.

None reported.

The past 66 million years: Mountain Building, 

Erosion, and Emergence of the Modern Landscape—

the Laramide orogeny raised the Rocky Mountains, 

including the Sangre de Cristo Mountains. Weathering and 

erosion carried silt, sand, and gravel down from the high 

mountains to the Pecos Valley. QTg deposited prior to the

establishment of the incised modern Pecos River and 

Glorieta Creek.

Ancha Formation

(QTa) 

QTa represents the uppermost basin fill unit in the Santa Fe 

embayment. Light-tan to pink and orange, thick-bedded silt, 

arkosic sand, and gravel. Mostly non-cemented and weakly 

consolidated. Sediments are composed of Proterozoic

granite, granite gneiss, and amphibolite gneiss derived from 

the southwestern flank of the Sangre de Cristo Mountains. 

Not mapped in Pecos National Historical Park. West of the 

park, in the Santa Fe embayment, the Ancha Formation 

constitutes an important hydrogeologic unit relied upon 

for groundwater by many local communities. 

The past 66 million years: Mountain Building, 

Erosion, and Emergence of the Modern Landscape—

the Laramide orogeny raised the Rocky Mountains, 

including the Sangre de Cristo Mountains. Weathering and 

erosion carried silt, sand, and gravel down from the high 

mountains to the Pecos Valley.

TERTIARY

Galisteo Formation

(Tg) 

Tan to white cross bedded massive sandstone interbedded 

with pink to red siltstone and mudstone. Contains 

conglomerate lenses. Limestone beds present in the lower 

portion of unit.

Paleontological Resources—Tg contains silicified logs up 

to 1 m (4 ft) in diameter and 9 m (30 ft) in length.

Gravel deposits, older

(Tgv) 

Fine- to coarse-grained deposits dominated by rounded 

Proterozoic boulder and cobble gravel with fewer Paleozoic 

clasts. Deposits are typically poorly exposed, but cobbles and 

boulders form highs in the landscape, up to 460 m (1,500 ft) 

above the modern Pecos River, and colluvium mantles 

hillsides. Thickness 0–37 m (0–120 ft). None reported.

Tesuque Formation

(Tt) 

Tan, pinkish-tan, and orange, thick-bedded, coarse-grained,

arkosic sandstone and some interbedded siltstone and 

pebble to boulder conglomerate. Moderately well cemented 

by calcium carbonate. Contains clasts of granite, granite 

gneiss, amphibolite gneiss, and the Madera Formation 

limestone.
Colored map units indicate those mapped within Pecos National Historical Park. Bold text corresponds to headings in the Geologic Features and Processes, Geologic Issues, and Geologic History sections of the report.

PECO Map Unit Properties Table, Page 3 of 6 

Age Map Unit

(Symbol) Geologic Description Geologic Features and Processes Geologic Resource Management Issues Geologic History

TRIASSIC

Chinle Group, 

undivided

(TRc)* 

Reddish-brown mudstone interbedded with lavender-gray 

sandstone. Contains pebble conglomerate lenses. 

*Nomenclature of the “Chinle Group” has changed over 

time. This report follows the “Chinle Group” nomenclature 

as per the source maps published in 1997 and 2002. More 

recently, the US Geological Survey and some state surveys 

prefer the “Chinle Formation” nomenclature. Refer to the US 

Geological Survey GEOLEX for additional information 

(http://ngmdb.usgs.gov/Geolex/Units/Chinle_4997.html, 

accessed 9 April 2015).

Mapped in the Cañoncito Subunit.

Glorieta Mesa—TRc is mapped on the western flank of 

Glorieta Mesa, in Galisteo Creek valley. 

Faults—only mapped west of the Glorieta Mesa fault 

zone.

Paleontological Resources—fossils may be present 

within TRc. 

Paleontological Resource Inventory, Monitoring, and 

Protection—a field-based survey could help determine 

presence of fossils in this formation in Pecos National 

Historical Park. 

252 million–66 million years ago: Sedimentary 

Basins—seas advanced over the land, eroding material 

from the underlying formations and depositing it just 

offshore in shallow water.

Santa Rosa Formation

(TRs) 

Brown sandstone alternating with red shale. 

Mapped in the Cañoncito Subunit.

Glorieta Mesa—Santa Rosa Formation is the youngest 

formation on the mesa. Caps the mesa, including the 

highest point, Cerro de Escobas. Erosion has removed 

much of this formation, leaving about 15 m (50 ft). In 

other places in New Mexico, the formation is as much as

140 m (450 ft) thick.

Paleontological Resources—fossil wood is common in 

the lower member of the formation. In the north and east 

sections of the formation contains petrified wood, which 

may have eroded from TRsl. 

Slope Movement Hazards and Risks—this formation 

has produced rockfalls in the past. Rockfalls are mapped as 

landslide deposits (Ql). 

Paleontological Resource Inventory, Monitoring, and 

Protection—a field-based survey could determine the 

source of the petrified wood and the presence of 

additional fossils in this formation. 

Land Subsidence—dissolution of the underlying San 

Andres Formation has caused this formation to collapse in 

locations outside Pecos National Historical Park. 

Santa Rosa 

Formation, upper 

member

(TRsu) 

Buff to dark brown, thick-bedded, fine-grained sandstone 

with limestone and chert-pebble conglomerates near base.

Cross-bedding is uncommon. Thickness 18–25 m (60–80 ft). 

Mapped in the Cañoncito Subunit. 

Santa Rosa 

Formation, middle 

member

(TRsm) 

Purple and green mudstone with nodular limestone, gray to 

buff, thinly laminated siltstone, and brown to buff, thickbedded, fine- to medium-grained sandstone. Thickness 

generally 30–40 m (100–130 ft). 

Mapped in the Cañoncito Subunit. 

Santa Rosa 

Formation, lower 

member

(TRsl) 

Light gray to buff, thick-bedded, fine- to coarse-grained, 

cross-bedded sandstone with conspicuous iron oxide 

staining. Limestone- and chert-pebble conglomerates present 

in most places, particularly along the southwestern edge of 

the quadrangle. Thickness 30–40 m (90–120 ft). 

Mapped in the Cañoncito Subunit. 

Moenkopi Formation

(TRm) 

Grayish red to purplish-red sandstones, conglomerates,

mudstones, and siltstones. Lacks gypsum. Sandstones are 

micaceous lithic arenites or lithic graywackes. Sandstones are 

moderately to poorly sorted, subangular to angular, and fine 

to coarse grained. Sandstones and conglomerates are 

generally trough cross-bedded and lenticular. Contact with

underlying Artesia Formation is disconformable. Thickness is 

up to 2,500 m (8,300 ft). 

Mapped in the Cañoncito Subunit.

Glorieta Mesa—TRm is mostly covered by vegetation and 

talus.

Paleontological Resources—no fossils documented from 

TRm in Pecos National Historical Park. Calcareous 

microfossils are in lacustrine and fluvial sediments of this 

unit in the Lucero Mesa area of Cibola County, New 

Mexico. Other TRm fossils include vertebrate bones and 

coprolites, footprints, charophytes, and ostracods.

Paleontological Resource Inventory, Monitoring, and 

Protection—a field-based survey could help determine 

presence of fossils in this formation in Pecos National 

Historical Park. 

252 million–66 million years ago: Sedimentary 

Basins—TRm consists of sands deposited by rivers and in 

lakes.

PERMIAN

Artesia Formation

(Pa) 

Orange to brick-red sandstones, siltstones, and gypsum. 

Sandstones and siltstones are quartzarenites and usually 

gypsiferous. Sandstones are well sorted, subrounded to 

subangular, and very fine to fine grained. Sandstones are

ripple laminated. Cross-bedding is rare. Mottled greenishwhite reduction spots are common. Thickness 10–20 m (32–

65 ft). 

Mapped in the Cañoncito Subunit. 

Glorieta Mesa—Pa is mostly covered by vegetation and 

talus.

Paleontological Resources—bioturbated beds are 

evidence of animal activity in the past. 

Geologic Resources in a Cultural Context—selenite 

used in window panes of mission churches may have come 

from an exposure of this formation southwest of the 

Canoñcito Subunit.

Paleontological Resource Inventory, Monitoring, and 

Protection—a field-based survey could help determine 

extent or significance of fossils in this formation in Pecos 

National Historical Park. 

542 million–252 million years ago: Rise and Fall of the 

Ancestral Rocky Mountains—sea level dropped, waves 

eroded the San Andres Formation and deposited the 

eroded material as the Artesia Formation. The Ancestral 

Rocky Mountains were largely worn down by this time. 
Colored map units indicate those mapped within Pecos National Historical Park. Bold text corresponds to headings in the Geologic Features and Processes, Geologic Issues, and Geologic History sections of the report.

PECO Map Unit Properties Table, Page 4 of 6 

Age Map Unit

(Symbol) Geologic Description Geologic Features and Processes Geologic Resource Management Issues Geologic History

PERMIAN

San Andres 

Formation 

(Psa) 

Buff to light gray, thin-bedded, dense, silty limestone and

interbedded calcareous sandstone. Reddish-orange chert or 

white aragonite satin spar (potentially after gypsum). 

Thickness is 5–13 m (15–45 ft). 

Mapped in the Cañoncito Subunit. 

Glorieta Mesa—Psa is commonly present near the 

contact with the underlying Glorieta Sandstone.

Cave and Karst Resources—may have developed karst 

topography that was then filled in with younger

sediments. 

Geologic Resources in a Cultural Context—Pueblo 

Indians may have used chert and chalcedony from this 

formation to make stone tools. Selenite used in the 

window panes of mission churches may have come from 

an exposure of this formation southwest of the Canoñcito 

Subunit.

Land Subsidence—dissolution of limestone and gypsum 

has caused land subsidence near Santa Rosa, New Mexico, 

which is approximately 145 km (90 mi) southeast of Pecos 

National Historical Park. 

542 million–252 million years ago: Rise and Fall of the 

Ancestral Rocky Mountains—sea level continued to rise 

and limestone was deposited in deeper areas.

Glorieta Sandstone

(Pg) 

Yellow to buff, thick bedded, fine- to medium-grained 

quartz sandstone. Grains are well rounded, well sorted, and 

frosted. Thickness 20–33 m (80–120 ft). 

Mapped in the Cañoncito Subunit. 

Glorieta Mesa—Pg is the most conspicuous formation on 

the mesa. Well cemented, very hard, resists erosion. Caps 

the mesa in most places. 

Paleontological Resources—fossils may be present 

within Pg.

Paleontological Resource Inventory, Monitoring, and 

Protection—a field-based survey could help determine 

presence of fossils in this formation in Pecos National 

Historical Park. 

542 million–252 million years ago: Rise and Fall of the 

Ancestral Rocky Mountains—rising seas allowed waves 

to erode the underlying Yeso Formation, producing quartz 

sand, which was deposited to form Glorieta Sandstone.

Yeso Formation

(Py) 

Reddish-orange to red, thick-bedded mudstone and 

interbedded buff to reddish-orange and yellow, fine-grained 

sandstone and some buff, coarse-grained, arkosic sandstone. 

Thin beds of gypsum have been observed in Py in a few 

locales, but not on Glorieta Mesa. Thickness 22–42 m (160–

200 ft). 

Mapped in the Cañoncito Subunit.

Glorieta Mesa—locally exposed along the base of 

Glorieta Mesa. 

Paleontological Resources—no fossils. Py is composed 

of dense, crystalline limestone.

None reported.

542 million–252 million years ago: Rise and Fall of the 

Ancestral Rocky Mountains—muds and sands 

accumulated in an intertidal environment.

PENNSYLVANIAN-PERMIAN

Sangre de Cristo 

Formation 

(PPNsc) 

Brownish red to purple, thick-bedded mudstone and 

interbedded mottled buff to dark brown and green, lensshaped coarse-grained arkosic conglomeratic sandstone. 

Contains some gray to purple, thin-bedded, dense, 

microcrystalline limestone and buff, thin-bedded, mediumgrained quartz sandstone. Thickness ranges from 900 m

(3,000 ft) near Cañoncito to 90 m (300 ft) to the south near 

Lamy.

Mapped in all units of the park.

Glorieta Mesa—oldest formation exposed on Glorieta 

Mesa. Upper 30 m (100 ft) is well exposed on the lower 

face of the mesa wall. Majority of thickness (4/5) occupies 

the valley floor where it is covered by thick red soil derived 

from the weathering of the formation itself.

Paleontological Resources—fossils documented from 

this formation in Pecos National Historical Park include a 

shark tooth, and amphibian and reptile bone fragments. 

Fossil vertebrates have also been found in this formation in 

Colorado. Marine fossils have been found in several 

limestone beds in the lower part of the formation in 

Colorado locations. 

Cave and Karst Resources—the Baca Cave developed 

between a sandstone and limestone layer in this 

formation. 

Geologic Resources in a Cultural Context—Pueblo 

Indians used conglomerate and sandstone blocks from this 

formation for the stone walls of Pecos Pueblo. Flagstone 

was quarried for walkways, steps, and shelves in Spanish 

mission churches. This formation contains the Baca mine 

which may have been opened in search of uranium or 

copper.

Disturbed Land Restoration—most of the areas of 

disturbed lands in Pecos National Historical Park are 

underlain by the Sangre de Cristo Formation.

Abandoned Mineral Lands Hazards—the Baca mine 

and surrounding area are closed with fencing and signs for 

visitor safety.

Paleontological Resource Inventory, Monitoring, and 

Protection—a field-based survey could help determine 

presence of additional vertebrate fossils in the park. 

Caves and Associated Landscape Management—a 

cave management plan could be developed for Baca Cave 

and small cave.

542 million–252 million years ago: Rise and Fall of the 

Ancestral Rocky Mountains—the shallow sea of 

Pennsylvanian time receded, giving way to terrestrial 

deposition. Rivers and streams deposited sediments eroded 

from the Ancestral Rocky Mountains in the Rowe-Mora 

basin. These sediments became the Sangre de Cristo 

Formation.
Colored map units indicate those mapped within Pecos National Historical Park. Bold text corresponds to headings in the Geologic Features and Processes, Geologic Issues, and Geologic History sections of the report.

PECO Map Unit Properties Table, Page 5 of 6 

Age Map Unit

(Symbol) Geologic Description Geologic Features and Processes Geologic Resource Management Issues Geologic History

PENNSYLVANIAN 

Madera Formation

(PNm) 

Gray to light-brown and purple, thick-bedded, dense, 

fossiliferous limestone with interbedded gray to brown,

calcareous sandstone and coarse-grained arkosic sandstone. 

Minimum thickness 250 m (820 ft). 

Paleontological Resources—the formation contains 

marine invertebrate fossils in locations throughout New 

Mexico and Colorado. 

Geologic Resources in a Cultural Context—Pueblo 

Indians used chert and chalcedony from this formation to 

make stone tools.

None reported.

542 million–252 million years ago: Rise and Fall of the 

Ancestral Rocky Mountains—interbedded offshore and 

land-derived sediments accumulated in the Rowe-Mora 

basin—a basin in a shallow sea trapped between ranges of 

the Ancestral Rocky Mountains.

Alamitos Formation

(PNal) 

Gray, medium to thin-bedded, fossiliferous marine limestone, 

interbedded buff to brown, coarse-grained arkosic 

sandstone, and dark gray to reddish shales. Contact with 

overlying Sangre de Cristo Formation is locally sharp just east 

of Pecos but is gradational or poorly exposed in most places. 

Basal feldspathic unit is cemented by silica or clay. Thickness 

approximately 450 m (1,500 ft). 

Mapped in the Pecos Unit.

Sangre de Cristo Mountains—this formation comprises

the foothills of the Sangre de Cristo Mountains in Pecos 

National Historical Park. 

Paleontological Resources—marine invertebrates such 

as bivalves, gastropods, ammonites, and crinoids are 

documented from this formation in the park. 

Paleontological Resource Inventory, Monitoring, and 

Protection—a field-based survey could help determine 

variety of species and abundance of fossils from this 

formation in Pecos National Historical Park. 

La Posada Formation

(PNlp) 

Gray, medium to thick-bedded, fossiliferous marine 

limestone with banded chert layers. Limestone is interbedded 

with thin shales and sandstones. Uppermost unit is quartz 

sandstone with calcite cement. Thickness approximately 300 

m (1,000 ft).

None reported.

Slope Movement Hazards and Risks—thick limestone 

beds form cliffs where PNlp is exposed near the State Fish 

Hatchery. 

Sandia Formation

(PNs) 

Buff to brown, fine-grained, thinly to thickly laminated

course-grained arkosic sandstone with interbedded gray 

shale, sandy shale, and lesser amounts of black, thin-bedded 

coaly shale. Thickness at least 45 m (150 ft). 

Paleontological Resources—fossil fragments are present 

in oolitic limestone layers outside of Pecos National 

Historical Park. 

None reported.

MISSISSIPPIAN

Tererro Formation

(Mt) 

Buff to dark brown, thick-bedded, coarse grained limestone 

breccia in a calcareous arkose matrix. Thickness 

approximately 15 m (50 ft). Restricted to local outcrops near 

railroad tracks between Lamy and Cañoncito in the Glorieta 

quadrangle. None reported.

542 million–252 million years ago: Rise and Fall of the 

Ancestral Rocky Mountains—landscape of New Mexico 

was subdued at this time and covered in a shallow sea 

Espiritu Santo where limestone and dolomite were deposited.

Formation 

(Mes) 

Gray to brown, dense, microcrystalline dolomite and 

interbedded sandy limestone. Thickness approximately 10 m

(33 ft). Restricted to local outcrops near railroad tracks

between Lamy and Cañoncito in the Glorieta quadrangle.

EARLY PROTEROZOIC EON

Granite to granitic 

gneiss

(Xgg) 

Medium- to coarse–grained, orange granite to gneissic 

granite consisting predominantly of plagioclase, potassium 

feldspar, quartz, biotite and iron oxides. The gneiss is 

strongly deformed. Contains small lenses and layers of 

amphibolite and is intruded by numerous pegmatites.

Mapped in the Cañoncito Subunit.

Sangre de Cristo Mountains—Xgg comprises the 

majority of the core of the Sangre de Cristo Mountains.

Geologic Resources in a Cultural Context—deposits in 

this unit may have been the target of the Old Bradley 

mine, which was north of the Pigeon’s Ranch Subunit.

None reported.

2,500 million–542 million years ago: Development of 

the Structural Foundation—Precambrian igneous and 

metamorphic rocks form the “basement” under all 

younger rocks in Pecos National Historical Park as well as 

the core of the mountains to the north.

Biotite granitoid

(Xg) 

Fine- to medium-grained, orange to pink to red granite. 

Ranges from strongly to weakly foliated. Faults—mapped just east of the Borrego fault zone. 

Granodiorite

(Xgr) 

Medium-grained, pink, unfoliated potassium feldspar, 

plagioclase, quartz and lesser amounts of biotite, chlorite, 

muscovite, epidote, and iron oxides. Xgr crosscuts both 

diorite (Xd) and leucogranite, and inclusions of both rock 

types exist within it. 

Faults—within and east of the Picuris–Pecos fault zone, 

where the rock is highly fractured and brecciated.

Megacrystic granitoid

(Xmg) 

Megacrystic quartz, plagioclase, potassium feldspar, biotite 

granitoid. Megacrysts of potassium feldspar are up to 5 cm 

(2 in) long.

None reported.
Colored map units indicate those mapped within Pecos National Historical Park. Bold text corresponds to headings in the Geologic Features and Processes, Geologic Issues, and Geologic History sections of the report.

PECO Map Unit Properties Table, Page 6 of 6 

Age Map Unit

(Symbol) Geologic Description Geologic Features and Processes Geologic Resource Management Issues Geologic History

EARLY PROTEROZOIC EON

Diorite 

(Xd) 

Medium- to fine-grained, dark greenish diorite with

hornblende, plagioclase, quartz, biotite, epidote, and iron 

oxides.

None reported.

None reported.

2,500 million–542 million years ago: Development of 

the Structural Foundation—Precambrian igneous and 

metamorphic rocks form the “basement” under all 

younger rocks in Pecos National Historical Park as well as 

the core of the mountains to the north.

Amphibolite

(Xa) 

Coarse- to fine-grained, black to green amphibolite with 

blue-green hornblende, plagioclase, quartz, and minor 

sphene. Includes amygdaloidal amphibolite and metavolcanic 

breccia. Continuous layers and discontinuous lenses occur 

within the gneissic granitoid (Xgg).

Quartz muscovite 

schist

(Xqms) 

Gray, fine- to medium-grained, well foliated muscovite-rich 

schist with variable amounts of plagioclase, biotite, quartz, 

and minor amounts of iron oxide and chlorite.

Geologic Resources in a Cultural Context—muscovite 

used in the window panes of mission churches was 

sourced from an exposure of this unit in the mountains 

north of Pecos National Historical Park.

Felsic schist and 

phyllite

(Xf) 

Generally fine-grained, orange, brown, or light-gray schist

and phyllite with primarily quartz, muscovite, microcline and 

plagioclase, and minor biotite and rare garnet. Amphibolite 

and quartzite layers are interlayered with felsic schists. Xf

interpreted as metamorphosed volcanic rocks. None reported.

Biotite and pelitic 

schist

(Xbp) 

A heterogeneous unit of biotite-rich schist, green-gray pelitic 

schist, mafic phyllite, and feldspar-quartz-magnetite phyllite..

Muscovite schist

(Xms) 

Strongly foliated and crenulated gray to brown muscovite 

schist.

Geologic Resources in a Cultural Context—muscovite 

used in the window panes of mission churches was 

sourced from an exposure of this unit in the mountains 

north of Pecos National Historical Park.