National Park Service

U.S. Department of the Interior

Natural Resource Stewardship and Science

Aztec Ruins National Monument

Geologic Resources Inventory Report

Natural Resource Report NPS/NRSS/GRD/NRR—2016/1245
ON THE COVER: Terraces cut by the Animas River dominate the landscape at Aztec Ruins National Monument. 

As mapped by Gillam (1998a), three primary terrace levels occur in the monument. The West Ruin is situated on 

the lowest and youngest terrace level. Two other terrace levels step up from this level and compose the “North 

Mesa” in the monument. That mesa covers the upper left corner of the photograph. Photograph by Kimberly 

Miskell-Gerhardt, taken from a plane piloted by Dan Gerhardt on 22 December 2002. Used with permission.

THIS PAGE: The bedrock of Aztec Ruins National Monument is the Nacimiento Formation, which consists of gray, 

green, and purple claystone, shale, and siltstone, as well as gray and yellow sandstone. The formation was used 

in construction of Aztec Ruins. In the West Ruin, green claystone makes up a distinctive band of a sandstone wall. 

National Park Service photograph available at http://www.nps.gov/media/photo/gallery.htm?id=572EFF35-155D451F-67AE9343238A8B8E.
Aztec Ruins National Monument

Geologic Resources Inventory Report

Natural Resource Report NPS/NRSS/GRD/NRR—2016/1245

Katie KellerLynn 

Colorado State University Research Associate

National Park Service Geologic Resources Division

Geologic Resources Inventory

PO Box 25287

Denver, CO 80225

July 2016

U.S. Department of the Interior 

National Park Service 

Natural Resource Stewardship and Science 

Fort Collins, Colorado
ii

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. 

This report is available from the Geologic Resources Inventory website (http://go.nps.gov/gripubs), 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:

KellerLynn, K. 2016. Aztec Ruins National Monument: Geologic Resources Inventory report. Natural Resource 

Report NPS/NRSS/GRD/NRR—2016/1245. National Park Service, Fort Collins, Colorado.

NPS 319/133454, July 2016
iii

Contents

Page

Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Products and Acknowledgments . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

GRI Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

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

Geologic Features and Processes . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Nacimiento Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Building Stone and Lithic Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Terraces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Alluvium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Eolian Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Geologic Resource Management Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Oil and Gas Development and Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Directional Drilling and Hydraulic Fracturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Adjacent Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Water Impacts on Monument Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Localized and Regional Subsidence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Piping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Bank Erosion and Landscape Restoration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Recreation and Land Use in the Animas River Corridor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Abandoned Mineral Lands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Paleontological Resource Inventory, Monitoring, and Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Geologic History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Laramide Orogeny . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Animas River on the Landscape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Glacial Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Eolian Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Ongoing Incision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Geologic Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Geologic Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Source Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

GRI GIS Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

GRI Posters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Map Unit Properties Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Use Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Additional References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Geology of National Park Service Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

NPS Resource Management Guidance and Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Climate Change Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Geological Surveys and Other Organizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

US Geological Survey Reference Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Appendix A: Scoping Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2007 Scoping Meeting Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2015 Conference Call Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
iv

Contents (continued)

Appendix B: Geologic Resource Laws, Regulations, and Policies . . . . . . . . . . . . . . . . . . . . . . . . . 43

GRI Posters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .in pocket

Map Unit Properties Tables . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .in pocket

Page

Figures

Figure 1. Photograph of the Great Kiva. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. Satellite imagery of Aztec Ruins National Monument . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Figure 3. Map of the Animas Glacier and Animas River valley. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 4. Geologic time scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 5. Illustration of San Juan Basin stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 6. Photograph of the San Juan Mountains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 7. Paleogeographic map of the Western Interior Seaway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 8. Photograph of the Great Kiva interior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 9. Map of lithic resources in the Jemez Mountains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 10. Photograph of terrace 5a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 11. Photograph of terrace 6a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 12. Photograph of terrace 7u . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 13. Photographs of alluvium on the North Mesa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 14. Photograph of Fee 4-A well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 15. Photograph of a soil pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 16. Photograph of rip-rap along the Animas River . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Figure 17. Photographs of fossils at Aztec Ruins National Monument . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 18. Photograph of the Animas River valley in 1897. The view is looking southeast over Animas City . . . . . . . . . . . . . . . . . . . 26

Figure 19. Schematic cross section of terraces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Page

Tables

Page

Table 1. GRI GIS unit correlation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Table 2. Terraces at Aztec Ruins National Monument . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Table 3. GRI GIS data for Aztec Ruins National Monument (Aztec quadrangle; azte_geology.mxd; Brown and Stone 1979) . . . . . . . . . . . 31

Table 4. GRI GIS data for Aztec Ruins National Monument (Flora Vista quadrangle; flvi_geology.mxd; Ward 1990) . . . . . . . . . . . . . . . 31

Table 5. GRI GIS data for Aztec Ruins National Monument (terrace map; azrt_geology.mxd; Gillam 1998a) . . . . . . . . . . . . . . . . . . . 31
v

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 inventory. This report synthesizes 

discussions from a scoping meeting for Aztec Ruins National Monument (New Mexico) on 13 February 

2007 and a follow-up conference call on 28 May 2015, which were held by the Geologic Resources 

Division to identify significant geologic resources and geologic resource management issues, as well as 

determine the status of geologic mapping. It is a companion document to previously completed GRI GIS 

data.

This GRI report was written for resource managers at 

Aztec Ruins National Monument to support scienceinformed decision making. It also may be useful for 

interpretation. The report was prepared using available 

geologic information, and the NPS Geologic Resources 

Division conducted no new fieldwork in association 

with its preparation. The report discusses distinctive 

geologic features and processes within the monument, 

highlights geologic issues facing resource managers, 

describes the geologic history leading to the present-day 

landscape, and provides information about the GRI GIS 

data that accompany this report. 

The GRI team used three source maps to compile the 

GRI GIS data for the monument. Data from Brown and 

Stone (1979) covers the western side of the monument; 

data from Ward (1990) covers the eastern side of the 

monument. These data are illustrated on a GRI poster 

(in pocket) and referred to as the “bedrock and surficial 

map.” Data from Gillam (1998a) covers the entire 

monument. A GRI poster (in pocket) illustrating those 

data is referred to as the “geomorphic map.” In addition, 

two Map Unit Properties Tables (in pocket) summarize 

report content for the GRI GIS data. One of these tables 

highlights bedrock and surficial deposits as mapped by 

Brown and Stone (1979) and Ward (1990); the other 

highlights geomorphic features, namely terraces and the 

modern floodplain, as mapped by Gillam (1998a). 

Aztec Ruins National Monument was established in 

1923 to preserve the remarkable remnants of a large 

ancestral Puebloan community, which flourished 

from 1050 to 1150 CE (common era [preferred to 

AD]). At that time, it was one of the largest Puebloan 

settlements of the American Southwest. The pueblo was 

strategically situated between Mesa Verde to the north 

and Chaco Canyon to the south.

Aztec Ruins National Monument is in the San Juan 

Basin—a structural depression that formed as the Rocky 

Mountains were rising during the Laramide Orogeny 

(75 million–40 million years ago). The bedrock of the 

monument—the Nacimiento Formation (map unit 

Tn)—lies near the top of a thick sedimentary package 

of rocks that fills the San Juan Basin. The strata are as 

much as 4,600 m (15,000 ft) thick. The sediments that 

make up the Nacimiento Formation were deposited 

64.5 million–61.0 million years ago (Paleocene Epoch) 

in floodplains, river channels, swamps, and lakes. 

The ruins themselves sit on a Quaternary terrace that 

was cut into the valley floor by the Animas River. The 

river appeared on the scene after the Laramide Orogeny 

but before the Pleistocene ice ages. The terrace upon 

which the West and East Ruins lies and two other 

terraces rise step-like above the modern floodplain and 

record the geologically recent but complex history of 

the Animas River valley. Meltwater from the enormous 

Animas Glacier in the upper valley transported glacial 

sediments, called “outwash,” downvalley, while the 

Animas River channeled into unconsolidated material 

and bedrock, creating numerous terraces in the process. 

At some counts, 34 different terrace levels were cut 

within the valley. Some rise as high as 660 m (2,170 

ft) above the valley floor. Terraces record climaterelated or other depositional cycles, probably reaching 

back at least 3 million years (late Pliocene Epoch). 

Terraces indicate that incision was not uniform but 

punctuated by periods when downcutting stopped and 

the river system either stayed at the same elevation or 

aggraded (built up). Each terrace level represents a past 

floodplain and marks the former course of the Animas 

River. These landforms dominate the monument and 

the surrounding viewshed.
vi

During the 2007 scoping meeting and 2015 conference 

call, participants (see Appendix A) identified 

the following geologic features and processes of 

significance for Aztec Ruins National Monument: 

● Nacimiento Formation. The bedrock underlying 

the monument lies towards the top of the San 

Juan Basin sequence of sedimentary rocks. It is 

the Paleocene Nacimiento Formation of primarily 

sandstone and shale. Elsewhere, the formation 

contains fossils significant for the beginning of 

the “Age of Mammals.” No such fossils have been 

found in the monument to date.

● Building Stone and Lithic Resources. Ancestral 

Puebloans used blocks of Nacimiento Formation 

in construction. They also used materials deposited 

by the Animas River that range in size from cobbles 

(used in foundations) to silt (used for mortar). 

Lithic resources found at the monument include a 

variety of local and nonlocal materials. 

● Terraces. As mapped by Gillam (1998a), the 

monument contains three primary terrace levels. 

Investigators have traced these terrace deposits to 

at least three ages of glacial moraines (ridge-like 

landforms composed of rock fragments deposited 

directly from glacial ice) at Durango, Colorado. 

The oldest terrace level (map unit Qt5a) formed 

approximately 340,000–250,000 years ago and is 

associated with the Durango moraines. The middle 

terrace level (Qt6a) formed approximately 160,000–

140,000 years ago and is associated with the Spring 

Creek moraines. The youngest terrace level (Qt7u) 

formed approximately 25,000–19,000 years ago and 

is associated with the Animas City moraines. These 

moraines developed during the pre-Bull Lake, Bull 

Lake, and Pinedale glaciations, respectively.

● Alluvium. The variety of rock types found in the 

monument reflects the course of the Animas River. 

Material transported and deposited by a river is 

referred to as “alluvium.” It ranges in size from 

boulders to silt. In addition to Animas River (“main 

river”) alluvium, tributary streams and arroyos also 

contributed alluvium to the landscape. Animas 

River and tributary alluviums comprise a complex 

record of fluvial activity that took place before, 

during, and after the Animas Glacier advanced and 

retreated in the Animas River valley.

● Eolian Features. Deposits of windblown dust, 

called “loess,” occur within the monument. These 

deposits may be as thick as 1 m (3 ft). Ancestral 

Puebloans mined the loess for its use as a primary 

ingredient in mortar and may have dug subsurface 

structures into thick loess layers. The Lava Creek 

B ash is another eolian feature of interest for the 

monument. This volcanic ash layer, which erupted 

from the Yellowstone caldera about 639,000 years 

ago, is a stratigraphic marker that investigators used 

to estimate the ages of terraces in the area. No ash 

occurs in the monument, but Gillam (1998b) found 

17 locales of Lava Creek B ash in Animas River 

valley. 

Geologic resource management issues identified during 

the GRI scoping meeting and follow-up conference call 

include the following. They are listed more-or-less by 

management priority:

● Oil and Gas Development and Production. 

Oil and gas development is a primary concern 

for resource managers at the monument. Four 

active, nonfederal oil and gas operations and 

two associated gathering lines occur within 

the boundaries. The source of the produced 

hydrocarbons is Upper Cretaceous sedimentary 

rocks, which lie below the surface. 

● Directional Drilling and Hydraulic Fracturing. 

Technological improvements in directional 

drilling and hydraulic fracturing, commonly called 

“fracking,” have spurred renewed interest by the oil 

and gas industry in the San Juan Basin. At present, 

all the wells in the monument are conventional, but 

directional drilling and fracking could be applied 

to enhance production from these wells in the 

future. Impacts to cultural resources, including 

archeological structures, as a result of vibrations 

from transportation and drilling are specific 

concerns. The approximate “safe distance” for 

directional drilling and fracking is unknown at this 

time.

● Adjacent Development. The monument is 

surrounded by the developing City of Aztec, 

resulting in a paucity of open space and concerns 

of trespass and access-related vandalism to 

monument resources. The primary area of concern 

is a proposed subdivision, Mesa Escondido, at the 

northern boundary of the monument. Impacts 

from development include loss of archeological 

resources, such as the destruction of a Chacoan 

road, as well as runoff-related erosion and visual 

impairments.
vii

● Water Impacts on Monument Resources. 

Structural deterioration associated with water at 

Aztec Ruins has been a problem for many decades. 

In an effort to create a mitigation plan for protecting 

archeological sites at the monument, a hydrologic 

study began in 2005. Investigators completed a 

final report in 2014. Issues include past irrigation 

of agricultural fields, transport of water through 

Farmers Ditch, and soil moisture related to 

groundwater levels.

● Localized and Regional Subsidence. Localized 

subsidence is occurring near East Ruin. The origin 

of these small depressions is not apparent, though 

some causes were proposed by GRI scoping 

participants. Additionally, since the late 1980s, 

methane has been produced from coal beds, 

predominantly within the Fruitland Formation. 

Large amounts of groundwater are extracted during 

methane production, causing centimeter-scale 

subsidence across the San Juan Basin. Because this 

type of subsidence does not produce differential 

surface movements over short distances, it is 

unlikely to directly affect the ruins.

● Piping. The formation of soil pipes, as a result 

of percolating subsurface water and the removal 

of soil material, is another hydrologic issue of 

concern. Soil pipes have formed in the banks along 

the Animas River. Piping could affect the stability 

of archeological sites via subsidence but may also 

have cultural significance. Ancestral Puebloans may 

have excavated soil pipes to create kivas (subsurface 

ceremonial chambers).

● Bank Erosion and Landscape Restoration. In 

order to prevent bank erosion (and deterioration 

of an archeological site), the former owner of the 

“Fallon property” installed concrete slabs as rip-rap 

along the Animas River channel. The National Park 

Service acquired this property in 2009. The goal 

for this property and other lands acquired within 

the monument’s legislative boundary is to restore 

natural function and appearance while protecting 

cultural resources. 

● Recreation and Land Use in the Animas 

River Corridor. A trail and bridge now connect 

downtown Aztec with the monument. Monument 

managers have added buck-and-rail fencing in this 

area to minimize the proliferation of social trails. 

Because no park infrastructure has been built on 

the floodplain, flooding of the Animas River is a 

minor management concern.

● Abandoned Mineral Lands. Resource 

management of abandoned mineral lands (AML) 

requires an accurate inventory and reporting of 

features in the servicewide AML database. During 

the scoping process, participants identified two 

abandoned gas wells at the monument. These 

wells were plugged in the 1970s. One additional 

plugged and abandoned gas well occurs within 

the expanded boundaries of the monument. The 

National Park Service has not yet acquired the land 

upon which this well is located, so no mitigation has 

been conducted to date.

● Paleontological Resource Inventory, Monitoring, 

and Protection. Aztec Ruins National Monument 

was included in the 2009 paleontological resource 

inventory and monitoring report for the Southern 

Colorado Plateau Network. In 2015, an investigator 

completed a reconnaissance level field survey at 

the monument. No fossils have been discovered 

within bedrock (Nacimiento Formation) at the 

monument to date, but elsewhere the formation 

has yielded fossils that are significant for the “Age 

of Mammals.” Limestone cobbles in the terraces 

contain reworked Paleozoic fossils, including 

brachiopods, crinoids, and horn corals. The 

monument’s collections contain petrified wood 

associated with archeological artifacts. Terrace 

deposits also contain pieces of reworked petrified 

wood of unknown origin.

ix

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 GRI 

products. This chapter 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 (NPS-75). The “Additional 

References” chapter 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://go.nps.gov/gri. 

The current status and projected completion dates of 

products are available at http://go.nps.gov/gri_status.

Acknowledgments

The GRI team would like to thank the participants 

of the 2007 scoping meeting and 2015 conference 

call (see Appendix A) for their assistance with this 

inventory. In addition, thanks to Edward Kassman, 

Jeremiah Kimbell, and Steve Simon (NPS Geologic 

Resources Division) for their input about oil and gas 

issues at the monument. Also, thanks to Phil Varela

(Chaco Culture National Historical Park) for his input, 

timely field survey, and photographs of paleontological 

resources. Finally, thanks very much to Mary Gillam

(independent consultant and researcher) for her 

follow-up on various geologic features and issues and 

her thorough review of the GRI report. Her comments 

greatly helped to clarify and improve the final product.

Review

Aron Adams (Aztec Ruins National Monument)

Mary Gillam (independent consultant and researcher)

Dana Hawkins (Aztec Ruins National Monument)

Jason Kenworthy (NPS Geologic Resources Division)

Jeremiah Kimbell (NPS Geologic Resources Division)

Phil Varela (Chaco Culture National Historical Park)

Editing

Rebecca Port (NPS Geologic Resources Division)

Report Formatting and Distribution

Jason Kenworthy (NPS Geologic Resources Division)

Rebecca Port (NPS Geologic Resources Division)

Source Maps

D. R. Brown and W. J. Stone (New Mexico Bureau of 

Mines and Mineral Resources)

M. L. Gillam (University of Colorado, dissertation)

A. W. Ward (US Geological Survey)

GRI GIS Data Production

Heather Stanton (Colorado State University)

Jason Isherwood (Colorado State University)

David Plume (NPS Geologic Resources Division)

Tim Cleland (Colorado State University)

Stephanie O’Meara (Colorado State University)

Georgia Hybels (Colorado State University)
x

GRI Poster Design

Georgia Hybels (Colorado State University)

GRI Poster Editing

Georgia Hybels (Colorado State University)

Rebecca Port (NPS Geologic Resources Division)

Jason Kenworthy (NPS Geologic Resources Division)
1

Geologic Setting and Significance

This chapter describes the regional geologic setting of Aztec Ruins National Monument and 

summarizes connections among geologic resources, other park resources, and park stories.

Aztec Ruins National Monument protects an 

exceptionally well-preserved great house community, 

also referred to as a “pueblo,” along the banks of the 

Animas River in northwestern New Mexico (National 

Park Service 2015). The monument, which provides 

opportunities for greater understanding of the evolution 

of the Chacoan culture, is nationally and internationally 

significant. It is listed in the National Register of 

Historic Places and is part of a world heritage site that 

includes Chaco Culture National Historical Park in 

the National Park System and five smaller Chacoan 

sites managed by the Bureau of Land Management. 

Located between two major cultural centers—Chaco 

Canyon to the south (see GRI report about Chaco 

Culture National Historical Park by KellerLynn 2015b) 

and Mesa Verde to the north (see GRI report about 

Mesa Verde National Park by Graham 2006)—Aztec 

Ruins was one of the largest ancestral Puebloan 

settlements in the Southwest (Thybony 1992). The 

complex consisted of several great houses, tri-walled 

and great kivas, small residential pueblos, earthworks, 

and roads. Far from being uncontrolled urban sprawl, 

however, the formal layout of the settlement, purposeful 

landscape modifications, and the orientation and visual 

relationships among the buildings all indicate planning 

of a grand design (National Park Service 2003). 

Contrary to the name of the monument, the Aztecs of 

central Mexico did not inhabit this place. Inspired by 

popular histories about Cortez’s conquest of Mexico 

and thinking that Aztecs built these structures, early 

Anglo settlers named the site “Aztec,” and the nearby 

city eventually took its name from the site (National 

Park Service 2003). The Aztecs lived centuries after 

the structures at “Aztec Ruins” were built. Ancestral 

Puebloans lived at Aztec for nearly 200 years, from 1098 

to 1265 CE. 

Since its proclamation in 1923, Aztec Ruins National 

Monument had has various boundary changes but now 

encompasses 129 ha (318 ac). The monument preserves 

a primary group of ruins consisting of the West Ruin 

and East Ruin complexes. The West Ruin encloses 

a central plaza that prominently contains the Great 

Kiva—a partly subterranean ceremonial structure

Figure 1. Photograph of the Great Kiva. The Great Kiva in the West Ruin plaza is the only reconstructed great 

kiva in the American Southwest. National Park Service photograph available at http://www.nps.gov/azru/learn/

photosmultimedia/index.htm.
2

Figure 2. Satellite imagery of Aztec Ruins National Monument. The City of Aztec, New Mexico, and urban development 

surround the monunment (green outline). Gillam (1998a) mapped three primary terraces in the monument: Qt5a, 

Qt6a, and Qt7u, as well as the modern floodplain (Qfp). A proposed subdivision, Mesa Escondido, is directly north of 

the monument. Four active nonfederal oil and gas wells (yellow outlines and labels) and two associated gathering 

lines (not shown on figure) are within or adjacent to the monument. Graphic compiled by Jason Kenworthy (NPS 

Geologic Resources Division). Imagery from ESRI “World Imagery” ArcMap basemap (accessed 28 April 2016) with data 

from Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, 

IGP, swisstopo, and the GIS User Community.
3

14 m (46 ft) across (fig. 1). This is the only Chacoan 

great kiva to have been completely reconstructed. 

The ruins also host original intact masonry, wood and 

earthen roofs, artifacts, and earthworks. Surrounding 

the ruins, prehistoric roadways and the overall 

landscape are other significant resources associated 

with the monument (National Park Service 2010).

The Animas River flows along the eastern boundary 

of the monument (fig. 2). Many modern Puebloans 

know the monument as the “Place by Flowing Waters,” 

though other names exist (KellerLynn 2007). Ancestral 

Puebloans took advantage of the accessible river 

water for farming and other uses. This source of water 

allowed for the development of fertile bottomlands 

during prehistoric times. The first European visitors to 

the area reported seeing signs of prehistoric irrigation 

features (Price 2010).

The Animas River valley is roughly 165 km (100 mi) 

long (Gillam 1998b), starting at the crest of the western 

San Juan Mountains in Colorado and ending where the 

Animas River joins the San Juan River (a major tributary 

of the Colorado River) near Farmington, New Mexico. 

The valley covers approximately 3,525 km2

 (1,360 mi2) 

(Denis et al. 1985).

The Animas River valley is significant as the site of 

one of the largest former glaciers in the US Rocky 

Mountains (fig. 3; Gillam 1998b). The glacier produced 

massive amounts of debris that the river reworked, 

sometimes aggrading (building up) and sometimes 

incising (cutting into), forming a series of terraces 

that dominates the landscape and viewshed of the 

monument. The morphologically complex terraces 

represent hundreds of thousands of years of fluvial and 

glacial activity. The highest terraces in the valley (not in 

the monument) developed before periodic glaciations 

began. The modern floodplain formed after the last 

glaciation ended.

The Animas River valley and Aztec Ruins National 

Monument are situated within the San Juan 

Basin, which is the dominant structural feature in 

northwestern New Mexico and southwestern Colorado. 

The basin is roughly 160 km (100 mi) long, north–south, 

and 145 km (90 mi) wide, east–west; it extends across an 

area of about 19,400 km2

 (7,500 mi2). The basin formed 

as the Rocky Mountains rose (and adjacent basins 

subsided) during the Laramide Orogeny (75 million–40 

million years ago; see fig. 4 for a geologic time scale).

In cross section, the asymmetrical San Juan Basin 

resembles a set of nested mixing bowls (fig. 5). The 

sequence of rocks is as much as 4,600 m (15,000 ft) 

thick (Fassett and Hinds 1971) and ranges in age from 

500 million (Cambrian Period; fig. 4) to 50 million years 

ago (Eocene Epoch; fig. 4). The strata dip from the 

margin toward the deepest part of the basin. Crystalline 

rocks more than a billion years old line the perimeter 

of the basin in uplifted mountain ranges such as the 

Nacimiento and Zuni mountains in New Mexico and 

the San Juan Mountains in Colorado (fig. 6). These 

mountain ranges have ancient cores, approximately 

1.7 billion to 1.4 billion years old (Proterozoic Era or 

Precambrian; fig. 4). Thus, the rocks are progressively 

older away from the center of the basin where the strata 

are thickest and the youngest strata are exposed. 

Bedrock at the monument consists of the 60-millionyear-old (Paleocene Epoch; fig. 4) Nacimiento 

Formation (Tn), which is one of the youngest 

sedimentary layers in the San Juan Basin. The youngest 

sedimentary-rock unit, the San Jose Formation (Tsj), is 

exposed in the vicinity of the monument (see GRI GIS 

data). The San Jose Formation contains a world-famous 

fossil record of early mammals and associated plants, 

fishes, and reptiles (Lucas 2010). Minor volcanic rocks 

of mid-Tertiary age (35 million–18 million years old) 

and various Quaternary deposits (less than 2.6 million 

years old) also are present in the basin (Levings et al. 

1990).

Upper Cretaceous rocks in the basin, which are more 

than 1,800 m (6,000 ft) thick, were deposited in the 

Western Interior Seaway, which spread across the North 

American continent from the Arctic to the Gulf of 

Mexico (fig. 7). The seaway advanced and retreated for 

30 million years (approximately 100 million–70 million 

years ago), depositing sediments in marine and coastal 

settings. Related continental deposits accumulated on 

surrounding lowlands. The final retreat of the seaway 

is represented by the Pictured Cliffs Sandstone, which 

crops out in Chaco Culture National Historical Park 

(see GRI report by KellerLynn 2015b). Excellent 

exposures of Upper Cretaceous rocks occur in Chaco 

Canyon. 

The remains of organisms (organic matter) deposited 

along with sediments in the Wester Interior Seaway 

would later be converted to hydrocarbons as the basin 

subsided during the Laramide Orogeny and more 
4

sediments accumulated atop previously deposited 

Upper Cretaceous rocks. By the end of the Laramide 

Orogeny, these rocks had reached a maximum depth of 

burial, and following the orogeny, regional heating of 

deeply buried organic matter resulted in the generation 

of oil and gas. The combination of thick Cretaceous 

source rocks and a large area covered by suitable 

reservoir rocks made the San Juan Basin one of the most 

important gas-producing basins in the United States 

(Engler et al. 2001; Brister and Hoffman 2002).

Figure 3. Map of the Animas Glacier and Animas River valley. The Animas River has its headwaters in the San Juan 

Mountains of Colorado and flows southward into New Mexico. During past glaciations, the upper valley contained 

the Animas Glacier. Its farthest advance is marked by ridge-like moraines at Durango, Colorado. The lower valley is 

filled with outwash transported beyond these glacial moraines. The Animas River episodically cut terraces into the 

valley floor/outwash plain. Graphic byTrista Thornberry-Ehrlich (Colorado State University) after Gillam (1998b, figure 

2.3). Imagery from ESRI “World Imagery” ArcMap basemap (accessed 28 April 2016) with data from Esri, DigitalGlobe, 

GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the 

GIS User Community. Shaded relief inset from ESRI “World Terrain Base” ArcMap basemap with data from Esri, USGS, 

and NOAA.
5

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 period are in parentheses. The 

bedrock underlying the monument was deposited during the Paleocene Epoch. The Animas River cut terraces into 

glacial outwash during the Pleistocene Epoch. The river continues to incise today (Holocene Epoch). The Pleistocene 

and Holocene epochs are in the Quaternary Period so the geomorphic map units within the monument begin with 

a “Q”. Compass directions in parentheses listed with “North American Events” indicate regional locations. 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).
6

Figure 5. Illustration of San Juan Basin stratigraphy. During the Cretaceous Period, the San Juan Basin occupied a 

small area on the western edge of the Western Interior Seaway where sediments were deposited in marine, coastal, 

and terrestrial settings and include the Crevasse Canyon Formation, Cliff House Sandstone, Menefee Formation, Lewis 

Shale, and Pictured Cliffs Sandstone (shown as Cretaceous rocks in light green on the illustration). These rocks are 

exposed along the Chaco River in Chaco Culture National Historical Park. During the Laramide Orogeny, the basin 

began to subside and thousands of feet of additional terrestrial sediment accumulated (shown as Tertiary rocks in 

orange). The Nacimiento Formation underlies Aztec Ruins National Monument. The San Jose Formation is exposed in 

the vicinity. Graphic by Trista Thornberry-Ehrlich (Colorado State University) after Lyford (1979, figure 3).

Figure 6. Photograph of 

the San Juan Mountains. 

A Precambrian core of 

ancient metamorphic 

rock (gneisses and schists) 

was lifted up during the 

Laramide Orogeny and is 

now beautifully exposed 

in the San Juan Mountains 

of Colorado. The Animas 

River has its headwaters in 

these mountains. Wikimedia 

Commons photograph by 

John Fowler [CC BY 2.0 (http://

creativecommons.org/licenses/

by/2.0)] available at https://

commons.wikimedia.org/wiki/

File%3ASan_Juans_north_of_

Durango.jpg.
7

Figure 7. Paleogeographic 

map of the Western Interior 

Seaway. During the Cretaceous 

Period, an expansive seaway 

spread across the North 

American continent. The 

seaway inundated New 

Mexico about 96 million years 

ago. This map represents 

the maximum extent of the 

seaway at about 85 million 

years ago. The red star 

indicates the approximate 

location of Chaco Culture 

National Historical Park, 

where Upper Cretaceous rocks 

are now exposed in Chaco 

Canyon about 85 km (53 mi) 

south of Aztec Ruins National 

Monument. These same rocks 

lie below the surface at Aztec 

Ruins National Monument. 

Graphic compiled by Jason 

Kenworthy (NPS Geologic 

Resources Division). Base 

paleogeographic map by Ron 

Blakey (Colorado Plateau 

Geosystems, Inc.) available at 

http://cpgeosystems.com/index.

html.

9

Geologic Features and Processes

This chapter describes noteworthy geologic features and processes in Aztec Ruins National Monument.

Geologic features and processes of significance for the 

monument include the following. They are discussed 

more or less stratigraphically (oldest to youngest).

● Nacimiento Formation

● Building Stone and Lithic Resources

● Terraces

● Alluvium

● Eolian Features

Information about many of these features came from 

the three source maps used in compiling the GRI GIS 

data (see “Geologic Data” section). Brown and Stone 

(1979; scale 1:62,000) mapped the Aztec 15' quadrangle, 

which covers the eastern side of the monument. Ward 

(1990; scale 1:100,000) mapped the Farmington 30' × 

60' quadrangle, which covers the western side of the 

monument. Both maps show bedrock (Nacimento 

Formation) and selected Quaternary deposits such 

as terraces and alluvium. Mapping by Gillam (1998a; 

working scale 1:24,000, compiled at 1:50,000) 

delineated all major terraces and the modern floodplain 

in the monument. 

Nacimiento Formation

The bedrock underlying the monument is the 

Nacimiento Formation (Tn), which was deposited 

about 64.5 million–61.0 million years ago (Paleocene 

Epoch; fig. 4). It consists of sedimentary rocks such as 

claystone, shale, siltstone, and sandstone (Brown and 

Stone 1979) of primarily continental origins such as 

floodplains, river channels, swamps, and lakes (Lucas 

and Ingersoll 1981a, 1981b; Williamson 1996). The 

sediments that make up these rocks were shed from 

the rising San Juan and Brazos–Sangre de Cristo uplifts 

to the north and east of the monument during the 

Laramide Orogeny (Lucas 1984; see “Geologic History” 

chapter). The Nacimiento Formation can be as much 

as 525 m (1,720 ft) thick (Williamson and Lucas 1992). 

Where exposed at the surface near the monument, 

the formation typically erodes to low, rounded hills or 

badlands (Levings et al. 1990). 

Although the Nacimiento Formation was mapped by 

Ward (1990) throughout much of the western portion 

of the monument (the Flora Vista quadrangle), the 

formation is almost entirely covered by alluvium (stream 

deposits), colluvium (foot-of-slope or cliff deposits), or 

terrace gravel (see “Terraces” and “Alluvium” sections). 

Christensen (1979) measured a depth to bedrock 

of 23 m (77 ft) in the water well at the monument’s 

visitor center. Ward (1990) disregarded many surface 

sediments in order to emphasize the underlying 

bedrock (Mary Gillam, independent consultant and 

researcher, written communication, 5 January 2016).

Noting exposures of bedrock is significant for the 

geologic resources inventory, especially in parks where 

they are rare. During the 2007 scoping meeting and 

site visit, participants thought that the Nacimiento 

Formation was exposed at only a single outcrop 

in the monument (KellerLynn 2007). Since then, 

however, additional exposures have been discovered; 

for example, during a field survey for paleontological 

resources, an investigator identified five exposures of 

the Nacimiento Formation in drainages on the mesa 

locally known as “North Mesa,” which rises to the 

north and west above Farmers Ditch in the monument 

(fig. 2). These “exposed areas” vary from 45 to 250 m2

(480 to 2,700 ft2) but in actuality are not completely 

bare; they are covered by patches of overlying sediment 

and vegetation. These areas are located in deeply cut 

drainages on North Mesa where erosion has exposed 

the bedrock (Phil Varela, Chaco Culture National 

Historical Park, paleontology technician, email 

communication, 2 February 2016).

With so few bedrock exposures in the monument, the 

likelihood of finding in situ fossils is slim (Tweet et al. 

2009), and no fossils have been found in the Nacimiento 

Formation within the monument to date (Phil Varela, 

Chaco Culture National Historical Park, paleontology 

technician, written communication, 5 November 

2015). Elsewhere in the San Juan Basin, however, the 

Nacimiento Formation yields a diversity of vertebrate, 

invertebrate, and plant fossils indicative of a variety of 

depositional settings (see Tweet et al. 2009). Because of 

its age, which is contemporaneous with the beginning 

of the “Age of Mammals,” the Nacimiento Formation is 

of interest for understanding the species that survived 

the Cretaceous–Tertiary (K–T) mass extinction event, 

which marks the loss of an estimated 50% of all 
10

species, including dinosaurs, living at that time. The 

mammals preserved in the Nacimiento Formation 

were mostly archaic forms that were later replaced 

(Williamson and Lucas 1992). They are important 

because they provide a record of mammalian 

evolution immediately after the K–T extinction, in 

an area with few other contemporaneous faunas 

(Weil and Williamson 2007).

Building Stone and Lithic Resources

Blocks of the Nacimiento Formation were used 

prehistorically in construction at Aztec Ruins. 

Ancient builders carried these blocks by hand from 

quarries several kilometers away, then used stone 

hammers, mauls, and pecking tools to break and 

dress (put the finishing touches on) the stone (Cajete 

and Nichols 2004). Ancient builders dabbled with 

the variety of finer and coarser grained rock types 

and colors that comprise the Nacimiento Formation. 

For example, the distinctive “greenstone” bed was 

incorporated as a decorative band in the wall of the 

West Ruin (see inside front cover). The greenstone 

was quarried outside of the monument at a site 

near Tucker Canyon, about 5 km (3 mi) northeast of 

the monument (KellerLynn 2007). Interestingly, outer 

walls were probably plastered with mud, obscuring any 

detailed stonework beneath, but this practice shielded 

mortar from the eroding forces of wind, rain, and snow 

(Cajete and Nichols 2004). 

The Great Kiva contains large limestone disks (fig. 8). 

Each disk weighs about 160 kg (355 lbs) (Cajete and 

Nichols 2004). The source of these disks is unknown, 

but in October 2015, Gary Gianniny (professor 

and carbonate rock expert at Fort Lewis College) 

visited the monument and conducted a cursory 

examination of these disks. Gianniny found the 

limestone comprising the disks “atypical” of marine 

limestone known from the Hermosa Formation and 

Leadville Limestone, which are exposed in the region. 

Furthermore, the continental Nacimiento Formation 

is not known to contain lacustrine limestone (Mary 

Gillam, independent consultant and researcher, written 

communication, 5 January 2016). With permission, 

Gianniny took a small chip from a broken interior edge 

of one of the disks to examine under a microscope; his 

findings are forthcoming (Mary Gillam, independent 

consultant and researcher, written communication, 5 

January 2016).

The terraces (see “Terraces” section) at the monument 

were a source of local material used in construction 

and tool making. This material was reworked from 

elsewhere and brought to Aztec Ruins by glacial 

meltwater and the Animas River. Cobbles in the 

terraces, which are from a variety of upstream sources 

(see “Alluvium” section), are plentiful: they were 

used for wall foundations (Cajete and Nichols 2004). 

Limestone cobbles in the terraces yield notable fossils 

(see “Paleontological Resource Inventory, Monitoring, 

and Protection” section).

Petrified wood of unknown origin occurs in the 

terraces. It was a lithic resource found along with other 

artifacts at the ruins (see “Paleontological Resource 

Inventory, Monitoring, and Preservation” section).

Nonlocal materials utilized by ancestral Puebloans 

include obsidian (“volcanic glass”) and chert 

(microcrystalline quartz; also known as “flint” or 

“jasper”). These lithic materials were traded throughout 

the Southwest and transported by humans to Aztec 

Ruins. Obsidian was used for making tools and 

projectile points. The conchoidal (smoothly curved like 

a “conch” shell) fracturing of chert made it suitable for 

cutting and scraping.

A likely source area of obsidian and chert was the 

Jemez Mountains, which make up a volcanic field that 

Figure 8. Photograph of the Great Kiva interior. Morning 

light breaks into the darkness of the Great Kiva. The kiva 

was excavated in 1921 and reconstructed in 1934. Note the 

limestone disks on either side of the staircase. National Park 

Service photograph available at http://www.nps.gov/azru/

learn/photosmultimedia/index.htm.
11

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 2015a). 

Three source locations of obsidian appear to have been 

the most important for lithic raw material:

(1) El Rechuelos, (2) Cerro Toledo, and (3) Valle Grande 

(fig. 9). Each has a distinctive chemical signature and 

can be differentiated by x-ray florescence or neutron 

activation (Baugh and Nelson 1987; Glascock and Neff 

1993). Some chert found in the monument probably 

came from Narbona Pass in the Chuska Mountains 

southwest of the monument (KellerLynn 2007). 

Another source of chert was Cerro Pedernal, meaning 

“Flint Hill” in Spanish, which is a mesa on the northern 

Figure 9. Map of lithic resources in the Jemez Mountains. The Jemez Mountains were a source area of lithic resources 

for ancestral Puebloans. The map shows source areas of obsidian (orange), including Cerro del Medio, Rabbit 

Mountain, and Obsidian Ridge at Valles caldera, as well as source areas of chert (pink). At the mouths of tributaries to 

the Rio Grande (within yellow area on the map), Cerros del Rio volcanic rocks yielded dacite (erroneously referred to 

as “basalt” in some archeological studies; see Shackley 2011) suitable for tool making. Bandelier National Monument 

contains a dacite quarry that exploited this material. Graphic by Trista Thornberry-Ehrlich (Colorado State University) 

using GRI GIS data for Bandelier National Monument and information from Baugh and Nelson (1987), Kilby and 

Cunningham (2002), Walsh (2005), Gauthier et al. (2007), and Civitello and Gauthier (2013).
12

flank of the Jemez Mountains (fig. 9; Kilby and 

Cunningham 2002). That chert was obtained from 

Miocene sedimentary deposits of the Santa Fe 

Group (Wilks 2005). 

Terraces

During the Pleistocene ice ages, the San Juan 

Mountains were draped in a 5,000 km2 (2,000 mi2) 

ice cap (fig. 3; Atwood and Mather 1932). The 

Animas Glacier drained into the Animas River valley 

and was the longest outlet glacier of that ice cap. The 

glacier produced huge amounts of debris, some of 

which built into ridge-like moraines (see “Geologic 

History” section). Downvalley of these moraines, 

meltwater streams spread debris across the valley 

floor. This material is known as “outwash” because 

it “washed out” beyond the terminus of the glacier. 

Layers of outwash, composed of mostly gravel but 

also larger and smaller rock fragments, once spread 

continuously southward from the moraines at 

Durango, Colorado.

In time, the Animas River cut down through the 

layers of outwash into bedrock to create broad 

terraces, which once extended 80 km (50 mi) 

downvalley to the junction with the San Juan River 

(Gillam 1998a, 1998b). Later, erosion removed 

parts of the outwash plain, including in places, the 

parts next to moraines, so these layers are now 

discontinuous. The landscape of the monument is 

dominated by three terraces. These features step 

up from the modern floodplain and represent 

downward incision of the Animas River through 

time. As such, the highest terrace level is the oldest. 

As mapped by Gillam (1998a; see fig. 2 and poster 

[in pocket]), the three principal terrace levels at the 

monument are terrace 5a (Qt5a; fig. 10), which is 

49–50m (160–165 ft) above the modern floodplain 

(Qfp); terrace 6a (Qt6a; fig. 11), which is 30–34 m 

(100–110 ft) above the modern floodplain; and 

terrace 7, undivided (Qt7u; fig. 12), which includes at 

least two benches with an elevation of roughly 3–6 

m (15–20 ft) above the modern floodplain (Mary 

Gillam, independent consultant and researcher, 

email communication, 4 June 2007). Numbers 

associated with terraces (e.g., 5, 6, and 7) indicate 

relative-age groups; the smaller the number, the 

older and higher the terrace above the present-day 

floodplain. A letter associated with a group number, for 

example “a,” indicates a single level within a group; “u” 

indicates an “undivided” group comprising at least two 

closely spaced levels. 

Figure 10. Photograph of terrace 5a. The highest and 

oldest terrace at Aztec Ruins National Monument is middle 

Pleistocene terrace 5a (Qt5a). It makes up the upper part 

of North Mesa in the monument. The Fee 4-A oil and gas 

operation, including well and pad (shown here), covers a 

portion of terrace 5a (Qt5a; see fig. 2). National Park Service 

photograph from Filippone and Martin (2014, photograph 10). 

Figure 11. Photograph of terrace 6a. The intermediate or 

middle-aged terrace at Aztec Ruins National Monument is 

the late–middle Pleistocene terrace 6a (Qt6a). It is shown 

at the center of the photograph (northwest side of Farmers 

Ditch). It makes up the lower bench of North Mesa. National 

Park Service photograph from Filippone and Martin (2014, 

photograph 11).
13

In the western part of the monument, Ward (1990) 

mapped two units of terrace gravel (Q4d and Q5b; table 

1); these correlate to the two higher of Gillam’s three 

terrace levels. Brown and Stone (1979) combined 

terrace deposits in the eastern part of the monument 

into a single unit—terrace and pediment deposits 

(Qtp). Neither Brown and Stone (1979) nor Ward 

(1990) mapped a low terrace level above the modern 

floodplain, similar to Gillam’s lowest terrace (Qt7u); 

instead, they grouped this material with younger valleyfloor alluvium (Qal; Brown and Stone 1979) or with 

younger tributary stream alluvium and bedrock (Qnt

and Tn, Ward 1990), respectively. These differences 

in map treatment arose because the earlier two 

maps show geologic units that either occur at the 

land surface or are shallowly buried by younger 

deposits. In contrast, Gillam’s map shows all terraces 

whether at the surface or more deeply buried (Mary 

Gillam, independent consultant and researcher, 

written communication, 5 January 2016). The three 

mapping projects also had different objectives and 

areas of emphasis. Table 1 provides a correlation of 

units as mapped by Brown and Stone (1979), Ward 

(1990), and Gillam (1998a); these three source maps 

comprise the GRI GIS data for the monument (see 

“Geologic Data” section).

Terraces in the monument range in age from 

approximately 340,000 to 19,000 years old (see 

“Geologic History” section). The age of a terrace 

reflects the timing of channel incision. Each 

individual terrace level may represent thousands 

of years or more of activity. The age of a terrace 

is inferred from its height above a river, relative 

dating methods that involve soil and weathering 

development, available numeric ages, and correlation 

to dated glacial deposits (Gillam 1998a, 1998b; Mary 

Gillam, independent consultant and researcher, written 

communication, 5 January 2016).

Alluvium

Since appearing on the landscape sometime between 

18 million and 3 million years ago (see “Geologic 

History” section), the Animas River has been eroding, 

transporting, and depositing material, referred to 

as “alluvium,” from its headwaters in the San Juan 

Mountains in Colorado to its junction with the San Juan 

River near Farmington, New Mexico. The variety of 

Figure 12. Photograph of terrace 7u. Upper Pleistocene 

terrace 7, undivided (Qt7u) is the lowest and youngest 

terrace in the monument. It is pictured on the far (southeast) 

side of Farmers Ditch. Terrace 6a (Qt6a) is directly above 

Farmers Ditch. A portion of terrace 5a (Qt5a) is shown in the 

foreground. National Park Service photograph from Filippone 

and Martin (2014, photograph 12). 

Table 1. GRI GIS unit correlation

Monument Landform Gillam (1998a) Ward (1990) Brown and Stone (1979)

Floodplain and active alluvial fans Qfp (modern floodplain) 

(Holocene)

Qcf (channel and floodplain 

alluvium) (Holocene)

Qal (alluvium)

(Holocene and late Pleistocene) Bench at level of East Ruin and 

West Ruin

Qt7u (terrace 7, undivided)

(Late Pleistocene)

Qnt (Naha and 

Tsegi Alluviums, 

ndifferentiated)

(Holocene)

Tn (Nacimiento 

Formation) 

(Paleocene)

North Mesa, lower bench Qt6a (terrace 6a)

(Late–middle Pleistocene)

Q5b (terrace gravel, unit 5, lowest 

terrace [of group 5]) (Pleistocene) Qtp (terrace and pediment 

deposits) (Pleistocene)

North Mesa, higher bench Qt5a (terrace 5a)

(Middle Pleistocene)

Q4d (terrace gravel, unit 4, 

intermediate terrace [of group 4]) 

(Pleistocene)
14

rock types at the monument reflects the course of 

the Animas River and the landscapes through which 

it flowed (fig. 13; Gillam 1998b; Scott and Moore 

2007). 

Quartzite (metamorphosed sandstone) is common 

in the terraces at the monument. This rock, which 

is 1.7 billion years old (middle Proterozoic; fig. 4), 

originated as part of the Uncompahgre Formation 

that crops out in the San Juan Mountains of 

Colorado. Also, rocks from the San Juan volcanic 

field in Colorado are common. This field was active 

about 35 million to 22 million years ago. These 

rocks are chiefly porphyry (an igneous rock with 

conspicuous crystals, called “phenocrysts,” in a 

fine-grained matrix) and volcanic tuff (consolidated 

volcanic ash) and lesser amounts of basalt. Varied 

sandstones are common near Durango but rarer 

downstream because soft rocks disintegrate readily 

when tumbled in a river with harder rocks. The 

alluvial mix also contains lesser amounts of the 

mineral quartz; sedimentary rocks such as limestone, 

mudstone, shale, chert, and jasper; metamorphic 

rocks such as hornblende gneiss, biotite schist, and 

some fine-grained varieties; and several different 

granitoid rocks that originated as igneous plutons 

below Earth’s surface (Mary Gillam, independent 

consultant and researcher, written communication 5 

January 2016).

In addition to alluvium transported by the Animas 

River, tributary channels and arroyos also deposited 

alluvium. The complex recent history of the river 

valley is recorded in main-river alluvium (outwash 

and post-glacial alluvium) and tributary or arroyo 

alluvium (see “Geologic History” section).

Brown and Stone (1979) recorded alluvium (Qal) as 

covering most of the eastern half of the monument. 

They described the alluvium as valley-fill deposits 

of gravel, sand, silt, and clay. Ward (1990) mapped 

channel and floodplain alluvium (Qcf) along the 

Animas River in the western part of the map area 

(see GRI GIS data). This material consists of yellowbrown and gray-brown, poorly sorted clay, silt and 

sand containing pebbles, cobbles, and boulders. 

Alluvium deposited as alluvial fans conceals most 

of the lowest terrace (Qt7u) mapped by Gillam 

(1998a) near the middle of the valley. During 

mapping, Gillam was able to identify exposed edges 

Figure 13. Photographs of alluvium on the North Mesa. Since 

appearing on the landscape between 18 million and 3 million 

years ago, the Animas River has transported colorful and 

varied alluvium that reflects the landscapes through which 

the river flowed. Note pen for scale in the upper photograph. 

Photographs by Katie KellerLynn (Colorado State University). 
15

of this terrace in some places but not in others (Mary 

Gillam, independent consultant and researcher, written 

communication, 5 January 2016). By contrast, Ward 

(1990) lumped fan, low terrace, and river floodplain 

deposits along the middle of the valley (unit Qcf), 

and Brown and Stone (1979) lumped deposits of 

various types and ages in both their “terrace” (Qtp) 

and “valley-floor” units (Qal) (Mary Gillam, written 

communication, 5 January 2016). Table 1 provides a 

correlation of these units.

Clarification of “Pediments”

As a point of clarification, unit Qtp of Brown and Stone 

(1979) was named “terrace and pediment deposits”; 

however, their use of “pediment” in this instance is 

not commonly accepted (Mary Gillam, independent 

consultant and researcher, written communication, 5 

January 2016). 

Pediments are a characteristic erosional feature of 

arid terranes (Wyckoff 1999). They are eroded into 

bedrock or occasionally into old alluvial deposits. 

They may be bare but are more commonly partly 

mantled by a thin, discontinuous veneer of alluvium. 

No bare bedrock or alluvium-mantled bedrock slopes 

matching this definition are within the area that Brown 

and Stone (1979) mapped as terrace and pediment 

deposits (Qtp) within and near the monument (Mary 

Gillam, independent consultant and researcher, written 

communication, 5 January 2016). That area essentially 

matches the distribution of higher terraces mapped by 

Gillam (1998a). Many parts of these terraces are buried 

by alluvial fans that mostly conceal the narrow bedrock 

slopes (former river cutbanks) between different terrace 

levels. Presumably, Brown and Stone (1979) interpreted 

the depositional alluvial-fan surfaces as erosional 

pediments because pediments do occasionally flank 

the upslope sides of older terraces in the region, for 

example, at the rear edge of terrace 3a (Qt3a) on the 

southern side of the Animas River downstream from the 

monument (Gillam 1998a; see GRI GIS data). However, 

this relationship does not occur at the monument 

(Mary Gillam, independent consultant and researcher, 

written communication, 5 January 2016).

Clarification of “Naha and Tsegi Alluviums”

In an area that Gillam (1998a) mapped as the upper 

Pleistocene terrace (Qt7u) in the western part of 

the monument, Ward (1990) mapped areas of Naha 

and Tsegi Alluviums, undifferentiated (Qnt) and the 

Nacimiento Formation (Tn) (see bedrock and surficial 

map, in pocket). Unit Qnt consists of well-stratified, 

yellowish gray and grayish brown silt and sand. As 

mapped by Ward (1990), the Qnt material in the 

monument appears to be a “finger” of the Estes Arroyo, 

probably consisting of locally derived fan alluvium that 

Ward (1990) interpreted as more similar to Qnt than to 

his other mapping units (Mary Gillam, independent 

consultant and researcher, written communication, 5 

January 2016).

The terminology “Naha and Tsegi Aluviums” originated 

in Arizona’s “Navajo Country” (Hack 1941). Hack’s 

system of alluvium in ascending order (oldest to 

youngest) is Jeddito, Tsegi, and Naha. According to 

Hack (1941), the Jeddito Alluvium contains remains 

of proboscidian (elephants and their extinct relatives) 

of late Pleistocene age, the Tsegi Alluvium contains 

evidence of human occupation deposited before the 

13th century, and the Naha Alluvium contains Pueblo 

IV pottery (i.e., older than 1300 CE). 

Hack’s terminology has been applied to areas in New 

Mexico (and Utah; e.g., Hereford 1988). Weide et al. 

(1980), Scott et al. (1984), and Mytton and Schneider 

(1987) used this terminology in the Chaco Canyon 

area (see GRI report about Chaco Culture National 

Historical Park by KellerLynn 2015b), and Ward 

(1990) used it in the monument and surrounding area. 

Correlation of this material to Hack’s type locality in 

northern Arizona is uncertain, and some investigators 

have questioned its use, preferring to apply local 

stratigraphic names (see Hall 1990, 2010).

Eolian Features

Throughout New Mexico, the wind has eroded bedrock 

and alluvium and deposited eolian (windblown) sand 

and silt on upland areas. Eolian sand covers significant 

portions of the San Juan Basin, where large dunes are 

still active. Eolian silt, referred to as “loess,” overlies 

much of the northern part of the San Juan Basin, where 

it forms sheets. Some of the best grazing and cropland 

on the uplands consists of loess (Scott and Moore 

2007). 

The uppermost deposits on most terraces and older 

moraines in the Animas River valley consist partly or 

entirely of loess. Scott and Moore (2007) observed that 

older loess has been strongly weathered so it contains 

more clay and is oxidized to a moderate reddish brown. 
16

Younger loess, where it is thick enough to form a 

distinct layer, contains less clay and is usually yellowish 

brown. Small deposits of dune sand also overlie 

outwash on some terraces (Gillam 1998b).

Deposits of Quaternary loess, as much as 1 m 

(3 ft) thick, occur within the monument (KellerLynn 

2007). Loess deposits have cultural significance; 

ancestral Puebloans dug subsurface structures into 

them (Paul Carrara, US Geological Survey, research 

geologist, personal communication, 8 March 2007). 

They also mined the loess for its use as a primary 

ingredient for mortar (Dana Hawkins, Aztec Ruins 

National Monument, biological technician, written 

communication, 20 October 2015). 

During scoping, participants suggested that findings 

of a study by the US Geological Survey of the surficial 

deposits at Mesa Verde National Park may be applicable 

to Aztec Ruins National Monument (see Carrara 2009, 

2012; and http://esp.cr.usgs.gov/archive/mverde/). The 

geologic resources of Mesa Verde National Park played 

a key role in the lives of the ancient people who lived 

there (see GRI report by Graham 2006). For example, 

thick, red loess on mesa tops was significant for 

prehistoric farming. The loess’ particle-size distribution 

has good moisture-retention properties. Soil that 

developed in this loess, along with seasonal rains, 

allowed ancient farmers to grow crops such as corn, 

beans, and squash (Carrara 2012).

Lava Creek B ash is another eolian feature of interest for 

the monument. This ash erupted from the Yellowstone 

caldera approximately 639,000 years ago (Lanphere et 

al. 2002) and was transported by the wind throughout 

the Southwest, southern Rocky Mountains, and Great 

Plains. Deposits of Lava Creek B ash serve as a timestratigraphic marker, that is, a distinctive, widespread 

layer of rock or sediment that has been numerically 

dated and helps to date other rocks and deposits with 

respect to their relative position to this layer. 

The Lava Creek B ash is not exposed within the 

boundaries of the monument (KellerLynn 2007), 

but remnants of the formerly widespread blanket of 

ash occur in the area. In the Flora Vista quadrangle, 

Ward (1990) mapped an “observation locality” of the 

ash about 11 km (7 mi) southwest of the monument 

near the Hargis Arroyo (see GRI GIS data). Gillam 

(1998a, 1998b) located lenses of Lava Creek B ash 

at 17 sites in the Animas River valley and used it to 

estimate the ages of some terraces in the Animas River 

valley (see Appendix C in Gillam 1998b). The nearest 

outcrops of Lava Creek B ash to the monument are 

the aforementioned Hargis Arroyo site and in Miller 

Canyon, which is on the south side of the valley about 

14 km (9 mi) northeast of the monument. 

Local residents can attest to ongoing eolian processes 

that transport sediment and induce dust storms (Mary 

Gillam, independent consultant and researcher, written 

communication, 5 January 2016). These modern 

processes, including potential “sand blasting” of 

the ruins, are not a management concern, however 

(KellerLynn 2007).
17

Geologic Resource Management Issues

This chapter 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 

Aztec Ruins National Monument. The NPS Geologic Resources Division provides technical and policy 

assistance for these issues.

During the 2007 scoping meeting (see scoping summary 

by KellerLynn 2007) and 2015 conference call, 

participants (see Appendix A) identified the following 

geologic resource management issues. They are listed 

more-or-less by management priority.

● Oil and Gas Development and Production

● Directional Drilling and Hydraulic Fracturing

● Adjacent Development

● Water Impacts on Monument Resources

● Localized and Regional Subsidence

● Piping

● Bank Erosion and Landscape Restoration

● Recreation and Land Use in the Animas River 

Corridor

● Abandoned Mineral Lands

● Paleontological Resource Inventory, Monitoring, 

and Protection

Resource managers may find Geological Monitoring

(Young and Norby 2009) useful for addressing these 

geologic resource management issues. The manual 

provides guidance for monitoring vital signs—

measurable parameters of the overall condition 

of natural resources. Each chapter of Geological 

Monitoring covers a different geologic resource and 

includes detailed recommendations for resource 

managers, suggested methods of monitoring, and case 

studies. An online version of Geological Monitoring is 

available at http://go.nps.gov/geomonitoring.

Oil and Gas Development and Production

Oil and gas development and production are principal 

resource management concerns because the monument 

is situated in one of the richest and most productive 

oil and gas provinces in North America—the San Juan 

Basin (Price 2010). The monument is on the edge of 

the Blanco Mesaverde oil field that produces from 

Cretaceous sandstones at a depth of 1,300 m (4,300 ft).

Four operating, nonfederal oil and gas wells—Bobbie 

Herrera #1, Fee 4-A (fig. 14), Fee 9Y, and Barbara K 

001 (fig. 2)—have the potential to affect monument 

resources. These wells were drilled through multiple 

producing zones, including the Upper Cretaceous 

Fruitland and Pictured Cliffs formations. The Fruitland 

Formation is a notable source of coal and coal-bed 

methane (see “Geologic Data” chapter). The GRI GIS 

data provide coal bed locations and thicknesses of the 

Fruitland Formation in the vicinity of the monument. In 

addition, the monument contains two active gathering 

lines. 

Private mineral ownership and the potential of 

undeveloped oil and gas resources beneath the 

monument create the possibility for additional drilling 

inside the monument. New operations would be 

subject to legal and policy requirements, including 

Code of Federal Regulations 36, Part 9, Subpart B (“9B 

regulations”; see Appendix B of this report). These 

regulations require oil and gas operators in National 

Park System units to submit a plan of operations for 

NPS approval. The plan must detail all activities of the 

oil and gas development, describe how reclamation will 

be completed, and provide the basis for performance 

bonds. The National Park Service uses this information 

to determine the effects of the proposed operation on 

the environment, park management, and visitor values. 

According to O’Dell (2001), well operations at the 

monument had caused localized and relatively minor 

impacts to natural resources. Vegetation and soils 

appear to be the most impacted natural resources. 

Existing and abandoned (see “Abandoned Mineral 

Lands” section) oil and gas operations impact 

approximately 2 ha (4 ac) of land within the monument 

(National Park Service 2010).

Past disturbances associated with wells and gathering 

lines have adversely affected cultural resources on the 

Hubbard property. The well on this property was the 

Moya-Hubbard #1, which was plugged and abandoned 

in 1977; the associated gathering line was abandoned 
18

in place (Jeremiah Kimbell, NPS Geologic Resources 

Division, petroleum engineer, written communication,

4 September 2015). 

Provided that operations are properly maintained and 

eventually reclaimed, the threat of additional damage 

to natural and cultural resources at the monument is 

minor. Monument staff works with operators to reduce 

the impacts of operations on monument resources and 

values (O’Dell 2001). 

Increased activity such as new drilling, reworking old 

wells, or replacing gas pipelines within the monument 

has the potential to damage resources if sites are not 

properly constructed, maintained, and operated. New 

drilling and production operations would be subject 

to the protective standards of the NPS oil and gas 

9B regulations, which require an operator to prevent 

or minimize damage to NPS resources and values. 

Resource protection related to the gas gathering system 

in the monument would be addressed through the NPS 

special use permit regulations at 36 CFR § § 1.6, 5.3, 

and 5.7 (O’Dell, 2001; Julia Brunner, NPS Geologic 

Resources Division, policy and regulatory specialist, 

personal communication, 22 March 2007).

Drilling outside the monument is occurring and is 

likely to continue in the future (GRI conference call, 

28 May 2015). In general, potential impacts include 

groundwater and surface water contamination, erosion 

and siltation, introduction of exotic plant species, 

reduction of wildlife habitat, impairment of viewsheds 

and night skies, excessive noise, and diminished air 

quality. Visitor safety, visual and sound intrusions on the 

cultural landscape, and overall degradation of the visitor 

experience are specific concerns at the monument. 

In the event of any future drilling and resource 

extraction adjacent to the monument, the National 

Park Service would work closely with representatives of 

the oil and gas industry to help insure that operations 

would be conducted in concert with NPS management 

goals and objectives and in a manner that minimizes 

impacts on park resources and visitor experience. If 

an activity outside park boundaries destroys, causes 

the loss of, or injures National Park System unit 

resources, the National Park Service has authority to 

seek recoveries for response costs and damages from 

a responsible party under “System Unit Resource 

Protection Act” at 54 U.S.C. §100721–100725 (formerly 

the Park System Resources Protection Act, 16 U.S.C. 

§19jj). This is a strict liability statute.

The NPS Geologic Resources Division is available 

to provide monument staff with policy and technical 

assistance regarding energy issues. The NPS Geologic 

Resources Division Energy and Minerals website, 

http://go.nps.gov/grd_energyminerals, provides 

additional information.

Directional Drilling and Hydraulic Fracturing

Improvements in directional drilling and hydraulic 

fracturing, commonly called “fracking,” have renewed 

industry interest in the San Juan Basin, in particular 

for the Mancos Shale and Gallup Sandstone (National 

Park Service 2014; see GRI report about Chaco Culture 

National Historical Park by KellerLynn 2015b). In 

conventional vertical wells, production of oil and gas 

is limited by natural fractures in the targeted layers. 

Natural fracturing took place during the formation 

of the San Juan Basin during the Laramide Orogeny 

(see “Geologic History” chapter). Directional drilling 

generally involves drilling vertically to or near the top 

of a target formation and then turning the drill bit 

horizontally into the target formation for the purpose of 

exposing more of the production zone to the wellbore 

(borehole) and intersecting vertical fractures to increase 

production. Recent technological innovations in drilling 

and steering the bit to stay within the target formation 

have increased the success rate of this process (Just et al. 

2013). 

Production is further enhanced by multiple-stage 

hydraulic fracturing in which a liquid, typically water, 

Figure 14. Photograph of Fee 4-A well. Oil and 

gas production is ongoing at Aztec Ruins National 

Monument. This photograph shows the Fee 4-A well in 

2001. National Park Service photograph. 
19

is mixed with sand and chemicals and then injected at 

high pressure into a wellbore to create more fractures in 

the reservoir rock. After hydraulic pressure is removed 

from the well, small grains of sand or aluminum oxide, 

called “proppant,” hold open these fractures and allow 

oil and gas to migrate into the wellbore (Just et al. 2013). 

The potential role of this technology in the San 

Juan Basin cannot be overemphasized (Engler et al. 

2001), and the size and scale of horizontally drilled, 

hydraulically fractured wells will likely dwarf anything 

seen in the area previously (National Park Service 

2014). Effects of horizontal drilling and hydraulic 

fracturing may include the following:

● Water contamination related to drilling and 

disposal of drilling fluids;

● Reductions in streamflow and groundwater levels 

from operational water requirements;

● Air quality degradation from internal combustion 

engines on drill rigs and trucks;

● Excess dust from equipment transportation;

● Disruption of solitude and night skies from 

operational lights or flaring; and

● Safety concerns and impacts to wildlife associated 

with the necessary transportation to support 

drilling operations (National Park Service 2014).

All the wells in the monument are presently 

conventional, but in theory a current operator could 

frack an existing well, assuming the operator had an 

approved plan of operation that included hydraulic 

fracturing (Jeremiah Kimbell, NPS Geologic Resources 

Division, petroleum engineer, written communication, 4 

September 2015).

Impacts from Vibrations

During the conference call, participants noted that 

horizontal drilling and hydraulic fracturing have 

the potential to impact cultural resources, including 

archeological structures, as a result of vibrations caused 

by drilling and from heavy vehicles. According to 

Jeremiah Kimbell (NPS Geologic Resources Division, 

petroleum engineer, written communication, 4 

September 2015), however, the only potential vibrations 

that could occur are limited, localized disturbances 

resulting from pumps and motors associated with 

surface equipment used to perform fracking. 

Furthermore, the hydraulic fracturing process itself is 

relatively short, typically less than one week (written 

communication, 4 September 2015). Also, because all 

existing wells in the monument are conventional, the 

number of fracturing stages would be only 1–2, rather 

than 10–25 (typical of horizontal wells).

In Chaco Culture National Historical Park (see 

GRI report by KellerLynn 2015b), vibration studies 

established guidance for “safe distances” from 

archeological structures. King et al. (1985) analyzed 

the seismic risk to the larger archeological structures. 

Some of their findings may be applicable to Aztec 

Ruins National Monument. Based on normal blasting 

practices in the Chaco Canyon area, conventional rail 

traffic, use of road building equipment, and vehicular 

traffic patterns, King et al. (1985) recommended that 

structures be a minimum of 1.2 km (0.7 mi) from 

blasting, 0.5 km (0.3 mi) from railroad traffic, 45 m (150 

ft) from road building, and 25 m (80 ft) from vehicular 

traffic. King et al. (1991) recommended that heavy 

vehicular traffic on a rough road be at least 30 m (100 ft) 

from a sensitive site. With respect to horizontal drilling 

and hydraulic fracturing in the vicinity of Aztec Ruins 

National Monument, a typical “safe distance” would 

be delineated by the wellsite pad (Jeremiah Kimbell, 

NPS Geologic Resources Division, petroleum engineer 

written communication, 4 September 2015).

Impacts from Injection Wells

Injection wells could be an issue for a number of 

reasons including high rates of casing failures, 

potentially leading to groundwater contamination, and 

permanent “noisy” facilities needed for the injection 

process (Jeremiah Kimbell, NPS Geologic Resources 

Division, petroleum engineer, written communication, 

4 September 2015). In addition, injection wells have the 

potential to induce earthquakes. Generation of such 

an earthquake is highly dependent on the geophysical 

properties of an injection site, the amount of fluid 

injected, and the number of injection wells at a site. 

Seismic activity is not the leading cause for concern 

regarding injection wells.

Adjacent Development

Aztec Ruins National Monument is surrounded by the 

developing City of Aztec (fig. 2). According to scoping 

participants, one of the most threatened resources for 

both the city and the monument is open space. As areas 

of open space become increasingly scarce, developers 

may view the monument as “available” for use in urban 

functions such as new road corridors, new pipeline 
20

corridors, and new sports areas for local schools. 

In the vicinity of Aztec Ruins National Monument, 

vandalism and trespass are issues of concern related to 

development and increased access to the monument. 

Such vandalism has been a particular concern at other 

parks including Petroglyph National Monument near 

Albuquerque, New Mexico (Dave Love, New Mexico 

Bureau of Geology and Mineral Resources, geologist, 

email communication, 24 April 2007). The National 

Park Service has erected barbed-wire fencing along 

the shared boundary between the monument and a 

proposed subdivision to the north (GRI conference 

call, 28 May 2015). Impairments to the monument’s 

viewshed are another concern related to adjacent 

development.

The primary area of concern for monument managers 

is the Mesa Escondido subdivision north of the 

monument (fig. 2). Development and construction 

at that site have impacted archeological features, in 

particular a stretch of prehistoric road, and their 

geologic contexts (Dave Love, New Mexico Bureau 

of Geology and Mineral Resources, geologist, email 

communication, 24 April 2007). 

Mesa Escondido is “upslope” from the monument, so 

runoff is a concern. Developed land has the potential 

to change the timing and duration of runoff events 

following storms because roads concentrate water 

(and concentrate water faster) than natural conditions, 

thereby increasing peak discharge. Greater discharge 

equates to greater erosive power, which could affect 

monument resources downslope. Also, future irrigation 

of lawns in the subdivision could contribute to 

runoff onto the monument property, as well as cause 

unnaturally high groundwater levels (KellerLynn 

2007). GRI conference call participants suggested that 

monument managers acquire a copy of the City of 

Aztec’s surface water management plan, which may be 

useful for planning purposes and provide guidance for 

addressing storm-water runoff from outside sources 

onto monument lands.

According to the monument’s foundation document 

(National Park Service 2015), the needs associated with 

adjacent development include updating the 1993 land 

protection plan and developing a mitigation plan with 

the New Mexico Historic Preservation Division. In 

addition, obtaining the most current LiDAR imaging 

of the area is needed to document and research access 

roads and perform a viewshed analysis. Planning needs 

include working with the NPS Intermountain Region 

Lands Office to determine the steps that the National 

Park Service and the land developer would take in 

obtaining access, securing appropriate development 

permits, and providing infrastructure for utilities that 

would probably have to cross monument lands. 

Water Impacts on Monument Resources

Structural deterioration associated with water at the 

monument has been a problem for many decades. 

Chapter 12 (titled “The High Cost of Water”) of Aztec 

Ruins National Monument: Administrative History 

of an Archeological Preserve (Lister and Lister 1990) 

contains a “long recital of the stabilization efforts 

at Aztec Ruins,” starting in 1916 with the American 

Museum of Natural History and followed by the 

National Park Service in 1923. Summer rains, winter 

snows, and impacts to natural hydrologic processes, 

including surface runoff from adjacent lands (see 

“Adjacent Development” section) and subsurface 

drainage from irrigated fields, have led to the “high 

cost of water.” In an effort to create a mitigation plan 

for protecting archeological sites at the monument, a 

hydrologic study began in 2005 (see Filippone et al. 

2007). Filippone and Martin (2014) completed a final 

report that interpreted groundwater conditions at the 

monument and provided conclusions and management 

recommendations. Monument managers are referred to 

that report.

Irrigation of Agricultural Fields

In 2000, the National Park Service acquired agricultural 

fields north of the monument and stopped irrigating, 

which resulted in greatly decreased local recharge to 

the groundwater system and subsequent lowering 

of the water table (Filippone and Martin 2014). The 

maximum water table elevation after irrigation stopped 

was several feet lower than the maximum water-table 

elevation in 2005–2012 when irrigation was occurring. 

Now, high water levels in the vicinity are generally 

more than 4.5 m (15 ft) below the bottom of the Great 

Kiva, indicating that high water-table conditions are no 

longer a significant source of soil moisture at the ruins 

(Filippone and Martin 2014).

Farmers Ditch

Scoping participants suggested that “Farmers Ditch”—

an irrigation ditch constructed in 1892 that runs 

through the monument (fig. 2)—may be artificially 
21

raising groundwater levels around the ruins. For many 

years, Farmers Ditch primarily provided water for 

irrigating agricultural fields in the Aztec area, but today, 

the ditch is also a conduit for municipal water to the 

City of Farmington. Water from Farmers Ditch is also 

used to irrigate the picnic area and heritage garden at 

the monument (GRI conference call, 28 May 2015). 

Until recently a head gate also routed irrigation water to 

an orchard at the monument. Irrigation of this orchard 

was discontinued in 2009 and the orchard was removed 

in 2013 (Filippone and Martin 2014).

Farmers Ditch transports water between March 

and November, with specific “on” and “off” dates 

determined by local hydrologic conditions and water 

supply needs of Farmington. When flowing at full 

capacity, the ditch conveys between 1.7 and 2.0 m3/s (60 

and 70 cfs) (Filippone and Martin 2014). 

Apparently when water is flowing in Farmers Ditch, 

it acts as a line source to the valley-fill aquifer, raising 

water levels between the ditch and the Animas River, 

and eventually discharging to the river. Filippone 

and Martin (2014) noted that local groundwater 

conditions as influenced by the ditch could be 

controlled by artificially lining the ditch. On the 

other hand, the cottonwood trees, which “contribute 

cool shade and a lovely sense of place,” (p. 15) may 

be negatively impacted if the ditch were to be lined 

where it crosses the monument. Filippone and Martin 

(2014) recommended that monument managers 

engage with the managers of Farmers Ditch for the 

purpose of communicating NPS interests. In particular, 

these investigators recommended that monument 

mangers suggest minimizing the disturbance of the 

low-permeability “natural” lining of the ditch during 

maintenance activities (cleaning and repair) as a 

desirable long-term management objective. Breaches 

to the lining can result in leaks and subsurface water 

movement to archeological sites.

Soil Moisture

Soil conditions near the surface at the monument vary 

widely within short distances (Filippone and Martin 

2014). The widespread presence of interfingered sandy 

to clayey soils from 5–9 m (15–30 ft) thick beneath the 

monument restricts deep percolation of infiltrated 

precipitation and creates localized perched watertable conditions and increases soil moisture along the 

western side of the West Ruin complex.

Localized and Regional Subsidence

During the scoping meeting and site visit in 2007, 

participants (see Appendix A) discussed localized 

subsidence, which is occurring near the East Ruin, 

which is located on the lowest terrace (Qt7u). The cause 

of these small depressions was not apparent. Some 

participants suggested that discontinuing irrigation 

of the fields on lands acquired in 2000 north of the 

core ruins may have affected the groundwater levels 

at East Ruin, which is downslope of the previously 

irrigated lands, contributing to ground instability and 

collapse (KellerLynn 2007). In addition to water table–

related subsidence, other possible causes of localized 

subsidence include deep-seated tectonic faulting or 

down warping, excessive precipitation or artificial 

wetting, and soil-related problems of shrinking and 

swelling clays or undercompacted soils (Shaw and 

Johnpeer 1985). Observed cracks (in November 2015), 

which had developed in the upper walls on opposite 

sides of the Great Kiva, may be a result of localized 

subsidence, potentially related to shrink and swell 

processes (Mary Gillam, independent consultant and 

researcher, written communication, 5 January 2016). 

Acquiring an understanding of soil-engineering 

properties near the East Ruin is important for the 

preservation of cultural resources (Mary Gillam, 

independent consultant and researcher, written 

communication, 5 January 2016). Some general 

information is currently available. For example, the 

Natural Resources Conservation Service (NRCS) in 

collaboration with the National Park Service completed 

a soil survey for the monument in 2006 (National Park 

Service 2006). These data are publically available at 

NPS Integrated Resource Management Applications 

(IRMA), https://irma.nps.gov/DataStore/Reference/

Profile/1048820, and the NRCS Web Soil Survey, 

http://websoilsurvey.nrcs.usda.gov/app/HomePage.

htm. Fruitland sandy loam, 0%–2% slopes (soil 

map unit Fr) underlies the East Ruin. However, sitespecific information is needed in order to address the 

underlying cause (or causes).

The NPS Geologic Resources Division provides 

assistance in evaluating and mitigating geologic hazards 

in National Park System units, including soil hazards 

and risks (see http://go.nps.gov/geohazards). Monument 

managers are encouraged to submit a technical 

assistance request to the division. GRD staff will either 

be able to address the request directly or serve as a 
22

liaison to other investigators, potentially including staff 

from the New Mexico Bureau of Geology and Mineral 

Resources, who have collaborated on GRI projects in 

the past.

In addition, minor regional subsidence occurs in 

association with coal-bed methane extraction in the San 

Juan Basin, but it is unlikely to affect the ruins (Mary 

Gillam, independent consultant and researcher, written 

communication, 5 January 2016). Since the late 1980s, 

coal-bed methane has been produced from coal beds, 

predominantly of the Fruitland Formation. The gas, 

which was adsorbed into the solid matrix of the coal, 

is extracted through the production of large volumes 

of water, which leads to a reduction of pore pressure 

within coal seams that in turn leads to the liberation 

of the methane, which can subsequently be produced. 

Findings by Katzenstein (2012) showed that enough 

groundwater production has taken place in the San 

Juan Basin to cause measurable (several cm) subsidence 

above the coal-bed methane–producing fields. 

Piping

Piping is “erosion by percolating water in a layer of 

subsoil, resulting in caving and in the formation of 

narrow conduits, tunnels, or ‘pipes’ through which 

soluble or granular soil material is removed” (USGS 

“Water Basics Glossary” at http://water.usgs.gov/waterbasics_glossary.html). Piping is a major management 

issue in anthropogenic areas such as earthen dams 

and raised roads. Piping also can occur in natural 

settings, typically when sheetwash (overland flow) 

erosion starts to concentrate into rill (channelized flow) 

erosion, entering a soil through cracks, animal burrows, 

fence-post holes, or excavations, and eventually 

moving through the subsurface to an exit point (Pete 

Biggam, National Park Service, soil scientist, email 

communication, 23 February 2007). 

Scoping participants identified piping as a minor 

geohazard at the monument. Soil pipes have developed 

in an eroding bank of the “Fallon property” along 

the Animas River (see “Bank Erosion and Landscape 

Restoration” section). Piping could affect the stability of 

archeological sites via collapse of overlying material into 

a pipe. 

Pipes also may have cultural importance. Ancestral 

Puebloans may have “enhanced” natural pipes for 

human use, even to the extent of excavating a kiva 

into a soil pipe (Pete Biggam, National Park Service, 

soil scientist, personal communication in KellerLynn 

2007, p. 11). A cultural connection between pipes and 

kivas has not been definitively established, however, 

because excavation would have destroyed the original 

pipe (Aron Adams, Aztec Ruins National Monument, 

Chief of Cultural Resources, email communication, 27 

January 2016).

Bank Erosion and Landscape Restoration

During scoping, monument staff identified erosion at 

an archeological site along the banks of the Animas 

River as a management concern. The site is part of the 

“Fallon property” acquired by the National Park Service 

in 2009. The property is along the river in a stretch with 

steep banks (approximately 60% slope). Elsewhere, the 

riparian corridor consists of sand or cobble beaches 

not particularly susceptible to slope movements. Before 

the National Park Service acquired this property, the 

land owner irrigated the pasture above the bank during 

summer months, which contributed to erosion. The 

pasture was heavily grazed, which denuded vegetation 

and compacted soil. During a moderate-intensity 

rainstorm, investigators observed that overland flow 

developed quickly, which may be a contributing factor 

to the rate of bank erosion (Johnson 2004). In addition, 

irrigation water that was allowed to slowly soak into the 

soil may have led to soil piping, which is evident along 

the bank (fig. 15).

Figure 15. Photograph of a soil pipe. Piping is occurring 

in an eroding bank along the Animas River at Aztec 

Ruins National Monument. The red mark on the ruler 

is at 1 foot. National Park Service photograph by Kim 

Johnson (taken in 2004).
23

The previous owner of the property added rip-rap 

consisting of cement slabs to protect the bank (fig. 16; 

Johnson 2004). The goal of the National Park Service 

for this property is to restore natural function and 

appearance while protecting archeological resources. 

As such, the National Park Service wants to remove the 

rip-rap at this site. In general, removal of debris that was 

used as stream-bank stabilization, including cement 

rip-rap and old cars, is a management goal for lands 

acquired within the monument’s legislative boundary. 

Since 2009, the National Park Service has removed a 

shed and fencing, conducted general clean-up, and 

taken steps toward eradicating invasive species on the 

property (GRI conference call, 28 May 2015).

To further address stream-bank erosion at the property, 

monument managers have submitted a technical 

assistance request to the NPS Water Resources Division. 

Monument managers expect assistance after the 2016 

spring runoff. This technical assistance request also 

includes remediation of an old lagoon site. An exotic 

plant management team (EPMT) will likely be involved 

with both projects. 

Recreation and Land Use in the Animas River 

Corridor 

The monument’s foundation document (National 

Park Service 2015) identified the Animas River as a 

fundamental resource and value. The monument is 

situated along the banks of the river, where the waters 

were once used for the benefit of ancestral Puebloans. 

Today, people enjoy kayaking, fishing, and canoeing. 

The river corridor also connects the monument to the 

nearby city (National Park Service 2015).

The City of Aztec recently built a bridge and trail in 

the riparian corridor, which allows pedestrians to 

cross the Animas River and enter the monument. 

Monument managers have added buck-and-rail 

fencing in this area in an effort to keep recreationists 

on the trail and thus minimize the proliferation 

of social trails, which causes soil compaction and 

erosion. Because no park infrastructure occurs 

within the floodplain, flooding of the Animas River is 

a minor management concern (Johnson 2004).

Abandoned Mineral Lands

During the scoping process, participants identified 

two abandoned gas wells: Rhodes Abram #1 and 

Moya-Hubbard #1 (see KellerLynn 2007). These 

abandoned wells were plugged in 1970 and 1977, 

respectively. 

According to the NPS Abandoned Mineral Lands 

(AML) database and Burghardt et al. (2014), the 

monument contains one (additional) plugged and 

abandoned gas well within the expanded boundaries 

of the monument—AZRU Gas Well-WE-01. The 

exact location of this well is unknown (Aron Adams, 

Aztec Ruins National Monument, Chief of Cultural 

Resources, written communication, 20 October 2015). 

The National Park Service has not yet acquired the land 

upon which this well is located, and no mitigation has 

been conducted to date.

Abandoned mineral lands can pose a variety of resource 

management issues such as visitor and staff safety and 

air, water, and soil quality. Spills or contamination from 

AML sites outside of the monument could also affect 

monument resources as evidenced by the August 2015 

spill of Gold King mine waste into the Animas River. 

See the NPS AML website, http://go.nps.gov/aml.

Paleontological Resource Inventory, 

Monitoring, and Protection

All paleontological resources are nonrenewable and 

subject to science-informed inventory, monitoring, 

protection, and interpretation as outlined in the 

2009 Paleontological Resources Preservation Act 

(see Appendix B of this report). Department of the 

Interior regulations associated with this act are under 

development. A variety of publications and online 

resources provide park-specific or servicewide 

information and guidance for paleontological resource 

management, for example, the NPS Fossils and 

Figure 16. Photograph of rip-rap along the Animas River. The 

steepest section of bank along the Animas River at Aztec 

Ruins National Monument is located along this property, 

which the National Park Service acquired in 2009. This 

photograph was taken in 2004. Since then, the National Park 

Service has removed the shed and fencing, but the rip-rap 

remains. National Park Service photograph by Kim Johnson.
24

Paleontology website, http://go.nps.gov/fossils_and_

paleo. Also, in Geological Monitoring, 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.

Tweet et al. (2009) completed an inventory and 

monitoring report about the paleontological resources 

of the Southern Colorado Plateau Network, including 

Aztec Ruins National Monument. That report provided 

geologic background information and discussed 

potential sources of paleontological material, including 

the monument’s bedrock and surficial deposits, 

monument collections, and cultural resource contexts. 

In October 2015, an investigator conducted a 

reconnaissance level, field-based paleontological 

survey for the monument. Field investigations were 

concentrated on North Mesa, focusing on five 

discovered outcrops of the Nacimiento Formation and 

the terraces adjacent to Farmers Ditch (Phil Varela, 

Chaco Culture National Historical Park, paleontology 

technician, written communication, 5 November 2015). 

Because no fossils were discovered in the Nacimiento 

outcrops at that time, a more thorough inventory is not 

presently recommended. However, erosion on North 

Mesa could expose more bedrock and further erode 

already existing outcrops, thus exposing new fossil 

discoveries in the future. 

Terrace deposits contain the most notable fossils 

found in the monument to date. These are Paleozoic 

brachiopods, crinoids, and horn corals in limestone 

cobbles (fig. 17). The cobbles are reworked; the 

source rock is most likely the Pennsylvanian (323 

million–298 million years ago) Hermosa Formation 

of southwestern Colorado, although the Leadville 

Limestone (Mississippian Period, 358 million–323 

million years ago) is another possibility. The recent 

field reconnaissance yielded five new specimens for 

the monument’s paleontology collection, for a total 

of 21 catalog numbers (Phil Varela, Chaco Culture 

National Historical Park, paleontology technician, email 

communication, 15 December 2015). Before these 

new specimens were added, fossils identified in the 

monument’s collections were mostly petrified wood 

(Tweet et al. 2009). Terrace deposits also contain pieces 

of reworked petrified wood of unknown origin.

Figure 17. Photographs of fossils at Aztec Ruins National 

Monument. Reworked limestone cobbles in the terraces 

at the monument contain fossils such as brachiopods 

(top; mostly brachiopods, but also some gastropod 

molds), crinoids (middle; crinoid stem segments mixed 

with brachiopods), and horn corals (bottom; cross 

section and longitudinal views of solitary rugose [horn] 

corals). Note the black-and-white centimeter scale. 

National Park Service photographs (taken in 2015) by 

Phil Varela (Chaco Culture National Historical Park).
25

Geologic History

This chapter describes the chronology of geologic events that formed the present landscape of Aztec 

Ruins National Monument.

Following an episode of mountain building that created 

the Rocky Mountains, the Animas River appeared 

on the landscape and began to incise episodically. 

Downcutting continues to the present day. Long-term 

channel incision was interrupted by intervals of stability 

or aggradation when voluminous deposits of glacial 

outwash accumulated on the floor of the Animas River 

valley. The river cut a flight of inset fluvial terraces into 

the valley floor, vertically eroding through each layer 

of outwash into underlying bedrock. Three primary 

terrace levels dominate the landscape of Aztec Ruins 

National Monument.

Laramide Orogeny

The Laramide Orogeny lasted approximately 35 million 

years (75 million–40 million years ago, Late Cretaceous 

Period to Eocene Epoch; fig. 4) and dramatically 

changed the landscape of western North America. This 

massive mountain-building event lifted up areas such 

as the Rocky Mountains while creating sags in Earth’s 

crust such as the San Juan Basin.

During the Laramide Orogeny, highlands, for instance 

in the vicinity of the San Juan Mountains (fig. 6), shed 

terrestrial sediments into the subsiding San Juan Basin. 

Cather (2004) argued for three periods of Laramide 

subsidence in the San Juan Basin: (1) Late Cretaceous 

(approximately 80 million years ago) when the basin’s 

oil and gas producing–rocks were deposited, 

(2) Paleocene Epoch when the Nacimiento Formation 

was deposited (64.5 million–61.0 million years ago), 

and (3) Eocene Epoch (55 million–50 million years ago) 

when the San Jose Formation was deposited. Within 

and in the vicinity of the monument, respectively, the 

Nacimiento and San Jose formations represent an 

overlying terrestrial package of sedimentary strata that 

was deposited atop downwarped Upper Cretaceous 

(100 million–66 million years ago) marine and coastal 

rocks, including the Menefee Formation and Cliff 

House Sandstone in Chaco Culture National Historical 

Park (see GRI report by KellerLynn 2015b). Upper 

Cretaceous marine and coastal rocks occur at depth 

below the monument (fig. 4).

Animas River on the Landscape

Long after the Laramide Orogeny, the Animas River 

began flowing from the San Juan Mountains, carving 

out what is now the Animas River valley. Incision of the 

valley began sometime between about 18 million and 

3 million years ago (Miocene and Pliocene epochs), 

that is, well before the advance of glacial ice into the 

valley but after mid-Tertiary volcanism, which occurred 

episodically from about 35 million to 18 million years 

ago. Areas with active volcanoes included the San 

Juan volcanic field to the north of the monument 

and the Mogollon–Datil volcanic field to the south 

(see GRI report about Gila Cliff Dwellings National 

Monument by KellerLynn 2014 for information about 

the Mogollon–Datil volcanic field). Volcanic activity in 

the San Juan Mountains produced an apron of volcanic 

material that probably covered the Aztec area and may 

have produced its own depositional sag to accumulate 

sediments (Smith et al. 2002).

Glacial Record

The timing of the first glacial advance into the Animas 

River valley is unknown, but ancient terraces provide 

some clues. The oldest terrace remnants composed of 

glacial outwash, which possibly represent this earliest 

glacial stage, are found along the southern side of the 

river near Farmington and north of the river near Flora 

Vista. This material is represented by terrace gravel, unit 

3, oldest (Q3a) of Ward (1990) and terrace 3a (Qt3a) 

of Gillam (1998a) (see GRI GIS data). The nearest 

remnants are 6 km (4 mi) west of the monument. Based 

on the height of these terrace remnants above the river, 

Gillam (1998b) estimated their age as roughly 800,000 

years old. This estimate closely matches the findings 

of Rogers et al. (1992), whose study site at Hansen 

Bluff (210 km [130 mi] northeast of the monument) 

provided a long alluvial record from the San Luis Valley 

of Colorado. This record indicated when significant 

cooling first affected the San Juan Mountains.

From oldest to youngest, the best-known glacial stages 

in the Rocky Mountains are pre-Bull Lake, Bull Lake, 

and Pinedale (table 2). Knowledge about the sequence 

and ages of Rocky Mountain glaciations is changing 

as moraines at Durango, Colorado, and elsewhere are 
26

Figure 18. Photograph of the Animas River valley in 1897. The view is looking southeast over Animas City. During the 

Pleistocene ice ages, the Animas Glacier built a series of moraines (m)—Durango (D), Spring Creek (SC), and Animas 

City (AC). Meltwater streams distributed outwash that covered the valley floor. Outwash deposits, which were cut by 

the Animas River into terraces (t), correspond to this series of moraines (see table 2). Label colors signify major age 

groups: purple (pre-Bull Lake glaciation), green (Bull Lake glaciation), and yellow (Pinedale glaciation). US Geological 

Survey photograph by A. C. Spencer, used as plate 25-A in Atwood and Mather (1932) available at http://library.usgs.

gov/photo/#/item/51dc378ce4b0f81004b7a516. Annotations provided by Mary Gillam (independent consultant and 

researcher, written communication, 6 March 2016).

Table 2. Terraces at Aztec Ruins National Monument

Age Estimate*

GRI GIS Map Unit (map symbol) Height above 

Modern 

Floodplain 

at the 

Monument

Associated 

Moraine/

Outwash

Gillam Glaciation

(1998a) Ward (1990) Brown and 

Stone (1979)

Late Pleistocene Epoch 

(approximately 25,000–19,000 

years ago)

Terrace 7, 

undivided 

(Qt7u)

No terrace No terrace <3 m (10 ft) Animas City Pinedale

Late–middle Pleistocene Epoch 

(approximately 160,000–140,000 

years ago)

Terrace 6a 

(Qt6a)

Terrace gravel, 

unit 5

(Q5b) Terrace and 

pediment 

deposits

(Qtp)

30 m (98 ft) Spring Creek Bull Lake

Middle Pleistocene Epoch 

(approximately 340,000–250,000 

years ago)

Terrace 5a 

(Qt5a)

Terrace gravel, 

unit 4

(Q4d)

43–46 m 

(141–151 ft) Durango Pre-Bull Lake

*From Gillam (1998a, 1998b; written communication, 5 January 2016); Pierce (2004); Guido et al. (2007).
27

studied and dated using more advanced techniques 

and multiple lines of evidence. The Wind River Range 

in Wyoming is the type area (reference location where 

these deposits were first studied) of these glaciations. 

Investigators have correlated glacial deposits such as 

moraines throughout the Rocky Mountains to that area. 

Very few numerical dates are available for moraines in 

Durango so their ages have been estimated mainly from 

their physical features and similarities to moraines in the 

Wind River Range. Local names for these moraines are 

from oldest to youngest: Durango, Spring Creek, and 

Animas City (fig. 18). Table 2 provides a correlation of 

terraces in the monument with moraines in Durango, 

outwash material downvalley of these moraines, and 

glacial stages.

One or more pre-Bull Lake glacial advances are the first 

for which moraines have been preserved in the Animas 

River valley (Richmond 1965). These moraines are 

composed of the Durango Till of Atwood and Mather 

(1932). Beyond these moraines, glacial meltwater 

deposited “Durango outwash,” which forms the highest 

terrace at Aztec Ruins—Q4d of Ward (1990) and Qt5a

of Gillam (1998a). The Animas River cut downward 

into this material to about 43–49 m (140–160 ft) above 

the present channel. Pre-Bull Lake outwash deposits 

can be dated by their relationship to outcrops of the 

Lava Creek B ash (see “Eolian Features and Processes” 

section); this approach suggests an age range of 

approximately 340,000 to 250,000 years ago (Gillam 

1998b).

The Bull Lake glaciation may have been the next to 

leave many moraines in the Animas River valley. Glacial 

ice of this age is thought to have deposited Spring Creek 

moraines; meltwater deposited Spring Creek outwash. 

At Aztec Ruins, the Bull Lake glaciation is represented 

by terrace 6a (Qt6a) of Gillam (1998a) or terrace gravel, 

unit 5 (Q5b) of Ward (1990). Approximately, 160,000–

140,000 years ago, the river cut into Spring Creek 

outwash to about 30 m (100 ft) above the level of today’s 

floodplain, creating this terrace. 

Dates from some recent investigations (Applegate 2005; 

Anderson and Kenney 2015; Passehl and Kenney 2015) 

suggest that some moraines previously identified as 

Figure 19. Schematic cross section of terraces. After a terrace is cut into outwash gravel, side streams deposit alluvial 

fans onto terrace benches, and wind deposits sand and silt in dunes and sheets of loess. Soil forms during periods 

of stability. Gravity causes colluvium to form on terrace risers and other steep slopes. Remnants of gravel, referred 

to as “lag,” serve as evidence of past floods of glacial meltwater. To provide a sense of scale on the landscape, the 

alluvial fan mapped by Gillam (1998a) in the vicinity of the monument is about 1 km (0.6 mi) long. Graphic by Trista 

Thornberry-Ehrlich (Colorado State University) after Gillam (1998b, figure 5.7).
28

Durango or Spring Creek may actually correlate with 

an intermediate glaciation that has not been widely 

identified in the Rocky Mountains. This glaciation is 

known as early Wisconsinan in the Midwest (Mary 

Gillam, independent consultant and researcher, written 

communication, 5 January 2016).

During the next glacial period, referred to as the 

Pinedale glaciation (table 2), Animas City moraines and 

outwash were deposited. According to Gillam (1998b), 

the Animas River incised into Animas City outwash 

creating terrace 7, undivided (Qt7u) about 25,000 years 

ago. Recent findings by Guido et al. (2007) suggest that 

parts of this terrace could be as young as 19,000 years 

old. This terrace level is less than 3 m (10 ft) above the 

present floodplain at the monument, but elsewhere is 

higher, approximately 15 m (49 ft) above. 

After the Animas River cut down through each outwash 

floodplain, creating a terrace, streams draining the 

valley sides deposited alluvial fans across the terrace 

tread, and colluvium formed on terrace risers and 

other steep slopes (fig. 19). With time and more valley 

incision, side streams began eroding the fan alluvium 

and eventually removed most of it from above the 

more resistant outwash gravel; side streams also 

removed some of the gravel, reducing the originally 

continuous terrace to discontinuous remnants (Mary 

Gillam, independent consultant and researcher, written 

communication, 5 January 2016).

All the terraces at the monument are overlain by varying 

amounts of modern sediment. Throughout the Animas 

River valley, such sediments may originate as fan 

alluvium, dune sand, loess, or colluvium. Thicknesses of 

overlying sediment are usually near zero at the riverfacing edge of a terrace but increase away from the river, 

commonly exceeding 10 m (30 ft) and rarely reaching 

45 m (150 ft) (Gillam 1998b).

Eolian Record

In the Animas River valley, buried soils within thick 

loess deposits, locally more than 6 m (20 ft) thick, 

indicate that several episodes of loess accumulation 

have occurred (Gillam 1998b; Scott and Moore 2007). 

Eolian deposition becomes significant when parts of a 

landscape become relatively stable, generally after fan 

deposition slows and in areas away from side streams. 

Maximum transport of windblown material coincides 

with availability of fine sand and silt in fluvial systems 

(Pazzaglia and Hawley 2004). Studies from several 

continents, including North America, show that most 

loess was deposited during glacial periods whereas soil 

formation occurred during interglacial periods. Glaciers 

are efficient producers of silt that makes up loess 

deposits; paleosols (old, buried soils) represent periods 

of landscape stability when loess deposition ceased or 

at least slowed significantly (Muhs et al. 2014). Loess is 

still accumulating in the Animas River valley today. 

Ongoing Incision

The final retreat of the Animas Glacier began about 

19,000 years ago and finished by about 12,000 years ago 

(Guido et al. 2007). By contrast, incision by the Animas 

River has continued to the present day. Terrace ages and 

early river diversions suggest that the overall incision 

rate (averaged across short-term cycles that produced 

terraces) has accelerated from late Pliocene time (fig. 4) 

toward the present (Gillam 1998b).
29

Geologic Data

This chapter summarizes the geologic data available for Aztec Ruins National Monument. Two 

GRI posters (in pocket) display the GIS data draped over imagery of the national monument and 

surrounding area. Two Map Unit Properties Tables (in pocket) summarize this report’s content for each 

geologic map unit. Complete GIS data are available at the GRI publications website:

http://go.nps.gov/gripubs.

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. The 

American Geosciences Institute website, http://www.

americangeosciences.org/environment/publications/

mapping, provides more information about geologic 

maps and their uses.

Source Maps

The GRI team digitizes paper geologic maps and 

converts digital geologic data to conform to the GRI 

GIS data model. The GRI map product includes 

essential elements of the source maps such as map unit 

descriptions, a correlation chart of units, a map legend, 

map notes, cross sections, figures, and references. 

The GRI team used the following source maps to 

produce the GRI GIS data for Aztec Ruins National 

Monument. These sources also provided information 

for this report. Data from Brown and Stone (1979) 

covers the western side of the monument. Data from 

Ward (1990) covers the eastern side of the monument. 

Data from Gillam (1998a) covers the entire monument. 

Brown, D. R., and W. J. Stone. 1979. Geologic map of Aztec 

quadrangle, San Juan County, New Mexico (scale 1:62,500). 

Hydrogeologic Sheet 1 (HS-1) in Hydrogeology of Aztec 

quadrangle, San Juan County, New Mexico. New Mexico 

Bureau of Mines and Mineral Resources, Socorro, New 

Mexico.

Ward, A. W. 1990. Geologic map emphasizing the surficial 

deposits of the Farmington 30’ × 60’ quadrangle, New 

Mexico and Colorado (scale 1:100,000). Miscellaneous 

Investigations Series Map I-1978. US Geological Survey, 

Washington, DC. http://ngmdb.usgs.gov/Prodesc/

proddesc_10056.htm.

Gillam, M. L. 1998a. Geomorphic map of the lower Animas 

River valley, San Juan County, New Mexico (scale 

1:50,000). Plate 1b in Late Cenozoic geology and soils of 

the lower Animas River valley, Colorado and New Mexico. 

Dissertation. University of Colorado, Department of 

Geological Sciences, Boulder, Colorado.

Varying objectives for each mapping project led to 

mismatched contacts along the boundary between 

the first two maps, which passes through the middle 

of the monument, as well as to differing distributions 

and terms for unconsolidated Quaternary deposits 

on all three maps. In addition, contacts between units 

on the first two maps are approximate and locally 

inaccurate. Although these maps provide a geologic 

overview, they are not detailed enough to fully support 

management decisions involving geologic issues (Mary 

Gillam, independent consultant and researcher, written 

communication, 5 January 2016).

In addition, the following maps by Dames & Moore 

(1979a, 1979b) provided coal-bed thicknesses of the 

Fruitland Formation in the vicinity of the monument:

Dames & Moore. 1979a. Coal resource occurrence and coal 

development potential maps of the Flora Vista quadrangle, 

San Juan County, New Mexico (scale 1:24,000). Open-File 

Report OF-79-1116. US Geological Survey, Washington, 

DC. http://pubs.er.usgs.gov/publication/ofr791116.

Dames & Moore. 1979b. Coal resource occurrence and coal 

development potential maps of the southwest quarter of 

the Aztec 15’ quadrangle, San Juan County, New Mexico 

(scale 1:24,000). Open-File Report OF-79-1117. US 

Geological Survey, Washington, DC. http://pubs.er.usgs.

gov/publication/ofr791117.

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://go.nps.gov/gridatamodel. 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 

the monument using data model version 2.1. The GRI 

Geologic Maps website, http://go.nps.gov/geomaps, 

provides more information about GRI map products. 
30

GRI GIS data are publically available through the NPS 

Integrated Resource Management Applications (IRMA) 

portal (https://irma.nps.gov/App/Portal/Home). Enter 

“GRI” as the search text and select a park from the unit 

list.

The following components are part of the GRI GIS 

data:

● A GIS readme file (azru_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 (see tables 3, 4, 

and 5);

● Federal Geographic Data Committee (FGDC)–

compliant metadata;

● An ancillary map information document (azru_

geology.pdf) that contains information captured 

from source maps such as map unit descriptions, 

geologic unit correlation tables, legends, cross 

sections, and figures; and

● An ESRI map document (.mxd) that displays the 

digital geologic data for each of the three primary 

source maps (tables 3, 4, and 5).

The GRI GIS data for the monument contain three 

data files that provide coverage of the monument and 

surrounding area. These data files are identified by the 

following prefixes: azte (GRI GIS data of the Aztec 

quadrangle from Brown and Stone 1979; see table 3), 

flvi (GRI GIS data of the Flora Vista quadrangle from 

Ward 1990; see table 4), and azrt (GRI GIS data of 

Aztec Ruins National Monument and vicinity from 

Gillam 1998a; see table 5).

GRI Posters

Two posters of the GRI GIS data draped over a shaded 

relief image of the monument and surrounding area 

are included with this report. One of the GRI posters 

highlights the bedrock and surficial geology and is 

referred to as the “bedrock and surficial map”; the other 

highlights terraces and the floodplain and is referred to 

as the “geomorphic map.” Not all GIS feature classes 

may be included on the GRI posters (see tables 3, 4, and 

5). Selected geographic information and park features 

have been added to the GRI posters. Digital elevation 

data and added geographic information are not 

included in the GRI GIS data, but are available online 

from a variety of sources. Contact the GRI team for 

assistance locating these data.

Map Unit Properties Tables

Two Map Unit Properties Tables, one for each GRI 

poster, list the geologic time division, symbol, and a 

simplified description for each of the geologic map units 

within Aztec Ruins National Monument. Following 

the structure of the report, the tables summarize the 

geologic features and processes, resource management 

issues, and geologic 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 the GRI team with any 

questions.

Minor inaccuracies may exist in the GRI GIS data 

and on the GRI posters with respect to the locations 

of geologic features relative to other geologic or 

geographic features. Based on the source map scale and 

US National Map Accuracy Standards, geologic features 

represented in the GRI GIS data are expected to be 

horizontally within the following distances of their true 

locations: At scale 1:24,000 (Dames & Moore 1979a, 

1979b), the horizontal distance is 12 m (40 ft). At scale 

1:50,000 (Gillam 1998a), the horizontal distance is 25 m 

(82 ft). At scale 1:62,500 (Brown and Stone 1979), the 

horizontal distance is 32 m (104 ft). At scale 1:100,000 

(Ward 1990), the horizontal distance is 51 m (167 ft).
31

Table 3. GRI GIS data for Aztec Ruins National Monument (Aztec quadrangle; azte_geology.mxd; Brown and Stone 1979)

Data Layer On GRI Poster?

Geologic Measurement Thickness (coal bed thicknesses, Fruitland Formation) Yes

Geologic Contacts Yes

Geologic Units Yes

Table 4. GRI GIS data for Aztec Ruins National Monument (Flora Vista quadrangle; flvi_geology.mxd; Ward 1990)

Data Layer On GRI Poster?

Geologic Observation Localities (Lava Creek B ash) No

Geologic Sample Localities (radiometric ages of Qnt deposits) No

Geologic Contacts Yes

Geologic Units Yes

Table 5. GRI GIS data for Aztec Ruins National Monument (terrace map; azrt_geology.mxd; Gillam 1998a)

Data Layer On GRI Poster?

Surficial Contacts Yes

Surficial Units Yes

33

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of Paleocene mammals of the Nacimiento Formation. Bulletin 8. New Mexico Museum of Natural History and Science, 

Albuquerque, New Mexico. 

Williamson, T. E., and S. G. Lucas. 1992. Stratigraphy and mammalian biostratigraphy of the Paleocene Nacimiento Formation, 

southern San Juan Basin, New Mexico. Pages 265–296 in S. G. Lucas, B. S. Kues, T. E. Williamson, and A. P. Hunt, editors. San 

Juan Basin IV. Fall Field Conference Guidebook 43. New Mexico Geological Society, Socorro, New Mexico.

https://nmgs.nmt.edu/publications/guidebooks/downloads/43/43_p0265_p0296.pdf.

Wyckoff, J. 1999. Reading the Earth: landforms in the making. Adastra West, Inc., Mahwah, New Jersey.

Young, R. and L. Norby, editors. 2009. Geological monitoring. Geological Society of America, Boulder, Colorado.

http://go.nps.gov/geomonitoring.

39

Additional References

This chapter lists additional references, resources, and websites that may be of use to resource 

managers. Web addresses are valid as of June 2016. 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://go.nps.gov/geology

● NPS Geologic Resources Division Education 

Website: http://go.nps.gov/geoeducation

● NPS Geologic Resources Inventory:

http://go.nps.gov/gri

● NPS Geoscientist-In-the-Parks (GIP) internship 

and guest scientist program: http://go.nps.gov/gip

● 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://go.nps.gov/views

● USGS Geology of National Parks (including 3D 

imagery): http://3dparks.wr.usgs.gov/

NPS Resource Management Guidance and 

Documents

● Management Policies 2006 (Chapter 4: Natural 

resource management):

http://www.nps.gov/policy/mp/policies.html

● 1998 National parks omnibus management act: 

http://www.gpo.gov/fdsys/pkg/PLAW-105publ391/

pdf/PLAW-105publ391.pdf

● 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://go.nps.gov/geomonitoring

● 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 Other Organizations

● 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/

● International Commission on Stratigraphy (ICS): 

http://www.stratigraphy.org/

● ICS chart/time scale: http://www.stratigraphy.org/

index.php/ics-chart-timescale

● United Nations Educational, Scientific and Cultural 

Organization (UNESCO) world heritage sites: 

http://whc.unesco.org/en/list/

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://pubs.usgs.gov/imap/i2720/

41

Appendix A: Scoping Participants

The following people attended the GRI scoping meeting for Aztec Ruins National Monument, held on 

13 February 2007, or the follow-up report writing conference call, held on 28 May 2015. 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.

2007 Scoping Meeting Participants

Name Affiliation Position

Gary Brown Aztec Ruins National Monument Chief of Cultural Resources

Dennis Carruth Aztec Ruins National Monument Superintendent

Tim Connors NPS Geologic Resources Division Geologist

Rich Friedman City of Farmington GIS Supervisor

Mary Gillam Independent Consultant and Researcher/Geologist

George Herring Aztec Ruins National Monument Interpretive Supervisor

Joe Hewitt Bureau of Land Management Geologist

Katie KellerLynn Colorado State University Research Associate/Geologist

Dave Love New Mexico Bureau of Geology and Mineral Resources Geologist

Terry Nichols Aztec Ruins National Monument Park Ranger

Lisa Norby NPS Geologic Resources Division Geologist

Phil Stoffer US Geological Survey Geologist

Heather Stanton Colorado State University Research Associate/Geologist

2015 Conference Call Participants

Name Affiliation Position

Aron Adams Aztec Ruins National Monument Chief of Cultural Resources

Mary Gillam Independent Consultant and Researcher/Geologist

Katie KellerLynn Colorado State University Research Associate/Geologist

Jason Kenworthy NPS Geologic Resources Division Geologist, GRI reports coordinator

Larry Turk Aztec Ruins National Monument and Chaco Culture National Historical Park Superintendent

Jim Von Haden Aztec Ruins National Monument and Chaco Culture National Historical Park Chief of Natural Resources

43

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 June 2016. Contact the NPS 

Geologic Resources Division for detailed guidance.

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 CFR § 2.1(a)(1)(iii) prohibits destroying, 

injuring, defacing, removing, digging or 

disturbing paleontological specimens or 

parts thereof.

Prohibition in 36 CFR § 13.35 applies 

even in Alaska parks, where the surface 

collection of other geologic resources is 

permitted.

Regulations in association with 2009 PRPA 

are being finalized (August 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 (including rock and mineral 

resources), unless otherwise authorized by 

law.

36 CFR § 2.1 prohibits possessing, 

destroying, disturbing mineral resources...

in park units.

Exception: 36 CFR § 13.35 allows some 

surface collection of rocks and minerals 

in some Alaska parks (not Klondike 

Gold Rush, Sitka, Denali, Glacier Bay, 

or Katmai) by non-disturbing 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.

None applicable.

Section 9.1.3.3 clarifies that only the 

NPS or its agent can extract park-owned 

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 parkwide borrow management plan;

-spoil areas must comply with Part 6 

standards; and

-NPS must evaluate use of external 

quarries.

Any deviation from this policy requires a 

written waiver from the Secretary, Assistant 

Secretary, or Director.
44

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 33 USC § 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.
45

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Mining Claims

Mining in the Parks Act of 1976, 16 

USC § 1901 et seq. authorizes NPS 

to regulate all activities resulting from 

exercise of mineral rights, on patented and 

unpatented mining claims in all areas of the 

System, in order to preserve and manage 

those areas.

General Mining Law of 1872, 30 USC 

§ 21 et seq. allows US citizens to locate 

mining claims on Federal lands. Imposes 

administrative and economic validity 

requirements for “unpatented” claims (the 

right to extract Federally-owned locatable 

minerals). Imposes additional requirements 

for the processing of “patenting” claims 

(claimant owns surface and subsurface). 

Use of patented mining claims may be 

limited in Wild and Scenic Rivers and 

OLYM, GLBA, CORO, ORPI, and DEVA. 

Surface Uses Resources Act of 1955, 

30 USC § 612 restricts surface use of 

unpatented mining claims to mineral 

activities.

36 CFR § 5.14 prohibits prospecting, 

mining, and the location of mining claims 

under the general mining laws in park 

areas except as authorized by law.

36 CFR Part 6 regulates solid waste 

disposal sites in park units.

36 CFR Part 9, Subpart A requires the 

owners/operators of mining claims to 

demonstrate bona fide title to mining 

claim; submit a plan of operations to 

NPS describing where, when, and how; 

prepare/submit a reclamation plan; and 

submit a bond to cover reclamation and 

potential liability.

Section 6.4.9 requires NPS to seek to 

remove or extinguish valid mining claims in 

wilderness through authorized processes, 

including purchasing valid rights. Where 

rights are left outstanding, NPS policy is 

to manage mineral-related activities in 

NPS wilderness in accordance with the 

regulations at 36 CFR Parts 6 and 9A.

Section 8.7.1 prohibits location of new 

mining claims in parks; requires validity 

examination prior to operations on 

unpatented claims; and confines operations 

to claim boundaries.

Nonfederal Oil and Gas

NPS Organic Act, 16 USC § 1 et seq.

authorizes the NPS to promulgate 

regulations to protect park resources and 

values (from, for example, the exercise of 

mining and mineral rights).

36 CFR Part 6 regulates solid waste 

disposal sites in park units.

36 CFR Part 9, Subpart B requires the 

owners/operators of nonfederally owned 

oil and gas rights to

-demonstrate bona fide title to mineral 

rights;

-submit a plan of operations to NPS 

describing where, when, how they intend 

to conduct operations;

-prepare/submit a reclamation plan; and 

-submit a bond to cover reclamation and 

potential liability.

Section 8.7.3 requires operators to comply 

with 9B regulations.

Nonfederal minerals other than oil and gas

NPS Organic Act, 16 USC §§ 1 and 3

Surface Mining Control and 

Reclamation Act of 1977, 30 USC § 

1201 et. seq. prohibits surface coal 

mining operations on any lands within the 

boundaries of a NPS unit, subject to valid 

existing rights.

NPS regulations at 36 CFR Parts 1, 5, 

and 6 require the owners/operators of 

other types of mineral rights to obtain 

a special use permit from the NPS as a 

§ 5.3 business operation, and § 5.7 – 

Construction of buildings or other facilities, 

and to comply with the solid waste 

regulations at Part 6.

SMCRA Regulations at 30 CFR Chapter 

VII govern surface mining operations on 

Federal lands and Indian lands by requiring 

permits, bonding, insurance, reclamation, 

and employee protection. Part 7 of the 

regulations states that National Park System 

lands are unsuitable for surface mining.

Section 8.7.3 states that operators 

exercising rights in a park unit must comply 

with 36 CFR Parts 1 and 5.
46

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Federal Mineral Leasing (Oil and Gas, Salable Minerals, and Non-locatable Minerals)

The Mineral Leasing Act, 30 USC § 181 

et seq., and the Mineral Leasing Act for 

Acquired Lands, 30 USC § 351 et seq. 

do not authorize the BLM to lease federally 

owned minerals in NPS units. 

Exceptions: Native American Lands Within 

NPS Boundaries Under the Indian Allottee 

Leasing Act of 1909, (25 USC § 396), 

and the Indian Leasing Act of 1938 (25 

USC §§ 396a, 398 and 399) and Indian 

Mineral Development Act of 1982 

(25 USC §§ 2101-2108), all minerals 

are subject to lease and apply to Native 

American trust lands within NPS units.

Federal Coal Leasing Amendments Act 

of 1975, 30 USC § 201 does not authorize 

the BLM to issue leases for coal mining on 

any area of the national park system.

36 CFR § 5.14 states prospecting, mining, 

and...leasing under the mineral leasing 

laws [is] prohibited in park areas except as 

authorized by law.

BLM regulations at 43 CFR Parts 3100, 

3400, and 3500 govern Federal mineral 

leasing.

Regulations re: Native American Lands 

within NPS Units:

25 CFR Part 211 governs leasing of tribal 

lands for mineral development. 

25 CFR Part 212 governs leasing of 

allotted lands for mineral development. 

25 CFR Part 216 governs surface 

exploration, mining, and reclamation of 

lands during mineral development. 

25 CFR Part 224 governs tribal energy 

resource agreements.

25 CFR Part 225 governs mineral 

agreements for the development of Indianowned minerals entered into pursuant to 

the Indian Mineral Development Act of 

1982, Pub. L. No. 97-382, 96 Stat. 1938 

(codified at 25 USC §§ 2101-2108).

30 CFR §§ 1202.100-1202.101 governs 

royalties on oil produced from Indian 

leases. 

30 CFR §§ 1202.550-1202.558 governs 

royalties on gas production from Indian 

leases. 

30 CFR §§ 1206.50-1206.62 and §§ 

1206.170-1206.176 governs product 

valuation for mineral resources produced 

from Indian oil and gas leases. 

30 CFR § 1206.450 governs the valuation 

coal from Indian Tribal and Allotted leases.

43 CFR Part 3160 governs onshore oil and 

gas operations, which are overseen by the 

BLM.

Section 8.7.2 states that all NPS units 

are closed to new federal mineral leasing 

except Glen Canyon, Lake Mead and 

Whiskeytown-Shasta-Trinity NRAs.
47

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 CFR 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; and

-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 319/133454, July 2016
National Park Service

U.S. Department of the Interior

Natural Resource Stewardship and Science 

1201 Oak Ridge Drive, Suite 150 

Fort Collins, Colorado 80525

www.nature.nps.gov

EXPERIENCE YOUR AMERICATM
Geologic Resources Inventory National Park Service rn-~t 

U.S. Department of the Interior - ~ 

Geology ofAztec Ruins National Monument, New Mexico 

0.25 0.5 

Tn 

D 

NPS Boundary Q5b Terrace gravel, unit 5, lowest terrace 

Q4e Terrace gravel, unit 4, youngest Infrastructure 

• Q4d Terrace gravel, unit 4, intermediate Structures 

-- Roads Terrace gravel, unit 4, intermediate 

Geologic Contacts Terrace gravel, unit 4, oldest 

-- known or certain Q3c Terrace gravel, unit 3, youngest 

- - approximate Q3b Terrace gravel, unit 3, intermediate 

~ quadrangle boundary Q3a Terrace gravel, unit 3, oldest 

Geologic Units Tsj San Jose Formation 

Qal Alluvium 

Tn Nacimiento Formation 

Qcf Channel and floodplain alluvium 

O w ater 

Ont Naha and Tsegi Alluviums, undifferentiated 

Qes Eolian sand 

Qtp Terrace and pediment deposits 

Tn ( 

/-...... / This map displays geologic map data compiled by the 


National Park Service Geologic Resources Inventory.


It is not a substitute for site-specific investigations. 


Source Maps Poster Layout 

Brown and Stone (1979): New Mexico Bureau of Georgia Hybels 


Mines and Mineral Resources, Socorro, New Mexico. (Colorado State University) 


Ward (1990): US Geological Survey, Washington, DC. 
 Poster Date 

See "Geologic Data" chapter ofreport for full May 2016 

citations. GRI Data Date 

March 2015 

Source Scales 1:62,500 and 1:100,000 Source Map Dates According to US National Map accuracy standards, 1979 and 1990 features are expected to be within the follow ing 

distances of their true locations: 

32 m (104 ft) for scale 1 :62,500 

51 m (167 ft) for scale 1:100,000. 

All Geologic Resources Inventory geologic map data and publications 

are available at http://go.nps.gov/gripubs. 

AZTEC j -----~--­

Qd Tn 

Tn Note: The GRI GIS data for the Aztec (GRI MapCode AZTE) 

and Flora Vista (FLVI) quadrangles were not compiled into 

one map because of significant differences in the source 

maps' scales and authors' choices of map units. 
Geology of Aztec Ruins National Monument, New Mexico 

Geologic Resources Inventory National Park Service 

U.S. Department of the Interior -

rn~t 

NPS Boundary 

D .,......,.,. Terrace 6 and Terrace Se, undivided 

Qt6b Terrace 6b 

Infrastructure Qt6a Terrace 6a 

Durango Moraines • Structures QtSeu Terrace Se -- Roads QtSa Terrace Sa 

Surficial Contacts Te rraces that proje<t above Durango Moraines 

-- known or certain - Terrace 4ex 

-- approximate Terrace 4e 

----- concealed Qt4d Terrace 4d 

+- Qt4c Terrace 4c approximate and queried 

Qt4b Terrace 4b • :;i. • • concealed and queried Other Surflclal Units 

-- map boundary Qaf Alluvial fans 

Surficial Units Qt3a Terrace 3a 

Qfp Modern floodplain Qt3b Terrace 3b Animas City Moraines 

Qft7u Terrace 7 and alluvial fans, Qp Pediment 

undivided QTtaf Stream terrace or incised alluvial fan 

Qt7u Terrace 7 undivided Tt1 u Terrace 1 undivided 

Spring Creek Moraines 

D water Qt6u Terrace 6 undivided 

This map displays geologic map data compiled by the National Park Service 

Geologic Resources Inventory. It is not a substitute for site-specific investigations. 

Source Map Poster Layout 

Gillam, M. L. 1998a. Geomorphic map of the lower Georgia Hybels 

Animas River valley, San Juan County, New Mexico (Colorado State University) 

(scale 1 :50,000). Plate 1 b in Late Cenozoic geology Poster Date 

and soils of the lower Animas River valley, Colorado May 2016 

and New Mexico. Dissertation. University of GRI Data Date Colorado, Department of Geological Sciences. March 2015 

Source Map Date Source Scale 1 :50,000 1998 According to US National Map accuracy standards, 

features are expected to be within 25 m (82 ft) of 

their true location. 

All Geologic Resources Inventory geologic map data and publications 

are available at http://go.nps.gov/gripubs. 
Bedrock and Surficial Map Unit Properties Table: Aztec Ruins National Monument

Bedrock and Surficial Map Unit Properties Table, Page 1 of 2 

Table shows units mapped by Brown and Stone (1979) on the eastern side of monument and Ward (1990) on the western side of monument. Colors in Map Unit column correspond to the bedrock and surficial map poster (in pocket). Bold text refers to sections 

in the GRI report. 

Age Map Unit

(Symbol) Geologic Description Geologic Features and Processes Geologic Resource Management Issues Geologic History

QUATERNARY

(Holocene)

Naha and Tsegi 

Alluviums, 

undifferentiated 

(Qnt)

Ward (1990)

Well-stratified, yellowish gray and grayish brown silt and sand. 

Mapped where Naha and Tsegi Alluviums are 

indistinguishable, but probably consists of mostly Tsegi 

Alluvium. In places, Qnt buries outwash deposited by the 

Animas River.

Alluvium—Qnt is alluvium of Hack (1941) with the type 

area in Arizona. Correlation to New Mexico is uncertain. 

Mapped as a “finger” of Estes Arroyo in the monument.

Abandoned Mineral Lands—abandoned gas well 

within the expanded boundaries of the monument. 

Associated map unit unknown.

occurs 

Glacial Record—correlates to terrace Qt7u of Gillam 

(1998a), approximately 25,000–19,000 years old.

Ongoing Incision—dissected by modern arroyos.

QUATERNARY 

(Holocene and Late Pleistocene)

Alluvium

(Qal)

Brown and Stone 

(1979)

Valley-fill deposits consisting of gravel, sand, silt, and clay in 

the modern floodplain, lowest outwash terraces, and overlying 

alluvial fans.

Alluvium—Qal

monument. 

covers most of the eastern half of the 

Localized and Regional Subsidence—small depressions 

have developed in the vicinity of East Ruin, which is 

situated on Qal of Brown and Stone (1979).

Piping—minor geohazard at the monument. Soil pipes 

have developed in an eroding bank of the Fallon property 

along the Animas River. Could affect the stability of 

archeological sites via subsidence. Pipes may have cultural 

importance as the beginning of an excavated kiva.

Bank Erosion and Landscape Restoration—the NPS 

goal is to restore natural function and appearance while 

protecting cultural resources along the banks of the 

Animas River.

Recreation and Land Use in the Animas River 

Corridor—no park infrastructure was built on the

Animas River on the Landscape—Qal records 

aggradation by the Animas River or its tributaries. 

Ongoing Incision—Qal represents continuing deposition 

along rivers and streams.

floodplain, so flooding is not a significant issue. 

Abandoned Mineral Lands—abandoned gas well occurs 

within the expanded boundaries of the monument. Map 

unit unknown.

QUATERNARY

(Pleistocene)

Terrace and 

pediment deposits

(Qtp)

Brown and Stone 

(1979)

Veneers of gravel and sand along valley sides and on mesa 

tops, mainly in terraces and alluvial fans. 

Terraces—Qtp is described as including both terrace and 

pediment alluvium. However pediments (gravel-covered 

erosional slopes) are absent or rare in the map area (Mary 

Gillam, independent consultant and researcher, written 

communication, 5 January 2016). 

Oil and Gas Development and Production—well 

operations (e.g., Bobbie Herrera #1) account for localized 

and relatively minor impacts to natural resources, including 

denuded vegetation and eroded and compacted soil on 

terrace surfaces. 

Abandoned Mineral Lands—abandoned gas well occurs 

within the expanded boundaries of the monument. 

Associated map unit unknown.

Animas River on the Landscape—formed as the Animas 

River was incising its channel and valley sides were 

retreating. 

Paleontological Resource Inventory, Monitoring, and 

Protection—terraces contain reworked Paleozoic 

invertebrate fossils in limestone cobbles.

QUATERNARY

(Pleistocene)

Terrace gravel, 

unit 5, lowest

(Q5b)

Ward (1990)

Terrace gravel consisting of glacial outwash and fan alluvium. 

Unit 5 has moderately developed soils that typically have stage 

II (Gile et al. 1966) carbonate. Lowest and youngest terrace 

that Ward did not include in other map units. 

Terraces—Q5b consists mostly of glacial outwash from 

the Animas Glacier. Stepping up from the modern 

floodplain (represented by Qal), terraces are past 

floodplains that mark former courses of the Animas River. 

Unit 5 includes subunits Q5b and Q5a, which represent 

the lowest and highest terraces of this group respectively. 

Abandoned Mineral Lands—abandoned gas well occurs 

within the expanded boundaries of the monument. 

Associated map unit unknown. 

Paleontological Resource Inventory, Monitoring, and 

Protection—terraces contain reworked Paleozoic 

invertebrate fossils in limestone cobbles.

Animas River on the Landscape—Q5b documents river

incision and represents a former Animas River floodplain.

Glacial Record—equivalent to terrace Qt6a of Gillam 

(1998a), approximately 160,000–140,000 years old.
Table shows units mapped by Brown and Stone (1979) on the eastern side of monument and Ward (1990) on the western side of monument. Colors in Map Unit column correspond to the bedrock and surficial map poster (in pocket). Bold text refers to sections 

in the GRI report. 

Bedrock and Surficial Map Unit Properties Table, Page 2 of 2 

Age Map Unit

(Symbol) Geologic Description Geologic Features and Processes Geologic Resource Management Issues Geologic History

QUATERNARY 

(Pleistocene)

Terrace gravel, 

unit 4, intermediate

(Q4d)

Ward (1990)

Terrace gravel consisting of glacial outwash and fan alluvium. 

Unit 4 has moderately developed soils that typically have stage 

III and locally stage IV (Gile et al. 1966) carbonate. Includes 

five subunits, from youngest to oldest Q4e, Q4d, Q4c, Q4b, 

and Q4a.

Terraces—Q4d consists of glacial outwash from the 

Animas Glacier. Stepping up from the modern floodplain, 

terraces are past floodplains that mark former courses of 

the Animas River. Q4d is an intermediate level terrace of 

Group 4, as mapped by Ward (1990). It is the highest and 

oldest in the monument.

Oil and Gas Development and Production—well 

operations (e.g., Bobbie Herrera #1) account for localized 

and relatively minor impacts to natural resources, including 

denuded vegetation and eroded and compacted soil on 

terrace surfaces. 

Adjacent Development—potential impacts include loss 

of archeological resources, runoff-related erosion, and 

visual impairments.

Abandoned Mineral Lands—abandoned gas well occurs 

within the expanded boundaries of the monument. 

Associated map unit unknown.

Paleontological Resource Inventory, Monitoring, and 

Protection—terraces contain reworked Paleozoic 

invertebrate fossils in limestone cobbles.

Animas River on the Landscape—incision of the Animas 

River valley began between about 18 million and 3 million 

years ago (Miocene and Pliocene epochs). Q4d documents 

river incision and represents a former Animas River 

floodplain.

Glacial Record—equivalent to terrace Qt5a of Gillam 

(1998a), approximately 340,000–250,000 years old.

Rocks or unconsolidated deposits from the Eocene through Pliocene epochs are not mapped within Aztec Ruins National Monument. 

PALEOGENE

(Paleocene) 

Nacimiento 

Formation 

(Tn)

Brown and Stone 

(1979)

Ward (1990)

Gray, green, and purple claystone, shale, and siltstone; gray 

and yellow, coarse conglomeratic cross-bedded and massive 

sandstone. Most areas that Ward mapped as Tn within the 

monument are covered by thin outwash alluvium and 

colluvium.

Nacimiento Formation—bedrock of the monument. 

Exposed in drainages on North Mesa.

Building Stone and Lithic Resources—ancestral 

Puebloans used Tn in construction. The distinctive 

“greenstone” bed was incorporated as a decorative band.

Oil and Gas Development and Production—well 

operations (e.g., Bobbie Herrera #1, Fee 9Y, and Fee 4-A) 

account for localized and relatively minor impacts to 

natural resources, including denuded vegetation and 

eroded and compacted soil on terrace surfaces. 

Adjacent Development—potential impacts include loss 

of archeological resources, runoff-related erosion, and 

visual impairments.

Localized and Regional Subsidence—small depressions 

have developed in the vicinity of East Ruin, which is 

situated on Tn of Ward (1990). 

Abandoned Mineral Lands—abandoned gas well occurs 

within the expanded boundaries of the monument. 

Associated map unit unknown.

Paleontological Resource Inventory, Monitoring, and 

Protection—elsewhere, Tn yields vertebrate fossils 

important for interpreting the beginning of the “Age of 

Mammals.” Also yields plant fossils, including petrified 

wood, and invertebrate fossils.

Laramide Orogeny—deposited during the mountainbuilding event that created the Rocky Mountains. Tn

consists of sediments shed from uplifts into a subsiding 

basin.
Terraces Map Unit Properties Table: Aztec Ruins National Monument 

Terraces Map Unit Properties Table, Page 1 of 1 

Table shows units mapped by Gillam (1998a) within Aztec Ruins National Monument. Colors in Map Unit column correspond to the geomorphic map poster (in pocket). Bold text refers to sections in report. 

Age Map Unit

(Symbol) Geologic Description Geologic Features and Processes Geologic Resource Management Issues Geologic History

QUATERNARY

(Holocene)

Modern floodplain

(Qfp)

Modern floodplain, including active alluvial fans graded to the 

floodplain.

Terraces—three primary terraces levels (Qt7u, Qt6a, and 

Qt5a) step up from the modern floodplain (Qfp). 

Alluvium—Qfp is composed of post-glacial alluvium.

Oil and Gas Development and Production—well 

operations account for localized and relatively minor 

impacts to natural resources, including denuded 

vegetation and eroded and compacted soil on terrace 

surfaces.

Adjacent Development—potential impacts include loss 

of archeological resources, runoff-related erosion, and 

visual impairments.

Localized and Regional Subsidence—small depressions 

have developed in the vicinity of East Ruin, which is 

situated on Qt7u.

Piping—primarily in Qfp and Qt7u, piping is a minor 

geohazard at the monument. Soil pipes have developed in 

an eroding bank of the Animas River. Could affect the 

stability of archeological sites via subsidence. Pipes may 

have cultural importance as the beginning of an excavated 

kiva.

Bank Erosion and Landscape Restoration—the NPS 

goal is to restore natural function and appearance while 

protecting cultural resources along the banks of the 

Animas River.

Recreation and Land Use in the Animas River 

Corridor—no park infrastructure was built on the 

floodplain (Qfp), so flooding is not a significant issue. A 

public trail along the river (Qfp and Qt7u) connects 

downtown Aztec to the monument; buck-and-rail fencing 

is meant to keep recreationists on the trail.

Abandoned Mineral Lands—abandoned gas well occurs 

within the expanded boundaries of the monument. 

Associated map unit unknown.

Paleontological Resource Inventory, Monitoring, and 

Protection—terraces contain reworked Paleozoic 

invertebrate fossils in limestone cobbles.

Animas River on the Landscape and Ongoing 

Incision—incision and aggradation of the modern 

floodplain by the Animas River is ongoing.

Glacial Record—Qfp developed after the final retreat 

(about 12,000 years ago) of the Animas Glacier.

QUATERNARY

(Late Pleistocene)

Terrace 7, undivided

(Qt7u)

Less than 3 m (10 ft) above the present floodplain at the 

monument, but elsewhere is higher (approximately 15 m [49 

ft] above). Terraces graded to the Animas City moraines in 

Durango, Colorado. Elsewhere, divided into three levels (Qt7a, 

Qt7b, and Qt7c).

Building Stone and Lithic Resources—ancestral 

Puebloans used terrace materials such as boulders and 

cobbles for foundations and silt to make mortar and 

plaster. 

Terraces—this terrace level was not mapped by either 

Brown and Stone (1979) or Ward (1990). 

Animas River on the Landscape—lowest and youngest 

terrace at the monument.

Glacial Record—Qt7u developed approximately 25,000–

19,000 years ago during the Pinedale glaciation.

QUATERNARY

(Late middle Pleistocene)

Terrace 6a

(Qt6a)

27–34 m (89–112 ft) high (above the modern floodplain). 

Qt6a grades to the Spring Creek moraines in Durango, 

Colorado.

Building Stone and Lithic Resources—ancestral 

Puebloans used terrace materials such as boulders and 

cobbles for foundations and silt to make mortar and 

plaster. 

Terraces—Qt6a is composed of glacial outwash from the 

Animas Glacier. It is the intermediate and middle-aged 

terrace level at the monument.

Animas River on the Landscape—represents the 

“middle-aged” floodplain cut by the Animas River at the 

monument.

Glacial Record—Qt6a developed approximately 

160,000–140,000 years ago during the Bull Lake 

glaciation. 

QUATERNARY

(Middle Pleistocene)

Terrace 5a

(Qt5a)

43–46 m (141–151 ft) high (above the modern floodplain). 

Graded to the Durango moraines in Durango, Colorado.

Building Stone and Lithic Resources—ancestral 

Puebloans used terrace materials such as boulders and 

cobbles for foundations and silt to make mortar and 

plaster. 

Terraces—Qt5a is composed of glacial outwash from the 

Animas Glacier. It is the highest and oldest terrace level 

within the monument.

Animas River on the Landscape—the river began to 

incise its channel following the Laramide Orogeny but 

before the pre–Bull Lake glaciation. Qt5a was the first 

terrace to be cut by the Animas River at Aztec Ruins.

Glacial Record—Qt5a developed approximately 

340,000–250,000 years ago during the pre-Bull Lake 

glaciation.