City of Rocks National Reserve 

Geologic Resources Inventory Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2010/191 

National Park Service 

U.S. Department of the Interior 

Natural Resource Program Center 
THIS PAGE: 

View from East Ridge to the east overlooking 

City of Rocks National Reserve. Features called 

“panholes” result from weathering processes 

and are found atop some of the granite 

exposures within the reserve. Panholes, such 

as this one, can fill with water following rains. 

ON THE COVER: 

Stripe Rock is one of numerous granite 

exposures in City of Rocks National Reserve. 

The large number and distribution of these 

features led emigrants to christen the area a 

“City of Rocks.” 

National Park Service photographs by Wallace 

Keck. 
City of Rocks National Reserve 

Geologic Resources Inventory Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2010/191 

Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, Colorado 80225 

April 2010 

U.S. Department of the Interior 

National Park Service 

Natural Resource Program Center 

Ft. Collins, Colorado
The National Park Service, Natural Resource Program Center publishes a range of reports that 

address natural resource topics 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 high-priority, current natural 

resource management information with managerial application. The series targets a general, 

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

management applicability. 

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 

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collection, analysis, or reporting of the data. 

Views, statements, findings, conclusions, recommendations, and data in this report are those of 

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

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

(http://www.nature.nps.gov/geology/inventory/gre_publications.cfm) and the Natural 

Resource Publications Management website (http://www.nature.nps.gov/publications/

NRPM). 

Please cite this publication as: 

Thornberry-Ehrlich, T. 2010. City of Rocks National Reserve: geologic resources inventory 

report. Natural Resource Report NPS/NRPC/GRD/NRR—2010/191. National Park Service, 

Ft. Collins, Colorado. 

NPS 003/102081, April 2010 

ii NPS Geologic Resources Division 
CIRO Geologic Resources Inventory Report iii

Contents

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

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

Acknowledgements...................................................................................................... vi

Credits........................................................................................................................................................................ vi

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

Purpose of the Geologic Resources Inventory ...............................................................................................................1

Park Setting .................................................................................................................................................................1

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

Geologic Issues .............................................................................................................. 5

Erosion and Slope Processes.........................................................................................................................................5

Joints and Fragile Features ...........................................................................................................................................6

Recreational Demands .................................................................................................................................................6

Mine-Related Issues and Disturbed Lands.....................................................................................................................7

Water Issues ................................................................................................................................................................7

Wind Erosion ...............................................................................................................................................................7

Seismicity.....................................................................................................................................................................8

Volcanism....................................................................................................................................................................8

General Geology and Research.....................................................................................................................................8

Geologic Features and Processes................................................................................ 11

Introduction...............................................................................................................................................................11

Joint Sets...................................................................................................................................................................11

Landform Types and Geomorphological Evolution ......................................................................................................11

Weathering Processes................................................................................................................................................13

Historical Connections with Geology ..........................................................................................................................14

Albion Mountains Metamorphic Core Complex ..........................................................................................................14

Basin-and-Range Features and Almo Fault..................................................................................................................15

Map Unit Properties .................................................................................................... 25

Map Units within City of Rocks National Reserve ........................................................................................................25

Geologic History .......................................................................................................... 29

Precambrian (prior to 542 million years ago)...............................................................................................................29

Paleozoic Era (542 to 251 million years ago)...............................................................................................................29

Mesozoic Era (251 to 65.5 million years ago)..............................................................................................................29

Cenozoic Era (65.5 million years ago until today)........................................................................................................30

Glossary........................................................................................................................ 35

Literature Cited............................................................................................................ 39

Appendix A: Overview of Digital Geologic Data....................................................... 43

Appendix B: Scoping Meeting Participants................................................................ 45

Attachment 1: Geologic Resources Inventory Products CD
iv NPS Geologic Resources Division

List of Figures

Figure 1. Maps of City of Rocks National Reserve and the surrounding area .....................................................................3

Figure 2. Rockfalls are a potential geologic hazard throughout City of Rocks National Reserve........................................10

Figure 3. These photos illustrate the use of restoration projects to restore disturbed areas..............................................10

Figure 4. Three mechanisms by which joints form in granitic rocks .................................................................................16

Figure 5. Aerial image of the central area of City of Rocks National Reserve....................................................................17

Figure 6. Aerial view of the Bread Loaves.......................................................................................................................18

Figure 7. Exfoliation joints in the Almo pluton................................................................................................................18

Figure 8. Modeled cross sectional view of the evolution of small-scale landforms on an emerging granitic spire..............19

Figure 9. Blisters are a type of “case hardening” on the surface of granite rocks in City of Rocks National Reserve ..........20

Figure 10. Window Arch is an example of fanciful features shaped by weathering of granite pinnacles...........................20

Figure 11. Panholes on Bath Rock at City of Rocks National Reserve ...............................................................................21

Figure 12. Tafoni visible on a pinnacle along the Geology Trail .......................................................................................21

Figure 13. The Twin Sisters are granite pinnacles in City of Rocks National Reserve .........................................................22

Figure 14. Emigrant signatures on Register Rock within City of Rocks National Reserve...................................................22

Figure 15. Generalized geologic map of the rock types surrounding City of Rocks National Reserve ................................23

Figure 16. Generalized geologic map and cross section of the City of Rocks National Reserve area .................................24

Figure 17. Geologic time scale .......................................................................................................................................32

Figure 18. Diagrammatic geologic cross sections depicting major events in southeastern Idaho ................................ 33-34
CIRO Geologic Resources Inventory Report v

Executive Summary

This report accompanies the digital geologic map for City of Rocks National Reserve in 

Idaho, produced by the Geologic Resources Division in collaboration with its partners. It 

contains information relevant to resource management and scientific research. This 

document incorporates preexisting geologic information and does not include new data or 

additional fieldwork.

City of Rocks National Reserve resembles a silent city on 

the landscape of southern Idaho, which is on the 

northern edge of the Basin and Range physiographic 

province. Within its stone monuments are some of the 

oldest rocks in North America—previously molten and 

metamorphosed rocks of the Green Creek Complex, the 

oldest of which are more than 2.5 billion years old. These 

are juxtaposed against a large body of much younger 

once-molten rock that intruded and cooled beneath the 

surface to form the granite of the Almo pluton. The Almo 

pluton is approximately 28 million years old, or 

Oligocene in age. This age gap, a tangible contact, 

represents billions of years of geologic history. The 

Green Creek Complex and Almo pluton met at depth

during mountain building and later Basin and Range 

extension when Earth’s crust began pulling apart. Uplift,

erosion, and movement along faults exposed them at the 

surface; then wind, water, and the elements began 

sculpting, chipping, cracking, exfoliating, hollowing, and 

molding the rock into an “urban” landscape. City of 

Rocks National Reserve is a showcase for granitic 

weathering processes. It encompasses a landscape of 

spires, fins, and domes in the midst of the southern 

Albion Mountains.

Geologic issues and concerns pertain to protecting the 

delicate geologic features at City of Rocks, as well as 

providing safe access for visitors to the reserve. Humans 

have modified the landscape surrounding City of Rocks,

and, consequently, have modified its geologic system. 

This system is dynamic and capable of noticeable change 

within a human life span (less than a century).

Additionally, current visitors are placing increasing 

demands on the resources available at City of Rocks 

National Reserve. The following features, issues, and 

processes were identified as having the most geological 

importance and the highest level of management 

significance for the reserve:

• Erosion and slope processes. Erosion and weathering 

have sculpted the rocks at the reserve. Deposition of 

eroded, unconsolidated sediments in valleys creates 

hazards when these rocks are saturated and exposed 

on a slope. Rockfall and slope failure are possible

almost everywhere along the roads and trails within 

City of Rocks National Reserve.

• Recreational demands. The park was created to 

preserve and protect a special landscape of geologic 

forms and some of the historic features of the pioneer 

experience during westward expansion of the United 

States. This area is also attractive to rock climbers, 

mineral collectors, and four-wheel-drive enthusiasts.

Activities related to climbing and use of roads and 

trails increase erosion, and modern graffiti obscure 

and mar the historic signatures and other human 

markings. 

• Mine-related issues and disturbed lands. Features such 

as mica mines, borrow pits, animal grazing areas, 

mining claims, woodcutting tracts, and other 

development adjacent to park lands create a situation 

where the resources of the park, including landforms, 

rock formations, vegetation, and viewshed, are 

jeopardized. Park management could establish 

cooperative relationships with the parties involved to

remediate and restore disturbed areas.

• Relation between historical landscape and present 

condition. Geology had a profound effect on the 

decisions of the early pioneers who charted a safe 

passage through the area. It presented challenges to 

crossing mountains and valleys. City of Rocks 

National Reserve is charged with preserving the 

historical landscape from the 1840s through the 

stagecoach era. This entails understanding and, where 

appropriate, mitigating the natural geological 

processes of erosion and weathering. 

Other geologic features and issues such as water, wind 

erosion and deposition, seismicity, volcanism, joints and 

fragile features, and geologic research were also 

identified as important resource management issues for 

City of Rocks National Reserve.

The Albion Mountains contain Granite Pass, a passage

for thousands of emigrants heading west on the 

California and Oregon trails. This silent city inspired 

many journal entries of early emigrants on their way to 

California and Oregon. The emigrants left their mark on 

the landscape with trails, roads, and graffiti, both carved 

into the rocks and smeared on with axle grease. This 

setting, defined by the geology, attracted 96,649 visitors 

in 2009. The stark beauty of the landscape with its 

pinnacles, fins, tors, spires, arches, tafoni, and panholes,

combined with the tangible history reflected by the 

names of pioneers etched in the pinnacles, are primary 

attractions for park visitors. The history of the area 

reflects the geology and anthropogenic changes to the 

landscape. The preservation of the historic context at 

City of Rocks and interpretation of geologic features and 

processes and their influences on the history merits 

attention to enhance the visitors’ experiences.

Geologic maps provide a fundamental tool for resource 

management and understanding the geology at City of 
vi NPS Geologic Resources Division

Rocks National Reserve. The “Map Unit Properties”

section details the different geologic units shown on the 

digital geologic map of City of Rocks National Reserve 

and potential resources, concerns, and issues associated 

with each mapped unit.

The glossary on page 35 contains explanations of many 

technical terms used in this report, including terms used 

in the Map Unit Properties Table.

Acknowledgements

The Geologic Resources Inventory (GRI) is one of 12 inventories funded under the National Park Service (NPS) 

Natural Resource Challenge. This program, administered by the Geologic Resources Division of the Natural 

Resource Program Center, relies heavily on partnerships with institutions such as the U.S. Geological Survey, 

Colorado State University, state geologic surveys, local museums, and universities in developing GRI products.

Special thanks to: Wallace Keck (City of Rocks National Reserve) for providing information and images for use 

in the report; Deanna Greco (NPS Geologic Resources Division) provide information and images regarding

hazards and disturbed land reclamation within the reserve; John Burghardt (NPS Geologic Resources Division) 

supplied information regarding Abandoned Mineral Lands; and Kevin Pogue (Whitman College) granted 

permission for reproduction of his figures illustrating the formation of geologic features in the reserve.

Credits

Author

Trista Thornberry-Ehrlich (NPS-Colorado State University)

Review

David Miller (U.S. Geological Survey)

Marsha Davis (NPS Pacific West Region)

Bruce Heise (NPS Geologic Resources Division)

Editing

Bonnie Dash (Envirocal)

Digital Geologic Data Production

Greg Mack (NPS Pacific West Region)

Stephanie O’Meara (NPS-Colorado State University)

Digital Geologic Data Overview Layout Design

Josh Heise (NPS Geologic Resources Division)

Georgia Hybels (NPS Geologic Resources Division)
CIRO Geologic Resources Inventory Report 1

Introduction

The following section briefly describes the National Park Service Geologic Resources 

Inventory and the regional geologic setting of City of Rocks National Reserve.

Purpose of the Geologic Resources Inventory

The Geologic Resources Inventory (GRI) is one of 12 

inventories funded under the National Park Service 

(NPS) Natural Resource Challenge. This program, 

administered by the Geologic Resources Division of the 

Natural Resource Program Center, is designed to 

provide and enhance baseline information available to 

park managers. The GRI team relies heavily on 

partnerships with institutions such as the U.S. Geological 

Survey, Colorado State University, state geologic surveys, 

local museums, and universities in developing GRI 

products.

The goals of the GRI are to increase understanding of the 

geologic processes at work in parks and to provide sound 

geologic information for use in park decision making. 

Sound park stewardship requires an understanding of 

the natural resources and their role in the ecosystem. 

Park ecosystems are fundamentally shaped by geology. 

The compilation and use of natural resource information 

by park managers is called for in section 204 of the 

National Parks Omnibus Management Act of 1998 and in 

NPS-75, Natural Resources Inventory and Monitoring 

Guideline. 

To realize these goals, the GRI team is systematically 

conducting a scoping meeting for each of the 270 

identified natural area parks and providing a parkspecific digital geologic map and geologic report. These 

products support the stewardship of park resources and 

are designed for nongeoscientists. Scoping meetings 

bring together park staff and geologic experts to review 

available geologic maps and discuss specific geologic 

issues, features, and processes. 

The GRI mapping team converts the geologic maps 

identified for park use at the scoping meeting into digital 

geologic data in accordance with their Geographic 

Information Systems (GIS) Data Model. These digital 

data sets bring an interactive dimension to traditional 

paper maps. The digital data sets provide geologic data 

for use in park GIS and facilitate the incorporation of 

geologic considerations into a wide range of resource 

management applications. The newest maps contain 

interactive help files. This geologic report assists park 

managers in the use of the map and provides an overview 

of park geology and geologic resource management 

issues.

For additional information regarding the content of this 

report and current GRI contact information please refer 

to the Geologic Resources Inventory web site 

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

Park Setting

City of Rocks National Reserve was established on 

November 18, 1988 to preserve the area’s historical and 

geological features, scenery, and recreational 

opportunities. It is managed cooperatively by the 

National Park Service and the Idaho Department of 

Parks and Recreation. The reserve is adjacent to Castle 

Rocks State Park to the north in Cassia County (fig. 1A). 

The reserve covers 5,709 ha (14,107 ac) of rocky 

scrubland in south-central Idaho, south of the Snake 

River Plain and near the town of Almo (fig. 1B). The 

place was named “City of Rocks” by early travelers on 

the Oregon-California trails for its unique geological 

formations and erosional features. Emigrants passed this 

way over a 50-year period in the nineteenth century on 

their way to the California Gold Rush beginning in 1849 

and more recently on the stagecoach route between Salt 

Lake City, Utah and Boise, Idaho. They scratched names, 

their signatures, and other graffiti on some of the 

pinnacles in the area (Haymond 1984; Bedford and 

Miller 1999).

Geologic Setting

City of Rocks is located in the southern part of the 

Albion Mountains, bordered on the east by the Raft 

River valley and on the north by the Snake River valley 

(fig. 1C). The Albion Mountains extend for 60 km (37 mi) 

and are the highest and westernmost mountains of a 

group of north-south-trending ranges in southeastern 

Idaho (Pogue 2008). The reserve centers on three upland 

valleys or basins (Circle Creek basin, Twin Sisters basin, 

and Emigrant basin). These basins are underlain by 

broad erosional plains (pediments) cut into the granite

and surrounded by high mountains to the east and west 

that are cut by narrow canyons (Miller et al. 2008). 

Headward erosion of Almo Creek, Circle Creek, and 

their tributaries created these basins, which are filled 

with granitic erosional remnants of City of Rocks and 

Castle Rocks (Pogue 2008). The primary rock types in 

the Albion Mountains in the vicinity of City of Rocks 

include very old metamorphic and igneous rocks, 

metamorphosed sedimentary rocks, and a large body of 

much younger granite, the Almo pluton. 

Local elevations rise from a low of 1,722 m (5,650 ft) 

where Circle Creek meets the boundary of the reserve to 

2,703 m (8,867 ft) at the top of Graham Peak, north of the 

park, to 3,151 m (10,339 ft) at the top of Cache Peak. The 

much smaller and lower Cotterel and Jim Sage mountains 

separate the Albion Mountains from the Raft River 

Valley to the east (Pogue 2008). East of the Raft River 

Valley are the Sublett, Deep Creek, Bannock, and 

Portneuf ranges as well as the Blue Spring Hills. These 

mountains average about 16 km (10 mi) in width 

(Cunningham 1971).
2 NPS Geologic Resources Division

In the vicinity of City of Rocks National Reserve, three 

geologic provinces—the western North American fold 

and thrust belt, the Basin and Range, and the Snake River 

Plain—intersect. The fold and thrust belt province 

formed during Mesozoic mountain-building events as 

east-west compressional (pushing together) stresses 

folded and thrust huge rock slabs tens of kilometers 

eastward. Exposures of the fold and thrust belt may be 

seen east of City of Rocks near the Idaho-Wyoming 

border; however local folds attest to Mesozoic 

compression at City of Rocks (Pogue 2008). 

City of Rocks National Reserve is located on the 

northern edge of the Basin and Range province. The 

principal ranges are between about 1,200 and 1,800 m 

(about 4,000 and 6,000 ft) above sea level and trend 

roughly north-south as parallel ranges separated by 

linear valleys. Normal faults, which allow rocks on one 

side of the fault to move down and away from those on 

the other side, separate the parallel ranges and valleys

(fig. 1C). These formed as a result of extensional (pulling 

apart) tectonic forces. The Albion Mountains were 

affected by a relatively greater amount of extension and 

uplift than was the rest of the Basin and Range. Because 

of the greater amount of uplift, erosion and normal

faulting removed a thick layer of sedimentary rock cover, 

exposing much older igneous and metamorphic rocks. 

The rocks of this hyper-extended subprovince of the 

Basin and Range compose a metamorphic core complex 

(Compton and Todd 1977; Pogue 2008). 

The Raft River and Goose Creek primarily drain the 

Albion Mountains and are in turn tributaries of the 

Snake River (Pogue 2008). The relatively flat, crescentshaped Snake River Plain dominates the southern half of 

Idaho (fig. 1C). It descends from 1,800 m (about 6,000 ft) 

above sea level near the Wyoming border to less than

760 m (2,500 ft) at the Oregon border, to the west. Thick 

lava flows that cover or underlie almost the entire plain

erupted from volcanoes associated with the eastward 

migration of the Yellowstone hot spot (Pogue 2008).

Among the volcanic features present are numerous 

thermal springs, flows, cinder cones, and high basaltic 

cliffs. Craters of the Moon National Monument and 

Preserve interprets the volcanic landscape and processes 

of the eastern Snake River Plain. The plain is far wider 

than the Snake River valley which runs along the 

southern part of the plain, and it extends northward for 

as much as 80 km (50 mi) from the river.
CIRO Geologic Resources Inventory Report 3

Figure 1. Maps of City of Rocks National Reserve and the surrounding area. A) Location map of City of Rocks National Reserve and Castle Rocks State Park in Idaho. B) Regional map of 

the area surrounding City of Rocks National Reserve with other public lands indicated. C) Map of the physiographic setting of City of Rocks National Reserve. Note the locations of City of 

Rocks and the Albion Mountains range. East of the Albion Mountains, the mountains are arranged in roughly parallel, linear chains characteristic of basin-and-range physiography.

A and B) National Park Service maps. C) Compiled by Phil Reiker (NPS Geologic Resources Division).

B

A C

Boise

Twin Falls

Salt Lake City

Great

Salt Lake

Craters of the Moon

NM and PRES

Yellowstone NP

City of Rocks

National Reserve

Hagerman

Fossil Beds NM

Albion Mtns

MONTANA

IDAHO

NEVADA

UTAH

Salmon River Mountains

Snake River Plain

CIRO Geologic Resources Inventory Report 5

Geologic Issues

The Geologic Resources Division held a Geologic Resources Inventory scoping session for 

City of Rocks National Reserve on June 16–17, 1999, to discuss geologic resources, 

address the status of geologic mapping, and assess resource management issues and 

needs. This section synthesizes the scoping results, in particular those issues that may 

require attention from resource managers. Contact the Geologic Resources Division for 

technical assistance.

Erosion and Slope Processes

City of Rocks is defined by its erosional features. 

Erosional processes transport the products of 

weathering and mass wasting away from their source,

and in the Albion Mountains water is the most effective 

erosive agent (Pogue 2008). Accompanying the 

weathering that created the hollow balls, pinnacles, and 

other features in the park are slope failures, rockfall and 

topple, debris flows, slides, creep, soil deformation, and 

colluvium deposits (fig. 2). The rate at which these 

processes occur ranges from gradual to catastrophic, but 

all rates of change can be equally destructive given time

(Miller et al. 2008). The most active areas of slope 

formation and erosion are in the low parts of the upland 

basins at City of Rocks (Miller et al. 2008). Slumps and 

scarps caused by mass wasting appear on the digital 

geologic map as colluvial and landslide units, most of 

which are geologically active (Miller et al. 2008).

In addition to the highly weathered rock, the 

unconsolidated material mantling the slopes (alluvial 

fans and colluvium) and present in the valleys and on 

much of the level surfaces at City of Rocks is especially 

susceptible to mass wasting in the form of debris flows, 

slumps, slides, and slope creep when saturated with 

water after a heavy rain or snow melt. The debris flows 

have the consistency of wet concrete and can rapidly and 

destructively transport objects at least as large as carsized boulders (Miller et al. 2008). Debris flows are most 

common at the base of steep slopes and near the heads of 

alluvial fans (Pogue 2008). Debris-flow deposits appear 

as conspicuous accumulations of boulders in deposits 

otherwise composed of sand-size grains in those parts of 

alluvial fans relatively distant from the mountain fronts 

(Miller et al. 2008).

Floods may create hazards in much of the park. Intense 

seasonal thunderstorms combined with a scarcity of 

stabilizing vegetation and cryptogamic crusts on slopes 

lead to sheetwash and flooding, sending torrents of water 

and debris down slopes and potentially damaging trails 

and other facilities. Nearly all Quaternary deposits in the 

reserve are subject to flooding and sheetwash (Miller et 

al. 2008)

Stream flow, concentrated as gully erosion following 

snowmelt, sustained precipitation, and intense storms, 

has occurred in many areas underlain by unconsolidated 

to moderately consolidated materials (alluvial fans, 

pediment veneers, and alluvium) and are also prevalent 

in weathered Archean-aged schist (Miller et al. 2008). As 

they are formed, the gullies can damage or destroy trails, 

buildings, and other resources and provide conduits for 

further floods. Active steam channels and steep colluvial 

slopes are sites of repeated destructive mass wasting and 

thus are unsafe and undesirable sites for visitor 

recreation or construction of facilities (Miller et al. 2008)

Erosion has been locally increased by drainage 

diversions where they cross roads and trails. Soil erosion 

is of particular concern at City of Rocks because the thin 

granitic soils of the area are being removed by storm 

washes and raindrop spatter (GRI scoping notes 1999). 

Loss of granitic soils affects the hard rocks because, it is 

believed, salts and expanding (shrink-and-swell) clays 

are intensely weathering much of the rock in specific 

places due to variations in moisture. The alteration of 

minerals by chemical weathering commonly produces 

minerals having a greater volume than does the original 

rock. For example, some clay minerals that result from 

weathering have a greater volume than the minerals in 

the rock from which it is derived. The growth of these 

new minerals wedges apart adjacent grains. Similarly, if 

water containing dissolved salts infiltrates tiny cracks 

and evaporates, the resulting precipitate crystallizes and 

exerts a strong force on the walls of the crack (Pogue 

2008). 

Inventory, Monitoring, and Research Suggestions for Erosion 

and Slope Processes

• Comprehensively study the erosion and weathering 

processes active at City of Rocks National Reserve, 

taking into account the different rock formations 

versus slope aspects, location, and likelihood of 

instability.

• Compile a rockfall-susceptibility map by plotting rock 

unit versus slope aspect in a GIS. Potential uses of such 

a map include determining future infrastructure

locations and resource management of trails, 

buildings, and recreation areas.

• Compare weathering rates between burned areas and 

unburned areas as well as between different geologic 

units to quantify the impact of vegetation in the park.

• Determine the outcrop extent (focusing on particular 

geologic units containing clays such as unconsolidated 

Quaternary deposits or units prone to weather to clay 

such as rhyolite and granite) of shrink-and-swell clays 

such as bentonite, which is unstable for building and 

roads. 

• Study Camp Rock (an exfoliating dome in close 

proximity to a road) for potential impacts that vehicle 
6 NPS Geologic Resources Division

exhaust may have on rock exfoliation (GRI scoping 

notes 1999).

• Study the weathering of granitic rocks in thin section 

and by geochemical analysis to better understand the 

grain-by-grain weathering patterns.

• Inventory and monitor potential for debris flow near 

visitor use areas; relate the data to slope and loose rock 

deposits.

• Inventory areas susceptible to flash floods; relate the 

data to climate and to confluence areas.

• Study the stability of trails and determine which trails 

are most at risk and in need of further stabilization.

• Study the role of enlarging fractures in the rock 

through solution weathering and freeze-thaw cycles.

• Study scarp retreat and subsequent pinnacle 

development. Use Ground Penetrating Radar to 

monitor subaerial pinnacle development.

• Study how different rock types weather and erode.

• Monitor effects of four-wheel-drive trails in the park 

related to erosion and slope stability.

Joints and Fragile Features

Some of the granitic rock spires exposed at City of Rocks 

National Reserve are among the oldest in North 

America. In their long history, these rocks were affected 

by multiple phases of deformation, resulting in folds, 

faults, joints, and other planes of weakness. These 

features compromise the strength of any rock unit and in 

many cases profoundly affect the geomorphological 

processes that shape the landscape at City of Rocks. 

Younger rock units, such as the granite of the Almo 

pluton, are also traversed by cracks, known as joints. 

Joints are fractures in rock produced by stresses that 

cause the rock on either side of the fracture to be 

displaced perpendicular to the fracture plane. The 

orientation and spacing of joints strongly affects the size, 

shape, and distribution of spires. As described in the 

“Geologic Features and Processes” section, joints within 

the granite at City of Rocks formed by contraction, 

extensional tectonic forces, and expansion (Pogue 2008).

Regional deformation is still occurring at City of Rocks. 

Basin-and-Range extension is ongoing. As erosion 

removes more material from the overlying surface, a 

release of pressure causes the rocks to expand and crack. 

Earth materials are responding to pressures within the 

earth, and recent small-scale fractures and joints attest to 

this. A newly discovered fault passes through the town of 

Almo, offsetting Pleistocene-aged alluvial materials as 

much as 5 m (16 ft) (Miller et al. 2008). Features such as 

these attest to active faulting processes and the potential 

for seismic activity in the City of Rocks area. As 

mentioned below under “Seismicity,” an earthquake 

could profoundly alter the fragile spires, fins, and arches 

at City of Rocks.

Understanding the nature of the deformational features 

at the reserve allows predictions of where weathering 

and erosion are likely to be concentrated and provides a 

sound basis for resource management. Recent mapping 

reveals that joints, crusts on rock outcrop surfaces, and

weathering are identifiable features for inventories of the 

fragility of the unique rock formations at City of Rocks 

National Reserve (Miller et al. 2008).

Inventory, Monitoring, and Research Suggestions for Joints 

and Fragile Features

• Study the role of jointing versus faulting (both strikeslip and thrust faulting) to determine control 

relationships on the formation of spires as well as 

understanding a future seismic potential that may pose 

a threat to fragile features within the reserve.

Recreational Demands

Recreational demands present resource management 

challenges at the reserve. With rock spires as much as 60 

stories tall, City of Rocks is a magnet for rock climbers. 

Since climbers discovered the area, more than 700 

different climbing routes have been scouted. Rock 

climbing that includes the use of power drills to set 

climbing gear has recently begun to raise questions as to 

how it may be degrading the resources. It is unknown 

exactly how much climbing has affected the rock spires. 

However, human use of roads and trails crossing the 

granitic sediments of the area beneath the spires results 

in localized erosion and diminishing wildlife and 

vegetation. The Climbing Management Plan outlines 

rules and regulations related to climbing within the 

reserve (National Park Service 1998). Climbing is not 

permitted on the Twin Sisters nor other pinnacles in the 

Research Natural Area. Installation of bolts via manual 

or power drills requires a permit.

Other public-use concerns are damages to pinnacles 

along easily accessible trails and vandalism in the form of 

spray-painted graffiti in various locations throughout the 

park (GRI scoping notes 1999). In addition to these 

threats within the park boundaries, woodcutting around 

the reserve leads to increased erosion and run-off 

problems that may extend into the park.

Mineral collecting is an important recreational pursuit in 

the City of Rocks area. The unique mineralogy at the 

park lends itself to collectors’ interests. Mineral 

specimens cannot be collected on federal land within the 

reserve.

Inventory, Monitoring, and Research Suggestions for 

Recreational Demands

• Study historic land-use practices and their effects on 

the modern environment.

• Study and determine ways to effectively remove vandal 

graffiti from the rocks in the park. Consult with 

organizations that restore vandalized granite statues 

and like items to determine the best ways to remediate 

without further impacting the geologic features.

• High resolution photodocumentation could be used to

record the historic signatures and preserve their 

record. Digital photographs could be used to build a 

virtual and searchable “Register Rock” for park 

visitors.
CIRO Geologic Resources Inventory Report 7

Mine-Related Issues and Disturbed Lands

Some anthropogenic features on the landscape at City of 

Rocks may be targets for remediation. These include a 

few prospect pits dug in exploration for pegmatite and 

skarn minerals (lime-rich silicates), mica mines, animal 

grazing areas, four-wheel-drive trails, development 

adjacent to NPS land, mining claims, and diverted 

culverts (GRI scoping notes 1999; Miller et al. 2008). The 

National Park Service Abandoned Mineral Lands 

database (accessed March, 2010) lists seven AML 

features at six sites including dozer trenches, surface 

mines, and one abandoned, plugged oil and gas well. 

Remediation of these features would help to fulfill the 

goal of the park to preserve and restore the historical 

landscape of early westward emigration. Figure 3

illustrates reclamation efforts at an abandoned aggregate 

“borrow pit” within the reserve. Morris (2006) and 

Historical Research Associations and Amphion (1996) 

contain brief descriptions of historical mining activity 

within the reserve.

Furthermore, if park boundaries expand in the future, 

knowledge of regional mining of nonmetallic and 

metallic minerals would be invaluable for park resource 

management. Regionally, quartzite deposits have been 

quarried for building stones, sulfide-mineralized veins 

were the target of small prospect pits, pegmatite micas 

attracted prospectors in search of insulation materials, 

and river aggregate and disintegrated granite have been 

used for local construction (Miller et al. 2008).

Inventory, Monitoring, and Research Suggestions for MineRelated Issues and Disturbed Lands

• Determine the status of agricultural and grazing areas 

as disturbed lands.

• Contact the NPS Geologic Resources Division for 

assistance with AML and disturbed lands issues.

Water Issues

In southern Idaho annual precipitation (rainfall and 

snowfall) varies greatly. It averages between about 200 

and 500 mm (between about 8 and 20 in.) in most valleyand-plain areas, including City of Rocks (Idaho 

Department of Water Resources 2009). Water is scarce in 

dry years at City of Rocks, which makes it a valuable and 

important resource to monitor and protect. 

Knowledge of how groundwater is moving through the 

subsurface is an important component of resource 

management at the reserve. The hydrogeologic system is 

likely highly influenced by the presence of prominent 

joint patterns within the bedrock. Open cracks within 

the bedrock are preferential pathways for groundwater 

flow. The unconsolidated surficial deposits likely host 

unconfined aquifers and this water is part of the 

subsurface differential weathering responsible for the 

topography at City of Rocks National Reserve (Pogue 

2008). Refer to the “Geologic Features and Processes” 

section for additional information.

Geology and geologic history of the area include a 

marked influence on the presence of hot springs and hot 

wells near the Albion Mountains. Such features are 

evidence of active crustal extension whereby the crust 

has thinned to such an extent as to allow for increased 

heat flow from the hotter than usual upper mantle 

underlying the area (Pogue 2008). Water likely flows 

down to depth along regional joints and fractures, then, 

once heated, quickly emerges back at the surface as hot 

springs. 

Water quality is a significant issue at City of Rocks, 

degrading significantly from Circle Creek to the visitor 

center, where it meets minimum drinking-water 

standards (Stan Lloyd, oral communication, 1999). This 

degradation is probably from the polluting impacts of 

livestock on surface water in surrounding areas. Baseline 

water quality data for the reserve was compiled by

National Park Service (1999). 

Inventory, Monitoring, and Research Suggestions for Water 

Issues

• Contact the NPS Water Resources Division for 

technical assistance regarding water issues.

• Survey water quality, addressing the down-gradient 

changes between Circle Creek and Emigrant and Twin 

Sisters basins. 

• Inventory springs within the reserve and note any 

instances of heavy metal concentrations within the 

water. 

Wind Erosion

In addition to water, wind is a major force that 

redistributes fine-grained materials, such as sand, soil,

and soil resources (e.g., litter, organic matter, and 

nutrients) within and among ecosystems. Erosion and 

deposition by wind are important at City of Rocks 

National Reserve and can be affected by human activities

that disturb stabilizing ground features. 

During dust storms, sand and soil from disturbed 

surfaces may blow onto undisturbed ground, burying 

and killing vegetation. Even biological cryptogamic

crusts can be damaged or destroyed by windblown 

sediment, exposing more soil to erosion (Miller et al.

2008). Human impacts within the parks that disturb the 

surface are primarily associated with off-trail hiking in 

high-use areas because park management practices limit 

or prohibit off-road travel. Soil erosion is accelerated

where livestock grazing or herding is still permitted.

A soils resource inventory of the reserve was completed 

in 2009 and includes a digital GIS soils map. The soils 

program, based at the NPS Geologic Resources Division, 

should be contacted for assistance with soil-related 

issues. 

Inventory, Monitoring, and Research Suggestions for Wind 

Erosion

• Determine geologic controls of parent material on soil 

type development using the park’s digital GIS soil and 

geologic maps.
8 NPS Geologic Resources Division

• Determine patterns of wind erosion and areas most 

susceptible to soil loss. 

• Study the distribution and species composition of 

biological soil crusts with the intent to create a plan to 

increase cover of the crust (D. Miller, written 

communication, 2009). 

Seismicity

The Basin and Range province is still seismically active.

Small-scale earthquakes happen in southeastern Idaho

and northern Utah almost every day. Most of these 

micro-earthquakes are too small to be detected by 

humans without a seismometer; however, there is 

potential for infrequent large-magnitude events

(Christenson et al. 1987; GRI scoping notes 1999). The 

region has been affected by considerable historical

seismic activity, and on November 18, 1937, an

earthquake (magnitude 5.4) occurred in the reserve area

in Junction Valley west of Almo (Christenson et al. 1987; 

Miller et al. 2008).

Faults mapped within the reserve tend to lie along steep 

slopes, commonly among outcrops of Elba Quartzite. 

These fault traces tend to trend north-south. The faults 

displace rocks downslope, toward the valleys, as normal 

faults (Miller et al. 2008). A Pleistocene fault, the Almo 

fault (described below under “Basin and Range 

Features”), extends south-southwestward along the east 

margin of the Albion Mountains and is traceable along a 

series of scarps showing downward displacement to the 

east. Springs and seeps occur along this and other local

faults (Miller et al. 2008). The presence of this fault and 

Pleistocene (and possible Holocene) faults to the south 

indicates continued potential for local seismic activity

(Compton 1972; Miller et al. 2008). 

The Wasatch Front, southeast of City of Rocks, is still 

considered seismically active. No historical earthquakes 

associated with surface rupture have occurred along the 

Wasatch fault zone since at least the 1870s. However, the 

recurrence interval, or time between major seismic 

events, for the entire Wasatch fault zone may be 50–430 

years (Swan et al. 1980). The Wasatch fault zone and 

related faults 50 km (30 mi) to the east project westward 

at moderate angles, presenting a potential for a major 

seismic event that could strongly shake the City of Rocks

region. In addition to hazards from ground shaking and 

surface rupture, lateral spreading and liquefaction could 

result from an earthquake (Miller et al. 2008).

The delicate geological features and historical landscape 

are the primary reasons the reserve was established.

Ground shaking could dislodge material from steep cliffs 

within the reserve, cause landslides in unconsolidated 

deposits, and damage the pinnacles and domes of the 

City of Rocks (Miller et al. 2008). The effect on the 

geomorphology at City of Rocks must be considered a 

significant resource management issue because of the 

potential for severe ground shaking in the area due to 

natural or man-made causes such as mine blasts (GRI 

scoping notes 1999). Seismic modeling could help 

estimate the intensity of ground shaking associated with 

various earthquake magnitudes possible in the City of 

Rocks area. Such modeling would help determine which 

features are particularly under threat of damage from 

seismicity. 

Inventory, Monitoring, and Research Suggestions for Seismicity

• Comprehensively study any faulting (start with those 

faults identified on the included GRI digital geologic 

data) and seismic processes active at City of Rocks

National Reserve, taking into account rock 

formations, slope aspects, location, and likelihood of 

instability. Such studies could be performed 

cooperatively though agencies such as the U.S. 

Geological Survey or the Idaho Geological Survey.

• Comprehensively study seismic activity and active 

faults in close proximity to the City of Rocks area,

including the mapping of small-scale faults and shear 

fractures.

• Real-time and historic earthquake information is 

available from the U.S. Geological Survey 

(http://earthquake.usgs.gov/earthquakes/).

• Inventory recent fault scarps in the area. These are 

commonly present in Quaternary (less than 2.6 million 

years old) surficial deposits.

Volcanism

The area of southern Idaho famous for Craters of the 

Moon, the Snake River Plain, and City of Rocks contains 

volcanic features formed during Miocene-aged volcanic 

events, the passing of the area over the Yellowstone hot

spot, and other intermittent volcanic episodes. There are 

numerous Pliocene and younger shield volcanoes north 

of the reserve along the southern flank of the Snake River 

Plain province (D. Miller, written communication, 2009). 

Though a distant possibility, volcanic events could affect 

the area in the future. Potential threats include ash falls 

downwind of volcanic eruptions and localized seismic 

events accompanying volcanism.

Inventory, Monitoring, and Research Suggestions for 

Volcanism

• Inventory and date numerous Pliocene and younger 

shield volcanoes north of the reserve along the south 

flank of the Snake River Plain to determine the timing 

of relatively recent local volcanic activity; focus on 

features likely to be upwind of the reserve during an 

eruption (D. Miller, written communication, 2009).

General Geology and Research

The possibilities for research arising from the geologic 

features at the park are endless. This section includes 

suggestions made by scoping meeting participants in 

1999 (Appendix B). An understanding of the geological 

processes and resources at City of Rocks is fundamental 

to management decisions. This report is intended to aid 

decision making at City of Rocks with further 

suggestions and baseline information, including the 

digital geologic map of the park to be incorporated into a 

natural resources GIS. The geology at City of Rocks

influenced westward expansion and the decisions made 

by emigrants as they passed through as described in 
CIRO Geologic Resources Inventory Report 9

“Geologic Features and Processes.” The tie between 

history and geology should be interpreted in the park.

Inventory, Monitoring, and Research Suggestions for General 

Geology

• Develop a detailed geology-themed brochure with 

roadlogs and park wayside exhibits for visitors to the 

park.

• Develop curriculum-oriented teaching guides 

featuring the geology of City of Rocks.

• Highlight geologic resources at the park’s visitor 

center by incorporating a three-dimensional, raised 

relief map of the park showing locations and features. 

A kiosk displaying GIS themes and geology in an 

interactive format could be featured. 

• One area well-suited for geology interpretation is Bath 

Rock. This area has good examples of rockfall, 

pediments, and inclusion in granite that typify the rest 

of the park.

• Twin Sisters is another site well-suited for geology 

interpretation. This feature displays the significant 

difference in rock ages between the two sisters and the 

relevance and cultural implications of the California 

Trail. 

• Additional sources of geologic information for use by 

interpreters include Pogue (2008) and Link and 

Phoenix (1996); the latter takes a wider regional 

approach and is also available online: 

http://imnh.isu.edu/DIGITALATLAS/geog/rrt/RRTfr.

htm. 

• Research the potential of a project using groundpenetrating radar to test the hypothesis that the City of 

Rocks dome forms originate under an alluvial and soil 

cover and then are exhumed. If possible, outsource 

work to Idaho State University as graduate student 

projects. 

• Investigate a geochemical project combining 

geochemistry, isotope studies, and thin section/x-ray 

work on mineralogy of the crust, comparing the 

honeycomb rocks with the case-hardened ones to 

determine controls on erodability of the rocks. 

• Initiate paleoecological studies of pack-rat middens 

and climate change relationships. 

• Use isotopes to study geochemistry and mineralogy on 

case hardening.

• Study panhole formation.

• Ensure that geologic features and materials are part of 

the inventory of any newly acquired lands in the park. 

• Map the Circle Creek area in geologic detail. 

• Conduct a detailed three-dimensional cartographic 

survey of the area that includes man-made features.
10 NPS Geologic Resources Division

Figure 2. Rockfalls are a potential geologic hazard throughout City of Rocks National Reserve. This photo shows broken down rubble 

from the west side of Steinfells Dome. National Park Service photograph by Wallace Keck (City of Rocks National Reserve).

Figure 3. These photos illustrate the use of restoration projects to restore disturbed areas within City of Rocks National Reserve. The 

2003 photo shows an abandoned aggregate borrow pit near the eastern entrance to the park. This man-made feature was visible from 

the California Trail, impacting the historic landscape of the area. The project restored natural contours to the site to establish a 

geologically and hydrologically stable landscape. Vegetation was subsequently planted on the restored area after the 2004 photo was 

taken. National Park Service photographs and information courtesy Deanna Greco (NPS Geologic Resources Division).
CIRO Geologic Resources Inventory Report 11

Geologic Features and Processes

This section describes the most prominent and distinctive geologic features and processes 

in City of Rocks National Reserve.

Introduction

The following discussion briefly introduces some of the 

more unusual features and processes in City of Rocks 

National Reserve. Readers are urged to refer to the 

report accompanying the geologic map by Miller et al. 

(2008), which served as the source of the GRI digital 

geologic map described under “Map Unit Properties” 

and is included on the enclosed CD. Another excellent 

source of descriptions and illustrations is Pogue (2008). 

Joint Sets

Joints are among the most conspicuous features of the 

granitic rocks exposed at City of Rocks National 

Reserve. Joints are cracks or planar zones of preferential 

weathering in pinnacles and eroded pediments at City of 

Rocks (Miller et al. 2008). In general, joints are fractures 

that display displacement perpendicular to the plane of 

fracture and form through myriad processes (Pogue 

2008). At City of Rocks, many of the joints or fractures 

resulted from (1) extensional tectonic forces, (2) 

contraction when the rock cooled, or (3) expansion 

when overlying materials eroded away and eliminated 

the confining pressure (fig. 4). The three or four joint sets 

(repeatedly occurring orientations of neighboring joints) 

in the area and their relative density are the most 

important factors in the spacing, distribution, and shape 

of the various rock outcrops (Pogue et al. 2001; Miller et 

al. 2008; Pogue 2008).

Tectonic joints—those resulting from extensional 

tectonics—commonly form distinct sets having a 

common orientation consistent with the regional stress 

field (Pogue 2008). Individual sets of joints may be 

related to separate tectonic events. One set, trending 

north-south and dipping to the east, is related to tectonic 

extension (pulling apart) during the Oligocene and 

displays hydrothermal alteration and mineralization. 

This set characteristically focuses and localizes the 

formation of gaps, or large “avenues,” of the City of 

Rocks region (Pogue et al. 2001). Other joints, at angles 

to the north-south trending set, allow wind and water to 

erode around pillars of rock, eventually forming the 

isolated pinnacles and fins in the granite (Miller et al. 

2008). The parallel orientation of these joints in large 

part determines the orderly distribution of the shapes 

and lends more of an organized “urban” look to the 

features (fig. 5). 

As the granitic rocks cooled slowly at depth, the 

subsequent reduction in mass during contraction caused 

a nearly vertical set of joints with random trends to 

develop (fig. 6). These joints vary in length from less than 

1 m to hundreds of meters (Cunningham 1971; Pogue 

2008). Another set, oriented parallel to Earth’s surface, 

weathers to produce a dome-like shape and is referred to 

as exfoliation joints (fig. 7). These joints are oriented 

subparallel to the surface topography and derive from 

the release of pressure as erosion removes confining 

overlying rock (Chapman and Rioux 1958; Barber 1962; 

Pogue 2008). 

Landform Types and Geomorphological Evolution

The overall geology at City of Rocks is characterized by 

the Oligocene-aged granitic intrusion into Precambrian 

rocks, such as schist, granite, amphibolite, and Elba 

quartzite, and the subsequent erosion of the intrusion.

This juxtaposition of rock ages, lithologies, and 

weathering properties provides an interesting and scenic 

contrast (Wells 1984). The bizarre shapes of City of 

Rocks are found in granitic rock and are developed by 

the processes of weathering along various joints as 

described above. Most of the fantastic rock shapes are 

developed through myriad processes, including chemical 

dissolution, frost wedging, mass wasting, mineral 

wedging, and exfoliation by which thin rock plates and 

scales of various sizes are sloughed off along joints in the 

rocks formed by stress associated with expansion.

Unconsolidated alluvial fans and colluvial deposits 

mantle the slopes below weathered bedrock outcrops 

(Miller et al. 2008).

Pinnacles

The most notable landform features of the park are the 

prominent, steep-sided, smooth, and rounded granitic 

pinnacles, also referred to specifically as “domed 

bornhardts,” “tors,” and “fins.” A bornhardt is an 

isolated domed granitic outcrop whose shape is 

controlled by upwardly convex exfoliation joints 

(Cunningham 1971; Bedford and Miller 1999; Pogue 

2008). Isolated blocky granitic outcrops whose shape is 

controlled by vertical and horizontal joint sets are tors 

(“towers” in the Cornish language) (Cunningham 1971; 

Pogue 2008). Fins are high and relatively narrow granitic 

outcrops that are elongated in one direction (Pogue 

2008). Pinnacles are most heavily concentrated along the 

west, south, and north reaches of the Circle Creek Basin. 

Groups of these define northwest-oriented spines that 

separate the three main upland basins in City of Rocks 

National Reserve (Miller et al. 2008).

Shapes of the pinnacles may be described as fins, loaves, 

spires, and domes. The name “City of Rocks” refers to 

this assemblage of rock formations, which resembles the 

assortment of tall and short buildings found in a city, 

especially when viewed from a distance. These rock 

features are as high as 60 stories and are found in a 

landscape that varies nearly 610 m (2,000 ft) in elevation 

from low basins to high ridges, but is mostly 

concentrated along the west, south, and north parts of 

the Circle Creek Basin. 
12 NPS Geologic Resources Division

Most pinnacles have formed in the granite of the 

Oligocene Almo pluton. Following the two-stage model 

of Linton (1955), geologists believe the shape and 

architecture of pinnacles are strongly influenced by 

joints. Parallel joint sets oriented nearly north-south are 

the most strongly developed in the area, causing many of 

the pinnacles to be elongate north-south. At roughly 

right angles to the dominant joint set is a secondary set. 

The formations developed due to deep weathering of 

adjacent rock in the unsaturated zone above the local 

water table. Water flowed along the joints, slowly 

dissolving the walls of the rocks away. Joints in effect 

guide the long disintegration process (Linton 1955; 

Miller et al. 2008). Differential weathering controlled by 

contrasts in joint density determines which bedrock 

surfaces remain to be exposed as granite spires (Pogue 

2008). Once the rock is disintegrated within the adjacent 

unconsolidated regolith, running water erodes the 

weathered material (grus) and exposes the resistant core 

mass (Linton 1955; Miller et al. 2008). This process is 

repeated to form very high pinnacles (Miller et al. 2008). 

See Cunningham (1971) and Pogue (2008) for more 

detailed descriptions of the formation of pinnacles and 

tors. Pogue (2008) presented a sequential model for the 

evolution of a granite spire, such as that at City of Rocks 

National Reserve (fig. 8). 

Loaf-shaped pinnacles form as a result of weathering 

along one prominent set of steep fractures or joints 

parallel to the long direction of the rock, whereas domes 

form from exfoliation joints in a rock mass with weakly 

expressed or widely spaced, steeply dipping joints 

(Miller et al. 2008). Named examples of loaf-shaped 

pinnacles include Bread Loaves (fig. 6), Stripe Rock, Bath 

Rock, and Elephant Rock. Named domes include 

Kaiser’s Helmet, Camp Rock, and Treasure Rock (Miller 

et al. 2008).

Most of the pinnacles are composed of the City of Rocks 

Adamellite (dated at ≈30 million years ago). This is a 

muscovite-biotite monzogranite that is relatively coarse 

grained and roughly equigranular. It is part of the large 

Oligocene Almo pluton. Other pinnacles are composed 

of Archean gneiss that is similar in composition to the 

Oligocene pluton but foliated and that has a porphyritic 

texture (large crystals surrounded by a much smallergrained matrix). Some pinnacles at City of Rocks may be 

as old as Miocene (Miller et al. 2008). 

Surface Crusts and Case Hardening

The surfaces of the granite pinnacles and other 

weathered formations display cavernous weathering and 

durable crusts. Once exposed, the tops of granitic rocks 

began to dissolve. Granular disintegration followed by 

case hardening by iron oxide and other mineral 

deposition form a relatively resistant outer shell. This 

chemical precipitate crust may be 1−3 cm (0.4−1.2 in.) 

thick (Pogue 2008). Observations of crusts produced by 

case hardening suggest the process combines several 

factors, including (1) deposition of chemical weathering 

residue left when water evaporates from the surface, (2) 

precipitation of mineral deposits transported to the 

outcrop by wind and rain, and (3) biochemical 

precipitation of silica by lichens (Pogue 2008). 

At City of Rocks, surface shells appear as blisters (fig. 9; 

dark, thin, reddish hardened surfaces with curled edges 

or domed shapes) and crusts (thicker, dark brown, with 

polygonal shrinkage cracks), as well as durable, darkcolored pitted surfaces (Maley and Randolph 1993; 

Miller et al. 2008). The pitted surfaces form on inclined 

and nearly horizontal surfaces and can be as much as

tens of centimeters thick. They may be the oldest casehardened surfaces. This type of surface is undulating, 

cemented and smooth with pocks as wide as 20 cm (8 in.) 

(Miller et al. 2008).

Surface crusts are brittle compared to the unmineralized 

underlying rock and more likely to crack under stress 

(Pogue 2008). If the crust is breached, then the softer, 

more erodable inner core is removed by wind and water

in the process of cavernous weathering. Small remnants 

of armored surface crust form knobs called 

“chickenheads” on otherwise featureless walls (Pogue 

2008). Removal of the inner core results in the caves, 

bathtubs, arches, and hollow balls found today (Maley 

and Randolph 1993). Arches are more commonly 

associated with eroding sedimentary rocks in the 

southwest and are especially rare in granitic terrains (fig.

10).

Cavernous Forms

There are more than 500 panholes in one small, 230-m2

(2,500-ft2

) area in the reserve. Panholes are basin-shaped 

hollows, usually about 0.3 m (1 ft) in diameter (see inside 

front cover and fig. 11) (Cunningham 1971). The most 

notable panhole is located on top of Bath Rock and 

frequently fills with water from rain or snow melt.

Panholes play a large role in shaping and lowering the 

rock outcrop surfaces. Their shape and size evolve over 

time in a predictable manner, but appear unrelated to 

joints unless in areas where they are elongated along the 

strike of a joint on which they originated (Cunningham 

1969; Pogue et al. 2001; Miller et al. 2008; Pogue 2008).

Panholes begin at small depressions or cracks where 

water and snow linger and promote weathering. Once a 

depression forms and holds water, the excavation of the 

panhole is enhanced. Biological activity from lichen and 

moss can accelerate weathering within the panhole 

(Pogue 2008). Nested panholes within larger panholes 

indicate a local drop in the water supplied to the larger 

panhole. When their bottoms (usually the hardened 

crusts) are breached, weathering of the interior of a 

dome is accelerated. The last remnants of hard coating 

left after panholes have coalesced and weathered away 

are spike-shaped features called “pickelhauben” (Pogue 

2008). 

Honeycomb weathering forms networks of marble-sized 

holes on the sides of pinnacles at City of Rocks National 

Reserve that are among the most fragile rock formations 

there. The honeycomb-like surfaces form when casehardened surfaces are breached and the granite rapidly 

disintegrates grain-by-grain (Miller et al. 2008). Hollows 
CIRO Geologic Resources Inventory Report 13

and cavities that form by cavernous weathering on the 

sides of granitic spires and boulders are called “tafoni”

and may be as large as a room (fig. 12). They typically 

have relatively narrow entrance holes armored by an

overhanging lip of case-hardened crust, and flat floors 

(Pogue 2008). Geologists surmise that these features 

form on areas protected from rainfall by the 

crystallization of salts within microcracks deposited by 

mist, fog, and wind-borne dust. This process is selfsustaining as salts are precipitated in repeated 

microcracks formed from previous cracking (RodriquezNavarro et al. 1999; Hejl 2005; Pogue 2008). The Great 

Salt Lake Desert, south of City of Rocks, is likely the 

source of the salt responsible for the fragile tafoni 

formations within the reserve area (Pogue 2008). 

Twin Sisters

When the tops of granitic exposures are weathered and 

dissolved by rainwater, minerals such as iron oxide are 

redeposited to form crustlike caps. These caps are more 

resistant to weathering than the underlying rock and are 

left standing as spires and pinnacles such as the Twin 

Sisters (fig. 13).

Juxtaposed with the weathered shapes of the Oligocene 

granite at City of Rocks are the harder Precambrian 

outcrops. These provide a contrast of sharper outlines.

Twin Sisters and Lost Arrow, are two examples of spires 

at City of Rocks National Reserve (Miller et al. 2008). 

The Twin Sisters—two adjacent rock towers on opposite 

sides of a contact between Precambrian gneiss to the 

southeast and Oligocene granite to the northwest—

represent each formation (Wells 1984).

The history of the Twin Sisters is in many ways 

representative of the evolution of the City of Rocks 

landscape. The geologic gap in time between them is 

more than two billion years of Earth history. The darker 

sister is rock from the Green Creek Complex. It is 2.5 

billion years old and is some of the oldest rock in North 

America. The lighter sister is rock from the far younger 

(≈28 million years old) Almo pluton. Both rock 

formations were once buried deep below other layers of 

rock. The Almo pluton intruded the Green Creek

Complex at a depth of approximately 10−14 km (6−9 mi) 

(Pogue 2008). Regional tectonic events, such as 

extension and uplift (described under “Geologic 

History”), caused large masses of rock to slide away from 

the core of the Albion Mountains. Masses of overlying 

rock slid east and west along large-scale normal faults. 

The removal of overburden and excess heat locally 

caused the rocks to rise. The uplifted rocks were 

intensely weathered, and erosion removed much of the 

overlying material. These processes acted in concert to 

expose the once-buried Almo pluton and Green Creek

Complex in the City of Rocks area. 

Other Weathering Features

Flat structures along the bases of many outcrops have a 

nonrandom distribution that can be attributed to 

parameters such as aspect, slope, and soil moisture 

(Pogue et al. 2001). In the upper parts of some drainage 

basins, erosional surfaces of granitic bedrock, called 

pediments, have very low relief. Their formation is 

somewhat enigmatic, but they may be produced by the 

erosion and retreat of a mountain scarp in the semiarid 

climate by a repetitive sequence of events (Strudley et al. 

2006; Pogue 2008). This sequence consists of sediment 

production by weathering of granite and subsequent

removal of the sediment by streams. For example, 

thinning of a pediment veneer by flowing water causes 

soil to develop more rapidly and thickens the veneer,

whereas thickening of the veneer when streamflow is 

reduced causes soil to develop more slowly. This 

repeated sequence creates smooth rock surfaces through 

grain-by-grain disintegration characteristic of granite 

(Strudley et al. 2006; Miller et al. 2008). The oldest 

landforms in the area are pediments preserved from 

erosion and weathering by overlying Miocene sediment 

and volcanic rocks (Miller et al. 2008).

Periglacial conditions that occur at high elevation 

promote the process of frost-wedging whereby rocks are 

broken apart by the expansion of water (≈9% volume 

change) freezing along fractures, cracks, and joints 

within the rock (Miller et al. 2008; Pogue 2008). 

Formations resulting from frost-wedging locally include 

patterned ground and scattered rocks in block fields on 

Graham Peak and other local ridges (Miller et al. 2008).

Many of the lower flanks of the granite spires in the 

Albion Mountains have smooth, concave, and locally 

overhanging walls ranging in height from 1 to 10 m (3 to 

33 ft). These flared surfaces are commonly covered with 

blisters and flakes produced by small-scale exfoliation 

because they are not exposed to precipitation that would 

form a surface crust. These features form prior to 

exhumation in the subsurface amidst water-saturated 

regolith and require prolonged periods of relative 

stability (Pogue 2008). 

Weathering Processes

A principle natural phenomenon at City of Rocks is the 

display of active weathering processes shaping the 

landscape (Jones 1973). The area is considered a type 

section for the origin of weathered granitic landforms 

(Cunningham 1971; Jones 1973). Two major categories 

of weathering—chemical and mechanical—alter the

rocks at City of Rocks and occur at all scales. Chemical 

weathering decomposes rocks at the molecular scale by 

decomposition through interactions between the rocks 

and water and dissolved chemicals. Mechanical 

weathering breaks rocks apart by applied physical force. 

The interplay of these two types of weathering is 

complex, and in granitic rocks the different reactions are 

controlled by factors such as (1) mineral composition of 

the rock, (2) chemistry of the water in contact with the 

rock, (3) regional climate, and (4) abundance of fractures 

or joints within the rock (Pogue 2008). Thoroughgoing, 

continuous fractures allow constant exposure of rock 

surfaces to water. Dissolved minerals are either flushed 

out of the rock (widening preexisting cracks) or 

redeposited as encrustations (Pogue 2008). Mechanical 

weathering processes such as frost wedging, mineral 

wedging, expansion from unloading, biomechanical 

weathering, thermal expansion, and root wedging expose 
14 NPS Geologic Resources Division

new fractures to further mechanical weathering or 

chemical dissolution (Pogue 2008). 

The different scales at which weathering occurs at City of 

Rocks demonstrate the pervasive influence of geologic 

processes on evolution of landforms. The entire Circle 

Creek Basin, at an elevation of 1,890 m (6,200 ft), in 

which City of Rocks is located, has developed through a 

complex erosional history consisting of discrete stages of 

stability followed by periods of intense erosion of the 

weathered granite. This erosion was accelerated as a 

result of tectonic uplift, climate changes, or both (Pogue 

et al. 2001). Most weathering occurs in the region below 

the surface where the rock and unconsolidated regolith 

are commonly saturated with groundwater (Pogue 2008). 

Erosion removes the mantling regolith, exposing the 

granitic bedrock. Weathering on this scale is in stark 

contrast to the molecular level of chemical dissolution 

along the tiniest fracture within an intact granitic 

outcrop. 

Historical Connections with Geology

City of Rocks became a prominent fixture on the 

California and Oregon Trails. Experienced mountain 

men, such as Joseph P. Chiles and Joseph R. Walker, led 

the early emigrant groups through the area. Granite Pass 

funneled traffic into the area because no road could be 

found south of the pass, and a northern detour would 

have been unnecessarily long. Granite Pass is a V-shaped 

notch in the middle of a ridge of Middle Mountain 

quartzite and gneiss. It is more than 2 km (1.25 mi) wide 

and more than 305 m (1,000 ft) deep. Because of the 

parallel, north-south-trending ridges across this part of 

the western United States, such notches and passes were 

vital to westward travel (Wells 1984).

After 1842, thousands of men, women, and children 

passed near City of Rocks to reach a passage through the 

Humboldt Valley of northern Nevada to the Willamette 

Valley in Oregon and the Sacramento Valley in 

California, as well as other destinations, such as the 

Mormon settlements in Utah (Cunningham 1971; Wells 

1984). In 1849 the California Gold Rush accelerated this 

traffic. In 1852 some 52,000 people passed through the 

City of Rocks area on their way west to the alluring 

California gold fields. These travelers often noted in 

journals and diaries the interesting geological features 

found in the area (Wells 1984).

"We encamped at the city of the rocks, a noted place 

from the granite rocks rising abruptly out of the ground," 

wrote James Wilkins in 1849. "They are in a romantic 

valley clustered together, which gives them the 

appearance of a city.” Wilkins was among the first wagon 

travelers to fix the name “City of Rocks” to what looked 

like "a dismantled, rock-built city of the Stone Age."

They scratched and spread axle grease (fig. 14) to mark 

names of rocks, their own names, and other graffiti on 

some of the pinnacles in the area, one of which is 

believed to have the largest historic graffiti collection in 

the United States (Ekman 1962; Jones 1973; Miller 1984).

It is called “Register Rock” (Miller 1984). The remnants 

of these features are in danger of eroding away by the 

geologic process of small-scale exfoliation, which has 

already removed much of this graffiti (Pogue 2008). Frost 

wedging, which occurs when water seeps into cracks on 

the sides of spires and then freezes (and expands), causes

great slabs of rock to crack off and has also removed 

some of the layers of rock bearing 150-year-old 

signatures left by the pioneers.

The overland wagon routes began to pass into history 

when the transcontinental railroad was completed in 

1869 at Promontory Point (now Golden Spike National 

Historic Site) in the Promontory Mountains ,south of 

City of Rocks. However, stagecoaches and wagons were 

in continued use on regional supply routes that spread 

out north and south from the railroad lines. A stage route

established by John Halley connected the railroad at 

Keton, Utah, with Boise, Idaho’s mining hub. The route 

passed through City of Rocks. A stone-built stagecoach 

station was set on the junction of the old California Trail 

and the Salt Lake Alternate on the Salt Lake City−Twin 

Falls stage route. The building was vacated in 1878 after 

the route changed. This historical treasure was bombed 

by vandals in the early 1980s (Miller 1984).

Settlers began to homestead southern Idaho in the late 

1800s. City of Rocks was attractive for dry farming,

which declined during the drought years of the 1920s 

and 1930s, but livestock ranching continues today. A vast 

and significant historical heritage is preserved in the 

rocks and landscape at City of Rocks (Historical 

Research Associations, Inc. and Amphion 1996). The 

scope and shape of the regional geology played a part in 

the western expansion of the United States. Granite Pass 

allowed relatively safe passage through the mountains.

Geologic features attracted the interest of travelers then 

as now, and this connection should be emphasized.

The earliest geologic studies of the area noted that the 

best use of the Silent City of Rocks was as a national 

monument (Anderson 1931). Although mineral 

resources such as skarn minerals, pegmatites containing 

muscovite and smoky quartz, hematite, sand and gravel, 

and disintegrated granite exist at the reserve, mining 

activity was not a significant part of the history of City of 

Rocks National Reserve and only a few old prospect pits 

were noted in recent mapping within the reserve (Miller 

et al. 2008). City of Rocks is a National Natural 

Landmark following recommendations and evaluations 

by Jones (1973). Many landmark studies on the exposure 

of metamorphic core complexes and the formation of 

eroded granitic pinnacles conducted at City of Rocks are 

now classic geology and are still widely cited by modern 

geologists. 

Albion Mountains Metamorphic Core Complex

The southern Albion Mountains are part of a larger 

region that is commonly termed the “Raft River−Grouse 

Creek−Albion Mountains metamorphic core complex.”

This is a complex of rocks that crop out in the Albion, 

Raft River, and Grouse Creek mountains and that share a 

common history of deformation and metamorphism

(Compton and Todd 1977; Miller et al. 2008).
CIRO Geologic Resources Inventory Report 15

A metamorphic core complex is characterized by highgrade metamorphic rocks overlain by shear zones 

(mylonite), other unmetamorphosed rocks and, in 

places, igneous intrusive rocks. The complexes often 

form as a result of overthickened crust that is later 

thinned. Massive lateral pulling apart of the crust (tens of 

kilometers of extension) and unroofing were required 

for the metamorphic core complex to be exposed at the 

surface (Compton and Todd 1977; Miller and Bedford 

1999).

The area was deformed during mountain building events 

(orogenies) during the late Jurassic(?) and Cretaceous. It 

consisted of eastward thrusting and thickening through 

metamorphism and emplacement of subsurface molten 

material (plutonism). Rocks buried deep below the 

surface were affected by ductile deformation and 

metamorphism. Regional folds, including a set trending 

north to northeast, formed during metamorphism 

(Compton and Todd 1977). The crustal thinning in the 

City of Rocks area was likely driven in part by thermallyweakened crust accompanying the intrusion of molten 

rocks as well as regional basin-and-range extension 

(Miller and Bedford 1999).

The rocks within the metamorphic core complex span 

more than two billion years of Earth history. The

≈2.5-billion-year-old Archean Green Creek Complex is 

white mica schist and amphibolite in a batholith of 

granite and gneissic granite that is overlain 

unconformably by the white, pure, thin-bedded basal 

member of the Elba Quartzite (Miller 1984; Miller and 

Bedford 1999; Miller et al. 2008). In the City of Rocks 

area these rocks, as well as other Archean-aged igneous 

and metamorphic rocks, Neoproterozoic-aged quartzite 

and schist, are capped everywhere by a major low-angle 

normal fault—the Mahogany Peaks fault. This fault is 

nearly parallel to bedding and places Ordovician 

Pogonip Group (youngest metamorphic rocks in the 

Albion Mountains) strata on the older metamorphic 

rocks. These rocks represent some of the oldest rocks 

found in North America. They are juxtaposed against 

much younger intruding granitic plutonic rocks—the 

Twin Sisters being a particularly dramatic example

(fig. 13) (Miller and Bedford 1999).

The Almo pluton is one of three Oligocene plutons that 

intruded the western part of the metamorphic core 

complex (figs. 15 and 16). The plutons intrude into shear 

zones and are themselves involved in ductile 

deformation in shear zones. Because of this, they provide 

key information as to the timing of the extensional shear 

that helped to create the complex (Miller and Bedford

1999). The largest exposure of the pluton is west of 

Almo, 24 km (15 mi) in length from north to south, and is 

approximately 8 km (5 mi) across (Cunningham 1971).

The Almo pluton consists of five domes of granite-like 

rock ranging from quartz-monzonite to biotitegranodiorite. This pluton forms a significant part of the 

plutonic/metamorphic infrastructure of the Albion 

Mountains metamorphic core complex. It has a complex 

relationship with the Archean gneiss, schist, and granite

that compose the rest of the core complex. Contact 

metamorphism surrounding the pluton locally 

recrystallized the older rocks. Some contacts between 

the two are orientated at an angle to the surrounding 

rock layers (discordant) other contacts are nearly parallel 

(concordant). In many places it appears that the 

metamorphic rocks collapsed into the pluton, forming a 

variety of subsidence structures and “stoped” blocks.

The emplacement of this pluton may have occurred 

along with tectonic extension about 28 million years ago 

(Miller et al. 2008). Sheared blocks incorporated into the 

pluton indicate that it postdates the Eocene-aged

extensional shear zone. This shear zone may have caused

a kink or anticline (arch) in the crust at the future 

location of the Almo pluton (fig. 16) (Miller and Bedford 

1999; Miller et al. 2008).

Then during the early Miocene (≈20−25 million years 

ago) nearby rocks to the west were sheared again, nearly 

parallel to the earlier Eocene shear zone. Elsewhere, 

undeformed dikes of granite cut previously sheared 

rocks. Thus, it is inferred that the pluton was emplaced

between extensional shear events (Miller and Bedford

1999). Other geologists interpret the doming of the 

Albion metamorphic core complex as the result of 

isostatic uplift and warping and uplift of the Almo pluton 

after the development of an east-west-trending shear 

zone (Bandy 1989).

Basin-and-Range Features and Almo Fault

City of Rocks sits on the northern edge of the Basin and 

Range physiographic province. City of Rocks displays 

many of the features that are characteristic of the 

province, including parallel ranges and valleys (fig. 1C).

High-angle normal faults cut all preexisting structures in 

the area surrounding City of Rocks National Reserve.

Most of these faults trend north-south, parallel to the 

dominant joint set. These faults are usually steeply 

inclined with dips greater than 50°. Many faults having

large inferred offsets are covered with unconsolidated 

Quaternary deposits, which make documentation and 

measurement difficult (Miller 1991).

Major low-angle normal faults typically occur (1) near 

the base of the Paleozoic rocks, (2) in Mississippian 

metamorphosed shales (not exposed at City of Rocks),

and (3) within Middle Permian formations (also not 

exposed at City of Rocks) (Compton and Todd 1977). 

Along steep slopes within the reserve, commonly among 

outcrops of the Elba Quartzite, most exposed faults 

displace rocks down toward the valleys with a normal 

sense of separation (rocks above the fault plane slide 

downwards with respect to rocks beneath the fault 

plane). Springs are common along the faults in the 

reserve (Miller et al. 2008). In many places these faults 

cut rocks along the crest of the range and strike generally 

north. Within the Almo pluton, one fault zone strikes 

N35°E and dips 75°E, cropping out as a brecciated zone 

nearly 3 m (10 ft) wide (Miller et al. 2008). 

Just east of City of Rocks National Reserve, the Almo 

fault passes through the town of Almo and extends in a 

south-to-southwest trend along the east margin of the 
16 NPS Geologic Resources Division

Albion Mountains as part of the Basin and Range 

geologic province. A series of co-linear scarps define the 

fault trace in outcrop (Miller et al. 2008). The scarps are 

rounded and consistently show displacement downward 

to the east (normal offset). The fault typically cuts 

Pleistocene alluvial-fan deposits, indicating the fault 

occurred after deposition of the alluvial-fan deposits. 

Along the road from Almo into the reserve, the fault 

deforms alluvium into a broad fold. A stage IV calcic 

horizon in the soil of the alluvium deposits indicates that 

this deposit is several hundred thousand years old 

(Machette 1985; Miller et al. 2008). Other local areas of 

alluvium having similar soil development do not contain 

scarps, an absence indicating either that (1) agricultural 

practices are obscuring the fault trace, (2) only discrete 

segments of the fault trace are visible, (3) the fault has 

varying vertical offset along strike, or (4) deposition of 

some alluvium postdates the last local fault rupture 

(Miller et al. 2008). 

Figure 4. Three mechanisms by which joints form in granitic rocks. A, joints formed by cooling; B, joints formed by tectonic forces; C, joints 

formed by exfoliation. Diagram is figure 13 from Pogue (2008), used with permission.
CIRO Geologic Resources Inventory Report 17

Figure 5. Aerial image of the central area of City of Rocks National Reserve. The light colored features are exposures of granite. Note the 

north-south orientation of many of the features, this orientation is influenced by one of the main joint sets that developed in the area. The 

arrangement of features separated by these large “avenues” and similarity to an “urban” landscape led emigrants to dub the area “City of 

Rocks.” Two named features are indicated on the image. The Twin Sisters (fig. 13) are located south of this image. Aerial imagery compiled 

from ESRI Arc Image Service, USA Prime Imagery.
18 NPS Geologic Resources Division

Figure 6. Aerial view of the Bread Loaves. The Bread Loaves are one type of granitic pinnacle found within the reserve. Joints parallel to the 

long axis of the “loaves” likely influenced their development. Other joints, perpendicular to the long axis of the loaves lend their “sliced 

bread” appearance. Note road and vehicle in right-center for scale. National Park Service photograph by Wallace Keck (City of Rocks National 

Reserve), available online: http://www.nps.gov/ciro/photosmultimedia/index.htm, accessed March 2010.

Figure 7. Exfoliation joints in the Almo pluton. As granite cools, it contracts forming joints. Exfoliation is just one of three processes 

responsible for the joints at City of Rocks National Reserve (fig. 4). National Park Service photograph by Wallace Keck (City of Rocks National 

Reserve), available online: http://www.nps.gov/ciro/photosmultimedia/index.htm, accessed March 2010.
CIRO Geologic Resources Inventory Report 19

Figure 8. Modeled cross sectional view of the evolution of small-scale landforms on an emerging granitic spire during a period of landscape 

degradation (net removal of material). Diagram is figure 21 from Pogue (2008). Used with permission.
20 NPS Geologic Resources Division

Figure 9. Blisters are a type of “case hardening” on the surface of granite rocks in City of Rocks National Reserve. The thin (few millimeters 

thick) reddish hardened surfaces form relatively quickly and are easily removed, exposing the lighter colored rock beneath. Crusts (not 

pictured) are much thicker and more durable. U.S. Geological Survey photograph from Miller and others (2008). 

Figure 10. Window Arch is an example of fanciful features shaped by weathering of granite pinnacles in City of Rocks National Reserve.

National Park Service photograph, available at http://www.nps.gov/ciro/photosmultimedia/index.htm, accessed March 2010. 
CIRO Geologic Resources Inventory Report 21

Figure 11. Panholes on Bath Rock at City of Rocks National Reserve. Panholes begin as shallow depressions in the hardened crusts that catch 

water or snow. Weathering accelerates when the crust is breached. The inside front cover of this report also shows panholes within the 

reserve. National Park Service photograph by Wallace Keck (City of Rocks National Reserve), available online: 

http://www.nps.gov/ciro/photosmultimedia/index.htm, accessed March 2010. 

Figure 12. Tafoni visible on a pinnacle along the Geology Trail. Tafoni is a form of cavernous weathering on the sides of granite pinnacles. 

Crystallization of salt (likely from the Great Salt Lake) in cracks contributes to the formation of tafoni. National Park Service photograph by 

Wallace Keck (City of Rocks National Reserve), available online: http://www.nps.gov/ciro/photosmultimedia/index.htm, accessed March 2010.
22 NPS Geologic Resources Division

Figure 13. The Twin Sisters are granite pinnacles in City of Rocks National Reserve. Despite their proximity to each other, the “twins” are 

actually separated by more than 2.5 billion years of geologic time. The darker-colored sister on the left is 2.5 billion-year-old granite—some 

of the oldest rocks in North America. The lighter-colored sister on the right is approximately 28 million years old granite associated with the 

Almo pluton. U.S. Geological Survey photograph, available online: http://3dparks.wr.usgs.gov/ciro/index.html, accessed March, 2010.

Figure 14. Emigrant signatures on Register Rock within City of Rocks National Reserve. The “city of rocks” was an important landmark along 

the Oregon and California trails. Emigrants left their names and other “historical graffiti,” often written in axle grease (as shown here), on a 

number of the granite pinnacles. National Park Service photographs courtesy Wallace Keck (City of Rocks National Reserve). 
CIRO Geologic Resources Inventory Report 23

Figure 15. Generalized geologic map of the rock types surrounding City of Rocks National Reserve. Note the location of the Almo pluton. 

Areas in white are underlain by valley fill and other unconsolidated deposits. Graphic adapted from Miller and Bedford (1999).
24 NPS Geologic Resources Division

Figure 16. Generalized geologic map and cross section of the City of Rocks National Reserve area, southern Idaho. The section illustrates the 

major arch in the southern Albion Mountains. Graphic adapted from Miller and Bedford (1999). 

Almo fault
CIRO Geologic Resources Inventory Report 25

Map Unit Properties

This section identifies characteristics of map units that appear on the Geologic Resources 

Inventory digital geologic map of City of Rocks National Reserve. The accompanying 

table is highly generalized and for background purposes only. Ground-disturbing 

activities should not be permitted or denied on the basis of information in this table.

Geologic maps facilitate an understanding of Earth, its 

processes, and the geologic history responsible for its 

formation. Hence, the geologic map for City of Rocks 

National Reserve provided information for the 

“Geologic Issues,” “Geologic Features and Processes,” 

and “Geologic History” sections of this report. Geologic 

maps are two-dimensional representations of complex 

three-dimensional relationships; their color coding 

illustrates the distribution of rocks and unconsolidated 

deposits. Bold lines that cross or separate the color 

patterns mark structures such as faults and folds. Point 

symbols indicate features such as dipping strata, sample 

localities, mines, wells, and cave openings. 

Incorporation of geologic data into a Geographic 

Information System (GIS) increases the usefulness of 

geologic maps by revealing the spatial relationships 

among geologic features, other natural resources, and 

anthropogenic features. Geologic maps are indicators of 

water resources because they show which rock units are 

potential aquifers and are useful for finding seeps and 

springs. Geologic maps are not soil maps, and do not 

show soil types, but they do show parent material—a key 

factor in soil formation. Furthermore, resource managers 

have used geologic maps to make connections between 

geology and biology; for instance, geologic maps have 

served as tools for locating sensitive, threatened, and 

endangered plant species, which may prefer a particular 

rock unit. 

Although geologic maps do not show where earthquakes 

will occur, the presence of a fault indicates past 

movement and possible future seismic activity. Similarly, 

map units show areas that have been susceptible to 

hazards such as landslides, rockfalls, and volcanic 

eruptions. Geologic maps do not show archaeological or 

cultural resources, but past peoples may have inhabited 

or been influenced by depicted geomorphic features. For 

example, alluvial terraces may have been preferred use 

areas and formerly inhabited alcoves may occur at the 

contact between two rock units.

The geologic units listed in the following table 

correspond to the accompanying digital geologic data. 

Map units are listed in the table from youngest to oldest. 

Please refer to the geologic timescale (fig. 17) for the age 

associated with each time period. The table highlights 

characteristics of map units such as: susceptibility to 

erosion and hazards; the occurrence of paleontological 

resources (fossils), cultural resources, mineral resources, 

and caves or karst; and suitability as habitat or for 

recreational use. Some information on the table are 

conjectural and meant to serve as suggestions for further 

investigation.

The GRI digital geologic maps reproduce essential 

elements of the source maps including the unit 

descriptions, legend, map notes, graphics, and report. 

The following reference is the source for the GRI digital 

geologic data for City of Rocks National Reserve: 

Miller, D. M., R. L. Armstrong, D. R. Bedford, and M.

Davis. 2008. Preliminary geologic map and digital data 

base of the Almo quadrangle and City of Rocks National 

Reserve, Cassia County, Idaho. Scale 1:24,000. OpenFile Report OF 2008-1103. Reston, VA: U.S. 

Geological Survey. 

The GRI team implements a geology-GIS data model 

that standardizes map deliverables. This data model 

dictates GIS data structure including data layer 

architecture, feature attribution, and data relationships 

within ESRI ArcGIS software, and increases the overall 

utility of the data. GRI digital geologic map products 

include data in ESRI personal geodatabase and shapefile 

GIS formats, layer files with feature symbology, Federal 

Geographic Data Committee (FGDC)-compliant 

metadata, a Windows help file that contains all of the 

ancillary map information and graphics, and an ESRI 

ArcMap map document file that easily displays the map 

and connects the help file directly to the map document. 

GRI digital geologic data are included on the attached 

CD and are available through the NPS Data Store 

(http://science.nature.nps.gov/nrdata/). 

Map Units within City of Rocks National Reserve

Early mapping efforts by Anderson (1931) and 

Armstrong (1968) distinguished granite, metamorphic 

rocks, and volcanic rocks at City of Rocks, as well as 

some of the rudimentary geologic structure and 

metamorphic history. Four unis—the Green Creek 

Complex, Elba Quartzite, schist of Upper Narrows, and 

Almo pluton—dominate the bedrock at City of Rocks 

National Reserve (Pogue 2008). The Archean Green 

Creek Complex of Armstrong and Hills (1967) includes 

amphibolite, granite and granite gneiss, and schist. The 

dark-brown, biotite-rich schist is the oldest rock of the 

complex (Miller et al. 2008; Pogue 2008). Granite 

intruded the schist and has a porphyritic texture. Local 

metamorphism and deformation (especially near the 

Almo pluton) converted some of the granite into granitic 

gneiss (Pogue 2008). The amphibolite appears as dark, 

linear bands within the granite, possibly in an intrusive 

relationship (Miller et al. 2008). 
26 NPS Geologic Resources Division

The Neoproterozoic Elba Quartzite of Armstrong (1968) 

is one of the most distinctive units in the Albion 

Mountains, having nearly pure quartz composition, 

zones of conglomerates, and very light color (Miller et al. 

2008; Pogue 2008). It nonconformably overlies the 

Archean rocks and has a gradational relationship with 

the overlying Schist of Upper Narrows (Miller et al. 

2008). Mud, silt, and carbonate sediments were 

deposited atop the quartz sands of the Elba Quartzite 

and metamorphosed into dark-brown and gray schist 

interlayered with quartzite and marble (Pogue 2008). 

Throughout the Albion and Raft River mountains are 

exposures of a thick sequence of metasedimentary rocks,

including the Quartzite of Yost, Schist of Stevens Spring, 

Quartzite of Clarks Basin, Schist of Mahogany Peaks, and 

the metamorphosed Pogonip Group (Miller et al. 2008; 

Pogue 2008). Within City of Rocks National Reserve, 

rocks of these units appear only in roof pendants and 

xenoliths in the Almo pluton, having been otherwise 

eroded away (Pogue 2008). 

From a distance, exposures of the Green Creek Complex 

granite and the Almo pluton granite appear similar; upon 

closer inspection, however, the Almo pluton granite 

lacks the strong porphyritic texture of the darker Green 

Creek Complex granite (Pogue 2008). The Almo pluton 

is of granitic to granodioritic composition with massive 

textures and common pegmatite-aplite dikes (Miller et 

al. 2008). 

The bedrock geology is strikingly displayed at City of 

Rocks as myriad geomorphologic forms indicative of the 

geologic history of the area, recently dominated by

regional erosion, uplift, and extension. The extension of 

the Basin and Range province followed the late 

Cretaceous to early Tertiary compressional Sevier–

Laramide orogenic events. The Miocene rhyolitic 

volcanic rocks, including welded tuff, erupted during the 

passage over the Yellowstone hot spot approximately 10 

million years ago to the north of the Albion Mountains 

(Miller et al. 2008; Pogue 2008). They were deposited as 

pyroclastic flows atop the weathered granite of the Almo 

Pluton and Green Creek Complex and now appear in 

small isolated outcrops within the reserve (Pogue 2008).

Sedimentary deposits from the Tertiary and Quaternary 

periods include cobbley sand (containing clasts of Almo 

pluton granite and Elba Quartzite), fluvial deposits, 

several generations of alluvial-fan deposits, talus, 

landslide deposits, alluvium, and colluvium (Miller et al. 

2008). These weathered from, and were deposited atop,

the older metamorphic and igneous rocks in small

valleys, at the base of slopes, and in Circle Creek Basin.

The various generations of unconsolidated deposits at 

City of Rocks record climate changes throughout the 

Cenozoic Era (Pogue 2008). Erosion upon continued 

uplift carved into these unconsolidated units, weathering 

them into the ravines, gulleys, sweeping valleys, and 

channels present at City of Rocks National Reserve 

today.
CIRO Geologic Resources Inventory Report 27

Map Unit Properties Table: City of Rocks National Reserve

Age Unit Name

(Symbol) Features and Description Erosion Resistance

Suitability for 

Infrastructure Hazards Mineral Specimens Habitat Recreation Geologic Significance

QUATERNARY

Alluvium

(Qal)

Younger alluvial fan 

deposits

(Qya)

Younger alluvial fan 

deposits over 

intermediate alluvial 

fan deposits 

(Qya/Qia)

Younger fluvial 

deposits

(Qyf)

Veneer on pediment 

(Qvp)

Unit Qal contains 2−3 m (7–10 ft) of dark-brown sand and mud along 

active washes and floodplains with some gravel deposited locally. Unit 

Qya includes dark-brown, organic-rich sand, gravel, and silt that form 

1- to 4-m- (3- to 13-ft)-thick alluvial fans from steep hills to lower 

gradient areas. Unit Qya/Qia is mixed thin (<2 m [7 ft] thick) sheets of 

younger and intermediate alluvial fan deposits. Unit Qyf contains mud, 

sand, and gravel associated with active river processes. Unit Qvp

consists of mixed colluvium, alluvium, and alluvial fan deposits as thin 

(<3 m [10 ft] thick) veneers on nearly flat erosional bedrock surfaces 

(pediment). 

Low unless 

carbonate 

cement 

present.

Units are associated 

with river 

environments, such 

as floodplains and 

stream banks; avoid 

for heavy 

development.

Units are prone to flooding and 

slumping when undercut. Stream 

deposits could be prone to 

liquefaction. May contain shrinkand-swell clays.

Sand, gravel

Units contribute to 

organic-rich soils 

along riparian zones;

some calcic horizons 

are present in unit 

Qyf.

Units are associated with 

active geomorphological 

processes and fragile 

riparian areas; avoid for 

any recreation beyond light 

seasonal use.

Units contain the Mazama ash layer 

(eruption that formed what is now 

Crater Lake in Oregon) dated at 

approximately 7,700 years old. Unit 

Qyf contains charcoal layers dated at 

8,265±35 years before present.

QUATERNARY

Landslide deposits 

(Qls)

Colluvium

(Qc)

Talus

(Qt)

Unit Qls contains bouldery, dark-colored deposits less than 10 m

(33 ft) thick displaying slump-block morphology in outcrop. Unit Qc is 

light- to dark-brown, unsorted and unconsolidated rubble of rock and 

soil on steep slopes less than 5 m (16 ft) thick. Unit Qt includes white, 

blocky boulder fields (<5 m [16 ft] thick) on steep slopes with angular 

edges and an overall lack of a finer grained matrix. 

Low

Units are associated 

with unstable and 

steep areas and 

should be avoided for 

development.

Units are associated with active 

mass-wasting processes, including 

repeated slumping, tumbling, 

rockfall, sheetwash, and slope 

creep. Some frost wedging also 

associated with these units.

Boulders

Units are associated 

with springs and 

dense vegetation 

locally. Unit Qls is 

mostly unforested.

Units form stepped slopes 

at the bases of steeper 

exposures and should be 

avoided for most 

recreational use.

Units are associated with prominent 

local features, including Twin Sisters.

QUATERNARY

Intermediate alluvial

fan deposits

(Qia)

Intermediate alluvial

fan deposits over 

older alluvial fan 

deposits

(Qia/Qoa)

Intermediate alluvial

fan deposits over 

Quaternary and 

Tertiary alluvium and 

landslide deposits

(Qia/QTa).

Unit Qia includes gravelly sand and sandy loam in medium- to darkbrown, 5- to 10-m-(16- to 33-ft) thick exposures as alluvial fans and 

some colluvium. Some loess deposits in 20- to 50-cm- (8- to 20-in.)-

thick lenses locally. Unit Qia/Qoa overlies older alluvial-fan deposits 

with a thin (<2 m [7 ft] thick) sheet of mixed sand and gravel. Unit 

Qia/Qta resembles unit Qia/Qoa in thickness and composition 

overlying alluvium and landslide deposits.

Low unless 

carbonate 

cement 

present.

Units are present as 

unstable alluvial fans 

prone to erosion, 

mass wasting, and 

flash flooding and are 

unsuitable for most 

development.

Units are associated with flashflood deposition and active erosion 

and are prone to gullying.

Gravel, sand, silt

Unit contains 

decomposed granite 

and one or two welldeveloped soils with 

argillic and calcic 

horizons. Stage II 

calcic soil and stage IV 

soil present. 

None documented Units record history of uplift and 

erosion throughout the area.

QUATERNARY

Intermediate fluvial 

deposits

(Qif)

Older alluvial fan 

deposits

(Qoa)

Older fluvial deposits 

(Qof).

Unit Qif contains sand, gravel, and mud with some Bt and calcic (stage 

II) soil horizons in uppermost layers of the deposit. Unit contains 

characteristic surficial pavements and gray mud beds atop well-sorted 

pebble to cobble gravel. Unit Qoa contains 15−25 m (49–82 ft) of 

medium-brown, gravelly sand deposited as alluvial fans and colluvial 

layers in steeper areas. Coarser cobbles and boulders form an erosional 

surface on rounded crests with sand and loam matrix. Unit Qof

contains rounded clasts, mixed with adjacent alluvium, and loess caps 

greater than 1 m [3 ft] thick. 

Low

Units are highly 

permeable and may 

be unsuitable for 

waste-water 

treatment-facility 

development.

Units are prone to slumping and 

slope creep if undercut or exposed 

on slopes.

Sand, gravel, mud, 

cobbles, pebbles, loam.

Unit Qoa contains 

well-developed calcic 

horizon in soils 

including more than 

1.4 m of stage IV 

calcic soil. Unit Qof

contains loess-rich 

areas.

Boulder fields are unstable 

and unsafe trail base and 

should be avoided for 

recreational use.

Unit Qif may contain glacial deposits 

recording activity in the Raft River 

Mountains. Unit Qoa contains soils 

that may be several hundred 

thousand years old. Units are used to 

determine erosional and depositional 

history of the area.

QUATERNARY

-TERTIARY

Alluvium and 

landslide deposits 

(QTa)

Fluvial deposits

(QTf).

Unit Qta includes boulder and gravel deposits forming high hills that 

are light colored in outcrop exposures as thick as 25 m (82 ft). Unit 

QTf is a 10-m-(33 ft) thick mixture of brown sand, gravelly sand, 

gravel, and silt with some bedding and cross-lamination depositional 

structures. Well to moderately sorted. Brown sand of this unit is of 

wind-blown origin and overlies river (fluvial) deposits. 

Moderately 

low.

Units are highly 

unsorted and 

heterogeneous and 

may be unstable if 

undercut for 

foundation 

construction.

Units contain large boulders atop 

less competent layers and may be 

prone to mass wasting during 

extreme precipitation.

Quartzite boulders, 

gravel, fine sand, silt.

Unit QTf resembles 

rounded terraces and 

contains stage III or 

stage IV calcic soil 

horizons.

Unit QTa is flanked by 

large boulders (1−5 m 

diameter) that may be 

unsuitable for trails and 

other uses.

Unit QTa associated with Smokey 

Mountain and Twin Sisters with 

deposits from now-eroded heights.
CIRO Geologic Resources Inventory Report 28

Age Unit Name

(Symbol) Features and Description Erosion Resistance

Suitability for 

Infrastructure Hazards Mineral Specimens Habitat Recreation Geologic Significance

TERTIARY

Rhyolite

(Tr)

Sedimentary deposits 

(Ts)

Granite

(Tg)

Unit Tr consists of bright-red, brownish-red, to gray welded rhyolite 

tuff with some black vitrophyre near the base and top, locally. Lithic 

fragments of granite as large as 1.5 cm (0.6 in.) within vesicular 

(flattened) central unit. Unit ranges in thickness from 9 m to 17−20 m

(30 ft to 56–66 ft). Unit Ts includes cobbly sand in light-brown, slopeforming deposits. Boulders from this unit are as large as 4 m (13 ft) in 

diameter in 5- to 10-m- (16- to 33-ft)-thick deposits. Unit Tg is white to 

light-gray granite of the Almo pluton. Compositions range from biotite 

granite, to biotite-muscovite granite, to muscovite granite in massive, 

banded outcrops. Dikes of pegmatite and aplite intrude locally. 

High for units 

Tr and Tg, 

low for unit 

Ts.

Igneous units may 

weather to radonproducing soils and 

should be evaluated 

prior to any 

infrastructure 

construction.

Unit Ts is prone to slumping and 

sliding if exposed on slopes or 

undercut. Unit Tg may be present 

as large blocks and slabs that may 

be subject to frost wedging and 

rockfall. Units may weather to 

produce shrink-and-swell clays.

Smoky quartz (3−6 mm), 

pumice (vesicles may be 

filled with secondary 

minerals locally), 

quartzite boulders, 

biotite, muscovite, 

garnet, granite, 

pegmatite.

None documented

Granitic exposures form 

much of the “City of 

Rocks” features. They are 

popular for rock-climbing.

Nearby rocks similar to unit Tr were 

dated to 8.8 million years old.

Unit Tg represents the Almo pluton

and has a Rubidium-Strontium 

radiometric age of 28.6±1.0 million 

years and forms the younger of the 

Twin Sisters.

ORDOVICIAN

Metamorphosed 

Pogonip Group

(Op)

Unit contains brown to bluish-gray, calcitic marble in 10-m- (33-ft)-

thick, narrow outcrops. Moderate

Unit may contain 

dissolved conduits 

and should be 

investigated before 

any infrastructure 

construction.

Unit could dissolve away from 

beneath intact rocks and present a 

rockfall hazard.

Brown ,micaceous 

marble; gray, pure 

marble; calcite;

dolomite; phlogopite.

Unit weathers to 

produce calcium- and

magnesium-rich soils. 

Dissolved crevices 

may provide bat 

habitat.

Unit may be subject to 

karst dissolution that may 

produce caves or small 

cavities that could attract 

visitors.

Unit represents clean and silty 

limestone deposition in a marine 

environment. 

PROTEROZOIC

Schist of

Mahogany Peaks

(Zmp)

Quartzite of

Clarks Basin

(Zcb)

Unit Zmp contains black schist with coarse-grained textures.

Unit Zcb includes thinly layered, flaggy 20-m- (66-ft)-thick quartzite 

with mica-rich joints or layers. Unit appears silvery white and tan to 

light gray. 

Moderate, 

moderately 

high for 

quartzite.

Heterogeneous and 

locally schistose 

nature of these units 

may render them 

unsuitable for 

infrastructure.

Mica-rich joints and layers may 

provide planes of weakness within 

resistant quartzite that may present 

rockfall hazards.

Quartzite None documented

Quartzites may form 

resistant ledges and ridges 

that may provide nesting 

habitat or attract climbers.

Units record mixed marine and nearshore depositional environments.

PROTEROZOIC

Schist of

Stevens Spring

(Zss)

Quartzite of Yost

(Zy)

Unit Zss is brown to silvery-brown schist with coarse-grained, wavy to 

knobby textures. Near the base of this unit is distinctive amphibolite. 

Unit is approximately 300 m (980 ft) thick. Unit Zy is pure white 

quartzite. Scant muscovite (mica) layers define the thin bedding of this 

40-m- (130-ft)-thick unit. 

Moderate to 

high for 

quartzite.

Coarse-grained schist 

may prove too friable 

locally for 

infrastructure.

Quartzite is susceptible to blockfall 

if undercut on slopes. Schist can 

pose mass-wasting problems,

especially if slope is aligned parallel 

to schistosity. 

Muscovite, sillimanite, 

biotite, garnet, 

andalusite, kyanite in 2-

to 7-cm- (0.8- to 2.8-in.) 

wide lenses, white 

quartzite.

None documented

Quartzite may form 

resistant ledges and ridges 

that may provide nesting 

habitat.

Unit Zss records a complex regional 

metamorphic history.

PROTEROZOIC

Schist of the

Upper Narrows

(Zun)

Elba Quartzite of 

Armstrong

(Ze)

Unit Zun includes thinly layered, quartz-rich muscovite-biotite schist 

and (2- to 12-cm- [0.8- to 4.7-in.]-thick) schistose quartzite that 

appears light to dark brown and grayish brown in outcrop. Unit is 

approximately 70 m (230 ft) thick. Pure limestone marble is present 

locally. Unit Ze is approximately 200 m (660 ft) of white to pale gray 

and grayish-brown quartzite in thick beds. Unit grades from vitreous

(glassy) quartzite beds at the base to darker, less pure, thick quartzite 

beds. Some tabular cross lamination and schist interbeds present 

locally. 

High

Layered and 

weathered nature of 

these units may 

render them 

unsuitable for 

infrastructure.

Quartzite is subject to blockfalls 

and frost wedging. The presence of 

unit Ze clasts in younger units 

suggest it is deeply eroded. Schist 

can pose mass wasting problems if 

slope is aligned parallel to 

schistosity.

Muscovite, biotite, 

quartzite, pure limestone 

marble, feldspar, 

epidote, actinolite.

Unit Ze tends to 

underlie well-drained 

high ridges due to its 

high resistance to 

erosion.

Unit forms resistant ridges 

that may be attractive to 

climbers.

Unit Zun may have been part of a 

shallow marine depositional 

environment with laminated sand 

and silt. In unit Ze, muscovite schist 

may represent metamorphosed 

argillaceous soil horizon above 

Archean rocks. 

ARCHEAN

Amphibolite

(Wga)

Granite and

granite gneiss

(Wgg)

Schist

(Wgs)

Unit Wga includes black hornblende-plagioclase and hornblendeplagioclase-garnet metamorphosed mafic rock with coarse- to 

medium-grained textures. Unit appears as linear, dark bands in granite 

and is a few tens of meters wide. Unit Wgg is porphyritic biotite granite 

that appears light to dark gray and coarse grained in outcrop. Large 

crystals are aligned as magmatic foliation. Granite is locally deformed 

to augen gneiss. Unit Wgs contains coarse-grained muscovite-biotite 

schist that appears dark brown in outcrop. Schist locally contains veins, 

stringers, and knots of feldspar and quartz and is intruded by unit Wgg. 

High to 

moderately 

high for schist.

Granitic unit may 

weather to produce 

radon-emitting soils 

and should be tested 

prior to basement

construction.

Unit subject to blockfall if 

undercut on slope. Weathered 

exposures can be friable and 

unstable on slopes. Mica-rich joints 

and layers may provide planes of 

weakness within resistant granite 

and quartzite that may present 

rockfall hazards.

Hornblende, plagioclase, 

garnet, quartz, 5x2-cm 

(2x0.8-in.) microcline 

phenocrysts, biotite, 

coarse granite.

None documented

Units are suitable for most 

recreation; large crystals 

may attract illegal sampling.

Archean rocks are among the oldest 

in North America. Unit Wga likely 

represents dike swarms intruding the 

granite. Unit Wgg is 2.5 billion years 

old and forms the older of the Twin 

Sisters.
CIRO Geologic Resources Inventory Report 29

Geologic History

This section describes the rocks and unconsolidated deposits that appear on the digital 

geologic map of City of Rocks National Reserve, the environment in which those units

were deposited, and the timing of geologic events that created the present landscape.

Precambrian (prior to 542 million years ago)

The rocks present within and surrounding City of Rocks 

National Reserve record a vast span of geologic history 

as part of the regional Raft River−Grouse Creek−Albion 

Mountains metamorphic core complex (see fig. 17 for a 

time scale). The oldest rocks provide a rare glimpse into 

the tectonic setting of the Archean Eon, more than 2.5 

billion years ago. They are among the oldest rocks in 

North America. The Green Creek Complex consists of a 

metamorphic rock called schist that formed from 

sediments originally deposited as mud, silt, and sand in a 

shallow sea (Pogue 2008). Following a metamorphic 

event, granitic and basaltic magma intruded the 

metamorphosed sediments around 2.5 billion years ago 

(Compton and Todd 1977; Miller and Bedford 1999; 

Miller et al. 2008). Subsequent metamorphism and 

deformation events (much later in the Mesozoic and 

Cenozoic eras) converted some of these rocks into gneiss 

and amphibolite and produced regional folds (Compton 

and Todd 1977; Pogue 2008). After the Archean-aged

metamorphic and intrusion events, significant uplift and 

erosion exposed these rocks at the surface during the 

Proterozoic (Miller et al. 2008; Pogue 2008). One of the 

Twin Sisters is composed of this Archean-aged granite

(fig. 13).

Late Proterozoic-aged rifting created a new continental 

margin along western North America. The thinned crust 

subsided, and shallow seas inundated the lowest areas. 

Sometime between 1,500 and 600 million years ago, pure 

quartz sediments derived from weathering of the ancient 

continent covered the eroded Archean-aged rocks 

(Pogue 2008). The City of Rocks area was located near

the western shoreline in this passive margin 

paleoenvironment, similar to today’s East Coast of North 

America. The Elba Quartzite is the widespread unit that 

records this environment. It is a prominent marker unit 

between the underlying Archean rocks and overlying 

Neoproterozoic-aged rocks such as the Schist of the 

Upper Narrows, Quartzite of Yost, Schist of Stevens 

Spring, Quartzite of Clarks Basin, and Schist of 

Mahogany Peaks, that were originally deposited as 

sandstone, siltstone, shale, and volcanic rocks (Wells et 

al. 1998; Miller and Bedford 1999; Miller et al. 2008).

During the Late Precambrian and Cambrian, deposits of 

shallow-water marine sediments thousands of feet thick 

accumulated along a passive plate-tectonic margin on the 

western side of the Transcontinental Arch. This arch 

was an upland area that stretched from northern 

Minnesota southwestward across Nebraska, Colorado, 

and northwestern New Mexico (Speed 1983; Sloss 1988).

As more sediment accumulated, the basin subsided and 

the shallow sea became deeper. At the beginning of the 

Cambrian, marine invertebrates with shells of calcium 

carbonate flourished in the local sea. Accumulations of 

their remains formed thick limestone deposits (Pogue 

2008). 

The Precambrian units now crop out at the surface at 

City of Rocks due to extensive erosion and normal 

faulting. The missing rock record makes it difficult to 

determine the paleoenvironments of millions of years at 

City of Rocks. Geologists refer to surrounding areas to 

determine the history in a regional context.

Paleozoic Era (542 to 251 million years ago)

Beginning in the Cambrian and throughout the Paleozoic

Era, cycles of erosion from small-scale tectonic uplifts 

contributed mud, silt, and sand lenses to the 

predominantly carbonate deposition within the marine 

basin. During the Late Cambrian, North America was

oriented so that the panhandle of Idaho was parallel to 

and very near the equator. Limy muds accumulated 

under these warm-water conditions in much the same 

way as they do in the broad lime-mud shoal-bank area in 

the Bahaman Islands today (Hintze 1988). The Paleozoic 

rocks elsewhere record remarkably thick accumulations 

of sediment, each unit being as much as 2,100 m (7,000 ft) 

thick in some places, in a marine environment 

(Cunningham 1971). For example, near Golden Spike 

National Historic Site, south of City of Rocks, dolomite 

and other carbonate rocks record broad carbonate 

shelves evolving into open and restricted marine 

environments and later near-shore marine 

environments. This was a time of shifting regional 

shorelines in a passive-margin setting. This relatively 

stable depositional environment persisted in southern 

Idaho for approximately 400 million years (Pogue 2008). 

Although most of the rocks from the Paleozoic and 

Mesozoic eras are missing, Paleozoic sediments, such as 

sandstone, limestone, and dolomite, deposited in a 

longstanding basin are present in the metamorphic core 

complex as the marble of the Pogonip Group, some 

Mississippian schist, and Pennsylvanian and Permian 

sandstone and limestone (Cunningham 1971; Miller et al. 

2008). Only the Pogonip Group appears in the mapped 

area of City of Rocks National Reserve, where it overlies 

the Precambrian rocks separated by the metamorphosed 

Mahogany Peaks fault contact (Miller et al. 2008). 

Mesozoic Era (251 to 65.5 million years ago)

At the beginning of the Mesozoic Era, the passive 

depositional setting of the marine basin began to change

(fig. 18A). During the Jurassic, the Farallon plate was

subducting beneath the western edge of North America. 

A chain of volcanoes developed over the subducting 
30 NPS Geologic Resources Division

plate, and compressional stress translated far into the

interior of western North America during several 

mountain-building events (orogenies), including the 

Sevier and Laramide orogenies. Between 170 and 60 

million years ago intermittent compressional stress 

forced kilometer-thick slabs of Proterozoic, Paleozoic, 

and Early Mesozoic sedimentary rocks to buckle and 

detach from the Archean-aged basement rocks of eastern 

Idaho and western Wyoming. They were then thrust up 

and over rocks to the east on a series of thrust faults

(fig. 18B) (Pogue 2008). 

The Sevier and Laramide orogenies were the last of the 

mountain-building episodes that formed the Rocky 

Mountains. The orogenies were characterized by two 

different styles of deformation. The Sevier orogeny was 

characterized by relatively thin slabs of older, upper 

Precambrian and lower Paleozoic sedimentary rocks 

being shoved eastward over younger, upper Paleozoic 

and lower Mesozoic rocks. In south-central Idaho,

compressional stress of the Laramide orogeny thickened 

the crust by thrust faults that were steep near the surface 

and shallower deep within the earth. Unlike the thrust 

faults of the Sevier orogeny, the Laramide faults cut into 

and thrust upward the Archean-aged basement rocks. 

The Laramide orogeny forming a mountain range and 

resulting in broad uplift, buckling, and fracturing of the 

crust along north to south-trending (potentially 

reactivated) faults (fig. 18C) (Cunningham 1971; Pogue 

2008). The Late Cretaceous to early Tertiary rocks 

exposed to the south of City of Rocks record the Sevierstyle deformation and its evolution into Laramide-style 

deformation (Goldstrand 1990).

The rocks exposed today in the Albion Mountains were 

deeply buried beneath overthrust slabs of rock during 

this mountain-building event. This deep burial caused 

widespread metamorphism through exposure to heat 

and intense pressure. The metamorphism formed gneiss, 

amphibolite, schist, and quartzite, as well as regional 

foliation and lineation sets and many small folds 

common in the Elba Quartzite (Compton and Todd 

1977; Miller et al. 2008; Pogue 2008). 

Cenozoic Era (65.5 million years ago until today)

Paleogene and Neogene periods (together called the 

“Tertiary;” 65.5 to 2.6 million years ago)

By the Paleogene, compressional (pushing together) 

stress tapered off along the western edge of North 

America, and by the middle Eocene extension (pulling 

apart) was the dominant tectonic force in southern Idaho 

(Pogue 2008). City of Rocks is very near the boundary of 

two major physiographic provinces—the Basin and 

Range and the Eastern Snake River Plain. The present 

landscape was influenced by the processes associated 

with the formation of both provinces. The Basin and 

Range is characterized by normal faults and pulling apart 

of Earth’s crust while the Eastern Snake River Plain 

characterized by volcanism associated with the 

Yellowstone hot spot.

Along the northern edge of the Basin and Range, City of 

Rocks National Reserve experienced three major 

extensional events during the Eocene (42−37 million 

years ago), the early Miocene (≈25−20 million years ago), 

and later Miocene (13−7 million years ago) (fig. 18D−G) 

(Compton and Todd 1983; Miller et al. 2008). Large slabs 

of rock moved down and laterally away from the 

thickened and heated crust of south-central Idaho 

(Pogue 2008). The net result of these extensional events 

was the vertical thinning and lateral extension of the 

rocks of the Albion Mountains metamorphic core 

complex (Miller et al. 2008). Uplift produced by excess 

heat in the mantle and extensional faulting caused the 

rocks, now exposed in the Albion Mountains, to slowly 

rise relative to the surrounding rocks. The first two 

thinning events moved upper rocks down to the west 

and northwest along west-dipping regional faults. This 

deformation was brittle closer to the surface, but at depth 

more ductile conditions prevailed and resulted in shear 

zones such as the Middle Mountain shear zone (Miller 

and Bedford 1999). 

The later Miocene extensional event displaced the upper 

rocks down to the east on a separate east-dipping 

detachment fault system (Miller et al. 2008). In the 

eastern Raft River Mountains near City of Rocks, the 

fault was ductile in character, forming mylonites—

elsewhere it was brittle in nature. This brittle 

deformation caused the rupture of the volcanic rocks in 

the Jim Sage Mountains, faulting them away from the 

Albion Mountains, moving down to the east. The rocks 

of the southern Albion Mountains bowed up as an archlike dome (Compton and Todd 1977; Pogue 2008). 

Extension during the Miocene also formed Junction 

Valley and the Raft River Valley Basin. Other deep basins 

related to the late Miocene extension lie east and west of 

the southern Albion Mountains. These basins are all 

filled with Miocene volcanic-rich sediment (Miller and 

Bedford 1999). 

Excess heat from the mantle and decompression of the 

rising dome in the Albion Mountains induced partial 

melting of the lower crust and produced a large magma 

body that would later become the Almo pluton. The less 

dense magma rose upward through the surrounding 

dense solid rock by intruding along cracks and 

incorporating blocks of surrounding country rock in a 

process called “stoping” (Pogue 2008). Once the magma 

reached a certain depth (between 10 and 15 km, or 

between 6 and 9 mi beneath the surface), cooler ambient 

temperature and lower pressures initiated crystallization 

and slowed upward movement (Pogue 2008). The mica 

minerals that attracted the interest of miners were 

emplaced in pegmatite dikes associated with the Almo 

pluton. The emplacement of the Almo pluton in the 

Oligocene (≈28 million years ago) and similar bodies 

farther south was in the midst of extensional events in 

the Tertiary (fig. 18D) (Miller et al. 2008). This intrusion 

was accompanied by contact metamorphism with the 

already metamorphosed country rock. This thermal

overprint is recorded in the Archean age units at City of 

Rocks (Miller et al. 2008). As the pluton cooled and 

solidified, joints formed throughout the rock as a result 
CIRO Geologic Resources Inventory Report 31

of contraction. Regional east-west extensional stress also 

formed joints in the new rock body oriented in a roughly

north-south trend.

Uplift, erosion, and tectonic thinning along extensional 

normal faults intermittently throughout the Tertiary 

caused the Almo pluton and surrounding metamorphic 

rocks in the City of Rocks area to rise quickly to the 

surface (fig. 18F−G) (Pogue 2008).The thick stack of 

rocks of the ductilely deformed metamorphic complex 

thinned and extended laterally (east-west). In the Albion 

Mountains, removal of about 14 km (8.7 mi) of rock that 

sat atop the pluton exposed the granite to erosion (Miller 

et al. 2008). 

Approximately 10 million years ago, several rhyolite ash 

flows were deposited onto the eroded metamorphic 

rocks at the surface as the North American plate passed 

over a stationary mantle plume (the Yellowstone hot

spot) (fig. 18G) (Miller et al. 2008; Pogue 2008). This 

mantle plume gave rise to the series of calderas and 

volcanic deposits of the Snake River Plain stretching 

from the Idaho-Oregon border to Wyoming (Pogue 

2008). The passage over the hot spot caused further 

uplift and exhumation due to increased heat and 

buoyancy of the mantle. The resulting regional volcanism 

spewed blankets of rhyolite ash flows over the entire area

(Baars 2000; Fillmore 2000). Within the Albion 

Mountains, solidified Miocene ash flows detached from 

the underlying metamorphic rocks and slid down along a 

low-angle normal fault during the later Miocene 

extensional event to compose the Jim Sage and Cotterel 

mountains due east and northeast of the reserve, 

respectively (Compton and Todd 1983; Miller et al. 2008; 

Pogue 2008). The last local volcanic event produced 

silica-rich lava flows that covered much of the surface 

west of the Cotterrel Range, exceeding 2,100 m (7,000 ft) 

in elevation (Cunningham 1971). The North American 

plate continued to migrate westward, and the hot spot is 

now fueling the geothermal features of Yellowstone 

National Park (Pogue 2008).

Throughout the Cenozoic, regional uplift and 

subsequent erosion of the ranges in the Basin and Range 

province largely gave rise to the topography of southern 

Idaho (Pogue 2008). Fault-bounded valleys filled with a 

variety of sediments in alluvial fans shed from the newly 

exposed highlands (fig. 18H−I). Conglomerate and 

reworked volcanic deposits were distributed across the 

region during periods of intense regional erosion (Baker 

and Crittenden 1961). The oldest alluvial deposits are 

remnants eroded to rounded caps on local ridges atop 

≈10-million-year-old volcanic rocks at City of Rocks. 

Relief on some of these ridges records at least several 

tens of meters of downcutting since their deposition 

(Miller et al. 2008).

Quaternary Period (2.6 million years ago until today)

The Pleistocene Epoch was a time when Earth’s climate 

cooled significantly. The increased precipitation and 

decreased evaporation created many lakes in the Basin 

and Range and caused glaciers to form on the highest 

peaks in the area, including Mount Harrison and Cache 

Peak (Pogue 2008). These glaciers carved bowl-shaped 

cirques on mountain slopes—now filled with water 

forming Lake Cleveland and Independence Lakes 

(Pogue 2008). The Pleistocene “ice ages,” characterized 

by multiple episodes of world-wide continental and 

alpine glaciation, caused great continental glaciers 

thousands of feet thick to advance and retreat over 

approximately 100,000-year cycles. Huge volumes of 

water were stored in the glaciers during glacial periods so 

that sea level dropped about 120 m (390 ft) (Clark and 

Mix 2002). In central Idaho north of City of Rocks 

National Reserve, glaciers rarely descended below

2,100 m (7,000 ft) (Cunningham 1971). Deposits of 

glacially-derived silt and loess in sediments from Raft 

River and Almo Creek record local pulses of glacial 

activity within the Raft River Mountains and Cache Peak 

(Miller et al. 2008). Moraine deposits locally record at 

least two periods of glaciation at approximately 140,000

and 20,000 years ago. Late Pleistocene deposits are 

widespread in the area and indicate periods of overall 

alluvial and fluvial deposition in most valleys and alluvial 

fans during that time (Miller et al. 2008).

During the Late Pleistocene large lakes flooded many of 

the valleys (basins) of the Basin and Range province 

throughout western Utah, Nevada, Idaho, and Oregon. 

Ancient Lake Bonneville, one of these lakes, covered an 

area of more than 51,800 km2 (20,000 mi2

) of Utah and 

parts of Nevada and Idaho, including the valleys between 

the parallel ranges surrounding City of Rocks 

(Cunningham 1971). Lake Bonneville was as much as 300

m (1,000 ft) deep and crested over a pass at Red Rocks 

into southern Idaho and City of Rocks approximately 

14,500 years ago. This breach sent a torrent of water out 

of Lake Bonneville and into the Snake River drainage to 

the north of City of Rocks. The water carved deep valleys 

and left deposits of sand, gravel, and even boulders as it 

rushed out. The Bonneville flood lasted less than one

year and lowered the lake level by 122 m (400 ft). As the 

region’s climate changed to become warmer and drier, 

Lake Bonneville slowly evaporated, leaving remnants 

now known as Great Salt Lake (fig. 1C) and Utah Lake.

Salt from the Great Salt Lake likely contributed to the 

formation of the tafoni features within the park (Pogue 

2008).

At City of Rocks National Reserve, surficial deposits, 

landforms, and the weathered rock formations attest to 

the recent geological evolution throughout the 

Quaternary. Erosion of Cenozoic granite such as the 

Almo pluton created upland basins, and the resistant 

Proterozoic quartzite persists as high ridges and peaks 

(Miller et al. 2008). The oldest surficial deposits are 

alluvial remnants left on ridges tens of meters above 

active stream erosion. Later terrace, fluvial, and alluvial 

deposits located adjacent to the modern mountain fronts 

indicate relatively stable mountain topography over the 

last 500,000 to 1 million years (Machette 1985; Miller et 

al. 2008). 

Global climate warmed and global sea level rose as 

glaciers retreated following the end of widespread 

glaciations about 19,000 years ago and the beginning of 

the Holocene Epoch about 12,000 years ago.
32 NPS Geologic Resources Division

Geologically, the landscape of the region around City of 

Rocks has not changed much during the Holocene. The 

entire area has been subjected to repeated earthquakes 

and extensional uplift and downdrop of the blocks on 

either side of regional normal faults, such as the Almo 

fault. The Almo fault cuts Pleistocene alluvial fans 

between Almo and the entrance to City of Rocks 

National Reserve (Pogue 2008). Major streams, such as 

Circle Creek and Almo Creek, have carved new 

landscapes since the end of the Pleistocene, eroding the 

bedrock and older stream deposits (Pogue 2008). 

Alluvial and fluvial deposits are set into channels 

downcutting through older Pleistocene deposits and 

spread into alluvial fans near Almo. Colluvium and talus 

that accumulated at the base of hill slopes throughout the 

region indicate active erosion of rocks, delivering 

sediments to streams below (Miller et al. 2008). Human 

activities in the area including mining and trail and road 

development, were influenced by the underlying 

geology. Those activities shaped the cultural landscape at 

City of Rocks National Reserve. 

Figure 17. Geologic time scale; adapted from the U.S. Geological Survey (http://pubs.usgs.gov/fs/2007/3015/). Red lines indicate major 

unconformities between eras. Included are major events of the geologic history of City of Rocks after Pogue (2008). Absolute ages shown are 

in millions of years (Ma, or mega-annum). 
CIRO Geologic Resources Inventory Report 33

Figure 18. Diagrammatic geologic cross sections depicting major events in southeastern Idaho between the Early Jurassic and the present.

Diagram is figure 7 from Pogue (2008), based on information from Covington (1983), Camilleri et al. (1997), and Miller (1991). Used with 

permission.
34 NPS Geologic Resources Division

Figure 18 (continued). Diagrammatic geologic cross sections depicting major events in southeastern Idaho between the Early Jurassic and the 

present. Diagram is figure 7 from Pogue (2008), based on information from Covington (1983), Camilleri et al. (1997), and Miller (1991). Used 

with permission.
CIRO Geologic Resources Inventory Report 35

Glossary

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

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

here please visit: http://geomaps.wr.usgs.gov/parks/misc/glossarya.html. Definitions are 

based on those in the American Geological Institute Glossary of Geology (fifth edition; 2005).

alluvial fan. A fan-shaped deposit of sediment that 

accumulates where a hydraulically confined stream 

flows to a hydraulically unconfined area. Commonly 

out of a mountain front into an area such as a valley or 

plain.

alluvium. Stream-deposited sediment.

aquifer. A rock or sedimentary unit that is sufficiently 

porous that it has a capacity to hold water, sufficiently 

permeable to allow water to move through it, and 

currently saturated to some level.

ash (volcanic). Fine material ejected from a volcano (also 

see “tuff”).

asthenosphere. Earth’s relatively weak layer or shell 

below the rigid lithosphere.

axis (fold). A straight line approximation of the trend of a 

fold, which divides the fold’s two limbs. “Hinge line” is 

a preferred term.

basement. The undifferentiated rocks, commonly 

igneous and metamorphic, that underlie rocks 

exposed at the surface.

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

rocks dip inward from all sides.

basin (sedimentary). Any depression, from continental to 

local scales, into which sediments are deposited.

batholith. A massive, discordant pluton, larger than 100 

km2 (40 mi2), and often formed from multiple 

intrusions of magma.

bed. The smallest sedimentary strata unit, commonly 

ranging in thickness from one centimeter to a meter or 

two and distinguishable from beds above and below.

bedding. Depositional layering or stratification of 

sediments.

bedrock. A general term for the rock that underlies soil or 

other unconsolidated, superficial material.

block (fault). A crustal unit bounded by faults, either 

completely or in part.

calcareous. Describes rock or sediment that contains the

mineral calcium carbonate (CaCO3).

caldera. A large bowl- or cone-shaped depression at the 

summit of a volcano. Formed by explosion or collapse.

cementation. Chemical precipitation of material into 

pores between grains that bind the grains into rock.

clastic Describes rock or sediment made of fragments of 

pre-existing rocks.

clay. Can be used to refer to clay minerals or as a 

sedimentary fragment size classification (less than 

1/256 mm [0.00015 in]).

cleavage (mineral). The tendency of a mineral to break 

preferentially in certain directions along planes of 

weaknesses in the crystal structure.

cleavage (rock). The tendency of a rock to split along 

parallel, closely spaced planar surfaces. It is 

independent of bedding.

colluvium. A general term applied to any loose, 

heterogeneous, and incoherent mass of rock fragments 

deposited by unconcentrated surface runoff or slow 

continuous downslope creep, usually collecting at the 

base of gentle slopes or hillsides.

concordant. Strata with contacts parallel to the 

orientation of adjacent strata.

conglomerate. A coarse-grained, generally unsorted, 

sedimentary rock consisting of cemented rounded 

clasts larger than 2 mm (0.08 in).

continental crust. The crustal rocks rich in silica and 

alumina that underlie the continents; ranging in 

thickness from 35 km (22 mi) to 60 km (37 mi) under 

mountain ranges.

continental shield. A continental block of Earth’s crust 

that has remained relatively stable over a long period 

of time and has undergone minor warping compared 

to the intense deformation of bordering crust (also see 

“craton”).

convergent boundary. An active boundary where two 

tectonic plates are colliding.

craton. The relatively old and geologically stable interior 

of a continent (also see “continental shield”).

cross-bedding. Uniform to highly varied sets of inclined 

sedimentary beds deposited by wind or water that 

indicate flow conditions such as water flow direction 

and depth.

cross section. A graphical interpretation of geology, 

structure, and/or stratigraphy in the third (vertical) 

dimension based on mapped and measured geological 

extents and attitudes depicted in a vertically oriented 

plane.

crust. Earth’s outermost compositional shell, 10 to 40 km 

(6 to 25 mi) thick, consisting predominantly of 

relatively low-density silicate minerals (also see 

“oceanic crust” and “continental crust”).

cryptogamic crust. The brown crust on sandy, desert 

soils that is composed of an association of algae, 

lichen, mosses, and fungi. Helps stabilize the soil.

crystalline. Describes a regular, orderly, repeating 

geometric structural arrangement of atoms.

debris flow. A moving mass of rock fragments, soil, and 

mud, in which more than half the particles of which 

are larger than sand size.

deformation. A general term for the process of faulting, 

folding, and shearing of rocks as a result of various 

Earth forces such as compression (pushing together) 

and extension (pulling apart).

dike. A narrow igneous intrusion that cuts across 

bedding planes or other geologic structures.

dip. The angle between a bed or other geologic surface 

and horizontal.
36 NPS Geologic Resources Division

dip-slip fault. A fault with measurable offset where the 

relative movement is parallel to the dip of the fault.

discordant. Having contacts that cut across or are set an 

angle to the orientation of adjacent rocks.

dome. General term for any smoothly rounded landform 

or rock mass. More specifically refers to an elliptical 

uplift in which rocks dip gently away in all directions. 

drainage basin. The total area from which a stream 

system receives or drains precipitation runoff.

ductile. Describes a rock that is able to sustain 

deformation (folding, bending, or shearing) before 

fracturing.

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

action of the wind. Also spelled “Aeolian.”

ephemeral stream. A stream that flows briefly only in 

direct response to precipitation in the immediate 

locality and whose channel is at all times above the 

water table. 

exfoliation. The breakup, spalling, peeling, or flaking of 

layers or concentric sheets from an exposed rock mass 

caused by differential stresses due to thermal changes 

or a reduction in pressure when overlying rocks erode 

away.

extrusive. Describes igneous material that has erupted 

onto Earth’s surface.

facies (metamorphic). The pressure and temperature 

conditions that result in a particular, distinctive suite 

of metamorphic minerals.

facies (sedimentary). The depositional or environmental 

conditions reflected in the sedimentary structures, 

textures, mineralogy, fossils, etc. of a sedimentary 

rock.

fault. A break in rock along which relative movement has 

occurred between the two sides.

felsic. Describes an igneous rock having abundant lightcolored minerals such as quartz, feldspars, or 

muscovite. Compare to “mafic”.

footwall. The mass of rock beneath a fault surface (also 

see “hanging wall”).

formation. Fundamental rock-stratigraphic unit that is 

mappable, lithologically distinct from adjoining strata, 

and has definable upper and lower contacts.

fracture. Irregular breakage of a mineral; also any break 

in a rock (e.g., crack, joint, or fault).

frost wedging. The breakup of rock due to the 

expansion of water freezing in fractures.

graben. A down-dropped structural block bounded by 

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

grus. A silica-rich sand derived from the weathering of a 

parent rock, usually granite.

hanging wall. The mass of rock above a fault surface 

(also see “footwall”).

hinge line. A line or boundary between a stable region 

and one undergoing upward or downward movement.

horst. Areas of relative “up” between grabens, 

representing the geologic surface left behind as 

grabens drop. The best example is the basin and range 

province of Nevada. The basins are grabens and the 

ranges are weathered horsts. Grabens become a locus 

for sedimentary deposition (also see “graben”).

igneous. Refers to a rock or mineral that originated from 

molten material; one of the three main classes of 

rocks—igneous, metamorphic, and sedimentary.

intrusion. A body of igneous rock that invades (pushes 

into) older rock. The invading rock may be a plastic 

solid or magma.

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

and parallel to a subduction zone.

joint. A break in rock without relative movement of 

rocks on either side of the fracture surface.

laccolith. A mushroom- or arcuate-shaped pluton that 

has intruded sedimentary strata and domed up the 

overlying sedimentary layers. Common on the 

Colorado Plateau.

landslide. Any process or landform resulting from rapid, 

gravity-driven mass movement.

lava. Still-molten or solidified magma that has been 

extruded onto the Earth’s surface though a volcano or 

fissure.

lithology. The physical description or classification of a 

rock or rock unit based on characters such as its color, 

mineral composition, and grain size.

lithosphere. The relatively rigid outmost shell of the 

Earth’s structure, 50 to 100 km (31 to 62 miles) thick, 

that encompasses the crust and uppermost mantle.

loess. Windblown silt-sized sediment, generally of glacial 

origin.

mafic Describes dark-colored rock, magma, or minerals 

rich in magnesium and iron. Compare to “felsic.”

magma. Molten rock beneath Earth’s surface capable of 

intrusion and extrusion.

mantle. The zone of Earth’s interior between crust and 

core.

matrix. The fine grained material between coarse (larger) 

grains in igneous rocks or poorly sorted clastic 

sediments or rocks. Also refers to rock or sediment in 

which a fossil is embedded.

mechanical weathering. The physical breakup of rocks 

without change in composition. Synonymous with 

physical weathering.

member. A lithostratigraphic unit with definable 

contacts; a member subdivides a formation.

metamorphic. Describes the process of metamorphism 

or its results. One of the three main classes of rocks—

igneous, metamorphic, and sedimentary.

metamorphism. Literally, a change in form. 

Metamorphism occurs in rocks through mineral 

alteration, formation, and/or recrystallization from 

increased heat and pressure.

mineral. A naturally occurring, inorganic crystalline solid 

with a definite chemical composition or compositional 

range.

mylonite. A fine-grained, foliated rock typically found in 

localized zones of ductile deformation, often formed at 

great depths under high temperature and pressure.

normal fault. A dip-slip fault in which the hanging wall 

moves down relative to the footwall.

oceanic crust. Earth’s crust formed at spreading ridges 

that underlies the ocean basins. Oceanic crust is 6 to 7 

km (3 to 4 miles) thick and generally of basaltic 

composition.

orogeny. A mountain-building event.

outcrop. Any part of a rock mass or formation that is 

exposed or “crops out” at Earth’s surface.
CIRO Geologic Resources Inventory Report 37

paleogeography. The study, description, and 

reconstruction of the physical landscape from past 

geologic periods.

Pangaea. A theoretical, single supercontinent that 

existed during the Permian and Triassic periods.

panhole. A flat-floored pit on the surface of an outcrop 

produced by weathering.

parent (rock). The original rock from which a 

metamorphic rock was formed. Can also refer to the 

rock from which a soil was formed.

passive margin. A margin where no plate-scale tectonism 

is taking place; plates are not converging, diverging, or 

sliding past one another. An example is the east coast 

of North America. (also see “active margin”).

permeability. A measure of the relative ease with which 

fluids move through the pore spaces of rocks or 

sediments.

phenocrysts. A coarse crystal in a porphyritic igneous 

rock.

plateau. A broad, flat-topped topographic high 

(terrestrial or marine) of great extent and elevation 

above the surrounding plains, canyons, or valleys.

plate tectonics. The concept that the lithosphere is 

broken up into a series of rigid plates that move over 

Earth’s surface above a more fluid asthenosphere.

pluton. A body of intrusive igneous rock that crystallized 

at some depth beneath Earth’s surface.

porphyritic. Describes an igneous rock wherein the rock 

contains conspicuously large crystals in a fine-grained 

groundmass, or matrix.

porosity. The proportion of void space (e.g., pores or 

voids) in a volume of rock or sediment deposit.

potassium feldspar. A feldspar mineral rich in potassium 

(e.g., orthoclase, microcline, sanidine, adularia).

progradation. The seaward building of land area due to 

sedimentary deposition.

provenance. A place of origin. The area from which the 

constituent materials of a sedimentary rock were 

derived.

regression. A long-term seaward retreat of the shoreline 

or relative fall of sea level.

reverse fault. A contractional high-angle (greater than 

45°) dip-slip fault in which the hanging wall moves up 

relative to the footwall (also see “thrust fault”).

rift valley. A depression formed by grabens along the 

crest of an oceanic spreading ridge or in a continental 

rift zone.

roundness. The relative amount of curvature of the 

“corners” of a sediment grain.

sand. A clastic particle smaller than a granule and larger 

than a silt grain, having a diameter in the range of 1/16 

mm (0.0025 in) to 2 mm (0.08 in).

sandstone. Clastic sedimentary rock of predominantly 

sand-sized grains.

scarp. A steep cliff or topographic step resulting from 

displacement on a fault, or by mass movement, or 

erosion. Also called an “escarpment.”

sediment. An eroded and deposited, unconsolidated 

accumulation of rock and mineral fragments.

sedimentary rock. A consolidated and lithified rock 

consisting of clastic and/or chemical sediment(s). One 

of the three main classes of rocks—igneous, 

metamorphic, and sedimentary.

sequence. A major informal rock-stratigraphic unit that 

is traceable over large areas and defined by sediments 

associated with a major sea level transgressionregression.

shale. A clastic sedimentary rock made of clay-sized 

particles that exhibit parallel splitting properties.

sill. An igneous intrusion that is of the same orientation 

as the surrounding rock.

silt. Clastic sedimentary material intermediate in size 

between fine-grained sand and coarse clay (1/256 to 

1/16 mm [0.00015 to 0.002 in]).

siltstone. A variably-lithified sedimentary rock with siltsized grains.

slope. The inclined surface of any geomorphic feature or 

rational measurement thereof. Synonymous with 

“gradient.”

slump. A generally large, coherent mass movement with a 

concave-up failure surface and subsequent backward 

rotation relative to the slope.

spring. A site where water issues from the surface due to 

the intersection of the water table with the ground 

surface.

strata. Tabular or sheet-like masses or distinct layers 

(e.g., of rock).

stratigraphy. The geologic study of the origin, 

occurrence, distribution, classification, correlation, 

and age of rock layers, especially sedimentary rocks.

stream. Any body of water moving under gravity flow 

and confined within a channel.

stream terrace. Step-like benches surrounding the 

present floodplain of a stream due to dissection of 

previous flood plain(s), stream bed(s), and/or valley 

floor(s).

strike. The compass direction of the line of intersection 

of an inclined surface with a horizontal plane.

strike-slip fault. A fault with measurable offset where the 

relative movement is parallel to the strike of the fault.

Said to be “sinistral” (left-lateral) if relative motion of 

the block opposite the observer appears to be to the 

left. “Dextral” (right-lateral) describes relative motion 

to the right.

subduction zone. A convergent plate boundary where 

oceanic lithosphere descends beneath a continental or 

oceanic plate and is carried down into the mantle.

subsidence. The gradual sinking or depression of part of 

Earth’s surface.

tafone (plural: tafoni). A large hollow in a rock produced 

by cavernous weathering.

tectonic. Relating to large-scale movement and 

deformation of Earth’s crust.

tectonics. The geological study of the broad structural 

architecture and deformational processes of the 

lithosphere and.

terrestrial. Relating to land, Earth, or its inhabitants.

thrust fault. A contractional dip-slip fault with a 

shallowly dipping fault surface (less than 45°

) where 

the hanging wall moves up and over relative to the 

footwall.

topography. The general morphology of Earth’s surface, 

including relief and locations of natural and 

anthropogenic features.

trace (fault). The exposed intersection of a fault with 

Earth’s surface.
38 NPS Geologic Resources Division

transgression. Landward migration of the sea due to a 

relative rise in sea level.

trend. The direction or azimuth of elongation of a linear 

geological feature.

tuff. Generally fine-grained, igneous rock formed of 

consolidated volcanic ash.

unconformity. An erosional or non-depositional surface 

bounded on one or both sides by sedimentary strata 

that marks a period of missing time.

uplift. A structurally high area in the crust, produced by 

movement that raises the rocks.

vent. An opening at Earth’s surface where volcanic 

materials emerge.

volcanic. Describes anything related to volcanoes. Can 

refer to igneous rock crystallized at or near Earth’s 

surface (e.g., lava).

water table. The upper surface of the saturated zone; the 

zone of rock in an aquifer saturated with water.

weathering. The set of physical, chemical, and biological 

processes by which rock is broken down.
CIRO Geologic Resources Inventory Report 39

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available from the National Park Service Geologic Resources Division.

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Appendix A: Overview of Digital Geologic Data

The following page is an overview of the digital geologic data for City of Rocks National 

Reserve. For a poster-size PDF of this overview and complete digital data, please see the 

included CD or visit the Geologic Resources Inventory publications web site 

(http://www.nature.nps.gov/geology/inventory/gre_publications.cfm).


CIRO Geologic Resources Inventory Report 45

Appendix B: Scoping Meeting Participants

The following is a list of participants from the GRI scoping session for City of Rocks 

National Reserve, held on June 16–17, 1999. The contact information and email 

addresses in this appendix may be outdated; please contact the Geologic Resources 

Division for current information. The scoping meeting summary was used as the 

foundation for this GRI report. The original scoping summary document is available on 

the GRI web site: http://www.nature.nps.gov/geology/inventory/gre_publications.cfm.

NAME AFFILIATION PHONE E-MAIL

Bruce Heise NPS, Geologic Resources 

Division (303) 969-2017 Bruce_Heise@NPS.gov

Joe Gregson NPS, Natural Resources 

Information Division (970) 225-3559 Joe_Gregson@NPS.gov

Tim Connors NPS, Geologic Resources 

Division (303) 969-2093 Tim_Connors@NPS.gov

Marsha Davis NPS, Columbia Cascades 

Support Office (206) 220-4262 Marsha_Davis@NPS.gov

Dave Miller USGS (650) 329-4923 Dmiller@usgs.gov

Paul Link Idaho State University (208) 236-3365 Linkpaul@isu.edu

Stan Lloyd CIRO cooperator (208) 638-5542

Ted Scherff CIRO/IDPR (208) 824-5519 Ted_Scherff@NPS.gov

Carolyn Firth College of Southern Idaho (208) 438-8215

John Russell CIRO/IDPR (208) 824-5519 Ciro_interpretation@NPS.gov

Greg Sprouse CIRO, SCA volunteer Gtsprouse@yahoo.com

Andy Millott CIRO, Americorps Aj.millott@eudoramail.com

David Landrum Lake Walcott State Park (208) 436-1258

Chris Gee CIRO (208) 824-5519

Whitney Warner CIRO (208) 824-5519

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