Mount Rushmore National Memorial 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/038 

National Park Service 

U.S. Department of the Interior 

Natural Resource Program Center 
THIS PAGE: THIS PAGE: 

View of George Washingt View of George Washington Carving, Mount Rushmore on Carving, Mount Rushmore 

National Memorial. National Memorial.

ON THE COVER: ON THE COVER: 

Memorial Carvings, Mount Memorial Carvings, Mount Rushmore National Memorial. Rushmore National Memorial.

NPS photos, cover photo by Sid Covington
Mount Rushmore National Memorial 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/038 

Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, Colorado 80225 

June 2008 

U.S. Department of the Interior 

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

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

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

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

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

Please cite this publication as: 

Graham, J. 2008. Mount Rushmore National Memorial Geologic Resource Evaluation Report. 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/038. National Park Service, Denver, 

Colorado. 

NPS D- 95, June 2008 
 MORU Geologic Resource Evaluation Report iii

Table of Contents 

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

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

Introduction ................................................................................................................... 2

Purpose of the Geologic Resources Evaluation Program...........................................................................................2

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

Park History................................................................................................................................................................3 

Geologic Issues............................................................................................................. 8

Structural Stability and Erosion...................................................................................................................................8

Water Resources........................................................................................................................................................8

Mining Impacts .........................................................................................................................................................10

Disturbed Lands .......................................................................................................................................................10 

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

Igneous and Metamorphic Features.........................................................................................................................11

Minerals....................................................................................................................................................................11

Cave and Karst Features..........................................................................................................................................11

Frost Wedging ..........................................................................................................................................................11 

Map Unit Properties .................................................................................................... 12 

Map Unit Properties Table........................................................................................................................................13 

Geologic History.......................................................................................................... 15

Precambrian History of the Black Hills Region .........................................................................................................15

Paleozoic History of the Black Hills Region ..............................................................................................................16

Mesozoic History of the Black Hills Region ..............................................................................................................17

Cretaceous – Tertiary Laramide Orogeny ................................................................................................................17

Quaternary History of the Black Hills Region............................................................................................................18 

Glossary....................................................................................................................... 29 

References................................................................................................................... 31

Appendix A: Geologic Map Graphic .......................................................................... 33

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

List of Figures 

Figure 1. Presidential faces of Mount Rushmore National Memorial carved into Harney Peak Granite.........................4

Figure 2. Regional location of Mount Rushmore National Memorial ..............................................................................5

Figure 3. Diagram of the Black Hills dome .....................................................................................................................6

Figure 4. Stratigraphic column for the Black Hills area...................................................................................................7

Figure 5. Geologic time scale.......................................................................................................................................20

Figure 6. Location of the Black Hills Uplift with regard to Precambrian terranes..........................................................21

Figure 7. Paleogeographic map of the Late Cambrian.................................................................................................22

Figure 8. Paleogeographic map of North America during the Early Mississippian .......................................................23

Figure 9. Paleogeographic map of North America during the Early Triassic ................................................................24

Figure 10. Paleogeographic map of North America during the Jurassic ......................................................................25

Figure 11. Western Interior Seaway during the Cretaceous.........................................................................................26

Figure 12. Major tectonic elements associated with the Laramide Orogeny ................................................................27

Figure 13. Diagram of Pleistocene terraces. ................................................................................................................28
 MORU Geologic Resource Evaluation Report 1

Executive Summary 

This report accompanies the digital geologic map for Mount Rushmore National Memorial 

in South Dakota, which the Geologic Resources Division produced in collaboration with its 

partners. It contains information relevant to resource management and scientific research. 

The iconic presidential faces of Mount Rushmore 

National Memorial are 18 meters (60 ft) high and carved 

into the Harney Peak Granite (fig. 1). The Harney Peak 

Granite is the primary component of the Precambrian 

pluton exposed in the core of the Black Hills in South 

Dakota. The Black Hills, a prominent 200- by 105-

kilometer (125- by 65- mi) elliptical dome, arched 

upward during the late Cretaceous to early Tertiary 

Laramide Orogeny. Precambrian mica schists, quartzites, 

and granite form the core of the Black Hills uplift while 

younger Paleozoic and Mesozoic sedimentary rocks rim 

the dome. 

The Harney Peak Granite was selected as the medium for 

the memorial sculptures because it is a massive, uniform 

granite, relatively free of joints and fractures. The 

location of the sculptures allows a southeast- facing 

exposure with optimal lighting. Work began on the 

sculptures in 1927 and was finished in 1941. The four 

presidents represented in the memorial are George 

Washington, Thomas Jefferson, Abraham Lincoln, and 

Theodore Roosevelt. These men served as president 

during significant junctures in the development of our 

nation and remain an inspiration to those who visit 

Mount Rushmore National Memorial. 

Geologic processes have played a significant role in 

developing the landscape of the Black Hills and in 

sustaining regional population growth. Tectonic 

upheaval deformed the rocks into a distinctive dome that 

drew Native Americans, trappers, and miners to the area 

from the surrounding plains. The discovery of gold led to 

a population boom in the late nineteenth century. 

Weathering and erosion, coupled with human activities, 

have resulted in geologic issues that are significant with 

regard to the management of Mount Rushmore National 

Memorial. These issues include: 

• structural stability and erosion, 

• water resources and regional aquifers, and 

• impacts from past mines and development in the park. 

Joints and fractures in the Harney Peak Granite, together 

with the processes of weathering and erosion, pose a 

threat to the sculptures of Mount Rushmore. Cracks in 

the granite must be continually monitored and repaired 

to maintain the integrity of the memorial. 

The complex geology of the Black Hills affects the 

hydrology of the region with regard to water quality and 

quantity. Currently, groundwater pumped from a well 

drilled into fractured mica schist and granite supplies 

water to the memorial. 

Research from 2000- 2002 by the U.S. Geological Survey 

documented the hydrology of the region. Groundwater 

quality from the major aquifers is excellent. Recharge of 

the aquifers supplies abundant water for the region. The 

primary aquifers for the Black Hills region are found in 

the Paleozoic and Mesozoic sedimentary rocks, which 

surround the core of the Black Hills granitic rocks and 

are not present at Mount Rushmore National Memorial. 

Although mining activity in the Black Hills has been high, 

mining impacts to the memorial from both historical and 

modern mining are slight. Nonetheless outstanding 

reclamation issues exist in the park. Three abandoned 

mines along with exploration pits are located within 

monument boundaries. In the past, pegmatite dikes were 

evaluated for potential sheet mica and potash feldspar. 

Mica was excavated from a small deposit called Mica Hill 

from 1943 to 1945. However, valuable mineral deposits 

were not identified in the memorial, which has been 

closed to mineral entry since its creation in 1925. 

In addition to the sculptures, there are other prominent 

and distinctive geologic features worth emphasizing in 

the memorial. Igneous and metamorphic features are 

exposed throughout the memorial. Metamorphic 

features include schistosity, a type of foliation that 

occurs in coarse- grained metamorphic rocks. Large 

crystals of beryl, garnet, and other minerals associated 

with igneous and metamorphic rocks are also present 

throughout the memorial. 

Geologic processes actively affect the landscape of the 

memorial. The fractures and joints present in the Harney 

Peak Granite are evidence that frost wedging, 

exfoliation, mineral expansion, and pressure release have 

weathered the rocks. These processes undermine the 

stability of the rock and of the memorial sculptures. 

Prior to the Laramide uplift of the Black Hills, the 

Paleozoic and Mesozoic sedimentary strata that now rim 

the Black Hills covered Precambrian rocks to depths of 

up to 14 kilometers (9 mi). Intense heat and pressure 

metamorphosed the Precambrian sediments, and 

igneous plutons intruded the rock units. Today, this 

complex geology is exposed in Mount Rushmore 

National Memorial.
 NPS Geologic Resources Division 2 

Introduction 

The following section briefly describes the National Park Service Geologic Resource 

Evaluation Program and the regional geologic setting of Mount Rushmore National 

Memorial 

Purpose of the Geologic Resources Evaluation Program 

The Geologic Resource Evaluation (GRE) Program is 

one of 12 inventories funded under the NPS Natural 

Resource Challenge designed to enhance baseline 

information available to park managers. The program 

carries out the geologic component of the inventory 

effort from the development of digital geologic maps to 

providing park staff with a geologic report tailored to a 

park’s specific geologic resource issues. The Geologic 

Resources Division of the Natural Resource Program 

Center administers this program. The GRE team relies 

heavily on partnerships with the U.S. Geological Survey, 

Colorado State University, state surveys, and others in 

developing GRE products. 

The goal of the GRE Program is to increase 

understanding of the geologic processes at work in parks 

and provide sound geologic information for use in park 

decision making. Sound park stewardship relies on 

understanding natural resources and their role in the 

ecosystem. Geology is the foundation of park 

ecosystems. 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 this goal, the GRE team is systematically 

working towards providing each of the identified 270 

natural area parks with a geologic scoping meeting, a 

digital geologic map, and a geologic report. These 

products support the stewardship of park resources and 

are designed for non- geoscientists. During scoping 

meetings the GRE team brings together park staff and 

geologic experts to review available geologic maps and 

discuss specific geologic issues, features, and processes. 

The GRE mapping team converts the geologic maps 

identified for park use at the scoping meeting into digital 

geologic data in accordance with their innovative 

Geographic Information Systems (GIS) Data Model. 

These digital data sets bring an exciting interactive 

dimension to traditional paper maps by providing 

geologic data for use in park GIS and facilitating the 

incorporation of geologic considerations into a wide 

range of resource management applications. The newest 

maps come complete with interactive help files. As a 

companion to the digital geologic maps, the GRE team 

prepares a park- specific geologic report that aids in use 

of the maps and provides park managers with an 

overview of park geology and geologic resource 

management issues. 

For additional information regarding the content of this 

report and up to date GRE contact information please 

refer to the Geologic Resource Evaluation Web site 

(http://www2.nature.nps.gov/geology/inventory/). 

Geologic Setting 

The 1,278.45- acre Mount Rushmore National Memorial 

is located in the Black Hills of South Dakota (fig.2). The 

Black Hills are the core of a prominent uplift associated 

with the Laramide Orogeny, a mountain- building 

episode that occurred about 65- 45 million years ago 

(Ma). Topographically detached from the Rocky 

Mountains by about 193 kilometers (120 miles), the Black 

Hills are part of the Great Plains physiographic province. 

Harney Peak is the highest peak in the Black Hills at 

2,207 meters (7,242 ft) above sea level. Mount Rushmore 

is 1,745 meters (5,725 ft) in elevation. The plains 

surrounding the Black Hills are about 915 to 1,070 meters 

(3,000- 3,500 ft) above sea level (Kiver and Harris 1999). 

The 18- meter (60- ft) high presidential memorial 

sculptures at Mount Rushmore are carved in the Harney 

Peak Granite (fig. 1) (Powell et al. 1973). 

The Black Hills form a 200- by 105- kilometer (125- by 

65- mi) elliptical dome (also called a doubly plunging 

anticline) that is elongated in a north- south direction 

(fig.3) (Kiver and Harris 1999). The forested mountain 

range in southwest South Dakota and northern 

Wyoming covers approximately 2 million acres. Erosion 

has removed Paleozoic and Mesozoic sedimentary rock 

from the central axis of the dome, exposing the 

Precambrian rock that makes up the core of the uplift. 

This exposure extends from near the towns of Lead and 

Deadwood, SD to just south of Custer, SD. 

Precambrian rocks buried to depths of approximately 

4,572 meters (15,000 ft) form the crystalline basement in 

the northern Great Plains (Driscoll et al. 2002). The 

Precambrian igneous and metamorphic rocks exposed at 

Mount Rushmore are highly variable in composition and 

are the oldest geologic units in the region (fig.4). They 

are composed mostly of sedimentary rocks (graywackes) 

that have been metamorphosed to schists or of granite. 

When exposed at the surface, granite is a much denser, 

more stable rock mica schist, which can be very brittle 

and unsuitable for sculpting. 

In the Black Hills, Paleozoic and Mesozoic sedimentary 

rocks appear as concentric rings around the Precambrian 

core (fig. 3). Although originally deposited over the 

Precambrian rocks as nearly horizontal beds throughout 

the area, the sedimentary rocks were tilted during the 

Laramide Orogeny and subsequently eroded from the 
 MORU Geologic Resource Evaluation Report 3

core of the Black Hills. The remaining strata now dip 

steeply (≈ 15o

 to 20o) away from the core of the uplift and 

flatten out with distance from the Precambrian core to 

less than 1o

 (Carter et al. 2002). 

Cambrian and Ordovician sandstone, shale, limestone, 

and dolomite unconformably overlie the Precambrian 

rocks. Another unconformity separates them from an 

overlying Devonian shale (fig.4). The Mississippian 

Madison Limestone is a major aquifer in the region and 

contains karst features in the Black Hills. The overlying 

Pennsylvanian and Permian Minnelusa Formation is also 

a major aquifer unit. Sandstone, shale, siltstone, and 

carbonate strata of Pennsylvanian and Permian age are 

overlain by red shale, siltstone, and evaporite deposits of 

Triassic age. Jurassic rocks are predominantly carbonate, 

shale, and calcareous shale. The Lower Cretaceous Inyan 

Kara Group forms another regional aquifer. Tertiary 

igneous and sedimentary rocks of the White River Group 

overlie the Cretaceous sandstones, shales, and minor 

carbonates. The youngest geologic units in the area are 

Quaternary alluvial deposits, which fill major drainages. 

Rivers flow radially from the Precambrian core across 

hogbacks and cuestas on the western slopes. Rivers also 

flow through the Red Valley, a scenic valley carved into 

soft Triassic shales and red sandstones of the Spearfish 

Formation (Kiver and Harris 1999). The Red Valley runs 

parallel to the adjacent hogbacks and rings the Black 

Hills dome (fig. 3). Most streams loose all or part of their 

flow becoming sinking streams as they cross exposures 

of the Madison Limestone and Minnelusa Formation. 

Dissolution of the limestone in these units has created 

underground conduits that funnel the stream flow. 

About 75% of the memorial is occupied by three 

topographic basins: Starling basin, Lafferty Gulch basin, 

and East Boundary basin. While separated by drainage 

divides on the surface, the basins may be hydraulically 

connected in the subsurface (Powell et al. 1973). Starling 

basin drains the west and southwestern part of the 

memorial, Lafferty Gulch basin drains the central and 

northern part, and East Boundary basin drains the 

northeastern section of Mount Rushmore National 

Memorial. 

Large, north–trending, granite sills, some of which are 

several hundred feet thick, form the more prominent 

hills and mountains in the memorial. About 1,700 Ma the 

large magma body that later crystallized into the Harney 

Peak Granite batholith squeezed and melted its way into 

the surrounding metamorphic rocks (Kiver and Harris 

1999). An eastward extending sill composed of Harney 

Peak Granite was injected into the schists in the Mount 

Rushmore area. Very coarse- grained granitic pegmatite 

sills and dikes are also common in the memorial (Powell 

et al. 1973). Light- colored pegmatite dikes can be seen 

extending through the sculpture at Mount Rushmore. 

The dips of the foliation and remnant bedding in these 

rocks increase from about 30o

 near the west border of the 

memorial to about 65o

 near the east border (Powell et al. 

1973). 

Numerous smaller structures–folds, faults, domes, and 

monoclines–also occur in the Black Hills. The 

deformation and uplift associated with the CretaceousTertiary Laramide Orogeny produced deep faults and 

fractures that acted as conduits for magma. Igneous 

intrusions were emplaced along those conduits on the 

northern flanks of the Black Hills uplift during the 

Tertiary Period. The intrusions formed sills, dikes, 

laccoliths, and small stocks of various compositions. 

These igneous features include the gray to greenish- gray 

phonolite of Devils Tower; tan to reddish- brown, ironstained trachyte in the Bear Lodge Mountains; dark- gray 

to greenish- gray latite with large crystals of andesine, 

oligoclase, biotite, hornblende, and sphene, and lighttan to light- gray rhyolite that forms laccoliths and stocks 

near the towns of Deadwood, Lead, Sundance, Sturgis, 

and Galena (DeWitt et al. 1989). 

The area around Mount Rushmore was not glaciated 

during the Pleistocene Ice Ages (Kiver and Harris 1999). 

The present rugged landscape in the core of the range is 

the result of differential weathering and erosion, mostly 

by running water. 

The continental climate of the Black Hills is 

characterized by generally low precipitation, hot 

summers, and cold winters, but also by extreme 

variations in both precipitation and temperature 

(Driscoll et al. 2002). As a prominent landform amidst 

the surrounding plains, the geography and topography of 

the Black Hills uplift influences spatial patterns of 

precipitation in the area, especially in the high- altitude 

areas near Harney Peak. Areas of relatively low 

precipitation occur in the lower altitudes. However, in 

most areas exceeding 1,800 meters (6,000 ft) above sea 

level, average annual precipitation is in excess of 48 

centimeters (19 in). Most of the precipitation occurs 

during May and June, with the smallest amounts 

November through February. 

Park History 

Native Americans did not live in the Black Hills, although 

the Lakota made frequent visits to the area (Powell et al 

1973). They called the Black Hills “Paha Sapa” meaning 

hills that are black. Members of the Francis and LouisJoseph Verendrye expedition in 1743 may have been the 

first Europeans to visit the Black Hills. Fur trappers and 

military expeditions traversed the area in later years, but 

the area was officially closed to settlers because the land 

was part of the Great Sioux Reservation established in 

the Fort Laramie Treaty of 1868. 

In 1874, General G.A. Custer led a party into the Black 

Hills and confirmed the presence of gold. In violation of 

treaty and Federal law, miners flocked to the area by the 

thousands. The resulting conflict led to the Black Hills 

War in which Tatanka Iyotake (Sitting Bull), Tasunka 

Witko (Crazy Horse), and their people waged war 

against the intruders and the United States. The war 

ended with a new treaty in 1877 in which the Sioux were 

forced to relinquish a strip of land along the western 

border of Dakota Territory 80 kilometers (50 mi) wide, 
 NPS Geologic Resources Division 4 

plus all land west of the Cheyenne and Belle Fourche 

Rivers, including all of the Black Hills in modern South 

Dakota. 

The first railroad reached the area in 1885 and various 

communities began promoting tourism. Devils Tower in 

Wyoming, the nation’s first National Monument, was 

established in the northwestern part of the Black Hills in 

1906. 

In 1924, Doane Robinson, then State Historian of South 

Dakota, had the idea of sculpting massive figures on 

some of the granite spires, or pinnacles, known as the 

Needles in the area south of Harney Peak. He contacted 

the well- known American artist, Gutzon Borglum, with 

the idea of creating a monument commemorating 

western heroes. Instead, Borglum urged that the figures 

be of nationally recognized historic individuals. 

Legislation to permit the sculpting in the Black Hills 

National Forest was passed by Congress in 1925 with the 

help of United States Senator Peter Norbeck and 

Congressman William Williamson, both of South 

Dakota. Mount Rushmore, named in 1885 for Charles E. 

Rushmore, a New York lawyer, was selected because its 

southeast face had the proper lighting and the monolithic 

granite mass was relatively free of joints and fractures 

and was large enough for the planned sculpture (Powell 

et al. 1973; Kiver and Harris 1999). 

Work began on August 10, 1927, the day President Calvin 

Coolidge dedicated the memorial. Due to a lack of 

dependable funding, sculpting continued intermittently 

for 14 years. After Borglum’s death in 1941, work ceased 

for about 8 months. His son, Lincoln, then became 

sculptor- in- charge. Over 400,000 tons of rocks were 

excavated during construction. The cost to complete the 

memorial was slightly less than one million dollars, about 

thirteen million dollars today (http://www.measuring 

worth.com, accessed June 28, 2007). 

Four presidents – George Washington, Thomas 

Jefferson, Abraham Lincoln, and Theodore Roosevelt –

were selected for the memorial because of the important 

roles they played in the history of the United States. 

Washington’s head was dedicated on July 4, 1930, by 

Doane Robinson. President Franklin D. Roosevelt 

dedicated the Jefferson head on August 30, 1936. The 

Lincoln head was dedicated by Senator Edward R. Burke 

of Nebraska on September 17, 1937. Roosevelt’s head was 

dedicated on July 2, 1939, by Governor Harlan J. 

Bushfield of South Dakota and William S. Hart, the 

silent- screen star. 

The original plan was to sculpt the figures from the waist 

up showing their hands and period clothing. This 

concept was abandoned during World War II and work 

on the memorial ended in October, 1941 (Powell et al. 

1973; Kiver and Harris 1999). 

Figure 1. Presidential faces of Mount Rushmore National Memorial carved into Harney Peak granite. From http://bensguide.gpo.gov/ 

images /symbols/mountrushmore.jpg (accessed February 8, 2006). 
 MORU Geologic Resource Evaluation Report 5

Figure 2. Regional location of Mount Rushmore National Memorial in the Black Hills area. From http://www.nps.gov/moru/ 

planyourvisit/maps.htm (accessed June 26, 2007). 
 NPS Geologic Resources Division 6 

Figure 3. Diagram of the Black Hills dome showing progressively younger sedimentary rock layers ringing the central Precambrian

core. Mount Rushmore National Memorial lies within the central crystalline core; Jewel Cave National Monument and Wind Cave 

National Monument lie within Paleozoic limestone; and Devils Tower is a Tertiary feature. Modified from Strahler, 1960. 
 MORU Geologic Resource Evaluation Report 7

ERA Period Stratigraphic Unit Thickness 

(feet) Brief Description 

Quaternary Undifferentiated 

alluvium & colluvium 0- 50 Sand, gravel, boulders, clay 

White River Group 0- 300 Clay, channel sandstone CENOZOIC 

Tertiary Igneous Rocks Rhyolite, latite, trachyte, 

Pierre Shale 1200- 2700 Dark shale; limestone lenses 

Niobrara Formation 80- 300 Impure chalk, calcareous shale 

Carlile Shale 350- 750 Shale with sandstone layers 

Greenhorn Formation 225- 380 Dark shale, impure limestone 

Belle Forche Shale 150- 850 Gray shale, bentonite 

Mowry Shale 125- 230 Siliceous shale, bentonite 

Muddy Sandstone/ 

Newcastle Sandstone 0- 150 Brown, yellow, and white sandstone 

Skull Creek Shale 150- 270 Dark siliceous shale 

 

10- 200 Sandstone 

Cretaceous 

Inyan Kara Group 

 Fall River Fm. 

 Lakota Fm. 35- 700 Sandstone, congl., siltst., coal 

Morrison Formation 0- 220 Shale, thin sandstone 

Unkpapa Sandstone 0- 225 Massive sandstone 

Sundance Formation 250- 450 Shale, glauconitic ss, ls. Jurassic 

Gypsum Spring Fm 0- 45 Siltstone, gypsum, limestone 

MESOZOIC 

Triassic 

Spearfish Formation 375- 800 

Silty shale, red sandstone, 

siltstone with gypsum, 

thin limestone beds 

Minnekahta Limestone 25- 65 Laminated limestone 

Opeche Shale 25- 150 Red shale and sandstone 

Permian 

Pennsylvanian Minnelusa Formation 375- 1175 Cross- bedded sandstone, 

limestone, anhydrite, shale 

Mississippian Madison (Pahasapa) 

Limestone 

<200-

1000 

Massive limestone, dolomite, 

caves in upper part 

Devonian Englewood Formation 30- 60 Limestone, local shale at base 

Whitewood (Red 

River) Formation 0- 235 Buff dolomite and limestone Ordovician 

Winnipeg Formation 0- 150 Green shale with siltstone 

PALEOZOIC 

Cambrian Deadwood Formation 0- 500 

Massive to thin- bedded 

sandstone, conglomerate, 

glauconitic shale 

Harney Peak Granite Layered granite, pegmatite 

granite, and pegmatite. 

PRECAMBRIAN Proterozoic Undifferentiated 

igneous & 

metamorphic 

rocks

Schist, slate, quartzite, ironformation and arkosic grit; 

intruded by diorite, granite. 

pegmatite 

PRECAMBRIAN Archean 

Figure 4. Stratigraphic column for the Black Hills area. Units present in the Mount Rushmore area are shown in red. Modified from 

Driscoll and others, 2002. 
 NPS Geologic Resources Division 8 

Geologic Issues 

A Geologic Resource Evaluation scoping session was held for Mount Rushmore National 

Memorial on June 12- 13, 2002.The following section synthesizes geologic issues identified 

during the scoping meeting, from discussions with scientists knowledgeable about the 

park, and from literature research. These issues may require attention from resource 

managers. 

Structural stability and erosion, water resources, and 

mining impacts are geologic issues relevant to Mount 

Rushmore National Memorial. Construction projects 

have also led to increased erosion in some areas (Wenk 

2000). 

Structural Stability and Erosion 

The Harney Peak Granite was deeply fractured during 

regional uplift associated with the Laramide Orogeny. 

The pinnacles and vertical slabs of rock that characterize 

the Needles area formed by weathering of the exposed, 

fractured, and weakened rock. However, the thick 

granite sill at Mount Rushmore has fewer joints and 

fractures than elsewhere. 

Wind, precipitation (rain and snow), chemical 

weathering, and frost wedging are the primary erosive 

forces acting on the rocks in Mount Rushmore today. 

The granite in Mount Rushmore National Memorial is 

relatively resistant to weathering and erosion. Erosion of 

the granite faces has been estimated at a rate of only 0.1 

inch per 1,000 years (Sever 2004; NPS 2006). 

Frost wedging is a process wherein water that has 

percolated into minute cracks in the granite freezes, and 

as it freezes, the ice expands and further opens fractures 

in the rock. Frost wedging poses a substantial threat to 

Mount Rushmore (Sever 2004). To combat this threat, 

the cracks on the memorial are caulked with a silicon 

solution that prevents water from entering the cracks. 

Every year a maintenance crew inspects the memorial 

and repairs cracks found in the faces (Sever 2004). 

Mount Rushmore National Memorial now has a “rock 

block monitoring” system that has been installed to 

detect temperature and any movement of the granite. A 

“rock block” is a distinct section of the Harney Peak 

granite defined by pegmatite intrusions and fractures in 

the granite. Twenty- one rock blocks have been 

identified on the sculpted faces with some rock blocks 

acting as “potential keyblocks.” Potential keyblocks must 

move before any other rock block can move. Instruments 

attached directly to the sculpture measure displacements 

of less than 0.0001 inches. Four times each day 

measurements are transferred directly into a central data 

acquisition computer. Since the installation of the rock 

block monitoring system, only slight shifting has 

occurred due to temperature changes. Interpretation of 

the accumulated data suggests that no immediate danger 

exists to the sculpture (http://www.nps.gov/archive 

/moru/park_history/carving_hist/mem_maint.htm, 

accessed July 3, 2007). 

Water Resources 

The average annual precipitation in the Black Hills is 

about 46 cm (18 inches).For comparison, Death Valley 

National Park receives about 4.9 cm (1.92 inches) of 

precipitation annually and the Hoh Rainforest of 

Olympic National Park receives 338 cm (133 inches) 

annually (NPS 2006). 

Groundwater is an important source of water in this 

region and the complex geologic history of the Black 

Hills is responsible for an equally complex hydrologic 

system. The Precambrian metamorphic and igneous 

rocks generally have low permeability, however local 

aquifers occur where the crystalline core has been 

extensively fractured and weathered. 

Major aquifers occur in the permeable sedimentary rocks 

that ring the core of the Black Hills. These include the 

Deadwood Formation, Madison Limestone, Minnelusa 

Formation, Minnekahta Limestone, and Inyan Kara 

Group, with the Madison and Minnelusa Formations 

most heavily used (fig. 4). These strata serve as excellent 

aquifers and are tapped extensively for water in and 

beyond the Black Hills. Karst features such as sinkholes, 

water- loss zones in streams, collapse features, solution 

cavities, and caves develop in limestones such as the 

Madison. Additionally, local shallow aquifers are found 

in alluvial deposits along streams (Carter et al. 2002). 

Layers of shale, siltstone, and gypsum act as barriers to 

the vertical flow of groundwater and separate the major 

aquifers in the region. The bedding of these confining 

layers and aquifers generally dips away from the flanks of 

the Black Hills, and thus in general, groundwater flows 

away from the central core of the Black Hills. 

In the memorial groundwater is recharged by 

precipitation that infiltrates into the fractured mica 

schist. The extensive slope- stabilizing tree and plant 

cover helps control runoff and increases infiltration to 

the subsurface. Of the three main drainage basins in the 

memorial, groundwater is most prevalent in Lafferty 

Gulch basin (Powell et al. 1973). 

Mica schist and granite are relatively impermeable and 

would yield little or no groundwater if they did not have 

extensive fractures and joints. Seeps and springs release 
 MORU Geologic Resource Evaluation Report 9

groundwater from fractures in the rock. The water wells 

drilled in Mount Rushmore National Monument, 

including the present well, are located along joint trends 

(Nonnast 2002). Joint trends tend to be oriented to the 

northwest from about 25°- 55o

 and to the northeast from 

about 45°- 65o. 

Until 1967, the total water supply for Mount Rushmore 

was obtained from springs. Increasing visitation required 

additional water sources so two wells were drilled in 

Lafferty Gulch basin and East Boundary basin. The well 

in Lafferty Gulch basin (well 3 of Powell and others, 1973) 

was drilled into fractured mica schist and granite and is 

61 meters (200 ft) deep. The East Boundary basin well 

(well 4 of Powell and others, 1973), also in mica schist and 

granite, is 152 meters (500 ft) deep. In 1968, well 3 was 

connected to the water distribution system at Mount 

Rushmore National Memorial and became the primary 

water source for the memorial (NPS 2003; see Plate 1 of 

Powell et al. 1973 for well locations). Today recycled 

effluent helps to maintain the memorial’s landscaping, is 

used to clean parking lots and vehicles, and may help 

with structural and wildfire suppression and mitigation. 

In places, granite or pegmatite sills act as barriers to 

down gradient groundwater flow. The localized 

accumulation of groundwater in the vicinity of well 3 is 

probably due to a confining pegmatite or granite sill. 

Water from well 3 flows even when it is not being 

pumped. Each year about 7 million gallons of water are 

pumped from the well, with most water being pumped 

between April and September. Currently, the NPS holds 

three water rights that provide water for the memorial: 

well 3, Mount Rushmore springs, and Grizzly Bear spring 

(NPS 2003). 

Population growth, resource development, and periodic 

droughts have the potential to adversely affect the 

quantity, quality, and availability of water within the 

Black Hills area and Mount Rushmore National 

Memorial (USGS 2002; Driscoll et al 2002). Water quality 

may also be degraded from mining activity outside the 

park, urbanization, irrigation, forest management 

practices, and recreational development. 

To address the water resources of the Black Hills, a longterm investigation assessing the quantity, quality, and 

distribution of surface and groundwater resources of the 

Black Hills area was initiated in 1990 and completed in 

2002. The study was a cooperative effort between the 

U.S. Geological Survey (USGS), the South Dakota 

Department of Environment and Natural Resources 

(DENR), and the West Dakota Water Development 

District. Detailed results of the study have been 

published in several reports for the Black Hills 

Hydrology Study, some of which are cited in this section. 

Additional information is available in two summary 

reports for the study by Carter and others (2002) and 

Driscoll and others (2002). These reports are available 

through the USGS or on- line at http://pubs. usgs.gov 

(accessed 9 Feb. 2006). 

Some of the study’s findings include: 

• The Madison and Minnelusa aquifers have a large 

recharge capacity and dominate the groundwater 

budgets for the Black Hills area. 

• Water levels do not decline from groundwater 

withdrawals in any of the bedrock aquifers. 

• The Madison and Minnelusa aquifers are hydraulically 

connected at some locations. 

• Artesian spring flow probably accounts for the largest 

percentage of groundwater leakage that occurs. 

Artesian spring flow is the single largest discharge 

component for the Madison and Minnelusa aquifers,. 

accounting for 38% of the total discharge from these 

aquifers. 

• Large- scale development of the Madison and 

Minnelusa aquifers has the potential to impact the 

complex hydrologic system in the Black Hills area that 

controls interactions between groundwater levels and 

artesian flow. 

• Streams provide an important source of recharge to 

the Madison and Minnelusa aquifers. Most streams 

generally loose all or part of their water when crossing 

outcrops of the Madison Limestone and Minnelusa 

Formation. 

• Very few health- related limitations for substances 

associated with radioactive decay, such as radon and 

uranium, exist for Black Hills groundwater 

(Williamson and Carter, 2001). Arsenic was detected in 

a few samples from the Minnelusa aquifer. 

• Most streams in the Black Hills area generally meet 

Environmental Protection Agency (EPA) water quality 

standards for designated beneficial uses. Exceptions 

are some streams in western South Dakota that are 

outside the Black Hills and which occasionally fail to 

meet EPA standards for temperature and dissolved 

oxygen during low- flow conditions. 

• Human influences that may potentially degrade 

groundwater quality include development on and upgradient of aquifer recharge areas and septic tanks. 

• Human activities that may potentially degrade surface 

water quality include various land- use practices such 

as large- scale mining activities (increased sediment 

loading into surface streams and subsequent acid mine 

drainage), agricultural practices, and urban and 

suburban development. 

• Of the average annual precipitation in the area, about 

91.6% is returned to the atmosphere via 

evapotranspiration, about 3.5 % recharges aquifers, 

and about 4.9% is runoff from the land surface. 

• Annual aquifer recharge rates were highly variable, 

ranging from about 62 cubic feet per second in 1936 to 

about 847 cubic feet per second in 1995. 

• Total consumptive use from both groundwater and 

surface water was estimated as 218 cubic feet per 

second. 
 NPS Geologic Resources Division 10 

Mining Impacts 

Wilson and DeWitt (1995) mapped 1,084 mines and 

prospects in 85 metallic mineral districts in the Black 

Hills. Three mines mapped are in Mount Rushmore 

National Memorial: (1) the Big Shot mine in section 7, 

Township 2 South, Range 6 East; (2) an unnamed mine in 

the same section; and (3) the Mica mine, a small deposit 

at Mica Hill in section 18, Township 2 South, Range 6 

East. 

Mineral deposits in the Black Hills range in age from 

Early Proterozoic or Late Archean to Quaternary. 

Potassium feldspar was mined from the pegmatite at the 

Big Shot mine. Most of the feldspar crystals in the 

pegmatites are intergrown with other minerals making 

them uneconomical to produce (Powell et al. 1973). From 

1943 to 1945, sheet mica was excavated from Early 

Proterozoic pegmatite at the Mica mine. 

Small prospect pits scattered across the eastern half of 

Mount Rushmore National Memorial indicate a history 

of exploration and prospecting (DeWitt et al. 1988). Road 

construction material was removed from two pits in mica 

schist, and flagstone used in the early construction of the 

memorial was quarried from two other pits. 

Prospect pits in pegmatite dikes were excavated 

primarily for sheet mica and potash feldspar (Powell et 

al. 1973; Wilson and DeWitt 1995). The other prospect 

pits in the area were probably excavated for gold (Powell 

et al. 1973; Wilson and DeWitt 1995), but all have proven 

uneconomic (Powell et al. 1973). All of the prospect pits 

are in the widest part of the Early Proterozoic 

metamorphosed iron- formation, which is unit Xif on the 

Map Unit Properties Table, and in hornblende schist 

where small aggregates of sulfide minerals were 

observed. 

Disturbed Lands 

Since 1925, development within the national memorial 

has occurred in order to provide for visitor access and 

services (Wenk 2000). These activities have led to 

increased erosion in some areas. Major road 

construction completed in 1990 on South Dakota 

Highway 244 generated several areas of erosion along the 

highway. Additional areas of disturbance resulted from 

major construction of visitor use and parking facilities 

completed in 1998. 

A rehabilitation plan during the fiscal years 2001- 2005 

was designed to include the following (Wenk 2000): 

• Inventory and map disturbed sites, 

• Replace lost top soil in areas worn to bedrock, and 

• Revegetate with native plants. 
 MORU Geologic Resource Evaluation Report 11

Geologic Features and Processes 

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

in Mount Rushmore National Memorial. 

Igneous and Metamorphic Features 

The presidential faces at Mount Rushmore are carved in 

an exposure of the Harney Peak Granite. The large 

granitic sills in the memorial are parallel or subparallel to 

the remnant bedding planes or metamorphic foliation of 

the surrounding rock. This is visible at Mount Rushmore 

where schist is exposed below the sculpture, and the 

granite contact is parallel to the foliation in the 

metamorphic rocks. Most of the sills are less than 91 

meters (300 ft) thick, but the sill south of Doane 

Mountain may be as much as 300 meters (1,000 ft) thick. 

Smaller feeder sills and dikes connect all the large sills to 

each other and to the batholith. Sills are more abundant 

than dikes in Mount Rushmore except in the northeast 

part of the memorial where most of the pegmatites are 

northeast striking vertical dikes (Powell et al. 1973). A 

single pegmatite may appear as a dike in one place and a 

sill in another. 

The foliation and remnant bedding in the metamorphic 

rocks are approximately parallel. In Mount Rushmore 

National Memorial, the strike is between N20o

E and 

N20oW, and the dip is 35°- 55o east. From Mount 

Rushmore to the northwest corner of the memorial, 

most dips are between 20o and 45o east and northeast, 

and most strikes are between due north and N80o

W 

(Powell et al. 1973). A few folds have been found along 

the contacts between mica schist units and quartzite, but 

exposures are so sparse that structural data are 

inconclusive. 

Minerals 

A variety of minerals are present in the metamorphic and 

igneous rocks exposed in Mount Rushmore National 

Memorial. Quartz, biotite mica, and muscovite mica are 

the primary minerals in the metamorphic rocks. 

Accessory minerals include plagioclase, garnet, 

tourmaline, sillimanite, staurolite, actinolite, and 

grunerite. The grain size of the minerals in the 

metamorphic rocks is quite small, ranging from 0.3- 1.0 

millimeters (0.01 –0.04 in) in length, but there are 

exceptions. Grunerite, an amphibole found in the 

metamorphosed Early Proterozoic iron formations, 

occasionally forms rosettes that are 2 millimeters (0.08 

in) in diameter. Red garnet crystals also may grow to 2 

millimeters (0.08 in) in length and some hornblende 

crystals are up to 3 millimeters (0.12 in) in length (Powell 

et al. 1973). 

Crystals are larger in the Harney Peak Granite and the 

many pegmatites present throughout the Black Hills. The 

granite is composed primarily of potassium feldspar, 

albite sodium feldspar, and quartz, with crystals that are 

approximately 3 mm (0.12 in) long. However, perthite 

crystals (K, Na feldspar that forms strings or laths) in the 

granite may be several centimeters to several meters long 

(Powell et al. 1973). The Mount Rushmore pegmatites are 

composed primarily of quartz, the feldspars albite and 

microcline, and muscovite mica. Some common 

accessory minerals in the pegmatite include tourmaline, 

biotite, garnet, and beryl. One bright- green beryl crystal 

measuring 33 centimeters (13 in) in length was discovered 

in a 0.6- 1.2 meter thick (2.3- 4.6- ft) perthite and quartzfilled fracture exposed on the sculpture between the 

figures of Lincoln and Roosevelt. A red- brown garnet 

crystal 7.6 centimeters (3 in) in diameter was found in the 

same fracture (Powell et al. 1973). However, federal 

mineral development is prohibited in the park along with 

the collection of mineral specimens with out a scientific 

permit. 

Cave and Karst Features 

No known caves exist in Mount Rushmore National 

Memorial, but cave and karst features are common in the 

Mississippian Madison Limestone elsewhere in the Black 

Hills. The Madison and other sedimentary strata, 

including other limestone formations, surround the 

metamorphic and igneous core of the Black Hills (fig. 3). 

Two of the largest caves in the area are National Park 

Service units - Wind Cave National Park and Jewel Cave 

National Monument. In addition to solution features 

such as caves, sinking streams, and dissolution holes, the 

Madison also commonly has collapse features and 

sinkholes. 

Frost Wedging 

Frost wedging, the freeze and thaw cycle, is a primary 

erosive force at the memorial. Over time, frost- wedging 

of small fractures and joints can exert intense pressure 

on the granite, wedging blocks of rock apart. Frostwedging plays a significant role in weathering the rock 

that forms the sculptures, requiring that the fractures in 

the rock be monitored and stabilized. 
 NPS Geologic Resources Division 12 

Map Unit Properties 

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

Evaluation digital geologic map of Mount Rushmore National Memorial. The 

accompanying table is highly generalized and is provided for background purposes only. 

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

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

files that accompany the digital geologic map or by contacting the National Park Service 

Geologic Resources Division. 

This section explains specific properties associated with 

different rock units that the park should be aware of 

when making resource management decisions. Geologic 

features and processes often occur in, or can be 

restricted to, a particular stratigraphic unit (group, 

formation, or member). The geologic units in the 

following table and their geologic features and properties 

correspond to the units found in the accompanying 

digital geologic data. Source data for the GRE digital 

geologic map are from: 

DeWitt, Ed, 2004, Geologic map of the Mount Rushmore 

and Rapid City 60’ x 60’ Quadrangle, South Dakota, 

USGS unpublished mylar maps scale 1:100,000. 

The following table presents a list of units exposed in 

Mount Rushmore National Memorial and features for 

each map unit. This table includes several properties 

specific to each unit present in the stratigraphic column 

including: age, map unit name and symbol, unit 

description, topographic expression, resistance to 

erosion, hazards, cultural resources, mineral occurrence, 

and suitability for development. 
 MORU Geologic Resource Evaluation Report 15

Geologic History 

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

geologic map of Mount Rushmore National Memorial, the environment in which those 

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

landscape. 

The Precambrian sediments and granitic rocks that are 

now exposed in Mount Rushmore National Memorial 

were once buried about 14 kilometers (9 mi) below the 

surface by younger sediments. Intense heat and pressure 

at these great depths changed the Precambrian sediments 

to metamorphic rocks such as schist. The metamorphic 

and igneous rock units arched upward during the 

Laramide Orogeny creating the Black Hills. The younger, 

overlying sediments were later eroded exposing the 

oval- shaped Precambrian core rimmed by Paleozoic and 

Mesozoic rocks (fig. 3). 

Precambrian History of the Black Hills Region 

The Precambrian rocks exposed in the Black Hills are 

predominantly Early Proterozoic (1,600- 2,500 Ma) 

metasedimentary units (fig. 4). Exposures show both 

northwesterly and northeasterly structural trends and a 

locally dominant northwest- trending metamorphic 

foliation. In the southern Black Hills, the Harney Peak 

Granite forms a dome that disrupts and modifies the 

local structure of the Proterozoic metamorphic rocks. 

The sheer number of Precambrian units mapped for the 

Black Hills region illustrates the complexity of the 

Precambrian depositional and tectonic history. 

Understanding the evolution of the Black Hills during 

the Precambrian is limited due to the complex geology, 

pervasive metamorphism, and deformation, as well as a 

lack of precise age control for metasedimentary rocks in 

the core of the Black Hills. However, some age control is 

possible due to radiometric age dates that are available 

for igneous rocks in the Black Hills. 

The following interpretations of Precambrian history are 

based on stratigraphic, depositional, tectonic, and 

chronologic studies of the Black Hills (Redden et al. 

1990). Correlation with units in surrounding areas is 

difficult because the Black Hills are more than 300 

kilometers (190 mi) from correlative outcrops in 

southern Wyoming and more than 1,100 kilometers (680 

mi) from similar age rocks in Manitoba and 

Saskatchewan. 

2,500 (Ma) 

Archean basement rocks underlie the Black Hills (fig. 5). 

These basement rocks consist of metamorphosed fineto coarse- grained epicontinental clastic rocks and 

granite. During deposition of these ancient sediments, 

Wyoming formed the southern margin of the landmass 

that later became the core of the North American craton. 

2,500 Ma to 2,170 Ma 

An extensional tectonic setting developed along the 

passive southern margin of the craton. Coarse- grained 

clastic rocks with local anomalous concentrations of 

uranium and minor amounts of chromite were deposited 

on the continental Archean basement. Pronounced 

changes in lithology, remnant sedimentary features, and 

faults that formed as the sediments were deposited 

indicate an alluvial fan depositional setting. Sediments 

deposited in the alluvial fan were eroded from a western 

source. Sediments become finer- grained towards the top 

and may preserve progressive deposition into a marine 

basin that had originally formed to the east. 

2,170 to 1,980 Ma Deformation Event 

Gabbro sills intruded into the Early Proterozoic strata 

and Archean basement about 2,170 Ma (Redden et al. 

1990). Folding occurred along north- northwesttrending axes with subsequent erosion. These events 

indicate a change from an extensional tectonic setting to 

one of compression along the cratonic plate margin. The 

deformation occurred between 2,170 Ma and 1,980 Ma 

and may record early activity associated with the 1.9 to 1.7 

billion- year- old Trans- Hudson Orogeny that resulted 

in more land being accreted to the Wyoming province 

(fig. 6). Sediments derived from the east were deposited 

in alluvial fans that connected to a marine shelf and tidal 

flat to the west and south. Folding caused by tectonic 

compression and increased sedimentation resulted in 

subsidence and the creation of the main Black Hills 

basin. Quartz- rich and feldspathic sediments deposited 

into the Black Hills basin eventually were 

metamorphosed to quartzite and mica schist. 

At about 2,000 Ma, black shale was deposited in a 

deepening Black Hills basin. This deposition was 

accompanied by the submarine eruption of basalt. 

Density currents (turbidites) transported sediment from 

more shallow, shelf environments into deeper water and 

deposited these sediments over the basalt. 

Two depositional settings have been proposed for the 

Black Hills area during this time. One setting is an 

intracontinental basin. Intracontinental basins form 

within the interior of the continent and away from plate 

boundaries. The other is a back- arc basin. Back- arc 

basins are located landward of a volcanic arc that forms 

above a subduction zone. The general composition of the 

turbidites, however, favors the interpretation of an 

intracontinental basin depositional environment 

(Redden et al. 1990). 
 NPS Geologic Resources Division 16 

While a marine basin formed in the Black Hills region, 

the area to the east was undergoing tectonic uplift and 

deformation as well as volcanism. Water depths 

increased from east to west and north to south. Source 

areas to the east supplied the sands that later became 

quartzites and silts and muds that later would be 

metamorphosed to mica schist. 

1,980 Ma to 1,900 Ma Depositional Break 

A depositional hiatus lasting about 90 my separates the 

turbidite deposits and basalt flows of the initial basin 

development and deepening from younger overlying 

conglomerates, debris flows, and scattered volcanic 

rocks. In some areas, the older rocks were folded and 

overturned prior to deposition of conglomerate and 

debris flow deposits. This suggests that a tectonic event 

may have occurred prior to the depositional break. Any 

deformation during this time is poorly understood. 

Although speculative, the deformation may be attributed 

to east- west closure of the Black Hills basin along part of 

what would become the axis of the Trans- Hudson 

Orogeny (Redden et al. 1990). 

1,900 to 1,710 Ma Deposition and Deformation Events 

A thick, widespread shale unit in the central Black Hills 

was deposited between 1,880 and 1,710 Ma. The main 

Black Hills basin continued to evolve as an 

intracontinental or back- arc basin from about 1,900 Ma 

to 1,800 Ma. 

The Archean basement and overlying Early Proterozoic 

strata were highly deformed during the Trans- Hudson 

Orogeny that occurred approximately 1.7 to 1.9 billion 

years ago (Hoffman 1989; Redden et al. 1990). This 

deformation event resulted from the collision of the 

Superior craton of eastern Canada with the Hearne 

craton in northern Saskatchewan and the Wyoming 

craton of the western United States. The stratified rocks 

and Archean basement were folded along northnorthwest trending axes and regionally metamorphosed. 

The orogeny included extensive folding and thrust 

faulting, metamorphism, and granitic intrusion along a 

belt that extended from Hudson Bay west through 

Saskatchewan and then south through the western 

portions of the Dakotas and Nebraska. 

Rubidium- strontium (Rb- Sr) age dates suggest that the 

metamorphism may have occurred about 1,840 Ma. 

Igneous activity associated with the Trans- Hudson 

Orogeny primarily occurred from about 1,910 Ma to 1,840 

Ma. 

1,710 Ma: Harney Peak Granite 

The Harney Peak Granite was likely derived from 

melting of the Archean crust and minor amounts of Early 

Proterozoic rocks that overlie the Archean basement. 

The intrusion of magma that created granite plutonic 

domes also deformed the previously folded structures. 

The 1,710 million- year- old Harney Peak Granite seems 

too young to be related to the 1,910- 1,840 Ma main phase 

of igneous activity associated with the Trans- Hudson 

Orogeny (Redden et al. 1990). Tectonic activity in the 

Central Plains Orogeny to the south is the event more 

likely associated with the generation of the Harney Peak 

Granite. 

During the Central Plains Orogeny, a series of island arcs 

were progressively added to the southern margin of 

North America from about 1,780 Ma to 1,610 Ma (Carlson 

and Treves 2001). The Central Plains Province (fig. 6) 

consists of metamorphic and granitic rocks in a belt more 

than 1000 kilometers (620 miles) long and at least 500 

kilometers (310 miles) wide that truncates the rocks of 

the Trans- Hudson orogen (Sims and Peterman 1986). 

Middle Proterozoic Uplift 

Metamorphosed volcanic tuff (metatuff) and 

metamorphosed greywacke (metagreywacke) record a 

metamorphic event that 0ccurred between 1,600 Ma and 

1,400 Ma (Redden et al. 1990). A northeast- trending 

metamorphic foliation that is found in the rocks of the 

Black Hills developed during this time. This foliation 

may have been part of a much larger structural regime, 

but the regional extent of this Middle Proterozoic 

tectonic activity is unknown. 

Paleozoic History of the Black Hills Region 

The Paleozoic strata that rim the core of the Black Hills 

record several episodes of marine deposition followed by 

subaerial exposure and erosion. Evidence of shallow 

marine depositional environments of a transgressive sea 

is preserved by the Cambrian and Ordovician sandstone, 

glauconitic shale, limestone, and dolomite units (Driscoll 

et al. 2002). Sea level rose in the latest Cambrian and 

flooded almost the entire North American continent, 

leaving a strip of land or a series of islands exposed along 

what is known as the Transcontinental Arch (fig.7). This 

arch is an upland that stretched from northern 

Minnesota southwestward across South Dakota, 

northwestern Nebraska, Colorado and northwestern 

New Mexico . Before the end of the Early Ordovician, 

the sea receded from the craton. Throughout the 

Devonian, erosion stripped much of the Cambrian and 

Ordovician strata from the area. Silurian age rocks are 

not present in the Black Hills area because of either 

erosion or nondeposition. 

From the Late Devonian to the Middle Mississippian, the 

western margin of North America was being deformed as 

the North American lithospheric plate collided with 

oceanic crust to the west (Johnson et al. 1991). This 

collision caused another west to east transgression of the 

sea onto the craton. In some intracratonic basins such as 

the Williston Basin in North Dakota and Montana, 

organic- rich marine shale accumulated in deep subtidal, 

dysaerobic (low oxygen) environments. These black 

shales are primary petroleum source rocks in these 

basins today. In the Black Hills, Devonian and 

Mississippian marine limestones overlie Devonian shales 

that were deposited in shallower, subtidal marine 

environments. Deposited in aerobic environments, the 

organic matter was biodegraded so that these shales did 

not become rich petroleum source rocks. 
 MORU Geologic Resource Evaluation Report 17

The Mississippian- age Madison (also called Pahasapa) 

Limestone records deposition in a warm, shallowmarine environment (fig.8). When the sea regressed 

again in the Late Mississippian, this limestone was 

exposed at or near the land surface. Extensive erosion 

and elaborate karst (solution) features developed in the 

Madison Limestone at this time. 

During the Pennsylvanian and Permian, the South 

American plate collided with the southern margin of 

North America, generating the forces responsible for the 

Ancestral Rockies in Colorado. The PennsylvanianPermian Minnelusa Formation unconformably overlies 

the Madison Limestone and is a coastal deposit with 

dune structures at the top of the formation that may 

preserve beach sediments (Driscoll et al. 2002). In 

general, the Pennsylvanian and Permian strata in the 

Black Hills record episodic transgressive and regressive 

cycles of the sea into the Black Hills basin. Marine 

sandstone, shale, siltstone, and limestones are overlain by 

evaporite deposits (anhydrite) and terrigenous red 

clastics. 

Mesozoic History of the Black Hills Region 

By the Early Triassic, the major landmasses had come 

together to form a supercontinent called Pangaea (fig.9). 

The red shale, siltstone, and evaporite deposits in the 

Spearfish Formation preserve a record of subaerial 

deposition in a continental setting. By the end of the 

Triassic, Pangaea was breaking apart (fig.5) and the 

relatively high velocities of plate movements caused a 

eustatic (worldwide) sea level rise in the Jurassic. 

To the west during the Jurassic, the Farallon plate was 

subducted beneath the North American plate as the early 

phases of the Sierra Nevada batholithic intrusions and 

related volcanic eruptions occurred (Brenner 1983). As 

relative sea level rose, the sea encroached southward 

from the Arctic and into the Western Interior of North 

American. Between the Early and Late Jurassic, the Black 

Hills area moved from 15o

 north latitude to about 35o

north latitude as the North American continent drifted 

northward (Kocurek and Dott 1983; Brenner 1983). The 

abundance of red beds, evaporites, and shallow water 

carbonates suggest that Jurassic paleoclimate in this area 

was generally warm and dry. The shale, glauconitic 

sandstone, and limestone of the Sundance Formation in 

the Black Hills preserve at least two transgressive - 

regressive cycles, including the last and most extensive 

transgression of the Jurassic. Extending as far south as 

present- day New Mexico, this broad epeiric sea covered 

much of the Western Interior of North America. The sea 

covered an area bordered by rising mountains to the west 

as the Sevier Orogeny began to deform the west coast 

and on the east by the stable mid- continent craton (fig. 

10). The widely distributed Morrison Formation records 

continental environments of deposition during the 

subsequent marine regression in the uppermost Jurassic. 

Although the subduction zone along the western margin 

of North America was far to the west of the Black Hills 

region, compressive forces caused by the collision were 

felt far inland. Several processes acted in concert to 

change the landscape of the Western Interior. As layers 

of rock were thrust up into mountains along the western 

margin, the land east of the concentrated mass 

responded by bending, folding, and flexing downward 

into an expanding foreland basin. In addition, as the 

mountains were thrust above sea level, weathering and 

erosion produced a vast amount of cobbles, pebbles, 

sand, silt and clay, which were deposited into this downflexing and expanding Western Interior basin. The 

sediments added more weight to the basin and resulting 

in further subsidence. 

As the mountains rose in the west and the roughly 

north- south trending Western Interior basin subsided, 

the Gulf of Mexico separating North and South America 

rifted open in the south, and marine water flowed into 

the basin. At the same time, marine water began 

transgressing from the Arctic region in the north. The 

Early Cretaceous sandstones, shales, minor carbonates 

and coal of the Inyan Kara Group were deposited in 

fluvial, floodplain, and marsh environments, but these 

depositional patterns changed in the Upper Cretaceous 

when four main transgressions and regressions of 

shallow seas encroached into the area (Driscoll et al. 

2002). 

The multidirectional transgressions of the Late 

Cretaceous produced the most extensive interior seaway 

ever to cover the North American craton (fig. 11). The 

Western Interior Seaway extended from today’s Gulf of 

Mexico to the Arctic Ocean, a distance of about 4,800 

kilometers (3,000 mi) (Kauffman, 1977). During periods 

of maximum transgression, the width of the basin was 

1,600 kilometers (1,000 mi). The basin opened into the 

Gulf of Mexico to the south and the Arctic Ocean to the 

north (Kauffman, 1977). The Skull Creek Shale and 

younger Cretaceous shale and sandstone beds in the 

Black Hills record cycles of relative sea level rise and fall 

of this epeiric seaway. 

Cretaceous – Tertiary Laramide Orogeny 

As the Cretaceous neared an end, the Western Interior 

Seaway regressed across the area of the future Black Hills 

uplift. The seas gradually receded, forming a sedimentary 

sequence that thinned upward. The offshore marine 

Pierre Shale was overlain by nearshore marine sediments 

of the Fox Hills Sandstone (not exposed in the Black 

Hills), which transition upward into the fluvio- deltaic 

Lance Formation and Hell Creek Formation (not 

exposed in the Black Hills). The Lance Formation and 

equivalent Hell Creek Formation were deposited in the 

last 3 million years of the Cretaceous. Streams flowed 

eastward from the east side of the Powder River Basin 

and across the area that would become the Black Hills. 

Not until the Paleocene would sedimentation in the 

Powder River Basin indicate an uplift to the east. By the 

Eocene, a radial drainage pattern had developed around 

the core of the Black Hills uplift (Lisenbee and DeWitt 

1993). 
 NPS Geologic Resources Division 18 

The combination of interaction of the subducted 

Farallon plate and northward movement of the Colorado 

Plateau geologic province initiated the Late Cretaceous 

to Early Tertiary Laramide Orogeny (Lisenbee and 

DeWitt 1993). The orogeny was marked by a pronounced 

eastward shift in deformation throughout Utah and into 

Colorado. Laramide thrust faults cut deeply into the 

earth’s crust, forcing ancient plutonic and metamorphic 

basement rocks to the surface. These thrust faults have 

steeply dipping fault planes at the surface that curve and 

flatten out in Precambrian basement crystalline rock at 

depths up to 9 kilometers (30,000 ft or 5.7 miles) below 

sea level (Gries, 1983; Erslev, 1993). 

The Laramide Orogeny formed the modern Rocky 

Mountains, crystalline - cored arches bounded by thrust 

faults and separated by deep sediment- filled basins. The 

north- south trending, doubly- plunging Black Hills 

anticline is the easternmost expression of this orogeny 

(fig. 12). The initial phase of uplift in the region occurred 

in the Paleocene about 62 Ma. Igneous activity 

accompanied deformation. Stocks, dikes, sills, and 

laccoliths intruded across the northern Black Hills uplift 

during this initial tectonism (Lisenbee and DeWitt 1993). 

Devils Tower exemplifies this Paleocene magmatic event. 

With uplift, erosion quickly produced sediments that 

were shed into the Powder River Basin and the Williston 

Basin. Erosion rates during the Paleocene averaged 10 to 

13 centimeters (4 to 5 in) per 1,000 years (Lisenbee and 

DeWitt 1993). 

A second phase of deformation related to the Laramide 

Orogeny affected the Black Hills region in earliest 

Eocene time (approximately 56 Ma). A second magmatic 

pulse followed at about 39 Ma. The plutons lie along a 

zone that offsets the boundary between the Wyoming 

Archean province and the Early Proterozoic TransHudson province (fig. 5). This zone, with associated 

plutons, can be traced northward into Montana. 

By the Late Eocene, the Laramide Orogeny was over. 

Erosion carved valleys into the uplift, creating the 

topographic form of the present Black Hills. These 

valleys were then filled by the post- tectonic White River 

Group. 

The Black Hills uplift is bounded on the west by northand northwest- trending monoclines, chiefly west facing. 

Smaller scale monoclines are common across the uplift 

where Paleozoic strata are exposed (Lisenbee and 

DeWitt 1993). Monoclines are inferred to overlie 

unexposed thrust faults formed during regional 

horizontal compression. Overlapping(en echelon) folds 

suggest an additional strike- slip component to faulting. 

Ancient Precambrian margins, structures, and fabrics 

involving the Wyoming Archean province and the Early 

Proterozoic Trans- Hudson province may have 

influenced Laramide folding, faulting, and magmatic 

activity (Lisenbee and DeWitt 1993). 

Quaternary History of the Black Hills Region 

Both the Cheyenne River that drains the southern part of 

the Black Hills and the Belle Fourche River that drains 

the northern part cut across north trending structures. 

These east- west flowing rivers incised their channels at 

such a rate that they were able to maintain their original 

course as erosion exposed the Black Hills uplift. The 

subradial drainage pattern in the Black Hills developed 

as many tributaries also incised their channels into 

bedrock. These tributaries are discordant to major 

structures and perpendicular to the bedding of the 

Paleozoic and Mesozoic strata that surround the uplift 

(Wayne et al. 1991). 

The Black Hills region was unglaciated during the 

Pleistocene so that many Pleistocene deposits are 

preserved along creeks in the Black Hills. Five terrace 

deposits have been identified in the Black Hills (fig. 13) 

(Wayne et al. 1991). The oldest and topographically 

highest terrace deposit is the Mountain Meadow 

erosional surface. The gravel beneath the surface is 

dominated by Precambrian siliceous clasts. A camel 

ankle bone from Gigantocamelus was found in the 

Mountain Meadow gravel just south of Rapid City. 

The Rapid terrace is composed of locally derived clasts. 

The terrace lies 55 meters (180 ft) above Rapid Creek, 

near Rapid City, and can be traced along the eastern 

flank of the Black Hills uplift. The Rapid terrace has been 

correlated with the Hot Springs mammoth site, a natural 

elephant trap formed by a karst depression containing a 

warm spring. A mammoth bone from this site yielded a 

radiocarbon date of 26,075 ± 880 years before present 

(ybp) (Wayne et al. 1991). 

Ages for the Sturgis, Bear Butte, and Farmingdale 

terraces have not been determined. The Sturgis terrace 

contains well cemented clasts of Permian Minnekahta 

Limestone and is at least 30 meters (100 ft) thick along 

Fall River. 

The Bear Butte terrace and lowermost Farmingdale 

terrace were identified below the Sturgis terrace along 

Bear Butte Creek. The Bear Butte terrace is a major 

terrace along the Belle Fourche River and along Bear 

Butte Creek as far upstream as Bear Butte. Farmingdale 

terrace extends along Rapid Creek from Farmingdale, 

South Dakota, east to the Cheyenne River. 

Three hypotheses have been proposed for the origin of 

the Rapid and younger terraces (Wayne et al. 1991). These 

include: 

• damming by glacial ice along the Missouri River 

• local tectonism 

• rejuvenated uplift of the Black Hills starting about 4.5 

Ma. 

Today, the Black Hills experiences an overall continental 

climate. Low precipitation, hot summers, cold winters, 

and extreme variations in both precipitation and 

temperatures impact erosion rates, vegetation growth, 

and sediment transport in the various drainage basins 
 MORU Geologic Resource Evaluation Report 19

(Carter et al. 2002). Lithologic heterogeneity of the 

Paleozoic and Mesozoic sedimentary strata, fracturing of 

Precambrian rocks, and differential weathering 

contribute to a complex groundwater system in the area. 

Diverse geologic conditions also influence stream flow 

and surface water quality. Alluvial gravel, sand, silt, and 

mud continue to be deposited by perennial and 

ephemeral streams while colluvium and talus slopes 

continue to be eroded and their sediment transported 

down slope. 

 
 NPS Geologic Resources Division 20 

Figure 5. 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 in life history and tectonic events occurring on the North American continent. 

Absolute ages shown are in millions of years. 
 MORU Geologic Resource Evaluation Report 21

Figure 6. Location of the Black Hills Uplift with regard to Precambrian terranes, northern Great Plains region. Modified from Lisenbee and 

DeWitt (1993). 
 NPS Geologic Resources Division 22 

Figure 7. Paleogeographic map of the Late Cambrian, about 500 Ma. The Transcontinental Arch is the narrow landmass that transects South 

Dakota, northwestern Nebraska, and Colorado. Relatively shallow water and nearshore environments occupy the region of the Black Hills 

while a deeper water basin (darker shade of blue) is forming in the area of the Williston Basin. From Dr. Ron Blakey, Northern Arizona 

University, http://jan.ucc.nau. edu/~rcb7/namC500.jpg (access February 27, 2006). 
 MORU Geologic Resource Evaluation Report 23

Figure 8. Paleogeographic map of North America during the Early Mississippian Period, about 345 Ma. Note that much of the conterminous 

United States is covered with a shallow sea, including the area of the Black Hills, and the Transcontinental Arch has been breached. The 

Williston Basin continues to be a physiographic feature at this time. From Dr. Ron Blakey, Northern Arizona University, http://jan.ucc.nau. 

edu/~rcb7/namM345.jpg (access February 27, 2006). 
 NPS Geologic Resources Division 24 

Figure 9. Paleogeographic map of North America during the Early Triassic Period, about 245 Ma. The landmasses had come together at this 

time to form the supercontinent, Pangaea. From Dr. Ron Blakey, Northern Arizona University, http://jan.ucc.nau. edu/~rcb7/namTr245.jpg 

(access February 27, 2006). 
 MORU Geologic Resource Evaluation Report 25

Figure 10. Paleogeographic map of North America during the Jurassic Period, about 170 Ma. The Western Interior of North America has been 

inundated by shallow marine environments. From Dr. Ron Blakey, Northern Arizona University, http://jan.ucc.nau. edu/~rcb7/namJ170.jpg 

(access February 27, 2006). 
 NPS Geologic Resources Division 26 

Figure 11. Western Interior Seaway during the Cretaceous, about 85 million years ago. Relatively shallow water covered the MORU region at 

this time. From Dr. Ron Blakey, Northern Arizona University, http://jan.ucc.nau. edu/~rcb7/namK85.jpg (access February 27, 2006). 
 MORU Geologic Resource Evaluation Report 27

Figure 12. Major tectonic elements and locations of Late Cretaceous to Early Eocene sedimentary, igneous, and metamorphic rocks

associated with the Laramide Orogeny. The Black Hills uplift is the easternmost expression of the Laramide Orogeny. The basin west of the 

Black Hills uplift is the Powder River Basin, a prolific hydrocarbon-producing basin. Modified from Miller and others, 1992. 
 NPS Geologic Resources Division 28 

Figure 13. Diagram of Pleistocene terraces in and near the Black Hills. Modified from Wayne and others, 1991. 
 MORU Geologic Resource Evaluation Report 29

Glossary 

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

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

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

alluvial fan. A fan- shaped deposit of sediment that 

accumulates where a high gradient stream flows out of 

a mountain front into an area of lesser gradient such as 

a valley. 

alluvium. Stream- deposited sediment that is generally 

rounded, sorted, and stratified. 

aquifer. Rock or sediment that are sufficiently porous, 

permeable, and saturated to be useful as a source of 

water. 

axis (fold). A straight line approximation that when 

moved parallel to itself generates the shape of a fold 

(see and use hingeline) 

basement. The undifferentiated rocks, commonly 

igneous and metamorphic, that underlie the rocks of 

interest. 

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

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

basin (sedimentary). Any depression, from continental 

to local scales, into which sediments are deposited. 

batholith. A massive, discordant pluton, greater than 100 

km2, (39.6 mi2) often formed from multiple intrusions. 

bed. The smallest sedimentary strata unit, commonly 

ranging in thickness from one centimeter to a meter or 

two and distinguishable from beds above. 

bedding. Depositional layering or stratification of 

sediments. 

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

completely or in part. 

calcareous. A rock or sediment containing calcium 

carbonate. 

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

clay. Clay minerals or sedimentary fragments the size of 

clay minerals (<2 cm). 

conglomerate. A coarse- grained sedimentary rock with 

clasts larger than 2 mm in a fine- grained matrix. 

continental crust. The type of crustal rocks underlying 

the continents and continental shelves; having a 

thickness of 25- 60 km (16- 37 mi) and a density of 

approximately 2.7 grams per cubic centimeter. 

craton. The relatively old and geologically stable interior 

of a continent. 

crust. The outermost compositional shell of Earth, 10- 40 

km (6- 25 mi) thick, consisting predominantly of 

relatively low- density silicate minerals (also see 

oceanic crust and continental crust). 

crystalline. Describes the structure of a regular, orderly, 

repeating geometric arrangement of atoms 

cuesta. An asymmetric ridge with one gently- sloping 

side (dip slope) and one steep side formed by the 

erosion of gently- dipping rock strata. 

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

rock and soil material involving a wide range of types 

and sizes. 

deformation. A general term for the process of faulting, 

folding, shearing, extension, or compression of rocks 

as a result of various Earth forces. 

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

where it flows into a lake or sea. 

dike. A tabular igneous intrusion that cuts across, or is 

discordant with, the country rock it intrudes. 

dip. The angle between a structural surface and a 

horizontal reference plane measured normal to their 

line of intersection. 

discordant. Having contacts that cut across or are set an 

angle to the orientation of adjacent rocks. 

dome. A doubly plunging anticline that dips radially in all 

directions. 

drainage basin. The total area from which a stream 

system receives or drains precipitation runoff. 

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

deposited by wind. epeiric sea. A sea on the 

continental shelf or within a continent. Also called an 

epicontinental sea. 

epeiric sea. An epicontinental sea. 

ephemeral stream. A stream that flows only in direct 

response to precipitation. 

epicontinental. Situated on the continental shelf or on 

the continental interior, as an epicontinental sea. 

eustatic. Relates to simultaneous worldwide rise or fall 

of sea level in Earth’s oceans. 

evaporite. Chemically precipitated mineral(s) formed by 

the evaporation of solute- rich water under restricted 

conditions. 

fault. A subplanar break in rock along which relative 

movement occurs between the two sides. 

foliation. A general term for a planar arrangement of 

textural or structural features in any type of rock, esp. 

the locally planar fabric in a rock defined by a fissility, 

a preferred orientation of crystal planes in mineral 

grains, a preferred orientation of irregular grain 

shapes, or from compositional banding. 

formation. Fundamental rock- stratigraphic unit that is 

mappable and 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, fault) 

frost wedging. The breakup of rock due to the expansion 

of water freezing in fractures. 

grabbro. A dark- colored, coarse- grained, intrusive 

igneous rock; the intrusive equivalent to basalt. 

graywacke. A dark gray well- indurated, coarse- grained 

sandstone with poorly sorted angular to subangular 

grains of quartz and feldspar, with a variety of dark 
 NPS Geologic Resources Division 30 

rock and mineral fragments in a compact clay matrix ; 

containing abundant very fine- grained illite, sericite, 

and chloritic minerals. 

hogback. Any ridge having a sharp summit and steep 

slopes formed by steeply inclined resistant rock and 

produced by differential erosion. 

igneous. Refers to a rock or mineral that originated from 

molten material; one of the three main classes or rocks: 

igneous, metamorphic, and sedimentary. 

intrusion. A body of igneous rock that invades older 

rock. The invading rock may be a plastic solid or 

magma that pushes its way into the older rock. 

joint. A semi- planar break in rock without relative 

movement of rocks on either side of the fracture 

surface. 

laccolith. A tack head- to arcuate- shaped, concordant 

pluton that domed or up- arched the overlying country 

rocks. 

latite. A porphyritic extrusive igneous rock with 

phenocrysts of plagioclase (Na,Ca feldspar) and K 

feldspar in about equal amounts. 

magma. Molten rock generated within Earth that is the 

parent of igneous rocks. 

member. A lithostratigraphic unit with definable contacts 

that subdivides a formation. 

metagraywacke. A metamorphosed greywacke. 

metamorphism. Literally, “change in form”. 

Metamorphism occurs in rocks with mineral 

alteration, genesis, and/or recrystallization from 

increased heat and pressure. 

metatuff. Metamorphosed tuff. 

monocline. A one- limbed flexure in strata, which are 

usually flat- lying except in the flexure itself. 

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

moves down relative to the footwall. 

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

Laramide orogeny). 

paleogeography. The study, description, and 

reconstruction of the physical geography from past 

geologic periods. 

Pangaea. A theoretical, single supercontinent that 

existed during the Permian and Triassic Periods. 

pebble. Generally, small, rounded, rock particles from 4 

to 64 mm in diameter. 

pegmatite. An exceptionally coarse- grained igneous 

rock, with interlocking crystals, usually found as 

irregular dikes, lenses, or veins, esp. at the margins of 

batholiths. 

permeability. A measure of the ease or rate that fluids 

move through rocks or sediments. 

phonolite. A fine- grained extrusive igneous rock 

composed of Na and K feldspar and a feldspathoid 

(usually nepheline). 

pluton. A body of intrusive igneous rock. 

plutonic. Describes igneous rock intruded and 

crystallized at some depth in Earth. 

recharge. Infiltration processes that replenish 

groundwater. 

red beds. Sedimentary strata composed largely of 

sandstone, siltstone, and shale that are predominantly 

red due to the presence of ferric oxide (hematite) 

coating individual grains. 

regression. A long- term seaward retreat of the shoreline 

or relative fall of sea level. 

sandstone. Clastic sedimentary rock of predominantly 

sand- sized grains. 

sediment. An eroded and deposited, unconsolidated 

accumulation of lithic and mineral fragments. 

shale. A clastic sedimentary rock made of clay- sized 

particles that exhibit parallel splitting properties. 

sill. A tabular igneous intrusion, concordant with the 

country rock, that forms when magma is intruded into 

fractures and cracks that are parallel to bedding 

silt. Clastic sedimentary material intermediate in size 

between fine- grained sand and coarse clay (1/256- 1/16 

mm). 

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

spring. A site where water flows out at the surface due to 

the water table intersecting the ground surface. 

stock. An igneous intrusion exposed less than 40 square 

miles at the surface. 

strata. Tabular or sheetlike masses or distinct layers (e.g., 

of rock). 

stratigraphy. The geologic study of the origin, 

occurrence, distribution, classification, correlation, 

age, etc. of rock layers, especially sedimentary rocks. 

strike. The compass direction of the line of intersection 

that an inclined surface makes with a horizontal plane. 

subduction zone. A convergent plate boundary where 

oceanic lithosphere descends beneath a continental or 

oceanic plate and is carried down into the mantle. 

tectonic. Relating to large- scale movement and 

deformation of Earth’s crust. 

terraces (stream). 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). 

thrust fault. A contractional, dip- slip fault with a 

shallowly dipping fault surface (<45o

) where the 

hanging wall moves up and over relative to the 

footwall. 

trachyte. A fine- grained, generally porphyritic, extrusive 

rock composed mainly of Na and K feldspar with 

minor amounts of biotite, amphibole, or pyroxene. 

transgression. Landward migration of the sea due to a 

relative rise in sea level. 

trend. The direction or azimuth of elongation or a linear 

geological feature. 

turbidite. A sediment or rock deposited from, or inferred 

to have been deposited from, a turbidity current, 

which is a density current in water, air, or other fluid, 

caused by different amounts of matter in suspension. 

unconformity. A surface within sedimentary strata that 

marks a prolonged period of nondeposition or 

erosion. 

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

movement that raises the rocks. 

weathering. The set of physical, chemical, and biological 

processes by which rock is broken down in place. 
 MORU Geologic Resource Evaluation Report 31

References 

This section lists references cited in this report. A more complete geologic bibliography is 

available from the National Park Service Geologic Resources Division. 

Brenner, Robert L. 1983. Late Jurassic Tectonic Setting 

and Paleogeography of Western Interior, North 

America. In Mesozoic Paleogeography of the WestCentral United States, edited by Mitchell W. Reynolds 

and Edward D. Dolly. Denver: Rocky Mountain 

Section, Society of Economic Paleontologists and 

Mineralogists (SEPM), Rocky Mountain 

Paleogeography Symposium 2, 119- 133. 

Carlson, Marvin P., and Samuel B. Treves. 2001. Central 

Plains orogen- accretioinary core of the North 

American transcontinental Proterozoic province. 

Boulder: Geological Society of America, Abstracts with 

Programs, Vol. 33, 9. 

Carter, Janet M., Daniel G. Driscoll, and Joyce E. 

Williamson. 2002. The Black Hills Hydrology Study. 

U.S. Geological Survey, Fact Sheet FS- 046- 02. Also 

available online at http://www.pubs.usgs.gov 

/fs/fs04602/pdf/fs04602.pdf (accessed February 9, 

2006). 

DeWitt, Ed, David Buscher, Anna Burack Wilson, and 

Tom Johnson. 1988. Map showing locations of mines, 

prospects, and patented mining claims, and 

classification of mineral deposits in the Mount 

Rushmore 7.5 minute quadrangle, Black Hills, South 

Dakota. U.S. Geological Survey, Miscellaneous Field 

Studies Map, MF- 1978- K, scale 1:24,000. 

DeWitt, Ed, J.A. Redden, David Buscher, and Anna 

Burack Wilson. 1989. Geologic Map of the Black Hills 

Area, South Dakota and Wyoming. U.S. Geological 

Survey, Miscellaneous Investigations Series, Map I1910, scale 1:250,000, reprinted 1997. 

DiFrancesco, Carl A., and James B. Hedrick. 2004. Mica 

(Natural), Scrap and Flake Statistics. U.S. Geological 

Survey, http://minerals_usgs.gov/ds/2005/140/mica.pdf 

(accessed February 22, 2006). 

Driscoll, Daniel G., Janet M. Carter, Joyce E. Williamson, 

and Larry D. Putman. 2002. Hydrology of the Black 

Hills Area, South Dakota. U.S. Geological Survey, 

Water- Resources Investigations Report 02- 4094. Also 

available online at http://www.pubs.usgs.gov 

/wri/wri024094/pdf (accessed February 9, 2006). 

Erslev, Eric A. 1993. Thrusts, Back- thrusts, and 

Detachment of Rocky Mountain Foreland Arches. In 

Laramide Basement Deformation in the Rocky 

Mountain Foreland of the Western United States, edited 

by C. J. Schmidt, R. B. Chase, and E. A. Erslev. Boulder: 

Geological Society of America, Special Paper 280, 339-

358. 

Gries, Robbie. 1983. North- south Compression of Rocky 

Mountain Foreland Structures. In Rocky Mountain 

Foreland Basins and Uplifts, edited by James D. Lowell 

and Robbie Gries. Denver: Rocky Mountain 

Association of Geologists, 9- 32. 

Hoffman, Paul F. 1989. Precambrian geology and tectonic 

history of North America. In The Geology of North 

America; An Overview, edited by Albert W. Bally and 

Allison R. Palmer. Boulder: Geological Society of 

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

513. 

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

Poole. 1991. Devonian lithofacies of western United 

States. In Paleozoic Paleogeography of the Western 

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

H. Stevens. Los Angeles: Pacific Section of the Society 

of Economic Paleontologists and Mineralogists, 83-

107. 

Kauffman, E. G. 1977. Geological and Biological 

Overview: Western Interior Cretaceous Basin. Denver: 

Mountain Geologist, Vol. 14, 75- 99. 

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

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

671- 677. 

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

paleogeography and paleoclimate of the central and 

southern Rocky Mountain region. In Mesozoic 

Paleogeography of the West- Central United States, 

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

Denver: Rocky Mountain Section, Society of 

Economic Paleontologists and Mineralogists (SEPM), 

Rocky Mountain Paleogeography Symposium 2, 119-

133. 

Lisenbee, Alvis L. 1985. Tectonic Map of the Black Hills 

Uplift, Montana, Wyoming, and South Dakota. 

Geological Survey of Wyoming, Map Series 13, scale 

1:250,000. 

Lisenbee, Alvis L., and Ed DeWitt. 1993. Laramide 

Evolution of the Black Hills Uplift. In Geology of 

Wyoming, edited by A.W. Snoke, J.R. Steidtmann, and 

S.M. Roberts. Geological Survey of Wyoming, Memoir 

No. 5, 374- 412. 
 NPS Geologic Resources Division 32 

Miller, E.L., M.M. Miller, Calviin H. Stevens, J.E. Wright, 

and R. Madrid. 1992. Late Paleozoic Paleogeographic 

and Tectonic Evolution of the Western U.S. 

Cordillera. In The Cordilleran Orogen: Conterminous 

U.S., edited by B.C. Burchfiel, P.W. Lipman, and M.L. 

Zoback. Boulder: Geological Society of America, The 

Geology of North America, Vol. G- 3, 57- 106. 

Nonnast, David. 2002. Geology and Joint Analysis of the 

South 1⁄2 of the Custer Quadrangle, South Dakota. 

Boulder: The Geological Society of America, Annual 

Meeting, Abstract. 

NPS. 2003. Mount Rushmore National Memorial 

Effluent Recycling System. U.S. Department of the 

Interior, National Park Service, Environmental 

Assessment. Available online at http://www.nps.gov/ 

moru/pphtml/documents.html (accessed February 23, 

2006). 

Powell, J.E., J.J. Norton, and D.G. Adolphson. 1973. 

Water Resources and Geology of Mount Rushmore 

National Memorial, South Dakota. U.S. Geological 

Survey, Water- Supply Paper 1865. 

Redden, J.A., Z.E. Peterman, R.E. Zartman, and Ed 

DeWitt. 1990. U- Th- Pb Geochronology and 

Preliminary Interpretation of Precambrian Tectonic 

Events in the Black Hills, South Dakota. In The Early 

Proterozoic Trans- Hudson Orogen of North America, 

edited by J.F. Lewry and M.R. Stauffer. Geological 

Association of Canada, Special Paper 37, 229- 251. 

Sever, Megan. 2004. Memorials in Stone. Geotimes. 

Available on line at http://www.geotimes.org 

/june04/feature- memorials.html (accessed February 

17, 2006). 

Sims, P.K., and Z.E. Peterman. 1986. Early Proterozoic 

Central Plains orogen: A major buried structure in the 

north- central United States. Geology, Vol. 14, 488- 491. 

Strahler, Arthur N. 1960. Physical Geography. New York: 

John Wiley & Sons, 2nd edition, 534 p. 

Wayne, William J., James S. Aber, Sherry S. Agard, Robert 

N. Bergantino, John P. Bluemle, Donald A. Coates, 

Maurice E. Cooley, Richard F. Madole, James E. 

Martin, Brainerd Mears, Jr., Roger B. Morrison, and 

Wayne M. Sutherland. 1991. Quaternary geology of the 

Northern Great Plains. In Quaternary Nonglacial 

Geology; Conterminous U.S., edited by Roger B. 

Morrison. Boulder: Geological Society of America, 

The Geology of North America, Vol. K- 2, 441- 477. 

Wenk, Daniel N. 2000. Strategic Plan for Mount 

Rushmore National Memorial. U.S. Department of the 

Interior, National Park Service. Available online at 

http://www.nps.gov/moru/pphtml/documents.html 

(accessed February 23, 2006). 

Williamson, Joyce E., and Janet M. Carter. 2001. Waterquality Characteristics in the Black Hills Area, South 

Dakota. U.S. Geological Survey, Water- Resources 

Investigations Report 01- 4194. Also available online at 

http://www.pubs.usgs.gov/wri/wri014194/pdf/wri01419

4.pdf (accessed February 9, 2006). 

Wilson, Anna, and Ed DeWitt. 1995. Maps Showing 

Metallic Mineral Districts and Mines in the Black 

Hills, South Dakota and Wyoming. U.S. Geological 

Survey, Miscellaneous Investigations Series, Map I2445, scale 1:100,000. 
 MORU Geologic Resource Evaluation Report 33

Appendix A: Geologic Map Graphic 

The following page is a preview or snapshot of the geologic map for Mount Rushmore 

National Memorial. For a poster- size PDF of this map or for digital geologic map data, 

please see the included CD or visit the Geologic Resource Evaluation publications Web 

page (http://www2.nature.nps.gov/geology/inventory/gre_publications). 

The Department of the Interior protects and manages the nation’s natural resources and cultural heritage; provides scientific and other 

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

Communities. 

Mount Rushmore National Memorial 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2008/038 

NPS D-95, June 2008 

National Park Service 

Director • Mary A. Bomar 

Natural Resource Stewardship and Science 

Acting Associate Director • Mary Foley, Chief Scientist of the Northeast Region 

Natural Resource Program Center 

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

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

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

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

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

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

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

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

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

national park system. 

Geologic Resources Division 

Chief • Dave Steensen 

Planning Evaluation and Permits Branch Chief • Carol McCoy 

Credits 

Author • John Graham

Review • Lisa Norby and Carol McCoy 

Editing • Trista Thornberry- Ehrlich and Melanie Ransmeier 

Digital Map Production • Heather Stanton and Giorgia de Wolfe 

Map Layout Design • Georgia Hybels
Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, CO 80225 

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

(303) 969-2090 

EXPERIENCE YOUR AMERICATM 

National Park Service 

U.S. Department of the Interior