National Park Service

U.S. Department of the Interior

Natural Resource Stewardship and Science

Fort Union Trading Post National Historic Site

Geologic Resources Inventory Report

Natural Resource Report NPS/NRSS/GRD/NRR—2015/1004
ON THE COVER 

Photograph of Fort Union Trading Post National Historic Site from the Bodmer Overlook. The overlook offers 

an excellent panoramic view of the bluffs (old alluvial terraces) visible across the river, the reconstructed fort, 

surrounding prairie, and Missouri River floodplain. National Park Service photograph by Emily Sunblade.

THIS PAGE

Photograph of Fort Union Trading Post National Historic Site during one of the annual summer rendezvous. 

National Park Service photograph by Emily Sunblade.
Fort Union Trading Post National Historic Site

Geologic Resources Inventory Report

Natural Resource Report NPS/NRSS/GRD/NRR—2015/1004

John P. Graham 

Colorado State University Research Associate

National Park Service Geologic Resources Division

Geologic Resources Inventory

PO Box 25287

Denver, CO 80225

August 2015

U.S. Department of the Interior

National Park Service

Natural Resource Stewardship and Science 

Fort Collins, Colorado
ii

The National Park Service, Natural Resource Stewardship and Science office in Fort Collins, Colorado, publishes a 

range of reports that address natural resource topics. These reports are of interest and applicability to a broad audience 

in the National Park Service and others in natural resource management, including scientists, conservation and 

environmental constituencies, and the public. 

The Natural Resource Report Series is used to disseminate comprehensive information and analysis about natural 

resources and related topics concerning lands managed by the National Park Service. The series supports the 

advancement of science, informed decision-making, and the achievement of the National Park Service mission. The 

series also provides a forum for presenting more lengthy results that may not be accepted by publications with page 

limitations.

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

credible, technically accurate, appropriately written for the intended audience, and designed and published in a 

professional manner. This report received informal peer review by subject-matter experts who were not directly 

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

Views, statements, findings, conclusions, recommendations, and data in this report do not necessarily reflect views and 

policies of the National Park Service, U.S. Department of the Interior. Mention of trade names or commercial products 

does not constitute endorsement or recommendation for use by the U.S. Government. 

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

Resource Publications Management website (http://www.nature.nps.gov/publications/nrpm/). To receive this report in 

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

Please cite this publication as:

Graham, J. P. 2015. Fort Union Trading Post National Historic Site: Geologic resources inventory report. Natural 

Resource Report NPS/NRSS/GRD/NRR—2015/1004. National Park Service, Fort Collins, Colorado.

NPS 463/129508, August 2015
iii

Contents

Page

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

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

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

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

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

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

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

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

Geologic Significance and Historical Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

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

River Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Fort Union Formation Type Section and Stratigraphic Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Bodmer Overlook and Surrounding Landscape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Paleontological Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Glacial Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Aeolian Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Volcanic Ash Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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

Impacts from the Construction of Fort Peck Dam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Missouri River Bank Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Flooding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Current and Potential Oil and Gas Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Slope Movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Earthquakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Abandoned Mineral Lands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Paleontological Resources Inventory and Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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

Prior to 541 Million Years Ago (Precambrian Time): Assembling the North American Craton . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

From 541 Million to 252 Million Years Ago (Paleozoic Era): Sediments Fill the Williston Basin . . . . . . . . . . . . . . . . . . . . . . . . . . 29

From 252 Million to 66 Million Years Ago (Mesozoic Era): North Dakota, Montana, and an Inland Sea . . . . . . . . . . . . . . . . . . . . . 30

From 66 Million to 2 Million Years Ago (Cenozoic Era): Paleocene Swamps and Floodplains and Climate Change . . . . . . . . . . . . . . . . 32

From 2 Million to 12 Thousand Years Ago (Cenozoic Era): Pleistocene Deep Freeze . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

The Past 12 Thousand Years (Cenozoic Era): Holocene Thaw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Geologic Map Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Geologic Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Source Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

GRI GIS Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

GRI Map Poster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Map Unit Properties Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

 Use Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

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

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Additional References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Geology of National Park Service Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

NPS Resource Management Guidance and Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Climate Change Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
iv

Contents (continued)

Geological Surveys and Societies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

US Geological Survey Reference Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Page

Appendix A: Scoping Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

2011 Scoping Meeting Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

2014 Conference Call Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Appendix B: Geologic Resource Laws, Regulations, and Policies . . . . . . . . . . . . . . . . . . . . . . . . . 53

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

Map Unit Properties Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .in pocket

Figures

Figure 1. Location map of Fort Union Trading Post National Historic Site. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. Geologic time scale. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Figure 3. The distinctive red-roofed Bourgeois House and visitor center at Fort Union Trading Post National Historic Site. . . . . . . . . . . . . . 3

Figure 4. Stratigraphic column for Fort Union Trading Post National Historic Site and vicinity. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Figure 5. Painting of Fort Union Trading Post by Karl Bodmer (ca. 1843). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 6. Arrowheads found at Fort Union Trading Post National Historic Site. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 7. Schematic illustrations of meandering and braided streams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 8. Illustration of the evolving Missouri River at Fort Union Trading Post National Historic Site. . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 9. Photograph of the Tongue River Member of the Fort Union Formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 10. Schematic illustration of depositional environments associated with a meandering stream. . . . . . . . . . . . . . . . . . . . . . 10

Figure 11. Photograph of the Sentinel Butte Member of the Fort Union Formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 12. Photograph of the fossil freshwater snail Melania tenuicarinata (now considered a species of Lioplacodes). . . . . . . . . . . . . . 11

Figure 13. Schematic illustration of glacial features associated with continental ice sheets. . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 14. Photograph of the south bank of the Missouri River. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 15. Map of erosion monitoring stakes along the south bank of the Missouri River.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 16. Photographs showing morphological impacts of bendway weirs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 17. Photograph of flooding at Fort Union Trading Post National Historic Site in 2011. . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 18. Map showing the extent of the Williston Basin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 19. Generalized stratigraphic column for the Williston Basin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 20. Geographic extent of the thermally mature shale member of the Bakken Formation (yellow). . . . . . . . . . . . . . . . . . . . . 23

Figure 21. Schematic illustration of hydraulic fracturing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Figure 22. Oil and gas well locations in western North Dakota and eastern Montana as of June 2013. . . . . . . . . . . . . . . . . . . . . . 24

Figure 23. Earthquake probability map for Fort Union Trading Post National Histroic Site (triangle). . . . . . . . . . . . . . . . . . . . . . . . 27

Figure 24. Photographs of the small gravel quarry just east of the reconstructed trade post. . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Figure 25. Map of land masses sutured together to form North America.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 26. Paleozoic paleogeographic maps of North America. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Figure 27. Mesozoic and Cenozoic paleogeographic maps of North America . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Figure 28. Illustration of relative global climate in the Cenozoic Era. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Page

Tables

Page

Table 1. Paleontological resources of the Fort Union Formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Table 2. Summary of Holocene (post-glacial) climate in the Fort Union region. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Table 3. GRI GIS data for Fort Union Trading Post National Historic Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
v

Executive Summary

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

(NPS) Inventory and Monitoring Program. The Geologic Resources Division of the NPS Natural 

Resource Stewardship and Science Directorate administers the GRI. This report synthesizes discussions 

from a scoping meeting for Fort Union Trading Post National Historic Site (Montana and North 

Dakota) on 17 August 2011 and a follow-up conference call on 7 October 2014, which were held by 

the Geologic Resources Division to determine geologic resources, the status of geologic mapping, and 

geologic resource management issues and needs. It is a companion document to previously completed 

GRI digital geologic map data.

This GRI report was written for resource managers to 

support science-informed decision making. It may also 

be useful for interpretation. The report was prepared 

using available geologic information; the NPS Geologic 

Resources Division conducted no new fieldwork in 

association with its preparation. Chapters of the report 

discuss distinctive geologic features and processes 

within Fort Union Trading Post National Historic Site, 

highlight geologic issues facing resource managers, 

describe the geologic history leading to the presentday landscape, and provide information about the GRI 

GIS data. A poster (in pocket) illustrates these data. 

The Map Unit Properties Table (in pocket) summarizes 

report content for each geologic map unit. 

From 1828 to 1867, Fort Union Trading Post served as 

a peaceful gathering place for at least nine American 

Indian tribes, European fur traders, explorers, and 

other travelers to and through the northern Great 

Plains. To commemorate the historical significance of 

this active fur trade post, Fort Union Trading Post was 

designated as a National Historic Landmark in 1961 and 

authorized as a National Historic Site and part of the 

National Park System in 1966.

The 180 ha (444 ac) of Fort Union Trading Post 

National Historic Site are part of the Great Plains 

physiographic province, a vast region that extends from 

Mexico into Canada and spreads east from the Rocky 

Mountains to the north-south stretches of the Missouri 

River and the eastern edge of the Texas Panhandle. 

The landscape surrounding Fort Union is a product of 

Pleistocene (ice age) glaciation and Holocene (post ice 

ages) fluvial (river) processes and consists of bluffs, river 

terraces, and a floodplain containing the meandering 

channel of the Missouri River.

When John Jacob Astor’s American Fur Company 

chose the site for Fort Union Trading Post in 1828, 

it was adjacent to the confluence of the Missouri 

and Yellowstone rivers, but the confluence has since 

migrated 3.6 km (2.2 mi) downstream from the historic 

site. The oldest rocks in the historic site belong to 

the Fort Union Formation and were deposited in 

floodplains, lakes, and swamps between 59 million and 

56 million years ago. 

The historic site also lies within the Williston Basin, 

an extensive area larger than the state of Texas that 

began forming hundreds of millions of years ago. 

Hydrocarbons in the Williston Basin have made North 

Dakota the second largest oil-producing state in the 

country. They have also raised issues for resource 

managers.

Geologic features and processes in the historic site 

include the following:

● River Features. The Missouri River contains 

features common to both meandering and braided 

streams, including point bars, cutbanks, midchannel bars, and islands. Fort Union Trading Post 

National Historic Site is adjacent to a relatively 

straight segment of the Missouri River that has 

characteristics of both braided and meandering 

streams. The Missouri River occupies only a small 

part of a much larger channel that formed from 

meltwater flowing from the most recent Pleistocene 

glacier.

● Fort Union Formation Type Section and 

Stratigraphic Features. The type section for the Fort 

Union Formation, which is the standard by which 

other exposures of the same strata are compared, is 

a compilation of many detailed descriptions from 

strata exposed in the Fort Union area. The Sentinel 
vi

Butte and Tongue River (Bullion Creek) members 

of the Fort Union Formation contain features such 

as cross-beds, rip-up clasts, lignite coal beds, and 

cycles of sandstone, siltstone, and mudstone that 

represent deposition in Paleocene river channels, 

floodplains, and swamps. 

● Bodmer Overlook and Surrounding Landscape. 

The panoramic view from the Bodmer Overlook 

and the rural landscape surrounding Fort Union 

Trading Post National Historic Site provide a 

glimpse of what the region looked like in the 1800s. 

● Paleontological Resources. A diverse assemblage 

of invertebrate, vertebrate, and plant fossils, 

including petrified wood, has been discovered 

from the Tongue River Member of the Fort Union 

Formation in or near Fort Union Trading Post 

National Historic Site. Studies of freshwater 

mollusks collected more than 150 years ago have 

helped revise upper Paleocene chronology and 

understanding of the epoch’s ecosystems. In 

addition to fossils from the Paleogene Period, fossils 

from the Quaternary Period have been discovered 

in the region. Fossils associated with cultural sites 

have also been found. Fossils were commonly 

used for tools and jewelry and traded amongst 

American Indians throughout the northern Great 

Plains. Silicified peat, known as Knife River Flint, 

was an exceptional source of tools and projectile 

points and was traded throughout North America. 

In the early days of the fur trade, the Chouteau 

family, managers of the American Fur Company’s 

Western Division, greatly contributed to scientific 

study of fossils from the Upper Missouri River 

area. Fossils that the fur traders and explorers 

sent back east became part of the collections of 

the Academy of Natural Science in Philadelphia 

and the Smithsonian Institution in Washington, 

DC. The Chouteaus helped outfit the Lewis and 

Clark Expedition, also known as the “Corps of 

Discovery,” and provided valuable information 

about the country beyond St. Louis.

● Glacial Features. Fort Union Trading Post National 

Historic Site was constructed near the terminal 

margin of the last major glacial ice sheet that 

advanced into North Dakota and Montana during 

the Pleistocene Epoch. The regional topography is a 

glaciated landscape that includes moraines and till. 

Prior to the Pleistocene Epoch, the Missouri River 

flowed to Hudson Bay, but the ice sheet diverted 

the river’s flow to the south. As the glacier melted, 

great quantities of meltwater carved broad valleys 

and floodplains that would eventually contain the 

much smaller modern channel of the Missouri 

River.

● Aeolian Features. Wind has removed fine-grained 

sediment from hilltops and from the base of 

glacial erratics in the region, creating depressions 

known as blowouts. Blowouts may be found on the 

Bodmer Overlook in the historic site. 

● Volcanic Ash Layers. The Sentinel Butte Member 

contains layers of volcanic ash that have altered 

to bentonite, a clay mineral that swells when wet 

and shrinks as it dries. This property is responsible 

for the popcorn-like texture on exposed surfaces 

and makes the clay slippery when wet, which 

could compromise the integrity of trails and roads, 

although this is not an issue in the historic site.

The primary geologic resource management issues 

identified during the GRI scoping meeting and followup conference call include the following:

● Impacts from the Construction of Fort Peck 

Dam. Stream power has increased below the dam 

because the dam captures all upstream sediment. 

Increased stream power has led to increased 

erosion, increased channel sinuosity, and the 

migration of the main channel towards the south 

bank of the Missouri River. Riparian vegetation has 

been negatively impacted and some habitats of fish, 

aquatic insects, and mussels have been destroyed. 

● Missouri River Bank Erosion. NPS staff has been 

monitoring erosion along the south bank of the 

Missouri River since 2000. Erosion appears to 

be triggered primarily by saturation levels of 

bank material in association with tension cracks. 

Recommendations for further monitoring include 

development of a Fort Union Trading Post 

National Historic Site Missouri River management 

plan, which would addresses bank protection 

and drainage control measures, as well as the 

installation of monitoring wells.

● Flooding. The upstream Fort Peck Dam regulates 

the flow to the Missouri River. However, in 

extremely wet years, such as 2011, flooding may 

occur and impact historic site infrastructure. 

Extremely wet springs are expected to increase in 

the northern Great Plains as global climate changes. 

Increased precipitation and flooding will influence 

sedimentation rates and bank erosion, which may 

affect the boundary definition of the historic site.
vii

● Current and Potential Oil and Gas Development. 

Extensive oil and gas production occurs in the 

Williston Basin. Currently, hydraulic fracturing 

(fracking) of the Bakken Formation is driving 

exploration and development of hydrocarbons. 

Exploration targets may be present beneath Fort 

Union Trading Post National Historic Site, where 

the United States does not own the subsurface 

mineral rights. Adverse effects from increased 

hydrocarbon extraction include increased 

population, increased traffic on local roads not 

designed for heavy truck traffic, and excessive 

water demands. In addition, wildlife, soil, and air 

resources may be negatively impacted by oil and 

gas development. Increased development may 

also degrade the scenic quality of the historic site 

and its cultural landscape, which is a fundamental 

resource for understanding the site’s historic and 

ethnographic contexts.

● Slope Movements. Minor slope movements occur 

within the historic site at Bodmer Overlook where 

colluvium may slump downslope.

● Earthquakes. Earthquakes have been felt in North 

Dakota, but they do not pose a major hazard 

for Fort Union Trading Post National Historic 

Site. There is a 0.00–0.01 probability (0%–1.0% 

“chance”) that a moderate 5.0 magnitude 

earthquake will impact the historic site in the next 

100 years. Disposal of waste water related to the 

hydraulic fracking process may lubricate fault 

planes and trigger earthquakes, but this has not 

been documented in North Dakota.

● Abandoned Mineral Lands. The NPS Abandoned 

Mineral Lands database documents three sites at 

Fort Union Trading Post National Historic Site. 

All three are sand and gravel quarries. Two of the 

sites have been leveled to form the upper (larger) 

and lower (smaller) parking lots at the historic 

site. A small sand and gravel quarry east of the 

reconstructed Fort Union is still used for historic 

site maintenance.

● Paleontological Resources Inventory and 

Monitoring. In situ fossils observed in the historic 

site are subject to inventory, monitoring, protection, 

and interpretation. A paleontological resource 

inventory and monitoring report for the Northern 

Great Plains Network provided recommendations.

Fort Union Trading Post was built on very young 

geological material. The unconsolidated sediments 

in the historic site are only as old as the last phase of 

Pleistocene glaciation, which ended about 12,000 years 

ago, while the oldest rocks on the slopes adjacent to 

the Missouri River floodplain are approximately 59 

million years old. In the subsurface, sediments began 

accumulating in the Williston Basin approximately 500 

million years ago. Although not exposed in the historic 

site, these rock layers tell a fascinating story of the 

advance and retreat of epicontinental seas and tectonic 

deformation resulting from continent-wide mountainbuilding episodes.

The modern Missouri River meanders through the 

historic site and past the reconstructed post. Older 

sediments are reworked and new alluvium is deposited 

in point bars and channel bars. The lateral migration 

of the Missouri River, bank erosion, and natural 

weathering processes continue to modify the landscape 

of Fort Union Trading Post National Historic Site. 

ix

Products and Acknowledgments

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

the US Geological Survey, state geological surveys, local museums, and universities to develop GRI 

products. This section describes those products and acknowledges contributors to this report.

GRI Products

The objective of the Geologic Resources Inventory is 

to provide geologic map data and pertinent geologic 

information to support resource management and 

science-informed decision making in more than 

270 natural resource parks throughout the National 

Park System. To realize this objective, the GRI team 

undertakes three tasks for each natural resource park: 

(1) conduct a scoping meeting and provide a summary 

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

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

provide a GRI report (this document). These products 

are designed and written for nongeoscientists.

Scoping meetings bring together park staff and geologic 

experts to review and assess available geologic maps, 

develop a geologic mapping plan, and discuss geologic 

features, processes, and resource management issues 

that should be addressed in the GRI report. Following 

the scoping meeting, the GRI map team converts the 

geologic maps identified in the mapping plan to digital 

geologic map data in accordance with the GRI data 

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

uses these data, as well as the scoping summary and 

additional research, to prepare the GRI report. 

The compilation and use of natural resource 

information by park managers is called for in the 1998 

National Parks Omnibus Management Act (section 

204), 2006 National Park Service Management Policies, 

and the Natural Resources Inventory and Monitoring 

Guideline (NPS-75). The “Additional References” 

section and Appendix B provide links to these and other 

resource management documents and information.

Additional information regarding the GRI, including 

contact information, is available at http://go.nps.gov/gri. 

The current status and projected completion dates of 

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

Acknowledgments

Thanks to Trista Thornberry-Ehrlich (Colorado State 

University) for drafting many of the graphics used in 

this report. Stephen Simon and Jeremiah Kimbell (both 

from NPS Geologic Resources Division) provided 

comments and information for the oil and gas sections 

of this report. Justin Tweet (NPS Geologic Resources 

Division) provided information regarding the fossils 

of the Fort Union area. Fred MacVaugh (Fort Union 

Trading Post NHS) provided additional photographs.

Author

John P. Graham (Colorado State University)

Review

Fred MacVaugh (Fort Union Trading Post NHS)

Andy Banta (Fort Union Trading Post NHS)

Eric Brevik (Dickinson State University)

Joseph Hartman (University of North Dakota)

Jason Kenworthy (NPS Geologic Resources Division)

Editing

Katie KellerLynn (Colorado State University)

Report Formatting and Distribution

Jason Kenworthy (NPS Geologic Resources Division)

Rebecca Port (NPS Geologic Resources Division)

Source Maps

R. N. Bergantino (Montana Bureau of Mines and 

Geology)

E. M. Wilde (Montana Bureau of Mines and Geology)

S. M. Vuke (Montana Bureau of Mines and Geology)

L. N. Smith (Montana Bureau of Mines and Geology)

GRI Digital Geologic Data Production

Greg Mack (NPS Pacific West Region)

Stephanie O’Meara (Colorado State University)

GRI Map Poster Design

Kari Lanphier (Colorado State University)

GRI Map Poster Review

Georgia Hybels (Colorado State University)

Rebecca Port (NPS Geologic Resources Division)

Jason Kenworthy (NPS Geologic Resources Division)

1

Geologic Setting and Significance

This chapter describes the regional geologic setting of Fort Union Trading Post National Historic Site, 

summarizes connections among geologic resources, other resources, and historic site stories.

Geologic Setting

The 180 ha (444 ac) of Fort Union Trading Post 

National Historic Site (fig. 1) are part of the Great 

Plains physiographic province a vast region that 

extends from Mexico into Canada and spreads 

east from the Rocky Mountains to the north-south 

stretches of the Missouri River and the eastern 

edge of the Texas Panhandle (Henry and Mossa 

1995). Straddling the border of North Dakota and 

Montana, Fort Union Trading Post National Historic 

Site is 580 m (1,900 ft) above sea level and lies on the 

southern edge of the glaciated portion of the Coteau 

Slope, an area that is near the maximum extent of 

the most recent Pleistocene glacial advance into 

North Dakota (Thornberry-Ehrlich 2012). 

During the Pleistocene ice ages, which began about 

1.6 million years ago (fig. 2), continental glaciers 

periodically advanced into and retreated from 

present-day North Dakota and eastern Montana, 

which were both colder and wetter than they are 

today. Thick sheets of ice extended to about 80 

km (50 mi) south of the present-day historic site, 

and a lobe of ice followed the Yellowstone River 

valley farther south and west all the way to Glendive, 

Montana, during the last (Wisconsinan) ice age. The 

glaciers smoothed North Dakota into a landscape of 

rolling hills and diverted many of the rivers, including 

the Missouri River, to the south. Prior to glaciation, the 

Missouri River flowed northeasterly toward Hudson 

Bay, but with its northerly route blocked by ice, the river 

began flowing to the southeast. 

Since the end of the Pleistocene Epoch, weathering and 

erosion have stripped much of the glacial sediments 

from the region, but glacial sediments (map unit Qgt) 

still cover some of the bluffs south of the historic 

site (see poster, in pocket). Unconsolidated alluvial 

sediments (Qal) define the present Missouri River 

floodplain, including the river’s north bank upon which 

the trade post, including the Bourgeois House and other 

buildings, was constructed (fig. 3). Alluvial sediments, 

which may include glacial outwash sand and gravel, 

form fan-shaped deposits or alluvial terraces (Qac) at 

the base of the bluffs north of the Trade House and 

adjacent to the floodplain. Erosion has carved a rugged 

landscape into these much older bluffs, which intersect 

the northern boundary of the historic site. The bluffs 

are composed of Fort Union Formation (Tfsb, Tftr) 

sediments.

The late Paleocene Epoch (approximately 59 million to 

56 million years ago) Fort Union Formation consists of 

two members, the Sentinel Butte Member and the older 

Tongue River Member (fig. 4). As much as 90 m (300 

ft) of interlayered gray shale and cross-bedded channel 

sandstone of the Sentinel Butte Member caps the bluffs 

(Vuke et al. 2003). Lignite coal beds are exposed near 

the base of the member. Several lignite beds also are 

prominent in the underlying Tongue River Member, 

which weathers to form badlands topography. The 

Tongue River Member contains silicified (“petrified”) 

wood, fossils of freshwater mollusks, and plant molds. 

The Tongue River Member has a maximum thickness 

of approximately 250 m (800 ft) (Bergantino and Wilde 

1998). 

1804

85

85

85

2 2

68

16

68

16

16

58

16

16

23

327

200

200

200

261 202

201

Yellowstone

Missouri

River

River

MONTANA NORTH

DAKOTA

Cartwright

Culbertson

Fairview

Sidney

Alexander

Trenton

Fort Buford

Bainville Williston

North

0 10 Kilometers

0 10 Miles

Fort Union Trading Post

Figure 1. Location map of Fort Union Trading Post National 

Historic Site. Note the meandering character of the Missouri 

and Yellowstone rivers and their confluence, which is now 

southeast of the trading post. National Park Service map, 

available online: http://www.nps.gov/hfc/cfm/carto.cfm.
2

Figure 2. Geologic time scale. The divisions of the geologic time scale are organized stratigraphically, with the oldest 

divisions at the bottom and the youngest at the top. Epochs of geologic time present in Fort Union Trading Post 

are indicated by green text. GRI map abbreviations for each time division are in parentheses. Compass directions in 

parentheses following items listed in the North American Events column indicate the regional locations of events. 

Boundary ages are millions of years ago (MYA). National Park Service graphic using dates from the International 

Commission on Stratigraphy (http://www.stratigraphy.org/index.php/ics-chart-timescale; accessed 7 May 2015).
3

Figure 3. The distinctive red-roofed Bourgeois House and visitor center at Fort Union Trading Post National Historic 

Site. A reproduction Red River cart is in the foreground. National Park Service photograph by Student Conservation 

Association intern Emily Sunblade, available online: http://www.nps.gov/fous/planyourvisit/index.htm (accessed 19 

September 2014). 

Age Map Unit (map symbol) Description Period Epoch

Quaternary

Holocene

Alluvium (Qal) Unconsolidated gravel, sand, silt, and clay deposited in stream channels and 

floodplains.

Alluvium and colluvium (Qac) Sand, silt, and clay deposited in glacial meltwater channels.

Alluvial terrace deposit (Qat) Gravel, sand, and silt deposited in terraces above modern rivers and 

streams.

Pleistocene

Till (Qgt) Mixture of gravel, sand, silt, and clay with rare to abundant cobbles and 

boulders deposited by glaciers.

Paleogene Paleocene Fort Union 

Formation

Sentinel Butte Member (Tfsb) Fossiliferous sandstone and mudstone deposited in channels and 

floodplains.

Tongue River Member (Tftr) Cyclic layers of claystone, lignite, fossiliferous sandstone, and sandstone 

deposited in floodplains.

Figure 4. Stratigraphic column for Fort Union Trading Post National Historic Site and vicinity. Colored rows are 

mapped on the surface in the historic site and correspond with the colors in the Map Unit Properties Table (in pocket). 

Nomenclature and descriptions are from Bergantino et al. (1998) and Vuke et al. (2003). Refer to the Map Unit 

Properties Table (in pocket) for more information.
4

The historic site and surrounding rocks straddle the 

state line between Montana and North Dakota, which 

creates some nomenclatural differences for the Fort 

Union Formation rocks. The Fort Union Formation 

rocks were named after exposures throughout the Fort 

Union area. This GRI report and the accompanying GRI 

GIS data follow US Geological Survey and Montana 

Bureau of Mines and Geology nomenclature, which 

refer to the Fort Union Formation and the Sentinel 

Butte and Tongue River members (fig. 4). Geologic 

maps for North Dakota refer to the same rocks as the 

Fort Union Group and the Sentinel Butte Formation. 

The Tongue River Member is known as the Bullion 

Creek Formation in North Dakota. 

The landscape of Fort Union Trading Post National 

Historic Site consists of river bluffs, river terraces, and 

a floodplain containing the meandering channel of the 

Missouri River. In 1828, John Jacob Astor’s American 

Fur Company chose the site for the Fort Union Trading 

Post because it was near the confluence of the Missouri 

and Yellowstone rivers. Today, the confluence of the two 

rivers is 3.6 km (2.2 mi) southeast and downstream from 

the historic site. Lateral migration of the river channel 

across its floodplain, erosion, and weathering processes 

continue to modify the landscape at Fort Union Trading 

Post National Historic Site.

Geologic Significance and Historical 

Connections

Pleistocene glaciers crafted a landscape that would 

allow Fort Union to become the most important 19th 

century fur trading post on the upper Missouri River. 

When Pleistocene continental ice sheets blocked the 

northerly flow of the Missouri River and diverted it to 

the east-southeast, the river channel incised into one of 

the broad valleys that had been carved by vast amounts 

of glacial meltwater. Today, the Missouri River flowing 

past Fort Union Trading Post National Historic Site 

drains 240,000 km2

 (94,000 mi2), an area comparable to 

the state of Michigan. (Simon et al. 1999; Ellis 2005).

Like the Missouri River, the Yellowstone River has a 

glacial past. Coincident with the continental ice sheets, 

alpine glaciers filled canyons in the Teton Range, 

and a 1,100-m (3,500-ft) thick ice cap covered the 

Yellowstone Plateau (Smith and Siegel 2000). When 

the climate warmed, torrents of glacial meltwater cut 

down through 250 m (800 ft) of volcanic rock to form 

the Grand Canyon of the Yellowstone. Fed by melting 

ice and punctuated by catastrophic floods when ice 

or debris dams broke, the Yellowstone River carved a 

northeasterly channel through Montana. It is currently 

the longest free-flowing river in the continental United 

States.

Sediments deposited since the most recent continental 

glacier retreated from present-day North Dakota and 

Montana provided the material for the lush grassland 

prairies of the Great Plains that supported immense 

herds of bison. The soils at Fort Union Trading Post 

National Historic Site developed from the complex 

interactions among glacial and alluvial deposits, time, 

climate, topography, plants, and animals. In 2006–

2007, a soils map and database for the historic site 

was completed by the NPS Soil Resources Inventory 

and is available from the NPS Integrated Resource 

Management Applications (IRMA) portal: https://irma.

nps.gov/App/Reference/Profile/1049047 (accessed 7 

November 2014). 

Searching for a location for a new American Fur 

Company post and at the request of the Assiniboine 

Nation, Astor’s company decided to build a trading post 

near the confluence of the Missouri and Yellowstone 

rivers (National Park Service 2014a). The relatively flat 

floodplain of the Missouri River made an excellent 

construction site. The trading post was built on alluvium 

and colluvium (Qac) that had been deposited by glaciers 

and reworked by modern fluvial processes. Glacial 

erratics—large stones transported by glaciers—may 

have been used as construction material, and the 

bluffs north of the post provided building stone (Fred 

MacVaugh, NPS Fort Union Trading Post National 

Historic Site, museum curator, written communication, 

2 February 2015).

In the 19th century, the Missouri River was a major 

transportation corridor. In the spring when the rivers 

were running high from Rocky Mountain snowmelt, 

keel and Mackinaw boats from St. Louis would dock at 

Fort Union to trade goods and collect buffalo skins and 

furs. The first steamboat arrived at Fort Union in 1832 

(Fred MacVaugh, written communication, 2 February 

2015). Between 1828 and 1867, at least nine American 

Indian tribes, including the Assiniboine, Blackfeet, 

Cree, Crow, Sioux, Hidatsa, Mandan, Arikara, and 

Ojibwa, traded buffalo hides, beaver pelts, and other 

furs for goods from at least eight countries. Fort Union 

exchanged more than 25,000 buffalo robes and more 
5

than $100,000 (approximately $3 million in 2013 

dollars) in merchandise on an annual basis.

Many well-known explorers, scientists, naturalists, and 

artists also travelled along the Missouri River, stopping 

at the fort. The list includes George Catlin, John James 

Audubon, and Prince Maximilian of Wied (Chaky 

1998). Karl Bodmer, the renowned Swiss painter, 

accompanied Prince Maximilian on his Missouri River 

expedition from 1832 to 1834, arriving at Fort Union in 

the summer of 1833. Bodmer chose the bluffs north of 

the trading post from which to sketch Fort Union and 

the Assiniboine people (fig. 5). Bodmer Overlook bears 

his name. 

In 1857, Ferdinand Vandeveer Hayden, a pioneering 

geologist of the Rocky Mountains, studied the 

geology of the area, and Fielding Meek described the 

Fort Union Formation’s classic fossil snails. In 1862, 

Hayden described a composite stratigraphic section 

of rock units from the area that included Fort Union 

and referred to this rock interval as the Fort Union 

Formation (Meek and Hayden 1862).

Before the Europeans arrived with steel, the Northern 

Plains tribes fashioned projectile points (arrowheads) 

by chipping, or knapping, flakes from a stone core with 

a harder rock. At Fort Union, archeologists discovered 

arrowheads made from Knife River Flint, which was 

quarried in the Knife River Valley in western North 

Dakota for thousands of years (fig. 6; Clayton et al. 

1970). Because of its high quality, Knife River Flint was 

traded throughout North America. Artifacts made from 

Knife River Flint have been found across the country 

in locations as geographically distant as New York, 

New Mexico, and northern Canada (Clayton et al. 

1970; Hoganson and Murphy 2003). The Mandan and 

Hidatsa, whose earthlodges are the focus of Knife River 

Figure 5. Painting of Fort Union Trading Post by Karl Bodmer (ca. 1843). The white bluffs on the horizon and those 

in the foreground are the fossiliferous coal-bearing strata of the Tongue River Member of the Fort Union Formation 

(Tftr). This view to the south is from what is now known as Bodmer Overlook. National Park Service image of Bodmer’s 

print (FOUS number 2761) available online: http://www.nps.gov/fous/historyculture/index.htm (accessed 7 November 

2014).
6

Indian Villages National Historic Site in North Dakota 

(see GRI report by Graham 2015 [in review]), traded 

Knife River Flint for such items as copper, obsidian, 

shells, beads, cloth, and metal tools. Even after the 

introduction of steel, fur traders and explorers used 

Knife River Flint in their flintlock weapons until the 

mid-19th century. Knife River Flint is still used today by 

those that knap flint for a hobby, and is offered for sale 

on many websites (Murphy 2014).

In 1866, the US Army built Fort Buford, 5 km (3 mi) 

east of Fort Union Trading Post, which was never a 

military fort. The following year, the Army purchased 

Fort Union from the North West Fur Company (the 

American Fur Company had folded in the 1840s) and 

dismantled the post. The salvaged timber and stone 

was transported to Fort Buford, where it was used 

as building material. Crews from passing riverboats 

scavenged the remaining wood for firewood to feed 

steam engines, and within a few years, only traces were 

left of Fort Union Trading Post. This once–dominant 

commercial hub had become a memory.

The landscape at Fort Union Trading Post National 

Historic Site proved to be ideal for the cultural exchange 

of ideas and material goods. In order to commemorate 

the rich history of Fort Union, the site was designated 

a national historic landmark on 4 July 1961, and on 20 

June 1966, it was authorized as a national historic site 

and part of the National Park System. The reconstructed 

structures were completed in 1991. In addition to 

the reconstructed trading post and American Indian 

encampment, the Missouri River, its watershed, and the 

surrounding prairie, as well as the cultural landscape 

and viewshed are fundamental resources that are 

essential to understanding the cultural resources of the 

site and provide a tangible reminder of what the habitat 

was like in the 1800s (National Park Service 2013).

Figure 6. Arrowheads found at Fort Union Trading Post 

National Historic Site. The upper three arrowheads are 

made from Knife River Flint. The lower three are iron, 

copper (“cuprous”), and brass. Penny for scale. National 

Park Service photograph available online:

http://www.nps.gov/fous/historyculture/index.htm

(accessed 19 September 2014).
7

Geologic Features and Processes

This section describes noteworthy geologic features and processes in Fort Union Trading Post National 

Historic Site

During the 2011 scoping meeting (Thornberry-Ehrlich 

2012) and 2014 conference call, participants (see 

Appendix A) identified the following geologic features 

and processes:

● River Features

● Fort Union Formation Type Section and 

Stratigraphic Features

● Bodmer Overlook and Surrounding 

Landscape

● Paleontological Resources

● Glacial Features

● Aeolian Features

● Volcanic Ash Layers

River Features

The Missouri River, watershed, and associated 

habitat are considered fundamental resources 

and values to Fort Union Trading Post National 

Historic Site (National Park Service 2013). 

In the 1940s, the Fort Peck Dam was built 

approximately 300 km (185 mi) upstream from 

the historic site. Prior to the dam, the Missouri 

River contained a high sediment load, and its 

channels meandered across a wide floodplain 

bracketed by high sandstone bluffs. Since the 

dam construction, channel migration across its 

floodplain has been minimal. The channel now 

meanders between the Missouri River’s banks, 

reworking sediment into channel bars, low 

banks, and an emerging inner floodplain (Ellis 

2005).

The Missouri River contains elements of both 

meandering and braided streams. Meandering 

rivers are sinuous and generally have a single 

channel (fig. 7). The meandering pattern 

reduces the river gradient and produces a 

deeper channel along the outer bank of a 

meander curve where the water’s momentum 

carries the mass of the water. The greater depth 

leads to higher velocity at the outer bank, and 

the increased velocity erodes an even deeper 

channel. Higher velocity also increases the 

river’s ability to carry larger-sized particles (known as 

a river’s “competence”). In contrast, a braided stream 

contains a higher sediment load, multiple channels, and 

steeper gradient (fig. 7). 

Geomorphic features common to meandering rivers 

include cutbanks and point bars (fig. 7). Meandering 

rivers whose floodplains consist of unconsolidated 

Cut bank

Levee

Point bar Channel

Thalweg

Oxbow lake

(abandoned 

meander) Upland

terrace

Back swamp

Old channel

Mud deposited

in abandoned

channel

Sand deposited in

active braided channels

Braided

channels

Figure 7. Schematic illustrations of meandering and braided 

streams. In a meandering stream (top), point bars are areas of 

deposition and cutbanks are areas of erosion. When a meander 

neck is cut off, an oxbow lake forms. The thalweg is the deepest 

part of a channel. Braided streams (bottom) have many channels. 

Sediment is deposited in mid-channel bars and islands, as well 

as in point bars. Fort Union Trading Post National Historic Site 

is adjacent to a relatively straight section of the Missouri River 

that contains features of both braided and meandering systems. 

Graphic by Trista Thornberry-Ehrlich (Colorado State University). 
8

sediment typically erode laterally, rather than vertically, 

as their main currents migrate from bank to bank. A 

cutbank forms on the outside of a meander loop where 

the current cuts into and erodes the unconsolidated 

sediments in the bank (fig. 7). Cutbanks typically 

expose alluvial sediments (map unit Qal) that have been 

deposited on the floodplain or in previous channels. 

The channel’s energy decreases on the inside of a 

meander loop, and sediment is deposited in a point bar 

(fig. 7). 

Mid-channel islands and bars are common features 

in braided rivers (fig. 7). When the sediment load 

overwhelms the transport capacity of a braided stream 

channel, deposition occurs, causing bars and islands to 

form and the channel to shift.

Fort Union Trading Post National Historic Site is 

adjacent to a relatively straight section of the Missouri 

River that contains features of both braided and 

meandering systems (Ellis 2005). As to be expected 

with such a river, the morphology of the channel has 

changed over time (fig. 8). At Fort Union Trading Post 

National Historic Site, mid-stream bars have become 

attached to the north bank, forming lateral bars that 

have been stabilized by vegetation, and lateral migration 

to the south has resulted in south bank erosion (Ellis 

2005, 2008). The channel may be switching from a 

Figure 8. Illustration of the evolving Missouri River at Fort Union Trading Post National Historic Site. As illustrated in 

the top row, the confluence of the Missouri and Yellowstone rivers has moved to the east. Green stars indicate the 

location of Fort Union Trading Post National Historic Site. The bottom row illustrates the southward migration of 

the south bank of the Missouri within the Fort Union Trading Post National Historic Site boundary and the changes 

in channel features. The yellow polygons represent bars and islands in the years indicated. The solid yellow line 

represents the channel pattern of the year indicated. The solid red line marks the approximate boundary of the 

historic site. Annotated photographs from Ellis (2008, figure 4.2). A complete set of aerial photographs for the years 

1939, 1949, 1958, 1966, 1980, 1983, 1995, 1998, and 2004 is available in Ellis (2005, figure 4.2).
9

predominately single channel (as in a meandering 

stream) to a multiple channel morphology (as in a 

braided stream), and new bars associated with an 

upstream point bar complex have been deposited 

farther into the channel and farther downstream (Ellis 

2008). An increase in the number of channel bars has 

resulted in an increase in the number of bends in the 

river (increased sinuosity) and decreased the distance 

from one bend to the next. 

The Yellowstone River also affects the Missouri River in 

the area of Fort Union Trading Post National Historic 

Site. In the spring of 1805, William Clark measured the 

Missouri and found it to be 300 m (990 ft) wide with a 

“deep” channel at the confluence with the Yellowstone 

(Ambrose 1996, p. 221). The Yellowstone River was 272 

m (891 ft) wide but had a maximum depth of only 3.7 m 

(12 ft). From 1828 to 1867, the Missouri River channel 

was deep enough to allow steamships to dock below 

the bluff upon which Fort Union Trading Post was 

built. One reason Fort Union was abandoned in 1867 

was because sandbars had formed along the Missouri’s 

northern bank and boats could no longer dock below 

the bluff (Ellis 2005). Fort Buford, the then-newly built 

military post 5 km (3 mi) to the east, replaced Fort 

Union as the area’s main trading post.

An 1891 cartographic survey shows the Missouri River’s 

main channel flowing along the southern bank and 

sandbars built out beneath Fort Union (Ellis 2005). 

The survey also shows the Yellowstone River flowing 

through an exceptionally sinuous meander before 

entering the Missouri River. Sometime after 1891, 

the Yellowstone River cut through this meander and 

relocated the confluence approximately 3 km (2 mi) to 

the south. Flooding of the sediment-laden Yellowstone 

may cause the Missouri River to back up, which may 

subsequently affect the morphology of the river at Fort 

Union Trading Post National Historic Site (Ellis 2005). 

The dynamic processes associated with meandering and 

braided rivers have caused issues for resource managers 

at the site (see “Geologic Resource Management Issues” 

chapter).

The Missouri River also may be considered an 

“underfit” stream (Dury 1964). Underfit streams appear 

to be too small for the valley in which they flow. Glacial 

meltwater, flowing from the terminal margin of the 

last melting ice sheet, carved wide valleys in the area 

of Fort Union Trading Post National Historic Site. The 

Missouri River now flows through one of these valleys, 

but its stream is far less voluminous than the previous 

glacial meltwater channel.

Fort Union Formation Type Section and 

Stratigraphic Features

A type section is the reference standard against 

which exposures of similar strata in other locations 

may be compared and correlated. Type sections are 

typically selected for layers of sedimentary rocks that 

share similar characteristics, such as rock type (e.g., 

sandstone, shale, siltstone), color, or distinctive features 

(e.g., fossils). Rock strata so recognized are called a 

“formation.” Geologists usually name formations after 

a geographic feature, such as a mountain, river, city, 

or, as in this case, a fort where the layers are best seen. 

Formations can be organized together into “groups” or 

subdivided into “members.” 

No single exposure serves as the type section for the 

Fort Union Formation. Rather, the initial detailed 

stratigraphic description of the Fort Union Formation 

came from a number of exposures in the Fort Union 

area that F. V. Hayden compiled in 1857 (Meek and 

Hayden 1862; Tweet et al. 2011). More than 700 

stratigraphic sections of this unit have been studied 

and described in the region (Hartman 1988). In their 

database of formation names, the U.S. Geological 

Survey credits the name to “exposures at Old Fort 

Union,” which they mistakenly state became Fort 

Buford (http://ngmdb.usgs.gov/Geolex/Units/

FortUnion_8173.html, accessed 20 November 2014). 

Fort Buford, however, was constructed downstream 

from Fort Union in 1866, four years after the Fort 

Union Formation had been officially named (Meek and 

Hayden 1862). 

Subsurface cores collected in the 20th century reveal a 

nearly complete stratigraphic section of the Fort Union 

Formation, which allowed for a detailed analysis of the 

Paleocene Epoch in North America (Hartman 1988). 

The stratigraphic features in the Tongue River (Tftr; 

Bullion Creek) and Sentinel Butte (Tfsb) members help 

define the depositional environments that existed in the 

region in the late Paleocene Epoch. 

Tongue River Member

At the base of the bluffs in the Fort Union Trading 

Post National Historic Site region, cycles of sandstone, 

siltstone, and mudstone beds in the Fort Union 
10

Formation’s older Tongue River Member (Tftr) 

suggest that sediments accumulated in a floodplain 

environment (fig. 9; Tibert 2001). The coarser sandstone 

beds mark old channels. When the channels were 

abandoned, finer-grained material (siltstone and clay) 

was deposited over the channel sand by floodwaters. 

Numerous lignite coal beds in the unit record the 

presence of swampy areas, and a rich species diversity 

of freshwater mollusks and other fossils have been 

found in lakes, floodbasins, and crevasse splays, which 

form when a stream breaks its levee and deposits 

sediment on a floodplain (fig. 10). 

At the time the Tongue River Member was deposited, 

the Cannonball Sea, a remnant of the vast Western 

Interior Seaway that developed in the latest 

Cretaceous Period (see the “Geologic History” 

chapter), lay to the east in central North Dakota. 

Trace fossils, the brackish-water bivalve Corbula 

mactriformis, other molluscan species that reflect 

varying salinities, and foraminifera found in the 

Tongue River Member suggest that estuarine, 

marginal marine, and open marine conditions were 

locally present (Tibert et al. 2001; Hartman and 

Kirkland 2002). 

Sentinel Butte Member

Fort Union Formation’s younger Sentinel Butte 

Member (Tfsb) caps the bluffs in the far northeast 

corner of Fort Union Trading Post National Historic 

Site. Stratigraphic features in the Sentinel Butte 

Member include cross-bedded sandstone, ripup clasts, and coal beds (fig. 11). Inclined beds of 

sandstone (cross-beds) indicate transportation of 

the original sand grains by wind or water. In the 

case of the Sentinel Butte Member, the size of the 

cross-beds and other features suggest that water 

produced the inclined beds of sand that filled the 

channel as it migrated across its floodplain. RipFigure 9. Photograph of the Tongue River Member of the 

Fort Union Formation. Sandstone, siltstone, and mudstone 

comprise the bluffs composed of the Tongue River Member 

just west of Fort Union in Montana. Photograph by Joseph 

Hartman (University of North Dakota).

Figure 10. Schematic illustration of depositional environments 

associated with a meandering stream. Crevasse splays, oxbow 

lakes, levees, and flood basins are associated with meandering 

rivers and contributed to the species diversity found in the 

Tongue River Member of the Fort Union Formation. Fossils 

were preserved in each of these settings. Graphic courtesy of 

Joseph Hartman (University of North Dakota).

Figure 11. Photograph of the Sentinel Butte Member 

of the Fort Union Formation. The lower part of 

the Sentinel Butte Member erodes into badlands 

topography. Photograph by Joseph Hartman 

(University of North Dakota).
11

up clasts are pieces of the underlying strata that have 

been literally ripped up from the surface. Rip-up clasts 

indicate a strong river current capable of scouring the 

channel bed. Coal beds form from the compaction 

and alteration of plants that were originally deposited 

in an anoxic environment, such as a marsh or bog that 

inhibited decomposition. These stratigraphic features 

in the Sentinel Butte Member are associated with river 

channels, floodplains, and swamps.

Bodmer Overlook and Surrounding 

Landscape

The cultural landscape of viewshed within and 

surrounding Fort Union Trading Post National Historic 

Site is a fundamental resource and value (National 

Park Service 2013). The Bodmer Overlook itself is 

considered an important resource for the historic site 

(National Park Service 2013). The rural landscape at 

the historic site provides a physical display and visual 

perspective of the habitat of the 1800s. North of the 

fort, Bodmer Overlook offers an excellent panoramic 

view of the bluffs (old alluvial terraces), surrounding 

prairie, and Missouri River floodplain (see cover). The 

1.6 km (1 mi) trail at the overlook crosses alluvium and 

colluvium (Qac) and the Tongue River Member of the 

Fort Union Formation (Tftr) as it climbs to the summit 

of the Bodmer Overlook at an elevation of 616 m (2,020 

ft) above sea level. From east to west, the panoramic 

view from the overlook spans the confluence of the 

Missouri and Yellowstone rivers to the Snowden Bridge, 

an historic 1913 structure that crosses the Missouri 

River near Mondak, the ghost town on the border of 

Montana and North Dakota that thrived from 1903 

to 1919 (National Park Service 2013). When it was 

constructed, the Snowden Bridge was the longest (353 

m [1,159 ft]) vertical-lift bridge in the world. The span 

of the bridge could be raised vertically, remaining 

parallel to the deck, to allow the occasional steamship 

to pass.

Paleontological Resources

Paleontological resources (fossils) are any evidence 

of life preserved in a geologic context (Santucci et al. 

2009). Body fossils are remains of an actual organism 

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

are evidence of biological activity; examples include 

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

NPS areas occur in rocks or unconsolidated deposits, 

museum collections, and cultural contexts, such as 

building stones or archeological resources. As of July 

2015, 260 parks, including Fort Union Trading Post 

National Historic Site had documented paleontological 

resources in at least one of these contexts. The NPS 

Geologic Resources Division Paleontology website 

(http://go.nps.gov/grd_paleo; accessed 15 June 2015) 

provides more information. 

As of 2011, fossils had yet to be documented from 

geologic units within Fort Union Trading Post 

National Historic Site (Tweet et al. 2011). However, 

researchers involved with the National Park Service 

paleontology synthesis project of 2012 discovered 

that in the mid 1850s, F. V. Hayden had collected the 

freshwater snail Melania tenuicarinata, now considered 

a species of Lioplacodes (fig. 12; Justin Tweet, NPS 

Geologic Resources Division, guest scientist, written 

communications, 28 October 2014 and 11 August 2015; 

see also Yen [1948] for taxonomic discussion). Previous 

specimens of M. tenuicarinata had been described from 

collections 5 km (3 mi) to 16 km (10 mi) from the fort, 

suggesting that Hayden’s sample came near or within 

Fort Union Trading Post National Historic Site. Hayden 

probably collected the fossil from the Tongue River 

Member of the Fort Union Formation (Hartman 1984; 

Hartman and Kihm 1992, 1995). 

In addition to fossil snails, petrified wood is commonly 

found in the Fort Union Formation. Recently, Fred 

MacVaugh, the museum curator at Fort Union 

Trading Post National Historic Site, found pieces of 

petrified wood at Bodmer Overlook (Fred MacVaugh, 

conference call, 7 October 2014). The off-white 

petrified wood is abundant in the Sentinel Butte 

Member (Joseph Hartman, University of North 

Figure 12. Photograph of the fossil freshwater snail 

Melania tenuicarinata (now considered a species of 

Lioplacodes). This snail was discovered by F. V. Hayden in 

the mid 1850s from at or near Fort Union Trading Post 

National Historic Site. National Park Service photograph 

by Vincent Santucci (NPS Geologic Resources Division) 

and Justin Tweet (NPS Geologic Resources Division).
12

Dakota, professor, written communication, 16 April 

2015). At Theodore Roosevelt National Park, which is 

about 80 km (50 mi) southeast of the historic site, the 

Fort Union Formation preserves a “petrified forest” 

(Fastovsky and McSweeney 1991; Justin Tweet, NPS 

Geologic Resources Division, guest scientist, written 

communication, 28 October 2014; see GRI report by 

KellerLynn 2007).

In the Fort Union Trading Post National Historic Site 

region, fossil collecting for scientific study dates back 

to the fur trade. The St. Louis–based Chouteau family, 

who for decades operated the Western Department 

of the American Fur Company, played a significant 

role in collecting fossils and sending them back east 

for paleontologists to study. At the turn of the 19th 

century, they helped outfit the Lewis and Clark “Corps 

of Discovery” and provided valuable information about 

the Upper Missouri country (Chaky 1998). George 

Catlin was indebted to Pierre Chouteau, Jr. for his safe 

passage in 1832 to Fort Union, the new post at the 

confluence of the Yellowstone and Missouri rivers. 

Catlin, an amateur naturalist and well-known painter 

of scenes of the American West, collected fossils along 

with Indian artifacts, minerals, and other items, and 

stored them in St. Louis (Chaky 1998). 

Pierre also befriended Prince Maximilian of Weid, 

who noted abundant shells in limestone beds near 

Fort Leavenworth and recognized the remains of a 

Mosasaurus that the Atkinson-O’Fallon Expedition to 

the Yellowstone River had noted in 1825 (Chaky 1998). 

Unfortunately, Maximilian’s collections were lost when 

the steamer Assiniboine sank in 1835 near present-day 

Bismarck, North Dakota.

Fossils collected from the Upper Missouri River 

area continued to find their way to the American 

Fur Company in St. Louis. French explorer Joseph 

Nicollet reported on abundant marine fossils to his 

report to Congress in 1843, and the discovery by St. 

Louis physician Dr. Hiram A. Prout of a jawbone of 

a large titanothere in the White River badlands drew 

the interest of Dr. Joseph Leidy of the Academy of 

Natural Sciences of Philadelphia (Tweet et al. 2011 [see 

Badlands National Park section]; Chaky 1998). 

When Maximilian traveled upriver in 1833, he was 

accompanied by Alexander Culbertson, a young man 

who had been hired to manage Fort Union for the 

American Fur Company. In 1843, Culbertson provided 

free passage and hospitality to John James Audubon 

(Chaky 1998). Audubon and his colleague Edward 

Harris collected fossils near the confluence of the 

Missouri and Yellowstone rivers. These were the first 

fossils scientifically collected in the vicinity of Fort 

Union and the first fossils collected from the American 

West (Hartman 1999; Tweet et al. 2011). 

Culbertson accompanied Audubon to Fort Pierre in 

August 1843 and then continued overland to Fort 

Laramie. On the trail, Culberson and his traveling 

companion, Army Captain Stewart Van Vliet, collected 

fossils that Dr. Leidy would describe in two papers 

published in 1848. These fossils, and others collected by 

his brother Thaddeus in 1850, would result in intense 

fossil collection in the vicinity of present-day Badlands 

National Park (Tweet et al. 2011; see GRI report by 

Graham 2008). 

Interest in Upper Missouri River fossils continued to 

grow. In 1853, much to the consternation of John Evans 

and Benjamin Franklin Shumard, famous geologist 

James Hall of Albany, New York, sent geologists 

Ferdinand V. Hayden and F. B. Meek to collect fossils in 

the White River Badlands. Evans and Shumard objected 

strenuously to sharing the badlands with Hayden 

and Meek. St. Louis scientist George Englemann and 

famed geologist Louis Agassiz mediated the dispute 

and convinced the two groups that the badlands held 

enough fossils for both parties (Chaky 1998; Hartman 

1999). 

In 1854, Hayden quit his job with Hall and returned 

to the West. For years he collected fossils in the Upper 

Missouri country. A large part of his collection was 

stored in the home of Pierre Chouteau, Jr.’s son, 

Charles (Chaky 1998). Fossil mollusks collected by F. V. 

Hayden and F. B. Meek between 1855 and 1860 became 

the “classic” Fort Union freshwater mollusk fauna 

(Hartman and Kihm 1992). 

Fossils of the Tongue River Member (Bullion Creek 

Formation)

The classic nonmarine freshwater mollusks collected 

by Meek and Hayden have since been determined 

to come from the upper Tongue River Member (Tftr; 

Bullion Creek Formation) and lower Sentinel Butte 

Member (Tfsb; Sentinel Butte Formation) of the Fort 

Union Formation (Hartman 1994, 1999, 2004; Kihm 

et al. 2004). Some of Hayden’s fossils may have come 

from exposures at or very near Fort Union Trading Post 
13

National Historic Site.

The Tongue River Member is extremely fossiliferous, 

containing a diverse assemblage of invertebrate, 

vertebrate, and plant fossils (table 1). More than 100 

taxa of fossil plants have been documented from the 

Wannagan Creek Quarry from the upper part of the 

member (Bullion Creek Formation; Erickson 1982, 

1999). The petrified wood at Bodmer Overlook may 

be from this unit. The unit also contains abundant 

terrestrial mollusks (Hartman and Kihm 1992, 1995; 

Hartman 1999) and other invertebrates, as well as 

vertebrates and microfossils. 

The flora and fauna record a depositional setting 

consisting of a low-relief coastal plain with rivers, 

lakes, ponds, swamps, and rainforests (Hoganson 

and Murphy 2003). The freshwater molluscan fauna 

described by Meek and Hayden is confined to strata 

that overlie the marine Cannonball Member of the 

Fort Union Formation. These strata include the Tongue 

River and lower part of the Sentinel Butte members, 

which represent the late Paleocene Epoch of North 

America (Hartman and Kihm 1995; Hartman 2004; see 

“Geologic History” chapter). 

Fossils of the Sentinel Butte Member (Sentinel 

Butte Formation)

The upper Paleocene Sentinel Butte Member (Tfsb) 

is lithologically similar to the Tongue River Member 

(Tftr), but clay content and channel-forms provide 

differences in unit color and landscape character. 

Lignite coal is found in both units, and reddish baked 

sediments resulting from spontaneously burned coal 

adds highlights to many viewsheds. In addition, both 

members contain a similar overall fossil assemblage 

(table 1; Tweet et al. 2011). The Sentinel Butte Member 

includes plant macrofossils and microfossils and 

freshwater and terrestrial gastropods and freshwater 

bivalves, along with invertebrate trace fossils. Vertebrate 

fossils are rare (Harrington et al. 2005), but microfossils 

and the largest mammal that lived during the Paleocene 

Epoch in North Dakota, Titanoides primaevus, was 

described from strata not far from the historic site 

(Hartman and Kihm 1991). The ecosystem in which 

these organisms lived consisted of more floodplain 

and fewer marsh environments than the Tongue River 

(Bullion Creek) ecosystem (Royse 1972; Daly et al. 1985; 

Clechenko et al. 2007).

Fossils of the Oahe Formation

The Quaternary Oahe Formation (Qal, Qac, Qat) 

encompasses all post-glacial deposits in the region 

of the historic site (Artz 1995). Fossils are rare in the 

formation, but the unit does contain charcoal, burrows, 

and vertebrate and invertebrate remains, such as bison 

bones (Clayton et al. 1976). 

The fossils of the Oahe Formation fall into one of 

two general categories: (1) Pleistocene megafauna, 

and (2) Holocene paleoenvironmental materials 

(Tweet et al. 2011). Extinct horses, ground sloths, 

mastodons, mammoths, and giant bison are among 

the Pleistocene megafauna. Pleistocene megafauna 

have been found near Watford City, 59 km (37 mi) 

southeast of the historic site, and on the shore of Lake 

Sakakawea, approximately 75 km (47 mi) southeast of 

Fort Union Trading Post National Historic Site (Tweet 

et al. 2011). A woolly mammoth molar recovered 

from the shoreline of glacial Lake Agassiz in western 

Cass County, North Dakota, (on the other side of the 

state from Fort Union Trading Post) suggests that the 

region consisted of tundra-like conditions about 11,500 

years ago (Harrington and Ashworth 1986). Holocene 

paleoenvironmental fossils consist of pollen, spores, 

aquatic microfossils, and other material that provides 

clues about past climatic condisions (Hoganson and 

Murphy 2003). 

Both categories of fossils are scarce, especially in the 

northern Great Plains. Because rivers have flowed 

through the region, less dense material is more likely 

to have been transported out of the area than large 

Quaternary fossils, such as petrified logs, mammoth 

teeth, or bison bones. The latter would have been 

transported only by high velocity flood waters.

Fossils and Cultural Resources

Knife River Flint is an example of a paleontological 

resource that is found in cultural settings (fig. 6). The 

brown, fine-grained flint formed between 50 million 

and 20 million years ago, although its exact age has not 

been determined (Clayton et al. 1970; Hoganson and 

Murphy 2003). It now occurs in all of the Quaternary 

units (Qal, Qac, Qat, and Qgt) in the historic site as a 

result of human use and transport. The flint was traded 

throughout North America in the 19th century. 

Knife River Flint consists of silicified peat, which is 

composed of fossil plants from ponds, swamps, and 
14

marshes. In these environments, anaerobic conditions 

would have decreased the rate of decomposition of 

organic material, allowing peat or lignite coal to form. 

Percolation of silica-rich groundwater altered the peat 

or lignite and produced the Knife River Flint.

Other fossils, such as shells, may also be found in 

archeological sites at the historic site, as they are at 

Knife River Indian Villages National Historic Site 

(Hoganson and Murphy 2003; see GRI report by 

Graham 2015 [in review]). American Indian tribes, 

including the Crow, Hidatsa, Mandan, and Sioux are 

known to have collected fossils, some of which were 

used for tools and jewelry (Hoganson and Murphy 

2003; Mayor 2005). An overview of fossils in the 

National Park System in cultural resource contexts is 

presented by Kenworthy and Santucci (2006). 

Glacial Features

Repeated glaciations (ice ages) during the Pleistocene 

Epoch (2.58 million to 11,700 years ago) scoured and 

reshaped the landscape of North Dakota and eastern 

Montana, including the area of present-day Fort Union 

Trading Post National Historic Site. Glacial features in 

the historic site fall into two major categories: (1) those 

created or carved by glaciers, and (2) those deposited by 

rivers flowing beneath or out of glaciers (“glaciofluvial”) 

or deposited in lakes near glaciers (“glaciolacustrine”). 

Fossils Tongue River Member (Bullion Creek Formation) 

(Tftr)

Sentinel Butte Member (Sentinel Butte 

Formation) (Tfsb)

Invertebrates Bivalves (freshwater)

Gastropods (freshwater and terrestrial)

Insects (beetles, butterflies, moths, dragonflies, damselflies)

Ostracodes

Arthropod trace fossils on plants

Crustacean and worm burrows

Bivalves (freshwater)

Gastropods (freshwater and terrestrial)

Ostracodes

Beetles

Trace fossils

Flora Petrified wood

Water ferns

Water lilies

Dawn redwood

Bald cypress

Katsura

Palms

Palmettos

Plane trees

Magnolia

Silicified peat

Roots, stumps, petrified wood fruits, leaves and other plant 

macrofossils

Vertebrates Freshwater sharks

Freshwater ray Myliobatis

Bony fish (including the gar Lepisosteus, bowfin Amia, pike Esox, 

osteoglossiforms, and long-snouted Belonostomus)

Giant salamander Piceoerpeton and Scapherpeton

Frogs

Turtles (snappers, pond tortoises, soft-shelled turtles)

Lizards

Snakes 

Small alligator (Wannaganosuchus)

Birds (Dakotornis, Presbyornis)

Diverse assemblage of early mammals (rodent-like arboreal 

forms, marsupials, early primate relatives, carnivorans)

Tracks of lizards, crocodilians, birds

Coprolites (fossil feces) mostly attributed to Borealosuchus

Rare vertebrate fossils:

Fish

Giant salamanders

Turtles

Champsosaurs

Crocodilians

Multituberculates

Marsupials

Primate relatives

Pantodont Titanoides primaevus

Microfossils Foraminifera (protists that secrete shells)

Diatoms (algae that secrete silica shells)

Spores

Pollen 

Fungal spores

Pollen

Table 1. Paleontological resources of the Fort Union Formation.

Compiled from information in Tweet et al. (2011).
15

Deposits and features that are the direct result of glacial 

ice moving across the landscape include till (Qgt), 

moraines, drumlins, kettles, glacial grooves, striations, 

roches moutonnées, and glacial erratics (fig. 13). 

Glaciofluvial or glaciolacustrine deposits and features 

include kames, eskers, and braided streams, as well as 

outwash fans, deltas, or plains (fig. 13). Many of these 

features occur throughout North Dakota and Montana, 

but not all of them are present in the historic site.

Fort Union Trading Post National Historical Site is 

approximately 80 km (50 miles) north of the terminal 

margin of the last major glacial ice sheet that flowed 

into North Dakota and Montana during the Pleistocene 

Epoch. North of the historic site, the topography 

reflects a glaciated landscape, including moraines and 

deposits associated with the margin of the retreating 

ice sheet. As the ice sheet melted, abundant quantities 

of meltwater flowed down a broad valley that would 

eventually contain the channel of the Missouri River. 

Some of the coarse sand and gravel in the alluvium and 

colluvium (Qac) in the historic site represent glacial 

outwash deposits. The coarse deposits that settled out 

of these braided meltwater channels indicate a close 

proximity to the glacial terminus (Thornberry-Ehrlich 

2012).

Aeolian Features

Strong winds that blow across 

North Dakota generate local 

landscape features known as 

blowouts. To form a blowout, 

the wind removes fine-grained 

sediments and leaves behind a 

depression floored by coarsegrained material too heavy for 

aeolian transport. Blowouts 

often occur on hilltops, such 

as the Bodmer Overlook, or 

next to large glacial erratics 

(boulders). In the past, bison 

would rub against these 

large boulders, removing the 

vegetation from the base of the 

erratic and exposing the fine 

sediment to the wind. Fossils 

(such as petrified wood) and 

resistant material (such as Knife 

River Flint) are commonly 

associated with blowouts 

(Thornberry-Ehrlich 2012). Blowouts are often found 

on hillslopes or hilltops, such as the Bodmer Overlook.

Volcanic Ash Layers

In the Sentinel Butte Member (Tfsb), layers of volcanic 

ash have been altered to bentonite, a type of clay 

that has a crystalline structure which accepts water 

molecules, so it swells when it becomes wet and 

shrinks when it dries out. This shrink-swell property 

makes the clay particularly slippery when wet and may 

compromise the integrity of roads and trails. It also 

creates a popcorn-like texture on exposed surfaces, 

typical of badlands topography. Bentonite occurs in the 

vicinity of Fort Union Trading Post National Historic 

Site, but it does not occur within the boundary of the 

historic site (conference call participants, 7 October 

2014). 

In addition, volcanic ash was deposited over the historic 

site as a result of the 1980 eruption of Mount St. Helens. 

The eruption spread volcanic ash as much as 1.3 cm (0.5 

in) thick across North Dakota. 

Retreating glacier

Braided stream

Drumlins

Terminal

moraine

Outwash 

plain

Ground Bedrock

moraine

Recessional

moraine

Future

kettle lake

Esker

Glacial

erratics

Kames

Kettle

lakes

Meltwater

terrace Outwash 

delta

Outwash Proglacial

fan lake

Figure 13. Schematic illustration of glacial features associated with continental 

ice sheets. Till and meltwater sediments are present in Fort Union Trading Post 

National Historic Site, and glacial erratics were used as building stone. Graphic by 

Trista Thornberry-Ehrlich (Colorado State University).

17

Geologic Resource Management Issues

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

for visitor safety, protection of infrastructure, and preservation of natural and cultural resources 

in Fort Union Trading Post National Historic Site. The NPS Geologic Resources Division provides 

technical and policy assistance for these issues.

During the 2011 scoping meeting (see ThornberryEhrlich 2012) and 2014 conference call, participants 

(see Appendix A) identified the following geologic 

resource management issues:

● Impacts from the Construction of Fort Peck Dam

● Missouri River Bank Erosion

● Flooding

● Current and Potential Oil and Gas Development

● Slope Movements

● Earthquakes

● Abandoned Mineral Lands

● Paleontological Resources Inventory and 

Monitoring

Resource managers may find Geological Monitoring

(Young and Norby (2009) useful for addressing some 

of these geologic resource management issues. An 

online version of Geological Monitoring is available at 

http://go.nps.gov/geomonitoring (accessed 16 June 

2015). The manual provides guidance for monitoring 

vital signs—measurable parameters of the overall 

condition of natural resources. Each chapter covers 

a different geologic resource and includes detailed 

recommendations for resource managers and suggested 

methods of monitoring.

Impacts from the Construction of Fort Peck 

Dam

Installation of the Fort Peck Dam across the Missouri 

River has caused significant impacts to downstream 

sections of the river (Ellis 2005). The dam captures 

all upstream sediment, allowing stream power to 

increase below the dam (Geologic Resources Division 

2012). Increased stream power has led to increased 

erosion (degradation) below the dam and subsequent 

downstream sedimentation (aggradation) in parts of the 

river where sediment load and channel velocity were 

previously at equilibrium. 

As a result of the dam, peak flows have decreased, but 

average discharges have increased. The dam also has 

changed the timing of seasonal flows. Winter flows 

that once occurred between April and July now occur 

in February and March. This change may enhance the 

erosive power of river ice and create backwater effects 

from late spring flooding on the Yellowstone River. In 

addition, sinuosity has intensified, and flow has become 

concentrated in a single channel. At Fort Union Trading 

Post National Historic Site, this has resulted in the 

attachment of channel bars to the north bank and the 

diversion of flow towards the south bank (fig. 8; Ellis 

2005).

The construction of Fort Peck Dam also has had 

adverse consequences to the Missouri River ecosystem. 

Lack of seasonal flooding limits the amount of 

sediment in the river and the transportation of seeds 

and nutrients. Pioneer communities of cottonwood 

(Populus deltoids) and willow (Salix hindsiana) have 

decreased because they require periodic inundation by 

floodwaters to become established. Roots from these 

trees help stabilize river banks. At Fort Union Trading 

Post National Historic Site, cottonwoods have locally 

stabilized the south bank, but erosion has increased 

where cottonwoods have been lost (fig. 8; Ellis 2008). 

The altered velocity, temperature, turbidity, nutrient 

concentrations, and trophic pathways of the river 

have also destroyed the habitat of many fish, aquatic 

insects, and mussel species that had adapted to pre-dam 

conditions (Ellis 2005). Water temperature in the river 

has decreased by as much as 12°C (10°F) because of 

water storage in the dam (Hoganson and Murphy 2003).

The upstream Fort Peck Dam and downstream 

Garrison Dam, which impounded Lake Sakakawea, 

altered the natural flow and processes of the Missouri 

River. Because the Missouri River is a fundamental 

resource in the historic site, resource managers are 

concerned that the altered river system, including 

regulated flow and bank erosion, will negatively impact 

visitor understanding and enjoyment (National Park 

Service 2013). 
18

For Fort Union Trading Post National Historic Site, 

the most damaging impact from the Fort Peck Dam is 

increased erosion of alluvium (Qal) from the Missouri 

River’s south bank. Erosion may degrade archeological 

sites as well as alter the river channel, which could lead 

to further loss of land and an eventual breach of the 

historic site’s southern boundary.

Missouri River Bank Erosion

Monitoring data suggest that bank erosion occurs 

episodically and catastrophically along the Missouri 

River—a fundamental resource (National Park Service 

2013)—in Fort Union Trading Post National Historic 

Site. Changes in precipitation, which influences bank 

saturation levels, river ice (see “River Ice” section), 

tension cracks, properties of bank sediments, and 

vegetation may trigger bank failure. According to Ellis 

(2008), increased saturation appears to be the main 

triggering mechanism for slope failure, which occurs 

along tension cracks (fig. 14). The failed slopes create 

a berm at the bank toe, but this berm does not prevent 

further slope failure. 

Staff at Fort Union Trading Post National Historic Site 

has been monitoring the south bank’s erosion since 

2000 using monitoring stakes. The initial five stakes 

placed 15 m (50 ft) from the bank in 2000 were replaced 

with new stakes by 2005, which were placed 15 m (50 ft) 

farther from the original stakes (fig. 15; Ellis 2005, 2008). 

Researcher Lucy Ellis recommended that historic site 

staff also install an off-set stake from which to measure 

a consistent angle to the bank (Andy Banta, Fort Union 

Trading Post National Historic Site, superintendent, 

written communication, 12 February 2015).

According to Ellis (2008), the following periods of 

significant erosion occurred along the south bank prior 

to 2008, with erosion shifting downstream following 

each event: 

● From September to December 2002, the bank 

between stakes 2 and 6 retreated an average of 4.6 

m (15 ft) because of a rapid rise in water level.

Figure 14. Photograph of the south bank of the 

Missouri River. Deep tension cracks (dashed red line) 

have developed approximately 2 m (7 ft) from the 

bank’s edge. Ultimately, bank failure will occur along 

this tension crack. National Park Service photograph 

taken on 9 March 2007 shortly before ice-out. 

Photograph from Ellis (2008, figure 4.3).

Figure 15. Map of erosion monitoring stakes along the 

south bank of the Missouri River. Staff at Fort Union 

Trading Post National Historic Site began monitoring 

bank erosion in 2000. Red dots indicate the locations of 

monitoring states. These are superimposed on a 2004 

aerial photograph. The red line marks the south and 

west boundaries of Fort Union Trading Post National 

Historic Site. Extracted from Ellis (2005, figure 4.4).
19

● Between July 2003 and March 2004, the bank 

retreated an average of 4.3 m (14 ft) between stakes 

5 and 9 because of a rapid fall in discharge from the 

dam.

● In March 2007, the same section of river bank 

(between stakes 6 and 9) once again retreated an 

average of 4.3 m (14 ft) because of rapid drawdown 

(described below). 

● In addition, an unusual amount of erosion occurred 

at stake 8 from October 2005 to April 2006. During 

these months, 8.5 m (28 ft) of the south bank fell 

into the river.

During floods, water enters the bank and the 

hydrostatic pressure increases bank stability. When the 

river stage falls, water is expelled and this drawdown 

decreases pore pressure and the banks may fail. 

Rapid drawdown may also be caused by a rapid fall in 

discharge from the dam. 

The average rate of erosion between stakes 6 to 11 is 

nearly 6 m (20 ft) per year. In 2008, the irrigation ditch 

in figure 16 was 40 m (130 ft) from the bank, and the 

south bank was expected to reach the ditch by 2015 

(Ellis 2008). As Ellis (2008) pointed out, however, little 

confidence can be placed in these types of estimations 

because the Missouri River is a dynamic system with 

many variables, so flow and effects are difficult to 

predict. As of 24 November 2014, the Missouri River 

was within 8.2 m (27 ft) of the irrigation ditch and is 

now expected to reach the ditch by 2020 (Andy Banta, 

written communication, 24 November 2014).

Global climate change predicts a rise in precipitation 

in the northern Great Plains, which will subsequently 

increase bank saturation (Karl et al. 2009; 

Intergovernmental Panel on Climate Change [IPCC] 

2014; Melillo et al. 2014). In the Fort Union Trading 

Post National Historic Site area, the annual average 

precipitation is projected to increase by 11% by the 

end of the century (IPCC 2007). The increase would 

be from 36.5 cm (14.4 in), which is the annual average 

precipitation from 1981 to 2010, to 40.5 cm (15.9 in) 

by 2100 CE (NOAA Satellite and Information Service 

2011). In addition to changes in average conditions, 

extreme events such as intense storms, floods, and 

periodic drought will multiply and impact historic site 

resources (National Park Service 2014b). Very heavy 

precipitation in the upper Great Plains increased by 

16% from 1958 to 2012, and the frequency of extreme 

daily precipitation events is projected to rise (Melillo et 

al. 2014).

In an attempt to keep open the nearby intake of the 

Buford-Trenton Irrigation District by shifting the 

thalweg away from the south bank and toward the 

north bank, the US Army Corps of Engineers installed 

bendway weirs downstream from Fort Union Trading 

Post National Historic Site in 1998 (fig. 16; Ellis 2005). 

A bendway weir is a rock sill that is placed at a bend in 

a river. It is angled from 20° to 30° upstream, redirects 

the thalweg away from the bank, and helps control 

current velocity. The US Army Corps of Engineers also 

expected the weirs to decrease erosion in the historic 

site. Unfortunately, the weirs were located too far 

downstream to prevent bank erosion in Fort Union 

Trading Post National Historic Site (Ellis 2005). 

Because high bank saturation triggers bank failure, any 

bank protection design should include drainage control 

measures that would reduce pore-water pressure. Ellis 

(2008) recommended two sets of monitoring wells to 

be drilled to measure the hydraulic conductivity (the 

ease with which water moves through rock or sediment) 

between the irrigation ditch and the bank. In 2010, the 

NPS Water Resources Division drilled three monitoring 

wells perpendicular to the bank between stakes 7 and 8 

and two wells between stakes 10 and 11 (Inglis 2010).

Ellis (2005, 2008) recommended continued monitoring 

of bank erosion by NPS staff and documentation of 

the date and location of bank failures. Because erosion 

may threaten the irrigation ditch, Ellis (2005, 2008) also 

recommended development of a Fort Union Trading 

Post National Historic Site Missouri River management 

plan that would include bank protection measures. 

As of November 2014, a river management plan had 

not been developed for the historic site (Andy Banta, 

written communication, 24 November 2014).

River Ice

Ice break-up (“ice-out”) in the spring has the potential 

to trigger erosion of floodplain alluvium (Qal). Except 

for the wet spring of 2007, however, large amounts of 

erosion do not appear to be associated with ice breakup at Fort Union Trading Post National Historic Site 

(Ellis 2005, 2008).

Ice break-up is episodic, depending on the formation 

of ice jams. Ice jams may cause abrupt changes in river 

levels. Water accumulating behind ice jams may overtop 
20

the river banks. When the ice jams break apart, ice may 

scour the banks (Simon et al. 1999). Total ice-out occurs 

near the end of March (Ellis 2008). Ice thickness, which 

is a factor in bank erosion, is variable. In 2007, the ice 

was relatively thin, but in some years, it is several feet 

thick (Ellis 2008). In 2014, ice levels were high on the 

bank, but no significant damage occurred (Andy Banta, 

conference call, 7 October 2014). 

Flooding

Fort Peck Dam regulates the flow of the Missouri River 

so that flooding has become less of a significant issue for 

Fort Union Trading Post National Historic Site, except 

in extreme flow years such as 2011. In extremely wet 

years, the tributaries below Fort Peck Dam contribute 

significant amounts of additional water (Andy Banta, 

written communication, 12 February 2015). In 2011, 

Montana recorded its all-time wettest spring, and North 

Dakota almost set a state record. The exceptionally 

snowy winter and spring flooded the Missouri River 

floodplain (National Park Service 2011). Major flood 

stage on the Missouri River near the historic site is 7.9 

m (26 ft), and by 18 June 2011, the river had risen to 

9.29 m (30.48 ft). Floodwaters inundated the Lewis 

and Clark Trail and the small, lower parking lot, as 

well as breached the south bank, sending floodwaters 

over the floodplain alluvium (Qal) (fig. 17). The river 

stayed high until August. Flooding on the undammed 

Figure 16. Photographs showing morphological impacts of bendway weirs. Installation of bendway weirs in 1998 

resulted in the development of mid-channel bars in front of the Buford-Trenton irrigation intake. Photographs from 

Ellis (2005, figure 4.7).
21

Yellowstone River may also back up into the Missouri 

River floodplain. 

Global climate change is expected to increase the 

frequency of exceptionally wet springs and produce 

more flooding in the northern Great Plains (IPCC 

2014; Melillo et al. 2014). In addition to an increase in 

precipitation, snowmelt is expected to begin earlier 

in the spring (Karl et al. 2009; IPCC 2014; Melillo et 

al. 2014). Heightened precipitation and flooding will 

influence sedimentation rates, cutbank erosion, and 

channel bars and islands. Severe flood events may not 

only inundate the lower parking lot, but also impact 

other infrastructure, such as low-lying park housing and 

maintenance facilities (Geologic Resources Division 

2012). The fort, however, was built on a bluff (old river 

terrace), a site selected to minimize flood damage.

Current and Potential Oil and Gas 

Development

The historic site lies near the center of the extensive 

Williston Basin. The basin includes western North 

Dakota and parts of South Dakota, Montana, and 

Canada (fig. 18). The basin covers about 780,000 km2

(300,000 mi2), an area significantly larger than Texas. 

As of 2014, hydrocarbons produced from the Williston 

Basin made North Dakota the second largest oilproducing state in the country behind Texas. 

Extensive oil and gas development in the Upper 

Devonian–Lower Mississippian Bakken Formation is 

driving the current boom in the Williston Basin (fig. 

19). The formation, which is widespread in the central 

and deeper parts of the basin, consists of three informal 

members: (1) lower shale member, (2) middle sandstone 

member, and (3) upper shale member (fig. 19). The 

shale members contain abundant organic matter (as 

much as 35% by weight), and although the formation 

is only as much as 49 m (160 ft) thick, it contains a 

tremendous amount of recoverable hydrocarbons 

(Bohrer et al. 2008; Pollastro et al. 2008, 2011). The 

geographic extent of Bakken Formation organic matter 

that has thermally matured into oil includes Fort Union 

Trading Post National Historic Site (fig. 20; Pollastro et 

al. 2008, 2011).

Figure 17. Photograph of flooding at Fort Union 

Trading Post National Historic Site in 2011. Floodwaters 

inundated the Lewis and Clark Trail on the north side of 

the Missouri River. National Park Service photograph by 

Andy Banta (Fort Union Trading Post National Historic 

Site).

Figure 18. Map showing the extent of the Williston 

Basin. The Williston Basin (purple) extends from the 

Dakotas in the United States into Saskatchewan and 

Alberta in Canada. The green star indicates the location 

of Fort Union Trading Post National Historic Site. The 

graphic also shows part of the Great Plains province 

of the United States (orange) and the approximate 

southern limit of the ice age continental glaciers 

(blue line). Map graphic by Trista Thornberry-Ehrlich 

(Colorado State University), with basemap by Tom 

Patterson (National Park Service), available online: 

http://www.shadedrelief.com/physical/index.html

(accessed 5 January 2015).
22

The Bakken Formation shale units are the principal 

source rocks in the hydrocarbon reservoir known as 

the Bakken-Lodgepole Total Petroleum System (TPS). 

This continuous reservoir includes a porous sandstone 

unit in the underlying Upper Devonian Three Forks 

Formation (the Sanish sand) and porous strata in the 

overlying Lower Mississippian Lodgepole Limestone 

Formation (fig. 19). In 2008, the US Geological Survey 

estimated that the Bakken-Lodgepole TPS contained 

undiscovered, technically recoverable resources of 3.65 

billion barrels of oil (Pollastro et al. 2008). 

Improved technology in the field of hydraulic fracturing 

(fracking) has led to the energy boom in the Bakken 

Formation. Fracking involves the injection of water, 

sand, and chemicals at high pressure into horizontally 

drilled wells, which may extend as far as 3,000 m 

(10,000 ft) below the surface (fig. 21). The pressurized 

mixture opens gaps and cracks in the shale. These 

cracks are held open by the sand particles, known 

as proppant, so that hydrocarbons can flow into the 

wellbore. Recovered water is stored then transported to 

a treatment plant.

Steel casing lines a hydrocarbon well and is cemented 

in place to prevent any communication between fluids 

in the wellbore and fluids, such as groundwater, in 

adjacent strata. Thousands of feet of impermeable rock 

also separate shallow formations holding freshwater 

that may be useful for farming or public consumption 

from the fractured shale.

The Bakken Formation is not the only Paleozoic Era 

unit that contains oil and gas in the Williston Basin 

(fig. 19). In the Buford Field, approximately 3 km (2 

mi) east of the historic site, one well produces from 

the Ordovician Period Red River Formation, and nine 

wells produce from the Mississippian Period Madison 

Formation (fig. 19; Geologic Resources Division 2006). 

Approximately 2,000 wells are producing oil and gas in 

the area adjacent to Fort Union Trading Post National 

Historic Site (fig. 22). The historic site is about 40 km 

System Rock Unit

Quaternary Pleistocene and Holocene

Paleogene and 

Neogene

(Teriary)

White River, Golden Valley, and Fort Union Group

Cretaceous

Hell Creek

Fox HIlls

Pierre

Judith River

Pierre

Eagle

Pierre

Niobrara, Carlile, Belle Forche, Mowry, Newcastle, 

Skull Creek, and Inyan Kara

Jurassic Swift, Rierdon, and Piper

Triassic Spearfish

Permian Minnekahta and Opeche

Broom Creek

Pennsylvanian Amsden 

Tyler

Mississippian

Otter

Kibby

Madison Group

Charles

Mission Canyon

Lodgepole

Bakken Formation

Upper shale

Middle sandstone

Devonian

Lower shale

Three Forks

Birdbear

Duperow

Souris River

Dawson Bay

Prairie

Winnipegosis

Ashern

Silurian

Interlake

Stonewall

Ordovician

Stony Mountain

Red River

Winnipeg Group

Deadwood Cambrian

Proterozoic (Precambrian)

Figure 19. Generalized stratigraphic column for the 

Williston Basin. Geologic formations shown in green 

produce oil; yellow units produce gas. The Cretaceous, 

gas-producing Judith River and Eagle formations 

“intertongue” in part with the Pierre Shale. Source: 

North Dakota Geologic Survey available online: 

https://www.dmr.nd.gov/ndgs/Resources/ (accessed 15 

September 2014).
23

(25 mi) southwest of Williston, North Dakota, the 

cultural hub of the Williston Basin (Thornberry-Ehrlich 

2012). Gas flares from a producing well 1 km (0.5 

mi) east of the historic site are visible from within the 

historic site (Geologic Resources Division 2012). In fall 

2011, drilling operations occurred 1.6 km (1 mi) north 

of the historic site and 

within 1.6 km (1 mi) 

west of the historic 

site (ThornberryEhrlich 2012). All 

these operations are 

within North Dakota. 

The Nohly Field, 3 

km (2 mi) southwest 

of the historic site, 

has the closest oil 

production within 

Montana. Twelve 

of the wells in the 

field produce oil 

from the Madison, 

Nisku, and Red River 

formations (fig. 20). 

Another 20,000 

Bakken Formation 

exploratory wells are 

proposed, including 

a horizontal well 

adjacent to the 

historic site boundary 

(Thornberry-Ehrlich 

2012). Much more drilling is occurring farther 

to the west in Richland County, Montana (Andy 

Banta, conference call, 7 October 2014). Richland 

County produces more oil and gas than any other 

county in Montana (Montana Board of Oil and Gas 

2014). As of 7 October 2014, no hydrocarbon wells 

in Richland County had been drilled within 11 km 

(7 mi) of the historic site (North Dakota Industrial 

Commission 2014). 

Although the United States owns nearly all of the 

surface rights within the boundary of Fort Union 

Trading Post National Historic Site, it does not 

own any of the subsurface mineral rights, meaning 

oil and gas beneath the site may be targeted for 

development. Nonfederal entities, however, must 

submit plans of operations for approval to the 

National Park Service for any proposed operations 

within the historic site’s boundary. (Geologic 

Resources Division 2006, 2012). The historic site 

Foundation Document (National Park Service 2013) 

identified oil and gas development as a potential threat 

to multiple fundamental resources and values. 

Figure 20. Geographic extent of the thermally mature shale member of the Bakken 

Formation (yellow). The blue line indicates the boundary of the Bakken-Lodgepole Total 

Petroleum System (TPS). Three NPS areas in North Dakota are labelled. Yellow arrows mark 

the probable oil migration pathways in the Williston Basin. Features on this map extend into 

Canada; however, the source graphic only covers area in the United States. Graphic by Trista 

Thornberry-Ehrlich (Colorado State University) after Pollastro et al. (2011, figure 15).

Figure 21. Schematic illustration of hydraulic fracturing. 

Although fracking processes occur thousands of feet below 

potable water sources, many safety measures are used to 

protect groundwater from contamination. North Dakota State 

Water Commission diagram available online: http://www.swc.

nd.gov/4dlink9/4dcgi/getcontentpdf/pb-2419/fact%20sheet.pdf

(accessed 17 November 2014).
24

With the increased development and exploration of the 

Bakken Formation in North Dakota and Montana, the 

possibility exists that further oil and gas exploration 

and production activities will occur in the immediate 

vicinity and possibly even inside Fort Union Trading 

Post National Historic Site (Geologic Resources 

Division 2006). Consequently, the geological units on 

the surface in the historic site (Qal, Qac, Tfsb, Tftr) may 

be affected by hydrocarbon exploration depending on 

the various geophysical and drilling techniques used 

and their related infrastructure, such as well pads, 

geophysical seismic techniques, and access roads. Such 

activities can compact surface materials or generate 

dust. 

Impacts from exploration and production activities 

inside Fort Union Trading Post National Historic Site 

may be limited on account of the historic site’s size. 

Because the historic site is small, drilling and production 

need not occur in the historic site. Directional drilling 

techniques can access any hydrocarbon exploration 

targets beneath the historic site. Also, well spacing is 

assigned by the state regulatory agency, and the present 

default spacing unit for new horizontal wells in the 

Williston Basin of North Dakota is 1,280 acres, though 

spacing units of 640 acres, 

320 acres, or smaller are not 

uncommon (Great Northern 

Energy 2009).

Recommendations for 

mitigation and management 

strategies, including legal 

options, are summarized in 

Potential for the Development 

of Oil and Gas Resources in 

and Adjacent to Fort Union 

Trading Post NHS, and 

Strategies for Addressing 

Such Development (Geologic 

Resources Division 2006). 

An online GIS tool on the 

North Dakota Oil and Gas 

Division website (http://

www.dmr.nd.gov/oilgas/; 

accessed 15 June 2015) shows 

the locations and horizontal 

extents of oil and gas wells. 

The NPS Geologic Resources 

Division Energy and Minerals 

website (http://go.nps.gov/grd_energyminerals; 

accessed 15 June 2015) provides additional information.

Fort Union’s cultural landscape and viewshed are 

fundamental resources, so preserving the scenic quality 

of the Fort Union area is a high priority for Fort Union 

Trading Post National Historic Site management 

(National Park Service 2013). Activities associated 

with hydrocarbon exploration and extraction that 

have decreased the scenic quality of the surrounding 

area include increased traffic on local roads, which 

are not designed for heavy truck traffic; construction 

of well pads, RV parks, and storage areas for oilfield 

materials; and the addition of a second rail line, which 

runs parallel to Highway 1804 along the historic site’s 

northern boundary. New rail loading facilities have 

been constructed 11 km (7 mi) east of the historic site 

and about 8 km (5 mi) south of the historic site. Trucks 

transport materials, such as well pipe, sand, load oil, and 

propane that arrive by rail (Thornberry-Ehrlich 2012). 

Other adverse impacts include excessive wastewater 

demands, impacts on wildlife populations and habitats, 

and negative impacts to soil, water, and air resources. 

Long-term adverse effects on natural resources from 

these activities are unknown.

Figure 22. Oil and gas well locations in western North Dakota and eastern 

Montana as of June 2013. Map created 6 January 2015 with GRD Energy and 

Mineral Resources Interactive Map Viewer using oil and gas well data from the 

Environmental Protection Agency and DrillingInfo, Inc., published June 2013 (most 

recent data available when map produced).
25

Other activities, such as mining sand and gravel for 

oil and gas operations, may also impact the historic 

site’s viewshed. Glacial deposits of sand and gravel 

cover about 75% of North Dakota, and sand and 

gravel mining is the third largest mineral industry after 

hydrocarbons and lignite (North Dakota Geological 

Survey 2014a). However, North Dakota sand is not 

suitable as proppant for hydraulic fracturing. Frac sand 

for use as proppant, currently comes primarily from 

west-central Wisconsin (Lindquist 2012). Gravel, on 

the other hand, is plentiful, but transporting gravel is 

expensive (North Dakota Geological Survey 2014a). As 

a result, gravel is typically mined near where it will be 

used, usually for roads and well sites. 

Water is a key limiting factor for oil and gas production 

via hydraulic fracturing in North Dakota. According 

to North Dakota Department of Natural Resources 

Director Lynn Helms, drilling the expected 2,000 new 

wells per year will require 42–45 million liters (11–12 

million gallons) of freshwater per day (Helms 2013). 

Water used for hydraulic fracturing in North Dakota, 

however, accounts for only 4% of the consumptive 

water use, calculated as water input less water output 

that is recycled and reused (North Dakota State Water 

Commission 2014). By contrast, irrigation in North 

Dakota accounts for 56% of consumptive water use, 

and the average daily flow of the Missouri River at 

Bismarck is enough water to hydraulic fracture 6,497 

wells (North Dakota State Water Commission 2014). 

In addition to water used for hydraulic fracturing, 

wells in North Dakota require water for maintenance 

activities. The primary maintenance activity water is 

used for is to keep salt from building up in the wellbore, 

which restricts the flow of oil and gas. Over the life of a 

well, which is typically 30–40 years, maintenance water 

can be 25–33 million liters (6.6–8.8 million gallons) 

(Kiger 2013). The 40,000–45,000 wells in North Dakota 

will require 64–106 million liters (17–28 million gallons) 

of water per day for maintenance (Helms 2013).

Although the Missouri River contains abundant water, it 

is not available to use for fracking or well maintenance. 

The Army Corps of Engineers, contending that they 

have authority through the Flood Control Act of 1944, 

restricted access to Missouri River water. The State of 

North Dakota opposes the Corps’ policy and maintains 

that enough water flows in the Missouri to satisfy users 

(North Dakota State Water Commission 2014). 

Freshwater used in fracking is also expensive. In North 

Dakota, freshwater delivered to a well site costs between 

10 cents and 14 cents per gallon, with the total water 

cost for hydraulically fracturing a well costing as much 

as $400,000 (Allison et al. 2012). In an effort to save 

costs, many oil and gas operators are beginning to use 

recycled water in hydraulic fracturing operations. Water 

that comes from a fractured well contains a variety of 

chemicals and salts and must be taken to a licensed 

disposal facility. Recycled frac water does not meet the 

water quality standards for drinking or growing crops, 

but it can be used to hydraulically fracture additional 

wells (Allison et al. 2012). 

Oil and gas operators balance the cost of using recycled 

water with the cost of drilling injection wells to dispose 

of waste water in the subsurface. Injection wells, 

however, may also impact the viewshed and cause 

additional problems (see the “Earthquakes” section).

A concern of historic site managers is construction of a 

water depot (a distribution center for frac water) within 

sight of Fort Union Trading Post National Historic 

Site. This water depot, if constructed, may impact the 

historic site’s cultural landscape (Fred MacVaugh, 

written communication, 9 February 2015).

The oil and gas boom in the Bakken Formation has 

contributed to the unprecedented hydrocarbon 

production in the United States and has significantly 

increased worldwide energy supplies (The Economist 

2014). This increase has led to a dramatic plummet in 

oil prices (Krauss 2015; InfoMine 2015; Said et al. 2014). 

Analysts do not believe falling prices will lead to a major 

bust in drilling, at least not in the near-term (Gunderson 

2014). Some producing fields with high-quality 

reservoir rock can turn a profit at an average breakeven price of $36/barrel (Zawadski 2013; Gunderson 

2014; The Economist 2014). If a well has already been 

drilled, average operating costs range from only $10/

barrel to $20/barrel (The Economist 2014). Although 

drilling permits have declined in North Dakota and 

service companies have begun to reduce staff, drilling 

is expected to continue into the foreseeable future (The 

Economist 2014). 

Many economic factors determine whether or not 

a company decides to invest in exploration of the 

Bakken Formation. Such a discussion is beyond the 

scope of this GRI. However, some factors include well 

costs, maintaining leased acreage positions, company 
26

debt, investor confidence, transportation costs, and 

technological developments. How these changes affect 

the viewshed and infrastructure of Fort Union Trading 

Post National Historic Site remains to be seen. The NPS 

Geologic Resources Division is available to provide site 

managers with policy and technical assistance regarding 

oil and gas–development issues. 

Slope Movements

Slope movements are the downslope transfer of soil, 

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

Soil creep, rockfalls, debris flows, and avalanches are 

common types of slope movements. These processes 

and the resultant deposits are also known as “mass 

wasting” and commonly grouped as “landslides.” Slope 

movements occur on time scales ranging from seconds 

to years. 

Only minor slope movements occur within the historic 

site boundaries (Thornberry-Ehrlich 2012). The slopes 

composed of Tongue River (Tftr) and Sentinel Butte 

(Tfsb) strata at the Bodmer Overlook are the steepest 

in the historic site (see poster, in pocket). Higher buttes 

occur several kilometers north and south of the historic 

site. In 2010, a minor slump exposed artifacts and an 

American Indian burial site. Some of the items were 

returned to the tribe and some were re-buried in the 

historic site (Andy Banta, written communication, 24 

November 2014). The slump had not been recognized 

as a potential hazard prior to its occurrence. The 

area was restored using an erosion mat followed by 

reseeding. The results of mitigation are not being 

monitored on a regular basis, but further slumping 

at the burial site has not occurred (Andy Banta, 

conference call, 7 October 2014). 

Earthquakes

Seismic activity results in ground vibrations—shaking—

that may occur when rocks suddenly move along 

a fault, releasing accumulated energy and causing 

earthquakes (Braile 2009). Earthquake intensity ranges 

from imperceptible by humans to total destruction of 

developed areas and alteration of the landscape. Until 

1979, earthquakes were measured using the Richter 

magnitude scale which is based on a logarithmic 

scale from 1 to 10. Seismologists currently measure 

earthquake magnitude using the moment magnitude 

scale, which is more precise than the Richter scale 

but retains the same continuum of magnitude values. 

The modified Mercalli scale is a measure of the effect 

of an earthquake on Earth’s surface. It consists of a 

series of key responses such as people awakening, 

furniture moving, chimneys damaged, and finally, total 

destruction. 

Earthquakes can directly damage NPS infrastructure, 

or trigger other hazards, such as slope movements that 

may impact NPS resources, infrastructure, or visitor 

safety. Should an earthquake occur, ground shaking may 

displace any of the geologic units (Qal, Qac, Tfsb, Tftr) 

exposed in Fort Union Trading Post National Historic 

Site. 

Although earthquakes have been felt in North Dakota, 

they do not pose a major hazard for Fort Union 

Trading Post National Historic Site (US Geological 

Survey 2014). The probability that an earthquake of 

magnitude 5.5 will occur at Fort Union Trading Post 

National Historic Site within the next 100 years is 

between zero and 1.0% (fig. 23; US Geological Survey 

2010). In 1909, a magnitude 5.5 (Richter scale) and 

Intensity VI (modified Mercalli scale) earthquake that 

was centered in Saskatchewan broke some windows 

and knocked articles off shelves in Dickinson, North 

Dakota (Bluemle 2002; US Geological Survey 2014). 

The 7.1 magnitude earthquake near Hebgen Lake, 

Montana, was felt in extreme western North Dakota 

in 1959. In 1968, dishes rattled and the State Capitol 

Building trembled (Intensity IV) from a magnitude 4.4 

earthquake centered near Huff, North Dakota (Bluemle 

2002; US Geological Survey 2014).

Earthquakes are primarily a result of movement along 

pre-existing faults. Earthquakes originating in North 

Dakota are probably related to deeply buried structures 

in the Precambrian basement rocks that make up the 

relatively stable North American craton (see “Geologic 

History” chapter). These highly deformed Precambrian 

rocks probably contain numerous faults, but because 

they are so deeply buried, their existence and location 

are not well defined. 

Recent surface tremors in Texas, Oklahoma, Colorado, 

Arkansas, and Ohio suggest that injection wells, used 

to dispose of fluids as part of the hydraulic fracturing 

process, may cause earthquakes (US Geological 

Survey 2011). The injection of produced water very 

likely triggered the majority of recent earthquakes 

in Oklahoma (Oklahoma Geological Survey 2015). 
27

However, the association between fracking and 

tremors in North Dakota has not been documented. 

Braile (2009), the NPS Geologic Resources Division 

Seismic Monitoring website (http://go.nps.gov/seismic_

monitoring.cfm); accessed 13 May 2015), and the US 

Geological Survey Earthquakes Hazards website (http://

earthquake.usgs.gov/; accessed 13 May 2015) provide 

more information.

Abandoned Mineral Lands

According to the NPS Abandoned Mineral Lands 

(AML) database, three AML sites occur within Fort 

Union Trading Post National Historic Site (Burghardt 

et al. 2014). All three were sand and gravel quarries. 

Two have been leveled and form the lower parking lot. 

Quarry operations had dug through the base of the 

old fort, but after the historic site was established, the 

slope was re-contoured to its pre-quarry appearance 

(Geologic Resources Division 2012). 

The third site is an abandoned gravel quarry of limited 

areal extent immediately east of the reconstructed Fort 

Union (fig. 24). The 

gravel deposits are 

most likely a mix of 

glacial, fluvial, and 

colluvial sediments 

(Qgt, Qal, Qac; 

Thornberry-Ehrlich 

2012). According to 

a 2001 National Park 

Service memorandum 

by Gregory Eckert, 

the quarry was carved 

out of the historic 

riverbank and has 

the same cut and 

slope as the bank 

(Andy Banta, written 

communication, 24 

November 2014). 

No visitor trail leads 

to the quarry, and it 

is unlikely to pose 

a safety issue. The 

quarry is overgrown 

with weeds, but it 

may continue to be 

used for maintenance 

purposes in the 

future.

Paleontological Resources Inventory and 

Monitoring

All paleontological resources are non-renewable and 

subject to science-informed inventory, monitoring, 

protection, and interpretation as outlined by the 

2009 Paleontological Resources Preservation Act (see 

Appendix B). As of August 2015, Department of the 

Interior regulations associated with the Act were still 

being developed.

Tweet et al. (2011) presented a number of 

recommendations regarding the paleontological 

resources at Fort Union Trading Post National Historic 

Site, including the following:

● Investigate Paleocene Epoch rocks (Tfsb, Tftr) and 

museum collections in the historic site to identify 

fossils. 

● Document in situ occurrences of fossils with field 

inventories and monitoring any significant sites.

Figure 23. Earthquake probability map for Fort Union Trading Post National Historic Site. A 

Richter scale magnitude 5.5 earthquake has a probability of 0.00–0.01 (0%–1% “chance”) of 

occurring near the historic site in the next 100 years. Map generated using the US Geological 

Survey Probabilistic Seismic Hazards Assessment (PSHA) mapping program available online: 

http://geohazards.usgs.gov/eqprob/2009/ (accessed 12 August 2015).

501-  ̊ '03 501-  ̊ '00 401-  ̊ '03 401-  ̊ '00 301-  ̊ '03 301-  ̊ '00 201-  ̊ '03 201-  ̊ '00 

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GMT 2015 Aug 12 15:22:07 EQ probabilities from USGS OFR 08-1128 PSHA. 50 km maximum horizontal distance. Site of interest: triangle. Fault traces are brown; rivers blue. Epicenters M>=5.0 circles.

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N O R T H D A K O T A

C A N A D A

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48 ̊ 00'

48 ̊ 30'

49 ̊ 00'

501-  ̊ '03 501-  ̊ '00 401-  ̊ '03 401-  ̊ '00 301-  ̊ '03 301-  ̊ '00 201-  ̊ '03 201-  ̊ '00 

74  ̊ '00 

47 ̊ 30'

48 ̊ 00'

48 ̊ 30'

49 ̊ 00'

501-  ̊ '03 501-  ̊ '00 401-  ̊ '03 401-  ̊ '00 301-  ̊ '03 301-  ̊ '00 201-  ̊ '03 201-  ̊ '00 

74  ̊ '00 

47 ̊ 30'

48 ̊ 00'

48 ̊ 30'

49 ̊ 00'

0.00

0.01

0.02

0.03

0.04

0.06

0.08

0.10

0.12

0.15

0.20

0.25

0.30

0.40

0.50

0.60

0.80

0.90

1.00

U.S. Geological Survey 2009 PSHA Model Site: WILLISTON ND . .

Probability

Williston
28

● Encourage historic site staff to observe, photodocument, and monitor any occurrence of fossils, 

while conducting their usual duties. Unless the 

fossils are subject to degradation by natural 

processes or human activities, they should be left in 

situ. Staff should also be encouraged to participate 

in any paleontological field studies with trained 

paleontologists.

● Document any fossil found in a cultural context and 

requesting the input of an archeologist.

● Request a trained paleontologist to be on site 

for any future archeological excavations or 

infrastructure developments.

The NPS Geologic Resources Division is available 

for assistance with any of these recommendations. 

In addition, the Geological Monitoring chapter about 

paleontological resources written by Santucci et al. 

(2009) described five methods and vital signs for 

monitoring in situ paleontological resources: (1) erosion 

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

catastrophic geohazards, (4) hydrology/ bathymetry, 

and (5) human access/public use.

Unless collected or stabilized, fossils exposed on the 

surface may be destroyed by erosion. Vital sign 1 focuses 

on geologic characteristics, such as lithology, bedding, 

slope of strata, and geochemistry of the rock unit that 

influence erosion rates. Vital sign 2 relates to external 

climatic factors, such as precipitation, temperature, and 

freeze-thaw processes (Santucci et al. 2009).

Geohazards (vital sign 3) consist of a variety of geologic 

processes and hazards that may threaten the stability of 

fossil sites. Some of these geohazards include local or 

regional volcanism, earthquakes, geothermal activity, 

glacial activity, and slope movements (Santucci et 

al. 2009). These potential catastrophic geohazards 

may pose a limited threat to Fort Union Trading Post 

National Historic Site.

Vital sign 4 pertains to fossil sites associated with areas 

submerged by or adjacent to water resources, such as 

the Missouri River in the case of Fort Union Trading 

Post National Historic Site. The stability of any fossil site 

associated with the Missouri River bank will depend 

on such hydrologic factors as fluctuating water levels, 

precipitation, bank saturation, and ice jams. Hydrologic 

processes may influence both sedimentation and 

erosion rates, which may impact the exposure and 

stability of fossil sites (Santucci et al. 2009).

Vital sign 5 focuses on human-related threats. Whether 

intentional or unintentional, human activities can 

threaten the stability of fossils sites. Santucci et al. 

(2009) offer procedures to monitor public use and 

prevent human impacts to in situ fossils.

Not all of these vital signs may be appropriate for Fort 

Union Trading Post National Historic Site. Resource 

managers may select from these vital signs and develop 

a monitoring program specific to the needs of the 

historic site.

Figure 24. Photographs of the small gravel quarry just east of the reconstructed trade post at Fort Union Trading Post 

National Historic Site. The left image shows the view from east to west across the quarry. The right view shows the 

view from west to east. National Park Service photographs by Andy Banta (Fort Union Trading Post National Historic 

Site).
29

Geologic History

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

Union Trading Post National Historic Site.

The landscape at Fort Union Trading Post National 

Historic Site is geologically very young (fig. 2). Most of 

the units were deposited between 70,000 and 12,000 

years ago, either during the last phase of Pleistocene 

glaciation (ice age) or after the last continental glacier 

retreated from North Dakota and Montana. But the 

historic site also rests in the center of the Williston 

Basin, and within the basin, the geologic history 

extends back in time to the formative years of the North 

American craton. Precambrian deformation influenced 

the evolution and sedimentation of the Williston Basin, 

which consists primarily of Paleozoic Era carbonate 

rocks and clastic sedimentary rocks deposited in the 

Mesozoic and Cenozoic Eras. The early history is briefly 

described below, followed by a description of the postglacial history that led to the current landscape.

Prior to 541 Million Years Ago (Precambrian 

Time): Assembling the North American 

Craton

A craton is an old, geologically stable interior of a 

continent. In the Precambrian, the Superior craton, 

which included present-day eastern North Dakota, 

collided with the Wyoming craton to the west, suturing 

the two cratons together to form the fledgling North 

American craton (fig. 25). The collision deformed a 

north–south trending region known as the “TransHudson orogenic belt,” which included what would 

become the Williston Basin and the present-day 

landscape of Fort Union Trading Post National Historic 

Site. A fault and lineament zone that was part of the 

Trans-Hudson Orogeny (mountain-building episode) 

formed the northeast–southwest-trending spine of 

the craton and was later singled-out and renamed 

the Transcontinental Arch (Anna et al. 2013). Today’s 

Williston Basin was established during the TransHudson Orogeny when Precambrian deformation 

formed normal faults along which the basin dropped 

down (Burrett and Berry 2000; Anna et al. 2013).

Precambrian faults were later reactivated by further 

tectonic activity to form buried north–south and 

northwest–southeast structures in the Williston Basin. 

These zones of weakness would become the Nesson, 

Cedar Creek, Little Knife, and Billings anticlines, 

which are convex, upside-down canoe-shaped folds 

(Gerhard and Anderson 1988; Kent and Christopher 

2008; Anderson 2009; Anna et al. 2013). These 

subsurface structures formed east and south of the 

current Fort Union Trading Post National Historic 

Site. During the Laramide Orogeny, approximately 70 

million to 5 million years ago (Late Cretaceous–Early 

Paleogene periods; fig. 2), reactivation of Precambrian 

faults produced additional folding and faulting. These 

structures would later become traps for oil and gas.

From 541 Million to 252 Million Years Ago 

(Paleozoic Era): Sediments Fill the Williston 

Basin 

By the end of the Precambrian, present-day North 

Dakota and eastern Montana lay far inland from the 

continental margin (fig. 26). In the Early Cambrian 

Period, however, sea level rose and drowned the 

present-day Fort Union region (fig. 26). Cambrian 

and Early Ordovician (approximately 541 million–471 

million years ago) sandstones and shale eroded from 

the Transcontinental Arch were deposited in near-shore 

marine environments. As sediment accumulated, the 

Williston Basin began to subside.

Sea level fluctuated throughout the Paleozoic Era, but 

near-shore and open marine environments persisted 

in the Williston Basin. During transgressions, when sea 

level rose, marine limestone was deposited in the basin, 

and as sea level fell (regression), calcareous mudstones, 

dolomite, and evaporates accumulated in subtidal to 

supratidal environments. Although the Williston Basin 

contains a relatively complete Paleozoic section, sea 

level regressions exposed some of the strata to subaerial 

erosion. 

One of the most extensive incursions of Paleozoic seas 

occurred in the Late Devonian–Early Mississippian, 

from approximately 385 million to 345 million years 

ago (fig. 26). The organic-rich marine shale of the 

Bakken Formation records deposition in a relatively 

deep, offshore marine environment. Since about 541 

million years ago (Cambrian Period; fig. 2), carbonate 

environments had dominated this area of western 

North America. The Bakken Formation documents 
30

the first input of clastic material into the Williston 

Basin following hundreds of millions of years of 

carbonate deposition (LeFever 1991; Anna et al. 

2013). The abundant organic matter that accumulated 

along with the fine-grained sediment of the Bakken 

Formation would eventually mature and become the 

source of oil that is produced from the Mississippian 

Period reservoirs in the Williston Basin (Gerhard 

and Anderson 1988; Anna et al. 2013; North Dakota 

Geological Survey 2014b).

By the Pennsylvanian Period, which extends from 

318 million years to 299 million years ago, presentday North Dakota and eastern Montana were once 

again inland of the continental margin. Throughout 

the Paleozoic Era (541 million–252 million years ago), 

continents collided and fused together to form the 

proto–North American continent. Along the eastern 

continental margin, several orogenies combined to close 

the Iapetus (pre-Atlantic) Ocean, build the Appalachian 

Mountains, and form the supercontinent Pangaea (fig. 

26; Hoffman 1997; Bradley 1997). By about 252 million 

years ago (Triassic Period; fig. 2), South America and 

Africa had sutured onto the North American continent. 

From 252 Million to 66 Million Years Ago 

(Mesozoic Era): North Dakota, Montana, and 

an Inland Sea

Clastic deposition in the Williston Basin continued in 

the Triassic and Early Jurassic periods. In the Triassic 

Period, the supercontinent Pangaea began to split apart, 

opening the Atlantic Ocean and the Gulf of Mexico. 

Continued regression led to subaerial exposure and 

subsequent erosion of Permian and older units (Anna et 

al. 2013). 

ATLANTIC

OCEAN

PACIFIC

OCEAN

NEW 

DENVER YORK

Archean

Churchill collision

Magmatic arc

Passive margin

Yavapai collision

Keweenawan rift

Grenville collision

Paleozoic orogens

Cordillera

Ouachitas

Appalachians

Cordillera

Yavapai Mazatzal

1.8–1.6

Grenville 1.4–1.0

Superior >2.5 Wyoming

Hearne

Rae

Slave

Trans-Hudson

Penokean

Taltson

Foxe

Iapetus

Tholon

2.0–1.8

1.9–1.8

1.8–1.6

2.0–1.8

Minnesota

 River 

 Valley

Figure 25. Map of land masses sutured together to form North America. The Trans-Hudson orogenic belt, which 

initiated the formation of the Williston Basin, formed when the Superior craton collided with the Wyoming craton. 

The green star shows the location of Fort Union Trading Post National Historic Site. The numbers represent isotopic 

ages in billions of years ago. Graphic by Trista Thornberry-Ehrlich (Colorado State University) using information from a 

US Geological Survey figure available online:http://commons.wikimedia.org/wiki/

File:North_america_basement_rocks.png (accessed 23 September 2014). Basemap by Ron Blakey (Colorado Plateau 

Geosystems) available online: http://cpgeosystems.com/ (accessed 5 January 2015).
31

North

American

craton

500 million years ago

Iapetus

Ocean Transcontinental 

E

arch 

q

uator

550 million years ago

360 million years ago 260 million years ago

Transcontinental arch 

North

American

craton Williston

Basin

Eq

uator

North Dakota was eroding at the end of the Proterozoic; Williston Basin accumulated sediments during the Cambrian

North 

America

Oceanic

Plate

North 

America

Oceanic

Plate

Africa

South

America

Sediments were still being deposited in the Williston Basin at the end of the Devonian; North America collided with an

oceanic plate and the supercontinent Pangaea formed at the end of the Paleozoic

Appalachian

Mountains

Figure 26. Paleozoic paleogeographic maps of North America. Approximately 550 million years ago (upper left), 

present-day North Dakota lay far inland from the craton’s margin. The darker brown color represents uplands. 

Present-day Fort Union Trading Post National Historic Site (star) was west of the northeast–southwest-oriented 

Transcontinental Arch, which formed the backbone of the North American craton. Approximately 500 million years 

ago (upper right), shallow inland seas had inundated all but the Transcontinental Arch in the southern part of the 

North American craton. Approximately 360 million years ago (lower left), active subduction along the margins of 

proto-North America caused another major incursion of the sea onto the continent. Approximately 260 million 

years ago (lower right), South America and Africa were colliding with proto-North America as the supercontinent 

Pangaea formed in the late Paleozoic Era and Early Triassic Period. Light blue indicates shallow marine areas; darker 

blue represents deeper marine areas. Graphic compiled by Trista Thornberry-Ehrlich (Colorado State University) with 

annotations by the author. Base paleogeographic maps by Ron Blakey (Colorado Plateau Geosystems, Inc.) available 

online: http://cpgeosystems.com/index.html (accessed 23 July 2014).
32

Throughout the Paleozoic and Mesozoic eras, 

subduction of the oceanic plate beneath the North 

American tectonic plate constructed mountains along 

the west coast, and seas variously transgressed onto, and 

regressed from, the continent’s interior (Brenner and 

Peterson 1994). In the Middle Jurassic Period, an inland 

sea spread from the north into North Dakota, Montana, 

and Wyoming (fig. 27). Normal marine limestone and 

shallow marine and marginal marine shale, siltstone, 

and evaporates were deposited in the Williston Basin. 

Plate collision and subduction along the western 

margin of North America stacked thrust sheets atop 

one another, causing the crust adjacent to this thrust 

belt to subside. With subsidence, sea water again filled 

the basin, although on a much grander scale. Episodic 

fluctuations in sea level occurred throughout the 

Cretaceous Period, culminating in the formation of the 

most extensive interior seaway ever to bisect the North 

American continent (fig. 27). The Western Interior 

Seaway extended from today’s Gulf of Mexico to the 

Arctic Ocean, a distance of about 4,800 km (3,000 mi; 

Kauffman 1977; Steidtmann 1993). During periods of 

maximum sea-level rise, the width of the basin reached 

1,600 km (1,000 mi) and inundated the entire state of 

North Dakota. 

The shallow marine sands and muds of the Western 

Interior Seaway became the Pierre Shale, which is about 

210 m (700 ft) thick at its type section near Pierre, South 

Dakota. The Pierre Shale contains abundant marine 

fossils, such as cephalopods and other mollusks, corals, 

sharks, bony fish, and large marine reptiles, including 

mosasaurs and plesiosaurs (Hartman and Kirkland 

2002; Hoganson and Murphy 2003). The Pierre Shale 

now lies about 500 m (1,700 ft) beneath Fort Union 

Trading Post National Historic Site, and it becomes 

progressively shallower to the west (Bergantino and 

Wilde 1998; Vuke et al. 2003).

When the Western Interior Seaway began to retreat, 

about 68 million years ago, rivers flowing from the west 

deposited sandstone, siltstone, and mudstone into large 

deltas (Murphy et al. 1999; Wilf et al. 2003). In western 

North Dakota, the deltaic wedge of sediments is as 

much as 129 m (423 ft) thick (Butler and Hartman 1999; 

Hartman et al. 2014).

Volcanic activity in Montana and Wyoming at the end 

of the Cretaceous Period contributed volcanic material 

to the deposits that would become the Hell Creek 

Formation (Butler and Hartman 1999; Hartman et 

al., 2014). The Hell Creek Formation includes famous 

dinosaur fossils, such as Triceratops and Tyrannosaurus

rex (Hoganson and Murphy 2003; Liggett 2014). 

Research into the plants and animals found in the Hell 

Creek Formation of Montana and North Dakota has 

helped define the ecosystem of the region at the end 

of the Cretaceous Period. Plants in the Hell Creek 

Formation are dominated by large-leafed angiosperms, 

which were adapted to a warm, wet climate. Freshwater 

sharks, bony fish-like gar, turtles, crocodiles, and 

amphibians inhabited coastal environments (Johnson 

2002; Nichols 2007; Liggett 2014).

A catastrophic extinction event marks the end of the 

Cretaceous Period. The Hell Creek Formation records 

the final days of the dinosaurs and many other animals 

and plants that went extinct about 66 million years 

ago. In the Fort Union Trading Post National Historic 

Site region, vegetation in the Hell Creek Formation 

represents an angiosperm-dominated woodland 

environment consisting of small- to medium-sized trees 

(Johnson 2002). Fossils found in the Cretaceous portion 

of the Fort Union Formation disappear immediately 

above the Cretaceous–Paleogene (K–P) boundary (Wilf 

et al. 2003). The catastrophic event eliminated the entire 

dominant megaflora in the upper Hell Creek Formation 

and 70%–90% of Cretaceous macrofossil species 

(Johnson 2002; Wilf et al. 2003; Nichols 2007). 

An abrupt change in global temperature also occurred 

at the K–P boundary. Studies of terrestrial plant fossils 

and planktonic marine foraminifera indicate that 

although climate warmed during the final 300,000 to 

500,000 years of the Cretaceous Period, reaching a 

maximum temperature between 65.8 million and 65.6 

million years ago, global climate cooled about 65.5 

million years ago, and this cooling trend continued 

across the K–P boundary (Johnson 2002; Wilf et al. 

2003). 

From 66 Million to 2 Million Years Ago 

(Cenozoic Era): Paleocene Swamps and 

Floodplains and Climate Change

Paleocene megaflora is less diverse and less 

heterogeneous than Cretaceous megaflora and is 

dominated by species that were common in Cretaceous 

bogs (Johnson 2002; Wilf et al. 2003; Bercovici et al. 

2008). The Cannonball Member of the Fort Union 

Formation, the remnant deposits of the last interior 
33

North

American

plate

Western Interior Seaway

CHICAGO NEW 

YORK

DENVER

Laurentide ice sheet

Cordilleran ice sheet

ATLANTIC

OCEAN

PACIFIC

OCEAN

126,000 years ago Rocky Mountains

360 million years ago 260 million years ago

150 million years ago 85 million years ago

60 million years ago

The supercontinent rifted apart; a transcontinental sea flooded the continent in the Late Cretaceous

Africa

South

America

North 

America

Subduction

zones

Kula

Plate

Farallon

Plate

North 

America

Laramide Orogeny built the Rocky Mountains; sea level fell during the Pleistocene glaciations

Figure 27. Mesozoic and Cenozoic paleogeographic maps of North America. Approximately 150 million years ago 

(upper left), an inland sea had spread into present-day North Dakota. Approximately 85 million years ago (upper 

right), the extensive Western Interior Seaway connected the Arctic Ocean with the Gulf of Mexico. Deltas, upon which 

the dinosaurs roamed, formed in western North Dakota, depositing sediments that would become the Hell Creek 

Formation. Approximately 60 million years ago (lower left), the Tongue River (Tftr) and Sentinel Butte (Tsb) formations 

formed on floodplains in the region of the present-day historical site. Approximately 126,000 years ago (lower right), 

continental glaciers flowed into North Dakota from the north. This was one of several episodes of glacial advance. 

The star represents the approximate location of present-day Fort Union Trading Post National Historic Site. Graphic by 

Trista Thornberry-Ehrlich (Colorado State University) with annotations by the author. Base paleogeographic maps by 

Ron Blakey (Colorado Plateau Geosystems, Inc.) available online: http://cpgeosystems.com/index.html (accessed 23 July 

2014).
34

sea, retreated from North Dakota and Montana about 

60 million years ago. In its wake, it left about 120–150 

m (400–500 ft) of sandstone and mudstone containing 

marine fossils (also called the Cannonball Formation 

in parts of North Dakota) (Hoganson and Murphy 

2003; Hartman 2004, 2015). Fossils in the Cannonball 

Formation represent life forms that are different from 

those that lived in the Cretaceous seas. The Cannonball 

Formation does not include large marine reptiles or 

ammonites, for example, and includes many different 

species of sharks, fish, and invertebrates.

The Paleocene Fort Union Formation (map units Tfsb, 

Tftr) is the oldest rock formation exposed in Fort 

Union Trading Post National Historic Site. Similar 

to the continental part of the Hell Creek Formation, 

the sandstone, siltstone, mudstone, and lignite beds 

of the Sentinel Butte and Tongue River (Bullion 

Creek) members represent deposition in channels, on 

floodplains and in lakes and swamps (fig. 27; Daly et 

al. 1985; Hoganson and Murphy 2003). Volcanic ash, 

drifting in from the west, altered to bentonite (swelling 

clay) in some areas. Peat accumulated in swampy areas 

and was eventually transformed into North Dakota’s 

vast lignite reserves.

According to Hartman (2004), mollusk diversity 

remained relatively low until the final regression of 

the Cannonball Sea in the upper part of the Paleocene 

Epoch, so that the classic terrestrial mollusk fauna, 

much of which was collected at exposures along the 

Upper Missouri River and near Fort Union Trading Post 

National Historic Site, is confined to strata overlying 

the marine Cannonball Member of the Fort Union 

Formation. Fauna in the Tongue River (Bullion Creek) 

and lower part of the Sentinel Butte members thus 

represents an upper Paleocene interval of time between 

about 61 million and 58 million years ago (Hartman 

2004; Kihm et al. 2004; Hartman 2015). The results of 

these investigations suggest that the duration of Tongue 

River deposition was much shorter than previously 

interpreted (Hartman and Kihm 1995; Hartman 2004). 

Fossils of alligators, crocodiles, lizards, and other 

reptiles, along with subtropical plants such as magnolia, 

bald cypress, and palm indicate that the region had a 

humid, subtropical environment during the Paleocene 

Epoch, similar to today’s climate in southern Florida 

(Hoganson and Murphy 2003). A distinct period of 

global warming, known at the Paleocene–Eocene 

Thermal Maximum (PETM) occurred 55 million years 

ago. The Sentinel Butte Member of the Fort Union 

Formation in the Williston Basin of western North 

Dakota spans the Paleocene-Eocene boundary, and 

vegetation in the unit records the PETM (Harrington et 

al. 2005). 

About 50 million years ago, the climate became 

cooler and drier. Plant and animal fossils from parks 

throughout the National Park System document this 

transition from a warm, “greenhouse” environment to 

a cooler, “icehouse” environment (fig. 28). Between 50 

million and 20 million years ago, silica-rich groundwater 

interacted with peat to form the Knife River Flint. By 

25 million years ago, the North Dakota climate was 

arid and cool. The landscape consisted of extensive 

grasslands with trees growing only along rivers 

(Hoganson and Murphy 2003).

From 2 Million to 12 Thousand Years Ago 

(Cenozoic Era): Pleistocene Deep Freeze

The continental glaciers that flowed into North Dakota 

and Montana beginning about 1.6 million years ago 

smoothed the landscape into rolling hills and scraped 

away whatever sediment had been deposited above the 

Fort Union Formation in the Fort Union area. Before 

glaciers blocked their path, most rivers in North Dakota 

flowed to the north on their way to Hudson Bay. The 

glaciers diverted the rivers, including the Missouri 

River, to the east and south (Hoganson and Murphy 

2003). Many glaciers advanced into present-day North 

Dakota and Montana during the early Pleistocene, but 

subsequent erosion has removed most evidence of these 

advances from the Fort Union area except for remnants 

of till (Qgt) and large boulders, known as glacial 

erratics, deposited when the glaciers melted.

The most recent major glacial advance into what is now 

North Dakota occurred between about 70,000 to 12,000 

years ago and is known as the Wisconsinan ice age (fig. 

27). Two or three glacial events occurred during the 

Wisconsinan (Hoganson and Murphy 2003). A pulse 

of glacial ice flowed into North Dakota about 40,000 

years ago, and a second event occurred about 25,000 

years ago. Ice ultimately withdrew from North Dakota 

for good about 12,000 years ago. The unconsolidated, 

heterogeneous mixture of clay, silt, sand, gravel, 

cobbles, and boulders (Qgt) mapped on the bluffs south 

of the historic site are remnants of the most recent 

glacial advance into the Fort Union area.
35

When the last continental glacier was at its maximum 

extent, about 14,000–12,000 years ago, spruce and 

deciduous forests grew as far south as the southern 

Great Plains. When the climate warmed and the glaciers 

melted, the trees retreated northward, leaving behind 

the vast grassland of the northern Great Plains. 

Ice dammed the Missouri River at least twice, causing 

glacial lakes to form. Glacial Lake McKenzie filled the 

Missouri River valley from the border of present-day 

South Dakota to Riverdale, North Dakota, in Mclean 

County (Hoganson and Murphy 2003). Remnants of 

Lake McKenzie are preserved in Knife River Indian 

Villages National Historic Site, approximately 220 km 

(135 mi) southeast of Fort Union Trading Post National 

Historic Site, where the McKenzie Terrace (Qtm) marks 

Relative Global Climate

Cooler/More Ice Warmer/Less Ice

S. Hemisphere Ice Sheet

N. Hemisphere Ice Sheet

60 50 40 30 20 10 0

Millions of Years Ago

Pliocene

Pleistocene

Today

Paleocene Eocene Oligocene Miocene

Tertiary

Paleogene Neogene

Quat.

Cenozoic

GREENHOUSE ICEHOUSE

Fossil

Butte

Knife River

Indian Villages

Fort Union

Trading Post

Tule Springs 

Waco Mammoth

Florissant

Agate

Hagerman

Badlands

T. Roosevelt John Day

Oxygen Isotope Ratio

5

4

3

2

1

0

-1

-2

Figure 28. Illustration of relative global climate in the Cenozoic Era. During the Paleogene and Neogene periods, 

global climate fluctuated, influencing landscape evolution. The red line was plotted using ocean temperature data 

from Zachos et al. (2001, 2008). The transition from global “greenhouse” conditions with minimal polar ice sheets 

to “icehouse” conditions with ice sheets at one or both poles occurred near the Eocene–Oligocene boundary. The 

yellow dots and horizontal bars indicate the geologic ages or ranges of ages of eight NPS units established to preserve 

scientifically Cenozoic Era fossils and strata: Fossil Butte National Monument in Wyoming, Florissant Fossil Beds 

National Monument in Colorado, Agate Fossil Beds National Monument in Nebraska, Hagerman Fossil Beds National 

Monument in Idaho, Badlands National Park in South Dakota, John Day Fossil Beds National Monument in Oregon, 

Tule Springs Fossil Beds National Monument in Nevada, and Waco Mammoth National Monument in Texas. The orange 

bars indicate the time spanned by Paleogene deposits in the North Dakota parks (Knife River Indian Villages and Fort 

Union Trading Post National Historic Sites and Theodore Roosevelt National Park). Holocene (post-glacial) deposits are 

not included. National Park Service graphic by Jason Kenworthy (NPS Geologic Resources Division) after Kenworthy 

(2010, figure 5.3).
36

the former shoreline. The undifferentiated Tongue 

River–Sentinel Butte (Bullion Creek) Members can 

be traced to the Knife River Indian Villages National 

Historic Site area, where similar Fort Union, it is the 

oldest unit in that historic site (Reiten 1983; see GRI 

report by Graham 2015 [in review]).

Warming produced vast amounts of meltwater, and 

the torrents of freshwater eroded many channels into 

Paleocene bedrock. Rivers cut downward through 

their floodplains, leaving them high and dry to become 

the bluffs visible from today’s Fort Union Trading Post 

National Historic Site. These processes also created 

the Pleistocene terraces mapped in Knife River Indian 

Villages National Historic Site (Reiten 1983; see GRI 

report by Graham 2015 [in review]). Alluvial terrace 

deposits (Qat) in the Fort Union Trading Post National 

Historic Site range in age from Pleistocene to Holocene; 

glacial meltwater may have formed some of them.

To the west, melting alpine glaciers fed rivers like 

the Yellowstone with vast amounts of meltwater and 

debris. Carving a northeasterly channel through eastern 

Montana, the Yellowstone River met the Missouri River 

channel just south of Fort Union Trading Post. 

The Past 12 Thousand Years (Cenozoic Era): 

Holocene Thaw

Alluvial and aeolian deposition and terrace 

development have been significantly influenced by 

climatic fluctuations since the end of the ice ages (table 

2). When climate was cool and wet, precipitation 

increased vegetation density, which stabilized hillslopes 

and decreased the amount of sediment eroded into 

rivers. As a consequence, the Missouri and Yellowstone 

rivers incised vertically into their floodplains. Increased 

precipitation promoted soil development, and increased 

vegetation stabilized sand dunes (Reiten 1983).

In contrast, when the climate was warm and dry, 

vegetation density decreased and hillslopes became less 

stable. Soil development slowed and more sediment 

was eroded from hillslopes by slope wash and aeolian 

activity (Reiten 1983). In the geologic record, grass and 

prairie paleosols replaced forest paleosols about 10,000 

years before present (BP) and the climate became 

warmer after 8,500 years BP (Reiten 1983).

Drier conditions occurred about 2,500 years BP. These 

climatic changes are recorded in the Holocene terraces 

in Knife River Indian Villages National Historic Site 

(Reiten 1983); similar evidence may be found in the 

area of Fort Union Trading Post National Historic 

Site. About 500 years ago, the Hidatsa and Mandan 

began building earthlodges on the terraces overlooking 

the Knife and Missouri rivers. In the 19th century, a 

profitable fur trade between American Indian tribes and 

Euro-American traders resulted in the construction of 

Fort Union Trading Post. Although its flow is currently 

regulated by Fort Peck Dam, the Missouri River 

continues to meander past Fort Union Trading Post 

National Historic Site, as it did in the 1800s, reworking 

older sediments (Qat, Qac) and depositing new alluvium 

(Qal) in point bars and floodplains.

Table 2. Summary of Holocene (post-glacial) climate in the Fort Union region. 

Age in Years 

Before Present

Climate Landscape

500–0 Fluctuations from cool and 

moist to warm and dry

Alternating between valley filling and valley incision. Paleosols formed and current soil 

developed.

2,500–500 Warm, dry Valley filling. Vegetation density decreased; hillslopes became less stable. Minor episodes of 

paleosol formation.

4,500–2,500 Cool, moist Incision of valley fill and deposition of fine-grained overbank sediment on floodplains. 

Vegetation density increased; hillslopes became more stable. Several paleosols formed. 

Floodplain forest fires occurred.

8,500–4,500 Warm, dry Valley filling. Vegetation density decreased; hillslopes became less stable. Active aeolian 

sand dunes occurred in the area of Knife River Indian Villages National Historic Site and 

possibly Fort Union Trading Post National Historic Site.

13,000–8,500 Cool, moist Vegetation density increased; hillslopes became more stable. A forest paleosol formed and 

was replaced by prairie grasses and soils about 10,000 BP.

Data and interpretations are from Reiten (1983).
37

Geologic Map Data

This chapter summarizes the geologic map data available for Fort Union Trading Post National 

Historic Site. A poster (in pocket) displays the map data draped over imagery of the historic site and 

surrounding area. The Map Unit Properties Table (in pocket) summarizes this report’s content for each 

geologic map unit. Complete GIS data are available at the GRI publications website: http://go.nps.gov/

gripubs.

Geologic Maps

Geologic maps facilitate an understanding of an area’s 

geologic framework and the evolution of its present 

landscape. Using designated colors and symbols, 

these maps portray the spatial distribution and 

temporal relationships of rocks and unconsolidated 

deposits. Geologic maps can be divided into two 

primary types: surficial and bedrock. Surficial 

geologic maps typically encompass deposits that are 

unconsolidated and formed during the past 2.6 million 

years (the Quaternary Period). Surficial map units 

are differentiated by geologic process or depositional 

environment. Bedrock geologic maps encompass 

older, typically more consolidated sedimentary, 

metamorphic, and/or igneous rocks. Bedrock map units 

are differentiated based on age and/or rock type.

Geologic maps often depict geomorphic features, 

structural interpretations (such as faults or folds), 

and locations of past geologic hazards that may be 

susceptible to future activity. Anthropogenic features 

such as mines or quarries, as well as observation or 

collection locations, may be indicated on geologic 

maps. The American Geosciences Institute website, 

http://www.americangeosciences.org/environment/

publications/mapping, provides more information 

about geologic maps and their uses. 

Source Maps

The GRI team digitizes paper maps and converts digital 

data to conform to the GRI GIS data model. The GRI 

digital geologic map product includes essential elements 

of the source maps such as map unit descriptions, a 

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

references, and figures. The GRI team used the 

following sources to produce the digital geologic data 

set for Fort Union Trading Post National Historic Site. 

These sources also provided information for this report.

Bergantino, R. N., and E. M. Wilde. 1998 (revised 2007). 

Geologic map of the Culbertson 30’ × 60’ quadrangle 

(bedrock emphasis), northeastern Montana (scale 

1:100,000). Open-File Report 359. Montana Bureau of 

Mines and Geology, Butte, Montana.

Vuke, S. M., E. M. Wilde, and L. N. Smith. 2003 (revised 

2011). Geologic and structure contour map of the Sidney 

30’ × 60’ quadrangle, eastern Montana and adjacent North 

Dakota (scale 1:100,000). Open-File Report 478. Montana 

Bureau of Mines and Geology, Butte, Montana.

GRI GIS Data

The GRI team implements a GIS data model that 

standardizes map deliverables. The data model is 

available at: http://go.nps.gov/gridatamodel. This data 

model dictates GIS data structure, including layer 

architecture, feature attribution, and relationships 

within ESRI ArcGIS software. The GRI team digitized 

the data for Fort Union Trading Post National Historic 

Site using data model version 2.1. The GRI Geologic 

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

more information about GRI map products. 

GRI digital geologic data are available through the NPS 

Integrated Resource Management Applications (IRMA) 

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

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

list.

The following components are part of the data set:

● A GIS readme file (fous_gis_readme.pdf) 

that describes the GRI data formats, naming 

conventions, extraction instructions, use 

constraints, and contact information;

● Data in ESRI geodatabase GIS format;

● Layer files with feature symbology (table 3);

● Federal Geographic Data Committee (FGDC)–

compliant metadata;

● An ancillary map information document (fous_

geology.pdf) that contains information captured 
38

from source maps such as map unit descriptions, 

geologic unit correlation tables, legends, cross 

sections, and figures;

● An ESRI map document (fous_geology.mxd) that 

displays the digital geologic data; and

● A KML/KMZ version of the data viewable in 

Google Earth (table 3)

Table 3. GRI GIS data for Fort Union Trading Post National 

Historic Site

Data Layer On Poster? Google Earth 

Layer?

Geologic Attitude 

and Observation 

Points

No No

Geologic Contacts No Yes

Geologic Units Yes Yes

Geologic structure 

contour lines (top of 

Pierre Shale)

No Yes

GRI Map Poster

A poster of the GRI digital geologic data draped over a 

shaded relief image of the historic site and surrounding 

area is included with this report. Not all GIS feature 

classes are included on the poster (table 3). Geographic 

information and selected historic site features have been 

added to the poster. Digital elevation data and added 

geographic information are not included in the GRI 

GIS data set, but are available online from a variety of 

sources. Contact GRI for assistance locating these data.

Map Unit Properties Table

The Map Unit Properties Table lists the geologic 

time division, symbol, and a simplified description 

for each of the geologic map units in the GRI GIS 

data. Following the structure of the report, the table 

summarizes the geologic features, processes, resource 

management issues, and history associated with each 

map unit. 

 Use Constraints

Graphic and written information provided in this 

report is not a substitute for site-specific investigations. 

Ground-disturbing activities should neither be 

permitted nor denied based upon the information 

provided here. Please contact GRI with any questions.

Minor inaccuracies may exist regarding the locations 

of geologic features relative to other geologic or 

geographic features on the poster. Based on the source 

map scale (1:100,000) and US National Map Accuracy 

Standards, geologic features represented in the geologic 

map data are horizontally within 51 m (167 ft) of their 

true locations.
39

Glossary

These are brief definitions of selected geologic terms relevant to this report. Definitions are based on 

those in the American Geosciences Institute Glossary of Geology (5th edition; 2005). Additional terms 

are defined at http://geomaps.wr.usgs.gov/parks/misc/glossarya.html.

ablation. All processes by which snow and ice are lost from 

a glacier, including melting, evaporation (sublimation), 

wind erosion, and calving.

aeolian. Describes materials formed, eroded, or deposited 

by or related to the action of wind. 

alluvial terrace. A stream terrace composed of 

unconsolidated alluvium produced by a rejuvenated 

stream via renewed downcutting of the floodplain or 

valley floor, or by the covering of a terrace with alluvium.

alluvium. Stream-deposited sediment.

alpine glacier. A small glacier in a mountain range (i.e., not 

an ice cap or ice sheet) that usually originates in a cirque 

and may flow down into a valley previously carved by a 

stream. Synonymous with “mountain glacier” and “valley 

glacier.”

angiosperm. A plant with true flowers, in which the seeds, 

resulting from double fertilization, are enclosed in an 

ovary, comprising the fruit.

anticline. A fold, generally convex upward (“A”-shaped) 

whose core contains the stratigraphically older rocks. 

Opposite of “syncline.”

avulsion. The sudden cutting off or separation of land by a 

flood or an abrupt change in a stream course.

badlands. Eroded topography characterized by steep slopes 

and surfaces with little or no vegetative cover; composed 

of unconsolidated or poorly cemented clays or silts.

bentonite. Soft clay or greasy claystone composed mostly 

of the clay mineral smectite, formed by the chemical 

alteration of glassy volcanic ash in contact with water.

bivalve. Having a shell composed of two distinct, but equal 

or nearly equal, movable valves, which open and shut.

blowout. A general term for a small saucer-, cup-, or troughshaped hollow or depression formed by wind erosion on 

a preexisting dune or other sand deposit, especially in 

an area of shifting sand or loose soil, or where protective 

vegetation is disturbed or destroyed. 

braided stream. A sediment-clogged stream that forms 

multiple channels that divide and rejoin.

calcareous. Describes a substance that contains calcium 

carbonate. When applied to a rock name it implies that as 

much as 50% of the rock is calcium carbonate.

carbonate. A mineral group composed of carbon and 

oxygen plus an element or elements; for example calcite, 

CaCO3

; and dolomite, CaMg(CO3)2.

clast. An individual constituent, grain, or fragment of a rock 

or unconsolidated deposit, produced by the mechanical 

or chemical disintegration of a larger rock mass.

claystone (sedimentary). An indurated rock with more than 

67% clay-sized minerals.

colluvium. A loose, heterogeneous, and incoherent mass 

of rock fragments and soil material deposited via surface 

runoff or slow continuous downslope creep; usually 

collects at the base of a slope or hillside, but includes 

loose material covering hillsides.

craton. The relatively old and geologically stable interior of 

a continent. 

cross-bed. A single bed, inclined at an angle to the main 

planes of stratification; the term is commonly restricted to 

a bed that is more than 1 cm (0.4 in) thick.

cross-bedding. Uniform to highly varied sets of inclined 

beds deposited by wind or water that indicate flow 

conditions such as direction and depth. 

cross section. A graphic interpretation of geology, structure, 

or stratigraphy based on mapped and measured geologic 

extents and attitudes, depicted in a vertical plane (i.e., a 

cut or profile view).

cutbank. A steep, bare, slope formed by lateral erosion of a 

stream.

delta. The low, nearly flat, alluvial tract of land at or near 

the mouth of a river, commonly forming a triangular or 

fan-shaped plain of considerable area; resulting from 

the accumulation of sediment supplied by the river in 

such quantities that it is not removed by tides, waves, and 

currents.

drift. All rock material (clay, silt, sand, gravel, and boulders) 

transported and deposited by a glacier, or by running 

water emanating from a glacier.

drumlin. A low, smoothly rounded, elongated oval hill, 

mound, or ridge of till that formed under the ice margin 

and was shaped by glacial flow; the long axis is parallel to 

the direction of ice movement.

esker. A long, narrow, sinuous, steep-sided ridge composed 

of irregularly stratified sand and gravel that was deposited 

by a subglacial or englacial stream flowing between ice 

walls or in an ice tunnel or a stagnant or retreating glacier, 

and was left behind when the ice melted.

estuary. The seaward end or tidal mouth of a river where 

freshwater and seawater mix.

fault. A break in rock characterized by displacement of one 

side relative to the other.

flood basin. The tract of land covered by water during the 

highest known flood.

floodplain. The surface or strip of relatively smooth land 

composed of alluvium and adjacent to a river channel, 

constructed by the present river in its existing regimen 

and covered with water when the river overflows its 

banks. A river has one floodplain and may have one or 

more terraces representing abandoned floodplains.

fluvial. Of or pertaining to a river or rivers.

fluvial channel. A natural passageway or depression 
40

produced by the action of a stream or river. lignite. A brownish-black coal that is intermediate in 

coalification between peat and subbituminous coal. footwall. The lower wall of a fault. Synonymous with “brown coal.”

foraminifer. Any protozoan belonging to the subclass lineament. A linear topographic feature of regional extent Sarcodina, order Foraminiferida, characterized by the that probably reflects an underlying crustal structure. presence of a test of one to many chambers composed Also, any extensive linear surface feature (e.g., fault lines, of secreted calcite (rarely silica or aragonite) or of aligned volcanoes, and straight stream courses). agglutinated particles; most foraminifers are marine 

but freshwater forms are known. Range: Cambrian to littoral. Pertaining to the benthic ocean environment 

Holocene. or depth zone between high water and low water; 

also, pertaining to the organisms of that environment. formation. Fundamental rock-stratigraphic unit that is Synonymous with “intertidal.” mappable, lithologically distinct from adjoining strata, 

and has definable upper and lower contacts. meander. One of a series of sinuous curves, bends, or turns 

in the course of a stream, produced by a mature stream fossil. A remain, trace, or imprint of a plant or animal that swinging from side to side as it flows across its floodplain has been preserved in the Earth’s crust since some past or shifts its course laterally toward the convex side of an geologic time; loosely, any evidence of past life. original curve.

glacial erratic. A rock fragment carried by glacial ice meandering stream. A stream containing meanders. deposited at some distance from the outcrop from which 

it was derived, and generally, though not necessarily, member. A lithostratigraphic unit with definable contacts; a 

resting on bedrock of different lithology. subdivision of a formation.

glacial groove. A deep, wide, usually straight furrow cut mollusk. A solitary invertebrate such as gastropods, bivalves, 

in bedrock by the abrasive action of a rock fragment and cephalopods belonging to the phylum Mollusca. 

embedded in the bottom of a moving glacier; it is larger Range: Lower Cambrian to Holocene. 

and deeper than a glacial striation, ranging in size from moraine. A mound, ridge, or other distinct accumulation a deep scratch to a glacial valley. Syn: groove (glacial of unsorted, unstratified glacial drift, predominantly till, geology). deposited mostly by direct action of a glacier.

glaciofluvial. Pertaining to the meltwater streams flowing morphology. The shape of the Earth’s surface. from wasting glacier ice, and especially to the deposits and 

landforms produced by such steams, as kame terraces and mudstone. An indurated mud having the texture and 

outwash plains. composition of shale, but lacking its fine laminations or 

fissility. glaciolacustrine. Pertaining to, derived from, or deposited 

in glacial lakes, especially referring to deposits and neritic. Describes the ocean environment or depth zone 

landforms composed of suspended material transported between low tide and 200 m (660 ft), or between low tide 

by meltwater streams flowing into lakes bordering a and approximately the edge of the continental shelf; also, 

glacier. describes the organisms living in that environment.

ground moraine. An accumulation of till after it has been oil field. A geographic region rich in petroleum resources 

deposited or released from the ice during ablation, to and containing one or more wells that produce, or have 

form an extensive area of low relief, produced, oil and/or gas.

hanging wall. The upper wall of a fault. orogeny. A mountain-building event.

hydraulic conductivity. The ease with which water moves outwash. Stratified detritus (chiefly sand and gravel) 

through spaces or pores in soil or rock. removed or “washed out” from a glacier by meltwater 

streams and deposited in front of or beyond the end incision. Downward erosion by a stream, resulting in a moraine or the margin of an active glacier. The coarser deepened channel and commonly a narrow, steep-walled material usually is deposited nearer to the ice. valley.

outwash delta. Outwash forming a delta in a body of intertonguing. The overlapping of markedly different rocks water. through vertical succession of wedge-shaped layers; 

results in the disappearance of sedimentary bodies in outwash fan. A fan-shaped accumulation of outwash 

laterally adjacent masses. deposited by meltwater streams in front of the end 

moraine of a glacier. kame. A low mound, knob, hummock, or short irregular 

ridge, composed of stratified sand and gravel deposited outwash plain. A broad, gently sloping sheet of outwash 

by a subglacial stream as a fan or delta at the margin of a deposited by meltwater streams flowing from the front of 

melting glacier; by a superglacial stream in a low place or or beyond a glacier, and formed by coalescing outwash 

hole on the surface of the glacier; or as a ponded deposit fans.

on the surface or at the margin of stagnant ice. paleoenvironment. An environment in the geologic past.

kettle. A steep-sided, usually basin- or bowl-shaped hole peat. An unconsolidated deposit of semicarbonized plant or depression, commonly without surface drainage, in remains in a water-saturated environment, such as a bog glacial-drift deposits, often containing a lake or swamp; or fen, and of persistently high moisture content (at least formed by the melting of a large, detached block of 75%). It is an early stage or rank in the development of stagnant ice (left behind by a retreating glacier) that had coal; carbon content is about 60% and oxygen content is been wholly or partly buried in the glacial drift. about 30% (moisture-free). 
41

point bar. A low ridge of sand and gravel deposited in a 

stream channel on the inside of a meander, where flow 

velocity slows.

proglacial lake. A lake formed just beyond the frontal margin 

of an advancing or retreating glacier, generally in direct 

contact with the ice. 

recessional moraine. An end or lateral moraine built during 

a temporary but significant pause in the final retreat of a 

glacier.

regolith. From the Greek “rhegos” (blanket) + “lithos” 

(stone), the layer of unconsolidated rock material that 

forms the surface of the land and overlies or covers 

bedrock; includes rock debris of all kinds, volcanic ash, 

glacial drift, alluvium, loess, and aeolian deposits, vegetal 

accumulations, and soil.

regression. Long-term seaward retreat of the shoreline or 

relative fall of sea level.

roche moutonnée. A glacially sculpted, elongated bedrock 

knob or hillock.

sandstone. Clastic sedimentary rock composed of 

predominantly sand-sized grains.

sedimentary rock. A rock resulting from the consolidation 

of loose sediment that has accumulated in layers; it may 

be “clastic,” consisting of mechanically formed fragments 

of older rock; “chemical,” formed by precipitation from 

solution; or “organic,” consisting of the remains of plants 

and animals. One of the three main classes of rock—

igneous, metamorphic, and sedimentary. 

sedimentation. The process of forming or accumulating 

sediment into layers, including the separation of rock 

particles from parent rock, the transportation of these 

particles to the site of deposition, the actual deposition or 

settling of the particles, the chemical and other changes 

occurring in the sediment, and the ultimate consolidation 

of the sediment into solid rock. 

siltstone. A clastic sedimentary rock composed of silt-sized 

grains.

strata. Tabular or sheetlike layers of sedimentary rock 

that are visually distinctive from other layers above and 

below. The singular form of the term is stratum, but is less 

commonly used.

stratification. The accumulation or layering of sedimentary 

rocks as strata. Tabular, or planar, stratification refers to 

essentially parallel surfaces. Cross-stratification refers to 

strata inclined at an angle to the main stratification.

stratigraphy. The geologic study of the origin, occurrence, 

distribution, classification, correlation, and age of rock 

layers, especially sedimentary rocks.

stream channel. A long, narrow depression shaped by the 

concentrated flow of stream water.

stream terrace. A planar surface alongside a stream valley 

representing the remnants of an abandoned floodplain, 

stream bed, or valley floor produced during a former stage 

of erosion or deposition.

striations (glacial). One of a series of long, delicate, finely 

cut, commonly straight and parallel furrows or lines 

inscribed on a bedrock surface by the rasping and rubbing 

of rock fragments embedded at the base of a moving 

glacier, usually oriented in the direction of ice movement; 

also form on the rock fragments transported by a glacier.

subaerial. Describes a condition or process that exists or 

operates in the open air on or immediately adjacent to the 

land surface.

subduction. The process of one lithospheric plate 

descending beneath another.

subduction zone. A long, narrow belt in which subduction 

takes place.

subtidal. Describes the ocean environment or depth zone 

between low tide and 200 m (660 ft), or between low 

tide and approximately the edge of the continental shelf. 

Synonymous with “neritic” and “sublittoral.”

supratidal. Pertaining to the shore area marginal to the 

littoral zone, just above high-tide level. Synonymous with 

“supralittoral.”

syncline. A generally concave upward fold of which the core 

contains the stratigraphically younger rocks. Opposite of 

“anticline.”

terminal moraine. The end moraine, extending across a 

glacial valley as an arcuate or crescentic ridge, that marks 

the farthest advance or maximum extent of a glacier.

terrace. Any long, narrow, relatively level or gently inclined 

surface (i.e., a bench or steplike ledge) that is bounded 

along one edge by a steeper descending slope and along 

the other edge by a steeper ascending slope, thus breaking 

the continuity of the slope; commonly occurs along the 

margin and above the level of a body of water, marking a 

former water level.

thalweg. The line connecting the lowest/deepest points 

along a stream bed; the line of maximum depth.

thrust belt. A series of mountainous foothills adjacent to an 

orogenic belt, which forms due to contractional tectonics.

thrust sheet. The body of rock above a large-scale thrust 

fault whose surface is horizontal or very gently dipping.

till. Unstratified drift deposited directly by a glacier without 

reworking by meltwater and consisting of a mixture of 

clay, silt, sand, gravel, and boulders ranging widely in size 

and shape.

trace fossil. A fossilized feature such as a track, trail, 

burrow, or coprolite (dung), that preserves evidence of an 

organism’s life activities, rather than the organism itself. 

Compare to “body fossil.”

transgression. Landward migration of the sea as a result of a 

relative rise in sea level.

trophic. Of or pertaining to nutrition.

underfit stream. A stream that appears to be too small to 

have eroded the valley in which it flows. Also, a stream 

whose volume is greatly reduced or whose meanders 

show a pronounced shrinkage in radius.

Wisconsinan. Pertaining to the classical fourth glacial stage 

of the Pleistocene Epoch in North America, following 

the Sangamonian interglacial stage and preceding the 

Holocene Epoch.

43

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Additional References

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

managers. Web addresses are valid as of August 2015. Refer to Appendix B for laws, regulations, and 

policies that apply to NPS geologic resources.

Geology of National Park Service Areas

● NPS Geologic Resources Division (Lakewood, 

Colorado) Energy and Minerals; Active Processes 

and Hazards; Geologic Heritage:

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

● NPS Geologic Resources Inventory: http://www.

nature.nps.gov/geology/inventory/index.cfm.

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

and guest scientist program:

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

● NPS Views program (geology-themed modules are 

available for Geologic Time, Paleontology, Glaciers, 

Caves and Karst, Coastal Geology, Volcanoes, and a 

variety of geologic parks):

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

NPS Resource Management Guidance and 

Documents

● Management Policies 2006 (Chapter 4: Natural 

resource management):

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

● 1998 National parks omnibus management act: 

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

pdf/PLAW-105publ391.pdf

● NPS-75: Natural resource inventory and 

monitoring guideline:

http://www.nature.nps.gov/nps75/nps75.pdf

● NPS Natural resource management reference 

manual #77: http://www.nature.nps.gov/Rm77/

● Geologic monitoring manual (Young, R., and 

L. Norby, editors. 2009. Geological monitoring. 

Geological Society of America, Boulder, Colorado): 

http://nature.nps.gov/geology/monitoring/index.

cfm

● NPS Technical Information Center (TIC) (Denver, 

Colorado; repository for technical documents): 

http://www.nps.gov/dsc/technicalinfocenter.htm

Climate Change Resources

● NPS Climate Change Response Program Resources: 

http://www.nps.gov/subjects/climatechange/

resources.htm

● US Global Change Research Program:

http://globalchange.gov/home

● Intergovernmental Panel on Climate Change:

http://www.ipcc.ch/

Geological Surveys and Societies

● Montana Bureau of Mines and Geology:

http://www.mbmg.mtech.edu/

● North Dakota Geological Survey:

https://www.dmr.nd.gov/ndgs/

● US Geological Survey: http://www.usgs.gov/

● Geological Society of America:

http://www.geosociety.org/

● American Geophysical Union: http://sites.agu.org/

● American Geosciences Institute:

http://www.americangeosciences.org/

● Association of American State Geologists:

http://www.stategeologists.org/

US Geological Survey Reference Tools

● National geologic map database (NGMDB):

http://ngmdb.usgs.gov/

● Geologic names lexicon (GEOLEX; geologic unit 

nomenclature and summary):

http://ngmdb.usgs.gov/Geolex/geolex_home.html

● Geographic names information system (GNIS; 

official listing of place names and geographic 

features): http://gnis.usgs.gov/

● GeoPDFs (download searchable PDFs of any 

topographic map in the United States):

http://store.usgs.gov (click on “Map Locator”)

● Publications warehouse (many publications 

available online): http://pubs.er.usgs.gov

● Tapestry of time and terrain (descriptions of 

physiographic provinces):

http://tapestry.usgs.gov/Default.html

51

Appendix A: Scoping Participants

The following people attended the GRI scoping meeting for Fort Union Trading Post National Historic 

Site, held on 17 August 2011, or the follow-up report writing conference call, held on 7 October 2014. 

Discussions during these meetings supplied a foundation for this GRI report. The scoping summary 

document is available on the GRI publications website: http://go.nps.gov/gripubs.

2011 Scoping Meeting Participants

Name Affiliation Position

Ken Anderson Fort Union Trading Post National Historic Site Maintenance worker

Andy Banta Fort Union Trading Post National Historic Site Superintendent

Eric Brevik Dickinson State University Associate professor of geology and soils

Tim Connors NPS Geologic Resources Division Geologist

Joseph Hartman University of North Dakota Chester Fritz Distinguished Professor, School 

director

Mine Hodgkinson Fort Union Trading Post National Historic Site Maintenance worker supervisor

Lisa Norby NPS Geologic Resources Division Geologist

Tim Shepherd NPS Northern Great Plains Network Data manager

Trista Thornberry-Ehrlich Colorado State University Geologist

Loren Yellowbird Fort Union Trading Post National Historic Site Chief ranger

2014 Conference Call Participants

Name Affiliation Position

Andy Banta Fort Union Trading Post National Historic Site Superintendent

Eric Brevik Dickinson State University Professor of geology and soils, department chair

John Graham Colorado State University Geologist

Joseph Hartman University of North Dakota Chester Fritz Distinguished Professor, School 

director

Edward Kassman NPS Geologic Resources Division Oil and gas policy and regulatory specialist

Jason Kenworthy NPS Geologic Resources Division Geologist, GRI reports coordinator

Fred MacVaugh Fort Union Trading Post National Historic Site Museum curator

Kerry Moss NPS Geologic Resources Division External energy and minerals program 

coordinator

Kara Paintner NPS Northern Great Plains Network Network coordinator

Carmen Thomson NPS Midwest Region Regional I&M coordinator

53

Appendix B: Geologic Resource Laws, Regulations, and Policies

The NPS Geologic Resources Division developed this table to summarize laws, regulations, and policies 

that specifically apply to National Park Service minerals and geologic resources. The table does not 

include laws of general application (e.g., Endangered Species Act, Clean Water Act, Wilderness Act, 

National Environmental Policy Act, or National Historic Preservation Act). The table does include the 

NPS Organic Act when it serves as the main authority for protection of a particular resource or when 

other, more specific laws are not available. Information is current as of July 2015. Contact the NPS 

Geologic Resources Division for detailed guidance.

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Paleontology

National Parks Omnibus Management 

Act of 1998, 16 USC § 5937 protects the 

confidentiality of the nature and specific 

location of paleontological resources and 

objects.

Paleontological Resources Preservation 

Act of 2009, 16 USC § 470aaa et 

seq. provides for the management and 

protection of paleontological resources on 

federal lands.

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

injuring, defacing, removing, digging or 

disturbing paleontological specimens or 

parts thereof.

Prohibition in 36 CFR § 13.35 applies 

even in Alaska parks, where the surface 

collection of other geologic resources is 

permitted.

Regulations in association with 2009 PRPA 

are being finalized (July 2015).

Section 4.8.2 requires NPS to protect 

geologic features from adverse effects of 

human activity.

Section 4.8.2.1 emphasizes Inventory and 

Monitoring, encourages scientific research, 

directs parks to maintain confidentiality of 

paleontological information, and allows 

parks to buy fossils only in accordance with 

certain criteria.

Rocks and 

Minerals

NPS Organic Act, 16 USC § 1 et seq.

directs the NPS to conserve all resources 

in parks (including rock and mineral 

resources), unless otherwise authorized by 

law.

36 CFR § 2.1 prohibits possessing, 

destroying, disturbing mineral resources...

in park units.

Section 4.8.2 requires NPS to protect 

geologic features from adverse effects of 

human activity.

Park Use of Sand and Gravel

Materials Act of 1947, 30 USC § 601

does not authorize the NPS to dispose of 

mineral materials outside of park units.

None applicable.

Section 9.1.3.3 clarifies that only the 

NPS or its agent can extract park-owned 

common variety minerals (e.g., sand and 

gravel), and:

-only for park administrative uses;

-after compliance with NEPA and other 

federal, state, and local laws, and a 

finding of non-impairment;

-after finding the use is park’s most 

reasonable alternative based on 

environment and economics;

-parks should use existing pits and create 

new pits only in accordance with parkwide borrow management plan;

-spoil areas must comply with Part 6 

standards; and

-NPS must evaluate use of external 

quarries.

Any deviation from this policy requires a 

written waiver from the Secretary, Assistant 

Secretary, or Director.
54

Resource Resource-specific Laws Resource-specific Regulations 2006 Management Policies Upland and Fluvial Processes

Rivers and Harbors Appropriation 

Act of 1899, 33 USC § 403 prohibits the 

construction of any obstruction on the 

waters of the United States not authorized 

by congress or approved by the USACE.

Clean Water Act 33 USC § 1342 requires 

a permit from the USACE prior to any 

discharge of dredged or fill material 

into navigable waters (waters of the US 

[including streams]). 

Executive Order 11988 requires federal 

agencies to avoid adverse impacts to 

None applicable.

Section 4.1 requires NPS to manage 

natural resources to preserve fundamental 

physical and biological processes, as well 

as individual species, features, and plant 

and animal communities; maintain all 

components and processes of naturally 

evolving park ecosystems.

Section 4.1.5 directs the NPS to reestablish natural functions and processes 

in human-disturbed components of natural 

systems in parks, unless directed otherwise 

by Congress.

Section 4.4.2.4 directs the NPS to allow 

natural recovery of landscapes disturbed by 

natural phenomena, unless manipulation of 

the landscape is necessary to protect park 

development or human safety. 

Section 4.6.4 directs the NPS to (1) 

manage for the preservation of floodplain 

values; [and] (2) minimize potentially 

hazardous conditions associated with 

flooding.

floodplains. (see also D.O. 77-2)

Executive Order 11990 requires plans for 

potentially affected wetlands (including 

riparian wetlands). (see also D.O. 77-1)

Section 4.6.6 directs the NPS to manage 

watersheds as complete hydrologic systems 

and minimize human-caused disturbance 

to the natural upland processes that deliver 

water, sediment, and woody debris to 

streams.

Section 4.8.1 directs the NPS to allow 

natural geologic processes to proceed 

unimpeded. Geologic processes...include...

erosion and sedimentation...processes. 

Section 4.8.2 directs the NPS to protect 

geologic features from the unacceptable 

impacts of human activity while allowing 

natural processes to continue.
55

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

Mining in the Parks Act of 1976, 16 

USC § 1901 et seq. authorizes NPS 

to regulate all activities resulting from 

exercise of mineral rights, on patented and 

unpatented mining claims in all areas of the 

System, in order to preserve and manage 

those areas.

General Mining Law of 1872, 30 USC 

§ 21 et seq. allows US citizens to locate 

mining claims on Federal lands. Imposes 

administrative and economic validity 

requirements for “unpatented” claims (the 

right to extract Federally-owned locatable 

minerals). Imposes additional requirements 

for the processing of “patenting” claims 

(claimant owns surface and subsurface). 

Use of patented mining claims may be 

limited in Wild and Scenic Rivers and 

OLYM, GLBA, CORO, ORPI, and DEVA. 

Surface Uses Resources Act of 1955, 

30 USC § 612 restricts surface use of 

unpatented mining claims to mineral 

activities.

36 CFR § 5.14 prohibits prospecting, 

mining, and the location of mining claims 

under the general mining laws in park 

areas except as authorized by law.

36 CFR Part 6 regulates solid waste 

disposal sites in park units.

36 CFR Part 9, Subpart A requires the 

owners/operators of mining claims to 

demonstrate bona fide title to mining 

claim; submit a plan of operations to 

NPS describing where, when, and how; 

prepare/submit a reclamation plan; and 

submit a bond to cover reclamation and 

potential liability.

Section 6.4.9 requires NPS to seek to 

remove or extinguish valid mining claims in 

wilderness through authorized processes, 

including purchasing valid rights. Where 

rights are left outstanding, NPS policy is 

to manage mineral-related activities in 

NPS wilderness in accordance with the 

regulations at 36 CFR Parts 6 and 9A.

Section 8.7.1 prohibits location of new 

mining claims in parks; requires validity 

examination prior to operations on 

unpatented claims; and confines operations 

to claim boundaries.

Nonfederal Oil and Gas

NPS Organic Act, 16 USC § 1 et seq.

authorizes the NPS to promulgate 

regulations to protect park resources and 

values (from, for example, the exercise of 

mining and mineral rights).

36 CFR Part 6 regulates solid waste 

disposal sites in park units.

36 CFR Part 9, Subpart B requires the 

owners/operators of nonfederally owned 

oil and gas rights to

-demonstrate bona fide title to mineral 

rights;

-submit a plan of operations to NPS 

describing where, when, how they intend 

to conduct operations;

-prepare/submit a reclamation plan; and 

-submit a bond to cover reclamation and 

potential liability.

Section 8.7.3 requires operators to comply 

with 9B regulations.

Nonfederal minerals other than oil and gas

NPS Organic Act, 16 USC §§ 1 and 3

Surface Mining Control and 

Reclamation Act of 1977, 30 USC § 

1201 et. seq. prohibits surface coal 

mining operations on any lands within the 

boundaries of a NPS unit, subject to valid 

existing rights.

NPS regulations at 36 CFR Parts 1, 5, 

and 6 require the owners/operators of 

other types of mineral rights to obtain 

a special use permit from the NPS as a 

§ 5.3 business operation, and § 5.7 – 

Construction of buildings or other facilities, 

and to comply with the solid waste 

regulations at Part 6.

SMCRA Regulations at 30 CFR Chapter 

VII govern surface mining operations on 

Federal lands and Indian lands by requiring 

permits, bonding, insurance, reclamation, 

and employee protection. Part 7 of the 

regulations states that National Park System 

lands are unsuitable for surface mining.

Section 8.7.3 states that operators 

exercising rights in a park unit must comply 

with 36 CFR Parts 1 and 5.

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

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

Island Communities.

NPS 463/129508, August 2015
National Park Service

U.S. Department of the Interior

Natural Resource Stewardship and Science 

1201 Oak Ridge Drive, Suite 150 

Fort Collins, Colorado 80525

www.nature.nps.gov

EXPERIENCE YOUR AMERICATM
Qal 

Qat 

NPS Authorized Boundary 

D 

rivers 

Infrastructure 

o cities 

railroads 

highways 

Geologic Units 

Qal Alluvium (Holocene) 

Qac Alluvium and colluvium (Holocene) 

Qat Alluvial terrace deposit (Holocene and Pleistocene) 

Qgt Glacial till (Pleistocene) 

' 

Tfsb Fort Union Formation, Sentinel Butte Member (Paleocene) 

Tftr Fort Union Formation, Tongue Ri ver Member (Paleocene) 

fli'lef 

.'15Hf1SR 

NORTH 

DAKOTA 

Alexander 

68 

This map was produced by Kari Lanphier (Colorado State University) in March 2014. It is an overview 

of compiled geologic data prepared as part of the NPS Geologic Resources Inventory. This map is not a 

substitute for site-specific investigations. 

The source maps used in creation of the digital geologic data were: 

~ Bergantino, R. N. and E. M. Wilde. 1998. Geologic map of the Culbertson 30' x 60' quadrangle 

(bedrock emphasis), northeastern Montana (revised 2007). 1: 100,000 scale. Open-File Report 359. 

Montana Bureau of Mines and Geology, Butte, Montana . 

Vuke, S. M., E. M. Wilde, and L. N. Smith. 2003. Geologic and structure contour map of the Sidney 30' 

x 60' quadrangle, eastern Montana and adjacent North Dakota (revised 2011). 1: 100,000 scale. 

Open-File Report 478. Montana Bureau of Mines and Geology, Butte, Montana. 

As per source map scale and U.S. National Map Accuracy Standards, geologic features represented 

here are within 50 m (166 ft) of their true location. 

All digital geologic data and publications prepared as part of the Geologic Resources Inventory are 

available at the NPS Integrated Resource Management Applications Portal (IRMA): 

httpsJ/irma.nps.gov/App/Reference/Search. Enter "GRI" as the search text and select a park from the 

unit list. 

Nort h 

Cf) 

0 2

•c::::::i••• Kilometers --c:::::::::J____ Miles 

0 2 
 Fort Union Trading Post National Historic Site Map Unit Properties Table, Page 1 of 3 

Map Unit Properties Table: Fort Union Trading Post National Historic Site 

Colored map units are mapped within Fort Union Trading Post National Historic Site. Colors correspond to those on geologic map poster. Bold text refers to sections in report.

Age Map Unit

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

QUATERNARY 

(Holocene)

Alluvium

(Qal)

Brown, olive, and gray boulders, cobbles, gravel, sand, silt, 

and clay deposited in stream channels and on floodplains. 

Deposits are poorly to well sorted, poorly to well stratified, 

and clasts may be as much as 0.6 m (2 ft) in diameter. 

Generally less than 6 m (20 ft) thick, but locally as much as 12 

m (40 ft) thick. 

River Features

Cutbanks, point bars, mid-channel bars, and islands.

Paleontological Resources

Potential to yield Pleistocene megafauna and Holocene 

paleoenvironmental fossils, including trace fossils 

(burrows), charcoal, vertebrate fossils, such as bison bones, 

pollen, spores, and aquatic microfossils. Qal contains Knife 

River Flint (silicified plant material) associated with 

archeological (cultural resource) sites. 

Missouri River Bank Erosion

Erosion along the south bank of the Missouri River has 

caused active slumping and may intersect an irrigation 

ditch in the future. The Missouri River may eventually 

breach the southern boundary of Fort Union Trading Post 

National Historic Site. 

Flooding

Flooding on the Missouri River during extremely wet years 

and backup from flooding on the Yellowstone River may 

inundate low-lying areas of the park.

Current and Potential Oil and Gas Development

Well pads, drill rigs, geophysical seismic activity, and access 

roads may erode and/or compact Qal. Drilling activities 

may also impact the viewshed.

Earthquakes

Potential earthquakes may occur due to secondary waste 

fluid disposal associated with the hydraulic fracturing 

process. These fluids may lubricate buried faults and cause 

the faults to slip, resulting in movement on the surface of 

Qal. This connection has not been demonstrated in North 

Dakota.

Abandoned Mineral Lands

A small abandoned quarry east of the reconstructed Fort 

Union contains a mix of Qal, Qac, and Qgt. The quarry is 

unlikely to pose a safety issue for visitors and is overgrown 

with weeds.

Paleontological Resources Inventory and Monitoring

Fossils found in the historic site, museum collections, or 

associated with archeological (cultural resource) sites

should be inventoried and/or monitored.

The Past 12 Thousand Years (Cenozoic Era): Holocene 

Thaw

The Missouri and Yellowstone rivers created modern 

floodplains and channels within the broad meltwater 

channels that formed at the end of the Pleistocene ice 

ages. 
Colored map units are mapped within Fort Union Trading Post National Historic Site. Colors correspond to those on geologic map poster. Bold text refers to sections in report.

 Fort Union Trading Post National Historic Site Map Unit Properties Table, Page 2 of 3 

Age Map Unit

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

Current and Potential Oil and Gas Development

Well pads, drill rigs, geophysical seismic activity, and access 

roads may erode and/or compact Qac. Drilling activities 

may also impact the viewshed.

Slope Movements

QUATERNARY 

(Holocene)

Alluvium and 

colluvium

(Qac)

Sand, silt, and clay in glacial meltwater channels. Colors reflect

that of older Quaternary and Paleogene units from which 

sediment was derived. May locally overlie glacial outwash 

deposits. Thickness not determined.

Bodmer Overlook and Surrounding Landscape

The trail to the overlook crosses Qac. 

Paleontological Resources

Potential to yield Pleistocene megafauna and Holocene 

paleoenvironmental fossils, including trace fossils 

(burrows), charcoal, vertebrate fossils, such as bison bones, 

pollen, spores, and aquatic microfossils. Qac contains 

Knife River Flint (silicified plant material) associated with 

archeological (cultural resource) sites.

Downslope movement of colluvium may occur.

Earthquakes

Potential earthquakes may occur due to secondary waste 

fluid disposal associated with the hydraulic fracturing 

process. These fluids may lubricate buried faults and cause 

the faults to slip, resulting in movement on the surface of 

Qac. This connection has not been demonstrated in North 

Dakota.

Abandoned Mineral Lands

The Past 12 Thousand Years (Cenozoic Era): Holocene 

Thaw

Sediments filled the broad meltwater channels that formed 

at the end of the Pleistocene ice age. 

Glacial Features

Coarse sand and gravel from glacial outwash.

A small abandoned quarry east of the reconstructed Fort 

Union contains a mix of Qal, Qac, and Qgt. The quarry is 

unlikely to pose a safety issue for visitors and is overgrown 

with weeds.

Paleontological Resources Inventory and Monitoring

Fossils found in the park, museum collections, or 

associated with archeological (cultural resource) sites 

should be inventoried and/or monitored.

QUATERNARY

(Pleistocene and Holocene)

Alluvial terrace 

deposit

(Qat)

Brown and gray, dominantly well-rounded gravel, sand, and 

silt in terrace remnants at elevations ranging from 1.5 m (5 ft) 

to 110 m (360 ft) above modern rivers and streams. Deposits 

are generally well sorted, but poorly to well stratified, with 

thicknesses generally less than 6 m (20 ft).

Paleontological Resources

Potential to yield Pleistocene megafauna and Holocene 

paleoenvironmental fossils. These may include trace fossils 

(burrows), charcoal, vertebrate fossils, such as bison bones, 

pollen, spores, and aquatic microfossils. Qat contains 

Knife River Flint (silicified plant material) associated with 

archeological (cultural resource) sites.

None documented.

From 2 Million to 12 Thousand Years Ago (Cenozoic 

Era): Pleistocene Deep Freeze

When the glaciers melted, vast amounts of meltwater 

spread gravel, sand, and silt across the region. Modern 

rivers and streams incised into these ancient deposits and 

left them as terraces above newly developed floodplains.

QUATERNARY

(Pleistocene)

Till

(Qgt)

Heterogeneous mixture of brown and gray clay, silt, sand, and 

gravel with rare to abundant cobbles and boulders. Locally 

contains lenses and stringers of moderately well sorted clay, 

silt, sand, or gravel. Thickness is generally less than 9 m (30 ft), 

but may be as much as 30 m (100 ft).

Paleontological Resources

Qgt contains Knife River Flint (silicified plant material) 

associated with archeological (cultural resource) sites.

Glacial Features

Heterogeneous mixture of sediment left behind when the 

glaciers melted.

None documented.

From 2 Million to 12 Thousand Years Ago (Cenozoic 

Era): Pleistocene Deep Freeze

The landscape was covered by a heterogeneous mixture of 

sediments when the last phase of Pleistocene glaciation 

ended with the melting of the continental ice sheet from 

western North Dakota and eastern Montana.
Colored map units are mapped within Fort Union Trading Post National Historic Site. Colors correspond to those on geologic map poster. Bold text refers to sections in report.

 Fort Union Trading Post National Historic Site Map Unit Properties Table, Page 3 of 3 

Age Map Unit

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

Fort Union Formation Type Section and Stratigraphic 

Features

PALEOGENE

(Paleocene) 

Fort Union Formation

Sentinel 

Butte 

Member

(Tfsb)

Tongue River 

Member

(Tftr) 

Dark-gray shale locally underlain by orange- brown or brown, 

iron oxide-stained, cross-bedded channel sandstone with 

medium- to coarse-grained, poorly sorted clasts. Gray or 

brown, easily eroded mudstone about 6 m (20 ft) thick 

underlies the sandstone locally. Erosion has removed the 

upper part of member. Thickness in the map area is about 90 

m (300 ft).

North Dakota Geological Survey assigns these rocks to the 

Sentinel Butte Formation of the Fort Union Group. 

Yellow, orange, or tan, fine- to medium-grained sandstone 

and thinner interbeds of yellowish-brown, orange, or tan 

siltstone and light-colored mudstone and clay. Consists of four 

basic stratigraphic layers, often in cycles. From lowest to 

highest, these cycles include: (1) gray claystone and siltstone, 

often containing plant and mollusk fossils; (2) a lignite layer; 

(3) yellow siltstone and sandstone, which may contain plant 

and mollusk fossils; and (4) sandstone deposits. The clay in the 

unit is not prone to swelling. Generally poorly cemented and 

weathers to badlands topography. Some relatively resistant 

sandstone beds form caprocks. Thickness is about 250 m (800 

Cross-bedded sandstone, rip-up clasts, and lignite coal 

beds.

Paleontological Resources

Abundant freshwater mollusks. Similar diverse fossil 

assemblage as Tftr, including invertebrate, vertebrate, and 

many plant fragments of petrified wood.

Aeolian Features

Blowouts on hilltops.

Volcanic Ash Layers

Volcanic ash altered to bentonite.

Fort Union Formation Type Section and Stratigraphic 

Features

Cycles of sandstone, siltstone, and mudstone; also 

contains lignite coal beds.

Bodmer Overlook and Surrounding Landscape

The summit of the overlook is on Tftr. 

Paleontological Resources

Abundant freshwater mollusks. Diverse assemblage of 

invertebrate, vertebrate, and plant fossils (table 1). Includes 

Current and Potential Oil and Gas Development

Well pads, drill rigs, geophysical seismic activity, and access 

roads may erode and/or compact these units. Drilling 

activities may also impact the viewshed.

Slope Movements

Downslope movement may displace these strata, which 

form the steepest slopes in the park.

Earthquakes

Potential earthquakes may occur due to secondary waste 

fluid disposal associated with the hydraulic fracturing 

process. These fluids may lubricate buried faults and cause 

the faults to slip, resulting in movement on the surface. 

This connection has not been demonstrated in North 

Dakota.

Paleontological Resources Inventory and Monitoring

Fossils found in the park, museum collections, or 

associated with archeological (cultural resource) sites 

should be inventoried and/or monitored.

From 66 Million to 2 Million Years Ago (Cenozoic 

Era): Paleocene Swamps and Floodplains and Climate 

Change

Once the Cannonball Sea retreated from the Fort Union 

region, sediments were deposited in river channels, 

avulsions, floodplains, lakes, and swamps. Peat that 

accumulated in swampy areas would eventually become 

part of North Dakota’s lignite reserves. Volcanic ash, 

drifting in from the west, would alter to bentonite, a clay 

mineral with shrink-swell characteristics.

ft). eroded pieces of petrified wood on Bodmer Overlook.

North Dakota Geological Survey assigns these rocks to the Aeolian Features

Bullion Creek Formation of the Fort Union Group. Blowouts on hilltops.