GEOLOGICAL SURVEY CIRCULAR 798 

Unstable Ground in 

Western North Dakota 

Unstable Ground in 

Western North Dakota 

By Donald E. Trimble 

GEOLOGICAL SURVEY CIRCULAR 798 

Distribution of landslides and subsidence in 

western North Dakota, and recognition of 

widespread earth slides in the unglaciated 

southwestern part of the state 

1979 
United States Department of the Interior 

CECIL D. ANDRUS, Secretary 

Geological Survey 

H. William Menard, Director 

Free on application to Branch of Distribution, U.S. Geological Survey, 1200 South Eads Street, Arlington, VA 22202 
CONTENTS 

Abstract-----------------------------------------------------------

Introduction-------------------------------------------------------

Page 

1 

1 

Large landslides of the slump-earthflow type----------------------- 1 

3 

3 

3 

6 

8 

8 

High buttes---------------------------------------------------

Des Lacs River------------------------------------------------

Missouri River------------------------------------------------

Little Missouri River-----------------------------------------

Heart River---------------------------------------------------

Cannonball River----------------------------------------------

Conclusions---------------------------------------------------

Soil slides--------------------------------------------------------

Subsidence---------------------------------------------------------

11 

11 

12 

14 

16 

19 

19 

Subsidence over mined-out areas-------------------------------

Subsidence over naturally burned-out coal seams---------------

Summary-------------------------------------------------------

References cited---------------------------------------------------

Figure 1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17 0 

18. 

19. 

20. 

21. 

22. 

23. 

24. 

25. 

ILLUSTRATIONS 

Page 

Location and type of unstable ground in western 

North Dakota----------------------------------------- 2 

Landslide on northeast side of HT (Black) Butte--------- 4 

Landslides on west side of North Chalky Butte----------- 4 

Landslides on northwest side of West Rainy Butte-------- 5 

Landslides on southwest side of Sentinel Butte---------- 5 

Landslides on east side of Killdeer Mountains----------- 6 

Large old landslide complex in Shipton Coulee----------- 7 

Recent landslide on north-facing slope south of 

Berg Coulee------------------------------------------ 7 

Tongue River Member of the Fort Union Formation--------- 8 

Large landslide complex on east side of the Missouri 

River------------------------------------------------ 9 

Landslide area on north side of Bear Den Creek---------- 9 

Landslides on south side of Little Missouri River------- 10 

Recent landslide south of Little Missouri River--------- 10 

Tilted slump blocks in Fort Union Formation------------- 11 

Badlands east of Little Missouri River------------------ 12 

Landslide along south side of the Heart River valley---- 13 

Sentinel Butte Hember of the Fort Union Formation------- 13 

Landslides along east side of Heart River valley-------- 14 

Soil slide scars on west-facing slope, south of 

Heart River------------------------------------------ 15 

Soil slide scars on north end of Chalky Buttes---------- 15 

Subsidence pits over old underground coal mine 

workings near Beulah, North Dakota------------------- 16 

Aerial view northwest across subsidence pits------------ 17 

Aerial view looking northwest across a subsidence area 

and an Arrowhead Coal Company strip mine-------------- 17 

View looking west to the Burning Coal Vein subsidence 

area-------------------------------------------------- 18 

Subsidence above a burning underground lignite bed------- 18 

III 

UNSTABLE GROUND IN WESTERN NORTH DAKOTA 

By Donald E. Trimble 

ABSTRACT 

Unstable ground in western North Dakota is 

mainly the result of mass-wasting processes. 

The units most affected are mudstones, 

siltstones, and sandstones of the Fort Union 

Formation. Ground instability generally is 

indicated by landslides, soil slides, or 

subsidence. 

Landslides are mostly of the slumpearthflow type and are localized along the 

flanks of the high buttes in southwestern North 

Dakota, including HT (Black) Butte, Chalky 

Buttes, Sentinel Butte, and East and West Rainy 

Buttes, and along parts of the valleys of the 

Des Lacs, Missouri, Little Missouri, and Heart 

Rivers. Landslides are sparse elsewhere. 

Soil slides are common in the areas south 

and southwest of the maximum position of the 

Pleistocene glacial ice margin on slopes of 15 

degrees or more, and have taken place on some 

slopes as gentle as five degrees. The 

weathered, exposed surface of the Fort Union 

Formation seems to be especially susceptible to 

soil slides. Soil slides constitute the major 

type of ground instability in southwestern North 

Dakota. 

Subsidence is of two types: (1) subsidence 

over old underground mine workings, and 

(2) subsidence over naturally ignited and burned 

underground coal beds. Major subsidence has 

taken place over old, underground workings near 

Beulah, Wilton, Lehigh, Haynes, and Belfield, 

and lesser subsidence near Scranton, and west 

and north of Bowman. Thickness of overburden 

above the coal in all these areas is believed to 

be less than 30m (100 ft). Subsidence has not 

taken place over old underground workings along 

the Des Lacs and ·Souris valleys northwest of 

~linot, where the thickness of overburden is more 

than 60 m (200ft). 

Spectacular subsidence has occurred over a 

burning underground coal bed at Burning Coal 

Vein Park near the Little Missouri River, 

northwest of Amidon. 

INTRODUCTION 

Huge landslides, abundant soil slide scars, 

and deep subsidence pits are convincing evidence 

of past ground instability in western North 

Dakota. These failures were mainly the result 

of mass-wasting processes affecting the 

mudstones and siltstones that form most of the 

Fort Union Formation, the bedrock for virtually 

the entire area. North of the Missouri River 

the bedrock is largely covered by glacial drift. 

Large slump-earthflow landslides are localized 

mainly along the valley walls of a few deeply 

entrenched streams. Small but abundant soil 

slides are widespread in the unglaciated areas 

of the southwest part of the State. Spectacular 

ground subsidence, both over old underground 

mines and over naturally ignited and burned coal 

beds, has occurred in many places in western 

North Dakota. 

This record of past failures is an 

indication that similar failures may occur in 

the future, and this potential for ground 

instability is a subject for consideration in 

planning for energy development as well as for 

highway, railroad, and transmission line 

construction. 

The information presented here is intended 

to inform developmental planners of the location 

and extent of past ground failures (fig. 1), and 

to provide information for evaluating the 

potential hazard related to future ground 

instability at localities in western North 

Dakota. 

LARGE LANDSLIDES OF THE 

SLUMP-EARTHFLOW TYPE 

Slumps are landslides characterized by 

shearing and rotational movement of an 

independent mass of rock or earth along a curved 

slip surface. Beds within the discrete mass 

become tilted backward and downward toward the 

slope (definition of slump modified from 

American Geological Institute, 1972). The 

distal parts of slump masses commonly become 

saturated and flow into lobate margins. This 
I 

I 

I 

I 

I 

EXPLANATION 

--'---'- Maximum extent of continental ice sheets 

~ Area of landslides of slump-earthflow type 

~ Subs~dence over underground coal mines * Subsidence over burning underground coal seam 

I~ 0<:> 

White ~th 

;Y 

.'-"'-

) 

w 

102° 

0 25 50 MILES 

0 25 50 KILO METERS 

A 

I /_7. 

---, .. , I 

I 

p r_j 

I 

Ol~ I 

101 ° 

Fi gur e 1.--Loca tion and t ype of unst abl e gr ound in west er n North Dakota. 

2 
combined form is termed a slump-earthflow type 

of landslide. 

Spectacularly large landslides of the 

slump-earthflow type occur mainly in five areas 

in western North Dakota. Four of these are 

along or near four major rivers--from north to 

south, the Des Lacs, Missouri, Little Hissouri, 

and Heart Rivers. The fifth area comprises the 

landslides flanking the high buttes of 

southwestern North Dakota. 

HIGH BUTTES 

A number of high buttes stand 122-153 m 

(400-500 ft) above the upland level, mostly in 

Slope and Golden Valley Counties. These include 

HT (Black) Butte (fig. 2), Chalky Buttes 

(fig. 3), White Butte, and East and West Rainy 

Buttes (fig. 4) in Slope County, and Sentinel 

Butte (fig. 5) and Flat Top Butte in Golden 

Valley County. These buttes are all erosional 

remnants of the Paleocene Fort Union Formation 

capped by the Oligocene White River Group. The 

basal part of the White River Group, the Chadron 

Formation, commmonly forms a resistant caprock 

that causes oversteepening of the slopes below. 

Average annual precipitation in most of this 

part of the state is between 38 and 41 em (15 

and 16 inches) a year. 

All of these buttes are almost completely 

flanked by landslide deposits that in many 

places head in huge amphitheaters having nearly 

vertical headwall scarps. In addition, abundant 

landslides flank the Killdeer "Mountains 

(fig. 6), south of the Little 11issouri River; 

Blue Buttes, north of the Little Hissouri River; 

and the Porcupine Hills, on the Standing Rock 

Indian Reservation west of the Hissouri River. 

The landslide deposits are the result of many 

coalescing and overlapping slump-earthflow 

landslides of a wide range in age. Although 

some are younger, most of them were probably 

formed during the Pleistocene, when 

precipitation was greater than it is now. 

DES LACS RIVER 

Few landslides have taken place in the 

drift-covered areas north of the Hissouri River. 

With the exception of a large landslide along 

the valley of the White Earth River, about 8 km 

(5 mi) north of White Earth, and some along the 

Little Knife River, the only important area of 

large landslides in the region north of the 

Missouri River is along the west side of the Des 

Lacs River and in some of its tributary coulees. 

At the mouth of Stony Creek, east of Bowbells, 

and for about 3 km (2-miles) to the south, there 

are large slump landslides along Upper Des Lacs 

Lake. The southernmost is at the north end of a 

valley-wall exposure of the Fort Union 

Formation. The drift cover at the other 

localities near here is presumably thin, 

3 

inasmuch as Lemke and Kaye (1953), who mapped 

these landslides in the Bowbells quadrangle, 

report "only till exposed in slide areas but 

failure of underlying beds of the Fort Union 

Formation is probably responsible for sliding". 

About 16 km (10 mi) farther south, along 

the west side of the lower end of Lower Des Lacs 

Lake and in every coulee tributary entering the 

Des Lacs Valley from the west between Lower Des 

Lacs Lake and Donnybrook, there are large 

landslides and landslide complexes of the slumpearthflow type, some nearly a mile long 

(fig. 7). Hany of these slides are quite old, 

perhaps Pleistocene, but others, especially a 

number of slides along the west wall of the Des 

Lacs Valley between Berg Coulee and Aennis 

Coulee (about a mile north of Donnybrook), are 

very recent (fig. 8). 

The Fort Union Formation is exposed at many 

places along the western wall of the Des Lacs 

valley north of Donnybrook (fig. 9), and this 

exposure of bedrock at the surface obviously is 

in part responsible for the prevalence of 

landslides in this area. Steep slopes 45-60 m 

(150-200 ft) high along the valley walls 

contribute to instability, and Lemke and Kaye, 

(1953) pointed out that in the Bowbells 

quadrangle the flow of ground water, in lignite 

beds of the Fort Union Formation and at the 

contact between the Fort Union and the overlying 

drift, was to the northeast. They suggest that 

this would cause saturation of the beds on the 

west wall of the Des Lacs valley, where the 

ground water emerged, and would probably 

contribute to the sliding. The situation 

undoubtedly is the same in the segment between 

Lower Des Lacs Lake and Donnybrook, which 

accounts for the abundance of landslides west of 

the river and the lack of them east of the 

river. Thus, it seems likely that a combination 

of many factors contributed to the formation of 

landslides along the Des Lacs valley, including 

the exposure of bedrock at the surface or at a 

shallow depth along valley walls, the steepness 

and height of valley wall or bluff, the 

hydrologic nature of the ground-water system, 

and the nature and amount of precipitation. 

MISSOURI RIVER 

Landslides occur along the Hissouri River--

mainly along two reaches, one extending about 32 

km (20 miles) downstream from Williston and the 

other extending about the same distance 

downstream from the bridge crossing of State 

Highway 23 near Newtown (fig. 10). The east 

valley wall of the Hissouri, south of the bridge 

near Newtown, is a landslide area about two 

miles long. A similar slide area is located 

along the west side of the reservoir north of 

the mouth of Bear Den Creek. A few other slides 

exist along the reservoir walls, but many lie 

near the heads of small reentrants or short 
Figure 2.- -Landslide complex on northeast side of HT (Black) Butte. Caprock is Ol igocene 

Chadron Formation. Most of slide mass is in Sentinel Butte Member of the Pal eocene Fort 

Union Formation. 

Figure 3.--Landslides on west side of North Chalky Butte, in Sentinel Butte Member of Paleocene 

Fort Union Formation capped by Oligocene Chadron Formation. Landslide mass covering most 

of slope is composed of a complex of slump-e a rthflow t ype landslides. 

4 
Figure 4.--Landslides on northwest side of West Rainy Butte. Entire slope below steep upper 

face is landslide complex. 

Figure 5.--Landslides on southwest side of Sentinel Butte . 

Formation overlying Sentinel Butte Member of the Fort 

below Chadron cliffs is landslide complex. 

5 

Caprock is Oligocene Chadron 

Union Formation. Nearly entire slope 
Figure 6.--Landslides on east side of Killdeer Mountains. Nearly all of the slope shown in this 

view is a landslide complex composed of slump-earthflow type landslides. 

tributaries to the reservoir in !1ountrail and 

McKenzie Counties, including several slides in 

the valley of Bear Den Creek, where it is 

crossed by State Highway 22 (fig. 11). 

Landslides along the river near Williston 

are few and smaller. One is on the south side 

of the river, directly south of Williston, and 

one is on the east side of the river, southeast 

of Williston. Only a few other large slides 

occur along the river downstream. The bluffs of 

the Missouri River, along both reaches 

containing large slump-earthflow type 

landslides, are about 91.5 m (300 ft) high, 

mostly in Fort Union Formation (Tongue River and 

Sentinel Butte l1embers). 

Other large landslides occur sporadically 

along the Missouri River between Bismarck and 

the South Dakota border. An especially large 

landslide complex exists on the west bank of the 

river near the South Dakota border. 

LITTLE MISSOURI RIVER 

The largest and most numerous landslides in 

the State of North Dakota are along the easttrending valley of the Little Missouri River 

between Redwing Creek (west of Theodore 

Roosevelt National !1e~orial Park, North Unit) 

and Lake Sakakawea (about 16 km, or 10 mi, 

downriver from Lost Bridge on State Highway 22) 

6 

where landslides are nearly continuous along t he 

valley walls (fig. 12). They occur on both the 

north and south sides as single slumps and as 

landslide complexes. Some are very old, perhaps 

dating back to the time when this part of the 

valley was cut as a diversion channel along the 

front of the last Pleistocene ice sheet, but 

others are very young--having formed only 

recently (fig. 13). Some rotated slump blocks 

in Theodore Roosevelt National Memorial Park, 

North Unit, are very large, and the rotation is 

so spectacularly illustrated by tilted blocks of 

the layered Fort Union Formation tha t they are 

the subject of one of the main point-of-interest 

stops in the park (fig. 14). 

This east-trending part of the Little 

!1issouri River valley is very narrow compared 

with the north-trending course, ranging from as 

little as 1.6 km (1 mi) to as much as 14.5 km 

(9 mi) in width. The valley walls, where 

unmodified by landslides, are nearly vert ical 

and 122-153.5 m (400-500 ft) high. 

Landslides are also abundant in tributary 

valleys south of the Little Missouri River in 

the southeastern part of McKenzie County, 

especially toward the heads of Bennet Creek, 

Corral Creek, and Crosby Creek. 

The only other area of extensive landslide 

activity along the Little l1issouri River is 
Figure 7.--Large old landslide 

Donnybrook, North Dakota. 

landslide complex. 

complex in Shipton Coulee, west of the Des Lacs River near 

Nearly all of the material below the level of the flat upland is 

._ 

Figure 8.--Recent landslide on north-facing slope south of Berg Coulee and north of Donnybrook, 

North Dakota, on the west side of the Des Lacs River valley. 

7 
Figure 9.--Tongue River Member of the Fort Union Formation at the mouth of Baden Coulee, west 

side of the Des Lacs River valley at the south end of Lower Des Lac Lake. Exposure about 

15 m (50 ft) high. 

along another east-trending reach in the 

northern part of Slope County. There too, the 

valley of the Little Missouri River is narrowed 

from its normal width along its north-trending 

course, where it commonly is 20- 32 km (15-20 mi) 

wide and deeply dissected into badlands 

(fig. 15) that are largely free of slumps. 

Local relief apparently was insufficient to 

induce slumping. The slides in this easttrending part of the Little Missouri River 

valley are small er than those farther north, and 

nearly all of them are on steep cutbanks, 

61-91.5 m (200-300 ft) high, at the bends of 

large meander loops of the Little t1issouri 

River. 

HEART RIVER 

Landslides occur along the Heart River from 

its junction with the Green River, about 16 km 

(10 mi) east of Dickinson, to the upper end of 

Lake Tschida, and from the lower end of Lake 

Tschida almost to Mandan. Above Lake Tschida, 

the slides are in the Tongue River Member of the 

Fort Union Formation, and they occur mostly on 

the northeast-facing bluffs on the bends of 

8 

meander loops (fig. 16). Only a few are on the 

southwest-facing bluffs on the north side of the 

river. The bluffs commonly are about 30.5 m 

(100ft) high (fig. 17). 

Between Lake Tschida and Mandan the slides 

are in the Cannonball Member of the Fort Union 

Formation. They are particularly abundant along 

the north-trending course in Mor ton County, 

especially on the west side of the river 

(fig. 18). The bluffs along this part of the 

river generally are 30.5-61 m (100-200 ft) high. 

Landslides also occur in the Cannonball 

Member along the Sweet Briar tributary of the 

Heart River between u.s. Highway 10 and the 

junction with the Heart River. 

CANNONBALL RIVER 

The only other area of large landslides in 

western North Dakota is along the Cannonball 

River from southeast of Elgin to just below the 

junction with Snake Creek. These few slides are 

mostly in the Cannonball Member of the Fort 

Union Formation and are smaller than those of 

other areas, perhaps because of lower height and 

gentler slopes of valley walls. 
Figure 10.--Large landslide complex on east side of the 11issouri River south of bridge on State 

Highway 23, west of Newtown, North Dakota. Several scarps at head of slumps are apparent 

above landslide debris in left half of picture. 

Figure 11.--Landslide area on north side of Bear Den Creek, west of State Highway 22. House is 

on stabilized landslide material, which also forms slope to base of light-colored bedrock 

exposure. 

9 
Figure 12.--Landslides on south side of Little Missouri River in Theodore Roosevelt National 

Memorial Park, North Unit. View from River Bend Overlook. Killdeer Mountains on skyline 

in left distance. Lower third of valley wall on opposite (south) side of river is 

landslide complex. 

Figure 13.--Recent landslide south of the Little Missouri River, west of U.S. Highway 85. 

10 
Figure 14.--Tilted slump blocks in Fort Union Formation in Theodore Roosevelt National Memorial 

Park, North Unit. Note horizontal bedding in undisturbed formation on skyline. 

CONCLUSIONS 

In summary, it appears that large 

landslides of the slump-earthflow type occur 

only where a special set of circumstances is 

present. A steep valley wall at least 30m 

(100 ft) high seems to be necessary in most 

areas, although locally, as along the Cannonball 

River, some sliding takes place where valley 

walls are not as steep or high. Exposed bedrock 

of all members of the Fort Union Formation is 

susceptible to landsliding when other conditions 

are met. Wetting of the mudstone and siltstone 

bedrock by ground water flow or surface water 

helps cause sliding by adding weight to the mass 

and by reducing internal friction. Thus, 

topography, lithology, and water all seem to be 

primary factors in causing large landslides of 

the slump-earthflow type in western North 

Dakota. 

SOIL SLIDES 

A soil slide is a downslope movement of a 

coherent thin plate of soil and weathered rock. 

The thickness of the plate seems to be 

determined by the depth to the B horizon in the 

11 

soil profile, where there is a high 

concentration of clay. The sliding takes place 

at the top of the clayey B horizon. The soil 

above the B horizon consists of the A and A 

horizons, which together essentially form t~e 

root zone of the grassy vegetation growing in 

this area. This part of the soil profile is 

open and porous, and rapidly becomes saturated 

during heavy precipitation. The less permeable 

B horizon, however, tends to divert downwardmoving soil water downslope. Saturation greatly 

increases the weight of the soil plate above the 

B horizon, and in many places this increased 

weight is sufficient to overcome the tensile 

strength of the material and the plate tears 

loose and slides downhill. This movement 

generally is only a few feet or less. 

A soil slide, then, is a type of landslide 

that moves primarily by gliding of a detached 

plate rather than by block rotation. Gravity is 

the primary cause of movement, but saturation of 

the mass is necessary for movement to occur. 

Because soil slides affect only near-surface 

soil and weathered rock, the depths affected by 

such slides are shallow. Large areas, however, 

such as entire hillsides, can be affected by 

multiple soil slides. 
Figure 15.--Badlands east of the Little Missouri River. Local relief is apparently too low to 

produce large slumps. View from Painted Canyon Overlook, north side of Interstate 

Highway 94 west of Belfield, North Dakota. 

Soil slides have affected large areas in 

southwestern North Dakota where the Fort Union 

Formation underlies the surface and was never 

mantled by Pleistocene ice. They are uncommon 

in the areas covered by glacial drift. Most of 

the areas of extensive soil sliding are in the 

unglaciated area south of the Little Missouri 

River in southern l1cKenzie County and to the 

south, and south of the Knife River to the 

southeast. The counties mainly affected are 

Golden Valley, Billings, Stark, Slope, 

Hettinger, Bowman, Adams, and the western parts 

of Morton and Grant. Soil slide scars are 

common on the upland along U.S. Highway 85 

southward from the Little Missouri River and 

along Interstate Highway 94 west of exit 25, 6 

or 7 miles west of New Salem. They are 

especially abundant south of the Heart River in 

Stark County (fig. 19), and are common on some 

lesser slopes near Chalky Buttes (fig. 20) and 

HT (Black) Butte in Slope County. Soil slides 

also have affected many slopes, some quite 

steep, in the badlands area of the Little 

Missouri River valley in Theodore Roosevelt 

National Hemorial Park, South Unit, and south of 

Medora. 

Soil slides in southwestern North Dakota 

take place locally on slopes that may be as 

gentle as 5 degrees, but most commonly occur on 

somewhat steeper slopes. Slopes of more than 

12 

10-15 degrees in the nonglaciated areas are 

especially susceptible to soil slides. 

Till of the extensive cover of ground 

moraine in the glaciated areas appears to be 

generally unaffected by soil slides. This 

likely is because the deposits are to young to 

have a well-developed soil profile with an 

appreciable B horizon. 

Although soil slides affect only the 

surficial materials to rather shallow depths, 

they constitute a widespread form of ground 

instability that can cause tremendous losses if 

it is not recognized and anticipated in planning 

and design stages. 

SUBSIDENCE 

Subsidence is the gradual downward sinking 

or settling of a part of the Earth's surface 

with little or no horizontal motion (definition 

of subsidence modified from American Geological 

Institute, 1972). Subsidence can be caused 

naturally by solution of soluble rocks and 

creation of a subsurface void into which the 

overlying beds can sink. Withdrawal of fluids 

from porous beds can cause compaction and 

subsidence. Removal of ore or coal from 

underground mines creates space into which 

overlying beds may subside if the lithostatic 
Figure 16.--Landslide along the south wall of the Heart River valley south of Taylor, North 

Dakota. 

Figure 17.--Sentinel Butte Member of the Fort Union Formation exposed in cutbank on north side 

of Heart River valley south of Taylor, North Dakota. Bluff 24-30 m (80-100 ft) high. Note 

thin lignite seams that probably contribute to wetting of the siltstones and claystones of 

the Sentinel Butte Member and facili t ate landsliding. 

13 
Figure 18.--Landslides along east side of Heart River valley east of Almont, North Dakota. 

Entire valley wall is landslide complex. 

load exceeds the shear strength of the rocks. 

Similarly, the burning of coal beds, which can 

occur naturally because of ignition on the 

outcrop by spontaneous combustion or lightning, 

creates a space or a void into which overlying 

beds may subside. 

Subsidence has occurred at several places 

in western North Dakota, both above old 

underground mines and above naturally burned-out 

coal beds. 

SUBSIDENCE OVER MINED-OUT AREAS 

Subsidence over old mine workings has taken 

place at several localities in western North 

Dakota. Some of these subsidence areas are of 

spectacular proportions, with subsidence pits 

outlining the underground rooms over tens of 

acres. Those at Beulah (fig. 21), Haynes 

(fig. 22), Wilton, Belfield, and Lehigh are 

especially impressive. Smaller areas of 

subsidence are located east of Belfield, at 

Scranton, and west and north of Bowman. 

Underground mining of lignite began early 

in North Dakota. Coal was mined near Minot in 

the 1880's (Andrews, 1939). The earliest year 

for which lignite production was reported in 

North Dakota was 1884, when 31,752 metric tons 

(35,000 English tons) was produced 

(Brandt, 1953, p. 57). Most early-day mining 

was underground, and in 1927, 146 underground 

mines produced 831,621 metric tons (916,690 

14 

English tons) while 81 strip mines produced 

555,627 metric tons (612,464 English tons) 

(Brandt, 1953, p. 58). Underground mining in 

North Dakota, then, was the predominant method 

of lignite mining through the early part of this 

century, but by 1958 only one underground mine 

remained in operation (Landis, 1973, p. 45). 

Many localities in the state have old 

underground workings beneath the surface, but 

subsidence has not taken place above all of 

them. 

Marked subsidence has taken place above 

large underground mines that were operating at 

the turn of the century at Lehigh (about 8 km, 

or about 5 mi east of Dickinson) and Wilton 

(east of the Missouri River north of Bismark) 

(Babcock, 1901; Wilder, 1902). The underground 

mining at Beulah and Haynes began later. The 

Washburn mine at Wilton, the largest mine 

operated in the state in 1902 (Wilder, 1902, 

p. 60), was an underground operation with an 

access shaft to a coal seam 18 m (60 ft) below 

the surface. The coal seam ranged from 2.4 to 

4 m (8 to 13 ft) in thickness, and was worked by 

the double entry and room and pillar system, the 

rooms being about 5.2 m (17 ft) wide 

(Wilder, 1902, p. 61). The coal seam in a strip 

mine currently operating near the subsidence 

area near Haynes, in southern Adams County, is 

only about 4.6 or 6 m (15 or 20 ft) below ground 

level, and the underground operation probably 
Figure 19.--Soil slide scars on west-facing slope, 3 miles south of the Heart River south of 

Taylor, No rth Dakota. Scars are apparent on both near and distant slopes. 

Figure 20.--Soil slide scars on the north end of Chalky Buttes, east of U.S. Highway 85 north of 

Bowman, North Dakota. 

15 
Figure 21.--Subsidence pits over old underground coal-mine workings near Beulah, North Dakota. 

Photograph by Richard c. Dunrud. 

had about 30 m (100 ft) or less of overburden 

over most of its extent (fig. 23). 

Many large underground mines were in 

operation along the Des Lacs and Souris Rivers 

around the turn of the century, but they had 

much more overburden over the mined coal beds: 

61-67 m (200-220 ft) at the North Dakota Lignite 

and Brick Company mine in sec. 10, T. 155 N., 

R. 83 ~. (Wilder, 1902, p. 115) and more than 

91.5 m (300ft) in the New Era Mine in sec. 15, 

T. 155 N., R. 84 W., near Minot (Wilder, 1902, 

p. 164). Subsidence pits have not formed where 

the overburden was thick. 

From these data, it is apparent that 

subsidence has taken place over old underground 

mines where the thickness of the overburden was 

30m (100 ft) or less, but that subsidence has 

not taken place where the overburden was 61 m 

(200 ft) or more in thickness. 

SUBSIDENCE OVER NATURALLY 

BURNED-OUT COAL SEAMS 

Widespread fused and oxidized layers of 

rock called clinker or scoria, that were 

16 

reddened and hardened by heat from burning coal 

beds, indicate that lignite coal beds have 

ignited and burned at many places throughout the 

centuries. When a thick coal bed burns, it is 

almost completely consumed, and only a little 

ash and a lot of space is left where formerly 

there was a coal bed. The overlying rock layers 

commonly collapse to fill the space formerly 

occupied by the coal. Surface expression of 

this collapse is subsidence. Such subsidence 

has happened many times in the past, and is 

still happening today. 

Subsidence over a currently burning coal 

bed is spectacularly illustrated near the Little 

Missouri River at Burning Coal Vein Park in 

Custer National Forest, northwest of Amidon 

(fig. 24). There the U.S. Forest Service has 

established a park and picnic ground, and an 

exhibit at a special observation area provides 

the public with an explanation of the ground 

failure caused by the burning of the subsurface 

coal bed (fig. 25). The fire has been burning 

in the Burning Coal Vein since before man came 

to the area, and has caused subsidence and 

rupturing of surface beds over many acres of 

land. 
Figure 22.--Aerial view northwest across subsidence pits over old underground coal mine workings 

east of State Highway 8 north of Haynes, North Dakota. Strip mine opera tion of Arrowhead 

Coal Company in foreground. 

Figure 23.--Aerial view looking northwest across subsidence pits and strip mine of Arrowhead 

Coal Company north of Haynes, North Dakota. High wall in strip mine above coal bed only 

4.6-6 m (15-20 ft) high. Note equipment for scale. 

17 
Figure 24.--View west to Burning Coal Vein subsidence area, Custer National Forest, northwest of 

Amidon, North Dakota. Forest Service observation area beyond curve in lower right. 

Figure 25.--Subsidence above burning underground lignite bed, Burning Coal Vein Park, Custer 

National Forest, northwest of Amidon, North Dakota. 

18 
SUMMARY 

Large slump-earthflow type landslides in 

western North Dakota occur mainly on the steep 

slopes of the high buttes (in southwestern 

North Dakota) and along parts of the entrenched 

valleys of the Des Lacs, Missouri, Little 

Missouri, and Heart Rivers. Because they are 

localized on steep slopes and in entrenched 

valleys, the landslides are not likely to be a 

major hazard to energy development. However, 

the possibility of reactivating an old, 

stabilized landslide mass by loading its head, 

excavating at its toe, or introducing water into 

the main mass of slide material, should be 

recognized when highway, railroad, or viaduct 

construction requires crossing an old, 

recognized slide area. 

Soil slides, though affecting only the 

upper part of the soil profile, constitute a 

greater potential hazard to energy development 

sites than the more impressive large landslides. 

Much of the unglaciated area south of the Little 

Missouri River in western North Dakota is highly 

susceptible to soil sliding, especially if much 

water is introduced, either artificially or by 

heavy precipitation. When colluvium and 

weathered Fort Union Formation become wet, even 

on gentle slopes, soil slides can occur, and 

this tendency for ground failure should be given 

careful consideration whenever any type of 

construction is to be undertaken in this region. 

Most large-scale underground coal-mining 

operations in western North Dakota ended more 

than 40 years ago and much subsidence has taken 

place over the old underground mines. The 

underground mines along the Des Lacs and Souris 

valleys northwest of Minot were abandoned even 

earlier, but the surface above them has not 

subsided in 60 or 70 years. The greatest 

19 

potential for damage in western North Dakota as 

a result of unstable ground conditions is, then, 

the .Potential for soil sliding as a result of 

saturation of colluvium and weathered Fort Union 

Formation on slopes exceeding 5° in the 

unglaciated areas south of the Little Missouri 

River. Care should also be taken, however, to 

ensure that old recognized landslides of the 

slump-earthflow type are not reactivated. 

REFERENCES CITED 

American Geological Institute, 1972, Glo<sary of 

geology, Gary, Margaret, McAfee, Rotert, 

Jr., and Wolf, C. L., eds., 805 p. and 

52 p. bibliography. 

Andrews, D. A., 1939, Geology and coal resources 

of the Minot region, North Dakota: U.S. 

Geological Survey Bulletin 906-B, p. 43-84. 

Babcock, E. J., 1901, First Biennial Report: 

North Dakota Geological Survey, 103 p. 

Brandt, R. A., 1953, Lignite resources of North 

Dakota: U.S. Geological Survey 

Circular 226, 78 p. 

Landis, E. R., 1973, Coal, in Mineral and water 

resources of North Dakota, Report for the 

Committee on Interior and Insular Affairs: 

United States Senate, Part I, p. 45-52. 

Lemke, R. W., and Kaye, C. A., 1953, Geology of 

the Bowbells quadrangle, North Dakota: 

U.S. Geological Survey Geologic Quadrangle 

Map GQ-26, scale 1:62,500. 

Wilder, F. A., 1902, Second Biennial Rep~rt: 

North Dakota Geological Survey, 262 p. 

«U.S GOVERNMENT PRINTING OFFICE.. 1979-6n.fJ26/37