Geology of the Fort Peck 

Area, Garfield, McCone 

and Valley Counties 

Montana

GEOLOGICAL SURVEY PROFESSIONAL PAPER 414-F

Prepared as a part of a program of the 

Department of the Interior for development 

of the Missouri River basin
Geology of the Fort Peck 

Area, Garfield, McCone 

and Valley Counties 

Montana

By FRED S. JENSEN and HELEN D. VARNES

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 414-F

Prepared as a part of a program of the 

Department of the Interior for development 

of the Missouri River basin

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEW ART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C., 20402
CONTENTS

Abstract_ _________-__-_.--___________--.

Introduction ______________________________

General location and purpose of the work. 

Geography and elimate_________________

Fieldwork and acknowledgments_______

Cretaceous stratigraphy ____________________

Montana group______________________

Post-Montana group formations_________

Subsurface bedrock formations __________

Exposed bedrock formations____________

Judith River formation _____________

Bearpaw shale.________-__--_.-_-_-

Fox Hills sandstone.-___-_-_____-__

Strata overlying the Montana group.____

Hell Creek formation_____________

Structure _________________________________

Regional structural setting._____________

Structural features in the Fort Peck area_ 

Deformation at Tiger Butte_________

Post-Cretaceous stratigraphy._______________

General statement _____________________

Formations antedating glaciation________

Tertiary deposits______________________

Flaxville formation_______________

Quaternary deposits______-__-_-_-____

Wiota gravels.

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Post-Cretaceous stratigraphy Continued

Glacial deposits_________________________________

General discussion. _________________________

Ground moraine. __ _________________________

Kintyre formation __________________________

Outwash channel deposits____________________

Outwash terrace deposits -____________._-__._

Postglacial Quaternary and Recent deposits________

Alluvium __________________________________

Intermittent pond deposits_____ ______________

Alluvial-colluvial deposits____ ________________

Geologic history of the Tertiary and Quaternary deposits. 

Preglacial history. ______________________________

Glacial history_________________________________

Regional background outlined by earlier studies. 

Glacial history of the Fort Peck area__________

Drainage history________________________________

Late-glacial and postglacial history of the Fort Peck

Summary of natural resources. _________________

Highway subgrade materials _______________

Sand and gravel __________________________

Riprap __________________________________

Ceramic clay_____________________________

Material for lining water-retaining structures_ 

Lightweight aggregate_____________________

Selected references. ___________________________

Index _______________________________________

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ILLUSTRATIONS

[Plates are in pocket]

PLATE 1. Geologic map and sections of the Fort Peck area.

2. Bedrock geologic map and columnar section.

3. Geologic map of Tiger Butte.

FIGURE 1. Index map of Fort Peck area, Montana.______________________________-_-_____-_-_-_-----_-----------

2. Stratigraphic diagram showing formations of the Montana group. ____________________----.----.-----------.

3. Badland topography in the Bearpaw shale on the lower slopes of Milk River Hill_______________--------_-_

4. Slacking and disintegrating of the Bearpaw shale exposed along the shoreline of the Fort Peck Reservoir.

5. Mechanical analyses of samples of weathered Bearpaw shale, Fox Hills sandstone, and the ground moraine. _ _

6. Atterberg limits and soils classification of test samples_______________________-_-_-_-_-_-_-_-----------

7. Mass wasting of the Fox Hills sandstone causing mantling of the Bearpaw shale___________--_----___------

8. Rimrock of the upper part of the Fox Hills sandstone_______________________-__--__--_----__-----------

9. Map showing thickness of the Fox Hills sandstone, including the Colgate member where present________-__-_

10. Contact of the Colgate member with the overlying Hell Creek formation________________-______--___-----

11. Resistant lenses of sandstone in the Hell Creek formation. ______________________-_--_-_-_-_--_----------

12. The Hell Creek formation capped by sandstone ledges.__________________________-___-_-______-_---_----.

13. Crossbedding and ovoid sandstone concretions in the Hell Creek formation__________________-___---------_.

14. Conglomeratic lenses of sandstone in the Hell Creek formation___________________-_-_-_---_-.-----------.

15. Soil profile of ground moraine._-__-______-_________-__________________-______-----------__----------.

16. Bedding of the Kintyre formation. ___________________________________________________________________

17. Sketch of the Kintyre formation___________________________________________________---_-__----------.

18. Mechanical analyses of alluvium and outwash of terrace gravel__________________--_-_-_-_-____----------.

19. Sketch map showing drainage history of the vicinity of Fort Peck area__________________________---------.

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SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGY OF THE FORT PECK AREA, GARFIELD, McCONE, AND VALLEY COUNTIES, MONTANA

By FRED S. JENS EN and HELEN D. VAENES

ABSTRACT

The Fort Peck area, in the central part of northeast Montana, includes a little more than three 15-minute quadrangles 

and covers an area of about 600 square miles. The area is in 

the northern part of the Great Plains physiographic province 

and is predominantly an undulating grassy treeless prairie 

upland which rises gradually to the north. The Milk and 

Missouri Rivers have carved their channels into this prairie 

upland, so that steep scarps as much as 200 feet high border 

the wide bottom lands. South and southwest of the rivers, the 

surface rises abruptly and irregularly through a belt of 

"breaks" to a rolling dissected prairie with badland areas 

along many of the drainage courses.

The exposed bedrock is Late Cretaceous in age; the oldest 

formation is the Judith River formation, which is exposed only 

in the complexly faulted Tiger Butte area. The overlying 

Bearpaw shale is the surface bedrock except for about 12 

square miles in the southern part and in the extreme northeast corner where the Bearpaw shale is overlain by the Fox 

Hills sandstone. The Hell Creek formation, generally considered to be latest Cretaceous in age, is the youngest bedrock 

in the Fort Peck area; it disconformably overlies the Fox Hills 

in an area of about 5 square miles.

The unconsolidated and semiconsolidated materials overlying the Cretaceous bedrock are Miocene or Pliocene or 

younger and represent later chapters in the long history of 

intermittent regional uplift that was characteristic of the 

Great Plains province since early in the Tertiary. The oldest 

are preglacial sediments deposited on stream-cut plains and 

terraces. Of these, the Flaxville formation (Miocene or Pliocene) is limited to a few small plateau remnants in the extreme northeast corner of the area. The younger Wiota gravels 

(Pleistocene) are extensively deposited, along the rivers and 

their major tributaries.

The surflcial material over more than half the Fort Peck 

area is ground moraine of Wisconsin (Pleistocene) age. At 

one time the ground moraine probably covered the entire area, 

but postglacial erosion has stripped it from major valleys and 

from the hilly belt of shale and sandstone in the southern and 

southwestern parts of the area.

The Kintyre formation, composed of fluviolacustrine silt and 

fine sand and clay, overlies the ground moraine in some parts 

of the Milk and Missouri River valleys. The Kintyre formation was deposited on stagnant ice of the wasting Wisconsin 

glacier. Glacial outwash is limited to widely separated deposits in the rolling prairie upland and to terrace deposits.

Since the disappearance of the glacier ice, erosion has been 

dominant except in the trenches of the Milk and Missouri

Rivers and along their major tributaries, where alluviation 

has been dominant.

INTRODUCTION 

GENERAL LOCATION AND PURPOSE OF THE WORK

The Fort Peck area (fig. 1) includes about 600 square 

miles in the central part of northeastern Montana in

1. uiasgow quadrangle

2. Nashua quadrangle

3. Frazer quadrangle

FIGURE 1. Index map of the Fort Peck area, Montana.

the northern part, of the Great Plains physiographic 

province. It consists of the Glasgow, Nashua, and 

Frazer 15-minute quadrangles and a small additional 

area of about 26 square miles southwest and southeast 

of the Fort Peck Dam.

The development of the natural resources of the Missouri River basin has been aided since the middle 

lOSO's by several government agencies, which are engaged in a long-term multiple-phase program. This 

program relates to mineral resources, efficient land use, 

and the construction of dams and canals for flood control, power, irrigation, and navigation.

Begun in 1933 and officially completed in 1941, the 

Fort Peck Dam, in the south-central part of the report 

area, stands 250 feet above the Missouri River. The 

reservoir extends nearly 180 miles upstream and has. a 

capacity of 20 million acre-feet of water.

Fl
F2 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

The purpose of the present investigation is to supply 

basic geologic data in support of Missouri River basin 

development program and to add to the geologic 

knowledge of an area where only reconnaissance geologic work has been done previously.

GEOGRAPHY AND CLIMATE

The Fort Peck area is typical of much of the northern Great Plains. It is a rolling upland dissected by 

the Milk and Missouri Rivers and their tributaries. 

The rivers have broad, flat flood plains. Most of the 

area is between 2,100 and 2,300 feet in altitude, although the high terrace remnants in the northeast corner locally are more than 2,700 feet above sea level. 

The gradient along the eastward-flowing Milk and 

Missouri Rivers averages only slightly more than 1 

foot per mile.

The Missouri River crosses the south-central margin 

of the area and then swings almost due east. The Milk 

River, which enters the area northwest of Glasgow, 

flows southeast to join the Missouri near the town of 

Nashua. Lewis and Clark, as well as the host of traders, trappers, steamboat men, and hunters who followed them, traveled up the Missouri River through 

the heavily wooded strip of country along the river. 

A reader of their journals might get an unbalanced 

picture of the plains of northeastern Montana were he 

not careful to note that beyond the flat bottom lands of 

the major rivers and creeks are grassy treeless plains 

rising slowly to the north in a series of irregular steps 

that culminate in broad mesalike plateaus. In the Fort 

Peck area the rivers have cut against these uplands so 

that abrupt scarps, locally as much as 200 feet high, 

border the valley floors. Southward from the Missouri 

River the surface rises both abruptly and irregularly 

through an area of "river breaks" to rolling prairies 

dissected into small badland areas along many of the 

drainage courses.

Strip cultivation and summer fallowing have permitted some successful wheat farming on the upland 

north of the Milk and Missouri Rivers. Part of this 

northern upland, however, and all the area south of the 

Missouri River, are used as grazing land.

During the early 1920's many of the homesteaders 

were near economic ruin because of dry years and attendant crop failures. The Federal government undertook a relocation program, installed irrigation systems 

in the Milk River bottom land, and resettled many 

homesteaders there. The N-bar-N Cattle Co. and other 

private concerns constructed irrigation systems on bottom lands of the Missouri and Milk Rivers, so that today much of this land has been cleared of cottonwood

groves to make way for irrigated fields of sugar beets, 

alfalfa, and other crops.

There are four towns in the area. Glasgow, the 

county seat of Valley County, has a population of 

6,398 (1960 census). It is situated on the Great Northern Railroad and U.S. Highway 2 and is the principal 

marketing center of the area. Nashua (population 

796) and Frazer (population 378) are along the highway and railroad east of Glasgow. Fort Peck, the second largest town in the area, has a population of 950 

(1960 census). It is under the jurisdiction of the U.S. 

Army Corps of Engineers and is the site of the offices 

and laboratories of the Fort Peck district.

The following tabulations summarize climatic conditions (U.S. Dept. of Agriculture Yearbook, 1941).

Temperature (°F)

Average, June to August-_-______-_-----_-_--- ---_- 65

Average, December to February______________________ 10

Summer maximum..______---.__-______--__-__----.- 110

Winter minimum. __________________________________ 60

Daylight 

Longest day__-____-----_-_--------__-_-_---_-_hr__ 16

Shortest day _ _ ____--_____---__--__---__ _hr and min. _ 8, 25

Sunshine:

In summer__________________-_______percent_- 65

In winter._________________________________do_ _ 50

Wind averages 10 to 12 miles per hour (in July from 

northwest; in January from southwest). The growing season averages 90 to 120 days without killing frost. 

Annual precipitation ranges from 10 to 15 inches, 

half of which falls in May, June, and July. There 

are about 40 days between April and September with 

0.01 inches or more of rainfall. Annual snowfall 

averages 30 inches. There is an average of 90 days per 

year with snow cover; first snowfall in autumn occurs 

on about October 8.

FIELDWORK AND ACKNOWLEDGMENTS

Fieldwork was done largely in 1948, 1949, and 1950 

by Fred S. Jensen assisted by Frederick T. Fisher and 

James O. Kistler. The geology was mapped on aerial 

photographs, mostly on a scale of 1:20,000, and then 

transferred by sketchmaster, at a scale of 1:31,680, to 

base maps compiled from topographic maps of the 

Nashua and Frazer quadrangles and from U.S. Bureau 

of Land Management township plats for the Glasgow 

quadrangle. Culture was interpreted from the photographs and in most places was field checked. Where 

topographic maps were not available or were not sufficiently accurate, altitudes were determined by Paulin 

altimeter and by telescopic alidade.

Geologic data were obtained from auger holes and 

shallow trenches to supplement information obtained 

from existing exposures of the rock strata. Informa-
GEOLOGY, OF THE FORT PECK AREA, MONTANA F3

tion on the rocks at greater depth was obtained from 

logs of water wells, test holes drilled by oil companies, 

and from numerous drill holes in the vicinity of the 

Fort Peck Dam put down by the U.S. Army Corps of 

Engineers. Other information on the Bearpaw shale 

and some surficial deposits was made available by the 

Fort Peck District of the Corps of Engineers at Fort 

Peck, Mont. The soils classification tests (fig. 6) and 

mechanical analyses (fig. 5 and 18) were made by the 

Montana State Highway Department.

In this report, where names of colors of geologic 

materials are followed by letter-number symbols in 

parentheses, both name and symbol are taken from the 

"Rock-Color Chart" of the National Research Council 

(Goddard, 1948). Where color names are used without the symbol, a more general meaning is implied.

Preliminary reports on the geology of the Nashua 

and Frazer quadrangles were prepared by Fred S. 

Jensen and released for open file in 1951. Jensen also 

prepared a preliminary draft for the chapters on the 

Cretaceous stratigraphy and structure in the present 

report. Helen D. Varnes prepared the rest of the report.

CRETACEOUS STRATIGRAPHY

Geographically and geologically the Fort Peck area 

is an integral part of the northern Great Plains; so, a 

sketch of the development of geological understanding 

in the region is included in this report.

The northern Great Plains are underlain by a great 

succession of nearly flat lying marine and fresh-water 

strata of Cretaceous and Tertiary age, interrupted in 

places by small groups of mountains of a more complex 

structure. The several sequences of fresh-water strata 

resemble each other in so many respects that this misidentification was an early source of confusion. Efforts 

to determine the correct interrelations resulted in much 

discussion as given in a very large body of literature.

MONTANA GROUP

The first published statements on the differences between the lower and upper parts of the Upper Cretaceous rocks of the Great Plains were made by Meek 

(1886, p. 32-33). He noted the magnitude of the 

paleontologic break between the two parts (later named 

the Colorado group and the Montana group, respectively). And stated that "the upper surface of the 

Niobrara beds [Colorado group] is, at several places 

on the Missouri, seen to have been eroded into irregularities, or depressions, previous to the deposition of 

the succeeding [Montana group] * * *." White (1878, 

p. 21, 22, 30) redefined the Colorado group, as the term 

is presently used, and Eldridge (1889) proposed the 

term Montana group in its present connotation.

The sequence and identification of formations in the 

Montana group were open to question for many years, 

principally because studies were begun in an area much 

complicated by folding and faulting. Stebinger (1914, 

p. 64) gave a concise account of this subject. The 

commonest error had been in the identification of the 

Judith River formation, which occupies a medial position in the Montana group, with other continental 

formations younger than the group. Careful study of 

this area (around the mouth of the Judith River, about 

150 miles west of Fort Peck) by Stanton and Hatcher 

(1905) established the correct sequence of formations. 

They found the fresh- and brackish-water clays, lignites, and sandstones of the Judith River formation to 

be both overlain and underlain by lead-gray shales 

resembling the "Fort Pierre shale" in physical appearance and fossil content. They gave the name Claggett 

shale to the shale below the Judith River formation, 

from old Fort Claggett at the confluence of the Judith 

and Missouri Rivers. The shale above the Judith 

River formation they named the Bearpaw shale, from 

typical exposures north, east, and south of the Bearpaw Mountains. Their report (Stanton and Hatcher, 

1905, p. 68) correlated the formations of the Montana 

group in central Montana with those in South Dakota. 

The following information is taken from their original 

chart (Stanton and Hatcher, 1905, p. 62) :

Group

Montana

South Dakota section

Fox Hills formation. _

Central Montana section:

Fox Hills (?) formation 

[Bearpaw shale 

1 Judith River formation

1 Claggett shale 

I Eagle sandstone

It remained for Stebinger (1914) to construct a unified picture for the area east of the Rocky Mountains 

in northwestern Montana by using the data gathered 

by earlier workers and supplemented by his own fieldwork. His stratigraphic diagram (Stebinger, 1914, p. 

67, fig. 9) is reproduced in figure 2 with slight modifications.

The diagram shows that two readvances of the sea 

took place during Montana time and that the second 

readvance, represented by the Bearpaw shale, was more 

extensive than the first, represented by the Claggett 

shale. Continental sediments, which include the upper 

part of the Eagle sandstone, the Judith River formation, and the Two Medicine formation, accumulated in 

areas from which the seas had withdrawn. Sandstones 

such as those that occur in the Virgelle sandstone member of the Eagle sandstone and the Fox Hills sandstone 

are strandline deposits.
F4 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

EXPLANATION

Plains of 

northwestern Montana

3000

2000'-

1000'-

Fresh- and J. c.^kish-water Marine sediment Direction of migration 

sediments of shoreline

Fort Peck area

Black Hills, S. Dak.

Pierre shale

Landward

7"

Base of Montana group Seaward

West East 

FIGURE 2. Generalized stratigraphlc diagram of the Montana group. (Adapted from Steblnger, 1914.)

C. F. Bowen did mapping in the Judith River area 

at about the same time that Stebinger was engaged in 

the work discussed previously. Bowen's (1915) work 

is important, because he collected and summarized the 

evidence that clarified the fundamentals of Montana 

group stratigraphy.

POST-MONTANA GROUP FORMATIONS

The thick fresh-water formations that overlie the 

Montana group are important because they include 

enormous tonnages of low-rank coal. The older postMontana group formations, the Fox Hills and Hell 

Creek of Cretaceous age, are exposed in the extreme 

northeast corner and across the southern part of the 

Fort Peck area. The overlying Fort Union formation 

of Tertiary age has been stripped entirely from the 

area.

Some of the earliest and most important contributions to post-Montana group stratigraphy were made 

by Barnum Brown who spent the summers of 1902 to 

1906 in the badlands around Hell Creek, a few miles 

southwest of the Fort Peck area, where he studied the 

stratigraphy and collected remains of many kinds of 

dinosaurs.

Among other salient points, Brown (1907) named 

and described the dinosaur-bearing "Hell Creek beds" 

as consisting of a lower sandstone portion, separated 

from the subjacent Fox Hills sandstone by an erosional 

unconformity, and an upper portion consisting of clays 

and sandstones. He stated that no lignite occurs in the 

lower portion and described none in the upper. He 

also described lignite-bearing beds above the "Hell 

Creek beds" as being entirely free of dinosaur remains. 

These beds have a lignite bed at the base and probably

are of Fort Union age. He unconditionally assigned 

the next higher strata to the Fort Union formation.

Barnum Brown returned to Montana during the 

summers of 1908 and 1909 to map the Upper Cretaceous 

rocks just to the south and southeast of Fort Peck. In 

his report on these studies Brown (1914, p. 357) stated 

that "marine and brackish-water' sandstones [of the 

Fox Hills sandstone] grade into the massive sandstones above [Hell Creek formation] without any sign 

of discordance."

In later years, coal-mapping parties working in eastern Montana generally failed to recognize an erosional 

break at the top of the Fox Hills sandstone and considered the separation of the Fox Hills and Hell Creek 

formations to be arbitrary. Some geologists considered the names inappropriate, and for a time the 

name Lance formation, which was being used in Wyoming, was applied collectively to the Fox Hills and Hell 

Creek strata in Montana (Collier, 1918, p. 31-32).

Several more papers subsequently presented evidence bearing on the Fox Hills-Hell Creek contact. 

Probably one of the more important papers resulted 

from a study by Bauer (1924) on the Freedom dome, 

some 65 miles south-southwest of Fort Peck. In his 

report Bauer described a persistent bed at the base of 

the Hell Creek member of the Lance (Barnum Brown's 

Hell Creek beds) that contains numerous quartzite and 

porphyry pebbles. He noted that the subjacent Fox 

Hills (?) ranges in thickness from 35 to 66 feet. Since 

his field investigation covered only a small area, he 

was not aware of how widespread these phenomena are. 

The most recent work (Jensen, 195 Ic, 1952b; Colton, 

1955a, b) on the stratigraphy of these beds show conclusively that a time break does exist.
GEOLOGY OF THE FORT PECK AREA, MONTANA F5

SUBSURFACE BEDROCK FORMATIONS

Except for one outcrop of the Judith River formation at Tiger Butte, the Bearpaw shale is the oldest 

exposed bedrock in the Fort Peck area. Beneath the 

Bearpaw are several thousand feet of older sedimentary 

strata. These rocks are relatively well known, because 

they crop out in many places to the southwest and west. 

Where they are not exposed, they have been penetrated 

by many oil and gas borings. Information from these 

borings indicates that, beneath the Bearpaw shale in 

the Fort Peck area, there is 7,000 to 7,500 feet of sedimentary strata that range in age from Late Cretaceous 

to Cambrian and rest unconf ormably on Precambrian 

crystalline rocks.

EXPOSED BEDROCK FORMATIONS

The bedrock formations exposed at the surface in 

the general vicinity of the Fort Peck area (pi 1) range 

in age from mid-Late Cretaceous to Tertiary (Paleocene) and include, from oldest to youngest, the Judith 

River formation, the Bearpaw shale, the Fox Hills 

sandstone, the Hell Creek formation, and the Fort 

Union formation (Ross, Andrews, and Witkind, 1955). 

The first three formations form the middle and upper 

parts of the Montana group; the Hell Creek is uppermost Cretaceous and the Fort Union formation is 

Tertiary. All were formerly present throughout the 

Fort Peck area (fig. 1), but extensive erosion occasioned 

by uplift of the Bowdoin dome to the northwest has 

removed the Fort Union entirely and much of the Hell 

Creek and the Fox Hills formations. These formations, however, remain in large nearby areas to the 

south, east, and north. The Judith River formation 

crops out immediately to the northwest, but only one 

small area of outcrop within the report area is identified with this formation (see p. F25). The bedrock 

formation at the surface in most of the Fort Peck area 

is the Bearpaw shale.

JUDITH RIVER FORMATION

The rocks provisionally assigned to the Judith River 

formation are in a fault-bounded sliver on Tiger Butte 

about 7 miles southeast of Glasgow (pis. 1 and 3). The 

material is predominantly soft gray-brown siltstone 

and sandy and silty shale but includes some layers of 

very fine grained thin-bedded sandstone. Although 

the lack of fossils prevented positive identification, indirect evidence, which is discussed more fully on page 

F25, indicates that this isolated sliver was upthrown 

from the upper part of the Judith River formation 

which, elsewhere near this point, lies more than 400 

feet beneath the surface.

BEARPAW SHALE

NAME AND DISTRIBUTION

The Bearpaw shale was first named and described 

by Stanton and Hatcher (1905) for typical exposures 

north, east, and south of the Bearpaw Mountains of 

central Montana. The Bearpaw shale is the surface 

bedrock in the Fort Peck area, except for a total area 

of about 12 square miles in the extreme northeastern 

and southeastern parts of the area.

The middle and upper parts of the formation are 

adequately exposed for study in the mapped area; 

satisfactory exposures of the lower beds were studied 

a few miles northwest of the mapped area in T. 29 N., 

R. 38 E.

TOPOGRAPHY

The Bearpaw shale is more easily eroded than any 

of the bedrock formations which overlie it. The slopes 

of the detrital shale commonly form smooth curves. 

In those few broad areas where the shale is the surface 

deposit, the Bearpaw forms either badlands (fig. 3) or

FIGURE 3. The Bearpaw shale on the lower slopes of Milk River Hill 

In sec. 4, T. 26 N., K. 42 E., erodes to badlands and supports only 

sparse vegetation. Scars of old landslides illustrate the Inherent 

instability of the Bearpaw even when undisturbed by human agency.

a terrain of small hills having gentle slopes. .The badlands formed from the Bearpaw are generally much 

less rugged than those formed from the Hell Creek and 

Fox Hills, because Bearpaw strata vary less in their 

resistance to erosion.

Structure contours on the bedrock surface in the 

Fort Peck reservoir area are approximately parallel 

to the long axis of the reservoir, so that the' same suite 

of bedrock strata form the shore throughout the extent 

of the reservoir. These strata are the upper part of 

the Bearpaw shale.

THICKNESS

Fieldwork in the mapped area indicates the Bearpaw shale to be about 1,140 feet thick. Electric logs 

of test wells for oil and gas that were drilled a few 

miles south and east of the Fort Peck area indicate a 

somewhat smaller thickness. This difference in thick-
F6 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

ness may be due to an actual northerly and westerly 

thickening of the formation, to errors in the surface 

measurements, or to inconsistencies in the choice of 

formation limits.

Tests wells near the Fort Peck area in which the entire Bearpaw shale was recorded on electric logs yield 

the following data:

Well

Stanolind-Amerada

1 N.P.R.R.

Socony- Vacuum,

Pegasus Div. F-ll20-P. Waller.

Amerada 1 Rock Creek

Unit.

Shell 22-35 "A"

N.P.R.R. 

Marigold Oil, Ltd.,

1 Farnham.

Location

SEMSEJi sec. 5, T.

20 N., R. 45 E.

NWMNWJi sec. 20,

T. 21 N., R. 46 E.

SEJ^SEJ^SWJi sec

10, T. 22 N., R. 44'

E.

SEMNWJ^sec.35, T.

22 N., R. 47 E. 

SW^SWJi sec. 14, T.

25 N., R. 48 E.

Bearpaw shale » (feet)

Top

1,283

Bottom

2, 193-2, 228

1, 457 ' 2, 193-2, 420

856-894

1,450

861-874

1, 890-1, 908

2, 354-2, 402

1,805

Thickness

910-945

935-963

996-1, 052

904-952

931-944

1 The different curves of the electrical survey of some individual wells indicate 

formation boundaries to be at somewhat different levels; where this ambiguity occurs 

the indicated range of values is given. Electric log interpretation by F. S. Jensen.

STRATIGRAPHIC POSITION

The Bearpaw shale lies conformably on the Judith 

River formation; the contact is transitional. It is 

overlain conformably by the Fox Hills sandstone; this 

contact also is transitional.

AGE

The Bearpaw shale, which is in the upper part of the 

Montana group, is of Late Cretaceous age. According 

to W. A. Cobban (oral communication, 1960), the Bearpaw of the Fort Peck area, referred to the European 

standard section, is of Maestrichtian and possibly very 

late Campanian age.

IITHOIOGT

Almost all the formation consists of dark-gray clayey 

shale. Bentonite occurs either as distinct beds or is 

disseminated in the shale. Concretions of several 

kinds, as well as some sandy shale and silty shale, also 

make up the formation. A composite measured section 

is presented in plate 2; all the measurements of units 

constituting the composite section were made in the 

Fort Peck area except for measurements of the lower 

two units, which were made northwest of Glasgow in 

T. 29 N., R. 38 E.

The Bearpaw shale is subdivided into seven units on 

the basis of lithology and on the nature and distribution of concretions and bentonite beds. Some units 

have been traced a few tens of miles, others less. It 

is not known how persistent or useful the units will 

prove to be for stratigraphic correlation in adjacent 

areas or whether any of them deserve member rank.

Bentonite, a common constituent, is disseminated in 

the shale and occurs as distinct beds. Bentonite beds 

are so abundant in some parts of the formation or are 

of such distinctive character or sequence that stratigraphic position within the formation is readily determined by them. Most are *4 inch to 2 inches thick; 

several are 2 to 10 inches thick, and a few are locally 

as much as 2 feet thick. Distinctive character depends 

on thickness, color, and presence or absence of mica. 

Colors range from pale green, pale yellow, or cream 

to neutral gray. The mica, probably biotite, where 

present, is more abundant near the base of individual 

beds, but in some places the mica is so abundant 

throughout that it darkens the color of the rock considerably. Lower contacts of the bentonite beds are 

sharp; upper contacts are gradational.

Distinctive concretions are characteristic of certain 

parts of the formation; in some places a particular 

kind of concretion is limited to a single bed, in other 

places a distinctive kind occurs in zones that are as 

much as a few hundred feet thick (pi. 2). Cone-incone structures, which occur as partial envelopes around 

concretions, are common in the Bearpaw shale. The 

concretions are classified as follows:

1. Clay-ironstone concretions (fig. 4), the most abundant, generally range from 6 inches to 2 feet in 

maximum diameter. The smaller concretions are 

generally oval both in plan and cross section. The 

large ones are oval in cross section but nearly circular in plan. They are nonseptarian and contain

FIGURE 4. Alternate wetting and drying, the result of fluctuating 

water levels, cause the Bearpaw shale exposed along the shoreline 

of the Fort Peck Reservoir to slack and disintegrate. Wave action 

then moves the comminuted shale to deeper water, leaving behind a 

"lag concentrate" of concretions.

very few fossils. Unweathered cores are light 

olive gray (5Y 6/1), and outer weathered parts 

are dusky red (WR 2/2) and dusky brown (5YR
GEOLOGY OF THE FORT PECK AREA, MONTANA F7

2/2). Clay-ironstone concretions and bentonitic 

shale are so commonly associated that a genetic 

relation is suggested.

2. Medium dark-gray (A"4) fossiliferous limestone 

concretions are second in abundance and are 

limited largely to the upper and middle parts of 

the formation. These are similar in size to the 

clay-ironstone concretions and are also nonseptarian. They generally weather yellowish gray

f 3. Ovoid and disk-shaped septarian concretions, some 

of which are fossiliferous, seem to be limited to a 

few stratigraphic positions within the formation. 

They are 3 to 8 feet in diameter, and exhibit, in 

different strata, all the colors noted for the other 

types of concretions. Locally, the concretions are 

enclosed in yellowish-brown "envelopes" several 

inches thick that are also of concretionary origin. 

The septaria are generally honey-yellow calcite 

although, at one horizon near the top of the formation, they are also composed of colorless or smoky 

barite.

4. "Tepee butte" limestone concretions of the type first 

described in the Pierre shale of southeastern 

Colorado by Gilbert and Gulliver (1895) occur in 

the Fort Peck area in a thin zone about 530 feet 

below the top of the Bearpaw. They commonly 

occur in this general stratigraphic position in 

many western interior States (Darton, 1905, p. 68, 

108, 168). In general, the "tepee butte" concretions of the Fort Peck area conform to Gilbert's 

original description (Darton, 1905, p. 108) which 

states that the concretions consist of "coarse, gray 

fossiliferous limestone, irregular or rudely cylindrical in form and standing vertical within the 

shale mass. Ordinarily they are from 5 to 30 

feet in horizontal diameter and their vertical extent is greater." Except at Twin Buttes, 2 miles 

southwest of the Glasgow quadrangle, the concretions in this area do not form prominent topographic features but occur as irregular masses only 

partially stripped of the enveloping shale. Most 

are found in the central part of T. 26 N., R. 39 E., 

and along the highway in sec. 29, T. 28 N., R. 42 

E. Only locally are the concretions of this zone 

large enough to constitute "tepee butte" limestone 

masses. The zone is represented laterally by concretions of gradually diminished size ; the smaller 

concretions of this zone are similar in size and 

shape to the fossiliferous-limestone concretions 

just described, although they are somewhat lighter 

in color.

WEATHERING

The shale at the surface is much weathered, and completely unweathered shale is exposed only at the baffles 

at the foot of the Fort Peck Reservoir spillway (locality inaccessible), where erosion is extremely rapid. 

The fresh shale is very dark gray and seems fairly well 

consolidated.

Weathering disaggregates the shale considerably, 

probably through leaching of colloidal matter. Corroded crystals of selenite form in the weathered zone. 

The scarcity or absence of these crystals on actively 

eroding slopes indicates that the crystals are entirely 

a product of weathering. Figure 5 shows two mechanical analyses of weathered shale, which is generally 

close to light olive gray (5Y 5/1 to 6T 5/2) though 

somewhat darker at places where less thoroughly 

weathered, such as in cutbanks that are being actively 

eroded.

Depth and thoroughness of weathering depend upon 

rate of erosion, composition of the beds, and intricacy 

of jointing. Thus, the thickness of the weathered zone 

ranges from several feet to several tens of feet. Other 

factors being equal, the weathered zone is relatively 

shallow over beds that contain appreciable quantities 

of bentonite dispersed in the shale or that include 

laminations of nearly pure bentonite. Initial weathering concentrates the bentonite as a surface rind that 

swells when wet and forms a seal that inhibits further 

weathering. When dry, the rind has a cracked and 

porous spongy surface and is easily distinguished from 

the soft flaky chips that mantle the nonbentonitic shale. 

Weathering is deepest in nonbentonitic shale where 

there is little erosion.

Abundant shallow fractures separate the weathered 

shale into small blocks. These fractures decrease in 

number downward and are comparatively rare in fresh 

shale. The comparatively few deep fractures found 

locally in the Bearpaw shale were produced by slumping of large blocks and, in a few places, by faulting. 

Because the shale is nearly impervious, except along 

fractures, deposition of mineral matter by percolating 

water has sealed many of the fractures.

FAUNA

A large marine fauna, mostly molluscan, has been 

collected from the Bearpaw shale of the Fort Peck 

area. This material was referred to William A. Cobban of the U.S. Geological Survey for identification; 

his faunal lists are given in the following table. 

Among his comments (written communications, 1952) 

on the material are the following:
F8

Cobbles

rlOOPebbles

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

Granules Sand Silt and clay

90

-80

-70

60

50

40

-30

-Alluvium of Porcupine Creek Valley (sample 23)

-^-JOutwosh terrace gravel (sample II)

Out wash terrace gravel (sample 12)

< -20

100 10 1.0 O.I 0.01

PARTICLE SIZE, IN MILLIMETERS 

FIGURE 5. Mechanical analyses of samples of weathered Bearpaw shale, Fox Hills sandstone, and till.

These collections are of * * * interest and value in that they 

contain excellent specimens of such rare species as Warren's 

Pecten saskatchewanensis, Whitfleld's Hoploparia ftrowni, and 

the young stages of Hyatt's Emperoceras.

Actinosepia canadensis Whiteaves is the pen of a cuttle-fish, 

described originally from the "Upper Montana" of Canada. 

This is the third record of the fossil.

[Collection 23509] containing Hesperorhynchia superba Warren, is of exceptional interest in that this brachiopod has never 

previously been reported from the United States.

The scaphite collections show the presence of two zones. 

The lower, which extends through most of the [Bearpaw] 

formation, is characterized by the genus Acanthoscaphites. The 

upper zone has DiscoscapMtes as its guide fossil.

The genus Baculites is well represented in the Bearpaw shale, and the several species present have proved 

to be very useful in zoning. Specimens are of varied 

abundance stratigraphically but are present throughout.

List of fauna collected from the Bearpaw shale of the Fort Peck area, Montana 

[Units within Bearpaw shale: L, lower unit; M, middle unit, U, upper unit; ?, unit uncertain]

USQS 

Mesozoic 

locality

Location

Units within Bearpaw shale (see pi. 2B; L, lower; U, upper)

1

L U

2

L U

3 4

L U

5

L M U

6

L M U 7

MOLLUSCA

Pelecypoda

Pecten nebrascenjsis Meek and Hayden

23361

23361

23361

23387

23366

23376

23360

22140

ODOtQ

23361

OOOCQ

23376

OOOQQ

23392

23502

EK sec 5, T. 26 N., R. 42 E. ..........

...-do...-..-...- .... . ... ..

.. _ do......... .. . . . .

SJ^ sec. 3, T. 33 N., R. 37 E...........

E}^ sec. 11 and NEJ£ sec. 26, T. 28 N.,

R. 41 E. Center sec. 16, T. 29 N., R. 43 E. _ 

EJ^ sec. 5, T. 26 N., R. 42 E...........

NEM sec. 12, T. 26 N., R. 41 E. __ ..

EH sec. 5, T. 26 N., R. 42 £..._. .._

... ..do........ .... . . .

SEM sec. 11, T. 28 N., R. 41 E........

SEMSWJi sec. 1, T. 26 N., R. 41 E ...

SEj| sec. 11, T. 28 N., R. 41 E.. ......

.. do........................-. .-. X

X

X

X

y

X

y

X

X
GEOLOGY OF THE FORT PECK AREA, MONTANA 

List of fauna collected from the Bearpaw shale of the Fort Peck area, Montana Continued

F9

USGS 

Mesozoic 

locality

Location

Units within Bearpaw shale (see pi. 2B; L, lower; U, upper)

1

L U

2

L U

3 4

L U

5

L M U

6

L M U ?

MOLLUSC A Continued 

Pelecypoda Continued

23362

23387

23366

23361

23371

23374

23388

23361

23363

23386

23366

23370

23389

23360

23363

23361

23376

23361

23365

23366

23369

23375

23380

23366

23376

23381

23359

23361

23366

23376

23376

23509

23361

23509

23361

23362

SEH sec. 5, T. 26 N., R. 41 E.. .... -

SEM sec. 5, T. 26 N., R. 42 E. ........

__ do __ . _ - - . _ -.-

.... .do... 

SEMSEM sec. 11, T. 26 N., R. 41 E 

WH sec. 2, T. 27 N., R. 40 E... .......

 .do 

...-do.....-.-...... ... ......... ...

. do 

Near SHsec. 3, T. 33 N., R. 37 E.. 

..... do.... ........................ .

SEMNWM sec. 29, T. 28 N., R. 42 E_.

Centersec. 34, T. 29 N., R. 41 E ......

.... .do..... .... .... .... .... 

-.-do ............................

 - do..... ..._ . 

.....do. ........... .............. .

.... .do 

NEM sec. 26, T. 28 N., R. 41 E........

.....do... 

.....do.... 

NEM sec. 36, T. 28 N., R. 41 E. .......

SWMsec. 13, T. 28 N., R. 41 E........

.___ .do.-... .

 .do 

Near SH sec. 3, T. 33 N., R. 37 E.. 

.... .do....... ... ...... ....... ... ...

.... .do .......... ......... ....

..... do.... ......................... 

 do.. _ . . 

..... do... ........... ........... .... ... .

 do 

 .do 

.... .do..... _ . 

Near SJi sec. 3, T. 33 N., R. 42E 

SEM sec. 5, T. 26 N., R. 42 E........ .

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

Gastropoda

23361

23360

23373

23376

23361

23363

23360

23361

23363

23370

23376

22140

23361

23381

23361

EH sec. 5, T. 26 N., R. 42 E...... _ .

 .do 

WH sec. 2, T. 27 N., R. 40 E... .......

 .do .

 do 

.... .do.....- ...... ... ... .... ... ....

... ..do -

...-.do -- ---------------------- --

--..-do -------------------------

.....do... -- - -. --- -----

-..-do ----------------------------

--..-do ---------------------- ------

-...-do ---------------------------

-..-.do- ------ ----------------------

--..-do --------------------------

--.-do ------------------- --------

X

X

X

X

X

X

X

X

X

X

X

X

23360

23361

23391

21381

23360

23370

23502

Scaphopoda

W>£ sec. 2, T. 27 N., R. 40 E.... ......

.... dO --------------------------

SEVisec. 5, T. 26 N., R. 42 E. -------

Cephalopoda

SEMsec. 5, T. 26 N., R. 42 E. __ 

Nautiloidea

SEM sec. 5, T. 26 N., R. 42 E... ......

-.-.do ---------------------------

.-..-do -------------------------- X

X

X

X

X

X
F10 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 

List of fauna collected from the Bearpaw shale of the Fort Peck area, Montana Continued

USGS 

Mesozoic 

locality

Location

Units within Bearpaw shale (see pi. 2B; L, lower; TJ, upper)

1

L U

2

L U

3 4

L U L

5

M U

6

L M U ?

MOLLUSCA Continued 

Ammonoidea

Discoscavhites n so

22145

22142

22141

22139

22140

22142

22143

22141

22140

22142

23381

23387

22141

23358

23364

23370

23371

23378

23379

23382

23384

23390

23393

23373

23385

23389

23368

23360

23359

23361

22144

23363

23391

22146

23391

22144

23373

23381

23382

23383

23384

23377

23366

23367

23502

9QQQO

23388

E^ sec. 5, T. 26N., R. 42E . 

SE^sec. 5, T. 26 N., R. 42 E... ...

SE^SE^i sec. 11, T. 26 N., R. 41 E___

SEi^NW^i sec. 29, T. 28 N., R. 42 E_.

NEJ£ sec. 12, T. 26 N., R. 42 E... .....

SE^sec. 5, T. 26 N., R. 42 E.. .......

--..do -----------------------------

-..- do -----------------------

--.-.do ----------- -----------------

.-.. do ... ---------------------- --_

--.--do ----------------------------

---do ------------------------

--.-do -------------- -------------

NW^sec. 1, T. 27 N., R. 42 E- 

 - -do ---------------------------

-.-do -------------------------

NWJ^SWJi sec. 8, T. 27 N., R. 42 E..

SW&NEjk sec. 14, T. 26 N., R. 41 E_.

Near Sy2 sec. 3, T. 33 N., R. 37 E_____ WH sec. 2, T. 27 N., R. 40 E.. ....

T. 34 N., R. 38 E -----------------

-. do ------------------------

WH sec. 2, T. 27 N., R. 40 E._ ...

--.-do ------------------------

NEJi sec. 26, T. 28 N., R. 41 E........

--.-.do -------------------------

--...do --------------------------

-.--do -- ---------------- --------

-.. ..do-...---- ----------------------

--.-do ---------------------------

SE^sec. 5, T. 26 N.,R. 42 E. ..----__

- ...do ------------ --------------

SEJ< sec. 5, T. 26 N., R. 42 E.. -------

-....do ------------------------

- ...do ---------------------------

--. do .--- -- 

NE^sec. 21, T. 28 N., R. 41 E ....

 do --------------------------

NEJi sec. 26, T. 28 N., R. 41 E 

NEJ£ sec. 26, T. 28 N., R. 41 E. ...

X

X

?

y

OQQ7C

23509

23371

21380

BRACHIOPODA 

Inarticulata

NEJ4 sec. 26, T. 28 N., R. 41 E 

Articulata

NEJ4 sec. 26, T. 28 N., R. 41 E........

ARTHROPODA 

Crustacea

NEJ4 sec. 26, T. 28 N., R. 41 E ....

WJ^ sec. 4, or E^ sec. 5, T. 26 N., R.

42 E.

X
GEOLOGY OF THE FORT PECK AREA, MONTANA Fll

Most fossils are in moderate-to-small-sized limestone 

concretions. A few are in the large septarian concretions, although they are somewhat less abundant in the 

concretions containing sufficient iron oxide to make the 

weathered outer portion dark reddish brown. Fossils 

are very sparse in clay-ironstone concertions.

ENGINEERING CONSIDERATIONS

Both unweathered and weathered shale are readily 

excavated with power tools, although some of the 

larger concretions might slow down progress. The 

network of small fractures in the weathered part facilitates excavation but necessitates timbering in tunnels 

and in large pits. Because the shale has a very low 

permeability, water seepage is negligible except along 

major fractures. Subsurface drainage is very poor, 

even in highly weathered shale.

Landslides, including large-scale slumping, are potential hazards wherever the shale is exposed to unusually rapid weathering and wherever construction 

involves cutting or building on steep slopes.

Unweathered shale slacks and loses strength very 

rapidly unless protected from wetting and drying. 

Wet shale reverts rapidly to plastic mud, so that unsurfaced roads on the shale are impassable in wet 

weather. Large quantities of coarse aggregate must be 

added to produce a satisfactory subgrade. Figure 5 

shows two mechanical analyses of the weathered shale. 

Soil classification and Atterberg limits are given in 

figure 6 for shale containing dispersed bentonite 

(sample 25), for nonbentonitic shale (sample 14), and 

for bentonite (sample 13).

The characteristics just mentioned are of considerable significance in regard to shoreline erosion, which 

is unusually active along the borders of the Fort Peck 

Reservoir (fig. 4). The upper part of the Bearpaw 

shale, which everywhere forms the shoreline, provides 

a soft, easily credible material that is readily removed 

by wave action, precipitation, and the slumping that results from fluctuations in reservoir level.

The hills that border the reservoir are in many places 

capped by the Fox Hills and Hell Creek formations. 

Where these more resistant strata have slumped, the 

Bearpaw shale slopes are partly but inadequately protected by the resulting debris (fig. 7) against shoreline 

erosion.

The rate of recession of the shoreline is a function of 

the ease with which the shale weathers (fig. 4) and of 

the frequency and magnitude of reservoir-level fluctuations. Weathering causes an intricate system of 

fractures to develop in the shale. Water enters these 

fractures and lubricates the fracture surfaces, thereby 

promoting landsliding. The water introduced into the

shale by precipitation and by infiltration below minimum reservoir level plays an important role in these 

processes, but particularly important are the rises in 

reservoir level which cause saturation of the contiguous 

parts of the shale slopes. The stability of the shale is 

greatly reduced during the ensuing drawdown because 

of the lubrication of the fracture surfaces and because 

of the weight of the added water. The net effect is to 

provide a large amount of slump debris at the shoreline, which is soon triturated by wave action and comminuted by wetting and drying. The resulting finegrained debris is moved to deeper water by gentle 

currents over low gradients; hence, the shore profile of 

equilibrium that is now developing will be a long slope 

gently concave upward.

FOX HILLS SANDSTONE

The Fox Hills sandstone was first described by Meek 

(1886) and Hayden (1862) from exposures along the 

bluffs of Fox Ridge in the area that is now northwestern Armstrong and southwestern Dewey Counties in 

northwestern South Dakota, about 350 miles southeast 

of the Fort Peck area.

DISTRIBUTION

The Fox Hills sandstone underlies 12 square miles in 

the south-central and extreme northeastern parts of 

the map area; locally, it is overlain by the younger 

Hell Creek formation or is partly concealed by surficial 

materials.

Studies of the Fox Hills were extended beyond the 

mapped area, even though all parts of the formation 

are well exposed here, in order to determine the persistence and lateral relationships of stratigraphic features within and bounding the formation. These 

studies were extended southwestward to include the 

Hell Creek-Devil Creek-Seven Blackfeet Creek drainage areas in Garfield County on the south side of Fort 

Peck Reservoir and the Larb Hills in Valley County 

on the north side; traverses were also made eastward 

into McCone County and northward to the Opheim 

area in Valley County; the Colgate sandstone member 

was examined at its type locality in Dawson County.

TOPOGRAPHY

The Fox Hills is well exposed in the badlands (fig. 

7) east of the Fort Peck Reservoir but is only poorly 

exposed on slopes of the hilly upland in the northeast 

corner of the map area. The number and quality of 

exposures differ according to the part of the formation 

involved. The lower beds are soft and largely uncemented, and most beds commonly weather to gentle 

or moderate slopes (fig. 7). The well-cemented parts
F12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

of the upper part of the Fox Hills sandstone stand out 

as rimrock (figs. 7 and 8) because of their superior 

resistance to erosion.

THICKNESS

The Fox Hills sandstone reaches a thickness of as 

much as 120 feet in the Fort Peck area. The lower 35 

to 40 feet is made up of beds transitional from the underlying Bearpaw shale to the overlying sandstone. 

The period of erosion in northeastern Montana that intervened between deposition of the Fox Hills and the 

overlying Hell Creek formation produced considerable local variations in the thickness of the Fox Hills, 

so that in and adjacent to the Fort Peck area the measured thickness ranges from at least 35 to 146 feet (fig. 

9).

STRATIGRAPHIC POSITION

The Fox Hills sandstone lies conformably on the 

Bearpaw shale; the contact is transitional (fig. 10). 

An erosional unconformity separates the Fox Hills 

from the overlying Hell Creek formation.

AGE

The Fox Hills sandstone, the uppermost formation 

of the Montana group, is of Late Cretaceous age. The 

formation is not of the same age throughout its area 

of occurrence in the northern Great Plains, inasmuch 

as it is a strandline deposit of an easterly retreating 

shoreline. W. A. Cobban (oral communication, 1952) 

said that the fauna near the type locality indicates a 

younger age than the fauna of the Fox Hills in northeastern Montana.

Sample

8

11

23

31

12

19

1

30

4

7

26

15

28

22

2

nit

10

21

14

3

25

13

Location

NEy4NWy4sec.29,T.28N.,R.42E.

NEy4SWy4Sec.36,T.28N.,R.41 E.

NEy4SEy4sec.23,T.28N., R.41E.

NWV4NWV4 sec. 9, T. 26 N.. R. 42 E.

Center sec. 35. T. 28 N..R. 41 E.

NEy4NEy4sec.29,T.29N.,R.41E.

SWy4SEy4sec.l7,T.28N.,R.41 E.

NEy4SEy4sec.8,T.26N.,R.41 E.

Center SWV4 s<£. 19, T. 27 N., R. 42 E.

SEy4NEy4sec.36,T.28N.,R.41 E.

NE%NEV4 sec. 1, T. 27 N.. R. 41 E.

Center sec. 6, T. 26 N., R. 41 E.

SEy4NEy4 sec. 20. T. 28 N., R. 42 E.

NEV4NWV4 sec. 10, T. 27 N., R. 41 E.

NWy4SWWsec.36,T.29N.,R.41E.

NEV4NEy4 sec. 13, T. 26 N.. R. 41 E.

SEy4NWy4 sec. 31, T. 28 N., R. 42 E.

NWV4NEV4 sec. 18, T. 27 N., R. 42 E.

NEy4NWy« sec. 28, T. 29 N., R. 41 E.

SWKNB4 sec. 11, T. 28 N., R. 41 E.

Center SWV4 sec. 19, T. 27 N., R. 42 E.

SEy4$Ey4 sec. 6, T. 26 N., R. 42 E.

Center sec. 35, T. 28 N., R. 41 E.

Classification1

A-l-a

A-l-a

A-l-a

A-3

A 1 a

A-4

A-2-4

A-2-4

A-6

A-6

A-7-6

-/-u

A-7-6

A-7-6

A-7-6

A-7-5

7

7

Map unit 

(

Wiota gravels (Qw)

Sandy gravel from alluvium (Qal) of Porcupine 

Creek valley

Loose sand from the Fox Hills sandstone (Kfh)

Wiota gravels (Qw)

Loose sand from the upper part of the Wiota

gravels (Qw)

Till from the ground moraine (Qgm)

Till from the ground moraine (Qgm)

Till from the ground moraine (Qgm)

Till from the ground moraine (Qgm)

Clay-silt-sand from upper part of alluvium (Qal)

of Porcupine Creek

Alluvium-colluvium (Qac)

Bearpaw shale (Kb), without disseminated bentonite

Shaly clay from the Kintyre formation (Qk)

Bearpaw shale (Kb), with disseminated bentonite

Moisture content, in percentage 

weight of oven-dried sample

) 10 20 30 40 50 60 70 80 90 100 iii i i i i i i

> . (Nonplastic)

(Nonplastic)

(Nonplastic)

Platfir . ,0ou^ limit ....... (P\\ v . .-,

(difference) LL-PL

^ *^-» "' :.-

 ...

-

 ' -

-""- ^

-

 '

............... ......^ .

.SSSS5J

m mm

m -- ' -

1 U.S. Bureau of Public Roads Classification of highway subgrade materials

FIGURE 6. Atterberg limits and soils classification of test samples. Data from results of tests by Montana State Highway Department.
GEOLOGY OF THE FORT PECK AREA, MONTANA F13

FIGURE 7. On Milk River Hill (sec. 4, T. 26 N., R. 42 B.) the upper part of the Bearpaw shale (foreground) is overlain by light-gray weathered 

beds composing the lower part of the Fox Hills sandstone. The resistant sandstones of the upper part of the Fox Hills cap the hill. 

Mass wasting has mantled the Bearpaw shale with debris of the Fox Hills sandstone.

FIGUBE 8. The rimrock formed by the upper beds of the Fox Hills sandstone is characteristically developed here on Milk River Hill in the 

SB cor. sec. 5, T. 26 N., R. 42 E. Flattened ellipsoidal concretionary masses, harder and darker than the sandstone matrix, form ledges 

and cap pedestals.

690-458 O-64-2
F14 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

T
GEOLOGY OF THE FORT PECK AREA, MONTANA F15

LITHOIOGY

The lower part of the Fox Hills is composed of 

noncalcareous thin beds of alternating soft claystone, 

siltstone, and very fine sandstone, the finer grained 

beds predominating near the base. Near the top of 

this sequence at most localities there are 1 or 2 discontinuous ledges of sandstone as much as 8 inches thick 

lithified with calcareous cement. These ledges show 

small-scale crossbedding, but other bedding in the 

lower part of the formation is parallel. Unweathered 

rocks beneath the surface are various shades of yellowish gray (5T 6/1, 6T 6/2, 5T 7/2) below and of 

light dusky yellow (5Y 7/4) and dark yellowish 

orange (10T7? 6/6) above. The lower part of the Fox 

Hills sandstone typically weathers light olive gray 

(5T 7/1) and light gray (vV7), colors that are easily 

distinguished from the underlying drab-gray Bearpaw 

shale.

The upper part of the Fox Hills is as much as 85 

feet thick. Most of it is fairly uniform very fine 

grained sandstone; the rest is lenticular thin-bedded 

shale and siltstone. Some of the sandstone is thin 

bedded; much is massive bedded, and a small part is 

crossbedded. Some of the crossbedding near the top is 

on a fairly large scale, approaching that seen in the 

overlying Hell Creek formation. Most of the weathered material is yellowish brown (10T7? 6/4) to dark 

yellowish orange (10T7? 6/6), but many colors from 

dark brown to pale gray are represented. Fresh sandstone is close to pale yellowish brown (10T7? 6/2). 

The sandstone ranges from very soft to hard owing to 

differential cementation by calcium carbonate.

At several localities in the vicinity of Fort Peck 

where tests were made, all the cemented Fox Hills 

above the transition beds, as well as the cemented sandstones within the Hell Creek formation, are calcareous. 

The transition beds and the Bearpaw shale are, on the 

other hand, noncalcareous. The fact that the cementing material is calcium carbonate contrasts with the 

findings of Thorn and Dobbin (1924, p. 492), who 

hold that the Fox Hills and the underlying strata are 

relatively free of lime, whereas the Colgate,1 Hell 

Creek, and overlying strata are markedly calcareous.

Concretionary masses (fig. 8) are common in the 

Fox Hills sandstone above the transition beds. Most 

abundant are thick lens- and log-shaped concretions 

cemented by calcium carbonate ranging from several 

feet to several yards in maximum dimension; these 

form the intricately eroded rim rock that is a conspicu1 Samples were collected by Jensen from the lower, middle, and 

upper parts of the Colgate member near its type locality in southeastern Montana, and from the Colgate member on Devil Creek in 

northwestern Garfield County, Mont. The material is uniformly noncalcareous.

ous feature of the Fox Hills sandstone in this area (fig. 

8).

Small spherical sandstone concretions cemented by 

pyrite are also common. They are pea-sized to golfball sized and occur alone or joined in pairs or chains. 

There are clay-ironstone concretions, generally smaller 

than those in the Bearpaw shale, that range from walnut size to slabs 6 to 8 inches thick and 2 to 3 feet long.

ORIGIN OF THE FOX HILLS

The composition and bedding of the lower part of 

the formation are consistent with the widely held view 

that the Fox Hills is predominantly marine, but an unequivocal statement as to the marine brackish- or 

fresh-water character of the Fox Hills in the Fort 

Peck area cannot be made on the basis of the present 

study.

No fossils, other than a few carbonized plant fragments near the top of the formation, were found during 

the present study; however, marine fossils have been 

reported from the Fox Hills in several other parts of 

Montana. Barnum Brown (1907, p. 827) collected 14 

species of marine invertebrates from the Fox Hills at 

a point on Hell Creek about 40 miles southwest of Fort 

Peck. Bowen (1915, p. 116) obtained a small collection of the marine brachiopod Lingula from a sandstone "probably near the top of the transition zone" of 

the Fox Hills at a point about 90 miles southwest of 

Fort Peck. Dobbin and Keeside (1929, p. 17) cited a 

measured section at a point on Devil Creek about 60 

miles west-southwest of Fort Peck, in which they list 

a 2-foot-thick bed of "hard brown sandstone [containing] Lingula nitida Meek and Hayden in abundance." 

This bed is 59 feet above the base of the Fox Hills and 

is 12 feet below the Colgate member (this member is 

present locally at the top of the Fox Hills sandstone in 

eastern Montana and is discussed subsequently). Colton (oral communication, 1952) has found a few specimens of Halymenites, a fucoid commonly considered 

marine, in the lower part of the Fox Hills at a point 

just south of the Missouri River and about 50 miles 

east of Fort Peck. Dobbin and Reeside (1929, p. 11) 

said that in southeastern Montana "the lower member 

of the [Fox Hills] formation contains a few marine 

invertebrates." Dobbin and Reeside (1929, p. 15) also 

reported "abundant remains of Halymenites major" 

from the Colgate member. In South Dakota, Wyoming, and Colorado, marine fossils are so characteristic 

that Lovering and others (1932, p. 702-703) have defined the top of the formation as the top of predominantly marine beds.

In eastern Montana, however, there is some basis to 

suggest that the upper part of the Fox Hills is non-
F16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

marine. R. W. Brown (1939, p. 239) described a flora 

from the Colgate member in southeastern Montana 

and, in noting other plant collections from the Fox 

Hills, said: "Plant fossils, although relatively scarce, 

have been reported occasionally from the continental 

and shore facies of the Fox Hills sandstone." Thorn 

and Dobbin (1924, p. 490) agreed with those specifying a nonmarine character for parts of the formation, 

for in describing the Colgate member "along the Missouri between Hell Creek and Musselshell River" they 

said that the member resembles "its type development 

in lithology, though probably of fresh-water rather 

than strand accumulation."

In the Fort Peck area the presence of plant fragments and sandstones in the top few feet of the Fox 

Hills suggests a continental origin for this part of the 

Fox Hills. Much of the continental material in this 

general area has probably been removed by pre-Hell 

Creek erosion, and surviving beds are apparently 

limited to places where the overall thickness of the Fox 

Hills sandstone is the greatest.

APPLICABIUTT OF THE TERM COLGATE MEMBER IN THE FORT PECK AREA

Collier and Knechtel (1939, p. 9-10, pi. 3) held that 

the Colgate member of the Fox Hills is present in McCone County. A part of the county is in the Fort Peck 

area, but no part of the Fox Hills formation cropping 

out in McCone County corresponds to the following 

description by Thorn and Dobbin (1924, p. 490) :

the conspicuous, white, upper sandstone of the Fox Hills, 

typically developed between Colgate station and Glendive, Montana, and extensively exposed along Cedar Creek anticline and 

elsewhere in eastern Montana. In addition to fossil leaves, 

the Colgate sandstone * * * contains abundant casts of Halymenites major in exposure along the Cedar Creek anticline and 

is gradational into the underlying marine strata on Little 

Beaver Creek. The Colgate is strikingly developed along the 

Missouri between Hell Creek and Musselshell River * * *.

In company of R. W. Brown, R. B. Colton, and J. O. 

Kistler, the senior author visited the area between Hell 

Creek and the Musselshell and attempted to trace the 

Colgate member eastward into McCone County and 

the Fort Peck area. Snowy white in its more westerly 

exposures, the member gradationally changes its character eastward and is indistiguishable as a lithologic 

unit before Hell Creek is reached. Though Collier 

and Knechtel (1939, p. 10) base their opinion on the 

work of Barnum Brown (1914), nowhere in his paper 

nor, as a matter of fact, in his earlier paper on this 

area (Barnum Brown, 1907) is there any mention of a 

white sandstone. It seems most probable that the 

upper part of the Fox Hills sandstone in the Fort

Peck-McCone County area is the stratigraphic equivalent of the Colgate member, but there is no reason for 

applying the member name here,

ENGINEERING CONSIDERATIONS

Weathering has not affected the strength of the 

material of the Fox Hills to the degree that it has 

affected the Bearpaw shale, though near-surface material has probably been somewhat softened by the leaching of cementing constituents. Surface drainage is 

good. Subsurface drainage varies with degree of cementation but is generally fair to good.

Probably very little ground water can be recovered 

from the Fox Hills sandstone within the Fort Peck 

area. There are no springs at the contact of the relatively pervious sandstone with the nearly impervious 

Bearpaw shale beneath. Minor perched water tables 

are likely to be present because of the wide local variations in degree of cementation within the formation.

The semiconsolidated lower part of the formation 

and the sand and soft sandstone of the upper part can 

be excavated with power tools. The discontinuous 

masses of hard sandstone, which are so common in the 

upper part, require blasting for removal.

Both shallow and deep cuts should stand well even at 

angles steeper than 45°, though some sloughing from 

cut surfaces should be expected. Large cuts extending 

through the formation into the Underlying Bearpaw 

shale might be subject to landsliding. Unsurfaced 

roads crossing the Fox Hills sandstone are passable in 

all weather.

The indurated sandstone might be useful as a construction material, but the irregular occurrence of these 

hard layers and the wide variations in quantity from 

one outcrop to the next would greatly raise the costs 

of development.

Soil classification and Atterberg limits of loose 

sand from the upper part of the formation are given in 

figure 6 (sample 31). A mechanical analysis of this 

sand is shown in figure 5.

STRATA OVERLYING THE MONTANA GROUP

One of- the most widely discussed questions of the 

early geologists of the western interior of the United 

States was the Cretaceous-Tertiary boundary (the subject was often referred to as "the Lance-Laramie 

Problem"). The strata principally involved in the 

discussions were the Fox Hills sandstone, the Hell 

Creek formation, the Fort Union formation, and the 

lateral time equivalents of these and other formations
GEOLOGY OF THE FORT PECK AREA, MONTANA F17

that were believed to chronologically intervene or to be 

lateral time equivalents.

At present the position of the Tertiary-Cretaceous 

boundary itself is no longer actively disputed and is 

placed at the contact of the Hell Creek formation (or 

the equivalent Lance) with the overlying Fort Union 

formation (Dorf, 1942). The nature of the contact between the Fox Hills and the Hell Creek was studied by 

Thorn and Dobbin (1924) and again by Dobbin and 

Reeside (1929), and they expressed the opinion that 

there was virtually no break in sedimentation between 

the Fox Hills and the Hell Creek. Eecent studies of 

the Fox Hills and Hell Creek formations in the general Fort Peck area establish that the two formations 

in that part of Montana are consistently disconformable (fig. 10), and that a significant break in sedimentation between the two there may be indicated.

HELL CREEK FORMATION

NAME AND DISTRIBUTION

The Hell Creek formation was named and first described by Barnum Brown (1907) for typical exposures along Hell Creek in Garfield County, 30 to 40 

miles southwest of the Fort Peck area.

The lower part of the Hell Creek formation is present in about 5 square miles in the northeastern and 

south-central parts of the area.

The Hell Creek formation was also examined beyond 

its scanty occurrences in the mapped area; these examinations were made in McCone County south and 

east of the mapped area, in the Valley County as far 

north as the Opheim area and as far west as the Larb 

Hills (fig. 13), and in Garfield County at the type 

locality along Hell Creek.

FIGURE 10. The contact of the white Colgate member of the Fox Hills sandstone with the overlying Hell Creek formation Is sharp and scalloped 

by pre-Hell Creek erosion. Photograph taken in the valley of the Seven Blackfoot Creek (T. 20 N., R. 34 B.). The Bearpaw shale exposed 

in the lower parts of the narrow canyon Is gradational upward into the lower member of the Fox Hills sandstone.
F18 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

TOPOGRAPHY

The Hell Creek formation underlies the Flaxville 

formation, which caps the isolated hills and the upland 

in the northeastern part of the Frazer quadrangle. In 

the area east of the Fort Peck Dam and south of the 

Missouri River, the Hell Creek formation and the Fox 

Hills sandstone form a rolling upland. This terrain 

is much dissected by steep-sided coulees (fig. 12) and 

otherwise interrupted by ledges of sandstone (fig. 11), 

which owe their prominence to their superior resistance 

to erosion. Growths of green ash and cottonwood trace 

the coulee bottoms and provide shelter for deer and 

cattle. Sandstone ledges, remnants of lenses well above 

the base of the formation, occur in buttes here and 

there on the upland (fig. 11).

Farther south, where it is more extensive, the Hell 

Creek formation is eroded into spectacular badlands 

in many places; the hard concretionary masses and

remnants of sandstone ledges cap pedestals, buttes, and 

various intricate landforms having sides or lower 

slopes cut, commonly in fluted fashion, in the softer 

interbeds. Bentonitic beds resist erosion more effectively than otherwise similar beds lacking disseminated 

bentonite, and many badland domical hills and roundtopped ridges, totally devoid of vegetation, owe their 

prominence to protective caps of bentonitic sand or 

shale.

THICKNESS

Except at Indian Hill (fig. 11) where about 140 feet 

of the Hell Creek formation is present, only the lower 

50 to 60 feet of the formation is preserved in the Fort 

Peck area. Barnum Brown (1907) reported 560 feet 

to be the total thickness of the formation at the type 

locality, and R. B. Colton (written communication, 

1952) reported that the Hell Creek ranges from 220 

to 280 feet in total thickness in the bluffs south of the

FIGURE 11. Indian Hill (sec. 28, T. 26 N., R. 42 E.) is typical of the hills and buttes capped by resistant lenses of sandstone in the Hell Creek 

formation in the southeastern part of the Fort Peck area. Hard, leglike sandstone concretions, such as the one pictured above, are common 

in these lenses. A small trenching shovel, 27 inches long, provides scale.
GEOLOGY OF THE FORT PECK AREA, MONTANA F19

Missouri River, about 45 miles east of Frazer between

Poplar and Brockton.

AGE

The Hell Creek formation is the youngest Cretaceous 

formation in the area, and it, as well as its lateral 

equivalent, the Lance (see p. FIT), is further considered to be latest Cretaceous in age. In its type area it 

carries a classic and abundant dinosaurian fauna, in 

addition to a sparse invertebrate fauna and substantial 

flora. Dinosaur bones and fossil plants are common 

in Hell Creek strata in most exposure in eastern Montana.

IITHOIOGY

The Hell Creek formation is of continental freshwater origin and consists dominantly of soft clay stone 

and shale (carbonaceous in part), siltstone, and silty 

fine- to medium-grained sandstone. The colors range 

through many shades of brown, gray, and violet (fig. 

12); the Hell Creek strata are often referred to in the

literature as the "somber beds." Some of the beds are 

bentonitic, and all are lenticular. Beds of soft and 

hard brown medium-grained sandstone occur at the 

base and as lenses higher in the formation. The basal 

sandstone also contains lenses of finer grained sediments which are more common upward, so as to be 

gradational into overlying beds. Lignite and lignitic 

beds are present although very sparse and discontinuous in the upper part of the formation.

In the Fort Peck area the basal sandstone is consistently present in the lower several tens of feet of the 

formation and is, moreover, fairly uniform in lithologic 

character. The sand is medium grained, fairly well 

sorted, rusty gray brown, and conspicuously speckled 

with black minerals (fig. 5). Large-scale crossbedding, 

including a type ascribed by Collier (1918, pi. 4Z>) to 

wind action, is conspicuous in this basal sandstone, as 

well as in most of the beds of well-sorted sandstone 

higher in the formation (fig. 13). Some of the sandFIGUEE 12. ^Except for basal sandstone beds and some stratigraphically higher local sandstone layers, the Hell Creek formation Is composed of 

clays, silts, and soft sllty sands, In part carbonaceous and in part having disseminated bentonite. Many hills eroded from beds of the Hell 

Creek are capped by sandstone ledges, such as this one in sec. 7, T. 26 N., R. 43 E.
F20 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

stone is cemented by calcium carbonate into hard ledgeforming rock. Some of this cementation affects whole 

sandstone lenses, but more commonly it seems to form 

concretionary structures around point ^ and linear centers yielding hard ellipsoidal and cylindrical masses 

(figs. 13 and 11). This differential cementation has 

limits, which in some places are abrupt and in others 

gradational.

Locally in the lower few feet of the formation there 

are lenses of poorly sorted conglomerate. The conglomerate occupies channels eroded, in some places, 

into the upper surface of the Fox Hills sandstone, 

and in other places into the lower beds of the Hell 

Creek formation (fig. 14). The lenses are generally 

less than 2 feet thick, even where the channels are 

deeper; in places are mere pebble lines. The conglomerate is poorly sorted, heavily iron stained, and on the 

whole weakly cemented, mostly by oxides of iron. 

None was found which is sufficiently hard to be topographically prominent. Most of the component fragments are subangular to subrounded and from y% inch

to 4 inches in long dimension; a very few are 6 to 9 

inches across. The fact that the chief constituent is 

fragments of concretions from the local bedrock may be 

the reason why these conglomerate lenses have gone 

unnoticed by earlier workers; the lenses are similar in 

appearance to the "trashy" concretionary layers commonly found higher in the formation. Minor constituents are surrounded pieces of lithified dinosaur bones 

and fossil wood and smooth well-rounded pebbles and 

cobbles of quartzite (fig. 16). Almost all the quartzite 

pebbles are light gray (A" 5.5 to N 8), though some are 

slightly stained by iron oxide and are yellowish gray.

This quartzite is dissimilar to the pebbles in the 

Wiota gravels or the Flaxville formation, and its 

source is not suggested by any evidence available in 

the Fort Peck area.

R. B. Colton (oral communication, 1958) found these 

conglomerate lenses, containing the same constituents, 

at and near the base of the Hell Creek in the area adjoining the Fort Peck area on the east.

FIGURE 13. Large-scale crossbedding and hard ovoid sandstone concretions are prominent features of the sandstone beds in the Hell Creek 

formation in the east-central part of Montana. Photograph taken in the Larb Hills in sec. 30, T. 24 N., R. 36 E. Hammer provides scale.
GEOLOGY OF THE FORT PECK AREA, MONTANA F21

Quartzite pebbles in this stratigraphic position have 

previously been reported by Bauer (1924) 65 miles 

south-southwest of Fort Peck:

one-half to three inches or more in diameter, at the base of the 

[Hell Creek] formation * * *. The pebbles are imbedded in 

impure limonite which also contains a small amount of coarse 

sand * * *. Most of the pebbles are of light-buff quartzite, but 

several porphyry pebbles were also found * * *. The contact 

does not seem very irregular at this place, and indications of 

channeling are rare, though the varying thickness [35 to 66 

feet] of the Fox Hills (?) indicates a slightly undulating surface upon which the earliest [Hell Creek] strata were deposited, suggesting a local unconformity at least.

The lower 50 to 60 feet of the Hell Creek formation 

in the mapped area consists for the most part of sandstone ; in addition, there are thin lenses of pebble-andcobble conglomerate and a variety of concretionary 

masses of fossil bones and fossil plants, including wood. 

The ledge weathers to yellowish brown (WYE 4.5/3); 

where unweathered, it is gray.

The beds above the basal sandstone are commonly 

very thin or absent in the mapped area except at Indian Hill. This hill is a butte standing about 100 feet 

above the surrounding upland. It is capped by a remnant of a 15-foot-thick hard sandstone lens; below the 

caprock the slopes of the butte are characteristic 

somber-colored clays and shales, some of which are 

carbonaceous.

DISCONFORMABLE CONTACT WITH THE FOX HILLS SANDSTONE

. The contact between the Hell Creek formation and 

the underlying Fox Hills sandstone was examined by 

the senior author in the area extending from the longitude of Seven Blackfeet Creek (fig. 10), in Garfield 

County, eastward to the longitude of Prairie Elk 

Creek, in McCone County, a distance of about 100 

miles. Wherever exposed, the contact is sharp (fig. 

10), and the upper surface of the Fox Hills is channeled in many places. Individual channels are generally less than 10 feet deep, but R. B. Colton (oral 

communication, 1958) reported a channel more than 

35 feet deep in northwestern Richland County in sec. 

26, T. 27 N., R. 52 E.

FIGURE 14. The Hell Creek formation locally contains conglomeratic lenses in channels at the base or within the lowest few feet of the formation. At the point of the pick is one of the smooth well-rounded gray quartzite cobbles which are a distinctive feature of these channel 

fills in the Hell Creek. Photograph taken by R. B. Colton in sec. 11. T. 26 N.. R. 47 E. The pick handle is 17 inches long.
F22 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Most of the channels are filled with medium-grained 

sandstone of the same composition as the rest of the 

thick basal sandstone of the Hell Creek; however, 

channels floored with conglomerate are not uncommon, 

and fine-grained carbonaceous "trash" fills some of the 

channels cut into the Colgate in Garfield County. 

Barnum Brown (1907, p. 827) found in the Hell Creek 

country that "there is usually a thin layer of ferruginous flat concretions of small size" capping the top of 

the Fox Hills.

Even where the actual contact cannot be examined, 

the following differences persist between the basal 

sandstone of the Hell Creek formation and the sandstone of the Fox Hills and allow ready identification 

of the contact.

1. The grain size of the basal sandstone of the Hell 

Creek is medium, distinctly coarser than the finegrained sandstone of the Fox Hills.

2. The sandstone of the Hell Creek formation has a 

conspicuous peppered appearance which the sandstone of the Fox Hills does not.

3. There is a difference in color, subtle only to the unpracticed eye; the sandstone of the Fox Hills is 

comparatively bright rusty yellow brown, whereas 

the sandstone of the Hell Creek formation is a 

more somber and darker rusty yellow brown.

4. Conglomerate beds, some containing a few quartzite 

pebbles and cobbles (fig. 14), are peculiar to the 

Hell Creek, except for a very few small lenses of 

edgewise conglomerate in the Fox Hills.

5. Dinosaur remains, fossil wood, and fossil plants are 

common in Hell Creek strata and are extremely 

scarce in Fox Hills strata.

6. Crossbedding in sandstone of the Hell Creek formation is on a relatively large scale (fig. 13) and can 

be measured in feet; crossbedding in the Fox 

Hills sandstone is, for the most part, on a relatively small scale and measurable in inches. Exceptions to these general conditions occur in the 

areas where pre-Hell Creek erosion has not too 

severely reduced the thickness of the Fox Hills. 

In these places, crossbedding in the upper part of 

the Fox Hills in similar to that in the Hell Creek. 

Where the white Colgate member is present and exposed, the use of the distinguishing features just mentioned is academic in locating the contact between the 

formations. The observed maximum and minimum 

thicknesses for the Fox Hills sandstone in this part of 

Montana (fig. 9) are 146 and 35 feet; some of the 

difference is probably due to original deposition, but 

pre-Hell Creek erosion is probably an important factor.

Two types of evidence thus are combined to indicate 

an erosional unconformity between the Fox Hills sandstone and the Hell Creek formation:

1. The sharp and channeled contact between the two 

formations (fig. 10) and the substantial local 

differences in thickness of the Fox Hills.

2. The abrupt and substantial differences across the 

contact in sedimentary and ecological characteristics.

The significance of the disconformity is difficult to 

assess; it may be a stratigraphic feature restricted to 

this part of Montana, and it may, moreover, represent 

an inconsequential gap in the geologic record. Almost 

certainly it means that the effects of the Laramide 

Revolution were being felt this far to the east, and hidden in the gap may be clues to changing ecological 

conditions that soon were to bring about extinction of 

the dinosaurs.

Some evidence suggests that the disconformity may 

be widespread. Bauer (1924, p. 344-346) found evidence of disconformity in Garfield County at Freedom 

dome, 65 miles south-southwest of Fort Peck, and 

Leonard (1908, p. 44-46, pi. 5) found an erosional unconformity at this horizon near Marmath, N. Dak., 

some 160 miles to the southeast.

Barnum Brown (1907, p. 829), in his original description and definition of the Hell Creek formation at 

its type locality 30 or 40 miles southwest of Fort 

Peck, said that "[the Hell Creek formation] unconformably overlies the Fox Hills * * *. This unconformity is erosional in character." In a later paper 

Brown (1914, p. 357) gave additional details:

On the east fork of Crooked Creek near the old Cook ranch, on 

the west fork of Crooked Creek near the Gus Colin claim, and 

on the east fork of Hell Creek near the EB cattle camp these 

marine beds [Fox Hills sandstone] have been eroded in places, 

sometimes to a depth of 10 feet, before the succeeding massive 

sandstones of the fresh-water "Lance" [Hell Creek formation] 

were deposited. The strata are, however, in all cases parallel 

to the bedding plane of the succeeding sandstones, and the 

break is evidently of local erosional character.

It was observation of these local erosional breaks in the 

vicinity of Hell Creek that led to the statement that the beds 

did not represent a continuous sedimentation from the marine 

Fox Hill. In view of later work, this statement must be 

modified.

Barnum Brown's concluding sentence here is unexplained; prossibly it relates in some way to the Cretaceous-Tertiary transition.

CONTACT WITH THE FORT UNION FORMATION

Although the Fort Union formation and the upper 

Hell Creek strata are missing in the Fort Peck area, 

the contact between these formations is well exposed
GEOLOGY OF THE FORT PECK AREA, MONTANA F23

less than 8 miles southeast of the Fort Peck Dam. 

K. W. Brown places the Tertiary-Cretaceous boundary 

at the contact between the Hell Creek formation and 

the overlying Fort Union formation. 2 He defines this 

contact as follows:

"The base of the Fort Union formation (Paleocene) is 

marked by a persistent lignitic zone or lignite bed, 

above which lignite beds are common and below which 

even discontinuous lignite beds are uncommon. At, 

or within, about 50 feet above the persistent lignite, 

the somber colors typical of the Hell Creek formation 

give way upward to a brighter yellowish-brown color 

typical of the lower part of the Fort Union formation. 

Dinosaur remains are abundant in the Hell Creek 

formation but are totally absent in the Fort Union 

formation."

ENGINEERING CONSIDERATIONS

Most of the statements made concerning the upper 

part of the Fox Hills sandstone apply equally well to 

those parts of the Hell Creek formation preserved in 

the Fort Peck area. The principal differences arise 

from the coarser grain size of the sand and the sandstone in the Hell Creek formation, which is more permeable and, hence, allows better surface and subsurface 

drainage.

There are no springs or wells in the Hell Creek formation within the Fort Peck area. It is inferred that 

only small quantities of ground water are recoverable 

because of the low annual precipitation, well integrated 

surface drainage, and dissection by erosion. As in the 

Fox Hills sandstone, discontinuous cementation probably causes any available water to be perched at different levels.

The unconsolidated sand of this formation might be 

used as fine aggregate for small concrete structures 

south of the Missouri River to avoid the expense of 

bringing in more suitable material from north of the 

river.

STRUCTURE 

REGIONAL STRUCTURAL SETTING

The major bedrock structures of eastern Montana 

are broad and open, dipping generally only a few tens 

of feet per mile. Dobbin and Erdmann's (1955) structure-contour map of the Montana plains shows that the 

attitudes of the beds in the Fort Peck area are influ2 The information presented here regarding the Hell Creek-Fort 

Union contact was originally collected and organized by Roland W. 

Brown of the U.S. Geological Survey, who was long a student of the 

stratigraphy and paleontology of the rocks near the Tertiary-Cretaceous boundary. The boundary near the Fort Peck area was exmained 

in the field by the senior author, in company with Brown, R. B. 

Colton, and others.

enced by three of these major structures: the Bowdoin 

dome, the Blood Creek-Sheep Mountain syncline, and 

the Williston basin.

The Fort Peck area lies on the southeast flank of the 

Bowdoin dome, about 50 miles from its structural summit. The Bowdoin dome is a nearly circular uplifted 

area that has about 600 feet of closure over a diameter 

of roughly 65 miles. It was originally described by 

Collier (1917), and its central part was later mapped 

in greater detail by Erdmann and Schwabrow (1941). 

The Fort Peck area also includes the north limb of the 

broad Blood Creek-Sheep Mountain syncline, an eastwest trending structure no more abrupt than the Bowdoin dome. The synclinal axis is about 60 miles south 

of the area and plunges gently eastward toward the 

largest structure in this part of the Great Plains, the 

Williston basin. Finally, the Fort Peck area is on the 

extreme west margin of the Williston basin, whose 

center is more than 100 miles to the east, in western 

North Dakota.

Between the crest of the Bowdoin dome and the 

deepest part of the Williston basin there is about 3,500 

feet of structural relief, as measured on the top of the 

Dakota sandstone. Known minor structures and the 

indications of others, which are as yet unmapped, show 

that the dip of the strata is not regular either in rate 

or direction over the great area encompassed by the 

three major structures.

STRUCTURAL FEATURES IN THE FORT PECK AREA

The structure-contour map (pi. 2A) compiled by 

the senior author is drawn on the top of the Judith 

River formation and shows only mild departures from 

the previous regional structure presented by Dobbin 

and Erdmann (1955). Gentle southeasterly dips from 

the crest of the Bowdoin dome and toward the Blood 

Creek-Sheep Mountain syncline and the Williston 

basin prevail across the entire mapped area. Other 

lesser structural features, including faults and in one 

small area at Tiger Butte steeply inclined beds, have 

been positively identified. The presence of similar 

features is suspected other places. These lesser structural features are noteworthy because (1) few conspicuous structural features have been reported so far 

in this part of the plains and (2) the involvement of 

the Wiota gravels in the faulted structures at Tiger 

Butte show that some of the structural movements 

have occurred as recently as late Quaternary, a circumstance heretofore almost unrecognized in the northern Great Plains.

Determination of structure was hindered by the 

character and discontinuous exposure of the Bearpaw
F24 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

shale, the only surface bedrock formation in most of 

the area. The Bearpaw is nearly homogeneous and 

subject to slumping; key beds had to be established at 

fairly short intervals, and evaluation of slumping 

effects was made wherever exposures were adequate.

About 50 control points were established for the 

structure-contour map by measuring with a Paulin 

altimeter the elevations of (1) key beds within and at 

the top and bottom of the Bearpaw shale (the Bearpaw-Judith River contact is exposed just northwest 

of the mapped area) and (2) the level of water in certain wells. Wells yielding water under artesian pressure penetrate the Judith River formation, and well 

records,3 therefore, provide an estimate of the depth 

to the top of Judith River. The map (pi. 2) shows the 

contours on the top of the Judith River formation.

The position of the several key beds above the top of 

the Judith River formation were determined by measurement of the intervals between them wherever useful 

exposures existed; the composite columnar section 

(pi. 2) is built from average thicknesses based on 1 

to 6 measurements per interval. The thickness of the 

beds not examined in detail (pi. 2) is inferred, because the propensity of these beds to slump prevented 

accurate measurement. The total thickness of the 

Bearpaw shale given here, 1,140 feet, is thus only approximate, and by the same token so also are the 

positions of the key beds.

The effects of faulting were not considered in drawing structure-contour lines. Except at Tiger Butte, 

the few faults observed have displacements of less 

than 20 feet and are visible only in the cutbanks of 

streams; therefore, their strikes are indeterminate. 

Presumably, undetected faults exist.

DEFORMATION AT TIGER BUTTE

Topographically, Tiger Butte (pi. 3; sees. 2 and 3, 

T. 27 N., R. 40 E.) is an irregular high area about 1 

mile in diameter that bears little or no resemblance to 

the form generally connoted by the word "butte." Its 

highest hill is near the center of the area and stands 

about 300 feet above the Milk River flood plain to the 

north and about 150 feet above the benchland to the 

south. Because Tiger Butte is flanked immediately on 

the north by the Milk River flood plain, it is probable 

that the northern part has been truncated by the river's 

activity. No topographic or geological elements of the 

remaining part suggest that Tiger Butte, is a remnant 

of a once larger feature having radial or bilateral 

symmetry.

3 Well records were obtained from the flies of the Montana State 

Board of Health at Helena, Mont.

Structurally, Tiger Butte is an "island" of strongly 

tilted and faulted strata (pi. 3) that rises abruptly 

from the broad gentle regional dips characteristic of 

eastern Montana. Tiger Butte is of especial interest, 

not only because such acute deformation is distinctly 

abnormal in this area but also because Pleistocene, as 

well as Mesozoic, deposits have been involved.

The Bearpaw shale is strongly tilted and faulted. 

If the gentle southesterly dip prevalent in the surrounding areas had not been severely disturbed, beds 

belonging to the middle part of the formation would 

crop out at Tiger Butte. Instead, the shale beds exposed there have the characteristics of the lower part 

of the Bearpaw shale (pi. 3; unit 2 and the upper 

two-thirds of unit 1). Fossils are comparatively 

scarce, but collections include Emperoceras and Nostoceras, two aberrant ammonite genera which W. A. 

Cobban (written communication, April 1951) specifies 

as characteristic of the lower part of the Bearpaw. 

The fact that many hundreds of feet of shale can be 

measured perpendicular to strike is attributed to repetition of beds resulting from the presence of overthrust 

bedding-plane faults, because all the beds of the Bearpaw in Tiger Butte apparently can be identified with 

the lower few hundred feet of the formation elsewhere.

The Wiotia gravels of Pleistocene age (see p. F28) 

are present at several places in the hills of Tiger Butte; 

near the summit they are clearly tilted and overlain by 

Bearpaw shale at an overthrust bedding-plane (?) 

fault. Elsewhere they seem to be involved in deformation in an equally integral manner, but their uncemented character makes objective observation difficult.

Lithologically the Wiota gravels of Pleistocene age 

(see p. F28) are identical with the Flaxville formation 

of late Tertiary age (see p. F27), but the gravel beds 

at Tiger Butte were assigned to the younger formation 

because of the following evidence: (1) There is no 

Flaxville formation present for many miles in any 

direction from Tiger Butte, but Wiota gravels are 

common in nearby areas; (2) in company with the 

senior author, Mr. Clarence Lohman found a mammoth 

tooth in the gravels a few feet from the highest peak 

of the hills (pi. 3, fossil loc. 5). Jean Hough (written communication, September 26, 1950) of the U.S. 

Geological Survey reported, in part, as follows:

The specimen is a lower molar of Mammtithns boreus Hay 

(MammntJuts primifjeniits Blumenbach Parelephas jeffersoni 

Osbqrne).

Since mammoths arrived in North America, as far as present 

knowledge goes, early in the Pleistocene, the age range can be 

Nebraskan to Wisconsin [any part of the Pleistocene]. If the 

tooth is correctly associated with the gravels these cannot be 

[so old as] Pliocene according to vertebrate evidence.
GEOLOGY OF THE FORT PECK AREA, MONTANA F25

Though the Wiota gravels were deposited discontinuously during much of the Pleistocene, it seems 

likely that the gravels involved in the Tiger Butte disturbance were deposited late in the Pleistocene, as indicated by the position of Tiger Butte relative to the 

old valley of the Missouri Kiver (fig. 19). This valley 

was floored with the youngest of these gravels and 

flanked by slightly older terrace gravels, which are 

also included in the Wiota gravels. Thus, the deformation can be stated to be definitely Pleistocene and probably late Pleistocene.

The third geologic formation in the Tiger Butte 

hills crops out in only one small area but is of unusual 

interest because it closely resembles certain beds of the 

Judith Kiver formation. The top of the Judith Kiver 

formation is calculated to be more than 400 feet beneath the Milk River flood plain elsewhere in this 

vicinity (pi. 2A). (The strata in question form a 

steeply dipping lens-shaped body about 15 feet wide 

and 50 yards long that crops out in a precipitous ravine 

about 300 yards north of the highest hill of Tiger 

Butte.)

Most of the lens consists of soft gray-brown siltstone 

and silty and sandy shale that strongly resemble, siltstone and sandy shale the transition beds from the 

Judith River formation to the Bearpaw shale. Some 

of the material is light-brown very fine grained sandstone, thin bedded and lithified, that closely resembles 

the ledge-forming beds of the upper part of the Judith 

River that are exposed on the northeast side of the 

Milk River flood plain 8 to 10 miles northwest of 

Glasgow. Assignment of these beds to the Judith 

River can be only provisional, because no fossils were 

found in the lens and because there are similar beds in 

the Fox Hills sandstone. Identification with the 

Judith River formation rather than the Fox Hills is 

preferred because:

1. The siltstone and sandy shale "transition" beds in 

the lens do not weather to the conspicuous light 

gray rock that is characteristic in this area of the 

lower part of the Fox Hills sandstone.

2. In the areas around Tiger Butte, the top of the 

Judith River formation is between 1,600 and 1,700 

feet above sea level (fig. 32A), and Fox Hills 

strata are not found below about 2,800 feet. By 

the time of deposition of the Flaxville formation 

(late Miocene or Pliocene), the land surface in 

this area was eroded to elevations lower than 2,800 

feet and was eroded still lower by the time of the 

Tiger Butte disturbance (Pleistocene ?). The Fox 

Hills sandstone, therefore, likely was not present 

to form part of the hills of Tiger Butte.

3. The presence of beds probably belonging in the 

lower part of the Bearpaw shale suggests the 

Tiger Butte area to be one of uplift rather than of 

subsidence.

Faults of two ages are present. The older faults are 

subparallel bedding faults produced by overthrusting 

from the north. They seem to account for repetition 

of beds within the Bearpaw shale and for emergence 

of the Judith River formation at the surface. A set of 

subparallel younger faults strike west-northwest and 

offsets both the strata and the older faults. The most 

prominent fault of this second set passes near the highest hill of Tiger Butte and divides the area of Tiger 

Butte into two nearly equal parts. On the north side 

of the trace of the fault, the strata strike about N. 58° 

E. and dip to the northwest at angles ranging from 

70° to nearly vertical. On the south side of the trace, 

the strata strike about N. 75° E. and dip to the north 

at much less steep angles. Rotation on the fault surface 

probably accounts for the differences in dip and strike. 

Slumping associated with modern geomorphic development results in superficial anomalies.

Although the geologic history leading to the structural pattern observed is not known, the following 

sequence of events is postulated as consistent with 

known facts:

1. Thrust faulting from north to south yielded imbricate structure, repetition of beds, and relatively 

steep dips. During this stage the Judith River 

formation was brought near the surface. Bedrock overrode the Wiota gravels and caused them, 

in part, to take on the lenticular character of fault 

gouge.

2. Thrust pressure dissipated and normal faulting 

was initiated. The normal faulting was accompanied by some rotation, bringing about the steep 

dips in those fault blocks in the northern half of 

the hills.

3. The modern topography evolved. Because Tiger 

Butte is made of weak material, slumping as well 

as erosion has played a part in shaping topography; slumping causes some aberrant surficial 

bedding attitudes.

Deep-seated, rather than surficial, causes are responsible for the deformation at Tiger Butte, as evidenced 

by:

1. The regularity of bedding attitudes in the incompetent Bearpaw shale. Although tilting and 

faulting are pronounced, there is a marked absence of contorted beds such as would be expected 

to occur in incompetent rocks deformed by glacial 

Hills sandstone therefore likely was not present 

action. Ice shove is likely to produce radically
F26 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

different structures, such as those in east-central 

Alberta described by Slater (1927). In that 

locality the beds have been contorted into sigmoid 

curves, drag and attenuated-flow overfolds, and 

diapir curves, all complicated by thrust faults 

that produced imbricate structure. Furthermore, 

Tiger Butte is situated in or near the center of 

the abandoned valley of the ancestral Missouri 

River (fig. 19), and, owing to this topographic 

situation, the site should have been "out of reach" 

of shoving glacial ice. Slumping as a prime cause 

of deformation is ruled out by the regularity of 

dips and strikes over most of the butte area.

2. The regularity in the distribution and attitude of 

the Wiota gravels of Pleistocene age involved in 

the deformation.

3. The appearance of beds at the surface probably belonging to the Judith River formation.

4. The presence of Bearpaw shale strata other than 

those to be expected if there had been no crustal 

disturbance in the local area.

Within the Fort Peck area there is other evidence 

that crustal adjustments occurred subsequent to the 

Tazewell(?) glaciation of the area. In the face of a 

steep cutbank of Porcupine Creek (SEi/4 sec. 11, T. 28 

N., R. 41 E.), a well-exposed fault offsets Bearpaw 

shale strata by about 8 feet. Near the top of the cutbank, the same fault offsets the overlying Pleistocene 

deposits, but the displacement is less than 2 feet. Thus, 

the fault was active both before and after the Pleistocene materials were deposited.

Abrupt crustal deformation in this part of the northern Great Plains is apparently not unique at Tiger 

Butte. Townsend (1950) reported folded and faulted 

Fort Union (Paleocene) strata in northwestern North 

Dakota extending over an area about 7 miles long and 

a mile wide and presents evidence indicating a wider 

distribution of the deformed strata. In further support of his qualified conclusion that the deformation is 

due to deep-seated causes, Townsend (1950, p. 1563- 

1564) wrote:

The Fort Union is generally an incompetent formation, most 

portions of which, unless firmly confined, would yield easily to 

the presumably lateral pressures of ice shove and result in a 

jumbled mass lacking continuity of bedding. Also, the structure near Lignite is at such an elevation and so related to the 

probable preglacial topography that the beds could hardly have 

constituted a prominence more vulnerable to ice deformation 

than surrounding undeformed beds. Deformation similar to 

that near Lignite but much smaller in scale and extent is conceivable. If evidence favoring ice shove were more abundant 

in the area described or in other related areas, more weight 

could be given this possibility. Lacking this, the weight of 

evidence favors structural deformation.

POST-CRETACEOUS STRATIGRAPHY 

GENERAL STATEMENT

The present configuration of the northern Great 

Plains of the United States and the adjoining Prairie 

Provinces of Canada is largely the work of two major 

forces operative during post-Cretaceous time: (1) 

Gentle but intermittent uplift that probably began in 

the late Eocene and may be still continuing into Recent 

time and (2) successive invasions and retreats of Pleistocene ice sheets. The deposits that resulted from 

these events are largely unconsolidated and can be 

compared with blankets of uneven thickness, discontinuous distribution, and varied composition. They 

may be divided into deposits antedating local glaciation, deposits resulting from local glaciation, and deposits formed after glaciation.

FORMATIONS ANTEDATING GLACIATION

The preglacial surficial materials of the Fort Peck 

area are part of a widespread group of Tertiary and 

Quaternary gravelly deposits laid down by streams 

draining east and northest from the Rocky Mountains. 

These deposits represent recurrent times of regional 

stability that alternated with times of gentle regional 

uplift, so that each succeeding deposit mantles a surface that is topographically lower than its predecessor. 

Because there is no apparent difference in lithology or 

method of deposition and because identifiable fossils 

are comparatively scarce, age differences in many 

places are deduced largely from elevations above sea 

level and from gradients of the erosion surfaces on 

which the gravel deposits lie.

The oldest Tertiary formation of the northern Great 

Plains area is the Cypress Hills gravel of Oligocene 

age, which is limited to southern Saskatchewan. Deposits assigned to other, possibly less important, times 

of planation and deposition which followed the Cypress Hills have also been found in southern Canada. 

Of these, the Wood Mountain gravel (Russell, 1853) 

of Miocene age is probably the most important. The 

oldest surficial formation in the Fort Peck area is the 

Flaxville formation of Miocene and Pliocene age (see 

p. F27) which is extensively distributed over large 

parts of northern Montana and eastern Saskatchewan 

but is present only in the extreme northeast corner of 

the Fort Peck area. The Wiota gravels, a younger 

preglacial fluviatile formation, is exposed along the 

Milk and Missouri Rivers and their major tributaries 

in the report area and consists of fluviatile material of 

Pleistocene age.
GEOLOGY OF THE FORT PECK AREA, MONTANA F27

TERTIARY DEPOSITS

FLAXVILLE FORMATION

NAME TYPE OF DEPOSIT DISTRIBUTION

The Flaxville formation originally described and 

named by Collier and Thorn (Collier, 1917, p. 194-195; 

Collier and Thorn, 1918, p. 179-184) is a fluviatile deposit that caps generally even-topped plateaus and 

benches sloping gently eastward from about 3,200 feet 

at the west margin of Blaine County, Mont., to about 

2,600 feet above sea level in the southeastern part of 

Sheridan County, Mont. Within the Fort Peck area, 

the exposures of the Flaxville are about 2,700 feet in 

altitude. Alden (1932, p. 13) estimated that the remnants of the Flaxville erosion surface total about 1,800 

square miles. R. B. Colton (oral communication, 

1958) mapped about 750 square miles of the Flaxville 

in the Fort Peck Indian Reservation, which includes 

parts of the area of this report. Within the area of 

this report, the Flaxville formation is limited to a few 

small plateau remnants near the northeast corner.

TOPOGRAPHY AND THICKNESS

The Flaxville formation was deposited in northeastern Montana on a well-developed pediplain cut across 

the Bearpaw, Fox Hills, Hell Creek (Lance), and Fort 

Union formations at altitudes ranging from 700 to 

1,500 feet below the level of the Cypress Hills plain 

(Alden, 1932, p. 13). The formation is very porous 

and has eroded slowly except along its margin where 

it sloughs off owing to the removal of the underlying 

soft shales and clayey sandstones. Hence, it forms 

short steep slopes bordering relatively flat uplands or 

benches. The average thickness of the Flaxville in the 

Fort Peck area is about 20 feet.

STRATIGRAPHIC POSITION AND AGE

In the Fort Peck area, the Flaxville formation was 

deposited unconformably on the Bearpaw, Fox Hills, 

and Hell Creek formations. The ground moraine, 

which overlaps the Flaxville as well as the older bedrock, is the only material overlying the Flaxville in 

this area. .Elsewhere in northeastern Montana, the 

Flaxville is locally covered by alluvial and eolian, as 

well as glacial, deposits. Numerous fragmentary vertebrate remains indicate that the Flaxville formation is 

probably lower Pliocene. The first important collections by Collier and Thorn (1918, p. 181) were pronounced by J. W. Gidley to be "not older than Miocene 

or younger than lower Pliocene."

R. B. Colton made fossil collections to the east that 

contained specimens identified by Jean Hough (written communication, 1950) of the U.S. Geological Survey as probably early Pliocene. Colton (written communication, 1959) also stated that "collections of 

vertebrate fossils from Flaxville made by R. W. Brown 

and the author were identified by Louis Gazin of the 

U.S. National Museum as probably early Pliocene or 

possibly late Miocene."

IITHOLOGY

Within the Fort Peck area, the Flaxville formation 

is composed predominantly of smooth well-rounded 

pebbles averaging 2 inches in maximum diameter. 

Most are brownish gray very fine to medium-grained 

quartzite, although pebbles of reddish argillite are 

common. Light-gray quartzite pebbles, such as those 

at and near the base of the Hell Creek formation, are 

scarce. The gravel is sandy and includes sand, silt, 

and clay in thin lenses. In a few places, it is discontinuously cemented to form conglomerate.

Collier and Thorn (1918, p. 181) described the Flaxville as

composed of yellowish to ash-gray gravel, clay and sand, but in 

some places it contains beds of white marl and volcanic ash. 

The gravel consists of well-rounded pebbles from less than an 

inch to a foot or more in diameter, of quartzite and argillite 

derived from the Rocky Mountains. Limestone pebbles from 

the same source may have been dissolved and the lime deposited as cementing material and beds of marl. The materials composing the Flaxville gravel are mostly noncoherent 

and are excavated easily by well diggers, though beds of hard 

sandstone and conglomerate cemented with calcite from 1 foot 

to several feet thick are encountered in most of the wells. In 

places the formation is very thoroughly cemented with calcite 

and forms prominently outcropping ledges of sandstone and 

conglomerate * * *.

Collier and Thorn (1918, p. 182) also gave a geneeralized section of the Flaxville formation, in sees. 

19, 20, and 29, T. 35 N., R. 43 E., with approximate 

thickness of each unit as follows:

Feet

Marl, containing a few scattered quartzite pebbles__ 15 

Sandstone cemented with calcite _____________ 30 

Volcanic ash, white to yellow, very pure but mixed with

the underlying gravel at the base ____________ 15 

Gravel, more or less cemented __ 20

Fort Union formation ________________ 80 

Thus the Flaxville of the Fort Peck area coincides 

reasonably well with gravel units discussed by Collier 

and Thorn but lacks the ash, sandstone, and marl units.

ENGINEERING CONSIDERATIONS

The Flaxville formation is a source of sand and 

gravel. Most of it is uncemented and is easily excavated ; however, crushing is necessary if angular material is desired. Chemically reactive material (hydrous 

silicates) probably is less than l 1/^ percent. Detailed 

prospecting is necessary to assess overburden.
F28 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

QUATERNARY DEPOSITS 

WIOTA GRAVELS

NAME, TYPE OF DEPOSIT, AND DISTRIBUTION

Although preglacial gravels that are post-Flaxville 

in age have long been recognized in eastern Montana, 

they were not given a definite name until 1951. In 

his report on the Nashua quadrangle Jensen (1952b, 

p. 45-50) grouped these deposits under the name of 

Wiota gravels after the Wiota railroad station on the 

Great Northern Eailroad at the junction of the U.S. 

Government's spur line leading to Fort Peck. According to Jensen's original definition,

Wiota gravels is the name here applied to gravels and associated sediments deposited by streams and rivers prior to 

glaciation of the sites of deposition of the gravels. The materials are predominantly of western provenance and discontinuously mantle of plain that forms the upland areas of the 

quadrangle. Wiota gravels underlying this upland are at altitudes ranging from about 2,100 feet to about 2,400 feet above 

sea level, but further east altitudes are 2,000 feet or less * * *. 

The Wiota gravels are typically exposed at several places 

within a radius of about 10 miles of Wiota railroad junction, 

which is in the eastern part of the quadrangle.

These gravels were deposited on a number of levels 

ranging from about 200 feet to more than 500 feet 

below the gravel plain of the Flaxville formation. It 

is the consensus of the workers that the gravels on 

these several benches and terraces in eastern Montana 

cannot be properly subdivided at the present time and 

that one formational name should be used to refer to 

them all.

The Wiota gravels are probably the most widespread 

of the preglacial fluviatile deposits. Field studies by 

Colton (1951a, b; 1953, 1954; 1955a, b), Witkind 

(1949), and Jensen (1951a, 1952b) show that the 

Wiota gravels are present over large areas in Valley, 

Roosevelt, and Sheridan Counties. The literature 

strongly indicates that they are also present over other 

large areas to the west in Montana and to the north 

in southern Saskatchewan.

Within the Fort Peck area, the formation is discontinuous but widespread beneath the glacial drift 

north of the Missouri River. It floors buried stream 

channels and mantles buried erosion surfaces, which 

generally slope toward the Missouri River. Extensive 

exposures border the valleys of the Milk River, Porcupine Creek, Cherry Creek, and Little Porcupine Creek.

TOPOGRAPHY AND THICKNESS

Most outcrops of the Wiota gravels are high on the 

valley sides. Small benches or topographic noses commonly mark the position of the formation, which is

generally more resistant to erosion than the overlying 

or underlying beds.

In most places the observed thickness is 6 to 20 feet, 

although the formation reaches a maximum thickness 

of about 30 feet in the vicinity of Wiota.

STRATIGRAPHIC POSITION AND AGE

The Wiota gravels immediately overlies bedrock. In 

the Fort Peck area the formation rests entirely on 

Bearpaw shale. Elsewhere in northeastern Montana, 

the underlying bedrock includes Cretaceous rocks 

younger than the Bearpaw, as well as the Fort Union 

formation of Paleocene age. In the Fort Peck area the 

Wiota immediately underlies ground moraine ; it crops 

out in only a few small areas other than the narrow 

bands exposed in hillsides.

Fossils are extremely scarce, but fortunately fhey are 

strongly diagnostic of the Pleistocene. Within the Fort 

Peck area, some horse teeth, resembling those of the 

modern Equus, were collected at the following localities : (1) About 4 miles west of Nashua, Valley County, 

Mont., in sec. 36, T. 28 N., R. 41 E. ; and (2) a gravel 

pit on the upland about 10 miles south-southeast of 

Glasgow, Valley County, Mont., in sec. 28, T. 27 N., R. 

40 E. C. L. Gazin ( J. B. Reeside, written communication, March 11, 1949) of the U.S. National Museum, 

who made the identifications at the latter locality, reported that both specimens "appear to be well fossilized 

and are almost surely Pleistocene in age. The species 

cannot be determined." A mammoth tooth was identified as Mam/mutJius primigenius by Jean Hough 

written communication, April 12, 1950) who commented as follows:

The tooth in question is a portion of an upper molar of 

muthus primigenius. This is the wooly mammoth which ranged 

over northern Europe, Asia and North America from Pleistocene to Recent times. However, the species (and genus) became extinct in the United States at about the time of the 

major retreat of the Wisconsin ice sheet. Your gravel is, therefore, almost certainly Wisconsin in age and probably late Wisconsin.

The early and middle Pleistocene mammoths, Mammuthus 

( Archidiskodon) imperator, were extremely large with very 

massive teeth. Your specimen is small, even for Mammuthus 

primigenius, and the tooth plate ridges narrow and closely 

spaced. Its referral to this species is pretty surely correct and 

the age determination can be depended upon.

In connection with this comment, however, G. E. 

Lewis of the U.S. Geological Survey provided the following oral communication:

Evidence in other portions of the Great Plains, however, indicates that Mammuthus primigenius is generally associated 

with pre-Wisconsin deposits. Schultz, Lueninghoener, and 

Frankforter (1951, table 1) point out that in Nebraska, Mam-
GEOLOGY OF THE FORT PECK AREA, MONTANA F29

muthus primigenius (Blumenbach) has a known occurrence in 

Yarmouth sediments and only a probable additional SangamonWisconsin range, whereas, Parelephas columbi (Falconer) is 

the known characteristic Wisconsin mammoth.

varies from outcrop to outcrop. In places, the upper 

sand is thin or absent. In others, the entire formation 

may be composed of sand. Locally, all or part of the 

formation changes laterally into a very poorly sorted 

colluvial facies. Most exposures exhibit crossbedding. 

The following sections are typical of the Wiota gravels 

in the Fort Peck area.

This tooth was found in a bed of the Wiota gravels exposed in a cutbank of the Missouri River in the SW^4 

sec. 5, T. 26 N., R. 44 E., Valley County, Mont, (see 

p. F30). Its presence demonstrates that the younger

members of the Wiota gravels could be as recent as L North wall of tributary to Porcupine Creekj NE%NE%sec. 29,

Wisconsin in age; even so, these younger members are T. £9 N., R. 41 E.

virtually nonglacial in the Fort Peck area. Ground moraine (Pleistocene) . Feet

The oldest and topographically highest deposits of Till_______-________-___-_____-_-__-____--____- 15+

the Wiota in the Fort Peck area are presumably only ==

slightly younger than the youngest part of the Flax- Wiota gravels (Pleistocene):

 T-IJ. ,. -vj.iu.Li j i. j-a Clay and silt; grade from dark brown at the base to ville formation, as indicated by the modest difference rJ ,. , ' & , ,, , in ' J light brown at the top__-_-_-____--_-_-_-__--__ 19

in altitude (about 200 feet) within the local area of Sandy silt; partly thin bedded____________________ 5

common occurrence of the two formations. No fossils, Sand, medium-fine, clean, light-tan__-___-___-__ 1

however, have been reported from either the youngest Sandy gravel; pebbles are as much as 7 inches in

part of the Flaxville formation or the oldest part of diameter but are predominantly 1-2% inches;

,, TTT. n n jn - , - . ,. pebbles are all derived from the Flaxville forma- the Wiota gravels, and therefore the age of the time tion, as there are no glacial erratics present. Some

boundary between the two formations remains con- thin discontinuous sand beds are present-------.- 5

jectural. Insofar as the bulk of the deposits is concerned, it seems more than likely that the Flaxville is Total Wiota-________________________________- 30

Pliocene and the Wiota is Pleistocene. It seems likely, , ,//*.,. s

. . J ' Bearpaw shale (Cretaceous):

however, that Wiota deposits range in age from early Shale, dark-gray __ _____ ____-__-__ __ 30+

to late Pleistocene. _ The extremely scarce glacial er- NOTE._Amtude of Bearpaw shaie-wiota gravels contact ta about 2,400 ft. 

ratios found locally in the younger (lower) part of the

Wiota gravels of the Fort Peck area probably repre- s- Hi^hwav cut near u&and level> NX cor- sec - ®9> T- ®8 N-> R- & Esent much attenuated drift from an ice sheet (Illinoian Ground moraine (Pleistocene): Feet

or lowan?) that did not advance as far south as the _T. Tlll---------:------------------------------- !°-15

 f -p i Wiota gravels (Pleistocene):

r ort .reCK area. Sandy gravel; contains numerous lenses, as much as

Collier and Thorn (1918, p. 182) regarded the older 1 ft thick, of pebbly to clean medium cross bedded

part of this formation as being late Pliocene or early sand._____________________________________ 22

Pleistocene. Alden (1932, p. 44-45; 59-60) recognized Bearpaw shale (Cretaceous):

j.i -i J.-.1 i T m i m Shale, dark-eray______------------------_-_--- 15+ these gravels in part, using the symbols T2 and T3 in ' & J

f , ,1 mi 111 u f i j. NOTE. The gravel contains no glacial erratics. The altitude of the Bearpaw shale- referring to them. These symbols have been found to Wlota gravels £ntact to about 2 2875 ft 

be incompatible with the results of later detailed mapping because of the multiplicity of levels on which the 3- Roadcut °f road leadin^ from U-s- Highway 2 north to gravel pit,

i i J -^^ j-a: i*. ^J-^ *. * SW%sec.86,T.!B8N.,R.41E. gravels occur, coupled with the difficulty of differentiating and correlating them. He regards the older part Outwnash *?""? grKavel (^efatocene): ^

. ,, . . . /m x n TO 11 Gravel with abundant glacial erratics____________ 12 +

of this formation (T.) as early Pleistocene, and the Ground moraine (Pleistocene):

younger part (T3 ) as older than the Wisconsin stage Till---__-_--__-_-___-------------------------- 7

of glaciation. Wiota gravels (Pleistocene):

IITHOIOGT Light-brown unit consisting of alternate beds of

A

. i i .-i , ,1 -rirr- , i -i clean fine-grained sand and thinly bedded clay _ _ _ 8

typical exposure shows that the Wiota has much , 6, , , ^w ^ *

JC T n 1-11 Gravel, poorly sorted; pebbles are as much as 5

in common with modern stream deposits and with the inches in diameter but are mostly 1-3 inches;

Flaxville formation. Generally, the lower part is glacial erratics are extremely scarce. Tooth of

moderately well bedded sandy gravel containing nu- Equus was found. -------- _________--____-_-- 4

merous lenses of medium sand. The upper part is com- ~~

, ... , ,. , F"., " , ,., Total Wiota..__-___-_-------------------_---- 12 monly silt and fine to medium sand with intercalated Bearpaw shale (Cretaceous):

clay lenses. Consistent with typical patterns of stream Shale, dark-gray.___________-__-_---_-_--------- 15 +

deposition, however, the Composition of the formation NOTE.-The altitude of the Bearpaw shale-Wiota gravels contact is about 2,100 ft.

690-458 O-64-3
F30 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

4. Gravel pit, center N/l 2 sec. 10, T. 27 N., R. 41 E.

Ground moraine (Pleistocene): Feet 

Tfll_________ _ --- -_ 3-12

Wiota gravels (Pleistocene):

Sand and sandy silt, medium- to fine-grained,

laminated and crossbedded, tan _____________ 4-5

Sandy gravel; pebbles are as much as 6 inches in 

diameter but are mostly 1-3 inches; commonly 

contains lenses of pebbly to clean sand. Erratics 

are extremely scarce__________________________ 11

Total Wiota ------------------------------ 15-16

Bearpaw shale (Cretaceous) :

Shale, dark-gray___-_______---_------------ pit bottom

NOTE. The altitude of the Bearpaw shale-Wiota gravels contact is about 2,100 ft.

5. Cutbank on north bank of Missouri River, SE% sec. 6, T. 26 N.

R. 44 E.

Kintyre formation (Pleistocene?): peet 

Fine sands and silts, tan to brown__ -_-____-______ 60 +

Ground moraine (Pleistocene):

Till.--__-___--_-_-____-.-------------------- 7-15

Wiota gravels (Pleistocene):

Sand and gravel. This unit is present from river level to 

22 ft above river level. At the east end of the exposure, 

the upper three-quarters of this unit consists of sand and 

silt, varying in grain size with the bed and showing crossbedding in places. The finer grained material is generally 

nearer to the top of the section. The lower part of the 

section consists of sandy gravel containing pebbles as 

much as 5 inches in diameter, although most are % inch 

to 2 inches; the lower part also contains a lew sand 

lenses. Toward the west end of the exposure, where 

sand lenses are more abundant, the gravel gradually increases in thickness and eventually constitutes the entire 

unit. No erratics were found. One horse (Equus) tooth 

and one mammoth (Mammuthus primigenius) tooth were 

found near east end of cutbank in SW% sec. 5___ 22

Bearpaw shale (Cretaceous) :

Shale, dark-gray (exposed during low-stage of river).

NOTE. The Wiota railroad junction is centrally located relative to these sections.

The pebbles in the Wiota gravels vary in lithologic 

character in northeastern Montana according to altitude and locality. In the Fort Peck area, the higher 

(older) parts consist entirely of reworked Flaxville 

formation; about 98 percent of the pebbles is smooth, 

well-rounded percussion-marked argillite and quartzite. These pebbles are as much as 7 inches in length 

and are various shades of brown, reddish brown, pink, 

green, and gray (the overall color of the formation is 

reddish brown). About 11/& percent is polished more 

or less pitted brown chert pebbles as much as 3 inches 

long. The other one-half of 1 percent is green tinguaite porphyry pebbles that presumably are all derived from the several isolated intrusions exposed in 

western Montana. The pebbles are similar in shape 

and size to the quartzite and argillite pebbles, though 

less smooth.

At lower (younger) levels in the Fort Peck area the 

pebbles are identical in origin and lithology, except 

that in a few places the gravels contain a small fraction 

of 1 percent of crystalline-rock erratics of apparent 

northern provenance. None of these erratics have been 

found west of the outcrop of Wiota gravels about a 

mile west of the town of Nashua. The gravels in this 

outcrop are presumed to have been deposited by the 

preglacial Porcupine Creek. Not all the lower level 

Wiota gravels east of this outcrop contain erratics.

Cream-colored limestone and dolomite, so common 

as erratics in the local drift, are absent from the Wiota 

gravels of the Fort Peck area. This absence is clean 

cut, as there is no known example of in-place erosion by 

leaching in the Fort Peck area to suggest that this 

absence is due to any cause other than nondeposition of 

these soluble rock types.

R. B. Colton (written communication, 1957) and 

Witkind (1949) found that east of the Fort Peck area 

there are gravels similar to the Wiota in stratigraphic 

position, age, mode or origin, and lithology in which 

erratics become increasingly abundant. Some lowlevel gravels in Sheridan County are reported (Colton, 

written communication, 1957) to contain 30 percent 

glacial-erratic pebbles. It is the intention here, however, to restrict the name Wiota gravels to deposits 

mostly nonglacial in origin.

Some scattered deposits of unconsolidated sediments 

in the southwestern part of the area have been designated on the map as Wiota gravels, although they 

differ considerably in appearance and lithology. Like 

the Wiota, these deposits overlie the Bearpaw shale 

and underlie glacial deposits. They consist primarily 

of poorly sorted sandy and clayey medium- to finegrained gravel and include abundant lenses and beds 

of sand, silt, and clay. All the pebbles are fragments 

of concretionary masses derived from the local Cretaceous bedrock formations. There are no pebbles of 

quartzite or of northern crystalline rocks. Iron-oxide 

coatings are prevalent, so that the material is light to 

dark rust brown. More extensive field investigations 

may result in separating this material as a distinct 

geologic unit.

ENGINEERING CONSIDERATIONS

Except for the poorly sorted low-strength material 

in the southwestern part of the area, the Wiota gravels 

are a source of road metal and aggregate. The pebbles 

are smooth and well rounded, so that crushing is necessary if angular material is desired; reactive material 

(amorphous and cryptocrystalline silica) composes 

about 1 percent. Detailed prospecting along the line 

of outcrop is necesary because of lateral changes in
GEOLOGY OF THE FORT PECK AREA, MONTANA F31

composition and because of the differing ratios in the 

thickness of gravel to overburden.

The materials composing this formation and the 

overlying glacial drift are easily worked with power 

tools. The Wiota is highly permeable, so that canals 

and small dams in this formation probably would require lining. Figure 6 (samples 1, 7, 8, and 19) gives 

typical soil classifications for this formation.

The Wiota gravels are a shallow source of ground 

water in upland areas. The water is low in dissolved 

solids and is excellent both for domestic use and for 

stock. The water percolates to the base of the gravels 

above the nearly impermeable underlying Bearpaw 

shale, escaping as intermittent springs at the contact 

where erosion has dissected the gravels. The quantity 

of water available differs markedly from place to place 

and is greatest near the centers of ancient drainage 

courses and negligible at the sites of drift-buried divides. Wells sunk into the Wiota gravels range from 

15 to 60 feet depth, which depends on the thickness of 

the overlying drift.

GLACIAL DEPOSITS

GENERAL DISCUSSION

Deposits of Pleistocene age, probably all related to a 

single phase of the Wisconsin glaciation, possibly the 

Tazewell, are the surface material covering a large 

part of the Fort Peck area. Ground moraine blankets 

the uplands north of the Milk and Missouri Rivers, and 

remnants of moraine have also been mapped south and 

west of the rivers. Next in abundance are the large 

patches of fluviolacustrine sediments that were laid 

down on tongues of stagnant ice in the old Missouri 

River trench during the wasting of the Tazewell (?) 

glacier. Glacial outwash deposits are comparatively 

small and widely scattered. Some mark channels 

carved by early melt water of the glacier but soon 

abandoned for courses that apparently conformed 

nearly to present-day drainage lines. The rest of the 

outwash occurs on discontinuous terrace remnants 

along the Milk River and some of the main tributaries 

of the Missouri River.

GROUND MORAINE

DISTRIBUTION

Ground moraine of Wisconsin age mantles more 

than half of the Fort Peck area. It has been eroded 

from the inner valleys of the Missouri and Milk Rivers 

and tlieir principal tributaries, and has been largely 

stripped from the hilly belt of Bearpaw shale in the 

south-central and southwestern parts of the mapped 

area. Beyond the area of more or less continuous

cover, there are scattered patches of ground moraine 

and isolated erratics resting on bedrock; this evidence 

suggests that the entire area was once glaciated. The 

ground moraine is predominantly till (the largely unsorted debris having been deposited directly by glacial 

ice), but it also includes sparse intercalated .bodies of 

rudely stratified silt, sand, and sandy gr,avel. Recent 

alluvium that floors the bottoms of the smaller drainage courses has not been differentiated from the ground 

moraine on the geologic map.

TOPOGRAPHY AND THICKNESS

The ground-moraine surface north of the Milk and 

Missouri Rivers is a relatively flat plain that slopes 

gently southward. Although there are some inconspicuous surface irregularities that include undrairied 

depressions, drainage is mostly integrated. The surface is commonly checkered by a characteristic network 

of vertical desiccation fissures that penetrate the upper 

few feet of the moraine.

The ground moraine is generally 5 to 20 feet thick 

except in the preglacial valleys. In the larger preglacial valleys it is as much as 170 feet thick, and in 

the smaller valleys it is a few tens of feet thick. Generally, however, it is not thick enough to completely 

obscure the configuration of the ancient valleys. In 

the northern part of the Frazer quadrangle, some small 

areas have topography similar to that of the end 

moraines; the thickness of the ground moraine in these 

areas reaches several tens of feet.

A network of low rounded till ridges, which apparently reflect a conjugate system of fractures formed 

in stagnant glacial ice, characterizes the ground-moraine surface east of Wiota. East of Frazer, ridges of 

this type are more numerous and longer. Colton 

(1958) traced these till ridges eastward as far as 

Westby in Sheridan County, a distance of 110 miles. 

He (Colton, 1958) stated that

The surface of the [ground moraine] in the valley of the Missouri River below an elevation of 2,400 feet is characterized by 

hundreds of ridges 50 to 100 feet wide, as much as 20 feet 

high, and 2% miles long. The longer ridges are parallel, 650 

to 1,200 feet apart and average five per mile. * * * the width of 

the belt ranges from 5 to 20 miles.

STRATIGRAPHIC POSITION AND AGE

The ground moraine was deposited in part on the 

Cretaceous bedrock and in part on the preglacial 

fluviatile materials. It is overlain by scattered outwash deposits, glaciofluvial deposits, probable eolian 

deposits, alluvium, pond deposits, and alluvium-col - 

luvium deposits. Throughout the area, the ground 

moraine is remarkably uniform in composition, soil
F32 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

profile, degree of dissection, and integration of drainage; this evidence indicates that the ground moraine 

is the product of a single ice sheet. However, in the 

western part of the Glasgow quadrangle south of the 

Milk River and west of Willow Creek, patches of 

ground moraine become sparse and thin and give way 

westward and southwestward to erratics lying free on 

bedrock. This area may represent simply an attenuated-drift border; on the other hand, this material 

may be drift of an older ice sheet.

Colton (1958) reported that the drift in the Fort 

Peck area continues eastward, and that in the vicinity 

of Medicine Lake in Sheridan County this drift is 

overlain by a younger moraine that he believed to be 

Gary in age. The ground moraine in the Fort Peck 

area overlies the Wiota gravels containing the mammoth tooth remains identified as "almost certainly Wisconsin in age and probably late Wisconsin." (See p. 

F28.) Presumably, if Colton is correct in his estimate 

of the age of the drift near Medicine Lake, the drift in 

the Fort Peck area is approximately Tazewell in age.

IITHOIOGY

The till constituting the ground moraine is a very 

compact, nearly impermeable moderately calcareous 

mixture of clay and lesser quantities of silt, sand, pebbles, cobbles, and boulders with traces of lignite. Its 

mechanical composition is much the same throughout 

the area. The clay and silt fractions make up 50 to 60 

percent of the total volume, the sand fraction constitutes 20 to 30 percent, and the rest is pebble sized and 

larger.

Sand sized and smaller grains, as well as some 

granule- and pebble-sized fragments of friable sandstone, shale, and lignite, were derived mostly from local 

bedrock of Cretaceous and Tertiary formations underlying much of this part of Montana. The silt and clay 

fractions probably were derived mainly from the Bearpaw shale, and the sand, sandstone, and lignite were 

derived mainly from the Hell Creek and Fort Union 

formations. In the coarse fraction, materials derived 

from the Flaxville and Wk>ta formations predominate, 

although the percentages vary from place to place. 

Pebbles, cobbles, and boulders derived from the Canadian Shield and the Paleozoic belts far to the northeast 

are also conspicuously present. Almost all bouldersized material is Canadian in origin. These erratics 

are much less smooth and rounded than the stones 

from the Flaxville and Wiota formations. They consist chiefly of cream-colored limestones and dolomites, 

various granitoid rocks, and a few schistose and other 

crystalline rocks. Although most of the erratics are

largely unaltered by weathering, those containing a 

large percentage of biotite have a disintegrated rind 

or, in the case of the smaller stones, are disintegrated 

to the center.

Percentages of the different kinds of pebbles in the 

ground moraine are given below. The figures represent averages taken from several pebble counts, in 

which the lower limit of size was one-half inch.

Percent

Upper Cretaceous and lower Tertiary bedrock (local 

origin) ______________________________ 3-4

Limestone and dolomite (northeast Canadian provenance) _______________________________ 26

Granitoid, schistose, and other crystalline rocks (northeast Canadian provenance) ________________ 15

Quartzite, amorphous and cryptocrystalline silica (from 

Wiota and Flaxville formations) _____________ 55

Large cobbles and boulders make up only a small percentage of the ground-moraine volume. Even so, boulders as large as 3 feet are not difficult to find. The largest boulder found, in NE^SW^ sec. 16, T. 28 N., R. 

40 E., was about 20 feet long before it was split by 

frost action.

WEATHERING

Where the ground moraine is less than 70 feet thick, 

it has all been oxidized yellowish brown; where it is 

thicker, the lower parts are gray to bluish gray.

Soil-forming processes have further altered the upper 5 or 6 feet. From the surface downward, a common soil profile (fig. 15) includes a dark-brown humus 

zone 6 to 10 inches thick, a lime-enriched zone about 2 

feet thick, and an underlying zone 2 feet or more thick, 

in which gypsum crystals have formed singly or in 

small clusters. It is not known whether the lowest 

zone has been enriched in gypsum or whether the crystals and nodules merely represent recrystallization of 

calcium sulfate originally disseminated in the till. 

Most pebbles within the upper two soil zones are coated 

with a thin layer of calcium carbonate on their undersurfaces.

The small patches of ground moraine shown on the 

geologic map in the vicinity of the Fort Peck Dam 

spillway and thence westward toward the dam itself 

are composed partly of till, but a considerable part of 

the material is a heterogeneous assortment of glaciofluvial deposits, other kinds of water-laid sediments, 

colluvium, and silty or sandy till. It is probable that 

the glacier involved in deposition of these materials 

was thin and more or less brittle, so that the effects of 

both ice shove and minor amounts of melt water have 

combined with slumping in both glacial and more recent times to produce the varied and nondescript materials present today.
GEOLOGY OF THE FORT PECK AREA, MONTANA F33

10 in -

36 in -

60 in

Humus zone, dark-brown; sandy silt, some 

clay pebbles

Caliche zone, light-brown; lime-enriched till. 

Lime as irregular pellets, stringers, and 

dispersed

Till, medium-brown; contains gypsum grains 

and nodules. Gradational into caliche 

zone above and unaltered till below. Base 

indefinite

FIGURE 15. Characteristic soil profile of the ground moraine in the 

Fort Peck area.

ENGINEERING CONSIDERATIONS

The till of the ground moraine is a dense, compact, 

nearly impermeable clay that is sticky when wet and 

hard and tough when dry. Figure 6 (samples 2, 15, 

22, 26, and 28) shows the soil classifications and Atterberg limits for the till. Figure 5 shows two mechanical 

analyses of till.

The ground moraine is worked easily with power 

tools, though the presence of a few large boulders 

might slow progress. Stability in cuts is proportional 

to their depth and steepness; cuts less than about 15 

feet deep stand well at angles exceeding 45°. The small 

slides that are common in open cuts are due more to the 

penetration of water- along the network of vertical 

fissures (see p. F31) in the till than to the weakness of 

the material.

In North Dakota northest of the mapped area there 

is a large number of coal-mine adits, about 6 feet high 

and 4 feet wide, that are partly excavated in ground 

moraine. These stand well for a number pf years without timbering. Inasmuch as the ground moraines here 

and in the mapped area are very similar, tunnels of 

comparable bore dug in till of the mapped area would 

probably stand as well.

Ground moraine is a stable foundation material, and 

settlement under heavy loads is negligible. It compacts well under optimum moisture conditions.

Unsurfaced roads on ground moraine are slippery 

and rut badly during wet weather; however, such 

roads in dry weather can be graded to a hard smooth 

surface.

Canals and other water-retaining structures constructed in ground moraine have negligible seepage 

losses. Till can be used as a nearly impermeable lining 

where such structures are constructed in pervious materials.

Because the ground moraine is so nearly impermeable, no appreciable quantities of water can be recovered from it. Even for the wells that happen to 

penetrate an intercalated lens of glaciofluvial sediment, 

the yield can be expected to be small.

Some shallow dug wells tapping this material produce small quantities of water, the amount varying 

with the season and weather.

KINTYRE FORMATION

NAME AND DISTRIBUTION

The Kintyre formation, here named for exposures 

near Kintyre, a siding on the Great Northern Railway about 5 miles west of the town of Frazer, is composed of fluviolacustrine sediments that were in large 

part deposited on stagnant ice in the old Missouri 

River trench (fig. 19). Although meanderings of the 

Milk and Missouri Rivers have cut away large parts, 

the formation still covers more than 70 square miles 

in the Fort Peck area and extends a considerable distance beyond both the east and west boundaries.

Large bodies of the Kintyre formation occur on the 

north side of the Missouri River east of Wiota and in 

a large triangular area west of the confluence of the 

Milk and Missouri Rivers and on the west bank of the 

Milk River southwest of Glasgow. Additional areas 

underlain by the formation are south of the Missouri 

River and east of Little Porcupine Creek. There is 

evidence that the Kinytre is also present in the Milk 

River valley northwest of the Glasgow quadrangle, 

and Colton (1958) mapped the formation east of the 

Fort Peck area.

TOPOGRAPHY AND THICKNESS

The Kintyre formation has an irregular gently rolling surface that differs from the ground-moraine surface chiefly in having greater local relief and smoother, 

longer hill-swale slopes. The topography is characterized by low gently rounded knobs, closed depressions, 

and poorly integrated drainage. It is notable also in 

that there are no erratics on the surface, except for a 

very few widely scattered glacial boulders.

The upland underlain by the Kintyre rises 75 to 150 

feet above the river flood plains. The formation forms 

nearly vertical cliffs where erosion is rapid, but it 

slumps to form irregular hummocky slopes where erosion is slow. The best sections were measured in gullies, which are now actively dissecting the cutbanks of
F34 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

the Missouri River near the town of Frazer and also 

about 6 miles south of the town of Nashua.

The formation ranges in thickness from 0 to more 

than 70 feet; in most areas of exposure the formation 

ranges in thickness from 20 to 70 feet. The variatior 

probably reflects to a large degree the irregularities of 

the underlying ice when the formation was deposited.

STRATIGRAPHIC POSITION AND AGE

The Kintyre is a Pleistocene formation laid down 

during the wasting of the Tazewell (?) ice sheet, which 

deposited the ground moraine in the Fort Peck area. 

The formation now rests directly on ground moraine 

in most places, but it rests on Bearpaw shale in some 

places south of the Missouri River. Generally it is the 

surface formation, although at a "few localities it is 

overlain by outwash terrace gravels, pitted outwash 

gravels, alluvium, and alluvial-colluvial deposits, or 

by patches of silt and clay deposited in intermittent 

ponds. South and west of Nashua the Kintyre formation is locally overlain by postglacial deposits of darkbrown sand that seem to be, at least in part, of eolian

origin.

LITHOLOGY

The lithology of the Kintyre formation varies considerably throughout the area. In most places the 

basal few feet consists of fissile olive-brown clay that is 

shalelike when dry. The overlying deposits are semiconsolidated interbedded fine-grained sands, sandy 

silts, clayey silts, and clay and include some small areas 

and isolated small lenses of unconsolidated mediumto coarse-grained sand. Broadly speaking, the lower 

half of the beds overlying the basal clay consists predominantly of light-tan silt and very fine sand interbedded with minor amounts of clay and silty clay. The 

upper half consist predominantly of dark-brown clay 

and silty clay. Locally, as in the bluffs north of the 

Missouri River in the central part of the Nashua quadrangle, the formation consists mostly of light-brown 

silt and silty very fine sand and includes dark-brown 

clay at the base.

Some sections of the Kintyre, which were measured 

near the town of Glasgow in the northwestern part of 

the area and near Frazer in the southeast, are composed only of dark-brown clay and silty clay from top 

to bottom.

For the most part, clays are massive, but silts and 

sands are delicately laminated and current bedded 

(fig. 16) similar to the structures observed in the Recent alluvium of the Milk and Missouri Rivers. Beds 

within the Kintyre are almost horizontal in many 

places, but in many others they have been folded,

broken, and otherwise contorted (fig. 17) on a scale 

that precludes frost action and under conditions that 

rule out modern landsliding action. The amount of 

folding and bending is probably an indirect reflection 

of the irregularities in thickness and shape of the ice 

upon which the beds of the Kintyre were originally 

deposited. Where the ice was thin or comparatively 

uniform in thickness, the sediments probably settled 

down onto the underlying materials with little or no 

distortion during ice wastage. .On the other hand, 

melting of large blocks or irregular masses of ice probably caused extensive readjustment and distortion .as 

the fluviatile cover settled.

ENGINEERING CONSIDERATIONS

Although all but the near-surface materials are compact or semiconsolidated, the Kintyre formation could 

be worked with either hand tools or power equipment.

Permeability is low in most parts of the formation, 

and it will be necessary to insure adequate drainage 

in any construction area. The average clay content is 

high, so that most materials found in the Kintyre are 

plastic when wet. Except where the dark-brown surficial sands are present, unsurf aced roads crossing the 

formation are nearly impassable in wet weather.

Unsupported shallow cuts are fairly stable at nearly 

vertical angles; cuts 20-30 feet deep stand well on 30° 

slopes where the material is predominantly clay. Where 

appreciable quantities of sand occur, slumping is prevalent.

Small quantities of ground water can be obtained 

from lenses of pervious materials. Subsurface circulation is limited by lateral f acies changes and by distortion of beds; therefore, groundwater is highly mineralized by soluble materials in the Kintyre formation and 

is generally unfit for domestic use. Seepage losses 

from small dams and earth tanks vary from negligible 

to serious, depending on locality and depth.

OUTWASH CHANNEL, DEPOSITS

DISTRIBUTION

Several widely separated deposits of glacial outwash 

have been mapped in the Fort Peck area. They consist predominantly of sandy gravel interbedded with 

lenses of sand, silt, and clay and are studded with 

numerous glacial erratics. A few outwash deposits are 

irregular in shape but most are long and narrow. 

They range from 1/4 mile to more than 3 miles in length 

and from a few tens of feet to more than a mile in 

width. Although a few show no apparent relation to 

modern drainage, most of the deposits in the Fort 

Peck area roughly parallel present-day valleys.
GEOLOGY OF THE FORT PECK AREA, MONTANA F35

FIGURE 16. In some places the beds of the Kintyre formation are definitely crosslaminated (photographed in sec. 23, T. 27 N., R. 41 B.). The 

similarity of this bedding to some in local alluvium of Quaternary age of the Missouri River supports a fluviatile origin for parts of the 

Kintyre formation. The white spots are calcium sulfate concentrations.

TOPOGRAPHY AND THICKNESS

Topographically, most of the outwash deposits are 

nearly identical with the surrounding ground-moraine 

plain. Surface appearance is distinctive only on the 

east side of Little Porcupine Creek, where the outwash 

deposits are marked by numerous irregular mounds 

and hollows that give them a pitted appearance.

Most outwash deposits in the map area are relatively 

thin and range from a few inches to 12 feet in thickness. The pitted outwash of Little Porcupine Creek, 

however, generally averages about 12 feet in thickness.

STRATIGBAPHIC POSITION AND AGE

The outwash gravels were deposited by the melt 

waters of the retreating Tazewell (?) ice. Most were 

laid down in flat-bottomed channels carved in the 

moraine. Locally, melt-water erosion cut channels 

completely through the ground moraine into the underlying Bearpaw shale. North of Frazer along the 

east fork of Charley Creek there are outwash deposits 

on Wiota gravels. Wherever present, the outwash deposits are at the surface.

EXPLANATION

FIGURE 17. Folds in the Kintyre formation exposed on the south 

side of the road at the curve in NE%SE% sec. 23, T. 27 N., R. 41 E.
F36 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

IITHOLOGY

The materials range from poorly sorted sandy 

gravel to well-sorted thin-bedded silt and clay. The 

gravel pebbles generally range from y2 to 4 inches in 

diameter. A large proportion of the pebbles is derived 

from the Flaxville and Wiota formations. Most of 

the rest are glacial erratics composed of limestone, 

dolomite, and crystalline rock; a few are derived 

from the underlying bedrock formations of Cretaceous 

age. Most of the erratics are pebbles, but a few are 

boulders and large cobbles. Most pebbles have a thin 

rind of caliche on their undersides, and some of the 

erratic pebbles that are rich in biotite are disintegrated.

ENGINEERING CONSIDERATIONS

The large body of outwash gravel on the east side 

of Little Porcupine Creek (sees. 16, 17, 20, 21, 28, 34 

of T. 28 N., R. 44 E., and sec. 3 of T. 27 N., R. 44 E.) 

constitutes the largest usable gravel reserve in the area. 

An estimated 5 million cubic yards is available; overburden is negligible, and excavation generally would 

be easy. Clay balls and reactive silica (hydrous silicates) are present in minor amounts. Most of the pebbles are rounded, so that crushing may be necessary if 

angular material is desired. Other outwash deposits 

are generally too thin or too poorly sorted to be of 

economic importance.

The outwash gravels are generally very permeable, 

and serious seepage can be expected to result from 

small dams and stock ponds that are built in these 

deposits.

OTJTWASH TERRACE DEPOSITS

DISTRIBUTION

Terrace remnants that consist of glacial-outwash deposits discontinuously flank the alluvial bottom lands 

of Little Porcupine Creek, Porcupine Creek, and Milk 

River. Outwash terraces that commonly are % to 1 

mile wide and 1 mile to more than 2 miles long are 

present north of Frazer and west of Nashua. Two 

long, narrow outwash terraces on the east side of Little 

Porcupine Creek range in width from a few tens of 

feet to one-fourth mile and are 4^2 and 3 miles long. 

A few small widely separated remnants are distributed 

on the west side of Little Porcupine Creek, along 

Porcupine Creek, and in the area between the Milk 

and Missouri Rivers. At least two periods of terrace 

development are represented, but no differentiation or 

correlation was attempted, because erosion has removed 

much of the terraces.

TOPOGRAPHY AND THICKNESS

Most of the outwash terraces have relatively even 

surfaces that slope gently downstream and also toward the streams that they border. Outwash terraces 

along Little Porcupine -Creek are commonly separated 

from the bottom land by scarps 5 to 20 feet high. The 

large outwash terrace north of Frazer, however, has 

no well-formed scarps and grades almost imperceptibly 

into bordering alluvium.

Scarps bounding the outwash terraces along the 

valleys of Porcupine Creek and Milk River are generally 40 to 60 feet high, although some are more than 

100 feet above the younger alluvial deposits.

The deposits range in thickness from 3 to 25 feet 

and average perhaps 12 feet. The larger remnants seem 

to be also the thicker deposits.

STRATIGRAPHIC POSITION

The outwash terrace deposits on the highland between the Milk and Missouri Rivers overlie the Kintyre 

formation. Elsewhere they rest on the Wiota gravels 

or the Bearpaw shale.

IITHOIOGY

Most of the material is sandy gravel (fig. 18) in 

lenticular beds, a few to several inches thick. Crossbedding is prominent in many places. Intercalated 

with the sandy gravel are some lenses that consist of 

sand and lesser amounts of silt and clay. Locally, 3 

to 5 feet of horizontally bedded and crossbedded fine 

sand and silt overlie the sandy gravel. The materials 

are unconsolidated, and the deposits, as a whole, are 

very pervious.

The rock types are the same as those in the ground 

moraine, except that hard clay balls are scattered 

through the outwash terrace deposits. Pebbles, generally ^2 to 3 inches in diameter, are the most common size fraction, although there are some cobbles and 

boulders.

ENGINEERING CONSIDERATIONS

The deposits are a source of sand and gravel, although the general lack of thick deposits probably 

limits exploitation to the terrace remnants just west of 

Nashua and north of Frazer. Detailed prospecting is 

necessary to determine the thickness ratios of overburden to usable material. The percentage of chemically reactive components is small, but friable pebbles 

and clay balls may limit the utility of the deposits. 

Because most pebbles are smooth well-rounded quartzite, crushing is necessary if angular material is required. For uses where cobbles and boulders are 

objectionable, the gravel must be screened. Acces-
GEOLOGY OF THE FORT PECK AREA, MONTANA F37

sible gravel deposits more than 6 feet thick and beneath 

less than 6 feet of overburden are estimated at about 

6 million cubic yards. Soils classification and Atterberg limits are given in figure 6 (samples 11 and 12). 

Mechanical analyses of these samples are shown in 

figure 18.

POSTGLACIAL QUATERNARY AND RECENT DEPOSITS

Erosion has been dominant since deglaciation except for deep alluviation in the trenches of the Milk 

and Missouri Eivers and along their major tributaries. 

Some fairly extensive colluvial deposits have accumulated along the lower slopes of these valleys. Pond 

and dune deposits are present, but only a few are 

large enough to map.

ALLUVIUM

DISTRIBUTION

Since the retreat of the last ice sheet, the rivers and 

streams have laid in their valleys extensive unconsolidated deposits of clay, silt, sand, and fine gravel; 

these deposits are everywhere characterized by both 

vertical and horizontal variations in composition and 

bedding. In general, the upper part of the alluvium 

is finer grained than the lower part.

Most of the mapped deposits are in the valleys of 

the Missouri and Milk Eivers and their larger tributaries; long, narrow bodies of alluvium floor some of 

the minor tributaries. Alluvium less than 3 feet thick 

was not mapped as a separate unit; thus no alluvium 

is shown on the map in many tributary valleys, in 

areas where thin alluvial deposits locally overlie 

glacial outwash, or in undrained depressions in the 

ground moraine.

TOPOGRAPHY

Most of the alluvial deposits of the Milk and Missouri Rivers valleys are comparatively flat except for 

shallow discontinuous meander scars. Many abandoned meanders are largely filled by slope-wash, eolian, 

and overbank deposits. Some of the more recent 

meander scars are 10 to 15 feet deep and contain intermittent ponds and swamps.

The surface of the alluvial fill of the principal tributaries is nearly level. Broad low alluvial fans have 

built up at many points where tributaries debouch onto 

the main flood plain; the fans are very gently convex 

and merge inconspicuously with the river alluvium.

Alluviation in this area has had a complex history, 

resulting in a generally greater rate of deposition durPebbles 

100

Granules Sand Silt Clay

90

80

70

60

50

Till from the ground

t.

40

Loose sand from the Fax Hills sandstone^

30

20

Weathered 

Bearpaw shale

Wet methods

1.0 O.I . 0.01 0.001 

PARTICLE SIZE, IN MILLIMETERS

*From data of the U.S. Army Engineers' laboratory at Fort Peck, Mont. 

fFrom data of the Montana Highway Department laboratory at Helena, Want.

FIGUBE 18. Mechanical analyses of alluvium and outwash terrace gravel samples (adapted from data of the Montana State Highway Department).. Sample numbers refer to figure 6.
F38 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

ing the recent past than at present. The older thicker 

fill forms terraces 5 to 25 feet above the present-day 

flood plains and is similar in composition to the younger 

thinner alluvium.

THICKNESS

Thicknesses of the alluvium in this area are difficult 

to determine. Exposures are limited to riverbanks and 

dredge cuts that stand less than 20 feet above the lowwater stage. Some information is available from logs 

of scattered drill holes and wells in the valleys, but, 

except for the area near the Fort Peck Dam, only a 

few borings penetrated to bedrock. Drill holes put 

down by the U.S. Corps of Engineers at and near the 

site of the dam show that alluvial fill is 122 to 164 feet 

deep near the center of the valley. A typical test hole 

by the U.S. Bureau of Reclamation at the center of 

the boundary between sec. 25, T. 27 N., R. 41 E., and 

sec. 30, T. 27 N., R. 42 E., showed that the fill is 114 

feet thick. A drill hole near the center of sec. 6, T. 26 

N., R. 43 E., entered the Bearpaw shale at 138 feet. 

R. B. Colton (written communication, 1959) stated 

that wells drilled in the Wolf Point and Poplar quadrangles to the east of the report area indicate that 

"more than 120 feet of alluvial fill is below the level 

of the present flood plain." These data support the 

general estimate that the alluvium along these segments of the Milk and Missouri Rivers ranges from 

90 to 130 feet in thickness.

The maximum thickness of the alluvium in Porcupine Creek northwest of Nashua is known from test 

borings to be about 50 feet. The alluvium is probably 

thinner upstream than downstream, if, as is supposed, 

the bedrock floor of this valley is graded to the bedrock 

floor of Milk River valley. Alluvial deposits of Little 

Porcupine Creek north of Frazer, Cherry Creek to the 

north of Glasgow, and Brazil and Willow Creeks south 

and west of Glasgow are presumed to have comparable 

thicknesses. The mapped alluvium in the smaller tributaries ranges in thickness from 3 feet to probably not 

more than 15 feet.

STRATIGRAPHIC POSITION AND AGE

During wasting of glacier ice from the trunk drainage lines of the Missouri and Milk Rivers, headwater 

regions supplied sufficient water, supplemented by local 

melt water, to initiate trenching of the main valleys 

and to the fix main features of the present drainage 

pattern. Subsequent aggradation began sometime after 

the Tazewell (?) glaciation. Thus, much of the alluvial 

fill in the principal valleys is probably Pleistocene in 

age, and, inasmuch as drill-hole data show that the 

basal fill is predominantly coarser grained than the 

upper alluvium, it may be glacial outwash.

Test holes in the valley bottoms and exposures along 

the walls indicate that the alluvium of the Milk and 

Missouri Rivers, as well as that which floors most 

tributaries, was laid directly on the Bearpaw shale. 

In some of the smaller tributaries, especially near their 

heads, the alluvium was deposited on the ground moraine and, locally, on the Wiota.

Although the alluvium is generally finer grained 

than the glacial outwash, the alluvium and outwash 

are so similar that contacts between them are drawn 

largely on the basis of topography.

The alluvial-colluvial deposits (p. F40) built out 

over the margins of the valley alluvium are the only 

deposits that are, at least in part, younger than the 

alluvium except for intermittent pond deposits in 

meander scars and in shallow depressions in ground 

moraine.

HTHOLOGY

The alluvium is composed of unconsolidated clay, 

silt, sand, and some gravel (fig. 18, sample 23). In 

nearly all deposits the upper beds have a higher percentage of fine materials than the lower beds. Lithologically, the alluvium is very similar to the finegrained fractions of the ground moraine and the 

Bearpaw shale from which it was derived.

Most of the alluvium is moderately to well sorted. 

Individual beds range in thickness from 1 inch to 4 

feet. All are lenticular, although some have a lateral 

extent of several hundred feet. Bedding structural 

features are varied and include lamination, thin to 

thick massive bedding, and crossbedding which ranges 

from delicate laminae having amplitudes of an inch 

or less to bold foreset beds 3 to 4 feet in amplitude.

The upper 15 to 20 feet of alluvium in the Milk and 

Missouri River valleys commonly has several fairly 

persistent dark beds a few inches to a foot thick, that 

are rich in humus. These dark beds are composed 

chiefly of poorly sorted mixtures of clay, silt, and fine 

sand and alternate with lighter nonhumic beds to produce conspicuously colored bands. The lighter beds 

also are composed of clay, silt, and sand but are generally better sorted. Unoxidized clay lenses that retain their original dark-bluish-gray color are common 

within the light-colored beds. The dark humic beds 

probably represent immature soil profiles formed when 

weathering was the dominant process in the cycle of 

alluviation. Because alternation of deposition and 

weathering both laterally and vertically would have 

occurred from the beginning of postglacial deposition, 

similar weathered zones probably are present to the 

base of the fills.

Test holes drilled by the U.S. Bureau of Reclamation show that as depth increases the sediments gen-
GEOLOGY OF THE FORT PECK AREA, MONTANA F39

erally increase in coarseness. A typical drill hole in 

sec. 36, T. 27 N., R. 42 E., shows the following profile:

Feet below surface

Silt _____________________________________ 7 

Loose fine sand __________________ 15 

Plastic clayey silt ________________ ___ 8 

Sand and gravel (maximum pebble diameter, 1 inch) 14

Total thickness _____________________ 44 

Bedrock (Bearpaw shale).

In the center of sec. 6, T. 26 N., K. 43 E., a test hole 

drilled 138 feet deep to the underlying Bearpaw shale 

passed through 10 to 15 feet of sandy clay, 85 feet of 

clayey silty sand, and 40 feet of sand and gravel. 

Without specific site investigations, however, it cannot 

be assumed that the alluvial sequence for any given 

location is as simple as has been indicated. Several 

borings by the U.S. Bureau of Keclamation and the 

U.S. Army Corps of Engineers have logged relatively 

thick bodies of gravel, sand, silt, or clay at various 

depths.

ALLUVIUM IN CREEK VALLEYS

The alluvium of Porcupine Creek, Little Porcupine 

Creek, and other tributaries is also divisible, in a general way, into a lower coarse-grained part and an upper fine-grained part. The lower coarse-grained part 

is 2 to more than 30 feet thick and is largely sand and 

gravel. Most of these deposits are poorly sorted, and 

crossbedding is common. The upper part is generally 2 

to 15 feet thick. It consists of small interfingering beds 

of fine sand, silt, and clay. In most tributaries, these 

beds are poorly sorted and also contain minor amounts 

of gravel which occur either as scattered pebbles or as 

small lenticular beds in addition to the finer material. 

Upstream, the sand fraction increases, and the clay 

and silt fractions are commonly very small. Downstream, as the tributaries approach the Missouri River 

flood plain, the silt and clay content increases, and the 

deposits are almost identical with the near-surface 

river alluvium.

ENGINEERING CONSIDERATIONS

The alluvium has considerably greater variation in 

permeability than most of the other unconsolidated 

materials. Permeability is low in the fine-grained 

parts of the river alluvium. As a result ground-water 

circulation is inhibited and the little water available 

in these fine-grained beds is unfit for drinking. The 

beds of both river and tributary alluvium are highly 

permeable where gravel and coarse sand are present, 

and they, as well as certain areas of Wiota and glacial 

gravels, are the best sources of potable water.

Topographically, alluvial bottom lands are favorable sites for highways, railroads, canals, irrigation 

projects, and towns. The physical properties of alluvium are therefore of unusual importance. These 

properties differ considerably from place to place, because the sediments composing the alluvium were deposited under such varying conditions of flow and load.

The materials are all unconsolidated and can be 

worked easily by hand or with power tools. As the 

water table is only 8 to 20 feet beneath the surface in 

most places, slopes are not stable below shallow depths. 

The near-surface materials erode to near-vertical slopes, 

but, because cohesion is low, cutbanks are subject to 

caving and slumping. Unsurfaced roads are satisfactory when dry but commonly become impassable when 

wet unless they are properly drained.

The basal beds of the alluvium along the tributaries 

are a source of sand and gravel, although the problem 

of removing the overlying fine-grained deposits limits 

their usefulness. Figure 18 shows a mechanical analysis of a sample (No. 23) collected in Porcupine Creek 

valley. Atterberg limits and soils classification for 

this sample are shown in figure 6 (sample 23). Soil 

classifications and Atterberg limits of other samples 

of alluvium, taken by channeling streambanks, are 

given in figure 6 (samples 6, 24, 30).

INTERMITTENT POND DEPOSITS

NAME AND DISTRIBUTION

There are a large number of small undrained depressions in the area. Most are on the ground moraine, 

and the rest occur on the Kintyre formation. Most of 

the depressions in the ground-moraine surface are kettles that formed when buried ice blocks melted; the 

others originated through the fortuitous deposition of 

glacial debris in such a way as to block the drainage 

of small areas. The depressions in the Kintyre are 

probably due to uneven settling during the wasting of 

the formerly subjacent glacial ice. Ponds, a few inches 

deep, form in them during wet weather but dry up 

soon afterward. Runoff from the ground immediately 

around the depressions washes silt and clay into them; 

the wind carries in a lesser amount of clay, silt,* and 

fine sand, especially during drought years.

TOPOGRAPHY AND THICKNESS

Except for a few saucer-shaped "blowouts," which 

contain no mappable deposits, the depressions have 

flat floors. A few have steep sides, 5 to 15 feet high, 

but most are bounded by very gentle slopes that rise 

only 3 to 8 feet above the flat pond deposits. The deposits themselves range in maximum thickness from 

6 to 12 feet and thin to a featheredge at the margins.
F40 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

STRATIGRAPHIC POSITION AND AGE

The filling of the depressions probably began soon 

after the glacial ice wasted away and has continued 

in most of them to the present. A few depressions 

have had their walls breached by erosion and no longer 

accumulate deposits.

IITHOIOGY

The pond deposits are predominantly plastic clay. 

A small amount of silt and fine sand is present either 

as thin laminae or admixed with the clay. The larger 

deposits overlying the ground moraine commonly have 

some coarse sand and fine gravel at the base of the clay. 

The uppermost foot of the deposits is a black clayey 

humus-rich soil. As the organic content decreases 

downward, the underlying material becomes denser 

and more compact and grades from dark gray below 

the humic soil zone to light brown 3 to 4 feet below the 

surface.

ENGINEERING CONSIDERATIONS

Both surface and subsurface drainage is very poor 

because of the flat surfaces and very low permeability 

of the deposits. Stability* of all but the shallowest cuts 

would be reduced by periodic inundation unless drainage facilities are provided. The deposits are handled 

easily by power tools. During wet weather unsurfaced 

roads are slippery and rut badly because of high plasticity of the deposits. Parts of the deposits may be 

suitable for ceramic clay. Figure 6 gives the soil classification and Atterberg limits of a sample (No. 10).

ALLTJVIAL-COLLTJVIAL DEPOSITS

DEFINITION AND DISTRIBUTION

Alluvial-colluvial deposits are deposits that have accumulated on the lower slopes of valley sides and over 

contiguous parts of the valley floors through creep, 

slope wash, and deposition by intermittent rivulets. 

These deposits therefore differ genetically from alluvial fans that accumulate by depositions from intermittent streams. The buoyant force acting on transported particles during alluviation derives from water, 

whereas in colluviation this force derives from the 

matrix of transported particles which is merely lubricated by water. Hence, in alluviation the role of 

water is direct and primary, and the role of gravity is 

indirect and secondary; in colluviation the reverse is 

more nearly true.

The term "alluvial-colluvial deposits" as used here 

connotes two things: a genetically intermediate origin 

for some of the deposits, and a commingling of the two 

types in other deposits where the scale of mapping 

makes it impossible to show them as separate units.

TOPOGRAPHY AND THICKNESS

Morphologically these deposits form broad, low fanshaped masses that, in many places, have coalesced to 

form aprons. The surface profiles are smooth concave 

curves that become progressively steeper upslope. 

Most are partly dissected by shallow steep-sided gullies. During dry periods the surfaces are checked by 

an intricate pattern of polygonal shrinkage cracks that 

commonly extend several inches into the ground.

Individual beds range in maximum thickness from 

10 to 20 feet. Near their margins, colluvial deposits 

diminish to a featheredge; downslope, they commonly 

grade imperceptibly into the valley alluvium. In general, map limits are arbitrarily drawn where the sediments thin laterally or upslope to less than 3 feet; 

downslope limits are based on topography.

STRATIGRAPHIC POSITION AND AGE

The colluvial deposits are contemporaneous with the 

more recent parts of the valley alluvium. No younger 

materials overlie the colluvium, and all animal remains discovered in them are those of living species.

IITHOIOGY

The composition and color of the colluvial deposits 

clearly reflect the parent materials. The Bearpaw 

shale, the Wiota gravels, and the ground moraine are 

the principal sources in the Fort Peck area. The colluvium derived from these deposits is predominantly a 

grayish-brown compact fairly tough mixture of clay 

and silt, but it includes minor amounts of sand and 

gravel that commonly occur as small scattered lenses 

in the upslope parts of the deposits.

In cross section, the material resembles till, but the 

presence of rude bedding and the arrangement of the 

flat surfaces of pebbles parallel to the colluvial surfaces gives a more pronounced fabric than that in till.

Figure 6 (sample 21) gives the soil classification and 

Atterberg limits of a colluvial deposit derived from 

the Bearpaw shale, the Wiota gravels, and the ground 

moraine.

ENGINEERING CONSIDERATIONS

The sloping surfaces and low permeability of the 

colluvial deposits combine to provide excellent surface 

drainage. Subsurface drainage is very poor, permitting little seepage loss from water-retaining structures. Alkali salts deposited by evaporting water 

make certain areas underlain by these deposits unfit 

for irrigation. The material is generally so tough that 

power tools are necessary for excavation. Cuts less 

than about 15 feet deep are stable at near-vertical
GEOLOGY OF THE FORT PECK AREA, MONTANA F41

angles. Deeper cuts at steep angles will initiate slumping.

Unsurf aced roads on such material are usually rough 

in dry weather and are slippery and subject to rutting 

when wet.

GEOLOGIC HISTORY OF THE TERTIARY AND 

QUATERNARY DEPOSITS

PBEGLACIAL HISTORY

Post-Cretaceous deposits older than early Pliocene 

or possibly late Miocene are not known in the Fort 

Peck area. To consider the Flaxville and Wiota formations in proper perspective, however, it is desirable 

to comment on the geologic history in the plains areas 

of Montana and the Prairie Provinces of Canada as 

far back as the approximate beginning of the Oligocene.

Three major drainage basins in this area seem to 

have been in existence continuously since early Oligocene time: the Yellowstone River drainage basin in 

southern Montana, the Missouri River drainage basin 

in central and northern Montana and in the southern 

part of the Prairie Provinces, and a drainage system 

in central and northern Alberta and Saskatchewan. 

In each basin prominent fluviatile deposits were laid 

down and, owing to differences in provenance, there 

are striking differences in lithology. The Missouri 

River has brought quartzite and argillite eastward 

from the Belt series, whereas the Yellowstone River 

has brought predominantly volcanic rocks.

From the beginning of deposition of the Cypress 

Hills gravel in southern Saskatchewan in the early 

Oligocene until continental glaciers invaded northeastern Montana late in the Pleistocene, pediplanation 4 

was a geologic process of prime importance in the Montana plains and neighboring parts of Canada's Prairie 

Provinces. During this time intermittent regional uplift and changes in stream regimen shaped the major 

features of the modern topography. This topography, 

broadly speaking, consists of a sequence of pediplains 

sloping eastward away from the Rocky Mountains, 

each younger pediplain at a lower elevation than its 

predecessor. Locally the slopes do not dip eastward,

* Pediplanation is the process yielding pediplains. King (1953) introduced the term "pediplain" into American literature. An individual pediplain may be defined as a simple or complex surface of 

regional extent consisting of either a single pediment or of a number 

of pediments forming an essentially continuous sequence of surfaces 

including no scarps other than local ones and no pronounced differences in profile form. Because pediplains enlarge by parallel retreat of 

headward scarps, more than one generation of retreating scarps may 

be present in an area at the same time. This multiple scarp condition 

holds in the northern Montana plains vicinity.

according to the positions and courses of the drainages 

that governed them. Remnants of old surfaces are 

present far from the mountains, and younger surfaces 

extend around and between them. For the most part 

the older surfaces are today the less extensive ones, but 

in the Fort Peck area this is not so. Here glaciation 

halted development of the lowest surface (Wiota age) 

shortly after its initiation, so that this surface is not 

much more than the "inner valley" of the preglacial 

drainage system.

A sedimentary mantle ranging from 0 to more than 

100 feet in thickness, but commonly from 5 to 35 feet 

in thickness, covers these pediplain surfaces. Sandy 

gravel, in some areas cemented to form conglomerate, 

is the predominant material of this mantle, which is 

distinctive lithologically as it corresponds to the provenance of the drainage basin. The youngest of the 

gravel was deposited prior to the arrival of continental 

glaciers at the site of deposition; the gravel is entirely 

nonglacial in character and underlies drift wherever 

drift is present.

The bench gravels of the ancient Missouri River system are dominated by characteristic pebbles and cobbles of smooth well-rounded chattermarked quartzite. 

The oldest bench gravel is preserved only atop the 

Cypress Hills in southwestern Saskatchewan and southeastern Alberta and was described by McConnell 

(1885). The formation has yielded Oligocene mammal remains. Some of the succeeding pediplains and 

their associated deposits blend almost imperceptibly 

into each other, as the difference in profile is so minor 

and the scarp between them is so inconspicuous. 

Others are readily separable. The two youngest (?) that 

are easily recognizable are the Flaxville formation and 

the Wiota gravels; a number of other units of subformational rank seem potentially recognizable. The 

Flaxville formation was named by Collier and Thorn 

(1918, p. 179) for typical exposures near the town of 

Flaxville in northeastern Montana. It covers large 

areas in this part of the State and contains Miocene or 

Pliocene fossils. Wiota gravels is the name introduced 

by Jensen (1951a) for gravel deposits younger than 

the Flaxville but older than local glacial deposits. 

They are widely distributed in this part of Montana 

and are known from fossil evidence to be, in part, as 

young as Wisconsin, probably late Wisconsin (Pleistocene).

In a few localities in the Nashua and Frazer quadrangles, the topographically lowest (that is, the youngest) parts of the Wiota gravels contain isolated glacial 

erratics. Jensen (1952b) therefore deduced that an ice
F42 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

sheet, older than the one (Tazewell?) that deposited 

the ground moraine of the Fort Peck area, advanced 

southward to within a few tens of miles of the area 

and that outwash from this ice sheet is responsible for 

the erratics. Porcupine Creek in pre-Tazewell (?) 

time apparently carried a small amount of this outwash, so that part of the Wiota gravels eastward 

(downstream) from Nashua also contains sparse erratics. No glacial erratics were found in the Wiota 

gravels west of the area near Nashua. The more northeasterly parts of Montana were evidently either closer 

to the margin of this ice sheet or were the sites of 

more important outwash drainage channels, for in 

these areas erratics are a conspicuous element in 

gravels of apparently equivalent age (Colton, 1951a, 

b; 1954; 1955a, b).

Jensen (1952b) believed that these gravel deposits 

associated with the development of the Tertiary and 

Pleistocene pediplains in central and northern Montana can be included in McConnell's (1885) South 

Saskatchewan group of Canada.

The term "South Saskatchewan group" was first 

used in 1885 by McConnell (1885) on a map of the 

Cypress Hills, Wood Mountain, and adjacent country 

showing reconnaissance geologic mapping in southern 

Saskatchewan. In his text McConnell (1885, p. TOc) 

used the term "South Saskatchewan gravels" for this 

map unit, the term being applied "as a general name" 

to deposits which he considered to be "not all contemporaneous." McConnell (1885, p. 45c, 5Tc, TOc) found 

these gravel deposits to be lithologically like those 

capping the Cypress Hills, to be derived from the 

gravels on the Cypress Hills, and to be widespread. 

In statements regarding age relations and age assignments, McConnell (1885, p. 33c, 36c, 45c, 57c, 59c, 70c) 

made it clear that his intent was to include in the two 

names "Cypress Hills gravel" and "South Saskatchewan gravels" all the preglacial quartzite gravels 5 in his 

map area, dividing them into two parts on the basis of 

age alone.

GLACIAL HISTORY 

REGIONAL BACKGROUND OUTLINED BY EARLIER STUDIES

Chamberlin (1888) was the first to recognize that 

northeastern Montana had been invaded by continental 

glaciers spreading southward from Canada. He drew 

the southern limit of this invasion a few tens of miles 

south of the Fort Peck area. A few years later Cal6 Calhoun (1906, p. 49-52), who coined the informal term "quartzite gravels" to describe deposits of the South Saskatchewan group 

in the northwestern plains of Montana, apparently chose not to use 

the available more formal term "South Saskatchewan," because McConnell (1885) and Dawson and McConnell (1895) had not defined 

clearly the exact scope of the term.

houn (1906, p. 32-45) undertook a study of the glacial 

geology of central western Montana, where he found 

evidence that led him to conclude that in preglacial 

times the Missouri Eiver flowed north around the 

Bearpaw Mountains and down the course of the present Milk Eiver.

In 1911, Alden undertook a more ambitious study, 

attempting to determine the later Tertiary and Pleistocene history of some 114,000 square miles in Montana, 

North Dakota, and Wyoming. He worked at this 

project intermittently until 1924, doing considerable 

fieldwork and collecting data gathered by other geologists working in this large area. His published 

work (Alden, 1932) is of necessity rather generalized.

GLACIAL HISTORY OF THE FORT PECK AREA

The ground moraine in the Fort Peck area is very 

uniform in composition, in soil-profile development, 

and in stage of slope dissection and drainage integration. No buried soils have been found within it. All 

evidence found during this study indicates the exposed 

ground moraine and associated drift to be the result of 

a single ice sheet. These characteristics are common 

to the ground moraine in the contiguous and nearby 

parts of the Fort Peck Indian Reservation to the east, 

to which E. B. Colton (written communication, 1959) 

assigned a Tazewell (?) age.

E. B. Colton (written communication, 1959) has 

found that large areas of the Flaxville plain north and 

northeast of the Fort Peck area are driftless areas in 

the otherwise drift-mantled plains of northeastern 

Montana. This evidence indicates that the Tazewell (?) 

ice sheet, in its general southerly or southwesterly advance, was separated into lobes by the Flaxville topographic prominences. Lobes of the glacier moved between and around the plateau remnants; as a result, 

the Tazewell (?) ice sheet advanced into the Fort Peck 

area from an easterly rather than a northerly direction 

and moved directly westward up the Missouri Eiver 

valley. It is likely that another lobe of this glacier 

moved westward around the north end of the driftless 

areas and, having passed this topographic barrier, 

turned south and southeast down the Milk Eiver valley 

toward Glasgow. The following evidence, although 

inconclusive, supports such a hypothesis: (1) Ground 

moraine capping low mesas on the east flank of the 

Bowdoin dome northwest of the Fort Peck area has 

the same characteristics as the ground moraine of the 

Fort Peck area and the Fort Peck Indian Eeservation 

area to the east; this evidence suggests that the ice 

which deposited the moraine near Bowdoin dome has 

much in common with the ice depositing the moraine 

of the Fort Peck area; (2) ground moraine of the Fort
GEOLOGY OF THE FORT PECK AREA, MONTANA F43

Peck area becomes attenuated westward from the approximate longitude of Glasgow. This fact indicates 

that the lobe that advanced westward up the Missouri 

River valley did not advance much beyond this longitude; (3) the interbedded ground moraine and gravel 

exposed in a gravel pit 3 miles northwest of Glasgow 

indicates a complex glacial history possibly attributable to interaction of multiple ice lobes of the same 

glaciation.

Evidence of the Tazewell(?) glaciation dominates 

the geologic scene, but other evidence here and in 

nearby areas shows that the Tazewell (?) ice was not 

the first glacier to advance over the Fort Peck area.

1. A few miles south and west of the Fort Peck Dam 

are the Larb Hills (fig. 9), whose flat tops are 

similar in elevation to many parts of the driftless 

areas of the Flaxville plain a few miles to the 

north and northeast of the Fort Peck area. On 

these hilltops are abundant granite erratics lying 

on the bedrock surface. The fact that the ice was 

not thick enough to override the Flaxville plain 

to the northeast shows clearly that the ice responsible for the drift that was on the Larb Hills 

could not have advanced from the northeast. The 

Larb Hills ice must have derived from the north 

or northwest and may have been the same ice that 

deposited the southeasterly aligned Snake Butte 

boulder train (Knechtel, 1942) in north-central 

Montana.

2. The few erratics in the Wiota gravels indicate the 

early presence of an ice sheet within at least a few 

scores of miles of the Fort Peck area.

3. Thickly scattered erratics rest on the bedrock surface of the high hills in the southernmost part of 

the Nashua quadrangle and in the westernmost 

parts of the Glasgow quadrangle. These represent either the distal parts of the attenuated 

Tazewell(?) drift margin or the much eroded 

remnants of the same drift that once covered the 

Larb Hills.

DRAINAGE HISTORY

Rivers and streams in the Fort Peck area were temporarily or, in some reaches, permanently displaced 

from their valleys by glaciation; commonly the axes 

of the modern valleys are offset to one side of preglacial-valley axes. The glaciation of the Larb Hills 

presumably affected drainage in the general area as 

much as did the subsequent Tazewell(?) ice, but data 

available on the Fort Peck area are too fragmentary 

to allow analysis of glacial-drainage history related to 

the earlier glaciation.

Drainage conditions just prior to the advance of the 

Tazewell (?) ice are evidenced by abundant data. The 

ground-moraine blanket is thicker in preglacial valleys 

than in preglacial divide areas but is not thick enough 

to obscure the positions or, in most places, the margins 

of these ancient valleys. Figure 19 shows the preTazewell (?) positions of principal streams in the Fort 

Peck area. Borehole data and natural exposures reveal 

no Wiota gravels in the Fort Peck area at altitudes of 

less than 2,030 feet in the western part of the area and 

of 2,005 feet in the eastern part of the area. It is 

therefore concluded that preglacial trunk drainage entered the Fort Peck area on the west at approximately 

2,030 feet above sea level and departed on the east at 

approximately 2,005 feet above sea level.

The valley now occupied by the Milk River and the 

part of the Missouri River valley downstream from the 

Fort Peck Dam represent the preglacial course of the 

Missouri River. This course is broader and morphologically more mature than the Missouri River valley 

upstream from the dam, which was established during 

the Pleistocene when advancing ice older than the 

Tazewell (?) ice blocked the old valley and forced the 

river into a new course. The topography bordering 

the Fort Peck Reservoir is characterized by dissected 

and commonly steep-sided ridges and hills; the tributary valleys have narrow V-shaped profiles. The main 

valley of the Missouri River itself, prior to the filling

FIGURE 19. Sketch map showing drainage history in the Fort Peck 

area and vicinity.
F44 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

of the reservoir, was considerably narrower and generally lacked the broad flood plain of the valley downstream from the dam. Surface exposures and drillhole data on the area near Fort Peck Dam show that 

the river valley near the dam is partly filled by Tazewell (?) moraine and, therefore, antedates the Tazewell( ?) glaciation. The fact that the Larb Hills west 

of Fort Peck was glaciated before Tazewell(?) time 

also indicates that the Missouri River was moved 

southward out of the Milk River valley to its present 

course prior to Tazewell (?) time.

Present principal drainage lines follow, in a general 

way, their pre-Tazewell (?) courses except for Willow 

Creek south of Glasgow. The chief difference is that 

the modern valleys are generally located at an edge of 

the preglacial valleys. Thus, the modern Missouri 

River flood plain is displaced to the south edge of the 

preglacial valley; the Milk River flood plain is generally north of the buried-valley axis; and Porcupine 

Creek is displaced a short distance to the west (fig. 19). 

This situation apparently is due to the fact that the 

thin ground moraine on soft Bearpaw shale at the old 

valley sides was more easily eroded than the thicker 

ground moraine in old valley centers.

In local areas the modern streams have abandoned 

preglacial courses entirely; for example, the lower 

reaches of Porcupine Creek and the Milk River downstream from Nashua. These changes probably began 

during meandering of the Milk River over a valley 

full of stagnant ice, the courses of the streams becoming fixed by the time the obstructing ice wasted from 

the original valley.

A drainage change of more significance occurred on 

Willow Creek, which in preglacial time flowed east in 

a broad valley across the southern part of the Glasgow 

quadrangle and emptied into the Missouri River. Willow Creek at present turns abruptly north in the southwestern part of the quadrangle, flows through a compartively steep-sided valley, and empties into the Milk 

River.

The Tazewell (?) ice lobe that advanced westward 

into the Glasgow quadrangle might have formed a 

glacial lake in the Willow Creek valley, but no field 

evidence was found to support this hypothesis. In the 

Milk River valley west and southwest of Glasgow are 

deposits mapped as Kintyre formation; these may 

represent a lake backed up by the Tazewell (?) ice. 

The canyon of Willow Creek was almost certainly a 

spillway channel from the indicated lake west of Glasgow into the postulated lake in the upper Willow 

Creek valley.

Willow Creek history may have been as follows: 

Ponding in the Milk River valley west of Glasgow

caused a divide at the position of the canyon to be 

overtopped; ponded water then spilled into the glacial 

lake in Willow Creek in which the water level may 

subsequently have risen until it too reached a spill 

point to the southwest (sec. 34, T. 24 N., R. 39 E.). As 

the ponding barrier formed by ice in the Milk River 

valley was reduced by melting and evaporation, the 

lake west of Glasgow drained away down the Milk 

River valley and the direction of flow in the Willow 

Creek spillway, was reversed, so that the glacial lake 

in Willow Creek drained into the Milk River. The 

flow continuously deepened the spillway in the process, 

resulting finally in the relatively steep-sided valley 

seen today.

As the Tazewell (?) ice advanced up the Missouri 

River valley into the Fort Peck area, the trunk drainage must certainly have been effectively blocked. Resultant ponding presumably backed water up tributaries leading into the river valley from the south and 

southwest until divides were overtopped. In this way 

the water gathered by the Missouri River drainage 

system was allowed to escape to the west. As the ice 

moved westward, these spillways would be overrun at 

their junctions with the main valley, and new spillways 

would be required; generally these would develop at 

different elevations because of the drainage-system 

gradient and the elevations of the lowest available 

spill-point divides. How many times this phenomenon 

occurred in the vicinity of the Fort Peck area is not 

known, but numerous spillway-channel remnants are 

known to exist south of the Missouri River in McCone 

County and in Valley County in the area between Willow Creek and the Fort Peck Reservoir to the west of 

the Fort Peck area.

LATE-GLACIAL AND POSTGLACIAL HISTORY OP 

THE PORT PECK AREA

The wasting of the Tazewell (?) glacier contributed 

to the formation of many of the present geologic features of the Fort Peck area. During wasting of the 

ice, the driftless-area nunataks enlarged and others 

formed in nearby areas; eventually, a stage was reached 

when much of the area north of the Missouri River 

valley was ice free, while the valley itself, where ice 

had been thickest, was still occupied by stagnant ice. 

As this ice thinned, it became less of a barrier to drainage, and in time the river overtopped this barrier and 

flowed on the stagnant ice. Irregularities in the ice 

surface included depressions that became the sites of 

ponds and lakes whose outlines and positions changed 

as deposition and melting proceeded. Melting of the 

stagnant ice was presumably slow, owing to insulation 

by fluvial and lacustrine deposits and may not have
GEOLOGY OF THE FORT PECK AREA, MONTANA F45

been complete until many years after the disappearance of ice elsewhere in this part of Montana.

The nunatak area to the north of the mapped area 

enlarged during wasting and eventually included the 

higher northern parts of the Fort Peck area-. Evidence for the ice-free condition of these northern areas 

during a time when ice still occupied the Missouri 

River valley to the south is found, among other places, 

in the northern part of the Frazer quadrangle. Here, 

melt waters flowed along the margins of the nunataks 

in channels trenched partly in ice-free ground and 

partly in ice, as indicated by discontinuous and fragmentary surface markings of the courses. Additional 

evidence lies in the trend of some of these channels (T. 

29 N., R. 43 E.), which is east normal to the southward 

slope of the ground-moraine plain; the scars of these 

channels are illuminating as to their origin in that the 

south sides are lower and less well defined than the 

north sides; indeed, in some places there is no evidence 

of a south side at all. Evidently the missing features 

in the channels were formed by the glacial ice, which 

has long since melted.

The East Fork of Charley Creek, a south-flowing 

stream near Frazer, provides additional evidence that 

much or all the northern part of the Frazer quadrangle 

was ice free at a time when ice still lay in the broad 

valley to the south. This creek follows a glacial-outwash channel which, though dissected, is topographically distinct except near its southern limit. Here the 

outwash deposits have no topographic boundary and 

are superposed on the irregular surface of the underlying ground moraine. This irregular surface is reflected in the uneven surface of the outwash deposits 

and is exposed in a highway cut in the SW^ sec. 20 

and NW*4 sec. 29, T. 27 N., R. 44 E. The deduction 

is that the water in the southern reaches of the channel 

flowed on stagnant ice lying in the valley of the Missouri River and that the channel deposits were lai«l 

down on the moraine as the ice melted away.

Collapse features within the superglacial fluviolacustrine deposits (Kintyre formation) are striking 

evidence of the persistence of underlying stagnant ice 

in the Missouri River valley long after the valley was 

reoccupied by trunk drainage. Folds, as great as 6 

feet in amplitude, formed by collapse are common 

for example, in the Missouri. River bluffs south of 

Frazer and at a highway cut about 4 miles south of 

Nashua (fig. 17). In the south bluff of the Missouri 

River south of Nashua near the common corner of Tps. 

26 and 27 N., Rs. 41 and 42 E., blocks of sand and silt 

as large as 30 feet on a side are locally enclosed in 

finer-grained sediments. Exposures in the gullies dissecting the bluffs on the other side of the river show

that the deposits are a jumbled mass of blocks and irregular stringers. The unconformable relations of the 

stratified eolian sand south of Nashua to the underlying superglacial deposits indicates that the structure 

of the superglacial deposits is not the result of slumping initiated by modern erosion of river bluffs. The 

bedding in this sand is not disturbed; therefore, the 

sand must have been deposited after deformation of 

the superglacial deposits. On the other hand, the bedding of the sand is unconformable with the present 

topography and, therefore, the sand must have been 

deposited prior to the existence of this topography.

Previous to the spilling of glacial Milk River onto 

the ice surface, a set of conjugate fractures opened in 

the stagnant ice in the central part of the Frazer quadrangle. These fractures became filled with till and 

their pattern consequently has been preserved on the 

ground moraine plain as a network of low ridges. In 

a few places these ridges have surface expression, even 

where covered by the Kintyre formation; this condition is consistent with the hypothesis that the Kintyre 

is a superglacial deposit that was draped over the ridge 

deposits as the intervening ice wasted away.

Outwash-gravel deposits, including outwash terrace 

gravels, and ice-contact gravel deposits are thin and 

sparse in the Fort Peck area compared with many 

other glaciated areas. This indicates a dearth of melt 

water and suggests that ice wasting was either slow 

or was accomplished less by melting than by evaporation. The fine-grained character of the Kintyre, which 

was certainly deposited during ice wasting, also supports the hypothesis that a slack-water low-volume 

drainage regimen existed during waning stages of the 

Tazewell (?) glaciation.

The hummocky pitted outwash on the east side of 

Little Porcupine Creek is probably a collapse deposit 

of stratified drift that was originally laid down by 

melt water on a thin tongue of stagnant ice.

As the glacial age progressed, downcutting began 

along drainage courses and continued until the valleys 

of the Milk and Missouri Rivers were some 160 feet 

deeper than they are today. A shift in regimen reversed the procedure, and valleys then began to be 

aggraded. It is not known at which point melt waters 

ceased to contribute a major portion of stream volume; 

however, the fact that the coarser material is largely 

confined to the lower part of the fill suggests that it 

was well before the end of aggradation. Aggradation 

in the valleys of the Milk and Missouri Rivers controlled deposition in Porcupine Creek, Little Porcupine Creek, and other large tributaries and continued 

until the flood-plain level in the main valleys was several feet higher than at present. The effect of ag690-458 O-64-4
F46 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

gradation was extended far up the tributaries, although it was most pronounced near the confluence 

with the main valley.

Within comparatively recent time, the rivers have 

resumed downcutting, which has lowered flood-plain 

levels about 25 feet below the highest terrace alluvium 

of the valleys.

SUMMARY OF NATURAL RESOURCES

Nonmetallic construction materials suitable mostly 

for concrete aggregate, fill, highway base course, or 

highway surfacing are the major natural resources, 

other than soil and water, known to be present in the 

Fort Peck area. There are no known deposits of metallic ore; the coal-bearing Fort Union formation, 

present in nearby areas, has been removed by erosion 

from the Fort Peck area. No oil or gas has as yet 

been discovered, although the subsurface formations include favorable stratigraphic units that have yielded 

both oil and gas in other parts of the State.

The following discussion summarizes and, in places, 

supplements the information presented under the 

heading "Engineering considerations" in the discussion of each geologic unit.

HIGHWAY SUB GRADE MATERIALS

Nearly all materials from the Tertiary and younger 

formations can be and have been used in the construction of the subgrade of primary highways. The entire 

road grade of secondary roads is usually constructed 

from the underlying material. The Bearpaw shale, 

particularly the bentonitic members, is generally considered unsuitable for road construction. Most of the 

poorly consolidated parts of the Fox Hills and Hell 

Creek formations could be used for surfacing and base 

course but have not been developed because more suitable materials are generally available nearer the highways.

SAND AND GRAVEL

The principal sources of sand and gravel include the 

Wiota gravels, the Flaxville formation, the outwashgravel deposits, and some stream alluvium. Varying 

thickness, lateral extent, and amount of overburden 

require detailed prospecting of each deposit as a prelude to exploitation. Most of the existing gravel pits 

are in the Wiota gravels. Poor accessibility has limited 

development of the Flaxville formation in the Fort 

Peck area. Except for a large deposit on Little Porcupine Creek, which represents the largest usable gravel 

reserve in the area, most of the outwash deposits are 

too thin or too poorly sorted to have any commercial 

value.

RIPRAP

Cobbles and boulders, 6 to 30 inches in diameter, 

have been gathered into heaps along some section lines 

on the ground-moraine plain; these and the harder 

ledge rock in the Fox Hills sandstone and Hell Creek 

formation might be of some economic value as riprap.

CERAMIC CLAY

The most promising source of ceramic clay seems to 

be those materials mapped as intermittent pond deposits. Because the ground moraine and Bearpaw 

shale consist very largely of clay, these formations 

might also merit testing.

MATERIAL FOR LINING WATER-RETAINING 

STRUCTURES

Bentonite is commonly used to prevent seepage loss 

from water-retaining structures. The Bearpaw shale 

contains a large number of bentonite beds, but all those 

exposed within the map area are too thin (V4 inch to 

19 inches) to be of economic value at present. The 

till of the ground moraine is so nearly impermeable 

that it might prove to be a satisfactory substitute in 

some uses.

LIGHTWEIGHT AGGREGATE

A major cement company has tested the Pier re shale 

in northeastern Colorado and has found it generally 

acceptable for the production of lightweight aggregate. Because the Bearpaw shale is the equivalent of 

the upper part of the Pierre and is similar in composition in most localities, the Bearpaw shale warrants investigation as a possible source of lightweight aggregate in eastern Montana. The expense of production 

often limits the use of lightweight aggregate; however, the small quantities of good natural aggregate 

and the abundance of the Bearpaw shale might justify 

the construction of a lightweight-aggregate plant in 

this area.

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GEOLOGY OF THE FORT PECK AREA, MONTANA F47

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 I952a, Structural contour map of part of the Williston 

basin in the Fort Peck area, Montana: U.S. Geol. Survey 

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U.S. Geol. Survey Misc. Geol. Inv. Map 1-129.

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F48 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

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Smith, J. F., Witkind, I. J., Trimble, D. E., and Duckworth, 

E. G., 1955a, Preliminary geologic map of the Goose Mountain No. 1 quadrangle, Montana: U.S. Geol. Survey openfile report.

 1955b, Preliminary geologic map of the Goose Mountain No. 2 quadrangle, Montana: U.S. Geol. Survey openfile report.

 1955c, Preliminary geologic map of the Lonesome

Lake No. 2 quadrangle, Montana: U.S. Geol. Survey openfile report. 

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Alberta and southern Saskatchewan : Canada Geol. Survey 

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INDEX

[Italic page numbers indicate major references]

Page 

Acanthoscaphites.............................. F8

Actinosepia canadensis....................__ 8

Aggregate, lightweight_____________ 46 

Agriculture..___________________ 2 

Alluvial-colluvial deposits_________ 31,38,40 

Alluvial fans_ _.______________ 37 

Alluvium. .__________._______ 31,57 

Artesian wells_________________ 24

Baculites..................................... 8

Badlands __.__.___.......___ 2,5,11,18

Bauer, C. M., quoted._____________ 21 

Bearpaw shale.. ________________ 3,5,24 

Benches.__....___.___________ 27,28,41 

Bentonite..__________________ 6,18,46 

Bibliography___________________ 46 

Blood Creek-Sheep Mountainsyncline...__ 23 

Bowdoin dome.________________ 5,23,42 

Brown, Barnum, quoted.___________ 4,22 

Brown, R. W., quoted...___________ 16

Ceramic clay._________________ 46 

Channelffll............................ 20,21,22,34

Claggett shale_________________ 3

Clay......................................... 46

Clay-ironstone concretions__________ 6,15

Climate___._.__._____.__.__ 2

Coal........................_... .-...... 4

Cobban, W. A., quoted..___________ 7 

Colgate member of Fox Hills sandstone_ 11, 16, 22

Collapse features...______________ 45

Collier, A. J., and Thorn, W. T., Jr., quoted... 27

Colluvium deposits______________ 31

Colorado group_________________ 3

Colton, R. B., quoted________.__._ 31 

Concretions..______________ 6,15,18,20

Cone-in-cone structures __________ 6

Coulees_____________________ 18

Cretaceous-Tertiary boundary, discussion of.. 16

Crossbedding_________________ 22

Cypress Hills gravel..__________._ 26

Dinosaurs__________________ 19,20,22 

Discoscaphites-.... ...._..___________ 8

Dobbin, C. E., and Reeside, J. B., Jr., quoted. 15 

Drainage history...__........__._.._ 43

Eagle sandstone________________ 3 

Emperoceras-................. ._______.. 8,24

Engineering considerations..._______ 11, 

16,23,27,30,33,34,36,39,40

Eolian deposits__.____________.. 31,34,45 

Equus.....................___________ 28

Erratics...................._...... 30,31,32,34,43

Faults..................................... 23,24,25

Fieldwork---.......____.______..__._.___.____ 2

Flaxville formation............... 18,20,24,26, «7,41

Fluviolacustrine sediments________ 31,33,45

Page 

Folds .-.. ..... - -... F45

Fort Pierre shale...___________... 3

Fort Union formation...._____.__ 4,5,17,22

Fossils ................ 7,15,16,19,20,21,24,27,28

Fox Hills sandstone.._____..___ 3,4,5, 11, 22 

Freedom dome........_...__.____ 4

Geography_____.______________ 2 

Glacial deposits...-_-.-.-- ..-........- 37,34,45

Glacial history._-....-................._-.- 42

Glacial lake.................................. 44

Glaciofluvial deposits...___________ 31 

Gravel................. ................ 36,45,46

Great P]atDa................................. 1,3,26

Ground moraine..- _--..-............__ 27,28, SI, 39

Ground water....................... 16,23.31,34,39

Hidymenites...................._....__.. 15

Halymenites major............ ......___... 15,16

Hell Creek beds.............................. 4

Hell Creek formation. _____.___.__ 4,5, 17 

Hesperorhynrhia superba........ ._...__... 8

Highway materials._______. _--.-. 46

Historical geology...---..----.__------. 41,42,43,44

Hoploparia f^rowni............................ 8

Hough, Jean, quoted.. __________.. 24.28

Ice shove____________________ 25,32 

Indian Hill................................... 21

Investigation purpose...._______.__ 2

Jensen, F. S., quoted______________ 28 

Judith River formation...__.......__ 3, 5, 24,25

Kettles---..-.___..-. ___...._ 39

Kintyre formation___________ ____ 33

Lance formation..__. 

Landslides.-...___. 

Laramide Revolution. 

LarbHmsice..... .

...._.___.__ 4

.-. ......... 11

. .- . - 22

.- ... .. 43

Late-glacial and postglacial history. _____ 44

Lewis, O. E., quoted______________ 28 

Lignite...... . .. . 4,19,23

Limestone concretions_____________ 7,11

Lingula....................................... 15

nitida -- ........................... 15

Lining for water-retaining structures_____ 46

Little Porcupine Creek_____________ 39

Location of area... -..-. . .. 1

Mammoths . . 24

Mammuthus (Archidiskodon) imperator........ 28

Mammuthus boreus........................... 24

Mammuthus primigenius..................... 24,28

Meander scars________________ 37

Meek, F. B., quoted.______.______ 3 

Milk River.. . 2,24,31,37

Page 

Missouri River......_......... F2,25,31,37,41

Montana group________.... S, 5

Moraine. See Ground moraine.

Natural resources. 

Niobrara.........

Nostoceras.........

Nunataks........

Outwash deposits............ ... . 31,34,36,45

Parelephas columbi............................ 29

jeffersoni.................................. 24

Pecten saskatchewanensis...................... 8

Pediplains._____....... . .. 27,41

Perched water tables...__.. 16 

Pond deposits . 31,38,39

Porcupine Creek.___--. _..----..--. 39,42

Precipitation__________________ 2

Preglacial history______-... 41

Quaternary deposits_____ 26,25,41

Recent deposits. 

Riprap.____.

37

Sand and gravel____________ 46

Sandstone concretions..____. 15

Septarian concretions_____ 7

Slumping_______________ 24

Snake Butte boulder train..__.. 43

Soils..-.... . - 32

Somber beds...._____.. 19

South Saskatchewan group 42 

Springs______....___...-_____ 16,23,31

Structural geology___ 25

Synclines ..... . . 23

Tazewell glaciation... 26,31,34,35,42,43 

Tepee butte concretions..___ 7 

Terraces 25,28,36,45,46

Tertiary deposits 26,27,41

Thorn, W. T., and Dobbin, C. E., quoted._ 16 

Thrust faults. . 25

Tiger Butte... 23,24

Till ridges 31

Townsend, R. C., quoted 26 

Two Medicine formation__ 3

Virgelle sandstone member of the Eagle sandstone__ - 3

Wells..... . 33

Wiffiston basin....... 23

Wiotagravels...________ 20,23,24,25,26,2S,41 

Wisconsin glaciation 31 

Wood Mountain gravel____ 26

Yellowstone River drainage basin. 41

F49