Bentonit€ Deposits in 

Marine Cretaceous, 

Formations, Hardin 

District, ,Montana and 

Wyoming 

GEOLOGICAL SURVEY BULLETIN 1023 

Bentonite Deposits in 

Marine Cretaceous 

Formations, Hardin 

District, Montana and 

Wyoming 

By MAXW~ELL M. KNECHTEL and SAM H. PATTERSON 

With a section on 

LABORATORY PROCEDU.RES USED FOR TESTING 

THE BENTONITES 

By SAM H. PATTERSON 

GEOLOGICAL SURVEY BULLETIN 1023 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1956 
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary 

GEOLOGICAL SURVEY 

W. E. Wrather, Director 

For sale by the Superintendent of Documents, U. S. Government Printinl;l Office 

Washington 25, D. C. 
CONTENTS 

Page 

Abstract__________________________________________________________ 1 

Introduction______________________________________________________ 1 

Location and field work_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 1 

Acknowledgments_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 2 

Geology__________________________________________________________ 4 

Surficial features______________________________________________ 4 

Bentonite-bearing formations _________________________________ - _ 5 

Lo\ver Cretaceous series____ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 6 

Cloverly formation_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 7 

Thermopolis shale_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 7 

Mowry shale__________________________________________ 9 

Upper Cretaceous series____________________________________ 11 

Colorado group__________ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 17 

Lower part of the Cody shale_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 17 

Montana group_______________________________________ 28 

Upper part of the Cody shale_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 28 

Judith River formation_____________________________ 34 

Bearpa w shale_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 36 

Hell Creek formation______________________________ 42 

Geologic structure_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 42 

Bentonite deposit.s_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 43 

Mineral content_______________________________________________ 44 

Characteristics of outcrops______________________________________ 44 

Effects of weathering on color and colloidal properties______________ 44 

Mining_______________________________________________________ 46 

Util~ation____________________________________________________ 50 

Foundry-sand bonding clay_________________________________ 50 

Rotary drilling-mud materiaL_______________________________ 51 

Clay Spur bentonite bed____________________________________ 52 

Soap Creek bentonite bed__________________________________ 52 

Bed M___________________________________________________ 53 

Bed R___________________________________________________ 53 

Bed U___________________________________________________ 54 

Bed V____________________________________________________ 54 

Bed VV___________________________________________________ 54 

Description by township and range_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ 54 

T. 1 N., Rs. 33 and 34 E___________________________________ 55 

T. 1 S., Rs. 30, 31, 32, and 33 E_____________________________ 56 

T. 1 S., Rs. 34 and 35 E____________________________________ 57 

T. 2 S., Rs. 30, 31, and 32 E________________________________ 61 

T. 2 S., Rs. 33 and 34 E_____________ ______________________ 62 

T. 2 S., R. 35 E___________________________________________ 62 

T. 3 S., R. 30 E ___________ ------------------------------- 66 

T. 3 S., R. 31 E___________________________________________ 67 

T. 3 S., R. 32 E___________________________________________ 68 

III 
IV CONTENTS 

Bentonite deposits-Continued 

Description by township and range-Continued Page 

T. 3 S., Rs. 33 and 34 E____________________________________ 70 

T. 3 S., R. 35 E___________________________________________ 70 

T. 4 S., R. 30 E___________________________________________ 72 

T. 4 S., R. 31 E___________________________________________ 72 

T. 4 S., R. 32 E___________________________________________ 74 

T. 4 S., R. 33 E. and part of R. 34 E______ _________________ 7.5 

T. 5 S., R. 30 E___________________________________________ 75 

T. 5 S., R. 31 E___________________________________________ 76 

T. 5 S., R. 32 E___________________________________________ 76 

T. 5 S., Rs. 33 ai1d 34 E ________________ ------~------------ 77 

T. 6 S., R. 31 E___________________________________________ 78 

T. 6 S., R. 32 E____________________________________ 7D 

T. 6 S., Rs. 33 and 34 E_________________ __ ________________ 8G 

T. 7 S., R. 32 E___________________________ ________________ 86 

T. 7 S., R. 33 E___________________________________________ 89 

T. 8 S., R. 33 E _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ~H 

T. 9 S., R. 34 E___________________________________________ 93 

T. 9 S., R. 35 E___________________________________________ 96 

T. 58 N., R. 88 W., Sheridan County, Wyo___________________ 98 

Laboratory procedures used for testing the bentonites, by Sam H. Patterson__________________________________________________________ 100 

Use as foundry sand-bonding clay_______________________________ 101 

Use as a constituent of rotary well-drilling mud___________________ 107 

Dilatancy (swelling capacity)_______________________________ 107 

V~co~ty_________________________________________________ 108 

Yield____________________________________________________ 109 

Gel strength (thixotropy)___________________________________ 110 

Wall-building efficiency_____________________________________ 110 

Content of nonclay material (including grit)______________________ Ill 

References cited___________________________________________________ 113 

Index____________________________________________________________ 115 

PLATE l. 

2. 

FIGURE l. 

2. 

3. 

4. 

5. 

6. 

ILLUSTRATIONS 

Page 

Map showing geology and bentonite deposits of the Hardin 

district, Montana and Wyoming _____________________ In pocket 

Columnar section of rocks exposed in Hardin district, showing 

stratigraphic positions of bentonite beds__ _ _ _ _ _ _ _ _ _ _ _ In poe ket 

Index map showing location of area mapped in the Ha.rdin diRtrict, Montana and Wyoming__________________________ 3 

View northea.stward from point in sec. 23, T. 9 S., R. 3-1: E., 

southeast of Little Bighorn River_______________________ 6 

Stratigraphic nomenclature employed on plate 1 and nomenclature favored by the authors_________________________ 12 

Belts and thicknesseR of Upper Cretaceous deposits of the 

western interior region of the United States______________ 13 

Schematic diagram 8howing position of facies at top of Frontier formation_________________________________________ 14 

Soap Creek bentonite bed locally thickened by flowage______ 19 
CONTENTS v 

Pag·c 

FIGURE 7. View southward along outcrop of bentonite bed Fin KW% 

TARLE 

sec. 17, T. 2 S., R. :35 E_______________________________ 37 

8. Closeup of large limestone concretions along outcrop of 

bentonite bed V in NWX sec. 17, T. 2 S., R. 35 E. (location 

same as shown in fig. 7) _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 37 

H. View southward along outcrop of bentonite bed W from point 

in SE% sec. 17, T. 2 S., R. 35 E________________________ 38 

10. Popcorulike clay bloom as shown on outcrop of Soap Creek 

bentonite bed in SE?~ sec. 15, T. 7 S., R. 32 E____________ 45 

11. Test data on tempered mixtures of molding sand (summarized) 

12. Curves for a sample of bentonite from the Soap Creek bed. 

A, Variation of green bonding strength according to percentage of tempering water in sand-clay mixture at time of test; 

B, variation of dry bonding strength according to percentage of tempering water in sand-clay mixture when cores 

were made______________________ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 12 

TABLE 

1. Results of tests of bentonite samples from Hardin district, 

Montana and ·wyoming, and of one sample from Black Hills 

Page 

district, South Dakota _____________________________ In pocket 

BENTONITE DEPOSITS IN MARINE CRETACEOUS FORMATIONS OF THE HARDIN DISTRICT, MONTANA 

AND WYOMING 

By MAXWELL M. l(NECHTEL and SAM H. PA'ITERSON 

ABSTRACT 

The bentonite deposits described in this report, which are roughly and tentatively estimated to include a minable reserve of 110 million short tons of montmorillonitic clay, are shown on a geologic map covering approximately 1,280 

square miles, mostly in the Crow Indian Reservation, Big Horn County, Mont., 

but extending a short distance southward into Sheridan County, Wyo. Bentonite has been mined in recent years from two small workings in Sheridan 

County, one of which is north of Gay Creek and the other near West Pass Oreek; 

the clay has been processed within the district in a plant at Aberdeen Siding, 

Mont., a shilJping point on the Chicago, Burlington and Quincy Railroad. 

The bentonite beds of minable thickness are interspersed among strata belonging to Cretaceous formations ranging from the Thermopolis shale to the 

Bearpaw shale. Two beds in the Cody shale and two other beds in the Bearpaw 

shale are extremely thick locally; these, and a number of other beds, are favorably situated for mining along some parts of their outcrops. 

Primarily, the beds are described with reference to their location, stratigraphic 

positions, thicknesses, geologic structural relations, accessibility and content of 

material that might be suitable for industrial use. Also outlined are the laboratory procedures employed in testing samples of the bentonite as molding-sand 

bonding materials for use by foundries and as ingredients of rotary oil well 

drilling muds. 

INTRODUCTION 

LOCATION AND FIELD WORK 

Extensive deposits of bentonite, a valuable rock Inaterial consisting 

essentially of clay derived from alteration of volcanic ash, are present 

in the Cretaceous sedimentary rocks of the western interior of the 

United States. A field and laboratory investigation of such bentonite 

deposits in several parts of the :Missouri basin lying in Montana, 

Wyoming and South Dakota, has been carried on intermittently 

since 1946 by the U. S. Geological Survey. Results of part of the 

work accomplished are embodied in the present report. The senior 

author has supervised the geologie field and office work since its inception, and the junior author has taken an active part in all phases of 

the work since 1947. Participation by other ·workers, including mem1 
2 BEN'l'ONI'l'E DEPOSl'rS IN MONTANA AND WYOMING 

bers of the lT. S. Geological Survey, the Illinois Geological Survey 

and the University of vVyoming Natural Resources Hesearch Institute, is acknowledged at appropriate places in the report. 

lTnusuaily thick beds of bentonite crop out in the plains that skirt 

the northeast flank of the Big I-Iorn ~fountains in :Montana and 

vVyoming. A segment of the belt of exposed Cretaceous rocks, in 

which these beds occur, is referred to in this report as the "Hardin 

district." It includes a large part of the tract of land de.scribecl by 

Richards and Rogers (1951) as the "IIardin area.'' The segment thns 

designated parallels the base of the mountain range and lies nwstly in 

the Crow Indian Reservation, in Big Horn County, Mont., but it extends for a short distance into the adjacent part of vVyoming (fig. 1). 

Large deposits of bentonite are present in the IIardin district at 

many sites where mining 1nay be feasible; some of these deposits 

contain clay that appears to be suitable for industrial uses. Because 

the bentonite deposits are of interest to the clay industry, to the 

Bureau of Reclamation, and to the Bureau of Indian Affairs, the 

deposits within the somewhat arbitrarily limited tract mapped on 

plate 1 have been the subject .of a study by the Geological Survey as a 

part of the program of the Department of the Interior for development of the natural resources of the Missouri River basin. This tract 

comprises about 1,280 square miles in Big Horn County, Mont., and 

about 8 square miles in Sheridan County, \Vyo. 

The I-Iardin district is crossed by the Chicago, Burlington & 

Quincy Railroad and by lT. S. Highway 87, and n1ost of the bentonite 

outcrops may be reached by motor vehicles over seeondary roads and 

trails. liardin, the seat of Big Horn County, is the largest town and 

the principal railroad shipping point in the area studied and its 

immediate vieinity; smaller centers of population farther·south along 

the railroad are Crow Agency, Lodge Grass, and Wyola; St. Xavier, 

another such center, can be reached by a hard-surfaced road that leads 

south from Hardin. 

In studying the bentonite deposits of the Hardin district, \vhere 

in reeent years a small tonnage has been mined by stripping, attention 

has been given primarily to their stratigraphic positions, thicknesses, 

geologie structural relations, accessibility, and content of material 

suitable for industrial uses. ~{ost of the samples and detailed 

measurmnents of the beds referred to in this report 'vere obtained by 

hand-boring with a 2-inch earth auger; some were obtained by 

trenehing. 

ACKNOWLEDGMENTS 

Field mapping of the bentonite deposits in relation to the geology, 

stream patterns, and eultnral features was performed directly on 

aerial photographs. The work was then transferred to a map pre-
INTRODUCTION 3 

pared from township plats of the U.S. Bureau of Land l\1:anagement, 

supplmnented by planetable \Vork by the Geological Survey and by 

land-survey data obtained from the Hardin, Mont., offices of the 

Agricultural Conservation Association, the County Engineer of Big 

EXPL:ANATION 

Outline of area shown on plate I 

N 

I 

~'il~~~~~0"~ Aberdeen Siding I 

45°---- MONTANA --------WYOMJN_G_--

10 

(.._ ___ 4501 I 

20 Miles I 

_ _j 

FmURE 1.-lllflPx map showing location of nrPa U!appe<l in the Har<lill <listrict, 

.!Uontana and \Vrnllling. 
4 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

IIorn County, and the State vVater Resources Survey Office n1 

Billings, Mont. 

A preliminary field study of this district was made in 19:1:6 in the 

course of a regional reconnaissance of bentonite deposits by M. M. 

Knechtel, who was assisted by I. G. Sohn. S. H. Patterson 1neasured 

~1nd sampled the bentonite beds in the late summer and autumn of 

1D±7 and mapped the southern and northeastern extremities of the 

llistriet in the summer of 1D±8 and autumn of 1951. W. A. Cobbai1 

studied the stratigraphy of the clistict in relation to the bentonite beds 

during the latter part of the summer of 1947; detailed sections 

measured by Cobban at that time, together with his paleontologic 

detenninations, are presented in this report in the chapters dealing 

with the stratigraphy of the bentonite-bearing formations. A geologie 1nap (Richards and Rogers, 1951), prepared by a party of the 

Geological Survey that was assigned concurrently to study the oil and 

gas possibilities of the Hardin district, has been utilized in the 

preparation of plate 1. 

The physical properties, whereby the usefulness of bentonites is 

judged, were investigated in the laboratory by Mr. Patterson at 

Urbana, Ill., in 1948 and 1949. The results relating to samples frmn 

the Hardin district are given in table 1 and are discussed at appropriate places in this report; the testing procedures employed are 

described by him in the final section. The authors are indebted for 

generous cooperation in this phase of the work to several members of 

the Illinois Geological Survey, including M. M. Leighton, chief of 

that organization, vvho placed its facilities and experience at our 

chsposal; R. E. Grim, petrographer and head of the section of Clay 

Resources and Clay Technology, who contributed much invaluable 

guidance, counsel, and advice relative to testing procedures and interpretation of results; and vV. A. vVhite, geologist, who gave helpful 

counsel on clay-study techniques. 

GEOLOGY 

SURFICIAL FEATURES 

The Hardin district is situated in moderately dissected plains that 

slope gently downward in a northeasterly direction from the foot of 

the Big Horn Mountains. The bedrock that underlies the district is 

largely concealed: ( 1) by deposits of alluvium, of late Tertiary (?) 

and Quaternary origin, on steplike terraces that represent a nmnber 

()f successive stages in the sculpturing of the land surface of the district; and (2) by rather extensive Quaternary and Recent alluvial 

deposits on the flood plains of Bighorn River and its larger tributaries. 

The classification of the various terraces, whose upper surfaces (pl. 1) 
GEOLOGY 5 

are coextensive with the deposits of Tertiary ( ~) and Quaternary age, 

js explained by Richards and Rogers (1951) as follows: 

Six main levels of stream terraces, ranging in height from 10 to 650 feet above 

the Bighorn River, occur in the plains area. The terraces along the Bighorn 

River are designated on the map by numbers from 1 to 6 according to their height 

above the river, and terraces along the tributary streams are correlated with 

them. Each terrace has been mapped as a surface, and its boundary line on the 

map represents the approximate outline of the top of the terrace. 

The terrace designated Qt 1, which is the lowest and youngest level, forms 

most of the valley floors in the area. The top of its streamward edge is commonly about 10 feet above the flood plain in the valley of the Bighorn River, 

but locally the Qt 1 terrace merges into the alluvial flood plain ( Qal) . Although 

Qt 1 is mapped as a separate surface in the Bighorn Valley, it is included in the 

Quaternary alluvium ( Qal) in all other valleys. This lowest terrace is a composite surface consisting of many small terrace surfaces left by the stream as it 

swung laterally at the time it 'vas in the process of forming the terrace. The 

surface of the terrace has a pronounced riverward slope that is accentuated by 

alluvial-fan material deposited on it by intf:rmittent tributary streams. 

Terrace Qt 2, which forms the top of the 125-foot bluffs along the Bighorn 

River, and terrace Tt 5, which forms the highest benches southwest of Hardin, 

ure the most extensive erosion surfaces above the valley floor surface. Terraces 

Qt 3 and Qt 4 are well preserved west of the Bighorn River west of St. Xavier 

but are represented only by small remnants elsewhere. The Tt 6 terrace occurs 

only in the area north of the mouth of the Bighorn Canyon, although a few high 

terraces along the Bighorn-Little Bighorn River divide may represent the same 

level. Some local surfaces which do not correspond in elevation to any of the 

main le,·els haYe been mapped separately and, depending on whether they are 

higher or lower than the nearest principal terrace level, are designated by a 

minus or plus sign. 

BENTONITE-BEARING FORMATIONS 

The rocks associated with the beds of bentonite that are exposed 

in this district are stratified sedimentary deposits belonging to formations that range in age from Early Cretaceous to Late Cretaceous 

(fig. 2). They tend to be exposed chiefly along the marginal escarpments of terraces and in the valleys of minor streams. A generalized 

composite columnar section of these formations, giving the stratigraphic positions of the principal bentonite beds, is shown on plate 2; 

more complete descriptions appear below and on following pages. 

The bentonite-bearing formations, with an aggregate thickness of 

about 4,400 feet, are made up almost entirely of fine-grained sedimentary material; about one-half is shale, about two-fifths is sandy 

shale, and the remainder is sandstone and calcareous shale in about 

equal amounts. The rock n1.aterials associated with the bentonite 

deposits are thus predominantly argi]]aceous, but they grade westward 

and southward into stratigraphica1Jy equivalent strata of a more sandy 

facies. The various stratigraphic units to which the exposed rocks are 

assigned crop out in relatively narrow belts (pl. 1), each of which 
{) BENTONI'l'l~ DEPO:::il'l'S IN MONTANA AND WYOMING 

:E'WURE 2.- View northeast"·ard from point in E: ec. 23, T . 9 S. , R. 34 E., southeast of Little 

Bighorn River. Low, rounded hill in fore.gTound consists of :\lowry shale; escarpment 

beyond has Clay Spur bentonite bed at base and f'hows seYeral higher bentonite beds, 

most prominent of which (long, white band) is Soap Creek bentonite bed. 

trends, in any given neighborhood, approxin'lately in the direction of 

strike, as indicated by structure contours. 

vVithin the Missouri R.iver Basin, the bentonite contained in the 

Thermopolis shale and the :Mowry shale is, in general, superior in 

quality to that contained in the younger Cretaceous formations. This 

is true with respect particularly to viscosity, thixotropy, and dilatancy (swelling capacity)-three of the properties of bentonites that 

are of value in the preparation of rotary \Veil-drilling fluids. That this 

generalization holds true for the Hardin district is indicated by the 

data in table 1 (in pocket) relating to bentonite beds in the Thermopolis shale and the Mowry shale as compared -vvith the data on beds 

in the Cody shale and the Bearpaw shale. 

LOWER CRETACEOUS SERIES 

The Lower Cretaceous series within this dish·ict comprises three 

formations. Frmn oldest to youngest these are the Cloverly formation, Thermopolis shale, and :Mowry shale. The Lower Cretaceous 

series thus includes younger strata than were assigned to it on earlier 

maps of this district, including a map by vV. T. Thorn and others 

( 1935), which showed only the Cloverly as Lower Cretaceous, and a 

map by Richards and Rogers (1951), on which only the Cloverly and 

Thermopolis were so designated. Paleontologic data recently adduced 

by Cobban and Reeside (1951) indicate that the Mowry shale is like-
GEOLOGY 7 

wise Lower Cretaceous. IIovven•r, the Early Cretaceous age assignment of the Mowry is questioned by Yen ( 1H54). 

The Lower Cretaceous strata of the Hardin district include many 

beds of bentonite, and some of the material of these beds is believed to 

be comparable in its physical properties to much of the high-grade 

drilling elay that is mined from the ~iowry shale in the Black Hills 

clistriet. Beeause of its unfavorable struetural and topographic relations, however, the bentonite in the Thermopolis and ~iowry strata of 

the Hardin district would be diffieult to mine. 

CLOVERLY FORMATION 

The Cloverly formation is the lowermost Cretaceous stratigraphic 

unit present in the Hardin district. The following generalized 

clescri ption of this formation is given by Richards and Rogers ( 1951) : 

Generalized section : In Limekiln Gulch and Grapevine Creek areas, north and 

south respectively of the month of the Big Horn River Canyon, the Cloverly 

formation consists of a lowermost 125 to 150 feet of conglomeratic sandstone and 

sandstone (Pryor conglomerate member) orerlain by about 50 feet of mostly 

concealed variegated beds and an upper -!0 feet of interbedded shale, siltstone, 

and sandstone beds which include 'rusty beds' : topmost thin calcareous sandstone 

b2d forms low ridges in covered areas ... A few miles farther south, near head 

of Soap Creek, the Pryor conglomerate is missing ... At Soap Creek dome the 

basal Cloverly sandstone is only 20 to 30 feet thick and is underlain by black and 

reel shales of the Morrison formation. 

Richards and Rogers also note that : 

A thick lenticular zone of bentonite and bentonitic shale in the undifferentiated 

Cloverly and Morrison formations is exposed north of Rotten Grass Creek in the 

central part of sec. 1, T. 8 S., R. 32 E. 

The Cloverly dips steeply at most of its exposures in the Hardin 

district and is not advantageously situated for strip mining; consequently, its lithologic features have not been examined as closely as 

those of overlying formations during the study of the bentonite 

deposits of this clistriet. 

THERMOPOLIS SHALE 

The Thermopolis shale, which rests on the Cloverly formation, consists ehiefiy of clark-gray shale with abundant ferruginous eoncretions. 

At Soap Creek don1e, where the thickness of the Thermopolis is 468 

feet, the shale of the lowermost 110 feet, below a sandstone that is 

correlated with the Birdhead sandstone, is almost wholly dark-gray 

and fissile; the shale higher in the formation, which is lighter in color 

and which tends to weather to clark-gray gmnbo, contains 1nany beds of 

bentonite; four of these have thieknesses of 2 feet or more. A few 

small sandstone dikes are present just below the middle of the forma-
8 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

tion. Fossils are searce, though the upper part of the Thermopolis 

contains a small undescribed invertebrate fauna. The Thermopolis 

shale was forn1erly regarded as Late Cretaceous in age, but it is now 

considered to be Early Cretaceous. 

The laboratory test results shown in table 1 indicate that son1e of the 

bentonite of beds A, B, 0, and D in the Thennopolis shale might he 

suitable for industrial use. However, these beds offer no sites favorable for strip mining anywhere in the Hardin district. The Thermopolis shows the following sequence of strata where it was measured and 

sampled in the escarpment carved from steeply upturned beds on the 

east flank of the Soap Creek dome, in the southeastern part ofT. 6 S., 

R. 32 E. Units are numbered in descending sequence. 

Rock Material 

1. Shale, dark-gray, soft, fissile; contains a few hard, thin, ripplemarked siltstone layers __________________________________ _ 

Ammonite 

2. Shale, dark-gray, soft, fissile; contains ferruginous concretions 

that weather purplish blaek to maroon _________________ _ 

Inocer·amus sp. 

Ammonites 

3. Shale, dark-gray, soft, fissile; contains near top a 1-in. hard, fossiliferous ferruginous layer consisting largely of tiny calcite 

prisms derived from Inoceramus_ ________________________ _ 

Oarclinrn? sp. 

EntoUnm? sp. 

Inocm·a.rnns sp. 

Ost1·ea sp. 

Pter··ia., n. sp. aff. P. nel!ra8censfs (Evans and Shumard) 

Gastropod 

Fragments of ammonites 

Fish scales 

4. Sllale, dark-gray; contains three bentonite beds. 1 to 9 in. thick __ 

5. Shale, dark-gray; contains a few hard, shaly siltstone beds ___ _ 

6. Bentonite, olive-yellow; weathers to gray ledge (bed D) ______ _ 

7. Shale, dark-gray, fissile, soft; upper part contains selenite 

crystals --------------·------------------------------------

8. Shale, dark-gray; some parts bentonitic; contains ferruginous 

concretions that weather purplish and dark brown and con- tain fish teeth and bones _________________________________ _ 

Oarchar·ias 1nacr·ota (Agassiz), 

Lamna appendimtl,ata Agassiz, 

Lepidotus sp. 

9. Bentonite, grayish-green, upper half is bentonitic shale (bed G)_ 

10. Shale, dark-gray; some parts bentonitic; contains silty and 

sandy streaks and a few ferruginous concretions ___________ _ 

Inoceranw.s sp. 

ll. Bentonite, olive-yellow--------------------------------------

12. Shale, dark-gray; contains silty aml sandy streaks and, in lower 

third, ferruginous concretions ____________________________ _ 

13. Shale, dark-gray, bentonitic; contains silty partings and at top 

a bed containing prominent dark ironstone concretions _____ _ 

Feet Inches 

16 0 

16 

37 

3 

9 

3 

4 

32 

3 

30 

1 

41 

13 

6 

0 

8 

0 

8 

6 

0 

0 

0 

2 

6 

0 
GEOLOGY 

Rock Material 

1-1. Bentonite, olive-green; weathers to bare, light-gray gumbo ledge 

(bed B) -------------------------------------------------

15. Shale, dark-gray, bentonitic; contains numerous ferruginous 

concretions and light-gray-weathering claystone nodules ____ _ 

16. Bentonite, olive-yellow to olive-green; weathers to prominent, 

bare, light-gray gumbo ledge (bed A)-----------------------

17. Shale, dark-gray, soft and fissile to chunky and bentonitic; contains light-gray-weathering claystone nodules and at least six 

beds containing prominent dark ironstone concretions. Unit 

is cut by sandstone dikes ________________________________ _ 

18. Bentonite, creatny-gray ____________________________________ _ 

19. Shale, clark-gray-------------------------------------------

20. Bentonite, gray; weathers to conspicuous, bare, light-gray gun1bo ledge ____________________________________________ _ 

21. Shale, dark-gray, bentonitic; contains small ironstone concretions near top; weathers brownish ________________________ _ 

Turtle bones undetermined 

Reptile teeth and bones 

Hybodus cf. clarkensis 

22. Sandstone, gray and tan, very fine grained, soft and argillaceous 

(Birdhead?) _____________________________________________ _ 

23. Bentonite and bentonitic shale, olive-green to gray and 

brownish ________________________________________________ _ 

2-1. Shale, dark-gray, bentonitic; weathers brownish gray; in middle 

contains ferruginous concretions and light-gray claystone nodules _________________________________________________ _ 

2G. Shale, dark-gray, fissile; contains ironstone concretions that 

weather purplish blaelc __________________________________ _ 

26. Shale, dark-gray; contains distantly-spaced ferruginous concretions, some of which contain teeth and bones of fish and 

black-coated phosphatic pebbles __________________________ _ 

27. Shale, dark-gray, fissile ____________________________________ _ 

28. Bentonite: cream _________________________________________ _ 

29. Shale, dark-gray, fissile; contains two brown ferruginous layers_ 

30. Shale, dark-gray, fissile; contains lenses composed of gypsum, 

sandstone, limonite, fish teeth, and tiny black phosphatic pebbles _________________________________________________ _ 

~n. Shale, dark-gray, fissile; contains a few ferruginous layers, ironstone concretions, sandy partings and, rarely, fish scales; 

9 

Feet Inches 

4 2 

31 

3 

so 

1 

2 

1 

7 

1 

3 

15 

50 

8 

13 

0 

10 

0 

0 

0 

9 

6 

6 

6 

0 

6 

3 

5 

4 

5 

(base of formation)-------------------------------------- 37 6 

Total measured thickness___________________________ 473 2 

MOWRY SHALE 

The Mmvry shale, a formation of widespread occurrence in Montana, Wyoming, and the western part of the Dakotas, crops out at 

several localities in the Hardin district. At Soap Creek dome, 1vhere 

the Mowry is approximately 400 feet thick and forms a prmninent 

hogback, it consists of hard gray sandy shale, dark-gray, hard siliceous 

shale, and many beds of bentonite; in 1nany localities the middle and 
10 BE.KTONI'l'E DEP08l'l'S IN MONTANA AND WYOMING 

npper parts of the formation contain thin-bedded, fine-grained gray 

sandstone. Scales and bones of fish, which occur at various horizons, 

are the only abundant fossils. ·until recently the Movvry was regarded 

as belonging to the Upper Cretaceous series. 

Bentonite beds E, F, a, and a bed believed to be the Clay Spur 

bentonite bed, crop out at a number of localities in the Hardin district 

and were 1neasured at exposures among steeply upturned Mowry beds 

on the east side of the Soap Creek dome. The bed believed to be the 

Clay Spur bed occurs just below the top of the Mowry shale at, or 

near, a stratigraphic position correlative \Vith that of the Clay Spur 

bed at its type locality in the Black Hills district, where it is extensively mined. In the Hardin district this bed, and the rocks immediately above and belmv it, closely resemble the Clay Spur and associated 

beds of many outcrops on the north side of the Black llills. Some of 

the laboratory tests indicate that much of the bentonite of exposures of 

the Clay Spur bed in the Hardin district is suitable for industrial 

use; this is true also of beds E, F, and a. However, the bentonite 

heels of the J\fowry are not known to crop out here unrler conditions 

favorable for strip mining. The follmYing detailed section of the 

Mowry was measured on the east side of the Soap Creek dmne, in 

the southeastern part of T. () S., R. :32 E. lTnits are nmnb3recl in descending sequence. 

Rock Mater-ial 

1. Sandstone and shale, interbedded : shale is gray and bard (top 

of forination) ___________________________________________ _ 

2. Bentonite, lower 5 ft cream yello'v; remainder is gray; weathers 

to very light gray bare gumbo (Clay Spur bentonite bed) ____ _ 

3. Shale. dark-gray, hard; contains thin, hard, laminated sandstone ___________________________________________________ _ 

4. Shale, dark-gray, hard, somewhat hackly; forms dark-bluishgray bare outcrop ________________________________________ _ 

5. Sandstone and shale; shale is gray, very fine grained. and hard 

and is interlaminated and interbedded with thin-bedded 

rivple-marked sandstone; shale contains fish scales _________ _ 

6. Bentonite, greenish-yellow grading up into gray (bed G) ______ _ 

7. Shale, clark-gray, hard; softer than underlying beds and forms 

back slope of hogback ____________________________________ _ 

8. Shale, weathers light gray, hard; forms crest of Movi"ry hogback_ 

9. Bentonite, light-gray; contains a siltstone parting in lower part 

(bed F)-------------------------------------------------

10. Shale; weathers light gray, hard; in lower part contains a 4-in. 

and a 6-in. bentonite becL _________________________________ _ 

11. Bentonite, light-green ______________________________________ _ 

12. Siltstone and shale interbedded and interlaminatecl: weathers 

light gray, hard. Several thin bentonite beds in lower 50 ft. 

Some interbedded fine-grained sandstone in upper 45 fL ____ _ 

13. Bentonite, yellow-------------------------------------------

14. Shale, dark-gray, silty; contains tv,To thin bentonite beds ______ _ 

J?eet Inche8 

.-., 

•J 6 

9 10 

8 9 

25 0 

42 0 

2 8 

<>•! fJ..o.. 0 

21 0 

9 9 

30 0 

2 0 

110 0 

2 0 

6 0 
GEOLOGY 11 

Rock Material Feet Inches 

15. Bentonite, creamy-yellO\Y and light-green_____________________ 1 0 

16. Shale, dark-gray, silty. Five thin bentonite beds in upper parL 29 0 

17. Bentonite, chiefly greenish__________________________________ 2 8 

18. Shale, dark-gray, silty: interlaminated with lighter gray, hard 

siltstone and very fine-grained sandstone___________________ 18 0 

19. Bentonite, creamy-gray------------------------------------- 1 0 

20. Shale, dark-gray, hard; contains fjsh scales; weathers to dark, 

bar~ steep slope_________________________________________ 38 0 

21. Bentontt~ gray____________________________________________ 4 

22. Shale, dark-gray, hard, fissile; contains fish scales: weathers to 

silvery-gray bare ledge____________________________________ 5 6 

23. Bentonite, gray to greenish-gray (bed E)______________________ 4 8 

24. Shale, dark-gray, hard, fissile; contains fish seales (base of formation) _____ -------------------------------------------- 8 

Total measurecl thickness___________________________ 400 4 

UPPER CRETACEOUS SERIES 

All the bedrock strata younger than the Mowry shale that crop Jut 

within the area studied are assigned to four fonnations of the Upper 

Cretaceous series. These formations-from oldest to youngest-are 

the Cody shale, Judith River forn1ation, Bearpaw shale, and Hell 

Creek formation; all of these, except the Hell Creek formation, are 

included in the Colorado and Montana groups. In other parts of the 

western interior of the Unit eel States (fig. 3) and on previously published maps of this area (Thom and others·, 1935; Richards and Rogers, 

1951), the Colorado and :Montana groups are separated, in accordance 

with established usage, by the contact bet\veen beds equivalent to the 

Niobrara formation and those of the next-younger stratigraphie unit. 

The separation in the area studied is accordingly made ·at the contact 

of the Niobrara shale 1nember of the Cody shale with the overlying 

Telegraph Creek member. The definitions of the formations and 

lesser subdivisions of the Colorado and Montana groups of this district have, however, been modified considerably, in part on the basis 

of information gathered in studying the bentonite deposits of this and 

other areas in Montana, W....-ymning, and South Dakota. 

On the whole, the changes that have been effected are believed to 

represent steps tmvard a satisfactory adjustment of the local terminology to that of equivalent rocks of other parts of the western interior 

of the lTnited States. Concerning the general character and interrelations of the l; pper Cretaceous sedimentary rocks of that region, 

Reeside ( 1944) has com1nented: 

In general, the Upper Cretnceons sediments in this region comprise an alternation of eoarser- and finer-grained rocks, all marine in the east but including 

progressively more nonmarine rocks and more coarse material westward. 

347624-56--2 
Southwest 

Bighorn Basin, Wyo. 

(Pierce and Andrews, !941) 

Meteetse formation 

Southeast 

Salt Creek oil field, Wyo. (Thorn and Spieker, 1931) 

Lewis shale 

[Teapot sandstone member 

-Es Unnamed 

~~ t---------- Mesaverde 

formation 

~ li~ Parkman sandstone 

:::el 

Cody shale 

li"' ~ 

lc?----, c...._, ? I I <.> 

Frontier 

formation 

Mowry shale 

Thermopolis shale 

member 

~ ~ """'"" i 'ShannOii saiidStone' (I) ___ _rn.!_mjte!_ _ _.. 

Unnamed 

N1obrara shale 

Carlile shale 

Frontier 

formation 

Mowry shale 

Thermopolis shale 

West 

South of Billings, Mont. (Knappen and Moulton, 1930) 

~ 

~ :::E 

~ 

Bearpaw shale 

Judith River formation 

Claggett 

formation 

E!B!_e sandstone 

Telegraph Creek formation 

Carlile and Niobrara shale (mapped together) 

8 f----? --

Frontie'r 

formation 

Mowry shale 

Thermopolis shale 

Plate I of this report 

Bearpaw shale 

Hardin district 

Favored by the authors 

Bearpaw shale 

Upper 

~ ~ ~ ~ 

~ ~ :::E :::E 

~ 

iii 

8 

.. 

0 

~ 8 

Claggett shale member ~ 

'unnamed Siiridy- j li ...._._s~li_~lf!l1!!r..._,., <n 

- T~~frea~e~~i~ 

Carlile and Niobrara shale members undifferentiated 

Gr;nr;;ncaiCa"'feous..._ 

.___s~l!.~e~b~_...--

Belle Fourche shale member 

Mowry shale 

Thermopolis shale 

Claggett shale member 

~-annameisand'Y" 

-~.J.h~!.e....!!li!JI.!?~!._ / Telegraph Creek shale member 

Carlile and Niobrara 

shale undifferentiated 

Greenhorn calcareous 

shale 

Belle Foorche shale 

Mowry shale 

Thermopolis shale 

~ 

8 

North 

"Accepted", central Montana (Reeside, 1944, slightly modified) 

Bearpaw shale 

Judith River formation 

Claggett 

formation 

Eagle sandstone 

Telegraph Creek formation 

Unnamed 

~ 

~ :::E 

East 

"Standard", northern Great Plains (Cobban and Reeside, 1952a, slightly· 

1 

modified) 

Unnamed 

r~- Monument Hillf.-_ ~n~n_i!i~m~m!.e!_ ~ 

iii I 

1i I Unnamed ~ 

~ r--:tt:~a~k--

~ 

shale member I 

G.:~:;;:~::-l member I 

Niobrara~_j 

Sage Breaks shale member 

iii 

5 ~1'5 .g <.> 

Turner sandy member 

E 

~ 

:s 

/--unnamed--, 1 8 t (Mosby sandstone 

'- _111~~r_!t~a~J../ 

Unnamed 

Mowry shale 

Thermopolis shale 

ll~named 

tjreenhorn limestone I 

Belle Fourche 

shale 

Mowry shale 

Newcastte saadstone aad Skull Cleek shale 

FIGIIRE 3.-Stratigraphic nomenclature employed on plate 1 and nom enclature favored by the authors; shown in relation to that of other 

parts of the western interior region of. the United States. 

f--4 

~ 

td 

M 

z 

!-3 

0 

z 

H 

!-3 

M 

ti 

M 

1-d 

0 

w 

H 

!-3 

w 

z 

~ 0 

z 

!-3 

~ > 

~ ~ 

~ 0 

~ 

z 

0 
---,-:---

: I I , : l I \ ---\ 

/ 

( 

( 

I 

GEOLOGY 

100 

EXPLANATION 

---2000---------

---3000---------

Isopach lines showing thickness of Upper Cretaceous deposits; 

interval 1000 feet; dashed where least de!inite 

13 

'~­

(,' 

------J 

\\ 

FIGURE 4.-Belts and thicknesses of Upper Cretaceous deposits of the western interior 

region of the United States. 

In figure 4, which is adapted from one of Reeside's maps, the large 

basin in which sedimentation took place in the western interior of the 

United States durjng much of Late Cretaceous time is shown as 

divisible, somewhat arbitrarily, into three great belts: (1) the Great 

Plains belt, which is characterjzed by "dominantly fine sediments," 

(2) the Rocky Mountain belt, where '"alternating coarse and fine sediments" predominate, and (3) the western 1narginal belt, where the 

sedimentary rocks are "dominantly coarse." 

However, the transition fron1 the lithologie facies characteristic of 

the Great Plains belt to that of the western 1narginal belt takes place 
14 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

West East 

ROCKY MOUNTAIN BELT SEQUENCE 

Mesaverde formation Judith River formation 

GREAT PLAINS "STANDARD" SEQUENCE 

Pierre shale (lower part) 

Carlile shale a. 

:;:J 

e 

bO 

0 

"0 

"' 0 

0 

(.) 

Mowry shale 

Belle Fourche 

shale 

Pmuam 5.-Schematic diagram showing position of facies at top of Frontier f:rmatior 

in different parts of Rocky Mountain belt in relation to ·•standard'' sequence of Great 

I'lains belt. 

in an irregular manner, and the sequences of stratigraphic unitE" 

recognizable \Yithin the ~Upper Cretaceous series of the western interior 

differ from one another markedly, not only from one belt to another 

but also from place to place ~within each belt. In the Hardin districL 

\vhich lies in the central part of the Rocky Mountain belt (fig. 4), the 

newly recognized sequence of subdivisions of the Colorado and 

l\fontana groups (fig. 3, col. 4) bears far more resemblance to thr, 

"standard" sequence cited by Reeside for the Great Plains belt (fig. 3, 

col. 7) than to the sequence he cited as representative of the equivalent 

stratigraphic interval in west-centrallTtah, within the western marginal belt (Reesicle, 1944). "'Vithin surrounding parts of the Rocky 

l\fonntain belt, the corresponding sequences in areas to the southeast 

dig. 3, col. 2) and north (fig. 3, col. 6) have more in common with the 

sequence in the Hardin district than does that of the Bighorn basin 

(fig. 3, col. 1), the northeast margin of which lies only 60 miles 

~:;outhwest. 
GEOLOGY 15 

Among the significant circumstanees leading to the changes in 

local nomenclature is the predominance (fig. 3, cols. 4 and 5) of 

shaly n1aterials in beds equivalent to the typical, prominent, readily 

mappable, cliff-forming sandstone strata of the Frontier and Eagle 

formations that are present only a few miles farther west (fig. 3, 

col. 3). As' a consequenee, the names "Frontier" and "Eagle~" whieh 

appear on the maps published by Thom and others (1935), Richards 

and Rogers (1951), and l(nechtel and Patterson ( 1952), have been 

eliminated from the nomenclature adopted for use in the present 

report, and the strata that ·were designated Frontier formation are 

assigned to the Belle Fourche member of the Cody shale. The Cody 

is now set up in this district as a formation that includes all of the 

nnits behveen the .!Howry shale and the .Judith Riv2r formation. 

Two other innovations are the recognition of the, Greenhorn ca 1-

ea,reous member as a nutppable unit ·within this district and the introduction of the name ''.Judith River formation'' to include, as 

members, the Parkman sandstone member and an unnamed overlying 

unit. 

Elimination of the name "Frontier~' in the Hardin district, as well 

as in central ~fontana and near the Black Hills, is consistent "\Yith 

the practiee "\vhereby the top of the interval to which this name refers 

is mapped at different stratigraphic horizons in different parts of 

the western interior region. Near Lander, \Vyo., for example (fig. 5), 

the uppermost strata of the Frontier formation include sandstone 

beds containing fossils of lower Niobrara age (Sharkey, 1946), 

\vhereas, farther east~ in the \Vind River Basin (Hares, 1946) and 

in the Salt Creek oil field, the top of the Frontier forms the top of 

sandstone strata equivalent to beds .. within the Carlile member of 

the Cody formation. In the area immediately west of the Hardin 

district (I{nappen and 1\foulton, 1930) the top of the Frontier is 

placed approximately at the top of sandstone beds that are correlated 

with the upper part of the Greenhorn ealcareous member (fig. 3, cols. 

3 and 4). In effeet, elimination of the name "Frontier'' as applied 

to the Hardin district may be regarded as placing the top of the 

:formation at the lowest possible position, making it eoincide with 

the top of the )fowry shale and thus redueing the thickness of the 

Frontier formation to zero. 

The reasons for eliminating the name "Eagle sandstone" for strata 

in the upper part of the Cody formation and for introducing the 

name "Judith River'' for younger beds are fundamentally the same 

reasons as those justifying elimination of the name "Frontier" and 

jntroduetion of the name "'Greenhorn calcareous member''. All of 

these ehanges in the localnmnenclature have to do with lateral variations in lithologic facies within the Cretaeeous roeks of this region. 
16 BENTONITE DEPOSI'l'S IN MONTANA AND WYOMING 

Thus, the local rocks that were formerly called Eagle sandstone (now 

assigned to an unnamed sandy shale 1nember of the upper part of the 

Cody) represent a shaly facies of strata that elsewhere in central 

Montana include the thick, massive sandstone beds that are typical 

of the Eagle sandstone. In a similar manner the sandy beds in the 

southern part of the district, which heretofore have been mapped as 

part of the BeaqJaw shale (Thmn and others, 1935) and which are 

now assigned to the upper member of the ,Judith River formation, 

are equivalent to the dark shales in the lower and middle parts of 

the Bearpaw of the neighborhood of the Custer battlefield and localities farther north. 

Following are three other changes in the local nomenclature that 

nave not been adopted by the Geological Survey, although they are 

favored by the authors: (1) Elimination of the name "Cody shale"; 

(2) introduction of the name "Steele shale" to designate a formation 

containing the three members of the Cody (Telegraph Creek Inember, "unnamed member," and Claggett n1ember) that fall within the 

Montana group ; and ( 3) promotion to formation rank of each of the 

three me:1:nbers of the Cody (Belle Fourche 1nember, Greenhorn member, and Carlile-Niobrara member) that are included in the Colorado 

group. 

Elimination of the name "Cody shale" from the local terminology 

would, in the authors' opinion, be consistent with the contrast in 

lithologic facies that exists between the Frontier-and-Cody sequence 

of the Bighorn Basin of Wyoming and the corresponding six unitsBelle Fourche, Greenhorn, Carlile-Niobrara, Telegraph Creek, 

unnamed, and Claggett-that are 1napped as Cody shale on plate 1. 

This change, and the other changes that are favored, would serve to 

eliminate a feature of the nomenclature employed in this report 

whereby a group-the Colorado group-is represented as containing 

only part of a single formation-the Cody shale; the nomenclature 

would then clearly and unquestionably conform to the rule by which 

a group "contains bvo or more for1nations (Ashley and others, 1933, 

p. 429) ." 

The marine Cretaceous strata of the Hardin district contain many 

beds of bentonite (pl. 2). Locally, these beds are exceedingly thick, 

contain 1nuch bentonite that appears to be suitable for use as sandbonding clay by foundries and steel works, and are so situated in relation to geologic strueture and topographic features that they offer 

excellent strip-mining sites. The results of many laboratory tests 

indicate, however, that little, or none, of the n1aterial at such sites is 

as well suited for use in rotary well-drilling 1nuds as is the bentonite 

1nined for that purpose from the Mowry shale of the Black Hills 

district. 
GEOLOGY 17 

COLORADO GROUP 

The base of the Colorado group "'as nutpped by Thom and others 

( 1935) at the contact between the Cloverly formation and the Thernwpolis shale. As already pointed out, hmvever, the Thermopolis 

and Mowry beds have come to be regarded in recent years as Lower 

Cretaceous, whereas the Colorado group is still generally understood 

to include only Upper Cretaceous rocks. Therefore, the Geological 

Survey's classification has recently be,en nwdifiecl to exclude the Thermopolis and Mmvry beds from the Colorado group. 

The Colorado group, as mapped on plate 1, comprises the lowermost three of the six members of the, Cody shale that are recognized in 

the Hardin district. As shown in figure 3, the sequence of three of 

these members-Belle Fourche, Greenhorn, and Carlile-Niobrara-is 

essentially equivalent to the sequence of strata between the Mowry 

shale and the Pierre shale in the vicinity of the Black Hills (fig. 3, col. 

7), to the Frontier and Carlile-Niobrara sequence of the Salt Creek oil 

field (fig. 3, col. 2), to the Frontier formation plus the lower part of 

the Cody shale of the Bighorn Basin (fig. 3, col. 1) , and to the ""\V arn1 

Creek shale of central Montana (fig. 3, col 6). 

LOWER P .ART OF THE CODY SHALE 

Belle Fmtrche shale 1nembe1·.-The strata that are 1napped on plate 

1 as Belle Fourche member of the Cody fonnation occupy essentially 

the same stratigraphic position as the Belle Fourche shale of the 

east side of the Powder River Basin and as the part of the Warm 

Creek shale that underlies the Mosby sandstone of north-central Montana. In their stratigraphic position the Belle Fourche strata are 

equivalent also to rocks, consisting in part of conspicuous cliff-forming 

sandstone beds, that are included in the Frontier formation of areas 

not far west (fig. 3, col. 3) and southeast (fig. 3, col. 2) of the Hardin 

district; in faet, heretofore the rocks of this interval have, in part, 

been designated Frontier formation in publications dealing with this 

district (Thom and others, 1035; Richards and Rogers, 1951; Knechtel 

and Patterson, 1952) . Unlike the typical Frontier strata, however, 

they consist almost wholly of shale, although a few obscure beds and 

lenses of sandy material are discernible upon close examination of 

some outcrops in the southeastern part of the district. Consequently, 

the entire interval between the lVIowry formation and the Greenhorn me1nber is shown on plate 1 as the Belle Fourche 1nember. As 

already pointed out, the thickness of the Frontier is thereby, in 

effect, reduced to zero for this district, as well as for central Montana 

and northeastern Wyoming (fig. 5). 

At the Soap Creek dmne, the Belle, Fourche member consists of 

three subdivisions, but the differences between these subdivisions are 

somewhat vaguely defined. The lower1nost subdivision, which is 82 
18 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

feet thick, consists of dark-gray shale, some of which is sandy with 

ferruginous concretions. The intPrmediate subdivision, the top of 

which is arbitrarily placed at the top of a thick bentonite layer ( referred to in this report as the Soap Creek bentonite bed), is approxinuttely 200 fee-t thick and consists of dark-gray shale, some of which 

]s sandy, with several intercalated beds of bentonite. The uppermost 

tiuhdivision, which is approximately HHl feet thick consists of darkgray shale, the basal ()() feet of ~which includes sandy beds; the top 

is marked by the base of bentonite bed ill. In the equiva.lent part of 

the Belle Fourche 1ne1nber in the southern part of the Hardin district, sandy beds are also readily discernible, though these are much 

less prominent than Cobban and Reeside ( 1952b, p. 1960) seem to imply 

in characterizing 25 feet of strata, including such beds, collectively as 

a "massive bed of sandstone." The aggregate amount of sandy ma.-

terial in the shaly strata of the lmver part of the upper subdivision 

is here much ,greater than the sand content in any comparable thick 

sequence of Belle Fourche strata exposed elsewhere vvithin this district; but, even here, these shaly strata, together with those of the 

underlying subdivisions, which also contain a small amount of sandy 

material~ bear only a remote resemblance to the massive, cliff-forming sandstone beds of the typical facies of the Frontier formation. 

In the field studies leading to the present report, mapping of the 

top of these sand-bearing shale strata has proved impracticable, and, 

as they are no more sandy than some of the typical beds of the Belle 

Fourche member in the vicinity of the Black Hills, they are 1napped 

(pl. 1) as part of the Belle Fourche 1nember of the Cody shale. 

Such sandy beds are not present at "'Voody Creek dome, 20 miles 

north of Soap Creek, where the uppermost subdivision of the Belle 

Fourche consists of dark-gray, fissile shale containing several layers 

of bentonite with many beds of gray- and brown-weathering calcareous concretions. According to Cobban and Reeside ( 1952b, p. 1960), 

the uppermost member of the Belle Fourche of this district contains, 

jn its lower part, the large aeant.hoceratid ammonite that is found in 

the Torchlight sandstone member of the Frontier in areas farther west 

and southwest, and it contains, in its 1niddle part, the ammonites 

1Jwn,veganocera8 pondi Haas and 111 antellicera8 ca;nita'urin,rttln Haas. 

The uppermost subdivision of the Belle Fourche was included by 

Thom and others (1935, p. 49-51) in the basal part. of the Carlile shale, 

Eotwithstancling the presence in the overlying deposits of fossils 

equivalent in age to those of the Greenhorn. 

Bentonite bed I is in the lower subdivision of the Belle Fourche 

member, between 12 and 25 feet stratigraphically above the Clay Spur 

bentonite bed. \V eathered outcrops of bed I are characteristically 

rusty in color, apparently from oxidation of a small amount of iron 

contained in the bentonite. Bed l, higher in that subdivision, ranges 
GEOLOGY 19 

IfiGCRE 6.-Soap Creek bentonite bed locally thickened by flowage, in )il"W7.4 sec. 31, T. 

7 S., R. 33 E. 

from about 3 to 7 feet in thiclrness and forms a narrow outcrop that 

can be traced for considerable distances, as shown on plate 1. Beds 

I and J are known to be present nuder thin overburden in only a few 

sma 11 areas. 

The thick Soap Creek bentonite bed contains a large part of the 

bentonite reserve of the cbstrict and is the only bed that has been 

mined here to date. This bed n1arks the top of the medial subdivision 

of the Belle Fourche. Buff calcareous concretions, 3 to 10 feet in 

diameter, are commonly stre\Yn along weathered surfaces of this bed 

and are particularly abundant in the southern part of the area shown 

on plate 1. Parts of the Soap Creek bentonite deposit consist of 

materi<tl that is suitable for use in preparing drilling muds, and nearly 

all of it is believed to have the properties of good foundry sand bonding cla:;· . The Soap Creek bed ranges from 12 to more than 30 feet 

in thickness in much of the area it occupies, and large tonnages are 

accessible to strip mining. Forty-five feet of bentonite was measured 

at point 37, sec. 31, T. 7 S., H. 33 E., but the thickness of the bed 

there has been augmented by flmyage of bentonite into a small dome 

(fig. 6), \vhich is associated with a minor fault; another measurement 

of the Soap Creek bed, obtained less than a mile away, shows 16 feet 

of bentonite, \Yhich is believed to represent approximately the average 

thickness in the neighborhood of the dome. It is possible that several 

such unusually thick sections reflect subsurface flowage or swelling 

of the bentonite at the outcrop. 

Bentonite bed L, in the uppermost subdivision of the Belle Fourche~ 

is relatively thick, contains a large amount of bentonite under light 
20 BENTON ITE DEPOSITS IN MONTANA AND WY OMING 

overburden, and has utilizable properties; in general, however, it is 

not as accessible as other beds and appears to be of secondary 

importance. 

The following section of the Belle Fourche member was measured 

in the southeastern part of T. 6 S., R. 32 E., on the east side of the 

Soap Creek dome. Units are numbered in descending sequence. 

Rock Material 

1. Bentonite, gray, impure (bed M in base of Greenhorn calcare- ous member) _____________________________________________ _ 

2. Shale, dark-gray (top of Belle Fourche member) ____________ _ 

3. Shale, dark-gray; contains three beds of light-gray and buffweathering calcareous concretions, septariate with darkbrown calcite ------------------------------------------- 4. Shale, dark-gray ___________________________________________ _ 

5. Shale, dark-gray; with concretions, light-gray-weathering, calcareous, septariate, distantly-spaced ______________________ _ 

6. Shale, dark-gray _______ ___ ______________________ __ _________ _ 

7. Shale, gray, bentonitic _____ ___ __ ___ __________ ______________ _ 

8. Bentonite, gray; contains brown-weathering fibrous aragonite 

21 in. above base; weathers to gray, bare, gumbo ledge 

(bed L) -------------- --- ------------ --------------------

9. Shale, dark-gray; contains very fine-grained sandy streaks ___ _ 

10. Shale, dark-gray, with concretions commonly measuring 11h by 

3 ft; brown-weathering, calcareous, septariate with white to 

pale-yellow calcite veins __________________________________ _ 

11. Shale, dark-gray; contains fine-grained sandy streaks and, 

rarely, thin sandstone beds and black chert pebbles. Locally at base a thin friable coarse-grained sandstone is 

present -------------------------------------------------

Aoanthoceras ? sp. 

12. Bentonite, greenish-yellow to gray; weathers to prominent, 

bare, light-gray gumbo ledge. (Soap Creek bentonite bed) __ 

13. Shale, dark-gray, sandy, bentonitic; contains gray-weathering 

calcareous concretions and fewer, but much larger (as large 

as 4 by 8 ft), brown-weathering calcareous concretions _____ _ 

Acanthoceras ? amphibolunt Morro·w 

14. Shale, dark-gray; contains sandy streaks and a few thin beds 

of shaly sandstone, particularly 15 to 20 ft above base ______ _ 

Inoceramus sp. 

15. Bentonite, gray _______ ____________________________________ _ 

16. Shale, dark-gray ; contains some sandy streaks and a few beds 

of bentonite less than 5 in. thick ________ __ _______________ _ 

17. Bentonite, light-gray to green; weathers to prominent, bare, 

light-gray gumbo ledge (bed J) - --- --------- - - - - -------- --

18. Shale, dark-gray ; contains some sandy streaks and a 6-in. 

bentonite bed ____ ____ _____ ______ ___ ______ ______________ __ _ 

19. Shale, dark-gray; contains some silty streaks and five or six beds 

containing ironstone concretions that are commonly 6 by 24 

in., weathering dark brown to dark maroon _______________ _ 

20. Bentonite, green to gray with limonitic streaks staining lower 

18 in. to rusty color (bed I) __________ -------- -------------

Feet Inches 

7 9 

19 0 

20 0 

37 0 

1 0 

28 0 

18 0 

7 0 

19 6 

1 6 

40 0 

12 0 

10 6 

73 0 

1 0 

67 0 

6 11 

31 0 

64 0 

0 
GEOLOGY 

Rock liiaterial 

:21. Shale, <lark-gray, bentonitic; contains three bentonite beds, none 

thicker than S in. Two beds contain ferruginous concretions __ 

22. Bentonite, gray; contains much brown-weathering aragonite __ _ 

23. Shale, dark-gray; lower half sandy _________________________ _ 

24. Bentonite, grayish-green; contains brown-weathering ara21 

Feet Inches 

7 

1 

l) 

6 

0 

gonite___________________________________________________ 2 0 

T1~al 1neasured thickness___________________________ 482 2 

Oreenlwrn calrareous membn·.-Rocks that are equivalent to the 

Greenhorn formation of the east side of the Rocky l\iountain belt form 

a mappable unit in the Hardin district. This unit, which was treated 

by Thom and others ( 1985) as an undifferentiated part of the Carlile 

shale, has been mapped on plate 1 as the Greenhorn calcareous member of the Cody forma6on. 

The Greenhorn in this area generally is readily distinguishable 

i'rom the overlying and underlying strata by its larger lime content. 

Ho·wever, it is much less calcareous than correlatives of the Greenhorn 

east of the Pmvder River Basin, 'vhere the Greenhorn formation contains beds of limestone and 1narl. The Greenhorn of the Hardin district is similar in lithology to beds of equivalent age forming the part 

of the Warm Creek shale immediately above the Mosby sandstone of 

areas farther north in Montana. A few miles west of this district, the 

material of the Greenhorn unit passes laterally into strata, including 

conspicuous sandstone units, that are mapped by l(nappen and 

~foulton ( 1980, pl. 1) as the upper beds of the Frontier formation. 

The Greenhorn of the Hardin district consists of dark-gray calcareous shale that weathers whitish. ·At Soap Creek, light-gray 

limestone concretions are numerous in the Greenhorn, and these concretions contain an invertebrate fauna that is characterized by the 

ammonite Y ascocera.s. Although a thick zone of bentonitic shale is 

present in the base, only a few thin layers of pure bentonite are present 

in the Greenhorn member of that vicinity. About 1 mile farther 

south, the basal zone is represented by a thick bed of bentonite; near 

Rotten Grass Creek~ this bentonite bed is more than 2'0 feet thick, and 

at Black Gulch dome it is about 25 feet thick. The Greenhorn calcareous member is approximately EiO feet thick on Black Gulch dome, 

and it is 115 feet thick at Soap Creek. A bentonite. bed, designated as 

Led JJJ, crops out locally at the base of the Greenhorn. At some localities its thickness is as great as 88 feet, and large amounts of the 

bentonite it contains are accessible to strip mining. However, in 

much of the area of outcrop of the Greenhorn, bed JJf is absent or is 

represented only by bentonitic shale. The bentonitic material of bed 

JJ[ varies greatly in quality, but some of it possibly could be used as 

molding-sand bonding clay in foundries and steel works. 
22 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

The following section of the Greenhorn calcareous n1ember was 

measured on the east side of ~oap Creek dome in the southeastern part 

o£ T. G S., R. 32 E. Units are numbered in descending sequence. 

Rock Material 

1. Shale, dark-gray, \veathering bluish and tlleu white, calcareous; 

contains a few limel'<tone coueretions 5 ft above base ________ _ 

2. Shale, dark-gray, with eoucretions; medium-gray, "'eathering 

light gray to lmff, calcareous _______________________________ _ 

3. Shale, clark-gray, vveatherin:; lJluish and then white, calcareous_ 

-±. Shale, clark-gray; vvith medium-gray concretions, weathering 

light gray to buff, calcareous, commonly n by 12 in.; some are 

somewhat geodal vvith calcite erystal-lined eaYities _________ _ 

Inoceramus labiatus Schlotheim 

sp. 

08trea n. sp. 

Plicatula? sp. 

Gastropod nndet. 

A.Jlocricera s 

P8u,edoastJidocera:sf sp. 

Pseudoti8sotia ( Ohoffaticems) sp. ·: 

Scaphitcs delicatulus 'Varren Yar. 

Ya8cocera8 moultoni Reesid·~ 

8tantoni Ref. side 

thomi Reesi1le 

lVatinocera8 t·eeclidei vVarrf'n 

Schloenbachia? >5P. 

Fish bones 

5. Shale: dark-gray, weathering bluish and theu white, calcareous_ 

G. Bentonite: gray, limonitic ___________________________________ _ 

7. Shale, dark-gray--------------------------------------------

8. Bentonite, g;ray, impure (bed ill)-----------------------------

Feet Inches 

28 

1 

2S 

1 

38 

1 

10 

7 

0 

0 

0 

0 

0 

0 

0 

9 

Total measured thielnwss______________________________ 11-± 9 

Octrlile-N-iobntra shale me,mber.-The strata in the Hardin district 

that belong to the Carlile-Niobrara member of the Cody shale are, as 

a rule, readily distinguishable from older and younger rocks. Locally, 

the Carlile and Niobrara portions of the member are separable from 

each other in map"ping, but the contact between then1 cannot everywhere be mapped satisfactorily. Follmving a precedent set by Thom 

and others ( 1935, pl. 1), they are shmvn on plate 1 as a single unit, 

which is referred to as "Carlile and Niobrara shale members) undifferentiated." In the present report, the composite subdivision is defined as resting on the Greenhorn member, whereas the undifferentiated Carlile and Niobrara, as mapped by those authors, included all 

of the Greenhorn beds and the uppermost beds of the Belle Fourche 

member. 

The Carlile shale member in the Hardin district comprises four 

lithologie units with a total thickness of approxin1ately 280 feet. The 

bwest unit consists of about 95 feet of clark-gray sandy shale with two 
GEOLOGY 23 

beds of bentonite, each 2 to 3 feet thick, in the upper part. This sandy 

shale is overlain by about 25 feet of clark-gray shale containing numerous flat clay-ironstone concretions that weather rusty to dark maroonbrmvn. About 30 feet of gray sandy shale makes up the third unit. 

The top unit consists of approximately 130 feet of dark-gray shale 

containing many beds of gray-\Yeathering calcerous concretions that 

are septariate \vith thick veins of dark-brown calcite. 

Fossils are rare in the lmver two units and none that have been found 

are diagnostic. The 30-foot gray sandy shale unit contains Prionocyclns wyomingens(s Meek and Inor'(Trtm.u.~ ptl'plr,,(~us 'Vhitfield. The 

top unit is poorly fossiliferous, but contains fragments of Prionocyclus 

and other fossils of Carlile age. 

Bentonite beds Nand 0 are in the ]oY,'er part of the Carlile member. 

These beds are known only at the locality at ~which the stratigraphic 

measurements given belmv were recorded and are believed to bs of 

little importance. 

The following section of the Carlile shale memb::>.r was measured 

on the east side of Soap Creek dome in the southeastern part ofT. 6 S., 

H. 32 E. lTnits are numbered in descending sequence. 

Rock Material 

1. Shale, dark-gray; readily breaks down into se~ft, li~~ht-g:ray soil; 

contains two beds of light-gray-\Yeathering calcareous concretions, septariate with yellow and dark-bro\Yll caleite yeins __ _ 

2. ShalE'. dark-gray: contains fragments of Lrg::-. thin-shelled 

Inoceranw8 encrusted with 08trea congcsta Conrad ________ _ 

3. Shale. dark-gray; readily breaks down into soft, l'ght-gray soil; 

contains calcareous septariate concretions that ~weather light 

gray to yellowish taiL ___________________________________ _ 

Inocera m'lls sp. 

4. Shale, clark-gray, weathering light gray _____________________ _ 

Inoceramu8 sp. 

Ostrea C011f}C8fa Conrad 

5. Septarian zone: Large calcareous concretions weathering light 

gray or buff, and septariate with thin yellow calcite veins; 

emlH'dded in <lark shale_---------------------------------

Inorcrumu8 sp. 

G. Shale, dark-gray; forms a dark, llare outcrop, IYith local alkali 

patches and selenite crystals; eontains thin sandy streaks, 

particularly in the middle, and a few calcareous septariate 

concretions ----------------------------------------------

Inoceramu8 Sfl. 

"11f(Wfc8ia" sp. 

08tren sp. 

renie7la sp. 

7. Shale, dark-gray; at base and top contains light-gray-weathering calcareous coneretions ser1tariate with thick, clark-brown 

and yellow caleite _______________________________________ _ 

Inoceramu8 sp. 

N1tcula sp. 

Priouocyclw;; sp. 

Feet Inclle.s 

13 6 

3 6 

3 6 

15 0 

5 0 

33 G 

7 0 
24 BEN'fONI'l'E DEPOSI'fS IN MONTANA AND WYOMING 

Rock material 

7. Shale-Continued 

Septariate coneretions, 16 ft above yellowish-tan-weathering 

concretions of unit 8 at Soap Creek, yielded the following 

fauna: 

"Oorbula" n. SIJ. 

Inoceramus n. SlJNucula, sp. 

Yoltlin sp. 

Turritella aff. T ~rhitci Stanton 

Denta1ium- sp. 

Baculites n. sp. 

Scaphites n. sp. 

Reesicle identified the following additional fossils collected by 

'l'hom's party, probably from this bed as \Yell as from the 

3'ellowish-tan concretions below: 

Anomia sp. 

Barbatia, sp. 

Callista tenuis Hall and Meek 

N cmodon n. sp. 

Gyrodes cf G. con;·afli Meek 

aff. G. pet1'0sa (l\forton) 

TurrUella n. sp. 

8. Shale, dark-gray: sonw beds weather light gray. In middle part 

it contains closely spaced ealeareous septariate concretions 

that weather light gray to yellowish tan __________________ _ 

''Jfarte8ia" Sfl. 

Tleniella cf Y. gonioplwra Meek 

Placenticems stantoni Hyatt 

Driftwood 

D. Shale, dark-gray, with large, yellowish-tan-weathering ealeareons concretions, commonly 111:? by 3 ft to 3 by 6 ft, septariate with thick, dark-brown and yE'llow ealeite veins. ]1'ossiliferous. (Base of Niobrara member-Thom and others, 1935) _ 

"CorlJula'' n. sp. 

('ras.<mtellltes ef. 0. reesidei Sidwell 

I11oreramus altus l\feek and Hayden? 

Yeniclla ef. r. gonioplwra Meek 

Tritonium? ef. T. ka11abense Stanton 

Gastropods indt. 

Baculites n. sp. 

Placcuticeras stantoni Hyatt 

Scaphttcs n. spp. 

10. Shale, dark-gray: contahm light-gray-weathering calearPons 

eoneretions. septariate with dark-brown caleite veins. In middle part it contains a bed with larger septarian concretions that 

weather yellowish tan _____________________________________ _ 

Membranipodna, sp. 

Inoceramu8 flacddu8 White 

n. spp. 

Nucula sp. 

O.<Jtrea sp. 

Bacullte8 n. sp. 

Prionocyclus H'YOtningensi8 Meek 

Scaphites n. sp. 

Feet Inches 
GEOLOGY 25 

Hock; 111atcrial Pf'('t Incltr·.~ 

11. Shale, dark-gray with thin gray :mndy streaks; readily weathers to soft, gray soiL_____________________________________ 30 0 

12. Conglomerate, compo~ed of light-gray-weathering pebbles of 

clayey siltstone, commonly lh in., but some as much as 3 in. 

long, that either lie in dark ~hale or form pebble l1ands of 

yell(nvish-tan-weathering caleareons concretions ____________ _ 

1:1. Rhale, dark-gray; contains some thin sandy streaks and 10 layer~ or mort> of- very hard gray ironstnnt> eoneretions that 

W(•ather hL"ight rusty orange atHl dark maroon brown. The 

eoncretious average from 12 in. to 15 in. and 2 in. to 3 in. 

in their lon~est and shortest dimensions. rt>sveetivt>ly, and 

are abundant. Some layers contain bright-tan-weathering 

clayey nodules __________________________________________ _ 

Inoceramus sp. 

Scaphites sp. 

14. Shale, dark-gray; contains sandy streaks ____________________ _ 

15. Bt>ntonite, gray _______ --------------------------------------

16. Shale, dark-gray; contains sandy streaks ___________________ _ 

17. Bentonite, gray, olive at base (bed 0) ------------------------

:18. Sandstone and shale, in thin bt>ds ___________________________ _ 

1 D. Bentonite, chiefly greenish gray (bed N) ---------------------

20. Shale, dark-gray ; contains very fine-grained sandy streaks ____ _ 

Inoceramus sp. 

l'rionocyclus sp. 

25 

6 

25 

3 

3 

3 

55 

0 

5 

6 

6 

2 

9 

1 

0 

Total measured thiekness_____________________________ 281 0 

The Niobrara shale member of the Cody shale of this district consists of approximately ±10 feet of dark-gray shale with many beds 

containing gray-weathering calcareous coneretions and numerous 

thin layers of bentonite. A 9-foot bed of calcareous shale is present 

75 feet above the base of the formation. 

Fossils are common in some of the concretions. The lower 75 feet 

of the formation contains Inocermnws dejm·1nis Meek. The overlying 

335 feet contains Baculites codyensis Reeside and 8caphites vennifm"lnis Meek and Hayden. 

Bentonite beds P and C2 crop out in the northwestern part of the 

area mapped on plate 1. 

The following section of the Niobrara 1nember was measured along 

the bluffs on the east bank of Bighorn River, starting 5 n1iles south 

of Hardin in the Elj2NEVJSE1;2 sec. 15, T. 2 S., R. 33 E., and extending nortlwastwarcl to the center of the north line of the NEVJ sec. 1, 

and thence northward to the NE1;4NE¥t, sec. 25, T. 1 S., R. 34 E., 

1 mile southeast of Hardin. lTnits are numebered in descending 

sequence. 
26 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Roc-k material Prct Incl/t'R 

1. Shale, dark-gray; coutains n•ry line grainPd saud,\' streaks aw1 

gray-\Yeatlwring cakan-'ou:-; <"oiH·rt->tions. Few fossils________ 3:2 0 

In accra ill IIi~ n. sp. 

Ostrcn sp. 

Pteria. aff. P. uclira.~cana (Evans and Slnunarcl) 

Baculitcs codycn.sis Reesh1e 

Bcaphites vermiform is ~leek 

2. Shale, dark: contains small grny-\VPatlleriug {':tl<-areons coll(•J't'-

tious and four thin lwntouitt> !H?<1 s __________________________ _ 

3. Bentonite, gray "·ith ettloresf·Pnt alkali_ _____________________ _ 

-1. Shale, dark-gray ___________________________________________ _ 

G. Shale, clarl>:-gray; contains small, rusty ironstone concretions __ _ 

6. Shale, dark-gray ___________________________________________ _ 

7. Shale, clark-gray; contains three 1 112-in. bentonite beds and a 

bed of calcareous eoncretions ______________________________ _ 

8. Shale, dark-gray; contains about 12 -rery thin bentonite beds, 

eommonly with aragonite----------------------------------

9. Bentonite, gray, streaked with yellow _______________________ _ 

10. Shale, dark-gray: contains two very thin bentonite beds _______ _ 

11. Shale, clark-gray; contains some crushed fossils ______________ _ 

1:2. Shale, dark-gray; contains four bentonite beds 1 to 3 in. thick __ 

1:1. Shale, dark-gray; eontains a few crnsl1ed foflsils _____________ _ 

Inoceramus spp. 

Baculites codycnsis Rfesicle 

Be a phites vermiform is Meek 

14. Bentonite and aragonite, gray--------------------------------

15. Shale, dark-gray; eontains a fe\Y ripple-marked, sandy layers 

in lower vart_ ___________________________________________ _ 

16. Bentonite, gray; eontains distantly spaeed lentieular calcareous 

concretions commonly measuring 4 by 36 in ________________ _ 

17. Shale, dark-gray--------------------------------------------

18. Shale: contains coneretions, gray-weathering. ealcareous, distantly spaced, commonly measuring 6 by 18 in ___________ _ 

1~ Shal~ dark-gray ___________________________________________ _ 

20. Bentonit~ gray ____________________________________________ _ 

21. Shale, clark-gray--------------------------------------------

22. Bentonite, gray, finely mieaceous ____________________________ _ 

23. Shale, dark-gray-------------------------------------------- 24. Bentonit~ gray ____________________________________________ _ 

25. Shale, dark _______________________________________________ _ 

26. Bentonite, gray and orange _________________________________ _ 

27. Shale, dark ________________________________________________ _ 

28. Bentonite, gray, finely mieaeeons ____________________________ _ 

29. Shale, dark _______________________________________________ _ 

30. Bentonite, gra~r, finely micaceous ___________________________ _ 

31. Shale, dark-gray; contains six bentonite beds, 1 to 6 in. thick __ _ 

Jnoce1·amus sp. 

32. Shale, dark-gray; with concretions, light-gray-\veathering, calcareous, commonly 8 by 18 ilL _____________________________ _ 

Anomia n. sp. 

Inoceratnus sp. 

Ost1·ea sp. 

Bamtlites n. sp. 

18 () 

4 

2 0 

1 0 

10 7 

7 9 

31 8 

4 

13 6 

18 (-) 

:20 7 

9 6 

4 

19 0 

1 

6 9 

6 

7 9 

9 

23 3 

2 0 

8 5 

6 

1 1 

4 

3 8 

1 6 

1 0 

1 0 

21 2 

8 
GEOLOGY 27 

Rock material Fc('t lncllo< 

33. Shale, llark-gray, faintly calcareous; contains erushe<l fossils__ 9 0 

Inoceramus spp. 

Osf1·ea n. sp. 

Baculites codyensis Reeside 

n. sp. 

Pelecypods inclt. 

:H. Shale, gray; contains concretions, light-grny-weatlwriug, calcareous. commonly 1 by 2 ft. clistautly-SJilH'PIL _________________ _ 

DG. Bentonite, gray, fit1ely nlieueecnu~~-----------------------------

:;6. Shale, dark-gray, slightly enleareous _______________________ _ 

Inoceramus sp. 

37. Shale, dark gray; contains concretions, light-gray-weathering, 

ealcareous, septariate, closely-spaced ______________________ _ 

38. Shale, dark-gray; lower part slightly calcareous; contains 

crushed fossils __________________________________________ _ 

Inocera-mus sp. 

Ostrea congesta Conrad 

Baculites n. sp. 

Fish scales 

39. Bentonite. gray, micaceous _________________________________ _ 

40. Shale, dark-gray, calcareous; contains crushed fossils; forms 

conspicuous yellowish-cream soil that supports greasewood on 

east side of Bighorn River from locality near Hardin southward to at least as far as Rotten Grass Creek ______________ _ 

Inocm·a.mus sp. 

Ostr·ea congesta Conrad 

V eniella-? sp. 

Ba-cuUtes n. sp. 

HcaphUes sp. 

Flsh scales 

41. Shale, dark-gray, slightly calcareous; contains two l-in. bentonite beds ---------------------------------------------------

42. Shale, dark-gray; shows yellow surfaces ____________________ _ 

43. Bentonite, gray with rusty spots ___________________________ _ 

44. Shale, dark ------------------------------------------------ 4~ Bentonit~ greenish ________________________________________ _ 

46. Shale, dark _______________________________________________ _ 

47. Bentonite, gray---------------------------------------------

48. Shale, dark-gray; eontains two thin bentonite beds __________ _ 

49. Shale, gray, bentonitic; contains distantly-spaced, large, 

buff-weathering calcareous concretions ___________________ _ 

50. Bentonite, light-gray--------------------------------------- 5!. Shale, dark _______________________________________________ _ 

52. Bentonite, gray--------------------------------------------- 53, Shal~ dark _______________________________________________ _ 

54. Bentonite. gray ____________________________________________ _ 

55. Shale, dark-gray; contains scattere-d biotite flakes ___________ _ 

Tnoceramtts sp. 

Ostrea congesta Conrad 

56. Bentonite, gray to greenish-gray, finely micaceous; eontains 

large, elongated, light-gray limestone concretions that 

'veather buff ____________________________________________ _ 

Inoceranws sp. 

34 7624-56--3 

1 

10 

29 

9 

7 

4 

1 

1 

13 

4 

1 

11 

1 

0 

0 

(j 

6 

0 

6 

2 

8 

0 

5 

3 

6 

7 

6 

0 

5 

5 

6 

3 

6 

6 

0 

6 
28 BENTONITE DEPOSITS IN :M:ON'l'ANA AND WYOMING 

Rock material Feet Inches 

57. Shale, dark-gray, faiutly eakareons__________________________ 2 6 

G8. Bentonite, greenish-gray------------------------------------ 1 

59. Shale, dark-gray; contains biotite___________________________ 6 

60. Bentonite, greenish-gray; weathers wry light gray; with efflorescent alkali_______________________________________________ 1 0 

61. Shale, dark-gray; 1-ft. layer in middle is ealcareous; contains a 

few sandy streaks in lower 11art and four thin bentonite beds 

in upper 3 ft ____________________________________________ _ 

Inoceram 1U5 sp. 

08f'rea con[!e8ta Conrad 

62. Concretions, caleareous, weather light gray, as large a:-; llj:.l by 

2 ft., embeclded in shale. Coneretions contain large Inoceran"t8 -----------------------------------------------------

Inoceramus deform i8 Meek 

Ostrea n. sp. 

Baculites n. sp. 

n. sp. aff. B. codycn8is Reeside 

Scaphite.s sp. 

Nautiloid 

Gastropocl 

G3. Shale, dark-gray; contains sandy streaks in micldle __________ _ 

Inoceramu8 deformis Meek 

n. sp. 

Baculite.s n. sp. 

Echinoid spines 

7 4 

1 6 

10 0 

G4. Shale, dark-gray; eontains very fine grained sandy streaks____ 8 9 

65. Bentonite, gray, shaly, micaceous; with concretions, light gray, 

weathering buff, calcareous_______________________________ 1 3 

Inoceramus defonnis Meek 

Baculite.s n. sp. 

Scaphites n. sp. 

Gastropods inclt. 

Total measurecl thickness____________________________ 408 1 

MONTANA GROUP 

UPPER PART OF THE CODY SHALE 

The Montana group, as 1napped on plate 1, comprises the uppermost 

three members of the Cody shale and two overlying formations, the 

Judith River formation and the Bearpaw shale. The Montana group, 

as thus constituted, appears to occupy very nearly the sa1ne stratigraphic interval as the Pierre shale; it is possible, however, that the 

uppermost part of the Bearpaw, as here mapped, may in part be 

stratigraphically equivalent to the Fox Hills sandstone, which over~ 

lies the Pierre in the vicinity of the Black Hills. 

In the lower part of the Montana group, between the Niobrara and 

~Tudith River units, are strata composed predon1inantly of shale, which 

are divisible, in son1e parts of the Hardin district, into three lithologic units. From the Montana-Wyoming boundary northward as 

far as the mouth of the Little Bighorn River (pl. 1), these strata are 
GEOLOGY 29 

mapped as a single unit of mmnber rank that forms the uppermost part 

of the Cody shale; in the area east of Hardin, these three lithologic 

units are shown separately as (1) the Telegraph Creek 1nember of 

the Cody; ( 2) an overlying unnamed shale member equivalent to the 

Eagle sandstone; and ( 3) the Claggett shale member, forming the 

top of the Cody. 

The strata in the interval between the Niobrara shale 1nember and 

the Judith River formation, as mapped on plate 1, occupy the same 

stratigraphic position as the Steele shale of areas farther southeast 

along the margin of the Powder River Basin (Wegmnann, 1918, p. 

13; Thom and Spieker, 1931, table 4). On the east side of that basin, 

near the Black Hills (fig. 3, col. 7), the beds of the lower part of 

the same interval are known as the Gammon ferruginous member and 

those of the upper part are known as the Mitten black shale member 

of the Pierre shale. A few miles west of the Hardin district the rocks 

of this interval pass laterally into strata anwng which are the prominent cliff-forming beds of the Eagle sandstone. 

Telegraph Creek 1n£1nber.-This member, which is largely buffweathering sandy shale, measures 867 feet in thickness where it is 

exposed along the Little Bighorn River near Hardin; Telegraph 

Creek, the intermittent stream from which the member derives its 

name, crosses U. S. Highway 87 about 15 1niles west of Hardin. Fossils, which are scarce, are n1arine forms of early Montana age. The 

only deposit of bentonite known to occur in the Telegraph Creek beds 

of this district is about 3 feet thick and crops out a few hundred yards 

in a northerly direction from the southeast corner ofT. 1 N., R. 33 E. 

This bentonite contains a good deal of sand and shaly material. It is 

not represented on plate 1. 

Uniw1ned sandy shale nwrnber.-An unna1ned member, with a thickness of about 375 feet (east of Hardin), is approximately equivalent 

to the Eagle sandstone of areas farther ·west and northwest, to the 

Groat sandstone bed of northea.stern Wyoming and, in part, to the 

Shannon sandstone 1nember of the Steele shale of central Wyoming. 

This men1ber, which consists largely of buff-weathering sandy shale, 

is much like the underlying Telegraph Creek mmnber; however, it 

can be identified by the presence of nmnerous beds of rusty-weathering ferruginous concretions, which are lacking in the Telegraph Creek 

member. In the northeastern part of the area shown in plate 1, 

the rocks of this unit are less sandy than those of the Telegraph Creek 

ntember, but farther south, in T. 9 S., R. 35 E., the. unit contains a 

thin, but conspicuous, layer of sandstone. The rock materials numbered 21, 22, and 23 in the measured section below form a prominent 

ledge. Marine invertebrate fossils, which are abundant in some beds, 

include Scapkites hippocrepis (De l{ay), Scaphites aquilaensis 
30 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Reeside, H amites novimexican!Us Reeside, and Baculites aqltilamu3is 

Reeside, all of which are common in the Eagle member. 

A conspicuous group of bentonite beds is present in the upper part 

of the unnamed sandy shale member. Bentonite bed R is in the lower 

part of the upper third. At point 1, see. 13, T. 1 S., R. 34 E., where 

this bed was measured and sampled, it is eomposed of three bentonite 

beds that are separated by thin shale strata. 

Because of the prevailing steep dip of the strata in the area oeeupied by the outcrop of bed R (amounting to 12 o or more at 1nost outerops), this bed lies under heavy overburden, exeept in very narrow 

belts along its outcrop. 

Claggett shale member.-Approximately 400 feet of strata forming the uppermost part of the Cody shale eomprise the following units, 

in ascending order, which are believed to represent the Claggett shale 

of eentral Montana: (1) About 100 feet of gray-·weathering bentonitic. 

shale, eontaining several beds of yellow bentonite and several beds with 

gray- and brown-weathering ealeareous septarian concretions; (2) 

about 200 feet of gray-weathering shale, many beds of which contain 

brown- and gray-weathering, ealcareous septarian eoneretions; and 

(3) about 100 feet of dark-gray, soft fissile shale forming dark outcrops and containing ealcareous septarian coneretions that weather 

yellow-tan to dark-brown. Marine fossils typieal of the Claggett 

shale of central Montana are fairly eomn1on in the uppermost dark 

shale unit. 

Bentonite beds S, T, and U are in the lower part of the Claggett 

shale member. Bed 8 has been used as a horizon n1arker for the base 

of that unit, and its location is indieated on the 1nap by the lower 

boundary of the Claggett 1nember. Each of these three beds was 

1neasurecl at only one loc.ality, and they are believed to have little eeonomic value. 

The following section of the upper Cody beds vvas measured 6 to 7 

miles east of Hardin, along a line more or less parallel to, and within 

half a mile of, the Hardin-Sarpy road. The 1neasured section of the 

lowermost 615 feet of the Telegraph Creek member extends from the 

base of a steep east-faeing hill in the SE~SEl;! see. 11, T. 1 S., R. 

34 E., due east to a prominent cut bank on the south side of Dry Creek 

near the eenter of the SlhSW~ see. 12. The uppernwst part of the 

Telegraph Creek, the entire Eagle equivalent, and the Claggett shale 

member were measured along the local drainage divide extending eastward across the northern part of sec. 13, T. 1 S., R. 34 E., to the base of 

the hills on which the Judith River fonnation erops out on the sa1ne 

divide in the NEl;JSvVl;JNvV~ see. 18, T. 1 S., R. 35 E. lTnits are 

numbered in descending sequence. 
GEOLOGY 

Rock material 

Claggett shale member: 

1. Shale, dark-gray: crops out as dark, soft, flaky shale, contrasting strongly with light-colored overlying sandy unit. 

Contains brown-weathering caleareous concretions, septariate with yellow calcite and white barite; contains same 

fauna as underlying fossiliferous concretion bed ________ _ 

2. Shale, dark-gray; with concretions, yellow-tan to bro>vnweathering, calcareous, quite fossiliferous, closely-spaced: 

septariate with thin, pale-yellow calcite veins; commonly 

10 by 18 in. to 15 by 30 in., but as much as 2 by 6ft_ _____ _ 

Oaprinella cor·aloidea Hall and Meek 

Inoceramus 7Jm·abini var. 'inflatitonnis D~uglas 

ba,rabini var. magniumbonatus Douglas? 

cf. I. sagensis Owen 

saskatchewanensis Warren 

'Oanuxemi Meek and Hayden 

Pteria cf. P. notukeuensis 'Van·en 

"Yoldia" sp. 

A_cantlwscapl!ite8 aff. ~J. breris Meek 

BacuUtesn. sp. aff. B. thomi Reeside 

3. Shale, dark-gray; contains a bPd of caleareous septariate 

concretions 28 ft. below top ____________________________ _ 

4. Shale, dark-gray; with concretions that weather rustybrown, tan and reddish, calcareous, very closely spaced, 

septariate with thin, pale-yt-llow calcite veins _______ _ 

5. Shale, dark-gray; contains a few brown-weathering calcareous concretions, septariate with yellow calcite. Crops 

out as dark, soft, flaky shale eontrasting with underlying 

gray-weathering shale ________________________________ _ 

Baculites cf. B. aquilaensis Heeside 

6. Shale, dark-gray; with concretions, bro>vn->veathering, c-alcareous, very closely spaced, septariate 'vith brown or white calcite veins _____________________________________ _ 

Baculite8 haresi Reeside 

7. Shale, dark-gray; weathers to gray, soft, grass-covered soil; 

eontains at least eight beds of brown-weathering calcareous eoncretions, septariate vdth yellow or brown calcite 

veins~-----------------------------------------------

Baeulites sp. 

8. Shale, dark-gray; weathers to bare. gray gumbo soil: contains a few small gray ealC'areous concretions __________ _ 

Baculites aqu-ilaensis var. separatus Heeside 

lHwesi Reeside 

9. Shale, concretion-bearing, and bentonite; two beds of shale, 

eontaining gray ealeareous eoneretions that weather light 

gray and brown, are separated by about 3 ft of gray 

bentonite and bentonitic shale. Concretions are very 

closely spaced and somewhat septariate, with veins of 

pale-yellow to brown caleite and, rarely, bluish 

chalcedony __________________________________________ _ 

10. Shale, dark-gray with bentonitic beds; weathers lighter gray 

with gumbo bands------------------------------------

11. Bentonite, yellow (bed U) -----------------------------

31 

Feet Inches 

16 

1 

42 

1 

40 

1 

156 

40 

4 

30 

2 

0 

6 

0 

0 

6 

0 

0 

0 

6 

0 

9 
32 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Rock material 

Claggett shale member-Continued 

12. Shale, dark-gray, bentonitic; weathers lighter gray ______ _ 

13. Bentonite, yello\v; forms bare, gray gumbo (bed T) ______ _ 

14. Shale, dark-gray; contains gray-weathering calcareous con- cretions at base ______________________________________ _ 

15. Bentonite, yellow (bedS at base of Claggett shale member)_ 

Total measured thickness ______________________ _ 

Unnamed sandy shale member equivalent to Eagle sandstone: 

16. Shale, dark-gray; weathers brownish; contains brown- and 

gray-weathering calcareous concretions, some of which are 

septariate with thin, white calcite veins ________________ _ 

Inoceramus cf. I. barabini Morton 

spp. 

Ba·cttlites aquilaensis Reeside 

var. separa.tus Reeside 

haresi Reeside 

minerensis Landes 

Placenticeras sp. 

17. Shale, dark-gray, weathering to brownish soil; contains calcareous ironstone concretions that weather rusty brown 

and reddish _________________________________________ _ 

Baculites sp. 

18. Shale, dark-gray, weathering brownish gray; contains some 

brown-weathering calcareous concretions, septariate with 

yellow calcite veins __________________________________ _ 

Goniochasma crockfordi Warren 

Inocer·amus barabini Morton 

saskatchewanensis Warren 

subdepressus Meek and Hayden 

n. sp. 

Pholadomya aff. P. subventricosa Meek and Hayden 

Baculites sp. 

Placenticeras planum, Hyatt 

19. Bentonite, yellowish-gray, grading upward into brownish 

gray. Contains much white aragonite (top of bed R) __ _ 

20. Shale, dark-bluish-gray, silicified _______________________ _ 

21. Bentonite, creamy-yellow grading upward into brownish 

gray ------------------------------------------------ 22. Shale, dark-bluish-gray, silicified _______________________ _ 

23. Shale, gray, bentonitic ________________________________ _ 

24. Bentonite, yellowish-cream, grading upward into gray; 

forms rather bare gray gumbo (base of bed R) _________ _ 

25. Shale, dark-gray, weathering to light-gray soil with bluish 

bands. Middle part contains numerous small calcareous 

concretions thta weather bluish gray tO. buff _________ _ 

BacuUtes aquilaensi.s var. separatus Reeside 

haresi Reeside 

Reptilian bones 

26. Shale, dark-gray, weathering lighter gray; contains very 

fine grained sandy partings and about 25 beds of closelyspaced ironstone concretions that weather rusty to dark 

maroon, embedded in shale __________________________ _ 

Baculites haresi Reeside 

Feet 

31 

1 

25 

2 

396 

27 

15 

46 

2 

1 

7 

1 

5 

6 

45 

50 

Inches 

6 

11 

0 

10 

6 

6 

0 

0 

9 

6 

0 

0 

0 

0 

0 

0 
GEOLOGY 33 

Rock material Feet Inches 

Unnamed sandy shale member equivalent to Eagle sandstone--Con. 

27. Shale, gray; with concretions, gray, weathering tan, rusty, 

and dark maroon, ferruginous, calcareous and silty, 

very closely spaced. Forms a more or less continuous 

ledge ------------------------------------------------ 1 0 

Baculites haresi Reeside 

28. Shale, gray, sandy; contains a few gray silty calcareous 

concretions and, locally, crossbedded sandstone concretions that weather buff gray--------------------------

29. Shale, gray; with concretions, gray-weathering, calcare<:>us, 

fairly closely spaced, usually somewhat septariate with 

thin white calcite veins. Some are fossiliferous carrying a prolific fauna typical of the Eagle _______________ _ 

Anomia sp. 

"C(tllista" cf. C. pellucida (Meek and Hayden) 

sp. 

Cardittm (Ethmocm·dium) H'hitei Dan 

Corbnlamella cf. 0. fJ1'CfJ(Wia (Meek and Hayden) 

Crenella cf. elegantula Meek and Hayden 

Oymbophora? sp. 

Cymella montanensis (Henderson) 

lnoceramns barabini Morton 

Leptosolen cf. L. cont·adi Meek 

Lin~a n. sp. 

Lithophaga n. sp. 

Pinna cf. P. dolosoniensis McLearn 

Syncyclonetn(t halli (Gabb) 

Spironema aff. S. tenuilinea1ta Meek and Hayden 

Volsella n. sp. aff. V. n1eeki (Evans and Shumard) 

Capultts? aff. C.? 1m'crostr'iatus Stephenson 

Drepanochilus aff. D. evansi Cossman 

Spit·onema aff. S. tenuilneata,. Meek and Hayden 

Other gastropods undetermined 

Baculites aquilaettsis Reeside 

tmr. separa.tus Reeside 

hm·esi Reeside 

var. (corrugated venter) 

thomi Reeside 

H amites novimea:icanus Reeside 

H elicoceras rubeyi Reeside 

Placenticeras meeki Boehm 

Scaphites aqnilaensis Reeside 

var. costatus Reeside 

var. nanu.s Reeside 

hippoct·epis (DeKay) 

var. crassus Reeside 

var. pusillus Reeside 

var. tenuis Reeside 

stantoni Reeside 

n. sp. 

Ammonite, possibly new genus 

Crustacean remains 

Fish scale 

5 0 

1 0 
34 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Rock material 

Unnamed sandy shale member equivalent to Eagle sandstone--Con. 

30. Shale, dark-gray, weathering buff gray, silty; contains ferruginous, tan-weathering, ca.lcareous concretions, so 

widely separate that they are rarely seen in exposures __ 

31. Shale, dark-gray, weathering light-gray, silty; contains a 

few brown-weathering calcateous ironstone concretions 

as much as 4ft in length ______________________________ _ 

Baculite8 siJ. 

B2. Be11to11ite, gray ancl ~vellovv ______________________________ _ 

:33. Shale, dark-gray; contains ironstone concretions _________ _ 

84. Bentonite, yellowisl1-ere-am; eontains much aragonite _____ _ 

gr,_ Shale, dark-gray, somewhat silty; contains numerous laye,rs 

of ironstone concretions that weather rusty brown and ~ark ntaroon ________________________________________ _ 

Total measured thiekness ______________________ _ 

Telegraph Creek sanely shale member: 

36. Shale, gray, sandy: contains gray-weathering calcareous 

coneretions, especially in lower 5 ft where they are septariate with orange calcite veins ____________________ _ 

Inoccramu8 sp. 

Baculites ltare8i Reesicle 

Scaphife8 sp. 

37. Sandstone and shale, gray, weathering light-tan-gray; contains small gray ealeareous concretions and large (1 by 

2ft to 3ft by 5 ft) yellcr\vish-tan-weathering, silty, lenticular, ealeareons concretions ___________________________ _ 

Baculites aqullaenBi8 Reeside 

hare8i Reeside 

38. Bentonite, cream-colored, eontains white aragonite _______ _ 

3!). Sandstone and shale: shale is gray, weathering light tangray, very fine grained. soft; sandstone is in thin layers 

commonly less than 1 in. thick, more or less ripple-marked, 

and separated by thin layers and laminae of clark-gray 

shale. Upper 50 ft contains a few calcareous concretions 

that weather gray or yellowish tan. Lower 100 ft contains ledge-forming, brown-weathering sandstone lenses 

that are commonly ripple-marked and crossbedclecL _____ _ 

Ostrea sp. 

Baculite8 hare8i Reeside 

8caphite8 sv. 

40. Sand and shale, as above _______________________________ _ 

Total measured thielmess ______________________ _ 

JUDITH RIVER FORMATION 

Feet Inches 

42 

72 

1 

17 

1 

2D 

376 

65 

17 

170 

614 

867 

0 

0 

6 

0 

0 

0 

3 

0 

0 

6 

0 

6 

0 

The Judith River formation (pl. 1), which rests with apparent conformity upon the Claggett shale member of the Cody, consists largely 

of light-colored sandy strata. As shown in figure 3 ( cols. 2 and 4) , it 

is approximately equivalent to the Mesaverde formation of the Salt 
GEOLOGY 35 

Creek oil field, Wymning, which lies farther southeast in the Powder 

River basin, and (fig. 3, col. 6) to the Judith River of central Montana. The Judith River, as 1napped near the Montana-Wyoming 

boundary (pl. 1), comprises two members-an unnamed n1ember and 

the Parkman sandstone member. Farther north within the Hardin 

district, however, the upper member loses its identity through a northward interfingering transition into dark, marine shale beds of the 

lower and 1niddle parts of the Bearpaw shale; north of T. 3 S., R. 

35 E., therefore, the ,Judith River formation is mapped as a single unit 

that is equivalent only to the lower n1ember, the Park1nan sandstone. 

The Judith River formation is about 255 feet thick as exposed east 

of Hardin, where, as has just been explained, it is equivalent to the 

Parkman sandstone of localities farther south. The lowermost beds 

near Hardin are buff-weathering sandy shale, which diminishes gradually upward for about 110 feet as it becomes intercalated with increasing amounts of sandstone similar to that of the overlying 145 

feet of beds. These overlying beds consist of massive, soft, buff, 

brownish and olive-green sandstone containing harder, brownweathering sandstone concretions and rusty, li1nonitic nodules. No 

fossils have been observed in the Judith River at this locality. 

The following section of the Judith River formation (the equivalent of Parkman sandstone n1ember) was measured 7 miles east of 

Hardin in the high hills north of the Sarpy road in the Elf2NE1;4SE1;4 

sec. 7 and S1;2N,VJ4SW:JA sec. 8, T. 1 S., R. 35 E. Units are numbered 

in descending sequence. 

Rock material 

1. Sandstone, buff, brownish and olive-green; commonly very soft 

with harder, brownish-weathering, concretionary sandstone 

Feet Inches 

and rusty limonitic nodules. Forms west-facing sandy hills__ 145 0 

2. Shale, buff-weathering; transitional upward from silty shale to 

very sandy shale. Contains four or five beds of silty calcareous 

concretions that weather gray, yellowish tan, or brown______ 109 0 

Total measured thickness____________________________ 254 0 

Parkman sandstone tnember.-Near the Montana-Wyoming boundary, where the Judith River formation con1prises two 1nembers, the 

lower (Parkman sandstone) 1nmnber is 1nade up of strata about 350 

feet thick, which are lithologically similar to the rocks described in 

the foregoing section of the Judith River as measured near Hardin. 

Upper ~member-The upper mmnber of the Judith River formation 

consists of about 700 feet of rather massive brown and gray sandstone, 

interbedded with green and light-brownish-gray sandy shale and 

clark-gray shale and, in the lower part, a fe\v beds of coal and clinker. 

Only the lower part of this member is mapped on plate 1. Farther 
36 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

north, as has already been stated, the beds of this men1ber interfinger 

with the dark marine shale beds o£ the Bearpaw shale; in T. 3 S., 

R. 35 E., they are represented only by a few thin sandy beds, and such 

Inaterial seen1s to be absent in the equivalent strata o£ the next township to the north. Thick beds of bentonite that occur in the part of the 

Bearpaw shale that is involved in this transition do not extend south-

'yard into the upper member of the ~Judith River; within this district 

the ~Judith River contains little or no bentonite. 

BEARP A W SHALE 

The Bearpaw shale, where it is exposed east of Hardin, is about 

865 feet thick. It consists largely of dark-gray shale, containing 

numerous brown-weathering calcareous concretions and much bentonite in its 1nidc1le part. The formation is divisible into three units 

on the basis of relative amounts of contained bentonite: a basal unit 

about 315 feet thick, with only a few thin bentonite beds; a medial 

unit about 240 feet thick, with many beds of bentonite, one of which 

has a maximmn thickness of 22 feet; and an upper unit about 310 feet 

thick, which, like the basal unit, contains little bentonite. 

The medial unit is believed to be stratigraphically equivalent to the 

Monument Hill bentonitic member of the Pierre shale of the Black 

Hills region and to a highly bentonitic part of the Bearpaw shale that 

is extensively exposed in Rosebud and Treasure Counties, Montana. 

Fossils are abundant in the Bearpaw shale, the most characteristic 

forms being Bamdites compress·us Say, "Acantlwscaphites nodosus" 

(Owen), Placenticeras ·meeki Boehm, and several species of I noceranYus. 

Bentonite beds V .. , TV, and X occur in the medial part of the Bearpaw 

shale. These beds are of particular interest because they include 

large deposits of bentonite that crop out in areas close to the railroad. 

A thickness of 2llh feet was measured for bed V at point 11, sec. 8, 

T. 2 S., R. 35 E., and this bed is believed to be similarly thick in the 

southern half ofT. 1 S., R. 35 E., and in the northern half ofT. 2 S., 

R. 35 E. The bed contains enormous tonnages of bentonite under less 

than 30 feet of overburden in the areas indicated on plate 1. Some of 

the bentonite of bed V has valuable foundry-sand bonding properties, 

but most of it probably does not have the properties desirable in clay 

that is tb be used for preparation of drilling mud. Calcareous concretions, which weather light-brown and which are 8 to 12 feet in 

dia1neter, are abundant on the exposed surfaces of this bed (figs. 

7and8). 
GEOLOGY 37 

FIGURB 7.-View southward along outcrop of bentonite bed V in NW:lpj sec. 17, T. 2 S., 

R. 35 E. ; note large limestone concretions. 

l!'JGURE 8.-Closeup of large limestone concretions along outcrop of bentonite bed V in 

NW~ sec. 17, T. 2 S .. R. 35 E . (location same as shown in fig. 7). 
38 BEN'l'ONITE DEPOSI'rS IN MON'l'ANA AND WYOMING 

FIGURE 9.- View southward along outcrop of bentonite bed W from point in SE :14 sec. 17, 

T. 2 S., R. 35 E. Bed appears as white material in for egnund and as long, white band 

in background. 

The most conspicuous outcrops of bentonite bed W are in T. 2 S., 

R. 35 E. (fig. 9). At most of its outcrops in this township, bed W 

consists of 3 to 8 feet of light-colored bentonite, overlain by a thick 

bed of dark-gray bentonitic shale. Parts of the light-colored matel'ial are believed to be suitable for use in drilling muds, and most of 

the bed, including some of the dark bentonitic shale, appears to contain good bonding material for synthetic molding sands. 

Bentonite bed X is 6 feet 10 inches thick at point 2, T. 1 S., R. 35 E., 

but it is much thinner 2 miles :farther south. The bentonite of bed X 

is believed to be suitable for sand bonding. 

The following section of the Bearpaw shale was measured from 

the center of the SW~ sec. 8 eastward through S¥2 sees. 8 and 9 to 

the base of a sandstone (believed to be in the Hell Creek formation) 

that is exposed at the Sarpy road in vV%NW~SYV~ sec. 10, T. 1 S., 

R. 35 E. 

Rock 1naterial 

UppeT unit: 

1. Shale, gray, sandy; interlaminated with very fine grained 

soft sandstone _______________________________________ _ 

2. Shale, dark-gray ________ _______________________________ _ 

3. Shale, dark-gray; with concretions, dark-brown-weather- ing, calcareous ___________ __________ _________________ _ 

Nucula planimarginata Meek and Hayden 

Feet Inches 

10 

25 

1 

0 

0 

0 
GEOLOGY 39 

Rock. material Feet Inches 

lJpper unit-Continued 

4. Shale, dark-gray_______________________________________ 8 0 

:>. Shale, dark-gray; with concretions, dark-brown-weathering 

calcareous -------------------------------------------

Protocardin subquadrata (Evans and Shumard) 

''Yoldia" evansi Meek and Hayden 

Baculites grandi8 Hall and Meek 

6. Shale, dark-gray ______________________________________ _ 

7. Bentonite, greenish-gray; with concretions, dark-brownweathering, calcareous _______________________________ _ 

8. Shale, dark-gray ______________________________________ _ 

9. Shale, dark-gray; largely concealed. Exposed along Bighorn River bluffs near old Nine Mile bridge ____________ _ 

Lucinid pelecypod, possibly new genus 

''A_canthoscapllltels'' nodosu8 (Owen) 

Baculites comprcs8u8 var. corrugafu8 Elias 

Pla.cenficera8 mecki Boehm 

planum Hyatt n. var. 

10. Shale, dark-gray with small gray-weathering, hard, nodular, calcareous concretions and large, odd-shaped, cavernous masses of limestone containing abundant Lucina 

occidentali8 (Morton)----------------------------------

Ohlamy8 nebra8ccn8i8 (Meek and Hayden) 

Inoceramu,<; baral1in1 var. infiatiformi8 Douglas 

Lucina occidentali8 (Morton) 

subumlata Hall and Meek 

08trea sp. 

Pteria linguae.form i8 (Evans and Shumard) 

Polinice8 aff. P. concinna (Hall and Meek) 

BacHlites compre88U8 var. corrugafu8 Elias 

Disco,<;caphife8 aff. D. 11icolleti (Morton) 

11. Shale, dark-gray; contains a few calcareous concretions __ _ 

12. Shale, dark-gray; contains calcareous concretions that are 

more or less ferruginous ______________________________ _ 

Baculites compre88118 corrngatu8 Elias 

13. Shale dark-gray; contains ealcareous concretions ________ _ 

Oymboplwra cf. 0. gntcili8 Ci\leek and Hayden) 

Inoceramus altus Meek 

Bac1tlites compre88us Say 

Plncenticeras meeki Boehm 

1 

21 

25 

150 

15 

35 

18 

0 

0 

8 

0 

0 

0 

0 

0 

0 

Total measured thickness_______________________ 314 8 

Bentonitic unit: 

14. Shale, dark-gray, poorly exposed; contains a few thin olivegreen bentonite beds and calcareous concretions_________ 28 0 

15. Bentonite, olive-green___________________________________ 1 0 

16. Shale, dark-gray--------------------------------------- 6 6 

17. Bentonite, greenish-gray________________________________ 1 0 

18. Shale, dark-gray; contains a few calcareous concretions____ 2 6 

19. Shale, dark-gray; with concretions, weathering brown or 

gray, calcareous, variable in size; some are fossiliferous__ 1 0 
40 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Rock ma.te1'ia1 Feet Inches 

Bentonitic unit-Continued 

20. Shale, clark-gray; contains two thin bentonite beds________ 4 6 

21. Shale, dark-gray; with concretions, weathering bright 

brown, calcareous, distantly-spaced. Smaller grayishbrown-weathering calcareous concretions are highly fossiliferous ------------------------------------------- 1 0 

Gu8piflaria aff. 0. tnorea•uemsi8 (Meek and Hayden) 

1"entt·ico8a, (Meek and Hayden) 

Gyml!ophora gracili8 (Meek and Hayden) 

Cymella meeki (Whitfield) 

Ge,rvillia 1·ectn Meek and Hayden 

I110ceramus tenuilineatus Hall and Meek 

sp. 

Lucina subundata Hall and Meek 

Osf1·ec~ cf. 0. subala.tn Meek 

Pteria cf. P. ZJ(trkensis (White) 

F:Jyncyclonenw halli ( Gabb) 

"r olclia!' evan8i Meek and Hayden 

aff. Y. ventricosa Hall and Meek 

sp. 

Acmaea occidentali8 (Hall and Meek) 

Dt·epanochilus evansi Cossman 

nebr·ascensis (Evans and Shumard) 

Fnsciolada aff. F. gracilenta Meek 

Haminen'! occidentnUs (Meek and Hayden) 

8Ubcylindric(( (Meek and Hayden) 

Polinice8 aff. P. concinna (Hall and Meek) 

Dentalium tJauperculum Meek and Hayden 

"Acanthoscaphites" bt·evis (.Meek) 

noclosus (Owen) 

qu((dmngt.tlat'is (Meek and Hayden) 

compressed variety 

Aptychi 

Baculites conrpressus Say 

Placenticems intercala,re Meek 

two varieties 

meek,i Boehm 

22. Bentonite, cream-colored _______________________________ _ 

22A. Shale, dark-gray--------------------------------------

22B. Bentonite, olive-yellow to gray--------------------------

23. Shale, clark ____________________________________________ _ 

24. Bentonite, olive-green to grayish-green __________________ _ 

25. Shale, contains concretions: weathering brown and gray, 

calcareous, closely-spaced ____________________________ _ 

Inoceramus vanuxemi Meek and Hayden 

''Acm~thoscaphites nodosus'' (Owen) 

Baculites comtJt'CS8U8 Say, var. 

26. Shale, dark-gray; contains calcareous concretions in 

middle ______________________________________________ _ 

27. Bentonite, olive-green __________________________________ _ 

28. Shale, dark-gray --------------------------------------- 29•. Bentonite, olive-green _________________________________ _ 

1 

10 

1 

2 

1 

1 

10 

3 

1 

0 

0 

6 

6 

6 

0 

9 

6 

0 

6 
GEOLOGY 

Rock materinl 

Ben toni tic unit-Continued 

30. Shale, dark-gray; contains calcareous concretions near 

middle ----------------------------------------------

Oymbophora gmcilis (Meek and Hayden) 

31. Bed X: 

Bentonite, yellow, grading upward into gray-------

Shale, dark-gray, somewhat sandy-----------------

Bentonite, yellow to yellowish green ______________ _ 

Shale, dark-gray, partly sandy and bentonitic _____ _ 

Bentonite, green, grading upward into gray ; locally 

contains large, gray, calcareous coneretions. Forms 

prominent, light-gray bare gumbo _______________ _ 

Shale, dark-gray. eontains gray, sandy, ealeareous 

concretions with silicified pelecypods _____________ _ 

CymbotJhora [rracili8 (Meek and Hayden) 

I noce·ramu8 sp. 

Bentonite, light-green grading upward into darkgreen ------------------------------------------

32. Shale, dark-gray, bentonitic; lower 9 ft. sanely ____________ _ 

33. Shale. dark-gray; with concretions, weathering dark-brown 

and purplish,· calcareous, sandy, ferruginous, closelyspaced ----------------------------------------------

Oyrnboph01·a gracili8 (Meek and Hayden) 

Inoceram U.8 sp. 

Baculites cornpr·es8U8 Say 

Placenticeras sp. 

34. Shale, dark-gray, soft, somewhat sandy; contains calcareous concretions 8 ft above base _______________________ _ 

Lucinid pelecypod, probably new genus 

"Acantho8caphite8" sp. 

Baculites cmnpres8us Say 

Placenticera8 sp. 

35. Shale, gray, bentonitic; eontains a few gray ealeareous 

concretions ------------------------------------------

36. Shale, gray, very bentonitic, weathering to gray, bare 

gumbo (bed W) _____________________________________ _ 

37. Shale, alternate units of gray bentonitic shale and darker, 

soft, fissile shale; poorly exposed ______________________ _ 

38. Bentonite, olive-green (bed Y) at base of middle uniL_. ___ _ 

Total mensured thkklH'SS _______________________ _ 

Lower unit: 

39. Shale, dark-gray; contains brown- and gray-weath{~riug 

septariate calcareous concretions 8 ft and 22 ft below 

the top _____________________________________________ _ 

40. Shale, dark-gray; contains several beds of calearj~ous, 

brown-weathering, ferruginous concretions _____________ _ 

41. Shale, dark-grey; eontains a few ealcareous eoneretions __ _ 

Inoceramus sp. 

Placenti.ce1·as rneeki Boehm 

J?eet 

12 

1 

2 

1 

2 

G 

2 

22 

1 

20 

35 

13 

42 

3 

241 

53 

20 

45 

41 

Inches 

6 

6 

6 

0 

0 

0 

6 

0 

0 

0 

0 

0 

n 

0 

0 

9 

0 

0 

0 
42 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Rock material Feet Inches 

Lower unit-Continued 

42. Shale, dark-gray; with concretions, brown-weathering, 

calcareous, septariate with yellow calcite veins. Commonly 1.5 by 3 ft, but as large as 2 by 10 ft______________ 1 6 

43. Shale, dark-gray ; contains a few brown-weathering 

calcareous concretions _______________________________ _ 

44. Shale, clark-gray; contains numerous brown-weathering 

calcareous concretions more or less septariate with yellow calcite veins ________________________________________ _ 

Inoceratnus cf. I. palliseri Douglas 

sagen8is Owen? 

saskatchewanen8is "\Van·en 

tenuilincatns Hall and Meek 

Ost1·ea sp. 

"Yoldia" sp. 

Baculites compressu8 Say 

45. Shale, dark-gray; contains a few small gray calcareous 

concretions -----------------------------------------

Inocemmus sp. 

Lucinasp. 

46. Shale, dark-gray; contains some bentonitic beds and, rarely, 

calcareous concretions _______________________________ _ 

47. Shale, dark-gray: with concretions weathering buff and 

63 0 

37 0 

20 0 

54 0 

brown, calcareous, closely spaced_____________________ 6 

Inoceramus bm·abini var. inflatifonnis Douglas 

Helicoceras simtJlicostatum Whitfield 

48. Shale, dark-gray_______________________________________ 15 0 

'l'otal measured thickness_______________________ 309 0 

HELL CREEK FORMATION 

The Hell Creek :formation, the uppermost Cretaceous stratigraphic 

unit exposed in the Hardin district, consists largely o:f massive, scarpforming sandstone beds and greenish-gray sandy clay. Some o:f the 

clay in the Hell Creek may be altered volcanic material, but this formation is not known to contain bentonite beds o:f commercial value. 

Thom and others (1935, p. 61) report that the Hell Creek o:f this 

locality, which is largely o:f fluviatile origin, contains "fresh-water invertebrate :fossils, frag1nents o:f turtle shells, and :fossil bones identified as belonging to the dinosaurian genus Trachodon." 

GEOLOGIC STRUCTURE 

The geologic structure o:f the Hardin district is illustrated (pl. 1) 

by means o:f contours drawn on the Clay Spur bentonite bed. The 

structure o:f the district is broadly homoclinal with the prevailing 

strike in a northwesterly direction and the overall dip northeastward, 

or a way :frmn the great uplift centering in the Big Horn Mountains. 

Therefore, the oldest rocks crop out along the southwestern side of the 

district, and the youngest crop out along the northeastern side. The 
BEN'l'ONITE DEPOSITS 43 

strata clip steeply in a belt along the southwestern margin of the clistriet, adjacent to the mountains, but they are less steeply inelinecl 

almost everywhere else. The homoclinal structure is interrupted 

loeally by synelinal and anticlinal flexures, mostly 'vith northwesterlytrending axes. There are also a few normal faults, most of which 

trend in northeasterly directions, approximately normal to the prevailing strike of the rocks. 

The anticlinal flexures include the Black Gulch dome, with its apex 

in the eastern part ofT. 9 S., R. 3± E.; the Rotten Grass dome, in the 

southern part of T. 7 S., R. 33 E.; the Soap Creek dome, with its 

apex in the southern part ofT. 6 S., R. 32 E.; the Woody Creek dome, 

with its apex in the southern part of T. 3 S., R. 31 E.; and the DunInore dmne, with its apex in T. 1 S., R. 34 E. Synclinal areas lie 

southwest of each of these antielinal folds. Further information on 

the structural features is given in the dosing chapter of the present 

report, where the geology is described, by township and range, in relation to the bentonite deposits. 

BENTONITE DEPOSITS 

The data presented below and on following pages, dealing primarily 

with the Hardin district, are, in part, contingent upon general inforlnation on bentonites (relating to their origin, manner of occurrence, 

composition, physical and chemical properties, and industrial testing 

procedures) that is contained in other reports, now in preparation, 

on bentonite deposits in the marine Cretaceous formations of various 

parts of the l\fissouri River basin. The bentonite deposits of this 

basin are similar to one another in so many respects that n1uch of the 

information to be presented in these other reports is applicable to the 

deposits in the Hardin district. 

A bentonite bed ordinarily consists of a number of layers of clay 

that differ frmn one another in one or more characteristics, such as 

co]or, texture, or collodial properties. As a rule, the bentonite of all 

these layers is light in color, with the exception of a top layer, which 

is dark -colored. The miners in the Black Hills district usually cast 

aside this top layer as waste material, though some of it is known to be 

suitable for use as foundry-sand bonding elay and as drilling-n1ud 

material. Most of the bentonite has a waxy consistency, and granular or other nonwaxy textures are commonly restricted to layers near 

the top and near the base of the light-colored material. 

The floor of most of the bentonite beds is 1narked by their sharp 

contact with the subjacent shale, which is loca1ly silicified just below 

the bentonite to form a thin, hard, chertlike layer. In general, the 

roof is less definite, and the bentonite of the upper part of the bed 

ordinarily grades upward through an indistinct zone of clark ben347624-56--4 
44 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

tonite and bentonitic shale into the overlying bedrock, which is most 

commonly dark-gray shale. 

MINERAL CONTENT 

The bentonites of the Hardin district belong in general to the gelforming types, although they differ considerably, from bed to bed 

and from place to place, in their behavior when immersed in water 

after drying. Results of differential thennal analyses, performed 

with a portable apparatus by Elizabeth A. Fischer, of the U.S. Geological Survey (informal communication, 1947), on several samples 

collected during the present investigation, indicate that the bentonite 

deposits of the llardin district consist predominantly of the clay mineral montmorillonite. They also commonly contain mica, feldspar, 

quartz, gypsum, soluble salts, and unaltered volcanic glass. Tiny, 

but plainly visible, residual particles of such non-clay constituents, 

many of them dark in color, are present in all the beds and are referred to collectively as "grit" by producers and buyers of bentonite. 

The grit content varies greatly from layer to layer, some layers showing conspicuous amounts and other layers showing little or none. 

CHARACTERISTICS OF OUTCROPS 

Freshly exposed bentonite ranges in texture from essentially holnogenous, firm, waxy clay to loosely compacted granular material, 

some of which resembles corn meal. When freshly exposed in mine 

excavations and prospect openings, nniCh of the bentonite quickly 

crumbles into grains, flakes and lumps of various sizes and shapes. 

The ground in the vicinity of the bentonite exposures is eommonly 

littered with residual fragments of selenite, calcite, or aragonite, 

and, in a few places, with conspicuous large li1nestone concretions 

(figs. 7 and 8) that have formed ·within the bentonite beds. During 

rainy periods, bentonite of n1oderate to high dilatancy soaks up many 

times its own volume of water, swells, and becomes a slippery thixotropic gel. On drying, the swollen clay eontracts to nearly its original volume, and, in so doing, it cracks and forms a peculiar popcornlike elay bloom (fig. 10). In general, the bentonite beds resist weathering to a slightly greater degree than the enclosing Cretaceous shales, 

and accordingly they tend to form conspicuous hillside ledges and 

buttresses. 

EFFECTS OF WEATHERING ON COLOR AND COLLOIDAL PROPERTIES 

The prevailing hues refleeted by most outcrops of the light-colored 

material of the bentonite beds are olive green, cream, and drab, representing discoloration of the bentonite upon exposure. Where the 
BENTONITE DEPOSITS 45 

FIGURE 10.-Popcornlike clay bloom as shown on outcrop of Soap Creek bentonite bed in 

SEI_4 sec. 15, T. 7 S., R. 32 E. 

beds pass under more than a few yards of cover, such colors are ordinarily supplanted by the shades of bluish-gray or white that prevail in subsurface sections of the light-colored material. The discoloration is an effect of weathering. Any concomitant discoloration 

of the darker clay that forms the uppermost parts of 1nost beds is so 

indistinct as to have escaped observation. 

In the Black I-Iills district, weathering of the bentonite is associated 

with an apparent enrichment of its valuable colloidal properties. 

Bentonite that is under more than 30 or 40 feet of overburden, and 

accordingly has the bluish-gray color of unweathered material, is 

considered, as a rule, to be less highly colloidal than the more shallow 

material, which is largely discolored. The data at hand do not reveal 

the extent to which this rule n1ay be applicable within the Hardin 

district. 

Experin1ents performed by ~1argaret D. Foster, of the U.S. Geological Survey (informal communication, 1948), on samples from the 

Black Hills district that were collected for the present study, confirm 

the popular view that the discoloration of the weathered bentonite is 

due chiefly to oxidation, from the ferrous to the ferric condition, of 

small amounts of iron in the clay. Undiscolored bentonite that was 

weathered experimentally under laboratory conditions over a period 

of 11 months became distinctly discolored as the iron rapidly oxidized. 

Weathering during the 11-month period also resulted in an appreciable increase in the dilatancy of the bentonite. 
46 BEN'l'ONITE DEPOSITS IN MONTANA AND WYOMING 

MINING 

A small bentonite-processing plant (the only one in ~fontana) has 

}Jeen operated by the Wyotana Mining Co. intermittently since 1946 at 

Aberdeen siding, 6 miles south of \iVyola. The fuel used at this plant 

.is crude oil. The clay, after passing through a rotary drier, is mechanically pulverized and packed in 100-pound paper sacks. The clay is 

supplied from the company's two small strip mines in \V"yoming, one 

on the north side of Gay Creek and the other on the west side of West 

Pass Creek; both n1.ines are close to the :Montana State line. 

If plans materialize for the construction of a large dam across the 

canyon of the Bighorn River, 35 miles south of Hardin, Mont., electric 

power may become available for openttion of processing plants. 

Although bedrock containing thick bentonite deposits crops out 

extensively in this district, the deposits are obscured in many places by 

surficial debris. Trenching or hand-boring is generally required for 

accurate measurement of their thickness and for sampling the clay. 

The beds of minable thickness, as already stated, are interspersed 

among strata that belong to Cretaceous formations ranging fron1 the 

Thermopolis shale upward to the Bearpaw shale. Nearly all the 

bentonite that has been mined in the principal producing districts of 

\Vyoming and South Dakota has been taken from the Clay Spur 

bentonite bed, just below the top of the Mowry shale. This bed is 

believed to have been recognized in the Hardin district, but it is not 

present here in situations where mining would be profitable; consequently, the possibility of producing bentonite in this district must 

depend almost entirely on the potentialities of other beds. Two beds 

of the Cody shale are especially thick and favorably situated for mining along parts of their outcrops-one of these beds is in the Belle 

Fourche shale member, and the other is in the Greenhorn calcareous 

member. All the bentonite mined by the \iVyotana Mining Co. has 

been taken from the Soap Creek bed of the Belle Fourche member. 

The bedrock above and below almost all the bentonite beds is shale. 

The bedrock immediately above the beds can in general be removed by 

stripping with bulldozers or other types of excavating equipment. 

The hard, chertlike, silicified shale of the floor below some beds should 

facilitate loading the bentonite with drag-lines or power shovels without admixture of the underlying n1aterial. Presumably, the large 

calcareous concretions (figs. 7 and 8) occurring in some beds would not 

give serious trouble. 

A few of the beds are persistent and can be traced for considerable 

distances, but the thickness of any given bentonite bed is by no means 

uniform from place to place, as shown by the sections recorded elsewhere in this report, which give detailed descriptions of the bentonite 
BEN'l'ONITE DEPOSITS 47 

de.posits by township and range. Some of the beds are lens-shaped 

deposits, which crop out locally with great thickness but which pinch 

out completely within a few miles. In some outcrops the typical, 

light-colored bentonite passes laterally into dark bentonitic shale, 

which grades, in turn, into shale similar to that of the enclosing 

bedrock. 

Outcrops of bentonite beds comparable in thickness with deposits 

mined in the Black Hills district are mapped on plate 1 in relation 

to their geologic structure. For several of these beds, the approximate 

areal extent of bentonite deposits under thin overburden also is indicated. For these beds, much of the bentonite lying between the line 

of outcrop and a roughly-sketched subsurface line, along which the 

overburden upon the deposits is estimated to be 30 feet deep, is considered to be suitably situated for strip-mining. The shallow deposits, 

thus outlined on plate 1, underlie belts of land that tend to be elongated approximately in the directions of strike of the beds. These 

belts vary in width because of changes in the dip of the bentonite beds 

and because of irregularities of the land surface. ...._1\_ccordingly, the 

belts underlain by shallow deposits tend to be widest, and tend to offer 

the most promising strip-mining sites, where the beds are horizontal, 

or nearly horizontal, and where the slope of the land is gentle. 

The dip of the strata is prevailingly gentle in several wide tracts 

along the axial portions of folds. 'Vithin some of these tracts, thick 

bentonite beds crop out in areas of n1any acres in which the land slopes 

gently, as, for example, in some parts of the syncline extending across 

T. 7 S., R. 32 E. Here, they are under a light cover of the superjacent bedrock, which consists generally of soft shale that could easily 

be removed; in such areas large amounts of bentonite could be mined 

by stripping. However, in some belts on the flanks of folds, the bentonite beds are steeply inclined, as, for example, near the base of the 

Big Horn Mountains in T. 5 S., R. 30 E. Here, beds could be stripmined only in narrow zones along their outcrops and would yield 

relatively little bentonite. 

Bentonite beds of this region range in thickness from a few inches 

to 1nore than 15 feet. Locally, one bed in the Hardin district (fig. 6) 

attains a thickness of ±5 feet because of flowage, and unusual thicknesses in a few other localities 1nay also be a result of flowage or 

swelling. In the present work, mn,phasis has been placed on beds 

comparable in thickness and quality to the Clay Spur bentonite bed 

in the Black Hills district of vVyoming and South Dakota, where 

the thickness of this bed ranges from 11j2 to 5 feet. As a rule, beds 

averaging less than 3 feet in thickness cannot be mined profitably, 

and attention has been restricted largely to beds with thicknesses 

averaging 3 feet or 1nore, among which are those mapped on plate 1. 
48 BENTONITE DEPOSI'l,S IN MON'l,AN A AND WYOMING 

Thinner beds have been 1neasured and sampled at some places, but 

these beds are not included in the following tonnage estimates. 

Within the area studied, beds of minable thickness lying in belts 

more than 50 feet wide under less than 30 feet of overburden are 

estimated to contain about 110 million short tons of bentonite. In 

computing this figure, the bulk density of crude bentonite, with its 

large moisture content, was assumed to be 100 pounds per cubic foot. 

The estimated total tonnage is distributed among the townships 

(pl. 1) as follows: 

Short tons 

Totvnship (thousands) 

T. 1 S., R. 35 E________________________________________ 7, 600 

T. 2 S., R. 35 E ________________________________________ _ 

T. 3 S., R. 31 E ________________________________________ _ 

T. 3 S., R. 32 E ________________________________________ _ 

T. 3 S., R. 35 E _______________________________________ _ 

T. 5 S., Rs. 33 and 34 E _________________________________ _ 

T. 6 S., R. 32 E ______________________________________ _ 

T. 6 S., R. 34 E·----------------------------------------

'1:. 7 S., R. 32 E _______________________________________ _ 

T. 7 S., R. 33 E _______________________________________ _ 

T. 8 S., R. 33 E _______________________________________ _ 

T. 9 S., R. 34 E ________________________________________ _ 

T. 9 S., R. 35 E _______________________________________ _ 

T'. 58 N., R. 88 W. (Wyoming)--------------------------

9,750 

9,600 

1,800 

1,400 

lG,OOO 

2,750 

300 

9,400 

15,700 

17,850 

12,850 

4,000 

2,000 

Total ______________________________________________ 110,000 

How much of the bentonite reserve covered by the foregoing estimates conforms to present specifications for foundry clay and drillingmud material can be determined only in a general way from data 

based on the small number of samples tested. Results of empirical 

tests of various physical properties of 94: sa1nples (table 1) nevertheless indicate that n1uch material of potential value is included. 

Apparently, all the beds contain some bentonite that is suitable for 

use as foundry elay and the reserve tonnage of such material is believed to be large. Some of the bentonite of this district also could 

be used as an ingredient of drilling mud, but most of it is not comparable in quality to the best grades of drilling clay mined in the 

Black Hills district. Part of the relatively small tonnage of whiteground bentonite produced to date by the Wyotana Mining Co. has 

been sold for use as filler for paper. Much of the bentonite of the 

various deposits described in this report would also be suitable for 

sealing reservoirs, irrigation canals and dams against leakage of 

water, and no doubt much of it could be adapted to a great variety 

of useful purposes, including the removal of impurities from public 

water supplies. It is improbable, however, that any of it would be 

suitable for use as a catalyst in petroleum refining or as a filtering 
BENTONITE DEPOSITS 49 

and decolorizing medimn for mineral and vegetable oils; supplies for 

these purposes are rarely, if ever, found in deposits of bentonite of 

the gel-forming types. 

The outlook for utilization of bentonite from the Hardin district is 

thus thought to depend primarily upon the practicability of mining 

large tonnages of this material for use as sand-bonding clay, or for 

uther uses that do not involve all of the essential properties of highgrade drilling clay. It is perhaps worthy of eomment that exploitation of the deposits of this distriet in eompetition with those of the 

bentonite-producing districts in the Black Hills and in southern 

Wyoming would be ham.pered somewhat by the greater cost of transporting the clay 200 to 300 miles farther to the foundries and steel 

works of the middle-western and eastern States; this disadvantage 

is likely, moreover, to continue for many years tu come, inasmuch as 

the reserves of foundry clay of the Black Hills and other bentoniteproducing distriets of the western interior region will probably last 

much longer than their reserves of drilling-mud bentonite. Much of 

the bentonite of the Hardin district n1ay nevertheless eventually be 

developed for use as foundry elay. 

As compared with its sand-bonding possibilities, the outlook for 

use of bentonite from the Hardin district as drilling-mud material 

appea,rs unpromising. Although the reserves are large the quality of 

nearly all of it, with reference to this use, is medium or low, and while 

much of it could no doubt be beneficiated to meet con1mercial specifications, the cost of production and refinement would make competition with other districts difficult. Nevertheless, the prospect for 

development of drilling-mud supplies will undoubtedly improve as 

the reserves of "com1nercial bentonite" in other districts become 

depleted, and, unless unforeseen changes in drilling technology 

w·eaken the demand, it appears probable that some deposits, in addition to those 1nined by the Wyotana :Mining Co., will eventually be 

developed for use in drilling fluids. 

The Soap Creek bed contains the largest accessible reserve of 

bentonite of this district that is suitable for use as foundry-sand 

bonding clay; other deposits that 1nay eventually be n1ined for this 

use are present in beds 111, R, U ?, Y, and lV. The bentonite that 

appears best suited for use in drilling muds occurs in the Clay Spur 

and Soap Creek beds and in bed TV. Drilling-mud bentonite of good 

quality is also present in several other beds-chiefly in the Thermopolis and Mowry shales; however, in this district, beds of such 

material crop out chiefly in zones of steeply dipping strata and where 

the topographic configuration is comparatively rugged. Because 

most of the beds are therefore covered by thick overburden, they 

cannot be considered as potential sources of cmnmercial supply. On 
50 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

following pages, further comments are offered concerning the potentialities of some of the principal bentonite beds for production of 

material to be used as foundry-sand bonding clay, as rotary drillingmud material, and for a few other purposes. 

UTILIZATION 

FOUNDRY-SAND BONDING CLAY 

Tempered mixtures of molding sand and clay that possess green 

compression strengths of at least 8 pounds per square inch (psi), 

and con1pression strengths of 50 psi. after drying, are comn1only 

regarded as suitable for preparing n1ost types of molds used by 

foundries and steel works in manufacturing metal castings. Measurements of the green and dry strength of many such mixtures in which 

the clay fraction was bentonite frmn the Hardin district (table 1) 

compare favorably with figures obtained in testing bentonites from 

the northern Black Hills district for use as bonding clay. In this 

connection, figures relating to a sample of commercial bonding bentonite from the latter district are included in table 1 for comparison. 

However, "conunercial" bonding clay is not susceptible of hard-andfast definition, and the commercia1 clay cited in table 1 does not 

represent a standard. Accordingly, samples whose test figures differ 

widely frmn those given for the elay cited may nevertheless represent 

valuable bonding-clay deposits. 

Perhaps a more satisfactory basis for cmnparison is provided by a 

smn1nary of significant bonding-test figures relating to the Hardin 

district (fig. 11-B) in comparison with those obtained in testing 49 

samples from the Clay Spur bentonite bed of the Black Hills district 

(fig. 11-A ), which is the chief source of a large tonnage of bentonite 

sold yearly as foundry-sand bonding clay. The figures for these 49 

samples involve a fairly broad representation of the various bondstrength characteristics of bentonites mined in the Missouri River 

basin for use by foundries and steel works. Significant circmnstanees 

brought out by this tabular summary are: ( 1) a wider range of figures relating to the bond-strength eharacteristies of samples from 

the beds in the Hardin district as compared with the eorresponding 

figures for the Clay Spur bed in the Black Hills distriet, and (2) the 

proportion of samples frmn the Hardin distriet having green strengths 

of 11 psi or more, and dry strengths of 50 psi or n1ore; this is nulCh 

higher than the proportion for the Black Hills district. Several of 

the samples having low dry strength proved to have exceptionally 

high green strength, and several of those having unusually high dry 

strength possessed only low green strength. The presence of both 

types of bentonite in the Hardin district 1nay enhance the eeonomic 

potentialities of both, inasmuch as blending can be considered. Also, 
Green compression strength (psi!, 

com puled on ba- sis of 1.4 percent 

tempering water 

BENTONITE DEPOSITS 

Dry compression strength (psi) computed on basis of 2 percent tempering water 

Summary of data on 49 mixtures containing bentonite from northern 

Black Hills distnct.Average strength (pSI): green, 9.5; dry, 56.7 

Green compression strength (psi 1, computed on ba- SIS of 1.4 percent tempenng water 

13.0--13.9 

12.0--12.9 

10.0--10.9 

A 

Dry compression strength (psi) computed on basis of 

2 percent tempering water 

Summary of data on 94 mixtures containing bentonite from the Hardin district 

B 

EXPLANATION 

Both green and dry strength greater than usual minimum requ~rements 

for foundry sand bonding clays 

m1ned in Missouri River basin 

Green strength adequate but dry strength less than generally required 

Dry strength adequate but green strength less than generally requ~red 

11111111111111 

51 

FIGURE 11 .. -Test data on tempered mixtures of molding sand from Illinois, with 

bentonite of (A.) 49 samples from Clay Spur bentonite bed in northern Black Hills district and (B) of 94 samples from various beds in Hardin district. 

both the green and dry strength of many sa1nples would be much 

greater if 6 percent by weight of elay were used instead of only 4 

percent, which 'vas the proportion used in all of the bonding tests 

performed in this investigation. 

ROTARY DRILLING-MUD MATERIAL 

In preparation of rotary well-drilling fluids, which has become a 

specialized field of engineering, various types of clay are mixed with 

other 1naterials. Bentonite of the Missouri River basin that has high 

dilatancy and conforms to certain specifications relating to viscosity, 

yield, gel strength (thixotropy), wall-building characteristics, and 

"grit"- or particles of nonclay 1ninerals, many of which are dark-is 

n1nch in demand for use as one of the ingredients. Tests for all of 

these properties (table 1) indicate that some of the bentonite of the 

Hardin district is suitable for use in the preparation of drilling mud. 
52 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Approximately 900,000 short tons of bentonite-about two-thirds 

of which was sold as drilling-mud 1naterial-was mined in 1952 from 

the Clay Spur bentonite bed in the Northern Black Hills district. 

Of 49 samples of bentonite from that district, secured during the present investigation and tested at Urbana, Ill., all but two samples swelled 

at least to 20 1nilliliters (ml) when 2 gran1s were added slowly to distilled water; these two samples were further tested for use in drilling muds. For these san1ples the computed yield (number of barrels of slurry ·with a viscosity of 15 centipoises that can be prepared 

fron1 1 short ton of clay) ranged from 55 to 125 bbl, with an average 

of 92.8 bbl. In the wall-building tests, the least, and therefore the 

n1ost favorable, water-loss volume (1neasured in milliliters of filtrate 

during a 30-minute period from a 6 pe.rcent suspension of clay in distilled water under a pressure of 100 psi) was 13.4 ml; the greatest 

\vater-loss volume, and therefore the least favorable, was 35.5 ml. 

Producers of clay that is to be shipped from points in the Missouri 

River basin for use as drilling-mud material commonly employ ruleof-thumb specifications, whereby the computed yield must be at least 

90 bbl and the filtrate must be no more than 16 1nl. However, a prodnet conforming to these requirements is often prepared by adding 

''high-gel" bentonite, or s1nall amounts of chemical reagents, to material that yields much less than 90 bbl; some of the material mined in 

the Black Hills district is given such treatment. The feasibility of 

beneficiating the bentonite of some of the deposits in the Hardin district 

might be worthy of investigation. 

CLAY SPUR BENTONITE BED 

Five samples of bentonite showing marked dilatancy that were 

obtained fron1 the light-colored portion of the Clay Spur bentonite 

bed have been tested for use in drilling fluids (table 1). The computed yield for these sa1nples ranged frmn 95 to 67 bbl per ton, and 

the 30-minute wall-building filtrate volume ranged from 14.2 to 17.8 

1nl. A sample from sec. 26, T. 6 S., R. 32 E., with a yield of 92 bbl 

and a water-loss of 14.2 ml, is judged to be one of the best drillingInud materials among the samples from this district listed in table 1. 

Some of the bentonite of the Clay Spur bed of this district also confornls to specifications for foundry-sand bonding clay that are employed elsewhere in the Missouri River basin. However, this bed lies 

mostly in positions unfavorable for strip-mining, and it contains a 

minable reserve of only a few hundred thousand short tons. 

SOAP CREEK BENTONITE BED 

The Soap Creek bentonite bed contains a 1nuch larger minable 

reserve than any other bed in the district. Tests of the foundry-sand 
BENTONITE DEPOSITS 53 

bonding efficiency of the bentonite of 23 samples from this bed 

showed green cmnpression strength ranging from 7.7 to 12.6 psi when 

1.4 percent of tempering water is present in the core mixture and 

showed a dry compression strength ranging from 35 to 78 psi with 

2.0 percent of tempering \Vater before drying. This bed is believed to 

contain a very large tonnage of bentonite that is suitable for use as 

foundry clay. 

All of the bentonite that has been mined by the vVyotana Mining 

Co. in Sheridan County, vVyo., close to the Montana State line (pl. 1), 

has come from this bed, and part of this material has been sold for use 

in drilling muds. Of 15 samples from the lower, light-colored part of 

the bed, 14 exhibit marked dilatancy and have been tested for use as 

mud 1naterials (table 1). The yield of these samples ranges frmn 80 

to 45 bbl per ton, with an average of 60 bbl; the \Vall-building filtrate 

volume ranges frmn 17.2 to 35 1nl, with an average of 22.2 ml. Judged 

as drilling-n1ud material, the average quality of the bentonite in the 

Soap Creek bed probably is inferior to that of the Clay Spur bed of the 

northern Black Hills district. However, inasmuch as bentonite frmn 

this bed has actually been n1ined in the Hardin district and sold as 

drilling clay, it is possible that detailed testing will reveal some better drilling clay in the Soap Creek bed than is indicated by the l! 

tests listed in table 1. 

Much of the lower, light-coloi·ed portion of the Soap Creek bed is 

nearly white when it is finely ground. Presumably, nulCh of this Inaterial would be suitable for use in pharmaceutical preparations and 

in other products requiring white clay for their manufacture-such 

as certain types of papm· filler, for which part of the vVyotana Mining Cmnpany's production has been sold. 

BED M 

The bentonite of bed Irl, which contains large minable tonnages in 

the southern half of the distriet, is chiefly clay of low or 1noderate 

dilatancy that typically contains relatively abundant grit and is discolored by impurities consisting largely of shale and organic matter. 

Consequently, this bed appears to eontain very little~ if any, material 

that conforms to the regional specifications for commercial drillingmud bentonite. \Vhile 1nuch of the material of bed Irf appears to be 

suitable for use as foundry-sand bonding clay, it is considered to be 

generally inferior for this purpose to the bentonite of the Soap Creek 

bed (table 1). 

BED R 

A sample of bentonite having low dilatancy and representing a 

considerable tonnage contained in a bed (tentatively correlated with 

bed R) that crops out in the area west of the Little Bighorn River 
54 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

shows exceptionally high green emu pression strength (table 1). The 

material of this deposit, which lies only D miles from the Chicago, 

Burlington and Quincy Railroad, possibly could be mixed with clay 

that is deficient in green bonding strength to produce marketable 

foundry clay at low cost. 

BED U 

Several hundred thousand tons of accessible bentonite are eon~ 

tained in a bed, tentatively correlated with bed U, that crops out in 

T. 9 S., R. 35 E. High to moderate dry compression strength is shown· 

(table 1) by a single sample, which is bentonite of low to moderate 

dilatancy. This bed is advantageously situated for mining because 

of its nearness to the processing plant of the vVyotana Mining Company and to the Chicago, Burlington and Quincy Railroad. 

BED V 

The bentonite of bed V conforms, in part, to present requirements 

for foundry-sand bonding clay~ but 1nost of it is unsuitable for commercial drilling-mud material. The bed contains large tonnages of 

bentonite in the area southeast of Hardin. The deposits are close to 

the railroad and are otherwise advantageously situated for 1nining. 

Presumably, stripping and loading would not be seriously hampered 

by the many large calcareous concretions they contain (figs. 7 and 8) . 

BED W 

Part of the large deposit of bentonite in bed TV has adequate green 

and dry cmn,pression strength; ho,vever, it also contains material that 

is deficient in green strengih and high in dry strength, as well as 

smne that is high in green strength and deficient in dry strength 

(table 1) . Careful control in mining would be necessary to maintain 

a unifonn product. The bed crops out (fig. 9) close to the Chicago, 

Burlington and Quincy Railroad, and some of the bentonite it contains could probably be 1nined at low cost and sold as foundry-sand 

bonding clay. Of five samples obtained from this bed, three were 

sufficiently dilatant to 'varrant testing for use in drilling fluids. The 

yield of these samples ranges from 86 to 78.7 bbl per ton, and the 

wall-building filtrate volmne ranges fron1 17.5 to 23.6 ml. For use 

as drilling-clay, the bentonite of these three samples compares favorably with that of the Soap Creek bentonite bed, but is of mediocre 

quality in comparison with most of the clay shipped from the northern 

Black Hills district and other localities in the Missouri River basin. 

DESCRIPTION BY TOWNSHIP AND RAN.QE 

Most of the 1neasured sections set forth in the following descriptions were obtained by boring with an auger, and the numerical order 
BEN'fONI'fE DEPOSITS 55 

in which the bentonite and associated rock materials are listed in each 

section corresponds to the dow1rward sequence in which they were 

penetrated in boring. Asterisks ( *) indicate san~pled materials for 

which test data are given in table 1. 

T. 1 N., RS. 33 AND 34 E. 

About 27 square miles have been 1napped (pl. 1) in the southern 

parts of these two townships. lTnimproved roads lead southward to 

the County road between Hardin and Sarpy. 

The land surface within this tract is moderately dissected as a 

result of long-continued erosive action by vvatercourses draining to 

Bighorn River, \Vhich flows along the \Vest side of the tract; the 

largest of these are Nine ~1:ile Coulee, which extends in a northwesterly direction across the central part, and Dry Creek, which cuts 

across the southwestern corner. Flood plains, underlain by Recent 

alluvium, occupy about 11/2 square miles along the river and along 

Dry Creek; older alluvial deposits underlie terraces, occupying a total 

of about 2 square miles, close to the river and near the divide between 

Nine :!\file and Dry Creeks. 

The tract is situated on a broad northeasterly-plunging anticlinal 

nose. The bedrock strata clip in a northerly direction nearly everywhere on the northwest side o£ the axis of this fold, and the di,p is 

prevailingly east-southeastward on its southeast side. The oldest 

strata exposed are those of the Telegraph Creek member of the Cody 

shale, which crops out in the south-central part of the tract; the 

youngest, in the lower part of the Hell Creek formation, crop out in 

the northeastern corner. Four intervening stratigraphic units-the 

uppe.rmost two members of the Cody shale, the tTudith River formation, and the Bearpaw shale-crop out in a broad, roughly arcuate belt 

that is convex toward the northeast; this belt extends from the 

western side of the tract to its southeastern corner and crosses the 

anticlinal axis in section 28. At a number of places on both sides of 

the plunging anticlinal axis, this belt is offset by normal tear faults, 

downthrown on the side toward the axis. Thus, the plunging nose 

is essentially a great arch that is broken into a series of down-faulted, 

wedge-sha,ped bloc.ks or grabens, one ~within another. 

Beds of bentonite oecur in this tract in the uppermost part of the 

Cody shale and in the Bearpaw shale. Two beds in the Cody-bed R, 

in the unnamed sanely member that is equivalent to the Eagle sandstone, and bedS~ at the base of the Claggett shale member-are comparable in thickness to the same two beds where they were 1neasured 

farther south, in T. 1 S., R. 3± E. ( q.v.). As exp'osed within this 

tract, however, the bentonite contained in both be.cls appears to have 

poor colloid-fonning properties and is probably of little value. Both 
56 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

beds, moreover, are cut by numerous faults, and, because they crop 

out in a belt wherein the strata clip rather steeply, their content of 

bentonite under overburden sufficiently light to favor strip-mining is 

restricted to narrow zones adjacent to their outerops. 

While bed lV, in the Bearpaw shale, has not been recognized within this tract, it is probably present as material that swells little and 

consequently does not crop out conspicuously; it may consist of bentonitic shale-as in T. 1 S., R. 35 E., immediately to the southeastor it may be represented only by shale containing little or no bentonite. 

Bed ...Y, in the Bearpa:w shale, crops out locally in sees. 26, 27, and 

35, R. 3-! E., but elsewhere in the tract this bed and the enclosing strata 

are eoneealed by soil. Small tonnages eould be mined in section 26, 

where the bed contains 4 or 5 feet of light-colored bentonite overlain 

by a thick zone of bentonitic shale. North of the fault that crosses 

the southeast corner of section 27, however, the bed and the enclosing 

strata dip steeply and crop out only on steep hillsides; consequently, 

within a few feet of the surface the overburden is so thick that stripmining would be impracticable. 

T. 1 S., RS. 30, 31, 32, .A.ND 33 E. 

A tract of about 86 square miles lying within these townships (pl. 

1) extends eastward from the center of T. 1 S., R. 30 E., to the east 

side of T. 1 S., R. 33 E.; the town of Hardin lies within the eastern 

end of this tract. The Chicago, Burlington and Quincy Railroad and 

U. S. Highway 87 pass eastward across the northern part; a paved 

highway leads south from Hardin, and unimproved roads and trails 

give access to various other parts. 

Bighorn River :flo\YS northward within the eastern margin of the 

tract. A divide extends in a northerly direction across the western 

half; beyond this divide the surface run-off drains westward into 

Fly Creek, a northward-flmving tributary of Yellowstone River, and 

nearly all of the land on the near side of the divide drains eastward to 

Bighorn River through vVilliams Coulee, Paritsa Creek, and their 

nmnerous branches. A small area extending along the south boundary 

of the tract lies within the basin drained by Two Leggin Creek, another minor affiuent of the Bighorn. 

Reeent alluvium underlies the broad flood plain along Bighorn 

River and fhe narrower plains along Fly Creek, "'\Villiams Coulee and 

Paritsa Creek. Older alluvial deposits occupy several areas of benchland within the eastern third of the tract; together, these older deposits represent a flood plain upon which Bighorn River flowed at 

some time during the Pleistocene epoch. Deposits that originated 

still earlier in the Pleistocene are present in a small area about 500 feet 

higher, in SE:IJt sec. 28, T. 1 S., R. 31 E. 
BENTONITE DEPOSITS 57 

All of the bedrock exposed 'lvithin this tract belongs to the CarlileNiobrara member of the Cody shale, except in small areas on the east 

side of Bighorn River, where undivided upper Cody beds crop out, 

and along the northern 1nargiu, where scattered areas aggregating 

about 6 square miles are occupied by outcrops of the Telegraph Creek 

member. 

The tract lies on the northern side of Two Leggin uplift, a broad, 

anticlinal nose, whose axis plunges at a low angle northward across 

the eastern half ofT. 1. S., R. 31 E. Three small normal faults have 

been mapped, two of which are in the southern part of that township; 

the third is in sec. 9, t· 1 S., R. 32 E. 

Bentonite beds P and Q, in the upper part of the Carlile-Niobrara 

mmnber of the Cody shale, crop out in the southeastern part ofT. 1 S., 

R. 32 E. These bentonite beds are well exposed where the gentlydipping strata enclosing thmn are dissected by local drainage. The 

thickness of each of the two beds averages more than 2 feet, and their 

clip is everywhere less than 2°; because of the large shale content, 

however, the swelling capacity of the bentonite is so low that it probably is unsuitable for the uses for which most of the bentonite produced in the Missouri River basin is marketed. Also, because beds 

P and Q crop out 1nostly on rather steep hillsides, relatively little 

of the material they contain lies under overburden sufficiently thin to 

favor strip mining. 

Outcrops of the bentonite beds can be reached over trails leading 

southward from U. S. Highway 87, at a point located a quarter of 

a n1ile southwest of the overhead bridge near Toluca, and from the 

railroad at Rowley siding. 

T. 1 S., RS. 34 AND 35 E. 

The rectangular tract comprising these two contiguous townships 

(pl. 1) centers about 7 1niles · east of the town of liar din. Bighorn 

Hiver, which meanders northward along the west margin of the tract, 

is joined near Hardin by Little Bighorn River and, farther north, by 

Dry Creek. U. S. Highway 87, and the Chicago, Burlington and 

(~uincy Railroad, cross the Bighorn on bridges about 1 mile east of 

Hardin and continue southeastward up the valley of the Little 

Bighorn. A graveled county road, branching o:ff at a point near the 

highway bridge, extenc~s across the northern part of the tract; uninlproved roads and trails give access to various other localities. 

Extending northward across the eastern end of the tract is the 

drainage divide between Bighorn River and Tullock Creek-a minor 

tributary of Yellowsto.qe River that flows northward across the next 

township to the east. vVest of this divide the run-off frmn the moderately dissected land surface is carried to Bighorn River by means 

of Dry Creek and its numerous branches. 
58 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Surficial materials deposited in Pleistocene and Recent time on the 

flood plains of Bighorn and Little Bighorn Hivers occupy a total of 

about 11 square 1niles in the southwestern part of T. 1 S., R. 34 E.; 

Recent alluvium also underlies areas along Dry Creek and several 

of its branches. 

_A_ subcircular dome "\vith a closure of less than 100 feet is centered 

in the south-central part of T. 1 S., R. 3-± E. The strata throughout 

the tract generally clip outward fro1n the apex of this dome. The 

angle of dip is less than 2° except in the middle of the tract, where, 

in a northward-trending belt of rather steeply tilted strata, the dip 

is as much as 22°. 

The Hell Creek formation, which is the youngest and highest bedrock unit exposed, crops out along, and east of, the drainage divide 

that crosses the eastern end of the tract. The oldest unit-the CarlileNiobrara member of the Cody shale-crops out in the bluffs along 

the southwestern side of the flood plain of Little Bighorn River, and 

it probably directly underlies the Recent alluvial deposits northeast 

of the bluffs. The Telegraph Creek 1nmnber of the Cody is exposed 

in the vicinity of the Little Bighorn and as far east as the central belt 

of steeply inclined rocks. Cropping out within that belt are the 

higher 1nembers of the Cody shale and the Judith River formation; 

the Bearpaw shale is at the surface in the broad area between the top 

of the Judith River and the base of the Hell Creek formation, which 

lies about 3 miles farther east. 

Detailed stratigraphic sections, appearing in this report under the 

heading "Upper part of the Cody shale" were measured near the 

middle of this tract, half a mile south of the Hardin-Sarpy road. 

The section given for the J uclith River formation was measured in 

sections 7 and 8, north of the road; the section for the Bearpaw was 

measured farther east, also close to the road. 

The principal bentonite beds exposed in this tract belong to the 

upper members of the Cody shale and to the Bearpaw shale. The 

principal bentonite beds in the Cody include bed R, in the unnamed 

shale member equivalent to the Eagle sandstone, and in beds S, T, 

and U, in the Claggett shale mmnber. Tests of samples from point 1, 

in sec. 18, T. 1 S., R. 34 E., indicate that beds R, S, T, and U contain 

some good foundry clay and that some of the material of bed R 1night 

be usable as drilling clay. However, no attractive mining sites occur 

along their outcrops because the beds are largely covered by surficial 

detritus and dip to the east as 1nnch as 22°. The following section 

of beds R, S, T, and U and the associated bedroek at point 1 is partly 

tl, repetition of data given in the stratigraphic seetion of the upper 

men1bers of the Cody shale, appearing else"\-vhe.re in this report. 
BENTONITE DEPOSITS 59 

Section measured at point 1, N E% sec. 18, T. 1 S., R. 34- E. 

Thickness 

All beds Pentonite only 

1\faterial JJ'eet Inches Feet Inches 

1. Shale, bentonitic, brown _____________ 0 6 I 2. *Bentonite, gray, impure, sticky when I 

Red U _____ t 3_ 

wet _____________________________ 1 10 } *Bentonite, cream-colored, granular, 2 9 

with much aragonite ______________ 11 

4. Shale, dark-gray, bentonitic in part ___ 31 0 

f 5. *Bentonite, yellowish-gray, granular ___ 6 Red T ____ __ l } 11 

6. *Bentonite, yellow, granular __________ 1 5 

7. Shale, gray, bentonitic in part ________ 25 0 

BedS ______ { 8. *Bentonite, yellow, granular: (base of 

Claggett member) _________________ 2 10 2 10 

9. Shale, dark-gray ____________________ 88 6 

10. *Bentonite, gray, granular_ __________ : } 11. *Bentonite, grayish-cream-colored, 2 9 

granular, with some aragonite ______ 2 

12. Shale, gra,y, hard ___________________ 1 6 

13. *Bentonite, brownish-gray, granular ___ 3 

-~ } Bed R _____ 14. *Bentonite, cream-colored ____________ 3 6 4+ 

115. Bentonite (not penetrated in boring) __ ? 

16. Shale, silicified, dark-bluish-gray ______ 1 0 

17. Shale, gra,y, bentonitic _______________ 5 0 

18. Bentonite, yellowish-cream-colored, 

grading: upward into gray __________ 6 0 6 0 

19. Shale, dark-gray. 

The Bearpaw shale contains numerous bentonite beds, but only the 

middle unit, which is eonsidered to be approximately equivalent to 

the Monument Hill bentonitic mmnber of the Pierre shale of the 

Black Hills region, contains deposits thick enough to represent a 

possible source of supply. This middle unit, in which beds V, W, 

and X are the thickest and 1nost prominent bentonite beds, crops out 

along a west-facing esearpment that extends southwaru from sec. 5 

to sec 31, T. 1 S., R. 35 E., crossing the Hardin-Sarpy road in the 

SE:IU sec. 8. These three bentonite beds clip about 3° to 5° eastward, 

except in sec. 31, where the dip is much steeper. Soft sandy shale 

forms the overburden upon all of the bentonite beds. 

Bed V averages nearly 20 feet in thickness in sections 17, 20, 29, 

30, and 31, and it forms striking outcrops that are littered with large 

calcareous concretions derived frmn the bed. In section 8, bed V 

thins considerably and seems to disappear farther north in section 

5, where its horizon is marked only by the characteristic large concretions. Within this tract, the reserve of minable bentonite from bed 

V is estimated at 7,550,000 short tons; it is believed that the large 

concretions would not seriously interfere with mining operations. 

\Vhen tested for use by foundries as sand-bonding clay, a composite 

sample of materials 2 to 4 of bed V, obtained at point 2 (pl. 1), 

347624-56--5 
60 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

showed high dry compression strength; a cmnposite of materials 1 

and 2 fron1 point 6 tested high in both green and dry strength. The 

sampled n1aterial from point 6 might also be adaptable for use as welldrilling clay, even though its quality seems inferior to that of the 

best grades of such clay 1ninecl in the Black Hills district. 

In this tract, bed lV is represented only by a bed of clark bentonitic 

shale, whereas farther south, in T. 2 S., R. 35 E., it is composed of 

light-colored waxy bentonite overlain by bentonitic shale. N otwithstanding the shaly character of bed W within this tract, the material 

it contains may be of some value because the one sample tested as 

foundry clay-material 2 frmn point 4-showed high dry compression 

strength. . 

Bed X is composed of several layers of bentonite separated by thin 

beds of soft shale. The most prominent of these layers ranges from 

6 to 7 feet in thickness in sections 9 and 15. In the southern part of 

the tract, bed X is much thinner and is largely covered by surficial 

detritus. A composite sample obtained at point 5, representing materials 5 to 7 of bed X, tested high in both green and dry strength. 

Following are 1neasurecl sections of the bentonite and associated 

rock materials cropping out at five numbered points in T. 1 S., R. 35 E. 

Sections measured in T. 1 S., R. 35 E. 

1\Iaterial 

Point 2. SE:x!SE:xi sec. 8: 

1. Shale, dark-brown, soft_ _____________ _ 

2. *Bentonite, dark-brownish-gray, granular, gypsiferous, with dark mineral 

particles; somewhat lighter colored in 

lower part _______________________ _ 

Bed y __ 3. *Bentonite, gray, cornmeal texture, with 

dark mineral particles _____________ _ 

4. *Bentonite, yellow, cornmeal texture, 

gypsiferous, with dark mineral parti- cles _____________________________ _ 

5. Shale, dark-gray, soft. 

Point 3. NW%,SWX sec. 9: 

1. Shale and sandy shale _______________ _ 

2. Bentonite, cream-colored, waxy, gypsif- erous ____________________________ _ 

3. Shale parting _______________________ _ 

4. Bentonite, dark-tan, waxy ___________ _ 

5. Bentonite, light-gray, waxy, with a few 

Bed X__ dark mineral particles and some gyp- sum _____________________________ _ 

6. Bentonite, gray, basal 2 in. limonitestained __________________________ _ 

7. Bentonite, light-green, waxy, with dark 

mineral particles __________________ _ 

8. Shale, brown. 

Thickness 

All beds Bentonite only 

Feet Inches Feet Inches 

0 5 

3 6 

3 

6 

4 6+ 

1 

2 

5 

1 

3 

9 

6 

0 

7 

3 

4 3 

1 3 

7 4 
BENTONITE DEPOSITS 61 

Sections measured in T. 1 S., R. 35 E.-Continued 

Point 4, S\V.%S\~l?{ sec. 9: 1\.faterial 

1. Shale, brownish-gray, soft ___________ _ 

B d TV { 2. *Shale, bentonitic, brownish-gray ____ _ 

e -- 3. Shale, bentonitic, (not penetrated in 

drilling) ________________________ _ 

Point 5, KE74NW~ sec. 16: 

1. Shale, gray, soft_ __________________ _ 

2. Bentonite, brown, waxy to granular_ __ _ 

3. Bentonite, brownish-yellow, granular, 

waxy in part ____________________ _ 

4. Shale, brown, soft, sandy ___________ _ 

5. *Bentonite, light-brownish-gray, granu-

·lar, waxy in part, with dark mineral 

Bed X_ _ particles ________________________ _ 

6. *Bentonite, light-brownish-gray, waxy_ 

7. *Bentonite, brownish-yellow, cornmeal 

texture _________________________ _ 

8. Shale, brown, hard _________________ _ 

9. Bentonite, brown, granular __________ _ 

10. Bentonite, light-brownish-gray, waxy to 

granular, gypsiferous _____________ _ 

11. Shale, brownish-gray, gypsiferous. 

Point 6, SE%,SE?{ sec. 30: 

(Top not exposed) 

{ 

1. *Bentonite, gray, waxy, gypsiferous ___ _ 

Bed V__ 2. *Bentonite, light-gray, waxy ________ _ 

3. Bentonite (not penetrated in boring) __ _ 

4. Shale, dark-gray, hard, siliceous, chertlike. 

T. 2 8., RS. 30, 31, AND 32 E. 

Thickness 

All beds Bentmite only 

Feet Inches Feet 

0 

6 

3 6 

1 10 6 

1 6 

3 0 

13 

4 

? 

4 } 1 

10 

Inches 

4 

10 

2 

All of T. 2 S., Rs. 31 and 32 E., together with 25 square miles 

mapped within T. 2 S., R. 30 E. (pl. 1), are here described as a single 

tract, elongated frmn west to east, having a width of 6 1niles and a 

maxinnun length of 18 miles. Unimproved roads and trails, giving 

access to various parts of this tract, branch southward frmn U. S. 

Highway 87 and westward from the St. Xavier-Hardin highway. 

A divide extends in a northerly direction across the tract, within 

its western boundary. "\Vest of this divide, the surface runoff drains 

to Fly Creek, a northward-flowing affluent of Yellowstone River; 

the east side of the divide lies within the watershed of Bighorn 

River, into which most of the runoff is carried by Two Leggin Creek 

and its many branches. 

Recent alluvimn underlies the flood plains along Fly Creek and 

along the lower part of Two Leggin Creek. Older alluvial deposits 

occupy areas of several square 1niles along the eastern 1nargin of 

the tract; these represent a flood plain on which Bighorn River 
62 BEN'l'ONITE DEPOSITS IN MONTANA AND WYOMING 

flowed at some time during the Pleistocene epoch. Some hundreds 

of feet higher, in scattered small areas of benchland elsewhere in the 

tract, are deposits that originated still earlier in the Pleistocene. 

All of the bedrock cropping out in this tract belongs to the CarlileNiobrara mmnber of the Cody shale, exeept for the outcrops in about 

1f2 square mile at the northwest eorner, which are assigned to the 

lower part of the overlying Telegraph Creek member of the Cody. 

This tract lies on the north side of the Two Leggin uplift-a broad 

anticlinal nose whose axis plunges at a low angle northward across 

the middle of the tract. Accordingly, the strata cropping out in the 

western half of the tract dip mostly in northwesterly directions, and 

those in the eastern half dip in northeasterly directions; the clip does 

not exceecl3° within this tract. 

Bentonite beds P and Q of the Niobrara shale crop out at the 

southwestern extremity of the tract, and within its northern part 

they crop out as far east as sec. 1, T. 2 S., R. 32 E. As a rule, neither 

of these beds exceeds 2 feet in thickness. Both beds are cmnposed 

of granular, gray bentonite, whose natural exposures show a moderately well-developed popcornlike clay-bloom. Both beds, however, 

are largely concealed by soil and other surficial material. 

Following are 1neasured sections of bentonite beds P and Q and 

the associated rock materials eropping out at two numbered points: 

Section measured in T. 2 S., Rs. 30, 81, and 82 E. 

Thickness 

JI.,Iaterial All beds Rentonite o-nly 

Point 7, sec. 18: Feet Inches Feet Inches 

1. Shale, soft, fissile, dark gray _________ _ 1 

Bed p __ 2. Bentonite, granular, gypsiferous, gray_ 1 10 1 10 

3. Shale, soft, dark-gray _______________ _ 7~+ 

Point 8, sec. 18: 

1. Shale, weathered, soft, dark-gray _____ _ 1 10 

{

2. Bento~ite.' granular: gray, with many 

Bed () _ _ dark .mmeral particles ___________ .- _ 

3. Bentomte, granular, gray, heavily 

stained by brown iron oxide _______ _ 

2 1 

} 2 2 

1 

4. Shale, moderately hard, dark-gray ____ _ 7~+ 

T, 2 S., RS. 33 AND 34 E. 

The rectangular tract of 72 square n1iles that cmnprises these two 

townships lies south of the neighborhood of Hardin (pl. 1). 

This tract has low topographic re.lief; Bighorn River flows northeastward across its western half, and Little Bighorn River flows 

northwestward across its eastern half. U. S. Highway 87, and the 

Chicago, Burlington and Quiney Railroad extend along the valley 

of Little Bighorn River. The paved highway between Hardin and 
BENTONITE DEPOSITS 63 

St. Xavier extends along the flood plain of Bighorn River and crosses 

the river on a bridge near the mouth of Two Leggin Creek. 

The bedrock of extensive areas within this tract is concealed under 

deposits of surficial materials, including Recent deposits occupying 

the wide flood plains along Bighorn and Little Bighorn Rivers and 

older deposits on broad areas of higher ground flanking those plains. 

The northern part of T. 2 S., R. 3± E. includes the southern flank 

of the shallow subcircular dome that centers in T. 1 S., R. 34 E. ( q. v.) 

and is separated by a shallow structural sag from a gently northeastward-dipping hmnocline that occupies the entire western half of the 

tract. In most of the remaindei· of the tract-that is, along its eastern n1argin and in 1nost of the southern part of T. 2 S., R. 34 E.-

the strata dip in east-southeasterly directions. In a monoclinal belt 

that here extends across the tract in a northeasterly direction, the 

strata dip at comparatively steep angles of as n1uch as 20°; everywhere else in the tract they eli p less than 2 o. 

The youngest exposed bedrock is that of the J uclith River fonnation, which crops out. along the monoclinal belt at the extreme eastern margin of the tract. The bedrock of all other exposures belongs 

to the Carlile-Niobrara and younger members of the Cody shale. 

These younger members-the Telegraph Creek, unnamed Eagle equivalent and Claggett-are mapped separately only in an area of about 

2 square miles east of Little Bighorn River; farther west, they are 

mapped as one unit. 

Bentonite bed R, in the unnamed member that is equivalent to the 

Eagle sandstone, is exposed for a distance of about 1 mile in sections 

6 and 7. The nearest exposure that has been measured and sampled 

in the course of the present investigation is at point 1, in see. 18, 

T. 1 S., R. 34 E. 

T. 2 S., R. 35 E. 

The southwestern corner of this township is less than 1 mile northeast of Crow Agency station of the Chicago, Burlington and Quincy 

Railroad, where an unimproved road leads to the southeastern part of 

the township; access frmn the north is provided by an unimproved 

road branching southward from the Hardin-Sarpy road. 

Little Bighorn River enters the township close to its southwestern 

corner and flows for a distance of 21h miles within its west boundary. 

Ash Creek, a branch of Tullock Creek which is a minor tributary of 

Yellowstone River, enters the township lh mile north of the southeast 

corner, bends northeastward in its central part, and leaves the township 1h mile south of the northeast corner. Extending across the 

township a short distance west of Ash Creek is the drainage divide 

between Tullock Creek and Little Bighorn River. Between this divide and the river, a westward-facing escarpment extends diagonally 
64 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

across the western half of the township. In general, the land surface 

rises gently eastward fron1 the top of this escarpment to the drainage divide and descends gently ·westward from the base of the escarpment to the river. 

Alluvial deposits occupy belts along Ash Creek and along Little 

Bighorn River and two of its small tributaries in the southwestern 

part of the. township. The bedrock dips toward the east. The unnamed 1nembers that are equivalent to the Eagle sandstone and the 

Claggett shale member crop out at the northwestern n1argin of the 

township. The Judith River fonnation, which crops out east of 

the Claggett shale 1nember extends southward as a narrow belt 

along the eastern side of the flood plain of Little Bighorn River. 

E:ast of the Judith River is a broad belt of Bearpaw shale, and, still 

farther east, the Hell Creek formation crops out in large areas along, 

and east of, the drainage eli vide. 

The principal bentonite beds exposed in this township belong to 

the uppermost members of the Cody formation and the Bearpaw 

shale. Those in the Cody include beds R and S, but these beds, as 

in T. 1 S., R. 34 E., dip steeply and are therefore believed to be of 

little value. For that reason, and because they were examined in 

detail at point 1, about 4 miles to the north, they were neither measured 

Hor sampled in this township. 

The Bearpa w shale contains numerous bentonite beds, but only the 

middle unit, which is regarded as approximately equivalent to the 

Monument Hill bentonitic member of the Pierre shale of the Black 

Hills region, contains deposits sufficiently thick to be of interest as 

possible sources of industrial supplies. The west-facing escarpment, 

already described, is a result of the greater degree of resistance to 

erosion that is offered by this middle bentonitic unit, as cmnpared 

\Yith that of the less bentonitic parts of the Bearpaw shale, both 

above and below. 

The most prominent and most resistant of the bentonite beds are 

beds 17 and TV. Bed 17 ranges from 16 to 22: feet in thickness in sections 6, 7, and 8, where its outcrop is conspicuously n1arked by large 

limestone concretions (figs. 7 and 8). Southward, bed 17 thins to less 

than 10 feet in the southern part of section 17; from section 20 

southward, both the bentonite and the characteristic large concretions 

are apparently absent. Tests of samples of bed TT, taken at point 9 

in section 6 and at point 11 in section 8, suggest the presence of 

foundry clay of fairly good quality; within this township, the bed 

:is estimated to contain approximately 4,200,000 short tons of bentonite 

under less than 30 feet of overburden. 

The outcrop of bed W (fig. 9) extends in a southerly direction 

across the township. Along most of its outcrop, this bed consists of 
BENTONITE DEPOSITS 65 

relatively pure, light-colored bentonite, with a n1aximum thickness of 

8 feet, which is overlain by a layer of bentonitic shale; in the northern 

part of section 6, the bed appears to contain only bentonitic shale. 

Tests of samples frmn point 10 in section 8, and from point 12 in 

section 20, indicate that the bed contains good foundry clay and some 

low-grade well-drilling clay. Within this township, bed W is estimated to contain about 5,550,000 short tons of bentonite under less 

than 30 feet of overburden. Attractive mining· sites occur on the 

nmnerous spurs and outliers in sections 20, 21, 28, and 29. 

Following are measured sections of the bentonite beds and assoCIated rock materials at four points in T. 2 S., R. 35 E. 

Sections measured -in T. 2 S., R. 35 E. 

llfaterial 

Point 9. SE7:1SE%: sec. 6: 

Bed lr __ {

1. 

2. 

3. 

(Roof not exposed) 

*Bentonite, gray, gypsiferous, with 

some dark mineral particles _________ _ 

Bentonite (not penetrated in boring) ___ _ 

Shale, dark-gray, hard, siliceous, chertlike. 

Point 10. NW%,SE%: sec. 8: 

1. Shale, brown, soft_ __________________ _ 

2. Bentonite, brown, impure, granular ____ _ 

3. *Bentonite, light-olive-green and gray 

B d lV interlaminated, waxy ______________ _ 

e . -- 4. *Bentonite, olive-green, waxy ____ ------

5. *Bentonite, gray, waxy ______________ _ 

6. *Bentonite, olive-green, waxy _________ _ 

7. Shale, bentonitic, brown, hard. 

Point 11. SEJ,iSW%, sec. 8: 

1. Sandstone, gray, coarse-grained, con-

{

·2. 

Bed V __ 3

_ 

4. 

5. 

taining plant remains and pelecypods_ 

*Bentonite, greenish-gray, waxy, gypsiferous, with dark mineral particles __ 

*Bentonite, gray, cornmeal texture ____ _ 

*Bentonite, light-gray, granular hard ___ _ 

Shale, dark-brown, sandy, siliceous, containing plant remains ______________ _ 

6. Shale, brown, sandy, soft. 

Point 12. SW%:NE%: sec. 20: 

1. Shale, bentonitic, brown ______________ _ 

(2. *Bentonite, brownish-gray, granular, · gypsiferous _______________________ _ 

3. *Bentonite, greenish-gray, waxy, gypBed W __ 4

_ 

5. 

6. 

siferous __________________________ _ 

*Bentonite, gray, waxy ______________ _ 

*Bentonite, light-gray, waxy __________ _ 

*Bentonite, gray, waxy ______________ _ 

,7. *Bentonite, light-gray, waxy __________ _ 

8. Shale, bentonitic, dark-brown. 

Thickness 

All beds Bentonite only 

Feet , : , Inches Feet Inches 

12 

? 

1 

1 

1 

2 

3 

6 

11 

1 

1 

2 

1 

1 

0 

2 

4 

7 

8 

2 

4 

0 

6 

8 

9 

3 

5 

0 7 

0 

9 

7 

1 

6 

4 
6G BEN'l'ONITE DEPOSITS IN MONTANA AND WYOMING 

'1'. 3 S., R. 30 E. 

The surface of the eastern half of this township, including 15 square 

miles shown in plate 1, is comparatively rough. This part of the 

township is crossed by the North and South forks of vVoody Creek, 

both of which are flanked by flood plains underlain by Recent alluvium; the alluvium also extends along the lower parts of the largest 

tributaries of these streams. 

In a small area west of a synclinal axis that extends northward 

across se~tion 34, the bedrock dips gently eastward; in the remainder 

of the area it dips gently westward frmn the Woody Creek dome, 

which has its apex in the next township to the east. The rocks exposed in this area include the uppermost part of the Belle Fourche 

shale, which erops out in about 3 square 1niles within the eastern 

boundary, and strata of the Greenhorn and Carlile-Niobrara lnembers, which crop out farther west. The exposed rocks are not known 

to contain bentonite deposits of value. 

T. 3 S., R. 31 E. 

This township, whose northeast corner is about 12 miles southeast 

of IIardin, can be reached by truck over unimproved roads and trails 

branching westward from a graveled road that crosses the next township to the east. 

The township has moderate topographic relief. Woody Creek, 

whose flood plain is underlain by Reeent alluYium, meanders aeross 

its central part. There are older alluvial deposits on stream terraces, 

the largest and highest of which are at the southern and northeastern 

margins of the township. 

The geologic structure is that of a broad dome of small closure, 

known as vVoody Creek dome, with its apex in the south-central part 

of the township. A normal fault, upthrown on its southeastern side 

with a vertical displacement of nwre than 100 feet, cuts northeastward across the apical part of this dome; a few normal faults with 

s1nall displacements are present in the north-eentral part of the 

township. 

The oldest strata exposed are those of the Mowry shale, whieh fonn 

two inliers on opposite sides of the fault that crosses the apex of the 

dome. The youngest rocks are those of the Carlile-Niobrara unit, 

which crop out in a broad belt a]ong the northern margin and in 

about ¥z square mile inside the southwestern corner of the township. 

The Greenhorn calcareous member crops out conspicuously in narrow 

belts bordering these two areas; the Belle Fourche me1nber forms 

t.he bedrock in broad areas intervening between the outcrops of Mowry 

and Greenhorn. Bentonite beds are present in the Mowry, Belle 

Fourche, and Greenhorn units. 
BENTONITE DEPOSITS 67 

The Clay Spur bentonite bed, in the topmost strata of the Mowry 

shale, is nearly horizontal as exposed in the two inliers in the central 

part of the W oocly Creek dome. According to laboratory tests of 

a sample fron1 point 16, this bed contains good fmmdry and drilling 

clay; it is only 1 foot, 7 inches thick at that point, however, and the 

topographic configuration there, and elsewhere along its outcrops, 

indicates that the bentonite lies under light overburden only in narrow zones; consequently, the reserve of 1ninable clay in the Clay Spur 

bed, as exposed in this township, is probably negligible. 

Within the township, on the north, west, and south flanks of the 

Woody Creek dmne, the Soap Creek bentonite bed crops out extensively with low dip. Thicknesses of 12 feet, 5 inches, 1neasured at 

point 14, and 10 feet, measured at point 13, are believed to be fairly 

representative of the thickness of this bed in the township as a whole; 

a sa1nple taken at the latter point gave good results when tested as 

foundry clay. North of vVoody Creek, in sections 24 and 13 and in 

the eastern half of section 14, the bed crops out in the basal part of 

a rather steep escarpment, and consequently it is present under light 

overburden in only a narrow zone. Elsewhere along its outcrop, the 

bed occurs at many sites that offer large tonnages under overburden 

thin enough to be removed easily. The total reserve available for 

mining is estimated at about 9,600,000 short tons. 

Bentonite bed L, in the Belle Fourche, 40 to 60 feet stratigraphically higher than the Soap Creek bed, also crops out extensively with 

low dip, but its outcrop in this township is largely concealed by surficial detritus. At point 15, the bed is 5 feet, 6 inches thick, and, according to tests of a single sa1nple, it contains material that could be 

used satisfactorily as foundry clay. Although the bed may offer a 

few fairly good 1nining sites along its outcrop, its reserves of minable 

material are small as compared with those in the Soap Creek bed. 

A bentonite bed, believed to be bed Jf, is present in the escarpment 

north of Woody Creek, but it has not been found elsewhere in the 

township and has not been measured or sampled. 

Following are measured sections of the bentonite beds and associated rock materials cropping out at four points in T. 3 S., R. 31 E. 

Sect£ons measured in T. 3 S., R. 31_ E. 

Thickness 

All beds Pentonite only 

~\Iaterial Feet Inches Feet Inches 

Point 13. SEX SE X sec. 8: 

1. Shale, dark __________________________ _ 11 

1 3 } 10 8 9 5 

Soap C k . {2 B t en omte, · b rowms · h -gray ____________ _ 

b:~~ _ _ _ 3.Bentonite, light-olive-green ___________ _ 

4. Shale, dark-gray, hard siliceous, cherttike. 
68 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Sections measur·ea in T. 3 S., R. 31 E.-Continued 

.AI at erial 

Point 14. SWXSE X sec. 13: 

(

1. *Bentonite, gray, granular ___________ _ 

Soap 2. *Bentonite, grayish-green, granular, with 

Creek dark mineral particles ____________ _ 

bed____ 3. *Bentonite, light-gray, waxy, with many 

dark mineral particles ____________ _ 

4. Shale, dark-gray, hard, siliceous, chertlike. 

Point 15. SW X SE X sec. 13: 

1. Shale, dark-gray. 

(

2. *Bentonite, greenish-yellow, with cornmeal texture; much selenite present_ 

Bed L ____ 3. *Bentonite, greenish-yellow, waxy, some 

iron stains in lower part; much selenite 

present _________________________ _ 

4. Shale, dark-gray. 

Point 16. SW X SW X sec. 21: 

1. Shale, dark-gray ____________________ _ 

Clay {2. Bentonite, dark-gray and olive-green inSpur terlaminated, waxy ________________ _ 

bed ____ 3. *Bentonite, olive-green, waxy ________ _ 

4. Shale, dark-gray, hard, siliceous, chertlike. 

T. 3 S.,R. 32 E. 

Thickness 

Feet Inches Feet 

2 

~: L~ 7 

2 

2 

:) 4 

2 

11 ---

d 1 

1 

Inches 

5 

6 

7 

The northeastern corner of this township is about 10 miles southsouthwest of Hardin and lies about 4 miles beyond the bridge over 

which the hard-surfaced Hardin-St. Xavier road crosses the Bighorn 

River near the mouth of Two Leggin Creek. The township can be 

reached over a gravel road that extends southwestward from a point 

near the bridge; it can be entered also over unimproved roads and 

trails approaching from other directions. 

The eastern half of the township lies almost wholly in a broad area 

of lowlands bordering Bighorn River, which enters from the south 

and flows for a distance of about 4 miles within the eastern boundary 

of the township. Here, the bedrock is aln1ost completely concealed by 

alluvial deposits underlying the flood plain of the river and capping 

broad, low terraces. In the western half of the township, which is 

hilly, alluvial deposits are restricted to the flood ,plain along Woody 

Creek and to several terraces in the southwestern and northwestern 

parts of the township; here, bedrock strata, dipping gently northeastward on the flank of Woody Creek dome, ar.e extensively exposed. The 

strata are displaced downward about 50 feet on the eastern side of a 

normal fault that trends south-southwestward in sections 18 and 19. 

The oldest rocks exposed in the township are strata in the vicinity 
BENTONITE DEPOSITS 69 

of Woody Creek that belong to the part of the Belle Fourche member 

that is below the Soap Creek bentonite bed. The Greenhorn and 

Carlile-Niobrara units crop out in much of the northwestern part of 

the township. 

The Soap Creek bentonite bed, which crops out in the southwestern 

part, offers attractive mining sites in sections 19, 20, and 31; it is estimated to contain a 1ninable reserve of 1,800,000 short tons within this 

township. At point 18, in section 19, on the east side of the abovementioned fault, this bed is 13 feet, 9 inches thick and dips about 2 

degrees north; at point 19, in section 28, it is 10 feet, 3 inches thick. 

Tests of a sample obtained at point 19 suggest that some of the bentonite would be marketable as foundry clay. 

At point 18, in section 19, bed Lis present in the Belle Fourche about 

40 feet higher, stratigraphically, than the Soap Creek bed, and in 

sections 19 and 20 a bed of bentonite about 4 feet thick occupies a 

position about midway between the Soap Creek bed and the base of 

the Greenhorn calcareous member. Neither of these beds is present 

at sites where mining would be feasible, and they have been neither 

measured nor sampled. 

At point 17, in section19, the thickness of bed JJJ, in the base of the 

Greenhorn calcareous member, is 7 feet, 9 inches, including 3 feet, 

11 inches of bentonitic shale in its upper part. The material of the 

bed as a whole, as represented by two samples taken at point 17, is 

evidently suitable for use as sand-bonding clay, because it tested high 

in green compression strength and the bentonitic shale showed exceptionally high dry strength. However, bed 111 crops out in this township only on steep hillsides, where the cost of 1nining would probably 

be prohibitive. 

Following are measurecl sections of bed ~I and the Soap Creek bentonite bed at three points in this township. 

Sections measured in T. 3 S., R. 32 E. 

J.Iaterial 

Point 17. NE%NE%, sec. 19: 

1. Shale, brown (light-gray when dry), cal- careous __________________________ _ 

2. *Shale, bentonitic, brown, limonitestained, gypsiferous _______________ _ 

3. *Bentonite, gray, granular, gypsiferous, 

B d ]}f with mica flakes __________________ _ 

e --- 4. *Bentonite, light-brown ______________ _ 

5. *Bentonite, brownish-yellow, granular __ 

6. *Bentonite, light-gray, granular, limonite-stained ______________________ _ 

7. Shale, dark-gray, soft. 

Thickness 

All beds Bentonite only 

Feet Inches Feet Inches 

3 

2 

1 

6 --- ----

11 

6 

3 

1 

0 

7 9 
70 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Sections measun:d in T. 3 S., R. 3.'2 E.-Continued 

l\Iaterial 

Point 18. NW7~SE7~ sec. 19: 

1. Shale, brownish-gray, soft ____________ _ 

2. *Bentonite, brownish-gray, flaky ______ _ 

3. *Bentonite, light-gray, flaky, gypsiSoap ferous ____________________________ _ 

4. *Bentonite, greenish-gray, lighter-colored Creek 

bed____ in lower part, flaky, granular in part, 

gypsiferous _____________________ -- _ 

5. *Bentonite, gray, waxy, with dark 

mineral particles __________________ _ 

6. Shale, brownish-gray, hard, siliceous, 

chert-like ________________________ _ 

Point 19. NW;,iNE?{ sec. 28: 

1. GraveL ____________________________ _ 

2. Shale, dark _________________________ _ 

Soap 13. Bentonite, dark-gray, waxy __________ _ 

Creek 4. Bentonite, light-gray-green, hard, 

bed____ chunky __________________________ _ 

5. Shale, dark-gray. 

T. 3 S., RS. 33 AND 34 E. 

Thickness 

All beds Bentonite only 

Feet Inches Feet Inches 

1 

1 

10 

20 

6 

9 

5 

7 

0 

13 9 

8 

6 

6 

0 

0 

10 

} 10 

3 

The rectangular tract encompassing these two contiguous townships includes the town of Crow Agency, which lies within its northeast corner on Little Bighorn River, and segn1ents of U. S. Highway 

87, and the Chicago, Burlington and Quincy Railroad. Bighorn 

River flows in a northeasterly direction within the northwestern corner 

of the tract. From a drainage divide that extends northward across 

the central part of the tract, the land slopes generally eastward to the 

Little Bighorn and westward to the Bighorn. In n1uch of the northeastern and extreme western parts of the tract, surficial deposits conceal the bedrock; the strata in all parts of the tract dip in westerly 

directions, and they include rocks belonging to the Judith River 

formation, the Carlile-Niobrara, and undivided upper members of the 

Cody shale. None of the rocks exposed here are known to contain 

valuable bentonite deposits. 

T. 3 S., R. 35 E. 

The Chicago, Burlington and Quincy railroad crosses this township in a northerly direction. The northwestern corner of the township is less than 1 mile northeast of Crow Agency station. Garryowen siding is in section 32. U. S. Highway 87 parallels the railroad 

and is joined about l:lh miles south of Crow Agency by State Highway 8, which extends eastward across the central part of the township. Near the junction of the highways, a hard-surfaced road 

branches southeastward frmn Highway 8, passes through the Custer 

Battlefield National Cemetery, and continues to the Reno Battlefield 
BENTONl'l'E DEPOSITS 71 

in section 34. An unimproved road leads from Crow Agency to the 

north-central part of the township. 

Little Bighorn River flows in a northwesterly direction midway between the Battlefield road and I-Iighway 87 and crosses the west 

township line at a point near the Cemetery, where it turns northward. 

Beyond Crow Agency, it re-enters the township and cuts across the 

northwestern corner of section 6. 

The surface of the township slopes downward from its eastern 

boundary to the flood plain of Little Bighorn River. Recent alluvial 

deposits underlie the flood plain and extend for several 1niles along 

two tributaries that enter the river from the east; older alluviu1n is 

present on low terraces bordering the flood plain. 

The exposed bedrock dips gently east,vard; it belongs to the Bearpaw shale everywhere except in an area underlain by the Hell Creek 

beds in the northeastern part of section 1, and in a belt of J uclith River 

formation extending northward from the Ce1netery, close to the river. 

The middle bentonitic unit of the Bearpaw shale crops out in al'1 area 

, within the Reno Battlefield and continuing northward across the 

central part of the township; this unit crosses Highway 8 in section 16 

and leaves the township at the north line of section 4. 

The Bearpaw shale contains numerous bentonite beds, but only bed 

TV, in the middle bentonitic unit of the fonnation, is thick enough to 

be considered minable within this township. Its thickness in section 

4 is nearly 10 feet, but it is considerably thinner in the southern half 

of the township. Tests o.f a sample obtained at point 20 suggest the 

presence of material that might be used as rotary well-drilling clay, 

and a sample from point 22 showed exceptionally high green strength 

when tested as fcundry clay. Bed lV is estimated to contain approximately 1,350,000 short tons of crude bentonite, under less than 30 feet 

of overburden, within T. 3 S., R. 35 E. The best mining sites are in 

section 4. 

Following are sections of bed lV and associated bedrock at three 

points along its outcrop. 

Sections measured in T. 3 S., R. 35 E. 

Thickness 

All beds Rentrmite only 

J.Iaterial Feet Inches Feet Inches 

Point 20. NEXNEX sec. 4: 

1. Shale, gray, soft _____________________ 2 1 

2. *Bentonite, gray, waxy _______________ 7 

3. *Bentonite, light-gray, waxy, gypsiferous_ 3 1 

4. *Bentonite, cream-colored, granular, 5 

gypsiferous 

Bed W __ 5. *Bentonite, light-gray, waxy, gypsiferous_ :3 5 9 11 

6. *Bentonite, dark-gray, granular_ _______ 6 

7. *Bentonite, cream-colored, waxy, 

gypsiferous ________________________ 9 

8. Bentonite, yellowish-gray, waxy _______ 2 

9. Shale, dark-gray. 
72 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Sections meas11/rea in T. 3 S., R. 35 E.-Continued 

Thickness 

All beds Bentonite only 

1\faterial Feet Inches Feet Inches 

Point 21. SEJ{SWX sec. 16: 

1. Soil and clay, light-brown ___________ _ 

{

2. Bentonite, cream-colored, waxy _______ _ 

Bed TV __ 3. Clay, li~ht-brown.-~- _________ ~- ______ _ 

4. Bentomte, yelloWltih-gray, waxy ______ _ 

5. Bentonite, cream-colored, waxy _______ _ 

3 5 ---

2 

-:n 1 5 2 

6. Shale, light-brown. 

Point 22. NEJ{NEX sec. 33: 

1. Shale, brownish-gray, somewhat sandy. 

Bed W __ 2. *Bentonite, gray, waxy ______________ _ 3 0 3 0 

3. Shale, brownish-gray, somewhat sandy. 

T. 4 S., R. 30 E. 

This township has a comparatively rough surface. The three largest streams-Beauvais Creek, Muddy Creek, and an aftluent of Muddy 

Creek-are bordered by narrow flood plains underlain by Recent alluvium. Older alluvial deposits lie on several terrace remnants in 

the east-central part of the township. 

In an arcuate 1nonoelinal belt extending southeastward from the 

northwestern corner of the township, the strata dip rather steeply in 

northeasterly to easterly directions ; throughout the rest of the township, on both sides of this belt, the dip is more gentle. A shallow 

syncline. parallels the monoclinal belt on its east side; the area on 

the west side of the belt has a rather uniformly gentle, but irregular, 

dip, as indicated by the structure contours on plate 1. The bedrock 

exposed within the syncline belongs to the Carlile-Niobrara sequence; 

the bedrock of the remainder of the township includes strata of the 

Thermopolis and ~fowry formations, as well as the Belle Fourche and 

Greenhorn members. Bentonite beds are present in this township 

but there are no known deposits that could be mined economically. 

T. 4 S., R. 31 E. 

The surface of this township is comparatively rough. Beauvais 

Creek flows eastward across the northern half, and Hay Creek flows 

for about 5 miles within the southern margin. The bedrock of the 

township is partly concealed by Recent aluvium underlying narrow 

f1ood plains that border these two streams and by older alluvial deposits lying on widely scattered terraces. The geologic structure of 

the township is that of an anticline, the axis of which extends in a 

northerly direction across its eastern half. The bedrock strata dip 

gently on both limbs of this anticline. The Mowry shale crops out 

within the east boundary of the township, where the anticlinal axis 

crosses Beauvais Creek, and the Belle Fourche member is extensively 
BENTONITE DEPOSITS 73 

exposed in the eastern and northern parts of the township. The 

Greenhorn and Carlile-Niobrara members are exposed in its south-

. western part and along its western margin. 

Bentonite beds in the Mowry shale and in the Belle Fourche member are conspicuously exposed in a steep southward-facing slope on 

the north side of Beauvais Creek. Tests of a sample of the Clay Spur 

bentonite bed, obtained at point 24, where it is 3 feet, 3 inches thick 

and dips gently eastward, indicate the presence of foundry clay having good dry compression strength. However, this bed and several 

others, including bed J and the Soap Creek bentonite bed, crop out 

only in hillside sites where only negligible amounts of clay occur under 

light overburden. Attractive mining sites have not been found in this 

township. 

Following are sections of the bentonite beds and enclosing bedrock, 

as measured at points 23 and 24. 

Sections measured in T. 4 S., R. 31 E. 

Thickness 

All beds Bentonite only 

Material Feet Inches Feet Inches 

Point 23. EH sec. 11: 

1. Covered interval to top of gravel terrace 28 5 

2. Bentonite, yellowish-green___________ 3 7 

3. Shale, dark-gray, fissile, with a few thin 

beds of sandstone_________________ 57 0 

4. Bentonite, cream-colored, waxy_______ 1 0 

5. Shale, dark, fissile___________________ 19 0 

6. Sandstone, thin beds and plates; intercalated shale, dark-gray___________ 3 0 

7. Sandstone, brown, soft, shaly _ _ _ _ _ _ _ _ _ 2 0 

8. Shale, dark; some layers consist of thin 

beds and laminae of bentonite and 

shale____________________________ 14 0 

9. Shale and bentonite, in alternating thin 

beds and laminae ________________ _ 

Bed J ____ 10. Bentonite, green; contains bluish-gray 

clay nodules _____________________ _ 

11. Shale, dark-gray, and siltstone, lightgray, in alternating plates and laminae 

12. Bentonite _________________________ _ 

13. Shale, dark-gray ___________________ _ 

14. Sandstone, dark-gray, fine-grained, ferruginous _________________________ _ 

15. Shale, gray, sandy _________________ _ 

16. Shale, reddish-brown, fissile _________ _ 

17. Shale, dark-gray, bentonitic _________ _ 

18. Bentonite, dark-greenish-gray _______ _ 

19. Bentonite, greenish-gray ____________ _ 

20. Shale, dark, fissile; sandy in upper part_ 

21. Bentonite, olive-green, waxy, with dark 

mineral particles _________________ _ 

5 0 

2 5 

25 0 

1% 

15 0 

6 

14 6 

2 9 

7 

2 4 

10 

21 0 

2 2 

3 7 

1 

2 5 

2 

2 2 
74 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Sections meastwed in T .. 1 S., R. 31 E.-Continued 

:Material 

Point 23. E% sec. 11-Continued 

22. Shale, dark-gray ___________________ _ 

23. Bentonite, greenish-yellow, flaky _____ _ 

24. Shale, dark-gray ___________________ _ 

25. Bentonite, dark-greenish-gray _______ _ 

26. Shale, dark-gray; makes conspicuous 

dark band on weathered surfaces ___ _ 

27. Bentonite, dark-grayish-green, flaky 

(base of Frontier) ________________ _ 

28. Shale, gray, fissile, siliceous _________ _ 

Cl~:,~~~: _ { 29. Bentonite, light-green, flaky .. _ - - - - - - -

30. Shale, gray, fissile, with a few partings 

of fine-grained platy sandstone ____ _ 

31. Shale, gray, with a few thin beds of 

sandstone in the upper part_ ______ _ 

32. Shale, olive-green __________________ _ 

33. Bentonite, bright-yellowish-green, waxy, 

with dark mineral particles ________ _ 

34. Bentonite, variegated orange, red and 

yellow __________________________ _ 

35. Sandstone, fine-grained, thin-bedded __ 

36. Shale, brownish-gray, bentonitic _____ _ 

.37. Sandstone, fine-grained, thin-bedded, 

with thin, fissile, gray shale partings_ 

38. Shale, dark-gray, fissile _____________ _ 

39. Bentonite, light-greenish gray, waxy __ _ 

40. Bentonite, green, with %-inch of yellow 

at base _________________________ _ 

41. Shale, dark-gray; silicified in upper 3ft; 

weathers light-bluish-gray _________ _ 

Point 24. SE%NEX sec. 11: 

1. Shale, light-gray, siliceous, fissile ______ _ 

Cl S 

{

2. *Bentonite, olive-green and gray interay pur . · b d lammated, waxy __________________ _ 

e ----- 3. *Bentonite, olive-green, waxy ___ ------

4. Shale, dark-gray, hard, siliceous, chertlike. 

T. 4 S., R. 32 E. 

Thickness 

All beds Bentonite only 

Feet Inches Feet Inches 

3 6 

4 4 

3 9 

4% 4% 

23 0 

8 8 

5 8 

3 

14 

25 

2 

6 

5 

7 

6 

1 

1 

14 

1 

1 

3 

0 

0 

0 

9 

5~~ 

0 

0 

0 

0 

6 

6 

0 

1 

4 

7 

3 3 

11 

The central and eastern parts of this township lie in the wide area 

of lowlands bordering Bighorn River, which flows in a northerly 

direction across the central part; the \Yestern half of the township 

includes a broad tract of the adjacent upland. Rotten Grass Creek, 

a tributary entering frmn the southeast, joins Bighorn River near 

St. Xavier, a small settlement in the southeastern part of the township; Beauvais and Hay Creeks flow eastward to Bighorn River in the 

central and southern parts. 
BEN·rONITE DEPOSI'TS 75 

The bedrock is largely concealed by Recent alluvium beneath the 

flood plains of the river and the three above-mentioned tributaries, 

and the bedrock is concealed also by older alluvial deposits on terraces that are widely distributed over the township. Nevertheless, the 

geologic structure is known to be essentially homoclinal, with a low 

dip toward the east. The Mowry shale crops out for a distance of 2% 

miles along Beauvais Creek, and the Belle Fourche member crops 

out extensively between the river and the west township line, in belts 

lying between areas occupied by older alluvial deposits. 

At a point a little more than 1 mile west of this township-point 

24, in T. 4 S., R. 31 E.-the Clay Spur bentonite bed, which is 3 feet, 

3 inches thick, contains good foundry clay. In T. 4 S., R. 32 E., the 

Clay Spur bed is well exposed north of Beauvais Creek. The Soap 

Creek bentonite bed crops out on the west sidB of section 7, but it is 

concealed farther east, partly by old alluvial deposits and partly by 

surficial detritus of Recent origin. Although both of these beds dip 

gently, they are exposed only on hillsides where the amount of bentonite under light overburden is small. 

T. 4 S., R. 33 E. AND PART OF R. 34 E. 

A rectangular tract of approximately 63 square miles that has been 

mapped within these two townships (pl. 1) ineludes the eastern margin of the broad area of lowland bordering Bighorn River in the 

vicinity of St. Xavier. The remainder of the tract is moderately 

rugged, and the drainage divide between Bighorn River and Little 

Bighorn River extends in a northerly dire.ction across the central part. 

The bedrock is concealed by Recent alluvium in the lowland near 

St. Xavier. On the adjacent higher ground to the east, it is concealed 

locally by older alluvial deposits on several small remnants of terraces, and a still older alluvial deposit occurs several hundred feet 

higher, in the S"\V:I4 see. 35. Outcrops of the Belle Fourche, Greenhorn and Carlile-Niobrara members, and undivided upper Cody strata 

occur in the hilly country between the weste.rn marginal lowland and 

the east end of the tract. No bentonite deposits of commercial value 

are known to be exposed within this tract. 

T. 5 S., R. 30 E. 

The surface of this township, which has an overall slope downward 

and toward the northeast, is rather rugged in the southwestern part 

and in and adjacent to the Big Horn Mountains, but it is less rugged 

farther to the northeast. The bedrock in the northern part is partly 

concealed by Recent alluvium underlying the narrow flood plain of 

Muddy Creek; in areas aggregating several square miles in the eastern 

part, it is concealed by older alluvial deposits. 

347624-56-6 
76 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

The geologic structure is essentially homoclinal with a northeasterly dip. Pre-Cretaceous rocks are exposed in the southwestern uplands, and Carlile-Niobrara beds crop out in an area of about 7 square 

1niles in the eastern part of the township. The intervening Cretaceous 

rock strata, comprising the Cloverly, Thermopolis, and Mowry forInations and the Belle Fourche and Greenhorn members, crop out in 

a belt extending diagonally southeastward across the township. The 

Cretaceous rocks include many bentonite beds, some of which are of 

minable thickness, but their dip is so steep that only small amounts 

of clay are present under light overburden; this township therefore 

contains no attractive mining sites. 

T. 5 S.,R. 31 E. 

In the north-central part of section 9 of this township, a good road 

leads northeastward toward Hardin from the Williams ranch office. 

Bighorn River flows in a northeasterly direction within the southeast margin of the township, and Muddy Creek flows within the northwest margin. Recent alluvium underlies flood plains along these 

streams, and alluvial deposits also conceal the bedrock within extensive areas occupied by remnants of ancient terraces. 

The geologic structure is somewhat obscured by the widespread 

alluvial deposits, but available evidence suggests that most of the 

township is occupied by a broad, shallow synclinal basin, the axis of 

which trends in a northwesterly direction across the western half 

of the township. However, the structure of areas north and south 

of the township suggests that an anticlinal axis extends northward 

within its eastern margin. 

The oldest bedrock strata exposed in the township belong to the 

Mowry shale, which crops out on the northwest side of Bighorn 

River, and the youngest strata belong to the Carlile-Niobrara sequence, which crops out along the marginal escarpment of a large 

terrace that occupies most of the land in the western half of the 

township. The Greenhorn calcareous member is poorly exposed in 

a narrow belt along this escarpment, and the Belle Fourche member 

crops out in a rather broad belt, largely covered by alluvial deposits, 

between the escarp1nent and the Bighorn River. 

Strata of the Mowry, Belle Fourche, and Greenhorn include many 

bentonite beds, some of which are of minable thickness. These beds 

have not been studied in detail because of the extensive cover of surficial 1naterial and vegetation; nevertheless, it is considered possible 

that, frmn section 27 northward, the Belle Fourche member may contain large quantities of bentonite Ull(;l~r light overburden. 

T. 5 S., R. 32 E. 

Thjs township can be reached from St. Xavier, which lies 2 miles 

from its north boundary, over a graded road that extends along the 
BENTONITE DEPOSITS 77 

southeastern side of the valley of Bighorn River. A road that 

branches off near the auxiliary airfield in section 29 leads to the Soap 

Creek oil field, in the next township to the south. 

The part of the township shown in plate 1lies largely in a broad 

area of lowland plains bordering the river, which flows for a distance of about 5 1niles within the west margin of the township. Recent alluvium underlies these plains and a narrow belt along Soap 

Creek, a tributary entering the river from the south; this material 

and the older alluvial deposits on extensive terraces coneeal the bedrock for many square miles on both sides of the river. 

The geologic structure of the southwestern quarter of the township 

is that of a broad anticlinal nose plunging in a northerly direction. 

The structure of the rest of the township is largely concealed by the 

extensive alluvial deposits, but there is evidence that all the strata 

dip gently eastward. 

The Thermopolis shales is exposed in a small area in section 32, 

within the anticlinal nose, where it is in contact on its eastern side 

with the Mowry shale, which crops out in an area largely covered 

by older alluvial deposits on the western side of the Soap Creek dome. 

The Mowry is exposed also in a small area on the western side of the 

Bighorn River in section 19. The Belle Fourehe member is exposed 

farther north along the river and in a broad belt within the northwestern margin of the township. The valley of Bighorn River is 

bordered on the east, as far north as section 1, by an outcrop of the 

Belle Fourche member extending along the lower slopes of the eastern hilly belt. The higher parts of this belt are underlain by beds 

of the Greenhorn meinber and the Carlile-Niobrara n1ember. 

The exposed bedrock formations include bentonite beds, but, because of the extensive alluvial cover, no deposits are present in this 

township at sites where mining would be feasible. 

T. 5 S., RS. 33 AND 34 E. 

A rectangular tract of approxi1nately 63 square miles has been 

mapped within these two townships (pl. 1). The western half of this 

tract is crossed by a graveled road that extends along Rotten Grass 

Creek; trails branching northward near the center ofT. 6 S., R. 34 E. 

give access to the eastern half. 

The lowest land within the tract lies along Rotten Grass Creek; 

the highest ground lies along the crest of a prominent cuesta that 

extends in a north-northwesterly direction across the central part, 

forming the drainage divide between Rotten Grass Creek and Little 

Bighorn River. 

Alluvial deposits of Recent age underlie the flood plain along 

Rotten Grass Creek; many remnants of older alluvial deposits lie 
78 BENTONITE DEPOSITS IN MON'l'ANA AND WYOMING 

at higher altitudes west of that stream, and still older aluvimn occupies a few sn1all areas east of the drainage divide. 

Throughout the tract,, the bedrock dips in easterly direotions at low 

angles; several small tear faults cross the central cuesta. The Greenhorn calcareous member crops out in a small area at the northwestern 

corner of the tract, burt ahnost everywhere else in the western third 

of the tract the bedrock belongs to the Carlile-Niobrara member of 

the Cody shale. The bedrock belongs to the undivided upper Cody 

everywhere east of a crooked line about half-way between Rotten 

Grass Creek and the boundary between rthe two townships. 

Strata of the part of the undivided upper member of the Cody 

shale that is thought to be stratigraphically equivalent to the Eagle 

sandstone crop ourt in a belt along the central cuesta. A bentonite 

bed that is tentatively correlated with bed R crops out prominently 

in the westward-facing escarpment of the cuesta; this bed also crops 

out in two areas in sections 29, 32, and 33, where the eastern, dipslope, side of the cuesta has been incised by minor streams to form 

inliers of the strata beneath the bed. Here the bentonite bed is 

underlain by a. thin bed of coarse sand and shale with abundant 

fragments. of silicified wood and rounded, wa,ter-worn fossil 

cephalopods. 

The bentonite contained in the bed is not of uniform thickness; 

however, a thickness of 9 feet, present at the locality in which the 

following section was measured, probably represents the approximate 

average thickness of the bed. The part of the bed at a depth of less 

than 30 feet below the land surface is estimated to contain a reserve of 

about 15 million short tons of bentonite, nwst of which occurs in 

areas broad enough to favor strip-mining. Although the bed has 

not been sampled here, test data on bed R, with which it is tentatively 

correlated, are given in table 1 for samples from T. 1 S.,. R. 34 E. 

Section measured in T. 5 S., Rs. 33 and 34 E. 

Jviaterial 

Point 26A, sec. 19: 

1. Soil and shale, bentonitic, grayish-brown, 

f2. 

Bed R'l __ _J 3

. 

l4. 

5. 

weathered _________________________ _ 

Bentonite, light-yellowish gray, granular, 

with traces of salenite _______________ _ 

Bentonite, light-yellow, granular _______ _ 

Bentonite, light-yellowish gray, granular_ 

Sand, dark-gray, shaly, containing cherty 

pebbles, water-rounded fragments of 

silicified wood and fossil cephalopods __ _ 

6. Shale, soft, dark-gray. 

Thickness 

All beds Pentonite only 

Feet Inches Feet Inches 

6 --- ----

8 

:J 9 ----

.... 

~t) --- ----
BENTONITE DEPOSITS 79 

T. 6 S., R. 31 E. 

The area within this township that is shown on plate 1 lies at the 

base of the Big Horn Mountains and includes in its northern part a 

segment of the Bighorn River immediately below the Yellowtail 

dam site. Steeply sloping land extends along the southwestern margin of this area, and a belt of upland lies along its eastern margin. 

The intervening land slopes northeastward to War Man Creek. Farther north, a broad belt of lowland extends along the river. Most of 

this lowland belt is occupied by Recent alluvium, which also extends 

along War Man Creek; it also. includes large areas underlain by older 

alluvial deposits. 

The geologic structure of the township is essentially homoclinal, 

with the over-all dip northeastward. Cloverly, and older, rocks crop 

out along the southwestern margin, and in a small area north of 

Bighorn River they are overlain by strata of the Thermopolis and 

1\fowry formations and the Belle Fourche member that dip rather 

steeply northward. Southeast of the river, a broad outcrop of 

Thermopolis shale lies between the Cloverly and strata of the Mowry 

that crop out with gentle dip on the upland at the eastern margin of 

the township. 

The Thermopolis, Mowry, and Belle Fourche strata include bentonite beds, but unfavorable geologic structure and topographic features probably rule out any possibilities for profitable mining in this 

township. T. 6 S., R. 32 E. 

This township is remote from rail transportation, but it is conYeniently accessible over a graveled road that enters from the north 

and extends to the Soap Creek oil field, which is situated within the 

south boundary. Soap Creek, a perennial stream, enters from the 

south and flows northward across the central part of the township 

along a flood plain bordered by low, stream-carved terraces; a large 

tributary, West Soap Creek, also enters from the south and joins the 

main stream near the center of the township. 

The part of the township lying west of Soap Creek and its large 

tributary is mostly upland, although Beaver Creek makes a long, deep 

incision extending northward from the southern boundary, and low 

terraces associated with Bighorn River occupy a small area at the 

northwestern margin of the township. The land on the eastern side 

of Soap Creek valley rises, in an escarpment of moderate steepness 

facing westward, to another belt of high land that extends along the 

entire eastern margin of the township. In the vicinity of the Soap 

Creek oil field, however, a conspicuous hogback extends in a northnorthwesterly direction between the creek and the main escarpment. 
80 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

The Soap Creek dmne, an elongated, asymmetrical fold having its 

steeper limb on the eastern side and its apex in the northern part of 

section 34, extends across the township in a north-northwesterly direction. On its western side, this dome is paralleled by a shallow synclinal belt that occupies most of the western half of the township. 

The oldest strata exposed are those of the Morrison formation, 

which forms a small inlier at the apex of the Soap Creek dome; the 

youngest strata belong to the Carlile-Niobrara unit, which is exposed in the upland at the east 1nargin of the township. Outcrops 

of all the intervening rock units are present between this upland and 

the inlier beds of the Morrison formation; the detailed stratigraphic 

sections of the Thermopolis, Mowry, Belle Fourche, and Greenhorn 

units, appearing in a preceding chapter of this report, were measured 

in this vicinity (in sections 35 and 36). Within this township, the 

bedrock exposed on the west limb of the Soap Creek dome belongs 

chiefly to the Thennopolis shale. It is succeeded farther west, in the 

adjacent broad synclinal belt, by the Mowry shale, which is the most 

extensively exposed bedrock formation in the uplands of the western 

part of the township. 

At least 15 bentonite beds with thicknesses of more than 18 inches 

crop out in this township, and several of these beds contain good 

foundry and well-drilling clay. 

Results of tests of a sample frmn the Clay Spur bentonite bed (in 

the topn1ost strata of the Mowry shale) at point 27 suggest that the 

material of that bed is probably the best drilling clay exposed in the 

township, though other test data indicate that beds A and B in the 

Thermopolis shale and bed L in the Belle Fourche shale member also 

contain clay suitable for drilling. Good sand-bonding clay is 

present in nearly all of the bentonite beds. Samples of the Soap 

Creek bed, obtained at point 29 where the bed is 12 feet thick, showed 

exceptionally high green strength and good dry strength. The Soap 

Creek bed is estimated to contain approximately 2,520,000 short tons 

of accessible clay in a sinuous belt, about 31;2 1niles long 'and averaging about 200 feet wide, that extends from section 15 to section 36. 

Nearby, bed 11! (at the base of the Greenhorn calcareous member) contains a minable reserve of clay estimated at 230,000 short tons; a 

san1ple of this material from section 36 consists of bentonitic shale that 

tested high in dry strength and showed moderate green strength. The 

structural and topographic features associated with the remainder of 

the bentonite beds exposed in this township indicate that the amount 

of clay under light everburden is present in extremely narrow belts 

and is too small for economical mining. 

Measured sections of the bentonite beds and associated rock materials cropping out at five numbered points in T. 6 S., R. 32 E. are given 
Bll:NTONITE DEPOSITS 81 

in the following table. The sections £or points 27, 28, and 29 consist partly o£ data that are given also in stratigraphic sections appearing elsewhere in this report. 

Sections measured in T. 6 S., R. 32 E. 

Aiaterial 

Point 25. SW~ sec. 4: 

1. Shale, alternating light-and dark-gray 

layers, containing a few thin bentonite beds and a few ferruginous con- cretions ________________________ _ 

2. Shale and bentonite, alternating thin 

beds __________________________ _ 

3. Shale, like No.1 (above) ___________ _ 

4. Bentonite ________________________ _ 

5. Shale, like No.1 (above) ___________ _ 

1

6. Bentonite, impure _________________ _ 

7. Bentonite, light-olive-green, waxy ___ _ 

Bed B____ 8. Bentonite, .reddish-brown and olivegreen, mottled, waxy ____________ _ 

9. Bentonite, greenish-yellow, waxy ____ _ 

1o. Shale, like No. 1 (above) ___________ _ 

Bed A____ 11. B en t o n i t e, light-greenish-yellow, 

darker in upper part, flaky ______ _ 

12. Shale, like No. 1 (above) ___________ _ 

13. B'entonite, dark, impure ___________ _ 

14. Bentonite, dark-gray, olive-green in 

lower 3~ in., waxy ______________ _ 

15. Shale, dark-gray __________________ _ 

16. Bentonite, dark-olive-green, lightercolored near base, waxy _________ _ 

17. Shale, dark, with thin plates of yellowish-green bentonite ______________ _ 

18. Shale, gray, hard, siliceous, chertlike_ 

19. Shale, gray _______________________ _ 

Point 26. NW~NW% sec. 22: 

Bed C { 1. Shale ____________________________ _ 

or 2. Bentonite, yellowish-green _________ _ 

Bed D 3. Shale. 

Point 27. NW~SW~ sec. 26: 

1. Shale, dark-gray, soft_ _____________ _ 

2. Bentonite, dark-gray, waxy ________ _ 

3. *Bentonite, olive-green, granular, with 

dark mineral particles ___________ _ 

4. *Bentonite, brownish-yellow, cornBed I____ meal texture, rusty-colored near middle _________________________ _ 

5. *Bentonite, greenish-gray, waxy, with 

dark mineral particles ___________ _ 

6. *Bentonite, rusty-colored, cornmeal 

texture ________________________ _ 

Thickness 

All beds Bentonite only 

Feet Inches Feet Inches 

20 

15 

27 

2 

35 

1 

40 

2 

80 

2 

1 

2 

20 

2 

3 

1 

0 

0 

0 

0 

0 

: ) 

8 

0 

0 

:~ } 

6 

6 

3 

0 

0 

0 

6 

6 

2 

1 

9 

3 

I 

I 

2 8 

2 8 

1 

1 

2 0 

5 9 
82 BENTONITE DEPOSITS IN MOKTANA AND WYOMING 

Sections rneasured in T. 6 S., R. 32 E.-Continued 

1\Iaterial 

Point 27. NWXSW%, sec. 26-Continued 

7. Shale, dark-gray, bentonitic, containing three very thin bentonite beds 

and some ferruginous concretions __ 

8. Bentonite, gray, containing much 

brown aragonite ________________ _ 

9. Shale, dark-gray, lower part sandy __ _ 

10. Bentonite, grayish-green, containing 

some brown aragonite (base of Frontier formation) _________________ _ 

11. Sandstone interbedded with shale, 

gray, hard _____________________ _ 

12. *Bentonite, dark-gray and brown, impure, granular __________________ _ 

13. *Bentonite, greenish-gray, waxy _____ _ 

Clay Spur 14. *Bentonite, light-olive-green, waxy, 

bed____ gypsiferous _____________________ _ 

15. ""Bentonite, greenish-gray, waxy, gypsiferous, with dark mineral particles __ 

16. *Bentonite, limonite-stained ________ _ 

17. Shale, dark-gray, hard, siliceous, chertlike. 

Point 28. NW.%, sec 35: 

1. Shale, bentonitic, dark-brown, granular_ 

r 2. *Bentonite, gray, waxy, with dark 

J 

mineral particles ________________ _ 

3. *Bentonite, olive-green, waxy, gypsiBed G ferous, with dark mineral particles __ 

----~4. *Bentonite, greenish-gray, waxy, with 

dark mineral particles ___________ _ 

5. *Bentonite, light-gray, waxy _______ _ 

6. Shale, dark-gray, hard, siliceous, chertlike ___________________________ _ 

7. Shale, weathers light-gray, hard; forms 

crest of Mowry hogback _________ _ 

8. *Bentonite, brownish-gray, granular, 

impure ________________ ----------

9. *Bentonite, olive-green and dark-gray, 

inter laminated, waxy ____________ _ 

10. *Bentonite, greenish-gray, waxy with 

dark mineral particles ___________ _ 

Bed F___ 11. *Bentonite, greenish-gray, waxy _____ _ 

12. Shale, weathers light-gray, siliceous __ 

13.*Bentonite, light-gray, granular ______ _ 

14. *Bentonite, light-gray (darker than 

above), waxy, with some dark mineral particles ___________________ _ 

15. *Bentonite, olive-green, waxy ______ _ 

Thickness 

All beds Bentonite only 

Feet Inches Feet Inches 

7 

1 

2 

2 

3 

4 

2 

2 

1 

1 

32 

21 

6 

6 

0 

0 

6 

6 

4 

11 

3 

2 

0 

0 

1 4 

8 

2 4 

3 

11 

2 9 

8 

10 

1 6 

2 0 

9 10 

2 8 

4 7 

3 
B1BNTONITE DEPOSITS 83 

SectZ:ons measured -in '1'. 6 S., R. 3B E.-Continued 

Thickness 

All beds Bentonite only 

Material 

Point 28. NW%, sec. 35-Continued 

Feet Inches Feet Inches 

16. Shale, dark-grayish-brown, hard, siliceous, chert-like ________________ _ 

17. Shale, weathers light-gray, with a 4-in. 

and a 6-in. bentonite bed in the 

6 

lower par1;______________________ 29 6 

18. Bentonite, light-green______________ 2 0 

19. Siltstone and shale, weathering lightgray, hard; interbedded and interlaminated. Several thin bentonite 

beds in lower 50 ft. Some interbedded fine-grained sandstone in 

upper 45 ft_____________________ 110 0 

20. Bentonite, yellow__________________ 2 0 

21. Shale, dark-gray, silty, with two thin 

bentonite beds___________________ 6 0 

22. Bentonite, yellow and light-green____ 1 0 

23. Shale, dark-gray, silty, with few thin 

bentonite beds in the upper part___ 29 0 

24. Bentonite, green___________________ 2 8 

25. Shale, dark-gray, silty; interlaminated 

with lighter-gray hard siltstone and 

very fine grained sandstone______ 13 0 

26. Bentonite, light-gray_______________ 1 0 

27. Shale, dark-gray, hard, containing 

fish scales; weathers to dark, bare, 

steep slope______________________ 38 0 

28. Bentonite, gray____________________ 4 

29. Shale, dark-gray, hard, fissile, containing fish seales, weathers to silverygray____________________________ 5 6 

30. Bentonite, dark-brown, impure, granular____________________________ 8 

31. *Bentonite, gray, waxy, limoniteBed E____ stained, gypsiferous, with dark 

mineral particles_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 3 

32. *Bentonite, reddish-brown, waxy, gypsiferous, with dark mineral particles_ 1 

33. Shale, dark-1~ray, hard, fissile, containing fish scales (base of Mowry shale) _________________________ _ 

34. Shale, siltstone and three thin bentonite beds__________________________ 79 

Bed D ____ 36· 

{

35. 

37. 

Bentonite, dark-gray, granular ______ _ 

*Bentonite, olive-green, limonitestained in lower part, waxy, with 

dark mineral particles ___________ _ 

Shale, dark-i'!;ray, bentonitic in part __ 

1 

1 

36 

0 

0 

8 

2 

9 

11 

6 

2 0 

2 0 

1 0 

2 8 

1 0 

4 

4 8 

} 3 8 
84 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Sections measured in T. 6 S., R. 32 E.-Continued 

.Afaterial 

Point 28. NW:!4, sec. 35-Continued 

38. Shale, bentonitic, dark-gray ________ _ 

39. Bentonite, greenish-gray, granular, 

gypsiferous, appears waxy when 

moist __________________________ _ 

*Bentonite, light-olive-green, waxy, 

Bed C____ 40_ 

41. 

gypsiferous _____________________ _ 

Shale with silty and sandy streaks; 

bentonitic in basal 13 ft; a 14-in. 

bentonite bed 30 ft. from top _____ _ 

*Bentonite, light-gray, waxy, limonitestained at base _________________ _ 

Shale, dark-brownish-gray, soft _____ _ 

*Bentonite, greenish-gray, waxy, with ! dark mineral particles ___________ _ 

42. 

Bed B ____ 43. 

44. 

45. 

46. 

Shale, dark-gray, bentonitic, with numerous ferruginous concretions and 

light-gray-weathering claystone nodules ___________________________ _ 

*Bentonite, light-gray, limonitestained, waxy, gypsiferous, with 

some dark mineral particles ______ _ 

*Bentonite, greenish-gray, waxy, with 

dark mineral particles ___________ _ Bed A ___ 47· 

48. Shale, bentonitic, dark-brown, gran- ular ___________________________ _ 

49. Bentonite, light-gray, waxy ________ _ 

50. Shale, bentonitic, brown ___________ _ 

51. Shale, dark-gray. 

Point 29. Sees. 35 and 36: 

Bed 0 ____ l 

1 

1. Shale, dark-gray __________________ _ 

2. *Bentonite, brown, granular; with thin 

beds of gray, waxy, gypsiferous 

bentonite in lower part __________ _ 

3. *Bentonite, greenish-gray, granular, 

gypsiferous _____________________ _ 

4. Sandstone ________________________ _ 

5. Shale, dark-gray, soft ______________ _ 

6. *Bentonite, dark-greenish-gray, waxy, 

with dark mineral particles _______ _ 

7. *Bentonite, dark-gray, waxy _______ _ 

Bed N _ _ _ 8. *Bentonite, dark-greenish-gray, waxy, 

with dark mineral particles _______ _ 

9. *Bentonite, light-gray, granular, limonite-stained, with dark mineral 

particles _______________________ _ 

10. Shale, dark-gray, with fine sandy 

streaks (base of Carlile shale membe~----------------------------

Thickness 

All beds Bentcnite only 

Peel Inches Feet Inchel.f 

1 5 

1 3 

4 

85 8 

1 

1 

2 

1 

0 

0 

31 0 

1 

1 

7 

3 

10 

2 

3 

1 10 

2 8 

6 

11 

9 

2 : l 

3 J 

35 0 

7 

1 1 

2 0 

2 10 

2 0 

3 2 

3 
BENTONITE DEPOSITS 85 

Sections measured in T. 6 S., R. 32 E.-Continued 

1\Iaterial 

Point 29. Sees. 35 and 36-Continued 

11. Shale, calcareous, dark-gray, weathering bluish and white, containing cal12. 

13. 

Bed .J\,1_ __ { 14· 

15. 

16. 

{

17. 

Bed L ____ 18_ 

19. 

careous concretions ______________ _ 

Bentonite, gray, limonite-stained ____ _ 

Shale, dark-gray __________________ _ 

*Shale, bentonitic, brown (base of 

Greenhorn calcareous member) ___ _ 

Shale, dark-gray __________________ _ 

Shale, gray, bentonitic _____________ _ 

*Bentonite, brown and light-brown, 

granular ________ .:c ______________ _ 

*Bentonite, light-brownish-gray, with 

dark mineral particles ___________ _ 

Shale, with sandy streaks; in a few 

places contains thin sandstones and 

black chert pebbles; locally at base 

a thin, friable, coarse sandstone ___ _ 

20. *Bentonite, brown, granular, impure_ 

21. *Bentonite, brownish-gray, granular to waxy _______________________ _ 

Soap Creek 22. *Bentonite, greenish-gray, granular, 

bed____ _ _ limonite stained in lower part _____ _ 

23. *Bentonite, olive-green, waxy, gypsiferous, with dark mineral particles __ 

24. *Bentonite, light-greenish-gray, waxy, 

with dark mineral particles ________ _ 

25. Shale, dark-gray, sandy, bentonitic; 

contains gray-weathering calcareous 

concretions and a few large brownweathering calcareous concretions __ 

26. Shale, gray; contains sandy streaks 

and a few thin beds of shaly sandstone __________________________ _ 

27. Bentonite, gray ___________________ _ 

28. Shale, dark-gray; contains some sandy 

streaks and a few beds of bentonite 

less than 5 in. thick _____________ _ 

29. *Shale, bentonitic, dark-brownish-gray 

30. Bentonite, dark-brown, waxy to granular, gypsiferous _________________ _ 

Bed J ____ 31. *Bentonite, gray, granular to waxy __ _ 

32. *Bentonite, brownish-gray, waxy, gypsiferous ________________________ _ 

33. *Bentonite, olive-green, waxy, gypsiferous with dark mineral particles __ 

34. Shale, dark-gray, hard, siliceous, chertlike. 

Thickness 

All beds Bentonite 011./y 

Feet Inches Feet Inches 

96 0 

1 

10 

0 

0 

7 9 

105 0 

18 0 

2 

4 

61 0 

11 

4 

8 8 

1 8 

5 

10 6 

73 

1 

67 

1 

1 

3 

0 

0 

0 

9 

6 

1 

6 

1 

1 0 

0 

12 0 

1 0 

5 2 
86 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

T. 6 S., RS. 33 AND 34 E. 

The rectangular tract comprising these two townships (pl. 1) is 

crossed by the graveled road that leads from St. Xavier to Lodge 

Grass; frmn this principal route, other parts of the tract may be 

reached over unimproved branch roads and trails. 

Lodge Grass Creek, which flows northward across the central part 

of the tract, divides it into two moderately dissected upland areas. 

Recent aluviun1 underlies the flood plain of Lodge Grass Creek; remnants of older alluvial deposits occur on higher ground west of that 

stream and elsewhere in the tract. 

The bedrock strata, which everywhere dip in easterly directions at 

low angles, belong to the Cody shale-except for an outlier of Judith 

River formation southeast of Good Luck Creek, in sec. 25, T. 6 S., R. 

34 E. A bentonite bed that crops out in the northern half of sec. 4, 

T. 6 S., R. 34 E. is tentatively correlated with bed R of the sandy shale 

unit equivalent to the Eagle sandstone. The exact thickness of this 

bentonite bed has not been determined, but 3 miles farther north, at 

point 24A, it is 9 feet thick. This bed is estimated to contain approximately 300,000 short tons of accessible clay. 

T. 7 S., R. 32 E. 

The southwestern part of this township, which lies within the Big 

Horn Mountains, is not shown on plate 1. The part that is shown on 

plate 1 can be reached conveniently by automobile over roads and 

trails branching from an unimproved road leading southward from 

the Soap Creek oil field, which lies less than 1 n1ile north of the township. 

The land surface has an overall northeasterly slope and lies almost 

entirely within the drainage area of Soap Creek, which flows in a 

north-northeasterly direction across the central part of the township. 

In the southern half of the township, a belt of rather rugged upland 

extends along the margin of the Big Horn Mountains; the relief is 

1nore moderate farther north. 

Recent alluvial deposits underlie the flood plains of Soap Creek, 

West Soap Creek, and their tributaries, and older alluvium lies on terraces at a number of places in the northern third of the township. 

The principal geologic structures are: ( 1) an anticlinal nose plunging southeastward into the northeastern part of the township from 

the Soap Creek dome, the apex of which lies just outside the northern 

boundary, and ( 2) a broad shallow syncline extending in a northwesterly direction and having its greatest structural depression near the 

southeast corner of section 8. Frmn that locality, the axis of the syncline rises to a point near the north line of section 23 ; beyond this point, 

i.t plunges southeastward into a more shallow structural concavity 
BENTONITE DEPOSITS 87 

in the next township to the east. The syncline is asymmetrical, with 

the rocks southwest of its axis more steeply inclined than those on its 

northeast side. 

Pre-Cretaceous rocks are at the surface on the southwest limb of 

the syncline, and the youngest rocks within the syncline belong to the 

Belle Fourche member, which is extensively exposed on both sides 

of the synclinal axis. Southeast of the part of Soap Creek that is 

above the mouth of Dry Soap Creek, the shale of the interval between 

the Mowry formation and the Greenhorn member contains, in general, 

more sand than does the shale of the same interval northwest of that 

locality. This is true particularly of the part of the interval below 

bed L. The Thermopolis and Mowry extend in narrow belts along the 

southwest limb of the syncline, where they dip steeply northeastward. 

The Mowry is also exposed in an area lying within the anticlinal nose 

and extending westward along the northern margin of the syncline. 

The Belle Fourche shale member crops out on the eastern side of the 

nose, and the Greenhorn and Carlile-Niobrara units are exposed in the 

northeastern quarter of section 1. 

Bentonite beds are present in the Thermopolis, Mowry, Belle 

Fourche, and Greenhorn units. Those in the Thermopolis, as exposed 

on the southwest flank of the syncline, dip steeply and are of little 

or no interest as possible sources of supply; they have, therefore, not 

been studied in detail. 

The Clay Spur bentonite bed crops out at 1nany places along the 

margins of the syncline; tests of a sample of this bed from point 32 

in section 10, where it is 6 feet, 5 inehes thick, indicate that it contains good foundry clay. However, the topographic and structural 

conditions along its outcrop are such that only small a1nounts of the 

elay lie under light overburden. 

Bed I, in the basal part of the Belle Fourche interval, is about 12~;2 

feet higher than the Clay Spur bed and is 4 feet, 8 inches thick at 

point 32, where the outcrop of the bed eneireles a large outlier of the 

Belle Fourche 1nmnber. Tests of a sample of this bed obtained here 

indicate that it contains excellent foundry clay and good drilling-mud 

material, but, like the Clay Spur bed, it does not offer attractive 

mining sites. 

The Soap Creek bentonite bed is extensively exposed in this township, where its thickness averages about 16 feet. Tests of samples 

obtained at points 30, 33, and 34 indicate that it contains excellent 

foundry clay. The becll]es under light overburden in rather broad 

belts at many places along its outcrops within the syncline and on the 

eastern side of the Soap Creek anticlinal nose. It is estimated that 

9,240,000 short tons of crude bentonite are present under less than 30 
88 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

feet of overburden in these belts, which include some of the most 

attractive mining sites in the Hardin district. 

Bed L, in the Belle Fourche 1nember, dips gently in sections 9, 15, 

and 16 and is more than 8 feet thiek at point 31 in section 9. However, 

this bed appears at the surface only on steep, grass-covered hillsides, 

where only small amounts of the contained bentonite are under light 

overburden. Tests of a sample from point 31 suggest, n1oreover, that 

this material would not be well adapted for use as drilling clay nor 

as foundry cia y. 

Bed 3/, at the base of the Greenhorn calcareous member, crops out 

for a distance of lh mile in section 1, where it is estimated to contain 

about 110,000 short tons of bentonitic shale under less than 30 feet of 

·overburden. Its thickness is more than 7 feet a few hundred feet 

north of the township, and a sample obtained there showed high dry 

cmnpression strength when tested as foundry clay; more complete 

data on this deposit are given in the preceding description of the 

bentonite deposits in T. 6 S., R. 32 E. 

Following are sections of the bentonite beds and enclosing bedrock 

at five points in T. 7 S., R. 32 E. 

Sections measured in T. 7 S., R. 32 E. 

:Material 

Point 30, SW7~SWX sec. 5: 

1. Soil, bentonitic, brownish-gray, soft_ __ _ 

~:e~k bed/2. Bentonite, gray, waxy, gypsiferous ____ _ 

(lower 3. *Bentonite, light-greenish-gray, with 

part dark mineral particles _____________ _ 

only) __ _ 

4. Shale, dark-brownish-gray, hard, siliceous, chert-like. 

Point 31, SE7~SW7~ sec. 9: 

1. Soil, brown, bentonitic _______________ _ 

Bed L __ __ 2. *Bentonite, brownish-gray, waxy _____ _ 

3. Shale, brown, hard, siliceous, chert-like. 

Point 32, NW7~NE7~ sec. 10: 

1. Shale, dark-gray, hard _______________ _ 

2. Bentonite, dark-gray and olive-green 

inter lamina ted____ _ _ _ _ ____________ _ 

Bed[_ ___ 3. *Bentonite, olive-green, waxy, limonitestained __________________________ _ 

4. *Bentonite, olive-green, darker in lower 

part 1 waxy _______________________ _ 

5. Shale, gray, soft ____________________ _ 

6. Shale, bentonitic, gray, soft __________ _ 

7. Shale, bentonitic, gray, hard _________ _ 

8. Shale, gray, soft (base of Frontier) ____ _ 

9. Shale, gray, sandy, hard _____________ _ 

Thickness 

All beds Bentonite only 

Feet Inches Feet Inches 

1 0 

2 

5 

1 0 

8 3 8 3 

3 ---

1 

: l4 8 

2 

1 0 

8 

8 

7 4 

3 0 
BENTONITE DEPOSITS 89 

Sections measured in T. 7 S., R. 32 E.-Continued 

~'fate rial 

Point 32, NW}~NE% sec. 10-Continued 

Clay Spur green laminae in lower part _______ _ 

{

10. *Bentonite, dark-gray, with a few olivebed ____ 11. *Bentonite, light-olive-green, waxy ___ _ 

12. *Bentonite, dark-greenish-gray, waxy __ 

13. Shale, dark-gray, hard, siliceous, chertlike. 

Point. 33, SW7~SE}~ sec. 15: 

1. Shale, dark-gray, soft_ _______________ _ 

2. *Bentonite, gray and greenish-gray interlaminated, flaky, lighter-colored in 

Soap lower part_ ______________________ _ 

Creek 3. *Bentonite, light-greenish-gray, with dark 

bed____ mineral particles; lower 6 in. darker in 

color and with many more mineral 

particles __________________________ _ 

4. Shale, dark-brownish-gray. 

Point 34, NW7~NE7~ sec. 17: 

Soap 

Creek 

bed ___ _ 

1. Shale, dark-gray, soft ________________ _ 

2. Bentonite, brownish-gray, granular, impure_ 

3. *Bentonite, light-greenish-gray, waxy __ _ 

4. *Bentonite, light-greenish-gray, waxy, 

with dark mineral particles _________ _ 

5. *Bentonite, bluish-gray, waxy, with dark 

mineral particles __________________ _ 

6. Shale, dark-gray, hard, siliceous, chertlike. 

T. 7 S.,R. 33E. 

Thickness 

All beds Bentonite only 

fleet Inches Feet Inches 

1 

1~ } 6 5 2 

1 10 

.5 

4 3 

16 8 

12 5 

6 

4 

4 5 

10 2 18 2 

3 3 

This township may he reached by driving 12 miles southwestward 

over a graded road from Lodge Grass, a small town on the Chicago, 

Burlington and Quincy Railroad; unimproved roads and trails give 

access to several parts of the township. 

The surface of the township is comparatively rugged. Rotten 

Grass Creek cuts northeastward across its southeastern part, and a 

tributary of Soap Creek flows northward along the western margin. 

Between these streams, the land surface rises to a high divide that extends from section 34 in a northerly direction across the township. 

Recent alluvial deposits underlie the flood plains of Rotten Grass 

Creek and a tributary of Soap Creek at the west margin, and older 

gravel deposits are ,present on remnants of dissected terraces at a 

number of localities in the intervening area. The overall dip of the 

bedrock strat is northwestward; therefore, the youngest strata, belonging to the upper members of the Cody shale, are exposed in the 

northeastern part of the township; the oldest strata are in the south-
90 BENTONITE DEPOSITS IN .MONTANA AND WYOMING 

western part. In the southwestern part, however, the structural uniforinity is interrupted by two flexures: (1) a sn1all northwestwardtrending anticline, known as the Rotten Grass dome, the apex of 

which is in the southern part of section 29; and (2) farther southwest, 

a small elongated structural basin, trending parallel with the anticline and forming part of a long syncline that extends northwestward 

into the adjacent township to the west. 

The oldest rocks exposed, which belong to the upper part of the 

Mowry shale, crop out at the axis of the Soap Creek dome in section 

29 as a small inlier within an area of about 7 square miles that is 

occupied by beds of the Belle Fourche. The interval is bordered 

on the east by a relatively narrow outcrop of the Greenhorn calcareous member. The Carlile-Niobrara strata crop out in a broad area 

lying between the Greenhorn and the area of upper Cody strata in 

the northeastern part of the township. 

Bentonitic material is present in all the formations exposed, but the 

only beds believed to be of interest as possible sources of bentonite 

supplies belong to the Belle Fourche and Greenhorn members. 

Bed J, in the Belle Fourche, ranges fron1 31;2 to 61;2 feet in thickness in exposures northwest and southeast of this townshi,p, and bed 

Lis at least 9 feet, 11 inches thick at a locality in the next township to 

the south. However, beds J and L are poorly exposed in this township 

and have not been studied in detail. 

The Soap Creek bentonite bed is here extensively exposed and is 

16 feet thick at point 38. In section 31, the bentonite of this bed is 

locally 45 feet thick in a. nearly circular dome (fig. 10) almost 125 

yards in diameter, which has been formed by lateral flowage of the 

bentonite (p. 19). Sand-bonding tests of samples of bentonite taken 

at points 37, 38, and 39 indicate that the bed contains material with 

high green compression strength and satisfactory dry strength. Some 

of it might also be usable as drilling clay, although the yield, viscosity 

and gel strength of samples that have been tested are somewhat lower 

than is desirable. It is estimated that 14,000,000 short tons of crude 

bentonite of this bed lie beneath less than 30 feet of overburden within 

this township. Along the outcrop of the Soap Creek bed are several 

good prospective mining sites; the most attractive are in sections 

30 and 31. 

Bed M, at the base of the Greenhorn, is 6 feet, 1 inch thick at 

point 36, where it is overlain by at least 1 foot, 2 inches of bentonitic 

shale. Bentonitic shale obtained from this bed, less than 1 mile 

from the northwest corner of the township, in section 36, T. 6 S., 

R. 32 E., showed high dry strength when tested for use as foundry 

clay. Within the township, 1,650,000 short tons of crude bentonite 

of bed 11/ are estimated to be present under less than 30 feet of overburden, but this bed offers no attractive mining sites. 
BEN'fONITE DEPOSITS 

Sections measured in T. 7 S., R. 33 E. 

:Material 

Point 35, SWXNE% sec. 18: 

1. Soil _______________________________ _ 

S {2. Bentonite, dark-olive-green, gypsiferous 0

d k 3. Bentonite, green, waxy, with dark 

1 r~e mineral particles; blue-gray streak 

)ec --- - 30 in. above base _________________ _ 

4. Shale. 

Point 36, NW%SE74 sec. 20: 

1. Shale, brown, bentonitic _____________ _ 

I

' 2. Bentonite, greenish-gray ______ --_-----

3. Bentonite, yellow ___________________ _ 

Bed l'.L- _14. Bentonite: upper half nearly white; 

lower half yellowish with orange 

mottling _________________________ _ 

5. Shale, brownish-gray. 

Point 37, NWXNW%, sec. 31: 

Soap 1. *Bentonite, in small dome into which 

Creek it has flowed; olive-green and darkbed. gray bentonite interlaminated and in 

irregular masses __________________ _ 

2. Shale, dark-gray, hard, siliceous, chertlike. 

Point 38, NE1-{NWX sec. 31: 

1. Shale. dark-gray, soft. 

r

2. *Bentonite, dark-gray and olive-green, 

Soap interlaminated, waxy ______________ _ 

Creek '\ 3. *Bentonite, olive-green, grayish in upper 

bed ___ -I and lower parts, waxy, with dark 

l mineral particles __________________ _ 

4. Shale, dark-gray, siliceous ____________ _ 

5. Shale, dark-gray: soft. 

Point 39, NWXNE% sec. 32: 

1. Shale, brown, soft. 

S 

{

2. Bentonite, dark-gray and olive-green 

oap C k mter · l amma · t d e , waxy ______________ _ 

b:~~ _ _ _ 3. *~entonite, olive-green, darker colored m upper part, waxy _______________ _ 

4. Shale, dark-gray, hard, siliceous, chertlike. 

T. 8 S., R. 33 E. 

91 

Thickness 

All beds Bentonite only 

Feet Inches Feet Inches 

1 

6 

1 

2 

2 

1 

45 

12 

4 

8 

6 

4+ 

1 

45 

5 

The northeastern corner of this township is about 14 miles southwest of Lodge Grass, a small town on the Chicago, Burlington and 

Quincy Railroad. A graded road leads from Lodge Grass to a point 1 

mile north of the township, and unimproved roads give access to its 

northern and eastern parts. 

A belt of rather rugged upland extends along the west margin, 

·within, and close to, the Bighorn Mountains, but the topographic re347()24-5!}--7 
92 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

lief is somewhat less pronounced in nwst other localities. The land 

surface is drained by Rotten Grass Creek and Lodge Grass Creek, 

which are northeasterly-flowing perennial streams with broad flood 

plains flanked by gravel-capped alluvial terraces, and by numerous 

tributaries of these strmuns. 

The northeastward dip of the bedrock is steep in a belt close to the 

n1ountains, but jt is more gentle farther east in the township. Rocks 

of pre-Cretaceous age are exposed in the upland belt at the western 

margin of the township, and Cretaceous rock formations crop out 

in northwesterly-trending belts farther east. The youngest bedrock 

strata are those of the Carlile-Niobrara sequence, which fonn an outcrop about 10 square 1niles in area in the northeastern part of the 

township. 

Bentonite beds of minable thickness are present in the Mowry, 

Belle Fourche, and Greenhorn units. The Clay Spur bed in sections 

17 and 21, and bed J in sections 6, 7, and 8, contain small tonnages of 

bentonite under light overburden. Bed Lis at least 9 feet, 11 inches 

thick at point 41, but elsewhere it is largely covered by vegetation and 

has not been examined. 

It is estimated that 11,250,000 short tons of accessible clay are contained in the Soap Creek bed. This bed, which is marked here by 

many large, brown calcareous concretions, averages about 25 feet in 

thickness. Particularly favorable mining sites occur on several spurs, 

which jut westward from an escarpment that crosses sections 9 and 

15, and in two outliers in section 27. At point 42, bed Mat the base 

of the Greenhorn member is more than 20 feet thick, and it retains 

this thickness throughout the township. In the southern part, however, bed 11/ contains a larger proportion of green waxy clay than is 

present at point 42. Probably about 6,600,000 short tons of clay could 

be mined fron1 bed jJf within this township. 

When tested as foundry clay, most of the samples taken from the 

Soap Creek bed and beds L and 11/ showed high green strength, and a 

few samples showed high dry strength; a sample taken from the Soap 

Creek bed at point 45 gave fairly encouraging results as drilling clay, 

but four other samples were inferior in quality. 

Sections measured in T. 8 S., R. 33 E. 

J.Iaterial 

Point 40, SE.%NWX sec. 6: 

1. Shale, brown, soft. 

Soap {2. *Bentonite, dark-gray:and"olive-green inCreek terlaminated, waxy ________________ _ 

bed____ 3. *Bentonite, olive-green, waxy ________ _ 

4. Shale, brown, hard, siliceous, chert-like. 

Thickness 

All beds Bentonite only 

Feet Inches Feet Inches 

10 

9 6 
BENTONITE DEPOSITS 93 

Sections measured in T. 8 S., R. 33 E.-Continued 

:Material 

Point 41, NW%SE~~ sec. 6: 

1. Shale, bentonitic, brown _____________ _ 

B d L {2. *Bentonite, brownish-gray, waxy _____ _ 

e ---- 3. *Bentonite, light-yellowish-green, ironstained __________________________ _ 

4. Shale, dark-gray. 

Point 42, SW%NE% sec. 6 

1. *Bentonite, dark-gray, waxy (lower 9% 

ft. not penetrated in drilling) _______ _ 

2. *Bentonite, greenish-gray, waxy, with 

dark mineral particles _____________ _ 

Bed lvf _ _ 3. *Bentonite, olive-green, waxy ________ _ 

4. *Bentonite, olive-green, waxy, with 

many dark mineral particles ________ _ 

5. *Bentonite, light-gray, with some orangered laminae, waxy, many dark mineral 

particles present __________________ _ 

6. *Shale, brown, soft. 

Point 43, NE}:i'NW7~ sec. 16: 

Soap 

Creek 

1. *Bentonite, dark-gray and olive-green 

interlaminated, waxy, gypsiferous ___ _ 

2. *Bentonite, light-olive-green, waxy, with 

dark mineral particles _____________ _ 

*Bentonite, olive-green (darker than 

bed ____ 3. 

abov~, waxy _____________________ _ 

4. Shale, dark-gray, hard, siliceous, chertlike. 

Point 44, NW~~NE7~ sec. 16: 

1. Shale, brown, soft ___________________ _ 

!

2. *Bentonite, brownish-gray, granular, 

Bed "AI___ gypsiferous _______________________ _ 

3. Bentonite (not penetrated in drilling) __ _ 

Point 45, SW7~NE7~ sec. 27: 

1. Shale, brown, soft. 

Q {2. *Bentonite, dark-grav and olive-green . ooap . . . -

C k mterlammated, waxy ______________ _ 

ree 3 *B t · · h ' ' 

1 d . en mute, gray1s -green, "axy ______ _ 

)e ---- 4. *Bentonite, olive-green, waxy ___ - - __ --

5. Covered. 

T. 9 S., R.34E. 

Thickness 

A.ll beds Bentonite onll' 

Feet 

9 

14 

2 

3 

15 

10 

3 

1 

10 

22 

2 

5 

Inches Feet Inches 

11 

: } 

6 

1 

5 

1 

8 

3 

9 

20 

6 

} 10 

g } 29 

10 

11 

5 

10 

The northeastern corner of this township is about 5 miles southwest 

of Wyola, a small town on the Chicago, Burlington and Quincy Railroad. Most of the township is accessible by gravel roads extending 

southwestward along the flood plain of Little Bighorn River and by 

unimproved roads and trails. The topography, both north and south 

of Little Bighorn River, is moderately rugged. 
94 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Terrace gravel is present on high ridges in sections 6 and 25, and 

gravel-capped terraces occur at lower levels north of Little Bighorn 

River. The bedrock beneath lands aggregating several square miles in 

area is concealed by gravels capping the terraces and by alluvial deposits underlying the flood plains of Little Bighorn River, several of 

its tributaries, and Gay Creek, in section 36. 

The principal structural features of this township are a large anticline that extends eastward across the central part and a syncline 

jn the southeastern part. The anticline is essentially a broad structural nose that plunges eastward, but it bulges locally at the eastern 

margin of the township to fonn the Black Gulch dome. A nearly 

vertical westerly-trending fault, downthrown on the southern side 

and having a maximum displacement of more than 1,000 feet, cuts 

the northwestern side of the Black Gulch dome, crosses the structural 

sag west of it, and continues westward for several miles on the southern flank of the large plunging anticline. The oldest rocks appearing 

at the surface in this arched structure are pre-Cretaceous strata that 

crop out in the southwestern and west-central parts of the township; 

farther east, along its crest and on its flanks, beds of Cr~taceous age 

(the youngest being those of the Carlile-Niobrara men1ber) crop out 

]n the synclinal area to the south and in two areas along the eastern 

and northern margins of the township. 

Bentonite beds are present in the Thermopolis, Mowry, Belle 

Fourche, and Greenhorn units. Some of these beds are so thick that 

large tonnages of bentonite are available for mining, notwithstanding 

unfavorable topographic relations along their outcrops (fig. 2), 

whereby deposits under light overburden occuring mostly in narrow 

belts. 

The Clay Spur bentonite bed, as measured at point 48 in section 

14, is 4 feet, 9 inches thick. Tests of a sample taken here indicate 

that this bed contains drilling and foundry clay that is comparable 

jn quality to much of the bentonite that is mined in the Black Hills 

district. The bed crops out in several )laces within the township 

but offers no attractive mining sites. 

Bed I, as measured at point 50 in section 14, is 7 feet thick and contains good foundry clay. This clay 1night be used as drilling-mud 

material, but, like the Clay Spur bed, bed I here contains only negligible reserves. 

Bed J, as measured at point 49 in section 14, is 6lf2 feet thick, and 

nt point 51 in section 34, it is 3"lh feet thick. At both points this bed 

contains good foundry clay. Although the bed crops out extensively 

in this township, it is not present at sites that could be mined 

economically. 
BENTONITE DEPOSI'J.iS 95 

The Soap Creek bentonite bed, as measured at point 47 in section 

13, is 22 feet thick and is overlain by approximately an equal amount 

of bentonitic shale. Brown calcareous concretions, as much as 8 feet 

}n diameter, are abundantly scattered along the weathered outcrop, 

especially in sections 13, 14, and 15. The bed contains good foundry 

clay. A reserve of about 7,900,000 short tons of accessible bentonite 

is estimated to be available \vithin this township. 

Bed lJJ, as measured at point 46 in section 13, is 33 feet thick. 

T'his bed clips about 10 degrees northeast and its weathered outcrops 

are strewn with light-gray calcareous concretions, many of which are 

about 3 feet in diameter. At point 46, the bed contains foundry clay 

having good green strength; in sections 13 ancl24, it is estimated that 

the minable reserve amounts to about 4,950,000 short tons of bentonite. 

Following are measured sections of the bentonite beds and associated rock materials at seven numbered points in T. 9 S., R. 34 E. 

SecHons measured in T. 9 S., R. 34 E. 

Alate rial 

Point 46, NE.%:SW7i sec. 13: 

1. Shale, brown, soft. 

1

2. *Bentonite, gray, granular (lower 10 ft 

not penetrated in drilling) __________ _ 

Bed 111 _ _ _ 3. *Bentonite, light-green, waxy (lower 

6% ft not penetrated in drilling) ___ _ 

4. *Bentonite, yellowish-green, waxy _____ _ 

5. Shale. 

Point 47, SW~iSWU sec. 13: 

1. Shale, bentonitic, brownish-gray, soft. 

S 

{

2. *Bentonite, olive-green and dark-gray 

oap C k m · · t er 1 amma · t d e , waxy ______________ _ 

b':~~ ___ 3. *Be~tonite, oli:e-green, waxy, with dark 

mmeral particles __________________ _ 

4. Shale, dark-gray, hard, siliceous, chertlike. 

Point 48, NE.%:SW7i sec. 14: 

Clay {1. *Bentonite, gray, waxy ______________ _ 

Spur 2. *Ben~onite, light-green, somewhat darkbed____ er m lower part, waxy _____________ _ 

3. Shale, gray, hard, siliceous, chert-like. 

Point 49, NEUSEU sec. 14: 

1. Shale, dark-gray, soft. 

{

2. *Bentonite, light-green ______________ _ 

Bed J _ _ _ _ 3. *Bentonite, dark-green, waxy _________ _ 

4. *Bentonite, gray, waxy ______________ _ 

5. Shale, dark-brownish-gray ___________ _ 

Thickness 

All beds Bentonite only 

Feet Inches Feet Inches 

16 

14 

8 

4 

2 

3 

6 l 

~ l 

33 0 

0 

6 
96 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Sections rneasured in T. 9 S., R. 34 E.-Continued 

Thickness 

All beds Bentonite only 

:Material 

Point 50, SE;!iSE;!i sec. 14: 

1

1. *Bentonite, dark-gray and olive-green 

interlaminated, waxy ______________ _ 

Bed [_ _ _ _ 2. *Bentonite, greenish-gray, waxy ______ _ 

3. *Bentonite, olive-green, waxy ________ _ 

4. *Bentonite, greenish-gray, waxy ______ _ 

Point 51, NW;!iNW;!i sec. 34: 

1. Shale, brownish-gray, sandy. 

Bed J ____ 2. *Bentonite, dark-greenish-gray, waxy __ 

3. Shale, brownish-gray, sandy. 

T.9 S.,R.35E. 

Feet Inches Feet Inches 

2 

~ l 7 ----

2 

1 

3 6 3 6 

U. S. Highway 87 and the Chic,ago, Burlington -and Quincy Railroad extend northward across the eastern part of this township, the 

southern boundary of which lies along the Montana-Wyoming State 

line; the northeast corner is about 3 miles southeast of the railroad 

station at the small town of Wyola. The bentonite processing plant 

of the Wyotana Mining Co. is at the Aberdeen siding of the railroad, 

in sec. 13, and a graded road that enters the township near its southwest corner has been used for hauling bentonite to this plant from 

the small mines in 1'. 58 N., R. 88 "'V., Sheridan County, Wyo. A 

graveled road, leading southwestward and up the valley of Little 

Bighorn· River from vVyola, crosses the northwest corner of the 

township. 

The surface configuration of this township is comparatively rugged, 

with a total relief of approximately 1,150 feet. The highest land 

lies along a divide that extends northeastward between the valley 

of Little Bighorn River and the valley in the central part of the township that is drained by Pass Creek and its tributary West Pass Creek. 

Beneath the flood plains of Little Bighorn River, Pass Creek, and 

their tributaries, the bedrock is concealed by Recent alluvial deposits. 

In the southern and eastern parts of the township, a few small tracts 

on strea1n terraces at higher levels are occupied by older alluvial 

deposits. 

The overall dip of the bedrock strata is eastward. The oldest 

strata exposed, belonging to the Mowry shale, occupy the central part 

of Black Gulch dome, the apex of which is at the western side of the 

township; the youngest strata belong to the Judith River formation, 

which extends northward through the easternmost tier of sections. 

The geologic structure of a tract that extends from Black Gulch 

dome eastward across the central part of the township is essentially 
BENTONITE DEPOSITS 97 

that of a broad, eastward-plunging anticlinal nose; the axis of this 

anticlinal nose lies about midway between two broad, shallow synclines, with axes also plunging eastward, which occupy large parts 

of the northern and southern halves of the township, -respectively . 

. A. normal fault, with a northeasterly trend, crosses the anticlinal 

axis diagonally near the center of the township, displacing the rocks 

on its southeasten1 side downward about 400 feet and thereby offsetting the southerly-trending belts of outcrop of the upper members 

of the Cody formation more than 1 mile to the southwest. The displacement on this fault decreases progressively, in opposite directions 

along its trace, from its interseetion with the anticlinal axis and is 

believed to die out within the east and west limits of the township. 

At point 48, about llh miles west of the township, the Clay Spur 

bentonite bed is 4 feet, 9 inches thick and contains clay of good quality. 

This bed is also exposed for a short distance in the southwest quarter 

of section 19, T. 9 S., R. 35 E., but, because of its steep clip, it contains 

only a small amount of minable clay. 

The Soap Creek bed and bed 111 crop out in section 19 on the steeply 

dipping eastern and northern flanks of Black Gulch dome. One mile 

west of this township, at point 47, the Soap Creek bed is 22 feet thick, 

and at point 46, in the same neighborhood, bed M is 33 feet thick. 

The beds at both of these points contain good foundry clay. Assuming 

that comparable thicknesses occur in section 19, T. 9 S., R. 35 E., 

it is estimated that the Soap Creek bed here contains about 150,000 

and that bed 11/ contains about 1,850,000 short tons of crude bentonite 

Gnder less than 30 feet of overburden. 

A bentonite bed that crops out intermittently in the north-central 

part of the township is nearly equivalent, stratigraphically, to bed R. 

The bentonite of this bed, as observed at its few exposures, is dark 

in color, and inasmuch as it supports a growth of grass along most 

of its outcrop, it probably has a high content of shaly impurities 

with correspondingly low colloidality. This bed seems to have little 

potential value. Because the bed was estimated to be only 11;2 to 2lf2 

feet thick, it was neither measured nor sampled within this township; 

however, its outcrop is mapped on plate 1. 

A bentonite bed at approximately the stratigraphic position of bed 

U crops out conspicuously in sections 2, 9, 10, and 15. The bentonite 

of the lower two-thirds of this bed weathers very light gray; its total 

thickness at point 51A is 13 feet. Assuming that 13 feet is the average 

thickness for the township, the bed here contains a reserve of about 

2,000,000 tons of bentonite under less than 30 feet of overburden. 

Test results indicate that the colloidality of this 1naterial is relatively 
98 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

low, but that it has potentialities for use by foundries as sand-bonding 

material. The best prospective mining sites along outcrops of this 

hed lie within 4 miles of the bentonite processing plant of the Wyotana 

l~ining Co. Following is a section that was measured at one of 

these sites: 

Section measured in T. 9 S., R. 35 E. 

Thickness 

All beds Bentonite only 

J.Iaterial Feet Inches Feet Inches 

Point 51A, SWH sec. 10: 

1. Bentonite, gray with laminae of darkgray shale (not drilled or sampled) __ _ 3 0 --- ---- 2. Bentonite, light-yellowish-gray, granular_ 10 6 

3. Bentonite, very light yellowish gray, 

Bed U? __ 4. Be~~:o:~~~~-~~~~--lf~ht-;~ll~~i~h--;r-a;, 2 0 13 0 

granular, with particles of biotite and 

other dark minerals _______________ _ 6 

5. Shale, soft, dark-gray. 

Clinker was quarried years ago for road metal from a small working 

in section 36, and a s1nall amount of coal fonnerly was mined in 

sections 26 and 36. A large amount of gravel has been hauled recently 

from terrace deposits in section 13 for use on U. S. Highway 87. 

T. 58 N., R. 88 W., SHERIDAN COUNTY, WYO. 

This township, which is bounded on the north by the Montana 

State line, lies directly south of T. 9 S., Rs. 34 and 35 E. It can be 

reached over a road leading southwestward from Aberdeen, Montana, 

a siding of the Chicngo, Burlington and Quincy Railroad 3 miles north 

of the State line. The part of the township shown on plate 1 is a strip 

of land about 1:1;3 miles wide, extending about 5 miles eastward from 

the west township line. 

The topographic configuration of this strip is moderaJtely rugged. 

Gay Creek, which flows northeastward across its western part, and 

West Pass Creek, which crosses the central part, have narrow flood 

plains flanked by tracts of hilly country. Recent alluvial deposits 

underlie the flood plains; older gravel caps a high terrace remnant 

in section 20. 

In the west-central part of the strip, the bedrock dips locally in a 

northerly direction toward the axis of an eastward-trending syncline 

that lies within rthe adjacent part of Montana. Everywhere else 

within the strip the dip is toward the northeast. The oldest exposed 

rocks belong to the Cloverly fonnation, which crops out in the south-
BENTONITE DEPOSITS 99 

west corner of the strip; the youngest bedrock is the unnamed Eagle 

equivalent, which was identified in the northeast corner of the strip. 

In the· intervening land,. strata of the Thennopolis and Mowry formations, as well as the Belle Fourche, Greenhorn, and CarlileNiobrara members of the Cody formation, crop out in belts trending 

northwestwa.rd. Bentonitic material is present in all these units, 

but the only beds considered to be possible sources of commercial 

bentonite are the Clay Spur bed, at the top of ~iowry, and rtwo bedsbed J and the Soap Creek bed-in the Belle Fourche member. 

At point 53, the Clay Spur bentonite bed crops out with a thickness 

of 5 feet, 1 inch. Tests of two samples obtained at this point indicate 

that the bed contains n1aterial well· adapted for use as foundry clay, 

and one sample, taken from the lower 3lj2 feet, showed possibilities 

as drilling clay. However, only small amounts of the bentonite of 

the Clay Spur bed lie under less than 30 feet of overburden, and there 

are no attractive mining sites within the area under discussion. This 

is also true of bed J, which is 31h to 6lf2 feet thick in the adjacent 

part of Montana. Bed J has not been measured nor sampled in this 

township. 

As measured at point 52, the Soap C~eek bentonite bed is 20¥2 feet 

thick, including 7 feet of dark, impure bentonitic material at the 

top of the bed. Two samples of bentonite from this point indicate 

the presence of good foundry clay and also material that can be 

used as drilling clay. The bed has been strip-mined at a small working of the Wyotana Bentonite Mining Co. in sections 17 and 18, 

where it crops out on a steep south-facing hillside overlooking the 

valley of Gay Creek and in a s1nall mine in section 21 on the west 

side of West Pass Creek. Owing to steepness of both the dip and 

the land surface at these sites, 1nining is limited to belts only a few 

yards wide along the outcrops. At both sites, 1nining has proceeded 

from east to west along the outcrops, and the crude bentonite has 

been hauled 10 to 12 miles by truck to a. small processing plant at 

Aberdeen siding. So far as known, all of the small commercial production of bentonite from the Hardin district has come from these 

two mines. Cmnparable sites for mining of the Soap Creek bed 

lie in sections 20 and 22, and it is estimated that this bed contains 

2,000,000 short tons of clay under less than 30 feet of overburden in 

the strip of land under discussion. 

Following are sections of the Soap Creek bentonite bed measured 

at point 52 and of the Clay Spur bed measured at point 53: 
100 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Sections measured in T. 58 N., R. 88 W. 

Thickness 

All beds Bentonite only 

Jl.1aferial Feet Inches Feet Inches 

Point 52, SW%SWX sec. 17: 

Soap 

Creek 

bed ___ _ 

1. Bentonite, dark-gray, with a few laminae of green waxy bentonite _______ _ 

2. *Bentonite, greenish-gray, waxy, gypsi- ferous ___________________________ _ 

3. *Bentonite, light-greenish-gray, waxy, 

gypsiferouR, with dark mineral particles ___________________ - - - - - - - - - -

4. *Bentonite, olive-green, waxy, with dark 

mineral particles _______ ~ __________ _ 

5. Shale, dark-gray, hard, siliceous, chertlike. 

Point 53, NWXNW%: sec. 20: 

1. Shale, brownish-gray ________________ _ 

2. *Bentonite, dark-brown, waxy ________ _ 

3. *Bentonite, light-yellowish-gray and 

Clay brown interlaminated, waxy _______ _ 

Spur 4. *Bentonite, light-yellowish-gray, waxy __ 

bed ____ 5. *Bentonite, light-bluish-gray, waxy ___ _ 

6. Bentonite, light-gray, waxy, limonitestained __________________________ _ 

7. Shale, dark-gray, hard, siliceous, chertlike. 

7 

4 

8 

1 

1 

1 

3 

.5 

6 

3 

6 

6 

6 

20 

3 5 

6 

4 

LABORATORY PROCEDURES USED FOR TESTING 

THE BENTONITES 

By SAM H. PATTERSON 

5 

1 

The purpose o£ this chapter is to outline several laboratory testing 

procedures which have been employed in evaluating bentonite £or use 

by the foundry industry as molding-sand bonding clay, and by the 

petroleum industry as rotary well-drilling clay. The resulting test 

data, together with geologic descriptions o£ the various bentonite 

deposits studied, are presented in other sections and in table 1 o£ this 

publication, and in other publications of the Geological Survey 

(Knechtel and Patterson, 1952, 1955). 

Modern research procedures that are employed in studying the 

identity, character, and mode of origin of clays include differential 

thermal analysis, X-ray diffraction, and electron-microscope studies 

as well as optical exa1nination, ion-exchange measurement, and chemical analysis. By such diverse means, a large amount of information 

has been gathered concerning the composition, geochemistry, and 

petrology o£ bentonite and related clay materials; much of this in£orInation has been summarized by Ross and Hendricks (1945, p. 1-79) 

and by Grim (1953). Resulting concepts of the composition and 
LABORATORY PROCEDURES USED FOR TESTING THE BENTONITES 101 

crystalline structure of clays aid in explaining some of the distinctive 

properties that determine the commercial value of bentonites. For 

example, such concepts are, in large part, the basis for conclusions 

reached by Grim and Cuthbert (1945, p. 1-55 and 1946, p. 1-36) regarding the factors affecting the bonding properties of clays used in 

preparing synthetic molding sands. 

While mineralogical and chemical information is important in any 

effort to explain such properties, commercial evaluation and selection 

of bentonite deposits to be mined are more largely based on descriptions of certain physical properties of the contained material as 

reve.aled by various laboratory tests. Thus, bentonite used by the 

foundry industry as sand-bonding clay must possess the required 

green or dry bonding strength (or both) , and bentonite used for 

drilling-mud must show adequate viscosity, yield, gel strength, and 

0ther colloidal properties. Whether or not the bentonite of any 

deposit possesses such properties in sufficient degree to be mined and 

sold at a profit is generally determined by means of tests of the type 

herein described as well as by its actual use in industrial operations. 

These tests are perfonned pri1narily for the purpose of evaluating 

bentonite for use in foundry-sand bonding 1naterials and in rotarywell drilling muds; however, the test results are also helpful in 

evaluating the bentonite for other uses. Drilling-mud tests, because 

they are related to dilatancy and viscosity, are thus helpful in evaluating bentonite for suspending agents, for water impedance, and for 

other uses. Color is a primary requirmnent for bentonite used in 

various products. The bentonite in phar1naceutical preparations, 

paper filling, and for other purposes must be nearly white. Color of 

the bentonite was determined by cmnparing small an1ounts of ground 

bentonite with the "Rock-color chart" (Goddard, 1948) . 

The samples of raw bentonite to be tested were air dried at room 

temperature, crushed in a small laboratory crusher to one-fourth of 

~tn inch or finer, and ground fine enough in a disc-type grinder to permit approximately 90 percent of the pulverized material to pass 

through a 200-mesh sieve. Some of the samples required drying with 

the aid of an electric fan, for periods ranging from 2 to 10 days before 

they could be ground. The pulvbrized samples 'vere reduced to con1-

parable moisture content by additional drying in an oven at 105° C. 

for 4 hours, and then were placed in desiccators and cooled to room 

temperature. 

USE AS FOUNDRY SAND-BONDING CLAY 

Tests having to do with use of the sampled materials in bonding 

foundry sands, as described in this report and in studies of the Northe-rn Black Hills district (Knechtel ani Patterson, 1955) were made 
102 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

according to the procedure set forth by the American Foundryman's 

Society (Anon., 1952, p. 17-28, 82-85) . 

In naturally bonded molding sands which were formerly used by 

all foundries in the 1nanufacture of metal castings, constituents such 

as clay and organic matter serve as the bonding material. Most of 

the sand now used in preparing molds for such use is synthetically 

bonded, and much of the bentonite that is produced in the Missouri 

River basin is marketed as sand-bonding clay. Synthetically bonded 

sand is prepared by a thorough mechanical mixing of clean sand with 

proper amounts of one or more carefully selected bonding agents such 

as underclay, bentonite, or certain substances present in cereals. The 

resulting mixture, when made ready for casting, must be sufficiently 

strong to withstand the erosive or cutting action of liquid metal as 

it flows into a mold, but it must not be so strong that it interferes with 

free contraction of the metal in contact with it during cooling and 

solidification; such interference might cause difficulty in removing the 

1nold from the hardened casting. 

The quality of bentonites in relation to their suitability for use as 

bonding clays cannot be determined satisfactorily by any known field 

tests and therefore 1nust be judged largely by the results of timeconsuming laboratory tests of samples, whereby their bonding strength 

rnay be compared with that of clays actually used in foundries. 

However, other significant properties of bonding clays are not revealed in the laboratory tests, and the need for final test runs in foundries has been emphasized by Grim (1950, p. 1) as follows: 

In order finally to prove the worth of a clay for foundry use, it must be tried 

in actual foundry practice by individual foundries, because each foundry has its 

own peculiar practices and because certain attributes of molding sand-clay 

mixtures show up only when metal is actually cast in them. On the basis of 

determinations of green and dry compression strength, it may be determined 

whether or not a clay has any potential use as a foundry bonding agent. If 

a clay shows strength value at least equal to those of clays currently used in 

foundries, obviously it has potentialities for this use and is worth actual foundry 

trial. 

In production of castings, the two molding processes employed 

are known as "green-sand molding" and "dry-sand molding." In 

green-sand molding, which is the process employed in making most 

iron castings, the metal is poured into moist sand 1nolds, whereas in 

dry-sand casting, which is employed commonly in the manufacture 

of large castings, the 1nold is dried thoroughly before the metal is 

poured. 

According to a publication of the American Foundrymen's Society 

(Anon., 1944, p. 73-74) : 

In many cases, the use of green sand has been found essential to the commercially satisfactory manufacture of steel castings of intricate designs. The 
LABORATORY PROCEDURES USED FOR TESTING THE BENTONITES 103 

resistance of dry sand molds and cores to the solid contraction of steel frequently 

prohibits their economieal applieation where light and intricate castings must 

be produced. Good green sand does not exert as much resistance to the normal 

contraction of the casting as does dry sand, and, consequently, severe strains 

which might result in hot tears or craeks often are avoided. 

In the bonding tests conducted during the investigations of the 

bentonite deposits described in the present bulletin and in studies of 

the Northern Black Hills district ( l{nechtel and Patterson, 1955) , the 

procedure outlined in detail in the Foundry Sand Handbook (Anon., 

1952, p. 17-28, 82-85) was followed closely except for the type of sand 

used. The handbook ( p. 28) defines standard testing sand in terms of 

U. S. Standard Series Sieve Nos. as follows: 

The standard sand shall consist of washed and dried, rounded-grain, silica sand. 

All grains shall pass through a No. 40 sieve, 95 pereent passing the No. 50 sieve 

and remaining on the No. 70 sieve. Any grains passing the No. 70 sieve shall 

remain on the No. 100 sieve. This sand shall have an A. F. S. fineness number 

of 50 plus or minus 1. 

However, the cost of preparation of standard sand 1nakes its use 

prohibitive for testing of nu1nerous samples. A sand produced in 

Illinois, which approximates the foregoing rigid specifications, was 

therefore substituted. The table below gives the results of a sieve 

analysis of this Illinois sand as compared with the grain-size requirelnents for standard sand. 

Percentages of standard and substituted sand reta£ned on five United States standard 

sieves 

Percent, by weight, retained 

United States standard sieve No. 

Substituted A. F. S. standard 

sand sand 

30 ___________________________________________ _ 0. 10 0 40 ___________________________________________ _ 16. 63 0 50 ___________________________________________ _ 56.41 0-5 70 ___________________________________________ _ 17.40 95-100 

100 __________________________________________ _ 7. 98 0-5 

Pan __________________________________________ _ 1. 46 0 

------- Total __________________________________ _ 99. 98 100 

The substituted sand, as shown by the table, differs rather widely 

from the standard sand in the percentages passing through and retained on the different sieves. The results of bonding tests in which 

the substituted sand is used nevertheless correspond closely to those 

obtained with the standard sand; the slightly excessive percentage 
104 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

of fines tends to give greater compression strength to sand-clay mixtures containing the substituted sand, but this tendency is nearly 

counterbalanced by the weakening effect of the excessive percentage 

of coarse grains. 

Progress toward an understanding of the bonding action of clays 

in foundry sands has resulted from the work of Grim and Cuthbert 

(1945, p. 1-55; 1946, p. 1-36) in the United States and from the 

work of Davies (1946, p. 6-46) in England. These authors have 

dmnonstrated that the green-compression strength of sand-clay mixtures differs from the dry-compression strength of the same mixtures 

in its manner of responding to variation in the amount of tempering 

water used in their preparation. Whereas the green-compression 

strength of a given mixture of sand and bentonite is reduced when 

water is added slightly in excess of an optimum amount (fig. 12A), 

maximum dry-compression strength is attained when much larger 

amounts of water have been used in preparation of the mixture for 

drying (fig. 12B). More than 95 percent of the several hundred mixtures tested during the investigations in the Hardin and Northern 

Black Hills districts reached their optimum green-compression 

strength when only 1.3 to 1.5 percent of tempering water ·was present. 

Nearly all the mixtures tested required the presence of n1ore than 

2.2 percent of tempering water to reach their maximmn dry-compression strength. }\fixtures of sand and clay that maintain high 

green-compression strength throughout a considerable range of tempering-water content and have high dry-cmnpression strength when 

low proportions of tempering water are used in their preparation 

are especially desirable. 

Because high dry-compression strength occurs only when considerably more tempering water is used than is necessary to attain maximum green strength, adequate evaluation of both green and dry 

strength requires preparation and testing of several mixtures of each 

bentonite sample that differ only in their moisture content. In an 

effort to dete;rmine approxi1nately the optimum attainable green 

strength for each san1ple tested, quadruplicate 2-kilogram mixtures 

were prepared containing 4 percent clay by weight, or 80 grams of 

the bentonite of each s:unple to 1,920 grams of sand. Occasionally 

a fifth mixture was added, as represented in figure 12A, in order to 

complete the curve. Accordingly each of the 2-kilogram mixtures 

was mulled for 2 minutes in a mechanical mixer. The required range 

of tempering water was then introduced by adding 35, 40, 50, and 60 

milliliters of water, respectively, to the four 1nixtures and each mixture was again n1ulled for 5 minutes. Immediately after mulling, 

each mixture was riddled through a screen of one-quarter-inch mesh 
LABORATORY PROCEDURES USED FOR TES'TING THE BENTONITES 105 

and kept in an airtight glass jar for at least 2 hours. Standard test 

specimens were formed from each mixture by weighing out the necessary amount, placing it in a core mold, and ramming it three times 

with an A.. F. S. standard ramming apparatus. The cores were then 

stripped from the mold and immediately tested for green compression strength in an apparatus of the dead-weight type. Because of 

appreciable variations in the figures obtained in testing different cores 

prepared fron1 the same mixture, the average for four to eight cores 

of each mixture was calculated and recorded as its green-compression 

strength. 

During the testing procedure, duplicate portions of each of the four 

mixtures were placed in weighing vials for use in detennining the 

percentage of tempering water. To obtain the data for calculating 

this figure, each duplicate portion was weighed, dried overnight at 

105°C, brought to room temperature in a desiccator, and reweighed. 

The percentage of tmnpering water and the green compression strength 

for each of the four mixtures were then plotted, as illustrated in figure 

12A, and a curve was drawn from which the strength for any given 

percentage of water could be estimated. 

For most of the samples tested, a sufficient increase in the percentage 

of tempering water would produce a dry-compression strength exceeding 100 psi, which is the maximmn stress that could be applied 

with the core-breaking apparatus employed. Inasmuch as 100 psi 

exceeds nearly all specifications for clay used by foundries in drysand molds, determination of maximmn dry-compression strength 

has not been attempted. 

The dry-compression tests for each sample of bentonite were made 

only on dried cores of the two wettest n1ixtures prepared for the 

green strength tests. Six cores, which were prepared from each of 

the two mixtures containing 50 to 60 milliliters of water, respectively, 

were dried in an oven at 105°C for 2 hours, were reduced to room 

temperature in desiccators, and were then tested for dry-compression 

strength in the same apparatus that was used in determining green 

strength. A.s a rule, three or four out of the six test readings for 

each mixture were in close agreement, whereupon the average for this 

group was calculated and recorded as the dry-compression strength. 

When the figures failed to fall ir-ito such a group, the average of all 

six readings was recorded. The dry-compression strength of the 

two mixtures was plotted against corresponding percentages of tempering water (same data as calculated for green strength) and curves 

were drawn which permitted .estimation of dry-strength values for 

amounts of tempering water ranging from 1.6 to 2.2 percent. The 

manner in which a dry-strength curve is prepared is illustrated by 
106 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

100 

:I: 90 ..... 

(!) 

z 80 w 

0::-

t; ~ 70 

~ g 60 

(f) a:: 

~~50 

0:: 

Cl. u) 40 ~m 

o....~ 

(.) -30 

>-

0:: 

0 20 

10 

J: 

I--

C) 

z 

w 12 

o::-

t;~ 

zo 

Q(/) 

(/)a: 8 

(/)LLJ 

~Cl. 

Cl.. 

~(/) 

om (.):::::!. 4 

z 

w 

w 

0: 

(!) 

1.0 

/' lb. 

, ~ ~ 

1.0 1.5 2.0 

TEMPERING WATER (PERCENT) 

A 

L 

/_ 

rl v 

/ 

v 

/ 

/ 

I 

I 

~ 

I 

1.5 2.0 2.5 3.0 

TEMPERING WATER {PERCENT) 

8 FIGURE 12.-Curves for a sample of bentonite from the Soap Creek bed. A, Variation of 

green bonding strength according to percentage- of tempering water in sand-clay mixture 

at time of test; B, variation of dry bonding strength according to percentage of tempering water in sand-clay mixture when cores were made. 

the curve shown in figure 12B. This curve represents a wider range 

of moisture content than was used in any of the other dry-compression 

tests performed during the present investigation and illustrates the 

manner in which the dry strength increases with increasing percentages of tempering water. 
LABORATORY PROCEDURES USED FOR TESTING THE BENTONITES 107 

USE AS A CONSTITUENT OF ROTARY WELL-DRILLING lVIUD 

Most of the tests relating to properties of significance in evaluating 

the samples as drilling-mud materials were made according to procedures employed by companies producing bentonite in the vicinity of 

the Black Hills. Most of these procedures have been outlined by Fisk 

(1946, p. 1-39). The apparatus used has been described by the American Petroleum Institute (Anon., 1942, p. 1-14), and by Stern (1941, 

p. 29--42). 

Many types of clay and other substances, including artificially prepared mixtures, are used in rotary well-drilling fluids; the preparation of these fluids has become a specialized field. Bentonite with a 

high dilatancy, in addition to various other colloidal properties, is 

much in demand for use as one of the ingredients because of its efficiency in performance of several useful functions. Its role as a 

drilling-fluid constituent is briefly smnmarized by Fisk (1946, p. 7) 

as follows: 

In the rotary drilling of wells, a "mud" is pumped down a drill pipe carrying the 

bit to wash away cuttings and to raise them to the surface through the annular space between the drill pipe and the walls of the hole. The mud also functions to seal the walls of the hole and prevent water loss, as well as ingress of 

water from the formations penetrated. Bentonite added to the drilling mud 

enhances its lubricating and wall-sealing properties and also, due to its thixotropic properties, assists in keeping cuttings from settling in the hole in the 

event of a shutdown of the drilling equipment. Where weighting additions to 

the mud such as barite are necessary to hold gas pressures, bentonite aids in 

maintaining the weighting material in suspension. 

Although opinions differ regarding the value of certain tests for 

determining the suitability of clays for use in preparing drilling 1nud, 

nearly all investigators agree that some t£lsts give useful results. In 

the wall-building tests, which are designed to determine the ability 

of a mud to form a thin, impervious cake under pressure, conditions 

in the testing apparatus differ somewhat from those encountered in 

the drilling of wells, but the results of these tests, as well as those 

relating to the characteristic known as ;'yield" are unquestionably 

valuable. The data on yield provide a basis for estimating the 

amount of any given clay that would be re·.quired for preparation of a 

drilling mud of any desired viscosity and volume. They also afford 

a rough idea of the gel strength of the n1ud, which tends to be greatest 

for clays that test high in yield; such clays moreover, generally contain 

the smallest percentages of grit. 

DILATANCY (SWELLING CAPACITY) 

All the bentonites from the Missouri River basin that are regarded 

as high-grade drilling-mud material possess a marked propensity 

347624-56--8 
108 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

to absorb water and thereupon to swell. However, some bentonites 

having ample dilatancy lack some of the other essential properties 

of good drilling-1nud 1naterial. Therefore, swelling tests were made 

on all of the samples under investigation and the samples that showed 

moderate-to-high dilatancy were tested further to determine whether 

or not they represent material that would be suitable for use as 

drilling-mud material. 

The procedure outlined by the Americ.an Colloid Co. (Anon., 1945, 

p. 1, 2) was followed in making the swelling tests. Two grams of 

ground bentonite were added slowly to 100 milliliters of distilled 

water in a graduate, by sprinkling very small amounts on the surface 

and allowing them to settle before adding more material. One hour 

nfter the last grains had been added, the volume of the resulting gel 

in milliliters (cubic centimeters) was recorded as the "swelling capacity" of the bentonite. 

A dilatancy of 20 milliliters was arbitrarily chosen as the minimum 

requirement in selecting clays to be tested further for use in drilling 

mud; only a few samples having lower dilatancy were tested for 

viscosity, yield, gel strength, and wall-building efficiency. Few, if 

any, of the samples that swell to less than 20 ml represent drilling-mud 

1naterial of the high quality demanded by present 1narkets, and the 

time consumed by extensive tests of such samples would therefore 

not be commensurate with the value of the results. However, some 

of these materials might be satisfactory under special conditions such 

as might exist if wells were being drilled close to the bentonite deposits, where the disadvantage of using large amounts of bentonite 

of an inferior grade might be offset by elin1ination of transportation 

costs. 

VISCOSITY 

Viscosity, which is an important property of drillings muds, is 

defined as a measure of a fluid's internal resistance to flow; it is usually 

expressed in centipoises. The standard apparatus for measuring the 

viscosity of drilling fluids is the Stormer viscometer (Anon., 1942, 

p. 3-6). The viscosity of an aqueous suspension of a clay depends 

on the proportions of clay and water used in its preparation; consequently, viscosity test data relating to different sa1nples are readily 

comparable only for suspensions containing the same proportions 

of clay and water. For each of the sa1nples to be tested for viscosity, 

a 6-percent-by-weight suspension of clay was prepared, as well as 

two or more suspensions containing other percentages of clay. These 

additional suspensions were tested primarily to obtain viscosity data 

for computing yield by the method to be described below, but the 

results of the three or more viscosity measurements also show the 
LABORATORY PROCEDURES USED FOR TESTING THE BENTONITES 109 

extent to which the viscosity of suspensions of each sampled clay 

varies in consequence of known variations in their clay content. 

In preparing the suspensions, the clay and distilled water were 

carefully weighed at room temperature; however, the water could 

have been measured volumetrically without serious loss of accuracy. 

Then, they were mixed in an electric blender for 3 1ninutes, with one 

brief interruption for scraping down the lumps from the sides of 

the cup with a spatula; immediately after mixing, the viscosity of a 

portion of the suspension was determined by means of a Stormer 

viscometer. 

YIELD 

Mainly in consequence of differences in the dilatancy of bentonites, 

equally viscous suspensilons prepared frmn equal weights of these 

clays contain variable amounts of water and may accordingly differ 

greatly in volun1e. The volume, in 42-gallon barrels, of an aqueous 

suspension with a viscosity of 15 ce·ntipoises that can be prepared 

from 1 short ton of clay is kno·wn as its yield ("\Villiams and others, 

1953, p. 7 ; Knechtel and Patterson, 1952, p. 5) . In general, clays 

showing satisfactory yield possess nwst of the properties that are considered desirable for preparation of drilling mud. 

The laboratory investigation of the bentonites from the Hardin 

district, described in this report, and from the northern Black Hills 

district (l(nechtel and ]Patterson, 1955) has been greatly facilitated 

through use of a diagram showing "Representative yield curves," 

'vhich was furnished by the Baroid Sales Division of the National 

Lead Co. The curves show the relation between viscosity and the 

proportions of clay and water used in preparing various drilling 

muds. Use of these curves permits a rough estimate of the percentage of the clay of each sample that will produce a suspension having 

any desired viscosity, after the viscosity of a suspension containing a 

known percentage of the clay has been determined. 

The viscosity figure that was obtained for each 6 percent-by-weight 

suspension described above was accordingly compared with this diagram and the percentage of clay of each sample that would be contained in a suspension with a viscosity of 15 centipoises was estimated. At least one additional suspension containing a little more, 

and at least one containing a little less, than this estimated percentage 

were now prepared and tested in an effort to produce at least one 

suspension with a somewhat higher, and another suspension with a 

slightly lower, viscosity than 15 centipoises. Then the three or more 

viscosity figures for each clay, including the figure relating to the 6-

percent suspension, were plotted on the "Representative yield curves" 

diagram against the corresponding clay percentages, and the yield 
110 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

of the clay, expressed in barrels per ton of a 15-centipoise suspension, 

was obtained from the diagram by interpolation. 

GEL STRENGTH (THIXOTROPY) 

Bentonite of high dilatancy when mixed with water, characteristically forms a fluid that becomes a jellylike mass if left undisturbed 

for a short period but again becomes a fluid when agitated. A 

teversible transformation of this kind, involving no change in temperature, is referred to as thixotropy or gelling. As indicated by 

Stern (19±1, p. 371), the ability of bentonites to fonn strong gels is 

not considered as important now as it once was, though it is still a 

much-desired property. The function of thixotropy in drilling is 

primarily to prevent the cuttings and weighting material, such as 

barite, frmn settling in the hole when the drill pipe and mud-circulating syste1n are not in n1otion. Most of the bentonitic clays from 

the Missouri River basin that are shown by the tests to have the yield, 

viscosity, and wall-building properties of good drilling-mud material 

also show satisfactory gel strength. 

The gel strength of a suspension of clay, as measured with a Stormer 

viscmneter, is expressed as the weight in gran1s that n1ust be placed 

on the scale of that instrument to sta1•t its rotor revolving against the 

resistance of the suspension after it has been allowed to gel. Two 

determinations of gel strength have been made for each of the 6-

percent-by-weight suspensions that were prepared for the viscosity 

and yield tests. Detern1inations for each of the two or more suspensions containing larger percentages of clay were also made in order 

to ascertain roughly the relation between gel strength and the amount 

of clay in suspension. For each suspension, gel strength was measured immediately following thorough agitation and once again after 

the suspension had remained undisturbed for a period of 10 minutes. 

WALL-BUILDING EFFICIENCY 

Some of the bentonites with high dilatancy are muong the 1nost 

effective of all known drilling-mud materia.ls for forming a thin, 

impervious layer, or wall, on the sides of holes bored with rotary 

well-drilling equipment. This wall tends to reduce loss of water 

from the n1ud, thus pennitting greater control over its viscosity, 

and prevents c.aving of the sides of the well, which sometimes occurs 

when soft rock 1naterials absorb moisture frmn drilling fluids. 

The wall-building propensities of samples of bentonite that swelled 

to more than 20 1nilliliters were tested with the aid of a filter press in 

accordance with the procedure outlined by the American Petroleum 

Ins~titute (Anon., 1952, p. 3, 11). In this procedure, a suspension of 
LABORATORY PROCEDURES USED FOR TESTING THE BENTONITES 111 

bentonite is filtered under a pressure o:f 100 pounds per square inch to 

form a filter cake comparable to the wall that would :form on the sides 

o:f a drill hole. The degree o:f imperviousness o:f the cake is indicated 

by the volume o:f filtrate that accumulates, the smaller volun1es indicwting the more impervious, and accordingly the better, wall-building 

material. 

Admittedly, the conditions under which this test is performed differ 

slightly :from those in a drill hole, because a suspension in a filter 

press is not in motion, whereas the mud in a drill hole circulates continuously while the drill is rotating. Because a suspension in the 

test apparatus is essentially motionless, some o:f the grit and smaller 

nonclay particles settle out whereas they would not do so if the suspension were in circulation. The rate o:f settling, which varies inversely with the viscosity and gel strength o:f the suspension, is most 

rapid at the beginning o:f the test period and slackens progressively as 

gelling sets in. The proportion of grit and smaller nonclay particles 

is nevertheless highest in the early accumulations o:f the filter cake, 

and these particles tend to reduce its imperviousness by interfering 

with the orientation and tight compaction o:f the clay particles. The 

actual wall-building efficiency o:f a clay-water suspension is therefore 

thought to oo greater than the efficiency indicated by the test results. 

The results are nevertheless useful because they serve as a basis :for 

comparing the wall-building characteristics o:f different clays. 

For the wall-building tests, a moistened W atman no. 52 filter paper 

was placed in the bottom o:f the cylinder o:f the testing apparatus. 

About 300 milliliters o:f a 6-percent-by-weight suspension o:f the clay 

to be tested was introduced into the cylinder, and the apparatus was 

assembled. A pressure o:f 100 psi was then applied to the suspension 

and the volume o:f filtrate, representing the water-loss, was recorded 

in milliliters at periods of 2, 15, and 30 minutes after the pressure was 

first applied. Immediately following the 30-minute period, the pressure was released, the apparatus was taken apa.rt, and the surplus 

suspension was removed :from the cylinder. The filter cake was gently 

flushed with water, and its thickness was measured and recorded in 

thirty -seconds o:f an inch. 

CONTENT OF NONCLAY MATERIAL (INCLUDING GRIT) 

The nonclay material present in bentonites is composed o:f grit, as 

well as smaller particles. The ter1n grit refers only to those nonclay 

particles, consisting o:f mica~ quartz, gypsum, calcite, feldspar, and 

various other minerals present in pulverized bentonite, that are too 

large to pass through a 325-mesh sieve. The finer constituents include 

particles o:f the same minerals as the grit, as well as softer nonclay 
112 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

particles, consisting partly of soluble salts and partly of insoluble 

matter. Because of the adverse effects of nonclay matter on the 

properties that give value to bentonites, the amount of such matter 

in samples is of interest in connection with efforts to explain differences in quality of the sampled clays. The total amounts of nonclay 

matter and the amount of grit were therefore estimated for the salnples from the Hardin and Black Hills distriots. 

In estimating the total content of nonclay material, each of the 

pulverized samples was examined 1nicroscopically, employing the 

liquid immersion method. Because of the great variety of nonclay 

minerals present and the impracticability of separating all of them 

from the clay, the estimated total percentages were based on their 

volume, rather than their weight; the procedure followed was similar 

to that described by Barrell ( 1901, p. 513) as follows: 

The field of view of the microscope is divided into quadrants by the cross hairs, 

and these are mentally divided into sectors which are thirds, fjfth~. or Hmall('r 

fraction. By taking such a power of objective that the component to be estimated is represented in the field of view by a fair number of crystals, the latter 

can be mentally collected together and packed into one quadrant, and the fraction which it fills estimated. 

Rectangular subdivisions were used instead of the sectors described 

by Barrell. Estimates were based on the fractions of subdivisions 

filled by clay and by nonclay 1naterial. Several random fields on 

each slide were carefully studied before estimates were made, and at 

least two estimates were 1nade for each sample. If the second of these 

estimates approximated the first, the average of the two estimates was 

assumed to be the correct figure. 1Vhen they differed by more than 

a few percent, repeated esti1nates on additional slides were made until 

consistent results were obtained. Estimates for samples containing 

less than 25 percent of nonclay n1aterial are believed to be no 1nore 

than 5 percent in error. The limit of error is somewhat greater for 

samples having higher percentages of nonclay constituents. 

To determine the approximate amount of grit, 25 grams of ground 

bentonite was dispersed in water, and the resulting suspension was 

washed through a 325-Inesh sieve. The grit, consisting of mineral 

particles remaining on the sieve, was washed into a watch glass and 

was dried and weighed; the weight of the grit was then computed and 

recorded as a percentage of the original 25 grams of bentonite. 

Most of the samples tested in the present investigation yielded less 

than 5 percent of grit; however, some of the san1ples showed much 

larger amounts. In general, samples containing more than 5 percent of grit proved to be deficient in the physical properties that give 

value to bentonites. It is possible to improve the quality of bentonite 

by removing grit, but the processes involved are costly. 
REFEHENCES CITED 113 

REFERENCES CITED 

Ashley, G. H., and others, 1933, Classification and nomenclature of rock units: 

Geol. Soc. America Bull., v. 44, pt. 2, p. 423-459. 

Barrell, Joseph, 1901, Microscopical petrography of the Elkhorn Mining district, 

Jefferson County, Mont.: U.S. Geol. Survey 22d Ann. Rept., pt. 2, p. 511-549. 

Cobban, W. A., and Reeside, J. R., Jr., 1951, Lower Cretaceous ammonites in 

Colorado, Wyoming, and Montana: Am. Assoc. Petroleum Geol'Ogists Bull., 

v. 35, no. 8, p. 1892-1893. 

--- 1952a, Correlation of the Cretaceous formations of the western interior 

of the United States: Geol. Soc. America Bull., v. 63, no. 10, p. 1011-1044. 

--- 1952b, Frontier formation, Wyoming and adjacent areas: Am. Assoc. 

Petroleum Geologists Bull., v. 36, no. 10, p. 1913-1961. 

Davies, W., 1946, The fundamental characteristics of molding sands: Jour. 

Iron and Steel Inst., v. 154, p. 1-46. 

Downs, G. R., 1949, Mesozoic rocks of the northern Powder River Basin, Wyoming: 

Wyoming Geol. Assoc., 4th Annual Field Conf., in the Powder River Basin 

Guidebook, p. 46-51. 

Fisk, H. G., 1946, Bentonite, with test methods and results of tests of Wyoming 

bentonite: Wyo. Univ. Natl., Res. Research Inst. Bull. 2, p. 1-39. 

Goddard, E. N., and others, 1948, Rock-color chart: Natl. Research Council, 

Washington, D. C. 

Grim, R. E., 1950, Olmsted fuller's earth as a bonding clay for foundry use : 

Ill. Geol. Survey Circ. 162, p. 1-7. 

---1953, Clay Mineralogy: New York, McGraw-Hill Book Co., Inc. 

Grim, R. E., and Cuthbert, F. L., 1945, The bonding action of clays : Part 1, Clays 

in green molding sands; Ill. Geol. Survey Rept. Inv. 102, p. 1-55: Part 2, 

1946, Clays in dry molding sands; Ill. Geol. Survey Rept. Inv. 110, p. 1-36. 

Hares, C. J., and others, 1946, Geologic map of the southeastern part of the 

Wind River Basin and adjacent areas in central Wyoming: U. S. Geol. Survey 

Oil and Gas Invest, Prelim. Map 51. 

Knappen, R. S., and Moulton, G. F., 1930, Geology and mineral resources of 

Carbon, Big Horn, Yellowstone and Stillwater Counties, Mont.: U. S. Geol. 

Survey Bull. 822-A, p. 1-70. · 

Knechtel, M. 1\L, and Patterson, S. H., 1952, Bentonite deposits of the Yellowtail district, Montana and Wyoming: U. S. Geol. Survey Circ. 150, p. 1-7. 

--- 1955, The northern Black Hills bentonite mining district: U. S. Geol. 

Survey M. F. Map 36. 

Pierce, W. G., and Andrews, D. A., 1941, Geology and oil and coal resources of 

the region south of Cody, Park County, Wyoming: U. S. Geol. Survey Bull. 

921-B, p. 99-180, pl. 14. 

Reeside, J. B., Jr., 1944, Maps showing thickness and general character of 

the Cretaceous deposits in the western interior of the United States: U. S. 

Geol. Survey Oil and Gas Invest. Prelim. Map. 10. • 

Richards, P. W., and Rogers, C. P. 1951, Geology of the Hardin area, Bighorn and 

Yellowstone Counties, Montana: U. S. Geol. Survey Oil and Gas Invest. 

Prelim. Map 111 (two sheets). 

Ross, 0. S., and Hendricks, S. B., 1945, Minerals of the montmorillonite group : 

U.S. Geol. Survey Prof. Paper 205b, p. 23-79. 

Sharkey, H. H. R., and others, 1946, Geologic and structure contour map of Sage 

Creek Dome, Fremont County, 1-Vyo.: U. S. Geol. Survey Oil and Gas Invest. 

Prelim. Map 53. 
114 BENTONITE DEPOSITS IN MONTANA AND WYOMING 

Stern, A. G., 1941, The role of clay and other minerals in oil well drilling fluids: 

U. S. Bur. Mines Rept. Inv. 3556, p. 1-88. 

Thom, W. r:r., Hall, G. M., Wegemann, C. H., and Moulton, G. F., 1935, Geology 

of Big Horn County and the Crow Indian Reservation, Mont., with special 

reference to the water, coal, oil and gas resources: U. S. Geol. Survey Bull. 

856, p. 1-200. 

Thom, W. T., and Spieker, E. M., 1931, The significance of geologic conditions 

in Naval Petroleum Reserve No. 3, Wyoming, with a section on the waters 

of the Salt Creek-Teapot Dome uplift: U. S. Geol. Survey Prof. Paper 163, 

table 4. 

'Vegemann, 0. H., 1918, The Salt Creek Oil field, Wyoming: U. S. Geol. Survey 

Bull. 670, p. 13. 

-Williams, F. J., Neznayko, l\L, and Weintritt, D. J., 1953, The effect of exchangeable bases on the colloidal properties of bentonite: Jour. Physical Chern., v. 

57, p. 6. 

Yen, Teng-Chien, 1954, Age of Bear River formation: Am. Assoc. Petroleum 

Geologists Bull., v. 38, no. 11, p. 2412-2413. 

Annoy111ous, 1942, Recommended practice on standard field procedure for testing drilling fluids: Am. Petroleum Inst., Code 29, p. 1-14. 

---1944, Oast metals handbook: Am. Foundrymen's Assoc. (Society). 

---1945, Laboratory methods for evaluating the physical properties of bentonite, Data 251, p. 1-2: Am. Colloid Co. 

---1952, Foundry sand handbook: Am. Foundrymen's Soc., p. 1-265. 
INDEX 

A Page 

Alluvial deposits_____________________ 4-5 

Anticlines _____________________ 43, 55, 62, 72, 7t3, 96,97 

B 

Bearpaw shale __________________________ 16,36-38,59 

Belle Fourche shale ________________________ 17,19-21 

subdivisions______________________________ 18 

Page 

Dome structures _____________________ 58, 62, 66, 80,86 

Drilling muds ______ 16,19, 49, 5G-51, 52, 60, 80,107-108 

gel strength _________________ - __ - ___ ------- 110 

wall building properties__________________ 111 

Dry-compression strength __________________ -- 104 

tests ________________________ 50, 52, 53, 60, 105-106 

Dry-sand molding __________________________ 102-103 

Bentonite, associated rocks___________________ 5 E 

commercial requirements _______________ 101,102 Eagle sandstone ______________________ _ 15 fresh______________________________________ 44 

general character _________________________ 43, 44 

investigations____________________________ 1-2 

quality ___________________ 6, 7, 16, 19, 21, 36, 38,49 

weathered ________________________________ 44-45 

bed L_ ---------------------- 18-19,81,87,93,95 

bed J __ ____________________ 18-19,73,85,90, 95,99 

bed L ______________________ 19-20,67,69,85, 88,93 

bed AL __________ 18, 21, 53, 69, 80, 90, 92, 93, 95, 97 

bed P _______ ______________________________ 57,62 

bed Q _____________________________________ 57,62 

bed R _____________________ 30,53-54,59,63, 64,86 

beds _____ -------------------------- 30, 55, 59,64 

bed T_ _ _ __ _ _ _ _ _ _ __ _ __ _ __ _ _ __ _ __ _ _ _ _ _ _ __ _ _ 30 

bed u ______________________________ 30, 54, 97,98 

bed F_ ____________________ 36, 37, 54,59-60,64,65 

bed w _____________________ 38, 54, 56, 60, 64, 65,71 

bed ){_ ________________________________ 38, 60,61 

Clay Spur bed __________________ 10, 46, 47, 49, 50, 

52, 67, 68, 74, 80, 87, 89, 94, 95, 99, 100 

floor ______________________________________ 43,46 

F 

Faults __________________________________ 43, 55, 66,97 

Field study ___________________________ ----- __ - 4 

Field mapping ________________________ ------- 2-3 

Fossils (see also Sections, measured) _____ ----- 10, 

15, 18, 21, 29-30, 36 

Foundry-sand bonding clay__________________ 19. 

36, 47, 5G-51, 87, 101-103 

requirements ___________________________ -- 102 

Frontier formation ___ ------------------------ 15, 17 

Future utilization of deposit~----------------- 49 

G 

Gel strength, tests_____________________ 110 

Green-compression strength_----------------- 104 

Greenhorn calcareous member ________________ 15,21 

Green-sand molding ________________________ 102-103 

Grim, R. E., and Cuthbert, F. L., cited______ 104 

GriL ________________________________ 44, 53, 112, 113 

flowage ___________________________________ 19,46 H 

general character_------------------ 43-44,46-47 Hardin district, bentonite, uses ______ -------- 48-49 

rooL ___ ---------------------------------- 43-44 bentonite mines ______ -------------------- 99 

Soap Creek bed __________ 18, 46, 49,52-53,54,67, homoclinal structures _____________________ 42-43 

68. 69, 70, 80, 86, 90, 93, 94, 97, 99, 100 rock facies________________________________ 14 

suitability for mining_____________________ 7, transportation____________________________ 2 

8,10, 47, 52, 54, 56, 57, 8.7, 90,94 Hell Creek formation_________________________ 42 

suitability for use ________ 8, 10, 16, 36, 48, 53, 54,94 

J c Judith River formation _________________ 15, 16,34-35 

Carlile-Niobrara shale member ____________ 22-23,25 Parkman sandstone member_____________ 35 

Claggett shale member----------------------- 30 upper member_ ___________________________ 35-36 

Clay bloom ___________________________________ 44,45 

Cloverly formation___________________________ 7 L 

Cobban, '\V. A., work cited___________________ Laboratory methods _____ -------------------- 100 

Cody shale ___________________________________ 16,17 Lower Cretaceous boundary ___________ ------ 6-7,17 

upper part, subdivisions__________________ 29 

(1rmc:retions, calcareous ____ -------------- 19, 36, 37,59 

Cretaceous rocks, nomenclature______________ 12 Mining methods______________________________ 46 

C1uthbert, F. L., and Grim, R. E., cited_____ 104 Montmorillonite_____________________________ 44 

D 

Mowry shale _________________________________ 9-10 

DiJatancy _________ ----------------------- 52 N 

:minimum requirement___________________ 108 Nonclay material, estimation ________________ _ 112 

related to weathering_____________________ 45 

tests ___ -__________________________________ 108 p 

and wall-building properties _____________ 54,110 Processing, bentonite ____________________ _ 46 

115 
116 INDEX 

R Page 

Reeside, J. B., cited__________________________ 11 

Reserves.--- 48, 59, 65, 67, 69, 71, 78, 80, 86, 87, 92, 97, 99 

Richards, P. W., and Rogers, P. W., work 

cited.-------------------- ________ 2, 5, 6 

Rogers, C. P ., and Richards, C. P., cited. ____ 2, 5, 6 

Rotten Grass dome___________________________ 90 

s 

Sections, measured, Bearpaw shale ___________ 38-42 

Belle Fourche shale _______________________ 21-22 

bentonite beds, Clay Spur bed___________ 68, 

74, 82, 89, 95, 100 

Soap Creek bed_ 68, 70, 85, 88, 89, 91, 93, 95,100 

beds A-C _____ ------------------------ 81,84 

beds D-E____________________________ 83 

beds F-G_____________________________ 82 

bed L ------------------------- 81, 89, 95,96 

bed J __________ ______ ------- _ 73-74, 85, 95, 96 

bed L________________________________ 84 

beds L-0 _______________________ ______ 84-85 

bed M----------------------------- 69, 90,93 beds P-Q_____________________________ 62 

bed R________________________________ 78 

Sections, measured-Continued Page 

Judith River formation___________________ 37 

Mowry shale _____________________________ 10-11 

Niobrara shale member ___________________ 21i-28 

Telegraph Greek member_________________ :H 

Thermopolis shale ___ -------------------- lHI 

unnamed sandy-shale member of Cody shale ___________________________________ 31-34 

Soap Creek dome_____________________________ 80 

Standard sand________________________________ 103 

Steele shale ___________________________________ 16, 2!J 

Swelling capacity. See Dilatancy. 

Synclines. ___________________ ------- _______ 43,86-87 

T 

Telegraph Creek member_____________________ 29 

Tempering water ___________________________ 104,105 

Tests, foundry-sand bonding-clay ___________ 102-106 

gel strength_______________________________ 110 

wall-building properties__________________ 111 

Thermopolis shale .. __________________________ 7-8 

Two Leggin uplift____________________________ 62 

u 

beds R-U____________________________ 59 Unnamed sandy-shale member of Cody shale. 29-30 

bed U_______________________________ 98 Upper Oretaceous rocks, facies ________________ 13-14 

bed V ________ --------------------- 60, 61,65 Uses of bentonite _____________________________ 48, 53 

bed w __________________________ 61, 65,71-72 

bed x ________________________________ 60,61 v 

Carlile shale member _____________________ 23-25 Viscosity ___________________________________ 108-109 

Claggett shale member.-- __ -- ____________ 31-32 related to water content._________________ 109 

Greenhorn calcareous member____________ 22 tests ____________________________________ 108-109