- ";; 

~ .bG 

0 

-

0 

~ 

i 

0 

lOo-t 

a: 

~ 

z 

~ 

0 

~ 

::&l 

< 

~ 

:a 

z 

~ 

~ 

~ 

;} 

z -~--- z 

~ 

Q 

z 

~ 

0 

C!l = 

-t::l:l 

IAI 

= E-1 

~ 

0 

~ 

~ 

0 

~ 

0 

~ 

C!l 

! 

~ 

~ 

Geology of the 

Bighorn CanyonHardin area, Montana 

and \Vyoming 

GEOLOGICAL SURVEY BULLETIN 1026 
Geology of the. 

Bighorn CanyonHardin area, Montana 

and Wyoming 

By PAUL W. RICHARDS 

GEOLOGICAL SURVEY BULLETIN 1026 

A description of the geography, stratigraphy, structure, geomorphology, and m.ineral resources of the area 

UNJTED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1955 
UNITED STATES DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary 

GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

First printing 1955 

Second printing 1983 

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304 
CONTENTS 

Page 

A bs t, r ac l ___ . ___ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 1 

In lroductiotL _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ~ 

Location and extent of.area_____________________________________ 3 

Purpose of report______________________________________________ 3 

Legal status of land____________________________________________ 4 

Field wot·k and acknowledgments________________________________ 4 

Previous work and publications_________________________________ 5 

GeograpllY-------------------------------------------------------- 5 

Surface features and relief______________________________________ 5 

Cli ~uaLc au(l vegetation______________________________ _ _ _ _ _ _ _ _ _ _ _ 7 

Drainage and water s11pply _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 7 

lnd H'SLry, settlement, and transportation_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 10 

StratigraphY------------------------------------------------------ 10 

General features_______________________________________________ 10 

Cambrian system_.____________________________________________ 11 

Oro::; Ventre fo11nation and Gallatin limestone_________________ 11 

Ordovician system_____________________________________________ 12 

Bighorn dolomite__________________________________________ 12 

Devonian system______________________________________________ 14 

.Jefferson limestone and Three Forks shale undifferentiated______ 14 

Carboniferous systems_________________________________________ 21 

Mississippian system_______________________________________ 21 

Madison limestone_____________________________________ 21 

Mississippian and Pennsylvanian systems_____________________ 26 

Amsden formation_____________________________________ 26 

Pennsylvanian system______________________________________ 30 

Tensleep sand-stone____________________________________ 30 

Permian system_______________________________________________ 32 

Em bar formation__________________________________________ 32 

Permian anrl Triassic systems___________________________________ 35 

Chugwater formation______________________________________ 35 

Jurassic system________________________________________________ 37 

:Middle Jurassic series______________________________________ 38 

Piper formation______________________________ _ _ _ _ _ _ _ _ _ 38 

Upper Jurassic series____________________ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 40 

Rierdon and Swift formations___________________________ 40 

Morrison formation____________________________________ 4i 

Cretaceous system____________________________________________ _ 42 

Lower Cretaceous series_____________________________________ 42 

Cloverly formation_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 42 

Thermopolis shale __________________ '___________________ 45 

Mowry shale_______ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 4 7 

Upper Cretaceous series_____________________________________ 49 

Frontier formation_____________________________________ 49 

Cody shale ____________________________ . . . . _ _ _ _ _ _ _ _ _ _ 50 

Parkman sandstone_. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 63 

Be~rpaw shale________________________________________ 63. 

JJI 
IV CON'l'EN'l'S 

Stratigraphy-Continued Page 

Tertiary(?) and Quaternary systems______________________________ 67 

Terrace gravels____________________________________________ 67 

Quaternary system____________________________________________ 67 

Landslide materiaL___ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 6 7 

All u vi urn___________ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 6 7 

Structure_________________________________________________________ 67 

General features______ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 6 7 

Bighorn Mountains____________________________________________ 69 

Anticlines and domes__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 72 

Faults________________________________________________________ 74 

Age of deformation_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 77 

Geonlorphology___________________________________________________ 78 

General features_______________________________________________ 78 

Terraces______________________________________________________ 80 

Pediments and undifferentiated surface~-_________________________ 82 

Economic Geology_________________________________________________ 82 

Oil and gas___________________________________________________ 82 

Mineral dcpo~its_____ _ __ ___ _ _ _ _ ___ ____ _ _ _ _ _ _ __ __ _ ___ _____ ___ __ _ 87 

Literature cited _________________ ---------------------------------- 88 

Index____________________________________________________________ 91 

ILLUSTRATIONS 

[All plates in pocket] 

PLATE 1. Geologic and structure contour map of the Bighorn Canyon-Hardin 

area, Montana and Wyoming. 

2. Generalized columnar section of rocks exposed between the Bighorn 

Mountains and Hardin, Mont. 

3. Log of lower formations in the Inland Empire Refinerie~ well 52-34, 

Crow tribal, Soap Creek dome, Montana. 

4. Surface and subsurface sections across the northern cud of the Bighorn Basin and the Bighorn Mountains from north of Cody, ·wyo., 

to Hardin, Mont. 

5. Cross section of Yellowtail dam site, Montana. 

6. Sketch map of the Bighorn Mountains and Great Plain~ area, Montana and 'Vyomiug, showing mountain.3, basins, and principal axe~ 

of folding. 

7. ~ketch map ~howiug disLribution of terrace gravels and alluvium in 

area between YellowLail dam site and Hardin, 'Mont. 

Page 

FIGUR~: l. Paleozoic strata in north wall of Devils Canyon of PorcupineCreek________________________________________________ 16 

2. Paleozoic strata in Bighorn Canyon near the crest of the Big- horn Mountians _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 17 

3. Erosional unconformity at the base of the Chugwater formation_ 30 

4. Correlation of Jurassic formations of south-central Montana 

with Upper and Middle Jurassic formations of other areas__ 38 

5. Aerial view of Bighorn Canyon____________________________ 68 

6. View eastward down Bighorn Canyon from ncar the crest of 

the Bighorn Mountains____ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 70 
CONTENTS v 

Page 

FIGURE 7. Steep western flank of the Bighorn Mountains ncar the junction of the Bighorn River and Dry Head Creek____________ 71 

8. Fault in Bighorn Canyon_________________________________ 76 

TABLES 

TABLE 1. Climatological data for Hardin and the surrounding area in 

Bighorn County, Mont_________________________________ 8 

2. Stream discharges________________________________________ 9 

3. Bighorn River terraces below the Yellowtail dam site_________ 81 

4. Selected oil and gas wells and dry holes_____________________ 84 
GEOLOGY OF THE BIGHORN CANYON-HARDIN AREA, 

MONTANA AND WYOMING 

By PAUL w. RICHALWS 

ABSTRACT 

This report de..;eribcs the geology of an area of about 1,300 :-;quare miles in 

so11th-ecntral Montana extending from Hardin, Mont., southward across part of 

the Great Plains to the Bighorn Mountains and th0nco sout.hwcstward through 

the mountains along Bighorn Canyon to the Bighorn Basin. The area is mostly 

within the Crow Iudian Reservation in southwestern Big Horn County, Mont., 

but it includes parts of Carbon County, Mont., and Big Horn County, Wyo., 

and less than 10 square miles of the southeast part of Yellowstone County, 

Mont. 

The only incorporated town within the area is Hardin, the count,y seat of Big 

Horn County, Mont., and the only other settlement is St. Xavier, Mont. The 

region is drained by the Bighorn River and its tributarie~. The main valleys 

arc cultivated. The proposed Yellowtail dam, in the canyon of the Bighom 

River ncar the cast edge of the Bighorn Mountain:-s, would create a reservoir 70 

miles long and provide irrigation water for extensive terrace lands along the 

Bighorn River. 

About 9,000 feet of sedimentary rocks, ranging in age from Cambrian to Late 

Cretaceous, arc exposed in the area. Pre-Cambrian granites and metamorphic 

rocks, which form the central parts of the Bighorn Mountains and arc separated 

from the overlying sedimentary rocks by an erosional unconformity, do not crop 

out in the area. In the deepest part of Bighorn Canyon approximately the upper 

700 feet of about 1 ,000 feet of shale and thin-bedded limestone that make up the 

Cambrian Gros Ventre formation and Gallatin limestone is exposed. Resistant 

limestone and dolomite which overlie the Gallatin comprise 400 feet of Bighorn 

dolomite of Ordovician age, 200 feet of undivided Jefferson limestone and Three 

Forks shale of Late Devonian age, and 700 feet of Madison li:ncstouc of 

Mississippian age. 

The overlying rocks belong to the Amsden formation, which is intcrgradational 

from t.hc Mississippian ( ?) to the Pennsylvanian and the Tenslcep sandstone of 

Pennsylvanian age. The Amsden formation lies unconformably on the Madison 

limestone, consists of red shales and siltstones, gray limestones an.d sandstones, 

and ranges from 230 to 280 feet in thickness. The Tenslcep sandstone, which 

is as much as 120 feet thick but is missing locally, consists of gray to yellowishgray sandstoue and a few thin beds of dolomite and chert. Overlying the Tensleep sandstone and separated from it by au erosional unconformity are the 

Permian l~mbar formation and the Triassic Chugwater formation. The Embar 

and Chugwater formations were mapped separately only in the southwestern 

part of the area where the Embar consists of 100 feet of dolomite, limestone, and 

red siltstone and sandstone. Elsewhere, a zone of redbeds, gypsum, and thin 

beds of litnc3tonc is seemingly the Embar equivalent, but the zone is too thin to 

1 
2 GEOLOGY OF THE BIGHORN CANYON-HARDIN AREA, MONT.-WYO. 

map as a formation and was included in the base of the Chugwater formation. 

The Chugwater formation consists of 350 to 500 feet of red sandstone and 

siltstone and gypsum. 

About 150 feet of red siltstone and sandstone, limestone, and gypsum belonging 

to the Piper formation of Middle Jurassic age overlies the Chugwater formation. 

In central Montana the Piper is separated from rocks as old as Mississippian by an 

extensive unconformity, but in the area adjacent to the Bighorn Mountains the 

Piper is seemingly conformable above the Chugwater. About 300 feet of light 

olive-gray to olive-brown calcareous shale belonging to the Upper Jurassic 

Rierdon formation overlies the Piper, and nearly 200 feet of yellowish-gray to 

grayish-green sandstone and siltstone and light olive-gray shale of the Upper 

Jurassic Swift formation overlie the Rierdon. Glauconite-bearing marine beds 

of the Swift formation grade upward into continental sandstones and siltstones 

of the overlying Upper Jurassic Morrison formation. The upper part of the 

Morrison is largely variegated shales which are commonly concealed, and the 

total thickness of the formation ranges from less than 100 feet to about 250 feet. 

The Lower Cretaceous Cloverly formation is 400 feet or leas thick. It contains 

a basal lenticular member named the Pryor conglomerate member, a middle zone 

of variegated beds, and an upper zone of interbedded siltstones, shales, and sandstones. Where the Pryor member is missing, variegated beds in the upper part 

of the Morrison cannot be distinguished from beds of similar lithology in the 

Cloverly, and the two formations were mapped together. 

The Lower Cretaceous Thermopolis shale, which is about 450 feet thick, 

consists of dark-gray shale and interbedded bentonite. A zone of gray sandstone 

dikes about 150 feet above the base is correlated with the Newcastle sandstone 

of the Powder River Basin and the Muddy sandstone in the Bighorn Basin. 

Overlying the Thermopolis shale is the Mowry shale which is characterized by 

abundant fish .,cales and numerous layers of siliceous shale and siltstone. The 

Mowry is 325 to 400 feet in thickness and contains several beds of bentonite. 

It is the uppermost formation in the Lower Cretaceous series. 

A conformable sequence above the Mowry shale of about 4,000 feet of shales, 

siltstones, and sandstones underlies the Great Plains region. Included iu thi::; 

::;equence arc the Frontier formation, Cody shale, Parkman sandstone, and 

Bearpaw shale, all of Late Cretaceous age. These formations belong to the 

Colorado and Montana groups. The basal few feet of the nonmarine Hell Creek 

formation, which is equivalent to the Lance formation and younger than the 

Montana group, occurs in the northeast corner of the mapped area. 

The Frontier formation, about 250 feet thick, comprises dark-gray partly 

sandy shale and interbedded bentonite. Overlying the Frontier is the Cody 

shale which consists of 2,600 feet of dark-gray concretionary partly sandy shale 

and interbedded bentonite. It has been divided into 7 members, the lower 

4 belonging to the Colorado group and the upper 3 to the Montana group. From 

base to top, the members of the Cody shale are (a) an unnamed member 200 

feet thick; (b) the Greenhorn calcareous member 60 to 100 feet thick; (c) the 

Carlile shale member about 280 feet thick; (d) the Niobrara shale member 400 

feet thick; (e) the Telegraph Creek member 750 feet to 850 feet thick; (f) a shale 

member equivalent in age to the Eagle sandstone about 375 feet thick; and 

(g) the Claggett shale member about 350 feet thick. Above the Cody shale is 

the Parkman sandstone which comprises 100 feet of silty and sandy shale overlain by 150 feet of massive and thick-bedded yellowish-gray and greenish-gray 

sandstone. The Bearpaw shale, which is the uppermost formation of the Montana 

group, is exposed only in the northeastern corner of the area. It is composed of 

dark-gray shale 850 feet thick that has many beds of bentonite iu the middle. 
INTRODUCTION 3 

A few feeL of greenicih sandstone, siltstone, and 5hale belonging to the Hell Creek 

formation overlie the Bearpaw in the northeastern corrier of the area. 

Six well-developed stream terraces, the highest two of which may be late 

Tertiary in age, extend along the Bighorn River. These and correlative terraces 

in tributary stream valleys have a 25-foot thick veneer of limestone and volcanic 

rock gravel. Several surfaces 1 which were mapped as pediments, in the southwestern part of the area slope rather steeply away from the Pryor Mountains 

# and arc covered with limestone gravel and boulders washed from the Pryor 

Mountains. Several erosional surfaces in the Great Plains area have little 

covering ot.her than reworked shale and gravel washed from nearby terraces, 

and these surfaces were mapped as undifferentiated Quaternary surfaces. 

The folding and main deformation of the mountains occurred during early 

Tertiary time after the dcpo.:;ition of all the sedimentary formations of the area. 

The domes and anticline.:; in the plains region may have formed during this same 

period of orogeny as they trend parallel to the northern end of 'ihe Bighorn 

Mountains which is itself the northward plunging end of a long anticline. Structural closure on the domes is small except for Soap Creek dorne, and faults are 

simple and of relatively small displacement except for the Sykes Springs fault 

zone ncar the southern tip of the Pryor Mountains. 

Black oil has been produced from the Soap Creek oil field, which was first 

drilled in 1921, and gas for the town of Hardin is supplied from the Hardin gas 

field. Thick beds of bentonite in the Cretaceous formations and limestone 

in the Carboniferous strata may some day be of economic importance. 

INTRODUCTION 

LOCATION AND EXTENT OF AREA 

This report and the accompanying maps describe the geology of an 

area of about 1,300 square miles which is mainly within southwestern 

Big Horn County, 11ont., but extends into the southeastern part of 

Carbon County, 11ont., and the north-central part of Big Horn 

County, Wyo. The location of the area is shown by the index map 

on plate l. About 1,125 square miles, or nearly one-third of the 

Crow Indian Reservation is included. Nearly 70 miles of the Bighorn 

Hiver ft·mn Crooked Creek in northern Wyoming to a point 5 miles 

below the 1nouth of the T ...ittle Bighorn River in 1tfontana, including 

the site of the proposed Yellowtail dam are shown. The maps cover 

the south half of the Fort Custet· and the north half of the St. Xavier 

30-minute quadrangles (published in 1894 and 1901 respectively) and 

an irregularly shaped area along Bighorn Cn,nyon from the Bighorn 

Basin through the Bighorn 11ountains. 

PURPOSE OF REPORT 

This report was prepared as part of the Interior Department program for development of the l\1issouri River basin. The fourfold 

purpose of the investigation wn,s (1) to 1nap and study the terraces 

below the 1nouth of Bighorn Canyon in order to provide information 

as to the type of aggregate available for construction of the proposed 

Yellowtail dam anrl show the distribution of terraces, smne of which 
4 GEOLOGY OF 'l'HE BIGHORN CANYON-HARDIN AREA, lVlO~'l'.-\VYO. 

would be irrigated by water diverted from the Bighorn River by the 

clam; (2) to map the geology of Bighorn Canyon and show formations 

that would be covemd in part by the Yellowtail reservoir; (:3) to locate 

and describe mineral deposits of possible economic importance; and (4) 

to map and describe the rock formations, some of which contain 

known or potential oil-bearing strata elsewhere in ~1ontana and 

Wyoming. These formations are exposed for detailed study in and ~ 

near the Bighorn Mountains. A map and brief text have been 

published describing the geology and geologic structure of part of the 

area (Richards and Rogers, 1951). 

LEGAL STATUS OF LAND 

The Crow Indian Reservation, which includes about 2,250,000 

acres of land, is made up of four classes of land ownership. Trust 

allotted lands, comprising 78 percent of the reservation, have been 

apportioned to individual members of the tribe with the title to the 

land held in trust by the United States Government. Tribal lands, 

making up about 12 percent of the reservation, are owned by the 

Crow tribe. Fee, or alienated lands, include about 10 percent of the 

reservation and have been patented to members of the tribe or to 

persons outside the tribe. Land owned by the United States Government makes up an almost negligible percentage of the reservation. 

Most of the land surrounding the Crow Indian Reservation is held in 

fee. 

FIELD WORK AND ACKNOWLEDGMENTS 

Field work was done during the months of June to October in 1947 

and 1948. The project was under the direction of C. P. Rogers, Jr. 

until November 1947 when the writer assumed charge and completed 

the work. Assistants were E. C. Beaumont and Z. S. Altschuler in 

1947 and Linn Hoover, Jr., W. J. Mapel, A. A. Meyerhoff, and G. E. 

Prichard in 1948. Members of the field parties worked alone much of 

the time, but each man worked in several areas and no index was made 

to show the parts of the area mapped individually. 

Geologic and topographic features were drawn in the field on aerial 

photographs, both contact prints and enlargements, and were trans- '» 

ferrecl to subsequently prepared base maps by the use of a vertical 

Sketchmaster for the plains area where the topographic relief is low, 

and by the use of a Kail plotter for the Bighorn Canyon area where ~ 

the relief is high. The base map of the plains area (pl. 1) was compiled from General Land Office plats on a plane table and telescopic 

alidacle triangulation net prepared by the writer. Vertical control 

for structure contouring was carried by plane table triangulation 

from Coast and Geodetic Survey benchmarks along the Chicago, 

Burlington & Quincy Railroad. The base map of the Bighorn Canyon 
GEOGHAPHY 5 

m·eu. (pl. 1) WitS taken from n, topographic mnp of the Y ellowtn.il 

reservoir site prepared for the Bureau of Reclamation by Fairchild 

Aerial Surveys, Inc. This topographic map was compiled by stereophotographic n1ethods, the triangulation net having been established 

by ground sui·vey. 

W. A. Cobban of the Geological Survey furnished several 

stratigraphic sections of Cretaceous formations measured and described by him that have appeared in genern.lized form (Knechtel and 

Patterson, 1952, pl. 1). Assistance in several ways was extended 

member·s of the party by offices of the Bureau of Reclamation and the 

Big Horn County Agricultural Conservation Association at Hardin, 

l\1ont. The Tribal Council of the Crow Indians granted permission 

to map within the reservation. Phillips Petroleum Co. kindly pt·ovided smnples and the electric log of the Soap Creek oil field (the 

lnlu.nd Empire Refineries well 52-34, Crow tribal); and H. H. Perrigo, 

distt·ict engineer of the Geological Survey at Billings, Mont., supplied 

t·ecorded data on wells. The writer expresses for himself and members 

of the party appreciation for the many courtesies given by residents 

of the area. 

PREVIOUS WORK AND PUBLICATIONS "1, 

The Bighor·n l\1ountains were mapped by Darton (1906a) between 

1901 and 1905. Thorn and l\1oulton (1921) made a report and 

structure contour map of Soak Creek dome, and Thorn (1922) made a 

t·epot·t on Soap Creek dome and other domes and anticlines in the area 

adjacent to the Bighorn l\1ountains. Thorn later ('l'hom and others, 

1935) completed the unpublished reports of several field projects in 

the Crow Indian Reservation, l\1ontana. The Hardin gas field was 

described by Perry (1937). Geology of the Pryor .lVlountains was 

described by Blackstone (1940). Several stratigraphic sections were 

measured along the flanks of the Bighorn l\1[ountains by L. S. Gardner 

and others (1946). 

GEOGRAPHY 

SURFACE FEATURES AND RELIEF 

The Bighorn Canyon-Hardin area lies along the border of the 

Great Plains and .1\lliddle Rocky l\1lountain provinces (Fenneinan, 

1931, p. 160). The Bighorn l\1ountains trend northwestward about 

150 miles fr01n north-central Wyoming into south-central .1\llontana. 

Their width decreases from about 30 miles in Wyoming to a point 

where they die out north of Bighorn Canyon in l\1Iontana. In the 

high central part of the mountains, glaciated peaks are as much as 

13,000 feet above sea level, but the crest of the range declines steadily 

northward to 8,000 feet at the .1\l!ontana-Wyoming border, 6,000 feet 
6 GEOLOGY OF THE BIGHORN CANYON-HARDIN AREA, MONT.-WYO. 

just south of Bighorn Canyon, and less than 4,500 feet at Grapevine 

dome at the northern tip of the Bighorns. 

In Montana the Bighorn 11ountains have been untouched by glaciation; so there are no peaks. A relatively smooth upland sm·face, 

which has been deeply incised by strea1ns, slopes from the crest to 

steep flanks. Although the streams are now in deep canyons, the 

intercanyon areas have been dissected so little that much of the sur- ~ 

face can be crossed by auto. Cattle, sheep, and buffalo that belong 

to the Crow Indians graze on the higher slopes. 

As the Bighorn Mountains decline in altitude northward, they 

take the outline of one's thumb and forefinger-the forefinger on the 

east and pointed northward. The western flank of the rangerepresented by the thumb-ends in Porcupine Creek anticline, almost 

on the Montana-Wyoming border, and the eastern flank-represented 

by_ the forefinger-runs a little more than 25 miles into Montana. 

The Pryor Mountains start just north of the west flank of the Bighorns and continue 30 miles northwestward into 11ontana. West of 

the Bighorn and Pryor Mountains is the Bighorn Basin, and east of 

the Bighorn Mountains is the Powder River Basin. These basins, 

which are roughly elliptical in outline and trend northward to northwestward, are more than 100 miles long. 

A smaller basin lies between the Pryor Mountains and the two 

northern prongs of the Bighorn Mountains. This basin is bisected 

by Bighorn Canyon; the area south of the canyon is called Garvin 

Basin, whereas the area north of the canyon is herein referred to as 

the Dry Head Creek area. 

Bighorn Canyon ranges from 200 feet to more than 1,000 feet in 

depth across the Garvin Basin-Dry Head Creek area. The canyon 

is narrow, and the walls are nearly vertical for most of the canyon's 

length. ·Livestock can cross the canyon and ford the river at only 

one locality, which is south of the mouth of Camp Creek in T. 8 S., 

R. 29 E. 

As the Bighorn River crosses the axis of the northern projection 

of the mountains, Bighorn Canyon changes from a 1,000-foot deep 

box canyon to a 2,200-foot deep gorge. The canyon declines steadily ;;; 

in depth from the axis of the 1nountains to the mouth, where the 

canyon walls are only a few feet high. There, the Bighorn flows onto 

the Great Plains and meanders through a broad valley to join the b 

Yellowstone River about 60 miles north of the mouth of Bighorn 

Canyon. Stream terraces in the Bighorn valley are as much as 700 

feet above the river. 
ti 

GI<~OGRAPHY 7 

CLIMATE AND VEGETATION 

'l'he climate of most of the region is scminrid, the average annual 

precipitation at Hardin from 1940 to 1947 being a little less than 15 

inches. 'J'he average annual precipitation is higher toward the Inountains, and at the Campbell farn1 camp, which is about 600 feet higher 

than fim·din, the average precipitation was 19 inches frmn 1942 to 

1947. Precipitation in the n1ountains is undoubtedly nuiCh higher. 

l\t[ay and June are the wettest months, and (July and August are the 

hottest. Temperatures as low as -46° F. and as high as 110° F. 

wm·e recorded at Crow Agency between 1897 and 1930. Climatological data for the area nrc summarized in table 1. 

'Rain and 1nelted snow support a good growth of grass in meadows 

of the Bighorn l\t[ountains and on high terraced lands of the plains 

region. Pine forests grow on the Bighorn 1\tiountains, scattered 

groves of pines Inm·k the outcrops of sandstone along the flanks of 

the 1nountains, and cottonwood and boxeldet· trees flourish in the 

valleys of perennial streams. Sagebrush, greasewood, small varieties 

of cacti, and sparse grass grow on thin soil that covers the dark shales 

of the plains and the gravels of the lower terraces. 

DRAINAGE AND WATER SUPPLY 

The Bighorn River, 1naster stream of the area, originates at the 

southern edge of the Bighorn Basin, 'Vyo., as a continuation of the 

Wind River. The Wind River crosses the western part of the Wind 

River Basin, Wyo., and then flows northward across the Owl Creek 

l\1ountains through Wind River Canyon. At the mouth of this 

canyon the strcnm changes in name from the Wind River to the 

Bighorn River, which crosses the Bighorn Basin, the northern end of 

the Bighorn l\t[ountains, Mont., and finally part of the Great Plains 

to the Yellowstone River about 30 miles north of Hardin, lVIont. The 

average rate of discharge of the Bighorn River is nbout 3,500 cubic 

feet per second (cfs) at a point just downstremn from the mouth of 

Bighorn Cayon and more than 4,000 cfs at Hardin. A high flow of 

37,400 cfs and a low of 228 cfs have been recorded at the gauge near 

the n1outh of Bighorn Canyon. 

The Little Bighorn River, which joins the Bighorn near Hardin, 

is the chief tributary stream within the area. A dozen smaller 

permanent streams, all of which head in the Bighorn and Pryor 

l\1ountains, enter the Bighorn River. A summary of stream discharges is shown in table 2. 

Irrigation water for the Bighorn River valley between the Bighorn 

l\1ountains and Hardin is carried in two canals. The Bighorn canal, 

with its headgate at the mouth of Bighorn Canyon, supplies water 

for the vnlley above Two Leggin Bridge about 8 1niles south of 
TABLE I.--Climatological data for Hardin and the surrounding area in Big Horn County, 111 ont. 

[From U. S. Weather Bureau reports] 

Station Period Jan. I Feb. I Mar. I Apr. i lVIay I June I July I Aug. I Sept. Oct. I Nov. I Dec. I Annual 

Average monthly and annual precipitation at Weather Bureau stations at and near Hardin (inches) 

Campbell farm camp ___________ 1940-47 __ ------------ 0. 59 0. 61 0. 93 1. 96 ., 2 45 4. 60 0. 78 Crow Agency 1 __________________ I881-90_ ------------- . 91 . 45 . 59 I. 13 I. 75 2 57 .87 

Do. 1 __ ---------------------- 1891-1900_- ---------- 2. 74 2. 46 2.82 2 I. 73 2 2 3:~ 2 3 08 2 I 68 Do. 1 ________________________ 1901-IO __ ------------ .58 . 85 1. 47 1.33 2 88 2. 48 . 98 Do. 1 ________________________ 1911-20 s _____________ 4 1. 05 4. i2 477 4]. 23 4 2. 26 4 2 27 3 I 52 Do.I ________________________ 1921-30_------------- .99 .84 1.12 I. 38 2. 74 2 01 1. 81 J)o.I ________________________ 1931-40 ____ ---------- .66 . 79 l. 56 1 61 2.03 l. 79 . 99 Do. 1 ________________________ 194I-47 -------------- . 58 . 41 IOl 1. 65 I. 93 4 16 . 61 Hardin _________________________ 1942-47-------------- . 51 .41 .82 . 84 1.90 3. 93 . 65 

-·- - ---------

Average monthly and annual temperature (° F.) 

Compb<'ll ''"" oomp ______ -----~1940-<L ________ ----~ ,. , I ~·I ''"I 47 'I M 7 I 00 'I 72.5 Crow Agency 1 __________________ 47 years 6 ____________ IS. 9 21.8 33.6 46.4 54.7 65.1 72.4 Do__________________________ 1931-47 ______________ 3 22 0 2 24 3 2 32 6 2 44.3 3 56.5 3 64 6 2 68 7 

Hardin______________________ 1942-47-------------- 23.4 2fl. 6 34 6 48.9 55 I Ill. 5 71. 7 

Average monthly and annual maximum temperatures (° F.) 

Crow Agency 1 _________________ ,47 years 6 ____________ , 31.61 a4 61 46.41 60.51 68 21 80.41 89.71 

Average monthly and annual minimum temperatures (° F). 

Crow Agency I _________________ ,47 years 6 ____________ , 6. 21 9 o 1 20. 91· 32.21 41.21 49.81 

1 Record from 1881 to October 1897, inclusive, for old Fort Custer, about 9 miles northwest of Crow Agency. 2 One ycar·s record missing. a Two years' records missing. • Three years' records missing. ; No record for I915--l6. 

"Through 19:30. 

~ 

-·~ 

55.1 1 

1.08 2.60 

1. 02 . 95 2. 72 . 8I 

1.02 I. 18 3.69 3 I 59 

. 64 l. 62 

.90 1. 09 1. 07 2. 37 

1.19 I. 89 

I 

W81 ,~,1 69 0 58.7 2 69.8 2 59.2 69.4 59.7 

86 41 75.41 

51.5 ! 42.0 

1. 40 1 06 0. 63 18.69 92 .39 .67 12.00 

1. 57 2 1 15 3. 34 4 16.07 

. 93 .88 . 97 15 5.5 3 1. 72 3 .83 3. 60 4 1.5. 68 1 49 1 2.3 .87 16.74 

1. 37 .66 . 79 14 22 1 36 .85 .87 16.86 

. 98 . .59 . 59 I4. 30 

5151 ''-'I W41 47.8 47.4 33.6 23.6 45.4 2 48.8 2:33.8 2 25 7 44() 7 

50.2 :H.5 26.2 4fi. 9 

fi2. 51 47. I I 35.51 59.9 

:~2. aj 20.1 1 II. 7 1 31.0 

f.J6. 

'l:J 

:;') 

8 

s 

0 

~ 

0 

~ 

'-3 

~ 

M 

to 

0 

~ 

0 

:::0 

z 

("') 

> 

z 

~ 

0 

z 

I 

:r 

> 

;;:) 

~ 

z 

~ 

;: 

~ 

0 

~ I 

~ 

~ 

·0 
... a-:-/ • 

TABLE 2 .-Stream discharges 

[Data in cubic feet per second for water years ending September 30. Sources: U. S. Geol. Survey, 1943 and 1947, Water Supply Papers 917 and 10.36; and unpublished records ofthe Water Resources Division, Billings, Mont.) 

Stream Location of gaging station 

Big~~~ Do -~!~~-r~~=== = = == == = = == = = = === === = ==1 ~a~!ie~,~~~ili~;,est -o{ si~ x~i:Vie-r~-Mon_t __ = = === == = = = = = = __________________________________ One-half mile above mouth of Little Bighorn River 

near Hardin, Mont. Little Bighorn River_ ___________________ Near Crow Agency, Mont_ ________________________ _ 

Rotten Grass Creek _____________________ ) Near. St. Xavier, Mont_ _______ ----------------------- Soap Creek _____________________________ _\ 10 miles southwest of St. Xaner, Mont_ ____________ _ 

I 

Period 

1928-45 

1934-45 

1917-24 

1929-32 

1928-29 

193D-31 

1931-32 

1938-46 

1914-li 

1939-47 

Average 

2, 319 3,556 

4, 941 4,055 

434 

153 

301 

285 

31 

33 

Discharge 

"£ . I , .... l'' ax1mum 1 ""l!mmum 

Amount I Date I Amount Dat< 

25,200 June 16, 19351 179 June 22, 1934. 37,400 June 16,19.35 228 Dec. 9, 1937. 

---------- --------------- ---------- --------------------

8, 200 July 23, 19231 None____ July 28-Aug. 6, 1921. 

~ ~~~~~~~ ~ ~~~~~~ ~ ~~ ~~~~ ,~~ ~ ~ ~ ~ ~ ~ ~~ ~ ~~~~~~~~ ~~~~~ ~ ~ ~ ~ ~ ~ ~- 1,060 May 13,1942 5 Jan. 18-19, 1940. 

0 

b 

0 

> "C 

;:c 

~ 

<:0 
10 GEOLOGY OF THE BIGHORN CANYON-HARDIN AREA, MONT.-WYO. 

Hardin. Two Leggin canal, with its headgate at the bridge, supplies 

the valley between Two Leggin Bridge and Hardin. Between the 

canyon and the bridge the river flows along the western edge of the 

valley, but it crosses to the eastern side below the bridge and flows 

northward past Hardin on the east edge of the valley. A reservoir 

impounded by the proposed Yellowtail dam would reach up the 

Bighorn River to near Kane, Wyo., on the west side of the Bighorn 

Mountains about 11 miles south of the Montana border. It would 

provide water to irrigate the terrace, now farmed by dry-land methods, 

125 feet above the Bighorn River on the west side of the valley below 

Bighorn Canyon. 

INDUSTRY, SETTLEMENT, AND TRANSPORTATION 

Ranching and farming are the principal industries in the area. 

Wheat is grown on the terraces, and cattle and sheep graze the grasslands. Sugar beets, which are refined at H·ardin, and hay are the 

chief crops of irrigated lands. Some oil has been produced from the 

Soap Creek field, and gas from the Hardin gas field supplies the town 

of Hardin and nearby ranches. -

Hardin, the county seat of Big Horn County, ~1ont., and the only 

incorporated town in the area, had a population of 1,886 in 1940. 

Most of the inhabitants of the area live on farms in the river valleys 

and on the lower terraces. Crow Agency, headquarters of the Crow 

Indian Reservation, is in the valley of the Little Bighorn River 

about 12 miles southeast of Hardin. 

Hardin is on the Chicago, Burlington & Quincy Railroad between 

Billings, Mont., and Sheridan, Wyo. U. S. Highway 87 is generally 

parallel to the railroad. The only other paved road in the area is 

between Hardin and St. Xavier. The history of exploration and 

early settlement of the area has been described by Thorn (Thorn and 

others, 1935, p. 5-7). 

STRATIGRAPHY 

GENERAL FEATURES 

About 9,000 feet of sedimentary rocks, predominantly of marine 

origin, is exposed within the area. Lying in a nearly conformable 

sequence, they represent every system from Cambrian to Cretaceous 

except the Silurian. Rocks older than the Upper Jurassic ~1orrison 

formation crop out only between and on the flanks of the Bighorn 

and Pryor Mountains. l\1orrison and younger rocks are exposed in 

the plains region east and north of the Bighorn and Pryor l\1ountains. 

Pre-Cambrian crystalline rocks and the lowermost Cambrian strata 

are not exposed within the area, and the only exposure of pre-Cambrian 

rocks in the ~1ontana part of the Bighorn ~1ountains is at Point 

Lookout, about 16 miles south of the mouth of Bighorn Canyon 

(Darton, 1906a, pl. 4 7). The pre-Cam brian crystalline rocks, which 

: 

I~ 

-

-~ 
STRATIGRAPHY 11 

fonn the core of the Bighorn 1\tlountains, arc mostly moderately 

coarse-grained rod granite and fine-grained gray granite which are 

transitional into each other (Darton, 1906a, p. 15-17). Some schist 

and gneiss are exposed in the core of the mountain, nnd 1nnny dikes of 

basic rock intrude the granite. All are truncnted by an unconfol'mity 

nt tho base of the Cambrian system (Darton, 1906a, pl. 6). 

,. Charactcl'istics of tho formations exposed in the area arc summarized 

in plate 2. Thicknesses of the formations were obtained froin surface 

sections and from records of wells drilled in the area. The formations, 

particularly those of the Paleozoic era, are traced westward into 

for1nations underlying the Bighorn Basin in plate 4. 

CAMBRIAN SYSTEM 

GROS Vl<JNTRE 1!-.0RMATION AND GALI...~ATIN I...~IMESTONB 

The basal 300 feet of Cambrian strata arc not exposed in the area, 

but as much as 700 feet of limestones and shales of Cambrian age 

crop out in the deepest part of Bighorn Canyon at the anticlinal 

axis of the Bighorn Mountains. Steep talus-covered slopes on these 

rocks rise frmn the river to the cliff-forming Bighorn dolomite of 

Ordovician age, and the Cambrian rocks can be seen only in isolated 

exposures. 

Darton included all Cambrian strata of the Bighorn 11ountains in 

the Deadwood formation, a name which he had earlier given to Cambrian rocks of the Black Hills area (Darton, 1901, p. 505). He 

described tho Deadwood of the Bighol'n l\1ountains as being about 900 

foot thick and consisting of a basal 10- to 200-foot zone of coarsegrained reddish-brown conglomeratic sandstone, an overlying 200-

foot zone of sandy shale and thin-bedded sandstone which merges 

upward into 400 feet or more of soft greenish-gray shale with a few 

limestone and sandstone beds, and an uppermost zone, 125 to 200 

feet thick, of gray slabby limestone, sandstone, and flat-pebble liincstone conglomerate (Darton, 1906a, p. 23-26). 

A two-fold division of the Cambrian strata into an upper prcdmninantly limestone formation and a lower predominantly shale and 

li1ncstone unit leads to a correlation of these strata in the northern 

end of the Bighorn Mountains with the Gros Ventre formation and 

Gallatin limestone of more westerly areas. 1'he usc of these names 

is to be preferred over using the Deadwood formation which was 

applied to beds in the Black Hills about 200 miles southeast of Bighorn Canyon. A correlation of the Cambrian strata across Bighorn 

Basin and the north end of the Bighorn Mountains is shown in plate 4. 

The uppermost 40 feet of the Gallatin in Bighorn Canyon:consists 

of thin-bedded gray limestone, thin-bedded mud-cracked, son1ewhat 

silty, grayish-green shale, thin stringers of sand, and thin beds of 

:334328-55-2 

r 
12 GEOLOGY OF 'l'HI<; BIGHORN CANYON-HAHlJlN AI{EA, .MON'l'.-WYO. 

rather inconspicuous flat-pebble limestone conglomerate. The underlying beds are mostly poorly exposed interb~ddccl shale and fiat-pebble 

or edgewise conglomerate. 

Cambrian strata, about 1 ,050 feet thick, in the Inland En1pire 

Refineries well 52-34, Crow tribal, at Soap Creek dome arc divisible 

into a lower 850 feet of what is predominantly shale and an upper 

200 feet of limestone. The lithology, based upon a study of cuttings 

from the well by G. E. Prichard, is shown in plate 3. The lower 

unit, which is probably the Gros Ventre formation, is made up of 

two main lithologie units, a low<'.!' 600 fed or more of dark-gray and 

dark-green shale, phyllitic in the basal part and variably silty, sn,ndy, 

and calcareous throughout, and an upper 2:3.5-foot unit composed of 

micaceous, fissile gray to green calcareous shale. The topmost 200 

feet of limestone is considered to be the Gallatin limestone. Some 

fragments of dark-green shale occur in t.he samples from this upper 

unit, but shale seems to be less abundant in the subsurface samples 

from the Gallatin than in the outcrop. The lower member of Darton's 

Deadwood formation, a conglomeratic sandstone, does not occur in 

the Inland Empire Refineries well, and the sandstone may be missing 

in the nearby mountains. 

Darton (1906a, p. 26) considered the Deadwood formation in the 

Bighorn l\1ountains to be l\~Iiddle Cambrian, but Nleyerhoff and 

Locbman (1938, p. 285) have found Upper Cambrian strata there, too. 

The Gt·os Ventre formation and the Gallatin limestone west of the 

Bighorn Basin are Middle Cambrian and Upper Cambrian respectively 

(Wilmarth, 1938, p. 883 and 793). 

Poor exposures of Cambrian strata in Bighorn Canyon and the 

absence of faunal data have led to mapping the two formations as 

one unit. The contact between the Gallatin limestone and the Gros 

Ventre formation in Soap Creek dome is based upon lithologic similarities with those formations west of Bighorn Basin (Leatherock, 

1950). As no fossils have been collected in the one area, part or all 

the 235 feet of calcareous shale at the top of the Gros Ventre formation 

in the Inland Empire Refineries well may be Late Cambrian in age. 

ORDOVICIAN SYSTEM 

BIGHORN DOLOMITE 

The Bighorn dolomite ranges in thickness from about 205 feet in ~ 

Black Canyon to 390 feet in Bighorn Canyon and 480 feet in Soap 

Creek dome. Within this area the Bighorn consists of a lower massive dolomitic limestone member and an upper thin-bedded dolomite 

and limestone member. The massive member is exposed in high cliffs 

which led Darton (1906a, p. 26) to say that the Bighorn is perhaps the 

most conspicuous sedimentary formation in the Bighorn l\1ountains. 
I~ 

S'l'HATIGHAPHY 13 

In the section ncar Lhc n1outh of Big Bull Elk Creek in Bighorn 

Canyon, a dcscripLion of which follo\v·s, the lower n1cmbcr of the 

Bighorn dolomiLc is light yellowish-gray dolomitic limestone thaL 

fonns a smooth vertieal cliff 180 feet high u,bovc a steep slope on the 

Cambrin.n sLrattt. Weathering nJong a bt~dd ing plmw n,bou L 45 feet 

below Lhe top has made Lhe only break in the cliff. Above this massive member, which forms Lhe cliff, are thin beds oflight-gray to white 

dolomite and dolomitic limestone, which form alternate slopes and 

ledges. A similar, though thinner section was measured by Gardner 

and ·_Rogers (Gardner and others, 1946, p. 90-91) at Blaek Canyon, 

about 9 1niles southeast of the Bighorn Canyon section. 

Sect·ion of Bi(Jhorn, dol01nite on north side of 8-iyhom River about 2 miles wef~t of 

·mouth of B1:g Bnll Elk Creek in N?~ sec. 5, T. ''( S., R. 80 B. (unsurveyed), Biy 

Horn County, JV/ont. 

[Units arc numbered as shown in plate 4, section 4] 

.Joffct·Ron limcRtone and Three Forks !:>hale undifferentiated: Feet 

Lime:sLone, light-brown, dolomitic, thick-bedded___________________ 23 

Bighorn dolomite: 

12. Limestone, light-gray, dolomitic, thin-bedded__________________ 22 

Lirne!:>tone, white, dolomitic, very finely erystn.lline, mediumbedded; forms prominent cliff______________________________ 80 

Lirnc:;;t,one, white, in part dolomitic, thin-bedded; forms ::;lopes___ 65 

13. Dolomite, whit.e, very finely CI'y:st.allinc, thick-bedded; forms 

small cliffl:\ locally but is genemlly concealed_________________ 40 

14. Limestone, yellowi:sh-gmy, dolomitic, finely erystallinc, massive; 

forms cliff wit.h underlying tmiL____________________________ 35 

Lime~t.one, ~imilar to overlying uniL; forms vel't.ic:d cliff________ 145 

Total, Bighom dolomite________________________________ 387 

Gros Ventre formation and Gallatin limestone undifferentiated: 

Limestone and :shale: gray thin-bedded ripple-marked finely crystalline 

limestone and green silty nmd-cracked shale in very thin beds and 

laminae; few thin bed:s of flat-pebble limestone conglomerate and a 

few streaks of sand___________________________________________ 40 

The Bighorn dolomite is fairly uniform in thickness along Bighorn 

Canyon, but, as shown in plate 4, it is thicker to the west beneath 

Lhc Bighon1 Basin and to the east in Son.p Creek dome. This range 

in Lhickncss may be due to an unconformity at the base of the Devonian systmn, bu L the mnounL is q ucstionable because of the uncerLainty 

in distinguishing Ordovician from Devonian stn1.ta. 1 Bedding in the 

u ppcr part of the Cmnbrian sLrata seems to be parallel with Lhc base 

of Lhe Bighorn dolomiLe, ot· neal'ly so, but. exposures of Lhc contaeL arc 

poot·, and Lhc exaeL relation heLwccn the Bighorn and Cambrian 

sLI'aLa wa.s noL detcl'1nincd. 

' 'l'hc occurn•ncc of Silurian rocks in t.he suhsurfact• at Soap Cn•ck donw was suggPsted shortly hcforr this 

n•pm·t wt•nt. to t.he printer. In the pre-Permian corrPiation chart. of the Billings 0Pological Socwty's 

l•'iftl1 Annual Field Confpr·tmcc Guidebook (1954) the rocks het.\\'I'Pn 2,\IO!i and :l,O!iO ft~t'l. dt•pt.h in the fnland 

l•:rnpirn ltrtinPI'ii'S \\'I'll at. Soap nri'Pk donw an• callt•d Silw ian('·') 
14 GEOLOGY OF 'l'HJD BIGHORN CANYON-HARDIN AREA, MON'l'.-WYO. 

All Ordovician strata of the Bighorn l\1ountains were included in 

the Bighorn dolomite by Darton (1906a). The two members of the 

Bighorn dolomite within the area are correlated with the Upper 

Ordovician beds described in the central latitudes of the Bighorn 

l\1ountains by Darton (1 906a, p. 28-29). At least 2 and probably 3 

units of the Bighorn dolomite in the Bighorn Mountains in Wy01ning 

do not oceur or arc poorly represented along Bighom Canyon. These 4') 

arc (1) a sandy dolomite in the base of the lower 1nassive men1ber, 

whieh Kirk (1930, p. 460-465) correlates with the Lander sandstone 

(l\1 illet·, 1 930), (2) the fine-grained white sandstone beneath the mas- = 

sive me1nber, which Kirk (1930, p. 460-465) correlates with the :l\1iddle Ordovician Harding sandstone of Colorado, and (3) red shaly and 

silty beds at the, top of the Bighorn, which was described briefly by 

Darton (1906a, p. 28) as "reddish clay, due to the weathering of the 

uppermost limestone." Whether these reddish silty beds are younger 

Lhan the highest beds in Bighorn Canyon or whether they represent 

a lateral change in lithology of the highest beds in Bighorn Caynon 

is not now known. These 3 units are well exposed along the South 

Fork of Rock Creek, Johnson County, Wyo., about 90 miles southsoutheast of the mouth of Bighorn Canyon. 

DEVONIAN SYSTEM 

JEFFERSON LIMESTONE AND THREE FORKS SHALE UNDIFFERENTIATED 

Rocks of probable Devonian age, which are herein mapped as 

undifferentiated Jefferson limestone and Three Forks shale, crop out 

in Bighorn Canyon between Black Canyon and Dry Head Creek, in 

Black Canyon, and in Devils Canyon of Porcupine Creek. Darton 

(1906, p. 14) did not recognize any formation of Devonian age in the 

Bighorn lVIountains. Both Stipp (194 7, p. 122) and Thomas (1 948, 

fig. 2), however, have indicated the possibility of Devonian rocks 

occurring in the northern part of the mountains, and earlier Wilson 

(1934) had mapped Devonian rocks along the west flank of the 

mountains a few miles east of Lovell in Big Horn County, Wyo. 

The names Jefferson limestone and Three Forks shale were first 

given to Devonian strata in Jefferson and Gallatin Counties, lVIont., 

about 200 miles west of Bighorn Canyon. These names are used in 

the Bighorn Canyon area for strata that are presumed to be at least 

in part the stratigraphic equivalents of the type Jefferson and Three 

Forks, even though the lithology of the beds along and near Bighorn 

Canyon is not wholly the same as the type formations. Further work, 

particularly subsurface studies of adjacent areas, may indicate that 

another name such as, perhaps, the Darby formation of Wyoming, is 

preferable. 
S'fRATIGRAPHY 15 

Jefferson and Three Forks rocks are 395 feet thick in a section 

1ncasured by W. G. Pierce and R. P. Bryson along Clarks Fork of the 

Yellowstone River in T. 56 N., R. 103 W., Park County, Wyo. 

(Lcathcrock, 1950). Two surface sections of the Devonian rocks 

were measured in the Bighorn Canyon area; one in Devils Canyon 

(fig. 1), where the formations arc 180 feet thick, and the other in 

Bighorn Canyon ncar the mouth of Big Bull Elk Creek (fig. 2), where 

the formations arc 220 feet thick. These two sections are described 

below. '"!"'he Devonian rocks include a lower limestone and dolomite 

unit containing s01ne greenish-gray shale, siltstone, and sand, and an 

upper very sandy limestone and dol01nitc unit. The sand grains arc 

well rounded, commonly one millimeter in diameter, and occur in 

stringers and disseminated throughout the limestone and dolomite. 

The greenish-gray shale is in very thin layers; some is so dolomitic 

that it could be called argillaceous dolomite. Li1nestones in the 

upper part of the Devils Canyon area contain many cal9ite-fillecl vugs. 

These and the abundant sand grains arc charactel'istie of the Darby 

formation of Wyoming (Thomas, 1948, p. 84). 

Section of undifferentiated Jefferson limestone and Three Forks shale on north side 

of the Bighorn Rive·r about 2 miles east of mouth of Big B1l.ll Elk Creek in N}~ 

sec. 5, T. 7 S., R. 30 E. ('tmsurveyed), Big Horn County, Mont. 

[Units arc numbered as shown in plate 4, section 4] 

Madison limestone: 

Limestone, gray, finely crystalline, medium-bedded; lower few feet 

dolomitic and sandy; thin-bedded chert in upper 40 feet; irregular 

contact with underlying strata __ ------------------------------

Jeffer~on limestone and Three Forks shale undifferentiated: 

9. Limestone, yellowish-gray, finely crystalline, aQd interbedded 

greenish-gray sH,ndy limestone in irregular beds; few inches at 

top consist of greenish-gray siltstone and sandstone with subrounded gray quartzite and limestone pebbles as much as linch in diameter _________________________________________ _ 

Limestone, gray, finely crystalline, and laminated greenish-gray 

sn,ndy limestone; a 3-foot bed of sandy limestone breccia about 

10 feet below top; lower 15 feet mostly concealed ____________ _ 

10. Limestone, brownh;h-gmy, granular, cliff-forming _______________ _ 

Concealed _________________________________________________ _ 

Limestone, brow11ish-gray, thin-bedded, cliff-forming ___________ _ 

11. Dolomite, gray, finely crystalline, hard; many vugs; thin zone uf 

green shale at base _______________________________________ _ 

Dolomite, gray, finely crystalline; some irregular zones of reddish 

dolomite in upper part; lower 5 feet thin bedded and platy; upper 

part weathers very blocky; much of unit weathers to very sharp rough surface ____________________________________________ _ 

Limestone, light brownish-gray, dolomitic, thick-bedded ________ _ 

Bighor·n dolomite: 

Feet 

84 

96 

30 

5 

10 

13 

17 

25 

23 

219 

Limestone, dolomitic, light-gray, thin-bedded_____________________ 22 
1() Gl~ULUGY OF '!'HE BIGHOHX CA~YOX-HAHDlN .HtEA, ~101\:'l'.-Wn>. 

Fwt-RE I.-Paleozoic strata in north wall of Drvils Canyon of Porcupine Creek. Madison limestone and 

undrrlying beds of probablf' Devonian age crop out about onr-thircl mile east of Bighorn Canyon. Porcupinr Creek, which is just bdow tlw lowest rocks ~l'l'll in photo, joins the Bighorn River in tlll' southwest 

corner of Big Horn County, :\lont. 
STHAT lG RAPHY 17 

FIGUHE 2.- Paleozoic strata in Bighorn Canyon nrar the crest of thr Bighorn Mountains. View e~JStwan.l 

across the Bigllom River from a point opposite the mouth of Big Bull Elk Creek. Paleozoic strata 

underlying the Amsclt>n formation are SPl'll in the far cliO's. TJ1e basal mPmht'r of tlw Bighorn 

dolomite and the underlying Cambrian thin-beddNllimestono and shales form the cliff in foreground. 

'The thickness shown for each formation was measured on near side of river, starting at. hase of Bighorn 

dolomite in foreground. 
18 GEOLOGY OF 'l'HE BIGHORN CANYON-HARDIN AREA, MON'l'.-WYO. 

Section of undifferentiated Jefferson limestone and Three Forks shale and upper 

part of Bighorn dolomite in Devils Canyon about one-half mile east of junction of 

Porcupine Creek and the Bighorn River in S.H sec. 25, T. 9 S., R. 28 E. (unsurveyed), Bia Horn County, Mont. 

[Units are numbered as shown in plate 4, section 3] 

Madison limestone: Feet 

Limestone, light-gray, dolomitic, hard ___________________________ _ 9 

.Jefferson limestone and Three Forks shale undifferentiated: 

7. Dolomite, brownish-gray, sandy, hard; vugs in upper part fil!ed by calcite __________________________________________________ _ 8 

Dolomite, greenish-gray, very silty; contains few thin rustyweathering sandy beds and beds of green shale; upper 6 inches 

somewhat brecciated, containing angular fragments of gray 

dolomite; lower 10 feet mostly concealed_____________________ 18 

Dolomite, light-brownish-gray, granular, partly silty_____________ 2 

Siltstone and fine-grained sandstone, greenish-gray; weathers 

green and brown; several thin layers of sandstone composed of 

well-rounded frosted quartz sand as much as 1 millimeter in 

diameter ________________________________________________ _ 

8. Limestone, gray, finely cryst!tlline. medium-bedded; ledge former; 

weathers brownish gray ___________________________________ _ 

Dolomite, gray; bedding in lower 4 feet very irregular; weather a blocky __________________________________________________ _ 

Dolomite, pale-brown, granular; at top is 3-inch bed of very porou~ 

limestone composed of calcite grains; ledge former ___________ _ 

Siltstone and sandstone, greenish-gray; sandstone has coarse wellrounded quartz grains in fine matrix; thin bed of yellowish-brown 

dolomite 2 feet below top has calcite crystals in vugs _________ _ 

Dolomite, dense and earthy, arid greenish-gray coarse-grained 

sandstone in which quartz grains are cemented ______________ _ 

Limestone, light-brown, dolomitic; numerous vugs filled with 

calcite crystals; channeling in top of unit; upper 2 feet very 

porous __________________________________________________ _ 

Shale, greenish-gray, dolomitic, hard, flaky; much sand in base __ _ 

Limestone, light-gray, dense, thin-bedded _____________________ _ 

Limestone, brownish-gray, finely crystalline; ledge former_ 

Dolomite, gray, sandy; vugs on surface _______________________ _ 

Sandstone, greenish-gray, shaly, mostly fine-grained and very 

fine grained; contains many !-millimeter quartz grains ________ _ 

Dolomite, light-gray, granular, weathers light gray _____________ _ 

Limestone, pale-yellowish-brown, very finely crystalline; ledge former __________________________________________________ _ 

Limestone, light-gray, weathers to pitted surfaces ______________ _ 

Limestone, gray; weathers brownish gray; ledge former _________ _ 

Dolomite, greenish-gray, silty; 1 inch of greenish-gray dolomitic 

shale at top _____________________________________________ _ 

Limestone, light-gray, earthy, thin-bedded ____________________ _ 

Limestone, brownish-gray, dolomitic; this and overlying unit 

weather to very rough corrugated surfaces __________________ _ 

Dolomite, gray ____________________________________________ _ 

Dolomite, greenish-gray, silty, soft; forms reentrant_ ___________ _ 

10 

4 

12 

1 

10 

1 

10 

5 

2 

.5 

3 

1 

2 

4 

3 

13 

1 

2 

2~~ 

2 

~~ 

7":' ~ 

;:, 
1,.. 

1 

STRATIGRAPHY 19 

Section of 'l.tndifferentiated Jefferson limestone and Three Forks shale and upper 

part of Bighorn dolomite in Devils Canyon about one-half mile east of j1.mction of 

Porwpine Creek and the Bighorn River in S% sec. 25, T. 9 S., R. 28 E. (nnsw·-

veyed), Big Horn Connty, Mont.-Continued 

Jefferson limestone and Three Forks shale undifferentiated-Continued 

Limestone and dolomitic lime'3tone, gray; ledge former; lower 4 to 5 Feet 

feet pitted ___ -------_____________________________________ 10 

Limestone and dolomite; dolomitic limestone at top and gray 

finely crystalline dolomite at base___________________________ 4 

Dolomite, silty and greenish-gray sandy dolomitic siltstone_______ 8 

Dolomite, gray, dense, hard, vuggy___________________________ 5 

Dolomite, gray, dense_______________________________________ 4 

Siltstone and shale, greenish-gray_____________________________ 3 

Dolomite and limestone, gray, thin- to medium-bedded__________ 7 

Limestone, browui3h-gray; ledge former________________________ 6 

Limestone, sanely, thin-bedded_______________________________ 7 

Dolomite and dolomitic limestone, gray, thin- to medium-bedded_ 8 

Concea~cl-------------------------------------------------- 4 

Total, Jefferson limestone and Three Forks shale, uncliffer- en t.iated _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 188 

Bighorn dolomite: 

9. Dolomite, light-gray; reddish brown in upper 10 feet; weathers 

brown and gray; sharp rough surfaces, finely crystalline; 2 or 3 

ledges in unit_____________________________________________ 25 

Dolomite, very light gray; weathers white; finely crystalline; 

gradational into underlying unit____________________________ 8 

Dolomite, light-gray to white; weathers to very sharp rough surface; caves develop at base and extend vertically, resulting in 

formation of pinuacles__ __ _ _ __ _ _ __ __ _ _ __ ___ _ __ __ __ ____ _ _ ___ 15 

Dolomite, light-gray, finely crystalline to dense, hard; forms small 

ledge____________________________________________________ 4 

Dolomite, light-gray, dense; forms two lodges___________________ 36 

Total, to level of Porcupine Creek ________________________ 88 

Devonian rocks arc 210 feet thick in the Barnsdall well 2, Dorothy 

Fox, sec. 7, T. 57 N., R. 97 W., Big Horn County, Wyo., on Sage 

Creek don1c in the Bighorn Basin. 'J'hey arc 185 feet thick in the 

Inland Empire Refineries well 52-34 Crow tribal, at Soap Creek dome, 

Big Horn County, 11ont., just cast of the Bighorn 11ountains (pl. 

3, and sec. 6, pl. 4). 

The contact between the Bighorn dolomite and the overlying 

Devonian strata is difficult to recognize. In this report it is so placed 

in the doubtful zone that the Bighorn dolomite includes most of the 

light-gray to white dolo~ites and the Jefferson and Three Forks 

strata contain the gray to brown dolomites and slightly shaly and 

sandy limestones. Bedding in the Ordovician and Devonian strata is 

seemingly pu.rullel and no erosional unconformity was seen in the 
20 GEOLOGY OF THE BIGHORN CANYON-HAl-WIN AHEA, MON'L'.-"\VYO. 

field. Dorf (1934, p. 727) described an erosional unconformity between 

the Ordovician and Devonian rocks at Beartooth Butte on the 

Beartooth Mountains north of Clarks Forie There, the Beartooth 

Butte formation was deposited during Early Devonian time in a deep 

channel cut into the Bighorn dolomite. 

Srveral fossils were collected from the Devils Canyon section by 

R. J. Ross, Jr., R. P. Kunkel, and the writer. Brachiopods in 

USGS collection D2 (SD) from brownish-gray limestones about 60 

feet below the ~1adison limestone have been described by A. J. 

Boucot of the Geological Survey (personal communication) as "finely 

ribbed brnchiopods referable to the genus Atrypa. Similarly ornamented members of this genus occur elsewhere in the liddle and 

Upper Devonian beds of the west." Corals in USGS collection D4 

(SD) were collected from silty limestones about 20 feet below the 

limestones that contained the above described brachiopods. These 

were examined by Jean l\1. Berdan of the Geological Survey who 

states (personal eommuni?ation) that 

Collection D-4 contains broken and sheared tetracorals which may be part of a 

loosely phacelloid colony, scraps of cladoporoid corals, and stromatoporoids. 

As some sections of the tetracorals .5how dissepiment;, and as cladoporoid corals 

are present, the collection may be dated as post-Ordovician. The presence of 

stromatoporoids suggests that it is pre-Mississippian. Although uoue of the 

fossils have been identified generically because of the poor preservation, the 

faunal assemblage suggests a Devonian age for the beds rather than Silurian, 

although on the basis of this collection the possibility of a Silurian age cam10t 

be eliminated. 

In Clarks Fork Canyon W. G. Pierce aud R. P. Bryson collected 

fossils from beds about 150 feet above the base of a rock sequence 

believed to be comparable to the Devonian of the Bighorn Canyon 

area. These fossils were examined by Edwin Kirk, who stated that 

they are probably of Jefferson age (Leatherock, 1950) and identified 

a brachiopod as Atrypa reticularis Linnaeus. The pelecypod fragments were indeterminate. Corals in the same lot have been restudied 

by Helen Duncan and Jean Berdan, who report (personal communication) that ·

1 

The dolomitic samples contain poorly preserved fragments l';uggesting "Favo:sites 

limitaris" and indications of forms that are probably disphyllids. Another 

specimen in a limestone matrix, although incomplete, is much better preserved. 

This coral, which may be the form called A mplexns in the previous report, is 

apparently an undescribed genus. It has a large columella and other characters 

that suggest relationships with Carboniferous zaphrentoidd. Inasmuch as 

corals comparable to this one have never been reported in the Dcvouiau and the 

specimeu occurs in a different lithology, derivation from higher bed:; is suggested. 

M. T. Rader and J. K. Osmond of the Continental Oil Co. collected 

an Atrypa n. sp. and Amplexiphyllum n. sp. from 40 feet below the 
S'I'HA'l'IGHAP.H Y 21 

~'lu.dison limestone and the same Atrypa n. sp. and Theodo&sia 11. sp. 

fr01n 6:3 feeL below the Nladison in Bighorn Canyon west of the 

1nouth of Big Bull Elk C1·eek (personal communication from Rader). 

These fossils, which canw from the same locality Umt was measured 

and described by the writer (pl. 4, section 4), were iden Lified by Dr. 

B. J. Chronic of the UniYersity of Colorado for the Continental Oil 

,,._. Co. tvlr. Rade1· staLes (personal communication) that forms similar 

to the AtryJJO 11. sp. were collected from Devonian strata at Beartooth 

Butte, Park County, vVyo., and in l\1Iontana from Shell ~'lountain in 

. Park County, the Little Rocky l\1ountains, and the Roeky !v[ountain 

front cast of Glacier National Parle 

In plate 4, sections 1 to 6, the writer has shown a correlation of 

what he considers to be Devonian strata in Bighorn Canyon and Bighorn Basin with the Devonian strata exposed west of Bighorn Basin at 

Clarks Forie Devonian strata extend northwestward fron1 Clal'l~s 

Fork around the Beartooth Mountains (Lovering, 1929, p. 27-28) 

and southward from Clarks Fork to Shoshone Canyon ncar Cody, 

'vVyo. (Johnson, 1934, p. 817-818), thence into vVind River I3a.sin, 

vVyoming (Thomas, 1948, p. 84). 

CARBONIFEROUS SYSTEMS 

MISSISSIPPIAN SYSTBM 

MADISON LIMESTONE 

Tho lVIadison limestone is 865 feet thick in the subsm·facc at Soap 

Creek dome, 7:35 feet in Bighorn Canyon, and 670 feet in Devils 

Cnnyon. ¥lestward from Devils Canyon it thickens to about 800 feeL 

in Sage Creek dome, vVyo., and nearly 1 ,000 feet at Clarks Fork of the 

Y cllowstono River, Wyo. In the higher parts of Bighorn Canyon the 

formation forms a stcplikc series of cliffs with pinnacles and spires 

common on strnta below the uppermost massive member. The 

Nladison limestone has been divided, for convenience of description, 

into four lithologic units which arc lettered A to D, from top to bottom. 

These units are easily recognized in exposures, and the thinnest, unit C, 

is a good marker in both surface and subsurface sections. They arc 

1: shown in the photograph of Devils Canyon (fig. 1). 

The lowest zone, unit D, is about 190 feet thick and consists chiefly 

of finely crystalline limestone and dolomite. A 40-foot zone of nodular 

chert 40 feet above the base of the l\i[adison is well exposed in Bighorn 

Canyon, and is l'Ccognitmblo in the Soap Creek dome and Sage CI'Cck 

dome wells. A mueh thinner bed of ehcrt oecurs at this horizon in 

Devils Canyon. 

Unit 0, which is about 75 feet thiek in sul'fa.ec scet.ions and somewhat thinner in the subsudaec, is the most conspicuous and persistcn L lithologie mal'lmt· in the formation. fL is grayish-purple to 
22 GEOJ.JOGY OF THE BIGHORN CANYON-HARDIN AREA, MONT.-,VYO. 

reddish, partly silty, limestone that contains thin layers of broken 

remains of brachiopods, bryozoans, and crinoid stems. It is readily 

recognized by its reddish color in subsurface samples and by its 

purplish color in weathered outcrops. 

Unit B is 265 feet thick at Devils Canyon, 310 feet near the mouth of 

Big Bull Elk Creek, and nearly 400 feet in the subsurface at Soap Creek 

dome. It is hard ~ncly crystalline light-gray limestone and dolomite. 

Prominent pinnacles on the canyon walls are erosional remnants of 

hods in this unit. The upper 40 feet of the zone in the lower Bighorn 

Canyon area and the upper 120 feet in the Devils Canyon area arc 

broken, possibly by collapse of solution caverns before deposition of 

the next younger unit in the :Niadison. 'L'his brecciated zone can be 

seen faintly in the wa1ls of Bighorn Ca.nyon near the mouth of Porcupine Creek. 

Small irregularities in the upper surface of unit B, observed in two 

a.ccessible but poorly exposed localities in the western end of Bighorn 

Canyon, arc filled with limestone fragments which arc lithologically 

similar to the limestone of the unit itself. These depressions, which 

are as much as a foot or two deep, are interpreted to be channels that 

have been filled by material eroded from the unit elsewhere and 

deposited in the channels. These channels seem to underlie and to be 

older than the breccias and redbeds in the overlying member of the 

Madison limestone. The breccia and the channel fills in unit B may 

be evidence of an unconformity above the unit, an unconformity that 

may follow in general but not in detail the contact of units A and B. 

Solution cavities lined with calcite crystals have formed in the 

upper part of this unit. Several cavities large enough to permit entry 

of a 1nan were intersected as much as 150 feet from the face of the 

canyon wall by tunnels at Yellowtail dam site. Although the cavities 

are undoubtedly the result of solution during a much later time than 

the solution that led to the brecciation of the upper part of unit B, 

their exact age has not been determined. Bureau of Reclamation 

geologists expect that a core-drilling program, which started late in 

1952 during the writing of this report, will show whether the cavities 

follow bedding planes in the Madison limestone or whether they 

follow some nearly level horizon. If they coincide with bedding planes, 

the cavities presumably are pre-Laramide folding in age, but if they 

follow a nearly level plane, the cavities may have formed in postLa.ramide time and have a genetic relationship to the Bighorn River 

when the river stood a.t smne higher level now marked by tcrra.ces in 

the Grea.t Plains. 

The upper member of the Madison limestone, unit A, is a.bout 145 

feet thick along Bighorn Canyon, and it forms a sheer cliff at the upper 

rim of the canyons in the northern part of the Bighorn Mountains. 

Much of the member is finely crysta.lline thin-bedded pa.le yellowish1 

) 
S'l'HA'L'IGHAP.H Y 23 

brown limestone. The unit is commonly thin bedded, although frmn a 

distance it appears to be massive. N umcrous zones of breccia in the 

unit, particularly at the top and the base, contain limestone fragments 

in a matrix of red claystone and mudstone typical of the basal part of 

the overlying Amsden formation. ~~Iany caves have weathered into 

the basal breccia zone, and collapse structures are common in the 

(.,._ brecciated li1ncstonc. 

Although unit A is reasonably uniform in thickness along Bighorn 

Canyon, its boundaries were presumably determined by erosion and 

not by deposition. That is, this member includes by definition the 

brecciated beds at the top of the Madison limestone, and the breccia 

has been described as a product of cavern collapse following solution 

of the top of the Madison limestone in a time of weathering that 

preceded or coincided with early Amsden deposition (Thorn and 

others, 1935, p. 37). If this theory of origin is correct, the top and 

base of unit A bear only incidental relation to bedding pianes in the 

1VIadison limestone, and the age of the limestone strata in unit A 

would differ fr01n place to place depending mainly upon the amount 

of folding of the IV[adison that preceded erosion. The outline of 

fillings of channels and sinkholes in the lVIadison limestone by reel 

Inuds, 1nuch of which may have been soil on the karst topography, 

is illustrated by the cross section shown in plate 5. Judging from its 

appearance in outcrops along Bighorn Canyon, unit A is nearly parallel 

to the underlying lVIadison strata. 

Unit A is correlated in plate 4 with the upper, brecciated mmnber 

of the ~1adison li1nestonc in the canyon of the Clarks Fork in Wyoming along the west edge of the Bighorn Basin (Leatherock, 1950). 

As has been pointed out in the preceding discussion, the uppcnnost 

beds in the ~1adison of the Bighorn Canyon and the Clarks Fork 

mn,y not be the san1c age; so the brecciated unit along the Clarks 

Fork 1nay be youngct· or oldm·, than unit A along the Bighorn River. 

The contacts of the ~1n,dison limestone with the underlying and 

overlying fornuttions arc well marked by clmnges in lithologic character 

and topography. ~'[assivc ledge-forming limestone at the base of the 

r, !v[ndison rests on sandy or shaly limestone or dolomite nt the top of 

the Devonian strata. The con tact is at a definite break in topography; 

the Dcvoninn strata have been eroded to steeply sloping surfaces, 

whereas the lississippian limestones occur as ledges and cliffs. 

Bedding of the two formations appears to be parallel, but a thin layer 

of breccia at the contact in Bighorn Canyon marks what may be an 

erosional unconformity. Redbeds at the base of the Amsden formation have been eroded to smooth slopes above the ~1adison cliffs. 

Thorn (Thom nnd others, 1935, p. 34-35) referred the lVIndison 

limestone to the lower lVlississippian. Additional paleontologic data 

may show the presence of upper lississippian strata, too. 
24 GJ<.;OLOGY OF TH~; BlGHOHN CANYON-HAHDlN ALWA, MONT.-WYO. 

Section of Madison limest.one on north side of B·iuhorn Canyon about 2 mdes east 

of the month of Biy Bull Elk Creek in S!~ sec. 32, T. 6' S., fl. 30 B. (unsu:rveyed), Biu Hom County, Mont. 

[Units are numbered as shown in plate 4, sedion 4] 

Arn~:;den formatiou: Coucealed, red soil. 

Unconformity. 

Madison limestone: 

Unit A: 

1. Limestone and breccia, light-brownish-gra.\·, finely crystaliine; 

hard thin-bedded limestone with a few thin layers of chert; 

breccia of limestone, red shale, and siltstone abundant uear 

top and base; forms massive vertical to overhanging cliff~:;; Feet 

contact with underlyiug unit is irregular_________________ 145 

Unit B: 

2. Limestone, very light gray, finely crystalline, medium-bedded; 

somewhat brecciated uear top; some chauneling at top with 

solution cavities along bedding planes; erodes to pinnacles__ 85 

3. Limestone, very light gray, dolomitic, fiuely crystalline to 

dense, hard,. thin- to medium-bedded; small vugs in upper 

part; lower few feet very dolomitic; forms pinnacles_______ 100 

4. Dolomite and limestone; thin-bedded gray dolomite and thicker 

beds of gray limestone; red-shale partings at base_________ 46 

5. Limestone, gray and pale-brown, finely crystalline; forms 

pinnacled cliff with overlying unit_______________________ 81 

Unit C: 

6. Limestone, grayish-red to grayish-red-purple, dolomitic, silty, 

medium- and thick-bedded.---------------------------- 45 

Limestone, grayish-purple; crystalline, partly clastic; contain~:; 

many red- and purple-silt stringers and thin layers of 

brachiopods, bryozoans, and crinoid stem fragments in 

lower part____________________________________________ 38 

Unit D: 

7 .. Limestone, light-brownish-gray, finely crystalline and dense, 

poorly bedded; forms ledges ___________________________ _ 

Limestone, light-brownish-gray, finely crystalline; forms massive overhanging ledge ________________________________ _ 

Limestone, light-gray, finely crystalline; weathert' to yellowish 

and light-gray blocky surface __________________________ _ 

Limestone, light-gray and gray, finely crystalline, vuggy ____ _ 

8. Limestone, gray, crystalline, medium-bedded; lower few feet 

dolomitic and sandy; thin-bedded chert in upper 40 feet; 

irregular contact with underlying unit __________________ _ 

29 

12 

20 

50 

84 

Total, Madison limestone _______ . __________ .. _ _ _ _ _ _ _ _ _ 735 

Jefferson limestone and Three Forks shale undifferentiated: 

Limestone, yellowish-gray, finely crystalline, and interbedded greenish-gray sandy limestone in irregular beds; few inches at top consist 

of greenish-gray siltstone and sandstone with subrounded gray 

quartzite and limestone pebbles as much as 1 inch in diameter____ 96 
S'l'RATIGRAPHY 25 

S(!ch:on o.f Mad,£son limestone in Devils Canyon of Porcupine Creek in, S}f Me. 25, 

T. 9 S., fl. 28 B. (nnsnrveyed) Biy Horn County, J1tfont. 

[IJnit.s are numlu:n~d as shown in plat.1\ 4, s1•et.inn :!) 

Amsden formation: Shale and ::;ilt:-)tone, red. 

Unconformity. 

Madison lime:-;tone: 

Unit A: 

l. Limestone, very light gray to light-brownish-gray. fine!~, 

crystalline to gmnulttr. partly dolomitic, thin- to thickbedded; fragmental and layered red and brown chert ncar 

Unit /3: 

top; basal 25 to 50 feet mostly breccia containing angular 

blocks and rounded pebbles of limestone in reel shale, siltstone. and sandstone; collapse structures in limestone. 

AnguhLr limestone blocks arc imbedded in red siltstone 

and sandstone 20 to 40 feet below top of unit. The unit 

forms ncar vertical cliff in Bighorn and tributary canyons __ 

Feel 

145 

2. Limc::;tonc, light-gray, finely cr·ystallinc and dense; wavy and 

broken bedding; contains bedded and fragmental chert____ 120 

Limestone, light-gray, finely crystalline, massive, ledgeforming______________________________________________ 14 

Limestone, light-gray, granular; fairly well bedded__________ 6 

3. Dolomite, gray to light-brownish-gray, dense, hard, mediumbedded; contains a few thin beds of dolomitic limestone___ 44 

Dolomite, gray to light-brownish-gray; some beds arc porous; 

thin beds of yellowish-gray chert in lower 3 feet__________ 12 

Dolomite, gray to light-brownish-gray; poorly preserved 

shells ncar top________________________________________ 32 

Dolomite, gray to light-brownish-gray, dense to finely crystalline, medium-bedded; contains few thin beds of limestone__ 36 

Unit C: 

4. Limestone and dolomite: lower 5 feet is mostly finely crystalline limestone, upper 12 feet is mostly dense gray dolomite 

with sevcml red-shale partings; top 1}~ feet is coarsegmincd limestone_____________________________________ l7 

Limestone, gray and purplish-gmy, partly dolomitic, mediumbedded, stylolitic, ledge-forming ___ --------------------- 18 

Limestone, gmy :tncl purplish-gmy; some interbedded gray 

dolomite contains red-shale stringers_________________________ 8 

Limestone, gray and purplish-gray; purplish zones contain 

many fragments of limestone and dolomite; numerous 

brachiopods__________________________________________ 22 

Limestone, gray and purplish-gray; thin, irregular bedding; 

forms base of overhanging ledge________________________ 3 

Dolomite and limestone, light-gray; contains laminae of reel 

silty material; many shell fragments: l-inch layer of coarse 

silty red limestone with crinoid stem fragments at top____ 4 

Unit D: 

5. Limestone, gray, hard, thin- to thick-bedded, ledge-forming__ 14 

Limc::;tone, gmy and dark-grny, dense to finely crystalline, 

thin- and medium-bed<icd___________________________ ___ 16 
26 GEOLOGY OF 'rHE BIGHORN CANYON-HARDIN AREA, MONT.-WYO. 

Section of Madison limestone in Devils Canyon of Porcupine Creek in S% sec. 25, 

T. 9 S., R. 28 E. (unsurveyed) Big Horn County, Mont.-Contiuued 

Madison limestone-Continued 

Unit D-Continued 

Limestone, gray (weathering bluish gray), dense, hard, thickbedded ______________________________________________ _ 

Limestone, gray, dense, thin-bedded ______________________ _ 

Limestone, brownish-gray (weathering black), dolomitic, 

granular, porous; upper 6 feet thin and medium bedded; 

has petroleum odor when freshly broken ________________ _ 

Limestone and dolomitic limeRtone, brownish-gray (weathering brown), porous ___________________________________ _ 

Limestone, gray, dense ___________________________________ _ 

Limestone, brownish-gray, dolomitic, granular, porous ______ _ 

Limestone, gray, finely crystalline; lower 4 feet has splintery fracture _____________________________________________ _ 

6. Limestone, gray, dolomitic, granular, porous; thin zone of 

gray chert nodules 27 feet above base ___________________ _ 

Limestone, gray, dolomitic, hard, massive _________________ _ 

Limestone, light-gray, dolomitic, hard ____________________ _ 

Total Madison limestone ____________________________ _ 

Unconformity. 

Jefferson limestone and Three Forks shale undifferentiated: 

Dolomite, brownish-gray, sandy; vugs in upper part filled with 

Feet 

7 

11 

20 

16 

2 

2 

9 

43 

30 

9 

660 

calcite______________________________________________________ 8 

Dolomite, green, very silty. 

MISSISSI.PPIAN AND PENNSYLVANIAN SYSTEMS 

AMSDEN FORMATION 

The Amsden formation, which ranges in thickness from 230 feet 

near the head of Bighorn Canyon to 280 feet on the eastern flank 

of the Bighorn Mountains, consists of interbedded sandstone, limestone, and reel shale and siltstone. The Amsden is separated from 

the underlying Madison limestone by an erosional unconformity but 

is gradational into the overlying Tensleep sandstone. 

The basal 50 feet of the Amsden is reel shale and siltstone with 

" 

intercalated limestone and dolomite resting upon channeled Madison ~ 

limestone. A sequence of interbedded red shale, limestone, dolomite, and sandstone comprises the rest of the formation. Thin beds 

of gray or red chert and cherty limestone form ledges about in the 
lj... 

r 

.. , 

STHA'l'IGRAPHY 27 

middle of the formation. Inasmuch as the contact of the Amsden 

formation with overlying Tensleep sandstone is gradational by inter 

fingering, it was found expedient at some. localities to map within 

the Amsden formation a few lenticular beds of sandstone similar to 

the Tensleep. 

Preceding or accompanying early Amsden deposition the upper 

part of the lVfadison limestone underwent solution with resultant 

bt·eeciation and filling by t·edbeds of the Amsden. There seems t.o 

be no divergence in bedding of the two formations however, except 

as noted by Thom at Shively Hill dome (Thom and others, 1935, 

p. 37). The nature of the complex con tact between the lVlad ison 

limestone and the Amsden formation and the size of the breceia masses 

in the upper member of the lVIadison' are shown by plate 4. 

1VI uch of the Amsden formation in the northern end of the Bighorn 

l\1ountains is Pennsylvanian in age. L. G. Henbest of the Geological 

Sut·vey collected and identified the following fossils from a 30-foot 

unit of chet·ty limestones about 100 feet above the base of the Amsden 

formation in the '"'7~ see. :35, '1'. 9 S., R. 28 E., Carbon County, l\tfont. 

USGS collection f8009: 

Sponge spicules. 

Calcitonwllids. 

Climacammina sp. 

Bradyi·na s p. 

Tetratax'is sp. 

Pseudosta:tfella sp. 

Pro.fusulinella sp. (highly specialized form) or possibly a 

species of Fusulinella.. 

Henbest states (personal communication) that 

Thcr·c is no record of these fusulinids in rocks that are known to be older than 

the Atoka; i. c., early middle Pennsylvanian. Occurrences in rocks known or 

thought to be of Atoka age arc numerous and widctSprcad. RetStriction of these 

particular forms to Atoka time .seems probable but range into early De:-; Moines 

time must be regarded as entirely possible. 

About 100 feet of red shale and siltstone and several beds of dolomite lie beneath the fusulinicl-bearing cherty limestones. No fossils 

were seen in these beds; so the age of approximately the 10\~er onethird of the Amsden within this area is Pennsylvanian or l\1ississippian . 

:::a4a2S-5G--3 
28 GEOLOGY OF 'L'H E BIGHOHN CANYON-.HAHDIN AHEA, MON'I'.-"\VYO. 

Section of Amsden formal?"on in canyon near Kane, Wyo.,-Dry Hearl, lltf ont., road. 

Upper 35 feet measnrerl in 8}~ sec. 84, 'l'. 9 8., R. 28 E., lower 196 feet measwwl 

near Bighorn River, opposite mou.th of Porwpine Creek in W}~ sec. 35, T. 9 8., 

R. 28 E. Carbon Coua.ly, 111 ont. 

I Plate 4, section :~1 

Ten~-;Jeep 3anch;tone: BaHal part. covered in most areas; where exposed, 

4 feet of ledge-forming clolomit.e with overlying sandstone. 

Amsden formation: 

Limestone, gray, sandy; 2 inches ot red and purple shale at top ___ _ 

Breccia; a 2-foot bed of sandy limestone at top is broken and partly 

fragmented and many pieces are imbedded in underlying clayey 

limestone; about 100 yards along outcrop the sandy limestone is 

in very irregular beds, but is only slightly broken _____________ _ 

Dolomite, red, silty __________________________________________ _ 

Shale and siltstone, red and purple; with a few thin beds of limestone _________________ ------------------------------------- Sandstone, gray, and red shale ________________________________ _ 

Dolomite, light-gray, and red shale; contains lenses of gray limestone, sandstone, and red chert ______________________________ _ 

Limestone, irregularly bedded, ledge-forming ____________________ _ 

Breccia; fragments of thin-bedded limestone enclosed in red mildstone _____________________________________________________ _ 

Shale, red __________________________________________________ _ 

Breccia, dolomite, and red shale; unit grades into clayey vuggy 

limestone __________________________________ _: ______________ _ 

Sandstone interbedded with dolomite, light-gray _________________ _ 

Shale, purple and red (weathering purplish); contains several lenses 

of white saud and, in lower 20 feet, 2 or 3 thin beds of dolomite __ 

Sandstone, light-gray, fine-grained, calcareous, cross-bedded, ledge- forming __________________________________________________ _ 

Sandy siltstone, yellowish-brown, calcareous, soft_ _______________ _ 

Dolomite, gray, Hoft; mostly covered ___________________________ _ 

Limestone and dolomite, light-gray, poorly bedded; contains some 

red shale; angular and nodular gray chert in upper part; middle 

mostly covered; Hen best collection, USGS f80mL _____________ _ Concea~cl __________________________________________________ _ 

Dolomite, light-gray, dense ___________________________________ _ 

Limestone, light-gray ________________________________________ _ 

Dolomite, light-gray, partly brecciated, irregularly bedded ________ _ 

Concealed ___________________________________________________ _ 

Dolomite, gray, variably calcareous, ledge-forming; upper part. 

:5iliceous, weathering to very sharp and irregular surface ________ _ Concealed __________________________________________________ _ 

Dolomite, white, dense ____ .. __________________________________ _ 

Siltstone and silty shale, red, slightly calcareou,;;; includes one thin 

bed of limestone ___________________________________________ _ 

Limestone and dolomite, light-gray, dense; red-silt stringers at base_ 

Concealed __________________________________________________ _ 

Pt in 

8 

2 8 

8 

6 0 

5 0 

6 0 

0 

4 () 

3 0 

2 0 

4 0 

50 0 

4 0 

3 0 

4 0 

31 0 

4 0 

3 0 

4 0 

3 0 

10 0 

9 0 

4 0 

0 

38 0 

24 0 

4 0 

Total, Amsden formation __________________________________ 231 0 

Unconformity; a few heels at top of Madison limestone are beveled. 

; 

l 
ti'L'HA'I'lGHAPH Y 29 

Sect£on of A '11/,l:itltm format:ion al1out 3 utile.~ ~:~outh of /Jiylwrn /liver t:anyon month 

lin :V E/~ sec. ,"J3, '/'. 6 8., fl. 31 /<:., Bi(f Horn County, Mont .. 

ll'latc 4, sect.ion 5. l'"lcasu1cd by L. S. Uardncr and C. P. Hogt•rs, .Jr.] 

Ten:-;leep :·mncbtone: Ft in 

Sandstone, white (weathering gra.v to yellowish gray), ft·iable, 

massive cro::;s-bedded _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 22 0 

j., Quart.J~ite, gray, fine-grained; capped by 8 inches of gray nodular 

::.;andstonc of light lavender and greenish tints__________________ 3 0 

Amsden formation: 

Shale, purplish-brown, calcareous, nnnresil5tant_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 2 

Sandstone, purplish-brown and white, quartzitic, locally cherty_____ 10 

Siltstone, dolomitic, light-purple, pink, and reel; in thin la•nina: iow; 

::.;eparat.ed by parting;;, of deep-red siltstone and platy shale_______ 7 0 

Silt::.;tone and :;hale, pmple, nonresi..,;tant. poorly exposed___________ 5 0 

Sandstone, white to gray (weathering browni:sh gray), calcareou~, 

tna::;::.;i ve _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 3 0 

Shale, dark-pmple, calcareous__________________________________ 6 

Sand~:;t,one, dark-red, fine-grained, ma::.;sive, wdl-cemented to friablp,·' 

nonresistant; has ::.;ome gray streak::; ___________________________ 6 6 

Concealed..___________________________________________________ 18 0 

Sandstone, light-reddish-brown, very calcareom;, medium- to finegrained, massive, friable _____________________________________ 11 0 

Sandstone breccia. light-pmple; contains many angular solution 

cavities as much as 2 inches acro:3s; composed of noncalcareous 

sandstone and purple shale; unit i::; nonresistant and becomes cavernous upon weathering __________________________________ 6 0 

Concealed___________________________________________________ 27 0 

Limestone, gray, mas~ive, sandy-textured; contains gray to brown 

and reddish-brown lensc::; or layer::; of chert; bed of chert /f to 2 

inches thick is in middle of unit_ _____________________________ 11 0 

Quartzi~e. gray, fine-grained, massive____________________________ l 6 

Shale, siltstone, and clay::;tone; dark-purple, calcareou::;, nome::;i::;tant; 

has some greeni::.;h-gray spot::; and irregular areas_______________ 1 6 

Concealed_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 6 0 

Chert, gray to light-purple, massive, bri t.t.le_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 5 0 

Concealed _____ :...------'""________________________________________ 7 0 

__ Chert.,-similar fo next chert above ______________________________ 3 0 

Concealed ___________________________________________________ 63 0 

Sancl::;tone, reel; made ttp of mediwn- to coarse-grainC>d subangular 

poorly sorted and ce:nented quartz ::;and grain::;; has some irregular-shaped areas of yellow sandstone _________________________ ·23 0 

Concealed; a 4-foot ledge of light-pmplish-gray lime~tone similar 

to next bed below i'3 exposed elsewhere at thi::; horizon ___________ 20 0 

Limestone, purplish-gray, weathers medium gray; forms single bed; 

resi::,tant but not commonly well exposed because it and other 

beds arc covered by slope wash____________________________ 5 0 

Concealed; re3idual soil i::; red ::;ilt., but it is mixed with coarse sand-

::;t.one from above___________________________________________ 50 0 

Total, Amsden formation__________________________________ 281 0 

Madison limestone: 

Limestone, gray, re::;istant, thin- t.o thick-berldecl_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 20 0 
3() (;I~OLO(;y OF Till~ BWHO!C\ (',L\YO~-HAHIJll'\ AHEA, :\10.:\T.-'\VYO. 

PE:-.I).;I•;;):'JNA:\'lAN SYSTI';l\1 

TENSLEEP SANDSTONE 

The Pclllls.Ylvanian Tt'nslPep sandstonP around tht> north l'ttll 

of tlw Bighorn :\fountains has an an'ntg<' thickness of about 75 

f<:d and consists of light-gray to ydlowish-gray cross-lwddcd sandstone and ~t littl0 inU'rbt'ddPd limcstOlH' and dolomite. Tlw sandstone contains \\Tl'll-roundl'd fint'- ttnd mt>dium-grairwd quartz sand. 

Grov<'s of pin<' trpcs mark outcrops of thr TPnslt':'p on thl' mountain 

slopl'S. 

Tlll' Tenslct-p is 110 f0d thick lH'ar tlH' head of Bighorn Canyon 

in Carbon County, :\Iont., ancl about. 90 f<'l't thick nNu· the mouth of 

Bighorn Can.\·on, but it is missing in a small arPn in the );'E?~ src . 

4, '1'. 8 :-:;., R. :32 E., about 12 mil0s southeast of the mouth of Bighorn 

Ctwyon. Although sonw of thr variation in thirklH'SS is clue to the 

gradational contact of tlH' 'l'<'nsl<'<'P "·ith tht> Amscl('n formation, 

much is clul' to nn erosional unconformity at the bust' of th<' Prrmiau 

strata. 'l'bis is shown by figun' :) which shows thl' unconformity in 

the ar<'a ncar the head of Soap Creek: on tlH' <'nst sidc of tlH' Bighorn 

:\Iountains. ThP uppermost :)0 fret of a surface Sl'ction near the 

Kan<'-Dryhl'tHI road (p. 31) was n'mov<'<l for n d istmwP of a fl'w hundrrcl feet b.'· erosion prior to thl' deposition of the Ernhar formation. 

'I'hl' Tenslt'ep sandstone thins to th t' north and is abs('llt from most. 

FWL' RE ~.-F.rosional llllC:onforrnity <ll tlw hasP or till' Ch·tg\\·at.(•r formation. Tlw Vll'\\. is north\\ttrd 

from nl•ar t ht> I wad of l:-ioa p Cr('l'k in t hl' :\ E ~ 1 Sl't. :n. '!'. 7 8., IL :)2 E. TlH' uppl'rmost 21l fl'l't of sandstone 

in tlw 'l'l·n~kt•p prl'Sl'nt in thl• hill on till' l"fl b mi,sing from till· hill on tlll' right. 'l'ht' dasht'd litH' inic·at<'S 

till' uneJnformity. \\'hill' an•as on llll' clopl'S in tlH· baekgru·uHI an• lwds or !!Y}lsum in the towN part of 

thl' Ch ll~\\·atrr formation . 'l'hl' kdgt' of linwstoJH' just hl'low thl' ;;k~ lim· is in llw uppl'r part of the Chug:· 

water forrnatio:1 an <I nw~· Ct' the .\le •va linwstonl' !ll('lll hPr. 
,ta 

S'l'HATIGRAPHY 31. 

of the area north of an east-west line through Billings, Mont .. (Rogers 

and others 1945). ' 

The age of the Tcnsleep sandstone is indicated by the following 

fusulinids collected b,v L. G. Hen best of the Geological Surve~r from 

a 1 0-foot unit of ehert and limestone 40 to 50 feet below the top of the 

Tenslccp sandstone near the southern tip of the Pryor ?vfountains 

(sec stratigraphic sect.ion p. 32). 

USGS collection f8008 

Sponge spicules. 

Cal citornellids. 

Climacammina sp. 

Bradyina s p. 

Pseudostaffella sp. 

Wedekindellina euthysepta (Henbest) 

W. excentrica (Roth and Skinner) 

F'usulina rockymontana? Roth and Skinner 

According to Henbest (personal communication) 

These fossils are preserved in chert. and though the shells have suffered some 

n.ltera.tion their identity se('ms to be definitely determinable. Early or early 

middle Des Moines age is indicated. Similar forms are present in collections 

from the Tensleep formation further south in the Bighorn Mountains. 

This fauna is widely distributed in the Rocky Mountain and Mid-Continent. 

regions, notn.bly in the McCoy formation and middle part of the Hermosa format.ion of western Colorado, and in the lower part of Division IT I of the Hart.ville 

group in southeastern Wyoming. 

Oil is obtained from the 'l'ensleep sandstone in several oil fields in 

the Bighorn Basin. A few exposures of the Tensleep sandstone along 

the eastern side of the Bighorn l\1ountains contain asphalt, and Thorn 

(Thorn and others 1935, p. 38) L'epor·ted "dead oil" in the '1\'nslecp 

pcnetratE~d by wells at Soap Creek dome. 

Secf.1'on of Tensleep sandstone in canyon just east of Kane, Wyo.,- Dry Head, Mont., 

-roarl in S}~ sec. 34, T. 9 S., R. 28 E., Ca-rhon County, Mont. 

[Plate 4, sec. :~] 

Embnr formation: Concealed; red soil. 

Unconformity; uppermost. 30 feet of Tensleep sandstone is bevelled in onequarter of a mile. 

Tensleep sandstone: 

Sandstone, yellowish-brown (weathering brown), calcareous, thinbedded, ledge-forming; includes one bed of white sandstone and, in Feet 

the upper part, thin lenses of calcite___________________________ 14 

Snndstone, white, fine-grained, porous, friable, crol:'s-beddcd. composed 

of clean well-rounded quartz gmins; weathers light gray and nodular 

(nodules about one-half inch in diameter)_______________________ 13 

Sn.ndstone, brown, medium-grained, calcareous, porous, slabby; a 

short distance to the north all overlying Tensleep rocks have been 

removed hy erosion and the Emhar formation lies on this unit____ 8 
32 CEOLOGY OF 'L'HE BlGHOHN CA~YON-HAHDIN AHEA, MON'I'.-WYO. 

Section of Tensleep sandstone in canyon just east of Kane, Wyo.,-Dry Head, 111 ont., 

road in S7~ sec. 34-, T. 9 S., R. 28 E., Carbon County, lvfont.-Continued 

Tensleep sandstone-Continued 

Dolomite, light-brown. finely crystnJline, granular; interbedded gray 

nodular chert (Hen best collection, USGS f8008) ________________ . 

Limestone, sandy; laminated calcareous siltstone _________________ _ 

Sandstone, white (weathering light gray), fine-grained, fri<:tble_ .. ____ _ 

Sandstone, brown, fine-grained, calcareous, thin-bedded ___________ _ 

Sandstone, white to light-gray (weathering light gray), mediumbedded and cross bedded, nodular, ledge-forming ________________ _ 

Sandstone, light-greenish-gray, calcareous, soft, platy; moRtly covered_ 

Sandstone, white (weathering gray), medium-grained, porous, cross- bedded ____________________________________________________ _ 

Sandstone, light-brown and gray, fine-gmined; upper part forms ledge, 

lower part mostly covered ___________________________________ _ 

Sandstone, light-gray (weathering brown), medium-grained, variably 

calcareous, locally quartzitic, ledge-forming ____________________ _ 

Sandstone and dolomite, gray, thin-bedded ______________________ _ 

SandRtone, light-gray (weathering gray), fine- and medium-grained, 

calcareous, thick-bedded, nodular _____________________________ _ 

Sandstone, white (weathering gray), fine-grained, slightly calcareous, 

porous ____________________________________________________ _ 

Shale, green, very silty ________________________________________ _ 

Sandstone, white to light-gray (weathering brown), medium-grained, 

friable, cross bedded, ledge-forming: calcareous at top ____________ _ 

Dolomite, light-gray, sandy _________________________ .. __________ _ 

Sandstone, greenish-gray (weathering light brown), medium-grained, 

slightly calcareous; cross bedded and nodular in upper half _______ _ 

Dolomite, purple to pmplish-gray; silty at base ___________________ _ 

Dolomite, light-purple (weathering brown), silty; is at base of prominent ledge-forming series of rock above the much less resistant 

Amsden formation; contact with Amsden formation covered in most 

Feet 

10 

3 

4 

2 

{i 

1 

3 

6 

4 

6 

] ] 

5 

5 

3 

2 

places______________________________________________________ ~ 

Total Tensleep sandstone ________________________________ .__ 110 

Amsden formation: Limestone and limestone breccia ______________ about 4 

PERMIAN SYSTEM 

EMBAR 1!-.0RMATION 

The Permian Embar formation, which was mapped separately 

from the Chugwater formation only in the area between Crooked 

Creek in Wyoming and Porcupine Creek in l\!fontana, is a sequence of 

limestones, dolomites, and red shales, siltstones, and sandston<'s 

which arc as mueh as 100 feet thick. 

Darton (1906b, p. 17-18) gave the name Embar to limestone, shale, 

and phosphatie rocks that arc <'Xposed on the north flanks of the 

Owl Creek l\1Iountains at t.hc southern edge of th<' Bighorn Basin. 

He thought the Embar was "1wither a development of the basal 

portion of the Red Beds nor of tlw ealearcous sandstone" at the top 

,._, 
S'I'HA'l'l G HAP.H Y 33 

of the 'l'ensleep sandstone (Darton, 1 906a, p. 35). He did not recogni7.e the formation north of No Wood, vVyo., west of the Bighorn 

lVlountnins or north of Bnrnum, enst of the mountains. 

fn 1918 B1nekwclder (p. 423-426) divided the Em bar in W('Stet·n 

\~Tyoming into the Permian Park City formation (eonsisting predominnntly of dol01nite, shale, and phosphatic. roek) and the Triassic 

Dinwoody formntion (made up of greenish-gray shale n.nd thinbedded sn.ndstone and dol01nite). Since that time the term Park 

City formntion hn.s been replaced in northwestern \Vyoming by the 

Phosphol'itt fonnu.tion, which wns nmned in a 1912 repol't by Richards 

n.nd .M nnsfield (p. 687) for Permian stratn in Idaho that al'e col'-

relative with the upper pal't of the Park City formn.tion. The 

gt·mtt vttrintions in the lithology of rocks neat· the boundary of the 

Permian n.nd Triassic systems hns l'esulted in conflicting conclusions 

by various geologists in tracing the beds from the type locality 

northeastward into the Bighorn .Mountains. 

'l'homas in 1934 studied the strntigraphic relations of the Phosphorin and Dinwoody formations with the overlying and underlying 

formations ft·onl the Owl Creek [ountains southeastwal'd into central 

vVyon1ing. He showed (1 934, fig. 3) that the Phosphoria nnd Dinwoody fol'lnations, which are predominantly limestone and shale, 

interfinger southeastward with redbeds. The unit above the Tensleep snndstone and below the Chugwater formation in central VVyoming, thus, consists of redbeds and intercalated n1aritH' limestone and 

sandstone tongues of the Phosphoria and Dinwoocly formations. 

'l'homn.s cnlls this unit "En1bar." 

'l'he na1ne En1bar hns been used along the northeastern side of the 

Bighorn Basin and in the Pryor lVlountains for a series of limestone, 

dolomite, n11d red shale and siltstone which lie unconformably on the 

'l'ensleep sandstone and are distinct in lithology from the overlying 

Chugwttter formation. Knappen and 11loulton (1 930, p. 14) said 

LhnL the "Em bat·('?)" in the area north of the Pryor ~1ountains consists of 10 to 15 feet of gray-white very porous thin- and wavybedded limestone. They found the limestone in only three localities 

n.nd nUributed its ttbsence e1sewhcm to nondeposition or l'<'moval by 

pre-Chugwater erosion. According to Blackstone (1 940, table 1) 

Lhe "Embn.r( '?)" of t.he Pt·yot· [ountnins is thickest on the southenstern fln.nks where it consists of 25 to 40 feet of white siliceous limestone underlain by 70 to 90 feet of red and yellow shales. Near 

the mouth of Shell Cttnyon, about 40 miles southeast of the mouth of 

Crooked Creek, Pierce (1 948) n1easured about 180 feet of Em bar, 

the basal part of which consists of 48 feet of 1naroon sandstone and 

siltstone with 2 to 5 feet of subangular chert conglo1nerate and sandstone nnd lies unconformably upon the Tensleep sandstone. This 
34 GEOLOGY OF THE BIGHORN CANYON-HARDIN AREA, .MON'l'.-WYO. 

rnaroon sandstone and siltstone is succeeded by 61 feet of maroon 

siltstone and pure white gypsum, and in turn by 73 feet of soft salmonred gypsiferous sandstone and gypsum with beds of dolomite at the 

base and top. The unit is conformable with the overlying Chugwater 

formation, the lower part of which contains thin beds of gypsum and 

dolomite similar to those in the Embar. 

Between Crooked Creek and the mouth of Porcupine Creek the 

upper dolomite or limestone beds in the Embar form bare white dip 

slopes, and basal beds of red siltstone lie on the eroded 'l'ensleep. 

Units of the formation thin a short distance north of Porcupine Creek, 

and in the area between Layout Creek and Dr.Y Head Creek the upper 

limestone is only 5 to 10 feet thick and the underlying red siltstone 

and sandstone about 25 feet thick. The limestone thins in places 

almost to zero eastward from Dry Head Creek to Grapevine Creek. 

North of sec. 26, T. 9 S., R. 28 E., opposite the mouth of Porcupine 

Creek, the limestone and underlying red shale and sandstone were 

included in the Chugwater formation in mapping, but a line representing the top of the limestone was drawn wherever the limestone is 

exposed. The eastward thinning of the Embar formation within the 

area may be due to decreased deposition inasmuch as both the dolomite 

and underlying siltstone unit are thinner east of the Bighorn Mountains 

than at the south end of the Pryor Mountains. The limestone and 

red siltstone and sandstone beds mapped as the lower part of the 

Chugwater formation north of Crooked Creek probably also belong 

to the Embar formation. 

Section of Embar formation between Bighorn River and Sykes Mountw:n in N% 

sec. 7, T. 57 N., R. 94- W., Big Horn County, Wyo. 

[Plate ~. sec. 3] 

Chugwater formation: Red gypsiferous siltstone. 

Em bar formation: 

Dolomite, light-gray (weathering very light gray), dense to finely 

crystalline: has variable porosity, nodular gray chert in basal 5 feet; 

20 feet of dolomite overlying chert is very gypsiferous; upper and 

lowPr beds form ledges; uppermost bed forms a dip slope and :-;upports a growth of juniper trees _______________________________ _ 

Concealed; red shales at same interval in nearby area _____________ _ 

Dolomite, porous, gray (weathering light gray); hard at top, non- resistant at base ____________________________________________ -

Sandstone, siltstone, and red shale; lower part is covered __________ . 

Limestone, gray and pink, dolomitic; has some interbedded red chert __ 

Concealed; green silty dolomitic limestone found by digging _______ _ 

Total Em bar formation __________ . _________________________ _ 

Unconformity. 

Tenslcep sandstone: Gray sandstone. 

Feet 

65 

15 

3 

12 

4 

6 

105 

.• 
STHATIGRAPHY 

FERMIAN AND TRIASSIC SYSTEMS 

CHUGWATJ<~R 1<-.0RMATION 

35 

'l'he Chugwater formation, which forms reel bluffs facing toward 

and encircling the Bighorn and Pryor Mountains, consists largel~r of 

red sandstone. The formation ranges in thickness from about 450 

feet along Bighorn Canyon to more than 650 feet near the head of 

.... Soap Creek. The sandstone is mostly fine to very fine grained and 

moderate to clark red. ;-Red siltstone and shale, in much smaller 

amount, arc interbedded with the sandstone. In the Chugwater 

gypsum commonly forms stringers and veinlets throughout, and thin 

beds occur in the basal part of the formation. The thickest beds of 

gypsum are in the vicinity of the head of Soap Creek. 'l'hin gmen 

streaks along small fractures in the Chugwater south of Crooked 

Creek, Big Horn County, Wyo., are undoubtedly due to chemical 

•·cd uction of ferric oxide (bleaching) in the rock adjacent to the fractures. Moulton (1926, p. 304-312) has attributed similar bleaching 

along the crests of minor folds and on the flanks of mountain folds to 

the reducing action of hydrogen sulfide upon the ferric oxide which 

coats sand and silt grains and gives the rock its red eolor. 

A 5- to 1 0-foot bed of limestone whieh is from 90 feet to 200 feet 

below the top of the Chugwater, is persistent throughout the area 

north and east of the Bighorn Mountains but is missing south of the 

Pryor 1\tlountains. The limestone is light gray with purple streaks, 

and fo•·ms eonspieuous but narrow white ledges. It may be equivalent 

to the Alcova limestone member in the Bighorn Basin and Wind River 

Basin of Wyoming first described by Lee (1927, p. 14) from exposures 

in the southern part of the Bighorn Basin. Limestone and dolomite 

also occur in a few thin beds in the lower part of the Chugwater formation. 

Section of Chu.{JWater formatwn between B1"ghorn River and Sykes Mountain in N~~ 

sec. 7, T. 57 N., R 94 W., Big Horn Cou.nty, Wyo. 

I Plate 4, SPC. :l) 

Gypsum Spring formation: Thick white gypsum. 

Chugwater formation: Feet 

Silt.stone, n~d; red fine-grained sandstone ________________ ·--------- 61 

Sandstone, red, very fine grained; some red siltstone_______________ 192 

s~\,ndstone, red, very fine grained, thin- and medium-bedded; contains 

till merous porous leached zo11es_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 85 

Sa11dstone, red, fitw-gmi11ed a11d very fine grained, thin- a11rl mediumbedded; ~omo red siltstone and shale _________ ------------------ 76 

Gyp~um, gray and pink, wavy- and thin-bedded___________________ 3 

Sandstone, red, predominantly very fine grained; red siltstone______ 4 

Dolomite, gra.y, platy__________ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 1 

Siltstone, red, gypsifer·ous_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 25 

Total Chugwater formation__________________________________ 44 7 

Em bar formation: Thick gmy dolomite. 
36 GEOLOGY OF THE BIGHORN CANYON-HAHDlN AlU~A, MON'l'.-'\VYO. 

Section of Chuuwater .formatwn south of Soap Creek in N E}~ sec. 4, '1'. 8 S., R. 32 E .. 

and SW}~ sec. 34. 'l'. 7 S., R. 82 E., Big Horn County, Mont. 

[Measured by W . .1. Mapel and A. A. Meyerhoff] 

Piper formation: 

Gypsum, white, granular, irregularly bedded; forms three ledges ____ _ 

Chugwater formation: 

Sancl~tone and silt;:;tone, red, ;;;haly and Rilt.y; siltstone at top grades 

clown into fine-grained silty ~andstone ________________ .. ________ _ 

Sa.ndstone, moderate-red, mostly fine-grained, locally cros .,bedded; 

forms three ledges ___________________________________________ _ 

Siltstone and fine-grained s9.ndstone, moderate-red; lower third is 

mostly sandstone; forms ledge, _______________________________ _ 

Sandstone, moderate-red, fine-grained, thick-bedded, ledge-forming_ 

f;iltstone, red, thin-bedded; interbedded moderate-red fine-grained 

ripple-marked sandstone; unit forms slopes _____________________ _ 

Limestone, white, hard, slabby; has cloRely spaced red laminations and 

shale partings; forms prominent ledge. Unit may be equivalent to 

the Alcova limestone member ________________________________ _ 

Siltstone, moderate-red, nodular; contain.;; 1 or 2 beds of fine-grained 

sandstone; forms reentrant ___________________________________ _ 

Sandstone, red, medium-gr::1ined, friable, thick-bedded, ledge-forming, somewhat cavernous _____________________________________ _ 

Sand::;tone. moderate-red, fine-grained, plai,y; interbedded reel thinbedded siltstone; forms slope _________________________________ _ 

Sandstone, moderate-red, fine-grained; lower 35 feet massive, upper 

one third ripple-marked; unit forms massive ledge _______________ _ 

Siltstone, maroon, well-cemented, thin-bedded, nodular; middla 7 feet 

is fine-grained thin-bedded red sandstone ______________________ _ 

Sandstone and siltstone. Sandstone is red, fine grained, thin to 

medium, interbedded with red thin-bedded siltstone. ___________ _ 

Sandstone, reel, fine-grained; silty in lower part becoming les.:; silty 

towards top; beds of reel fine- to medium-grained thin-bedded reF;istant sandstone in upper 10 feet ________________________________ _ 

::lilts tone, red; very fine grained thin-bedded reel sand:.,; tone; numerous 

layers and stringers of white crystalline gypsum as much as 1 inch 

thick; unit slightly calcareous; fotms .3lope _____________________ .. 

Gypsum, white with closely spaced pink laminations, thin-bedded; 

lower 4 feet forms ledge; upper 3 feet is soft and impure with thin 

lensea of reel siltstone _______________________________________ _ 

Siltstone, red, and flue-grained reel sandstone; contain.;; stringers and 

thin beds of gypsum ________________________________________ _ 

Covered; probably all red beds _________________________________ _ 

Gypsum, white, granular, thick-bedded; a 5-foot interval about 13 

feet above base has interbedded red siltstone and forms small ledge_ 

Covered; few exposures of red shaly siltstone _____________________ _ 

Gypsum, white, granular, medium- to thick-bedded _______________ _ 

Si!Lstone, red, thin-bedded, fine-grained calcareous red sandstone ___ _ 

Covered; elsewhere the lower 2 or 3 feet is red sandstone and siltstone 

containing numerous angular chert fragments as much as 2 inches 

in dian1eter ________________________________________________ _ 

Feet 

10 

65 

25 

70 

22 

ll 

5 

3 

24 

71 

50 

16 

34 

48 

51 

7 

f) 

46 

38 

28 

8 

10 

5 

1~ 

~ 

.. ~ 
• 

ti'I'HA'l'lGHAPH Y 37 

Sectiou of Chugwater fonnal-ion ;;outh of Soap Creek in N /<J}~ sec. ;,_, ·1'. 8 S., U. 32 E. 

a.nd SW}~ sec. 31,., T. 7 S., fl. 32 E., Bt:g Horn County, Mont.-Continued 

Chugwater formation-Continued 

Sandstone, white, locally :;tained red and purple, mo:;t,ly fine-grained, 

partly leached and brecciated, locally cavernottH; haH red and gray 

chert fragments in upper· 3 feet. This unit is placed at base of 

Chugwater formation and is considered to be locally reworked sand 

of the Tensleep. A feather edge of Tensleep sand.5tone cr OJB out Feet 

nearby at about this horizon ______________________ ----------- 28 

Total Chugwater formation_________________________________ 674 

Unconformity. 

Amsden formation: 

Shale and silty shale, rad and purple, thin-bedded; top 2 feet dolomitic; 

mostly covered______________________________________________ 13 

A widespread erosional unconformity separates the Chugwn.tcr 

formation from the Tcnslcep sandstone, but the bedding in the lower· 

part of the Chugwater closely parallels the bedding of the 'l'enslecp 

sn.ndstonc. Thin beds of chert-pebble conglomern.tc arc common in 

the basal part of the Chugwater in areas where the Chugwater overlies 

the Tensleep sandstone. L. G. Henbest collected a Pennsylvn.nin.n 

fauna from gray chert pebbles in a basal conglomeratic sandstone along 

Grapevine Creek; the chert presumably was eroded from Ten sleep 

sandstone or possibly Amsden formation during early Chugwater time. 

Although the Chugwater in this area is seemingly conformable with 

the overlying ~1iddle Jurassic Piper formation, an unconformity at 

the bn.se of the tJ urassic strata has removed the Chugwater in central 

lVIontn.na, and the Chugwater is only about 200 feet thick under the 

liard in n.t·cu. (Rogers, Gardner, and Hadley, 1945). It was almost 

com plctely removed prior to Jurassic deposition north of the Yellowstone River in central ~1ontanu. (Rogers, Gardner, and Hadley, 1945). 

The thin limestone, gypsum, and redbed series at the base of the 

Chugwater east of the Pryor ~'lountains is very likely equivalent to 

the Eml.Htr formation and was induclccl in t.he Chugwater because it 

is too thin to map separatdy. Tints the Chugwater in the vicinity of 

Big:hor·r1 Canyon includes rock of Triassic and probably Permian nges. 

JURASSIC SYSTEM 

The Jurassic systcrn includes four formations, of which three, the 

~'liclclle Jurassic Piper and the Upper Jurassic Rie1·don and Swift, 

constitute the Ellis group (Imlay and others, 1948). The ~1orrisou 

fornuttion overlies the Ellis group. 

The grayish-green and olive-gray sandstones and shales of the 

Ril't·don and Swift formations n.ncl the redbeds and limestones which 
38 <;EOLOUY OF 'L'I:-I.E. BWHOHN CANYON-HAHDIN AH.E.A, MON'l'.-WYO. 

mnke up the Piper formation formerly were called Ute Ellis formation 

in central and north-central l\1ontana. The Piper is, at least in 

part, equivalent to the Gypsum Spring formation of Wyoming. 

Thorn (Thorn and others, 1935, p. 39-40) placed all known Jurassic 

strata below the Morrison formation in the Sundance formation which 

included the upper one-half of the Piper formation and the Rierdon 

and Swift formations. Figure 4 has been taken from the nomenclature t; 

and correlation table prepared by Imlay and others (1948). 

Upper 

Jurassic 

<I) 

t;i 

Middle 

Jurassic 

Formations in 

south-central 

Montana 

Morrison 

formation 

Swift 

formation 

Rierdon 

formation 

Red shale 

and siltstone 

... Gray shale Q) 

and limestone .So 

Po. 

Black H11ls of western 

South Dakota and 

northeastern Wyoming 

Morrison 

formation 

Redwater 

shale 

member 

Wind River Basin of 

central Wyoming 

Morrison 

formation 

"Upper 

Sundance" 

Fir:UHE 4.-Conelation of .Jurassic formations of south-central Montana with Upper and Middle Jurassic 

formations of other areas. 

MIDDLE .JURASSIC SERIES 

PIPER FORMATION 

The Piper formation eonsists of red sandstone and siltstone, gray 

limestone, and gypsum, and it is 150 feet thick or more in outcrops 

alon~ the Bighorn Mountains. It was named by Imlay in 1948 for 

exposures near Piper, Fergus County, Mont. The formation was 

defined to include all redbeds, gypsum, and associated normal marine 

beds of Middle Jurassic age in Montana cast of the Sweetgrass-Big 

Belt uplift. Some of the Piper is equivalent in age to, and the 

remainder is younger than, the Gypsum Spring formation of Wyoming. 
• 

S'L'RA'l'IGHAPHY 39 

A section typical of Piper in south-central l\1ont.ana is exposed 

along the north side of Grapevine Creek where the Piper is 150 feet 

thick and consists of three principal units. The lower unit consists of 45 feet of thick-bedded gypsum and thin layers of red and 

green shale. This gypsum is the base of the Piper formation throughout the area covered by this repot·t. The middle unit, the bottOin of 

which wa.s mapped as the base of the Sundance formatiqn in this 

area by Thom ('I' hom and others, 1935, p. 39-40), is gray argilln.cr.ous limestone with intercalated green and reel shale. It erodes 

to form prom in en t light-gray ledges, below which stand bright-red 

blufl's of Piper and Chugwater strata. The upper unit of the Piper 

consists of reel shale and siltstone but is covered at most locali tics 

by debris on a dip slope above the limestone of the middle unit. The 

contact of the Piper with the ovel'lying Rim·don formation commonly 

is concealed in strike valleys. 

The Piper formation is about 200 feet thick at the eastern foot of 

Sykes lVlountain a short distance south of Crooked Creek, Big Horn 

County, vVyo.; 175 feet thick near the head of t:;oap Creek about 14 

miles southeast of the Grapevine Creek section; and 90 feet thick at 

Shively dome in T. 5 S., R. 27 E. north of the Pryor l\1ountains, Big 

Horn County, lVlonL., (Imlay and others, 1948). Imlay e.:x:plained 

the wide variation in thickness of the Piper in lVIontana as being 

clue to deposition upon an irregular surface. In central and northcentral lVfontana, the Piper rests on a pre-Jurassic erosion surface 

which cuts across the l\1Iaclison limestone, Amsden formation, Tensleep sandstone, and the Big Snowy group, all of Carboniferous age. 

Secl.·ion of P£peT foTuwtion east. c.f Gra?Jev£ne Creek 1:-n center of sec. 6, '1'. 6 S., R. 

31 l!J., B1:g Horn Co·11nly, Mont. 

[Plate 4, seetion .'i] 

Rierdon formati'on: Lower part covered by valley fill. 

Piper formation: 

Siltstone and sanddtone, red; unit. is mostly covered and contact with 

Ricrdon formation is concealed. in strike valley, about ___________ _ 

Limestone, light-gray, finely crystalline, thin-bedded, moderately 

resistant; caps some ridge.s; contains a few poorly preserved molds 

of fo.ssils ___________________________________________________ _ 

Shale, red and green, mostly covered ____________________________ _ 

Limestone, gray, ::trgillaeeous; abundant molds of small pelecypod:--; 

thin st.rcak':l of green calcite along fracture planes _______________ _ 

Limestone, argillaceous; red and gray Hhalc ______________________ _ 

Limestone, gnty, argillaccom;, ledge-forming; red Hhalc Htrcaks in 

lower 3 f~ct and thin dolomite bed at base: in 4- to ()-iJtch bedH ___ _ 

Lirne~tonc, light-gray, argillaceous; a few beds of reel shale 3 to 12 

inchPs chick; 10 inches of brown Hhalc at top; thi'3 and overlying 

limestone beds form ledges whore clipo;; arc gentle and ridges where 

clips arc s t.ecp ______________________________________________ _ 

Feet 

20 

8 

8 

3 

]() 

13 

8 
40 GEOLOGY OF '!'HE BIGHOHN CANYON-1-IAHDIN AHEA, 1VION1'.-·WYO. 

Section of Piper formation east of Grapevine Creek in center of sec. 6, T. 6 S., R. 

31 E., Big Horn County, Mont.-Continued 

Piper formation-Continued Feet 

Shale, red; gray thin-bedded limestone___________________________ 6 

Shale, red; siltstone near base; contains numerous gypsum stringers; 

mostly covered __________________________________________ .. ___ 27 

Gypsum, white, granular; in 2- to 4-foot beds separating?~- to 4-inch \ 

bed3 of red shale or thin gray dolomitic shale___________________ 47 

Total Piper formcttion_____________________________________ 150 

Chugwater formation: 

Sand;,;tone, red, silt,y and red gypsiferous siltf;tonc__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 26 

UPPBR .JURASSIC Sl<JRIES 

RIERDON AND SWIFT FORMATIONS 

The Rierdon and Swift formations consist of Upper Jurassic marine 

fossiliferous grayish-green and olive-gray sandstones and shales that 

lie between the redbeds of the Piper formation and the continental 

sandstones and shales of the Morrison formation. Total thickness of 

the Ricrclon and Swift formations is about 500 feet along the eastern 

side of the Bighorn Mountains, but their band of outcrop is narrow 

because dips arc steep, and the two fonnations have been mapped as 

one unit. 

The Rim·clon formation consists of nonresistant calcareous olivegray shale as much as 300 feet thick north of Grapevine Creek but 

much thinner south of the Bighorn River. The shale weathers 

readily and is commonly covered by several inches of residual soil 

which contains many Belemnites and Gryphaea shells. At the top of 

the Rim·don is a 5- to 10-foot bed of sanely "oolitic" fossiliferous 

ledge-forming limestone that overlies 5 to 10 feet of highly calcareous 

yellowish-gray very fine grained sandstone. This sandy unit forms 

long low ledges on an otherwise fairly smooth steep slope cut on the 

Rierdon and Swift formations. During a visit to the area in 194 7, 

Imlay stated that the sandy unit is correlative with the Hulett 

sandstone member of the Sundance formation of the Black Hills area. 

The Swift formation consists of grayish-green sandstone, siltstone, 

and shale which together range from about .180 feet to a little more 

than 250 feet thick along the cast front o(the Bighorn l\1ountains. 

The sandstone contains glauconite, small chert pebbles, and layers of 

fossil shells. There are lenticular fossiliferous sandstones near the 

top of the Swift, which arc overlain by nonmarine beds of the l\1[orrison formation. The contact of the two formations was drawn above 

the highest bed that contains marine Jurassic fossils. 

Imlay (Imlay and others, 1948) has described an angular unconformity between the Rierdon and Swift formations in north-central 

l\1ontana with the S\'.rift overlapping the Ricr·don and Piper formations 

... 
S'l'RA't'lGRAPHY 41 

and resting on Paleozoic strata. However, within this area the 

Riet·don and Swift arc seemingly conforma.ble. 

Sect·ion of Rierdon and Swift .format-ions exposed north of month of Bighorn Canyon 

·in sec. 6, T. 6 S., U. 31 E., Biu Horn County, 1\font. 

l Plate 4, section 5] 

• .1. M O!Tison formation: Concealed; ~ome green ~ilt~tone exposed. 

• 

Swift formation: 

Sandstone, greeni~h-gray and brown. fine- and medium-grained, 

poorly exposed; contains few fo~sils; thin ledge of glauconitic Peet 

coarse-grained sandstone at top__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 25 

Limestone, greenish-gray, sandy, cro~~bedded, very fossiliferous, 

ledge-forming_______________________________________________ 15 

Concealed; probably largely shale________________________________ 63 

Concealed; some discontinuous exposure~ of calcareous light-brown 

shale and gray siltstone ncar middle; light-brown shale at top_____ 64 

Total, Swift formation_____________________________________ 167 

Riet·don formation: 

Limestone, oolitic, very sandy, very fossiliferou~; ledge forming, 

weathers dark brown_________________________________________ 9 

Sandstone, light-brownish-gray, weathering tan; fine grained, calcareous. Similar sandstone may be concealed at top of underlying 

unit________________________________________________________ 9 

Concealed; a few expo~ures of soft light-brown to light-greenishbrown calcareous clay shale; numerous Belemm'les, fragments of 

~tar-shaped crinoid ~tcms, and Gryphaea shells in soil; base concealed beneath alluvium in valley bottom, about________________ 300 

Total Hicrdon fornmtion___________________________________ 318 

Piper formation: Concealed, probably mostly redbeds. 

MORRISON FORMATION 

The U ppcr Jurassic l\1orrison formation consists of grayish-green 

siltstone and sandstone and variegated shale. Locully, .as in the 

ccntm· of Soap Creek dome, the vai·icgatcd shales arc conspicuous, 

but at 1nost localities along the mountain front the l\1orrison is 

gru.yish-grccn sandstone and siltstone. Some of the lower beds that 

n.rc crossbcddcd and calcareous arc not easily distinguished frmn 

beds in the Swift formation. 

The l\'[orrison ranges in thickness between 140 and 280 feet in the 

area. 'l'his pronounced change in thickness is due largely to gradation and interfingering downward into the Swift formation and 

upward into continental deposits in the lower part of the Cloverly 

formn.tion. The l\1[onison and Cloverly forn1ations are mapped as 

units wherever a basal cotiglomeratic sandstone occurs in the Cloverly 
42 GEOLOGY OF THE BIGHORN CANYON-HARDIN AHEA, MONT.-WYO. 

formation. Elsewhere the l\1orrison cannot be differentiated from 

the Cloverly and- the two are mapped as one unit.. 

Section of Morris on formation along Lime K tln Creek in N W}~ sec .. fd 1, 7'. 7 S 

R. 81 E., Big Horn County, .Mont. 

[Plate 4, section 5. Measured by L. S. Gardner and C. P. l:{ogers, .Jr.] 

Cloverly formation: 

Conglomerate, gray to light-brown, massive; made up of pebbles of 

sandstone, quartz, and chert as much as 2 inches in diameter in a 

matrix of fairly well cemented medium-grained quartz sand _____ _ 

Morrison formation: 

Siltstone, greenish-gray, clayey, highly calcareous, platy. friable, 

poorly exposed _____ . _______________________________________ _ 

Sandstone, white, calcareous, friable, massive, nonresistant ________ _ 

Siltstone, medium-gray (weathering light greenish gray), poorly exposed; contains some platy gray shale _________________________ _ 

Limestone or calcareous siltstone, light-gray, silty, clayey, nonresistant ____________________________________________________ _ 

Siltstone and shale, gray. Siltstone grades into fine-grained sandFeet 

22 

38 

14 

28 

stone_______________________________________________________ 92 

Concealed____________________________________________________ 89 

Siltstone and claystone, yellowish-brown, friable, nonresistant_______ 5 

Sandstone, dark-gray, highly calcareous, platy, nonresistant ________ _ 

Siltstone or claystone, yellowish-brown, friable, nonresistant________ 15 

Total, Morrison formation__________________________________ 283 

Swift formation: 

Sandstone, greenish-gray, very calcareous, massive, crossbedded; 

contains layers of shells near top______________________________ 7 

CRETACEOUS SYSTEM 

LOWER CRETACBOUS SBRIES 

CLOVERLY FORMATION 

The Cloverly formation, exposed along the northern and eastern 

sides of the Bighorn Mountains and around Soap Creek dome, generally consists of a discontinuous basal conglomeratic sandstone 

member, a middle variegated shale member, and an upper shale, 

siltstone, and sandstone member. The formation ranges somewhat 

in thickness, but its average thickness is between 300 and 400 feet. 

The Pryor conglomerate member at the base of the Cloverly is 

composed of resistant sandstones, commonly with chert-pebblebearing conglomerates in the lower part. The Pryor member ranges 

in thickness fr9_!!1 ~bout 30 feet at Soap Creek dome, where it is not 

conglomeratic, to 150 feet south of the mouth of Bighorn Canyon, 

except in the area from sec. 7, T. 7 S., R. 31 E. southeastward to 

Rotten Grass Creek where the member is missing. The Pryor 

reappears south of Rotten Grass Creek in see. 8, T. 8 S., R. 33 E. and 

-t" 

~ 

.. \ 
STRA 'l'IG RAPHY 43 

thickens considerably within a few hundred feet southward. Prominent hogbacks held up by the Pryor conglomerate member 

have formed in the belL of steeply dipping strata along the eastern 

side of the mountains. Differences in thickness of the Pryor conglomerate member account for much .of the range in thickness of the 

Cloverly formation. 

Poorly exposed reel and gray shales, and some interbedded siltstones, constitute the middle unit of the Cloverly. 'I'his member 

ranges from a. few feet to a bout 50 feet in thickness except in the 

area north of Rotten Grass Creek where 140 feet of vm icga.ted sha.le, 

bentonite, and bentonitic shale, which are exposed in sec. 1, T. 8 S., 

R. 32 E., represents the middle and basal Cloverly units and, perhaps, 

the upper part of the lV[orrison formation. Reel shales at this outcrop conta.in numerous gastroliths and thin beds of light-gra.y to 

light-red limestone and chert. 

The upper part of the Cloverly formation is about 250 feet thick 

along the east side of the Bighorn 1\tlountains. The unit consists 

of interbedded siltstones, dark-gray shales, and thin-bedded "fta.ggy" 

sandstones. The siltstones weather brown, and thin ironstone laye1~s, 

most of which are in the lower part, have a pronounced rusty weathered 

color. Numerous dahllite concretions (1t1cConnell, 1935, p. 593..:... 

698) occur about 125 feet below the top of the formation along the 

mountain flank, but none were seen at Soap Creek dome. 

'J'he Birdhead sandstone, which was tentatively correlated with 

the Newcastle sandstone of the Black Hills and Powder River Basin 

areas and the Muddy sandstone of the Bighot·n Basin and included 

in the Thermopolis shale by Thom (Thon1 and others, 1935, p. 47; 

also Richards and Rogers, 1951), is the top of the Cloverly formation 

as herein described. The Birdhead sandstone member is mostly 

fine-grained sandstone and is in part silty and argillaceous. It is 

not sharply set off from the underlying be.ds and is commonly 10 

or 15 feet thick. Locally, a thin brown-weathering limestone at the 

top of the Birdhead n1ember makes ledges. Overlying the Birdheacl 

is a zone of black shale with hard ironstone concretions. This shale 

is the basal unit of the Thermopolis shale. A few feet of shale and 

siltstone which occurs l0cally between the Birdhead and the blaek 

shale is included in the Cloverly formation. 

The Cloverly formation was I~amed by Darton (1904, p. 398-

399) for exposures on the east side of the Bighorn Basin, vVyoming. 

In his report on the Bighorn 11ountains, Darton (1906a, p. 50-51) 

descr·ibed the Cloverly as consisting of a conglomeratic sandstone 

member aHd an overlying clay member, a few feet of massive sandstone occuring locally above the clay. Darton (1906a, p. f'l3) thought 

the Cloverly to be equivalent to the Lakota sandstone, Fuson forma33-1328-55--4 
44 GEOLOGY OF THE BIGHORN CANYON-HARDIN AREA, MONT.-WYO. 

tion, and Dakota sandstone (now Fall River sandstone) of the Black 

Hills region, but he noted that the Dakota member is either not well 

developed or is represented by transition shales and thin sandstones 

at the top of the Cloverly. The section of the Cloverly formation 

in its type area as described by Darton (1906a, p. 52) follow-s. 

Section of Cloverly formation 17~ miles west of Cloverly, Wyo. 

Feet 

Light-buff sandstone (overlain by Benton shale)_______________________ 10 

Tan-colored sandstone_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 10 

~aroon clay _____________________________________________________ _ 

Reddish and tan-colored sandy clay _________________________________ _ 

Drab sandy clay __________________________________________________ _ 

Deep-maroon sandy-clay __________________________________________ _ 

Hard tan-colored sandstone ________________________________________ _ 

Deep-maroon to purple variegated clay ______________________________ _ 

Lenses of maroon sandstone ________________________________________ _ 

Deep-maroon sandy clay ________________________________ . _________ _ 

Olive-green, soft, cross-bedded sandstone, with hard layers (lying on 

maroon and drab-gray ~orrison shale) ____________________________ _ 

4 

10 

10 

20 

3 

12 

3 

20 

11 

Total______________________________________________________ 113 

Lupton (1916, p. 168) called the 20 feet of sandstone at the top of 

Darton's type Cloverly formation the Greybull sand, a name which 

originated as a driller's term and was reported by Hintze in 1915 

(Wilmarth, 1938, p. 878). Since then the Greybull sandstone has 

been a member of the Cloverly formation but not always the top 

member. Overlying the Greybull san5lstone member in the Bighorn 

Basin is a unit of shales and rusty-weathering sandstones which Darton 

(1906a, p. 54-55) called the "rusty series" and "rusty beds." Near 

the former post office of Cloverly, Wyo., which was 9 miles northnorthwest of the present town of Shell, Wyo., the basal part of the 

Colorado formation, now the Thermopolis shale, was described by 

Darton (1906a, p. 55) as consisting of 

about 100 feet of dark-gray to black shales with layers of thin brown sandstone 

weathering to a rusty color. The shales usually begin abruptly on top of the 

Cloverly sandstone, but without sign of unconformity. The characteristic 

globular [dahllite] concretions averaging an inch in diameter occur in a few feet 

of the lower shales about 60 feet above their base. Above this "rusty" series are 

about 200 feet of black fissile shales ... 

The so-called "rusty beds" are a poorly defined stratigraphic uriit, 

but "rusty beds" were placed at the base of the Thermopolis shale, 

which overlies the Cloverly formation, by Washburne (1908, p. 350), 

Hewett and Lupton (1917, p. 19), and Knappen and Moulton (1930, 

p. 27-28). "Rusty beds" were included in the Greybull sandstone 

member of the Cloverly formation by Lee (1927, p. 64), and a thinner 

unit of "rusty beds" was placed above the Greybull sandstone but at 

•) 
lo 

S'l'RATIGRAPHY 45 

the Lop of the Cloverly by Pierce (1948) and Rogers and others (1948). 

"Rusty beds" lut,ve also been placed near the top of the Cloverly 

fonnation east of the Bighorn :rviountains in Wyoming (Niapel, 1954). 

In localities east of the Bighorn Basin, such as Soap Creek dome, it 

is not wholly clear which beds represent the "rusty beds" and the 

Greybull sandstone of the basin. The black shale at the base of the 

Thermopolis shale in the Bighorn Canyon-Hardin area is considered 

by W. A. Cobban (personal co1nmunication) of the Geological Survey 

to be the stratigraphic equivalent of the Skull Creek shale of the Black 

Hills region. As the Skull Creek shale overlies the Fall River smidstone ncar the Black Hills, the Cloverly formation in the Bighorn 

Canyon-llardin area represents the Lakota sandstone, Fu~on shale, 

and Fall River sandstone of the Black Hills region-the formations 

that Darton thought were equivalent to the type Cloverly formation. 

However, if the Cloverly formation from the Birdhead sandstone 

member down to the variegated beds is correlative with Darton's 

"rusty beds," the Cloverly in the Bighorn Canyon-Hardin area includes strata younger than the type Cloverly formation. 

The inclusion of "rusty beds" in the Cloverly formation in this and 

other areas constitutes a redefinition of the Cloverly described by 

Darton. Such a redefinition is justifiable because Darton's upper 

unit of the Cloverly, the Greybull sandstone member, is not readily 

separated in many localities from the overlying "rusty beds." The 

top of the Cloverly formation-if it is to occur at a lithologic break of 

considerable areal extent-should be placed either below the Greybull sandstone member and above the variegated beds or below the 

black Skull Creek shale (or its stratigraphic equivalent) and above the 

so-called "rusty beds." Inasmuch as Darton considered the upper 

part of the Cloverly to be a correlative of the Fall River sandstone, 

the top of the Cloverly as described is as close as practical to its originally defined position. THERMOPOLIS SHALE 

'fhc Thermopolis shale is about 425 feet thick and consists largely 

of dark-gray shale with many beds of bentonite and ironstone concretions. 'J'ln·ee or four beds of bentonite near the middle of the formation am 2 to 4 feet thick. A basal unit of dark-gray to black shale, 

about 150 feet thick, is part of the black shale widely distributed in 

lVIontana that is the stratigraphic equivalent of the Skull Creek shale of 

the Black Hills area (Cobban, 1951, p. 2175-2179). Overlying the black 

shale is dark-gray shale 50 feet or more thick that is cut by gray sandstone dikes. The sandstone dikes range from 1 to 8 inches in thickness 

and fr01n 5 to 15 feet or more in length. Many of the dikes are nearly 

perpendicular to bedding planes in the shale, but others deviate 

several degrees therefrom. The sandstone is fine grained, non-
46 GEOLOGY OF THE BIGHORN CANYON-HARDIN AREA, MONT.-WYO. 

calcareous, and in part crossbedded with respect to the walls of the 

dikes. Sand for the dikes may have been derived from thin layers of 

sand in and above the zone of the sandstone dikes prior to the consolidation of the beds. The dikes, judging from the presence of sandstone in strata adjacent to the dikes and from the crossbeclded nature 

of the sand in the dikes, seemingly represent filling of fissures in the 

shale. •• 

Section of Thermopolis shale on east side of Soap Creek dome in SW}~ sec. 35, T 

6 S., R. 32 E., Bi(J Horn County, Mont. 

[Plate 4, section 7. Measured by C. P. Rogers, Jr. and P. W. Richards] 

Mowry shale: 

Bentonite, gray; few inches of dark-gray shale with fish scales at base_ 

Thermopolis shale: 

Shale, dark-gray; contains few thin siltstone layers and thin beds of 

bentonite __________________________________________________ _ 

Bentonite ____________________________________________________ _ 

Shale, dark-gray ______________________________________________ _ 

Bentonit~ gray _______________________________________________ _ 

Shale, dark-gray ______________________________________________ _ 

Shale, dark-gray; contains layers of thin-bedded brown siltstone and 

two thin beds of bentonite ___________________________________ _ 

Bentonite------------------------------------~---------------­

Shale, dark-gray; contains a few thin beds of gray sandstone and 

manganiferous-stained concretionary siltstone beds in lower half __ _ Bentonite ____________________________________________________ _ 

Shale, dark-gray; contains several thin layers of concretionar.v 

manganiferous-stained siltstone and gray sandstone; fine-grained 

gray sandstone dikes cut nearly perpendicularly across the unit_ __ _ 

Bentonite and shale ____________________________________________ _ 

Shale, dark-gray; contains thin beds of rusty-weathering siltstone ___ _ 

Bentonite ____________________________________________________ _ 

Shale, dark-gray; contains numerous thin beds of siltstone, some 

sandstone, a few thin beds of bentonite, and scattered ironstone 

concretions _________________________________________________ _ 

Total, Thermopolis shale __________________________________ _ 

Cloverly formation: Sandstone and siltstone, the Birdhead sandstone 

member, overlain by a few feet of silty shale. 

Feet 

3 

65 

1 

57 

1 

2R 

31 

2 

21 

3 

58 

5 

6 

4 

143 

425 

The contact between the Thermopolis shale and the overlying 

Mowry shale is gradationaL Although close to the base of the lowest 

occurrence of abundant fish scale impressions and siliceous shale and 

siltstone of the Mowry shale, the contact has been mapped at the 

base of a 2- to 4-foot bed of bentonite below about 50 feet of clarkgray shale typical of the Thermopolis shale. This bentonite bed is 

near the middle of the steep, nearly bare slopes beneath escarpments 

held up by the l\1owry shale. 

The Thermopolis shale is now known to be of Early Cretaceous 

~ 

.., 
S'l'HATIGRAPHY 47 

age (Cobban and Recsidc, 1951, p. 1892-1893). The Birdhead 

sandstone member, which is at or just below the top of the Cloverly 

formation as herein described, is not a correlative of the Newcastle 

sandstone as thought by T'hom (Thorn and others, 1935, p. 4 7), but 

the zone of sandstone dikes is about the stratigraphic equivalent of 

the Newcastle sandstone (W. A. Cobban, personal communication). 

This correlation is based upon the lithologic similarity of the black 

shale beneath the zone of sandstone dikes with the Skull Creek shale 

of the Black Hills area and the similarity of the sandstone dikes with 

dikes which occur locally in a stratigraphic zone where the Newcastle 

sandstone is either thin or missing in the Black Hills region. 

MOWRY SHALE 

The ~1owry shale is the most conspicuous formation of the Cretaceous system because of the cuesta-like escarpments that have formed 

of it. The Mowry is exposed in an almost continuous cuesta extending along the eastern edge of the Bighorn Mountains from Rotten 

Grass Creek at the northeast corner of T. 8 S., R. 32 E. northward 

to sec. 7, T. 6 S., R. 32 E. south of the Bighorn River and from Muddy 

Creek in the north-central part ofT. 5 S., R. 30 E. northward to the 

South Fork of Woody Creek in the northeast corner of T. 4 S., R. 

29 E. 

The Mowry is 350 to 400 feet thick and consists of dark-gray 

shale and light-gray siltstone and sandstone. Some of the shale and 

much of the siltstone in the formation is hard and resistant to erosion 

because it is siliceous (Rubey, 1929). The more siliceous beds 

characteristically weather light gray and light bluish gray. l\1any 

beds contain abundant fish scale impressions which are particularly 

characteristic of the l\1owry shale. The base of the Mowry shale is a 

4-foot bed of bentonite at the bottom of 50 feet of dark-gray-weathering shale that commonly forms a band almost barren of vegetative 

cover about midway in the escarpment slopes of the Mowry cuestas. 

Ridges or hogbacks common on the l\1owry shale are supported 

by thin beds of sandstone about 150 feet below the top of the formation. Similar beds of sandstone about 50 to 60 feet below the top 

of the formation are exposed in the SE% sec. 28, T. 3 S., R. 31 E., 

next to a fault which transects Woody Creek dome. The Clay 

Spur bentonite bed 1narks the top of the l\1owry and is a source of 

commercial-grade bentonite in northeastern Wyoming (Rubey, 

1930, p. 4). The contact of the l\1owry shale and the overlying Frontier formation is commonly at the foot of long dip slopes held up by 

the upper beds of the Mowry shale. 

The Mowry shale has long been considered to be Late Cretaceous 

in age, but accumulating knowledge now ~n~~cates that the M.owry 

/" 
,, 

48 GEOLOGY OF THE BIGHORN CANYON-HARDIN AREA, MONT.-WYO. 

fauna is Early Cretaceous (Cobban and Reeside, 1951 and 1952, 

chart lOb). 

Section of Mowry shale south of the South Fork of Woody Creek in W~~ sec. 2, T. 4 S., 

R. 29 E., Big Horn County, Mont. 

Frontier formation: 

Shale, dark-gray, fissile; containing 4 or 5 beds of ironstone concretions_ 

Mowry shale: 

Bentonite, greenish-gray. This is the Clay Spur bentonite bed _____ _ 

Shale, dark-gray; containing a few very thin sandstone beds with 

mud-cracked shale partings ______ ~ ___________________________ _ 

Shale, gray, siliceous; with numerous ripple-marked sandstone 

stringers; silty in upper few feet; mostly covered ________________ _ Bentonite, gray _______________________________________________ _ 

Sandstone; gray with "salt-and-pepper" appearance; mostly finegrained, thin-bedded, crossbedded, hard; forms ledges and ridges __ Covered; shale and siltstone float _______________________________ _ 

Bentonite, gray _______________________________________________ _ 

Siltstone, gray and gray siliceous shale; contains some fine-grained sandstone __________________________________________________ _ 

Siltstone, gray, platy __________________________________________ _ 

Shale, gray, siliceous __________________________________________ _ 

Shale and siltstone, gray, siliceous in part, hard, platy; weathers to 

light bluish gray or silvery gray ______________________________ _ 

Shale, gray, chippy, poorly exposed _____________________________ _ 

Sandstone, gray, fine- to medium-grained, shaly and silty, thin-bedded; 

locally forms top of prominent hogback; elsewhere the top of the 

hogback is an overlying unit and this sandstone forms a small ledge 

Covered; siliceous shale and siltstone in float _____________________ _ 

Bentonite, gray; probably very impure __________________________ _ 

Sandstone, gray, fine-grained, silty; contains shale partings ________ _ 

Siltstone, silty shale, and fine-grained thin-bedded sandstone; a 6-inch 

bed of green bentonite is 10 feet below top; upper part of unit mostly covered ____________________________________________________ _ 

Bentonite, green ______________________________________________ _ 

Siltstone and shale, gray (weathering bluish gray); small ledges form 

at top of unit ______________________________________________ _ 

Siltstone, gray, sandy; weathers bluish gray, forms small ledge, con- tains numerous fish scales ____________________________________ _ 

Bentonite, light-gray; upper 6 inches shaly _______________________ _ 

Shale, dark-gray; sandy in upper part, forms steep slope devoid of 

vegetation _________________________________________________ _ 

Bentonite, gray; forms small shoulder ___________________________ _ 

Shale, gray, hard; weathers bluish gray __________________________ _ 

Bentonite, gray _______________________________________________ _ 

Shale, dark-gray ______ -- _____ -_------------------------------ 1 -

Bentonite, yellow-green; rather granular _________________________ _ 

Peel 

28 

2 

16 

32 

3 

3 

5 

1 

47 

4 

5 

10 

17 

1 

14 

1}~ 

2 

95 

1 

19 

2 

3 

49 

2 

2 

1}~ 

4 

1 

Total Mowry shale _________________________________________ 343 

Thermopolis shale: 

Shale, gray; contains a few thin beds of ironstone aud sandstoue_ _ _ _ _ 88 

.• 

'· 
STRATIGRAPHY 49 

UPI~J<JR CRETACEOUS SERIES 

FRONTIER FORMATION 

The Frontier formation, about 260 feet thick, forms a much wider 

outcrop belt around the Bighorn and Pryor 11ountains than do the 

underlying formations. Although the Frontier contains a few lenses 

of sandstone, some of which arc mostly composed of small black chert 

pebbles similo.1· to those that characterize the Frontier sandstone in 

the Bighorn Basin, the fonnation consists predominantly of dark-gray 

concretionary, sandy shale with interbedded bentonite. It is, in 

fact, very similar to the lower part of the overlying Cody shale. 

The basal 70 feet of the Frontier contains several beds of bentonite 

and ferruginous concretions in clark-gray partly bentonitic shale. 

At least one of these bentonite beds in each exposure is stained red 

fron1 a layer of li1nonite. Where this basal unit has not been protected by vegetation, it has eroded to badland topography. 

The white-weathering Soap Creek bentonite bed (Knechtel and 

Patterson, 1952, pl. 1), the topmost unit of the Frontier of this report, 

is fron1 5 to 10 feet or more thick over much of the area. Brown 

calcareous concretions, locally as much as 8 feet in diameter, arc very 

co1nn1on in the shale beneath the Soap Creek bed, and the association 

of the thick bed of bentonite and exceptionally large concretions 

provides an easily mapped horizon. The top of the Frontier formation might be difficult to recognize in subsurface and it probably 

would be placed at the top of the highest sandstone or sandy shale in 

wells which penetrate the Frontier. Inasmuch as lenses of sandstone 

and sandy shale are locally common in the lower part of the overlying 

Cody shale, the contact as selected in the subsurface might range 

s01newhat up or down in the stratigraphic section from well to well. 

Section of the Frontie1· formation on the north flank of Rotten Gmss dome in the N7'2 

sec. 29, T. 7 S., R. 38 E., Big Horn County, Mont. 

(Measured by S. ll. Patterson and P. W. Richards] 

Cody shale: Shale, gray, sandy 

Frontier formation: 

Bentonite, olive-green, waxy: Soap Creek bentonite bed ___________ _ 

Shale, gray, partly sandy; mostly covered ________________________ _ 

Bentonite, greenish-gray, waxy _________________________________ _ 

Sandstone and shale; consists predominantly of gray poorly consolidated sandstone in upper 25 feet and gray shale in lower 45 feeL 

Bentonite, green, waxy·----------------------------------------

Shale, dark-gray, sandy; has 2 thin beds of bentonite near the middle_ 

Shale, dark-gray; weathers gray; sandy in upper 10 feet, bentonitic in 

lower part; has bed of ferruginous and manganiferous siltstone at 

top and at base _____________________________________________ _ 

Bentonite, greenish-gray, waxy; mostly stained a rusty color_ ______ _ Shale, dark-gray ______________________________________________ _ 

Feet 

8 

62 

7~ 

70 

2 

45 

60 

H~ 

2' 
50 GEOLOGY OF 1'HE BIGHORN CANYON-HARDIN AREA, MONT.-"\VYO. 

Section of the Frontier formation on the north flank of Rotten Grass dome in the N% 

sec. 29, T. 7 S., R. 33 E., Big Horn County, Mont.-Continued 

Frontier formation-Continued 

Bentonite, dark-gray, waxy; has rusty weathered streak in middle Feet 

from interbedded ferruginous materiaL ________________________ _ H 

Shale, dark-gray ______________________________________________ _ n~ Bentonite, dark greenish-gray __________________________________ _ 1 

Shale, dark-gray; sandy in lower half; 8- to 12-inch bed of gray claystone concretions near middle ________________________________ _ 6 

Total Frontier formation ____________________________________ 260 

CODY SHALE 

The Cody shale includes 2,600 feet of dark-gray partly sandy shale 

which underlies much of the plains region in south-central Montana. 

The Cody is conformable above the Frontier formation and under 

the Parkman sandstone and includes rocks of the Colorado and 

lVfontana groups. Several members of the Cody shale were mapped 

by Thorn (Thorn and others, 1935, p. 49-58) as formations. 

The Cody shale is a thick, mappable formation composerl of seven 

members. Although lithologic characteristics of the members enable 

the mapping of individual units in much of the area, the difl'erences 

in lithology are commonly too little to make possible the distinction 

of individual members where the shale is not well exposed east of the 

Bighorn River. The lower members of the Cody shale in the Colorado group are an unnamed basal member, the Greenhorn calcareous 

member, and the combined Carlile and Niobrara shale member~, the 

total thickness being about 1,000 feet. Members of the Cody shale 

which belong to the ~1ontana group are the Telegraph Creek member, a shale unit equivalent to the Eagle sandstone, and the Claggett 

shale member. Rocks in the Montana group part of the Cody shale 

are about 1,600 feet thick. 

Inasmuch as the dark-gray shale of the Cody shale is relatively 

homogeneous in composition and uniform in its resistance to erosion, 

low rounded hills are typical of Cody outcrops in the Hardin area. 

Some high topographic relief has resulted on the plains, however, 

where terrace gravels in high-elevation remnants have preserved the 

underlying shale from erosion. Very little vegetation grows ou the 

Cody shale except where silt has been deposited on it by streams or 

wind. 

The Cody shale was measured and fossils collected and identified 

by W. A. Cobban. Fossils have been previously collected by W. T. 

Thorn, Jr., and identified by J. B. Reeside, Jr. (Thorn and others, 

1935, p. 50-58). 

Lower member of the Cody shale.-The lower member of the Cody 

shale consists of concretionary dark-gray shale and has several thin 

lenticular beds of sandstone in its lower part. The member is about 

. .-

_, 
S'l'HATIGRAPHY 51 

200 feet thick in the southern part of the area and is somewhat thicker 

in Woody Creek valley. On the east side of Soap Creek dome an 8-foot 

bed of bentonite occurs about nO feet above the base of the formation, 

but in Woody Creek valley the interval between the Soap Creek 

bentonite bed at the top of the Frontier formation and the next 

higher bentonite layer is only 30 feet. The concretions found throughout the Cody shale commonly are septarian, calcareous, and brown 

to gray in <.:olor. 'I'his member and the underlying Frontier formation 

ar·e equivalent to the Belle Fourche shale in the Black Hills region. 

Sect·ion of lower member of Cody shale on east flank of Soap Creek dome in SW~~ 

'sec. 36, '1'. 6 S., R. 82 l!J. Big Horn Co11nty, Mo·nt. 

[Plate 4, Section 7. Measured by W. A. Cobban] 

Cody shale: 

Greenhorn calcareous member: Feet 

Shale, calcareous, dark-gray; weachering bluish-white__________ 38 

Bentonite, gray, limonitic __________________________________ _ 

Lower member: 

Shale, dark-gray___________________________________________ 10 

Bentonite, gray, impure____________________________________ 3 

Shale, dark-gray __________________________________________ . 19 

Shale, clark-gray: contains three beds of light gray- and buffweathP.ring calcareotB concretions with dark-brown calcite 

septarian vei nlet<S_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 20 

Shalt:!, da.r·k-gray _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 37 

Shale, clark-gray; contains calcareous septarian concretions in 

upperfoot ______________________________________________ 29 

Shale, gray, bentonitic_____________________________________ 18 

Bentonite, gray; contains brown--weathering fibrous aragonite 

21 inches above base; weathers to gray bare gumbo ridge_____ 8}~ 

Shale, dark-gray; conta,ins very fine sandy streaks; contains Cttlcareous septarian concret.ions at ba:se, commonly 1}~ by 3 feet 

with white to pale-yellow calcite veins at top________________ 21 

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

of sandstone with black chert pebbles; has thin bed of friable 

coarse-grained sandstone locally at base; contains Acanthoceras? 

sp. (loc. 24059) _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 40 

Total, lower member, Cody shale ______________________ 205% 

Frontier format,ion: Soap Creek bentonite bed. 

Greenhorn calcareous member.-The Greenhorn calcareous member 

of the Cody shale ranges in thickness from about 60 feet on the north 

side of Woody Creek valley to nearly 100 feet on the east flank of 

Soap Creek dome. It is composed of dark-gray very calcareous shale 

which weathers to nearly white soil that is conspicuous on dry sunny 

days. Its base is marked by a bed of gray limonitic bentonite and its 

top by a change from calcareous chunky shale in the Greenhorn 

member to noncalcareous fissile shale in the Carlile shale member. 
52 GEOLOGY OF THE BIGHORN CANYON-HAHDIN AREA, MON'l'.-WYO. 

Thorn (Thorn and others, 1935, p. 49-51) collected several forms of 

the ammonite genus Vascoceras from concretions 40 feet above the 

base of the Greenhorn east of Soap Creek dome. Reeside (1 925, 

p. 25-33) identified them as the only specimens of the genus found m 

North America outside the State of Coahuila, l\1exico. 

Section of Greenhorn calcareons member on east flank cf Soap Creek dome in 8 W% sec. 36, 1'. 6 S., R. 32 E. Big Hcrn County, Mont. 

[Plate 4, sec. 7. Measured by W. A. Cobban.] 

Cody shale: 

Carlile shale member: Feet 

Shale, dark-gray; contains very fine ::;andy streaks ____________ _ 56 

Greenhorn calcareous member: 

Shale, dark-gray, calcareous; weathers blui::;h gray to white; 

contain.3 a few limestone concretions 5 feet above base_______ 28 

Shale, dark-gray, calcareoll3; contains light-gray to buff cal care- -

ous concretions__________________________________________ 1 

Shale, dark-gray, calcareow;; weathers bluish gray to white_____ 28 

Shale, dark-gray, calcareous; contains light-gray to buff limestone concretions commonly 9 by 12 inches with calcite 

crystal-lined cavities; contains I noceram1ts labiatns (Schlotheim), Inoceramus sp., Ostrea n. sp., Plicatnla? sp., gastropod 

indet., Allocrioceras pariense (White)?, Pseudaspidoceras sp., 

Pse1tdotissotia (Choffaticeras) sp. ?, Scaphites delicatulus Warren 

var., Vascoceras mmdtoni Reeside, Vascoce1·as stantoni Reesidc, 

Vascoceras thomi Reeside, Watinoceras reesidei "Tarren, fish 

bones (Iocs. 10755, 10890, 20!)35)__________________________ 1 

Shale, dark-gray, calcareous; weathers bluish gray to white____ 38 

Bentonite, gray, limonitic __________________________________ _ 

Total, Greenhorn calcareous member_____________________ 97 

Lower member: 

Shale, dark-gray___________________________________________ 10 

Carlile shale member.-The Carlile shale member is about 280 

feet thick and consists chiefly of dark-gray shale, which is sandy 

in the lower part. Two zones of concretions, one about 7 5 to 100 

feet above the base, and one about 90 feet below the top, crop out 

widely within the area. Concretions of the lower zone occur in thin 

beds in a stratigraphic interval of about 25 feet. They are thin, 

very hard, gray on fresh surfaces, red to brown on weathered surfaces, and soluble in hot. hydrochloric acid. This zone is an easily 

recognized stratigraphic horizon because the "rusty" concretions 

are the only ones of their kind below the Telegraph Creek member 

and are conspicuous on the shale surfaces. 

The upper concretionary zone contains light-brown to grayishorange septarian concretions and is well exposed near the top of the 

hill in the SW7~ sec. 36, T. 6 S., R. 32 E. Elsewhere in the area the 

zone forms small ledges. This horizon was n1apped by Thorn (Thorn 
S'l'RATIGRAPHY 53 

and others, 1935, p. 52) as the base of the Niobrara shale, but Cobban, 

after examining the fauna he collected, placed the contact nearly 

100 feet above the zone of concretions. 

Composite section of Carlile .~hale member of Cody shale, Big Horn County,- Mont. 

[Piut.e 4, sec. 7. Measured by W. A. Cobban. Upper 95 feet measured on cast side of Bighorn valley in 

NW~~SWHSW~4 scc.I5and SEHSEH sec. )(j, 'l'. 3 S., R. a3 E. and in SWJ,~SWH sec. 26, T. 2 S., R. 3a 

E.; t.he lower 185 feet measured on cast flank of Soap Creek dome in SWH sec. 36, 1'. 6 S., R. :l2 E.] 

Cocty shale: 

Niobrara shale member: 

• Shale, dark-gray, sandy; lJcd of concretions in bentonitic shale 

at base _______________________________________________ _ 

Carlile shale member: 

Shn le, dark-gray; contains septarian concretions at base ______ _ Shale, dark-gray _________________________________________ _ 

Shale, dark-gray; contains numerous fragments of large thinshelled Inoceram:us encrusted with Ostrea congesta Conrad ___ · 

Shale, dark-gray; contains <'alcareous septarian concretions 

which weather light gray to yellow tatL __________________ _ 

Shale, dark-gray; weathers light gray; contains fragments of 

large Inoceram7tS encru.:;ted with Ostrea congesta Conrad (loc. 

20945)-------------------------------------------------

Shale, dark-gray; contains Ostrea congesta Conrad ____________ _ 

Shale, dark-gray; contains large calcareous septarian concretions which weather light gray or buff and contain thin yellow calcite veinlets ________________________________________ _ 

Shale, dark-gray; contains thin sandy partings in middle and 

few gray calcareous septarian concretions; fossils are rare; 

contains Inoceramus sp., Ostrea sp., Veniella sp ____________ _ 

Shale, dark-gray; contains distantly spaced gray calcareous 

septarian concretions at lJase; fossils scarce; contains Inoceraums sp., Pricnocyclus sp. (loc. 20944) _________________ _ 

Shale, dark-gray; gray calcareous septarian concretions in 

middle; fossils scarce; contains V en1:ella cf. V. goniophora 

Meek, Placenticeras stantoni Hyatt (loc. 20943) ___________ . 

(Underlying beds measured east of Soap Creek dome) 

Shale, dark-gray, weathers buff; large gray calcareous concretions at top weather yellow tan, and smaller septarian concretions weather light gray or buff; fossiliferous; contains 

"Corbula" n. sp., Crassatellites cf. C. reesidei Sidwell, Inoceramus altus Meek and Hayden?, Inoceramus n. sp., Veniella 

cf. V. goniophom Meek, 1'ritoniwn? cf. T. kanabense Stanton, 

BaC'ulites cf. B. beMi1·iei Collignon, Placenticeras stantoni 

Hyatt, Scaphites corvensis Cobban, ScaphitPS corvensis var. 

Pt in 

10 0 

5 6 

8 0 

3 6 

3 6 

10 6 

4 6 

5 0 

39 0 

1 6 

14 3 

bighornensis Cobban (loc. 2093!))__________________________ 5 0 

Shale, dark-gray; contains gray-weathering septarian concretions, many of which are fossiliferous; dark-gray calcareous concretions at base; shale contains M embraniporina 

sp., Inoceramus cf. I. flaccidus \Vhite, Inocemmus n. sp., 

Nncula sp., Ostrea sp., Baculites cf. B. besa·i·riei Collignon, 

Pn'onocuclu.s wyomingensis Meek, Scaph1:tes corvensis var. bi(Jhornensis Cabban (loc. 20938) __________________________ 16 6 
\ 

54 GEOLOGY OF 'l'HE BIGHORN CANYON-HARDIN AREA, MONT.-WYO. 

Composite seclwn of Carlile shale member of Cody sh[t[c, Big Horn County, Mont.-

Continued 

Cody shale-Continued 

Carlile shale member-Continued 

Shale, dark-gray; contains gray-weathering calcareous septarian 

concretions with dark-brown calcite veins; contains Inoceramus 

sp., Prionocyclus wyomingensis Meek, Scaphites nigricollensis 

var. meeki Cobban (loc. 20937)---------------------------

Shale, dark-gray; with thin sandy streaks; weathers to gray 

soft soiL ______________________________________________ . 

' Conglomerate; very light gray-weathering pebbles of clayey siltstone commonly one-half inch in length but as much as 3 inches 

in length in a matrix of dark shale; or forms a band of pebbles 

through the middle of calcareous concretions which weather 

yellowish tan __________________________________________ _ 

Shale, dark-gray; contains some streaks of sand and at least 10 

layers of very hard gray ironstones weathering bright orangerust and dark maroon-brown which average 12 to 15 inches 

by 2 to 3 inches and contain bright tan weathering claystone 

nodules; ~ontains Inoceramus sp., Scaphites sp. (loc. 20936) __ 

Shale, dark-gray: contains sandy streaks ____________________ _ 

Bentonite, gra.:r- __________________________________________ _ 

Shale, dark-gray; contains sandy streaks__ _ ________________ _ 

Eentonite, gray with base olive, nonswelling _________________ _ 

Sandstone and shale alternating in thin beds ________________ _ 

Bentonite, greenish-gray, nonswelling _______________________ _ 

Shale, dark-gray; contains very thin streaks of sand and contains Inoceramus sp., Prionocyclus sp _____________________ _ 

Ft in 

12 u 

30 0 

4 

25 0 

6 6 

() 

25 6 

3 0 

3 n 

2 6 

55 0 

Total Carlile shale member_ ___________________________ 280 10 

Greenhorn calcareous member: 

Shale, dark-gray, calcareous; weathers bluish then white; contains a few limestone concretions 5 feet above base_________ 28 0 

Niobrara shale member.-The Niobrara shale member, which is a 

little more than 400 feet thick where measured by Cobban on the 

east side of the Bighorn River south of Hardin, crops out along the 

northern and eastern edge of the area. It consists of dark-gray shale, 

many thin beds of bentonite, and several beds of septarian concretions. 

A 10-foot bed of calcareous shale about 90 feet above the base of the 

n1ember weathers to form a band of yellowish-colored soil extending 

along the east side of the Bighorn valley from near Two Leggin 

Bridge southward past St. Xavier into the Rotten Grass Creek valley. 

This same bed forms a thin band of white soil south of Beauvais 

Creek and east of the airplane beacon in sec. 30, T. 4 S., R. 31 E., 

where it looks very much like the Greenhorn calcareous member. 

As the zone can be traced throughout most of the area, it was mapped 

wherever possible. 
I 

S'l'HA'l'J GHAPHY 55 

Unless exposures arc good, the Carlile and Niobrara shale members 

arc ha.rcl to distinguish so the two members were mapped together. 

The contact of the Niobrara shn.le member with the ovcl'lying Telegraph Ct·eck member is gradational between shale in the Niobrara and 

sandy shale in the Telegraph Creek member. The contact is concealed in much of the area, but can be mapped because the sandy 

shale of the Telegraph Creek member forms yellow soil, whereas the 

Niobrara shale member forms gray soil. 

Sectwn of Niob·rara shale member of Co~y shale in bluffs on east bank of 

Bighorn· River 

[Plat.e 4, sect.ion 8. lVIeasurcd by W. A. Cobban. Meastired from pomt in E~~NE~·iSEH sec. 15, T. 2 S. 

H. :i:l g, to a point in NE~r.NEH sec. 25, '1'. I S., R. :~;{ E , I ~11le southeast of Hardin, Big Horn C?unty, Mont] 

Cody shale: 

Telegraph Creek member: 

Shale, dark-gm~', sanely. 

Niobrara shale member: 

Shale, clark-gray; contains some very thin streaks of sand and 

some gray-weathering calcarcotis concretions; few fossils; 

contains Inoceramus n. sp., Ostreq,''sp., Pteria aff. P. nebrascana 

(Evans and Shumard), Baculites codyensis, Reesidc, Clioscaphites venmjonnis (Meek ancl.Haydcn) (loc. 20953) ______ _ 

Shale, dark-gray; contains small gray-weathering calcareous concretions and four.thin bentonite beds _____________________ _ 

Bentonite, gray __________________________________________ _ 

Shale, clark-gray _________________ ' ________________________ _ 

Shale, dark-gray; contains small rusty ironstone concretions __ _ Shale, clark-gray _________________________________________ _ 

Shale, clark-gray; contains three 17~-inch beds of bentonite and 

a bed of calcareous concretions __ : ________________________ _ 

Shale, dark-gray; contains nearly a dozen very thin layers of 

bentonite, commonly with aragonite _____________________ _ 

Bentonite, gray, streaked with yellow ______________________ _ 

Shale, clark-gray; contains two very thin layers of bentonite ___ _ 

Shale, clark-gray; contains some cn'tshcd fossils ______________ _ 

Shale, cln.rk-gray, and four 1- to 3-ipch layers of bentonite ____ _ 

Shale, clark-gray; contains a fe\~' crushed fos~ils including 

lnoceTamus sp., Bacultles codyensts Reesicle, Clioscaphites 

venni.fonnis ( M cek and Hayden) '(Joe. 20952) ______________ _ 

Bentonite and aragonite, gray _____________________________ _ 

Shale, clark-gray; contains a few layers of ripple-marked sand 

in lower part __________________ ~ _______________________ _ 

Ben'tonite, gray; widely spaced calcareous concretions commonly 4 by 36 inches arc developed along the bed _________ _ 

Shale, dark-gray; contains calcareous widely spaced grayweathering concretions, commonly 6 by 18 inches at base ___ _ Shale, dark-gray _________________________________________ _ 

Bentonite, gray __________________________________________ _ 

Shale, dark-grn.y _________________________________________ _ 

Bentonite, gray, finely micaceous.--------------------------

Ft rn 

32 

18 0 

4 

2 0 

1 0 

10 7 

7 9 

31 8 

4 

13 6 

18 6 

20 7 

!) {) 

4 

H) 0 

7 3 

7 9 

9 

23 3 

2 0 
56 GEOLOGY OF THE BIGHOHN CANYON-HAHDIN AHEA, MONT.-WYO. 

Section of Niobrara shale member o.f Cody shale in bluffs on east bank of 

Bighorn River-Continued 

Cody shale-Continued 

Niobrara shale member-Continued 

Shale, dark-gray and interbedded bentonite in thin beds ______ _ 

Shale, dark-gray; contains' six thin bedR of bentonite; calcareous 

concretions that weather light gray at base contain A n01nia 

n. sp., Inoceramus sp., O._trea sp. Baculites sp. (Joe. 20951) __ 

Shale, dark-gray, slightly calcareous; contains crushed fossils; 

a zone of calcareous widely spaced concretions that weather 

light gray and are commonly 1 by 2 feet at base; shale contains 

Inoceramus sp., Ostrea n. sp., Baculites codyen/)is Reeside, 

Baculites sp., pelecypods indet. (loc. 20950) _______________ _ 

Bentonite, gray, finely micaceous __________________________ _ 

Shale, dark-gray, slightly calcareous; contains light-gray weathering calcareous septarian concretions at base ______________ _ 

Shale, dark-gray; lower part slightly calcareous; contains crushed 

fossils; contains Inoceramus sp., Ostrea conger,ta Conrad, 

Baculites sp., fish scales _________________________________ _ 

Bentonite, gray, micaceous ________________________________ _ 

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

conspicuous yellowish-cream soil supporting greasewood east 

of Bighorn River from near Hardin and south at least as far 

as Rotten Grass Creek; contains Inoceramus sp., Ostrea 

congesta Conrad, Veniella sp., Baculites sp., Scaphites sp., 

fish scales (loc. 20949) _________________________ l, ________ _ 

Shale, dark-gray, slightly calcareous; includes t\vo l-inch 

bentonite layers _____________________ ------ ____________ _ 

Shale, dark-gray, and thin beds of bentonite ________________ _ 

Shale, dark-gray; contains scattered biotite flakes ____________ _ 

Bentonite, gray to greenish-gray, finely micaceous: contains 

long light-gray, buff-weathering limestone concretions _____ _ 

Shale, dark-gray, and thin beds of bentonite; bed of light-gray 

weathering calcareous concretions as large as 1% by 2 feet 

at base; concretions contain Inoceramus de.formt's Meek, 

Ostrea n. sp., Baculites mariasensis Cobban, Baculiles sweetgrassensis Cobban, Scaphites sp., nautiloid indet., gastropod 

indct. (Joe. 20948) __________________ -- ____ - ___ -- __ - ____ _ 

Shale, dark-gray; contains sandy streaks in middle and fossiliferous calcareous concretions at base; concretions contain 

Inoceramus defonnis Meek, Inoceram'ttS n, sp., Bnculites mnriasens'tS Cobba11, echinoid spin(l~ (loc. 20947) ________________ _ 

Shale, clark-gray; contains very thin streaks of sand; at base a 15-

inch micaceous bentonite bed along which are developed lightgray calcareous buff-weathering concretions containing 

Inoceramu.c~ deform£s Meek, Scaphites impendicostatur, Cobban, 

Baculites mariar,ensis Cobban (loc. 20946) _________________ _ 

Ft in 

17 (i 

21 10 

10 0 

0 

11 0 

29 0 

6 

9 2 

7 8 

28 10 

11 0 

1 6 

12 11 

10 !) 

10 0 

Total, Niobrara shale member ________________________ 408 10 

Carlile shale member: 

Shale, dark-gray. 

' ..... 

~ 
S'L'HA'L'IGHAPHY 57 

Telegra.1Jh Creek member.-Tho Telegraph Creek member crops 

out along tho northern and eastern edges of tho area. It was first 

named tho Telegraph Crook formation by 'I' hom (1922, p. 38), who 

sLated thu.t tho fol'lnation consists of 320 feet of light-colored sandy 

shales that cr·op out in T. 2 S., Rs. 28 and 29 E. between the top of 

tho Niobrara shale and tho base of tho Virgolle sandstone member of 

tho Eagle sandstone. Tho writer and members of his party measured 

a total thickness of 785 feet of what is considered to be an equivalent 

unit, as follows: tho upper 440 feet in sec. 6, '1'. 2 S., R. 29 E., tho 

middle 140 foot along tho drainage divide in soc. 17, 'J'. 2 S., R. 29 

E., and tho lower 205 foot in tho east-central part ofT. 1 S., R. 30 E. 

Tho Telegraph Creek member is 850 feet thick where measured by 

Cobban cast of Little Bighorn River in T. 2 S., R. 34 E. 

Throughout tho vicinity of Hardin, tho Telegraph Creek member 

is sandy shale that contains a few thin beds of sandstone and calcareous concretions. 11ost of the sand occurs in laminae and thin 

sLringor·s in tho shale; some of it is calcareous. Weathered surfaces 

on tho Telegraph Ct·eok member are yellowish gray, and they contr·ast Inodoratoly with tho weathered gray shale of the Niobrara. 

However, tho con tact of tho two membot·s in tho shale bluffs along 

tho Bighorn Rivm· cast of Hardin is not discernible from a distance, 

because the basal part of the Telegraph Creek is neither sufficiently 

sandy nor sufficiently weathered to have a color different from that 

of the underlying shale. 

The Telegraph Cl'Cek men1ber was described by Reeside (Thorn 

and others, 1935, p. 54) . as containing a n1ixed Eagle and Niobrara 

fauna; nevertheless the Telegraph Creek is considered to be the basal 

part of the 11ontana gl'Oup. Cephalopods from the Telegraph Creek 

have been described and figured by Reeside (1927). 

Sect.·ion of Telegraph Creek member of Cc,dy shale extending from east bank o.f Little 

B·iohorn !liver in cente·r of sec. 2 southeastward one and a half miles across the 

87~ sec. 1, '1'. 2 S., R. 34- E., Big· Horn Co1mty, Mont. 

LPlatc 4, Section 8. Measured by W. A. Cobban] 

• Cody shale: 

Shale member equivalent to Eagle sandstone: 

Shale, weathers gray; contains rusty-colored ironstone concretions. 

Telegraph Creek member: 

I Shale, dark-gray, light-gray weathering; contains gray calcareous 

concretions; bed of calcareous yellowish-tan weathering concretions at base contains Baculites .sp., Scaphites hippocrepis Feet 

(DeKay) var. temtis Reeside (Joe. 21201)__________________ 122 

Concealed: buff-colored sandy soil; contains calcareous tan- to 

orange-weathering concretions at base; concretions contain 

Ost.rea sp., Baculites SP----------------------------------- 90 
58 GEOLOGY OF THE BIGHORN CANYON-HARDIN AREA, MONT.-WYO. 

Section of Telegraph Creek member of Cody shale extending from east bank of Litttt: 

Bighorn River in center of sec. 2 southeastward one and a half miles across the 

S% sec. 1, T. 2 S., R. 34 E., Big Horn C01mty, Mont.-Continued 

Cody shale-Continued 

Telegraph Creek member-Continued 

Sandstone and shale, interlaminated and interbedded gray. 

buff-weathering; sandstones are very fine grained, commonly Feet 111111 

ripple marked, and occur in layers less than 1 foot thick ____ . _ 96 

Sandstone and shale, interlaminated and interbedded gray, buffweathering; sandstones are very fine grained, non-re.,;istant, 

and are in layers less than one-half inch thick______________ 68 ~ 

Sandstone, gray, very fine-grained, shaly, hard, tan-weathering; 

ledge forming __________________________________________ _ 

Sandstone and shale; similar to second unit above; lower 60 feet 

exposed in river bluff______________________________________ 490 

Total Telegraph Creek member__________________________ 867 

Niobrara shale member: 

Shale, dark-gray; thin beds of bentonite; 10 feet below top arc 

calcareous concretions which weather light gray____________ 11 

Shale member equivalent to the Eagle sandstone.-The Eagle sandstone forms prominent ledges and cliffs north of U. S. Highway 87 

in T. 1 8., R. 29 E., but it gr~des eastward into shale and becomes 

inconspicuous east of the Bighorn River in T. 1 N., R. 33 E. A 

unit of shale at a corresponding stratigraphic horizon north of the 

Little Bighorn River and east of the Bighorn River contains fossils 

considered by Cobban to be typical of the Eagle fauna. This unit 

has been mapped as a distinct member of the Cody shale north of 

the Little Bighorn River, but it could not be mapped separately 

south of the Little Bighorn. 

The shale member is 375 feet thick where measured by Cobban 

in sec. 13, T. 1 N., R. 34 E. It consists of three units; 50 feet of 

dark-gray shale at the base with several beds of ironstone concretions 

_generally similar in appearance to those in the Carlile ,shale member, 

215 feet of yellowish gray-weathering silty shale with many ironstone 

co_ncretions in the middle, and 90 feet of dark-gray shale, which 

weathers brownish gray and contains some septarian and ironstone 

concretions. A 20-foot zone of bentonite and bentonitic shale at 

the top of the middle unit forms a gray band across the surface. A 
•' 

I 

STRATIGRAPHY 59 

thin sandy zone, which makes small ledges about 160 feet above the 

base, is a good horizon marker in the area north of the Little Bighorn 

River and has been shown on the geologic map. 

Section of shale member equivalent to Eag~e sandstone south of Dry Creek in the 

north-centml part of sec. 13, T. 1 S., R. 34 E., Big Horn County, 111ont. 

[Plate 4, section 8. Measured by W. A. Cobban] 

Cody shale: 

Claggett shale member: Ft in 

Bentonite, yellow_________________________________________ 1 6 

Shale member equivalent to Eagle sandstone: 

Shale, dark-gray, brownish-weathering; contains brown- and 

gray-weathering calcareous concretions; contains Inoceramus 

cf. I. barabini Morton, Inoceramus sp., Baculites aq1tilaensis 

Reeside, Bac1.lites aquilaensis var. separat1ts Reeside, Baculites 

haresi Reeside, Baculites minerensis Landes, Placenticeras sp. (loc. 21210) ___________________________________________ _ 

Shale, dark-gray; weathen to browrlish soil; contains calcareous 

rusty-, brown-, and reddish-weathering ironstone concretions_ 

Shale, dark-gray; weathers brownish gray; contains some brownweathering calcareous concretions with yellow calcite veins; 

contains Goniochasma crockfordi 'Varren, Inoceramus barabini 

:Morton, Inoceramus saskatchewanensis vVan·en, Inoceramus 

subdepressus Meek and Hayden, Inoceramus1n. ap., Pholad01nya 

atf. P. snbventricosa Meek and Hayden, Baculties sp., Placenticems planum i-Iyatt (loc. 21209) _____________________ _ 

Bentonite, yellowi~h-gray grading upward into brownish-gray; 

contains much white aragonite __________________________ _ 

Shale, dark bluish-gray, silicified ___________________________ _ 

Bentonite, creamy-yellow grading upward into brownish-gray __ 

Shale, dark bluish-gray, silicified ___________________________ _ 

Shale, gray, bentonitic ___________________________________ _ 

Bentonite, yellowish-cream grading upward into gray; forms 

gray bare gumbo soiL __________________________________ _ 

Shale, dark-gray; weathers to light-gray soil with bluish bands; 

middle part contains numerous omall calcerous concretions 

which weather .bluish gray to buff; contains Baculites aquilaensis var. separatus Reeside, Baculites haresi Reeside, Ieptilian 

bone3 (loc. 21208) _____________________________________ _ 

Shale, dark-gray; weathers lighter gray; contains very fine sandy 

partings and about 25 beds of closely spaced ironstone concretions which weather rusty to dark maroon;·contains Baculites haresi Reeside (loc. 21207). Bed of gray calcareous and 

ferruginou.s silty ledge-forming concretions at base _________ _ 

334328-55--5 

27 6 

15 0 

46 0 

3 0 

1 6 

5 0 

1 0 

5 0 

6 0 

45 0 

51 0 
60 GEOLOGY OF THE BIGHORN CANYON-HARDIN AREAJ MONT.-WYO. 

Section of shale member equivalent to Eagle sandstone south of Dry Creek in the 

north-central part of sec. 13, T. 1 S., R. 34 E., Big Horn County, J1font.-Con. 

Cody shale-Continued 

Shale member equivalent to Eagle sandstone-Continued 

Shale, gray, sandy; contains a few gray silty calcareous concretions and locally crossbedded sandstone concretions. Includes 

a bed of calcareous gray-weathering concretions that are fairly 

closely spaced and commonly septarian with thin veins of 

white calcite; some concretions carry a prolific Eagle fauna, 

Anomia sp., "Callista" cf. C. pellucida Meek and Hayden, 

"Callista" sp., Cardium (Ethmocardium) whitei Dall, Corbulamella cf. C. gregaria Meek and Hayden, Crenella cf. C. 

elengantula Meek and Hayden, Cymbophora? sp., Cymella 

montanensis Hender.5on, Inoceramus barabini Morton, Leptosolen cf. L. conradi Meek, Lima n. sp., Lithophaga n. sp., 

Pinna cf. P. dolosoniensis McLearn, Syncyclonema halli (Gabb) 

Tellina cf. T. scitula Meek and Hayden, Velsella n. sp. aff. V. 

meeki Evans and Shumard, Capulus? aff. C.? microstriatus 

Stephenson, Drepanochilus aff. D. evansi Cossman, Spironema 

aff. S. temtilineata Meek and Hayden, other gastropods indet., 

Baculites aquilaensis Reeside, B. aquilaensis var. separatus 

Reeside, Baculites haresi Reeside, B. haresi var. (corrugated 

venter), Baculites th(.mi Reeside, H amites novimexicanus 

Reeside, H elicoceras rubeyi Reeside, Placenticeras meeki Boehm, 

Scaphites aquilaensis Reeside, S. aquilaensis var. costatus 

Reeside, S. aqttilaensis var. nanus Reeside, Scaphites hippocrepis (DeKay), S. hippocrepis var: crassus Reeside, S. 

hippocrepis var. pusillus Reeside, S. hippocrepis var. tenuis 

Reeside, Scaphites stantoni Reeside, ScapMtes n. sp., Scaphites 

sp., crustacean remains, fish scales (loc. 21206) ____________ _ 

Shale, dark-gray, silty; weathers buff gray; contains a few ferruginous tan-weathering calcareous concretions ____________ _ 

Shale, dark-gray, silty; weathers light gray; contains a few 

brown-weathering calcareous ironstone concretions as long as 4 feet_____________________ _ _______________________ _ 

Bentonite, gray and yellow _____________________ -- ____ ---- _ 

Shale, dark-gray; contains ironstone concretion.L ____________ _ 

Bentonite, yellowish-cream; contains much aragonite _________ _ 

Shale, dark-gray, somewhat silty; contains many layers of ironstone concretions which weather rusty, brown, and dark 

Ft in 

6 

42 0 

72 0 

1 6 

17 0 

1 0 

maroon________________________________________________ 29 0 

Total, shale mem}Jer equivalent to Eagle sandstone __ • ____ 374 6 

•> 

Telegraph Creek member: \ 

Shale, gray, sandy; contains gray-weathering calcareous concretions especially in lower 5 feet, where they are septarian 

with orange calcite veins_________________________________ 65 0 
.i 

STRATIGRAPIIY 61 

Claggett shale member.-Tbe Claggett, topmost member of the Cody 

shale, is about 350 feet thick north of the Little Bighorn River in the 

only part of the Bighorn Canyon-Hardin area in which it was mapped 

as a separate unit. It also crops out south of the Little Bighorn 

River, but there much of it is concealed and it cannot be separated 

from the underlying shale. The Claggett consists of dark-gray shale 

that is bentonitic in the lower part and concretionary throughout. 

Two beds of gt·ayish-yellow bentonite at the base are overlain by 

nearly 70 feet of shale, which weathers to a lighter color than the 

underlying shale and which contains several thin beds of bentonite. 

I'he shale above the bentonite zone contains a nmnber of calcareous 

septarian concretions. 

Large forms of Baculites and Inoceramus occur in reddish-brown 

septarian concretions near the top of the rrwmber at some localities. 

The contact between the Claggett shale member and the overlying 

Paekman sandstone is the base of sandy shales in the Parkman. 

Section of Claugett shale member of Cody shale along drainage divide south of Sarpy 

road in NE}~ sec. 13, 'L'. 1 S., R. 34 E. and NW7~ sec. 18, 'P. 1 S., R. 35 E., Big 

Hom C otmly, il1 ont. 

[Plato 4, section 8. Measured by W. A. Cobban] 

Parkman sandstone: 

Shale, buff-weathering; transitional upward from silty shale 

into very sandy shale; contain 5 or 6 beds of silty calcareous J~'t in 

concretions that weather gray, yellowish tan, or brown _____ 109 0 

Cody shale: 

. Claggett shale member: 

Shale, dark-gray; crops out as dark soft flaky shale which contrasts strongly with overlying light sandy unit; contains fossiliferous brown-weathering calcareous septarian concretions 

with yellow calcite and white barite; base marked by yellowtan to brown-weathering calcareous concretions which arc 

quite fossiliferous, closely spaced, and septarian with thin 

pale-yellow calcite veins, commonly 10 by 18 inches to 15 

inches by 3 feet but as much as 2 by 6 feet; concretions contain 

Captinella coraloidea Hall and Meek, Inoceramus bambini var. 

injlatiformis Douglas, Inoceramus barabini var. magniumbonatus Douglas, Inoceramus cf. I. sagensis Owen, Inoceramus 

saskatchewanensis Warren, Pteria cf. P. notukeuensis Warren, 

I nocerarnus vanuxemi Meek and Hayden. "Y oldia" sp., Acanthoscaphites aff. A. brevis Meek, Baculites asperiformis Meek 

(loc. 21214)-------------------------------------------- 17 6 

Shale, clark-gray; contains a bed of calcareous septarian concretions 28 feet below top; concretions at base are calcareous, 

very closely spaced; concretions weather rusty brown, tan, 

and reddish and are septarian with thin veins of pale-yellow 

calctie_________________________________________________ 43 0 

334828-65-6 
62 GI!:OLOGY OF' THE BIGHOI{N CANYON-HAltDLN AREA, MONT.-WYO. 

Section of Claggett shale member of Cody shale along drainage divide south of Sarpy 

road in N EX sec. 1.'3, T. 1 S., R . .'34 E. and N W% sec. 18, T. 1 S., fl. 35 E, Big 

Horn County, 1\.font.-Continued 

Cody shale-Continued 

Claggett shale member--Continued 

Shale, dark-gray; contains a few brown-weathering calcareous 

concretions; septarian with yellow calcite; crops out. a.~ dark 

soft flaky shale in contmHt with undel"lying gray-weat.lwring 

shale; contains Bacnl-ites cf. B. aquilaensis Hcc~ide (loc. 21213). 

Concretions at baHe arc brown weathering, calc.a.reou.:;, very 

closely spaced, septarian with brown or white calcite Yeins; Ft tn 

contain Baculiles haresi H.eeside (Joe. 21212) _ _ _ _ _ __ __ _ _ _ _ _ _ _ _ 41 6 

Shale, dark-gray, gray-weathering; forms grass covered slopes; 

contains at least 8 beds of brown-weathering calcareous concretions that are septarian with yellow or brown calcite veim;_ 156 0 

Shale, dark-gray; weathers to gray bare gumbo soil; contains a 

few small gray calcareow; concretions; contain:-; Raculiles 

aquilaensis var. separatus Reesidc, Bac·ttlile.s haresi Reesidc 

(loc. 21211) _________________ .. _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ 40 0 

Bentonite and concretions. Two beds of gray calcareous concretions, weathering light gray and brown, separated by 

about 3 feet of gray bentonite and bentonitic shale. Concretions are very closely spaced and somewhat septarian 

with veins of pale-yellow to brown calcite and, rarely, bluish 

chalcedony ___________________________________________ _ 

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

with gumbo bands _____________________________________ _ 

Bentonite, yellow ________________________________________ _ 

Shale, dark-gray, bentonitic; weathers lighter gray ___________ _ 

Bentonite, yellow; forms gray bare gumbo ___________________ _ 

Shale, dark-gray; contains gray-weathering calareous concre- tions at base ___________________ .. ______________________ _ 

Bentonite, yellow____ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ______________ _ 

4 6 

30 0 

3 0 

24 6 

3 0 

3 0 

1 6 

---- Total, Claggett shale member_ _______________________ 367 6 

----

Shale member equivalent to Eagle sand.stone: 

Shale, dark-gray, brownish-weathering; contains gray- and 

brown-weathering calcareous concretions__________________ 27 6 

•J 
I 

~'L'BA'rlt:HAPI-l Y 

PARKMAN SANDSTONE 

'.I.' he Pnl'knutn sandstone consists of nbou L 250 feeL of sandy shale 

nncl snndstone. The only completely exposed section in the area is 

north of the Little Bighorn River, where it is made up of n lower 110-

foot unit of shnle (which is silty in the lower part and sandy in the 

upper part) nno an upper 140-foot unit of massive grayish-yellow to 

olive. snndstonc (which contnins many hard limonitic nodules). 

Dm't.on (190Qu,, p. 58-59) nnmcd the Pn,rkman snndstone after the 

t,own of Parkmnn, Sheridnn County, \.Yyo., whct·e he found about 

~50 feet of buff-eolored massive nonresistant sandstone with hard 

dark concretionary portions. '.l'he base of the formation is sharply 

delineated in some areas and tra11sitional in others. Fossils collected fr01n the Pal'kman by Darton were identified by T. W. Stanton 

as being n1arinc fonns of Late Ct·etaecous age. 

Thorn ('.L'hmn and others, 19;35, p. 59) states that in the arcn of 

Pine Ridge, in '.L'. 1 N., Rs. ;n and :)2 E. northwest of Hardin, the 

beds above the basal sandstone of the Parkman resemble the Judith 

River formation and arc a continuation of the fresh-water and 

brackish-water beds in U10 J uclith River formation that occur in the 

Huntley field (Hancock, 1920, p. 1 21-124). 

BEARP A W SHALE 

Overlying the massive sandstone of the Parkman sandstone is 

about 850 feet of dark-gray marine shale which const,itutcs the Bearpaw shale. Cobban has Clividccl the formation into three members, 

a lower 300 feet of dark-gray concretionary shale, a middle 250 feet 

of dark-gray concretionary shale with numerous beds of bentonite, 

and an upper 300 feet of dark-gray concretionary shale which is sandy 

in the upper few feet. 'l'he formation is exposed only in the Ninemile area where it forms saddles and small valleys between ridges of 

Parkman sandstone and pine covered hills of Lhc Hell Creek formation. 

The Bearpaw is the uppermost formation of Lhe lV[ontana group in 

the Hardin area and is, according to Thorn (Thorn and others, 1935, 

p. 61) apparently conformable with the overlying Hell Creek formation, the base of which is largely greenish-gray shale, siltstone, and 

sandstone that supports a growth of pine trees in the Ninemile area. 
64 GEOLOGY OF THE BIGHORN CANYON-HARDIN AREA, MONT.-WYO. 

Section of Bearpaw shale along the Sarpy road between the center of the SW7~ sec. 8 

and the SW7~ sec. 10, T. 1 S., R. 35 E, Big Horn County, ~font. 

[Plate 4, section 8. Measured by W. A. Cobban] 

Hell Creek formation: Sandstone at base. 

Bearpaw shale: 

Upper member: 

Shale, gray, sandy; interlaminated very fine grained soft sand- Ft in -fl 

stone__________________________________________________ 10 0 

Shale, dark-gray; bed of dark-brown weathering calcareous concretions at base contains N'ucula planimarginata Meek and 

Hayden (Ioc. 21228)------------------------------------ 26 0 ~ 

Shale, dark-gray; fossiliferous dark-brown weathering calcareous 

concretions at base; contains Protocardia subquadrata Evans 

and Shumard, "Yoldia" evansi Meek and Hayden, Baculites 

grandis Hall and Meed (loc. 21227) _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 9 0 

Shale, dark-gray; dark-brown weathering calcareous concretions 

developed along greenish-gray bentonite at base__________ 21 8 

Shale, dark-gray__________________________________________ 25 0 

Shale, dark-gray, largely concealed; exposed along Bighorn 

River bluffs near old Nine Mile bridge where the following 

fossils were collected: Lucina sp., Acanthoscaphites nodosus 

(Owen), Baculites compressus Say, Placenticeras meeki Boehm, 

Placenticeras planum Hyatt (Ioc. 21363). A bed of small 

gray-weathering hard, nodular calcareous concretions and 

large cavernous masses of "tepee butte" limestone containing 

abundant Lucina occidentalis (Morton) at base; concretions 

and limestone contain Chlamys nebrascensis Meek and Hayden, 

Inoceramus bambini var. injlatiformis Douglas, Inoceramus 

sp., Lucina occidentalis (Morton), Lucina subundata Hall and 

Meek, Ostrea sp., Pteria linguaeformais (Evans and Shumard), 

Polinices aff. P. concinna (Hall and Meek), Baculites compressus Say, Discoscaphites aff. D. nicolletii (Morton) (loc. 21226) ________________________________________________ 155 0 

Shale, dark-gray; a few calcareous concretions______________ 15 0 

Shale, dark-gray; contains calcareous, more or less ferruginous, 

concretions; contains Baculites compressu,s Say____________ 35 0 

Shale, dark-gray; contains calcareous concretions and Cymbophora cf. C. gracilis (Meek and Hayden), Inoceramus 

altus Meek, Baculites compressus Say, Placenticeras meeki 

Boehm (Ioc. 21225) ____________________________________ _ 

Bentonitic member: 

Shale, dark-gray, poorly exposed; contains a few thin beds of 

olive-green bentonite and some calcareous concretions _____ _ Bentonite, olive-green ____________________________________ _ 

Shale, dark-gray---- _____________________________________ _ 

Bentonite, greenish-gray __________________________________ _ 

Shale, dark-gray; a few calcareous concretions; a bed of calcareous concretions at base weathers brown or gray; some con- cretions arc fossiliferous _______________________________ .. 

18 0 

2R 0 

1 () 

6 6 

1 0 

3 G 

•J 

lj 
I 

STRATIGRAPHY 65 

Section of Bem·paw shale along the Sarpy road between the center of the S W}~ sec. 8 

and the SW}~ sec. 10, T. 1 S., R. 35 E., Big Horn County, Mont.-Con. 

Beu.rpu.w shale-Continued 

Bentonitic member-Continued 

Shale, dark-gray; contains two thin beda of bentonite; calcareous 

widely spaced concretions at base weather bright brown; 

smaller grayish-brown weathering calcg,reous concretions are 

highly fossiliferous and contain Cttspidaria aff. C. moreauensis 

(Meek and Hayden), Cttspidaria ventr-icosa (Meek and Hayden), Cymbophora gracilis (Meek and Hayden), Cymella 

meeki Whitfield, Gervillia 1·ecla Meek and Hayden, Inoceramus 

tenuilinealtts Hall and Meek, Inoceramus sp., Lucina sub-

'l.tndala Hall and Meek, Ostrea cf. C. subalala Meek, Pteria 

cf. P. parkensis White, Syncyclonema halli (Gabb), "Yoldia" 

cvansi Meek and Hayden, "Yoldia" aff. Y. ventricosa Hall 

and Meek, A cmaea? occidenlalis (Hall and Meek), Drepanochil11.s evansi Cossmann, Drepanochilus neb:ascensis (Evans 

and Shumard), /t'asciolaria aff. F. gracilenta Meek, Ellipsoscapha occidenlalis (Meek and Hayden), Ellipsoscapha s·u.bcylindn:ca (Meek and Hayden), Polinices aff. P. concinna 

Hall and Meek, Dentalium pattperculum, Meek and Hayden, 

Acanthoscaphites brevis (Meek), A canlhoscaphiles nodosus 

(Owen), Acanthoscaphites quadrangularis (Meek and Hayden), 

Bac1tlites compressus Say, Placenticeras intercalare Meek, 

P lacenticeras meeki Boehm (loc. 21224) _ _ _ _ _ _ _ _ _ _ _ _____ _ 

Bentonite, cream-colored _________________________________ _ 

Shale, dark-gray _________________________________________ _ 

Bentonite, olive-yellow to gray ____________________________ _ 

Shale, dark-gray--------- ________________________________ _ 

Bentonite, olive-green to grayish-green; bed of calcareous 

brown and gray weathering closely spaced concretions at 

base; contains Inoceramus vanuxem,i .Meek and Hayden, 

Acanthoscaphiles nodostts (Owen), Baculiles c01npressus Say (loc. 21223) _____________________________ .-,. _. ___________ _ 

Shale, dark-gray; contains calcareous concretions in middle ___ _ Bentonite, olive-green ________________________________ .:: ___ _ 

Shale, dark-gray _________________________________________ _ 

Bentonite, olive-green ____________________________________ _ 

Shale, dark-gray; contain8 calcareous concretions near middle; 

concretions contain Cymbophora gracilis (Meek and Hayden) 

(loc. 21222) ___________________________________________ _ 

Bentonite, yellow, grades 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 concretions; forms conspicuous lightgray bare gumbo soiL __________________________________ _ 

Shale, dark-gray; contains gray sandy calcareous concretions 

with silicified pelecypods; concretions contain Cymbophora 

gracilis (Meek and Hayden), Inoceramus sp. (loc. 21221) ___ _ 

Pt In 

5 6 

1 0 

10 0 

1 6 

2 6 

2 6 

10 9 

6 

3 0 

6 

12 6 

1 6 

2 6 

1 0 

2 0 

5 0 

6 
()() GIWLO(;Y Of' 'J'HI·~ BWHOI{N CANYON-HAHDIN ArU:A, MON'l'.-'VYO. 

Section of Bearpaw shale along the Sarpy road between the center of the S W.X sec. 8 

and the SW.X sec. 10, T. 1 S., R. 85 J!7., R1:g Horn County, Mnnt.-Con. 

Bearpaw shalu-Co11Lir111<!d 

Bc11t.onitie rnombor·-- Cont.rrltiCd Ft In 

Bent.onite, light.-grcl)ll, grading upward into dark-green _______ _ 

Shale, dark-gray, bento11itic; lo\\'er !) feet sandy; contains bed 

of calcareous, sandy, ferrtrginou:-5 closley spaced concretions 

that weather dark brown and purplish at base; concrei,ions 

contain Cymbophora gracilis (Meek and Hayden), Inoceramus 

sp., Baculites compressus Say, Placent~ceras sp. (loc. 21220) __ 

Shale, dark-gray, somewhat sandy, nonresistant; contains 

calcareous concretions 8 feet above base; concretions contain 

Lucina sp., Acanthoscaphites sp., Baculites compressus Say, 

Placenticeras sp. (loc. 21219) _____________________________ _ 

Shale, gray, bentonitic; eontains a few gray ealcareous eon- cretions _______________________________________________ _ 

Shale, gray, very bentouitic; weathers to gray gumbo soiL ____ _ 

Shale, gray, bentonitic, poorly exposed; interbedded with 

darker colored fissile shale ______________________________ _ 

Bentonite, olive-green _____ . ____________________________ .. __ _ 

2 0 

23 0 

20 0 

35 0 

13 0 

42 0 

3 0 

Lower member: 

Shale, dark-gray; contains brown- and gray-weathering septarian calcareous concretions 8 feet and 22 feet below top _______ _ 

Shale, dark-gray; contains several beds of calcareous brownweathering ferruginous concretions. ______________________ _ 

Shale, dark-gray; contains a few calcareous concretions; contains Inoceramus sp., Placenticeras m.eeki Boehm (loc. 

21218) includes bed of calcareous brown-weatheriug coucretions that are septarian with yellow calcite veins, com53 0 

20 0 

monly 17~ by 3 feet but as large as 2 by 10 feet at base________ 46 6 

Shale, dark-gray; contains a few brown-weathering calcareous 

concretions _____________________________________________ 63 0 

Shale, dark-gray; contains many brown-weathering calcareous 

concretions more or less septarian with yellow calcite veins: 

contai11s Inoceramus cf. I. palliseri Douglas, Inoceramus 

sagensis Owen?, Inoceramus saskatchewanensis vVarren, lnoceramns tenuilineatus Hall and Meek, Ostrea .sp., "Yoldia" sp. 

Baculites compressus Say (loc. 21217)______________________ 37 0 

Shale, dark-gray; contains a few small gray calcareous concretions; contains Inoceramus sp., L·ucina sp. (loc. 21216) _ _ _ 20 0 

Shale, dark-gray; contains some bentonitic beds and a few 

calcareous concretions; at base a bed of calcareous closely 

spaced concretious that weather buff and brown; concretions 

contain Inoceramus bambini var. injlatifcrmis Douglas, 

Emperoceras simplicostatum, (Whitfield) (loc. 21215)________ 54- 6 

Shale, dark-gray _____________ .. __________________ .. _________ 15 0 

Total, Bearpaw shale __________________________________ 865 5 

Parkman sandstone: Sandstone, buff, brownish, and olive-green; forms 

hills. 

<i}l 

~· 
S'J'H.UCTUR FJ 67 

TERTIARY(P) AND QUATERNARY SYSTEMS 

'l'BB.RACB OHAVJ<JLS 

.l~xLcnsivc gravel dcposit,s of Qunt.ernary and m part, perhaps, of 

'l'crtin,t·y age lie adjacent t,o the larger sLrcarns cast of the Bighorn 

?vfounLnins. These tcrrncc deposits, which occur as high as 650 feet 

above Lhc Bighorn River, nrc about 25 feet thick and contain a variety 

of rock maLcrial derived fr01n the nearby Bighorn l\1ountains and fr01n 

Lhe Absaroka l\tfountains on the west side of the Bighorn Basin. A 

description of the gravel deposits and the correlation of surfaces is 

discussed under "Geomorphology" on pages 78-82. 

QUATERNARY SYSTEM 

LANI)SLIDI<J MA'l'J<JRIAL 

Several smnll landslides, each as much as one-fourth square mile 

in area, occur ncar Lhe head of Soap Creek and along the high divide 

mtsL of Rotten Grass Creek. Landslides, some of very recent date, 

rcsulL fron1 hcadward erosion by small streams into shale that is 

covered by protective layers of tenace gravel. The slides always 

occur in the Cody shale and take place irrespective of the attitude of 

Lhc bedding. Slides conLinue to develop in tLn area until the proLccLivc covering of gravel on the stream divides is destroyed and 

ovct·sLcepcning is no longer possible. 

ALLUV.lUM 

Alluvium on .the flood plains which border each stream consists of 

silt, sand, and grnvel. In Lhc Bighorn River valley the alluvium 

shown on the geologic 1nap includes only the present clay flood plain, 

but in all other valleys Lhe alluvium includes the flood plain, and 

Lhc lowest terrace level, which generally is 5 Lo 20 feet above the 

flood plaii1. Alluvial flats in the valleys of Bighorn River, Little 

Bighorn River, Rotten Grass Creek, and Soap Creek arc occupied 

by irrigated farms. 

GENERAL FEATURES 

'I'he area covct·ecl by this repot·t includes the northern end of the 

1 Bighorn l\tfountains-an anticlinal uplift which exposes rock formaLions of Paleozoic age-and the adjacent foothills and plains, on which 

nrc exposed l\tfesozoic sLrata that clip at low degrees away frorn the 

n1ountains. Geologic structures in the area arc simple folds that are 

little affected by faults, and the chief visible difference between the 

north end of the Bighorn l\1Iountain anticline and the anticlines in 

Lhe n.djaecnL plains area is in size. 
FIGURE 5.-Aerial view of Bighorn Canyon. The view is southwestward toward the Pryor :'viountains which make the distant skyline. 

'fhe light-colored cliff near the rim of the canyon is the uppermost member of the Madison limestone, and the slopes above it are on the Amsden 

formation. Trees in the left center grow on a dip slope of the Tensleep sandstone. A hogback on the Pryor conglomerate member of the Cloverly 

formation is visible in left foreground . 

.,. 

~ 

00 

0 

M 

0 

~ 

0 

0 

-< 

0 

!-,:j 

~ t:':: 

ttl 

H 

a 

~ 

0 

::0 

z 

() 

> 

~ 

~ 

0 

z 

I 

::0 

> ::0 

0 

z 

> :::0 

t:':: 

> 

~ 0 

z 

;3 

~ ~ 

0 
STRUC'l'URE 69 

The strucLurnl relief between the vicinity of I-Iardin and the crest 

of the Bighorn lV[ountnin anticline, where the anticline is breached 

by Bighorn Canyon, is about 6,000 feet. Between Hardin and Soap 

CL'cck dome, which is the highest structural point east of the .mountains, the strueturul relief is a.bou t 2,000 feeL. 

~ BIGHORN MOUNTAINS 

The northern end of the Bighorn Mountains is a northward plunging anticline with a rathel' broad top nnd steep limbs. These features 

are seen in figure 5. The crest is ncar the western lilnb, and surface 

strata, whieh arc the uppern1ost beds of the Madison limestone and 

Ute lower beds of the Atnsden fonnation, dip about 5° eastward across 

Lhc top of the fold and then steepen to 40° on the eastern limb. An 

eastward view down Bighorn Canyon and a view of the western limb 

of the lnotmtains are shown in figures 6 and 7. West ward from the 

ct·est, the sn.tnc strata dip at a low angle across the top of the fold to 

the western lirnb where they st)eepcn abruptly to about 50° and 

flu.Ltcn just as abruptly 1 ,500 feet lower. 

The area between the Pryor and Bighorn Mountains is a structural 

and topographic basin, elliptical in outline and trending northeastwat·d. There arc two geographic divisions, Garvin Basin south of 

the Bighorn River and the Dry Head Creek'area north of the river. 

Garvin Basin is bordered on the east and south by the Bighorn 

11ountains and on the west by Porcupine Creek anticline, which is 

a northward-plunging fold that forms the northernmost part of the 

western flank of the Bighorn 11ountains. Porcupine Creek anticline dies out just north of the Bighorn River at faults which mark 

the southern end of the PL'yor 11ountains. The Dry Head Creek 

u.rcn. is bordered on the west by the Pryor 11ountains, on the east by 

the northernmost tip of Lhc Bighorns, and on the north by a ridge of 

Triassic and J urn.ssic rock that connects the Pryor and Bighorn 

1vlountains. Although the Garvin Basin-Dry Head Creek area is 

strucLurn.lly rnuch lower thn.n the surrounding mountains, the top of 

the 11Iu.dison limestone is n.bout 4,700 feet higher than it is in the subsurface at Hn.rdin. The Pennsylvanian Amsden forma.tion forms 

most of the surfn.cc in the Garvin Basin-Dry Head area. 

Fort Smith n.nticline, which is centered in sees. 3 and 11, T. 7 S., 

tr R. 31 E., occupies a unique position on the cast flank of the Bighorn 

Mountn.in anticline. Its eastern limb coincides with the eastern 

limb of the Bighorn Mountains, but it is separated from the broad 

Bighorn 11ountain anticline to the west by a northward-plunging 

syncline. The Fort Smith anticline is unfaulted and has more than 

200 feet of structural closure. -The resistant Tensleep sandstone 

and the uppermost part of the Amdsden formation are at the surface 
:FrGrRE 6.-View eastward down Bighorn Canyon from near tlw crest of the Bighorn !-.fountains. Fort Smith anticline forms the skyline at the right. Photograph 

by G. E. Prichard. 

-1 

0 

c 

h 

0 

1---1 

b 

c 

;.<: 

0 

'=: 

~ 

,..... 

~ 

~ 

to 

,.....; 

0 

~ 

L 

u 

> 

z 

~ 

0 

~ 

i 

~ 

> 

~ 

b 

H 

z 

> 

~ 

trj 

> 

~ 0 

z 

1-3 

·, _ 

~ 

~ 
71 

, 
72 GEOLOGY OF THE BIGHORN CANYON-HARDIN AREA, MONT.-WYO. 

and thus the topographic form of the fold reflects closely the structural 

form. 

Triassic and Jurassic formations near the eastern limb of the Bighorn mountains dip even more steeply eastward than does the 

Pennsylvanian Tensleep sandstone on the mountain flank. The dip 

of strata in the Ellis group south of the mouth of Bighorn Canyon is 

as much as 70° eastward, but within a mile to the east the dip has 

flattened to 10° or less in the shales of Late Cretaceous age. Throughout the plains portion of the Bighorn Canyon-Hardin area these 

shales dip about 2° away from the mountains, except locally where 

they are folded into anticlines and domes. The relation between the 

folds of the plains area and the axis of the Bighorn lV[ountains is shown 

by plate 6. 

ANTICLINES AND DOMES 

Soap Creek anticline.-Soap Creek anticline extends from Rotten 

Grass Creek in the southern part of 'l'. 7 S., R. 33 E. northwestward 

to the Bighorn River in sec. 25, 'l'. 5 S., R. 31 E., north of which it is 

concealed by extensive deposits of terrace gravel. Rotten Grass 

dome is near the south end of the anticline and Soap Creek dome 

near the middle. 

Soap Creek dome.-Soap Creek dome, which is the only oil-producing 

structure in the area, was first described by Thorn and Moulton 

(1921). The d01ne is elongate and the axis trends northward across 

sec. 34, T. 6 S., R. 32 E. The 1\1orrison formation is exposed along 

Soap Creek at the highest structural point on the dome, which has 

more than 500 feet of structural closure at the surface. Formations 

above the Morrison are well exposed on the steeply dipping east flank 

of the dome, but they are concealed beneath terrace deposits on the 

gently dipping west flank. The history of drilling and of oil production in the Soap Creek field is discussed under "Economic Geology" 

on pages 82-83. 

Rotten Grass dome.-Rotten Grass dome, in the SE~~ sec. 29, 

T. 7 S., R. 33 E., has a structural closure at the surface of less than 

200 feet. Rocks exposed in and around the dome range from the top 

of the Mowry shale to the Greenhorn calcareous member of the Cod~ 

shale. The asymmetry of the dome is opposite to that of Soap 

Creek dome, and beds which dip less than 5° on the east side of the 

dome dip as much as 25° on the west side. The Western States Oil i 

and Land Co's. well No. 1, in the SWXSW~~ sec. 28, T. 7. S., R. 33 

E., was drilled in 1921 to the Morrison formation and abandoned as 

a dry hole. It encountered water in the Cloverly fonnation. 

Reed dome.-Reed dome is in the SW~~ sec. 35, T. 7 S., R. 32 E. and 

the NWX sec. 2, T. 8 S., R. 32 E. It is west of Rotten Grass dome and 

is at the eastern edge of the Bighorn Mount.ains. Rocks from the 
STRUCTURE 73 

Chugwater formation to the Morrison formation are exposed in the 

uplift, and although the clip of beds on the flanks is steep, the structural closure is less than 200 feet at the surface. A well described 

only as being in sec. 2, T. 7 S., R. 32 E., was drilled to a depth of 

100 feet and abandoned as a clry'hole. 

liVoody Greek dome.-Woody Creek dome is centered in theN}~ sec. 

33, T. 3 S., R. 31 E., south of Woody Creek. The dome is cut by a 

northeast-trending fault and has about 200 feet of closure on the southcast side and a little more than 100 feet of closure on the northwes~ 

side of the fault. Mowry shale is exposed along the fault, and the 

Frontier for1nation and the lower part of the Cody shale crop out 

around the dome. The axis of the anticline on which the dome is 

located continues southeastward and disappears under extensive 

terraces near the Bighorn River. 'I'his axis, however, is not in line 

with that of the Soap Creek anticline, the north end of which is also 

concealed beneath the terraces. A northward continuation of the 

axis trom the W oocly Creek dome forms the 'rwo Leggin uplift. Wells 

which have been drilled at Woody Creek are described on page 83. 

Beauvais Creek uplijt.-'l.'he Beauvais Creek uplift, which is north of 

Grapevine dmne, represents the northernmost recognized structural 

feature of the northward-plunging Bighorn Mountain anticline. It 

is traversed by the eastward-flowing Beauvais Creek and outlined on 

the northern and eastern sides by Mowry shale hogbacks. Reversals 

of the general northward dip in the area have made several small 

domes. Of these, Point Creek don1e, in NW}~ sec. 29, T. 4 S., R. 30 

1~., has structural closure of less than 100 feet. Another small dome, 

south of Point Creek, also has less than l 00 feet of surface closure. 

Several wells drilled in T. 4 S., Rs. 29-30 E. encountered water in the 

Tcnslcep sandstone and were abandoned as dry holes. These are 

discussed under "Economic Geology" on pages 83 and 86. 

[-Jardin gas field.-The Hardin gas field, in and around the town of 

Hm·din, yields dry gas from sandy shale at an average depth of about 

725 feet in what has been called the Frontier fonnation. The sandy 

shale, however, probably is somewhat higher stratigraphically than the 

top of the Frontier formation as mapped in this report. Elevations 

on the top of the producing zone in the wells indicate a low northward 

dip, similar to dips observed in shale outcrops east and west of Hardin. 

The surface formation, the Cody shale, is concealed by terrace gravels 

at most places other than in river bluffs, but no faults that might have 

caused a trap for the gas were found in or near Hardin. The reservoir 

probably is a stratigraphic trap caused by sandy shale changing to 

shale or pinching out in the updip, southerly, direction. 

Ninemile and l-Ianlin areas .-'rhe Ninmnile area lies along Dry 

Creek and Nine 11ile Coulee north of the Little Bighorn River and 
74 G~OLOGY OF TFIF: BfGf-IOHN CANYON-HAHDlN AHEA, MON'I'.-WYO. 

cast of the Bighorn Rivet·. The steeply dipping faulted Parlnnan 

sandstone extends from the Little Bighorn River northward along 

the eastern edge of sec. 12, 'I'. 1 S., R. 34 E. to sec. 35, '1'. l N., R. 34 

E. and thence westward to the Bighorn River in sec. 23, T. 1 N., R. 

33 E.; it mttrks in general the northern and eastern boundary of the 

Ninemile area. The Nincmile area lies mainly 011 a poorly de:fine.d 

anticlinal nose that plunges northward cast of Hardin. North of the 

Little Bighorn River Llwrc are no anLielincs with surfaee elosurc of 

100 feet or more. 

The Hardin gas field is centered in the town of Hanlin, about 6 

miles south of the mouth of Nine lVIile Coulee. The area surrounding 

Hardin, including the gas field, is calleclloosly the Hardin area, and 

there is no well-defined border between the Ninemilc and Hardin 

areas. The 11arcus Snyder field was mtmcd upon the discovery of 

oil during 1952 in the G. J. Greer No.2 Kendrick well just cast of the 

Parkman sandstone in the NE}~NW?~NW% sec. 6, T. 1 S., R. 35 E. 

No distinct boundary separates this field from the Ninemile and 

Hardin areas. An area of about 6 or 7 square miles within the 1,800-

foot structure contour lies about 5 miles southeast of Hardin and 

along the Little Bighorn River. This small anticlinal structure is 

on the broad anticlinal nose that strikes northward from the Little 

Bighorn River in the south-central part of T. 1 S., R. 34 E. through 

the Ninemile area, and southwestward from the Little Bighorn River 

toward Soap Creek anticline. 

The Parkman sandstone around the northern and eastern flanks 

of the Ninemile area dips locally as much as 20° northward and 

eastward. Steeper dips are common along faults. These steep 

dips persist along the outcrop of the Parkman sandstone just east 

of the mapped area. Westward dips in the Cody shale are low along 

the western flank of the anticlinal nose that plunges northward through 

the Hardin and Ninemile areas, commonly less than 3°. There are 

no closed anticlines in the Ninemile area and only one small closed 

anticline in the Hardin area along the Little Bighorn River. Northward thinning of the Chugwater formation may increase the closure 

on the top of the Tensleep sandstone or l\1adison limestone in the 

Little Bighorn River area. This thinning is at a rate of about 10 

feet per mile from the latitude of Bighorn Canyon northward to the 

H. L. Hunt No. 1 Kendrick Cattle Co. well in the SE?~SE}~ sec. 24, 

T. 1 N., R. 34 E., Big Horn County, Mont. 

FAULTS 

The faults in the Bighorn Canyon-Hardin area are simple and have 

small displacement except for two fault zones, the Lake BasinHuntley zone northeast of Hardin and the Sykes Spring zone at the 
S'l'HlJC'I'{J RI~ 7.5 

south end of the Pryor i\tlountains. [n the plains arm1. north and 

northeast of the Bighot·n ~!fountains, isolated small faults were found 

only because they at·c visible on aerial photos. 

Sykes Spn:nu fault zone.-The Sykes Spring fault z.one, named by 

Bhwkstonc (1940, p. 60:3) for the springs which rise along the faults, 

extends southward from East Pryor ~'lountain, through sees. 27 and 

:34, T. 9 S., R. 28 E., Carbon County, ~'font., and sees. 23 and 26, 

'1'. 58 N., R. 95 W., Big Honl County, 'fVyo., to Sykes ~'lountain south 

of Crooked Creek. The zone is about 5 Tniles long and not over onequarter mile wide and consists of two or tht·ec northward-striking 

faults with associated shorter in echelon faults. The Sykes Spring 

ftuilts transect rocks from Lhc [adison limestone in East Pryor 

l\1[ountain to the Cloverly formation in Sykes [ountn.in. 

Bh1.ckstonc ( l940, p. 612), describing the Sykes Spring fn.ults, 

stn.tes that they mn.y be controlled by a zone of struetural wen.kness 

in the basmncnt rod~s that is eommon to the Sykes Spring faults 

and the Dry Hm1.d fault whieh lies along the cast flank of East Pryor 

Iv[ountain n.nd north of the Sykes Spring fault zone. The Sykes 

Spring faults forn1cd as pat·t of the Pryor ~'fountain struetures, but 

their origin seems to have been associated with, or is in part a result of, 

the fonning of Porcupine Creek anticline which is an unfaulted northwestward-plunging extension of the western flank of the Bighorn 

1ountains. Porcupine Creek anticline abuts the Pryor ~'[ountains 

opposite the 1nouth of Poreupine Creek. 

Bighon~ Canyon area.-A nem·-vertical fault at least 6 miles long 

strikes eastward across Bighorn Canyon tlu·ough 'l'. 8 S., Rs. 28 and 

29 E. 'I'hc fault, which has a downthrow of about 225 feet on the 

south where it crosses Bighorn Canyon, is shown in figure 8. This 

is the only fault that was observed to cross Bighorn Canyon. Bedding 

plane faults may occur in steeply dipping beds of the west flank of the 

Bighorns along the eastern edge of T. 7 S., R. 29 E. 

GTapevine Creek m·ea.-An easterly trending fault, in sees. 24 and 

25, T. 5 S., R. 29 E. and sees. 20 and 30, T. 5 S., R. 30 E., cuts the 

north and northen.st sides of Grapevine dome. Chugwater strata 

that dip about 30 degrees eastward arc offset 1,100 feet to the east 

on the south side of the fault, and the Chugwater formation is thinned 

consiclcrn.bly on the north side of the dome. A short fault trends 

northward across the outcrop of the basal sandstone of the Cloverly 

fonnation in the southeastern part of sec. 6 to the basal part of the 

Chugwater formation in sec. 7, T. 6 S., R. 31 E., a short distance 

north of the 1nouth of Bighorn Canyon. Both of these faults in the 

Grapevine Creek area are near lines of change in strike and dip of the 

strata on the edge of the Bighorn ~!fountains, and they undoubtedly 
76 GEOLOGY OF 'l'HE RIG HORN CANYON···-HARDIN ARRA, MON'r.-WYO. 

· .. ..,. 

FIGUitE 8 ... --Fault in Bighorn Canyon. View eastward from near mouth of Camp Creek in T. 8 8., R. 29 E . 

showing a nearly vertical fault that brings the lower part of the Amsden formation on tho right opposite 

the uppermost part of tbe Madison limestone. The west flank of the Bighorn Mountains appears in the 

background. 

were forn1rd during the deformation that CTC't1tNl the 1nountain 

structure. 

Woody Creek dome. -A IH'ar-vertical faull rross<'s ·\Voody ( 'r<'<'k 

dome, trending from see. ~;), T. :3 S., R. :r1 E. into s<'C. I 1. This 

fault dies out in a very short distance in the Cody shale' south of th<· 

dome and has a maximum vertical displacement of about 100 ft><'L. 

A similar fault in sees. 3 and 9, west of the anticlinal axis extending 

northward from Woody Creek donw, has prominent surfae<' Pxpression, as on the north side of Woody Creek valley it displacrs the 

white-weathering Greenhorn calcareous shale member of the Cody 

shale nearly 100 feet. Several other s1nal!P.r faults on the north side 

of the valley arc en echelon to the \Voody Creek dome fault, and they 

occur in a belt parallel to the axis of the northward-plunging Two 

Leggin uplift. Structural closure along the faults is less than 100 f(•et. 

Ninemile area.- Ten faults ranging from one-half mile to 2% miles 

in length cross the Parkman sandstone in sees. 23, 24, and 25, T. 1 ~-, 

R. 33 E. and the southwestern part ofT. 1 N., R. 34 E. These faults 

generally trend perpendicular to the strike of the Parkman sandstone, 

and the faults die out shortly after entering the shales that overlie 

and underlie the sandstone. The maximum horizontal displacement 

of the Parkman sandstone is about 3,000 feet. These faults are at 

the eastern encl of the Lake Basin-Huntley fault zone, a narrow belt 

of northeastward-trending fnults extending for about 100 miles north-
... 

..... 

STRUCTURE 77 

westward through the town of Huntley and north of Billings to the 

Ln.ke Basin area in Golden Valley County (IIancock, 1918, 1920). 

As the trend of most of the faults in the Ninemile area is perpendicular to, rather than parallel to, the strike of the formations, there 

is little or no structural closure along the faults. There seems to be 

a definite association of these faults with the Parkman sandstone, 

and it seems probable that the faults follow the Parkman sandstone 

downdip and continue to die out in the shale above and below the 

sandstone. If the deforming forces broke the sandstone beds and 

deformed the shale plastically, other sandstones-such as the Pennsylvanian 'I'ensleep sandstone-which are stratigraphically lower 

than the Parknut.n may be cut by an entirely different set of faults 

tlu1n is seen in the surface fonnations. 

AGE OF DEFORMATION 

All fonnations in the Bighorn Canyon-Hardin area are older than 

the Laramide mountain-making deformation of the region, so the 

deformational history must be detennined from other parts of the 

Bighorn ~1ountains and the adjacent Bighorn Basin. Ample evidence in the Bighorn Basin (Hewett, 1926, p. 68; Rogers and others, 

1948) shows that slight folding preceded deposition of the oldest 

~rertiary strata, which belong to the Paleocene Fort Union formation, 

and that 1nore intense folding preceded deposiLion of the next younger 

strata of the Willwood (Wasatch) formation. 

Along the eastern edge of the Bighorn Mountains, the Jeformation continued well into Eocene tin1e. In Sheridan and Johnson 

Countids, Wyo., the IGngsbury conglomerate (Darton, 1906a, p. 60-

62; Brown, 1948. See bibliography of latter paper) is separated from 

undedying Mesozoic and Paleozoic rocks by a pronounced angular 

unconformity and fr01n the overlying ~1oncrief g1·avel (Sharp, 1948, 

p. 1-15) of probable Eocene age, by another angular unconformity. 

Darton's IGngsbury conglomerate is composed n1ainly of limestone 

boulders derived from the Paleozoic limestones in the Bighorn ~1ountnins, whereas the ~1oncrief gravel (of Sharp) is composed mainly of 

boulders derived from the pre-Cambrian crystalline core of the 

1nountains. Stratigraphic and structural relations of the lGngsbury 

conglomerate and Moncrief gravel to older formations led Sharp 

(1948, p. 14) to conclude that at least three episodes of Laramide 

orogeny are recorded: Uplift and attendant erosion which led to the 

deposition of the IGngsbury conglomerate, uplift and deformation 

of the l{ingsbury and erosion which resulted in the deposition of the 

Moncrief gravel, and the action of compressional forces which thrust 

the Paleozoic limestones onto the Moncrief gravel along the present 

east front of the Bighorn Mountains. These episodes of deformation 

334328-55-7 
78 GEOLOGY OF THJ;j BIGHOHN CANYON-HARDIN ARJ<~A) MON'I'.-WYO. 

were preceded in Lhc Bighorn l\1oun Lain ama by at least one other, 

folding, during pre-Fort Union tin1e. 

According to Mackin (I 937, p. 819, 824, 892; 1947, p. 103-120) the 

Bighorn Basin remained near sea level during the Laramide uplift 

of the mountains, but the basin and adjacent mountains were elevated approximately to their present position during a regional uplift which was initiated in middle Tertiary time. The course of the 

Bighorn River across the northern end of the Bighorn Mountains 

rather than out of tbe open north end of the Bighorn Basin a few miles 

to the north\vest has been cited as evidence (Fenneman, 1931, p. 166; 

Mackin, 1937, p. 821) that the river was superimposed from a surface 

on Tertiary formations in the Bighorn Basin that was at least partly 

continuous with the Tertiary cover of the Great Plains region (.l\1ackin, 

1937, p. 821). 

Post-Laramide uplift in the region is thought by Th01n (Thorn and 

others, 1935, p. 84) to have continued through late Tertiary and 

Quaternary time in "n1ore or less periodic upwarping or upbowing 

of the Big Horn arch and the adjacent region, attended by downcutting and terrace development." .l\1

fackin, in considering thf' 

problem of regional uplift in the Bighorn Basin, notes that stream 

terraces formed during pauses in downcutting of the stream need 

not be explained solely by regional uplift of the area. He points out 

(Mackin, 1937, p. 887) that fluctuations of sea level in late Tertiary 

and Pleistocene time and the changes in the course of the Missouri 

River during the Pleistocene undoubtedly affected the length and 

declivity, and, consequently, the load and volume of the .l\1issouri, 

Yellowstone, and Bighorn River systems. These factors as well 

as the possibility of regional uplift must be considered when seeking 

an explanation of the origin of the Bighorn River terraces. 

GEOMORPHOLOGY 

GENERAL FEATURES 

The Bighorn Canyon-Hardin area lies athwart the boundary of 

the Middle Rocky .l\1ountains and the Great Plains provinces (Fenneman, 1931, p. 160), and much of its geomorphic history is reflected 

in surface features associated with or directly caused by the Bighorn 

River, the master stream of the area. The Bighorn River meanders 

across Bighorn Basin in a broad, shallow terraced valley cut into 

nonresistant Cretaceous and Tertiary formations, but it crosses the 

Bighorn Mountains in meanders incised to canyon depth in resistant, 

slightly folded Paleozoic limestones. The canyon ranges between 

200 feet to a little more than 1,000 feet deep from Porcupine Creek 

anticline at the southern tip of the Pryor Mountains eastward across 

Garvin Basin and the Dry Head Creek area of the Bighorn Mountains 
;;· 

G J<:OMOR PHOLOGY 79 

to the nol'thcl'n end of the antielinal {trch of the Bighorn l\1Iountains. 

The top of the l\1aclison lin1cstone generally forms the edge of the 

canyon and the overlying nonresistant redbeds in the Amsden formation have been stripped back from the lVIadison limestone. As 

shown by figure 7, meander spurs in the canyon have not been greatly 

I'Cduccd, even where the canyon is l ,000 feet deep. 

The Paleozoic limestones rise steeply, and abruptly, up the west 

flank of the Bighorn l\1ountain anticline a short distance east of the 

mouth of Dry Head Creek. Where the river meets the flank of this 

great fold, it leaves a canyon one-quarter 1nile wide and 1,100 feet 

deep to enter a gorge five times as wide and twice as deep. In the 

axial part of the anticline the Bighorn River has exposed Cambrian 

shales and thin lirnestones that are much less resistant than the 

overlying 1nassivc limestones, and for 6 miles in this part of the 

co.nyon the river has no incised meanders. Steep talus-covered 

slopes cut on nonresistant Cambrian strata rise as much as 750 feet 

fro1n the l'iver's edge to neal'ly vertical limestone cliffs that arc as 

nuwh as 1,300 feet high. East of the mountain crest, however, the 

eastward dip of the strata is greater than the gradient of the river, 

and the canyon again narrows about 4 miles northeast of the axis, 

and the river flows to the mouth of Bighorn Canyon in incised meandm·s that show little regard for the bedrock structure. 

The Bighorn River thus flows through incised meanders except 

across the axial part of the Bighorn lVIountains where it follows a 

straight course in shale of Cambrian age. Undoubtedly, incised 

meanders existed when the river was cutting in limestone of Paleozoic age at higher levels, but as the river reached shale of Cambrian 

age, its meanders were able to migrate downstream, thereby undercutting and rmnoving meander spurs. Evidence of former meanders 

has been destroyed as the cross-sectional profile of the valley widened 

above stream level to the angle of repose of the shale. There is no 

valley floor, other than the stream channel, and there is no sign of 

former stream deposits on the valley walls, because weathering and 

rill wash are constantly eroding the shale of the valley sides. Incision of the meanders may have started at a level considerably 

higher than the Madison limestone in the Dry Creek-Garvin Basin 

area, but owing to the alternately resistant and nonresistant character 

of the strata overlying the Madison limestone these beds have been 

largely stripped off or weathered back from the edge of the Madison 

cliffs. Terrace gravels have been almost completely removed from 

the same area. 

The Bighorn River flows northeastward from the mountains along 

the west edge of its valley to Two Leggin Creek where it crosses the 

valley and continues northward past Hardin. Through this part 
80 GEOLOGY OF THE BIGHORN CANYON-HARDIN AREA, MONT.-WYO. 

of its course the river cuts at the base of nearly vertical shale bluffs 

125 feet high, first on the west side of the valley, then on the east. 

The shale bluff is overlain by terrace gravels. Between Rotten 

Grass Creek and Hardin, the Bighorn fiows parallel to the strike 

of the shnle of Lnte Cretaceous nge. The strenm divide to the east 

is a westward-facing cuesta that is gradually shifting eastward. 

Numerous small valleys on the east slopes of the cuesta have been 

beheaded during the eastward migration of the divide. 

The Little Bighorn River and Rotten Grass Creek rise on the east 

slopes of the Bighorn Mountains and flow eastward onto the plains, 

where they turn northward parallel to the strike of the bedrock. This 

pattern may have developed at a higher level as subsequent streams 

in the Cretaceous shales worked headward and captured stremns 

flowing eastward from the mountains. Stream piracy in more recent 

times is well exemplified by the capture of the headwaters of Hay 

Creek by Muddy Creek. This capture, which occurred in the NW}~ 

sec. 23, T. 4 S., R. 30 E., left Hay Creek valley with virtually no 

stream. Typical reversed streams at the hend of Hay Creek join 

Muddy Creek at the elbow of capture. 

TERRACES 

Six principal terrace levels nncl n few intermediate ones thnt nre 

cut on beveled strata along the Bighorn River and its tributary streams 

may be recognized east of the Bighorn 1\tfountains. The most extensive terraces were n1ade by the Bighorn and Little Bighorn Rivers. 

The terraces along Bighorn River, which range from 10 to 650 feet 

or more in elevation above the river, are designated by numbers from 

1 (the lowest and youngest) to 6. Terrnces of tributary streams have 

been correlated with them at points as close as possible to the junctions 

of the streams. 

The terraces are underlain by generally unconsolidated silt, sand, 

and gravel, which total about 25 feet thick on the lower levels. The 

gravel is composed of limestone, basalt, andesite, and a subordinate 

amount of sandstone, quartzite, granite, gneiss, and chert. Near the 

mouth of Bighorn Canyon the coarser aggregate in terrace Qt1 ranges 

from 2 to 40 inches in diameter, and is· predominantly limestone, 

whereas material smaller than 2 inches in diameter is chiefly volcanic 

rock. Near Hardin, however, limestone is subordinate to the volcanic 

rocks in all sizes of aggregate. Characteristics and correlations of the 

terraces are summarized in table 3. Distribution of terrace gravels 

is shown on plate 7. 

The lowest terrace, designated Qt1, covers most of the valley floors 

in the area. Its forward edge is about 10 feet above the narrow flood 

plain in Bighorn River valley, but the two surfaces merge locally. 
io· 

~-

GEOMORPHOLOGY 81 

T ABT ... E 3.-Bighorn R-iver te1·races below the Yellowtail dam site 

[Symbols: Qal, Quaternary alluvium; Qt, Quaternary terrace; 'I't, Tertiary terrace 

Symbo 

'l't.!i ____________ _ 'l't5 ____________ _ Qt4 ____________ _ Qta ____________ _ Qt2 ____________ _ 

Qui Qtl ------------ ____________ _ 

A vcrago ."M'aximum 

height diameter 

above river of gravel (feet) (inches) 

H50 

550 

:J50 

250 4 120-140 

20-90 

0-15 

10 

12 

12 

13 

16 

~ 40 

Average 

diameter 

of gravel 

(inches) 

Correlation 

'l'homl Aldcn 2 

Rock types of aggregate (most 

abundant named 

first) 

4 or no. 1. gravel. Limestone, sand- 3 Flaxville(?) F 1 ax vi lie ) 

4 } {No.2(?)____ stone, basalt, an- 3 No.2andno.3. desitc, chert, a No.2________ granite, and 5 Qai6 ___________ QaL_________ qnartzitc.3 5 QaL __________ QaL _______ _ 

-----~---~--_:----~-------------------

1 U. S. Cleo!. Survey Bull. 856, plate 3. ~ V. S. Cleo!. Survey Prof. Paper 174, pin te 1. a Volcanic rock is most abundant in Qtl and tlal gravel north of Two Leggin Creek and in gravel of 1 to a-inch diameter in almost all terraces. 4 120 feet above river ncar mouth of BighOI n Canyon; downstream the interval increases. 6 Ncar the mouth of Bighorn Canyon. 

6 Except ncnr the mouth of Bighorn Canyon where 1t is included in nos 2 nnd 3. 

AlLhough Qtl is mapped as a separate surface in the Bighorn River 

valley, it is included in the alluvium, Qal, in nll other valleys. The 

Qt1 is a composite surface of 1nany small terraces left by streams that 

swung laterally as they cut downward. It has a pronounced riverwat·d slope that is accentuated by alluvial fan material deposited on 

its outer edge by intennittent tributary streams that do not reach the 

main stream. 

Terrace number Qt2 caps the l 25-foot bluffs along the Bighorn 

River and is one of the most prominent terraces in the area. It is 

well preserved on the northwest side of the Bighorn River, but it has 

been largely destroyed on the southeast side. Terraces Qt3, about 

250 feet above the river, and Qt4, about 100 feet higher, are well 

preserved on the west side of the Bighorn River west of St. Xavier 

but n.re represented elsewhere only by small remnants. 

The fifth terrace, Tt5, fonns high benches southwest of Hardin and 

northeast of the 1nouth of Bighorn Canyon. It is 'fl.bout 550 feet above 

the river and is perhaps the 1nost conspicuous terrace in the area. 

The highest terrace, Tt6, which is about 650 feet above Bighorn 

Hiver is represented by a few remnants north of the mouth of Bighorn 

Canyon. A few rCinants of high terraces on the Bighorn-Little Bighorn River divide that are Inarked "Tt" represent Tt6 or perhaps an 

even higher level. 

No fossils were found in the terrace deposits, and no glaciers extended frmn the Bighorn 11ountains into this area to leave deposits 

that would aid in dating the terraces. The two highest terraces, 

Tt6 and Tt5, were tentatively correlated by Alden (1 932, p. 28) with 

the Flaxville gravel of northern 11ontana, which Collier and Thorn 

(1 918, p. 179-184) considered to be upper ~1iocene or Pliocene in age. 

The lower terraces are Quaternary in age. 
82 GEOLOGY OF THE BIGHORN CANYON-HARDIN AREA, MONT.-WYO. 

PEDIMENTS AND UNDIFFERENTIATED SURFACES 

_ Several pediments in the Dry Head Creek area slope away from the 

Pryor Mountains toward Bighorn River and Dry Head Creek. These 

surfaces occur at more than one level and are covered by poorly sorted 

boulders and smaller debris, mostly limestone, derived from the 

Pryor Mountains. The pediments slope as much as 3° riverward 

within the area mapped. 

A number of surfaces that bevel shale and are covered by reworked 

shale have been mapped as part of the shale bedrock formation in the 

Plains area. Elsewhere, as in the area between Little Bighorn and 

Bighorn Rivers in T. 1 S., R. 34 E., terraces have been partially or 

wholly destroyed and their gravels redistributed on a younger poorly 

developed surface. All such surfaces are identified on the geologic 

map as "Qu." 

ECONOMIC GEOLOGY 

OIL AND GAS 

Small quantities of oil and gas have been found in the area. Oil 

has been produced periodically from the Soap Creek field and sufficient 

gas for the town of Hardin is obtained from the Hardin gas field. Oil 

was discovered during the latter part of 1952 in the Ninemile area. 

Exploratory drilling in several other areas has failed to find commercial 

amounts of oil or gas. Significant wells in the Hardin area are listed 

and described in table 4. 

Hardin gas field.-Gas was discovered near Hardin in 1913 and was 

first piped into Hardin in 1929 (Perry, 1937, p. 55). The gas, which 

is mostly methane, comes from depths of from 67 5 to 825 feet. 

Although 103 wells had been drilled by June 1, 1949, only 50 were in 

production on that date, and most of these were in T. 1 S., Rs. 33 and 

34 E. A check of the records of 90 wells in T. 1 S., Rs. 32-34 E. shows 

that about 70 percent had initial daily production of 30,000 to 80,000 

cubic feet, and that the maximum daily production was about 180,000 

cubic feet. Rock pressure in most wells now producing is between 

120 and 140 pounds. 

'I'he Daniels Petroleum Corp.'s No. 1 Nlay, in Hardin was started 

in 1935 and abandoned as a dry hole at total depth of 4,195 feet. Oil 

showings were reported in the upper part of the l\1adison limestone, 

which, depending upon interpretation of the driller's log, was penetrated 

as much as 250 feet but not more than 600 feet. 

Soap Greek field.-Oil was discovered in Soap Creek dome in February 1921 (Thorn and Moulton, 1921). Seven of 16 wells drilled on 

the dome have yielded oil. These were located close to the axis of 

-~ 
ECONOMIC GEOLOGY 83 

the dome in sees. 27 and 34, T. 6 S., R. 32 E. Oil and water were 

recovered from the Amsden formation in 5 wells, and from the upper 

part of the Madison limestone in 2 wells. Initial tests of the Inland 

Empire Refineries well 52-34, Crow tribal, which was drilled in 1'948, 

recovered oil with no water from the inLerval beLween 120 and 140 

feet below the top of the Madison limestone. The gravity of Amsden 

oil is about 19° and Madison oil is 20°. Total production of the field 

as of July 1, 1948 was nearly 150,000 barrels. 

Records of the Soap Creek wells are tabulated in table 4. Driller's 

logs of several wells have been published by Thorn (Thorn and others, 

1935, p. 111-119). A log of the lower formations penetrated by the 

Inland Empire Refineries well 52-34 (pl. 3) was made by G. E. 

Prichard from a study of chip samples of cores and cuttings. 

Woody Creek dome.-Two wells have been drilled on Woody Creek 

dome, one on each side of the fault that strikes northward through the 

dome. The Haskell and others well 1, Crow allotted, in the SE}~SE}~ 

sec. 29, '1'. 3 S., R. 31 E., west of the fault, reached the Chugwater 

formation and was abandoned as a dry hole at total depth of 2,000 

feet but was later completed as a flowing water well. The 11idNorthern Oil Co.'s. well 1, Crow tribal, in the NE%NE% sec. 33, T. 

3 S., R. 31 E. was drilled on the east side of the fault to a total depth 

of 2,755 feet, stopping in the 11ndison limestone. It obtnined no 

oil and was cmnpleted as n flowing wnter well. Both wells were drilled 

in 1923 and presently provide a small flow of water. 

A driller's log of the 11id-Northern Oil Co.'s well 1, Crow tribal, 

indicates the top of the ~1mTison is at n depth of 1,270 feet, the top 

of the 1'ensleep sandstone at 2,332 feet, and the 11[ndison limestone at 

2,704 feet. 

Beauvais Creek uplift -Four wells have been drilled in '1'. 4 S., 

Rs. 28-29 E. on the Beauvais Creek uplift. All were drilled in 1922 

and were abandoned as dry holes. They started near the top of the 

Cloverly formation and penetrated not more thnn 100 feet into the 

Amsden formation. All found water in the upper 15 feet of the Tensleep sandstone and, with the possible exception of the '1'. S. Hogan 

well 1, all found water in the upper part of the Amsden formntion. 

Drillet·'s logs of the "56" Petroleum Co.'s well 1, NW%SW}~NE7~ 

see. 28 and well 2, SE%NW7~ sec. 9, T. 4 S., R. 29 E., were published 

by Thorn (Thorn nnd others, 1935, p. 109-110). The only well which 

lies within the area of the accompanying geologic map is the T. S. 

Hogan well 1, Crow tribal. Formation contacts hnve been indicated 

by the writer in the driller's log of the well, as given on page 86. 
TABLE 4.-Selected oil and gas wells and dry holes 

(Land status: P, Patented (lessee of patented land in parentheses). Location: Townships marked with (*) indicate north, all others south; all ranges are east. Remarks: IDP, initial 

daily production; M, thousand cubic feet; RP, rock pressure of gas wells in pounds per square inch.] 

No. 

on 

pl. 1 Operator 

1 Mackinnie Oil & Drilling Co ___ _ 2 Superior Oil & Coal Co _________ _ 3 E. A. Lammers _________________ _ 

4 Marcus Snyder------------------ 5 H. I. Crowe ____________________ _ 

6 Marcus Snyder _________________ _ 7 Warren & Sons _________________ _ 

8 Yellowstone Oil & Gas Co ______ _ 9 Hardin Gas & Fuel Co _________ _ 10 Yellowstone Oil & Gas Co ______ _ 

11 Big Horn Oil & Gas Develop- ment Co. 12 Daniels Petroleum Corp ________ _ 

13 Big Horn Oil & Gas De>elop- ment Co. 14 _____ do __________________________ _ 

15 _____ do __________________________ _ 16 _____ do __________________________ _ 17 _____ do __________________________ _ 18 _____ do __________________________ _ 

19 _____ do __________________________ _ 20 Ursulla Zelka ___________________ _ 

21 Big Horn Oil & Gas Develop- ment Co. 22 Yellowstone Oil & Gas Co ______ _ 23 Mackinnie Oil & Drilling Co ___ _ 24 _____ do ____________ ---------- ____ _ 

25 _____ do __________ ----------------_ 

26 Big Horn Oil & Gas Develop- ment Co. 27 _____ do _____ ------------------ ___ _ 28 C. H. Butler ____________________ _ 

29 Warren & Sons _________________ _ 

30 Big Horn Oil & Gas Develop- ment Co. 

.~ 

Well 

no. Land status 

P-(Douglas) _____ ------------

p ----------------------------- P-(E. A. Lammers) _________ _ 

P-(Buzzetti-Corkin) ______ --- P-(Snyder-Reed) ___________ _ 

1 I P-(E. Kelly) ________________ _ 3 P-(Warren & Sons) _________ _ 3 P-(Sike~ga) _________________ _ 1 P-(Feans) __________________ _ 

1 P-(Grand Junction Sugar 

Co.). P-(McKay) _________________ _ 

1 P-(A. May) _________________ _ 6 P-(L. Mead) ________________ _ 

15 P-(Ottun) __________________ _ 

41 State of Montana ____________ _ 45 P-(Cox) _____________________ _ 

17 P-(Big Horn County Bank). 

19 P-(Houghton) ___ ------------ 23 P-(Luther) __________________ _ 1 P-(U. Zelka) ________________ _ 

24 P-(Peck) ____________________ _ 

2 P-(Newkirk) ________________ _ 4 P-(Douglas) ________________ _ 5 _____ do ____________ -------- ____ _ 

2 _____ do ________ -----------------

49 P-(Eckroy) ------------------

51 _____ do __________________ ------_ 

1 P-(C. H. Butler) ____________ _ 1 P-(Warren & Sons) _________ _ 56 P-(Warren) _________________ _ 

~· 

Location 

DrillFraction 

Sec- ing tion T. R.l ceased 

SWUNEU 32 *1 34 

SWUSEUNEU 33 *1 34 

SEUSWUNWU 14 1 32 NEUNEU 16 1 32 SEUSE~NWU 23 1 32 

Center, lot 3 24 1 32 

NWUSEUSWU 8 1 33 NWU 8 1 33 

SEUNWU 9 1 33 

swu 10 1 33 

SW!4NWUSEU 11 1 33 

NWUSEUSEU 13 1 33 

NEUNEUSWU 14 1 33 

Lot 7 15 1 33 

EY2NEUSWU 16 1 33 

NY2NEUSWU 20 1 33 

NY2NEUNEU 21 1 33 Lot 6 22 1 33 

NEUNEUSEU 22 1 33 Lot 2, block 16, 23 1 33 

Old Town, Har- din. 

NWUNEUNE,U 26 1 33 

NEUNEU 27 1 33 NEUNEU 3 1 34 NWUNWU 3 1 34 NW~SWU 4 1 34 

EY2SEUSWU 17 1 34 

NEUNW~SWU 17 1 34 

SWUSWUSEU 18 1 34 

NWUNWUSE,U 19 1 34 NWU 21 1 34 

1946 

1922 

1936 

1946 

1950 

1945 

1940 

1938 

1925 

1913 

1928 

1938 

1929 

1930 

1939 

1942 

1930 

1931 

1935 

1936 

1935 

1915 

1946 

1946 

1946 

1945 

1945 

1937 

1937 

1946 

Total 

depth 

Lowest 

formation 

reached 

Remarks 

1, 046~----------------1 Trace of gas at 720 feet. 2,504 Cloverly(?) __ _ 750 Frontier ______ _ 800 _____ do ________ _ 

4, 000 Madison _____ _ 

IDP 135M gas. IDP 60M gas. Top of Madison 3,489 ft. 

663 ' Frontier_ _____ _ 830 _____ do ________ _ 850 _____ do ________ _ 800 _____ do ________ _ 

2, 210 Cloverly _____ _ 

Abandoned. 

IDP 40M gas. IDP 30M gas, RP 60. Water and some gas. Show of gas. 

780 Frontier _____ __! IDP 105M gas. 

4,195 Madison(?) __ _ 730 ·Frontier_ ______ IDP 50M gas. 

720 _____ do _________ IDP 143M gas. 765 _____ do _________ IDP 68M gas, RP 120. 

690 _____ do _________ IDP 60M gas. 693 _____ do _________ IDP 161M gas. 642 _____ do ________ IDP 115M gas, RP 120. 676 _____ do _________ IDP 150M gas, RP 135. 

692 _____ do _________ IDP 40M gas. 

750 _____ do_________ Show of gas. 

1, 190 _____ do_________ Do. 428 _____ do_________ Dry hole. 

2, 982 Ellis__________ Do. 

882 Frontier------- Do. 761 ____ _do _________ IDP 92M gas, RP 132. 

805 _____ do _________ IDP 122M gas, RP 132. 

721 _____ do _________ IDP 42M gas. 

727 _____ do_________ IDP 40M gas, RP 135. 

845 _____ do _________ IDP 53M gas, RP 134. 

.! i· 

00 

~ 

0 

tr.1 

0 

tot 

0 

0 

t-q 

0 

l:lj 

~ tr.1 

td 

H 

0 

~ 

0 

lJj 

z 

n 

~ t-q 

0 

z 

~ > lJj 

t;j 

z 

~ tr.1 

...> 

~ 

~ 

~ 
~ "' 

31 Massey Oil & Gas Co ____________ 1 P-(Massey Oil & Gas Co.) ___ 32 Haskell and others _______________ 1 Crow allotted _________________ 

33 Mid-Northern Oil Co ____________ 1 Crow tribaL __________________ 

34 T. S. Hogan _____________________ 1 _____ do ________________________ 

35 B. E. La Dow & Co _____________ 1 _____ do _______________________ --

36 Winnie & Richards ______________ 1 _____ do _________________________ 

37 Detroit-Wyoming Oil Co _________ 1 Crow allotted _________________ 

38 Mule Creek Oil Co ______________ 6 P-(P. R. Krone) _____________ 

39 Phillips Petroleum Co ___________ 3 Crow allotted _________________ 

40 _____ do_. ___ --------------- _______ 1 Crow tribaL __________________ 

41 _____ do .• --------- ________________ 1 _____ do _________________________ 

42 _____ do .• _________________________ 4 _____ do _________________________ 

43 Western States Oil & Land Co __ 7 Crow allotted _________________ 

44 Phillips Petroleum Co ___________ 2 _____ do _________________________ 

45 _____ do ___________________________ 3-8 Crow tribaL __________________ 

46 Inland Empire Refineries, Inc ___ 52-341-- ___ do _________________________ 

47 Dox Oil Co ______________________ 1 P-(C. E. Wolf) ______________ 

48 Thermopolis-Cat Creek Oil Co __ 1 _____ do _________________________ 

49 Rice-Hoffman ___________________ 1 Crow tribaL __________________ 

50 Western States Oil & Land Co ___ 1 _____ do ___________ .. ____________ 

51 G. J. Greer, trustee ______________ 1 P-(Ottun) ___________________ 

52 _____ do .• _________________________ 1 P-(Kendrick) ___ . ____________ 

53 _____ do ___________________________ 2 _____ do _________________________ 

NOTE.-Hardin gas field: 

Producing wells: 50 on June I, 1949. 

Wells drilled: 103 to June 1, I949. 

Soap Creek oil field: 

ProductiOn: 

127,500 bbl, 1921-38. 

20,400 bbl, 1947-July 1, 1948. No production on June 1, 1949. 

SE%'SW%'SE%' 11 

W~SEXSE%' 29 

NWXNEXNE%' 33 

SWXSWXNWX 19 

SWXNEXSEX 16 

SEXSWX 21 swxswxswx 26 E~SWXNWX 27 W~SEXSWX 27 

SWXSWXSEX 27 

SWXSWXNEX 34 

SWXNEX 34 

NW%'SWX 34 

SEXSEXNWX 34 

NWXSWXNE%' 34 NWXNEX 34 

NWXNWXNWX 2 N~NE%'SWX 2 NEXNEX 4 swxswxswx 28 

SWXNEX 32 

SEXSEXSWX 34 NEXNWXNWX 6 

2 31 

3 31 3 31 4 30 

6 32 6 32 6 32 6 32 6 32 6 32 6 32 6 32 6 32 6 32 6 32 6 32 

7 32 7 32 7 32 7 33 

*1 34 *1 34 1 35 

1940 

1923 

1923 

1922 

1922 

1922 

1922 

1922 

1921 

1944 

1921 

1921 

1922 

1922 

1934 

1948 

1921 

1922 

1922 

1922( ?) 

1952 

1952 

1952 

2 150 2:ooo 

2, 755 1, 435 

2, 260 2,030 

1, 200 

1, 970 

1, 964 

1, 826 

1, 966 

I, 656 

2,115 

I, 652 

1, 658 

4, 470 

2,168 

2,017 

2,345 

2, 6I5 

. I 

Cloverly. ____ _ Chugwater ___ _ Madison _____ _ Amsden ______ _ 

_____ do ________ _ 

Madison _____ _ Chugwater ___ _ Madison _____ _ _____ do ________ _ _____ do ________ _ _____ do ________ _ 

Amsden ______ _ Madison _____ _ 

Amsden(?) ___ _ _____ do ________ _ 

pre-Cambrian_ 

Amsden ______ _ Chugwater ___ _ Amsden ______ _ 

Chugwater ___ _ 

5, i27 ·~ Cambrian ____ _ 2. 700 Morrison _____ _ 4, 588 Tensleep _____ _ 

\) 

Flowing water, 1,900--1,943 ft. 

Completed as water well. Do. 'Yater from Tensleep, 1,270-

1,283 ft; flowing water well. 

Fresh water in Amsden. Water in Tensleep. 

IDP 28 bbl, Amsden. IDP 200 bbl, Madison. 

Shut in. IDP 400 bbl, Amsden. 

IDP, 1,000 bbl, Amsden. Water in Madison. IDP I,500 bbl, Amsden. IDP 65 bbl oil, 650 bbl water. 102 bbl oil, no water pumped 

in 6 hr from 1,990-2,010 ft, 

Madison. 

Show of oil in Amsden. . Water in Cloverly, 550-575 ft. 

Show of oil in Tensleep. . ' Water in Cloverly, 1,575-

1,705ft. 

Abandoned. Do. Discovery of low-gravity oil in 

Tensleep. 

trJ 

0 

0 

z 

0 

r5 

c 

0 

trJ 

0 

t"l 

0 

0 

~ 

00 

Ot 
86 GEOLOGY OF 'J'F-11<; BIGHORN CANYON-HAnDJN AR~A, :MON'l'.-\VYO. 

T. S. Hogan wellf, Crow tribal, Beanvais Creek -npli[t, SW~4SWJ,4Nl.VH sec. 19, T. 4 S, R. 30 E., Big Horn Connty, ll£ont. 

I 

'I~~~~;- I Depth 'l~~~~;- Depth 

----------------~fcP.t,)_ -~~ --------------------~(;t) ~fePt)- Unclassified· Unclassified-Continued No record __ ------------------ 400 400 Limestone, sanely _____________ ll 1, 254 Sandstone. ___________________ 10 410 Shale, red ____________________ 16 1, 270 Sandstone; water _____________ 10 420 'l'enslcep sandstone: Shale, blue ___________________ 100 520 Sandstone; 200 barrels of Limestone; water_ ____________ 20 540 water _____ ------------------ 1:3 1, 28:3 Shale, light-blue ______________ 20 560 Sandstone; finer toward bot- Lime shelL __________________ 9 569 tom, water __ --------------- 77 1, :loO Shale, blue, red, and pink ____ :m6 875 Amsden formation: Limestone, blue ______________ 2a 898 Limestone, shelL _____________ 15 1, 37.~ Shale and shells, red __________ 113 1, 011 Limestone ___________ --------- 7 1, 382 Sandstone ____________________ 14 

I 

1, 025 Sandstone, white, soft_ _______ 5 1, 387 Redbeds ______________________ 218 1, 243 Limestone, white, hard _______ 48 1, 435 

The small structural closure of the domes in the Beauvais Creek 

area and the proximity of outcrops of all underlying sedimentary 

formations minimize the likelihood of a well obtaining oil in the 

Beauvais Creek area. 

Two Leggin uplijt.-One well has been drilled on the Two Leggin 

uplift, the northward-plunging anticline that is a continuation of 

Woody Creek dome. This well, the Massey Oil and Gas Co. well 1, 

SE7~SW7~SEX sec. 11, T. 2 S., R. 31 E., Big Horn County, Mont., 

started drilling at about the concretion zone at the base of the Niobrara 

shale as defined by Thorn (Thorn and others, 1935, p. 52) and was 

abandoned as a dry bole when it reached a total depth of 2,150 feet. 

The top of the Morrison formation is at a depth of 1,943 feet, judging 

from the driller's log. 

Ninemile area and Marcus Snyder field.-Discovery of oil just east 

of the Ninemile area came during 1952 when low gravity oil was 

recovered from the Tensleep sandstone at a depth of 4,556 feet. The 

discovery well, the G. tT. Greer, No. 2 Kendrick, is in 

the NE7~NW7~NWX sec. 6, T. 1 S., R. 35 E., in the newly named 

Marcus Snyder field. Several older shallow wells have been drilled 

in the Ninemile area; none of them reached formations below the 

Ellis group. Records for four of these wells are listed in table 4. 

The G. J. Greer, No. 1 Ottun well, in the SW7~NEX sec. 32, T. 1 

N., R. 34 E., Big Horn County, Mont., which was drilled late in 

1952, was dry and abandoned. 2 

2 '!'he Carter No. 1 Rides Bear, which was drilled during 1953 in the mrddle of NE~4SEH sec. 27, '1'. 1 

S., R. 34 E. on the small structural closure shown on plate 1, was also dry and abandoned. No oil or gas 

shows were reported for this well which reached Cambrian strata. Sulfur water reportedly flowed from the 

'l'ensleep sandstone. A second Carter test, the Carter Oil No.1 Crow tribal in the NEHNE~4SWH sec. 7, 

'!'. 1 S., R. 35 E., was abandoned in 1954 reaching the Tensleep sandstone, where "dead oil" was 

encountered. 
ECONOMIC GEOLOGY 87 

MINERAL DEPOSITS 

Bentonite.-N umcrous beds of be11LonitP, l'anging in t.hiclnwss from 

less thu.n 1 ineh tio llcn.dy 20 feet, ocelli' in shale of CI'eLaceous age. 

The benLoniLe is o[ Lhe swelling vn.l'ieLy, n.lHl iLs colol' l'n.ngcs ft"ont 

light. gray to yellowish u.nd gl'eenish gl'ay. Scvel'al thiek beds are 

well exposed in see. 35, '1'. G S., R. 32 E. on the cast flank of Soap 

Creek don1c. I-ICI·c the Clay Spur bentonite bed at the top of the 

.lV[owry shale is 7 feet thick; the Soap Creek bentonite bed at the 

Lop of the Frontier formation is about 10 feet thick, and a bed in 

Lhc lower part of the Cody shale is 8 feet thick. These beds are much 

thinner along Woody Creek, but the Soap Creek bed is locally more 

than 10 feet thick south of Soap Creek dome. 

A bed of bentonite in the upper part of the Cody shale is 14 or 

more feet thick where exposed along the drainage divide cast of 

HoLten Grass Creek between sec. 18, T. 5 S., R. 34 E. and sec. 4, 'I'. 

6 S., H.. 34 E. Two thick beds at the base of the Claggett shale 

n1cmbcr and a thick zone of bentonite about 100 feet below the top 

of the shaie 1ncmbcr equivalent to the Eagle sandstone are well 

exposed south of Dry Creek in the S% sec. 12, T. 1 S., H.. 34 E. A 

thick lenticular zone of bentonite and bentonitic shale in the undifferentiated Cloverly and Morrison formations is exposed north 

of H.otten Grass Creek in the central part of sec. 1, T. 8 S., H.. 32 E. 

1'he thicker and more extensive beds of bentonite have been 

mapped and studied in detail by M. M. l(nechtel and S. H. Patterson 

of the Geological Survey, and the results of their investigations have 

been published separately (I(nechtel and Patterson, 1952). 

Gypsum.-Dcposits of gypsum in the basal part of the Piper formation arc well exposed in many scarps that face toward the Bighorn 

11ounLains. Along the north side of Grapevine Creek, north of the 

Bighorn River, and in the northern part ofT. 6 S., R. 30 E. and in 

the northwestern part of 'r. 6 S., R. 31 E., the gypsum is about 50 

feet thick and occurs in beds about 4 feet thick separated by 2 or 3 

inches of reel ot· green shale. Gypsum also occurs commonly in the 

lower part of the Chugwater forrnation. The thickest beds are in 

the area ncar the head of Soap Creek, in the south-central part of 

T. 7 S., R. 32 E. and the north-central part ofT. 8 S., H.. 32 E., where 

several beds are from 5 to 40 feet thick. 11any of them contain thin 

scmns of red shale. 

Limestone.-The 11adison limestone, which is well exposed in 

Bighorn Canyon and in several smaller canyons on the flanks of the 

Bighorn 11ountains, is a huge deposit of limestone. The uppermost 

150 feet is brecciated and, contains zones of red shale mixed with 

blocks of limestone. Many of the beds in the Madison, particularly 

in the lower part, are notably dolornitic. 
88 GEOLOGY OF THE BIGHORN CANYON-HARDIN AREA, MONT.·-WYO. 

Limestone deposits in the upper part of the Piper formation are 

in several beds that range from nearly pure to very clayey limestone. 

They are well exposed along the north side of Grapevine Creek. 

A 5-foot bed of hard and slabby limestone about 100 feet below the 

top of the Chugwater formation has been quarried in Lime Kiln 

Creek in sec. 21, T. 6 S., R. 31 E. 

Shale and clay.-An inexhaustible source of clay rock is the Cretaceous dark-gray shales, most of which are noncalcareous, and which 

are exposed over hundreds of square miles east of the Bighorn l\1ountains. The clay shales in the Ri01·don formation are highly calcareous, 

and claystones and shales in the Cloverly and l\1orrison formations 

nre t.hin or very lenticular. 

Alden, W. C., 1932, Physiography and glacial geology of eastern Montana and 

adjacent areas: U. S. Geol. Survey Prof. Paper 174. 

Blackstone, D. L., Jr., 1940, Structure of the Pryor Mountain::;, Montana: .Jour. 

Geology, v. 68, p. 590-618. 

Blackwelder, Eliot, 1918, New geological formations in western ·wyoming: 

Washington Acad. Sci. Jour., v. 8, p. 417-426. 

Brown, R. vV., 1948, Age of the Kingsbury conglomerate is Eocene: Geol. Soc. 

America Bull., v. 59, p. 1165-1172. 

Cobban, vV. A., 1951, Colorado shale of central and northwestern Montana 

and equivalent rocks of Black Hills: Am. Assoc. Petroleum Geologists Bull., 

v. 35, p. 217D-2198. 

Cobban, W. A., and Reeside, J. R., Jr., 1951, Lower Cretaceous Ammonites in 

Colorado, Wyoming, and Montana: Am. Assoc. Petroleum Geologists Bull., 

v. 35, p. 1892-1893. 

--- 1952, Correlation of the Cretaceous formations of the western interior 

of the United States: Geol. Soc. America Bull., v. 63, p. 1011-1044. 

Collier, A. J., and Thom, vV. T., Jr., 1918, The Flaxville gravel and its relation to 

other terrace gravels of the northern Great Plains: U. S. Geol. Survey Prof. 

Paper 108, p. 179-184. 

Darton, N. H., 1901, Geology and water resources of the southern half of the 

Black Hills and adjoining regions in South Dakota and Wyoming: U. S. 

Geol. Survey 21st Ann. Heport, pt. 4, p. 489-599. 

--- 1904, Comparison of the stratigraphy of the Black Hills, Bighorn Mountains, and Rocky Mountain Frqnt Hange: Geol. Soc. America Bull., v. 15, 

p. 379-448. 

--- 1906a, Geology of the Bighorn Mountains: U. S. Geol. Survey Prof. Paper 

51. 

--- 1906b, Geology of the Owl Creek Mountains: 59th Cong. 1st Sess., S. 

Doc. 219. 

Dorf, Erling, 1934, Stratigraphy and paleontology of a new Devonian formation 

at Beartooth Butte, Wyoming: Jour. Geology, v. 42, p. 720-737. 

Fenneman, N. M., 1931, Physiography of western United States: 534 p., 

New York, McGraw-Hill. 

Gardner, L. S., Hendricks, T. A., Hadley, H. D., and Rogers, C. P., ~r., 1946, 

Stratigraphic sections of Upper Paleozoic and Mesozoic rocks in southcentral Montana: Montana Bur. Mines and Geology Mem. 24. 

.... 

., 
LITERATURE CITED 89 

Hancock, E. T., HH8, Geology and oil and gas prospects of the Lake Basin field, 

Montana: U. S. Geol. Survey Bull. 691-D, p: 101-147. 

--- 1920, Geology and oil and gas prospects of the Huntley field, Montana: 

U. S. Geol. St1rvey Bull. 711-G, p. 105-148. 

Hewett, D. F., 1926, Geology and oil and coal resources of the Oregon Basin, 

Meeteetse, and Grass Creek Basin quadrangles, ·wyoming: U. S. Geol. Survey Prof. Paper l 45. 

Hewett, D. F., and Lupton, C. T., 1917, Anticlines in the southern p~1rt of th~ 

Bighorn Basin, ·wyoming: U. S. Geol. Survey Bull. 656. 

Tmln.v, H.. \V., Gardner, L. S., Rogers, C. P., Jr., and Hadley, H. D., 19411, Marinn 

.Jmassic formations of Montana: U.S. Geol. Survey Oil and Gas Inv., Pff~lim. 

Chari, 32 . 

.Tohmwn, G. D., 1934, Geology of the mountain uplift transected by the Shoshone 

Canyon, vVyoming: Jour. Geology, v. 42, p. 809-838. 

I<ir·k, Edwin, 1930, The Harding sandstone of Colorado: Am. Jour. Sci., 5th ser., 

v. 20, p. 456-466. 

I<nappen, R. S. and Moulton, G. F., 1930, Geology and mineral resources of parts 

of Carbon, 13ig Horn, Yellowstone, and Stillwater Counties, Montana: U. S. 

Geol. Survey Bull. 822, p. 1-70. 

Knechtel, M. M. and Patterson, S. H., 1952, Bentonite deposits of tho Yellowtail district, Montana and vVyoming: U. S. Geol. Survey Circ. 150. 

Lcatherock, Constance, 1950, Subsurface stratigraphy of Paleozoic rocks in 

southeastern M.utana and adjacent parts of \Vyoming and South Dakota: 

U. S. Geol. Survey Oil and Gas Inv. Chart OC 40. 

Leo, \V. T., L927, Correlation of geologic formations between east-central Colorado, 

central \Vyoming, aud southern Montana: U.S. Geol. Survey Prof. Paper 149. 

Lovering, T. S., 1929, The New vVorld or Cooke City mining district, Park 

County, Montana: U. S. Geol. Survey Bull. 811, p. 1-87. 

Lupton, C. T., 1916, Oil and gas ncar Basin, Big Horn County, Wyoming: U. S. 

Geol. Survey Bull. 621, p. 157-190. 

Mackin, J. H., 1937, Erosional history of the Bighorn Basin, ·wyoming: Gcol. 

Soc. America Bull., v. 48, p. 813-8'94. 

---194 7, Altitude and local relief of the Bighorn area during the Cenozoic, in 

Univ. of \Vyo., \Vyo. Geol. Assoc., Yellowstone-Bighorn Research Assoc., 

Guidebook, Field conference in the Bighorn Basin, p. 103-120. 

Mapel, \V. J., 1954, Geology and coal resources of the Lake DeSmet area, Johnson 

County, vVyoming: U.S. Geol. Survey Coal Inv. Map C23. 

McConnell, Duncan, 1935, Spherulitic concretions of clahllite from Ishawooa, 

\Vyoming: Am. Mineralogist, v. 20, p. 693-698. 

Meyerhoff, H. A. and Lachman, Christina, 1938, Cambrian formations in the 

northern Bighorn Mountains (Abstract): Geol. Soc. America Proc. 1937, 

p. 285. 

Miller, A. J<., 1930, The age and correlation of the Bighorn formation of northwestern United States: Am. Jour. Sci., 5th ser., v. 20, p. 195-213. 

Moulton, G. F., 1926, Some features of red-bed leaching: Am. Assoc. Petroleum 

Geologists Bull., v. 10, p. 304-312. 

Perry, E. S., 1937, Natural gas in Montana: Mont. Bur. Mines and Geology 

Mem. 3. 

Pierce, \V. G., 1948, Geologic and structure contour map of Basin-Greybull 

area, Big Horn County, Wyoming: U. S. Ceol. Survey Oil and Gas Inv. 

Prelim. Map 77. 
90 G~~OLOGY 01!' 'l'H~ BIGHORN CANYON-.HARDlN AH.F..;A, MON'r.-WYO. 

Rem;ide, J. B., Jr., 1925, A new fauna from the Colorado group of ::>outhern 

Montana: U. S. Geol. Survey Prof. Paper 132, p. 25-33. 

--- 1927, The cephalopods of the Eagle sandstone and related formatious 

in the western interior of the United States: U. S. Geol. Survey Prof. :Paper 

151. 

Richards, P. W. and Rogers, C. P., Jr., 1951, Geology of the Hardin area, Big 

Horn and Yellowstone Counties, Montana: U. S. Geol. Survey Oil and Gas 

Inv. Map OM 111. 

Richards, R. W. and Mansfield, G. R., 1912, The Bannock overthrust, a major 

fault iu southeastern Idaho and northeastern Utah: Jour. Geology, v. 20, 

p. 681-709. 

Rogers, C. P., Jr., Gardner, L. S., and Hadley, H. D., 1945, Map showing thickness 

and general distribution of Mesozoic and Paleozoic rocks in south-central 

Montana: U. S. Geol. Survey Oil and Gas Inv. Prelim. Map 43. 

Rogers, C. P., Jr., Richards, P. W., Conant, L. C., and others, 1948, Geology of 

the Worland-Hyattville area, Big Horn and ·washakie Counties, Wyo.: 

U. S. Geol. Survey Oil and Gas Inv. Prelim. Map 84. 

Rubey, W. W., 1929, Origin of the siliceous Mowry shale of the Black Hills 

region: U. S. Geol. Survey Prof. Paper 154, p. 153-170. 

--- 1930, Lithologic studies of fine-grained Upper Cretaceous sedimentary 

rocks of the Black Hills region: U. S. Geol. Survey Prof. Paper 165. 

Sharp, R. P., 1948, Early Tertiary fanglomerate, Bighorn Mountains, ·wyoming: 

Jour. Geology, v. 56, p. 1-15. -., 

Stipp, T. F., 1947, Paleozoic formations of the Bighorn Basin, ·wyoming, in Univ. 

of Wyo., vVyo. Geol. Assoc., and Yellowstone-Bighorn Research Assoc., 

Guidebook, Field conference in the Bighorn Basin, p. 121-130. 

Thom, W. T., Jr., 1922, Oil and gas prospects in and near the Crow Indian Reservation, Montana: U. S. Geol. Survey Bull. 736, p. 35-53. 

Thom, W. T., Jr., and Moulton, G. F., 1921, The Soap Creek oil field, Crow 

Indian Reservation, Montana: U. S. Geol. Survey Press Bull. mimeo., 

Dec. 5, 1921. 

Thom, W. T., Jr., Hall, G. M., Wegemann, C. H., and Moulton, G. F., 1935, 

Geology of Big Horn County and the Crow Indian Reservation, Montana: 

U. S. Geol. Survey Bull. 856. 

Thomas, H. D., 1934, Phosphoria and Dinwoody tongues in lower Chugwater 

of central and southeastern vVyoming: Am. Assoc. Petroleum Geologists 

Bull., v. 18, p. 1655-1697. 

--- 1948, Summary of Paleozoic stratigraphy of the Wind River Basin, 

Wyoming, in Wyo. Geol. Assoc., Guidebook, Third Annual Field Conference, 

Wind River Basin, p. 79-95. 

vVashburne, C. W., 1908, Gas fields in the Bighorn Basin, Wyoming: U. S. Geol. 

Survey Bull. 340, p. 348-363. 

Wilmarth, M. G., 1938, Lexicon of geologic names of the United States: U. S. 

Geol. Survey Bull. 896. 

Wilson, C. W., Jr., 1934, A study of the jointing in the Five Springs Creek are3 

east of Kane, Wyo.: Jour. Geology, v. 42, p. 498-522. ~-
INDEX 

Page 

Abstract. __ --------------------______________ 1-3 

Acknowledgments ___________________________ _ 

Alluvium. ___ -------------------------- _____ _ 

4-5 

(17 

Amsden formation, fossils. ___ . _ _ _ _ _ _ ___ __ ____ 27 

lithology ___ ---------- ________________ -· __ 26-27 

st.ratlgraphic section ______________________ zg-29 

Anticli.ne, Porcupine CtecK ------------------ 75 

Soap Creek .. _-----------------_---------- 72 

Hcarpaw shale, fossils ________________________ f\4-GG 

lithology ___ ---------------_______________ G3 

stratigraphic section ____ .. ________________ G4-GG 

Beauvais Creek uplift, description____________ n oiL.-------- ____ --_----- __________________ 8:1, 8t1 

Hen t.onitc deposits ________ • ____ . ______ ._ .. ___ 87 

Dcrclan, Jean M., quoted_____________________ 20 

13ighorn Canal, irrigation supply _____________ 7,10 

Bighorn Canyon, Cmnhrian roeks. ---------- 11-12 Devonian rocks. _________________ .. _.____ 14 

fault area·-------------------------------- 75 

Mississippian rocks. _______ ---------- ____ 21-2G 

Ordovician roc}{s ___________________ .. ____ 12-14 

Pennsylvanian rocks __ .__________________ ao Bighorn dolomite, lithology __________ -------- 12-14 

Cretaceous rocks. See Cloverly formation; 

Thermopolis shale; Mowry shale; 

Frontier formation; Cody shale; 

Parkman sandstone; Bcarpaw 

shale. 

Dakot.tt sandstone, etluivalent of Clo\·erly 

formation ____ --------------------

Darby formation, suggested name for Devonian rocks ______ ------ ___ ---------

Darton, N. II., quoted.----------------------

Deadwood formation, Darton's description. __ _ Deformation, age oL ________________________ _ 

Devonian rocks. See Jefferson limestone, 

'I'hrec forks shale. 

Puge 

44 

14 

44 

11 

77-78 

l>ome, Grapevine. ___ ---- ______ .. __ . ____ .____ 75 

Pomt Creek. _____________ --------------- 73 

Reed_------·------- ____ . ___ . .. __ . _______ 72-73 

Rotten Grass. __________________ . _ __ ____ 72 

Soap Creek. ___________________________ 73,82-83 

Woody Creek_. ____ ---------- .. ------- 73, 76,83 

I >rainage of thll area ___________ .. ______ . __ . __ 7, 9-10 

Dry Head fault, location_.___________________ 75 

Duncan, Helen, quoted _______________ -------- 20 

stratigraphic scctlott.______________ _ __ __ __ 13 

Bighorn Mountains, Cambrian rocks _________ 11-12 Eaglesandstone,shalemcmberequtvalcntto .. .58-60 

Drvonlan rocks. ___ ----------------______ 14 Economic geology ____ ------------------------ 82-88 

Jurassic rocks____________________________ 38 Em bar formation, ltthology ----- ---- _________ . 32-34 

Ordovician rocks _______ ---------- ________ 12, 14 

Pennsylvanian rocks_---------------- ____ 30,31 

structural features ________________________ 69-72 

Bighorn River, discharge_____________________ 7,9 

path _____ -------- ____ ---- _________ ._. ____ 79-80 

Blrdhead sandstone member, lithology_______ 43 

Cambrian rocks. See Gallatin limestone; Gros 

Ventre formation. 

Carboniferous rocks. See Mississippian rocks; 

Pennsylvanian rocks. 

Carillo shale member, fossils __________________ 53-54 

lithology ___ ---- ______ ---- ______ . _________ 52-53 

stratigraphic section ______________________ 53-54 

Chugwt>tcr formation, lithology ______________ 35,37 

stratigraphic section ___ ------ __________ 35,36-37 

Claggett shale member, fossils ________________ (il-62 

lithology ___ --------------________________ 61 

stratigraphic section ______________ -------- Gl-62 

Clay deposits. __ -----------------.___________ 88 

Climate._--------------______________________ 7, 8 

Cloverly formation, lithology _________________ 42-45 

relation to Morrison formation ___________ 41-42 

stratigraphic section _____________ . _______ • 44 

Cody shale, general lithology _________ . ______ • 50 

lower member, lithology __________________ 50-51 

stratigraphic section__________________ 51 

See also Greenhorn calcareous member; 

Carlile shale member; Niobrara 

shale member; 'l'clcgraph Creek 

member; Shale member equivalent 

to tho Eagle sandstone; Claggett 

shalo member. 

stratigraphic section______________________ 34 

Fall River sandstone. See Dakota sandstone; 

Cloverly formation. 

Fault zones, location _____ ------ __ . __________ . 74-7.5 

Faults, Bighorn Canyon area_________________ 75 

Dry Head-------------------------------- 75 

Grapevine Creek area ________ ------- _____ 75-76 

Lake Basin-Huntley zone ________________ 76-77 

Ninemilc area.------------ _______________ 7fJ-77 

Sykes Spring zone________________________ 75 

Woody Creek dome______________________ 76 

l<'ossils, Bearpaw shale ________________________ 64-66 

Carlile shale member.-------------------- 53-54 

Claggett shale member ______ . _____________ t\J_-62 

Greenhorn calccrous member_____________ 52 

Mississippian and Pennsylvanian rocks___ 27 

Niobrara shale member_-------------- ____ b5-5G Rierdon formation ________________________ 40,41 

shale member equivalent to Eagle sandstone.----------- _________________ 59-fiO 

Swift formation___________________________ 40 

Telegraph Creek member. ____ ------------ 57 

Frontier formation lithology __ ---------------- 49 

stratigraphic section ______________________ 49-50 

Fuson formation, partial equivalent of Cloverly formation. __________________ 43,45 

Gallatin limestone, lithology __________________ 11-12 

Gas field, Hardin __________________________ 73, 74,82 

Gas wells, records ____________________________ 84-8fi 

Geomorphology, general features. ____________ 78-80 

Grapevine Creek area, faults ___ . ____ --------- 75-76 

91 
92 INDEX 

Page Page 

Greenhorn calcareous member, fossils_________ b2 Oil wells, records.---------------------------- 84-85 

lithology--.------------------------------ 51 Ordovician rocks. See Bighorn dolomite. 

stratigraphic section______________________ 52 Gros Ventre formation, lithology_____________ 12 Parkman sandstone, lithology _______________ _ Gypsum deposits .. ___________________________ 87 Pediments, occurrence .... ___________________ _ 

63 

82 

Gypsum Spring formation, partial equivalent Pennsylvanian rocks. See Tenslecp sandof the Piper formation________________ 38 stone. 

Hardin, gas field--------------------------- 73. 74.82 

general area. __ .-------------------------- 74 Henbest, L. G., quoted .. _____________________ 27,31 

Industry. __ ------------------------------____ 10 

Irrigation water, supplY---~------------------ 7,10 

Jefferson limestone, fossils .. ------------------ 20 lithology ______________________________ 14-15,19 

stratigraphic sections .. ---------------- 15,18-19 

Jurassic rocks, general description ____ ------ __ 37-38 

See al.~o Piper formation; Rim·don formation; Swift formation; Morrison 

formation. 

Kingsbury conglomerate, lithology _________ _ 77 

Lake Basin-Huntley fault zone, location ______ 76-77 

-Lakota sandstone, partial equivalent of Cloverly formation ______________________ 43,45 

Landslide materiaL_________________________ 67 

Laramide Revolution, factors affecting the 

Bighorn Mountains. ___ --------- 77-78 

Legal status of land. ___ ---------------------- 4 

Limestone deposits--------------------------- 87-88 

Little Bighorn River, discharge_------------- 7, 9 

path ____ -------------- ____ ---------------_ 80 

Location of area. __________ ----------------- 3 

Madison limestone, lithology _________________ 21-23 

stratigraphic section.------------------ 24, 25-26 

Marcus Snyder field, oiL--------------------- 86 

Mineral deposits, bentonite___________________ 87 

clay-.----------------------------------.. 88 

gypsum _______ --------------____________ 87 

limestone ... -------------------------- ____ 87-88 

shale _____ ------------------------------- 88 

Mississippian rooks. See Madison limestone. 

Mississippian rocks and Pennsylvanian rocks. 

See Amsden formation. 

Moncrief gravel, lithology____________________ 77 

Morrison formation, lithology _________ ------ 41 

relation to Cloverly formation ____________ .. 1-42 

stratigraphic section______________________ 42 

Mowry shale, lithology ___ ----------------___ 47 

stratigraphic section______________________ 48 

Pennsylvanian rocks and Mississippian rocks. 

See Amsden formation. 

Permian rocks. See Embar formation. 

Permian rocks and T1 iassic rocks. See Chug 

water formation. 

Piper formation, lithology ______________ . _____ 38-39 

stratigraphic section .. ____________________ 39-40 

Point Creek dome.__________________________ 73 

Population.----------------__________________ 10 

Porcupine Creek anticlinP,locatlon___________ 75 

Previous publications. _______ •• _____________ _ 

Pryor conglomerate member, lithology________ 42 

Purpose of report.____________________________ 3-4 

Quaternary rocks. See Terrace gravels; Alluvium; Landslide material. 

Reed dome, description ___________________ 7:.!-73 

References .... -------- ________________________ 88-90 

Relief of area ________ --------------------______ 5-6 

Rierdon formation, fossils--------------------- 40,41 

lithology ____________ ------ ___ .____________ 40 

stratigraphic section______________________ 41 

Rotten Grass Creek, discharge________________ 9 

path______________________________________ 80 

Rotten Grass dome, description______________ 72 

Sections, stratigraphic .. 13, 15 18-19 24 25-26 28 

29, 31-32, 34, 35, 36-37, 39-4o, 4i, 42: 44, 4B, 48: 

49-50, 51, 52, 53-54, 55-56, 57-58, 59-60, 61-62, 

64-66. 

Shale deposits. ______________________________ _ 88 

Shale member equivalent to Eagle sandstone, 

fossils ..•.. ____ -- __________________ 59-60 

lithology--------------____________________ 58 

stratigraphic section ______________________ 59-60 

Soap Creek, discharge________________________ 9 

Soap Creek anticline, location________________ 72 

Soap Creek dome, descriptiOn________________ 72 

oiL.------------------------ _________ . ____ 82-83 

Stream discharge, Bighorn River_____________ 7, 9 

Little Bighorn River_____________________ 9 

Rotten Grass Creek______________________ 9 

Soap Creek_______________________________ 9 

Stream piracy, effect on Hay Creek valley.... SO 

Stratigraphy, general features _________________ 10-11 

Muddy Structural features, Bighorn Mountains ______ 69,72 sandstone, correlated with Birdhead 

sandstone._----------------- ____ _ 43 Structural geology, general features .. _________ 67,69 

Newcastle sandstone, correlated with BTrdhead 

sandstone.----------------------- 43,47 Ninemile area, faults ________________________ 76-77 

description. ___________ . ______ ----------__ 74 

oiL_. __ ------------- ____________ -------__ 86 

Niobrara shale member, fossils _______________ 55-b6 

lithology-----------------_. _____ . ____ . ___ 54-55 

stratigraphic section __________ ---- ___ . ____ 55-56 

Oil, Marcus Snyder field _____ --------------- 86 

Soap Oreek field .•..• --------------------- 82-83 

Surface features of area. __ -------------------- 5-6 

Swift formation, fossils________________________ 40 

lithology------------------________________ 40 

stratigraphic section______________________ 41 

Sykes Spring fault zone, location______________ 75 

Telegraph Creek member, fossils.------------- 57 

lithology ___ -------------- ___ ._____________ 57 

stratigraphic section ____________ .--------- 57-58 

Tensleep sandstone, fossils •... ---------------- 31 

lithology--------- _______ ------- _____ ------ 30 

stratlgrapblc sectlon------------------ ____ 31-32 

...) 

111 
lNDEX 93 

Page 

Terrace deposits, age of. ______________________ li7, 81 

lithology------_--- ______ ---- ______________ 80-81 

Tertiary rocks. See Terrace deposits. 

Thermopolis shnlc, lithology __________________ 45-41i 

stratigraphic section _________ .• _______ ---- 4fi 

'l'hrcc Forks slwlo, fossils.-------------------- 20-21 

lithology------ __ ------------ ___________ 14-15, 19 

st.rati)!raphic SI'Ction. ______ . _. ______ .• _ 15, 18-1!J 

Transportation. ____ -------------- _____ ------- 10 

Tt iassic r:>eks nnd Permian rocks. See Chu~-

wntPr formation. 

'l'wo Leggin cnnal, irrigation supply-------- __ 

'l'wo Leggin uplift, cont.inu:ttion of \\'oody Creek dome _____________________ _ 

Vt~getat.ion. _____ ---------- ____________ ----- __ _ 

Page 

10 

73 

Water supply ________________________________ 7, 9--10 

\Veil log, Beauvais Creek uplift..______________ 8G 

Woody Ct·cck dome, description ____ ---------- 73 faults •. ___________________________________ 71i 

oiL.-- _____________ --- ________ ------ __ ---- 8:l 

0 

~U.S. GOVERNMENT PRINTING OFFICE·l983-381-614/587