Geology of the

Stanford-Hobson

Area

Central Montana

GEOLOGICAL SURVEY BULLETIN 1027-J

Prepared in cooperation with the U. S. 

Bureau of Reclamation, as a part of a 

program for the geologic mapping and 

investigation of mineral resources in the 

Missouri River basin

Geology of the 

Stanford-Hobson

Central Montana

By JAMES D. VINE

A CONTRIBUTION TO ECONOMIC GEOLOGY

GEOLOGICAL SURVEY BULLETIN 1027-J

Prepared in cooperation with the U. S. 

Bureau of Reclamation, as a part of a 

program for the geologic mapping and 

investigation of mineral resources in the 

Missouri River basin

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

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

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

Washington 25, D. C. - Price $1.25 cents (paper cover)
CONTENTS

	Page 

Abstract__ _ _ _______--__----__-__-__-____-_-____-___----_---_____ 405

Introduction._____-____--____-__-___-___-___-________----_-_______ 406

Location _____________________________________________________ 406

Purpose and scope of report. ____._________-__-_-___-_---________ 407

Previous investigations.___-____-_-___--____-______-----________ 407

Field work and acknowledgments ___._______________-___________'_ 408

Geography__ ____________________________________________________ 408

Topographic and physiographic features. __________-__---_-_-______ 408

Transportation facilities and accessibility.._________-_--__--___-___ 411

Climate and vegetation.._______________________________________ 411

Utilization of the land __________________________________________ 414

Stratigraphy. _____________________________________________________ 414

Madison limestone__----_-----_-_--_-------------_-----------__ 417

Big Snowy group ____-____--_--__-___--_______--__--_-_________ 417

Kibbey sandstone_ _ ______________________________________ 421

Otter formation ___________________________________________ 422

Heath shale____--___-___--__-___________-_____--____._____ 422

Amsden formation _____________________________________________ 424

Ellis group_ ______--_-_______-__________--__---_-_-__________ 434

Morrison formation ____________________________________________ 436

Kootenai formation ____________________________________________ 437

Colorado shale ________________________________________________ 439

Rocks equivalent to Thermopolis shale________________________ 439

Rocks equivalent to Mowry shale, Belle Fourche shale, and Greenhorn limestone._________________________________________ 441

Rocks equivalent to Carlile shale and Niobrara formation. _ ______ 445

Telegraph Creek formation____________________________________ 445

Eagle sandstone.______________________________________________ 446

Claggett shale. ________________________________________________ 447

Judith River formation.________________________________________ 447

Terrace gravels and alluvium________________________________________ 447

General description ____________________________________________ 447

Age of the terrace surfaces.. _____________________________________ 449

Origin of the gravels_ _ __-_-_-__-____-_-__--______-__-----_____ 451

Economic importance._________________________________________ 452

Igneous rocks.____________________________________________________ 453

Structural relations of the igneous rocks ___________________________ 453

Petrographic relations of the igneous rocks_ ______________________ 454

Igneous rocks in the Stanford-Hobson area, by W. T. Pecora__________ 454

Groupl_____-__-___-________________.____________________ 454

Group 2___________________________^______________________ 455

Structure. ________________________________________________________ 455

Structural setting._____________________________________________ 455

Individual anticlinal structures._________________________________ 456

Faults.-__-_--_---__---_-____-____-______-_______-_--__--__-__ 460

Secondary structural trend. _____________________________________ 460

Formation thicknesses used in computing structure contours. ________ 461

HI
IV CONTENTS

	Page 

Mineral resources,. _ _________-__-----__--________-_-___-_-_________ 462

Oil and gas___________________________________________________ 462

Coal__-___---_----------------------------------------------- 462

Gypsum_ _ ____________---_-------__--_-_____-________-______ 464

Gravel deposits._______________________________________________ 465

Bentonite__ _-____-__--_------------__-__-_-_-__-___----____ 465

Literature cited_____ _______________________________________________ 466

Index ______-_-------_-------------_--------------------_------..-- 469

ILLUSTRATIONS

[Plates 45-47 in pocket]

PLATE 45. Geologic map and section of the Stanford-Hobson area, central Montana.

46. Generalized columnar section of rocks exposed in the Stanford-Hobson 

area.

47. Columnar sections of the Big Snowy group, Amsden formation, and 

the Ellis group.

Page

FIGURE 52. Index map of Montana showing location of the Stanford-Hobson area, Montana.___________________________________ 406

53. Sketch map of the Stanford-Hobson area ___________________ 409

54. View of the Madison limestone, which forms steeply dipping

hogbacks.___________________________________________ 417

55. The Madison limestone and Kibbey sandstone in the canyon of

Antelope Creek_____________________________________ 418

56. The Otter formation and Swift sandstone exposed along a

fault-..---------------------------------------..--.- 423

57. Graphic diagram showing the relationship of the Heath shale,

the Amsden formation, and the Ellis group______________ 425

58. Thickness map of the Amsden formation ___________________ 431

59. Hogback formed by the Amsden formation _________________ 43 2

60. Three units in the Eagle sandstone. _______________________ 446

61. Involution, surficial deformation due to frost action. _________ 450

TABLES

Page 

TABLE 1. Temperature and precipitation data for the Stanford-Hobson

area ___________________________________________________ 412

2. Samplelog of the Pacific Western Oil Co. Todd No. 1 well.____._ 415

3. Comparison of names used for Upper Jurassic and Lower Cretaceous formations in central Montana._____________________ 436

4. Comparison of designations for terrace surfaces _______________ 448

5. Thickness of Kootenai and pre-Kootenai formations ___________ 461

6. Records of wells drilled for oil and gas in the Stanford-Hobson

area..____________-_-_---__-__-_____-______-__-_-_______ 463
A CONTRIBUTION TO ECONOMIC GEOLOGY

GEOLOGY OF THE STANFORD-HOBSON AREA, 

CENTRAL MONTANA

By JAMES D. VINE

ABSTRACT

The Stanford-Hobson area comprises an area of about 1,300 square miles on 

the northern flank of the Little Belt Mountains and in the southern part of the 

Judith Basin, in Judith Basin and Fergus Counties, Mont. As a part of the 

program of the Department of the Interior for the-'development of the Missouri 

River basin, studies were made of the lithology, thickness, and distribution of 

12 formations of Paleozoic and Mesozoic age cropping out in the area. The 

structure and stratigraphic relationships of these formations were studied in 

sufficient detail to permit an evaluation of the mineral resources, with particular 

reference to the oil and gas possibilities and the coal resources of the area.

The bedrock formations range in age from the Madison limestone of Mississippian age to the Judith River formation of Cretaceous age. Much of the area 

of the younger rocks, outside of the mountain area, is covered by extensive 

terrace deposits and alluvium of Quaternary age. These deposits form the most 

valuable agricultural land of the region.

Stratigraphically the lowest formation mapped was the Madison limestone 

of Mississippian age. It consists of gray, dense limestone that forms massive 

cliffs and hogbacks along the front of the Little Belt Mountains. It is overlain 

unconformably by the formations of the Big Snowy group: the Kibbey sandstone 

(190 feet thick), the Otter formation (545 feet thick), and the Heath shale (435 

feet thick). Overlying the Big Snowy group is the Amsden formation of Mississippian and Pennsylvanian(?) age. The Amsden consists of 575 feet of sandstone 

and shale overlain by about 135 feet of gray dense limestone containing red shale 

partings near the top.

The Ellis group, of Middle and Late Jurassic age, which unconformably overlies 

the Amsden formation and older beds, is comprised of the Piper formation at the 

base, overlain successively by the Rierdon and Swift formations. The Ellis 

group is nearly 500 feet thick in a well in the basin part of the area, but in the 

outcrops the group is represented by only 60 feet of sandstone assigned to the 

Swift formation.

The Morrison formation, overlying the Swift formation, is composed of from 

150 to 300 feet of varicolored shale and siltstone interbedded with gray limestone 

and gray to green shale. A bed of coal is near the top.

Unconformably overlying the Morrison is the Kootenai formation of Early 

Cretaceous age, 550 feet thick, consisting of a basal sandstone and overlying 

interbedded red shale and brown sandstone. Conformably, above the Kootenai 

and constituting the lower part of the Colorado shale is 720 feet of dark-gray to 

black shale containing thin beds of sandstone and bentonite that is considered 

equivalent to the Thermopolis shale of the Bighorn Basin. This is overlain by 

135 feet of bentonite, shale, and sandstone, equivalent to the Mowry shale.

405
406 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Overlying the Mowry shale equivalent is shale of Late Cretaceous age, 100 feet 

thick, which contains lenses and dikes of sandstone and thin bentonite beds and 

is considered the equivalent of the Belle Fourche shale. Above this is a section 

consisting chiefly of shale and including 85 feet of strata equivalent to the Greenhorn limestone, 310 feet of strata equivalent to the Carlile shale and 410 feet of 

strata equivalent to the Niobrara formation, which is the uppermost unit of the 

Colorado shale. The Telegraph Creek formation of the Montana group, 160 

feet thick, was mapped with the underlying Colorado shale, because it was not 

possible to separate beds of these groups in the field. The uppermost beds of 

Mesozoic age consist of 250 feet of the Eagle sandstone and unmeasured thicknesses of the Claggett shale and the Judith River formation of the Montana group.

Four sequences of terraces and alluvium of the flood plains were mapped. 

Landslides, consisting chiefly of shale and angular slump blocks, were also mapped 

separately.

Several domes and anticlines are present and may have possibilities for production of oil and gas, because seven formations that produce oil in surrounding 

areas,are presumed to underlie this area also. Other materials of possible economic 

importance that occur in the area include coal in the Morrison formation, gypsum 

in the Piper formation and in all three formations of the Big Snowy group, bentonite in the Mowry shale, and gravel deposits on Quaternary terraces and 

alluvial plains.

INTRODUCTION 

LOCATION

The Stanford-Hobson area is located in Judith Basin and Fergus 

Counties, Mont., and includes approximately 1,300 square miles in 

the southern part of the Judith Basin (fig. 52). The mapped area is 

nearly triangular, being bounded on the east by the meridianal line 

109°40', on the north by the north line of township 18 (approximately

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FIGURE 52. Index map of Montana, showing the location of the Stanford-Hobson area, central Montana.
GEOLOGY OF STANFORD-HOBSON' AREA, MONTANA 407

parallel 47°11 / ), on the west by the meridianal line 110°30/ , and on 

the south by an irregular line trending generally northwest near the 

northern base of the Little Belt Mountains. It is 40 miles long on the 

north side and 46 miles long on the east side and about 45 miles long 

on the southwest side.

PURPOSE AND SCOPE OF REPORT

The Stanford-Hobson area project was undertaken by the United 

States Geological Survey, in cooperation with the United States 

Bureau of Reclamation, as part of a program for the geologic mapping 

and investigation of mineral resources in the Missouri River basin. 

The field work that is the basis of the ensuing report consisted of 

mapping the geology and determining stratigraphic relationships in 

sufficient detail to evaluate the mineral resources, especially the oil 

and gas possibilities in the area.

PREVIOUS INVESTIGATIONS

The principal geologic features of central Montana are well known, 

as the result of a number of previous geologic investigations, several 

of which are truly classical studies. The first comprehensive description of the geology and economic resources of a part of this area 

appeared in the Little Belt Mountains and Fort Benton folios of the 

U. S. Geological Survey (Weed, 1899a and 1899b) and in a description 

of the geology of the Little Belt Mountains (Weed, 1900). This was 

followed by more detailed geologic mapping and detailed descriptions 

of the coal resources in the areas adjacent to the Little Belt and Big 

Snowy Mountains by Calvert (1909) and by Fisher (1909). Still 

later, Reeves' geologic mapping of the plains adjacent to the Highwood 

Mountains (Reeves, 1929) and of the Big Snowy Mountains (Reeves, 

1930) added to our knowledge of areas to the north and to the east of 

the Stanford-Hobson area. A study of the physiography and glacial 

geology of eastern Montana by Alden (1932) includes a discussion of 

the three chief terrace levels in this area. Meanwhile, continuing 

investigations by paleontologists and stratigraphers contributed 

greatly to our knowledge and understanding of the sedimentary rocks. 

Of particular significance to the geology of this area are the studies of 

the Mississippian and Pennsylvanian rocks by Scott (1935 and 1942), 

and by Perry (1943); the studies of the Jurassic rocks by Cobban 

(1945), by Brown (1946), by Imlay (1945), and by Imlay and others 

(1948); and the studies of the Cretaceous rocks by Cobban (1951), 

and by Cobban and Reeside (1951). These studies have defined and 

described the sedimentary formations as the}7 are mapped in the 

Stanford-Hobson area.
408 CONTRIBUTIONS TO ECONOMIC GEOLOGY 

FIELD WORK AND ACKNOWLEDGMENTS

Field investigation of the Stanford-Hobson area was undertaken by 

the U. S. Geological Survey during the summers of 1948 and 1949. 

A map and short report of the findings of the first summer's field work 

(Vine and Hail, 1950) and a similar map and report of the second 

summer's work were published in advance of this report (Vine and 

Johnson, 1954.) -The areal geologic mapping was done^on aerial 

photographs at a scale of approximately 1:20,000 and transferred 

to the base by means of a vertical sketchmaster. Horizontal and 

vertical control for the mapping was established by plane table 

triangulation and adjusted wherever possible to U. S. G. S. benchmarks and triangulation stations.

The writer is indebted to William J. Hail, Jr., who assisted with 

the geological mapping during the summer of 1948 and William D. 

Johnson, Jr., who assisted during the summer of 1949. Both helped 

in the compilation of maps in the office. Robert H. Dott, Jr., served 

as a field assistant during the summer of 1948. William A. Cobban 

visited the party during the summer of 1949 and aided in selecting the 

formation boundaries within the Colorado shale.

The writer also wishes to express his sincere thanks to the many 

residents in the area for the kindnesses and courtesies that facilitated 

the work in the field.

GEOGRAPHY

TOPOGRAPHIC AND PHYSIOGRAPHIC FEATURES

The Stanford-Hobson area of central Montana (pi. 45) is located in 

the southern part of the Judith Basin, a topographic depression in 

an unglaciated part of the northern Great Plains that is partly enclosed 

by several isolated mountain ranges. A sketch map, fig. 53, shows 

the principal physiographic features in the area. Five mountain 

ranges, the Highwood, Little Belt, Big Snowy, Judith, and Moccasin 

Mountains nearly enclose the Judith Basin, except on the north. 

The two principal streams, Judith River and Arrow Creek, have 

sources in these mountains, drain the Judith Basin, and flow northward along generally parallel courses to join the Missouri River as it 

flows eastward across central Montana.

Rolling plains from 3,500 to 4,500 feet above sea level make up the 

greater part of the area. The five mountain ranges that nearly 

enclose Judith Basin rise abruptly from the plains to elevations of 

6,000 to 9,000 feet above sea level. The plains consist chiefly of 

gently sloping, gravel-covered terraces that are dissected by steepsided stream valleys that range in depth from a few feet near the heads 

of small coulees to several hundred feet along the major stream
FIGCBE 53. Sketch map of the Stanford-Hobson area, cent
410 CONTRIBUTIONS TO ECONOMIC GEOLOGY

courses. However, an observer standing on one of these terraces sees 

an apparently uninterrupted and featureless plain extending to the 

foothills of the mountains. Deep dissection along Arrow Creek and 

its tributaries has resulted in areas of badlands unsuited for cultivation 

and very inadequate for grazing. A belt of foothills in which gently 

dipping bedrock crops out generally separates the terraced plains 

from the main flanks of the mountains. Locally, terraces that 

resemble remnants of pediments extend to the steep slopes of the 

mountains and lie on steeply tilted and truncated strata.

The Highwood Mountains, northwest of the area, are a highly 

dissected mass of volcanic rocks and laccoliths. They are separated 

from the Little Belt Mountains on the south by about 12 miles of 

rugged uplands that form the western drainage divide of Arrow Creek. 

The Little Belt Mountains are structurally complex, with sedimentary 

rocks uplifted along a northwest-trending axis. On the northern flank 

of the range these rocks are disturbed by a series of stocks, other igneous 

intrusions that arch the sedimentary rocks, small domelike uplifts, 

faults, and minor folds. This range forms the southwestern limit of the 

area mapped. The Little Belt Mountains are separated from the Big 

Snowy Mountains to the east by a structural and topographic saddle 

or divide known as Judith Gap. The Big Snowy Mountains are 

composed of sedimentary rocks uplifted along a northwest trending 

axis that is slightly offset from the axis of the Little Belt Mountains. 

The Judith and Moccasin Mountains lie to the north of the Big Snowy 

Mountains and are separated from them by a lobe of the Judith Basin. 

The Judith and Moccasin Mountains consist of igneous intrusions 

that are probably stocks or laccoliths over which the sedimentary 

strata are domed.

Two prominent topographic features interrupt the continuity of the 

terraced plains in the vicinity of Stanford. About 2% miles west of 

the town, a group of gravel-capped remnants of a high terrace surface 

consititute Stanford Buttes. About 5}£ miles southwest of Stanford, 

Skull Butte towers nearly 1,000 feet above the adjacent plains and 

is the topographic expression of a structural dome. Other named 

landmarks in the area lie near the mountains. Wolf Butte is a prominent mountain peak that stands out somewhat in front of the main 

flank of the Little Belt Mountains about 10 miles south of Geyser. 

Square Butte and Round Butte are prominent buttes at the east 

end of the Highwood Mountains and are about 8 miles north of the 

area mapped. West of Antelope Creek the north flank of the Little 

Belt Mountains is characterized by a prominent hogback to which 

no general name has been applied.
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 411 

TRANSPORTATION FACILITIES AND ACCESSIBILITY

Nearly all the towns are located on one or the other of the two 

railroads that cross the area. A branch of the Great Northern Railway with a terminus in Great Falls enters the area at Geyser and 

passes south through Judith Gap, with a branch line extending east to 

Lewistown. The Chicago, Milwaukee, St. Paul and Pacific Railroad 

has its route along the north side of the Highwood Mountains and 

enters the Stanford-Hobson area near the town of Arrow Creek. 

A branch line of the latter railroad passes south through Judith Gap 

to Harlowton.

Two paved highways provide easy access to most of the area: 

U. S. Highway No. 87 between Lewistown and Great Falls crosses 

the center of the area from northwest to southeast; near Hobson it 

is joined by Montana State Route No. 19, which comes north from 

Harlowton through Judith Gap. A secondary highway across the 

northern part of the area through the towns of Denton and Coffee 

Creek has been graded and partly graveled. Many other secondary 

roads have been graded and graveled and, hence, are passable in nearly 

all kinds of weather. A close network of secondary roads and trails 

permits access by car in good weather to within one or two miles of 

any part of the area.

The nearest commercial air fields are those in Lewistown and Great 

Falls, but 3 small private fields at Moore, Stanford and Denton 

provide facilities for light airplanes. An emergency landing field, 

maintained by the Civil Aeronautics Administration, is located about 

2 miles east of Geyser. Small airplanes are flown by several ranchers 

and land on cleared areas on their ranches.

CLIMATE AND VEGETATION

The plains in the Stanford-Hobson area, under semiarid climate, 

supported a thick growth of grass before the modern period of excessive 

grazing and cultivation, and grass still covers areas left uncultivated. 

Conifers are abundant on the slopes of the mountains and the thickest 

stands are found on the northwest-facing slopes. On the plains, trees 

grow only locally. Thus conifers grow along the outcrop of sandstone 

beds, such as the basal part of the Kootenai formation and the Eagle 

sandstone. Other evergreens also grow along some of the sheltered 

coulees and steep-sided gulches that are tributary to Arrow Creek. 

Deciduous trees, such as cottonwoods and aspens, are found in some 

sheltered valleys where more moisture is available than on the open 

plains.

Heavy rains occur in June and during that month many secondary 

roads are impassable. Afternoon thunder showers are common
412 CONTRIBUTIONS TO ECONOMIC GEOLOGY

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Average precipitation (inches) 

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414 CONTRIBUTIONS TO ECONOMIC GEOLOGY

during July and August. Light snow or frost may occur during any 

month of the year but only rarely during the summer months. In 

September there is usually an increase in precipitation, but during 

October and November the weather is generally dry and mild.

Temperatures are moderate in the summer and there are rarely a 

few days of uncomfortably warm weather. The table (table 1) was 

compiled from temperature and precipitation data for the StanfordHobson area as recorded in the U. S. Weather Bureau Office, Helena, 

Montana.

Moderately strong, cool winds blow almost continuously, and the 

nights are usually cool, even in midsummer. The winters are severe.

UTILIZATION OF THE LAND

Wheat, cattle, and sheep are the chief economic products of the 

farms and ranches in the area. The gravel-covered terraces are usually 

mantled by two or three feet of fine, loamy soil, and a large part of 

these uplands is cultivated and planted in wheat. In addition, parts 

of the more gently sloping valley lands and of the rolling foothills 

region surrounding the mountains are also cultivated and planted in 

wheat. Some of the highest yields of wheat are obtained along the 

foothills of the mountains where the soils are derived from decomposed 

sandstone and red shale and in the valley lowlands where the soils 

are derived from sandy black shale. Nearly all the remaining area, 

which is not cultivated, is used as grazing land for cattle and sheep 

or is reserved for hay meadows and winter pasture. The largest cattle 

and sheep ranches are found along the foothills of the mountains or 

along the broad alluvial river plains, which are too moist for the cultivation of wheat.

Coal mining was for a time an industry of secondary importance in 

the area. The abandoned town of Lehigh, about 3% miles southwest 

of Windham, was once a thriving coal mining camp. The coal was 

used by the Great Northern Railroad, but about 1921 most of the 

mines were closed and have never reopened.

STRATIGRAPHY

Sedimentary strata ranging in age from Mississippian to Recent are 

present in the Stanford-Hobson area (pi. 45). Older Paleozoic formations present in the Little Belt and Big Snowy Mountains include 

about 850 to 1,300 feet of Cambrian rocks and 130 to 165 feet of 

Devonian rocks that have been described by Weed (1899a, 1899b) but 

were not investigated for this report. The younger Paleozoic formations and the Mesozoic formations to the Judith River formation, 

totaling as much as 6,500 feet of strata, are described herein. The 

rocks from the Madison limestone of Mississippian age to the Eagle
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 415

sandstone of Late Cretaceous age are represented graphically on 

plate 46, which, is a composite of surface sections measured in the area. 

A comparison with the sample log of the Pacific Western Oil Co. 

Todd no. 1 well, table 2, from the northern part of the area indicates 

variations in the stratigraphic sequence within the area that will be 

discussed in detail. The only rocks overlying the Mesozoic strata 

are Pleistocene and Recent terrace gravels and alluvium.

TABLE 2. Sample log of the Pacific Western Oil Co. Todd No. 1 well

Location: SWJ.NWJ* sec. 32, T. 17 N., R. 14 E.

Drilling commenced: September 15, 1948

Drilling completed: October 29, 1948

Total depth: 3356 feet

Surface formation: Colorado shale

Microscopic study of well samples made by William J. Hail, Jr.

Lithologic description

Thickness 

(feet)

Depth 

(feet)

Colorado shale (as penetrated):

No samples___________________-________.___________ 40 4.0

Siltstone, gray, with some gypsum___________-_---____ 75 115

Limestone, light-gray_______________________________ 15 130

Shale, gray and dark-gray; bentonitic in lower part_____ 70 200

Shale and siltstone, gray; bentonite from 230 to 240 ft.__ 110 310 

Shale, gray, and white bentonite; high concentration of 70 380

bentonite between 340 and 360 ft___________________

Siltstone and shale, gray; in part sandy_________----___ 70 450

Shale, gray to dark-gray; in part silty and sandy. ______ 220 670

Siltstone, gray; sandy in upper 20 ft.________________ 75 745

Sandstone, light-gray, very fine grained, friable; in part

silty__.__--_--_-_----________-__--___-_---_-.-__ 45 790

Shale, gray_-__-_-_______________________-_---___._ 10 800

Sandstone, gray, very fine grained ____________________ 30 830

Shale, gray; sandy from 870 ft to 940 ft.__ _.__-_____ 160 990

Sandstone, gray, friable, very fine grained to fine

grained; interbedded with some gray shale._-__-_--__ 80 1070 

Kootenai and Morrison formations, undifferentiated:

Shale, red, grayish-red, and gray; in part silty__---_____ 50 1120 

Sandstone, grayish-yellow-green, medium-grained to

silty_--_-_----_-_-----_________-______-_--_----_ 75 1195

Limestone, pale-yellowish-brown, varicolored _________ 20 1215

Shale, red, gray, varicolored; in part sandy.___________ 35 1250

Limestone, grayish-orange-pink and light brown, dense. _ 25 .1.275 

Siltstone, gray, sandy; interbedded with gray shale_____ 20 1295

Limestone, pale-yellowish-brown______________________ 15 1310

Siltstone, gray, sandy; interbedded with varicolored

shale__________________________----.__. 85 1395

Limestone, yellowish-gray; siltstone at base____________ 25 1420

Siltstone, light-gray; interbedded with gray shale; sandy

at the base_____________________________________ 30 1450

Shale, dark-gray and red; loose sand grains.___________ 125 1575

Sandstone, light-gray, very fine grained to silty_______ 25 1600

Shale, dark-gray, red, and varicolored; carbonaceous

near the top; in part sandy________________________ 115 1715

Limestone, light-olive-gray; interbedded with gray shale_ 30 1745 

Shale, dark-gray, red, and varicolored; interbedded with

siltstone and sandstone---.-------.------.-..----.--- 50 1795
416 CONTRIBUTIONS TO ECONOMIC GEOLOGY

TABLE 2. Sample log of the Pacific Western Oil Co. Todd No. 1 well Continued

Lithologic description

Thickness 

(feet)

Depth 

(feet)

Ellis group:

Sandstone, light-gray, glauconitic; shaly toward the base. 50 1845 

Shale, gray and grayish-red.___-_________--_--__---__ 20 1865

Shale and sandstone, interbedded, gray and grayishred, glauconitic__-____-_-_-_-_-___-___-_-_____---_ 50 1915

Limestone, light-gray, silty; some siltstone and shale.___ 30 1945 

Siltstone, light-gray; interbedded with red shale._______ 30 1975

Shale, gray and dark-gray; some limestone near top_-__- 60 2035 

Shale, red and gray, and calcareous siltstone.__________ 35 2070

Shale, gray, and anhydrite; interbedded_______________ 45 2115

Shale, siltstone, and limestone, interbedded, gray and

varicolored_______________________________________ 50 2165

Siltstone and shale; interbedded, varicolored; some anhydrite--________-___-._________.______________ 90 2255

Limestone, pale-yellow-brown, dense._________________ 10 2265

Shale, gray and dark-gray to red, varicolored_________ 25 2290

Heath shale:

Shale, gray to grayish-black__________________________ 15 2305

Shale and limestone interbedded-______________________ 30 2335

Shale, grayish-black_______________________________ 20 2355

Shale and siltstone, gray and varicolored._____-_-_-___ 35 2390

Shale, grayish-black_______________________________ 10 2400

Shale, dark-gray, interbedded with gray siltstone_______ 20 2420

Shale, dark-gray; siderite pellets with magnetite centers.. 5 2425 

Sandstone, pale-brown, medium-grained.______________ 15 2440

Shale and siltstone, dark-gray and varicolored._________ 120 2560

Otter formation:

Shale, green, gray, and varicolored, silty; thin limestone

bedsat 2600 and 2630 -------------------------- 110 2670

Limestone, gray, silty_____-_-___.----------_________ 30 2700

Shale, gray, greenish-gray and red____________________ 30 2730

Limestone, light gray, dense.__._---_--____-_-______- 20 2750

Shale, gray, green, and varicolored-.__________________ 10 2760

Limestone, gray, shaly______________-_--_____----___ 10 2770

Shale, gray, grayish-red, greenish-gray, and grayish-black. 40 2810 

Limestone, light gray, silty, and shaly.________--_--___ 10 2820

Shale, gray, grayish-black and varicolored.____________ 15 2835

Limestone, gray to light-gray, shaly_________________ 10 2845

Shale, light-gray to dark-gray, and varicolored._________ 60 2905

Kibbey sandstone:

Siltstone, light-gray, shaly__..______________. 25 2930

Shale, light-gray to dark-gray________________________ 25 2955

Sandstone, white, fine- to medium-grained_____________ 15 2970

Shale, light- to dark-gray, and varicolored, sandy.______ 45 3015

Sandstone, pale brown, very fine-grained and silty______ 30 3045

Shale and anhydrite, interbedded-____________________ 10 3055

Shale gray, dark-gray, and varicolored, silty._______-_- 40 3095

Sandstone, siltstone and shale; interbedded, pale-yellowbrown to pale-red_______________-___-_-___-.__-_ 90 3185

Madison limestone (as penetrated):

Dolomite, light-gray________-_-_-_-_---_---------_-- 5 3190

Limestone, light- to dark-gray, oolitic________________ 30 3220

Limestone, anhydrite and shale, interbedded ___________ 75 3295

Siltstone and anhydrite, sandy and shaly______________ 10 3305

Shale, gray to dark-gray______________________ 15 3320

Limestone, light-olive-gray, to pale-yellowish-brown; 

interbedded with some gray shale___--_-__---_--__-_ 36 3356 

(Bottom of well)
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 417

MADISON LIMESTONE

The Madison limestone is the most conspicuous, resistant formation 

exposed in the area. It is about 1,000 feet thick (Weed, 1899b, p. 2) 

and forms steep hogbacks on the tree-covered flanks of the Little Belt 

and Big Snowy Mountains (see figs. 54 and 55). It also caps many

FIGURE 54. The Madison limestone, in foreground, which forms the steeply dipping hogback along the 

north flank of the Little Belt Mountains. View north along Antelope Creek.

high peaks in the mountains. The top of the Madison limestone was 

the oldest contact mapped, and the formation was not studied in the 

same detail as the overlying formations. The upper part of the 

Madison limestone, where mapped, consists of gray, dense limestone, 

weathering light gray. Locally, the upper part of the formation is 

fractured and brecciated.

BIO SNOWY CJROTTP

The term Big Snowy group was first applied to strata above the 

Madison limestone and below the Amsden formation by H. W. Scott 

(1935, p. 1023). Three formations from oldest to youngest, the 

Kibbey sandstone, the Otter formation, and the Heath shale comprise the group.

Before the introduction of the term Big Snowy group, all the strata 

in central Montana above the Madison limestone and below the Ellis 

group were assigned to the Quadrant formation by Weed (1899b, p. 2) 

and others. Strata thus included in the Quadrant formation in the

354332 56 3
418 CONTRIBUTIONS TO ECONOMIC GEOLOGY

FIGUKE 55. The Madison limestone and the Kibbey sandstone in the 

canyon of Antelope Creek where the creek breaches the hogback of the 

Madison limestone. The contact of the formations is at the top of the 

ledge on which the men stand. View east across the canyon.

type area at Quadrant Mountain in northwestern Yellowstone Park 

are of middle Pennsylvanian age. However, the term Quadrant 

formation as formerly used in central Montana was applied chiefly to 

rocks of Mississippian age, although rocks of Pennsylvanian and 

Permian ages may possibly have been included. Scott (1935, p. 1022) 

also extended the use of the term Amsden formation by applying it to 

strata in central Montana that lie beneath the Ellis group and above 

the Big Snowy group. In contrast to earlier usage of the term Quadrant formation, the term Quadrant quartzite is restricted at the present 

time to beds in southwestern Montana that overlie the Amsden formation and are approximately equivalent to the Tensleep sandstone. 

In that area there are probably no strata present that are equivalent 

in age to the Big Snowy group of central Montana. A comparison of 

these terms as used by various authors in central Montana is shown 

in the chart on the following page.
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 419

Comparison of names used for upper Paleozoic formations in central Montana 

(Dates refer to entries in bibliography)

According to Scott (1935, p. 1031) the Big"Snowy group is not older 

than Warsaw nor younger than upper Chester series and probably 

ranges in age from middle Valmeyer to middle Chester. An erosional 

unconformity separates the Big Snowy group from the underlying 

Madison limestone. The following descriptions of stratigraphic sections (1 and 2) of the Big Snowy group and of the Amsden formation 

indicate the characteristics of these strata in the southwestern part 

of the Stanford-Hobson area.
420 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Stratigraphic section 1. The Big Snowy group north of Woodhurst Mountain, sec.

2, T. 14 N., R. 11 E. 

Amsden formation. 

Big Snowy group:

Heath shale: Feet 

Shale, black; mostly covered______________________________ 175

Igneous rock in sill, mafic alkalic, deeply weathered__________ 40

Shale, black; mostly covered; loose fragments of aragonite on

covered slope________________________________________ 134

Igneous rock in sill, mafic alkalic, deeply weathered________ 60

Shale, black; mostly covered-_____________________________ 127

Total, Heath shale..-..,-__-___.___-_____________ 536

Otter formation:

Limestone, gray, dense; shale partings________-_---_______ 9. 5

Covered slope.__________________________________________ 86

Shale, light greenish-gray; interbedded with beds of thin gray 

limestone. __-___-_--______-_________________-_--___.__ 9,0

Limestone, gray, dense____________________-__--__________ 4

Shale, light-gray, light purplish-gray, light greenish-gray____ 47

Limestone, dark-gray, dense, bedded-______________________ 14. 5

Shale, gray; partly covered.______________________________ 38

Covered slope.__________________________________________ 120

Covered slope with limestone fragments in the soil___._______ 137

Total, Otter formation.____________________________ 546

Kibbey sandstone:

Sandstone, partly covered, yellow and yellowish-gray, finegrained sandstone fragments exposed on soil covered slope. _ 125 

Siltstone, red; mostly covered_______-______----------__--_ 65

Total, Kibbey sandstone.__________-_-___--____---_ 190

Total, Big Snowy group_____.-___---____.______ 1,272

Unconformity. 

Madison limestone.

Stratigraphic section 2. Big Snowy group and Amsden formation on the southeast 

side of the Blacktail Hills, sees. 19 and SO, T. 15 N., R. 11 E.

Ellis group.

Unconformity.

Amsden formation:

Limestone, gray to light-brown, dense; in beds as much as 3 ft thick, 80 

Covered slope.___--____-_--____-______-_____---_-----_-------- 330

Sandstone, light-brown, friable, thin-bedded to massive_____________ 90

Covered slope.________________________________________________ 80

Sandstone, light-brown, friable, massive._________________________ 120

Total, Amsden formation.________________________________ 700
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 421

Stratigraphic section 2. Continued 

Big Snowy group: 

Heath shale:

Shale, black; mostly covered, but exposures are occasionally

found in animal diggings; fragments of aragonite veins are Feet 

scattered on the ground._________________________________ 470

Total, Heath shale__.-----_..--._________________ 470

Otter formation:

Shale, pale-green and moderate-green; with limestone beds 2 to

6 in. thick at intervals of 5 to 10 ft-_-_-_________________ 220

Shale, light-gray and pale-green; with limestone beds as much as 3 

ft thick; 3-ft limestone bed at the top; grayish-yellow limestone 

beds interbedded with dark-gray shale and grading downward 

into the light-yellow sandstone beds of the Kibbey sandstone 

at the base._____________-------_-__-_--_-___----_---___ 200

Total, Otter formation_-_--__---_--_----_-----_----_- 420

Kibbey sandstone: Sandstone, light-yellow, fine-grained; mostly

covered, the basal 25 to 50 ft is a red siltstone_____-_-____-_-___ 250

Total, Kibbey sandstone.________________________________ 250

Total, Big Snowy group.___---___--__-_-__--__-_-________ 1, 140

Unconformity. 

Madison limestone.

KIBBET SANDSTONE

The Kibbey sandstone consists of red siltstone, sandstone, and 

shale in the lower part of the formation and of yellow to white sandstone or siltstone in the upper part and ranges from 150 to nearly 

300 feet in thickness. A fine- to medium-grained quartz sandstone 

in some exposures of the upper part of the Kibbey is friable and 

porous and is a potential reservoir rock for oil or gas. A bed of gypsum 20 feet thick, about 25 feet below the top of the Kibbey, is 

exposed in a prospect pit north of Wolf Butte (sec. 21, T. 16 N., 

R. 10 E.). The extent of the deposit is unknown. Elsewhere, thin 

lenses of gypsum were found in the Kibbey. The red sandstone, 

siltstone, and shale in the lower part of the Kibbey may represent the 

residual products formed by the erosion of the Madison limestone.

The Kibbey sandstone is formed from the first sediments to be 

deposited on the Madison limestone in this area after an interval of 

erosion, and variations in thickness of the Kibbey sandstone may be 

due in part to the irregular surface of the Madison limestone. The 

actual contact of the Madison and Kibbey is seldom visible, owing 

to the dense growth of conifers on the outcrops of these formations, 

so the physical evidence for an unconformity is rarely seen.
422 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Completely exposed sections of Kibbey were not found in the area, 

and the formation has been widely mapped on the evidence of red or 

yellow sandstone fragments on the soil-covered slopes. The contact 

of the Kibbey and Otter formations, where visible, is gradational 

through a vertical interval of 10 to 15 feet. Thus, thin-bedded 

yellow sandstone grades upward into gray and black shale, interbedded with thin layers of impure yellow limestone. Where poorly 

exposed, the contact of the Kibbey and Otter formations was established by the highest occurrence of yellow sandstone fragments in the 

soil at approximately the correct distance above the Madison limestone.

OTTER FORMATION

The Otter formation consists of 400 to nearly 500 feet of shale and 

limestone. Grayish-green to moderate-green shale is very characteristic of the Otter formation. The formation also includes some 

light-gray to purplish shale and many thin beds of gray limestone, 

which are generally less than 3 feet thick. The limestone beds, some 

oolitic, commonly occur in the lower half of the formation. Prominent beds of green shale, described by some geologists as "vivid green 

shale," usually occur in the upper 200 to 300 feet. Similar green shale 

does not occur in any of the younger formations in this area, and therefore the top of this interval is an important horizon marker. A bed 

of black shale about 8 feet thick is present in the upper part of the 

Otter formation in the East Buffalo Creek section, (stratigraphic 

section 9, p. 428) and suggests interfingering of the typical Heath 

shale with beds typical of the Otter formation. Between Running 

Wolf Creek and Lone Tree Creek in the western part of the area the 

contact of the Otter and Heath is marked by a thin, gray, silty limestone that forms a minor ledge on soil-covered slopes.

Few good exposures of the Otter formation can be found, because 

it is composed of a nonresistant sequence of strata and the outcrops 

are usually mantled with soil. The Otter formation and the Heath 

shale form valleys and gentle slopes between more prominent outcrops of the Madison limestone and the Amsden formation. East of 

the Judith River most of the outcrops of the Otter formation occur 

in zones of faulting or deformation, and most of the measurements 

of sections of these strata are not reliable (fig. 56).

HEATH SHALE

Heath shale is the term applied to the youngest division of the Big 

Snowy group. At its type locality in the Big Snowy Mountains 

(Scott, 1935, p. 1028) the formation consists of black petroliferous 

shale and has several sandstone beds in the upper part. The formation in the area mapped includes, in addition to these strata, dense,
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 423

FIGURE 56. The Otter and Swift formations exposed along a fault. The Otter formation forms the lightcolored slopes on the right and is apparently overlain by a ledge formed by the Swift formation above 

the fault. View looking across Antelope Creek in sec. 16, T. 12 N., K. 13 E.

black and gray limestone beds, granular, brown sandy limestone beds; 

north of Wolf Butte it contains gypsum. Typically a soft, slopeforming formation, the Heath is generally covered with soil and is 

commonly partly concealed by talus and landslide material from the 

Amsden formation above it. The thickness of the Heath shale where 

measured along the north flank of the Little Belt Mountains west of 

the Judith River ranges from 536 to 470 feet (stratigraphic sections 

1 and 2). The only exposure of Heath north of this outcrop belt is 

in the Skull Butte dome, where it is 466 feet thick as shown in stratigraphic section 3.

Stratigraphic section 3 Heath shale and Amsden formation at Skull Butte dome, 

sec. 6, T. IB N., R. 12 E.

Swift formation.

Unconformity.

Amsden formation: Feet 

Limestone, gray, dense, bedded______-_-.-_____-______.__________ 51. 5

Covered slope; red soil and red shale in the soil____________________ 220

Sandstone, red, arkosie; contains coarse grains of quartz, shell fragments, chert pebbles and limestone pebbles______________________ 15. 5

Covered slope.__-________-_-__-___--____-____-____--____--____ 67

Sandstone, brown, ferruginous, somewhat friable. _________________ 5

Total Amsden|formation_________________._______________ 359
424 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Stratigraphic section 3. Continued

Heath shale (Big Snowy group): Feet

Covered slope.________________________________________________ 57

Shale, black, fissile; lower part mostly covered____________________ 26

Sandstone, yellow to white, fine-grained, friable.__________________ 16

Shale, black; partly covered__-__---___-_________________________ 98

Sandstone, dark-brown, medium-grained; forms minor ledge__--___-_ 5

Shale, black, partly covered__.____________-_-___---_____________ 34

Limestone, gray._______________________________________________ 1

Shale, black; partly covered_____________________________________ 75

Igneous rock in sill, deeply weathered, mafic alkalic rock __________ 27

Shale, black; interbedded with thin beds of gray chert; partly covered. 32

Igneous rock in sill, deeply weathered, mafic alkalic rock_ __________ 1

Shale, black; with several thin beds of gray chert; partly covered..-. 38. 5

Limestone, dark-gray __________________________________________ 0. 5

Shale, black; partly covered_______________-_--__-_-_____-______.- 55

Total, Heath shale.-______.________.__..__._.__...________ 466

Otter formation.

Near the intrusive^igneous'rocks of the Little Belt Mountains, the 

Heath shale contains at least two sills of igneous rock. The sills, 

where not too. deeply weathered, form ledges on the soft-shale slopes 

and from a distance resemble sandstone beds. They may be traced 

along the outcrop for many miles. Although not included in the 

columnar section (pi. 46), the sills are mentioned in the descriptions 

of the stratigraphic sections.

Layers of aragonite, as much as 2 inches thick, that display columnar crystal structure were commonly observed in the Heath shale. 

The fact that unweathered fragments of these layers were found with 

brachiopods and other fossils attached to the upper surfaces, suggests 

that the layers are a primary depositional feature. The presence of 

such aragonite fragments on soil-covered slopes provides a good criterion for the identification of the Heath shale.

A relatively thin unit of Heath shale, only 270 feet thick, is overlain 

directly by an unusually thick unit of strata of the Ellis group where 

these formations were penetrated by the drill in sec. 32, T. 17 N., 

R. 14 E. This is interpreted as being due to pre-Ellis erosion which 

removed not only the Amsden formation, but also part of the Heath 

shale (fig. 57).

AMSDEN FORMATION

The type section of the Amsden formation, named by N. H. Darton 

(1906, p. 31-33), is in the Bighorn Mountains of Wyoming, where 

rocks of Mississippian(?) and Pennsylvanian ages are exposed, resting 

on the Madison limestone and overlain by the Tensleep sandstone. 

The term has since been applied by Scott (1934, p. 1020-1023) to 

rocks in a somewhat similar stratigraphic position in central Montana. 

Many geologists believe that the Amsden in the Stanford-Hobson
 v >______

Heath shale Amsden formation Pip

Big Snowy group 

Mid

MISSISSIPPIAN PENNSYLVANIAN 

CARBONIFEROUS
426 CONTRIBUTIONS TO ECONOMIC GEOLOGY

area is of Mississippian age, but the writer does not have sufficient 

evidence to prove or disprove this age assignment.

The Amsden formation in the Stanford-Hobson area is a mappable 

unit not defined by time boundaries and includes all the strata above 

the Big Snowy group and below the Ellis group. This definition is 

rather broad and includes strata of widely differing types and thicknesses. Stratigraphic sections of the Amsden formation measured in 

the Stanford-Hobson area range in thickness from 159 feet to 917 

feet. Stratigraphic sections (4-10 and pi. 47) show the changes in 

thickness and lithologic character of the formation.

Stratigraphic section 4- Upper part of Heath shale and Amsden formation east 

of Lone Tree Creek, sec. 3, T. 16 N., R. 10 E.

Swift formation.

Unconformity.

Amsden formation: peet 

Covered slope; reddish-orange soil containing limestone fragments.___ 88 

Limestone, gray, dense; massive near the base___________________ 27

Covered slope; soil contains pale-yellow, dense, thin-bedded limestone 

fragments __________________________________________________ 28

Covered; red soil with red shale in the soil________________________ 77

Sandstone, red, ferruginous; partly covered _______________________ 38

Sandstone, white, red, and yellow, crossbedded, friable; forms ledge. 60

Total, Amsden formation_______.____..____^.__ 318 

Heath shale (Big Snowy group):

Shale, black; mostly covered-.-----------.-------.-------.--- --. ... 171

Gypsum, gray; forms ledge,____________________________________ 22

(The lower part of the Heath shale was not measured)

Total, incomplete section, Heath shale.____________________ 193

Stratigraphic section 5. Upper part of the Heath shale, the Amsden formation and 

lower part of the Ellis group, northwest of Surprise Creek, sec. 12, T. 16 N., R. 10 E. 

Ellis group:

Sandstone, brown; contains chert pebbles and oyster shells (Not

measured) Feet 

Shale, brown; mostly covered_-____---_-_----___----_-----_---_- 11 

Limestone, brown, dense. ______________________________________ 2

Total, incomplete section, Ellis group ______________________ 13

Unconformity.

Amsden formation:

Covered.___-___-___-____-_____-----__--__--_-_---_-_---_-_-_- 26

Shale, red__.__-____-_-___-____---___-__________________ 16

Limestone, gray to white, dense.__________________----__----__-- 3

Covered.____-____________________--___-_-_______-___----___-- 5

Limestone, very light-gray to pale-yellow, fine-grained crystalline.___ 16 

Covered; limestone fragments on the ground-_____________________ 40

Sandstone, red, coarse-grained, crossbedded; forms ledge_________ 62

Total, Amsden formation_________________________________ 168
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 427

Stratigraphic section 5. Continued

Heath shale (Big Snowy group): Feet

Covered; sandstone talus from Amsden above.____________________ 16

Shale, black; mostly covered------------------------------------ 44

Limestone, brown, impure; weathers like sandstone.____-__-___-__- 1

Shale, black, fissile___________________________________________ 31

Limestone, brown, impure; contains silt and sand grains.___________ 1

Shale, black, fissile; covered at the base__________________________ 77

(The lower part of the Heath shale was not measured.)

Total, incomplete section Heath shale_____-________________ 170

Stratgraphic section 6. Amsden formation south of Surprise Creek, sec. 18,

T. 16 N., R. 11 E. 

Ellis group. 

Unconformity.

Amsden formation: peet 

Covered slope.________________________________________________ 72

Limestone, gray, dense; forms a ledge____________________________ 5

Covered slope._______________________________-_-__-_-___------ 11

Limestone, crystalline; weathers brown___________________________ 1. 5

Shale, red; with some yellow and brown siltstone; mostly covered__ 32. 5 

Conglomeratic sandstone, brown, thin-bedded.____________________ 1

Sandstone, red, locally mottled with white; ferruginous, crossbedded; somewhat friable_-_-_-___-_-_-______________-________ 36

Total, the Amsden formation. ____________________________ 159

Stratigraphic section 7. Upper part of the Amsden formation along Dry Wolf Creek,

sec. 5 T. 15 N., R. 11 E. 

Ellis group.

Unconformity. v

Amsden formation: Feel 

Limestone, gray, dense.________________________________________ 22

Shale, red_.-__.____________________________________ 28

Limestone, gray, dense.________________________________________ 1

Shale, red______-_--_______--__--_-_____________________ 56

Sandstone, red________________________________________________ 3

Sandstone, yellow, thin-bedded, ripple-marked; some red shale

partings.___________________________________________________ 22

Covered._____________________________________________________ 50

Sandstone, yellow to white, medium-grained, massive to crossbedded, somewhat friable; forms ledge________________________ 22

Sandstone, brown; in beds 0.5 to 3 ft thick with black shale partings

up to 4 in thick_____________________________________________ 54

Sandstone, brown to white, massive to crossbedded, fine-grained, 

somewhat friable____________________________________________ 34

Total, incomplete section, Amsden formation, base not exposed. 292

Stratigraphic section 8. Upper part of the Heath shale and the Amsden formation,

northwest side of Willow Creek, sees. 34 and 35, T. 15 N., R. 11 E. 

Ellis group. 

Unconformity. 

Amsden formation:

Limestone, gray, dense; in beds from 5 to 10 ft thick, with thin partings of red shale___________________________________________ 135
428 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Stratigraphic section 8. Continued

Amsden formation Continued Feet 

Covered; red soil with red shale in the soiL_______________________ 60. 5

Sandstone, white to orange, friable.___-___--___--________._____ 5

Covered._____________________________________________________ 56

Sandstone, white, orange and red, medium-grained, bedded; color

varies rapidly both horizontally and vertically.___________________ 64

Covered.. ______________________________________________ 102

Sandstone, gray, in part massive, friable__-_____--_-______________ 18

Covered___-_________-______________________________________ 133

Sandstone, brown, medium- to coarse-grained; weathers into large

rough-surfaced blocks ________________________________________ 15

Covered slope._________________________________________:______ 82

Sandstone, red, coarse-grained, locally conglomeratic, hematitic;

breaks into angular fragments... __________ ___________________. 38. 5

Total, Amsden formation.._______________________________ 709

Heath shale:

Covered slope; talus from the sandstone beds of the Amsden formation above-___________-__-_______-_-___--______-___-__-_.___ 77

Shale, black, fissile; partly covered__----_-----_-_-------_________ 131

Limestone, black, dense._______________________________________ 10

(The lower part of the Heath shale is covered.)

Total, incomplete section Heath shale-__-___-_______________ 218

Stratigraphic section 9. Ellis group, Amsden formation, and Otter formation 

(Big Snowy group), on East Buffalo Creek, sees. 11 and 14, T. 12 N., R. 16 E

Morrison formation. 

Ellis group:

Swift formation: Feel 

Sandstone, brown, fine-grained, glauconitic___________________ 75

Siltstone, grayish-yellow_--__--_-_-_-_-_-_-_-_-___----______ 5

Total, Ellis group___________________________________ 80

Unconformity. 

Amsden formation:

Limestone, light-gray to pale grayish-orange, coarsely crystalline,

fossiliferous. ____________-_______-__-____-_---_-_-_---_-_-___ 152

Covered slope..__________--___-_-____-_-_-_--_--_--_-----_-_-_ 55

Limestone, light-gray, dense to fine-grained, granular._______________ 10

Covered slope.________________________________________________ 90

Limestone, light-gray, dense to fine-grained, granular; contains

brachiopods _______________________________________________ 25

Covered slope.________________________________________________ 160

Sandstone, yellowish-orange, fine-grained, thin-bedded ______________ 9

Siltstone, red, and red, coarse-grained, arkosic and conglomeratic

sandstone; contains fossil tree trunk __ ________________________ 50

Limestone, light-gray, dense; forms ledge_________________________ 4

Shale, red; with thin beds of red hematitic sandstone._____________ 50

Sandstone, grayish-orange, very fine grained___._______-____-_-_-- 6

Shale, red, and red siltstone.._.__________-__--________---___---_ 106
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 429

Stratigraphic section 9. Continued

Amsden formation Continued Feet

Sandstone, grayish-orange, very fine grained______________________ 22

Covered.. _____________-----_-_______--_______-_______ 105

Shale, red, and red silt; mostly covered__________________________ 70

Sandstone, grayish-orange, quartzose, scattered limonite stains._____ 3

Total, Arusden formation.. __________ _____________________ 917

Otter formation (Big Snowy group):

Covered slope.____________-_-_--_-_-______-_-_____---_________ 90

Shale, light-green; interbedded with thin limestone beds____________ 132

Shale, black, fissile; contains pyrite nodules weathering to limonite_ 8 

Limestone, gray; in part oolitic, in part fragmental._______________ 2. 5

Shale, moderate-green; lower part mostly covered_______________ 47. 5

Limestone, light yellowish-gray, massive, dense__________________ 10

Covered slope.________________________________________________ 20

Limestone, light-gray, o6litic__-___-_-___________________________ 5

Covered slope.________________________________________________ 25

Limestone, light yellowish-gray, somewhat mottled________________ 8

Total, incomplete section, Otter formation._________________ 348

. (The lower part of the Otter formation is absent, owing to 

faulting.)

Stratigraphic section 10. Upper part of the Otter formation and the Amsden formation, exposed on Antelope Creek, sees. 9 and 16, T. 12 N., R., 13 E. 

Ellis group. 

Unconformity.

Amsden formation: peet 

Covered slope.________________________________________________ 100

Limestone, light-gray, dense; in beds 3 to 6 ft thick with red shale

partings; partly covered-_.___________________________________ 310

Sandstone, brown and yellow, medium-grained, friable, crossbedded;

partly covered._____________________________________________ 70

Sandstone and conglomerate, red; partly covered. _________________ 110

Limestone, gray, dense, cherty______.____.______________________ 30

Total, Amsden formation....___________________________ 620

Otter formation (Big Snowy group):

Shale, green, olive-green, and brown, interbedded-.________________ 5

Shale, green, purple, red, and gray, interbedded; and one thin bed of

green limestone____________________________________________ 15

Covered slope.________________________________________________ 95

Shale, moderate-green; interbedded near the top with thin beds of

purplish limestone that contain pyrite crystals __________________ 185

Limestone, light-gray to purplish-gray, ripple-marked; in part a breccia,

contains fossil fragments.-'__________________________________ 3

Shale, moderate-green______________________________________ .___ 27

Total (incomplete thickness), Otter formation.______________ 330

(Lower part of the Otter formation is not exposed, owing to 

faulting.)
430 ' CONTRIBUTIONS TO ECONOMIC GEOLOGY

Two distinct lithologic units are recognizable in most stratigraphic 

sections of the Amsden formation that were measured in the StanfordHobson area. A lower, clastic unit contains red shale and siltstone, 

and red, brown, or white sandstone and conglomerate, and an upper 

unit contains thick beds of dense, gray limestone with some gray 

and red shale partings. Two additional units are present along Antelope and Wait Creeks, in T. 12 N., R. 13 E. At the base of the 

Antelope Creek section (sectkm|lO) a marine limestone underlies the 

lower, clastic unit, and at the top of the Wait Creek section (section 

11) another clastic unit] containing white sandstone and red shale 

is present. Locally, only one unit is present. In the area just west 

of Dry Wolf Creek, the upper unit (the top unit in stratigraphic 

section 7) thins westward and locally is completely missing. Where 

the hogback of the Amsden formation is crossed by Surprise Creek 

the lower, clastic unit consists of a red sandstone bed 62 feet thick 

(lowest unit in stratigraphic section 5). About half a mile south of 

Surprise Creek this red sandstone unit thins to 36 feet (lowest unit 

in stratigraphic section 6) and about 1 mile south of Surprise Creek 

the upper limestone unit is separated from the Heath shale by only 

a thin, covered interval. This thinning or disappearance of the 

lower, red sandstone may be evidence of an unconformity within 

the Amsden, or it may be due to nondeposition.

An unconformity that marks an important hiatus separates the 

Amsden formation from the overlying Ellis group. Over a large 

area this unconformity is characterized by progressive overlap of 

Jurassic rocks over older rocks of the Amsden formation, Big Snowy 

group, and Madison limestone. The unconformity does not show 

as an angular discordance at any exposure in the Stanford-Hobson 

area but was recognized by the differences in the lithology and fauna 

of the Amsden formation and the Ellis group. Pre-Ellis erosion has 

removed the Amsden formation from most of the area north of a line 

drawn just north of the towns of Geyser, Stanford, Moccasin, and 

Moore, as shown on figure ,58. The Ellis group rests on the Heath 

shale in exposures along South Moccasin Mountain and also in the 

Pacific Western, Todd no. 1 well, sec. 32, T. 17 N., R. 14 E. (see 

table 2, p. 416). Data from the drillers' logs of other wells in the area 

are of questionable reliability, but it is estimated that from 180 to 

210 feet of Amsden strata were found in the Nesson Rock Creek 

wvells in sec. 36, T. 14 N., R. 16 E. A thickness map of the Amsden 

formation (fig. 58), based on the limited data now available, shows the 

distribution and thickness of the Amsden formation in the area mapped.

The Amsden formation is a resistant unit that holds up the outer 

hogback along the northeast flank of the Little Belt Mountains. At 

the eastern end of the Little Belt Mountains, at the western end of
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 431

R. 1O E. R. II E. 

EXPLANATION

X 585 T

Location of measured thickness 12

FIGURE 58. Thickness map of the Amsden formation.

the Big Snowy Mountains, and along the Little Belt Mountains west 

of Antelope Creek the limestone beds of the upper unit of the Amsden 

formation generally form the highest ridges stratigraphically above 

the Madison limestone (fig. 59).

It is sometimes difficult to determine accurately the boundary of 

the Big Snowy group and the Amsden formation, because the units 

do not have constant lithologic characteristics. The contact of the 

Heath shale and the Amsden was generally chosen so as to place 

all thick units of black shale in the Heath. In the stratigraphic 

section exposed at Skull Butte (section 3, p. 423) a sandstone bed 16 

feet thick and similar to sandstone beds in the Amsden formation is
432 CONTRIBUTIONS TO ECONOMIC GEOLOGY

FIGURE 59. Hogback formed by the Amsden formation. In the center of the picture the i, : . .ni. t 

limestone beds in the upper part of the Amsden formation dip steeply to the right (north). View is west 

across Willow Creek in sec. 10, T. 18 N., R. 16 E.

overlain by a partly covered, but apparently thick, unit of black shale. 

It was decided to place both the sandstone and the black shale in 

the Heath shale, in order to preserve the lithologic definition of the 

Heath shale.

In some exposures the contact between the Heath shale and Amsden formation is not readily apparent. This suggests that the two 

formations may be transitional or interfingering. Walton (1946, 

p. 1304), following a different line of reasoning, considers that the 

absence of the Heath shale in localities near Judith Gap is a result 

of erosion after gentle folding and before the deposition of the Amsden. 

He thus places an erosional unconformity between the Heath and 

Amsden. Within the area studied, no physical evidence other than 

the local thinning and disappearance of Heath black shale below 

typical Amsden was found to confirm Walton's interpretation. Other 

structural and sedimentary explanations for the local thinning and 

disappearance of the black shale may be postulated. At East Buffalo 

Creek (stratigraphic section 9, p. 427), though no black shale is exposed, the lower Amsden is unusually thick, and it may be that part 

of the lower Amsden is equivalent to the Heath. There may even 

be units of black shale represented by the covered intervals in the 

Amsden section at this locality. Thinning and local absence of 

black shale at the east end of the Little Belt Mountains and in the 

Antelope Creek sections (stratigraphic section 10, p. 429) may be
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 433

due to intense structural deformation that caused the nonresistant 

black shales to be either squeezed out or faulted out.

Though the contact of the Amsden formation and the Ellis group 

is usually easy to distinguish, in an area west of Antelope Creek and 

on both sides of the Judith Kiver the contact is problematical. Stratigraphic section 11 (below) was measured along a, branch of Wait 

Creek and illustrates this difficulty. In this section the upper part of 

the Amsden formation consists of limestone and sandstone beds that 

are overlain by 5 feet of conglomerate and by 43 feet of dark-red shale 

that contains 2 thin beds of limestone. This in turn is overlain by 

5 feet of fossil-fragmental limestone and 15 feet of brown shale that 

contain Jurassic fossils. It is not certain that the conglomerate and 

the dark-red shale are correctly assigned to the Amsden. Imlay 

(Imlay and others, 1948, locality 20) recognized similar beds in the 

Piper formation along the Judith River, 3% miles southwest of Utica, 

but he collected no fossils. This interval of redbeds has been tentatively included in the Amsden formation in the present report and is 

differentiated as Amsden(?) formation in stratigraphic section 11.

Another interpretation of this section is that the white friable sandstone at the top of the known Amsden formation is equivalent to the 

Tensleep sandstone, and the dark-red shale above this sandstone represents the Chugwater formation. These formations extend into 

southern Montana from Wyoming, but are not known to be present 

in the Judith Basin.

Throughout most of central Montana pre-Eilis erosion removed 

the Tensleep and the Chugwater, but it is quite possible that there 

are some remnants. This interpretation was applied by Gardner and 

others (1946, p. 50, 52) to similar strata exposed on the south flank of 

the Big Snowy Mountains.

Stratigraphic section 11. Ellis group and the upper part of the Amsden formation

on a branch of Wait Creek, sec. 1, T. 13 N., R. 12 E. 

Morrison formation. 

Ellis group:

Swift formation: peet 

Sandstone, brown, medium-grained, unfossiliferous.-_._________ 15

Sandstone, gray, medium- to coarse-grained, crossbedded, glauconitic; contains chert pebbles and oyster shells. ____________ 20

Sandstone, brown, fine- to medium-grained, thin-bedded, glauconitic.__---_-_-___-_-_________---_____________________ 23

Conglomerate, brown; with chert pebbles, oyster shells and 

belemnites in a matrix of sand. ___________________________

Rierdon formation: Shale, brown, fossiliferous___________________

Piper formation: Limestone, brown, granular, consisting of broken 

fossil fragments.____________________________________________

Total, Ellis group__________________________________ 85

Unconformity.

354332 56 5
434 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Stratigraphic section 11. Continued

Amsden(?) formation: peef 

Shale, dark red_-_----_-_-_-_-________-_______--___-_-_-_-___ 13

Limestone, gray, dense._-___-__-______________'__------_-_______ 0. 5

Shale, dark-red._______________________________________________ 5

Limestone, gray, dense.________________________________________ 0. 5

Shale, dark-red______________________________________________ 24

Conglomerate, brown, angular chert and quartz pebbles in a matrix 

of sand_____________________________________________________ 5

Total, beds of Amsden(?) age. ____________________________ 48

Amsden formation:

Sandstone, white, fine-grained, friable. ___________________________ 3

Limestone, pale yellowish-gray to white, dense ____________________ 1

Sandstone, white, fine-grained, friable, bedded_____-____-__________ 18

Limestone, pale yellowish-gray to white, dense. ___________________ 30

Sandstone, white, fine-grained, friable, only the top exposed. 

(The lower part of the Amsden formation was not measured.)

ELLIS GROUP

Use of the term Ellis for rocks of Jurassic age dates back to some of 

the earliest geologic mapping in western Montana (Peale, 1893, 1896, 

Iddings and Weed, 1894). The name is taken from old Fort Ellis, 

near Bozeman, Montana, which is now considered to be the type locality. It is applied to marine Jurassic strata in Montana and adjacent areas of Wyoming. Cobban (1945) raised the Ellis formation 

of north-central Montana to group status and divided it into the 

Sawtooth, Rierdon, and Swift formations. Imlay and others (1948) 

applied the name Piper formation to strata in central Montana and 

showed that the Piper is correlative with Cobban's Sawtooth formation but of a different facies.

Stratigraphic section 12. Swift and Morrison formations on the east side of Skull 

Butte dome, Sec. 5 T. 15 N., R. 12 E.

Kootenai formation.

Unconformity.

Morrison formation: Feet 

Coal. ________________________________________________ 3

Covered, gray shale in the soil_ _________________________________ 80

Siltstone, brown, ferruginous, interbedded with gray shale. _________ 5

Covered, gray shale in the soil. _________________________________ 13

Limestone, brownish-gray, impure.______________________________ 1

Covered, gray shale in the soil_ _________________________________ 11

Sandstone, brown, fine-grained._________________________________ 3

Shale, greenish ________________________________________________ 5

Covered, red shale in the soil____________________________________ 22

Limestone, gray, dense, interbedded with gray shale.______________ 3

Shale, varicolored, mostly covered.______________________________ 26

Total, Morrison formation________________________________ 172
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 435

Stratigraphic section 12. Continued

Swift formation: pcet 

Sandstone, brown, fine- to medium-grained, speckled with limonite__ 49 

Sandstone, gray to brown, coarse-grained, speckled with dark minerals, including, probably, glauconite _________________________ 10

Total, Swift formation. __________________________________ 59

Unconformity. 

Amsden formation.

Stratigraphic sections of the Ellis group where it was measured 

north of the Little Belt Mountains indicate that the sequence consists 

chiefly of medium- to coarse-grained brown glauconitic sandstone belonging to the Swift formation. Locally the sandstone contains chert 

pebbles as much as one inch in diameter and, in some exposures, an 

abundance of fragmental shells of oysters and other pelecypods. In 

exposures on a branch of Wait Creek (Stratigraphic section 11) and 

on Judith River, from 15 to 20 feet of brown fossiliferous shale and 

brown fragmental limestone that lie below the Swift formation probably represent the Bierdon and Piper formations. In exposures along 

the north flank of the Little Belt Mountains, the Ellis group is commonly about 60 feet thick and is rarely more than 100 feet thick. 

About 250 feet of Ellis was found in the Nesson Rock Creek wells, sec. 

36, T. 14 N., R. 16 E.; 440 feet is exposed on the north side of South 

Moccasin Mountain (Calvert, 1909, p. 21); and 495 feet was found in 

the Pacific Western, Todd No. 1 well, sec. 32, T. 17 N., R. 14 E. (table 

2, p. 416). These changes in thickness of the Ellis group are explained by the fact that the Ellis was deposited on an irregular surface that resulted from a long interval of erosion. The oldest Ellis 

strata were deposited on the low parts of the old erosion surface in 

the northern part of the area, and only the younger beds in the Ellis 

group covered higher parts of the surface, near the site of the present 

Little Belt and Big Snowy Mountains.

To the north the Ellis group rests on 'successively older beds of 

the Amsden formation and the Big Snowy group. For several miles 

north of the Little Belt Mountains the combined thickness of the 

Amsden formation and the Ellis group probably remains nearly constant, because the Ellis group thickens toward the north, whereas the 

Amsden formation thins northward and disappears. At some localities north and northwest of the area mapped the Ellis group rests 

directly on the Madison limestone. Although the occurrence of a 

hiatus is unquestioned, no angular unconformity was observed at the 

base of the Ellis and the only local criteria that indicates the position 

of the uncomformable contact are the marked faunal change between 

the Ellis and underlying beds and a conglomeratic sandstone at the 

base of the Swjft formation where it rests on pre-Jurassic rock.
436 CONTRIBUTIONS TO ECONOMIC GEOLOGY

MORRISON FORMATION

The Morrison formation, an Upper Jurassic continental deposit, is 

recognizable over large areas in the Rocky Mountain Region. The 

name is derived from the town of Morrison, near Denver, Colorado, 

where the type section is exposed. The name was first used in central 

Montana in 1909 by Calvert (1909, p. 22-24) and Fisher (1909, p. 

28-30), though in a more restricted sense than it is today. The top 

of the Morrison as then mapped was from 60 to 90 feet below the coal 

bed that is now included in the Morrison (Cobban, 1945, p. 1269; R. 

W. Brown, 1946). A comparison of the terms used by other geologists in mapping the strata above the Ellis and below the Colorado 

shale is shown in table 3.

TABLE 3. Comparison of names used for Upper Jurassic and Lower Cretaceous 

formations in central Montana

Weed, 1899

Colorado formation

Dakota formation

Cascade formation

Ellis formation

Calvert, 1909 and 

Fisher, 1909

Colorado shale

Kootenai formation

Morrison formation

Ellis formation

Reeves, 1930

Colorado shale

Kootenai and 

Morrison formations 

undifferentiated

Ellis formation

Cobban, 1945, and 

present report

Colorado shale

Kootenai formation

Basal sandstone of 

Kootenai formation

Morrison formation

Ellis group

In the Judith Basin area the Morrison formation consists of 150 to 

300 feet of variegated shale and siltstone, thin nodular limestone, 

white and brown sandstone, black shale, and, near the top, coal or 

carbonaceous shale. Owing to the lenticular nature of the freshwater deposits in the Morrison, only such a general description of the 

lithologic character of the formation can be made.

Dinosaur bones, gastroliths, and fossils of fresh water clams are 

common in the Morrison formation. Stratigraphic sections 12 and 13 

show the thickness and lithologic character of the formation at two 

localities in the area.
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 437

Stratigraphic section 18. Morrison formation on the southeast side of Dry Wolf 

Creek, sees. 8 and 9, T. 15 N. R. 11 E.

Kootenai formation.

Unconformity.

Morrison formation: feet 

Covered slope; sandstone talus from the basal Kootenai above______ 16

Shale, gray; partly covered.____________________________________ 19

Sandstone, brown, silty______________.__________________________ 2

Shale, gray; partly covered-____________________________________ 12

Limestone, gray, impure____-___________________________________ 1

Shale, gray___--_-____-____-_____-_--_________-______-___-____ 42

Sandstone, reddish-brown, silty________________________________ 3

Shale, reddish and greenish _____________________________________ 6

Siltstone; brownish-orange and thin gray limestone________________ 5

Limestone, gray; somewhat chalky_______________________________ 4

Shale, reddish and greenish; partly covered _______________________ 2

Sandstone, brown, well-cemented, medium-grained______:________ 1

Shale and siltstone, interbedded; mostly covered__________________ 4.7

Total, Morrison formation________________________________ 160

Swift formation.

The coal bed in the upper part of the Morrison has an average 

aggregate thickness of from 3 to 6 feet of coal, but locally is absent. 

A detailed description of the coal beds and of the mining districts 

once active in this area, is presented in the reports by Calvert (1909, 

p. 56-78) and Fisher (1909, p. 50-81). In these reports the coal bed 

was included in the Kootenai formation. In the area south and west 

of Skull Butte, a sandstone below the coal bed in the Morrison cannot 

be distinguished in mapping from the overlying basal sandstone of the 

Kootenai.

The Morrison formation generally forms gentle slopes and valleys 

between the ridges formed by the underlying Swift formation and the 

overlying basal sandstone of the Kootenai. For this reason, it is 

rarely well exposed. Where these bounding sandstone beds are 

steeply dipping, as along the mountain front, the Morrison is usually 

thin, probably because of squeezing that accompanied the structural 

deformation.

An erosional unconformity separates the Upper Jurassic Morrison 

formation from the Lower Cretaceous Kootenai formation. The 

relief on the surface of the unconformity is not great, but it is sufficient to cause the stratigraphic interval between the coal bed and the 

Kootenai formation to vary in thickness as much as 15 feet. Locally 

the coal bed is missing, owing to erosion.

KOOTENAI FORMATION

The use of the term "Kootenai" was extended into Montana from 

Canada, where it had long been applied to the basal lower Cretaceous 

strata. In central Montana the Kootenai formation is a continental
438 CONTRIBUTIONS TO ECONOMIC GEOLOGY

deposit and includes all strata above the Morrison formation and 

below the marine Colorado shale. The Kootenai formation as 

mapped by Calvert (1909, p. 24-28) and Fisher (1909, p. 30-36) 

included the coal bed that in this report is placed in the Morrison and 

also from 60 to 90 feet of strata below the coal. Reeves (1930, p. 139), 

on the other hand, recognized the term Kootenai, but mapped the 

Morrison and the Kootenai formations as one undifferentiated unit. 

The Kootenai formation consists of a sequence of red shale, siltstone, 

thin nodular freshwater limestone, and yellow and brown sandstone of 

continental origin that extends from the base of a prominent sandstone 

bed, about 100 feet thick which overlies the coal bed in the Morrison 

to the base of the thin-bedded brown sandstone and black, sandy shale 

that characterize the lower part of the Colorado shale. In some areas 

limestone pebble and siltstone pebble conglomerate beds are abundant 

in the Kootenai. The observed thickness ranges from 350 to 550 

feet. Stratigraphic section 14 shows the detailed character of the 

formation. Dip readings at the base of the section measured 22 

degrees, whereas at the top of the section they measured 16 degrees. 

The thickness of the section was calculated on the assumption that the 

dip gradually changed from the maximum to the minimum angle.

Stratigraphic section 14- Kootenai formation exposed on east side of Skull Butte 

dome, sec. 5, T. 15 N., R. 12 E.

Colorado shale.

Kootenai formation: Feet 

Shale, red; mostly covered; red soil with red shale in soil. __________ 185

Shale, brown, calcareous; interbedded with limestone, impure gray___ 10. 5 

Covered;red, sandy soil_______________ _________________________ 52

Sandstone, light brown; speckled with limonite; partly covered______ 10

Covered slope;red soil- --______-_____________--_--___-_---___-_ 31

Sandstone, brown, fine- to medium-grained; containing siltstone and 

limestone pebbles; partly covered-_______-_-____--.__-_---_---- 5

Covered slope; red soil-----____-_---____--_-_-----_---__------- 19

Sandstone, brown______________________________________________ 0. 5

Covered slope; red soil- ________________________________________ 41

Sandstone, brown, medium- to coarse-grained; containing pebbles of 

limestone and siltstone___________________________________-_-_ 5

Covered slope.________________________________'____________-___ 33

Sandstone, brown, fine-grained, thin-bedded. _____________________ 4. 5

Red shale; mostly covered; red soil__ ____________________________ 15

Limestone; gray; with limestone-pebble conglomerate at top. _______ 3

Siltstone, gray, calcareous, thin-bedded; weathers brown____________ 2. 5

Covered-_ ___-___--_-___-____________________--------__--_---- 13

Sandstone, gray, "salt and pepper" appearance, medium- to coarsegrained, crossbedded; weathers into large blocks.-_______________ 122

Total, Kootenai formation, ,,,,,,__,,-,-,-,-,--,,---.------ 552

Unconformity. 

Morrison formation.
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 439

The most prominent bed persistent enough to be useful in mapping 

the Morrison and Kootenai part of the stratigraphic column is a sandstone bed averaging 100 feet in thickness that is found at the base of 

the Kootenai. For convenience in mapping and in order to obtain the 

control that is necessary to determine geologic structure this basal 

sandstone bed was mapped as a separate unit. It consists of a 

medium- to coarse-grained, impure, dark-brown to gray, crossbedded 

sandstone and chert-pebble conglomerate, which weathers into large 

angular blocks. It is continental in origin and somewhat lenticular 

in nature, but is persistent enough in the area of outcrop to be readily 

identified. A similar sandstone of approximately the same age, in the 

Cat Creek oilfield is known by drillers as the Third Cat Creek sand, 

and another in the Cut Bank oilfield is called the Cut Bank sand. 

Sandstone was not reported at this zone in the drill cuttings from the 

Pacific Western, Todd no. 1 well, sec. 32, T. 17 N., R. 14 E., (table 2) 

but this may be due to poor samples, because the electric log of this, 

well shows a deflection characteristic of a sandstone at this horizon.

Many of- the early oil test wells in this area were drilled only deep 

enough to test the Kootenai, because the sandstone beds in the 

Kootenai were known to produce oil in the Cat Creek area and the red 

shale of the Kootenai made a good horizon marker that was readily 

recognized by the drillers.

COLORADO SHALE

The Colorado shale and the Telegraph Creek formation underlie 

most of the Stanford-Hobson area. The soft shale strata that charac 

terize these formations are easily weathered, and as large parts of the 

outcrop area are blanketed by deposits of terrace gravel, there are 

few good exposures of the bedrock. In other areas, where well exposed, the Colorado shale can be subdivided into smaller units of 

formational rank (Cobban, 1951). However, in the Stanford-Hobson 

area the unit is poorly or not at all exposed in the uplands, and it was 

not feasible to map individual formations.

The Colorado shale includes strata of both Early and Late Cretaceous age. Cobban and Reeside (1951) have demonstrated the 

existence of an Early Cretaceous fauna in the Mowry shale and 

equivalent rocks in Colorado, Wyoming and Montana. This places 

the Early-Late Cretaceous boundary between the rocks probably 

equivalent to the Mowry shale and the rocks probably equivalent to 

the Belle Fourche shale; the boundary is about 850 feet above the 

base of the Colorado shale in the Stanford-Hobson area.

ROCKS EQUIVALENT TO THERMOPOLJ8 SHALE

The lowest 720 feet of strata in the Colorado shale consist of darkgray to black fissile shale, sandy shale, and thin beds of brown sand-
440 CONTRIBUTIONS TO ECONOMIC GEOLOGY

stone that are generally not very fossiliferous and are similar in 

lithology and probably equivalent in age and stratigraphic position 

to the Thermopolis shale in the Bighorn Basin of northwestern Wyoming. The contact of the Colorado shale and Kootenai formation 

is marked in the Stanford-Hobson area by a thin-bedded sandstone 

at the base of the strata equivalent to the Thermopolis shale. This 

sandstone is very fine-grained, with thin yellow- and brown-colored 

laminae and prominent fucoidal markings. It is generally underlain by 

red shale and overlain by black shale. Where well exposed, this sandstone forms a minor ledge, but it was frequently necessary to search for 

fragments of this sandstone in the soil in order to locate the Kootenai 

and Colorado shale contact.

The following stratigraphic sections (15 and 16) of strata equivalent 

to the Thermopolis shale are composite sections made up of several 

incomplete sections measured west and northwest of Geyser.

Stratigraphic section 15. The part of the Colorado shale probably equivalent to the 

Thermopolis shale and the lower part of the Mowry shale, sees. 26, 34, and 35, 

T. 18 N., R. 9 E.

Colorado shale:

Rocks probably equivalent to the lower part of the Mowry shale: feet 

Tuff, bentonitic, white to light gray, massive___________ _______ 8

Covered slope; soft, gray clay soil____-_-__-__-_-__-___-_____ 24

Bentonite, pale-olive-yellow-________________________________ 8

Total, incomplete thickness, rocks probably equivalent to 

part of the Mowry shale____________________________ 40

Rocks probably equivalent to the Thermopolis shale:

Shale, black, partly covered__-_--_--_-___--_-____-_-______ 11

Shale, silver-gray; contains fish scales; resembles the Mowry 

shale,__________________________________________________ 2

Covered slope.-_-______--_-_-__-____----______--___-_-____ 11

Igneous rock in sill; deeply weathered mafic alkalic_ ___________ 2

Shale, black; mostly covered--___-__________________________ 66

Shale, light-gray, bentonitic______-_____--..__-_--____________ 4. 5

Shale, dark-gray_______--___________--_-___-_--____________ 6. [

Sandstone, gray, mottled; medium-grained; forms ledge_______ 3

Shale, dark gray; interbedded with thin sandstone beds, mostly 

covered ____________________________________________________ 80

Sandstone, brown, medium-grained, impure; forms ledge____________ 2. 5

Shale, gray, sandy__--_____-____________-__-__-__-_____-____--_ 8

Sandstone, brown, medium-grained, impure; forms ledge_-____-_'-___ 6. 5

Covered slope; talus from overlying sandstone beds________________ 207

Shale, black, fissile______________________________________-_____- 58

Igneous rock in sill; deeply weathered mafic alkalic________________ 3

Shale, dark-gray; interbedded with thin sandstone beds; partly 

covered ____________________________________________________ 12

Sandstone, thin hard___________________________________________ 1

Covered slope; probably dark-gray shale__________--____---_-___ 15. 5

Sandstone, gray and brown mottled, fine-grained._________________ 0. 5
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 441

Stratigraphic section 15. Continued

Colorado shale Continued Feet 

Covered; probably dark gray shale_____________________________ 21

Igneous rock in sill; fine-grained, hard-_-----_--__---_-_-_---_-__- 3 

Shale, dark-gray; interbedded with thin sandstone beds; partly

covered.___________________________________________________ 30. 5

Sandstone, gray and brown mottled; contains 7nocera?miS___________ 1

Shale, black, interbedded with thin sandstone beds; partly covered. _ 104 

Igneous rock in sill; deeply weathered mafic alkalic________________ 3

Shale, black, sandy; interbedded with thin beds of laminated brown

and yellow sandstone; mostly covered_________-_-_-__--________ 52

Sandstone, brown and yellow laminae, thin-bedded, fine-grained,

fucoidal markings_--__--_--------_---_--__---_-_---_----_--__ 2. 5

Shale, dark-gray, fissile_--__________________________'____________ 8. 5

Sandstone and sand, gray, massive, friable__--_--__--_____-_______ 5. 5

Total, 1 rocks probably equivalent to the Thermopolis shale. _ _ 720 

Kootenai formation.

1 Does not include thickness of sills of igneous rock.

Stratigraphic 16. The part of the Colorado shale probably equivalent to the Lower part 

of the Mowry shale and the upper part of the Thermopolis shale, sees. SO and 31, 

T. 18 N., R. 10 E.

Colorado shale:

Rocks probably equivalent to the lower part of the Mowry shale: Feet 

Tuff, bentonitic; weathers white_____________________________ 7. 5

Bentonite, gray.__________________________________________ 15. 5

Shale, gray, bentonitic.____________________________________ 4

Bentonite, gray; weathers white.____________________________ 9

Total incomplete thickness, rocks probable equivalent to 

part of Mowry shale__-------____-__--___-_--______ 36

Rocks probably equivalent to the upper part of the Thermopolis shale:

Covered slope_____________________________________________ 46

Sandstone, medium-grained, crossbedded.____________________ 2

Shale, dark gray, partly covered__.__________________________ 18. 5

Bentonite, gray_____-__-______---_-__-_-__---_-___-____-_ 15. 5

Covered interval; overlies a sandstone bed believed to be equivalent to the 2.5-foot thick ledge forming sandstone about 186 

feet below the top of the Thermopolis shale equivalent in Stratigraphic section 15_--__-_-_---_--__-__--__--.__--__--_--__ 118

Total, incomplete thickness, rocks probably equivalent to 

upper part of the Thermopolis shale _________________ 200

ROCKS EQUIVALENT TO MOWRY SHALE, BELLE FOURCHE SHALE, AJSTD 

GREENHORN LIMESTONE

Rocks probably equivalent to the Mowry shale, the Belle Fourche 

shale, and the Greenhorn limestone overlie the Thermopolis-shale 

equivalent in the Stanford-Hobson area. The Mowry shale at its 

type locality in Wyoming consists of silicious shale that weathers 

silver gray and contains numerous fish scales and some thin beds of
442 CONTRIBUTIONS TO ECONOMIC GEOLOGY

bentonite. Exposures of the Mowry equivalent in the north bank 

of Arrow Creek northeast of Geyser, however, consist of approximately 

40 feet of bentonite, tuffaceous shale and porcelanite in the lower part, 

overlain by about 95 feet of gray and brown soft interbedded shale, 

sandstone and bentonite in the upper part. The predominantly 

bentonitic lower part weathers to a distinctive white or light-gray 

outcrop what was mapped locally for structural control. W. A. 

Cobban measured a section (stratigraphic section 17) of Mowry shale 

equivalent in 1949 on the north side of Arrow Creek, about 5 miles 

northeast of Geyser, Montana. In 134 feet 5 inches of section only 

3 beds in the upper part correspond in lithology to the Mowry shale 

of the type locality.

Stratigraphic section 17. Part of the Colorado shale measured on the north side of 

Arrow Creek, five miles northeast of Geyser, Montana, sees. IS and 24, T. 18 N., 

R. 10 B.

(Measured by W. A. Cobban)

Colorado shale: Feet inches 

Rocks probably equivalent to lower part of Carlile shale" (Blue 

Hill shale member):

Shale, dark gray, noncalcareous, poorly exposed (not

measured).

Rocks probably equivalent to Greenhorn limestone and basal 

Carlile shale (Fairport chalky shale member):

Shale, dark gray, weathering orange brown, soft; forming 

slope; contains several thin beds of creamy-gray bentonite and a few thin shaly lenses of limestone weathering 

orange brown; limestone contains fish scales, and a small 

smooth oyster was found----_.,_---_-----_--------- 14 6

Shale, gray, weathering orange-tan, highly calcareous; 

some chalky layers; contains thin lenses of gray limestone; forms a prominent cliff; unit contains fish scales, 

shark teeth, a small smooth oyster, and fragments of 

Inoceramus labiatus (Schlotheim)___________________ 10 0

Shale, dark-gray, calcareous._______________________ 17 0

Shale, dark-gray, calcareous; grading upward into bluishblack, fissile, less calcareous shale.-_________________ 25 6

Bentonite, light-olive; passing upward into medium-gray

bentonite; inconspicuous_____---_-___--_----_------ 3 6

Shale, calcareous, dark-gray, weathering medium-gray.- 13 6 

Limestone concretions, medium-gray; weathering to 

lighter gray; closely spaced; highly septarian with thick 

veins of brown and pale-yellow calcite_____________ 1 3

Total, rocks probably equivalent to Greenhorn

limestone and basal Carlile__________________ 85
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 443

Stratigraphic section 17. Continued

Colorado shale Continued Feet inches 

Rocks probably equivalent to the Belle Fourche shale and 

lower part of Greenhorn formation:

Shale, dark bluish-gray; lower part fissile; upper part 

sandy and massive containing hard, tan-weathering 

lenses of fine- to very fine-grained sandstone as much 

as 3 inches in thickness. Near top was found one 

fragment of Metoicoceras sp_______________-______-_ 30 0

Shale and mudstone, dark-gray to bluish-gray, more or less

sandy, firm; cut by thin sandstone dikes___________ 26 0

Bentonite, creamy__________________________________ 1 0

Shale, dark bluish-gray, base and top sandy_i________ 8 9

Bentonite, creamy; micaceous at base_________________ ____ 6

Sandstone and shale, gray to dark-gray, very interbedded. 6 6 

Bentonite, yellow cream_____________________________ 1 0

Sandstone and shale, weathers, bluish; much interbedded; 

resistant, forming ledge_________________________ 3 0

Shale, dark bluish-gray; contains thin lenses of hard, very

fine-grained sandstone,.___________________________ 12 9

Bentonite, creamy__________________________________ ___.. 6

Shale, dark bluish-gray; contains thin lenses of hard, very

fine-grained sandstone_____________________________ 7 9

Shale, brown, bentonitic; locally, creamy bentonite_____ 1 0

Total, rocks probably equivalent to the Belle 

Fourche shale and lower part of Greenhorn 

formation__________________________________ 98 9

Rocks probably equivalent to Mowry shale: " 

Shale, dark-gray, weathering light bluish gray, hard,

finely sandy; forms ledge. _________________________ 2 6

Bentonite, creamy gray_______j_____________________ 2 9

Shale, dark-gray, hard_____________________________- 0 8

Bentonite, creamy__________________________________ 0 7

Sandstone, gray, weathering to prominent brown ledge, 

very fine-grained and hard, shaly, ripple-marked; contains abundant fish bones and teeth_________________ 1 6

Shale, dark-gray, hard, finely sandy; weathers light gray, 

contains some fish scales and 2 thin layers of bentonite___________________________________________ 10 6

Sandstone, salt and pepper, coarse-grained, massive, hard;

contains abundant fish bones; dip 3° 15' N__________ 0 3

Bentonite, creamy-gray.. ___________________________ 1 0

Sandstone, gray; weathering to brownish ledge, fine- to 

very fine-grained; in thin ripple-marked layers separated by films of dark shale________________________ 2 6

Sandstone, gray, fine- to very fine-grained, soft, thinbedded.____________________________ 6 0

Bentonite, creamy-gray__ ___________________________ 1 0

Sandstone, gray, fine- to very fine-grained, soft, thinbedded_-.________________________ 18 0

Sandstone and shale, gray to brownish, soft; much interbedded; contains minute carbonaceous flakes._____-_- 27 0
444 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Colorado shale Continued

Rocks probably equivalent to Mowry shale Continued feet Inches 

Bentonite, creamy__________________________________ 0 4

Shale, brownish to gray, spft, finely sandy; much interbedded shaly very fine-grained sandstone and 2 thin layers 

of bentonite______________________________________ 16 3

Shale, dark-gray, weathering light gray; very finely sandy,

hard; forming ledge; contains a few fish scales___.-___ 2 9 

Shale, dark-gray, firm, very finely sandy.____________ 2 7

Bentonite, creamy gray to medium gray_______________ 6 3

Bentonite, tuff, and porcellainite, creamy, massive; forms

prominent white outcrop; contains very few fish scales. 25 0 

Bentonite, olive grading up into yellow and cream; forms

prominent light-gray barren gumbo_________________ 9 0

Total, rocks probably equivalent to the Mowry

shale__________________________________ 134 5

Rocks probably equivalent to the Thermopolis shale: ===== ===

Shale, dark bluish-gray, hard, very finely sandy_ _______ 11 0

Sandstone, gray to brownish, fine- to very coarse-grained; 

some parts containing black chert pebbles as much as 

three-fourths of an inch in diameter; friable, porous, 

crossbedded; in lenses as much as 1.5 feet thick separated by dark-gray shale; forms a ledge _____________ 2 9

Shale, dark bluish-gray, hard, very finely sandy._______ 12 6

Sandstone, gray, weathering brown, fine-grained; with 

very fine to coarse streaks; thin-bedded, ripple-marked, 

crossbedded, hard; forms ledge; contains some fish 

bones.__________________________________________ 1 3

Shale, dark bluish-gray, hard, very finely sandy; contains 

many lenses of very fine-grained sandstone (not 

measured).

Overlying the Mowry shale equivalent is a unit consisting of 98 

feet and 9 inches of dark-gray or dark bluish-gray fissile shale containing several thin bentonite beds and, near the top, thin mudstone 

beds that are cut by fine-grained sandstone dikes. This unit is considered by Cobban (oral communication) probably to be equivalent 

to the Belle Fourche shale and basal part of the Greenhorn formation 

in the Black Hills.

Rocks equivalent to the Greenhorn limestone and basal Carlile 

shale (Fairport chalky shale member) are included by Cobban in one 

unit 85 feet 3 inches thick. The upper 24}£ feet of this unit consists 

of gray calcareous shales and thin lenses of gray limestone which 

weather into conspicuous orange-colored outcrops that are locally 

mappable. The lower part of the unit consists chiefly of dark-gray 

calcareous shale with a minor amount of bentonite and concretionary 

limestone. Cobban (1951, p. 2190) reports that shale similar to the 

upper 14K feet (stratigraphic. section 17) of this unit yields fossils of 

Fairport age near Mosby, Montana.
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 445

ROCKS EQUIVALENT TO THE CARLrLE SHALE AND NIOBRARA FORMATIONS

Rocks probably equivalent to the rest of the Carlile shale and to 

the Niobrara formations were not measured in the Stanford-Hobson 

area, because the low dips, wide outcrop bands, and lack of continuous 

exposures make detailed measurements impractical. Sections measured by W. A. Cobban indicate that 310 feet of Carlile shale and 402 

feet of Niobrara formation are present near Alosby, Montana, and from 

157 to 180 feet of Carlile shale and as much as 635 feet of Niobrara 

formation are present in northwestern Montana (Cobban, 1951, p. 

2190-2193). Though the Mosby section is nearly 90 miles east of the 

Stanford-Hobson area, measurements from wells of the latter area 

indicate that similar thicknesses of Carlile acd Niobrara equivalents 

are present here. The Carlile-shale equivalent consists chiefly of 

dark bluish-gray to black fissile shale. From 30 to 45 feet above the 

base of the section of equivalent strata there is a zone of red ironstone 

concretions. Still higher in the section, there is a zone of light-yellow 

limestone concretions, and near the top, a zone of gray limestone 

concretions occurs. The formation is sparsely fossiliferous. The 

Niobrara formation equivalent consists chiefly of dark-gray fissile 

shale that is calcareous in the upper part, contains many thin beds of 

bentonite, and yields Baculities codyensis Reeside and fragments 

of a large Inoceramus commonly having attached clusters of Ostrea 

congesta Conrad.

TELEGRAPH CHEEK FORMATION

The name Telegraph Creek formation is applied to transitional beds 

between the underlying Colorado shale and the overlying Eagle 

sandstone. The formation is 160 feet thick in the northeastern part of 

the Stanford-flobson area and consists of yellow-weathering dark-gray 

to dark-brown shale and sandy shale. Because the base of the Telegraph Creek is seldom exposed, the formation was not separated from 

the Colorado shale in mapping. Faunal studies, however, indicate 

that the Telegraph Creek is more closely allied to the overlying Eagle 

sandstone than to the Colorado shale and it is therefore assigned to 

the Montana group.

Stratigraphic section 18. Telegraph Creek formation on the west side of the Judith 

River, sec. 5, T. 18 N., R. 16 E.

Eagle sandstone. 

Telegraph Creek formation:

Shale, dark brown, slightly sandy; a 4-inch bed of aragonite with Feet 

cone-in-cone structure at the base. ____________________________ 50

Shale, grayish brown, soft; interbedded with several thin beds of hard, 

brown siltstone._____-_-_---___________--____-__-_____-______ 33

Siltstone, brown, hard___--_--_.______-_--______-___--_-___-----_ 5. 5

Shale, dark gray_____________________________________________ 30. 5
446 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Stratigraphic section 18. Continued

Telegraph Creek formation Continued

Sandstone, white, medium-grained, thinly laminated, forms ledge_.__ 

Sand, soft interbedded with shale, brown ___________ ___--________.

Total, Telegraph Creek formation. 

Colorado shale.

EAGLE SANDSTONE

Feet

I

40

160

The Eagle sandstone, which was first named by Weed (1899a) for 

exposures along the Missouri River near the mouth of Eagle Creek, 

is well exposed in the northeastern part of the Stanford-Hobson area 

(fig. 60). The measured section (section 19) that is given is exposed

FIGURE 60. Three units in the Kagle sandstone. The top unit i.- \ . <!LM.i -forniiii:-; >:iiulstone; the 

middle unit is a slope-forming shale; and the basal unit is a second ledge-forming sandstone, which here 

overlies the slope formed by the Telegraph Creek formation. View is north in sec. 10, T. 18 N., R. 16 E.

in cliffs along the west side of the Judith River. There, the Eagle 

consists of a lower, white and brown, concretionary, cliff-forming 

sandstone, 72.5 feet thick; a middle, slope-forming unit of gray shale, 

sandy shale, and thin lignite seams, 66 feet thick; and an upper, 

brown and white, cliff-forming sandstone 110.5 feet thick, making a 

total of 249 feet. The sandstone beds are medium- to coarse-grained, 

soft, and friable and weather readily into grotesque monuments and 

pinnacles. The following section shows the detailed character of the 

formation.
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 447

Stratigraphic section 19. Eagle sandstone on the west side of the Judith River,

sec. 6, T. 18 N., R. 16 E. 

Claggett shale.

Eagle sandstone: Feet 

Sandstone, brown and white, massive, friable.____________________ 55

Coal.._..___-_-__-__________________________________ 0. 5

Sandstone, brown and white; the upper part forms a massive white 

cmT________________________________________________________ 55 '

Shale, gray; interbedded with sand; partly covered._______________ 22

Shale, gray; interbedded with coal seams; partly covered. __________ 14

Covered;forms a slope.________________________________________ 30

Sandstone, brown and white, fine- to medium-grained, friable; contains a few black chert grains in addition to quartz; forms a 

prominent cliff._____________________________________________ 72. 5

Total, Eagle sandstone.__________________________________ 249

Telegraph Creek formation.

CLAGQETT SHALE

The Claggett shale is present in the vicinity of Judith Gap, and 

there is one small exposure in the northeastern part of the area. 

However, the exposures are inadequate for accurate measurement of 

the thickness or for a detailed study of the lithology. The Claggett 

consists of a dark gray or brown marine shale that forms valleys 

and weathers into gentle slopes. Near Mosby the formation was 

measured by Cobban (oral communication), and its thickness was 

determined to be 430 feet. Bowen states (1915, p. 98-99) that the 

Claggett is about 500 feet thick along the Missouri River.

JUDITH RIVER FORMATION

The lower part of the Judith River formation is present in Judith 

Gap, but the exposures are so poor and scattered that mapping it is 

rather indefinite, and measurement of the section is impossible. 

Where visible, it consists of beds of soft, friable, gray sandstone similar 

to those in the Eagle sandstone. To the east, where Cobban (oral 

communication) measured this formation near Mosby, he found it to 

be 273 feet thick. To the north, along the Missouri River, Bowen 

(1915, p. 100-101) reports a thickness of about 620 feet.

TERRACE GRAVELS AND ALLUVIUM

GENERAL DESCRIPTION

Gravel deposits that represent a succession of several terrace surfaces 

are present in the interstream areas of the Judith River and Arrow 

Creek drainage basins. A comparison of the terrace designations for 

gravel deposits that mantle the succession of terrace surfaces as 

applied in this report and by Alden (1932) is shown in table 4.

Gravel deposits that are small isolated remnants of the highest and
448 CONTRIBUTIONS TO ECONOMIC GEOLOGY

oldest surface, the number 1 terrace, are present at scattered localities 

in the area. These remnants consist of coarse, consolidated gravel 

beds and at a former time may have been part of an extensive terrace 

surface that covered a large portion of the area. The second oldest 

deposit of gravel, the number 2 terrace, is the most extensive of those 

preserved in the area and, at one time, must have blanketed nearly 

all the lowlands of Judith Bosin. Local variations of the main terrace 

surface, some which can be seen to merge with the main surface, are 

designated by a plus (+) or a minus ( ) sign following the terrace 

number, according to whether they are higher or lower than the main 

surface. The number 2 terrace is so well preserved that many places 

show no signs of erosion. Younger deposits that occur on lower 

terrace surfaces, the number 3 and 4 terraces, are locally preserved

TABLE 4. Comparison of terrace designations as used in this report with those 

used by W. C. Alden in 1932

Present report

Number 1 terrace 

(Gravel deposits on the highest and oldest terrace 

surface. Map symbol: Qti.)

Number 2+ terrace

(Gravel deposits slightly higher than the main 

second highest or most extensive terrace surface. 

Map symbol: Qtz+.)

Number 2 terrace

(Gravel deposits on the main second highest or 

most extensive terrace surface. Map symbol: 

Qti.)

Number 2~ terrace

(Gravel deposits slightly lower than the main 

second highest or most extensive terrace surface. Map symbol: Qt2-.)

Number 3 terrace

(Gravel deposits on the third highest terrace 

surface. Map symbol: Qts.)

Number 4 terrace

(Gravel deposits on the main fourth or lowest 

terrace surface. Map symbol: Qt4.)

Number 4~ terrace

(Gravel deposits slightly lower than the main 

fourth or lowest terrace surface. Map symbol:

Qt4-.)

Alluvium

(Gravel deposits in the flood plains of the modern 

streams. Map symbol: Qal.)

Alden (1932)

Number 1 bench 

(Remnants of first or high main bench of stream 

terraces and alluvial fans. Map symbol: 

Tt».)

Number 2 bench

(Remnants of second or medial bench of stream 

terraces and alluvial fans. Map symbol: 

QT».)

Number 3 bench

(Remnants of third or lowest bench of stream 

terraces and alluvial fans. Map symbol: 

Qt».)

Alluvium

(Modern stream gravels and alluvial fans. 

Map symbol: Qal.)
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 449

along the main streams, such as Judith River, Ross Fork, Wolf Creek 

and Arrow Creek. These terraces were never much more extensive 

than at the present. The youngest and lowest deposit of gravel 

found in the flood plains of the modern streams is mapped as alluvium. 

Along parts of the valley of the Judith River, the two or three minor 

terrace surfaces too small to be mapped separately are included with 

the alluvium.

The lithologic composition and texture of the gravels of the terraces 

are functions of their [proximity to the mountains and of the lithologic 

types exposed in the adjacent parts of the mountains. Opposite the 

higher parts of the Little Belt Mountains where intrusive igneous 

rocks are exposed, the gravels contain as much as 60 percent of 

igneous rocks. Parts of the Little Belt Mountains where no igneous 

rock is exposed yield gravels containing as much as 100 percent of 

limestone boulders. Sandstone boulders make up less than 25 

percent of most gravel samples. North of Arrow Creek and opposite 

the Highwood Mountains dark-colored weathered igneous rocks 

make up nearly 100 percent of the gravels. The size of the gravel 

boulders varies locally but, in general, increases toward the mountains 

and decreases from them. Except near the mountains and along some 

stream channels, the gravels generally have less than 10 percent of 

the material larger than 6 inches in diameter.

Long narrow gravel ridges, occurring in some localities, are from 3 

to 5 feet above the general level of the number 2 terrace surface and 

from 30 to 50 feet wide. These features were first observed on the 

aerial photographs, where they appear as straight lines trending 

northeast-southwest and parallel to the trend of igneous dikes that 

occur in the same general] area. On [photographs ''the] similarity in 

appearance of these gravel ridges to igneous dikes led to an investigation that failed to demonstrate any definite relationship between the 

two features. The origin and significance of the gravel ridges is 

not clear.

AGE OF THE TERRACE SURFACES

Evidence in this area for the exact age determination of the various 

terrace surfaces is scanty. Alden (1932 p. 13) correlates the highest 

terrace surface with the surface on the Flaxville gravel in northeastern 

Montana, which he considers to be late Tertiary in age. There is no 

paleontological evidence in the Stanford-Hobson area for the age 

of this or for any of the lower surfaces. Because of the difficulty of 

correlating the high terrace in this area with anything so far away as 

the Flaxville gravel, no attempt has been made to distinguish Tertiary from Quaternary terrace surfaces. All surfaces have been 

assigned to the Quaternary.
450 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Physical evidences that at least the number 2 terrace is preWisconsin in age are shown by involutions. Involutions are a type 

of surficial deformation in which the stratified material to a depth of 

5 or 10 feet below the surface of the ground is distorted into irregular 

structures, some of which resemble anticlines and synclines. Examples 

of these structures are shown in figure 61. In roadcuts along State

FIGURE 61. Involution, a type of surflcial deformation that is due to frost action. Deformation has 

affected both the terrace gravel and the underlying Eagle sandstone, as seen in 10-foot deep roadcut, 

about 3 miles northeast of Denton, in sec. 7, T. 18 N., E. 15 E.

Route 19 west of Moore the soil and gravel on the number 2 terrace 

have been deformed to a depth of 6 or 8 feet. In roadcuts about 3 

to 4 miles northeast of Denton the soil, gravel, and the underlying 

Eagle sandstone were observed to be deformed to a depth of about 

8 feet. The areas in which these structural features were observed 

have not been subjected to continental glaciation, recent tectonic 

deformation, landslide movements, or to swelling of beds of bentonite 

or gypsum: causes that might account for the observed structures. 

It is believed that these involutions were caused by intense frost 

action during the stages of ice advance in Pleistocene time. Similar 

structural features are not being formed in the modern soils and 

gravels at this elevation in Montana. However, it is believed that 

deformation could take place under the influence of the colder climate 

that existed during the periods of ice advance in the region peripheral 

to the continental glacier. In such a periglacial region a layer of 

perennially frozen ground is likely to be formed which would inhibit
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 451

the drainage of ground water and facilitate the freezing and expansion 

of surface water causing the deformation of the surficial strata.

Involutions in this and adjacent areas have been described by 

J. P. Schafer (1949, p. 164) who cites evidence that they were probably 

formed during the younger, or Wisconsin, ice advance. If the deformation of the gravels took place during the Wisconsin stage of 

glaciation, the gravel itself is most likely pre-Wisconsin in age. 

Because the number 2 terrace is the youngest surface on which these 

structures were observed, it is reasonable to tentatively correlate this 

surface with the pre-Wisconsin, or Sangamon, iriterglacial stage.

The number 1 terrace is older than the number 2 terrace and is 

represented only by widely separated remnants. If this surface ever 

was as extensive as the number 2 terrace, it was extensively eroded 

before the gravel of the number 2 terrace was deposited. If this is 

true, the number 1 terrace is early Pleistocene in age or, perhaps, 

even Pliocene. The number 3 terrace is younger and lower than the 

number 2 terrace and is only developed locally along the major 

streams. Its surface probably formed immediately after the Wisconsin stage of glaciation. The lower terraces and the alluvium are 

undoubtedly local post-Pleistocene surfaces that may be due to 

changes in climate or to successive lowerings of the base level by the 

Missouri Kiver.

ORIGIN OF THE GRAVELS

The cobbles and boulders that make up the terrace gravels are 

comprised of limestone, igneous rock, and quartzite similar to those 

which crop out in the adjacent mountain areas. This fact and the 

areal distribution of the terraces and modern streams converging 

toward Arrow Creek and Judith River from the surrounding mountains provide good evidence that the terrace gravels were deposited 

by former local streams converging on Judith Basin from the mountains in a manner similar to the pattern of the modern streams. The 

former streams probably differed from the modern streams in some 

respects: at times they must have carried a greater volume of water, 

capable of transporting a larger amount of coarse material from a 

readily available source. Their base level was more distant or considerably higher. These conditions can be explained by the glacial 

climate that prevailed during the Pleistocene epoch. Small glacial 

cirques in the high peaks of the Little Belt Mountains are evidence 

that a rigorous climate accompanied the glacial stages and caused 

an increase in the rate of erosion in the mountains and a seasonal 

swelling of the streams that enabled them to transport a larger volume 

of material. The number 2 terrace is only about 200 feet above the 

present streams near the Little Belt Mountains but is about 600 feet 

above the present streams near the junction of Surprise and Arrow
452 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Creeks. Thus it is evident that the streams at the time the alluvium 

of the number 2 terrace was deposited had a lower gradient than the 

present streams; therefore base level must have been higher or more 

distant.

The following hypothetical set of conditions is proposed to explain 

the origin of the extensive number 2 terrace.

1. At the end of an early stage of Pleistocene ice advance the physiographic setting for the Judith Basin was similar to that at the present. 

However, there were remnants of only one major terrace above the 

flood plains of the streams, and the streams were all descending from 

the mountains onto a plains country of much less topographic relief 

than exists today, for none of the streams were deeply incised.

2. The climate, although quite rigorous in the whiter, was warm 

enough during the short summer to melt nearly all the snow which 

accumulated during the long whiter, and the resulting meltwater 

reached the plains through swollen streams as floods which spread out 

onto coalescing alluvial plains.

3. Annual and daily cycles of freezing and thawing in the mountains 

caused a rapid disintegration of the rocks and supplied a large volume 

of boulders, which the streams in flood stage carried out onto the 

flood plains, depositing a mantle of cobbles as the stream velocity 

decreased farther from the mountains.

4. The process of transportation and deposition of the cobbles continued for a sufficiently long period of time to permit gravel deposits 

from the Highwood, Little Belt, Big Snowy, Judith and Moccasin 

Mountains to coalesce in the center of the Judith Basin to form a 

nearly continuous deposit, interrupted only by isolated hills of bed 

rock, such as Skull Butte and the gravel-capped hills that are remnants 

of the number 1 terrace deposit.

5. The resulting surface was not absolutely smooth or flat, for it was 

interrupted by meander scars of the many stream channels that crossed 

the area, and there was a gentle slope from the mountains toward the 

center of the basin and from the center of the basin toward the north 

where the streams eventually joined the ancestral Missouri River, 

probably near the bend of the present Missouri River at Virgille.

ECONOMIC IMPORTANCE

The terraces underlain with gravel and overlain by a thin veneer of 

soil are important as agricultural and grazing land. The deposits are 

an important source of road building and construction material. The 

highly permeable nature of most of the gravel makes it a good reservoir 

for water and provides a means of drainage. The fine loamy soil 

overlying the gravel on most of the terraces is ideal for agriculture. 

The terraces have very little relief and can be irrigated, and the gravel
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 453

provides the good underground drainage that is necessary for continued irrigation. All the terraces are used for agriculture, but only 

the number 2 and lower terraces are commonly irrigated.

The gravels of the terraces are used throughout the area as local 

sources of road-building material, and it is rarely necessary to transport gravel more than short distances. There is an adequate source of 

gravel for all road construction and any other type of large scale construction in which gravels are needed.

The gravels form a permeable layer that is commonly water bearing. 

Most of the small towns and farms located on terrace gravels obtain 

water from wells dug to the base of the gravel deposit. Irrigation 

projects on the terrace gravels are generally more successful than those 

on bed rock, because the gravels provide a means of drainage for the 

water, thereby preventing the accumulation of saline minerals at the 

surface of the ground through evaporation.

IGNEOUS BOCKS 

STRUCTURAL RELATIONS OF THE IGNEOUS ROCKS

Intrusive igneous rocks are found at several places in the Stanf ordHobson area. They occur in five principal types of igneous rock 

structures: dikes, sills, stocks, concordant intrusions or laccoliths, and 

combinations of these. Weed (1900, p. 317-343) describes in detail 

the petrology and structural relations of the igneous rocks within the 

Little Belt Mountains adjacent to the Stanford-Hobson area. Igneous dikes are common features, especially in the plains areas. The 

most striking examples are dikes of dark-colored igneous rock found in 

the Colorado shale where differential erosion has preserved parts of the 

dikes as ridges of igneous rock that stand out above the soft shale. 

Nearly all the dikes that occur in the plains area (with the exception 

of those near the Highwood Mountains) have a northeast-southwest 

trend. Sills composed of dark-colored igneous rock along the north 

flank of the Little Belt Mountains and in Skull Butte dome occur 

chiefly in the Heath shale. Both sills and enclosing sedimentary 

rocks are deformed. Sills and dikes of dark-colored igneous rocks 

also occur in the Colorado shale south of the Highwood Mountains. 

Two major stocks composed of light-colored igneous rocks, are present 

along the north flank of the Little Belt Mountains: one occurs on the 

east side of Running Wolf Creek, about 12 miles southwest of Stanford, 

and the other forms the peak of Wolf Butte, 12 miles south of Geyser. 

Windham dome, about 5 miles north of Windham, contains a concordant intrusion composed of light-colored igneous rocks. The 

igneous intrusion has arched the Colorado shale, and erosion has 

removed enough of the sedimentary strata to expose the igneous core. 

Because no base for the igneous rock is exposed, the structure is con-
454 CONTRIBUTIONS TO ECONOMIC GEOLOGY

sidered to be an intrusion in which the igneous rock is concordant 

with the arched sedimentary strata, rather than to specify that it is a 

laccolith. Structural features similar to the Windham dome occur 

along the north flank of the Little Belt Mountains, but are complicated 

by faulting or by containing partly discordant, light-colored igneous 

intrusive bodies. The dome-shaped structure in which Woodhurst 

Mountain is the highest peak is probably of this type.

Other structural features in which only sedimentary strata are 

exposed may be the result of igneous intrusions. Skull Butte dome, 

Blacktail Hills dome, and the small round dome in the Lone Tree dome 

are examples that probably have concealed igneous cores.

PETBOGBAPHIC RELATIONS OF THE IGNEOUS BOCKS

The igneous rock types that occur in the Stanford-Hobson area 

have not been studied in as great detail as the sedimentary rocks. 

Specimens of representative igneous rocks were collected from a 

number of localities in the area, and from these, 10 were selected for 

more detailed examination by thin-section methods. Localities in 

which the 10 selected specimens were collected are marked on the 

map by the designations 1-1 through 1-10. The thin-sections were 

examined by W. T. Pecora who made the following report:

IGNEOUS BOCKS IN THE STANFOBD-HOBSON ABEA

By W. T. PECOBA

For purpose of mapping, and in" the absence of other criteria, the 10 

rocks may be classed into 2 petrographic groups:

1. Quartz monzonite porphyry group (light-colored, quartz-bearing, 

fine-grained porphyritic intrusive rocks)

2. Mafic alkalic intrusive rocks (dark-colored, quartz-free, finegrained to coarse-grained intrusive rocks with abundant potash 

feldspar and ferromagnesian minerals)

Group 1 includes specimens from 1-1 to 1-6.

Group 2 includes specimens from 1-7 to 1-10.

GROUP 1

Phenocrysts. By volume the phenocrysts make up from 5 to 30 

percent of the group and attain a maximum size of 2 centimeters. 

Plagioclase (ranging in composition from calcic oligoclase to calcic 

andesine) is the principal mineral, and quartz and sanidine are subordinate. Resorbed quartz phenocrysts are common only in specimen 1-5.

Groundmass. The groundmass is fine grained in all the rocks of 

Group 1, and is composed of quartz, potash feldspar, and plagioclase
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 455

(calcic albite to calcic oligoclase). Specimen 1-5 is richest in quartz 

(40 percent of groundmass).

Ferromagnesian minerals. Biotite is the principal dark mineral and 

makes up less than 5 percent of the rocks.

Alteration. Calcite, chloritized biotite, and sericitized feldspar are 

the principal alteration minerals.

Nomenclature and Comments. Rocks in Group 1 range in composition from dioritic monzonite (1-6) to calcgranite (1-5). The assignment of "quartz monzonite" as a field name for mapping purposes and 

group description is, however, justifiable. Specimen 1-5 is characterized by drusy vugs that are probably miarolitic in origin and by 

dark quartz grains that probably are the remnants of resorbed hexagonal intratelluric crystals. The group is similar to rocks known in 

several mountain uplifts in north-central Montana.

GROUP 2

The four rocks of this group (1-7 to 1-10) show some fundamental 

differences, but all are characterized by an abundance of ferromagnesian minerals (augite, biotite, olivine) and a high proportion of 

potash feldspar. Specimens 1-10 and 1-9 are fine-grained shonkinite; 

1-8 is a plagioclase shonkinite; and 1-7 is a mafic alkalic monzonite. 

These rocks are common in the central Montana petrographic province.

The plagioclase in 1-8 is labradorite and makes up only a small 

amount of the rock. In 1-7 the plagioclase is andesine, and the 

proportion of sanidine to andesine in the rock is about 2:1.

The ferromagnesian minerals make up from 30 to 50 percent of the 

rocks. Augite is the principal dark mineral, and resorbed olivine and 

biotite are subordinate. Alteration of 1-10 has resulted in bleached 

biotite and calcite.

If definite rock names are required for each of the specimens, those 

of the foUowing list can suffice:

Quartz monzonite porphyry: 1-1, 1-2, 1-3, 1-4

Calcgranite: 1-5

Quartz calcmonzonite: 1-6

Shonkinite: 1-9, 1-10

Plagioclase shonkinite: 1-8

Mafic alkalic monzonite: 1-7

Inasmuch as remarks on field relations were not submitted with the 

specimens, the chronological position of these rocks in the Montana 

province cannot be given. They are, however, characteristic rocks

of the province.

STRUCTURE

STRUCTURAL SETTING

The Stanford-Hobson area is situated to the southeast of the 

Sweetgrass arch, a major structural element in'Montana from the
456 CONTRIBUTIONS TO ECONOMIC GEOLOGY

vicinity of Great Falls north to the Canadian border. The regional 

dip in the area is toward the northeast, and the strike is a.continuation 

of the northwest-southeast trend which forms the eastern flank of the 

Sweetgrass arch. This gentle dip of the sedimentary rocks to the 

northeast is modified by uplifted strata on the flanks of several 

mountain ranges. Thus, the beds dip steeply on the flanks of the 

Little Belt, Big Snowy, Judith, and Moccasin Mountains, although 

in the Highwood Mountains there has been little structural arching. 

The resulting structure is a basin, enclosed along the southwest by 

the Little Belt Mountains, on the southeast by the Big Snowy Mountains, on the east by the Judith and Moccasin Mountains, but open 

on the north and northwest. Judith Gap is a structural (and topographic) saddle between the Little Belt and Big Snowy Mountains. 

The configuration of this structural basin is shown on the geologic 

map (pi. 45) by structure contours that are drawn at 100 foot intervals 

on the surface that is formed by the contact of the Kootenai formation and the Colorado shale.

The lack of data for strata below the pre-Ellis unconformity makes 

it impossible to determine accurately the elevation of the Paleozoic 

rocks. This limiting factor was the chief reason for making the 

structure contour map of the top of the Kootenai formation rather 

than of some older surface such as the top of the Madison limestone.

The regional structural features are greatly modified locally by 

small faults and domes. In fact, the northern flank of the Little 

Belt Mountains is so modified by many small structural features 

(Weed, 1900 p. 384-400) that the major structural trend of the 

mountains is obscured. It is, however, these small structural features 

that are of primary interest in the search for oil and gas, because 

they may serve as structural traps in which oil and gas may accumulate. Each of these possible structural traps is described below with 

respect to the type of structure, the history of exploratory drilling, 

and the future possibility of finding oil and gas.

INDIVIDUAL ANTICLINAL STRUCTURES

Garneill nose. Garneill nose is a structural feature located on the 

west side of Judith Gap, the structural saddle between the 

Little Belt and the Big Snowy Mountains. It is a dome-shaped 

structure slightly more than a mile in diameter and forms the end 

of the nose protruding from the northern flank of the Little Belt 

Mountains. Several oil and gas test wells were drilled on this dome 

in the early 1920's, and the driller's log shows indications of oil and 

gas in the Otter formation. This fact, plus statements from local 

residents that oil flowed from the well for several years after it was 

drilled, interested D. C. Shay of the Chief Exploration Co., Billings,
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 457

Mont., sufficiently to make a study of this dome in 1950. In February 

of that year he drilled 4 shallow exploratory holes to ascertain the 

amount of structural closure on this dome. An examination of the 

drill cuttings from these 4 test holes indicated at least 110 feet of 

structural closure. In August 1950 an oil and gas test well, the T. W. 

Doswell, Dolgan No. 1, was drilled to the rocks of Cambrian age 

without obtaining commercial production.

Buffalo dome. A dome on the north flank of the Little Belt Mountains, is about ten miles northwest of Garneill nose. In plan it is a 

semioval structure and is flattened on the south side by the north 

flank of the Little Belt Mountains. It is asymmetrical, the gentle 

side dipping 4 to 5 degrees southward to the mountains, and the steep 

side dipping 15 to 17 degrees northward to the basin. The dome is 

dissected on the north side by two branches of Buffalo Creek; the 

western one has uncovered the top of the Ellis group. Outcropping 

strata on the south flank, where data are needed to determine the 

amount of structural closure, are obscured by a cover of soil and 

grass. The dome probably has less than 100 feet of closure, but the 

south flank is complicated by faulting that may make the effective 

closure greater. A test well was drilled on Buffalo dome in 1925 to a 

depth of 1,110 feet. The driller's log of this well shows a number of 

indications of oil and gas. Though the closure on this dome is not 

great, more thorough testing in a well located higher on the structure 

may be worthwhile.

Antelope Creek dome and smaller domes. A small anticline that is 

exposed in the valley of Antelope Creek about 4 to 5 miles northwest 

of Buffalo dome is called the Antelope Creek dome. It is a small 

feature, being slightly more than a mile long and less than a mile 

wide. From the exposures on three sides, it is estimated to have at 

least 100 feet of closure. About 2 miles west of Antelope Creek 

dome are several other small flexures. These folds are in an area 

of complicated structure, the major structural trends being determined by a fault that terminates as an eastward pitching anticline 

south of Antelope Creek dome. An extension to the south and west 

of the area of complicated structure was not mapped. Until this 

adjacent area has been investigated, the structural relations cannot 

be fully understood. No test wells have been drilled in this area 

to date. Although the anticlines are small, a test well on Antelope 

Creek dome will have to be drilled only to a depth of about 1,500 

feet to enter all the strata above the Madison limestone.

Skull Butte dome. Skull Butte dome is about 4 or 5 miles southwest of Stanford and north of the main flank of the Little Belt Mountains; it is a circular dome about 2 miles in diameter in which differential erosion has formecj a central topographic basin about 1
458 CONTRIBUTIONS TO ECONOMIC GEOLOGY

mile in diameter. Within this central basin, which locally is called 

Cayuse Basin, erosion has exposed the top of the Otter formation. 

Limestone beds of the upper part of the Amsden formation encircle 

the basin and form a hogback that is breached by a stream at 

the northeast side. The steep-sided, circular plan of this dome, 

closely resembling those of Windham dome and the dome-shaped 

structures along the north flank of the Little Belt Mountains, suggest the possibility of an intrusive core. It is difficult to prove or 

disprove the presence of such a core, however, until a test is drilled. 

In 1929 a well was drilled to a depth of 500 feet in the SWK sec. 6, 

T. 15 N., R. 12 E. and produced a flow of water. As no other information is available, the sequence of rocks penetrated is unknown. 

Although the exact position and elevation of the well is unknown, 

it is likely that it entered the Kibbey sandstone and the top of the 

Madison limestone. Skull Butte dome has several hundred feet 

of closure, and if there is no igneous core or if the igneous core is 

quite deep, it may have served as a structural trap for oil and gas. 

However, part of the Otter formation, the Heath shale, and the 

Amsden formation, are exposed so the only possible oil-producing 

formations that it would be feasible to test are the Madison limestone and the Kibbey sandstone. Further testing of this dome 

may be unsuccessful because of the likelihood of finding an igneous 

rock core. Such cores are not known to be productive of oil or gas.

Blacktail Hills dome. This dome, a circular uplift about 2% to 3 

miles in diameter, is similar structurally to Skull Butte dome, but 

the upper surface of the Madison limestone forms a domelike hill 

surrounded by a ring-shaped valley eroded from the less-resistant 

strata of the Big Snowy group. The hill is dissected on the northwest 

side by Dry Wolf Creek. The circular form of this uplift, so similar 

to that" of Skull Butte dome and the proximity of a large igneous 

mass less than one mile to the west are evidence for the belief that th.e 

core of the dome is a mass of igneous rock. Because all the possible 

oil-bearing strata younger than Madison limestone have been stripped 

from this dome, and because of the possibility of an igneous core, 

a test of this structure seems inadvisable.

Lone Tree dome. North of Wolf Butte an uplifted area more than 

5 miles in diameter is called the Lone Tree dome. On the dome is a 

smaller circular dome, 1 to IK miles in diameter and having about 

350 feet of closure. In diameter the smaller dome is about onehalf that of Blacktail Hills dome, butjresembles it in structure and 

topography. The reasons that make it unadvisable to test the 

Blacktail Hills dome for oil and gas apply with equal logic to this 

small dome.

The anticlinal fold that is located about 1 mile west of the locality
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 459

where Surprise Creek breaches the Amsden hogback is an eastward 

plunging fold with little or no closure. It is bounded on the north 

and east by the main hogback of the Amsden formation and on the 

south by a plunging syncline in which isolated blocks of the Amsden 

formation and the Ellis group are much lower than [can be explained by the folded structure. It is apparent that these blocks have 

been lowered, but neither the strata above nor below the blocks show 

evidence of being displaced; therefore the blocks were mapped as 

landslide material. A third anticline that resembles a structural 

nose or terrace occurs within the Lone Tree dome in the broad area 

in which the Otter formation crops out. This area is so broad that 

there may be one or more gentle flexures not recognizable, owing to 

the lack of mappable horizons within the Otter formation. In 1929 

a dry hole was drilled to a depth of 1,140 feet from a position near the 

topographic high point on the Otter outcrop band. Although the 

stratigraphic position of the top of the well within the Otter formation 

is unknown, it is probable that in drilling to a depth of 1,140 feet 

the well penetrated several hundred feet into the Madison limestone. 

One dry hole does not condemn the entire area within the Lone Tree 

dome; however, the prospects of finding oil within the area are limited 

to a relatively few stratigraphic horizons, namely the Madison limestone, and the Kibbey sandstone.

Windham dome. An isolated circular dome rising abruptly above 

the gently dipping Cretaceous strata about five miles north of Windham is called in this report the Windham dome. This structural 

feature differs from most of those previously described, because it is 

more than 10 miles from the mountain front, yet erosion has exposed 

a core of igneous intrusive rock. Cretaceous strata probably equivalent to the Mowry shale are exposed on the flanks of the dome, dipping 

10 to 15 degrees from the center of the dome, and erosional remnants 

of the same strata are exposed on the crest of the dome. The presence 

of an igneous core in this dome is added evidence for the hypothesis 

that several of the other circular domes in the area have igneous cores 

at depth.

Hidden structural features. These domes and anticlines do not 

represent the only possible oil and gas producing structures in the area. 

As a large part of the area mapped is covered by gravel that obscures 

the structure of the underlying rocks, it is possible that additional 

anticlines or domes are present which can be discovered only by 

geophysical or other methods. At least five of the test wells in parts 

of the area that have no clearly defined anticlinal structures had 

indications of oil and gas. (See table 6.) These indications may or 

may not be evidence for the presence of hidden structural features; 

however, they definitely show that the Stanford-Hobson area is in a
460 CONTRIBUTIONS TO ECONOMIC GEOLOGY

petroliferous province where oil or gas is likely to be present if favorable structural and stratigraphic conditions exist.

FAULTS

Faulting and thinning of the beds accompany the intense deformation along the north flank of the Little Belt Mountains west of Garneill 

nose. Locally the strata are overturned and the Madison limestone 

is faulted against the Amsden formation. The displacement may be 

due to a high angle reverse fault.

Strata on the south side of Garneill nose are dislocated by one large 

and several small faults. These appear to be tears with nearly vertical 

fault surfaces, but nearly all displacement is horizontal. As such, 

they are not likely to form oil or gas traps.

Several minor faults and one major fault are present in the area 

south and west of Antelope Creek dome. The east-west fault about 

1% miles south of Antelope Creek dome is a major fault that has its 

greatest displacement just west of the area mapped and that passes 

into a plunging anticline and syncline to the east. Because the 

western end was not mapped, the importance of this structure in 

localizing the accumulation of oil and gas is not known. The other 

faults in this area are small and probably have little or no significance 

as oil or gas traps.

About 2K miles northwest of Utica a large fault trends northeastsouthwest. Although the fault trace is largely concealed by the cover 

of gravel, the influence of the fault on the continuity of the outcrop 

bands is readily seen on the map. This fault may be somewhat 

similar to the one south of Antelope Creek, but'because it is so poorly 

exposed, the detailed relationships cannot be given. For the same 

reason, it is not possible to say how important it may be in localizing 

the accumulation of oil and gas.

Along the northern border of the area, southeast of the Highwood 

Mountains, there are two grabens in which the Eagle sandstone is 

in contact with Colorado shale. Owing both to the difficulty in 

recognizing faults in the dark Colorado shale and to the extent of the 

gravel cover, it is certain only that the areas mapped as Eagle sandstone must be within the down-faulted block of the grabens. Indications of oil and gas have been reported in rocks adjacent to 

similar faulted structures several miles north of the area mapped.

SECONDARY STRUCTURAL TREND

Superimposed upon the major northwest-southeast regional strike 

is a secondary northeast-southwest structural trend prominent 

throughout the area of the Judith Basin. This trend is reflected by 

the large fault northwest of Utica; by a minor fault or flexure in tho
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 461

Colorado shale in sec. 21, T. 15 N., R. 13 E.; by the northeast strike 

of many dikes in the area; and by the stream pattern of most of the 

streams in the area. The geologic map shows a conspicuous northeast-trending pattern of lines, including both drainage features and 

geology.

This structural trend might be significant if oil were discovered in 

the Judith Basin, for it is possible that any structural traps in the area 

would be complicated by secondary features in this general direction, 

and even a stratigraphic trap might be limited by a fault or dike 

parallel to this direction. This structural trend is subordinate to 

and probably later than the major structural features of the mountains, but it is nevertheless significant in the search for oil.

FORMATION THICKNESSES USED IN COMPUTING STRUCTURE

CONTOURS

Structure contours were computed and drawn at 100-foot intervals 

on the top of the Kootenai formation. The formation thicknesses 

used in computing the structure contours for the formations above 

the Kootenai formation remained constant throughout the area. 

Owing to the many changes in thickness of the formations below the 

Colorado shale, it was necessary to calculate changes in thickness for 

use in different parts of the area. The following list and table 5 

show the intervals used for purposes of calculating the structure 

contours in the area.

Unit

Thickness of intervals from base 

of Colorado shale to tops of 

various formations mapped

Claggett shale__._..___._..____._._.____._____._._.._._.__.___ 2600

Eagle sandstone._______________________________________________ 2180

Telegraph Creek formation, _____________________________________ 1920

Colorado shale.______-_-_-_-------_--_-_-__-_--__-----_-_-____- 1760

Greenhorn-formation equivalent-_____________________________ 1040

Mowry-shale equivalent (bentonite zone)______________________ 760

Kootenai formation.____________________________________________ 0

TABLE 5. Variations in thickness of Kootenai and pre-Kootenai intervals

Unit

Basal sandstone of the Kootenai fm ......

Lone 

Tree 

Creek

350 

450

700

775

1,095

1,420

1,920

2,070

Surprise 

Creek

350 

450

700

775

945

1,270

1,770

1,920

Dry Wolf 

Creek and 

Running 

Wolf 

Creek

350 

450

700

775

1,475

1,945

2,365

2,615

Willow 

Creek

350 

450

700

775

1,485

1,920

2,465

2,655

Skull 

Butte

450 

550

715

775

1,134

1,570

Antelope 

Creek and 

Wait 

Creek

250 

350

700

800

1,400

1,800

2,100

Ross 

Fork

275 

375

650

710

1,400

1,800

2,100
462 CONTRIBUTIONS TO ECONOMIC GEOLOGY

MINERAL RESOURCES 

OIL AND GAS

From 1921 to 1950 sporadic well-drilling activity has resulted in 

the completion of at least 21 test wells in or near the Stanford-Hobson 

area. (See table 6.) Of these, 9 had indications of oil or gas, and 

the rest were dry holes or were completed as water wells. Most of 

these wells were located and drilled by small local companies. The 

log of a sample study of the Pacific Western Oil Company, Todd no. 1 

well is given in table 2.

The Judith Basin area has not received as much attention from the 

petroleum industry in the past several years as have the nearby areas 

east of the Big Snowy Mountains, which are characterized by welldefined anticlinal structures. The known anticlinal structures in the 

Stanford-Hobson area are much smaller than those to the east, and 

some of them probably possess an igneous core. Though some of 

these anticlinal structures may eventually become a source of oil or 

gas, it is unlikely that they will become important fields. Probably 

the best possibility of finding an important oil or gas field in the 

Judith Basin is to discover either a stratigraphic trap or a large 

structural feature that, if present, is concealed by terrace gravel. 

Geophysical methods of exploration will probably have to be employed 

in either instance. Several possibilities for stratigraphic traps exist 

in this area. The Ellis group thins from nearly 500 feet in the 

Pacific Western, Todd no. 1 well north of Benchland, to about 60 

feet along the front of the Little Belt Mountains. Because the Ellis 

group thins updip against the unconformably underlying Amsden 

formation, ideal conditions for a stratigraphic trap exist if any oil or 

gas is locally present in the lower part of the Ellis group as is true 

at Bowes dome north of the Bearpaw Mountains. Other possibilities 

for stratigraphic traps are lenticular sand bodies, which are present 

in the Kibbey sandstone, the Heath shale and the Amsden'formation. 

The discovery of an oil pool in these sand bodies/ however, will be 

fortuitous, until considerably more subsurface geologic data is available than at the present time.

COAL

During the first quarter of the present century, coal mining was an 

industry of secondary importance in the Stanford-Hobson area. A 

number of small mines were operated along the north flank of the 

Little Belt Mountains, and there was one mining camp, Lehigh, about 

3 miles southwest of Windham. Much of the coal was used by the 

Great Northern Railway and the rest was sold and used locally. 

Most of the mines were closed about 1921, and many of them were
TABLE 6. Records of wells drilled for oil and gas in the Sta

(Deepest formation source of information: a, driller's log; b, sample log

Name

T. W. Doswell, Dolganno. 1 __ . __ 

Minnesota-Montana Petroleum Co. 

no. 1. 

Minnesota-Montana Petroleum Co. 

no. 2. 

E. E. Chard and L. H. Harvey no. 1-- 

E. E. Chard and L. H. Harvey no. 2.. 

Buffalo Dome Oil Co. no. I--- ... _ .. 

Triple Dome Oil Co. Fulton no. 1 - 

F. E. Hunt, Oregon Mortgage Co. 

no. 2. 

F. E. Hunt, Oldham no. 1 __ ._ ___

F. E. Hunt, et al., Bone no. 3 ____

Reck Creek Bench Oil Co., Nesson no. 1 - - 

Rock Creek Bench Oil Co., Nesson 

no. 2, North line. 

John Carter. ___ .- ____ - __ -.. 

Windham-South Dakota Oil Co.. . 

Ooyt, Montana, no. 1.---. . ----,--_

National Refining Co. no. ! ----_ _ 

Pacific Western Oil Corp., Todd no. 1. 

Montana Drilling Equipment Co., 

Blackwood no. 1. 

Montana Drilling Equipment Co., 

Blackwood no. 2.

Location (section, township, range)

JSTEJ<NWJiSEM 12, 11 N., 15 E 

SWMSEM 12, 11 N., 15 E - - 

NWJ^SEM 12, 11 N., 15 E. ...........

NEMNWJi 9, 12 N., 14 E.... ......... 

NEJiSW}^ 9, 12 N., 14 E. ......-. .

NEJ^NWKSWJi 21, 12 N., 14 E...... 

SWJiNWMNEJi 6, 13 N., 14 E....... 

NEMSW^SEJi 20, 14 N., 15 E___ .

SEMNW^NWM 21, 14 N., 15 E .

NWJ<SEKNEJ< 30, 14 N., 15 E.......

NE}iNEJ<CNWK 36, 14 N., 16 E -

NEMNWJi 36, 14 N., 16 E... .........

SWJiSWJi 6, 15 N., 12 E_- 

SE^NEM 24, 15 N., 12 E 

SWMSWJ^NWJi 24, 15 N., 15 E...... 

SEMNEJ4SWJ4 15, 16 N., 10 E.......

30, 16 N., 12 E....... ........ ..........

SW^SWJiSWM 20, 16 N., 15 E... 

SW&NWJi 32, 17 N., 14 E..... .......

NEKNEJi 12, IS N., 15 E. ........... 

SEMNEM 12, 18 N., 15 E... ..........

Date 

drilled

1950 

1922

1922

1925 

1925 

1922 

1923 

1934

1933 

1935 

1947 

1948

1929 

1921 

1921 

1930 

1928 

1922 

1948 

1923

1925

Surface formation

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

 -do. 

- .do... .. .

 do... ... ...

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

 do . 

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

- do 

. .do - ...

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

-- do ... ..

. do .. .......

De

Ott

Ot

Ma

Ko

Elli
464 CONTRIBUTIONS TO ECONOMIC GEOLOGY

never reopened. Since that time small amounts of coal have been 

mined intermittently by local operators.

The coal is mined from the bed of subbituminous to bituminous 

coal with an aggregate thickness of from 5 to 6 feet that occurs at or 

near the top of the Morrison formation in this area. Locally, the coal 

bed was eroded before the Morrison formation was covered by the 

sediments that formed the Kootenai formation. Impure coal or 

carbonaceous shale also occurs in the Eagle sandstone, but the coal 

bed in the Morrison formation is the only one in this area that has 

been mined commercially. The outcrop pattern of the coal bed in 

the Morrison formation closely parallels the Morrison-Kootenai 

boundary as shown on the geologic map (pi. 45). No attempt was 

made to obtain detailed information on the thickness and grade of 

the coal, as most of the mine openings are now caved and the workings 

flooded. The locations of coal mines and prospects, nearly all of 

which have been abandoned, are shown on the geologic map (pi. 45). 

Descriptions of the mines, coal sections, and coal analyses for the 

Sage Creek area are given by Fisher (1909, p. 71-80) and for the 

Buffalo Creek and Utica areas by Calvert (1909, p. 57-59 and 72-75).

GYPSUM

Gypsum and anhydrite, the mineral forms of calcium sulphate, 

occur at several stratigraphic zones in the Stanford-Hobson area. 

East of the Stanford-Hobson area, gypsum in the Piper formation of 

Middle Jurassic age is mined at Hanover, on the south side of the 

Moccasin Mountains, and also at Heath, several miles southeast of 

Lewistown. There are no outcrops of the Piper formation in the area 

mapped, but it was found in the Pacific Western, Todd No. 1 well 

(table 2), sec. 32, T. 17 N., R. 14 E., and thus is present at depth in 

the northern part of the area.

Gypsum in beds of Big Snowy age crops out in the southwestern 

part of the area. One such occurrence is from 100 to 150 feet above 

the base of the Otter formation where at least from 15 to. 20 feet of 

white gypsum is exposed on the east end of a tree-covered ridge on 

the south side of the Blacktail Hills dome, sec. 25, T. 15 N., R. 10 E. 

The thickness and extent of the bed could not be determined. In the 

Lone Tree dome all three formations of the Big Snowy group contain 

gypsum. A 20-foot bed of impure gypsum occurs in the Heath shale 

from 170 to 190 feet below the Amsden formation in sections 3 and 4, 

T. 16 N., R. 10 E. It is more resistant than the shale and forms a 

gray ledge along the hogback. A 12-foot bed of gypsum that is interbedded with shale and limestone occurs from 100 to 112 feet above 

the base of the Otter formation in sec. 16, T. 16 N., R. 10 E. A 

20-foot bed of white gypsum occurs from 25 to 45 feet below the top
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 465

of the Kibbey sandstone and is exposed in a prospect pit at the boundary between section 20 and 21, T. 16 N., R. 10 E. The lateral extent 

of the bed is unknown.

GRAVEL DEPOSITS

The general distribution, age, mode of origin and economic significance of the gravel deposits in the area are discussed under the heading 

Terrace gravels and alluvium, on page 449. There is a supply of gravel 

in the area adequate for all foreseeable future uses.

BENTONITE

Bentonite deposits of possible commercial value occur in the Mowry 

shale equivalent. The bentonite zone is shown on the geologic map 

wherever it is exposed well enough to map. Probably the best exposures of this zone may be seen along the north bank of Arrow Creek, 

northeast of Geyser, and also along the east side ot Stanford-Buttes.

M. M. Knechtel (written communication) who made a brief examination of the bentonite outcrops about 3% miles north of Geyser 

has supplied the following information:

"The following section of the bentonite beds and the enclosing 

bedrock was measured in NE# sec. 19, T. 18 N., R. 10 E. The upper 

of the two beds listed may be equivalent to the Clay Spur bentonite 

bed, in the upper part of the Mowry shale, which is mined extensively 

in the Black Hills district. The northward dip is gentle.

Bentonite beds near Geyser, Mont.

Bed

Clay Spur (?).._._

Bed, 7 feet, 11 

inches thick.

Description of material

at top of Mowry. 

Bentonite- ____--_-_____--_.-_____-_

chert. 

[Bentonite, yellowish-gray, weathers 

1 white. 

| Bentonite, yellow-green; contains dark 

[ mineral particles. 

Shale, black, somewhat silicified; makes

hard floor; grades downward to softer 

dark-gray shale.

Thickness 

Ft In

1 6

1 10

21 0

2 5 

5 6

Sample 

No.

1

2 

3

The bentonite of these beds may be a possible source of clay for use 

in preparation of rotary oil-well drilling fluid. The suitability of the 

materials of the two beds for use by foundries as molding-sand bonding 

clay should also be tested."
466 CONTRIBUTIONS TO ECONOMIC GEOLOGY

LITERATURE CITED

Alden, W. C., 1932, Physiography and glacial geology of eastern Montana and

adjacent areas: U. S. Geol. Survey Prof. Paper 174. 

Bowen, C. F., 1915, The stratigraphy of the Montana group with special reference

to the position and age of the Judith River formation in north-central

Montana: U. S. Geol. Survey Prof. Paper 90-1, p. 95-153. 

Brown, R. W., 1946, Fossil plants and Jurassic-Cretaceous boundary in Montana

and Alberta: Amer. Assoc. Petroleum Geologists Bull. v. 30, no. 2, p. 238-248. 

Calvert, W. R., 1909, Geology of the Lewistown coal field, Montana: U. S. Geol.

Survey Bull. 390. 

Cobban, W. A., 1951, Colorado shale of central and northwestern Montana and

equivalent rocks of Black Hills: Amer. Assoc. Petroleum Geologists Bull.

v. 35, no. 10, p. 2170-2198. 

Cobban, W. A., 1945, Marine Jurassic formations of Sweetgrass arch, Montana:

Amer. Assoc. Petroleum Geologists Bull, v 29, no. 9, p. 1262-1303. 

Cobban, W. A., and Reeside, J. B., Jr., 1951, Lower Cretaceous ammonites in

Colorado, Wyoming, and Montana: Amer. Assoc. Petroleum Geologists

Bull., v. 35, no. 8, p. 1892-1893. 

Darton, N. H., 1906, Geology of the Bighorn Mountains: U. S. Geol. Survey Prof.

Paper 51. 

Fisher, C. A., 1909, Geology of the Great Falls coal field, Montana: U. S. Geol.

Survey Bull. 356. 

Gardner, L. S., Hendricks, T. A., Hadley, H. D., and Rogers, C. P., Jr., 1945,

Columnar sections of Mesozoic and Paleozoic rocks in the mountains of

south-central Montana: U. S. Geol. Survey Oil and Gas Investigations

Prelim. Chart 18. 

 1946, Stratigraphic sections of Upper Paleozoic and Mesozoic rocks in

south-central Montana: Montana Bureau of Mines and Geology, Mem. 24,

100 p. 

Hadley, H. D., Gardner, L. S., and Rogers, C. P., Jr. 1945, Subsurface

stratigraphy of Lower Mesozoic and Upper Paleozoic formations in the basin

area of south-central Montana: U. S. Geol. Survey Oil and Gas Investigations

Prelim. Chart 19, 1945. 

Iddings, J. P., and Weed, W. H., 1894, Livingston, Montana: U. S. Geol. Survey

Folio 1. 

Imlay, R. W., 1945, Occurrence of Middle Jurassic rocks in the western interior

of the United States: Amer. Assoc. Petroleum Geologists Bull. v. 29,

p. 1019-1027. 

Imlay, R. W., Gardner, L. S., Rogers, C. P., Jr., and Hadley, H. D., 1948, Marine

Jurassic formations of Montana: U. S. Geol. Survey Oil and Gas Investigations Prelim. Chart 32. 

Peale, A. C., 1893, The Paleozoic section in the vicinity of Three Forks, Montana:

U. S. Geol. Survey Bull. 110.

Peale, A. C., 1896, Three Forks, Montana: U. S. Geol. Survey Folio 24. 

Perry, E. S., and Sloss, L. L., 1943, Big Snowy group: Lithology and correlation

in the northern Great Plains: Amer. Assoc. Petroleum Geologists Bull.

v. 27, p. 1287-1304. 

Reeves, Frank, 1929, Thrust faulting and oil possibilities in the plains adjacent

to the Highwood Mountains, Montana: U. S. Geol. Survey Bull. 806-E,

p. 155-195. 

Reeves, Frank, 1930, Geology of the Big Snowy Mountains, Montana: U. S.

Geol. Survey Prof. Paper 165-D, p. 135-149.
GEOLOGY OF STANFORD-HOBSON AREA, MONTANA 467

Rogers, C. P., Jr., Gardner, L. S., and Hadley, H. D., 1945, Thickness and general 

distribution of Mesozoic and Paleozoic rocks in south-central Montana: 

U. S. Geol. Survey Oil and Gas Investigations Prelim. Map 43.

Schafer, J. P., 1949, Some periglacial features in central Montana: Jour. Geology 

v. 57, no. 2, p. 154-174.

Scott, H. W., 1935, Some Carboniferous stratigraphy in Montana and northwestern Wyoming: Jour. Geology v. 43, no. 8, pt. 2, p. 1011-1032.

Scott, H. W., 1942, Ostracodes from the Upper Mississippian of Montana: 

Jour. Paleontology, v. 16, no. 2, p. 156-164.

Stebinger, Eugene, 1914, The Montana group of northwestern Montana: U. S. 

Geol. Survey Prof. Paper 90-G, p. 61-68.

Vine, J. D., and Hail, W. J., Jr., 1950, Geologic map of the Hobson area, central 

Montana: U. S. Geol. Survey Oil and Gas Investigations Prelim. Map 108.

Vine, J. D., and Johnson, W. D., Jr., 1954, Geology of the Stanford area, Judith 

Basin and Fergus Counties, Montana: U. S. Geological Survey Oil and Gas 

Investigations Map OM 139.

Walton, P. T., 1946, Ellis, Amsden, and Big Snowy group, Judith Basin, Montana: 

Amer. Assoc. Petroleum Geologists Bull. v. 30, no. 8, p. 1294-1304.

Weed, W. H., 1899a, Fort Benton Montana: U. S. Geol. Survey Folio 55.

Weed, W. H., 1899b, Little Belt Mountains, Montana: U. S. Geol. Survey 

Folio 56.

Weed, W. H., 1900, Geology of the Little Belt Mountains, Montana: U. S. Geol. 

Survey Ann. Rept. 20, pt. 3, p. 257-461.

INDEX

Page 

Agriculture . . .. 414

Alden, W. C., cited.... - 448,449 

Alteration. ........ -..-. -... 455

Amsden formation.... ... 418,426,430,432,435

doubtful strata. ---------------------- 433

Antelope Creek dome.-... ....___ 457

Aragonite-... -_-____.. ....._... 424

Belle Fourche-shale equivalent..-----.. 439,441,444 

Bentonite........_.- .. .-. 442,465

Big Snowy group____-----_____- 417-24,464 

Big Snowy Mountains-..-. - - 410 

Blacktail Hills dome.-. 458 

Buffalo dome...----------------------------- 457

Carlilc-shale equivalent.. -------------- 444,445

Chert.... 435

Claggett qhale_........._......-.---.----- 447

Coal bed, Morrison formation....- 436,437,438,464

Coal mines, abandoned......___----- 414,462-63

Cobban, W. A., cited..... 408,431,444,455

Colorado shale.....------------------------- 439-445

Cores of igneous rock in domes.. ........ 458

Dikes-.-.-- ------------------ -- 449,453

Domes.-------- -------- - 453-54,457-59

Dott,R.H., work cited.. ------------ 

Drainage.._............ ...... ... 408,451

Eagle sandstone_____ . 446,464 

Earlier studies_.__... 407 

Ellis group 434-35

Faults . -. 460

Field work.-----.-----.-------------------- 408

Fossils^ --------------------- 424,436,445

Gardner, L. S., work cited -. - 433 

Garneill nose. . . . 456 

Gravel.. - 447-49,451,453-54 

Greenhorn-limestone equivalent- - ___... 441,444 

Gypsum.... 421,423,464

Hail, W. J., Jr., work cited--.- 408,415 

Heath shale.. 417,423-25,432,464 

Highways- ---- 411 

Highwood Mountains 410 

Hogbacks-.---- -- 410,417,430,431,458,459

Igneous rocks. ... - 424,453-55

Imlay, R. W., work cited. ----------------- 433

Involutions -- ------------ 450-51

Judith Basin.- -- 408,456

Judith Gap...- .... 410,456

Judith Mountains_._ _____ 410 

Judith River formation... ... .. 447

Kibbey sandstone------------- 417,421-22,459,464

Kncchtol, M. M., work cited --------------- 465

Kootenai formation_ _ -------- 437-438

basal sandstone___ __.... 438,439 j

Page 

Laccoliths.--.--------- 453-54

Limestone.. 417,422,430,431,433,435,437,438,445,464 

Little Belt Mountains..----.---..--.-.. 410,430

Lone Tree dome____ -________. _ 458-59

Madison limestone..___......._ 417,421-22,459

Minerals..__..-..___ . _ __ 454-55 

Morrison formation___.................. 436-37

Mo wry-shale equivalent___._. .__ 439,441-42

Niobrara-formation equivalent . ........ 445

Oil and gas fields, possibilities for_ 457,458,461,462 

Otter formation......---.....-..... 417,422,464

Phenocrysts-. - - .- _-__ 454 

Piper formation.. .............. 433,434,435,464

Pleistocene glacial deformation.. .. ____ 450-51

Quadrant formation... .__......._._ 418

Quartz monzonite..- -__.................. 455

Reeside, J. B., cited.__._.__.____.... 439

Rierdonformation_..--__....._____ 434,435

Rock units, contacts .- 422,

431,432,433,435,437,439,464

topographic expression.. . ... 417,

422,423, 431,437,439,447

Sandstone.. . 421,422,423,430,437,438,440,446,447 

Sawtooth formation......___.____... 434

Schafer, J. P., work cited-__ __.__ 451 

Scott, H. W., work cited....... ......... 418,425

Shale - - -.-. 421,

422,423,430,433,437,439,440,441,445,447,464 

Sills 424,453

Skull Butte dome.. ..__ 457-58

Skull Buttes-- - - 410

Stanford Buttes -----__ 410

Stanford-Hobson area, climate --------_ 411

location._ .. __ 408

Stock...-. . . . ... 453

Stratigraphic sections, Amsden formation__ 421, 

423-24,426-27, 428,434

Belle Fourche-shalc equivalent..____ 443 

Big Snowy group...---------..-.- 416,420,421

Carlile shale.------- ..... .......... 442

Colorado shale.....--------- ._.__ 415,442

Eagle sandstone.. _ 447 

Ellis group. - 416,426,427,428,433,435,437 

Greenhorn limestone................_. 442-43

Heath shale.. .......... 416,420,421,426,427

Kibbey sandstone-------_...__ 416,420,421

Kootenai formation.... _... .__ 415,438

Morrison formation. . .. . 415,434,437 

Mowry-shale equivalent_. 440,441,442,443-44 

Otter formation.---- . 415,420,429

Telegraph Creek formation.. __ 445-446 

Thermopolis-shale jequivalent__... 440-41,444

469
470 INDEX

Page

Structure... . ... ...- ------- 455

Structure contours, data__.. ..,__ 461 

Structures, hidden ..- .------- - 459-60

Sweetgrass arch...__----------------__ 455-56

Swift formation .. _____ __ 434,435

Telegraph Creek formation.._------------ 439,445

Tensleep sandstone...--.------.- ...--.- 418

Terraces. 408,409,414,447-53

Test wells for oil and gas..-.. . ..___ 415, 

439,456-57,459,462, 463

Page

Thermopolis-shale equivalent__ .... 439-40,444 

Thinning of rock units. .-..-. 430,432,437,460,462 

Trends, structural____________.__ 460-61

Unconformities, pre-Ellis_. ______ 430,432,435 

Vegetation..... _...... . 411

Walton, P. T., cited 432

Water . ... 453

Windhamdome___.___._____ 453-54,459

U. S. GOVERNMENT PRINTING OFFICE: 1956