ANTICLINES IN THE BLACKFEET INDIAN RESERVATION,

MONTANA.

By EUGENE STEBINGER.

INTRODUCTION.

The discovery of small quantities of oil and gas in several anticlines 

along the eastern front of the Kocky Mountains west of Calgary, 

Alberta, has aroused interest in the possible oil and gas resources of 

the formations in this general region, especially of the southern 

extensions of the folded and faulted beds that were productive in 

Canada. This interest has led to drilling for oil and gas at places 

within a few miles of the northern boundary of the Blackfeet Indian 

Reservation, on the west edge of the plains. A similar interest, in 

1900-1902, led to drilling in the Cretaceous rocks at several places a 

few miles west of the west boundary of the reservation, close to the 

mountain front.

A report on the possibilities of finding oil and gas in a large area in 

north-central Montana east of the Blackfeet Indian Reservation has 

recently been issued,1 and the Blackfeet Reservation itself, shown in 

figure 35, has now become the center of similar inquiries. The purpose of this paper is to give a brief account of the geologic formations in the reservation and of their lay or geologic structure, and a 

more detailed description of the anticlines and of the formations that 

appear to contain oil or gas in southern Alberta and northern Montana. An intelligent search for oil or gas in the reservation must be 

based on a knowledge of these geologic facts. Reports that describe 

coal 2 and low-grade iron ore 8 and the general features of the geology 

of the reservation have already been published, and it is now considered desirable to summarize the facts bearing on the oil and gas 

prospects.

The general conditions in this region suggest that drilling in the 

Blackfeet Indian Reservation would have about the same chance of

1 Stebinger, Eugene, Possibilities of oil and gas in nortn-central Montana: U. S. Geol. 

Survey Bull. 641, pp. 49-91, 1916 (Bull. 641-C).

2 Stebinger, Eugene, Geology and coal resources of northern Teton County, Mont.: 

U. S. Geol. Survey Bull. 621, pp. 117-156, 1916.

3 Stebinger, Eugence, Titaniferous magnetite beds on the Blackfeet Indian Reservation, 

Mont.: U. S. Geol. Survey Bull. 540, pp. 338-342, 1913.

43768° Bull. 641 17 19 281
282 CONTEIBUTIONS TO ECONOMIC GEOLOGY, 1916, PAKT II.

success as in the adjacent region in southern Alberta, extending from 

the international boundary northward to Calgary a region in which 

drilling by over 40 companies during the last three years has been 

slightly successful. The similarity of the geologic features in these 

two regions may be summarized as follows:

1. The Colorado shale and the Kootenai formation, which yield 

most of the oil and gas so far found in southern Alberta and northern 

Montana, underlie the greater part of both regions.

2. The belt of folded and faulted strata along the front of the 

Rocky Mountains is continuous from one to the other of these regions, 

and in this belt the productive anticlines near Calgary and the folds 

in the Blackfeet Indian Reservation are both in places somewhat 

compressed and in others are associated with faults.

FIGURE 35. Index map showing location of Blackfeet Reservation, Mont.

3. In the area east of the disturbed belt in both regions great thicknesses of nearly horizontal strata that have no strongly marked 

structural features, such as anticlines and synclines, overlie the 

possible productive formations.

LOCATION AND SURFACE FEATURES.

The location and area of the Blackfeet Indian Reservation are 

shown on the index map (fig. 35). The reservation lies wholly in 

Teton County, occupying nearly all its northern half, and contains 

about 2,425 square miles. The international boundary between Alberta and Montana forms its northern limit. The country is a. 

gently sloping, nearly treeless plains region, which is everywhere 

easily accessible, presenting few obstacles to the construction of
ANTICLINES IN BLACKFEET INDIAN RESERVATION, MONT. 283

railways or roads. The plains slope gently upward to the west to 

the base of the mountains, which rise abruptly without marked foothills. The average elevation of the plains districts ranges from 

3,800 feet on the east edge of the reservation to about 5,000 feet near 

the base of the mountains. The mountains rise with AV all-like abruptness from 4,000 to 5,000 feet above the general level of the plains.

The main line of the Great Northern Railway crosses the reservation from east to west. Cut Bank, Browning, and Glacier Park are 

the principal stations. Browning is the seat of the Indian agency 

for the reservation. Settlement on the Indian reservation has so 

far been limited almost entirely ^ to its west half, where Indian 

ranches along the principal streams are devoted to the raising of 

horses and cattle. The extensive interstream stretches remain an 

open range, without fences or other improvements.

In the western part of the reservation, near the mountains, there 

are high, level plains, Avhich comprise numerous isolated tracts, from 

a fraction of a square mile to 40 square miles or more in extent, and 

Avhich are doubtless remnants of an older, almost perfectly formed 

plain. They are covered Avith a veneer of gravel, made up of kinds 

of rock found in the aHjacent mountains, and have remarkably 

smooth surfaces and slope A^ery regularly to the northeast, aAvay from 

the mountains. Milk River Ridge and St. Mary Ridge are typical 

of the larger of these areas. Below these higher plains there are 

much more extensive lower plains, which, Avhere best developed, also 

carry a veneer of gravel and slope evenly eastward, away from the 

mountains, to merge into the Great Plains. The best examples of 

the loAver plains are Carlow Flat, an extensiAre and very evenly 

graded surface crossed by the Great Northern Railway west of Cut 

Bank, and the equally Avell developed plain betAveen Cut Bank and 

Greasewood creeks, a few miles northwest of Browning.

FIELD WORK.

The field work on which this report is based was done by the 

Avriter alone, during a part of each of the field seasons of 1911, 1912, 

and 1913. The main object of the Avork was to classify the lands in 

the reservation Avith respect to their mineral value, and it was therefore necessary to make a regional examination of the geology and a 

close inspection of all possibly mineral-bearing tracts.

Good topographic maps were available for all the area examined 

and Avere used in the field as base maps on which to plot the geologic 

data. The land corners in all parts of the region are Avell marked 

and easily found. Iron pipes marking the boundaries of the lands 

allotted to the Indians have been placed at many of the corners.
284 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1916, PART II.

GEOLOGY.

STRATIGRAPHY.

GENERAL FEATURES.

The geologic formations east of the mountains in the Blackfeet 

Indian Eeservation are all sedimentary in origin and range in age 

from Lower Cretaceous to probably ba,sal Tertiary. (See PL 

XXIV.)" All the beds are apparently conformable and afford no 

evidence of break or disturbance between any of them or any indication of an interval of erosion. The sequence of the formations in the 

Upper Cretaceous part of the section is different from that so far described for any other locality in Montana, owing to the fact that in 

this region the marine invasion which is represented by the Claggettshale did not extend so far west as the present position of the Rocky 

Mountain front between latitudes 48° and 49° N. An account of 

these relations has already been published.1

Two of the formations, the Colorado and Bearpaw shales, are unquestionably of marine origin and were laid down in separate epochs, 

during which a comparatively shallow sea covered the entire region. 

The remaining formations are mainly of continental origin that is, 

they are irregularly bedded rocks that were for the most part deposited by streams and winds on land areas that were only slightly above 

sea level.

The succession of formations and the principal features of the 

rocks are outlined in the table on page 285.

1 Stebinger, Eugene, The Montana group of northwestern Montana: U. S. Geol. Survey 

Prof. Paper 90, pp. 61-68, 1915.
U. S. GEOLOGICAL SURVEY BULLETIN 641 PLATE XXIV

Two Medicine 'formation

HORIZONTAL SCALE SAME AS MAP; 1/EffTICfli. SCALE INCREASED S TIMES

GEOLOGIC SKETCH MAP OF BLACKFEET INDIAN RESERVATION, MONT., AND VICINITY, SHOWING SECTIONS AND WELLS DRILLED FOR OIL OR GAS.
ANTICLINES IN BLACKFEET INDIAN RESERVATION*, MONT. 285

Formations occurring east of the mountains on the Blaclcfeet Indian Reservation, Mont. ,

System.

Quaternary.

Tertiary (?).

Cretaceous.

Series.

Recent.

Pleistocene.

Eocene (?).

Tipper Crelaceons.

Lower Cretaceous.

0

Group and formation.

Alluvium.

Glacial drift.

Willow Creek formation .

St. Mary River formation (coal bearing).

Montana group.

Horsethief sandstone.

Bearpaw shale.

Two Medicine formation (coal bearing).

Virgelle sandstone.

Colorado shale.

Kootenai formation.

Thickness in 

feet.

720+

980

225-375

490

I

1,950

220

1, 500±

2, 000±

Character of the rocks.

Deposits of small extent found along 

flood plains of the larger streams.

Bowlder clay, gravel, and lake silt and 

clay. Contains bowlders and cobbles 

of granite, gneiss, quartzite, etc., 

transported from other regions. 

Deposits are of several stages not 

distinguished in this report.

Variegated clay and soft sandstone, 

chiefly maroon to chocolate-brown 

with subordinate gray, yellow, and 

greenish-gray beds. Fragments of 

fossil bones common. Clay in places 

contains thin lenticular beds of purplish-gray limestone. Forms a red 

soil over large areas. Top not seen .

Alternating clay, clay shale, and soft 

sandstone; sandstone much crossbedded and ripple marked. Colors 

gray to greenish gray; a few layers of 

clay arered. Contains a few thin lenticular limestones, many fragments 

of dinosaur bones, and fossil shells.

Gray to greenish-gray sandstone, 

weathering buff. Thin bedded and 

somewhat shaly in lower half. In 

upper half generally massive and 

concretionary, weathering in castellated forms. In places near the top 

contains titaniferous magnetite. . H as 

many shell beds, mainly of oysters.

Dark-gray clay shale with a few limestone concretions. Contains abundant marine shells. Forms subdued, 

rounded topography.

Gray to greenish-gray clay and soft, 

irregular sandstone, which is most 

abundant in the lower 250 feet. In 

places thin beds of red clay and 

nodular limestone. Contains an 

abundant reptilian fauna of Judith 

River types, besides leaves and shells. 

Coal beds near base and at top.

Gray to bufE coarse-grained, much 

cross-bedded massive sandstone, 

with many ferruginous concretions in 

upper half. In lower half slabby 

gray sandstone, becoming shaly 

toward the base. Contains gas in the 

Havre field and at Medicine Hat.

Bluish-gray shale with a few limestone 

- concretions. Contains an abundance 

of marine shells. Forms a subdued 

and rounded topography. Complete 

undisturbed section not present. 

May contain oil and gas in areas of 

favorable structure.

Gray sandstone and shale, alternating 

with maroon clay shale. Some of the 

sandstone massive. Conglomerate 6 

to 50 feet thick near center. Carries 

a few leaves and fresh-water shells. 

Complete undisturbed section not 

found. May contain oil and gas in 

areas of favorable structure.
286 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1916, PAET II. 

FORMATIONS THAT MAY IN PLACES CONTAIN OIL OR GAS.

Kootenai formation. The Kootenai formation is the oldest formation seen in the plains portion of the Blackfeet Indian Reservation. 

It is exposed only in the belt of disturbed rocks adjacent to the mountains but underlies all the plains region. In this disturbed belt 

folding and especially thrust faulting have so greatly deformed the 

rocks that their exact sequence and thickness can not be determined, 

but the exposures are good and the general character of the formation is well known. A few miles beyond the west border of the area 

the Kootenai overlies a dark marine shale carrying Jurassic fossils. 

This relation, together with the fact that the Kootenai immediately 

underlies the Colorado shale, gives the formation approximately the 

stratigraphic position of the Kootenai of the Great Falls region, 

150 miles to the southeast, which has in turn been correlated with 

the Kootenai formation of British Columbia and Alberta by means 

of the characteristic fossil plant remains, mainly leaves, found at 

both localities.

The Kootenai is composed essentially of alternating dark greenishgray, black, and maroon shales and of gray sandstones that are in 

many places gritty and conglomeratic. A few beds of black carbonaceous shale and some thin streaks of coal are the only indications 

of coal found in the area, although to the* north, in Alberta, and 

to the south, in the vicinity of Great Falls, the formation contains 

a large quantity of coal. An irregular bed of conglomerate, which 

is at some places as much as 50 feet thick, crops out at a horizon 

believed to be near the middle of the formation. Beds of sandstone, 

conglomerate, and grit, which must be considered the more likely 

reservoirs for oil or gas in the Kootenai, are distributed through it, 

generally in beds that range in thickness from 20 to 70 feet. Limestone in concretions and thin lenticular beds occurs sparingly in the 

shale. *

The thickness of the Kootenai is not determinate at any point 

along its outcrop in the Blackfeet Indian Reservation because of 

its frequent duplication by faulting and'because neither its base nor 

its top is anywhere visible. However, its minimum thickness can 

be determined by examining faulted blocks of the formation in which 

no duplication appears. Near Badger Creek (sec. 34, T. 30 N., 

R. 11 W.) 1,200 feet of beds that clearly belong to the Kootenai 

were measured in a single block of the formation that shows no 

repetition by faulting. On comparing this thickness with the measurement obtained in a well bored about 15 miles west of the reservation (see well log^itecl on p. 305), showing about 580 feet of beds 

that have been assigned to the Kootenai, it is evident that the 

formation becomes thinner toward the east in this region. At Great
ANTICLINES IN BLACKFEET INDIAN RESERVATION, MONT. 287

Falls, where the Kootenai is best known in Montana, it is between 

400 and 500 feet thick.

No escaping gas or residues or seepages of oil were noted at any 

point in the Kootenai formation in the area here described. Small 

quantities of oil and considerable flows of gas are reported from 

beds in Alberta that are believed to be equivalent to the Kootenai of 

this region. A small flow of gas was also obtained from sands in 

this formation in a well in the Sweetgrass district.

Colorado shale. The next unit in the strata overlying the 

Kootenai formation in this re'gion is the Colorado shale, a great 

body of bluish-gray to black shale containing sandstone beds in its 

lower half. This formation is about 1,600 feet thick in the eastern 

part of the Blackfeet Indian Reservation and underlies nearly all 

the area. It is of marine origin that is, it was laid down on a sea 

bottom and contains fossil sea shells in great variety. The rock 

deposited during this marine submergence continues northward into 

Canada and southward into Wyoming and beyond.

The Colorado shale seems to be the most probable source of oil and 

gas in this region. The only surface showings of oil so far found 

on the plains of northern Montana seep from the lower part of this 

formation, and pay sands that are either in the formation or are 

associated with it are found in adjoining areas both north and south 

of the State. The principal production from the Salt Creek field in 

northern Wyoming, and of all the fields in the Bighorn Basin comes 

mainly from the sands in the lower half of this group of rocks. The 

production of the important gas field at Bow Island, east of Lethbridge, in the province of Alberta, comes from the top of a sand that 

is supposed to lie immediately under this formation. This sand, 

however, is probably in the lower part of the Colorado shale. Likewise, as reported by Dowling,1 a small production-of high-grade oil 

from several wells in two anticlines lying southeast of Calgary, in 

the same Province, comes from sands at the same position with 

respect to this shale.

There are many exposures of the Colorado shale in the western part 

of the Blackfeet Indian Reservation, especially from Two Medicine 

Creek southward, in the belt of disturbed rocks, but the beds are invariably much crushed and crumpled. These crumpled beds of 

bluish-gray to black shale, which contain limestone in concretions 

and thin irregular beds, can not be measured, because they are at 

many places repeated by faulting, and therefore no accurate estimate 

of the thickness of this formation in the western part of the reservation can be made. The most extensive exposures appear along Two 

Medicine Creek frorrTits junction with the South Fork in sec. 29, T.

1 Dowling, D. B., Correlation and geological structure of the Alberta oil fields : Am. 

Inst. Min. Eng. Bull. 102, pp. 1360-1364, .19.15.
288 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1916, PAET II.

31 N., E. 11 W., westward for 8 miles, to the vicinity of Glacier Park 

station. In this stretch continuous exposures show the shale crumpled 

and repeated by faults in a remarkable manner. The upper beds of 

the Colorado shale are exposed at the head of Marias River, on the 

eastern edge of the reservation in sec. 35, T. 32 N., R. 5 W., and from 

this point extensive exposures of the upper beds of the formation, 

which lie nearly horizontal, extend for many miles downstream. The 

log of a well (p. 305) which was drilled through these horizontal 

rocks near Kevin, about 15 miles east of the Indian reservation, shows 

1,060 feet of shale which doubtless belongs to the Colorado. In addition to this thickness, about 580 feet of shale is exposed between the 

site of this well and the base of the overlying Virgelle sandstone, in 

the escarpment lying to the west, making about 1,640 feet in all. 

This is the only available estimate of the thickness of the formation 

in this region. According to the well record mentioned, three sandstones containing gas and water are reported from the lower part of 

the Colorado shale. As these sandstones are overlain by great thicknesses of shale they seem to be favorable receptacles for the accumulation and retention of oil and gas, which may have accumulated in 

them in commercial quantities in places where anticlines have permitted their concentration.

Additional evidence of the presence of petroleum in the lower 

part of the Colorado shale in this region was obtained by the writer 

during the summer of 1916 in the course of field work in the area 

adjoining the Blackfeet Indian Reservation on the south, extending 

across Teton County and into the northern part of Lewis and Clark 

County. The disturbed belt adjacent to the mountains in the Blackfeet Indian Reservation (see PI. XXIV) continues* southward across 

this area and, as in the reservation, affords many exposures of the 

Colorado shale, for the most part much folded and faulted. The 

lower part .of this shale formation was found to be petroliferous in 

every extensive exposure for a distance of about 35 miles, extending 

from Dupuyer Creek to Sun River.

All the " showings " seem to occur at a single horizon in the lower 

part of the formation arid are in the form of impregnations of a 

soft tarry bitumen along fracture planes in thin beds of impure 

limestone occupying a zone about 12 feet thick, in bituminous shale. 

The limestone is in beds as much as 8 inches in thickness. It has 

been irregularly fractured by the intense deformation in the disturbed belt, and the fractures are wholly or partly filled with calcite. 

The openings in the calcite veinlets thus formed are generally filled 

with a black tarry substance of high luster, readily soluble in carbon 

disulphide, and soft enough to flow when the thicker accumulations 

are tilted. A thin film of the bitumen is transparent, and it has a
ANTICLINES IN BLACKFEET INDIAN KESERVATION, MONT. 289

deep brownish-red color. It is clearly an impregnation from a 

source outside the limestone, the distance the bitumen was able 

to travel along individual veinlets being readily determinable in 

many hand specimens. The shale inclosing these fractured limestone 

beds is a dark compact bituminous rock giving a distinct though 

transient odor of kerosene, especially after being gently heated. 

Hand specimens of this material yielded on distillation between 1 

and 2 gallons of petroleum to the ton, and this shale is doubtless 

the source of the tarry material found in the limestone. The total 

thickness of this low-grade oil shale averages about 50 feet.

The horizon at which these bituminous limestones and shales 

occur is near the base of the thick body of shale making up the 

upper two-thirds of the Colorado shale in this region and just above 

its lower sandy portion, which contains as many as six sandstone 

beds from 10 to 50 feet thick intercalated with the shale and carrying 

Inoceramus and other marine fossils. The principal localities at 

which these bituminous rocks were noted are as follows: On the 

south bank of the Middle Fork of Dupuyer Creek, in the SW. £ 

SW. £ sec. 18, T. 27 N., E. 8 W.; on the South Fork of Dupuyer 

Creek, in the 'NW. J sec. 36, T. 27 N., K. 9 W.; on the North Fork of 

Willow Creek, in the NW. £ sec. 9, T. 24 N., E. 8 W.; on Deep Creek, 

in the NW. £ sec. 28 and in sec. 16, T. 23 N., E. 8 W.; and on 

Sun Eiver, in the SW. i sec. 29, T. 22 N., E. 8 W.

No seeps or other indications of oil were found on the outcrops 

of the Colorado shale in the Blackfeet Indian Eeservation. HOAVever, the only noteworthy showings of oil so far reported from 

northern Montana occur in the lower half of the Colorado shale. 

W. A. English,1 of the United States Geological Survey, reports an 

occurrence on the north side of West Butte in the Sweetgrass Hills, 

a, short distance south of the Canadian boundary. At this locality, 

in a spring on the Eoscoe ranch near the line between sees. 11 and 12, 

T, 37 N., E. 1 E., there are slight showings of a light-greenish oil, 

which collects to the extent of about a teaspoonful in every 24 hours. 

Mr. English also reports that extraction tests made on samples from 

outcrops of the sandstone in the lower part of the Colorado on West 

Butte give a good show of oil. That the lower part of the Colorado 

shale in this part of Montana carries petroleum seems to be clearly 

proved by this occurrence.

Virgelle sandstone. The Virgelle sandstone overlies the Colorado 

shale conformably. The transition from one to the other is marked 

by alternating beds of shale and sandstone lying between the characteristic rocks of the two formations. The principal member of the 

Virgelle formation is a thick bed of rather coarse-grained, massive

1 English, W. A., personal communication.
290 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1916, PART II.

sandstone which forms its upper half. In places it is much crossbedded and contains large ferruginous concretions. At the mouth 

of Cut Bank Creek this formation is 220 feet thick. Near the center 

of T. 35 N., E. 4 W., it is 225 feet thick. Both these measurements 

 were made on the eastern edge of the Blackfeet Indian Reservation 

and therefore do not apply throughout the reservation. The Virgelle 

was not measured along the outcrops in the disturbed belt of rocks, 

but in the west half of the area its thickness apparently does not 

differ greatly from that measured to the east.

The Virgelle sandstone contains large amounts of. gas at Havre, 

Mont., along the axis of a folded and faulted uplift. 1 This formation 

is also gas bearing at Medicine Hat and other places in southeastern 

Alberta and may therefore contain oil and gas in this general region.

FORMATIONS ABOVE THE VIRGELLE SANDSTONE.

The succession of formations above the beds that may contain oil 

and gas in this region includes the Two Medicine formation, Bearpaw shale, Horsethief sandstone, St. Mary River formation, and the 

Willow Creek formation, presenting an aggregate thickness of about 

4,400 feet of strata, composed of clay, shale, and sandstone. None 

of these beds has yet yielded favorable showings of oil or gas in, 

southern Alberta and northern Montana. They have been drilled 

through in many places in southern Alberta with uniformly negative 

results except for small puff's of gas in rocks that are equivalent to 

the Two Medicine formation. The Bearpaw is a marine shale that 

is roughly equivalent to the upper part of the Pierre shale, which 

in Wyoming and Colorado, notably in the Florence oil field of Colorado, has yielded oil in commercial quantities.

The prospects for oil and gas in the formations above the Virgelle sandstone are meager, so the oil prospector is mainly interested 

in their thickness, the best available estimates of which are given 

in the table on page 285.

COMPARISON WITH CANADIAN CLASSIFICATION OF FORMATIONS IN 

SOUTHERN ALBERTA.

The general series of formations that occur in northern Montana 

extends northward into Canada. The changes in the Montana group 

and the thinning out and disappearance of the Claggett shale from 

east to west, which became apparent in the section of Montana, persist into southern Alberta, where they have been recognized by 

Dowling and are in every way similar. The differences in the for1 Stebinger, Eugene, Possibilities of oil and g;as in north-central Montana: TJ. S. Geol. 

Survey Bull. 641, pp. 49-91, 1916.
ANTICLINES IN BLACKFEET INDIAN RESERVATION, MONT. 291

mation names used in Canada for the probable oil and gas bearing 

formations relate chiefly to the Colorado shale and the Virgelle sandstone, which the Canadian geologists have classified into three units, 

the Dakota sandstone, Benton shale, and Niobrara sands. The 

"Niobrara sands" are identical with the rocks described as Virgelle 

sandstone in this report. The fact that all the evidence afforded by 

fossils is adverse to the use of the name Niobrara for these sandstones 

was .long ago emphasized by Stanton.1

The Dakota has not been recognized in northwestern Montana at 

any point where the strata in which it should be present have been 

examined, and it is "believed that its existence is equally uncertain in 

southern Alberta, where geologists have been inclined to assign' to 

the Dakota any sandy beds that occur at its horizon. The sandstone 

beds at Bow Island, elsewhere referred to as Dakota, are probably the 

sandstones in the lower part of the Colorado shale, as stated in this 

report. It is very suggestive that well logs at Bow Island show dark 

shale beneath the pay sands called the Dakota. Marine fossils found 

in the basal sandstones of the Colorado shale in the section in central 

Fergus County in Montana seem to indicate the marine origin of all 

these sandstones, which are very similar to the productive sands in 

the Colorado shale in the oil fields of Bighorn Basin, Wyo. The -use 

of the name Benton shale for shale overlying the supposed Dakota 

in southern Alberta .is also inadvisable, for no evidence afforded 

either by fossils or lithology is important enough to warrant the 

recognition of the Benton and Niobrara divisions of the Colorado. 

The simple expedient of applying the term Colorado shale to strata 

in northern Montana, which include both Benton and Niobrara, has 

long been in use.

The Two Medicine formation, as well as the underlying Virgelle 

sandstone, when traced northward appears to be identical with the 

rocks known to the Canadian geologists as the Belly River series. 

In Avestern Montana, however, the Virgelle sandstone is easily distinguished from the overlying rocks and has been mapped over considerable areas as a distinct unit, so that the term Belly River can not 

be conveniently used as a formation name in the area here described.

STRUCTURE. 

EFFECT OF STRUCTURE ON THE ACCUMULATION OF OIL OR GAS.

It is generally conceded that the accumulation of oil in pools is due 

to some irregularity in the rock bed. If the beds lie flat or if they dip 

slightly but regularly in a certain direction, the oil that circulates 

through the pores of the rock will not tend to accumulate in any par1 Stanton, T. W., and Hatcher, J. B., Geology and paleontology of the Judith River 

beds:,U. S. Geol. Survey Bull. 257, p. 64, 1905.
292 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1916, PART II.

ticular place, but if the beds have been disturbed that is, bent into 

folds or broken the oil may find a resting place and form a pool.

The structural form that is most favorable to the accumulation of 

oil is an upwarp or bulge, a feature that is generally known as an 

anticline, but if the anticline has about the same width as length it-is 

generally called a dome. Other structural forms that are less favorable to the accumulation of oil are terraces, monoclines, synclines, or 

tilted blocks of rock that have been separated from the adjapent 

strata by faults. The degree of deformation of rock beds, however, 

varies greatly. In some places, as in the Kansas, Oklahoma, and 

Texas fields, it is so slight as to be imperceptible to casual inspection 

and requires close instrumental work for its detection; in others, as in 

the California and Wyoming fields, the tilting may be strongly developed, and can be ascertained at a glance by a trained observer. '

The presence or absence of water in the " sands " is a further qualifying factor in determining the location of pools. The following 

statements concerning the presence of water have been verified again 

and again in developed fields:

1. If the pay sands are saturated with water, the gas and oil, 

which are lighter than the water, will be found above it in the crests 

of domes or anticlines or on the flats or terraces of monoclinal 

structures.

2. If only parts of the sands are saturated with water, the gas and 

oil will be found at the upper level of saturation, and this level may 

be on the crest of the anticline or may be part way down its sides.

3. If the sands are dry, the oil will have migrated downward either 

to the troughs of the synclines or to a point where further shifting 

has been prevented by the closeness of the sand.

4. If oil and gas occur in the same stratum, gas, which is1 the lighter 

substance, will generally, though not invariably, be found above 

the oil.

GENERAL FEATURES OF STRUCTURE IN THE BLACKFEET INDIAN

RESERVATION.

The area described in this report can be divided into two large 

structural units, which differ greatly in the degree to which the strata 

have been deformed. A slightly curving line (shown on the geologic 

sketch map, PL XXIV) , extending northward from a point on Birch 

Creek in T. 37 N., E. 9 W., to a point on the Canadian boundary in 

T. 37 N., E. 12 W., would mark a sharp transition from an area on the 

east in which the beds are nearly horizontal and undisturbed to an 

area on the west, adjacent to the mountain front, in which the rocks 

have been extensively faulted and folded. The eastern part of the 

area shown on the map is characterized by simple broad belts of out-
ANTICLINES IN BLACKFEET INDIAN RESERVATION, MONT. 293

crop that trend regularly northward and the western area by a relatively complex arrangement of formations.

In the area where the beds are nearly horizontal they dip in general slightly westward at angles ranging from nearly 0° to 5° in 

gentle undulating folds whose average dip is not over 2°. As a result of this average westward dip of the bedrock formations the 

youngest or the highest stratigraphically, the Willow Creek formation, lies farthest west in these nearly horizontal beds, and the 

oldest, the Colorado shale, lies farthest east on the eastern edge of 

the tract. Thus one who is traveling westward up any of the principal valleys, as, for instance, from Marias River up Cut Bank Creek 

or Two Medicine Creek, would cross all the formations in sequence 

from older to younger in a distance of 35 to 50 miles. Between the 

south slopes of Milk River Ridge and the international boundary a 

broad, open syncline, whose steeper limb is on the west, lies on the 

west border of the area of nearly horizontal rocks. The axis of this 

syncline is between 1 and 2 miles east of the line that marks the 

transition to the disturbed belt of rocks and closely parallels that 

line. Farther north, in Alberta, this syncline broadens into a much 

more extensive feature, which reaches to Calgary and beyond. On 

the south it is practically coextensive with the outcrop of the Willow 

Creek formation and does not reach beyond Cut Bank Creek.

Minor undulations of the strata in the area of nearly horizontal 

rocks can be seen in detail only along the principal stream valleys. 

In places these undulations have produced structural terraces that 

is, the general westward dip of the beds is interrupted locally by 

flats or benches. Elsewhere dips to the east in this area of generally 

westward-tilted strata indicate slight anticlinal folds. Both these 

types of structure may or may not have acted as reservoirs or traps 

for oil and gas if these substances were once in any of the underlying rocks. The areas occupied by these structures seem to be mOst 

favorable for obtaining oil and gas in the eastern part of the reservation. The detailed descriptions which follow treat of twelve 

structural terraces and anticlines that were noted in the course of 

the field work in the area of horizontal rocks. (See map, PI. XXV.) 

The change in structure from the nearly horizontal beds in the eastern half of the reservation to the steeply dipping disturbed beds 

in the other half is very abrupt. Where exposures are good, especially along the major stream valleys, this change can be seen to 

occur within a few feet, there being no intermediate zone of gentle 

folds.

The belt of disturbed beds adjacent to the mountains that occupy 

the western third of the region here described is a small part of a 

structural area from 15 to 20 miles wide, which extends at least 80 

miles southeast along the east front of the Rocky Mountains to and
294 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1916, PART II.

beyond Sun River and a much greater distance northwest across 

Alberta. Throughout this belt the rocks have been intensely folded 

and faulted by thrust stresses that acted from the southwest. In 

many places the individual formations are so much crushed and 

broken that it is impossible to identify them with certainty. The 

only constant feature in this whole disturbed area is the general 

northwesterly strike of the rocks. Because of this parallelism of 

strike the more resistant sandstones of the several formations appear 

as numerous strike ridges, and in many places the same hard beds 

are repeated by faulting within short distances. In the Blackfeet 

Indian Reservation the deformation of the disturbed belt accompanied the overthrusting that produced the Lewis overthrust, a great 

fault which can be traced along the eastern face of the mountains in 

the Glacier National Park, just off the west edge of the reservation.1 

The folds and faults in the belt of disturbed rocks along three 

well-exposed lines transverse to the general trend of the belt are 

shown graphically in the structural sections given on Plates XXIV 

and XXV. The only part of this area where large folds were formed 

lies along its eastern edge, which was farthest removed from the 

center of disturbance in the mountains. In the western two-thirds 

o'f this disturbed belt the beds are so much faulted and crushed 

that they could hardly serve as reservoirs for oil or gas. Large 

anticlines, which are described below, occur at seven localities along 

the east side of the disturbed belt.

DETAILS OF ANTICLINES AND STRUCTURAL TERRACES. 

ANTICLINES IN BELT OF DISTURBED HOCKS.

Milk River anticline. An anticline, here called the Milk River 

anticline, whose axis can be traced for more than 12 miles in the 

disturbed belt, extends northwestward across the south, middle, and 

north forks of Milk River. (See PI. XXV.) This is the largest 

and apparently the most promising anticline in the reservation for 

prospecting for oil and gas in the belt of disturbed rocks. The Two 

Medicine is the lowest formation exposed in this anticline, and on its 

crest in each of the river valleys crossed all but about 300 to 500 

feet of these beds have been removed by erosion, so that it is possible 

to explore the underlying Colorado shale and the upper part of the 

Kootenai formation by drilling to depths of not more than 2,500 

feet. The best exposures in this large fold appear in the valleys of 

the forks of Milk River.

The northernmost outcrops of the Milk River anticline are in the 

valley of North Fork of Milk River, in the southwestern part of

1 Willis, Bailey, Stratigraphy and structure of Lewis and Livingston ranges, Mont.: 

Geol. Soc.. America Bull., vol. 13, p. 331, 1902.
ANTICLINES IN BLACKFEET INDIAN RESERVATION* MONT. 295

T. 37 N., K. 12 W. The axis of the anticline at this locality is fairly 

well defined and extends through sees. 31 and 32, cutting the section 

line between these two sections about 1,500 feet north of the quarter 

corner. The dips on the east limb of the fold are best exposed in 

the N. |- sec. 32, where there are a series of outcrops of sandstone and 

shale whose structure can be accurately determined. The dips in this 

tract range from 30° to 51° NE. The west limb of this fold is less 

steeply tilted than the east limb, so the anticline is somewhat unsyrnmetric. About 1,500 feet west of the east quarter corner of sec. 31 

the beds exposed dip to the southwest at an angle of not over 4°. 

Farther west on North Fork of Milk River the Two Medicine 

beds are continuous for about 1-| miles upstream, the westward dip 

apparently continuing through all this stretch to the W. £ sec. 36, 

where the overlying Bearpaw shale enters in normal sequence and 

it in turn is overlain by the Horsethief sandstone, with its characteristic oyster beds. The stratigraphic sequence here shown seems to 

prove conclusively that the beds exposed along the crest of the Milk 

River anticline at this point belong to the Two Medicine formation. 

In sec. 6, T. 36 N., R. 12 W., numerous exposures in a tributary coulee 

to Milk River also show the westward-dipping rocks on this fold. 

The low dips on the west limb of this anticline near the axis of the 

fold again appear at this point, where they are only 7° to 8°, but 

farther west the beds are inclined at angles as great as 51°.

The northward extension of the Milk River anticline is apparently 

interrupted by a cross fault, which trends about N. 80° E., cutting 

through the S. $ sees. 29 and 30, T. 37 N., R. 12 W., and extending 

westward into the adjoining township. In the W. \ sec. 29 the dips, 

instead of being to the east, as they would be if the east limb of the 

Milk River anticline continued through this locality, are to the west, 

the maximum angle being 60°. From this locality westward the dips 

are all to the west, but instead of the low dips of 4° seen on the 

west limb of the Milk Riyer anticline the dips that would appear 

normally in this position on the fold the dips are as great as 60°. 

The lack of accordance of dip in this tract with that in the area to 

the southeast, along the anticline, seems to prove that the anticline 

does not continue through this area but has been interrupted by 

faulting. About 6 miles farther northwest, however, on St. Mary 

River in Canada, a broad anticline occurs almost exactly on the prolongation of the axis of the Milk River anticline. This is doubtless 

an extension of the same fold.

The rocks south of the exposures on North Fork of Milk River 

are very poorly exposed for about 5 miles, but in the valley of 

Middle Fork of Milk River, in the south half of T. 36 N., R. 12 W., 

the anticline is again exposed and its axis, which is fairly well
296 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1916, PART II.

defined, passes through sees. 22, 27, and 34 with a slight curve. 

Here, as on North Fork of Milk Kiver, the fold is not symmetric, the 

steeper limb being on the east. Near the center of sec. 22, about 

1,500 feet east of the crest of the fold, a dip of 56° NE. was noted. 

About 2,000 feet southwest of this locality the dip is 4° SW. A 

quarter of a mile farther southwest it has increased to 18°, and it 

gradually becomes steeper until in the N. $ sec. 29 the strata are 

tilted at an angle of about 40°, and again the dark marine shale of 

the Bearpaw is overlain by the Horsethief sandstone in normal succession. In this area on the whole the succession of the strata and 

the character of the folds are identical with those in the area to the 

northwest and seem to prove conclusively that the Milk Biver anticline is continuous through the intervening area.

On the south side of the valley of Middle Fork of Milk Kiver the 

dips outline a fold that is also in harmony with the structure to the 

north. In sees. 33 and 34 southwestward dips of 24° to 34° are 

noted, and the Bearpaw shale and Horsethief sandstone enter in 

normal succession above the Two Medicine formation. The crest of 

the fold, however, is more compressed than it is to the north, the dips 

near the north line of sec. 34 being in opposite directions, those on 

the west having an angle of 58° and those on the east an angle of 

65°. Throughout the W. ! sec. 34 and in the E. £ sec. 35 dips of 27° 

to 44° were measured at a number of points, indicating that in this 

locality the east limb remains much as it is farther north. In the 

SE. £ sec. 26, however, there are several small cross faults, which are 

indicated by westward dips of about 25° throughout about 50 acres. 

This is the only cross fault noted on this anticline. Others are 

probably present and must be considered in prospecting for oil or gas.

In T. 35 N., E. 12 W., the axis of the Milk Kiver anticline is still 

well outlined by opposite dips in the Two Medicine formation. The 

anticline continues southward beyond South Fork of Milk Kiver 

through sees. 2, 11, and 14, and probably as far as the south side of 

sec. 25. In sees. 3 and 4 the westward dips are much the same as 

those farther north on Middle Fork. Near the center of sec. 15 of 

the same township the Horsethief sandstone, which lies on the continuation of its exposures to the north and dips about ^0° W., indicates that the fold still continues in this locality. South of Milk 

River a few exposures of the same sandstone in sec. 26 show that the 

anticline is continuous for at least a mile south of South Fork. 

This fold, however, if extended farther south meets the extensive 

gravel-covered area that forms Milk Kiver Ridge. On the south 

slope of this ridge, in sees. 8, 16, 17, and 18, T. 34 N., K. 11 W., on 

the line of the trend of the Milk River anticline, the exposures are 

fairly good and indicate no anticlinal structure, the beds all dipping 

westward. At this place, therefore, there is probably a cross fault
ANTICLINES IN BLACKFEET INDIAN BESERVATION, MONT. 297

like the one that limits the Milk River anticline on the northwest, as 

already described. The southern limit of this fold must consequently remain indefinite, as the surface indications do not prove 

its existence for a distance greater than a mile southwest of Middle 

Fork of Milk River. All told, this anticline can be followed for 

about 12 miles, and is the largest single fold found in the Blackfeet 

Reservation. The lowest stratigraphic horizon reached along its 

crest, and therefore the position that is nearest to the underlying, 

possibly productive sands in the Colorado shale and Kootenai formation, is in the bottoms of the valleys of the major streams crossed 

by the axis of the fold. These localities include parts of sees. 31 

and 32, T. 37 N., R. 12 W.; sees. 27, 34, and 35, T. 36 N., R. 12 W.; 

and sees. 2, 3,11,13, and 14, T. 35 N., R. 12 W. The most favorable 

locations for drilling appear to be in these sections. Because of the 

high dips in the east limb of the fold it seems best to drill on the 

west limb, near the axis of the anticline.

South Fork anticline. A large fold, the second in size and probably in value as a possible reservoir of oil or gas, lies east of and 

parallel to the Milk River anticline and can be traced for about 6 

miles across the principal branches of South Fork of Milk River 

in T. 36 N., Rs. 11 and 12 W., and in T. 35 N., R. 11 W. For con- 

 venience of description in this report this fold is described as the 

South Fork anticline.

This anticline is the only extensive one in the belt of disturbed 

rocks on the. reservation which is not bounded on both sides by 

faults. It is bounded on the west by a fault along its entire length, 

but the beds on the east extend into the area of nearly horizontal 

rocks without a break. The desirability of prospecting in this anticline, however, is somewhat uncertain because of the great thickness 

of rocks that overlies the possible oil and gas sands. At least 400 

feet of the beds of the St. Mary River formation occur at the lowest 

stratigraphic point on the crest of the fold, so that the depth to 

the Virgelle sandstone is over 3,000 feet and the depth to the base 

of the Colorado shale is nearly 5,000 feet. However, if other anticlines in this region should prove productive drilling might be done 

on this fold. On Middle Fork of Milk River the South Fork anticline is best exposed along the south side of the valley in sec. 36, 

T. 36 N.1, R. 12 W., and sec. 31, T. 36 N., R. 11 W. The crest can 

be located only approximately between the eastward dips along the 

west line of sec. 31 and the westward dips, which occur about 2,000 

feet to the west in sec. 36. The exposures on the north side of the 

valley are very poor, and the continuation of this anticline northward 

can not be proved from the surface indications.

Between South Fork and Middle Fork of Milk River the area in 

a line prolonged from this fold affords only a few exposures, but

43768° Bull. 641 17 20
298 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1916, PART II.

on South Fork of Milk Eiver the anticline is again indicated by 

opposite dips in the cut bank along the south side of that stream. 

Near the center of sec. 8 the dip in the Willow Creek formation is 

]GC NE., which accords very well with the dip of 18° along the 

same line of strike on the Middle Fork. In the NW. £ sec. 17, close 

to the west line of the section, the rocks are tilted 30° SW., the 

anticlinal axis cutting through the W. £ sec. 8 apparently in close 

alignment with the trend of the fold as outlined on Middle Fork of 

Milk River.

The southward continuation of the South Fork anticline is very 

indefinite, the area south of the river affording few exposures from 

which its position can be determined. On the south side of Milk 

Kiver Eidge, along the line of G-D on Plate XXV, the crest of a 

fold lying close to the fault block that bounds the Blackfeet coal 

field on the east probably represents a southeastern extension of this 

anticline, but if so its axis in this area has shifted westward to a 

point nearer the fault just mentioned than it is in the area farther 

north. The lower part of the Willow Creek formation outcrops on 

the axis of the fold at this locality.

Blackfeet anticlines. A fault block lying between the Milk River 

and South Fork anticlines and extending southward beyond the 

limits of those folds contains rocks that are closely folded, for the 

most part in two parallel anticlines. Coal is exposed on the flanks of 

these folds for about 12 miles along the fault block, and the area has 

been described in a previous report as the Blackfeet coal field.1 Seclions A-B and C-D (PI! XXV) intersect these folds and give a general idea of their relations to the surrounding structural features. 

They seem to offer little chance of finding oil or gas, because the 

' faults on both sides probably limit the area which could serve as collecting ground for oil or gas. Wells drilled at the lowest stratigraphic points on these folds to the highest possibly productive sand 

would have to extend through nearly all the Bearpaw shale in addition to the full thickness of the Two Medicine formation, a depth of 

nearly 2,500 feet, and would reach the base of the Colorado shale at a 

depth not much less than 4,000 feet.

Anticlines on Cut Bank Creek. On Cut Bank Creek, about 2 miles 

west of Cut Bank School, in sees. 10,11,14, and 15, T. 33 N., R. 11 W., 

there are two parallel anticlines whose axes trend about N. 18° W. 

The western of these two folds is closely compressed, the dips on the 

west limb being as high as 70° and those on the east about 60°, the 

axis intersecting a point very close to the southwest corner of sec. 11. 

The axis of the east fold crosses the creek in the southwest quarter of 

the same section. The anticline there is much more open than the one

1 Stebinger, Eugene, Geology and coal resources of northeastern Teton County, Mont.: 

U. S. Geol. Survey Bull. 621, pp. 148-153, 1916.
ANTICLINES IN BLACKPEET INDIAN RESERVATION, MONT. 299

to the west and exposes the Bearpaw shale along its crest, as indicated 

by the cut-bank exposures on either side of the creek. The east limb 

of this fold near the axis, on the south side of the creek, has a dip of 

about 20°. The west limb, on the north side of the creek, has a dip 

of about 37°, and the St. Mary Eiver formation appears in normal 

succession above the Horsethief sandstone, which in turn lies above 

the Bearpaw shale. The rocks exposed in the west fold at this point 

are identical with those of the St. Mary Eiver formation and apparently belong to that formation, as is shown by the sequence just 

cited and by the absence of indications of faults between these two 

folds.

Anticline on Two Medicine Creek. In the valley of Two Medicine 

Creek, near the mouth of Little Badger Creek, in sees. 22 and 27, T. 

31 N., R. 10 W., there is an anticline which lies between steeply dipping faulted beds on either flank. The axis of this fold can be located approximately by opposite dips on the north side of the valley, 

in the NE. £ sec. 22. Near the center of this section the strata dip 

30° SW., whereas about 2,000 feet to the east similar beds are tilted 

64° NE., this steep dip continuing to the faulted ground in the W. £ 

sec. 23. The exposures clearly indicate a moderately compressed 

fold, as shown on cross section E-F (PL XXV), but it was impossible to determine the formation to which the beds belong. Their 

lithologic appearance indicates that they can be assigned either to the 

St. Mary River or the Two Medicine formation, but as the rocks are 

not in normal sequence and the fossil shells and dinosaur bones found 

in them are indeterminate, it is not yet possible to decide this question.

Anticline on. Badger Creek. On Badger Creek, in the east half of 

T. 30 N., R. 10 W., there is a fairly open fold which exposes the St. 

Mary River formation along its axis. The axis trends about N. 15° 

W. and is fairly well defined on both sides of the creek. Its northwest and southeast limits, however, are indefinite because of the cover 

of glacial drift, which conceals the rocks in the upland areas on both 

sides of the valley. The dips on the east limb of this fold range from 

15° to 30° and those on the west limb from 30° to 60°. Beds of sandstone in the St. Mary River formation arch over the crest of the 

anticline and are strikingly shown on the north side of the valley. 

The small size of this fold and the great depth to the underlying 

sands that may contain oil or gas seem to offer comparatively little 

inducement for prospecting.

ANTICLINES AND STRUCTURAL TERRACES IN THE AREA OF NEARLY HORIZONTAL

ROCKS.

Anticline on North Fork of Milk River. In T. 37 N., R. 11 W., 

there is an anticline in very gently folded rocks whose axis passes 

through sees. 17, 20, and 21 ? but can not be more definitely locate^,
300 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1916, PART II.

The dips that outline this fold are exposed only along the sides of 

the valley of North Fork of Milk Eiver. Near the center of sec.

16. on the northwest side of the valley, the beds dip 1£° NW. The 

strata southwest of this point lie horizontal, as is shown by exposures 

on both the north and the south side of the river. The west limb of 

the fold is indicated in the SW. % sec. 20 by dips of about 2° SW. 

The strata southwest of this point lie horizontal, as is shown by exposures on both the north and the south sides of the river. The 

west limb of the fold is indicated in the SW. £ sec. 20 by dips of 

about 2° SW.

The strata exposed in the vicinity of this anticline belong to the 

Willow Creek formation, and a well drilled to the nearest productive sand yet found in this region would necessarily penetrate all the 

intervening strata between the Virgelle sandstone and the Willow 

Creek rocks, which would include all of the St. Mary Eiver formation, Horsethief sandstone, Bearpaw shale, and Two Medicine formation a total of nearly 4,000 feet, in addition to an undetermined 

thickness of Willow Creek beds probably about 500 feet. The 

extension of this fold can not be certainly determined from the surface exposures, as the outcrops of the bedrock formations away from 

the river valley are very meager. In the valley of South Fork of 

Milk Eiver, in sees. 27 and 28 of the township adjoining to the south, 

and nearly on the line of strike, there is no anticline, as the beds dip 

continuously westward.

Anticline on Cut Bank Creek. On the south side of the valley of 

Cut Bank Creek, in sec. 34, T. 34 N., E. 9 W., exposures indicate a 

gentle anticline whose axis extends north-northwestward, the axis 

lying approximately along the center of the valley of Willow Creek, 

which at this point enters Cut Bank Creek from the south. The 

Bearpaw shale is the surface formation on the fold. In the W. £ 

sec. 34, in a steep bank on the south side of the creek, the clay shale 

beds belonging to this formation dip westward at an angle of about 

2°. On the east side of the same section beds of similar shale dip 

eastward at an angle somewhat under 1°. This fold does not reach 

as far north as the south edge of Eimrock Butte, near the center of 

this township, as is indicated by excellent exposures of the rocks, 

which show a uniform westward dip. The southward extension is 

indefinite because of the poor exposures along the lower part of the 

valley of Willow Creek. Nearly the full thickness of the Bearpaw 

shale is exposed along the crest of this fold on Cut Bank Creek, and 

the overlying Horsethief sandstone lies above the shale less than 1 

mile to the west. The depth here to the top of the Virgelle sandstone is about 2,400 feet.

Anticline on Willow Creek. On Willow Creek, in sees. 8, 16, and

17. T. 33 N., E. 9 W., there are excellent exposures of the crest of a
ANTICLINES IN BLACEFEET INDIAN RESERVATION, MONT. 301

broad, gentle anticline outlined by dips for several miles along the 

valley of the creek. The east limb of the fold shows dips as great as 

4° on the south bank of the creek, near the north line of the section, 

in the NW. £ sec. 16. A few hundred yards to the west the beds have 

flattened to a dip of only |°, and where the creek crosses the line 

between sees. 16 and 17 the beds are horizontal. The next exposures 

to the southwest, along the valley of the creek, show rocks that are 

tilted uniformly at an angle of about 2° SW. for about 1,500 feet, 

as far as the exposures continue. The dips indicate that an anticline 

extends in a northwestward direction across the creek near the line 

between sees. 16 and 17. Nearly the full thickness of the Horsethief 

sandstone is exposed along the crest of this anticline, so that in order 

to reach the underlying Virgelle sandstone the drill would have to 

penetrate about 2,700 feet of rocks.

Small structural terraces on Two Medicine Creels. On the lower 

part of Two Medicine Creek there are four localities where a flattening of the dip in beds that are generally inclined westward produces 

small structural terraces. At each locality these features can be 

seen only in the badland exposures along the sides of the valley of 

the creek, the north and south extent of the structural terraces being 

indeterminate because of the lack of exposures. In the N. \ sec. 4, 

T. 31 N., R. 6 W., the rocks of the Two Medicine formation lie horizontal, and the flat or terrace structure apparently continues northward through the center of sec. 33. Near the east line of the section the beds dip about 2° W., and near the center of sec. 3 they 

again lie practically flat over an area of several hundred acres. 

Still farther east, in sec. 2, these beds dip westward, and this dip 

continues for several miles downstream. The dips therefore seem 

to show the presence of two small structural terraces, which are 

indicated on the map (PL XXV). Three miles to the southwest, 

in sees. 11 and 12, T. 31 N., R. 7 W., there is another structural 

terrace. In the W. \ sec. 12 the beds lie nearly horizontal. In the 

E. I sec. 12 they dip about 1£° W. and then flatten in the E. £ SW. \ 

sec. 11, west of which they again assume the uniform westward dip 

which continues for many miles up the valley of the creek. These 

four structural terraces on the lower part of Two Medicine Creek 

are all in the lower half of the Two Medicine formation. The 

eastern pair of terraces is probably not more than 400 or 500 feet 

above the top of the Virgelle sandstone, whereas the two farther 

west lie nearly 1,000 feet above the Virgelle.

Structural terraces in Tps. 29, 30, and 31 N., R. 9 W. Between 

Badger Creek and Two Medicine Creek, in sees. 29 and 32, T. 31 N., 

R. 9 W., and sees. 5 and 8, T. 30 N., R. 9 W., there is a structural 

terrace that occupies about 3 square miles. In sees. 21, 22, and 28
302 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1916, PAET II.

the Horsethief sandstone and St. Mary Eiver formation are fairly 

well exposed and dip uniformly 2° SW. Toward the southwest 

these inclined beds flatten first to a dip of about £° in the NE. £ sec. 

29 and then to practical horizontality near the center of the section. The flat-lying beds, as indicated by exposures of rocks in the 

coulees, extend southeastward in a belt that reaches nearly' to Badger 

Creek. The southward extent of this structural terrace could not 

be definitely ascertained, because the exposures along Badger Creek, 

on what appears to be its trend, are very poor. In sees. 7 and 8, 

T. 30 N., E. 9 W., on the north side of Badger Creek valley, the 

Horsethief sandstone dips 1° W., and this dip is probably the continuation of the general westward dip on the west side of the structural terraces referred to. Farther north, in sec. 19, T. 31 N., R. 9 

W., in the line of strike of the inclined strata last mentioned, the 

rocks dip as much as 3° NE., and this dip suggests that the horizontal strata near the center of sec. 29 occupies the basin of a 

shallow syncline. Farther south the dips, however, clearly indicate 

the presence of the structural terrace. Nearly the full thickness of 

the Horsethief sandstone and a few feet of St. Mary Eiver beds 

appear at the surface in this terrace.

In an area of about 3 square miles that lies at the south edge of 

T. 30 N., E. 9 W., and extends southwestward into the adjoining 

township, there seems to be a second structural terrace. The beds 

lie flat throughout all of sec. 34 and parts of the adjoining sections, 

and exposures in the N. -| sec. 10, T. 29 N., E. 9 W., indicate that this 

area of horizontal rocks extends at least that far south. To the 

west, as indicated by the numerous dip symbols shown on the map 

(PI. XXV), the rocks dip uniformly to the west. Dips of 4° to 5° 

in sec. 9 of the same township, together with those of 2° to 3° in 

sec. 32, T. 30 N., E. 9 W., indicate that the beds are uniformly tilted 

to the west on the west side of this structural terrace. In the SW. £ 

sec. 26 a dip of 7° NE. was noted, which suggests that there is at 

that locality a sharp reversal of the normal southwestward dip.

Anticlines on Blacktail Creek. In the valley of Blacktail Creek, 

in the southeast quarter of T. 30 N., E. 8 W., a gentle fold is clearly 

outlined in the upper part of the Two Medicine formation. The 

dip of the beds on the west side of this fold is in places as great as 

8°, and many exposures show that the beds are uniformly tilted to 

the west for a distance of about 4 miles along the side. The dip of 

the beds on the east side of the fold is lower but equally persistent. 

Flat beds were noted in the W. \ NW. \ sec. 28, and the axis of the 

fold that extends in a northwest direction crosses near this locality. 

In order to reach the base of the Colorado shale on the crest of this 

fold the drill would have to penetrate about 3,800 feet of rocks. The
U. S. GEOLOGICAL SURVEY BULLETIN 641 PLATE XXV

Features m area of nearly horizontal rocks

Anticline, location ofaxris 

not knowsn. North and south 

extent indefinite

Structural terrace, area in which 

the beds that generally c?if> 

westward ar-e horizontal

Features in belt of disturbed rocks

Anticline and antic/ins! axis.

North and soijih extent of fold indefinite

rn which 770 large folds sre known. 

Dips are generally westward'and/suits numerous

Bedrock generally concealed

^f 

B -owning Sta?

Scale of sections twice that of map

R.l

;kfpot South For) 

Icline anticJina

.?,.. > . -r 1

^5Ss 

2 -' ' S *£^ r ^^ / / ^-J t

Key to formations : r, Koocenai formation; £, Colorado sha/e; 3, l/irge//e sandstone; -vt TwoMec/ic/ne formation; 5, Bearpaw sfy&te; 6,HorsetS?/ef 'sane/stone; 7,St,M<sry ffii/er formation; a, either Two Medicine ~" ' "y ftii/er formation; S, W///QW Cr^ek formation.

from U.S.G.S.topoc/raphic maps, General Land Off ice plats 

&no! authors fie/d sheets MAP SHOWING ANTICLINES ON THE BLACKFEET INDIAN RESERVATION, MONTANA

By Eugene Stebinger
. ANTICLINES IN BLACKFEET INDIAN RESERVATION, MONT. 303

southeast extension of the anticline just described can not be accurately determined because of the lack of exposures in the level areas 

southeast of Blacktail Creek. Where the extension of this fold 

should cross Birch Creek the exposures are also very poor, and no 

definite indications of aii anticline were noted.

Eocks dipping in opposite directions on Blacktail Creek, near the 

north quarter corner of sec. 13, T. 29 N., R. 9 W., indicate a narrowcrested fold extending in a direction about N. 15° W. The dips on 

the east flank are about 1° and those on the west flank about 2°. 

Bearpaw shale outcrops along the crest of this fold.

Anticlines on Birch Creek. There is a small, narrow-crested anticline on the west edge of the area of nearly horizontal rocks on Birch 

Creek, in sees. 4 and 9, T. 28 N., R. 9 W. This fold is exposed only 

along the sides of the creek in outcrops near water level. The beds 

on the west limb of the anticline dip between 8° and 10° and to the 

southwest are cut off sharply by a fault within half a mile of the 

axis of the fold. The beds on the east limb dip about 1°, thus showing a decidedly unsymmetric fold with a steep flank on the southwest. The St. Mary River formation is exposed on the crest of the 

anticline and at water level along the creel?: there are a few feet of 

the Horsethief sandstone.

Two miles farther up Birch Creek fairly good exposures indicate 

a second anticline, whose axis, which trends N. 12° W., crosses the 

creek about 900 feet west of the line between sees. 17 and 18, T. 28 N., 

R. 9 W. The Bearpaw shale and Horsethief sandstone are exposed 

along the crest of the fold, and on the west flank a considerable thickness of the St. Mary River formation overlies the Horsethief. This 

anticline is also unsymmetric, but the steeper flank is on the northeast, the-dips approaching the vertical. On the southwest side for 

a mile from the axis the dips do not exceed 30°, and for nearly 

half Jbhis distance the beds dip gently at angles not much over 8°.

RESULTS OF DRILLING.

No drilling had been done on the Blackfeet Indian Reservation 

up to the time of the preparation of this report. Borings, however, 

have been made on the east, north, and west sides of the reservation, all of them with negative results. These borings suggest that 

in this region it is useless to drill either in highly faulted rocks, such 

as those in the west half of the disturbed belt near the mountain 

front, or in areas where the rocks lie very flat.

The only well so far drilled on an anticline near the reservation 

was located near the crest of a large fold on the line of the Milk 

River anticline, on St. Mary River about 1£ miles north of the boundary (locality 4, PI. XXIV). Information concerning this boring,
304 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1916, PAET II.

which was drilled since the writer examined the region, was kindly 

furnished by Dr. James S. Stewart, of the Geological Survey of 

Canada. Only a few hundred feet of the upper part of the Colorado shale were penetrated according to latest reports, and therefore the boring can not be considered a fair test of the possibility 

of obtaining oil along this fold. The boring was made with a diamond drill and reached the Colorado shale at a depth of about 300 

feet, after passing through the sandy beds of the lower part of the 

Two Medicine formation and the Virgelle sandstone. By September, 1915, the drill had reached a depth of 560 feet, and operations 

ivere suspended. The shale at the bottom of the hole, as shown by 

the drill core, is crumpled and dips at a high angle, although the, 

overlying sandstones on the crest of the anticline are practically 

flat. Although the soft shale of the upper part of the Colorado has 

been thus crushed and crumpled, the more rigid sandstones in the 

lower part of the formation and in the Kootenai, like those that are 

exposed at the surface of the anticline, may have been strong enough 

to withstand the stresses that caused the deformation without crumpling and breaking. It is therefore probably in general true that the 

presence of crumpled shale'in the larger anticlines in the belt of disturbed rocks in this region does not necessarily condemn these folds 

for oil prospecting.

About 15 miles off the east edge of the Blackfeet Reservation, near 

Kevin, Toole County, on the James Miller ranch, a boring 1,755 feet 

deep, sunk in search of oil, passed through the lower two-thirds of 

the Colorado shale and all of the Kootenai formation, and probably 

entered the Jurassic. This well is in the midst of a large area whose 

rocks lie horizontal, as shown by exposures in the vicinity and also 

by the Virgelle sandstone in the prominent escarpment west and 

north of the locality. The boring was therefore made at a place 

where the structure was unfavorable to the accumulation of gas or 

oil. Small flows of gas, however, were encountered at three horizons. 

The log of this well, furnished by H. C. Price, of Great Falls, Mont., 

was as follows:
ANTICLINES IN BLACKFEET INDIAN BESEKVATION, MONT. 305

Log of loett at the James Miller ranch, in the NW. % sec. 25, T. 34 N., R. 4 TP-

[Elevation, 3,360 feet.]

Probable formation.

Glacial drift.

Colorado shale.

Kootenai formation.

Jurassic.

Driller's description of the rock.

Lime shell. . .....................................

Black shale.. . ...................................

Hard dark shale. ................................

Black shale. .....................................

Hard shell:.. ........... J^ff^./.. .............

Shell.............................................

Thickness.

Feet. 

40

120

2

153

5

100

10

30

10

180

70

10

40

80

100

95

20

5

30

15

35

45

5

25

5

70

60

30

10

60

40

10

40

50

50

5

20

5

50

25

Depth.

Feet. 

40

160

162

315

320

420

430 4 fin

470

650

720

730

770

850

950

1,045

1,065

1,070

1,100

1,115

1 1 Kfl

1 1QR

1,200

1 99^

1,230

1,300 i wn

1,390

1,400

1,460

1,500

1,510

1 550 1,'eoo

1,650

1,655

1,675

1,680

1,730

1,755

Borings were also made just off the west edge of the reservation 

in three places at the base of the mountain front in the Glacier National Park (localities 1, 2, and 3, PI. XXIV). All these borings 

seem to have been dry or to have struck only very small amounts 

of gas. The Colorado shale is the surface rock at each of these places 

and the greater part of the beds penetrated belong to that formation. At locality 2, near Sherburne Lake, several holes were drilled 

to a depth of about 1,600 feet entirely in the Colorado shale. These 

three places are only a few miles from the trace of the Lewis overthrust, and the whole of the Colorado shale is tilted to the west and 

has been much thickened by crumpling and repeated faulting. The 

negative results obtained at these widely scattered places along the 

base of the mountain front and the geolqgic evidence that the formations on the west edge of the reservation are much disturbed indicate that this part of the region is unfavorable for oil and gas 

prospecting.