GEOLOGY OF NORTHEASTERN MONTANA. 

By ARTHUR J. CoLLIER. 

INTRODUCTION. 

REGION EXAMINED. 

A large region in northeastern Montana has 

never been thoroughly explored by geologists, 

owing to the fact that it is a part of the Great 

Plains and the belief that it is too monotonous 

and uninteresting to tempt anyone to turn aside 

from the pronounced geologic features a little 

farther west, for which Montana is noted. 

This region includes parts of Sheridan, Valley, 

Phillips, and Blaine counties. Its investigation was begun by Smith 1 in 1908, when he 

made a geologic survey of the Fort Peck Indian 

Reservation. Beekly 2 explored a strip of land 

along the Montana-North Dakota line from 

Missouri River to the international boundary, 

and Bauer 3 examined the townships in which 

Plentywood and Scobey are situated. 

Their results are here included with those of 

the writer, who during the field seasons of 1915 

and 1916 was engaged in an investigation of 

the lignite resources of the remainder of this 

region, which extends from a line ~ithin 12 

miles of the Montana-North Dakota boundary 

westward about 200 miles. In the western 

half of the region very little lignite was found, 

and the work was of the nature of a hasty 

reconnaissance extending from the . international boundary to Missouri River. A few 

townships are covered by old land surveys, and 

the corners are located with difficulty, other 

townships are still unsurveyed, and in some 

the work hs.s not yet been completed; though 

the comers were found in the field, no plats 

were available. The townships in the eastern 

1 Smith, c. D., The Fort Peck Indian Reservation lignite field,. Mont.; 

U. S. Geol. Survey Bull. 381, pp. 40-59, 1910. 

z Beekly, A. L., The Culbertson lignite field, Valley County, Mont.: 

U. S. Geol. Survey Bull. 471, pp. 319-358, 1912. · · 

a Bauer, C. A., Lignite in the vicinity f Plentywood and Scobey, 

Sheridan County, Mont.: U.S. Geol. Survey Bull. 541, pp.293-315, 1914. 

part of the region have been surveyed by the 

General Land Office within the last six or seven 

years. In the examination of the lignite be~s 

in this ·area the plane table and telescopic 

alidade were in constant use, and the locations 

of the outcrops were tied to the section comers, 

which are usually marked by iron posts and are 

easily found. 

ACKNOWLEDGMENTS. 

W. T. Thorn, jr., and R. F. Baker were 

assigned to the field party in 1915, and H. R. 

Bennett accompanied the writer in 1916. Edwin T. Conant was employed as teamster and 

assistant during both seasons. Much of the 

information contained in this report is the result of the faithful work done by these men. 

W. C. Alden, who in 1916 began an investigation of the glacial features of eastern Montana, 

visited the party in the field and has frequently 

been consulted in the office. He has mooe 

many valuable suggestions, used in the preparation of this paper. 

Acknowledgment is also 9-ue to Mr: Barnum 

Brown, of the American Museum of Natural 

History, for suggestions made in the course of 

the field season of 1916. 

As the work in many parts of the region 

was more or less of a reconnaissance nature 

the only available maps were township plats 

made by the General Land Office. The Bowdoin Saco Hinsdale, and Cherry Ridge topographic m~ps of the United States Geological 

Survey; the maps to be published by the International Boundary Commission; the topographic maps of the Fort Peck Indian R.eservation made by the 9-eneral Land Office In connection with the township surveys and published by the Geologica.! Survey; and the rna ps 

published by the Missouri River Commission 

have all been used in preparing this report. 

17 
18 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1918. 

The base marp (Pl. I) is compiled from the 

township plats, with additions from the few 

topographic maps available and the field notes 

made by th~ writer and his assistants. 

PREVIOUS PUBLICATIONS. 

1875. 

Dawson, G. M., Geology and resources of the 49th parallel, British-North American Boundary Commission. 

1886. 

McConnell, R. G., On the Cypress Hills, Wood Mountain, and adjacent country: Geol. Survey Canada Ann. 

Rept., vol. 1. 

Cope, E. D., The vertebrates of the Swift Current Creek 

region of the Cypress Hills: Idem, pp. 79-85. 

McConnell discovered the interesting Oligocene mammal remains in the Cypress Hills, a few miles north of the 

international boundary, and Cope's report is on the fossil 

remains of the region. 

1891. 

Cope, E. D., The species from the Oligocene or lower 

Miocene beds of the Cypress Hills: Canada Geol. Survey 

Contr. Paleontology. 

1896. 

Weed, W. H., and Pirsson, L. V., Geology of the Little 

Rocky Mountains: Jour. Geology, vol. 4, pp. 399-428. 

1906. 

Calhoun, F. H. H., The Montana lobe of the Keewatin 

ice sheet: U. S. Geol. Survey Prof. Paper 50. 

1907. 

Brown, Barnum, The Hell Creek beds of the Upper Cretaceous of Montana: Am. Mus. Nat. Hist. Bull. 23 . In 

this paper Brown refers to an extension of the ''Hell Creek 

beds" northward across Missouri River, and hence in this 

field. 

1909. 

Dowling, D. B., Coal fields of Manitoba, Saskatchewan, 

Alberta, and eastern British Columbia: Canada Geol. Survey Pub. 1035. 

1910. 

Pepperberg, L. J., The Milk River coal field, Mont.: 

U.S. Geol. Survey Bull. 381, pp. 82-107. 

Smith, C. D., The Fort Peck Indian Reservation lignite 

fields, Montana: U. S. Geoi. Survey Bull. 381, pp. 40-59. 

. 1912. 

Beekly, A. L., The Culbertson lignite field, Valley 

County, Mont.: U.S. Geol. Survey_Bull471, pp. 319-358. 

1914. 

Rose, Bruce, Willowbunch coal area, Saskatchewan: 

Canada Geol. Survey Summ. Rept. for 1913, pp. 153-164. 

Bauer, C. M., Lignite in the vicinity of Plentywood and 

Scobey, Sheridan County, Mont.: U.S. Geol. Survey Bull. 

541, pp. 293-315. 

1915. 

Rose, Bruce, The ·wood Mountain coal area, Saskatchewan: Canada Geol. Survey Summ. Rept. for 1914, pp. 

64-67. 

1916. 

Stebinger, Eugene, Possibilities of oil and gas in northcentral Montana: U. S. Geol. Survey Bull. 641, pp. 49-61. 

Rose, Bruce, Wood Mountain-Willowbunch coal area, 

Saskatchewan: Canada Geol. Survey Mem. 89. 

1917. 

Collier, A. J., The Bowdoin dome, Montana, a possible 

reservoir of oil or gas: U. S. Geol. Survey Bull. 661, pp. 

193-209 (Bull. 661-E). 

Collier, A. J., and Thorn, W. T., jr., The Flaxville gravel 

and its relation to the other high-terrace gravels of the 

northern Great Plains: U. S. Geol. Survey Prof. Paper 

108, pp. 179-184 (Prof. Paper 108-J). 

SURFACE FEATURES. 

The region here described is a part of the 

Great Plains of North America. By this designation it is not intended to imply that the 

whole area consists of only one plain, for in 

fact there are at least three levels at which 

remnants of fairly well marked surfaces approaching plains can be discovered and between 

which there are more or less irregular slopes 

and escarpments. It can be assumed that at 

some very remote time the whole of the Great 

Plains constituted one plain, which was subsequently elevated and probably warped. T!w 

increased slope made it possible for the streams 

to sink their channels into the plain and as 

time went on to destroy most of its even surface by the headward cutting of the tributaries. 

Other periods of uplift and erosion have made 

possible the production of partial plains below 

the original one, until the region to-day is 

marked by a series of terraces or remnants of 

these plains. Adjacent to the southwest cotner 

of the region described are the high peaks of 

the Little Rocky Mountains, which owe their 

origin to a totally different cause, namely, an 

intrusion of igneous rocks with which the 

underlying strata not elsewhere exposed are 

brought to the surface. Except for the Little 

Rocky Mountains the relief between the lowest 

point and the highest in this region is about 

1,400 feet. 

The highest poin.ts are in a series of plateaus 

near the Canadian line which are capped and 

protected from erosion by gravel. The plateau 

in the western part of the region stands at 
~ 

QUATERNARY TERTIARY 

~ 

TERTIARY (?) 

~ 

F-ount (f l 

CJ [!] DD D .A Uuvium Flnx:ville grarel ort · niOJl Lance foT"ma ion Rearpaw hal formation 

CRET,t,CEOUS 

&.J .. ~ 

2>---===----====---0=============='>"MIICS 

Geology co mp: 1ed from the worl< of C. D. Smit h 

A. L. Be ly, C. A Sa uer nd A. J. Co. •er 

Surve ed •n 1908·1 6 
GEOLOGY OF NORTHEASTER.N MONTANA. 19 

3,000 to 3,400 feet above the sea, as is indicated on the Cherry Ridge topographic map of 

the Geological Survey; east of it, for about 30 

miles, no remnants of the high plateau are 

found, but in the neighborhood of Opheim there 

is another extensive plateau at an altitude of 

about 3,100 feet, east of which the elevation 

declines to about 2, 700 feet in a plateau south 

of the town of Redstone. 

The Larb Hills, 1 near :Missouri River also c. on- ' sist of an undulating plateau about 3,000 feet 

in altitude but differing from the plateau farther north in not having a thick gravel cover. 

Outside of the areas occupied by these plateaus there is a marked bench of rolling land 

from 200 to 500 feet below the higher level. 

This bench can be seen to good advantage 

from Big Muddy Creek, northeast of Redstone, 

to the North Dakota line, as shown in the maps 

of the Boundary Commission, and ranges in 

altitude from 2,300 to 2,500 feet. At the 

international boundary the Big Muddy Valley 

has steep walls rising 260 feet to the comparatively level plain. West of Big Muddy Creek 

this bench continues with varying altitudes, 

and along the forks of Poplar River it consists 

of long slopes extending from the upper escarpment down within 200 feet of the river level. 

West of Opheim it consists of level-topped 

areas, of sloping grass-covered hills, and of 

badlands in which the shale is exposed. It 

ranges in altitude from 2,5.00 feet near Milk 

River to 3,000 feet at the international boundary near the crossing of Frenchman Creek. 

The valleys of Rock, Frenchman Whitewater 

Cottonwood, ' ' and other creeks cut across this 

bench and are especially deep near Milk River. 

A view of the gorge of Rock Creek is given 

in Plate II, A, and the valley of Cottonwood 

Creek is shown in Plate III, A. South of Milk 

River the elevated areas are generally somewhat lower, and the streams cutting across 

them occupy broad valleys from 100 to 400 

feet in depth. 

1 The name Larb Hills has been applied to the uplands immediately 

south of Saco and has been published on the Bowdoin, Saco, and Hinsdale topographic maps of the United States Geological Survey. That 

this is an error is affirmed by many of the old inhabitants of the region. 

Messrs. Henry Carpenter and L. W. Gibson, who resided near the 

~issouri for many years, assert that the name was first applied to a relatlve~y small area eas: of Ti~ber Creek . . It is applied to the whole group 

of highlands ncar M1ssour1 River on the map by Calhoun published in 

U. S. Geol. Survey Prof. Paper 50. 

The valleys of Missouri and Milk rivers, 

ranging in altitude from 1,900 to 2,300 feet 

and lying from 100 to 500 feet below the level 

of the bench just described, together with the 

valleys of the smaller streams, constitute the 

lowest land in this region. The valley of Milk 

River is bordered by pronounced bluffs. It is 

in places several miles in width and was probably formed by a much larger stream than 

that which now occupies it. Calhoun 2 is 

authority for the statement that this valley 

was once occupied by Missouri River and that 

it was completely filled with ice during the 

glacial epoch so that the river was forced to 

cut for itself a new valley (the present course) 

50 miles to the south. 

The present valley of Missouri River near 

the mouth of the Musselshell is cut from 300 

to 500 feet in the bench described and is not 

more than 3 miles across from rim to rim. 

(See maps published by the Missouri River 

Commission.) The river flows eastward in 

h. I 

t IS narrow valley for many miles to the 

mouth of Big Dry Creek, in whose former 

valley it flows back to its original channel at 

the mouth of Milk River. 

The effect produced by the moving of the 

continental ice sheet across this region is interesting and varied. In some areas the evidence 

of glaciation is almost entirely lacking, but 

even in such areas, if the surface has not been 

covered by deposits made since the glacial 

epoch, boulders of granite brought from the 

far-away shores of Hudson Bay, or large 

bodies of boulder clay, in part from the same 

source, can be found here and there. In addition to this evidence there are several glacial. 

moraines marked by small irregularly placed 

lake beds separated by rounded hills. There 

are two large areas of this kind near the Canadian boundary-one in the eastern part of the 

region and the other extending from Whitewater Creek to the west side of the region 

mapped. 

A description of the surface would not b.e 

complete without some reference to the Little 

Rocky Mountains, adjacent to the southwestern part of the region. These mountains, as has 

2 Calhoun, F. H. H., The Montana lobe of the Keewatin ice sheet: 

U. S. Geol. Survey Prof. Paper 50, pp. 38-42, 1906. 
20 SITORTER CONTRIBUTIONS T'O GENERAL GEOLOGY, 1918. 

beAn shown by Weed and Pirsson, 1 are of a 

peculiar type in that they have a central core 

of very old rocks covered by a mass of porphyry 

(a laccolith) intruded at some time very much 

later than the formation of the sedimentary 

rocks and are flanked by the upturned edges 

of the ·deep-seated strata. They furnish the 

only clue that can be obtained in this region 

to the strata that lie far beneath the surface. 

The Black Hills of South Dakota and Wyoming 2 present a much better kno·wn example 

of this form of mountain uplift, and the Little 

Rocky Mountains can be described as a miniature reproduction of the Black Hills. On the 

Little Rocky Mountains there is a comparative 

abundance of timber, not only yellow pine. and 

cedar but in many places fir, tamarack, and 

lodgepole pine. 

SETTLEMENT OF THE REGION. 

From the days of Lewis and Clark (1804-

1806) to the completion of the Northern Pacific 

Railway (1883) Missouri River was the great 

northern route of communication between the 

East and the West. Steamboats plied regularly on its muddy waters, bringing in supplies 

of all kinds and taking out pelts that the great 

fur companies had collected from the Indians 

and trappers. At that time many forts were 

built on the banks of the river to guard against 

attacks by the Indians, and one of the most 

important of these was Fort ·Union, which 

stood in this area opposite the mouth of Yellowstone River. 

The building of the first railway ended the 

river traffic, and as the region adjacent to the 

-international boundary was out of the ordinary 

line of travel, it was little visited until the 

completion of the Great Northern Railway 

about 1889. Up to that time the only inhabitants were Indians,, trappers, traders, and 

woodcutters for the river steamers. After- the 

building of the Great Northern Railway the 

country was practically given over to the 

raising of cattle and sheep. The railway stations became 4istributing points and have 

been the principal settlements for many years. 

Small irrigation plants have been established 

1 Weed, W. H., and Pirsson, L. V., Geology of the Littl,e Rocky 

Mountains: Jour. Geology, voL 4, pp. 399-428, 1896. -

2 Darton, N.H., U.S. Geol. Survey Geol.Atlas,Sundance folio (No. 

127), 1905. 

at many of these places, the principal crop 

being blue-joint hay. The Milk River irrigation project of the Reclamation Service, begun 

many years later, has improved large areas of 

low-lying land. About seven years ago a tide 

of new life in the guise of "dry land" homesteaders set in and is still swelling, though the 

more desirable land has been occupied. To 

meet this sweep of immigration, townships 

have been surveyed, branch railroads built or 

projected, and new towns established. The 

best land, from the farmer's point of view, is 

that on the flat surfaces of the higher plateaus, 

where the soil is easily worked and produces 

good crops of grain and flax. The bench lands 

are usually more rolling in character and are 

generally strewn with glacial boulders which 

must be gathered up before plowingo The soil 

on the benches in the eastern part of the region 

is derived largely from sandy formations and 

is easily cultivated and fully equal to that of 

the plateaus. (See Pl. II, B.) Large areas 

in the western part are underlain by shale, 

which forms a gumbo soil difficult to cultivate 

and on which the surface forms are more varied, but there are at many places large areas 

in which the invasion of ice during the glacial 

epoch has left a sandy soil that is easily tilled. 

The new homestead law, passed in 1917J which 

enables the settler to acquire 640 acres of land 

for grazing, will certainly encourage further 

settlement and will force the abandonment of 

the methods of the old stock-raising days. 

The principal ·towns in this region are Culbertson, Poplar, Wolf Point, Glasgow, Vandalia, Hinsdale, Saco, and Malta, on the main 

line of the Great Northern Railway; Plenty~ 

wood ano Scobey, on a branch of the Great 

Northern; Outlook and Whitetail, on the St. 

Paul, Minneapolis & Sault Ste. Marie Railway; 

and Glentana and Opheim, on proposed railroads not yet built. 

STRATIGRAPHY. 

GENERAL SECTION. 

The sedimentary rocks range from the preCambrian core of the Little Rocky Mountains 

~o the Recent alluvium of the existing streams. 

The source of the information regarding the 

rock from the pre-Cambrian to the Carbonifer-
U , S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 120 PLATE II 

A. GORGE OF ROCK CREEK, VALLEY COUNTY, MONT. 

B. GRAIN FIELD NEAR SCOBEY, MONT. 

C. CAMP IN THE FOOTHILLS OF THE LITTLE ROCKY MOUNTAINS, PHILLIPS COUNTY, MONT. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 120 PLATE III 

A. VIEW UP THE VALLEY OF COTTONWOOD CREEK, PHILLIPS COUNTY, MONT. 

B. BADLANDS OF BEARPAW SHALE NEAR LISMAS, MONT. 
GEOLOGY OF ORTHEASTERN MONT'ANA. 21 

ous is an article by Weed and Pirsson,t from 

which the following remarks on the rocks -

owing to undetected variations in the dip. 

The areal distribqtion of the formations is 

the Carboniferous are adapted. The shown in figure 2. 

from the Carboniferous to the Claggett 

the Cretaceous I system was examined 

The descriptions of the formations above the 

Claggett shale are the results of work done by 

29 

32 

Terrace 

gravels 

Ellis 

formation 

32 33 

R 25 E R 26 E. 

;-1 

/~.+~~~~~~~~~~ 

z 

25 

34 35 36 

M1les 

~-----L----~~----~----~----~ 

2 3 4 

~~ 

PRE-CAMBRIAN 

,-----"-----. 

Limestone 'mestone,lime- Crystalline 

1 

neconglomerate, schists 

le,and guartzite 

Lower 

Cretaceou.s 

r----"----

Porphyry 

FIGURE 2.-Sketch map of the ofT. 25 N., Rs. 25 and 26 E. principal meridian, Mont. 

the field season of 1916. Only one m ure- the writer and his assistants in 1915 and 1916, 

ment of the section exposed was made, and it and the areal distribution of these formations is 

may be found to be more or less in curate shown in Plate I. The geologic section is as 

1 Weed, w. H., and Pirsson, L. v., op. cit., given on page 22. 
22 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1918. 

Section of formations in northeastern Montana. 

System. Seri.es. I Group and forma- Thickness Character. tion. (feet). 

Sandd soil derived from Flaxville gravel, Fort Union, Lance, an Judith Ri.ver formations. Gumbo soil derived from 

Recent. the Bearpaw and Claggett shales. 

Quater- Alluvium deposited in the flood plains of streams. nary. ------Unconformity 

Scattered boulders, t~ll , and moraines. Pleistocene. Gravel deposited in Milk River valley by preglacial }.~iss uri 

River. 

Unconformity 

Quater- Pleistocene or Gravel and silt north of Malta. Gravel on benches around nary or late Plio- Little Rocky Mountains. Tertiary. cene. Unconformity 

Early Pli- ocene or Flaxville gravel. Q-100 Gravel, sand, and clay, in some places cemented with calcite. 

Miocene. Tertiary. Unconformity 

Eocene. Fort Union for- o-1,400 Yellowish sandstone and shale carrying beds of lignite. mation. 

18Q-200 Somber-colored shale and sandstone ; lignite beds. Tertiary)?). Eocene(?). Lance formation. 

8Q-200 Yellowish sandstone and shale. ............ .............. 

Bearpaw shale. 

0.. 

8oo-1,ooo Dark-gray shale; marine fossils; gumbo soil. . 

;:i Judith River Light-colored shale and sandstone; marine and fresh-water 0 40o-425 60 formation. fossils; a somewhat sandy soil. <:ll 

~ Claggett shale. 750 Dark-gray shale, marine fossils, gumbo soil. .s 

~ 

Upper 0 Eagle (?) Light-colored sandstone present in Little Rocky Mountains 

Creta- ~ sandstone. 100 but probably absent in Bowdoin dome and points west. 

Creta- ceous. ceous. 0.. 875 Bluish-gray to black shale. 

;:i 

Light-colored sandstone capped by thin layer of fossiliferous 0 

60 60± limestone. 0 

"0 

<:ll 325 Dark-blue shale. "" 0 

0 

0 Mowry shale. 100 Platy siliceous shale; weathers into porcelain-like debris. 

LowerCre- Kootenai ( ?) for- 825 Variegated shale interbedded with yellow and brown sandtaceous. mation. stone. 

Upper 200 Massive cross-bedded sandstone. 

Jurassic. Ellis formation. Jurassic. 20Q-300 Thin-bedded limestone and shale. 

Carboni£- Mississip- Unconformity 

Madison lime- Massive fossiliferous limestone . erous. pian. stone. Thin-bedded limestone.a 

Devonian 

(?) and Dark-gray and black fetid limestone.a Sil urian(?). 

Thin-bedded limestone.a 

Cambrian. Limestone conglomerate. 

Green shale. 

Quartzite. 

Pre-Cambrian. I 

Black crystalline amphibole schist.a 

a Adapted from We~d, W. H., and Pirsson, L. V., Geology of the Little Rocky Mountains: Jour. Geology, vol. 4, pp. 399-428, 1896. 
GEOLOGY OF NORTHEASTERN MONTANA. 23 

PRE-CAMBRIAN ROCKS. 

The core of the Little Rocky Mountains is 

composed of crystalline schist (fig. 3). The 

type most usually seen is a fine-grained, dense, 

compact black crystalline amphibole schist or 

amphibolite, which splits into fragments with 

bright surfaces. These rocks are reported by 

Weed and Pirsson as being clearly altered 

sandstones. They are certainly older than the 

Cambrian. 

PORPHYRY LACCOLITH. 

Overlying the pre-Cambrian core of the 

Little Rocky Mountains is a great lenslike mass 

or laccolith of porphyry (see fig. 3), which was 

intruded at a much later date and which 

arched the overlying rocks in ~o a dome. The 

w .. 

Fossils collected by I Weed and Pirsson from 

the limestone mentioned in the section were 

examined by Walcott, who identified Ptychoparia oweni Hall and 1

0bolella nana Meek and 

Hayden. Both are Middle Cambrian forms. 

SILURIAN AND DEVONIAN SYSTEM~(?). 

Above the Cambrian rocks (figs. 2 and 3) 

are slate-colored and ~ black fetid limestones 

possessing the general characteristics of the 

Silurian and Devonian rocks as developed in 

the Rocky Mountain region to the west. No 

fossils were found in these rocks, and their 

tentative reference to I the Silurian and Devonian systems by Weed and Pirsson is based on 

their lithologic character and position between 

the Cambrian and Carboniferous rocks. 

Anto-ine Butte E. 

Granite Butte 

3,000 f'eet 

above sea level 

FIGURE 3.-Sectiox;_ of the laccolith of the Little Rocky Mountains, Mont. (After Weed and Pirsson.) a, Crystalline schists; b, granite porphyry; c, Cambrian; d, Silurian ( ?)and Devonian(?); e, Carboniferous; f, Mesozoic. 

exact thickness of the porphyry has not been 

measured, but it is known to be 300 or 400 

feet on the principal summits, where it rests 

upon black mica schist into which the streams 

have cut their gorges. The outlying buttes, 

like those shown in figure 3, probably represent 

local thickenings or possibly separate intrusions 

of porphyry from which, in many places. the 

overlying rocks have not been eroded. 

CAMBRIAN SYSTEM. 

Camb!.'ian rocks, of which Weed and Pirsson 

give the following section, lie upo11 the laccolith just described. 

Section of Cambrian rocks in Little Rocky Mountains, 

Mont. 

Thin-bedded fissile limestone, pure limestone with 

shell remains alternating with impure sandy and 

more or less conglomeratic beds; general color 

Feet. 

CARBONIFEROUS SYSTEM. 

MADISON LIMESTONE. 

I 

The Carboniferous rocks (fig. 2) consist of 

several hundred feet of somewhat thinly 

bedded limestone at the base, capped by 

several hundred feet of massive, thick-bedded 

limestone which appe~rs ·to be characteristic 

of the Mississippian Madison limestone throughout the Rocky Mountain region. From this 

formation Weed and Pirsson report five species 

of fossils collected b3"" E. 8. Dana and pronounced lower Carboniferous by Whitfield.1 

Several collections of fossils, made principally by H. R. Bennett, have been examined 

by G. H. Girty, who has identified at least 

22 species and who declares that the formation 

is identical with the Madison limestone of 

other parts of Montana. His report follows: 

gray ...................................... ····· 

Limestone conglomerates and gray shale, greenish 

sandy layers alternating with pure argillaceous 

beds. There are some thin beds of limestone 

25 Sec. 33, T. 25 N., R. 24 E. qot 19): 

from which a few fossils were taken ......... . . . 

Green shale carrying limestones and conglomerates. 

Green or copperas-colored shale . . .... ___ .. _ ... _ .... . 

Interval in which the beds are not exposed ___ .. ___ .. 

Quartzite changing to conglomerate near the base, 

generally flesh-colored , mostly brecciated and 

rusty .............................. . ............ . 

40 

50 

30 

300 

75 

Zaphrentis sp. 1 

Syringipora surcularia. 

Rhipidomella aff. R. burlingtonensis. 

Schuchertella chemuJgensis? 

Productus ovatus. 

Productus aff. P. viminalis. 

Productus gallatinensis? 

Productus (Strophalosia?) sp. 

Spirifer aff. S. incertus. 

1 Ludlow, William, Reconnaissance from Carol, Mont., to Yellow" 

520 stone NatiOnal Park, p. 129, Washington, 1876. 
24 SHORTER CONTRIBUTIONS TO GENERAL GEOLO-GY, 1918. 

Sec. 33, T. 25 N., R. 24 E.-Continued. 

Brachythyris suborbicularis? 

Martinia? sp. 

Syringothyris carteri. 

Athyris lamellosa. 

Euomphalus sp. 

Sec. 36, T. 25 N., R. 15 E. (lot 21): 

Zaphrentoid corals. 

Euomphalus sp. 

Same location as above but lower in section (lot 22): 

Triplophyllum sp. 

Schuchertella che:~rningensis. 

Chonetes ornatus. 

Spirifer centronatus. 

Reticularia cooperensis. 

Sec. 27, T. 25 N., R. 26 E. (lot 25): 

Leptaena analoga. 

Sch uchertella chemungensis. 

Chonetes loganensis. 

Chonetes ornatus. 

Chonetes logani. 

Pustula n. sp. 

Productus ovatus. 

Productus arcuatus? 

Productus sp. 

Camarotoechia metallica. 

Camarotoechia sp. 

Dielasma aff. D. burlingtonense. 

Spirifer centronatus. 

Syiingothyris carteri. 

Martinia rostrata. 

Spiriferina solidirostris. 

Reticularia cooperensis. 

Composita humilis. 

Cliothyridina cf. C. crassicardinalis. 

Platyceras sp. 

Same location as above but higher in the section (lot 26): 

Triplophyllum sp. 

Schuchertella che~ungensis. 

Chonetes logani. 

Chon"etes ornatus. 

Spirifer centronatus. 

Spiriferina solidirostris. 

Sec. 22, T. 25 N., R. 25 E. (lot 36): 

Triplophyllum sp. 

Fenestella, several'sp. 

Pinnatopora sp. 

Batostomella sp. 

Rhombopora sp. 

Cystodictya sp. 

Schuchertella chemungensis? 

Schizophoria? sp. 

Chonetes ornatus. 

Chonetes logani. 

Chonetes loganensis. 

Productus arcuatus? 

Pustula n. sp. 

Camarotoechia aif. C. herrickana. 

Camarotoechia metalHca. 

Spirifer centronatU:s. 

Camarotoechia mysticensis. 

Camarotoechia sp. 

Spiriferina solidirostris. 

Sec. 22, T. 25 N., R. 25 E.-Continued.• 

Reticularia cooperensis. 

Martinia rostrata. 

Composita sp. 

Cliothyridina incra~sata? 

Eumetria aff. E. vera. 

Entolium shumardianum. 

Platyceras sp. 

Griffithides? Elp. 

It is probable that all six collections are of lower Mississippian age and belong to the Madison limestone, though 

they show seveml different faunal facies. Lots 22, 25, 26, 

and 36 are typical Madison faunas. .Lot 21 shows too little 

for definite identification of the horizon. Lot 19 suggests 

a typical Burlington more than a typical Madison fauna, 

yet the Madison is in part of Burlington age, and collections 

from it sometimes show a markedly Burlington facies, 

like this one. 

The Madison limestone is the most striking 

formation surrounding the Little Rocky Mountains. It is the encircling girdle of the mountains, in which the picturesque canyons are 

cut. Seen from a distance it appears like a 

great horizontal bed, but on closer examination 

it is found to be turned up at high angles and 

its edge cut by deep canyons... The three 

small outlying domes shown in figure 2 are 

made up wholly of this limestone. 

JURASSIC SYSTEM. 

ELLIS FORMATION. 

Overlying the Madison limestone in the 

Little Rocky l\1ountains is a formation made up 

of shale and shaly and sandy limestone (fig. 

.2), which was described by Weed and Pirsson. 

A fauna from it, determined by T. W. Stanton 

as Jurassic, consisted of the following forms: 

Ammonite fragments of undetermined species. 

Belemnites densus Meek and Hayden. 

Pleuromya subcompressa Meek. 

Astarte meeki Stanton. 

Modiola subimbricata Meek. 

Gryphaea calceola var. nebrascensis Meek and 

Hayden. 

Larger collections from this formation were 

~ade by H. R. Bennett during the field season 

of 1916 and were identified by J. B. Reeside, jr., 

as follows: 

9832. Sec. 27, T: 25 N., R. 26 E. From shale above thinbedded shale and limestone which o':erlies the Madison 

limestone: 

Ostrea strigilecula White. 

Belemnites den,sus Meek and Hayden. · 

Pleuromya sp.? 

Indeterminable casts of pelecypods. 
GEOLOGY OF NOR.THEAST·ERN MONT AN A. 25 

9832. Sec. 27, T. 25 N., R. 26 E.-Continued. 

Small ammonite distinct from described American 

forms but evidently a cardioceratid. 

9833. Same locality as 9832 but lower in the section: 

Gryphaea calceola var. nebrascensis Meek and 

Hayden. 

Ostrea strigilecula White. 

Pleuromya subcompressa Meek. 

Astarte? sp. 

Rhynchonella sp. undescribed. 

Belemnites densus Meek and Hayden. 

Serpula sp. undet. 

Fragment of an undescribed ammonite, suggesting 

Perisphinctes. 

·9834. Sec. 7, T. 25 N., R. 26 E. From thin-bedded limestone above the Madison limestone : 

Fragments of an ammonite suggesting P erisphinctes. 

Gryphaea calceola var. nebrascensis Meek and 

Hayden. 

Pleuromya subcompressa Meek. 

Pholadomya kingi Meek. 

Lima cinabarensis Stanton. 

Cucullaea? sp. cf. C. haguei Meek. 

Camptonectes extenuatus Meek and Hayden. 

Thracia? sp. like T. arcuata Meek but very much 

larger. 

Rhynchonella sp. undescribed. 

Belemnites derisus Meek and Hayden. 

Astarte meeki Stanton. 

Cyprina? sp. 

9835. Sec. 7, T. 25 N., R. 26 E. From shale above thinbedded limestone which overlies the Madison limestone: 

Dosinia? sp. undet. 

Eumicrotis curta Hall. 

Ostrea strigilecula White. 

Belemnites densus Meek and Hayden. 

Weathered casts of undetermined pelecypods. 

Small ammonite distinct from described forms but 

evidently a cardioceratid. 

These fossils are Upper Jurassic. :Most of 

the species are known in the Sundance and 

Ellis formations, but several ammonites and 

a Rhynchonella are unlike any known Sundance 

forms. 

The detail of part of the Ellis formation 

which has been identified paleontologically 

is as follows: . 1 

Section of Ellis formation. 

Feet. 

Shale with interbedded sandstone and limestone .. 100± 

Limestone, thin bedded, shaly, and calcar~ous 

shale •.......... · . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 

300± 

Above the shaly portion there is. a more or 

less massive sandstone, which is tentatively 

considered as also belonging to the Ellis, 

though no · fossils were found in it. It presents the following character in sec. 7, T. 25 N., 

R.26E.: 

Section of the sandstone in the upper part of the Ellis 

formation. 

Sandstone, massive, white, cross-bedded ......... . 

Shale, variegated .. .. ............................ . 

Sandstone, yellowish, and thinner bedded ........• 

Feet. 

50 

50 

100 

200 

The formation is almost identical with the 

Sundance formation of the Black Hills and the 

Ellis of other parts of Montana. As the 

Madison limestone is of Mississippian age·, 

there is necessarily an unconformity between 

it and the Ellis (Upper Jurassic), but the 

actual contact of the two formations has not 

been recognized. A photograph (Pl. II, 0) 

taken in the flanks of the Little Rocky Mountains in sec. 23, T. 25 N ., R. 25 E., shows the 

camp situated on the outcrop ·of the 'Ellis 

formation iri the foreground . and the outcrops 

. of the older formations ·in the background.. 

CRETACEOUS SYSTEM. 

KOOTENAI ' ( ~) FORMATION. 

The sandstone just described is overlain in 

the Little R.ocky Mountain section by variegated shale interbedded with yellow and brown 

sandstones, which are believed to represent the 

Kootenai formation (Lower Cretaceous) of 

other parts of Montana, though they may possibly include the Morrison. The shale is in 

places blue, green, red, and gray. The approximate thickness, as determin~d by only one 

measurement, is 825 feet. From 100 to 200 

feet above the massive sandstone there is an 

exposure of carbonaceous sandstone and sandy 

shale containing woody fragments and impressions of poorly preserved stems which may 

represent the coal beds occurring in the Kootenai in the neighborhood of Great Falls or the 

corresponding Lakota of the Black Hills. No 

fossils were found in this part of the section 

during the season of 1916, but Weed and 

Pirsson report that it contains Goniabasis subZaevis Meek and Hayden, Oorbicula cytheriforrnis 

Meek and Hayden, and an Ostrea. 

COLORADO GROUP. 

MOWRY SHALE .. 

Above the Kootenai ( ~) formation is a well-:-

marked siliceous shale (fig. 2) which has been 

recognized at several localities and is the highest formation described by Weed and Pirsson 

in their paper on the geology of the Little 
26 SHOR.TER CONTRIBUTIONS TO GENERAL GEOLOGY, 1918. 

Rocky Mountains. When freshly broken it is 

brown or black, but on weathering it becomes 

light colored and breaks into porcelain-like 

debris. It contains numerous fish scales and 

bones, some of which can be found on nearly 

every piece examined, and on some specimens 

the trails of small marine animals, probably 

crustaceans, were also found. Thin sections 

under the microscope show that the material 

of this shale consists of small, angular grains of 

quartz interbedded with a dark amorphous 

substance, probably carbon. Two distinct 

forms of Infusoria were seen, but their nature 

could not be determined in the material at 

hand. In appearance this shale resembles the 

shale of the Green River formation (Eocene) 

in Colorado and Utah, from which oil may be 

obtained by distillation, but a small fragment 

tested by distillation yielded only a trace of oil. 

On the basis of its lithologic similarity and content of fish scales, this shale is correlated with 

the Mowry shale, which occurs at several localities in Wyoming and southern Montana. 

Around the Little Rocky Mountains its outcrop is marked by a low ridge covered with 

pines. The thickness of the formation is about 

100 feet. 

COLORADO GROUP ABOVE THE :MOWRY SHALE. 

Above the Mowry shale, in the region surrounding the Little Rocky Mountains, there is 

about 325 feet of dark-blue shale which differs 

slightly in color from the dark-gray gumbo 

shale higher in the section. The exposures are 

not continuous, and little can be said of the 

details of this part of the section. 

fossils found in it were baculites, a few of which, 

collected from limy concretions, were submitted 

to J. B. Reeside, jr., who reports upon them as 

follows: 

Baculites sp., usually referred to B. aspe:r Morton. It is 

quite distinct from the related Pierre B. aspe:r and from B. 

anceps obtusus. It has been collected from several localities in the Benton of Montana. (Collection 9841.) 

MONTANA GROUP. 

EAGLE (1) SANDSTONE. 

The sandstone that lies above the shale just 

described in the region surrounding the Little 

Rocky Mountains is marked by a ridge about 

30 feet high (fig. 2), the top of which is covered 

with large limestone concretions resting on 

sandstone. One poorly preserved specimen of 

Nemodon, referable to an undescribed· species 

from the Claggett near Havre, was found (collection 9842). The same formation was seen 

at a locality about 3 miles southeast of the 

point where 'the section was measured, and 

although it was not closely studied it was 

thought to be about 100 feet thick. This 

s·andstone is about 750 feet below the base of 

the Judith River formation, and if it is the 

Eagle sandstone it proves that the overlying 

formation varies a great deal in thickness, for 

the Eagle sandstone is described by Stebinger 1 

as being from 350 to 500 feet below the base of 

the Judith River formation. Reports of drilling northeast of Malta, in the Bowdoin dome, 

seem to indicate that the Eagle sandstone is 

not present there as a mappable unit. 

CLAGGETT SHALE. 

This shale is overlain by a sandstone capped The Claggett shale represents part of a 

by a foot or more of impure limestone contain- marine formation called the Pierre shale in · 

ing fossil gastropods and pelecypods which have I regions to the east and southeast, which to the 

been identified by T. W. Stanton as of Benton west is represented by four formations-the 

· age. J. B. Reeside, jr., who examined the Eagle sandstone, Claggett shale, Judith River 

collection, reports the following species: Oallista formation, and Bearpaw shale. Of these formaorbiculata Hall and Meek, Pachymelania? sp. tions the Judith River is of fresh-water origin. 

undescribed, Amauropsis? sp. undescribed. The Claggett shale on the east side of the 

The stratigraphic position of this sandstone is Little Rocky Mountains, where its top and 

not far from that of the Frontier formation of bottom are exposed, measures approximately 

Wyoming and southern Montana. Its thick- 750 feet. In the center of the Bowdoin dome 

ness could only be estimated and is given in the north of Saco about 500 feet of the upper part 

table as about 60 feet. of the formation is exposed. Stebinger, workThe sandstone noted above is overlain by ing in the vicinity of Havre, about 90 miles to 

about 875 feet of bluish-gray to black shale, the west, reports the thicknes~ of this formation 

much of which is concealed and in which sandy 1 Stebinger, Eugene, Possibilities of oil and gas in north-central 

facies if present were not recognized. The only Montana: u.s. Geol. survey Bull. 641, p. 53, 1917. 
GEOLOGY OF NORTHEASTERN MONT:ANA. 27 

as from 350 to 500 feet. It consists of a darkgray shale which on weathering forms typical 

gumbo soil. Scattered through the formation 

in many places are concretions or masses of 

impure limestone, many of which contain 

marine fossils, or of aragonite, a mineral resembling fossil wood. The formation also contains large flakes of selenite or transparent 

gypsum, and water in wells, springs, or small 

stre"ams in the area of its outcrop is always 

more or less alkaline. This shale forms the 

lower slopes of the valley of Milk River between 

Hinsdale and Malta. Two collections of fossils 

from it were referred to J. B. Reeside, jr., who 

identified one species, Baculites compressus Say, 

regarded as characteristic of the Pierre shale. 

About 70 feet of strata in the upper part of 

the Claggett shale appear to form a transition 

zone between the Claggett and the Judith 

River formation. This zone is found on the 

west side of the Bowdoin dome, north of Malta. 

On the north side of the dome, near Whitewater post office, it does not contain the characteristic sandstone, and it can not be recognized at all on the east side near Hinsdale. 

The following sections will illustrate this peculiarity: 

Sections of the transition zone of the Claggett and Judith 

River formations. 

West' of the Bowdoin dome near the center ofT. 31 N., R. 30 E. 

Typical Judith River formation. Feet. 

Shale resembling the Claggett. .................... 50± 

Sandstone resembling Judith River formation containing abundant marine fossils .................. 20± 

Typical Claggett formation. 

70± 

North of the Bowdoin dome in Tps. 34 and 35 N., R. 32 E. 

Typical Judith River formation. 

Shale containing fossils similar to those of the marine 

sandstone of the preceding section. . . . . . . . . . . . . . . . 50± 

Typical Claggett shale. 

The following lots of fossils collected from 

the upper part of the Claggett shale have been 

identified by Mr. Reeside, who finds that they 

are marine forms characteristic of the Pierre 

shale. 

10016. Sec. 32, T. 35 N ., R. 32 E.: 

Lunatia subcrassa Meek and Hayden . . 

10017. Sec. 25, T. 33 N., R. 30 E.: 

Tancrediaamericana Meek and Hayden (fragments). 

Lunatia subcrassa Meek and Hayden. 

9829. North half ofT. 31 N., R. 30 E.: 

Cardium speciosum Meek and Hayden. 

Tancredia americana Meek and Hayden. 

Lunatia subcrassa Meek and Hayden. 

Haminea subcylindrica Meek and Hayden. 

9830. Western part ofT. 31 N., R. 30 E., south of Assinniboine Creek: 

Anatina sp. undescribed. 

Mactra alta Meek and Hayden. 

Tancredia americana Meek and Hayden. 

Cardium speciosum Meek and Hayden. 

Anchuria sp. undet. 

Lunatia subcrassa Meek and Hayden. 

Placenticeras intercalare Meek. 

Baculi tes com pressus Say. 

JUDITH RIVER FORMATION. 

Above the Claggett shale lies the Judith 

River formation, estimated from several imperfect measurements to range from 400 to 425 

feet in thickness. The outcrop of this formation surrounds the Bowdoin dome, standing 

out as a light-colored formation in the 

hill south of the railroad from Malta to 

Vandalia and north of the railroad near Valleytown and Lovejoy. It also crops out in 

one of the surrounding sandstone ridges of the 

Little Rocky Mountains. It probably extends 

east of the Bowdoin dome beneath the surface, 

at least as far as the mouth of Milk River, 

where it has probably been reached in an artesian well at a depth of 750 feet. To the southeast the Judith River formation is not recognizable as a distinct unit, as shown by the gas 

wells sunk on the Glendive anticline. A characteristic view of the Judith River formation 

is given in Plate III, .A. The formation consists of extremely variable beds of hard brown 

sandstone, soft, · friable light-colored sandstone, and light-gray shale; and on account of 

their variability no beds were found that would 

serve as horizon markers. In describing this 

formation Stanton and Hatcher 1 say: 

A detailed section taken at any point is of little value, 

since a similar section made at a distance of only a mile or 

two would give a quite different sequence of the alternating strata of sandstones and shales. 

Some of the details of this formation may be 

seen from two sections which were measured 

in its upper part on the east side of Rock Creek, 

as follows: 

IStanton, T.W., and Hatcher, J. B., Geology and paleontology of 

the Judith River beds: U.S. Geol. Survey Bull. 257, p. 34, 1905. 
28 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1918. 

Sections of par.t .of the Judith Ri'l!er formation . 

On east canyon wall of Rock Creek, in sec. 20, T. 34 N., R. 36 E. 

Bearpaw shale abo~eJudith Rive~ f~rmation. Feet· 

Sandstone, _ p1as'sive, grayis.h, with iron concretions 

scattered or in peds; weathe~s yeliowish'; ~~me 

· red band~ cross-bedded ..... ~~ ... ... . . ...... ·.. 32 

Shale, brown, in places approaching lignite .... .' .. _:" · · · 2 

S~nd.stone, soft, yellowish gray, ~nd gray shale.... 23 

Lignite ....... , ...... .... . . . . . . . . . . . . . . . . . . . . . . . . . ~ 

Shale, brown ....................... ·. . . . . . . . . . . . . 1 

Shale, gray .... ~ ..... ~ .. -............ · ........... ·.. 4 

Shale, dark, carbonaceous.. . . . . . . . . . . . . . . . . . . . . . . ! 

Shale, gray, and sandstone ........... ~....... ..... 5 

Lignite ................. · ..... ~ .... ·. ;· ... ~ ... :,· ..... : l 

Shale, brown ................. : . ." .... . .... : . : . . . . i 

Shale, gray .... -.1•· ..... , ., .. ,. J··· · ···:• .... _................ 6 

Shale, brown........................................ 3 

~Jj!.~t.<>ne, ~oft, with iron concretion,s...... ... . . . .. .. 1 

~}t#e sand-y;~ br<nyn .... ~ .............. · ..... ·. . . . . . 2, 

~~dstone, soft, g~ay .............. ·, . .. ........... \ 6 

Clay, shale gray .......... ....... _._.: .· ............ ·. 4 

Shale·, brown .. ~ ... : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

Sandstone, soft, gray, and sandy shale ... ·.·......... . 9 

Sandstone, concretion bed........ . . . . . . . . . . . . . . . . . 1 

Shale, soft, sandy, gray ........ _.................... 5 

Sandstone, iron-concretion bed......... . . . . . . . . . . . 1 

Shale, blue a'nd gray ................ .' ......... :. .. 4 

Concretions....................................... 1 

Shale, gray..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Shale, brown ...... ...... ............ -. . . . . . . . . . . . 2 

Sandstone, soft, gray and Y.ellow.................. 17 

Sandstone, massive; weathers brown; arkosic, containing much feldspathic and · ferromagnesian 

material. This bed forms conspicuous ledges.... 7 

Sandstone, soft, gray; some shale.................. 20 

Base of formation not exposed. 

165 

East canyon wall of Rock Creek, near line between sec. 32, T. 34 N ., 

R. 36 E., and sec. 4, T. 33 N., R. 36 E. 

Glacial gravel. Feet. 

Sandstone, massive, gray; weathers yellow or reddish; some thin beds; cross-bedded .. ~.......... 47 

Shale, gray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

Sandstone, soft, gray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Shale, gray .......................... ~........... 4 

Shale, brown ................................... ·. 3 

Shale, gray.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2· 

Shale, brown... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Sandstone, soft, gray............. . . . . . . . . . . . . . . . . . 5 

Shale, brown, with 3 inches of lignite............. 2 

Shale, gray...................................... 5 

Shale, brown ................................ ·.·... 6 

Shale, blue .................................... ·. . . 2 

Sandstone, soft. ................. :. . . . . . . . . . . . . . . · 9 

Shale, brown.................................... 1! 

Sandstone, soft, gray..... . . . . . . . . . . . . . . . . . . . . . . . . . 3 · 

Shale, blue-gray ................. ·. . . . . . . . . .. . . . . . 5 

Concretions................ . . . . . . . . . . . . . . . . . . . . . . . 6 

Sandstone, soft, massive, gray........... . . . . . . . . . . . 18 

Shale, brown ........... : ....................... . 

Lignite .......................................... . 

ShaJe, brown .................................... . 

Shale, blue.~gray ................................ . 

Shale, brown ............ : ...................... . 

Shale, sandy .... ~ .......... ~ .................... . 

SandstOne, concretions .......................... . 

Shale, gray .......... ....... ... ... .............. . 

Sandstone, soft, gray ............................. . 

Shale, dark .. " ...... · ............................ . 

Feet. 

i 

l 

5 

12 

1 

6 

1 

10 

13 

3 

Sandstone, soft, gray, and sandy shale.............. '14 

Shale, brown .. · .. ·. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Sandstone, massive, soft, gray or yellowish, weathering yellowish; certain beds weather out as massive reddish layers or as huge concretions; shows 

some cross-bedding .... . ........................ . 

Base of formation not exposed. 

101± 

296± 

The base of thi~ formation is not very definitely _marked and can not everywhere be 

separated from the upper part of the Claggett 

shale, but its top can be readily recognized in 

the few places where it is well exposed. The 

upper part of the Judith River formation consists of a 'massive sandstone in the two sections 

cited and in the neighborhood of the Little 

Rocky Mountains, and is overlain by character-

. i~tic Bearpaw shale. At other places in the 

region the massive sandstone is not prominent, 

and in one or two localities a small bed of lignite 

within a few feet of the top has been mined. 

Stanton and Hatcher report a layer of shell 

breccia consisting of shells of Ostrea subtrigonalis 

near the top of the Judith River formation as its 

most persistent horizon marker, but such beds 

were not seen in the neighborhood of the 

Bowdoin dome. Invertebrate fossils collected 

· from the formation during the season of 1916 

have been examined by Mr. Reeside, whoreports that six lots consist of marine forms and 

one lot Erom the flanks of the Little Rocky 

Mquntains contains fresh-water forms of Judith 

River types. The species identified are as 

follows: 

9826. Sec. 36, T. 33 N., R. 37 E., on road: 

Plates of an undescribed barn.acle. 

Tancredia americana Meek and Hayden. 

Cardium speciosum Meek and Hayden. 

Protocardia sp. ?, large form. 

Mactra sp.? 

Inoceramus sp.?, very young individual. 

Nucula sp.? 

Lunatia subcrassa Meek and Hayden. 
GEOLOGY OF NORTHEASTERN MONTANA. 29 

9826. Sec. 36, T. 33 N., R. 37 E., on road-Continued. Sections of Judith River formation in northeastern Montana. 

Cf. Trachytriton vinculum Meek and Hayden. 

Scaphites sp., fragment suggesting S. nicolleti 

Morton. 

Baculites sp.?, fragment. 

Most of this lot consists of casts lacking essential 

details. The Protocardia?, Lvnatia, and Scaphites suggest Bearpaw rather than typical Judith River. 

9843. SW. t sec. 21, T. 33 N., R. 37 E., on Ea.gle Creek; 

upper part of Judith River formation: 

Liopistha undata Meek and Hayden. 

Pteria nebrascana Edwards and Solander. 

Marine; Pierre fauna. 

41. Sec. 4, T. 33 N., R. 37 E., on Bitter Creek; upper part 

of Judith River formation: 

Natica sp.? 

Mactra sp.? 

Serpula sp.? 

The only fresh-water forms are found in the 

following lot: 

9837. Sec. 21, T. 25 N., R. 26 E., sandstone about middle 

of Judith River formation at east end of Little Rocky 

Mountains: 1 

Unio cf. U. primaevus \Vhite. 

Unio cf. U. supragibbosus \\'hiteaves. 

This lot consists of fragments which suggest a reference 

to the Judith River but are too poor to qe conclusive 

Unio-supragibbosus occurs in the Canadian Belly River 

and in the Judith River formation. U. prtmaevus is a 

Judith River species. 

Inasmuch as this lot of fresh-waterforms is 

from the middle of · the formation, while the 

majority of the fossils found are marine, it is 

probable that the Judith River formation of 

this area is ·marine at the. top and bottom. 

Bones of fresh-water dinosaurs were collected 

at several localities and were examined by C 

W. Gilmore, of the National Museum, who 

found them too fragmentary for specific determination. 

, I 

Location. 

Sec. T.N. R.E. 

- - --

21 33 37 

------

4 33 37 

------

5 33 37 

------

15 32 37 

I 

------

16 32 37 

------

7 34 36 

------

12 34 35 ------ 32 34 35 

------ 6 33 35 

------ Specimens of petrified wood, described by 20 34 34 

F. H. Knowlton as that of cone-bearing trees 

were collected at several places in the Judith 

River formation, but no determinable fossi 

leaves were found. 

II __ 

5 ----

35 34 

This formation contains small lenticular beds 

of lignite or subbituminous coal which are too 

thin to. be of value. Local mines from which 

the farmers have obtained a small amount of 

fuel were examined at the points where the following sections were measured: 

28288°-19-3 

--

9 --

9 

9 

35

__ !34 , __ 

28 

~ 34 

Section. 

Ft. in. 

Bearpaw shale. 

Sandstone. __ ........... __ ... 20± 

Lignite. ______ .. _ ...... _ 8 

Sandstone· and shale ........ 15± 

Shale, carbonaceous, · and 

lignite .................... 2± 

Bearpaw shale. 

Sandstone and shale ........ 59 0 

Lignite ................. 6 

Sandstone and shale ........ 22 0 

Shale, carbonaceous ........ 2 0 

Lignite __ ................ 8 

Shale, carbonaceous ..... _ .. 2 0 

Shale, blue ................ 4 6 

Shale, carbonaceous ........ 6 

Lignite ................. 1 2 

Shale, carbonaceous ........ 6 

Sandstone .................. 10 0 

Lignite ................. 1 4 

Shale, sandy gray .......... 3 0 

Lignite, a few inches. 

Shale, gray ................. 5 0 

Lignite .. _ ........... _ .. 1 4 

Shale, brown ............... 1 0 

Lignite ....... ~ ......... 6 

Shale, gray ................. 5 0 

Shale, brown ............... 2 

Lignite .............. _ .. 8 

Shale, brown ............... 4 

Shale, gray. 

Shale, gray ................. 5 0 

Lignite ................. 10 

Shale, brown ............... 8 

Shale, gray. 

Shale ....................... 10 0 

Lignite ........... ~ ..... 6 

Shale, brown. 

Shale gray I+ 

Lignite ..... ~ .. . . . .. .. .. 6 

Lignite ..... __ .. .. .. . .. .. 8 

Lignite._ .... __ .....• _... 6---10 

Bearpaw shale. 

Judith River formation .. __ . 50± 

Clay,_ bl~ck, carbonaceous.. 1 6 

L1gmte ............. _ .. . 10 
30 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1918. 

BEARPA W SHALE. 

The Bearpaw shale, from 800 to 1,000 feet 

thick is, as shown on Plate I, the most widespread formation exposed in the region described, and overlies the Judith River formation. It consists essentially of dark-gray shale 

which forms a gumbo soil and presents an uninteresting and monotonous landscape. This 

shale is exposed in many places along escarpments and canyons and forms badlands which 

lack the variety due to the presence of harder 

beds. A characteristic view of the badlands of 

the Bearpaw shale is given in Plate III, B. The 

fossils are all of marine types; Baculites, Inoceramus, and oysters are very abundant, and occasionally the remains of a gigantic marine saurian 

are found. The formation in places contains 

limy concretions, in which the lime is in the 

form of aragonite showing cone in cone structure or appearing like the stumps of large trees. 

Calcite also is present in many of these concretions, and smaller masses of barite were 

noted in several localities. The formation contains more or less gypsum, which at some 

places l.s transparent and at others occurs in 

uncrystallized forms replacing the calcite of 

oyster shells. The water from the Bearpaw 

shale, like that from the Claggett, is usually 

alkaline. 

· The following invertebrate fossils were collected during the field season of 1916 and were 

identified by J. B. Reeside, jr.: 

9825. SW; t sec. 22, T. 33 N., R. 37 E.: 

Inoceramus sagensis Owen. 

Heteroceras sp. related ·to H. newtoni Meek but 

probably undescribed. 

Pierre fauna; fossils not distinctive of Bearpaw shale. 

9827. Sec. 36, T. 36 N., R. 36 E.: 

Ostrea patina Meek and Hayden. 

So far as known this form is confinetl to the Bearpaw 

shale. 

9828. Sec. 5, T. 36 N., R. 26 E.: 

Chlamys nebrascensis Meek and Hayden. 

Baculites compressus Say. 

Of. Trachytriton vinculum Say. 

Membranipora sp. undescribed (identified by R. S. 

Baseler). 

Pierre fauna. 

9838.' Sec. 21, T. 25 N., R. 26 E.: 

Ostrea patina White 

9839. Sec. 21, T. 25 N., R. 26 E.: 

Inoceramus sagensis Owen. 

Baculites sp., fragments. 

Pierre fauna. 

10020; Sec. 10, T. 23 N., R .. 36 E. , 150 to 200 feet below 

top of Bearpaw shale : ' 

Scaphites nodosus var. plenus Meek. 

Baculites compressus Say. ~ 

Cyprimeria? sp., single weathered individual. 

Pierre fa una. 

10021. Southwestern part of T. 23 N. , R. 36 E. , 50 to 100 

feet below top of Bearpaw shale: 

Baculites compressus Say; 

Pierre fauna. 

10024. Northeastern part ofT. 23 N., R. 34 E., 50 feet be-: 

low top of Bearpaw shale, on Timber Creek: 

Baculites compressus Say. 

Pierre fauna. 

10025. T. 26 N., R. 34 E., along Larb Creek; middle of 

Bearpaw shale: 

Baculites sp., fragments. 

Pierre fauna. 

10028. T. 24 N., R. 41 E., near top of Bearpaw shale: 

Inoceramus sagensis Owen. 

Protocardia subquadrata Edwards and Solander. 

Anomia sp., fragments. 

Yoldia evansi Meek and Hayden. 

Lunatia sp., fragments. 

Dentalium gracile Meek and Hayden. 

Anchura sp. , fragment. 

Actaeon attemiatus Meek and Hayden. 

Scaphites sp., probably young S. nodosus plenus 

Meek. 

Bearpaw shale. 

Parts of skeletons of two large marine reptiles 

collected in this region, one found several 

years ago by Ira Taylor; were . submitted to 

C. W. Gilmore, who identified one of them 

(from sec. 23, T. 37 N., R. 35 E.) as consisting 

of numerous parts of . vertebrae, limb . bones, 

fragmentary teeth, jaws, and other fragments 

of a large plesiosaurian reptile; and the other 

(from sec. 23, T. 34 N., R. 38 E.) a~ consisting 

of numerous. vertebrae and other parts of the 

skeleton of a mososauri.an reptile. Neither of 

these forms was otherwise determinable. The 

remains of both animals are found . only in 

marine or brackish-water formations 

TERTIARY (1) SYSTEM. 

LANCE FORMATIO,N. 

The Lance formation, which overlies the 

Bearpaw shale, as here identified, is co~posed 

of two more or less distinct members in the 

eastern and southern parts of the region 

described. In the northwestern part, in the 

Cherry Ridge quadrangle~ the exposures are 

too meager to indicate their char~cter. 
GEOLOGY OF NORTHEASTERN MONTANA. 31 

The lower member has been described by 

Dawson/ who estimated the thickness at 80 

feet and referred it to the Fox Hills sandstone. 

Figure 4, reproduced from Dawson's report, 

shows the relation of the two members near 

the international boundary northwest of 

Opheim. The lower member is here composed 

of yellowish and rusty sands grading into 

arenaceous clay toward its base. It is characterized by large, irregular sandy concretions 

which in places approach a more or less spherical form and are slightly darker than the prevailing color of the formation. So far as the 

writer knows marine fossils have not been 

obtained from this member within the region 

described. Rose, 2 in his description of beds 

he designated Fox Hills sandstone in the 

Wood Mountain district of Canada, gives a 

list of the fossils found by R. G. McConnell in 

beds designated Pierre and Fo~· Hills formaSouth Dakota. Small exposures of this member are found in T. 37 N., R. 40 E., and T. 35 

N., R. 44 E., where it is similar in lithologic 

character to the rocks northeast of Opheim. 

In the Larb Hills, in the southern part of the 

region described, a similar member having a 

thickness of about 100 feet where measured 

in sec. 13, T. 24 N., R. 32 E., rests upon the 

Bearpaw shale and is capped by a harder 

sandstone. The same succession of beds is 

exposed south of Missouri River, where Brown 3 

reports collecting 14 species of marine invertebrate fossils, identified as the Fox Hills fauna, 

from concretions found on Hell Cr~ek. He 

further reports ~ that in the same valley, 20 

feet below the base of the sandstone he calls 

Fox Hills, the remains of a large trachodon t 

dinosaur were discovered. The specimen was 

found in place partly embedded in a large 

calcareous concretion that was weathered out 

F!G"UnE 4.-8ection in badlands south of Wood Mountain, Saskatchewan, Canada, showing the relation of Fort Union formation (a), upper 

part of Lance formation (b), lower part of Lance formation (c), and Bearpaw shale (d). 

tions in the Cypress Hills, but as no effort is 

made to distinguish between the formations 

the list is probably of little value. 

. A very good exposure of yellowish sandsto e, resembling that at the boundary, covers 

2 or 3 square miles in and around sec. 34, T. 

36 N., R. 39 E. In this sandstone, which 

surely lies only a short distance above the 

Bearpaw shale, was found a well-preserved 

fossil bone which Gilmore identified as "the 

proximal half of the left ischium of a trachodont 

dinosaur. So far as it can be compared it 

appears identical with the same element in 

Trachodon annectens Marsh, from the Lance 

formation." The finding of this fossil justifies 

the belief that the lower part of the Lance is a 

fresh-water deposit and should not be classified 

as the Fox Hills sandstone, which in the type 

locality is marine. It may, however, be contemporaneous with the Fox Hills sandstone of 

1 Dawson, G. M., Geology of the 49th parallel, British North American 

Boundary Commission, Montreal, 1875. 

2 Rose, Bruce, Wood Mountain-Willowbunch coal areas, Saskatchewan: Canada Geol. Survey Mem. 89, p. 32, 1916. 

and broken. The humeri were still covered 

with shale and well preserved. 

This member northwest of Opheim is probably the same as that found on Missouri River 

in the southeastern part of the region here 

described and called the Fox Hills sandstone 

by Smith. 5 (See Pl. IV, A.) It is described 

in a later report by Beekly 6 as the Colgate 

sandstone member of the Lance formation, 

consisting of 50 to 150 feet of buff sandstone 

interstratified with somber-colored and yellow 

clay. The type locality is the vicinity of Colgate station on the Northern Pacific Railway, 

described by Calvert.7 

The upper member of the Lance formation is. 

composed of shale and argillaceous sandstone 

and is characteristically of somber cqlor. Its 

a Brown, Barnum, T:q_e Hell Creek beds of Montana: Am. Mus. Nat. 

Hist. Bull. 23, p. 827, 1907. 

t Idem, p. 826. 

5 Smith, C. D., The Fort Peck Indian Reservation lignite field, Mont.: 

U.S. Geol. Survey Bull. 381, p. 42, 1908. 

6 Beekly, A. L., The Culbertson lignite field, Valley County, Mont.: 

U.S. Geol. Survey Bull. 471, pp. 329,330, 1912. 

7 Calvert, W. R., Geology of certain lignite fields in eastern Montana: 

U.S. Geol. Survey Bull. 471, pp. 194, 195, 1912. 
32 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1918. 

thickness is from 180 feet northwest of Opheim 

to 250 feet where measured in the Larb Hills. 

At some places the argillaceous sandstone shows 

cross-bedding, due to wind action while it was 

being deposited (Pl. IV, B). Its erosion results in many picturesque badlands or isolated 

pinnacles capped by hard concretions (Pl. 

V, A) or interesting corrugated forms resembling organ pipes (Pl. V, B). The Lance formation in many places contains loglike sandstone 

concretions and carries near its top one or more 

somewhat irregular lignite beds. The bed 

mined northeast of Opheim has a maximum 

thickness of about 6 feet; that near Daleview 

is over t"2 feet thick and extends with varying 

thickness throughout the outcrop of the Lance 

formation on Big Muddy Creek. In othe! 

parts of the region there are lignite beds too 

thin to be of commercial value. 

Brown 1 has described numerous fossil vertebrates, mostly dinosaurs, the most striking of 

which is the large three-horned Triceratops, 

which he collected from the south side of Missouri River opposite the Larb Hills. Fragments of dinosaur bones collected in 1916 from 

the Lance formation in the Larb Hills were not 

good enough to be specifically determined. 

C. W. Gilmore reports that a small collection 

made northwest of Opheim in sec. 23, T. 37 N., 

R. 39 E., consists of 

foot bones, vertebra, chevrons, etc., of Thescelosaurus 

neglectus Gilmore; caudal vertebra of a carnivorous dinosaur, not determinable; two fragments of a turtle shell, 

probably pertaining to the genus Aspidontes. The presence of Thescelosaurus neglectus would indicate that these 

fossils came from the Lance formation, for the four or five 

specimens of the genus known at. this time are all from 

the Lance. 

A single vertebra collected near the top of 

the Lance formation ·in sec. 27, T. 37 N., R. 

46 E., is that of Ohamposaurus sp., characteristic of both the Lance and Fort Union formations. 

Several small collections of invertebrate 

fossils, made in the Larb Hills, have been identified by Mr. Reeside as follows: 

10022. Sec~ 29, T. 24 N., R. 35 E. 

Unio brachyopisthus. 

Unio holmesianus. 

1 Brown, Barnum, op. cit., pp. 823-845. 

10023. Sec. 29, T. 24 N., R. 35 E. 

Unio cf. U. brachyopisthus White. 

Sphaerium subellipticum Meek and Hayden. 

Viviparus sp., fragment. 

10026. Sec. 29, T. 24 N., R. 35 E. 

Tulotoma thompsoni White. 

10027. Sec. 25, T. 25 N., R. 34 E. 

Sphaerium subellipticum Meek and Hayden. 

Campeloma multilineata Meek and Hayden. 

All are probably from the Lance formation. 

Fossil wood from the Lance formation has 

been identified by Knowlton as that of coniferous trees, but no identifiable fossil plants were 

collected. 

TERTIARY SYSTEM. 

FORT UNION FORMATION. 

Overlying the somber-colored shale of the 

Lance formation is the Fort Union formation, 

of Eocene age, which is exposed along the 

North Dakota line and extends more or less 

continuously westward as far as Opheim. 

It carries the principal lignite beds of the region 

and also contains widely distributed beds of 

clay and shale 2 suitable for the manufacture of 

brick and other clay products. The formation 

is composed of fine-grained sandstone and 

shale and is distinguished from the Lance 

formation by the prevailing light-yellow color. 

Like the Lance formation, it has, especially in 

its lower part, concretions of hard sandstone 

resembling fossil logs. (See Pl. VI, 0.) Observations made along the contact of the Lance 

and Fort Union formations indicate that the 

change in color ia not always noticeable at 

exactly the same horizon. The mappin~ of 

the contact of these formations on Plate~ is 

based on the judgment of the geologists making 

the examination in each locality. According 

to Beekly 3 the thickness of the Fort Union 

remaining at the southeast end of the region 

mapped is ,about 1,400 feet. The lower part 

is probably equivalent to the Lebo shale member of the Fort Union, described by Rogers.4 

The following section of the Lance and Fort 

2 Rose, Bruce, Wood Mountain-Willowbunch coal areas, Saskatchewan: Canada Geol. Survey Mem. 89, pp. 69-83, 1916. Bauer, C. M., 

Clay in northeastern Montana: U.S. Geol. Survey Bull. 540, pp. 369-372, 

1914. 

a Beekly, A. L., The Culbertson lignite field, Valley County, Mont.: 

U.S. Geol. Survey Bull. 471, pp. 326, 331, 1912. 

4 Rogers, G. S., The Little Sheep Mountain coalfield, Dawson, Custer, 

and Rosebud counties, Mont.: U.S. Geol. Survey Bull. 531, pp. -12, 13, 

1913. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 120 PLATE IV 

A. LOWER PART OF THE LANCE FORMATION, SHOWING 

SPHERICAL CONCRETIONS. 

B. ARENACEOUS SANDSTONE, SHOWING EOLIAN CROSS-BEDDING. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 120 PLATE V 

A. PINNACLE OF SOFT SANDSTONE OF LANCE FORMATION. 

B. EROSION FORM OF LANCE FORMATION. 
GEOLOGY OF NOR.T"HEASTERN MONTANA. 33 

Union formations exposed from Redstone to 

Plentywood (Pl. I) is much generalized: Sandstone with some shale_------------------- ~ Lignite _____________________ --------------

Ft. in. 

121 0 

1 0 

Generalized section of the Lance and Fort Union formations Sandstone and shale_·_------- -- - - --- - - - - - - - - - - 24 0 

between Redstone and Plentywood, :Mont. Lignite, bed D D-------- ______________ . __ _ 4 4 

Sandstone and shale. _______ . _ . ______________ " 

Lignite, bed CC. ________ ---- _- __________ _ 

Sandstone and clay ________________________ _ 

Lignite. _____________ . _________________ _ 

Fort Union formation: 

Sandstone and shale. __________________ ... 

Lignite _____________________ - - _ - - -- - - -

Sandstone and shale. ___________ .... __ ... _ 

Ft. in. 

81 0 

6 

67 0 

70 0 

2 5 

130 0 

5 7 

Shale __________________ . ________ . _______ _ 52 0 416 8 

Lignite, Richardson bed. _____________ _ 

Sandstone and shale more or less concealed 

6± 

320± Fort Peck field west of Big Muddy Creek. 

Lignite, upper Eagles Nest bed._ . ___ _ 

Shale and sandstone, yellowish. ____ .... __ 

Lignite, lower Eagles Nest bed. _____ _ 

Shale and sandstone, yellowish. _____ . __ .. 

Lignite .... __ . ________ ... _ . _ . ______ _ 

Shale and sandstone, yellowish. _ . _______ _ 

Lignite .•• ___ . _____ .... _______ ____ .. _ . · 

Shale and sandstone, yellowish ________ . __ 

Lignite ...... ______ ... _ ... ___ . _____ _ 

Shale, grading upward into sandstone, 

yellowish . __ .. _____ .. ___ . _ . _ . _______ . _ 

Lignite _____ . _____ .. _______________ _ 

Shale and sandstone, yellowish __ ... ___ . __ 

6 3 

50± 

4 0 

70 0 

10 

19 0 

1 2 

18 0 

4 

20 0 

1 6 

60± 

[C. D. Smith, 1908.1 

Clay ____________ ------· --------------------- Lignite, bed D __________ __ __ . __ . __ .. __ . __ 

Sandstone and clay_ . ____ .. __ . __ .. __ .. __ ... _ 

Lignite, bed C. _________ ._. ________ -- __ _ 

Sandstone anc1_ clay _________ . _ . _____ .. ____ . _ 

Lignite, bed B _ . _ ... __ .. _______________ . 

Sandstone and clay ________________________ _ 

Lignite _____ . _ . _ . _ . ____________________ _ 

Clay ___ -·----------------------------------- Lignite, bed A ______________ . ______ .. __ _ 

Somber-colored beds (Lance) .. 

6 0 

9 0 

95 0 

2 0 

115 0 

3 0 

275 0 

5 0 

15 0 

7 7 

532 7 

Lignite, Redstone bed ______________ ._ 6 0 1,390 3 

Shale and sandstone, yellowish. _ . ______ . _ 22 

Lignite _____________ ..... _ ... ____ . __ 2 

Lance formation: 

Shale and s~ndstone, somber color ___ . __ 30 

Lignite _____ ._ ... _______ .... ___ .. __ _ l 

Shale and sandstone, somber color _____ _ 

0 

0 

0 

10 

The following section, measured in sec. 18, T. 

24 N., R. 33 E., shows the relations of the Fort 

U:nion, Lance, and Bearpaw formations in the 

L&rb Hills: 

27 0 

Lignite . - _ . _ .. - -.. - - - - . - - - - - - - - - - - - _ 6 0 Section showing the relation of the Fort Union, Lance, and 

Bearpawformations in the Larb Hills, Mont. 

872± Fort Union formation: 

The following generalized section of the Fort 

Union and its lignite beds as exposed on Missouri River in the southeastern part of the 

region is taken from Beekly's report: 1 

Sandstone, yellowish. ____ .. ______ .. _. __ _ 

Lignite. ____ . _______ --. ___ .. ___ -- __ . 

Sandstone, soft, yellowish_ . _______ ... __ _ 

Lignite ___________ .. -.. _ - _ . _ .. _____ _ 

Sandstone, soft, yellowish._. ___________ _ 

Lignite. __ .. ________ ---- ______ . ____ _ 

Generalized section showing the distribution of lignite beds 

in the Fort Union formation. 

Sandstone, soft, yellowish. _______ . _____ _ 

Lignite ____________ . - ______ - - _____ . _ 

Culbertson field, 2 miles east of Lakeside. 

Ft. in. 

Clay and shale ....... _ .. _____ . ______ ... _. ___ _ 125 0 Lance formation: 

Lignite, bed H .. __ . _ ... _____________ .. _ . 4 0 Shale and sandstone, somber color _____ . __ _ 

Clay and shale. ____ . __ . ______________ .. __ . __ _ 70 0 Lignite. _________________ .... ____ . __ 

Lignite, bed G _. _________________________ _ 4 0 Shale and sandstone, somber color; con- Clay and sandy shale. ________________________ _ 150 0 tains dinosaur bones. __ . ____ .. __ . ___ . __ . 

Lignite. _______________ . ____ ---_._ ... ____ _ 1 10 Concealed. __ . __ . ____________ ._. _____ ... _ 

Sandstone and clay ..... , ____________________ _ 30 0 Shale, sandy ___ . __ . ___ ..... ______ . ______ _ 

Lignite. ________ . _______ .. _______________ _ 1 2 Bearpaw shale. 

Sandstone and shale .. __________ . ____________ _ 55 0 

441 0 

Ft. in. 

50 0 

3 

20 0 

1 0 

11 0 

1 0 

20 0 

6 0 

109 3 

50 0 

6 

100 0 

100 0 

100 0 

350 6 

459 9 

The relations of these sections are shown Culbertson field, sec. 3, T. 27 N., R. 56 E. 

Lignite, bed F _______ .. _________________ _ 

Sandstone and shale .. __________ .. ________ . __ _ 

Lignite, bed E _____ . ___ . ________ . _ .... ___ _ 

1 Beekly, A. L., op. cit., p. 331. 

7 

45 

6 

graphically in figure 5. 

o Full descriptions of the lignite beds in each 

O township are given in other reports. 2 

4 

2-q. S. Geol. Survey Bull. 381, pp. 40-59, 1910; Bull. 471, pp. 319-358, 

1912; Bull. 541, pp. 293-315, 1914; and a report not yet published. 
84 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1918. 

The following small collections of fossil 

plants have been examined by F. II:. Knowlton, 

who reports that they are of. Fort Union age: 

7001. Sec. 3, T. 35 N ., R. 44 E.: 

Sequoia nordenskioldii Heer. 

A 

Richardson bed 

Upper Eagles Nest 

Lower Eagles Nest 

Redstone bed 

Contact 

B 

Bed H 

Bed G 

Bed F" 

Bed E 

Bed DO 

Bed CC 

Bed D 

Bed C 

Bed B 

Bed A 

Onoclea sensibilis fcissili ewberry. 

Trapa microphylla Lesquereux of Ward. 

Selaginella collieri Knowlton,! n. sp. 

Potamogeton n. sp. 

7005. Canada, near the north line of sec. 4, T. 37 N., R. 

49 E.: 

c 

I 

Ft. Union formation 

(. sandstone1 shale,and lignitej yellow beds) 

Phillips bed 

Thin bed 

Lance formation Lance formation 

Colgate(?)sandstone 

member 

Lance formation 

(somber sandstone and shaie, and lignite above) 

Colgate sandstone member Yellow sandstone and shale (Colgate(?)sandstone member) 

Bearpaw shale 

100 0 100 200 300 400 soo Feet 

FIGURE 5.-Generalized sections of Fort Union and Lance formations, showing their relation to ~he Bearpaw shale from its outcrop northwest of 

Opheim to Plentywood, M:ont. (A); north of Missouri River from Brockton to the Dakota line (B); and in the Larb Hills, in T. 24 N., R. 33 

E.(C). 

Glyptostrobus ungeri Heer. 

Dicotyledon, gen. and sp.? 

7002. NE.! sec. 31, T. 36 N., R. 45 E.: 

Sequoia nordenskioldii Heer. 

7004. Sec. 33, T. 37 N., R. 47 E.: 

Alga. 

. Onoclea sensibilis fossilis Newberry. 

Taxodium occidentale Newberry. 

Sequoia nordenskioldii Heer. 

Platanus sp. 

1 Knowlton; F. H., A new fossil Selaginellx from the lower Tertiary of 

Montana: Torreya, vol. 16, pp. 101-103, 1916. 
GEOLOGY OF NORTHEAST'ERN MONT'ANA. 35 

7505. Canada, near the north line of sec. 4-Continued. 

Grewia sp. 

.Persea sp. 

7006. SE. i sec. 21, T. 36 N., R. 52 E.: 

Taxodium occidentale Newberry~ 

Sequoia nordenskioldii Heer. 

Grewia pealei Ward. 

7272. NE. i sec. 36, T. 37 N., R. 39 E.: 

Sequoia nordenskioldii Heer. 

Glyptostrobus europaeus Unger. 

Fragments of a dicotyledon. 

· Collections of invertebrates, on which T. W. 

Stanton has reported, were made at the following localities: 

9837. SW. i sec. 6, T. 35 N., R. 45 E.: 

Unio ep. fragments. 

Campeloma producta White. 

Campeloma multilineata Meek and Hayden. 

Fort Union. 

9380. T. 36 N., R. 46 E.: 

Viviparus sp. 

Probably Fort Union. 

9381. Sec. 13, T. 34 N., R. 50 E.: 

Unio sp. 

Campeloma multilineata Meek and Hayden. 

Fort Union. 

FLAXVILLE GRAVEL. 

With the Fort Union the conformable series 

stops, and the formations representing the later 

history· of the region are surficial deposits of 

rather small extent laid down during favorable 

intervals between great time breaks when 

. erosion was prevalent and no sediments were 

deposited. 

The Flaxville gravel, 1 shown in several areas 

along the international boundary, is a few feet 

to 100 feet thick and caps a series of plateaus 

upon eroded surfaces of the Fort Union, Lance, 

and Bearpaw formations at altitudes ranging 

from 2,600 feet at the east end to 3,200 feet 

at the west end of the region described, near the 

international boundary. It is composed of 

gravel, sand, clay, volcanic ash, and marl and 

in places is cemented with calcite. The gravel 

consists of pebbles derived from the quartzite 

and argillite of the Rocky Mountains. 

Fragments of fossil vertebrates found at 25 

widely distributed localities have been submitted to . J. W. Gidley, who states that the 

beds which contained them can not be older 

than Miocene or younger than early Pliocene 

and that they are probably upper Miocene. 

1 Collier, A. J., and Thorn, W. T., jr., The Flaxville gravel and its 

relation to other high terrace grovels of the northern Great Plains: U.s. 

Geol. Survey Prof. Paper 108, pp. 179-184, 1917 (Prof. Paper 108-J). 

I The time break between the Fort Union formation (Eocene) and the Flaxville gravel (Miocene) is in part represented by the Oligocene 

deposits found on a still higher plateau in the 

Cypress Hills in Canada, 50 miles away. 

TERTIARY OR QUATERNARY SYSTEM. 

LATE PLIOCENE OR EARLY PLEISTOCENE 

DEPOSITS. 

Below the' Flaxville level there are extensive 

areas across which the present streams have 

eroded valleys from 100 to 500 feet in depth. 

Stratified silt and gravel are to be found in a 

very few places on this level, and from one of 

these exposures of .gravel in sec. 23, T. 33 N., 

R. 35 E., a single fossil tooth was collected 

which Dr. Gidley identified as that of a horse 

resembling the living species. As the gravel 

from which it came is clearly older than the 

last advance of ice during the glacial epoch, the 

formation is regarded as of either late Pliocene 

or early Pleistocene age. 2 

Extensive benches slope gently away from 

the Little Rocky Mountains at several levels. 

These benches are capped with gravel containing many pebbles of limestone and porphyry 

derived from the neighboring mountains and 

are almost surely of Tertiary age, for in some 

places the gravel on them is overlain by glacial 

drift. The lowest bench is probably equivalent 

to the late Pliocene or early Pleistocene bench 

in the region north of Milk River with which 

it is tentatively correlated. The presence of 

undissolved limestone pebbles seems to. forbid 

the assignment of these benches to an earlier 

period. 

QUATERNARY SYSTEM. 

ALLUVIUM OF PREGLACIAL RIVERS. 

Since-late Pliocene or early Pleistocene time 

streams have been at work eroding the present 

valleys. The effects of this erosion can be seen 

in the escarpments north and south of Milk 

River between Tampico and Malta, where at 

one place 300 feet of beds belonging to the 

Claggett and Judith River formations, capped 

by late Pliocene or early Pleistocene gravel, 

are exposed. The cutting of the river valley 

must have occurred early in Pleistocene time, 

when the valley of Milk River was occupied by 

2 Idem, p. 18Z. 
36 SHORT'ER CONTRIBUTIONS TO GENERAL GEOLOGY 1 1918. 

~ n1uch larger river, the Missouri/ and along 

1ts banks and flood plains gravel, silt, and 

sand were deposited. Some of this material 

can be seen in pits northwest of Saco and north 

of Malta. The gravel here differs from the 

Flaxville gravel in that it contains numerous 

fragments of porphyry. 

Similar deposits occur at several points in 

the first benches above the flood plain of the 

Missouri between Milk River and the North 

Dakota line, and are best exposed ·where gravel 

pits are operated by the Great Northern Railway. In the summer of 1917 a pit was operated at Wolf Point, where a new freight division of the railway was being installed and 

where the photograph reproduced in Plate 

VI, B, was taken. Wolf Point lies about 25 

feet above the Missouri, and the gravel pit is 

in the second bench, about 20 feet higher. 

·The section .exposed is about as follows: . 

Section exposed in gravel pit at Wolf Point. 

Soil, with a few scattered granite bo~lders on the Feet. 

surface ............................... _ . ___ . _ _ _ 1 

Glacial till, containing granite cobbles._. __ , __ . __ ._ 5 

Gravel, chiefly quartzite but containing fragments 

of sandstone, porphyry, and near the bottom shale. 13 

Shale, Bearpaw. 

19 

The gravel is interstratified with lenses of 

sand, and the bedding planes are nearly horizontal. The gravel consists for the most part 

of well-rounded pebbles of quartzite, probably 

derived from the Flaxville gravel, but it contains some pebbles of porphyry and fragments 

of sandstone and of fossiliferous Bearpaw shale 

near the bottom. A thorough search failed to 

reveal any glacial material such as granite 

pebbles. It was evidently deposited by Missouri River in preglacial time, for it is overlain 

by glacial till containing many more or less 

angular fragments of granite. Another gravel 

pit 2 miles west of Brockton shows finely laminated sand. The bedding at the west end of 

the pit lies nearly horizontal, whereas that at 

the east end dips . east about 70°. The surface has been eroded irregular-ly and is covered 

with glacial till to a maximum depth of about 

15 feet. The bottom of this pit is about 40 

feet above the present river level. 

1 Calhoun, F. H. H., The Montana lobe of the Keewatin ice sheet: 

U.S. Geol. Survey Prof. Paper 50, pp. 34-45, 1906. 

GLACIAL MORAINES AND DRIFT. 

Overlying the gravel in the Milk River 

valley as well as the Flaxville gravel and all 

the higher benches described there is in many 

places a sprinkling of glacial boulders, among 

which specimens of limestone from Lake Winnip_eg and of granite from the Hudson Bay 

regwn are abundant. One large limestone 

erratic about 20 feet long was found by 

W. C. Alden and the writer just north of 

Nelson Reservoir. At the time this glacial 

drift was introduced the Missouri was probably 

forced out of the Milk River valley and compelled to flow through a narrow valley south 

of the Larb Hills. When the glacial epoch 

was over and the ice sheet had retreated for 

the last time the waters that could collect in 

~he old channel formed Milk River, which flows 

1n a very meandering course over the old valley 

floor. In addition to the scattered boulders 

there are in the northern part of this region 

accumulations of glacial material in which the 

topography is suggestive of moraines. There 

are ·also in the southern part of the region some 

large areas covered by till or boulderclay. 

ALLUVIUM. 

Soil of various kinds, brought in since the 

glacial epoch, has been deposited along Milk 

River and other streams of the region. On the 

flood plains of Milk River there is usually a 

gumbo soil, derived from the shale of the 

country, which forms a sticky mud when wet. 

The river ~as brought down a small quantity 

of sand in some places, and where this is 

deposited the soil is more easily cultivated than 

the gumbo. · 

Milk River since the retreat of the ice has 

trenched its flood plain about 10 or 12 feet, 

developing jntricate meanders, and back of the 

river there. are . oxbow lakes which show the . 

extent of the flood plain. 

Big Muddy Creek, in the eastern part of the 

region, flows in a valley cut from 100 to 300 feet 

below the late Pliocene or early Pleistocene 

bench. Its flood plain is about a mile wide 

an~ is covered with fine silt, which provides 

t~e material that doubtless suggested the 

name for the creek and makes it difficult to 

cross on account of the muddy banks. The 

gravel terraces; from 20 to 50 feet above the 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 120 PLATE VI 

GEOLOGY OF NORTHEASTERN MONTANA. 37 

flood plain, were probably formed by the large 

river that occupied the Big Muddy Valley and 

received its water and load of sand and gravel 

directly from the front of the glacier to the 

north, near the end of the glacial epoch. 1 

Along Eagle, Beaver, and Whitetail creeks, 

tributaries of the Big Muddy, there are at many 

places beaver dams, above which are long strips 

of flood plain called beaver-dam land. 

_The allu:vium along the forks of Poplar 

River consists largely of reworked Flaxville 

gravel, and because of this gravel the streams 

can be easily forded at nearly all places. 

Rock Creek has a long, narrow canyon so 

precipitous that it was considered to be too 

rough to survey by the surveyors of the Land 

Office. The upper end of this canyon is near 

Barr post office, above which the ·valley is 

broad and filled with alluvium. 

·Frenchman Creek, by far the largest stream 

entering Milk River from the north, rises far 

to t~e north~est in Canada and after crossing 

the InternatiOnal boundary flows in a comparatively nan:ow valley whibh has probably 

been eroded since the last glacial advance. 

The flood plain along it is in general narrower 

than the flood plains of most of ·the other 

streams. 

Whitewater Creek, farther west than Frenchman Creek, was probably situated in about its 

pr~sent ?ourse _before the advance of the large 

Wisconsm glacier. Its valley is at least a mile 

wide. and is filled by an extensive glacial 

moraine. Cotto~wood Creek in its lower part 

has a compa~atlvely narrow flood plain; the 

upper part of Its course is called Woody Island 

Coulee. . The water in this coulee apparently 

at one time flowed eastward and entered the 

valley of Whitewater Creek, but its old channel 

was blocked with glacial d~ift, and only one or 

two lakes are left to indicate its former position. 

Beaver Creek, which enters Milk River from 

the south, occupies a broad valley bedded with 

silt, making the stream difficult to ford. The 

lower part of this valley coincides with that of 

the Musselshell, which before the advance of 

the Wisconsin glacier flowed into the Missouri 

in the valley of Milk River. The upper part 

of the valley of Beaver Creek from T. 27 N., 

R. 30 E., to the Coburn ranch, in T. 25 N., R. 

26 E., is largely filled with glacial till, in which 

the creek is at places intrenched about 20 feet. 

In preglacial time it probably reached the old 

channel of the Missouri by way of Alkali Creek. 

For about 12 miles from T. 27 N., R. 30 E., to 

T. 26 N., R. 32 E., Beaver Creek occupies a 

narrow valley cut in the Bearpaw shale. 

Larb Creek, east of Beaver Creek, occupies a 

very broad valley, and its bed is cut in alluvium. There is an easy pass from its head 

over the divide to the present course of the 

Missouri, as there is also from Beaver Creek. 

SOIL OF THE UPLANDS. 

The soil on the plateau surfaces of the region 

varies with the character of the underlying 

rock. The most interesting soil is that which 

caps the Flaxville gravel. This soil is from a few 

inches to several feet thick and has a slightly 

dark color. It is bound together by grass 

roots in its upper part and contains the bones 

of the bison. With the exception of that on 

the Boundary Plateau, this soil does not appear 

to contain much glacial material. South of 

Glasgow there is an extensive upland surface 

beneath which lies the Bearpaw shale, but 

~ere there is a sandy soil which owes its origin 

In part to the advance of the ice. Over much 

of the western part of the area there is a gumbo 

soil derived directly from the Bearpaw shale 

which it overlies. The soil on the Larb Hills is 

sandy, being derived from the disintegration 

of. the Lance formation underlying it, mixed 

with a small amount of glacial material. The 

soil overlying the Judith River formation is 

sandy and not quite so good as that which 

overlies the Fort Union and Lance formations. 

In the parts of the region covered by glacial 

moraines many small lakes have been filled 

since the retreat of the ice. The filling is gen-

~rally grass and other plants that have grown 

In the water, together with materials that have 

blown from the sides. These old lake beds are 

in many places used for farm lands. In the 

large moraine-covered area, which has its center around Whitewater Creek a great many of 

these lakes are still unfilled, and water persists 

in them, throughout the average summer. 

STRUCTURE. 

The Bowdoin dome, 2 northeast of Malta, in 

the western part of the region, is the most 

obvious structural feature, and from it the 

1 Rose, Bruce, Wood Mountain-Willow bunch coal area, Saskatchewan: 2 c u· Canada Geol. Survey Mem. 89, p. 50, 1916. . o Ier, A. J., The Bowdoin dome, Montana, a possible reservoir of 

ml or gas: U. S. Geol. Survey Bull. 661, pp. 193-209, 1917 (Bull. 661-E). 
SHORTER CONTRIBUTIONS T'O GENERAL GEOLOGY, 1918. 

strata dip away in all directions. The dips are 

usually less than 1°, and where they have been 

determined by careful instrumental surveys 

they are best recorded in feet to the mile. The 

Little Rocky Mountains, about 40 miles southeast of the Bowdoin dome, present an abrupt 

uplift of the strata accompanied by an intrusion of igneous rocks, forming a laccolith, but 

this disturbance does not extend far from the 

base of the mountain. The beds around this 

uplift are tilted at angles in some places as high 

as 70°. The relation of the Bowdoin dome to 

the structure prevalent in the Little Rocky 

Mountains is shown in figure 6. 

In the area immediately surrounding the 

Bowdoin dome the field work has not been in 

sufficient detail to ascertain whether or not 

there are minor structural features, but such 

features are known to exist in the eastern part 

of the region. The Poplar don1e, on Missouri 

River, described by Smith/ is evidently such a 

SE. 

ing a maximum width of about 2 miles is present. In the valley of Big Muddy Creek similar variations occur in the strike and dip. 

Such features probably are present in the western part of the region but they are very difficult to find because most of the rocks. exposed 

are shale, in which it is practically impossible 

to trace individual beds for any distance. 

Two small faults having a displacement of 

less than 15 feet each may be seen on the north 

side of Coal Creek in T. 37 N., R. 45 E., and a 

third fault of similar proportions displaces one 

of the lignite beds near the northwest corner 

of T. 36 N., R. 52 E. No other faults are 

known in either the eastern or the western part 

of the area of flat-lying beds, though they may 

be present. In the escarpments of the Flaxville plateau and the late Pliocene or early 

Pleistocene bench large blocks of strata may 

break away fron1 their natural position and 

slump, gi¥ing the appearance of a condition 

Bowdoin Dome NW, 

FIGURE 6.-Relations ofthe Bowdoin dome to the structure prevalent in the Little Rocky Mountains, Mont. a, Madison limestone; b, Ellis 

formation; c, Kootenai(?) formation; d, Colorado group; e, Eagle(?) sandstone; j, Claggett shale; g1 Judith River formation; h, Bearpaw shale. 

feature, as it brings up a large elliptical area of which might result from faulting. This has 

Bearpaw shale nearly surrounded by the Lance occurred .at many places in the Judith River 

formation. The formations immediately con- formation around the Bowdoin dome. Faults 

cerned dip away from it in all directions. The of great displacement were seen in · the Little 

lignite beds along the river east of Brockton dip Rocky Mountains and in the writer's opinion 

eastward at about 100 to 135 feet to the mile. are common there, but .they can not be deIn directions other than east of the dome, scribed without further work in the field: 

where its presence only intensifies the general A word should be said in regard to the strucdip, its effect does not reach farther than 15 ture of the Larb Hills. Calhoun 2 makes the 

miles away and probably the dips are much statement that these hills represent a dome 

lower than those to the east. Similar small whose character is similar to that of the Little 

undulations of the strata can be inferred from Rocky Mountains but which is not eroded sufthe distribution of the Lance and Fort Union ficiently to expose the underlying rocks. This 

outcrops north of the Poplar dome, and several statement is incorrect, for the Larb Hills are 

such features, which are not readily detected, . simply an outlier of the large area of Lance and 

were fou~d in the course of mapping the lig- Fort Union rocks exposed south of Missouri 

nite beds~ River, and the strata here have a uniformly 

In the vicinity of Scobey, where the general low dip to the south. It is assumed, theredip of the rocks is toward the east, there is a fore, that Calhoun did not make a persona] 

westward dip for about a mile. Northeast of examination of these hills but relied on infor~ 

Scobey, south of Coal Creek, a small dome hav- mation obtained from some other source. 

I Smith, C. D., The Fort Peck Indian Reservation lj.gnite field, Mont., 

U.S. Geol. Survey Bull. '381, p. 44, 1910. 

2 Calhoun, F. H. H., op. cit., p. 12. 
GEOLOGY OF NORTHEASTERN MONTANA. 39 

The major part of the present structure in 

both the large flat-lying portions of the formations and the highly upturned portions in the 

Little Rocky Mountains was introduced sub-

·sequent to the deposition of the Fort Union 

formation and before that of the Oligocene 

gravel in the Cypress Hills, for this gravel rests 

unconformably upon the eroded surfaces of 

the Fort Union, Lance, and Bearpaw formations indiscriminately. In other words, this 

-revolution occurred in late Eocene time. 

That a minor part of the folding may have 

been accomplished since the deposition of the 

Flaxville gravel is indicated by the fact that 

some of the streams in the eastern part of the 

region occupy synclinal valleys and by the ab-

:sence of the Flaxville gravel between Opheim 

. and the Boundary Plateau, it being assumed 

that an uplifted portion of the plateau would 

be more subject to erosion than the lowerlying part. 

ADJUSTMENTS OF DRAINAGE. 

Most stream adjustments in northeastern 

Montana are due to the invasion of ice during 

the glacial epoch. One of the most obvious 

features of this kind can be seen on Wolf 

Creek, a tributary of the Big Muddy, for its 

headwaters have been diverted and captured 

by the South Fork of Eagle Creek, largely 

because of glacial drift piled across its valley 

in sees. 8 and 9, T. 34 N., R. 51 E. Reference 

has been made to the change in the course of 

Woody Island Coulee, in T. 36 N., R. 28 E ., by 

the filling up of its valley with glacial drift 

and to a possible old course of the headwaters 

of Beaver Creek by way of Alkali Creek to 

Milk River, a short distance above Malta. 

The most pronounced stream adjustment 

in this region, however, is the diversion of 

Missouri River from the Milk River valley, 

which Calhoun 1 has described. The present 

valley of the Missouri south of the Larb Hills 

is practically a canyon 800 feet deep and not 

more than 2 miles across from rim to rim, 

whereas its old valley, now occupied by Milk 

River, is much broader and shallower. Before 

the river was diverted Musselshell River flowed 

1 Calhoun, F. H. H., op. cit., pp. 34-45. 

into it by way of the lower portion of the 

Beaver Creek valley. By this change about 

70 miles of the lower part of Musselshell River 

was diverted. Larb Creek was probably a 

much larger stream heading south of the 

Missouri and passing through the Larb Hills 

by way of the Timber Creek valley. The 

glacier from the north filled the whole of the 

Milk River valley and was thick and active 

enough to cover the Larb Hills with glacial 

drift. At its front the great volume of water 

due to melting ice and an attendant moist · 

climate easily eroded a new channel in the 

soft Lance formation and Bearpaw shale. A 

view south across the gorge of the Missouri, 

700 to 800 feet in depth, is shown in Plate 

VI, A . 

An old channel of the Missouri from Culbertson to Lakeside, which is followed by the 

Great Northern Railway, and a broad depression extending from the Big Muddy Valley 

at Medicine Lake northwestward, crossing the 

State boundary about 6 miles south of the 

Canada line, have been described by Beekly.2 

The course o'f the depression· is marked by a 

chain of shallow intermittent lakes. Bauer 3 

has assumed that this feature also is an old 

channel of the Missouri, in which it flowed 

northward to Hudson Bay by way of Souris 

and Assinniboine rivers before the glacial 

epoch. The south end of this depression 

near Poplar has recently been inspected by 

Alden, who states 4 that its floor is about 200 

feet above that of the present valley of the 

:.Missouri, and that its topography blends 

with the long slopes which characterize the 

early Pleistocene or late Pliocene bench of this 

region. Alden suggests that the river probably 

. occupied this valley as long ago as the Pliocene epoch and that it was diverted at the 

time of the first invasion of ice early in the 

Pleistocene. The late Pliocene or early Pleisto<;ene gravel described on pages 35-36 was 

probably laid down in or near the channel of. 

one of the streams which contributed to form 

this river. 

2 Beekly, A. L., The Culbertson lignite field, Mont.: U.S. Geol. Survey 

Bull. 471, pp. 320-323, 1912. 

a Bauer, C. M., A sketch of the late Tertiary history of the upper Mis .. 

souri River: Jour. Geology, vol. 23, pp. 52-58, 1915. 

• Alden, W. C., oral communication.