GRADATIONS FROM CONTINENTAL TO MARINE CONDITIONS OF DEPOSITION IN CENTRAL MONTANA DURING THE EAGLE AND JUDITH 

RIVER EPOCHS. 

By C. F. BowEN. 

INTRODUCTION. The field sheets thus prepared and the land 

The large amount of geologically unexplored plats obtained from the General Land Office 

territory in western Rosebud and Dawson furnished the data from which the accompanycounties, Mont., led t.o an examination by the ing base map (Pl. IV) was prepared. This map 

writer of a part of this territory for the purpose shows the principal streams and roads of the. 

of getting general information on its stratig- area, as well as the geologic formations exraphy and structure. The region proved to be posed. The formation boundaries are repreone of considerable geologic interest. sented by solid lines where they are accurately 

The work demonstrated (1) that pro- determined and by broken lines where, because 

nounced dome structure exists in a region of lack of exposures or for other reasons, they 

heretofore supposed to be underlain by nearly could be located only approximately. 

flat-lying beds; (2) that the Eagle sandstone, The writer was assisted in the field by C. A. 

whieh is a thick formation farther west, thins Bonine, to whom mueh credit is due for the reeastward and disappears near the western suits obtained. 

border of the area herein described; (3) that the SURFACE FEATURES. 

Judith River formation, previously known only 

as of fresh-water and brackish-water origin, The area is a part of the Missouri Plateau of 

grades into a marine formation in passing the Great Plains, and its surface features are 

eastward fron1 the shore line of the sea along those produced by the erosion of such a region 

whieh it accumulated; and (4) that east of the · in a semiarid climate. Erosion has changed a 

meridian of lOT., 30' (approximately) the comparatively level surface into one dissected by 

deposition of marine sediments was continuous streams and surmounted by numerous buttes. 

throughout the Colorado and Montana epochs This sculpturing is most. pronouneed along 

of Upper Cretaceous time in this part of the Musselshell River, where in a belt 1 to 3 miles 

Great Plains region. wide intricately dissected badlands have been 

The area herein considered comprises about formed. In most parts of the area the sand1,200 square miles in Rosebud and Dawson stones of the Judith River and Lance formacounties and extends from Forsyth northward tions form conspicuous ridges that rise abruptly 

for a distance of 40 miles, thence westward from the lower and more nearly level surfaces 

to Musselshell River. Its geographic position occupied by the shale formations. The irregis shown on the key map (Pl. IV). ularities of the surface are due chiefly to difThe area was mapped on a scale of 1 mile ferenees in erosion induced by the unequal 

to the inch. Horizontal control was based on resistance of the formations. 

the land survey of the General Land Office 

and was obtained by stadia and triangulation. 

A stadia traverse along the Bearpaw-Lance 

boundary was earried throughout the field. 

From this traverse triangulation points were 

established whieh furnished control in other 

parts of the field. Elevations were obtained by 

means of vertical angles, the initial elevation 

being a point on the Chicago, Milwaukee & 

St. Paul Railway a few miles west of Forsyth. 

131049°-20-2 

GEOLOGY. 

STRATIGRAPHY. 

GENERAL SECTION. 

The formations exposed in this area range 

in age from Upper Cretaceous to Tertiary ( n 

and inelude the Colorado shale (upper part), 

Eagle sandstone, Claggett and Judith River 

formations, Bearpaw shale, and Lance formation. With the exception of the Eagle sand11 
12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1919. 

stone, which does not extend east of Mussel- in the immediate vicinity, but have been peneshell River, these formations are believed to be trated by the drill to a depth of about 2,500 

continuous throughout the area, but the Judith feet below the top of the Colorado shale in a 

River changes from a fresh-water formation in deep well put down at Vananda, in T. 7 N., 

the western part of the area to a marine forma- R. 38 E., by the Chicago, Milwaukee & St. 

tion in the eastern part. In other words, Paul Railway Co. The greater part of this 

marine conditions prevailed over the eastern thickness consists of the dark-gray to black 

System and series. Group. 

Quaternary. 

Tertiary ( ?) (Eocene?). 

Geologic formations in the area herein discussed. 

Formation. 

Lance formation. 

Thickness 

(feet). Character. 

Alluvial gravel, sand, and silt along Yellowstone 

and Musselshell rivers and some of the smaller 

streams. 

Brown, irregularly bedded sandstone, alternating 

with "somber" gray shale. Upper part (25 

feet or more) is somewhat sandy, forming a transition to the Lance. This transition phase 

is probably what has been recognized as Fox 

Hills on Hell Creek about 35 or 40 miles farther 

north . 

.. - ....... - .... ---. ---1·-------- 1-----1--------------------

Upper Creta- ceous. 

Lower Creta- ceous (?). 

Bearpaw shale. 

Judith River formation. 

Claggett formation. 

Eagle sandstone. 

Colorado shale. 

Kootenai formation(?). 

900-1,100± Dark-gray shale in which occur calcareous concre- tions containing marine invertebrate fossils. 

Upper member, light-brown to light-gray massive 

sandstone. Middle member, light-gray to darkgray shale. Lower member, sandstone, weather100-200± ing brown and giving rise to large boulder-like masses. The formation is of fresh-water origin 

in the western part of the field and of marine 

origin in the eastern part. 

Dark-gray shale; contains numerous large cal- careous concretions in upper part, some of 

600-700± which show well-developed cone-in-cone structure; marine fossils similar to those in the 

Bearpaw. 

0-21 

2,300± 

Buff massive sandstone; does not occur east of 

Musselshell River. 

Dark-gray to black fissile shale, with thin beds 

of sandstone and sandy shale in upper part and 

thin beds of limestone and sandstone near the 

base; shale is only slightly plastic when wet. 

Contains marine fossils. 

Consists predominantly of red and white shale, 

157+ overlain by a thin bed of limestone and sandstone. 

part throughout the Montana epoch, whereas 

in the western part a temporary recession of 

the sea allowed the accumulation of freshwater deposits. These conditions are illustrated in the diagrammatic and generalized 

sections shown on Plate IV. 

shale of the Colorado, but the lower 157 feet 

seems to represent some underlying formation, 

probably the Kootenai. 

The accompanying table presents in condensed form the sequence, character, and thickness of the exposed formations and of those 

penetrated by the drill in the well at V ananda, 

a detailed log of which is given on page 13. 

Rocks below the upper part of the Colorado 

shale are not exposed in this field or anywhere 
U. S. GEOLOGICAL SURVEY 

NW. 

Genereliud &ec:tion e t north end of 

Judith Mounteins 

ff:E"T 

6,000 A 

5.000 

3.000 

z.ooo 

500 

Section on 

Musselehe ll River Section a t west side of Porcupine dome Section at 

8 

.F 

east side of' Porcupine dome 

g C.twrt .R,U.~G<o!Oillc•l Su<Vey lluii.J501 p30, 1909 

(/)) Section ooserved by Bowen but nOttme.et~ured 

C B~ ed on Jog of dtep wellet Vannnde 

EXPLANATION 

L m ~ ·• fo,·ma.tion }~ ;: 

D CJ ~get1. and <'olorndo h-tics 

0 

Str ike and dip 

D 

Landcorncrfound 

"' 

0 " ... 

u 

"' ... 

... 

tr 

u 

·--- ~--

All monne deposit-3 

SE. 

If 

- - -----

-----=-====== :::.-=..-::::.-::: =-=~-= z=--~~~~---~ 

All mc'Jr/n~ 

,_ _______ 48miles ------- +--------- 47miles -------"*,__ - +------------.96tnol., ____ _ ________ ~ 

SOUTH EAST TO ILLUSTRATE CHANGES OF SEDIMENTS DUE TO OSCILLATIONS OF UPPER CRETACEOUS SEA DIAGRAMMATIC SE CT ION FROM NORTHWEST TO ( • dopled f room ~ene St eblnner, U. S. Geol. Survey PcQf . Pope, 90, flgu r<t 9, p . 67, 191 5) Thlokneaaea baaed on datal obtained by C. F. Bowen &xcept •• otherwl•• al ated In te:.ct. ,.., .,. ., 

• .000 

J,OOO 

z,ooo 

1,000 

o S~U:llt!l 

Columnar section West end of field 

Columnar sect ion East end of field 

Creteceous 

l t~nce rOrmebon 470 '.: (l"re:Jh 8f.~r) 

Bea rpaw sha le 1,100 '.,. 

(mt~rin~ 

----

- Judith River f;r:ation 18s':t .,.-...(Fresh wBler) 

Claggett fbr t~ ion 600' (marine) 

Eagle sandslone 0· 2 1' 

(msri"ne) 

Coloc8do s hele (mari'neJ 

lance for·mation 350' ± (fresh Ylft r) 

Bear paw shele 900' ± 

(m~>n;,e) 

Judith River formation 100 -~25' 

(mllrine) 

Cleggettand Colorado shales 3,000' ± 

(marina) 

GEOLOGIC MAP OF THE .fORDAN COAL FIELD, MONTANA, vVITH COLUMNAR SECTIONS, STRUCTURE SECTIONS, AND DIAGRAMMATIC EOTION 

PROFESSIONAL PAPER 126 PLATE IV 

E. 

T. 7 N . 

'-•• 
GRADATIONS FROM CONTINENTAL TO MARINE CONDITIONS IN CENTRAL MONTANA. 13 

Log of well drilled by the Chicago, Jlf:ilwaukee & St. Paul Railway Co. at Vananda, in sec. 5, T. 7 N., R. 38 E., Jffont., 

between October 23, 1913, and October 17, 1914. 

[Elevation of mouth of well, 2, 704 feet above sea level.] 

Formation. a Driller's description. 

Shale, sandy ______________________ ... --------------------- Sand; 30 gallons of water an hour. ____________________ .... --- Shale, yellow _______________________________ ._.------------

Sand. ______ ---------------------------------------------- Shale and sand _________________________________ . _ . _ .. --- - -

Judith River formation.b Sand. ______ ------------------- _______________ ------------- Shale, yellow _________________________________ . _ . _ .. _ ..... -

Sand. ______ ---------------------------------------------- Sand and shale _____________________ . _____ . _ . _ .. _ . __ . _ .... -

Shale, blue _______________________________ . ___ . _ .. -. -- - --- -

Sand; flow of alkaline water, 2, 400 gallons an hour .. - - - - - - - - -

Claggett shale. c Shale; disintegrates when exposed to air ________ ..... __ .. _ ... 

Thickness. 

Feet. 

23 

7 

16 

11 

24 

8 

16 

14 

24 

46 

26 

685 

Depth. 

Feet. 

23 

30 

46 

57 

81 

89 

105 

119 

143 

189 

215 

900 

----------(d)-----------1---------------------------------------------- - - ----1-------

Shale, e black surface; whitens on exposure to air ......... - - - - Shale __________ . _________________________________________ _ 

Oyster-shell rim, fossil determined by former State geologist 

of South Dakota as Inoceramus labiatus. 

Shale, dark brown ........................................ - Hard rock_ . _____________ . ___________________ . ___ ........ -

Shale, soft; color changes on exposure from dark to light brown. 

Shale, light brown .. _ ........... _ . _ . _ . __ ............. -... - - -

Slate. ___ .. _ . .............................. -... -.---- ... -- Soapstone. __ . _ .. ~ ........ _ ..... _ . _________ ... _ . _________ . __ 

Shale, white ...... _ ......................... -------------- Shale, dark ..... __________ . ____________ . _ ..... _ ........ -.- Shale, black __ . _________________________ . _ . _ ... _ .......... . 

Colorado shale. Shale, brown. __________ ........ _ .... _ .......... -- - - - - - - - - - - Shale, dark, caving._. __ . _________________________ . ___ ._ .. . 

Slate, caving. _ .... _ . ___ . _ . _________ . _ . __ .......... _ ...... . 

Limestone ................................... - - - - . - - -.... -

Shale ___ . __ ..... - -... --- . - - . -.... - - - - - - - - - - - - - - - - - ----- - - - Shale and lime shells ....... ___ . _______ ... ___ . _ ..... _ ...... . 

Shale, soft, caving ....... _ ..... _ .......... -.- ............. . Shale and lime shells ... _ ........ _ ............. -. -......... - Shale, soft, caving_ . _ . ___________ ... _ .... _____ ..... _ ... __ .. 

Shale, dark .. _ ... _ ........... -.-.-.- .. ------.----.-------- Shale and lime shells _ .. ___ ... ____ ... _ .................. _ . __ 

Sandstone, hard __ .. _ .. } { Limestone, black, sandy May represent the Mowry s,hale IDemLimestone ...... __ .. _ ber. 

Slate. _ . __ .. _________ _ 

Sand, water bearing; water rose within 50 feet of surface. ___ _ Sandstone, compact. ___________________ . _ . ________________ _ 

Limestone ........ __ . _ .. ___ ......... _ ... - - - -. - - -- - - - - - - -.. 

Kootenai (?). 

Shale, white. ____________________________________________ _ 

Shale, red. _ . _ .. _ . _______________________________________ _ 

Shale, white ........ ____________ . ______ .. ____ .... -- ...... . Shale, red ...... ______________________ . _____________ . ____ _ 

Shale, white ....... _____________________ . __ .. _ ....... - -... . 

Shale, red ..... _ ..... __ .. _______ . ____ ... _ . _ . ___ ........... . 

a Correlation revised by author. 

140 

907 

5 

107 

29 

32 

40 

40 

13 

23 

45 

19 

100 

31 

19 

41 

239 

15 

35 

20 

105 

35 

103 

32 l8 

42 

55 

10 

30 

19 

23 

7 

8 

27 

8 

35 

1,040 

1,947 

1,952 

2,059 

2,088 

2,120 

2, 160 

2.200 

2;213 

2, 2.36 

2,281 

2,300 

2,400 

2,431 

2,450 

2,491 

2, 730 

2,745 

2,780 

2,800 

2,905 

2,940 

3,Q43 

3,075 

3,093 

3, 135 

3,190 

3,200 

3,230 

3,249 

3,272 

3,279 

3,287 

3,314 

3,322 

3,357 

b Called Lance in log furnished by railway company. c Called Pierre in log furnished by railway company. dIn view of the lithologic similarity between the Claggett and Colorado the position of this boundary must be regarded as uncertain. e Called Niobrara in log furnished by railway company. · 
14 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1919. 

CRETACEOUS SYSTEM. 

KOOTENAI (1) FORMATION. 

That part of the Kootenai ( ~) penetrated by 

the drill in the V ananda well consists in descending order of 40 feet of sandstone, 19 feet 

of limestone, and 98 feet of red and white shale 

in alternate layers. There is no information 

available as to whether these beds are of fresh 

water or marine origin; in lithologic aspect, 

however the beds, especially the red and white ' . shales, are similar to the fresh-water Kootenai 

(Lower Cretaceous) of the Great Falls field. 

In that field the Kootenai as described by 

Fisher 1 is 400 to 500 feet thick and consists 

predominantly of clay, with thin intercalated 

beds of sandstone and limestone, but contains 

valuable beds of coal in the lower part and a 

basal sandstone from which building stone is 

obtained. 

COLORADO SHALE. 

The Colorado shale is the oldest formation 

exposed in this area. It consists of dark-gray 

to black fissile shale, with some thin bands of 

sandstone and sandy shale in its upper part, 

and according to the record of the V ananda 

well has more or less sandstone and a thin bed 

of limestone at the base. The shale contains 

large calcareous concretions in which fossils 

occur· some of the sandstone bands are also ' 

therefore, on purely paleontologic grounds to 

separate the Color!ldo of this area into the 

Benton and Niobrara, as·in Nebraska, Colorado, 

South Dakota, and Wyoming. 

In the easternmost areas iust mentioned the 

Niobrara formation consists . chiefly of limestone; farther west it is a calpareous buff shale.; 

and apparently still farther west it loses its 

calcareous aspect, becoming a typical clay 

shale that is somewhat arenaceous. This is the 

sort of change to be expected in a formation of 

this ·kind when traced toward the source of its 

constituent materials. 

The Colorado differs from the overlying 

Claggett in being darker and more fissile and in 

becoming only slightly plastic when wet; because of this last characteristic the local term 

"gumbo" is not generally applied to it. 

The Colorado occupies an area of about 350 

square miles in the central part of the Porcupine dome, which lies principally west and 

south of the eastern part of this area. It also 

occupies another small dome exte·nding westward from a point a few miles east of Musselshell River. Except in the small area west of 

the Musselshell, where the Eagle can be recognized, the Colorado and Claggett have not been 

differentiated on the accompanying map. 

EAGLE SANDSTONE. 

distinctly fossiliferous. The formation has a The Eagle, which forms a prominent and 

total thickness, as interpreted from the log of persistent lithologic unit farther w~st, is-re_rrethe Vananda well, of -about 2,300 feet. It sented in this area by a buff medium-grained 

probably includes the time equiv~lents of bot.h sandstone consisting of quartz, feldspar, and 

the Benton and Niobrara formatwns. In this black chert with a small amount of mica inpart of Montana there appears to be no recog- closed in a calcareous matrix. It ranges from 

nizable lithologic distinction between these two a knife-edge to about 20 feet in thickness. The 

formations, and they are therefore grouped sandstone is recognizable as a distinct unit at 

together under the name Colorado shale. It the western margin of the area, from which 

may be, however, that detailed paleontologic its outcrop has been traced northwestward and 

work would show the existence of a Niobrara southwestward for many miles. In these mote 

fauna in the upper part of the Colorado shale, western areas the formation ranges from 200 to 

even though no lithologic distinction exists. 300 feet in thickness, but it thins eastward, so 

~ed 2 and Stanton and Hatcher 3 have h t 'd f T 15 N R 29 E o::; that near t e eas si e o . ., . <~., 

already pointed out a general resemblance be- where the sandstone is exposed in a clean-cut 

tween the fauna of the upper part of the Colo- face it is only 21 feet thick, with marine shale 

rado in the vicinity of Fort Benton and the both above and below. From this place the 

fauna of the Niobrara. It might be possible, formation can be traced eastward for about 2 

1 Fisher, c. A., Geology of the Great Falls coal field, Mont.: u.s. miles and in that distance the sandstone is seen 

Geol. survey Bull. 356, pp. 22, 30-35, 1909. . to be~ome more and more in terlamina ted with 2 Weed, w. H., u. s. Geol. Survey Geol. Atlas, Fort Benton folio . . l t Th 

(No. 55), p. 2, 1899. 1 thin bands of shale In Its ower par . e 3 Stanton, T. w., and Hatcher, J. B., Geology and paleontology of the I sandstone passes under an old river terrace 

Judith River beds: U. S. Geol. Survey Bull. 257, pp. 63, 64, 1905. 
GRADATIONS FROM CONTINENTAL TO MARINE CONDITIONS IN CENTRAL MONTANA. 15 

about halfway between the western margin of 

the area and Musselshell River and is not exposed farther east, but at the Musselshell, 

where there is a clean-cut exposure from the 

Lance down in to the Colorado, the Eagle is 

either lacking or is represented by a sandstone 

3 feet . thick. 

The pinching out of the Eagle is not due to 

an unconformity but to the seaward thinning 

of a shoal deposit laid down during a recessional stage of the Cretaceous sea and later 

covered by an off -shore deposit as the sea 

deepened and the shore line was shifted farther 

west. 

CLAGGETT FORMATION. 

The Claggett consists of 600 to 700 feet of 

dark-gray shale containing numerous calcareous 

concretions in which invertebrate fossils occur. 

The shale is but slightly consolidated and 

becomes very plastic when wet, so that it is 

commonly referred to as "gumbo." Crystals 

of selenite are more or less abundant in the 

shale and in places accumulate in considerable 

quantities on weathered slopes. At the top of 

the formation there is a zone that is especially 

prolific in large calcareous concretions, many 

of which show a well-developed cone-in-cone 

structure. This feature, so far as the writer is 

aware, is peculiar to the Claggett throughout a 

large part of Montana and does not occur in 

either the Colorado or the Bearpaw. It therefore constitutes one criterion for the recognition of the Claggett in central Montana. 

• JUDITH RIVER FORMATION. 

massive brownish, poorly cemented sandstone, 

a middle division of ash-gray shale, and an 

upper division of grayish-white to brown massive to heavy-bedded sandstone. The total 

thickness of this facies of the formation is 

about 185 feet. 

The marine fiacies likewise has a threefold 

character which corresponds in every way to 

the fresh-water divisions except that the middle 

shale member is darker. The upper sandstone 

is of a light-brown color in the eastern part of 

the field but farther west changes to light gray · 

or dirty white; the lower sandstone is prevailingly brown on weathered •surfaces. The 

marine facies has a thickness of about 125 feet 

near the head of Big Porcupine Creek; it is 

reported as 200 feet thick in the V ananda well 

and may be thicker than that, as it is believed 

that the mouth of the well is below the top of 

the formation; but on the east side of the field 

the thickness probably does not exceed 100 

feet. 

Detailed sections of these two facies are 

given below. 

Section of Judith River formation (marine) in sec. 8, T. 12 N., 

R. ssE. 

Feet. 

Sandstone, massive, yellow to brown (marine shells 

and Halymenites) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

Talus, sandy, probably sandstone................... 30 

Shale, gray to yellow...... . . . . . . . . . . . . . . . . . . . . . . . . . 20 

Sandstone, massive, gray at top and full of Halymenites, lower part not well exposed.................. 45 

115 

Section of Judith River formation (fresh-water) on west bank 

of Musselshell River, in sec. 16, T. 15 N., R. 80 E. 

Character· and thickness.-The Judith River Sh 1 B f · . . a e, earpaw. Feet. 

ormatwn, as originally defined, 1 includes fresh Sandstone....................................... 29 

and brackish water beds lying above the marine 1 Shale, light gray................................. 15 

Claggett and below the Bearpaw. This defini- Shale, carbonaceous ................ :. . . . . . . . . . . . . 8~ 

tion was given before the full extent of the Sandstone.···············-············------···· 8 

formation was known. The work of the season Shale, gray; carbonaceous in upper part........... 13 

Sandstone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

of 1918 has shown that in the area described Shale. ___ ............. _. . . . . . . . . . . . . . . . . . . . . . . . 35 

the fresh-water deposits when traced eastward Sandstone....................................... 13 

grade into marine deposits of the same litho- Shale, light gray................................. 15 

logic character. In this paper the term Judith Sandstone .... ··································· 2 

River is used to include both the fresh-water Shale.·····································--·· 3 

deposits exposed in the western part of the ~!:~e;t~:~~~~~~~~~---_·_· _" ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ area and their equivalent marine deposits in Shale, Claggett, dark gray, sandy at top, transition. 

the eastern part. 189t 

The fresh-water facies may be considered as Composition.-As revealed by a study of 

made up of three parts-a lower division of thin sections the sandstones of both the freshwater and marine facies of "he formation are IStanton, T. W., and Hatcher, J. B., Geology and paleontology of . . • . . 

the Judith River beds: u. s. Geol. Survey Bull. 257, pp. 33-34, 1905. 1 as s1m1lar In miCroscopiC appearance as they 
16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1919. 

are in outward physical appearance. They 

are arkosic and consist of angular, subangular, and rounded grains of orthoclase, plagioclase, quartz, and black chert, with small 

amounts of muscovite and biotite, inclosed in 

a matrix or cement of calcite, which is more 

or less stained with iron oxide. Grains of 

limestone are numerous in one specimen of 

fresh-water origin and are rather well rounded; 

but they are not observed however in any of 

the other specimens examined~ A rather 

surprising feature is the slight alteration of a 

considerable proportion of the feldspar. Many 

of the grains, especially of the plagioclase 

variety, are perfectly fresh and clear and 

show no sign of kaolinization. In the sections examined the calcite constitutes 50 per 

cent or more of the bulk of the rock; of the 

granular material feldspar is in general the most 

abundant, followed by black chert and quartz, 

which vary in amount, in some specimens 

the one and in some the other predominating. 

Another interesting feature is the small proportion of well-rounded grains. The grains of 

chert and quartz are subangular to rounded, 

whereas those of feldspar are predominantly 

angular and give to the thin sections the 

appearance of being made up largely of angular to sub angular fragments. This marked 

angularity does not accord well with the 

highly assorted condition of the rna terial, 

which is remarkably uniform in size and free 

from silt or fine particles. This highly assorted condition suggests considerable agitation, either by waves or by currents, whereas 

the angularity of the grains suggests but a 

moderate amount of abrasion. It is probably 

to be attributed to the cleavage of the feldspar 

and the smallness of the grains rather than to 

the amount of abrasion of the particles. The 

texture is fine and varies somewhat in the 

different specimens, the grains ranging from 

about 0.075 to 0.2 millimeter in average 

diameter. 

Relations.-The Judith River is conformable with both the underlying and overlying 

formations. In the western part of the area 

the fresh-water facies overlies about 600 feet 

of the Claggett, including a sandy transition 

zone about 35 feet thick, and is in turn overlain 

by 1,000 to 1,100 feet of Bearpaw shale. The 

marine facies overlies 700 feet (Vananda well 

record) of the Claggett and is overlain by 

about 900 feet of the Bearpaw. In other 

words, the marine and fresh-water facies are 

at the same stratigraphic horizon, and the 

overlying shale is a unit from the locality of 

the one to that of the other. The marine 

phase represents the seaward extension of the 

continental deposits and marks the maximum 

retreat of the sea during this stage of minor 

oscillation. 

BEARPAW SHALE. 

The Bearpaw shale includes the marine 

shale lying between the Judith River and 

Lance formations. It consists chiefly of darkgray shale that is similar in lithologic character 

to the shale of the Claggett except that so far 

as observed it contains no concretions showing cone-in-cone structure. In the upper part 

of the formation there is a sandy zone 25 feet 

or more thick which consists of sandy shale 

and some thin sandstones forming a transition 

to the Lance. This is probably the representative of the sandy shale from which, on Hell 

Creek, 35 or 40 miles farther north, Barnum 

Brown obtained a Fox Hills fauna and which 

he therefore designated the Fox Hills formation.1 A small collection of marine fossils, 

all representing a single species of Lingula, 

was obtained by the writer in a sandstone that 

caps a small outlier in the SW. l- sec. 1, T. 14 

N., R. 32 E. This sandstone is probably near 

the top of the ~ransition zone just described. 

The species of Lingula obtained is not sufficiently diagnostic for purposes of close time 

correlation. It therefore does not establish 

the existence of the Fox Hills at this place, 

but it indicates that the sandy transition zone 

at the top of the Bearpaw is marine. · 

The thickness of the Bearpa w ranges from 

1,000 or 1,100 feet in the western part of the 

area to about 900 feet in the eastern part. As 

determined by the fossils which occur in it, the 

formation is of Montana age and is equivalent 

to a part of the Pierre and possibly includes 

also the representative of the Fox Hills. Its 

correlation in this area is rendered certain not 

only by its included fossils but by its continuity 

with the undoubted Bearpaw of other regions. 

1 Brown, Barnum, The Hell Creek beds of the Upper Cretaceous of 

Montana: Am. Mus. Nat. Hist. Bull., vol. 23, p. 829, 1907. 
GRADATIONS FROM CONTINENTAL TO MARINE CONDITIONS IN CENTRAL MONT AN A. 17 

TERTIARY ( ~) SYSTEM. 

LANCE FORMATION. 

Section of a part of the Lance formation exposed in Table 

Butte, in theSE. t sec. 32, T. 7 N., R. 41 E. 

Feet. 

Sandstone, massive, cross bedded, containing clay Character and thickness.-The section given balls ... _______________ _________________________ -

25 

below is fairly representative of the lower part Shale, becoming sandy toward top................. 80 

of the Lance formation, the only part exten- Shale, carbonaceous ....... _ ................... :.... 1 

sively represented in this area. This part con- Shale, yellow to drab ............. -. __ ..... _,_....... 70 

sists mainly of brownish massive to heavy- Sandstone, ma~sive, buff; contains bone fragments and 

bedded sandstone and interbedded light-gray to : small scattermg pebbles.·····--·······--------·· 30 

Shale, buff...................... . ................. 20 

dark-gray shale. Some of the sandstone layers Sandstone, massive, brown. ___ .... ______ . ___ ...... 10 

are well consolidated and form protecting caps "Somber"andyellowclayshale,alternating ......... 60 

to the softer underlying beds. Large rustybrown loglike concretions are common. Though 

brown is the prevailing color, some of these 

sandstones are dirty white to gray. Ripple 

marks and cross-bedding are common structural features. 

The interbedded shale is light-gray to leadgray and contains some carbonaceous members, 

though these are rare. The shale is unconsolidated and weathers rapidly where not protected by the more resistant sandstone. Thin 

layers of impure fossiliferous limestone occur 

in a very few places. 

In the northeast corner of the area the beds 

above described are overlain by a zone consisting chiefly of "somber" gray shale with 

interbedded layers of carbonaceous shale and 

yellow and brown sandstone. Some of the 

sandstone layers, which are as much as 20 feet 

thick, are well consolidated and resemble in 

every way the sandstones in the lower part of 

the formation. 

There is no abrupt change between this member arid the lower member described above. 

They seem to represent continuous deposition 

and a gradual change from the conditions 

favoring the accumulation of sandstone to those 

favoring the accumulation of shale. This shale 

member, although not definitely included in the 

Lebo shale member of the Little Sheep Mountain field, seems to be identical in character with 

it. So far as the writer was able to ascertain, 

this member pinches out or at least thins 

markedly farther west; however, more detailed 

work on the overlying beds and a careful study 

of the extent and character of the various 

phases are necessary in order to establish their 

continuity and determine their relations. 

296 

Oomposition.-The sandstones of the Lance, 

including . those interstratified in the darkshale member described above, as well as 

those which make up the bulk of the lower 

distinctly sandy portion of the formation, are 

arkosic and are shown on microscopic examination to consist of the same material as the 

sandstones of the Judith River formation, 

the only difference being the higher degree of 

oxidation of the material of the Lance. In 

all the thin sections examined the principal 

granular minerals present are feldspar, chert, 

quartz, muscovite, and biotite, named in the 

order of their abundance; these are embedded 

in a matrix of calcium carbonate, which constitutes 50 per cent or more of the bulk of the 

rock. Grains of limestone are also sparingly 

present in some specimens. The feldspar, 

including orthoclase, plagioclase, and microcline, is commonly the predominant granular 

mineral. Many of the grains are comparatively fresh and unaltered and show cleavage 

or crystal boundaries and beautifully marked 

striations; others are more or less-cloudy and 

opaque from the development of kaolin. 

Most of them are angular or subanglilar, 

well-rounded grains being exceptional. The 

chert, which in the hand specimen is gray 

or black, is invariably present. In the thin 

section it is clear and transparent and exhibits 

a cryptocrystalline texture. It is variable 

in amount, being in some specimens almost 

as abundant as the feldspar but in others 

less abundant than the quartz. The grains 

of chert show ·a· much more marked tendency 

toward rounding than those of feldspar. 
18 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1919. 

The quartz occurs in subangular to rounded' 

grains. It is variable in amount, being almost 

wholly absent in one specimen and in others 

consti_tuting 25 per cent or more of the granular 

material. Muscovite is present in notable 

amount and is easily recognized in the hand 

specimens; biotite is also sparingly present. 

The calcite, which constitutes the matrix of 

the rock, shows a finely crystalline texture. 

It is commonly more or less stained and 

obscured by limonitic material, which in 

most of the spec~mens also occurs to a greater 

or less degree in distinct grains. Like the 

sandstones of the Judith River, those of the 

Lance are medium grained, and their grains 

are of fairly uniform size, showing a marked 

degree of assortment. 

Age and correlation.-Paleontologists are 

not agreed as to the age of the Lance formation, the invertebrate and vertebrate fossils 

being regarded as related to Cretaceous types, 

whereas the plants are regarded as of Fort 

Union age. Under these circumstances the 

~eological Survey has only provisionally assigned the Lance ·to the Tertiary system. 

The formation is identified in this area 

and its relations are established by (1) 

the finding of Triceratops remains near the 

base of the formatio.n at Castl-e Butte, northwest of Forsyth, and the occurrence in it 

of many fragments of dinosaur bones in other 

parts of the area; (2) its stratigraphic position 

between the Bearpaw shale and the Fort 

Union formation; and (3) its continuity with 

beds that undoubtedly belong to the Lance 

in other parts of Montana. 

Distribution and surface features.-The 

Lance formation occurs at the southern ' eastern, and northern margins of this area 

and extends beyond the area in each of these 

directions. In general it occupies a zone from 

1 mile to several miles broad, its width depending on the attitude of the beds and the stage 

of erosion. 

The outcrop of the formation has a marked 

effect on land forms. Its lower part produces 

a prominent ridge which rises abruptly, in 

some places almost precipitously, from the 

lower areas occupied by the Bearpaw. From 

the margin it extends as a dissected upland 

studded with buttes, pinnacles, and other 

features characteristic of the weathering of 

moderately consolidated beds in a semiarid 

regwn. 

STRUCTURE. 

In its broad general relations the structure 

in this part of Montana is an tielinal and represents the southeastward extension of the 

Big Snowy anticline. More locally the eastern part of the area occupies the eastern and 

no~thern flanks of the Porcupine dome, around 

whiCh the beds have a quaquaversal dip and 

are therefore in this area inclined southward ' eastward, and northward. As shown on the 

accompanying map, the dips in this part of 

the area range from 1° or 2° to 40°. This 

dome is separated by a broad, shallow transverse syncline from a pronounced anticline in 

the vicinity of Musselshell River. On the 

west side of the syncline the strata dip 15°-

25 ° E.; on the east side the westward dip is 

not more than 2 ° or 3 °. West of this cross 

syncline the Big Snowy anticline is the dominant structural feature. Around the point of 

this anticline the Judith River formation dips 

from 5° on the south limb to 45° on the north. 

~hese steep dips do not continue for any considerable horizontal distance in the direction 

of the dip, as the overlying Lance rarely shows 

dips of more than 10° along its border and 

within half a mile lies essentially flat. It is a 

noteworthy fact throughout the area that 

the steeply dipping beds become approximately 

horizontal within short distances. The features above described are shown graphically by 

the structure sections along the lines ABC and 

EF on Plate IV. 

Besides the main structural features outlined above there are local irregularities, such 

as the anticlines in T. 14 N., Rs. 32 and 33, 

37, and 38 E. 

SOURCE OF THE SAND. 

The feature that most impresses one who 

studies the sandstones of the Lance and Judith 

River, and, so far as can be judged by a microscopic examination, this applies also to the 

Eagle sandstone, is their identitv in mineral 

composition and the relative abundance of 

the predominant mip,erals. The abundance of 

feldspar, the invariable occurrence of black 

chert, commonly in considerable amovnt, together with quartz and mica, and the almost 

complete absence of other granular constituents strongly suggest derivation either from a 

common source or at least from rock of 

similar composition, and the uniformity in 
GRADATIONS FROM CONTINENTAL TO MARINE CONDITIONS IN CENTRAL MONT AN A. 19 

texture and assortment certainly points to 

similar physiographic conditions of accumulation. This striking resemblance is still more 

remarkable in view of the fact that the formations are separated from one another by 

several hundred feet of marine shale. The 

conclusion is inevitable that the oscillations 

which give rise to the alternate deposits of 

shale and sandstone during the Upper Cretaceous were of a gentle, rhythmic type. 

This conclusion at once raises a question as 

to the age of the Rocky Mountain orogenic 

disturbance. The petrographic evidence may 

ultimately have more weight in deciding this 

question than has yet been assigned to it. At 

present this evidence seems to warrant the 

belief that there was no great physical break 

between the deposition of the formations underlying the Lance and that of the Lance and 

to corroborate the evidence afforded by the 

attitude of the Lance and Fort Union formations, namely, that the present Rocky Mountains are of post-Fort Union rather than preLance age. 

As to the probable source of the material, it 

is hardly profitable in our present state of 

lmowledge to speculate further than to say 

that it was evidently derived from the land 

mass to the west that is supposed to have been 

in existence during the Cretaceous period. It 

would yield interesting data, however, if 

observations of this character were extended 

over the entire region involved, and it might 

then be possible to form more definite conceptions, not only as to the source of the material, but also as to the conditions of its accumulation. 

UPPER CRETACEOUS HISTORY. 

Variations in the character of sediments involving changes from sandstone to shale and 

from marine to fresh-water deposits imply cor-

'responding changes in physiographic conditions 

during the period of their accumulation and 

therefore furnish a basis for the interpretation 

of the past history of the region in which they 

occur. 

Such changes occurred in north-central Montana during the Upper Cretaceous epoch. The 

area herein described happens to be favorably 

situated in that it furnished more exact evidence than has heretofore been available as to 

the magnitude of some of these oscillations of 

the sea and as to the extent and character of 

the formations involved. 

An understanding of these conditions will be 

facilitated by a study of the diagrammatic 

section on Plate IV which extends from Black 

Butte, at the north end of the Judith Mountains, about 48 miles west of this area, southeastward to the longitude of Glendive, a total 

distance of about 210 miles. Section A is 

compiled from measurements made by W. R. 

Calvert in the Lewiston field and those made 

by the writer in 1912; sections B, F, and G are 

compiled from data collected by the writer 

while examining the area herein described; and 

information regarding conditions at Glendive 

is based on the record of a well put down near 

that place by theN orthern Pacific Railway Co. 

It is not possible to plot this record because 

the exact horizon of the mouth of the well is 

not known. The well was begun, however, 

somewhere near the top of the Pierre shale and 

continued to a depth of 2, 710 feet, which would 

bring it near the base of the Colorado shale. 

Upper Cretaceous tin1e seems to have been 

ushered in by a widespread transgression of the 

sea extending westward beyond the eastern 

front of the present Rocky Mountains. During 

the Colorado epoch great quantities of mud 

and silt were spread out over the floor of this 

epicontinental sea, and its waters teemed with 

life, the remains of which were in part entombed 

in the accumulating muds that now constitute 

the Colorado shale. The fine texture and 

homogeneous character of this shale and its 

freedom from notable amounts of sandy material, except perhaps near its base, indicate 

that it was laid down under very uniform conditions. This implies that the relative elevations of land and sea must have remained 

nearly constant throughout the Colorado epoch. 

So the sea was filled to a depth of more than 

2,300 feet by the accumulated mud and as the 

land surface was necessarily lowered by the 

removal of this material, it seems necessary to 

infer that the relations between land and sea 

level could not have remained constant unless 

the sea floor was being gradually depressed at 

about the same rate as that at which the 

sediments accumulated. 

This gradual submergence was arrested and 

the Colorado epoch was brought to a close in 

the western part of the epicontinental sea by 

an uplift of the western land mass. As a result 
20 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1918. 

of this uplift the sea became shallow and the Judith River formation contain the reprobably withdrew from local areas, and the mains of land and fresh-water vertebrates and 

accelerated streams brought down great quan- invertebrates, leaves, stems, trunks of trees, 

tities of sand, which were spread over the shal- and beds of lignite; whereas east of point E 

low sea floor and now constitute the Eagle these deposits contain the remains of marine 

sandstone. As the uplift progressed the sea organisms. 

withdrew from its former western shore to a How far eastward this Judith River uplift 

point, in the latitude represented by the dia- was effective is not known. Sections A toG in 

grammatic section on Plate IV, a few miles Plate IV show that the formation gradually 

east of section A, for from that place westward thins from west to east, and apparently it 

the upper part of the Eagle sandstone contains either disappears or is represented by not 

thin beds of carbonaceous shale and lignite. more than 15 feet of sandstone at point H, in 

The distance seaward to which this uplift was the longitude of Glendive. The well drilled by 

effective in producing a change of sedimenta- the Northern Pacific Railway Co. jn sec. 20, 

tion is 2 or 3 miles east of section B, Plate IV, T. 14 N., R. 55 E., about 10 miles southwest of 

or about the present site of Musselshell River in Glendive, was begun near the top of the Pierre 

the western part of the area here discussed. shale and penetrated a bed of sandstone 15 

At that place the Eagle sandstone feathers out feet thick at a depth of 7 45 feet and another 

between the Colorado and Claggett formations bed 6 feet thick at a depth of 900 feet. The 

and thence eastward the deposition of shale remainder of the well log is given as shale down 

was continuous through the Colorado, Eagle, to a depth of 2,710 feet, or near the base of the 

and Claggett epochs. Colorado. It is evident, therefore, that in the 

Another submergence in the Claggett epoch longitude of Glendive conditions of sedimenrestored the sea over at least a part of the tation were practically uniform throughout 

area from which it had been withdrawn, and Colorado and Montana time and that the oscilthe deposition of shale was extended west- lations which gave rise to the Eagle and Judith 

ward, covering the Eagle sandstone in this River deposits to the west did not extend that 

part of Montana. This submergence did not far east. 

extend as far northwest, however, as that of the A renewed submergence brought the Judith 

Colorado epoch, because, as shown by Stebin- River epoch to a close and restored the sea 

ger/ the entire interval between the Colorado over the entire known area of the Judith River 

and Bearpaw epochs in the vicinity of the front formation. During this, the last widespread 

range of the Rocky Mountains was character- submergence, 1,000 feet or more of marine 

ized by the deposition of sediments that were shale was laid down, constituting the Bearp~w 

mainly of continental origin. formation. This submergence was followed by 

After 500 to 700 feet of marine shale had a widespread emergence that resulted in the 

been laid down another emergence set in ·which withdrawal of the sea from this part of the 

marked the beginning of the Judith River epoch. continent, which has since remained above sea 

This emergence was more widespread than level except in small areas. This uplift was 

that of the preceding Eagle epoch, and during probably of epeirogenic character, because the 

its progress the sea was withdrawn from the next succeeding deposits seem to have suffered 

area herein considered to about the longitude the same amount of orogenic deformation as 

of 107° 30'-that is, to some position between the underlying Bearpaw shale. 

the points indicated by D and E in the dia- On the lowlands produced during this uplift 

grammatic section on Plate IV. The clay and the continental sediments and associated coal 

sand not transported by the streams from the beds of the Lance and Fort Union were laiG. 

adjacent highlands were in part spread out as down apparently without interruption and 

continental deposits on the emerged sea floor under only such changes in conditions as reand in ·part carried out to the sea and depos- suited in the alternate deposition of sand and 

ited beneath its surface. Westward from mud and the more or less extensive accumulapoint B the resulting sandstones and shales of tion of vegetable growth. This period of depo-

. . · sition was ·brought to a close by a widespread I Stebmger, Eugene, The Montana group of northwestern Montana: I u.s. Geol. survey Prof. Paper 90, pp. 62-63, 1914. orogenic disturbance, accompanied by folding 
GRADATIONS FROM CONTINENTAL TO MARINE CONDITIONS IN CENTRAL MONTANA. 21 

and thrusting, which produced the principal 

tectonic features of the Rocky Mountain system. Since that time erosion has reduced the 

land surface to its present form, though it may 

be that between the epochs of deformation and 

erosion the Oligocene and Miocene deposits now 

found farther east were deposited over parts of 

this region as well. 

By way of summary it may be said that 

eastern Montana remained beneath the sea and 

was not affected by any oscillations of land and 

sea level throughout the Colorado and Montana 

epochs, whereas in the central part of the State, 

at least as far west as the front of the present 

Rocky Mountains, there were oscillations of 

land and sea level produced either by rhythmic 

gentle depressions of the sea floor or by uplifts 

of the land mass to the west. The first of these 

uplifts, which resulted in the deposition of the 

Eagle sandstone, was of the shortest duration 

and was effective in the area here described for 

only a few miles east of its western boundary. 

The uplift during the Judith River epoch was 

of longer duration, to judge by the thickness of 

material deposited, and was considerably more 

widespread. As a result o:f this uplift the sea 

withdrew to about the longitude of 107° 30' 

and the site of sedimentation was shifted to 

some point east of the longitude of Forsyth, 

perhaps nearly as far east as Glendive. Because of this change the ~[ontana group is in 

the central part of the State divisible into the 

Eagle, Claggett, Judith River, and Bearpaw 

formations but farther east consists of a single 

lithologic unit-the Pierre shale-to which a 

thin representative of the Fox Hills sandstone 

is added near the eastern boundary. In this 

eastern region also there is little if any lithologic . 

distinction between the shnles of the Colorado 

and Montana epochs.