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Geology of the Varney 

and Gameron Quadrangles, 

Madison County > Montana

GEOLOGICAL SURVEY BULLETIN 1459

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Geology of the Varney 

and Cameron Quadrangles, 

Madison County, Montana

By JARVIS B. HADLEY

With a chapter on PALEONTOLOGY AND CORRELATION 

OF THE MADISON GROUP ON BALDY MOUNTAIN

By WILLIAM J. SANDO and J. THOMAS DUTRO, Jr.

GEOLOGICAL S U R V E Y BULLETIN 1459

Description of the Paleozoic, Mesozoic, 

and Cenozoic stratigraphy and structure 

of the northern Gravelly Range, Montana

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1980
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Hadley, Jarvis Bardwell.

Geology of the Varney and Cameron quadrangles, Madison County, Montana.

(Bibliography: 110 p. Geological Survey bulletin ; 1459)

1. Geology Montana Madison Co. 2. Paleontology Montana Madison Co. 3.

Stratigraphic correlation Montana Madison Co. I. Sando, William Jasper, joint

author. II. Dutro, J. Thomas, joint author. III. Title. IV. Series: United

States. Geological Survey. Bulletin ; 1459.

QE75.B9 no. 1459 [QE134.M25] 557.3'08s 78-606068 [557.86'663]

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

Washington, D.C. 20402

Stock Number 024-001-03229-5
CONTENTS

Page

Abstract ____________________________________________ 1

Introduction __________________________________________________ 1

Acknowledgments __________________________________________ 2

Geologic setting ______________________________________ 2

Paleozoic rocks ___________ ______________________ _____________ 11

Flathead Sandstone and Wolsey Shale ___________________ 11

Meagher Limestone _______________________ ____ ____ _____ 12

Park Shale _________________________________ 15

Pilgrim Dolomite or Limestone ______ _____-- 16

Red Lion Formation and Bighorn Dolomite _____-________ 20

Jefferson Dolomite or Limestone ______ _____________ ___ 22

Three Forks Formation ___________________________________ 29

Madison Group _______________________________________ 31

Lodgepole Limestone _______________ ____ ______ _____ 31

Mission Canyon Limestone ___ ________ __ __ __ 32

Paleontology and correlation of the Madison Group on Baldy

Mountain, by William J. Sando and J. Thomas Dutro, Jr __ 33

Big Snowy Group and Amsden Formation _____________________ 46

Quadrant Quartzite or Formation ___________________ ________ 57

Shedhorn Sandstone _________________________________________ 58

Mesozoic rocks ________________________________________________ 62

Dinwoody Formation __________________________________ 62

Ellis Group ________________________________ 63

Morrison Formation _____________________________ 68

Kootenai Formation _______________________________ 69

Thermopolis Shale ______________________________ 72

Frontier Formation ________________________________________ 75

Upper Cretaceous volcanic and volcaniclastic rocks _____________ 78

Cenozoic rocks ________________________________ 81

Sphinx Conglomerate ______________________________________ 81

Tertiary volcanic rocks and gravel ________________________ 83

Lower Tertiary erosion surface and colluvial deposits __________ 88

Rhyolite ash-flow tuff ____________________________ 89

Freshwater limestone ______________________________________ 91

Quaternary deposits _________________________ _ _ 93

Glacial deposits _________________________________________ 93

Boulder deposits of uncertain origin _____________________ 96

Alluvial fans and gravel of the Madison Valley ____________ 97

Gravel of the Ruby River valley __________________ 98

Landslide deposits _________________________________ 99

m
IV CONTENTS

Page

Structural geology _______________________________ 100

Laramide structural features _______________________ 100

Cenozoic normal faults _____________________________ 103

Selected references _______________________________ 105

ILLUSTRATIONS

Page

FIGURE 1. Columnar sections of the Bighorn Dolomite, Red Lion Formation, and Pilgrim Dolomite or Limestone, Varney quadrangle _____________---________________ 18

2. Columnar sections of the Amsden Formation and underlying beds, Varney quadrangle __________ 48

3. Columnar sections of the Shedhorn Sandstone and Phosphoria Formation, Varney quadrangle _______ __ 60

4. Columnar sections of the Ellis Group and the upper part

of the Dinwoody Formation _______________________ 66

5. Columnar section showing position of numbered fossil collections from Cretaceous rocks, Varney quadrangle ____ 74

6. Photograph showing Tertiary volcanic rocks, Varney quadrangle ________________________________ 84

7. Photograph showing faulted rhyolite ash-flow tuff, Madison

River valley _________________________ 90

8. Photograph showing Tertiary limestone in lower canyon of

Wigwam Creek, Varney quadrangle ______________ 92

9. Map showing relation of the Greenhorn and Snowcrest

faults in the vicinity of Ruby River Canyon __________ 101

TABLES

TABLE 1. Stratigraphic units in the Varney quadrangle ___________

2. Stratigraphic units in the Cameron quadrangle ________

3. Invertebrate fossils collected from the Madison Group on 

Baldy Mountain __________________________

Page

3

7

40
GEOLOGY OF THE

VARNEY AND CAMERON QUADRANGLES, 

MADISON COUNTY, MONTANA

By JARVIS B. HADLEY*

ABSTRACT

In the Gravelly Range, west of the Madison Valley in southwestern Montana, Paleozoic rocks about 1,500 m thick range in age from Middle Cambrian to Permian; Mesozoic rocks about 1,400 m thick range in age from 

Early Triassic to Late Cretaceous. The lithology, thickness, and age of each 

of 21 mapped Paleozoic and Mesozoic units and the extent of several locally 

distributed Tertiary and Quaternary units are summarized.

Variations from the usual Paleozoic sequence in southwestern Montana 

show that 245 m of Middle and Upper Cambrian and Ordovician rocks were 

eroded from a local uplift in pre-Late Devonian time and that 200 m of Upper 

Mississippian rocks (Big Snowy Group or its equivalents), present in the 

western part of the area, are absent in the eastern part, probably because of 

uplift and erosion before deposition of the Amsden Formation.

The structure of the western part of the area is dominated by Laramide 

folds and thrust faults, including low-angle faults on which Precambrian 

and Paleozoic rocks have been carried several kilometers eastward over 

Upper Cretaceous rocks. The eastern part of the area is dominated by highangle faults associated with downfaulting of the adjacent Madison Valley in 

late Tertiary and Quaternary time. Tertiary rocks, mainly cobble gravel and 

various volcanic rocks probably of Eocene to Miocene age, were deposited on 

a surface now tilted northeastward from the crest of the Gravelly Range to 

near the present level of the Madison River. Farther south, the Madison 

Valley seems to have subsided mainly by faulting on both sides.

INTRODUCTION

The northern part of the Gravelly Range and adjacent areas 

were studied as a part of the U.S. Geological Survey's investigations ;of the structure and stratigraphy of the disturbed belt in 

western Montana. Geologic mapping of the Varney and Cameron 

quadrangles began in 1956 and was essentially complete by 1960. 

Final field checking of the geologic maps was completed in 1965. 

A preliminary report on the geology of the area was published

* Deceased.
2 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

as a part of the Billings Geological Society's symposium on the 

West Yellowstone-earthquake area (Hadley, 1960). The 1:62,500- 

scale maps of the two quadrangles, containing geologic structure 

sections, were published in 1969 (Hadley, 1969a, b).

This report, left in nearly completed manuscript form at the 

time of the writer's death in 1974, was revised and completed for 

publication by J. T. Dutro, Jr.

ACKNOWLEDGMENTS

The writer was assisted during the field-mapping phase of the 

work by G. H. Haddock, R. W. Kistler, Harold Masursky, and 

R. W. Swanson in 1956; J. J. Branco and E. H. McKee in 1957; 

P. W. Birkeland in 1958; W. D. Long in 1959; and E. J. Young 

in 1965.

This report has benefited from lively discussions in the field, 

and during the compilation stages, with M. R. Klepper, J. D. Love, 

W. J. McMannis, John Montagne, G. M. Richmond, G. D. Robinson, and I .J. Witkind, all of whom were deeply involved in similar 

work in nearby areas at the time.

The writer was visited in 1956, 1957, and 1958 by J. T. Dutro, 

Jr., and W. J. Sando, who measured the sections of the Madison 

Group and the Big Snowy-Amsden interval on Baldy Mountain. 

The results of their studies have been incorporated as a chapter 

in this report. W. A. Cobban helped with the field studies of the 

Cretaceous rocks and collected fossils from them in 1958, and 

R. W. Imlay gave similar assistance with the Jurassic strata in 

1959.

Fossils from other stratigraphic units were identified by R. W. 

Brown, W. A. Cobban, H. M. Duncan, J. T. Dutro, Jr., W. H. 

Hass, R. W. Imlay, Estella Leopold, A. R. Palmer, John Pojeta, 

Jr., W. J. Sando, N. J. Silberling, I. G. Sohn, D. W. Taylor, R. H. 

Tschudy, J. A. Wolfe, and E. L. Yochelson. Thanks are expressed 

to all these people for their insights into the age significance and 

environmental implications of the fossils.

GEOLOGIC SETTING

The stratigraphic sequences in the Varney and Cameron quadrangles are similar, but the two areas complement each other so 

as to provide a complete geologic history of the region (see tables 

1 and 2). For example, most of the Tertiary units are best developed in the Cameron quadrangle, and the thick sequence of 

Upper Cretaceous volcanic rocks is not present in the Varney
GEOLOGIC SETTING 3 

TABLE 1. Stratigraphic units in the Varney quadrangle

Thickness 

(meters)

Quaternary System:

Till and cobble gravel. 

Tertiary System:

Limestone fanglomerate, unconsolidated cobble gravel, basalt and 

basaltic andesite flows and associated tuff, fresh-water limestone 

and associated tuffaceous sediments, rhyolite ash-flow tuft0. 

Unconformity. 

Cretaceous System:

Upper Cretaceous Series:

Frontier Formation: Medium- to fine-grained sandstone and 

grayish-olive to brownish-gray mudstone, minor shale and 

a few thin lenticular limestone and coal beds. Coarser sandstone, irregularly bedded, forms units 1.5 to 9 m thick at 

approximately 60-m intervals. Abundant plant debris in 

lower part. A unit 60 to 90 m thick, about 60 m above the 

base of the formation, contains bentonitic or zeolitic claystone; sandstone containing reddish-orange heulandite; also 

thin beds of porcellanite interbedded with greenish-gray 

claystone and sandstone ____-____-__----_-_________-__-_ 900±

Lower Cretaceous Series:

Thermopolis Shale (110 m):

Upper sandstone member: Sandstone, gray mediumgrained, argillaceous, and somewhat calcareous, irregularly and discordantly thin- to thick-bedded _______ 15-25

Middle shale member: Silty clay shale, dark-gray, moderately fissile, thin-bedded, contains thin layers, lenses, and 

nodules of dark-gray iron-carbonate rock, and minor 

thin beds of quartzose sandstone ___________________ 60-80

Lower sandstone member: Sandstone, fine- to mediumgrained, quartzose, commonly quartzitic and rusty, thinbedded and commonly finely current-bedded with ripple 

marks. Interbeds of shale in upper and lower parts, 

abundant worm burrows in upper part ______________ 12-15

Kootenai Formation (120-170 m) :

Shale member: Light- to moderate-gray silty clay shale, 

variably fissile, increasingly sandy toward top ________ 12-23

Limestone member: Limestone, microcrystalline to finegrained, commonly oolitic, medium-bedded (5-60 cm), 

thin shale beds in upper part. Several beds are crowded 

with small gastropods _____________________________ 17

Claystone member: Claystone; lower part red, containing 

minor beds of argillaceous limestone and limestone 

pebble conglomerate; upper part gray, containing large 

nodules of olive- to reddish-gray, very fine grained 

limestone ___________________________ 80-90 

Basal sandstone member: Light-gray, medium- to coarsegrained argillaceous sandstone containing abundant 

chert grains, locally pebbly, or chert-pebble conglomerate, irregularly bedded, lenticular ________________ 4.5-60
4 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT. 

TABLE 1. Stratigraphic units in the Varney quadrangle Continued

Thickness 

(meters)

Unconformity. 

Jurassic System:

Upper Jurassic Series:

Morrison Formation: Silty claystone or mudstone, gray, green, 

maroon, olive-brown, or brownish-black; upper part light to 

moderate gray and carbonaceous, containing minor sandstone, mostly fine grained, but locally like overlying unit __ 60-80 

Unconformity. 

Middle and Upper Jurassic Series:

Ellis Group: Mainly limestone, medium- to thick-bedded, 

commonly sandy, calcarenitic, or quartzose; indistinct fine 

current-bedding; interbeds of silty shale and argillaceous 

limestone; uppermost beds finely oolitic. About half a meter 

of limestone- and chert-pebble conglomerate locally at the 

base of the unit _____________________________________ 40-45

Disconformity. 

Triassic System:

Lower Triassic Series:

Dinwoody Formation: Dolomite, shale, and sandstone; 

lower part is thin-bedded yellow-gray dolomite interbedded with silty or finely sandy shale; upper part is 

medium-bedded yellowish-gray dolomite thin- to thickbedded, commonly finely to coarsely current-bedded ______ 70-115

Permian System:

Shedhorn Sandstone: Mostly sandstone, fine- to mediumgrained, gray, medium- to thin-bedded; subordinate chert, 

thin-bedded, containing shaly partings, to massive, containing remnants of partly replaced sandstone; dolomite, 

light-gray, fine-grained, cherty, medium-bedded; minor 

dark-gray phosphatic shale ___________________________ 60-72

Pennsylvanian System: ,

Quadrant Quartzite: Quartz sandstone, fine-grained, white, 

yellow, or orange, generally calcareous but locally quartzitic, indistinctly thick-bedded, locally coarsely crossbedded; 

lower part is locally a breccia containing abundant fragments of light-gray compact dolomite in a fine sandstone 

matrix _____________________________ 80-150

Pennsylvanian and Mississippian Systems:

Amsden Formation: Limestone and dolomite, gray, yellow, 

or pink, cherty, in part argillaceous, medium-bedded, moderately fossiliferous; minor beds of gray, red, or purple 

shale in lower part; a basal unit, 6 to 45 m thick, of red 

or orange calcareous mudstone or fine quartzose sandstone, has a few lenses of limestone- and chert-pebble 

conglomerate containing shell fragments and fish teeth __ 50-75 

Unconformity. 

Mississippian System:

Upper Mississippian Series:

Big Snowy Group, undivided: Red and gray calcareous 

shale, siltstone, and sandstone overlain by limestone, fineto medium-grained to coarsely bioclastic, medium-
GEOLOGIC SETTING 5

TABLE 1. Stratigraphic units in the Varney quadrangle Continued

Thickness 

(meters)

Mississippian System Continued

Upper Mississippian Series Continued

to thick-bedded, commonly very fossiliferous; locally darkgray nodular chert and beds of dark-gray fissile shale; 

absent east of Greenhorn fault ________________________ 200

Lower and Upper Mississippian Series:

Mission Canyon Limestone: Limestone, microcrystalline to 

medium-grained or coarsely bioclastic, locally oolitic, generally thick-bedded (0.6-9 m) ; locally dolomite or dolomitic limestone; weathers light gray; large pockets of 

solution breccia in upper part ________________________ 290-380

Lower Mississippian Series:

Lodgepole Limestone: Limestone, medium-bedded (8-30 

cm), in part finely current-bedded containing partings and 

beds of yellowish-brown shale, highly fossiliferous; commonly contains a 9-m unit of thick-bedded, medium- to 

coarse-grained limestone about 30 m below top _________ 190-240

Mississippian and Devonian Systems:

Three Forks Formation (70-90 m) :

Sappington Member: Argillaceous sandstone and sandy 

siltstone, yellowish-orange, calcareous, and indistinctly bedded, grading to shale below _____________ 20-25

Trident Member: Dark fossiliferous shale and interbedded limestone ______________________ 23-28

Logan Gulch Member: Olive-gray limestone, 3 to 21 m, 

microcrystalline to very fine grained, thick-bedded, 

commonly brecciated; underlain by dolomite and limestone breccia that has abundant red argillaceous 

matrix, 15 to 30 m, nonbedded; probably an evaporite 

breccia. 

Devonian System:

Upper Devonian Series:

Jefferson Dolomite: Dolomite, medium-grained, moderate to 

dark-yellowish-brown, thick-bedded, vuggy, and petroliferous; interbedded with dolomite, compact, finely crystalline, moderate yellowish-brown to yellowish-orange, thinbedded, locally current-bedded; minor shale and dolomite 

breccia; some limestone in lower 30 m _________________ 110-115

Unconformity. 

Ordovician System:

Bighorn Dolomite: Dolomite, light-gray to white, coarsely

mottled, thick-bedded; absent east of Greenhorn fault ___ 18-43 

Cambrian System:

Upper Cambrian Series:

Red Lion Formation: Basal red and green shale overlain by 

dolomite, silty or finely sandy, quartzose, thin-bedded and 

current-bedded, containing prominent partings and beds 

of red, yellow, green, or purple shale; many beds of flat-
6 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT. 

TABLE 1. Stratigraphic units in the Varney quadrangle Continued

Thickness 

(meters)

Cambrian System Continued

Upper Cambrian Series Continued

pebble conglomerate; some fine-grained quartz sandstone

in lower part; absent east of Greenhorn fault __________ 34-58

Pilgrim Dolomite: Mostly dolomite, fine-grained, moderate- to pale yellowish-gray; lower part thin-bedded and 

containing many greenish or reddish shaly partings, 

minor silty shale, and intraformational conglomerate; 

upper part mostly thick- to medium-bedded, fewer partings, uppermost 6 to 18 m consisting of sandy dolomite 

containing lenses and thin beds of fine-grained white or 

pink quartz sandstone; absent east of Greenhorn fault __ 110-130 

Middle Cambrian Series:

Park Shale: Green or red, fissile, noncalcareous shale, 

minor thin beds of limestone and intraformational limestone conglomerate; upper part red, silty, and calcareous; 

absent east of Greenhorn fault _______________________ 46-53

Meagher Limestone: Mostly dolomite, mottled light- to moderate-yellowish-brown, fine- to medium-grained, thin- to 

thick-bedded, locally weakly petroliferous; upper and 

lower parts locally are limestone, olive-gray, mottled yellowish-orange, or red, thin-bedded, shaly partings ______ 110-120

Wolsey Shale: Green fissile shale and sandy shale containing thin beds of glauconitic sandstone; upper part is 

yellowish-brown or buff, fine-grained, highly calcareous 

sandstone grading to limestone of overlying unit ______ 15-60

Flathead Quartzite: Yellow, orange, red, or white calcareous 

to quartzitic or hematitic sandstone medium-bedded and 

current-bedded, containing partings and thin beds of finegrained sandstone or greenish-gray sandy shale ________ 30

Unconformity. 

Precambrian rocks:

Gneiss, schist, dolomite, marble, quartzite, amphibolite, phyllite, granite, pegmatite, and mafic intrusive rocks.
GEOLOGIC SETTING 7 

TABLE 2. Stratigraphic units in the Cameron quadrangle

Thickness 

(meters)

Quaternary System: 

Pleistocene:

Till, cobble gravel, and colluvial deposits. 

Tertiary System: 

Pliocene (?).

Limestone: Pale-yellowish-gray, porous, and poorly bedded limestone; contains fresh-water pelecypods and 

algae ___________________________ 15 

Rhyolite ash-flow tuff: Compact to welded rhyolite 

crystal tuff and poorly consolidated tuff; locally 

contains fragments of Precambrian rocks __________ 18

Eocene:

Andesite and basalt flows: Compact to vesicular olivine, 

andesite, and basalt ______________________ 30

Gravel and tuff: Pebble and cobble gravel containing 

cobbles and small boulders of Precambrian rocks and 

sparse fragments of Paleozoic limestone, indurated 

tuff, and fossil wood; overlain locally by weakly indurated tuff _____________________________ 45

Sphinx Conglomerate: Coarse limestone fanglomerate 

that has silty calcareous matrix __________________ 300

Unconformity. 

Cretaceous System:

Upper Cretaceous Series:

Volcanic conglomerate: Well-rounded pebbles, cobbles, 

boulders of various volcanic rocks and sparse pebbles 

of chert, jasper, mudstone, sandstone, and quartzite 

in volcanic sandstone matrix. Subordinate volcanic 

breccia and few beds of volcanic sandstone or mudstone _____________________________ 180

Volcanic breccia: Polylithic breccia of various andesitic rocks, clasts generally 5 to 30 cm in matrix of 

similar composition, nonbedded. Includes some monolithic breccia and volcanic conglomerate, also a few 

thin andesite flows ________________________ 300

Sandstone unit: Volcanic sandstone and conglomerate 

containing rare plant-bearing layers, medium- to 

dark-greenish gray, medium- to very coarse-grained, 

thin- to thick-bedded and locally current-bedded. Conglomerate beds 0.6 m to 9 m thick contain wellrounded pebbles of chert, quartzite, and sparse volcanic rocks, mostly 2.5 to 5 cm in diameter. Unit 

gradational with underlying unit over a few tens of 

meters ___________________________ 90-150

Frontier Formation and Colorado Group undivided: 

Shale, mudstone, calcareous sandstone, and minor 

sandy limestone. Sandstone, light-gray, thick- to thinbedded, commonly lenticular, locally coarsely cross-
8 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

TABLE 2. Stratigraphic units in the Cameron quadrangle Continued

' Thickness 

(meters)

Cretaceous System Continued

Upper Cretaceous Series Continued

bedded. Shale and mudstone, medium- to dark-gray, 

poorly bedded to laminated. Sparse plant and wood 

debris _____________________________ 1,375 

Lower Cretaceous Series:

Thermopolis Shale: Dark-gray silty shale and underlying 

thin-bedded quartzose sandstone. Exposed mainly along 

Tolman Creek ___________________________ 23 

Kootenai Formation: Coarse-grained argillaceous sandstone

and conglomerate, claystone, and limestone _________ _ 54

Jurassic System:

Upper Jurassic Series:

Morrison Formation: Varicolored mudstone and sandstone.

Poorly exposed _______________________________ 70

Unconformity.

Upper and Middle Jurassic Series:

Ellis Group: Sandstone, limestone, and shale including

parts of the Swift, Rierdon, and Sawtooth Formations ___ 55 

Disconformity. 

Triassic System:

Lower Triassic Series:

Dinwoody Formation: Sandstone, dolomite, and dolomitic

shale __________________________________ 60 

Permian,System:

Shedhorn Sandstone and Phosphoria Formation: Cherty 

sandstone, dolomite, shale, bedded and nodular chert, 

oolitic phosphate rock _____________________________ 60

Pennsylvariian System:

Quadrant Quartzite: Quartzite and calcareous quartz sandstone, white or yellow, coarsely crossbedded. Breccia of 

dolomite blocks near base ____________________________ 120(?)

Pennsylvanian and Mississippian Systems:

Amsden Formation: Shale, dolomite, and limestone; little

exposed ________________________________ 30(?) 

Unconformity. 

Mississippian System:

Lower and Upper Mississippian Series:

Mission Canyon Limestone: Thick-bedded limestone and dolo^

mite, minor nodular and bedded chert __-___________ 300

Lower Mississippian Series:

Lodgepole Limestone: Medium- to thin-bedded limestone

containing shaly interbeds and partings ______________ 240

Mississippian and Devonian Systems:

Three Forks Formation: Mainly shale. Very little exposed 30 

Devonian System:

Jefferson Dolomite: Medium- to thick-bedded dolomite ranging from dark brown and porous to light brown and 

compact. Sparsely fossiliferous __________________ 90
GEOLOGIC SETTING 9 

TABLE 2. Stratigraphic units in the Cameron quadrangle Continued

Thickness 

(meters)

Unconformity. 

Ordovician System:

Bighorn Dolomite: Medium-bedded, light-gray, compact

dolomite. Exposed only at eastern edge of quadrangle __ 7.5 

Cambrian System:

Upper Cambrian Series:

Red Lion Formation: Red calcareous sandstone and red and 

green sandy shale. Exposed only at east edge of quadrangle __________________________ 18

Pilgrim Limestone: Dolomitic limestone, thin-bedded, nodular to laminated, commonly mottled. Few beds of compact 

gray dolomite __________________________________ 120

Middle Cambrian Series:

Park Shale: Fissile green micaceous shale, has abundant 

thin limestone layers in lower part; minor nodular limestone containing green shale partings and beds in upper 

part ______________________________ 3

Meagher Limestone: Thin-bedded slabby limestone, bluish 

gray, with orange argillaceous mottles. Locally dolomite 

replaces limestone _________________________ 120

Wolsey Shale: Sandstone, argillaceous and micaceous, containing abundant worm burrows, commonly highly glauconitic; green ishale partings in lower part. Upper part 

strongly calcareous, grading to overlying unit _______ 38

Flathead Sandstone: Quartz sandstone, slightly feldspathic, 

medium- to coarse-grained and locally pebbly, mediumbedded, containing green shale partings; commonly crossbedded _______________________________ 7.5

Unconformity. 

Precambrian System:

Cherry Creek Formation:

Quartzite unit: Quartzite, gray, biotitic, variously feldspathic, locally white or green, medium- to thinbedded, minor interbedded quartz-mica schist _______ 300

Upper dolomite unit: Dolomite marble in units 30 to 

90 m thick, interbedded with kyanite-staurolite-biotite 

schist and gneiss and feldspathic quartzite _______ 460

Quartzite and schist unit: Mica schist, phyllite, and 

quartzite _______________________________ 460

Lower dolomite unit: Dolomite marble, fine- to coarsegrained, locally contains thin layers of quartzite or 

micaceous schist _________________________________ 1,500

Lower schist unit: Micaceous schist and phyllite, muscovitic to iron-rich, locally calcareous, in part of volcanic origin. Many thin persistent quartzite beds. 

Minor schistose volcaniclastic rocks, actinolitic green 

schist, dolomite, and thin-bedded magnetite-rich 

quartzite (iron formation) _______________________ 2,450
10 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

quadrangle. On the other hand, certain Paleozoic units (such as 

the Big- Snowy Group and the Three Forks Formation) are well 

exposed in the Varney quadrangle but are absent or poorly exposed in the Cameron quadrangle.

Mesozoic and Paleozoic formations are similar in the two areas, 

but average thicknesses vary considerably for some units. The 

Thermopolis Shale (of former usage)' thickens westward from 

23 m to about 110 m. A parallel thickening of the Kootenai Formation takes place from about 53 m in the Cameron quadrangle 

to more than 150 m in the Varney quadrangle.

The Morrison Formation, Ellis Group, Dinwoody Formation, 

Shedhorn Sandstone, and Quadrant Quartzite show similar lithologic characteristics and variations in thickness across the region. 

The Amsden Formation is poorly developed in the Cameron quadrangle, and the Big Snowy Group, so well represented in parts 

of the Varney quadrangle, is not found to the east in the Cameron 

quadrangle.

The Madison Group, with its consistent Lodgepole and Mission Canyon Limestones, maintains a fairly consistent lithology 

and thickness throughout the region. The Three Forks Formation 

is well exposed on Baldy Mountain but is thinner and very poorly 

exposed in the Cameron quadrangle. The Jefferson Dolomite 

thickens slightly from east to west, and the Bighorn Dolomite is 

found only west of the Greenhorn fault in the Varney quadrangle 

and at the east edge of the Cameron quadrangle.

Most of the Cambrian units show similar lithologic and thickness patterns throughout the area, except for the Park Shale, 

which is absent east of the Greenhorn fault in the Varney quadrangle and is only about 3 m thick where it occurs in the Cameron 

quadrangle.

The earliest of these variations appears in the thinning of the 

Flathead Sandstone and Wolsey Shale (both Middle Cambrian) 

from a combined thickness of more than 90 m in the northern 

part of the Varney quadrangle to about 20 m near its southeastern corner. The Wolsey Shale, here about 15 m thick, is 

largely calcareous sandstone; it represents the upper part of the 

formation farther north and lies on 5 m or less of Flathead Sandstone. The thinning is probably due to overlap onto a rising slope 

of Precambrian rocks, the thin Flathead facies occupying a higher 

stratigraphic position here than elsewhere.

1 The Thermopolis Shale is now geographically restricted from Montana by Rice (1976) 

for U.S. Geological Survey usage; its rocks are now replaced in Montana by the Muddy 

Sandstone, the Skull Creek Shale, the basal part of the Mowry Shale, or the Fall RivetSandstone. Hadley's mapping was completed before this restriction.
GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT. H

PALEOZOIC ROCKS

FLATHEAD SANDSTONE AND WOLSEY SHALE

Basal Cambrian strata throughout the Varney and Cameron 

quadrangles are quartzite and sandstone characteristic of the 

Flathead Sandstone, as recognized throughout southwestern Montana and adjacent areas (Klepper and others, 1957). The lower 

part of the formation consists of red-, orange-, pink-, or brownweathering medium- to coarse-grained orthoquartzite, medium 

bedded and commonly crossbedded; the upper part consists largely of finer grained calcareous sandstone, thinner bedded, and containing partings and thin beds of green micaceous shale. Red 

colors, presumably due to syngenetic hematite, are more pronounced in the lower beds; most of the higher beds are pale yellowish gray or white. Layers and lenses of quartz-pebble conglomerate 30 cm or less thick occur mostly in the lower part of 

the formation. Rare fragments of quartz, as much as 8 cm across 

and generally more angular than the smaller fragments, are confined to the basal meter of the formation, especially where it 

rests on Precambrian granitic rocks containing pegmatite. Such 

large clasts may be accompanied by large fragments of feldspar, 

but the quartzite and sandstone beds are not conspicuously feldspathic and nowhere contain more than 10 percent feldspar. The 

upper part of the Flathead is generally calcareous sandstone, 

medium to fine grained, locally quartzitic in beds 10 to 30 cm 

thick. Shaly partings are commonly coated with fine clastic mica 

and marked by abundant worm castings and trails.

The base of the Wolsey Shale is marked in most places by a 

poorly exposed interval containing float of greenish fissile to silty 

shale and thin-bedded fucoidal glauconitic sandstone. Rarely, a 

little dark-red glauconitic limestone appears in this interval. The 

upper beds of the Wolsey throughout the Varney quadrangle are 

dominantly sandstone, pale orange to yellowish gray and thin 

bedded. They are everywhere moderately to strongly calcareous 

and locally grade to slabby, very sandy limestone. Sandstone and 

sandy limestone of this type occupy most of the upper half of the 

formation throughout the eastern part of the Gravelly Range.

On Baldy Mountain and southward in the Greenhorn Range, 

the upper part of the Wolsey consists of 18 m or more of finegrained sandstone, which is argillaceous and dolomitic rather than 

calcareous, and moderate to dark yellowish orange. It locally contains abundant glauconite, forming greensand beds several centimeters thick. Similar beds of greensand in the Wolsey on Indian
12 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

Creek in the eastern part of the Cameron quadrangle are as much 

as 30 cm thick.

Because the upper limit of the Flathead Sandstone is rarely exposed, the thickness of the formation is not precisely known. 

Measurement of the beds in the better exposures along the eastern 

side of the Gravelly Range indicates that the Flathead is at least 

19 m thick near the mouth of Dry Hollow and decreases in thickness southeastward to less than 6 m on Hyde Creek in the southwest part of the Cameron quadrangle. Less well exposed sections 

to the north and west indicate a thickness of as much as 27 m on 

Baldy Mountain and only slightly less throughout most of the 

Greenhorn Range. In the two best exposed sections on the western 

flank of the Madison Range in the eastern part of the Cameron 

quadrangle, the Flathead is 7.5 to 12 m thick.

The full thickness of the Wolsey Shale is not revealed in any 

exposures in the Varney or Cameron quadrangles, but, in general, 

its thickness varies with that of the underlying Flathead. Its 

maximum thickness is about 75 m near Dry Hollow and farther 

north; thickness decreases southeastward to a minimum of about 

15 m at Cherry Creek and at Hyde Creek. On Baldy Mountain 

and in most of the Greenhorn Range, the Wolsey is 60 m or less 

thick; in the Cameron quadrangle, it is estimated at 20 to 37 m.

MEAGHER LIMESTONE

The Wolsey Shale is succeeded by 105 to 120 m of carbonate 

beds that are assigned to the Meagher Limestone (Deiss, 1936). 

This formation consists of two> contrasting lithologic fades a 

thin-bedded nodular limestone facies and a brown, less distinctly 

bedded dolomite facies. The dolomite facies is confined to the 

upper part of the formation on the eastern side of the Gravelly 

Range in the Varney quadrangle, but it occupies most of the formation throughout the Greenhorn Range.

The limestone facies of the Meagher consists largely of thin- to 

medium-bedded, fine-grained to aphanitic limestone, light to 

moderate bluish, olive, or reddish gray characterized by pale-yellowish-orange or yellowish-brown argillaceous mottles. It is variably argillaceous and much of the limestone consists of layers of 

small carbonate lumps 2.5 to 5 cm across in a matrix of argillaceous limestone or dolomitic shale. Local thin units of limestone 

are evenly thin bedded or laminated and contain thin beds of 

fossiliferous calcareous shale. (See stratigraphic section 1.)

The dolomite facies consists typically of light- to moderatebrownish-gray, yellowish-brown, or, rarely, dark-brown dolomite,
PALEOZOIC ROCKS 13

Stratigraphic section 1

(.Flathead Sandstone and Meagher Limestone, north side of Dry Hollow, sec. 1, T. 8 S., R. 

2 W., Varney quadrangle. Measured by J. B. Hadley |

Thickness 

(meters)

Basal Devonian beds 

Unconformity. 

Meagher Limestone:

27. Limestone, light-olive to brownish-gray, bright-orange 

silty mottles, very fine grained, thin-bedded, and 

somewhat nodular. Most beds partly, many beds 

wholly, altered to dolomite preserving the orange 

mottling. Upper 0.5 m is limestone containing abundant Girvanella ___________________ _ _ 3

26. Dolomite as in the two underlying units, but including 

large remnants of limestone, yellowish-orange-mottled, thin-bedded, nodular. Units as much as a meter 

thick pass laterally from limestone to dolomite within 

a few centimeters _____________________- 8.5

25. Dolomite, moderate-brownish-gray with pale yellowishbrown mottling, thin-bedded, nodular; bedding similar 

to that of limestone of lower units _______________ 5

24. Dolomite, dark-brown, coarsely crystalline, in calcareous

matrix _______________________________________ 0.6

23. Dolomite, light- to moderate-brown, medium- to very

fine grained, thin- and evenly bedded _____________ 4.5

22. Dolomite, grayish- to yellowish-brown, weathering gray, 

calcareous, mostly massive, little bedding; darker 

medium-grained dolomite surrounded by finer grained 

lighter colored dolomite _____-____--_____- 16

21. Dolomite, in alternating layers 2 to 35 cm thick, finegrained, light-grayish-brown and medium-grained 

dark-reddish-brown; moderately calcareous. Most of 

interval shows indistinct thin bedding ___________ 7

20. Dolomite, moderate-yellowish-brown, fine-grained; bedding indistinct and masked by dolomitization. The 

yellowish silty mottling and the bedding characteristics of the limestone are faintly preserved. Grades 

into underlying limestone through 30- to 50-cm 

interval __________________________ 8

19. Limestone, like underlying unit, but containing many 

ramifying bodies of brownish-gray dolomite. Abundant "tube structures" _________________ 2.7

18. Limestone, light- to moderate-olive-gray mottled with 

pale yellowish orange, grading to medium yellowish 

brown; largely massive and having indistinct bedding 8.8

17. Limestone, moderate-olive-gray, little mottling, mediumbedded to 25 cm _____________________ 2.7

16. Limestone, very thin bedded and containing abundant

yellow shale beds; abundant trilobite fragments ____ 0.9

15. Limestone, mottled, thin-bedded, nodular; abundant

trilobite fragments _____________________________ 3.7
14 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

Stratigraphic section 1 Continued

Thickness 

(meters)

Meagher Limestone Continued

14. Limestone, nodular, very shaly ______________________ 0.9

13. Limestone, mottled, thin-bedded, nodular, contains several beds of reddish-gray aphanitic limestone 15 to 

30 cm thick ___________________________ 3.7

12. Limestone, light-olive-gray with light- to dark-orangeyellow mottles, fine-grained to aphanitic; thin-bedded, 

strongly nodular near base; lower 1.2 m shaly, containing more scattered limestone nodules ______ 24

11. Covered. Thickness probably 9 to 15 m _______ ___ 12

10. Limestone, mottled light-olive-gray and pale-reddishbrown, weathers light olive gray with dark-yellowish 

orange mottles; thin-bedded (2-15 cm), finely crystalline; bedding even, finely nodular; abundant trilobite 

fragments _________________________-__-_-_------ 7.6

Total thickness Meagher Limestone __________ 120

Covered. Largely Wolsey Shale ____--_____-__________________ 80

Flathead Sandstone:

9. Poorly exposed. Sandstone, like underlying unit, but 

somewhat thinner bedded. Near top of interval, 2 to 

5 cm of red, coarsely crystalline limestone contains 

hematite ovoids and glauconite. Glauconitic sandstone 

float _____________________________ 4.6

8. Sandstone, very pale orange with limonite spots, thinbedded (2-8 cm), fine-grained, somewhat calcareous; 

partings micaceous, pale-green, shaly, containing 

abundant worm casts. Bedding surfaces very lumpy 3.7

7. Covered. Float is thin-bedded quartzitic sandstone containing chips of green shale ______________________ 1.2

6. Sandstone, pale-orange quartzitic, medium-bedded, to 

coarse-grained. Clayey sandstone pellets form lenses 

or pockets as much as 30 cm long ________________ 3

5. Sandstone, very fine grained, silty, micaceous and fissile;

wavy bedded ____________________________________ 1.5

4. Orthoquartzite __________________________________ 0.3

3. Granule sandstone containing angular fragments of

quartz and minor feldspar to 15 mm in diameter ____ 0.6

2. Quartz sandstone and quartzite, white to very pale 

orange, variably carbonatic, fine- to medium-grained; 

beds 15 cm to 1 m thick __________________________ 2.5

1. Quartz sandstone, grayish-orange, fine- to mediumgrained, variably siliceous cement; basal 0.6 m hematitic, moderate reddish brown, micaceous partings __ 1.5 

Total thickness Flathead Sandstone _________ 19

Unconformity. 

Precambrian basement:

Calcite marble, upper part decomposed and reddish; poorly 

exposed.
PALEOZOIC ROCKS 15

showing variously preserved remnants of sedimentary structures 

characteristic of the limestone facies. The characteristic mottling 

is commonly preserved as lighter brown mottles in darker brown 

dolomite. Bedding is, however, commonly obscured so that the 

dolomite facies appears thicker bedded and is more resistant than 

the limestone facies. Most fossils have been obliterated in the 

dolomite facies, but characteristic tubes or branching twiglike 

structures filled with white calcite are abundant in some beds.

From a mapping standpoint, the contacts between the two facies 

are fairly well defined, although the thickness of the limestone 

facies of the Varney quadrangle varies from about 15 m in the 

southeastern part to somewhat more than 60 m in the east-central 

part. The limestone is about 45 m thick on Baldy Mountain but 

appears to be only 9 to 15 m thick where sandwiched between 

dolomite units in the lower part of the formation, farther south 

in the Greenhorn Range. Limestone appears at or near the top 

of the Meagher in several places throughout the Varney quadrangle. Where the rocks of the two facies are in contact, there is 

abundant evidence of progressive replacement of limestone by 

idiomorphic saccharoidal dolomite. Limestone units 3 to 6 m thick 

are penetrated by irregularly ramifying bodies of dolomitized 

and partly dolomitized rock in which dolomite rhombohedra are 

increasingly abundant in the more calcic or aphanitic parts of the 

limestone beds, and the finer grained and lighter colored argillaceous material remains.

PARK SHALE

The name Park Shale was given by Weed (1899) and emended 

by Deiss (1936) to designate a unit between the Meagher Limestone and the overlying Pilgrim Dolomite or Limestone in the 

Little Belt Mountains of west-central Montana. The term has since 

been applied to a remarkably persistent shale unit of Middle and 

Late Cambrian age throughout southwestern Montana. This unit 

succeeds the Meagher Limestone throughout the Greenhorn Range 

and in the Madison Range; it is absent, however, in the northern 

part of the Gravelly Range east of the Greenhorn fault, where it 

and the overlying Cambrian and Ordovician rocks were removed 

by erosion before deposition of the Jefferson Dolomite in early 

Late Devonian time.

The Park Shale is poorly exposed in most places but is approximately 55 m thick in the least deformed sections, where its 

thickness could be estimated. On the east ridge of Baldy Mountain, 

the lower part of the unit consists largely of dark-greenish-gray
16 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

noncalcareous clay shale containing many thin beds of purplishor reddish-gray limestone and a few beds of intraf ormational 

limestone conglomerate and brownish-gray limestone containing 

abundant fragments of a small linguloid brachiopod. The upper 

part of the unit in this section is dominantly red and green silty 

shale that becomes calcareous upward.

An unusually complete exposure of the Park Shale on the northern slope of Sheep Mountain (sec. 21, T. 8 S., R. 3 W.) shows 30 

m of fissile noncalcareous grayish-green shale, red in the upper 

4.5 m. The upper 9 to 12 m of this section contains nodular layers 

of dark-brown dolomitic siltstone less than 2.5 cm thick. In the 

only good exposure of the formation seen in the Cameron quadrangle on the north side of the canyon of Indian Creek, the Park 

Shale is much reduced in thickness by faulting. What is left consists of green fissile shale that contains many regular beds less 

than 2.5 m thick of nodular limestone as well as thin-bedded 

nodular limestone containing green shale partings.

PILGRIM DOLOMITE OR LIMESTONE

A thick carbonate unit overlying the Park Shale is referred to 

the Pilgrim Dolomite or Limestone in accordance with recent 

usage in western Montana (Hanson, 1952; Klepper and others, 

1957; Robinson, 1963). The Pilgrim is a conspicuously exposed 

formation' 110 to 120 m thick in the western part of the Varney 

quadrangle and near the eastern border of the Cameron quadrangle. It is absent, however, throughout the eastern flank of the 

Gravelly Range, where Devonian rocks rest directly on the 

Meagher Limestone. Although the Pilgrim consists largely of 

limestone in its type locality and throughout much of western 

Montana (Deiss, 1936, p. 1333-1335; Hanson, 1951, p. 56; 

McMannis, 1955, p. 1394-1395), it is nearly all dolomite in the 

Varney quadrangle.

On Baldy Mountain and throughout the Greenhorn Range, the 

Pilgrim can be divided somewhat loosely into four lithologically 

distinct parts: (1) a lowermost unit consisting of thin-bedded 

silty or shaly dolomite, (2) a middle unit of limestone, (3) an 

upper medium- to thick-bedded dolomite, and (4) an uppermost 

unit of quartzose sandy dolomite.

The lowermost unit consists of thin-bedded, light-yellowishgray or light-yellowish-brown dolomite containing greenish- or 

yellowish-gray silty laminae or partings and weathering to platy 

fragments. Medium-bedded mottled dolomite forms subordinate
PALEOZOIC ROCKS 17

units in some sections of the lowermost unit, and the basal 3 to 

9 m is commonly of this type. Thin beds of flat-pebble conglomerate containing fragments of dolomite 0.8 cm thick and 6.5 cm 

long are associated with sandy and silty, current-bedded dolomite 

in the upper part of this unit on Baldy Mountain. The thickness 

of the lowermost, thin-bedded dolomite ranges from 43 m to 72 m 

(see fig. 1).

The upper dolomite is 12 to 50 m thick and is characterized by 

beds of light-yellowish-gray dolomite, 0.3 to 0.9 m thick, locally 

interbedded with thinner bedded or laminated dolomite. Thicker 

beds commonly have a distinctly mottled or brecciated appearance 

on weathered surfaces. This unit also shows small-scale current 

bedding at many places.

The dolomite of both these units is generally finely crystalline 

and compact and rarely shows the coarse idiomorphic dolomite 

replacement texture characteristic of the Meagher Limestone. 

Details of sedimentation, such as lamination and current bedding, are better preserved, and the rocks are nowhere dark or 

petroliferous.

The middle limestone unit consists of modular or thin-bedded 

blue-gray limestone mottled orange or yellowish orange, much 

resembling parts of the Meagher Limestone and dissimilar to most 

of the Pilgrim in the area. Silty, shaly, or fine sandy interbeds 

are somewhat coarser than the detrital material in the Meagher. 

The limestone beds are generally confined to a unit 15 to 25 m 

thick in the middle and lower parts of the formation (fig. 1). They 

first appear about 60 m above the base of the formation in the 

northeastern part of sec. 4, T. 8 S,, R. 3 W. Farther southwest, 

the limestone is much lower in the formation, but a well-exposed 

section of the Pilgrim in sec. 4, T. 9 S. P R. 3 W., shows no limestone at all.

The uppermost part of the Pilgrim throughout the Greenhorn 

Range is characterized by medium- to thick-bedded dolomite containing fine- to medium-grained quartz sand in widely varied 

amounts. Sandy beds range from slightly sandy dolomite to dolomitic quartz sandstone in beds or lenses a few centimeters to a 

meter in thickness. They are conspicuously crossbedded in many 

places. This sandy interval increases in thickness southward from 

6 m on Baldy Mountain to 21 m in sec. 22, T. 8 S., R. 3 W. A 

similar unit can be recognized farther south in the Greenhorn 

Range but is not sufficiently exposed there to provide estimates 

of its thickness.
18 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

1 2 3 

JEFFERSON 

DOLOMITE

METERS

100 -i

75-

50 -

o7 / / /

PILGRIM 

DOLOMITE

OR 

LIMESTONE

/ / /

EXPLANATION

Sandstone and 

dolomite sandstone

Limestone

Dolomite, thick to 

thin bedded and 

irregularly bedded

/ a /

Dolomite, intraformational 

conglomerate

Dolomite, sandy, 

shaly partings

Shale

FIGURE 1. Columnar sections of the Bighorn Dolomite, Red Lion
PALEOZOIC ROCKS 

5

LOCATION OF SECTIONS

1. Baldy Mountain, sec. 27, T. 7 S., R. 3 W.

2. Steep gully in west half, sec. 27, T. 7 S., R. 3 W.

3. Baldy Mountain, west ridge, sec. 33, T. 7 S., R. 3 W.

4. NEK sec. 4, T. 9 S., R. 3 W.

5. NWV4 sec. 23, T. 8S...R. 3 W.

6. SEV4 sec. 20, T. 8 S., R. 3 W.

19

JEFFERSON 

DOLOMITE

PILGRIM 

DOLOMITE

OR 

LIMESTONE

PARK SHALE

Formation, and Pilgrim Dolomite or Limestone, Varney quadrangle.
20 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

In the limited exposures of the Pilgrim along the eastern 

border of the Cameron quadrangle, the formation has about the 

same thickness as in Varney quadrangle but is composed almost 

entirely of limestone. The limestone is generally medium to thin 

bedded and nodular, locally laminated by beds of dolomitic siltstone or silty dolomite. Yellowish-orange or pale-red mottling is 

common, and a few thin beds in the lower part of the formation 

contain abundant glauconite.

The basal contact of the Pilgrim on the underlying Park 

Shale is not exposed in the Varney quadrangle, but the presence 

of thin limestone beds in the upper part of the Park, as indicated 

by float at several places, suggests a transition from shale to 

carbonate deposition. At altitude 2,200 m, on the north wall of 

the canyon of Indian Creek in the Cameron quadrangle, the contact is fully exposed and shows a transitional interval at least 

12 m thick in the upper part of the Park Shale.

RED LION FORMATION AND BIGHORN DOLOMITE

Immediately above the Pilgrim Dolomite or Limestone on Baldy 

Mountain and throughout the Greenhorn Range is about 30 m 

of thin-bedded dolomite, calcareous siltstone, and dolomitic pebble 

conglomerate, underlain by a thin basal red or green shale. This 

sequence contains trilobite and brachiopod fragments of Cambrian aspect and is considered to be the equivalent of the Red 

Lion Formation of the Philipsburg district (Emmons and Calkins, 

1913, p. 61-63) and the Snowy Range Formation recognized in 

south-central Montana (Dorf and Lochman, 1940, p. 545-547; 

Grant, 1965). According to recent usage in southwestern Montana, the unit is assigned to the Red Lion (Hanson, 1952, p. 17, 

30, 37; Klepper and others, 1957, p. 12-14). This unit is overlain throughout most of its extent by a conspicuously exposed 

thick-bedded to massive light-gray dolomite, which has been assigned to the Bighorn Dolomite of Middle and Late Ordovician 

age by Hanson (1952, sec. 9, p. 30, sec. 22, p. 37, and sec. 8, p. 28). 

Like the other Upper Cambrian rocks, the Red Lion and the 

overlying dolomite are absent throughout the eastern flank of the 

Gravelly Range, having been removed by pre-Late Devonian 

erosion.

Although the basal beds of the Red Lion Formation are generally concealed, they are everywhere represented by abundant 

chips of red and green fissile to sandy shale in the soil cover. The 

interval is estimated to be 3 to 10 m thick.
PALEOZOIC ROCKS 21

The main part of the Red Lion in the Greenhorn Range consists of variegated light-yellowish, pinkish, or brownish-gray 

dolomite, thin bedded, commonly silty or fine sandy, and weathered 

to slabby or slaty fragments. Greenish, tan, or reddish-gray shaly 

interbeds and partings are characteristic of the unit, as are crinkly or strongly wavy bedding surfaces. Many thin beds of dolomite pebble conglomerate occur in the upper 9 to 12 m of the 

formation in the vicinity of Baldy Mountain. The lower part tends 

to be sandy and includes a few beds of glauconite-bearing quartzose sandstone or dolarenite (fig. 1). Fragments of trilobite molts 

and shells of Billingsella and Eoorthis were found in shaly beds 

in most sections of the formation. According to A. R. Palmer 

(written commun., 1959), these genera are associated characteristically in beds of early Franconian (middle Late Cambrian) 

age.

These rocks differ considerably from the limestone and interbedded shale characteristic of the Red Lion and Snowy Range 

Formations in their type areas, and correlation with subdivisions 

such as the Sage or Grove Creek Limestone Members of the 

Snowy Range does not seem feasible. The basal red and green 

shale, however, is probably the same as the Dry Creek Shale 

Member of the Snowy Range, as defined by Lochman-Balk (1950) 

and by Grant (1965, p. 10-12). The rest of the formation more 

nearly resembles, in its sandy and silty beds, the dominantly 

clastic sequence described as Red Lion Formation in the southern Elkhorn Mountains some 100 km to the north (Klepper and 

others, 1957, p. 10-12).

The rocks in the Greenhorn Range provisionally assigned to the 

Bighorn Dolomite of Middle and Late Ordovician age by Hanson 

(1952, p. 22) are massive to medium-bedded, light-gray or white 

dolomite, commonly irregularly mottled darker gray. Beds a few 

meters thick of laminated nonfisslle dolomite also occur. They 

rest directly on thinner bedded variegated dolomite of the Red 

Lion and form a prominent bedrock rib between the less resistant 

rocks of the Red Lion and the lower part of the Jefferson Dolomite. The exposed thickness of the unit ranges generally from 18 

to 43 m; the 128 m assigned to the Bighorn on Sheep Mountain 

in sec. 21, T. 8 S., R. 3 W., by Hanson (1952, p. 30) represents 

the maximum thickness of 43 m repeated by faults, together with 

infaulted beds of the overlying Jefferson Dolomite.

No fossil evidence indicating the age of this dolomite unit has 

been found, and correlation with the Bighorn was made apparently on the basis of its stratigraphic position and lithologic similar-
22 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

ity to the rocks in the Madison Range to the east, previously regarded as marking the western limit of the Bighorn in southcentral Montana (Sloss and Moritz, 1951, p. 2147-2148). Both 

the Bighorn and the Red Lion are missing, apparently because of 

pre-Jefferson erosion, in the narrow belt of Paleozoic rocks in 

the vicinity of Indian Creek in the eastern part of the Cameron 

quadrangle. In the northeastern corner of sec. 32, T. 8 S., R. 2 E., 

a few hundred feet east of the quadrangle boundary, however, the 

Pilgrim is overlain by 18 m of poorly exposed red calcareous sandstone and green sandy shale and 7.6 m of medium-bedded lightgray dolomite that probably represent the Red Lion and Bighorn, 

respectively.

JEFFERSON DOLOMITE OR LIMESTONE

The rocks immediately overlying the Cambrian sequence and 

the Bighorn Dolomite in the Varney and Cameron quadrangles 

are regarded as belonging to the Jefferson Dolomite or Limestone 

of Late Devonian age. Originally described from exposures near 

Three Forks, Mont., 65 km north of the present report area, the 

Jefferson has been recognized throughout a wide area in Montana 

and adjacent parts of the neighboring States (Sloss and Laird, 

1947). In the present area, the Jefferson is 75 to 120 m thick 

and consists largely of dolomite and minor interbedded shale, 

claystone, and limestone. Beds equivalent to the Maywood Formation (Middle and Upper Devonian) of the Philipsburg area 

may be present, but they are lithologically distinct from the Maywood as described in nearby areas and are here included in the 

Jefferson.

The Jefferson consists mainly of yellowish or brown dolomite, 

medium to thick bedded, commonly unusually dark and petroliferous. Much of it forms alternating sequences of light-yellowishgray compact dolomite and contrasting dusky-brown dolomite 

that is medium to coarsely saccaroidal, porous, and strongly 

petroliferous (stratigraphic section 2). The latter rock type 

weathers to large float blocks that mark the presence of the Jefferson in many places where the other rocks are not exposed. 

Calcite-filled solution cavities, lenses of collapse breccia, and sparse 

ovoid or irregularly bedded dark chert are other features that 

serve to distinguish the Jefferson from the Cambrian dolomites. 

The fomation is moderately fossiliferous, although dolomitization 

has obscured or destroyed organic remains in many places. The 

most commonly observed forms are Amphipora and Phillipsastrea, 

which can be found in almost any large exposure of the forma-
PALEOZOIC ROCKS 23

Stratigraphic section 2

[Three Forks Formation and Jefferson Dolomite, north side of Ruby Creek near junction 

with Beartrap Canyon, Varney quadrangle. Measured by .1. B. Hadley and W. D. Long |

Thickness 

(meters)

Lodgepole Limestone (incomplete) :

Calcarenite, coarsely crystalline, containing fragments of yellow

sandy siltstone; sharp contact with underlying unit ________ 1.0

Three Forks Formation: 

Sappington Member:

44. Siltstone, dark-yellow-orange, argillaceous and calcareous, becoming more sandy upward, indistinctly 

bedded, partings 2.5 to 25 cm apart _________________ 9.0

Trident Member:

43. Poorly exposed. Few outcrops of limestone like unit 42 29.0 

(Fault, nearly vertical, northwestern side down about 

3 m) 

42. Limestone, pale-grayish-yellow, argillaceous, thinly and

indistinctly bedded, platy weathering _____________ 4.0

Logan Gulch Member:

41. Limestone, moderate-grayish-brown, microcrystalline, 

nonbedded, much broken and slumped into underlying breccia _________________-_--__-______---__- 3.0

40. Breccia, coarse, chaotic, consisting of fragments of yellowish-brown and yellowish-gray dolomite, both compact and thin bedded, in a grayish-yellow, argillaceous, silty, and calcareous matrix. Fragments are 

2.5 to 60 cm in diameter; many in upper half of unit 

are limestone similar to overlying unit ____________ 15.0

Total thickness Three Forks Formation ____ 61

Jefferson Dolomite:

39. Covered. Probably similar to underlying unit ________ 5.2

38. Dolomite, moderate-grayish-orange to pale-red, argillaceous and calcareous, thin-bedded and platy containing argillaceous partings ______________________ 1.5

37. Dolomite, light-yellowish-gray, compact, nonbedded,

fractured and rubbly weathering __________________ 9.9

36. Mostly covered. Probably shale, lower part red ______ 4.6

35. Shale, red and yellow, dolomitic, 0.4 m yellow argillaceous dolomite ____________________________ 1.3

34. Dolomite, pale- to moderate-yellowish-brown; lower 1.5 

m compact and blocky; upper 2.1 m porous, thickbedded, and indistinctly current bedded; similar to 

unit 28 ____________________________ 3.6

33. Dolomite, dark-yellowish-brown, indistinctly bedded,

blocky weathering, moderately petroliferous _______ 4.6

32. Dolomite, grayish-yellow, argillaceous, grading upward

to more compact dolomite as in unit 1 __________ 1.2

31. Claystone, grayish-yellow to yellowish-gray, dolomitic, 

poorly exposed; contains brachiopod fragments; forms 

slope below 8-m cliff ___________________ 3.6

30. Dolomite, like unit 28 _____________________ 1.4
24 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT. 

Stratigraphic section 2 Continued

Thickness 

(meters)

Jefferson Dolomite Continued

29. Poorly exposed. Lower part is dolomite, moderate-reddish-orange to very pale olive-gray, microcrystalline, 

argillaceous; middle part is moderate to dark-brown 

dolomite; upper part is dolomitic claystone and shale 2.9

28. Dolomite, moderate-yellowish-brown, medium crystalline; 

lower part compact and brecciated, upper part friable, 

laminated; sandy texture is due to leaching of calcium 

carbonate ______________________________________ 1.3

27. Dolomite, compact like unit 1, blocky weathering _ 2.1

26. Dolomite, compact, nonbedded, weathering to irregular

rubble _____________________________ 0.9

25. Dolomite, pale-yellowish-brown, compact, 25-cm beds,

jointed like unit 1 _______-_____---______-_------- 0.9

24. Dolomite, moderate- to dark-yellowish-brown, finely 

crystalline, mostly thin bedded, partly porous and 

indistinctly bedded, weathers to small blocks ________ 5.5

23. Shale and claystone, red and yellow as in basal part 

of unit 21, dolomitic; basal contact sharp, top sharp 

but undulating ________________________________ 1.2

22. Dolomite, moderate- to dark-yellowish-brown, poorly 

bedded, locally a breccia. Thirty-centimeter bed 0.6 m 

above base contains abundant poorly preserved 

corals and brachiopods __________________________ 3.0

21. Dolomite, yellowish-brown, finely crystalline, grading 

downward to pale-yellowish-brown argillaceous dolomite and this in turn to a basal 30 cm of red and 

yellow silty shale containing fragments of yellow 

dolomite and dark chert -_--____-_______________ 1.4

20. Dolomite breccia consisting of fragments of darkbrown, medium-crystalline dolomite in a matrix of 

dark-yellowish-orange dolomite. The basal 30 cm is 

very fine grained, pale-yellowish-brown dolomite ____ 1.0

19. Dolomite, dark-yellowish-brown, medium crystalline, irregularly laminated, containing abundant egg-shaped 

chert nodules 4 cm in diameter. Lower part is finely 

crystalline compact dolomite like unit 1, brecciated, 

argillaceous and yellow at base. Base of unit contains 

lenses as much as 10 cm thick of fragmental microcrystalline quartz ____________________________ 1.4

18. Dolomite, like unit 16 but lacks chert; somewhat brecciated and fossils less abundant; abundant algal 

structures in top 30 cm grades into unit 17 ________ 3.7

17. Dolomite, moderate-yellowish brown, finely crystalline. 

Lower part is thin bedded, containing lenses of argillaceous siltstone; middle is dolomite breccia containing 

dark-brown chert nodules as much as 13 cm in diameter; upper is compact like unit 1 _________________ 1.2
PALEOZOIC ROCKS 25 

Stratigraphic section 2 Continued

Thickness 

( meters)

Jefferson Dolomite Continued

16. Dolomite, moderate- to dark-yellowish-brown, medium 

crystalline, porous, locally vuggy and strongly petroliferous; lower part medium bedded; upper part 

current bedded. Basal 1.5 m contains sparse chert 

nodules as much as 5 cm diameter and abundant 

corals __________________________________________ 4.2

15. Dolomite, pale-yellowish-gray, very finely crystalline, 

blocky weathering; two beds 20 cm thick containing 

parting of pale-greenish-yellow shale ____________ 0.4

14. Dolomite, dark-yellowish-brown, medium crystalline, 

compact to moderately porous, moderately petroliferous; impersistent partings 5 to 10 cm apart, 

blocky weathering; contains poorly preserved Phillipsastrea. Upper 30 cm is laminated and current 

bedded and grades into unit 13 ____________________ 1.5

13. Shale and dolomite. Shale is pale-greenish-yellow (10Y 

8/4)', dolomitic, contains fragments and thin beds 

of pale-red argillaceous dolomite, grades upward 

into compact dolomite, variegated yellowish gray, 

orange, and brown, finely crystalline _____________ 0.8

12. Dolomite, dark-yellowish-brown to grayish-brown (5YR 

3/2), finely crystalline, moderately porous and petroliferous, beds 2 to 15 cm thick, blocky weathering; 

upper meter lighter colored and more compact ______ 4.7

11. Dolomite, pale-yellowish-gray (5Y 7/4), finely crystalline, compact ___________________________________ 0.3

10. Dolomite, finely crystalline to microcrystalline; alternating laminae yellowish-brown and grayish-red argillaceous dolomite, in part brecciated, platy weathering ________________________________ 1.0 

9. Dolomite, moderate yellowish-brown (10F.R 5/4) to 

pale-yellowish-brown (1QYR 7/2), finely crystalline, 

thin-bedded to laminated, porous, thin lenses of fine 

breccia _____________________________ 1.4

8. Dolomite, grayish-red (10/2 4/2), finely crystalline,

laminated _______________________________ 0.5

7. Dolomite, pale-yellowish-orange, yellowish-brown and 

moderate-reddish-orange, finely crystalline, bedding 3 

to 25 mm; irregular lenses of reddish dolomite breccia 0.5 

6. Dolomite, mostly like unit 2 but more compact; top 8

cm laminated _________________________________ 0.6

5. Dolomite, moderate-brown (5F.R4/2), finely crystalline,

laminated by reddish partings; grades into unit 4 __ 0.4 

4. Dolomite, dark-yellowish-brown (1QYR 3/2), finely

crystalline, compact, bedding thin and indistinct ____ 0.9

3. Dolomite, dark- to moderate-yellowish-brown, finely 

crystalline, compact, medium-bedded, vertically 

jointed ___________________________________ 0.8

1 Color designations are based on the "Rock-Color Chart" of the National Research Council 

(Goddard and others, 1948).
26 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT. 

Stratigraphic section 2 Continued

Thickness 

(meters)

Jefferson Dolomite Continued

2. Dolomite, dark-yellowish-brown, finely crystalline, 

partly thin bedded and compact, partly porous and 

brecciated, the latter type forming lenses and irregular crosscutting bodies 2.5 to 30 cm thick; 

moderately petroliferous _________________________ 1.0

1. Dolomite, yellowish-brown, compact, finely crystalline, 

beds 15 to 25 cm thick, close vertical joints, weathers 

to small smooth-surfaced blocks and chips _________ 0.3

End of good exposure. 

Partial thickness Jefferson Dolomite __________________ 82.7

tion. Brachiopod remains, crinoid columnals, and columnar algal 

stromatolites appear in some of the better exposed sections.

Claystone or shale, generally associated with argillaceous dolomite, is present in units 0.3 to 4.6 m thick throughout the middle 

and upper parts of the formation but appears rarely in outcrops. 

In an unusually well exposed section of the middle and upper parts 

of the formations on Ruby Creek in the southeastern corner of 

the Varney quadrangle (stratigraphic section 2), shale and claystone amount to nearly 15 percent of the formation. These 

argillaceous beds are calcareous and generally thin; they weather 

prominently red, yellow, or brownish gray. They commonly grade 

to argillaceous dolomite or limestone.

Limestone is rare in most of the Jefferson in the Varney quadrangle, but it is an important constituent of the lowermost 30 m 

of the formation along the eastern slope of the Gravelly Range. 

These basal beds are largely concealed on gentle slopes below the 

more resistant and typical dolomite beds of the Jefferson and 

above the cliff-forming dolomite of the Meagher Limestone. Reasonably complete exposures of this unit were found, however, on 

the crests of two narrow ridges on the northern side of Morgan 

Gulch in SW^ sec. 13, T. 8 S., R. 2 W., and south of Cherry 

Creek in SW^4 sec. 6, T. 9 S., R. 1 W. (stratigraphic section 3). 

In these three sections, the basal unit of the Jefferson is 27 to 38 

m thick and consists largely of interbedded dolomite and limestone. The dolomite beds are compact, fine to medium crystalline, 

light yellow to medium dark brown, generally thin bedded or 

laminated, and commonly silty. The limestone is light colored, 

yellowish, olive, or brownish gray, or faintly pink; it is medium 

to thin bedded or platy and forms units 0.3 to 6 m thick, locally 

altered to dark-brown petroliferous dolomite like the typical Jef-
PALEOZOIC ROCKS 27 

Stratigraphic section 3

[Lower part of Jefferson Dolomite, north side of Morgan Gulch, SW. corner, sec. 13. T. 8 S., 

R. 2 W., Varney quadrangle. Measured by J. B. Hadley]

Thickness 

(meters)

Top of section (incomplete) :

14. Dolomite, dark-brown, moderately well bedded 

13. Limestone, medium-olive-gray with pink tinges, indistinctly bedded; contains a few thin-shelled brachiopods _________________________________________ 5.5

12. Covered ______________________________ 4.6 

11. Dolomite, dark-brownish-gray, petroliferous, indistinctly

bedded/beds 25 to 45 cm thick __________________ 4.6

(Section offset 150 m)

10. Dolomite, light-yellowish-brown mottled with light 

brownish-gray, finely crystalline, medium-bedded, 

blocky weathering _______________________________ 2.1

9 Limestone, moderate-olive-gray, medium-bedded ____ 3.4 

8. Dolomite, very light yellowish gray, laminated, slabby

to blocky weathering _____________________________ 3.4

7. Limestone, like unit 4 ______________________________ 0.9

6. Limestone, light- to moderate-brownish-gray, finegrained, laminated to thin-bedded. Top bed is limestone, 15 to 25 cm thick, microcrystalline, indistinctly 

fragmental, dark-olive-gray __________________ 3.7

5. Largely covered interval. Few small exposures of limestone like unit 4 _________________________________ 6.1

4. Limestone, light-brownish- to yellowish-gray with 

abundant pink and orange tints, fine-grained to microcrystalline, medium- to thin-bedded and platy; 7.5-cm 

chert layer near top; a few thin beds in upper part 

contain Amphipora ------------------------------ 4.0

3. Dolomite like unit 2, but includes beds 15 cm thick of

laminated and lighter colored dolomite ____________ 2.1

2. Dolomite, medium- to dark-brownish-gray, medium

crystalline, nonbedded, strongly petroliferous ______ 3.7

1. Dolomite, yellow, fine-grained, platy-weathering (float

only) _______________________________ 3.0 

Partial thickness of Jefferson Dolomite ______ 47

Unconformity 

Meagher Limestone:

Dolomite, pale- to moderate-grayish-yellow, thin-bedded, 15-cm

layer containing Girvanella at top _________________ 3.4

ferson. The limestone is found throughout all but the basal 6 to 

9 m of the unit and makes up 35 to 60 percent of the three best 

exposed sections. Amphipora, thin-shelled brachiopods, and dark 

chert, all characteristic of the Jefferson, occur in small quantities 

in various parts of the unit.

In a well-exposed section on Baldy Mountain, the lower part 

of the Jefferson is entirely dolomite; the basal bed lies, with an
28 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

essentially conformable contact, on 23 m of thick-bedded, lightgray dolomite assigned to the Bighorn. Farther south in the 

Greenhorn Range, the lower 30 m of the Jefferson is very poorly 

exposed, but no evidence of limestone was seen.

According to J. T. Dutro, Jr. (written commun., 1957, 1975), 

four collections of fossils from the Jefferson Dolomite in the 

Varney quadrangle indicate a Late Devonian (Frasnian) age for 

the formation. Three of these collections are from the section on 

the north slope of Baldy Mountain (NWVi sec. 27, T. 7 S., 

R. 3 W.)- Amphiporat is abundant 10 m above the base of the 

formation, thamnoporoid and alveolitid corals are found about 18 

m above the base, and the colonial rugose coral Phillipsastrea was 

collected 66 m above the base. Brachiopods from low in the Jefferson on the northern side of Morgan Gulch (SWJ/4 sec. 13, 

T. 8 S., R. 2 W.) include Atrypa sp. and Cyrtospirifer sp. No occurrences of Atrypa above the Frasnian are known, and Phillipsastrea is a common Frasnian coral in many parts of western North 

America.

Devonian rocks underlying the Jefferson Dolomite or Limestone 

have been reported from several areas in southwestern Montana, 

but they are difficult to correlate because they are poorly exposed 

and their thickness and lithologic character vary considerably 

from place to place. A sequence of light-gray and buff-weathering 

limestone and shale 60 to 90 m thick, which has a prominent sandstone near the base, was termed the Maywood Formation in the 

Philipsburg district by Emmons and Calkins (1913, p. 64-65). 

The upper part of this sequence resembles the basal part of the 

Jefferson unit described herein, but the latter contains no sandstone and, unlike the Maywood, does contain fossils and chert 

like those in the overlying Jefferson. In the Three Forks area, 

beds assigned to the Maywood by Robinson (1963, p. 24-26) consist of 9 to 30 m of highly colored siltstone and mudstone and interbedded limestone in the upper part. The lower part contains 

thin lenses of siltstone pebble conglomerate in a few places. As 

in the southern Elkhorn Mountains, where 12.5 m of silty dolomite, dolomitic siltstone, shale, sandstone, and very minor limestone were assigned to the Maywood (Klepper and others, 1957, 

p. 13), the Maywood is mostly a clastic rather than a carbonate 

unit and is not readily correctable on lithologic grounds with 

the basal part of the Jefferson in the Varney quadrangle. No dense 

dark limestone comparable with that found in the main part of the 

Jefferson at other localities, such as at Logan, Mont. (Sloss and
PALEOZOIC ROCKS 29

Laird, 1947, p. 1409), occurs in the lower part of the Jefferson 

sequence in the Varney quadrangle.

THREE FORKS FORMATION

The rocks that overlie the Jefferson Dolomite or Limestone 

throughout the Varney quadrangle are assigned to the Three 

Forks Formation as denned by Sandberg (1965). Subdivisions 

proposed by Sandberg include the Logan Gulch Member, which 

lies directly on the Jefferson Dolomite, followed in ascending order 

by the Trident and Sappington Members. All three of these members can be recognized in the Varney quadrangle, where they have 

a combined thickness of about 90 m.

Logan Gulch Member. The Logan Gulch Member iof the Three 

Forks Formation consists of a lower unit of limestone and dolomiite breccia about 30 m thick, overlain by an easily recognizable 

brownish, purplish-gray, or chocolate-colored limestone generally 

15 to 22 m thick. The breccia unit consists of abundant fragments 

of pinkish- or reddish-orange limestone and light-yellowishgray or grayish-orange dolomite 30 cm or less to a meter in 

diameter, randomly embedded in a matrix of coarse calcite and 

red argillaceous nonbedded limestone. Even where it is not exposed, the presence of the breccia is commonly marked by red 

soil. Large fragments of the overlying brown limestone appear in 

the upper part of the breccia unit in the better exposures, supporting interpretations by previous workers that the breccia unit is a 

collapse breccia caused by removal of easily soluble evaporite 

minerals. The presence of similar though much thinner breccia 

beds in the upper part of the Jefferson (unit 20, stratigraphic 

section 2) also supports Sloss' contention that the red breccia 

represents a continuation of sedimentation in restricted basins 

under the conditions of semiaridity that characterize parts of the 

Jefferson and contemporaneous formations in the northern Rocky 

Mountains (Sloss and Laird, 1947, p. 1410).

In contrast to the red breccia, the limestone of the Logan Gulch 

Member is commonly exposed and forms a conspicuous narrow 

ridge between the breccia and the overlying Trident Member. In 

most exposures, the limestone unit is recognized by its finely 

crystalline to aphanitic texture and characteristic purplish- or 

chocolate-brown color. It is generally massive, in part thin bedded 

or laminated, and is commonly brecciated and veined by white 

calcite. The limestone unit ranges in thickness from a minimum 

of 13 m at Dry Hollow to 22 m on the northwestern slope of Baldy 

Mountain. It is nearly unfossiliferous, although brachiopods, iden-
30 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

tified as Cyrtospirifer by J. T. Dutro, Jr. (oral commun., 1958), 

were found at one locality on the north slope of Baldy Mountain. 

Trident Member. As denned by Sandberg (1965), the Trident 

Member of the Three Forks Formation includes dark fossiliferous 

shale and interbedded limestone that were the basis for the 

original description of the Three Forks (Peale, 1893, p. 29-32). 

The only good exposure of the Trident in the report area is in a 

roadcut on Call Road at the north side of Dry Hollow, where 

approximately 12 m of greenish- to dark-gray shale and interbedded fossiliferous limestone overlies about 15 m of poorly exposed light-yellowish-gray or yellowish-orange platy argillaceous 

limestone that, in turn, overlies the chocolate-brown limestone of 

the Logan Gulch Member. Somewhat farther south, in Morgan 

Gulch, the yellow limestone in the Trident is 22 m thick and also 

lies on the limestone of the Logan Gulch. The Trident, very rarely 

exposed in the report area, is usually represented by a covered 

interval in which fragments of the platy yellow limestone are 

the only indication of its presence. Because of lack of exposure, 

the thickness of shale and interbedded limestone in the Trident 

is not well known but is probably not much greater than that at 

Dry Hollow.

Sappington Member. The upper part of the Three Forks Formation throughout the report area consists of distinctive yellowish-orange calcareous siltstone and fine calcareous and silty 

sandstone corresponding to the Sappington Sandstone Member of 

earlier reports (Sloss and Laird, 1947, p. 1411; Sandberg, 1962). 

Except for the limestone unit of the Logan Gulch Member, the 

Sappington Member is the most commonly exposed part of the 

Three Forks in the Varney and Cameron quadrangles and is 

commonly recognized as float in poorly exposed intervals. It consists largely of yellow-weathering, strongly calcareous or argillaceous siltstone and fine silty quartz sandstone, locally grading to 

very argillaceous limestone. The sand component typically increases upward; the limestone is generally found in the lower part 

and resembles the yellowish-orange limestone in the lower part 

of the Trident. The base of the member is not exposed in the report 

area, but in the better exposed sections, the Sappington is at least 

18 m thick. It is overlain by the basal beds of the Madison Group, 

and this contact is exposed at several places on Baldy Mountain 

and in the section on Ruby Creek (stratigraphic section 2). In 

these exposures, the uppermost sandy beds of the Sappington are 

overlain by coarse bioclastic calcarenite 0.6 to 0.9 m -thick that 

locally contains small fragments of the underlying sandstone. The
PALEOZOIC ROCKS 31

black shale, which in other areas intervenes between the Sappington and the Madison Group (McMannis, 1955, p. 1397; Sloss and 

Laird, 1947, p. 1411), is not present.

MADISON GROUP

The Three Forks Formation is overlain by a very thick succession of carbonate rocks known throughout the region as the 

Madison Limestone or Group. These rocks are well exposed in the 

Varney quadrangle, notably on Baldy Mountain and in the lower 

part of the canyon of Wigwam Creek. They also form the high 

peak in sec. 21 and 22, T. 8 S., R. 3 W., and are prominent in the 

belt of Paleozoic rocks at the eastern edge of the Cameron quadrangle. Their average thickness in these areas is about 550 m. 

Following the usage established by Sloss and Hamblin (1942), the 

Madison Group in the Varney and Cameron quadrangles is divided 

into a lower thinner bedded formation, the Lodgepole Limestone, 

and an upper thicker bedded formation, the Mission Canyon Limestone. The type sections of the Madison Group and its constituent 

parts were recently described in detail by Sando and Dutro 

(1974).

LODGEPOLE LIMESTONE

The Lodgepole Limestone consists of thin- to medium-bedded, 

bluish- to brownish-gray limestone characterized by thin interbeds 

and partings of yellowish- or brownish-gray calcareous shale or 

argillaceous limestone. Thicker beds are generally 0.3 to 0.9 m 

thick, are commonly calcarenitic,.and contain bioclastic material, 

especially in their lower parts. Thinner beds are 2.5 to 12 cm thick 

and generally consist of calcilutite and fine calcareous sandstone. 

Although the lower parts of many beds are somewhat coarser than 

the upper parts, graded bedding like that in turbidites is not 

common; the dominant bedding consists of parallel beds of laminae and small-scale cross lamination indicating deposition by 

rather weak currents. Shaly beds are generally 1 to 10 cm thick, 

but, locally, shale and (or) argillaceous limestone form stratified 

units as much as 1 m thick. Chert is not abundant anywhere in 

the formation; where present, it is commonly porous rather than 

compact and occurs as irregular ramifying masses or small discontinuous bodies along the bedding.

Argillaceous beds diminish in number and thickness upward in 

the Lodgepole, and intervals 1 to 9 m thick of thicker bedded, 

coarser limestone are found in the upper 30 m or so on Baldy 

Mountain and elsewhere. One such unit, 9 m thick and 38 m below
32 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

the top of the formation, is recognizable in several exposures in 

the northeastern part of the Varney quadrangle. The top of the 

Lodgepole is placed, for convenience, at the upper limit of conspicuously thin- or medium-bedded limestone containing shaly 

partings, and the transition to the thicker bedded, essentially nonargillaceous limestone of the Mission Canyon is generally abrupt. 

The base of the Lodgepole is well defined at many localities where 

the basal bed is a moderately coarse bioclastic limestone 0.6 to 

0.9 m thick that lies directly on yellow calcareous sandstone or 

siltstone of the Sappington Member of the Three Forks Formation. 

Thickness1 of the Lodgepole, as calculated from map data in the 

better exposed areas, ranges from 190 m in the southern part of 

the Greenhorn Range and 200 m at Morgan Gulch to 240 m at 

Baldy Mountain and in the Wigwam Creek Canyon. The last figure 

agrees well with the thickness of 230 m reported by Sando and 

Dutro (1960, pi. 1) in a measured section on Baldy Mountain. The 

Lodgepole is also about 240 m thick at the eastern edge of the 

Cameron quadrangle.

MISSION CANYON LIMESTONE

The Mission Canyon Limestone is conspicuously thicker bedded 

than the Lodgepole, lacks argillaceous beds and partings, and has 

mostly coarser beds. Thin-bedded limestone or dolomite appears 

locally but is generally not argillaceous. Many of the thicker beds, 

1 to 1.5 m thick, are bioclastic 'or epiclastic calcarenite that may 

be cross laminated in parallel or concave sets 30 cm or more thick. 

A few beds in the middle and upper parts of the formation are 

oolitic. In contrast to the Lodgepole, a considerable part of the 

Mission Canyon is dolomite, generally somewhat darker and 

browner than the normal limestone but usually detectable only by 

chemical means. Nearly one-third of the formation, as recorded 

by Sando and Dutro (1960), is dolomite, as is a large part of both 

thin- and thick-bedded rocks in the section in Wigwam Creek 

Canyon. Nodular or ovoid chert is sparingly present in the lower 

part of the formation but is more abundant and more regularly 

bedded in the upper part. Lenses of solution or collapse breccia 1 

to 6 or more meters thick are fairly common in the uppermost 30 

to 60 m of the formation, especially in the vicinity of Wigwam 

Creek. They are commonly partly filled with reddish argillaceous 

deposits and are considered to have been produced during erosion 

of the Madison Group before deposition of the overlying Amsden 

Formation.
PALEOZOIC ROCKS 33

The thickness of the Mission Canyon, calculated from map 

measurements, ranges from about 300 m on Baldy Mountain and 

305 m on Wigwam Creek to 370 m or 380 m in the central part of 

the Greenhorn Range and on Ruby Creek in the southeastern part 

of the Varney quadrangle. It appears to be about 300 m thick in 

the Cameroon quadrangle.

PALEONTOLOGY AND CORRELATION OF THE 

MADISON GROUP ON BALDY MOUNTAIN

BY WILLIAM J. SANDO AND |. THOMAS DUTKO, JR.

An excellent section of the Madison Group is exposed on the 

north flank of Baldy Mountain in the northwestern quarter of the 

Varney quadrangle. This section was studied in detail in the 

summer of 1958 as a part of biostratigraphic studies of the Madision, in the northern Rocky Mountain region. A description of the 

section is presented below.

Section of Madison Group on Baldy Mountain

('Section begins at base of Lodgepole Limestone exposed in SE 1/, NW/i sec. 27, T. 7 S., R. 

3 W., Madison County, at an altitude of about 2,700 m and proceeds upspur above Garrison 

mine to unit 48 in Mission Canyon exposed at crest of ridge. Units 48-51 measured in S'X> 

sec. 26. Section measured by W. J. Sando and J. T. Dutro, Jr., 1958. U.S. Geol. Survey 

upper Paleozoic fossil localities (USGS -PC; see table 3) given in parentheses |

Thickness 

(meters)

Big Snowy-Amsden sequence:

52. Covered. Red-weathered, fine-grained sandstone and 

siltstone intermingled with limestone float from 

below; one bed seemingly in place at contact; 

about 9 m of relief noted along contact. 

Madison Group:

Mission Canyon Limestone:

51. Limestone, fine-grained, like unit 12; probably 

most of unit is pelletal; crinoidal and other 

bioclastic debris 5 percent or less; chert about 

20 percent; beds 0.3 to 0.9 m thick; forms massive benches on slope (USGS 17493-PC from 

lower 6 m) ______________________ 38.0

50. Covered. Limestone and chert float from unit below 24.0

49. Limestone, dark-yellowish-brown, fine-grained, 

weathering medium light to light gray; oolitic 

in part, very little bioclastic debris; about 20 

percent orange-weathering irregular chert 

sheets; beds 1 to 1.5 m thick; forms massive 

bench (USGS 17492-PC, 21 m above base- __ 16.0

48. Limestone, dark- to pale-yellowish-brown, weathering medium light to light gray, fine-grained, no 

bioclastic debris; 10 percent brown-weathering 

chert nodules; beds 0.6 to 1.5 m thick; unit partly
34 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT. 

Section of Madison Group on Baldy Mountain Continued

Thickness 

(meters)

Madison Group Continued

Mission Canyon Limestone Continued

brecciated, breccia is dominantly limestone having a carbonate matrix; forms south slope of 

ridge (USGS 17963-PC, 15 m above base; 17964- 

PC from lower 6 m) _________________ 16.6

47. Limestone breccia; limestone fragments 3 cm or 

less in diameter, fine-grained, weathering medium 

light gray in yellowish-weathering matrix; 

about 90 percent covered ___________________ 11.3

46. Dolomite, like unit 45; about 90 percent covered __ 19.8

45. Dolomite, pale- to dark-yellowish-brown, weathering yellowish gray to pale yellowish brown, 

mostly fine-grained, finely laminated; bioclastic 

debris 5 to 10 percent; indistinct beds; chert 

about 20 percent, consists of brownish to jaspery 

nodules, feathery stringers, some subspherical 

nodules having concentric bands; beds 3 to 30 

cm thick, platy; much brecciation and evidence of 

faulting _________________________ 26.0

44. Limestone, medium- to coarse-grained, crinoidal, 

like unit 42; chert 10 percent; upper contact 

gradational; top of unit is at crest of ridge, cairn 

marks top (USGS 17961-PC 0.6 m above base) _ 3.7

43. Dolomite, dark-yellowish-brown, weathering light 

olive gray to yellowish gray, fine- to medium 

grained, scattered coarse crinoidal debris; about 

20 percent brown and pink smooth chert nodules; 

beds 6 to 30 cm thick; both contacts gradational 3.0

42. Limestone, colors like unit 41, medium- to coarsegrained, crinoidal; chert as in unit 41 about 20 

percent, crossbedded _______________________ 7.0

41. Limestone, mostly dark-yellowish-brown, weathering medium light to light gray, fine-grained; 

about 10 to 20 percent coarse bioclastic debris; 

10 percent smooth brown-weathering chert nodules and tabular lenses; scattered brownishweathered dolomitic nests; beds 1 to 1.5 m thick; 

forms massive cliffs (USGS 17959-PC, float from 

lower 15 m; 17960-PC, 9 to 15 m above base, 

17962-PC, 9 m above base) ____________ 21.4

40. Dolomite, colors like unit 34; fine-grained, very 

little bioclastic debris; about 5 percent smooth, 

brown-weathering chert nodules; beds 0.3 to 0.6 

m thick; forms prominent notch below top of 

ridge __________________________ 16.2

39. Limestone, medium- to coarse-grained, crinoidal,

like unit 22, massive ________________ 10.0
PALEOZOIC ROCKS 35

Section of Madison Group on Baldy Mountain Continued

Th iclcncsn 

(meters)

Madison Group Continued

Mission Canyon Limestone Continued

38. Dolomite, colors like unit 34, medium- to coarsegrained, crinoidal; beds 0.3 m thick ____ __ 2.4

37. Limestone, medium- to coarse-grained, crinoidal, 

like unit 22; about 10 percent smooth brownweathering chert nodules; beds 1 to 1.5 m thick; 

travertine-filled joints; forms massive cliffs 

(USGS 17958-PC, 0.6 m below top) ________ 11.0

36. Dolomitic limestone, medium- to coarse-grained,

crinoidal; upper contact gradational _____ 0.6

35. Limestone, medium- to coarse-grained, crinoidal, 

like unit 22; upper contact gradational (USGS 

17957-PC, from upper half) ____________ 0.6

34. Dolomite, dark-yellowish-brown, weathering light 

olive gray to yellowish gray, fine- to mediumgrained ; many beds have 1- to 5-mm lamination; 

worm burrows common; bioclastic debris 5 percent or less; about 10 percent brownish-weathering chert in smooth and feathery irregular nodules, mostly in lower half; beds 0.3 to 0.6 m 

thick __________________________ 4.6

33. Limestone, medium- to coarse-grained, crinoidal, 

like unit 22; a 0.3-m bed of fine-grained dolomitic 

limestone 1.5 m below top; about 10 percent 

jaspery, brown-weathering irregular chert nodules; crossbedded ____________________ 7.0

32. Dolomite, like unit 30; crinoidal and bryozoan 

debris throughout and concentrated in lower 1.2 

m; 5 percent brownish chert nodules and stringers; beds 0.3 to 0.9 m thick (USGS 17956- 

PC, float at middle) _________________ 20.7

31. Limestone, coarse-grained, crinoidal, like unit 22; 

crossbedded; a single bed that forms massive cliff; 

gradational into unit above ___________________ 5.5

30. Dolomite dark-yellowish-brown, weathering light 

olive gray to yellowish gray, fine- to mediumgrained; interbedded micrite and bryozoancrinoidal debris; less than 5 percent dark-brown 

chert nodules and stringers; beds 9 to 60 cm 

thick; forms rubble-covered notch (USGS 17955- 

PC, float from unit) _________________ 15.5

29. Limestone, medium- to coarse-grained, crinoidal, 

like unit 22; crossbedded; beds 0.2 to 1.1 m thick; 

grades into unit above ______________ 2.7

Total Mission Canyon Limestone ______ 284

Lodgepole Limestone:

Woodhurst Limestone Member:

28. Limestone, like unit 25 but bedding averages a 

little thinner (3 to 9 cm) and nodular bedding 

is more common; upper 1.5 m dolomitic; unit
36 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT. 

Section of Madison Group on Baldy Mountain Continued

Thickness* 

(meters)

Madison Group Continued

Lodgepole Limestone Continued

Woodhurst Limestone Member Continued

forms massive cliff because silty beds do not 

weather free ________________________________ 14.5

27. Limestone, like unit 25 (USGS 17953-PC, from

float throughout; 17954-PC, 0.6 m below top) __ 21.4

26. Limestone, like unit 25; bedding deformed by fault 

that cuts across bedding at an angle of 40° to 

60° _____________________________________ 3.4

25. Limestone, fine- to coarse-grained; fine-grained 

limestone beds 6 to 15 cm thick compose about 80 

percent of unit; yellowish- to purplish-weathering silty partings are about 10 percent of unit; 

coarse-grained crinoidal limestone beds as much 

as 30 cm thick are about 10 percent of unit; 

fossils mostly in silty partings, Zoophycos and 

other trace fossils common (USGS 17951-PC, 

4.3 to 4.6 m above base; 17952-PC, 10 m above 

base) __________________________ 12.5

24. Limestone, medium- to coarse-grained, crinoidal, 

like unit 22; beds 0.3 to 0.9 m thick (USGS 

17950-PC, 1.5 to 2.4 m above base) _________ 2.7

23. Limestone, fine- to coarse-grained, like unit 20;

forms notch ________________________ 2.0

22. Limestone, dark- to pale-yellowish-brown, weathering medium light to light gray, medium- to 

coarse-grained; consists of medium- to coarsegrained crinoidal debris in micrite matrix; 

crossbedded; beds 15 to 60 cm thick; forms 

prominent cliff ____________________________ 4.3

21. Limestone, like unit 20, but coarse bioclastic beds 

constitute about 30 percent of unit; many yellowish-weathering silty beds (USGS 17947-PC, 

3 m above base; 17948-PC, 4.3 m above base; 

17949-PC, 1.8 m below top) ____________ 9.0

20. Limestone, fine- to coarse-grained, like unit 18; 

includes several 30-cm beds of coarse bioclastic 

limestone; nodular-bedded in part; bedding 

planes weather yellowish and purplish; abundant 

worm tracks; many cross joints filled with dogtooth spar; upper half mostly covered by talus 

(USGS 17945-PC, 5.8 m above base; 17946-PC, 

float 25 m above base probably from bed in upper 

meter) _________________________ 27.4

19. Limestone, fine- to coarse-grained; micrite to fine 

calcarenite (50 percent), silty beds (10 percent), 

coarse bioclastic beds (20 percent); about 20 

percent chert as in unit 17; beds 6 to 30 cm 

thick (USGS 17944-PC, 1.2 to 1.5 m above base) 5.6
PALEOZOIC ROCKS 37

Section of Madison Group on Baldy Mountain Continued

Thickness 

(meters)

Madison Group Continued

Lodgepole Limestone Continued

Woodhurst Limestone Member Continued

18. Limestone, fine- to coarse-grained; micrite to fine 

calcarenite (70 percent), silty beds (10 percent), 

coarse bioclastic beds (20 percent); beds 6 to 30 

cm thick (USGS 17941-PC, from lower 30 cm; 

17942-PC, 7.3 m above base; 17943-PC, 9.5 m 

above base) _________________________________ 11.0

17. Limestone, olive-gray to light-olive-gray, weathering medium to light gray, fine- to coarse-grained; 

micrite to fine calcarenite (65 percent), silty, 

yellowish-weathering beds (10 percent), coarse 

bioclastic beds (5 percent) ; brownish-weathering chert in spongy irregular sheets and nodules 

(20 percent) ; worm tracks; forms prominent 

bench (USGS 17939-PC, 0.9 m above base; 

17940-PC, 4.6 m above base) __________ 7.8

16. Limestone, fine- to coarse-grained; micrite to fine 

calcarenite (50 percent); silty, argillaceous platy 

beds (20 percent), coarse bioclastic beds (30 percent) ; bedding planes weather yellowish and 

purplish; limestone is olive gray, weathering 

medium light and light gray; Zoophycos and 

other trace fossils; unit is poorly resistant 

(USGS 17936-PC, 1.2 m above base; 17937-PC, 

float 1.5 m above base; 17938-PC, from upper 

0.6 m) __________________________ 7.9

15. Limestone, like unit 14 but contains more bioclastic beds (about 20 percent) and about 10 

percent fuzzy tabular lenses of incipient brown 

chert; forms bench; first occurrence of Vesiculophyllum 0.6 m below top (USGS 17930-PC, 

from lower 30 cm; 17931-PC, 3 m above base; 

17932-PC, float throughout; 17933-PC, 5.2 m 

above base; 17934-PC, 1.5 m below top; 17935- 

PC, 0.6 m below top) ________________ 12.2

14. Limestone, fine- to coarse-grained; interbedded 

micrite to calcarenite (90 percent) and coarse 

bioclastic debris in thin beds and lenses (10 percent) ; platy beds weather yellowish and purple; 

Zoophycos and other trace fossils common; beds 

6 to 30 cm thick, bedding irregular; forms cliffy 

slope (USGS 17927-PC, float 3 m above base; 

17928-PC, 7.6 m above base; 17929-PC, 10.7 m 

above base) ________________________________ 12.5

13. Limestone, medium-gray to olive-gray, weathering 

medium light gray with much orange stain, 

coarse-grained, bioclastic; contains about 10 percent pebbles of fine-grained limestone; cross-
38 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

Section of Madison Group on Baldy Mountain Continued

Thickness 

(meters)

Madison Group Continued

Lodgepole Limestone Continued

Woodhurst Limestone Member Continued

bedded; first occurrence of Homalophyllites 

(USGS 17926-PC, from throughout unit) ___ __1.5 

Total Woodhurst Limestone Member of

Lodgepole Limestone ____________ 155.8

Paine Shale Member:

12. Limestone, fine-grained, argillaceous, like unit 4; 

well-laminated, faint irregular laminae 1 to 4 mm 

thick; contains mud-ball structures, worm tracks 

poorly exposed, forms rubbly slope ________ 10.4

11. Limestone, fine-grained, argillaceous, like unit 3; 

weathers platy and hackly; laminae 1 to 2 mm 

thick; forms prominent bench ______________ 1.8

10. Limestone, fine- to medium-grained, argillaceous; 

interbedded micrite, like unit 4 (85 percent), and 

fine calcarenite, like unit 5 (15 percent); upper 

15 m poorly exposed; forms rubbly slope (USGS 

17920-PC, 0.3-0.6 m above base; 17921-PC, 3 m 

above base; 17922-PC, 6 m above base; 17923- 

PC, float from lower 6 m; 17924-PC, float 

throughout unit; 17925-PC, float from upper 

16 m) __________________________ 22.5

9. Limestone, medium-grained, like unit 5; 10 percent 

incipient ferruginous chert in layers; mudcracks^) _______________________ 0.9

8. Limestone, fine-grained, like unit 4 (USGS 17919-

PC from throughout unit) ____________________ 1.5

7. Limestone, mottled orange, medium-grained, like

unit 5, crossbedded ______________________ 0.2

6. Limestone, fine-grained, like unit 4; purple bedding 

surfaces common in silty beds (USGS 17918-PC, 

1.8 m above base) ___________________________ 3.7

5. Limestone, grayish-brown, weathering brownish 

gray, medium-grained; laminated, crossbedded 

fine calcarenite; small nodules of orange-weathered ferruginous chert; one bed ______________ 0.2

4. Limestone, fine-grained, argillaceous, like unit 3 but 

more conspicuously bedded; beds 6 to 15 cm 

thick, separated by 3- to 12-cm beds of silty, 

purplish platy limestone; limonite concretions: 

spaghettilike worm markings; forms rubbly slope 

(USGS 17916-PC, 4.5 m below top; 17917-PC, 

upper 0.9 m) _____________________ 13.7

3. Limestone, olive- to light-olive-gray, weathering 

medium light to light gray, fine-grained, argillaceous; micrite and silt-sized carbonate; beds 

9 to 30 cm thick; silty layers define bedding 

planes but are not conspicuous; about 5 percent
PALEOZOIC ROCKS 39

Section of Madison Group on Baldy Mountain Continued

Thickness 

(meters)

Madison Group Continued

Lodgepole Limestone Continued

Paine Shale Member Continued

scattered hematite nodules 3 cm in average diameter; forms prominent hackly bench (USGS 

17911-PC, from lower 0.6 m; 17912-PC, 2 to 3 

m above base; 17913-PC, 5.5 to 7 m above base; 

17914-PC, 8.2 to 10 m above base; 17915-PC, 

11 to 14.5 m above base) __________________ 16.8

2. Limestone, olive-gray, weathering medium to 

medium light gray, mottled orange; coarsegrained, crinoidal; beds 30 cm thick __________ 0.9

Total Paine Shale Member of Lodgepole

Limestone _________________________ 73.7

Total Lodgepole Limestone _____________ 229.5

Total Madison Group __________________ 513.5

Three Forks Formation:

Sappington Sandstone Member:

1. Quartz sandstone, dark-yellowish-brown, finegrained fucoidal; beds 30 cm thick; mostly 

covered.

A rich invertebrate fauna collected from the Baldy Mountain 

section permits biostratigraphic zonation of the Madison Group 

according to the scheme originally published by Sando and Dutro 

(1960). The results of subsequent study, by B. L. Mamet, of 

foraminiferal samples from this locality were incorporated in 

Sando, Mamet, and Dutro (1969). A list of megafossils from 

Baldy Mountain, showing their distribution in the Madison, is 

given in table 3.

All the lithic divisions of the type Madison are recognized 

in sequence, and representatives of Kinderhookian, Osagean, 

and lower Meramecian divisions of the type Mississippian 

have been established by means of the fossils (table 3). Zone 

boundaries are recognized principally on coral ranges. Zone A, 

originally reported by Sando and Dutro (1960) in the section, is 

no longer recognized here because of the lack of certain critical 

coral species. No brachiopods characteristic of Zone A occur in 

the section, and the presence of Leptagonia cf. L. analoga 

(Phillips) in the lowest collection confirms the evidence of the 

corals that the earliest fossils here are assignable to Zone B.

Characteristic Zone B brachiopod assemblages are found in the 

strata identified as this zone on the basis of corals. However, 

ranges of some brachiopod species are greater than those found 

in other sections in southwestern Montana. For example, Cyrtina
40 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

TARLE 3. -Invertebrate fossils collected from the Madison Group onBaldy

[Fossils collected by W. J. Sando and J. T. Dutro, Jr., and identified by Helen Duncan (bryozoans), J. T. Dutro, Jr. (brachiopods and

MEGAFOSSILS

PAINE SHALE MEMBER

ZONE B

Corals:

Cyathaxonia cf. C_. tantilla (Miller)

Permia? sp.

"Amplexus" sp.

Zaphrentites spp.

Homalophyllites spp.

"Menophyllum" cp.

Canadiphyllum? sp.

Vesiculophyllum sp.

Horn corals, indet.

Diphyphyllum sp.

Lithostrotionella microstylum (White)

Michelinia cf. M. expansa White

M.? sp.

Aulopora sp.

Syringopora aculeata Cirty

S^. surcularia Girty

S. aff. s. surcularia Girty

3. sp. indet.

3yringoporoid coral 

Bryor.oans:

Cliotrypa? sp.

F.ridopora sp.

Fistuliporoid bryozoan

Ramiporalia sp.

Fenestella cf. F. tenax Ulrich

F. spp.

Polypora sp.

Ptiloporella sp.

Fencstrate bryozoan

Penniretepora sp.

Ptilopora sp.

"Rhombopora" sp.

Streblotrypa? sp.

Rhomboporoid bryozoan

Cystodictya sp.

Dichotrypa sp.

Trepostomatous bryozoan, ramose form

Bryozoan, indet.

X XXX

XXX X 

XXX

XX

XX 

X

X

xxxx

XX

XX X

xxx

X XX X
PALEOZOIC ROCKS 41

Mountain (NW1/* sec. 27 and sec. 26, T. 7 S., R. 3 W., Madison County, Montana)

echinoderms), W. J. Sando (corals), and E. L. Yochelson (mollusks). Collections are arranged with lowest on left and highest on right.]

L 0 G E p Q L::MESTONE

WOODl tUS LIM] !STONE ME) IER

I MISSION CANYON 

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ZONE C: ZONE P

US( IS Upper : 'aleozoic :\>S8J1_Localities (-]>Cj

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X X XX X*

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x x

X

xxxxxx

x?xxx

XX

X X

XX XXI

XX 

X
42 GEOLOGY, VARNEY AND CAMERON QUADRANGLE©,

TABLE 3. -Invertebrate fossils collected from the Madison Group on Baldy Mountain

MEGAFOSSILS

PAINE SHALE MEMBER

ZONE

Kchinoderms:

Cryptoblastus cp.

Aclinocrlnitid crlnoid columnals and plates

Platycrinit.es sp.

Platycrlnltld crinoid columnals and plates

Echlnoid plateo 

Brachiopods:

Philhedra sp.

Rhipidomella sp.

Schizophoria cf. S. swallovi (Hall)

Leptafionia cf. L. analoga (Phillips)

Schuchertella sp.

Jchcllwjenella sp.

Orthotctid brachiopocl

Cacnanoplia? logani (Norwood and Pratten)

Hucosochonetes Igcanensis (Hall and Whitfield)

i^. np.

Chonetoid brachlopod

Orbinaria sp.

Marcinatia cf. M. burlingtonensis (Hall)

M_. cf. M.. fcrnfLlenensi's" (Weller)

M. CP.

nuxtonia? cf. B.? viminalis (White)

^j. ? ap.

Ovatia sp.

Echinpconchus cf. E. alternatus (Norwood and Pratten)

Dictyoclostid brachiopod

Productoid brachiopod

Axiodeaneia platypleura Clark

"Camarotoechia? sp.

Tetracamera? sp.

Khynchonelloid brachiopod

L\unetria sp,

Actinpconchus? sp.

Composita madisonensis (Girty)

C. humilis (Glrty)

C. sp.

Cleiothyriilina cf. £ tenuilineata (Rowley)

C. cf. C. incrassata (Hall)

C. aff. C. obmaxima (McChesney)

C.? up.

CrurithyrluV sp.

7

X

?x

X X

XX

XX
PALEOZOIC ROCKS 43

(NW1/* sec. 27 and sec. 26, T. 7 S., R. 3 W., Madison County, Montana) - Continued

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44 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

TABLE 3. - Invertebrate fossils collected from the Madison Group on Baldy Mountain

MEGAFOSSILS

PAINE SHALE MEMBER

ZONE

rowleyi Weller 

'S." blplicotdes Weller

Ambocoelid brachlopod 

Cyrtlna cf. C.. burlingtonensis Rowley 

C.? sp.

Unispirifer cf. tJ. centronatus (Winchell) 

U. op..

Amcsopleuru? cf. A.? semifurcatus (Girty) 

A.? sp. 

Imbrcxia sp. 

Spirifer cf. S.

"Spirit'rr" cf. _ ________ 

S. sp. (small)

Aiit-hracospirifer aff. A. keokuk (Hall) 

A. up.

Tylothyris? sp. 

Martinia rostrata Girty 

Torynifer cf. T. cooperensis (Swallow) 

T. '. sp.

Bracnythyris cf. B. suborbicularis (Hall) 

J3. sp.

Spiriferold brachlopod 

Syringothyris sp.

Punctosplrifer cf. P. solidirostris (White) 

P. sp.

Hustedia? sp.

Beecheria? chouteauensis (Weller) 

B. ? sp.

Terebratuloid brachlopod 

Brachiopod fragments, indet. 

Gastropods:

Straparollus (Euomphalus) cf. S. (E.) subplanus (Hall)

S. (E.) sp. Indet.

Rhineoderma cf. R. pealeana (Girty)

R. sp. indet.

Pleurotomarlacean gastropod cf. Eotrochus

Mourlonia sp.

"Pleurotomaria" cf. "P." br&zeriana Girty

Pleurotomariacean gastropod cf. Worthfnia

Platyceras (Platyceras) n. $p. (splno^e)

P. (P.) spp.

P. (Orthonychia) sp.

High-spired gastropod cf. Lpxonema

Meekospira? elegans (Girty)

x XXK

x x

X X

XX

XXX

XX 

XX 

X

X 

XX

X X
PALEOZOIC ROCKS 45

(NWk sec. 27 and sec. 26, T. 7 S., R. 8 W., Madison County, Montana)-Continued

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46 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

cf. C. burlingtonensis Rowley, previously thought to be confined 

to Zone B, ranges upward as high as USGS 17940-PC (U.S. 

Geological Survey upper Paleozoic fossil locality).

Zone Ci marks the advent of the corals Homalophyllites and 

Vesiculophyllum in the sequence as well as Lithostrotionella microstylum (White) and Michelinia cf. M. expansa White, which are 

indices of the zone. This zone in this section has most of the 

brachiopod taxa that characterize the zone in many places in the 

northern Rockies, but, again, some of the ranges are modified. 

Rugosochonetes loganensis (Hall and Whitfield) begins lower in 

this section than normal, being found as low as USGS 17917-PC. 

However, it does range above Zone C,, and the Ci-C2 boundary is 

a sharp one.

An interesting rhynchonelloid species, common in this section 

but rare elsewhere, is Axiodeaneia platypleura Clark. This elongate and straight-'sided shell, first described from the Baldy 

Mountain locality, ranges through the upper part of Zone B into 

the lower part of Zone d (USGS 17919-PC through 17932-PC).

Zone C2 is characterized by continuation of the corals Homalophyllites and Vesiculophyllum. Zone D is recognized by the 

appearance of the new coral elements Diphyphyllum and Zaphriphyllum. Zones C2 and D have very few brachiopods, but those 

that do occur are indicative forms. Anthracospirifer cf. A. keokuk 

(Hall), Cleiothyridina aff. C. obmaxima (McChesney), and a 

large Spirifer are found in Zone C2 ; Anthracospirifer sp., Tetracameral sp., and a large orthotetid brachiopod occur in Zone D.

Zone B is regarded as Kinderhookian in age. Osagean-age 

beds include Zones C, and C2 . Zone D is of early Meramecian 

(Salem) age.

BIG SNOWY GROUP AND AMSDEN FORMATION

The rocks that overlie the Madison Group1 in central and western 

Montana consist of a varied assemblage of limestone, shale, and 

red beds, whose age and stratigraphic relations have been debated 

for many years. In central Montana, the rocks in question have 

been generally assigned to the Big Snowy Group (Scott, 1935; 

Gardner, 1959); in Wyoming, they usually have been referred to 

the Amsden Formation and the Sacajawea Formation of Branson 

(1937).

In the Varney quadrangle, the rocks between the Madison Group 

and the Quadrant Quartzite are represented by two quite different 

sequences. One sequence, exposed in the Gravelly Range east of the 

Greenhorn fault, is less than 75 m thick, appears to be largely of
PALEOZOIC ROCKS 47

Early Penngylvaniatt age, and is tentatively correlated with the 

upper part of the Amsden Formation of north-central Wyoming. 

The other sequence, exposed in the vicinity of Baldy Mountain 

ftflfi farther south in the Greenhorn Range, is nearly 275 m thick 

afid includes beds of Pennsylvanian age as well as beds of Late 

Mississippiaii &<g@ f Which are correlated with the Big Snowy 

Group.

Rocks assigned to the Big Snowy Group can be traced for several kilometers in the Greenhorn Range along the ridges leading 

east and south from Baldy Mountain and across the floor of the 

cirque at the head of Arasta Creek. Measured sections in this belt 

are shown in figure 2. The lower 60 m or so of the Big Snowy 

Group is generally covered, but appears to consist largely of silty 

clay shale arid calcareous sandstone and minor interbedded greenish-gray gherty limestone. The clastic rocks are characteristically 

red, although gf6ehish-gray shale is locally abundant. Sandstone 

and red color are more common in the lower part of this interval; 

interbedded limestone is more common in the upper part. These 

dominantly clastic rocks are overlain by a sequence 76 to 110 m 

thick of thin- to thick-bedded limestone, generally pale olive gray 

and commonly very fossiliferous. A unit of coarse bioclastic limestone 7.6 to 12 m thick (unit 7, stratigraphic section 4) was traced 

for several miles, and a bed containing abundant corals of the 

species Caninia excentrica in growth position, identified by W. J. 

Sando, was found in two of the sections studied. The interbedded 

shale in this dominantly limestone interval is silty clay shale, 

nowhere sandy, generally dark greenish or olive gray, and lacking 

megafossils.

Little correlation could be established with the six formations 

of the Big Snowy Group as described in the Big Snowy Mountains, 

275 km northeast of the Varney quadrangle (Gardner, 1959). The 

basal 60 m of red silty shale and sandstone, however, is similar to 

the Kibbey Sandstone or Formation that rests unconformably on 

the Madison Limestone in the Big Snowy Mountains (Scott, 1935, 

p. 1026). The overlying sequence of fossiliferous limestone and 

shale in the Varney quadrangle may contain equivalents of the 

Otter and Heath Formations, but direct correlation has not been 

made.

Fossil collections by J. T. Dutro, Jr., and W. J. Sando from parts 

of section 147 (fig. 2) contain an abundant shelly fauna of Late 

Mississippian age (J. T. Dutro, Jr., written commun., 1970).

The thinner post-Madison rocks east of the Greenhorn fault 

(fig. 2, columnar sections W. C., 166, 170, 242) consist largely of
48 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

W.C. 166 170

QUADRANT QUARTZITE OR FORMATION

242

4-

3-

2-

0-1

17969 

float

MISSION

CANYON

LIMESTONE

LOCATION OF SECTIONS

W.C. North side Wigwam Creek canyon, Gravelly Range

166. 1.6 km northwest of Crockett Lake, Gravelly Range 

170. 1.6 km southwest of Crockett Lake, Gravelly Range 

242. Head of Middle Fork, Warm Springs Creek, Gravelly Range 

147. Floor of cirque on Arasta Creek, Greenhorn Range 

161. South ridge Baldy Mountain, Greenhorn Range

152. South ridge Baldy Mountain, Greenhorn Range 

» USGS Upper Paleozoic locality number

FIGURE 2. Columnar sections of the Amsden For

varicolored limestone, dolomite, and shale; shale becomes increasingly abundant in the lower part of the unit. The top of the unit is 

fairly well denned by the appearance of clean, well-sorted white
PALEOZOIC ROCKS 49

147 161 152 

QUADRANT QUARTZITE OR FORMATION

\ 

\

\

:

/ / /

i i i

i i

r^

J i

i i_

i IPI

V-T

AMSDEN FORMATION

-17502 ., 

^^

p 17501 1.^

- 17500 Caninia torquia

/

Caninia excentrica

/

/ 

/

X 

X

X / 

-17498 Caninia 

excentrica

/ /

/ / / / / /

i ' ! '

r r J

 '-r

-H-H

i i

LMi i

z_-

"."- -i

^ ;^

iV'-V.

_-

_ _

»i

~ ~_"

TO

 ' -1

"- -

4-

1 , 1

1 ! 1

1 1

E

? 

JIG !

Hr-jr

/

i /

m i i

~.~~.

W:#

:-$&.

' * ! '& 

__ ?

SNOWY GROUP

Fault 

EXPLANATION

Sandstone

Shale 

Dolomite 

Limestone

Sandy shale

i i

----

-Z--Z--

i-^.~:

i i

i i

£p-H-r

Conglomeratic sandstone

Shaly sandstone

mation and underlying beds, Varney quadrangle.

quartz sandstone of the Quadrant Quartzite or Formation, which 

commonly contains unoriented fragments of lightrgray dolomite. 

The clastic rocks of the lower part of the unit are dominantly red,
50 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

sandy, or silty shale or mudstone and fine-grained red silty sandstone. They lie directly on the Mission Canyon Limestone ; a variation in thickness apparently reflects karst topography. This relation is well exposed on the north wall of the Wigwam Creek 

Canyon (fig. 2, columnar section W. C.), where the basal clastic 

unit, 1 to 6 m thick, which consists of unsorted reddish-brown 

mudstone and scattered small fragments of chert, appears to represent a residuum accumulated on the underlying limestone and 

little reworked during deposition of the overlying rocks. Elsewhere, the clastic beds are a good deal thicker and more distinctly 

bedded (fig. 2, columnar sections 166 and 170). Interbeds of 

medium- to fine-grained calcareous sandstone are locally current 

bedded and contain well-rounded pebbles of chert and various 

types of limestone along with waterworn shells and fish teeth. 

Fossil collections from two of these sections (fig. 2, columnar sections W. . and 242) yielded only Early Pennsylvanian species, 

with the single exception of a chaetetiform bryozoan, possibly of 

Mississippian age ( J. T. Dutro, Jr., written commun., 1970) .

Correlative sections west of the Greenhorn fault (fig. 2, columnar sections 152 and 161) lie on beds assigned to the Big Snowy 

Group, as previously described. These sections contain significantly 

more shale in their upper parts than do the sections to the east, 

but they otherwise contain similar varicolored limestone and dolomite. Fossils collected from beds 52 to 72 m below the Quadrant 

Quartzite (columnar section 147) are Early Pennsylvanian (J. T. 

Dutro, Jr., and W. J. Sando, written commun., 1970) .

Although the upper part of the post-Madison, pre-Quadrant 

sequence in the Varney quadrangle is clearly of Early Pennsylvanian age, correlation with the type Amsden Formation of northcentral Wyoming is questionable. Possibly one or more of the thin 

sandstone units shown in columnar sections 161, 166, 170, and 152 

(fig. 2) may be the Darwin Sandstone Member, considered to be 

the basal member of the Amsden in nearby parts of northwestern 

Wyoming (Sando and others, 1975), but which, if any, of these 

thin sandstone units may have regional significance is not known.

Stratigraphic section 4

[Amsden Formation and Big Snowy Group, NW^ sec. 35 and SW1/^ sec. 26, T. 7 S., R. 3 

W., Varney quadrangle. Measured by J. B. Hadley and E. H. McKee]

Thickness 

(meters)

Quadrant Quartzite (incomplete) :

20. Dolomite, massive, brecciated, medium to coarsely crystalline, calcareous; in part contains abundant white quartz 

grains and fragments of light-gray fine-grained dolomite and white quartzite ______ _ ___________________ 12
PALEOZOIC ROCKS 51

Stratigraphic section 4 Continued

Thickness 

(meters)

Quadrant Quartzite (incomplete) Continued

19. Sandstone, white, weathering light brown, calcareous, poorly bedded, abundant well-rounded grains.

18. Covered ____________________________-__ 18 

Amsden Formation:

17. Dolomite, light-yellowish-gray to pale-yellowish-orange, 

medium- to coarse-grained; in part thin bedded (1-10 

cm); in part massive and brecciated, containing fragments of limestone and dolomite in limestone matrix. 

Upper meter is limestone, aphanitic, containing prominent calcite grains, light olive gray __________________ 4.6

16. Covered _________________________________ 26

15. Limestone, light-yellowish-gray, and dolomite, fine- to 

medium-crystalline, highly calcareous; thick to thin 

bedded, containing beds of fine sandstone, moderate reddish brown to pale red and dolomitic, and grayish-pink 

sandy dolomite. Thin bed of white quartz sandstone in 

upper part. Top bed of unit is light-gray limestone, 55 

cm thick, like unit 13. Fossil collection USGS 17052-PC, 

1.5 m below top of unit ______-______-_____-__------- 6

14. Lower 4.5 m poorly exposed, probably mudstone; chips in 

soil are fine sandy mudstone, pale green, purplish gray, 

and red, dolomitic. Upper part is limestone, light-yellowish-gray to pale-yellowish-orange variegated, fine-grained 

to aphanitic, thin- to medium-bedded. Considerable compact chert, reddish to brownish, in nodules and thin 

lenses. One 60-cm bed bioclastic calcarenite. Fossil collection USGS 17501-PC, 3.7 to 7 m above base of unit.__ 11

13. Limestone, yellowish-gray to pale-brown, aphanitic to finely 

crystalline, medium-bedded; minor yellowish- to reddishbrown compact chert in lenses 5 cm and less thick. Fossil 

collection USGS 17500-PC, 3.7 m above base of unit __ 4.6

12. Covered _______________________________ 50 

Big Snowy Group:

11. Limestone ____________________________________________ 3

10. Covered _______________________________ 6

9. Limestone, lower part olive gray to brownish gray, medium 

bedded, containing thin argillaceous beds; upper part, 

medium dark gray to olive gray, medium bedded, finely 

crystalline, considerable olive- to dark-gray chert lenses 

and nodules. Fossiliferous, 30- to 60-cm Caninia reef. 

Fossil collection USGS 17498-PC, from beds in upper 

part of unit ________________________________ 6

8. Covered ______________________________ 7.5 

(Section offset 1.2 km to northwest)

7. Limestone, thick-bedded, in part cross laminated, coarsely 

crystalline and bioclastic, locally aphanitic; forms promi-
52 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

Stratigraphic section 4 Continued

Thickness 

(meters)

Big Snowy Group Continued

nent ridge. Fossiliferous, prominent zone of horn corals 

3 m below top of unit. Fossil collections USGS 17496-PC 

and 17497-PC ____________________________ 11

6. Limestone, grayish-orange to pale-brown, mainly thin 

bedded, platy, in part argillaceous, interbedded with 8- 

cm beds olive-gray limestone; like unit 4, but less 

argillaceous _______________________________________ 7

5. Poorly exposed on crest of ridge. Limestone, yellowishbrown and dark-gray, medium to moderately thin bedded, 

ramifying chert; upper 0.6 to 6 m coarsely bioclastic. 

Fossil collection 4.6 m below top, USGS 17495-PC ___ 20

4. Limestone, light-olive-gray, medium bedded, aphanitic to 

medium crystalline containing beds of yellowish-gray, 

yellowish-orange, and reddish variegated argillacous limestone, few nodules dark-gray chert in lower part. Fossil 

collection from upper 7 m, USGS 17494-PC _______ 15

3. Shale, dark- to dusky-yellowish-brown, calcareous, silty, 

fissile, interbedded with olive-gray argillaceous, mediumbedded limestone. Bedding disturbed; thickness approximate ________-________________________________ 15

2. Largely covered. Includes at least 30 m clastic red beds ___ 55 

Mission Canyon Limestone:

1. Limestone, moderate-yellowish-brown and aphanitic to lightbrownish-gray and medium to coarsely crystalline, some 

bioclastic. Two coral zones 40 to 45 m below top ________ 84

The following fossil collections were made by J. T. Dutro, Jr., 

and W. J. Sando from beds between the Mission Canyon Limestone 

and the Quadrant Quartzite in the cirque valley at the head of 

Arasta Creek, just east of Baldy Mountain. The corals were identified by W. J. Sando; bryozoans, by Helen Duncan; pelecypods, by 

John Pojeta, Jr.; gastropods, by E. L. Yochelson; and brachiopods, 

by J. T. Dutro, Jr., all of the U.S. Geological Survey.

Upper Mississippian:

USGS 17494-PC. Big Snowy Group about 30 m above the basal redbed unit. Unit 4, Stratigraphic section 4.

trepostomatous bryozoan (encrusting form), undet.

Fenestella sp.

Septopora sp.

rhomboporoid bryozoan, genus indet.

Orbiculoidea sp.

Orthotetes! sp.

Nix angulata Easton

Ovatia cf. 0. ovata(Hall)

Flexaria sp.

Inflatia cf. I. inflata (McChesney)
PALEOZOIC ROCKS 53

Upper Mississippian Continued 

USGS 17494-PC Continued 

Crurithyrisl sp.

Punctospirifer transversus (McChesney) 

Limipecten sp. 

Aviculopecten sp. 

IPteronites sp. 

Myalina sp. 

Cypricardella sp. 

Nuculopsis sp. 

Phestia sp. 

Septimyalina sp. 

Parallelodon sp. 

ISphenotis sp. 

crassatellacean, indet. 

Platyceras sp. 

Bellerophon sp.

Straparollus (Euomphalus) sp. 

pleurotomarian gastropod, indet. 

Euphemites cf. J£. sacajawensis Branson 

"Bulimorpha" sp. 

Knightites (Retispira) sp. 

Paleostylus (Pzeudozygopleura) sp.

USGS 17495-PC, Big Snowy Group about 45 m above the basal redbed unit. Unit 5, stratigraphic section 4.

crinoid columnals, indet.

echinoid plates and spines, indet. cidarid

Zaphrentitesl sp.

Inflatia sp.

Anthracospirifer sp.

Composita sp.

Crurithyris sp.

Punctospirifer transversus (McChesney)

Reticulariina spinosa (Norwood and Pratten)

Eumetria sp.

Cranaenal sp.

terebratuloid, indet.

USGS 17496-PC, Big Snowy Group about 60 m above the basal redbed unit. Unit 7, stratigraphic section 4.

crinoid columnals, indet.

echinoid plates and spines, indet. cidarid

Pentremites sp.

Zaphrentitesl sp.

Michelinia cf. "Pleurodictyum meekanumt" of Nelson

(1962)

ramose and fenestrate bryozoans, undet. 

Orthotetes sp. 

Nix angulata Easton
54 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

Upper Mississippian Continued 

USGS 17496-PC Continued

chonetid, indet.

Inflatia sp.

Flexaria sp.

Ovatia sp.

Avonial sp.

productoid fragments, indet.

Anthracospirifer sp.

Composite, sp.

Eumetria sp.

Punctospirifer transversus (McChesney)

Reticulariina spinosa (Norwood and Pratten)

Girty ellal sp.

terebratuloid, indet.

Cypricardella sp.

Parallelodon sp.

INuculopsis sp.

Septimyalina sp.

Cypricardinia sp.

nuculoid pelecypod, indet.

pelecypods, indet.

Euphemites sp.

Bellerophon sp.

cf. Pleurotomaria brazeriana Girty

Knightites"! sp.

Naticopsis sp.

Agnesial sp.

Straparollus (Euomphalus) sp.

gastropod, indet.

orthoconic cephalopod, undet.

USGS 17497-PC, Big Snowy Group. Unit 7, stratigraphic section 4; 

essentially same level as USGS 17496-PC.

echinoderm columnals, indet.

Pentremites sp.

Zaphrentitesl cf. Z.? spinulosa Milne-Edwards and Haime

Michelinia cf. "Pleurodictyum meekanuml" of Nelson

(1962)

chaetetiform bryozoan, undet. 

Inflatia sp. < 

Flexaria sp. 

Krotovial sp. 

Avom'a? sp.

Spirifer brazerianus Girty 

Anthracospirifer aff. A. curvilateralis (Easton) 

Composita sp. 

Toryniferl sp. 

Septimyalina sp.
PALEOZOIC ROCKS SB

Upper Mississippian Continued 

USGS 17497-PC Continued

Phestia sp.

cf. Palaeoneilo sp.

cf. Similodonta sp.

aff. Nuculopsis sp.

"Phanerotrema" brazeriana Girty

cephalopod, indet.

bone and tooth fragments, indet.

USGS 17498-PC, Big Snowy Group about 75 m above the basal redbed unit. Unit 9, stratigraphic section 4.

Caninia excentrica (Meek)

Fenestella sp.

rhomboporoid bryozoan, undet.

Chonetes sp.

Inflatia spp.

Flexaria sp.

Krotovial sp.

Spirifer brazerianus Girty (abundant)

Anthracospirifer n. sp.

Cleiothyridina aff. C. sublamellosa (Hall)

Composita cf. C. subquadrata (Hall)

Torynifer cf. T. setigera (Hall)

Reticulariina spinosa (Norwood and Pratten)

Dielasma! sp.

Cranaenal sp.

Conocardium sp.

Straparollus (Euomphalus) sp.

bellerophontacean gastropod, indet.

trilobite pygidium, undet.

Lower Pennsylvanian:

USGS 17500-PC, Amsden(?) Formation about 46 m below the Quadrant Quartzite. Unit 13, stratigraphic section 4.

Caninia cf. C. torquia (Owen)

Antiquatonia cf. A. pernodosa Easton

Rugoclostusl sp.

Linoproductus cf. L. nodosus (Newberry) of Easton

Anthracospirifer cf. A. occiduus Sadlick

USGS 17501-PC, Amsden(?) Formation, about 43 m below the Quadrant Quartzite. Unit 14, stratigraphic section 4.

echinoid plates and spines, indet.

trepostomatous bryozoan, indet.

Derbyia sp.

orthotetid, indet.

Antiquatonia cf. A. pernodosa Easton

Linoproductus cf. L. nodosus (Newberry) of Easton

Rugoclostusl sp.

productoid fragments, indet.

Anthracospirifer sp.

Composital sp.
56 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

Lower Pennsylvania!! Continued

USGS 17502-PC, Amsden(?) Formation, about 30 m below the Quadrant Quartzite. Unit 15, stratigraphic section 4.

Fasciculiamplexus cf. F. contortus Easton 

Schizophoria cf. S. depressa Easton

The following fossil collections were made from the Amsden( ?) 

Formation at other localities. Identifications were made as indicated above.

USGS 17506-PC, Amsden(?) Formation 33 m below Quadrant Quartzite, north side of Wigwam Creek Canyon, Varney quadrangle, 

stenoporoid and rhomboporoid bryozoans, undet. 

echinoderm debris, indet. 

echinoid plate and spine, indet. 

orthotetid, indet.

Chonetes cf. C. pseudoliratus Easton 

Antiquatonia sp.

Linoproductus cf. L. nodosus (Newberry) of Easton 

Anthracospirifer cf. A. occiduus Sadlick 

Composita cf. C. sulcata Weller 

cf. Wilkingia sp. 

pteriacean pelecypod, indet. 

trilobite pygidia, indet.

USGS 24075-PC, Amsden(?) Formation 24 m below Quadrant Quartzite, SE corner, sec. 8 (unsurveyed), T. 9 S., R. 2 W., Varney quadrangle.

echinoderm debris, indet.

ramose bryozoans, undet.

Orbiculoidea sp.

Chonetes pseudolirata Easton

Linoproductus cf. L. nodosus (Newberry) of Easton

Antiquatonia sp.

Rugoclostusl sp.

Anthracospirifer cf. A. occiduus Sadlick

cf. Septmyalina sp.

straparollid gastropod, undet.

The overall aspect of the faunas in the Big Snowy Group is that 

of the assemblage found in the Heath Formation in the type area 

in the Big Snowy Mountains (Easton, 1962). An exception is the 

fauna of unit 9 (USGS 17498-PC) which has, in addition, two 

major elements from the upper part of the Monroe Canyon Formation of southeastern Idaho. This locality and a similar occurrence 

of these western faunas at Indian Creek, Mont., were reported by 

Dutro and Sando (1963). Thus, this sequence on Baldy Mountain
PALEOZOIC ROCKS 57

is of considerable importance in regional correlations, for it suggests interrelationships between central Montana platform deposits of the Big Snowy Group and the thicker miogeosynclinal 

sequence of southeastern Idaho.

The Pennsylvanian faunas of the Amsden Formation contain a 

distinctive assemblage that includes Caninia cf. C. tor quid 

(Meek) and Anthracospirifer cf. A. occiduns Sadlick. The age of 

these faunas is Middle Pennsylvanian, probably pO'st-Morrowan.

QUADRANT QUARTZITE OR FORMATION

The beds provisionally assigned to the Amsden ^Formation in the 

Varney quadrangle are succeeded by 110 m of clean quartz sandstone or quartzite referred to throughout the region as the Quadrant Quartzite or Formation of Pennsylvanian age. The larger 

part of the formation consists of quartz sandstone, variably calcareous or quartzitic, weathering white or pale yellowish or pinkish gray. The rock consists almost entirely of quartz grains and 

calcareous or siliceous cement and is generally fine to very fine 

grained, although a few quartz pebbles as much as 1 cm in diameter were seen in the basal beds at one or two places. It is typically thick to medium bedded and commonly contains thick sets of 

steeply inclined parallel crossbeds or laminae. Characteristic exposures are ledges or large blocks rounded and pitted by solution 

of the carbonate cement.

The lower part of the formation includes pale-yellowish-gray or 

white, medium- or thin-bedded dolomite, either as interbedded 

units 0.6 to 3 m thick or as detached blocks chaotically embedded 

in structureless sandstone. In one of the better exposed sections of 

the Quadrant, on Wigwam Creek, interbedded dolomite amounts to 

60 percent of the lower 25 m of the formation. Elsewhere, as in 

most exposures in the vicinity of Baldy Mountain, dolomite in the 

lower part of the Quadrant occurs largely as blocks, probably fragmented before lithification of the enclosing sandstone. Similar dolomite-sandstone breccia also occurs near the top of the formation, 

notably on the ridge northwest of Crockett Lake, where the basal 

beds of the overlying formation are also dolomite.

The contact between the basal part of the Quadrant and the 

variegated dolomite of the underlying Amsden Formation appears 

to be conformable and is marked by the abrupt appearance of one 

or more beds of clean sandstone 0.3 m or more thick, above which 

similar sandstone is abundant if not the dominant lithologic type. 

In contrast to the generally fossiliferous Amsden, the Quadrant is 

essentially devoid of megafossils in the Varney area.
58 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT:

The Quadrant throughout the Varney quadrangle is 9(3 to llO hi 

thick. It is somewhat thicker in localities a few kilometers to the' 

south and southwest, where 113 m of Quadrant is reporteo! in' tne 

Snowcrest Range and 163 m, in the southern part of the Gravelly 

Range (Mann, 1960, p. 117).

SHEDHORN SANDSTONE

Rocks of Permian age, 43 to 72 m thick, overlie the Quadrant 

Quartzite and are overlain conformably by the Dinwoody Formation of Triassic age throughout the Varney and Cameron quadrangles. The rocks in this stratigraphic interval in southwestern 

Montana and adjoining areas have received much detailed study 

because they contain phosphate and other minor elements. From 

these studies, two systems of stratigraphic nomenclature have 

emerged; in one system, the rocks between the Quadrant and the 

Dinwoody are assigned to unnamed lithostratigraphic subdivisions of the Phosphoria Formation (Cressman, 1955; Sheldon, 

1957) ; in the other, the rocks of the same interval are assigned 

to the Phosphoria Formation, Park City Formation, and Shedhorn Sandstone (McKelvey and others, 1956). In the Varney and 

Cameron quadrangles, these rocks are less than 75 m thick in 

contrast to much thicker sections in nearby parts of western 

Montana and southeastern Idaho. The rocks are largely sandstone, chert, and minor dolomite belonging to the upper unnamed divisions of the Phosphoria, that is, to units C, D, and E 

of Cressman (1955), or to the Shedhorn Sandstone (Cressman 

and Swanson, 1959). Correlated sections of the Shedhorn Sandstone and Phosphoria Formation are shown on figure 3. Dark 

mudstone and phosphate rocks similar to the Phosphoria of 

southeastern Idaho are thin and inconspicuous in the Varney and 

Cameron quadrangles.

Most of the Shedhorn Sandstone in these two quadrangles is 

pale- to moderate-gray, yellowish-gray, or grayish-orange sandstone, variably cherty, calcareous, or quartzitic. It is mostly fine 

to medium grained and well sorted, although fine- to coarsegrained, poorly sorted or pebbly sandstone occurs sparingly near 

the base and top of the formation. It is variably medium to thin 

bedded or indistinctly thick bedded, especially where cherty. The 

sandstone of the Shedhorn differs from that of the Quadrant by 

its darker color, more varied grain size and sorting, abundant 

dark grains of chert and other minerals, and yellowish-brown 

chert replacing the sandstone in various degrees. In many samples of the cherty sandstone or sandy chert, tiny rodlike sponge
PALEOZOIC ROCKS 59

spicules are visible when a hand lens is used, and thin sections 

also reveal variously shaped grains and ooliths of phosphorite 

and chert.

Bedded or massive chert, in 3- to 15-m-thick units, makes resistant beds, which are exposed as low ridges and cliffs within 

the formation throughout the Varney quadrangle. The chert is 

light gray, reddish or brownish gray, or dusky yellow, massive to 

thinly and irregularly bedded, and more or less sandy. Massive 

chert and partly chertified sandstone are commonly full of voids, 

which are apparently caused by weathering of the parts of the 

sandstone that retained a carbonate cement; this feature gives 

to outcrops a disorganized rubbly appearance. All gradations 

exist, from calcareous sandstone to sandy chert containing 5 to 

20 percent of corroded quartz grains. In thin section, the chert 

is seen to have replaced quartz grains, ooliths, and tubular structures probably of organic origin.

Dolomite is confined to the lower part of the formation, where 

it occurs in units a meter to as much as 15 m thick including 

minor sandstone beds. The dolomite is mostly light gray, compact 

and finely crystalline, and medium bedded. In the southwestern 

end of the Greenhorn Range, dolomite is interbedded with chert.

Reasonably complete exposures of the Shedhorn Sandstone are 

found on the north wall of the Wigwam Creek Canyon just south 

of sec. 33, T. 7 S., R. 2 W., on the ridge northwest of Crockett 

Lake in the southwestern corner of sec. 12, T. 8 S., R. 3 W., and 

on the northern side of Warm Springs Creek in sec. 22, T. 9 S., 

R. 3 W. Detailed study of these sections and other exposures in 

the Varney quadrangle and published sections of the formation 

on Indian Creek just east of the Cameron quadrangle (Cressman, 

1955, pi. 1) and at Lazyman Hill (Swanson and others, 1953, p. 

13-15), 2 or 3 km south of the Varney quadrangle, indicates that 

the formation in the Varney and Cameron quadrangles includes 

units , D, and E of the Phosphoria Formation. At Warm Springs 

Creek, 8 m of unit A and 0.6 m of mudstone and phosphate rock 

of unit B have been reported (Swanson and others, 1953, pi. 12), 

but these units were not found elsewhere in the area.

Unit E, 29 to 40 m thick in the Varney quadrangle, consists of 

sandstone, cherty sandstone, and chert. The most conspicuous 

chert unit is 12 to 18 m thick and occurs at the base of unit C; it 

consists of well-bedded chert in the lower part, overlain by increasingly sandy and less well bedded chert, grading upward to 

cherty sandstone. A thinner discontinuous unit, about 3 m thick, 

of thin-bedded chert containing shaly partings, is found 8 to 11
60 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

1 

Indian Creek

N W O O

Wigwam Creek 

D Y

METERS 

60^

45-

30-

15-

O-1

Part of 

Upper Shedhorn

Sandstone

Tosi Chert 

Member of 

Phosphoria Formation

Retort Phosphatic Shale

Member of

Phosphoria Formation /

Lower part of

Grandeur Tongue

of Park City 

Formation

"' 0 c, tj 

<><, 0 c, 

0 <i f > f <)

c/->V(/ J

j £,($'//

tffifflft

"i7'"7r

(. '... <

'>tt '-H/J

O Oot)

 ;.- - x

?

^ 

^____ - ^^"

_____ --^^"

- 

 ' :; ' ''

F^^

 ( : . tjr, i

'v'-'^.'-'v

 ^ 'V'tit

.V/.'^y'/

Oo<j 0

n rr

7 V

/ /

; ' ^ y

/ /

^

^P^

"" ~ __?

 ^^^

^^^^

^^^^

^^-^

^^~ ^

QUADRANT QUARTZITE OR FORMATION 

EXPLANATION

Sandstone 

Cherty sandstone 

Cherty

Dolomite 

Mudstone 

Phosphate rock

FIGURE 3. Columnar sections of the Shedhorn Sand

m below the top of the formation at Wigwam Creek. Mapping 

in the Varney quadrangle indicates that the thickness of both 

these chert units is variable and that massive chert replacing
PALEOZOIC ROCKS 61

Northwest of 

Crockett Lake

Warm Springs 

Creek

Lazyman Hill

FORMATION

-? -?- _ ? _

~~ -- __^

>.}/; <; iv

' ( \\<lj-<-'>-

>o(>o(

( i 0 "-'

O-trO.f-1

^^

rgT.-^f-

,i.y '/,',].(

»V$'

(' (i ';!)(

-J^r; ?/ :

iffii^Vj

O'jf) 1 J^

^i^l^fliJI

>y 'i i> n

', M , M , ,

^

ft

'aAW/ii

^&^.

LOCATION OF SECTIO

1. Type section of Shedhom Sandstone, Indie 

20, T. 8 S., R. 2 E.

2. Canyon of Wigwam Creek, north wall, sec. 

Measured by R. W. Swanson, J. B. Hac 

and G. H. Haddock, 1956

3. South edge of sec. 12, T. 8 S., R. 3 W. 1 

Hadley, 1957

~

__ . ~ - =-

NS 

n Creek, SEVi sec.

33,T.7S.,R.2W. 

iley, H. Masursky,

\

Measured by J. B. \ 

\

^

V.V.o.'',1

 : ;.' /.-; ;/.' 

l^.

1 1

1 1

II 1

1 '/ I-, 1

 ' 1 /

^^

^^-^ -^-T^:

^_:l^^_

 i i 

4. North side of Warm Springs Creek, sec. 22, T. 9 S., R. 3 W. 

From Swanson and others (1953, p. 11-12)

5. Lazyman Hill, crest of Gravelly Range 2.4 km south of 

Vamey quadrangle. From Swanson and others 

(1953, p. 13-15)

stone and Phosphoria Formation, Varney quadrangle.

sandstone occurs at other straitigraphic positions in unit E. Sandstone in the upper 9 to 15 m of unit E includes poorly sorted, 

thin-bedded, and crossbedded sandstone and sandy dolomite, com-
62 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

monly containing glauconite and well-worn fragments of bryozoans and molluscan shells.

The beds immediately below the main chert of unit E are 

rarely exposed except by trenching. At Warm Springs Creek, they 

are reported as including 3 m of phosphate rocks and mudstone 

assigned to unit D (Swanson and others, 1953, p. 11-12). Similar 

rocks were found in small landslide deposits or float in the concealed interval at a few other places in the quadrangle, but none 

were exposed in place. Six m of dark mudstione and 15 cm of 

phosphatic sandstone were found near the crest of the southern 

end of the Greenhorn Range just west of the Varney quadrangle 

boundary 1.5 km or so north of the Ruby River water gap. Darkbrown oily shale 0.6 m thick and a few centimeters of oolitic 

phosphate rock were found in the canyon of Tolman Creek (sec. 

10, T. 7 S., R. 1 E.) near the northern border of the Cameron 

quadrangle. Presumably these beds represent unit D, but in both 

places, the beds are strongly overturned, folded, or faulted; 

hence, their stratigraphic position is uncertain.

Because of insufficient exposure of unit D and the underlying 

beds, the lower part of the Shedhorn Sandstone is not well denned in the Varney quadrangle. It consists largely of gray sandstone and light-gray, variously calcareous dolomite in isolated 

beds or units as much as 15 m thick at or near the base of the 

formation. These rocks are lithologically similar to those of unit 

C, as described from localities not far west and southwest of the 

quadrangle (Cressman, 1955). According to the terminology of 

McKelvey and others (1956), the thicker dolomite units are considered to represent tongues of the Park City Formation in or 

below the Shedhorn Sandstone.

MESOZOIC ROCKS

DINWOODY FORMATION

The Shedhorn Sandstone is overlain throughout the Varney and 

Cameron quadrangles by the Dinwoody Formation of Early 

Triassic age. The formation consists largely of interbedded silty 

shale, argillaceous and calcareous sandstone, and yellow-weathering, platy to blocky dolomite or silty limestone, all poorly exposed 

in most of the area,

The lowermost part of the Dinwoody in most localities is easily 

recognized platy, grayish-yellow dolomite, commonly containing 

specimens of Lingula on the bedding surfaces; this dolomite is 

found as float in an interval that is probably mostly shale. Higher
MESOZOIC ROCKS 63

are thigkgr bedded dolomite that becomes increasingly argillaceous and calcar-e,oii§ upward anjl is interbedded with silty shale 

or mudstone and very fine grained argillaceous and dolomitic 

sandstone. In the section on the North Fork of Warm Springs 

Creek, several cyclic units appear, in each of which mediumfy&d{led dolomite is succeeded gradationally by thinner bedded 

dolomite, which becomes increasingly argillaceous and sandy and 

interbedded with sandstone toward the top of the unit. The uppermost b&ds, gp§ cpmmpnjy medium-bedded argillaceous sandstone 

pyerlain, with sharp contact, by dolomite of the next cycle. Some 

f the thicker dolomite beds are bioclastic and show fine current

the Icjwer boundary of the Dinwoody is quite well 

defined in the Varney quadrangle, the upper boundary is not. 

For example, in areas immediately south of the Varney quadrangle, the Dinwoody is overlain by red silty sandstone, siltstone, 

and shale of the Woodside Formation (Moritz, 1951, p. 1791; 

Mann, 1960, p. 117-120). Although similar beds appear in the 

Ruby River water gap at Canyon Camp just west of the Varney 

Qua.drangle boundary, no red beds occur in the few exposures of 

this interval in the quadrangle. The upper 12 to 30 m of the Dinwoody in the central part of the Varney quadrangle and on Indian 

Creek, although not red, is largely yellowish-brown sandstone 

and shale that might represent a featheredge of the Woodside 

Formation.

Mollusks in the Dinwoody are sparse and poorly preserved. All 

those collected were long-ranging species found in rocks of late 

Paleozoic and Early Triassic age (N. J. Silberling, written 

commun., 1958). Linguloid brachiopods are in general agreement 

with Lingula borealis Bittner, described by Newell and Kummel 

(1942) from the Dinwoody of western Wyoming, southeastern 

Idaho, and southwestern Montana. The Lingula zone was established by Newell and Kummel as the lowermost Triassic zone in 

this area. Nevertheless, these brachiopods are also similar to representatives of Lingula ranging from Cambrian to Holocene. 

Lingula, in the absence of normal marine forms such as articulate 

brachiopods and cephalopoda, is commonly used as an indicator 

of brackish or poorly oxygenated water (N. J. Silberling, written 

commun., 1956).

ELLIS GROUP

A succession of sandy 'siltstone, shale, and interbedded limestone, 45 to 60 m thick, that overlies the Dinwoody throughout the
64 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

Varney and Cameron quadrangles represents the Ellis Group of 

Middle and Late Jurassic age. All three formations of the group, 

as defined in northwestern Montana some 325 km northwest of 

the report area (Cobban, 1945; Moritz, 1951, p. 1802-1813), can 

be recognized in the Varney quadrangle.

The uppermost of these formation®, the Swift Formation or 

Sandstone, consists of massive grayish^orange, fine-grained sandstone, 4.6 m thick in the northern part of the Gravelly Range and 

12 m thick in the Greenhorn' Range. The basal few centimeters 

commonly contain small well-rounded pebbles of chert and rest 

with sharp contact on a unit of light-brown oolitic limestone and 

interbedded clay shale 4.6 to 6 m thick. The oolitic limestone is 

characteristic of the Rierdon Formation. The remainder of the 

Ellis Group in the Varney quadrangle consists of 40 to 55 m of 

interbedded argillaceous and sandy limestone, shale, and sandstone, most of which has been assigned to the Sawtooth Formation by previous workers in the region (Moritz, 1951, p. 1804). 

Shale and sandstone predominate in the lower part of the formation, limestone and shale, in the upper part, which is generally 

better exposed. In the section on French Gulch (fig. 4, columnar 

section 2), the base of the Sawtooth is assumed to be a 3-m bed 

of brownish-gray limestone containing abundant angular granules 

and pebbles of chert in the basal 0.3 to 0.9 m, which rests on 

medium-bedded grayish-orange dolomitic sandstone. The base of 

this chert-pebble bed may represent the regional disconformity 

between the Ellis Group and the underlying Dinwoody or Woodside. Elsewhere in the report area, hlowever, no clear lithologic 

break marks this boundary.

In the absence of such a break, the base of the Ellis was taken 

at the upper limit of the cyclic dolomite and sandstone beds of 

the upper part of the Dinwoody.

The limestone of the Ellis Group in the Varney quadrangle is 

generally fossiliferous and yielded the following species, identified by R. W. Imlay (written commun., 1958), probably all from 

the Sawtooth:

Pentacrinus asteriscus Meek and Hayden 

Pleuromya subcompressa (Meek) 

Astrocoenia hyatti Wells

Lima (Plagiostoma) occidentalis Hall and Whitfield 

Trigonia americana Meek 

Camptonectes plates siformis White

Unidentified species of Ostrea, Pinna, Isognomon, Quenstedtial, and worm tubes
MESOZOIC ROCKS 65

In his regional study of the Twin Creek Limestone, Imlay 

(1967) discussed and correlated all the related Middle and lower 

Upper Jurassic units in the central and northern Rocky Mountains. Imlay considered the lower part of the Sawtooth Formation to be of late Bajocian age, the upper part of the Sawtooth 

to be of Bathonian age, and the Rierdon Formation to be 

of late Bathtonian and early Callovian age.

Stratigraphic section 5

[Ellis Group and Dinwoody Formation, north side of French Gulch near middle of sec. (> 

(unsurveyed), T. 9 S., R. 2 W., Varney quadrangle. Measured by P. W. Birkeland and J. B.

Haclleyl

ThickneBH 

(meters)

Ellis Group, near base of Morrison Formation:

21. Limestone, upper 2.8 m, moderate-brownish-gray, finely 

oolitic, bedding indistinct 10 to 25 cm, forms cliff along 

crest of ridge. Lower 1.8 m, dark-greenish-gray to yellowish brown, silty to finely sandy, variable to large 

amounts of fine quartz sand and minor sandy and 

argillaceous beds, thinly current bedded _______________ 4.6

20. Mostly covered. Upper part is clayey, calcareous, yellowishbrown siltstone ____________________________________ 2.4

19. Limestone, moderate-yellowish-brown, massive, calcarenitic.

Lower part abundantly fossiliferous; forms low cliff ___ 1.5

18. Covered. Upper part, exposed by digging, is moderately fissile argillaceous limestone ________________-_-_-----__ 2.1

17. Limestone, moderate- to dark-yellowish-brown, finely crystalline to fine sandy, fine quartz sand throughout, bedding 

medium to thick, few impersistent shaly partings, abundant fine current lamination on weathered surfaces. Finely laminated calcilutite at top and oolitic limestone at 

base; forms cliff ________________________________ 8.8

16. Covered. Probably mostly yellowish-brown, finely current

bedded siltstone _______________________________ 2.8

15. Limestone, light-brownish-gray, very fine sandy, quartzose, 

massive, indistinct current lamination. Upper 30 cm is 

dark brownish gray, oolitic, petroliferous _______-- 2.1

14. Mudstone, moderate-yellowish-brown, fine sandy, mostly covered _______________________________________ 1.5

13. Limestone, massive, finely clastic, conchoidal fracture. Fine

calcarenite to calcilutite. Many fossil fragments ______ 8.5

12. Mostly covered. Probably argillaceous limestone and shale. 

Basal 3 m exposed in offset part of section 75 m to the 

north is dark- to light-brownish-gray, fine- to mediumgrained, porous and somewhat petroliferous, poorly 

bedded limestone. Basal 30 cm is breccia, including small 

angular chert fragments and rounded claystone fragments in limestone matrix, in sharp contact with underlying beds __________________________________ 12

Total thickness of Ellis Group ____________ 47
66 gEQLQGY, VARNEY AND CAMERON QUADRANGLES, MONT.

.1 

Section 24 French Gulch

ML 27031^ 

Ml 2703|j

METERS

15-

0 i

ML 27028 27029' 

ML 27027-

"!', I','

m

^iiJr 

hVV^VT!

T .V".1.';'-'*

fr* ' ' 'r.4i'

,_ ,-,-_

~' '7~ ' 7' '-T

SAWTOOTH FORMATION 

7

QINWOQQY FORMATION

LOCATION OF SECTIONS

SE%, sec. 24, T. 8 S., R. 3 W., Vamey quadjsured by J. B..Hadley and P. W. Birkeland,

:rench Gulch, middle of sec. 6, T. 9 S., R. 2 W., 

drangle. Measured by J. B. Hadley and P. W. 

1Q58

^

CT

=4r=

i=

rl 4 ..l. r.

S/iife

?^-j'.U

t f 1 ,( ,

"ZclfZf,'

c-2--y.

'^ :..<'.<.

fe^

p

Fault? 

? 

1958

Birkeland, 1958

3. North side of Wigwam Creek, sec. 4, T. 8 S., R. 2 W., Vamey 

quadrangle.. Measured by J, B. Hadley, E. W. McKee, and 

J. J. Branco, 1957

4. South side of Arasta Creek, WVfe sec. 36, T. 7 S. ; R. 3 W., 

Vamey quadrangle. Measured by Harold Masursky, J. B: 

Hadley, and G. H. Haddock, 1956

5. Canyon of Indian Creek, sees. 21 and 22, T. 8 S., R. 2 E., 

Sphinx Mountain quadrangle. Gardner and others (1946)

FIGURE 4. Columnar sections of the Ellis Group
MESOZOIC ROCKS 67

3 4

Wigwam Creek Arasta Cr 

MORRISON FORMATION ......

7 .

!

I

L

i 

E

i

3

5

n 

J

¥

E^

m\ 

\

EXPLANATION

Sandstone

>] Cherty sandstone

B

:-T^-=-r_

| i 1 i j|

--=Tr

c/ /

**TSl*

/ /

/ /

S*^**

. / . / :

^.^y ?.f.:f>Y

5 

eek Indian Creek 

___J\/!ORRISON FORMATION

SWIFT FORMATION

RIERDON 

FORMATION

SAWTOOTH 

FORMATION

^,^^

Hi

S3§ 1,1,

=W=n

^F

1

;ML 19619

Fault? 

DINWOODY FORMATION 

Fault?

SHEDHORN SANDSTONE

Dolomite 

Limestone

pl d Mudstone or shale

|rl£f2^ Sandy shale 

ML 27027 USGS Mesozoic locality number

and the upper part of the Dinwoody Formation.
68 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

Stratigraphic section 5 Continued

Thickness 

(meters)

Dinwoody Formation:

11. Sandstone, lower 3 m fissile, remainder grayish-orange, 

light-yellowish-gray, and pale-orange, medium-bedded 

(5-60 cm), fine-grained, dolomitic, forms cliff _________ 13

10. Dolomite, lower part interbedded with equal amount of 

fissile dolomitic sandstone; upper 4 m light-gray, thickbedded to massive, porous, and containing poorly preserved fossil fragments; forms cliff _________________ 7

9. Poorly exposed. Basal 1.8 m is sandstone, thin bedded and 

laminated like unit 5; top meter is more fissile sandstone; 

remainder is probably fissile sandstone and silty shale__ 9

8. Mostly covered. Massive dolomite 1.5 m thick, like unit 10 

in lower part; remainder probably fissile dolomitic sandstone and shale __________________________________ 7

7. Sandstone, dolomitic, fine-grained, and quartzose, thick 

bedded and current laminated, some crossbed sets 20 cm 

thick; forms cliff ________________________________ 3.3

6. Mostly sandstone, dolomitic, very fine grained, and fissile. 

Two or more beds, 38 cm thick, of yellowish- to lightgreenish-gray dolomite; one contains small poorly preserved gastropods and is somewhat petroliferous _______ 5.5

5. Sandstone, light-yellowish-brown, fine-grained, beds 5 to 30 

cm; in part finely laminated and current-bedded; a few 

partings coated with very fine grained muscovite; forms 

cliff ___________________________________ 8

4. Largely covered. One outcrop plus float indicates silty to 

fine sandy, thin- to very thin bedded, micaceous and finely 

laminated, pi aty-weather ing dolomite. Considerable silty 

shale probably present. Contains poorly preserved specimens of Myalina __________________________________ 7.6

3. Dolomite, like unit 1, somewhat thicker bedded (2-30 cm) __ 2.1

2. Covered. Probably dolomite shale and thin-bedded dolomite 2.8

1. Dolomite, pale-yellowish-brown to yellowish-gray, finely 

crystalline, thin-bedded; two beds in lower part are 30 cm 

thick; argillaceous partings common, also thin interbeds 

dolomitic mudstone _______________________________ 6.7

Partial thickness of Dinwoody Formation _______ 72

Base of measured section about 45 m above Shedhorn Sandstone

MORRISON FORMATION

Immediately succeeding the Ellis Group is a thick unit of nonresistant clayey rock referred to the Morrison Formation of Late 

Jurassic age. It is characteristically very poorly exposed, but, because of its highly plastic and unstable consistency, it can be 

commonly recognized by a distinctive hillock-and-sag-pond topography a result of gravity sliding. In several places, especially in
MESOZOIC ROCKS 69

the drainage of the South Fork and Middle Fork of Warm Springs 

Creek, the sag ponds have been enlarged by beaver dams.

Judging from the few good exposures, such as those just northwest of Moose Lake in sec. 25, T. 8 S., R. 3 W., on the north slope 

of the ridge 1.3 km west-southwest of bench mark 8523 on the 

Gravelly Ridge road, and on the ridge south of Arasta Creek in 

sec. 36, T. 7 S., R. 3 W, the Morrison Formation consists mainly 

of dark-greenish-gray to reddish-brown or maroon claystone and 

mudstone, poorly bedded, and containing sparsely distributed calcareous concretions and thin indurated beds of calcareous mudsrtjone. Thin beds of fine- to very fine grained orange-weathering 

sandstone are commonly present in the middle and upper parts 

of the formation; sandstone seems to be unusually abundant in a 

poorly exposed section on the north side of the Wigman Creek 

Canyon southeast of Horse Hill.

No fossils were found in the formation, but distinctive wellrounded and polished chert pebbles, found nowhere else in the 

area, are sparsely scattered throughout its outcrop area. A few 

large dinosaur bones displayed by residents of Ennis are said to 

have been found in the Morrison farther south in the Gravelly 

Range.

The Morrison Formation lies on various beds of the Ellis 

Group, apparently on sandstone of the Swift Formation in the 

vicinity of Wigwam and Arasta Creeks and on the Rierdon or 

Sawtooth elsewhere in the area. Thickness of the Morrison is 

estimated at about 75 m in most places.

KOOTENAI FORMATION

The Kootenai Formation, widely distributed and having many 

distinctive lithologic features, marks the beginning of Cretaceous 

time throughout much of western Montana and adjacent areas. 

The formation is exposed at many places along the crest and 

western slopes of the Gravelly Range and locally in the Greenhorn and Madison Ranges. Throughout this extent, four welldefined members can be recognized a basal sandstone unit, succeeded by a thick unit of clayey mudstone, a thinner unit of 

limestone, and a top unit of variably fissile clay shale and sandstone. The formation as a whole is 140 to 170 m thick.

Basal sandstone. The lowest member of the Kootenai consists 

of light-gray, medium- to coarse-grained argillaceous sandstone 

and chert-pebble conglomerate, marking an abrupt change from 

the dark clay of the underlying Morrision Formation. The sandstone is typically composed of quartz grains and large quantities
70 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

of dark chert grains that give the rock the distinctive "salt and 

pepper" appearance commonly reported in the literature. The 

largely white clay matrix contains little or no carbonate cement. 

The lower beds' of the unit are typically medium t|o thick bedded 

(1-3 m) and commonly show abundant steeply inclined planar 

crossbeds. Lenses and thin layers of conglomerate contain chert 

pebbles 5 to 25 mm in diameter. The upper beds of the unit are 

generally somewhat finer grained, more argillaceous and quartzose, and thinner and more regularly bedded. At a few places they 

clontain small particles of coaly material. In some sections, for 

example, the section exposed just southeast of Moose Lake, the 

basal sandstone unit has two major upward-fining sequences, 

each about 15 m thick. Impressions of leaves and stems of plants 

were found in the unit in a few places; in exposures northwest 

of Moose Lake, compressed tree trunks are represented by fluted 

molds as much as 2 m long and 22 cm wide, marked by narrow 

ridges 2 to 5 cm apart at right angles to the fluting.

Thickness of the basal Kootenai sandstone unit is generally 

about 30 m, although it ranges from less than 15 m in some 

places to more than 38 m in others.

Mudstone unit. The basal sandstone unit is overlain by poorly 

to moderately bedded mudstone and claystone ranging from light 

olive gray to dusky red or reddish brown. The lower part of this 

unit is commonly mudstone containing scattered grains of quartz 

and dark chert like that in the underlying beds; also present 

are thin units 'of thin-bedded sandstone, sandy or silty limestone, 

and limestone conglomerate. The upper part of the unit is generally somewhat more clayey, is indistinctly bedded in layers 0.6 

to 0.9 m thick, and is generally gray rather than red. An unusually good exposure of the lower 43 m of the mudstone unit was 

found near the top of ithe ridge about 1 km north of the canyon 

in the lower part of the Warm Springs Creek (sec. 22, T. 9 S., 

R. 3 W.). Thickness of the unit as a whole is estimated at about 

75 m in most places.

Limestone unit. Above the mudstone unit lies a persistent and 

easily recognizable unit about 15 m thick, consisting almost wholly 

of pale-yellowish-brown or yellowish-gray limestone, thin to 

medium bedded and somewhat argillaceous. Some of the limestone beds are composed of limestone fragments ranging in size 

from coarse s'and to small pebbles; others are conspicuously 

oolitic. Still lothers are crowded with small gastropods of onie or 

two species, resulting in a distinctive rock recognized throughout 

large areas where the Kootenai Formation is known. The lime-
MESOZOIC ROCKS 71

stone unit is 17 m thick in the best exposed section, just west of 

the canyon of Warm Springs Creek (stratigraphic section 6), 

and is about this thick at most other places in the area, except 

on the ridge south of Arasta Creek, where it is about 25 m thick.

Stratigraphic section 6

(Limestone unit in Kootenai Formation1, north side Warm Springs Canyon, NW corner sec. 

22, T. 9 S., R. 3 W., Varney quadrangle. Measured by J. B. HadleyJ

ThiclcncsH 

(meters)

Covered.

8. Limestone, like unit 6, medium- to thick-bedded, lacking 

shale interbeds; abundant gastropods in several beds; top 

bed 45 cm thick is microcrystalline to oolitic. Upper 

1.5 m partly covered, may be shaly ________________ 4.9

7. Limestone, microcrystalline, like units 2 and 4 __________ 0.6

6. Limestone, moderate-brownish-gray, medium- to thickbedded (5-60 cm), finely crystalline; contains beds of 

shale 5 to 30 cm thick showing crumpling and duplication by low-angle reverse faulting. Upper 30 to 60 cm 

is highly oolitic; at base is 60 cm moderately gray fissile 

shale containing thin limestone beds _________________ 5.3

5. Limestone, like unit 3; upper 60 cm highly oolitic ____._ 1.7

4. Limestone, like unit 2, medium-bedded (8-35 cm) ________ 2.1

3. Limestone, moderate-brownish-gray, darker than unit 2, 

microcrystalline to finely crystalline and oolitic, mediumbedded (8-30 cm), abundant gastropods _______________ 1.2

2. Limestone, moderate-brownish-gray, strong close jointing, 

at right angles to bedding, weathers to wedge-shaped 

chips _______________________________ " 0.8

Mudstone unit: 

1. Mudstone, light-brownish-gray, calcareous, nonbedded ____ 1.5

A fossil collection from the upper part of the limestone unit, 

375 m east-northeast of the southwest corner of sec. 13, T. 9 S., 

R. 3 W. (fig. 5), USGS paleont. loc. D-1765, yielded the following lacustrine species identified by W. A. Cobban and I. G. Sohn 

(written commun., 1958,1959) :

Pelecypoda Unio farri, Stanton

Gastropoda Reesidella montanensis Stanton

Ostracoda Cyprideal anomala"! Peck

Shale unit. Above the limestone unit and below a characteristic 

marine sandstone at .the base of the Colorado Group, is a littleexposed interval, 15 to 30 m thick, of dark-brown to mediumgray clay shale and claystone, variably fissile and sandy. It is 

well exposed only on the Warm Springs Creek road just east of 

Davis Creek in sec. 22, T. 8 S., R. 3 W., where a small collection
72 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

of lacustrine ostracodes and pelecypods was made, I. G. Sohn and 

W. A. Cobban (written commun., 1958, 1959) give the following 

species:

Ostracoda Cypridea wyomingensis Jones, sensu Peck 1941

Cypridea anomala Peck 1941 

Pelecypoda Eupera onestae (McLearn)

Three samples of shale from the upper shale unit on the Warm 

Springs Creek road 0.16 km east of Davis Gulch (fig. 5, USGS 

paleobot. loc. D-1830) yielded abundant gymnosperm pollen along 

with the fresh-water alga Botryococcus; no angiosperm pollen 

was present (R. H. Tschudy, written commun., 1962). Each of 

four samples from the mudstone unit of the Kootenai proved to 

be barren of both pollen and spores.

THERMOPOLIS SHALE 1

Above the gray shale and sandstone of the upper part of the 

Kootenai Formation is a unit about 110 m thick, consisting largely of dark-gray shale. This unit is recognized throughout northern Wyoming, where it is referred to as the Thermopolis Shale; 

it is mapped as far east as the Black Hills, where it is called the 

Skull Creek Shale. It grades downward into thin-bedded rusty 

quartz sandstone, marked by worm casts and trails, and lenticular, 

clean, current-bedded quartz sandstone, which is taken as marking the base of the formation. In northern Wyoming, the rusty 

sandstone beds are generally included in the upper part of the 

Cleverly Formation, which is equivalent to the Kootenai Formation of Montana. Above the dark shale is 15 m or less of dirty, 

coarser grained, thicker bedded sandstone that has been mapped 

with the Thermopolis Shale, but it equally well might be considered the basal unit of the overlying Frontier Formation.

A conspicuous characteristic of the basal sandstone of the 

Thermopolis in the Varney quadrangle is the presense of 6 m or 

less of clean quartz sandstone or quartzite that is strongly current bedded, overlain by thin-bedded gray or rusty sandstone 

marked by shaly partings and abundant worm casts and burrows. 

These basal sandstone beds are a minor ledge maker and can be 

recognized at many places, either in outcrop or in float. On Arasta 

Creek, they overlie 13 m of fine-grained fissile argillaceous sandstone and sandy shale assigned to the upper part of the Kootenai

'The name Thermopolis Shale is now geographically restricted from Montana by Rice (1976) 

for U.S. Geological Survey usage; rocks in Montana formerly called Thermopolis Shale are 

now called the Muddy Sandstone, the Skull Creek Shale, the basal part of the Mowry Shale, 

or the Fall River Sandstone. Hadley's mapping was completed before this restriction.
MESOZOIC ROCKS 73

Formation. Although unfossiliferous, the sandstone at the base 

of the Thermopolis and in the uppermost part of the Kootenai 

appears to indicate a rather abrupt transition from nonmarine to 

marine conditions of deposition.

Although nonresistant and rarely well exposed, the dark-shale 

unit can be seen near the confluence of the Middle and South Forks 

of Warm Springs Creek and on the west bank of Davis Creek just 

above its junction with Warm Springs Creek. The shale is generally medium gray to very dark gray and moderately fissile; its 

upper part is commonly silty or sandy and less fissile. Dark-red 

ironstone has resulted from weathering of thin lenses and nodules 

of ferruginous carbonate. Thickness of the shale unit is estimated 

at 75 to 82 m.

The dark-shale member is overlain throughout the Varney quadrangle by a prominent sandstone unit approximately 15 m thick, 

which is similar in lithologic character and stratigraphic position to the Muddy Sandstone farther east and northeast in Montana. In the Varney quadrangle, this sandstone is a minor ledge 

maker; it is medium gray, medium grained, argillaceous, and 

calcareous and is generally "dirty," containing abundant dark 

grains. Clay galls and fragments are common in some beds. Bedding characteristics range from thin bedded and flaggy weathering to thick bedded and coarsely current bedded. This sandstone 

unit is similar in many respects to those in the overlying Frontier 

Formation.

The Thermopolis is sparingly fossiliferous. Palynomorphs 

found in three samples from near the base, middle, and top of 

the dark-shale unit (fig. 5, USGS paleotoot. loc. D-1422, 1423, 

1424)were identified by E. B. Leopold as follows:

Classopolis cf. torosus (Reiss) Couper (the dominant

species)

Inaperturopollenites dubius Pot. 

Pityosporites labdacus Pot. 

Abietineaepollenites cf. dunrobensis Couper 

Gleicheniidites senonicus Ross 

Monocolpopollenites (Gingko type) 

Eboracia cf. lobofolia (Phill.) Thomas 

Stereisporites psilatus Ross

"The assemblage is identical in all three samples. The sediments contain about 300 pollen and spore grains per gram. The 

forms present are land plants of the gymnosperm and fern 

groups, and no higher plant (dicot) pollen could be found. I conclude the materials are of Lower Cretaceous age and that the
74 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT. 

DESCRIPTION OF SAMPLES

D-1828-1 Hard gray silty shale interbedded with sandstone

D-1828-2 Fine carbonaceous sandstone, 

coal prospect

D-1828-3 Carbonaceous siltstone 

D-1766 Argillaceous sandstone

D-1828-4 Gray silty shale interbedded 

with fine sandstone

D-1828-5 Mudstone containing coaly 

fragments

D-1829-2 Claystone immediately overlying 

coaly bed

D-1829-1 1.8-m coaly shale bed

D-1768 Thin-bedded argillaceous sandstone 

containing abundant carbonized plant 

fragments

VQ-58-19 Float of carbonized plant 

fragments, in argillaceous sandstone 

and claystone

D-1764, 1767 Argillaceous sandstone 

D-1422, 1423, 1424 Dark shale

D-1830-1 Clay shale containing ostracodes "

D-1830-2 Thin sandstone bed just below 

shale

D-1830-3 Dark shale 45 cm below gray 

shale

VQ-58-1 Shale 4.5 m above limestone 

member

D-1765 Thin-bedded limestone near top of 

limestone member

" D-1828-1, 2 - 

D-1828-3\_ 

D-1766 

D-1828-4 

D-1828-5/

D 1R9O_9

o-ms-r

D-1768

VQ-58-19,

D-1764, 1767. 

hD-1424\\_ 

D-1422A" 

|_D-1423\V~~

D-1830-1 

" ' 1. 2, 3 1 

VO458-1J /-

D-1 765

JLJS] FORMATION FRONTIER 

claystones) (Volcanic 

THERMOPC 

SHALE

KOOTENAI FORMATION

zP^>- 

H "^r£Z

^SH:

-z^-z^

"FvJ^I

 ~ ~r 

I ^-

^H^fx

Hz ~T 1

T7; , .-r-T

 ^ _ 

iSriz^iH

rL-^r.^:

 * 

'^~^1-

,^;ri'~r

=S^

' ^' ' ::^::.

-METERS 

-900

-600 

-300

-0

FIGURE 5. Columnar section showing position of numbered fossil collections 

from Cretaceous rocks, Varney quadrangle.
MESOZOIC ROCKS 75

depositional environment could be either nearshore marine or 

continental lacustrine." (E. B. Leopold, written commun., 1959). 

Fossils collected from the upper sandstone member of the 

Thermopolis Shale near the NW corner of sec. 23, and southeast 

of the NW corner of sec. 15, both in T. 9 S., R. 3 W., (fig. 

5, USGS paleont. loc. D-1764, 1767) consist entirely of the 

shallow-water marine species Ostrea anomioides Meek (W. A. 

Cobban, written commun., 1958.)

FRONTIER FORMATION

The rocks above the Thermopolis Shale in the Varney and 

Cameron quadrangles constitute several thousand meters of sandstone, mudstone, and shale and minor coal and limestone, of 

mixed fluviatile and shallow-marine or estuarine origin. Because 

rocks of nonmarine or estuarine origin appear to be dominant, 

they are assigned to the Frontier Formation 'of Late Cretaceous 

age. Bentonitic claystone, thin beds of porcellanite, and pink or 

reddish zeolite-bearing beds indicate 'a strong volcanic contribution in the basal 110 m of the formation. The best exposures are 

along the Ruby River road west of Warm Springs Creek and in 

low bluffs along the west side of Warm Springs Creek west and 

southwest of Warm Springs Cow Camp.

Most of the Frontier Formation in the two quadrangles consists of gray mudstone or claystone and drab or olive-gray sandstone, fine to medium grained, in beds 30 to 90 cm thick. Coarser 

sandstone occurs in discontinuous units 3 to 12 m thick every 

few hundred meters throughout the formation. This sandstone is 

typically medium to coarse grained, is coarsely and irregularly 

crossbedded, and appears to be fluviatile. Clay galls and rare 

chert pebbles occur in some beds. Much of the finer grained sandstone and mudstone is highly calcareous and grades locally to 

finely sandy or argillaceous limestone in lenses 30 to 60 cm thick. 

Much of the finer noncalcareous sandstone, especially in the lower 

600 m, contains abundant coalified plant debris, and thin coalbearing beds in the middle and upper parts of the formation have 

been explored in prospect workings at two places near the Ruby 

River road.

The basal unit, which is rich in volcanic materials (stratigraphic section 7), consists largely of soft-grayish-olive and 

greenish-gray claystone and subordinate argillaceous sandstone. 

Much of the claystone shows "popcorn" weathering surfaces, resulting from swelling of bentonite, or is variegated salmon pink, 

pale orange, or reddish brown because of the presence of minute
76 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

crystals of heulandite. Thin resistant beds in the claystone are 

silicified tuffaceous layers (porcellanite) or reddish indurated 

zeolitic sandstone containing abundant highly altered shards. 

Most sandstone of this unit is quartz-poor and contains abundant 

angular clasts of plagioclase, feldspar, chert, biotite, and, rarely, 

pyroxene. Similar bentonitic and zeolitic rocks are reported in 

the basal part of the Frontier or in the underlying Mowry Shale 

and equivalent rocks at localities in northern Wyoming and northwestern Montana (Love and others, 1948; Cobban, and others, 

1959).

The stratigraphy of the upper part of the Frontier Formation 

in the Gameron quadrangle is complicated by the presence of 

many concordant and semiconoordant intrusive sheets of andesite 

porphyry abundantly exposed on Cedar Mountain and adjacent 

ridges. Thick shale units on some of these ridges in the upper 

part of the sequence on Cedar Mountain may be more properly 

correlated with members of the Colorado Group farther east.

Stratigraphic section 7

[Basal volcanic unit of Frontier formation, west side of Warm Springs Creek, W1/^ sec. 14, 

T. 9 S., R. 3 W., Varney quadrangle. Measured by ,1. B. Hadley and P. W. Birkeland]

Thickness 

(meters)

End of exposure.

12. Clay, grayish-olive (10F 4/2) to dark-yellowish-brown and 

zeolitic or grayish-brown (5YR 4/2') ; probably bentonitic 

with "popcorn" surface ___________________________ 7.6

11. Sandstone, moderate- to pale-reddish-orange, medium- to finegrained, indistinctly current laminated. Coarser material 

contains abundant heulandite and possibly shards. Blocky 

weathering, resistant ___________________ 1.2 

10. Claystone, olive-gray, soft. Thin sandstone bed at top ______ 3.0

9. Lower 1.2 m is sandstone containing abundant dark mica, 

overlain by clayey reddish-brown sandstone, purplish 

(zeolitic) toward top. Upper part is mainly olive-gray to 

reddish claystone. At top is dark-gray claystone 0.6 m 

thick containing abundant heulandite, overlain by 25 cm 

medium-dark-gray porcellanite, weathering light gray, 

irregularly laminated ____________________________ 5.8

8. Claystone, variegated reddish-brown, olive-gray, and darkgray with red flecks; mostly zeolitic _________--__----___ 4.6

(Section offset 120 m to south at top of unit 7)

7. Claystone, olive-gray and greenish-gray, mostly nonresistant. 

Two darker gray resistant beds contain cavities filled with 

red heulandite and green sepiolite ____________________ 4.9

6. Sandstone, 1.5 m thick, medium- to fine-grained, argillaceous, 

micaceous, and fissile, overlain by olive-gray claystone that 

has minor fissile sandstone in lower part ______________ 8.8

5. Covered ___________________________ __ _______ 4.0
MESOZOIC ROCKS 77

Strati graphic section 7 Continued

Thickness 

(meters)

4. Claystone and sandstone like unit 2, nonresistant; 15 cm 

resistant dark-greenish-gray claystone at top; similar bed 

2.8 m below top _________________________ 8.2

3. Claystone, medium-dark-gray, resistant, interbedded with

softer claystone ______________________________________ 1.5

2. Claystone as in unit 1, interbedded with sandstone, argillaceous, nonresistant, grayish-yellow (5F 7/4). Bedding 

units 5 cm to 1.2 m thick. Includes several thin beds of 

resistant silty or fine sandy claystone; "popcorn" surface 18.0

1. Claystone, light-olive-gray to olive-black (5GY 5/2 to 5Y 

2/1), silty. Interbeds 8 to 15 cm thick of siltstone and finegrained calcareous "salt-and-pepper" sandstone. Upper 3 m 

is fine-grained argillaceous friable sandstone, moderately 

fissile, nonresistant __________________________________ 10.7

Upper sandstone unit of Thermopolis Shale.

The easily deformed rocks of the Frontier Formation underlie 

major thrust faults in both the Varney and Cameron quadrangles, 

so that it is difficult to make proper allowance for repetition by 

folding and thrusting in estimating the thickness of the formation. Approximate measurements along the Ruby River road suggest that beds at least 1,070 m thick are present above the Thermopolis Shale. The formation may be thicker in the Cameron 

quadrangle, where it is overlain by Upper Cretaceous volcanic 

and volcaniclastic rocks. A thickness of 1,100 m has been estimated in the Sphinx Mountain area immediately east of the 

Cameron quadrangle (Beck, 1960, p. 131).

Both plant and invertebrate animal remains are found throughout most of the Frontier Formation in the Varney quadrangle. 

A collection from argillaceous sandstone containing abundant 

plant debris, about 300 m above the base of the formation in 

Golden Sucker Gulch (fig. 5, USGS paleont. loc. D-1768), yielded 

the following (W. A. Cobban, written commun., 1958) : Ostrea 

sp.; Anomia aff. A. micronema Meek; Corbicula sp.; Corbula 

n. sp. Another collection, about 900 m above the base (fig. 5, 

USGS paleont. loc. D-1766) northwest of Schoolmarm Gulch, 

yielded the following species: Ostrea soleniscus Meek; Corbicula 

sp.; Corbula n. sp. Cobban reported that these collections represent brackish-water faunas, the lower one of late Colorado age 

and the upper one of probable late Colorado age.
78 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

A small collection of plant leaves and stems of early Late Cretaceous aspect, associated with silicified logs in the lower part 

of the volcanic unit at the base of the formation near Vigilante 

Ranger Station (fig. 5, field loc. VQ-58-19), was studied by R. W. 

Brown, who listed the following (written commun., 1958) : 

Anemia fremonti Knowlton; Cladophlebis readi Brown; Gleichenia nordenskioldi Heer; Sequoia or Araucarites; coniferous 

wood; dicotyledonous leaves.

Samples of claystone and shale from several horizons in the 

Frontier Formation in the section exposed along Ruby River in 

sees. 17 and 20, T. 9 S., R. 3 W. (fig. 5, USGS paleobot. loc. D1828, -1829), were studied for palynomorphs by R. H. Tschudy 

(written commun., 1962). Of 14 samples collected, 7 were barren; 

these included 3 samples of silty claystone and shale from the 

middle of the Frontier section and all 4 samples of claystone from 

the basal volcanic unit. Samples of silty shale, mudstone, and fine 

argillaceous sandstone, especially those containing carbonized 

plant debris, yielded sparse to abundant angiosperm pollen as well 

as considerable amounts of gymno'sperm pollen and pteridophyte 

spores. Three samples of silty shale and fine sandstone from the 

upper part of the section yielded histrichospheres, suggesting, according to Tschudy, marine conditions of deposition. In only one 

of the seven productive samples (D-l828-5) were the materials 

sufficiently abundant and well preserved to permit grain counts. 

Palynomorphs in the other six samples were either too poorly 

preserved or too sparse to reveal more than the presence of a 

few forms.

UPPER CRETACEOUS VOLCANIC AND VOLCANICLASTIC ROCKS

No Mesozoic rocks younger than the Frontier Formation appear in the Varney quadrangle; in the northeastern part of the 

Cameron quadrangle, however, the Frontier is succeeded by more 

than 600 m of volcaniclastic rocks of Late Cretaceous age. These 

rocks are well exposed on the ridges south of Cedar Mountain 

and along the several forks of Bear Creek and extend several 

kilometers eastward into the Sphinx Mountain quadrangle, where 

they have been correlated with the Livingston Group (Beck, 

1960, p. 131-132). In the Cameron quadrangle, these rocks can 

be readily divided into a basal unit of volcanic sandstone, siltstone, and pebble conglomerate; a middle unit of polymictic volcanic breccia; and an upper unit of coarse volcanic conglomerate. 

They are overlain unconformably by the Sphinx Conglomerate of 

Eocene age.
MESOZOIC ROCKS 79

Volcanic sandstone unit. The base of the volcanic sandstone 

unit is marked by the abrupt appearance, above the light-colored 

"salt-and-pepper" sandstone of the Frontier Formation, of darkgreenish-gray or brownish-gray, medium- to very coarse-grained 

sandstone, consisting largely of crystals, crystal fragments, and 

volcanic rock fragments similar to those in the overlying breccia 

and conglomerate. Angular fragments and nearly euhedral crystals 

of twinned and zoned plagioclase are the most abundant constituents; other minerals are clinopyroxene, magnetite, hornblende, 

and biotite in order of decreasing abundance. Chert and quartz 

grains are generally rare, and the ferromagnesian minerals are 

commonly wholly or partly altered to chlorite.

The sandstone is generally well bedded and commonly crossbedded, showing sorting and concentration of the heavier minerals, especially magnetite, by current action. Lenses of pebble 

conglomerate containing subangular clasts of dark chert and wellrounded pebbles of various volcanic rocks and some quartzite occur throughout the unit. Volcanic clasts are generally fewer in 

the stratigraphically lower lenses but become predominant in the 

higher ones. Units 15 or more meters thick of lighter colored 

feldspathic sandstone of Frontier aspect occur within the basal 

unit at several places. Some beds also contain abundant carbonized 

plant debris and impressions of leaves and stems of several species 

of plants.

Beds of soft, white-weathering tuff containing quartz or biotite crystals were found in a few places in the volcanic sandstone unit. Three such beds, 30, 60, and 150 cm thick, occur in 

the upper part of the unit on the ridge between North and Middle 

Forks of Bear Creek.

The basal volcanic sandstone unit is generally too poorly exposed or too much deformed to permit detailed measurement of 

its thickness. The thickness as measured from map data ranges 

from 90 to 210 m.

Volcanic breccia unit. The volcanic sandstone unit is abruptly succeeded by a thick mass of poorly bedded rock composed 

largely of angular fragments and blocks of andesite or basaltic 

andesite. The fragments range from less than 3 cm to a meter 

or so in diameter and include several types of pyroxene-, hornblende- , or biotite-bearing andesite firmly cemented in a matrix 

consisting of finer particles of the same materials. These rocks 

are dark greenish gray, olive gray, or reddish gray depending 

upon their state of oxidation. Units 15 m or more thick commonly show little or no sorting or bedding, but interbeds of finer
80 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

grained tuff-breccia or coarse volcanic sandstone are fairly common. Most fragments are highly angular and show little abrasion, but well-rounded cobbles and small boulders are locally 

intermixed, and layers composed largely of rounded fragments 

appear locally, especially in the upper part of the unit.

Thin sections of 12 samples of the larger fragments in the 

breccia unit were taken from, fresh talus along the North Fork 

of Bear Creek in the northwest corner of sec. 31, T. 7 S., R. 2 E. 

The rocks studied are all porphyritic; nearly all contain 10 to 

35 percent phenocrysts of calcic andesine or labradorite, and most 

contain 3 to 20 percent phenocrysts of augite. A few contain 

phenocrysts of hornblende rather than augite, and some contain 

small amounts of highly altered biotite in addition to augite. The 

moderately felsic matrix contains abundant plagioclase laths 0.1 

mm long and finer grained ferromagnesian constituents. Some 

reddish-gray samples are so strongly oxidized that most of the 

ferromagnesian minerals have been replaced by hematite and 

quartz. Many samples contain collapsed vesicles filled by chlorite 

and chalcedonic quartz.

In a few places in the breccia unit, thick bodies of unusually 

uniform and little brecciated pyroxene andesite may represent 

lava flows in place. At several places, the breccia unit is cut by 

dikes of amygdaloidal andesite porphyry characterized by prominent tabular plagioclase phenocrysts 3 to 10 mm long. No welldefined or mappable units were found, however.

Thickness of the breccia unit, as estimated from map measurements, is 300 to 400 m.

Volcanic conglomerate unit. Overlying the volcanic breccia 

unit is a few hundred meters of cobble and boulder conglomerate 

essentially composed of the various kinds of volcanic rocks that 

make up the breccia. The clasts are generally well rounded, and 

their common large size in a given bed ranges from 12 to 30 cm. 

Boulders 60 to 90 cm in diameter are fairly common, however, 

and a few are as large as 150 cm in diameter. The matrix is 

coarse, dark-greenish- or reddish-gray volcanic sandstone, like 

that in the basal sandstone unit but less well sorted. Conglomerate 

forms beds several meters thick of poorly sorted material like 

that in mudflows or alluvial-fan deposits, containing much matrix 

separating adjacent clasts. Beds of volcanic sandstone and siltstone and lenses of pebble conglomerate that have mixed volcanic 

and nonvolcanic components are locally present, as are minor 

amounts of breccia like that in the underlying unit.
MESOZOIC ROCKS 81

Maximum thickness of the conglomerate unit is about 180 m.

Correlation and age. Although Peale (1896) and Beck (1960) 

correlated the volcaniclastic rocks of the Madison Range with the 

Livingston Group (Livingston Formation of Weed, 1893), the 

unusually coarse texture and domrnantly volcanic composition of 

these rocks suggest that they are part of the Elkhorn Mountains 

volcanic field and are best correlated with the lower member of 

the Elkhorn Mountains Volcanics in the southern Elkhorn Mountains, 100 km north-northwest of the Cameron quadrangle 

(Klepper and others, 1957, p. 32-33). This unit is described as 

900 m of fragmental andesitic rocks, mainly poorly sorted to unsorted breccia, and volcanic conglomerate, whose fragments are 

commonly 30 cm or less in diameter. The rocks in the Cameron 

quadrangle seem to fit this description, whereas rocks found 

throughout the Livingston Group in its type area are finer grained 

and contain greater proportions of nonvolcanic material (Roberts, 

1963; Klepper and others, 1957, p. 40).

No new information on the age of these volcanic rocks was 

found in the Varney-Cameron area, and the age assignment of 

Klepper and others (1957) is followed. The basal volcanic sandstone unit seems to be an integral part of the overlying depositional sequence, but whether it lies conformably or unconformably on the Frontier Formation is not clear in the area studied because of the lack of detailed stratigraphic information in the 

upper part of the Frontier. The abrupt appearance of pebble conglomerate in the basal unit, however, suggests a radical change 

in regional stream gradients, presumably resulting from uplift tp 

the north and northwest in Late Cretaceous time.

CENOZOIC ROCKS

SPHINX CONGLOMERATE

The oldest of the Cenozoic deposits in the Varney-Cameron area 

is a distinctive red limestone fanglomerate, the Sphinx Conglomerate, conspicuously exposed on Sphinx Mountain, 1.6 km east of 

the Cameron quadrangle. About 600 m of this rock is exposed in 

The Helmet (sec. 5, R. 8 S., T. 2 E.; not the prominent hill of 

Precambrian dolomite so labeled on the geologic map (Hadley, 

1969a)). Three much smaller remnants of the Sphinx Conglomerate were found on the crest of the Gravelly Range in the southern part of the Varney quadrangle. Apparently the unit was once 

far more extensive and has been largely removed from the Varney-Cameron area.
82 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

The Sphinx Conglomerate typically consists of subrounded to 

well-rounded cobbles and boulders of various Paleozoic and 

Mesozoic rocks in a matrix of coarse, pale- to moderate-reddishbrown, calcareous and argillaceous sandstone or mudstone. The 

common large size of the rock fragments is 20 to 25 cm in the 

Gravelly Range and 60 cm on The Helmet; the largest boulders 

seen were about 90 cm in diameter. Sorting is poor and bedding 

indistinct except for a few thin lenses of pebbly sandstone here 

and there. In extensive exposures in the cirque wall on the northeast side of Sphinx Mountain, Beck (1960, p. 132-133) described 

large-scale planar cross stratification of units as much as 90 m 

thick and ascribed this structure to coalescing alluvial fans along 

an ancestral mountain front.

Although fragments of many of the Paleozoic and Mesozoic formations are represented in most exposures of the fanglomerate, 

their proportions vary from bed to bed and from place to place 

within the formation. An exposure of 18 m of the basal part of 

the formation in the Gravelly Range, for example, contains 

abundant fragments of limestone of the Madison Group and 

Amsden Formation and sandstone or quartzite of the Quadrant 

Quartzite and Shedhorn Sandstone. Fragments of the Meagher 

Limestone and Flathead Quartzite are present in small amounts, 

and a very few small fragments of phyllite and other Precambrian 

rocks also appear in this exposure. The basal fanglomerate in The 

Helmet contains many large cobbles and boulders, including sandstone of the Quadrant, the Shedhorn, and the Kootenai and dolomite or limestone of the Mission Canyon, the Amsden (?), and the 

Shedhorn (?). Other Paleozoic rocks are poorly represented, and 

no Precambrian rock types were seen there. This kind of variation suggests a strong influence of local sources on the fanglomerate deposits.

The Sphinx Conglomerate was somewhat arbitrarily assigned 

to the Eocene by Peale (1896, p. 3), who reported no fossil evidence ; little has been found since to improve on that assignment. 

The formation in the Cameron quadrangle and on Sphinx Mountain unconformably overlies sharply folded and thrust-faulted 

rocks, here correlated with the Elkhorn Mountains Volcanics, yet 

the fanglomerate on The Helmet is folded in a syncline which has 

beds dipping at least 25° and subordinate folds of even sharper 

flexure. Thus, the Sphinx Conglomerate is clearly contemporaneous with Laramide regional deformation but, unfortunately, has 

provided no biostratigraphic evidence on which to date more precisely these Laramide structures.
CENOZOIC ROCKS 83 

TERTIARY VOLCANIC ROCKS AND GRAVEL

Volcanic rocks. The Gravelly Range owes its name to patches 

of unconsolidated cobble gravel found at many places along its 

crest. These patches are commonly associated with basalt lavas 

and tuff and are remnants of once more extensive deposits laid 

down probably in Eocene to early Miocene time. They appear to 

record a period of lower altitude and relief and a more humid 

climate in the region than those that existed there during earlier 

Tertiary or later Cenozoic time.

The best exposed rocks of this sequence are basalt lava flows 

which occupy a large area in the north-central part of the Varney 

quadrangle and a similar area in the adjacent Virginia City 

quadrangle. The rocks in the Varney quadrangle consist of about 

300 m of interbedded compact and vesicular olivine basalt, vesicular basalt breccia and agglomerate, and sandy tuff. A continuously 

exposed section 75 m thick in the cliffs above Blue Lake, at the 

northern edge of the main basalt area, includes at least six separate flows that have weakly amygdaloidal bases and highly vesicular and fragmental tops, separated by 0.6 to 3 m of indurated 

vitric tuff. The individual flows are 9 to 18 or more meters thick; 

many of the thicker flows form low scarps on the gentler slopes. 

Interbedded, less resistant, parts of the sequence consist of thick 

layers of amygdaloidal basalt breccia and agglomerate filled and 

cemented by snow-white calcite (fig. 6).

Outcrops of compact basalt in the central parts of the flows 

commonly show an indistinct and variably developed platy structure that in most places dips gently northeast. This structure is 

parallel to flattened amygdules seen near the bases of the better 

exposed flows; thin-section studies indicate that it represents a 

fluidal structure marked by alinement of plagioclase crystals. 

Other outcrops of compact basalt are parts of dikes a few tens of 

meters thick, in which the flow structure is parallel to more steeply dipping walls. Such dikes cut the flows at several places in sees. 

21, 23, 26, and 27, T. 7 S., R. 2 W.

The flows and dikes consist of uniform porphyritic olivine 

basalt, which contains 10 to 15 percent olivine and nearly equal 

amounts of augite, 70 percent calcic plagioclase, and 2 to 5 percent magnetite-ilmenite. The phenocrysts, most of which are 

olivine, amount to 5 or 10 percent of most samples studied. Small 

amounts of alkali feldspar or feldspathoids and traces of biotite 

were found in several samples of both intrusive and extrusive 

basalts. The lava of the two areas on the north side of Johnny 

Gulch in the Varney quadrangle is unusual in that it is highly
84 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

FIGURE 6. Tertiary volcanic rocks, S% sec. 23, T. 7 S., and R. 1 W., Varney 

quadrangle. Photographs by Harold Masursky. A, Interbedded highly amygdaloidal basalt and scoria breccia. B, Detail of contact between amygdaloidal basalt and overlying breccia. White matrix is mainly crystalline 

calcite. Scale is 18 cm long.
CENOZOIC ROCKS 85

porous, low in olivine, and rich in opaque iron oxide, apparently 

replacing olivine and possibly other iron-silicate minerals. Chemical analysis shows that the lava is andesitic or latitic in composition, but it appears to have been considerably altered, possibly 

by gas fluxing during eruption.

Marvin and others (1974) presented K-Ar age determinations 

on igneous rocks from the nearby Virginia City and Black Butte 

areas. Ages range from 50 m.y. for Eocene andesite-dacite 

porphyry to 33 m.y. 34 m.y. for the overlying Oligocene basalts 

to 23 m.y. for the lower Miocene basalt plug near Black Butte, 

just south of the Varney quadrangle. Concerning these ages, the 

writers (p. 17) stated:

The K-Ar ages reveal a more complex Tertiary igneous history 

than was previously known. In the Virginia City area, the difference 

of about 16-17 my between the Eocene andesite-dacite porphyry 

(50 my) and the overlying Oligocene basalts (33-34 my) was not 

apparent from geologic mapping. The basalts occurring at various 

places along the crest 'of the Gravelly Range, from the area of 

Virginia City southward to the vicinity of Black Butte, would have 

been considered contemporaneous; however, if the Black Butte plug 

was a source of adjacent basalt flows, the volcanic activity spanned 

about 10 my from mid-Oligocene (34 my) to early Miocene (23 my).

In most of the areas described, the lava lies on gravel or tuff, 

the tuff being more widespread than the gravel. Tuff is especially 

abundant in, the vicinity of the Axolotl Lakes, which are sag ponds 

on a thick sliding blanket of tuff beneath the main lava sequence. 

Good exposures of the tuff are rare, but its presence is commonly 

revealed airound small animal burrows. In the better exposures, 

it is a light-gray rock with orange or pink tinges, composed of 

fine sand- to silt-sized particles. It is generally poorly sorted and 

lacks stratification, but generally not indurated, and weathers to 

smiall blocky fragments in a viscous clayey mud, which flows 

readily. In a few places, the tuff is distinctly bedded and contains 

thin lenses of pebble gravel.

Samples of tuff examined under the microscope consist of 

shards and other glassy material (70-98 percent) and angular 

grains of various minerals of volcanic and detrital origin (2-25 

percent). Plagioclase, olivine, and augite, which are approximately the same ratio as in the lava, make up most of the crystalline component. Other minerals, present only in trace amounts, include hornblende, biotite, muscovite, epidote, tremolite, and tourmaline, mostly derived from Precambrian rocks. Most of the nonvolcanic components are fine particles presumably mixed with the 

tuff during eruption rather than during deposition.
86 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

In general, the tuff appears to represent ash falls deposited at 

a considerable distance from eruptive sources and locally reworked by water. Most of it appears to have fallen before the 

eruption of the main lava sequence and to have filled depressions 

between ridges of eroded Paleozoic and Mesozoic rocks. Essentially similar tuff between individual flows, however, indicates that 

explosive volcanism continued during the accumulation of basalt 

flows.

Gravel. On the basis of geographic location and local stratigraphic relations, the Tertiary gravel can be assigned to three 

groups of deposits. One is found in a narrow linear belt at 2,500 

to 2,600 m altitude, trending north-northwest along the crest of 

the Gravelly Range across the Greenhorn fault into the northern 

part of the Greenhorn Range. Another group, at lower altitudes, 

is north of Wigwam Creek and west of the large area of tuff and 

basalt in the northern part of the Varney quadrangle. The third 

group is in low hills nearly buried by alluvial fans at the foot 

of the Madison Range in the southern part of the Cameron 

quadrangle.

The gravel in the linear belt along the crest of the Gravelly 

Range consists of well-rounded cobbles and boulders, some as 

much as a meter in diameter, mainly of Precambrian granitic and 

gneissic rocks similar to those exposed northwest of the Greenhorn fault. Other rock types in this gravel are Paleozoic quartzite and sandstone and especially well rounded cobbles of hard 

gray quartzite, possibly derived from the Belt Supergroup, whose 

nearest exposures are now nearly 80 km north or west of the 

Varney quadrangle. Minor amounts of local limestone are represented in the gravel; however, in several exposures, the gravel 

contains largely resistant quartzite, chert, and sandstone, and 

limestone fragments are conspicuously absent. At two places in 

this belt, the gravel is associated with blocky accumulations and 

jumbled outcrops of olivine basalt that are topographically higher 

than the gravel and apparently overlie it. At another place, 300 m 

southwest of the junction of Warm Springs Road and Gravelly 

Ridge Road, the gravel is apparently overlain by tuff. An exceptional exposure in a landslide scar at the northwest edge of the 

gravel hill west of Crockett Lake revealed the following section 

of gravel and interbedded volcanic rocks:
CENOZOIC ROCKS 87

Stratigraphic section 8

| Tertiary gravel and interbedded volcanic rocks, east side sec. 25, T. X S., R. tt W., Varney

quadrangle)

Thickncsit 

(meters)

8. Gravel containing well-rounded pebbles, cobbles, and boulders 

as large as 1.5 m; mainly Precambrian rocks, such as quairtzite, 

granite, and gneiss; few large angular blocks Kootenai sandstone and Shedhorn cherty sandstone; no limestone. Smaller 

clasts of deeply weathered sandstone and volcanic rocks. Matrix 

is coarse yellow clayey sand, noncalcareous, leached and oxidized. No sign of glacial origin ___________________________ 30

7. Basalt flow, compact base, vesicular top ____________________ 6

6. Andesite(?) porphyry, purplish-gray, weathered, possibly intrusive _____________-______.__________-__ 6(?) 

5. Volcanic breccia interbedded with compact olivine basalt in

layers 30 to 90 cm thick ______________-__ __-- 8 

4. Tuff, light-gray, sandy; plant fragments __-_---_-__---_------ 1.5

3. Bentonitic clay _____________________________ 0.3

2. Sand, light-gray, tuffaceous, moderately coarse, well-bedded ____ 1.2

1. Cobble gravel, well-rounded clasts in coarse sand matrix, poorly

bedded; common large size smaller than in upper gravel ______ 15+

Outcrops of compact olivine basalt in the gravel on the narrow 

ridge in the northwestern part of sec. 9, T. 8 S., R. 3 W. appear to 

be part of a dike that cuts the gravel, confirming evidence elsewhere that the gravel, in part at least, is older than the olivine 

basalt flows.

The outcrops east of the Madison River in the Cameron quadrangle show a similar sequence of gravel overlain by tuff and 

olivine basalt flows. Pebbles and small cobbles in this gravel include several kinds of Precambrian granitic and gneissic rocks 

as well as basaltic lava, tuff, and some Paleozoic limestone. This 

gravel is considered to be equivalent in age to the gravel on the 

crest of the Gravelly Range.

A sequence of boulder and cobble gravel overlain by tuff and 

agglomerate as much as 150 m thick, which in turn is overlain 

by olivine basalt flows, was reported by Mann (1954, p. 37-43) 

from localities farther south along the crest of the Gravelly 

Range. Vertebrate fossils in the tuff from these localities are 

early Oligocene in age, according to Mann.

The gravel north of Wigwam Creek is more problematical; it 

is poorly exposed, and its relation to the adjacent volcanic rocks 

is obscure. It consists almost wholly of fragments of locally derived Precambrian rocks and contains only minor amounts of 

younger rocks, which are commonly large angular blocks or slabs 

of Cambrian quartzite or limestone, obviously of very local origin.
88 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

Some large blocks of Precambrian rocks are 1.5 to 3 m in longest 

dimension and cannot have traveled far from their sources, presumably in the adjacent Precambrian terrane. Cobbles and small 

boulders of basalt appear in this gravel at several places. In the 

only good exposure (NE corner sec. 7, T. 7 S., R. 3 W.), large 

pebbles and small cobbles are embedded in a matrix of yellow, 

variably clayey sand, which is poorly sorted and poorly stratified. 

The exposed thickness of the gravel at this locality may be as 

much as 30 m.

The stratigraphic relation of this gravel to the adjacent tuff 

and basalt flows is not clear from exposures seen in the Varney 

quadrangle. Abundant cobbles and small boulders lie on the olivine 

basalt of the isolated outcrop areas in sees. 7 and 31, T. 7 S., 

R. 3 W., prompting an earlier conclusion that the gravel was interbedded in the lower part of the basalt sequence. However, the 

gravel is not known to underlie any of the basalt flows and does 

not appear at the margin of the flows or tuff northeast or south 

of the main basalt pile. Rounded pebbles, small cobbles, and a few 

large boulders are scattered here and there in the large area of 

tuff in the Axolotl Lakes area, but whether these fragments represent gravel beneath the tuff or whether they have been moved 

down from the gravel areas to the west during the extensive landsliding of the tuff is not clear. On balance, the evidence seems 

to favor the interpretation that this gravel is younger than the 

volcanic rocks and occupies valleys cut perhaps in later Tertiary 

time. The large blocks may even have been deposited by local 

glaciers during the Pleistocene.

LOWER TERTIARY EROSION SURFACE AND COLLUVIAL DEPOSITS

The gravel and volcanic rocks previously described were deposited on a surface of folded and eroded Precambrian, Paleozoic, 

and Mesozoic rocks whose relief was considerably less than that 

of the present landscape and which has been tilted or downwarped 

northeastward since the lower Tertiary materials were laid down. 

This surface extends for long distances at altitudes of 2,500 to 

2,600 m along the crest of the Gravelly Range; it appears to have 

continued as a valley floor across the Greenhorn fault about 360 

m below the highest summits of Baldy Mountain and the peak in 

sec. 28, T. 8 S., R. 3 W. Beneath the large area of basal flows and 

tuff, the surface slopes northeast toward the Madison Valley at 

50 to 75 m/km; a similar slope is found by comparing the altitudes of gravel, tuff, and lava remnants in the southern part of 

the Varney quadrangle. As this slope is considerably less than
CENOZOIC ROCKS 89

the average or even the minimum dip of the flow structure in 

the basalt, it seems likely that the basalt pile was thickest near 

its northwestern edge and that the direction of flow was generally northeastward.

Massive movements of blocky colluvium appear to have taken 

place on this surface somewhat later than the deposition of the 

gravel and volcanic rocks. These movements are principally recorded in two anomalous areas of very large blocks of chert and 

cherty quartzite of the Shedhorn Sandstone. The larger of these 

areas occupies about 0.6 km 2 south of Horse Hill on the north 

side of Wigwam Creek Canyon. Blocks as much as 18 m long 

litter this area, lying in red clayey soil on the mudstone of the 

Kootenai Formation. The probable bedrock source for these blocks 

is the outcrop belt of Shedhorn across the canyon to the southeast, 

and the blocks presumably moved by gravity northwestward on 

the precanyon slope.

RHYOLITE ASH-FLOW TUFF

Extensive remnants of rhyolite in the southern parts of the 

Varney and Cameron quadrangles record the arrival of what was 

probably a large sheet of ash-flow tuff that spread northward 

from a source somewhere near the margin of the Yellowstone 

volcanic field. The field evidence indicates that most of the deposits were produced during a single eruption; they filled valleys 

whose floors were about 150 m above the present levels of Ruby 

Creek and Ruby River.

Along the west side of the Madison River in the southwestern 

part of the Cameron quadrangle the rhyolite is at least 18 m 

thick, although the better exposures include only 7 to 11 m of 

the more resistant lower and middle parts of the sheet (fig. 7). 

The main part of the sheet is compact lithic rhyolite containing 

conspicuous crystal fragments of sanidine and quartz. It is chalky 

and pinkish gray or pale red to medium brownish gray or reddish 

brown. Some layers are strongly spherulitic and contain elliptical 

voids, 1 to 2 cm long, flattened parallel to a megascopic platy 

structure in the more compact rhyolite. The lithic parts of the 

sheet are underlain in several places by a basal layer 30 to 90 

cm thick of medium- to dark-gray glassy rock, which also contains abundant crystal fragments. Upper parts of the sheet grade 

upward to less compact tuff, part of which has been stripped 

away, leaving broad benches like that in sees. 35 and 36, T. 9 S., 

R. 1 W. In several places, the more resistant parts of the tuff lie 

on crudely bedded vitric tuff containing abundant rounded frag-
CD

O

o

M 

O 

f 

O O

a o

2 

Of

M OT

FIGURE 7. Faulted rhyolite ash-flow tuff exposed in bluffs along west side of the Madison River valley, sec. 1, T. 10 S., R. 1 

W., Cliff Lake quadrangle. View west from highway 0.8 km south of Cameron quadrangle.
CENOZOIC ROCKS 91

merits of pumice and subangular fragments of various local Precambrian rocks, 0.5 to 6.5 cm in diameter. On Johnny Gulch in 

El/2 sec. 9, T. 9 S., R. 1 W., the rhyolite lies on approximately 

.18 m. of volcanic mudflow deposits, including vesicular and compact olivine basalt.

A few smaller remnants of essentially similar rhyolite were 

found in the valley of Ruby River in the southwestern part of 

the Varney quadrangle. In the larger remnants, compact crystallithic tuff at least 12 m thick is underlain by 30 to 60 cm of darkgray glassy rock, which lies directly on bedrock.

The rhyolite is everywhere composed of shards, small pumice 

fragments, and angular crystal fragments of sanidine, quartz, 

sodic plagioclase, and, rarely, pyroxene. The rock ranges from 

loosely cemented nan-welded to extremely compact welded tuff. The 

light-colored lithic parts are partially to completely devitrified, 

but the dark obsidianlike basal layers, although highly welded, are 

still largely glass. In some specimens of the basal welded tuff, the 

shards have been folded on a microscopic scale, apparently by 

small movements within the flow after deposition. Devitrification 

has produced a variety of axiolitic and spherulitic textures similar to those described by Ross and Smith (1961) in ash-flow tuff 

from many localities throughout the world.

The eruption of the flow took place when the topography of the 

region was similar to that of today but before maximum downfaulting of the Madison Valley and deposition of the Quaternary 

alluvial gravels on the valley floor." Fragments of olivine basalt 

in and beneath the base of the tuff sheet indicate that the rhyolite 

is younger than the basalt lavas in the Gravelly Range and on 

the floor of the Madison Valley. However, moraines and outwash 

gravels, probably of Bull Lake age, are found along Ruby Creek 

and Ruby River in terrace remnants only 60 m lower than the 

rhyolite, which, therefore, may be not greatly older than the Bull 

Lake Glaciation. Although a late Tertiary age has been assumed 

for the rhyolite, it may be as young as early Pleistocene.

FRESHWATER LIMESTONE

An extensive unit of freshwater limestone is exposed in a 

series of low hills just west of the Madison River in the northeastern part of the Varney quadrangle and the northwestern part 

of the Cameron quadrangle. This rock is easily recognizable, pale 

yellowish brown to light brownish gray, moderately to poorly 

indurated, crudely bedded, and commonly porous and permeated 

by small solution cavities along the bedding. It commonly con-
92 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

FIGURE 8. Tertiary limestone in lower canyon of Wigwam Creek, near line 

between sees. 24 and 25, T. 7 S., R. 1 W., Varney quadrangle. Ridge on 

center skyline is same limestone uplifted and tilted eastward 11°.

tains lenses and fragments of crustiform or banded limestone or 

flowstone within more massive limestone, as well as layers of 

spongy tufalike material. The best exposure of the unit (fig. 8) 

is in a narrow canyon cut below the fan gravels of Wigwam 

Creek in sees. 24 and 25, T. 7 S., R. 2 W., where a section 21 m 

thick was measured (stratigraphic section 9).

A collection of snails from unit 2 of this section was described 

by Dwight L. Taylor as follows (written commun., 1959) :

Freshwater snails:

Valcata cf. V. lewisi Currier

Stagnicola sp.

Physa sp. 

Land snail:

cf. Euconulus fulvus (Muller)

On account of the poor preservation of the fossils, specific identification is impossible. So far as the material goes, however, it may all 

represent species living locally. This evidence tends to suggest a 

Pleistocene rather than Pliocene age. No late Pliocene fossils from 

the area are known for comparison, but the middle Pliocene mollusks from the northern Rocky Mountains are strongly distinct. 

The habitat represented is a perennial fresh water body, either a 

lake or perhaps a gently flowing stream. The single land snail is 

recorded by only one specimen, which might have been transported 

for some distance from the original habitat.
CENOZOIC ROCKS 93

Stratigraphic section 9

[Tertiary limestone, SE^ sec. 24, T. 7 S., R. 2 W. t Varney quadrangle]

Thickness 

(meters)

5. Limestone, like unit 3, more weathered, abundant solution 

cavities; much flowstone and tuf alike material; overlain by 

gravel _________________________ ___ 7.6

4. Limestone, light-brownish-gray, nonresistant, laminated with

cross-fibrous calcite in straight even layers ______ _____ 1.5

3. Limestone, similar to unit 2, less compact and less distinctly 

bedded; locally platy or laminated; contains porous interlayers _____________________________ 3.7

2. Limestone, mostly light yellowish brown, porous and vuggy; 

many solution cavities to 5 cm in diameter. Bedding indistinct but even, locally curved and truncated by overlying beds. Locally spongy and tufalike, containing hollow 

tubes 4.5 or less in diameter. Lower part: more compact 

and resistant; includes fossiliferous lens 30 cm by 4 m 

near base of unit ____________________________________ 6.8

1. Limestone, light-brownish-gray, porous to compact; locally 

includes flowstone mantling more compact limestone. Contains rounded pebbles 0.5 to 13 cm in diameter of upper 

Paleozoic and Mesozoic sandstone and quartzite, upper 

Paleozoic limestone and chert, Precambrian granite, and 

olivine basalt _____________________________ 2,1

Base not exposed.

Bedding in the limestone unit dips generally eastward 3° to 

12°; locally, however, beds dipping as much as 20° were seen. 

The variations probably represent originally nearly horizontal 

beds tilted eastward by late Cenozoic faulting in the Madison, Valley. In the excellent exposures on Wigwam Creek, however, dip 

variations of as much as 10° occur within the unit; structural 

relations suggest that the steeper dips may have resulted from 

fault movements during deposition of the limestone.

The geomorphic and structural relations of the Tertiary limestone are similar to those of the ash-flow tuff, which, therefore, 

may have been deposited ait about the sarnie time. Both limestone 

and tuff contain fragments of olivine basalt presumably derived 

from the lava nearby to the west, and both have been displaced 

about the same amount by the normal faults of the Madison Valley floor.

QUATERNARY DEPOSITS

GLACIAL DEPOSITS

Direct glacial deposits in the Varney and Cameron quadrangles 

consist of till deposited by valley glaciers flowing from cirques on 

isolated summits in the Gravelly and Greenhorn Ranges and from
94 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

several strongly glaciated valleys in the Madison Range. They 

may also include some considerably older deposits of bouldery 

material found in unusual topographic situations on the eastern 

flank of the Gravelly Range. Glaciers on Baldy Mountain in the 

northwestern part of the Varney quadrangle and on Specimen 

Butte at the head of Ruby Creek just south of the quadrangle 

boundary were all short, originating in cirques at about 2,750 m 

altitude and descending the northern and eastern slopes for distances of 1.5 to 6.5 km, Glaciers in the Madison Range were somewhat longer; those in the canyons of Wolf Creek and the South 

Fork of Indian Creek flowed 11 to 13 km from cirques at about 

3,000 m in altitude and reached the range front at 1,890 to 1,950 

m. A glacier of similar length occupied the valley of Noman 

Creek just east of the Cameron quadrangle and flowed into the 

main valley of Indian Creek, ending at an altitude of 1,950 m 

about 3 km short of the range front.

The extent of the ice is recorded by many discontinuous lateral 

moraines in the lower parts of the valleys and by well-developed 

lobate end moraines of those glaciers that reached the alluvial 

floor of the Madison Valley. The surfaces of the morainal deposits 

are generally characterized by more or less well-rounded boulders, 

some as much as 3 to 4.5 m in size, derived from bedrock sources 

within the glaciated valleys. For example, the moraines on the 

headwaters of Wigwam Creek on the east side of Baldy Mountain contain only Paleozoic and Mesozoic rocks, whereas the adjacent Tertiary gravel contains abundant Precambrian clasts. 

However, a few Precambrian clasts derived from the gravel were 

incorporated in the basal parts of these moraines. The deposits 

s'how well-developed morainal ridges as much as 18 m high, with 

intervening swales and a few undrained depressions in spite of 

their relatively steep overall slopes.

End-moraine complexes at the mouths of the canyons of Wolf 

and Indian Creeks in the Cameron quadrangle are especially well 

preserved. As Alden (1953, p. 180-181) pointed out, the moraines 

on Indian Creek were deposited by a glacier descending the South 

Fork while the lower part of the main valley remained unglaciated. These morainal complexes contain many well-defined lobate 

morainal ridges, unbreached depressions, and large blocks scattered about their surfaces.

On both Wolf Creek and the South Fork of Indian Creek, subdued lateral moraines :along the canyon walls reach altitudes of 

150 to 180 m above the present valley floors. They represent ice 

either at a somewhat higher and older level of erosion, or thicker
CENOZOIC ROCKS 95

than that which produced the end moraines, for the gradients of 

these higher moraines suggest that their terminal lobes would 

have extended farther out on the alluvial valley floor than the 

existing end mioraines. Older, more extensive end moraines may 

have been depressed by faulting and partly buried beneath the 

alluvial fan. This is best shown at the mouth of Wolf Creek 

canyon, where a lateral moraine at an altitude of 2,200 m on the 

north wall appears to be cut off abruptly at the canyon mouth 

more than 120 m above the surface of the younger end moraine 

on the valley floor just outside the canyon.

Evidence for older and younger, or higher and lower level, 

glaciations is not so clear in the other glaciated valleys of the 

area. Lateral moraines 120 to 135 m above Ruby Creek in the 

southeastern part of the Varney quadrangle may correspond to 

the higher lateral moraines in the Madison Range. The ice on 

Ruby Creek did not extend beyond the mapped moraines, for the 

lower part of the canyon shows no evidence of glacial action. 

Remnants of bouldery terrace deposits on the north wall of the 

canyon about 45 m above the present streambed suggest that the 

valley floor at the time of maximum glaciation was graded approximately to the surface of the alluvial fan at the mouth of the 

valley.

The existence of mo-re than one stage of glaciation in the Madison Range was suggested by Alden (1953, p. 63, 180-181), who 

assigned ntost of the moraines on Indian Creek and elsewhere in 

the range to lowan or Illinoian and higher level deposits to early 

Pleistocene glaciation. Subsequently, Richmond (1964, p. 225- 

227) recognized deposits in the nearby Yellowstone Park area as 

belonging to the Bull Lake and Pinedale Glaciations, as well as 

to one or moire pre-Bull Lake glaciations. He suggested that the 

Pinedale is approximately contemporaneous with the Wisconsin 

Glaciation in the midcontinent region and that the Bull Lake 

Glaciation is either early Wisconsin or represents a pre-Wisconsin, post-Illinoian glaciation not recognized in the midcontinent. 

Whether the higher and lower moraines I have mapped as older 

and younger till in the Madison Range represent two glaciations 

separated by an interval of extensive deglaciation is debatable, 

but they were formed probably during one or both of the two 

stades of the Bull Lake Glaciation recognized by Richmond. Comparable morainial deposits in the valley of Beaver Creek, a tributary to the Madison River just northwest of Hebgen Lake in the 

adjacent Hebgen Dam quadrangle, were assigned to the older 

stade of Bull Lake Glaciation by Richmond. These moraines are
96 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

smoothed and resemble the older or outer moraines for example, 

those on Baldy Mountain and the older moraines in the Madison 

Range. Moraines of the Pinedale Glaciation, according- to Richmond, are confined to the upper 3 to 5 km of glaciated valleys in 

the Hebgen Lake area. Nevertheless, the state of preservation 

of the end moraines on Indian and Wolf Creeks suggests that 

these may be of Pinedale age (see Richmond, 1965, p. 224).

BOULDER DEPOSITS OF UNCERTAIN ORIGIN

Unusual deposits of subangular blocks in a dark-brown clayey 

matrix occupy interfluve areas extending in a horseshoe-shaped 

pattern across the headwaters of Morgan Gulch, Cherry Creek, 

and Johnny Gulch in the east-central part of the Varney quadrangle. The deposits consist of angular to subangular blocks and 

moderately rounded cobbles of various Paleozoic and Mesozoic 

rocks, among which the most conspicuous are cherty sandstone 

of the Shedhorn Sandstone, Quadrant Quartzite, sandstone and 

conglomerate of the Kootenai Formation, Flathead Quartzite, and 

much weathered Precambrian granite and gneiss. The material 

is not well exposed, but the matrix appears to be very clayey and 

the deposits unsorted. Most of the fragments are of local origin. 

Blocks of the Kootenai Formation are abundant in the western 

part of the deposits on Morgan Gulch, but they become obviously 

fewer eastward, as do, in succession, blocks of the Shedhorn and 

the Quadrant. In the eastern part of the deposit farther down 

Morgan Gulch, Precambrian granitic blocks, some as much as 

4.6 m long, are abundant as well as blocks of Flathead Quartzite. 

Near the head of Johnny Gulch, the bouldery deposits contain 

many small fragments of dark-gray phyllite and iron-rich quartzite that can have come only from areas of Precambrian rocks at 

lower altitudes to the east.

The surface of these deposits, especially in the headwaters of 

Morgan Gulch, includes small undrained depressions resembling 

subdued knob-and-kettle topography comparable with the more 

recent morainal deposits southeast of Baldy Mountain. Although 

landsliding might explain the depressions, the clear evidence of 

transportation westward from topographically lower source areas 

and the horseshoe shape of the deposits as a whole strongly suggest that they are morainal material deposited by a broad tongue 

of ice that moved westward from highlands in the Madison 

Range. Presumably this took place before the present Madison 

structural valley was formed, for there is no evidence of similar 

deposits on the floor of the valley, and moraines in the Madison 

Range barely reach the present range front.
CENOZOIC ROCKS 97

Remnants of formerly extensive glacial deposits at levels 300 

or more meters above present valley floors at many places in the 

Madison Range and nearby areas have commonly been referred 

to the "Buffalo Till," presumably of pre-Wisconsin age. Those 

in the northern part of the Madison Range have been described 

by Hall (1960, p. 197-199) as being on the eastern slope of the 

range, 30 to 50 km east of the deposits under discussion, and 

having come from source areas nearer the crest of the range to 

the west. Hall also pointed out that similar bouldery deposits on 

the crest of the Gravelly Range near Black Butte contain striated 

stones and lie on a striated pavement that indicates ice movement 

in the direction N.65°W. These were described by Atwood and 

Atwood (1945) as till of Eocene age and later by Mann (1954, p. 

37-43; 1960, p. 122-123) as representing mudflow deposits of 

probable Oligocene age. Whether the deposits in these widely separated places are closely related in time and process is still conjectural. As Hall (1960) pointed out, they may have resulted 

from widespread glacial activity, probably of pre-Wisconsin age, 

whose deposits have been radically dismembered by late Cenozoic 

normal faulting and canyon cutting in the region.

ALLUVIAL FANS AND GRAVEL OF THE MADISON VALLEY

Most of the floor of the Madison Valley is occupied by variably 

coarse cobble gravel carried into it by the larger streams on either 

side. These deposits have formed a series of partly coalescing 

alluvial fans on both sides of the valley, which originally merged 

with a narrow northward-sloping alluvial plain in the axial part 

of the valley. Although this surface of alluviation has been 

eroded and dissected in many places, large remnants are preserved in the north-central part of the Cameron quadrangle in 

what is called the Cameron Bench and in many of the fans on 

the west side of the river. Because the altitude is higher and the 

relief is greater in the Madison Range to the east than in the 

Gravelly Range to the west, the fans east of the river have grown 

larger than their counterparts west of it, producing the present 

asymmetrical position of the river. Since construction of the 

Cameron Bench, the Madison River has cut its channel nearly 60 

m below its former alluviated valley floor. Concurrently, Indian 

Creek, Wolf Creek, Ruby Creek, and other tributaries have become entrenched in their respective fans, and the Madison has 

carved a distinctive set of erosional terraces. Most of the larger 

fans east of the river have been eroded to some degree, leaving 

remnants of the original constructional surfaces 1 to 6 m or more 

above the present fan surfaces. Most of this erosion appears to
98 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

have resulted from lowering the bed of the Madison River; some 

erosion near the fan heads may be due to relative uplift of the 

mountain block along the Madison Range fault system.

A series of somewhat older and steeper fans and aprons, of 

mixed alluvial and colluvial material, mantle the lower slopes of 

the Madison Range front. All these have been eroded, some rather 

deeply for example, those south of Bear Creek and adjacent to 

Corral Creek. These deposits appear to have been brought down 

by short streams descending the mountain front on steep gradients and to have been tectonically raised relative to the axial part 

of the valley. They are surmounted and locally overlapped by 

landslide material from the actively rising bedrock slopes of the 

mountain front.

Alden (1953, p. 71) thought that the Cameron Bench was early 

Pleistocene in age, thereby implying that the associated fan 

gravels were early Pleistocene or older. Although this may be 

true for much of the underlying valley fill, relations between the 

larger fans and the glacial deposits in the Varney and Cameron 

quadrangles indicate that construction of the fans and the 

Cameron Bench continued at least until the beginning of Bull 

Lake Glaciation, which Richmond (1970, p. 15) believed followed 

the Sangamon Interglaciation 120,000 to 130,000 years ago. The 

younger end moraines on Indian Creek and on Wolf Creek appear 

to have been laid down on the apices of the fans shortly before 

the erosion previously described. Thus, the age of the Cameron 

Bench and the associated alluvial fans of the Madison Valley is 

probably at least as young as Sangamon in terms of the American 

midcontinent Pleistocene sequence. If the end moraines produced 

at older and higher ice levels have indeed been dropped and partly buried by the fans, this fact would suggest that building of the 

fans continued into early Wisconsin time.

GRAVEL OF THE RUBY RIVER VALLEY

Thin gravel caps spurs and dissected terraces adjacent to Ruby 

River and Warm Springs Creek in the southwestern part of the 

Varney quadrangle. This deposit is largely cobble gravel and a 

few small boulders composed mainly of Paleozoic sandstone and 

limestone. The gravel along Warm, Springs Creek also contains 

clasts of Precambrian rocks and Tertiary basalt and rhyolite. The 

Precambrian rocks and basalt presumably were derived from 

Tertiary gravel and lava in the Gravelly Range. The gravel generally is not more than 6 or 9 m thick and appears to be remnants 

of a sheet that sloped steeply toward Ruby River and Warm
CENOZOIC ROCKS 99

Springs Creek and whose lowest points were about 30 m above the 

present streams. Prevalence of Quadrant Quartzite in the gravel 

south of the confluence of Warm Springs Creek shows that this 

particular gravel came largely from the Snowcrest Range west of 

Ruby River.

Remnants of a depositional surface not eroded or covered by 

the gravel are preserved on several steep spurs sloping eastward 

from the Greenhorn Range along Timber Creek and on a similar 

spur sloping westward along the south side of Squaw Creek. In 

these places, the surface is mantled by large and small colluvial 

blocks derived from the surmounting slopes. By analogy with 

the Quaternary deposits in the Madison River valley, the gravel 

in the Ruby River valley probably was also deposited near the 

time of the latest glacial maximum and has been undergoing erosion since then.

LANDSLIDE DEPOSITS

Evidence of large-scale mass movement of exposed rocks down 

the present topographic slopes is widespread in the Gravelly 

Range and along the steep faulted front of the Madison Range. 

Th!e results range from the irregular ridges and sag ponds, characteristic of outcrop areas of the Morrison Formation, to visible 

landslide scars surmounting chaotic masses of disturbed bedrock. 

Where landslide movements have been sufficient to carry significant amounts of one formation over another, these deposits have 

been mapped as colluvial or landslide deposits. Such deposits include earthflows of mudstone of the Morrison Formation in the 

South Fork of Warm Springs Creek, of Kootenai Formation and 

Thermopolis Shale east of Romy Lake, and a mudflow of softened 

Sphinx Conglomerate in Grindstone Gulch, all in the Gravelly 

Range. Also included are a landslide mass of Precambrian dolomite just north of the Bear Creek Ranger Station and several 

earthflows of Cretaceous mudstone and sandstone farther north 

along the front of the Madison Range.

A similar origin may be ascribed to an unusual pile of angular 

blocks of the basal sandstone of the Kootenai Formation 2.5 km 

north of the Crockett Lake Ranger Station in the Gravelly Range. 

Many of the blocks in this area are 3 m in diameter, and some are 

as large as 9 m. They are adjacent to two outcrop areas of the 

Kootenai Formation, from which they were probably derived by 

slumping of the underlying Morrison Formation.

Some of the landslides have obviously taken place within the 

past decade or two, but most are grassed over or forested. Most
100 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

of the activity, therefore, probably coincided with the colder 

climate during glacial advances in. the region, when melting of 

accumulated ice and snow provided excessive soil moisture and 

opportunities for intensive frost action.

STRUCTURAL GEOLOGY

The structural pattern in the Phanerozoic rocks of the Varney 

and Cameron quadrangles has resulted from two distinct periods 

and contrasting types of deformation. Episodes of Laramide 

orogeny, essentially compressional in character, produced two 

major belts of thrust and tear faults along with northwest-trending folds and associated high-angle thrust faults in the Paleozoic 

and Mesozoic rocks. Later erogenic events were, by contrast, 

largely extensional. They resulted in relative depression of a 

major structural block in the Madison Valley, complementary 

strong uplift of the western side of the Madison Range, and attendant steep normal faults in both the Madison and Gravelly 

Ranges. Most of this deformation took place in late Cenozoic time, 

and it continues to the present.

LARAMIDE STRUCTURAL FEATURES

Laramide structural features are concentrated in two belts of 

faults and folds, a western belt in the Greenhorn Range, continuing northeastward across the northern end of the Gravelly 

Range in the Varney quadrangle, and an eastern belt along the 

west front of the Madison Range in the Cameron quadrangle. The 

eastern belt is much obscured by normal faulting and is covered 

by basin fill in the northern part of the Cameron quadrangle, but 

it becomes more visible as it trends away from the range front 

in the east-central part of the quadrangle and continues southeastward in the adjacent Sphinx Mountain and Hebgen Dam 

quadrangles.

A major fault zone extends along the east side of the Greenhorn, Range from a point about 1.6 km northeast of Ruby River 

to the northern part of the Varney quadrangle, where it disappears beneath gravel and volcanic rocks of Tertiary age. This 

fault was first recognized by Mann (1954, p. 52), who called it 

the Greenhorn thrust. It represents a major structural break 

bringing complexly deformed Precambrian and Paleozoic rocks 

eastward over less deformed Upper Cretaceous rocks. The minimum stratigraphic displacement is 3,000 m, and the horizontal 

displacement is probably 8 km or more. Attempts to contour the 

fault surface along the outcrop belt between Ruby River and 

the North Fork of Wigwam Creek suggest that much of its ir-
STRUCTURAL GEOLOGY 101

regular trace results from local variations in attitude. The strike 

of the fault along segments 1.5 to 3 km long appears to vary 

from nearly due north to N. 40° E., and the dip may range from 

less than 20° to considerably more at various places. The fault 

trace is offset by right-lateral tear faults, notably just west of 

the quadrangle boundary, where the Greenhorn fault is offset 

more than 1.5 km on a steeply dipping fault trending N. 30° W. 

(%. 9).

112°

45°05'

45W

EXPLANATION

UPPER CRETACEOUS 

ROCKS

TRIASSIC, JURASSIC AND 

LOWER CRETACEOUS 

ROCKS

PALEOZOIC ROCKS

p PRECAMBRIAN ROCKS

?=- Thrust fault T on upper plate; 

dashed where inferred. Arrows 

show relative movement

0 2 5 KILOMETERS

FIGURE 9. Relation of the Greenhorn and Snowcrest faults in the vicinity

of Ruby River Canyon.
102 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

The Greenhorn fault has not been located north of the cover of 

Tertiary rocks in the northern part of the Varney quadrangle. 

One intriguing clue to its location, however, is provided by the 

small outcrops of Precambrian dolomite surrounded by tuff in 

sec. 8, T. 7 S., R. 2 W. If these outcrops are part of the Greenhorn thrust block, the northern continuation of the fault must be 

between them and the outcropping Jurassic rocks nearby to the 

east. The fault has also not been traced in detail southwest of the 

Varney quadrangle. It may continue across the Ruby River into 

the Snowcrest Range, or it may be overlapped by a second major 

fault, the Snowcrest fault, which has been mapped for many kilometers along the west side of the Snowcrest Range (fig. 9).

Southwest of Ruby River, the Thermopolis Shale, the Kootenai 

Formation, and older Mesozoic and Paleozoic formations appear 

at the northern end of a belt of steeply dipping and overturned 

strata that extends southwestward the full length of the Snowcrest 

Range (Klepper, 1950, pi. 16). This belt represents the western 

limb of a major syncline whose eastern limb includes the gently 

dipping rocks of the southern part of the Gravelly Range and 

whose axis is approximately along the upper course of the Ruby 

River.

East of the Greenhorn fault in the Varney quadrangle, two 

major northwest-trending anticlines appear in the Paleozoic and 

Mesozoic rocks the Warm Springs anticline and a larger anticline near the center of the quadrangle. Both folds are cut longitudinally by high-angle thrust faults; similar faults cut the westdipping beds throughout the central part of the quadrangle. Small 

compressional folds a few meters in amplitude and minor beddingplane thrust faults are found locally almost to the eastern limit 

of the Paleozoic rocks in the Gravelly Range.

Structural features of the eastern Laramide fault belt are similar to those of the western belt. They are best seen in the ridges 

on either side of Indian Creek, in the Cameron quadrangle, where 

steeply dipping and strongly overturned Paleozoic rocks are associated with a major thrust fault that brings Precambrian gneiss 

and dolomite eastward over rocks as high in the section as the 

Frontier Formation. Beds immediately beneath the fault are locally overturned as much as 60°; beds farther east are folded and 

sliced by subsidiary thrust faults. Similar relations north of Bear 

Creek are obscured by younger normal faults which give rise to 

peculiar structural relations along the Madison Range front. For 

example, a small block of Precambrian granitic rock exposed between Burger and Mill Creeks (sees. 22 and 27, T. 7 S., R. 1 E.)
STRUCTURAL GEOLOGY 103

apparently represents part of the overthrust dropped by normal 

faulting against the much younger Cretaceous rocks beneath the 

fault. How complex these relations can get is well shown in the 

exposures on the range front at Tolman Creek near the northern 

quadrangle boundary.

The youngest formation affected by the Larami.de deformation 

is the Sphinx Conglomerate of Eocene age. Although this unit lies 

unconformably on folded and faulted rock ranging from Mississippian to Late Cretaceous age, including the volcanic and volcaniclastic rocks in the Cameron quadrangle, the Sphinx Conglomerate at the eastern border of the quadrangle is strongly 

folded on northwest-trending axes parallel to other folds of late 

Mesozoic to early Tertiary age.

CENOZOIC NORMAL FAULTS

The eastern part of the Gravelly Range and the west front of 

the Madison Range are dominated structurally by normal faults 

resulting from regional deformation beginning in middle or late 

Tertiary time and continuing to the present. In contrast to the 

compres'sional deformation of the Laramide orogeny, the late 

Cenozoic deformation has resulted in crustal extension and relative depression of major structural features like the Madison Valley. It was accompanied by extensive volcanism not far south of 

the Varney and Cameron quadrangles.

The master feature in this area is the Madison Range fault, 

recognized and named by Pardee (1950, p. 369) as a system of 

en echelon faults along the western front of the Madison Range 

and the eastern border of the Madison Valley. The course of the 

frontal fault system is marked by easily visible recent fault scarps 

on the gravel fans and colluvial slopes throughout the Cameron 

quadrangle and for many kilometers north and south of it. Detailed mapping in the Cameron quadrangle shows that the rangefront fault system consists of three parallel overlapping segments 

about 5 km apart, each trending north-northwest along the northtrending range front. Each of the downdropped blocks bounded 

by these faults has been tilted northward and probably westward, 

like a series of piano keys hinged at their southern ends. The 

three main fault segments are largely concealed by the accumulated surficial deposits along the foot of the range, but their traces 

are outlined by prominent fault scarps in these deposits. In particular, the location of the western fault is indicated by two distinct scarps that were overlooked during the fieldwork but are 

readily seen on air photographs. One of these scarps trends
104 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

N. 10 E. through sees. 7 and 18, T. 7 S., R. 1 E.; the other trends 

N. 5° W. in the western part of sees, 9 and 16, T. 8 S., R. 1 E.

Only one locality provided, bedrock exposures of any of the 

faults. A 9-m zone of brecciated granitic gneiss trending N. 40° W. 

can be traced along the western slope of The Wedge in the southeastern corner of the Cameron quadrangle. Abundant slickensiided fault surfaces within the breccia dip steeply westward and 

show striae about parallel to their dip. Angular fragments of 

gneiss and fault gouge are cemented by carbonate, amorphous silica, and iron oxide. Some fragments also show argillic 

alteration.

Estimates of the amount of movement on the faults of the 

Madison Range system are difficult because no well-defined features can be correlated across them. Tertiary gravel and volcanic 

rocks that appear on the downdropped blocks along the eastern 

side of the Madison Valley may have been eroded from some of 

the higher blocks, but similar deposits have not been found in 

the Madison Range in the vicinity of the range front. Nevertheless, cumulative depression of the valley floor on these faults in 

the northern part of the Cameron quadrangle must amount to 

a few thousand meters.

Much smaller displacements are found on many of the faults 

that cut the Paleozoic rocks along the west side of the Madison 

Valley. Stratigraphic displacements on these faults are easily 

measurable and range from less than 30 m to a maximum of 750 

m on the faults shown near the west end of structure section 

B-B' on the geologic map of the Cameron quadrangle (Hadley, 

1969a).

Many of these faults are in a zone trending about N. 40° W., 

extending roughly parallel to the major segments of the Madison 

Range fault system into the north-central part of the Varney 

quadrangle. Others form a set of faults trending east-northeast, 

parallel to foliation and lithic boundaries in the adjacent Precambrian rocks. Relative displacement is down on the east side 

of most of these faults, contributing toward depression of the 

western margin of the Madison Valley. In general, these displacements are larger on the southern faults' of the group, indicating 

increasing downfaulting of the western side of the valley in this 

direction.

The normal faults displace the youngest bedrock units in the 

area, including the upper Tertiary (?) limestone and ash-flow 

tuffs. The recent fault scarps show that major movements have 

taken place on some 50 km of the Madison Range fault probably
SELECTED REFERENCES 105

within the past 1,000 years. Minor movements took place on the 

same fault system several kilometers south of the Cameron quadrangle during the Hebgen Lake earthquake of 1959 (Myers and 

Hamilton, 1964, p. 77-78). Relations, of faulted moraines at the 

range front on Wolf Creek in the southeastern part of the 

Cameron quadrangle, previously described, indicate downfaulting of the floor of the Madison Valley by as much as 120 m since 

Bull Lake Glaciation about 30,000 years ago.

SELECTED REFERENCES

Alden, W. C., 1953, Physiography and glacial geology of western Montana and 

adjacent areas: U.S. Geol. Survey Prof. Paper 231, 200 p.

Atwood, W. W., and Atwood, W. W., Jr., 1945, The physiographic history of 

an Eocene skyline moraine in western Montana: Jour. Geology, v. 53, 

no. 3, p. 191-199.

Beck, F. M., 1960, Geology of the Sphinx Mountain area, Madison and Gallatin Co., Montana, in West Yellowstone Earthquake area: Billings 

Geol. Soc., llth Ann. Field Conf., Sept. 1960, [Guidebook], p. 129-134.

Blake, 0. D., 1959, Big Snowy stratigraphy in the area adjacent to the Rocky 

Mountain front, in Sawtooth-Disturbed Belt area [Montana]: Billings 

Geol. Soc., 10th Ann. Field Conf., August 13-15, 1959, [Guidebook], p. 

64-68.

Branson, C. C., 1937, Stratigraphy and fauna of the Sacajawea Formation, 

Mississippian, of Wyoming: Jour. Paleontology, v. 11, no. 8, p. 650-660.

Burger, H. R., Ill, 1969, Structural evolution of the southwestern Tobacco 

Root Mountains, Montana: Geol. Soc. America Bull., v. 80, no. 7, p. 

1329-1342.

Cobban, W. A., 1945, Marine Jurassic formations of Sweetgrass arch, Montana: Am. Assoc. Petroleum Geologists Bull., v. 29, no. 9, p. 1262-1303.

 1951, Colorado shale of central and northwestern Montana and equivalent rocks of Black Hills: Am. Assoc. Petroleum Geologists Bull., v. 35, 

no. 10, p. 2170-2198.

Cobban, W. A., and Reeside, J. B., Jr., 1952, Correlation of the Cretaceous 

formations of the Western Interior of the United States: Geol. Soc. 

America Bull., v. 63, no. 10, p. 1011-1044.

Cobban, W. A., Erdmann, C. E., Limbe, R. W., and Maughan, E. K., 1959, 

Revision of Colorado group on Sweetgrass arch, Montana: Am. Assoc. 

Petroleum Geologists Bull., v. 43, no. 12, p. 2786-2796.

Cressman, E. R., 1955, Physical stratigraphy of the Phosphoria Formation in 

part of southwestern Montana: U.S. Geol. Survey Bull. 1027-A, p. 

A1-A31.

Cressman, E. R., and Swanson, R. W., 1959, Shedhorn sandstone, in McKelvey, 

V. E., and others, The Phosphoria, Park City, and Shedhorn Formations 

in the western phosphate field: U.S. Geol. Survey Prof. Paper 313-A, 

p. A31-A33.

Deiss, C. F., 1936, Revision of type Cambrian formations and sections of Montana and Yellowstone National Park: Geol. Soc. America Bull., v. 47, no. 

8, p. 1257-1342, 2 pis., 10 figs.

Dorf, Erling, and Lochman, Christina, 1940, Upper Cambrian formations in 

southern Montana: Geol. Soc. America Bull., v. 51, no. 4, p. 541-556.
106 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

Dorr, J. A., Jr., 1954, Very late Miocene (late Barstovian) mammal fauna 

from the Madison Valley Formation, Montana [abs.]: Geol. Soc. America 

Bull., v. 65, no. 12, pt. 2, p. 1245.

Dutro, J. T., Jr., and Sando, W. J., 1963, Age of certain post-Madison rocks 

in southwestern Montana and western Wyoming: U.S. Geol. Survey Prof. 

Paper 475-B, p. B93-B94.

Easton, W. H., 1962, Carboniferous formations and faunas of central Montana: U.S. Geol. Survey Prof. Paper 348, 126 p., 13 pis.

Emmons, W. H., and Calkins, F. C., 1913, Geology and ore deposits of the 

Philipsburg quadrangle, Montana: U.S. Geol. Survey Prof. Paper 78, 

271 p.

Gardner, L. S., 1959, Revision of Big Snowy Group in central Montana: Am. 

Assoc. Petroleum Geologists Bull., v. 43, no. 2, p. 329-349.

Gardner, L. S., Hendricks, T. A., Hadley, H. D., and Rogers, C. P., Jr., 1946, 

Stratigraphic sections of upper Paleozoic and Mesozoic rocks in southcentral Montana: Montana Bur. Mines and Geology Mem. 24, 100 p.

Goddard, E. N., and others, 1948, Rock-color chart: Washington, D.C., Natl. 

Research Council, 6 p. (Republished by Geol. Soc. America, 1951; reprinted 1970.)

Grant, R. E., 1965, Faunas and stratigraphy of the Snowy Range Formation 

(Upper Cambrian) in southwestern Montana and northwestern Wyoming: 

Geol. Soc. America Mem. 96, 171 p.

Hadley, J. B., 1960, Geology of the northern part of the Gravelly Range, 

Madison County, Montana, in West Yellowstone Earthquake area: Billings Geol. Soc., llth Ann. Field Conf., Sept. 1960, [Guidebook], p. 149-153.

 1969a, Geologic map of the Cameron quadrangle, Madison County, Montana: U.S. Geol. Survey Geol. Quad. Map GQ-813, scale 1:62,500, sections. 

-1969b, Geologic map of the Varney quadrangle, Madison County, Montana: U.S. Geol. Survey Geol. Quad. Map GQ-814, scale 1:62,500, sections. 

Hall, W. B., 1960, Multiple glaciation in the Madison and Gallatin Ranges,

southwestern Montana, in West Yellowstone Earthquake area: Billings

Geol. Soc., llth Ann. Field Conf., Sept. 1960, [Guidebook], p. 191-199. 

Hanson, A. M., 1951, Distribution and origin of Cambrian limestone and

dolomite: Montana Acad. Sci. Proc., v. 10, p. 53-67.

 1952, Cambrian stratigraphy in southwestern Montana: Montana Bur. 

Mines and Geology Mem. 33, 46 p.

Heinrich, E. W., 1950, Sillimanite deposits of the Dillon region, Montana:

Montana Bur: Mines and Geology Mem. 30, 43 p. 

Imlay, R. W., 1967, Twin Creek Limestone (Jurassic) in the Western Interior

of the United States: U.S. Geol. Survey Prof. Paper 540, 105, p., 16 pis.,

18 figs. 

Klepper, M. R., 1950, A geologic reconnaissance of parts of Beaverhead and

Madison Counties, Montana: U.S. Geol. Survey Bull. 969-C, p. C55-C85. 

Klepper, M. R., Weeks, R. A., and Ruppel, E. T., 1957, Geology of the southern Elkhorn Mountains, Jefferson and Broadwater Counties, Montana:

U.S. Geol. Survey Prof. Paper 292, 82 p. 

Lochman-Balk, Christina, 1950, Status of Dry Creek Shale of central Montana:

Am. Assoc. Petroleum Geologists Bull., v. 34, no. 11, p. 2200-2222. 

Love, J. D., and others, 1948, Stratigraphic sections of Jurassic and Cretaceous

rocks in the Jackson Hole area, northwestern Wyoming: Wyoming Geol.

Survey Bull. 40, 48 p.
SELECTED REFERENCES 107

Lowell, W. R., and Klepper, M. R., 1953, Beaverhead Formation, a Laramide 

deposit in Beaverhead County, Montana: Geol. Soc. America Bull., v. 64, 

no. 2, p. 235-243.

Mann, J. A., 1954, Geology of part of the Gravelly Range, Montana: Yellowstone-Bighorn Research Proj., Contr. 190, 92 p.

 1960, Geology of part of the Gravelly Range area, Montana, in West 

Yellowstone Earthquake area: Billings Geol. Soc., llth Ann. Field 

Conf., Sept. 1960, [Guidebook], p. 114-127.

Mairvin, R. F., Wier, K. L., Mehnert, H. H., and Merritt, V. M., 1974, K-Ar 

ages of selected Tertiary igneous rocks in southwestern Montana: 

Isochron/West, no. 10, p. 17-20.

McKelvey, V. E., Williams, J. S., Sheldon, R. P., Cressman, E. R., Cheney, 

T. M., and Swanson, R. W., 1956, Summary description of Phosphoria, 

Park City and Shedhorn Formations in western phosphate field: Am. 

Assoc. Petroleum Geologists Bull., v. 40, no. 12, p. 2826-2863.

McMannis, W. J., 1955, Geology of the Bridger Range, Montana: Geol. Soc. 

America Bull., v. 66, no. 11, p. 1385-1430.

Moritz, C. A., 1951, Triassic and Jurassic stratigraphy of southwestern Montana: Am. Assoc. Petroleum Geologists Bull., v. 35, no. 8, p. 1781-1814.

Myers, W. B., and Hamilton, Warren, 1964, Deformation accompanying the 

Hebgen Lake earthquake of August 17, 1959, in The Hebgen Lake, Montana, earthquake of August 17, 1959: U.S. Geol. Survey Prof. Paper 

435-1, p. 155-198.

Newell, N. D., and Kummel, Bernhard, Jr., 1942, Lower Eo-Triassic stratigraphy, western Wyoming and southeast Idaho: Geol. Soc. America 

Bull., v. 53, no. 6, p. 937-995, 3 pis., 5 figs.

Pardee, J. T., 1950, Late Cenozoic block faulting in western Montana: Geol. 

Soc. America Bull., v. 61, no. 4, p. 359-406.

Peale, A. C., 1893, The Paleozoic section in the vicinity of Three Forks, 

Montana: U.S. Geol. Survey Bull. 110, 56 p.

 1896, Description of the Three Forks sheet [Montana]: U.S. Geol. 

Survey Geol. Atlas, Folio 24, 5 p., maps.

Reid, R. R., 1957, Bedrock geology of the north end of the Tobacco Root 

Mountains, Madison County, Montana: Montana Bur. Mines and Geology 

Mem. 36, 27 p.

Rice, D. D., 1976, Revision of Cretaceous nomenclature of the northern Great 

Plains in Montana, North Dakota, and South Dakota: U.S. Geol. Survey 

Bull. 1422-A, p. A66-A67.

Richmond, G. M., 1964, Glacial geology of the West Yellowstone basin and 

adjacent parts of Yellowstone National Park, in The Hebgen Lake, Montana, earthquake of August 17, 1959: U.S. Geol. Survey Prof. Paper 

435-T, p. T223-T236.

 1965r Glaciation of the Rocky Mountains, in Wright, H. E., and Frey, 

D. G., eds., The Quaternary of the United States: Princeton, N.J., Princeton University Press, p. 217-230. 

-1970, Comparison of the Quaternary stratigraphy of the Alps and

Rocky Mountains: Quaternary Research, v. 1, no. 1, p. 3-28. 

Roberts, A. E., 1963, The Livingston Group of south-central Montana: U.S.

Geol, Survey Prof. Paper 475-B, p. B86-B92. 

Robinson, G. D., 1963, Geology of the Three Forks quadrangle, Montana: U.S.

Geol. Survey Prof. Paper 370, 143 p. 

Ross, C. S., and Smith, R. L., 1961, Ash-flow tuft's Their origin, geologic

relations and identification: U.S. Geol. Survey Prof. Paper 366, 81 p.
108 GEOLOGY, VARNEY AND CAMERON QUADRANGLES, MONT.

Sandberg, C. A., 1962, Stratigraphic section of type Three Forks and Jefferson Formations at Logan, Montana, in Three Forks-Belt Mountains area, 

and symposium The Devonian system of Montana and adjacent areas: 

Billings Geol. Soc., 13th Ann. Field Conf., 1962, [Guidebook], p. 47-50.

 1965, Nomenclature and correlation of lithologic subdivisions of the 

Jefferson and Three Forks Formations of southern Montana and northern 

Wyoming: U.S. Geol. Survey Bull. 1194-N, 18 p.

Sando, W. J., and Dutro, J. T., Jr., 1960, Stratigraphy and coral zonation of 

the Madison Group and Brazer Dolomite in northeastern Utah, western 

Wyoming, and southwestern Montana, in Overthrust belt of southwestern 

Wyoming and adjacent areas: Wyoming Geol. Assoc., 15th Ann. Field 

Conf., 1960, Guidebook, p. 117-126.

 1974, Type sections of the Madison Group (Mississippian) and its subdivisions in Montana: U.S. Geol. Survey Prof. Paper 842, 22 p.

Sando, W. J., Gordon, Mackenzie, Jr., and Dutro, J. T., Jr., 1975, Stratigraphy and geologic history of the Amsden Formation (Mississippian and 

Pennsylvanian) of Wyoming: U.S. Geol. Survey Prof. Paper 848-A, 83 

p., 11 pis. 22 figs.

Sando, W. J., Mamet, B. L., and Dutro, J. T., Jr., 1969, Carboniferous megafaunal and microfaunal zonation in the northern Cordillera of the United 

States: U.S. Geol. Survey Prof. Paper 613-E, 29 p.

Scholten, Robert, Keenmon, K. A., and Kupsch, W. 0., 1955, Geology of the 

Lima region, southwestern Montana and adjacent Idaho: Geol. Soc. 

America Bull., v. 66, no. 4, p. 345-404.

Scott, H. W., 1935, Some Carboniferous stratigraphy in Montana and northwestern Wyoming: Jour. Geology, v. 43, no. 8, pt. 2, p. 1011-1032.

Sheldon, R. P., 1957, Physical stratigraphy of the Phosphoria Formation in 

northwestern Wyoming: U.S. Geol. Survey Bull. 1042-E, p. E105-E185.

Sloss, L. L., 1950, Paleozoic sedimentation in Montana area: Am. Assoc. Petroleum Geologists Bull., v. 34, no. 3, p. 423-451.

Sloss, L. L., and Hamblin, R. H., 1942, Stratigraphy and insoluble residues of 

Madison Group (Mississippian) of Montana: Am. Assoc. Petroleum 

Geologists Bull., v. 26, no. 3, p. 305-335, 11 figs.

Sloss, L. L., and Laird, W. M., 1947, Devonian system in central and northwestern Montana: Am. Assoc. Petroleum Geologists Bull., v. 31, no. 8, 

p. 1404-1430.

Sloss, L. L., and Moritz, C. A., 1951, Paleozoic stratigraphy of southwestern 

Montana: Am. Assoc. Petroleum Geologists Bull., v. 35, no. 10, p. 

2135-2169.

Swanson, R. W., Cressman, E. R., Jones, R. S., and Replogle, B. K., 1953, 

Stratigraphic sections of the Phosphoria Formation in Montana; pt. 2, 

1949-50: U.S. Geol. Survey Circ. 303, 21 p.

Wanless, H. R., Belknap, R. L., and Foster, Helen, 1955, Paleozoic and 

Mesozoic rocks of Gros Ventre, Teton, Hoback, and Snake River ranges. 

Wyoming: Geol. Soc. America Mem. 63, 90 p., 13 pis.

Weed, W. H., 1893, The Laramie and overlying Livingston Formation in 

Montana: U.S. Geol. Survey Bull. 105, p. 10-41.

 1899, Description of the Fort Benton quadrangle [Montana]: U.S. 

Geol. Survey Geol. Atlas, Folio 55, 7 p., maps.

 to U.S. GOVERNMENT PRINTING OFFICE: 1980 O 311-344/18