Geology of the 

Three Forks Quadrangle 

Montana 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 370 
Geology of the 

Three Forks Quadrangle 

Montana 

By G. D. ROBINSON 

With sections on 

Petrography of Igneous Rocks 

By H. FRANK BARNETI 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 370 

A comprehensive restudy of a classic a_rea of 

Rocky Mountain geology at the head of the 

Missouri River 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director 

The U.S. Geological Survey Library has cataloged this publication as follows: 

Robinson, Gershon Du Vall, 1918-

Geology of the Three Forks quadrangle, Montana. With sections on petrography of igneous rocks, by H. Frank Barnett. 

Washington, U.S. Govt. Print. Off., 1963. 

iv, 143 p. illus., maps, diagrs., tables. 29 em. (U.S. Geological Survey. 

Professional paper 370) 

Part of illustrative matter folded in pocket. 

Bibliography: p.135-140. 

1. Geology-Montana-Three Forks quadra.ngle. 2. Petrology-Montana-Three Forks quadrangle. 3. Rocks-Analysis. I. Barnett, Henry 

Frank, 1921- . II. Title: The Three Forks quadrangle, Montana. 

(Series) 

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

Washington 25, D.C. 
CONTENTS 

Abstract----------~--------------------------------

Introduction ______________________________ ~--------

Previous work ___ ______________________________ _ 

Acknowledgments ______________________________ _ 

GeographY-----------------~----------------------- Terrain _______________________________________ _ 

Flora and fauna _______________________________ _ 

Climate ______________________________________ _ 

History and settlements ________________________ _ 

Travel ________________________________________ _ 

Precambrian metamorphic rocks __ =- _______ _ __________ _ 

Gneiss and amphibolite _______________________ __ _ 

Hematitic phyllonite ___________________________ _ 

Origin ________ ________________________________ _ 

Age ___ _______________________________________ _ 

Regional relations ______________________________ _ 

Sedimentary rocks ______________________ __ _________ _ 

Precambrian-Belt series _______________________ _ 

North Boulder formation __________________ _ _ 

Paleozoic ____________ ______________ ___ ________ _ 

Cambrian ___________ ___________ ___________ _ 

Names and correlations of Cambrian formations ____ _____ ___________________ _ 

Flathead sandstone ____________________ _ 

Wolsey shale _____ _____________________ _ 

Meagher limestone _________ ____ ________ _ 

Park shale ___________________ ________ _ _ 

Pilgrim limestone ____________________ __ _ 

Devonian and Lower Mississippian ___________ _ 

Maywood formation _________________ __ _ 

Jefferson dolomite ___________________ __ _ 

Three Forks shale ______________________ _ 

Carboniferous __________ __ _________________ _ 

Names and general correlation of Carboniferous formations ___________ __________ _ 

Madison group ______________ __________ _ 

Lodgepole limestone _______________ _ _ 

Mission Canyon limestone __________ _ 

Big Snowy formation _______ ___ ________ _ _ 

Amsden formation _____________________ _ 

Quadrant formation ___ _______ __________ _ 

Permian _____ _______ ___ ___________________ _ 

Phosphoria formation _______ __________ _ _ 

Mesozoic ________________ ___________ __________ _ 

Jurassic ____________ ______________________ _ 

Ellis formation ______________ ________ __ _ 

Morrison formation __________ __________ _ 

Cretaceous __ . ___________ __ _______________ -.-_ 

Kootenai formation ____________________ _ 

Cenozoic ______________________________________ _ 

Tertiary-Bozeman group ______________ ____ _ 

Names and correlation of units __________ _ 

Sphinx conglomerate _______ ____________ _ 

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Sedimentary rocks-Continued 

Cenozoic-Continued 

Tertiary-Bozeman group-Continued 

Milligan Creek formation _______ -_- ___ ---

Climbing Arrow formation ________ - _____ -

Dunbar Creek formation ________________ _ 

Middle and upper Tertiary graveL _______ - Quaternary _______________________________ _ 

Old alluvium __________________________ _ 

Rounded gravel _____________ - __ - - - -

Subrounded graveL __________ - _____ -

Origin of old alluvium ______________ _ Fan gravel ____________________________ _ 

Silt __________________________________ _ 

Young alluvium _______ ------ __ -_-------

Age and stratigraphic relations of Quaternary deposits __________ -- ____ _ --_----- Igneous rocks _____________________________________ _ 

Upper Cretaceous and lower Tertiary _____ --_-----

Extrusive rocks-Elkhorn Mountains volcanics_ 

Petrography, by H. Frank Barnett _______ -

Andesite _________ ------------------

Porphyritic perlite _________________ -

Correlation and age ____ -_-- ___ ---- __ ----

Intrusive rocks ________ -_------_-----.:------

Quartz monzonite and related ~ocks ___ ~ __ - 10Npluton _______________________ _ 

Thick sill west of Spring Creek ______ -

Petrography, by H. Frank Barnett_ __ _ Age ______________________________ _ 

Latite ____ - -------------------- -------- Pluglike bodies ______________ - _____ -

Thick sills ___________ - ___ -- - - -- - ---

Thin sills __________ - - - _ - - - - - - - - - - - -

Petrography, by H. Frank Barnett_ __ _ Age ______________________________ _ 

Dacile ________________________________ _ 

Buttleman laccolith _________ .:. _______ _ 

Petrography, by H . Frank Barnett ___ _ Age ______________________________ _ 

Andesite _______ ---------- ---- ----------

Sills south of Milligan Canyon _______ _ 

Intrusive rocks north of Milligan 

Canyon _________________________ _ 

Petrography, by H. Frank Barnett ___ _ 

Age ______ ------------------'-------

Summary of age relations _________ - _____ _ 

Igneous rocks intrusi v~ into Tertiary rocks_- ______ -

Olivine basalt_ __________ -------------------

Petrography, by H . Frank Barnett _______ _ 

Age __________________________________ _ 

Some petrologic relations ____ -- --- -- ----- --------

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IV CONTENTS 

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Structure__________________________________________ 102 Geomorphology ____________________________________ _ Page 

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Summary of structural history____________________ 102 

Structure of Precambrian crystalline rocks_________ 102 

Structural significance of Precambrian sedimentary rocks _____________ : __________________________ 103 

Laramide structural features_____________________ 103 

Early folding_______________________________ 103 

Steep faults older than thrusting______________ 105 

Thrust faults-Sixteenmile thrust zone________ 105 

Lombard thrust________________________ 106 

Highway thrust________________________ 106 

Jefferson Canyon thrust_________________ 107 

Minor thrusts_______ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 107 

Thrust displacement_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 107 

Time relations of folding and thrusting____ 108 

Steep faults in upper plate of Highway thrust__ 108 

Other steep faults closely related to thrusting__ 109 

Steep faults younger than thrusting___________ 110 

Origin of Cenozoic basin___________________ _ _ _ _ _ _ 111 

Post-Laramide structural features_________________ 112 

Hills _________________________________________ _ 

Benches---------------------------------------

Strea~---------------------------------------

Origin of benched terrain _______________________ _ 

Geologic history_.;, ____ · ________________ - ____________ _ 

Economic geology __________________________________ _ 

Constrliction rna terials _________________________ _ Glass sand ____________________________________ _ 

Mica _________________________________________ _ 

Phosphate rock ________________________________ _ 

Iron and mang!}.nese ____________________________ _ Copper _______________________________________ _ 

Outlook for future discoveries of ore deposits ______ _ 

Petroleum possibilities __________________________ _ 

Measured sections in pre-Tertiary rocks ___________ ~ __ _ Section A _____________________________________ _ 

Section B _____________________________________ _ 

Section C _____________________________________ _ 

Section D _____________________________________ _ 

References ________________________________________ _ 

Index ____________________________________________ _ 

ILLUSTRATIONS 

[Plates are In pocket] 

PLATE 1. Geologic map and sections of the Three Forks quadrangle, Montana. 

2. Data map, Three Forks quadrangle, Montana. 

3. Structural diagram, Three Forks quadrangle, Montana. Page 

F:iGURE 1. Index maps, Three Forks quadrangle, Montana _____________________________________ -------- ___ - __ ------ 4 

2. Key map for identifying areas of pre-Tertiary sedimentary rocks------------------------------------------ 11 

3. Measured sections and fauna in upper Three Forks shale------------------------------------------------ 36 

4. Limestone breccia in upper Mission Canyon limestone ___________________ ------------------------------- 42 

5. Comparison of correlations of the Big Snowy west of Eustis station, Gallatin County, Mont_-_---_----------- 46 

6. Sphinx conglomerate of Eocene age, above Milligan Canyon--------------------------------------------- 62 

7. Mechanical analysis of typical tuffaceous limestone, Milligan Creek formation------------------:------------ 65 

8. Climbing Arrow formation in type area ______ .:__________________________________________________________ 69 

9. Popcorn surface on weathered bentonitic siltstone of Climbing Arrow formation _____________ --------------- 70 

10. Mechanical analyses of some clay-rich rocks of the Climbing Arrow formation______________________________ 72 

11. Mechanical analyses of some tuffaceous siltstones, Climbing Arrow and Dunbar Creek formations_____________ 74 

12. Lime-armored Qua'ternary graveL __________________________________________________________ ---------- . 83 

13. Silica-variation diagram for chemically analyzed igneous rocks, Three Forks quadrangle, Montana____________ 90 

14. lOS thrust, exposed on Highway lOS, 3}~ miles southwest of Three Forks Junction----------~--------------- 107 

15. Milligan Creek entrenched in its flood plain above Milligan Canyon______________________________________ 116 

16. Jefferson River superposed on limestone of the Madison group, 414 miles above Willow Creek ___ -_---_-_------- 117 

TABLES 

Page 

TABLE 1. Generalized stratigraphic succession, Three Forks quadrangle, Montana ____________________________ ------ 10 

2. Check list .of brachiopods and cephalopods in well located collections from Three Forks shale, Three Forks quad- rangle, ·Montana __________________________ '" ____________________________________________________ In pocket 

3. Chemical analysis of'typical tuffaceous limestone, Milligan Creek formation________________________________ 66 

4. Check list of gastropods in Tertiary rocks of Three Forks quadrangle, Montana ___________ ~ ___ -_----------- 68 

5. X-ray mineralogy of clay and silt fractions of some rocks of the Climbing Arrow formation__________________ 71 

6. Chemical data on representative clay-rich and glass-rich rocks, Climbing Arrow formation__________________ 73 

7. Chemical analyses and norms of representative tuffaceous siltstones, Dunbar Creek formation________________ 78 

8. X-ray mineralogy of clay and silt fractions of represenj;ative tuffaceous siltstones, Dunbar Creek formation_--- 79 

9. Chemical analyses and norms of some igneous rocks, Three Forks quadrangle, Montana____________________ 91 
GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

:By G. D. RoBINsoN 

ABSTRACT 

The Three Forks quadrangle at the head of the Missouri 

River, has a long stratigraphic succession. The oldest rocks 

are Precambrian gneiss, derived from arkosic sandstone, and 

amphibolite, probably altered from mafic sills and lava flows. 

The unmetamorphosed sedimentary rocks range in age from 

late Precambrian to Recent, have an aggregate thickness of 

about 11,000 feet, and underlie about 85 percent of the quadrangle. The only systems not represented are Ordovician, 

Silurian, and Triassic. 

The oldest unmetamorphosed sedimy'1tary rocks are at least 

4,000 feet of marine micaceous arkose and conglomerate, the 

North Boulder formation of the Belt series of late Precambrian age. The formation is a wedge arkose controlled by a 

buried east-west fault of pre-Belt or Belt age here named the 

Willow Creek fault. 1 

The rocks of the North Boulder formation are disconformably succeeded by about 1,500 feet of marine strata of Middle and Late Cambrian age separated into five conformable 

formations: the Flathead sandstone, 0-100 feet thick (oldest) ; the Wolsey shale, 200--400 feet thick; the Meagher 

limestone, 300-500 feet thick ; the Park shale, 100-200 feet 

thick ; and the Pilgrim limestone, 300--450 feet thick (youngest). 

No rocks of latest Cambrian, Ordovician, Silurian, and earlier Devonian age are recognized. The Upper Cambrian Dry 

Creek shale and Pebbly limestone of Peale, that overlie the 

Pilgrim nearby, are not present here. Rather, the Pilgrim 

is disconformably overlain by the marine Maywood formation, 

generally less than 50 feet thick, but as much as 200 feet 

thick, of Late (?) Devonian age. 

Succeeding the Maywood conformably is the marine Jefferson dolomite, 350-650 feet thick, of Late Devonian age. 

Dolomite is a major component of the Cambrian and Devonian carbonate rocks. The dolomite in mottled carbonate 

rocks is plainly of replacement origin. The mottling itself, 

widespread in nonmagnesian limestone, is primarily a structural and not a chemical phenomenon. Those dolomite rocks 

which are extensive, homogeneous, and have sharp contacts 

with neighboring limestone seem to have been deposited directly. 

Conformably above the Jefferson is the marine Three Forks 

shale, 100-400 feet thick, of Late Devonian and Mississippian 

age. Orange calcareous siltstone and sandstone at the top of 

the Three Forks were separated as the Sappington formation 

of Mississippian age by Berry. The Early Mississippian age 

of part of these strata is confirmed. The orange siltstone is 

gradational with the underlying green shale and is in sharp 

contact with the overlying Lodgepole limestone. They are 

retained in the Three Forks, rather than transferred to the 

Lodgepole, on grounds of priority, lithologic community, and 

cartographic practicality. 

Succeeding the Three Forks are about 2,400 feet of marine 

Mississippian and . Pennsylvanian rocks classed together as 

Carboniferous because the Mississippian-Pennsylvanian boundary has not been located even approximately. From bottom 

to top, the formations include the Lodgepole limestone, 350-

650 feet thick, and the Mission Canyon limestone, 600-1,400 

feet thick, both of the Madison group of early Mississippian 

age; the Big Snowy formation, 0--400 feet thick, and the 

Amsden formation, 250-600 feet thick, mapped together and of 

late Mississippian and early Pennsylvanian age; and the 

Quadrant formation, 200-400 feet thick of presumed Pennsylvanian age. Field evidence suggests that the Quadrant may 

be partly of early Permian age. A slight break in sedimentation preceded the Lodgepole, and more significant erosional 

intervals preceded the Big Snowy and the Amsden. In the 

pre-Big Snowy interval an extensive zone of solution breccia 

and locally of karst topography was developed in the upper 

part of the Mission Canyon limestone. 

The Permian is definitely represented only ·by the marine 

Phosphoria formation, 75-200 feet thick, that conformably 

ann probably gradationally succeeds the Quadrant. 

The Mesozoic record is fragmentary. Triassic time is represented by an unconformity that is covered by the Ellis formation, generally less than 150 feet thick but locally more 

than 350 feet thick, of Jurassic age. Two other Meso'l.oic 

formations have been recognized. The oldest is the landlaid 

Morrison formation 100 to 250 feet thick, of Late Jurassic 

age, which conformably(?) succeeds the Ellis and is gradationally overlain by the Kootenai formation, as much as 650 

feet thick, of Early Cretaceous age. The Kootenai is succeeded, with probable angular unconformity, by the Elkhorn 

Mountains volcanics, a thick sequence of andesitic flows and 

volcanic breccia of Late Cretaceous age. Great thicknesses 

of marine and continental sedimentary rocks of Late Cretaeeons and Paleocene age occupy large areas near the quadrangle but do not crop out within it. 

The Tertiary is represented by thick basin deposits of the 

Bozeman group (herein named, formerly Bozeman lake beds) 

more than 2,000 feet thick and divided into four formations: 

Sphi,nx conglomerate, 0-100 feet thick of Eocene age; the Milligan Creek formation (new name) , 0-300 feet thick of Eocene age; Climbing Arrow formation (new name), 750-1,000 

feet thick, of late Eocene and early Oligocene age; and the 

Dunbar Creek formation (new name), 250-800 feet thick, of 

Oligocene age. The Sphinx conglomerate is an alluvial apron: 

The Milligan Creek formation is largely lake limestone. Tne 

Climbing Arrow formation is mainly streamlaid bentonitic 

clay and sand. The Milligan Creek and Climbing Arrow contain much volcanic ash, and the Dunbar Creek is made up 

largely of such debris, blown or washed into the Three Forks 

basin. Included in the Bozeman group as defined are middle 

l 
2 GE·OLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

and upper Tertiary gravels, widespread nearby but present 

here only in small unmappable patches. 

A wide variety of unconsolidated Quaternary deposits completes the stratigraphic succession. Broad benches in the 

southern and central parts of the quadrangle are capped by 

rounded gravel deposited in early to late Pleistocene time by 

the ancestral Jefferson and Madison Rivers. Valleys· tributary to the ancient master streams, in the northern part, are 

filled with subrounded gravel. Fan gravel, of late Pleistocene and Recent age, mantles many benchlands north of the 

Jefferson River, and floors many short tributary valleys. Late 

Pleistocene windblown silt thinly coats large upland areas. 

The present broad flood-plain and channel deposits are largely 

of very late Pleistocene age. 

Masses of calcalkaline intrusive rocks occupy large _ areas in 

the quadrangle. All but two are of very Late Cretaceous to 

very early Tertiary (pre-Bozeman) age. They are mainly 

monzonitic, dacitic, latitic, and andesitic intrusive rocks in 

the form of sills, laccoliths, and one semiconcordant pluton 

emplaced during or after regional folding. Some were probably being intruded while the Elkhorn Mountains volcanics 

were being extruded. The exceptions are two small bodies 

of olivine basalt, intrusive into limestone conglomerate at the 

base of the Bozeman group and probably of early Tertiary 

age. 

The structural history is complex. The earliest recorded 

structural event was the pre-Belt folding and metamorphism 

of the lower Precambrian crystalline rocks. Much later, but 

still in Precambrian time, the southern limit of the embayment that received sediments that formed the Belt series became established apparently through faulting. This buried 

structural feature, the Willow Creek fault, exerted a powerful 

influence on much subsequent deposition and deformation. The 

quadrangle was structurally quiet, though far from dormant. 

from late Precambrian to Late Cretaceous time. 

Most of the folding and faulting were in response to compression along north-south axes in a comparatively short 

time: very late in the Cretaceous to middle or late Eocene. 

During this time, referred to as the Laramide interval, largescale folding occurred first ; major thrusting followed, perhaps after a period of quiescence. 

A broad belt of thrusting, called the Sixteenmile thrust 

zone, developed, but only the uppermost thrust is exposed. 

This thrust consists of the well-known Lombard thrust that 

trends north-south across the northeast part of the quadrangle, and the equally well known Jefferson Canyon thrust that 

trends east-west across the west-central part of the quadrangle, linked together by a thrust of intermediate trend, 

first described in this report and called the Highway thrust, 

in the center of the quadrangle. The Lombard-HighwayJefferson Canyon thrust dips northwestward at angles of 

40°-60°. Rocks as old as the Belt series are thrust eastward over rocks as young as the Elkhorn Mountains volcanics and even younger latite intrusive rocks; not less than 

10,000 feet of stratigraphic throw is involved. Their total 

displacement is probably greater than the stratigraphic throw 

because the fault dips in the same direction and at about the 

same angle as the beds. A minimum displacement of 3 miles 

seems certain; maximum displacement may be much greater. 

Principal relative movement on the thrust system seems to 

have been from west to east so that the Jefferson Canyon 

thrust segment has probably experienced mostly strike-slip 

displacement. A cluster of tilted fault blocks above the thrust 

surface, and several high-angle faults below, are probably 

contemporaneous with the thrust advance. 

Only two steep faults are known or inferred to be distinctly 

younger than the thrusting but older than the Tertiary basin 

deposits. Possibly, however, many developed and controlled 

the general dimensions of the Tertiary basin. . If so, they are 

masked by continental deposits shed from the fault scarps. 

All but two of the many intrusive masses were emplaced 

during or shortly after folding. Time relations between intrusions and thrusting are unknown ; a reasonable guess is 

that the period of intrusion overlapped the period of thrusting. 

The Tertiary basin is structurally controlled at least to the 

extent that it lies on a zone of repeated faulting and its borders cut sharply across deformed older rocks but there is no 

deciding to what extent it is due directly to faulting or to 

what extent to erosion of unresistant rocks exposed by diastrophism. There is no evidence to support the theses of 

Atwood that the region was peneplaned between Laramide 

deformation and creation of the basin, and that the basin 

formerly drained southwestward to the Snake River. 

Deformation during the Tertiary seems to have been mostly 

in the form of gentle eastward tilting, recurring or continuing from late Eocene until the close of the Tertiary, and 

punctuated by shallow local folding. · Quaternary deformation has apparently been limited to slight uplift with northward or northwestward tilt. 

At the end of Tertiary time the quadrangle seems to have 

been an area of low relief with scattered hills protruding no 

more than a few hundred feet above a graveled plain. The 

development of the present physiography is the history of the 

dissection of that surface and of the exhumation of a very 

early Tertiary surface of mature relief that had been buried 

under the Tertiary basin deposits. The main agent of dissection has been a stream -system -of large perennial rivers, which 

derive their discharge from a persistently humid region far 

beyond the Three Forks basin, and their tributaries, which 

vary from perennial to nearly dry as the basin climate has 

varied. The former flood plains of the through streams remain as terrace remnants capped by rounded gravel, and the 

courses of their tributaries are marked by subrounded gravel 

fills. Broad benchlands in the northern part of the area that 

are virtually bare-rock surfaces are identified as pediments. 

Apparently they have long been isolated from integrated 

streams and have been slowly lowered by mainly colluvial 

processes. 

The dissection has been episodic, presumably with long periods of lateral cutting and filling by the streams alternating 

with short ones of vigorous downcutting. The prime driving 

force in dissection seems to have been uplift of the mountain 

ranges to the south, with climatic change playing a secondary role. Although uplift must have caused the dissection, 

discontinuities in uplift were not necessarily responsible for 

the discontinuities in dissection revealed by the benched topography. The main proximate cause for the development of 

some or all of the benches may have been d~ffering resistance 

of strata at the outlet canyon of the Missouri River; major 

Quaternary climatic changes doubtless contributed, but to 

what degree is unknown. Climatic change in the past century probably is responsible for deep trenching of the flood 

plains of tributary streams. 

The Three Forks quadrangle has yielded no mineral products other than small quantities of sand and gravel and of 

glass sand. Its petroleum possibilities have not been tested 

but they do not seem promising. 
GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 3 

INTRODUCTION 

The region around the headwaters of the Missouri 

River rich in agricultural and mineral resources, in 

handsome scenery and complex geology, has long attracted geologists. More than 70 years ago, the classic 

Paleozoic stratigraphy of the northern Rocky Mountains was established in this vicinity by A. C. Peale, 

and many later geologists contributed to the regional 

literature. Few large-scale studies, however, have 

been published. The present report portrays the general geology of the Three Forks 15-minute quadrangle, which is at a scale of 1 inch to 1 mile and 

covers 205 square miles. The quadrangle takes its 

name from the town of Three Forks, which in turn 

is nam~d for the three rivers-Jefferson, Madison, and 

Gallatin-that join nearby to make the Missouri. The 

quadrangle is bounded by latitudes 45°45' and 

46"0' N. and by longitudes 111 °30' and 111 °45' W . 

(See fig. 1.) The quadrangle was mapped during the 

summers of 1952, 1953, and 1954. In 1952, I was assisted by Thomas P. Lovett, and in 1953 and 1954, 

by JohnS. Bader, Jr. 

Geology was plotted on 1 : 24,000 preliminary topographic maps, prepared by multiplex methods, and 

transferred photographically to publication scale (pl. 

1) . Most geologic contacts were traced on foot. Color 

terms used in measured sections follow the "RockColor Chart of the National Research Council" 

(Godc!trd and others, 1948), but this usage is not 

consistently followed in the rest of the text. 

R.eferences to the neighboring Toston quadrangle 

(see fig. 1B) are based on my work there during the 

su~mers of 1955-1957. (See Robinson, 1959, for interim report.) 

An effort is made to view the Three Forks quadrangle in its regional setting, as part of the Rocky 

Mountains in western Montana. Little is known 

about the northern half of this geologic entity, so 

that the region considered is limited to southwestern 

Montana; more specifically, :Montana south of the latitude of Helena ( 46°40') and west of the longitude of 

the Bridger Range (110°50'). (See fig. 1A.) 

PREVIOUS WORK 

The Three Forks region was one of the first areas 

studied in the Northwest, perhaps because of Captain 

Meriwether Lewis' enthusiastic accounts. F. V. 

Hayden, as a member of the Raynolds Expedition, 

made the first geologic observations here in 1860 

(Hayden, 1869, p. 90). Geologists of the Hayden 

Survey began working in the region as early as 1868 

and a brief account of their findings was published 

by Hayden ( 1873, p. 65-85) 5 years later. Paleozoic 

and Cretaceous fossils collected by Hayden and his 

colleagues were discussed by Meek (1873). In 1883, 

members of the northern Transcontinental Survey 

hastily examined the north half of the quadrangle 

(Pumpelly and others, 1886). From 1883 to 1889 

Peale mapped the 1 o quadrangle ( 45°-46° N. and 111°-

1120 W.) called the Three Forks sheet, of which the 

present Three Forks quadrangle is a part (see fig. 

lB) ; all unqualified references in this paper to the 

Three Forks quadrangle are to the 15-minute sheet. 

Peale published a report on the Paleozoic stratigraphy (1893), and another on the general geology 

( 1896) in folio form at a scale of 1 : 250,000. While 

Peale was making his areal map for the Geological 

Survey, Earl Douglass (1899 and later publications) 

was independently studying the Tertiary deposits and 

their vertebrate fossils. Matthew (1903) also collected vertebrate fossils from the Tertiary rocks. The 

Upper Devonian and Lower Mississippian faunas in 

the hills north of Three Forks were investigated by 

P. E. Raymond (1907, 1909, 1912) and somewhat later 

by W. P. Haynes (1916a). As a byproduct of his 

paleontologic research, Haynes ( 1916b) discovered the 

Lombard thrust. 

The Permian phosphatic rocks in the southern part 

of the quadrangle were traced by D. D. Condit (1918) 

but not specifically discussed. Frenzel and Mundorf 

(1942) examined the phosphatic rocks in the northern 

part of the quadrangle and found diagnostic Permian 

microfossils in them. The Pre.cambrian crystalline 

rocks at the south end of the quadrangle were visited 

by Tansley and Schafer (Tansley, Schafer, and Hart, 

1933, p. 8-9, pl. 1). Wood ( 1933) described a Miocene rhinoceros found a few miles south of Three 

Forks. Scott ( 1935) briefly discussed some of the 

Carboniferous rocks of the quadrangle. 

By the late 1930's it was evident that Peale's work, 

though reconnaissance of a high order, was too generalized for modern use: and several detailed areal 

mapping and stratigraphic projects were undertaken 

in the region. Only one of these, however, by G. W. 

Berry (1943), produced new mapping in the Three 

Forks quadrangle. Berry's map, covering most of 

the Jefferson Island and Three Forks 15-minute quadrangles (see fig. 10), is a distinct advance over 

Peale's. Unfortunately, however, Berry had no topographic base map and had to publish his work at 

1 :125,000 in black-and-white on a poor planimetric 

base. His text is brief and barely touches most aspects of the geology. Its main contribution lies in 

the application of modern names and definitions to 

many formations. 
4 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

,--..,.. _______ ....___ ____ _ I 

I 

I 

I 

\ 

\ 

c. ,, \ 

\ 

•Kalispell 

-------------------------~ 

' I 

I 

I 

\ 

I 

I 

\ 

I 

I 

\ 

--------- ______ j 

B 

FIGURE 1.-lndex maps of the Three Forks quadrangle, Montana. A. Outline of Montana, showing southwestern Montana (ruled) 

and Three Forks quadrangle (black). B. Southwestern Montana, showing Cenozoic basins, ranges, rivers, and topographic quadrangles mentioned in this report; basins are &haded and the Three Forks basin is outlined heavily. Key to topographic quadrangles: P, Philipsburg; W, Willis I; CF, Canyon Ferry; Td, Townsend; R, Radersburg; T, Toston; Mw, Maudlow; J I, Jefferson Island; TF Three Forks; 

M, Manhattan; H, Harrison; N, Norris; A, Anceney; E, Ennis; SM, Sphinx Mountain. 
GE'OGRAPHY 5 

Since Peale's ·day there has been difficulty in dating 

precisely the rocks near the Devonian-Mississippian 

boundary. Berry's attempted solution to this problem 

-establishment of the Sappington formation of supposed Mississippian age-led to field examinations by 

Sloss and Laird (1946, 1947) and by Holland (1952). 

Partial stratigraphic sections in the quadrangf'D 

have been published by Gardner and others (1946f, 

Imlay and others (1948), and Moritz (1951). 

ACKNOWLEDGMENTS 

M. R. Klepper and E. T. Ruppel of the Geological 

Survey introduced me to western Montana stratigraphy. I am indebted for other sti,mulating discussions in the field to C. S. Denny, V. L. Freeman, 

James Gilluly, and 0. M. Hackett, all of the Geological Survey; to vV. E. B. Benson, then of the 

Geological Survey; to former professors 0. D. Blake, 

W. J. _McMannis, and Forbes Robertson of the Montana School of Mines; to Professor C. C. Bradley of 

Montana State College; to C. H. Falkenbach of the 

American Museum of Natural History; and to Peter 

Verrall, then of Princeton University. H. F. Barnett 

studied the igneous rocks in thin section, and wrote 

the paragraphs on igneous petrography. Among 

many other Geological Survey colleagues who aided 

in office and laboratory discussions, I feel obliged to 

mention especially Clyde P. Ross and C. B. Hunt. 

The fieldworl\: was made agreeable by the cooperation 

of many ranchers and other residents of the area, 

whose aid is gratefully remembered. The specific 

help of many others is acknowledged at appropriate 

places in the text. 

GEOGRAPHY 

The Three Forks quadrangle is in the southeastern 

part of the Northern Rocky Mountains province of 

Fenneman. Consisting of deeply dissected mountain 

uplands separated by broad benchlands, it is a typical, 

if somewhat subdued, fragment of the province. The 

brief discussion of geogi'aphy which follows is designed as background for the geologic description. 

Further consideration of the lands<;.ape is in a later 

discussion of geomorphology. 

TERRAIN 

The Three Forks quadrangle is in the northwest 

part of an irregularly sha peel intermontane basin of 

about 1,000 square miles at the head of the Missouri 

River. Unlike most of the intermontane basins in 

Montana, which trend north-south, the Three Forks 

basin, as it is commonly known, is elongated east-west. 

(See fig. 1B.) It is bounded on the southwest by 

the Tobacco Root Mountains (also called the J efferson Range), on the northwest by the Elkhorn Mountains, on the north by the Horseshoe Hills, on the 

east by the Bridger Range, and on the south by the 

Madison and Gallatin Ranges. 

The landscape of the quadrangle is mainly a series 

of broad benchlands, from which rise only a few 

rugged ridges and hills. The total relief is about 

1,800 feet, from 4,035 feet at the head of the Missouri River in the northeast corner to 5,800 feet at 

the west-central edge on the ridge south of Dry Hollow. The lowest benchlands are the flood plains of 

the Jefferson and Madison Rivers and their tributaries; the broad flood plain of the Jefferson is interrupted by a short canyon reach near its entrance to 

the quadrangle. A series of benches rising steplike 

above the flood plains make up the greater part of 

the area. South of the Jefferson River the benches 

tend to be broad and smooth topped, and rise 1,000 

feet in 5 steps to form the highest surfaces. Those 

north of the river are small, irregular, and overshadowed by hills. 

The few rugged hills are rarely more than 1,000 

feet high, with narrow but flattish summits. In most 

places, the hills are arranged in long parallel ridges 

separated by narrow valleys or saddles. A few low 

hilly areas, however, are rolling, without well-defined trends. 

With -minor exceptions, there is close correspondence between topographic expression and lithology. 

The long ridges are held up by Paleozoic and Mesozoic carbonate rocks and quartzitic sandstone, the 

intervening narrow valleys by Paleozoic and Mesozoic 

clay-rich rocks, the rolling hills by large masses of 

igneous rocks, and the benchlands by Cenozoic deposits. 

FLORA AND FAUNA 

The Three Forks area is dry country; the vegetation is meager and the characteristic landscape color 

is dull grayish yellow. Green patches of brush and 

woods are scattered about the flood plains of the Jefferson and :Madison Rivers as well as along many 

minor stream courses at lower altitudes. The more 

common trees are willow, aspen, cottonwood, and 

poplar; conifers are rare. Some patches of low grassy 

swamp on the flood plains, especially near the Missouri, mark areas of recurrent flooding and recently 

abandoned stream channels. Most of the flood plains 

of the Jefferson and Madison Rivers and Willow 

Creek are wet farmed, mainly for wheat and alfalfa. 

Most of the benches are dry farmed, largely for 

wheat. Uncultivated benches, principally those un-
6 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

derlain by gravel, are thinly strewn with sagebrush, 

cactus, bunch grass, tumbleweed, and an · occasional 

pine. On steeper slopes, such as bench fronts, valley 

sides, and hill sides, are many bare rock outcrops 

broken by sagebrush and sparse grasses. 

The area is in a game preserve and for this reason 

wild animals are more abundant than in similar areas 

nearby. Deer families and small herds of antelope 

are common as are rabbits, chipmunks, and ground 

squirrels. A few porcupines, badgers, skunks, and 

rattlesnakes live near streams. Birds are plentiful 

aud varied. 

CLIMATE 

The climate is north temperate semiarid, characterized by long windy intensely cold snowy winters, 

short wet warm springs, hot dry summers, and cool 

autumns with long spells of "Indian summer." From 

late June through August midday temperatures are 

high, often above 90°F and occasiona1ly above 100°F, 

but nights are cool, and freezing temperatures have 

been recorded in every month. Maximum daytime 

temperatures in the winter months are commonly near 

freezing; minimum winter temperatures are often below -l5°F, though rarely below -30°F. Mean annual temperature is near 44.5°F. The highest temperature recorded is 105°F, the lowest -49°F. 

Average annual precipitation is between 10 and 15 

inches, of which a large fraction falls as snow during 

the winter and early spring. Maximum rainfall is in 

late May and June .. The remainder of the summer 

and the autumn are likely to be dry, with light rains 

or snowfall in September and October. Heavy snowfalls are not the rule until November or December. 

Geologic fieldwork is often feasible from late April 

through November but is generally comfortable only 

from early June through mid-October. 

These climatic notes are based on the U.S. Weather 

Bureau's "Climatological Summary" (to 1930) and 

"Annual Climatological Data" (after 1930). The 

Weather Bureau maintained a station at Three Forks 

from 1910 to 1930, and has operated one at Trident 

on the Missouri, 6 miles northeast of Three Forks, 

continuously since 1938. 

HISTORY AND SETTLEMENTS 

The party of Lewis and Clark were the first white 

men to enter the Three Forks quadrangle. They 

reached and named the Three Forks of the Missouri 

in July 1805. Several hunters, traders, and adventurers are known to have crossed the area in later 

decades, but the first settlement was not made until 

1862 (by another account 1864) when Gallatin City 

was founded just east of the junction of the Madison 

and Jefferson Rivers. The settlers apparently thought 

that they were at the head of Missouri navigation 

and that a mercantile center might be developed 

there to serve nearby mining camps as well as the 

agricultural communities which might be expected 

to devel~p in the fertile surrounding valleys. When 

the putative merchants learned about the Great Falls 

far downstream, Gallatin City was largely abandoned. 

In 1882 a small settlement was started a mile southwest of Gallatin City, between the Madison and the 

Jefferson, and called Three Forks. In 1908 the site 

of Three Forks became a railroad division. point and 

the town began to grow. It soon became clear that 

the townsite was too low and too close to the river 

junction for safety and about 1912 the town was 

moved 11j2 miles southwest to its present site. A community was planned for several thousand people, with 

carefully laid out streets and concrete sidewalks. In 

the 1930's and early 1940's the town declined, and 

many of its houses were moved away. On the present 

outskirts it is surprising to see neat sidewalks framing vacant pasture. The present population is about 

900, largely railroad employees, and ranchers who 

like living in town and commuting to their ranches. 

The only other settlement is Willow Creek, 7 miles 

to the southwest, a farming community with less than 

100 inhabitants. 

Interesting, occasionally conflicting, discussions of 

local history are in Vestal ( 1945), Campbell and 

others (1915), and Federal Writers Project (1939). 

These sources were used :for history before 1900. History after 1900 is based on these works modified by 

conversations with elder inhabitants. 

TRAVEL 

The road system is excellent. Three Forks is connected with Bozeman and the east, with Butte and 

the west, and with Helena and the north by first-class 

heavy-duty U.S. Highway 10. From a junction 21j2 

miles west of Three Forks and northwest of the Jefferson River the highway branches north and west. 

East of the junction the road is designated simply 

U.S. Highway 10. The branch leading north through 

Helena is called lON; the branch leading west through 

Butte is called lOS. At the west edge o:f the quadrangle, paved State Highway 1, which goes south to 

Ennis and beyond, joins lOS. Good graveled county 

roads make access easy to every part of the quadrangle. 

Both the Northern Pacific and the Chicago, Milwaukee, and St. Paul railroads :follow the Jefferson 

Valley to Three Forks and there turn southeast. Only 
PRECAMBRIAN METAMORPHIC ROCKS 7 

the Northern Pacific, however, normally stops at 

Three Forks. Small planes may land on an airstrip 

southwest of town. The nearest commercial airfield 

is outside Belgrade on Highway 10 about 20 miles to 

the southeast. There is no commercial traffic on the 

,Jefferson or l\fadison Rivers~ and even sportsmen 

rarely try to navigate the swift and dangerous currents. 

PRECAMBRIAN METAMORPHIC ROCKS 

Metamorphic rocks of Precambrian age include 

gneiss, derived from sedimentary rocks, amphibolite, 

probably altered from mafic sills and lava, and hematitic phyllonite. These rocks have no known unmetamorphosed equivalents. 

GNEISS AND AMPHIBOLITE 

Gneiss and amphibolite underlie about 3 square 

miles of the southwest corner of the quadrangle. 

They tend to be deeply weathered, so that outcrops 

are few, small, and very likely not representative. 

The best exposures are near contacts with the overlying resistant Flathead sandstone and in Willow 

Creek canyon (sees. 7 and 18, T. 1 S., R. 1 E.). 

Typically, layers many feet thic.k of light-colored 

gneiss alternate with similarly thick layers of dark 

amphibolite. Near the Flathead, gneiss is the main 

rock type; at the south edge of the quadrangle, 

amphibolit.e dominates. Banding strikes N. 70°-

900 W. and dips 50°-70° N; foliation is parallel to 

banding. The overlying Flathead sandstone has 

similar strike but its dip is much lower, 10°-40° N., 

indicating great angular unconformity. 

The dominant rock type is light-colored mediumgrained biotite · gneiss. Mineral proportions range 

widely ; the specimens studied contain 40-60 percent 

quartz, 25-45 percent feldspar, 5-20 percent biotite, 

and 1-2 percent of accessory minerals. Mosaic or 

sutured texture is typicaf of quartz aggregates, and 

almost every quartz gr~in shu\vs pressure shadows. 

The main feldspar is oligoclase, which is mostly untwinned and not strikingly oriented. Many of the 

rocks also contain colorless microcline, some as much 

as 20 percent. The biotite . is brown or olive and is 

scattered in small bundles and individual flakes. Common accessory minerals are apatite, clinozoisite, 

sphene, and opaque oxides. Many of the grains of 

. accessory minerals are rounded. A few biotite gneiss 

layers contain from 5 to 25 percent of faintly pink 

garnet ; except for the garnet these rocks are similar 

to the other gneisses. Microcline and garnet do not 

occur together in the small suite examined. 

The interbedded amphibolite is dark, almost black, 

medium grained, and gneissic rather than schistose, 

despite its large proportion of hornblende. It is mineralogically very simple, averaging about 60 percent 

hornblende, about 35 percent plagioclase (generally 

rather calcic labradorite), 1-2 percent each of quartz 

and of sphene, and a few percent of opaque oxides. 

Wide departures from these proportions are common. 

The hornblende is green, moderately pleochroic, and 

in rather stubby prisms. Feldspar grains are generally clear and little twinned; the edges of some, 

however, are crowded with sericite flakes and a few 

are thoroughly saussuritized. Quartz is interstitial 

and lacks pressure shadows; it developed late, perhaps long after the foliation. The texture of the 

amphibolite varies a good deal, from definitely nematoblastic to mosaiclike granoblastic. In the granablastic rocks the feldspars are less twinned, and large 

porphyroblasts of sphene are common, suggesting a 

slightly higher metamorphic grade than in the nematoblastic ones. 

Biotite-hornblende gneiss, mineralogically transitional between biotite gneiss and amphibolite, crops 

out sporadically. Its relation to the main types is not 

known owing to poor exposures. 

The stratigraphic thickness represented by the metamorphic rocks is not known. At least several hundred 

feet and probably several thousand feet of strata are 

represented. 

Small dikes of aplite and granite pegmatite, and 

veins of milky quartz . are widespread both along and 

across the banding in all the metamorphic rocks. 

Most of them are but a few inches thick and imper-

-sistent, but one large mica pegmatite body (see p. 

122) crops out just below the Flathead sandstone in 

the southeast corner of sec. 9, T. 1 S., R. 1 W. None 

of these transgressive masses is visibly metamorphosed. Many of them occur near the Flathead contact, but none of like character invade Paleozoic strata. 

HEMATITIC PHYLLONITE 

Hematitic phyllonite, markedly different from the 

coarse-grained metamorphic rocks, forms small scat- . 

tered outcrops between the coarse crystalline rocks 

and the Flathead sandstone in sees. 8 and 9, T. 1 S., 

R. 1 W. These rocks, less than 100 feet thick, were 

not mapped separately. 

The contact of the phyllonite with the Flathead is 

an angular unconformity. The phyllonite strikes 

roughly parallel to the Flathead, but dips much more 

steeply : near the contact, dips in the Flathead are 

20°-30° N., whereas banding and foliation in the 
8 GEOLOGY OF THE THREE FORKS QUADRANGLE,. MONTANA 

phyllonite dip 60° N. to vertical. Contacts between 

phyllonite and the main mass of gneissic rocks are 

not exposed, but in places the phyllonite contains a 

few thin bands of light-colored gneissic rock much like 

the underlying biotite gneiss. 

The phyllonite is a dark fine-grained rock, composed mainly of hematite, quartz, chlorite, and sericite, but there is much diversity and no two outcrops 

are identical. The individual bodies differ mainly in 

degree of foliation. 

Most of the outcrops are of medium-dark-gray hard 

massive faintly foliated spotted rock. The spots are 

subrounded to angular sand-size clusters of tiny 

grains of quartz or hematite, or of limonite or calcite 

replacing hematite. The groundweb consists of 

microcrystalline hematite, chlorite, sericite, and quartz 

in laminae, streaks, and patches. 

The hematite, variably altered to limonite, is in 

cross fractures and also forms cement around quartz 

grains and sericite aggregates, indicating late development. The sericite shows no preferred orientation, 

even in the more highly foliated types. Rarely present are tiny remnants of feldspar grains, riddled with 

sericite flakes, and small rounded grains of sphene and 

epidote-group minerals. 

A rare type is strikingly banded, and has half-inch 

wide light-olive-gray layers, rich in sericite, that alternate with medium-bluish-gray layers, rich in hematite and chlorite; within the color layers, mineral 

banding is developed down to microscopic scale. 

The texture of the banded rock is cataclastic; 

shearing is less evident in th~ massive types. 

ORIGIN 

The gneiss and amphibolite are the product of. moderately high grade regional metamorphism, as indicated by their mineralogy and their setting as part 

of the huge mass of similar rocks that forms the core 

of the Tobacco Root Mountains and the Madison and 

Gallatin Ranges. 

The gneiss has developed from arkosic sandstone 

and conglomerate, as indicated by outlines of relict 

pebbles and cobbles, and by relict graded bedding and 

crossbedding; sedimentary origin is further suggested 

by the roundness of such refractory accessories as 

sphene and by the very high quartz content of many 

layers. 

The amphibolite may represent silica-poor, iron-rich 

shale or tuff, but more probably was once mafic lava 

or sills. Hayden's comment (1872, p. 67) on similar 

rocks exposed in ~fadison Canyon nearby attests to 

the antiquity of this interpretation : "Masses of a 

very compact black hornblende gneiss lie between the 

strata as if they were old intrusions of trap." 

The phyllonite seems · to be the product of local 

cataclastic retrograde metamorphism of gneiss (thus 

the choice of name; see Williams, Tt~rner, and Gilbert, 1954, p. 199-228). Details of mineralogy and 

texture leave some questions, but the presence within 

the phyllonite of gneiss bands, apparently representing unsheared parts, seems conclusive. The phyllonite 

could scarcely have reached its present state during 

the same metamorphic cycle that produced the interleaved and bordering gneiss. The fact that the phyllonite belt is in an important tectonic zone active in 

Precambrian times (pl. 3) encourages the view that 

th~ phyllonite is the product of local shearing. The 

"spots" and the persistently unoriented sericite both 

suggest an episode of recrystallization after shearing 

and at higher temperature. 

AGE 

These metamorphic rocks cannot yet be dated 

closely. At only three places in southwestern Montana have ancient crystalline rocks been reported ·in 

depositional contact with the next younger rocks of 

the Belt series. Two of these reported occurrences, 

one south of the Rochester mining district ( Sahinen, 

1939, p. 14) and the other in the Tendoy ~fountains 

(Scholten, Keenmon, and Kupsch, 1955, p. 353), are 

reconnaissance observations, and may not hold up 

under detailed mapping. Only the third occurrence, 

6 miles southeast of vVhitehall (Lowell, 1956b), seems 

\Yell established. In many places, however, rocks of 

the Belt series are faulted against pre-Belt metamorphic rocks, or crop out within a few miles of the 

older rocks. At or near depositional contacts the 

Belt is well indurated but unmetamorphosed, or 

nearly so, whereas the crystalline rocks are high-grade 

metamorphic products. The crystalline rocks are 

therefore not metamorphosed Belt rocks bvt are older 

by one or more major metamorphic and erosional 

cycles. 

The shearing which made phyllonite from gneiss is 

younger than the regional metamorphism and older 

than the Flathead sandstone. Probably it is pre-Belt, 

but satisfactory evidence for this is lacking. 

The age of the many small quartz-rich dikes and 

veins is uncertain. Because no such bodies oc.cur in 

the nearby Paleozoic rocks, which contain many intrusive masses of a different character, a Precambrian 

age may be inferred. Because these ·rocks are not 

metamorphosed, they are presumably late Precambrian. Pebbles of vein quartz in the conglomerate 

of the Belt series show that some quartz veins are 
PRECAMBRIAN METAMORPHIC ROCKS 9 

pre-Belt. Neither aplite nor pegmatite is represented in the local conglomerate of the Belt, but this 

negative fact does not prove their post-Belt age. 

REGIONAL RELATIONS 

In 1873, Hayden (p. 67) said of.the crystalline rocks 

of the Missouri head waters region : "The work of reducing these metamorphic strata to a system, and connecting them over extended areas, has not yet been 

attempted, and it seems to me an almost hopeless as 

well as fruitless task." The task has still not been 

attempted. 

The great bulk of the metamorphic rocks in this 

region, including those described here, were mapped 

as a single unnamed unit-gneiss and schist-and assigned an Archean age by Peale ( 1896, p. 2) . In a 

small area bordering the Madison River, southeast of 

Virginia City, Peale recognized another, younger 

group of Precambrian metamorphic rocks-marble, 

mica schist, and gneiss-which he named Cherry Creek 

formation and assigned to the Algonkian. 

The metamorphic rocks of the same general area 

were later subdivided by Tansley (Tansley, Schafer, 

and Hart, 1933) into two series, the Pony (older) and 

the Cherry Creek, and these terms have been widely 

used. The Pony is described as "gneiss and schist 

of igneous and sedimentary origin;" the Cherry Creek 

is "garnetiferous gneiss, schist, quartzite, and limestone * * * whose sedimentary origin is unquestionable." The Pony is said to contain no limestone or 

quartzite, and comparatively little garnet-bearing 

rock. Much of Peale's unnamed Archean unit is included in Tansley's Pony series. Tansley's application of the name Cherry Creek differs so widely from 

Peale's (compare their maps) that Tansley's use of 

the name seems unwise. 

The metamorphic rocks of the Three Forks quadrangle are included with the Pony series by Tansley 

(1933, pl. 1, p. 6); the nearest rocks assigned to the 

Cherry Creek series are 15 miles southwest. If they 

must be assigned to one of these series, the Pony is 

preferable. But the Pony and Cherry Creek series of 

Tansley are based on rapid reconnaissance work, and 

are likely to be radically modified or abandoned when 

studied in detail. Reid (1957), for instance, stated 

that the Pony rocks near their type locality are 

younger than Cherry Creek rocks. Consequently, the 

metamorphic rocks of the Three Forks quadrangle are 

assigned no formal name. 

SEDIMENTARY ROCKS 

The sedimentary rocks of the Three Forks quadrangle range in age from Precambrian to Recent, have 

an aggregate thickness of around 11,000 feet, and underlie about 85 percent of the quadrangle. The sequence 

is summarized in table 1. The only systems not represented are Ordovician, .S_ilurian, and Triassic. 

All the Jurassic and older sedimentary rocks are 

marine. All younger deposits are mostly, if not entirely, continental. Thick sequences of Upper Cretaceous and Paleocene marine and continental sedimentary rocks occupy large areas just outside , the 

quadrangle but do not crop out within it. 

The outcrops of pre-Tertiary sedimentary rocks are 

largely confined to three widely separated areas in the 

northeastern, west-central, and southwestern sectors 

(fig. 2). A major thrust fault system (see pl. 3) cuts 

across the quadrangle from northeast to southwest, 

splits the northeast pre-Tertiary mass, and skirts the 

south edge of the west-central mass. Thus there are two 

areas of upper-plate rocks, northeast and west-central, 

and two areas of lower-plate rocks, northeast and southwest. For convenient reference, these masses are designated as follows : Milligan Creek area, west-central 

upper plate; Mud Spring area, northeast upper plate; 

Hossfeldt Hills area, northeast lower plate; Willow 

Creek area, south west lower plate. 

The upper or northwest plate has moved unknown 

distances to the south and east relative to the lower 

or southeast plate. The pre-Tertiary formations in 

the upper plate differ, in some instances strikingly, 

from their representatives in the lower plate. Ultimately some of these differences may help determine 

the amount of fault displacement and the character of 

the original depositional sites. No conclusions as to 

these matters are reached in this paper, but with structural implications in mind the formations are discussed 

not in generalities but with emphasis on the similarities and differences within units from place to place. 

The pre-Cretaceous marine rocks contain many fossils and the Tertiary beds have a few. With this aid 

jt, has been possible to date practically all the preQuaternary sedimentary rocks rather closely. Fossilcollecting localities of special interest are shown on 

plate 2 where each is identified by a brief field number; in the text, a field number may be followed by a 

formal Geological Survey collection number, in brackets, if one has been assigned. 

For simplicity, the thicknesses of formations and 

their subdivisions are rounded off to the nearest 50 

feet, wherever practicable. Measured sections with 

finer. subdivisions and closer measurements are on 

pages 124-135 ; their locations are shown on plate 2. 

PRECAMBRIAN-BELT SERIES 

U nmetamorphosed Precambrian sedimentary rocks, 

mapped as a single unit, the North Boulder formation, 

underlie 3 sauare miles o£ the west-central part of 
10 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

TABLE !.-Generalized stratigraphic succession, Three Forks quadrangle, Montana 

"' rn Formation in feet Description ~ -~ I o.

0

p Thickness, 

--~---~--~~--------------------------I---------I-----------------------------------------------------------------------

C) 

~ 0 

5 

0 

Young alluvium G-50(?) Hounded gravel and finer deposits on present flood plains of master streams; subangular gravel along tributaries. 

Age relations Fan gravel G-50(?) Fan-shaped sheets of colluvial unsorted angular limestone gravel on benches, and wedges of 

complex; many alluvial subangular gravel in tributary valleys. . disconformities. I------------------------I----------I----------------------------------------'--------------------------

Silt and fine sand G-15 Unstratified angular silt and fine sand on benches. Eolian. 

Old alluvium Q-400 Deposits of the ancestral Jefferson and Madison Rivers. Rounded gravel on former flood plains Rounded gravel member o-50 of master streams; subangular gravel in tributary valleys. , Subangular gravel member (}-150 -----l--------.-----l------------------------l·---------l------------------------------ Disconformity --------------------------- Miocene 

and 

Pliocene 

-----

Miocene 

~ 

------- ;:., s "' 

~ Q) 

N 

Oligocene 0 

1:;> r:Q 

E-< 

------

Eocene 

Gravel (?) 

Gravel (?) 

Gravel? (?) 

Stream gravel (known from one locality only) 

Stream gravel (known from one locality only) 

Deposit not seen in this study. Inferred from report of articulated remains of Diceratherium armatum Marsh from one locality. I----------------------l--------l---------------------------- Disconformity -----------------------

Dunbar Creek formation 

Climbing Arrow formation 

Milligan Creek formation 

25G-> 800 Light-colored tuffaceous siltstone, partly lacustrine, partly eolian, and subordinate stream- channel sandstone and conglomerate. 

75G-1, 000 Olive bentonitic clay and coarse yellow sand. Alluvial. 

G-300 Light-colored tuffaceous lake limestone snd interfingering stream-channel sandstone and 

conglomerate. 

Sphinx conglomerate o-100 Pebbles and cobbles of Paleozoic limestone in a reddish-orange calcareous matrix. Alluvial. 

------------I--------------------l---------l---------------------- Angular unconformity--------------------

:§ Upper 

Creta- 0 

Q) ceo us 

"" ----- .s 

Q) Lower 

"" 0 Creta-

*§ ceo us --------- &1 Upper 

~ Juras-

-~ sic -------

"' Middle 3 

..... & Upper Jurassic 

Elkhorn Mountains volcanics 

---

Kootenai formation 

Morrison formation 

Ellis formation 

> 1, 000(?) Andesite lava flows and volcanic breccia. 

Angular unconformity(?) 

650(?) Orange sandstone with subordinate "salt-and-pepper" sandstone, and varicolored mudstone, 

siltstone, and limestone. Continental. 

10(}-250 Red, yellow, and brown mudstone, siltstone, and sandstone. Continental. 

75-350 Varied lithology. Basal brown pebble conglomerate common. Other common types are brown sandstone, gray limestone, and yellow siltstone. Marine. 

----------- -----I---------------------I·---------I---------------------------- Disconformity --------------------------- Lower 

Permian 

Phosphoria formation 75-200 Varied lithology. Common types are brown chert, dark phosphatic sandstone, yellow siltstone, 

light-colored quartzitic sandstone, and dolomitic limestone. Marine. -------------l------------------------l--------l-------------------------------------------------------------------

Quadrant formation 

Amsden formation 

200-400 Light-colored quartzitic sandstone and subordinate dolomitic limestone. Marine. 

25Q-600 Red sandstone and mudstone grading upward into pink and gray limestone and quartzitic sandstone. Marine. ----I-------------------l--------l'----------------------- Disconformity --------------------

Big Snowy formation Q-400 Varied lithology. Common types are brown and gray siltstone, gray limestone, and dark shale. 

Marine. -----------I---------------------I·--------I-------------------------- Disconformity ------------------------- ~ 

Lower iE 

Missis- ;a 

Mission Canyon limestone 60o-1, 400 Gray thick-bedded limestone. Forms cliffs. Marine. 

sippian ~ Lodgepole limestone 350..650 Dark-gray thin-bedded limestone. Basal beds form ledge. Marine. 

---------------l----------------------l---------1--------------------------------------------------------------------

Upper 

Devonian 

Three Forks shale 

Jefferson dolomite 

Maywood formation 

10(}-400 

35Q-650 

G-200 

Upper orange siltst?ne; medial green shale; lower orange limestone and siltstone. All marine. 

Brown fetid dolomite with subordinate black and gray dolomite and gray limestone. Forms 

cliffs. Marine. 

Yellow, orange, and red calcareous siltstone, grading upward into gray limestone. (Some basal 

siltstone may be uppermost Cambrian.) Marine. 

-------------I-------------------+-------+------------------------- Disconformlty ------------------- Upper 

Cam- Pilgrim limestone 35Q-450 Gray and orange dolomite, subordinate gray limestone. Forms cliffs. Marine. 

brian 

Middle 

Cambrian 

Park shale 

Meagher limestone 

Wolsey shale 

Flathead sandstone 

10o-200 Greenish gray shale. Marine. 

300..500 Gray and black limestone with orange mottling. Forms cliffs. Marine . 

20G-400 Olive and brown micaceous shale. Many interbeds of glauconitic sandstone near base and lime- stone near top. Marine. 

o-100 Pink quartzitic sandstone. Forms low ledge. Marine. 

---------~-------------------I·---------I------------------------- Disconformity --------------------

Belt North Boulder formation > 4,000 Brown coarse micaceous arkose and conglomerate. Marine. 

----------+-------------1-----11--------------- Angular unconformity-------------- Gneiss and amphibolite Unknown 

but 

great 
SEDIMENTARY ROCKS 11 

111 °45' 

EXPLANATION 

Outcrop of pre-Tertiary rocks 

~ ... _,~ 

Main trace of Sixteenmile. 

thrust system 

FIGURE 2.-Key map for identifying areas of pre-Tertiary sedimentary 

rocks. A, Milligran Creek area; B, Mud Spring area; C, Hossfeldt 

Hills area; D, Willow Creek area. 

the quadrangle, and are more than 4,000 feet thick. 

Made up of 75 percent sandstone, 20 percent siltstone, 

and 5 percent conglomerate, they are nearshore marine 

deposits of geosynclinal or unstable-shelf type, and 

are assigned to the Belt series though thoroughly 

untypical of regional Belt lithology. 

NORTH BOULDER FORMATION 

Rocks assigned to the North Boulder formation 

crop out mostly at the west edge of the quadrangle 

north of Highway lOS, where they form a continuous 

section dipping northward at angles ranging from 5° 

to 40 o, averaging 30 °. They are also exposed in a 

narrow strip north of the highway in center sec. 5, 

T. 1 N., R. 1 E. 

The North Boulder rocks yield a thin rubbly darkreddish-brown soil which supports a scrubby growth 

of conifers and sagebrush through which many low 

outcrops protrude. The rough and somber landscape 

thus developed characterizes the formation. 

Rocks believed to be Belt were penetrated in at 

least one of a series of borings put down recently by 

the Montana Power Co. in the Madison River flood 

plain and the bordering terraces southeast of Three 

Forks. A boring in NW1;4SW%, sec. 19, T. 1 N., 

R. 2 E., started at 4,390 feet, passed through 1,120 

feet of Cenozoic continental deposits, entered thinbedded greenish-gray calcareous arkosic sandstone ::w.d 

chlor.itic argillite at altitude 3,270 feet, and bottom~d 

in these beds at altitude 3,191 feet. Such rocks are 

common in the Belt formations of the Horseshoe Hills 

nearby to the north, 1 but are unknown in other terranes of the region. I am indebted to Montana Power 

Co. for copies of well logs and for a large core sample 

of the rocks similar to those of the Belt series. 

The North Boulder formation has an exposed thickness of more than 4,000 feet. The local base is the 

Jefferson Canyon thrust fault. The top is a disconformity, overlain by the Middle Cambrian Flathead 

sandstone or, where the Flathead is missing, by the 

Wolsey shale. Consequently, the full thickness of the 

North Boulder is not present. 

The exposed strata are mostly thick-bedded darkyellowish-brown and reddish-brown coarse micaceous 

feldspathic sandstone. The average grain size, which 

approaches very coarse near the local base, decreases 

upsection and downdip (that is, northward) to medium near the top of the brown sandstone sequence. 

Many layers show graded bedding, and some others 

contain numerous flat pebbles parallel to bedding. 

Poor sorting and poor rounding are characteristic. 

The sandstone is mineralogically . complex, averaging 

about 40 percent glassy quartz, 30 percent white or 

colorless plagioclase in the range ol!goclase-andesine, 

15 percent orange-pink microcline, 5 percent mica, and 

10 percent matrix, which is largely greenish-gray 

chlorite with varying proportions of limonite-hematite, 

clay minerals, and, locally, carbonate. Most of the 

quartz grains are strained. The plagioclase is twinned 

but rarely zoned. The colorless plagioclase grains are 

unaltered; the white ones are mostly argillized on the 

surface and along openings but some are more severely 

altered to saussurite and even to fine-grained mixtures 

of epidote-group minerals and calcite. Microcline is 

commonly unaltered and clear, indicating that the pink 

color is a body color and is not due merely to super1 Verrall, Peter, 1955, Geology of the Horseshoe Hills area, Montana: Princeton Univ., Ph.D. thesis (available on microfilm) . 
12 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

ficial alteration or coating; furthermore, microcline 

ground to powder retains the color. As with most 

colored feldspars the source of the color is unknown ; 

if it is due to hematite dust, a reasonable possibility, 

the dust particles must be submicroscopic for none are 

visible under high magnification ( X 344). The mica 

is mainly biotite, variably bleached and chloritized, but 

includes a little muscovite. Sparse accessory minerals 

are zircon, apatite, sphene, and opaque oxides. By 

most definitions this rock is an arkose. 

Thin laminae of micaceous olive-gray siltstone separate many sandstone beds, especially in the upper part 

of the sequence. The siltstone laminae have about the 

same minerals as the sandstone but in different proportions: micas, chlorite, and clay minerals are dominant, 

quartz subordinate, and feldspar rare. 

Lenses, a few feet thick, of subrounded cobble conglomerate are . abundant near the base. The cobbles 

are mostly vein quartz with lesser proportions of 

quartzite, granite pegmatite, diorite, gneiss, amphibolite, and diabase. 

A distinctive green-flecked reddish-brown to grayish-red coarse sandstone, 100-250 feet thick, is at the 

top of the exposed section. Good outcrops are in 

Nlj2 sec. 9, T. 1. N., R. 1 W., and along Highway lOS 

at the southwest end of the strip of North Boulder 

formation in sec. 5, which consists entirely of this 

rock. Its general characteristics and mineralogy are 

much like those of the underlying b_rown sandstone 

but the red rock has less mica, more ch~orite, and more 

clay-sized material, perhaps as much as 25 percent in 

places, and the matrix is much richer in limonitehematite. Its plagioclase is thoroughly argillized, 

though the microcline is usually about as fresh as that 

in the brown rocks. Along the highway in sec. 5 the 

rock is especially rich in hematite and is garishly red. 

The rock is here classed as an arkose though it approaches graywacke.· 

ORIGIN 

The North Boulder formation clearly was derived 

from the mechanical weathering of rocks like the 

nearby crystalline rocks, and shows every sign of 

short transport and rapid deposition. The textures of 

the coarser fractions suggest transportation by turbidity currents. The landmass from which these sediments were worn must have been mountainous, though 

not necessarily large. The North Boulder strata are 

one-cycle immature rocks of the sort common in marine 

geosynclines, or, as Peale (1896, p. 2) described them, 

they are "beds of littoral formation." 

Southward coarsening of the North .Boulder rocks 

plainly suggests derivation from that direction.. The 

crystalline terrane immediately to the south seems an 

obvious source. It is not a wholly satisfactory one, 

however; for one thing, the crystalline rocks lack pink 

microcline. so abundant in the North Boulder formation. The nearest known adequate source of pink 

microcline seems to have been the granite gneiss near 

Dillon 60 miles or more to the southwest (see .Scholten 

and others, 1955, p. 351) ; at least, pink microcline has 

not been reported by the several workers in intervening areas. There also seems no nearby source for the 

diabasic fragments in the North Boulder (reported 

also from the Jefferson Canyon area by Alexander, 

1955, p. 29) but this may be due merely to failure of 

other workers to recognize or mention small masses of 

such rocks, rather than to their absence. Unmetamorphosed basaltic intrusive bodies are known in Precambrian crystalline rocks near Sheridan about 40 mUes 

to the southwest.2 

Understanding the origin of the North Boulder formation is complicated by the fact that near Three 

Forks and almost everywhere else the base of the formation is a thrust fault of evidently large but indefinitely known displacement. . Perhaps these rocks 

were deposited far from their present position. As 

later discussion of the \Laramide thrust faults brings 

out, movement along th~ Jefferson Canyon thrust, the 

local base of the North Boulder rocks, was probably 

mostly strike slip, with the upper plate having moved 

relatively east. If so, the mass of North Boulder 

rocks may be many miles· east of its depositional site, 

but not far south of it. 

N4.ME 

The rocks here described are at the east end of a 

large mass of coarse feldspar-rich clastic rocks that 

extend westward for 15 miles, nearly to Cardwell. 

Similar rocks crop out sporadically in a narrow eastwest belt for 75 miles from near Melrose on the Big 

Hole River to the west front of the Bridger Range. 

These rocks are unique among pre-Flathead strata, 

but Pe.ale ( 1896, p. 2) assigned them to the Belt 

formation. Later, the Belt having been raised to 

series rank, these rocks were named North Boulder 

group by Ross (1949), who reported their thickness 

as more than 6,000 feet at their type locality. Because 

these rocks have not been subdivided in this area or 

nearby, the name North Boulder formation seems 

preferable in this area. 

Alexander (1955, p. 17-3.6) studied these rocks in 

some detail in the type area, and called them the 

LaHood formation after LaHood Park, a small settle2 Levandowski, D. W., 1956, Geology and mineral deposits of 

Sheridan-Alder area, Madison County, Montana: Univ. Michigan, 

Ph.D. thesis (available on microfilm). 
SEDIMENTARY ROCKS 13 

ment near the mouth of the Boulder River in the J,efferson Island quadrangle. 

AGE AND REGIONAL RELATIONS 

On local evidence the North Boulder formation can 

be dated only as older than the Flathead and Wolsey 

formations, which overlie it disconformably, and much 

younger than the Precambrian metamorphic rocks to 

the south. As the Flathead grades into the Wolsey 

and the Wolsey has Middle Cambrian fossils, the 

North Boulder formation is Middle Cambrian or older. 

The fact that the Flathead and younger Paleozoic formations overlie thousands of feet of North Boulder 

rocks north of Highway lOS, but lie directly on preNorth Boulder crystalline rocks a few miles to the 

south, without extreme differences in facies or thickness, indicates an important erosional, if not diastrophic, interval between the North Boulder and the 

Flathead. This strongly suggests that the North 

Boulder is Precambrian. 

The regional relations of the North Boulder rocks 

emphasize their probable Precambrian age. North of 

Cardwell, the coarse clastic rocks are reported to intertongue northward with the Greyson shale, high in the 

Belt series (Alexander, 1955, p. 14) ; about 17 miles east 

of Three· Forks, in the Horseshoe Hills, the arkosic material, according to H. D. Klemme 3 intertongues laterally and grades vertically with the Spokane shale which 

overlies the Greyson; according to Verrall 4 ( 1955) 

the intertonguing in the southern part of the Horseshoe Hills is with the Newland limestone and Chamberlain shale, which successively underlie the Greyson, 

but are still high in the Belt series. These interpretations are not necessarily in conflict; they may all be 

correct, leading to the conclusion that rocks of North 

. Boulder aspect were being deposited near shore, no 

doubt intermittently, over a long time interval during 

which perhaps as much as 10,000 feet of finer grained 

sediments were accumulating farther from shore. It 

should, however, be noted that Klemme's reconnaissance interpretation was not confirmed by my detailed 

mapping in the Toston quadrangle, which indicates 

that coarse arkosic sandstone similar to that of the 

North Boulder formation is interbedded with rocks 

low in the Grey son shale and high in the New land 

limestone rather than with the Spokane shale. At 

present, then, there is no unequivocal evidence that 

rocks of North Boulder facies were being deposited 

after Greyson time. The several thick Belt formations 

with which the North Boulder rocks are intimately 

related, and other Belt formations, are truncated 

3 Klemme, H. D., 1949, Geology of Sixteenmile Creek area, Montana: Princeton Univ., Ph.D. thesis (available on microfilm). 

4 See footnote, p. 11. 

654729 ~63----2 

regionally by the Flathead, in places with marked 

angular unconformity (Deiss, 1935). It seems safe to 

conclude that the North Boulder formation is to be 

correlated with the upper part of the Belt series, and 

is of late Precambrian age. 

The outcrops of Belt rocks on the north side of 

the Three Forks basin, and the near-certain presence 

of Belt rocks directly below Tertiary rocks near the 

south edge of the basin suggest that Belt rocks may 

be widespread beneath the Cenozoic cover. Precambrian rocks, partly crystalline and partly Belt, bound 

the Gallatin Valley on the north and south and dominate the western flanks of the Bridger Range, which 

forms the east boundary of the Three Forks basin. 

.... ~s first noted by Peale ( 1896, p. 3) , the probability 

is strong that Precambrian, largely Belt, rocks are 

the principal ones to be expected below the Tertiary 

deposits throughout the Three Forks basin. · 

The paleogeologic significance of the North Boulder formation and the sharp southern edge of Belt 

deposits which the formation defines have been widely 

discussed. Agreement is general that an ancient eastwest zone of tectonic activity determined the distribution and affected the lithologic nature of the North 

Boulder and other Belt rocks of the region; some 

plainly label it a fault zone, others are less explicit. 

It is agreed that the source of Belt sediments lay to 

the south, but opinions diverge widely on such matters as the size, relief, and tectonic habit of this landmass, whether it was ever covered by Belt sediments, 

and the degree and . significance of the unconformity 

between Belt and Flathead. Recognizing that the 

Paleozoic formations were deposited over both Belt 

and pre-Belt crystalline rocks, most authors regard 

this east-west lineament as structurally unimportant 

in post-Belt time, hut Sloss ( 1950, p. 430) believed 

that it "was an important tectonic boundary . in Precambrian and Paleozoic time and remains active today," and McMannis (1955, p. 1425) found evidence 

o£ important fault movements along it in the Bridger 

Range "during Laramide deformation." In this report the southern boundary is interpreted as a steep 

fault zone, and named the Willow Creek fault. 

The sole contribution of this study to the confused 

regional Belt problem is to call attention to a further 

element of confusion; namely, the possibility that the 

North Boulder sequence may be largely allochthonous. 

This is not merely a local possibility. Everywhere 

west of Three Forks, the base of the North Boulder, 

where not veiled by Cenozoic deposits, seems to be a 

northward-dipping thrust fault, with the probable exception of a small area 6 miles southeast of Whitehall where Lowell (1956b, p. 71) found conglomerate 

J 
14 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

of the Belt series lying on rugged topography cut in 

Precambrian crystalline rocks. The mass east of Divide, .shown on the "Geologic Map of Montana" 

(Ross and others, 1955) as in depositional contact with 

crystalline rocks, is actually in thrust contact (W. B. 

Myers, oral communication, 1955). East of Three 

Forks, the only North Boulder rocks not basally 

masked by Cenozoic detritus are on the west side of 

the Bridger Range, where they are in fault (though 

not necessarily thrust) contact with Precambdan 

meta~orphic rocks (McMannis, 1955, pl. 1, fig. 5,. p. 

1421). In part, the thrusting seems to represent a 

reversal of movement on the Willow Creek fault. If 

these rocks have moved far on a thrust system; they 

may tell little about conditions in the apparent source 

area directly to the south. Other aspects of these 

relations are discussed unde~ the heading "Structure." 

PALEOZOIC 

The Paleozoic rocks, subdivided into 13 map units, 

are 4,000-5,000 feet thick. The Paleozoic section is 

made up of about 60 percent limestone, 10 percent 

dolomitic limestone and dolomite, 20 percent siltstone 

and shale, and 10 percent sandstone; the clastic ratio 

is about 0.4. Typically, very thick sequences of carbonate rocks, mainly chemical but partly clastic, alternate with thinner sequences of carbonate-poor clastic rocks. .The earliest Paleozoic rocks are of Middle 

Cambrian age. A substantial part of the Paleozoicfrom very late Cambrian through Ordovician and Silurian to Middle or Late Devonian time-is represented by an erosion surface with little relief. Above 

this surface the rest of the Paleozoic is represented · 

with only minor breaks. No angular unconformities 

have been · detected. Viewed in the large, the assemblage seems to have been formed under conditions 

of deposition, and nondeposition, that remained stable 

for very long periods. In the near view, however, the 

impression of stability disappears. Within a few miles 

nearly every formation of Paleozoic age, carbonate as 

well as noncarbonate, exhibits striking changes in 

thickness and lithology, only part of which can be laid 

to later deformation. It would be misleading, therefore, to call them stable-shelf deposits, if this implies 

strata of monotonous uniformity over large areas. 

The clastic components of several of the pre-Car- . 

boniferous Paleozoic formations seem to have been 

derived from land to . the west, presumably not far 

from the present Idaho-Montana border. This landmass was recognized long ago by Willis (1907) and 

Walcott ( 1915), and its existence has been confirmed 

by the more recent work of Ross ( 1934, 1935), Deiss · 

(1941), and others. The evidence has, however, been 

overlooked or otherwise interpreted by contemporary 

paleogeographers, who have visualized geosynclinal 

conditions in the probable source areas through much 

if not all pre-Carboniferous Paleozoic time (-Rocky 

Mountain trough of Eardley, 1947; Millard belt of 

Kay, 1951, p. 7-14; Cordilleran geosyncline of Sloss, 

1950). 

Notwithstanding some difference of opinion regarding conditions near the Idaho-Montana border, the 

above-cited paper by Sloss and its companion piece 

(Sloss and Moritz, 1951) offer broad views of the regional stratigraphy and depositional environments 

during the Paleozoic era that are invaluable in orienting the small segment that is the Three Forks quadrangle. 

The Paleozoic rocks are much deformed.· In most 

places their dips are moderate to steep .(20°-60°), but 

over large areas they are vertical or overturned. Low 

dips characterize only the Paleozoic rocks at the east 

side of the Milligan Creek area, and in the axial 

parts of folds. Strike faults are many, but only the 

largest can be traced. Dip faults are rare, appearing 

only in the Milligan Creek sector. 

CAMBRIAN 

The Cambrian rocks, separated into 5 formations, 

are 1,200-1,600 feet thick. They are about 3·5 percent . 

limestone, 25 percent dolomitiG limestone and dolomite, 30 percent siltstone and shale, and 10 percent 

sandstone; the clastic ratio is about 0.7. 

Not all of Cambrian time is represented. Lower 

Cambrian rocks have not been recognized. Four of 

the mapped Cambrian formations are of known or 

probable Middle Cambrian age: Flathead sandstone, 

Wolsey shale, Meagher limestone, and Park shale. 

Only the fifth, the Pilgrim lime.stone, is of Late, but 

not latest, Cambrian age. Some unfossiliferous strata 

above the Pilgrim may be of very late Cambrian age, 

but are here included with similar strata of Devonian 

age. 

Comprehensive discussions of Cambrian stratigraphy in southwestern Montana have been presented 

by Deiss (193·6) and Hanson (1952). Cambrian paleoecology in the region has been treated by Lochman 

(1957). 

NAMES AND CORRELATION OF CAMBRIAN FORMATIONS 

Peale ( 1893, p. 20-25) subdivided the Cambrian system of the Three Forks region into . two formations, 

the Flathead and the ·Gallatin. He further subdivided 

the Flathead into two members and the Gallatin into 

five members, which he did not map separately. His 

Cambrian section is shown on the next page. 
SEDIMENTARY ROCKS 15 

Devonian: Jefferson limestone. Thicknes3 (feet) 

!

Pebbly limestones ______ _ 

Gallatin forma- Dry Creek shales ___ ____ _ 

tion Mottled limestones _____ _ 

Cambrian Obolella shales 

Trilobite limestone ____ _ _ 

Flathead for- {Flathead shales 

mation Flathead quartzite ____ ~ __ 

145 

30 

260 

280 

120 

290 

125 

Total Cambrian___ 1, 250 

All of Peale's members are readily recognized in 

the Three Forks quadrangle although some of his 

measurements of thickness cannot be confirmed· nevertheless, hi~ names are not' used. In regional usage, 

most of his names have been avoided or drastically 

modified, bec~use they are not carefully defined, or 

not etyi?ologiCally acceptable. Accordingly, to conform with current usage, terms are brought in from 

ne~ghboring a:eas. Names of the five definitely Cambrian formatwns are those first applied by Weed 

( 1899a, 1899b) in . the Little Belt Mountains, 80 miles 

northeast of Three Forks, and redefined by Deiss 

(1936). 

These formations seem safely correlated with 

Peale's five lower members, despite many differences 

in thickness and lithologic detail. Perhaps the most 

extreme disparity is between the Pilgrim of the Little 

Belt Mountains and equivalent rocks (mottled limest~n~s of Peale) at Three Forks. rhe Pilgrim' as 

originally defined by Weed is largely interbedded 

flat-pebble limestone conglomerate and gray-green 

shale, less than 100 feet thick, hardly . comparable with 

the massive dolomite and the banded limestone-anddolomite sequences, around 400 feet thick, assigned to 

the Pilgrim in this report. J?ut the rocks fit fairly 

closely the redefined Pilgrim of Deiss ( 1936, p. 1280-

1281), though they largely lack the intraformational 

conglomerates emphasized by him (p. 1334) as characteristic of the formation, and are mostly dolomite 

rather than limestone. If the ~arne Pilgrim is not 

used for these rocks, the other a vail able choices are 

.Hasmark, from the Philipsburg area (Emmons and 

Calkins, 1913), 80 miles northwest of Three Forks or 

Maurice, from the Yellowstone Park area (Dorf ~nd Lochman, 1940), 100 miles to the southeast As the 

Hasmark of Emmons and Calkins ·probably includes 

beds elsewhere assigned to the Meagher and Park 

formations (Hanson, 1952, p. 15 and 17) it is not a 

desirable term, notwithstanding its acceptance for regional use by Sloss and Moritz (1951, p. 2144-2145). 

Use of Maurice presents some practical problems, too 

(see Hanson, 1952, p. 16; Sioss and Moritz, 1951, p. 

2144-2145), in addition to the obvious problem of 

priority. Consequently, the name Pilgrim is used in . 

this paper. 

Only Late ( ? ) Devonian rocks of the Maywood formation were definitely identified in the interval in 

which Peale mapped his two uppermost Cambrian 

units, the Dry Creek shales and Pebbly limestones. 

Equivalents of part of the Dry Creek may be present, 

but cannot be distinguished from the Maywood and 

therefore are mapped and discussed with it. 

FLATHEAD SANDSTONE 

The lowest Cambrian rocks are nea!lY pure quartzitic sandstone assigned to the Flathead sandstone. 

The Flathead is exposed only in the Milligan Creek 

area where it overlies the North Boulder formation, and in the Willow Creek area where it rests on 

Precambrian crystalline rocks. In the Willow Creek 

area, the Flathead crops out almost continuously in 

a low ridge for more than 6 miles. Its outcrop width 

varies greatly: where it is overlain by Wolsey shale, 

in the west half of the strip, it forms a broad dip 

slope; where plutonic rocks have taken the place of 

the Wolsey, in the east half, the dip slope is narrow. 

Near the Madison-Gallatin County line, in sees. 9 and 

10, T. 1 S., R. 1 W., the Flathead outcrop swells to 

a width of half a mile. 

In the Milligan Creek area, the Flathead is largely 

missing, apparently through nondeposition rather 

than pre-Wolsey erosion. A small lens of Flathead 

sandstone is well exposed in a creek bed at the east 

edge of sec. 10, T,. 1 N., R 1. W. The Flathead also 

appears, along the strike of this lens, at the west edge 

of the quadrangle, and continues for several miles to 

the west in the Jefferson Island quadrangle. Where 

the Flathead is missing, a persistent low ridge, stratigraphically below the Wolsey and thus seemingly representing the Flathead, is actually formed by the upper red arkose of the North Boulder formation. 

In most of the Willow Creek area, the Flathead is 

30-50 feet thick. Its broad outcrop mostly reflects 

its tendency to form a dip slope. The very broad 

outcrop near the county line, however, can only in part 

be ascribed to parallelism between bedding dip and 

surface slope. The apparent thickening may be due 

to deformation, suggested by abrupt strike changes, 

but it is possible that the formation actually thickens 

locally to several hundred feet. The lens west of 

Milligan Creek is less than 50 feet thick. 

The formation is of uniform lithology, consisting 

mainly of thick beds of medium- to coarse-grained 

subrounded sandstone, cemented by quartz and tinted 

pinkish or brownish by films of iron oxides. Locally, 
16 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

bedding is thin and in such places crossbedding is 

common. Lentils of pebble conglomerate, only a few 

pebbles thick, are 'abundant near the base and common elsewhere. Rarely, the sandstone is deeply colored in shades of red and brown, due to coating of 

grains by iron oxides. A few laminae of dark shale 

appear near the top of the formation, which is placed 

below the first shale sequence more than 5 feet thick, 

assigned to the Wolsey shale. This contact is everywhere conformable, and in some places, where sandstone and shale are thinly interbedded for a few feet 

above thick-bedded Flathead sandstone, is gradational. 

Where shales do not crop out, the contact is arbitrarily placed at the base of the dip slope on the 

stratigraphically highest ledge of Flathead sandstone. 

Two measured sections of the Flathead are given on 

pages 129 and 135. 

Quartz makes up more than 98 percent of the rock, 

even where iron oxides seem abundant, and approaches 

100 percent in many places. The quartz cement is in 

clear overgrowths, optically continuous with the clastic grains; typically, it only partly fills the pores. 

Other constituents are wisps of clay and a few rounded 

grains of zircon and tourmaline. The rock, while 

firmly cemented, generally breaks around the grains. 

The Flathead and Wolsey formations are closely 

related and their origin, age, and regional relations 

are discussed jointly in the next section. 

WOLSEY SHALE 

Rocks assigned to the Wolsey shale lie on the Flathead sandstone in the Willow Creek and Milligan 

Creek areas, and on the Belt rocks where the Flathead is missing. Outcrops are rare and the trace of 

the formation is usually marked by a grassy depression. A few good exposures are in steep gulches 

in SW1/4NW1/4 sec. 11, and SE1/4NE1/4 sec. 10, 

T. 1 N., R. 1 W., west of Milligan Creek; near the 

hjghway in the SW% sec. 5, T. 1 N., R. 1 E., east 

of Milligan Creek; and just west of the quadrangle, 

along the main northwestward-draining creek in the 

NW% sec. 8, T. 1 S., R. 1 W. (Jefferson Island quadrangle). 

In this study, the base of the Wolsey is placed at 

the first shale beds more than 5 feet thick overlying 

the massive Flathead sandstone, or overlying the upper red Precambrian arkose where the Flathead is 

missing. In the Willow Creek area, sandstone bodies 

thick and continuous enough to be mapped as members appear low in the Wolsey, underlain , by perhaps 

as little as 20 feet of shale. These sandstone beds 

generally contain a little glauconite; otherwise, they 

closely resemble the Flathead. In places the thickest 

of these sandstone bodies, near the base of the formation, is thicker and makes a more impressive outcrop 

than nearby sandstone mapped as Flathead, and the 

temptation is strong to include such sandstones and 

the shale beneath them in the Flathead, as Alexander 

(1955, p. 39) seems to have done. To do so, however, 

would lead to unilluminating complexities in the Wolsey-Flathead contact, which would jump up and down 

in the section to enclose each thick sandstone lens. 

This problem does not appear in the Milligan Creek 

area, where the rocks mapped as Wolsey contain no 

prominent sandstone similar to that of the Flathead. 

In the Milligan Creek area the formation ranges in 

thickness from 200 to 400 feet; it is thinner where 

the Flathead is absent. In the Willow Creek area, 

the formation seems to be consistently around 400 

feet thick, but measurements are inexact because the 

formation is riddled with sills. The Wolsey section, 

typical of both areas, consists of: 

Thickness 

(feet) 

Thinly interbedded olive-gray limestone and subordinate 

olive shale_________________________________________ 100 

Olive micaceous shale with thin interbeds of olive-gray limestone ___________________ _______________________ 200 

Grayish-green and yellowish-brown micaceous shale, with a 

few beds, some very thick, of reddish glauconitic sandstone _______ ______________________________________ ~ tOO 

Where the formation is thinner, the lower part seems 

to be absent. · 

The micaceous material in the shale is largely 

chlorite, but includes considerable biotite, usually 

showing signs of marginal alteration to chlorite. 

Though glauconite is abundant only in the sandstone 

low in the formation, it is a common minor constituent in sandstone and limestone throughout. 

In the Willow Creek area the lower part of the 

Wolsey is slaty and dark reddish brown, due to. high 

local content of iron oxides. This part of the Wolsey 

contains many sills. The cleavage and reddening are 

probably intrusion effects, although the color change 

may reflect local initially high content of oxidizable 

iron, as in the red and brown phases of the Flathead. 

Bedding planes between many of the micaceous 

shale beds that make up the bulk of the formation 

are strewn with flattened cylinders of shale or fine 

sand, lh,-1j2 inch wide and 1-3 in. long, that taper 

slightly at the ends; these forms, widely distributed 

in early Paleozoic shales, for example, the Cambrian 

Ophir shale of northern Utah (Calkins and Butler, 

1943, p. 12-14 and pl. 9), are generally interpreted 

as worm trails or casts. While the shale beds themselves rarely crop out, their presence is made plain 

by soil glittering with flakes of golden-brown miCa, 

and by float fragments bearing worm casts. 
SEDIMENTARY ROCKS 17 

The upper quarter of the formation, in which shale 

becomes subordinate to limestone, may well be thought 

of as tral1sit.ional into the overlying Meagher lime7 

stone, and on strictly lithologic grounds the beds 

should no doubt be assigned to the Meagher, as some 

workers, such as Sloss and Moritz (1951, p. 2142) and 

McMannis (1955, p. 1393), have done. This partly 

accounts for the discrepancy between the thickness of 

the W alsey as reported by these workers, 125-200 

feet, and the much greater thickness reported here. 

Exposures of the transitional beds are rare; at most 

places the entire W alsey is covered and the first outcrop upsection from the Flathead sandstone is thickbedded limestone from near the base of the Meagher. 

In mapping, therefore, it is practical to place the 

contact between the Meagher and · the W alsey slightly 

below the base of this limestone ledge and arbitrarily to include the transitional beds with the Wolsey, as Klepper and others (1957, p. 8) have done. 

For detailed measured sections of the Wolsey shale, 

see pages 129 and 135. 

AGE AND ORIGIN OF FLATHEAD AND WOLSEY FORMATIONS 

Many fragments of a species of Ehmania, a characteristic Middle Cambrian trilobite (A. R. Palmer, 

written communication, June 30, 1955) have been recovered from a limestone bed about 200 feet below 

the top of the W alsey in the Milligan Creek area 

(loc. 2, near SV2NW1,4 sec. 11, T. 1 N., R. 1 W., pl. 2,). 

Nodular shaly limestone beds 30 feet below the top 

of the Wolsey, in the Jefferson Island quadrangle (loc. 

449 [2145-EO], south side of 4698 hill, NEl)tNWl)t 

sec. 8, T. 1 S., R. 1 W., pl. 2) about 1,000 feet west 

of the Three Forks quadrangle, yielded a definitive 

middle Middle Cambrian fauna (A. R. Palmer, written 

communication, October 22, 1956) including a "distinctive and apparently undescribed phosphatic brachiopod 

resembling Pegrnat'tY(,ta" and a trilobite assemblage, 

listed below, with forms closely related to those reported by Deiss from the Pagoda limestone in north-

. west Montana : 

Tonkinella cf. T. stephenensis Kobayashi 

Kootenia sp. (with 10 long marginal spines) 

Bolaspis sp. indet. 

Glyphaspis robusta Deiss 

These are the only local age data for both Flathead 

and Wolsey. Because sedimentation was apparently 

continuous from the base of the Flathead to the fossiliferous beds, it is a reasonable inference that both 

Flathead and W alsey are Middle Cambrian, though 

an Early Cambrian age is not ruled out for the lower 

part of the sequence. Fossil -evidence from other 

nearby areas (Hanson, 1952, p. 13-14; Alexander, 

1955, p. 40) supports a Middle Cambrian age for the 

entire W alsey shale and strengthens the case for assigning the Flathead sandstone to the Middle Cam- · 

brian. 

The Flathead sandstone represents slow but steady 

marine invasion across a subaerial erosion surface. 

This surface represents the removal of very great 

thicknesses o:f Belt rocks regionally, as several workers 

have shown (for example, see Clapp and Deiss, 1931) . . 

The source area had considerable relief judging from 

the general coarseness of grain and the widespread 

occurrence of conglomerate beds. 

In the region around Three Forks the Flathead 

must have been derived either as first-cycle deposits 

from erosion of early Precambrian gneiss and schist, 

or as second -cycle deposits from reworking of Belt 

sedimentary rocks, or both. In any event, the winnowing process that yielded pure quartz sand near the 

shore must have concentrated the waste from ferromagnesian and feldspathic components of the source 

rocks further offshore, as clays or muds. The lower 

part of the W alsey shale is, then, probably the offshore 

time-equivalent of the Flathead sandstone. The local 

concentration of red hematitic pigment in both Flathead and W alsey probably represents stripped regolith. 

A local topographic high, or island, in the Flathead and early W alsey sea is suggested by the virtual 

absence of Flathead and thinning of W alsey rocks 

west of Milligan Creek; this island was earlier recognized by Hanson ( 1952, p. 13) and by Alexander 

(1955, p. 36). Similar thickness variations in the Flathead are also reported in the Philipsburg area (Emmons and Calkins, 1913, p. 51-52} and in the Gravelly 

Range (J\1ann, 1954, p. 6); very likely they are typical 

of the Flathead throughout western Montana. 

Shoreline oscillations are indicated by the reappearance of sandstone~ similar to the Flathead in the lower 

'Volsey, but dominant marine transgression and retreat of the shoreline during this episode is clearly 

shown by the virtual disappearance of sandstone from 

the upper W alsey and concomitant increase in the 

number and thickness of limestone beds. Local evidence of the direction of transgression is lacking, but 

recent regional studies suggest that it was from west 

to east (Lachman, 1957, figs. 1-3; Hanson, 1952, p. 13), 

and not east to west as proposed earlier by Emmons 

and Calkins ( 1913, p. 64). 

MEAGHER LIMESTONE 

The Meagher limestone is the lowest of the great 

carbonate formations that dominate the Paleozoic column, not only in the Three Forks region but throughout western Montana. The Meagher is exposed in the 

Milligan Creek and Willow Creek areas, but is absent 
18 GEOLOGY OF THE mREE FORKS QUADRANGLE, MONTANA 

elsewhere in the quadrangle except for a small thrust 

wedge in the northeast corner (sec. 19, T. 3 N., 

R. 2 E.). It forms most of the cliffs skirted by Highway lOS in the center of the quadrangle and the most 

southerly row of cliffs west of Milligan Creek. 

Near Milligan Creek the Meagher limestone is 

around 500 feet thick and typically consists of· 

Thickness 

(feet) 

Thick-bedded gray limestone; forms ledge________________ 50 

Thin-bedded gray limestone; with mottles and bands of 

orange silty limestone, poorly exposed ____ ._____________ 250 

Thin-bedded orange calcareous siltstone and gray dolomite, 

generally covered_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 50 

Thick-bedded nearly black limestone with crinkled laminations, and abundant grains of feldspar and quartz sand; 

forms ledge_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 100 

Thin-bedded gray limestone with yellow shaly partings, 

generally covered _____________________________ ~ _ _ _ _ _ 50 

Brecciated, locally cavernous, zones, marked at the 

surface by dark iron-stained outcrops and slight depressions, appear high in the Meagher at the east side of 

Milligan Creek sector (sees. 32 and 33, T. 2 N., R. 1 E.). 

These follow bedding, and are probably due to postdepositional ground-water solution. 

In the Willow Creek area the gross thickness and 

lithologic character are similar, but the typical section 

is notably different, except for the basal limestone and 

shale unit: 

Thickness 

(feet) 

Thin-bedded gray limesto:q.e with many interbeds of orange 

silty limestone, poorly exposed__________ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 200 

Thick-bedded gray limestone; forms ledge ________________ 150 

Thin-bedded limestone, mottled in gray and orange, poorly exposed ____________________________________________ 100 

Thin-bedded nodular gray and orange mottled limestone and 

shale______________________________________________ 50 

Particularly noticeable in the vVillow Creek area 

is the absence of the great black limestone ledge that 

persists near the base of the section in the Milligan 

Creek area. 

Perhaps the most distinctive characteristic of the 

Meagher, aside from its stratigraphic position as the 

lowest Paleozoic carbonate formation, is the mottling 

of gray microcrystalline limestone with coarser 

grained, more porous, orange or gray limestone. Rock 

of this sort was once quarried near Radersburg for 

decorative stone under the trade name "black and gold 

marble" (Hanson, 1952, p. 14 and photograph, pl. 

3H), and geologists in the region usually refer to this 

facies informally as mottled blue-and-gold, mottled 

black-and-gold, or mottled black-and-tan. The orange 

(equals gold or tan) mottles, which rarely comprise 

more than 30 percent of any sequence of beds, range 

from partings in the bedding, to irregularly ramifying networks only crudely related to bedding, to 

scattered isolated blobs. Individual orange zones are 

rarely more than half an inch thick. The orange 

color seems due to films of ferruginous clay. Contacts 

between the black and the gold phases are sharp ; 

typically, they are not flat but undulating in smooth 

waves a fraction of an inch in length and amplitude. 

Several other features are characteristic of the thin-

. bedded parts of the Meagher. Thin lenses· of yellowish calcite sandstone, locally cross-stratified and laden 

with fossil fragments, are especially common. Abundant, too, are thin to thick beds of conglomerate 

made up of flat, angular to subrounded limestone 

pebbles in a matrix of · calcite sand; most of the 

pebbles lie approximately in the plane of bedding 

but many are steeply inclined, though without obvious preferred orientation. Rarely, thin zones of 

edgewise or shingled flat-pebble conglomerate appear 

· near the top of the formation. 

Both thin-bedded -and thick-bedded parts of the 

formation have several features in common. Many 

beds contain abundant rounded grains of glauconite. 

Oolitic zones are common, though rarely more than 

a few feet thick or traceable more than 100 feet 

laterally; the oolites, of slightly darker limestone 

than their matrix, are of sand size, rarely · as much 

as 1/15 inch long. Much rarer are pisolitic zones, of 

similar dimensions, but with ovoids ranging from 

Ys to 1j2 inch long (for photograph see Hanson, 1952, 

pl. 3B.). Oolites and pisolites do not occur in the 

same zone. Many of the darker beds have a distinct 

petroliferous odor when freshly broken. Characteristic, too, of many of the gray and black beds of the 

Meagher is a distinctly bluish tinge on weathered 

surfaces. · 

Where exposures permit, the base of the Meagher 

is mapped at the contact of thin-bedded limestone 

with interbedded limestone and shale of the uppermost Wolsey; elsewhere, it is carried about 50 feet 

stratigraphically below the base of the lowest limestone ledge, in order to include the basal thin-bedded 

limestone. 

The top of the upper thick-bedded limestone makes 

a dip slope into a narrow grassy valley or saddle underlain by the Park shale. Exposures of the contact 

are rare, but it nevertheless seems safe to assume that 

the contact throughout is sharp but conformable, for 

there are no prominent transitional limestone interbeds in the basal Park, but the Park seems to lie on 

the same set of Meagher beds everywhere. On the 

map the contact is arbitrarily placed where the slope 

made by the upper part of the Meagher limestone 

begins to flatten. 

For detailed measured sections of the Meagher limestone, see pages 128-~29, 135. 
SED~ENTARY ROCKS 19 

AGE AND ORIGIN 

The Middle Cambrian age of the Meagher limestone 

near Three Forks is well established by fossil collections from several horizons. An especially well preserved assemblage from the lowermost 10-30 feet of 

the formation in the Willow Creek area (locs. 435 and 

450 [2144 £0], NW14NW14 sec. 9, E. 1 S., R. 1 

W., pl. 2) contains the following forms (A. R. Palmer, 

written communications Oct. 7, 1954, and rOct. 22, 

1956) : 

Ptychagnostus atavus ( Linnarson) 

Tomagnostus sp. 

Bathyurisous cf. B. formosus Deiss 

Elrathina sp. 

Peronopsis cf. P. scutalis (Hicks) 

ct. Elrathiella sp. 

Ptychoparioid gen. and sp. indet. 

Prototreta flabellata Bell 

The Meagher seems to have been deposited in shallow marine water, but for the most part far from 

sources of noncarbonate detritus. The widespread 

pebble beds, clastic zones, and oolitic zones indicate 

shallow-water deposition despite the thickness of the 

formation. Probably a large fraction of the formation is of bioclastic origin. It seems likely that the 

water in which the Meagher was deposited was no 

deeper than that in which the Wolsey shale accumu-

.lated. The change in sedimentation was perhaps due 

largely to the reduction of relief in the source area 

that had supplied the detritus in the Wolsey. 

Whereas the oolites probably formed as a result 

of alternate chemical deposition and mechanical rolling in disturbed water, the pisolites, though superficially similar, are probably of very different origin: 

according to A. R. Palmer (oral communication, 

1956) they are the remains of ·stromatolites of the 

genus Girvanella. Alexander ( 1955, p. 40) also recognized these forms to be Girvanella. 

The origin of the mottling in the black-and-gold 1 

limestone, which is not confined to the Three Forks 

quadrangle but is regionally distributed, has not ·yet 

been systematically studied. The nearest approach is 

an interesting but inconclusive investigation by Hanson (1951). 

In many places, particularly west of the Three 

Forks area, the gold mottles in the Meagher are . 

dolomitic limestone or dolomite (Hanson, 1952, p. 14; 

Klepper and others, 1957, p. 9) . Similar mottles and 

bands in thin-bedded parts of the next Cambrian 

carbonate unit, the Pilgrim limestone, are consistently 

of dolomite in the Three Forks quadrangle and else-. 

where (Klepper and others, 1957, p. 11; McMannis, 

1955, p. 1394-1395). The temptation is strong to regard the mottling as an early product of the dolomitization process. But the absence or paucity of dolomite 

in the mottled Meagher near Three Forks and in 

other areas, particularly to the east (see chemical 

analyses quoted by Hanson, 1952, p. 42) shows that 

the mottles are not the result of dolomitization; rather, 

the coarser, more porous gold-colored zones probably 

served as channelways in some places (but not near 

Three Forks) for postdepositional dolomitizing solutions. Similar channeling of late dolomitizing solutions has been often interpreted, notably for the 

Platteville limestone by Griffin (1942) and £or certain Alpine carbonate rocks by Sander (1936) who 

coined the term "belteroporic" to define the process 

and resultant £abric. 

Hanson stated (1952, p. 14) that "the mottling is 

apparently a primary feature." This interpretation is 

supported not only by the absence o£ dolomite but 

also by the presence o£ clay in the mottles and by 

the sharp contacts between the black and the gold 

phases. 

But if primary, meaning depositional, why does the 

mottling cut across the bedding in so many places and 

why are many of its contacts wavy? An attractive 

explanation depends on different lithification rates 

for the two types of sediment involved; it might 

therefore be called a diagenetic hypothesis. Suppose 

that initially thick layers of fine-grained, largely 

chemical calcium carbonate mud (black) are deposited 

alternately with thin layers of coarse clayey, largely 

clastic calcium carbonate mud (gold), and that the 

original contacts are horizontal and flat. Long after 

deposition-that is, after removal from the water interface-the sediments will still be wet' and plastic. 

The more porous gold layers could be expected to 

harden more slowly and, if thick enough, to act as 

highly viscous liquids after the black layers have 

begun to act as so£t solids. Depending on the viscosity and elasticity relationships that develop, the 

more competent black layers might merely buckle and 

sag into the gold ones, thus forming wavy contacts; or 

they might break up and settle piecemeal, producing 

a breccia; or a flat-pebble gravel might form if the 

still-soft black fragments are rounded a little by 

streaming or by chemical reaction with the gold fluid. 

This process could operate in a number of layers simultaneously, and requires no external dynamic force 

other than gravity. It could, therefore, operate £ar 

from shore and in horizontal materials without the 

aid of subaqueous sliding, subaerial desiccation, waves 

or currents. Angular slabs of carbonate rock in a 

carbonate mud matrix have been assigned a similar 
20 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

origin and called "inhomogeneity breccia" -by Sander 

(1936). 

Intraformational conglomerate, of the sort common 

in the thin-bedded parts of the Meagher, has generally been ·interpreted as due either to subaqueous 

sliding or to penecontemporaneous fragmentation by . 

waves or currents. It seems clear that the nonshingled conglomerate in the Meagher formed far from 

shore an.d on a horizontal bottom, as most workers in 

the same or closely similar ro_cks have concluded 

(McKee, 1945, p. 65--'69; Lochman, 1957, p. 143-146). 

Under such circumstances, subaqueous sliding on a 

large scale seems out of the question. McKee by inference and Lochman explicitly favor fragmentation 

by waves or currents. Worth considering, however, 

is the possibility that some of the nonshingled intraformational flat-pebble conglomerate in the Meagher 

formed as a result of differing rates of lithification 

in thin-bedded strata, and thus might be called, to 

extend Sander's usage, inhomogeneity conglomerate. 

Specifically excluded from such an origin, however, 

are t!te rare layers of shingled or edgewise conglomerate, for their high degree of orientation athwart .the 

bedding planes indicates development at a water-sediment interface in the presence of strong currents or 

waves. 

The origin of the few dolomite beds in the Meagher 

is moot. Some of their field aspects and relations 

favor direct precipitation. ¥or example, the persistent 

dolomite beds above the lower black limestone ledge 

are finely crystalline, even in texture, and without 

mottling, although the calcite sandstone with which 

they are interbedded is similar to ·that in the gold 

mottles higher in the section. Contacts between clastic calcite rock and seemingly chemical dolomite are 

sharp. Here the porous sandstone has evidently not 

acted as a passageway for dolomitizing · solutions. 

The problem is discussed more fully later, in connection with the dominantly dolomitic Pilgrim and J efferson formations. 

Although most of the Meagher was laid down far 

from the sources of noncarbonate material, there must 

have been a nearby source for the feldspar and quartz 

sand common in the lower ledge of the Meagher limestone in the Milligan Creek area. The sand was 

doubtless derived from the same island responsible 

for the absence of Flathead sandstone and thinness of 

Wolsey shale in that area, though its shoreline was 

presumably farther away than in Wolsey time. ·The 

absence of sand in the Meagher of the Willow Creek 

area coupled with the normal development of Flathead and Wolsey there implies that the island was 

to the west or north rather than south or east. No 

such island is indicated by the work of Klepper and 

others ( 1957) to the north so it must have been to 

the west. Middle Cambrian islands may be inferred 

in or near the Philipsburg quadrangle (Emmons and 

Calkins, 1913, p. 51-53) 70-100 miles to the west. 

There, the Flathead and the Silver Hill formation 

(apparently equivalent to the Wolsey formation although Calkins did not make this correlation) exhibit rapid variations in thickness, especially in the 

southeast corner of the quadrangle where in places 

the Flathead is absent, and abnormally thin Silver 

Hill rocks lie directly on Belt rocks. The principal 

island mass would thus seem to have been between 

Three Forks and Philipsburg, where possible evidence for or against it is conveniently concealed by 

younger igneous rocks. 

PARK SHALE 

The Park shale, which conformably overlies the 

Meagher limestone in the Milligan Creek and Willow 

Creek areas, crops out even more rarely than does the 

Wolsey shale, but there are good, if incomplete, exposures east of Milligan Creek in the center of the 

Wlj2NE1_4 sec. 5, T. 1 N., R. 1 E., on the west bank 

of the main valley there ; and at two places west of 

Willow Creek: · in the only canyon in the NE% 

NW1_4 sec. 12, T. 1 S., R. 1 W., and on the north 

bank of the creek along the Madison-:Gallatin County 

line · between sees. 4 and 9, T. 1 S., R. 1 W. 

In the Milligan Creek area the Park shale is 150-

200 feet thick; in the Willow Creek area it is somewhat thinner. 

The formation is almost wholly greenish-gray clay 

shale. A few beds are calcareous and many bedding 

surfaces are strewn with flakes of chlorite and altered 

biotite. Near the base are scattered layers, less than 

half an inch thick, of yellow and orange calcareous 

siltstone and fine-grained clayey limestone. The upper 

part in the Willow Creek area includes much darkyellowish-brown mudstone and shale. For measured 

sections, see pages 128 and 135. 

Where exposed, the contact of the Park shale with 

the overlying Pilgrim limestone is sharp but apparently conformable. In most places, the contact is obscured by float from the basal ledge of the Pilgrim, 

and on the map it is arbitrarily placed at the base 

of this ledge. 

The Park is a typical marine shale. No fossils were 

recovered from it in the Three Forks quadrangle, but 

its 'sparse fauna from neighboring areas indicates 

latest Middle Cambrian age (Deiss, 1936, p. 1333; 

Lochman and Duncan, 1944, p. 5; Hanson, 1952, p. 16). 

In seeming to lack thick transitional zones with the 

overlying and underlying carbonate formations, the 

Park shale in this quadrangle differs frolll that de-
SEDIMENTARY ROCKS 21 

scribed in neighboring areas (Hanson, 1952, p. 15). The generalized section in the Milligan Creek area is: 

If exposures were better, perhaps the apparent differ- Thi~ie~~) 

ences would disappear. Thick-bedded dolomite mottled in tones of yellowish gray 

The Park shale reflects a brief regressive episode with a few beds of light-gray fossiliferous limestone; 

in the dominantly transgressive history of the Cam- ledge ______________________________________________ 200 

brian formations of western Montana Sloss, 1950, Thinly interbedded gray limestone and orange dolomite; 

p. 432). The nearest likely source area was to the D~~:~;~l~i~~v;~~~;di-;;t~-ii~;st~~~-lfk~-;~~~r-~~it~~~~~~ :gg 

west, where a Cambrian positive element in the gen- In the Pilgrim near Willow Creek the main rock types 

eral vicinity of the Idaho batholith was recognized are much the same, but their thicknesses are notably 

half a century ago by Willis (1907, p. 399) noted different: 

subsequently by Walcott (1915, p. 198), and dis- Thickness 

cussed in some detail -by Ross ( 1935) and by Deiss Mottled dolomite, ledge _______________________________ u~3 

(1941, p. 1095-1097). More specifically, central Idaho Thin-bedded limestone and dolomite ____________________ 200 

offers a possible source. Large areas of Belt rocks Mottled dolomite, ledge _____________________________ -- 150 

there could have been exposed to erosion at the re- For more detailed measured sections, seep. 128, 134-

quired time, as, according to Ross ( 1934) , the basal 135. 

Paleozoic rocks are of Ordovician age (Kinnikinic The Pilgrim near Willow Creek is consistently 

quartzite). If the source area was to the south or darker with many yellowish-brown and brownish-gray 

southwest, it had little relief, as the Park shale appar- beds, and includes many thin beds of flat-pebble conently does not grade to a sand facies in these direc- glomerate and prominent oolitic and pisolitic zones 

tions. On the contrary, it tends to pass southward rare in the Milligan Creek vicinity. Common also 

into a limestone facies (Mann, 1954, p. 8). Further, in the Willow Creek ledges are scattered twiglike 

it does not notably thicken toward a supposed south- bodies of earthy yellowish-gray calcite, presumably 

western shore (see Hanson, 1952, pl. 7 B). Thus, a the remains. of stromatoporoids or algae. The partial 

more distant easterly source may just as reasonably Mud Spring section is much like that near Milligan 

be inferred. Such a source would have been farther Creek, though the upper dolomite ledge is thinner. 

east than the Big Snowy Mountains, where Park The partial section in the Hossfeldt Hills shares the 

equivalents are known to be thick (Hanson, 1952, pl. brownish tones, the twiglike structures, and the con9) but perhaps still within Montana, as the Park glomerate and oolitic beds of the Willow Creek secseems to be missing in the Williston basin (Sloss and tor. (For photographs of several of these textural 

Moritz 1951 p. 2140). features, see Hanson, 1952, pls. 2 and 3.) 

' ' The thin-bedded middle part has distinctive color 

PILGRIM LIMESTONE and compositional banding. Layers from % inch to 

Carbonate rocks assigned to the Pilgrim limestone 4 inches of gray microcrystalline limestone alternate 

crop out in all four areas of pre-Tertiary rocks. Com- with t~inner layers of coarser grained, porous, clayey, 

plete sections, however, traceable for long distanGes, orange or brown dolomite, giving a ribboned effect 

appear only in-the Milligan Creek and Willow Cr~ek and leading to the informal field name of "banded 

areas. In the Mud Spring area, the Pilgrim lies on black-and-gold" or "banded blue-and-gold". Mottled 

the Lomba;d. thrust, which cuts out the lower one- black-and-gold facies like those so common in the 

quarter to one-half of the formation. In the Hoss- Meagher occur also in the Pilgrim, but are rare. 

feldt Hills area (sec. 20, T. 3 N., R. 2 E.), the upper Where present, the gold portions are generally dolomite or dolomitic limestone, and the black, limestone. 

part only of the Pilgrim appears in the core of an Exposures of the contact between the Pilgri!filimeoverturned anticline, here named the Hossfeldt anti- stone and the succeeding Maywood formation are poor 

cline. and in the 16 miles of contact traced, no suggestion 

The Pilgrim, 350-450 feet thick, consists of per- was found either of transitional relation or of angular 

sistent upper and lower sequences of ledge-forming unconformity. The Pilgrim is overlain by the Dethick-bedded mottled dolomite with an intermediate vonian Jefferson dolomite in three places for a total 

sequence of poorly resistant thin-bedded limestone and of about 2y2 miles: above the Lombard thrust in 

dolomite; but the thickness of the several facies varies Wy2 sec. 30, T. 3 N., R. 2 E.; below the thrust on 

widely as do many lithologic details. Characteristic the east flank of the Hossfeldt anticline in the center 

of the Pilgrim throughout are distinctive yellowish of sec. 20, T. 3 N., R. 2 E.; and west of Willow Creek 

pitted weathered surfaces. from NE14NE%, sec. 11 to center NE14 sec. 12, 
22 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

T. 1 S., R. 1 W. In none is the Pilgrim deeply 

eroded. The absence of Maywood rocks in these places 

suggests a disconformity due to initial nondeposition 

of the Maywood, on high parts of a post-Pilgrim, 

pre-Maywood erosion surface. It is possible that the 

rocks mapped as Maywood include some Late Cambrian beds, conformable with the Pilgrim, and that 

the disconformity is within the Maywood as mapped 

rather than between it and the Pilgrim. On the 

map the Pilgrim-Maywood contact where not exposed 

is arbitrarily placed where the dip slope on the top 

of the upper dolomite ledge begins to flatten. 

The Pilgrim is like the Meagher in many ways. 

In structurally complex situations or where exposures 

are few it may be difficult to tell them apart without 

the aid of fossils. In the Three Forks quadrangle 

the acid-bottle is the most useful separatory tool as 

the Pilgrim is largely dolomite while the Meagher is 

almost wholly limestone, but this distinction assuredly 

does not hold regionally. Brownish or yellowish 

weathered tones suggest Pilgrim, whereas a bluish 

cast suggests Meagher. Thick sequences of banded 

black-and-gold beds suggest Pilgrim, whereas much 

mottled black-and-gold suggests Meagher. Limestone 

pebble beds are much more common in ~he Pilgrim. 

AGE AND ORIGIN 

The Late Cambrian age of the Pilgrim formation 

. in the Three Forks quadrangle is established by the 

presence of the trilobites Oedaria sp., and K ormagnostus sp. [D177 £0] identified by A. R. Palmer 

(written communication, April 6, 1955) from the lower 

and middle parts of the sequence. In the absence of 

fossil collections from high in the Pilgrim, it is/ possible that the uppermost part may be of Early Ordovician age, as is the, uppermost Pilgrim in southern 

Montana (Lochman and Duncan, 1944). The possibility is remote, however, as Cambrian fossils have 

been recovered from beds above the Pilgrim variously 

assigned to Peale's Dry Creek shale, Calkins' Red Lion 

formation, or Lochman and Duncan's Snowy Range 

formation not far from the Three Forks quadrangle 

(Deiss, 1936, p. 1312 and 1316; McMannis, 1955, p. 

1395; Klepper, 1950, p. 32; Klepper and others, 1957, 

p. 13). 

The Pilgrim resembles the Meagher and no doubt 

formed under similar conditions. Like the thin-bedded 

phases of the Meagher, the thin-bedded parts of the 

Pilgrim must ha~e been deposited in disturbed, presumably shallow water, but the thick-bedded, finely 

homogeneous, clastic-free portions of both formations 

suggest quiet, presumably deeper, water. It is noteworthy that quiet-water phases of both formations are 

much better developed in the Milligan Creek area 

than in the 'Villow Creek area, and that this reflects 

the regional pattern (Hanson, 1952, p. 14-17, and 

pl. 7-8) , for quiet-water phases of both formations 

are dominant west of the Three Forks area to beyond 

Philipsburg, and disturbed-water lithologies are dominant for great distances to the east. Both formations 

also thicken rapidly west of Three Forks, and in both 

dolomite becom~s the dominant carbonate to the west; 

further, the shale and limestone-pebble conglomerate 

-beds that are important constituents of the Pilgrim 

nearby to the east (Verrall ;

5 McMannis, 1955, p. 1394-

1395) are thin or absent to the west, and the Park 

shale is unrecognizable in the Philipsburg area (Hanson, 1952, p. 15). These relations, incidentally, are 

just the opposite of those often cited for other regions, 

where, to quote a recent summary (Cloud and Barnes, 

1957, p. 185) "rocks that are dolomites near ancient 

lands are prone to grade to limestones away from the 

old shore lines"-but the quotation may apply to the 

western shoreline. The features cited suggest a genetic 

relation between conditions favoring at least one class 

of dolomite deposition and quiet, clear, but not necessarily deep water. It has been' contended (Hanson, 

1952, p. 14, 17) that disturbed-water features and fossils. were initially abundant in the Cambrian carbonate 

rocks west of Three Forks but have been obliterated 

by regional dolomitization. Far simpler, however, to 

accept the direct suggestion of quiet clear water, 

poorly populated perhaps because of· its high magnesian content. Initial quiet-water conditions to the 

west are implied in the westward thinning and disappearance of the shales in and between the Meagher 

and Pilgrim, which could hardly be the result of 

dolomitization. Further, in westerly occurrences of 

these formations disturbed-water features and fossils 

are just as rare in the few thick dolomite-free limestone beds as in the dolomites. 

The dolomite in the banded and mottled beds of the 

Pilgrim seems to have formed during diagenesis, 

partly by precipitation of dolomite in openings in the 

porous gold layers, but mostly by replacement of calcite. Structurally the banded and mottled beds in the 

Pilgrim are identical with those in the Meagher. The 

fact that the porous mottles are dolomitic limestone in 

the Pilgrim but are limestone in the Meagher suggests 

that the mottling developed similarly in both formations, but that dolomite was introduced into the Pilgrim at a later stage in the lithification process. Whatever the source of the dolomite-bearing solutions which 

invaded the mottled beds, the introduction of dolomite 

5 s~ footnQte, p. 11. 
SEDIMENTARY ROCKS 23 

·was so widespread that special sources such as juvenile 

hydrothermal solutions seem out of the question. Most 

likely the solutions were connate waters mobilized during compaction and diagenesis. How magnesium was 

concentrated in this environment, and why dolomite 

was more stable in it than calcite are unknown. 

Although the dolomite in the banded and mottled 

beds seems to be of postdepositional origin, it does not 

necessarily follow that the dolomite of the thickbedded phases represents the same replacement process 

gone to completion. The field evidence of extensive 

thick beds of seemingly almost pure dolomite, much of 

it no coarser in grain than intercalated beds of limestone, and showing only trivial visible signs of intermediate stages of alteration that might be ascribed to 

dolomitization, suggests that these beds were deposited 

initially as dolomite. 

The primary origin of any stratigraphic dolomite is 

open to question and with good reason (for recent discussions see Cloud and Barnes, 1957, p. 182-186 and 

Fairbridge, 1957). In the laboratory, calcite is readily 

deposited from sea water or similar solutions under 

conditions expectable in the ocean, but dolomite is not 

(VanTuyl, 1916, p. 297-306). In several warm shallow parts of the open ocean (for example, the Bahama 

banks as discussed by Smith, 1940) limestone has been 

found now forming as a chemical and organo-chemical 

precipitat.e, but dolomite has not. (It has, however, 

been reported as a "definite authigenic mineral" from 

the North Atlantic Ocean, both in nearshore deposits 

in the Gulf of Guinea and in pelagic deposits in the 

Cape Verde Basin, by Correns (1939, p. 385), and 

dolomite is forming today on a large scale in coastal 

saline lakes and a nearby shallow inlet of the sea in 

the south-east Province of South Australia (Alderman 

and Skinner, 1957). It is plain, too, that dolomitization has occurred in many mottled carbonate rocks 

and on a large scale. Further, as Rubey (1952) has 

reasoned, there have probably been no major shifts in 

the composition of ocean water since far back into the 

Precambrian, so that it is hard to visualize physicochemical changes in the open ocean small enough to be 

reasonable but large enough to cause the deposition of 

vast amounts of primary dolomite rather than calcite. 

It becomes tempting, then, to push the problem of 

stratigraphic dolomite into the postdepositional history of the rock, though not too far; a penecontemporaneous origin is often postulated, for reasons well 

expressed by Blackwelder long ago ( 1913, p. 619-624) 

based largely on volume considerations. But to decide, as did Blackwelder, and Daly before him (1909, 

p. 170) and many others after, that dolomite has 

formed on a vast scale by wholesale replacement of 

part of the calcium from calcium carbonate "jelly" 

just after precipitation and ~efore deep burial is 

merely to veil the problem: the old questions are still 

there, and some new ones. How did the magnesium, 

evidently not concentrated enough to precipitate when 

the calcite jelly was deposited, become sufficiently concentrated to displace part of the calcium merely by 

being covered with a little very wet sediment? The 

rise of temperature attending burial could not be expected appreciably to modify the relative solubilities 

of dolomite and calcite. Did the jelly selectively remove magnesium by sorption from the water that was 

being squeezed out of the accumulating sediment? 

Very likely the controlling mechanism was not the 

ordinary solubility product. Perhaps such agencies as 

microorganisms or natural earth-currents significantly 

modified the chemical relationships of calcium and 

magnesium below the depositional interface.. If the 

magnesium trapped in the jelly was simply the usual 

magnesium of average sea water, how have any chemical marine carbonates escaped dolomitization? If the 

substitution of magnesium for calcium happens so 

readily, how is magnesium sometimes saved from interaction long enough to produce dolo~ite mottles 

later in the lithification process, as in/ much of the 

Meagher limestone? 

And what becomes of the replaced calcium? In 

this part of the problem there is a little negative field 

evidence. In the great stratigraphic dolomites calcite 

is not often trapped in the rock as cement, it does not 

form large or numerous veins, and it does not, apparently, return to the ocean floor to produce limestone 

interbeds in a volume proportional to dolomite. Perhaps the freed calcium promptly reacts with pore 

water to form new calcite that in turn reacts with rest 

magnesium, on an ever-decreasing scale like the mirrors in Flemish paintings. 

Other queries arise but the point is made without 

them : ignorance about the · mechanism of dolomite 

formation is profound. Under the circumstances, the 

thesis of direct deposition of stratigraphic dolomite 

need not yet be abandoned. So little is known about 

the chemistry of the carbonates, especially in their 

natural habitat,. that current chemical inferences about 

the origin of dolomite need not be taken as conclusive. Further, the fact that dolomite is not forming 

in today's oceans may not be especially relevant. An 

important point in this connection, noted first, perhaps, by Tarr ( 1920), is that the great dolomite masses 

of the Paleozoic were not deposited in the open ocean, 

but rather in bodies of salt water which may have had 

limited connection with the ocean. The dolomite now 

forming extensively in coastal South Australia (Alder-
24 GEOLOGY OF TIIE THREE FORKS QUADRANGLE, MONTANA 

man and Skinner, 1957) is an impressive if rare 

modern example. Epeiric seas like those in which the 

great stratigraphic dolomites were deposited do not 

exist today. The closest parallel might be along the 

northwestern coast of Australia, on -the Sahul shel:f 

and related shelves, which is little known with respect 

to sedimentation (see Fair bridge, 1953, for summary 

of present knowledge). 

The direct physical evidence on record, both microscopic and macroscopic, is that dolomite deposits share 

every feature of limestone deposits though perhaps 

not in the same proportions. In the only exhaustive 

published investigation of the petrography of interbedded limestone and dolomite rocks known to me, 

Sander (1936) found that in the rhythmically bedded 

Triassic carbonate rocks of the Alps every fabric developed is present in both limestone and dolomite, and 

that, as far as the petrographic evidence goes, dolomite 

may form in all the ways that calcite may. 

DEVONIAN AND LOWER MISSISSIPPIAN 

The Devonian rocks, 600-1,000 feet thick, are separated into three formations: The Maywood formation, 

at the base; the J eft'erson dolomite; and the Three 

Forks shale. Some of the unfossiliferous beds at the 

base of the Maywood as mapped are conceivably preDevonian, and near the top . of the Three Forks shale 

beds containing a Late Devonian fauna are transitional with and cannot be mapped separately from 

beds containing an Early Mississippian fauna. Thus, 

neither the upper nor lower limit of the Devonian 

section is accurately located, but the uncertainty involves only a few tens of feet of strata at each end. 

The Devonian rocks are about 50 percent dolomite, 

20 percent dolomitic limestone and limestone, 20 percent shale, and 10 percent siltstone and sandstone; the 

clastic ratio is about 0.4. Included high in the section 

is a small thickness of evaporite beds, the oldest such 

deposits in the quadrangle. 

Although the Devonian has not received nearly so 

much attention as the Cambrian, regional treatment of 

Devonian stratigraphy has been given by Sloss and 

Laird (1947) with emphasis on correlation, and by 

Andrichuk (1951) with emphasis on the sedimentational environment. 

MAYWOOD FORMATION 

Assigned to the Maywood formation (Emmons and 

Calkins, 1913, p. 64-65; age emended from Silurian ( ? ) 

to Devonian after Lochman, 1950, p. 2213) are poorly 

exposed yellow, orange, and red calcareous clastic rocks 

locally grading upward into, and interbedded with, 

light-colored limestone that occupies a narrow grassy 

valley or saddle between the upper dolomite ledge of 

the Pilgrim limestone and the ledge-forming brown 

dolomite· at the base of the J eft'erson dolomite. In the 

13 linear miles along which this stratigraphic interval 

has been traced, outcrops and float fragments suggest 

that it is occupied entirely by brightly colored silt-_ 

stone and mudstone for about 9 miles, and that limestone appears in the upper part of the interval along 

about 4 miles. 

The interval is markedly thicker and contains much 

more limestone in the Milligan Creek and Mud Spring 

areas. In these areas its thickness commonly ranges 

from 30 to 100 feet, tl~ough it is missing for about 

half a mile in sec. 30, T. 3 N ., R. 2 E., and swells to 

about 200 feet in the southwest corner of sec. 31, 

T. 3 N., R. 2 E. In the Hossfeldt Hills area it is confined to the crestal zone of the anticline (sec. 20, 

T. 3 N., R. 2 E.). On the upright limb to the west 

of the crest the sequence i~ persistent but less than 

50 feet thick, and lacks prominent limestone beds; on 

the overturned limb it is absent, perhaps due to squeezing rather than to nondeposition or erosion. In the 

Willow Creek area the Maywood is missing for a mile 

west of the Principal meridian and is generally less 

than 30 feet thick elsewhere; limestone is virtually 

absent. 

In only two places is the entire sequence well exposed. The best exposure is one-third. mile east of 

Milligan Creek on the west wall of the only large 

canyon in the NE1/4NE1/4 sec. 1, T, 1 N., R. 1 W., 

where 60 feet of these beds appear in a small cliff 

(measured section E, pl. 2). The sequence may conveniently be summarized as: Thickne&& (feet) 

Limestone and siltstone. Thin-bedded fossiliferous yellowish and bluish-gray limestone and orange calcareous 

siltstone___________________________________________ 20 

Mudstone and limestone. Alternating beds 1-3 feet thick of 

calcareous mudstone, mottled in orange and . red, and 

medium-gray limestone ____________ .,_________________ 15 

Mudstone and fine sandstone. Beds 1-4 feet thick, calcareous, vuggy; orange with scattered reddish mottling__ 25 

The clastic beds throughout are much alike and there 

are no overt breaks in the sequence. 

The . second good . exposure is 1113 miles northeast of 

the first; in SW'-4NW'-4 sec. 32, T. 2 N., R. 1 E. Here, 

on the north ·wall of ·the eastward-draining gulch at 

the south base of hill 4 797, the interval is 30 feet thick. 

The upper 12 feet consists of thinly interbedded gray 

limestone and yellowish-gray shale. The -lower 18 feet 

is yellowish-gray siltstone and shale, mottled in 

orange, with 1 foot of dark-purplish-gray shale at the 

top. These rocks, though broadly similar to those in 

the first exposure, are markedly harder, more fissile, 

and more somber, perhaps due to differences in weathering rather than in initial sedimentation. 
SEDIMEN.Tt RY ROCKS 25 

In the area of maximum thickness (sec. 31, T. 3 N., should be used even for Cambrian rocks above the 

R. 2 E.), the upper half of the sequence is almost Pilgrim is debatable. (See Lochnian, 1950, p. 2204-

entirely thick-bedded light-yellowish-gray limestone, 2205.) 1 

and the lower half, poorly exposed, seems to be inter- Above the Dry Creek shale and below the black 

bedded brownish-gray limestone, and yellowish-brown carbonate rocks of the Jefferson formation, Peale 

and yellowish -gray siltstone and shale. Scattered in ( 1893, p. 24-25) recognized an even later Cambrian 

the lower half are thin lenses of conglomerate, the unit, the "Pebbly limestones" (member of the Gallatin 

pebbles of which are red siltstone. formation). This is the only stratigraphic unit of 

The contact relations of the Maywood formation Peale that might be applied to the limestone parts of 

are not wholly clear. The unit as mapped overlies the the Maywood. There is little lithologic similarity bePilgrim limestone with erosional unconformity. The tween Peale's description and the limestone of the 

upper contact with the Jefferson dolomit~ is rarely Maywood, and it is more likely that Peale simply did 

exposed and then only where there is limestone in the not see any of the rare outcrops of Maywood limestone 

Maywood. At such places the contact is conformable. in his widely spaced traverses. This is not to deny 

In the few places where light-colored limestone alter- the existence of any Cambrian rocks equivalent to the 

nates with dark dolomite near the base of the J effer- Pebbly limestones, as Deiss ( 1936) did. A more 

son, the contact might even be called gradational. The reasonable interpretation (Sloss and Laird, 1947, p. 

local absence of Maywood between the Pilgrim and 1409) is that the Pebbly limestones of Peale exist, at 

the Jefferson may indicate overlap of the sea during least on Dry Creek where Peale measured them, but 

Jefferson time on pre-Devonian topographic high are lacking in many other places including the Three 

places not buried by the Maywood, but could be due Forks quadrangle owing to pre-Maywood erosion. 

to local or widespread post-Maywood pre-Jefferson 'Vhile assigning the entire sequence to the Maywood, 

erosion due to uplift. Conformity with overlap seems is reasonable, it may oversimplify the situation. The 

the better explanation. limestone-bearing upper parts of the interval are all 

On the map the upper contact where not exposed is no doubt part of the same depositional sequence of 

arbitrarily drawn at the break in slope between the Late (?) Devonian age. It is possible, however, that 

lowest outcrop of fetid dolomite, regarded as Jefferson, in places the clastic rocks of the sequence are partly, 

and the highest appearance of red, orange, or yellow or even wholly, of Cambrian age and that an unclesiltstone float. tected irregular erosion surface separates similar rocks 

For measured sections of the Maywood, see pages of two systems of widely different age. An extreme 

128, 131, and 134. interpretation is that only the prominent limestones 

AGE, NAME, AND coRRELATION are Devonian and that the bp.lk of these rocks is CamThe only identifiable fossil from the Maywood are 

brachiopods "similar to the genus Allanaria * * * most 

likely of Late Devonian age" (A. J. Boucot, written 

communication, Apr. 14, 1955) recovered from the 

limestone beds 12-16 feet below the top in the exposure one-third of a mile east of Milligan Creek 

(loc. 74 [D38 (SD) ], p. 2). As all the beds assigned 

to the Maywood seem to reflect unbroken deposition, 

they are presumably of the same age. Pending better 

faunal evidence, the age is tentatively considered 

Late ( ? ) Devonian. 

The clastic rocks of this sequence are those designated Dry Creek shale (member of the Cambrian 

Gallatin formation) by Peale ( 1893, p. 24; 1896, 

p. 2), but this name is not well applied to rocks with 

Devonian fossils. This is not to deny the existence of 

Peale's Dry Creek shale, for Cambrian fossils have 

been found in beds of Dry Creek lithology above the 

Pilgrim limestone nearby (Deiss, 1936, p. 1312 and 

1316; McMannis, 1955, p. ·r395; Klepper, 1950, p. 32; 

Klepper and others, 1957, p. 13). Whether the name 

brian. If this is so, pre-Jefferson Devonian rocks may 

be confined to the upper plate of the thrust system in 

the Three Forks quadrangle. 

ORIGIN 

I The Maywood formation is apparently marine, but 

both the texture and color of its clastic rocks reflect a 

I 

geologic history different from that of. the Cambri~n 

marine clastic Wolsey and Park formatiOns. The d1f-

' ferences may date from the time of deposition or may 

be the result of weathering long after deposition. 

'Vhether or not any Dry Creek equivalents occur in 

I 

the quadrangle itself, fine-grained red, orange, and 

yellow clastic rocks of Late Cambrian age are ~e-

' spread nearby. Probably, as Lochman (1950, 1957) 

suggested, the Dry Creek was deposited as gray and 

green shale like the Park and Wolsey formations. 

I 

Impersistent shales in the Dry Creek, much like those 

in the Park and Wolsey, crop out in the Toston quad-

' rangle and were also reported by . McMannis (1955, 

p. 1395) and by Sloss and Laird (1947, p. 1411-1413). 
26 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

Later the Dry Creek was partly eroded and deeply 

weathered to its present colors and textures during 

continuous or recurrent emergences 'in the well-known 

stillstand in the region during Ordovician, Silurian, 

and earlier Devonian time. Reworking of the weathered Dry Creek strata by the transg~ssing later 

Devonian sea may have produced the clastic beds of 

the Maywood. As the shoreline retreated, in a direction as yet unknown, limestone deposition gradually 

prevailed over clastic deposition. Such a history could 

explain the similarity between the clastic facies of the 

Dry Creek and Maywood despite their great difference 

in age. 

It is possible, of course, that the similarity is fortuitous, and that the Maywood clastic deposits were 

derived from other, more remote, sources. A reconstruction of pre-Middle Devonian paleo geology by 

Sloss (1950, fig. 6) indicates that non-Cambrian rocks, 

mostly Ordovician, were exposed in pre-Maywood 

time within a hundred miles of Three Forks to the 

west, south, and east, but not to the north. These 

rocks to the south and east are almost wholly carbonate 

(Big Horn dolomite) unsuitable as clay sources. The 

Ordovician of central Idaho, however, contains a high 

proportion of suitable source material (Saturday 

Mountain and Kinnikinic formations of Ross, 1934). 

JEFFERSON DOLOMITE 

A thick sequence of dolomite and limestone assigned to the Jefferson dolomite (Peale, 1893, p. 27-

29) crops out in all the areas of pre-Tertiary rocks. 

The lower part of the Jefferson, except where dips 

are nearly vertical, makes the highest of the series of 

cliffs that begin in the Cambrian limestones; the upper 

part of the formation descends in a long dip slope, 

broken by low ledges, to a broad valley under lain by 

Three Forks shale. 

The Jefferson is a distinctive formation although 

it varies considerably in lithologic detail and in thickness. The dominant rock type is rather coarsely 

crystalline sugary fetid dark-brown dolomite in beds 

a few feet thick. The color and odor are due to hydrocarbons in films on individual grains and in tiny 

interstitial pellets. At the top of most sections is 

distinctive light-colored coaTse-grained dolomite 30-

60 feet thick. Typical sections also include many 

thick to thin beds of black dolomite, of yellowishgray dolomite, and of gray limestone and dolomitic 

limestone that are fi:n.er in grain than the dominant 

dolomite type and that tend to form ledges. 

Widespread, too, though of small volume are layers 

of dolomite pebble conglomerate, pebble breccia, and 

"spaghetti beds." The pebble conglomerate typically 

consists of discoidal subrounded pebbles of dark sugary 

dolomite, generally black or brown, but rarely red, in 

. a lighter colored matrix of finer grained dolomite ·or 

dolomitic limestone. The pebbles are oriented parallel to bedding and generally do not touch but float 

in the matrix. Much less common though widely 

present are thin beds with pebble-size. fragments so 

angular that the rock must be termed breccia. The 

pebbles typically are of dark dolomite and are 

crowded together with only rude orientation in a 

light-colored limestone matrix. Possibly the conglomerate and breccia grade into each other but intermediate types were not seen. The "spaghetti beds" 

consist of closely packed slender tubes, about lf2 inch 

long and 1/20 inch across, of grayish-yellow finely 

crystalline calcite imbedded in a brown dolomite 

or dolomitic limeston~ matrix of coarser grain. The 

tubes are patently organic, but in the few thin sections 

seen, any cell structure that they may have had has 

been obscured by recrystallization. The "spaghetti" 

structure has been variously interpreted as representing algal colonies, worm burrows, and' poorly preserved corals of the genus Oladopora. According to 

Helen Duncan (written communication, June 28, 

1956) however, the "spaghetti beds" of the Jefferson 

were probably formed by dendroid stromatoporoids 

of the genus Amphipora (Lecompte, 1952, p. 321-331, 

pis. 67-70). 

The proportions of limestone, conglomerate, and 

"spaghetti" beds vary widely. Limestone is markedly 

more abundant in the lower part of the formation in 

the Milligan Creek and Willow Creek areas, but in 

the Mud Spring area it is more abundant in the upper 

part. Conglomerate and "spaghetti" beds are rare in 

the Milligan Creek and Mud Spring sectors, but 

prominent in the Willow Creek area. 

The Jefferson dolomite is thickest in the Milligan 

Creek area, where it averages about 600 feet. Apparent thicknesses are locally as great as 750 feet, but 

in such places some duplication l;>y faulting is suspected. 

The typical section in this area includes: 

Thickness 

(feet) 

Grayish-yellow thick-bedded dolomite___ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 50 

Interbedded grayish-yellow dolomite and yellowish-brown 

dolomite, base forms ledge___________________________ 300 

Gray limestone thinly interbedded with brownish-gray 

dolomite, poorly exposed __ _:_ __________________________ 200 

Dark-yellowish-brown dolomite, forms ledge______________ 50 

Limestone interbeds are considerably more abundant 

east of Milligan Creek than west. Zones of coarse 

solution breccia, widespread in the Jefferson elsewhere, are prominent in the Three Forks quadrangle 

only in the northeast side of the Milligan Creek sector, especially sec. 29, T. 2 N., R. 1 E. 
SEDIMENTARY ROCKS 27 

In the Mud Spring area the formation seems much 

thinner, averaging around 400 feet, and less magnesian, but here the J effer.son is much affected by 

folding and faulting, and the differences may be more 

apparent than real. Furthermore, the formation has 

been widely altered by igneous intrusions in the northeast part of the Mud Spring area. Near the 10N 

pluton in sec. 25, T. 3 N., R. 1 E., the Jefferson is 

bleached to pale tones of yellowish gray and is not 

fetid. On the other hand the Jefferson in the high 

ridge east of this intrusive is unusually dark and 

petroliferous, probably because of condensation of 

hydrocarbons distilled from the formation close to 

the intrusion. This sensitivity to intrusive igneous 

rocks is apparently a regional characteristic of the 

Jefferson (see Emmons and Calkins, 1913, p. 66 ; Klepper and others, 1957, p. 15; Calkins and Butler, 1943, 

p. 20). 

Seemingly straightforward sections in the Mud 

Spring area (SEcor., sec. 36, T. 3 N., R. 1 E., and NE 

cor., sec. 1, T. 2 N., R. 1 E.) show: 

Thickness 

(feet) 

Thick-bedded yellowish-gray and yellowish-brown dolomite 

(2/3), and yellowish-gray limestone, spottily dolomitic 

(1/3), poorly exposed on dip slope ____________________ 300 

Thick-bedded dark-yellowish-brown dolomite, upper part 

forms ledge ___________ ______ _____ _____ ,... _ _ _ _ _ _ _ _ _ _ _ _ _ 100 

In the Hossfeldt Hills the Jefferson appears only 

near the crest of the overturned anticline. Exposures 

are poor, but the formation seems to be about 500 feet 

thick and not notably different from the 11ud Spring 

area. 

The Jefferson dolomite in the Willow Creek vicinity 

is comparatively thin, rarely exceeding 350 feet, and 

typically consists of: 

Thickness 

(feet) 

Grayish-orange and yellowish-gray thin- to thick-bedded 

dolomite _________________ _____ ___ ____________ __ ____ 100 

Yellowish-brown and brownish-gray thick-bedded dolomite 

with "spaghetti" and pebble congl_omerate beds; forms 

ledge ____________________ _____ _________________ ___ _ 100 

Orange dolomitic limestone with pebble conglomerate beds, 

poorly exposed______ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 50 

Olive to brownish-gray thick-bedded dolomite, forms ledge_ 50 

Yellowish-brown dolomitic limestone with many chert 

masses, poorly exposed_ __ ___ ________ ___________ __ ___ 50 

Although the Jefferson dolomite as a unit is distinctive and easy to recognize, similar rock types occur sparingly in other formations. For example, in 

the Willow Creek sector, and also east 'of the Lombard thrust in the neighboring Toston quadrangle, 

the upper part of the Pilgrim is dark, coarse grained, 

and fetid with many twiglike bodies. Dark, fetid, 

dolomitic beds also occur locally in the Meagher limestone and in the Lodgepole limestone of the Madison 

group. Consequently, strata should not be assigned 

to the Jefferson on the basis of single small exposures, 

however '"typical" they might appear. 

The contact with the overlying Three Forks shale 

is sharp but conformable. Where exposures permit, 

it is placed between the light-colored thick-bedded 

dolomite at the top of the Jefferson and the thinbedded orange siltstone and limestone at the base of 

the Three Forks. This is generally close to the valley 

bottom, for stream courses tend to follow the unresistant and intricately fractured lowest beds of the 

Three Forks. Where the contact is not exposed it is 

arbitrarily placed at the foot of the dip slope on the 

uppermost Jefferson ledge. 

Detailed measured sections of the Jefferson are given 

on pages 127-128, 130-131, and 134. 

NAME, AGE, AND CORRELATION 

In the more casual usage of his day, Peale (1893) 

ref~rred to these rocks variously as Jefferson forma- · 

tion and Jefferson limestone although the chemical 

analyses he quoted show the rock to be dolomite. The 

term "Jefferson formation" is widely used, but because dolomite is the dominant rock type in the Three 

Forks . quadrangle and regionally, I prefer the name 

Jefferson dolomite. 

Except for the Amphipora beds, and a few recrystallized corals, fossils were not recognized in the 

Jefferson dolomite. The formation is lean in fossils 

everywhere, a feature no doubt closely related to its 

high magnesian content. Despite this, its age is regarded as definitely Late Devonian, for the underlying Maywood formation has a probable Late Devonian 

fauna and the overlying Three Forks shale has a rich 

Late Devonian fauna. The presence of Amphipo"n,f, 

according to Helen Duncan (written communication, 

June 28, 1956}, permits but does not demand a Late 

Devonian age: 

* * * [Amphipora] are a characteristic feature of the upper Middle and lower Upper Devonian dolomites in western 

United States. In the Great Basin, banks of Amphipora are 

such distinctive · lithologic features that they are widely referred to as "spaghetti beds." Similar beds have not been described in the older literature on the Jefferson of southwestern Montana, but Jean M. Berdan and I made a point of looking for Amphipora when we examined the Logan section in 

1951. We found several impersistent beds of this fossil in 

the Jefferson at that locality and have also identified Amphipora in samples collected from the Jefferson near Melrose 

in Silver Bow County, and in the Madison Range, Ennis 

quadrangle. Amphipora beds probably are far more widely 

developed in the Jefferson of the region than collections in 

hand indicate. 

Ordinarily these fossils were not collected because, until 

recently, most American geologists and paleontologists thought 

they were corals (especially Cladopora), sponges, bryozoans, 
28 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

or some other organism too poorly preserved for identification 

and therefore of no value for age determination. In spite of 

their unpromising appearance, specimens rarely are too thoroughly recrystallized for generic identification. The general 

prevalence of Amphipora in calcareous facies of the Devonian 

all ovm• the world makes this one of the more useful fossils 

in stratigraphic work. 

The Late Devonian age of the Jefferson is accepted 

by most workers (see Devonian correlation chart of 

Cooper and others, 1942; Sloss and Laird, 1947) .although both Berry (1943, p. 10-14) and Verra11 6 favor 

late Mid~le Devonian age for the lower part of the 

Jefferson dolomite in this vicinity. 

According to Sloss and Laird (1946; and 1947, p. 

1407-1410) "'* * * at its type locality near Logan, 

Montana and elsewhere, the Jefferson is divisible into 

an upper dolomite member and a lower limestone 

member." The two members, however, have not been 

recognized in the Three Forks quadrangle, ·.in the 

Bridger Range (McMannis, 1955, p. 1397), in the Elkhorn Mountains (Klepper and others, 1957, p. 14), or 

even in the southern Horseshoe Hills that include 

Logan (Verrall) . 

7 On the other hand, the lower limestone unit is reported in sections measured in several 

areas by Sloss and Laird, and by Andrichuk (1951, 

p. 2371-237 4) ; Klemme 8 noted that limestone dominates the lower part of the Jefferson in some parts of 

the Sixteen Mile Creek area (Toston and Maudlow · 

quadrangles) but is subordinate in other nearby parts; 

th~e formation is dolomitic limestone in the Canyon Ferr-y quadrangle (Mertie, Fischer, and Hobbs, 

1951, p. 25) and far south in the Gravelly Range 

(Mann, 1954, p. 10) it is described as limestone 

throughout; and in the Three Forks quadrangle limestone is an important component of the lower Jefferson in the hills east of Milligan Creek, though a minor 

component west of the creek. These observations 

suggest that the Jefferson is characterized by rapid 

lateral changes in the relative propqrtions of limestone and dolomite, and that dividing the formation 

in the Three Forks region into limestone and dolomite members is at best of strictly local value. 

The boundaries of the Jefferson dolomite chosen 

in this report are believed to be those of Peale ( 1896, 

p. 2), who described the Jefferson as "black or mudcolored limestone, which is generally crystalline and 

magnesian from top to bottom." 'There is room, however, for other choices in placing the upper boundary 

of the Jefferson, as Peale's description of the rocks 

near the contact with the Three Forks shale is somewhat cloudy and his reported thicknesses are hard to 

6 See footnote, p. 11. 7 See footnote, p. 11. 

s See footnote, p. 13. 

confirm. Thus, Sloss and Laird (1947, p. 1409-1410) 

felt impelled to assign to the Jefferson the unresistant 

red, yellow, and orange "breccias and associated 

shales," about 100 feet thick, above the massive dark 

dolomite, despite their closer physical affinity to the 

clastic beds of the Three Forks, on the grounds that 

to do otherwise would violate Peale's original classification. This approach led Sloss and Laird to report significantly greater thicknesses of Jefferson formation than those stated here, and correspondingly 

smaller thicknesses of Three Forks shale. 

ORIGIN 

The Jefferson dolomite is a marine deposit probably laid down in shallow water subject to considerable current action, but far from sources of land-derived sediment. In thickness and in many lithologic 

detail~ the Jefferson is similar to the Meagher and 

Pilgrim limestones, but in several respects it is strikingly different and must have had a correspondingly 

divergent origin or history. The most vivid differences are the sugary texture, the rich content of hydrocarbons reflected in color and odor, and the color 

contrasts between limestone and dolomite. 

The sugary texture may result from recrystallization, but more likely it is a clastic fabric due to reworking of preexisting dolomite. A clastic origin 

is favored by the occasional presence of cross-stratification and channeling in the sugary dolomite beds. 

The sugary texture and the rich content of hydrocarbons may be related, for in general the most petroliferous parts of the formation are the most sugary. 

There are exceptions, however, particularly in the 

upper part of the formation where many beds of 

sugary dolomite are very pale and poor in hydrocarbons. If texture and hydrocarbon content are related, 

the relation probably is epigenetic and structural 

rather than syngenetic. Very likely the volatile hydrocarbons, whatever their origin and initial distribution in the sediment, have tended to migrate during 

lithification (and possibly tectonism) into the porous 

sugary parts. of the formation. 

Surprisingly, the Jefferson has little of the mottling 

so widespread in the Cambrian carbonate rocks; in-

. dividual beds retain their color, texture, and composition for long distances. Whether the individual bed 

is limestone, dolomitic limestone, or dolomite, it seems 

to have reached a stable composition during or soon 

after deposition, and was only locally subjected to 

postlithification attack by migrating dolomitizing solutions. Although homogeneous fabric in dolomites may 

mean that the metasomatic process has gone to completion, this is questionable where intermediate products _of the , process are rare or lacking. 
SEDIMENTARY ROCKS 29 

The "spaghetti beds" offer some evidence on the 

problem of dolomite origin. The stromatoporoid tubes 

are porous and are finer grained than their dolomite 

matrix. Had dolomitizing solutions entered the rock 

after deposition, they would presumably have ent.ered 

the tubes, but the "spaghetti" is wholly calcite. This 

suggests that the dolomite of the matrix was not 

produced by postdepositional reactions but was already 

dolomite when it enclosed the Amphipora tubes. The 

point is weakened, however, by the possibility that the 

sugary dolomite is a clastic rather than a direct ·chemical deposit. 

Beales (1953) studied the widespread dolomitic 

mottling in the Palliser limestone in Alberta, which 

resembles the Jefferson and is also of Late Devonian 

age. In beds exactly like those here called "spaghetti 

beds" (see his fig. 4), tubes which Beales ~scribed to 

worm burrows and algal colonies are dolomite, and 

served as centers from which dolomitizing solutions 

worked outward into the surrounding limestone, just 

the reverse of conditions at Three Forks. The ready 

conclusion at this time is that stratigraphic dolomites 

can form in more than one way. There seem to be 

stratigraphic dolomites and stratigraphic dolomites. 

The origin of a rock mass which is here limestone 

and there dolomite, with a zone of mottling or intercalation or imperceptible chemical gradation between 

can be approached in the same way as the origin of 

granite: as a problem in contact relations between 

initially unlike materials, or as a problem in the transformation of a single original material. The dramatic 

appeal of transformation. or replacement hypotheses 

and their current popularity makes it easy to neglect 

other possibilities, but the state of knowledge does not. 

At this writing, it is at least as reasonable to visualize 

simultaneous deposition of some limestone and dolomite in separate semiconnected parts of a sea, with 

both lateral and vertical gradations between due to 

shifts of chemical and physical variables, as to picture 

all stratigraphic dolomite as due to postdepositional 

replacement of limestone. 

The thick, extensive, and well-exposed Devonian 

and Cambrian carbonate formations of the region 

around Three Forks are well suited for detailed study 

of the dolomite problem. I suspect, though, that field 

studies, however detailed and supported by petrographic and petrochemical data, will bring little progress unless further supported by experimental data. 

In whatever ways dolomites form, it must be under 

conditions well within the means of modern laboratories to duplicate. Is it not time to try again to make / 

laboratory analogs of stratigraphic dolomite~ 

654729 0--63----3 

The differences in detail between the Jefferson and 

the Cambrian carbonate formations may be related 

to gross differences in the environment of deposition. 

Regional studies indicate that the Cambrian limestones pass shoreward into normal marine clastic rocks. 

Much of the Jefferson, on the other hand, grades, at 

least northward from the Three Forks area, into 

thick evaporite sequences (Potlatch anhydrite of 

P~rry, 1928, and equivalent units) and the Jefferson 

in the Three Forks 1 quadrangle is conformably succeeded by evaporitic rocks. If shoreline environments 

were in marked contrast, then offshore depositional 

conditions probably differed too, but how is not yet 

known. 

THREE FORKS. SHALE 

The Three Forks shale (Peale, 1893, p. 29-32), a 

highly varied assemblage of clastic and carbonate 

rocks, appears in all the areas of pre-Tertiary rocks. 

Composed mainly of clay shale, the formation is unresistant and forms deep valleys between ridges of 

Jefferson dolomite and Lodgepole limestone. Outcrops are rare, but the formation is nevertheless one 

of the most readily recognized in the region, even 

when faulted out of its normal stratigraphic position, 

because of its unique succession of rock types artd its 

unique richness in invertebrate fossils, both apparent 

from float where outcrops are lacking. 

Throughout the quadrangle, the Three Forks consists of a lower unit of thin-bedded, crumpled, and 

brecciated orange limestone and siltstone, a medial 

unit of shale with many thin limestone beds, and an 

upper unit of flaggy orange siltstone, but the thickness and lithologic detail of each of these units range 

widely. Generally, a valley bottom follows the lower 

brecciated unit, a gently rolling grassy slope marks 

the medial shale, and a steep slope is developed on the 

upper unit, partly because it is more resistant and 

partly because it is protected by the basal ledge of 

Lodgepole limestone. 

In the Milligan Creek area the formation is exceptionally well exposed, particularly along Dry Hollow. 

A generalized section in the Milligan Creek vicinity 

where the Three Forks is 300--400 feet thick comprises: 

Thickness 

(feet) 

Thin- to thick-bedded orange siltstone and sandstone _- --- 50 

Thinly interbedded dark-green shale and dark varicolored 

limestone with some layers of orange siltstone in upper 

part _____ ____ ___ _________ ___ ____ __ ________ ___ ______ 100 

Olive shale with subordinate thin beds of orange siltstone_- 150 

Thin-bedded orange and yellow limestone and mudstone, 

brecciated and contorted _________ ----- ____ ---- - ----- 50 
30 GEOLOGY OF THE mREE FORKS QUADRANGLE, MONTANA 

The basal breccia is riddled with cavities; its fragments are generally no more than a few inches long, 

angular to subangular, and spottily cemented by a 

carbonatic clay. It grades laterally into and is interbedded with crumpled beds of similar color and composition, with contortions of wavelength and amplitude ranging from a 'few inches to several feet. Red 

layers were not seen in outcrop, but bits of red mudstone appear sporadically in the float from this interval and red soil is developed on it. 

The gree;n and olive shale that makes up the bulk 

of the formation is almost all calcareous clay shale, 

with little of the mica and glauconite that characterize 

the Cambrian green shales. The dominant green and 

olive types grade locally into black, purple, brown, 

and orange varieties, presumably related to local concentrations of organic and ferriferous material. Black 

shale is especially common in the green-shale interval 

in sec. 31, T. 2 N., R. 1 E., east of Milligan Creek, 

where many prospect pits have been dug, apparently 

in the hope of finding coal or possibly oil shale. Black 

shale is also rather common just west of the Lombard 

thrust in the Mud Spring sector. 

All the shale varieties are locally rich in fossils, 

especially small thin-shelled brachiopods; a few dark 

shale beds, lean in brachiopods, contain nautiloid and 

gonia.tite cephalopods. The brachiopod shells are 

almost entirely carbonate, with fillings of finely crystalline carbonate or of shale, whereas nearly all the 

cephalopod tests are replaced by dark-reddish-brown 

to black hematite with fillings of hematite, or, rarely, 

of pyrite. Many of the replacements are beautiful 

pseudomorphs with smooth shiny surfaces. More 

commonly, though, replacement has gone beyond the 

pseudomorph stage to yield, first, shells partly crusted 

with crystals of hematite, and finally reniform masses 

of hematite as much ~s 3 inches long, with rough surface coatings of tiny cubes and octahedra of hematite. 

The cores of these masses are gen.erally hematite and 

hydrous iron oxides, or, uncommonly, finely crystalline 

pyrite. Associated with the fossil pseudomorphs and 

nodules are cubes and octahedra of hematite as much 

as half an inch in diameter, and small clusters of such 

crystals. Evidently the crystals, and the hematitized 

fossils too, were formerly pyrite, now replaced by 

hematite and limonite. 

These occurrences are not confined to the Three 

Forks shale of the Thr~e Forks quadrangle, as "pyritized fossils, chiefly cephalopods" have been collected 

from the adjoining Toston quadrangle (Haynes, 1916a, 

p. 16) and "pseudom6rphic crystals of limonite after 

pyrite are common" in the Spokane Hills 50 miles to 

the north (Mertie, Fischer, and Hobbs, 1951, p. 26). 

The interbedded siltstone is usually a few inches thick 

and contains about equal amounts of angular grains 

of quartz and rhombs of calcite. The siltstone is 

porous but is· firmly cemented with iron-stained carbonatic clay. 

The limestone beds intercalated with the clay shale 

are generally less than a foot thick, are colored in 

dark tones of red, brown, and gray, and are largely 

made up of fossil :fragments and calcite sand in a 

matrix of clayey · finely crystalline limestone. Some 

are so fossiliferous as to be classed as coquina. As in 

the shale, the dominant type of fossil material is small 

thin -shelled brachiopods but the limestone is rich also 

in larger, thicker shelLed, more tumid brachiopods, as 

well as bryozoan and crinoid fragments; cephalopod 

remains are sparse. Widely scattered in both shale 

sequences are lenses a few inches to a few feet thick 

of yellow and orange limestone breccia, like that in 

the basal unit. 

In the upper 30 to 50 feet of the green shale sequence, siltstone beds like those in the olive shale 

interval become increasingly common, thicker, and 

coarser in grain, eventually grading into the upper 

siltstone and sandstone unit. This upper unit ranges 

widely in thickness; just east of Milligan Creek (center 

sec. 36, T. 2 N., R. 1 W.) it is nearly 100 feet thick, 

and at the east edge of the Milligan Creek sector it 

is 76 feet thick, but it is much thinner between these 

areas, and also thins west of Milligan Creek, so that 

it is less than 30 feet thick at the west edge of the 

quadrangle. These changes may be due not so much 

to decrease in the total amount of siltstone in the 

upper part of the formation as to interruption of the 

light siltstone succession by interbeds of dark shale. 

The upper siltstone and sandstone is mineralogically 

much like the siltstone lower in the formation, composed of about half quartz and half calcite, but individual beds tend to be thicker and .the average grain 

. size is greater' with many beds in the fine sand-very 

fine ::;and range. As is generally true of rocks containing both silt and sand fractions, the sand is better 

rounded than the silt. Both cross-stratification and 

graded bedding are rare. Most of these beds lack 

fossils, but a few contain assemblages dominated by 

large tumid thick-shelled brachiopods. 

_ ·The Three Forks i~ intruded by andesite and diorite 

sills east of Milliga~ Creek. ·. The sills plainly do not 

replace but displace the shale, 'for the stratigraphic 

·interval between the J e:fferso:ri dolomite and the Lodgepole limestone increases to m·atch the thickness of the 

sills. The shale is unaffected by the sills, except 'for 

slight hardehing and reddening within a few feet of 

an intrusive contact; the upper siltstone is locally 

bleached and epidotized. 
SEDIMENTARY ROCKS 31 

In the Mud Spring sector, the Three Forks is poorly 

exposed except for a short stretch in the center of 

sec. 1, T. 2 N., R. 1 E. Here, with a thickness close 

to 300 feet, the Three Forks includes: 

Thickness 

(feet) 

Thick-bedded yellowish-brown siltstone__________________ 50 

Shale, mostly olive gray with thick lenses of black shale, 

and lentils of red limestone coquina; a few orange siltstone 

layers near the top __________________________________ 150 

Very thin bedded yellowish-gray and orange siltstone and 

limestone, with minor gray shale; crumpled and brecciated_ 100 

This section, much like that in the Milligan Creek 

area, differs in having markedly less dark-green shale, 

more black shale, and fewer transitional siltstone beds 

between the medial shale and the upper siltstone. 

It is unfortunate that the Three Forks interval in 

the Nlj2 sec. 36, and NE'1;4 sec. 25, T. 3 N., R. 1 E., 

is almost unexposed, for the rare outcrops are of rock 

types unique in the formation, apparently recrystallized during emplacement of the neighboring large 

monzonite mass and its apophyses. In the center of 

NE'1;4 sec. 36, exposed in the bed of the largest northward-flowing stream, are a few feet of olive-gray to 

dark-greenish-gray impure chert or microcrystalline 

quartzite with splotches of grayish · red and yellowish 

green, crowded with ovoids that are lighter in color 

and, being slightly more resistant, weather into seedlike lumps. Here and there slaty partings separate 

ovoid-bearing layers. The ovoids range in length 

from about Vso to 1;3 inch, but are mostly in the range 

1/s to 1;4 inch. They have nearly equal intermediate 

axes about half as long. In most of the ovoids the 

long axis is subparallel to bedding, with random 

orientation in the bedding plane. In most places the 

ovoids cut across fine details of bedding; in a few, 

bedding elements bend around the ovoids. Oolitic or 

ovulitic beds are unknown in unmetamorphosed Three 

Forks, and the ovoids are probably incipient crystals 

of some prismatic metamorphic mineral. The rock 

seems to be recrystallized and perhaps silicified 

quartzose shale. · 

Exposures in the Three Forks interval in sec. 25 are 

confined to a few outcrops of greenish-gray finely 

banded hornfels and a single thick, massive outcrop 

of pinkish-gray quartzite. The hornfels seems composed largely of microcrystalline clay minerals and 

calcite, the paper-thin banding reflecting varying proportions and grain sizes of the two main constituents· . . ' 

It was probably originally calcareous shale. The 

qqartzite consists of subrounqed to subangular quartz 

sand, with calcite in scattered grains and as intergranular cement. This rock is recrystallized orange 

sandstone or· siltstone. , S~ch rocks ~n · tp.ick beds are 

most common in the upper unit of the Three Forks, 

but this outcrop is near the base of the formation. 

The V-shaped valley cut in folded Three Forks shale 

in the Hossfeldt Hills area is perhaps the most striking topographic feature in the quadrangle. Outcrops along the valley are sufficient to identify the 

formation, but not continuous enough for detailed 

measurements of the section, which is around 300 feet 

thick. The section seems much like that in the Mud 

Spring sector with two differences: the orange siltstone at the top is more clayey and the sequence is 

distinctly thinner; and a gray limestone sequence, 

thick and resistant enough to produce a sharp ridge, 

appears between the lower orange siltstone and the 

olive shale in the extreme northeast corner of the 

quadrangle (sec. 20, T. 3 N., R. 2 E.). This ledgeforming limestone characterizes the lower part of 

the Three Forks in the Logan area (Verrall) 9 and 

farther east (McMannis, 1955, p. 1397). 

The Three Forks shale in the Willow Creek vicinity 

differs markedly from that in other sectors and is so 

variable that a "typical'~ section cannot be given. Here 

it is much thinner, being rarely more than 200 feet 

thick and in places, such as SE1;4 sec. 3, T. 1 S., R. 1 

W., less than 100 .feet thick. The lower contorted 

orange siltstone and limestone unit is poorly developed 

throughout and is missing for long stretches; masses 

of thick-bedded orange and gray limestone with many 

solution breccia zones sporadically overlie the basal 

unit near the west edge pf the quadrangle but do not 

appear in most of the sector. The olive and green 

. shale sequences that dominate the formation elsewhere are reduced to thicknesses of as little as 20 feet 

in places and largely lack the dark fossiliferous limestone interbeds so common elsewhere. The upper 

siltstone unit is relatively thick, as much as 100 feet, 

near the west edge of the quadrangle but is thin or 

missing to the east near the Principal meridian, and 

it is distinctly more clayey and less quartzose than 

in other sectors. 

For , detailed measured sections of the Three Forks 

shale ·see pages 127, 130, and 134. 

The lower boundary of the Three Forks, discussed 

previously, is obscure, but the upper boundary, the 

contact with the overlying Lodgepole limestone, is 

widely . exposed especially in the Milligan Creek sector. In all exposures, dark thin-bedded Lodgepole 

limestone· seems to lie conformably on the orange 

siltstone or sandstone of the upper part of the Three 

F'orks. The only direct sign of hiatus is a little local 

channeling and grooving of the top of the Three 

Forks, to depths of perhaps an inch. The variable 

9 See footnote, p. 11. 
32 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

thickness of the upper unit is probably due more to 

primary depositional factors than to post-Three Forks 

erosiOn. 

NAME 

The name Three Forks shale in this paper follows 

Peale's original usage, . although limestone and siltstone are important in the sequence and justify the 

more general term "Three Forks formation," as first 

used by Haynes (1916b, p. 281) and subsequently by 

several others (such as, Sloss and Laird, 1946 and 

1947; Andrichuk, 1951; Berry, 1943; McMannis, 1955). 

The general sequence within the formation as described here is identical with Peale's description as 

to lithology, but not as to thickness: present total 

measurements are consistently greater than Peale's, 

which total less than 150 feet. As noted before, 

difficulties in reconciling thickness measurements are 

largely responsible far wide divergences in published 

descriptions of relations in the upper part of the 

Jefferson and lower part of the Three Forks. Haynes' 

measured sections (1916b, p. 281), made a few miles 

east of the Three Forks quadrangle, give thicknesses 

of 222 and 287 feet, which are comparable to those 

of this repbrt. The "seven fairly distinct lithologic 

divisions" he recognized in the region were not, how--

ever, identified in the Three Forks quadrangle. Despite the considerable difference in reported thickness 

and in details of description, Peale and Haynes chose 

the same, thoroughly practicable, boundaries for the 

Three Forks and these are the boundaries used here. 

FOSSILS 

Parts of the Three Forks shale are richly fossiliferous, and paleontological literature about the Three 

Forks is extensive. The stratigraphy and age of the_ 

up,per part of the formation have, nevertheless, been , 

in debate ever since Berry (1940) concluded that the 

upper siltstone is of Early Mississippian age and 

separated it, as the Sappington sandstone, from the 

wholly Devonian Three Forks shale (restricted). Extensive faunal collections were made for this report 

partly in an effort to illuminate if not to eliminate 

this controversy, which is discussed later. The reasonably well identified components of collections 

whose stratigraphic position is certain to within a 

few feet are listed in table 2. Collections Mil-l 

through Mil-6 were made by R. J. Ross, Jr. in June 

1953; collection 57MDu35 was made by J. T. Dutro, 

J _r. in July 1957; the remainder were made by me in 

the period 1952-54. 

Brachiopods and-cephalopods are the most abundant 

faunal element~. The brachiopods were studied by 

J. T. Dutro, Jr. (written communications June 16, 

1958, and July 6, 1959). The cephalopods were studied 

by Mackenzie Gordon, Jr. (written communications 

August 22, 1955, and August 24, 1955). In addition, 

pelecypods, ostracodes, and fragments of crinoids and 

bryozoans are present but were not studied systematically for this report. Present, but rare, are conodonts, not further identified, and bits of primitive 

vertebrate jaws (in collection 23; see table 2, and 

pl. 2), identified as such by David Dunkle, U.S. National Museum (written communication, Jan. 26, 1955). 

Table 2, used with plate 2, is largely self-explanatory, but a few comments may be helpful. . 

The interval represented by each collection is given 

in feet below the base of the Lodgepole limestone, and 

also in feet above ( + ) or below (-) the base of the 

upper siltstone unit if exposures permit; single numbers indicate an interval of 1 foot or less. Great 

significance should not be attached to either of these 

reference levels, as the upper one is at a slight disconformity and the lower is gradational, and thus 

rather subjective. . 

The best collections of Three Forks faunas, from the 

standpoint of stratigraphic control, are from the east 

side of the ~1ud Spring sector, in sec. 1, T. 2 N., 

R. 1 E., (loc. 252, pl. 2; the stratigraphic position 

of each element of collection 252, a through i, is shown 

in measured section B, p. 130. 

In the strip of Three Forks shale on the west limb 

of the Hossfeldt anticline, a few of the rare outcrops 

provided excellent collections, mostly from the upper 

siltstone unit (collections 425, 426, 233, 244). The 

shale below the siltstone in this strip has yielded vast 

numbers of fossils in float, but few in place. An exception, the same locality that produced collections 

425 and 426 from the siltstone, p~ovided three collections-232, 351, and 424. 

A few well-located collections have been made in 

the upper .Three Forks shale of the Milligan Creek 

area. Only one locality west of Milligan Creek has 

yielded good material from both the upper siltstone 

and the medial shale units. This is locality 25, on the 

north bank of Dry Hollow ( SEl;iSWl)t NE1)t sec. 3, 

T. 1 N., R~ 1 W.). Other localities upstream have 

yielded fossils only from the shale. Collection A-29, 

made from the south bank of Dry Hollow, downstream from locality 25 (Wlj2 sec. 2, T. 1 N., R. 1 W.), 

is from the top 30 feet of unit A-29 in measured 

section A (seep. 127) 200-230 feet below the Madison 

and squarely in the middle of the Three Forks, which 

may be partly duplicated by faulting. 

Just west of Milligan Creek, in sec. 36, T. 2 N., 

R. 1 W., are fine exposures of the upper siltstone unit, 
SEDIMENTARY ROCKS 33 

here 100 feet thick (see fig. 3, column 2). This is the 

type area of Berry's Sappington sandstone. The 

upper .60 feet yielded only a few specimens, collection 

348, of one species, from 40-50 feet below the top. 

The lower 30 feet yielded the rather sparse collections 

56, 55, 348, Mil-l, Mil-2, Mil-3, and 57MDu35. Collection Mil-2 is omitted from table 2 because it is 

contaminated by float; collection 56 includes the same 

interval but shares only a few forms. The exposures 

had been previously sampled, which may account for 

the relative paucity of fossils. 

Collection Mil-4, from 12 feet of greenish-gray 

shale just below the yellow siltstone unit, is omitted 

'from table 2 because of contamination by float; these 

strata, unfortunately, are not otherwise represented at 

this locality. 

Collection 50c, NW14SE14 sec. 36, is included although comprised solely of float specimens; they must 

have come from beds no lower stratigraphically. The 

specimens of collection 50a were collected in place, 

but indiscriminately from both shale and siltstone. 

The same is true of collection· 54, from 300 feet farther 

east. 

The Three Forks shale in the vicinity of collections 

81-83 from Slj2SE14 sec. 30, T. 2 N., R. 1 E., is slightly 

silicified and the strata look a little different from 

their unaltered equivalents. 

In the Willow Creek sector, the only good exposures 

of fossiliferous beds in the Three Forks are in Slj2Nlj2 

sec. 4, T. 1 S., R. 1 W., in a stretch about 500 feet 

long in the valley of the creek that flows northeastward across the section. The calcareous siltstone and 

nodular clayey limestone that yielded collections 

284-287 are continuously exposed for a thickness of 

about 60 feet from the base of the Madison to the creek 

bed (this is the upper 2h of unit D-38, measured 

· section D, p. 134) . The strata are mainly orange, but 

a few near the base are black, gray, or reddish brown. 

More clayey and less quartzose than the upper siltstone unit in other sectors, they resemble the common 

upper siltstone in color, general texture, and apparent 

stratigraphic position. The base of the upper siltstone unit seems to be about 30 feet lower, to judge 

by the position of nearby outcrops of shale (D-37 

in measured section D, p. 134. 

Collection 286 is from a prominent zone, 3 to 6 feet 

thick, rich in potato-shaped limestone nodules that 

are 1-4 inches long and are reddish brown on the 

outside and gray inside. According to R. C. Gutschick 

(oral communication, 1958) these nodules are fossils, 

too; some are spongiostromatid algae, and others are 

isolated sponges of the class Calcispongea ( ? ) . 

AGE 

These collections contain both Late Devonian and 

Early Mississippian elements. Fixing the DevonianMississippian boundary with respect to either physical 

stratigraphy or morphologic paleontology is not, however, easy. 

Collections regarded as assignable to the Early 

Mississippian include: 252i, 426, 425, 233, 244, 25c, 81, 

and 285. Without exception, they are from lightcolored siltstone or fine sandstone. Most are from 

near the middle of the upper siltstone unit; none is 

less than 20 feet below the Madison and most are 

more than 20 feet above the base of uninterrupted 

siltstone. 

Collections assignable to the Late Devonian include: 

252g, ·252f, 252d, 252e, 260a, 252c, 252b, 252a, 232, 351, 

424 25a, 23, 27, A-29, 56, 348, 55, 39, 50, 54, 83, 284, 

286:10 287, 288, 435T6, 57MDu35, Mil-l, Mil-3, Mil4.5, Mil-5, and Mil-6. Practically all the cephalopods 

are from dark shale, but the brachiopods are not 

closely confined to a particular rock type. Most of 

the Devonian brachiopod collections are from dark 

shale and associated limestone 60 feet to as much as 

230 feet below the Lodgepole and below the sequence 

of uninterrupted light-colored siltstone,. but several 

are from the upper siltstone (256, 284, 286, 287, 

57MDu35, Mil-l, Mil-3), and others (252a, 50a, 54, 

Mil-6) are from siltstone or thinly interbedded siltstone and dark shale from well below the upper 

siltstone unit. An especially striking instance in which 

the same brachiopod assemblage occurs in two distinct lithofacies is in the type area of the Sappington 

sandstone of Berry, where collection 57MDu35 from 

the lower part of the orange siltstone unit contains 

many of the same species as collection Mil-5 from 

dark shale 50 feet lower. 

According to R. J. Ross, Jr. (written communication, Nov. 30, 1954), several of the brachiopod genera 

and probably even so:rne of the species are present 

in the Percha shale of New Mexico and the Ouray 

limestone of southwestern Colorado. Both of these 

formations are of latest Devonian age. 

According to Mackenzie Gordon, Jr. (written communication, Aug. 22, 1955), the goniatite cephalopods 

listed are all members of a well-known Late Devonian 

Rocky Mountain fauna; all the known species are 

present. Raymondiceras and Tornoceras are not regarded as of value for precise dating, but the representatives of Platyclymenia are related to Platycly10 R. c. Gutschick (oral communication, 1958) regards the fossil 

algae and sponges from the same stratum as of Early ~iss.isslppian 

(Kinderhook) age, but the age significance of such fossils IS •"4-ebatable. 
34 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

menia ( Pleuroclymenia) crass a Schindewolf, which occurs in · the so-called Prolobites-Platyclymenia Stu:fe 

of the ·German Upper Devonian; two goniatite zones, 

(not represented in the Three Forks shale) the Orthoclymenia· and W ocklumeria, lie between the ProlobitesPlatyclymenia zone and the Gattendorfia zone which 

has been accepted by the Heer len Congress as the 

base of the Lower Carboniferous in Europe. 

In discussing the ages of the U.S. Geological Survey 

collections from the type area of the Sappington 

sandstone of Berry, NE cor. SW~ sec. 36, T. 2 N., 

R.1 E., Dutro (written communication, June 16, 1958) 

stated: 

There is a gradual change in the composition of the assemblages * * * up the section~ Mil-7 represents the typical molluscan assemblage . described by Raymond ( 1909). It is this 

fauna that has been correlated with the Prolobites-Platyclymenia zone of the German usage (approximately middle 

Famennian). 

Collections Mil-6 upward through Mil-4.5 contain about the 

s~me fossils and are representative of the "Three Forks fauna" 

described by Haynes ( 1916) . 

In Mil-3, and Mil-l, the cyrtospiriferids appear to be progressively less like C. whitneyi and ·C. monticola. Two types 

of syringothyrid occur in Mil-l. Nudirostra is found in Mil-3. 

A different species of Schuchertella, from that found below 

in Mil~4.5 and Mil-6, appears in Mil-l. 

These changes are what I would expect to find in progressively younger Famennian faunas. 

Early Mississippian conodont faunas have been reported 

from shaly zones below the Lodgepole at several places in the 

northern Rockies. A similar situation may exist in or near 

Milligan Canyon. Fossils of undoubted Early Mississippian 

age do occur in the Sappington lithology nearby (coils. 25c 

and 81). Therefore, the Devonian-Mississippian bourid.ary 

lies somewhere in the silty Sappington interval. Exactly 

where is of no particular importance. 

The sparse :faunas in collections 252h and 82 are 

insufficient to permit firm age assignment. 

In all but one of the few places ·where both Mississippian and Devonian faunas have been recovered 

from the same Ideality, all of the Mississippian :faunas 

overlie all the Devonian. Generally in such localities, 

several tens of feet of unfossiliferous rocks intervene 

between the lowest occurrence of definitely Mississippian faunas and highest occurrence of Devonian 

faunas. 

An exception to the customary sequence appears in 

the Willow Creek area, where Mississippian collection 

285 was recovered from beds 20-25 feet below Devonian collection 284. This reversal of the customary 

faunal sequence was noted in the field, confirmed in 

preliminary paleontologic reports; and checked in the 

field by several of the paleontologists who are cited. 

Conceivably, the inversion is due to faulting, but 

there are no overt signs of deformation, nor, more 

'significantly, of the repetition of specific identifiable 

beds. The strata seem indeed to be in the sequence 

of deposition. The many gaps in the Three Forks 

section may conceal similar alternations elsewhere in 

the quadrangle. . 

The anomalous position of collection 284 is possibly the result of reworking of Devonian :fossiliferous 

strata by Mississippian ocean currents. The fossils 

themselves, however, show no signs of special wea-r, 

either in surface abrasion or in degree of disarticulation. 

A better possibility is that the alternation of faunas 

reflects an alternation in primary depositional conditions during a time when earliest Mississippian and 

latest Devonian faunas coexisted, each adapted to a 

slightly differing environment~ There are several 

known instances of alternating faunal zones where biota 

sensitive to lithofacies-"facies fossils"-are involved; 

there is no reason why they should not occasionally 

develop at conformable systemic boundaries. Under 

such circumstances, the systemic boundary might be 

selected at the first (lowest) appearance of the 

"younger" fauna, the last (highest) appearance of 

the "older" :fauna, or regarded as a zone between these 

two limits. 

If it should be argued that collection 284 is not 

really Mississippian but Devonian, or that collection 

285 is not Devonian but Mississippian, the :fact of 

faunal alternation is not changed but the matter becomes another possible case of facies control within 

a system. 

Whatever is made of the relations in the Willow 

Creek sector, the paleontologic evidence presented :for 

the quadrangle as a whole shows that the DevonianMississippian transition occurs high within the Three 

Forks shale. In the few places where exposures and 

fossil collecting are both fairly good, the transition 

seems to occur well above the base of the upper siltstone. If the siltstone grades laterally into dark shale, 

however, as seems likely, there must be places where 

the transition is accomplished wholly within shale 

and others where the lithologic boundary coincides 

with the time boundary. 

THE SAPPINGTON PROBLEM 

Difficulties with the exact placing of the DevonianMississippian boundary in southwestern Montana have 

been recognized and disc~ssed ever since the pioneer 

work of Peale ( 1893). Us'!ally, questions about the 

age of a formation arise from too little paleontologic 

eviden!3e, but in the Three Forks shale the pr9blem 

stems partly from an embarrassment of fossil riches. 

Peale (p. 31-32) noted that the prolific fauna in the 

upper part of the Three Forks shale contains many 

Upper Devonian organisms and a few Carboniferous 
SEDIMENTARY ROCKS 35 

ones. This led him (1896, p. 2) to conclude that "the 

shales are near the border line of the Devonian and 

Carboniferous * * * but the predominance of evidence 

is in favor of their Devonian age." 

The first workers who followed Peale recognized 

the age problem but were not troubled by it. Raymond (1907, 1909) clarified and delimited the unspecified "mingling" of Devonian and Carboniferous 

forms noted by Peale. Raymond demonstrated that 

the "mingling" is practically confined to a few tens of 

feet of poorly exposed yellow sandy and silty beds 

at the top of the Three Forks, and interpreted the 

relations to mean simply a transition from Devonian 

to Mississippian high within the established Three 

Forks shale. Haynes (1916a, p. 27-28) confirmed 

this stratigraphic and faunal transition. 

The age of the upper part of Peale's Three Forks 

became a "problem" when Berry (1943, p. 14-16) 

raised the "yellow sandstone" at the top to formation 

rank, on the ground that it "is both lithologically 

and faunally a recognizable stratigraphic unit" of 

Early Mississippian, probably Kinderhook, age, and 

named it the Sappington sandstone, with a thickness 

of 60 feet at the type locality (NWl!t, sec. 36, T. 2 N., 

R. 1 w.; according to Berry, but the outcrops he 

described are actually in NElli, SW111,) in the Three 

Forks quadrangle. Berry did not describe a type 

section. The fossil evidence cited is a Mississippian 

Syringothyris fauna, confined to the yellow sandstone, 

which has no species of the Devonian Oyrtospirifer 

fauna so abundant in the green shale below. He 

noted the possibility of a transitional fauna in 30 feet 

of "argillaceous limestone and carbonaceous shale" 

(his fig. 4, p. 17) between the base of the Syringothyris 

zone and the top of the Oyrtospirifer zone, but assigned these intervening beds to the Three Forks. 

Although it was not possible to do so without gross 

exaggeration, Berry separated the Sappington on his 

geologic map (pl. 1, scale about 1 :125,000). 

Several other local studies in and near the Three 

Forks area (Holland, 1952, p. 1704-1790; McMannis, 

1955, p. 1396-1399; Verrall 11 ) followed Berry in recognizing a separate Sappington sandstone of Mississippian age, though each introduced modifications, 

and the formation and its implications were accepted 

in the regional synthesis of Crickmay ( 1952, p. 588-

589). 

Holland (1952) interpreted the Sappington to be 

about 100 feet thick in the type locality (fig. 3, column 

1). He not only found Syringothyris in orange siltstone more than 30 feet lower than any found by 

Berry, and below the 30 feet of "argillaceous limestone 

uSee footnote, p. 11. 

and carbonaceous shale" assigned to the Three Forks 

by Berry, but also discovered some decorticated specimens of Rhipidomella "believed to be R. missouriensis 

(Swallow)" and of Mississippian age in his opinion, 

near the base of a sequence of nodular rubbly yellowish-gray calcareous sandstone about 10 feet lower than 

the lowest occurrence of Syringothyris. Below the 

presumed Mississippian Rhipidomella-bearing sandstone is a thick covered interval with scattered outcrops of orange siltstone; rarely exposed within this 

interval is brownish black shale, a few :feet thick. 

Holland (1952, p. 1705 and figs. 5 and 6) placed the 

base of the Sappington and the base of the Mississip-

- pian as well below this rarely . seen shale, a choice of 

minimum usefulness for mapping, whatever its paleontologic virtues. Holland (1952, p. 1707) reported 

the yellow and orange sandstone at the top of the 

Sappington to be overlain with erosional unconformity 

by the gray crinoidal limestone of the Lodgepole. 

If Holland's identifications and age assignments 

are accepted, he found Mississippian :forms below some 

of the assemblages reported here as Devonian ( collections 56, Mil-l, and 57MDu35, on fig. 3). This applies even if the decorticated Rhipidomellas are disregarded. Holland himself found no - Oyrtospirifer 

faunas above Syringothyris faunas, but would not 

change his selection of a boundary if such were found, 

on the premise (paraphrasing Caster) that "it is the 

appearance of the new elements, not the last vestige of 

the old, _ that dates the strata" (Holland, 1952, p. 1709-

1710). 

McMannis (1955) accepted the brownish-black 

shale as the base of the Sappington, but extended the 

Sappington of Holland upward to include a few feet 

of conodont-bearing black shale which is widespread 

in the region (Sloss and Laird, 1946; 1947, p. 1411-

1418) . Insisting on the persistent presence, not only 

in the Bridger Range but throughout the "type Three 

Forks region" of black shale between beds typical of 

the Sappington and the "typical Three Forks of 

Haynes," and claiming a sharp :faunal break marked 

also by this black shale, McMannis denied either a 

lithologic or faunal transition between the Three 

Forks formation and his Sappington sandstone. Somewhat surprisingly, however, he did not insist on a 

Mississippian age :for the Sappington, recognizing the 

possibility that "these beds transcend the systemic 

boundary." 

The idea of a sharp faunal and lithologic break 

between the siltstone of the Sappington and the underlying shale of the main body of the Three Forks, 

independently advanced by McMannis, was first proposed on a regional basis by Crickmay (1952). He 

concluded that the Three Forks shale near Three 
36 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

2 

HOLLAND, 1952 

(p. 1706-1709, fig. 5) 

Lodgepole limestone 

Eroded 

~ 

top of 

Sappington 

sandstone 

of Berry 

Yellow and orange 

sandstone, 38 ft 

Gray shale, 30 ft 

Grayish-orange 

siltstone, 5lf2 ft 

. Yellowish-gray 

sandstone, 

19 ft (nodular in lower 

4ft) 

Base of Sappington sandstone of Berry and of 

--~i_s~~~ippian system 

Top of Three Forks 

shale and of Devonian system 

F-1 

A mbocoelia minuta White 

Camarophorella? cf. C. 

buckleyi (Rowley) 

Camarotoechia cf. C. 

tuta(Miller) 

Composita sp. 

Dictyoclostus cf. D. arcuatus Hall 

Productella sp. 

Spirifer cf. S. marionensis Shumard 

Streptorhyncus? sp. 

Syringothyris cf. S. 

hannibalensis 

(Swallow) 

Platyceras sp. 

L-H 

Syringothyris cf. S. 

hannibalensis 

(Swallow) 

L-0 

to be R. missouriensis (Swallow)" 

L-E 

AmbocoeliagregariaHall 

Cyrtospirifer monticola 

(Haynes) 

Cyrtospirifer whitneyi? 

(Hall) 

Nudirostra 

madisonense? 

(Haynes) 

Nudirostra mesicostale 

(Hall) 

. THIS REPORT 

(Measured by R. J. Ross, Jr., July 1953; 

for more data on collections see text) 

57MDu35 

Schizophoria sp. 

Camarotoechia sp. 

Nudirostra aff. N. 

ventricosa (Haynes) 

Cyrtospirifer cf. C. 

whitneyi (Hall) 

Cyrtospirifer cf. C. monticola (Haynes) 

"Spirifer" cf. "S." 

raymondi (Haynes) 

"Cleiothyridina" ct. "C" 

coloradoensis (Kindle) 

Crurithyris? sp. 

Platyclymenia 

(Pleuroclymenia) 

americana (Raymond) 

Mil-l 

Schuchertella sp. 

"Productella" sp. 

"Avonia" arcuata (Hall)? 

Cyrtospirifer sp. 

"Spirifer" sp. 

Athyris? sp. Camarotoechia sp. 

"Syringothyris "sp. 

Syringothyris? sp. 

· Mil-3 

Rhipidomella sp. 

Nudirostra sp. 

Cyrtospirifer cf. C. 

whitneyi (Hall) 

Cyrtospirifer cf. C. 

monticola (Haynes) 

Crurithyris? sp. 

Mil-4.5 

Schuchertella cf. S. arctostriata (Hall) 

Cyrtospirifer 

monticola (Haynes) 

Mil-5 

Schizophoria cf. S. 

australis Kindle 

Nudirostra sp. 

"Productella" cf. "P." 

coloradoensis (Kindle) 

· Cyrtospirifer 

whitneyi (Hall) 

Cyrtospirifer 

monticola (Haynes) 

"Spirifer" cf. "S" 

raymondi (Haynes) 

"Cleiothyridina" 

devonica (Raymond) 

Mil-6 

All species reported from 

Mil-5, and also: 

Schuchertella sp. 

CamarQtoechia aff. C. contracta (Hall) 

Nudirostra cf. N. 

dunbarense (Haynes) 

"Productella"cf. "P." 

spinigera (Kindle) 

Ambocoelia? aff. A? 

gregaria Hall 

} 348 Nudirostra 

ventricosa (Haynes) 

56 

Schizophoria sp. 

Schuchertella? sp. 

Camarotoecliia sp. 

Cyrtospirifer cf. C. 

animasensis (Girty) 

Cyrtospirifer 

monticola (Haynes) 

"Spirifer" sp. 

A mbocoelia? 

gregaria Hall 

55 

Schizophoria 

australis Kindle 

Nudirostra 

jejfersonense (Haynes) 

Nudirostra sp. 

Camarotoechia aff. C. 

contracta (Hall) 

"Productella" cf. "P." 

coloradoensis (Kindle) 

Cyrtospirifer cf. C. 

animasensis (Girty) 

Cyrtospirifer 

gallatinensis (Haynes) 

Cyrtospirifer 

monticola (Haynes) 

Cleothyridina 

devonica Raymond 

Crurithyris? sp. 

Tornoceras 

crebriseptum 

(Raymond) 

Platyclymenia 

(Pleuroclymenia) 

americana (Raymond) 

Lodgepole limestone 

Yellowish-brown 

siltstone and 

sandstone, 35ft 

Olive and orange 

siltstone and 

shale, 6ft 

Yellowish-gray siltstone, 21 ft 

Orange, brown, and 

red shale and 

siltstone, 8 ft 

Grayish-orange siltstone, 7 ft 

Grayish-orange and 

yellowish-gray 

siltstone, 11 ft 

Yellowish-gray silty 

nodular limestone, 12 ft 

Covered, 12 ft 

(green shale 

nearby) 

Dark-gray limestone, 1 ft 

Green shale and 

siltstone > 15 ft 

.t:: 

0 

8 

.-t::! 

c 

::l 

Q) 

c 

·v; 

~ 

Q; 

a. 

a. 

::::> 

·c: 

::l 

Q) 

ro 

~ 

:0 "' 

Q) 

~ 

FIGURE 3.-Measured sections and fauna in upper part of TQree Forks shale, NE cor. SW:JA, sec. 36; T. 2 N., R. 1 E. (type loe!altty 

of Sappington sandstone of Berry). 
SEDIMENTARY ROCKS 37 

Forks does not carry a Oyrtospirifer fauna at all, but 

that the forms commonly identified as such are species 

of the superficially similar genus OyrtiopsUi, distinctly 

younger than the Oyrtospirifer fauna but wholly older 

than the Syringothyris fauna of the Sappington, from 

which it is separated by 1 to 19 feet of black shale 

without megafossils (Crickmay, 1952, p. 588). Crickmay assigned to the Mississippian all the rocks above 

the black shale which are similar to the Sappington, 

and implied that the black shale and possibly even 

some of the beds below it are also Mississippian. 

Later (Crickmay, 1956, p. 57-58) he specifically correlated this black shale with the Exshaw shale of 

Mississippian age in Alberta, but favored a Devonian 

age for the strata below. 

A different interpretation is that of Sloss and Laird 

(1947, p. 1411), who said that "it can be demonstrated that the normal shale facies of the Three 

Forks, bearing the typical Oyrtospirifer fauna, is interbedded with the sandy facies bearing Syringothyris," and that the sandy facies grades laterally 

into typical shale of the Three Forks formation. These 

writers considered the "Sappington sandstone" to be 

a "local member of the Three Forks formation" and 

assigned it an unequivocal Late Devonian age, despite 

the Mississippian aspects that had impressed all previous worker~. Such an assignment had, however, been 

foreshadowed by the hesitations of the committee that 

prepared the Devonian correlation chart (Cooper and 

others, 1942) : they assigned the "Three Forks with 

Syringothyris" an equivocal Devonian or Mississippian age and remarked (p. 1785) that this zone may 

ultimately be assigned definitely to the Devonian. The 

opinions of Sloss and Laird as to the age of the Sappington have been reiterated by Sloss and Moritz 

(1951, p. 1252), Andrichuk (1951), Mann (1954, p. 

11), Alexander (1955, p. 45-49), and others. 

In the Three Forks quadrangle, an alternation of 

orange siltstone and sandstone beds of "Sappington 

type" with green shale and dark limestone beds of 

"Three Forks type" is unquestionably present, as 

Berry and Holland also recognized. Black shale intervenes locally between the orange siltstone and green 

shale but its persistence has not been demonstrated, 

despite the contrary claim of McMannis and Crickmay. 

Although alternation of lithofacies is proved, accompanying faunal alternation of the sort described by 

Sloss and Laird has not been confirmed. The anomalous relations west of Willow Creek lend support, 

but the fact that "Oyrtospirifer faunas" seem to have 

tolerated both muddy and silty environments does not. 

Whether or not a faunal alternation occurs and regardless of the exact ages of the faunas, the sequence 

of clastic orange siltstone, too thin to map by itself 

at the scale of this report, is better mapped with the 

unresistant, dominantly clastic Three Forks shale, 

with which it grades, than with the ledge-forming 

Lodgepole limestone. 

The physical stratigraphy of the Three Forks shale 

in the Three Forks quadrangle thus supports the descriptions of Sloss and Laird. Whether or not they 

are correct about alternation of faunas closely controlled by lithofacies, however, their conclusion seems 

untenable that the entire formation is of Devonian 

age. In this matter the present work confirms the 

majority opinion, dating from Raymond and Haynes, 

that the Devonian-Mississippian boundary is high 

within these rocks, though just where is uncertain. 

The Devonian-l\1ississippian transition at the type 

locality of the Sappington sandstone of Berry is within 

or possibly even at th'e top of the sequence of yellow 

siltstone, to judge by Dutro's interpretation (quoted 

earlier) of the Geological Survey collections reported 

on table 2 and figure 3. He does not accept any of 

Holland's or Berry's Mississippian age assignments. 

The crux of the matter is the time significance of 

Syringothyris. Dutro rejects the idea that "Syringo-· 

thyris means Mississippian." In taking this position 

he is not merely considering the recorded range of the 

genus, which of course has long been recognized as 

Late Devonian to Mississippian. Rather, he opposes 

the view that any single species or genus can be 

meaningfully chosen as a systemic index, and bases 

his ideas of age on an evaluation of the whole fauna. 

The Devonian-Mississippian transition in this locality 

is placed by Dutro 60 to 100 feet above the boundary 

selected by Holland, and 20 to 60 feet above that 

chosen by Berry. The top of his zone of transition 

coincides with the boundary proposed by Sloss but 

only because fossils are lacking at this locality, for, 

on evidence from other nearby collections, he would 

expect the transition to be som~what lower. ' 

Despite all the attention given it, the inconsequential "Sappington problem" is about the same as when 

Haynes left it in 1916. 

Cartographically, the concept of the Sappington 

sandstone, whether as a Mississippian formation or 

_a member of a Devonian formation, is of little value 

in the Three Forks quadrangle. This is not to minimize the potential usefulness of the unit for mapping 

in other areas, however, particularly where the orange 

siltstone is thick and is set off by black shale, for 

beds lithologically similar to those of the Sappington 

near the Devonian-Mississippian boundary are now 

known from a rather large area: they appear as 

far west as the middle of the Jefferson Island quad-
38 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

rangle (shown me by W. R. Lowell), though not as 

far west as Philipsburg, where the entire intBrval 

of the Three Forks is absent (Emmons and Calkins, 

1913, p. 65), nor, apparently, as far southwest as the 

Blacktail and Snowcrest Ranges (Klepper. 19f>O. p. 

60); at least as far south as the Gravelly Range 

(Mann, 1954, p. 11) and perhaps even into northern 

Utah (Holland, 1952, p. 1719-1723), but the rocks 

of this interval have not been traced across intervening Idaho; as far east as the Bridger Range but 

seemingly not as far southeast as Yellowstone Park 

(Hague, Weed, and Iddings, 1896, p. 4); at least as 

far north west as the Helena quadrangle (A dol ph 

Knopf, oral communication, 1957) but not as far 

due north as the Canyon Ferry quadrangle (Mertie, 

Fischer, and Hobbs, 1951; p. 26). The impression is 

that rocks of Sappington lithology are somewhat less 

extensive than are the green shales, and that where 

limestone dominates the formation the upper siltstone is likely to be absent. 

The thin black shale that generally intervenes between the Three Forks and the Lodgepole in central 

Montana and has been designated the Little Chief 

Canyon member of the Lodgepole limestone by 

Knechtel and others (1954) does not appear at any 

place where the contact is exposed in the Three Forks 

quadrangle. The shale is not reported by Ber~y 

(1943) or Alexander (1955) in the adjoining Jefferson 

Island quadrangle, or by Klepper and others ( 1957) 

in the Elkhorn Mountains to the northwest, or by 

Mann (1954) in the Gravelly Range to the south, and 

I have not seen it in the Toston quadrangle to the 

northeast. It crops out as near by as Logan, however, 

according to V errall 12 and in the Bridger Range 

(McMannis, 1955, p. 1397), though only a few feet 

thick. This shale may have been stripped off the 

orange siltstone by pre-Lodgepole erosion, or it may 

merely wedge out near the Missouri River. 

ORIGIN 

Most of the Three Forks seems to be of shallow, 

nearshore marine origin. The dark shale with pyrite, 

in the form of crystals and fossil replacements, suggest sharply restricted communication with the ocean. 

The basal crumpled and brecciated thin-bedded mudstone and limestone, with their bright colors, appear 

to be residual from originally much thicker evaporite 

sequences, deeply leached, as recognized by Sloss and 

Laird (1946) and discussed by Andrichuk (1951). 

Though superfically similar to the basal evaporites, 

the upper orange and yellow siltstone and sandstone 

probably formed under much different conditions, 

1 2 See footnote, p. 11. 

presumably close to shore in shallow, well-aerated 

waters in free _communication with the ocean. Very 

likely part at least of this nearshore facies was de- . 

posite~ contemporaneously with and grades into offshore dark ~hale, as Sloss and Laird (1947) concluded. 

The dominance of free-swimming cephalopods 

among pyritiz~d fossils in the dark shale beds, whereas 

brachiopods are the overwhelmingly dominant · nonpyritized forms elsewhere in the formation, suggests 

that the cephalopod faunule is a thanatacoenose or 

death assemblage, for living mobile cephalopods 

would scarcely inhabit sulfide-rich muds, whereas the 

brachiopod faunule may approach a biocoenose or 

life assemblage (Wasmund, 1926) . 

The source of the clastic components of the Three 

Forks is hard to visualize. It may have been a 

southern extension of the uplift postulated by Deiss 

( 1933) in parts of the Swan, Flathead, and Lewis 

Ranges, to account for a hiatus between the upper 

part of the Jefferson and lower part of the Madison. 

An eastern source is not out of the question, as 

Devonian rocks are absent in southeastern Montana 

and adjacent parts of "Tyoming, and seem never to 

have been deposited there (Sloss, 1950, p. 439, fig. 7). 

Low relief of a possible eastern source is suggested 

by the dominance of nonelastic rocks in the Devonian 

sequence to the shoreline (Sloss, 1950). 

CARBONIFEROUS 

Rocks mapped as Carboniferous, divided into four 

map units, are 1,600 to 3,500 feet thick. They comprise about 80 percent limestone, 10 percent siltstone 

and sandstone, 5 percent dolomite, and 5 percent shale; 

the clastic ratio is less than 0.2. The boundary between the Mississippian and the Pennsylvanian is 

not yet clear in western Montana. The two great 

limestone formations, Lodgepole and Mission Canyon, 

of the Madison group near the base of the Carboniferous can safely be assigned to the Mississippian, the 

Quadrant formation at its top is assigned to the 

Pennsylvanian (though it may in part be as young 

as Early Permian) and the Mississippian-Pennsylvanian transition comes within the rocks here mapped 

together as the Big Snowy and Amsden formations. 

The Carboniferous of. western Montana, though not 

as thoroughly studied as the early Paleozoic, has been 

approached on a regional scale in three papers: the 

Madison group is treated by Sloss and Hamblin 

( 1942) ; the Big Snowy group by Perry and Sloss 

( 1943) ; and the Big Snowy, Amsden, and Quadrant 

by Scott ( 1935) . None of these studies is both detailed and comprehensive, and none has much to 

say about western Montana, as their emphasis is on 
.SEDIMENTARY ROCKS 39 

oil-productive central Montana. Despite evident possibilities for useful contributions in this part of the 

geologic column, the Carboniferous rocks are cursorily 

treated here : the bulk of strata in the interval is 

limestone of the Madison group with monotonously 

uniform texture and composition, that discourages 

detailed scrutiny by ordinary methods; and the remaining interbedded clastic and carbonate rocks are 

on the whole too poorly exposed to justify elaborate· 

treatment. The data at hand, · though meager, lead 

to some thoughts that may be of interest on certain 

aspects of correlation and origin. 

NAMES AND GENERAL CORRELATION OF CARBONIFEROUS 

FORMATIONS 

Peale ( 1893, p. 32-33) assigned to two formations 

the rocks he recognized as Carboniferous, which at 

that time included the Permian: the Madison formation, consisting wholly of limestone, 1,250 feet thick; 

and the overlying Quadrant formation, consisting of 

interbedded limestone and quartzite, "the latter predominating at the top," 300-400 feet thick. "A prominent bed of quartzitic sandstone/' 40-60 feet thick, 

capping his Quadrant he assigned to the J uratrias 

Ellis formation, though recognizing its possible Carboniferous age (Peale, 1896, p. 2) . From his brief 

but clear descriptions, it is plain that he recognized 

all the rock types found by later workers, though the 

thicknesses he reported are puzzlingly small, but the 

scale of his mapping did not allow further subdivision. 

The Madison was raised to group rank and divided 

into two formations, a lower thin-bedded unit named 

the Lodgepole limestone and an upper thick-bedded 

one named the Mission Canyon limestone by Collier 

and Cathcart ( 1922, p. 173) , the usage now commonly 

followed throughout the region and in this report. 

The Lodgepole was further subdivided into a lower . 

shaly member, the Paine, and an upper wholly limestone member, the W oodhurst, by Sloss and Hamblin 

( 1942, p. 313-318) ; these members can be identified 

locally in the Three Forks quadrangle but not in 

enough places to permit mapping. 

Although Peale was the first to use Quadrant formation in ·print ( 1893), the term originated with Weed 

contemporaneously working in Yellowstone Park. 

Weed restricted the name to post-Madison, prePermian rocks. Not recognizing any Permian in his 

area, Peale included the entire post-Madison preMesozoic sequence in his Quadrant. Condit (1918) 

traced the Permian Phosphoria formation (Richards 

and Mansfield, 1912, p. 683-689) into the Three Forks 

region, and rest-ricted the Quadrant there to rocks 

fitting Weed's original definition. 

Later, Scott (1935) proposed radical changes of 

nomenclature for the beds included in Weed's Quadrant. He ·limited the Quadrant to the quartzitic 

sandstone beds just below the Phosphoria, correlating 

the restricted Quadrant with the Tensleep sandstone 

(Darton, 1904, p. 394-401) or" Pennsylvanian age. 

The beds below the restricted Quadrant, and above 

the Madison, he subdivided into four conformable 

formations (in ascending order) : Kibbey sandstone 

(Weed, 1899b), Otter shale (Weed, 1892, p. 30!), 

Heath shale (new name) , and Amsden formation 

(Darton, 1904, p. 394-401), all of late Mississippian 

(Chester) age. The three lower units were placed 

in the Big Snowy group, confined to central Montana, 

with the Amsden separately classified, presumably because of its differing, much wider, distribution. The 

stratigraphic proposals of Scott represent a distinct 

advance. His Big Snowy, Amsden, and Quadrant 

have · been widely accepted and are used in this report, 

although it could be argued that the name Quadrant 

has lost its value through modifications, and should 

be abandoned. 

The "red limestone" in the lower part of Peale's 

Quadrant ( 1896, p. 2) is clearly the Amsden of Scott. 

In most of the area studied by Peale, including much 

of the Three Forks quadrangle, the Amsden formation 

lies directly on the Madison group, and the Big Snowy 

js absent. In parts of the Three Forks quadrangle, 

however, rocks typical of the Amsden are underlain 

by as much as 400 feet of strata of the Big Snowy 

group. The beds contain fossils characteristic of the 

upper part of the Big Snowy, but lithological~y they 

are only vaguely similar to type Big Snowy units and 

are not subdivided into established formations; consequently, they are designated Big Snowy for~ation 

rather than group. Because the Big Snowy IS generally thin, and its contact with the Amsden formation hard to locate in most places, the two units are 

combined on the geologic map. 

MADISON GROUP 

LODGEPOLE LIMESTONE 

Assigned to the Lodgepole limestone of the Madison 

group is a thick succession of thin-bedded limestone 

and silty limestone. Characteristically, the Lodgepole 

outcrops beo-in with one or two ledges, 10 to 20 feet 

high, succe:ded by a long grassy valley slope, with 

onlv scattered outcrops; the slope ends in a great ledge, 

as ~uch as 100 feet high-the base of the very thick · 

bedded Mission Canyon limestone. Best exposures of 

the form.ation as a whole are on the north side of 

Dry Hollow in sees. 2 and 3, T. 1 N., R. l W., and 

on the east side of Milligan Creek, just below the 

canyon in sec. 36, T. 2 N., R. 1 E. 
40 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

In contrast to the older limestones, the Lodgepole 

has rather constant thickness, as well as it can be determined, and constant gross lithology throughout the 

quadrangle. Thickness in the Milligan Creek and 

Mud Spring areas is in the range 600-700 feet; widely 

differing apparent thicknesses, from as little as 200 

feet to as much as 700 feet, measured at close intervals 

in the Hossfeldt Hills and Willow Creek vicinities 

suggest unrecognized near-bedding faults and make 

estimates of average thickness hazardous, but it is 

perhaps not far from 600 feet in each sector. The 

main rock type is dark-gray microcrystalline limestone, in beds 4 inches to 2 feet thick, that weathers 

yellowish gray to light olive gray. More coarsely 

crystalline beds or zones are rare near the base but 

are progressively more prominent upsection; many 

of these coarser phases are medium gray and olive 

gray, have sugary textures suggestive of clastic fabric, 

and a fresh petroliferous odor. Good casts and molds 

of brachiopods and solitary corals are widespread in 

the microcrystalline limestone, and fragments of 

brachiopods, corals, crinoids, and bryozoans are · 

abundant in the sugary zones. Partings of yellowishbrown . and grayish-orange calcareous mudstone, having sharp contacts with the limestone, are abundant 

in the lower 200 feet or so, becoming fewer but thicker 

upsection (though nowhere more than · a few inches 

thick). Mudstone seems to be somewhat more common in the Milligan Creek and Mud Spring areas 

than in the other two pre-Tertiary sectors. In the 

upper 100 feet, a few very thick-bedded sequences, 

10-20 feet thick, appear. For crudely measured sections, see pages 127, 130, 132, and 134-135. 

In the northeastern part of the Mud Spring area 

(sees. 24 and 25, T. 3 N ., A. 1 E.), the Lodgepole has 

been altered as a result of the intrusion of a nearby 

monzonitic pluton and its apophyses. Most of the 

altered rocks are merely bleached nearly white and 

recrystallized to a coarse fabric approaching marble. 

Rarely exposed in shallow gulches near the north edge 

of sec. 25, however, are variably coarse calcsilicate 

rocks of varied composition, permeated with ramifying hornblende diorite dikelets. The commonest type 

is very coarse, with crystals as much as 2 inches across 

of white and pale-pink calcite intimately intergrown 

with grayish-olive to dark-yellowish-brown idocrase 

and subordinate similarly colored garnet. On casual 

· inspection, the idocrase looks much like garnet but 

it has many envelope-shaped crystals, with well-developed bipyramids separated by narrow four-siqed 

prisms (resembling a common zircon form) ; identity 

of the mineral with idocrase has been confirmed in 

thin-section and by oil immersion. methods. In a few 

places these rocks have green stains suggesting copper 

salts, but have no readily recognized copper . minerals. 

A rare, and confusing, calcsilicate type is a dense 

hard fine-grained yellowish-gray rock studded with 

equant bluish-black crystals %0 to %0 inch across. 

The large crystals look like spinel, and the general 

association leads to the idea that the rock is an 

altered dioritic dikelet. The rock, however, actually 

consists of large zoned garnets, brown in thin section, 

in a fine groundmass of colorless garnet-diopside, and 

minor amounts of wollastonite, allanite ( ~); and a 

micaceous mineral; it presumably is a contact-altered 

sedimentary rock. 

These rocks, from close to the base· of the Lodgepole, 

probably represent original silty strata that have 

formed new mineral combinations without additions 

of anything but heat and perhaps water from the 

intrusions. 

Much of the Lodgepole resembles thin-bedded parts 

of the Meagher and Pilgrim formations. Distinction 

is readily made on the basis of fossils, which can 

usually be found even in small exposures of the 

Lodgepole and recognized even by novices in paleontology : cup corals, segments of crinoid stems, and 

a varied brachiopod community conclusively indicate 

that a questionable outcrop is Lodgepole rather than 

a Cambrian limestone. 

The Lodgepole, as noted earlier, overlies the Three 

Forks shale with seeming conformity, though the 

absence of correlatives of the Little Chief Canyon 

shale member suggest a brief hiatus. Its upper contact with the Mission Canyon limestone is conformable 

and transitional, being placed where very thick bedded · 

sequences begin to dominate the section. In a few 

places, such as the mouth of Milligan Canyon, the 

transition is accomplished within a few feet, but in 

most places the combination of partial exposures and 

minor thick-bedded -sequences within the upper part 

of the Lodgepole makes it difficult to place the contact 

closely. Consequently a gradational contact is drawn 

throughout. Some o-f the difficulties with the thickness 

of the Lodgepole may be due, not to structure as suggested above, but, to unconscious changes in the choice 

of the upper boundary, or real original variations 

in the stratigraphic level at which thinner bedded 

strata give way to thicker. This point could perhaps 

be resolved if the thickness of the Mission Canyon 

limestone could be measured with fair accuracy and 

its changes compared with those of the Lodgepole but 

this, as shown below, is not feasible. 

AGE AND ORIGIN 

The Lodgepole limestone is a typical marine carbonate, laid down in rather shallow water (sublittoral 
SEDIMENTARY ROCKS 41 

to shallow neritic). Its thin but even stratification 

suggests a gently oscillating sea. It is largely of 

chemical origin, but includes some clastic layers, probably. reworked from nearby parts of the Lodgepole 

itself. The fact that mudstone interbeds are fewer 

but thicker upsection, where:;ts thick-bedded limestone 

is increasingly common, suggests a gradual slowing 

of the rate of oscillation, no doubt accompanied by 

slow reduction of borderlands, rather than shoreline 

retreat. The same data, considered along with the 

decrease in proportion of clastic limestone and in 

numbers of fossils upsection, suggest increasing average water depth during Lodgepole time. 

The Lodgepole is highly fossiliferous, so that systematic collecting could have produced fossil lists of 

staggering length. However, the fauna is thoroughly 

known, and the age of the formation is established 

as Early Mississippian. Five collections, taken from 

various levels merely to confirm the stratigraphic assignment and age, and examined by J. E. Smedley 

(written communication, May 28, 1954) are listed 

below: 

1. From uppermost 25 feet of the formation in Milligan Canyon [16430-PC] : 

Oamarotoechia sp. indet. 

Spiriter centronatus Winchell 

Schuchertella cf. S. chemungensis (Qonrad) 

Chonetes loganensis Hall and Whitfield 

Productus gallatinensis Girty 

A Vf)nia? sp. indet. 

Spiriferina? sp. indet. 

Dielasma sp. indet. 

Platyceras sp. indet. 

2. From 15-20 feet below top, south o£ Jefferson River 

in SW%,NE%, sec. 3, T. 1 S., R. 1 W. [16434-PC] :-

Fenestella sp. undet. 

Spirifer centronatus Winchell 

Schuchertella sp. indet. 

Leptaena? sp. indet. 

Chonetes cf. C. loganensis Hall and Whitfie~d 

Schizophoria compacta Girty 

Spiriferina sp. indet. 

Platyceras sp. undet. 

Low spired gastropod frag. gen. indet. 

3. From the middle Lodgepole In Milligan Canyon 

[16432-PC]: 

Camarotoechia metallica White 

Axiodeaneia sp. indet. 

Spirifer centronatus Winchell 

Torynifer cooperensis (Swallow) 

Composita madisonens.is ( Girty) 

Chonetes loganensis Hall and Whitfield 

Linoproductus sp. indet. 

Platyceras sp. undet. 

Proetid trilobite frag. gen. undet. 

4. From the middle Lodgepole in center, sec. 36, 

T. 3 N., R. 1 E. [16433-PC]: 

Fenestella sp. indet. 

Camarotoechia metallica White 

Spirifer centronatus Winchell 

Composita madisonesis ( Girty) 

Chonetes loganensis Hall and Whitfield 

Ckiothyridina aff. C. incrassata (Hall) 

Leptaena analog a (Phillips) 

Spiriferina cf. S. solidirostris White 

Orthotetes sp. indet. 

Platyceras sp. i:ndet. 

Straparolid gastropod gen. indet. 

Proetid trilobite gen. indet. 

5. From 20-25 feet above the base of the formation 

in Dry Hollow, center Slj2NWl,4 sec. 3, T , lN., 

R. 1 W. [16431-PC] : 

Fenestella sp. indet. 

Schizophoria sp. indet. 

Schuchertella? sp. indet. 

Chonetes loganensis Hall and Whitfield 

Chonetes logani Norwood and Pratten 

Spirijer centronatus Winchell 

Spirifer striatlus var. madisonensis of Girty 1899 

Straparollus sp. indet. 

Platyceras sp. indet. 

Paracyclus sp. indet. 

MISSION CANYON LIMESTONE 

The Mission Canyon is the last of the mountainforming Paleozoic limestones. Great ledges carved 

from its basal beds form the highest and boldest ridge 

crests throughout the quadrangle, though they are only 

slightly higher than many ridges formed by basal 

beds of the Jefferson dolomite. Where dips are moderate, most of the Mission Canyon, like the J e~erson, 

is poorly exposed, and forms long grassy dip slopes 

broken by a few low ledges. The formation is magnificently exposed where dips are very steep, as in 

the Willow Creek area east. of Ingleside Quarry and 

the eastern part of the Hossfeldt Hills. The most accessible continuous exposures are in Milligan Canyon 

and in the southeast-draining canyon in SW cor. sec. 

29, T. 3 N., R. 2 E. 

Throughout the quadrangle the Mission Canyon, 

like the Lodgepole, is lithologically constant. The 

main rock type is gray microcrystalline limestone 

much like that in the Lodgepole, but lighter in color. 

The Mission Canyon contains a few beds of mediumgrained crystalline sugary limestone, like that in the 

Lodgepole, mostly near the base, but lacks mudstone 

layers and is but rarely fossiliferous. 

The only striking compositional difference between 

the two formations of the Madison group is the 

abundance of chert in false-nodules, nodules and lentils 
42 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

in the Mission Canyon, especially the upper half. 

Most of the chert is grayish black to olive black, but 

some is much lighter, in the range of pale yellowish 

browns and grays. The nodules tend to be almond 

shaped and an inch or two long. Their long axes 

are usually parallel to bedding, and some grade into 

lentils as much as .5 feet long. The structures called 

false-nodules look like ordinary nodules, but consist 

of a thin skin of chert surrounding lumps of iimestone indistinguishable from the country limestone. 

Gradations from false-nodules to true do not occur, 

and the two types seem to be of different origin. The 

general richness of the formation in silica is apparent 

011: weathered surfaces, which in many places have 

thin rough crusts of finely crystalline quartz, easily 

mistaken for chert. 

The distinguishing feature of the Mission Canyon 

is its thick, commonly indistinct, bedding. _Individual 

beds 3-5 feet thick are the rule, and beds more than 

10 feet thick are numerous ; sporadically' however' 

thin-bedded sequences appear. 

The· apparent thickness of the Mission Canyon 

varies widely within individual pre-Tertiary areas. 

In the Milligan Creek area west of Milligan Canyon 

the formation is rather consistently 600-700 feet thick, 

whereas in seemingly undisturbed sections only 2 miles 

east of the Canyon measured thicknesses are 1,000-

1,100 feet. In Milligan Canyon itself and for more 

than a mile to the northeast, the formation is so 

thickened by a combination of cross faults and unmappable near-bedding faults that measurements are 

precluded. 

In the 3-mile strip of continuous Mission Canyon 

on the east flank of the Hossfeldt anticline, the apparent thickness varies rapidly, ranging between 700 

and 1,000 feet. In the Willow Cr.eek vicinity, the 

apparent thickness also varies widely, from 1,000 

fe·et to more than 1,400 feet, within another 3-mile 

continuous strip without mapped faults. The problem of thickness variation does not arise in the Mud 

Spring sector for the full Mission Canyon is nowhere 

exposed; it is at least 700 f~et thick in the trough 

of the syncline east of Dunbar Creek. 

For crudely measured sections see pages 127 and 132. 

The great local variations in thickness may be 

ascribed largely to erosion or to known but unmappable deformation. Rocks of the Big Snowy formation 

are missing west of Milligan Creek, where the Amsden 

lies on deeply eroded Mission Canyon, but the full 

Mission Canyon section is ·preserved beneath the Big 

Snowy east of Milligan Creek. No such explanation 

will serve for the Hossfeldt Hills and Willow Creek 

sectors, where the Big Snowy formation overlies the 

~fission Canyon throughout; in these areas, nearbedding faults, which are visible in many transverse 

canyons but of unknown displacement and untraceable 

beyond the canyon walls, are a possible explanation. 

In the upper two-thirds of Milligan Canyon, where 

the canyon trends north-northeast, a thick zone of 

limestone breccia within the Mission Canyon limestone is spectacularly exposed (fig. 4) . The breccia 

is an unresistant interval that here controls the 

course of Milligan Creek. The stratigraphic thickness 

of the zone changes abruptly from less than 10 feet 

to more than 40 feet; these changes largely reflect 

irregularities in the top, for the base seems to stay 

within !).arrow stratigraphic limits about 200 feet 

below the top of the formation. The zone is loosely 

filled with unsorted angular to subangular fragments 

of limestone, which range from granule to giantboulder size but are mostly in the cobble to small 

boulder size. Most of the breccia is cemented by 

calcareous siltstone and claystone, colored orange or, 

rarely, brownish red; in some places, spaces between 

stones are open, or lined with coarsely crystalline 

calcite. Rarely, spaces that seem to have been tubelike, of unknown, possibly great length along the 

dip but only a few feet wide and high, are filled with 

laminated to very thin bedded calcareous siltstone 

like the matrix material of the breccia. 

This breccia zone continues east of the canyon, 

where it dominates the long arm of Mission Canyon 

rocks east of Milligan Creek (NWlJ.t, sec. 25, T. 2 N., 

R. 1 W.), but few signs of it appear farther to the 

east, though exposures are fairly good. To the west, 

the breccia zone can be followed to about the center 

FIGURE 4.-Limestone breccia in upper Mission Canyon limestone, 

Milligan Canyon . . View at right angle to strike of formation ; dip 

35° away from viewer. Note partly filled triangular cavity above 

man, and laminated siltstone layers above him to his right. 
·sEDIMENTARY ROCKS 43 

sec. 35, T. 2 N., R. 1 W., where it seems to disappear 

against the Amsden contact. Breccia zones, similar 

lithologically but much smaller in all dimensions, 

occur somewhat lower in the Mission Canyon in this 

vicinity, and in the upper few hundred feet of the 

Mission Canyon in all the other pre-Tertiary sectors. 

A thin limestone conglomerate, 5-10 feet thick, is 

exposed high in the formation in one canyon , ( SW 

cor. sec. 29, T. 3 N., R. 2 E.). This facies, consisting 

of pebble-to-cobble size subrounded clasts of typical 

Mission Canyon limestone, solidly cemented by gray 

clayey, sandy limestone, could not be traced beyond 

the canyon walls. 

In the Mud Spring area, Mission Canyon rocks 

form the west and north margins of the monzonitic 

pluton that intrudes Lodgepole rocks at its east n1argin. Curiously, the pluton despite its size and the 

vigorous effects · it exerted on the Lodgepole limestone, had virtually no effect on the chemically similar 

Mission Canyon. The Mission Canyon at the north 

border gives no hint of an intrusive contact nearby, 

but the contact itself is not exposed; at the west 

border, the contact is almost continuously visible for 

nearly a mile, and marked by a discontinuous zone, 

in places several feet thick, of siliceous iron and 

manganese oxide deposits, but the limestone is hardly 

more than bleached, and that only for a few feet. 

The Mission Canyon where fully preserved is easy 

to identify on the basis of its great thickness alone. 

In faqlted t~rranes, it can be confused with the 

thick and indistinctly bedded parts of the Meagher; 

just such a situation appears in the Lombard thrust 

zone in sec. 19, T. 3 N., R. 2 E., where Meagher is 

thrust over Mission Canyon. In the absence of fossils, 

the bluish tinge of the weathered Meagher distinguishes it from the yellowish-gray surface of the 

Mission Canyon; in the instance cited, the assignment 

is confirmed by the presence of characteristic Meagher 

blue-and-gold strata nearby. 

The Mission Canyon, as noted in the discussion of 

thickness, is succeeded in most parts of the quadrangle 

by the Big Snowy formation. Pre-Big Snowy disconformity is suggested by signs of weathering, such 

as discoloration, pitting, and slight channeling in 

the Mission Canyon just below the Big Snowy con:-

tact, by chips of Mission Canyon limestone just above 

the contact, and by the great variety of Big Snowy 

rock types in contact with the Mission Canyon. In 

the adjoining Toston quadrangle, disconformity is 

dramatically shown in fossil karst topography on the 

Mission Canyon, where local relief is as much as 100 

feet along 300 feet of contact with basal Big Snowy 

rocks. 

"\Vest of Milligan Creek and in the Mud Spring 

sector, Big Snowy rocks are missing and the Amsden 

overlies the Mission Canyon. Thickness measurements 

and truncation of the limestone breccia zone demonstrate deep pre-Amsden erosion of the Mission Canyon 

west of Milligan Creek. The contact itself is nowhere 

well enough exposed to give added evidence of disconformity; this is true also of the Mud Spring area. 

The uppermost part of the Mission Canyon is more 

resistant than either the basal Amsden or basal Big 

Snowy, so that the top of the Mission Canyon is a 

dip slope into a depression or valley. Commonly, 

the succeeding rocks, whether Amsden or Big Snowy, 

are red and yield red soil. Where exposures are 

lacking, therefore, the contact is drawn at the change 

of slope between identified Mission Canyon and 

younger Carboniferous strata, just downsection from 

red soil. AGE AND ORIGIN 

Systematic fossil collections were not made from 

the Mission Canyon. Its Mississippian age, however, 

is established by collections of Early Mississippian 

fossils from the underlying Lodgepole limestone and 

Late Mississippian forms from the overlying Amsden 

and Big Snowy formations. Closer dating has been 

attempted by several regional workers, who generally 

assign it to the Early Mississippian. It may be, 

however, that the uppermost beds are middle Mississippian (Gardner and others, 1945) . 

The fairly deep neritic conditions that prevailed 

at the end of Lodgepole deposition continued while 

the Mission Canyon was being deposited, but with far 

fewer and fainter oscillations. Despite the extreme 

stability thus suggested, on the order of 1,000 feet of 

highly uniform limestone was laid down in water 

that was never more than a few hundred feet deep. 

Limestone breccia in the upper part of the Mission 

Canyon is not merely a local phenomenon. It is a 

prominent feature as far north as the Limestone 

Hills, near Townsend (shown me by E. T. Ruppel); 

as far west as the southern Elkhorn Mountains (Klepper and others, 1957, p. 19-20) ; as far south as the 

Gravelly Range (Mann, 1954, p. 12); and as far 

east as the Bridger Range (McMannis, 1955, p. 1400-

1401) . The thickest and most persistent breccia in 

the region around Three Forks is in places where 

the Big Snowy is spotty or'lacking. This observation 

certainly does not apply beyond the Three Forks 

region, for neither Big Snowy rocks nor breccia 

zones are present in south-central Montana (Gardner 

and others, 1945, fig. 2), in the Willis quadrangle in 

southwesternmost Montana (W. B. Myers, oral com-
44 GEOLOGY OF THE THREE FORKS QUADRANGLE, · MONTANA 

munication, 1957), or, by inference, in the Philipsburg 

quadrangle (Emmons and Calkins, 1913, p. 67-71). 

Berry (1943, p. 16), apparently the first to publish 

a description of a persistent fragmental zone in the 

upper part of the Madison of western Montana, interpreted it to be a marine conglomerate "indicating a 

significant stratigraphic break.'' It is hard, however, 

to visualize a way in which ordinary submarine 

processes, acting far from known shores, could pro_. 

duce thin but areally extensive sheets of coarse unsorted angular limestone detritus in the midst of a 

completely conformable flat-lying limestone sequence. 

It is possible that Berry mistakenly included with the 

Madison red -cemented limestone conglomerate of 

Tertiary age that c~ops out at both ends of Milligan 

Canyon (see fig. 6) and was thus misled as to the 

nature of the process that formed the fragmental 

Madison rocks. Later workers in western Montana. 

agree in general with the interpretation of Thorn and 

others (1935, p. 27), made about similar fragmental 

rocks in the Madison of south-central Montana, that 

these rocks are solution or collapse breccia developed 

by cave-making processes above sea level. In this 

explanation, the rare lenses of conglomerate and wellbedded siltstone represent subterranean stream deposits. 

If this is the correct mode of origin, the time of 

origin must be post-Mission Canyon, rather than intraMission Canyon as Berry concluded. The breccias 

are evidently older than the oldest folding, which is 

Late Cretaceous, for they are confined to a rather 

narrow stratigraphic range without regard for kind 

or degree of deformation, or for present topography. 

If due to underground _ solution, they must l1ave been 

formed during one or more of the several known 

episodes of post-Mission Canyon pre-Laramide uplift 

and erosion. In the southern Elkhorn Mountains, 

where the Big Snowy is absent, Klepper and others 

, ( 1957, p. 20) refer formation of the main breccia 

zone to a post-Mission Canyon pre-Amsden erosion 

interval. In the Three Forks quadrangle, the presence 

of some Big Snowy rocks leaves only pa~t of Big 

Snowy time for breccia development. Farther east, 

in the Bridger Range, the basal Kibbey sandstone of 

the Big Snowy group lies on the Mission Canyon, 

leading McMannis ( 1955, p. 1401) to state that "the 

breccias developed prior to Big Snowy deposition, 

rather than prior to Amsden deposition." A reasonable inference is that the emergence which produced 

the restricted water body in which the main mass of 

the Big Snowy group was deposited also started 

solution breccia forming to the west. Where emergence was short-Fved, breccia developed poorly; westward, longer emergence encouraged growth of thicker 

and more persistent solution breccia. Thus, both 

Klepper's and McMannis' interpretations would seem 

literally correct for each area. 

BIG SNOWY FORMATION 

Assigned to the Big Snowy formation is a bewildering variety of rock types whose unity lies largely in 

their stratigraphic position, between the relatively persistent lithologies of the lVIission Canyon and Amsden 

formations. Mostly fine-grained clastic rocks and impure thin-bedded carbonate rocks, the Big Snowy is 

unresistant, and forms grassy topographic sags of 

irregular width, broken by a' few low ledges. 

Best, and least equivocal, exposures are in the canyon across the east limb of the Hossfeldt anticline in 

SW cor. sec. 29, T. 3 N., R. 2 E. There, 400 feet of 

strata of the Big Snowy consist of: 

Thickness 

(feet) 

Yellowish-gray siltstone and dark-gray shale, largely 

covered________________________ _______________ _____ 25 

Thick-bedded olive-gray limestone, forming ledge _________ 50 

Thick beds of alternating yellowish-brown sandstone and 

fragmental limestone___ ____ _______ __ ________ ________ 25 

Largely covered interval, probably dark shale, with a few 

thin fossiliferous dark-gray limestone beds near base___ 150 

Yellowish-brown thin-bedded siltstone and mudstone______ 150 

From this canyon the Big Snowy seems to swell 

and pinch irregularly both northeast and southwest, 

with rapid facies changes. The most persistent strata 

of the Big Snowy throughout the Hossfeldt Hills 

sector are the yellowish-brown siltstones at the base. 

In the adjoining Mud Spring area, not more than 100 

feet of rocks of the Big Snowy formation are present. 

They consist of a few ledges of thick-bedded dark-gray 

microcrystalline limestone, aggregating about 20 feet, 

separated by rarely exposed grayish-olive shale and 

dark clayey limestone. 

In the Milligan Creek sector, the Big Snowy is confined to a strip about 6,000 feet long, east of Milligan 

Creek. The beds of the Big Snowy and lower beds of 

the Amsden are contorted through much of this strip 

into tight folds with amplitudes of a few hundred feet, 

and exposures are spotty, so that it is hard to get a 

clear picture of the stratigraphy. In a Big SnowyAmsden section apparently 800 feet thick, a maximum 

of 200 feet seems assignable to the Big Snowy, and of 

this, float in the covered upper half includes some red 

sandstone that may be Amsden. Including the questionable strata, the Big Snowy of this sector comprises: 

Thickness 

(feet) 

Largely covered. Rare outcrops are of thin-bedded lightgray dolomite and limestone; float includes some red 

sandstone ___ ______________________________ _________ 100 

Yellowish- and greenish-gray sandstone, with a little thinbedded light-colored mudstone and carbonate rock______ 50 

Grayish-yellow siltstone and yellow-to-orange dolomite, 

poorly exposed __________ __ _____________ ___ __ ___ ___ _ 50 
SEDIMENTARY ROCKS 45 

In the Willow Creek area, rocks of the Big Snowy 

are only about 50 feet thick but persistent, assuming that 

the base of the Amsden is correctly located below a thin 

sequence of red hematite-rich sandstone. Less than 

half the interval yields outcrops; these are of olivegray and pale-yellowish-brown dolomite. The concealed part is probably rmderlain by dark shale. 

For detailed measured sections of the Big Snowy 

formation, see pages 126---127, 130, 132, and 133. 

Disconformity between the Big Snowy and the 

Amsden is suggested by rapid local changes in the 

thickness of both, by the great variety of rock types 

overlain by the basal part of the Amsden, and by the 

basal hematitic sandstones of the Amsden themselves, 

which seemingly represent worked regolith. All these 

relations, however, can be interpreted to indicate 

offlap in the Big Snowy and overlap of the basal part 

of the Amsden. The point is moot within the Three 

Forks quadrangle. In the neighboring Toston quad-: 

rangle, however, rocks of the Big Snowy group are 

thick in the southwestern part, but absent from the 

eastern part where they should be even thicker as 

the main basin of deposition of the Big Snowy is 

approached; this strongly suggests pre-Amsden erosion rather than offlap-overlap. 

RELATION TO ESTABLISHED FORMATIONS OF THE 

BIG SNOWY GROUP 

It seems futile to attempt correlations of these 

highly varied rocks from sector to sector, and even 

more so to force correlations at the formation level 

with other areas. Berry (1943, p. 18) handled the 

problem by recognizing no rocks of the Big Snowy 

group in his map area "except in the hills north of 

Three Forks where 3 feet of green shale, which may 

be Otter, separates the Madison and Amsden." Correlation of the Big Snowy of the Hossfeldt Hills with 

Scott's formations has, however, been attempted by 

Scott himself (1935, fig. 1, columnar section 7) and 

by Hendricks and Gardner (in Gardner and others, 

1945, p. 20-21). Neither of these reports locates the 

section precisely, but there is no doubt that the 

"Eustis" section of both deals with the same outcrops 

described on page · 44 and in section C ( p. 132) . The 

location of the section of Big Snowy measured by 

Hendricks and 'Gardner cannot be as described. They 

place it "about 1 mile west of Eustis [a Jlag /station] 

* * * in sees. 7 and 8, T. 2 N., R. 2 E. in the Lombard 

Hills about 4 miles northeast of Three Forks," but 

Eustis is nearly 8 miles northeast of Three Forks, and 

sees. 7 a,nd 8 are wholly covered by Quaternary deposits. However, their description of the terrain near 

the measured section, their statement that "the locality 

is within 1 mile of the Lombard overthrust," and de654729 0--63----4 

tails of the stratigraphy make it clear that the correct 

location is near that of the Hossfeldt Hills section of 

this paper, and would better be given as something like 

2 miles southwest of Eustis and 6 miles northeast of 

Three Forks, in sees. 29 and 32, T. 3 N., R. 2 E. 

A comparison of their interpretations with each 

other and with mine is illuminating. (See fig. 5.) 

The entire Big Snowy recognized by Hendricks and 

Gardner (see column D, units 1 and 2, fig. 5) includes 

about the same rocks as the lower three-fourths of the 

Big Snowy of this report (column C, units 3-6, fig. 5) 

therefore, they place in the Amsden about 100 feet 

of strata that are here regarded as Big Snowy. The 

strata in question are foreign to the basal part of the 

Amsden of the quadrangle artd the region, which consistently is dominated by red clastic rocks, so that this 

part at least of their interpretation is untenable. 

Scott's Big Snowy, on the other hand, while thinner 

than that of Hendricks and Gardner, paradoxically 

embraces the same span of strata as does the Big 

Snowy of this report, as shown by the similarity o:f 

Scotfs Heath and Amsden :formations (column B, 

units 3, 4, and 5, fig. 5) to the upper part of the Big 

Snowy :formation and the Amsden formation of this 

paper (column C, units 7b-10, fig. 5). The differences 

in thicknesses among all three measurements does 

not mean significant operator error, but rather reflects measurements at different places of rocks with 

much initial lateral variation, and much disturbed by 

minor strike faults. 

Comparison between Scott's interpretation and that 

of Hendricks and Gardner indicates that the Otter 

formation as used by Scott (column B, unit 2) comprises the same rocks as the Heath as used by Hendricks and Gardner (column D, unit 2); and the 

Kibbey formation as used by Scott (column B, unit 1) 

is the "Otter formation and perhaps Kibbey sandstone" as used by Hendricks (column D, unit 1). 

If a choice had to be made, Scott's interpretation 

seems the better. On lithologic grounds his usage of 

Heath seems acceptable enough. But even if no importance is attached to the :fauna identified by Smedley 

(see below) as similar to that of the Heath :formation 

from about 150 :feet above the base of the Big Snowy, 

and therefore far down in the Otter as used by Scott, 

the lithologies of Scott's Otter and Kibbey here are 

little like those of well-developed Otter and Kibbey 

northeast of Lombard, only 10 miles a way (Scott, 

1935, fig. 1, and p. 1026), that in turn resemble Scott's 

type sections (p. 1024-1025) generalized in column A, 

figure 5. Consequently, it seems best to treat the rocks 

of the Big Snowy group of the Three Forks quadrangle as a single unit of formation rank. 
46 GEOLOGY OF THE mREE FORKS QUADRANGLE, MONTANA 

A. Big Snowy Mts. 

(after Scott, 1935) 

HEATH FORMATION 283 ft 

Block shale 

ond sandstone 

B. 

Scott, 1935 

(fig. I, column 7 and p. 1021) 

r 

C. 

This report. 

(Measured section C, 

units 10-3) 

AMSDEN FORMATION 250ft 

C-10 

D. 

(Gordner and others, 1946) 

I Gray limestone ond 

c- 9 orange siltstone 215ft 

5 AMSDEN FORMATION 200 ft 

I 

I 

I 

r 

C-9o 

C-8o,b Red sandstone 35ft 4 

I 

J 

I 'r C-7b,c ~~~flto~~ole and 29ft 

AMSDEN FORMATION 

(total .not measured) 

Limestone 175 ff 

/ 

I I r { :; C-7a Gray limestone 42ft '-

o c 6 Brown sanpstone 31ft Gray limestone >20ft 

OTTER FORMATION 334ft 

Gray and green 

shale with minor 

gray limestone 

KIBBEY FORMATION 70ft 

Red shaly sandstone 

4 

3 

2 

Red sandstone 25 tt I 

HEATH FORMATION shale 3ottUI 

OTTER FORMATION 160 ft J 

Shale with minor ../ 

limestone, mostly 

at top 

KIBBEY FORMATION 75ft -

Sandstone 

II 

I 

/ 

~ - and limestone 

z-----

0 Dark shale i= 

<( (covered) with 141ft~ 

~ minor limestone 

r ~ C-5 u.. C-4 

>- 3: Covered (probably 

~-----brown siltstone) 82ft 

(/) r-

(? 

_ID C-3 Brown siltstone 91 ft 

2 

-- HEATH SHALE 161ft 

Shale, mostly block; 

minor dark gray 

to block limestone 

I OTTER FORMATION AND 

- - PERHAPS KIBBEY SANDSTONE 

153ft 

Ton to g roy s i Its tone 

and dark shale 

Limestone of MADISON GROUP 

F IGURI<J 5.-Compa rison of correlations of the Big Snowy west ()f Eustis Station, Gallatin County, Mont. 

AGE AND ORIGIN 

The age of the Big Snowy formation in the Three 

Forks quadrangle is only roughly determinate. Fossils are few, fragmentary, and extensively replaced 

by coarsely cryst alline silica, making precise identification difficult. Only one locality, 150 feet above 

the base of the sequence in the Hossfeldt Hills sector 

(loc. 236 [16440-PC], pl. 2; center SE14SW%, sec. 29, -. 

T. 3 N., R. 2 E.) has yielded enough material with 

~dequate stratigraphic control, to be regarded as definitive. Smedley (written communication, May 28, 1954), 

who has also collected from this locality lists the 

following fauna: ' 

Crinoid columnals 

Chonetes sp. undet. 

Dictyoclostus inflatus (McChesney) 

Buxtonia sp. undet. 

Linoproductus sp. indet. 

N eospirifer aff. n. sp. of Easton 

Oomposita ozarkana Mather 

Punctospiriter sp. 

Proetid trilobite frags., gen. indet; 

and says: 

The species identified in this small faunule are similar to 

forms identified by Easton from the Heath formation in its 

type area. The N eospirifer corresponds to a new species being 

described by Easton from the Heath and the D. inflatus is 

similar to a new sub-species being described by him from the 

same unit. This faunule is of Late Mississippian age. 

Crudely silicified corals from the s9me strata, studied 

by Helen Duncan (written communication, June 29, 

1956), included bits of a favositid, tentatively identified . 

as Michelinia?, and horn eorals. 

Two collections from beds probably of the Big Snowy 

formation in the Milligan Creek sector (loc. 43, SE~~ 

SW7~, sec. 24, and loe. 102, center E%NE% sec. 25, 

T. 2 N., R. 1 W., pl. 2) contain the following forms 

suggestive of Late Mississippian age, according to 

Smedley: 

locality 43 

[16436-PC] l

Schuchertella sp. indet. 

Stenocisma? sp. indet. 

Spirifer aff. S. braterianus Girty 

Cleiothyridina hirsuta Girty . 

Girtyella cf. G. woodsworthi Clark 

locality 102 {Fenestella sp: indet. · [

16437_PC] Camarotoechw aff. C. herrickana Girty 

Eumetria cf. E. verneuiliana Hall 

The lowermost beds of the Big Snowy in this area 

may be of late Early Mississippian age, if fossili:ferous 

thin-bedded gray limestone and orange siltstone, just 
SEDIMENTARY ROCKS 47 

below the Lombard thrust in the NW%, sec. 12, T. 2 N., 

R. 1 E., (loc. 254 [16441-PC], pl. 2) are correctly 

assigned to the Big Snowy, but it is possible that 

these rocks are from the Madison group, in an undetected fault sliver. Of fragmental fossil material 

from these rocks, Smedley said: "It is reasonable to 

believe that the collections are of Mississippian age, 

and very likely Early Mississippian (Madison)". 

Forms include : 

Crinoid columnals 

Rhipidomella sp. indet. 

Chonetes sp. indet. 

Productus cf. P. gallatinensis Girty 

LinoprodJ.uctus? sp. indet. 

Productid, gen. indet: 

Spirifer oentronatus Winchell 

Punctospiriferoid frag., gen. indet. 

The uppermost beds recognized as Big Snowy may 

be of Early Pennsylvanian age. They have yielded 

no diagnostic fossils, but the lowest fossiliferous strata 

of the Amsden cannot be dated more closely than 

Late Mississippian or Early Pennsylvanian (see later 

discussion), and the strata of the Big Snowy could 

cross the systemic boundary. 

To summarize, rocks of the Big Snowy formation in 

the Three Forks quadrangle are mostly of Late 

Mississippian age, but may include strata as young as 

Early Pennsylvanian and as old as late Early Mississippian. 

The clastic rocks of the Big Snowy formation are 

mostly if not entirely nearshore marine deposits. 

Possible exceptions are the basal siltstones which could 

be flood-plain deposits. Much of the rapid lateral 

variation in the Big Snowy may. be due to the effects 

of the irregular weathered surface of Mission Canyon 

limestone on which it was deposited. Plainly · suggested is a nearby land source of varied lithology, 

but the nearest such source seems to have been far to 

the southwest. In Big Snowy time, limestones of the 

Mission Canyon or its equivalents must have been the 

only exposed rocks for not less than a hundred miles 

southwest of Three Forks and for much greater 

distances in other directions (see, for instance, Sloss, 

1950, fig. 8). Conscious of the distances involved, 

Sloss (1951, p. 56) nonetheless favored a source to 

the southeast on the "Wyoming Shelf" or even farther 

south in the "Ancestral Rockies." Part of the clastic 

components, especially near the base, were doubtless 

derived locally by accumulation of insoluble residues 

from the erosion of the Mission Canyon. 

AMSDEN FORMATION 

The Amsden formation, which crops out in all four 

pre-Tertiary areas, is a heterogeneous assemblage of 

thin-bedded vividly colored clastic and carbonate 

rocks. Rather poorly sorted mudstone and sandstone, 

dominant at the base, give way to limestone and dolomite upsection. The clastic phases change characte-r 

as well as volume upsection, shifting to progressively 

better sorted siltstone and quartzitic sandstone. The 

unifying lithologic trait of the Amsden is its redness, 

which ranges from blackish red in the basal sandstones 

to delicate pink tints in the upper carbonate beds; 

where outcrops are poor, residual soils are typically 

reddish brown. · Many beds are other colors, with 

yellow and orange especially common in the siltstone, 

and gray in the carbonate rocks, but the formation as 

a whole has a distinctive reddish cast. The base of the 

formation is invariably in a soil-covered saddle or 

valley; upsection, exposures are progressively better 

and relief is accentuated as resistant carbonate beds 

become more numerous. 

In the Milligan Creek area, the Amsden is 500-600 

feet thick, both west of the creek, where it lies on 

Mission Canyon limestone, and east of the creek, where . 

it succeeds the Big Snowy formation. East of the 

creek, the sequence may be summarized as: 

Thickness 

(feet) 

Interbedded gray dolomitic limestone and orange to reddish-brown sandstone _________________________ ---- -- 100 

Dark-gray limestone, in a few thick ledges, separated by 

orange siltstone ____________________ - ____ - ______ - - - ~- 50 

Thin-bedded sandstone, mudstone, limestone, and dolomite, varicolored red, purple, pink, orange yellow; poorly exposed ____________________________________________ 450 

West of the creek, exposures are too poor for detailed 

measurements, but it is plain that the basal unit is 

much thinner and the upper units concomitantly 

thicker. Almost every sandstone and mudstone has 

abundant carbonatic cement. An interesting feature 

of some of the brown sandstone beds in the upper unit 

is the presence of several percent of chlorite, determined in immersion liquids and checked by X-ray. 

The Amsden appears in the Mud Spring area only 

in a short strip cut by Mud Spring Gulch. The full 

section is present for only half a mile north of the 

gulch, and much of that is covered. Here the Amsden 

is 300 feet thick, comprising: 

Thickness 

(feet) 

Thick-bedded gray limestone and dolomite, forms ledges_-_ 200 

Thin-bedded red and yellow limestone and siltstone, poorly exposed ____________________________________________ 100 

On the east flank of the Hossfeldt Hills anticline the 

Amsden is even thinner, being rather close to 250 feet 

thick, and likewise divisible into two convenient units: 

Thickness 

(feet) 

Thin-bedded gray limestone and orange siltstone ______ ---- 200 

Thin-bedded red and greenish-gray mudstone and sandstone, poorly exposed ___________________________ ----- 50 
48 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

In the Willow Creek area the Amsden is generally 

around 400 feet thick, though in places it is more than 

500 feet thick. Typical _sequences include: Thickness 

(feet ) 

Thin-bedded yellowish-gray dolomite and limestone, with 

subordinate orange and red siltstone ____ ________ ____ __ 100 

Thin-bedded pinkish-gray quartzitic sandstone and yellowish 

siltstone _____________________________ __ ____________ 200 

Thick-bedded gray, orange, and brown sandstone, with 

reddish-black hematitic sandstone at base ______________ 100 

For detailed measured sections see pages 126, 130, 132, 

and 133. 

Thickness of the middle unit varies erratically, 

whereas the upper and lower units are persistent. 

The Amsden strata of the Milligan Creek and Willow 

Creek sectors are rather alike as are the Amsden of the 

Mud Spring and Hossfeldt Hills sectors, but the 

Amsden of the two southwestern sectors contrasts 

markedly with that of the two northeastern sectors. 

The contrasts are mostly with respect to clastic com-

. ponents. The carbonate rocks are rather uniform, 

being mostly microcrystalline and even textured, wl.th 

considerable silica in the form of nodules of black or 

olive chert, scattered grains of quartz, and, in the 

limestone, thoroughly silicified fossils. The few fossiliferous beds have sugary coarser textures, and coarse 

sugary dolomite beds are rather common near the top 

of the formation. The Amsden and Big Snowy are 

unique among the Paleozoic rocks in containing many 

silicified brachiopods and corals; in older strata, these 

forms are replaced by carbonate or, rarely, iron oxides. 

Despite the contrasts within the respective sections, 

each begins with basal hematite-rich sandstone or 

siltstone, and ends by grading into the overlying 

Quadrant formation through an alternation of thick 

beds of gray limestone or dolomite and pale quartzitic 

sandstone. In the Willow Creek and Milligan Creek 

areas, the passage from dominant limestone to dominant sandstone involves 50 feet or more of transitional 

beds in which the two rock types are about equal, so 

that the contact is mapped · as gradational. In the 

Mud Spring and Hossfeldt Hills sectors, the transition is generally accomplished within 20 to 30 feet 

stratigraphically and the symbols for ordinary contacts are used. Where outcrops are poor, the lowest 

ledge ( 2 feet or more thick) of sandstone is arbitrarily 

used as the base of the Quadrant; the same stratum 

is surely not being chosen throughout but the errors 

are small. This criterion has been widely used in 

western Montana (for example, Gardner and others, 

1946, p. 4-5; Klepper and others, 1957, p. 20). An 

obvious alternative, to choose the top of the Amsden 

as the highest thick limestone or dolomite bed, is not 

practicable, as thick carbonate strata appear throughout the Quadrant. 

AGE AND ORIGIN 

Fossils from the Amsden ai-e few, poorly preserved, 

and ~onfined to the middle of the formation. They 

indicate either Late Mississippian or Early Pennsylvanian age. Only the Milligan Creek and Willow 

Creek areas have yielded fossil material from the 

Amsden. The six collections cited below, from the 

3-mile strip of Amsden at the north side of the Willow 

Creek area (see pl. 2) , represent no strata lower than 

150 feet above the base or higher than 100 feet below 

the top. · 

1. Collection 323 [16366-PC]from gray silty limestone 150-175 feet above the base (J. E. Smedley, 

written communication, Dec. 6, 1954): 

Cleiothyridina sp. 

Dictyoclostus sp. indet. 

Echinoconchus sp. indet. 

Linoproductus sp. 

2. Collection 315 [16444-PC] from olive-gray limestone 350-400 feet above the base (J. E. Smedley, 

written communication, May 28, 1954) : 

Composita cf. a. subtilita (Hall) 

A.ntiquatonia sp. 

Rhipidomella? sp. indet. 

Terebratuloid brachiopod n. gen.? 

3. Collection 316 [16445-PC] from orange siltstone 

20-30 feet below 315 ( J. E. Smedley, written 

communication, May 28, 1958) : 

Spirifer cf. S. increbescens Hall 

Composita sp. indet. 

Orthotetid brachiopod gen. indet. 

Productid gen. indet. 

Punctospiriferoid gen. indet. 

Cypricardella sp. indet. 

4. Collection 321 [16446-PC] from 25 feet of yellowish gray and pale red siltstone, 275-300 feet above 

the base ( J. E. Smedley, written communication, 

May 28, 1958) : 

Fenestellid bryozoan gen. undet. 

Spirifer aff. S. inc1·ebescens Hall 

Composita aff. C. sulcata w ·eller 

Punctospirifer sp. indet. 

5. Collection 322 [16447-PC] from 25 feet of strata 

like those of 321 and directly .below 321 ( J. E. 

Smedley, written communication, May 28, 1954): 

Fenestrate bryozoan cast indet. 

Fistuliporoid bryozoan gen. indet. 

Camarotoechia? frag. sp. indet. 

Spirifer increbescens Hall 

Composita cf. C. subtilita (Hall) 

Orthotetid brachiopod frag. gen. indet. 

Chonetes sp. indet. · 

Linoproductus sp. i.ndet. 

Productid frag. gen. indet. 

Rhipidomella sp. 
SEDIMENTARY ROCKS 49 

6. Collection 309 [16443-PC] from light-colored, 

mostly yellowish gray silty limestone, dolomitic 

limestone and siltstone 150-250 feet above base 

( J. E. Smedley, written communication, May 28, 

1954) : 

Fenestellid bryozoan gen. indet. 

Spiriter cf. S. increbescens Hall 

Chonetes sp. indet. 

Derbya sp. indet. 

Dictyoclostus cf. D. injlattus (McChesney) 

Linoproductus sp. indet. 

Myalina? sp. indet. 

The Milligan Creek area furnished two Amsden 

fossil localities, both from east of the creek and from 

the middle limestone unit. One of these (loc. 106 

[16439-PC], pl. 2; NE14NW14 sec. 25, T. 2 N., R. 1 

W.) contains a faunal assemblage much like that in 

the Willow Creek localities ( J. E. Smedley, written 

communication, May 28, 1954) : 

Echinoid plate gen. indet. 

Spirifer cf. S. increbescens Hall 

Oomposita cf. 0. subquadrata Hall 

Dictyoclostus cf. D. injlatus (McChesney) 

Straparollus (Euomphalu s) sp. indet. 

and fragments of punctospiriferoid and productid 

brachiopods. 

The other locality, 4,500 feet farther east (loc. 103 

[16438-PC], pl. 2; SW cor. sec. 19, T. 2 N., R. 1 E.) 

has yielded, according to Smedley, two of the forms 

present in the other localities listed: 

Spirifer cf. S. increbescens Hall 

Oomposita sulcata Weller 

and also several forms known from the upper part 

of the Big Snowy group in its type area: 

Antiquatonia cf. n. sp. of Easton 

Spirifer aff. n. sp. of Easton 

Cleiothyridina hirsuta Girty 

Eumetria aff. E. acuticosta Weller ; 

and also, according to Duncan, fragments of a coral 

of a "manuscript genus" known from the Otter formation. Like the fauna from the bulk of the underlying 

Big Snowy, the Amsden faunas can be dated no closer 

thai_I- Late ~ississippian and (or) Early Pennsylvanian. Tlus means that the systemic assignment 

of 300 to 800 feet of fossiliferous beds is in question. 

Furthermore, no fossils have been recovered from the 

overlying, supposedly Pennsylvanian Quadrant formation, leaving in question the age of 300 feet more o£ 

strata. Plainly, subdivision of the Carboniferous 

systems into Mississippian and Pennsylvanian in this 

area must await further work. The apparent occurrence of typical Big Snowy fossils high in the Amsden 

is pu~zli~g, but can hardly be profitably discussed 

now In VIew of the meager local and regional evidence. One safe conclusion is that the erosional 

unconformity, if any, between the Amsden and Big 

Snowy represents a short time. 

The Amsden is mostly a transgressive shallow-water 

marine formation, no doubt under climatic and orographic conditions much like those which led to deposition of the Flathead sandstone and Wolsey shale. 

The basal hematitic sandstones are mineralogically 

like the Flathead, though less well sorted and rounded, 

suggesting a s}rorter period of beach reworking. The 

differences between the finer grained parts of the 

Amsden and the Wolsey are mainly mineralogic, and 

reflect differences in the mineralogy of source rocks 

of the clastic components. As the source of the clastic 

rocks of the Flathead and Wolsey was Precambrian 

crystalline rocks like those to the south, and Belt 

rocks derived from such crystalline rocks, the source 

rocks of the bulk of the Amsden must be sought in 

other terranes. Part of the clastic rocks of the 

Amsden were no doubt derived from the erosion of 

nearby Carboniferous rocks. The chlorite in the 

upper sandstone of the Amsden, however, cannot have 

had a local source. Perhaps it" and, of course, other 

detrital components as well were supplied by the 

erosion of Paleozoic, Belt, and Precambrian crystal-: 

line rocks exposed far to the west (but not necessarily 

farther th~n the vicinity of the Idaho border where 

the previously mentioned Cambrian land barrier recognized by Willis (1907), Walcott (1915), and others, 

could have persisted into, or been revived in, late 

Carboniferous time.) 

The depositional environment of the interbedded 

carbonate rock and quartzitic sandstone in the upper 

part of the Amsden resembled that of the overlying 

Quadrant formation (p. 51~52). 

QUADRANT FORMATION 

The Quadrant formation, mostly variably cemented 

sandstone, crops out in all four areas ?f pre-Tertiary 

rocks. Where the sandstone is well cemented by 

silica, and is appropriately called quartzitic, it tends 

to be resistant to erosion, and forms prominent ledges 

and ridge crests. Where the rock is cemented by 

calcite, it crops poorly and yields a thin rubbly soil. 

The kind and degree of cementation varies greatly 

in individual beds, so that ledges are discontinuous, 

though the strata that form them may not be. Best 

exposures of the Quadrant are east of Milligan Creek, 

along the section line between sees. 24 and 25, T. 2 N., 

R.1 W. 

Although. its thickness varies considerably, the gross 

lithology of the Quadrant is rather uniform throughout the quadrangle. The main rock type is thick-
50 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

bedded light-colored medium-grained rounded quartz 

sandstone. Cementation is mainly by quartz overgrowths, but a little carbonatic cement is present in 

nearly every stratum, and in many carbonate is the 

principal cement. Rarely both fine-sand and coarsesand phases occur. Cross-stratification occurs, but is 

not common. Thick interbeds of light-colored even:-

grained sandy dolomitic limestone, abundant in the 

lower part, are progressively rarer upsection. 

In the Milligan Creek area, the Quadrant averages 

about 300 feet thick, though it is 400 feet thick where 

detailed measurement, summarized below, was made 

east of Milligan Creek: Thickness (feet) 

Thick-bedded quartzitic sandstone, yellowish-gray, pinkishgray, very pale orange; forms ledge ___________________ 100 

Sandstone like that above but with calcareous cement 

poorly e~posed ____________________________________ ~ 150 

Alter~~g thick beds of light-colored calcareous sandstone .. I . , 

(2/3) and of sandy and lolomi~ tone (1/3), poorly 

exposed ______ ~------------------------- --- ------- 150 

Gardner and others ( 1945, p. 18-20) measured the 

same section described here, but- reported only 72 

feet of Quadrant. At the base of their measured 

Quadrant is 10+ feet of resistant light-gray limestone 

that they assigned to the Amsden, so there is no way 

of knowing how they interpreted the 300 feet of 

quartzitic sandstone and limestone below. The impression is that they abandoned their own criterion, 

and used the top of the highest thick limestone rather 

than the base of the lowest thick sandstone for the 

Amsden and Quadrant contact. Their Quadrant, thin 

as it is, nevertheless includes 32 feet of strata assigned 

to the Phosphoria formation in this study, based on 

the presence of bedded chert and of early Permian 

fossils (see p. 52-53) . · 

The comparatively great thickness of the Quadrant 

east qf Milligan Creek -may be initial, or may be due 

to unrecognized strike faults like the fault which 

duplicates the Phosphoria and Ellis just upsection in 

the same vicinity, or may reflect varying amounts of 

pre-Phosphoria erosion. The last of these possibilities seems remote; evidence is inadequate for 

choosing between the other two. 

The Quadrant crops out extensively on both sides 

of Mud Spring Gulch, but due to folding and accidents of exposure there is no section suitable for 

measuring. Minimum thiGkness is 250 feet; average 

thickness may be much greater, as several map measurements, not worth much consideration sin~ly, consistently are close to 500 feet. The basal 100 feet or 

so is thick-bedded yellowish-gray and yellowish-orange 

quartz,itic sandstone, without carbonate interbeds. At 

least 100 feet of similar beds are at the top of the 

formation. Some covered zo:q.es within the formation 

are probably underlain by sandstone with calcareous 

cement. 

In the Hossfeldt anticline: the Quadrant is comparatively thin, rarely xceeding 150 feet: Thickness (feet) 

Quartzitic sandstone like that below, but lacking limestone, 

brecciated and sheared___ _____ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 50 

Thick-bedded white to yellowish-gray quartzitic sandstone; 

a few beds 1-3 feet thick of grayish-yellow limestone ____ 100 

As the description suggests, the formation may be 

thin due, in part at least, to faulting. Scott (1935, p. 

1017) reported a thickness of 160 feet here ("west of 

Eustis"). 

In the Willow Creek sector, the Quadrant is 200 feet 

thick, and consists of: Thickness (feet) 

Thin- to thick-bedded yellowish-gray quartzitic sandstone, 

and a few thin beds of white limestone _____ ___ __ __ ___ __ 150 

Alternating -thick beds of light-gray dolomitic limestone and 

quartzitic sandstone ___ __ __ __ _____ _______ _____ ·____ ___ 50 

The Quadrant is conformably overlain by the Phosphoria formation. Gradational relations are suggested 

by the appearance throughout the Phosphoria of 

quartzitic sandstone and sandy limestone strata identical with those in the Quadrant. Overlap of the 

Phosphoria is suggested by variations in lithology of 

the basal beds of the Phosphoria, but there is no hint 

of pre-Phosphoria subaerial erosion, except possibly 

in the erratic thickness of the Quadrant formation. 

In the Milligan Creek and Hossfeldt Hills areas, 

where the basal . part of the Phosphoria is resistant 

thick-bedded carbonate rock, the contact has little 

topographic expression; elsewhere, the basal strata of 

the Phosphoria are unresistant chi ppy siltstone and 

bedded chert, and the contact is marked by a shallow 

and narrow depression. 'Vhere the contact is not 

exposed, it is arbitrarily placed at the downsection 

base of the lowest slope bearing outcrops or float of 

any distinctive Phosphoria rock type: chippy_ yellow 

siltstone, bro,:vn or gray chert, or phosphatic sandstone. 

Detailed measured sections .are given on pages 126, 

130, 132, and 133. 

AGE AND ORIGIN 

No fossils were found in the Quadrant. On local evidence . the Quadrant may be as young as ear~y Permian, for Foraminifera from the base of the Phosphoria (see p. 53) are no older than middle Wolfcamp; it may be as old as early Pennsylvanian, for 

no fossils from the underlying local Amsden are 

dated as younger than early Pennsylvanian, if that 

young. .Scott ( 1935, p. 1031) regards the Quadrant, 

redefined, as "the westward equivalent of the Ten-
SEDIMENTARY ROCKS 51 

sleep sandstone" and of "basal Pennsylv~nian" age. 

Early Pennsylvanian (Des Moines) fusulinids have 

been reported from near the top of the Tensleep as 

far west as the Pryor Mountains, 150 miles southeast 

of Three Forks (Richards, 1955, p. 31). But if the 

Quadrant of southwestern Montana is truly gradational with the Phosphoria, as suggested not only in 

this paper but also by Klepper and others (1957, p. 

22) and by 'Scholten and others (1955, p. 366) then 

at least part of the Quadrant is late. Pennsylvanian 

or even early Permian as Sloss and Moritz (1951, p. 

2165), and later Scholten and others ( 1955), recognized. Furthermore, Verville ( 1957) presented evidence that the Tensleep in the Big Horn Mountains, 

Wyoming, is of Permian (Wolfcamp) age. On the 

opposite side, Mann (1954, p. 18) found the Phosphoria disconformable on the Quadrant in the Gravelly Range. 

If the Quadrant near Three Forks is indeed the 

strict lithologic equivalent of the Tensleep of eastern 

Montana, it too may be of Pennsylvanian and Permian age. Sandstones like the Quadrant are deposited 

slowly, and the Quadrant lithofacies may well rise 

in time westward. Until definitive faunas are discovered in the western Quadrant, however, it seems prudent to assign the Quadrant near Three Forks the 

customary Pennsylvanian age. 

The thick-bedded, locally cross-stratified sandstone 

of the Qua-drant, almost monomineralic and thorollghly worked, suggests shallow neritic or beach conditions on a stable shelf, dominated by slow marine 

transgression. The sandstones are much like those of 

the Flathead, and conditions of deposition were presumably similar. The interbeds of limestone, much 

of it clastic, but not bioclastic, and with a persistent 

fraction of quartz sand grains, suggest the same depositional environment. 

A nearby land source seems required. It is not 

necessary, of course, for the landmass to have been 

reduced to base level to account for each limestone 

bed, or even for the shoreline to have retreated. A 

sufficient inference is that local shifts of current caused 

sand to bypass a particular part of the basin long 

enough for limestone to accumulate, partly by direct 

chemical deposition, and partly by current reworking 

of poorly consolidated chemical limestone. 

While it is not hard to account for the limestones, 

absence of intermediate fine-grained clastic rocks is a 

real problem, and one of more than local dimensions, 

for similar accumulations of interbedded quartzitic 

sandstone and limestone are widespread, particularly 

in the Pennsylvanian and Permian of western United 

States; for example, Oquirrh formation of central 

northern Utah (Gilluly, 1932), Wells formation of 

northeastern Utah, eastern Idaho, and southwestern 

Wyoming (Richards and Mansfield, 1912). 

The coarse clastic rocks of the Quadrant were derived from a source either of varied rock types, or of 

rocks much like the Quadrant in composition and 

grain size. If the source was a normally varied assemblage of older rocks, then accumulation of nearly 

pure quartz sand in one place must inevitably have 

been attended by accumulation of clay rocks elsewhere. If the Quadrant is genetically like the Flathead sandstone and the offshore time-equivalent of 

the Flathead is the Wolsey shale, what rocks of an 

age overlapping that of the Quadrant are like the 

Wolsey~ If the age of the Quadrant throughout is 

limited to Early Pennsylvanian as it seems to be in 

eastern Montana, there are no shaly rocks in the 

region which can play the part. But if the western 

Quadrant is younger, as suggested earlier, then a possible candidate for this role is the thick sequence of 

fine-grained clastic rocks in the lower part of the Phosphoria formation, far to the south. · 

Such interpretation implies a northern source. One 

possible source is in central Montana, roughly on the 

site of the old Big Snowy basin, as reconstructed by 

Scott, (1935, fig. 3). The Quadrant thins abruptly 

northeastward from Three Forks, being less than 50 

feet thick in much of the Toston quadrangle, and absent from central Montana (Scott, 1935, p. 1015) . It 

may never have been laid down there, as Scott implied; its absence can, however, also be interpreted 

as due to pre-Ellis (Jurassic) erosion, rather than to 

nondeposition (see, for example, Vine, 19[56, p. 433). 

But although the former Big Snowy basin may have 

been an island or peninsula contributing detritus to 

the Quadrant sea, its dimensions do not seem· to have 

been large enough to provide material for all the 

Quadrant of southwestern Montana, which blanketed 

many thousands of square miles to average depths of 

200 or 300 feet, and is 1,000 feet thick in places 

(Condit, 1918, p. 111). 

The other possibility, namely that the sandstone of 

the Quadrant was derived from rocks consisting almost 

wholly of coarse quartz sandstone, poses equal problems. The only rocks of this sort in the regional 

geologic column are the Flathead sandstone and the 

quartzites in the Belt series. The Flathead is plainly 

too thin to have been a major contributor to the Quad- · 

rant. In most places, the Belt series has too high a 

proportion of clay-rich strata, and its quartzite is 

too fine grained, to qualify. In northern Beaverhead 

County, a thick sequence of coarse-grained quartzite 

(W. B. Myers, oral communication, 1957) would seem 

lithologically suitable, but it is apparently a local 

phase of inadequate volume to supply a significant 
52 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

part o.f the Quadrant. The thick and extensive Hoodoo quartzite and. its equivalents in south-central 

Idaho (Ross, 1935, p. 15-19) would appear suitable 

in nearly every respect, but are rather distant. 

Derivation of the local Quadrant clastic rocks by 

winnowing of sand from a northerly landmass of 

varied lithology seems the better interpretation at present, but there are many uncertainties. The Quadrant 

of southwestern Montana clearly invites detailed 

study. 

PERMIAN 

Rocks of demonstrably Permian age are 75 to 200 

feet thick, and are assigned to a single formation, 

the Phosphoria. They are made up of about 40 percent chert, 40 percent sandstone and · siltstone, and 

20 percent carbonate rock, mostly dolomitic limestone; 

the clastic ratio, assuming that chert is nonelastic, is 

near 0.7. The Permian rocks are unique in this region 

in containing substantial amounts of bedded chert and 

of phosphatic material. 

Because of the economic importance of its phosphatic 

rocks, the Permian of the Cordilleran region has been 

thoroughly studied, mostly during the past decade by 

the Geological Survey. McKelvey and others (1956) 

summarized the regional stratigraphic relations and 

proposed an interesting nomenclatural scheme. Cressman (1955) described the physical stratigraphy of the 

Permian rocks of southwestern Montana. 

PHOSPHORIA FORMATION 

The Phosphoria formation, which crops out in all 

four areas of pre-Tertiary rocks, is a heterogeneous 

assemblage of clastic and chemical rocks, displaying 

almost as much variety as the Big Snowy formation. 

Best exposures are east of Milligan Creek in sec. 24, 

T. 2 N., R. 1 W. Limestone like that of the Quadrant 

and quartzitic s,andstone dominate the section in some 

places; in other places, dark phosphatic sandstone and 

yellow siltstone prevail. The main unifying factor is 

the presence of bedded chert throughout. Considering 

the great internal variety and the presence of an important unconformity above, the variations in thickness of the Phosphoria are surprisingly small. 

Topographic expression of the Phosphoria varies 

with the dominant lithology. Where rocks similar to 

those of the Quadrant are the main component, they 

yield low discontinuous ledges, ~nd there is little topographic evidence of change from the Quadrant. In 

some places tl~e contact appears inconspicuously on a 

back-slope valley wall; in others, on a steplike dip 

slope. Where unresistant rock types dominate, the 

basal Phosphoria forms a ·low grassy depression and 

rises upsection toward the overlying resistant basal 

rocks of the Ellis formation. 

In the 11illigan Creek area, the Phosphoria is at its 

thinnest; it is nowhere more than 150 feet thick and in 

places is as thin as 75 feet. A typical section includes: 

Thickness 

(feet) 

Thick-bedded brown chert, and thinly interbedded chert, 

yellow very fine sandstone, and greenish-gray phosphatic 

sandstone______________ ____________________________ 50 

Dolomitic limestone, like basal unit; forms white ledge___ __ 25 

Orange quartzitic sandstone and brownish-gray chert, 

poorly exposed ________ __________________ __ -._________ 25 

Yellowish-gray dolomitic limestone, with many pods of olive 

chert________________ __________ ___ _________________ 25 

Most of the chert is very thin bedded to laminated, 

in sequences a few inches thick. The brittle chert, 

intricately fractured, rarely yields good outcrops, but is 

readily traced by its abundant angular float. Near the 

top of the formation are a few thick-bedded sequences 

of chert that form bold outcrops. There, chert is 

invariably somber, mostly in muted tones of green and 

brown. A little "of it is phosphatic, as shown by a 

bluish-white weathering bloom. 

Although the quartzitic sandstone and some of the 

limestones are indistinguishable ·from those of the 

Quadrant, many of the limestones differ somewhat. 

They have considerable clay that produces a dull 

earthy surface, even when freshly broken, and distinctive knobby pods, commonly 4 to 8 inches long 

and a quarter as thick, of dark chert. Discontinuously present at the very base of the formation, east 

of Milligan Creek, is a bed, rarely as much as 3 

feet thick, of gray limestone like that of the Quadrant that is riddled with .lentils and partings of 

chert, and contains many silicified fusulinids; a few 

similar lenses are scattered in the lower unit of the 

formation. 

The greenish-gray phosphatic sandstones in the u_pper part of the formation, though of small vohime, 

are unusual enough to warrant description. They 

are rarely as much as 1 foot thick or traceable for 

more than a few hundred feet laterally; the thickest 

lens exposed, 31j2 feet above the base of the upper 

unit in SvV1/4SW1/4 sec. 24, T. 2 N., R. 1 W., is 

now here more than 11j2 feet thick. Three or four 

similar, but much thinner, lenses typically appear 

higher in the unit. The rock is medium to coarse 

grained, rather poorly sorted, and well cemented, in 

places by carbonate, in places by silica. The sand 

grains are of rounded to subangular chert, . phosphatic oolites and pellets, shell fragments (some of 

which seem to be phosphatic), and iron-stained clay 

pellets, in widely varying proportions. The phosphatic grains weather bluish white, making a distinctive speckled surface. 
SEDIMENTARY ROCKS, 53 

Gardner and others ( 1945, p. 19~20) describing 

the same section, reported a thickness of only 84 

feet. As noted earlier, they included with the Quadrant 32 feet of chert-rich clayey carbonate strata 

that are here classed as basal Phosphoria. 

In the Mud Springs sector, the Phosphoria is exposed · 

only along two short ~tretches at the west edge, and 

there poorly. The section is about 150 feet thick and 

comprises: 

Thickness 

(feet) 

Thick-bedded yellowish-gray quartzitic sandstone_________ 50 

Olive-gray thin-bedded chert____________________ _______ 50 

Thinly interbedded chert, carbonatic siltstone and phosphatic sandstone, largely concealed _________ ~________ _ 50 

Limestone, so prominent near Milligan Creek, is 

absent here, and phosphatic sandstone is near the base 

rather than the top. 

In the Hossfeldt Hills, the Phosphoria is about 200 

feet thick, and fairly well exposed. It consists of: 

Thickness 

(feet) 

Thick-bedded yellowish-gray quartzitic sandstone_______ __ 50 

Thinly interbedded dolomitic limestone, siltstone, and chert, poorlyex~osed _____________________________ __ ______ 25 

Thick-bedded light-olive chert, forms ledge____ ________ ___ 50 

Quartzitic sandstone, limestone, and chert, largely covered__ 50 

Yellowish-gray limestone with large pods of chert_________ 25 

This section is much more like that near Milligan 

Creek, 10 miles away, than that near Mud Spring Gulch, 

6 miles away, except for the upper unit. It differs 

notably from both in lacking phosphatic sandstone. 

In the Willow Creek area, the full Phosphoria section 

is present only along a 1,000-foot strip in the NWXSEX 

sec. 34, T. 1 N., R. 1 W.; elsewhere, it is covered by 

Cenozoic deposits. Exposures in this strip are good, 

though possibly not typical. They com prise: 

Thickness 

(feet) 

Thick-bedded yellow quartzitic sandstone _____ ,___________ 25 

Chippy yellow and orange siltstone; minor chert and phosphatic sandstone, poorly exposed_____________________ 50 

Olive-gray chert, poorly exposed, with three 3-foot beds of 

greenish-gray phosphatic sandstone near top_ _ _ _ _ _ _ _ _ _ _ 50 

Orange siltstone and dolomite, and olive-gray chert, poorly 

exposed_ _____ __________ ______ ______________________ 25 

The Phosphoria of this area largely lacks carbonate 

rock, and in this respect reseinbles that of the Mud, 

Spring area; in general, it is much more like the two 

distant sectors than the nearby Milligan Canyon sector. 

Measured sections of the Phosphoria are given on 

pages 125-126, 130, 131, and 133. 

The Phosphoria is disconformably overlain by 

rocks of the Jurassic Ellis formation. In most places 

the basal beds of the Ellis are pebble and cobble 

conglomerate composed of chert of the Phosphoria, 

plain evidence of erosional unconformity. In the few 

exposures of the contact there is no hint of angular 

unconformity. Although the exact contact is rarely 

exposed, definitive exposures of both formations are 

numerous enough to fix the contact everywhere within a few hundred feet horizontally. (In the recently 

published Jurassic folio (McKee and others, 1956) 

plate 2, showing pre-,J urassic geology, is in minor 

error in showing the Phosphoria absent below the 

Ellis in ·the northeastern part of the Three Forks 

quadrangle. In fact, Phosphoria rocks, though thin, 

persist northeastward throughout all but the northeasternmost part of the Toston quadrangle, and the 

Phosphoria-Quadrant boundary of pre-Ellis time was 

thus on the order of 20 to 25 miles farther northeast 

than drawn.) 

NAME, AGE, AND CORRELATION 

Peale did not recognize Permian rocks in any part 

of the Three Forks 1° sheet. Most of the Permian 

rocks make . poor outcrops. and .have few fossils; those 

that crop out well are unfossiliferous strata, like the 

Quadrant, and it is not surprising that Peale included 

them with the Quadrant. Condit (1918) was the 

first to recognize that lithologic equivalents of the 

Permian Phosphoria formation, with type area in 

southeastern Idaho and northeastern Utah, occur 

throughout the Three Forks quadrangle. Nearly all 

subsequent workers in southwestern Montana have 

applied that name to all the rocks, of whatever 

lithology, between the Quadrant formation and known 

Mesozoic rocks. Berry ( 1943, pl. 1, p. 20- 21) included 

in the Ellis formation rocks regarded by Condit 

(1918), by Cressman (1955, pl. 4), and in this paper 

as Phosphoria equivalents. Faunal evidence that some 

of these rocks in the Three Forks quadrangle are of 

early Permian age and therefore at least rough timeequivalents of the Phosphoria was presented by Frenzel and Mundorf (1942). 

Fossils collected during this study from the base of 

the Phosphoria in the strip in 81/2 sec. 24, T. 2 N., 

R. 1 W. (loc. 115 [f97901], pl. 2), apparently also 

the locality of Frenzel and Mundorf, have been examined by L. G. Hen best (written communication, 

May 19, 1954), who identified the following forms: 

Pseudoschwagerina sp. cf. P. montanaensis Frenzel and 

Mundorf 

Pseudoschwagerina or Schwagerina sp. 

Pseudofusulina (or ParafuSiuUna?) 

indicative of middle or late W olfcamp (or possibly 

early Leonard) age; that is, very early but not earliest 

Permian. The unfossiliferous upper part of the formation may, of course, be younger by an epoch or 

more but there is no reason to suspect that any part 

of the formation is younger than Permian, for lower 

Triassic rocks, though absent from the Three Forks 
54 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

quadrangle itself and from the region to the west, 

north, and east, overlie the Phosphoria equivalents 

not far to the south (see, for example, Klepper, 1950, 

p. 66; Moritz, 1951, fig. 11; Mann, 1954, p. 20-22). 

The name Phosphoria is assigned to the entire Permian sequence even though phosphatic shale and 

chert, the characteristic lithofacies of the type Phosphoria, do not dominate the local sequence. There 

is much merit in the proposal of McKelvey and his 

associates ( 1956) that the name should be limited to 

rocks of a single lithologic community, the chertmudstone-phosphorite facies, and that different names 

should be given to sequences of Phosphoria age but 

different lithofacies: Park City formation (of Boutwell, 1907) for carbonate facies, and Shedhorn sandstone (new name) for quartz sandstone facies. 

The Three Forks quadrangle contains rocks that 

might be assigned to all three of McKelvey's formations, and in roughly equal proportions, but the scale 

of mapping prevents their separation, if all _three formations are really present. The rocks must be mapped 

as a unit, preferably with a convenient unit name. 

This might as well be the time-honored Phosphoria 

formation, rather than such unwieldy, though more 

precise, constructions as "rocks of the Phosphoria interval," "rocks of Phosphoria age," or, in the usage 

of Williams (McKelvey and others, 1956, p. 2856-

2857), "rocks of Park City age." 

ORIGIN 

The light-colored quartzitic sandstone and sandy 

limestone of the Phosphoria no doubt formed like 

similar rocks in the Quadrant; that is, not far from 

shore under conditions of slow marine transgression. 

The dark phosphatic sandstone and yellow siltstone 

bear the stigmata of shallow-water deposition also, 

but their poor sorting and high organic content suggest regression, and at a comparatively rapid rate. 

Evidence in this area is insufficient to indicate whether 

transgression and regression were cyclic, as concluded 

on regional evidence by McKelvey and others ( 1956, 

p. 2827-2829) for the western field as a whole, and 

by other authors, and if so, how many cycles are represented. The regional studies by Cressman ( 1955, 

p. 22-25) and McKelvey and others ( 1956, especially 

their fig. 1) suggest a land source of varied rocks 

northeast of Three Forks-perhaps about the same 

landmass which has been visualized as supplying the 

sand of the Quadrant. 

As for the bedded chert nothing can ,be said, on 

the basis of this study, about the source of its silica, 

and little about the conditions of its deposition. Its 

uniform anisotropic texture indicates chemical deposition. Its intimate association with all the other 

Phosphoria rock types, including pebbly phosphatic 

sandstones, strongly suggests that it too was deposited 

in shallow water, but in _quiet places bypassed by 

clastic material. This is the same environment 

sketched for the limestones of the Phosphoria and the 

Quadrant as well. The variables which controlled the 

proportions of limestone and chert in such environments offer an interesting field for investigation. 

MESOZOIC 

The exposed Mesozoic sedimentary rocks, subdivided 

into three map units, are 500 to 1,100 feet thick, but 

occupy less than 1 square mile. Comprising but a 

small part of the known Mesozoic column in adjoining areas, they are not a representative sample of 

the region. Indeed, exposures are so poor and fragmentary that they may not even be representative of 

the quadrangle. If they are representative, sandstone 

and siltstone make up the great bulk, perhaps threequarters, of the deposits of the era with the remainder comprised of small volumes of limestone, 

mudstone, and conglomerate. 

Rocks of Triassic age are absent. The oldest Mesozoic rocks are ne~rshore marine deposits of Late 

Jurassic age; these are the youngest mappable marine 

deposits in the quadrangle. Rocks of latest Jurassic 

and Early Cretaceous age are largely stream deposits. 

The Mesozoic rocks are structurally conformable 

with the Paleozoic rocks, sharing their intense deformation. 

Of probable Mesozoic (Late Cretaceous) age are 

a thick sequence of mainly andesitic extrusive rocks, 

that can readily be placed in the geologic column 

and could be treated as stratigraphic units. They are, 

however, discussed separately along with the intrusive 

rocks. Their stratigraphic position is shown in table 1. 

JURASSIC 

The Jurassic system is represented by 300-500 feet 

of highly varied rocks, separated into two map units, 

the marine Ellis formation, and the overlying continental Morrison formation. The Jurassic-Cretace-

·ous time boundary, following regional custom (for example, see Imlay and others, 1948; Klepper, 1950, p. 

67; Scholten and others, 1955, p. 367; McMannis, 

1955, p. 1406), is arbitrarily placed at the lithologic 

boundary between the rather similar Morrison and 

Kootenai formations. As diagnostic fossils have been 

recovered from neither formation in the quadrangle, 

and only from the top of the thick Kootenai in neighboring areas, the system boundary may not coincii 

with the rock boundary. 
SEDIMENTARY ROCKS 55 

The Jurassic rocks of the quadrangle are confined to 

eight scattered, mostly covered strips aggregating little 

more than 3 miles in length. With such information, 

only a minimum of discussion is in order. Fortunately, added information on the Jurassic of the 

Three Forks quadrangle is not urgently needed, as 

adequate statements concerning most elements of Jurassic geology have recently appeared at every thinkable 

lever from the regional (Moritz, 1951), to the State 

(Imlay and others, 1948), to the provincial (Schmitt, 

1953) to the national (McKee and others, 1956) and, 

finally, to the world (Arkell, 1956). On all significant 

points, the Three Forks quadrangle has been properly 

treated, considering scale. 

ELLIS FORMATION 

Conglomerate, limestone, and sandstone assigned to 

the Ellis formation crop out in all four pre-Tertiary 

areas. Reasonably good exposures of the full sequence 

are confined to the Milligan Creek and Hossfeldt Hills 

sectors; best exposures are in sec. 24, T. 2 N., R. 1 W. 

The basal beds of the Ellis are about as resistant as 

the upper ones of the Phosphoria, and there is little 

topographic expression of the contact. In most places, 

where the Phosphoria appears in a steplike dip slope 

below a Quadrant-capped ridge, the Ellis continues the 

steplike terrain, ending in a sag that marks the base of 

the unresistant Morrison formation; in a few places, as 

east of Milligan Creek, the lower part of the Ellis caps 

a ridge, and only the upper part forms a dip slope. 

The ,Ellis of the Milligan Creek area is 75 to 150 feet 

thick. A typical section includes: 

Thickness 

(feet) 

Very thin bedded yellowish-gray limestone, with minor 

brown sandstone, poorly exposed_______ ______ ____ ____ 50 

Thick-bedded brown sandstone, forms ledge_ _ _ _ _ _ _ _ _ _ _ _ _ 25 

Brown, orange, and gray thick-bedded limestone__________ 10 

Brown pebble conglomerate, forms ledge___ ____ __ ____ ___ _ 10 

The conglomerate is made up of subangular to 

rounded pebbles of dark chert and subordinate orange 

siltstone, derived from the underlying Phosphoria formation, in a brown matrix of quartz and chert sand. 

The rock is well cemented, partly by silica, partly by 

calcite. Although its lithology is rather constant, its 

thickness is not. In some places, it is more than 30 

feet thick; in others, it is absent and limestone is at 

the base. 

In contrast to the conglomerate, the limestone In 

the lower part of the Ellis i~ rather persistent in 

thickness but highly variable in lithology. Within 

a few hundred feet laterally it grades from a brown 

or orange fragmental variety--containing many grains 

of chert and quartz sand, a few chert pebbles, and 

bits of pecten shells-to a dense finely crystalline 

medium-gray variety. 

The sandstone weathers brown to a depth of several 

inches. The unweathered rock is gray. Subrounded 

medium quartz sand is the main component; subangular dark chert sand is abundant, and grains of 

limonite are common. Imlay and others (1948, loc. 

42) describe the sandstone of the Ellis east of Milligan Creek as glauconitic, but glauconite was- not mentioned by Hendricks and Gardner (in Gardner and 

others, 1946, p. 19) who measured the same section, 

nor recognized in this study. The rock is well cemented, partly by silica, partly by calcite. The sandstones thus are much like the matrix of 'the conglomerates. The sandstone strata are broken into blocks 

by persistent rectangular joints spaced several feet 

apart. On steep dip slopes, as in SE1/4 sec. 24, T. 2 

N., R. 1 "\V., the blocks tend to slide downhill, forming spectacular talus jumbles at the base. Alexander 

(1955, p. 59) published excellent photographs of such 

piles. 

The limestone at the top of the Ellis is variably 

microcrystalline to finely crystalline, and has a high 

proportion of sand and clay. The weathered rock is 

flaggy, with an earthy appearance and grayish-yellow 

color more like siltstones than typical limestones. This 

led Hendricks and Gardner to characterize this interval as 49 feet of "brittle calcareous sandstone * * * 

with some calcareous siltstone." The sequence includes 

a few thin beds of brown sandstone, much like that 

below. 

In the Mud Spring sector, the Ellis formation is exposed only in two short strips in sees. 20 and 29, T. 

3 N., R. 1 E. Though only half a mile apart, they 

are very different. In sec. 29 the upper and lower 

limits of the formation are easily located and its thickness is on the order of 200 feet; in sec. 20 the top of 

the Ellis is covered by Quaternary gravel, but the partial sequence exposed is close to 350 feet thick. The 

reasons for this discrepancy are not known. Exposures in sec. 29 are poor, so that knowledge of the Ellis 

in the Mud Spring sector rests, shakily, on observations in sec. 20, where the formation seems to comprise: 

Thickness 

(feet) 

Thinly interbedded varicolored clayey limestone and calcareous siltstone, poorly exposed____________________ 225 

Hard brown cherty sandstone, poorly exposed_--------- 35 

Orange fragmental limestone ________________ - - ------- 20 

Mostly covered; rare outcrops are of chert pebble conglomerate_ ___ _____________________ _______________ 30 

Thick-bedded olive-gray limestone, with chert pebbles-- 30 

The lower half of this section seems to correlate 

fairly well with the entire Milligan Creek section, 

leaving the upper half with no correlatives at :NElligan 

Creek. 
56 GEOLOGY OF THE mREE FORKS QUADRANGLE, MONTANA 

The Ellis formation in the Hossfeldt Hills is 100-200 

feet thick. The typical section comprises: 

Thickness 

(feet) 

Covered interval. Rare outcrops are yellowish-gray siltstone and limestone_________________________________ 75 

Thick-bedded brown chert pebble conglomerate_________ __ 25 

Thick-bedded yellowish-gray limestone, forms ledge___ ____ 25 

Locally, as much as 30 feet of uncemented breccia 

composed of boulder-size angular masses of chert and 

quartzitic sandstone separate the limestone from the 

underlying Phosphoria. This breccia may be tectonic 

but more likely is a rubble reflecting pre-Ellis weathering. The Ellis rock types in the Hossfeldt Hills all 

have their counterparts in the Milligan Creek and 

Mud Spring sectors, but the distinctive thick-bedded 

brown-weathering sandstones of those areas are lacking here. ' 

In the Willow Creek area, Ellis rocks are exposed 

only in N1/2SE1/4 sec. 34, T. 1 N., R. 1 W. Topographic relations suggest that at least 100 feet of 

Ellis rocks are present, but detritus from the Phosphoria formation masks all but 3 feet of chert pebble 

conglomerate at the base. 

Measured sections of the Ellis formation are given 

on pages 125, 130, 131, and 133. 

Because the upper part of the Ellis and the lower 

part of the Morrison are alike unresistant, exposures 

of the contact are rare, and their relations are in 

doubt. The two formations are presumed conformable 

here, being conformable in nearby areas (Klepper and 

others, 1957, p. 23; McMannis; 1955, p. 1406) and 

regionally (Imlay and others, 1948). 

NAME AND CORRELATION 

The name Ellis formation was first used by Peale 

in 1893 (1893, map) for the rocks above his Quadrant 

formation and below the Cretaceous, but the rocks 

themselves were not described. The formation is 

briefly described later in his Three Forks folio (1896, 

p. 2--3) as b~ing 300--400 feet thick, with thick upper 

and lower quartzitic sandstone separated by fossiliferous argillaceous limestone ; he recognized that the 

basal sandstone may be Carboniferous.· This basal 

sandstone, as noted earlier, is included in this report 

with the Quadrant formation, and part of the middle 

argillaceous limestone unit of Peale is the Phosphoria 

formation of this report. It is difficult to compare 

the rest of the Ellis as recognized here with Peale~s 

generalized descriptions. 

Berry ( 1943, p. 20-21) described and figured the 

Ellis formation from near Milligan Creek as being 

170 feet thick, and including all the strata from the 

"white sandstone, mostly silicified" at the top of the 

Quadrant to a thick; "brown calcareous sandstone" at 

the base of the Kootenai formation. I assign all but 

the upper 50 feet to the Phosphoria formation, and 

the brown calcareous sandstone that is Berry's basal 

Kootenai is here considered part of the Ellis. 

In the Sweetgrass Arch, Cobban (1945, p. 1263) 

raised the Ellis to group rank and divided it into 

three forn1ations, roughly comparable to the three informal divisions of Peale: sandstone-and-siltstone 

units at the base (Sawtooth formation) and at the top 

(Swift formation) separated by a limestone-and-shale 

unit (Rierdon formation). Imlay and others (1948, 

loc. 42) assigned the entire Ellis interval near Milligan Creek to the Swift formation, with the concurrence of Cobban (R. W. Imlay, written communication, Jan. 6, 1954). Thus there are grounds for applying the term Swift to all the rocks of the Ellis in the 

Three Forks quadrangle, but I hesitate nevertheless, 

because the lithologic succession can .. be_ otherwise interpreted (see, for example, Moritz, 1951, p. 1803, fig. 

4, who figures an Ellis section from the "Three Forks 

area" as about 125 feet thick and consisting about 

equally of all three of Cobban's formations) and because needed faunal evidence is lacking. Consequently, 

the name Ellis formation is used in this report. 

AGE AND ORIGIN 

The only fossils recovered from the Ellis are fragments of the pelecypod Oampt.oneot'es studied by Imlay (written communication, Jan. 6, 1954), and. of 

oysters from limestone near the base. According 

to Imlay, ' "Oamptonectes is evidence for Jurassic . age. " 

If only the Swift formation is present, the rocks are 

pres~mably of Late Jurassic age; if equivalents of the 

Rierdon and Sawtooth also occur, then part of the 

Ellis is of Middle Jurassic age (McKee and others, 

. 1956, table 2). . 

T~e rocks of the Ellis formation are evidently of 

shallow marine origin. The flaggy . yellowish rocks 

of mixed composition and fine grain in the upper part 

may be brackish water or estuarine deposits, heralding 

the retreat of marine waters from the quadrangle and 

the deposition of the landlaid Morrison formation. 

As first recognized by Cobban (1945), aceepted generally by reeent workers, and shown in the maps of 

the Jurassic. folio (MeKee and others, 1956, espeeially 

pl. 8) a low island persisted in eentral Montana during 

Eliis time. The western limit of the Ellis sea was, 

according to the folio, close to the Idaho-Montana state 

line. The Three Forks quadrangle is near the southwest margin of the island, and probably received at 

least part of its Ellis clastic rocks, particularly the 

ehert-pebble eonglomerates, from the northeast. A 

westerly souree .for much or all the clastic roeks of the 

Ellis cannot, however, be ruled out. 
SEDIMENTARY ROCKS 57 

MORRISON FORMATION 

Fine-grained clastic rocks assigned to the Morrison 

formation crop out north of the Jefferson River and 

form an irregular but subdued topography. Good exposures are restricted to the Milligan Creek sector. 

Elsewhere, the Morrison is marked by a distinctive 

reddish-brown soil. 

The Morrison seems to be rather constant in thickne~s. and lithology wherever it is exposed. In the 

Milligan Creek sector it is 200-250 feet thick. It 

~e~ms about as thick in the Mud Spring sector, where 

It IS largely covered by Quaternary gravel, but somewhat thinner, 100-150 feet thick in the Hossfeldt 

anticline. ' 

Dominantly reddish-brown mudstone near the base 

the formation becomes better sorted and coarse; 

grained upsection, with siltston~s and fine sandstones 

prevailing near the top, and the red-brown tone gives 

way t~ y~llows and yellowish browns. This general 

trend IS Interrupted by sporadic yellowish-gray siltstones near the base and a few reddish-brown mudstone strata near the top; near the base, too, are lenses 

of dark-gray shale that weather bright bluish gray. 

Many mudstone beds, mostly reddish brown, are mottled or varicolored locally, in-somber shades of red 

brown, orange, and yellow in many combinations: 

Lenses of olive-gray microcrystalline limestone a few 

feet thick and a few tens or hundreds of fee~ long, 

are scattered throughout the formation. The sandstone beds near the top are generally a few feet thick 

and made up largely of quartz sand, with a little 

clay, chert, and limonite. All the clastic rocks have 

calcare~us cemen~. Carbonaceous shale and lignitic 

coal, widespread In the uppermost Morrison in much 

?f Montan_a. (~oritz, 1951, p. 181~-1811) and present 

In the adJOining Elkhorn Mountains (Klepper and 

others, 1957, p. 24) and Toston quadrangle are not 

exp~sed in the Three Forks quadrangle, but' may under he covered areas. Two measured sections are described on pages 125 and 131. 

. The contact with the overlying Kootenai formation 

IS conformable, perhaps gradational. The base of the 

Kootenai in this region is generally taken as the base 

of the lowest thick stratum of coarse sandstone rich 

in grains ·of dark chert and widely called "salt-andpepper" sandstone (see for example, Gardner and 

others, 1946, . p. 2; Kl,epper and others, 1957, p. 

24; McMannis 1955, p. 1406; McKee and others, 

1956, ~· 3). Whe~e the basal salt-and-pepper sandstone IS well cemented, the contact with the Morrison 

is easy to locate. In some places, however, this sandstone is absent, or unresistant and thin, and choice of 

a contact is difficult, as the Kootenai contains many 

fine sandstone and siltstone beds much like those in 

the Morrison. Along such stretches the contact is 

merely guessed. 

NAME AND CORRELATION 

The Morrison and Kootenai formations of this report were placed by Peale ( 1896"; 'P· 3) in the Dakota 

formation, of Cretaceous age. Since then, the term 

"Dakota" has been restricted to Upper Cretaceous 

rocks that apparently do not occur in southwestern 

Montana. Berry ( 1943, p. 20-21) placed the same 

rocks wholly in the Kootenai formation, of Early 

Cretaceous age, but noted that the "lower part of the 

section * * * may be equivalent to the Morrison and 

Cloverly formations" recognized nearby. Imlay 

( 1948) measured the Jurassic section east of Milligan 

Creek in the course of a state-wide Jurassic study, 

and assigned "231 feet, siltstone and claystone, varicolored" above the Ellis to the Morrison formation of 

Eldridge (Eldridge and Emmons, 1896). As noted 

above, other workers have done the same with nonmarine dominantly red strata above the Ellis and below the lowest thick salt-and-pepper sandstone. 

AGE AND ORIGIN 

Although these rocks seem appropriately named 

Morrison formation, it does not necessarily follow 

that they are wholly of Late Jurassic age, as is the 

Morrison of the type area in faraway central Colorado 

(Simpson, 1926). Diagnostic fossils have not been 

found in the ,Morrison or in the lower several hundred feet of the Kootenai in the Three Forks quadrangle, or elsewhere in southwestern Montana. In the 

Sweetgrass Arch, more than 100 miles north of Three 

Forks, Cobban ( 1945, p. 1269-1270, 1281) found an 

erosional unconformity between the two formations, 

and Brown (1946, p. 246-247) found that the unconformity separates Jurassic plant remains from Cretaceous ones. At the southern end of the Big Belt 

Range/3 and also in the Gravelly Range (Mann, 

1954, p. 30-31) the Morrison is overlain with angular 

unconformity by rocks of Kootenai lithology (called 

Cloverly formation by Mann). McMannis (1955, p. 

1406) also recognized a disconformity in the nearby 

Bridger Range but found no fossils to aid in determining whether or not it is sensibly contemporaneous 

with that in the Sweetgrass Arch country. It is thus 

possible that in and beyond the Three Forks quadrangle the arbitrary lithologic boundary between Morrison and J(ootenai formations does not coincide· with 

the equally arbitrary time--line . between Jurassic and 

Cretaceous. In the absence of definitive new data ' 

Ia See f()otnote, p. 13. 
58 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

however, the age of the Morrison formation is taken 

as Late Jurassic. 

The Morrison exposures reveal little about their 

depositional environment, but the prevalence of poorly 

sorted red-brown mudstones and of siltstones, and the 

lenticularity of many layers, combine to suggest a nonmarine, largely fluvial, environment. The extremely 

fine grained limestones may well have been deposited 

in fresh-water overflow ponds or lakes. 

Convincing reconstructions of the paleogeography 

of Morrison time (Kimeridgian according to Arkell 

( 1956, p. 20) but commonly spelled Kimmeridgian in 

this country) by Imlay (McKee and others, 1956, pl. 

8) indicate that the Three Forks quadrangle was then 

near the west edge of a great basin of continental 

deposition, that extended southward into Arizona and 

New Mexico, eastward into the Dakotas and Nebraska, 

and narrowed northward near the Canadian border. 

The provenance of the Morrison clastic rocks is therefore reasonably sought to the west, probably beyond 

Montana, as 250 feet of Morrison formation is present as far west as Drummond (Imlay and others, 

1948, loc. 17). It should be noted that this view conflicts with the earlier interpretation of Schmitt (1953, 

p. 389), based on far less data: "* * * cessation of 

marine Jurassic deposition was accomplished by gradual withdrawal of the marine waters to the west into 

the Utah trough. As the waters withdrew, continental 

deposition occurred over a very extensive area as 

represented by the Morrison formation * * *" inescapably inferring an eastern source for the Morrison. 

CRETACEOUS 

In the Three Fdrks quadrangle, Cretaceous sedimentary rocks are exposed only north of the J e:fferson 

River where they reach a maximum thickness of 650 

feet. They are confined to a single landlaid formation, the Kootenai of Early Cretaceous age. This is 

iri striking contrast to the Cretaceous sedimentary 

column of nearby areas, which is very thick and 

mostly of Late Cretaceous age : as much as 3,000 feet 

thick in the Elkhorn Mountains to the northwest 

(Klepper and others, 1957, p. 24); moJ;e than 12,000 

feet thick near Whitehall to the west (Alexander, 

1951, fig. 3) ; more than 2,500 feet thick in the Gravelly 

Range to the south (Mann, 1954, p. 31-32) ; not less 

than 10,000 feet thick in the Bridger Range to the· 

east (McMannis, 1955, p. 1406-1409) ; and more than 

3,000 feet just across the Missouri River in the Horseshoe Hills<14 (Verrall; 1951). 

Studies of the Cretaceous, comparable to those for 

most of the older systems, have not been published on 

any regional scale that includes southwestern Montana. 

14 See footnote, p. 11. 

KOOTENAI FORMATION 

Sandstone and siltstone, with subordinate mudstone 

and limestone, assigned to the Kootenai formation, 

crops out in the three more northerly sectors of preTertiary rocks. A complete section of the Kootenai is 

exposed at the east edge of the quadrangle in the Hossfeldt Hills area (sees. 29 and 32, T. 3 N ., R. 2 E.). 

In the Mud Spring and Milligan Creek areas, only 

the lower Kootenai is exposed, in six scattered strips 

with an aggregate length of about lY2 miles, and in 

five of these the rocks have been altered, at least in 

color, due to igneous intrusions. Within these strips, 

the rocks are fairly well exposed, the resistant sandstones forming ledges and low ridges, separated by 

shallow depressions thinly mantled with reddishbrown soil. The fine-grained clastic rocks of the Kootenai, like those of the Wolsey shale and Three Forks 

shale, are preferred hosts for tabular intrusive rocks. 

The Kootenai formation at the east edge of the 

Hossfeldt Hills area is 650 feet thick, but only 500 feet 

is present within the quadrangle. The formation includes: 

Thickness 

(feet) 

Thick-bedded mottled orange to gray coarse gastropod 

limestone, forms ledge ________________ --------------- 10 

Covered_______________ __ ____________________________ 75 

Thick-bedded yellowish-brown sandstone and light-olivegray quartzitic sandstone, forms ledge ____ ___ ------ ----- 10 

Largely covered. Scattered outcrops are of gray and red 

limestone ; main float is red and orange mudstone ___ ------ 200 

Thick-bedded orange sandstone, with lenses of chert pebble 

conglomerate_ ______ _____ ___ ________________ ________ 50 

Covered_____________ ________________________________ 50 

Interbedded orange sandstone and olive-gray quartz~tic 

sandstone__________________________________ ________ 50 

Largely covered. Scattered outcrops are brown limestone, 

·gray claystone, red siltsto]le and sandstone ___ ----------- 17 5 

Thick-bedded orange sandstone, with much chert, forms 

ledge ________________________ ----------- --------- 25 

For more detailed sections. see pages 124--135. 

Although sandstone is only about 20 percent of the 

formation, it provides most of the outcrops. The 

main variety is grayish orange and is composed of 

medium to coarse subangular quartz sand that contains many chert grains and chips of orange siltstone 

and is firmly cemented by iron-stained clay and calcite. Less abundant is an olive-gray variety that is 

similar in o-rain size and in rounding but lacks silt- b •• 

stone chips and is cemented largely by s1hca overgrowths· this is the common salt-and-pepper sandstone of 'the Kootenai throughout the region. Because 

this variety does not appear in the basal sandstone, 

but is 200 feet higher and above a thick covered interval that produces reddish soil, it might be argued 

that the two lower units of this section belong in the 

Morrison formation. Most of the other Kootenai ex-
SEDIMENTARY ROCKS 59 

posures in the quadrangle, however, have no gray chertbearing ·Sandstone whatever, but all have basal sequences of orange or yellowish-gray chert-rich coarse 

sandstone. Orange or yellowish-gray chert-bearing 

sandstone must be accepted as diagnostic of the Kootenai in the Three Forks quadrangle, or the formation cannot be mapped except in the Hossfeldt Hills. 

The limestone at the top of the formation, though 

thin, is persistent in the low hills just east of the 

quadrangle; similar gastropod-bearing limestone is at 

or near the top of the Kootenai in most of the region 

(Peale, 1896, p. 3, under heading Dakota formation; 

Klepper and others, 195'7, p. 25; Freeman, 1954, p. 

10-14; Scholten and others, 1955, p. 356) but not in 

the Bridger Range (McMannis, 1955, p. 1406). 

In even the best exposures, at least .'70 percent of the 

formation is covered. Float and scattered small outcrops suggest that the covered intervals are underlain mostly by thin, impersistent brown and red calcareous mudstone and siltstone, and subordinate red 

fine calcareous sandstone, brown coarsely crystalline 

sandy limestone, and gray claystone. Gray claystone 

is associated with lignitic coal in the upper part of the 

Kootenai in the Toston quadrangle, but no coal is 

exposed in this interval in the Hossfeldt Hills. 

In the Milligan Creek sector, the lower 100 to 200 

feet of the Kootenai is fairly well exposed. The 

lower 100 feet is largely thick-bedded faintly laminated yellowish-gray chert-rich medium-grained sandstone, with calcareous cement, much like the basal 

sandstone in the Hossfeldt Hills. The succeeding 100 

feet, exposed only in sees. 24 and 13, T. 2 N., R. 1 W., 

includes similar sandstone, but in thin beds, intercalated with thin lenses of yellow- and reddish-brown 

siltstone and fine sandstone. Much of the sequence 

is reddened and hornfelsed by a large andesitic intrusive. The entire Kootenai- is missing for about 500 

feet in SEl/4 sec. 24, whe-re the andesite is in contact 

with the upper part of the Morrison. 

Several hundred feet of Kootenai appear no be pressent at the west edge of the Mud Spring area ( SW14 

sec. 20, T. 3 N., R. 1 E.) but exposures are poor, and 

the rocks are disturbed and altered by a nearby intrusive mass. Principal outcrops and float are greenish-gray hornfels, presumably altered mudstone or siltstone. If these were originally red or brown, the 

common color of such rocks in the Kootenai of the 

Hossfeldt Hills, their colorants, presumably hydrous 

iron oxides, have evidently -been reduced owing to heating by the intrusion, just the opposite of the oxidizing 

(=reddening) effect of the intrusion near Milligan 

Creek. 

Just east of the quadrangle, the Kootenai is overlain conformably by varicolored sandstone and siltstone of the Upper Cretaceous Colorado shale. (Peale's 

geologic map (1896) shows large areas of "Colorado 

and Montana formations" of Cretaceous age along upper Milligan Creek, lower Mud Spring Gulch, and in 

the hills in the northwest corner of the quadrangle. 

The supposed Cretaceous rocks in the two valleys are 

fossiliferous Tertiary deposits, and those in the hills 

are igneous rocks assigned to the Elkhorn Mountains 

volcanics.) Within the quadrangle, the Kootenai is 

overlain with angular uneonformity by lower Tertiary 

rocks of the Bozeman group. 

NAME, AGE, AND ORIGIN 

As noted earlier, the rocks divided into Morrison 

and l(ootenai formations in this report were classed 

together as Dakota formation by Peale but as Kootenai formation by ·Berry. In this, Berry followed 

Fisher (1908) who extended the term from Canada 

into northern Montana. Fisher (1908, p. 18-19) recognized the presence of probable Morrison formation 

below his Kootenai in northern Montana, and Berry 

suspected it near Three Forks. Restriction of the term 

as made in this report is consonant with current regiona.l usage. 

No diagnostic fossils were recovered from the Kootenai. The gastropods in the uppermost limestone ar~ 

numerous but invariably replaced by coarsely crystalline caleite and by silica in a manner that destroys 

their characterizing internal structure. Samples from 

uppermost limestone of the Kootenai in the Toston 

quadrangle were studied by Reeside (written communication, Nov. 23, 1955) who was able to find only a few 

roughly determinable shells: 

U~io sp. 

Reesidella montanensis (Stanton) ? 

Gyraulus sp. 

These are not definitive as to age, but serve to confirm the formation as Kootenai, which is demonstrably 

of Early Cretaceous age as nea~ by as the Great Falls 

region (Fisher, 1908, p. 20), the Harlowton area 

(Yen, 1951, p. 1-3) , and the vicinity of Dillon (Yen, 

1951, p. 1-3). While at least part of the Kootenai 

near Three Forks is assuredly of Early Cretaceous· 

age, the possibility remains, as discussed earlier, that 

the lower part of it may be of Late Jurassic age; or, 

conversely, that part or all the Morrison formation 

may be of Early Cretaceous age. In the absence of 

significant new data, however, the Kootenai is tentatively assigned the customary Early Cretaceous age. 

The rocks of the Kootenai are much like those of 

the Morrison, and seem to have been deposited under 
60 GEOLOGY OF THE TIIREE FORKS QUADRANGLE, MONTANA 

similar nonmarine conditions. The thick and extensive coarse sandstones in the lower part of the Kootenai do not have their counterpart in the Morrison, 

and indicate a distinct but mild orogenic interruption 

in a general history of fluvial lowering of a region of 

low relief during Morrison and Kootenai time. 

CENOZOIC 

Cenozoic sedimentary rocks, subdivided into nine 

map units, dominate the quadrangle, underlying 150 

square miles. The Cenozoic units, all of nonmarine 

origin, have a maximum aggregate thickness of 3,000 

feet, but the total local thickness is probably nowhere 

greater than 2,500 feet, and is generally less than 

1,000 feet. The Cenozoic column is made up mostly of 

lower Tertiary rocks, but the surface exposures are 

largely of thin unconsolidated Quaternary deposits. 

Long ago, Peale ( 1896, p. 3) recognized the broad 

geologic similarity among the Cenozoic intermontane 

basins of western Montana : 

· The movements that began during the latter part of .the 

Cretaceous period _ultimately resulted not only in the folding 

of the strata but also in the formation of the numerous basins 

or valleys found throughout the mountainous portions of 

Montana. 

The early paleontologic work of Douglass (especially 

his papers of 1899, 1902, and 1909) and of Matthew 

( 1903) confirmed and strengthened this concept. But 

no systematic study of the Cenozoic geology of any 

part of southwestern Montana has yet appeared. The 

nearest approach to a systematic local study has been 

in the Canyon Ferry quadrangle, where Mertie (1951, 

p. 5-6, 29-43, 55-56, 84-85) subdivided the Cenozoic 

into six mapped units-four Tertiary and two Quaternary. Extensive fossil collections have been made by 

inembers of the Carnegie Museum of Pittsburgh, the 

Frick Laboratory, Princeton University, the University of Michigan, University of Chicago, and other organizations, but little publication has yet resulted. 

Some fairly detailed work on local Tertiary deposits 

and history appears in recent · unpu,blished doctoral 

theses.15 This report attempts to treat the littleknown Tertiary rocks in somewhat more detail than 

the older rocks. The Quaternary rocks, though mapped 

in considerable detail, are otherwise . summarily treated. 

Large-scale syntheses of Cenozoic history in western Montana have been ventured by Atwood (1916), 

Pardee (1950, based on work done before 1947), and 

Alden (1953, based on work before 1948), and briefer 

summaries have been given by Pardee and Schrader 

( 1933, p. 3-6, 24-28) and Lowell ( 1956a). 

15 See footnotes, p. 11, 13, and 111. 

, 

TERTIARY-BOZEMAN GROUP 

The Three Forks basin is ,carved largely in soft, 

little-deformed Tertiary rocks that were laid down in 

a basin of dimensions similar to the present one. The 

Three Forks quadrangle, in the northwest part of 

the modern basin, is also near the northwest edge of 

the Tertiary basin, as shown first by Peale ( 1893, 

pl. 1) . In the quadrangle, continental sedimentary 

rocks of known or probable Tertiary age are exposed 

over 35 square miles and are thinly mantled by unconsolidated Quaternary deposits over 40 square miles 

more. Subdivided into . four formations, they have a 

maximum aggregate thickness near 2,500 feet, but 

such a figure means little, as the pre-Tertiary surface 

had much relief. 

Principa_l rock types are siltstone, 30 percent; sandstone, 25 percent; conglomerate, 20 percent; clay, 20 

percent; limestone and marlstone, 5 percent. About 

ha1f of the Tertiary deposits is derived from erosion 

of pre-Tertiary rocks, and half is penecontemporaneous pyroclastic debris. Few beds are free of Tertiary 

volcanic ash, and many strata are made up wholly of 

such m~terial. The Tertiary rocks are of Eocene, 

Oligocene, late Oligocene or .early Miocene, late Mio,. 

cene, and early Pliocene age, but only the Eocene 

and lower Oligocene rocks are shown on the map (pl. 

1) ; the outcrops of upper Oligocene-lower Miocene, 

upper Miocene, and Pliocene rocks are too small to_ 

map. 

NAMES AND CORREL_ATION OF UNITS 

The Tertiary rocks of the Three Forks quadrangle 

were designated Bozeman lake beds by Peale (1893, 

pl. 1; 1896, p. 3), who regarded them as deposits 

in one of a series of coexisting lakes widespread in 

western Montana in "N eocene" (that is, post-Eocene, 

pre-Pleistocene) time. He concluded that some of the 

detritus was derived by erosion of the pre-Neocene 

rocks bordering each lake but that the bulk of the 

deposits is "fine volcanic dust" blown into the basin 

by winds; that a little fell on the water and settled 

to form "pure white sediments;" but that most of it 

fell on land and was washed into the lake along with 

a small proportion of foreign fragments, to :form 

layers of "rusty color." In ascribing a lacustrine origin to these deposits he was following the ideas of 

his colleague Hayden, . who, as early as. 1869 (p. 114-

115), stated that "from the dawn of the Tertiary 

period, even up to the commencement of the present, 

there was a continuous series of fresh-water lakes all 

over the continent west of the Mississippi River," 

and, more specifically of the headwaters of the Missouri: "These * * * broad valleys have all been lakebasins during the last portion of the Tertiary period'' 
SEDIMENTARY ROCKS 61 

(Hayden, 1872, p. 147). Peale's lacustrine thesis was 

accepted by his contemporaries Weed and Iddings 

(1894) who used the name Bozeman lake beds, and 

Douglass (1899), who did not use the name. Beginning with Matthew (1899), however, all other writers 

have opposed the idea that the basin deposits are 

largely lacustrine, favoring instead various combinations of fluviatile and eolian processes (see Davis 1900, 

and Osborn, 1909, p. 24-28 for summaries of early 

views), and the established name Bozeman lak~ beds 

has been avoided by the use of informal terms such 

as Tertiary deposits, or "basin beds" (Scholten and 

others 1955, p. 368) or used reluctantly, with actual 

or implied deprecatory quotation marks. 

In the Three Forks quadrangle, rocks that Peale 

assigned to the Bozeman lake beds have been divided 

in this report into three gradational map units, designated as new formations. In ascending order, they are 

Milligan Creek formation, mainly limestone, of Eocene 

age; Climbing Arrow formation, mainly bentonitic 

clay and sand, of late (and middle~) Eocene and early 

Oligocene age; and Dunbar Creek formation, mainly 

tuffaceous siltstone, of Oligocene age. The Thompson 

Creek beds of Douglass (1902) and the Titanotherium 

beds of Matthew ( 1903) are part of the Climbing 

Arrow formation. The Oreodon beds of Matthew are 

difficult to locate from his description, but probably 

are from the lower part of the Dunbar Creek formation. The Milligan Creek and Dunbar Creek formations are largely lake beds with high proportions of 

volcanic ash; the Climbing Arrow . is mostly stream 

laid; a very small fraction of the Climbing Arrow 

and possibly a larger fractio'n of the Dunbar Creek 

may be tuff, windborne into the basin and deposited 

on the dry basin floor. The three formations are here 

regarded as local, though it may eventually be feasible 

to trace them into adjoining basins. 

Sharing many features, it is fitting that these rocks 

be designated a group; in deference to Peale~s prior 

usage but to avoid debatable genetic implications, the 

name Bozeman group is proposed. Also regarded as 

part of the Bozeman group are small bodies of limestone conglomerate, with distinctive orange matrix, 

that gradationally underlie the Milligan Creek formation, are probably of Eocene age, and are assigned 

to the Sphinx conglomerate (Peale, 1896, p. 3) ; and 

unmappably small patches of Miocene and early Pliocene gravel. The limestone conglomerate is lithologically like parts of the "Laramide" (largely Paleocene) 

Beaverhead conglomerate of Lowell and l{lepper 

(1953). The Miocene and early Pliocene gravel 

patches are apparently outliers of the thick units of 

similar age exposed in the east bluffs of the Madison 

654729 0--63----5 

River and called Madison Valley formation by Douglass (1907); they have been recently discussed by 

Dorr (1956). 

In the Three Forks quadrangle, the Bozeman group 

overlaps an erosion surface that developed during 

and soon after Laramide deformation, which cannot 

be more closely dated than Late Cretaceous-early 

Eocene. The youngest rocks below this surface are the 

Elkhorn Mountains volcanics, of Late Cretaceous age. 

At the top of the Bozeman group, upper Tertiary 

gravels lie with great erosional, and possibly some 

angular, unconformity on the Climbing Arrow formation. This unconformity, involving many hundred 

feet of Climbing Arrow and Dunbar Creek rocks, 

and unknown thicknesses of any other units which 

may once have intervened, might at first glance seem 

a natural top for the Bozeman group. But because 

basin deposits of Miocene and Pliocene age are thick 

and extensive just across the Madison River and were 

likely once abundant in the Three Forks quadrangle 

too, before Quaternary uplift and erosion, their re_pnants should be included with the Bozeman group. " To ,\. 

omit them would be to bisect a useful genetic unit in I 

both time and space, and place an undesirable restric- 1 

tion on Peale's original concept. 

To recognize that Tertiary deposits are thick and 

have a wide range in age is :r:tot to imply that the 

Tertiary history ·of the Three Forks basin was largely 

depositional. All the mappable deposits in the western 

part may well have been laid down in a brief episode 

that extended from late Eocene or at most middle 

Eocene to early Oligocene time. The interval from 

middle Oligocene through middle Miocene time is 

represented by virtually no deposits anywhere in the 

basin and was likely a time of slow erosion. Upper 

Miocene deposits in the eastern part are thick and extensive but are largely tuffaceous sediments and may 

have been deposited fast. Pliocene deposits are thin 

and apparently very local. Thus the Tertiary was 

more a time of erosion than of deposition. 

The rocks of the Bozeman group, laid down in a 

·basin, are confined to the low parts of an existing 

basin, and have obviously not been deformed on a 

scale remotely approaching that of the pre-Tertiary 

rocks. They have, nevertheless, been considerably deformed. In fine-grained, even-bedded strata, presumed to have been horizontal, common dips are 

5°-10°; rarely such rocks dip as steeply as 25°, and 

dips as steep as 60° are recorded. Many conglomerates 

and sandstones dip 15°-35°, but a large fraction of 

such dips is probably initial. No faults have been 

mapped in the Bozeman group, though at least one 

is suggested by relations near Dunbar Creek, and 
62 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

others may be concealed. At any rate, there are no 

faults profound enough visibly to affect the surrounding pre-Tertiary rocks. 

The Bozeman gruup is on the whole poorly consolidated, and unresistant to erosion. Broad•benches 

have been cut across it, and thin but continuous soils 

have been developed on these benches, where they 

have not been covered by Quaternary deposits. Consequently, although exposures are generally fair on 

valley walls, they are poor elsewhere. 

Owing to the discontinuous exposure and inconstant 

dips, only the grossest units and structures can be 

traced. . 

It is hoped that the Bozeman group-defined as 

the Tertiary fluvial, eolian and lacustrine rocks which 

accumulated in the basins of western Montana after 

the Laramide orogeny-may become a useful regional 

term. If the concept is to be useful it must have 

rather elastic age limits, for it is not only reasonable 

but known that the earliest and latest deposits from 

basin to basin are not of identical age. Upper Eocene 

basin deposits have long been known from Sage Creek, 

in southwester_nmost Montana (Douglass, 1903, p. 

145-146; recently restudied by H~ugh, 1955) and now 

from the Three Forks quadrangle, but in most places 

the oldest basin fills seem to be lowest Oligocene, as 

indicated in the most recently issued Tertiary correlation chart (Wood and others, 1941). The youngest 

fossiliferous basin deposits in the Three Forks vicinity 

are of Pliocene age, but in basins farther west the 

youngest dated deposits of the basin-filling sequence 

seem generally of late Miocene age, though middle 

Pliocene basin deposits occur in Deer Lodge Valley 

(Konizeski, 1957). Both the earliest and latest time 

of filling may in many places not be datable due to 

lack of fossils or exposures. 

In several places in southwestern Montana, the 

oldest Tertiary sedimentary rocks are coarse conglomerate whose potential correlation with the Bozeman group is debatable. In Beaverhead County such 

rocks are named Beaverhead formation and are shown 

to have formed in Late Cretaceous to early Eocene 

time during the late stages of the Laramide orogeny 

(Lowell and Klepper, 1953). The Beaverhead and 

similar rocks could be considered the earliest deposits in the Tertiary basins, and thus classified as 

basal units of the Bozeman group, as I have done 

with the Sphinx conglomerate. A different but tenable 

view is that they are synorogenic rather than postorogenic and are better kept separate. 

Certain other rocks associated with the intermontane 

basins are plainly not referable to the Bozeman group 

as defined above. Glacial deposits, even if of Tertiary 

age, like the supposed Eocene Black Butte till (Scott, 

1938) should be excluded, as should mappable igneous 

flow rocks. In Quaternary time, low-level lakes existed at several places in valleys cut into the Tertiary 

basin deposits and older rocks. Such a lake occupied 

part of the Madison River valley near Ennis, according to R. E. Swanson (oral communication, 1955). 

The deposits in and on the margins of this and similar 

lakes naturally resemble the Tertiary basin deposits 

(so much so that Peale, 1896, mapped those near 

Ennis with the Neocene Bozeman lake beds) but 

should, of course, be distinguished from the Bozeman 

group wherever possible. 

The high-level gravel that overlies the Bozeman 

group cannot be closely dated. It may be of very 

late Tertiary age, but in this report it is arbitrarily 

assigned to the Quaternary' as are certain other somewhat younger fossiliferous deposits. 

'SPHINX CONGLOMERATE 

At the base of the Bozeman group and assigned to 

the Sphinx conglomerate is limestone conglomerate 

with a reddish-orange calcareous matrix that appears 

in small outc~ops at many · places in the quadrangle. 

Every such exposure is near the present basin floor 

at the foot of a limestone ridge. Most of the bodies 

are too small to map separately. As seen in valley 

walls, most of the masses are less than 10 feet thick 

and less than 100 feet long; their third dimension is 

unknown but probably small. Four large patches are, 

however, shown on the geologic map: two at the ends 

of Milligan Canyon (fig. 6) and two on the south side 

of Jefferson River valley above Willow Creek. The 

largest masses have exposed thicknesses of around 

100 feet. The subsurface extent of one, the mass above 

FIGURE 6.-Sphinx conglomerate of Eocene age, above Milligan Canyon. East side of valley, SE~4SW~ sec. 25, T. 2 N., R. 1 W. 

Thin dark layers are reddish brown bentonitic mudstone. Compare with figure 4, showing limestone breccia of cave origin in 

Mission Canyon limestone, half a mile downstream. 
, SEDIMENTARY ROCKS 63 

Milligan Canyon, is partly known as a result of 

drilling by the Bureau of Reclamation (Harold C. 

Elliott, oral communication, Aug. 5, 1957). This 

body is 30-50 feet thick below about 30 feet of 

alluvial cover for not less than 600 feet upstream from 

the northern end of the exposure; and it lies on pre: 

Tertiary rocks. 

Just east of the quadrangle, in NW%,NW1;4NW1;4 

sec. 28, T. 1 N., R. 2 E., a test well (Ryan, Tom Tice 

no. 1) penetrated a thick mass of conglomerate · that 

is probably referable to this unit. The well penetrated 

a sequence of more than 300 feet of "colored" conglomerate, mostly orange, beneath a succession 2,000 

feet thick of "sand" and "shale" with streaks of coal, 

presumably strata equivalent to the Dunbar Creek and 

Climbing Arrow formations. 

The conglomerate is composed mainly of subangular 

pebbles and cobbles of dark fine-grained Paleozoic 

limestone. Larger clasts are rare but boulders as 

much as 3 feet long occur. A few of the stones are 

light-colored Paleozoic limestone, siltstone, and quartzitic sandstone. In addition, in the conglomerate above 

Willow Creek, gneiss and schist pebbles are common, 

and rarely present are hematite pebbles derived either 

from hematitic phyllonite of Precambrian age or 

hematitic layers in such Paleozoic formations as the 

Wolsey, Big Snowy, or Amsden. In places the stones 

are angular, and the rock might better be called 

breccia. In others, many of the stones are subl'Ounded. Whether rounded or angular, the conglomerate is everywhere poorly sorted and obscurely 

bedded. The matrix, which makes up 20-40 percent 

of the rock, is earthy, porous, and generally reddish 

orange, but in places is reddish brown or yellowish 

gray. The sand-sized components are mostly quartz, 

but include feldspar, altered mica, and bits of noncarbonate rocks, firmly cemented by clay minerals 

and by powdery calcite. The orange color is confined 

to the matrix, the gravel presenting only the normal 

colors of the parent formations. The matrix color 

seems due to staining by finely disseminated ferric 

oxides and not to the body colors of component grains 

as in the somewhat similar Beaverhead conglomerate 

(Lowell and Klepper, 1953, p. 239). Conglomerate 

in the main mass above Milligan Canyon (trenched 

by the Bureau of Reclamation) contains layers a few 

inches thick of dark-reddish-brown bentonitic sandy 

siltstone and clay. 

The conglmnerate is variably deformed. Near 

Milligan Canyon the unit has low dips, conforming 

to those of neighboring Tertiary rocks. In the small 

mass in SE cor., sec. 25, T. ·1 N~, R. 1 W., the strata 

dip 25° northward, apparently passing below Milligan 

Creek formation to the north; but the contact area 

is concealed by alluvium. Bedding elsewhere is so 

poor that attitudes are unknown. 

The conglomerate lies with angular unconformity 

on the Mission Canyon limestone. Below Milligan 

Canyon, the upper part of the limestone conglomerate 

is interbedded with tuffaceous limestone and coarse 

channel sandstone of the Milligan Creek formation, 

demonstrating gradational relations. 

AGE AND ORIGIN 

The conglomerate is younger than the Carboniferous 

Mission Canyon lime!;)tone it lies on. It is older than 

the Milligan Creek formation but not greatly. As the 

Milligan Creek formation, discussed later, is of demonstrated middle or late Eocene age, the conglomerate 

is also of Eocene age. 

Similar rocks are widespread though not abundant 

in the Quaternary and Tertiary deposits of this region, and they are probably common throughout the 

Rocky Mountains. Composed mainly of limestone, 

and with a calcareous matrix, they differ greatly 

from the widespread and thick red-banded, but carbonate-poor, rocks of the Rocky Mountain region that 

have been much discus~ed (for summary and analysis 

see Van Houten, 1948). The fact that the conglomerates are mostly limestone pebbles, · little rounded, 

demonstrates that they were not deposited in standing. 

water or handled by perennial streams. Their mixed 

composition and slight rounding show that they are 

not merely talus, though they have surely not moved 

far. Their topographic position and texture suggest 

deposition by intermittent streams or by mudflows in 

fanlike forms. Except for the matrix, they are like 

ordinary alluvial cones or fans. 

But the reddish-orange calcareous matrix offers 

problems, for redness and high carbonate content seem 

generally antithetic in land-laid sediments, though 

common enough in marine ones. Very likely the 

two properties originated separately. Perhaps the 

orange clayey fraction of the matrix represents lateritic soil developed on- local uplands of low relief 

during warm and wet episodes in earliest Tertiary 

time. Such an origin would be in keeping with the 

manner and conditions of formation postulated by 

Van Houten ( 1948) for the red material in the early 

Cenozoic deposits of the region generally. Later, accelerated erosion might have stripped the clayey soil 

and mixed it as it moved downslope with limestone 

fragments shed from steep valley walls free of residual 

soil. The . mixture might then have been deposited 

at lower altitudes, orange in color, but with little interstitial carbonate; the carbonate matrix might have 
64 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

been deposited later by ground water. Such a process 

would be easier to accept if any remnants of earliest 

Tertiary upland paleosols were known. Considering 

the subsequent history of the region it would be 

astonishing if any were ever identified, however widespread they may once have been. Suggestive of an 

upland paleosol of suitable age nearby is the "pebbly 

red mantle as much as 30 or 40 feet thick" at the 

base of the Tertiary tuffaceous sequence along the 

Boulder River in the Devils Fence quadrangle 

(Klepper and others, 1957, p. 42). 

Worth considering is a reverse possibility-that the 

orange color developed later, after deposition under 

semiarid conditions of gray fan gravel with a limerich matrix. Perhaps the. orange color was developed 

by ground water, partly by oxidation of ferrous iron 

from the carbonate rocks, and partly by physical 

redistribution of ferric iron washed from hematitic 

pebbles derived from a variety of iron-rich preQuaternary rocks. This concept, while not requiring 

vanished upland paleosols, is opposed by the sharp 

contacts of gray limestone pebbles with the orange 

matrix. If· the color is postdepositional, the surfaces 

of the pebbles might be expected to be discolored as 

well as the matrix. 

The Sphinx conglomerate may well have formed a 

widespread apron at the foot of the mountains that 

were the surface expression of Laramide uplift and 

accelerated erosion. Deposits formed under such conditions are likely to be stripped as erosion proceeds 

so that only a few fortuitously covered and preserved 

fragments tend to remain in the geologic column. 

Detailed study of such remnants on · a regional basis 

would surely be rewarding. 

MILLIGAN CREEK FORMATION 

NAME AND DISTRIBUTION 

The Milligan Creek formation crops out only in the 

southwest part of. the quadrangle, on both sides of 

the lefferson River valley. It is here named for 

Milligan Creek, a southward-flowing tributary of the 

lefferson, which near its mouth cuts through and exposes rocks typical of the unit. The Milligan Creek 

formation is made up of light-colored fine-grained 

tuffaceous lake deposits, mainly limestone but ranging 

from limestone through marlstone to calcareous mudstone, and interfingering stream-channel sandstone 

and . conglomerate. 

The formation is exposed over an area of 3 square 

miles and underlies a thin covering of Quaternary 

deposits in perhaps 2 square miles more. Its original 

extent was less than 100 square miles, perhaps less 

than 50. West of Three Forks, distribution and topographic setting of exposures indicate that the formation was never more than 6 miles broad and suggest 

that its western margin may have been within the 

quadrangle, though it may have extended a mile or 

two into the Jefferson Island quadrangle. The formation probably does not extend more than a few miles 

east of its -most easterly outcrop (sec. 16, T. ~ N., 

R. 1 E.) as several borings near the east edge of the 

quadrangle penetrated little or no limestone, the characterizing rock type of the Milligan Creek ; there is 

no reason to think it ever extended farther east. The 

formation may have extended south a few miles beyond the southwest corner of the quadrangle to the 

base of high hills in the Precambrian crystalline rocks. 

In the north half of the quadra.ngle the Milligan Creek 

. formation · does not appear between the younger Ter-

. tiary units and the pre-Tertiary rocks. It might have 

been eroded before deposition of the younger units, or 

been laid down only locally and not yet exposed, but 

the simplest interpretation is that no Milligan Creek 

rocks were ever deposited. 

A very small patch of Milligan Creek formation 

may, however, -be represented by limestone (called 

"Cambrian" on the driller's log) in the bottom of a 

test well (Dunbar Oil Co., . Dunbar no. 3) 534 feet 

deep in NE%,NE:14 sec~ 11, T. 2 N. R. 1 W., on the 

west flank of an anticline that exposes the lowest 

recognized Climbing Arrow strata; similar limestones 

were not met in much deeper tests a few miles to the 

west and east. 

J;t would be unsafe to predict the presence of much 

Milligan Creek formation beneath the Jefferson River 

flood plain. In the few places where attitudes can be 

measured near the riverward edges of the formation, 

the rocks are horizontal or dip away from the river. 

Possibly, the Jefferson . River has completely removed 

the Tertiary rocks in this stretch, and filled the cavity 

with Quaternary alluvial deposits. 

TOPOGRAPHIC EXPRESSION 

The Milligan Creek formation appears in nearwhite dissected benchlands, thinly mantled with lightcolored sandy soil, between the rugged hills of preTertiary rocks and the tTefferson River flood plain. 

The limestone of the formation, unlike that of the preTertiary rocks, is not very resistant to weathering, 

owing to its high proportion of altered volcanic glass 

and clay, and the fine-grained lake clastic rocks are 

even less resistant. Where the fine-grained rocks dominate, as east of Willow Creek, exposures are poor. 

Where the formation has a high proportion of conglomerate, as near Milligan Creek and west of Willow 
SEDIMENTARY ROCKS 65 

Creek, it is more resistant, producing fairly prominent 

outcrops. Throughout, the fine-grained strata wear 

back to produce wide steps capped by sandstone and 

conglomerate. 

LITHOLOGY AND THICKNESS 

The bottom of the Milligan Creek basin is nowhere 

exposed, being covered by Quaternary alluvium or 

perhaps never reached by prealluvium downcutting. 

The exposed gradational contact with underlying 

Sphinx conglomerate, mentioned earlier, is at the edge 

of the basin and the strata there are probably unlike 

the lowest beds farther out. The original top of the 

formation is visible only in the NW1,4 sec. 16, T. 1 N., 

R. 1 E., and there poorly; elsewhere, it has been removed by erosion, and in many places the formation is 

unconformably covered by Quarternary gravel or silt. 

The best exposures of ·the formation are in the low hills 

north of Highway lOS and west of Milligan Creek. 

This vicinity (specifically, E1j2 sec. 11, NW1,4 sec. 12, 

and SW1,4 sec. 1, T. 1 N., R. 1 W.) is therefore designated the type area of the formation. 

A composite partial section, about 300 feet thick, 

from the type area, includes: 

Top of hill. 

Thickness (feet) 

Limestone: thick-bedded massive, yellowish-gray to very 

pale orange; glassy, hard, sugary, with many tiny irregular 

vugs; scattered earthy yellowish-gray rounded sand and 

granules of limestone and clay_____ ______________ ___ ___ 25 

Limestone: thin- to thick-bedded, platy to flaggy, yellowishgray to white; earthy, hard, microcrystalline; much clay, 

mostly in silt-size fragments; small irregular silicified 

patches. A few lenses, as much as 3 feet thick, of subangular 

pebble conglomerate and nearly white subangular sandstone __ _______________ _________ __ __ _______________ _ 60 

Madstone and limestone: thick-bedded light-olive-gray 

earthy madstone, weathering white, interbedded with subordinate amounts of dusky-yellow and yellowish-gray finely 

crystalline clayey limestone. All beds contain from traces 

to several percent of clay pellets mostly in silt and sand 

sizes ____________ _____ _______________________ ______ 100 

Limestone: . thin-bedded, white, clayey, hard, microcrystalline, with many tiny vugs; small irregular silicified patches. 

Many sand- to granule-size pellets of white clay; a few 

rounded grains of glassy quartz sand. A few layers, as 

much as 3 feet thick, of light-olive-gray madstone and 

of white coarse subangular sandstone____________ _______ 60 

Limestone and madstone: thick- to thin-bedded, lightyellowish-gray and yellowish-brown fine-grained strata 

ranging from calcareous mudstone to clayey limestone. 

Many sand-size pellets of clay______ _________ ___ ______ _ 50 

Bottom of gulch. 

Some of the strata here called marlstone may contain enough clay and silt to be called calcareous mudstone. Locally, the ljm~stone is replaced by laminae 

of white, red, or green chert. 

A typical earthy tuffaceous limestone from the upper part of the formation west of Willow Creek 

(sample 13, pl. 2; NE1,4NW1,4 sec. 6, T. 1 S., R. 1 E.) 

consists mostly of large fragments in a very fine 

matrix. As figure 7 shows, more than a quarter of 

the rock is coarser than coarse sand and more than 

half is silt and clay; the four intervening sand classes 

comprise less than a fifth. 

The large fragments, rounded to subangular, are 

mostly clay with a little coarse vitric ash (these and 

subsequent terms for pyroclastic material are used as 

defined by Wentworth and Williams, 1932) variably 

altered to clay; the sand and silt fraction consists 

mostly of aggregates of tiny calcite crystals with 

subordinate clay aggregates; and the clay. fraction is 

mostly clay minerals but with a heavy sprinkling of 

calcite crystals. Minor sand and silt components are 

angular fragments of light-green hornblende, brown 

biotite, chlorite, quartz and feldspar, both twinned and 

untwinned. The large clay fragments resemble the 

ash fragments in shape and color and there are all 

gradations between fairly fresh ash and pure clay. All 

the large clay fragments are probably thoroughly 

altered ash, and at least some of the clay of the matrix 

is also altered ash. Glass in the ash has a refractive 

index near 1.50. 

Two X-ray analyses (one [Lab No. 141369] by C. J. 

Parker; the other [Serial No. 226415] by A. J. Gude, 

III) also show that the clay and silt fraction of the 

rock contains mostly calcite and montmorillonite with 

very small amounts of feldspar (labradorite, according 

to Gude) , chlorite, and quartz. 

30 

1- 20 z 

w 

u 

0:: 

w 

a.. 10 

0 

PERCENT 

FIGURE 7.-Mechanical analysis of typical tuffaceous limestone from the Milligan Creek formation. Sample from 

locality 13 (see pl. 2) ; laboratory no. 141369. Grainsize classification according to Wentworth scale. Analysis by P. R. Blackmon. 
66 GEOLOGY OF THE TIIREE FORKS QUADRANGLE, MONTANA 

A chemical analysis 16 (column A in table 3) is not 

· easy to interpret in terms of mineral composition, even 

with the information given by the other methods. Assuming that essentially all the C02 in the analysis 

represents calcite, the rock has about 66 percent of 

this mineral, which is somewhat more than the visual 

estimate. But neither the microscope nor X-ray analysis reveals any mineral phase that would. account for 

the rather high phosphate content : presumably the 

phosphate is present submicroscopically in organic 

matter amorphous to X-rays. If the remaining material is wholly volcanic ash, it should be possible to 

approximate its chemical and thence its normative 

composition by subtracting calcite, phosphate in the 

form of normative apatite, and excess water, and then 

recalculating. This operation gives the compositon 

shown in columns B and 0 of table 3. 

TABLE 3.-Chemical analysis of typical tuffaceous limestone from 

the Milligan Creek formation, locality 13 (see pl. 2) 

Si02---------------------

A~Oa--------------------

Fe203-------------------- Fe0 ____________________ _ ~gO ____________________ _ 

N CaO------~-------------- a

2

0 ___________________ _ 

K20---------------------

Ti02---------------------

P20s--------------------- Mn0 ___________________ _ H2o ____________________ _ co2-____________________ ~ 

Sum __ ____________ _ 

A 

19.8 

4. 5 

1.8 

. 10 

. 52 

39. 9 

. 41 

. 50 

. 18 

. 30 

. 06 

3. 9 

28. 9 

101 

B c 

57. 5 64. 7 

13. 0 14. 6 

5. 2 5. 7 

.3 .3 

1.5 1.7 

7. 8 8. 7 

1.2 1.3 

1.4 1.5 

.5 .6 

. 8 . 9 

11. 3 ----------

100 .. 5 100 

A. Analysts, P. L. D. Elmore, and K. E. White (rapid method) . B. Recalculated free of CaCOa and Caa(P04)2. C. Recalculated free of water. 

Even with allowances for reasonable variation this 

rock fits none of the average rocks of the common igneous groups as calculated by N ockolds ( 1954), nor 

the glass-rich rocks of the Climbing Arrow formation 

(columns 7 and 8 in table 6) nor those of the Dunbar 

Creek formation (columns 9, 10, 11, table 8B). The 

principal difficulty is in the CaO content, far too high 

for rocks of comparable Si02 and Al20 3 content; part 

of the calcium may be present as sulfate, for many 

similar rocks contain visible gypsum, · but this point 

cannot now be established as 804 was not determined. 

The total iron is also too high, and N a2 0 and K20 

too low. The noncarbonate part of this rock thus 

16 This analysis and all subsequent chemical analyses of rocks cited 

were made by rapid methods similar to those described by Shapiro 

and Brannock (1956) . The results, summed to the nearest percent, 

a-re entirely adequate for the purposes of this report. Calculations 

made on the basis of such analyses are, of course, correspondingly 

inelegant but serviceable. 

seems to be polygenetic. It is nevertheless likely that 

the ash fraction is chemically similar to the ash component of tbe Climbing Arrow formation, as the refractive index of their glass is similar. The ash of 

the Climbing Arrow is in the range of calc-alkali 

rhyolite in N ockolds' classification. 

Some limestone and marlstone strata in the formation contain tests and ·casts of small gastropods and 

clams, as well as charophyte oogonia, and, rarely, 

ostracodes. 

The conglomerate interbeds, typically 1-3 feet thick 

and a few tens or hundreds of feet long, are made up 

of quartzitic sandstone, vein quartz, and much lesser 

amounts of intermediate coarse-grained igneous rocks. 

Rare components are chert and mud balls. The matrix 

is mostly subangular quartz sand, with a little feldspar, chert, and clay. Induration varies from poor, 

where the rock is loosely bound by clay and a little 

carbonate, to excellent, where the rock is solidly cemented with silica, usually clear and crystalline but in 

places milky and chertlike. The sandstone layers are 

similar in composition to the matrix of the conglomerate, but are rarely well indurated, being loosely cemented by clay and carbonate. . 

The outcrops on the east side of Milligan Creek are 

much like those of the type area, but with a higher 

proportion of platy rather than massive limestone and 

of conglomerate and sandston·e lenses. 

Between these two masses of mostly fine grained 

beds is an area underlain by a thick body of crossstratified conglomerate and sandstone, best exposed on 

the north side of the eastward -draining gulch in 

Slj2NW1h, sec. 1, T. 1 N., R. 1 W. The strata, commonly a few feet thick and dipping 5°-10° SE., are 

alternating coarse pebbly sandstone and subround to 

round pebble conglomerate, well cemented with silica. 

Fragments of many of the pre-·Tertiary sedim~ntary 

formations above the Madison group and of a variety 

of igneous rocks are present. The principal rock types 

are quartzitic sandstone, andesitic volcanic, and monzonite. Limestone is rare; shale, siltstone, and chert 

are absent. Because of its position and its lithologic 

similarity to the sandstone ond conglomer·ate interbedded with fine-grained Milligan Creek facies, this 

mass is regarded as part of the Milligan Creek forma- . 

t.ion. Actual interbedding with known Milligan Creek 

rocks is, however, not visible, and the conglomeratesandstone body may be a part not of the Milligan 

Creek formation but of the Climbing Arrow formation, deposited in a deep channel cut into the Milligan 

Creek formation. 

The formation west of Willow Creek differs from 

the type area in several respects. Fine-grained rocks, 
SEDIMENTARY ROCKS 67 

including a few beds of bentonitic siltstone, mudstone, 

and shale, in addition to the dominant limestone and 

marlstone, make up only about 40 percent of the sequence, the remainder being beds a few feet thick of 

poorly rounded rudely cross-stratified coarse sandstone and pebble conglomerate, locally cemented with 

·silica and very hard. The sandstone and conglomerate are much like their counterparts across the river, 

dominated by quartz and quartzitic sandstone, but differ in having considerable chert and a few fragments 

of gneiss and amphibolite, and in lacking andesitic 

and monzonitic debris. . ' 

East of Willow Creek, coarse layers are rare, and the 

principal rock types are thin-bedded yellowish-gray 

earthy tuffaceous calcareous mudstone and lightbrownish-gray clayey limestone. Present here are a 

few lenses, less than a foot thick, of rounded limestone-pebble conglomerate, with yellowish-gray muddy 

matrix. 

The maximum exposed thickness of the Milligan 

Creek formation is 300 feet. Although the base is not 

exposed, it probably is not far below the lowest exposures, and the total thickness is not much greater 

than the exposed thickness. In the type area and east 

of Willow Creek the exposed thickness is about 300 

feet; west of Willow Creek and east of Milligan Creek 

less of the formation is present, perhaps no more than 

200 feet. The conglomerate-sandstone body west of 

Milligan Creek is as much as 150 feet thick. 

The Milligan Creek formation seems to be conformably overlain by the Climbing Arrow formation but 

exposures of the contact, limited to a few places southwest of the Buttleman Ranch buildings, are too poor 

to establish this with certainty. Both formations in 

this vicinity are nearly horizontal and their lithologies 

appear gradational, for the Milligan Creek has some 

bentonitic strata much like the prevalent rock type of 

the Climbing Arrow, and white limestones like those 

of the Milligan Creek are abundant in the lower part 

of the Climbing Arrow. 

A possibility, hard to evaluate, is that part of the 

Climbing Arrow formation in the north half of the 

quadrangle was formed contemporaneously with all or 

part of the Milligan Creek formation. Such a relation is favored by the fact that the Climbing Arrow 

formation overlies limestone conglomerate above Milligan Canyon, whereas the Milligan Creek formation 

overlies limestone conglomerate below the canyon. 

FOSSILS AND AGE 

The Milligan Creek formation has yielded a variety 

of fossils but none are diagnostic of age. Because the 

Milligan Creek formation is seemingly gradational 

(and partly contemporaneous~) with the Climbing 

Arrow formation, and because the lower part of the 

Climbing Arrow is definitely of rather late Eocene 

age, it is assumed that the Milligan Creek formation 

is also of Eocene age. 

The formation contains many snails and a few clams 

and ostracodes. The only floral remains recognized 

are charophyte oogonia. The gastropods and pelecypods were studied by D. W. Taylor (written communication, May 29, 1957), the ostracodes by I. G. Sohn 

(written communications, Jan. 5 and March 24, 1955) ; 

the charophytes, examined by R. E. Peck of the University of Missouri, proved to be too poorly preserved 

to permit study. 

Snails are the most abundant fossils. Unfortunately, they are poorly preserved; external structures 

are obscured by the coarse texture of the replacing 

calcite, and internal structures are practically obliterated by calcite fillings, so that close identification 

is rarely possible. Nevertheless, because little is known 

of North American continental Tertiary gastropods, 

the scant data on those from Three Forks seem worth 

giving. Gastropod forms identified from the Milligan Creek and the other formations of the Bozeman 

group by Taylor are listed by him in table 4 for convenient reference. They include Lymnaea of indeterminate species ranging from Triassic on; Planorbidae of the subfamily Segmentininae indeterminate 

as to genus and similar to forms described from both 

the late Eocene Tepee Trail formation of Wyoming 

and the Oligocene Florissant formation of Colorado; 

representatives of fm~r genera of the family Pupillidae 

known throughout t'he Tertiary in Europe but little 

studied in the pre-Pliocene of the United States; and 

a new species of Oreohelim ( Radiocentrum). 

The pelecypods are all of the wide-ranging genus 

Pisidium. 

The ostracodes include two small smooth forms, one 

of which is questionably assigned to Oypridopsis j the 

other is indeterminate even as to genus. · 

According to Taylor, all snails identified from the 

Milligan Creek outcrops north of the Jefferson River 

and most of those from the south side are fresh-water 

forms of pond or lake habitat. Those from south of 

the river include both fresh-water and land snails. 

Most of the land snails are of genera that seem to 

adapt to a wide variety of environments. Oreohelim, 

however, is characteristically a mountain snail. 

ORIGIN 

Peale's concept of eolian-lacustrine origin for the 

entire Tertiary basin sequence applies well to the Milligan Creek :formation. 
68 GEOLOGY OF THE mREE FORKS QUADRANGLE, MONTANA 

TABLE 4.- Check list of gastropods in Tertiary rocks of Three Forks quadrangle, Montana 

[Localities shown on data map, pl. 2] 

Milligan Creek formation '-.: ~ Climbing Arrow 

formation 

Dunbar Creek formation 

20002 20003 20004 20005 20006 20007 20008 20009 20010 20011 20012 20013 20014 20015 20016 20017 20018 20019 

--~~-------·-------'- ·-- ----------------------------------

Fresh-water snails: 

Lymnaea________________________ X X X X X X ---- ---- X ---- ---- ---- ---- ---- ---- ---- X X 

A ustralorbis pseudoammonius (Schlotheim) _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ X ___________________________________ _ 

Planorbidae, Segmentininae ___________ X ________ X ________________________ -.--- ---- ---- ---- ---- _______ _ Planorbidae indet ____ .:. ___________ X ____ X ____________________ X ____________________ ---- ____ ____ X 

Physidae ___________________________________________________ X _______________________________________ _ 

Land snails: 

Pupillidae A--------------------- ____________________ X _____________ . __________________________________ _ 

B __________________________ ---- ---- ---- ________ X ---- ____ ---- ---- ---- X ---- ---- ---- ---- ---- ---- c __________________________ ---- ---- ---- ---- ---- X ---- ---- ---- ---- X X ---- X ---- X ---- ---- D __________________________ ---- ---- ---- ---- ---- X ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----

~==================~======= G __________________________ ==== ___ ==== ==== ==== ==== ==== ==== -~- ==== r== -x- -x- ==== -x- -x- ==== ==== ==== .:. ________________________________ ---- X ---- ---- ---- ---- ---- ---- ----

H __________________________ ---- ---- ---- ________________ ---- ---- ---- ---- X ---- ---- ---- __ ..:_ ---- ----

! _______________________ -:_ __ -------------------------------- __ _:_ ---------------- X X X -------- indet _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ X _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ 

Polygyrella ______________ . _______________________________ X X __________ · __________________ ---- _______ _ 

Oreohelix n. sp _______________________________________ X ____________________ X X X X X _______ _ Helminthoglyptidae n. gen_________ ____ ____ ____ ____ ____ ____ ____ ? ____ X ____________ X X ___________ _ 

Indeterminate___________________ _ _ _ _ X ____________________________ --- _ ---- ---- ---- ---- ---- ---- ---- ----

The number given for each locality is that of the U.S. Geological Survey Cenozoic 

series. Altitudes are correct to ::l::lO ft. All localities are in the Three Forks quad- rangle (1950) 1:62500. 20002. NWUSE%' sec. 11, T. 1 N., R. 1 W. Altitude 4,350 feet. 20003. E7':;NE%' sec. 11, T. 1 N ., R. 1 W. Altitude 4,430 feet. 20004. Same locality as 20003, but 5-10 feet topographically and stratigraphically lower. 20005. SWUSW%' sec. 1, T. 1 N., R. 1 W. Altitude 4,470 feet. 20006. NWUNW%' sec. 7, T. 1 N., R. 1 E. Altitude 4,185 feet. 20007. NWUSW%' sec. 36, T. 1 N., R. 1 W. Altitude 4,260 feet. . 20008. SE%'NEUNE34" sec. 10, T. 2 N., R. 1 W. Altitude 4,600 feet. 

20009. NEUNEUNE~ sec. 11, T. 2 N., R. 1 W. Altitude 4,580 feet. 

The limestone and other fine-grained rocks of the 

Milligan Creek formation, as attested by their lithology and fossil content and distribution, were deposited in a lake. The thick bedding suggests that 

the lake was perennial rather than seasonal. The 

lake basin seems to have existed at about the position 

of the present Jefferson Valley. The coarse marginal 

deposits and the presence of the mountain snail Ore.oheliw suggest that the basin lay in terrain not much 

different from the present. 

The volcanic ash fragments no doubt were partly 

airborne into the lake and partly washed in, after 

brief periods of subaerial weathering: The alteration to clay seems to have occurred mainly after the 

fragments reached the lake. Favoring sublacustrine 

alteration is the common presence of patches and 

streaks of silica, presumably released in the devitrification of nearby glass. Further, the coarse ash must 

have been mainly glass when it entered the lake: 

that which was airborne into the lake could scarcely 

have been much altered, and that washed into the 

lake could not have floated out and been deposited 

eventually in a very fine matrix if its pumiceous character has been destroyed by argillization. 

20010. Center sec. 12, T. 2 N., R. 1 W. Altitude 4,430 feet. 20011. SEUNW%' sec. 6, T. 2 N., R. 2 E. Altitude 4,360 feet. 20012. NWUNW%' sec. 5, T. 2 N., R. 2 E. Altitude 4,350 feet. 20013. NWUSW%' sec. 3, T. 2 N:, R. 1 E. Altitude 4,230 feet. 20014. Same location as 20013 but 18 feet stratigraphically higher. 20015. NWUSW%' sec. 3, T. 2 N. R. 1 E. Altitude 4,235 feet. 20016. S7':;NE%'SE%' sec. 19, T. 3 k, R. 1 E. Altitude 4,450 feet. 20017. NWUSWU sec. 3, T. 2 N., R. 1 E. Altitude 4,235 feet. 20018. S WUNE%' sec. 6, T. 1 N ., R. 2 E. Altitude 4,225 feet. 20019. Sec. 29, T. 1 N., R. 2 E. 600 feet E., 0 feet N. of SW corner. Altitude 

4,230 feet. 

Perhaps repeated lapilli-ash showers, by interfering with normal soil formation on the lake margins, 

were responsible for the restriction of life to the sturdy 

Charophyta and rare ostracodes. 

The vents from which the pyroclastic debris in the 

Milligan Creek and later Tertiary formations were 

erupted are unknown and likely long to remain so, 

mainly because early Tertiary volcanism was so widespread in southwestern Montana and adjoining regions. 

Source areas for the coarse interbeds are more 

readily. identified. The composition of the conglomerate and sandstone indicates that those on the north 

side of the Jefferson River, lacking metamorphic rock 

fragments, came from farther north and those south 

of the river, rich in gneiss, came from farther south. 

Their degree of rounding and sorting, their common 

cross-stratification, and their lack of limestone pebbles show that they were not shed directly off the limestone-dominated slopes on which they lie but were 

deposited by streams after considerable transport. 

None have the sorting and rounding nor . the steep 

foreset bedding expectable in deltas. The simplest 

interpretation is that these masses are deposits made 
SEDIMENTARY ROCKS 69 

in stream channels scoured in the lake deposits when 

the lake was low or dry. But surely the streams also 

flowed when the lake was high, and a large fraction 

of the sand and gravel deposits must have been made 

where streams entered the lake. The absence of welldeveloped standing-water features in any of the sandstone and ·conglomerate may be due to rapid deposition, resulting from steep gradients and perhaps from 

significant transport only during floods. 

The climate of Milligan Creek time is hard to visualize from the data at hand. The thick bedding in 

the formation suggests that the lake was perennial 

and this in turn favors a fairly humid climate. Conversely' the coarseness of conglomerate at the margins, 

suggesting torrential deposition, and the absence of 

fossil wood faintly hint at semiarid conditions. 

CLIMBING ARROW FORMATION 

NAME AND DISTRIBUTION 

The Climbing Arrow formation is made up of olive 

thick-bedded sandy bentonitic clay and coarse sand 

with subordinate light-colored siltstone, sandstone, 

conglomerate, and limestone. The formation is here 

named for the Climbing Arrow Ranch, of which the 

principal buildings (SW cor. sec. 8, T. 1 N., R. 2 E.) 

are on fairly typical outcrops of the formation though 

rather remote from the main areas of exposure. The 

Climbing Arrow, except for Quaternary alluvium, is 

the most extensively exposed unit in the quadrangle; 

it crops out over 27 square miles and is obscured 

only by a thin mantle of Quaternary detritus over 

20 square miles more. Rocks of similar lithology 

underlie large areas in the adjoining Radersburg quadrangle (V. L. Freeman, written communication, 

Mar. 4, 1954) and some northward extension of the 

formation may ultimately be feasible. 

TOPOGRAPHIC EXPRESSION 

The formation in general is unresistant, producing 

subdued landscapes. In the northwest part of the 

quadrangle it tends to form low but distinct rounded 

hills, banded in tones of light gray and white, and 

separated by broad smooth valleys. These hills are 

held up by resistant limestone, sandstone, and conglomerate strata in the lower part of the formation. 

The bulk of the formation, poorly consolidated and 

rich in clay, tends to develop somber badland topography (fig. 8) with bare clay-capped hills, rounded on 

top but with ~teep sides, separated by grassy poorly 

drained lowlands. In the lowlands, the local drainage pattern has been disrupted by many small-scale 

earth flows. 

FIGURE 8.-Climbing Arrow formation in type . area. View southeast 

from road in NW cor. sec. 12, T. 1 N., R. 1 E. 

LITHOLOGY AND THICKNESS 

The base of the Climbing Arrow is definitely known 

only from the poor exposures south of the Buttleman 

Ranch. Exposures in the minor anticline in sees. 1 

and 12, T. 2 N., R. 1 W., are presumably dose to 

the base. The top is much better known from good 

exposures east of the Mud Spring Gulch road near 

Highway 10N and along the Madison Bluffs in sec. 

6, T. 1 N., R. 2 E. Within the formation it is hard 

to establish even the general stratigraphic position 

of many exposed segments. The composite section 

below, though surely containing gross errors, gives 

the general tenor north of the-Jefferson River where 

the formation is not less than 750 feet thick, and 

may be considerably more than 1,000 feet thick. 

(Total thicknesses greater than 1,500 feet can be interpreted if use is made of . driller's logs of test wells 

in sees. 7 and 8, T. 2 N., R. 1 E., but such interpretations are of little value as the logs are terse, incomplete, and unsupported by cuttings.) 

Upper dark unit: thick-bedded olive bentonitic 

sandy clay with many lenses of dark coarse sand 

and a few layers of white coarse tuffaceous siltstone and sandstone; near basin edges, thick 

tongues of dark coarse pebbly sandstone and 

Thicknu8 

(feet) 

conglomerate_______________________________ 200 to 300 

White unit: thick-bedded white ·tuffaceous siltstone and finely crystalline porous limestone 

with a few thick interbeds of olive bentonitic 

sandy clay and poorly consolidated dark sandstone______________________________________ 100 to 200 

Lower dark unit: like upper dark unit but with 

thin lenses of white limestone and thick lenses of 

poorly consolidated sandstone and pebble 

conglomerate throughout____________________ >500 

Base concealed. 

The three units are gradational and contacts between 

them can only locally be drawn. 
70 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

The lower dark unit is especially well exposed at 

the west edge of the quadrangle near the common 

corner of sees. 8, 9, 16, and 17, T. 2 N., R. 1 W. The 

white unit is best seen in the syncline south of the 

Silver Sage Ranch in sees. 3 and 10, T. 2 N., R. 1. W.; 

it is also well exposed along the county road at the 

south edge of SW1/4 sec. 8, T. 2 N., R. 1 E.; and 

the isolated areas of Climbing Arrow rocks in sees. 

22 and 27, T. 3 N., R. 1 W., and centering in NE1/4 

sec. 34, T. 2 N., R. 1 W. are also t>robably part of this 

unit. The upper dark unit is well exposed in NW1/ 4 

se?. 16 and NE1/4 sec. 17, T. 2 N., R. 1 E., and along 

Highway 10N, about a mile south of its junction with 

.Mud Spring Gulch road. Basin -edge tongues of 

coarse detritus in the upper units are well seen near 

the mutual corner: of sees. 17, 18, 19, and 20, T. 2 N., 

R. 1 E. 

The three units, only broadly recognizable north 

lof the Jefferson River, cannot be distinguished south 

of the river. There, the formation, about 700 feet 

thick, is dominated throughou't by olive clay and dark 

sand, with limestone prominent only near the base 

and white tuffaceous sjltstone numerous only near the 

top. Excellent continuous exposures of nearly 350 

feet of the section appear in the hills east of the county road in :SE1/4 sec. 11, and W1j2 sec. 12, T. 1 N., 

R. 1 E. This area (shown in fig. 8) is designated the 

type area of the formation, as it offers thick and continuous exposures of the dominant clay-and-sand lithology. For a full view of the formation, however it . ' 

IS necessary to examine the subordinate limestone, siltstone, sandstone, and conglomerate north of the Jefferson River in. the places listed previously. A sequence about 100 feet thick exposed below the summit 

bench in center "\iV1/ 2 sec. 12 ( :rpeasured section F, pl. 

2) is representative of the type area: 

Gravel-covered bench, top of hill. Thickness 

(feet) 

Clay, thick-bedded, grayish-olive, bentonitic, and a few 

streaks of clayey fine subangular sand _____________ '"___ 12 

Sand, white to yellowish-gray, clayey, medium to coarse, 

subangular; one bed_________________________________ 1 

Clay, thick-bedded, grayish-olive, bentonitic_ _ _ _ _ _ _ _ _ _ _ _ _ 5 

Clay and sand; alternating lenses and beds, 4 inches to 1 foot 

thick, of olive-gray clayey sand and pale-olive sandy clay_ 25 

Sand, thick-bedded, yellowish-gray, fine to coarse, subangular_____________________________________ _______ 6 

Clay, thin-bedded, pale-olive, sandy; includes several 

reddish-brown and grayish-olive bentonitic zones a few 

inches thick __________ :- _____ .,_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 4 

Sand, thin-bedded, white to yellowish-gray, clayey, coarse, 

subangular_________________________________________ 8 

. Clay, thick-bedded, olive; grades into underlying sandstone______________________________________________ 5 

Sandstone, grayish-olive, medium to fine, subrounded, 

cemented with clay; one bed __________________ -,- _ _ _ _ 3 

Thickness 

(feet) 

Clay, grayish-olive to dusky-yellow, markedly bentonitic; 

onebed____________________________________________ 3 

Clay, thick-bedded, grayish-olive, sandy, bentonitic______ 7 

Sand, thick-bedded, pale-olive to yellowish:-gray, coarse, 

feldspathic and biotitic, subangular to rounded_________ 7 

Clay, grayish-olive, very sandy, bentonitic; one bed_______ 1 

Sand, thick-bedded, pale-olive to yellowish-gray, medium to 

coarse, feldspathic, biotitic, subangular; grades into overlying cl~Y-------- -- -------------- ----------------- 6 

(Sequence starts at bench at _4,330 ft alt.) 

The relative volumes of rock types in the Climbing 

Arrow, based on several crudely measured partial sections and with some assumptions as to subsurface 

thinning and thickening of strata with distance from 

the hill . fronts, are estimated at bentonitic clay, 45 

percent; quartzose sand, 25 percent; quartzose sandstone and conglomerate, 10 percent; tuffaceous siltstone 

and sandstone, 10 percent; limestone, 10 percent. 

The bentonitic clays that dominate the formation are 

somber rocks, mostly colored olive tones of gray, but 

some are olive brown and reddish brown. Most of 

them swell appreciably when wetted, but rarely more 

than double their original volume. Unlike nonswelling clays, they tend. to maintain outcrops, producing steep backslopes and gentle dipslopes, alike barren of vegetation and mantled with loose popcornlike 

lumps of dark clay. (See figs. 8 and 9.) Although 

clay is the dominant material, few beds are pure. 

Most contain from traces to several percent of subrounded sand and angular silt grains of quartz and 

clotted clay minerals, and, less commonly, of biotite, 

feldspar, dark aphanitic volcanic rocks, and volcanic 

glass. Crystals of gypsum, evidently authigenic, are 

scattered through many clay strata, and, rarely, gypsum is a major component, forming plates, as much 

as an inch thick and a foot across, of large intricately intergrown crystals. 

Sand, yellowish-gray, · coarse, feldspathic and biotitic, FIGURE 9.-Popcorn surface on weathered bentonitic siltstone of 

subrounded; one bed---------------------·----------- 3 Climbing Arrow formation. 
SEDIMENTARY ROCKS 71 

The foregoing field observations have been supplemented by laboratory study of six representative samples. The obvious presence of small amounts of 

coarse material is quantitatively shown by mechanical 

analysis of the six samples (fig. 10). 

In thin section, the six Clays are similar. By reflected light they are varicolored pale red, yellowish 

gray and light gray, rather than the olive tones of 

the fresh outcrop. This is presumably a dusty film 

for, by transmitted light, the clays are largely dark 

colored, mostly greenish brown and translucent but 

locally dark brown, grayish red, and pale red. They 

consist mostly of closely packed unoriented splotchy 

masses of dark translucent clay, generally in the claysilt range but locally as coarse as fine sand, feathering 

out into lighter colored clear clay at the edges. In 

places dark translucent .and light transparent clay are 

in alternating bands. Some grains are separated by 

reticulating veinlets of clear opal, and a few contraction fractures are lined with opal. Under crossed 

nicols, the dark clay masses appear as groups of 

cloudy anhedral grains with yellowish-brown ( anomalous low second order?) polarization colors; the clear 

clay is microcrystalline, with myriad unoriented 

straight-extinguishing length-slow fibers giving gray 

and white first-order colors. The dark clay is of 

slightly lower maximum refractive index than the 

clear clay which in turn is slightly lower than balsam. 

Apparently' two clay species are represented but they 

are here both regarded simply as montmorillonite. 

Within and between the clay masses are many angular 

bits of quartz, ranging in size from clay to fine sand, 

and in places making up as much as 10 percent of the 

rock. Much less abundant, and not present in all 

specimens, are slightly bleached greenish-brown biotite, clayey muscovite, sodic plagioclase, bubble-rich 

clear glass in both curved and equant shapes, epidote, sphene, hydrated iron oxides, and black opaque 

minerals. Carefully sought but not found were relict 

pyroclastic textures of the sorts reported from literally 

hundreds of occurrences of montmorillonite rocks and 

summarized by Ross and Hendricks ( 1945, p. 64-65, 

pis. 1-5). 

The identification of the clay minerals as montmorillonite is based on · X-ray analyses of the six 

samples. The X-ray studies, tabulated in rows 1-6 

of table 5, show that the rocks consist almost wholly 

of montmorillonite, with minor amounts of quartz, 

feldspar, mica, and, in one only, kaolinite. Not only 

is the clay fraction ( 5A) dominated by montmorillonite but the silt fraction as well ( 5B). 

Chemical analyses of the combined clay and silt 

fractions are presented in table 6A. The chemical 

compositions of the six clays, from widely separated 

parts of the formation, are remarkably similar. They 

are well within the chemical ranges of the sedimentary 

montmorillonites studied by Ross and Hendricks 

(1945); 34 analyses of reasonably comparable material 

have been selected from the group presented by Ross 

and Hendricks and summarized as to median and 

range of oxides, for comparison with the median and 

range of the analyzed Climbing Arrow samples. 

TABLE 5.-X-ray mineralogy of clay and siU fractions of some rocks of the Climbing Arrow formation 

[Analysts: J. C. Hathaway, samples 1-5 and 7; C. J. Parker, samples 6 and 8. 

Localities shown on pl. 2. For laboratory numbers, see table 6] 

Samplol Po.oont ·IM~~tl ":o'!:'~te I Fcl"'PM ~!l~\:J,",gl ~~~~ 

A, Clay fraction 

!_ ________ _ 

2 _________ _ 3 _________ _ ------------ ------------ X ------------ 4 _________ _ ------------ X 5 _________ _ 

X 

X 

X ------------

X 

X 

X 

X 

X 6 ______ ___ _ 

7----------

69.1 

86. 4 95.2 

26.8 

60.7 

77.9 

27.0 

22.3 

============ ::::=:::::=: =:::::::==== _____ x ____ _ g ___ ______ _ 

!_ ___ _____ _ 

2 _________ _ 3 _________ _ 

4 __ __ _____ _ 5 _________ _ 

6----------

27. 6 10. 1 

B, Silt fraction 

X 

X 

X ------------ ------------ ------------ X 

7 .. ________ _ 

4. 2 70.5 

30.3 

10.2 

66.8 

27. 9 ------------ ------------ ------------ ------------ X 

X 8 _________ _ X ------------------------------------------------

a Percent of rock in size class, from figure 10 for samples 1-{i; from figure 11 for 

samples 7 and 8. ' . ., Glass is major component. (Glass not reported but probably present m most other silt and clay samples.) &-only component reported . . .6.-major component (>20 percent). 

X -minor component, generally only a trace. 

The lenses of yellowish-gray incoherent sand that 

alternate with the clay strata are typically several 

feet thick, several hundred feet broad, and, to judge 

by low ridges they produce on the surface, a few 

thousand feet long; their trends, though diverse, are 

consistently easterly. The sand is mostly coarse, subangular to subrounded, and composed of quartz with 

subordinate feldspar (mostly plagioclase but partly 

alkali feldspar), biotite, and dark fine-grained volcanic rocks. A pecularity of the biotite is that it occurs not only as flakes but also in equidimensional 

booklike aggregates of sand and granule size. The 

thick tongues of sandstone and conglomerate, near 

basin edges, are probably lateral equivalents of the 

sand lenses, cemented mostly by clay but partly by 

silica with the aid of ground water. 
72 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

Iz 

L..LJ 

u 

0:: 

L..LJ 

c... 

Iz 

L..LJ 

u 

0:: 

L..LJ 

c... 

90 90 90 

80 80 80 

>. 

ro 

70 

0 70 70 

60 60 60 

50 50 50 

40 40 40 

30 30 30 

20 20 20 

10 10 10 

0 0 0 

PERCENT PERCENT PERCENT 

1 2 3 

90 90 90 

80 80 80 

+-' 

i:/5 

70 70 70 >. 

ro 

0 

60 60 60 

50 50 50 

40 40 40 

30 30 30 -o 

c: 

ro 

en 

20 20 20 Q) 

~ 

ro 

0 

10 10 10 u 

>. > Q; ro 

> 0 

0 0 0 

1.5 0.8 

PERCENT PERCENT PERCENT 

4 5 6 

FIGURE 10.-Mechanical analyses O<f clay-rich rO<cks from the Climbing Arrow f ormatiO<n. Sample numbers correspond. to those in table 6 

where laboratory num~rs and chemical analys€S are given. Grain-size classificati()n according tO< Wentw()rth scale. Analyses by P. R. 

Blackmon. 
SEDIMENTARY ROCKS 73 

TABLE 6.- Chemical data on representative clay-rich and glass-rich rocks, Climbing Arrowjormation 

[Rapid-method analysis by P. L. D . Elmore and K. E. White. Localities from which samples where collected are sho~ on pl. 2. Laboratory numbers are last 3 digits of 6-digit numbers of which the first 3 digits are 141. Rc, range of samples 1-6; Me, median of samples 1-6; Mr, median of 34 analyses of sedimentary montmorillonites, and Rr, range of 34 analyses of sedimentary montmorillonites, published by Ross and Hendricks, 1945, p. 34-35, analyses 1-5, 7-12, 14-16, 18-20, 23, 25-27, 3G-32, 34-35, 37, 38, 46, 48, 5Q-53] 

Sample____________________________ ___ _ 1 Laboratory No______________ ___ _______ 360 2 

361 

Clay-rich rocks 

3 

362 4 

363 5 

364 6 

366 

Glass-rich rocks 

7 

365 8 368 

Rc Me Mr Rr 

A. Chemical composition 

Si 02----- _ ------ _ ------------ __ -------- 57.7 56. 1 55. 1 57. 5 59.4 56.0 68. 6 67.8 55.1 -59.4 56.8 50.4 44.0 - 55. 4 

Ah03--- ------------------------------ 15.6 16. 4 19.0 15.0 16. 6 14.7 14. 0 13.6 14.7 -19.0 16.0 17.8 11. 7 -28.9 

Fe203- -------------------------------- 5. 6 5.9 2.9 5. 9 5. 2 5. 2 1.5 2.0 2.9- 5.9 5.4 2.8 .8 - 7. 3 

FeO -------- _ ------------------ _ ----- __ .16 . 06 .17 . 22 . 22 .20 .19 .14 .06- . 22 .18 0 0 - 1. 4 

MgO -----_---------------------------- 2.4 2. 8 2. 1 2.8 2.0 3.0 .88 .64 2. 0- 3. 0 2. 6 4. 1 .so- 8.6 

CaO---------------------------------- 2. 2 1.8 1.8 1. 9 1.3 1.9 . 81 . 80 1.3-2.2 1. 85 1.2 0 - 3. 2 

Na20 ___ ------------------------------ 1.5 . 92 . 96 1. 0 . 72 1.2 2. 8 2.0 . 72- 1. 5 .98 .17 0 - 3.8 K20 ____ _ ------------------------------ 2. 0 1.4 1.5 1. 2 2. 8 1.5 4.4 4. 6 1.2- 2.8 1.5 .35 0 - 2.3 T i 02- _ --_- _ --______ -- __ __ ____ _________ .62 . 54 . 60 .68 . 74 .66 . 18 .10 .54- . 74 .64 .11 0 - .80 

P20s---------------------------------- .19 .09 .05 .06 .08 .12 . 04 .02 . 05-- .19 .085 0 0 - . 24 Mn 0 _____ ..: _________ ------------------ .04 . 16 .03 .18 .05 .06 . 04 .06 . 03- . 18 . 055 0 0 - .18 H,o __________________ ----------------- 12.3 14. 1 15. 4 13.0 11.1 15.2 6. 7 8.0 11.1 -15.4 13.6 20. 9 12.4 -24. 1 

c 02----------------------------------- < . 05 <.05 < . 05 .08 <.OS . 18 < . 05 < . 05 <.05- .18 <.05 0 0 - .37 

Sum ___________________ ________ _______ --------- 100 100 100 100 100 100 100 100 -------------- ---------- ---------- ------------

B. Chemical composition, recalculated free of water and C02 

8102----------------------------------- 65. 8 Als03--- ------------------------------ 17. 7 Fe203- -------------------------------- 6. 4 Feo__________________________________ . 2 

MgO---------------------------------- 2. 7 

cao_--------------------------------- 2. 5 Na20- ____ ---------------------------- 1. 6 K,o_________________ ______________ __ __ 2. 3 

Ti02---------------------------------- . 7 P20s --------~--------- -- -~--------- . 2 MnO_- _ ------------------------------ ----------

65.3 

19.1 

6. 9 

. 1 3. 2 2.1 

1.1 

1.6 

. 6 

.1 

.2 

65.5 

22. 5 3.4 

.2 

2.5 

2.1 

1.1 

1. 8 

.7 

. 1 

66. 4 17.4 

6. 8 

.3 

3. 2 2.2 

1.1 

1.3 

. 8 

. 1 

. 2 

66.7 

18.7 

5. 8 

.3 

2. 2 1. 4 . 

.8 

3. 2 

.8 

.1 

. 1 

66.2 

17. 3 6.1 

• 2 3. 5 

2. 2 

1.4 

1. 8 

. 8 

.2 

.1 

73.4 

15.0 

1.6 

.2 

. 9 

. 9 3. 0 

4. 7 

.2 

73.7 

14.8 

2.2 

.2 

.7 

.9 

2. 2 5.0 

. 1 

-------------------------1-----1--------1---- Sum____ ________ ______________________ 100.1 100.3 99.9 99. 8 

The white siltstone and sandstone are composed essentially of poorly sorted fragments of volcanic glass. 

The refractive index of the glass (9 samples studied) 

ranges from near 1.500 to 1.505; most of it seems to 

be close to 1.502. A little is in curved bubble-rich 

shards but most of it is in equant fragments, angular 

to rounded, riddled with cracks in a rudely rectangular pattern. From 10 to 20 percent of the equant 

fragments are altered to montmorillonite, mostly the 

pale-green or greenish-brown semiopaque variety described earlier from the Milligan Creek formation, 

but including lesser amounts of the clear fibrous type. 

The curved shards are typically unaltered. Minor 

components are angular to subrounded grains of 

quartz, alkali feldspar (mostly orthoclase), plagioclase (albite to andesine) , brown biotite, light-green 

hornblende, angular opaque minerals, and dark aphanitic volcanic rocks. Some of these white rocks contain interstitial calcite; a few contain large quantities 

of it, and grade into tuffaceous limestone. Mechanical, 

X-ray, and chemical analyses of a typical carbonatefree white tuffaceous· siltstone, from the white unit in 

SE1;4NE1;4NE1;4 sec. 10, T. 2 N., R. 1 \V., appear 

as sample 7 in tables 5 and 6 and figure 11. 

100. 1 99.8 99. 9 99.8 -------------- ---------- ---------- ------------

Several thin-sections and many grain mounts of 

white siltstone that seems to be nearly pure volcanic 

ash contain intimately admixed, beautifully preserved 

organisms (identified by Estella Leopold and Richard 

Rezak) including pollen, pelagic diatoms, non pelagic 

diatoms, filamentous algae, and blue-green algae. 

The white limestone of the formation is similar in 

all important respects to that of the Milligan Creek 

formation and needs no further description. 

Very rare are thick lenses of white conglomerate 

made up of unsorted subrounded fragments of pumice 

as much as 2 inches long, cemented by white silt and 

clay. One of these lenses, which furnished sample 8 

(tables 5 and 6 and fig. 11), is well exposed in a 

prospect pit in NW1)tNW% sec. 32, T. 3 N., R. 1 E. 

In addition to the micro-organisms in the siltstones, 

the formation contains fossil evidence of a rich fauna 

and flora that included-in decreasing size-brontotheres, tapiroids, oreodonts, turtles, rodents, lagomorphs, insectivores, marsupials, gastropods, and ostracodes; and ·coniferous and deciduous trees, flowering plants, and charophytes. The vertebrate remains 

are generally in the bentonitic clays. The gastropods 

and ostracodes occur in both the white siltstone and 
74 GEOLOGY OF THE THREE FORlCS QUADRANGLE, MONTANA 

the limestone. The fossil wood is largely in the 

sandstone and conglomerate lenses. r:rree and plant 

leaves have been found only in clays, and charophyte 

remains only in limestone. Algae,. pollen, and diatoms 

occur widely in white limestone, as well as in siltstone. 

By far the most abundant fossil material is silicified 

wood. Some fragments are rather large: a stump 

5 feet across and a single log 26 feet long and 9 feet 

across have been observed. These and many other 

60 

fZ50 

w 

() 

0::: 

~40 

30 

20 

10 

0 

PERCENT 

7 

PERCENT 

9 

40 

30 

20 

10 

0 

PERCENT 

8 

90 1--f---ii-----j 

40 

30 

PERCENT PERCENT 

10 11 .l/AI I grains larger than silt classed simply 

as sand. Fraction contains much si lt and clay 

in aggregates that could not be dispersed with 

sodium metaohosohate. 

FIGURE 11.-Mechanical analyses of tutraceous slltstone from Climbing Arrow (7, 8) and Dunbar Creek (9, 10, 11) formations. Sample numbers correspond to those in tables 6 and 8 where laboratory numbers and chemical analyses are given. Grain-size classification according to Wentworth scale. Analyses by P. R. Blackmon (7, 8, 9) and J. C. Hathaway (10, 11). 

smaller fragments are largely yellowish-brown chalcedony and bluish-black opal. Many cavities and 

fractures are lined with earthy greenish-yellow jarosite that looks disturbingly like carnotite. Jarosite 

is also common in the cement of the associated sandstone and conglomerate. In a different type of fossilization the wood is replaced by translucent to 

opaque yellowish-gray microcrystalline quartz that 

has well-developed prismatic jointing. Small pencillike fragments of this replaced wood are easily mistaken for crystals of some hard authigenic mineral. 

The Climbing Arrow grades upward into the Dunbar Creek formation over a stratigraphic interval -of 

perhaps 50 feet by a·n alternation of dark bentonitic 

clay with the white and yellow tuffaceous siltstones 

that characterize the basal Dunbar Creek. 

PEBBLY SAND MEMBER 

Included with the Climbing Arrow formation, 

though not without hesitation, is a large body, not 

less than 150 feet ·thick, of cross-stratified yellowishorange pebbly subrounded sand and lesser amounts 

of pebble gravel, that is exposed above the east bank 

of Willow Creek. A small mass of similar sand is 

well exposed above the opposite bank. The cross 

strata in the sand have northerly or northeasterly 

dips of a few degrees. The sand, mostly quartz, also 

contains much feldspar and a little biotite, hornblende, and dacitic rock. · 

The sand seems to have been locally derived from 

crystalline rocks to the south and deposited by a 

predecessor of Willow Creek. It is entirely possible 

that the predecessor was a . late Quaternary stream 

rather than an early Tertiary one, for the sand lackr.1 

fossils, and its obscure stratigraphic relations permit 

either interpretation. 

FOSSILS AND AGE 

The age of the Climbing Arrow formation ranges 

from middle or late Eocene to early Oligocene. The 

age of the lower part of the Climbing A_rrow formation has been established as no younger than late 

Eocene on the basis of small but diagnostic vertebrate 

and invertebrate f~unas. These Eocene faunas are of 

more than routine interest as only one other Eocene 

loc~lity has heretofore been known in southwestern 

Montana-near Sage Creek, 100 miles to the southwest. The vertebrate fauna is from a single locality 

in SE%,NW14SE!4 sec. 3, T. 2 N., R. 1 W., from 

an area sera ped smooth by bulldoz~rs (in 1954) at . 

the south abutment of th~ small dam downstream from 

the Silver Sage Ranch buildings (loc. 443, pl. 2 [ (red) 

D111]). The fauna, studied by G. E. Lewis (written 
SEDIMENTARY ROCKS 75 

communication, Jan. 19, 1956), includes the following 

forms: 

Hyopsodus cf. H. uintensis Osborn; right maxilla, PLM3 

[(red) Dlll-1] 

Dilophodon sp. or Isectolophus sp.; right ramus, P3-M2 

[red) Dlll-2] 

Protoreodon sp.; two upper molars [(red) Dlll-3] 

Lewis concludes that this assemblage is probably 

of Uinta age. The vertebrate locality is within 100 

feet of. the top of the lower dark unit, so that on this 

evidence at least 400 feet of the formation is o:f late 

Eocene age or older. 

The stratigraphically lowest Oligocene fossils were 

recovered from near the top of the white unit, just 

north o:f the county road at the south edge of sec. 8, 

T. 2 N., R. 1 E., 1,600 :feet east o:f southwest corner 

(loc. 158, pl. 2 [(red) D104]). These include two 

rami and a cheek tooth o:f Paleolagus temnodon 

Douglass, o:f earliest Oligocene (Pipestone Springs) . 

age, according to Lewis (written communication, Jan. 

19, 1956). If the stratigraphy is correctly understood, therefore, the transition from Eocene to Oligocene is now located within less than 300 :feet o:f strata. 

(This conclusion would be more convincing i:f both 

Oligocene and Eocene :fossils had been recovered :from 

a single continuous section.) 

. A single locality in center sec. 12, T. 2 N., R. 1 W. 

(20010, pl. 2) has yielded :fresh-water gastropods of 

rather definite Eocene age, along with other snails o:f 

wide or indeterminate range in age. The snails are 

in strata :far down in the lower dark unit, exposed in 

a small anticline, and therefore are several hundred 

feet stratigraphically below the Eocene vertebrate 

locality. The :fauna, studied by D. W. Taylor (written communication, May 29, 1957) and summarized 

in table 3, includes Lymnaea and Plan01'bidae of indeterminate species, that occur in both the Milligan 

Creek and Dunbar Creek :formations as well and are 

o:f little chronologie value, and the fresh-water planorbid A ustralorbis ps.eudoammonius ( Schlotheim). According to Taylor, this species is widely known in 

Europe where it is restricted to the Eocene. In the 

United States it is more widely distributed in middle 

Eocene than in younger deposits, but is known, from 

at least three deposits dated as late Eocene on verte-:.. 

brate evidence (Kishenehn :formation o:f northernmost 

Montana and southeastern British Columbia; Tepee 

Trail :formation, Wind River basin, Wyo.; and an unnamed unit at Beaver Divide, Fremont County, Wyo. 

The species is also known :from one locality (Beaver 

Divide conglomerate member of N ace, 1939, Fremont 

County, Wyo.) dated as very early Oligocene on sparse 

vertebrate evidence. On its own merits, then, the occurrence of .this species is strong but not indisputable 

evidence o:f Eocene age. 

The upper dark unit north o:f the J e:fferson River 

contains . the following forms identified by Lewis 

(written communication, Jan. 19, 1956) : 

brontotheres cf. Allops sp. or Brontotherium sp. left metacarpal IV [(red) D107]; right radius [(red) . D108-1] 

Merycoidodon cf. M. gracilis Leidy; right ramus [(red) 

D106]; right M3 [red) D108-2] 

These collections came from the NW~ sec. 16, T. 2 N., 

R. 1 E. (locs. 344-346, 362, pl. 2). 

M erycoidodon remains have been found only in the 

upper 200 feet o:f the formation north o:f the J e:fferson 

River, and in the succeeding basal 200 :feet o:f the Dunbar Creek formation (loc. 409, pl. 2). Based only on 

oreodont evidence, this part of the Climbing Arrow 

formation might be regarded as of middle Oligocene 

(early Brule) age. Entire large brontothere bones 

have, however, been recovered :from within 100 :feet 

of the top of the Climbing Arrow and these are regarded, by both Hough and Lewis, as diagnostic o:f 

early Oligocene (Chadron) age. Fragments o:f brontothere bones have also been recovered :from the Dunbar Creek :formation (NE1;4 sec. 6, T. 2 N., R. 2 E., 

loc. 250, pl. 2) so that the upper part of the Climbing 

Arrow, and at least part of the Dunbar Creek as well, 

is o:f early Oligocene age. 

The only vertebra'te remains recovered from the 

Climbing Arrow formation south of the J e:fferson 

River are fragments of brontothere teeth, identified 

in the field by Lewis, from about 350 feet below the 

top, in NE1;4SE1;4 sec. 11, T. 1 N., R. 1 E. They ~u:ffice, however, to date this sequence as o:f early Oligocene age and to confirm its unity with the strata of 

similar lithology across the river. 

The fossil wood and leaf remains offer little aid in 

age determination but are o:f some paleoecologic interest. Ten collections of fossil wood were studied by 

Elso S. Barghoorn of Harvard University (written 

communication, Jan. 14,. 1955). Most of the reasonably well preserved . material he found to be coniferous wood assignable with more or less definiteness to 

the family Cupressaceae. For most of the material, 

generic determination is not feasible but at any rate 

the wood is not that of Juniperus and is probably 

either Thuja plicata (Western Red Cedar),· Ohamaecyparis lawsoniana (Port Orford Cedar) , or Libocedr.u.s decurrens (Incense Cedar) . The very large 

stump, mentioned previously, is that of a dicotyledonous tree which is "in all reasonable probability that 

of Acer (Maple)." "Growth ring~ are normal and 

well developed in all specimens, .indicating seasonal 

climatic conditions." Most of the woods identified 
76 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

are from the lower dark unit of the Climbing Arrow 

and are thus probably of Eocene age, but the age evidence from the woods themselves is not more closely 

determinable than Tertiary, according to Barghoorn. 

A collection, made by Harold Masursky, of plant 

material from the lower dark unit of the Climbing 

Arrow formation (NE1;4NE1t4 sec. 12, T. 2 N., R. 1 

W.) was studied by R. W. Brown (written communication, Oct. 12, 195Q). He identified the following 

forms, to which he was unable to assign an age: 

Equisetum sp. (horsetail) 

Typha sp. (cattail) 

~ragment of a dictoyledonous leaf 

Part of a fruit, undetermined 

Of great promise for future age and ecologic interpretations are the abundant well-preserved microorganisms in the white rocks of the formation. The 

obviously varied · assemblage of pollen, eharaceous remains, algae, and diatoms invites systematic collecting and study. 

ORIGIN 

The Climbing Arrow formation is largely the product of an aggrading stream system. The lenses of 

yellowish-gray quartzose sand, sandstone, and conglomerate spread throughout the formation, and the 

thick mass of pebbly sand flanking Willow Creek are 

clearly stream-channel deposits. Most of the streams 

were probably small, as suggested by the rather low 

degree· of rounding and sorting, and the rather poor 

bedding. The gravels are consistently finer grained 

than those on the present flood plains of tributary 

streams, suggesting that the relief of the time was 

more subdued than the present. 

The rudely east-west orientation of the channels, 

the increase in proportion and coarseness of conglomerate westward, and the general distribution of the 

Climbing Arrow formation suggests that the· streams 

flowed eastward. The suggestion is strengthened by 

the lithology of the coarse deposits, which are dominated by debris from biotite-rich intermediate plutonic rocks, and dark aphanitic volcanics, a combination characteristic of the southern Elkhorn Mountains 

that form the west edge of the Three Forks basin. 

M~re distant transport cannot, of course, be ruled out. 

The dark bentonitic clay strata, with their persistent fraction of silt and rounded sand and their intimate association with channel deposits are evidently 

overflow deposits that accumulated on the flood plains 

in short-lived ponds and lakes. If the coarse sediments were shed from the ancestral Elkhorn Mountains, it is reasonable to expect that the associated 

fine sediments were similarly derived. The situation 

is complicated by the · mineralogy of the clay and by 

the known contemporaneous pyroclastic volcanism, for 

most large masses of montmorillonite rock are derived 

from the alteration of volcanic glass, as Hewitt ( 1917) 

long ago recognized and as subsequent work (summarized by Ross and Hendricks, 1945) abundantly 

confirmed. The possibility must be considered that · 

some, perhaps most or even all, of the montmorillonite 

in the clays and tuffaceous siltstones is altered con-

. temporaneous pyroclastic debric like the clay pellets 

in the Milligan Creek formation. If so, textural con-

.firmation is lacking. As Ross and _Hendricks noted 

( 1945, · p. 64) "thin sections show that bentonites 

[their term for montmorillonite or beidellite rock] 

commonly retain the structure of volcanic ash with 

surprising perfection." None of the six thin sections 

of· clays of the Climbing Arrow show any relict ash 

textures, suggesting that whatever the origin of the 

clay, it was already montmorillonite before deposition. 

The general absenc.e of diagenetic silica in the 

Climbing Arrow is strong negative evidence that 

alteration to montmorillonite with concomitant release of silica occurred before deposition. 

Very likely, the montmorillonite is poly genetic : 

some is residual clay from normal weathering of the 

monzonitic (and andesitic?) rocks of the clastic source 

area and some is altered latitic volcanic ash, perhaps 

ash first deposited in headwater areas during the. 

earlier eruptions of Milligan Creek time and altered 

in place and in transit. Contemporaneous ash, whether 

falling on surrounding slopes and washed into the 

clay ponds, or falling directly into them from the air, 

may not have had time or appropriate conditions to 

alter much and may be represented by the glass fraction of the clay-rich strata. 

The white siltstone, sandstone, and conglomerate 

made up largely of fresh vitric, crystal, and lithic ash 

fragments appear at first glance to be tuffs deposited 

on dry land. The tuff was calcalkaline rhyolite as 

·indicated by comparison of the chem~eal composition 

of two analyzed specimens (recalculated free of water 

and C02 in columns 7 and 8 of table 6B) with 

Nockolds' tabulation (1954, p. 1012). A few of these 

strata may be tuff in the strictest sense, especially 

those composed only of angular fragments, but most 

of them were water laid as indicated by their lenticularity, their small but persistent content of rounded 

sand grains and their varied population of well-preserved pollen, diatoms, and algae. These strata probably represent rhyolitic ash that fell on or near the 

flood plains and was only slightly reworked by the 

meandering streams. That their glass is scarcely 

altered does not weaken the ease for water deposition, 

even if the clay-rich strata should be shown to be 

direct deposits of latitic ash in standing water, for 

the difference in composition of the ash is apparently 
SEDIMENTARY ROCKS 77 

an important element in determining the course and 

rate of alteration. As Ross and Hendricks (1945, 

p. 65) point out: "It is not uncommon to find geologic sections of volcanic rocks where the tuffs of 

feldspathic composition have altered to bentonite, 

whereas the rhyolitic rocks in the same section are 

almost unaltered." 

Even those few strata composed wholly of volcanic 

detritus may be flood-plain deposits. Sufficiently thick 

and extensive ash falls could have provided streams 

and ponds with an exclusively pyroclastic load for 

a time, leading to flood-plain deposits of volcanic 

siltstone and clay and with channel fills of waterrounded lapilli and coarse ash. 

The limestone of the Climbing Arrow presumably 

formed in ponds isolated from frequent flooding. The 

rather inhomogeneous and porous texture and the 

presence of characeous remains suggest that they are 

at least partly organic. 

The abundant and varied flora and fauna show that 

the climate was warm and humid; according to Taylor (written communication, May 29, 1957) Australorbis pseudoammonius indicates tropical climate. The 

climate inferred from flimsy local evidence is consonant with the climate inferred from large-scale studies. As summarized by Barghoorn ( 1953, p. 244) : 

During the early Cenozoic the northern mid-latitudes were 

covered by a vegetation, the botanical equivalent of which is 

now largely confined to subtropical and even tropical climates * * *. It seems quite unlikely that truly tropical conditions occurred in mid-latitudes during the Eocene. It is 

more probable that the absence of pronounced winter freezing allowed an unusual extension poleward of tropical plants 

* * *. Hence the absence of cold * * * rather than extreme 

warmth is indicated. 

DUNBAR CREEK FORMATION 

NAME, DISTRIBUTION, AND TOPOGRAPHIC EXPRESSION 

The Dunbar Creek formation crops out in the northeast and southeast parts of the quadrangle. The 

.spectacular bluffs that rise on the west bank of the 

Madison River are carved largely in this unit. The 

formation is here named for Dunbar Creek, a southward-flowing tributary of Mud Spring Gulch, that 

flows, occasionally, along the east edge of the wedge 

of Dunbar Creek rocks east of Highway 10N. The 

Dunbar Creek formation is mainly white to grayishyellow thick-bedded tuffaceous siltstone, partly lacustrine and partly eolian, intricately laced with fluviatile sandstone and conglomerate. Rare components 

are dark bentonitic clay and white limestone. In 

color and general appearance, the Dunbar Creek resembles the Milligan Creek formation, but the Dunbar 

Creek contains little limestone, the main rock type 

in the Milligan Creek. 

654729 0--63----6 

The formation is exposed over an area of 6 square 

miles and is thinly mantled by Quaternary deposits 

over more than 20 square ·miles more. The northern 

and southern parts, widely separated, are correlated 

on the basis of lithology, stratigraphic position, and 

the ages· of ·contained fossils. Easterly dips carry 

the formation below the Madison River; similar strata 

cropping out on the east bluffs are mostly, if not 

wholly, of much younger age (late Miocene and early 

Pliocene according to most workers) . 

Rocks very much like the Dunbar Creek formation 

crop out extensively in the Townsend Valley and 

Clarkston Valley portions of the Radersburg and Toston quadrangles. Detailed study may eventually show 

that the Dunbar Creek formation is widely mappable 

in these valleys. 

The Dunbar Creek formation develops topography 

much like that of the Milligan Creek-white dissected 

benchlands rising steplike from the flood plain, gashed 

with many steep-walled canyons. Because the rocks 

are well consolidated and yield little soil, outcrops are 

plentiful throughout. 

LITHOLOGY AND THICKNESS 

The base of the Dunbar Creek is well exposed in 

sees. 3 and 4, T. 2 N., R. 1 E., flanking Highway 10N. 

It is also visible at the north end of the Madison 

Bluffs in sec. 6, T. 1 N., R. 2 E. The top is nowhere 

exposed in the quadrangle. The stratigraphically 

highest exposures are in the bluffs at the southeast 

corner. On the flanks of the Hossfeldt Hills no more 

than 250 feet of the formation are preserved but in 

the southeast corner of the quadrangle not iess than 

800 feet and perhaps more than 1,000 feet are present. 

Near Dunbar Creek the formation is mostly a monotonous succession of thick-bedded white to grayishyellow tuffaceous siltstone. The sequence includes a 

few beds, several feet thick, of white tuffaceous limestone, many tongues of poorly sorted, poorly rounded 

carbonate-rich gravel that thicken hillward, and a few 

thin lenses of slightly bentonitic gray clay. 

South of Three Forks, the lower 300 feet of the 

formation is much like the section near the Hossfeldt 

Hills, though with fewer limestone beds and more 

clay beds; conglomerate beds are better sorted than 

those near Dunbar Creek, and lack carbonate pebbles. 

The upper part of the formation has· little yellow 

siltstone and no limestone; it consists largely of thickbedded white tuffaeeous siltstone and fine sandstone, 

and many thin lenses and a few very thick ones of 

well indurated, rudely cross-stratifieq, yellowish-gray 

and yellowish-brown coarse to fine . pebbly sandstone. 

It also contains a few thick sequences of dark clayrich siltstone and bentonitic clay. 
78 GEOLOGY OF THE THREE FORKS QUADRANGLE, . MONTANA 

Chosen as the type area of the formation is the 

E~~ sec. 7, T. 1 S., R. 2 E., for its exceptionally good 

and continuous exposures of about 315 feet of strata 

in the upper part of the formation. The lower 180 feet 

of the section in the type area is at the base of the 

bluffs in the southeast corner of the section; the upper 

135 feet is exposed in the most northerly gulch in sec. 7 

!measured section G, pl. 1 2). This partial section 

comprises: 

Q Thickness uaternary gravel: (feet) 

Gravel, pebble to cobble, rounded, largely quartzose 

rocks but with some volcanic and gneissic rocks, 

heavily cemented by calcium carbonate____________ 4 

Dunbar Creek formation: · 

16. Sandstone, thick-bedded, yellowish-gray, pebbly, fine, 

subangular, tuffaceous, porous____________________ 18 

15. Siltstone, thick-bedded, very pale orange, pebbly, micaceous, tuffaceous______ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 29 

14. Siltstone like that in unit 4_________________________ 20 

13. Sandstone, thick-bedded, faintly cross-stratified, grayish-yellow, subangular, medium___________________ 5 

12. Claystone, thick bedded; like unit 9 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 12 

11. Sandstone; like unit!______________________________ 2 

10. Sandstone; like that in unit 4_______________________ 36 

9. Claystone, yellowish-gray with distinctive subconchoidal 

fracture; one bed_______ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 2 

8. Siltstone; like that in unit 4 ________________________ 12 

(break in section at gravel covered bench) 

7. Siltstone; like that in unit 4 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 36 

6. Sandstone; like unit L _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 10 

5. Covered (probably sand)___________________________ 55 

4. Siltstone and fine sandstone; thick-bedded, yellowishgray to white, subangular, tuffaceous______________ 38 

3. Sandstone; like unit!______________________________ 12 

2. Sandstone, thick-bedded, white, fine, micaceous, tuffaceous________________________________________ 6 

1. Sandstone, interbedded, moderate - yellowish - brown, 

pebbly, coarse, subangular cross-stratified sandstone 

and yellowish-gray, medium subangular micaceous 

sandstone______________________________________ 22 

Bottom of gulch. 

Thickness of Dunbar Creek formation _____________ 315 

Not exposed in the type are are any limestone or 

carbonate-cemented clastic rocks, fairly common 

nearer the base. These rocks, and the basal part of 

the formation, are best seen in SWl~ sec. 3, T. 2 N., 

R.1 E. 

The formation as a whole is made up of about 80 

percent tuffaceous siltstone and' sandstone; 15 percent 

quartzose sandstone, sand, and conglomerate; and 5 

percent limestone, clay, and claystone. 

The tuffaceous rocks are composed almost wholly 

of fine volcanic ash. The chemical composition of 

three representative samples of tuffaceous siltstone is 

reported on table 7 A ; recalculated to standard oxides 

7 B, these rocks are nearest to adamellite (quartz monzonite) or dellenite (quartz latite) in N ockolds' classification (1954, p. 1014). That they are lower in 

total alkalis and higher in alumina than their presumed source rocks is an expectable consequence of 

the presence of a large clay fraction. The clay and 

silt fractions of these same rocks are comprised overwhelmingly of montmorillonite (table 8). They are 

virtually identical with those of the Climbing Arrow 

formation in appearance and similar in chemical composition (compare the recalculated analyses of table 

1B with analyses 7 and 8, table 6B). · The refractive 

index of the glass (21 samples studied) seems to have 

a slightly wider range than that of the Climl'-ing 

Arrow, from 1.498 to 1.506 ; most of it is close to 1.501. 

The younger tuffaceous rocks also seem to have a 

wider range of common accessory minerals, including 

labradorite, yellow-brown chlorite, and sphene, as well 

as all the minor components of siltstone of the Climbing Arrow, but less c.lay. The greater v~riety in the 

siltstone of the Dunbar Creek may merely reflect the 

fact that many more samples of the Dunbar Creek 

have been examined. The white sandstone seems idenTABLE 7.-Chemical analyses and norms of representative samples of tuffaceous siltstone, Dunbar C,reek formation 

[Rapid-method analysis of sample 9 by P. L. D. Elmore and K. S. White; samples 10 and 11 by P. L. D. Elmore, S.D. Botts, and M.D. Mack] 

A. Chemical composition 

Sample __ --------------------- 9 10 11 

B. Chemical composition, recal- culated free of water and C02 

9 10 11 

Si02-------------------------- 63. 2 65.2 66.4 69.9 71.7 72-7 Q-------------------------

Ah03------------------------- 14.8 13. 5 14. o 16. a 14.9 15.4 or------------------------- Fe20a------------------------- 2. 6 3. 3 1. 9 2. 9 3. 6 2.1 ab __ ---------------------- FeO__________________________ . 55 . 07 . 81 . 6 .1 . 9 an __ ---------------------- MgO_________________________ 1. 2 1. 3 1.1 1. 4 1. 4 1. 2 {WO ---- - -- -- ---- cao__________________________ 3. 4 1. 9 2. 1 3. 7 2. 1 2. 3 di efsn ___ ~--- _-_--_-__ =-__ =_--_=_-_- -_-_=_-_-_-__ - Na2o._ ----------------------- 1. 8 1. 6 1. 6 2. o 1. 8 1. 7 

K20-------------------------- 2. 6 3. 5 3.1 2. 9 3. 8 3. 4 by {~sn ________ --__ -_--_-__ --_-_-_-_-_-_-__ --__ - P205-------------------------- . 35 . 44 . 28 . 4 . 5 . 3 , MnO -·------------------------ . 08 . 00 . 00 .1 . 0 . 0 mt_ ______________________ _ 

TinO------------------------- . 04 . 06 . 04 ------------ . 1 ------------ iL------------------------- H20-------------------------- 9. 1 8. 8 8. 6 ------------ ------------ ------------ ap __ ---------------------- C02 .. ------------------------ . 39 <. 05 <. 05 ------------ ------------ ------------ hm _______________________ _ 1----1---------1-----1-----1----1 c ________________________ _ Snm____________________ 100 100 100 100.2 100 100 ru ________________________ _ 

plagioclase __ --------------

C. Norms 

9 

37.6 

17.2 

16.8 

17.5 

.o 

.0 

.0 

3. 5 

.o 

.7 

.8 

. 3 

2.4 

3. 5 

.o AbtGAn5t 

10 

40.2 

22.2 

15.2 

10.6 

.0 

.o 

.o 

3.5 

.o 

.o 

.3 

. 0 

7. 7 4.0 

:a 

Ab5gAnu 

11 

43.3 

20.00 

14.2 

11.4 

.0 

.o 

.0 

3.0 

.0 

2.1 

.6 

.o 

.8 

4.8 

.o 

Ab65Ant5 
.SEDIMENTARY ROCKS 79 

TABLE 8.---:X-ray mineral.ogy of clay and silt fractions of repre- . sentatwe tuffaceous stltstones, Dunbar Creek formation 

[Analysts: J. C. Hathaway, samples 10 and 11; C. J. Parker, sample 9. Localities 

shown on pl. 2. Laboratory Nos. 141367, 152106, 152107] 

I Monti?oril-~ Kaolinite I I Quartz andl Chlorite Sample Percent a ~~~~ group Feldspar cristobalite and mica 

9.--------1 lQb ______ _ 

Ub ______ _ 

9---------1 10b ______ _ Ub __ ____ _ 

14.51 2. 7 6.3 

60.31 42.2 

54.0 

X 

X 

6. 

A. Clay fraction 

1

----------- -----------·I----------- - ------------·I----------- -------------I------------ ------------ ------------ ------------ ------------ ------------

B. Silt fraction 

1

----- ------------1 ------- ------------ 1

-----6-----1 ======== ==== ------------ ------------

•Percent of rock in size class, from figure 11. bQla~s is major component. (Glass not reported but probably present in most other silt and clay samples.) A-only component reported. 6.-major component (>20 percent). X-minor component, generally only a trace. 

tical mineralogically with the white siltstone. The 

sandstone not only is interbedded with the siltstone 

but intergrades with it, both vertically and laterally. 

Like their counterparts in the Climbing Arrow formation, the tuffaceous rocks locally contain a rich and 

varied assemblage of micro-organisms. 

In the tuffaceous rocks calcite generally occurs as 

scattered interstitial aggregates of small clear crystals. 

In a few places, howev~r, calcite is very abundant in 

rocks that look like carbonate-poor siltstone, but are 

harder and contain many tiny vertical tubelike openings. In such beds, calcite forms as much as half the 

rock, surrounding the ash fragments and replacing 

and embaying their margins. Such calcite-rich rocks 

are rarely more than 3 feet thick, with gradational 

lower contacts into calcite-poor siltstone or sandstone 

but sharp upper contacts. Similar calcite-cemented 

tu~aceous sandstone, from much younger rocks on the 

east side -of . Madison Valley, has been described by 

Howard (1932, p. 11-12). 

The other rock types closely resemble their lithologic counterparts in the older formations of the Bozeman group, except that the rocks of the Dunbar Creek 

formation despite their lesser age are somewhat better 

indurated, so that the sand of the Climbing Arrow 

corresponds to the yellow sandstone of the Dunbar 

Creek, and some of the Dunbar Creek clay is indurated enough to have subconchoidal fracture and be 

classed as claystone. 

Fossils are not uncommon in the formation north 

of the Jefferson River. In addition to micro-organisms, the limestone and calcite-rich siltstone locally 

contain many gastropods and a few ostracodes. The 

yellowish siltstone and sandstone contain scattered 

brontothere, turtle, oreodont, and rodent remains . 

South of the Jefferson River macrofossils of all kinds 

are rare, but parts of an anthracothere have been recovered from quartzose sandstone, and some of the 

white siltstone contains gastropods. 

The Dunbar Creek formation is overlain only by 

Quaternary deposits, with marked angular unconformity, except in sec. 3, T. 2 N., R. 1 E., where a 

small mass of gravel that has yielded the remains of 

a late Miocene or early Pliocene mastodon lies on the 

Dunbar Creek. This gravel, discussed later, is no 

more than 250 feet above the base of the Dunbar 

Creek formation, demonstrating deep middle Tertiary erosion. AGE 

The Dunbar Creek formation is at least in part of 

early Oligocene age. Early Oligocene brontothere and 

oreodont remains, mentioned earlier in connection with 

the age of the Climbing Arrow formation, have been 

recovered from all parts of the Dunbar Creek formation flanking . the Hossfeldt Hills. The only vertebrate materials from the formation south of the Jefferson River are many articulated trunk and limb 

bones of an anthracothere (identified by G. E. Lewis, 

written communication, Apr. 12, 1957) from hard 

pebbly sandstone about 400 feet above the base of the 

formation in the bluffs facing the Madison River about 

18 feet below the rim in NW%,SW14 sec. 29, T. 1 N., 

R. 2 E. (loe. 384, pl. 2). Unfortunately the bones, 

though numerous, include none sufficiently diagnostic 

to permit assigning a more specific name, or a more 

precise age than Oligocene. 

Poorly preserved gastropods are widespread in the 

formation. According to D. W. Taylor (written communication, May 29, 1957), who studied nine collections, none are diagnostic as to age. Of some interest, however, in connection with depositional environment, the gastropods are discussed later under the 

heading of origin. 

Thus, no more than 400 feet of the formation can 

be dated as specifically as Oligocene and only the 

lower 250 feet as early Oligocene. Possibly, part of 

the formation may eventually prove to he of middle 

Oligocene age but not younger. Deposits with late 

Oligocene or early Miocene fossils, discussed later, are 

known to lie on the eroded surface of the Climbing 

Arrow formation and it is unlikely that the Dunbar 

Creek continued to be d~posited during the pre-ht.t.F 

Oligocene or early Miocene hiatus. For the present. 

the Dunbar Creek is assigned simply an Oligocene 

age. 
80 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

ORIGIN 

The rock types in the Dunbar Creek are nearly all 

the same as those in the Climbing Arrow and presumably are of like mixed origin : partly fluviatile, 

partly eolian, partly lacustrine. Their very different 

proportions, however, indicate a marked shift in depositional ~nvironment. In Dunbar Creek time, drainage rarely passed out of this part of the Three Forks 

basin, but was largely interior, to judge by the nature 

of the dominant rock type of the formation. The 

well-sorted thick-bedded laterally persistent white 

sandstone and siltstone, formed largely of poorly 

rounded, little altered rhyolitic ash, are plainly not 

deposits made in stream channels or flood plains, but 

rather are indicative of lake or bolson deposition. 

If the ash was not brought in to the basin by 

streams, it must have blown in. But whether ~he ash 

of a particular stratum fell on dry land or in standing water is not always easy to determine. The ash 

in many strata certainly fell in standing water, as 

demonstrated by the presence of intimately mixed, 

abundant, and varied quiet-water micro-organisms. 

Ash-rich strata in the Madison Bluffs have also yielded 

fresh-water snails (loc. 20018-20019, table 3 and pl. 2). 

On the other hand; samples from many other ashrich beds lack micro-organisms and most of the gastropods from the formation north of the Jefferson 

River are land snails (locs. 20011-20017, table 3 and 

pl. 2). This, however, is purely negative evidence 

and of little value .. 

Another bit of negative evidence is the absence of 

upside-down graded bedding. Theoretically, this 

should develop, with finer ash grading upward into 

coarser, when pumiceous ash of varying grain size 

falls into standing water, because larger pieces of 

pumice, with more air holes, should float longer than 

smaller ones. At least one such occurrence is known 

(Bateman, 1953). Possibly, this type of gradation 

is actually present in some beds, but was not detected 

in casual examination owing to the small total range 

in ash· size. 

Better evidence, favoring at least a modest proportion of bolson deposition for the white strata, comes 

from the fact that they are everywhere channeled by 

sands and gravels derived from the neighboring highlands and containing little contemporaneous pyroclastic. debris. Such channels appear wherever the formation is exposed for as much as 50 feet vertically or a 

few hundred feet horizontally. If the bulk of the 

white rocks were deposited in a lake, it must have 

been shallow and often dry. 

The paucity of identifiable pyroclastic material in 

the coarse channel clastic rocks is puzzling. Rhyolitic ash surely fell on the surrounding hills at the 

same time it fell on the basin floor, and much of the 

stream load must have been contemporaneous pyroclastic debris. The crystal fraction of the ash is of 

minerals common in the pre-basin rocks, and obvious 

signs of their pyroclastic origin disappeared when 

they entered the stream system. The rhyolitic glass 

that fell on valley sides, presumably as slow to alter 

to clay as glass of similar composition in a lake, was 

probably transported as glass, rapidly reduced to siltsize in transit and eventually deposited with the fines 

outside the channels. Glass that fell on interfluves 

and long remained there may have altered to clay 

before entering a stream and in transit lost all overt 

signs of its pyroclastic origin. 

The rare thin porous limestones that grade upward 

from tuffaceous siltstones, but have sharp tops, may 

well be evaporitic rocks reflecting periodic drying; 

or they may be fossil caliche horizons, even more suggestive · of general bolson rather than lake conditions. 

The fabric, however, might have been produced by 

circulating ground water (Howard, 1932, p. 21). 

The climate of Dunbar Creek time is as uncertainly 

known as the mode of deposition of the tuffaceous 

rocks, and for much the same reasons. The idea that 

conditions were less humid than in Climbing Arrow 

time naturally follows from the suggestion of dry-land 

deposition. If there was a major shift in climate from 

Climbing Arrow to Dunbar Creek time it was not 

sudden, as the two units are gradational and the 

lower part of the Dunbar· Creek contains the same 

sort of vertebrate population as the Climbing Arrow. 

MIDDLE AND UPPER TERTIARY GRAVEL 

Fossil evidence indicates that deposits of late Oligocene-early Miocene, late Miocene, and possibly 

early Pliocene age occur in the quadrangle, but the 

rocks containing the· fossils are not mappable. 

Articulated remains of a · late Oligocene or early 

Miocene rhinoceros, Dieerathe'l'ium armatum Marsh, 

from a few miles south of Three Forks, have been 

described by Wood (1933, 1938). Wood did not himself collect the material, and the locality data furnished . 

are contradictory (2 miles southwest of Three Forks 

in ~he 1933 report; 4 miles south of Three Forks in 

the 1938 abstract) but it seems that the locality was 

in coarse sandstone overlying the bentonitic clays of 

the Climbing Arrow formation (called Thompson 

Creek beds by Wood), and probably therefore from 

between the Climbing Arrow formation and Quaternary bench deposits. Other remnants of Miocene ·deposits may be preserved similarly, appearing only at 

bench rims. In this category may be ch~nnel deposits, consisting of 10-25 feet of cross-stratified peb-
SEDIMENTARY ROCKS 81 

ble conglomerate and fine sandstone, exposed for a 

few hundred feet in the bluffs in the NW~ sec. 29, T. 

1 N., R. 2 E., below a thin capping of Quaternary 

cobble gravel. These rocks are stained dark yellowish orange, are variably cemented with clay and silica, 

and contain many fragments of opalized wood but no 

other fossils. They resemble some of the channel deposits in the Climbing Arrow formation, and the resemblance is strengthened by the fact that they overlie grayish-olive bentonitic clay and grayish-yellow 

bentonitic siltstone. The bentonitic rocks here, however, are less than 10 feet thick, and -overlie a thick 

succession of white glass-rich siltstones and fine sandstones, typical of the Dunbar Creek formation. The 

orange conglomerate and sandstone are much younger 

than Climbing Arrow and could be Miocene. Too 

small to ma·p separately, the exposures are included 

with the Dunbar Creek formation. 

Fragments of a horse tooth, identified as M eryehippus sp. of late Miocene age by Lewis (written communication, Apr. 12, 1957) were collected from rounded 

clay-matrix pebble gravel capping a low hill in SE1,4 

NW1JtSE1_4 sec. 3, T. 2 N., R. 1 W., 400 feet southwest of the Eocene vertebrate locality in the Climbing 

Arrow formation, and 15 feet higher (loc. 444, pl. 2 

[(red) D276]. The gravel is much like that designated old Quaternary alluvium, widespread in this 

vicinity, and it well may be that some of the alluvium 

mapped as Quaternary is really of late Tertiary age; 

or, conversely, the gravel may indeed be Quaternary 

and the tooth reworked from older rocks. 

About 1947, most of the skull, several teeth, and 

some other bones of a mastodon were found west of 

the junction of Highway 10N and Mud Spring Gulch 

road by Dr. Nick Helburn, Professor of Geography 

at Montana State College, and a group of students. 

The remains were taken to the college campus where 

I examined them. The exact locality is not known, 

but seems to have been within NE1/4SE1j4 sec. 3, 

T. 2 N., R. 1 E., just below the rim of the bench, 

capped with angular gravel, that occupies the northeast part of sec. 3. Lenses of subrounded gravel crop 

out at several places around the rim, between the angular-gravel cap and the Dunbar Creek formation, 

and the mastodon remains presumably came from 

one of these, for many subrounded pebbles were 

packed in the skull cavity when I saw it. Through the 

generosity of Professor Helburn, the remains of two 

cheek teeth were made available for study. Jean 

Hough (written communication, Mar. 4, 1954) found 

the material to be too incomplete for certain generic 

identification, but definitely "to belong to a primitive 

mastodon and not to the more common upper Pliocene or Pleistocene species. More complete material 

is needed to determine to which of the Miocene or 

Pliocene genera they should be referred." G. E. Lewis 

also examined this collection (written communication, 

Jan. 19, 1956); he described the animal as "a gomphotheriid mastodon [(red) D109], possibly referable to 

a Rhynehotherium such as that from the Deep River 

fauna of Montana. The age is certainly not older than 

late Miocene, and n1ight be early Pliocene." 

The distribution and stratigraphic relations of these 

later Tertiary gravels plainly indicates great disconformity with the Eocene and some Oligocene rocks. 

In terms of time, the shortest interval separates the 

late Oligocene or early Miocene deposits with Dieeratherium from the early Oligocene part of the Climbing Arrow formation; longer is the interval between 

the mastodon-bearing latest Miocene or early Pliocene gravel and the underlying Oligocene Dunbar 

Creek formation; longer still the interval between 

the M eryehippus-bearing late Miocene gravel and the 

underlying late Eocene part of the Climbing Arrow 

formation. In terms of structure, the younger Tertiary gravels chfl,nnel the older formations; the scale 

of channeling, however, seems no greater than that 

of the sandstone and conglomerate lenses within the 

older formations, so that significant angular unconformity is not suggested. In terms of thickness, it is 

reasonable to assume that many hundreds of feet of 

Climbing Arrow and Dunbar Creek rocks have been 

removed hy intra-Tertiary erosion, but due to the inconstant initial thicknesses and lateral variation of 

the units, and their uncertain structural history, the 

assumption is hard to fortify with even crude measurements. 

Little can be said about the origin of these elusive 

later Tertiary deposits. The gravels are much like 

those in the older rocks of the Bozeman group, suggesting similar sources and regimens. The meager 

faunal evidence suggests a more humid climate with 

a narrower temperature range than that of the present. 

QUATERNARY 

Rocks of known or probable Quaternary age, subdivided into six map units, are thick and continuous 

enough to map over more than 110 square miles, and 

over a vertical range of more than 1,000 feet. The 

thickest and most extensive Quaternary deposits are 

those made by the present streams and their predecessors. The stream deposits flooring valleys, whatever 

their textural variations, are combined in a single 

map unit designated young alluvium. Older stream 

deposits, graded to surfaces above the present flood 

plains, constitute a second unit of formational rank, 

named old alluvium. It is subdivided over large areas 

into two members, rounded gravel, on valley plains, 
82 GEOLOGY OF THE TIIREE FORKS QUADRANGLE, MONTANA 

an.d subrounded gravel, in tributary valleys. Also 

mapped are thin but extensive deposits of angular 

and subangular fan gravel and sheets of eolian silt. 

The old alluvium, the fan gravel and the eolian 

silt have intricately overlapping relations and cannot 

readily be discussed in chronological order. They are 

described, therefore, in order of abundance, followed 

by a brief description of the young alluvium and a 

discussion of the ages and stratigraphic relations of 

all the Quaternary deposits. 

Although the limits of the Quaternary deposits 

were mapped with care, the rocks within each unit 

have been but casually studied. If the preceding discussion of the Tertiary rocks may fairly be called a 

start. toward their understanding, the ensuing discussion of Quaternary deposits must be regarded as 

merely a sympathetic gesture in their direction. 

OLD ALLUVIUM 

Old alluvium, graded to levels above the flood plains 

of the Jefferson and Madison Rivers and actively 

being eroded, is exposed over 30 square miles. Rem- . 

nants at low altitudes, on the sides of Jefferson Valley, 

are as much as 400 feet thick; at higher altitudes 

they are much thinner, generally less than 15 feet 

thick on high benches. 

The formation lies mostly on soft, easily eroded 

Tert.iary rocks. In some places, however, particularly in the southwest and northwest parts of the quadrangle, it lies in shallow valleys cut on all sorts of 

hard bedrocks ranging from Precambrian gneiss and 

arkose to Paleozoic limestone and quartzitic sandstone, 

Upper Cretaceous volcanics and Cenozoic basalt. 

The old alluvium almost everywhere dips at angles 

of less than 2° toward the junction of the Madison 

and Jefferson Rivers. 

Most of the old alluvium is gravel. The higher an 

alluvial remnant, and the farther from the present 

rivers, the greater its proportion of gravel: the highest old alluvium, capping the 5,050-foot bench at the 

south edge of the quadrangle, is all gravel; the lowest, 

just above the flood plain of the Jefferson River, contains large volumes of sand and silt; the old alluvium 

on the sides of the valleys of the present tributaries 

has increasingly large proportions of sand and silt 

downstream. 

The main size-grades in the gravel are large pebbles 

and small cobbles, though boulders as much as 2 feet 

long are present. The gravel tends to coarsen somewhat away from the present master streams, but the 

rate of coarsening seems much less per unit distance 

than in the young alluvium. The average degree of 

rounding in the stones varies markedly among gravel 

bodies. Large volumes in the southern part of the 

quadrangle are dominated by rounded gravel, whereas 

other large volumes, mostly north of the Jefferson 

River, are mainly subrounded gravel. This is an overall aspect quite apart from the downstream increase 

in degree of rounding to be expected in any stream 

gravel, and is the basis for recognizing two gravel 

members, rounded gravel and subrounded gravel. 

The gravel is dominated by noncarbonate rocks of 

low porosity: quartzitic sandstone, medium- and 

coarse-grained quartzite, ·vein quartz, quartz-rich 

gneiss,_ and dark volcanic rocks. All the abundant 

rock types are common in the surrounding hills. 

ROUNDED GRAVEL 

The rounded gravel member has been mapped over 

an area of 16 square miles south of the Jefferson 

River. Rounded, well-sorted pebble and cobble gravel 

mantles most of. the -broad· smooth- benches south of 

the river and drapes discontinuously over the intrabench slopes. Benches are numerous north of the river 

but, remarkably, no similar deposits cap them, unless 

some are concealed by soil on the benches in the large 

area of undifferentiated old alluvium across the river 

from Three Forks. It is unlikely that rounded gravels 

were once extensive north of the river, but have been 

utterly stripped, while their counterparts to the south 

remained. Rather, rounded gravel seems never to 

have been deposited on the northern benches. The 

gravel capping evidently retards erosion of the southern benches while the northern ones, unprotected, are 

beihg rapidly removed. 

Many gravel patches are large, covering more than 

a square mile, and the gravel cover on the highest 

( 5,050-foot) bench extends continuously for 7 miles 

south of the Three Forks quadrangle. The gravel 

is by no means the continuous and presumably thick 

mass it appears to be, however, in the 9-mile stretch 

south of Fairview Cemetery, where it blankets benches 

and slopes alike. Actually, the gravel is thick only 

on the benches. On the slopes Tertiary strata make 

innumerable small outcrops and have been encountered at many places by digging through a gravel film 

a stone or two thick, but the Tertiary outcrops are 

rarely continuous enough to map. The gravel film, 

no doubt let down from the bench above, is not distinguishable from gravel in place and is thus mapped 

on many slopes. The original relation between gravel 

and terrain is well shown at the base of the highest, · 

oldest bench where streams have become integrated 

enough to sap the colluvial cover and reveal the Tertiary rocks continuously along the deeply dissected 

bench front, with gravel confined to the bench surface. 

Although the gravel protects the underlying bench, 

even the youngest. lowest bench is dissected enough to 
SEDIMENTARY ROCKS 83 

demonstrate that the gravel is thin in comparison with 

its areal extent, although close measurement of thickness is made difficult by the tendency of the uncemented round stones to roll downhill even on low 

slopes. Where gullies intersect locally cemented 

gravel, however, the thickness can readily be measured: in such places it is generally on the order of 

5 feet and rarely exceeds 15 feet. Probably, it averages around 10 feet thick and has a maximum thickness of less than 50 feet. The gravel at the head of 

the highest bench, 17 miles south of Three Forks, is 

thicker, about 100 feet thick according to Alden (1953, 

p. 35). 

Judging by the surface layer only, the gravel at each 

bench level differs markedly in coarseness, sorting, 

and lithology from other bench levels. The surface 

of the highest gravel body is smoothly paved with 

rounded egg-shaped or discoidal cobbles, mostly 4 to 

8 inches long, of quartzite or quartzitic sandstone, 

colored pale grayed tones of orange, brown, or red. 

Nearly every cobble has a yellowish-brown skin, whatever the color of the rock, and many are studded with 

neatly curved percussion marks. The surface layer 

on each successively lower bench exhibits similar large 

cobbles but has successively higher proportions-of small 

cobbles and of pebbles, so that the average grain 

size is progressively smaller and the sorting poorer; 

also, the prevailing quartzite lithology is progressively 

diluted with other rock types, particularly gneisses 

and volcanic rocks. 

A foot or two below the surface, however, these 

apparent differences disappear. If there are any systematic lithologic or textural differences from bench 

to bench, that might be demonstrated by detailed 

counts or other measurements, they are small. Viewed 

in three dimensions, the rounded gravels from highest 

to lowest can be categorized as indistinctly bedded 

sheetlike pebble and cobble strata, with a little interstitial sand, dominated by quartzite rocks but with 

large proportions of other crystalline rocks. 

Though generally unconsolidated, the rounded 

gravel in several places is thoroughly cemented by 

white chalky calcite. Lime-cented zones occur in the 

highest rounded gravel but are most abundant at intermediate levels. None have been found at altitudes 

below 4,440 feet. As exposed in stream cuts, the cemented zones consist of strips, a few tens or hundreds of feet wide and the entire thickness of the 

gravel, that are thoroughly impregnated with calcite, 

in places so much so that the pebbles are separated 

an inch or. more (fig. 12). The distribution of lime 

cement seems to be channellike rather than blanketlike but the trends of the channels are unknown as 

the cemented zones cannot readily be traced away from 

FIGURE 12.-Lime-armored Quaternary gravel, capping bench in NW~ 

NW% 8.ec. 18, T. 1 S., R. 2 E. 

the outcrop. Transitions to uncemented gravel are 

abrupt, occurring within a foot or two laterally. The 

calcite is apparently not the product of evaporation but rather appears to have been deposited from 

laterally moving channeled ground water or perhaps 

from hot-spring water rising along fractures. 

SUBROUNDED GRAVEL 

Subrounded pebble gravel is mapped separately 

from the other deposits of old alluvium in many large 

lobate areas riorth of the Jefferson River, and in the 

drainage of Willow Creek. Covering about 15 square 

miles, the bodies of subangular gravel differ from the 

bodies of rounded bench gravel in being relatively 

thick in comparison with their width, and in having 

rudely triangular rather than sheetlike cross sections. 

Only locally are they distinctly benched. In many 

places, they reach almost or quite to the top of the 

valleys they occupy but do not spread over the 

bordering benches, suggesting that their original extent was never much greater than their present extent. 

The base of the subangular gravel is rarely visible 

so its thickness can ohly be estimated. On the unsafe 

assumption that the valleys holding these gravels had 

cross sections and long profiles shaped like those of 

present valleys, the wedges of subangular gravel appear to be more than 50 feet thick over long stretches 

and exceptionally as thick as 100 or even 150 feet. 

A thickness of at least 60 feet of old alluvium, in a 

patch too small to map, is penetrated by a pumping 

water well at the head of the valley in Three Forks 

shale in NW1JtSE1;4 sec. 19, T. 3 N., R. 2 E. 

Most of the gravel is in the range of pebbles and 

small cobbles, but small boulders are common. Surface layers contain little fine material but excavation 

reveals a large sand and silt fraction. Although most 

stones are subrounded, many are subangular. 
84 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

The lithology of the sub rounded gravel varies 

widely. In the northwestern part of the quadrangle, 

it is dominated by dark andesitic volcanic rocks. On 

the north side of. Mud Spring Gulch, it contains large 

proportions of coarse monzonitic rocks. Near the 

southwest corner of the quadrangle it is heavily 

charged with fragments of gneiss and of vein quartz. 

ORIGIN OF OLD ALLUVIUM 

The form and distribution of the deposits show that 

the rounded gravel and most of the undifferentiated 

old alluvium near the present Jefferson River are 

valley-plain deposits and that the subrounded gravel 

is fill in tributary valleys. 

That the rounded gravel was deposited by a major 

perennial river is indicated by its rounding, its sorting, and its considerable volume and extent. All the 

gravel could have been derived from the basin margins, to judge by lithology. More distant transport 

for much of it is suggested by the fact that the 

gravels are about as well rounded and sorted at the 

remote edges of the ba~in as they are near the outlet 

at the head of the Missouri. The gravels are little 

different from highest to lowest except for the surface 

layer, indicating that the regimen was about the same 

during the cutting and filling of the entire flight of 

terraces capped by rounded gravel. The differences 

between surface layers are regarded as lag phenomena. 

Most of the rounded gravel, as noted earlier, is in 

the southern part of the quadrangle : the patches 

descending from the Madison Range are broad and 

long; those descending to the Jefferson River from 

the north are narrow and short. This asymmetry of 

distribution, apparently a consequence of northerly 

or northwesterly tilting, is discussed later. 

The distribution and texture of the subrounded 

gravel show that it was deposited by tributary streams. 

That the tributaries were short is suggested by the 

lithology of the gravel, everywhere directly reflecting 

the kind of resistant rocks that crop. out upstream. 

Presumably, deposition of subrounded gravel in 

tributaries accompanj~d deposition of all the rounded 

gravel. The visible s-.;tbrounded gravel, however, seems 

to represent only the later part of the long · history 

indicated by the size and distribution of the rounded 

gravel. No subrounded gravel is preserved above 

4,700 feet. None is strikingly benched, and it is recognized only in valleys that are the sites of present 

streams. Possibly, older remnants exist, but were not 

recognized because they are not dissected. More likely, 

the older subrounded gravel has been eroded. Lacking discharge for effective sidecutting, the tributaries 

became fixed at an early stage; the older fills were 

swept out during episodes of downcutting and only 

the younger valley fills are preserved. 

Certainly the landscape and probably the climate 

were greatly different from the present when the 

highest gravel was laid down. The relief within the 

quadrangle must have been far more subdued than 

now. Only 10 square miles of very resistant bedrocks 

are now topographically higher than the highest 

rounded gravel on the 5,050-foot bench, and less than 

1 square mile is as much as 500 feet higher. Relief 

between ridge tops and the flood plains matching that 

of today-800 to 1,300 feet-would require removal 

of a volume of resistant rocks inconceivable in the 

brief time available, even if the oldest old alluvium 

is of late Pliocene age. Lower relief than at present, 

or at any rate lower average stream gradient, is directly suggested by the fact that the average and 

maximl}m sizes of clasts in the old alluvium are distinctly smaller than in the young alluvium. In turn 

the young alluvium was formed under geomorphic 

conditions much like those of the present if not necessarily under the present climate. Successively lower 

deposits of the old alluvium were of course deposited 

under physiographic conditions that successively approached those of the present. Their distribution 

indicates increasing relief and narrowing flood plains 

during old-alluvium time. 

As for climate during that time, little positive evidence exists. Ancient soil horizons are lacking, or at 

any rate not notably developed; lacking, too, are comparable features such as rotted pebbles or authigenic 

growths of such climate-sensitive ·minerals as iron 

and manganese oxides. The local lime-cementation 

has little climatic implication if correctly interpreted 

as of ground water or spring rather than evaporativecapillary origin: The varying lag concentrations of 

quartzite cobbles seem to be a function of time rather 

than of climate. 

The textures and structures of the subrounded gravel 

compared with those of young alluvium in the same 

valleys imply a much more humid climate than now. 

Not only are the older gravels much better sorted and 

rounded than adjacent young alluvium but they virtually lack torrential false-bedding, common in the 

youngest fill, implying that they were the products 

of streams with more and steadier discharge than the 

present intermittent tributaries. 

FAN GRAVEL 

Widespread north of the Jefferson River, but rare 

to the south, are extensive fan-shaped deposits of 

angular and subangular gravel. Only the largest are 

rna pped. They consist mostly of unsorted slabs of 

limestone heavily encrusted with caliche on the base, 
SEDIMENTARY ROC:rts 85 

and rarely on other sides. Most of the deposits are 

sheetlike, commonly no more than 2 or 3 stones thick; 

a few are wedge-shaped and much thicker. The clasts 

are generally in the pebble and small boulder range, 

but rarely are as much as 3 feet long. In places the 

stones are in a sand or mud n1atrix but usually they 

are loosely strewn about the surface. The older the 

deposit, as indicated by its degree of dissection and 

of topographic isolation, the smaller the proportion 

of interstitial fines and the thicker and more uniform 

the caliche crusts. 

Most of the fan gravel mantles benchlands, below 

altitudes of 4,900 feet, between the limestone ridges 

and the flood plains. In plan, the shapes of the 

benchland deposits vary somewhat but they are an 

rudely fanlike, broadening downhill, or consist of 

scattered patches of gravel that seem to be the dis.,. 

sected remnants of an originally fanlike gravel film. 

The larger deposits are draped over several benches 

and the intervening steeper slopes. Blanketing the 

base of the Hossfeldt Hills is a virtually continuous 

apron of subangular gravel, with a copious sand 

matrix, that extends to the Jefferson River flood plain 

on the southeast and to the flood plain of Mud Spring 

Gulch on the southwest. North of Highway lOS near 

the west edge of the quadrangle are scattered patches, 

with an aggregate fan shape, of angular limestone 

gravel that lies on a variety of rocks ranging from 

Belt arkose to old alluvium; the distal end of this 

tattered fan is more than 100 feet above the J e:fferson 

River flood plain. A large area of angular gravel 

east of Willow Creek is mostly fragments of dacite. 

Smaller strips or caps of angular limestone gravel lie 

on benches in the center of the quadrangle, west of 

Three Forks Junction; about llj2 miles west of Silver 

Sage Ranch at the edge of the quadrangle; and above 

Fairview Cemetery a mile southeast of Three Forks. 

Thicker deposit~ of subangular gravel occupy the 

valleys of a few short tributaries. Such deposits are 

at the north edge of the quadrangle in. sees. 20, 21, 28 

and 29, T. 3 N., R. 1 E.; between the monzonite pluton 

and Copper City in sees. 25 and 26, T. 3 N., R. 1 E.; 

east of the mouth of Milligan Canyon; and on tributaries of Sand Creek in sec. 33, T. 1 N., R. 1 W., and 

sec. 4, T. 1 S., R. 1 W. These valley-filling gravels 

seem slightly better rounded than those on the benchlands and have relatively little caliche. They tend 

to be composed of whatever well-indurated rock crops 

upslope. This is generally limestone but in places is 

quartzitic sandstone or coarse-grained igneous rock. 

Exposures reveal only the upper few feet of these 

deposits; in places they are no doubt many tens of 

feet thick. 

ORIGIN 

The texture, lithology, and topographic position of 

the unsorted angular limestone gravel on the benchlands suggest colluvial rather than stream origin. 

The rudely fanlike ground plans, and the plain evidence of miles of movement of large fragments over 

low slopes, especially well shown in the fan remnants 

lying on North Boulder formation north of Jefferson 

School, favor transportation by sheetflow or, better, 

mudflow . rather than by unlubricated creep or mass 

flowage. It well may be that such deposits are not 

the results of torrential rains or thaws but instead 

may require the ge;n.tle and protracted wetting of 

material of appropriate texture lying at the base of 

steep slopes. One trouble with a torrential origin 

is that there are too few mudflow deposits, given the 

many hundreds of torrential rains that must have 

occurred in the many thousands of years during which 

the benchlands have been exposed. Perhaps the operative process was akin to fluidization processes in 

gas-solid systems, now widely used in industry (summarized by Reynolds, .1954). The essential element 

in connection-with gravity-driven water-solid systems 

like mudflows is that optimum small amounts of water 

may act to reduce the coefficient of friction of the 

mud, to permit flow on low slopes, but not reduce its 

viscosity to a point below which it cannot move large 

fragments. The water requirements for inducing and 

maintaining flow in a gravel-soil mixture may have 

so narrow a range that exactly the right rate and 

duration of precipitation (or thaw) on properly prepared materials and slopes is statistically improbable, 

thus accounting for the paucity of colluvial gravels. 

However transported, the angular gravels are plainly 

the products of an arid or semiarid climate like the 

present. 

. The subangular gravels in valleys and, mantling 

the Hossfeldt Hills are probably partly colluvial, but 

mostly alluvial. That their stones are subangular and 

include much limestone reflec~s minimal transport of 

the materials in running water, a simple consequence 

of the shortness of the depositing streams. These deposits appear to be genetically intermediate between 

the angular colluvial benchland gravels and the subrounded valley fills of old alluvium. 

SILT 

Light-colored silt of probable eolian origin is widespread in the benchlands. Such silt is distributed over 

perhaps 30 square. miles, of which about 10 square 

miles are covered continuously enough to map. There 

are three large general areas of silt; one at the north 

edge of the quadrangle, one at the west-central edge, 
86 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

and one south of Three Forks. The deposits along 

the north edge are at the south margin of a great 

silt sheet that cloaks the broad divide between the 

Townsend and Three Forks valleys. Those at the west 

edge seem to be a part of a long strip of similar material that extends 3 or 4 miles farther west into the 

Jefferson Island quadrangle. Thickness of the silt, 

hard to measure without much digging, is estimated 

to be less than 5 feet over most of the mapped areas 

and only exceptionally more than 15 feet. It weathers 

readily to soil, and most of the silt areas are in active 

cultivation. 

The silt lies at varying altitudes and on a variety 

of rocks and surfaces. A unifying relation is that 

all the mapped silt bodies are south of extensive bare 

exposures of poorly consolidated Tertiary rocks rich 

in clay and volcanic glass. 

The unit consists mostly of yellowish-gray to very 

pale orange angular silt and clay with scattered grains 

and streaks of rounded fine sand: Several inches at 

the top are commonly altered to a yellowish-brown 

clayey soil. No stratification is visible, even in continuous vertical exposures as much as 10 feet high, 

as in the silage pit in SW14NW1;4 sec. 2, T. 1 S., 

R. 1 E., and along the former course of Highway 10N 

(west of the present highway) in NW14 sec. 7, T. 3 N., 

R. 1 E. The dominant materials are volcanic glass, 

quartz, and clay minerals with minor amounts of mica, 

feldspar, and calcite. The silt is riddled in many 

places with thin vertical tubular openings, presumably 

root scars. Despite its lack of ·cement the silt, presumably because of the angularity of its grains, is 

able to maintain vertical walls for many years in 

artificial cuts, a feature well shown at the localities 

mentioned. 

The deposits have the classic features of loess. Their 

eolian origin seems certain. Distribution of the loess 

suggests that northerly winds swept the grains off 

bare outcrops of the Climbing Arrow formation and 

similar units, eventually dropping them either at the 

windward base of high obstructions, like the Madison 

limestone ridge above Dry Hollow, or in the lee of 

low obstructions. Part of the loess, especially that 

south of Three Forks, may have been '.blown off aggrading flood plains. 

YOUNG ALLUVIUM 

Stream deposits graded to or below the present 

flood plains occupy 60 square miles. Regardless of 

their texture, they are indiscriminately shown as young 

alluvium. Exposures are abundant, owing to a very 

recent, possibly still continuing, episode of rejuvenation that has led to as much as 15 feet of trenching 

by tributaries. (See fig. 15.) The deposits exposed 

along present channels of the Jefferson and Madison 

Rivers are dominantly subrounded to rounded, clean, 

fairly well sorted gravel and sand. The gravel is 

but rarely coarser than cobble size. Large areas of 

swampy or soil-covered ground are probably underlain by silt and mud but little of such finer material 

is exposed. Along the tributaries the dominant visible 

material is poorly sorted subangular silty and sandy 

gravel containing many boulders. Rarely, lenses a 

few feet thick of better sorted material-sandy cobble 

gravel, pebble sand, and reddish-brown clayey siltappear. Judging by a few tributaries followed to 

their mouths, this poorly sorted, poorly rounded alluvium extends along the tributary channels almost 

if not quite to their junctions with the through rivers. 

With more observations it would have been feasible 

to subdivide the young alluvium in the same way as 

the old, into two gradational lithologic units in characteristic settings: poorly sorted, poorly rounded rna- · 

terial, in tributary valleys; and much better sorted 

and better rounded detritus on the flood plains of the 

through rivers. 

The thickness of the young alluvium is unknown 

but is probably less than 50 feet throughout. In the 

tributaries the thickness is greater than the depth of 

recent trenching-15 feet maximum-and may be 

several times this thickness; above Milligan Canyon, 

drilling by the Bureau of Reclamation (H. C. Elliott, 

oral communication, 1957) has shown that the valley 

fill is rarely more than 30 feet thick. Presumably it 

is not greatly thicker under the flood plains of the 

Jefferson and Madison Rivers, for the tributaries are 

accordant. Many water wells have been drilled in 

the flood plains but it is hard to tell from driller's 

log where the wells leave young alluvium and enter 

poorly consolidated Tertiary deposits. 

Very little of the young alluvium was deposited 

under present climatic conditions. Under the present 

climate, the str~ams are primarily intrenching themselves in their earlier deposits; such deposits as they 

make a:r:e scanty and confined to mere patchy films 

on valley floors or sides. The bulk of the young 

alluvium was surely laid down under more humid 

climate than now prevails, and most of it is of 

Pleistocene rather than Recent age. 

AGE AND STRATIGRAPHIC RELATIONS OF QUATERNARY 

DEPOSITS 

Fossils have been recovered only from the young 

alluvium so that the age of the other Q,uaternary 

deposits must be approached indirectly. 

The age of the young alluvium is late Pleistocene 

and Recent. The right scapula of a Bison antiqulusf 
SEDIMENTARY ROCKS 87 

of Mankato or Cary age (identified by the Upper 

Cenozoic Research Group, over the signature of Edward Lewis, written communication, Nov. 18, 1955) 

was excavated from alluvium graded to the Jefferson 

River flood plain 2 miles southeast of the Buttleman 

Ranch in center NW1;4 sec. 23, T. 1 N., R. 1 E. (loc. 

394, pl. 2). The alluvium here is deeply trenched by 

the present stream. The scapula, showing little sign 

of erosion, was found embedded in the east· bank of 

the trench 31j2 feet below the surface. Because no 

other bones of the animal were attached or nearby, 

there is some possibility that the bone was reworked 

from Quaternary gravel that crops out upstream and 

uphill. 

Remains of modern cow (identified in the field by 

Jean Hough) were recovered f~om the upper foot or 

two of alluvium or associated colluvium in several 

places. The presumption is that modern cow did not 

appear in the area until the first settlement by white 

people around 1860, and thus that significant flooding 

and deposition has occurred in the area within the 

past century (the trenching is not necessarily younger 

than burial of the cows but may be contemporaneous) . 

Neglecting the scant paleontological evidence, it 

seems reasonable to assume that the flood plains of 

the Jefferson and Madison Rivers, 2 to 4 miles brbad, 

and the great volume of deposits beneath them required far longer than the Recent to form. In developing the present flood plains something on ·the 

order of 400 to 600 feet of deposits, mostly old alhivium, had to be removed from the Jefferson Valley. 

This is indicated not only by the height of paired 

bench gravels above the rivers but is also required to 

achieve superposition of the present Jefferson channel 

upon at least 450 feet of cover between the Madison 

limestone ridges 41j2 miles southwest of vVillow Creek 

(see fig. 16). It also must have taken far longer 

than the Recent to cut the towering cliffs below 

Milligan Creek. Of course both of these latter erosional feats may have been performed by early Tertiary rivers, and the Jefferson's task may have been 

the somewhat easier one of exhuming the evidence by 

removing soft earlier Quaternary deposits. 

The old alluvium represents a long time interval, as 

implied by its vertical distribution of 1,000 feet, and 

by the large dimensions of individual gravel masses. 

Downcutting to each successively lower bench level 

may have happened quickly, but the extensive lateral 

shifting necessary to cut and fill terraces miles wide 

must have required much time. Of course, the entire 

present width of each terrace need not have been developed before incision of the next lower one; part 

of the widening has doubtless been by mass-wasting 

processes long after the rivers had begun working 

at lower levels and could only affect higher terraces 

by narrowing them. 

The old alluvium is no younger than late or middle 

'Visconsin for it is disconformably succeeded by young 

alluvium. Part of the old alluvium could be of Pliocene age because the latest fossil-=the gomphotheriid 

mastodon-locally found in the preceding deposits 

is no younger than early -Pliocene or possibly latest 

Miocene, and no demonstrably younger rocks have 

been reported in the vicinity. A Pliocene age would, 

of course, give more time to develop the long flight 

of terraces and also the quartzite lag gravel at the 

surface of the higher deposits, which closely resembles 

the widespread Flaxville gravel of Collier and Thorn 

(1918) of Miocene and Pliocene age. 

Probably the old alluvium is entirely of Pleistocene 

age. The Madison Valley formation of Douglass 

(1903) forms the upper part of the east Madison 

bluffs and dips gently northeastward (mostly in Manhattan quadrangle; see fig. 1B). The plain that slopes 

northeastward at about 150 feet per mile from the top 

of the bluffs approximates a dip slope on the Madison 

Valley formation, according to Dorr (1956, p. 72). 

The Madison Valley formation is generally regarded 

as of Miocene and Pliocene age (Wood and others, 

1941 ~ Charles H. Falkenbach, oral communication, 

1954) aithough there is some doubt as to the location 

of the time boundary. Dorr (1956, p. 73) concluded 

that "Most if not all of the Madison Valley formation 

is Miocene rather than early Pliocene in age" but 

Falkenbach cited recovery of Pliocene oreodonts from 

the uppermost gravels on the bluffs as convincing 

evidence of Pliocene age for part of the formation. 

As Dorr presented no fossil evidence that the uppermost bluff-making rocks are pre-Pliocene it seems 

premature to abandon the long accepted view that 

some at least are Pliocene. The dip of the Madison 

Valley formation projects it far above the Three 

Forks quadrangle. The formation has evidently been 

truncated by the erosion which preceded the earliest 

deposits of old alluvium, and removed practically all 

post-Oligocene Tertiary rocks from the quadrangle. 

The oldest old alluvium, with its much lower and 

more northerly dip~ appears to be structurally unconformable with the Madison Valley formation. This 

interpretation of unconformity, highly questionable 

on local evidence because the rocks involved are not 

in contact, is supported by direct evidence from the 

Manhattan quadrangle, sec. 26 and 35, T. 1 N., R. 2 E. 

(shown me by 0. M. Hackett). There, the northeastdipping l\:fadison Valley formation is unconformably 

overlain by north-dipping gravel, identical lithologi-
88 GEOLOGY OF THE mREE FORKS QUADRANGLE, MONTANA 

cally with the oldest old alluvium and at the same 

altitude, about 5,050 feet. 

If sufficient time is allowed for the g~eat amount 

of erosion represented by the unconformity between 

the Pliocene part of Douglass' Madison Valley formation and the rounded gravel, and at the opposite end 

enough time is allowed to develop the quartzite laggravel surface on the old alluvium, it is a reasonable 

working hypothesis that none of the old alluvium is 

older than Pleistocene, but that some of it may be 

earliest Pleistocene. 

The fan gravels, to judge by their greatly varying 

degrees of dissection, have a rather wide age range. 

All, however, extend to such low altitudes as to show 

that they are younger than the bulk of the old alluvium. The youngest fan gravel is in the great 

blanket swathing the Hossfeldt Hills. This blanket, 

extending to the flood plain of the Jefferson - and 

Madison Rivers and scarcely touched by erosion, is 

probably younger than all but a small fraction of 

the young alluvium. The oldest fan gravel would 

appear to be the most tattered fans of angular gravel 

at the highest altitudes, but because the deposits are 

colluvial and not graded to any stream, this sort of 

evidence is weak. The valley-filling fans seem younger 

than most of the old alluvium but older than most of 

the young alluvium. 

If the fan gravels are the products of semiarid 

climate, they were not laid down simultaneously with 

any of the humid-climate old alluvium. Probably one 

or more of the episodes of cutting that produced the 

successively lower terraces on which old alluvium was 

deposited was accompanied by climatic changes (interstadial intervals ~ ) , so that fan gravels were deposited 

alternately with old alluvium as the climate oscillated 

in later Quaternary time. (It does not necessarily 

follow, however, that climatic change was responsible 

for any of the terraces involving old alluvium.) 

The stratigraphic relations of the small area of 

angular fan gravel above. Fairview Cemetery are 

unclear. This angular gravel is older than rounded 

gravel on the low-level bench to the east but its low 

altitude suggests that it is not among the oldest 

Quaternary deposits. The nearest 'bedrock source for 

its limestone blocks, and the nearest deposits of similar 

material, are 4 miles to the west across the Jefferson 

Valley. Pet·haps this gravel is the remnant of a mudflow that started on the northwest valley wall at a 

time when the valley was filled with alluvium to the 

level of the 4,400-foot bench. The presep.t downhill 

northward extension of the gravel to below 4,200 feet 

is probably due to postdepositional movement in response to erosion of the northern base of the deposit. 

The eolian silt may represent more than one depositional event. The silt along the north edge of the 

quadrangle may be older than the subrounded gravel, 

for the silt is cut by the valleys in which the subrounded gravel is exposed. Furthermore, considerable 

antiquity is suggested by the development of extensive arable soil on the silt. But although the silt is 

older than the present valley incisions, it could be 

younger than the subrounded gravel, now being exhumed simultaneously with the silt. The other areas 

of silt all seem to be younger than most of the old 

alluvium and older than most of the young alluvium. 

The silt south ·of the Jefferson River overlies and is 

thus younger than the rounded gravel on the 4,400-

foot bench, but is older than the young alluvium in 

the valley below. The windblown silt south of upper 

Milligan Creek over lies and is younger than the old 

alluvium· there; it is probably a little older than the 

nearby young alluvium. 

There is no direct evidence of the time relations 

between eolian silt and fan gravel. Presumably windblown deposits are more likely to form in a dry 

climate, so that the silts were probably deposited 

concurrently with some of the fan gravels. 

IGNEOUS ROCKS 

Igneous rocks in the quadrangle occupy 14 square 

miles and are divided into five main map units. They 

are · mostly of Late Cretaceous to early Tertiary (preBozeman group) age and fall into two categories: 

extrusive rocks-the Elkhorn Mountains volcanicsof much deformed andesitic lava flows and volcanic 

breccia that were emplaced before or in the early 

stages of regional folding, and an array of mon~onitic, 

dacitic, latitic, and andesitic intrusives, emplaced 

during or after folding in the form of sills, laccoliths, 

and one semiconcordant pluton. Some of the intrusive rocks, particularly the andesite and latite, may 

be contemporaneous with the volcanic rocks; most 

of them, however, seem to be a little younger. Igneous 

rocks that do not fit these categories. are pockets of 

Precambrian pegmatite in the Precambrian crystalline rocks, noted in the section on the crystalline rocks, 

and two small masses of olivine basalt, intrusive into 

rocks near the base of the Bozeman group and of 

probable early Tertiary age. Not classed as igneous 

rocks, though they might be, are the large volumes of 

Tertiary sedimentary rocks made up mainly of volcanic ash but here included with the ~ozeman group. 

The igneous rocks are much less resistant to erosion 

than the pre-Tertiary carbonate rocks and quartzitic 

sandstone but more resistant than the pre-Tertiary 

clay-rich rocks or the poorly consolidated Cenozoic 

sediments. vVhere extensive enough, they ' develop 
I(m":EIOUS ROCKS 89 

low rolling somber-hued topography with even less 

vegetation than the surrounding rocks, but with a 

thick carapace of weathered material that in most 

places effectively conceals the nature of the rock and 

its relations. 

About 80 thin sections were studied by H. Frank 

Barnett who wrote ·the petrographic descriptions that 

follow. The local~ty and rock type represented by 

each thin section studied is identified by a numb~r 

and symbol on the data rna p, plate 2. The thin se ctions were examined by routine petrographic microscope methods. Mineral volume percentages were 

estimated against comparison charts; they are least 

accurate in heavily altered and in very fine grainE~d 

. rocks. Plagioclase compositions were mostly inferred 

from measurements of extinction · angle compared 

with the high-temperature curves of Troger (1952); 

a few untwinned feldspars were studied in immersion 

liquids and further checked' by staining with sodium 

cobaltini trite. 

Rock names follow the classifications. of Kemp and 

Louderback as modified by R. C. Mielenz (1948). 

Sufficient information is given for the common rocks 

so that other classifications could readily be applied. 

Quartz is considered essential if it amounts to more 

than 5 percent of· the total rock volume. Rocks are 

described as porphyritic if phenocrysts aggregate 

from 2 percent to 50 percent, or porphyries if phenocrysts exceed 50 percent. 

Eleven typical igneous rocks have been analyzE~d 

chemically. The localities represented by analyzed 

rocks are shown on plate 2. Results of these analyses, 

and the norms calculated from them, are presented 

in table 9; the analyses are the basis for a silica-variation diagram (fig. 13). 

UPPER CRETACEOUS AND I.JOWER TERTIARY ROCE:S 

EXTRUSIVE ROCKS-ELKHORN MOUNTAIN'S 

VOLCANICS 

Assigned to the Elkhorn Mountains volcanics of 

Klepper and others (1957, p. 31-41) are stratified, 

deformed andesitic volcanic rocks that crop out over 

more than 6 square miles and are the most extensive 

igneous rocks in the quadrangle. They appear in 

two areas: in the northwest corner of the quadrangle, 

and below the Jefferson Canyon thrust west of Jefferson School. In both areas, the formation yields 

low rolling barren topography that is a distincthre 

steely purplish gray when viewed from a distanc:e. 

Close at hand, purplish tones are not so apparent and 

the weathered surfaces are mostly yellowish brown 

or olive brown. Freshly broken surfaces are mostly 

medium dark gray though a greenish or bluish cast 

is not uncorr .. mon. 

The dominant rocks are porphyritic andesite with 

many somewhat indistinct stubby phenocrysts of feldspar and scattered long ones of pyroxene. Nearly all 

are compact, with few vesicles or· amygdules, and their 

groundmasses are fine grained to medium grained. 

Many fractures are lined with coarsely crystalline 

yellowish-gray stilbite in stellate clusters. The andesite layers are generally a few tens of feet thick. 

Presumably, most of them are lava flows but a few 

may be shallow penecontemporaneous sills. Interbedded with them are a few thin layers of latite, most 

of which are no doubt lava flows also ; some have a 

streaky banded structure suggesting welded tuff. 

In the northwest corner, the formation includes 

much volcanic breccia in units as much as 500 feet 

thick; their lateral extent is unknown but may well 

be miles. The breccia is an interesting and confusing 

rock. It is polylithic, with crowded subangular to 

subrounded fragments, generally 3 to 15 inches long 

but in places several feet long, of a variety of extru- ·~ 

sive and hypabyssal rock types including biotite dio- • 

rite, andesite, latite, and quartz latite, in a holocrystalline andesitic matrix exactly like the dominant layered andesite of the formation. There are no obvious 

signs of interaction between the matrix and any of 

the clasts, and no marked change in grain size in the 

matrix near clasts. These features, combined with the 

range in texture, composition and degree of roundness 

· of the fragments suggest mudflow origin. But the 

uniformly dense holocrystalline porphyritic texture 

of the matrix, an appearance maintained in thin section, seems unequivocally to indicate a fluid-poor melt 

rather than a watery mud. Tentatively, these rocks 

are interpreted to be lava flows, and the large clasts 

to be xenoliths, largely cognate and in equilibrium 

with the andesite matrix. 

The sequence west of J e:fferson School includes one 

especially interesting and unusual layer, apparen_t~y 

a flow but conceivably a welded tuff, of porphyntic 

perlite (loc. 16, pl. 2: NE1;4NE%,NE1;4 sec. 21, T. 1 

N., R. 1 W.). It forms a layer 5-20 feet thick continuous for at least 400 feet between massive ledges 

of latite; it does not thin out but continues beneath 

cover at either extremity. The rock is very dark gray 

with a vitreous lustre and subconchoidal, almost 

hackly fracture. Scattered throughout the glassy 

groundmass are large grains of feldspar, golden biotite, and green mafic minerals exhibiting . rude flow 

banding. 

The total thickness of these layered volcanics is a 

few thousand feet. The main body of Elkhorn Mountains volcanics is probably more than 10,000 feet thick, 

according to Klepper and others ( 1957, p. 32). Near-
90 GEOLOGY OF mE TIIREE FORKS QUADRANGLE, MONTANA 

FIELD NO. 

18 

17 

16 

15 

14 

9 

8 0 

7 Fe2o3 0 

t FeO 

6 

5 

4 

3 

2 

~ 9 

z 

w I 

u 8 I a:: MgO AI w 7 Q. • CoO ------- ./' Na2o 

6 -------

/ 

Na + 2o } _ Ka2o 

5 + .-- K20 

4 

3 

2 

5 

4 

3 Na2o 

2 K20 .-- Ti02 

55 

Si02 , IN PERCENT 

FIGURE 13.-Silica-variation diagram for chemically analyzed igneous rocks, Three Forks quadrangle, Montana. 
IGNEliOUS ROCKS 91 

TABLE 9.~Chemical analyses and norms of 80me igneous rocks, Three Forks quddrangle, Montana 

[Localities and rock types shown on pl. 2] 

Laboratory Sample._ -------- No. 152 ----------------1 ____________ _ 121 095 161 096 140 I 097 1921 098 1961 099 211 I 100 2161 101 2241 102 3261 103 3341 104 339 

105 

Chemical composition 

[Rapid-method analysis by P. L. D. Elmore, S.D. Botts, and M.D. Mack] 

Si02.---------------------------- AhOa. _-------------------------- Fe20a ... --- _ --------------------- FeO. __ --------------------------

57. 9 16. 8 

5. 9 

1. 7 2. 3 5. 2 

3. 7 3. 0 

63. 6 15.2 

1.8 

1. 8 

1. 2 

3.0 

3. 9 

3.1 

58.5 

16.8 

2. 7 4.1 

2. 5 6.1 

3.3 

2. 8 

59. 4 62. 1 58. 4 59. 7 65. 1 54. 4 70. 2 71. 4 16. 7 16. 2 16. 3 16. 9 15. 8 14. 2 15. 9 15. 9 2. 0 2. 7 3. 8 3. 4 2. 9 1. 4 1. 5 1. 0 

MgO ___ ----------------------- __ 4. 5 1. 8 3. 9 3. 5 1. 3 7. 5 1. 1 1. 0 

CaO _______ --------- ____________ _ 2. 9 1. 4 2. 8 2. 7 1. 4 7. 3 . 37 . 42 

Na20. --------------------------- K20- _ ---------------------------

6. 2 4. 2 6. 2 5. 8 3. 2 6. 5 1. 7 1. 6 3. 9 3. 5 3. 2 3. 4 4. 1 3. 0 4. 2 4. 0 

Ti02. __ ---- ____ ------------------ 1.:3 3. 4 2. 8 2. 8 4. 8 2. 2 3. 2 3. 1 

P20s ____ -------------------------

. 64 

. 37 

.14 

.71 

. 10 

.10 

. 50 

.30 

. 20 

.46 .38 .66 .53 .36 1.5 .20 .17 

Mno_ --------------------------- H20-- --------------------------- C02.----------------------------

. 27 . 18 . 31 . 31 . 16 . 30 . 14 . 02 . 16 . 12 . 20 . 18 . 11 . 16 . 06 . 08 1.6 

< . 05 5. 0 < . 05 1.0 

.07 1. 9 2. 2 . 88 . 78 . 42 1. 4 . 91 1. 2 

<.W .m <.W <.M <.M <.M < . M <.M 

Sum _________ -------- ___ ________ _ 99 100 99 100--I----9-9--I----9-9--I---1~---I----1-00---I---1-00---J---~100---J----1~00~ 

Norms 

[Norms calculated by G. A. Macdonald, and Elizabeth Jaff] 

Q __ ------------------------------ or ______ -- ________ ---_-- __ --_--- __ a b. ______________________ _______ _ an ______ ___ __ ___________________ _ 

di {:~ ~========================= by {~~-- ~=~====================== mt..----------------------------- iL ____ ----- _ --- __ ---------- _____ _ 

ap_------------------------------ hm. __ -------------------------- _ 

c --------------------------------

cc.------------------------------- plagioclase _____ ---- _____________ _ 

12.0 

17. 8 31.5 

20. 3 

1.2 

1.0 

0 

4. 7 0 

4.2 

1.2 

1.0 

3.0 

0 

0 

Ab61 

20. 3 18.4 

33. 0 14. 7 

. 1 

. 1 

0 

2. 9 

1.0 

2.6 

1. 4 

0 

0 

0 

0 

Ab6~ 

11.5 

16. 7 27.8 

22. 8 

2. 3 

1.2 

1.1 

5. 0 4.1 

3. 9 

. 9 

. 7 0 

0 

0 

Abss 

13. 1 7. 8 33.0 

24. ~ 

2.0 

1.0 

. !} 

6.2 

5. 1 3.0 

. !) 

.:7 0 

0 

0 

Abss 

by to the west, Alexander (1955, p. 61-73) described 

a sequence of similar rocks, which he assigned to the 

Livingston formation, as being more than 9,600 feet 

thick. Here, near the southeastern limit of the formation, the maximum thickness is likely to be mueh 

less. No units seem to be repeated in the steeply dipping sequence west of Jefferson School; if so, the 

thickness there approaches 1,000 feet. 

Within the qua~rangle, no depositional contacts of 

the Elkhorn Mountains volcanics are exposed. In the 

Elkhorn Mountains (Klepper and others, 1957, p. 24:, 

40) the volcanics in places gradationally overlie the 

Slim Sam formation, of Late Cretaceous age, and in 

other places lie on an erosional unconformity that 

represents removal of the Slim Sam and other formations at least as far down section as the Morrison formation; a few miles west of the Three Forks quadrangle, volcanic rocks of the same sequence lie with 

angular unconformity on rocks as old as the Madison 

group (Alexander, 1955, p. 67). In the quadrangle, 

the volcanics are unconformably overlain by rocks as 

old as the Eocene part of the Climbing Arrow formation. 

19. 5 20. 0 29.9 

15.3 

0 

0 

0 

3. 5 

. 8 

3.9 

.8 

.3 

0 

1.1 

1. 7 Ab66 

12. 1 16.7 

27.3 

21.7 

3.0 

1.9 

.9 

5.0 

2.6 

5. 6 1. 2 

.7 

0 

0 

0 

Ab6«1 

12. 9 16.7 

~J 1.7 

1.1 

.5 

5.6 

2.8 

4. 9 

1.1 

. 7 0 

0 

0 

Abs6 

15.3 

28.4 

34. 6 10.6 

1.9 

1.6 

0 

1.9 

0 

3. 7 

.8 

.3 

0 

0 

0 

Ab77 

PETROGRAPHY 

2. 5 12.8 

25.2 

18.9 

4.8 

2.8 

1.7 

15. 4 

8.8 

2.1 

2. 9 

. 7 

0 

0 

0 

Abs7 

By H. FRANK BARNETT 

29.4 

18.9 

35.7 

7. 5 

0 

0 

0 

.9 

.7 

2.1 

. 5 

.3 

0 

2.8 

0 

Abs3 

31.5 

18.4 

34.1 

8. 1 0 

0 

0 

1. 0 

1.1 

1.4 

.3 

.0 

0 

3. 0 

0 

Ab11 

The Elkhorn ] ~ountains volcanics are represented 

by 13 thin section::>, located on plate 2; 2 typical andesites (192, 196) a 

1

e also represented by chemical analyses on table 9. The matrix of the breccias seems 

identical with the andesite flows, and the descriptions _ 

apply to both. T e rarelatite in the formation (represented by speci en 160, pl. 2) is similar to that of 

large masses map ed separately and described later; 

it will not be des 1 ribed here. 

ANDESITE 

All the 11 ander· tes represented by thin sections are 

porphyritic, with phenocrysts, commonly 0.5 to 1.5 

mm long, averagi g 30 percent. Of the phenocrysts, 

two-thirds, or 2 percent of the rocks, are zoned 

plagioclase, rangi g from oligoclase-andesine to sodic 

labradorite. Local y, feldspar phenocrysts are clumped 

in crude "sunbur t" patterns. 

The remaining henocrysts, 10 percent of the rock, 

are mafic minera s. Half are colorless to very pale 

green clinopyroxe e, probably diopsidic augite. The 
92 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

clinopyroxene is generally in long prisms, randomly 

scattered, but in places it oc.curs as spheroidal clumps 

of stubby crystals accompanied by much magnetite. 

The other half is· pale-green orthopyroxene (probably 

hypersthene), greenish-brown hornblende and brown 

biotite, in varying proportions. 

Plagioclase microlites with weak fluidal texture 

comprise about half of th8 groundmass. The microlites appear to be more sodic than the phenocrysts but 

are too altered for sure determination. Alkali feldspar, about a quarter of the groundmass, is mostly 

anhedral, filling many interstices; less commonly, it 

assimilates plagioclase and encloses other minerals. 

Rarely, alkali feldspar appears in well-formed crystals showing Carlsbad twins. Mafic. minerals, a quarter of the groundmass, are anhedral and indeterminate. 

Accessory minerals are apatite, magnetite, and, rarely, 

sphene. 

All the rocks are somewhat altered, with sericite, 

clay, calcite, epidote, chlorite, and secondary magnet-

. ite present in amounts approaching 5 percent. Calcite 

commonly replaces pyroxene, amphibole, and the cores 

of plagiocl~se phenocrysts; rarely, it fills c.racks. Clay, 

sericite, and epidote are secondary aft8r both phenocrystic and groundmass feldspar. Chlorite and magnetite marginally replace the mafic minerals. Small 

clear spherical masses of low birefringent material 

are probably chalredonic silica but possibly a zeolite. 

PORPHYRITIC PERLITE 

In thin section (no. 16, pl. 2), the porphyritic perlite is vividly perlitic with contorted fluidal structure. 

It is in part porphyritic and in part fragmental. 

Phenocrysts, generally 0.5 to 1.5 mm long, comprise 

15 percent of the rock. Half are fresh plagioclase, 

zoned froni andesine-labradorite to mid-andesine; 

locally, they are em bayed by glass. Other phenocrysts 

are biotite and clinopyroxene, probably augite, each 

forming about 2 percent. Magnetite, in large euhedra, 

is nearly as abundant as the mafic minerals; large 

crystals of apatite are sporadically present. 

The bulk of the rock, 70 percent, is light-brown 

completely isotropic glass with many perlitic cracks. 

The refractive index is near 1.512. 

Fragments floating in the glass amount to 15 percent of the rock. Many are broken bits of plagioclase 

of the same composition as the phenocrysts. Others 

are chunks of glass that differ somewhat from the 

groundmass: some are . darker brown with contorted 

banding and fillings of a zeolite or silica mineral in 

perlitic cracks and small vesicles; others are light 

brown, with crowded alined plagioclase microlites. 

A chemical analysis (no. 16, table 9) is not as revealing as it might be, for no attempt was made to 

separate the fragments or phenocrysts from the glass. 

Nevertheless, the rock, with due allowance for its 

water content, is clearly of a composition close to 

Nockolds' (1954, p. 1014) average rhyodacite. 

CORRELATION AND AGE 

The volcanic works in the northwest corner of the 

quadrangle are continuous with the very large body 

of similar rocks mapped and described by Klepper 

and his associates (1957, p. 31-41) on the east flank 

of the Elkhorn Mountains and named by them Elkhorn Mountains volc.anics. Klepper (oral communication, August 1957) believes that the volcanic rocks 

in the Three Forks quadrangle are in the basal part 

of the formation. The volcanic rocks west of J efferson School are correlated with them, despite the considerable distance separating the two, not only because of their· ·general petrographic and chemical 

similarity but also because they are similarly deformed . 

On Peale's geologic map ( 1896) neither of the two 

areas involved is shown as c.ontaining mappable igneous rocks. The mass in the northwest corner is shown 

as sedimentary rocks of the Colorado and Montana 

formations. That west of Jefferson School is shown 

as Bozeman lake beds, but similar rocks on strike to 

the west are shown as part of the Livingston formation, described as conglomerate, sandstone, and andesitic tuff. 

Berry ( 1943, p. 22-23, pl. 1) assigned both bodies 

to the Livingston formation but, anticipating Klepper's work, noted that they are mainly andesitic lava 

flows and "coarse conglomerates derived from them" 

and probably "should be correlated with similar rocks 

south of Helena," rocks that are part of the Elkhorn 

Mountains volcanic field of Klepper. 

Klepper (1957, p. 38-40) decided against using the 

term "Livingston formation" because the Livingston 

is by definition a water-laid sedimentary unit whereas 

the volcanics, more than 10,000 feet thick, in the Elkhorn Mountains field contain little sedimentary material and therefore need a separate name, even though 

the two sequences are at least partial age equivalents. 

A distinction between them is especially desirable because much of the Livingston formation is derived 

from erosion of the Elkhorn Mountains volcanics. 

The textures, structures, and mineralogy of the Elkhorn Mountains rocks are much like those of the very 

Late Cretaceous trachybasalts from the Adel Mountains about 80 miles north of Three Forks and midway 

between Helena and Great Falls (Lyons, 1944, especially p. 460-461). Especially striking is the asso-
IGNrEOUS ROCKS 93 

ciation in both sequenoos of alkali feldspar (which 

Lyons designates simply as orthoclase) and calcic 

plagioclase. The Adel Mountain rocks differ, however, 

in having much more calcic plagioclase (calcic. labradorite) and a little modal olivine. 

The Elkhorn Mountains volcanics are older than 

the Climbing Arrow formation and are presumably 

younger than the Kootenai formation, which is the 

youngest exposed pre-Tertiary formation that lacks 

volcanic or igneous components. In the neighboring 

Elkhorn Mountains, Klepper and others ( 1957, p. 37-

38) have been able to fix the age more closely. They 

cite varied, if scanty, faunal and floral evidence that 

the volcanics are wholly of Late Cretaceous age; their 

-range cannot be delimited more closely than very late 

Niobrara or Telegraph Creek to Judith River or 

younger. 

The porphyritic perlite, if a flow, welded tuff, or 

penecontemporaneous sill and thus also of Late Cretaceous age, is among the oldest glassy rocks on record. 

Obsidians and related siliceous rocks older than Mio- ,, cene are rarely reported. There is only one other 

occurrence of glassy rocks as old as Late Cretaceous 

known to me: it happens to be in this general region, 

near Wolf Creek, 35 miles north of Helena (Barksdale, 1951, called to my attention by Prof. W. H. 

Mathews). 

INTRUSIVE ROCKS 

Intrusive igneous rocks that are contemporaneous 

with or younger than the Elkhorn Mountains volcanics and the regional folding, but older than the 

Bozeman group, occupy 8 square miles. They are 

divided into four compositional types: quartz monzonite and related, rather coarse-grained, rocks; latite; dacite; and andesite. The monzonitic and dacitic rocks tend to form large and thick masses: plutons 

or laccoliths. The latite and andesite appear both 

in extensive thin sills and in large thick bodies. 

QUARTZ MONZONITE AND RELATED ROCKS 

Monzonitic rocks are the mo~t abundant intrusive 

rocks in the quadrangle. Most of them are _ lightcolored equigranular medium- to coarse-grained quartz 

monzonite, but they grade into and include substantial volumes of monzonite, quartz diorite, and diorite. 

Mapped with them is a rich assortment, but small 

volume, of spatially associated textural and compositional variants. These rocks are largely confined to a 

single mass-the 10N pluton-at the north edge of the 

quadrangle, but include a large sill in the Willow 

Creek sector, and a few unmappably small sills east 

of Milligan Creek. 

654729 0--63----7 

ION PLUTON 

Much the largest intrusive mass in the quadrangle 

is a quartz monzonitic pluton at the north edge 

of the quaqrangle. Within the quadrangle it extends for mbre than 3 miles east-west, between Mud 

Spring Gulch and Dunbar Creek, and crops out for 

more than 2[ miles north-south. It is exposed for at 

least 2 more Files into the Radersburg quadrangle and 

may extend ~everal miles farther north and south beneath younger deposits. It is here named 10N pluton, 

for Highw~1 10N which cuts through its center. 

Merrill (18 5, p. 663) described this body as diorite 

porphyrite. Berry (1943, p. 23) described it as "a 

small stock ith average composition of a quartz monzonite" and noted that it _ includes monzonite, quartz 

monzonite pprphyry, and, in the western part, quartz 

latite porphyry. 

The plutof, is made up ?f a wide va:iet! of rock 

ty. pes but is minated .by 1. Ight-colored pinkish quartz monzonite. eathering usually darkens the rock only 

slightly, to 1 ale grayish brown or rarely to moderate 

·brown and produces rolling terrain with scattered 

low r~unde, outcrops protruding from light-colored 

sandy soil. Exposures are too poor to reveal the 

shapes of a~y but the grossest petrographic units or 

to show w ether contacts between rock types are 

sharp or gr dational and th~s whether it is a compound or cobposite body. 

Within t~F pluton three subunits, based primarily 

on texture, ~ave been ro~ghly delineated: t~o easte:n 

masses of ~oughly equigranular quartz monzonite 

rocks, one edium grained and one coarse graine~; 

and a west rn mass of mixed rocks, slightly more 

mafic than he quartz monzonite and characterized 

by porphyrific texture. . . 

Considering the size a.nd siliceous composition of 

the pluton, t contains amazingly few xenoliths even 

near intrusi e contacts, and has had little effect on 

the intrude rocks beyond bleaching and ·local recrystallizati n of some of the carbonates, effects discussed earlitr in connection with the sedimentary 

rocks. One !large xenolith or pendant of green thinbeaded medium-grained sandstone, most resembling _ 

Kootenai or !Morrison rocks but conceivably from one 

of several her older units, is exposed in a small 

~~~ey in th, southeast corner of sec. 28, T. 3 N., R. 

The plutop is largely intrusive into the Madison 

group, wher1 its original contacts are not concealed by 

Cenozoic covrer; at its eastern edge, however, the pluton or its a~ophyses intrude the Jefferson and Three 

Forks form;Jtions. 
94 GEOLOGY OF 'THE mREE FORKS QUADRANGLE, MONTANA 

To· judge by the attitude of the bordering limestone, 

the pluton is a steep-sided but rather flat'-topped 

structure that is roughly concordant on its upper 

surface but discordant on its sitles. With little information in the vertical plane, it can only be guessed 

whether the pluton is stocklike, as it casually appears, 

or sheetlike, roughly parallel to bedding. On the 

weak ground that other intrusive rocks of the suite 

are sills. sheetlike structure seems slightly favored; 

the stru~ture is so drawn in section D-D' (pl. 4) .

11 

It might be ~rgued that an· intrusive structure of 

this form, intruded into a sedimentary column evidently no thicker than a few thousand feet (even if 

duplicated by deformation), is not well termed a pluton. In practice, however, pluton is often used for 

coarse-grained masses of large area, the third dimension being largely conjectural except for the upper 

few tens or hundreds of feet known due to erosion or 

subsurface exploration. Less committal than the 

terms stock or sill, it seems useful in just such equivocal situations as the present one. As so little is known 

about the depths at which large bodies of coarsegrained igneous rocks crystallize, the use of the name 

pluton surely does not necessarily imply much in the 

way of depth. 

THICK SILL WEST OF SPRING CREEK 

At .the opposite corner of the quadrangle from the 

10N pluton is another large monzonitic mass. There, 

a large sill, 2 miles long and · as much as 200 feet thick, 

. intrudes the basal Wolsey shale west of Spring Creek. 

The sill is mostly of monzonite, but locally is quartz 

monzonite. It resembles the main medium-grained 

rocks of the 10N pluton in color, texture, and visibl~ 

mineralogy. The western part of the sill is intruded 

by several thin dikes of latite. 

For much of its length the sill displaces essentially 

all the shale strata in the lower part of the Wolsey, 

between the Flathead sandstone and the sandstone 

member of the Wolsey. The sill steadily thickens 

from west to east, until it abruptly ends, but it is uncertain whether this reflects forcible spreading in the 

eastern part or permissive displacement of a s~ale 

interval that steadily thickens eastward. Permissive 

emplacement is favored by the fact that the siU continues without marked change in thickness eastward 

from the pinchout of the sandstone member, so the 

body is not regarded as a laccolith; that is, a concordant injected mass that lifted its roof by arching. 

17 Preliminary study of gravimetric and magnetic surveys of the 

pluton made in 1959 (Isidore Zietz, written communication, January 

1960) in,dicat~ that1 at least the upper surface of the body plunges 

to the southe!af,•.·and .. 1mggest that the pluton may well be bottomed 

at a depth of several thousand feet. · 

Berry (1943, p. 23 and 'pl. 1) mapped this sill as 

syenite. PETROGRAPHY 

By H. FRANK BARNETT 

Though separated on the geologic map, the mediumand coarse-grained nonporphyritic quartz monzonite 

(represented by six thin sections in plate 2 and by 

chemical analyses 216, 224 on table 9) are described 

together because they differ notably only in grain 

size. Grains in the medium variety are mostly in 

the range 0.5 to 1.5 mm. In the coarse variety grains 

are nearly twice as large. . 

The typical quartz monzonite is hypidiomorphic 

granular with faint but definite parallelism of plagioclase and mafic laths. Some is vaguely porphyritic. 

The plagioclase is zoned from sodic labradorite in the 

cores to mid -andesine, and makes up 30 to 50 percent 

of the rock. Quartz, 5 to 10 percent, is interstitial 

to the plagioclase, is anhedral, and is usually slightly 

strained. Fine-mosaic aggregates of quartz are developed along many grain boundaries and fractures. 

Twinned potash-rich alkali feldspar, comprising 30 

to 50 percent of the rock, replaces plagioclase and is 

poikilitic in mafic minerals. The fine-mosaic quartz 

seems to have overlapping relations with alkali feldspar, here veining it, there apparently assimilated 

by it. 

Green hornblende is the most abundant mafic mineral, averaging 5 percent. It commonly is fractured, 

and healed with mosaic quartz. Brown biotite and 

pale-green augite are about half as abundant. Biotite 

appears as plates and shreds generally within hornblende laths. Augite is widely replaced by hornblende. Accessory minerals include abundant apatite, 

le~ser amounts of magnetite and sphene, and, rarely, 

zircon. Chlorite locally replaces biotite along cleavages. · A little clay, sericite, and iron oxides appear 

in weathered zones. 

Other rock types in the nonporphyritic part of th~ 

pluton have similar textures and minerals bu~ the 

proportions of minerals differ. Two rocks (speCimens 

272 and 222, pl. 2) in the northern part of the mass, 

classed as granodiorites, have 20 percent subhedral 

alkali feldspar and 60 percent plagioclase. The outlier 

at the Copper City workings (specimen 269) has little 

quartz and is called monzonite. A thin apophysis 

(specimen 230) in limestone a quarter of a mile north 

of the workings contains strongly zoned. augite as its 

only mafic mineral, lacks quartz and has less than 

15 •percent alkali feldspar; it is called diorite. 

. In the monzonitic sill west of Spring Creek ( represented by specimens 298, 304, and 293, pl. 2) the 

dominant mafic mineral is biotite, deeply chloritized, 
IGNEOUS R'OC.KS 95 

rather than hornblende; the plagioclase grains commonly show little zoning; the alkali feldspar is in 

distinct anhedral grains rather than irregular ramifications; and quartz is rare. 

The rocks in the western subunit of the 10N pluton, 

represented by six thin sections on plate 2 and by one 

chemical analysis (211) in table 9, range even more 

widely than indicated by the thin sections, no two of 

which are assigned the same name. In addition to 

the coarse-grained diorite of 211, monzonite, quartz 

diorite, and granodiorite, the group of thin sections 

includes fine-grained quartz latite and andesite. Present, too, are outcrops, not represented by thin sections, 

of granite aplite or quartz monzonite aplite, of alaskite, and of hornblende diorite porphyry. 

In this perplexing mixture by far the most common 

types seem to be diorite and quartz diorite that are 

allied in containing persistent but subordinate arnounts 

(3 to 10 percent) of large hornblende phenocrysts 

and very small amounts of alkali feldspar, in contrast 

to the plentiful alkali feldspar of the nonporphyritic 

phases. Their general mineralogy is similar to that 

of the nonporphyritic phases. 

AGE 

The youngest rocks intruded by the main mass of the 

10N pluton are Mission Canyon limestone of early 

Carboniferous age. The oldest rocks in sedimentary 

contact with the pluton are those of the Dunbar Creek 

formation of Oligocene age. The range can be narrowed greatly, however, if the small mass of porphyritic diorite in SE14SW14 sec. 20, T. 3 N., R. 1 E., 

is accepted as part of the pluton, for it intrudes and 

alters rocks as young as the Lower Cretaceous Kootenai 

formation. The age of the · pluton, and presumably 

of the monzonitic sill west of Spring Creek also, thus 

is post-Kootenai and pre-Dunbar Creek. 

The range can be narrowed still further if the 

monzonitic roc.ks can be dated with respect to regional folding, which began slightly before but ended 

after the Elkhorn Mountains volcanics were erupted 

in late Late Cretaceous time. Dating the pluton in 

this way is difficult. On the one hand, it may be 

viewed as of prefolding age, with its steep western 

side and faulted eastern side giving evidence of crumpling and failure of the less competent Paleozoic 

strata against the monolithic pluton during compression. On the other hand, the main body of the pluton 

shows no signs of response to major compression, such 

as shearing or the development of zones of retrograde 

metamorphism, and this suggests postorogenic age. 

The field relations of the monzonitic sill west of 

Spring Creek, however, strongly favor postfolding 

or intrafolding age. The sill persists for 2 miles 

654729 ~63----8 

across a gentle syncline and along the low-dipping 

west limb of an asymmetric anticline; in fact, it 

thickens toward the anticlinal axis. But it ends 

abruptly where the steeply dipping east limb begins. 

Not only does the monzonite fail to continue on the 

east limb, but that limb holds no sills of any kind, 

though latite sills are abundant in the low-dipping 

strata to east and west. The monzonite and the other 

sills as well seem to have been emplaced after asymmetric folding had progressed far. The low-dipping 

segments were intruded because these beds were easy 

to spread apart, while the steep-dipping segments 

acted as shoulders and remained free of sills. Intrusion might have come long after folding ceased, but 

it seems more likely that folding, differential separation of beds, and intrusion formed a continuous process. By this reasoning, the monzonitic intrusive 

rocks, assuming they are of a single age, are most 

likely post-Elkhorn Mountains and pre-Bozeman 

group ; that is, latest Cretaceous to middle Eocene. 

The rocks of the pluton are much like those of the 

Boulder batholith, 25 1niles to the west; similar rocks, 

in similar settings, are widespread in the intervening 

area (Klepper and others, 1957, p. 48-52). Probably 

all these monzoni tic and related calcalkaline intrusive rocks are of about the same age. Most modern 

workers (Klepper and others, 1957, p. 44, 60; Chapman and others, 1955, p. 607) have recognized that 

the possible age range of these rocks, based on stratigraphic evidence, is post-Niobrara (middle Late Cretaceous) to pre-early Oligocene. Evidence from this 

study suggests a somewhat earlier upper limit, for 

detritus from the batholith is a major component of 

the Eocene and Oligocene Climbing Arrow formation, 

and supports the conclusion of J(lepper and others 

(p. 60) that "a very late Cretaceous or Paleocene date 

is most probable." 

A still narrower range is suggested by recent age 

determinations of Boulder batholith rocks by the 

Larsen method (Chapman and others, 1955). Five 

determinations ranging between 61 and 72 million 

years, and averaging 68 million years, suggest emplacement "at or near the close of the Cretaceous" 

(Chapman and others, 1955, p. 609). The utility of 

the Larsen method is still, however, in debate. 

LATITE 

Latite, though of small volume, is the most widespread intrusive rock type. Thin sills of it occur in 

each of the areas dominated by other intrusive rocks, 

as well as in the Elkhorn Mountains volcanics, but 

only five bodies of intrusive latite are large enough 

to map. All the latite is medium gray to medium 

dark gray, weathering to steely-purplish tones like 
96 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

the Elkhorn Mountains volcanics, with sparse and 

small phenocrysts of feldspar and pyroxene in a dense, 

nonvesicular holocrystalline matrix. The forms and 

settings of the bodies, however, range widely . . Two 

are pluglike masses, surrounded by Elkhorn Mountains volcanics ; two are thick sills in the Wolsey and 

basal Meagher; and one consists of many closely 

spaced thin sills interleaved with basal str~ta of the 

Meagher. PLUGLIKE BODIES 

The largest body of latite is at the west edge, north 

of Highway lOS; it centers in NW!4 sec. 21, T. 1 N., 

R.'· l W. Exposed for a length of a mile, it is no 

doubt much larger beneath gravel cover to the south 

and the Jefferson Canyon thrust to the north. It is 

represented by specimen 12 on plate 2, and chemical 

analysis 12 in table 9. In places, the rock is coarse 

enough to be called monzonite, but most of it is not 

greatly coarser than the andesite and latite volcanic 

rocks to east and west. Its general field relations 

suggest that it is intrusive into the Elkhorn Mountains 

volcanics, but there is no positive evidence of intrusion. 

The other pluglike mass, much smaller, is north of 

Cowan Spring in SW1,4, sec. 33, T. 3 N., R. 1 W.; it 

is represented by specimen 168 on plate 2. Finegrained throughout, it is separated with difficu!ty 

from the surrounding andesitic volcanic :r:ocks, from 

which it visibly differs only in lacking signs of stratification and in being faintly lighter in color. Its 

intrusive origin, too, is not demonstrated but must be 

inferred from its rudely circula~ outline and seeming 

transgressive relations to the volcanic rocks. 

THICK SILLS 

Two thick sills of latite appear between Willow 

Creek and Spring Creek, in sec. 11 and 12, T. 1 S., 

R. 1 W. The larger, traced for nearly a· mile in 

SEI4 sec. 11 and SW% sec. 12, is more than 300 feet 

thick; it is represented by specimen 300 on plate 2. 

The smaller, represent~d . by specimen 301, is more 

than 50 feet thick and ·1,700 feet long. Neither is 

well exposed. . 

The larger mass .may be a laccolith. It not only 

displaces virtually all the shale between the top of 

the Flathead sandstone and . the base of the quartzitic 

sandstone member of the Wolsey, but it seems to have 

increased greatly the apparent stratigraphic separation between the two sandstones. Scattered outcrops 

suggest tnat the intrusive is fairly homogeneous. It 

differs from the other latites in having many miarolitic 

cavities. These are lined with small quartz crystals 

that in turn are thinly coated . with sooty black 

hematite (?). 

The smaller sill seems to have wedged its way into 

the eon tact between ·wolsey shale and Meagher limestone. The rock has a much higher proportion of 

feldspar phenocrysts than the other latites; in places 

it approaches latite porphyry. 

THIN SILLS 

Layers of latite, generally 1 to 4 feet thick, are 

abundant in the southwest eorner of the quadrangle; 

they are represented by specimens 296, 306, and 289 

on plate 2. Several appear in the western part of the 

thick monzonite sill that displaces the basal Wolsey; 

others are scattered in the adjacent remaining shale 

strata, and one i~ in gneiss (west bank of most southeasterly gulch in sec. 15, T. 1 S., R. 1 W.). In the 

monzonite, the latite bodies must be called dikes 

though they are rudely parallel to the outer walls. 

It was apparently one of these which Merrill (1895, 

p. 664) described as "brownish coarsely porphyritic 

andesi~ * * * extruded through the porphyrite sill" 

(here, monzonite). In the shale, gneiss and limestone, 

the latite bodies are sills. 

The only latite in this area that is mappable, however, is a swarm of at least 15 latite layers more than 

a mile long in the basal 100 feet of the Meagher 

limestone. The silled zone is well exposed along the 

creek that flows west across the N¥2 sec. 9, T. 1 S., 

R. 1 W. The sills have thin glassy selvages and are 

separated by a few inches to several feet of slaty 

black limestone. Surprisingly, these blackened slaty 

beds can be traced along the strike into typical banded 

blue-and-gold beds. 

PETROGRAPHY 

By H. FRANK BARNETT 

The latite varies considerably in texture, from 

rather coarsely fluidal to vitrophyric, and from poorly 

to richly porphyritic. The bulk mineral compQsition 

is !airly con~tant, however, and most of the variant 

textures seem to represent quantitatively insignificant 

phases. The main rock type has a fluidal texture · with 

sparse phenocrysts, less than 5 percent, of mafic 

minerals. Most are stubby prisms of clinopyroxene, 

probably augite but too extensively altered ·for clear 

determination; the re~t are green hornblende, in part 

as kelyphitic rims on pyroxene; 'bro'\'Vn biotite; and, 

r~rely~ phlogopite'. · · · 

¥ic~olites 'of p_lagioclase, ranging from' calcic oligoclase to andesine, make up as much' as 80 percent of 

the rock. , Many ·grains ·are thinly iin;mied by alkali 

feldspar, probably albite; late-formed alkali 'feldspar 

also occupies many interstices, but rarely totals as 
IGNEIOUS ROCKS 97 

much as 5 percent. Quartz is rare, occurring only 

as rounded xenocrysts and in the lining of a few 

cavities. Subhedral clinopyroxene, hornblende and 

biotite appear in the groundmass to the extent of 

perhaps 10 percent. Apatite and fine-grained magnetite are persistent accessories; sphene is rare. The 

mafic minerals are extensively altered to chlorite and 

coarse-grained magnetite. Other common alteration 

products are calcite,-sericite, limonite, and clay. 

In a rather common variant, ·. the rock has similar 

texture and groundmass but the phenocrysts are 

largely feldspar in the andesine range, and many have 

thin rims of clear albite. 

AGE 

Assuming that all the latite is of a single age, the 

unit is no older than very late Cretaceous, for it apparently cuts Elkhorn Mountains volcanics. It may, 

of course have been intruded while the Elkhorn 

Mountains volcanics were being erupted. In the 

Elkhorn Mountains there are many sills of quartzpoor intermediate rocks that were emplaced about at 

the same time as the volcanic rocks (Klepper and 

others, 1957, p. 44-45); such rocks, however, are 

largely diorite porphyry and andesite porphyry, little 

like the slightly porphyritic latite of the Three Forks 

quadrangle. Its upper age limit can with certainty 

be placed no lower than Quaternary, as latite is 

nowhere demonstrably overlain by Tertiary rocks. 

Latite, however, intrudes and is younger than the 

monzonitic sill \vest of Spring Creek. Like the monzonitic sill, the latite is probably younger than the 

folding but not greatly so. 

DACITE 

Dacite is mostly confined to a single large mass, the 

Buttleman laccolith, cut by Willow Creek. Unmappably small sills of dacite also appear in the large . 

andesite mass above Milligan Canyon. 

BUTTLEMAN LACCOLITH 

The Buttleman laccolith, flanking Willow Creek 

near the south edge of the quadrangle, is more than 

4,000 feet thick and 3 miles long. It is named for the 

Buttleman Ranch on which it crops out. Peale ( 1896, 

p. 4) and Merrill (1895, p. 664) classed the rock as 

hornblende porphyrite; Berry ( 1943, p. 23 and pl. 1) 

described the mass as "an intrusion of quartz latite 

and quartz latite porphyry." 

The great bulk of the laccolith is dacite, represented 

by six thin sections on plate 2 and by chemical 

analyses 334 and 339 in table 9, but at its east edge 

it is somewhat more siliceous and alkalic, grading into 

quartz latite (represented by specimens 340, 341, 

pl. 2). 

The laccolithic rocks are light colored, mostly 

medium light gray. Weathering generally bleaches 

them even lighter but locally they have yellowishbrown or yellowish-green splotches due to oxidation 

of iron-bearing minerals. They are uniformly fine 

grained and porphyritic, with many stubby prisms 

of feldspar, scattered needles of hornblende, and flakes 

of biotite. 

The base of the laccolith is the Flathead sandstone_ 

throughout; its top has varying stratigraphic position, 

but rises no higher than the base of the Meagher limestone. At its western limit the laccolith has simply 

displaced the lower shales of the Wolsey · between the 

Flathead and the quartzitic sandstone member of the 

lower part of the Wolsey, in the fashion of the sills 

farther west. The apparent stratigraphic interval 

between the two sandstones increases rapidly eastward, 

presumably due to forceful widening by the intrusion. 

Near the west wall of Willow Creek, the sill thicke_ns 

abruptly where the sandstone member of the Wolsey 

seems to thin out, cutting across a thick interval of 

shale to bottom against another thick sandstone (or 

possibly the same one offset by an unrecognized preintrusion fault). East of the creek the outcrop of the 

laccolith widens tremendously to 1% miles, but apparently the only rocks displaced are those of the 

Wolsey, for the northeast edge of the laccolith is near 

the base of the Meagher limestone. A thick mass of 

Wolsey rem~ins as a giant xenolith or pendant within 

the intrusive east of Willow Creek (mostly in NW 

cor. sec. 8, T. 1 S., R. 1 E.). 

Despite its size and siliceous composition the laccolith has had essentially no effect on its wall rocks. 

Where shales are preserved near contacts they are 

slightly hardened and reddened, but the bordering 

sandstones and limestones are not visibly changed. 

Xenoliths are few. The missing shale seems to have 

been bodily pushed aside by the intruding dacite, but 

not to have interacted with it. 

PETROGRAPHY 

By H. FRANK BARNETT 

In thin section, the typical dacite is holocrystalline 

porphyritic, with 10 to 30 percent of small phenocrysts 

in a microcrystalline, distinctly fluidal groundmass. 

Of the phenocrysts, 95 percent are zoned plagioclase 

in the andesine range. Remaining phenocrysts are 

mafic minerals, largely needlelike prisms of green 

hornblende but including small amounts of biotite 

and traces of clinopyroxene. The mafic minerals are 

much altered, the hornblende to magnetite, sphene, 
98 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

and calcite; the biotite to chlorite. Some feldspar 

phenocrysts also are replaced by calcite. 

The groundmass consists mostly-as much as 80 

percent-of feldspar. In some places, the feldspar is 

wholly plagioclase, in the andesine range, in dense 

felted aggregates of microlites; in a few places it is 

late-formed alkali feldspar (no more calcic than 

albite-oligoclase) in cloudy anhedral masses; most 

commonly, both types appear, in varying proportions. 

The only other prominent mineral in the groundmass 

is quartz, in both anhedral interstitial grains and 

tiny well-formed crystals, averaging 10 percent. 

Apatite, magnetite, . and sphene are the common 

accessories. Calcite, in addition to replacing many 

phenocrysts, also replaces groundmass feldspar and 

quartz. Sericite, clay, and limonite are sparingly 

developed along fractures. 

AGE 

On direct stratigraphic evidence, the age limits of 

the B:uttleman dacite are broad indeed. It is younger 

than the Wolsey shale but older than Quaternary 

gravel. Yellow sands assigned to the Climbing Arrow 

formation contain dacite pebbles but these strata are 

not firmly established as early Tertiary; they may be 

Quaternary . . 

A reasonable assumption is that the Buttleman 

dacite is temporally, if not genetically, related to the 

latite sills on strike with it, and shares their intraor post-folding age. If so, the dacite is of late Late 

Cretaceous to pre-early or middle Eocene age. 

ANDESITE 

Wit_Qin a radius of 2 miles of Milligan Canyon are 

a nu~er of intrusive bodies mapped together in view 

of their generally andesitic composition and geo.., 

graphic po~ition, though they are by no means similar 

in all other respects. They .fall naturally into two subgroups : one; south of the canyon, made up mostly 

of thin andesitic sills in the Three Forks shale; the 

other, north of the canyon, consisting of large masses 

of andesite intruded into late Mesozoic rocks. 

Berry ( 1943, p. 23 and pl. 1) designated the sills 

south of the canyon as syenite. Those north of the 

canyon were mapped .as porphyrite by Peale (1896) 

and as andesite by Berry ( 1943) . 

SILLS SOUTH OF MILLIGAN CANYON 

Many thin sills of andesite appear · in a narrow 

stratigraphic interval east of Milligan Creek below 

Milligan Canyon. Where· unaltered, the andesite is 

niedium grayi~h green, fine grained, and very dense, 

with scattered needles and flakes of mafic minerals 

and many small feldspar phenocrysts. In most places, 

however, particularly near the creek, the andesite is 

deeply altered and has a thick porous dusky-yellow 

to olive crust of secondary minerals. 

The andesite sills, represented by five specimens on 

plate 2, are nearly all intruded into the Three Forks 

shale; one is in the overlying Lodgepole limestone. 

They have an interesting relation to the host rocks 

and the pair of northward-trending faults that offset 

them. West of the faults, only a single, seemingly 

discontinuous sill, rarely as much as 30 feet thick, 

appears in the shale, just below the orange siltstone 

unit. Between the faults there is also only one sill; 

it, however, is continuous and thicker, averaging perhaps 40 feet. Just east of the faults, the upper half 

of the Three Forks contains at least three thick sills, 

and sills are progressively more numerous eastward, 

so that where the Three Forks passes under Quaternary gravel in center sec. 31, T. 2 N., R. 1 E., the 

shaly parts of the formation are ribbed with sills, 

totaling at least 31 in number and averaging 3 feet 

thick, with somewhat thicker septa of baked,. reddened, 

hornfe1sed, and locally slaty shale between. None of 

the Three Forks is missing; rather, the interval between the Jefferson and the Lodgepole formations has 

spread to accommodate the intrusive rocks. In this 

segment, too, there is a sill as much as 8 feet thick 

and at least 200 feet long in the Lodgepole limestone, 

25 feet above the base. 

Where the Three Forks interval emerges north of 

the gravel patch its character is much different: the 

Three Forks, all present, has been nea.tly split by a 

single concordant intrusive 300 feet thick and at least 

3,500 feet long. This intrusive seems properly called 

a laccolith. It is recognizable for miles from the 

north and northeast because it forms a distinctive 

rounded ridge just below a huge letter T (for Three 

Forks High School) on the limestone mountain above. 

The main mass of the laccolith (represented by specimen 77 on pl. 2) is fine-grained diorite that differs 

from the andesite sills farther west only in texture; 

the borders are andesite (represented by specimen 69 

onpl.2). 

Plainly, the sills are not offset by the faults, as 

they casually seem · to be, but are controlled by them 

and therefore are younger than the faults. 

INTRUSIVE ROCKS NORTH OF MILLIGAN CANYON 

Above Milligan Canyon are two roughly elliptical 

masses of fine-grained medium-gray porphyritic andesite : a large one east of the creek, centering in 

Nlj2 sec. 24, T. 2 N., R. 1 W., and a much smaller one 

west of the creek, straddling the line between sees. 22 

and 27. · The typical andesite contains many small 
IGNEOUS ROCKS 99 

phenocrysts of both feldspar and mafic minerals, only 

slightly larger than the groundmass, that give the 

rock a salt-and-pepper granularity. Weathering produces pitted dusky-yellow to yellowish-brown surfaces. 

Locally, the rock is more coarsely porphyritic latite, 

and in a few places it is so rich in mafic phenocrysts 

as to be classed as latite porphyry (represented by 

specimens 126, 134-, 14 7 on pl. 2). A little dacite 

(represented by specimen 281 on pl. 2) occurs in the 

western mass. Exposures are too poor to tell whether 

the latite and dacite are gradational variants or dikes. 

Lacking any signs of extrusive origin such as layering, oxidized surfaces or vesicularity, these masses are 

hesitantly interpreted to be shallow-seated intrusive 

rocks. Their only exposed contact with older rocks 

is transgressive, the southern border of the large 

mass east of Milligan Creek cutting at a small angle 

across bedding so that it is in contact with varying 

levels in the Kootenai formation in most places and 

for a short distance cuts across the Morrison formation. The Mesozoic strata are hardened and reddened 

for a few tens of feet from the contact. No apophyses 

crop out in the underlying strata and there are few 

sedimentary xenoliths in the andesite. All other 

margins of the large intrusive and the entire perimeter 

of the small one west of the creek are blanketed by 

Cenozoic sediments, the oldest of which are low in 

the Climbing Arrow formation. The two masses are 

probably plugs that remained as hills in the Tertiary 

basin after the less resistant sedimentary deposits, 

which they intruded, had been stripped. 

PETROGRAPHY 

By H. FRANK BARNETT 

The andesite south of the canyon is described separately from that to the north, to bring out some 

differences between them. 

The andesite south of the canyon (represented by 

five specimens on pl. 2) has a porphyritic faintly 

fluidal texture, generally veiled by alteration. The 

phenocryst content ranges widely but averages 30 

percent. At least 80 percent of the phenocrysts are 

small laths, no longer than 1.5 millimeters, of plagioclase ranging from calcic oligoclase to calcic andesine. 

Some display oscillatory zoning and most have distinct rims of alkalic feldspar, probably albite. The 

remaining phenocrysts, aggregating less than 10 percent of the total rock, are yellow-brown biotite, palegreen hornblende, and colorless augitic clinopyroxene, 

in order of decreasing abundance. Present throughout are fresh euhedra of magnetite and apatite, mostly 

in association with mafic minerals or their altered 

remains. 

The original groundmass was presumably largely 

feldspar; now it is mostly saussuritic material, of 

which some is sufficiently crystallized to yield determinable grains of clinozoisite and calcite. Reeognizable in the groundmass are minute euhedra qf 

untwinned alkali feldspar, probably albitic, that locally total as much as 15 percent of the rock, and a 

little interstitial quartz. 

Plagioclase phenocrysts are extensively replaced by 

caleite and clinozoisite, and the mafic minerals are 

replaced by calcite and chlorite, in many places 

pseudomorphously. In the western sills, ealcite and 

subordinate clinozoisite total as . much as 65 percent 

of the rock. Elsewhere, these alteration products, and 

chlorite, rarely aggregate 20 percent, and elinozoisite 

is more abundant than calcite. The widespread development of well-crystallized clinozoisite and the 

abundance · of· unaltered magnetite combine to suggest 

that much of the alteration is deuteric rather than 

related to '-veathering. Ascribable to weathering, 

however, are widespread limonite and variable amounts 

of a cryptocrystalline yellowish brown interstitial 

material. 

The northern andesite (represented by 7 specimens 

on pl. 2, and analysis 140 . in table 9) averages 

20 percent of small phenocrysts in a fluidal groundmass. · Plagioclase, zoned normally from andesinelabradorite to sodic andesine, makes up 80 percent 

of the phenocrysts. Thin clear outer rims on many 

grains are even more sodic and may approach albite. 

The remainder of the phenocrysts is mostly colorless 

to yellowish-green augite in well-formed but usually 

altered crystals. Some specimens eontain brownishgreen hornblende and others reddish to yellowishbrown biotite. A few contain scattered grains of 

pale-green orthopyroxene. 

The groundmass seems initially to have been almost 

wholly composed ·of microlites of plagioclase with 

mere traces of mafic minerals, but it is saussuritized 

throughout so that its fresh composition is conjectural. 

Alkali feldspar, much less altered, appears sporadically in the groundmass as roundish crystals and 

ramifying irregular masses. Magnetite and apatite 

are abundant accessories, commonly reaching 2 percent each. 

The augite crystals are extensively replaced by mixtures of magnetite, hornblende, and chlorite. Hornblende and biotite are also somewhat altered to magnetite and ehlorite. In places, the groundmass is reerystallized to determinable elinozoisite and epidote. 

Sericite, clay, and calcite are widely distributed and 

many fractures are filled with zeolites (mostly stilbite) 

or chalcedonic silica. 
100 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

The northern andesite is distinctly more mafic than 

the southern, as the plagioclase is much more calcic, 

and augite is the main ferromagnesian mineral rather 

than a subordinate one. In addition, the southern 

andesite is characterized by intense deuteric alteration, whereas the northern andesite is but mildly 

modified. 

AGE 

On stratigraphic grounds only, the andesite sills 

south of Milligan Canyon can be dated no more closely 

than as younger than the Lodgepole limestone, which 

they locally intrude, and older than overlying Quaternary gravel. The andesite masses north of the canyon 

can be more closely dated : they are younger than 

the Early Cretaceous Kootenai formation which they 

intrude or unconformably overlie and older than the 

Climbing Arrow formation, 'which overlies them unconformably and to which they contribute detritus. 

Their petrographic similarity to the Elkhorn Mountains volcanics leads readily to the idea that some o:f 

the andesite bodies may have been intruded at the 

same time as the volcanic rocks were extruded. 

Structural evidence permits somewhat narrower age 

assignment. The relation of the southern sills to the 

faulting in the Lodgepole and Three Forks vigorously 

suggests that these sills are younger than the faults, 

that in turn are younger than regional :folding, i:f 

but slightly. The andesites, then, seem to be no 

older than late Late Cretaceous or younger than 

middle Eocene. 

SUMMARY OF AGE RELATIONS 

On direct stratigraphic evidence, and assuming only 

that all the intrusive rocks of a particular rock type 

are substantially contemporaneous, the monzonites are 

of post-Kootenai, pre-Dunbar Creek age; the latites 

are of post- or intra-Elkhorn Mountains pre-Quaternary age ; the dacites are of post-Wolsey, pre-Climbing 

Arrow ( ~) age; and the andesites o:f post-Kootenai, 

pre-Climbing Arrow age. 

Given an additional assumption, that intrusion did 

not begin until regional :folding began, the lower 

limit o:f the period o:f intrusive activity can be raised 

to latest Cretaceous. The upper limit, of course, is 

not ehanged by this assumption. 

With a third assumption, that all the intrusive rocks, 

having evident close structural relations, are temporally related too, and were emplaced in essentially 

a single irruptive episode, their aggregate age can be 

:further narrowed to intra:f<?lding and pre-Climbing 

Arrow, or latest · Cretaceous to middle Eocene. 

IGNEOUS ROCKS INTRUSIVE INTO TERTIARY ROCKS 

OLIVINE BASALT 

Olivine basalt erops out in two small areas near 

the Jefferson River in sec. 34, T. 1 N., R. 1 W. Much 

of the largest mass is on the north bank in NW·%, 

see. 34 where it :forms a conical hill mantled at the 

base by Quaternary gravel. The mass exhibits rude 

vertical flow banding striking N. 50° W. It is complexly jointed; one main joint set seems concentric 

to the rounded outcrop shape, another tangential, but 

this impression was not eonfirmed by systematic measurement. Most of the rock is eompact but vesieular 

strueture appears loeally. The homogeneous flow 

bands are rarely interrupted by thin and impersistent 

zones of flow breeeia. 

Across the river in eenter Ey2 sec. 34, is a remnant 

o:f basalt less than 100 feet long and no more than 

20 feet thiek eutting Sphinx eonglomerate. (The 

size of this body is exaggerated on the geologie map.) 

The basal few feet of the basalt is pock-marked with 

large ealeite amygdules, and many pebbles :from the 

conglomerate are incorporated in it. A way :from the 

eontaet the roek is dense and only spottily vesicular. 

The basalt in both masses is similar, with scattered 

small phenoerysts of mafic. minerals set in a eompaet 

dark-gray mieroerystalline to glassy groundmass. 

Weathered sur:faees are grayish brown to brown and 

·pitted where the phenocrysts have weath-ered out. 

The small mass is intrusive into the limestone eonglomerate. The relation o:f the large mass to older 

roeks is unknown, but it is evidently related to the 

small mass and presumably contemporaneous with it. 

Both are probably plugs. 

By H. FRANK BARNETT 

A single thin seetion represents eaeh mass ( 326 

and 320, pl. 2). The larger mass is also represented 

by ehemieal analysis 326 in table 9. Both roeks 

are mueh alike mieroseopieally, with around 10 percent of small phenoerysts, mostly euhedral olivine, 

in a fluidal groundmass of pale-greenish-brown glass 

studded with tiny grains of plagioclase and clinopyroxene. A :few clinopyroxene phenocrysts, probably diopsidic augite, also oeeur in the smaller mass. 

Both have seattered xenocrysts o:f quartz, rounded 

and with thiek reaction rims. 

Groundmass plagioclase, which makes up 30 percent 

of the rock, is in grains too small for aeeurate determination by the methods used. Its indices are well 

above balsam and it is probably near ealcie andesine 

or labradorite. Groundmass olivine and clinopyroxene 
IGNEIQUS ROCKS 101 

each contribute perhaps 5 percent. Present also is 

a little microcrystalline pleochroic brown material 

that is probably biotite. Magnetite is abundant, in 

small euhedra and widely disseminated dust. The 

rest of the rock, near 50 percent, is glass with index 

well below that of balsam. 

The olivine in both phenocrysts and groundmass is 

intricately cracked and much altered to bow lingite ( ~). 

Locally, the glass is slightly devitrified. Otherwise, 

the rock is unaltered. 

AGE 

The basalt is the youngest igneous rock in the 

quadrangle, for it is the only one that intrudes rocks 

of Cenozoic age. If the intruded limestone conglomerate is correctly dated as slightly older than 

the middle Eocene or late Eocene Milligan Creek 

formation, the basalt may be only a little younger than 

the other intrusive rocks. It may, however, be much 

younger, though older than the older Quaternary 

gravel that mantles it. For convenience, it is placed 

at the oldest possible position on the geologic n1ap 

explanation. 

SOME PETROLOGIC RELATIONS 

The petrologic succession is obscure. Most of the 

intrusive ro.cks, including intrusive andesite, seem to 

be younger than the extrusive andesitic volcanics of 

the Elkhorn Mountains, and monzonite is intruded by 

latite west of Willow Creek; otherwise, no sequence 

is indicated. 

The field evidence supports the view that all the 

intrusive rocks are magmatic and rose to the present 

level of exposure in a fluid but very viscous condition, 

with little superheat. Only the highly varied rocks 

in the western part of the 10N pluton suggest hybridization; if hybridization occurred, it must have 

been at far deeper levels than are now exposed, for 

the visible signs are of only trivial interaction between magma and wall rocks. 

The inference is strong that all the igneous rocks 

are genetically related. The most direct, and perhaps 

the only convincing, evidence is the simple fact that 

the main rock types, though each is dominant in certain areas, are each widespread in the quadrangle, and 

. were apparently emplaced in a single time span of 

no great geologic length. Some sort of genetic relation may, with reservations, be inferred from the close 

chemical relation of the igneous rocks, as shown by 

the shape and smoothness of the curves they yield in 

a silica-variation diagram (fig. 13), prepared from 

the analyse.s of table 9, recalculated free of H 2 0 and 

of CaC03, on the assumption that all the C02 reported is in carbonate form. (Incidentally, the :fact 

that the porphyritic obsidian (no. 16) fits the curves 

may be taken as a faint confirmation of its contemporaneity ·with the Elkhorn Mountains volcanics; 

but the same sort of reasoning probably does not 

apply with equal force to the age of the basalt plugs 

(represented by no. 326), simply because basalts of 

similar composition are common throughout the Cenozoic column in the region.) Petrography and chemistry combine tQ demonstrate that all the igneous 

rocks involved are normal components of the calcalkalic group (Peacock, 1931); the alkali-lime index 

of the analyzed group cannot be given as a single 

figure because the curve for N a2 0 + 1\:2 0 intertwines 

rather than intersects with the curve for CaO (see 

fig. 13) ; the silica range in the zone of intertwining 

is 59.5 to 61.8, which is within the range of the calca,lkalic group as defined by Peacock. To say that a 

group of rocks are genetically related is not to say 

much about their genesis-but is all that is warranted 

at present. 

The Three Forks quadrangle is close geographically 

to the petrographic province of central Montana, as 

defined by Larsen ( 1940, p. 888-890) . Chemically 

and mineralogically, however, the Three Forks igneous 

rocks are little like the orthoclase-rich subsilicic rocks 

that characterize the province, even from as close 

by as the northern Big Belt Mountains (Lyons, 1944). 

Instead of having a high potash-to-soda ratio, characteristic of all but the Crazy Mountains subprovince 

of the central Montana province, the Three Forks 

igneous rocks have potash-soda ratios close to those 

of average calcalkaline rocks of similar general composition, as tabulated by N ockolds ( 1954) ; that is, 

they have distinctly more soda than potash or, in the 

case of quartz monzonitic rocks ( adamellites of N ockolds), slightly more potash. It should be noted that 

the Three Forks dacites (nos. 334, 339) are not appropriately compared with Nockolds' dacites (1954, 

p. 1015) from which they differ radically, but with 

his depenites (p. 1014). The latites of the Three 

Forks quadrangle, in fact, have less potash than soda, 

if analysis 12 is representative, whereas N ockolds' 

average latite (p. 1017) has more potash. At the 

latitude of Three Forks, therefore, the western limit 

of the central Montana petrographic province is east 

of the Missouri River. 

The Three Forks igneous rocks are, however, almost 

identical with certain seemingly contemporaneous rocks 

in the Elkhorn Mountains to the west and north. 

For example, virtually every chemical analysis in 

table 9 can be matched by analyses from the southern 

Elkhorn Mountains (l{lepper and others, 1957, tables 

1-3) . As might be expected, however, from the far 
102 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

greater area covered in the Elkhorn Mountains report, the extrusive and intrusive rocks of that area 

are far more diverse. 

STRUCTURE 

Structural events are implicit in many of the facies 

changes and virtually all of the unconformities recorded in the bedded rocks, and in the locations and 

shapes of igneous masses. For present purposes, however, they are sufficiently noted in the discussions of 

the rocks they affect. 

Discussed here, in chronologie order, are those 

structural features-folds, faults, tilted blocks-whose 

character and origin do not appear in the discussions 

of the rocks themselves. Unfortunately, only a fraction of the structures developed before deposition of 

Cenozoic sediments are visible. A wholly satisfactory 

local structural synthesis is thus out of the question, 

much less integration with the little-known regional 

structure. 

Structural elements for which there is reasonably 

good evidence are sketched and named on plates 1 and 

3. Plate 3 shows how few these are. 

SUMMARY OF STRUC.TURAL HISTORY 

The earliest recorded structural event was the folding and metamorphism of the earlier Precambrian 

rocks. Much later, but still in Precambrian time, the 

southern limit of the embayment that received Belt 

sediments became established at about the latitude of 

Willow Creek village apparently through faulting. 

This discontinuity exerted a powerful if intermittent 

influence on subsequent deposition and deformation. 

From late Precambrian to Late Cretaceous time, 

the quadrangle was structurally quiet, though far 

from dormant. Most of the folds and faults formed 

in response to regional compression applied recurrently during very Late Cretaceous to middle or late 

Eocene time. For convenience, this time interval is 

referred to as the Laramide interval or Laramide 

time, and the structural events within it as the Laramid~ orogeny, without any implication that deformation elsewhere assigned to the Laramide orogeny 

necessarily has the same time limits. 

In the Laramide orogeny large-scale folding along 

roughly north-south axes occurred first. Major thrusting followed, perhaps after a period of quiescence. 

A broad belt of thrusting developed, but only the 

uppermost thrust is exposed in the Three Forks quadrangle. A cluster of tilted fault blocks above the 

thrust surface probably was formed during thrust 

advance. Some high-angle faults below the uppermost 

plate cannot be definitely dated -in relation to thrusting but are likely also contemporaneous with the 

thrusting. Only two steep faults are known or inferred to be distinctly younger than the thrusting 

but older than the Tertiary basin deposits; possibly, 

however, many developed and controlled the general 

dimensions. of the Tertiary basin, but if so they are 

masked by continental deposits. All but two of the 

many intrusive masses were emplaced during or 

shortly after folding and before deposition of the 

Boze:tnan group. Time relations between intrusions 

and thrusting are unknown but a reasonable guess is 

that the period of intrusion overlapped the period of 

thrusting. 

The Tertiary basin is structurally controlled at 

least to the extent that it lies on a zone of repeated 

faulting, and its borders cut sharply across deformed 

older rocks. But there is no deciding to what extent 

it is due directly to diastrophism and to what extent 

indirectly, by erosion of unresistant rocks exposed 

by diastrophism. It is not, however, a product of 

dissection of an Eocene peneplane, as has been postulated. 

Deformation during the Tertiary seems to have 

been mostly in the form of gentle easterly tilting, 

recurring or continuing from late Eocene until the 

close of the Tertiary, and punctUated by shallow local 

folding. 

Quaternary deformation seems to · have been limited to slight northward tilting. 

STRUCTURE OF PRECAMBRIAN CRYSTALLINE ROCKS 

The shape of the surface of the Precambrian crystalline mass below the younger rocks and the distribution 

and nature of mobile units within it have no doubt 

been basic elements in the structural development of 

the area, as Peale ( 1893, p. 14) early noted and as 

several later workers (see, for instance, Bucher, Thorn, 

and Chamberlin, 1934) have hypothesized for the 

entire Cordilleran region. They must, however, be 

left to conjecture. All that is known of the Precambrian crystalline rocks in the quadrangle is based 

on a few poor exposures that do little to brighten 

the corner where they are. Banding and foliation 

strike N. 70°-90° W. and dip 50°-70° N. Northsouth compression older than the North Boulder formation is indicated if the crystalline rocks have acted 

orily as buttresses during subsequent compressional 

deformation. Perhaps they have, but to judge by the 

patterns made by Paleozoic rocks folded within Precambrian crystalline masses in southwestern Montana, 

as shown on the current geologic map of the State 

(Ross and others, 1955), the Paleozoic rocks are 

molded smoothly around crystalline cores in neat 
'STRUCTURE 103 

anticlinal form, suggesting that the crystalline rocks 

have responded to some extent by folding. Thus, the 

structural trend of the small body of Precambrian 

crystallines here, which is parallel to that of overlying 

Paleozoic rocks, may be in part of post-Paleozoic 

age and cannot be confidently taken as the trend of 

pre-Belt structure. 

STRUCTURAL SIGNIFICANCE OF PRECAMBRIAN 

SEDIMENTARY ROCKS 

The North Boulder formation was deposited at a 

time of active tectonism, as indicated regionally by 

its coarse texture, great thickness, and linear dis--

tribution. The small volume of North Boulder rocks 

in the quadrangle and their position on the upper 

plate of a Laramide thrust zone of unknown but P?Ssibly great displacement make it difficult to appraise 

their structural significance on local evidence, but on · 

a regional scale the meaning of the elongate wedge 

of coarse clastic rocks of which the North Boulder 

rocks are a part seems fairly clear. Peale (1893, 

p. 14) early interpreted the texture and distribut~on 

of these rocks to indicate an east-west Precambrian 

(Belt) shoreline at about the latitude of Bozeman. 

Most later workers agree that the shoreline probably 

is a fault or fault zone, downthrown on the north in 

pre- and intra-North Boulder time. (Deiss, 1935; 

Hinds, 1935; Berry, 1943, p. 7; Sloss, 1950; Alexander, · 

1951· McMannis, 1955, p. 1390-1392; and Verrall.18 ) 

The 'projected trace probably passes close to the village of Willow Creek, and the structure is therefore 

named the Willo:w Creek fault. 

As Sloss and McMannis recognized, the influence 

of this ancient fault zone did not end with deposition 

of the North Boulder formation. In post-North 

Boulder pre-Flathead time, further uplift along this 

zone, by arching or tilting rather than faulting, is 

suggested by the fact that southward the Flathead 

overlaps lower and lower strata in the Belt series, and 

ultimately lies on pre-Belt crystalline rocks : in the 

northeastern part of Canyon Ferry quadrangle (see 

fig. 1B) the Flathead lies on Helena limestone, in 

the southwestern part on Empire shale (Mertie, 

Fischer, and Hobbs, 1951, pl. 1); in the northern 

part of the Toston quadrangle it lies on Spokane 

shale, farther south on Greyson shale; in the Manhattan quadrangle it rests on rocks as old as the 

New land limestone.19 

The fault zone seems to have had little effect _on 

the deposition of Paleozoic or Mesozoic sedimentary 

rocks of the Three Forks area, which lie without ex·-

1s See footnote, p. 11. 

19 See footnote, p. 11. 

ceptional differences in thickness or lithology on both 

sides of the zone. It seems, however, to have reappeared as an important influence in Late Cretaceous 

and early Tertiary time, as it apparently limited the 

Elkhorn Mountains volcanics on the south and provided the southern terminus of the great thrust system 

that includes the Lewis thrust. It is no coincidence 

that the Three Forks basin overlies this zone and is 

elongated parallel to it, thus being one of the very 

few intermontane basins in Montana that do not 

trend north-south. 

Ironically, this important structure cannot .be. located closely except where it inferentially COinCides 

with the Jefferson Canyon thrust. 

LARAMIDE STRUCTURAL FEATU.RES 

EARLY FOLDING 

Folding on a large scale was the first major local 

event in the Laramide orogeny. The folding, presumably with" concurrent erosion at the surface as 

sketched by Lowell ( 1956a), began before deposition 

of the Elkhorn Mountains volcanics, for beyond the 

quadrangle, as noted earlier, they rest on an angular 

unconformity representing the removal of thousands 

of feet of earlier Cretaceous, Mesozoic, and late 

Paleozoic rocks. But folding continued after the 

vol~anic rocks were erupted, for the volcanics are 

folded to about the same degree as the older rocks in 

most places; this is true of the main mass of Elkhorn 

Mountains volcanics also (I\::Iepper and others, 1957, 

p. 32, 55) . Large-scale folding ended before thrusting began, as thrust faults that shear off several large 

folds are not themselves folded. 

The largest fold for which there is evidence is the 

great crumpled Radersburg syncline or synclinorium 

that, plunging northward, oc~upies most of the northwest quarter of the quadrangle and extends across 

and beyond the Radersburg quadrangle to the north, 

between the elongate dome at the east edge of the 

southern Elkhorn Mountains (Klepper and others, 

1957, p. 56 and pl. 2) and the northward-trending 

anticline whose west flank forms the Limestone Hills, 

a few miles southwest of Townsend (Ruppel, 1950). 

This fold, recognized but not named by Klepper and 

by Ruppel, seems appropriately called the Radersburg 

syncline. The horizontal distance between the trough 

of the syncline and the crest of each bordering anticline 

is 5 or 6 miles at the latitude of Radersburg and 

seems twice this at the latitude of Three Forks. The 

syncline involves the entire column of pre-Tertiary 

sedimentary and volcanic rocks present in the quadrangle, from the North Boulder formation to the 

Elkhorn Mountains volcanics. Most of the area 
104 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

underlain by the fold is unconformably covered by 

younger deposits, and the fold structure in much of 

the rest has been complicated by faulting so that only 

its general outlines can be sketched, and it is impractical to show its axis on the geologic map. The 

syncline is abruptly cut off by the Jefferson Canyon 

and Highway thrusts at the south; the remnants of 

steeply dipping contorted Paleozoic and Mesozoic 

rocks east of Mud Spring Gulch road are all that is 

visible of its east flank; on the west the structure 

continues at least 7 miles into the Jefferson Island 

quadrangle. The average dip on the flank is about 

30° and the plunge perhaps 10° northward, but local 

dips are in almost any amount and direction due to 

minor folds and to the influence of faults. The gross 

structure trends slightly east of north, as Klepper 

and Ruppel noted for the bordering anticlines, but 

the trend within the quadrangle seems to be a little 

west of north. Sections B-B' and 0-0' illustrate 

the southern part of the syncline. 

The pre-Tertiary sedimentary ·rocks south of the 

Jefferson River, below the thrusts that limit the 

Radersburg syncline, are regarded as separate structurally, though with similar general trends. Folds, 

though well formed in places, change trend and die 

out rapidly. Faulting seems to have been the main 

method of structural adjustment. The structure of 

this block is shown in section A-A'. 

The most pronounced fold . here is an asymmetric 

anticline. It forms the northerly bulge in the contact 

between Flathead sandstone and Precambrian crystalline rocks in NEJ4 sec. 10, T. 1 S., R. 1 W. From 

the Flathead, it plunges first north and then northwest, disappearing under Quaternary gravel in NEl4 

sec. 4, T. 1 S., R. 1 W. Its western limb, interrupted 

by a long strike fault, has low to moderate dips; its 

eastern limb, broken by two strike . faults, is steep. A 

few thousand feet to the west is a poorly defined syncline of similar trend and of low amplitude. To the 

east, no large well defined folds are apparent. 

The tight, locally overturned and broken anticline 

in N~12 sec. 12, T. 1 S., R. 1 W., though small, is worth 

noting. In it are unusually good exposures of Park 

shale in con1plex relation to the Pilgrim 1imestone. 

The fold illustrates that thoroughly indurated rocks 

can pass from open to intensely deformed structure 

and back again, within very short distances. 

These folds probably are related to the Radersburg 

syncline. Their axial trends diverge from that of the 

Radersburg syncline due partly to the influence of 

the underlying crystalline rocks and perhaps partly 

to refolding by thrusting, as hinted by the curving 

axes. Conceivably, the folds may be open in part 

because of "unfolding" (Kelley and Del Mar, 1957) 

related to thrusting. 

The most spectacular fold, shown in sections D-D' 

and E-E', is in the northeast corner, where all the 

pre-Tertiary rocks are involved in a great overturned 

plunging anticline, here named the Hossfeldt anticline. The lowest formation exposed in the core of 

the fold is the Pilgrim limestone. On the west limb, 

cut off by the Lombard thrust, the h~ghest formation 

is the Amsden; on the east limb, the Kootenai. The 

structure continues eastward into the Manhattan 

quadrangle, as shown by Verrall,20 ultimately involving rocks high in the Colorado shale of Late Cretaceous age. The horizontal distance from the crest of 

the Hossfeldt anticline to the trough of the parallel 

Eustis syncline of Verrall is about 6,500 feet. Northnortheastward, the structure continues for at least 7 

miles in the Toston quadrangle, where it disappears 

beneath the northern extension of the Lombard thrust. 

The anticline is overturned to the east, with moderate 

dips, mostly 30° to 40°, on the upright western limb,. 

and steep dips, commonly 60° (that is 130~) to vertical, on the overturned limb. The axis plunges S. 30° 

W. at angles of 15° to 30°. Peale (1896) chose this 

structure for the west end of a section ( B-B') but did 

not recognize the thrust fault and was led to interpret 

the entire structure east of the large intrusive ( 10N 

pluton of this report) as an unfaulted syncline, overturned to the east. Haynes ( 1916b), in his original 

description of the Lombard thrust (or overthrust as 

he called it), recognized the fold structure to be an 

overturned anticline and illustrated it with a photograph (his fig. 9) ; his map (fig. 1) shows no rocks 

older than Jefferson dolomite or younger than Madison limestone on the flanks. Berry ( 1943, pl. 1) 

mapped the structure similarly . and identified Pilgrim 

limestone in the core, but no rocks younger than 

Madison limestone on the flanks. 

A smaller open synclinal fold, involving only rocks 

between the upper part of the Pilgrim limestone and 

the lower part of the Mission Canyon limestone, lies . 

between the Lombard thrust and Dunbar Creek. The 

limbs are roughly symmetrical with average dips of 

about 40°, but the fold is complicated by many minor 

crenulations with dips ranging from 15 ° to vertical. 

It plunges S. 35° W. at about 20°. At the east edge 

of this syncline, against the Lombard thrust~ is a 

small but striking asymmetric, in part overturned, 

anticline involving the uppermost Pilgrim, the Maywood and the basal Jefferson formations. Its axis, 

which can be traced for about 1,500 feet, plunges 

steeply southwestward. Possibly it is a large drag 

ro See footnote, p. 11. 
STRUCTURE 105 

fold and contemporaneous with thrusting rather than 

with the older folding. East of the Radersburg syncline, a broad shallow anticline, or dome, on the order 

of 3 miles broad, is the main structure in the preTertiary rocks, mostly limestone of the Madison, intruded by the lON pluton. On its west flank, where 

it passes into the Radersburg syncline, the structure 

develops steep dips and many compl~x minor folds. 

Section D-D' attempts to show part of this· :fold. 

All of the folds could have been developed in a 

single episode of folding with roughly east-west compression. Most axial planes dip westward, suggesting 

a vertical couple with the western element :moving 

relatively upward. This is part of a widely noted 

regional pattern, suggesting that large-scale tangential compression was the principal type of deformation, rather than local readjustments controHed by 

gravity. 

The Madison group played a special · role in folding 

(Berry, 1943, p. 24). It acted locally much the same 

as the Precambrian crystalline mass acted regionally, 

by serving as a buttress against which the thinly 

bedded, less competent overlying aiid underlying formations were crumpled. More distant units, both 

competent and incompetent, were more gently folded. 

North of Mud Spring Gulch the Madison is folded 

but elsewhere it is broken by closely spaced faults, 

both along and across the bedding. These faults individually are small but their aggregate displacement 

is great and causes large changes in apparent thickness. The tremendous apparent thickness of the 

Madison east of Milligan Canyon is largely due to 

such faulting, and in the valley on the northwest side 

of this thickened mass is an example of the buttreSs 

effect. There the Big Snowy and basal Amsden rocks 

have been intricately contorted. The folds have amplitudes up to 200 feet but are too small to show on 

the geologic map. The competent formations above 

and below the crumpled rocks maintain consistent 

strike and dip. 

STEEP FAULTS OLDER THAN THRUSTING 

Two small steep faults are overridden by a thrust, 

and are therefore of pre-thrust age. They are exposed under the Highway thrust in the low hills west 

of Highway lOS in SW cor. sec. 33, T. 2 N., R. 1 E., 

and in NE corner of adjoining sec. 3, T. 1 N., R. 1 E. 

The fault in sec. 33 trends slightly west of north 

and repeats part of the Pilgrim limestone. The fault 

trace is marked by discordant attitudes of the strata 

on opposite walls. The relative movement is not 

known because banded blue-and-gold beds, present at 

several levels in the formation, appear on both walls; 

the throw cannot exceed a few hundred feet. Along 

the nearly parallel fault in sec. 3, lower Meagher limestone on the 'Yest has been dropped a few hundred 

feet against Pilgrim limestone on the east. 

THRUST FAULTS-SIXTEENMILE THRUST ZONE 

Thrust faults cut the masses of pre-Tertiary rocks 

in the northeastern, central, and west-central parts of 

the quadrangle. The thrust in the northeastern part 

-the Lombard thrust of Haynes ( 1916b) -trends 

north -south and dips west. That in the west-central 

part-the Jefferson Canyon fault of Peale ( 1896) and 

Berry (1943)-trends east-west and dips north. 

These faults are linked in the center of the quadrangle 

by the Highway thrust of intermediate attitude, and 

appear to be elements of a single thrust along which 

gently to moderately folded rocks as old as Precambrian moved relatively eastward over tightly folded 

rocks as young as Late Cretaceous or even very early 

Tertiary. In turn, this thrust is not isolated but is 

the uppermost plate in a broad zone of thrusting, 

named the Sixteenmile thrust zone (Robinson 1959), 

most of which is concealed by Cenozoic deposits in the 

Three Forks quadrangle. The concealment is not 

fortuitous but reflects controls on Cenozoic sedimentation exerted by the thrusts and related steep faults, 

both activ~ly by providing gradient through the vertical component of displacement, and passively by 

providing fractured rocks for easy erosion. 

A few miles to the northeast this broad zone of 

thrust faulting emerges from beneath the Cenozoic 

cover. In the Toston quadrangle, the Lombard thrust 

is the most westerly of three westward-dipping thrusts 

in a strip some 8 miles wide, each of which has similar 

maximum stratigraphic throw: Belt series rocks on 

J(ootenai formation. Another, similar thrust probably extends the thrust zone a mile or two farther 

east into the Maudlow quadrangle (Klemme's work 21 

in the same area does not agree in ail particulars but 

gives the same general tectonic' picture). All the 

thrusts east of the Lombard appear in the northern 

part of the Manhattan quadrangle, where, according 

to Verrall,22 they swing into southwesterly trends. 

The easternmost thrusts die out in the sharp, steeply 

plunging folds of the Horseshoe Hills but two thrusts, 

the Trident and Green, 7,000 feet apart, continue 

throughout the hills and disappear beneath Cenozoic 

deposits above the mouth of the Gallatin River. With 

the· Lombard thrust, they form a zone of known but 

largely covered thrusting more _than 31j2 miles wide 

at the east edge of the Three Forks quadrangle. 

21 See footnote, p. 13. 22 See footnote, p. 11. 
106 GE.OLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

This thrust system was not named by previous 

workers, although among Peale, Berry, Klemme, and 

Verrall all the individual thrusts comprising it have 

been named~ The probability is good that the system 

is the southern extension of the Lewis thrust zone, as 

Clapp (1932) suggested, but the linkage is far from 

complete and a local name is desirable. Because most 

of the system is well exposed along Sixteenmile 

Creek, where much of it was first mapped by Klemme, 

I have proposed the name Sixteenmile thrust zone 

(Robinson, 1959). 

LOMBARD THRUST 

In the northeastern part of the quadrangle, rocks 

as old as the Middle Cambrian Meagher limestone are 

thrust over rocks as young as the early Carboniferous 

Mission Canyon limestone. This is the southern end 

of the Lombard overthrust of Haynes (1916b). The 

trace of the thrust, curving but with an average strike 

near N. 20° E., is exposed continuously for 41j2 miles, 

:from the north edge of the quadrangle, in sees. 19 

and 20, T. 3 N., R. 2 E., to sec. 12, T. 2 N., R. 1 E., 

where it disappears beneath Quaternary gravel. For 

much of its length the thrust places Pilgrim limestone in the western upper plate on Mission Canyon 

limestone in the eastern lower plate. Toward the 

south, :formations in both plates are progressively 

younger so that where the fault disappears below 

gravel Jefferson dolomite is thrust on Amsden formation. For most of its length the thrust has a single 

trace parallel to the bedding above and below. At 

the north edge of the quadrangle, however, it splits 

into four branches with an aggregate width of more 

than half a mile. Here, splits introduce slivers of 

Meagher and of Jefferson formation below the Pilgrim on the uppermost plate. The lowermost trace 

cuts sharply across the west limb of the Hoss:feldt 

anticline and down section, so that Jefferson is thrust 

on Jefferson at the quadraligle edge. Another smaller 

split, into two branches, appears along 2,000 feet of 

the thrust in the southern part, mostly in sec. 6, T. 

2 N., R. 2 E. This split introduces a sli~er of J efferson dolomite between Mission· Canyon limestone in 

the lower plate, and Pilgrim, Maywood, and Jefferson 

rocks in the upper plate. 

The trace of the thrust is marked by a slight de-

. pression and can easily be followed, but exposures of 

the fault surface are rare and the dip of the fault has 

been measured only at one point, where it is 30° · to 

the west. The average dip is probably closer to 45° 

judging by the bends in the trace across rugged 

topography. This is about the average dip of the 

:formations bordering the thrust. Signs of deformation such as drag folds, breccia, gouge, or mylonite 

are virtually absent. In many places on and downslope from the sag made by the thrust trace are concentrations of fragments of orange chert, apparently 

related to the fault surface but not seen in place. The 

maximum stratigraphic throw, Meagher limestone on 

Mission Canyon limestone, is 4,000 feet. 

Haynes (1916b, p. 271, fig. 1) traced the Lombard 

thrust for 13 miles, with an average strike of N. 30° 

E., from 3 miles north of Three Forks across the 

double-horseshoe bend of the Missouri River near 

Lombard and 2 miles beyond. The stratigraphic 

throw increases northward with the result that near 

Lombard Haynes reported that the fault, dipping 

about 40° to the west, "has brought strata of the Belt 

series over strata of Cretaceous age" so that the maximum displacement "is · approximately 2 miles and 

strata which are stratigraphically about 6,800 feet 

apart are here in contact." The northern 10 miles of 

the thrust has been recently mapped· in detail (Robinson, 1959, pl. 2). The thrust is overlapped by Tertiary deposits 3 miles north of the horseshoe bend; thus 

it has a continuous length of 14 miles. Detailed mapping confirms Haynes' interpretation as to scale of 

throw and displacement. ~lore precisely, strata as 

old as the Greyson shale are thrust over strata as 

young as the uppermost part of the Kootenai formation, with a stratigraphic throw of more than 8,000 

feet. 

HIGHWAY THRUST 

In the center of the quadrangle a thrust :fault, offset by a high-angle oblique :fault, places wolsey shale 

and Meagher limestone on complexly faulted Meagher 

and Pilgrim formations east of the high-angle fault, 

and uppermost part of the Wolsey and Meagher on 

North Boulder formation west of the high-angle 

fault. The thrust, exposed for 2 miles at the base of 

the hills that rise above Highway lOS (fig. 14), strikes · 

northeast and dips northwest; the dip is at angles of 

10° to 20° to judge by the sinuosity of the thrust 

trace and by one exposure in the east branch of the 

gulch that drains south along the west edge of SW1;4 

sec. 33, T. 2 N., R. 1 E. The thrust, not previously 

recognized, is here named the Highway thrust. 

The trace of this thrust, unlike that of the Lombard 

thrust, is marked by several striking, if small, disturbed features. For example, in SE cor. sec. 33, 

Mea<Yher limestone 200 feet above the thrust trace is b . 

dragged into a tight asymmetric anticline with a vertical southeast limb. At several places, especially 

where red arkose of the North Boulder formation is 

at the top of the lower plate, the rocks for 2 to 3 feet 
STRUCTURE 107 

FIGURE 14.-Highway thrust, exposed on Highway lOS, 314 miles 

southwest of Three Forks junction. North side of highway at 

turnout in SW14- sec. 5, T. 1 N., R. 1 E. Thrust fault indicated 

by 'I'. Geologist stands on upper red arkose of North Boulde r 

formation (p£n). Thrusting has reduced thickness of Wolsey shale 

(£w) normally 200-400 f eet, to 20 feet. P a rt of Meagher limestone (£m) is also thrust out above upper thrust slice, as basal 

Meagher beds are missing. Thick ledge above geologist is glauconitic quartzite, low in the Wolsey shale. 

on either side of the fault trace are microbr~:,cciated, 

slightly recrystallized, and bleached almost white. 

Where the thrust is in Wolsey shale much of the 

chlorite is locally reoriented parallel to the thrust 

surface. The maximum stratigraphic throw is less 

than 500 feet. 

JEFFERSON CANYON THRUST 

From the west edge of the quadrangle a large 

thrust fault trends east parallel to and a mile north 

of Highway lOS. This thrust, which dips 40° N. 

where exposed in the creek valley near the south edge 

SE1;4 sec. 16, T. 1 N., R. 1 W., places Precambrian 

North Boulder formation on steeply dipping Upper 

Cretaceous Elkhorn Mountains volcanics and on a 

latite mass intrusive into the volcanic rocks. The apparent stratigraphic throw is on the order of 10,000 

feet. The only signs of deformation along the trace 

are a few small drag folds, with steeply dipping south 

limbs, in the conglomeratic North Boulder rocks at 

the very edge of the quadrangle. 

This fault is the eastern end of a thrust system that 

was traced first by Peale ( 1896, p. 5) and later by 

Berry ( 1943, p. 24-25) for at least 9 miles eastward 

from the mouth of South Boulder River; on Peale's 

map the east end of the fault is placed a mile west of 

the Three Forks quadrangle. Both Peale (his section A) and Berry (his section B-B') interpreted the 

fault zone as a thrust system dipping 45°-60° N. 

Berry (1943, p. 25) estimated its stratigraphic throw 

at 12,000 feet and named it the Jefferson Canyon 

fault. It seems more evocative, however, to call it the 

Jefferson Canyon thrust. 

The thrust is on the trend of the buried Willow 

Creek fault of Belt age, and may be the same fault, 

revived with opposite throw. In the Bridger Range 

the presumed same Belt fault zone has been interpreted as the locus of a similar reversal of throw in 

Laramide time (Pass fault of McMannis, 1955, p. 

1420-1421). 

MINOR THRUSTS 

Two very small thrust slices flank Milligan· Creek 

above Milligan Canyon. East of Milligan Canyon in 

SE1;4 sec. 24, T. 2 N., R. 1 W., a thrust dipping northwest more steeply than 40° repeats the Phosphoria 

formation and much of the Ellis formation. West of 

Milligan Canyon, in Nlj2 sec. 35, T. 2 N., R. 1 W., a 

thrust that dips about 30° N. repeats most of the 

Amsden formation and the basal Quadrant formation. 

Each of these small thrusts has a few hundred feet 

of stratigraphic throw, is covered by Cenozoic deposits at one end, and disappears into bedding at the 

other. 

THRUST DISI'LACEMENT 

A start toward estimating thrust displacement is 

to compare it with the measurable stratigraphic 

throw, llj2 to 2 miles. The dip is roughly parallel to 

bedding, and the displacement may be many times 

the throw. But, the rocks wi';ITe folded before thrusting and possibly the displacement is less than the 

stratigraphic throw. Because folding is open in most 

of the upper plate, minimum displacement should 

much exceed the stratigraphic throw and 3 miles 

would seem a reasonable estimate of minimum displacement. 

To estimate maximum displacement is harder. The 

absence of Paleozoic and Mesozoic windows west of 

the thrust system foredooms attempts to estimate 

maximum displacement by direct measurement. A 

laborious kind of stratigraphic approach through the 

construction of palinspastic maps (Kay, 1945) might, 

however, ultimately succeed in yielding rude estimates. Its basic element would be an attempt to restore the deformed rocks to their prefold and prethrust positions with the aid of contour or isopleth 

maps of such variables as thickness, clastic ratio, dolomite-calcite ratio, or coarse-to-fine ratio of formations 

or systems or other rock or time-rock groupings. The 

four sets of detailed measured sections ( p. 124-135) \vere 

made with a reconstruction of this sort in mind, but 

it was abandoned when rough comparisons among the 

summaries of these measurements (spread through 

the discussions of the sedimentary rocks) indicated 
108 GE.OLOGY OF THE THREE FOR~S QUADRANGLE, MONTANA 

that there is about as much stratigraphic variation 

within each plate as there is across the thrust. The 

measurements reveal a surprising amount of lateral 

variation in supposedly uniform Paleozoic shelf rocks 

but help little with the problem of displacement. 

With the poor data at hand it seems prudent to conclude merely that displacement on the LombardHighway-Jefferson Canyon thrust is probably on the 

order of a few miles: Displacement on the whole Sixteenmile thrust system is presumably several times 

greater than that on the Lombard thrust alone. 

Casual measurements suggest that far more crustal 

shortening is due to the Laramide folds. 

If the eastward-trending Jefferson Canyon and 

northward-trending Lombard thrusts are part of the 

same thrust fault, it is necessary to reconcile their 

great apparent difference in relative direction of 

movement. If only one fault is involved, and the 

different traces are due to accidents of erosion, was 

the relative movement of the upper plate southeasterly, or did one direction of relative transport, eastwest or north-south, dominate so that the movement 

on one of the segments was largely strike slip? 

Regional evidence suggests that the dominant rela-

. tive transport was east-west. The Sixteenmile thrust 

zone seems to be at the southern end of a sinuous belt, 

more than 350 miles long, of thrust faults that dip 

westerly (Clapp, 1932). The zone can be traced with 

scarcely a break from southern Alberta to the Canyon 

Ferry quadrangle (see fig. 1B) east of Helena, but a 

·stretch of about 25 miles from Canyon Ferry to the 

north end of the Toston quadrangle is covered by 

Cenozoic deposits so that structural continuity, 

though reasonably certain, cannot be proved. Assuming continuity, the zone near Three Forks curves 

abruptly to the southwest and then disappears beneath younger deposits. The overall movements on 

the great plates involved were surely about at right 

angles to the northerly regional trend of the . thrust 

belt, which means a principal eastward rather than 

southward component. No thrusts that might be direct southern continuations of the thrust zone have 

been recognized in 'the Precambrian crystallines south 

of the curving end of the known thrust belt. It seems 

likely that the faulted no:t;thern margin of the ·Precambrian crystalline rocks was the southern limit of 

the thrust belt and that the crystalline body acted as 

a buttress that absorbed any southward components 

of movement in the production of minor folds and 

faults. Berry (1943, p. 23) and Alexander (1955, 

p. 99) made similar interpretations. If this reconstruction is correct, movement on the Jefferson Canyon thrust was largely strike slip. 

The problem of the direction and amount of displacement should not be left without reference to a 

bolder concept than I have ventured. Certain geographically discordant stratigraphic sequences in a 

reconstruction of the initial distribution of facies in 

the Upper Cambrian formations, led Lochman and 

Duncan (1944, p. 16) to conclude that southwestern 

Montana has been subjected to post-Cambrian crustal 

shortening in a northeasterly direction of at least 50 

miles and to imply that the shortening reflects Laramide thrusting on the Lewis-Sixteenmile system; 

merely to flatten Laramide folds would not suffice. In 

my opinion their evidence, supplied by a few widely 

spaced measured sections, is insufficient, but their conelusion may nevertheless be correct. 

TIME RELATIONS OF FOLDING AND THRUSTING 

The thrust system is younger than the folds confined to pre-Tertiary rocks, for the thrusts shear off 

several folds but are not significantly folded themselves. If these were thrusts of Alpine type, in which 

thrusting is the . end result of folding continued beyond the ·rupture point (break or stretch thrusts), 

the thrusting followed directly on folding. But in 

such thrusts folds both above and below a thrust 

surface should be typically tight and asymmetrically 

steepened or overturned in the direction of thrusting. 

Although the Hossfeldt anticline in the lower plate is 

tight and overturned, the larger structures in the 

upper plate nearby are open and only moderately 

asymmetric. Consequently, the thrusts seem to be 

subsequent shear thrusts. . 

If so, questions arise as to the age relations among 

thrusting, Laramide igneous intrusions, and related 

hydrothermal ore deposits. The intrusive rocks and 

any ore deposits genetically related to them · are o:f 

intra- or post-folding age. It is conceivable that some 

or all of the thrusting is younger than the intrusives 

and their attendant ore deposits, though generally 

considered older (see, for example, Klepper and 

others, 1957, p. 60). For example, the 10N pluton 

and any buried ore deposits that might be associated 

with it may be cut off a few thousand feet down by 

the Lombard thrust. 

STEEP FAULTS IN UPPE-R PLATE OF HIGHWAY 

THRUST 

From the roads that flank it, the hilly mass that 

lies above the Highway thrust appears to be a 

straightforward pile of little-deformed strata, nearly 

horizontal above Milligan Creek and with moderate 

to low northwesterly dips elsewhere. Actually, the 

mass is broken by steep faults into a mosaic of tilted 

blocks. The individual faults, most of which are fol-
STRUCTURE 109 

lowed faithfully by valleys or by marked topographic 

sags, can generally be traced for no more than a few 

thousand. feet. They are e;ut off by or merge with 

other faults, or trend beneath Cenozoic deposits, or 

simply die out in the thick carbonate units. All the 

faults, even those with curving traces, seem to be vertical or nearly so. Maximum stratigraphic throw 

and net slip on most of them is on the order of 500 

feet, rarely as much as 1,000 feet, so that Upper Cambrian rocks are thrown against Devonian ones, and 

Devonian against lower Carboniferous, but there are 

no larger stratigraphic throws. All the faults are 

probably oblique slip but with large vertical components. Significant horizontal offset is indicated 

only on the fault that, striking N. 15°W., cuts across 

the Jefferson-Broadwater County line in sees. 36, T. 2 

N., R. 1 W., and 31, T. 2 N., R. 1 E., where drag and 

shearing in Three Forks shale indicate that the east 

wall moved south. 

These faults are most readily interpreted as gravity 

faults. Little crumpling, gouge, or breccia is associated with them, except where three faults merge 

near the township corner half a mile east of the 

mouth of Dry Hollow. There, promine:r:tt ridges of 

Jefferson dolomite are intensely brecciated. 

Most of the blocks bordered by these faults have 

been tilted perhaps 10° to 20° southward, opposing 

the general moderate northwesterly dip pattern. The 

small rectangular block that centers in. the NW center 

sec. 6, T. 1 N., R. 1 E., however, seems to have popped 

up like a cork. 

Included in this group of steep faults is the long, 

gently curving fault that bisec.ts sec. 31 from northeast to southwest. The map pattern of this fault suggests that it is a minor thrust parallel to the Highway 

thrust and much like the two small slices farther upsection, but excellent exposures show conclusively that 

the fault is a trivial, near-vertical one, that for all 

but a small fraction of its length drops beds close to 

the base of the Three Forks shale on the west against 

strata at the top of the Jefferson dolomite, with a 

throw of less than 50 feet. Near its western end, it 

rises slightly in section and repeats basal beds of the 

Three Forks. 

None of these minor faults are large enough to have 

been shown by Peale. Berry (1943, pl. 1, p. 24-25) 

generalized the pair of faults that affect the Madison 

limestone as a single "high-angle thrust" with horizontal offset of nearly a mile, but recognized none of 

the other faults nearby. 

These faults presumably formed during development of the Highway thrust ·but their contacts with 

the thrust are concealed. Like the thrust, these faults 

are older than the Bozeman group for they are overlain by unfaulted rocks of the Milligan Creek formation. They are also older than the basin occupied by 

the Milligan Creek rocks for there was a reversal of 

topography after faulting and before the Tertiary 

basin existed. The upthrown sides of these faults 

along the east side of the valley now occupied by 

Milligan Creek had to be reduced by postfaulting 

erosion to produce this part of the Tertiary basin. 

Simple differential erosion contemporaneous with initial faulting will not suffice, for the carbonate rocks 

on the opposite sides of the faults are of equal resistance to weathering. Like the thrust faults, too, 

the steep faults are younger than the pre-Tertiary 

folding for they show no signs of being folded themselves. A reasonable inference is that these structures 

formed as blocks jostled on the shallow upper plate as 

thrusting proceeded. The absence of similar complications above the Jefferson Canyon and Lombard 

thrusts possibly reflects a greater thickness of cover. 

On the other hand, the two slices higher in the pile 

may simply not have moved far enough to develop 

jostled blocks. 

OTHER STEEP FAULTS CLOSELY RELATED TO 

THRUSTING 

Three long faults roughly parallel to bedding have 

been mapped in upper Paleozoic rocks south of the 

Jefferson River. The longest extends for more than 

21j2 miles from NW 1,4 sec~ 1, T. 1 S., R. 1 W., to SE1_4 

sec. 33, T. 1 N., R. 1 W.; it may be much longer, as it 

is concealed by Quaternary deposits at each end. For 

most of its length it raises rocks in the lower part of 

the Lodgepole limestone on the north against strata 

low in the Mission Canyon limestone on the south. 

Along two stretches, however, the north wall is raised 

enough to expose distinctive upper strata of the Three 

Forks. vVith the upthrown side downdip, the dip of 

the fault must be greater than that of the strata, which 

ranges widely along the fault but averages about 60°. 

The stratigraphic throw is perhaps 900 feet, a few 

hundred feet more than the thickness of the Lodgepole, presumed to average about 600 feet in this vicinity. The dip slip necessary to achieve this much 

stratigraphic throw is 2,000 feet. The amount of 

strike slip, though unknown, is probably small. 

The parallel fault to the south has several similarities but is not identical. At its east end in center 

SE1_4 sec. 2, T. 1 S., R. 1 W., the fault raises beds low 

in the Jefferson dolomite on the north wall against 

upper Lodgepole limestone on the south. West ward, 

the upthrown side gradually rises in section so that 

both walls are in Lodgepole limestone in SE1,4 sec. 3, 

and the fault can no longer be recognized. The maxi-
110 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

mum stratigraphic throw at the east end is around 

800 feet. Because the dips of beds along the fault 

average about 75° the theoretical dip slip is more than 

3,000 feet. At first glance, this fault seems to be rotational, with the actual displacement declining westward to zero within a little more than a mile. But 

eastward the displacement seems to decline to zero 

rapidly, too, as the fault, covered by Quaternary valley deposits at the east end, does not reemerge in the 

hills only half a mile away. 

The third fault mapped in this area apparently 

joins the first described one under Quaternary gravel 

in SE cor. sec. 33, T. 1 N.; R. 1 W. Trending westsouthwest, it crosses N:Y2 sec. 5, T. 1 S., R. 1 W., and 

leaves the quadr~ngle to pass und~r Cenozoic deposits 

along upper Sand Creek. Whether or not it continues 

on the west side of the creek is not known. It is exposed at the south edge of Ingleside Quarry, where 

its dip is near vertical. Its curvature nicely reflects 

the syncline it crosses but it departs far from bedding, have Lodgepole in the south wall and Mission 

Canyon in the north wall at its east edge and the reverse situation where it leaves the quadrangle. It 

appears, therefore, to be a rotational fault, hinged 

somewhere in the NEl4NWli.J, sec. 4 where both walls 

are in Mission Canyon limestone. The north wall has 

moved relatively up at the west and down at the east. 

Maximum throw at either end is a few hundred feet. 

This fault is not merely a curving continuation of 

the fault with which it merges for that fault must 

continue toward the mouth of Sand Creek to explain 

the relations between Mission Canyon and Lodgepole, 

north of the fault junction. 

Like the block faults across the river, these faults 

are not only older than the Tertiary basin deposits, 

but are also older than the basin itself. The evidence 

is the same as for the block faults: the upthrown sides 

of the faults, with equally resistant units on both 

sides, face the present valley, which was also the Tertiary valley, and must have been reduced by erosion 

(or reversal of movement for which there is no evidence) in the development of the basin. 

STEEP F .A ULTS YOUNGER THAN THE THRUSTING 

Only one fault is demonstrably younger than the 

thrusting and a second is interpreted to be younger. 

Neither . of these faults can be traced for as much 

as a mile before passing beneath Cenozoic deposits. 

Nevertheless, they may be symptomatic of a major 

stage in the t~ctonic history of the quadrangle : an 

episode of post-thrust steep faulting that did much 

to define the Cenozoic basins. That such faults, if 

present, are rarely exposed seems due to rapid retreat 

of initial scarps in the humid earliest Tertiary climate 

and to the mantling of the fault trace~ by the resulting debris. 

The single certain post-thrust fault splits the Highway thrust in the center of the quadrangle. Curving 

gently in plan, with an average trend close to due 

north, the fault is in Meagher limestone for most of 

its exposed length of 4,000 feet but at its southern end 

it drops the upper(~) part of Meagher on the ,east 

wall against Wolsey shale on the west. Its stratigraphic throw and dip slip are both on ·the order of 

a few hundred feet. Comparable displacement is also 

indicated by the vertical separation in the projected 

dip of the thrust plane. Considerable strike slip, with 

the east side having moved relatively south perhaps 

more than 1,000 feet, is indicated by lateral offset of 

the Wolsey and by drag folds. This fault is marked 

by a bare ridge of contorted limestone trending 

athwart the normal consequent drainage to the Jefferson River. A remarkable box canyon, as much as 

200 feet deep and not much wider at the top . (but 

shown on the .topographic map as simply a fla~ing 

gulch) follows the fault trace in SEl4SEli.J, sec. 32, 

T. 2 N., R. 1 E., and NEl!.t,N.El!.J, sec. 5, T. 1 N., R. 1 

E.; making a right angle bend across the fault ridge, 

the canyon widens abruptly in Wolsey shale above the 

Highway thrust only 600 feet from the Jefferson 

River. 

This fault is pre-Quaternary. It is older than old 

alluvium which covers it and it does not seem to offset any Quaternary landforms. There is no direct 

evidence of its age relations to Tertiary basin deposits 

but it seems reasonably regarded as of pre-basin age. 

The second fault ·in this category is exposed for 

half a mile near the mine workings in sec. 25, T. 3 N., 

R. 1 E. This fault, striking N. 45° E., disappears to 

the north in a poorly exposed dip slope of Jefferson 

dolomite; to the south, it passes under Quaternary 

fan gravel. Downthrown on the northwest, it brings 

Lodgepole limestone against Jefferson dolomite on the 

southeast with a stratigraphic throw of perhaps 500 

feet. The net slip may be much less because the fault 

is oblique to bedding. Exposures are inadequate to 

show whether monzonite, intrusive into the Lodgepole 

and bottoming on the fault, is offset by the fault or 

controlled by it. It is easy to imagine that this fault 

continues for several miles, more or less following 

Dunbar Creek, determining the contact between Lodgepole limestone and Tertiary deposits and, initially or 

by recurrent movement, being responsible for the abnormally high dips in the isolated mass of Dunbar 

Creek rocks in SEl!.J, sec. 3, T. 2 N., R. 1 E. The fault 

is regarded as of postthrusting age only because it 
S'I'RUCTURE 111 

resembles the one known postthrusting fault in being 

accordant with basin topography; it may even be 

post-Tertiary, but older than the Quaternary fan. 

ORIGIN OF CENOZOIC. BASIN 

When deposition of the Bozeman group began, the 

topography of the quadrangle seems to have been 

much like that of the present, with steep ridges separated by broad valleys. How that terrain developed 

is largely conjectural. Strong diastrophic movements 

must in some way have been involved in the creation 

of the Three Forks basin, whose borders cut abruptly 

across faulted and folded older rocks, whose early 

record is of interior drainage in a humid climate, and 

whose shape closely follows known fault trends. It 

is surely no accident that the basin lies on and shares 

the trend of the east-west Precambrian Willow Creek 

fault and the Jefferson Canyon thrust, and that the 

adj.oining Clarkston and Townsend basins trend at 

right angles and lie on the Sixteenmile thrust zone. 

Assuming this fundamental structural control, the basin might have developed by differential erosion of 

unresistant Precambrian rocks (and shaly Upper Cretaceous ones east of Three Forks) exposed as a result 

of structural events, or it might have originated as a 

direct expression of structure, from the downdropping 

or tilting of blocks on steep faults younger than the 

thrusting but genetically related to it. 

If differential erosion were the dominant element, 

it should be possible to detect large-scale patterns in 

which the basin in earliest Tertiary time was integrated with drainage to the sea, for vast amounts of 

rocks had to be removed. Atwood (1916) tried to 

demonstrate an erosional origin of all the in termontane basins of southwestern Montana. He concluded 

that all the basins initially developed from the dissection of an Eocene peneplane by consequent streams 

that drained southward, from a Continental Divide 

at the present eastern front of the Rocky Mountains 

to the ancestral Snake River. The "lower Bozeman 

beds," in his nomenclature, were deposited in the valleys of this stream system which was closed in late 

Oligocene time by the' "great outpouring of lava in 

the Snake River region." Subsequent drainage 

changes he ascribed in part to structural events, but 

stream erosion was named the primary agent in creating the intermontane troughs. Original drainage 

to the south was proposed to explain "exceedingly 

peculiar" drainage conditions off the present Continental Divide involving alternating basin and canyon 

stretches along major streams, barbed tributaries, 

abandoned valleys, great wind gaps, and the like. 

654729 0--63----9 

Atwood's reconstruction, though ingenious, does not 

seem adequate, . as Blackwelder (1917) early pointed 

out. A glance at Atwood's illustrations (pl. 27, fig. 

52) shows that the explanation creates about as many 

peculiar physiographic conditions as it resolves. Further, the early Tertiary Continental Divide could not 

have been as far east as postulated by Atwood, as 

J. J. Tanner 23 and H. D. Klemme 24 both showed that 

early Tertiary drainage across the Big Belt Mountains 

and Maudlow Basin was eastward rather than westward, and Billingsley ( 1915, p. 32), McMannis ( 1955, 

p. 1412) and Klepper and others (1957, p. 38-40) 

have shown that much of the Late Cretaceous and 

Paleocene Livingston. formation was derived from the 

eastward transport of detritus from the ·Elkhorn 

Mountains volcanics. 

More signific~ntly, there now appears to be little 

basis for an Eocene peneplane on which to develop 

integrated consequent drainage, to the south or in any 

direction. The principal evidence advanced by Atwood for the peneplane is the accordance of many 

summits in the Butte region. Because basins below 

those summits were then known to contain deposits 

as old as Oligocene( actually, Eocene, for the deposits 

near Lima had been described by Douglass in 1903) , 

Atwood reasoned that the peneplane formed before 

Oligocene time but after Late Cretaceous deformation, and thus was Eocene. But the considerable 

Cenozoic folding and faulting now known in the region (see Pardee, 1950, for partial summary) make it 

most unlikely that remnants of a low surface dating 

back to the early Tertiary could be preserved .in accordant summits~ The summits may signal the existence of a former extensive surface of little relief, but 

if so, it is of much later age. 

The foregoing objections to an Eocene peneplane 

are largely negative; in addition, there is some opposing positive evidence. The extensive and thick 

Beaverhead conglomerate (Lowell and Klepper, 1953), 

at least part of which is of Paleocene age and which 

is itself folded and overridden by pre-Oligocene 

thrusts, and the thick Eocene basin deposits near 

Lima and Three Forks, combine to indicate a degree 

of early Tertiary crustal unrest and surface irregularity hardly compatible with the idea of a peneplane. 

There seems no reason to assume that the Three 

Forks basin is fundamentally the product of an integrated drainage system that flowed south or west, and 

was later blocked by deformation or lava damming. 

If the basin is mainly erosional, northerly or easterly 

~Tanner, J . J., 1949, Geology of the Castle Mountain area, Montana: Princeton Univ. Ph.D. thesis (available on microfilm). 

~4 See footnote, p. 13. 
112 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

drainage is far more compatible with the apparent 

distribution and height of · the pre-Tertiary ridges, 

much like the present ones. Easterly drainage during the early Tertiary is particularly favored by the 

pattern of gra vei channels in the Climbing Arrow 

formation, and by the fact that an eastward-flowing 

stream system is known to have existed as late as 

Paleocene time, to transport debris from the Elkhorn 

Mountains volcanics to the Maudlow and Livingston 

basins, to provide the andesitic components of the 

Li~ingston formation. 

Possibly, the earliest Tertiary drainage was not 

simply subsequent on uplifted unresistant strata but 

directly consequent on topographic irregularities produced by rapid Laramide faulting. The meager direct evidence, in the form of the Sphinx conglomerate 

apron and of the lake limestone of the Milligan Creek 

formation, supports . the idea that the Three Forks 

basin was born closed. If so, its initial relief was 

primarily tectonic and not erosional. 

It will probably never be possible to decide whether 

contemporaneous deformation or simple stream erosion was the main mechanism of basin formation, if 

either was. An unspectacular but reasonable view is 

that tectonism was generally slow enough and erosion fast enough so that both played important, complexly interrelated parts in the genesis of the basin. 

POST-LARAMIHE STRUCTURAL FEATUR.ES 

However the Tertiary basin formed, recurrent diastrophism was probably responsible for its recurrent 

existence as the site of thick continental sedimentation during the dominantly warm and humid Tertiary 

climate. Often, exterior drainage systems to the east 

must have threatened capture of the basin; the long 

hiatus in the middle Tertiary record suggests that 

they sometimes succeeded. To maintain aggrading 

conditions in early and late Tertiary time, capture of 

the basin must have been defeated by diastrophism, 

for extensive and thick lava flows that might have 

dammed its natural eastern outlet .are unknown. As 

suggested by McMannis (1955, p. 1426-1427), discontinuous relative uplift of the Bridger Range during 

the Tertiary offers a persuasive mechanism for maintaining a depositional environment. The barrier, 

whether this or some other, was apparently operative 

from very early Tertiary to middle Oligocene time, 

and again in late Miocene and at least part of Plio-· 

cene time, but ineffective in t~e long interval between. 

Though the structures that influenced depo~ition of 

the basin deposits are obscure, some of the ·structures 

. imposed on the deposits themselves are fairly plain. 

The gross structure of the Tertiary rocks is that 

of an easterly tilted block that has been thrown into 

several broad gentle folds of diverse trend. Regional 

eastward dip has been noted by Peale ( 1896, p. 3), 

Pardee ( 1950, p. 365), McMannis ( 1955, p. 151), and 

others. Easterly tilt of a few degrees is shown not 

only by the dominance of dips in that direction, but 

also by the steady rise in stratigraphic position and 

age of the rocks eastward and southwestward without 

regard for intervening masses of older rocks. This 

asymmetry in the distribution of the Tertiary formations is apparent throughout the Three Forks basin. 

At the west edge of the basin, the exposed Tertiary 

rocks are tuffaceous sediments of early Oligocene age 

(Klepper and others, 1957, p. 42; Alexander 1955, p. 

76). At the east edge, as noted earlier, the Tertiary 

rocks are of late Miocene and Pliocene age. A similar 

distribution prevails in the ~djoining Townsend Valley. At the west side, the Tertiary rocks are eastward-dipping lower Oligocene strata (V. L. Freeman, 

written communication, Mar. 22, 1954). Just east of 

the Missouri River in the Toston quadrangle the 

Tertiary strata dip mainly east and are mostly of 

early ( n and middle Oligocene age (dated on the 

basis of vertebrate remains identified by G. E. Lewis, 

written communication, Mar. 12, 1957). At the east 

edge of the valley are Miocene (and Pliocene~) gravels 

(dated on the ·basis of vertebrate remains identified 

by ·G. E. Lewis, written communication, May 6, 1957) 

and · dip eastward into the west frontal fault of the 

Big Belt Mountains. 

It seems clear that the eastern edge of the Three 

Forks basin is a fault at the west edge of the Bridger 

Range; its throw has been estimated (Pardee, 1950, 

p. 379-381) as 3,000 feet or more. The same fault 

zone forms the east edge of Townsend Valley. But 

downfaulting does not explain the dips of the Tertiary strata in the Three Forks quadrangle into 

islands of pre-Tertiary rocks, because there is no 

repetition of beds on the east sides of the islands. 

Tilting after basin filling is also inad~quate to account for the eastward rise in stratigraphic position, 

though it can account for the regional dip. 

Eastward tilting, beginning after depositio~ of the 

Milligan Creek formation and continuing or recurring throughout deposition of the rest of the Bozeman 

group, with progressively shifting depositional troughs 

eastward, seems the most satisfactory interpretation. 

A consequence of such deformation is that the entire 

thickness of basin deposits at any poip.t may be only 

a fraction of the apparent total thickness derived by 

adding the thicknesses of individual units at their 

outcrops. The tilted block may have been the eastern 

flank of a huge anticlinorium trending north-north-
GEIOMORPHOLOGY 113 

east, the core of which was the Boulder batholith, long 

since crystallized, and this rising fold might have 

provided both gradient and material for the alluvial 

part of the Bozeman group. 

At the scale of the geologic map, the wrinkles on 

the tilted block are more apparent than the block 

itself. Perhaps the largest of these wrinkles is a 

northeasterly trending anticline underlying the J efferson River valley and largely concealed by Quaternary 

alluvium. Its dimensions are vague but measurable 

in miles. The few dips visible on its flanks are mostly 

under 5° but in a few places are as high as 15 °. The 

fold is doubtless partly responsible for the exposure 

of the early Tertiary rocks along the river, both by 

uplifting the rocks and by localizing the course of 

the river. How much of the persistent southeasterly 

dips in the Tertiary rocks south of Buttleman Ranch 

are to be ascribed to this anticline and how much to 

regional tilt is moot. North of this anticline the 

Milligan Creek formation is folded into an open syncline. To the east of the large anticline, another 

vaguely defined but apparently broad and low anticline trends east-west or northwest-southeast, with its 

crest emerging on the Madison bluffs about a mile 

north of Climbing Arrow Ranch. This anticline, with 

limbs dipping generally 5° to 10°, seems to plunge 

gently southeastward and disappear beneath the Madison Valley. Up plunge, it disappears under Quaternary alluvium. 

Many smaller folds appear in the Tertiary rocks 

north of the Jefferson River, but only two have been 

traced for as much as a mile along .the axis. These 

are the anticline and syncline previously mentioned in 

discussion of the Climbing Arrow formation. Most 

clearly defined is the syncline south of the Silver 

Sage Ranch, whose axis trends north-northwesterly 

across the Wi4 sec. 10, T . 2 N., R. 1 W., disappearing to north and south under Quaternary alluvium. 

The fold is asymmetric, the west limb gentle with dips 

of 10° or less, the east limb steeper with common dips 

of 15 ° to 25 °. The anticline, whose crest trends 

roughly parallel, across the WV2 sec. 10, T. 2 N., R. 1 

W., is a much gentler fold with opposing dips of 3° 

or less. 

These folds are much too large to be slump structures or the result of differential compaction of underlying strata. Possibly they are the products of regional compression but their divergent trends invite 

the interpretation that they are shallow reflections 

of local readjustments of underlying Laramide fault 

blocks. 

No faults profound enough visibly to affect older 

rocks transect the Bozeman group. Many minor faults 

are probably present, as suggested by linear trends 

athwart bedding visible on aerial photographs, but 

they could not be found on the ground. As noted 

earlier, the fault in Paleozoic rocks on upper Dunbar 

Creek may have initially or by recurrence deformed 

the Tertiary rocks, too, but the evidence is concealed 

by Quaternary gravel. If faulting occurred after 

the Dunbar Creek ·formation was deposited, the total 

displacement was too small to produce r~cognizable 

repetition of Tertiary beds on opposite sides of the 

Hossfeldt Hills. 

The Quaternary rocks are nowhere markedly deformed except for scattered slump structures on steep 

valley walls. Slight Quaternary uplift with northerly 

tilt offers adequate explanation for at least two major 

geomorphic events : the capture of the basin, formerly 

closed or draining east, by the Missouri River; and the 

development of the series of Quaternary benches. Discussion of these events and their probable tectonic 

origin is deferred to the section on geomorphology. 

GEOMORPHOLOGY 

Near the close of Tertiary time, the Three Forks 

quadrangle seems to have been an area of low relief, 

with scattered hills protruding no more than a few 

hundred feet above a graveled plain. The development of the present topography is the history of the 

dissection of that surface under a climate that has 

alternated between humid and semiarid, and of the 

exhumation of a very early Tertiary surface of mature relief, that had been buried by the Tertiary basin 

deposits. 

The main agent of dissection has been a stream system consisting of large perennial rivers, deriving their 

discharge from a persistently humid region far beyond the Three Forks basin, and their tributaries, 

varying from perennial to nearly dry as the basin 

climate varied. The former flood plains of the through 

streams remain as terrace remnants, capped by rounded 

gravels ) the courses of their tributaries are marked 

by subrounded gravel fills. In the Mud Springs 

Gulch drainage, broad benchlands that are virtually 

bare-rock surfaces are identified as pediments; apparently they have long been isolate~ from integrated 

streams and have been slowly lowered by mainly 

colluvial processes. Some of the detritus used as tools 

in dissection remains behind, evidence alike of the 

cutting processes and of their inefficiency, but the 

volume of Quaternary deposits is small in comparison 

with the amount of material removed. The dissection was episodic, presumably with long periods of 

lateral cutting-and-filling by the streams alternating 

with short ones of vigorous downcutting. 
114 . GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

Although uplift probably caused the dissection, discontinuities in uplift were not necessarily responsible 

for the discontinuities in dissection revealed by the 

steplike benches. Proximate cause for the development of most benches, and perhaps for all, may be 

differing resistance of the strata at the outlet canyon 

of the Missouri River from the Three Forks basin. 

The lowest benches may be nonorogenic. They 

may be climatic terraces or the products of base level 

changes stemming from the growth and drainage of 

glacial Lake Great Falls. The present downcutting 

in tributary valleys is of probable climatic origin and 

has continued for only a short time; the flood plains 

above incised stream courses may therefore be regarded as climatic terraces. 

HILLS 

Most of the hills and ridges of the quadrangle, 

though neither high nor extensive, are rugged, rising 

abruptly from the benchlands to summits that are 

generally 800 to 1,300 feet ab~ve the valley floors, and 

thus to altitudes ranging from 5,000 to 5,400 feet; 

a few ridges rise several hundred feet higher. Most 

summit areas are flattish, though narrow. The prominent ridges of the quadrangle are without exception 

strike ridges of Paleozoic carbonate rocks. Lower 

ridges are developed on quartz-cemented sandstones of 

the Flathead, Quadrant, Ellis, and Kootenai formations. 

Low hills of irregular shape are developed on large 

intrusive masses and on the Elkhorn Mountains volcanics. 

The hills are exhumed from a late mature early 

Tertiary topography that was much like the present 

topography. The Tertiary deposits, however deformed, faithfully follow the details of the present 

hills and valleys. The surface below the Tertiary 

deposits, wherever it is visible in stream cuts or has 

been intersected by the drill, is a smooth continuation 

of the hill slope above. There is only a small notch 

where the subaerial and subsurface slopes meet, indicating that the modern hill fronts are not only parallel to the early Tertiary ones but not far from them. 

This slight amount of retreat further suggests that 

most of the present hill slopes were buried by Tertiary 

deposits until well into Quaternary time. 

Added indication that the hills are exhumed is given 

by the Tertiary deposits t_hemselves. They invariably 

coarsen toward the hills. The lithology and attitudes 

of many border sandstones and conglomerates demonstrate that they were shed from the hill directly above. 

The coarseness of the very early Tertiary limestone 

conglomerate suggests mountainous relief. The later 

Tertiary conglomerates at hill edges are, however, 

distinctly finer grained than the modern gravels in 

similar positions, suggesting somewhat less relief than 

at present, and general lowering of relief during the 

Tertiary as the . basin deposits grew thicker and the 

summits were worn lower. This general tendency 

niay have been reversed for a time during the middle 

Tertiary hiatus. Low relief in late 'Fertiary or earliest 

Quaternary time is suggested by the fineness of the 

oldest Quaternary subrounded gravel, by the rude 

accordance of summits, and by the r<?undness of crests 

and divides. 

How the hills initially formed is unknown. Most 

likely it was by a combination of Laramide faulting 

and differential stream erosion, du:ring and soon after 

Laramide deformation. 

BENCHES 

Most of the quadrangle is a series of broad benches 

that slope toward the junction of the Madison and 

Jefferson Rivers. The benches are especially well defined south of Three Forks, just above th~ Madison 

River, where five distinct levels appear. The lowest 

is about 40 feet above the Jefferson flood plain threefourths of a mile southeast of Three Forks; northeastward, it merges with the flood plain within a mile; 

southwestward, it continues unbroken for 3 miles. A 

second bench .level is about 150 feet higher; it is prominent east of. Fairview Cemetery and between Willow 

Creek and the Buttleman Ranch. The third level, 

about 100 feet higher and 300 feet above the flood 

plain, forms the extensive benchlands between Willow 

Creek and the Climbing Arrow Ranch, and is the 

only prominent . level west of Willow Creek. About 

180 feet higher. is a fourth bench, conspicuous only 

near the Mad~son River. The fifth bench, more than 

500 feet higher, and 1,000 feet above the flood plain, 

forms the h~hest surface at the southeast edge of the 

quadrangle; this is the north tip of an extensive 

bench that continues for many miles southward into 

the Norris quadrangle. The road that leads southeast from Three Forks to Fairview Cemetery, and 

then follows section lines for 61j2 miles south, intersects the leading edges of three of these benches : the 

lowest at approximately 4,100 feet, the third at about 

4,350 feet, and the fourth at about 4,530 feet. East 

of Fairview Cemetery the east edge of the second 

bench is close to 4,240 feet ; between Willow Creek and 

the Buttleman Ranch its leading edge is slightly 

above 4,260 feet. The lower edge of the highest bench 

is not far from 5,050 feet. 

Most interbench slopes are moderate, with grades 

perhaps two or three times that of the bordering 

benches. The highest bench, however, is bordered by 
GEiOMORIPHOLOGY 115 

steep slopes throughout and for long stretches the 

faces between other benches are steep also. 

North of the river, benches are not nearly so well 

developed. Around the mouth of Mud Spring Guleh 

the two lower benehes are extensively developed, and 

the third level is prominent west of Milligan Creek, 

but elsewhere in the Mud Spring Guleh drainage and 

along upper Milligan Creek benches are numerous but 

small, much dissected, and diffieult to eorrelate with 

each other or with the extensive surfaees south of the 

river. These northern benches are irregularly rolling 

and poorly drained. 

Most of the broad benehes south of the Jefferson 

River are thiekly eapped by rounded gravel. The 

lowest bench north of the river is similarly capped, 

as is the distal end of the very broad bench above 

it. These surfaees were evidently eut by the same 

stream or streams that deposited the rounded gravels 

upon them. They are valley-plain terraces of the 

aneestral Jefferson and Madison rivers. 

The highest gravel-capped terrace seems to be in 

the lowest part o£ a gently rolling surface elsewhere 

defined by the summits of the bedrock ridges. This 

surfaee apparently had less than 500 feet of relief, 

but the eoarseness of the gravel on the terrace shows 

that mountainous terrain existed not far away. 

The old valleys filled with subrounded gravel no 

doubt represent the primary consequent tributaries of 

the ancestral river system. 

All but the lowest of the northern benches la<~k 

caps of rounded gravel, or any vestiges of such caps. 

Doubtless they are far more dissected than the southern benches owing to the lack of a p1·otective cover 

of coarse gravel. Most of these benches are cut across 

the edges of folded strata of the Bozeman group, and 

therefore they are not stratum benches as defined by 

Rubey ( 1952, p. 113). All contain patches of fan 

gravel and windblown silt, and a few are mantled 

widely by such materials. Evidently they were never 

part of the valley plain of perennial rivers nor of the 

flood plain of perennial tributaries of the sort that 

deposited the subrounded gravel. Presumably thE~Y 

are pediments initially cut by sheetflow or by intermittent tributaries, left as benches after the master 

streams and their accordant tributaries were incised 

to lower levels, and subsequently lowered by colluvial 

processes. Details of their origin and history are, 

however, hard to visualize. If slow colluvialprocesses 

have indeed been major agents in their genesis and 

subsequent lowering, a necessary implication is that 

the surfaces have be.en effectively isolated from the 

much more rapid attack of integrated stream systems. This could have been achieved if the Jefferson 

River had flowed consistently south of its present 

course in earlier times, so that the northern part of 

the quadrangle remained a divide area. Later discussion of the asymmetry of terrace distribution leads 

to this very suggestion. 

The origin of seemingly contemporaneous river terraces and pediments is one of the intriguing general 

problems not yet studied in those extensive parts of 

the arid West crossed now, or formerly, by perennial 

through rivers. . 

The manner of retreat of the fronts of both terraces and pediments seems fairly clear. The frontal 

scarps, steeper on terraces capped with resistant gravel 

than on unprotected pediments cut on soft rocks, were 

formed as the products of stream downcutting and 

maintained by stream sidecutting until a new episode 

of incision. Thereafter, their retreat has presumably 

been mostly by mass-wasting processes. Whatever the 

mechanism of retreat, it is evident from inspection 

of the frontal scarps that they have retreated parallel 

to their original slopes, for the ·scarps are of about 

equal average declivity from highest terrace to lowest 

and from highest pediment to lowest. 

STREAMS 

The Jefferson River meanders across the quadrangle 

in a broad flood plain interrupted by a short canyon 

near the west edge. The flood plain slopes from 

slightly below 4,200 feet altitude at the west edge to 

4,040 feet at the east edge; the river has a channel 

gradient of 5.6 feet per mile. No measurements are 

available of width, depth, or velocity. For a stream 

meandering on a broad flood plain, however, the Jefferson flows remarkably fast. 

The Madison River flows back and forth across the 

east edge of the quadrangle in a flood plain less broad. 

The slope of this flood plain is somewhat greater than 

that of the Jefferson, from 4,240 feet at the south edge 

to 4,040 feet at the river mouth. Unlike the Jefferson, 

the Madison has a comparatively straight but braided 

course, and its channel gradient is much steeper, 12.5 

feet per mile. No measurements of width, depth, or 

velocity are at hand; the impression is that the bankfull Madison within the quadrangle is a little broader, 

much shallower, and about as swift as the Jefferson. 

The two rivers join at the east edge of the quadrangle to form the Missouri River; above the june-

. tion is a long reach of distributaries. The Missouri 

leaves the quadrangle at an altitude of about 4,035 

feet and is joined by the ,Gallatin River a few hundred 

yards east. 

To reach the quadrangle, each river passes through 

alternating reaches of wide flood plain and of narrow 
116 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

canyon. The rivers head within a few miles of each 

other in the high humid Yellowstone Plateau and are 

· perennial from head to mouth. The Jefferson, flowing in a broad arc convex to the west, drains about 

9,200 square miles, and has average discharge near 

Three Forks of around 2,300 second-feet; the Madison, 

fl,owing pretty much straight north, drains only 2,500 

square miles and has an average discharge of about 

1,600 second-feet. (For detailed discharge data, see 

U.:S. Geol. Survey, 1953, p. 19, 71, 76; 1954, p. 40, 43, 

46.) 

Only a few of the tributaries are perennial. Willow 

Creek has a small annual flow that is maintained by 

dams upstream from the quadrangle. Segments of 

Milligan Creek and of two or three other northern 

tributaries of the Jefferson are perennial, being fed 

from springs, but their dry season flow is rarely 

greater than a trickle. Most water courses are dry 

most of the year. 

The primary and secondary tributary valleys are 

mostly flat-floored, even near their heads. Exceptions 

are the short tributaries of the Madison at the east 

edge: they are V -shaped canyons from mouth almost 

to head. 

Along much of their courses the larger tributaries 

are markedly incised in their flood plains. The Madison and Jefferson are not strikingly entrenched nor 

are the tributaries at their mouths, but beginning a 

few hundred feet upstream the tributaries are progressively more deeply incised, to depths of as much 

as 15 feet (see fig. 15). The entrenchment commonly 

persists for a few miles, then gradually dies out in 

headward areas. 

Drainage throughout the Quaternary seems to have 

had the same general pattern as now to judge by the 

distribution of older alluvial deposits, and by the deFIGURE 15.-Milligan Creek entrenched in its flood plain above Milligan ·canyon. 

tailed drainage relations of through streams, primary 

tributaries and secondary tributaries. 

In general, the present drainage pattern seems consistent enough with the interpretation that the through 

rivers and the primary tributaries developed as consequent streams with dendritic pattern in soft relatively 

undeformed Quaternary and Tertiary deposits with an 

initial northeastward slope, and that the secondary 

tributaries, including such as Dry Hollow and Dunbar Creek, are rudely trellislike subsequent streams 

controlled by exhumed Laramide structures. 

But some stream features require more involved 

explanations. The Jefferson River and each of its 

main tributaries run for a small part of their courses 

within a comparatively narrow hardrock valley or 

steep-walled canyon that flares out into broader valleys upstream and downstream. At the present level 

of exposure, soft unresistant Cenozoic rocks nearby 

seemingly offer far easier paths. As there is no indication that these restricted stretches have been subjected to localized Quaternary uplift, the ready conclusion is that the streams have been superposed from 

a continuous cover of virtually unconsolidated, unresistant Cenozoic deposits. The position of Tertiary 

outcrops and of terrace gravels suggest that such a 

blanket may well have existed to altitudes of slightly 

above 5,000 feet, adequate to account by superposition 

for all the anomalous canyon reaches within the quadrangle. 

At one or two of the canyons the tributary streams 

may at times have been unable to maintain their course 

across hard rocks. The valleys just above Milligan 

Canyon and above the Buttleman laccolith on Willow 

Creek flare broadly and some of the deposits preserved on their sides are unusually. fine grained and 

well sorted. These relations suggest that the streams 

were dammed and built lake deposits, over which they 

ultimarely flowed to complete cutting the present 

canyons. Though resembling the process of anteposition as visualized by Hunt (1956, p. 65-67) there is 

no evidence of active deformation during stream evolution, as required for anteposition. 

The aggradation that produced the present flood 

plains is presumably due to the same tectonism or 

downstream control by differential hardness that was 

responsible for the higher benches. Why flood plains 

are absent from the Madison tributaries is unclear. 

Perhaps rapid westward sidecutting by the Madison 

is responsible. In the present climate, the stream 

mouths, under the influence of the perennial Madison, 

may be migrating westward faster than their heads. 

The overall gradient may, therefore, be increasing 
GEIOMOR'PHOLOGY 117 

and any deposits that may be made after floods are 

quickly removed. 

Why the lower Madison is straight and braids but 

the lower Jefferson meanders is not clear. The two 

rivers are much alike in many respects, including 

the important ones of source area and types of rocks 

crossed, but the Jefferson has much lower average 

gradient than does the Madison, and much greater · 

discharge. To make any headway on this problem, 

far more information is hecessary on such_ vaFi-ables 

as bed load and channel shape. 

ORIGIN OF BENCHE.D T'ERRAIN 

Preceding discussion of the landscape in the benchlands and low lands has touched mostly on the development of the near-horizontal surfaces-terraces, pediments, flood plains. The purpose now is to consider 

the origin of the intervening steeper surfaces. Involved are at least four widespread sets of steep 

surfaces below benches, with heights of as much as 

500 feet, in a total vertical interval of 1,000 feet. 

The mere scale of these features suggest strongly that 

uplift was their prime cause. The nature of the uplift, however, is far from clear. Was it all at the 

start of the bench-forming episode or did it continue 

throughout bench development? If it continued, was 

it smoothly slow or spasmodic? Was it verticnJ or 

tilted? 

As to the timing and duration of uplift, continuing but spasmodic upward movement offers, of course, . 

the simplest answer: one bit of uplift for each paired 

bench. Certain simple regional evidence, however, 

opens the field to other explanations. The number 

and size of valley-plain terraces, and the height and 

continuity of interterrace scarps vary widely and 

abruptly from basin to basin in the Missouri headwaters region. In general the number, complexity, 

and vertical spacing of benches increases upstream. 

In the valley of the Missouri . above the Gates o:E the 

Mountains and below Canyon Ferry only two broad 

low benches seem to be developed, judging by a rapid 

automobile reconnaissance. In the Townsend Valley (Pardee, 1925, oP· 6-8) are three well-developed 

bench levels, in a vertical interval of about 300 feet. 

In the Three Forks basin are five main levels over a 

vertical range of 1,000 feet. In the middle Madison 

Valley above Ennis are at least six conspicuous benches 

over a range of more than 2,000 feet. This simple 

picture is complicated by the existence of many fragmentary benches that are slip-off slope terraces or 

local rock-defended terraces rather than basinwide 

valley-plain terraces ; further, some of the benches 

noted may not be stream terraces at all, but pedin1ents 

or structural benches. Nevertheless, the generality 

seems to hold. This indication of a measure of independence in bench development from basin to basin 

suggests that differential resistance of folded and 

faulted hard rocks encountered by the rivers in the 

canyons between basins may have been an important 

element in valley sculpture. Thus, steady smooth uplift or a single early upward movement might both 

have resulted in alternating benching and scarping 

!n a basin as the master streams met rocks of varying resistance in the downstream canyon. In this 

view, even the most extensive paired bench might be 

regarded as a species of rock -defended terrace and the 

whole flight of terraces visualized as the products of 

"continuous valley excavation during [variably] restrained downcutting," to modify the description of 

Davis ( 1909, p. 514-586). 

The uplift, whether smooth or jerky, seems to have 

been not vertical but tilted northward, on an axis 

somewhere north of the Gates of the Mountains, as 

suggested by the increase in number of benches from 

basin to basin upstream (southward) and the increasing total relief within the benched interval upstream. 

Several other lines of evidence of varying validity 

lead to the same idea. The course of the Jefferson 

River, to judge by the distribution of valley-plain 

terraces, has been shifting progressively northward 

throughout the Quaternary. Small remnants of paired 

benches show that the river has in the past flowed 

north of its present path but not far and not for long. 

Its present course is on its north bank across most 

of the quadrangle; presumably it would be there 

throughout if it were not superposed on limestone of 

the Madison group at the south side of the valley 

above Willow Creek. (See fig. 16.) 

These observations are unimpressive alone, but begin to be more meaningful when taken in regional 

context, for the entire headwaters region of the Missouri shows strong signs of having developed initially 

FIGURE 16.-Jefferson River superposed on limestone of the Madison. 

group, 4% miles above Willow Creek. View is due west from 

Pleistocene terraces, visible in center left. Course of river is 

marked by line of trees in middle right. 
118 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

by consequent flow down a regional northward slope 

and of having experienced northward, or better northwestward, tilting late in its history. Initial northerly 

slopes, prior to superposition, are suggested by the 

overall north-pointing dendritic pattern of the master 

streams and primary tributaries. Northwesterly tilting is favored by the persistent tendency of the 

through rivers, where in broad valleys, to flow against 

their western and northern valley walls, clearly shown 

by examination of 15-minute and 7lj2 -minute quadrangles in the region, but not very satisfactorily on 

smaller scale maps. 

The dips of benches seem to support the idea of 

northerly tilt. The benches in the quadrangle are 

graded to the Jefferson and Madison Rivers and their 

initial surface dips were presumably about the same 

in amount though differing in direction. The impression is strong that the benches north of the Jefferson 

River now have lower dips, ~n the undissected parts 

of their surfaces, than do those south of the river. 

But measurements testing the impression have not been 

made and are probably not feasible, owing to the considerable dissection of the unprotected pediments that 

dominate the northern benches. 

Perhaps the greatest virtue of the concept o·f continuing or repeated northerly tilt during the Quaternary is that it accounts for the capture of the eastward-draining Tertiary basin by the present northward -draining system. 

Although all the terraces can be rationalized as the 

product of continuing or recurrent uplift, possibly 

complicated by differential resistall\Ce of rocks in outlet canyons, at least one other ide)t of origin should 

be considered. The terraces were formed coinci- ; 

dentally with some of the ma~or climatic changes of 

the Quaternary and perhaps some of the terraces are 

climatically controlled. Given enough initial uplift, 

it is not inconceivable that climatic change dictated· 

the entire sequence of alternate downcutting and sidecutting, with differ~ntial resistance at outlets accounting for the differences in terrace development from 

basin to basin. Climatic terraces (Bryan, 1925) result 

when flood plains built during periods of relatively 

humid climate are cut into as the climate shifts 

toward aridity, protective vegetation dies out, and 

gullying ensues. The concept has often been invoked 

to account for low terraces produced by the "Recent 

epicycle of erosion," first recognized by Bryan and 

thoroughly described by Hack ( 1942), in the arid 

Southwest. Whether the idea can be applied to features as large as the high terraces is conjectural. 

Whether or not climate played an important part in 

forming the high terraces, it probably controlled incision of the flood plains of present tributary streams. 

A marked shift toward aridity during the past century 

is well documented in Western United States, presumably including Montana (see Bryan and Hack fdr 

data), and the present trenches have probably been 

cut entirely within that time. The perennial rivers, 

not much affected by the climatic change because they 

derive their flow from the persistently humid Yellowstone Plateau, have not become entrenched, and have 

limited the entrenchment of tributaries near their 

mouths. 

Conceivably, some of the minor low terrace remnants may be neither tectonic nor climatic in the 

direct sense, but related to the growth and drainage 

of glacial Lake Great Falls (Calhoun, 1906). Glacial 

Lake ·Great Falls, ice-dammned at the north and 

penned by the Highwood, . Big Belt, and Little Belt 

Ranges on the south, maintained itself with an upper 

surface on the order of 800 feet above the prelake 

channel of the Missouri long enough to produce recognizable shoreline features and, partly drained, had 

another steady stage at about 400 feet above the prelake channel. Such lakes certainly must have been 

effective temporary base levels for the stretch of the 

Missouri just upstream, but it is uncertain that even 

deep lakes existing for no 1nore than a few hundreds 

or thousands of years could significantly affect the relation between downcutting and sidecutting almost 

150 miles upstream and across several reaches of hardrock canyons. 

GEOLOGIC HISTORY 

Whatever has seemed worth saying about the origin 

and history of each element of the geology of the 

Three Forks quadrangle has been said earlier in this 

report, but under topical headings. The object of this 

chapter is to place all the main known and inferred 

deposits and events in their chronologie order. Many 

uncertainties and gaps in the record, brought out previously, are here arbitrarily resolved and filled, to 

smooth the story. 

The known geologic record of the area begins in 

early Precambrian time with th~ piling up of thick 

arkosic sand and gravel in shallow marine or continental waters. Mafic lava occasionally poured out on 

the clastic sediments and was injected into them as 

sills. This sequence was indurated, folded, and recrystallized to form gneiss from the sandstone and 

amphibolite from the igneous rocks. The metamorphic 

cycle ended with the formation of small masses of · 

granite pegmatite and of vein quartz. 

By late Precambrian time the area was part of a 

large low landmass contributing fine detritus to the 
GEOLOGIO HISTORY 119 

shallow sea not far to the north and west in which 

the late Belt rocks of the Missoula group were accumulating. By Newland time, or perhaps as early 

as Chamberlain time, the crystalline rocks along the 

southern edge of the quadrangle began to rise rapidly 

along an east-west fault zone at about the latitude of 

Willow Creek village-the Willow Creek fault-and 

shed coarse feldspathic detritus northward to intertongue with the fine deposits that continued to thicken 

farther north. Repeated recurrence of movement on 

the fault zone produced coarse arkosic sand and gravel 

-the North Boulder formation-throughout Newland 

and probably well into ·Greyson time. As part of 

the orogenic disturbance, small masses of diabase 

were ~rupted near or at the surface, and fragments 

worn from the diabase were incorporated with the . 

sand and gravel that accumulated to thicknesses of 

more than 4,000 feet. As faulting proceeded, hematitic phyllonite was produced in the gneiss along the 

fault zone. 

Before Spokane time, movements on the Willow 

Creek fault zone apparently ceased. The landmass 

·was reduced and only fine detritus was shed into the 

Spokane sea. Little or none was deposited south of 

the fault z6ne. The shallow sea may have persisted 

long after the end of Spokane time and the other 

formations of the Missoula group above the Spokane 

may have been deposited in this area. 

In very latest Precambrian or Early ·Cambrian 

time, the sea retreated northward or westward and 

a long period of subaerial erosion followed. The retreat and erosion were probably due to another uplift on the Willow Creek fault zone. A low rolling 

surface was -carved that transgressed progressively 

older Belt rocks southward and extended eventually 

across the fault zone and onto the gneiss and amphibolite mass. 

Across this surface the Middle Cambrian sea advanced slowly from the west eroding Belt and crystalline rocks alike, dropping a veneer of coarse quartz 

sand-the Flathead sandstone-near shore and around 

the bases of low islands, and thickly spreading· the 

clayey and silty waste-the Wolsey shale-farther 

from shore and in places atop surfa-ces too high to 

be mantled by sand. In areas bypassed by currents 

carrying clastic debris, thin layers of limestone were 

deposited. 

~s the shoreline gradually transgressed far beyond 

the quadrangle layers of limestone became thicker and 

more numerous, terrigenous strata thinner and fewer 

until a thick sequence of largely carbonate beds-the 

Meagher limes.tone-was formed. 

The shoreline retreated briefly in latest Middle Cambrian time and fine-grained clastic sediments were 

again washed into the area to form the Park shale. 

In Late Cambrian time eastward transgression of 

the shoreline was resumed. Conditions much like 

those of Meagher time were restored and the thick 

Pilgrim limestone was formed. 

The region began to rise gently in very Late Cambrian time and the sea to withdraw. Thin regressive 

shale, siltstone, and clastic limestone-the Dry Creek 

shale and Pebbly limestones of Peale-were probably 

deposited at this time but were stripped or so deeply 

weathered that their original character disappeared 

in the ensuing vast interval of Ordovi-cian, Silurian, 

and earlier Devonian time in which the area stood 

close to sea level, receiving no new deposits and losing very little of its old ones. 

In Middle or Late Devonian time the sea again 

began to advance across the area from an unknown 

direction, but probably the west, reworking the 

weathered Dry Creek strata and redepositing the 

debris in hollows, as the basal mudstone and sandstone of the Maywood formation. As transgression 

continued, the waters cleared and quieted. Chemical 

carbonate deposition began to prevail over clastic 

deposition and the upper limestone of the Maywood 

formed. 

Later, in the early part of Late Devonian time, the 

sea continued shallow, but persistently agitated, and 

the largely clastic petroliferous Jefferson dolomite 

was deposited. 

Well into Late Devonian time, the body of marine 

water overlying the area, already in limited communication with the open sea, was cut off from the sea, 

and the basal muddy evaporitic beds of the Three 

Forks shale resulted. 

When restricted communication with the sea was 

restored, the dark pyriti-c shale and thin muddy red 

limestone of the bulk of the Three Forks shale were 

laid down. Renewed transgression at the end of Devonian time ended these semieuxenic conditions. Along 

the advancing shoreline the upper orange siltstone and 

sandstone of the Tl}.ree Forks shale were laid down 

contemporaneously with dark shale offshore. 

As the sea advanced farther, clastic deposition in 

the area virtually ceased and limestone, mostly of 

chemical origin, was laid down. A time of gentle 

oscillations is represented by the thin-bedded Lodgepole limestone. Extremely stable conditions later led 

to formation of the thick-bedded Mission Canyon 

limestone. 

Shortly after the Mission Canyon was deposited, 

the area emerged from the sea, probably due to east-
120 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

erly tilting in this region, leaving a small restricted 

depositional basin-the Big Snowy basin-to the 

northeast. The exposed Mission Canyon strata in 

the Three Forks area were deeply weathered. Cave 

systems developed in the upper part of the Mission 

Canyon and a . red regolith formed at the surface. 

Solution- and collapse-breccias eventually filled the 

caves, probably not long after they were formed. 

Very near to the quadrangle, perhaps locally within 

it, the solution networks rose to the surface, producing karst topography. Later, the Big Snowy sea 

began to spread westward, eventually filling and leveling the irregularities on the eroded Mission Canyon 

surface with a heterogeneous assemblage of nearshore 

mudstone, sandstone, and carbonate rocks-the Big 

Snowy formation. 

Another episode of uplift, late in Big Snowy time, 

led to deep erosion of the Big Snowy rocks in most 

of the quadrangle, and their complete removal from 

the west-central part. 

In :r:niddle or late Carboniferous time, transgressive 

marine conditions were reestablished. The Amsden 

formation was deposited, beginning with muddy red 

strata derived from the pre-Amsden erosional mantle 

and later, as the water cleared, passing into carbonate 

rocks and subordinate pale quartzitic sandstone. In 

late Carboniferous and possibly early Permian time, 

sandstone deposition prevailed over limestone, and 

the Quadrant formation resulted. Later, but still in 

the early Permian, dominantly regressive shallow 

marine conditions became the rule, and the cherty and 

phosphatic sandstone, siltstone, and dolomitic limestone of the Phosphoria formation developed. Regression was probably southward due to gentle uplift 

on the north, perhaps on the Sweetgrass Arch. 

Before the start of Triassic time, the sea had receded south of the quadrangle and did not return 

again, apparently, until Middle or Late Jurassic time. 

The area was slightly emergent during at least part 

of this interval and chert fragments worn from the 

Phosphoria became incorporated in the base of the 

succeeding Ellis formation, a heterogeneous complex 

· of clastic limestone, sandstone, and conglomerate laid 

down under unstable shoreline conditions. The Ellis 

sea had advanced from the east; it retreated toward 

the east, still within Jurassic time. - In its wake, 

streams eroded lowlands rising slowly to the west and 

laid down the continental deposits of the Morrison 

formation in latest Jurassic time. A surge of more 

rapid uplift to the west in Early Cretaceous time 

produced the basal coarse continental sandstone of 

the Kootenai formation. Sluggish ·streams deposited 

fine-grained strata during most of Kootenai time; 

occasionally, they were rejuvenated and laid down 

coarse sandstone. 

In Late Cretaceous time the sea again invaded the 

area, depositing thick shale and sandstone of the 

Colorado formation and probably younger units. 

There were probably several advances and retreats 

of the sea during very late Cretaceous time, but their 

record is not preserved in the quadrangle. 

The deformation that drove the Late Cretaceous sea 

from the . area for the last. time marked the local start 

of the Laramide orogeny. This deformation was 

probably in the · form of gentle folding along northsouth axes. The rising folds were deeply eroded, and 

on the irregular surface thus formed the Elkhorn 

Mountains volcanics were erupted in Late Cretaceous 

time. · Compression continued and the folds became 

asymmetrically steepened or overturned toward the 

east. 

As the folds developed they provided space for a 

host of igneous intrusions of calcalkaline composition: 

mostly andesite, dacite, latite, monzonite, and quartz 

monzonite. Most of the intrusions came in as thin 

sills in the shales of the Wolsey, Three Forks, Amsden, and Kootenai formations. Some . made larger 

masses : dacite and andesite formed large laccoliths 

in shale; monzonite formed one co:mpl~x semiaccordant pluton that mostly displaces Paleozoic carbonate 

rocks; and large pluglike bodies of andesite and latite 

were intruded into late Mesozoic rocks. 

After folding, and probably during the long inter~ 

val of intrusion, a broad system of thrust faults-the 

Sixteenmile thrust zone-developed diagonally across 

the quadrangle from northeast to southwest. On the 

thrust system, dipping 30°-50° northwesterly, rocks 

as old as the Belt series were pushed a few miles relatively eastward over rocks as young as the Kootenai 

formation. Thrusting was accompanied by complex 

block faulting in one segment of the upper plate and 

by steep faulting in a lower plate crushed against the 

Precambrian crystalline rocks. In late Paleocene or 

early Eocene time, many block faults possibly provided the framework of the Tertiary basin, but only 

two minor faults of the sort required are known. 

Others, if they exist, are concealed by the Cenozoic 

debris they gave rise to. 

While deformation and intrusion were proceeding 

at depth, an eastward drainage system developed on 

the surface. The Elkhorn Mountains volcanics were 

deeply eroded and the fragments carried to the Maudlow and Livingston basins where they were deposited 

as part of the Livingston formation. 

Diastrophism, probably the first uplift of the 

Bridger Range, interrupted eastward drainage in mid-
ECONOMIC GEOWGY 121 

dle or late Eocene time, and a brief episode of interior 

drainage ensued. At an early stage the basin floor 

was dry, and limestone gravel that became the Sphinx 

conglomerate was deposited as an alluvial apron 

around many hills. At this time, or perhaps somewhat later, olivine basalt plugs were intruded. A lake 

developed in the southwestern part of the quadrangle 

and the Milligan Creek formation accumulated in it. 

The lake .received not only limy muds offshore and 

coarse hill wash nearshore but also much volcanic ash, 

blown into the area from distant vents. 

Elsewhere in the quadrangle, streams continued to 

carry material eastward, but perhaps slowed or 

blocked by the rising Bridger Range began to deposit 

gravel and sand in their channels and montmorillonitic 

mud in overflow ponds on the flood plains. These deposits-the Climbing Arrow formation-were largely 

detritus from the ancestral Elkhorn, Madison, and 

Tobacco Root Mountains, but were partly contemporaneous volcanic ash. After the Milligan Creek lake 

was filled or breached, fluviatile Climbing Arrow rocks 

began to accumulate thickly in the southern part of 

the quadrangle, too. 

Climbing Arrow sediments cmitinued to pile up until well into early Oligocene time. Then by a combi- · 

nation of eastward tilting in the area and further 

uplifts to the east, another basin like that of Milligan 

Creek time was created but with its center farther 

east, outside the quadrangle. In this basin, occupied 

only intermittently by lakes, grew the Dunbar Creek 

formation, formed mostly of airborne volcanic ash 

with only small amounts of evaporitic limestone and 

of streamlaid ash and mountain waste. 

. Continued eastward tilting shifted the area of deposition out of the quadrangle, and by middle or late 

Oligocene time it was no longer receiving significant 

volumes of deposits. Beginning in middle or late 

Oligocene time and continuing intermittently until 

late Miocene time, easterly exterior drainage resulted 

in the removal of large volumes of Eocene and Oligocene deposits. 

In late Miocene and early Pliocene time, a closed 

basin like that of Dunbar Creek time again developed, 

but centering east of the quadrangle. In it the Madison Valley formation of Douglass (1908) was deposited, largely from fine pyroclastic materials and under conditions reminiscent of the Dunbar Creek formation. The quadrangle was apparently on the western margin of the Madison Valley basin and received 

coarse clastic channel, fan, and delta deposits during 

late Miocene and early Pliocene time. By late Pliocene time the Madison Valley basin, and probably the 

entire Three Forks basin as well, was filled, and sluggish exterior drainage to the east was reestablished. 

The Three Forks quadrangle was a graveled plain 

with a few low hills rising above it. 

In latest Pliocene or earliest Quaternary time a 

northerly draining consequent stream system , developed, probably initiated by uplift with northwesterly 

tilt in the ranges to the south, and the formerly eastward-draining or closed basin became part o{ the 

Missouri River system. The ancestral Madison and 

Jefferson rivers and their primary tributaries cut 

rapidly down through the soft Tertiary rocks and 

became superposed on the rugged pre-Tertiary surface. The cutting- proceeded in a series of giant terrace steps controlled by discontinuities in the rate of 

tilting, or by varying resistance 9f rocks encountered 

in· the downstream canyon outlet, or by major climatic 

changes, or varying combinations of these. Rounded 

gravels were deposited on the terraces by the master 

streams, and subrounded gravels in the tributary valleys. In dry intervals, in very late Pleistocene and 

Recent time, thin deposits of fan gravel and windblown silt formed on the benchlands. The present 

flood plains were built mostly in relatively humid 

Quaternary time. In the past century a semiarid 

climate developed ; vegetation died out, and the flood 

plains of tributaries were deeply trenched, becoming 

climatic terraces. Little affected by the changing 

climate, because supplied by the persistently humid 

Yellowstone Plateau, the master streams did not become entrenched. 

ECONOMIC GEOLOGY 

CONSTRUCTION MATE.RIALS 

The flood plains of the through rivers offer endless 

sources of gravel and sand, though much testing 

would be necessary to outline suitably large bodies of 

these materials that would meet the requirements for 

construction of large dams and heavy-duty roads. A 

few small gravel and sand pits have been opened but 

none have produced large tonnages to judge from the 

small size of the pits, and none was in continuous 

operation during the period 1952-57. Pits at floodplain level are of course somewhat undesirable because 

of the danger of recurrent flooding by seepage or 

overflow, and the necessity of lifting the materials. 

More economic sources for sand and gravel would be 

older stream materials of Quaternary or even Tertiary 

age, exposed in the scarps between benches. Little 

sand, however, is likely to be preserved at bench 

fronts and sand is better sought on the flood plains. 

A few desultory attempts have been made to develop 

the higher gravels but were quickly abandoned, pos-
122 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

sibly due to lack of demand rather than to poor 

quality. 

· Two main ingredients in cement manufacture, limestone and quartz s~nd, are availaqle in large quantities of high purity. Parts of the Paleozoic limestones, especially the Mission Canyon limestone, offer 

unlimited amounts of cement-grade limestone, and 

much of the white quartzitic sandstone of the Flathead and Quadrant formations could supply suitable 

sand. 

The best local source of riprap is probably the 

brown sandstone of the Ellis formation because of its 

natural tendency to separate into blocks with dimensions of a few feet. The most favorable localities are 

east of Milligan Creek. With less predictable splitting properties, the sandstones of the Flathead and 

Quadrant are not so promising but should be suitable. 

The bentonitic clays of the Bozeman group appear 

to have swelling ratios and other properties far below 

specifications for permanent canal linings, but might 

be useful tern porary sources in emergencies. 

The white tuffaceous siltstones of the Bozeman 

group contain too many impurities in the form of 

· admixed sand and thin interbeds of other materials 

to be promising sources of commercial pumicite. 

GLASS SAND 

Very small fractions of the sandstone of the Quadrant and Flathead formations may be potential sources 

of glass sand. At the south end of the hill of Quadrant that occupies Elj2NE1M sec. 5, .T. 2 N., R. 1 E., 

is a small quarry reported to have produced glass 

sand. It was not in operation during any of several 

visits in the period 1953-56. However pure the sand 

from these formations may be, it is invariably cemented enough so that some grinding would be necessary to prepare it for use as glass sand. 

MICA 

A little book mica occurs in one area of Precam-

-brian gneiss. The largest books of muscovite are 8 

inches across and 3 inches thick. Some books of biotite are even broader and thicker. Most of the mica, 

however, is in much smaller aggregates. The mica 

is intimately associated with both granite pegmatite 

and vein quartz that cut the gneiss. Mica-bearing 

pegmatite is exposed in a single zone in SEl!.t,SElM 

sec. 9, T. 1 S., R. 1 W., near the base of the Flathead 

sandstone. The zone, trending N. 86° W., makes a 

small angle with the trace of the Flathead so that at 

its west end it passes below the Flathead. Book mica 

occurs notably only at this end, where it is exposed 

sporadically by a shallow trench 150 fe~t long, trending N. 15° W., and by a few small pits nearby. No 

mica books crop out elsewhere but the associated pegmatite continues eastward for 600 feet. A vein of 

milky white quartz that lies next to the mica and the 

pegmatite in the trench is traceable by prominent 

outcrops and float for at least a mile farther eastward. 

The trench exposes a small anticlinal fold, plunging 

steeply about N. 50°-60° W., in gneiss. The schistosity, striking N. 85° W. and dipping 80° S., trends 

unbroken across the crest of this fold. The crestal 

zone is intricately brecciated; among horses of gneiss, 

are masses several feet long of coarse biotite, muscovite, fel¢lspar, granite pegmatite, and milky quartz. 

The distribution of mica seems to be controlled by the 

schistosity rather than by the fold. 

There is no information that any mica has been 

shipped. 

PHOSPHATE ROCK 

The phosphatic sandstone and shale of the Phosphoria formation are mined in many parts of southwestern Montana. The Phosphoria in the Three Forks 

quadrangle, however, is dominated by chert and limestone and contains little phosphatic material. A few 

strata, especially in the Milligan Creek sector, are 

seemingly of commercial grade but are far too thin 

and impersistent to warrant mining. 

IRON AND MANGANES.E 

Small bodies of hematite and related iron and manganese oxides crop out at many places in the quadrangle. All have been prospected without leading to 

production. They are of two main types: red sedimentary deposits of hematite-rich sandstones at the 

base of the Amsden and Big Snowy formations; and 

reddish-black hydrothermal deposits of mostly specularitic. iron oxides and sooty black manganese oxides, 

with associated silica, in limestone and; rarely, quartzite at and near contacts with monzonitic. intrusive 

rocks. In none is the body of ore minerals more than 

a few feet thick or continuous along strike for more 

than a few tens of feet. None has been followed for 

more than a few tens of feet downdip. 

COPPER 

No minable copper deposits are known in the quadrangle and no c~pper production has been recorded. 

Along the northern and eastern edges of the 10N pluton in both the monzonite and intruded carbonate 

rocks are many small showings of copper, largely in 

the · form of surface stains of blue and green copper 

salts. All have been prospec.ted. Copper sulfides 

appear in only one prospect, in north-central part 
EC.ONOM~C GEOLOGY 123 

sec. 26, T. 3 N., R. 2 E. (See pl. 1.) Here, a shear 

zpne, 3-5 feet wide, striking N. 88° E. and dipping 

83 ° N. at the contact between quartz monzonite and 

a xenolith or pendant of limestone, has been explored 

by a deep shaft. To judge by material on the dump 

beside the shaft, the zone is mineralized with a little 

chalcocite, covellite ( ~) , and magnetite. The sulfides 

are thinly coated with malachite, azurite, chrysocolla, 

and a moderate yellowish-green mineral, in radiate 

clusters of curved brittle fibres, that is probably a 

copper salt. Monzonite of the footwall is for the most 

part remarkably fresh; locally, it is chloritized. The 

hanging wall is shot with veinlets of cross-fibre carbonate minerals. 

At the Copper City workings southeast of the center of sec. 25, T. 3 N., R. 2 E., many hundreds of feet 

of shafts and drifts have been driven but there is no 

sign of copper ore . beyond faint copper-salt stains on 

any of the dumps or outcrops, and no record of production. The prospect seems to have been last worked 

in 1949 and the claims patented by Mr. Herbert Dunbar, a rancher resident near Three Forks, as of July 5, 

1950. The shaft, well timbered, had a hoist stuck 

about 50 feet down during several attempts at examination so I was unable to go underground. A few 

outcrops and material on the dumps indicate that 

the workings are in recrystallized white Lodgepole 

limestone, permeated with dikelets and with irregularly shaped masses of hornblende- and biotite-bearing monzonite. Loc.ally, the limestone is altered to 

spotted calc-silicate rock, as described previously. 

OUTLOOK FOR FUTURE DISCOVERIES OF ORE 

DEPOSITS 

By current standards, the Three Forks quadrangle 

is an unpromising place to seek ore deposits. But an 

intriguing idea offers hope of discoveries in the distant future when outcropping ore bodies have been 

exhausted and new discoveries, if they are to come 

at all, must come from locating completely buried 

deposits. The idea follows from the now commonplace observation that individual ore deposits tend 

to be controlled by loc.al structures in the country 

rocks, and that ore districts are controlled by larger 

scale structures. Pre-ore and intra-ore folds and, 

especially, faults of ·suitable orientation have commonly determined the loci of minable deposits. Especially favorable sites for ore deposition are the junctions of two or more sets of faults or persistent 

fractures of appropriate age. 

The ore deposits on the east flank of the Boulder 

batholith, as shown by many workers (for example, 

Billingsley, 1915; Pardee and Schrader, 1933), are 

closely associated in time and space with the monzo- · 

nitic intrusive rocks of the batholith. Appearing 

within intrusive rocks as well as in the surrounding 

rocks, the ore deposits are somewhat younger than 

the intrusions. The intrusive rocks were emplaced 

during and after Laramide folding. Their relation 

to major faulting is not clear, either locally or regionally, but is probably an overlapping one; in at least 

one place in southwestern Montana-the Willis quadrangle (W. B. Myers, oral communication, 1957) -a 

large Laramide monzonitic intrusion definitely cuts 

a major thrust. Most workers (for example, Klepper 

and others, 1957, p. 44, 60) regard the main episode 

of intrusion as later than the main period of folding 

and faulting. Thus the ore deposits of the region 

were probably deposited late in or after a major interval of faulting. 

It is,. then, conceivable that the juncture between 

the north-south Sixteenmile zone of Laramide faulting, and the east-west Willow Creek fault zone, was 

the locus of an ore district that is concealed by the 

Tertiary and Quaternary deposits of the Three Forks 

basin. 

How might this hypothesis be tested~ Random 

drilling through the Cenozoic cover, even though it 

probably averages little more than 1,000 feet thick in 

the target area, would be prohibitively expensive. 

Perhaps geochemical prospecting methods could be 

employed. From time to time, deep water wells and 

oil tests are drilled in the basin areas. At such times 

cuttings and fluids might be tested by available quick 

and cheap field tests for metallic anomalies, for it is 

well known (Hawkes, 1957, p. 267-289) that geochemical halos migrate upward into younger overburden. If striking anomalies appear they might 

form the basis for a drilling program. Surface or 

subsurface geophysical approaches might also be tried, 

but would seem less promising in view of the thickness of the cover. 

PETROLEUM POSSIBILITIES 

The geology of the Three Forks quadrangle, and 

its regional setting, has not encouraged petroleum 

exploration, and as a result its oil and gas possibilities are largely untested. No experienced exploration 

company has ever drilled a test well in or near the 

quadrangle, and locally sponsored tests, of which 

seven are recorded (see pl. 2), have yielded little information other than some terse drillers' logs with 

highly questionable stratigraphic interpretations or 

none. (I am indebted to C. E. Erdmann, of the Geological Survey, for copies of these logs.) None of 

these tests was as deep as 2,300 feet, three were less 
124 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

than 1,000 feet deep, and all were collared in Cenozoic deposits, so that in the absence of properly made 

·Jogs it is impossible to determine what pre::.Tertiary 

formations were eventually penetrated, if any. 

About the only encouraging element from the standpoint of petroleum is. the rapid thinning and disappearance, by wedgeout and erosion combined, of the 

Big Snowy formation within the quadrangle. The 

Big Snowy is a producing interval farther east in the 

State, and in the adjoining Toston quadrangle many 

of its outcropping strata ,are notably petroliferous. 

The associated solution breccia zone in the upper part 

of the Mission Canyon limestone has evident possibilities as a reservoir but there is no positive reason 

to think it acts as one in this region. 

Parts of the Radersburg syncline offer the only 

places where the Big .Snowy might be tested at reasonable depth and with reasonable hope of avoiding unfavorable structural complexities or large intrusions. 

Such places are upsection from the Big Snowy outcrops in the Milligan Creek sector, but well away 

from the andesite masses. There is little reason to 

suspect that this part of the Tertiary basin is fault 

controlled, so that tests might start in Cenozoic deposits. 

Attempts to test the Big Snowy beneath the Lombard thrust within the quadrangle would seem inadvisable because the formation is presumably involved 

in unfavorably steep folding. Northeast of the quadrang!~, where the remnants of Big Snowy rocks have 

moderate to low dips in certain thrust slices, the situation is perhaps a little more inviting. 

- Worth noting here is a thin zone of hydrocarbon 

enrichment in bleached Jefferson dolomite that might 

be called a dried oil seep. The occurrence is on the 

north wall of the west-draining gulch in NW1;4NW1;4 

sec. 36, T. 3 N., R. 1 E., half a mile south of Copper City. The zone appears as a vivid irregular 

dark streak in a low-dipping fracture in white limestone of the Jefferson dolomite, bleached (and dedolomitized) due to intrusion of the 10N pluton. For 

a length of about 100 feet, the limestone for 8 inches 

to 2 feet on both sides of the fracture is darkened by 

hydrocarbons. In places hydrocarbon rich "dikes" 

extend as much as 4 feet above and below the central 

fracture. Perhaps this is an orthodox seep, but the 

setting suggests that it is a place where hydrocarbons 

driven away from the Jefferson during intrusion 

seeped backed down after the intrusive and the country 

rocks cooled. 

MEASURED SECTIONS IN PRE-TERTIARY ROCKS 

As complete a section as practicable waas measured 

in each area of pre-Tertiary rocks : Section A in the 

Milligan Creek sector; seCtion B, Mud Spring Gulch 

sector; section C, Hossfeldt Hills sector; section D, 

·Willow Creek sector. Each section - is located on 

plate 2. 

Total formation thicknesses were measured by 

planetable methods, except for the Mission Canyon 

and North Boulder formations, the thicknesses of 

which were estimated from a series of map measurements. Thicknesses of lithologic units within each 

formation were measured with steel tape and Brunton compass. Within :formations, taped thicknesses 

are not corrected for curvature or bedding slip. Total 

formation thicknesses are adjusted to an arbitrary 

_ round number to allow for structural thickening and 

at the same time give a figure which is both reasonable and easily handled in discussion. For convenient 

reference, each measured interval has been given a 

number, prefixed by the symbol for its ~ection (A, 

B, C, or D) ; each number also represents a specimen 

collection. 

The Milligan Creek area offers much the best conditions in the quadrangle for measuring the pre-Tertiary section; elsewhere, in fact, confidence in measurements is weakened because of steep · dips and other 

structural complications. Accordingly, section A is 

more detailed and, in general, more accurate than 

sections B, C, and D. 

In section A, effort was made to record in the field 

the following features for the main rocks in each 

measured interval : common name; fresh color; weathered color if much different from fresh; type of stratification and of parting if prominent, based on McKeeWeir scale ( 1953) ; degree of roundness; grain size, 

based on Wentworth scale for clastic rocks and on 

resolution with 10-power hand lens for crystalline carbonate rocks; composition; and any special characteristics useful in identifying the unit. In sections 

B, C, and D, much of the petrographic detail is omitted or generalized. 

SECTION A 

Above Jefferson Canyon thrust, west-central part of 

quadrangle, in the drainage of Milligan Creek. From 

upper part of North Boulder formation to large andesite intrusive in lower part of the Kootenai formation. Formations from the North Boulder through 

the Lodgepole are represented by essentially continuous measurements which begin in the bed of the 

large creek that flows south along the east edge of 

sec. 10, T. 1 N., R. 1 W. Here, the beds strike N. 75°-

800 W. and dip 30°-35° N. Upsection above the 

Wolsey strikes swing more northerly and dips decline 

slightly so that in sec. 2 the strike averages N. 85° 

W., and dips are 25°-30° N. 
MEASURED !SECTION'S IN PRE-TERTIARY ROCKS 

Morrison formation-Continued 

and medium light gray; irregularly thin 

to thick bedded; a few lenses, as much as 

2 ft thick, of argillaceous microcrystalline 

grayish-olive limestone. In lower 40ft, a 

few beds 1-2 ft thick of yellowish-gray 

125 

Unit thickness 

(feet) 

Above the Mission Canyon the measured section 

shifts 3 miles northeast, where the remaining measurements are in two segments. The first begins at 

the top of the Mission Canyon and goes to the base 

of the Phosphoria across the NE corner of sec. 25, T. 

2 N., R. 1 W. and into the SE corner of sec. 24. 

Beds in the lower 200 feet of this segment (Big Snowy 

and Amsden<·formatiori.s) are somewhat crumpled and 

the reported thicknesses may be greatly in error; an 

average strike of N. 40° E. is assumed. Di:r,:>s, except 

in the crumpled zone, are near 30° N. The second 

segment begins 3,200 feet west of the upsection end 

of the first, and extends from the base of the Phosphoria into the lower part of the Kootenai; . in the 

Phosphoria and Ellis the strike is N. 70° E., swinging 

easterly in the Cretaceous formations to N. 85° E.; 

dips in the lower part average 32° N., flattening to 

about 29° N. in the upper part. 

calcareous siltstone _________________ -_- 90 

Andesitic intrusive. 

Kootenai formation: 

A-62. Poorly exposed. Sandstone and siltstone, 

thin-bedded, reddened and hornfelsed 

near plutonic contact. Main rock types, 

in scattered outcrops and float, are 

moderate- to pale-yellowish-brown calcareous subrounded fine sandstone; yellowish-gray salt-and-pepper subrounded 

medium sandstone, weathering grayish 

orange; dark-yellowish and reddish-brown 

siltstone ____________________________ _ 

A-61. Sandstone, thick-bedded, faintly laminated. 

Basal 3 ft is grayish-orange calcareous 

limonitic subangular medium sandstone, 

like that in the Morrison formation 

(A-60), grading into yellowish-gray 

medium subrounded calcareous salt-andpepper sandstone, with scattered grains 

of orange earthy limonite and chips of 

shale; a few thin lenses of pebble conglomerate. Lower 35 ft forms ledge ___ _ 

Total adjusted thickness __________ _ 

Morrison formation: 

A-60. Siltstone and fine subrounded sandstone, 

medium-bedded, yellowish-gray, calcareous, with a few lenses as much as 2ft 

thick of mottled reddish-brown, olive-gray 

argillaceous microcrystalline limestone. 

In lower 30 ft are several· thin beds of reddish-brown mudstone. Light-olive-gray 

limestone, like A-59, in beds 2-3 ft thick, 

at 15ft and 40ft from top _________ ___ _ 

A-59. Limestone and siltstone. Three beds, each 

2ft thick, of light-oliYe-gray microcrystalline limestone, separated by 4-ft beds of 

yellow calcareous siltstone ____________ _ 

A-58. Mudstone, like A-58 below but thin bedded_ 

A-58. Mudstone. Mainly reddish brown, but in 

many places grades laterally and vertically into light olive gray, yellowish gray 

Total adjusted thickness___________ 220 

Ellis formation: 

A-57. Limestone, slabby, yellowish-gray, and lightolive-gray microcrystalline, weathering 

grayish-yellow, with a few thin beds of 

pale-reddish-brown calcareous micaceous 

medium sandstone, weathering moderate 

yellowish brown. Several short covered 

intervals are probably on siltstone______ 36 

A-56. Sandstone, moderate-yellowish-brown to 

light-brown, hard, medium, calcareous. Unit Lower 7 ft forms ledge; rest of unit yields thickness 

(feet) dip slope covered with large slabs______ 22 

A-55. Limestone. Grades laterally from paleyellowish-brown to grayish-orange coarse 

reef calcite sandstone with many chert 

and quartz grains and fragments of 

pectens, cemented with calcite, to dense 

laminated finely crystalline medium-gray 

limestone. Basal 2 ft has many chert 

pebbles. Thin hard sandstone lenses in 

upper 4 ft; top channeled by overlying 

69 sandstone A-56_______________________ 9 

A-54. Chert pebble conglomerate. Subangular to 

rounded pebbles of dark chert and yellowish-orange siltstone in slightly calcareous 

sand matrix; weathers moderate yellowish 

brown, forms distinct ledge_ _ _ _ _ _ _ _ _ _ _ _ 8 

Total adjusted thickness __________ _ 

Phosphoria formation: 

88 A-53. Chert and sandstone. Ledges a few feet 

thick of massive brownish-gray to 

150 yellowish-brown chert separated by 

poorly exposed intervals of similar thickness of thinly interbedded chert, platy 

yellowish-orange and yellowish-gray very 

fine sandstone, and dark-greenish-gray 

medium to coarse phosphatic sandstone. 

The phosphatic sandstone is made up of 

rounded to subangular chert grains, 

phosphatic oolites, shell fragments and 

iron-stained clay, cemented in places by 

calcite, in places by silica. A phosphatic · 

74 bed 1}~ ft thick crops out 3.% ft above the 

base of this unit; concentrations of float 

suggest 2 or 3 similar but much thinner 

beds in the upper part of the unit. 

14 Phosphatic grains weather bluish white, 

48 .making distinctively speckled surfaces. 

Phosphatic parts have strong sour odor 

when wetted with HCl or struck sharply. 

Striking whitish outcrop ______________ _ 

75 

49 
126 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

Phosphoria formation-Continued 

A-52. Dolomitic limestone, with chert nodules, 

Unit 

thickness 

(feet) 

like A-50 .. ------------- ---------~--- 26 

A-51. Largely covered. Sparse outcrops are of 

thin-bedded nodular grayish-orange fine 

quartzite, weathering medium yellowish 

brown; float suggests most of interval is 

thin-bedded brownish-gray chert __ ............ 22 

A-50. Dolomitic limestone, light-olive-gray to 

yellowish-gray, finely crystalline, with 

faint irregular bedding planes a few inches 

apart. Many pods and lenses of lightolive chert, locally rich in fusulinids__ ___ 23 

Total adjusted thickness .... _________ 120 

Quadrant formation: 

A-49. Quartzitic sandstone, thick-bedded blocky 

jointed, yellowish-gray, pinkish-gray and 

very .. pale orange weathering yellowish 

gray and yellowish orange; medium to 

coarse, with mixture of silica and carbonate cement; rounded. Forms ledge_____ 85 

Largely covered. Few outcrops are sandstone like A-49; float is of similar sandstone, with a little silty limestone .. _ .. ___ .. 150 

Largely covered. Sparse outcrops and float 

suggest interval made up of light-colored 

arenaceous limestone and dolomitic limestone (perhaps %), and calcareous sandstone (perhaps%) in beds a few feet thick.. 148 

A-48. Sandstone and limestone. Beds a few feet 

thick of sandstone like that of A-49 above 

alternating with limestone like that of 

A-47 and A-46 below. Sandstone ledge 

9ft thick at top. This is gradation zone 

between Quadrant and Amsden formations________________________________ 24 

Total adjusted thickness .. __________ 37 5-400 

Amsden and Big Snowy formations: 

[NOTE.-The Amsden-Big Snowy contact is much in doubt 

here; perhaps it is best chosen in or at base of unit A-41] 

A-47. Limestone, olive-gray and light-olive-gray, 

fine to medium crystalline, in beds 5-10 

ft thick, with interbeds 1-3 ft thick of 

yellowish-gray medium crystalline dolomitic limestone; forms low ledge________ 36 

A-46. Sandstone and limestone. Mostly thickbedded grayish-orange calcareous fine to 

medium subrounded sandstone, with thin 

interbeds of pale-olive medium crystalline arenaceous dolomitic limestone, palebrown medium subangular calcareous 

chloritic sandstone, and medium-gray 

microcrystalline limestone. At base, 7 -

ft of fetid medium crystalline yellowishgray limestone________________________ 41 

A-45. Sandstone, pale-reddish-brown, thin-bedded, 

fine calcareous subangular. Near middle 

of interval, 2ft bed of medium-light-gray, 

medium crystalline fetid limestone, and 7 

Amsden and Big Snowy formations-Continued 

ft dioritic sill (nearby, this sill thickens to 

as much as 20ft); sandstone is hornfelsed near sill (excluding sill) _______________ _ 

Covered. Float is largely yellowish-orange 

and grayish-yellow siltstone, with impersistent reddish-brown soil at top (red color 

here may be due to Quaternary weathering rather than to originally red rocks in 

this interval) ________________________ _ 

A-44. Limestone. Like A-43. Ledge former ___ _ 

Unit 

thickness 

(feet) 

58 

28 

17 

Poorly exposed. Mostly thin-bedded silty 

and arenaceous limestone, in yellowish · 

and greenish tones, interbedded with 

dark-gray limestone .. _____________ .. _ .. __ 

A-43. Limestone, very thick bedded dark-gray 

fetid microcrystalline; weathers yellowish 

gray; many nodules and lentils of oliveblack chert; locally rich in giant cup corals 

and biohermal debris; few interbeds of 

olive silty limestone. Ledge former ____ _ 

A-42. Limestone, dolomite, and mudstone. Thinbedded, medium-gray nodular .limestone 

and · pale-red slabby finely crystalline 

dolomite interbedded with lenses of grayish-red-purple calcareous mudstone and 

fine sandstone. Dolomite layers weather 

to reddish-brown soil. Dioritic sill 3 ft 

thick at 15 ft below top (excluding sill) .. 

A-41. Sandstone, mudstone, limestone, and dolomite. Alternations of thin beds of 

mottled grayish-yello'w, yellowish-gray, 

grayish-orange-pink mudstone, yellowishgray medium crystalline limestone, moderate orange-pink finely c-rystalline dolomite, and blocky mottled pale-red, 

grayish-orange-pink, very pale orange 

fine subangular calcareous sandstone ___ _ 

Covered in section. Good outcrops in this 

interval 300 ft to east show it to be much 

like A-42 above _____________________ _ 

A-40. Limestone, olive-gray, weathering light 

gray, with films of grayish-orange chert 

and a few nodules of olive-gray chert; one 

ledge-forming bed _______ .. ______ -------

Covered. Mostly thin-bedded light-gray 

finely crystalline dolomite and grayishyellow arenaceous medium crystalline 

dolomiti-c limestone, with some grayishred fine subangular sandstone, judging 

from float _____________________ -----.. -

A-39. Sandstone, thin-bedded, yellowish-gray, 

medium- to coarse subrounded, calcareous 

near base grading upward into fine subangular reddish-brown silty sandstone 

and greenish-gray medium subrounded 

quartzitic sandstone with a few dark 

chert grains; 3 ft of medium-grained 

medium-gray arenaceous limestone at 

top ________________________________ _ 

A-38. Poorly exposed. Thinly interbedded lightcolored calcareous mudstone, flaggy dolomite, and banded dolomitic limestone __ _ 

24 

18 

70 

233 

147 

4 

79 

19 

7 
MEASURED !SECTIONS IN PRE-TE'RTIARY ROCKS 

Amsden and Big Snowy formations-Continued 

A- 37. Sandstone, thick-bedded, pale-yellowishgray, medium subrounded, calcareous, 

with a few dark chert grains __________ _ 

A- 36. Dolomite and siltstone. Thin-bedded, 

light-olive-gray finely crystalline dolomite at base (19 ft) and thin-bedded 

mottled grayish-orange and yellowishgray finely crystalline dolomite at top 

(4 ft) with largely covered interval 

between of dolomite interbedded with 

grayish-yellow siltstone (48ft) _________ _ 

Total adjusted thickness __________ _ 

Mission Canyon limestone: 

Largely covered. Base of formation forms massive 

ledge (A-34) 50-100 ft thick which caps cliff in 

Lodgepole formation. Bulk of formation underlies long grassy dip slope broken by a few low, 

much weathered ridges. Basal ledge, top ledge 

(A-35), and rare outcrops are of thick-bedded 

dark-gray to medium-gra y microcrystalline 

limestone weathering yellowish gray to light 

olive gray, locally containing many dark chert 

nodules; fairly common are lenses of coarsely 

crystalline olive-gray limestone rich in fragments 

of crinoid stems, brachiopods, cup corals, and 

bryozoans. Thickness estimated _____ _______ _ 

Lodgepole limestone: 

Largely covered. Ledges, 20-30 ft thick near base 

(A-32) and about 450 ft above base (A-33) are 

mostly of thin-bedded dark-gray to medium-gray 

microcrystalline limestone, weathering yellowish 

gray to light olive gray; a few beds are of coarsely 

crystalline olive-gray limestone containing many 

fragments of crinoid stems, brachiopods, and cup 

corals. Covered parts, as indicated by float and 

by exposures on strike within a few thousa~d 

feet of this section, are of similar dark thinbedded limestone, with partings of yellowishbrown and grayish-orange calcareous shale. In 

the lower 200 ft, sllaly partings are numerous 

and comparatively thin; they become fewer and 

thicker upward. Thick-bedded sequences appear 

in the upper 200 ft, and ultimately the formation 

grades into the thick-bedded Mission Canyon 

limestone. Fossiliferous beds are especially rich 

and abundant in the transition zone. Total 

thickness ________________ _________________ _ 

Three Forks shale: 

[NOTE.-This thickness, though measured on well-exposed 

beds without visible structural complications, is great for 

the formation in this vicinity: 6 sections measured on the 

map at 1-mile intervals east and west of section· A ranged 

from 200 to 350 ft. A good average thickness for this vicinity is 300 ft. The great measured thickness here may 

be due to local thickening of the lower ·shales (A-29 and 

A-30) or to near-bedding faulting; bec~use the measured 

Jefferson here also seems too thick, duplication by 

unrecognized faults is the more reasonable explanation] 

Unit 

thickness 

(feet) 

Three Forks shale-Continued 

A-31. Sandstone and siltstone, thin-bedded, locally 

flaggy; pale-yellowish-orange, dark9 yellowish-orange and grayish-orange; 

weathered rock darker and more brown; 

subrounded quartz grains and angular 

orange-stained calcite grains with a little 

clay in a calcite matrix. Near base, 

exposures poor, but sequence includes 

at least 3 beds 1-3 ft. thick of olive-gray 

clay shale, separated by a few feet of 

71 orange siltstone, as well as sporadic lenses 

of grayish-red, dark-reddish-brown, and 

800 grayish-yellow fossiliferous limestone; upsection, the beds are thicker, the clay 

content decreases, the quartz content 

increases, the average grain size increases_ 

Covered. In basal 24 ft. a few exposures of 

olive-gray clay shale and masses of orange 

argillaceous limestone breccia like the 

basal breccia (A-28); rest of interval 

seems to be largely shale like A-29, with 

thin limestone beds like those near the 

base of A-31; top 1Q-20 ft. may include 

much orange siltstone like that of A-3L_ 

A-30. Limestone, irregularly bedded mottled argillaceous finely crystalline, yellowish-gray 

and olive-gray weathering pale yellowish 

± 900 brown and pinkish gray, fossiliferous~ __ _ 

Covered (creek bed). Probably largely 

shale like A-29 _ -.- ____________ - - - -- - - -

A-29. Shale, thick-bedded, olive-gray, weathering 

grayish yellow to dus):cy yellow; many 

nodules of iron oxides and fossils replaced 

by iron oxides (fossil loc. A- 29 from 

upper 30ft.) -- ---------- ----------- --

A-28?-t Shale (4/5) thinly interbedded with siltstone (1/5) dark-yellowish-orange, calcareous; much float, but no outcrop, of 

light-gray microcrystalline limestone __ _ 

A-28. Limestone breccia, yellowish and grayishorange, weathering pale yellowish orange, 

thinly interbedded with grayish-orange 

medium crystalline silty limestone, 

weathering grayish orange pink ______ __ _ 

Total adjusted thickness ______ ____ _ 

Jefferson dolomite: 

A-27. Poorly exposed. Dolomite in alternating 

± 600 dark and light sequences 20-40 ft thick. 

Light sequences grayish yello~, fine to 

medium crystalline, with a few layers of 

limestone; dark sequences pale yellowishbrown medium crystalline, fetid. Grayish-yellow beds at top ___ __ ___ _____ ----

A-26. Dolomite, pale-yellowish-brown, finely crystalline mostly thick-bedded; upper part 

has indistinct bedding planes a few inches 

apart; fetid; few thin lighter colored 

limestone beds, especially near top. Some 

dolomite beds finely streaked with lime127 

Unit 

thickness 

(feet) 

44 

78 

4 

81 

154 

28 

17 

400 

180 
128 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

Jefferson dolomite-Continued 

stone. Ledge former. Note: Some beds 

recorded in A-26, A-27 may be duplicated 

by near-strike fault ___________________ _ 

Covered----------------------- ------~

A-25. Dolomitic limestone and limestone, thickbedded, light-hrownish-gray to lightolive-gray, microcrystalline ___________ _ _ 

A-24. Dolomite, with minor limestone, like A-22_ 

A-23. Limestone and dolomite. Alternating beds, 

a few feet thick of several varieties of 

limestone (% of interval) and dolomite 

(.Ya). Limestone mostly light-gray microcrystalline and yellowish-gray medium 

crystalline. Dolomite medium crystalline, light-brownish-gray with streaks of 

yellowish-gray limestone ______________ _ 

A-22. Dolomite, medium-bedded, light-brownishgray to brownish-gray, medium crystalline, weathering yellowish brown. Few 

beds slightly lighter colored dolomitic 

limestone and limestone _______ ___ ____ _ 

A-21. Dolomite, thick-bedded, pale-yellowishbrown and dark-yellowish-brown, weathering pale-yellowish-brown; ledge former; 

medium crystalline, fetid. Few lenses, 

as much as 3 ft thick, of stromatoporoidal 

pale-yellowish-brown dolomitic limestone, near base _____________________ _ 

Total adjusted thickness ______ ____ _ 

Maywood formation: 

Largely covered. Few outcrops of paleyellowish-brown medium crystalline locally fetid limestone __________________ _ 

A-20. Dolomitic limestone, thin-bedded, lightolive-gray and grayish-orange, interbedded with grayish-yellow fiaggy 

calcareous siltstone. Sequence includes 

a few layers of pale-reddish-brown and 

moderate-reddish-orange calcareous siltstone __________________ ~-------------

A-19. Largely covered. Probably much like A-20 

-above. At 13 ft above base, 2 ft outcrop 

of thin-bedded grayish-orange and palereddish-brown medium crystalline silty 

limestone ___________________________ _ 

·· ··--·Total adjusted thickness __ ________ _ 

Pilgrim limestone: 

A-18. Dolomite and subordinate limestone, thickbedded; ledge former. Medium.;.lightgray microcrystalline limestone at base, 

grading in less than 50 ft into yellowishgray medium crystalline dolomite. 

Lower part of dolomite sequence weathers 

mottled yellowish gray, with the slightly 

. darker patches coarser grained and less 

resistant than the lighter _____________ _ 

Unit 

thickness 

(feet) 

246 

68 

27 

52 

24 

23 

47 

650 

13 

36 

48 

100 

226 

Pilgrim limestone-Continued 

A-17. Largely covered. Small outcrops and float 

indicate interval is mostly banded gray 

limestone and orange dolomite, like A-16, 

but with some dark-yellowish-orange calUnit 

thickness 

(feet) 

cite sandstone__________________ ____ __ 58 

A-16. Limestone and dolomite, thin-bedded, with 

wavy bedding surfaces. Layers of 

medium-light-gray limestone, finely crystalline, %-3 in. thick, alternating with 

silty medium-grained grayisl:-orange 

dolomite bands, less than 7~ in. thick; 

rarely, dolomite is pale reddish brown___ 70 

A-16. Dolomite, thick-bedded mottled, mediumgrained, yellowish-gray. Sporadic thin 

beds of light-olive-gray oolitic limestone, 

with fragments of trilobites and straight 

cephalopods___ _______________________ 91 

Total adjusted thickness___________ 450 

Park shale: 

A-15. Largely covered. Scattered outcrops are of 

dark-greenish-gray to pale-olive clay 

shale, with a few thin beds of fine sandstone near base_______________________ 165 

Total adjusted thickness___________ 150 

Meagher limestone: 

[NOTE.-Much of the formation has mild petroliferous odor] 

A-14. Limestone, medium-gray, finely crystalline, 

thick- to thin-bedded, with few lenses, 

less than 6 in. thick, of oolitic limestone 

and limestone-pebble conglomerate. 

Ledge former ________________________ _ 

A-14. Limestone, massive-looking but laminated 

where weathered, finely crystalline, 

mottled medium dark gray and grayish 

orange, with sporadic sequences a few feet 

thick of thin bands of modular mediumdark-gray fine-grained limestone alternating with bands of silty mediumgrained grayish-orange limestone _______ _ 

A-13. Largely covered, but with poor outcrops of 

banded gray and orange limestone like 

upper 20ft of A-12 __________________ _ 

A-12. Limestone, mottled; medium-dark-gray 

fine-grained limestone grading irregularly 

into coarser silty grayish-orange limestone. Ledge former _________________ _ 

A-12. Limestone much like A-10, but with mottling slightly more evident, laminae 

thinner, slightly dolomitic in middle of 

sequence ____________________________ _ 

-t\.-11. Mostly covered. Sporadic outcrops of thin 

beds of calcite sandstone and siltstone, 

dark-yellowish-orange, alternating with 

medium-gray finely crystalline dolomite . 

(Concealed beds probably like A-10) ___ _ 

40 

40 

46 

20 

137 

51 
MEASURED 1SECTIONS IN PRE-TERTIARY ROCKS 129 

Mf;)agher limestone-Continued 

A-10. Limestone, olive-gray to olive-black, 

microcrystalline in very thick beds; ledge 

former. Weathers light olive gray, showing thin crinkled laminae, and faint mottling in upper 100 ft. Scattered darkyellowish-orange chert concretions and 

trilobite fragments. Ledge maker _____ _ 

A-9. Limestone, microcrystalline, dark-gray, 

faintly banded with darker and lighter 

layers 1-4 in. thick. In lower half, a few 

bands of silty moderate-olive-brown limestone, weathering yellowish gray _______ _ 

Total adjusted thickness __________ _ 

Wolsey shale: 

A-8. 

A-7. 

A-6. 

A-5. 

A-4. 

· A-3. 

Covered (upper 25-75 ft may include beds 

gradational between Wolsey and Meagher 

lithologies. Rest of interval probably 

Cl~;e s~~~1,- ~ll;litl_y_-~i~~~~;~s-;- ~~i;e~~;~; weathering pale olive; few thin nodular 

limestone beds _______________________ _ 

Shale (2/3) and limestone (1/3). B-eds as 

much as 1 ft thick of medium crystalline, 

hard light-olive-gray and greenish-gray 

limestone, weathering pale brown to dark 

yellowish brown, separated by poorly 

exposed beds, a few inches to 3 ft thick, of 

olive-gray to grayish-olive micaceous arenaceous shale _____________________ _ 

Shale (4/5) and sandstone (1/5). Alternating beds 1-4 in. thick of shale, grayisholive with many worm casts on bedding 

surfaces, micaceous arenaceous calcareous, and pale-olive to grayish-olive sandstone, fine glauconitic micaceous 

calcareous subangular, weathering dark 

yellowis)l brown. Grades upward into 

A-7 ________________________________ _ 

Covered (probably much like A-5) _______ _ 

Argillaceous limestone, medium-gray, speckled with light olive, weathering medium 

gray with grayish-orange splotches; nodular thin-bedded, finely crystalline. A 

few beds are coarsely crystalline, pale yellow brown, with many trilobite fragments. 

A few shale partings, weathering grayish 

orange _______________________________ _ 

Covered (probably 45 ft shale like A-3 overlain by 13 ft limestone like A-5) ___ ____ _ 

Clay shale, fissile, pale-green to grayishgreen, with lentils of moderate-yellowishbrown sandstone _____________________ _ 

Sandstone, thin-bedded, grayish-red to moderate-grayish-orange, coarse, quartzitic, 

subrounded __________________________ _ 

6:54729 0-63-10 

Unit 

thicknesi 

(feet) 

136 

22 

500 

192 

44 

42 

28 

27 

13 

58 

7 

14 

Wolsey shale-Continued 

A-2. Clay shale, fissile, grayish red, with streaks 

of grayish-green shale; interval divided 

into 3 equal parts by 2 beds, each 1 ft 

thick, of pale-reddish-brown glauconitic 

Unit 

thickness 

(feet) 

sandstone _______________________ ~----- 14 

Total adjusted thickness___________ 400 

Flathead sandstone: 

A-1. Quartzitic sandstone, very thinly crossbedded, 

in part fiaggy, varicolored moderate-orange 

pink, grayish-pink, pale-reddish-brown, 

and grayish-red, coarse to medium; 

rounded; nearly pure quartz sand, cemented with quartz and tinted with iron 

oxi.des. Includes a few conglomerate lentils. Forms ledge_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 9 

A-0. Sandstone, much like unit above but coarser 

grained and less well cemented, pale-reddish-brown and grayish-pink____________ 26 

Total adjusted thickness 

North Boulder formation: 

Sandstone, grayish-red to dark-reddish-brown, 

coarse, subangular, micaceous arkose, with 

grayish-olive micaceous siltstone laminae; many 

lenses of gray pebble-to-boulder conglomerate 

35 

in lower par,t. Measured only on map _______ >4, 000 

SECTION B 

Above Lombard thrust, northeast part of quadrangle, in drainage of Mud Spring Gulch. Maywood 

formation through most of Ellis formation. Formations from the Maywood through the · Lodgepole are 

represented by a continuous section which begins in 

the NE%,NE%, sec. 1, T. 2 N., R. 1 E., on the east 

side of 4,889-foot crest. The beds here strike N. ·10° 

E. and dip 27° W. Due to folding and intrusion 

there is no full section of the Mission Canyon in this 

sector. Big Snowy and Amsden formations are measured in SE1_4NE1,4 sec. 32, T. 3 N., R. 1 E., beginning on the north side of the only prominent northwest-southeast depression in this quarter-section. The 

strike is N. 25 ° W., and the dip is vertical. 

Because of folding and poor exposure, the Quadrant 

formation was not measured. 

The Phosphoria and Ellis formations were measured in SE%, sec. 20, T. 3 N., R. 1 E., beginning in 

the small saddle southwest of hill 4720. The strike 

averages N. 35° E.; dip 10° W. 
130 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

Qua ternary gravel. 

Ellis formation: 

B-33. Poorly exposed limestone and siltstone, 

thinly interbedded; varicolored fine- and 

microcrystalline clayey limestone, with 

subordinate cal,careous siltstone that 

rarely crops out. At top, 5 ft of darkgreenish-gray mudstone __________ ____ _ 

B- 32. Mostly covered. Scanty . outcrops are of 

hard iron-stained sandstone, rich in chert_ 

B- 31. Limestone, yellowish-gray, sandy, clastic ; 

many pelecypod fragments ___________ _ 

Covered, except for small exposure of chert 

pebble conglomerate 8 ft below top ___ _ 

B- 30. Limestone, thick-bedded, olive-gray, finely 

crystalline, with scattered chert pebbles_ 

Total adjusted thickness _______ ___ _ 

Phosphoria formation: 

Covered _______ ____ ______________ _____ _ 

B-29. Quartzite and quartzitic sandstone, thickbedded, yellowish-gray; a few thin interbeds of olive chert ___ __ ______________ _ 

B-28. Chert~ thin-bedded, olive-gray _______ ____ _ 

B-27. Quartzite, yellowish-gray, one prominent bed ________________________________ _ 

Covered saddle. Slope below known Phosphoria formation covered with chert and 

calcareous siltstone float, pr~bably from 

overlying beds, but also has phosphatic 

sandstone float of Phosphoria type not 

present in outcrop above. (Opposite 

slope of saddle has only quartzitic sandstone float and is assigned to Quadrant 

formation) ____ __ __ ______________ ____ _ 

Total adjusted thickness __________ _ 

Quadrant formation (individual units within formation 

not measured) : . 

"Quartzitic sandstone, thick-bedded, yellowish-gray 

and dark-yellowish-orange, fine grained ____ ___ _ 

Amsden formation: 

B-26. Limestone and dolomite, thick-bedded, paleolive and medium-gray, microcrystailine; 

forms ledge _______________ ___ __ ______ _ 

B- 25. Limestone, thick-bedded, medium-darkgray, microcrystalline; fetid, with many 

chert nodules ________________ __ ______ _ 

B- 24. Partly covered. Much reddish-brown soil 

at top; scattered exposures of reddishbrown and grayish-yellow clayey limestone and calcareous siltstone __________ _ 

Total adjusted thickness_:. __ ______ _ 

Big Snowy formation: 

B-23. Largely covered. Two 14-ft covered intervals separated and bounded by 1-2 ft 

limestone ledges. Limestone is. in single 

beds, light olive, microcrystalline, fetid; 

many dark chert nodules and fossil 

fragments _____ __ ____ __ _____ __ _______ _ 

Unit 

thickness 

(feet) 

224 

32 

21 

31 

30 

Big Snowy formation- Continued 

B-22. Shale, fissile, grayish-olive _________ ______ _ 

Covered. Probably shale like B-22_ ~ ____ _ 

B-21. Limestone, thick-bedded, dark-gray, microcrystalline, fetid; fossiliferous __________ _ 

Covered. Probably shale like B-22 ______ _ 

Total adjusted thickness _____ _____ _ 

Mission Canyon limestone, not measured. 

Lodgepole limestone: 

Largely covered. Ledge at base. of section and 

scattered outcrops above are of thick-bedded 

dark-gray microcrystalline limestone with a few 

silty partings ____ ~ ______________ __ __ _______ _ 

Unit 

thickness 

(feet) 

5 

18 

13 

52 

100 

709 

Total adjusted thickness _________________ 700 

8 Three Forks shale: 

52 

51 

4 

60 

150 

>80 

108 

82 

98 

300 

[NOTE.-Details of Three Forks shale from NE HSWH sec. I, 

about 3,200 ft southwest of main measured section] 

B- 20. Siltstone. Like B-19 but thick-bedded and 

flaggy. (Fossil loc. 252i, 35-40 ft below top) _____ __ ________ - - - - - - - - - - - - - -

B-19. Siltstone, thin-bedded, pebbly, ferruginous 

and calcareous, dark-yellowish-brown to 

dusky-brown, locally petroliferous. (Fos-

. sil lac. 252h.) Pale-yellowish-brown 

clayey limestone bed at base ____ ______ -

Covered. Float is mostly black shale __ __ _ 

B-18. Shale, clay, olive-gray, with scattered lentils 

of dark limestone. (Fossilloc. 252g from 

top of interval; 252f from 10 ft below top)_ 

B-17. Poorly exposed. Scattered outcrops are 

grayish-black to greenish-black fissile clay 

shale, with lenses of dark-reddish-brown 

limestone coquina. (Fossilloc. 252e from 

2- 5ft above base; 252d from basal 2 feet)_ 

B-16. Shale, clay, olive-gray to brownish-gray, 

with a few lenses of dark-reddish-brown 

limestone coquina. (Fossil lac. 252c from 

10-20 ft below top; 252b from basal 5 ft)_ 

B-15. Largely covered. Scattered exposures are 

mostly of very thin-bedded yellowishgray calcareous siltstone and olive-gray 

clay shale, with lenses 1-4 in. thick of 

fossiliferous dark-reddish-brown to pale- , 

yellowish-brown calcareous siltstone. At 

base, a few thin crumpled layers of 

yellowish-gray to very pale orange 

medium crystalline clayey dolomitic limestone. (Fossilloc. 252a from 90 ft above 

base.) ____ _____ __ _________ __________ ,_ 

Total adjusted thickness __ ____ -----

Jefferson dolomite: 

B-14. Poorly exposed. Long dip slope with a few 

steplike ledges of thick-bedded yellowishgray medium crystalline dolomite and 

31 dolomitic limestone-· - ___ - - - - - - - - - - - - - -

69 

6 

23 

45 

20 

32 

127 

300 

30 
MEASURED 'SECTIONS IN PRE.-TE.'RTIARY ROCKS 

Jefferson dolomite-Continued 

B-13. Dolomite and dolomitic limestone. About 

% yellowish-gray and yellowish-brown 

dolomite, and 7~ yellowish-gray limestone 

or dolomitic limestone ________________ _ 

B-12. Dolomite, thick-bedded, dark-yellowishbrown, coarsely crystalline ____________ _ 

Covered. Probably much like B-12 _____ _ 

Total adjusted thickness __________ _ 

Maywood formation: 

B-11a. Poorly exposed. Scattered outcrops are of 

light-olive-gray and yellowish-gray thickbedded medium crystalline limestone. 

Covered intervals probably underlain by 

yellowish-gray calcareous siltstone ____ _ 

B-11 b. Largely covered. 2-ft beds at top and 

bottom of yellowish-gray medium crystalline limestone. Covered remainder 

probably underlain by yellow siltstone __ 

B-llc. Grayish-yellow calcareous siltstone ______ _ 

Total adjusted thickness ___ ___ ___ _ 

Pilgrim limestone: Dolomite, thick-bedded, yellowishgray. 

SECTION C 

Unit 

thickness 

(feet) 

Kootenai formation-Continued 

C-25. Quartzitic sandstone, thick-bedded, medium; yellowish-gray, forms ledge _____ _ 

C-24. Partly covered. Mostly reddish-brown and 

260 dark-yellowish-orange mudstone with 

scattered outcrops of dark sandstone and 

66 limestone in beds 1-3 ft thick ________ _ 

21 C-23. ·Quartzitic sandstone, thick-bedded, yellowish-brown, with many chert fragments 

400 and lenses of pebble conglomerate ____ _ 

Covered. Interval probably underlain by 

reddish-brown mudstone ______________ _ 

C-22. Sandstone, thick-bedded, yellowish-brown, 

with much chert_ ___________________ _ 

C-21. Largely covered. Scattered outcrops of 

reddish-brown, olive-gray, and' light-gray 

32 mudstone and fine sandstone _______ ::.. __ _ 

26 

C-20. Sandstone, thick-bedded, locally cross-stratified, yellowish-gray; medium, much 

chert; calcareous cement; ledge former __ 

11 Total adjusted thickness __________ _ 

70 Morrison formation: 

Covered. Reddish-brown and yellowish-brown 

soil and siltstone float_ ____________________ _ 

Total adjusted thickness ______________ _ 

Ellis formation: 

Largely covered. Lacks red detritus and 

soil, typical of adjoining covered Morrison 

interval; probably underlain by yellowish-gray siltstone and limestone ________ _ 

C-19. Conglomerate. Rounded chert pebbles, 

dark-yellowish-brown _________________ _ 

C-18. Limestone, thick-bedded, yellowish-gray, 

dense microcrystalline to medium 

crystalline with scattered pebbles; ledge 

former ______________________________ _ 

C-17. Breccia zone. Uncemented angular masses 

of chert and quartzite. (Probably repre131 

Unit 

thickness 

(feet) 

12 

198 

43 

53 

47 

180 

32 

>600 

132 

130 

74 

26 

20 

Below Lombard thrust, northeast corner of quadrangle. From Lodgepole formation through Kootenai 

formation. Measurement is essentially along the east 

wall of a canyon which begins at the west edge of 

the SWl)tSW'%, sec. 29, T. 3 N., R. 2 E., and trends 

SE into the NEl)tNWl)t sec. 32, T. 3 N., R. 2 E. 

Strike averages N. 30° E. All the strata are overturned, with dips ranging from 40° N. to 80° N. 

Because the rocks are overturned and with variable 

dip, it is obvious that thickness measurements are not 

to be taken very seriously. Formations between the 

Lodgepole and the Pilgrim crop out in this sector but 

not well enough for satisfactory measurement. 

sents a weathered pre-Ellis surface.)____ 26 

Covered saddle (near top of Kootenai formation) ______ _ 

Kootenai formation: 

C-27. Sandstone and siltstone. Many ledges 1-2 

ft thick of several sandstone varieties, 

separated by thick beds of siltstone. Sandstone types include hard light-gray 

quartzitic, soft-grayish-yellow calcareous, 

and dark-greenish-gray arkosic. Many 

worm casts on siltstone bedding surfaces_ 

C_:26. Limestone, thick-bedded, mottled yellowishorange and yellowish-gray, coarsely crystalline, with abundant gastropods; forms 

ledge _______________________________ _ 

Covered. Distinctive reddish-brown soil 

in upper 14 ft; interval probably underlain by siltstone ______________ ~ ______ _ 

Unit 

thickness 

(feet) 

>100 

>140 

7 

72 

---- Total adjusted thickness___________ 150 

Phosphoria formation: 

C-16. Quartzitic sandstone, thick-bedded, yellowishgray, with a few thin beds of dark chert __ 

C-15. Partly covered. Scattered outcrops are of 

yellowish-gray medium crystalline lime- · 

stone. Covered interval probably siltstone and chert ______________________ _ 

C-14. Chert, light-olive, thick-bedded, forms ledge_ 

C-13. Quartzite, thick-bedded, fine-grained, paleyellowish-brown _____________ - __ ------

Largely covered. Scattered outcrops are of 

yellowish-gray medium crystalline limestone and dark chert ___________ - - - - - - -

C-12. Limestone, thick-bedded, yellowish-gray 

medium crystalline, with large irregular 

54 

37 

49 

10 

46 

lentils of dark chert and quartzite __ ..:_-- 15 ---- Total adjusted thickness___________ 200 

=== 
132 GEOLOGY OF THE mREE FORKS QUADRANGLE, MONTANA 

Quadrant formation: 

Brecciated and sheared sandstone and 

Unit 

thickness 

(feet) 

quartzite, like C-16 above______________ 30 

C-11. Quartzitic sandstone, thick-bedded, white 

to yellowish-gray, medium-grained; 

includes a few grayish-yellow limestone 

beds 1-3ft thick______________________ 100 

Total adjusted thickness--_.:_______ 150 

AmRden formation: 

l"!-1 0. Siltstone and limestone. Thin-bedded, yellowish-gray siltstone and some fine 

sandstone with calcareous cement, interbedded with subordinate olive-gray 

microcrystalline limestone ______ _______ _ 

C-9. Limestone, thick-bedded, light-olive-gray, 

with a few thin beds of moderate yellowish orange siltstone ___________ _____ ___ _ 

~""~9a. Siltstone and _limestone like C- 10, but 

proportions of two types about even. 

At base 8ft blackish-red microcrystalline 

coralline limestone _____ ____ __________ _ 

C-8b. Mudstone, thick-bedded varicolored, mostly 

brownish-red and dark-bluish-gray _____ _ 

C-8a. Sandstone and pebble ·conglomerate, thickbedded, blackish red ____ _____________ _ 

Total adjusted thickness ___ _______ _ 

Big Snowy formation: 

C-7 c. Quartzitic sandstone, yellowish-gray ______ _ 

C-7b. Shale, dark-greenish-gray ___ ____________ _ 

Largely covered. Float suggests main rock 

· type is yellowish-gray calcareous silts.tone; 

some dark-gray shale exposed by digging __ 

C-7a. Limestone, thick-bedded, light-olive-gray, 

microcrystalline, with a few thi~ silty beds; 

medium crystalline near base; forms ledge_ 

C-6. Sandstone and limestone. Alternating t}J.ick 

beds of yellowish-brown calcareous sahdstone and yellowish-brown fragmental 

sandy limeston.e ______________________ _ 

Largely covered. Probably dark shale, except for 12-ft bed near middle of yellowishbrown fragmental limestone like that in 

C-6 _________________________________ _ 

C-5. Limestone, thin-bedded, olive gray, with 

mottled weathering surfaces in yellowishgray and yellowish-orange. Fossiliferous, 

especially in basal 3 ft_ ________________ _ 

Q-4. Poorly exposed. Scattered outcrops of . black shale ___________________________ _ 

Covered. Probably same as C-3 _________ _ 

C-3. Siltstone .and mudstone, thin-bedded, yellowish-brown, calcareous _______________ _ 

Total adjusted thickness __ _____ ______ _ 

Mission Canyon limestone: 

C-2. Conglomerate and breccia, subrounded to 

angular "fragments of limestone _________ _ 

114 

27 

51 

18 

17 

250 

3 

3 

23 

42 

31 

116 

17 

8 

82 

91 

400 

4 

Mission Canyon limestone-Continued 

C-1. Limestone, very thick-bedded, dark-gray and · 

grayish-olive microcrystalline to fine crystalline; forms monolithic outcrop cut by 

many small faults subparallel to bedding. 

Unit 

thicknest 

(feet) 

Many thin zones of sclution breccia______ 950 

Total adjusted thickness______________ 950 

Lodgepole formation: 

C-0. Limestone, thin- to thick-bedded; dark-gray 

microcrystalline, with a few yellowishbrown silty partings near base___ _ _ _ _ _ _ _ _ 295 

Total adjusted thickness______________ 300 

SECTION D 

Below the thrust zone in southwest corner of the 

quadrangle. From Precambrian hematitic phyllonite 

and gneiss to base of Ellis formation. From the 

crystalline .rocks to the base of the Meagher formation the measured section is in sec. 8, T. 1 S., R. 1 W., 

partly in the Jefferson Island quadrangle. The section begins 15 ft northeast of the large north-flowing 

creek in E1j2 of· the section and roughly follows the 

creek. The beds here strike N. 65° W. and dip 36° N. 

To avoid structural complications, the section beginnirtg with the basal Meagher shifts about 2,000 ft 

east· to the NW1;4NW1;4 sec. 9, T. 1 S., R. 1 W., 

Three Forks quadrangle, beginning in the bed of the 

only west-flowing creek in ·this part of sec. 9. From 

this point to the top of the Jefferson 1 dolomite the 

section is continuous and the strike and dip throughout are close to N. 85° E., 25° N. At the top of the 

Jefferson, there is an offset of 300 feet east to take 

advantage of better exposures in the Three Forks 

shale. The overlying Lodgepole limestone was also 

measured on this offset. 

This part of the section ends in the SW1;4NW1;4 

sec. 4, T. 1 S., R. 1 W., at the top of the Lodgepole. 

The Mission Canyon limestone was not measured with 

the planetable, but estimated from map measurements. 

The remainder' of the section from the top of the 

Mission Canyon to the base of the Ellis was measured 

in NW1;4SE1;4 sec. 34, T. 1 N., R. 1 W., about 11.;4 

miles northeast 9f the end of the preceding part of 

the section. This segment begins on the southwest 

side of the only" prominent saddle in center SE1;4, 

sec. 34 and continues almost to the Jefferson River 

flood plain. The strike is N. 55° W., dips are vertical. 

Exposures of the Phosphoria formation in this interval are poor and have been supplemented by lithologic details from measurements 4,500 ft to the eastsoutheast in the SW1;4SE%, sec. 35 . . 
MEASURED 1SECTIONS IN PRE.-TERTIARY ROCKS 133 

Quaternary alluvium and colluvium. 

Ellis formation: 

D-61. Chert-pebble conglomerate _____________ _ 

Total adjusted thickness __________ _ 

Phosphoria formation: 

D-60. Quartzitic sandstone, thick-bedded, slightly 

calcareous, grayish-yellow at base grading to pale-yellowish-brown at top _____ _ 

D-59. Siltstone, pale-yellowish-orange, chippy; 

with a little pale-yellowish-brown chert, 

olive-gray chert, and phosphatic sandstone like that in b-58. (Firm outcrops 

of siltstone are not present, but the float 

is so abundant and the lithology so unique 

that the siltstone must be in place.) ____ _ 

D-58. Phosphatic sandstone and chert. Three 

beds of medium to fine phosphatic sandstone, medium-gray to olive-gray, averaging 3 ft thick, separated by paleyellowish-brown chert beds, 2 ft thick. 

Phosphate is in scattered ovoid and irregularly shaped grains _______________ _ 

D-57. Chert. Like D-54 ____________________ _ 

D-56. Sandstone, grayish-yellow, calcareous cement; with many rounded shale pebbles __ Covered ______________________________ _ 

D-55. Calcareous siltstone and dolomite, thinly 

. interbedded, pale-yellowish-orange to 

very pale orange, flaggy _____ -t ________ _ 

D-54. Chert, light-olive-gray to yellowish-gray, in one bed ____________________________ _ 

Total adjusted thickness __________ _ 

Quadrant formation: 

D-53. Quartzitic sandstone, thin'- · to thick-bedded, 

yellowish-gray, medium; faintly crossstratified locally. Main cement is quartz, 

but most beds also have a little calcareous 

cement. Includes a few thin beds of white 

to yellowish-gray limestone ___________ _ 

D-52. Dolomitic limestone, light-olive-gray and 

yellowish-gray, in 3 bands, each 5-10 ft 

thick, separated by two covered intervals 

6 ft. thick probably underlain by quartzitic sandstone _______________________ _ 

Total adjusted thickness __________ _ 

Amsden formation: 

Covered (except for 1-ft bed of olive dolomite in middle). Probably underlain 

mostly by thin-bedded siltstone and 

quartzite ______________________ __ ___ _ 

D-51. Dolomite and dolomitic limestone, thickbedded, yellowish-gray. A few thin beds 

of very pale orange calcareous siltstone __ 

D-50. Limestone and dolomitic limestone, thin- to 

thick-bedded, grayish-yellow; includes a 

few thin beds of pale-red calcareous siltstone and of brownish-gray very fine 

quartzite; forms low ledge ____________ _ 

Unit 

thickness 

(feet) 

3 

19 

59 

157 

38 

Amsden formation-Continued 

D-49. Calcareous siltstone, thin-bedded, grayishorange with a few pale-red beds. Upper 

part contains a few brachiopods _______ _ 

D-48. Limestone, laminated, light-olive-gray, 

medium crystalline; one bed forming low 

ledge ______________ _________________ _ 

Covered. Float is yellowish-brown, pale--

yellowish-orange and grayish-orange calcareous siltstone, and probably represents composition of intervaL _________ _ 

D-47. Limestone, light-olive-gray to light-gray, 

weathering slightly darker, locally pinkish; one ledge-forming bed __________ __ _ 

Covered. North side of saddle covered by 

moderate orangish-pink soil to base of 

next unit ___________________________ _ 

Covered. South side of saddle covered by 

float of pinkish-gray quartzite and yellowish-gray and grayish-orange siltstone_ 

Quartzitic and calcareous sandstone, thickbedded, pinkish-gray, dark-yellowishorange and pale-yellowish-brown, medium; cement alternately quartz and calcite. The quartz-cemented beds contain 

ovoid mottles _______________________ _ 

Quartzitic sandstone, thick-bedded yellowish-orange, medium, with a few pebbles 

of yellowish-gray siltstone _____________ _ 

Calcareous siltstone, thick-bedded, palereddish-brown and light-brown, locally 

mottled to yellowish orange. A few thin 

discontinuous bands of nearly black hematite and silica ______________________ _ 

Unit 

thickness 

(feet) 

31 

3 

21 

4 

55 

170 

79 

8 

6 

Total adjusted thickness___________ 500 

Big Snowy formation: 

Co~ered------- -- ------------------~--- 9 

D-43. Largely covered. Scattered outcrops of 

dolomite, light-olive gray to pale-yellowish-brown, flaggy. Unexposed beds 

probably shale_______________________ 58 

Total adjusted thickness___ ______ _ 60 

Mission Canyon limestone: 

200 D-42. Limestone, light-olive-gray, thick-bedded, 

fine crystalline ___ __ _________________ _ 

D-41. Limestone, thick-bedded, dark-gray, microcrystalline with a few yellowish-gray 

silty partings ________________________ _ 

(Remainder of Mission Canyon limestone 

37 not measured.) 

17 

Total adjusted thickness __________ _ 

Lodgepole limestone: 

D-40. Limestone, thin-bedded like D-39; abundant brachiopods and crinoid fragments_ 

Covered. Probably thin-bedded limestone 

64 with many silty partings ______________ _ 

20 

134 

>150 

190 

218 
134 GE-OLOGY OF THE THREE FORKS QUAD-RANGLE, MONTANA 

Unit 

thickness 

(feet) 

Lodgepole limestone-Continued 

D-39. Limestone, thin- to thick-bedded; olivegray, many layers contain coral and 

crinoid fragments; forms ledge _________ _ 

Total adjusted thickness ________ _ _ 

Three Forks shale: 

D-38. Calcareous siltstone and nodular limestone, 

thin-bedded. Beds progressively thicker, 

more sandy, and less clayey toward top; 

most beds pale orange to grayish orange; 

some limestone layers yellowish-gray, 

grayish-red and dark-reddish-brown; entire sequence fossiliferous, especially the 

red and brown beds _____ _____________ _ 

D-37. Clay shale, fissile, yellowish-gray to paleolive-gray _______ _ -. ____ ___ __ ___ ___ __ _ 

D-36. Siltstone and limestone. Thinly interbedded grayish-orange calcareous siltstone and olive-.gray limestone, with 

sea ttered zones of limestone breccia ____ _ 

D-35. Limestone. Lower half thick-bedded, 

grayish-orange, medium crystalline, 

porous, with zones of solution breccia 

containing angular fragments of dark 

microcrystalline limestone, grading into 

upper . half, thick-bedded light-olive-gray 

microcrystalline limestone which forms 

ledge __ ___________ ___ ___ _____ ______ _ _ 

D-34. Limestone and mudstone, thick-bedded, 

light-brown, grayish-orange, and darkyellowish-orange, clayey limestone and 

calcareous mudstone with lentils of lightgray shale, intricately contorted in places; 

locally, is a breccia with fragments several 

inches long ___ ____ ______ ___ ________ _ _ 

Total adjusted thickness _________ _ _ 

Jefferson dolomite: . 

[NOTE.- Practically every bed has strong fetid odor] 

D-3~. Dolomite. Like D-32 but beds are thinner_ 

D-32. Dolomite, thin:-bedded, grayish-orange, 

coarsely crystalline _____ __ ___ __ _ . _____ _ 

D-31. Dolomite, thin-bedded, yellowish-gray, 

medium crystalline _____ ____ ___ __ ___ _ _ 

D-30. Dolomite, medium-light-gray 6-ft ledge at 

base, grading upward into beds 1-3 ft 

thick medium dark gray; medium 

crystalline ______ _____ ______ .. __ ___ ___ _ 

D-29. Dolomite. Alternating thick beds of paleyellowish-brown and yellowish-gray, 

grades into D-30 ___ ____ __ ______ ____ _ _ 

. D-28. Dolomite, like D-25 ____ _____ ______ ___ _ _ 

D-27. Poorly exposed. Outcrops are very pale 

orange dolomitic limestone and lenses of · 

dolomite-pebble conglomerate ·· as much 

as 4ft long and 1ft thick ___ ..: ____ __ ___ _ 

D-26. Dolomite, thick-bedded, olive-gray, fine 

crystalline; forms ledge ___ ____ __ __ ____ _ 

64 

450 

92 

19 

8 

19 

Jefferson dolomite-Continued 

D-25. Dolomite, thick-bedded medium-gray to 

brownish-gray, medium crystalline. 

Scattered "spaghetti beds" consisting of 

dark crystalline dolomite surrounding 

closely packed irregular tubes, % in. to 

2 in. long and averaging % in. thick, of 

earthy yellowish-gray limestone, presumably stromatoporoid zones. Includes 

a few thin beds of light-olive-gray dolomite and dolomite-pebble conglomerate __ 

Covered ______ ----------- --- - ---- ---- -

D-24. Dolomitic limestone, thick-bedded, paleyellowish-brown at base grading rapidly 

upward to dark yellowish brown, medium 

crystalline. Contains many irregular 

pods and masses of black and yellow 

chert, mostly less than an inch thick, 

but nearby are single masses of chert as 

much as 10 ft thick and 40 ft long ____ _ 

Total adjusted thickness ___ _______ _ 

Maywood forma tion: 

D-23. Calcareous siltstone, yellowish-gray, paleyellowish-orange and dusky-yellow ___ __ _ 

Covered. Float is reddish and yellowish 

siltstone ______________ __ :.. ___________ _ 

Total adjusted thickness __________ _ 

Pilgrim limestone: 

D-22. Dolomite and dolomitic limestone, thickbedded, yellowish-gray, dusky-yellow, and 

light-olive-gray with scattered orangepink splotches; slightly porous _________ _ 

D-21. Dolomite, thick-bedded, mottled light-olive54 gray to olive-gray, medium to coarse 

crystalline. Rock markedly porous and 

200 resembles sandstone. Slight fetid odor. 

Forms top of cliff _______ -.- ___________ _ 

D-20. Dolomitic limestone and dolomite, thinbedded; dolomite content varies rapidly 

26 both laterally and vertically. Least 

dolomitic material is yellowish-gray fine 

27 crystalline; most dolomitic is brownish 

gray and medium crystalline __________ _ 

24 D-19. Dolomitic . limestone ,and dolomite, thinbedded, light-brownish-gray, locally mottled with pale yellowish brown. Includes 

thin ,zones of cop.glomerate with flat sub33 angl)lar dolomite pebbles, and slabs as much as 4 ft long __________________ __ _ 

D-18. Limestone and dolomite. Alternating lam41 ,inae of gray limestone and orange dolo55 mite with thin layers of . dolomite pebble 

conglomerate like that in_ D-19 ________ _ 

Covered. (Float is mostly dark-gray, 

. coarsely crystalline, fetid dolomite, but 

49 interval probably largely underlain by 

banded dolomit~ and limestone like D-17 

22 and D-18.) _________________________ -

Unit 

thickness 

(feet) 

,;_;. 

28 

40 

18 

350 

3 

9 

10 

28 

29 

32 

21 

6 

60 
MEASURED 1SE.CTIONS IN PRE.-TERTIARY ROCKS 135 

Pilgrim limestone-Continued 

D-17. Limestone and dolomite. Thin layers of 

medium-grav, finely crystalline limestone 

alternating ~ith generally thinner ones of 

pale-yellowish-brown clayey dolomite; 

some beds nodular. Many thin dolomite 

streaks within limestone beds. Many 

chert stringers and lentils. Includes a 

few layers of dolomite-pebble conglomerate, many wit h reddish dolomitic 

cement. Forms ledge ___________ _____ _ 

Covered. Probably thick-bedded dolomite 

like D-16 ____ ____________________ ___ _ 

D-16. Dolomite, thick-bedded, light-gray to medium-gray, microcrystalline, with thin 

zones of oolites; lower 34 ft poorly exposed, upper 12 ft forms ledge with distinctly mottled surfaces due to slightly 

darker oolitic zones which grade into 

lighter crystalline material; rare laminae 

of yellowish-gray limestone; many films 

of yellowish-orange chert on bedding 

planes and in joints ____________ _____ _ 

Total adjusted thickness _________ _ 

Park shale: 

D-15. Shale, dark-greenish-gray to dusky-yellowish-green, weathering light greenish 

gray to pale olive, locally calcareous ___ _ Covered _______ ____ ________________ ___ _ 

Total adjusted thickness __________ _ 

Meagher limestone: 

D-14. Limestone like D-13, but beds somewhat 

thinner and with more orange clayey layers ___ ____________ _____ _____ __ ___ _ 

Covered __ __ __ ________ _______ ___ __ __ __ _ 

D-13. Limestone, alternating thin beds of medium-gray crystalline limestone and 

grayish-orange clayey limestone __ ___ __ _ 

D-12. Limestone, thick-bedded, medium-gra) with 

many fine grayish-orange silty streaks __ _ 

Covered. Rare outcrops nearby and float 

indicate this interval largely mediumgray, thin-bedded limestone, mottled pale 

orange to grayish orange, with many films 

and lentils of grayish-orange chert_ ____ _ 

D-11. Limestone, thin-bedded, medium-darkgray, nodular, grading into calcareous 

mudstone. Locally, dusky yellow to 

yellowish gray silty streaks. (Sequence 

is broken into units a few feet thick by 

11 latite sills ranging in thickness from 4 

in. to 3ft; sills not included) ____ _____ _ _ 

Unit 

thickness 

(feet) 

92 

77 

46 

400 

73 

30 

100 

93 

22 

93 

130 

107 

22 

Total adjusted thickness __ .. _________ _ 500 

Wolsey shale: 

Unit 

thickness 

(feet) 

D-10. Covered. Probably mostly gray shale_-__ 16 

D-9. Limestone, thin-bedded, light-olive-gray 

and grayish-orange, clayey, nodular. 

Orange strata fossiliferous ___________ - _ _ 86 

D-8. Shale, thick-bedd~d pale-brown to paleyellowish-brown and light-olive-gray, micaceous; a few yellowish-brown limestone 

laminae. (Latite sill, 5 ft thick, near base 

notincluded)_________________________ 91 

D-7. Shale and mudstone. Like D-5 but lacking 

limestone and quartzite ___ ------------- 64 

D-6. Shale and mudstone. 7 ft grayish-red shale 

impregnated with iron oxide (D-6a); 

overlain by 11 ft pale-reddish-purple 

mudstone flecked with white calcite crystals (D-6b) _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 18 

D-5. Mudstone and shale. Pale-yellowish-brown 

mudstone and medium-gray micaceous 

shale, slightly hornfelsed, with a few thin 

beds of ferruginous clayey limestone and 

pale-yellowish-brown glauconitic quartzite_ 81 

(Monzonite sill about 200 ft thick.) 

D-4. Largely covered. Small exposures of thickbedded, medium- to dark-gray hornfelsed 

shale and thin-bedded quartzitic sandstone 

like D-3------------------------------ 50 

Total adjusted thickness____________ 400 

Flathead sandstone: 

D-3. Quartzitic sandstone, thick-bedded, varicolored grayish-orange, pink to pinkishgray and yellowish-gray; largely medium, 

but coarse near top. Near top, a few thin 

laminae of yellowish-gray hornfelsed shale. 

Forms low ledge_______________________ 38 

Covered________________________________ 6 

D-2. Pebble conglomerate, subrounded____ _ _ _ _ _ _ 3 

Total adjusted thickness___________ _ 50 

Precambrian crystalline rocks: 

D-1. Alternating hematitic phyllonite and gneiss. 

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138 GEOLOGY OF THE THREE FORKS QUADRANGLE, MONTANA 

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A Page 

Acknowledgments_____________________ 5 

Age, andesite_ __ ______________ _____ ___ _ 100 

dacite ____ ------------_____________ 98 

Dunbar Creek formation_________ _ 79 

basalt___ ________ _______ ___________ 101 

latite ______________________________ .. _ _ _ __ _ 97 

Precambrian and metamorphic rocks_____ 8-9 

quartz monzonite____ ____ ____ _____ ___ ____ __ 95 

Three Forks shale _________________ __ ______ 33-34 

Age and correlation, Elkhorn Mountains volcanics _____ ----------------·· __ ____ 92-93 

Age and fossils, Cli.ri:J.bing Arrow formation __ 74-76 

Milligan Creek formation__________ ____ ____ 67 

Age and origin, Amsden formation ____________ 48-49 

Big Snowy formation _____________ ___ __ ____ 46--47 

Ellis formation____________________ _____ ___ 56 

Flathead and Wolsey formations_____ _____ 17 

Lodgepole limestone ___ __ _____ _____________ 40-41 

Meagher limestone __ ___________ ___ . _ _ _ _ _ _ 19-20 

Mission Canyon limestone _______ _________ 43-44 

Morrison formation -------------- _______ 57- 58 

Pilgrim formation ________________ ________ 22-24 

Quadrant formation __________ ___ ____ ___ __ 50- 52 

Sphinx conglomerate ________ _____ ____ ____ 63- 64 

Age and regional relations, North Boulder formation ____________________ ___ _____ 13-14 

Age and stratigraphic relations, Quaternary 

deposits __ ------------------------- 86-88 

Age, name, and correlation, Jefferson dolomite 27-28 

Maywood formation__ __ ___________ ________ 2~ 

Phosphoria formation ________ ____ __________ 53- 54 

Age, name, and origin, Kootenai formation __ 59-60 

Alexander, R. G. Jr., cited__ _____ ____ _________ 12 

Algae ___ __________________ _________ ____ ··- ___ __ 33, 74 

Allanaria _____ ______________________ ---··---- __ 25 

Allops sp______ __ ____ ___ ______ ________ _ ___ ____ 75 

Alluvium, old___ ___ _____________ _____ ___ ___ __ _ 82 

Alluvium, young_ __ ______________ __ ____ 86 

Amphibolite ____ ________ ____ ________ __ _ 

Amphipora_--------------------------- 26 

beds____ _____ __ ____ __________ ___ ___ 27 

Amsden formation ___ ____________ __ ____ ·- 47, 120, 122 

Andesite __ __________ ____ __ __ ____ ___ ___ . ___ __ 98-100 

Andesite, Elkhorn Mountains volcanics ______ 91-92 

Antiquatonia n. SP---------------------··------ 49 

SP---------------------------------··------ 48 Australorbis pstudoammonius ___ ___ ________ 68, 75,77 

Avonia SP----- ------------------------- _______ 41 

Axiodeaneia SP------------------------- 41 Azurite___ __ ___ _______ ____ _______ ______ 123 

B 

Barghoorn, Elso S., cited_ ___ __________ ____ ____ 75 

Barnes, V. E., with Cloud, P. E., quoted____ 22 

Bathyuriscus formosus_ ---------------- 19 

Beales, F .. W., cited_ _____ ____ __ ______ _________ 29 

Belt formation________________________ ___ ______ 12 

Belt series, Precambrian __ ______ ____ _____ __ ___ 9-14 

Belteroporic _______ ______________ __ ___ _ .. __ _ __ _ 19 

Benched terrain, origin oL __ . _________ ___ ____ 117-118 

Benches _____ ____ __ _____ _ "- ___________ _ ··-- __ 114-115 

Berry, G. W., quoted_----------------· 44, 56, 57,92 

Big Snowy basin_________ ____ _____ ___________ 120 

Big Snowy formation ___ _____________ _ <[4-45,122,124 

Bison antiquus________________________ __ ___ ___ 86 

INDEX 

Page 

"Black-and-gold marble"--------------------- 18 

Blackmon, P.R., analyst_ ________________ 65, 72,74 

Bolaspis sp_ _ _ __ _ _______ _ ___ ____ ____ _ ______ _ __ 17 

Botts, S.D., analyst_ ________________________ 78,91 

Boucot, A. J., quoted__________________ _______ 25 

Bozeman group, Tertiary rocks ________ c______ 60 

Brachiopods ___ ---------------------- 30, 32, 38, 40, 48 

Bridger Range________________________________ 13 

Brontotheres_ -------------------------------- 73,79 

Brontotherium sp ___ ---------------------- ____ 75 

Brown, R. W., cited___ _______ _________ _____ __ 76 

Bryozoans __ __ __ ________ c _______________ 30, 32, 40,49 

Buttleman laccolith_------------------------- 97 

Buxtonia SP----------------------------------- 46 

c 

Calcispongea_ __ ___ ________ ___ ___ _____ __ ____ __ 33 

Calcium carbonate"jelly"-------------------- 23 

Camarotoechia __ ----------------- _____ __ __ ____ 48 

h.errickana _______ --------•- __ _ ____ ____ __ _ _ 46 

mttallica______ _ _______ __ __ __ __ ___ __ __ ___ __ 41 

sp _ _ ___ _ _ _ _ _ __ ____ _ ________ __ _ __ _ _ _ _ _ __ _ __ 41 

Cambrian sedimentary rocks_________________ 14 

Carboniferous sedimentary rocks _____________ 38-39 

Cedaria sp __ ---------------------------------- 22 

Cenozoic basin, origin___ ______ ____ __ __ ____ ___ 111 

Cenozoic sedimentary rocks__________________ 60 

Cephalopods __ ---------------------------- 30, 32,38 

Chalcocite_ ___________ _____ ____________ ___ ____ 123 

Chamaecyparislaw~oniana ____ __ _____ ~-------- 75 

Chamberlain shale___________________________ 13 

Chapman, R. W., and others, quoted_ _______ 95 

Charophyte oogonia__________________________ 67 

Charophytes_ _ _ ______ __________ _____ _______ __ 73 

Cherry Creek series _________________________ _ 

Chonetes logan en sis __ ------------------------- 41 loganL ___ __ _ _ __ _ _ _ _ _____ __ _ __ _ ______ _ __ _ _ 41 

SP---------------------------------- 46, 47, 48,49 

Chrysocolla __ -------------------------------- 123 Cladopora_ _ ___ _ _ _ _ __ __ _______ _ _______ ___ ____ _ 26 

Clams---------------------------------------- 67 

Cleioth.yridina hirsuta _ -------------- __________ 46, 49 

incrassata___ __ _ __ _ _ __ _ ___ _ __ __ __ _ ___ _ __ _ __ 41 

SP- --------------------------------------- 48 

Climate: ------------------------------------- 6 Climbing Arrow formation _____ ___ __ __ ___ 61,69,121 

Cloud, P. E., and Barnes, V. E., quoted _____ 22 

Colorado formation_ ___________________ _______ 120 

Composita madi8onensis__ ______ __ ______ _______ 41 

ozerkana___ _ __ _ _ _ __ _ __ ___ _ _ __ _ _ ___ __ __ __ _ _ 46 

subquadrata___ __ _ __ _ _____ __ ___ __ ___ _ __ _ __ _ 49 

subtilita_ ___ _ __ _ _ ___ ________ _ __ _ __ _ _ __ ___ _ _ 48 

sulcata __ ___ ----------------------- ________ 48,49 

SP------------------------------ : ___ -- 48 

Construction materials ___ __ ---------------- 121-122 Copper ___ __ __ ---_____________________ __ _____ 122 

Copper City prospect___ ___________ _______ ___ 123 

Coquina limestor;e_ -------------------------- 30 Corals. ___ __ __ ____ ____ __ ____ ____ _______ ___ __ __ 26, 40 

Correlation and age, Elkhorn Mountains 

volcanics ____ ___ __ ___ _____ _____ ___ 92-93 

Corrt lation and name, Ellis formation________ 56 

Morrison formation_______________ ___ ____ 57 

Correlation and names. Bozeman group ______ 60-63 

Cambrian formations ______ ________ _______ 14-15 

Page 

Correlation, name, and age, Jefferson dolomite ______________________________ 27-28 

Maywood formation______________________ 25 

Phosphoria formation ___ __ __ _______ ___ ___ 53-54 

Covellite _________ ----------------------------- 123 

Cretaceous rocks, Upper _________________ ___ _ 89-100 

Cretaceous sedimentary rocks _____________ ___ 58 

Crickmay, C. H., cited ________ ___________ ___ _ 35,37 

Crinoids----------------------------- 30, 32, 40, 46,47 

Cupressaceae __ ------------------------------- 75 

Cypricardella sp_ ----------------------------- 48 Cypridopsis__ __ _ __ ____ _____ __ _ _ _ __ _ __ _ __ _ _ __ __ 67 

Cyrtospirifer fauna __ _____ _________ _____ ____ __ - 35,37 

D 

Dacite _______ _________________________________ 97-98 

Derby asp ___ ______________________ -----------

Devonian and Lower Mississippian rocks- ---

Diatoms--------------------------------------

49 

24 

74 

Diceratherium ___ _____________ ___ ___ ___ __ -__ -- 81 

armatum _ -------------------------------- 80 

Dictyoclostus infiatus ___ ____ ___ ____ ___ _____ ---- 46, 49 

sp_ ---------------------------------------

Dielasma sp _- --------------------------------

Dikes ________ __ -------------------------------

Dilophodon sp_- ------------------------------

Dip faults ___ __ ______ -------------------------

Distribution and name, Climbing Arrow formation_-------------- ___________ _ 

Distribution, name, and topographic expres48 

41 

7 

75 

14 

69 

sion, Dunbar Creek formation__ __ 77 

Dolomite origin ___________________________ 22-24, 29 

Dry Creek shales _____________ ______ __ 15, 25, 26, 119 

Dunbar Creek formation ___ _______________ 61, 77-80 

Duncan, Helen, quoted ________ ______________ _ 27 

Dutro, J. T., Jr., quoted ____ __ ____ _______ ____ _ 34 

E 

Echinoconchus SP----------------------------- 48 Echinoids _____ _____________ ____ _______ ------- 49 

Economic geology _____________________ ~ __ -- 121-123 

Ehmania ___ ---------------------------------- 17 

Elkhorn Mountains volcanics __ _______ __ _ 89-92, 120 

Ellis formation __ ______________ ___ ____ 52, 53, 55, 120 

Elmore, P . L. D., analyst_ ___________ _ 66, 73, 78, 91 

Elrathiella sp_ -~- --------- --- ---------- ----- 19 

Elrathina sp __ ---- ---------- ---- -------~- ---- 19 

Equisetum SP--------------------------------- 76 

Eumetria acuticosta___________________________ 49 

verneuiliana -------------------------------- 46 

Exshaw shale_____________________________ ___ _ 37 

Extrusive rocks------------------- ----- -- -- 89-92 

F 

Faults ____ __ _ 12, 13-14,21,27,34, 40, 42, 103-113, 120 

block __________________________ --- _____ --- 110 

steep, closely related to thrusting_________ 109 

in upper plate of Highway thrust_____ 108 

older than thrusting__________________ 105 

younger than the thrusting___________ 110 

thrust, Sixteenmile thrust zone___________ 105 

See also Structure. Fault system ___________ ___ __ ___ ------- __ -----

Fenestella SP---------------------------------- 41, 46 

141 
142 

Page 

Fenestellid bryozoans ___ --------------------- 48 

Fistuliporoid bryozoan gen. indet_____________ 48 

Flathead formation ____ ___ _______ _________ 13, 14, 15 

Flathead quartzite __ _ ------------------------ 15 

Flathead sandstone ___ ____ 7, 11, 14, 15--16, 17, 20, 119 

Flathead shales_______________________________ 15 

Flora_________________________________________ 5-6 

Folding, early Laramide ____________________ 103-105 

Folding and thrusting, time relations_________ 108 

Folds ________________________________ 27, 50, 103, 120 

Set also Struct1ire. 

Fossils ______ -----------------------------_____ 5-6, 

9, 17, 19, 22, 24, 25, 25, 27-28, 29, 30, 32, 40, 41 

46, 47, 48, 49, 53, 56, 73, 74, 79. 

Fossils and age, Climbing Arrow formation ___ 74-76 

Milligan Creek formation ___ ------------- 67 

G 

Gallatin formation __________ _________ ______ 14, 15,25 

Gallatin Range_______________________________ 8 

Gardner, L. S., and others, cited _____________ 50,55 

Gastropods __________________________ 41, 67, 68, 73,79 

Gattendorjia zone ___ --------------------______ 34 

General correlation and names of Carboniferous formations____________________ 39 

Geography __________________________________ _ 

Geologic history---------------------------- 118-120 

Geomorphology __ -------------------------- 113-117 

Girtyella woodsworthL_______________________ _ 46 

Gir'Oantlla ____ -------------------------------- 19 

Glass sand ________ ----------------- ---- ---- 122 

Glyphaspis robusta_ --------------------- ____ __ 17 

Gneiss- --------------------------------------- 7 Gravel, fan___________________________________ 84 

rounded_______________________________ ___ 82 

subrounded __ ---------------- _ ____ __ __ ___ 83 

Greyson shale_-------------__________________ 13 

Gyraulus sp_ -----------------------~--------- 59 

H 

Hanson, A. M., quoted _______________________ 19,22 

Hasmark_ ------------------------ _ _ _________ _ 15 

Hathaway, J. C., analyst ____ ____ _____________ 71,79 

Hayden, F. V., quoted_______________________ 8,9 

Haynes, W. P ., cited ________ 30,32,34,35,37,104,106 

Helburn, Nick, cited______ ___________ ________ 81 

Helminthoglyptidae n. gen___________________ 68 

Highway thrust_ _______________________ 106-107, 108 

Hills_ ------------------------- ___ ----------__ 114 

History __ ------------------------------------ 6 Cenozoic basin___________________________ 111 

geologic ________ ---------------- ________ 118-121 

previous work _____________ ___________ __ __ 3-5 

structural________________________________ 102 

Hossfeldt anticline _______________ 21, 32, 42, 47, 50,104 

Hossfeldt Hills area __ ------------------------ 9 

Hough, Jean, cited__ __ _________ ______________ 87 

Hyopsodus uintensis __ ------------------------ 75 

Igneous rocks ____ ___ ______________ ____ _______ 88-102 

Inhomogeneity breccia_______________________ 20 

Insectivores__________________________________ 73 

Introduction _________ ------------------------ 3 

Intrusive rocks ______________ ---------------- 93-100 Iron __ ~_______________________________________ 122 

Isectolophus sp ________ --------- __ ____ _ __ ______ 75 

J 

Jaffe, Elizabeth, norm calculated by____ __ ____ 91 

Jefferson Canyon thrust_ ____ ____ ___ ____ ____ 107, 111 

Jefferson dolomite __ ___________________ 24, 25,26-29 

Jefferson limestone___________________________ 15 

Juniperus __ ------------ __________ : ___________ 75 

Jurassic sedimentary rocks ___ ---------------- 54 

INDEX 

K Page 

Kinnikinic formation _________________________ 26 

Klemme, H. D., cited____ __________ ________ __ 13 

Klepper, M. R., cited ___ ._____________________ 92 

Kootenai formation _________________________ 58, 120 

Kootenia SP-------------------"--------------- 17 

Kormagnostus sp ___ -------------------------- 22 

L 

Lagomorphs__ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ __ _ ___ _ _ _ 73 

LaHood formation ____________________________ 12-13 

Laird, W. M., and Sloss, L. L., quoted_______ 28 

Laramide deformation________________________ 13 

Laramide orogeny_____ _____ ______________ ____ 120 

structural features ______________________ 103-110 

Latite ________________________________________ 95-97 

Leptaena analoga ___________ _ ----------------- 41 sp ____ -_ _ __ _ __ _ _ _ _ _ _ _ _ _ _ _ _ ___ _ _ _ __ _ __ _ _ _ _ _ 41 

Levandowski, D. W., cited___________ ______ __ 12 

Lewis, G. E., cited ___ ----------------- 74-75, 79, 81 

Libo,ctdrus decurrens __ ------------------------ 75 

Linoproductus _ ------------------------------- 48 

SP------------------------------ 41, 46, 47, 48, 49 

Lithology and thickness, Climbing Arrow formation_-------------------------- 69 

Dunbar Creek formation_________________ 77 

Milligan Creek formation_________________ 65 

Lodgepole limestone ___ ------------------- 29, 39-41 

Lombard thrust _____________ 21,104, 105,106, 108,124 

Lowell, W. R., cited__________ _______ _____ ____ 8,13 

Lower Mississippian and Devonian rocks_____ 24 

Lymnaea ____ --------------------------------- 68, 75 

M 

Macdonald, G. A., norm calculated by_______ 91 

Mack, M. D., analyst ________________________ 78,91 

McMannis, W. J., cited ___ __________ ____ __ __ _ 35,44 

Madison group _____ ________ ____ ______________ 39-52 

Madison Range __ ---------------------------- 8 

Madison Valley formation____________________ 87 

Magnetite____________________________________ 123 

Malachite____________________________________ 123 

Manganese___________________________________ 122 

Marsupials__________ ____ _____________________ 73 

Maywood formation _______ ______ __________ 21,24-26 

Meagher formation_-- ------------------------ 15,22 

Meagher limestone ____________________ 14, 17-18, 119 

Measured sections __________________________ 124-135 

Merychippus SP------------------------------- 81 

Merycoidodon gracilis ___ ---------------------- 75 

Mesozoic sedimentary rocks__________________ 54 

Metamorphic rocks--------------------------- 7 Mica ________________ --------_________________ 122 

Middle and upper Tertiary graveL __________ 80-81 

Milligan Canyon, intrusive rocks north oL __ 98-99 

Milligan Creek area__________________________ 9 

Milligan Creek formation _____________ ____ ___ 61, 121 

Mission Canyon limestone _______ 41-44, 119, 120, 124 

Morrison formation _______ ___________ _____ ___ 57, 120 

Mottled limestones--------------------------- 15 Mud Spring area_____________________________ 9 

Myalina sp ____________ ----------------- ______ 49 

N 

Name, North Boulder formation ___________ ___ 12-13 

Three Forks shale________________________ 32 

Name, age, and correlation, Jefferson dolomite_ 27-28 

Maywood formation______________________ 25 

Phosphoria formation ___ _ ---------------- 53-54 

Name, age, and origin, Kootenai formation ___ 59-60 

Name and correlation, Ellis formation________ 56 

Morrison formation_______________________ 57 

Name and distribution, Climbing Arrow formation ___ -------------___________ 69 

Milligan Creek formation ______ ____ ____ o__ 64 

Page 

Name, distribution, and topographic expression, Dunbar Creek formation____ 77 

Names and correlation, Bozeman group ______ 60-62 

Cambrian formations _______ __ ______ ______ 14-15 

Names and general correlation of Carboniferous formations____________________ 39 

Nautiloids ----------------------------------- 30 Neospirifer n. sp.:_____________________________ 46 

Newland limestone___________________________ 13 

North Boulder formation ___________________ 9, 11-12 

0 

Olivine basalt_------------------------------- 100 

Ore deposits, future discoveries_______________ 123 

Oreodon beds of Matthew-------------------- 61 

Oreodonts ________ _____ ______ ----------------- 73, 79 

Oreohelix (Radiocentrum)______________________ 67 

n. SP-------------------------------------- 68 

Origin, Climbing Arrow formation___________ 76 

Dunbar Creek formation_________________ "80 

fan graveL----------------~-------------- 85 Jefferson dolomite_________________________ 28 

Maywood formation______________________ 25 

Milligan Creek formation_________________ 67 

North Boulder formation_________________ 12 

Phosphoria formation ____ ---------------- 54 

Precambrian metamorphic rocks_________ 8 

Three Forks shale_________________________ 38 

Origin, age, and name, Kootenai formation ___ 50-60 

Origin and age, Amsden formation ___________ 48-49 

Big Snowy formation _____________________ 46-47 

Ellis formation___________________________ 56 

Flathead and Wolsey formations._------- 17 

Lodgepole limestone ______________________ 40-41 

Meagher limestone ___________ ____________ 19-20 

Mission Canyon limestone _______________ 43-44 

Morrison formation _______________________ 57-58 

Pilgrim formation ________________ __ ______ 22-24 

Quadrant formation _______________ _______ 50-52 

Sphinx conglomerate ._------------------- 63-64 

Orthoclymenia ____ ---------------------------- 34 

Orthotetes sp ________ ----------- _ -------------- 41 

Orthotetid brachiopod________________________ 48 

Ostracodes ___ __ ________ _: ______________ 32, 67, 73,79 

p 

Paleolagus temnodon__________________________ 75 

Paleozoic sedimentary rocks__________________ 14 

Paracyclus SP--- ___________ _ ------------------ 41 

Park formation __________________ 14, 15, 18, 20-21, 25 

Parker, C. J., analyst_ _______________________ 71, 79 

Peale, A. C., quoted__________________________ 9, 

12, 13, 14, 28, 35, 39, 60, 92, 104 

Pebbly limestones----------------------- 15, 25, 119 

Pebbly sand member, Climbing Arrow formation______________________________ 74 

Pegmatite ____________________________________ 7, 122 

Pelecypods___________________________________ 32 

Camptonectes _ ____ ______ _ _ _______ __ _ ______ 56 

Permian sedimentary rocks___________________ 52 

Peronopsis scutalis. _____ ------------- ____ ----- 19 

Petrography, andesite _______________________ 99-100 

dacite ________________ --------------- _____ 97-Y8 

Elkhorn Mountains volcanics _______ ._____ 91 

igneous rocks----------~--- ____ __ _______ 100-101 

latite ___________ ____ ____ ---------- __ ______ 96-97 

quartz monzonite _________________________ 94-95 

Petroleum possibilities ____ ------------------- 123 

Petrologic relations, igneous rocks__________ 101-102 

Phosphate rock_______________________________ 122 

Phosphoria formation ____ ------------------- 52, 120 

Phyllonite, hematitic___ ______________________ 7-8 

Physidae_ __ _ __ ___ _ _ __ _ __ ___ _____ _____ ________ 68 

Pilgrim limestone _________ 14, 15, 20, 21-24,25,27, 119 

Pisidium __ ----------------------------------- 67 Planorbidae indet_ _____ ______________ ____ ____ 68,75 

Platyceras sp __________________________ -_- _ _ _ _ 41 
Page 

Platyclymenia ____ -------------------------- __ 33 

(Pleuroclymenia) crassa___________________ 34 

Pluglike bodies of Jatite_______ ________________ 96 

Pluton, lON ----------------------- 93-94,95,101, 122 Pollen ________________________________ -------_ 74 

Polygyrella ___ ---------------------- __________ 68 Pony series ________ __________________________ _ 

Porphyritic perlite, Elkhorn Mountains 

volcanics.------------------------ 89,92 

Post-Laramide structural features __ ---------- 112 

Precambrian, Belt series---------------------- 9-14 

crystalline rocks, structure________________ 102 

sedimentary rocks, structural significance_ 103 

Productids _________ -------------- __ __ ____ _ _ 47, 48 

Productus gallatinensis ________________________ 41 , 47 

Proetid trilobite______________________________ 41 

Prolobites-Platyclymenia_ _ __ ________ ____ ______ 34 

Protoreodon SP-------------------------------- 75 

Prototreta ftabellata ___ ---------- _____ ------ ___ 19 

Psuedofusulina Parafusulina__________________ 53 

Psuedoschwagerina montanaensis______________ 53 sp _ _ _ _ _ _ _ _ _ _ __ _ _ __ _ _ _ _ _ _ _ _ __ _ _ __ _ _ _ _ _ _ _ _ __ 53 

Ptychagnostus atavus_________________________ _ 19 

Ptychoparioid gen. and SP-------------------- 19 

Punctospirifer SP------------------------------ 46, 48 Punctospiriferoids _____________ _______________ 47,48 

Pupillidae________ __ ________ ____ _______ _ __ ____ 68 

Q 

Quadrant formation __________ _______________ 49,120 

Quartz monzonite____________________________ 122 

Quartz monzonite and related rocks __________ 93-95 

Quaternary rocks----------------------------- 81-86 

R 

Radersburg syncline ____________________ 103,105, 124 

Railroads--------------------_________________ 6 

Raymondiceras ... _ ---------------------------- 33 

Reesidella montanensis___________ _____________ 59 

References .. __________________________________ 135 

Regional relations, Precambrian metamorphic 

rocks_____________________________ 9 

Regional relations and age, North Boulder formation ____________ " ____________ 13-14 

Relation of established formations of Big 

Snowy group to Big Snowy formation_____ _________________________ 45 

Rhinoceros ___________________________________ 80 

Rhipidomella SP------------------------------- 47,48 

Rhipidomella missouriensis____________________ 35 

Rhynchotherium_____ ___ _________ ____ _______ ___ 81 

Rierdon formation·-----------------------~ -- 56 Roads ________________________________ -·--____ 6 

INDEX 

Page 

Rodents _____________ ------------------------_ 73, 79 

Ross, C. 8 ., and Hendricks, S. B., cited______ 76 

s Sappington problem __________________________ 34-38 

Sappington sandstone_----------------------- 33 

Sawtooth formation__________________________ 56 

Schizophoria compacta_________________________ 41 

sp__ __ __ _ _ __ _ ___ _ _ ________ ___ _ _________ _ __ 41 

Schmitt, G . T., quoted_______________________ 58 

Schuchertella _____ _______ ------------ _______ ___ 34 

chemungensis ________ -------------- ______ _ 41 

SP---------------------------------------- 41, 46 Sch wagerina sp _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 53 

Section A _______ ________ ----------____________ 124 

Section B_________________ ___________________ _ 129 

Section C_____ _____ _ __ _ ______ _ ____ _____ _______ 131 

Section D _ _ _ _ ____ __ __________ __ __ ___ _______ _ _ 132 

Sedimentary rocks---------------------------- 9-88 

Settlements ____________________ ---------_____ 6 

Sill west of Spring Creek____ _________________ 94 

Sills, andesite, south of Milligan Canyon_____ 98 latite_ _ _ ______ _______ _____ ____ ___ ___ _____ _ 96 

Silt, eolian ______________________ ______________ 85-86 

Sixteenmile thrust zone _______ __________ 105,120, 123 

Sloss, L. L., quoted__________________________ 13 

Sloss, L . L., and Laird, W. M., quoted _______ 28,37 

Smedley, J. E., quoted ____________________ 41, 46,48 

Snails. __ ------------------------------------- 67 Spaghetti beds. __ ------------ ________________ 26, 29 

Sphinx conglomerate _____________________ 62-64,121 

Spirifer brazerian'Us _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 46 

centronatus _______________________________ 41, 47 

increbescens _______ ------------ ____________ 48, 49 

striatw! madisonensis______________________ 41 

n. sp ___ . ______ _ ------------------------- _ 49 

Spiriferina solidirostris _______ -------------- __ _ 41 

SP---------------------------------------- 41 

Spokane shale _________ ----------------------- 31 

Stenocisma SP--------------------------------- 46 

Straparolid __________ --------------------------- 41 

Straparollus (Euomphalus) SP----------------- 49 sp__ ___ __ _ __ _ ___ __ _ _________ ___ _ __ ______ _ _ 41 

Stratigraphic relations and age of Quaternary 

deposits.------------------------- 86-88 

Stratigraphic succession ______ ---------------- 10 

Streams _______________________ ------------- 115-117 

Strike faults__________________________________ 14 

Structure ___________________________________ 102-113 

Summary of age relations, intrusive rocks___ __ 67, 

-- 68, 75,77 

Sweetgrass Arch _____________________________ 56, 120 

Swift formation __ _____________________ -------- 56 

Synclines____________________ _________________ 104 

Syringothyris fauna __ ------------------------- 35,37 

143 

T Page 

Tansley, Wilfred, quoted_____________________ 9 

Tapiroids ___________________ -------- ___ __ _ __ __ 73 

Tarr, W. A., cited ___ -------------------------- 23 

T aylor, D. W., cited_________________________ 79 

Tertiary rocks, Bozeman group_______________ 60 

Tertiary rocks, lower_ _______________________ 89-100 

Thickness and lithology, Climbing Arrow 

formation ____ -------------------- 69 

Dunbar Creek formation_________________ 77 

Thin sills ___________ ------------______________ 96 

Thompson Creek beds of Douglass___________ 61 

Three Forks basin, creation oL_______________ 111 

Three Forks formation_______________________ 120 

Three Forks shale ________________ 29-38, 101, 119, 120 

Thrust displacement. __ ---------------------- 107 

Thrusting and folding, time relations_________ 108 

Thrusts, minor_ ___________________ ----------- 107 

Thuja plicata_ -------------------------------- 75 

Titanotherium beds of Matthew·------------- 61 Tobacco Root Mountains ___________________ _ 

Tomagnostus sp ___ c___________________________ 19 

Tonkinella stephenensis ________________ _"______ 17 

Topographic expression, Climbing Arrow formation__ _________________________ 69 

Milligan Creek formation_________________ 64 

Topographic expression, name, and distribution, Dunbar Creek formation____ 77 

-Tornoceras ________ ------------------- _ _ ______ _ 33 

Torynifer cooperensis __________________ --------- 41 

Trilobites ______________________________ 17, 22, 41.46 

Trilobite limestone___________________________ 15 

Turtles ________ ----------------- ______________ 73, 79 

Typha sp____ _ ______ _ __ _ _____ _ _ ___ ___ _ ______ __ 76 

u 

Unio SP-------------------------------------- 59 

Uplift. _________ ------------- ___ ------------ 117, 120 

v 

Verrall, Peter, cited ____________ 11,13,28,31,35,38 ,58 

w 

White, K. E., analyst ________ __ ____________ b6, 73,78 

Willow Creek area____________________________ 9 

Willow Creek canyon _______________________ _ 

Willow Creek fault_ ________ __________ 13, 14,111,119 

Wocklumeria_ ----------------------- _________ 34 

Wolsey formation __ --------------·- ------ 13, 25, 120 

Wolsey sea___________________________________ 17 

Welsey shale _____________ 11, 14, 15, 16-17,20, 119-120 

Wood, H. E., cited___________________________ 80 

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