The Lewis Thrust Fault and 

Related Structures in the 

Disturbed Belt, 

Northwestern Montana 

G E 0 L 0 G I CAL SURVEY P R 0 FE S S I 0 N A L PAPER ll74 

The Lewis Thrust Fault and 

Related Structures in the 

Disturbed Belt, 

Northwestern Montana 

By MELVILLE R. MUDGE and ROBERT L. EARHART 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1174 

M ajar structures reflect large-scale 

easterly displacement of 

early Tertiary age 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:l980 
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary 

GEOLOGICAL SURVEY 

H. William Menard, Director 

Library of Congress Cataloging in Publication Data 

Mudge, Melville Rhodes, 1921-

The Lewis thrust fault and related structures in the disturbed belt, northwestern Montana. 

(Geological Survey Professional Paper 11 74) 

Bibliography: p. 17 

Supt. of Docs. no.: I 19.16:1174 

1. Faults (Geology)-Montana. 2. Geology-Montana. I. Earhart, Robert L., joint author. 

II. Title. III. Series: United States Geological Survey Professional Paper 1174. 

QE606.5.U6M82 551.8'7'09786 80-607023 

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

Washinton, D.C. 20402 
CONTENTS 

Page 

Abstract 

Introduction .............................................................................................................................................. 1 

Geology ...................................................................................................................................................... 6 

Lewis thrust fault ...................................................................................................................................... 6 

Displacement along the Lewis thrust fault ........................................................................................ 11 

Age of Lewis thrust fault ........................................................................................................................ 14 

Structures related to the Lewis thrust fault ........................................................................................ 15 

Hoadley thrust .................................................................................................................................. 15 

Continental Divide syncline .......................................................................................................... 16 

Summary .................................................................................................................................................... 16 

References cited ·····················--------------------------------------------------······················------------------------------------------- 17 

ILLUSTRATIONS 

Page 

FIGURE I. Index map of the area showing the Lewis, Hoadley, and Eldorado thrust faults from Glacier National Park south to 

Canyon Creek, northwestern Montana............................................................................................................................................ 2 

2. Geologic maps of the Lewis and Eldorado thrust plates from the southern part of Glacier National Park to Steamboat 

Mountain ............................................................................................................................................................................................ 3 

3. Correlation chart of Precambrian rocks of the Belt Supergroup in th~ eastern part of the Rocky Mountains in northwestern Montana ................................................................................................................................................................................ 7 

4. Photograph showing the Lewis thrust at the south end of Glacier National Park .................................................................... 7 

5. Closeup aerial photograph of the Lewis thrust fault above folded and faulted Lower Cretaceous rocks .............................. 7 

6. Photograph showing the Lewis thrust fault at the south end of Glacier National Park .......................................................... 8 

7. Photograph showing the Lewis thrust at the base of deformed Altyn and Greyson Formations at the south end of 

Glacier National Park ...................................................................................................................................................................... 8 

8. Cross section of the Lewis and Eldorado thrust plates -------------·················································----------------------------------······················ 9 

9. Photograph showing the Lewis thrust fault from Pot Mountain north along the west side of Dolly Varden Creek .......... 9 

10. Photograph showing the Lewis thrust near the head of Moose Creek ------·--------------------------------···············------------------------------------ 9 

11. Cross sections of the Hoadley and Eldorado thrust plates ................................................................................................................ 10 

12. Photograph showing southern limit of the Lewis thrust at the southwest side of Steamboat Mountain .............................. 12 

13. Map showing projected strikes of strata in the Lewis and Hoadley thrust plates ........................................................................ 13 

14. Chart showing thickness of Precambrian units in the Lewis and Eldorado thrust plates .......................................................... 14 

TABLE 

Page 

TABLE 1. Stratigraphic throw and estimated slip of the Lewis thrust from the Dearborn River north to Glacier National Park 14 

III 

THE LEWIS THRUST FAULT AND RELATED STRUCTURES IN THE 

DISTURBED BELT, NORTHWESTERN MONTANA 

By MELVILLE R. MUDGE and ROBERT L. EARHART 

ABSTRACT 

The classical Lewis thrust fault in Glacier National Park has 

now been mapped 125 km south of the park to Steamboat 

Mountain, where the trace dies out in folded middle Paleozoic 

rocks. The known length of the fault is 452 km, extending 

northward from Steamboat Mountain to a point 225 km into 

Canada, where the fault also dies out in Paleozoic rocks. 

At the south end, the surface expression of the Lewis thrust 

begins in a shear zone in folded Mississippian rocks. To the north, 

the thrust progressively cuts downsection into Proterozoic Y (Belt) 

rocks near Glacier National Park. 

Displacement on the Lewis plate increases northward from 

approximately 3 km on an easterly trending hinge line at the West 

Fork of the Sun River to a postulated 65 km at the southern edge of 

the park, where the stratigraphic throw is about 6,500 m. Present 

data indicate the thrust formed during very late Paleocene to very 

early Eocene time. 

The Lewis thrust and related structures, the Hoadley thrust and 

the Continental Divide syncline, probably formed concurrently 

under the same stress field. The northern limit of the trace of the 

Hoadley thrust is within the lower portion of the Lewis plate, 

about 28 km north of where the Lewis thrust develops, and the 

Hoadley extends for at least 125 km to the south. Displacement of 

the Hoadley increases southward from about 1 km at the hinge 

line to an inferred 70 km near its known southern extent. If our 

inference is correct, the Hoadley is nearly the southern mirror 

image of the Lewis to the north. The Continental Divide syncline, 

a doubly plunging, broad, northerly trending open fold that is 

about 120 km long, is a major fold within the Lewis plate. 

INTRODUCTION 

The Lewis thrust fault was named in 1902 by Willis 

from exceptional exposures along the east and south 

sides of Glacier National Park in northwestern 

Montana. Since then it has been considered a 

classical thrust fault, and it is discussed in most 

textbooks on structural geology. 

Prior to our studies, the Lewis thrust fault had been 

mapped for 327 km in Glacier Park and adjacent 

parts of Canada. The fault is exposed at many places 

for approximately 80 km along the front of the Lewis 

Range from near Marias Pass to the Canadian 

border (Campbell, 1914; Billings, 1938; and Ross, 

1959). North of the border, the fault has been traced 

along the Clarke Range, the Canadian continuation 

of the Lewis Range (Clark, 1954); north of the Clarke 

Range, the trace of the fault approximately parallels 

the British Columbia-Alberta provincial boundary 

and terminates in Paleozoic rocks at least 225 km 

beyond the U.S. border (Douglas, 1958; Price, 1965). 

This paper discusses new descriptive data of the 

Lewis thrust fault and related structures south from 

Glacier National Park for 125 km to Dearborn River 

canyon, where the trace of the Lewis developed from 

fractures in a fold (fig. 1). The Hoadley thrust 

originated in the Lewis plate, about 28 km north of 

the point of origin of the Lewis, and extends at least 

125 km to the south. 

The Lewis thrust fault and related structures form 

the western structures of the disturbed belt of 

Montana. To the north the Lewis is the westernmost 

thrust fault, whereas to the south the Hoadley is 

westernmost. They are west of the intricately thrust 

faulted and folded Paleozoic and Mesozoic rocks that 

compose the eastern and central parts of the 

disturbed belt south of Glacier National Park. In the 

park most of these structures plunge beneath the 

Lewis thrust plate; some extend northwest just east 

of the park. 

The disturbed belt lies west of the Sweetgrass arch, 

a structurally high area that consists of the south 

arch and Kevin-Sunburst dome (Dobbin and Erdmann, 1955; Alpha, 1955). The Sweetgrass arch was 

active tectonically at various times from Precambrian time to near the end of the Cretaceous (Mudge, 

1972b). The position and configuration of the Sweetgrass arch influenced the structural development of 

the disturbed belt, which was formed in early Tertiary time. 

Three major structures along the west side of the 

arch (Dobbin and Erdmann, 1955) affected the 

deformation pattern of the disturbed belt. A broad, 

1 
2 LEWIS THRUST FAULT, NORTHWESTERN MONTANA 

~o~~~oo_o·-----------,----------~3o~·--------,--.----------~11~3~~o~·----------r-----------~3r0' __________ -, ___________ 1112~0' 

48°00' 

30' 

47°00' 

GLACIER NATIONAL PARK 

Littie Dog 

Bob 

Wilderness 

boundary 

10 

j .___, __ / \" 

\) 

... ....__,! 

20 30 KILOMETERS 

EXPLANATION 

.3 MEASURED SECTJON-11 West Fork of the 

Sun River; 21 Middle Fork of the Flathead 

River; 31 Camp Creek; 41 Ahorn Creek; 51 

Bungalow Mountain; 61 Pivot Mountain; 

7, southwestern Glacier National Park; 

81 Flathead Range; 91 Swan Range 

~ THRUST FAULT-Sawteeth on upper plate 

_J...._ NORMAL FAULT-Bar and ball on 

downthrown side 

A--A' LINE OF SECTION-A-A' in figure 8; 8-8' I 

C-C' I and D-D' in figure 11 

STRUCTURE SYMBOLS 

Marshall 

Wilderness 

boundary 

Anticline 

Syncline 

BRITISH ~ ALBERTA 

COLUMBIA\ 

-~tiN~CAN,-~~~fu; ', Study 

\, area 

IDAHO'~ MONTANA 

c. Lincoln 

46°45' L__ _____ L_ _____ .L_ _____ l_ _____ L_ _____ .L_ _____ J._ _____ _L__ ____ __. 

FIGURE 1.-Index map of the area showing the Lewis, Hoadley, and Eldorado thrust faults from Glacier National Park south 

to Canyon Creek, northwestern Montana. 
INTRODUCTION 3 

~0',-----~13T~=o~----------------------------~15~----------------------------~1~13T~=O_' __________________ , 

15' 

48"00' 

GLACIER NATIONAL PARK 

• • 

EXPLANATION 

CRETACEOUS ROCKS 

MISSISSIPPIAN ROCKS 

DEVONIAN ROCKS 

CAMBRIAN ROCKS 

PROTEROZOIC Z DIORITE SILLS 

PROTEROZOIC Y ROCKS 

ROCKS OF BELT SUPERGROUP 

UNDIFFERENTIATED 

Garnet Range Formation 

McNamara Formation 

Bonner Quartzite 

Mount Shields Formation 

Shepard Formation 

Snowslip Formation 

Helena Formation 

Empire and Spokane Formations 

Spokane Formation 

Greyson Formation 

CONTACT 

FAULTS 

Thrust-Sawteeth on upper plate 

Normaf--Bar and ball on downthrown side 

STRUCTURES 

Anticline 

Syncline 

A--A I LINE OF SECTION 

A. SOUTHERN PART OF GLACIER NATIONAL PARK AND DRAINAGES OF 

THE MIDDLE FORK OF THE FLATHEAD RIVER 

FIGURE 2.-Geologic maps of the Lewis and Eldorado thrust plates from the southern part of Glacier National Park to Steamboat 

Mountain. Cross-section A-A' shown in fig. 8. Cross-sections B-B' and C-C' shown in fig. 11. 
4 LEWIS THRUST FAULT, NORTHWESTERN MONTANA 

10 15 KILOMETERS 

B. SPOTIED BEAR RIVER AREA TO THE WEST FORK OF THE SUN RIVER 

FiGURE 2.-Continued. 

gently westerly dipping structural low area extends 

beneath Glacier National Park (Mudge, 1977). The 

structurally high south arch extends west beneath 

the central part of the disturbed belt. Farther south, a 

broad, southwest-dipping structural low area 

extends beneath the southern part of the disturbed 

belt. 

The structures composing the disturbed belt of 

Montana are a result of an early Tertiary orogeny 

and are equivalent in age only to the easternmost 
INTRODUCTION 5 

47°1 

C. WEST FORK OF THE SUN RIVER TO STEAMBOAT MOUNTAIN 

FIGURE 2.-Continued. 

thrust faults in the Idaho-Wyoming thrust belt and 

to the foothills and possibly the eastern main range 

structures in Alberta and British Columbia. 

Previously, the location and character of the Lewis 

thrust south of Marias Pass were uncertain because 

of the lack of adequate geologic mapping. The Lewis 

thrust was traced 21 km south of Marias Pass by 

Childers (1963), at which point he shows the thrust 

terminated by a normal fault. Earlier investigators 

(Clapp, 1932, Deiss, 1943a, b; and Ross, 1959) 

speculated that the Lewis continued to the southeast 

for an undetermined distance. In the Sun River area 

they applied that name to the thrust mapped as the 

South Fork thrust zone by Mudge (1966a and b; 

1972a and b), and Clapp (1932) connected the Lewis 

with the Lombard thrust to the southeast in the 

Three Forks area. Their speculation was based on 

the observation that Precambrian rocks are thrust 
6 LEWIS THRUST FAULT, NORTHWESTERN MONTANA 

over rocks of late Paleozoic to Cretaceous age, at 

these places, as they are along the Lewis thrust in 

Glacier National Park. Mudge (1972b, p. B27-B28) 

described more than one thrust fault to the south that 

juxtaposes Precambrian rocks on Paleozoic and 

Mesozoic rocks and also noted that the Precambrian 

rocks in the other plates to the south are a nearshore 

facies, whereas rocks of the Lewis plate are of a 

basinward facies. As discussed by Mudge (1972b, p. 

B27), the data at that time were insufficient to 

determine the relationship, if any, between the 

Lewis, Lombard, and South Fork thrusts. Our 

studies show the Lewis to be a thrust fault west of the 

South Fork and Lombard thrusts. Mapping 

southeast of the area by M.W. Reynolds (oral 

commun., 1978) has established the South Fork 

thrust of Mudge (1972b) as the Eldorado thrust. We, 

therefore, adopt the name Eldorado for the South 

Fork thrust. 

Our study of the Lewis fault started in 1968 south 

of Glacier National Park in conjunction with 

mineral resource evaluations, including 

reconnaissance geologic mapping, of the Bob 

Marshall, Great Bear, and Scapegoat Wilderness 

areas and their proposed additions. We plotted all 

geologic contacts and structures of most of the area 

shown in figure 1 on 1:24,000-scale topographic 

maps from mapping traverses 3 to 6 km apart that 

were supplemented by aerial mapping from a 

helicopter. These data were then compiled on a single 

1:125,000 topographic map. The area studied, 

approximately 8,100 km2, extends from the northern 

part of the Flathead Range, west of the park, south to 

Lincoln, Mont. (190 km), and east from the Swan 

River valley to the eastern edge of the Sawtooth 

Range (84 km)-thus including an area of intense 

deformation called the northern disturbed belt in 

Montana. 

GEOLOGY 

The Lewis thrust plate consists entirely of 

Precambrian rocks in Glacier National Park (fig. 

2A), but farther south, in the Bob Marshall, Great 

Bear, and Scapegoat Wilderness Areas, it also 

contains Paleozoic rocks as young as Mississippian 

in age (figs. 2B, 2C). The nomenclature of the 

Precambrian units now used in this region is 

compared with older nomenclature in figure 3. The 

results of our mapping of the Sun River Canyon area 

confirmed the correlation of the Precambrian units 

as projected by McGill and Sommers (1967), and 

Mudge (1972a). The units are described in Glacier 

National Park by Willis (1902) and Ross (1959 and 

1963), in the Marias Pass area by Childers (1963), in 

the Wood Creek Hogsback-Camp Creek area by 

McGill and Sommers (1967), in the Sun River 

Canyon area by Mudge (1972a), in the Scapegoat 

Wilderness by Mudge and others (1974), and in the 

Swan, Flathead, Lewis and Clark, and Sawtooth 

Ranges by Mudge and Earhart (1978). Cambrian 

rocks in the thrust plates have been described 

thoroughly by Deiss (1933, 1939, 1943a), and the 

Devonian and Mississippian rocks by Sloss and 

Laird (1945) and Mudge (1972a). 

LEWIS THRUST FAULT 

The geology of the Lewis thrust plate in Glacier 

National Park has been described by Willis (1902), 

Campbell (1914), Billings (1938), and Ross (1959). 

The erosional front of the Lewis thrust at Glacier 

National Park is along the eastern edge of the 

mountains, but both north and south of the park it 

swings abruptly westward into the mountains (Ross, 

1959, p. 76). In Canada the westerly bend in the 

thrust trace begins about 40 km north of the 

international boundary (Price, 1965, fig. 1). At the 

south edge of the park the westerly bend begins 

about 74 km south of the international boundary 

(Ross and others, 1955; Ross, 1959). Everywhere in 

Glacier National Park the Lewis thrust plate 

contains Precambrian rocks and overlies Cretaceous 

mudstones and sandstones. The Lewis thrust has an 

average strike of N. 30° W. and dips west at a low 

angle, mostly less than 10° (Ross, 1959, p. 76). Within 

the Lewis plate in the central part of the park, the 

west limb of a large, broad, northwesterly trending, 

doubly plunging syncline (Mudge, 1977) was 

displaced by a large normal fault, variously called 

the North Fork normal fault, the Flathead fault, and 

the Blacktail fault (Childers, 1963, p. 159). 

Discussions of the geology at the southern edge of the 

park by Childers (1963), Mudge, Earhart, and Rice 

(1977), Mudge and others (1977), and Mudge (1977) 

are pertinent to this report. 

The Lewis fault is well exposed along the south 

edge of the park for a distance of 19 km (figs. 4, 5, 6, 

and 7). The dip of the fault plane along most of the 

trace is between 15° and 25° NW.; it strikes N. 20° E. 

(fig. 4). At the west end of a salient, south of Elk 

Mountain, the fault surface dips 15° to 40° SW.; 

broad, open, southwesterly plunging folds cause 

variations in its strike (Childers, 1963, p. 158). 

The Precambrian Altyn and Greyson Formations 

compose the Lewis thrust plate at the south end of 

the park. The Altyn Formation (figs. 4, 5, 6, and 7) 

forms the sole of the plate as far southwest as 1.6 km 

southwest of Little Dog Mountain. Along the 

southern part of the park the Altyn and Greyson are 

complexly folded, brecciated, and in places repeated 
LEWIS THRUST FAULT 7 

c: 

~ "' 

Sun and 

Dearborn Rivers1 

Flathead Quartzite 

E 

Ll .-1----V-.__..---._.~ uNCONFORMITY 

:; "' 

0 

.!!1 "'

~ 

c: 

"' 

~ 

E 

"' e " 

a.. 

-

"' > 

a: "'

-

'iii 

> 

a: "' ,j, 

a: 

Garnet Range Formation 

McNamara Formation 

Bonner Quartzite 

Mount Shields Formation 

Shepard Formation 

Snowslip Formation 

Helena Dolomite 

Empire and 

Spokane Formations 

Greyson Formation 

Thrust fault 

Marias Pass 

(Childers, 1963) 

Red Plume Quartzite 

Shields Formation 

Shepard Formation 

Snowslip Formation 

Siyeh Formation 

Grinnell Formation 

Appekunny Formation 

Thrust fault 

1 McGill and Sommers, 1967; Mudge, 1972a; and Mudge and others, 1974. 

2McGi ll and Sommers, 1967; Mudge, 1972a; Harrison, 1972; Mudge and 

others, 1974; Mudge, Earhart, and Rice, 1977; and Mudge and Earhart, 1978. 

Glacier Park 

(Willis, 1902) 

Kintla Formation 

Shepard Quartzite 

Siyeh Limestone 

Grinnell Argill ite 

Appekunny Formation 

Altyn Limestone 

Thrust fault 

Northeastern 

Rocky Mountains2 

McNamara Formation 

Bonner Quartzite 

Mount Shields Formation 

Shepard Formation 

Snowslip Formation 

Helena Formation 

Empire Formation 

Spokane Formation 

Greyson Formation 

Altyn Formation 

Thrust fault 

FIGURE 3.-Correlation of Precambrian rocks of the Belt Supergroup in the eastern part of the Rocky Mountains in northwestern 

Montana. 

FIGURE 4.- View northeast of the Lewis thrust at the south end of 

Glacier National Park. The light-gray band in the middle of 

the photo is deformed Altyn Formation, which overlies the 

Lewis thrust. The Greyson Formation overlies the Altyn. 

Folded and faulted Cretaceous rocks underlie the Lewis 

thrust. 

by small easterly and westerly dipping thrust faults. 

The Greyson Formation overlies the Altyn, and 

farther south it is the oldest unit in the plate. 

Along the southern edge of the park, the Lewis 

plate everywhere rests on Cretaceous rocks (figs. 4, 5, 

FIGURE 5.-Closeup aerial view of the Lewis thrust fault above 

folded and faulted Lower Cretaceous rocks south of Little Dog 

Mountain, south end of Glacier National Park. 

6, and 7). Southeast of Elk Mountain, the plate 

mostly overlies the Lower Cretaceous Kootenai 

Formation, which commonly dips 45° to the 

southwest and is repeated by numerous steep, 

westerly dippling thrust faults. East of this area, the 

Lewis plate rests on progressively younger 

Cretaceous rocks that are repeated by many westerly 

dipping low-angle thrust faults. East of Little Dog 
8 LEWIS THRUST FAULT, NORTHWESTERN MONTANA 

FIGURE 6.-Aerial view of the Lewis thrust fault at the south end of 

Glacier National Park. Note the small imbricate thrust that 

repeats the Altyn and Greyson Formations (light and dark 

units, respectively). 

FIGURE 7.-Aerial view to the north showing the Lewis thrust at 

the base of deformed Altyn (light) and Greyson (dark) 

Formations at the south end of Glacier National Park. Thrustfaulted and folded Cretaceous rocks are below the Lewis but 

are covered here by talus. Note the easterly dipping low-angle 

normal faults, one of which extends to the Lewis. 

Mountain, the Lewis overlies an imbricate 

thrustfault zone in the westerly dipping Blackleaf 

Formation (Lower Cretaceous) that strikes N. 30° to 

50° E. and dips 15° to 50° NW. (Mudge and others, 

1977; Mudge, 1977). The Blackleaf is repeated by 

westerly dipping (30° to 40° W.) thrust faults and 

folds. The thrust-fault imbrications originate from a 

low-angle, northwesterly dipping thrust underlain · 

by the same complexly deformed Upper Cretaceous 

rocks that underlie the Lewis farther east. 

The trace of the Lewis thrust is almost completely 

exposed in the Lewis and Clark Range from a point 

about 3 km south of Glacier National Park to the 

vicinity of the Dearborn River, 125 km farther south, 

where it dies out in folded Paleozoic rocks (fig. 1). 

South of the park, the Lewis plate contains 

Precambrian and Paleozoic rocks that form a broad 

open fold, the Continental Divide syncline (figs. 2 

and 8). West of the synclinal axis, the upper plate of 

the Lewis thrust cuts upsection from the Spokane 

and Empire Formations to within the Mount Shields 

Formation. The thrust plane is more than 3,200 

meters stratigraphically lower on the west limb than 

it is on the east. An unfaulted section including rocks 

as old as the Helena Formation crops out on the west 

flank, whereas Mount Shields is the oldest unit 

exposed on the east flank (fig. 8). 

The trace of the Lewis is concealed for 3 km south 

of the park by Quaternary deposits in the vicinity of 

Giefer Creek, north and south of U.S. Highway 2. In 

this interval, the fault probably either connects with 

the Blacktail fault or is displaced by it (Childers, 

1963, pl. 1). The first exposure of the thrust south of 

the covered interval is near a ridge southeast of 

Giefer Creek. Here the Greyson Formation forms the 

basal unit in the upper plate, and the Kootenai 

Formation is the uppermost unit in the lower plate, 

the same relationship as at Elk Mountain in the 

southern part of the Park. This observation confirms 

Childers' (1963, pl. 1) identification of the Lewis 

thrust in the Giefer Creek area. The trace of the Lewis 

thrust can be extended southeast from the Geifer 

Creek area with certainty. We essentially agree with 

Childers' (1963, pl. 1) map except that we show the 

Lewis continuing southeast from the Morrison Creek 

area (fig. 2A) instead of being cut by the Blacktail 

fault. We also mapped the southern trace of the 

Blacktail fault differently. 

In the Morrison Creek area, the Lewis thrust cuts 

southward upsection from the Spokane Formation 

through the Helena, Snowslip, and Shepard 

Formations and into the lower part of the Mount 

Shields Formation. A similar change in the 

stratigraphic position of the Lewis thrust fault was 

noted by Price (1965, p. 100-101) at the boundary 

between the Clarke and Flathead Ranges in British 

Columbia, northwest of Glacier National Park. 

There the fault position changes abruptly northward 

from the Altyn to the Siyeh or Helena Formation. 

Price relates the change to transverse structures 

above and below the thrust. He (1965, p. 100) states 

that a northwest-dipping monocline is superposed 

across the fault and is reflected by the transverse 

step of the Lewis. The relatively abrupt change in 

stratigraphic position is comparable to a tear fault 

except that the "tear fault" does not cut strata above 

or below the thrust. Northeast of Morrison Creek, the 
LEWIS THRUST FAULT 9 

A 

METERS 

3000 

2000 

1000 

SEA LEVEL 

1000 

2000 

3000 

Yes and Yg 

CONTINENTAL 

DIVIDE 

SYNCLINE 

A' 

EXPLANATION 

DEVONIAN ROCKS 

CAMBRIAN ROCKS 

PI'!OTEROZOIC Y ROCKS 

~ Garnet Range Formation 

McNamara Formation 

Bonner Quartzite 

Mount Shields Formation 

Shepard Formation 

Snowslip Formation 

Helena Formation 

Empire and Spokane Formations 

Greyson Formation 

FIGURE B.-Cross-section A-A' showing the Lewis and Eldorado thrust plates. Approximate position of line of section is shown in 

figure 2B. The Belt units between the Eldorado and Lewis thrusts are shown individually on the section but are consolidated on the 

geologic map. 

FIGURE 9.-Aerial view of the Lewis thrust fault from Pot 

Mountain north along the west side of Dolly Varden Creek. 

The thrust fault juxtaposes light-gray Mississippian carbonate rocks and the dark-gray rocks of the Mount Shields 

Formation. View is northwest. 

westerly trend of some folds noted by us and the 

series of northeasterly trending normal and tear 

faults mapped by Weimer (1955) and by Mudge, 

Earhart, and Rice (1977) may reflect the 

northeasterly trend of a pre-thrust structure that 

may have influenced the stratigraphic level of the 

Lewis south of the park. 

Between Morrison Creek and the head of Dolly 

Varden Creek the Lewis thrust rests at progressively 

FIGURE 10.-View west at the Lewis thrust near the head of Moose 

Creek. The Precambrian Mount Shields Formation rests on 

Devonian carbonate rocks. Cambrian rocks of the upper plate 

form the cliffs along the skyline known as the Chinese Wall. 

higher stratigraphic levels in the lower plate. In 

Morrison Creek the Lewis cuts the Eldorado thrust 

plate, and to the north it rests on Mississippian and 

Devonian rocks. To the south, the Eldorado thrust is 

east of and subparallel to the Lewis thrust (fig. 2). 

Both plates contain the same Precambrian 

formations, but the rock facies differ in that those of 

the Eldorado plate are a thin shoreward facies, 

whereas those in the Lewis plate are a thicker 

basin ward facies. South of Morrison Creek the Lewis 

rests at progressively higher stratigraphic levels 

from Precambrian to Mississipian rocks. 
10 

B 

METERS 

3000 

2000 

1000 

SEA LEVEL 

1000 

2000 

3000 

c 

METERS 

3000 

SEA LEVEL 

D 

METERS 

3000 

1000 

SEA LEVEL 

1000 

2000 

LEWIS THRUST FAULT, NORTHWESTERN MONTANA 

Steamboat 

Mountain 

STEINBACH 

THRUST 

Faulted and folded Paleozoic 

and Mesozoic rocks 

Faulted and folded Paleozoic 

and Mesozoic rocks 

Line of section shown on figure 1 

B' 

C' 

D' 

FIGURE ll.-Cross sections of the Hoadley and Eldorado thrust plates, not shown to scale. Approximate position oflines of 

section are shown in fig. 1; those of section B-B' and C-C' are also shown on fig. 2C. The Belt units in the thrust plates are 

shown individually on the sections, but are consolidated on the reduced geologic map. 

The Lewis is in the lower part of the Mount Shields I Creek (figs. 1 and 2), a distance of about 87 km. As far 

Formation from Morrison Creek south to Straight south as the West Fork of the Sun River, the Lewis 
DISPLACEMENT ALONG THE LEWIS THRUST FAULT 11 

.. X.~ ~ 

Ybo 

Yms 

Ysh 

Ysn 

=.'Y;ti:: 

Yes 

Ye 

Ys 

Yg 

EXPLANATION 

CRETACEOUS ROCKS 

CRETACEOUS AND JURASSIC ROCKS 

MISSISSIPPIAN ROCKS 

DEVONIAN ROCKS 

CAMBRIAN ROCKS 

PROTEROZOIC Z DIORITE SILLS 

PROTEROZOIC Y ROCKS 

McNamara Formation 

Bonner Quartzite 

Mount Shields Formation 

Shepard Formation 

Snowslip Formation 

Helena Formation 

Empire and Spokane Formations 

Empire Formation 

Spokane Formation 

Greyson Formation 

FIGURE 11. Continued. 

rests mostly on Paleozoic rocks in the Eldorado plate 

(figs. 2, 9, and 10). At the West Fork of the Sun River, 

the Lewis cuts downsection across the underlying 

Paleozoic rocks and rests on Precambrian strata. 

Along the southerly trace of the Lewis, strata 

adjacent to the thrust are mostly undisturbed. 

However, in a cirque northeast of Argosy Mountain, 

the thrust truncates diagonally across Devonian 

strata that form overturned isoclinal folds. 

From the Middle Fork of the Flathead River to the 

West Fork of the Sun River, the Lewis thrust has 

nearly the same strike as the strata in the upper and 

lower plates. However, the dip of the fault plane 

commonly differs from that of the strata. The 

average strike of the fault and of the strata in both 

plates is N. 15° W. From the Middle Fork of the 

Flathead River to Moose Creek the fault plane dips 

40° to 45° W., whereas south of Moose Creek it dips 

mostly 20° W. The dip of the Mount Shields in the 

upper plate ranges between 25° and 45° W., whereas 

that of the Paleozoic strata in the lower plate is 10° to 

15° W. Locally, strata oftheupperplatedipthesame 

as the fault plane. 

At the West Fork of the Sun River, the strata in 

both plates change strike abruptly to about N. 55° W. 

Mudge (1972b, p. B34) noted that the West Fork of the 

Sun River was incised along an east-west line of 

flexure along which all strata and structures 

abruptly change strike. In this area, Mudge (1972b, 

p. B32) described the West Fork thrust zone (herein 

redesignated the Lewis) as containing three thrust 

faults that merge to the north and south into a single 

thrust fault. The lowest and largest thrust in the zone 

dips about 25° SW. and has a stratigraphic throw of 

about 457 m. 

The Lewis thrust cuts upsection from Straight 

Creek southward, and its trace extends into folded 

middle Paleozoic rocks (fig. 11, section B-B' and 

C-C'). At Straight Creek, the Lewis cuts upsection 

into Cambrian rocks which are thrust on younger 

Cambrian and Devonian rocks (fig. 2C). About 6 km 

farther south, near Pass Creek, Devonian rocks are 

at the base of the plate. Southeast of Pass Creek, the 

Lewis thrust dissipates in a zone of brecciated, much 

deformed Mississippian rocks for a distance of about 

3 km along strike. The last vestiges of offset are 

traceable southward into a chevron fold in the 

Eldorado thrust plate (fig. 12) about 2.4 km 

southwest of the lookout tower on Steamboat 

Mountain. Within a kilometer, the folding changes 

from a chevron fold to an open fold (fig. 11, section 

C-C') to unfolded westerly dipping rocks. During 

reconnaissance studies, Mudge and others (1968, pl. 

1, and 1974, pl. 1) originally mapped the beginning of 

the Lewis (West Fork) in the canyon on the west side 

of Steamboat Mountain and the Mississippian rocks 

there as a klippen. Our subsequent mapping 

demonstrates that the chevron fold marks the 

termination of the Lewis thrust. 

DISPLACEMENT ALONG THE 

LEWIS THRUST FAULT 

The eastward displacement along the Lewis thrust 

fault increases northward from zero near Steamboat 

Mountain to possibly more than 65 km at the south 

end of Glacier National Park (fig. 13). In the park, the 

measured minimum net slip is 24 km (Campbell, 

1914). At the south edge of the park, the minimum net 

slip is 19 km (Clapp, 1932, p. 25; Childers, 1963, p. 

157). In southeastern British Columbia, northwest of 

Glacier National Park, Price (1965, p. 115) measured 

51 km of minimum displacement. Thus the 

displacement seems to be rotational eastward in the 

park relative to a hinge at the West Fork of the Sun 

River. 

Our computation of displacement along the Lewis 

thrust is based on the relative difference between the 

projected average strike of strata south of a hinge 

line that crosses the fault at the West Fork of the Sun 

River and the average strike north of the hinge line. 

Regional structural trends, including fault traces, 
12 LEWIS THRUST FAULT, NORTHWESTERN MONTANA 

FIGURE 12.-Southern limit of the Lewis thrust at the southwest 

side of Steamboat Mountain. The thrust developed from 

fractures in folded Mississippian strata. View to the northwest. Yms, Precambrian Mount Shields Formation· £u 

Cambrian rocks; Du, Devonian rocks; Mu, Mississi~pia~ rocks. 

fold axes, and attitudes of all strata, change from 

about N. 45° W. south of the line to about N. 15° W. 

north ofthe line (fig. 13). The computation of 65 km of 

displacement assumes that strikes were about the 

same to the north and south of the hinge line prior to 

the formation of the Lewis. We recognize that any 

rotational movement on the previously formed 

Eldorado thrust fault would introduce a small 

unknown element of error by this method of 

estimation. 

Large eastward transport of the Lewis plate is also 

indicated by comparing its stratigraphic sequence 

with that in the underlying Eldorado thrust plate 

(fig. 14). At Morrison Creek, 18 km south of the park, 

the Lewis thrust truncates all strata in the Eldorado 

thrust plate. Farther south, most units in the Lewis 

plate are more than twice as thick as their 

counterparts in the Eldorado plate. Data are not 

available to determine the distance that originally 

separated the plates. The Eldorado thrust plate also 

is warped about the hinge line at the West Fork of the 

Sun River. 

As shown in figure 14, the Helena Formation and 

younger Belt rocks, in general, thicken westward 

and southwestward from the Sun River area in both 

the Lewis and Eldorado plates. The eastern edge of 

the Belt basin had a northwest trend in this area 

(Harrison, 1972; Harrison and others, 1974). 

Therefore, the northerly increase in thickness of 

units in the Lewis plate suggests an increase in 

displacement northward from the hinge point. 

A comparison of stratigraphic data from the 

southwest corner of Glacier National Park with that 

from near the West Fork of the Sun River support the 

hypothesis of increasing eastward displacement to 

the north. These data are insufficient, however, to 

determine the amount of displacement, because 

basinward thickening of the Precambrian 

formations was irregular. Formations in the Glacier 

National park sequence are thicker than those at the 

West Fork of the Sun River, but somewhat thinner 

than those at the Camp Creek section southwest of 

the West Fork section (fig. 14). From the West Fork of 

the Sun River the Bonner Quartzite thins northward, 

whereas the McNamara Formation thickens. The 

MeN amara also shows a noticeable change in facies 

to the north. In the southern section, greenish-gray 

argillite, siltite, and quartzite compose the lower half 

of the McNamara, whereas reddish-brown quartzite 

composes the upper half. To the north the 

MeN amara is almost all greenish-gray argillite and 

siltite with negligible amounts of interbedded 

quartzite. Thus the northern sequence was originally 

deposited more basinward (farther west) than the 

southern one. 

The amount of stratigraphic throw along the 

Lewis thrust fault has little meaning unless 

considered with other geologic relations in the upper 

and lower plates (Table I). For example, in the lower 

part of Morrison Creek, the Mount Shields 

Formation is the basal unit in the Lewis thrust plate 

and the upper unit in the Eldorado plate. Here the 

stratigraphic throw could be interpreted as being 

minor, whereas the slip actually is quite large. 

A series of well-exposed sections of the rock units 

in the Lewis and Eldorado thrust plates illustrates 

the increase in stratigraphic throw to the north (fig. 

14). The estimated displacement, determined 

graphically, is shown for each locality. From the 

Dearborn River to the West Fork of the Sun River, 

both the stratigraphic throw and the slip are small. 

At the hinge line, near the West Fork of the Sun 

River, the amount of stratigraphic throw is about 460 

meters and the slip is about 3,000 meters. North of 

the hinge line, toward the structural low to the north 

of the south arch, both the stratigraphic throw and 

the slip increase dramatically. As the thrust cuts 

upsection through Belt rocks to a position near the 

top of the Cambrian rocks, the stratigraphic throw is 

disproportionately greater than the slip. Farther 

north, the amount of slip increases more rapidly 

than the amount of stratigraphic throw. 

The results of preliminary paleomagnetic studies 

on Belt rocks in the region by D. P. Elston (oral 
114°15' 48"30' 114°00' 

DISPLACEMENT ALONG THE LEWIS THRUST FAULT 

30' 113°00' 

~ ~ Present east edge of --------r Lewis thrust in Glacier 

~<(' National Park 

~<;, ~ <;,(:;) '4l x Marias Pass 

o.J. i:l' strata in Lewis 

30' 

~ ~ \_\Average strike of 

~~;);/·~ ~~ , plate-N. ,5. w. ~~ ~ i\ "?"~'Q0 .L'oo 

~- " 

48"00' 

30' 

47°00' 

-~'"~ ",~Projected strike of strata 

"-, , at an early stage of 

',, '- Lewis thrust development 

',,__ ", 

', " --,,, ", 

', " 

'-,,, ", 

'',,, ", 

', " ', " ',, " 

', " ',, " 

', " ',, " 

Projected strike of strata/',, '-, 

prior to the Lewis thrust ',,, "- \ 

',,_ ", ',, " \ 

',, " a'~ -,,_ '-~?J 

',, " Northern limit of trace 

of Hoadley thrust 

', " 

', " 

'-------~-·-- ... -...~- \'',~ 

:1 ' 

:I 

\1 ~ :r--

:I 

:I 

:I 

:I 

:I 

\I 

Average strike of strata 

in Lewis plate-N. 45• W. 

: 1 Average strike of 

; I strata in Hoadley __)\ 

: I plate-about N. so• W. 

Southern limit of trace 

of Lewis thrust 

Measured section of the 

Helena Formation in 

Dearborn River I I : 1 "' Present east edge 

\ 1 ?J~ ~ / A'- of Hoadley thrust 

Projected strike of strata~ 1 ~ -~~" 

prior to the Hoadley thrust : ~ ~e~sured section of c:;,~ \ 1 the Helena Formation '\ 

: I " X : I '-R~~ : I Pass 

: I : I : r-Projected strike of strata 

\ 1 at early stage of Hoadley 

: \ thrust development 

: \ : I : I 

: \ : \ : I : I : I 

\~\ ,o~ : I 

\.6.5 km 

10 20 30 KILOMETERS 

112"00' 

~~O'L---------~------------------~--------------------~------------------~------------------~ 

FIGURE 13.-Projected strikes of strata in the Lewis and Hoadley thrust plates north and south of the hinge points before and 

after faulting. 

13 
14 LEWIS THRUST FAULT, NORTHWESTERN MONTANA 

ELDORADO THRUST PLATE LEWIS THRUST PLATE 

Southwest South North Northwest West 

1 2 3 4 5 6 7 8 9 Formation West Fork of Middle Fork Camp Creek A horn Bungalow Pivot Southwest Flathead Swan 

the Sun River of the (McGill and Creek1 Mountain1 Mountain1 Glacier Park Range1 Range 

area Flathead Sommers, 1967) (Childers, 1963) area1 

(Mudge, 1972a) River area1 

Garnet Range Formation ~~~ ~~.~~~ 225-300 ~~ t/////////~ V///0////// 

~ V/////~ 274 

%;:;8~~:~ 61+ 61± 61+ 

McNamara Formation ~~.:?:;:~:;; 810 747-808 1128 1647 1464 915 Q-175 Q-183+ 30+ 

Bonner Quartzite 335 259± 386 427 320 259 241 259 351 

Mount Shields Formation 555 442 833 366+ 824+ 930+ 778 961 2181 

Shepard Formation 248 183 604 473 762 900 

Snowslip Formation 107 214 682 488 976 1662 Thrust fault Thrust fault Thrust fault 

Helena Formation 191 451+ 1159 1967 2852 

Thrust fault Spokane and 366+ Covered 610+ 1403+ Empire Formations Thrust fault 732 (incomplete) 

1Unpub. mapping by Mudge and Earhart, 1969-1979. 

FIGURE 14.-Thickness of Precambrian units (in meters) in the Lewis and Eldorado thrust plates. Location of sections 

shown in figure 1. 

AGE OF LEWIS THRUST FAULT commun., 1978) support field evidence indicating 

rotational movement on the Lewis thrust. Elston 

measured the orientation of paleomagnetic poles in 

Precambrian rocks in the upper plate of the Lewis in 

Glacier Park and in equivalent rocks near the 

Dearborn River. His comparison suggested the rocks 

in the park had rotated to the east. 

The Lewis thrust fault has not been dated 

precisely; present data suggest a very late Paleocene 

to very early Eocene age. Deiss (1943a, p. 256) 

interpreted the thrust faults in the eastern part of the 

disturbed belt as being older than the Lewis because 

TABLE I.-Stratigraphic throw and-estimated slip of the Lewis thrust from the Dearborn River north to Glacier National Park, showing 

the formations above and below the Lewis thrust 

Location 

Dearborn River----

Straight Creek----

South of West Fork 

of Sun River. 

North of West Fork 

of Sun River. 

Moose Creek-------

Spotted Bear River 

Upoer Morrision 

Creek. 

Glacier Park------

Stratigraphic unit 

Amount 

throw 

Above thrust Below thrust (m) 

Allan Mountain Limestone 

(Mississippian). 

Cambrian rocks----------

Snowslip Formation 

(Precambrian) . 

Mount Shields Formation 

(Precambrian). 

-----do-----------------

-----do-----------------

Helena Formation 

(Precambrian). 

Altyn Formation 

(Precambrian) . 

Allan Mountain Limestone 0 

(Mississippian). 

Cambrian rocks---------- 155 

Mount Shields Formation 460 

(Precambrian) . 

Devils Glen Dolomite 2,290 

(Cambrian). 

Lower member, Jefferson 2,775 

Formation (Devonian). 

Three Forks Formation 3,570 

(Devonian). 

Castle Reef Dolomite 4,R80 

(Mississipoian). 

Blackleaf Formation 6,560 

(Cretaceous). 

Estimated 

slip 

(km) 

0 

1.6-3.2 

3.2 

8 

14 

47 

65 

Distance 

between 

locations 

(km) 

0 

21 

13 

6.5 

14.5 

11 

43 

16 

Cumulative 

distance 

(Ian) 

0 

21 

34 

40.5 

55 

66 

126 
STRUCTURES RELATED TO THE LEWIS THRUST FAULT 15 

the Lewis truncates many of them. The relationship 

of the Lewis to these thrusts is well documented 

along the south edge of Glacier National Park 

(Childers, 1963; Mudge, 1977). In the Sun River area, 

Mudge (1972b, p. B34) noted that deformation in the 

disturbed belt proceeded from east to west. However, 

some workers believe that deformation along the belt 

in Canada proceeded from west to east (Hume, 1957; 

Scott, 1951; Bally and others, 1966; Price and 

Mountjoy, 1970; Jones, 1971). Childers (1963, p. 160) 

and Jones (1971, p. 305) believe the cutting of eastern 

thrusts by western thrusts is dependent on the 

duration of the movement of the western thrust. 

Childers further suggested that the thrust faults of 

major displacement, such as the Lewis, may have 

started to move eastward earlier than the thrusts 

east of it and were active after the thrusts formed on 

the west side of the Sawtooth Range. 

The age of the orogeny that formed the disturbed 

belt is geperally accepted by most workers as 

probably beginning no later than Late Cretaceous 

and ending no later than late Eocene (Mudge, 1972b, 

p. B46). Recent studies on bentonites in Cretaceous 

rocks that were metamorphosed by burial beneath 

thrust plates have now further restricted the period 

of thrust faulting. Hoffman, Hower, and Aronson 

(1976) report a K-Ar radiometric age of 72 to 56 

million years B.P. for such bentonites. The bentonite 

beds are in folded and faulted Lower and lower 

Upper Cretaceous strata that lie east of the Lewis 

thrust in the disturbed belt in the Sun River Canyon 

area, and south of the Lewis in the Marias Pass area. 

The metamorphism of these beds was very likely 

caused by the increased load and higher thermal 

gradients resulting from the overriding thrust 

plates. 

Recent K-Ar dates of igneous rocks in 

northwestern Montana establish the younger age 

limit for thrust faults in the eastern and central parts 

of the disturbed belt in that area. A hornblende 

monzonite dike about 24 km east of Rogers Pass cuts 

a major thrust fault called the Eldorado by Schmidt 

and Strong (1972). According to R.G. Schmidt (oral 

commun., 1975) the K-Ar date on hornblende from 

the dike is 46.3 m.y. H.H. Mehnert and R.G. Schmidt 

(U.S. Geol. Survey, 1971, p. A37) also obtained a K-Ar 

of 58.3 m.y. on biotite from a quartz monzonite 

porphyry sill that intruded along the Steinbach 

thrust fault (fig. 1), incorrectly called the Eldorado by 

Schmidt (1972), near Wolf Creek; this provides an 

upper limit on the movement of the thrust in this 

area. They believe that fault formed at a late stage in 

the orogenic history of the region (U.S. Geol. Survey, 

1971, p. A37). These data indicate that thrusting in 

this part of the disturbed belt occurred prior to midEocene time, in agreement with the radiometric ages 

on bentonites reported by Hoffman, Hower, and 

Aronson (1976). 

The Hoadley, Lewis, and Eldorado thrust faults 

are west of the Steinbach thrust plate (formerly 

called the Eldorado) and are younger than it, 

inasmuch as the Eldorado thrust as used in this 

report cuts the Steinbach about 4 km northwest of 

the Steamboat Mountain lookout (fig. 1). However, 

we believe these faults are older than the sill (58.3 

m.y. old) that intruded the Stein bach thrust, because 

dikes and sills of similar composition intrude the 

Hoadley plate in the Rogers Pass area. 

STRUCTURES RELATED 

TO THE LEWIS THRUST FAULT 

The Hoadley thrust and the Continental Divide 

syncline are in the upper plate of the Lewis thrust 

and very likely were formed by stresses that formed 

the Lewis. 

HOADLEY THRUST 

The Hoadley thrust is the westernmost major 

thrust fault in the northern disturbed belt. It 

originates near the upper reaches of the West Fork of 

the Sun River and continues south at least to the 

Canyon Creek valley (figs. 1, 11, and 13), a distance 

of about 125 km. The amount of displacement along 

the Hoadley appears to increase southward over the 

structurally low area south of the south arch. Thus, 

in many aspects the Hoadley thrust is a near mirror 

image of the Lewis. 

the Hoadley thrust, like the Lewis, hinges at the 

West Fork of the Sun River, and its average strike of 

N. 50° W. approximately parallels that of the Lewis. 

Its thrust plane dips 25° to 50° W. In the upper plate, 

Mount Shields strata dip between 15° and 30° W., 

whereas those in the lower plate dip 25° to 45° W. 

Farther south, the rocks in the lower plate mostly dip 

20° and 25° W. (Lange, 1963, pl. 1). As far south as the 

Dearborn River canyon, the Mount Shields 

Formation is the basal unit in the Hoadley plate (fig. 

2). Between the Dearborn River and Falls Creek, the 

thrust cuts downsection to within the Spokane 
16 LEWIS THRUST FAULT, NORTHWESTERN MONTANA 

Formation. Farther southeast, between the east side 

of Rogers Pass and the southern reaches of the South 

Fork of the Dearborn River, the Greyson Formation 

is at the base of the Hoadley plate. From Rogers Pass 

south to the Canyon Creek valley, the Spokane and 

at places the Greyson form the basal unit of the 

Hoadley plate. The location and extent of the thrust 

to the south of Canyon Creek valley has not been 

determined. The Hoadley is the Lyons thrust fault 

mapped west of Wolf Creek by Schmidt and Strong 

(1972). 

Strata beneath the Hoadley plate are similar to 

those beneath the Lewis plate. In most places, almost 

as far southeast as Lewis and Clark Pass, the upper 

plate is thrust onto locally folded Paleozoic rocks. 

Farther southeast, to Rogers Pass, the Hoadley rests 

on strata as old as the Helena Formation. 

At the head of Lyons Creek the Hoadley splits into 

two thrust faults that repeat the Spokane and 

Greyson Formations. In most places, the eastern 

thrust dips between 60° and 90° SW., whereas the 

western thrust dips less than 50° SW. We infer that 

the western thrust is the main Hoadley thrust. 

We postulate that, from the hinge point at the West 

Fork of the Sun River, the lateral displacement on 

the Hoadley increases southward to possibly as 

much as 70 km near Canyon Creek valley; however, 

additional studies of the fault and of stratigraphic 

relations along the fault are needed to better estimate 

the lateral displacement. South of Lyons Creek, the 

displacement appears small because the Spokane 

Formation is repeated by a thrust. However, in the 

vicinity of Rogers Pass, the Spokane in the Hoadley 

thrust plate contains a thick, basin ward tidal facies, 

where as that in the Eldorado thrust plate is a thinner 

near-shore facies (J.W. Whipple, oral commun., 

1978). Thus it appears the Hoadley thrust 

significantly foreshortens the facies relations. 

A significant amount of lateral shortening 

between the two plates is also suggested by 

comparing the thickness of formations in the plates 

(fig. 11). For example, the Helena Formation is 174m 

thick in the Eldorado plate on the Dearborn River 

east of Steamboat Mountain, but 13 km southwest, in 

the Hoadley plate, it is 1,090 m thick (line of section 

C-C', fig. 11). The Helena also thickens southward in 

the Eldorado plate. At 16.8 km to the southeast (line 

D-D', fig.11)itisabout488mthick, which suggests a 

thickening rate of about 1.8 m per kilometer. If this 

rate of thickening is applicable to the Helena to the 

southwest, then the section in the Hoadley plate has 

been transported more than 48 km from the 

southwest. However, stratigraphic data suggest that 

the rate of thickening of the Helena to the southwest 

is greater than to the southeast; therefore, a more 

conservative estimate of about 38 km translation, as 

shown on fig. 12, may be more accurate. 

CONTINENTAL DIVIDE SYNCLINE 

The Continental Divide syncline is a broad, open, 

doubly plunging structure that extends from the 

Middle Fork of the Flathead River south to the 

Dearborn River, a distance of 116 km. The axis 

strikes southerly through Trinity and Silvertip 

Mountains, along the White River, to Junction 

Mountain (fig. 2). From Junction Mountain 

southward, the axis follows along the Continental 

Divide through Sugarloaf and Scapegoat 

Mountains. The axis of the syncline strikes 

subparallel to the strike of the Lewis and Hoadley 

thrust faults. Rocks as young as Mississippian occur 

in the central part of the fold (fig. 2). At its northern 

end, the fold is truncated by a high-angle fault 

northwest of Trinity Mountain. The Hoadley thrust 

marks the southern terminus of the syncline at 

Whitetail Creek, 5 km south of Steamboat Mountain 

lookout. The Continental Divide syncline may have 

been formed by stresses that produced the Lewis 

thrust. 

The east limb of the syncline comprises the upper 

plate of the Lewis thrust to the north and that of the 

Hoadley thrust to the south. The west limb was 

displaced by a longitudinal normal fault (South Fork 

normal fault) that hinges at Scapegoat Mountain 

(fig. 2). The easterly dipping strata of the Swan 

Range are in the downthrown block of the normal 

fault and form part of the west limb of the syncline 

(Mudge, 1970). 

SUMMARY 

The trace of the Lewis thrust fault in Montana has 

been mapped over a distance of about 227 km. It 

originates in sheared and folded Mississippian rocks 

in the Dearborn River canyon, and toward the north 

it progressively cuts downsection to lower 

Proterozoric Y rocks near Glacier National Park. 

We postulate that the Lewis thrust moved 

eastward more than 65 km at the south edge of the 

park and that the stratigraphic throw there is about 

6,500 m. The Lewis plate is rotated about an easterly 

trending hinge line at the West Fork of the Sun River, 

and its displacement increases northward from that 

point. The Hoadley thrust originates from a point 

about 28 km north of where the Lewis thrust develops 

and extends southward for at least 125 km. The 

Hoadley appears to be a near mirror image of the 
REFERENCES CITED 17 

Lewis, as it is rotated eastward at the same hinge line 

and its displacement increases southward. At the 

southern end of its known trace, it may have moved 

eastward more than 70 km. Present data in the 

northern disturbed belt indicate that the Lewis and 

the Hoadley are the youngest of the thrust faults in 

the area; we believe they formed during very late 

Paleocene to very early Eocene time. The Lewis 

thrust, the Hoadley thrust, and the Continental 

Divide syncline probably formed concurrently in the 

same stress field. 

REFERENCES CITED 

Alpha, A.G., 1955, The Genou Trend of north central Montana: 

Am. Assoc. Petroleum Geologists, Rocky Mountain Sec., 

Geological record, Feb. 1955, p. 131-138. 

Bally, A.W., Gordy, P.L., and Stewart, G.A., 1966, Structure, 

seismic data, and orogenic evolution of southern Canadian 

Rocky Mountains: Bull. Canadian Petroleum Geology, v. 14, 

no. 3, p. 337-381. 

Billings, Marland, 1938, Physiographic relations of the Lewis 

overthrust in northern Montana: Am.Jour.Sci., 5th ser., v. 35, 

no. 208, p. 260-272. 

Campbell, M.R., 1914, The Glacier National Park-A popular 

guide to its geology and scenery: U.S. Geol. Survey Bull. 600, 

54 p. 

Childers, M.D., 1963, Structure and stratigraphy of the southwest 

Marias Pass area, Flathead County, Montana: Geol. Soc. 

America Bull., v. 74, no. 2, p. 141-164. 

Clapp, C. H., 1932, Geology of a portion of the Rocky Mountains 

of northwestern Montana: Montana School Mines Mem. no. 4, 

30p. 

Clark, L.M., 1954, Cross-section through the Clarke Range of the 

Rocky Mountains of southern Alberta and southern British 

Columbia: Alberta Soc. Petroleum Geologists Guidebook, 4th 

Ann. Field Conf., Aug. 1954, p. 105-109. 

Deiss, C.F., 1933, Paleozoic formations of northwestern Montana: 

Montana Bur. Mines and Geology Mem. 6, 51 p. 

____ 1939, Cambrian stratigraphy and trilobites of northwestern Montana: Geol. Soc. America Spec. Paper 18, 135 p. 

____ 1943a, Stratigraphy and structure of southwest Saypo 

quadrangle, Montana: Geol. Soc. America Bull., v. 54, no. 2, 

p. 205-262. 

---- 1943b, Structure of central part of Sawtooth Range, 

Montana: Geol. Soc. America Bull., v. 54, no. 8, p. 1123-1167. 

Dobbin, C. E., and Erdmann, C. E., 1955, Structure contour map of 

the Montana plains: U.S. Geol. Survey Oil and Gas lnv. Map 

OM 178 A. 

Douglas, R.J.W., 1958, Mount Head map-area, Alberta: Canada 

Geol. Survey Mem. 291, 241 p. 

Harrison, J.E., 1972, Precambrian Belt basin of northwestern 

United States-its geometry, sedimentation, and copper 

occurrences: Geol. Soc. America Bull., v. 83, no. 5, p. 

1215-1240. 

Harrison, J.E., Griggs, A.B., and Wells, J.D., 1974, Tectonic 

features of the Precambrian Belt basin and their influence on 

post-Belt structures: U.S. Geol. Survey Prof. Paper 866, 15 p. 

Hoffman, Janet, Hower, John, and Aronson, J.L., 1976, Radiometric dating of time of thrusting in the disturbed belt of 

Montana: Geology, v. 4, no. 1, p. 16-20. 

Hume, G.S., 1957, Fault structures in the foothills and eastern 

Rocky Mountains of southern Alberta: Geol. Soc. America 

Bull., v. 68, no. 4, p. 395-412. 

Jones, P.B., 1971, Folded faults and sequence of thrusting in 

Alberta foothills; Am. Assoc. Petroleum Geologists Bull., v. 

55, no. 2, p. 292-306. 

Lange, S.S., 1963, The geology of the Lewis and Clark Pass area, 

Lewis and Clark County, Montana: Missouri Univ. unpub. 

M.A. thesis. 144 p. 

McGill, G.E., and Sommers, D.A., 1967, Stratigraphy and correlation of the Precambrian Belt Supergroup of the southern 

Lewis and Clark Range, Montana: Geol. Soc. America Bull., v. 

78, no. 3, 343-352. 

Mudge, M.R., 1966a, Geologic map of the Pretty Prairie quadrangle, Lewis and Clark County, Montana: U.S. Geol. Survey 

Geol. Quad. Map GQ-454. 

____ 1966b, Geologic map of the Glenn Creek quadrangle, 

Lewis and Clark and Teton Counties, Montana: U.S. Geol. 

Survey Geol. Quad. Map GQ-499. 

____ 1970, Origin of the northern disturbed belt in northwestern Montana: Geol. Soc. America Bull., v. 81, p. 377-392. 

____ l972a, Pre-Quaternary rocks in the Sun River Canyon 

area, northwestern Montana: U.S. Geol. Survey Prof. Paper 

663-A, 142 p. 

____ l972b, Structural geology of the Sun River Canyon and 

adjacent areas, northwestern Montana: U.S. Geol. Survey 

Prof. Paper 663-B, 52 p. 

____ 1977, General geology of Glacier National Park and 

adjacent areas, Montana: Bull. Canadian Petroleum Geology, 

v. 25, no. 4, p. 736-751. 

Mudge, M.R., and Earhart, R.L., 1978, Geology of the Bob 

Marshall Wilderness and study areas, Chapter A of Mineral 

resources of the Bob Marshall Wilderness and study areas, 

Lewis and Clark, Teton, Pondera, Flathead, Lake, Missoula, 

and Powell Counties, Montana: U.S. Geol. Survey Open-File 

Rept. 78-295, p. 1-51. 

Mudge, M.R., Earhart, R.L., Cobban, W.A., and Rice, D.D., 1977, 

Glacier National Park and areas to the south and east, in 

Geological guide for the Canadian Soc. Petroleum Geologists 

1977 Waterton-Glacier Park field conference: p. 41-62 and 

75-82. 

Mudge, M.R., Earhart, R.L., and Rice, D.D., 1977, Preliminary bedrock geologic map of part of the northern disturbed belt, Lewis 

and Clark, Teton, Pondera, Glacier, Flathead, and Powell 

Counties, Montana: U.S. Geol. Survey Open-File Rept. 77-25 .. 

Mudge, M.R., Earhart, R.L., Watts, K.C., Jr., Tuchek, E.T., and 

Rice, W.L., 1974, Mineral resources of the Scapegoat Wilderness, Powell and Lewis and Clark Counties, Montana, with a 

section on Geophysical surveys, by D.L. Peterson: U.S. Geol. 

Survey Bull. 1385-B, 82 p. 

Mudge, M.R., Erickson, R.L., and Kleinkopf, Dean, 1968, Reconnaissance geology, geophysics, and geochemistry of the 

southeastern part of the Lewis and Clark Range, Montana, 

with Spectrographic data by G.C. Curtin and A.P. Marranzino, and a section on Isotopic composition oflead by R.E. 

Zartman: U.S. Geol. Survey Bull. 1252-E, 35 p. [1969]. 

Price, R.A., 1965, Flathead map-area, British Columbia and 

Alberta: Canada Geol. Survey Mem. 336, 219 p. 

Price, R.A., and Mountjoy, E.W., 1970, Geologic structure of the 

Canadian Rocky Mountains between Bow and Athabasca 

Rivers-A progress report: Geol. Assoc. Canada Spec. Paper 

no. 6, 25 p. 

Ross, C.P., 1959, Geology of Glacier National Park and the Flathead region, northwestern Montana: U.S. Geol. Survey Prof. 

Paper 296, 125 p. 
18 LEWIS THRUST FAULT, NORTHWESTERN MONTANA 

____ 1963, The Belt series in Montana: U.S. Geol. Survey 

Prof. Paper 346, 122 p. [1964]. 

Ross, C.P., Andrews, D.A., and Witkind, I.J., 1955, Geologic map 

of Montana: U.S. Geological Survey. 

Schmidt, R.G., 1972, Geologic map of the Comb Rock quadrangle, 

Lewis and Clark County, Montana: U.S. Geol. Survey Geol. 

Quad. Map GQ-976. 

Schmidt, R.G., and Strong, C.P., Jr., 1972, Geologic map of the 

Roberts Mountain quadrangle, Lewis and Clark County, 

Montana: U.S. Geol. Survey Geol. Quad. Map GQ-977. 

Scott, J.C., 1951, Folded faults in Rocky Mountain foothills of 

Alberta, Canada: Am. Assoc. Petroleum Geologists Bull., v. 

35, no. 11, p. 2316-234 7. 

Sloss, L.L., and Laird, W.M., 1945, Mississippian and Devonian 

stratigraphy of northwestern Montana: U.S. Geol. Survey Oil 

and Gas Inv. Prelim. Chart 15. 

U.S. Geological Survey, 1971, Dating the Eldorado thrust, in Geological Survey research 1971: U.S. Geol. Survey Prof. Paper 

750-A, p. A37. 

Weimer, R.J., 1955, Geology of the Two Medicine-Badger Creek 

area, Glacier and Pondera Counties, Montana, in Billings 

Geological Society Guidebook, 6th Ann. Field Conf.: p. 143-

149. 

Willis, Bailey, 1902, Stratigraphy and structure, Lewis and 

Livingston Ranges, Montana: Geol. Soc. America Bull., v. 13, 

p. 305-352. 

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