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5-1950 Structure of Southwestern Montana Donald W. Levandowski
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Recommended Citation Levandowski, Donald W., "Structure of Southwestern Montana" (1950). Bachelors Theses and Reports, 1928 - 1970. 319. http://digitalcommons.mtech.edu/bach_theses/319
This Bachelors Thesis is brought to you for free and open access by the Student Scholarship at Digital Commons @ Montana Tech. It has been accepted for inclusion in Bachelors Theses and Reports, 1928 - 1970 by an authorized administrator of Digital Commons @ Montana Tech. For more information, please contact [email protected]. STRUCTURE OF SOUTHWESTERN MONTANA
by Donald We Levandowski
A Thesis Submitted to the Department of Geology Ll partial fUlfillment of the RAqoirements for the degree of Bachelor 'of Science in Geological Engineering
I MONTANA SCHOOL· OF MINES BUTTE 9 MONTANA May, 1950 , - NI'ANA :sCHOOL OF. M!l\l:'.S UBRAR.l: BUTTE STRUCTURE OF SOUTHWESTERN MONTANA
by Donald W. Levandowski
21983
A Thesis SUbmitted to the Department of Geology in partial fulfillment of the Requirements for the degree of Bachelor of Science in Geological Engineering
MONTANA SCHOOL OF MINES BUTTE, MONTANA May, 19.50 TABLE OF CONTENTS
Page
Introduction - - - - - 1
General Statement - 1
Locality ------1
Acknowledgement - 2
General Geology - - - - 3
Regional Features - - - - 3 Stratigraphy -- 4
Geologic History 9
Structure and Related Features - 11
General Structural Features 11 Character and Distribution of Igneous Rocks - IS Relation of Igneous Bodies to Structure ·16
Mechanics of Folding 17
Conclusion - - 20
Bibliography - - 21
LIST OF ILLUSTRATIONS
Figure 1. Structural Divisions of Montana and Principal Topographic Features of Western Montana 2
Plate 1. Distribution of Igneous and pre-Cambrian Rocks in Montana - 7
Plate 2. Distribution of Paleozoic and Mesozoic Rocks in Montana ------8
Plate 3. Principal Structural Features of Southwestern Montana ------_ 12
Plate 4. Geologic Map With Structure Sections of Southwestern Montana - - (Inside of Pocket) STRUCTURE OF SOUTHWESTEm~ MONTANA
by
Donald W. Levandowski
INTRODUCTION
GENERAL STAT~mNT
Intrusions of granitic rocks on a large scale are commonly
found in the central part of folded mountain systems. Igneous rocks,
intrusive and extrusive, are widespread in the mountains of south-
western Montana. An examination of the structural pattern of this
area indicates that the fold trends form a radial pattern. How and why this pattern formed and its relation to the igneous activity in
the area have not yet been discussed in the literature. The author has prepared this paper and the accompanying map in an effort to
emphasize the fact that such an area exists, and that there is a
relation between the igneous activity and the structure.
LOCALITY
The map area embraces most of southwestern Montana (See Plate h).
This portion of the state is a rugged, mountainous region. The many mountain ranges have a generdl nortn~rly trend; however, local marked variation in the trel~<.lmay be noted. Some of ~the larger ranges are the Bitterroot, I'obaccc Root, }f.adisoi'l;and Gal.Lcb'in mountains.
Continuity of thie mountainous landscape is broke~ by occasional wide Lnt.ermorrtanevalleys with nearly flat floors lying from 2000 to
5000 feet below the crests of the enclosing mountains. These large
1 valleys are as characteristic of the topography of the western part of the state as are the mountain ranges. They are 10 miles or more in width and have a much greater length. Approximate locations of some of the principal intermontane valleys and mountain ranges are shown in Figure 1.
I I ,f ?-'I..J
I·~/;·~;.~'::I//ViFRIV/OK'T"1/N YA,u"EYS
-PITllYe/P/lL., ,q/{IY'::;~J
Figure 1. Structural Divisions of Montana and Principal Topographic Features of Western Montana.
ACKNOWLEDGEMENT
The sources of information for this problem are the Geologic Map of Montana and the literature. Since no fieldwork has been done, the author makes no claim for originality in field observations for any portion of the text or accompanying maps and illustrations.
Work in preparation of this thesis was facilitated by the assist- ance and counsel of Dr. Eugene S. Perry, Department of Geology, Montana
2 School of NUnes. Much information has been made available to the
author by Dr. Perry in personal communication, and by an unpublished
manuscript (8).
GENERAL GEOLOGY
REGIONAL FEATURES
The geologic structure of Montana may readily be divided into
three arbitrary divisions: (1) Eastern, (2) Central, and (3) Western.
See Figure 1, page 2.
Perr,y (8) divides Montana's mountains, the western division, on
the basis of major structure into three divisions: (1) the Northern
Rockies, composed of parallel, northwest trending folds and faults;
(2) the Southern Rockies, composed of complex folds and faults trending
in a dive~sified direction; and (3) an eastern marginal area composed
of (a) isolated igneous mountains and (b) east-west trending geanticlines
and geosynclines. An examination of the inclosed map or Plate 3 will
show a definite line between the northern and southern parts. This
line is indicated on Figure 1, page 2.
Igneous rocks, both intrusives and extrusives, are widespread in
the southern portion. This feature is in marked contrast to the limited extent of such rocks in the northern portion of the mountains.
Plate I-A illustrates this distribution. Practically all the igneous bodies in Montana are late Cretaceous or early and middle Tertiary~
in age. Although great areas of extrusive rocks are older than the folding, most of the intrusives appear to have ascended in early
Tertiary time after the initial intense folding and faulting of the
Rocky Mountain revolutiono Much of the extrusive material was erupted
3 in middle and late Tertiary time after the mountains and their intruded
granites had been planed by erosion to a mature surface. The pre-
granite lavas are extremely folded and faulted, but the post-granite
lavas lie as a blanket on the eroded granite. The igneous rocks,
intrusive and extrusive, are as irregular in form as they are in dis-
tribution. The intrusives occur as batholiths, as much as 60 miles
long; stocks; bosses; dikes; and sills. The extrusives occur as true
flows, agglomerates, and tuffs.
The relation of these masses to the major structure of the area
is the thesis of this paper.
STRA'l'IGRAPHY
The sedimentary formations listed in the following table are
exposed within the area represented by the accompanying map. Since
there are only a few formations found in western Montana that are not
found within the map area, the table is fairly representative of the
stratigraphy of the Rocky Mountains in Montana.
SEDIMENTARY FORMATIONS IN SOUTHWESTERN MONTANA
Designation Geologic Age Formation Thickness Character on Ma (feet)
Alluvium Sand, gravel, boul- Quaternary ders, and clay Glacial Drift and moraines Cenozoic drift of sand and gravel Unconformity ~~~'-~~ Tertiary
Unconformity
Upper Cretaceous
Mesozoic Unconformity
4 Designation Geologic Age Formation Thickness Character on Ma (feet) Mesozoic .Unconformity (Contd) Lower Kootenai 800-1000 Sandstone and shale Cretaceous with thin band of gastropod limestone near middle Morrison ? Varie~ated shales Jurassic Ellis 300- 00 Arenaceous and ar- gillaceous lime- stone and quart- zite Unconformity Permian Phosphoria 100-350 Shale, quartzite, limestone; beds of oolitic phosphate rock Pennsylvanian Quadrant 300-700 Mainly quartzite, some black and cherty shale, and limestone; red band at base MisSissippian Madison 1200- Massive bluish-gray 1500 limestone contains cherty bands; sandy at base Paleozoic Three Forks 0-300 Orange, green, and black shales, car- Devonian bonaceous in Elaces Jefferson 700-1 00 Massive black mag- nesium fetic lime- stone and some shale Unconformity Gallatin 420-1000 Trilobite limestone mottled limestone, pebbly limestone separated by shale Cambrian bands Flathead P~' quartzite and conglomerate, also some shale ~ Unconformity. Proterozoic Belt 5000 Fine-grained shale underlain by green- gray arkose and arkosic conglomerate Cryptozoic Unconformity
5 Geologic Age Formation Character
Unconformity Cryptozoic Cherry 8000 Hornblende-garnet (Contd) Creek gneiss, crystal- line limestone, quartzite, mica schist, and quartz feldspar gneiss of Aftcheozoic sedimentary origin Unconformity Pony Dark and light colored gneiss and schist, sedimentary and igneous origin
IGNEOUS ROCKS
Intrusive Late Cretaceous Intrusive Quartz Monzonite, granite, or quartz diorite, granodiorite, Early Tertiary andesi tepo_!'P!Wry Extrusive Extrusive Rhyolite, trachyte, dacite, , quartz latite, andesite, augenite, basalt
The distribution of these formations and rocks are shown on the map and on Plates 1 and 2.
•
6 \'
• Havre
.~o .:7 .K&llspell "~'."~ o Glugo" "bJ' . ~ ~' Great FaUa ~ , ._,. olAwiatown
,.1188 City
• Billing.
k',":1 Extrusive _ Intrusive
A. Distribution of Igneous Rocks
.Ha.Yn
• GlasgoW'
. Great JlI.lla
• Lewi.tcnm
Plil.. Cit,.
•Billing.
1:.'.:.] sa... l, H18hly UetUIOrphoaed OOliplex l1li Lat. Proteroaoio Belt Seri ••
B. Distribution of Pre-Cambrian Rocks
DI8TI1IDUTIOU OF IG'r:oUS AND"Jtl'~,....r,~T13RIAN ROCKS ru HONT1IJJA
1 p
• HaTl'e Kalispell
o Glalgow
. Uilea ctiy
BUlingl
A. Distribution of Paleozoic Rocks
Kal1s.po 11
I.I1l11oula.
B. Distribution of UC"IO ic ~or'cs
T)ISTRlBU'TION OF Prl.V-;;OZOIG~· W ',07 TC «»: Dr JOUTAU/\.
8 .. GEOLOGIC HISTORY
The variety of complexly deformed rocks of southwestern Montana is the result of a long and complex geologic history. The metamorphic character and structural complexity of the Pony groups obscure much of the record of their history. These complex schists and gneisses were probably sedimentary rocks that were folded, metamorphosed, and in- truded by granite, diorite, basalt, and pegmatites before the depos- ition of the Cherry Creek (11). The folded mountains of Pony rocks were eroded and depressed beneath the seas to receive the sedimentary detritus and limy muds which formed the Cherry Creek formation.
Another period of deformation folded the Cherry Creek beds, converted them into schists, gneisses, marbles, and quartzites, and elevated them into mountains which were then exposed to weathering and erosion.
The land was again depressed, and it appears as if the Belt series
Was deposited during Proterozoic time in a relatively shallow sinkirtgJ basin. This sedimentation of the Belt preceded without interruption until toward the close of Algonkian time when the water was withdrawn and the Belt sediments of northwest Montana were tilted but not signif- icantly deformed. However, in the southeast portion of western Montana, the Belt was greatly deformed, uplifted, and largely eroded before the deposition of the overlying rocks. Clapp (3) points out that the lineament between the northern and southern geologic divisions of western Montana may have been produced at this time or it may have been the result of still older deformation, as indicated by the occurrence of Archean rock in the Little Belt Mountains.
During Middle Cambrian time, Paleozoic seas invaded the region
9 along the eastern part of western Montana and along the lineament between the geologic divisions of western Montana. These embayments rapidly deepened, and limestones were deposited during most of the remainder of the period. It appears as if the seas were withdrawn during
OrdoVician and Silurian time for the sediments of these periods are absent. There is some evidence that the Cambrian sediments were tilted slightly before the deposition of the Devonian marine sediments.
FolloWing another withdrawal of the seas and some erosion, the
MiSSissippian marine limestones and shales -were deposited on the undisturbed Devonian sediments. Oscillation of the seas took place during the latter part of the Paleozoic period, and the sandstone,
Shale, and limestone of the Quadrant and Phosphoria formations were deposit~d during this period.
Prior to the invasion of the Mesozoic seas, the Paleozoic rocks were eroded but not greatly tilted. The record of the Mesozoic reveals a sUccession of periods of uplift, erosion, submergence, and sedimenta- tion. The Mesozoic seas were smaller, in this area, than the Paleozoic seas, and the rocks are essentially shallow water deposits, many of
Which are freshwater continental deposits, such as the Kootenai. During
Upper Cretaceous time a series of volcanoes developed and lavas (the
LiVingston formation) were spread throughout the area.
The Rocky Mountain or Laramide revolution, beginning after
LiVingston time, crumpled and faulted the rocks deposited during the
Proterozoic, Paleozoic, and Mesozoic eras. During the latter stages of thereyolution, the Boulder and Tobacco Root batholiths and many other igneous bodies intruded, pushed aside, and replaced the
10 sedimentary rocks.
The history follow'ing the Rocky Mountain revolution can be briefly summarized. Volcanism continued throughout most of Cenozoic time. and resulted in the deposition of Tertiary lavas. Faults and uplift damm~d the streams flowing southwestward, and formed intermontane lakes in which the Bozeman Lake Beds accumulated. Continued faulting caused the elevation of large earth segments, and alluvial fans were built up at the foot of slopes. Pleistocene cold brought glaciation. The great ice masses have only recently left the mountains; vigorous stream erosion has only begun the task of removing the morainal eVidence of g+aciation.
STRUCTURE AND RELATED FEATURES
GENERAL STRUCTURAL FEATURES
As already noted, the structure of western Montana is exceedingly complex and shows a large diversity. It is not proposed to delve too deeply into the details of structure or the mechanics of mountain making in this report for the author feels that a much greater detailed field study of western Montana must be made. However, certain general- izations of the structure can be made from map studies, and the author shall attempt to hypothesize the mechanics.
Dr. C. H. Clapp (3) has graphically described the structure of northwestern Montana. He found that northeast dipping rocks are re- peated in Successive fault blocks formed by longitudinal thrust faults dipping southwest more steeply than the rocks. The front ranges are
SYnclinal, and are sharply limited by the eroded fault scarp of the
11 PLATE ,, \ ,, ,, \
I I ,I 13 , \ \ f Great Falls 1 1 \ \ , ...... ""'- ...... , , \ ...... __ \ '- ...... _ \
',-", .. ...Jissoula --'\ ,'-_ \ --_...., -----:_" "'./"', ~,,,- ":..-~ ..... ~ <, \ ( \ ., \ Vlhite Sulphur ) Springs } ~ Butte J .. Three Forks ---
Q Bozeman 13 -, Livin~ston \
..
", -" ----
PRTIJrIPAL [)T UCTUru L F :ATtTR"'..S OF SOUTTIi ,STm J },lOUTANA
12 Lewis Overthrust, a major tectonic feature of North America. East of the Lewis Overthrust, he observed a number of southwest dipping, parallel, longitudinal thrust faults. Hence a series of wedge shaped fault blocks have been produced in this region. Clapp suggests that a tangential force, acting from the southwest, first uplifted and folded the rocks and that these rocks broke along the steeply dipping thrust faults to form the series of downward-pointing wedges. Continued deformation produced the overthrusting. The part of the map area north of Helena was covered by Clapp in his report.
Contrasting the parallel alignment of structure in the north, the map (Plates 3 and 4) reveals a diversely oriented set of structures in the southern area. At first, the structures appear as a confused unrelated group. However, an examination of the fold-trends reveals a radial pattern with the center about 20 miles southwest of Three
Forks. The major structures are complicated by many minor folds and by faulting. In some instances, the folds are limited on one side by faulting. At least 8 major anticlines and 8 major synclines can be counted.
The map and Plate l-A emphasize the absence of igneous activity in the northern area and the abundance of it in the southern area.
A close examination reveals that the igneous activity is most marked in the area of the radial pattern. However, it must be remembered that the main intrusive masses of Montana came after the development of the major structural pattern or at least after its initiation aCcording to Perry (8). Therefore, the structural pattern seems to be independent of igneous intrusion; although the localization of
13 igneous activity in the area may have resulted from the same causes that produced the diversified trend of folding. The author believes that the igneous activity developed shortly after the initiation of the forming of the structural pattern and that it may have had some control of the final pattern.
The center of the radiating structure is a high range of pre-
Cambrian gneissic rocks known as the Tobacco Root Mountains. Perry (8) considers that the Tobacco Root region is probably one of the most extensive anticlinal uplifts in Montana. It has a vertical component of uplift greater than 20,000 feet.
In describing the area, Perry states that the diameter of a circle surrounding the area of diversified fold-trends is approximately 150 miles. The individual folds of the pattern are 20 to 50 miles long and from 1 to 20 miles wide. The vertical component of deformation
(the vertical stratigraphic distance from trough of syncline to crest of anticline) is measured from thousands of feet to 2 miles. The displacement along faults is also in some places more than 2 miles.
The rocks exposed on the crests of most of the anticlines are
Archeozoic in age, while the rocks exposed in the troughs of the syncline are usually Cretaceous in age. A comparison of the mountain ranges in Figure 1, page 2, and the folds on Plate 3 shows that the trends of the mountains do not necessarily coincide with the trend of major folds; in fact, some folds trend obliquely across the ranges.
A line (See Figure 1) connecting Missoula and White Sulphur Springs defines the lineament which has already been mentioned, and villich divides the structural patterns of northwestern and southwestern
14 Montana. In this area east of the Rocky Mountain Front, there are two major structural elements which also trend eastward in central Montana.
The Little Belt-Big Snowy geanticline, which is over 100 miles long and 30 miles wide and with a vertical component of deformation of approximately 12,000 feet according to Perry (8), is north of Vfuite
Sulphur Springs. See Plate 4. Westward, upon approach to the main ranges of the Rockies, this great structure merges into the Big Belt
Mountain anticlinorium and is lost. South of the Little Belt-Big
Snowy geanticline is the Bull Mountain-Crazy Mountain geosyncline, about 150 miles long and 25 miles wide. Westward, this structure is lost where it enters the Rocky Mountains, although the synclinal condition near Butte and Deer Lodge, where Cretaceous formations are widespread, suggests a continuation of it.
The east-west trend of these large structures is practically at right angles to the trend of the Rocky Mountain deformation. Another interesting feature is that the-western projection of the synclinal structure crosses the area of diversified folds and the western projection of the anticlinal structure marks the northern limit of the same area; that is, it follows the lineament.
CHARACTER AND DISTRIBUTION OF IGNEOUS ROCKS
Again let it be repeated that the greater portion of igneous rocks in Montana are confined to this diversified fold-trend area and that the structural pattern was initiated before the intrusions of igneous rocks.
Both intrusive and extrusive rocks are widely distributed in
15 irreg1.1larlyshaped, discontinuous areas. Lavas are more widely dis- tributed than the intrusives. The intrusives are usually elongated but no general trend is noticeable. These intrusives are most plenti- ful south of Missoula and Helena and north of Dillon, except for small stocks and dikes. The intrusives have generally been considered batholiths because no floor has been discovered, and because the side- wall contacts are steeply inclined or vertical.
In composition, the intrusive bodies in the diversified fold-trend area are granitic, usually quartz monzonite, and the various bodies appear to be related. Frequently the walls of the intrusion are brecciated and the granite extends into the host rock as apophyses.
A province of soda-rich igneous rocks (syenites) border the area on the north, and extend far out into central Montana. These syenite intrusives are small in area, widely distributed, definitely localized.
They have domed the Mesozoic and Paleozoic strata above them. In places, the liquid magma broke through the sediments without deforming them and heaped piles of lava thousands of feet high.
RELATION OF IGNEOUS BODIES TO STRUCTURE
What relation do the igneous bodies bear to the structure? After examination of the map and upon singling out individual intrusive masses, one can find no definite relationship except that axes of some folds do more or less parallel the long axes of some of the igneous bodies or radiate from the igneous body. Some igneous bodies are in anticlinal areas, some are in synclinal areas, some are intermediate, and some cut indiscriminately across both. The Boulder Batholith, largest of these bodies, is in a synclinal area. The Tobacco Root
16 Batholith, about 100 square miles in area, is near the center of the
radial pattern.
Speculation concerning the depth of folding as compared to the
depth of origin of the igneous bodies suggests that the igneous bodies
originated at a depth below that at which folding can take place.
All of the preceding evidence suggests to the writer that a
relationship does exist between the igneous activity and the structure.
MECHANICS OF FOLDING
The development of the radial pattern must have been caused by the diversification in direction of the forces causing folding. That is, there must have been a complex of forces from several directions, rather than merely two opposing forces as Clapp (3) suggests for de- formation in the northwestern part of the state. It is conceivable that the lithologic character of a large segments of the earth, if somewhat different from that of an adjoining segments, might influence fold trend. This principal does not seem to apply in southwestern
Montana for the sediments there are similar to those in adjacent less folded areas (at least those sediments in the upper two miles of the earth's crust).
Searching for an explanation of a combination of complex forces,
Perry (8) considers crustal shortening to produce compression along a single line with deformation produced normal to this line. Meanwhile, another crustal shortening would occur in a different direction, and a different deformation at variance with the first set would be produced. These two deformation lines would develop at their point
17 of intersection a complex of forces, the complexity depending on the rock type and principally on the combination of the two sets of opposing forces of compression. It is possible that at this intersection the resultants of the complex forces may produce.a radial pattern.
The major deformation in central and western Montana indicates two elements of forces: one exerting pressure northeastward and another in the form of a couple working east to west. The intersection of these trends of forces probably was in southwestern Montana and may have produced the radial pattern.
An explanation of the cause of the forces in the preceding dis- cussion involves a complex combination of deep seated earth stresses.
The "Theory of Laterally Spreading Batholiths" advanced by R. T.
Chamberlain and T. A. Link (2) seems to explain to some degree the origin of the forces. By a series of experiments, these authors were able to determine the mechanics of intrusion without the complicating factor of lateral compression. These experiments showed that liquids forced upward through artificial strata followed inclined pathways rather than vertical ones, that they spread out latera~ly, and that in the process of spreading produced a radial pattern of tension cracks and wedge shaped blocks. This evidence along with the fact that batholi ths develop characte:.d.stice.lJ.yLn the hearts of strongly folded mountains indicated t.c Chamberlain and Link t~iat igneous activity was definitely related to many mour.ta in s:.rUCT,ures. They concluded that deformation of +-118 overlying rocks during igneou5 nctivity allowed lateral spreadine; as the magIIladppruauhed the sur:LD.ce. Two conditions necessary for the above theory to apply are the following: (1) that
18 the belt of folding resulted primarily from regional lateral compressive stresses which represent crustal shortening (a condition which both
Perry and Clapp stress for the origin of western Montana structure) and
(2) that the principal igneous masses were intruded while folding and faulting were in progress. (The relationship discussed in this report indicates that igneous activity became apparent after the initiation of the structure but not necessarily after the completion.)
From the evidence already stated, it seems that the initial forces which folded the Rocky Mountains in Montana acted from the southwest and preceded igneous activity. However,-in the early stages of this compressive folding, pressures on the magma beneath the strata were relieved, and hence an accumulation of magmatic forces took place in this area. The combination of the transverse forces and those of the magma finished folding the area. If the batholiths entering the diversified fold-trend area entered into the basement complex below the depth of folding as a long narrow east-west belt, they would exert an east-west pressure. This pressure could have supplied the east-west couple which intersected the southwest force to produce the radial pattern. In the northern region where overthrust faulting rather than folding predominated, the magmatic rocks would have an extra amount of strata to contend with, and hence would find no surface expression. An article by Keith (5) in which he describes the fan structure of the Appalachians supports this conclusion although he bases his conclusion largely on the fact that the fan structures are only found in regions dominated by batholiths. 21983
19 CONCLUSION
In conclusion, the writer would like to emphasize that this paper was principally designed to explain the map and to give some indication of the relation of the igneous activity to the structure. Although he has attempted to explain the mechanics of folding, much work remains to be accomplished before the actual mechanics of the formation of such structural patterns are known. It is hoped that this report will stimulate an intense study of this problem coupled with detailed structural field work in an area which may be considered to be com- paratively geologically unknown. Such a study will go far toward explaining the character and mode of operation of those forces which have gone into the building of the Rocky Mountains.
20 BIBLIOGRAPHY
1. Calkins, F. C. and Emmons, W. H., U. S. Geol. Surv. Geol. Atlas, Philipsburg Folio No. 156, 1915.
2. Chamberlain, R. T. and Link, T. A., The Theory of Laterally Spreading Batholiths: Journal of Geology, Vol. 35, 1927, pp. 319-352.
3. Clapp, Charles H., Geology of a Portion of the Rocky Mountains of Northwestern Montana: Montana Bureau of Mines and Geology, Memoir No.4, 1932.
4. Flint, Richard F., A Brief View of Rocky Mountain Structure: Journal of Geology, Vol. 32, 1924, pp. 410-431.
5. Keith, Arthur, Structural Symmetry in North America: Geol. Soc. Am. Bull., Vol. 39, 1928, pp. 356-359.
6. King, Philip B., An Outline of the Structural Geology of the United States: International Geological Congress XVI session, Guidebook 28, 1933, pp. 1-57.
7. Olson, Stanley G., Geology of Montana, B. S. Thesis, Montana School of Mines, 1948, pp. 1-30.
8. Peale, Albert Charles, Description of the Three Forks Sheet (Montana), U. S. Geol. Surv., Geol. Atlas, Three Forks Folio, No. 24, 1896.
9. Perry, Eugene S., Physiography and Ground Water Supply in the Big Hole Basin, Montana: Montana Bureau of Mines and Geology, Memoir No. 12, 1934, pp. 1-34.
10. Perry, Eugene S., Relation of Batholiths to Major Structure In the South Montana Rockies: Unpublished Manuscript, 1933, pp.1-7.
11. Tansley, Wilfred and Schafer, Paul A., A Geological Reconnaissance of the Tobacco Root Mountains, Madison County, Montana: Montana Bureau of Mines and Geology, Memoir No.9, Part I, 1933, pp. 8-22.
12. Weed, Walter Harvey and Iddings, J. P., U. S. Geol. Surv. Geol. Atlas, Livingston Folio No.1, 1893.
13. Weed, W. H., U. S. Geol. Surv. Geol. Atlas, Little Belt Mountains Folio No. 56, 1898.
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AFTeR. GEOLOGIC MAP OF MONTANA, U.S.G.S. &. IdB.M. ~G'I/.944 BY ow.~EVANOOWSK:~_:~N~~~A __s:_OO(._ OF" MtNES, /sso _j
SECTIONS o F SOUTHWESTERN MONTAN