USGS Logo Geological Survey Professional Paper 296
Geology of Glacier National Park And the Flathead Region, Northwestern Montana



The present report is a composite one in that it presents data on two adjacent and slightly overlapping regions in northwestern Montana. The more northerly and better known of these comprises Glacier National Park plus small areas on either side of the park. This region, shown in plate 1, is an irregular mountain mass bounded on the north by the international boundary, on the east by the Great Plains, and on the west by the valley of that part of the Flathead River locally called the North Fork. Plate 1 extends eastward to longitude 113°10' and thus includes some of the plains. The southern boundary of the park corresponds essentially to parts of the channels of the Middle Fork of the Flathead and of its tributary, Bear Creek, but rather than stop at this irregular line the map extends southward almost to latitude 48°10'.

The second region here described is the one in which the present project was begun because of the extreme lack of available geologic information in regard to it. For convenience this latter is here termed the "Flathead region." Much of it is in the drainage basin of the Flathead River, and a large part of the Flathead Range is included. The Flathead region, shown in plate 2, comprises parts of the Heart Butte, Marias Pass, and Nyack 30-minute quadrangles (p1. 3). It is north of latitude 48° N. west of longitude 112°30' E., east of longitude 114°00' and south of Glacier National Park except that the southern tip of the park has been included.

The position of the Glacier National Park and Flathead regions with respect to the rest of northwestern Montana is shown in plate 3. Together these regions embrace the northwestern corner of Teton County, the western end of Pondera County, the western part of Glacier County, the northern tip of Lake County, and much of the mountainous part of Flathead County. They include parts of the Lewis and Clark and Flathead National Forests.


The data on the Flathead region presented in the present report were gathered mainly during a 54-day geologic reconnaissance in the summer of 1948 and a 60-day reconnaissance in the summer of 1949. During this time a little more than 1,300 square miles within the area shown on plate 2 was covered. The information obtained has been supplemented on the peripheral parts of plate 2 by the results of previous mapping by Stebinger, Alden, Campbell, Erdmann, and others, as noted in more detail below. Much of the data on Glacier National Park assembled below and shown on plate 1 came from unpublished material gathered in and near Glacier National Park by Geological Survey parties under the direction of M. R. Campbell and east of the park boundary by Stebinger, Alden, and others. These data are supplemented, coordinated, modified, and, in places, corrected on the basis of 64 days of fieldwork in the summer of 1950 plus a little of the field time in 1948 and 1949. Some fieldwork was done in the park in the summers of 1951, 1952, and 1953 mostly in connection with the study of stromatolites. Plate 1 represents an area of about 2,375 square miles, but only about 1,000 square miles of this was studied in any detail during the 1949 and 1950 seasons. Most of the park not thus studied was visited by the writer or his assistants, which has aided in the interpretation of the field notes and maps of the previous workers. Plates 1 and 2 include index maps that show the areas mapped during each of the three field seasons; they also show areas mapped by previous workers and not remapped. These index maps are referred to in the rock descriptions as convenient means of indicating the particular areas described.

The investigation was started for several reasons: the need for information on this region of complex geology for use in the compilation of a new geologic map of Montana, then well advanced and later completed (Ross, Andrews, and Witkind, 1955); and the opportunity presented by the western part of the Flathead region for the study of the Belt series.

To prepare an adequate geologic map of western Montana, it was essential to decide on major stratigraphic subdivisions of the Belt series that could be mapped throughout western Montana. However, the geologic and economic interest in the Belt series extends far beyond problems related directly to the State map. Fieldwork in Glacier National Park in 1950 and 1951 coupled with the unpublished data on the geology of the park in the files of the U. S. Geological Survey yielded much information on the Belt series and modified some of the ideas previously held in regard to these rocks. The Belt series is better exposed in and just south of Glacier National Park than in many of the other areas in which it has been studied. Such data on the series as apply directly to the areas mapped are summarized below, but study of these rocks is still in progress, and there are certain lines of evidence and methods of attack that were opened during the fieldwork of 1948-51 that are too incomplete to be discussed adequately as yet. The problems of nomenclature and subdivision of the Belt series throughout Montana and adjacent regions are beyond the scope of the present paper.

Fossil names used to describe the zones in the present paper are those used by Richard Rezak in his work on algae in the Belt series (Rezak, 1957). Detailed descriptions of these fossils appear in his report. Rezak has also checked the stromatolite zones that appear on the map and mapped a new zone in the Missoula group over a small area in the southwest part of plate 1 and the corresponding part of plate 2.

Another objective was the study of the structure of a region that contains a segment of one of the greatest and earliest recognized overthrusts in the Western United States (the Lewis overthrust). In earlier geologic works, the Lewis overthrust was regarded as an erosion thrust that emerged at and moved over the surface of the ground close to the present mountain border. The results of the present investigation do not support this concept. If the Lewis overthrust did reach the surface, it was probably far to the east. Similarly little support is found for the idea of a sole with undeformed strata below, which was based on comparison with structural features in the Scottish Highlands rather than on evidence available in northern Montana. Steeply inclined faults in the western part of the region here described have hitherto been supposed to be in accord with the wedge theory of diastrophism. The evidence now at hand is inconclusive but favors the idea that the faults result from relatively minor adjustments and are probably normal instead of the thrusts required by the wedge theory. Deformation appears to have recurred at least as recently as the Pleistocene.

The field mapping was carried out, rather rapidly, on a scale of 1:125,000. This was necessitated in part because of the lack of modern, large-scale topographic base maps suitable for detailed studies. All studies of the Belt series have contained generalizations not adequately supported by geologic maps. Even now large parts of the broad regions in which the series occurs are covered only by geologic maps of exploratory type, and the present project has been planned as a contribution toward more adequate mapping. The components of the series are so thick and contain so few recognized horizon markers that subdivisions can be established and lateral facies changes recognized only on the basis of regional mapping.

Certain of the other geologic problems in this part of Montana, such as the geomorphologic history, can be adequately attacked only on a regional basis. It is hoped that the generalized data assembled in the present, primarily descriptive, report will be tested, modified, and refined by future detailed work. Detailed studies will yield their richest rewards only if made after a general setting, such as is provided by this report, is available. For much of the remaining parts of northwestern Montana, too little is yet known to give such a setting. General studies should be undertaken initially for the mountains between the Flathead region and the general vicinity of Missoula and Helena and later extended westward to connect with the detailed studies being carried on in mining districts in Idaho. The initial objective, correlation of widely separated units of the Belt series, would be aided greatly by publication of maps by various geologists for which much of the field work has already been done.

While the facts and concepts regarding such things as the stratigraphy of the Belt series and the character of the Lewis overthrust should be of interest in connection with economic studies throughout western Montana and adjacent regions, the two regions here reported on contain so little in the way of known mineral deposits of apparent economic value that little time was devoted to this aspect of the investigation. A little lignitic coal appears in the master stream valleys of the two regions, and years ago prospecting for copper and other metals was undertaken in the mountains, with such slight success that it has almost entirely ceased. Drilling for oil is being actively carried on in areas east of the mountains, but very little has been undertaken as yet within the areas represented on plates 1 and 2. The mountainous parts of the country covered by the report seem geologically unfavorable to the presence of oil pools of commercial interest, even though small oil seeps have been found there.

Part of the Flathead region has been flooded by the reservoir created by the Hungry Horse Dam, for which concrete began to be poured in 1949. The present report has no direct bearing on this project, and fieldwork was begun after the site of the dam had been selected. Such economic advantages as may result from the construction of the Hungry Horse Dam will be felt mainly in the Flathead Valley and more distant areas.


Throughout the investigation progress was greatly facilitated by the cooperation of National Park Service and U. S. Forest Service officers. Aid in various ways was furnished by people living in the vicinity of the regions mapped. The spirit of helpfulness manifested by most of those with whom contact was made contributed greatly to the speed with which the fieldwork was carried out.

In 1948 Harold Masursky was the geologic assistant, and his careful work contributed substantially to the results. He became especially familiar with the lower Paleozoic formations, and most parts of plate 2 in which these units are shown reflect his efforts. In 1949 the geologic assistant was John Hensley, who also contributed materially to the success of the undertaking. In the fieldwork of 1950, highly efficient assistance was rendered by Edward C. Stoever, Jr., who contributed much to the geologic mapping, especially in the southern part of Glacier National Park. During the same season Stephan Nordeng devoted his attention mainly to a study of the stromatolitic remains in the Belt rocks of the park. In 1951 Richard Rezak, working alone, took up this phase of the investigation and continued it through 1953 with the assistance of Robert N. Oldale. Rezak has added materially to knowledge of the stromatolites and is continuing his investigations of them. The data relative to stromatolites in the present report were provided by Rezak, and a more detailed treatment of the subject by him has been published (Rezak, 1957).

During the early part of the 1948 season the pack train was furnished and managed and camp work done by David W. Stonehouse. During the latter part of that season, Herbert Moore performed these functions. In 1949 the pack and camp work was done by Mr. and Mrs. Harland Knowlton. In 1950 this work was done by Athylone J. French. All these packers proved to be skillful and experienced, and it is a pleasure to acknowledge the indispensible assistance they furnished.

Little could have been accomplished in regard to the geology of Glacier National Park in the short time devoted thereto if it had not been for the large amount of pertinent but unpublished material already available. In 1911 through 1914 field parties of the Geological Survey made extensive geologic studies and mapped the greater part of the park. The geologic personnel involved in that work included M. R. Campbell (in charge), W. C. Alden, T. W. Stanton, Eugene Stebinger, J. R. Hoats, J. E. Thomas, C. S. Corbett, E. M. Parks, C. R. Williams, and H. R. Bennett. Much of the material they gathered was available for use in the present investigation. Data credited to any of these men in the present report came from their field notes. The geologic map of Glacier National Park (pl. 1) is largely the result of their efforts, although it covers a wider area and is modified in various respects. A map embodying their work was compiled some years ago by W. C. Alden, and copies of it were furnished to the National Park Service for their use. Plate 1 of the present report incorporates much the same material as Alden's compilation but was taken, so far as possible, directly from the original field notes and maps of M. R. Campbell and the men who worked with him plus the results of the field work of 1949 and 1950 with minor corrections in 1951. Much of the data obtained by Campbell's parties was available for the compilation of plate 1, but some items are missing. Similarly the current work in the Flathead region was supplemented in the plains in the eastern part by data assembled by Eugene Stebinger and others, parts of which were not published.


Several valuable papers dealing with features of the geology of Glacier National Park are available, but they are not accompanied by geologic maps. Very little information on the geology of the Flathead region has been published. The presence of great faults along the eastern border of the mountains, glaciation within the mountains, and some of the other major geologic features of the two regions were appreciated more than 60 years ago. Some of the scenic features of the Glacier Park region were noted by explorers even earlier (Culver, 1892; Davis, 1886, p. 712; Pumpelly, 1918, p. 641-646; Stevens, 1860). Pumpelly's trips were made in 1882 and 1883 and this may have been the earliest visit of a geologist to the area of Glacier National Park. Near the beginning of the 20th century, data on the character and origin of the topographic features began to appear. One of the earliest summaries was by Elrod (1903), who, however, was concerned mainly with the vicinity of Flathead Lake. Matthes (1904) published a nontechnical description of the topography of Glacier National Park but barely touched on the origin of the features he described. His paper is based on observations he made as one of the topographers who surveyed and prepared maps of the area of the park for the U. S. Geological Survey. These maps, made in 1900-1904 and 1907-12, are still in use, with minor revisions, mainly with respect to cultural features. The three quadrangle maps of the area immediately to the south, used as base maps for the present work on the Flathead region, are of similar date, having been surveyed during 1907-14, with minor subsequent revisions. Detailed and comparatively recent maps of the principal forks of the Flathead River are available, (U. S. Geol. Survey 1937, 1939, 1947, 1950) and various maps of the U. S. Forest Service, as well as one by that Service in cooperation with the National Park Service, issued in 1938, have been of great assistance, particularly with respect to roads and trails. Data from these have been incorporated in plates 1 and 2.

Chapman (1900), a topographer of the U. S. Geological Survey who did exploratory work in the eastern part of the mountains from latitude 47° N. to the southern border of Glacier National Park, contributed valuable information and ideas in regard to this region, which includes the eastern part of the Flathead region of the present report.

Willis (1902) made the first contribution to the geology of the area of Glacier National Park by a valuable and fundamental paper to which is added an appendix on the igneous rocks by his associate, G. I. Finlay (1902). Willis discovered the Lewis overthrust, discussed it in some detail, and recognized the Precambrian age of the rocks now called the Belt series. He thought the Lewis was an erosion thrust and referred to the valley of the Flathead River west of the park as a graben. In those earlier reports on northern Montana in which an age assignment was hazarded, the Belt strata were thought to be Cambrian, although in the original definition of the series in the vicinity of Three Forks (Peale, 1893) in southern Montana the age was tentatively given as Algonkian.

Calhoun (1906) on the basis of work done in 1901, 1902, and 1903 presented comprehensive data on the glacial history of the eastern side of the mountains and of the plains east of them. Many of his generalizations are still accepted, and his clear descriptions are of much value in gaining a perspective as to the geomorphic features of the region. At about this time Walcott (1906, p. 18) made a reconnaissance across the Mission and Swan Ranges and the mountains east of them and published the first correlation table of the Belt series in northwestern Montana.

Alden, in part in cooperation with Stebinger, has written several papers dealing with glaciation in and near Glacier National Park (Alden, 1912, Alden and Stebinger, 1913, Alden, 1914 a and b, Alden, 1932). Campbell (1914, 1921) has prepared two papers intended to aid visitors to understand the geology of the park.

Powers and Shimer (1914) made the suggestion that the large overthrust described by Willis in the paper cited above divides into several smaller thrusts before reaching Sun River. They also listed fossils of Cretaceous, Jurassic, Mississippian, and Devonian age collected near the Sun River by W. O. Crosby.

Davis (1916, 1921) contributed interesting data on areas just west of that here described. He laid stress on the work done by large valley glaciers and thought that although the straight, steep fronts of the Mission and Swan Ranges had probably resulted from faulting the evidence is obscured by the effects of later glaciation to which the existing scarps are largely due. This conclusion is in accord with such general observations as were made during the present investigation.

Stebinger and his associates (Stebinger, 1916, 1917, 1918, Stebinger and Goldman, 1917) did much work in the plains east of Glacier National Park and plains east of the mountains south of the park. This work, which has been extensively drawn upon in the present report, established the presence of thrusts and tight folds in a zone that extends for some miles east of the mountain front. He also worked around Flathead Lake, and his field notes on that area have been utilized in the present report.

A highly condensed but very valuable paper by Clapp (1932) gives a summary of information on an area that extends from the Canadian boundary to latitude 46°30' and from longitude 114°30' eastward to the mountain front, based largely on field work carried on by him or under his supervision during the 12 years preceding publication of the memoir. Those working with Clapp at various times included G. S. Lambert, Arthur Bevan, R. A. Wilson, C. F. Deiss, and others. While the map in the memoir includes almost the entire area described in the present report, it was necessarily based on scanty information for the greater part of that area. Clapp made reconnaissance trips in parts of Glacier Park and the Flathead region and had access to an unpublished thesis by C. M. Langton based on more detailed mapping in the vicinity of Schafer Ranger Station on the Middle Fork of the Flathead River. Bevan (1929), who worked in several places along and east of the front of the Rocky Mountains, has summarized data on that area gained from his own observations and other sources.

Several reports dealing with geologic features in Glacier National Park have been published in comparatively recent years. C. L. Fenton and M. A. Fenton (1931, 1933, 1937) have written papers, of which the principal ones are here cited. Their papers deal mainly with organic remains in the Belt series and the stratigraphy of that series, and constitute some of the most noteworthy contributions yet made to these subjects. They include discussion of details of sedimentary structures and paleontology, many of which are not repeated here. In addition to the comparatively technical papers just cited, C. L. Fenton (1939) has published an interesting popular account of some features of the geology of Glacier National Park. Billings (1938) has presented evidence to show that the Lewis overthrust is not an erosion thrust as Willis had supposed it to be. Two recent papers by Dyson (1949a, b), prepared for the use of visitors to Glacier National Park, are useful brief summaries. Two other papers by officers of the Park Service include road and trail logs in which attention is called to geologic features (Ruhle 1949, Beatty 1947).

Dam-site investigations carried out some years ago by Erdmann (1944, 1947) yielded facts and concepts of interest to all concerned with geologic matters in the drainage basin of the Flathead River. One of the dam-site studies he made, near the western border of the Nyack quadrangle, yielded data that have been incorporated in the maps accompanying plate 2 of the present report.

South of the eastern 2 of the 3 quadrangles included in plate 2 much work has been done by C. F. Deiss. This work was begun in 1933 in association with Clapp under the auspices of the State of Montana and continued at intervals until 1941. The last part of the investigation was carried out as a member of the U. S. Geological Survey but was interrupted when wartime needs diverted Deiss' activities. The investigation extended over large parts of the Saypo, Silvertip, Ovando, and Coopers Lake quadrangles, which together constitute a block immediately south of the Flathead region (pl. 3), covered in the present report. Deiss (1933, 1935, 1938, 1939, 1941, 1943a, b) has published reports based in whole or in part on his investigations in these quadrangles. While the published reports do not include geologic maps, Deiss has very kindly made his field maps available for examination in connection with the present report. His reports and maps furnish an invaluable basis for the study of the stratigraphy and structure in the regions here described. His contributions to knowledge of Paleozoic stratigraphy in northwestern Montana are especially outstanding. Sloss and Laird (1945), whose work included traverses across parts of the region mapped by Deiss, have supplemented his work with additional observations, but differ from him in regard to certain details of stratigraphic subdivision that have only slight bearing on the present study.

Recent papers (Imlay, 1945, p. 1019-1027; Cobban, 1945, p. 1262-1303; Imlay and others, 1948; Imlay, 1948, p. 13-33) give details in regard to the Jurassic strata in and near the regions here considered. Some of the observations reported on in these papers were made in the Heart Butte and Marias Pass quadrangles, others in the Saypo quadrangle. The subdivisions of the Jurassic rocks set up in these reports have been found to be present within the regions here reported on, although the scale of the work makes it quite impractical to show them on the map.

Reports dealing with areas in Canada north of Glacier National Park and with the Rocky Mountain Trench and related features in Montana west of the park have been consulted mainly in connection with the interpretation of structure. Daly's report (1912) on the geology along the international boundary provided the basis for subsequent work in the vicinity of that line. Among other things, he emphasized the striking character of the Rocky Mountain Trench and defined that feature. His work was necessarily reconnaissance but much of it has stood the test of time. Even where further work has modified some of his conclusions, his discussion of the problems involved continues to be a stimulus to those who follow him. Among those Canadians who have made notable contributions in the present connection are Schofield (1914), (1915), (1920), MacKenzie (1916; 1922, p. 97-132), Hume (1932, p. 1-20), and Link (1932, p. 786-798; 1935, p. 1464-1466; 1949, p. 1475-1501). Incidental mention of others will be found in the body of the present report. Among those who have dealt particularly with the structure of the Rocky Mountain Trench and related features are Evans (1933), Shepard (1922), (1926), Flint (1924), and Chamberlain (1925, 1945).

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Last Updated: 08-Jul-2008