GEOLOGY OF GLACIER NATIONAL PARK AND THE FLATHEAD REGION NORTHWESTERN MONTANA
By CLYDE P. ROSS
This report summarizes available data on two adjacent and partly overlapping regions in northwestern Montana. The first of these is Glacier National Park plus small areas east and west of the park. The second is here called, for convenience, the Flathead region; it embraces the mountains from the southern tip of Glacier Park to latitude 48° north and between the Great Plains on the east and Flathead Valley on the west. The fieldwork under the direction of the writer was done in 1948, 1949, 1950, and 1951, with some work in 1952 and 1953.
The two regions together include parts of the Swan, Flathead, Livingstone, and Lewis Ranges. They are drained largely by branches of the Flathead River. On the east and north, however, they are penetrated by tributaries of the Missouri River and in addition by streams that flow into Canada. Roads and highways reach the borders of the regions; but there are few roads in the regions and only two highways cross them. The principal economic value of the assemblage of mountains described in the present report is as a collecting ground for snow to furnish the water used in the surrounding lowlands and as a scenic and wildlife recreation area. A few metallic deposits and lignitic coal beds are known, but these have not proved to be important and cannot, as far as can now be judged, be expected to become so. No oil except minor seeps has yet been found, and most parts of the two regions covered do not appear geologically favorable to the presence of oil in commercial quantities. The high, Hungry Horse Dam on which construction was in progress during the fieldwork now floods part of the Flathead region and will greatly influence the future of that region.
The rocks range in age from Precambrian to Recent. The thickest units belong to the Belt series of Precambrian age, and special attention was paid to them. As a result, it is clear that at least the upper part of the series shows marked lateral changes within short distances. This fact introduces complexities into stratigraphic correlation and should be remembered wherever the series is studied. The stromatolites, or fossil algae, in the Belt series, although still imperfectly understood, give clues with respect to problems of ecology and stratigraphy.
The subdivisions of the Belt series within the areas covered by the present report are, in ascending order, Altyn limestone, Appekunny argillite, Grinnell argillite, Siyeh limestone, and Missoula group. Local subdivisions of the Missoula group are possible in certain areas, and all the units just named are expected to be subdivided when detailed studies are undertaken.
In the Glacier National Park and Flathead regions together, it is probable that between 25,000 and 30,000 feet of beds belonging to the Belt series, possibly more, are present. These consist largely of quartzitic argillite, quartzite, and carbonate rocks, mostly dolomitic. Small gabbroic and diabasic intrusive bodies and, at one horizon, basaltic lava are associated with the Belt series. Above the Belt series is a thick sequence of Cambrian, Devonian, and Carboniferous strata, in which limestone is dominant, followed by strata of Jurassic and Cretaceous age, largely limestone and shale and partly of terrestrial origin. Slightly consolidated gravel, sand, and silt of Tertiary age are preserved in some valleys and as erosional remnants on the plains close to the mountain border. Pleistocene and Recent glacial and fluviatile deposits are plentiful in mountain valleys and on the plains east of the mountains.
Sufficient crustal movements took place during the latter part of Belt time to produce tension cracks that permitted some intrusion and related extrusion to occur. Broad crustal warping probably took place at intervals during the Paleozoic era, but these successive movements left little record other than the absence of sedimentary rock units that might otherwise have been deposited. The same can be said of much of the Mesozoic era, but the uplift that was to give rise to the mountains may have begun during the Cretaceous period.
In or shortly after the late Paleocene, conditions changed drastically. Thrust and normal faults of major magnitude, preceded and accompanied by folds and minor fractures, resulted from a series of violent crustal movements that have not yet entirely ceased. The master structural feature produced during this deformation is known as the Lewis overthrust. Much remains obscure, but the concepts resulting from the present study are that the Lewis thrust originated fairly deep in the earth's crust and that the overthrust block moved many miles in a direction somewhat north of east over a mass of relatively incompetent rocks that were intricately folded, overturned, and overthrust to depths thousands of feet below present sea level before and during the advance of the main thrust. If, as seems quite possible, the thrust plane ever emerged at the surface, it was at some place so far to the east that erosion has since removed the evidence. The concept of a sole with which the exposed thrusts merge at depth and which is underlain by undisturbed rocks is not supported by evidence at hand. The block above the Lewis overthrust was itself deformed but, in the part now remaining, much less intensively than the rocks below. Adjustments that may have begun during the overthrusting and continued to the present have fractured the overlying block.
A series of geomorphic events not clearly recorded in the present topography reached a culmination in a mature surface (the Blackfoot surface) near the end of the Tertiary period. Several stages of glaciation with an intermediate stage in which rejuvenated stream erosion cut deep gorges have modified and, over large areas, obliterated the Blackfoot surface. Nevertheless, the present topography still reflects the structure of the underlying rocks. Since the close of the Pleistocene epoch, renewed erosion has caused the streams to cut small inner gorges in the valley floors. Also small glaciers reappeared in the uplands carved by the far larger Pleistocene ice streams, and some of these minor glaciers persist to the present day.
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.