Geological Survey Bulletin 611 Guidebook of the Western United States: Part A

SHEET No. 19. (click on image for an enlargement in a new window)

Below Clinton the character of the canyon is much the same as it is above that place. The hills range from 1,500 to 2,000 feet in height above the stream, and the slopes are everywhere strewn with the debris of the red shale of the Spokane.

Bonner (see sheet 19, p. 144), at the mouth of Blackfoot River, is noted for its lumbering industry, being the location of some large sawmills. The river has been dammed below the mouth of the Blackfoot, affording about 4,000 horsepower, which is converted into electricity and transmitted to Missoula and the towns of the Bitterroot Valley.

At Bonner the traveler again comes upon the route of Lewis and Clark, for on his return trip Lewis ascended Hell Gate Canyon as far as this point and then turned to the north up Blackfoot River. Six miles west of Bonner, Hell Gate Canyon terminates abruptly,¹ and a short distance beyond this termination is situated the flourishing town of Missoula.

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¹The east wall of the valley at Missoula is so abrupt and regular that it at once suggests a fault—that is, the mountain has risen with relation to the valley or the valley has dropped with relation to the mountain. Farther south, as Waldemar Lindgren has shown, the Bitterroot Valley is bounded on the west by a great fault along which the rocks of the mountain have been raised or those on the east depressed, forming the long, straight Bitterroot Valley. The effect of the movement on these two fault planes, if they are continuous along both sides of the valley, is the depression of the block of strata between them forming the floor of the Bitterroot Valley, as illustrated in figure 23 (p. 112). As the hard rocks under the valley are poorly or not at all exposed, the evidence of the fault at Missoula is to be found only in the topography.

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FIGURE 29.—Horizontal beach lines on Mount Jumbo, as seen from railway station at Missoula, Mont.

From the station at Missoula a good view may be obtained of the steep side of the valley, which rises like a mountain on the east. The knob north of Hell Gate Canyon is Jumbo Mountain, and the larger mass south of the canyon is University Mountain. The slopes of these mountains are free from trees and brush, and on looking closely it will be seen that they are marked by many horizontal lines (fig. 29) which become very prominent when they are covered by a slight fall of snow. These lines have attracted general attention, and many theories regarding their origin have been suggested. Some have supposed that they are stock trails, but it is now generally agreed that they are undoubtedly beach lines cut by a body of water that occupied the broad valley in which Missoula is situated and also many other valleys in this part of the mountains. According to the markings on the valley walls, the water must have been nearly 1,000 feet deep where Missoula now stands.²

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²The horizontal beach lines that are so well shown along the railway at Missoula, in the Jocko Valley and at Plains, below Trout Creek, in the valley of Clark Fork, and also off the railway in the Bitterroot Valley and across the divide north of Ravalli were undoubtedly formed by a continuous body of water that at some recent geologic date occupied these valleys. On account of the excellent development of the beaches of this lake at Missoula. it has been named Lake Missoula. (See map on sheet 19, p. 144.)

Lake Missoula must have occupied the valley at a very recent date, for the faint shore lines would have been entirely obliterated if the lake had been here long ago, and little or nothing would have remained to tell of its existence. However, although it was geologically recent, it existed many, many years ago, probably long before the Indians began to roam over these hills and mountains.

Lakes are transient features and are due to some interference with the normal development of the drainage system of the region. What then occurred in this region to change the drainage and to cause the ponding of the streams at Missoula to a depth of 1,000 feet? The altitude of the highest beach line that has been observed is about 4,200 feet at Stevensville, in the Bitterroot Valley south of Missoula, and as the altitude of Missoula is about 3,200 feet, the depth of the water must have been about 1,000 feet.

When the beaches are traced northward and westward, they are found to terminate just in front of the southernmost extent of the great glacier that came down from the north. As the beaches thus show a definite relation to the ice front and as they seem to correspond in time with the glacial epoch, it seems altogether probable, if not certain, that Lake Missoula was due to the damming of Clark Fork by the ice. The great glaciers that swept down from Canada at this time are known to have occupied all the mountain valleys to the north, filling them to depths which range from a few hundred to perhaps thousands of feet. One lobe of this mass of ice came down the Flathead Valley as far as the Northern Pacific Railway at Dixon, and another down the broad valley from Bonners Ferry, on the Kootenai, by Sandpoint to the vicinity of Spokane. The intermediate valleys in the Cabinet Range have not been examined in sufficient detail to say whether or not they were also filled with ice, but it seems probable that at least some of them afforded avenues for the southward flow of small tongues of ice nearly or quite to Clark Fork.

All the evidence points to the conclusion that the main valley of Clark Fork in the vicinity of Pend Oreille Lake was effectively blockaded by the ice, and that the low valleys to the north were shut off as avenues of escape for the waters of the upper valleys. Such a dam would necessarily be inconstant, allowing the depth of water to fluctuate considerably, and consequently many shore lines would be cut on the rocks; but none of them would be strongly marked, as the water was not held long enough at any one level to permit deep cutting. The shifting of the positions of the several ice lobes would also tend to produce a difference in level of the outlet and a corresponding change in level of the surface of the water. As the glacial epoch waned the ice probably grew thinner and thinner and the lake shrank in a corresponding manner, until at last the present outlet was opened and the water disappeared.

Although the general history of Lake Missoula is about as sketched, a number of facts now known indicate that many modifications may be necessary when the final history of the lake is written. The most difficult to harmonize with the theory given above is the difference in the height of the beach lines in the several valleys. Thus at Stevensville, in the Bitterroot Valley, they extend up the valley wall to an altitude of 4,200 feet; north of Dixon beach lines are well developed up to 3,950 feet; at Plains they can be traced up to an altitude of 3,100 feet, but above that level the hills break away and it seems certain that the uppermost terraces are not represented; near Trout Creek they apparently cease at 3,500 feet; and on St. Regis River no beach lines have been found, but extensive terraces that probably record the height of the water and should be correlated with the uppermost beach lines in other valleys are well developed at Haugan and Saltese, at an altitude of 3,450 feet. It is true that some of these altitudes have not been accurately determined, but there seems to be a gradual decrease in the altitude of the terraces toward the northwest that indicates a depression of the earth's crust in that direction since the beaches were formed, or a rise in the surface toward the southeast. Such a movement is also indicated by the recent canyon cut by Clark Fork between Missoula and the mouth of St. Regis River.

Glacial Lake Missoula had so transient an existence that very little of the sediment deposited in its waters can now be identified, and it is possible that some of the sand and clay noted as Tertiary lake beds were laid down in Lake Missoula.

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Missoula, one of the most important towns of western Montana, is situated on a broad plain at the lower end of Bitterroot Valley, which extends southward for a distance of at least 75 miles. It is the junction of a branch line of the railway which runs up the Bitterroot Valley to Stevensville, Hamilton, and Darby. At Missoula is located the University of Montana, and a little below the town, on the opposite side of the river, is Fort Missoula, one of the principal military posts in the mountain region.

The first permanent settlement in this region was made in 1841, when Father De Smet founded the Mission of St. Mary at the point where Stevensville is now located. He established the mission for the Salish or Flathead Indians, who then occupied the valley but who later were transferred farther north to a reservation which is crossed by the Northern Pacific in the vicinity of the towns of Ravalli and Dixon. Father De Smet was joined in 1843 by Father Anthony Ravalli, who labored faithfully with the Indians throughout a long and busy life. These two priests had great influence on the early settlement of this region, and their services have been commemorated by the naming of towns in their honor.

It was to the entrance of the canyon above Missoula that the name Hell Gate was first applied. The Blackfeet Indians, residing on the plains east of the mountains, were noted fighters; and many were the forays they made through this canyon on the more peaceful Flatheads on the west. The French traders and trappers, on account of the devastation wrought by the marauding parties that emerged from the mouth of the canyon, called it Porte d'Enfer, which may be translated Hell Gate.

The isolation of Missoula in the early days and its distance from the outside world are well illustrated by the slowness of returns from some of the elections; thus it is reported that the settlers in the Bitterroot Valley who voted in the presidential election of November, 1856, did not know the result until April, 1857, when an Oregon paper describing how Buchanan had been elected was brought into the valley.¹

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¹Oregon, which was organized as a Territory by act of Congress in August, 1848, included what is now the county of Missoula, Mont. By an act of Congress approved March 2, 1853, the Territory of Oregon was divided, and the country including Missoula County became a part of the Territory of Washington. In December, 1860, Spokane County, which had included this region, was divided, and Missoula County was organized, with the county seat at the store of Worden & Co. Missoula County remained in Washington Territory until Idaho was organized, on March 3, 1863, when it became a part of Idaho Territory. On the organization of Montana, in 1864, Missoula County became a part of that Territory.

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As the train leaves Missoula, the traveler can obtain on the left (south) a good view of Lolo Peak, a high summit of the Bitterroot Range, which lies south of the Lolo trail that played so important a part in the early exploration of this country. He can not, however, see much of the Bitterroot Valley, for the view is obscured by some low hills on the south side of the river.

The railway runs through a broad valley, with low, rolling hills on the right composed of Tertiary lake beds in which, near milepost 121, low-grade coal is being mined in a small way. The faint beach lines of glacial Lake Missoula, which are so prominent on the side of Mount Jumbo, can be followed with the eye along the north side of the valley for several miles.

At De Smet, 7 miles west of Missoula, the road branches, one line turning to the left (west) and following Clark Fork to Paradise, with a branch across the mountains to the Coeur d'Alene mining district, and the other, the old main line, turning sharply to the right and reaching Jocko Valley through the Coriacan Defile. This narrow pass is reported to have been an Indian highway and it takes its name from Chief Coriacan, of the Flatheads, who was surprised and killed here by the savage Blackfeet.

The railway winds around the hills, through cuts in the Tertiary lake beds, and passes over the Marent viaduct, which has a height of 226 feet. It continues up through a narrow gulch in the Belt series until finally it reaches a broad flat at Evaro, near the summit of the ridge. This place was formerly on the boundary of the Flathead Indian Reservation. A few years ago each Indian was allotted a certain amount of land, and the remainder of the reservation was thrown open to settlement. On this summit and in the descent on the farther side the road runs through the pine forest that formerly covered much of the country, but it soon emerges into the broad, flat Jocko Valley, in which there are some fairly good farms. At milepost 16 an excellent distant view can be obtained of the terminal moraine which once marked the extremity of a small glacier that descended from the canyon in the range to the right. The plan of the moraine can not be seen from the train, but close inspection would show that the ridge of rocky fragments comes down from the canyon wall on one side and loops around and unites with the wall on the opposite side of the creek. As can be seen from the train, the moraine is built up to a height of about 100 feet. The mountains on the right (east), though not so high as the Mission Range, which can be seen farther on, are steep and rugged, towering above the valley to the height of several thousand feet.

Arlee is one of the towns that have begun to grow since the reservation was thrown open to white settlers. It lies in a broad valley containing rich agricultural land and will doubtless in time become an important farming center. A familiar scene in this valley is shown in Plate XIX, A (p. 119).

PLATE XIX.—A (top), SUMMER CAMP OF THE FLATHEAD INDIANS, A FAMILIAR SCENE IN THE JOCKO VALLEY, MONT. B (bottom), GLACIER ON THE NORTH SLOPE OF McDONALD PEAK, MONT. Photograph by C. D. Walcott.

Just below Arlee faint beach lines can be seen on the right (east) near the base of the hill, and a short distance farther on a terrace of fine light-colored sediment is prominent on the same side of the road. This terrace can be followed with the eye as far as the canyon by which the river escapes from the valley. It is composed of brownish clay and sand and is supposed by some to be the sediment deposited at the bottom of Lake Missoula, or it may have been deposited by the present stream when it was ponded by a greater volume of water flowing down Flathead River from the melting glaciers to the north.

The broad valley in which the railway is situated is surrounded on all sides by rocky walls, through one of which the stream draining the valley has cut a deep gorge. Such a basin is seldom, if ever, produced in the normal development of a stream, but is common in the mountainous part of Montana. It is supposed to have been formed by the depression of the bottom of the basin, thus leaving the walls standing high above the valley floor.¹

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¹It is possible to account for the formation of the basin in this manner, but how did the stream cut the canyon at the outlet? There are three possible answers to this question. (1) The movement of deformation was so slow that the stream cut the rock faster than it was uplifted. (2) The basin was formed so rapidly that the water was ponded, forming a lake. This lake rose until the water flowed over the rim, and the stream thus formed cut the present gorge, permitting the water to escape. (3) The valley was filled with sediment, and the stream draining it simply cut down through the soft material and trenched the barrier of hard rock below. During this period the stream removed the great bulk of sediment with which the basin had been filled. Which of these explanations fits the case in hand can not be told without a detailed study of the region, but each process should have left certain marks, if it has occurred, and it is through the study of these characteristic marks that the question can be answered.

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The rocks exposed in the walls of the canyon belong to the Belt series and consist largely of quartzite and argillite. In many protected places in the canyon the white sand and clay deposited by the flooded Flathead River can be seen, showing that this body of water filled not only the valleys where they are wide, but also the narrow canyons connecting them.

At Ravalli the valley is narrow, but the hills are smooth and comparatively low. A stage line runs from this place north 30 miles to Polson, at the lower end of Flathead Lake, where connection is made with boats for Kalispell and other towns on the Great Northern Railway. Ravalli is the distributing point for a large part of the Flathead Reservation. Much of the best land for agriculture lies across the ridge north of the station and extends from St. Ignatius to Flathead Lake. A large part of this area is to be irrigated by the United States Reclamation Service.¹

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¹Nestling between the towering peaks of two ranges of the Rocky Mountains, the Flathead Indian Reservation occupies one of the world's most beautiful valleys, A few years ago the allotment of land to the Indians was completed, and the remaining agricultural lands were opened to white settlement and nearly all filed on.

The Reclamation Service is building an irrigation system to cover about 150,000 acres of land in this region. The average elevation is 3,000 feet above sea level, and the temperature ranges from 30° below zero to 100° above zero. The soil is clay, gravelly loam, and forest loam, and fair crops of hay, grain, and fruits are frequently produced on it without irrigation, the average annual rainfall being about 16 inches. With irrigation, alfalfa, all kinds of grain, vegetables, and fruits in great variety suited to this elevation and latitude are produced in abundance.

The Indians are allowed to sell a portion of their allotments, and farms may also be purchased from white settlers at fair prices. The lands bordering Flathead Lake, which has more than 50 miles of shore line within the reservation, have been subdivided by the Government into summer-resort tracts of 2-1/2 to 5 acres, and many of these tracts have been sold. This valley undoubtedly has a great future as a residential section. It is located near the south end of Glacier National Park, between two great transcontinental railway lines. In summer the valley is gay with flowers. The mountain slopes are covered with fir, larch, and pine trees; the glaciers on their summits sparkle in the sunshine, and at their bases lakes of emerald and sapphire delight the eye and provide the angler with his favorite sport.

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St. Ignatius, 4 or 5 miles northeast of Ravalli, is one of the Catholic missions early established in this region.² It was originally located farther down Clark Fork, but in 1854 was removed by Father Hoecken to its present position, on a fertile plain at the foot of the Mission Range, which affords an abundant supply of good water for household use and for irrigation.

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²The first Catholic mission in this region was established in 1841 at St. Marys, in Bitterroot Valley. The next was the Coeur d'Alene mission, founded in 1842 in the Coeur d'Alene Valley, some 100 miles to the west. St. Mary's mission suffered severely from the raids of the Blackfeet Indians, and in 1850 the property was sold and the mission abandoned. The next mission to be established was that at St. Ignatius.

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Half a mile north of Ravalli is the southeast corner of the Montana National Bison Range, which is surrounded by a specially woven wire fence that can be seen for several miles north of the track. The range, which is under the management of the Biological Survey of the United States Department of Agriculture, includes both prairie and mountains and is over 30 square miles in extent. The buffalo herd now numbers 75, besides which there are a few antelope, deer, and other large game animals.

Just beyond Ravalli, at milepost 31, the traveler can obtain a charming view of the rugged tops of the Mission Range on the right (northeast). This view includes the highest peaks of the range and is the most nearly alpine scenery to be found along the Northern Pacific Railway in the Rocky Mountains.

The terraces of soft material, resting against slopes composed of the Belt series, show almost continuously along the valley of Jocko River, first on one side and then on the other, growing stronger downstream. They reach their best development at Dixon, where the Jocko joins Flathead River, which comes down from the north. This river drains Flathead Lake, which lies 26 miles to the north and is one of the largest bodies of fresh water lying wholly within the United States. The river is navigable for small steamers from Dixon up to a point within a short distance of the lake, where rapids stop further progress.

At Dixon the material deposited in Lake Missoula is fine white sand and clay, being the "rock flour," or fine rock powder, which a glacier grinds from its rocky bed and which is carried off by the streams, giving them a milky appearance. It was brought down by Flathead River from the immense glacier that long ago occupied the entire valley of Flathead Lake and the country farther north, where the town of Kalispell now stands. This material was deposited in the waters of the lake in thin layers (laminae) that give to the cut edges of the material a banded appearance. It is probable that the glacier occupying Flathead Valley reached at its greatest extension nearly or quite to the place now occupied by the town of Dixon, but there is no evidence that it passed farther down the valley.

At milepost 40 a more extended view than that obtained farther up the valley can be had of the Mission Range, including its highest summit, McDonald Peak, and a small glacier lying in a deep amphitheater on the north face of the peak, where the ice is sheltered from the rays of the midday sun. This is the only glacier in the Rocky Mountains that can be seen from the Northern Pacific trains. (See Pl. XIX, B, p. 119.) This noble range marks the eastern boundary of the Flathead Reservation and is the western limit of a broad wilderness of mountain ranges that extend to the margin of the Great Plains and include farther north the rugged mountains of the Glacier National Park. The Mission Range was named from the Roman Catholic mission established at St. Ignatius, near its base, in the early fifties. The range can be seen by the traveler from a point a little west of Dixon to McDonald, and if the weather is clear the view can be relied on to hold the attention, especially from early in October until late in June, when the rugged outlines of the range are veiled under a shining cover of snow. The nearest peaks are about 24 miles away and rise to heights of a little more than 10,000 feet above sea level, or about 7,500 feet above the railway.

This chain of mountains lies on the east border of the broad grassy plains of the Flathead Valley and extends in a remarkably straight line due north and south for nearly 60 miles. The straightness of the mountain front and the sheer abruptness of its rise to heights of 7,000 feet above the low, flat plains at its base make a topographic contrast that is rarely equaled in any other part of the Rockies. This remarkably abrupt front is due to the mountain range being a single huge block of the earth's crust raised and tilted to the east and broken away from the block underlying the lowlands of the Flathead Valley, the unusually straight front of the mountains corresponding closely with the plane along which this break took place.

The hills on both sides of the railway are made up of rocks of the Belt series, which show little variety in the different beds of which it is composed or in the positions in which they lie. The valley walls are generally dark, and they are fringed on one side or the other by remnants of the lake terrace, which can always be identified by their light color.

In the lower part of the valley, between Dixon and Paradise, the railway follows the banks of the river for several miles, and the traveler can obtain many attractive views of the broad river and its wooded islands, set off by the dark background of the rugged hills.

Near Perma the river turns to the left and cuts a narrow canyon through the ridge which above this place bounds the valley on the left. In this part of its course the Belt series is cut by many dikes and sheets of igneous rock (diorite), showing that at one time there was considerable disturbance in the region. The most prominent of these igneous sheets shows in a bold hill on the north side of the river, nearly opposite milepost 50. The igneous material was intruded between the layers of the sedimentary rocks, which subsequently have been turned on edge, and the diorite now forms a conspicuous outcrop for a number of miles to the north.

At Perma another sill of the same sort as that described above crosses the river almost at the station, making a loop through the cliffs immediately to the left and then crossing the river about 1-1/2 miles farther west. This loop is caused by the folding of the sheet of igneous rock, together with the inclosing sedimentary beds, into a great anticline, but the bedding is so obscure that the fold can not be traced from the train.

Twenty miles to the north is Camas Hot Springs, a small settlement where bath houses and hotels are maintained for the use of summer visitors who wish to bathe in the warm mineral water.

Below Perma the canyon is deep and narrow, and its walls are very precipitous. Just beyond milepost 55 the railway crosses to the north side of Flathead River, at a point where several diorite sills are conspicuously exposed. From this point to Paradise the valley has high, rocky walls that rise 1,500 to 2,500 feet above the valley floor. The rocks are dark brownish red, but the large masses of broken rock below the cliffs are a much brighter red and give to the valley the appearance of being decorated with great red banners that are caught up at the base of the cliffs and stream down to the valley bottom in long, graceful curves. The walls are rugged and picturesque, but there is little or no variety, and one soon tires of watching the selfsame combination of river, talus slopes, and cliffs. The river, however, is really worth attention and presents many charming views of the clear water; almost turquoise-blue, sweeping around willow-covered islands and between the stately pines that dot the river's bank. (See Pl. XX.) The terrace of soft material doubtless once continued throughout the canyon, for here and there can be seen remnants of the white clay that vary the monotonous red of the valley walls.

PLATE XX.—VIEW DOWN FLATHEAD RIVER FROM KNOWLES, MONT. Remnants of terraces on both sides of the stream afford a pleasing contrast to more rugged slopes above. Clark Fork enters the valley from the left. Photograph by Haynes, St. Paul, Minn

At milepost 62 Flathead River unites with Clark Fork, sometimes called Hell Gate or Missoula River. The low-grade line of the railway, which leaves the main line at De Smet, follows the winding course of Clark Fork and at Paradise, a short distance below this milepost, connects with the main line. Paradise is a district terminal and so becomes the stopping place of many of the freight crews. The traveler, like others who have gone over this road before him, may wonder for what reason the name Paradise was given to this narrow, rugged valley. The writer has no suggestions to offer, except that even this valley may have looked like a paradise to some unfortunate individual who had been obliged to cross the adjacent mountain country, or who had perchance been lost in the intricate mazes of its deeply cut ravines.

At Paradise the change is made from Mountain to Pacific time, one hour earlier.

The valley of Clark Fork between Paradise and Plains has about the same character as that of Flathead River above Paradise. The rocks are similar, belonging to one of the oldest sedimentary formations known in this country (Prichard). The low terrace which is so conspicuous at Dixon and which is there composed largely of fine material scoured from the bedrock by the glacier that came down Flathead Valley still persists below Paradise, or, rather, remnants of it can be seen here and there on both sides of the valley; but the material is pink, partaking more of the color of the local rocks and in places containing gravel beds of considerable thickness.

To the traveler interested in the geologic history of this region some of the most instructive features of the topography are small deltas or terraces in the side gulches at a height of fully 400 feet above the level of the track. A typical example can be seen on the right (north) just beyond milepost 1. Evidently these deltas mark the mouths of small streams that at one time flowed into a lake whose surface was at the level of the terrace. The lake must have been very transient to have left no other evidence of its existence, and probably it was merely a low stage of the body of water called glacial Lake Missoula.

Plains, formerly known as Horse Plains because it is situated in the midst of a broad prairie that was used as a pasture ground for horses belonging to the trading posts of the Hudson's Bay Co., is a sort of oasis in the desert of rocky canyons along Clark Fork. Here the valley opens and terraces are well developed, especially one about 170 feet above river level. East of the station the terrace shows on the north, but west of the town a large remnant of the terrace, equally well developed, can be seen across the river on the south.

Faint beach lines also appear on the high, smooth hill slope back of Plains, but it is probable that the lines visible from the railway are not the highest in this region, the others being obscured by the timber and brush growing on the higher hills.

One of the most striking features of the valley of Clark Fork is the fairly regular succession of narrow canyons and broad valleys, without any apparent reason so far as the action of the stream is concerned. These changes are not due to differences in the hardness of the rocks, for as a rule all the formations of the Belt series have about the same degree of resistance to erosion; but they are due to great breaks or faults in the rocks. These faults have broken the crust of the earth into huge blocks, some of which have been raised, some lowered, and some tilted over so that one edge is very much lower than the other. These dropped or tilted blocks form the broad valleys, and the raised blocks or the upper edges of the tilted blocks have proved to be serious obstructions in the pathway of the river, which has succeeded in cutting only narrow canyons through them. This explains the broad valley at Plains and the narrow canyons which lie directly above and below that place.

The breaking into blocks and then the tilting of these blocks into various attitudes seem to have been the movements that gave to this region its distinguishing structural features.¹ In some of these blocks, as, for example, the one which lies between Plains and Thompson Falls (see sheet 20, p. 152), the rocks are slightly bent into broad, open folds. The structure in this block is represented by figure 31.

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¹East of the Mission Range the rocks are thrown into great folds or tilted along faults having a general northwesterly trend, but west of that range the structure is less regular and the folds and faults do not have a common direction, as they do farther east. In the area about Plains the structure is broadly simple and yet is rendered complex in detail by minor faults and folds that trend in different directions. As a rule, the rocks are not very severely folded, and most of the faults are of the type called normal faults, due to tension or stretching of the earth's crust, but some of them are distinctly of the overthrust type, due to horizontal stresses of compression. The two kinds of faults are illustrated in figure 30.

FIGURE 30.—Normal faults (A) and overthrust fault (B) Arrows indicate direction of movement.

In a normal fault the rocks slip in such a way that they occupy a greater horizontal space after the movement than they did before the faulting occurred, as shown by the diagram. An overthrust fault is generally produced by the breaking of a fold. The fold and fault are due to compression in the earth's crust, and the result of the movement is that the older rocks are shoved upward and forward over the younger rocks, thus giving them an inverted relationship. Another result is that the faulted mass occupies less space than it did before the movement began.

In the region west of Missoula there have probably been two principal movements—(1) a movement of compression, which threw the rocks into broad folds, the compression in some places, as in the Glacier National Park, being so intense as to produce a great overthrust fault; and (2) a movement of tension or stretching, by which the somewhat folded mass of rock was broken by a great many normal faults, a few of which are shown on the accompanying maps.

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The rocks in sight east of Plains belong to the Prichard formation, which dips to the west and passes below water level, and at Plains the thin-bedded gray quartzite and argillite of the overlying (Ravalli) formation come into view. This formation continues with fairly regular dip to a point about one-half mile beyond Weeksville. At this place the Newland limestone, overlying the Ravalli, appears, dipping in the same direction and at about the same angle as the Ravalli. Within a short distance the dip flattens, and at milepost 15 is reached the point toward which the beds dip from both directions; that is, the axis of the syncline. The beds here are nearly horizontal, but toward the west they begin to rise, and near milepost 18 the Newland limestone disappears from track level, though still present in the tops of the hills, and the Ravalli formation beneath it again comes into view. From this place past Eddy and Frost to the mouth of Thompson River, near milepost 26, the cliffs are made up of Ravalli rocks thrown into folds or wrinkles too small to be shown in the diagram (fig. 31). The walls in this part of the canyon are probably more rugged and more nearly vertical than those of any other part of its course.

FIGURE 31.—Great folds in the rocks between Plains and Thompson Falls, Mont.