Geological Survey Bulletin 600 The Glacier National Park: A Popular Guide to Its Geology and Scenery

CONDITIONS WHEN THE ROCKS WERE LAID DOWN.

Although the mountain rocks are very, very old, they show by their form and composition that originally they were sediments laid down in water, either in large lakes or in the sea. The evidence of this origin consists principally of ripple marks, which were made by the waves when the material was soft sand along the beach, and of sun cracks and rain prints, which show that the mud of the bottom was from time to time exposed to the drying action of the air or to the storms that beat upon the coast. The presence of casts of salt crystals in some of the upper rocks seems to indicate that during at least part of the time the water was salt.

Most of the rocks were therefore deposited in a shallow sea or lake and consequently must have been nearly horizontal. As they are now far above sea level and not at all horizontal, it is manifest that great changes have occurred since they were laid down. It is impossible to say how many and what earth movements have taken place in this region, but the geologist is fairly certain that the movement which distorted the rocks and tilted them up at high angles was comparatively recent and that the movements which preceded this violent disturbance were merely broad uplifts or subsidences, which left the beds of rocks in much the same positions as those they originally occupied.

During each uplift the sea probably receded from this region, but it returned as soon as the land sank below the level of its surface. The last great inroad of the sea occurred at the time the rocks of the plains were being deposited. Then the waves swept across what is now the park, and no Front Range was in existence. Soon after that time, however, the sea was driven out by an uplift of the land, and the region has since been continuously above sea level. The traveler may wonder how many years have elapsed since that uplift, but the geologist must confess his inability to answer the question. He knows, however, that it has been a very long time—possibly several millions of years—and when in imagination one goes back in the history of the world several millions of years, it matters little whether he thinks of one million or twenty million, for both are inconceivable to human intelligence.

ROCKS UPLIFTED AND FAULTED.

Next came a time when all was movement and change. Deep seated forces in the earth had been gathering energy until finally the stresses became so great that the rocky crust began to move. It is not definitely known what causes such stresses in the rocks. They may be produced by the shrinking of a slowly cooling globe; but be that as it may, there is positive evidence to show that after the rocks of the plains were laid down a great pressure developed in the mountain rocks, which caused them to tend to move toward the plains. The rocks of the plains were, however, immovable and as the stresses accumulated they found relief by the folding of the rocks. Here again is something just as inconceivable as the length of geologic time—the power necessary to move a great mass of rock thousands of feet in thickness and wrinkle it up as sheets of paper can be wrinkled in the hand. The geologist learns to accept such things without question, for although he may not be able to realize fully the forces involved in a movement of this sort the evidence of it is so plain as to be incontestable.

The probable results of the movement in the crust of the earth are shown in the accompanying diagram (fig. 1). Section A represents the edges of the rock strata in the condition in which they were originally laid down, before they had been deformed in any way. The force which affected them came apparently from the southwest, and as there was no escape from it, except by bending and wrinkling, it is supposed that one large fold and several small ones were produced, as shown in section B. The pressure, although slightly relieved by the corrugation, still persisted and the folds were greatly enlarged, as shown in section C. At this stage the folds had nearly reached their breaking limit, and when the pressure continued as time went on the strata broke in a number of places along the lines of least resistance, as indicated in the diagram, and the rocks on the west side of the folds were pushed upward and over the rocks on the east, as shown in section D. The mountain rocks (represented by patterns of cross lines) were shoved over the rocks of the plains (represented in producing an overthrust fault.

FIGURE 1.—Diagram illustrating manner in which the Lewis overthrust was produced and its effect on the aspect of the front range.

As the rocks on the west were thrust eastward and upward they made, in all probability, a greatly elevated region, but they did not at any time project into the air, as indicated in section D, because as soon as the rocky mass was uplifted above drainage level streams began to wear it away and to cut deep canyons in its upland portion, and they also reduced the soft rocks of the plains to a nearly even surface. The rocks of the mountains, owing to their more resistant character, still tower above the plains, and where they overlie the soft rocks the mountains are terminated by precipitous walls of limestone, as shown in section E. This explains the absence of foothills that is so conspicuous a feature of this mountain front and one in which it differs from most other ranges.

Naturally on these abrupt and exposed slopes the streams have cut deep gorges through the hard mountain rocks and down into the soft rocks of the plains, so that the actual trace of the fault on the surface is an irregular line zigzagging from spur to valley.

In places along the fault line the streams have cut through the overthrust mass, leaving isolated outliers of the hard mountain rocks far from the main line of the range. The most noted example of this kind is Chief Mountain, on the northeastern boundary of the park, which is formed of a single block of the mountain limestone completely isolated from other rocks of its kind and resting directly on the soft sandstone and shale of the plains. The mountain stands as a single monolith 1,500 feet high, facing the plains as if it were a sentinel standing guard over the hunting ground of the red man. The Indians called this mountain, from its commanding attitude, the "Old Chief," and it is still known by that name.

As shown on the map (P1. XIII, in pocket), the fault separates the mountains from the plains along the east front throughout the extent of the park. It crosses the Great Northern Railway at Fielding, on the west side of the summit, and it passes into Canada where the North Fork of Belly River crosses the international boundary. Although it is present everywhere along the mountain front it is much more conspicuous in the northern than it is in the southern part of the park, because in the north the limestone which immediately overlies it is much thicker than it is in the south. In fact, at some places along the mountains just north of the railroad the limestone is lacking, and at other places it is so inconspicuous that it may not be noticed by the traveler if his attention is not directed to it.

Although the evidence regarding the presence and character of this fault is incontrovertible, there is still one question unanswered—How far has the overthrust mountain mass moved? To those who think of the "everlasting hills" as one of the immutable features of the earth such a question may seem startling indeed, but to the geologist it is the normal question, for to him all natural features are in a state of change, though the action is so slow that it is in most things imperceptible to the eye.

That the mountain mass of the park has been thrust far to the northeast from its original position, as illustrated by the diagram, is clearly shown, but the full extent of that movement may never be known, for it is difficult, if not impossible, to locate the place from which the mass was overthrust. Nevertheless, some idea of the extent of the movement may be obtained by measuring along the railroad the distance from the fault line where it crosses the valley at Fielding to the east boundary of the park, which agrees in a general way with the eastward extent of the overthrust mass. This distance is about 15 miles. Thus it is certain that the whole mass of rocky strata thousands of feet in thickness, weighing incredible millions of tons, has been shoved toward the northeast at least 15 miles, and were the original position of the mountain mass known the distance might prove to be much greater. On account of the great movement and the excellence of the exposures, this great fault, known to geologists as the Lewis overthrust, is destined to become a classic in geologic literature.

THE UPLIFTED MASS CARVED BY WATER.

Coincident with or consequent upon the overthrust came uplift of the region. This uplift stimulated all the streams to increased activity, and the great mass of the mountains was deeply scored by the canyons they cut. Running water is the most powerful agent known in carving mountains and other features of high relief. Where the streams have a sharp descent the cutting is rapid, but the force slowly becomes less effective as the grade is reduced, until the stream becomes sluggish, and then its cutting power ceases and it builds up instead of wearing down its channel. This process, which is known as erosion, may be witnessed by anyone in small rills or brooks after a hard rain. Each stream is swollen with water which, if the ground or rock over which the stream passes is soft, is heavily charged with mud and sand that act as cutting and scouring agents, effectively rasping the walls and deepening the channel of the stream. Where it reaches a lowland or pond the coarser sediment carried by the water is deposited, being spread out over the flat land or built out as a delta in a pond or lake.

The process of stream erosion may seem to many readers to be slow, but in reality it is rapid, for the streams, especially in a mountain country, are constantly charged with abrasive sand or gravel and so are always at work. There is no cessation, no relief from the endless rasping of the grains of sand on the beds of the streams, and as a result the hardest and most massive rocks are rapidly worn away. The streams cut deep gorges, which at first may have nearly vertical walls and be true canyons, but which in a region of considerable precipitation will sooner or later take on, in cross section, the form of a V.

As time goes on the streams cut farther and farther back into the mountain mass until they completely dissect it, leaving instead of an upland plateau a region of serrate ridges and sharp peaks. In this manner most of the mountains of the world have been produced—not by volcanic eruption and sudden movement but by gradual uplift and the dissection of the uplifted mass into rugged mountain forms by the streams.

It will be seen that the conspicuous features of the park are due to inequalities in the hardness and differences in the positions of the rock strata, which are composed of sediments originally laid down beneath the level of the sea, and to the work of water and ice acting on these uplifted strata. In this respect the Glacier National Park contrasts somewhat strongly with the Yellowstone National Park where the most interesting features have resulted from volcanic phenomena and where water and ice have acted mainly on great lava flows and ash beds spread out from time to time on ancient land surfaces.

THE UPLIFTED MASS MODIFIED BY GLACIAL ICE.

Nearly all the valleys in the Glacier National Park show by their forms that they have been occupied by ice, although in many of them no glaciers exist at the present time. The form of a valley after it has been modified by moving ice is well illustrated by figure 2, B. It is known as a U-shaped valley, though the U may be very broad. Such valley forms could have been produced only by great masses of ice which filled the valleys to a depth of at least 2,500 feet and covered most of the lower slopes of the mountains. This of course means that immense glaciers must have originated in these mountains and moved out in all directions, extending 20 or 30 miles over the Great Plains on the east and down the valley of Flathead River on the west. The present glaciers are only the diminutive remnants of the earlier ones, and if the mean annual temperature were raised slightly or the amount of precipitation decreased they would probably disappear. As it is, they cling to the north and east sides of the high ridges and peaks, where the winter snows find a lodging place and where the summer sun has little effect upon them. There are no longer any great bodies of ice in this area, but the work they accomplished is still visible and lends beauty to the scenery, for the rugged mountain tops are accentuated by the rounded, graceful forms of the valley sides.

FIGURE 2.—Diagrams showing form of a stream-cut valley (A) and of the same valley after it has been occupied by a glacier (B).

The glaciers have had an even more marked effect upon the topography than that of smoothing the valleys down which they extended. They had the power to cut into the mountain slopes at their heads, forming basins called cirques, which add greatly to the ruggedness and variety of the scenery. The process of cirque cutting is not well understood, but at or near the point where the névé or snow field changes into the moving ice of the glacier the cirque is formed. Blocks of rock are evidently plucked out by the ice, even back to the névé, and this tends to give to the cirque a nearly level floor. The effect of the backward plucking is to undermine the bordering walls, and these break down, giving to the excavation a circular or semi-circular form with nearly vertical walls from a few hundred to more than 3,000 feet in height.

The glaciers have not only produced the cirques, but are the direct cause of the formation of most of the lakes, large and small, which are without doubt one of the most attractive features of the region. Some of the lakes, such as McDermott, Red Eagle, Ellen Wilson, and Two Medicine, occupy rock basins that were scoured out by the old glaciers in places where the rocks were slightly softer than they were lower down the valleys. Such basins may be distinguished by the rocky barriers that cross them at the outlets. These ledges were not thrust up like dikes, as might perhaps be supposed. They have remained in their original positions, but the softer rocks farther up the valleys have been carried away, leaving basins in which the waters of the lakes accumulated.

Other lakes, such as Bowman and Upper Quartz, are held in place by great glacial moraines—accumulations of bowlders, mud, and other rock débris laid by the ice at the margin of the glacier—which have been deposited across their valleys like huge dams. Many of the small lakes and ponds also are rimmed about by moraines.

Lake basins of a third class have been produced by the glaciers, but in these the dam was not formed by a moraine, but by the outwash of sand and gravel from the end of the ice. The basins of Logging, Lower Kintla, Lower Two Medicine, and McDonald lakes are supposed to have been formed in this manner. At the extremities of these lakes there is no visible evidence of barriers, but the valleys below the lakes are filled deeply with coarse but well-rounded gravel which the streams flowing away from the ice carried and deposited, forming dams just as effectively as if moraines had been built around the ice front.