MOUNTAIN BUILDING AND SCULPTURE
There is a common misapprehension in the mind of the public regarding the mode of origin of mountains. Many people think of mountains as the results of great upheavals of nature by which the jagged peaks were forced up from the depths into their present positions, like the "spine" of Mount Pelee, or were formed by volcanic outbursts in which lava and ashes were poured out upon the surface, building up great chains and rugged peaks. Indeed, so common is this idea that almost every mountain of unusual form or composed of exceptionally red or black rocks is regarded as of volcanic origin.
It is true that many mountains in the United States show evidence of having been produced in this way, but when compared with the whole number of ranges or peaks, those of volcanic origin are few indeed. Among the most prominent may be mentioned the great landmarks of the Cascade RangeMounts Baker, Rainier, St. Helens, Hood, Jefferson, and Shasta. Most of the other great mountain masses, such as the Rocky Mountains from the Canadian line to northern New Mexico, the Sierra Nevada, and the ranges in the Great Basin have been subjected to little or no volcanic activity, and their present forms are due entirely to other causes and conditions.
Although mountains are not generally the direct result of upheaval, they owe their form and height indirectly to movements in the earth's crust. In other words, there could have been no mountains if a portion of land had not been lifted above the surrounding region. Such uplifts may have taken place in one of three ways: (1) By the uplifting of a large region, like the entire Rocky Mountain province; (2) by a great arching of the rocky strata comprising the earth's crust; or (3) by the uplifting and tilting of a huge block of strata that previously had been broken and separated from the adjacent areas, much as a block of rock is broken from a cliff and tilted out of its original position. Each of these uplifts except the first seems to imply great disruption at the surface, but it is generally believed that the movement took place so slowly that erosion nearly or quite kept pace with it, resulting in little apparent local surface displacement or disturbance, but a general elevation of the region. It is probable, however, no matter how slowly the readjustment took place, that the crust of the earth was affected by earthquakes, which are the outward expression of slight movements of the rocks below. As soon as the mountain mass was lifted above the surrounding region streams began to cut channels in its upland surface. At first these were small and extended backward only a short distance into the range, but as time went on the streams became more active, cutting great gashes or V-shaped canyons and extending their activities almost to its center. A picture of the range in that stage of its dissection would show a net-work of deep branching canyons whose slopes were made up of straight lines from the tops of the spurs to the bottoms of the canyons with no trace of curves to relieve the angularity of their profiles. Between the canyons there would be left great residual masses of the mountain projecting as huge peaks with rugged barren slopes. From the outline given above it will be seen that mountains as such are not the direct result of uplift, but are brought out by the cutting around them of deep canyons or valleys in the uplifted mass.
As the elevation of the mass increased, the moisture-laden winds striking the slopes were forced up into the higher and cooler atmosphere and much of the moisture was precipitated in the form of snow. Where the conditions were such that the amount falling each winter was in excess of that which melted during the succeeding summer the snow accumulated and changed to granular ice, forming glaciers, which, by their erosive action, materially changed the form and contour of the mountains. At the head of nearly every tributary of these glaciers an amphitheater or cirque was cut out of the solid rock by the ice. These, as their name implies, are generally semicircular, have flat bottoms, and are bounded on the back by nearly vertical walls. Their form and position on the side of mountain ridges are well illustrated by figure 2, which represents a small cirque containing a glacier near the head of Cut Bank Creek.
Space does not permit of a detailed description of the manner in which such cirques are excavated by glaciers, but a description of the glaciers and glacial phenomena of the park will be found in a corresponding paper by Mr. W. C. Alden, entitled "Glaciers of Glacier National Park."1 The long-continued action of a glacier resulted in its cirque being greatly enlarged and cut back far into the mountain mass. Where two glaciers were located on opposite sides of a dividing ridge the mountain crest between them was reduced in places to a thickness of a few feet, forming an exceedingly rugged saw-tooth ridge, or was cut through, forming a comparatively low pass.
The great rivers of ice flowing down the mountain valleys scoured them out, changing them from the sharp V-shaped canyons that result from stream erosion to rounded U-shaped valleys, as shown in figure 3.
The glaciers also caused the formation of many lakes and ponds, either by the gouging out of rock basins or by the damming of valleys or other depressions in the surface. Of the rock basins the greater number lie in the cirques and are due to the wonderful power that a glacier has to dig out the floor of the cirque below the level of its "lip" or outlet. Other lakes in rock basins occur farther down the mountain slopes, where, owing to the less resistant character of the rock, or to the peculiar configuration of the valleys, the erosive action of the ice streams was more intense than in adjacent parts of the valley, and rock basins were formed. In all cases as soon as the ice melted away the basins were filled with water-forming ponds or lakes.
As a rule the large lakes in a mountain country are the result of damming of valleys by outwash of sand or gravel from the ice or by the terminal moraine or ridge of coarse material which a glacier almost invariably builds around its outer extremity. Such dams are in many places of great height and extent, and large deep lakes are formed behind them as soon as the ice disappears.
All lakes, however, are transient features, as geologists measure time, and sooner or later their basins are drained and the lakes disappear by the cutting of stream channels through the obstructions that hold the water in place. Although this process may be a slow one when measured in years, it must be remembered that the streams are always active; they are at work day and night cutting the rocks over which they flow. At times of low water the streams carry only a small amount of sediment and they have little cutting power, but in times of flood they are extremely active. As the volume of water and velocity increase the carrying power of the streams is greatly augmented, and they pick up immense quantities of mud, sand, and gravel and even roll along by the force of the current large blocks of rock that the water can not lift. All this rock material acts like a huge rasp cutting the rocky ledges over which the streams flow, no matter how hard or massive they may be.
The constant tendency of streams and glaciers in a mountainous region is to deepen the valleys, and thus to make the region more rugged and picturesque and to increase the apparent heights of the mountains. Many persons think that it is the real height of mountains above sea level that makes them impressive, but such is not generally the case, unless the observer can view them from the ocean shore. The magnitude and grandeur of a mountain are determined almost entirely by the height of its summit above the adjacent low land, for this is the height that is apparent to the eye and by which the amount of relief of the mountain is measured. Nothing can be more impressive than the view of the east end of Mount Gould from one of the lakes farther down the valley, as shown in figure 4. Although this mountain is not classed among the high ones of the park, it is more imposing in this view than many of the highest summits, for the reason that it stands 4,700 feet above water level, and the distance between it and the lake is so small that it seems to rise directly out of the water.
In viewing mountain scenery, except in volcanic regions, the traveler should remember that many, if not all, of the causes that have produced it are active to-day and are going on before his eyes. All that is required to produce the stupendous results is time, and nature has been in no hurry.
During the early stages of mountain sculpture the effect of erosion is to increase the relief, but although at first the process is comparatively rapid it is gradually retarded, and, finally, if no change occurs in the crust of the earth, the process becomes inactive, and then the tendency is to reduce the relief and to obliterate the results previously attained. Hence the stage of reduction that is marked by the most beautiful results is an intermediate one, in which the mountain forms are still in their youthful ruggedness without any trace of the softening influence of old age that marks the later stage of their existence.
Last Updated: 09-Nov-2009