Popular Science Monthly/Volume 78/May 1911/The Formation of North American Natural Bridges
|THE FORMATION OF NORTH AMERICAN NATURAL BRIDGES|
By Professor HERDMAN F. CLELAND
ALTHOUGH there are more than fifty natural bridges of considerable size in North America, comparatively few persons have ever seen one, the reason being that, with the exception of the Virginia bridge and the natural bridge in North Adams, Mass., most of them are more or less inaccessible.
A bridge, according to the usual definition, is a structure that permits one to pass from one side of a depression to another, whether that depression be a railroad cut, a street or a river. As used in this article a natural bridge is a natural stone arch that spans a valley made by running water; a natural arch being a structure that does not span a valley of erosion.
Although a number of descriptive articles on natural bridges have
Fig. 1. Block Diagram illustrating the Formation of a Natural Bridge in Limestone by the Partial Caving in of the Roofs of Tunnels.
appeared in recent years, the explanation of their origin has, for the most part, been omitted or has been unsatisfactorily given. In this paper an effort is made to show how the more important North American natural bridges were formed.
In the older geologies and geographies we were taught that all natural bridges were formed in one and the same way. According to this time-honored theory natural bridges resulted from the partial caving in of the roof of an underground tunnel or cavern, the portion of the roof left spanning the chasm being a natural bridge. That natural bridges must occasionally be formed in this way is evident. For example, in Edmonson County, Kentucky, where the Mammoth Cave is situated, it is estimated that there are 100,000 miles of underground passages. In the course of time these passages will be widened and the rocks above them will be worn down by surface erosion until, at length, the roofs will almost completely disappear, leaving portions standing here and there as natural bridges. What is happening in Kentucky now has been going on for countless ages in limestone regions in other parts of the world with the possible formation (Fig. 1) and later destruction of natural bridges. It is a rather curious fact, however, that although many small natural bridges have this history, as, for example, a number of bridges in Florida, Iowa, Missouri and other states, yet, as far as known at present, none of the world's great natural bridges has this origin.
The Virginia natural bridge may be taken as a type of natural bridge formed by solution aided by cracks (joints). This can best be explained by a theoretical case. Let us suppose that a short distance—100 or 200 feet—above the brink of Niagara Falls the water of the river should find a crack athwart its course in the limestone bed of the river and that the water seeping through this crack should flow along the top of a lower layer, and reappear underneath the fall as a spring. In the course of years this underground water might eat out a channel which, in time, might accommodate a small portion of the volume of the river. This might eventually be enlarged to such an extent that all the water of the river would pass under the old bed of the river at the fall, leaving the present brink dry land; in other words forming a natural bridge under which the river would flow. The conditions above described have never been fulfilled in the case of the Niagara River and probably never will be, but they were completed in the formation of the Virginia natural bridge, and a bridge of this sort is actually in the process of formation in Two Medicine River, Montana. The height of a bridge of this origin will depend both upon the height of the original fall and upon the amount the stream deepens its valley after the formation of the bridge. The Virginia natural bridge is more than 200 feet high, but the original fall was probably less than that, since the stream has cut down its bed to some extent subsequent to the formation of the bridge.
Within the city limits of the manufacturing city of North Adams, situated in a valley which is beautiful in spite of the efforts of man to render it unsightly, is a natural bridge which well repays a visit. It is one of the most picturesque of natural bridges (Fig. 2) composed, as it is, of white marble with nearly vertical walls. It is small as natural bridges go, the top being but 44 feet above the stream bed and the cavity beneath only about 10 feet wide and 25 feet long. This bridge was formed somewhat as the one just described but differs in some important particulars.Across the Kicking Horse River in the Canadian Rockies, a short distance from Field, B. C, amid some of the grandest scenery on the continent, within sight of primitive forests and glaciers, is a curious natural bridge and one which, at first sight, does not fulfill our conception of such a structure. In this case the opening is almost too small for the volume of the river, so that during floods the water probably flows over the top. The path which one follows in crossing the bridge is almost a horizontal S (see Fig. 3). This bridge was formed largely by "pot-hole" action. Almost everyone in New England has seen those interesting round holes which have been formed in the beds of swift streams by the whirling of pebbles in a permanent eddy until, after many years, a hole is bored which may be several feet in diameter and many feet deep. In the Kicking Horse River there was formerly a rapid or fall on which pot-holes were developed. These holes deepened and broadened at their bottoms until at length (Fig. 4) the walls of two of them were worn through near their bases and permitted the water of the river to flow through the opening thus made. In other words, the natural bridge across the Kicking Horse River is the sides of which were worn through so that the holes opened into one another.
Fig. 3. The Natural Bridge across the Kicking Horse River near B. C.
Formed by "pot-hole" action.
There are probably many small streams that are spanned by bridges of this sort, but few of them have been reported. Two such occur in Vermont.
In Kentucky are several arches with an unusual origin which should perhaps be included under the term natural bridge. They were formed in a plateau composed of horizontal sandstone and limestone by the cutting back of the heads of two streams flowing in opposite directions in deep valleys. The streams continued to cut back until only a narrow ridge or divide separated their basins. This divide was in time perforated by the action of water, wind and frost until at length a fine bridge resulted. One of these (Fig. 5) near the station of Natural Bridge on the Lexington and Eastern Railroad, is 32 feet high and 66 feet wide. There are three bridges, or arches, of this origin within a radius of three or four miles.
In narrow mountain valleys natural bridges are sometimes formed by a large rock falling down the mountainside and wedging into the valley. In Switzerland two bridges of this sort are actually in use by pedestrians, but none has been reported in this country, though many doubtless exist. An unusual bridge formed by gravity (Fig. 6) is one consisting of a large slab which was separated from one side of a valley and fell to the other side. When the crack was filled with debris a usable bridge resulted.
In the Yellowstone National Park a natural bridge (Fig. 7) composed of a lava made up of vertical plates of compact and porous rock spans Bridge Creek near Yellowstone Lake. The bridge, although only forty feet high, is very interesting, both because of its rugged beauty and of its unique origin. An examination shows that the bridge is made of two vertical slabs of lava, one two feet and the other four feet thick, separated by an opening two feet wide. The bridge was formed as follows:
at one time the stream flowed over a fall which is now represented by the top of the bridge; in the course of time the freezing of the water between the thin vertical plates of which the slabs are composed at the foot of the fall forced them apart, making it easy for the water from the fall to wear away the rock at its foot and to excavate a cave. This cavity was gradually extended up stream until a porous layer was encountered through which the water of the stream poured into the cavity, thus forming a bridge of the first of the two slabs. The same process was continued with the undercutting of the second slab. In this way a natural bridge was formed. It is hardly probable that another structure made in the same way is in existence.
It does not seem possible at first thought that a natural bridge more than 125 feet high could be formed by the deposit of lime from water (travertine), but such a bridge (Fig. 8) occurs near the little Mormon
village of Pine about 90 miles south of Flagstaff in Arizona, and can be reached by a horseback journey of three days from that city. As one travels south from Flagstaff through the forest reservation he passes for miles over a lava plateau which, were it not for the pines and cedars, would be called a desert. The forest does not fulfill the usual conception of a wooded region since the trees, though gigantic, are far apart and underbrush is entirely absent. After many hours of travel over a
Fig. 8. Block Diagram to show the Formation of the Travertine Natural Bridge at Pine, Arizona. This bridge was formed by the deposits of travertine derived from the springs on the east. (In the diagram the height of the bridge is exaggerated.)
country of this sort, the brilliant green of the irrigated valley of which the natural bridge is a part seems remarkably beautiful and is a sight not soon forgotten. Not only is the bridge composed of this limestone
deposit (travertine) but also the whole of the cultivated valley of about 25 acres. The bridge has a span of 140 feet, a height of more than 125 feet and a width of about 400 feet. In fact the bridge is so large that the visitor is likely to walk over it, as the writer did, without knowing he is on it. The top of the bridge is under irrigation and produces crops of alfalfa.
The formation of the bridge is simple and the process can sometimes be seen when moist, drifting snow forms a bridge across a small valley. Several large springs that empty into the valley on the east contain lime in solution which, upon evaporation or loss of carbonic acid gas, is deposited. For many years this deposit has been accumulating and with sufficient rapidity to force the stream (Pine Creek) to the west side of the valley. In one place the deposition was rapid enough and the travertine strong enough to arch over the creek and buttress itself on the opposite bank, thus forming a natural bridge. The deposition does not seem to be going on as actively now as formerly, but in one place the lime is being deposited so rapidly that hats, shoes* and other objects left in the spray are coated over with a thick layer of lime in a few months. Underneath the bridge are caves of considerable size adorned with stalactites and stalagmites.
The greatest natural bridges of the United States, and of the world, are found in southeastern Utah. These were formed in a manner so simple that the explanation may, at first, seem inadequate. The streams which they now span have great bends and formerly had greater ones. As they deepened their beds they kept cutting away on the inside of these curves. In some cases the streams probably cut through the necks of these meanders without the formation of bridges, but in four instances the stream perforated the neck of the bend, forming natural bridges. These Utah natural bridges are enormous, varying in height from 108 to 308 feet, and in length of span from 186 to 275 feet. The bridges are made of red sandstone and occur in a high plateau in which the streams have cut canyons hundreds of feet deep.
An interesting bridge of a similar origin spans Swifts Camp Creek (Figs. 9 and 10) in the mountains of eastern Kentucky. Although the top of the bridge is but 15 or 20 feet above the surface of the stream and the length of the span is only 50 feet, yet it illustrates as well as the great Utah natural bridges the manner in which the stream has worked to accomplish this result. The greatest natural bridge in Europe, the Pont D'Arc across the Ardêche has a similar origin, as can readily
be seen by a study of a map of the region. Another Kentucky bridge formed by the lateral erosion of a tributary stream has produced a perforation (Figs. 11 and 12). The water flows through the opening into the tributary stream when the river is high and from the tributary to the river when the water in that is high. One at least of the Utah bridges may have been formed in this way.
On the coast of California is a natural arch (Fig. 13) made by the beating of the waves against a cliff of soft shale. The top of the arch is so level that a team of horses can be driven across it in safety. It was formed by the partial falling in of the roof of a sea cave. Openings of a similar nature are not uncommon on rough and stormy coasts but it is seldom that a structure so perfect as this is formed.
It will be seen from the above that natural bridges are formed in many ways; that they are not confined to any particular kind of rock, nor are they restricted to any particular region but are found alike in deserts and fertile lands, in mountains and on plateaus.
Natural bridges are short lived, geologically considered. The marble natural bridge in North Adams, Mass., for example, was formed many years after glacial times, but already a portion of the bridge has fallen in. Since the great ice sheet is believed to have disappeared from Massachusetts between 20,000 and 80,000 years ago and since this bridge was not formed until long after it had vanished, it will be seen that the life is not long as time goes. Nevertheless, short lived as they are geologically, some of them probably were in existence when the human race was very young.
- For a more complete discussion see "North American Natural Bridges, with a Discussion of their Origin," Bulletin of the Geographical Society, Vol. 21, pp. 313-338, July, 1910.