Outlines of the Earth's History: A Popular Study in Physiography

by Nathaniel Southgate Shaler

When the raindrops coalesce on the surface of the earth, the rôle of what we may call land water begins. Thenceforward until the fluid arrives at the surface of the sea it is continually at work in effecting a great range of geological changes, only a few of which can well be traced by the general student. The work of land water is due to three classes of properties—to the energy with which it is endowed by virtue of its height above the sea, a power due to the heat of the sun; to the capacity it has for taking substances into solution; and to its property of giving some part of its own substance to other materials with which it comes in contact. The first of these groups of properties may be called dynamical; the others, chemical.

The dynamic value of water when it falls upon the land is the amount of energy it can apply in going down the slope which separates it from the sea. A ton of the fluid, such as may gather in an ordinary rain on a thousand square feet of ground in the highlands of a country—say at an elevation of a thousand feet above the sea—expends before it comes to rest in the great reservoir as much energy as would be required to lift that weight from the ocean's surface to the same height. The ways in which this energy may be expended we shall now proceed in a general way to trace.

As soon as the water has been gathered, from its drop to its sheet state—a process which takes place as soon as it falls—the fluid begins its downward journey. On this way it is at once parted into two distinct divisions, the surface water and the ground water: the former courses more or less swiftly, generally at the rate of a mile or more an hour, in the light of day; the latter enters the interstices of the earth, slowly descends therein to a greater or less depth, and finally, journeying perhaps at the rate of a mile a year, rejoins the surface water, escaping through the springs. The proportion of these two classes, the surface and the ground water, varies greatly, and an intermixture of them is continually going on. Thus on the surface of bare rock or frozen earth all the rain may go away without entering the ground. On very sandy fields the heaviest rainfall may be taken up by the porous earth, so that no streams are found. On such surfaces the present writer has observed that a rainfall amounting to six inches in depth in two hours produced no streams whatever. We shall first follow the history of the surface water, afterward considering the work which the underground movements effect.

If the student will observe what takes place on a level ploughed field—which, after all, will not be perfectly level, for all fields are more or less undulating—he will note that, though the surface may have been smoothed by a roller until it appears like a floor, the first rain, where the fall takes place rapidly enough to produce surface streams, will create a series of little channels which grow larger as they conjoin, the whole appearing to the eye like a very detailed map, or rather model, of a river system; it is, indeed, such a system in miniature. If he will watch the process by which these streamlet beds are carved, he will obtain a tolerably clear idea as to that most important work which the greater streams do in carving the face of the lands. The water is no sooner gathered into a sheet than, guided by the slightest irregularities which it encounters, it begins to flow. At first the motion is so slow that it does not disturb its bed, but at some points in the bottom of the sheet the movement soon becomes swift enough to drag the grains of sand and clay from their adhesions, bearing them onward. As soon as this beginning of a channel is formed the water moves more swiftly in the clearer way; it therefore cuts more rapidly, deepening and enlarging its channel, and making its motion yet more free. The tiny rills join the greater, all their channels sway to and fro as directed this way and that by chance irregularities, until something like river basins are carved out, those gentle slopes which form broad valleys where the carving has been due to the wanderings of many streams. If the field be large, considerable though temporary brooks may be created, which cut channels perhaps a foot in depth. At the end of this miniature stream system we always find some part of the waste which has been carved out. If the streamlet discharges into a pool, we find the tiny representative of deltas, which form such an important feature on the coast line where large rivers enter seas or lakes. Along the lines of the stream we may observe here and there little benches, which are the equivalent in all save size of the terraces that are generally to be observed along the greater streams. In fact, these accidents of an acre help in a most effective way the student to understand the greater and more complicated processes of continental erosion.

A normal river—in fact, all the greater streams of the earth—originates in high country, generally in a region of mountains. Here, because of the elevation of the region, the streams have cut deep gorges or extensive valleys, all of which have slopes leading steeply downward to torrent beds. Down these inclined surfaces the particles worn off from the hard rock by frost and by chemical decay gradually work their way until they attain the bed of the stream. The agents which assist gravitation in bearing this detritus downward are many, but they all work together for the same end. The stroke of the raindrop accomplishes something, though but little; the direct washing action of the brooklets which form during times of heavy rain, but dry out at the close of the storm, do a good deal of the work; thawing and freezing of the water contained in the mass of detritus help the movement, for, although the thrust is in both directions, it is most effective downhill; the wedges of tree roots, which often penetrate between and under the stones, and there expand in their process of growth, likewise assist the downward motion. The result is that on ordinary mountain slopes the layer of fragments constituting the rude soil is often creeping at the rate of from some inches to some feet a year toward the torrent bed. If there be cliffs at the top of the slope, as is often the case, very extensive falls of rock may take place from it, the masses descending with such speed that they directly attain the stream. If the steeps be low and the rock divided into vertical joints, especially where there is a soft layer at the base of the steep, detached masses from the precipice may move slowly and steadfastly down the slope, so little disturbed in their journey that trees growing upon their summits may continue to develop for the thousands of years before the mass enters the stream bed.

Although the fall of rocks from precipices does not often take place in a conspicuously large way, all great mountain regions which have long been inhabited by man abound in traditions and histories of such accidents. Within a century or two there have been a dozen or more catastrophes of this nature in the inhabited valleys of the Alps. As these accidents are at once instructive and picturesque, it is well to note certain of them in some detail. At Yvorgne, a little parish on the north shore of the Rhône, just above the lake of Geneva, tradition tells that an ancient village of the name was overwhelmed by the fall of a great cliff. The vast débris forming the steep slope which was thus produced now bears famous vineyards, but the vintners fancy that they from time to time hear deep in the earth the ringing of the bells which belonged to the overwhelmed church. In 1806 the district of Goldau, just north of Lake Lucerne, was buried beneath the ruins of a peak which, resting upon a layer of clay, slipped away like a launching ship on the surface of the soft material. The débris overwhelmed a village and many detached houses, and partly filled a considerable lake. The wind produced by this vast rush of falling rock was so great that people were blown away by it; some, indeed, were killed in this singular manner.

The most interesting field of these Swiss mountain falls is a high mountain valley of amphitheatrical form, known as the Diablerets, or the devil's own district. This great circus, which lies at the height of about four thousand feet above the sea, is walled around on its northern side by a precipice, above which rest, or rather once rested, a number of mountain peaks of great bulk. The region has long been valued for the excellent pasturage which the head of the valley affords. Two costly roads, indeed, have been built into it to afford footpaths for the flocks and herds and their keepers in the summer season. Through this human experience with the valley, we have a record of what has gone on in this part of the mountain wilderness. Within the period of history and tradition, three very great mountain falls have occurred in this field, each having made its memory good by widespread disaster which it brought to the people of the chalets. The last of these was brought about by the fall of a great peak which spread itself out in a vast field of ruins in the valley below. The belt of destruction was about half a mile wide and three miles long. When the present writer last saw it, a quarter of a century ago, it was still a wilderness of great rocks, but here and there the process of their decay was giving a foothold for herbage, and in a few centuries the field will doubtless be so verdure-clad that its story will not be told on its face. It is likely, however, to be preserved in the memory of the people, and this through a singular and pathetic tradition which has grown up about the place, one which, if not true, comes at least among the legends which we should like to believe.

As told the present writer by a native of the district, it happened when, in the nighttime the mountain came down, the herdsmen and their cows gathered in the chalets—stout buildings which are prepared to resist avalanches of snow. In one of these, which was protected from crushing by the position of the stones which covered it, a solitary herdsman found himself alive in his unharmed dwelling. With him in the darkness were the cows, a store of food and water, and his provisions for the long summer season. With nothing but hope to animate him, he set to work burrowing upward among the rocks, storing the débris in the room of the chalet. He toiled for some months, but finally emerged to the light of day, blanched by his long imprisonment in the darkness, but with the strength to bear him to his home. In place of the expected warm welcome, the unhappy man found himself received as a ghost. He was exorcised by the priest and driven away to the distance. It was only when long afterward his path of escape was discovered that his history became known.

Returning to the account of the débris which descends at varied speed into the torrents, we find that when the detritus encounters the action of these vigorous streams it is rapidly ground to pieces while it is pushed down the steep channels to the lower country. Where the stones are of such size that the stream can urge them on, they move rapidly; at least in times when the torrent is raging. They beat over each other and against the firm-set rocks; the more they wear, the smaller they become, and the more readily they are urged forward. Where the masses are too large to be stirred by the violent current, they lie unmoved until the pounding of the rolling stones reduces them to the proportions where they may join the great procession. Ordinarily those who visit mountains behold their torrents only in their shrunken state, when the waters stir no stones, and fail even to bear a charge of mud, all detachable materials having been swept away when the streams course with more vigour. In storm seasons the conditions are quite otherwise; then the swollen torrents, their waters filled with clay and sand, bear with them great quantities of boulders, the collisions of which are audible above the muffled roar of the waters, attesting the very great energy of the action.

When the waste on a mountain slope lies at a steep angle, particularly where the accumulation is due to the action of ancient glaciers, it not infrequently happens that when the ground is softened with frost great masses of the material rush down the slope in the manner of landslides. The observer readily notes that in many mountain regions, as, for instance, in the White Mountains of New Hampshire, the steep slopes are often seamed by the paths of these great landslides. Their movement, indeed, is often begun by sliding snow, which gives an impulse to the rocks and earth which it encounters in its descent. At a place known as the Wylie Notch, in the White Mountains, in the early part of this century, a family of that name was buried beneath a mass of glacial waste which had hung on the mountain slope from the ancient days until a heavy rain, following on a period of thaw, impelled the mass down the slope. Although there have been few such catastrophes noted in this country, it is because our mountains have not been much dwelt in. As they become thickly inhabited as the Alps are, men are sure to suffer from these accidents.