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

by Nathaniel Southgate Shaler

The tendency of the débris-making influences along the coast line is to fill in the irregularities which normally exist there; to batter off the headlands, close up the bays and harbours, and generally to reduce the shores to straight lines. Where the tide has access to these inlets, it is constantly at work in dragging out the detritus which the waves make and thrust into the recesses. These two actions contend with each other, and determine the conditions of the coast line, whether they afford ports for commerce or are sealed in by sand bars, as are many coast lines which are not tide-swept, as that of northern Africa, which faces the Mediterranean, a nearly tideless sea. The same is the case with the fresh-water lakes; even the greater of them are often singularly destitute of shelters which can serve the use of ships, and this because there are no tides to keep the bays and harbours open.

The Ocean Currents.

The system of ocean currents, though it exhibits much complication in detail, is in the main and primarily dependent on the action of the constant air streams known as the trade winds. With the breath from the lips over a basin of water we can readily make an experiment which shows in a general way the method in which the winds operate in producing the circulation of the sea. Blowing upon the surface of the water in the basin, we find that even this slight impulse at once sets the upper part in motion, the movement being of two kinds—pulsating movements or waves are produced, and at the same time the friction of the air on the surface causes its upper part to slide over the under. With little floats we can shortly note that the stream which forms passes to the farther side of the vessel, there divides, and returns to the point of beginning, forming a double circle, or rather two ellipses, the longer sides of which are parallel with the line of the air current. Watching more closely, aiding the sight by the particles which float at various distances below the surface, we note the fact that the motion which was at first imparted to the surface gradually extends downward until it affects the water to the depth of some inches.

In the trade-wind belt the ocean waters to the depth of some hundreds of feet acquire a continuous movement in the direction in which they are impelled by those winds. This motion is most rapid at the surface and near the tropics. It diminishes downwardly in the water, and also toward the polar sides of the trade-wind districts. Thus the trades produce in the sea two broad, slow-moving, deep currents, flowing in the northern hemisphere toward the southwest, and in the southern hemisphere toward the northwest. Coming down upon each other obliquely, these broad streams meet about the middle of the tropical belt. Here, as before noted, the air of the trade winds leaves the surface and rises upward. The waters being retained on their level, form a current which moves toward the west. If the earth within the tropics were covered by a universal sea, the result of this movement would be the institution of a current which, flowing under the equator, would girdle the sphere.

With a girdling equatorial current, because of the intense heat of the tropics and the extreme cold of the parallels beyond the fortieth degree of latitude, the earth would be essentially uninhabitable to man, and hardly so to any forms of life. Its surface would be visited by fierce winds induced by the very great differences of temperature which would then prevail. Owing, however, to the barriers which the continents interpose to the motions of these windward-setting tropical currents, all the water which they bear, when it strikes the opposing shores, is diverted to the right and left, as was the stream in the experiment with the basin and the breath, the divided currents seeking ways toward high latitudes, conveying their store of heat to the circumpolar lands. So effective is this transfer of temperature that a very large part of the heat which enters the waters in the tropical region is taken out of that division of the earth's surface and distributed over the realms of sea and land which lie beyond the limits of the vertical sun. Thus the Gulf Stream, the northern branch of the Atlantic tropical current, by flowing into the North Atlantic, contributes to the temperature of the region within the Arctic Circle more heat than actually comes to that district by the direct influx from the sun.

The above statements as to the climatal effect of the ocean streams show us how important it is to obtain a sufficient conception as to the way in which these currents now move and what we can of their history during the geologic ages. This task can not yet be adequately done. The fields of the sea are yet too imperfectly explored to afford us all the facts required to make out the whole story. Only in the case of our Gulf Stream can we form a full conception as to the journey which the waters undergo and the consequence of their motion. In the case of this current, observations clearly show that it arises from the junction near the equatorial line of the broad stream created by the two trade-wind belts. Uniting at the equator, these produce a westerly setting current, having the width of some hundred miles and a depth of several hundred feet. Its velocity is somewhat greater than a mile an hour. The centre of the current, because of the greater strength of the northern as compared with the southern trades, is considerably south of the equator. When this great slow-moving stream comes against the coast of South America, it encounters the projecting shoulder of that land which terminates at Cape St. Roque. There it divides, as does a current on the bows of an anchored ship, a part—rather more than one half—of the stream turning to the northward, the remainder passing toward the southern pole; this northerly portion becomes what is afterward known as the Gulf Stream, the history of which we shall now briefly follow.

Flowing by the northwesterly coast of South America, the northern share of the tropical current, being pressed in against the land by the trade winds, is narrowed, and therefore acquires at once a swifter flow, the increased speed being due to conditions like those which add to the velocity of the water flowing through a hose when it comes to the constriction of the nozzle. Attaining the line of the southeastern or Lesser Antilles, often known as the Windward Islands, a part of this current slips through the interspaces between these isles and enters the Gulf of Mexico. Another portion, failing to find sufficient room through these passages, skirts the Antilles on their eastern and northern sides, passes by and among the Bahama Islands, there to rejoin the part of the stream which entered the Caribbean. This Caribbean portion of the tide spreads widely in that broad sea, is constricted again between Cuba and Yucatan, again expands in the Gulf of Mexico, and is finally poured forth through the Straits of Florida as a stream having the width of forty or fifty miles, a depth of a thousand feet or more, and a speed of from three to five miles an hour, exceeding in its rate of flow the average of the greatest rivers, and conveying more water than do all the land streams of the earth. In this part of its course the deep and swift stream from the Gulf of Mexico, afterward to be named the Gulf Stream, receives the contribution of slower moving and shallower currents which skirted the Antilles on their eastern verge. The conjoined waters then move northward, veering toward the east, at first as a swift river of the sea having a width of less than a hundred miles and of great depth; with each step toward the pole this stream widens, diminishing proportionately in depth; the speed of its current decreases as the original impetus is lost, and the baffling winds set its surface waters to and fro in an irregular way. Where it passes Cape Hatteras it has already lost a large share of its momentum and much of its heat, and is greatly widened.

Although the current of the Gulf Stream becomes more languid as we go northward, it for a very long time retains its distinction from the waters of the sea through which it flows. Sailing eastward from the mouth of the Chesapeake, the navigator can often observe the moment when he enters the waters of this current. This is notable not only in the temperature, but in the hue of the sea. North of that line the sharpness of the parting wall becomes less distinct, the stream spreads out broadly over the surface of the Atlantic, yet its thermometric effects are distinctly traceable to Iceland and Nova Zembla, and the tropical driftwood which it carries affords the principal timber supply of the inhabitants of the first-named isle. Attaining this circumpolar realm, and finally losing the impulse which bore it on, the water of the Gulf Stream partly returns to the southward in a relatively slight current which bears the fluid along the coast of Europe until it re-enters the system of tropical winds and the currents which they produce. A larger portion stagnates in the circumpolar region, in time slowly to return to the tropical district in a manner afterward to be described. Although the Gulf Stream in the region north of Cape Hatteras is so indistinct that its presence was not distinctly recognised until the facts were subjected to the keen eye of Benjamin Franklin, its effects in the way of climate are so great that we must attribute the fitness of northern Europe for the uses of civilized man to its action. But for the heat which this stream brings to the realm of the North Atlantic, Great Britain would be as sterile as Labrador, and the Scandinavian region, the cradle-land of our race, as uninhabitable as the bleakest parts of Siberia.

It is a noteworthy fact that when the equatorial current divides on the continents against which it flows, the separate streams, although they may follow the shores for a certain distance toward the poles, soon diverge from them, just as the Gulf Stream passes to the seaward from the eastern coast of the United States. The reason for this movement is readily found in the same principle which explains the oblique flow of the trades and counter trades in their passage to and from the equatorial belt. The particle of water under the equator, though it flows to the west, has, by virtue of the earth's rotation, an eastward-setting velocity of a thousand miles an hour. Starting toward the poles, the particle is ever coming into regions of the sea where the fluid has a less easterly movement, due to the earth's rotation on its axis. Consequently the journeying water by its momentum tends to move off in an easterly course. Attaining high latitudes and losing its momentum, it abides in the realm long enough to become cooled.

We have already noted the fact that only a portion of the waters sent northward in the Gulf Stream and the other currents which flow from the equator to the poles is returned by the surface flow which sets toward the equator along the eastern side of the basins. The largest share of the tide effects its return journey in other ways. Some portion of this remainder sets equatorward in local cold streams, such as that which pours forth through Davis Strait into Baffin Bay, flowing under the Gulf Stream waters for an unknown distance toward the tropics. There are several of these local as yet little known streams, which doubtless bring about a certain amount of circulation between the polar regions and the tropical districts. Their effect is, however, probably small as compared with that massive drift which we have now to note.