Читать книгу Walks and Talks in the Geological Field - Alexander Winchell - Страница 21

When the road-side pool left by the last shower dries away a film of fine sediment remains. This once hung in suspension in the water; it was gathered up from the land by the eddies born of rain. We shall see that this simple observation is the key to an explanation of many of the grandest facts in the world’s history.

A few years ago, in ascending the valley of the Aar, in Switzerland, I enjoyed an extraordinary opportunity to observe the action of moving water. The Aar is a turbulent stream issuing from the foot of the Aar glacier of the Jura Mountains. It comes out of its ice-roofed cavern milky white with the clay sediment which results from the scouring of the rocks by the sliding glacier. The sharp collision of transported rock-fragments accompanies the loud roaring of the impetuous stream. On this occasion, the white streamlet, always rapid, had been swollen to a furious torrent by a recent cloud-burst. The torrent, in its rage, had rent all barriers, and coursed over the adjacent lands. Stones, up to several tons in weight, had been hurled right and left, as the autumn wind disperses the light leaves of the maples along the street. Hundreds of acres lay buried beneath sand and mud, cobble-stones and massive rocks. The rough and rocky slope had received its deposits; the late goat pasture lay concealed beneath a bed of stones, and the grassy flat was hidden by a blanket of gravel and slime.

Observe the power of assortment exerted by the moving water. The heavier rocks were left where the most precipitous hill-side graduated into the sharp slope. Here was the first abatement of the force of the stream. It dropped what could no longer be moved by the diminished power of the torrent. The smaller rocks lay next in order. Where the sharp slope passed into a gentler grade, the still waning force of the maddened stream became insufficient to bear them on. Still beyond, on the lower levels, the flood was widened, its velocity slackened, and its transportative power so abated that the average sized cobble-stones had to be left. Still went on the gravel, and found pause only on the pastures where domestic animals had been grazing. But the sand was borne to the level, and spread itself out over many an arable field and fragrant meadow; while the fine alluvial mud had floated with the tired waters, which sought out sheltered nooks and depressions in which to rest.

This was yesterday. This morning the lesson lay before me. Here were effects of a geological cause on whose action the startled peasant yesterday gazed despairingly. He needs no theory to convince him of the nature and mode of action of the forces which devastated his fields; and I, who found in Switzerland many windrows of gathered cobble-stones and pebbles, and sheets of assorted sands and mud, miles in extent, felt that it was scarcely a theoretical view to attribute these larger results of the same kind to a geologic agency of a similar nature, though it had acted unknown ages before human eyes had been created.

Not far from the home of my boyhood was the mill-pond, dear to every school-ward trudging urchin who had to pass it, and a Saturday resort for many others who lived in the adjoining “district.” Here we bathed; here we fished; here we risked our lives in shaky skiffs, and astride of unmanageable logs. The water was deep and clear. Last summer I visited the old pond. Like the anxious parents, who shared with mill-pond the affection of which boyish hearts are susceptible, the scene of so much truant enjoyment was changed almost beyond recognition. The deep, clear water was silted up, and flags were thrusting their brown noses up, in the sites where I used to swim in summer and skate in winter. Sedges fringed the borders; bulrushes, to their knees in water, were holding possession of land that was expected to be, and the encroaching marsh threatened to corner the anxious perches and sunfishes in the last lingering bowl of clear water close by the decrepit old dam. This, I thought, is a picture of the history of the world. How long, I queried, before this mill-pond will be a swamp? Is this the impending fate of all our ponds and lakelets? Johnny, do you think your favorite skating place will over come to this?

The first land-surveyors of the territory of Michigan laid down on their plats an extraordinary number of swamps and bogs. It is true they greatly overdid the swamp-land business; but swamps are there in plentiful abundance; and swamps properly drained and tilled are the richest lands in the state. But the early settlers of Michigan found many of the swamps non-existent; some were grassy plains; some were quaking bogs, and others were part marsh and part lakelet. During sixty years, many of the quaking bogs have become solid meadows; and many of the marsh-side lakelets have totally disappeared, under the encroachments of the growing marsh. These are geological changes, and the geologist’s eye looks about for the causes. It is not a far-fetched solution to see in the hillside wash a source of silt, which annually diminishes the depth of water to a certain extent. And it requires but ordinary sagacity to notice each decaying crop of grasses, sedges, and rushes as the source of the dark peaty deposit which displaces the last water, when other causes have produced the requisite shallowness. We have caught the marsh-making business in the midst of its accomplishment. Short as our lives are, each life falls within the geologic age in which vast results are actually working out. All these marshes have been lakes. If we dig in them we find the bleached relics of the very shells which held animated tenants of the vanished lakelet. Thus, gathering sediments add sheet after sheet to the deposits which are filling the larger as well as the smaller bodies of water which rest on the earth’s surface.

All great rivers are enormous mud-carriers. The Nile, the Amazons, the Ganges, the Hoang Ho, the Mississippi, are great vehicles for the transport of earthy substances from the higher to the lower levels. Like the Tiber, their waters are all “yellow.” The Chinese have surpassed all other nations in making a proper name of the generic description of muddy rivers. What a potion is a glass of Mississippi water, placed by the side of one’s plate in the cabin of the steamer! In thirty minutes it holds a deposit of impalpable sediment, which is simply mud. Think of the entire breadth and depth of this mighty stream charged with earthy materials to such an extent. What must be the total amount of matter carried down to the Gulf annually? The engineers of the United States have attempted to answer this question. They say that if the annual discharge of mud were brought together and dried, it would form a block a mile square and two hundred and seventy-eight feet high. Imagine that block lying on the surface of some level township. Then think another block on the top—the result of another year’s transport. Recall the fact that the Mississippi has been at this business at least five or six thousand years. Put five or six thousand such blocks together; the aggregate would be a mountain range.

There are seasons when the proud river climbs over its bounds—climbs over the artificial restraints which have been imposed in the form of levées. Water and mud spread over hundreds of plantations. Then, as in the overflowing torrent of the Aar, the slackened motion of the water allows the fine sediment to subside. Corn lands and cotton lands receive a new contribution of fertilizing material. Such service the Nile performs for Egyptian agriculture—under the rule of the Khedives, as during the reigns of the Pharaohs. Thus the deltas of the great rivers are formed. Still the great preponderance of river silt passes on to the outlets. Not only the floating sediment, but a large amount of bottom mud, too thick to float and too loose to lie unmoved. This the stream pushes along into the sea—year by year into deeper and deeper water, as the shallower shore region becomes silted up. This is the bar. By the annual extension of the bar, the delta gradually protrudes a tongue of land into the sea. Look at a map of the mouth of the Mississippi, or the Nile, or the Ganges. Often the piled up bar-material so obstructs the exit through the main channel, that the water sets back up the stream during some flood, overflows its banks, and seeks a new route to the sea. This may be many times repeated. So these great rivers acquire numerous outlets. Look at the map again. The bar at the mouth of the Mississippi extends three hundred and thirty-eight feet into the Gulf annually.

Much of the Mississippi sediment, therefore, lies somewhat permanently on the Gulf bottom, near the shore. Through this Engineer Eads has staked out a channel, to which the current of the Mississippi is confined after entering the Gulf, until deep water is reached. Its velocity is thus preserved, and its mud is carried beyond into the deeper basin. Before this improvement, the water spread out fan-like, and slackened its velocity to such an extent that the mud was deposited in a region where the water was already so shallow that navigation became seriously obstructed.

Still, some of the sediment floats on beyond the bar. There is a current in the Gulf which sets eastward along the northern border, and bears Mississippi sediment as far as the straits of Florida. The fine impalpable dust finally comes to rest on the bottom of the Gulf.

A thousand rivers thus are bringing their contributions to the sea. Around ten thousand miles of coast, the sea itself is battering down the land. The coarser fragments are left along the beach. The enfeebled action of the retreating surf bears some distance seaward the smaller fragments and the pebbles—rolled and rounded on the beach. The finest sediments have no opportunity to subside till floated far from shore. Thus the same assortment is exerted which we saw effected by the torrent of the Aar. The ocean’s bottom lies covered to a vast extent with sheets of sedimentary materials which, near the shore, are coarse, and remoter from shore are progressively finer, as far as the finest sediments are floated. This process goes forward before our eyes; it has been continued during all the thousands of ages past, since the ocean first came into existence. How many layers must there be? How many feet of sediments have been piled up? What conditions have they assumed while the geologic æons have rolled by?