Читать книгу Walks and Talks in the Geological Field - Alexander Winchell - Страница 23

Whence come the sediments which muddy the rivers and fill the lakelets, and make even the oceans shallower? The query must have occurred to you as we talked about the abundance of sediments; yet, simple as it is, comparatively few people have considered it. These sediments must all come from some source where they existed as solid, massive constituents of our planet. They are portions of the planet transported from one position to another. Their transportation changes the figure of the planet. Every film of sediment proclaims that the fashion of the planet has been worked over to some extent. The making of the planet has been merely a progressive changing of the fashion of the materials of which it is composed. If the completed planet as we see it is the product of geological forces, then the work of sedimentation proceeds by means of forces which are geological. The least mentionable portion of that work is performed by such force. The filling of boyhood’s mill-pond was a geological work. The slime settled by the roadside is a geological phenomenon. These are results accomplished; let us see how far we can trace them to their causes, and thus unfold a bit of the world’s history.

The sediments have been brought by moving waters; we must therefore trace the waters to their sources; we must retrace their course from the higher level. Obviously, the roadside slime has descended the rill-ways from the middle of the street; from the hill-slope down which a portion of the water descended. Some water flowed down the field-slope, moved under the fences, and found its course to the road-side pool, bringing as much sediment with it as it had power to bear. The corn-fields have been taxed; the earth built into the highway has been stolen; the form and bulk of the hill have been changed. So the farmer’s fields contributed the material which lies in the bottom of the mill-pond. To some extent, the fields have been scraped down and impoverished. There lies the farmer’s property spread over a surface which forms the floor of the sunfishes’ home.

Over every square mile flows some stream. The smallest stream, as well as the largest, occupies a valley; and down its slopes descend the sediment-laden drainage waters which seek the stream to join in its journey to the lower levels. Follow the streamlet. Along every rod of its course we find discharged during a rain the muddy washings of the land. The streamlet grows. Many a lateral rill brings in its contribution from the fields which stretch in another direction. Our streamlet flows on, and sooner or later it discharges its burden in some larger stream, which has already grown to its present volume by the contribution of a score of streamlets higher up the valley. All are merged together; but we are sure the water and the mud from our own village—our own farms—are there with the rest. The stream moves on—it never rests—and it grows as it moves. It courses across a State; it marks a boundary between States. Men have made it a vehicle for floating logs; a highway for skiffs and barges. Now, the more pompous stream styles itself a river. It hastens to join the Ohio and share in the dignity of floating steamboats and carrying on the commerce of a populous valley. The Ohio has even surpassed the tributary by which we have been led, in taking on its cargo of mud. We stand in the middle of the suspension bridge at Cincinnati and look down on the yellow surface of the great stream. There go the contributions from half a dozen States. There goes the soil filched from our garden, or torn from our new-made road, two hundred miles away. We know it is there.

Look on the map and notice how many rivers are bringing their sediments to the Ohio. Trace these tributaries to their sources. From how wide a territory is the mud gathered which thus rushes down with the main river? Notice that the Ohio carries its burden to the Mississippi. Look again upon the map and see how many other great rivers bring the mud from other far-off regions to concentrate it all in the mighty Father of Waters. Here float sediments from western New York, from West Virginia, from the Ozark Mountains, from the Cumberland Table Land, from Minnesota, and the Indian Territory. Here in this resistless tide floats the identical soil which was washed from Farmer Jones’s potato field.

In this view, consider the great Missouri. It pours its yellow stream into the clearer tide of the Mississippi a few miles above St. Louis. I have stood on the deck of a steamer between Alton and St. Louis and looked down on the Missouri’s turbid volume pushing far into the Mississippi, and retaining for miles a distinct boundary between the waters of the two rivers. It appears that the contributions from the far northwest exceed all those from the east. Follow the whirling tide of the Missouri upward toward its sources. There stand great cities on its alluvial banks. The crumbling bluffs by spells slide into the river. Above the limits of city populations the river is already gathering in the mud destined to journey to the Gulf of Mexico—mud which has already been floated from some remoter region and deposited here at times of overflow. Here comes the Niobrara, with slime from the prairies of Nebraska; the Cheyenne, with washings from the mining camps in the Black Hills; the Little Missouri and Yellowstone, with sands worn from the Big Horn, the Wind River, and the Snow Mountains; here, on a grassy plain, unite the Jefferson, Madison and Gallatin tributaries, which bring the dust of the continent from the high watershed of the Red Rock Mountains, which parts the continental drainage to opposite points of the compass. It is a bewildering breadth and complexity of operations. Over every foot of this wasting expanse the land is yielding to the corrosive action of rivers and rains and frosts. The proud mountain domes and pinnacles are coming down to acknowledge the supremacy of the powers of denudation. The Rocky Mountains have begun their journey to the Gulf of Mexico. Cubic miles of their granitic substance are buried in the delta of Louisiana and the bar of the Mississippi.

Now that we discover in action the forces which could transform the face of the land in some hundreds of thousands of years, we take a new view of the aspects of the terrestrial surface which had already been acquired when man came into existence. We discover that the face of the earth had already been transformed before we began our observations on it, and by means of agencies which corroded the rocks and carried away the materials precisely as the forces of nature are wasting the continent before our eyes. We have already recognized the fact that aqueous erosion cut through the Straits of Mackinac and chiseled down the steep sides of the monumental island in the Straits. We have seen the deep precipitous-walled gorge of the Niagara, and the rock-bluffs bounding on certain sides the basins of the great lakes—the works of rivers and waves.

Every river, in its search for a resting-place, has cut a way of even grade across the inequalities of the land, and the rubbish has been dumped somewhere—in alluvial border or broad delta, or seaward rolling bar. The Yampa has sawed a broad gash through the Uinta range on its way to the Green river. The Green has cut a dark chasm down through the plateaus of Colorado to the river whose colored waters, poured in from the snow-born floods of the Rocky Mountains, gave name to the river and the state. The Colorado, with augmented force, has dug a deeper and a wider cañon through the shattered terraces of the southern half of the state. The “Grand Cañon” sinks vertically six thousand feet through the rocks—a terrific gash, like a sabre-cut from some of the powers of Nature.

“It looks as if broken by bolts of thunder, Riven and driven by turbulent time.”

So a hundred rivers of the far west have scored the land. So the Cumberland, the Kentucky, the Hudson, the James, the Mississippi, by gentle worrying of the underlying rocks, have plowed out channels whose steep walls rise as high as the smoke from the steamer which utilizes the water-way. We have not seen these works begun; but we see them in progress; and we feel bound in reason to infer that the rivers have worked in the distant past as they are working before our eyes.

There are other erosions, however, which were effected not only before human times, but by agencies which have disappeared from existence. There are the Catskill Mountains—essentially a mere wall of horizontally laid slabs of red sandstone. We have not detected Nature anywhere raising such a wall. These mountains must be a remnant of a broad formation once stretching far east and west. The forces of erosion have worn away the formation on both sides, and the Catskills stand forth a feature of relief, as the statue emerges from the block of stone under the chisel of the sculptor. Such, too, is the Cumberland Table Land, high up-raised like a mountain, but yet not uplifted. It is a mere salience resulting from the vast erosions that have taken place along its western border. In central Tennessee, indeed, this erosive process has excavated a basin a hundred miles in diameter, bounded on all sides by the ragged edges of the formations which were left.

So this completed work of erosive powers which have retired from action is commemorated in many a monument-like outlier in Wisconsin and Minnesota. A great formation which once overspread many a township has all been carried away, save here and there an isolated remnant which lies like an island in the midst of geology of a different character. It is the Potsdam Sandstone which has been thus eroded; but wide areas still remain, and underlie portions of those states. Similar are the columns in Monument Park, and the ruins in the “Garden of the gods.” Like the great basin of central Tennessee are many of the excavations in the Bad Lands of the Upper Missouri and in New Mexico. We shall have other occasion to talk about these; for they are burial places of the brute populations which held possession of America before the advent of man.

These two great processes, erosion and sedimentation, must be vividly appreciated. The whole history of the visible land has consisted chiefly of up-building and destruction, rebuilding and disintegration, by the action of forces which have left gigantic monuments of their former power, and are even in our times, working on a scale large enough to illustrate to us how the foundations of the land were laid, and how the face of the earth has been carved into the fashion it presents to our interested eyes.

In another walk we must follow the sediments, under the sea, and try to learn what goes on in the mysterious abysses through which no highway has been opened.