Earth and Sky Every Child Should Know Part 5

The hard water, that comes through limestone rocks, adds lime in solution to the ocean water. All the sh.e.l.lfish of the sea, and the creatures with bony skeletons, take in the bone-building, sh.e.l.l-making lime with their food. Generations of these inhabitants of the sea have died, and their sh.e.l.ls and bones have acc.u.mulated and been transformed into thick beds of limestone on the ocean floor. This is going on to-day; but the limestone does not acc.u.mulate as rapidly as when the ocean teemed with sh.e.l.l-bearing creatures of gigantic size. Of these we shall speak in another chapter.

The fine dust that is blown into the ocean from the land, and that makes river water muddy, acc.u.mulates on the sea bottom as banks of mud, which by the burden of later deposits is converted into clay. Sandstone is but the compressed sand-bank.

In the study of mountains, geologists have discovered that old seash.o.r.es were thrown up into the first great ridges that form the backbone of a mountain system. The Rocky Mountains, and the Appalachian system on the east, were made out of thick strata of rocks that had been formed by acc.u.mulations of mud and sand--the washings of the land--on the opposite sh.o.r.es of a great mid-continental sea, that stretched from the crest of one great mountain system across to the other, and north and south from the Laurentian Hills to the Gulf of Mexico. The great weight of the acc.u.mulating layers of rock materials on one side, and the wasted land surfaces on the other, made the sea border a line of greatest weakness in the crust of the earth. The shrinking of the globe underneath caused the break; mashing and folding followed, throwing the ridge above sea-level, and making dry land out of rock waste which had been acc.u.mulating, perhaps for millions of years, under the sea. The wrinkling of the earth's crust was the result of crushing forces which produced tremendous heat.

Streams of lava sprang out through the fissures and poured streams of melted rock down the sides of the fold, quite burying, in many places, the layers of limestone, sandstone, and clay. Between the strata of water-formed rocks there were often created chimney-like openings, into which molten rock from below was forced, forming, when cool, veins and dikes of rock material, specimens of the substance of the earth's interior.

Tremendous pressure and heat, acting upon stratified rocks saturated with water transform them into very different kinds of rock. Limestone, subjected to these forces, is changed into marble. Clays are transformed into slates. Sandstone is changed into quartzite, the sand grains being melted so as to become no longer visible to the naked eye. The anthracite coal of the Pennsylvania mountains is the result of heat and pressure acting upon soft coal. a.s.sociated with these beds of hard coal are beds of black lead, or graphite, the substance used in making "lead"

pencils. We believe that the same forces that operated to transform clay rocks into slate, and limestone into marble, transformed soft coal into hard, and hard coal into graphite, in the days when the earth was young.

The word _sedimentary_ is applied to rocks which were originally laid down under water, as sediment, brought by running water, or by wind, or by the decay of organic substances. _Stratified_ rocks are those which are arranged in layers. Sedimentary rocks will fall into this cla.s.s.

_Aqueous_ rocks are those which are formed under water. Most of the stratified and sedimentary rocks, but not all, may be included under this term. Rocks that are made out of fragments of other rocks torn down by the agencies of erosion are called _fragmental_. Wind, water, and ice are the three great agencies that wear away the land, bring rock fragments long distances, and deposit them where aqueous rocks are being formed. Volcanic eruptions bring material from the earth's interior.

This material ranges all the way from huge boulders to the finest impalpable dust, called volcanic ashes. Rivers of ice called glaciers crowd against their banks, loosening rock ma.s.ses and carrying away fragments of all sizes, in their progress down the valley. Brooks and rivers carry the pebbles and the larger rock ma.s.ses they are able to loosen from their walls and beds, and grind them smooth as they move along toward lower levels.

The air itself causes rocks to crumble; percolating water robs them of their soluble salts, reducing even solid granite to a loose ma.s.s of quartz grains and clay. Plants and animals absorb as food the mineral substances of rocks, when they are dissolved in water. They transform these food elements into their own body substance, and finally give back their dead bodies, the mineral substances of which are freed by decay to return to the earth, and become elements of rock again.

The decay of rock is well shown by the materials that acc.u.mulate at the base of a cliff. Angular fragments of all sizes, but all more or less flattened, come from strata of shaly rock, that can be seen jutting out far above. A great deal of this sort of material is found mingled with the soil of the Northeastern States. Round pebbles in pudding-stone have been formed in brook beds and deposited on beaches where they have become caked in mud and finally consolidated into rock. If the beach chanced to be sandy instead of muddy, a matrix of sandy paste holds the larger pebbles in place. Limestone paste cements together the pebbles of limestone conglomerates.

In St. Augustine many of the houses are built of coquina rock, a ma.s.s of broken sh.e.l.ls which have become cemented together by lime mud, derived from their own decay. On the slopes of volcanoes, rock fragments of all kinds are cemented together by the flowing lava. So we see that there are pudding-stones of many kinds to be found. If some solvent acid is present in the water that percolates through these rocks it may soften the cement and thus free the pebbles, reducing the conglomerate again to a mere heap of sh.e.l.l fragments, or gravel, or rounded pebbles.

The story of rock formation tells how fire and water, and the two combined, have made, and made over, again and again, the substance of the earth's crust. Chemical and physical changes constantly tear down some portions of the earth to build up others. The constant, combined effort of wind and water is to level the earth and fill up the ocean bed. Rocks are constantly being formed; the changes that have been going on since the world began are still in progress. We can see them all about us on any and every day of our lives.

GETTING ACQUAINTED WITH A RIVER

I have two friends whose childhood was spent in a home on the banks of a n.o.ble eastern river. Their father taught the boy and the girl to row a boat, and later each learned the more difficult art of managing a canoe.

On holidays they enjoyed no pleasure so much as a picnic on the river-bank at some point that could be reached by rowing. As they grew older, longer trips were planned, and the river was explored as far as it was navigable by boat or canoe. Last summer when vacation came, these two carried out a long-cherished plan to find the beginning of the river--to follow it to its source. So they left home, and canoed up-stream, until the stream became a brook, so shallow they could go no farther. Then they followed it on foot--wading, climbing, making little detours, but never losing the little river. At last they came to the beginning of it--a tiny rivulet trickled out of the side of a hill, filling a wooden keg that formed a basin, where thirsty pa.s.sers-by could stoop and drink. They decided to mark the spring, so that people who found it later, and were refreshed by its clear water, might know that here was born the greatest river of a great state. But they were not the original discoverers. Above the spring, a board was nailed to a tree, saying that this is the headwater of the river with the beautiful Indian name, Susquehanna.

It was a dry summer, and the overflow of the basin was almost all drunk up by the thirsty ground. They could scarcely follow it, except by the groove cut by the rivulet in seasons when the flow was greater. They followed the runaway brook, through the gra.s.s roots, that almost hid it.

As the ground grew steeper, it hurried faster. Soon it gathered the water of other springs, which hurried toward it in small rivulets, because its level was lower. Water always seeks the lowest level it can find. Sometimes marshy spots were reached where water stood in the holes made by the feet of cattle that came there to drink. The water was muddy, and seemed to stand still. But it was settling steadily, and at one side the little river was found, flowing away with the water it drew from the swampy, springy ground. All the mud was gone, now; the water was clear. It flowed in a bed with a stony floor, and there were rough steps where the water fell down in little sheets, forming a waterfall, the first of many that make this river beautiful in the upper half of its course. To get from the high level of that hillside spring to the low level of the sea, the water has to make a fall of twenty-three hundred feet, but it makes the descent gradually. It could not climb over anything, but always found a way to get around the rocks and hills that stood in its way. When the flat marsh land interfered, the water poured in and overflowed the basin at the lowest margin.

In the rocky ground the two explorers found that the stream had widened its channel by entering a narrow crevice and wearing away its walls. The continual washing of the water wears away stone. Rocks are softened by being wet. Streaks of iron in the hardest granite will rust out and let the water in. Then the lime in rocks is easily dissolved. Every dead leaf the river carried along added an acid to the water, and this made easier the process of dissolving the limestone.

Every crumbling rock gives the river tools that it uses like hammer and chisel and sandpaper to smooth all the uneven surfaces in its bed, to move stumbling blocks, and to dig the bed deeper and wider. The steeper the slope is, the faster the stream flows, and the larger the rocks it can carry. Rocks loosened from the stream bed are rolled along by the current. Then bang! against the rocks that are not loose, and often they are able to break them loose. The fine sand is swept along, and its sharp points strike like steel needles, and do a great work in polishing roughness and loosening small particles from the stream bed. The bigger pebbles of the stream have banged against the rock walls, with the same effect, smoothing away unevenness and pounding fragments loose, rolling against one another, and getting their own rough corners worn away.

The makers of stone marbles learned their business from a brook. They cut the stone into cubical blocks, and throw them into troughs, into which is poured a stream of running water. The blocks are kept in motion, and the grinding makes each block help the rest to grind off the eight corners and the twelve ridges of each one. The water becomes muddy with the fine particles, just as the drip from a grindstone becomes unclean when an axe is ground. Pretty soon all the blocks in the trough are changed into globes--the marbles that children buy at the shops when marble season comes around.

I suppose if the troughs are not watched and emptied in time, the marbles would gradually be ground down to the size of peas, then to the size of small bird shot, and finally they would escape as muddy water and fine sand grains.

Sure it is that the sandy sh.o.r.es that line most rivers are the remnants of hard rocks that have been torn out and ground up by the action of the current.

Not very many miles from its first waterfall the stream had grown so large that my two friends knew that they would soon find their canoes.

The plan now was to float down the curious, winding river and to learn, if the river and the banks could tell them, just why the course was so crooked on the map. They came into a broad, level valley where streams met them, coming out of deep clefts between the hills they were leaving behind them. The banks were pebbly, but blackened with slimy mud that made the water murky. The current swerved from one side to the other, sometimes quite close to the bank, where the river turned and formed a deep bend. On this side the bank was steep, the roots of plants and trees exposed. On the opposite side a muddy bank sloped gently out into the stream. Here building up was going on, to offset the tearing down.

The sharp bends are made sharper, once the current is deflected from the middle of the stream to one side. At length the loops bend on each other and come so near together that the current breaks through, leaving a semicircular bayou of still water, and the river's course straightened at that place. It must have been in a spring flood that this cut-off was made, and, the break once made was easily widened, for the soil is fine mud which, when soaked, crumbles and dissolves into muddy water.

Stately and slow that river moves down to the bay, into which it empties its load. The rain that falls on hundreds of square miles of territory flows into the streams that feed this trunk. The little spring that is the headwater of the system is but one of many pockets in the hillsides that hold the water that soaks into the ground and give it out by slow degrees. Surface water after a rain flows quickly into the streams. It is the springs that hold back their supply and keep the rivers from running dry in hot weather.

Do they feel now that they know their river? Are they ready to leave it, and explore some other? Indeed, no. They are barely introduced to it.

All kinds of rivers are shown by the different parts of this one. It is a river of the mountains and of the lowland. It flows through woods and prairies, through rocky pa.s.ses and reedy flats. It races impetuously in its youth, and plods sedately in later life. The trees and the other plants that shadow this stream, and live by its bounty, are very different in the upland and in the lowland. The scenery along this stream shows endless variety. Up yonder all is wild. Down here great bridges span the flood, boats of all kinds carry on the commerce between two neighbour cities. A great park comes down to the river-bank on one side. Canoes are thick as they can paddle on late summer afternoons.

No one can ever really know a river well enough to feel that it is an old story. There is always something new it has to tell its friends. So my two explorers say, and they know far more about their friendly river than I do.

THE WAYS OF RIVERS

A ca.n.a.l is an artificial river, built to carry boats from one place to another. Its course is, as nearly as possible, a straight line between two points. A river, we all agree, is more beautiful than a ca.n.a.l, for it winds in graceful curves, in and out among the hills, its waters seeking the lowest level, always.

No artist could lay out curves more beautiful than the river forms.

These curves change from year to year, some slowly, some more rapidly.

It is not hard to understand just why these changes take place.

Some rivers are dangerous for boating at certain points. The current is strong, and there are eddies and whirlpools that have to be avoided, or the boat becomes unmanageable. People are drowned each season by trusting themselves to rivers the dangerous tricks of which they do not know. Deep holes are washed out of the bed of the stream by whirling eddies. The pot-holes of which people talk are deep, rounded cavities, ground out of the rocky stream-bed by the scouring of sand and loose stones driven by whirling eddies in shallow basins. Every year deepens each pot-hole until some change in the stream-bed shifts the eddy to another place.

No stream finds its channel ready-made; it makes its own, and constantly changes it. The current swings to one side of the channel, lifting the loose sediment and grinding deeper the bed of the stream. The water lags on the opposite side, and sediment falls to the bottom. So the building-up of one side is going on at the same time that the tearing-down process is being carried on on the other. With the lowering of the bed the river swerves toward one bank, and a hollow is worn by slow degrees. The current swings into this hollow, and in pa.s.sing out is thrown across the stream to the opposite bank. Here its force wears away another hollow; and so it zigzags down-stream. The deeper the hollows, the more curved becomes the course, if the general fall is but moderate.

It is toward the lower courses of the stream that the winding becomes more noticeable. The sediment that is carried is deposited at the point where the current is least strong, so that while the outcurves become sharper by the tearing away of the stream's bank, the incurves become sharper by the building up of this bank.

The Mississippi below Memphis is thrown into a wonderful series of curves by the erosion and the deposit caused by the current zigzagging back and forth from one bank to the other. Gradually the curves become loops. The river's current finally jumps across the meeting of the curves, and abandons the circular bend. It becomes a bayou or lagoon of still water, while the current flows on in the straightened channel.

All rivers that flow through flat, swampy land show these intricate winding channels and many lagoons that have once been curves of the river.

No one would ever mistake a river for a lake or any other body of water, yet rivers differ greatly in character. One tears its way along down its steep, rock-enc.u.mbered channel between walls that rise as vertical precipices on both sides. The roaming, angry waters are drawn into whirlpools in one place. They lie stagnant as if sulking in another, then leap boisterously over ledges of rock and are churned into creamy foam at the bottom. Outside the mountainous part of its course this same river flows broad and calm through a mud-banked channel, cut by tributary streams that draw in the water of low, sloping hills.

The Missouri is such a wild mountain stream at its headwaters. We who have seen its muddy waters from Sioux City to St. Louis would hardly believe that its impetuous and picturesque youth could merge into an old age so comfortable and placid and commonplace.

This thing is true of all rivers. They flow, gradually or suddenly, from higher to lower levels. To reach the lowest level as soon as possible is the end each river is striving toward. If it could, each river would cut its bed to this depth at the first stage of its course. Its tools are the rocks it carries, great and small. The force that uses these tools is the power of falling water, represented by the current of the stream.

The upper part of a river such as the Missouri or Mississippi engages in a campaign of widening and deepening its channel, and carrying away quant.i.ties of sediment. The lower reaches of the stream flow through more level country; the current is checked, and a vast burden of sediment is laid down. Instead of tearing away its banks and bottom, the river fills up gradually with mud. The current meanders between banks of sediment over a bottom which becomes shallower year by year. The Rocky Mountains are being carried to the Gulf of Mexico. The commerce of the river is impeded by mountain debris deposited as mud-banks along the river's lower course.

Many rivers are quiet and commonplace throughout their length. They flow between low, rounded hills, and are joined by quiet streams, that occupy the separating grooves between the hills. This is the oldest type of river. It has done its work. Rainfall and stream-flow have brought the level of the land nearly to the level of the stream. Very little more is left to be ground down and carried away. The landscape is beautiful, but it is no longer picturesque. Wind and water have smoothed away unevennesses. Trees and gra.s.s and other vegetation check erosion, and the river has little to do but to carry away the surface water that falls as rain.

But suppose our river, flowing gently between its gra.s.sy banks, should feel some mighty power lifting it up, with all its neighbour hills and valleys, to form a wrinkle in the still unstable crust of the earth.

Away off at the river's mouth the level may not have changed, or that region may have been depressed instead of elevated by the shrinking process. Suppose the great upheaval has not severed the upper from the lower courses of the stream. With tremendous force and speed, the current flows from the higher levels to the lower. The river in the highlands strikes hard to reach the level of its mouth. It grinds with all its might, and all its rocky tools, upon its bed. All the mud is scoured out, and then the underlying rocks are attacked. If these rocks are soft and easily worn away, the channel deepens rapidly. One after another the alternating layers are excavated, and the river flows in a canyon which deepens more and more. As the level is lowered, the current of the stream becomes slower and the cutting away of its bed less rapid.

The stream is content to flow gently, for it has almost reached the old level, on which it flowed before the valley became a ridge or table-land.

The rivers that flow in canyons have been thousands of years in carving out their channels, yet they are newer, geologically speaking, than the streams that drain the level prairie country. The earth has risen, and the canyons have been carved since the prairies became rolling, level ground.

[Ill.u.s.tration: This little pond is a basin hollowed by the same glacier that scattered the stones and rounded the hills]

[Ill.u.s.tration: Every stream is wearing away its banks, while trees and gra.s.s blades are holding on to the soil with all their roots]

The Colorado River flows through a canyon with walls that in places present sheer vertical faces a mile in depth, and so smooth that no trail can be found by which to reach from top to bottom. The region has but slight erosion by wind, and practically none by rain. The local rainfall is very slight. So the river is the one force that has acted to cut down the rocks, and its force is all expended in the narrow area of its own bed. Had frequent rains been the rule on the Colorado plateau, the angles of the mesas would have been rounded into hills of the familiar kind so constantly a part of the landscape in the eastern half of the continent.

The Colorado is an ancient river which has to carry away the store of moisture that comes from the Pacific Ocean and falls as snow on the high peaks of the Rocky Mountains. Similar river gorges with similar stories to tell are the Arkansas, the Platte, and the Yellowstone. All cut their channels unaided through regions of little rain.