Conservation Reader Part 6

When it rains, water creeps into the tiny crevices. The water carries with it a little carbonic acid which the raindrops took from the air.

This substance aids in dissolving some of the rock materials. If the nights are very cold, the water in the crevices freezes and opens them a little wider, for ice, as you know, takes up a little more room than it did when it was water.

Plants also aid in breaking the rock. Often seeds are dropped by the wind, and the rootlets of some of these seeds, when they sprout, may find a crevice large enough and deep enough for them to push their way into the rock. In these crevices they find a little food and slowly grow larger and stronger. By and by some of the roots are strong enough to push apart large pieces of rock.

If the rock which we are studying is granite, we shall after a time be able to pick out the different minerals of which it is composed. We can tell the grains of quartz, because they look gla.s.sy and remain very hard. Other grains, which we call _feldspar_, soften and change into clay, which makes the water muddy as it runs over the rocks. We see also little scales of yellow mica, sometimes called "fool's gold," and a few grains of iron. There are tiny quant.i.ties of other things which we shall not be able to see, for the rainwater dissolves them and carries them away.

As the rock slowly crumbles to sand and clay, the bacteria begin to make their home in it. Hardy plants, that are not particular about what they grow in, get a foothold, and when they die their stems and leaves decay and mix with the rock particles until at last this material begins to look like soil. It has become dark in color and rich in plant food.

Then, many other plants that require a good soil take root there. The rock has at last completely disappeared under the layer of soil and its carpet of vegetation.

Suppose, now, that we dig down and find how deep the soil is and what lies below it. When we have gone down two feet the soil is harder and of a lighter color, for there are fewer plant remains in it. This poorer, lighter-colored soil we call _subsoil_. If we dig a little deeper, we shall find pieces of rock in the subsoil. Below these we come to soft, crumbling rock and last of all the solid rock.

The soil that is found resting on the rocks from which it was formed is known as _residual soil_. This name is given to such soil, because it is what remains after long years of rock decay during which the rains have washed away a part of the finer material.

What has become of the soft earth that the water washed away? The muddy rivulet has already told us its interesting story. We have learned that a part of this earth (or soil) is borne to the distant ocean. There it is forever lost unless the sea bottom should some day become dry land.

Stranger things than that have happened on this ancient earth of ours.

The part of the soil which the water carried away to form the rich valley lands and deltas is known as _alluvial soil_.

[Ill.u.s.tration: _U. S. Department of Agriculture_ A flood plain, where alluvial soil has been deposited by the river.]

Long ago the northern part of our country was covered with a sheet of ice. This ice crept slowly southward, and as it moved along it tore off all the soil and loose rocks on the surface of the earth over which it pa.s.sed. When it melted it left them spread roughly over the country.

Such material forms _glacial soil_. It is often deep but not very rich.

[Ill.u.s.tration: _U. S. Geological Survey_ Soil brought by a glacier and deposited as the ice melted.]

There is another kind of soil, formed by the wind. If you have ever been in a dust storm you have seen the fine, powdery substance that settles over everything and creeps into the smallest cracks. In some countries where there are strong winds and not much rain there is little vegetation on the surface to hold the soil. Year after year the winds pick up particles of the dusty soil, whirl them high in the air, and do not let them down again until they have been carried many miles. In some far-off land where the winds go down the dust particles settle again to the earth. After a long, long time, enough dust collects to form a thick layer of the richest soil. This is called aeolian soil, from the word _aeolus_, meaning the "wind."

There is one more kind of soil which we ought to know about; that is _peat soil_. It is found in marshy or swampy lowlands and is formed largely of plant remains. When lands with such soil are drained, they prove very rich.

CHAPTER ELEVEN

HOW VEGETATION HOLDS THE SOIL

[Ill.u.s.tration: _H. W. Fairbanks_ What the rivulets did to the hillside pastures where the gra.s.s was destroyed.]

A walk up the mountains on a rainy day is not a pleasant one. There are mud and water under our feet, and overhead are the dripping branches which, if touched, send down a shower of drops. But if we keep our eyes open we shall learn something which will be of great value to us. We shall learn how it is that Nature holds the soil on the slopes--the wonderful soil which it takes her so long a time to make and which is the source of all our wealth.

Our way up the mountains is by a winding road. We first pa.s.s the foothills upon which there are scattered oaks. The rain is steadily pouring down and rivulets loaded with mud are eating little gullies all over the slopes. Along the roadside, where they have united, the rivulets form a torrent which is making a deep ditch that threatens to render the road impa.s.sable.

These slopes were once covered with gra.s.s and the rivulets ran down them without doing any harm. But so many sheep were pastured here that the gra.s.s was killed. The roots, which once formed a thick protecting sod, are now decaying. How quickly the rivulets have taken advantage of the unprotected slopes!

The road leads still upward until it brings us to where there were once pine forests. The lumbermen cut off all the trees, and then fire came and burned the decaying vegetation which once lay spread over the ground. Now all that remains is bare earth and blackened stumps.

What are the raindrops doing here? They gather in rivulets just as they do on the once gra.s.sy hillside; but because there are so many roots still remaining in the ground they have not done much work. They are not loitering, however, and by and by, when the roots have rotted, they will seize their chance and begin tearing away the soil from the mountain side.

But this is not the end of the road. Farther up we come to the primeval forests, where the giant trees stand just as they did before men came.

Here we can see how the slopes are protected, for in making the road the workmen cut deep into the hillside. They first removed a layer of pine needles and decaying branches. Then they cut through a layer of soil about two feet thick which was completely filled with little roots of trees and bushes. Below this they came to the soft subsoil, which contained only a few roots, and at the bottom they reached the solid rock.

The layer of roots and soil at the top of the bank, you can see from the picture, now overhangs the road, because the raindrops which beat against the bank have washed away all that they could reach of the unprotected earth at the bottom. How plainly we can see the network of roots. What a hard task it must be for the water to get at the soil in which these roots are growing.

[Ill.u.s.tration: _H. W. Fairbanks_ The layer of roots holds the soil on the mountain side.]

We will now leave the road and, although it is still raining hard, we will walk a distance through the forest and see if there is anything more that we can learn. We are soon in the deep woods where, perhaps, no one has ever been before. Around us are trees of all ages and sizes, from little seedlings to great giants six feet through. Among them are the crumbling stumps of trees long dead. Their trunks lie on the ground, and many are so soft and rotten that we can kick them to pieces with our feet.

As we walk our feet never touch the real earth. It is always on the soft, yielding leaves and crumbling branches that we step. These leaves and branches form a thick layer completely hiding the soil. But the strangest thing is that, although the rain is still falling, we can discover no rivulets. What, then, becomes of the water? The soft, decaying vegetation on which we are walking and the rotting stumps and logs act like a great sponge. As long as this sponge can take up the falling drops, none have a chance to run away. If it rains a very long time and the sponge becomes saturated, the drops that creep away and finally unite in rivulets in the hollows do no harm to the soil, for they cannot get at it.

[Ill.u.s.tration: _H. W. Fairbanks_ The roots of the tree grip the soil like the fingers of a great hand.]

Long after the storm has pa.s.sed, the earth underneath the trees remains wet, while the ground out in the open has become dry. A part of the water held by the decaying vegetation evaporates. Another part creeps down through the earth to the crevices in the rocks and feeds the springs.

Let us now put aside our storm clothes and journey, in imagination, far away to where it seldom rains--to that land which we call the desert.

Here the bare rocks of the mountain slopes are burned brown by the hot sun. Here there is little soil and only a few little bushes that somehow manage to live. Why does not the soil gather over the rocks as it does in other places? The rocks are surely crumbling, for we can crush some of the pieces in our hands.

Once in a long time it rains in this desert. Then the drops descend furiously. The water gathers in rivulets and these turn to torrents which sweep down the slopes. They carry away the particles of sand and clay which would in time, if there were plant roots to hold them, turn to soil.

The winds also help keep the desert rocks bare and free of soil. Have you ever been in a dust storm or have you read of caravans caught in such storms in the Sahara Desert? The fierce wind picks up the particles of sand and clay from the bare earth and sweeps them along as it does the snow in winter, or it whirls them in clouds high in the air. The dust clouds are often so dense that they hide the sun and all landmarks by which the traveler can guide his way. But have any of us ever seen the winds pick up much dust from the green fields where the vegetation protects the surface?

[Ill.u.s.tration: _H. W. Fairbanks_ The vegetation prevents the wind from blowing the sand away, so that wherever the roots obtain a hold there a little mound is formed.]

If we turn now to a very wet country, such as that upon our northwest coast, where often nearly eight feet of rain falls in a year, we shall find the vegetation so dense that it hides both soil and rocks. Here water can do little in wearing away the soil, even upon the steepest slopes, while the wind cannot get a peep at the earth.

Does it not seem strange that where little rain falls the earth washes a great deal faster than where it rains very heavily? The reason is that the more it rains the more dense becomes the carpet of vegetation. If we wish to preserve the soil, we must preserve the natural growth on the hillsides.

CHAPTER TWELVE

WHAT HAPPENS WHERE THERE IS NO PROTECTING CARPET OF VEGETATION

Not all of the muddy streams are due to the carelessness of men. It is the business of some of the servants of Nature, as we have already learned, to tear down the mountains and fill up the hollows in the earth. It is the business of others to spread a carpet of vegetation over the surface, and wherever they have already succeeded in their work the waters run clear most of the time.

Where it is dry so much of the time that few plants can live, the destructive servants have their own way when the occasional rains come.

Where there is a warm sun and frequent rains, a green carpet is spread over all the slopes. But when men destroy the carpet and take no care of the soil underneath, the raindrops are able to do as much damage as they do during the cloudbursts in the deserts.

The Colorado is one of those rivers in the basin of which few people live. Much of its journey is through a land in which there is little vegetation. Here, the waters from the melting snows upon the lofty mountains about the basin and those of the occasional heavy rains have things their own way. They are always yellow with mud. The amount of mud which this river carries has been measured. You will hardly believe me when I tell you that it amounts to sixty-one million tons every year.

This is enough to cover 164 square miles one foot deep. We might call this the cream of the soil from all the slopes of the great basin of the Colorado River.

In other parts of our land, where abundant rains fall, the streams tell a different story. Before men came the water of these streams was clear throughout the greater part of the year. It was only when the rains were very heavy that the soil washed away, for the vegetation held it well.

Now the gullies on the hillsides and along the roads tell us as plainly as though they could speak that our country is losing wealth here.

[Ill.u.s.tration: _H. W. Fairbanks_ The roots of the tree form a wonderful network underground from which the water cannot tear the soil.]

The soil is our most valuable possession. The people of many lands are suffering from poverty today because their forefathers did not take care of the soil as they should. In such lands the people who live on the mountain sides are poor, because the best of their soil has been washed away. Those who live in the valleys are often poor because of the sands and gravels which floods have spread over their fertile fields.