Part 6 (1/2)
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.