A Curious Rock greatly revered by the Natives
This is the Dance Rock of the Walpi Indians of Arizona. Its curious shape is the result of weathering.
What goes on along the coast finds a parallel in the interior of continents where, as in Arizona, in America, or by the desert of Gobi, in Asia, or in the Karroo of South Africa, or in Central Australia and Africa, there is great dryness of climate and a continual disintegration of the surface rocks. Sometimes the dust or sand remains and gradually consolidates or hardens. More often it is only a temporary visitor. Wind and rain are continually removing it, sometimes in vast quantities, into the sea; and in the course of time the most astounding changes are wrought in the surface and appearance of the land. The softer rocks are worn down; the harder ones are left sticking out. Gradually the surface is carved out into heights and hollows. The harder rocks become the hills and ridges; the softer rocks are worn into valleys and plains. If there were no water left on the earth's surface a great deal of this process would still go on. In some respects it might become more violent, for owing to the absence of moisture the winds of the earth would always be laden with fine particles; and every one who has seen a "sand-blast" at work, or even the modified sand-blast which is sometimes used for cleaning the stonework of some of our cities, will appreciate what a tornado laden with sand grains might do in the way of destroying the surface of any rock on which it was playing. But, as a matter of fact, the action of water in carving the surface of the earth is the most important of all the factors we have at present to consider.
As rain falls from the clouds it absorbs the gases of the air, including oxygen and carbonic acid. Now both these are what we call corroding agents. If water is allowed to fall on a steel knife the knife rusts; but it has been shown by Dr. Gerald Moody, during the last few years, that if there were no acid gas present, the rusting would not take place. Oxygen and carbonic acid will rust other things beside metal; they will rust stone. Moreover, when the rain reaches the earth it absorbs any other acids of the soil which rotting vegetation may afford, and reinforced by these it goes on to attack the stones over which it flows. When it rolls along as a brook or a river it is no doubt attacking in this way the rocks and stones of its channel, though this action is not very strikingly shown. But sometimes the rusting or dissolving action of water is very evident. When it issues from a peat bog, for example, and is consequently highly charged with acid, it will make a very great impression on any limestones it may encounter; for as any schoolboy knows who has ever put a piece of chalk in vinegar, or in any of the stronger acids of the school laboratory, all the limestones are peculiarly susceptible to this form of chemical attack. Peat-water eats into limestone rapidly, while the limestone above the stream escapes, though it is a little (and much more slowly) dissolved by rain. Hence arise some curious features in the scenery of limestone districts. The walls of limestone above the water are not eaten away so fast as their base over which the water flows. Consequently they are undermined and are sometimes cut into tunnels and caverns and caves.
The rivers carry away the dissolved material. The carbonate of lime is taken to the sea; and this substance, of which sea shells, for example, are principally formed, is constantly supplied to the sea by the rivers that transport it from the land. The rivers of Western Europe have been known to convey one part of dissolved mineral matter in every 5000 parts of water, and of this mineral matter one half is carbonate of lime. The Rhine alone bears enough carbonate of lime to the sea every year to make 332,000,000,000 oyster shells of the usual size. The Thames conveys 180,000 tons of sulphate of lime past London every year. It has been computed that more than 8,000,000 tons of dissolved mineral matter are removed from the rocks of England and Wales in one year. That is equivalent to a general lowering of the surface of the country, by chemical solution alone, at the rate of one foot in 13,000 years. That is not much, it may seem; but in a million years, which is not a long period in geological time, half the present towns of England would be sunk under water by this cause alone.
CHAPTER IV
RECORDS LEFT BY RIVERS
When we come to examine more closely the work which rivers do in removing mineral substances from the land by washing particles of them from the surface, we find that the records they leave in geological history must be plainly marked. Every stream, large or small, is always busy carrying mud, sand, or gravel. Rivers are the "navvies" of geology. When they are swollen by rain they sweep large stones away with them. If we look at the bed of a mountain torrent we shall often see huge blocks of stone that have fallen from the cliffs on either side blocking the pathway of the stream. To all appearance the stream is quite powerless to remove these blocks, and has to circumnavigate them. But visit such a torrent when the snows are melting, or heavy rain has fallen, and you will hear the stones knocking against each other or on the rocky bottom as they are driven downwards by the flood. It is not easy to estimate the driving power of water. M. Gustave le Bon has furnished an illustration of its power which is very curious. In the south of France a stream is led downwards from the mountains to drive the turbine of some machinery at a manufactory. It comes down several thousand feet. In the manufactory there is a vent-hole, out of which the water can be allowed to shoot. The vent-hole is about an inch in diameter; and the water rushes out with such swiftness and force that the water-jet becomes as rigid as steel. It is impossible to cut through this water-jet; and if any one were to try to do so with a sword, the sword might break but it could never pierce or pass through those swiftly moving particles of water. A more commonplace illustration is the use that is sometimes made of water-jets to break up the surfaces of rock in quarries; nor must it be forgotten that horse-power of great value and extent for electric lighting and other purposes is always being drawn from waterfalls. Thus as a mechanical force merely the river can be immensely powerful; and must leave marks of its power on the rocks.
The aspect of its force with which we are, however, most concerned is that which is directed to lowering gradually the surface of the land. In the last chapter we showed how much mineral might be dissolved in the waters of rivers. If we are to include also the amount of mud, sand, and other things classed altogether as silt which a river carries down, the figures become much more imposing. Sir Archibald Geikie says that, taking the Mississippi as a typical river (it is as good an example as would be found, because in its great length it passes through many different kinds of land, soil, and climate), we may assume that the average amount of sediment carried down by a river is one part of sediment to every 1500 parts of water.
If now, says he, we assume that all over the world this is the amount carried down, we can see how seriously the level of the land is lowered by rivers. The Mississippi carries from the land it drains every year the 1/6000th part of a foot of rock. If we take the general height of the land of the whole globe to be 2100 feet, and suppose it to be continuously wasted at this rate, then the whole dry land would be carried into the sea in 12,600,000 years. Or if we assume the average height of the continent of Europe to be 940 feet, and to be lowered by its rivers at the same rate, then the last vestige of Europe would have disappeared in 8,640,000 years. Such figures are of course not exact; and it must always be remembered that the rivers are merely robbing Peter to pay Paul, and whatever they take away are always putting somewhere else, but we may learn from the foregoing considerations that the lowering of the land is much more rapid than is sometimes supposed. Another thing about the excavating work of rivers has to be remembered. The torrents carry sand, shingle, and rock with them, and these very materials act as agents of destruction on the beds of the water-courses. If we want to polish brass or steel we mix emery powder (or something finer or coarser) with the polishing liquid. The torrent or river uses sand or shingle as its polishing powder. It then wears out the rock over which it travels, and sometimes carves it into holes or caverns, gorges or ravines. Sometimes the process is varied, as when a stream finds its way over a hard rock which overlies a softer rock. If the arrangement is like that of a series of steps (there may be only one or two steps) it is possible for the river as it foams in a waterfall over the hard step at the top to eat its way into the lower softer step. The lower softer step will gradually disappear, and then the waterfall, still eating its way in, will begin to undermine the hard top step, and when that has gone on long enough the hard top step will fall down and the waterfall will have to begin a little farther up the stream. In this way a waterfall, gorge, or ravine can be constructed by a river.