Читать книгу Walks and Talks in the Geological Field - Alexander Winchell - Страница 15

Where goes the rain which falls upon the earth? If the surface were completely level, and all the water should stand which comes from the clouds in the form of rain and snow, it would be everywhere about forty inches deep. In some parts of the United States it would be more, and in others less than this. Such an amount of water would be 34,480 barrels on every acre. What becomes of all that water?

Part of it runs off, you say; and part of it soaks in the ground. True, and part of it evaporates, and is afterward condensed and rained down again. Also, part of that which soaks in the ground returns when the surface becomes dry, and is evaporated. But, not to be too precise, let us attempt to follow the water which soaks in. First of all, it must have dissolved some substances with which it came in contact at the surface. These substances must be, to a limited extent certain mineral constituents of the Drift; but the Drift has been so many thousand years exposed to rains, that all its readily soluble constituents have been dissolved away from the surface. The chief agencies which supply soluble matters to the surface are man and animals. The underground waters, therefore, carry with them a certain amount of solutions of organic and inorganic origin, and are not absolutely pure, like carefully distilled water. They may even be poisonous and unsanitary.

Following these waters in thought, beneath the surface, we see them percolating through the sands and gravels, which we have found to make up the principal part of the upper Drift. Through layer after layer they continue to descend. If any obstruction is encountered, they are quickly deflected around it, and so continue to settle toward the impervious Bowlder Clay at the bottom of the Drift; or, if that is absent, the waters settle to the bed-rock. We will not attempt, at present, to follow them in the rocks.

Now, we know that the Drift contains sheets of impervious clay. Of course, then, these intercept the descending water. The water arrested by a clay-bed saturates the overlying sand, and gradually flows along the surface of the clay to a lower level. But we have seen that all these Drift beds are of quite limited extent. The water, therefore, soon reaches the edge of the clay-bed and escapes down to a lower level. Probably it is again intercepted by a deeper clay-bed. Along this it flows in a similar way, and so continues—always approaching nearer and nearer to the lower limit of the Drift. Some of the clay-beds are concave upward, and thus form real dishes or cisterns, which remain full.

Suppose we dig a well. While passing through the sandy strata, from which the water drains away, no supply will be struck. As soon, however, as we reach one of the subterranean basins or cisterns, a supply is found. Should we dig a hole through the bottom of the cistern, we would, of course, lose much if not all of the water. But we might continue down to the next water-basin.

Let us suppose another well is needed, a few rods away. We must not be too sanguine in the expectation of getting water at the same depth. Perhaps the new well is beyond the limits of the higher water-basin; we must then dig to some lower one. Perhaps the new well is on higher ground; it does not follow that we must dig to the level of the basin in the first well. In the higher ground may be a higher water-basin; and so the second well, though several feet higher than the first, may not require to be so deep.

Do not suppose these water-beds are everywhere of such limited extent. There are districts where the same bed may be traced one or two miles. The bed, in such cases, is nearly horizontal; and that condition of the underground structure is indicated by a level condition of the surface.

Now, how are springs produced? Suppose a river valley has cut through a deep mass of the Drift, must it not cut the water-bearing sheets with the rest? And when that is done, will not the water flow out? Certainly, just as when we knock a hole in a cistern. So a hill-side spring is nothing but a leak in one of nature’s cisterns. The water in escaping from the cut edge of the sheet finds some spot where least resistance is experienced, and there it escapes in largest quantity. It forms a sort of stream, and by degrees wears a little channel, which extends back into the bank, opening at its mouth in a little arch under which the water finally escapes. Of course, all the work was accomplished before we ever saw the spring. A well is an artificial spring.

Generally, the water of a hill-side spring is allowed to flow off to a brook or rivulet. In the course of a number of miles, scores or hundreds of springs may discharge their contributions into the stream. In fact, the greater part of the water in the stream is supplied by springs. It gets directly from rains only so much as flows from the surface of the basin which the river drains. Most of the rain falling within the basin, however, sinks into the ground, and finds its way into the stream only in the form of spring water. But when a stream flows over a drift-formed bed, much water wastes away. Besides this, many deep water-basins convey their contents under the river. So the river never contains the whole amount of water which falls within the basin which it drains.

Suppose all the water-basins under a township or a county should cease to exist, what would become of wells and springs? You understand at once that they would dry up. Therefore the streams would dry up. The water would settle to the Bowlder Clay or the bed-rock, and there would be the only accumulation. Every well must then be sunk to that depth—even if it were two hundred feet. And wells would be the only resort, for of springs there would be none; of brooks there would be none; of ponds and lakelets there would be none. Then, again, the Drift sands being so dry, little evaporation would take place from the earth’s surface. The air would be dry; no dew would condense; no clouds would form, and so the rains would stop descending, unless some other region could supply us with clouds. How beneficent, then, are the clay-beds! Literally, they are all which saves many a fertile region from becoming a desert and an uninhabitable waste. We looked carelessly at these courses of sands and clays exposed in the railroad cut, and thought, perhaps, they only served to form a pile of earth for the railroad builder to cart into the neighboring filling. How admirably the constitution of the Drift is suited to human wants! To us it looks as if it had been an intentional preparation for man. There are persons, however, who prefer to say it is not so; but man is here only because the situation is one which permits him to be here. But we are sure, at least, that a happy coördination exists between our necessities and our surroundings; and the constitution of things which brings enjoyment out of the coördination is a beneficient constitution.

In regions of deep Drift and abundant water-basins, the supplies of spring-water are sometimes sufficient to meet the demands of towns and cities. The city of Ann Arbor, with its ten thousand of population, is thus supplied with nearly five hundred thousand gallons daily. This is obtained from two groups of springs, and distributed through the city in the usual way. Five times this amount could be had, if needed.

Now let us consider springs in another light. We have already reflected that the percolating water takes some substances in solution from the surface. It must take up much more in leaching through the sands. This is the reason why most sands are composed chiefly of insoluble constituents. Their soluble constituents have been leached out. But there remain still, in many regions, some soluble limestone pebbles or larger masses which have not yet been dissolved, and the water is continually diminishing the amount of these. Now, first of all, consider that this little fact is a demonstration that the present order of things has not existed from eternity. If it had, all the soluble substances would have disappeared long ago—in fact, an eternity since. The time, therefore, since this Drift was brought here is only a finite number of years. That is a positive datum.

Water that has dissolved much limestone is “hard.” Hence, many spring and river waters are hard. The water of a pond may be softer, because a large proportion of it has been directly rained in, or supplied by surface drainage from the surrounding land. Of course, the hardness of underground water depends upon the amount of limestone pebbles and grains with which it has come in contact. Aside from any supply of limestone from neighboring ledges, the amount of limestone in the Drift depends on the amount transported from the northern regions which supplied the material of the Drift at each particular place. Some northern regions supplied much limestone, and others little. Hence, in Michigan, Ohio, and Indiana, well and spring waters are hard, while in New England and Pennsylvania they are comparatively soft.

Under-ground water often experiences great pressure. In this state it dissolves more limestone than it can hold in solution after the pressure is relieved. Hence many springs throw down a calcareous deposit which in the open air hardens into tufa and travertine. It often incrusts mosses and forms what is called “petrified moss.” The vegetable matter perishes and disappears by degrees, but the form of the moss remains. Calcareous springs flowing into ponds cause a deposit of chalky matter on the bottom, which is called marl. It is generally mixed with earthy substances washed in from the surrounding slopes.

In precisely the same way certain springs deposit peroxide of iron, which is yellowish or red. Iron is also often transported to bogs and ponds, and there undergoes deposition. Thus bog iron ore is formed.

Now, I wish to ask Johnnie if he understands what has been said in this Talk. I hope he will think these matters over. They will help him to understand, by and by, some matters which are far more interesting. At least, I think he will find them so.