The mighty deep

BETWEEN Land and Ocean a ceaseless contest is carried on.

The land is always endeavouring to keep its present state unchanged. The ocean is ever trying to tear down and demolish the land. Yet the very object of the ocean in so doing seems to be that new land may be raised up with the stolen materials.

There are Continents and Islands, moulded long ago, which we see as finished structures, that have reached the meridian of their existence and entered upon their decadence. With many of these Ocean’s efforts are chiefly employed for destruction. Indeed, in all cases, the power which once erected an island or a continent is at work upon it still, not always and only for destruction, but sometimes for modification, altering its shape, making it larger or smaller, rounding a curve here, adding a protuberance there, giving finishing touches, and in the end, if not from the beginning, acting also as spoliator.

But the work of building has not ceased; it never does cease. Through the ages it has gone on, and it goes on yet. There are embryo islands, perhaps embryo continents, being now slowly fashioned in Ocean’s workshops, not ready for use.

A great part of the work of land-building takes place in darkness. It is carried on, not merely from day to day, but from century to century, never hasting, never pausing. The materials are brought often from far distances, not by men, but by streams, by rivers, by glaciers, by ocean-waters; and the weight of those waters helps to weld the gathered materials into a solid whole.

Anybody who watches the results of even one heavy downpour of rain on land may gain some notion of what is going on everywhere.

If your house has a soft-water tank, you can hardly have failed to note how black the water becomes, after a pelting shower, following upon a drought.

No wonder, for the rain has washed the roof, and has carried off a supply of collected dust and soot. Not the roofs of houses alone, but the streets, the walls, the pavements, in a town—the trees, hedges, grass, roads, in the country—are all relieved of dusty layers, while, if the rain be heavy enough, sand and soil also are swept away.

Sometimes they are only taken to a lower level in the same neighbourhood, but large quantities find their way into the drains or streamlets, which join bigger brooks, and thus reach the nearest river, finally joining the sea.

Any moving stream is able to carry along floating materials, which in still water would sink to the bottom. The faster the current the heavier is the weight which it can support. A mountain torrent conveys pebbles, grinding them to a smaller size as it rushes along; and some of the more powerful torrents transport even great stones.

If one is sceptical about this lifting power of water, and if one cannot go to the mountains to see for oneself, the next best thing is to watch the breaking of waves upon a shingly beach. The pebbles in rough weather are caught up and whirled about like grains of sand; and after a severe gale the whole look of a beach is often completely changed. The shingle may have been piled to an unusual height, or it may have been borne away. So it is easy to recognise that water has no mean carrying strength.

In the last chapter we saw what huge gifts of water are handed over, year by year, from the Land to the Ocean. But the Rivers, acting as handmaidens to carry these offerings, do not bestow water only. The water-gifts are laden with solid materials, to be used for building purposes.

A river tears earth and sand from its own banks, and wears down the stones and rocks in its bed; and to this growing collection, as it flows, it adds contributions of earth and sand, brought from higher reaches by its tributary streams. Most of those materials are kept afloat.

To make sure that they are so, we only have to examine the mouth of a river. Floating mud and sand may not be apparent in the river itself; but just where the river joins the sea, just where the outflowing stream encounters the inflowing ocean-tides, a bank or banks of mud and sand may be seen. These are a common feature of river-mouths.

Many a larger river has at its mouth an enormous network of banks, divided by streams, and the whole is described as the “delta” of that river; its shape being roughly like the Greek letter “delta,” or like an opened fan. Everybody has heard of the Delta of the Nile, the Delta of the Ganges, the Delta of the Mississippi. But hardly a river exists, no matter how small, which on joining the sea does not make its own little delta.

And the formation of these deltas comes about in a very simple manner.

So long as a river flows onward, at a fair pace, it is able to hold up its floating materials, to keep the earth and sand in its embrace from sinking to the bottom. But when it reaches the sea, its progress is checked; and no sooner does it slacken speed, than some of its buoyed-up materials begin to sink. The heavier kinds are the first to touch bottom, getting piled up so as to form a bar or bank; and farther out, as the stream grows yet more slow, lighter substances find their way downward, making another bank.

In fact, the river, with Ocean’s help, is busily building Land, forming an island or a group of islands, either of pebbles or sand or mud. In the case of a great river such islands are often very extensive.

The flow of a small river is quickly stopped by the sea, but such powerful streams as the Ganges or the Nile pour onward for many many miles, before they begin to mingle with the salt water, their speed slackening gradually. Not only do they form great deltas of islands, but they carry vast supplies of material far into the ocean.

Recently an examination was made into the waters of nineteen important rivers, to discover what quantity of material was carried by each. The result of this examination was somewhat startling.

We have seen already how many cubic miles of water a river may give over yearly to the great deep. It was found that, on an average, these rivers may be reckoned to give over also, with each cubic mile of water, more than seven hundred and sixty thousand tons of material, torn from the land. These enormous collections, together with vast quantities which the waves have dug and wrenched from beaches and cliffs, are dropped to Ocean’s floor.

Not indeed generally to her deepest parts. So far as we yet know, the Ocean does not build continents in those black profound abysses, miles deep, flat-floored, calm, unchangeable. No traces have been there discovered of such strata as form the Continental parts of Earth’s Crust. Ocean’s main work, as builder, goes on in “The Transitional Area;” and most so in that part of it which is called “The Continental Shelf,” where the depth does not exceed about six hundred feet.

“Transitional” regions are so named because it is believed that some of them may once have been Continents, that others of them may yet become Continents. There was a time, in the long past, when ocean-waters flowed over all those parts of Earth which now are known to us as dry land. There may be a time, in the far future, when areas of comparatively shallow water will become dry land.

Through ages the task of taking materials from the land for building purposes has been carried on by the Ocean, and it goes on still. The Continents are being steadily worn away; the Islands are becoming continually smaller.

Were this to continue always, with no counteracting forces, islands and continents must in the end disappear. It would take a very long time; probably well over six millions of years, at the present rate of demolition. Still, however long deferred, the end would be sure.

But counteracting forces exist.