Abridged from Science Primer on Physical Science, by Prof. Geikie.

III.—RIVERS AND GLACIERS.

We have found that the water of the river is largely derived from springs, and that all spring-water contains more or less mineral materials dissolved out of the brooks. Every river, therefore, is carrying not merely water, but large quantities of mineral matters into the sea. It has been calculated, for instance, that the Rhine in one year carries into the North Sea lime enough to make three hundred and thirty-two thousand millions of oyster shells. This chemically-dissolved material is not visible to the eye, and in no way affects the color of the water. At all times of the year, as long as the water flows, this invisible transport of some of the materials of rocks must be going on.

But let us now again watch the same river in flood. The water is no longer clear, but dull and dirty. You ascertained that this discoloration arises from mud and sand suspended in the water. You see that over and above the mineral matter in chemical solution, the river is hurrying seaward with vast quantities of other and visible materials. And thus it is clear that at least one great part of the work of rivers must be to transport the mouldered parts of the land which are carried into them by springs or by rain.

But the rivers, too, help in the general destruction of the surface of the land. Of this you may readily be assured, by looking at the sides or bed of a stream when the water is low. Where the stream flows over hard rock, you find the rock all smoothed and ground away; and the stones lying in the water-course are all more or less rounded and smoothed. When these stones were originally broken by frosts or otherwise, from crags and cliffs, they were sharp-edged, as you can prove by looking at the heaps of blocks lying at the foot of any precipice, or steep bank of rock. But when they fell, or were washed into the river, they began to get rolled and rubbed, until their sharp edges were ground away, and they came to wear the smooth rounded forms which we see in the ordinary gravel.

While the stones are ground down, they, at the same time, grind down the rocks which form the sides and bottom of the river-channel over which they are driven. You can even see in some of the eddies of the stream how the stones are kept moving round until they actually excavate deep round cavities, called pot-holes, in the solid rock.

Now, it is clear that two results must follow from this ceaseless wear and tear of rocks and stones in the channel of a stream. In the first place, a great deal of mud and sand must be produced; and, in the second place, the bed of the river must be ground down so as to become deeper and wider. The sand and mud are added to the other similar material washed into the streams by rain from the mouldering surface of the land. By the deepening and widening of the water-courses, such picturesque features as gorges and ravines are excavated out of the solid rock.

Look, again, at the channel of a river in summer. You see it covered with sheets of gravel in one place, beds of sand in another, while here and there a piece of hard rock sticks up through these different kinds of river-stuff. Note some portion of the loose materials, and you find it to be continually shifting. A patch of gravel or sand may remain for a time, but the little stones and grains of which it is made up are always changing as the water covers and moves them. In fact, the loose materials over which the river flows are somewhat like the river itself. You come back to its banks after many years, and you find the river there still, with the same ripples, and eddies, and gentle murmuring sound. But though the river has been there constantly all the time, its water has been changing every minute, as you can watch it changing still. So, although the channel is always more or less covered with loose materials, these are not always the same. They are perpetually being pushed onward, and others, from higher up the stream, come behind to take their place.

It is not in the bottoms of the rivers, then, that the material worn away from the surface of the land can find any lasting rest. And yet the rivers do get rid of a good deal of this material as they roll along. You have, perhaps, noticed that a river is often bordered with a strip of flat plain, the surface of which is only a few feet above the level of the water. Most of our rivers have such margins, and, indeed, seem each to wind to and fro through a long, level, meadow-like plain. Now this plain is really made up from the finer particles of decomposed rocks which the river has carried along. During floods, the river, swollen and muddy, rises above its banks, and spreads over the low ground on either side. Whenever this takes place, the overflowing water moves more slowly over the flats; and, as its current is thus checked, it can not hold so much mud and sand, but allows some of these materials to settle down to the bottom. In this way the overflowed tracts get a coating of soil laid over them by the river, and when the waters retire this coating adds a little to the height of the plain. The same thing takes place year after year, until by degrees the plain gets so far raised that the river, which all this while is also busy deepening its channel, can not overflow it even at the highest floods. In course of time the river, as it winds from side to side, cuts away slices of the plain and forms a newer one at a lower level. And thus a series of terraces is gradually made, rising step by step above the river.

Still the laying down of its sand and mud by a river to form one or more such river-terraces is, after all, only a temporary disposal of these materials. They are still liable to be carried away, and in truth they are carried off continually as the river eats away its banks.

When the current of a river is checked as it enters the sea or a lake, the feebler flow of the water allows the sand and mud to sink to the bottom. By degrees some portions of the bottom come in this way to be filled up to the surface of the river, and wide flat marshy spaces are formed on either side of the main stream. During floods these spaces are overflowed with muddy water, in the same way as in the case of the valley plains just described, and a coating of mud or sand is laid down on them until they slowly rise above the ordinary level of the river, which winds about among them in endless branching streams. Vegetation springs up on these flat swampy lands; animals, too, find food and shelter there; and thus a new territory is made by the work of the river.

These flat river-formed tracts are called deltas, because the one which was best known to the ancients, that of the Nile, had the shape of the Greek letter ? (delta). This is the general form which is taken by accumulations at the mouths of rivers; the flat delta gets narrow toward the inland, and broader toward the sea. Some of them are of enormous size; the delta of the Mississippi, for example.

Each delta, then, is made of materials worn from the surface of the land, and brought down by the river. And yet vast though some of these deltas are, they do not show all the materials which have been so worn away. A great deal is carried far out and deposited on the sea-bottom; for the sea is the great basin into which the spoils of the land are continually borne.

Having now followed the course taken by the water which falls on the land as rain, we come to that taken by snow.

On the tops of some of the highest mountains in Britain snow lies for great part of the year. On some of them, indeed, there are shady clefts wherein you may meet with deep snow-wreaths even in the heat of summer.

But in other parts of Europe, where the mountains are more lofty, the peaks and higher shoulders of the hills gleam white all the year with unmelted snow.

Let us see why it is that perpetual snow should occur in such regions, and what part this snow plays in the general machinery of the world.

You have learned that the higher parts of the atmosphere are extremely cold. You know also that in the far north and the far south, around those two opposite parts of the earth’s surface called the Poles, the climate is extremely cold—so cold as to give rise to dreary expanses of ice and snow, where sea and land are frozen, and where the heat of summer is not enough to thaw all the ice and drive away all the snow. Between these two polar tracts of cold, wherever mountains are lofty enough to get into the high parts of the atmosphere where the temperature is usually below the freezing-point, the vapor condensed from the air falls upon them, not as rain, but as snow. Their heads and upper heights are thus covered with perpetual snow. In such high mountainous regions the heat of the summer always melts the snow from the lower hills, though it leaves the higher parts still covered. From year to year it is noticed that there is a line or limit below which the ground gets freed of its snow, and above which the snow remains. This limit is called the snow-line, or the limit of perpetual snow. Its height varies in different parts of the world. It is highest in the warmer regions on either side of the equator, where it reaches to 15,000 feet above the sea. In the cold polar tracts, on the other hand, it approaches the sea-level. In other words, while in the polar tracts the climate is so cold that perpetual snow is found even close to the sea-level, the equatorial regions are so warm that you must climb many thousand feet before you can reach the cold layers of the air where snow can remain all the year.

There is, you see, one striking difference between rain and snow. If rain had been falling for the same length of time, the roads and fields would still have been visible, for each drop of rain, instead of remaining where it fell, would either have sunk into the soil, or have flowed off into the nearest brook. But each snowflake, on the contrary, lies where it falls, unless it happens to be caught up and driven on by the wind to some other spot where it can finally rest. Rain disappears from the ground as soon as it can; snow stays still as long as it can.

You will see at once that this marked difference of behavior must give rise to some equally strong differences in the further procedure of these two kinds of moisture. You have followed the progress of the rain; now let us try to find out what becomes of the snow.

In such a country as ours, where there is no perpetual snow, you can without much difficulty answer this question. Each fall of snow in winter-time remains on the ground as long as the air is not warm enough to melt it. Evaporation, indeed, goes on from the surface of snow and ice, as well as from water: so that a layer of snow would in the end disappear, by being absorbed into the air as vapor, even though none of it had previously been melted into running water. But it is by what we call a thaw that our snow is chiefly dissipated; that is, a rise in the temperature, and a consequent melting of the snow. When the snow melts, it sinks into the soil and flows off into brooks in the same way as rain.

In the regions of perpetual snow the heat of summer can not melt all the snow which falls there in the year. What other way of escape, then, can the frozen moisture find?

You will remember that the surplus rainfall flows off by means of rivers. Now the surplus snow-fall above the snow-line has a similar kind of drainage. It flows off by means of what are called glaciers.

When a considerable depth of snow has accumulated, the pressure upon the lower layers from what lies above them squeezes them into a firm mass. The surface of the ground is usually sloped in some direction, seldom quite flat. And among the high mountains the slopes are often, as you know, very steep. When snow gathers deeply on sloping ground, there comes a time when the force of gravity overcomes the tendency of the pressed snow to remain where it is, and then the snow begins to slide slowly down the slope. From one slope it passes on downward to the next, joined continually by other sliding masses from neighboring slopes until they all unite into one long tongue which creeps slowly down some valley to a point where it melts. This tongue from the snow-fields is the glacier. It really drains these snow-fields of their excess of snow as much as a river drains a district of its excess of water.

But the glacier which comes out of the snow-fields is itself made not of snow, but of ice. The snow, as it slides downward, is pressed together into ice. You have learned that each snowflake is made of little crystals of ice. A mass of snow is thus only a mass of minute crystals of ice with air between. Hence when the snow gets pressed together, the air is squeezed out, and the separated crystals of ice freeze together into a solid mass. You know that you can make a snowball very hard by squeezing it firmly between the hands. The more tightly you press it the harder it gets. You are doing to it just what happens when a glacier is formed out of the eternal snows. You are pressing out the air, and allowing the little particles of ice to freeze to each other and form a compact piece of ice. But you can not squeeze nearly all the air out, consequently the ball, even after all your efforts, is still white from the imprisoned air. Among the snowfields, however, the pressure is immensely greater than yours; the air is more and more pressed out, and at last the snow becomes clear transparent ice.

A glacier, then, is a river, not of water, but of ice, coming down from the snow-fields. It descends sometimes a long way below the snow-line, creeping down very slowly along the valley which it covers from side to side. Its surface all the time is melting during the day in summer, and streams of clear water are gushing along the ice, though, when night comes, these streams freeze. At last it reaches some point in the valley beyond which it can not go, for the warmth of the air there is melting the ice as fast as it advances. So the glacier ends, and from its melting extremity streams of muddy water unite into a foaming river, which bears down the drainage of the snow-fields above.

A river wears down the sides and bottom of its channel, and thus digs out a bed for itself in even the hardest rock, as well as in the softest soil. It sweeps down, too, a vast quantity of mud, sand, and stones from the land to the sea. A glacier performs the same kind of work, but in a very different way.

When stones fall into a river they sink to the bottom, and are pushed along there by the current. When mud enters a river it remains suspended in the water, and is thus carried along. But the ice of a glacier is a solid substance. Stones and mud which fall upon its surface remain there, and are borne onward with the whole mass of the moving glacier. They form long lines of rubbish upon the glacier, and are called moraines. Still the ice often gets broken up into deep cracks, opening into yawning clefts or crevasses, which sometimes receive a good deal of the earth and stones let loose by frost or otherwise from the sides of the valley. In this way loose materials fall to the bottom of the ice, and reach the solid floor of the valley down which the ice is moving; while at the same time similar rubbish tumbles between the edge of the glacier and the side of the valley.

The stones and grains of sand which get jammed between the ice and the rock over which it is moving are made to score and scratch this rock. They form a kind of rough polishing powder, whereby the glacier is continually grinding down the bottom and sides of its channel. If you creep in below the ice, or catch a sight of some part of the side from which the ice has retired a little, you will find the surface of the rock all rubbed away and covered with long scratches made by the sharp points of the stones and sand.

You will now see the reason why the river, which escapes from the end of a glacier, is always muddy. The bottom of the glacier is stuck all over with stones, which are scraping and wearing down the rock underneath. A great deal of fine mud is thus produced, which, carried along by streams of water flowing in channels under the glacier, emerges at the far end in the discolored torrents which there sweep from under the ice.

A glacier is not only busy grinding out a bed for itself through the mountains; it bears on its back down the valley enormous quantities of fallen rock, earth and stones, which have tumbled from the cliffs on either side. In this way blocks of rock as big as a house may be carried for many miles, and dropped where the ice melts. Thousands of tons of loose stones and mud are every year moved on the ice from the far snowy mountains away down into the valleys to which the glaciers reach.

The largest glaciers in the world are those of the polar regions. North Greenland, in truth, lies buried under one great glacier, which pushes long tongues of ice down the valleys and away out to sea. When a glacier advances into the sea, portions of it break off and float away as icebergs. So enormous are the glaciers in these cold tracts that the icebergs derived from them often rise several hundred feet above the waves which beat against their sides. And yet, in all such cases, about seven times more of the ice is immersed under water than the portion, large as it is, which appears above. You can realize how this happens if you take a piece of ice, put it in a tumbler of water, and watch how much of it rises out of the water. Sunk deep in the sea, therefore, the icebergs float to and fro until they melt, sometimes many hundreds of miles away from the glaciers which supplied them.

You will come to learn afterward that, once upon a time, there were glaciers in Britain. You will be able with your own eyes to see rocks which have been ground down and scratched by the ice, and big blocks of rock and piles of loose stones which the ice carried upon its surface. So that, in learning about glaciers, you are not merely learning what takes place in other and distant lands, you are gaining knowledge which you will be able by and by to make good use of, even in your own country.