But the main work of carving out the hills and mountains of the world is done by streams, rivers, and glaciers; and so we now pass on to consider how they perform their tasks. Water by itself, even when flowing fast, would be powerless to carve gorges and valleys in the solid rock; but the stones which torrents and streams carry along give them a marvellous grinding power, for with such material a stream continually wears away its rocky bed. Moreover, the stones themselves are all the while being rubbed down by each other, until finally they are ground down to fine sand and mud, which help in the work of erosion.
Every mountain stream or torrent runs in a ravine or valley of some sort; and any traveller who will take the trouble to watch what goes on there may easily convince himself that the ravine, gorge, or valley has been carved out by the stream, aided by the atmospheric influences to which we have already alluded.
But perhaps some may be inclined to look upon the ravine as a chasm produced by some violent disturbance from below, whereby the rocks were rent asunder, and that the stream somehow found its way into the rent. A little inquiry will dispel this idea. In the first place, such catastrophes are quite unknown at the present day; and as we have more than once pointed out, the geologist's method is to apply a knowledge of processes now in operation to the phenomena of the rocks, in order to read their history. Secondly, no conclusion can be accepted which is not supported strongly by evidence.
If such a rending of the rocks had taken place, there would assuredly be some evidence of the fact. We should expect to find a great crack running all along the bed of the stream; but of this there is no sign. Go down in any weather when the stream is low, and look at the rocks over which it flows, and you will search in vain for such evidence. Instead of being broken, the rocks extend continually across. You would also expect to find the strata "dipping," or sloping away from the stream on each side, if they had been rent by such an upheaval; but here again we are met by a total want of evidence. Thirdly, a crack might be expected to run along more or less evenly in one direction. But look at the ravine, follow it up for some miles, and you will see that it winds along in a very devious course, not in a straight line.
For these reasons, then, we must conclude that the ravine or valley has been carved out by the stream; but perhaps the most convincing arguments are afforded by the furrows and miniature ravines so frequently met with on the sides of all mountains; and it is impossible to examine these without concluding that they have in every case been cut out of the solid rock by the little rapid torrents that run along them after heavy rain. If we are fortunate enough to see them on a thoroughly rainy day, we may derive much instruction from watching the little torrents at work as they run down the mountain-side, here and there dashing over the rocks in little cascades, and bringing down to the base of the hill much of the débris that forms higher up. In this way Nature gives us an "object lesson," and seems to say: "Watch me at work here, and learn from such little operations how I work on a larger scale, and carve out my ravines and big valleys. Only give me plenty of time, and I can accomplish much greater feats than this."
The question of time is no longer disputed; and all geologists are willing to grant almost unlimited time, at least periods of time that seem to us unlimited. Most streams have been flowing for thousands of years; and when once we grant that, we find no difficulty in believing that all valleys are the work of rain and rivers. Surely no one would argue that the furrows on a mountain-side are all rents which have been widened by the action of water; for if they were rents, each must have been caused by some disturbance of the rocks composing the mountain, and we should of course be able to see the cracks for ourselves, and to find that the rocks had in some way been disturbed and rent open.
Even the rain which falls on the road in a heavy shower teaches the same simple but important lesson, as it runs off into the gutters on each side; and we may often find the road furrowed by little miniature rivers, that carve out for themselves tiny valleys as they run off into the gutter, bringing with them much débris in the form of mud and sand.
Sometimes a stream encounters in its course a layer of rock that is harder than the rock underlying it. In this case the softer rock is worn away faster, and the hard layer forms a kind of ridge at a higher level; the result is a waterfall. Waterfalls are frequently found in mountain streams. In this case, it is easy to trace the ridge of harder rock running unbroken across the path of the stream, showing clearly that it has not been rent in any way. First it showed merely as a kind of step, but gradually the force of the falling water told with greater effect on the softer rock below, wearing it away more rapidly than that above, and so the depth of the waterfall went on increasing year by year; and at the same time the hard layer was slowly worn away until the stream sawed its way through.
Some river valleys are steep and narrow; others are broad, with gently sloping sides. A careful study of the different valleys in any large country such as Great Britain, shows that their forms vary according to the nature of the rocks through which rivers flow. Where hard rocks abound, the valleys are steep and narrow; where soft rocks occur, the valleys are broad and low. This is only what might be expected, for hard rocks are not easily worn away; a river must cut its way through them, leaving cliffs on either side that cannot be wasted away by rain. But in a district where clay or soft sandstone occurs, the rain, as it finds its way to the valley, will wash them away and give a smooth gentle slope to the sides of the valley.
It is very instructive to notice how the scenery of any district depends on the nature of its prevailing rocks. Hard rocks give bold scenery with steep hills and rocky defiles; while soft rocks make the landscape comparatively flat and tame, though often very beautiful in its way, especially where a rich soil abounds, so that we see pleasant woods, rich pasture-land, and heavy crops in the fields.