The Story of the Hills: A Book About Mountains for General Readers.

by H. N. Hutchinson

Compare, for instance, the scenery of Kent or Surrey with that of the Lake District or the west of Yorkshire. The difference is due chiefly to the fact that in Kent and Surrey we have rocks that succumb more easily to the action of rain and rivers, and consequently are worn away more rapidly than the harder rocks in the north country. Geologists have a word to express the effects of this wear and tear; namely, "denudation," which means a stripping off, or laying bare.

In Kent and Surrey the agents of denudation (rain and rivers, aided by the effects of the air, of heat and cold, and so on) wear away the whole surface of the county in a tolerably even and uniform manner, because there are no hard rocks for them to contend with. In this case rain washes away the sides of the valleys faster than the river can carve its bed, consequently the valleys are shallow compared to their width. And so the streams have broad valleys, while the hills are smooth and gently rounded. Chalk, clay, and soft sandstone abound there. The two latter rocks are washed away with comparative ease, and the chalk is dissolved; whereas in the Lake District we have very much harder and older rocks, that require to be split up and broken by the action of frost, while every stream carves out for itself a steep valley, and great masses of hard rock stand out as bold hills or mountains, that seem to defy all the agents of denudation. Here the opposite is the case, and the valleys are deepened faster than they are widened. But for all that, a vast amount of solid rock has been removed from the surface there, of which the mountains are, as it were, but fragments that have escaped the general destruction. Moreover, the rocks in this region have been greatly disturbed and crumpled since they were first formed, and thereby thrown into various shapes that give certain peculiar structures more or less capable of resisting denudation.

Very effective illustrations of the power of rain by itself are afforded by the "earth pillars" of the Tyrol, and "cañons" of Colorado. The material of which they consist is called conglomerate, because it is composed of stones and large blocks of rock with stiff earth or clay between. All the taller ones have a big stone on the top which protects the softer material below from being washed away by heavy rains; and it is easily perceived that each pillar owes its existence to the stone on the top, which prevents the soft materials below it from being washed away. When, after a time, the weathering of the soft strata diminishes the support of the capping boulders, these at last topple over, and the pillar, thus left unprotected, becomes an easy prey to the rain, and is rapidly washed away. Some of the pillars are over a hundred feet in height. But it is only in places where heavy rains fall that these interesting monuments of denudation are to be seen.

By way of contrast we may turn now to a district in which very little rain falls, but where the streams have a considerable slope, and so can wear away, or erode, their valleys much faster than rain and frost, etc., can bring down the rocks of which the sides are composed.

The river Colorado of the West, which runs from the Rocky Mountains to the Gulf of California, flows for nearly three hundred miles at the bottom of a profound chasm, or cañon, being hemmed in by vertical walls which in some places are more than a mile in depth. The tributary streams flowing into the river run through smaller ravines forming side cañons; and there is no doubt that these wonderful chasms have been, in the course of ages, slowly carved out by the river Colorado and its numerous tributary streams. Sometimes the walls of the cañon are not more than fifty yards apart, and in height they vary from three thousand to six thousand feet.

Far above the level of the highest floods patches of gravel are found here and there on the sides, which must have been left there by the river when it had not cut its way so far down. These cañons afford striking testimony to the erosive power of running water, of which they are the most wonderful illustration in the world.

But water, even when in the form of ice, has more or less power to wear away solid rock; and the glaciers that we see in Switzerland, Norway, and other countries must slightly deepen the rocky valleys down which they flow. Let us see how this can be accomplished.

The snow that falls in the High Alps, impelled by the weight of fresh layers of snow overlying it, and by the slope of the mountain-sides, gradually creeps down into the valleys. Owing to the pressure thus put upon it, and partly to the melting power of the sun's rays, it assumes the form of ice; and glaciers are composed of solid ice. The downward motion is so slow that a glacier appears quite stationary; and it is only by putting in stakes and watching them change their positions that it can be shown to be moving.

In all respects except speed, glaciers flow like rivers, for ice is a viscous body, behaving partly like a fluid and yet partly like a solid substance; but it will not endure a sharp bend without snapping. Hence, a glacier in traversing a valley frequently gets split. The cracks thus formed widen by degrees until they expand into chasms, or "crevasses." Like rivers, glaciers transport a large amount of rocky matter to lower levels, and at the same time wear away and deepen their rocky channels.

Let us see how they do this twofold work of transportation and erosion. In the first place, a large amount of débris falls onto the sides of a glacier from the peaks, precipices, and mountain-side along which it flows. Some stones, however, fall down crevasses, and so reach the bottom, where they become cemented in the ice. In this way they are slowly carried down over the rocky floor of the valley, until at last they reach the end of the glacier, where in the warmer air the ice melts just as fast as it creeps down; and there they will be left to form a heap of stones, sand, and mud.