In these vast folds it sometimes happens that portions of older (and lower) strata are caught up and so embedded among those of newer rocks. It will therefore be readily perceived that to unravel the geological structure of a great mountain-chain is no easy task. We need not then be surprised if in some cases the arrangement of the rocks of mountains is not thoroughly understood. The wonder is, when we think of the numerous difficulties which the geologist encounters,—the arduous ascents, the precipices, glaciers, snowfields obscuring the rocks from his view, the overlying soil of the lower parts, and the steep crests and dangerous ridges that separate the snowfields,—that so much has already been discovered in this difficult branch of geology.
However, the general arrangement of the rocks of which many mountain-chains are composed has been satisfactorily made out in not a few cases. Let us look into some of these and see what has been discovered.
You will remember the structure of the Weald, described in chap. vii., pp. 235-238, and how we showed that a great low arch of chalk strata has been entirely removed over that area, so that at the present time only its ends are seen forming the escarpments of the North and South Downs. This area, then, is now a great open valley, or rather a gently undulating plain enclosed by low chalk hills. Now, an arch of this kind is called an "anticline," and it might have been expected that it would have remained more or less unbroken to the present day. Why, then, has it suffered destruction?
In the first place, chalk is a soft rock, and one that rain water can dissolve; but more than that, its arch-like structure was against it, and its chance of preservation was decidedly small. In architecture the arch is the most firm and stable structure that can be made; but not so with strata, and this is the reason. Such an arch was not made of separate blocks, closely fitting and firmly cemented together; on the contrary, the arch was stretched and heaved up from below. It therefore must have been more or less cracked up; for rocks are apt to split when bent, although when deeply buried under a great thickness of overlying rocks, they will bend very considerably without snapping. But this was not the case here. And so the forces of denudation set to work upon an already somewhat broken mass of rock. Try to picture to yourself this old low arch of chalk as it was when it first appeared as dry land. Probably some of it had already been planed away by the waves of the sea, and what was left was by no means well calculated to withstand the action of the agents of denudation. If you look back to the figure, you will see the dotted lines showing the former outline of this anticline, or arch, and you perceive at once that the strata must have been sloping outwards away from the middle. Now, this one fact greatly influenced its fate, for an anticline cannot be regarded as a strong or stable arrangement of strata. It is easy to see why; suppose a little portion were cut away on one side at its base by some stream. It is clear that a kind of overhanging cliff would be left, and blocks of chalk would sooner or later come rolling down into the valley of the little stream. When these had fallen, they would leave an inclined plane down which others would follow; and this would continue to take place until the top of the arch was reached. The same reasoning applies to the other side. It is very seldom that arches, or anticlines, can last for a long time. The outward slope of the strata and their broken condition are against them.
But when the rocks dip inwards, to form a kind of trough or basin, it is just the opposite. Such basins are known as "synclines;" and a structure of this kind can be shown to be much more stable and permanent than an anticline. The strata, instead of being stretched out and cracked open, have been squeezed together.
It is very important to bear this in mind, and to remember how differently anticlines and synclines are affected; for this simple rule is illustrated over and over again in mountain-ranges:—
Anticlines, being unstable, are worn away until they become valleys.
Synclines, being stable, are left and frequently form mountains.
Now look at the section through the Appalachian chain (see Fig. [1]), and you will see that each hill is a syncline, and the valleys between them are anticlines. This happens so frequently that almost every range of mountains furnishes examples; but as every rule has its exceptions, so this one has, and we may find an example in the case of the Jura Mountains outside the Alps.
It will be seen from the section that the ridges are formed by anticlines, and the valleys by synclines. But on looking a little more closely, we see that the tops of the former have suffered a considerable amount of erosion (as indicated by the dotted lines). Now, the reason why they have not been completely worn down into valleys is that these rocks were once covered by others overlying them, so that this outer covering of rocks had first to be removed before they could be attacked by rain and rivers. These wave-like ridges of the Jura are being slowly worn down; and the time must come when they will be carved out into valleys, while the synclines between them will stand out as hills. It is simply a question of time. But many mountain-chains have a far more complicated structure than that of the Appalachians, and consist of violently crumpled and folded strata (see section of Mont Blanc, Fig. [3]).