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.