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?