Fig. 1. SECTION ACROSS THE WEALD OF KENT AND SURREY.
Fig. 2. THE HIGHLANDS OF SCOTLAND ON A TRUE SCALE (after Geikie).
The dotted lines in the figure show us their former extent; but the vertical height is exaggerated, for otherwise the hills would scarcely be seen.
These lines simply follow out the curves taken by the strata at each end of the denuded arch, and therefore rightly indicate its former height. By making such a drawing on a true scale, geologists can easily measure the former height of the surface of this old arch, or "anticline," of chalk, greensand, and other strata, just as an architect might restore the outlines of an old traceried window from a few portions left at the sides.
This very useful and instructive method is much employed in drawing sections through mountain-chains, in order to gain some idea of the amount of denudation which they have suffered.
Let us see how much has been removed from the present surface of the Weald. First there is the chalk, which we may put down at six hundred feet at least; then there is the lower greensand, say, eight hundred feet; and below that, and forming the lowest ground in the Weald, is the Weald clay, which is one thousand feet thick, and being softer, was more rapidly borne away. Along the centre runs a ridge of Hastings sand, forming higher ground on account of its greater hardness, but this formation is not much denuded. However, adding together the thicknesses of the others, we arrive at the conclusion that about twenty-four hundred feet of chalk and other strata has been removed from the present surface of the Weald. And all this denudation has probably been effected by rain and rivers, for it is very doubtful whether the sea had any share in this work.
But in other parts of our own country we find proofs of denudation on a much grander scale than this; for example, in North Wales there are rocks now lying exposed at the surface which are of a very much greater antiquity than any that may be seen in the Wealden area, belonging to the very ancient periods known as the Cambrian and Silurian. These have evidently been exposed for a much longer time to the action of denuding forces; and the Welsh hills, as we now see them, are but fragments of what they once were. After carefully mapping out the rocks in the neighbourhood of Snowdon, noting their thickness, the directions in which they slope, or "dip," so that the structure of this region might be ascertained, as in the case of the Weald, it was found, on drawing sections of the rocks there, and putting in dotted lines to continue the curves and slopes of the strata as known at or near the surface, that from fifteen thousand to twenty thousand feet of solid rock must have been removed (see diagrams, chapter ix., p. [307]). Applying the same method to the Lake District, it has been calculated that the amount of denudation which that beautiful country has suffered may be represented by twenty-six thousand feet. Turning to the other side of the Atlantic, we find the American geologists estimate that a thickness of five miles has been removed from a large part of the Appalachian chain of mountains (near their east coast), and that at least one mile has been eroded from the entire region between the Rocky and Wahsatch Mountains (see chapter [ix.]).