repose upon their flanks. The Ochils of Kinross afford us a good example. (Fig. 24.) Here we have an underlying series of hard igneous rocks, a, folded along an axis from which they dip away on both sides below overlying sheets of red sandstone. These red sandstones almost certainly at one time extended across the anticline, which has thus been

much denuded.

But, owing to the greater durability of the igneous rocks, the anticline, of which they form the axis, continues to show as a prominent elevation.

Hitherto we have been considering the surfaceforms assumed by gently folded strata in regions

which have been subjected for a more or less prolonged period to subaërial denudation.

In areas

where deformation of the strata has been effected within geologically recent times, not infrequently some coincidence may be observed between the undulations at the surface and the underground struct

FIG. 26. ANTICLINAL RIDGE, GREEN RIVER PLAINS. (Powell.)

ure. The Colorado district we have described as a region of practically horizontal strata. Here and there, however, the rocks are more or less folded, and when such is the case they often give rise to corresponding folds at the surface. In the region traversed by Green River, for example, the horizontal strata occa

sionally show anticlines and synclines, as in the following sketches from Major Powell's description of the Cañon country, where the synclinally arranged beds in Fig. 25 form a valley, while the anticlinal strata in Fig. 26 appear as a swelling ridge.

Such coincidence of underground structure and superficial configuration, however, is not always to be traced even in so young a land as the Cañon district, while, as already remarked, it is of very uncommon occurrence in lands of high geological antiquity.

CHAPTER V

LAND-FORMS IN REGIONS OF HIGHLY FOLDED AND DISTURBED STRATA

TYPICAL ROCK-STRUCTURES IN REGIONS OF MOUNTAIN-UPLIFTGENERAL STRUCTURE OF MOUNTAINS OF UPHEAVAL-PRIMEVAL COINCIDENCE OF UNDERGROUND STRUCTURE AND EXTERNAL CONFIGURATION-RELATIVELY WEAK AND STRONG STRUCTURES-STAGES IN THE EROSION OF A MOUNTAIN-CHAINFORMS ASSUMED UNDER DENUDATION-ULTIMATE FATE OF

MOUNTAIN-CHAINS.

WE

E have now to study the various land-forms that characterise regions where highly folded strata occur. Deformation of the crust has taken place in all ages of the world's history. In some countries rock-plication and folding date back to the earliest period of which geologists have any certain knowledge. In other places the deformations belong to relatively recent times. Again, we find evidence to show that certain areas have experienced such changes at many successive periods. As might have been expected, the oldest rock-folds have suffered excessive erosion, while the youngest have experienced less. We are thus able to study in different countries. the successive phases through which a region of highly

disturbed strata must necessarily pass. We see it in its youth in such mountains as the Alps, the Himalayas, the Cordilleras, and in its old age in the Appalachians and the mountains of Scandinavia and Britain.

Let us now briefly consider some of the typical kinds of structure presented by the more steeply inclined strata. In regions of moderately inclined rocks the folds, as we have seen, are symmetrical anticlines and synclines. the axes of which are vertical, the beds

AVAVAVAAY

FIG. 27. ISOCLINAL FOLDS.

Axes moderately inclined from the vertical.

dipping away from or towards the axes at approximately equal angles. (See Fig. 22, p. 87.) Folds of this kind, however, are not restricted to areas of moderately inclined strata; they are met with also in regions where the rocks as a rule dip steeply. But in such regions. the anticlines and synclines are usually more or less unsymmetrical—their axes are inclined. In Fig. 27 we have represented a series of moderately inclined folds. In Fig. 28 the inclination of the axes is still greater. As the folds in these two diagrams all lean in one direction, they are said to be isoclinal. Very frequently the inclination of the axes increases to such a degree that one fold may come to lie almost hori