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When a series of undulating strata contains no intercalated hard beds, but is of much the same consistency throughout, the synclines still offer the stoutest resistance to denudation, anticlines being relatively weak structures. In the former the strata are not liable to be undermined and displaced by the
F1g. 23. Escarpment H1lls And Syncl1nal H1ll.
e *, hard bed; sl s* sJ, synclinal troughs; b £, base-level.
action of springs. In the latter, however, the strata hang away from the axis, and water percolating through them, and coming out along the beddingplanes, tends to their demolition. But this is a matter which will be considered more fully when we come
OcM H.m Wtrf tinni
F1g. 24. Sect1on Across West Lomond H1ll And The Och1ls.
a, igneous rocks; b, red sandstones, etc.; c, basalt.
to consider the surface-forms yielded by steeply inclined and highly folded strata.
In regions long exposed to denudation all weakly built hills tend to disappear. Hence in such countries anticlinal hills are of very rare occurrence. Now and again they do occur, but only when they happen to be composed of more durable rocks than those which 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
Fig. 25. Synclinal Valley West Of Green River. (Powell.)
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 subaerial 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
F1g. 26. Ant1cl1nal R1dge, Green R1ver Pla1ns. (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 occasionally show anticlines and synclines, as in the following sketches from Major Powell■s description of the Canon 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 Canon district, while, as already remarked, it is of very uncommon occurrence in lands of high geological antiquity.
LAND-FORMS IN REGIONS OF HIGHLY FOLDED AND DISTURBED STRATA
TYPICAL ROCK-STRUCTURES IN REGIONS OF MOUNTAIN-UPLIFT— GENERAL 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-CHAIN— FORMS ASSUMED UNDER DENUDATION—ULTIMATE FATE OF MOUNTAIN-CHAINS.
WE 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