panying section (Fig. 22) it will be observed that synclines coincide with hills, and anticlines with valleys. This configuration has been determined by the geological structure. In each hill we have practically two escarpments placed back to back. The beds hh are relatively harder than others in the series. Had no

such beds occurred the synclines would probably not have been so strongly emphasised by elevations. But the presence of one or more hard beds in series of undulating and relatively soft strata does not necessarily give rise to synclinal hills. The hard beds in such a series would no doubt in time crop out at the surface

d

FIG. 22.

SYNCLINAL HILLS AND ANTICLINAL VALLEYS. ss, synclines; a a, anticlines; hh, relatively hard beds.

and project above the base-level of the district; but if in the synclinal troughs they descended below that level, they could have no influence upon the surface. Thus in the section (Fig. 23) a relatively hard bed crops out and forms escarpments at ee, but it descends below the base-level, b b, in the two synclinal troughs (s1 s2), which remain unaffected by it. In the third trough (s), however, it remains above the base-level, protecting the underlying softer beds, and thus forming a hill.

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

FIG. 23.

ESCARPMENT HILLS AND SYNCLINAL HILL.

e e, hard bed; s1 s s3, synclinal troughs; bb, 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

FIG. 24.

SECTION ACROSS WEST LOMOND HILL AND THE OCHILS.
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 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.