A region of folded strata is, in the first instance, thrown into a series of ridges and valleys, the ridges formed by anticlines and the valleys by synclines; in other words, the topography is tectonic in character and determined by dias-trophic movements. If the folding be of moderate degree, so as to produce undulations of sweeping and gentle curves, the tendency of denudation is to reverse the original topography and convert the anticlines into valleys and the synclines into ridges. This apparently paradoxical result is found, when examined, to be natural and simple enough. The crests of newly formed anticlines have been subjected to tensile stresses which open the joints in the strata and render them an easy prey to the denuding agents. The surface of the synclines, on the contrary, has been tightly compressed, and their joints are closed by crowding. Aside from this, another factor tends to produce the same result. In a folded series of alternating harder and softer beds denudation is most rapid on the exposed anticlines, and in them the hard strata are first reached and cut through.

When an underlying mass of soft strata is reached, it is rapidly trenched into valleys which may soon be excavated below the level of the synclinal troughs.

Fig. 235. - Anticlinal ridge, Big Horn Mts., Wyo., hard beds in relief. (U. S. G. S).

If the folds originally made by the force of lateral compression be steep and high, as in mountain ranges, the anticlines persist longer as ridges, but the wearing away of their summits gives rise to subordinate ridges and valleys within the limits of each anticlinal arch. Here also the ridges are the outcropping harder beds, and the valleys are cut in the softer ones. Even in mountain ranges denudation may reverse the original structural topography and give rise to anticlinal valleys and synclinal mountains.

Fig. 236. - Truncated anticlinal ridge, Montana. (U. S. G. S).

If a region of folded rocks has once been planed down to base-level or to a peneplain, and then reelevated and subjected to denudation, the resulting topography will be determined by the same laws. Indeed, this is a frequent method in which regions of tilted or inclined strata are produced, for, as we saw in Chapter XII (The Structure Of Rock Masses - Stratified Rocks) (p. 327), inclined beds are very often parts of truncated folds. In. such regions drainage is first in accordance with the slopes of the planed and tilted surface, but as denudation proceeds, the structure and arrangement of the rocks make themselves felt, and bring about changes and adjustments of the drainage to the structure, as will be more fully explained in a subsequent chapter.