We have seen that any region, however lofty and rugged, must eventually be worn down to base-level, provided only that the country remain stationary with reference to the sea until the process of degradation is complete. It is doubtful, however, whether any extensive region of hard rocks has ever been absolutely reduced to base-level: the usual result is the formation of a peneplain, a low-lying, featureless surface of gentle slopes and with only occasional eminences rising above the general level. Reelevation of such a peneplain at once revivifies the streams and gives all the destructive agencies new powers. The peneplain is attacked and carved into valleys and hills, the valleys being rapidly cut down to base-level, while the divides and hills are much more slowlv removed. If time enough be granted, the rugged country formed from a dissected peneplain is in its turn worn down to a second peneplain at a lower base-level. This alternate upheaval and wearing down together constitute a cycle of denudation, from base-level back to base-level. A complete cycle is one in which the whole region is reduced to a peneplain before the reelevation occurs, and a partial or incomplete cycle is one which is interrupted by upheaval before the region is cut down, and only small and local peneplains have been formed.

From a study of an old region several cycles of denudation may frequently be made out, represented by the remnants of dissected peneplains at different levels preserved in the harder rocks. The successive adjustments of the drainage system are a valuable auxiliary in working out the history of the cycles.

As an excellent example of these cycles of denudation whose marks are preserved in the structure of the rocks, we may take the Appalachian Mountains, which have been studied with great care. The cycles have been worked out elaborately, but only an outline of the more striking events can be given here.

These mountains began as a great geosyncline in which throughout the vast lengths of the Palaeozoic era were accumulated enormously thick masses of shoal-water sediments. Toward the close of that era a number of crustal movements set in, crushing the sides of the geosynclinal trough, and crumpling the mass of strata contained in it into a series of roughly parallel, closed, inclined, or overturned folds, forming doubtless a very lofty range of mountains. During the long ages of the Mesozoic era the mountains were attacked and worn down by the destructive agencies; and by the time the Cretaceous period was reached the range had been reduced to a peneplain, with only a few hills rising above its almost featureless level, - hills which are now the peaks of western North Carolina, the highest points of the range at present. The present height of these peaks is due to subsequent reelevation. This plain is called the Kittatinny peneplain, because the ridge of that name in Pennsylvania and New Jersey is one of the remnants of it. To the observer who can overlook the billowy ridges of the present range their even sky-line is very striking, and these ridges are all composed of the hardest rocks, which all rise to nearly the same level.

To reproduce the plain it would be necessary to fill the valleys between the Blue Ridge on the east and the plateau on the west up to the level attained by the hard ridges, and this would give- a gently arched surface, sloping very gradually to the Mississippi Valley and the Atlantic. On this peneplain were already established the great streams which flow to the ocean, such as the Susquehanna and the Potomac.

Next the peneplain was raised very gradually to a height of 1400 feet in Virginia, diminishing in both directions from this point, and the denuding forces once more attacked and dissected the plateau, the larger streams holding their transverse courses and sawing through the hard strata, which were left standing as ridges by the cutting of the longitudinal valleys along the more destructible beds. Denudation had cut down the softer beds to one general level, called the Shenandoah peneplain, the period of rest being long enough to bring all the areas of soft and soluble beds to this level, but not materially to lower the ridges of the more resistant strata.

"The swelling of the Appalachian dome began again. It rose 200 feet in New Jersey, 600 feet in Pennsylvania, 1700 feet in southern Virginia, and thence southward sloped to the Gulf of Mexico. ... In consequence of the renewed elevation, the streams were revived. Once more falling swiftly, they have sawed, and are sawing, their channels down, and are preparing for the development of a future base-level." (Willis).