=The final product of river sculpture—the peneplain.=—When maturity has been reached in the history of a river, its energies are devoted to a paring down of the valley slopes and crests so as to reduce the general level. From this time on hill summits no longer fall into a common level—that of the original upland—for some mount notably higher than others, and with increasing age such differences become accentuated. There is now also a larger aggradation of the valleys to form the level floors of flood plains, out of which at length the now slight elevations rise upon such gentle slopes that the process of land sculpture approaches its end. Gradually the vigor of the stream has faded away, and can now only be renewed through a fresh uplift of the land, or, what would amount to the same thing, a depression of the base level. Upland and river have reached old age together, and the approximation to a new plain but little elevated above base level is so marked that the name _peneplain_ is applied to it. Scattered elevations, which because of some favoring circumstance rise to greater heights above the general level of the peneplain, are known as _monadnocks_ after the type example of Mount Monadnock in New Hampshire (Fig. 179). [Illustration: FIG. 180.—Comparison of the cross sections of river valleys for the different stages of the erosion cycle.] =The river cross sections of successive stages.=—To the successive stages of a river’s life it has been common to carry over the names from the well-marked periods of a human life. If neglecting for the moment the general aspect of the upland, we fix our attention upon the characteristic cross sections of the river valley, we find that here also there are clearly marked characters to distinguish each stage of the river’s life (Fig. 180). In infancy the steep, narrow, and sharp-angled cañon is a characteristic; with youth the wider V-form has already developed; in adolescence the angles of the cañon are transformed into well-rounded shoulders, and the valley broadens so as in the lower reaches to lay down a flood plain; in maturity the divides and the double curves of the line of beauty appear; while in the decline of old age the valleys are extremely broad and flat and are floored by an extended flood plain.