Fig. 177.—View of a maturely dissected upland from one of its hilltops, Klamath Mountains, California (after a photograph by Fairbanks).
Viewed from one of the hilltops, the landscape of this stage bears a marked resemblance to a sea in which the numberless divides are the crests of billows, and these, as distance reduces their importance in the landscape, fade away into the even line of the horizon ([Fig. 177]).
The Hogarthian line of beauty.—Since the youthful stage of the upland, when the lines of its landscape were straight, its character rugged, and its rivers wild and turbulent, there has been effected a complete transformation. The only straight line to be seen is the distant horizon, for the landscape is now molded in softened outlines, among which there is a repeated recurrence of the line of beauty made famous by Hogarth in his “Analysis of Beauty.” As well known to all art students, this is a sinuous line of reversed or double curvature—a curve which passes insensibly at a point of inflection from convex to concave ([Fig. 178)]. The curve of beauty is now found in every section of the hills, and it imparts to the landscape a gracefulness and a measure of restfulness as well, which are not to be found in the landscapes of earlier stages in the erosion cycle. In the bottoms of the valleys also the initial windings of the rivers within their narrow flood plains add silver beauty lines which stand out prominently from the more somber background of the hills.
Fig. 178.—Hogarth’s line of beauty.
Considered from the commercial viewpoint, the mature upland is one of the least adaptable as a habitation for highly civilized man. Direct lines of communication run up hill and down dale in monotonous alternation, and almost the only way of carrying a railroad through the region, without an expenditure for trestles which would be prohibitive, is to follow the tortuous crest of a main divide or the equally winding bed of one of the larger valleys.
Fig. 179.—View of the old land of New England, with Mount Monadnock rising in the distance.
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]).