Fig. 67.—A gully developed by a single shower. (Blackwelder.)
The cycle begins with the beginning of valley development, and at that stage drainage is in its infancy. The type of the infant valley is the gully or ravine (Figs. [67] and [68]). It has steep slopes and a narrow bottom. [Fig. 1 of Plate IV] represents similar, or rather older, ravines in contour (shore of Lake Michigan, just north of Chicago). With age, the valley widens, lengthens, and deepens, and passes from infancy to youth. In this stage also the valleys are relatively narrow, and the divides between them broad. They may be deep or shallow, according to the height of the land in which they are cut, and the fall of the water flowing through them; but in any case the streams flowing through them have done but a small part of the work they are to do before the country they drain is base-leveled. Figs. [69] and [70], respectively, represent youthful valleys in regions of moderate and great relief. [Fig. 2, Plate IV], shows a youthful valley in a region of slight relief (near Casselton, N. D., Lat. 46° 40′, Long. 97° 25′). The uppermost line in [Fig. 64] likewise represents topographic youth, as shown in cross-profile.
Fig. 68.—A gully somewhat older than that shown in [Fig. 67]. (Alden.)
Fig. 69.—A young valley in a region of slight relief.
Not only are narrow valleys said to be young, but the territory affected by them is said to be in its topographic youth, since but a small part of the time necessary to reduce it to base-level has elapsed. An area is in its topographic youth when considerable portions of it are still unaffected by valleys. Thus the areas (as a whole), as well as the valleys, represented on [Plate IV], are in their topographic youth. It is often convenient to recognize various sub-stages, such as early, middle, and late, within the youthful stage of valleys or topographies. The different parts of the areas shown on [Plate IV], for example, represent different stages of youth.
Youthful streams, as well as youthful topographies, have their distinctive characteristics. They are usually swift; their cutting is mainly at the bottom rather than at the sides, and their courses are often marked by rapids and falls.
As valleys approach base-level they develop flats. As the valleys and their flats widen, and as their tributaries increase in numbers and size, a stage of erosion is presently reached where but little of the original upland surface remains. The country is largely reduced to slopes. In this condition the drainage and the topography which it has determined are said to be mature. Mature topography is shown in contours in the figures of [Plate V], and in the northern part of [Plate VI], where slopes, rather than upland or valley flats, predominate. [Fig. 1 of Plate V] represents an area in southeastern Kentucky (Lat. 37° 12′, Long. 83° 10′); [Fig. 2], an area in western Virginia. [Plate VI] represents an area in southern California, somewhat west of San Bernardino. The three areas are alike in representing mature drainage, though not of equal stages of advancement. The striking differences of topography of the three areas are the result of differences in rock structure and altitude, and will be considered later. Mature topography is also shown in [Fig. 71], where the relief is moderate, and in Figs. [72] and [73], where it is great. Figs. [72] and [73] illustrate clearly the universal tendency of rivers in regions of notable relief to develop new flats well below the old surface of the region. At the same time that these low-lying flats are developing, tributary drainage is dissecting and roughening the upper surfaces. This process is well shown in [Fig. 73]. In both Figs. [72] and [73] the summits of the mountains on either side of the valleys appear to have had about the same elevation. The new flat is therefore developed at the expense of the old flat. As will be seen in the sequel, the first flat which a stream develops along its course is usually somewhat above base-level. It is a graded flat.