The age of valleys in terms of erosion is also expressed more or less perfectly by their cross-sections. The line 1–1 (and 1′-1′) of Fig. 64 represents in cross-section a narrow V-shaped valley. Such a section is always indicative of youth. The stream which developed it cut chiefly at its bottom, not at its sides. It was therefore rapid, and rapid streams are young. The line 2–2, (2′-2′) (Fig. 64) shows the same valley at a later and maturer stage when downward cutting has nearly ceased. The widening of the valley by slope wash has become relatively more important than before, and the stream has so far lost velocity as the result of diminished gradient as to be unable to carry away all the detritus washed down from the sides. As a result of deposition at the bases of the side slopes, a concave curve has been developed. Up the valley from the point where such a section as is represented by 2–2 occurs, the valley may still have a section similar to that represented by 1–1. [Illustration: PLATE VII. Fig. 1. KANSAS. U. S. Geol. Surv. Fig. 2. KANSAS. U. S. Geol. Surv.] [Illustration: PLATE VIII. ABOUT 15 MILES SOUTHWEST OF ST. LOUIS, MISSOURI. U. S. Geol. Surv.] Still later stages of development are represented by the cross-sections 3–3 and 4–4. Not only has the valley become larger, but the stream has deposited detritus (not shown in the figure) in the bottom of its valley, developing an alluvial flat. On this flat the stream meanders, and the valley may be widened by the undercutting of the bluffs wherever the stream in its wanderings reaches them (Pl. VIII, near St. Louis). A valley might possess the characteristics shown by the cross-sections 3–3, 2–2, and 1–1, Fig. 64, in its lower, middle, and upper courses, respectively. The preceding discussion, and the illustrations which accompany it, give some idea of the topography which characterizes an area in various stages of its erosion history. Whether the valleys are deep or shallow, and the intervening ridges high or low, depends on the original height of the land and its distance from the sea. The higher the land, and the nearer it is to the sea, the greater the relief developed by erosion. A plateau near the sea may become mountainous in the mature stage of its erosion history, while a plain in the same situation would only become hilly. A plateau in the heart of a continent would have less relief in its maturity than one of equal elevation near the sea, since the grade-plain in the former position is higher than in the latter. Plates IV and IX show youthful topography where the relief is relatively slight, and Plate X shows youthful topography where the relief is great. Similarly, Plates V and VI show mature topography where the relief is great, and Fig. 1, Plate III, shows mature topography where the relief is relatively slight.