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. [Illustration: PLATE V. Fig. 1. KENTUCKY. U. S. Geol. Surv. Fig. 2. VIRGINIA. U. S. Geol. Surv.] [Illustration: PLATE VI. PARTS OF LOS ANGELES AND SAN BERNARDINO COUNTIES, CALIFORNIA. U. S. Geol. Surv.] [Illustration: +Fig.+ 70.—The valley (canyon) of the Yellowstone. A young valley in an elevated region.] [Illustration: +Fig.+ 71.—Mature erosion topography in a region of slight relief, Iowa. (Calvin.)] [Illustration: +Fig.+ 72.—Mature erosion in a mountain region. From mouth of Gray Copper Gulch, Silverton, Colo., quadrangle. (Cross, U. S. Geol. Surv.)]