86 GEOLOGY. 



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 moder- 

 ate 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. 



Not only are narrow valleys said to be young, but the territory af- 

 fected 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 dif- 

 ferent parts of the areas shown on Plate IV, for example, represent 

 different stages of youth. 



Youthful streams, as well as youthful topographies, have their dis- 

 tinctive 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 fiats 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. L 

 of Plate V represents an area in southeastern Kentucky (lat. 37° 12'^ 

 long. 83° 10'); Fig. 2, an area in western Virginia. Plate VI repre- 

 sents 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 topog- 

 raphy of the three areas are the result of differences in rock structure 

 and altitude, and will be considered later. Mature topography is also 



