CYCLE OF EROSION 71 



in a region of great relief. The uppermost line in Fig. 54 likewise 

 represents topographic youth, as shown in cross-section. 



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 V, are in 

 their topographic youth. It is often convenient to recognize 



Fitf. 60. A valley much older than that shown in Fig. 

 southwestern Colorado. (U. S. Geol. Surv.) 



, Gray Copper Gulch, 



various sub-stages, such as early youth, middle youth, and late 

 youth, within the youthful stage of valleys and topographies. 



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 

 valleys and their flats widen, and as their tributaries increase in 

 number and size, a stage of erosion is presently reached in which 

 but little of the original upland surface remains. The country 

 is reduced largely to slopes, and in this condition the drainage and 

 the topography which it has determined are said to be mature. 

 Mature topography is shown in contours in Fig. i, PI. VI, where 

 slopes rather than upland or valley flats, predominate. Mature 



