THE WORK OF STREAMS 147 



when their nearly level, wide bottoms are bounded by low, 

 gently sloping sides (Fig. 135). Figure 136 shows the chang- 

 ing shape of a valley in its advance from youth to old age. 



The terms youth, maturity, and old age are applied also to 

 rivers and to the topography of drainage basins. Figures 137, 

 138, and 139, and Plates III, IV, and V, show young, mature, 

 and old topographies. In Figure 137 the valleys are few in 

 number and have the characteristics described above as dis- 

 tinctive of youth. The region is poorly drained, broad upland 

 areas between the valleys being as yet untouched by erosion. 

 The task of carrying to the sea all the material above base 

 level has scarcely begun. The area shown in Figure 138 has 

 been eroded into a rough hill-and-valley country. Only nar- 

 row ridges remain to indicate the position of the once broad 

 inter-valley uplands, while the larger valleys have nearly 

 reached base level. The region, therefore, possesses greatest 

 relief at this stage. Slope is at a maximum, and every part 

 of the area is now reached by drainage lines. When the 

 rivers have reduced a region to old age (Fig. 139), it again 

 approaches flatness at a level as low as running water can 

 bring it. Such a plain, if perfected, would be a base-level 

 plain. 



It is especially important to note that youth, maturity, 

 and old age are terms which, as used in geolog}^, indicate 

 stages in development and not periods of years. It is per- 

 fectly possible, for example, for a large river working on 

 weak material to bring its valley to old age in the same or 

 less time than that required for a smaller stream, opposed 

 by stronger rocks, to develop a mature valley. 



Peneplains and monadnocks. Areas have been rarely, 

 if ever, absolutely base-leveled. The time required is very 

 great, and before the rivers have accomplished the task, the 

 area is likely to have been elevated with reference to sea 

 level and the quickened streams started upon the new task 

 of reducing it again. Extensive areas ha*ve, however, been 

 reduced nearly to base level. Such plains are called pene- 



