THE WORK OF RUNNING WATER. 



89 



The same processes which have made young valle^^s mature will in 

 time work further changes. When the gradients of the valleys have 



Fig. 73. — Mature erosion in a mountain region, Silverton, Colo. (Cross, U. S. 



Geol. Surv.) 



become low and their bottoms wide, and when the intervening ridges 

 and hills have become narrow and small, the drainage and the drainage 

 topography have reached old age, and the streams are m a condition 

 of senility. This is illustrated by Fig. 1, Plate VII (central Kansas), 

 and in section by the third and lower lines in Fig. 64. Topographic 

 old age sometimes has a different expression; this is shown in Fig. 74, 

 where most of the surface has been brought low. The elevations 

 which rise above the general plain are small in area, but have abrupt 

 slopes. This phase of old-age topography is usually the result of the 

 unequal resistance of the rock degraded. The effects of unequal rock- 

 resistance will be considered later. 



The marks of old streams are as characteristic as those of young 

 ones. They have low gradients and are sluggish. Instead of lower- 

 ing their channels steadily they cut them down in flood, and fill them 

 up when their currents are not swollen. They meander widely in 



