THE WORK OF RUNNING WATER. 



81 



in time the surface will approach a plain. In this condition it is known 

 as a peneplain (an '^almost-plain"). Finally, v/hen running water has 

 done its utmost, the ridges will be essentially obliterated and a base- 

 leveled plain (e, e', e'') results. The figure expresses the fact that the base- 

 level develops laterally from the axis of the valley. It also develops 

 headward from the seaward end of the valley. Similarly, taking into 

 account all the valleys which affect it, the seaward margin of a base- 

 leveled plain is developed first, and thence it extends itself inland. 



Tributaries are tolerably sure to develop along each main valley. 

 The heads of the tributaries work back across the uplands between 

 the main valleys, dissecting them into secondary ridges (Fig. 65). 

 Tributaries will develop on the tributaries, and these tertiary valley j^ 

 dissect the secondary ridges into those of a lower order. This process 



Fig. 65. — Diagram showing the dissection of the upland shown in Fig. 64 by 



tributary valleys. 



of tributary development goes on until drainage lines of the fourth, 

 fifth, sixth, and higher orders are formed (Fig. 66). Since the process 

 of valley development under such circumstances is also the process of 

 ridge dissection, a stage is presently reached where the ridges are cut 

 into such short sections that they cease to be ridges, and become 

 hills instead. Even then the processes of erosion do not stop, for the 

 rain-water falling on the hills washes the loose material from their sur- 

 faces, and starts it on its seaward journey. Thus the '^ everlasting 

 hills" themselves are lowered, and, given time enough, will be carried 

 to the sea. Under these conditions, as under those already discussed, 



