68 



WORK OF RUNNING WATER 



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

 water falling on the hills washes the loose material from their 

 surfaces, and starts it on its seaward journey. Thus the "ever- 

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

 to the sea. 



The base-leveled surface is not absolutely flat. The area 

 reduced by each stream will have a slight slope down-stream, and 

 from its sides toward its axis. The low divides between streams 

 flowing in the same direction may, however, disappear altogether, 

 for when valleys have reached their limits in depth, their streams 



do not cease to cut laterally. 

 Meandering in their flat- 

 bottomed valleys, they may 

 reach and undercut their 

 divides (PL IV, and Fig. 57). 

 By lateral planation, there- 

 fore, the divides between 

 streams may be entirely 

 eaten away. 



The terms "grade," and 

 "graded plain," and "base 

 level" and "base-leveled 

 plain," are somewhat vari- 

 ously, and therefore some- 

 what confusingly, used. "A 

 graded valley is one in which there is a condition of essential balance 

 between corrasion and deposition." 1 Its angle of slope is variable 

 and is dependent on the capacity of the stream for work, and on the 

 work it has to do. A small river must have a higher gradient than a 

 large one; a stream with much sediment must have a higher gradi- 

 ent than one with little, and a stream with a load of coarse material 

 must have a higher gradient than one with a load of fine. Thus 

 the graded valley of the lower Mississippi has an inappreciable angle 

 of slope; but the graded valleys of some of its small mountain 

 tributaries have slopes of hundreds of feet per mile. Since both the 

 size of the stream and the amount and coarseness of its load at a 

 given place vary from time to time in the course of a cycle of erosion, 

 it is clear that the inclination of the graded valley in a given place 

 must vary from time to time. With the changing conditions of 

 1 Davis. Jour, of Geol., Vol. X, p. 87. 



Fig. 57. Diagram showing streams in 

 adjacent valleys, undercutting the divide 

 between them. They may, in time, cut the 

 divide away. 



