128 GEOLOGY 



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

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

 do not cease to cut laterally. Meandering in their flat-bottomed I 

 valleys, they often reach and undercut their divides (PI. V, and 

 Fig. 94). By lateral planation, therefore, the divides between! 

 streams may be entirely eaten away. The time involved in thej 

 reduction of a land area to base-level is a cycle of erosion. 



The terms " grade," " graded plain," and " base-level" and 

 "base-leveled plain," are somewhat variously, and therefore some- 

 what confusingly used. "A graded valley is one in which therel 

 is a condition of essential balance between corrosion and depo-l 

 sition." 1 Its angle of slope is variable and is dependent on thej 

 capacity of the stream for work, and on the work it has to do. A 

 weak river must have a higher gradient than a strong one; a stream 

 with much sediment must have a higher gradient than one withj 

 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 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 a graded valley must vary. \Yith 

 the changing conditions of advancing years, the slope of a graded 

 valley normally decreases. The same principles apply to graded 

 surfaces outside of valleys. 



In the continual readjustment of grades incident to a river's 

 normal history, the land is brought nearer and nearer to sea-level 

 When the inclination of a graded surface becomes so low that it i- 

 sensibly flat, the surface may be said to be at base-lcirl, ultlmugl 

 this does not mean that the surface can never be degraded more 

 If the term be used in this way, it is clear that there is no sliarj 

 line of distinction between a graded surface and a base-leveled sur-i 

 face. A base-leveled surface is necessarily a graded surface 

 and all graded surfaces of low gradients are base-levels. 



The ocean may be looked upon as a barrier which in a gener.-i 

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



