THE WORK OF RLWXIXG WATER. 83 



tion.'' ^ A graded valley is one in which deposition and corrasion are, 

 in the vertical sense, balanced. Its angle of slope is most variable, and 

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

 has to do. A Aveak river must have a higher gradient than a strong 

 one; a stream with much sediment must have a higher gradient 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 many of its 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, it is clear 

 that the inclination of a graded valley must vary also, and further, that 

 it must be in process of continual readjustment. With 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 nor- 

 mal history the land is brought nearer and nearer to sea-level without 

 ceasing to be at grade. When the inclination of a graded surface be- 

 comes so low that it is sensibly flat, the surface may be said to be at 

 hase-level, although this does not mean that the surface can never be 

 degraded further. If the term be used in this way, it is clear that there 

 is no sharp line of distinction between a graded surface and a base-leveled 

 surface, and as the terms are now commonly applied no such distinc- 

 tion exists. ■ 



If the term base-level Avere made synonymous with sea-level, as has 

 been proposed,^ the term might as well be discarded, for sea-level could 

 always be used in its stead. Furthermore, streams often erode below 

 t:ea-level. The bottom of the channel of the Mississippi is below sea- 

 level for some 400 miles above its debouchure, and locally (Fort Jack- 

 son) it is nearly 250 feet below. This deep channel is the result of the 

 erosive acti^dty of the stream, not of subsidence. Again, the sea-level 

 is itself inconstant. The extent of its changes cannot now be measured, 

 but they have probably been more considerable in the course of geo- 

 logical history than has been commonly recognized. It is true that they 

 take place slowly, as far as knoAAii, but it is also true that the duration 



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

 - Ibid., p. 77 et seq. 



