THE WORK OF RUNNING WATER. 



69 



by the side cutting of the stream, by the wash of the rain which falls 

 on its slopes, and by the action of gravity which tends to carry down 

 to the bottom of the slope the material which is loosened above by any 

 process whatsoever. If there be but one valley in a land area its limit- 

 ing width is scarcely less than the width of the land itself. 



Had "there been several initial meridional depressions instead of one 

 in the island, or had there been several meridional belts where the mate- 

 rial of the surface was less resistant than elsewhere, several valleys 

 would have been developed, converging toward the center (Fig. 47). If 

 the conditions were such as to allow of the equal development of valleys 

 on all sides of the island, each would be lengthened by head erosion until 

 it reached the center of the island, where the permanent divide between 

 their heads would be established. Each would be widened by all the 

 processes which widen valleys, and their widening would narrow the in- 

 tervening areas (Figs. 48 and 49). Under conditions of equal erosion 

 the limits of width for each valley would be the centers of the ridges on 

 either side, and here the divides between them would be permanently 

 estabhshed. Though erosion would continue even after the crest of the 

 ridge had been narrowed to a line, the permanence of the divide would 

 follow from the fact that erosion would be equal on both sides of this 

 line, and its effect wOuld be to lower 

 the divide, but not to shift it hori- 

 zontally (Fig. 50). The hmits in 

 length and width are therefore not the ^^^- 50. -Diagram to illustrate the 



in lowering of a divide without shifting 



same where there are several valleys ^ The crest of the divide is at a, b, 



as where there is but one. The limit and c successively. If erosion were 



in depth, however, remains the same, unequal on the two sides, the divide 



1 ,1 n 1 li r • would be shitted 



and the final result oi erosion, pro- 

 ceeding along these lines, would be the base-leveling of the land, leaving 

 a plain but slightly above sea-level. The plain would not be absolutely 

 flat, though its relief would be very slight, and the higher parts would 

 be along the lines of the divides between the streams (Fig. 51. Com- 

 pare also Fig. 42). Many valleys would occasion more rapid degra- 

 dation than few, and the period of base-leveling would be correspond- 

 ingly shortened. 



Had the initial depressions which gave origin to the valleys had posi- 

 tions other than meridional, the valleys would have had other and less 

 regularly radial courses, but the final result of their development would 

 have been the same. 



