DEVELOPMENT OF VALLEYS 





parallel mountain ridges, leaving a depression between (Fig. 46). 

 I >rainage will appropriate such a valley, so that it becomes in some 

 sense a river valley; 

 but it is not a river 

 valley in the sense in 

 which the term has 

 been used in the pre- 

 ceding pages. It is 

 rather a structural val- 

 ley. A river valley 



may be developed ^ ^ structural valley with a river valley 

 in its bottom (a, Fig. developing in its bottom. 

 46) and it may be in 



process of development throughout the whole length of the struc- 

 tural valley at the same time. 



These illustrations do not exhaust the list of conditions under 

 which valleys develop, but they suffice to show that valleys origi- 

 nate and develop in different ways. 



Limits of growth. There are limits in depth, length, and width, 

 beyond which a valley does not grow. A stream flowing to the sea 

 'tends to erode its valley to sea-level, 1 but actually reaches the sea- 

 level only near the coast. In length, the valley will grow as long 

 as its head continues to work inland. If but a single valley affected 

 a land area, the limit in length toward which it would tend would 

 be the length of the land area in the direction of the valley's axis. 

 In general, valleys are limited in length by other valleys. The 

 head of a valley works back until it reaches a point where erosion 

 toward the valley in question is equal to erosion in the opposite 

 direction. Here the divide 

 becomes permanent (Fig. 47). 

 The width of a valley is in- 

 creased chiefly by the side cut- 

 ting of the stream, by the wash 

 of the rain which falls on its 

 slopes, and by the action of 

 gravity which tends to carry 



Fig. 47. Diagram to illustrate the 

 lowering t :i divide without shifting it. 

 The crest of the divide is at a, b, and c 

 successively. If the erosion was unequal 

 on the two sides, the divide would be 

 shifted. 





down to the bottom of the slope the material which is loosened above 

 by any process whatsoever. The widening of valleys is limited 



1 Great rivers, like the Mississippi, cut their channels somewhat below sea-level, 

 for miles above their debouchures. 



