LAND WATERS STREAMS 121 



Limits of growth. In all cases there are limits in depth, length, 

 and width, beyond which a valley may not grow. A stream flow- 

 ing to the sea tends to erode its valley to sea-level, l but actually 

 reaches the sea-level only at the coast. The lowest level to which- 

 running water can wear a land surface is a base-level. 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 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 per- 

 manent (Fig. 82.) In width, a valley is increased chiefly by the 







Fig. 82. Diagram to illustrate the lowering of a 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. 



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. The widening of valleys is limited 

 much as their lengthening is. Adjacent valleys grow wider until 

 the tops of the intervening divides are reduced to lines. Then, 

 if erosion is equal on the two sides, the divide is lowered without 

 shifting, so far as slope wash is concerned. 



The development of tributaries. Most considerable valleys 

 have numerous tributaries. So soon as a gully is started, the 

 water flowing into it from either side wears back the slopes. A 

 slight inequality of slope, or a slight variation in the character of 

 the material, is sufficient to make the erosion of the slopes un- 

 equal at different points, and unequal erosion in the slopes results 

 in the development of tributary gullies. Tributary gullies develop 

 into ravines and valleys, the same as their mains. Every new 



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

 sea-level, for miles above their debouchures. 



