64 WORK OF RUNNING WATER 



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 

 being shifted in position. 



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. Any 

 slight inequality of slope or material makes the erosion of the slopes 

 unequal at different points, and unequal erosion in the slopes 

 results in the development of tributary gullies. Some of these 

 gullies develop into ravines and valleys, the same as their mains. 

 Every new valley facilitates the run-off of the water which falls on 

 the land, and so helps along erosion. 



Struggle for existence among valleys and streams. It is not 

 to be inferred that every gully becomes a valley, nor that every 



small valley becomes 

 a large one. The 

 number of little gul- 

 lies which develop on 

 a slope may be very 

 large (Fig. 41); but 

 Fig. 48. Diagram illustrating how one gully takes the history of many 

 others as a result of lateral erosion. The lines 1-4 o f them is short If 

 represent, in cross-section, four stages in the develop- ,. ,,. 



ment of gullies a, b, and c. adjacent gullies are 



of unequal depth, the 



growth of the larger finally removes the divide between them, and 

 they become one (Fig. 48). Again, a good map of the north shore 

 of Lake Superior or the west shore of Lake Michigan shows a large 

 number of small valleys and gullies (PI. III). No equal stretch of 

 coast has so great a number of large valleys. It therefore seems 

 evident that of these many small valleys a few only will attain 

 considerable size. 



Some young valleys work their heads back into the land faster 

 than others, because of inequalities of slope and material. If 

 valleys develop in ways other than by head erosion, the chances 

 are also against their equal growth. If two streams, such as a 

 and c, Fig. 49, develop faster than the intermediate stream b, it 

 is clear that their tributaries may work back into the territory 

 which at the outset drained into b, so as to cut off the supply of 

 water from the latter stream (compare a'b'c', Fig. 50). As a result, 



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