122 



GEOLOGY 



valley facilitates the run-off of the water which falls on the land, 

 and so helps along erosion. 



The struggle for existence among valleys and streams. \\ \< 

 not to be inferred that every gully becomes a valley, nor that every 

 small valley becomes a large one. The number of miniature gul- 

 lies which develop on a slope may be very large (Fig. 73) ; but the 

 history of many of them is ephemeral. If two adjacent ones are 

 of unequal depth, the widening of the deeper narrows and finally 

 eliminates the divide between them, and the two become one 

 (Fig. 83). 



Any good map of the north shore of Lake Superior, or the west 

 shore of Lake Michigan shows a large number of small streams 



Fig. 83. Diagram illustrating how one gully takes another as a result of 



lateral erosion. 



and gullies (PL III). No equal stretch of coast has a number of 

 large valleys comparable to that of the small ones along such shores. 

 It therefore seems evident that of these many small valleys a few 

 only will attain considerable size. 



Some of the 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. 84, develop faster than the intermediate stream />. 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. 85) . As a result . 

 the growth of b will be checked, and ultimately stopped. Sim- 

 ilarly other valleys, such as / (Fig. 84), will get the better of their 

 neighbors, and many of the competitiors, as b', d f , e', and <;' (1 'IL: s ~> 

 will soon drop out of the race. Between the stronger streams 





