68 



PHYSIOGRAPHY 



one shower is made deeper, wider, and longer by the next. Year 

 by year, as the result of repeated showers and repeated meltings of 

 snows, the gully grows to be a ravine, and later, a valley. 



Not all gullies, however, become valleys. On a steep slope, 

 many gullies may start; but as they grow, some are so widened as 

 to take in others (Fig. 60), and the number is reduced. But a small 



Fig. 60. Diagram illustrating how one gully takes another as a result of 

 lateral erosion, a, b, and c, represent the cross sections of three young 

 gullies. By growth they become one, as shown. 



proportion of all that start become ravines, fewer still become small 

 valleys, and the number of valleys which attain great length is very 

 small. As valleys develop from gullies, the heads of some work 

 back faster than others, with the result that many valleys are 

 arrested in their development early, as shown in Figs. 61-63. For 

 example, c, Fig. 61, will grow in length little more, because the water 

 which falls on the land above its head flows off through some other 

 valley to the sea. Later stages in the growth of the valleys shown 

 in Fig. 61 are illustrated by Figs. 62 and 63. 



The courses of valleys. The headward growth of a gully is due 

 chiefly to the erosion of the water which flows into its upper end. 

 If all the material about the upper end of a gully is equally hard, 

 its head works back in the direction whence most water comes 

 (Figs. 64-66). But the head of the gully keeps advancing, and if 

 the surface about its head is uneven, more water may flow in, now 

 from one direction, and now from another. The result is that the 

 head of the gully is rarely worn back in a straight line. 



If the soil or rock about the head of a gully is harder at some 

 points than at others, the head of the gully is likely to advance on 

 that which is most easily worn. Inequalities of slope or material, 

 therefore, cause the head of a gully to turn now to one side, and now 

 to the other, as it advances, and where the head of the gully goes, 



