78 GEOLOGY. 



from some direction other than that in the direct hne of its extension. 

 In Fig. 59, for example, more water may enter from the direction of h 

 than from that of a. Since most wear is hkely to be affected along 

 the line of greatest inflow, the head of the gully will be turned in that 

 direction (Fig. 60). Started in this course it will continue in the new 

 direction so long as erosion in this line is greater than that elsewhere; 

 but whenever the configuration of the surface causes more water to 

 enter the head of the gully from some direction other than that in 

 which it is headed, the line of axial growth is again changed, as toward c, 

 Fig. 60. Since new land surfaces are probably more or less undula- 

 tory, crookedness should be the rule among valleys developed from 

 gullies by head erosion. Streams and valleys the courses of which 

 are determined by the original slope of the land are said to be consequent. 

 Inequalities of material, leading to unequal rates of erosion, effect 

 the same result, in the absence of inequalities of slope. If at any stage 



of a valley's development erosion 

 were equal in two directions at its 

 head, and at the same time greater 

 than at points between, two gulUes 

 would result (Fig. 61) diverging from 

 the point in question. 



In the case of a valley developed 

 Ftg. 61.— Diagram illustrating the de- by overflow from a lake its course 

 velopmentof two equal gullies from ^^ determined by the lowest line of 



the head of one. *^ 



flow to which the water has access. 

 If this line be straight the valley will be straight ; if it is crooked, as it 

 generally is, the valley is crooked also. 



The development of tributaries. — Thus far valleys leading immediately 

 to the sea have been considered, and no account taken of tributaries. 

 As a matter of fact most considerable valleys have numerous tribu- 

 taries. It is now in order to inquire into their mode of develop- 

 ment. 



So soon as a gully is started, the water flowing into it from either 

 side wears back the slopes. The least inequality of slope, or the least 

 variation in the character of the material, is sufficient to make the lateral 

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

 results in the development of tributary gulhes. The oldest tributaries 

 may be nearly as old as the main which they join, and from which they 

 developed, for the possibilities of unequal side erosion exist as soon as a 



