748 N. M. FENNEMAN 



apparently a tacit assumption that the gully differs only in degree 

 from the rill-mark, or what amounts to the same thing, that the 

 behavior of the water in gully-making is the same in kind but differing 

 in degree from the behavior of water in rills. The attendant assump- 

 tion is that the change from the earlier condition to the later is gradual. 

 A point to be emphasized is that the two conditions differ funda- 

 mentally, not in degree but in kind, and that the change from one to 

 the other is sudden. It will be seen that this harmonizes with the 

 common observation that heads of gullies in homogeneous uncon- 

 solidated material and having a simple history, are perfectly definite 

 as to form and location. 



All streamlets above the point where continuous valley-cutting 

 begins, are here conceived of as overloaded, hence wandering, braid- 

 ing, etc., according to well-known habits. Their union is, however, 

 to some extent progressive, and when a certain stage is passed, the 

 power is more than necessary for the load, and then definite, pro- 

 gressive down-cutting begins. It is unnecessary to show just how 

 such down-cutting favors further concentration of rills at that point 

 and therefore when once begun, goes on at an increasing rate. It 

 may, however, be said that the progressive union of overloaded rills 

 is to a large extent fortuitous. 



Terms. — -The characteristic suddenness of the change from the 

 overloaded condition of the small streamlets to the cutting condition 

 after a certain degree of concentration has been reached, makes it 

 desirable that these conditions should be designated by distinctive 

 names. No such distinctive names are in use, for the good reason 

 that the distinctive character of the streamlets previous to this degree 

 of concentration, seems not to be generally recognized. In the vast 

 majority of cases the word "rill" seems to be apphed to such cases, 

 but no essential characteristic is implied by that term except smallness. 

 In the absence of a more specific term, and for the purposes of this 

 discussion, the word "rill" will be used to indicate such a streamlet 

 in an overloaded condition, that is previous to the degree of con- 

 centration necessary to cut a gully. It is not necessary to specify the 

 want of permanence since that is a necessary corollary. The con- 

 dition succeeding that of the rill is equally without a distinctive term 

 and there seems to be nothing to do but to use the word "gully stream' 



