SOME FEATURES OF EROSION BY UNCONCENTRATED 



WASH 



N. M. FENNEMAN 

 University of Cincinnati 



Erosion without valleys appears frequently to be regarded as due 

 to the absence of all initial inequalities which might tend toward 

 concentration of the run-off. It should follow as a corollary (and 

 this too seems quite as often to be tacitly accepted), that such erosion 

 cannot of itself perform a great geologic task, for, in nature, condi- 

 tions of absolute equality are rare. If exemption from valley-cutting 

 is due to such an exceptional condition, the wonder is that any broad 

 slopes remain and that valleys do not branch indefinitely. Yet the 

 observation is common that in loose and homogeneous material, 

 the head of a gully is a perfectly definite thing, and that while some 

 large gullies do arise from the union of smaller ones, this subdivision 

 in a headward direction is not carried to microscopic dimensions. 



If a broad slope without valleys is possible only with a nice equality 

 of rills, its existence must be highly precarious, but it must be recog- 

 nized that for every dissecting land surface there is a degree of minute- 

 ness beyond which dissection will not go, and when this degree is 

 reached the slopes are in no danger whatever of further dissection. 

 The exact degree of minuteness of dissection (or what has sometimes 

 been called the texture of the topography) depends on a number of 

 factors not here discussed. The purpose of this paper is to point 

 out and account for that limitation and to examine some of the 

 topographic effects of erosion without valleys. It should be made 

 clear that such erosion is not in the main dependent on equality of 

 conditions, and that among rills or runnels on a broad hillside there 

 is not always a tendency to grow larger, hence not always a contest 

 for mastery or a struggle for existence. 



Conditions assumed. — The simplest and typical case for the study 

 of this principle is that of the plowed field or other surface of homo- 

 geneous material. A sod cover may temporarily hold its own against 



746 



