106 W JMCGEE — SHEETFLOOD EROSION. 



trict. A third condition is that the soil shall be readily pervious only 

 in limited degree or to limited depths ; and this condition is met on the 

 lightly veneered baselevels adjacent to the mountains, where the mantle 

 only is porous and the under rocks sound and hard ; it is not met in the 

 deeper central portions of the valleys, where the permeable sands are of 

 considerable depth. There are also other conditions which need not be 

 noted in detail. 



The second requisite is that there shall be abundant detritus, whereby 

 the moving water may be readily loaded to the full limit of its capacity; 

 and this requisite also involves several conditions: An important con- 

 dition is absence of sward or turf to hold the earth-particles in union ; 

 this condition is fulfilled by the bare sands and naked rocks making up 

 nine-tenths or more of the Sonoran surface. It is probable that another 

 condition resides in chemical inertia of the mechanically comminuted 

 rock-matter in the dry, coupled with some chemic activity promoting 

 miscibility when wetted ; and this condition is found in the friable sands 

 and silts of the region, which form a tenaceous mud on saturation and 

 a viscid slime on flooding. A third condition is dimensional heteroge- 

 neity in the debris, so that every part of the sheetflood may be loaded 

 to its full capacity, whether its movement be swift or slow ; and this 

 condition is fully met on the upper plains, where silt, sand, gravel, and 

 boulders in all sizes and shapes are intermingled, though it is less per- 

 fectly met in the valley interiors, where the materials are more com- 

 pletely assorted. Other conditions exist, but they are apparently of 

 minor importance. 



A third requisite is that the slope of the surface shall be of a certain 

 somewhat variable value (not yet determinate save empirically as say 75 

 to 200 or possibly 300 feet per mile). The optimum or most efficient 

 slope is evidently conditioned by thickness of the detritus mantle, which 

 would appear to be considerable on the higher slopes, less on the lower 

 slopes, and so great as to be an obstruction toward the valley interiors ; 

 by coarseness of the detritus, which always is greatest on the higher 

 slopes ; by dimensional heterogeneity, which in like manner culminates 

 on the higher slopes ; by porosity and friability, and by various other 

 conditions. With slopes above the limit of efficiency, sheetflooding does 

 not occur; the detritus is simply swept away, and the under-loaded 

 storm-waters gather into streams, which carve channels. When the 

 sLope is below the limit of efficiency, the mechanism becomes clogged, 

 the declivity and consequent velocity do not permit the incipient stream 

 to overload itself quickly, and there is a tendency to assume the habit 

 of streaming rather than that of sheetflooding. The various conditions 

 of slope requisite for sheetflooding are strikingly met in the Sonoran dis- 



