EROSION IN ARIZONA BOLSON REGION 145 
that everything loose on the surface is swept down into the 
steep canyon. The grade of the canyon decreases below, and 
deposition commences, boulders first and the rest following in 
decreasing size according to the v® formula for mass to velocity. 
Here the ordinary backing-up effect is not important. Permanent 
streams generally develop deposition at a certain point, and this 
works back, the current damming itself up. The torrent, however, 
drops its coarse at the top and the rest continuously on its downward 
path, resembling a single wave rather than a continuous current, 
so that the effects of deposition are not readily transmitted back- 
ward. Wash deposits often show simultaneous dropping of coarse 
and fine together, as a result of rapid checking of velocity, the forma- 
tion consisting of a conglomerate of pebbles or boulders set in a 
matrix of sand or mud, the boulders carried down by the deepest 
rush of the torrent, and filled in with sand by the subsiding water. 
Such a deposit indicates clearly the agency of torrential flood. Far- 
ther down there may be a spreading-out of the flood as a widening 
sheet of mud and water. This is the flood sheet, and it may be 
either the expanded lower portion of the torrent from far above, or 
the even sheet of run-off from a shower on the lower detrital slope. 
The coarse material has been dropped, and the fine is now carried 
wholly in suspension. 
The ordinary conception of a loaded stream is one that is picking 
up material to its full capacity, and also one that has the fine material 
at hand so that the picking-up and the laying-down are in equilibrium. 
To increase the capacity of a stream to carry material in suspension 
therefore, it is necessary to increase either (a2) the number of bottom 
subwhirls, or (0) the number of particles cast upward in unit time 
by each subwhirl, or (c) the average upward throw of the bottom 
whirls. 
Let these factors be represented respectively by m, n, and d, and 
L the carrying capacity of a stream for fine material suspended by 
subcurrents. 
(1) L co mnd. 
We have assumed certain whirls distributed over the bottom, 
Tepresenting the average upward throw of the stream, and that fine 
