148 C. F. TOLMAN 
friction, the final layer of mud being left at or toward the bottom 
of the slope. 
I have described the ideal case in which the descending water 
spreads out into the flood sheet. This particular phase, described 
by McGee, and emphasized by Keyes,* while not uncommon, has 
hardly the importance attached by those observers. It is in every 
case a depositing and not an eroding sheet, save when confined to a 
valley where it can do some undercutting, and it is strange that the 
term “sheet-flood erosion”? should have been allowed to go unchal- 
lenged so long in geological literature. Final deposition on the 
margins of the bajada is more often by distributaries, as described 
by Johnson,’ although again his statement that the latter is the only 
method is not in accord with the conclusions reached here. 
In any case torrential action develops the three topographic 
surfaces mentioned above. ‘The occasional storm will accomplish 
more work, in comparison to the precipitation involved, than more 
frequent cloudbursts, as the first can carry off the entire supply, 
leaving the following storms to rework its deposits. Provided that 
there is no marked change at the foot of the slope, such as earth 
movements affecting the same, damming-back of the bajada drainage 
by expanded water sheet, or a quickening of the temporary “through 
drainage,” in the case of the semi-bolson, the initial angle of deposition 
depends upon a number of minor factors, a few of which are stated 
below in about the order of their relative value: (1) Size of abundant 
jragments. The relation of upper slope to the coarse material 
occurring in large amounts, is noticeable everywhere; (2) Density 
of precipitation, and (3) Supply of detritus. The largest bajadas 
should be developed with steepest initial slope where concentrated 
torrential precipitation is separated by intervals of sufficient time 
for the development of a large supply of detritus; (4) Height of 
mountains above foot of bajada. Attention will be again turned to 
the upper margin after climate has been analyzed. 
Wind action.—The wind has been given the leading roéle among 
t McGee, ‘‘Sheetflood Erosion,’’? Bull. Geol. Soc. Am., Vol. VII, pp. 87-112; 
Keyes, op. cit., pp.78-82. 
2 Johnson, ‘‘The High Plains and Their Utilization,” Twenty-first Annual Report . 
U.S. G.S., pp. 612-22. 
