EROSION IN ARIZONA BOLSON REGION 161 
Many of these seas were sufficiently 
each time forming a “sea. 
long-lived to develop excellent strand phenomena, and yet the basin, 
over which these advances and stays of the water took place, shows 
a most insignificant proportion of lake deposits compared with the 
detrital outwash from the mountains. It is little wonder that the 
early explorers, attributing all the depositional work to stan ing 
water, formed a very erroneous idea of the relative importance of 
lake deposition, and extended the sphere of its action far beyond that 
warranted by facts. 
EFFECT OF CLIMATE ON THE EROSION OF THE BAJADAS 
Turning again from facts to theory, let us investigate the effect of 
climatic variation on the erosion of the upper margin of the bajada. 
dese Space forbids a discussion of all the possible 
variations of climate on desert erosion, and 
moreover the profit resulting from such a 
theoretical discussion is very problema- 
tical. Even the limited inves- 
tigation presented is more for 
the purpose of showing how 
Fic. 3.—Ideal profile 
of a bolson. Curves b and 
c show the different angles ge Sy Sie tee 
~_=- ee 
ae ee Se KS 
of slope of the upper portion ES SSPE at ee a Sa eS es 
OH {dae lomievels, Ghigguisreal io! a) ye 8 eee ete SS re ers en Coa 
the text. 
complicated are the relations obtaining, and to emphasize the quota- 
tion which appears at the beginning of this article, rather than to 
develop working criteria. 
Assuming that a playa is unaffected by earth movements that 
might displace it relatively to the surrounding bajadas, or by erosional 
attack of through drainage discharging to a lower level, a gullying 
from top to bottom would hardly be expected, and a minor incision 
especially of the upper portion would be more common. The initial 
slope is profiled by curve a, Fig. 3. Curve 6 shows the equilibrium 
