162 C. F. TOLMAN 
line with a lower angle of slope, a state of affairs that will result in 
the gullying of the bajada, while curve ¢ indicates a condition that 
would result in rapid deposition. That such an erosion of the upper 
edge takes place irrespective of the conditions at the foot of the slope, 
is indicated by the fact that there are gullies which develop in the up- 
per slope, but fade out before the bottom of the outwash is reached. 
Furthermore, an important criterion is discovered at this point. 
Should the bajada be gullied sharply from top to bottom, the cause 
may well lie in some change affecting the level of the playa, or the 
outwash drainage of the semi-bolson. North of Tuscon, in the Santa 
Catalina Mountains, there are remnants of an older and much higher 
bajada, than the well-developed lower and gullied slope. Prelimi- 
nary studies suggested that this erosion might be connected with cli- 
matic change, but later it became evident that this was directly 
connected with the drainage developing at the foot of the slope. 
Theoretical analysis, then, teaches us to look for variations of 
climate written in the upper slopes of the bajadas. The following 
is a list of those that ought to be most easily recognized: 
Change in daily temperature difference variable, other factors 
remaining constant. 
Change in torrential concentration variable, other factors remain- 
ing constant. 
Change in plant covering variable, other factors remaining constant. 
More effective still would be the immediate result of a decided 
swing, say from an effective daily temperature difference, to a marked 
torrential concentration, or the destruction of a heavy plant covering 
by aridity and a torrential concentration. Such possible factors 
were considered in attempting to explain the formation of the decided 
gullies that issue from the larger canyons of the Santa Catalina 
Mountains, and have incised the recent bajada,* but on close con- 
sideration it seemed more probable that these could best be explained 
by the fact that they drain a large area back in the mountains. The 
higher watershed receives more of the non-torrential winter rains, 
while the lower lands receive a larger proportion of their precipitation 
in the summer months and of a markedly torrential variety. The 
winter streams maintain a constant flow for several months, and are 
t See the Topographic Map, Tucson Quadrangle, Ariz., V.S. G. S. 
