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50 BULLETIN 54, U. S. DEPARTMENT OF AGRICULTURE. 
THE TROUGH VALLEYS OF ARIZONA AND SONORA. 
Arizona south of the Gila River and the northern and western 
portions of Sonora form another region of great parallel ranges and 
valleys essentially similar to the Great Basin though somewhat more 
complex in the details of its structure. The trough form of the valleys 
is especially well developed north of the international line, being 
typified by the Lechuguilla and Tule “Deserts” and the Mohawk 
and Ajo Valleys to the west; the Quijotoa, Baboquivari, and Santa 
Cruz Valleys in the center, and the San Pedro, Arivaipa, and San 
Simon Valleys to the east. South of the line the topography is less 
simple and the dendritic drainage of the Altar River has cut trans- 
versely across range and valley in a way which strongly suggests 
the character of the Quaternary Mojave. 
The great troughs of the northern section resemble those already 
discussed in that they are usually higher in the middle than at the 
ends, thus creating in each a water parting north of which drainage 
was once to the Gila, while southward it joined the Altar or flowed 
directly to the Gulf. Without exception the troughs are essentially 
open in one direction or the other and in the whole region there is no 
known basin of structural origin. Furthermore, most of the drain- 
age lines are still open and, paradoxically, because the aridity has 
been too complete. The process of alluvial damming so character- 
istic of the troughs of the Great Basin has been impossible because 
the rainfall has been too meager to move the alluvium. Even the 
minimum of rainfall necessary for the formation of local playas has 
been lacking. Two streams, the San Pedro and the Altar, have 
their sources in higher and better watered regions, and manage to 
maintain a precarious existence over part of thei former channels. 
The Sonoita, the Santa Cruz, and a few other streams have a transient 
and truncated wet-weather flow. With these rare and shrunken ex- 
ceptions there is no drainage at all. An occasional cloudburst in the 
mountains is imperceptible a dozen miles below. Yet because of the 
very paucity of drainage the region is not one of great salt accumula- 
tion. It is too arid to be saline. The drainage has not decayed but 
vanished, and there is water neither for chemical rock decay and salt 
solution, nor for the carrying to areas of concentration of such salts 
as do chance to be freed. Such salt accumulations as there are are 
in the better watered valleys rather than in the worse. 
The Quaternary history of this region is a field for speculations of 
peculiar interest, and not without their present importance. Cli- 
matic changes have been continent-wide and probably world-wide, 
and the evidences of a previous lesser aridity are unmistakable in the 
region to the north. Is it not probable, therefore, that the present 
unmitigated aridity of this southern area has replaced a time of less 
extreme conditions when a more moderate desiccation permitted and - 
