60 BULLETIN 54, U. S, DEPAETMENT OF AGEICULTUEE. 



THE TROUGH VALLEYS OF ARIZONA AND SONORA. 



Arizona soiitli 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 Ime, 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 diainage 

 was once to the Gila, whUe southward it joined the Altar or flowed 

 dijectly 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 

 theii' sources in higher and better watered regions, and manage to 

 maintain a precarious existence over part of theii- 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 lather 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 



