Dry-Farming in Arizona 



By A. M. McOmic 



Assisted by C. R. FILLERUP and L. L. BATES 



Edited and Revised by H. C. HEARD 



GEOLOGY AND SOILS 



Arizona is divided into two main physiographic provinces, the 

 Colorado Plateaus and the Arizona Highlands; but some phy- 

 siographers recognize a third, including the southwestern part of 

 the State that lies within the Colorado Desert. Because of the very 

 limited rainfall in this last division it cannot be considered a region 

 with dry-farming possibilities ; consequently, only the first two 

 provinces are discussed in this bulletin. 



THE COLORADO PLATEAUS PROVINCE 



The Colorado Plateaus Province includes northeastern Ari- 

 zona, southeastern Utah, southwestern Colorado, and northwest- 

 ern New Mexico. In Arizona its southwestern border runs fairly 

 straight from the northwest corner of the State to a point slightly 

 south of the middle of the eastern boundary. The Colorado Pla- 

 teaus Province consists, mainly, of a series of table lands 5000 or 

 more feet above sea level with occasional intrusions and extrusions 

 of igneous rocks, particularly in the southeast portion. 



In the geologic history of the region three erosion cycles are 

 evident. The first, a monoclinal folding and faulting, gave a pro- 

 nounced relief to the region. This deformation formed a series of 

 plateaus, descending from west to east, which were subsequently 

 eroded to a peneplain. The second period of faulting left features 

 which are still prominent and reversed the general slope, causing 

 the descent of the series of plateaus from east to west. In the sub- 

 sequent erosion there was no great vertical reduction of surface, the 

 chief result being the denudation and horizontal stripping back of 

 surface strata. In this period a widespread system of shallow but 

 mature valleys was formed. In the third erosion cycle deep can- 

 yons and gorges were formed, the most interesting example being 



