27. 



SOUTHERN ARIZONA ROCKIES 



PHYSIOGRAPHIC CHARACTERISTICS AND DIVISIONS 



Arizona is divided into three physiographic units : the Colorado Plateau 

 on the northeast, the Sonoran Desert region on the southwest, and the 

 Mountain Region between them. (See index map, Fig. 27.1.) The Moun- 

 tain Region and Sonoran Desert have been included in the Basin and 

 Range physiographic province (Butler and Wilson, 1938). 



The Mountain Region (also referred to as the Mexican highland) forms 

 a belt 60 to 100 miles wide and contains most of the large ore deposits of 

 the state. It is characterized by many short ranges nearly parallel to each 

 other and to the margin of the Plateau. The individual ranges are sepa- 

 rated by valleys deeply filled with fluviatile and lacustrine deposits which 



are now generally being eroded to widespread pediments capped by 

 veneers of gravel. 



The southern margin of the Colorado Plateau is usually taken as the 

 erosional escarpment of the Kaibab limestone and Coconina sandstone, 

 called the Mogollon Rim, but lower Paleozoic beds extend southward in 

 certain summit areas well within the Mountain Region. The Mogollon 

 Rim is at an altitude of 8000 feet or slightly less, and Phoenix near the 

 junction of the Mountain Region and the Sonoran Desert is 1100 feet 

 above sea level. The elevation of Tucson is 2372 feet and Bisbee in the 

 Mountain belt is 5490 feet. The Sonoran Desert (also called the desert 

 region) is characterized by a great preponderance of broad desert plains 

 over mountain ranges. The ranges are relatively short and far apart, and 

 generally have lower elevations than those of the Mountain Region. 



According to Ransome ( 1933 ) : 



Probably the most impressive feature of the landscape, to one who sees it 

 for the first time, is the sharp contrast between steep and rugged mountains 

 and wide expanses of desert plain. It is true that the plains merge impercep- 

 tibly with smooth, evenly graded alluvial slopes, which may attain considerable 

 altitude where they meet the mountain fronts, but the presence of these great 

 ramps of detritus scarcely detracts from the general striking contrast between 

 mountain and lowland. Such topography is, of course, characteristic of most 

 mountainous desert regions, in which the greater part of the debris washed 

 from the mountains is deposited in the adjacent valleys, these gaining in extent 

 and becoming more plainlike as the minor eminences are worm down and 

 buried. 



The surface forms of the Sonoran Desert may be classified into three 

 groups, according to Gilluly (1937c), which are: 



... (1) The mountains, commonly rugged and steep-sided, with either bare 

 rock at the surface or only a thin cover of talus; (2) the pediments, smooth 

 carved-rock plains that generally border the mountains and are strewn with a 

 thin but discontinuous mantle of gravel; and (3) the bajadas, smoothly rounded 

 alluvial aprons that slope forward into the axes of the "valleys." Of these, the 

 mountains and pediments are chiefly carved by erosion; the bajadas are chiefly 

 depositional. 



A glance at the Geologic Map of the United States will show that the 

 ranges of the southern California and Arizona and southwestern New 

 Mexico are smaller, more irregular in shape, less linear and parallel, and 



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