18 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



to the southwest, are of disordered structure and consist of masses of granites and gneisses 

 flanked by sedimentary rocks of Paleozoic or later age. The majority of the ranges, 

 however, are composed of Paleozoic sedimentary or metamorphic rocks, together with 

 later lavas. Most are fault blocks which have been uplifted on the southwest side of lines 

 of faulting running northwest and southeast, and have been tilted in such a way that the 

 back of the block slopes toward the southeast. The structure is not regular, for there 

 has been a large amount of secondary faulting. As none of the faulting is recent, the 

 mountains are maturely dissected. This does not mean that sharp forms of peak and 

 cliff are rare. On the contrary, many of the fault-block ranges are carved into the most 

 striking forms, and all the mountains display a great amount of naked rock. The little 

 Tucson Range, for instance, which lies just to the west of Tucson, and is composed largely 

 of andesite and other eruptives, presents one of tlie most jagged sky-lines to be found 

 anywhere in America, a striking sight against the clear sunset sky. The Sawtooth Moun- 

 tains, a few miles to the west, are of the same structure, and are, if anything, still more 

 jagged. The granitic mountains, on the other hand, are not characterized by pronunent 

 peaks. From a distance they present the appearance of great solid masses, but near at 

 hand are seen to be full of splendid deep canyons, often with precipitous walls of naked rock. 



The rockiness of the mountains speaks strongly of arid climatic conditions. Mountains 

 in a similar stage of dissection in a moist climate would be covered with soil and would 

 present graded slopes for the most part. In Arizona the slopes are largely washed bare of 

 soil because lack of moisture restricts the growth of plants and prevents the accumulation 

 of roots and fallen leaves which would hold the soil in place when heavy showers tend to 

 wash it down. The truth of this statement is apparent from the fact that the low mountains, 

 under 5,000 feet or so in height, are more rocky and on the whole more rugged than those 

 which rise liigher. The high mountains, such as the Catalinas, which, as we have seen, 

 rise to a height of 9,000 feet, enjoy a much greater rainfall than the lower portions of the 

 country, at least twice as nuich apparently. They are also cooler, so that evaporation is 

 far less active than in the hot regions of lower elevation. Accordingly the supply of 

 moisture available for plants is far in excess of that below, and the mountains above 5,000 

 feet are covered with forests. At the lower levels oaks and bushy trees of the smooth- 

 barked manzanita and its allies prevail, while, higher up, the mountains are densely clothed 

 with splendid forests of juniper and pine. In the mountains of moist lands the amount 

 of soil commonly decreases from the bottom upward. In southern Arizona the case is 

 different; from the base of the hills, at an elevation of approximately 3,000 feet, the amount 

 of soil decreases in the normal fashion at first, but after 1,000 or 2,000 feet it begins to 

 increase, and at a height of 6,000 or 7,000 it is much greater than at the base. Such con- 

 ditions can occur only in an arid climate among mountains rising high enough to receive a 

 considerable rainfall. 



(2) The Bahadas, or Piedmont Gravel Deposits. — The second element in the landscape 

 in the basin region is the vast accunmlation of gravel, sand, and silt which flanks the 

 mountains on every side. This accumulation of detrital material slopes gently away, 

 mile after mile, becoming flatter and flatter, until many of the slopes merge into level 

 playas. The name "bajada" has been appUed to such slopes by Tollman.* The Span- 

 iards use the word "bajada" to designate any sort of descent, including the process of 

 descending, but in the absence of any other appropriate term in EngUsh, I feel constrained 

 to adopt it. The word is pronounced "bahadtha," the sound of the d being neither d nor 

 th exactly. The a's have the French sound and the accent is on the second syllable. In 

 defiance of all rules I venture to write the word with an h instead of a,j, because otherwise 

 it is sure to be mispronounced. Geneticallj' it belongs to the same class as mesa, butte, 

 arroyo, playa, and others in common use. 



* C. F. Tollman: Erosion and Deposition in Soutliprn Arizona Rolson Region. .Tour. Geol., vol. xvii, 1909, p. 142. 



