THE CLIMATIC THEORY OF TERRACES. 25 



than the interior, and to show that the bahada is decidedly older than the alluvial plain 

 at the base of the terrace which borders it. The development of the calcareous deposit 

 known as caUche is also a sign that the materials of the bahada are older than those of 

 the alluvial plain. 



To the superficial eye it seems as if the bahada which stretches from Tucson south- 

 eastward in a splendid green slope for a score of miles toward the mountains were the uni- 

 form product of a single period of deposition. Nevertheless, at various points other 

 deposits belonging to higher terraces rise above it. For instance, at the end of the Speed- 

 way which runs seven miles eastward from the city, a line of low hills projects above the 

 plain. They are composed of gravel of the same nature as that of the surrounding portion 

 of the main bahada, but of somewhat coarser texture. On every side they are isolated 

 from the mountains by means of the plain of the main bahada; yet it seems fairly certain 

 that they were originally part of a higher, older bahada which bore the same relation to 

 the one on which Tucson hes as that bears to the alluvial plain, or as the alluvial plain 

 bears to the foot or two of materials which have been laid down in the river channel since 

 the last flood. A few miles to the north of the hills, that is, on the north side of the sandy 

 channel of Rillito Wash at the base of the Santa Catalina Mountains, a much more distinct 

 portion of the old bahada is seen. It here takes the form of a regular terrace of the same 

 form as the lower terrace on which Tucson is built. Above it rises still another terrace 

 whose gently sloping fiat top presents the unmistakable appearance of a bahada. It is 

 composed of gravel, very coarse here because of proximity to the mountains. Evidently 

 it once extended unbroken to the mountains on the one hand, and far out to the center of 

 the valley on the other. Now it forms a flat-topped and almost isolated plateau connected 

 with the mountains only at one or two points. At a still liigher level, traces of a fourth 

 terrace and bahada appear, but are not sufficiently distinct to allow of certain identification. 

 Similar terraces occur along every stream which I saw anywhere near the mountains 

 during three months' stay in the country. On the minor tributaries and far downstream 

 on the main drainage lines only one terrace is commonly visible, but higher up the number 

 increases along streams of any size. In many cases, similar to that just described, only 

 a single terrace appears at first, but careful examination, even without going farther 

 upstream, usually shows that there are several. As a rule the older terraces are so com- 

 pletely dissected that they have disappeared except in a few favored spots, where they are 

 preserved either as stumps, so to speak, on the lower flanks of the mountains high above 

 the limits of the lowest main terrace, or else as isolated islands in the form of flat-topped 

 hills, such as those along the southwest base of the Catalina Mountains. 



Back in the mountains toward the heads of the main streams the number of terraces 

 is conmionly four or five. Thus at the head of the Caiiada del Oro, on the northeast side 

 of the Santa Catahna Mountains, Dr. MacDougal states that there are five distinct terraces. 

 Lower down in the same valley I saw three, very perfectly developed. On the opposite 

 side of the same mountains the three, or possibly four, main terraces of the Rilhto have 

 already been described. In addition to these there is a minor terrace bordering the present 

 flood plain. In the upper valley of the Pantano, a main tributary which joins the Rillito 

 from the south just before that waterway unites with the Santa Cruz, numerous terraces 

 can be seen to the south and southwest of the Empire Ranch at the eastern base of the high 

 Santa Rita Mountains. In Gardner Canyon, for instance, the upper terrace appears as a 

 broad bahada lying close to the base of the mountains, while farther out in the main valley 

 of the Pantano portions of it can be seen as isolated hills with flat tops; like all the rest of 

 the terraces it is composed of coarse gravel full of cobblestones and small boulders. The 

 next terrace is very pronounced, and can be seen over a wide range of country. Where I 

 climbed it, the height amounts to 30 or 40 feet, and the fairly steep slope leading up to the 

 broad flat bahada is covered with loose boulders and cobbles. The third is less pronounced 



