as Haynes Well, now caved. In 1919 a well was dug at San Vicente which is 75 feet deep in alluvium and does not penetrate rock. It supplies a windmill. In the valley to the north water is obtained with deep wells, as at the Santa Rosa ranch and at Aguirre. These wells, supplemented by represos, such as that at Compartidero, are probably ample to furnish water for stock. EASTERN PORTION LOCATION, TOPOGRAPHY, AND GEOLOGY The eastern portion of the Papago country consists of the area east of the Baboquivari and Silver Bell mountains. The plains in the southern part of this area stand at altitudes of 3,000 to 3,500 feet and consequently have a somewhat greater rainfall than other parts of the Papago country. The mean annual rainfall in the Santa Cruz Valley decreases from south to north, being 14.15 inches at Nogales, 11.85 inches at Tucson, and 6.57 inches at Casa Grande. The station at the Ronstadt ranch, in the Altar Valley, has a mean of 17.65 inches for an incomplete five-year record. The area contains the Tumacacori Mountains, which consist of several more or less independent units, and the Cerro Colorado, Sierrita, and Tucson mountains. The valleys are the Altar, Abra, and Santa Cruz valleys; the last includes the valley of Nogales Wash. MOUNTAINOUS AREAS TUMACACORI MOUNTAINS The Tumacacori Mountains are a complex group consisting of several more or less independent units and not sharply separated from the Sierra del Pajarito in Sonora. North of the international boundary the mountain area is about 25 miles from east to west and 20 miles from north to south. The eastern portion consists of a ridge with a definite northerly trend, which rises about 1,000 feet above the dissected slopes that border the valleys of Santa Cruz River and Nogales Wash. Atascosa, Sardina, and Diablito mountains are well-defined summits on this ridge. Atascosa Mountain is the highest point in the area, having an altitude of about 6,500 feet. Through this ridge there are a number of practicable passes. West of the eastern ridge is a tangled mass of mountains drained by deep canyons that lead north to Arivaca Creek and south into the headwaters of Altar River. In a general way the summits reach a general level except for Chiminea Mountain, Bartolo Mountain, Montana Peak, and Fraquita Peak, which rise 250 to 500 feet higher in flat, cliff-encircled summits. With the exception of these cliffs, the slopes of this part of the mountains are smooth and soil covered, with a good growth of grass and scattering oak trees. The deep soils are developed principally on tuffs, which form the tops of the mountains. In the canyons crystalline rocks and a great series of arkosic sandstones are exposed (see p. 57), and the slopes are steeper and more broken. West of Coches Ridge the irregular clump of hills sometimes called the San Luis Mountains are bare and rocky. North of the mountains just described is a great area of dissected pediment, which forms the headwater basin of Arivaca and Sopori creeks (p. 89 and Pl. XV, 4). The spirelike summits of the Cerro Colorado Mountains close in this basin on the north. On the west the mountains southwest of Arivaca Creek and the Guijas Mountains north of the creek shut off the basin from Altar Valley. These small mountains are composed largely of crystalline rocks and have a well-defined northwest trend. This trend in the Guijas Mountains is probably of structural origin, for Guijas Wash follows a fault line that separates Tertiary volcanic rocks from the crystalline complex on the southwest.52 CERRO COLORADO MOUNTAINS The Cerro Colorado Mountains are an irregular clump of reddish peaks about 5 miles in diameter. The peaks rise from a conical platform of sloping plains, which are presumably pediments. These mountains are thought to be made up chiefly of Tertiary volcanic rocks. SIERRITA MOUNTAINS The Sierrita Mountains consist of a roughly circular area of mountainous country about 7 miles in diameter. Adjacent to this area of the sierra type of topography are a number of hills and groups of hills, which are residual eminences above the pediments that surround the main mass. The highest peak rises about 2,500 feet above the upper part of the pediment. The mountain slopes are without forests. According to Ransome, the Sierrita Mountains have an intrusive core of fine-grained biotite granite which forms the main mass. On the west the granite is intruded in a great series of schists with limestone lenses. This rock series, of undetermined age, rests unconformably on granite presumed to be pre-Cambrian. The foothills and much of the mountain pediment on the west side is underlain by these rocks. On the east side of the mountains the granite is intrusive into pre-Cambrian granite, Paleozoic limestone and quartzite, and a series of red and green shales and thin quartzites of presumed Mesozoic 52 Larsen, E. S., personal communication. 53 Ransome, F. L., Ore deposits of the Sierrita Mountains, Pima County, Ariz.: U. S. Geol. Survey Bull. 725, p. 409, 1922. age. Andesite also occurs. An extensive pediment extends from the mountains eastward to Mineral Hill, Twin Buttes, and the Tinaja Hills. It is described by Ransome as follows: For at least 5 or 6 miles from the mountain front this plain had in general a rock surface thinly covered here and there by alluvial detritus. The plain is not perfectly even but is trenched by shallow arroyos and is surmounted by clusters of conspicuous hills, some of which rise as much as 700 feet above its general surface. TUCSON MOUNTAINS The Tucson Mountains, including the Black Hills, at the south end, extend about 25 miles in a direction slightly west of north. They have an irregular border, particularly on the west side, and there are numerous passes and detached hills. The width ranges from half a mile to 6 miles. The most striking topographic features of these mountains are the irregular and precipitous summits of the main range, with cliffy slopes developed on tilted rhyolitic and andesitic lava flows and plugs. This wild and desolate topography contrasts sharply with the flat-topped somber Black Hills and Tumamoc Mountains, capped by basalt flows. The eastern slopes of the mountains are slightly less steep than the western, in harmony with the prevailing dip of the lava flows that make up the main mass. The western front of the mountains consists of Paleozoic limestone and quartzite and Mesozoic shale and quartzite,55 which are intruded by granite rocks and rhyolite plugs. Wasson (Amole) Peak, the highest point, is composed of one of these rhyolite plugs. On the west side of the mountains is a narrow pediment veneered by gravel above which rise a number of outlying knobs, of which the Picacho de la Caleria, northwest of the mountains, and Snyders Hill, to the southwest, are the most notable. On the east is Tumamoc Mountain, capped by horizontal basalt and tuff and having somber talus-covered slopes. The Black Hills and the buttes near San Xavier Mission are composed largely of horizontal basalt flows and have a similar topography. VALLEY AREAS ALTAR VALLEY The Altar Valley is about 45 miles long and from 10 to 15 miles wide. The Pozo Verde, Baboquivari, and Coyote mountains form a continuous mountain barrier on the west side. On the east side the Op. Cit., p. 412. Jenkins, O. P., and Wilson, E. D., A geological reconnaissance of the Tucson and Amole mountains: Arizona Univ. Bull. 106 (Geol. ser., No. 2), 1920. Tumacacori, Cerro Colorado, Sierrita, and Tucson mountains, separated by broad passes, form a discontinuous boundary. The southern part of the valley drains to Altar River in Mexico. The broad divide south of Buenos Aires and the adjacent parts of the valley are underlain largely by hard rocks with a veneer of gravel. North of the divide the axial stream runs north to Abra Valley in an inner trench described on page 109. The grassy plains of this valley are in marked contrast to the scrub-covered plains of other valleys. The value of the forage has led to the establishment of numerous ranches. (See pp. 374-376.) ABRA VALLEY Abra Valley is a broad plain that lies between the Roskruge, Waterman, and Silver Bell mountains on the west and the Tucson Mountains on the east. It has an indefinite boundary on the south, where the Tucson-Ajo road is usually considered the arbitrary dividing line. On the north it has an equally indefinite boundary in the slopes that lead down to Santa Cruz River. The axial stream of Altar Valley passes into Abra Valley with a very indefinite channel, but farther north, where it forms the axial stream of Abra Valley, it is dissecting the plain in places and continues through the valley to a junction with Santa Cruz River. SANTA CRUZ VALLEY The Santa Cruz Valley is a complex unit lying east of the Tumacacori, Cerro Colorado, Sierrita, and Tucson mountains. The San Cayetano, Santa Rita, and Santa Catalina mountains, which form the eastern boundary, are high, continuous, and relatively wellwatered ranges that are in marked contrast to the ranges on the west. Santa Cruz River carries relatively large quantities of water and in some parts of its course is perennial. (See p. 119.) The plains and terraces of the valley are described on page 109. The flood plain of the inner trench has a shallow water table, and here are located most of the farms and ranches. Above Calabasas Santa Cruz River and its tributary Nogales Wash are bordered by grassy meadows. The hills are covered with perennial grass, and springs and wells are numerous. GROUND-WATER RESOURCES The deep wells of Altar Valley are discussed on page 185. They are doubtless sufficiently numerous to water all the cattle the valley will support, but if necessary other equally successful wells can be drilled along the trough of the valley. In order to use all the range successfully, however, additional water supplies near the mountains are necessary. The present dug wells at the mouths of canyons in the narrow pediment on the east side of the Baboquivari Mountains are evidence that additional wells can be obtained. There are also a number of places where small reservoirs can be constructed. On the Sasabe Flat, south of Buenos Aires, deep drilled wells will probably be unsuccessful, but dug wells in the rock plains or in the beds of ephemeral streams will obtain water. There are excellent places for reservoirs where rock spillways can be obtained, as at San Fernando. On the east side of the valley the pediments are extensive, and dug wells can be sunk with success in the beds of all the larger washes. For several miles north of Guijas Wash the rocks are mostly lava, and it is possible that properly located drilled wells will obtain larger and more permanent supplies. Abra Valley has a meager vegetation, and it may be that the wells at the Robles, Abra, and Cocoraque ranches and along Santa Cruz River are sufficient for the cattle the range will support. Deep drilled wells will obtain water in the center of the valley. There are now at least two represos in the valley, and improvement of these structures may be sufficient to permit the use of the range. The western slope of the Tucson Mountains should furnish sites for dug wells, but there are large areas of limestone which may be drained of water by underground outlets. In this vicinity there are numerous old mines and prospects, some of which may furnish enough water to serve as wells. The ground-water problems of the Santa Cruz Valley are outside the scope of this report. Increase of irrigation and of population in this valley should, however, direct increasing attention to the full use of adjacent range. Numerous wells will then be dug along the mountains. The extensive pediment east of the Sierrita Mountains is favorable for such wells, and there are numerous abandoned mining shafts that could be used as wells. GUIDE TO WATERING PLACES DISTRIBUTION AND TYPES OF WATERING PLACES The watering places of the Papago country differ widely in the quantity and character of water and in the facilities they afford travelers. Certain types of watering places are almost peculiar to the region; they are rare elsewhere. The valley of Santa Cruz River and its tributary Nogales Wash from Nogales north to Tucson is an almost continuous watering place, containing many wells. During a large part of the year the bed of Santa Cruz River and the Gila Valley from Florence west to Gila Crossing and from Buckeye to Gila Bend carry water. In the irrigated area around Phoenix, which is supplied with water 104211-25-18 |