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Show of the entire range is Fortification Hill- a lava- capped mesa of light- colored fanglomerate, near Hoover Dam. East of Black Mountains, across Detrital Valley, are three other prominent ranges of somewhat similar character. From north to south, they are the White, Cerbat, and Hualpai Mountains. The last- named is the largest and highest, reaching an altitude of about 7,500 feet, and has an abrupt western front caused by a large fault. Already, advantage has been taken of the summit's cool climate and good views with the development of a county recreational area. The broad geologic features are simple in most parts of this area where many flows of black basalt may be seen, one upon another or resting on an old eroded surface of granite and schist. On the other hand, details of structure in the older rocks are difficult to work out for they are much altered by metamorphism and intrusion. The region as a whole is inhospitable both because of the very hot summer climate and because of the barrenness of most of the ranges. Some geological features, such as a prominent lava dike that cuts through gravels along the highway to Hoover Dam, might be made interesting to visitors, but opportunities are limited. East- west trending ranges of middle western Arizona.- In contrast to the north- south trend of most mountains throughout the basin and range province, those ranges forming a belt across middle western Arizona are oriented in an east- west direction. These mountains are largely of very ancient crystalline rocks, although some also contain remnants of sedimentary strata of Paleozoic age. Furthermore, late basalts are common in places. Most of these ranges have complex structure, resulting from a long history of crustal disturbances. Extensive overthrusting of great mountain blocks has been recognized in various localities. Prominent among the east- west trending ranges of western Arizona are the Bucksin, Harquahala, and Huarcuvar Mountains in Yuma County, and the Vulture Mountains in Maricopa County. All of these are typical desert ranges of considerable interest from the standpoint of geologic structure and mineral wealth. The structure is very complex but in other respects there is nothing outstanding about the mountains. In general, they are relatively low in altitude. The Mohave Mountains in southern Mohave County probably also should be included in this group. The western part of these mountains is formed of volcanic rocks and in them the " Needles" - irregular pinnacles of erosion near the Colorado River- have been carved. Southeastern Arizona ranges.- Satisfactory generalizations concerning the many mountain ranges of central and southern Arizona are difficult to make. These mountains have in common a general north- south or north- west- southeast trend and most of them rise abruptly to considerable altitudes from the low, flat, alluvial plains of the desert. A majority are composed chiefly of sedimentary strata that are steeply tilted, but there are notable exceptions. The Santa Catalinas, as exposed today, are mostly metamorphic. The Galiuro and some others are largely volcanic. Each separate mountain range has a distinct and, in most cases, complicated structural history. Some involve folding, and some have their fronts raised along huge normal faults. Cross faults may or may not be present, and overthrusts are responsible for some of the ranges. In brief, a description of these ranges, adequate to illustrate the origin of each, would require a series of individual treatments and, in many instances, sufficient information is not yet available. The more prominent of the mountains in this region are the Pinal, Mescal, Dripping Springs, Apache, Santa Catalina, Santa Rita, Wetstone, and Huachuca. Mining operations are, or have been, active in a majority of these. Other types of development are negligible except in the foothills and most development is handicapped by the general lack of permanent water. Of the more prominent scenic attractions are such high peaks as Mount Graham ( 10,100 feet), Mount Turnbull ( 7,800 feet), Mount Lemmon ( 9,500 feet), and Baboquivari ( 7,500 feet). Also of note are the great steepwalled canyon of the Gila River ( 2,200 feet) in the Mescal Mountains and the weird monuments of erosion at Chiricahua National Monument. Several caverns, including Colossal Cave and Onyx Cave, are known, but none of outstanding character. 52 |