OCR Text |
Show The principal subdivisions of the basin and range province as recognized here are ( 1) the sedimentary ranges of the Arizona- Nevada corner; ( 2) the desert ranges south of Lake Mead; ( 3) the east- west trending ranges of middle western Arizona; ( 4) the southeastern Arizona ranges; ( 5) the southwestern Arizona ranges; and ( 6) the Colorado River delta, and Salton sink area. Arizona- Nevada corner.- That portion of the basin and range province that includes the southern part of Nevada, northwestern Arizona, and southwestern Utah differs from neighboring parts of the province in the Colorado River drainage in that most of its ranges are formed largely of the same sedimentary strata found in the adjoining Colorado Plateau. These ranges, in general, have a north- south trend, but they differ considerably in structure. Some are relatively simple, being formed by the tilting of rock layers through movement along normal faults, but others are extremely complex and involve folding, thrust faulting, and several stages of normal faulting. The Muddy Mountains and the Spring Mountains represent the more complex types. Over- thrusting of rock strata on a huge scale is especially well illustrated and easy to see in the Muddy Mountains and might well be featured as an educational exhibit. Folding, compound normal and reverse faults forming wedges of strata, and examples of rock replacement near faults can be well illustrated in both ranges. Examples of mountains developed with simple structure are the ranges through which have been carved Iceberg and Grand Wash Canyons on Lake Mead. These are particularly good illustrations of the effects of crustal disturbances, both because of the spectacular character of the ranges and because of their easy accessibility by boat. They also offer unusually good opportunities for illustrating to the layman the basic geologic principles involved in contrasting types of deposition, cyclic sedimentation, and differential erosion, all of which are shown especially well by the colorful beds forming the walls of these canyons. Other ranges in this area which are relatively simple in structure, and therefore easy to understand, are the Frenchman, the Virgin, and the Beaver Dam Mountains. Steeply tilted layers of rock are readily apparent in all of these and usually attract the attention of the visitor. The Frenchman Mountains as seen from the air near Las Vegas Airport are especially striking. In addition to large- scale structural features of the mountain ranges, this portion of the basin and range province contains an abundance of other geological features having interest and significance, many of which should be considered in developing a recreational program. Among these are ( 1) Gypsum Cave, Nev., where traces of early man were found associated with remains of extinct animals; ( 2) extensive lake deposits resting high above Lake Mead and illustrating clearly the former presence of a large natural lake in this area; ( 3) dikes of volcanic rock intruded into other rocks as illustrated especially well by the black dike in gravels by the highway south of Hoover Dam; ( 4) excellent illustrations of angular unconformities to be found in several places along Lake Mead and elsewhere in the region; ( 5) erosional controls in forming spectacular mesas of gravel as shown by the Temple, Napoleon's Tomb, and other hills near Lake Mead; ( 6) the remarkable erosional features developed in red sandstone at the Valley of Fire; and ( 7) deposits containing mammoth remains east of Las Vegas. Desert ranges south of Lake Mead.- A large section of the basin and range province in northwestern Arizona contains north- south trending mountains composed partly of very ancient crystalline rocks and partly of geologically young ( Tertiary) volcanic rocks in varying proportions. Sedimentary strata are absent. This type of desert topography extends over a large area from Lake Mead on the north to the vicinity of Topock on the south; from the Grand Wash Cliffs on the east to and beyond the Colorado River on the west. It is a region of rugged mountain ranges with mining operations in many places. The Black Mountains are the largest desert range in this area. They are 100 miles long and reach heights of more than 5,000 feet at Mount Perkins and Mount Wilson. At their northern end, where they are cut through by the Colorado River, Hoover Dam has been constructed. From this locality southward, the range is paralleled on the west by the Colorado. Perhaps the most unusual element 51 |