| OCR Text |
Show Southwestern Arizona ranges.- In the southwestern portion of Arizona is found the driest, hottest part of the State. The nearly parallel desert ranges that slant diagonally across this area from the northwest to southeast differ from those to the north in their orientation and from those to the east in being smaller and less covered with vegetation. Furthermore, they are almost entirely lacking in sedimentary rocks such as occur in the adjoining regions. Mountain ranges in this region are of two principal types- those composed dominantly of ancient granites and schists, and those of relatively recent volcanics. Because these two varieties of rock weather very differently, mountains developed from each have distinctive shape and topographic expression. On the other hand, some ranges are combinations of both types of rock, with volcanic materials partially covering and concealing the older granites. In such cases, the resulting mountain types have mixed characteristics. In the extreme southwestern part of this area the mountains are almost all of the granitic type. They include the Tinajas Altas, the Gila Mountains, the Cabeza Prieta, Mohawk, and Sierra Pintas. All are low and characteristically weather into blocks. In the northeastern part of the area other ranges, especially the Sierra Estrella and the Maricopa, are also of this type. Elsewhere volcanic rocks predominate and tend to make the ranges rougher and more impressive. Especially rugged and massive are the Kofa Mountains, northeast of Yuma, which rise with a sheer western front high above the alluvial plain at their base and which have several narrow, spectacular gorges cut into them, including Palm Canyon. Other smaller mountains of the group are the Castle Dome and Plomosa in the west and Growler and Sand Tank farther east. The Whipple Mountains on the California side of Lake Havasu just to the northwest of Parker Dam are scenically spectacular because of their brilliant coloring, picturesque canyons and peaks of deeply eroded volcanic rocks, and the striking desert vegetation. The Eagle Tail Mountains in the north central part of this area form a narrow, prominent ridge that is remarkable because of its picturesque peaks known as Eagle Tail and Court House Rock. These mountains are formed of a fine succession of early volcanic rocks. One of the most interesting geologic features of this region is the Pinacate lava field at the international boundary southeast of Yuma. It is part of a very large area of volcanism extending southward into Mexico and is significant because of its recency. Lava is 50 to 100 feet thick with well- defined margins. Many small craters are present, all of them apparently very young. Through this rough but awe- inspiring terrain the famous Camino del Diablo passes. Colorado Delta and Salton Sink.- The delta of the Colorado River is formed in, and controlled by, one of the most remarkable structural troughs on the earth's surface. It is a depression resulting from the subsidence of a long narrow crustal block between faults or breaks, comparable to the trough of the Dead Sea and Jordan Valley in Palestine. Extending for more than one hundred miles in a north- south direction, it is submerged at one end beneath the Gulf of California and is occupied at the other by the Salton Basin which is partly below sea level. In the central part, where the Colorado has for ages been piling up its load of sediment, the history has been that of the constructional activities of the river versus the downsinking of the trough. Although much of the Colorado delta is in Mexico and therefore beyond the scope of this report, its effect on that part of California within the Salton Basin has been profound. There is evidence to indicate that despite recent subsidence, concurrent building of the delta has successfully kept the upper end of the trough blocked off from the Gulf of California. As a result, the Salton Sea is about 250 feet below sea level and has a depth of 24 feet, yet the river delta is built up to slightly above sea level. Numerous features of unusual geologic interest are found in this area, but there is some question as to what extent they may be developed for educational or other purposes because of the inhospitable character of much of the delta area. Mud volcanoes occur on the west side of Volcano Lake in Mexico, and hot springs issue forth along the San Jacinto fault. At Travertine Point, a spur from the Santa Rosa Mountains that projects toward the Salton Sink from the west shore, are seen evidences of 53 |
