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Show explosion was responsible, the crater is unique in character. The Mogollon Plateau.- The Mogollon Plateau is a southward and a southeastward extension of the Coconino Plateau. These two sub- provinces are similar in geologic structure and age, but are separated by the San Francisco volcanic field. ( See PL 13, Sec. 12, in pocket.) In both, the surface is formed largely of resistant limestone of Permian age and the strata are essentially horizontal, being disturbed only by a few breaks or normal faults and by monoclinal folds. The Mogollon Plateau ends abruptly on the south and west where high escarpments form what is known as " the Rim." Beyond this, low valleys give a very different aspect to the country. Most of the Mogollon Plateau is covered with forests which, together with the cool summer climate, make it favorable for recreational development. Springs are not abundant but are scattered throughout the area. Relatively small, but narrow and attractive little canyons such as Chevelon, Black, Clear Creek, and others, dissect its surface. Cutting back into it from the adjoining valleys to the south and especially to the west are some large and colorful canyons that are very scenic. One of these- Oak Creek Canyon- is readily accessible by a main road and is a very popular resort area offering fishing, swimming, and exceptional scenery. Some of the others, such as Beaver Creek and Fossil Creek, are attractive and interesting, but less known. Of special geologic significance in the Mogollon Plateau area are coal deposits located in the eastern part of the area near to the rim, and " the Sinks" which are to the north of this area. " The Sinks" consist of 30 or 40 bowl- shaped depressions in limestone, ranging from a few yards to about one hundred yards in diameter, formed by the solution work of water. Lesser features of interest are the spectacular dikes of black lava that extend through red sandstone in the walls of Oak Creek Canyon and the examples of columnar jointing in lava at the head of this canyon. Defiance Plateau. - Immediately west of the Arizona- New Mexico line from the vicinity of Houck on Highway 66 almost to the border of southern Utah is an upwarped area named the 818271° 50- 5 Defiance Plateau. Strata of Paleozoic and earlier age, comparable to those on the Mogollon and Coconino Plateaus to the south and west, appear here at the surface, folded into a large anticline along a north- south axis. ( See Plate 13, Sec. 11, in pocket.) Near the center of the fold, in Quartzite Canyon, very ancient quartzites project through the red shales and sandstone, showing that for vast ages an old land mass remained in this area while seas came and went around its borders. The magnitude of the uplift, together with the massive character of the sandstone involved, resulted in the erosion of numerous deep and spectacular canyons in the flanks of the anticline. Most notable are Canyons de Chelly and del Muerto, although lesser canyons such as Nazlini are also colorful. From a geological standpoint these and related features of erosion compete with th^ antiquity and history of the quartzite core for m : or interest. Southwest of Fort Defiance is a natural bridge of spectacular character carved in the red de Chelly sandstone. It is one of many scenic features sculptured by erosion along the flanks of this plateau. Zuni Mountains.- The Zuni Mountains of New Mexico are interesting geologically because they represent an uplifted area the surface features of which have been considerably modified through erosion. Structurally the area is similar to the Defiance Plateau to the northwest in Arizona in that there is a massive core of very ancient rock flanked by younger strata. In the Zunis, granite forms the core and red sandstone of Permian age has been deposited around the margins. This structure is significant from the standpoint of historical geology for it represents a positive area or region that stood above sea level during millions of years ( Paleozoic) while marine waters from several directions periodically advanced toward and retreated from it. EASTERN BORDER LANDS OF COLORADO PLATEAU The Green River Basin, the Uinta Mountain region, and the Colorado Plateau, which constitute by far the greatest and most characteristic part of the vast region drained by the Colorado River north of central Arizona, are relatively simple in topographic expression, physiographic expression, 43 |
