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Show essentially flat- lying strata of sedimentary rock that have been uplifted as a block or series of blocks along great fault lines that traverse the region with a general north- south trend. The largest of these faults are the Grand Wash and Aubrey, although others of considerable magnitude also cross the region. The surface of the Hualpai Plateau, about 5,000 feet in elevation, is somewhat lower than that of neighboring plateaus because the several uppermost formations in the others have here been stripped away by long erosion. On the other hand, the surface of the Hualpai is everywhere 2,000 to 3,000 feet higher than the desert valleys to the west and south, causing it to have a temperate climate and to be favorable for year- round recreational use. The geological features of this region are primarily those of the Grand Canyon, which forms its northern boundary. Surface rock over most of the area is massive gray limestone of the uppermost Redwall ( Mississippian), although locally beds of red Supai sandstone ( Permian) remain above this, and elsewhere Devonian strata have been faulted up to the surface. Where the river has cut through these strata, forming Grand Canyon, there is a drop of about 3,000 feet and the canyon walls have the appearance of being nearly sheer. This accounts for many fine and spectacular views along the rim, as at Quartermaster Canyon and Bridge Canyon in the west and above Granite Park farther east. Geology of the Hualpai Plateau is similar and related to that of eastern Grand Canyon, yet it is different enough to be worthy of special consideration. In addition to the broad aspects of stratigraphy and structure, there are such special features as Rampart Cave, where prehistoric animals, notably the extinct ground sloth, are buried in great numbers. The green and purple shales at Columbine Falls also contain quantities of well- preserved Cambrian trilobites. Coconino Plateau.- The Coconino Plateau, extending from Grand Canyon south to the San Franciscan volcanic field ( see PI. 13, Sec. 12, in pocket) and from the Aubrey Cliffs east nearly to the Little Colorado, is the best known of the plateau blocks witAin the Grand Canyon province. It is composed of essentially flat- lying strata with resistant Kaibab limestone of Permian age forming the surface. Along the eastern margin the strata are folded into a monocline, causing them to dip under more recent rocks of the Painted Desert. Elsewhere in this plateau and along its western margin, strata have been broken by faults of varying magnitude. In general, however, the geologic structure is very simple. Eastern Grand Canyon, of which the Coconino Plateau forms the southern rim, is the outstanding feature of this area. Included is the most frequently visited and best known portion of Grand Canyon. As part of a national park, its recreational aspects have been carefully studied and partly developed. Throughout the Coconino Plateau are many minor features of geologic significance some of which, through proper development as places of interest to the visitor, would add to the recreational assets of the area. Included in this category are solution phenomena formed in the surface limestone by rain and snow waters. Examples are the Citidel fissure and Doney fissure northeast of the San Francisco Mountains, Bottomless Pits near Walnut Canyon, and the caverns south of Ashfork. Another type of feature is represented by Grand Falls of the Little Colorado. It is formed where an ancient lava flow blocked the stream course and caused the river to drop over a limestone cliff, making an inspiring sight, especially in flood season. Farther downstream is the canyon of the Little Colorado, remarkable for its proportions. It stands out in a region of canyons as one of the narrowest and most sheer- walled. Excellent views into its depths may be obtained at a few places along the Grand Canyon- Cameron highway, but much more might be done to take advantage of the canyon's inspirational and educational possibilities. Salt Springs near its mouth, where Hopi Indians obtained their main supply of salt for years, is a little- known place of considerable historic interest. East of the San Francisco Mountains toward Winslow is Meteor Crater- one of the most remarkable features in the United States and one that is internationally known. Today, a preponderance of evidence seems to favor the theory that this pit is the result of impact and explosion of a meteorite from outer space, but whether this be true or, as some scientists have suggested, a volcanic steam 42 |