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Show plateau district, a separate treatment will be given the volcanic areas. Plateaus on the north side of Grand Canyon. - The general character and geologic structure of the plateau blocks that form the north wall of Grand Canyon have been well- known since the early surveys made by Powell, Dutton, and others 60 to 70 years ago. The remarkable manner in which great segments of the earth's crust have been elevated along faults or fractures to form a series of steplike platforms, rising from west to east ( PL 13, Sec. 10, in pocket), has been the subject of much speculation and has greatly influenced the development of ideas concerning the causes of mountain uplift. Approached from the west along the Colorado River, the plateau province begins in Arizona, not far from the Nevada line, where a great cliff has been formed as a result of vertical movement of the area to the east along the Grand Wash fault. This upraised block is known as the Shivwits Plateau and extends from west to east for 30 miles. Its flat surface terminates abruptly against the face of a cliff formed by the Hurricane fault, and east of this is the higher Uinkaret Plateau. Two other faults, the Toroweap and Kanab, mark the western boundaries of successively higher plateaus still farther to the east. These are the Kanab and Kaibab. Finally the eastern end of the Kaibab Plateau, which has elevations up to 9,000 feet, is reached where the normally flat- lying rock strata are bent steeply downward in a monocline to the low level of the Marble platform. These four great blocks or plateaus form the northern rim of Grand Canyon, which is more than 100 miles long in air- line distance. The Kaibab Plateau is by far the best known of the areas bordering Grand Canyon on the north. This is partly because it is more accessible and therefore better known than the plateaus farther west, and partly because its higher altitude affords a more attractive summer climate. Significant, however, is the fact that while many of the finest views of Grand Canyon are to be had from points along the margin of the plateau, such views cover only one part of Grand Canyon, whereas many superlative views of other and different parts can be obtained only from the more western areas. Furthermore, though the climate of the Kaibab favors summer recreation, it prohibits any appreciable winter usage, whereas the lower areas to the west can be visited during almost any season. Geologically each of the plateau blocks is a modified repetition of the next. It is composed of essentially flat- lying strata such as are seen in the walls of Grand Canyon, but varying in proportion and in detailed character so that over many miles the surface expression of the rocks is quite distinct. Thus, the wide, green bench of shale that forms the well- known Tonto platform of eastern Grand Canyon gradually disappears westward, whereas the broad, red esplanade of western Grand Canyon is a negligible feature in the east as a result of the thinness of the Hermit shale there. The significant point, therefore, concerning the Grand Canyon as a great spectacle with unusual inspirational value is that it is distinctly different in different parts, yet large and impressive throughout. Notable exceptions to the general uniformity in geological features of the plateau are those developed by volcanic activity. In both the Uinkaret and Shivwits divisions, extensive flows of lava and numerous small craters and cinder cones blanket large parts of the surface, thus giving a new character and added interest to the region. In Toroweap Valley on the Uinkaret Plateau, this display of vol- canism is especially fine and constitutes an exhibit of exceptional value which has as yet been but little appreciated. The freshness and recency of some flows, the symmetry of many craters, the spectacular manner in which lavas have cascaded over canyon walls, examples of lava dikes leading upward from the depths, and, above all, the complex interrelationship between erosion of the canyon on the one hand, and the piling up of volcanic matter on the other, make this area outstanding as a place for stimulating the mind and furnishing inspiration. The Hualpai Plateau.- The southwesternmost section of the Colorado Plateau, bounded by the Colorado River on the north, the Grand Wash and Aubrey Cliffs on the west and east, respectively, and the Juniper Mountains on the south, is commonly termed the Hualpai Plateau. The Hualpai Indian Reservation occupies a large part of this area. In geologic structure the Hualpai Plateau is similar to the adjoining plateaus. It is formed of 41 |
