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Show the symmetrical Gallup syncline has prevented the destruction of a commercially valuable coal field. The Navajo country, substantially the Navajo Indian Reservation, is the home of approximately 56,000 Navajo Indians, most of them nomadic sheepmen who follow their flocks from place to place in search of forage and water, and of 3,700 Hopis who occupy ancient villages on mesa tops and derive their support from cultivated crops, chiefly corn. A prehistoric pueblo population is represented by cliff houses, especially well preserved in Canyon de Chelly and Tsegi Valley. The region is known chiefly to professional geologists, archeologists, and graziers. The absence of paved automobile highways and the restrictions imposed by the Bureau of Indian Affairs are deterrents to general travel. Passable roads connect the administration centers but reach few places of special scenic interest. In fact, some areas of considerable size await exploration. San Juan Basin- At the eastern edge of the plateau province southward from the foothills of the San Juan Mountains at Durango, Colo., the land surface descends into the San Juan Basin- a topographic and structural depression that occupies about 12,000 square miles of northwestern New Mexico and adjacent parts of Colorado drained by the San Juan River system. The southern part of the basin is floored with nearly flat- lying Cretaceous sandstones, above which rise a few conspicuous mesas capped by rocks of Tertiary age; the northern part by southward and westward dipping Cretaceous sandstones overlain by Tertiary rocks that include shales, conglomerates, sandstones, and as much as 2,000 feet of volcanic debris. The strata in the basin have been deformed by several small folds and faults. The basin is conspicuously walled in on the west by the Hogback Ridge- a huge monocline, in which the Cretaceous beds are abruptly bent downward at angles exceeding 30 degrees. ( Plate 13, Sec. 9, in pocket.) Regional features of southern portion.- As defined in this report, the southern portion of the Colorado Plateau is that area, extending both north and south from the Grand Canyon and southeastward to the Arizona- New Mexico boundary, in which the rock sequence is chiefly of Paleozoic age. The plateau province to the north and east, including both the Navajo country and the canyon region of southern Utah differs from this southern portion chiefly in the character and sequence of the rocks. In both regions strata are flat- lying with the result that the processes of erosion have sculptured innumerable straight- walled canyons, mesas, and tablelands. In the southern area, however, the rocks are of an older group ( Paleozoic) than those upstream ( Mesozoic) and basic differences in these strata have profoundly affected various aspects of the landscape. Contrast between physical features of the Grand Canyon district and those of the basin and range province to the south and west is great. The characteristically horizontal strata of the former are in marked contrast to the steeply tilted or much- folded layers of the other. Flat- topped hills and level skylines are replaced by jagged ranges and irregular peaks. Even where the same strata that form the walls of Grand Canyon are represented in mountains to the south and west, features of the landscape are totally different because of differences in rock structures. The southern portion of the plateau province is divisible into 10 principal sections, separated one from the other by major lines of displacement such as faults or folds. Each section is in reality a block or segment of the earth's crust raised more or less vertically to its particular elevation high above sea level. North of the Colorado River at Grand Canyon are five of these segments or plateaus as shown on Plate 13, Section 10 ( in pocket). Two others lie south of Grand Canyon, and an eighth extends southeastward forming the southern margin of the Colorado Plateau in this area. ( PI. 13, Sec. 12, in pocket.) Finally, near the New Mexico- Arizona boundary two additional sections are assigned to this province because of the similar rock types involved, even though the strata are more folded than elsewhere, making dome structures. ( Plate 13, Sec. 11, in pocket.) Within the plateau province and along its borders are areas of varying size, the surface features of which have been formed through the agencies of volcanism. Some of the features are spectacular; all of them have profoundly modified the general appearance and character of the landscape. Because they differ so from other features of the southern 40 |
