OCR Text |
Show and Navajo Creeks. At the north, overlooking the San Juan and Colorado Rivers, are the Tsegi Mesas - enormous blocks of red sandstone separated by profound canyons- and west of them is the conspicuous Navajo Mountain, whose dome- like summit ( altitude 10,416 feet), 7,000 feet above the floor of Glen Canyon, is the highest land within an area of many thousand square miles. There seems reason to believe that the prominent, detached high- lying sedimentary rocks in the Navajo country are remnants of once widely extensive formations that owe their positions to a regional uplift near the close of the Cretaceous period and that, during Tertiary and Recent times, the landscapes of pre- Tertiary times have been completely remodeled, chiefly by stream erosion. From a former general surface that stood perhaps 9,000 feet above sea level, the region has been worn down to an average altitude of about 5,000 feet, parts of it to less than 4,000 feet, and in consequence nearly all the Tertiary, most of the Cretaceous, large parts of the Jurassic, and some of the Triassic and Permian strata have been swept away. The volcanoes have been reduced to stumps, the lava flows to their feeding dikes, and deep- seated folds have been uncovered. Coincident with the removal of enormous masses of sedimentary and igneous rock, differential erosion has produced prominent topographic inequalities; flat surfaces that end abruptly at the base of terraced escarpments; mesas that rise at various stratigraphic levels above plateau surfaces, and streams that follow roundabout courses, in and out of canyons, across folds and hogbacks, with seeming disregard for rock composition, structure or regional slopes. Continuous vigorous erosion has exposed complete sections of the Permian, Triassic, Jurassic, and Cretaceous rocks and partial sections of the Tertiary and Carboniferous. In fact, the type sections for many Permian and Mesozoic formations are in the Navajo country. Furthermore, in consequence of regional denudation, igneous rocks, particularly intrusives, are unusually well displayed - seemingly extraneous features of a landscape modeled almost wholly in 38 Figure 25.- Elaborately carved frontal escarpment, Bryce Canyon National Park, Utah. |