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Show unique in geologic history, topographic form, and scenic grandeur. Within it are displayed the oldest and youngest rocks exposed on the North American Continent, and the major subdivisions of the geologic time scale are represented in orderly succession. It is a vast expanse of plateaus, terraces, mesas, and cliffs that seem to have an unlimited range in form, size, and color. Over an area of about 130,000 square miles in Utah, southwestern Colorado, northwestern New Mexico, and northern Arizona, plateaus that rise 5,000 to more than 10,000 feet above sea level are so numerous and prominent that the region well merits the various applied names- the Colorado Plateau province, the plateau province, the Colorado plateaus, or, considered as an areal unit distinct from surrounding regions, the Colorado Plateau. Outstanding features are the widespread Triassic, Jurassic, and Cretaceous strata in approximately horizontal position, the gigantic cliffs and the multitude of canyons that carry the perennial, intermittent, and ephemeral run- off. The sedimentary strata that lie above the meagerly exposed Paleozoic rock extend hundreds of miles with little change in character. They include series of shales 1,400 to 2,200 feet thick, of limestones 1,000 to 1,500 feet thick, of volcanic tuff 6,000 feet thick, and many single beds of sandstone 200 to 1,500 feet thick. In the walls of Zion Canyon the remarkable Navajo sandstone measures 2,280 feet- the thickest unit sedimentary bed so far known. In succession, above the strongly predominant sedimentary rock extensive sheets of lava characterize the landscape in widely scattered areas. The edges of both these sedimentary and igneous beds are fully exposed in vertical, unscalable cliffs of seemingly interminable length that advance in headlands and retreat in bays, tower above the adjacent lower lands, and from their crests extend backwards to the bases of similar escarpments. Substantially the rocks in each of the great cliffs are of different geological age and are marked by distinctive colors- Lower Triassic, brown and light red; Upper Triassic, vermilion or brilliantly variegated; Jurassic, uniformly white or red, or regularly banded; Cretaceous, gray; Tertiary, pink, extending up and into white; and, the Tertiary and Recent capping lavas, black. The major features of the plateau topography are so consistent that in distant views the long stretches of even skyline give an impression of extensive flat surfaces that terminate in lines of cliffs. The plateau country in reality is intricately dissected and closer inspection reveals a ruggedness possessed by few if any other regions. Over large areas the canyons are so narrow, so deep, and so thickly interlaced that the region seems made up of gorges, cliffs, and mesas and platforms, intimately associated with a marvelous variety of minor erosion forms. Some of the topographic features are developed on a scale that in other regions would justify the term " mountains." However, the plateau is essentially devoid of high peaks and narrow, serrate ridges. Like a deeply engraved intaglio, its departures from the regional surface are downward rather than upward. For making a land of canyons, the physiographic conditions in the plateau province are especially favorable. The region lies at high altitudes; its climate is generally arid; its major rock strata are thick, resistant, and lie nearly flat; its longer streams receive abundant water from the adjacent humid Rocky Mountains and are at the stage in their life history where their ability to grind up rock and to transport the resulting debris is greatest. Consequently, innumerable deep, narrow trenches are cut into strata that otherwise remain intact, and the great beds of sandstone and limestone terminate in vertical walls. In striking contrast to the wide open valleys, the graceful slopes, and the rounded hills of most other regions, the topography of the plateau lands is abruptly angular, even in minor details. Though the Colorado Plateau is generally floored with horizontal or slightly inclined sedimentary strata, its surface has been roughened in places by folding, faulting, the building of volcanoes, and the intrusion of igneous rocks- geologic activities that have modified the otherwise normal drainage pattern and the orderly development of erosion features. Of the folds, many are merely " rock wrinkles" of slight importance, but the larger upwarps and downwarps have produced elongated domes 90 to 150 miles long and 40 to 75 miles wide that in the present topography stand 1,000 to more than 3,000 feet above the adjacent land. Particularly conspicuous are the mountainous upwarps that brought into existence the isolated Uncompahgre Plateau, the San Rafael Swell, the Kaibab Plateau, and the Waterpocket Fold, the Defiance upwarp, and the Escalante anticline. Most of these folds are steeper 30 |
