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Show drained by the Dirty Devil and San Rafael Rivers. Farther upstream the Colorado River is bordered on the north by the Roan Plateau and on the south by the Uncompahgre Plateau and Grand Mesa. In valleys tributary to the Colorado River, plateau topography persists. In Green River Valley, East Tavaputs and West Tavaputs Plateaus, parts of an otherwise continuous highland, are separated only by the profound Gray Canyon; in the San Juan Valley, Grand Gulch and Chaco Plateaus, and the plateau- like Mesa Verde are prominent features; and above the floor of the Little Colorado River, continued as the Rio Puerco, stands the Moenkopi, Manuelito, and Dutton Plateaus. In topographic expression these extensive river- border platforms, which lie at relatively low altitudes, are similar and all are remnants of high- lying lands but, in consequence of crustal deformation and varying stages of erosion, they display rocks of different ages and in somewhat different attitudes. Thus the eastward tilted Shiv- wits, Uinkaret, and Kanab Plateaus are outlined by faults, and the Kaibab Plateau is a sequence of bowed strata bordered by monoclines. As these four plateaus occupy the area of greatest uplift, and consequently of unusually vigorous erosion, they have been worn down the most- to rocks of Permian age that farther north in the plateau country are exposed only in canyons. The adjacent East Kaibab monocline and, a short distance beyond, the Echo monocline, have so lowered the strata that along Glen Canyon, rocks younger than the Triassic are buried. In fact, throughout the Colorado drainage basin the rocks in most of the plateaus are of Triassic, Jurassic, Cretaceous, or Tertiary age. The Uncompahgre Plateau, 20 to 30 miles wide and nearly 100 miles long, with summit altitude of 9,000 to 10,000 feet, is a flat- topped anticline in which the core of Pre- Cambrian granite is largely covered by Triassic and Jurassic rocks, which are magnificently displayed in Colorado National Monument. In forming the plateau, the strata of Paleozoic age were stripped away from a former highland mass before younger sediments were deposited. Throughout its life the Uncompahgre seems to have determined the local drainage pattern. On approaching it, streams from the Rocky Mountains and the northern San Juan Mountains are deflected; they follow the edge of the upwarped highland. Even the powerful Colorado, which once made its way through the barrier, has chosen an easier course - a great curve around its north end. Its abandoned runway through the deep, narrow Unaweap Canyon, 53 miles in length, is now an automobile highway. It seems strange that this outstanding illustration of stream adjustment has received so little attention. Grand Mesa, " the world's largest flat- topped mountain," is an isolated outlier of the Roan Plateau, 50 miles distant across the Colorado. In it the Tertiary and older rocks are preserved by a thick cap of basalt. Its glaciated lake- dotted surface covers an area of 900 square miles at an altitude of 10,300 feet- 5,000 feet above the surrounding lowlands. North of it stands the smaller Battlement Mesa- similar in form and geologic history. Mesa Verde, 250 square miles in area, is a portion of a widespread highland, in essence a promontory extending from the San Juan Mountains, from which it is separated by the broad valleys of the Mancos and Upper Dolores Rivers. In distant view, its surface, at an altitude of 8,000 feet, appears to be smooth and continuous and, in conformity with the dip of the strata, to slope southwestward. In reality the mesa is ragged. Except along its northern edge, it is so elaborately dissected by scores of south- flowing, box- headed canyons and gulches that the interstream spaces are reduced to narrow strips. The whole frayed highland has been likened to a " worm- eaten log." The mesa is surfaced with resistant sandstone about 1,000 feet thick ( the Mesa Verde group) and underlain by the easily eroded Mancos shale, 1,800 to 2,000 feet thick, thus providing conditions favorable for the rapid making of great cliffs at the edge of the strata while their surfaces were but slightly abraded. The northern face of the mesa is an escarpment 1.500 to 2.000 feet high. In sequence of geologic events, a regional uplift permitted vigorous erosion of the ancient highlands; the more exposed and the softer rocks were stripped clown to the massive sandstones. In late Tertiary times the mean surface was doubtless coextensive with the peneplained surface of the southwestern San Juan and adjacent areas in the plateau province. Renewed uplift and erosion brought the structure to its present form. The high plateaus of Utah.-- Of the 10 great plateaus that dominate the topography of south 36 |
