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Show verdam and Virgin Mountains; and the Colorado itself flows squarely across the Grand Hogback and the Waterpocket Fold and has cut a mile- deep trench across the Kaibab Plateau. Obviously, these streams, so strangely out of place in the present landscape, are superposed; they have inherited courses established on higher, less complex surfaces. They illustrate the superposed stream pattern characteristic of the entire Colorado drainage basin. In developing their runways by down cutting, headward cutting, and the sluicing- out of rock waste, the Colorado and its scores of tributaries have removed from their drainage basin rocks with an average thickness of about 5,000 feet, and during this process have exposed all the major geologic formations. Along the Grand Canyon, where vigorous erosion has continued longest, rocks of Pre- Cambrian and Paleozoic age lie at the surface; northward across Utah, Mesozoic rocks predominate; and in southwestern Wyoming the floor of Tertiary limestones and shales is almost continuous. In southern and western Arizona, below the great plateau region, the Colorado River and its tributaries drain an area totally different from that to the north. Rugged mountain ranges rise above broad, flat valleys like islands in the sea. The mountains contain rocks of many geologic ages and their structural histories are complex. Surrounding plains are in reality deep valleys that have been filled to thousands of feet with debris worn off the mountains within relatively recent geologic time. The present drainage follows the gentle surfaces of these valley floors between the ranges, flowing over sediments at low elevations. The Colorado River drainage basin comprises areas of unlike physiographic and geologic history. For convenience of description they are listed as provinces, each of which is discussed as a separate unit. They are the Green River Basin, the Uinta Mountain Region, the Colorado Plateau ( north and south portions), the eastern and western border lands, the Arizona mountain region, the Arizona volcanic areas, and the basin and range province. ( Plate 3, in pocket.) GREEN RIVER BASIN PROVINCE In its broad relations, southwestern Wyoming and an adjacent narrow strip in northeastern Utah, together with an area of considerable size in northwestern Colorado, comprise a topographic depression of about 30,000 square miles, shown on modern maps as the Green River Basin or the western part of the Wyoming Basin. The basin is completely enclosed except for a deep trench in its south rim, through which the waters carried by the Green River and its large tributaries- Sandy Creek, Little Snake River, Yampa River, Vermillion Creek, La Barge Creek, and Blacks Fork- pass southward and finally reach the master stream- the Colorado. ( Plate 3, in pocket.) The general floor of this regional basin consists of flat stretches and broad slopes of gentle gradient trenched by relatively broad valleys and shallow canyons, but is roughened by ridges and elongated domes sufficiently high and resistant to erosion to outline somewhat poorly defined subordinate units. The Rock Springs Dome and the White Mountains separate the Great Divide Basin- 4,200 square miles in extent- from the Bridger Basin, the latter extending along both sides of the Green River, from the foothills of the Wind River and Gros Ventre Mountains southward for 150 miles to the base of the Uinta Mountains. The lofty cuestalike Lany Rim and Cathedral Bluffs mark the boundary between the Bridger and Great Divide Basins and the Washakie Basin, and farther south the anticlinal Cherokee Ridge separates this structural depression from the much larger Yampa Basin. Though its average altitude exceeds 6,500 feet, the Green River Basin is surrounded by mountains 10,000 to 12,000 feet high. At its northeastern border stands the massive Wind River Range. On its southwest side, the broadly exposed granite core of the range rises abruptly from the flat lands of Bridger Basin along a fault, and northeastward is flanked by the truncated edges of steeply inclined strata. The Wind River Range originated as an uplift in late Cretaceous time, during which thick, horizontal Paleozoic and Mesozoic strata were bent upward in elongated anticline. Then during a long period of erosion the up- arched strata were worn down to Pre- Cambrian rocks and the mass uplifted again and reexposed to erosion by the present- day powerful streams. The southeastern border of the Green River Basin is defined by the Park Range, which essentially is a mass of ancient granite; the basal rock of 23 |