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Show and physiographic history. Bordering this central region on the east and still within the Colorado drainage basin is a belt of unusual topography developed on rock complex in structure, composition, and relationship. Particularly noteworthy are the lands along the Upper Colorado, the Gunnison Valley, and the San Juan Mountains. Upper Colorado River Valley. - In geologic make- up, scenery, and recreational resources, the valley of the Colorado River in north central Colorado is totally unlike its expression elsewhere. The landscape is that characteristic of the Rocky Mountains rather than of the plateau country. In the place of flat lands, vertically walled canyons, and cliff- bound mesas and plateaus, fairly simple and regular in topographic form and stratigraphic details, there is an aggregation of saw- toothed ridges, ragged peaks, abraded highlands, glacial cirques, and flat- floored " parks" whose origin and interrelations are difficult to interpret. The Upper Colorado and its tributaries flow in a succession of wide and narrow canyons and valleys with flaring sides, over plains, across anticlines, synclines, and faults, and from place to place trench schists, lavas, sedimentary rocks, and alluvium of varied composition and attitude. The larger tributaries have cut their way far into the granite core rocks along the Continental Divide- past the axis of the Rocky Mountain uplift - and thus bring into the Colorado drainage basin most of the water that falls on the lofty, glaciated Park Range, Gore Mountains, and the Sawatch Range, which culminate in the highest peaks of the Rockies. In the Sawatch Range about 25 peaks exceed 13,000 feet, and 10 exceed 15,000 feet. The Roaring Fork has cut from the granitic Sawatch mass the equally high Elk Mountains - a much deformed anticline of Carboniferous rocks - into which many dikes and sheets of igneous rock have been intruded. Because developed in rocks of different hardness, attitude, and stratigraphic sequence- flat lying and tilted sandstone, limestone, shale, and lavas- the remarkably sinuous streams flow swiftly between close- set vertical rock walls, between slopes gashed by ravines, or meander leisurely across wide alluvial flats. A traverse of Eagle River, the Roaring Fork, or the Colorado above the brightly colored canyon at Glenwood Springs reveals a succession of fascinating vistas, tumbling waters, quiet waters, pasture land, grain fields,, farmhouses, and above the valley walls, snowcapped mountains. An outstanding feature of the Upper Colorado Valley is the White River Plateau, about 40 miles in diameter and 10,000 to 12,000 feet above sea level ( PI. 13, Sec. 6, in pocket)- a flat- topped highland bordered by steep slopes and structurally an uplifted mass in which the deformation of its component strata is restricted to its rims. At its western edge the Grand Hogback, composed of thick Cretaceous and thinner Jurassic and Triassic strata with dips as great as 30 degrees, emerges from the adjacent Tertiary floor and stands as prominent ridges, 600 to 1,000 feet high. From the hogback across the plateau top, nearly horizontal Paleozoic rocks form the surface, and at their eastern limit are overlapped by steeply tilted Mesozoic beds, the remnants of strata that once overarched the plateau and joined those in the Grand Hogback. To attain their position and altitude, the sedimentary rocks inside the tilted borders of the plateau must have been uplifted en masse as much as 14,000 feet ( early Tertiary time), then stripped down to a somewhat uneven surface over which lava in extensive sheets was poured out ( Quaternary time). At the present time the plateau is a single topographic unit, the source of streams that flow to the Yampa, the White, and the Colorado, but as a regional structure once extended southwestward across the Colorado River, Eagle River, and Roaring Fork. Its major characteristic features in modified form are represented in Red Table Mountain and adjacent lava- capped mesas and its western terminus, the Grand Hogback, is traceable for 80 miles. Gunnison Valley.- South of the Elk Mountains the landscape presents features not represented elsewhere along the eastern edge of the Colorado drainage basin. Above a general surface, developed on Cretaceous rocks at an average altitude of about 8,500 feet, rise low domes of metamorphic and igneous rocks - the uneroded remnants of a former peneplain. In the West Elk Mountains, masses of intrusive porphyry culminate in isolated peaks that rise as much as 1,500 feet above the upturned edges of the strata that once arched over them. The dominating surface features are thick, widespread accumulations of volcanic breccia and tuff- in one 44 |
