OCR Text |
Show bordering structural valleys and its principal fault lines trend east- west and, unlike that which characterizes the mountainous masses along the eastern edge of the Colorado Basin, the topography is in close accord with the structure. As a major feature of the regional landscape, the present Uinta Mountains differ little except in height from their ancestral forms; the crestline seems always to have been approximately the drainage divide and the bordering slopes to have retained their original position. The widely exposed core rock of the Uinta Mountains is the brick red, thickly bedded quartzite and sandstone of the Pre- Cambrian Uinta formation. On the flanks of this central mass, Paleozoic and Mesozoic formations in normal sequence are everywhere steeply tilted and, on the north and east sides of the mountain, broken by faults, some of great magnitude. ( Plate 13, Sections 3 and 4, in pocket.) The rocks of the Uintas contain a rich fossil fauna. The bones of dinosaurs are particularly abundant in the Jurassic Morrison formation at Dinosaur National Monument. The major events in the geologic history of the Uintas include two periods of crustal movements, two of widespread denudation, and an epoch of glaciation. Soon after the close of the Cretaceous period the rocks of the region were pushed upward in the form of an elongated fold, thus bringing strata previously below sea level to altitudes exceeding 20,000 feet ( Powell's estimate is 30,000 feet). This great up- arching was associated with down- warps paralleling its borders- on the north the Bridger Basin, on the south the Uinta Basin, and on the east the Yampa Basin. Then followed a period of erosion, an estimated 45,000,000 years, during which the range was greatly reduced in height. The bordering strata were truncated, and rock thousands of feet in thickness eroded from the highlands, the sediments being deposited in the adjacent basins, forming thick extensive deposits of sands and muds, now represented by the formations of early and middle Tertiary age. Before denudation ceased, the original lofty mountains had been worn down to a low- lying surface, probably of inconsiderable relief. In late Tertiary time, crustal upthrust was renewed, the remnant of the original arch was raised, and the edges of the flat- lying surrounding sediments were bent upward. There was thus introduced a second major cycle of erosion 818271° 50- 4 which has not yet come to an end. The new conditions have been favorable for roughening the surface of the mountain mass by cutting deep canyons and outlining peaks and ridges, especially along the borders. Twice during the glacial period the summit mountain surfaces were covered by ice. Thirty distinct glaciers, the smallest l]/ 2 miles long and the largest 27i/ 2 miles long, extended down the highland valleys. Over an area exceeding 1,000 square miles only a few peaks and narrow divides escaped burial. The peculiar work done by these stationary ice caps and slowly moving ice streams is plainly recorded by striated rocks, cirques, U- shaped valleys, moraines, and glacial lakes- about 400 of them large enough to be shown on maps. Most of the streams in the Uinta region are in accord with the topography, and also generally with the structure. They occupy synclinal troughs or descend the flanks of anticlines, and locally follow courses determined by the relative hardness and attitude of the rocks encountered. In other words, they are consequent or subsequent streams. A conspicuous exception to the normal arrangement is the Green which, after flowing southward for 150 miles over soft flat- lying rocks, abruptly turns east, crosses a great fault, meanders through the hard, steeply upturned rocks along the flanks of the Uintas, and passes entirely through the range in spectacular canyons. To account for this abnormal course involves the supposition, based on a study of the regional geology, that the ancestral Green River was established on a surface that lay far above its present canyon walls at a time when the eastern end of the Uintas was buried beneath strata of late Tertiary age. In consequence of readjusted aline- ment caused by the growth of its tributaries and by local tilting and faulting, the stream became so well established that it maintained its course, cutting through the soft rocks and on into the complex hard rocks- its present floor. Like many rivers in surrounding regions, the Green through the Uintas is superposed; its course has been inherited. Uinta Basin.- Among the physiographic features of outstanding interest in northeastern Utah and northwestern Colorado, the Uinta Mountains are matched by the Uinta Basin which covers an area of about 8,000 square miles in Utah and 7,000 square miles in Colorado drained by the Green 27 |