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Show an anticlinal uplift that in Pre- Tertiary time was deeply mantled with sedimentary rocks. In consequence of erosion, a second uplift, and recent dissection by streams, the originally arched surface of the range has been worn down to a plateau of moderate relief, above which in places rise craggy, glaciated alpine ridges to heights exceeding 12,000 feet. The eastern border of the Green River Basin between the Park Range and the Wind River Mountains is poorly defined. For a distance of 125 miles the rocks characteristic of the crests of the high ranges along the Continental Divide occupy a huge structural sag, where they are covered by sediments of younger age. In the place of the usual topographic drainage divide, the margin of the basin is a sinuous line across featureless plains where the intermittent and ephemeral streams tributary to the Green River seem to be interlaced with those flowing eastward to join the Platte. The western border of the Green River Basin is a belt of parallel mountain chains- the prominent Wyoming Range, Salt River Range, and Oyster Range- that rise 8,000 to 11,000 feet above the intermontane valleys, and southward decrease in height to less than 1,000 feet. Their component strata, chiefly sedimentary, are closely folded- much like those of the Appalachian Mountains- and are broken by faults of enormous dimensions. In the Bannock fault, 150 miles long, the rocks have been overthrust toward the east, possibly as much as 35 miles. At Aspen Ridge ( Aspen Station, Union Pacific Railroad), an anticline is crossed by three long faults, in one of which ( the Absaroka) the strata were raised more than 15,000 feet, bringing Jurassic rocks to the level of the Tertiary. These border ranges are famous as illustrations of geologic structure, and if made more accessible, doubtless would be visited each year by many students. ( Plate 13, Sections 1 and 2, in pocket.) The drainage pattern of the Green River Basin is peculiar; to an extent not duplicated elsewhere in the United States, the alinement of rivers and creeks is out of accord with the topography. Bitter Creek goes through, instead of around, the Rock Springs Dome, crosses a valley, and then plunges directly into the face of White Mountain. In the present topography an unobstructed course lies 1,000 feet below. Equally remarkable is the drainage of the Great Divide Basin, particularly the Red Desert, where the streams have no outlet. On a surface of flat land, ridges, and remnant mountains they terminate in alkaline lakes or die out on mud flats in the midst of brightly colored sand dunes- the all- year home of antelope and a winter grazing ground for sheep. The interpretation of the anomalous stream pattern- its wholesale disregard of present day mountains- involves the assumption that the Pre- Tertiary rough floor of the Green River Basin- its highlands and lowlands alike- was completely buried by younger sediments and that the courses of streams established on the new- made surface have been maintained regardless of the structure and composition of the exhumed mountains encountered in down cutting. In geologic terms, the entire drainage system is superposed. Within the Green River Basin most of the exposed rocks are sedimentary in origin and geologically very young. Lavas cap a few mesas and, at Leucite Hills, lavas, volcanic tuffs, and ash occupy about 750 square miles; in a few places Upper (?) Jurassic rocks are in sight; and Upper Cretaceous beds crop out in some valleys and inter- stream areas and are uptilted along folds and faults. But the basin is almost completely filled with Tertiary strata in nearly horizonal position. ( PI. 13, Sees. 1 and 2, in pocket.) The hard Cretaceous rocks form hogback ridges, while the poorly consolidated Tertiary strata erode into banded cliffs and picturesque castles, towers, " pulpits," and " witch rocks," in places into extensive badlands. From them have been obtained the fossil mammals, fish, birds, and plants exhibited in American and foreign museums. Yampa River Valley.- The salient Yampa River Valley topographic and geologic features of the Green River Basin in Wyoming extend southward into northwestern Colorado where an area of about 2,000 square miles drained by the Yampa River, particularly its chief tributary, the Little Snake River, is surfaced with rocks of Tertiary age. This plainlike expanse, most of it north of the Yampa, is trenched by shallow streams, and its easily eroded soft rocks have been carved into attractive small- scale badland forms. Locally, the Tertiary beds are overlain by lavas or so far stripped away as to expose Cretaceous sandstones, igneous dikes, and vol- 24 |
