OCR Text |
Show canic necks. At the Elkhead Mountains, a lone mass of metamorphic and igneous rock that culminates in Hahns Peak ( altitude 10,824 feet) interrupts the continuity of the flat- lying Tertiary beds. Near its mouth the Yampa has cut deeply into the steeply tilted rocks of the eastern Uinta Mountains, exposing the edges of Paleozoic and Mesozoic strata. The Yampa also transects Cross Mountain and, farther east, Juniper Mountain, which are short domelike masses that rise abruptly from the regionally flat surface. Like Douglas Mountain and Spring Mountain north of the river, these conspicuous landmarks are anticlines once arched over by Cretaceous rock but now so deeply eroded that the Cambrian and Pre- Cambrian core rock is exposed, and against it the Paleozoic and Mesozoic formations lie in orderly sequence. In shape and structure these isolated masses are so like the Uinta Mountains that they have been dubbed " Little Uintas." Also suggestive of the Uinta structures is the remarkable Axial Basin- a valley formed by stripping away the crest of an anticline- which crosses the Yampa River and farther southeast merges with folds of the White River Plateau. The sequence of strata shows that the anticline is post- Eocene in age; the date of the second up- arching of the Uintas. South of Juniper Mountain, beyond the Axial Basin, the land rises to the anticlinal Danforth Hills, which stand 2,000 feet above the bordering lowland and outline the drainage divide between the Yampa and White Rivers. Here in the Cretaceous rocks, Goodsprings, Wilson, and Morgan Creeks, and Maudlin and Temple Gulches have cut deep channels across hogback, ridges, and the Collom syn- cline. From Juniper Mountain to Steamboat Springs, Colo., except near the mouth of Elkhead Creek, where Teritary rocks are exposed, the Yampa and its southern tributary flow on Cretaceous rocks, developing broad valleys and rounded hills in soft shales, and rugged landscapes of ridges, mesas, and " pagodas" in the most resistant sandstone. The many- branched Williams Creek, Trout Creek, and the Upper Yampa start abruptly at the Flat Tops- extensive sheets of basalt and andesite. East of the latitude of Steamboat Springs, characteristic landscapes of the Yampa Basin entirely disappear. In the topography the plainlike surfaces developed on approximately horizontal beds are replaced by rugged mountains trenched by steeply inclined streamways and, as stratigraphic units, the dominant Tertiary formations are replaced by schists, gneisses, and granites. In other words, the southeastern edge of the Green River Basin- the northeastern edge of the Colorado River drainage basin- lies on the west slope of the Park Range and its features are those of the Rocky Mountains rather than of the plateau lands farther southwest. If approach roads were available, the deep meandering canyon of the lower Yampa and a large part of the ruggedly carved landscape along its upper tributaries, now accessible only on foot or horseback, would add much to the recreational resources of northwestern Colorado ( Plate 13, Section 5, in pocket.) White River Valley.- In transporting White River Valley surface water and rock waste from northwestern Colorado to the Green River, the streams that drain the Yampa River Basin are assisted by those within the similar, but much smaller, White River Basin. Like the Yampa, the meandering White and its major tributaries have developed most of their channels in nearly horizontal Tertiary strata and have cut into Cretaceous and older rocks only where anticlines lie athwart their long- established route. In the soft Tertiary rocks, the streams have cut innumerable runways and thus have carved the surface into flat- topped ridges, mesas, and towers which are well displayed at Cathedral Bluffs, Gray Hills, Powell Park, and Douglas Peaks- features of the " Goblin City" mentioned by diarists of the second Powell Expedition. ( Plate 13, Section 6, in pocket.) UINTA MOUNTAIN PROVINCE Uinta Range.- As a highland belt 25 to 40 miles wide with an average elevation exceeding 11,000 feet- 2,000 to 6,000 feet above the peripheral lowlands- the Uinta Mountains are the dominating topographic feature of northeastern Utah between the Wasatch Mountains and the Green River. In some respects the Uinta Mountain region- the lofty Uinta Range and the Uinta Basin along its southern base- is unique. In contrast with the general north- south alinement of similar topographic features elsewhere in the Colorado drainage basin, in fact, within the United States, the mountain mass, its 25 |