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Along the south-central side of the basin is a prominent topographic feature, the Axial Basin, formed by an uplift whose axis has been deeply eroded exposing underlying soft rocks and forming a sharply outlined trough. Two isolated mountains rise abruptly from the floor of this trough, Juniper Mountain and Cross Mountain. The Yampa River cuts through both of these mountains in deep canyons. Along the south side of the Axial Basin is a low range of hills, the Danforth Hills, lying about 2,000 feet above the adjacent valleys. Northeast of the Axial Basin, the Williams Fork Mountains are a high hogback ridge formed by resistant sandstone layers. The Elkhead Mountains in the east-central part of the basin consist mainly of flat-lying soft sedimentary rocks protected by basalt flows. The highest peaks reach elevations of from 10,000 to 11,000 feet. Geology Rocks ranging in age from Precambrian through Quaternary are exposed in the Yampa River Basin (Figure 3 - Generalized Bedrock Geology Map). They consist of crystalline rocks of Precambrian age, volcanic rocks of Tertiary age, and a thick sequence of sedimentary rocks of Paleozoic, Mesozoic, and Cenozoic age. Areal distribution of these rocks is con- trolled by four major structural features, the Washakie Basin, the Park Range uplift, the White River uplift, and the Uinta Mountains uplift. These structural features were formed gradually over an interval begin- ning near the end of the Mesozoic and continuing through the early Tertiary. The Washakie Basin occupies all of the central and northern parts of the Yampa River Basin. It is a broad structural depression which was also the original basin of deposition for early Tertiary sediments. On the east it is flanked by the Park Range uplift, on the southeast it is flanked by the White River uplift, and on the southwest by the Uinta Mountains uplift. The oldest part of the basin geologically is in the Park Range and Uinta Mountains uplifts. Precambrian rocks make up most of the Park Range and consist of metamorphic schist and gneiss which are extensively intruded by granitic igneous rocks consisting mainly of quartz monzonite. The Precambrian rocks of the Uinta Mountains uplift consist primarily of red siliceous sandstone and quartzite of the Uinta Mountain Group. These are overlain by more than 3,000 feet of lower Paleozoic sedimentary rocks including quartzite, sandstone, limestone, and shale. Among the more prominent Paleozoic formations are the limestones of Mississippian age, the Morgan Formation and Weber Sandstone of Pennsylvanian age, and the Park City Formation of Permian age. - 6 - 1 I I '. 1 I. II I I I 1 I I 1 I I II I