[Illustration: The Grand Hogback is a good example of the type of geologic structure known as a _monocline_. The hogback ridge is formed by differential erosion, where soft layers wear away more easily than hard layers.] Simple folds and faults have given the mesas different elevations. Thus the average elevation of the White River Plateau is 11,000 feet, that of the Roan Plateau 9,500 feet, and that of Mesa Verde only 7,000 feet. West of Durango the plateaus dip gently southward, as can be seen at Mesa Verde. Igneous intrusions and extrusions have altered plateau topography in some areas. West of Mesa Verde, for instance, an intrusive stock forms a prominent dome in the Southern Ute Indian Reservation. West of the northern Colorado mountains, and north and west of the White River Plateau, a rolling upland extends from Colorado into Utah and Wyoming. It is interrupted by the Uinta Mountains and a number of smaller related uplifts such as Juniper Mountain and Cross Mountain. South of the Uinta axis the area is known as the Uinta Basin. The northern part of this area is structurally the south edge of the Green River or Washakie Basin in Wyoming. The Rangely anticline, in the northeastern corner of the Uinta Basin, is one of Colorado’s richest oil fields; it is discussed in Chapter III. Although surfaced with much younger sediments than the rest of the Plateau Province, this area is structurally similar. On the whole, sedimentary layers are relatively flat-lying, and where they are uplifted they are deeply sculptured by streams and rivers. The sedimentary rocks in this region contain uranium and placer gold in addition to great oil and gas deposits. The southeastern part of the Uinta Basin, usually called the Piceance Basin, is the site of a great deposit of oil shale (see Chapter III). The term “basin” may here seem unusual to the casual observer, for the oil shales occur on the Roan Plateau at places well over 10,000 feet in elevation. However, the entire region was basin-like—lower than the surrounding ranges—for many millions of years, and during Tertiary time thousands of feet of valley and lake deposits were laid down in it. The White River Plateau, north of Glenwood Springs, is composed of almost horizontal Paleozoic sedimentary rocks that fold downward sharply along its south and west edges. The fold is 135 miles long and is clearly marked by the Grand Hogback, the eroded edge of hard Cretaceous and early Cenozoic rock layers. Shale and coaly layers involved in the same fold have eroded more readily, leaving the resistant sandstone as a prominent ridge.