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.
The Uncompahgre [Plateau], southwest of Grand Junction, is structurally very like the White River Plateau. Its features can be well observed in Colorado National Monument. It has been elevated several thousand feet more than the Book Cliffs and Grand Valley areas to the north. Sharp folding and faulting near the Colorado River at the north boundary of the National Monument show that differential movement between the two regions was sharp and localized.
A series of northwest-trending [anticlines] along the Utah border in southwestern Colorado are of special geologic interest. They represent peculiar structures in which salt and gypsum have played a major part. These minerals were deposited in thick layers late in Paleozoic time; subsequently they were covered by thousands of feet of sand, shale, and limestone. Because of their low density and high plasticity they have since crept upward along weak spots in the overlying sediments, often contorting these rocks as they moved. Breaking through to the surface, the salt and some of the gypsum washed away more rapidly than the surrounding rock, leaving long faulted troughs such as Gypsum Valley and Paradox Valley. In most of these structures the gypsum can still be seen, although the more soluble salt has eroded away. Oil wells in this part of Colorado and in adjacent parts of southeast Utah have penetrated thousands of feet of [evaporites], including pure salt, gypsum, and potassium salts.
