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Show on their east sides where, in consequence of erosion, the strata that once overarched them are represented by remnant beds steeply inclined along their flanks. Few features in the plateau are more conspicuous than the upturned strata in the high, rugged hogbacks that characterize the Waterpocket, Comb, Echo, and East Kaibab monoclines. The larger folds, perhaps all the ridge- making folds, in the plateau country are the result of compressive forces directed horizontally from the east and confined to the sedimentary strata. Another type of upwarp in which the strata were raised by the intrusion of igneous rocks- laccoliths- is also exceptionally well represented. The loftv Navajo, Henry, Abajo, La Sal, Ute, and Carrizo Mountains, and similar structures in the Rico and La Plata Mountains, are essentially igneous masses that once were completely overlain by sedimentary rocks. In all of them except Navajo Mountain, which retains its cover, erosion has exposed the igneous cores. Similar to the laccolithic mountains in manner of formation are the peculiar faulted domes in the vicinity of the La Sal Mountains, where the strata have been deformed by the intrusion and expansion of masses of salt and gypsum. In the southwestern part of the plateau province, where conspicuous folds are rare, the strata have been deformed by faults so large in dimensions as to completely remodel the landscape. Thev have sliced the region into enormous earth blocks, re- alined streams, and formed cliffs comparable in height and length to the towering escarpments developed by erosion. In roughly parallel position the Paunsaugunt, Sevier, and Hurricane faults, each nearly 200 miles in length, and the shorter Grand Wash fault, extend from south central Utah into Arizona and southward across the Colorado River. Along the east side of these fractures the strati- graphic formations have been raised a few hundred to as much as 7.000 feet above their counterparts on the west. The effects of faulting are typically expressed in the Zion National Monument, where the sharply defined Hurricane Cliffs, produced by the Hurricane fault, separates the flat- lying cultivated plain about New Harmony and Kanarra- ville from the uninhabitable Kolob Plateau, which stands more than 3,000 feet higher. The remarkable unevenness of the regional surface produced by great faults has been intensified by the building of volcanoes and the outpouring of lavas during Tertiary and Recent times. Above the general surface of the Uinkaret Plateau in the southern portion of the province more than 150 ash and cinder cones rise to heights of 50 to 500 feet, and the tabular masses of lava that cap Trumbull, Logan, and Emma Mountains rise to heights of 1,000 to 2,000 feet. Similar igneous prominences characterize the adjoining Shivwits Plateau, and cones with attendant lava flows are conspicuous features of the St. George Basin, the Little Creek and the Kolob Terraces, and the valleys of Kanab and Johnson Creeks. Many craters are of such recent origin as to have retained their original features; others are worn to stumps or volcanic necks that, particularly in the Navajo country, rise sheer from the surface as towers of black rock. Likewise, some of the lava fields have been little changed, but others have been cut into fragments and, in the existing topography, are represented by the basaltic cap rock of many mesas and ridges and the thick sheets of andesitic lavas that cover Pine Valley Mountain, the Aquarius Plateau, and Grand Mesa. Considered as a whole, the land forms that characterize the Colorado Plateau comprise a single physiographic province readily distinguishable from other parts of the North American Continent. However, the component plateaus vary so much in origin, altitude, degree of isolation, and amount of dissection that in geological literature they are classified by groups or sections. Certain areas of special scenic interest have been further segregated as national parks and national monuments. Plateaus adjoining the Colorado River.- On both sides of the Colorado River from Lake Mead to Grand Junction, Colo., and for many miles along the tributary Green. San Juan, and Little Colorado Rivers, plateau topography is dominant. In the Grand Canyon section the Shivw; ts, Uinkaret, Kanab. Kaibab. and Coconino Plateaus terminate at the rim of the world's most spectacular gorge. In corresDonding positions along Glen Canybn lie the moderately elevated Glen Canyon platform, the Paria, Rainbow, and Grand Gulch Plateaus, and the Kaiparowits Plateau which stands nearly 4,000 feet above the river at its base. The trenchlike Cataract Canyon separates Dark Canyon Plateau from the equally lofty Standing Rock Plateau 35 |