The Geologic Story of Canyonlands National Park

by Stanley William Lohman

Not exposed in the area but present far beneath the sedimentary cover, and exposed in a few surrounding places, are examples of the other two principal types of rocks: (1) igneous rocks, solidified from molten rock forced into or above younger rocks along cracks, joints, and faults and (2) much older metamorphic rocks, formed from other pre-existing rock types by great heat and pressure at extreme depths. The particles comprising the sedimentary rocks were derived by weathering and erosion of rocks of all three types in the headwater regions of the ancestral Colorado River basin. Igneous rocks of Tertiary Age ([fig. 80]) form the nearby La Sal, Abajo, and Henry Mountains ([fig. 7]).

SHALLOW INLAND SEA which covered Canyonlands and vicinity during Middle Pennsylvanian time. (Fig. 8)

Second among the main events leading to the formation of the canyon country was the raising and buckling of the Plateau by earth forces so that it could be vigorously attacked by various forces of erosion and so that the rock materials thus pried loose or dissolved could eventually be carted away to the Gulf of California by the ancestral Colorado River. Some idea of the enormous volume of rock thus removed is apparent when you look down some 2,000 feet to the river from any of the high overlooks, such as Dead Horse Point ([fig. 15]) or Green River Overlook ([fig. 23]), or when you lay a straightedge across the three high mesas in [figure 10] and note the large volume of missing rocks below. Not so apparent, however, is the fact that some 10,000 feet of younger Mesozoic and Tertiary rocks that once overlay this high plateau also has been swept away. In all, the river has carried thousands of cubic miles of sediment to the sea and is still actively at work on this gigantic earthmoving project. In an earlier report (Lohman, 1965, p. 42) I estimated that the rate of removal may have been as great as about 3 cubic miles each century. For a few years the bulk of it was dumped into Lake Mead, but now Lake Powell is getting much of it. When these and other reservoirs ultimately become filled with sediment, for reservoirs and lakes are but temporary things, the Gulf of California will again become the burial ground.

ROCK COLUMN OF CANYONLANDS NATIONAL PARK. One foot equals 0.305 meter. (Fig. 9)

AGE (millions of yrs ago) GEOLOGIC AGE NAME OF ROCK FORMATION KIND OF ROCK AND HOW IT IS SCULPTURED BY EROSION THICKNESS (feet) NAMED FOR OCCURRENCE AT OR NEAR

150 Jurassic Entrada Sandstone Crossbedded white fine-grained sandstone at top (Moab Member); salmon colored to pink, fine-grained, generally crossbedded sandstone in middle (Slick Rock Member); red earthy sandstone and siltstone at base (Dewey Bridge Member, grades into Carmel Formation west of Green River). Forms steep-sided buttes north, east, and west of park. 400-500 Entrada Point, San Rafael Swell, Utah 175 Jurassic & Triassic(?) Glen Can Group Navajo Sandstone Crossbedded buff to gray sandstone, some red sandstone, and thin beds of limestone. Residual rounded patches on highest mesas. 325-550 Navajo Country, Ariz., New Mexico, Utah Late Triassic(?) Kayenta Formation Irregularly bedded stream-laid gray, buff, lavender and red fine-to-coarse-grained sandstone and siltstone. Caps most high mesas and forms tops of highest cliffs. Contains fresh-water fossils. 160-300 Glen Canyon, S. Utah, Kayenta, Arizona 200 Late Triassic Wingate Sandstone Buff and light red generally crossbedded medium-grained sandstone. Forms highest cliffs, many of which are coated with black desert varnish. 210-340 Ft. Wingate, New Mexico Chinle Fm. Unnamed upper member Reddish siltstone, mudstone, and sandstone locally bleached to bluish or greenish gray, and few thin beds of limestone. Forms steep slopes at base of highest cliffs. Contains some fossil wood and reptile bones. 205-740 Chinle Valley, Ariz. Moss Back Mbr Gray, brown, and gray-green sandstone and conglomerate. 0-80 Moss Back Ridge, Utah 215 Middle(?) and Early Triassic Moenkopi Fm. Unnamed upper member Brick red, reddish-brown, and brown mudstone and sandstone, and some conglomerate and gypsum. Forms slopes broken by thin ledges. 250-940 Moenkopi Wash, Ariz. Triassic(?) Hoskinnini Tongue Pale-brown fine-to-coarse-grained sandstone; forms ledges. 0-120 Hoskinnini Mesa, Ariz. 250 Permian Cutler Fm. Undivided Cutler Formation in northeastern part of area is composed of buff, red, and purple arkosic sandstone and conglomerate. South of Indian Creek is the thick Cedar Mesa Sandstone Member, composed of massive mainly crossbedded white to pale red sandstone with thin beds of cherty limestone. Forms needles, arches and other erosional features. Thickening southwestward is the White Rim Sandstone Member of white crossbedded sandstone. 800-1,000 Cutler Creek, Colo. White Rim Sandstone Member White Rim, Wayne Co., Utah Organ Rock Tongue Organ Rock, Utah Cedar Mesa Sandstone Member Cedar Mesa, S. E. Utah Rico Formation Buff, red, and purple arkosic sandstone and conglomerate containing several thin beds of marine fossiliferous limestone. Forms moderately steep slopes. 250-585 Rico, Colo. 300 Pennsylvanian Hermosa Formation Hermosa Creek, Animas River Valley, Colo. Unnamed upper member Blue, greenish, and gray fossiliferous limestone interbedded with white, gray, and greenish sandstone and gray to green shale. Lower part known only from deep wells. Forms steep canyon walls. 900-1,800 Paradox Member Salt, gypsum, and anhydrite with interbedded black and brown shale; some limestone. ? Paradox Valley, Colo.

Last but far from least among the factors responsible for the grandeur of the canyon country is the desert climate, which allows us to see virtually every foot of the vividly colored naked rocks and has made possible the creation and preservation of such a wide variety of fantastic sculptures. A wetter climate would have produced a far different and smoother landscape in which most of the rocks and land forms would have been hidden by vegetation. In the canyon lands the vegetation is mainly on the high mesas and on the narrow flood plains bordering the rivers, but scanty vegetation does grow on the gentle slopes or flats.

The desert climate has combined with the nearly flat lying layers of sediments of different character, hardness, and thickness to produce steep slopes having many cliffs and ledges and generally sharp to angular edges rather than the subdued rounded forms of more humid regions. This has led geologists to refer to such terrain as having “layer-cake geology,” and this is brought out by the profile in the rock column ([fig. 9]), by the cross section ([fig. 10]), by [figure 15], and by many of the other photographs. But the baker of this cake was rather careless—not only do the layers range widely in thickness and character, but some are wedge shaped, thick on one side of the cake but thin or absent on the other. Then too, when he ran out of icing in the midst of a layer, he was apt to finish with a different kind or color, for no inspector was on the job to insure orderly construction.