In Middle Jurassic time the land now called central Utah, which then was the eroded surface of the Navajo Sandstone, sank beneath an arm of a shallow sea that came in from the north, and most of the area remained beneath this sea until Late Jurassic time. Sediment carried into this sea and into bordering lagoons and estuaries later hardened into the sedimentary rocks of the Carmel Formation, Entrada Sandstone, and Curtis and Summerville Formations. The Carmel and Curtis contain abundant marine fossils of Middle Jurassic age, and in central Utah the intervening unfossiliferous Entrada also is believed to have been deposited in or near the sea, and the unfossiliferous Summerville Formation probably was deposited upon a tidal flat that was submerged part of the time.
Deposits and Events East of the Sea
In eastern Utah, east of the ancient Jurassic sea, the Entrada Sandstone is entirely unfossiliferous, was partly water laid and partly wind blown, and has been divided into three distinctive parts, which in ascending order are the Dewey Bridge, Slick Rock, and Moab Members.[25] In and near the Colorado National Monument, the long period of erosion discussed in a preceding section probably continued well into the Jurassic, so only the upper part of the Slick Rock Member and the overlying Moab Member were deposited on the eroded surface of what little remained of the Kayenta Formation ([fig. 14]).
The Slick Rock Member was named from its occurrence at and near the mining town of Slick Rock, Colo., which originally was named after the appearance of the rock because it generally forms slick, smooth cliffs. It reminds one of the chicken and egg conundrum. The Slick Rock is composed mainly of sand dunes that were piled up on the eastern shore of the Jurassic sea by winds blowing from the northeast. Occasional rainy spells created lakes and ponds in which some of the sand was laid down in level beds. This pile of sand later hardened into the cliff-forming Slick Rock Member, which looks something like the Wingate but is generally only half as thick, weathers into less abrupt cliffs, is mostly salmon red, and is almost free of joints. The joints in the Wingate ([fig. 11]) probably resulted from the uplift and tilting of the Plateau before the long period of pre-Entrada erosion; whereas the land seems to have been more stable during Entrada time. The Slick Rock is cemented with calcium carbonate (CaCO₃), which is soluble even in weak acid, such as rain or snow water containing dissolved carbon dioxide. For this reason solution openings or pits occur in some of the cliff faces, the most striking of which are those shown near the top of [figure 15].
The Slick Rock Member of the Entrada Sandstone forms a line of cliffs and isolated monoliths that are second in height and grandeur only to those of the Wingate. The Member is best displayed southwest of Rim Rock Drive between the Visitor Center and the Coke Ovens and along the western arm of Ute Canyon ([fig. 16]). It also forms the Saddlehorn just south of the camp and picnic grounds near the Visitor Center ([fig. 50]). Most of the smooth cliff faces show both the steeply dipping crossbeds of the old sand dunes and the flat-lying beds of the lake or pond deposits.
ENTRADA SANDSTONE, just above normally dry waterfall in west arm of Ute Canyon. Note smooth unjointed cliff of Slick Rock Member protected at left by overhanging basal bed of Moab Member, which forms about lower half of slope in distance. Upper part of distant slope is the Summerville Formation overlain by Salt Wash Member of the Morrison Formation. Note Slick Rock at left resting upon eroded crossbedded sandstone in Kayenta Formation, in which the canyon was cut. (Fig. 16)
The overlying Moab Member of the Entrada is a white level-bedded sandstone that generally weathers into stairsteps or ledges. One of the best exposures of the Moab Member is shown in [figure 17], but good exposures also are seen along the west side of Rim Rock Drive just northeast of the Coke Ovens Overlook. In some places the Moab Member forms cliffs continuous with those of the underlying Slick Rock Member. It appears to consist of hardened beach or lagoon sand that was deposited along the eastern shore of the sea, which suggests that the sea extended farther east during Moab and Summerville times than it did during Dewey Bridge and Slick Rock times. Like the Slick Rock, the Moab also is cemented by calcium carbonate, but the lower sandstone ledges of the Moab Member are more resistant to erosion than the Slick Rock Member, so the Moab helps preserve the underlying cliffs. The top of the Moab Member forms patches of bare pavement east of the Monument, known as the “Slick Rim,” which may be observed from the Little Park Road.
