KAYENTA FORMATION, showing lenses of hard channel sandstones and wedge of red siltstone and mudstone. Along road cut of Rim Rock Drive near head of main stem of Ute Canyon. Vertical grooves remain from drill holes used in blasting roadcut. (Fig. 14)
Beautifully sculptured remains of the Navajo are featured attractions at Zion, Capitol Reef, and Arches National Parks, Rainbow Bridge, Navajo, and Dinosaur National Monuments; border many miles of beautiful Lake Powell; and form the eastern flank of the San Rafael Swell. For reasons to be explained, this sandstone thins to the northeast, and is absent entirely at about the Utah-Colorado State line, some 35 miles southwest of the Monument. Thus, in the Monument, the Navajo, most of the Kayenta, and the lower part of the Entrada Sandstone are missing at another gap in the rock record, as shown in [figure 15].
GAP IN THE ROCK RECORD, between Kayenta Formation below 3½- × 6-inch green notebook and Slick Rock Member of Entrada Sandstone above. The reasons for this gap are given in the text on [page 38]. That this is an erosional unconformity is clearly indicated by the uneven top of the Kayenta, particularly to the left of the notebook. Note solution pits and openings in the Entrada near top of photograph. (Fig. 15)
How is it possible that the Navajo Sandstone is more than 2,200 feet thick in Zion National Park, is several hundred feet thick in much of the Plateau in Utah and parts of southwestern Colorado, yet is absent entirely, together with a considerable thickness of younger rocks in and near Colorado National Monument? How much of the missing strata once were present in the Monument is not known, but it seems clear that at least part was present but was eroded away before the Entrada Sandstone was deposited. There is evidence[21] that following the deposition and consolidation of the Navajo Sandstone the Plateau and adjacent areas were uplifted, tilted gently westward, and eroded for a considerable period of time. Erosion naturally was most pronounced in the eastern areas, including the Monument, where the uplift was greatest. Thus, in the northeastern part of the Plateau all the Navajo and most of the Kayenta were eroded away, and erosion continued there while the lowest member of the Entrada, the Dewey Bridge Member, and the lower part of the overlying Slick Rock Member were being laid down in the Moab, Utah, area.[22] This old erosion surface is clearly visible in many places along the cliff wall on the southwest side of Rim Rock Drive between the Visitor Center and Kissing Couple.
The reduction in thickness of the Navajo Sandstone from southwest to northeast and absence of the Navajo and some younger rocks in and near the Monument are shown on an isometric (three dimensional) block diagram prepared by artist John R. Stacy and me, which is displayed in the Museum of the Visitor Center. This block diagram portrays the surface and subsurface rocks from Zion National Park, Utah, to Black Canyon of the Gunnison National Monument, Colo., via Capitol Reef National Park, the Henry Mountains, and Colorado National Monument. Throughout the Plateau and parts of adjacent areas, the erosion surface on top of the Navajo Sandstone is covered by scattered pebbles of chert—a hard variety of silica (SiO₂) derived from cherty beds of freshwater limestone in the Navajo.[23] Where the Navajo has been completely eroded away and the ancient erosion surface is on the Kayenta Formation, as in Colorado National Monument, scattered pebbles (some of which are chert) derived from the conglomerate lenses in the Kayenta are found locally on the old surface.[24]
Because of this gap in the rock record we will continue part of our story farther west, where the rock record is more nearly complete.
The Sea to the West
