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Zion's oldest sedimentary rock is the relatively thick Moenkopi layer, which is composed of sandstone and shale. The composition and depth of these rocks indicate that approximately 230 million years ago the Colorado Plateau was covered by a vast, gentle sea. During the Triassic period (225 to 190 million years ago), as the land began to thrust upward, the sea gave way to shallow rivers that steadily increased in velocity and carried away the rock. Trees toppled, while volcanoes erupted and spewed forth millions of tons of ash. This slow, natural holocaust laid down the Chinle Formation, a remarkable assemblage of shale, sandstone, and petrified wood. The rivers persisted for several million years and, in their wake, left large lakes. Deposits formed the Moenave layer, reddish sandstone in which fossils of fresh water fish and other creatures are evident, and streams laid down sandstone sediments that would later compose the Kayenta Formation. Dinosaurs plodded through these streams; one track in the Kayenta Formation is displayed in the park Visitor Center. Late in the Triassic period and early into the Jurassic period (190 to 136 million years ago), the Colorado Plateau became distinctly arid. Sand-laden wind smothered the verdant land and gradually laid the most visible of Zion's formations the Navajo Sandstone, as much as 2,220 feet thick in some areas of the park. The great temples, towers, and cliffs of Zion Canyon were formed from this sandstone. The Temple Cap Formation is another sandstone layer, some 20 to 200 feet thick, with a high concentration of iron oxide that streaks the cliffs red when dissolved by rain. The most recent mineral layer is the Carmel Formation, deposits of limestone formed by seas that once again flooded the land. Fragments of the Carmel Formation are found high in the canyon on the East Temple and West Temple. About 13 million years ago, the entire area of southern Utah and northern Arizona began to rise. Pressure on the layered rocks was so great that they broke into great plateaus, seven of which are located in southwestern Utah. Two of these are the Paunsaugunt Plateau, of which Bryce Canyon is part, and the Markagunt Plateau, which encompasses Zion. The uplift caused rivers to run fast and strong; the Virgin River, for example, began to cut into its present bed. A very powerful process of erosion was quite pronounced 1 million years ago. Water Erosion The steep walls of the North Fork of the Virgin River are the result of water cutting into soft sandstone. So soft is this sandstone and so abrasive is the action of the gushing waters and their accumulated particles, that erosion is quick and decisive. Naturalists call the North Fork an "endless belt of sandpaper." Zion Canyon is still being formed. Erosion by the Virgin River begins on the Markagunt Plateau at an elevation of 9,000 feet, where the river begins picking up sediment. Downstream, the Virgin combines with tributaries and gathers momentum. Its cuts become deeper, and in some places the river drops 80 feet each mile, a fall ten times as great as that of the Colorado River in Grand Canyon National Park. As it leaves the Narrows, the river opens into a broad canyon at the Temple of Sinawava, where it fans out and loses some of its momentum. Continuing on, it flows southwest for 200 miles until it reaches Lake Mead at an elevation of less than 1,000 feet. Occasionally when the river leaves the Narrows, it floods the river bottom, churning sediments and toppling trees in a few awesome moments. In 1954, its volume increased fiftyfold in fifteen minutes, and the Virgin became an enormous force that ripped through the landscape. Many other valleys and canyons at Zion also have been carved by erosion, but some have not experienced the force of the Virgin River and have been left hanging, so to speak. These "hanging valleys" stand at 1,000 to 1,300 feet above the river; examples are found between the Mountain of the Sun and Twin Brothers. The widening of Zion Canyon is the result of water percolating through the porous Navajo Sandstone until it reaches the impervious shales of the Kayenta Formation. Then the water accumulates and flows laterally, forming a natural watercourse that over time erodes the shales, which support the Navajo layer. Once undermined, the sandstone gives way, and the canyon widens. This process is dramatically illustrated near the Temple of Sinawava, where the Narrows of the Virgin River begin. The percolating action of water also has been responsible for the formation of the Great Arch of Zion, and the cliff along the Weeping Rock Trail. Stream channels, choked with sand, forced the river to meander, removing shales from the Kayenta Sandstone. Eventually, slabs of undermined Navajo Sandstone gave way, and the arch emerged. These forces are still at work, and one day a patch of blue may appear beneath a semicircular ribbon of vermilion rock. Extract from "The Sierra Club Guides To The National Parks Desert Southwest". Published by Stewart, Tabori & Chang. Distributed by Random House. 1984. |
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