BRYCE CANYON A Geologic and Geographic Sketch of Bryce Canyon National Park

A GEOLOGIC AND GEOGRAPHIC SKETCH OF BRYCE CANYON NATIONAL PARK

THE PAUNSAUGUNT PLATEAU

In strong contrast to the intricately dissected, brightly colored, nearly bare lands "under the rim" in Bryce Canyon National Park, the surface back from the rim is nearly flat, dull-colored, forested, and without conspicuous cliffs and canyons, even sharp-cut gullies or terraced hills; truly remarkable features in a region of rugged topography. (See Fig. 2 and 6) In place of the short, swift streams that carry water through defiles from the lower lands to the Paria, the plateau top is drained by the slow-moving Sevier River that wanders across the surface in a broad, shallow valley between inconspicuous cliffs and rugged ridges. The divide between the two drainage systems is very narrow. From one edge of a belt 5 feet wide along the plateau rim, rain water goes to the Colorado and on to the Pacific Ocean; from the other edge it follows a long route (Sevier River) through central Utah only to disappear in the desert flats of the Great Basin. It is interesting to note that none of the valleys in which water flows northward is complete. They begin in broad swales (passes) that are abruptly cut off along the southern plateau rim. The more vigorous tributaries to the Paria have cut off the heads of slower-moving north-flowing tributaries of the Sevier.

Figure 7. Generalized diagrams illustrating the effect of faulting and representing stages in the development of the Paria amphitheater and the canyons, cliffs, and gorges of Bryce Canyon National Park. (a), a short time after movement along the Paunsaugunt fault had raised the rocks on the east side about 2,000 feet above those on the west; the drainage was northward. (b), the Paria River in eroding headwards has begun the removal of the rocks on both sides of the fault, but more on the higher east side where the streams have steeper descent and therefore are more powerful. (c), the Paria and its tributaries have broken up and carried to the Colorado River large portions of the limestones, sandstones, and shales of the Tertiary, Cretaceous, and Jurassic formations down to the massive Navajo sandstone. At their heads the tributary streams are eating into the hard Wasatch limestone which forms the surface of the Paunsaugunt Plateau and Table Cliffs. Not much more headward erosion is needed for the Paria to capture the waters of the upper East Fork of Sevier River. N, Navajo sand stone; UJ, upper Jurassic limestones, sandstones, shales and gypsum; D, Dakota conglomerate; T, Tropic shale, including coal; WS, Wahweap and Straight Cliffs sandstones and shales; K, Kaiparowits formation; W, Wasatch formation—eroded to form the Pink Cliffs. (click on image for an enlargement in a new window)

In a sense the surface of the Paunsaugunt Plateau is monotonous, but it is full of meaning. It is the present-day chapter in a history of erosion that spans more than a million years. Above the surface once stood mesas and buttes overlooking gorges in which snow-fed streams ran swiftly. As erosion progressed, the land was lowered, the inequalities were removed, and the drainage channels became less steep until finally the main streams were unable to cut deeper grooves and were barely able to carry away the small amount of rock debris brought in by their tributaries. Most of the weathered rock now remains where it is formed, unlike the bare rock in much of the surrounding region, the surface is mantled with soil. In geological parlance, the surface of the Paunsaugunt Plateau has attained "old age" and its bordering lands are "youthful." (See Fig. 8)