Geologic History.—A large part of geologic history is revealed in the canyon walls of Zion and Bryce Canyon National Parks. Just as Grand Canyon is the best known record of ancient geologic history, Zion Canyon records most clearly the events of medieval geological time, and Bryce reveals much of modern geologic history. The story of Zion begins where that of Grand Canyon ends, and ends where Bryce begins. The rocks exposed in these three national parks incorporate the records of a billion years.
Generalized section of sediments in Zion and Bryce Canyons
| Age of rocks | Formation | ||
|---|---|---|---|
| lava | |||
| Eocene | Wasatch: Pink Cliffs | ROCKS IN BRYCE CANYON PARK | |
| MESOZOIC | |||
| Cretaceous | Undifferentiated: Gray Cliffs | ||
| sandstone | |||
| shale | |||
| coal | |||
| Jurassic | |||
| gypsum | |||
| Carmel | limestone | ||
| Navajo sandstone: White Cliffs | cross-bedded sandstone | ROCKS IN ZION PARK | |
| Kayenta-Wingate | |||
| Triassic | |||
| Chinle: Vermilion Cliffs | shale, sandstone, ash and limestone | ||
| fossil wood | |||
| Shinarump conglomerate | |||
| Moenkopi: Belted Cliffs | shale | ||
| sandstone | |||
| oil | RIM OF GRAND CANYON | ||
| Permian | Kaibab limestone | ||
A study of the rocks of Zion and Bryce Canyon shows that during the last 200,000,000 years the region comprising the parks has witnessed many changes in landscape and climate. At times it was covered by the sea, at other times broad rivers traversed its surface, and at still other times it was swept by desert winds. Most of the rocks were laid down by water as gravel, sand, mud, and limy ooze. They have been converted into solid rock by the weight of layers above them and by lime, silica, and the iron that cement their grains. Embedded in the rocks are fossil sea shells, fish, trees, snails, and the bones and tracks of land animals that sought their food on flood plains, in forests, or among sand dunes. The most conspicuous remains are those of dinosaurs—huge reptiles that so dominated the life of their time that the Mesozoic is known as the “age of dinosaurs.”
The accumulation of some 8,000 feet of strata (Mesozoic and Tertiary) on top of 4,000 feet of older (Paleozoic) beds, which are exposed in Grand Canyon, may be considered the first of three major events in the development of the marvelous landscapes of the Zion-Bryce region; it provided the material from which the huge scenic features were later carved. The second event was a regional uplift which elevated the previously low-lying top beds of the series (Wasatch, Pink Cliffs) to a height of nearly 2 miles above sea level. As a result of this movement, the earth’s crust was broken into huge rectangular blocks by north-south fractures or faults. Three of these great faults can be seen in the vicinity of the parks: the Hurricane fault in the Hurricane Cliffs, west of Zion; the Paunsaugunt fault in the cliffs of Bryce; and the Sevier fault along the Mount Carmel Road between the two parks.
In consequence of the uplift the third major event, the present cycle of erosion, was initiated. The streams became strong and swift and so were able to cut deeply into the underlying rock and carry away the land waste. In this process the streams have removed many cubic miles of rocks, which, if replaced, would fill the present canyons and build up their bordering land to the level of the lofty Markagunt and Paunsaugunt Plateaus. The gigantic features of Zion and Bryce Canyon National Parks mark a stage in the process of erosion that began long ago and which, if continued without interruption, will convert the present rugged landscape into plains near sea level.
Distinctive Features of Bryce Canyon.—In its regional setting Bryce Canyon National Park is the southeastern border of the extensive Paunsaugunt Plateau. It comprises two areas of strongly contrasted topography that meet in a line that marks the rim of the plateau and the top of its bordering cliffs. Back from the rim the plateau surface on which the park buildings and the automobile roads have been constructed is generally flat land traversed by broad, shallow valleys in which erosion is inconspicuous. Below the rim erosion has produced a region of remarkable ruggedness. It might naturally be supposed that the beautiful amphitheater at the head of Bryce Canyon had been carved by waters pouring down from the plateau above. But the plateau streams contribute nothing; they flow away from the rim. The chief sculpturing agents are the rain and snow that fall directly into the canyon. In producing the amazing variety of erosion features the streams have been aided by frost that pries fragments from the cliffs and by chemical agencies that decompose the rock and supply its vivid color.
The development of the park landscape was made possible by movements within the earth’s crust which brought originally low-lying strata to an altitude exceeding 8,000 feet. Streamways on the flat top of this uplifted block were little affected, but with greatly steepened gradients the streams around the borders of the newly made plateau became powerful. The Paria and its swiftly flowing tributaries have cut deeply into the face of the Paunsaugunt and carried billions of tons of ground-up rock to the Colorado. At the present time erosion by Bryce Creek, Yellow Creek, Willis Creek, Podunk Creek, and many smaller streams, is causing the plateau face to retreat northwestward, and these streams are taking into their drainage areas channels that formerly carried waters northward to Sevier River.
The drab-colored landscape along the southeast border of the park has been developed in alternating beds of shale, coal, and sandstone (Cretaceous age); in the resulting erosion forms, slopes, mounds, rounded ridges, and valleys with inclined or steplike sides predominate. The brightly colored rocks that form the topmost cliffs and terminate abruptly in such headlands as Steamboat Mountain, Bryce Point, and Rainbow Point are resistant limestones of Tertiary age (Wasatch formation). They erode as vertical walls (Pink Cliffs), which weathering has developed into the astonishing display of architectural forms that make Bryce Canyon distinctive.