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

A GEOLOGIC AND GEOGRAPHIC SKETCH OF BRYCE CANYON NATIONAL PARK

UPLIFT AND EROSION

This explanatory description of the features of Bryce Canyon National Park and of the processes active in making them leaves unanswered the questions, "How did Bryce Canyon originate? Why has this particular region so many great alcoves, lofty mesas, plateaus, and vertical cliffs?" The complete answer involves a knowledge of a long train of events in earth history and of forces and processes as yet not fully understood. In brief the answer is: The rise of the land with respect to sea level; the development of the park landscape was made possible by movements within the earth's crust. At a time long past, estimated at 13,000,000 years ago, all of southern Utah and adjoining regions began to rise and continued its upward movement slowly and intermittently until the lands once near sea level attained altitudes exceeding 10,000 feet. During this great uplift that brought the former low-lying plains of the Bryce Canyon region to a position nearly two miles above sea level, the beds of rock were broken into earth blocks many miles in length and width. In its regional relations, Bryce Canyon National Park is the eastern edge of the Paunsaugunt Plateau—an enormous mass of sedimentary rock—one of the nine great blocks the constitute the High Plateaus of Utah. It is separated from its neighbors, Aquarius Plateau on the east and Markagunt Plateau on the west, by great cracks (faults) that extend scores of miles and along which the rocks on one side have been raised or on the other side lowered a few hundred to more than 2,000 feet. In consequence of this faulting, the plateau is bordered by cliffs of commanding height which, though originally straight lines or broad curves, have through the lapse of millions of years lost their simple structure. In place of an original simple escarpment directly determined by a fault, the boundary walls of Paunsaugunt Plateau stand some distance back of the fault line and are sinuous and crenulated to a remarkable degree. (See Fig. 7). The western border of the Paunsaugunt block is the great earth fracture known as the Sevier fault that crosses the Panguitch-Bryce highway at the mouth of Red Canyon and extends southward through Alton, Glendale, Mt. Carmel, and on into Arizona. The original wall initiated by this fault remains as the Sunset Cliffs, now broken into sections by canyons and much decreased in height by the removal of rocks at their tops. The cliff that long ago marked the eastern border of the Paunsaugunt Plateau as a wall that rose some 2,000 feet above its surface has been almost entirely destroyed. (See Fig. 7c). A remnant stands at Table Cliffs, the southern salient of Aquarius Plateau, but southeastward along its ancient trend nothing remains of its former grandeur. In the rugged landscape along the southern border of the park the position of the Paunsaugunt fault is shown only by a break in the continuity of the strata; beds of sandstone abut against beds of limestone or of shale. The effect of faulting is clearly revealed at view-points on the eastern rim of the plateau. Thus at Sunset Point, the bed of rock on which the observer stands 8,000 feet above sea level on the western side of the fault is the same bed that forms the cap of Table Cliffs at 10,000 feet on the east side of the fault. (See Fig. 7).

Figure 8. View from Inspiration Point towards Sunset Point. Typical erosion in the Wasatch (Pink Cliffs) formation. Note in order the plateau, rim, pinnacles, and spires. (Photo by Zion Picture Shop)

The uplift of the ancient lowlands of southern Utah and breaking them into blocks is the first great event in the making of the plateau landscape. The second event is the erosion of the blocks into their present scenic forms. This regional uplift introduced a long period of time during which the conditions have been favorable for erosion. On the original lowlands before it was broken into blocks, streams did little work. They flowed in broad shallow valleys of gentle gradient like those along the road from Red Canyon to the park headquarters. In consequence of the uplift which steepened their paths, the streams became powerful agents of erosion. Their accelerated speed permitted them to cut trenches in solid rock, and as the land rose progressively higher, to develop their trenches into the present profound canyons and to reduce the inter-stream lands to mesas and long flat-topped ridges. Though the plateau blocks were raised, their tops remained comparatively level. (See Fig. 2) Hence the streams on the surface of the newly made plateaus were little affected by the uplift, but those on its precipitous edges flowed in channels steeply inclined. The Sevier River on top of the plateau has for long stretches a gradient of less than 15 feet a mile and has cut its bed but slightly. In contrast, the tributaries to the Paria after thousands of years of activity descend 1,000 to 1,500 feet a mile, and have cut deeply into the south and east faces of the Paunsaugunt Plateau and at present the erosion by Bryce Creek, Yellow Creek, Willis Creek, Riggs Creek, and many similarly placed streams are taking into their drainage areas channels that formerly carried waters northward to Sevier River.

In the Bryce Canyon region, the whole bewildering landscape is but the slow work of familiar agents—streams, rain, and frosts—continued for long periods of time. It is the product of erosion, the commonplace term which here seems to have a new meaning. As erosion has been continuous ever since the plateau lands were raised above sea level, the present erosion forms merely mark the present stage of a long train of geologic events. Formerly the layers of limestone that make the Pink Cliffs at Bryce Canyon extended eastward farther than the eye can reach, and a thousand cubic miles of rock have been worn away in forming the lower lands, and still the region is geologically young. Though the work accomplished by the Paria and its tributaries is enormous, these streams have not completed their ultimate task of wearing down the Paunsaugunt Plateau to near sea level. The bed of the streams might be sunk 1,000 feet deeper and still have slopes sufficient to carry silt-laden water. They have been at work only for the few million years that make up the last chapter in a billion years of geologic history.

In rock erosion, as in carving by human hands, time is a factor in producing forms. Progressively on the untrimmed block, new grooves and chipped surfaces appear. But unlike the human sculptor who finally completes his work, erosion though at times rapid and at other times slow knows no stopping place. In unnoticed ways, it daily modifies the form of canyons and cliffs and, continuing its task, rearranged the elements of a landscape until their individuality is lost. The resulting new landscape likewise eventually gives place to another. In the Bryce Canyon region the ancient landscapes bear little resemblance to the present surface forms, and future landscapes are likely to differ even more widely.