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

A GEOLOGIC AND GEOGRAPHIC SKETCH OF BRYCE CANYON NATIONAL PARK

STORY OF THE ROCKS

Bryce Canyon National Park is not only a region of fascinating landscapes—masses of rock wonderfully carved and painted. It is a book whose illuminated pages reveal much of geologic history. Just as Grand Canyon is the best-known record of ancient geologic history (Paleozoic), and Zion Canyon records most clearly the events of medieval time (Mesozoic), the cliffs and canyon walls of Bryce Canyon National Park reveal much of modern geologic history (Cenozoic). The story of Bryce begins where that of Zion ends, and Zion in turn, where Grand Canyon ends. In the 16,000 feet of sedimentary rock exposed in these three parks are incorporated the records of a thousand million years of geologic history. (See Fig. 9) As dated by geologists, the pink rocks near at hand were formed 55,000,000 years ago. And these are late events, for underlying the gray sandstones at the mouth of Bryce Canyon are progressively older beds, leading down to sediments that include the earliest known life forms. A study of these rocks shows that the region comprising the park has witnessed many changes in landscape and climate. At times it was covered by the ocean, at other times it was a seashore with bays and estuaries, and at still other times its surface was traversed by rivers and dotted by shallow lakes. The rocks were laid down in water and gravel, sand, mud, limy ooze and fine silt. They have been converted into solid rock by the weight of layers above them and by lime, silica, and iron that cemented their grains. Embedded in the rocks are fossil sea shells, trees, turtles, snails, and the bones of land animals that sought their food in flood plains, in forest and uplands, and along shores. 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 fossils found in the rock layers in the park record the closing chapters of the "age of reptiles" and the beginning chapters in the "age of mammals."

Figure 9. Generalized cross section of the geologic formations exposed in Bryce Canyon National Park and the adjoining region. (click on image for an enlargement in a new window)

As treated by geologists, the rocks exposed in Bryce Canyon National Park are assigned to the Cretaceous and the Tertiary periods. (See Fig. 7 and Fig. 9) The drab-colored landscape along the southeast border of the park has been developed in rocks of Cretaceous age-alternating beds of shale, coal, and sandstone. The brightly colored rocks that form the topmost cliffs and terminate abruptly in such headlands as Boat Mountain, Bryce Point, and Rainbow Point are limestones of Tertiary age.

The enormous pile of rocks that constitutes the formations of sediments of Cretaceous and Tertiary ages was not continuously deposited. At times the sea bottoms, the low-lying plains, and the inland basins which served as resting places for stream-borne materials were uplifted and erosion replaced deposition; the consolidated rocks laid down in one period were worn into sands, muds, and gravels, and redistributed to form rocks of a later period. Particularly toward the end of the Cretaceous periods the landscape underwent profound changes. The highest lands were leveled off, new mountains were formed, the sea was replaced by land, and the ancient plants and animals gave way to forms much like those of today.

For convenience of study and description, the rock strata that constitutes geologic eras are subdivided into groups known as "formations" which differ from each other in such features as color, mineral content, thickness, areal extent of individual layers, and kinds of fossils. Each formation, therefore, reveals somewhat in detail the physical geography, the climate, the fauna, and the flora of the time of its origin.

Of the formations prominently displayed in the park, the youngest in the Wasatch limestone of Eocene (?) Tertiary age which forms the surface of Paunsaugunt Plateau, the Pink Cliffs, and the picturesque erosion forms in the alcoves cut into the plateau rim.

WASATCH:—Though in general view the Wasatch formation seems uniform in color and composition, an examination of the rocks along the rim road and the trails reveals a somewhat surprising variation of color tones and in the kind and the arrangement of material. In places the vertical succession of massive red limestone beds is interrupted by layers of firm, thin, shale-like limestone mottled deep red, pink, and white; by sheets of gray sandstone, and by irregular masses of gravel tightly cemented with lime. Generally the top of the formation as exposed in Boat Mountain and Whitemans Bench is more sandstone and conglomerate than limestone and the bottom is an assemblage of pebbles and boulders of quartz, quartzite, limestone, sandstone and igneous rocks. The hard cobbles from this remarkable basal conglomerate are strewn over the lower slopes in the park and in flood seasons are carried along the tributaries of the Paria, even to the Colorado. Embedded in the limestones are land shells of three kinds and in the shales are impressions of leaves. The style of bedding and the fossils make it possible to know the geography in the long-ago time when the rocks that make up the Wasatch were laid down as limy ooze, silt, sands, and gravels. The finest material must have been deposited in lakes and ponds or in other bodies of quiet water, the coarser debris along streams. The beds that in ancient times probably overlaid the Wasatch have been entirely removed by erosion and the thickness of the formation itself has been reduced by this process, in some places considerably. Thus at Bryce Canyon the formation, once probably 2,000 feet in thickness, has been reduced to 1,300 feet, at places along the rim to 1,000 feet or less, and even more in the gaps (passes) where deep valleys have been formed.

Figure 10. Parallel canyons and remarkable erosional forms in the Silent City as viewed from near Inspiration Point. (Photo by Zion Picture Shop)

The Wasatch is not only the youngest series of sedimentary beds in the Bryce region and the strongest cliff maker, but it is also the most prominent because of its coloring. In fact "Wasatch" and "Pink Cliffs" are nearly synonymous terms. The color of the fresh (unweathered) limestone is pale pink; of the sandstone and grits, nearly white. The rocks thus record the amount of iron—the chief coloring matter—in the sands and silts from which the present beds have been formed. Weathering has caused the iron to change its chemical state and to be more widely distributed; the various tones of red, pink, yellow, and tan record the kind and stage of oxidation. The reddest, densest, and most completely calcareous rocks contain the most iron; such sandy porous white rocks as form the mesa tops and many knobs on canyon walls are nearly free of iron, and doubtless part of the iron once present has been removed by leaching.

KAIPAROWITS:—Next below the red and pink rocks of the Wasatch are the dark-colored sandstones and shales of the Kaiparowits formation deposited by streams in late Cretaceous time. As exposed in the park, the Kaiparowits has a maximum thickness exceeding 1,000 feet and a minimum of less than 500 feet—a great range in thickness that represents the degree to which the top beds were eroded before the overlying formation was deposited.

As a whole the formation is readily distinguished. Even in distant views it appears as a dark-gray or yellow-brown band below pink rocks and above light-gray rocks. On the east face of the Paunsaugunt Plateau it appears as a slope broken here and there by irregularly placed benches, terminated downward by terraces of sandstone and upward by vertical cliffs of limestone.

In composition the Kaiparowits is dominantly quartz sandstone, much of it so poorly consolidated as to weather into loose sand, forming slopes nearly free of broken blocks and cobbles. Mingled with the rounded quartz grains are feldspar, mica, and also much iron which together with calcite serves as cement and which gives to the rock its yellow-brown tones, in places nearly black. Some of the sandstone ledges consist of evenly laid beds of uniform composition with thicknesses of 20 to 50 feet, but generally a section of the hillside includes not only dark sandstone but also white sandstone, blue-gray clay shale, and lime shales. As the material was laid down by streams as sand bars, river flats, and local deltas, few of the beds are continuous for long distances and most of them are very irregular in form. They contain plasters of impure limestone, sand balls of various sizes, and masses of ironstone that on weathering remain as knobs on steep slopes or as the caps of towers and buttes. In the firm limestone and the yellow-tan weakly cemented sandstone are embedded the fossil bones of dinosaurs, crocodiles, turtles, and fresh-water shells, also fossil wood and leaves. The Kaiparowits formation is well exposed along the "under the Cliffs" trail from Rainbow Point eastward.

WAHWEAP AND STRAIGHT CLIFFS:—The Kaiparowits formation is underlaid in turn by the Wahweap and the Straight Cliffs formation which east of the Paria River have distinguishable characteristics, but within Bryce Canyon National Park intergrade to such an extent that their boundaries in many places are obscure. In these formations that together have an average thickness of about 800 feet, the most conspicuous features are the beds of buff sandstone 30 to 150 feet thick and continuous for miles, and which weather as nearly vertical walls. Along the edge of the park, layer on layer of thick sandstone separated by thin beds of the same composition stand on the general slope as huge steps that combine to make unscalable walls. As shown by fossils in the Straight Cliff sandstone, the sediments were deposited in a sea and in lagoons of brackish water.

TROPIC:—In downward succession the sandstones of the Straight Cliffs give way to clayey shales, designated as the Tropic formation, an unmistakable assemblage of dark, drab, thin, fossiliferous marine beds that have a thickness of 600 to 1400 feet. This formation contains the coal mined at Tropic and Henrieville,

DAKOTA:—Without any sharp separation, the Tropic shale is underlaid by beds of conglomerate sandstone rarely more than 50 feet thick, known as the Dakota formation.

Thus as units in the stratigraphic series exposed in Bryce Canyon National Park, the Wasatch formation is the highest (youngest) and the Dakota is lowest (oldest). (See Fig. 7 and Fig. 9). Sedimentary beds that once overlay the Wasatch have been removed by erosion. Lava (basalt) rests on them at Red Canyon and north of the park igneous rocks are widespread. The more recent deposits—the sands and gravels along the streamways and the jumbled materials about the base of cliffs—have not as yet been converted into solid rock. Southward from the park formations older than the Dakota are prominently displayed in the White Cliffs and the Vermilion Cliffs, visible from the rim of the Paunsaugunt Plateau. (See Fig. 2).

The Wall of Windows. (Photo courtesy of the Utah Magazine)