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

A GEOLOGIC AND GEOGRAPHIC SKETCH OF BRYCE CANYON NATIONAL PARK

MAKING OF THE PARK LANDSCAPE

After feasting their eyes on the painted alcoves adorned with huge rock carvings, few visitors to Bryce Canyon National Park fail to ask the question, "How was Bryce Canyon formed?" Many, unfamiliar with geologic processes, view the seemingly disordered assemblages of tables, gorges, ridges, and spires as evidence of a mighty upheaval and shattering of the earth's crust. Others, perhaps comparing it with valleys in the Rocky Mountains or the Sierra Nevadas, which have been deepened and widened by glaciers, think of it as the product of glacial erosion. Still other think of the wind as at least a contributing force. But the rocks in view are not a jumbled mass; they are horizontal beds arranged in orderly succession; those on one wall of a gorge correspond layer by layer with those on the opposite wall and would meet if extended across. There is no evidence of glaciation nor of large-scale wind erosion, improbable as it may seem, the alcove at the head of Bryce Canyon is chiefly the work of Bryce Creek and its myriad tributaries, the stream that now flows from it. Likewise, the similar alcoves and bowls at the head of Campbell Creek, Yellow Creek, Pasture Creek, Bridge Creek, Black Birch Creek, and the great amphitheater drained by Riggs Creek were formed by the streams that occupy them. These streams are the direct cause of the depth of the alcoves, and, in cooperation with other agencies, of their width. The fresh, sharp, angular profiles of mesas, ridges, and canyon walls, and the extensive areas of bare rock are maintained by the rapid down-cutting and prompt removal of the resulting debris; the deepest canyons are those carved by the most vigorous streams at work for the longest time. In down-cutting, the streams derive their working power chiefly from the fragments of rock which they transport. Dragged along downstream, every grain of sand and lime—that was once part of the cubic miles of sandstone and limestone removed in excavating the alcoves—has aided in cutting away rock. Each day that the streams are running, even when they seem clear, some scouring material is carried along, and because the tributaries run on bare rock and thus are unimpeded by soil and vegetation, they quickly carry sediment-ladden water to their master stream. Though relatively small and few of them perennial, the streams in the alcoves have steep gradients and are therefore powerful agents of erosion, especially in times of flood when they are supplied with large amounts of disintegrated rock material swept from the ledges by torrential rains. In this region, floods follow every shower and many times each year the large streams and small streams are given sufficient volume and speed to carry forward the newly made loose material about as fast as formed; to tear up and take away the rock wastes stranded along their courses, and also to dig into solid bed-rock. Even if the most vigorous stream had lowered its bed but a fraction of an inch each year, its life of hundreds of thousands of years is amply sufficient for the work it has accomplished. Flowing on very steep gradients, the tributaries of the Paria furnish a large part of the load of the master streams which carry to the Colorado each year many tons of rock waste. Thus during their long life, each of the streams that heads in the park has been busy with two tasks—cutting gorges and transporting the rock waste supplied to it.

Figure 4. Hindu Temples. Peculiar erosional forms near the head of Bryce Canyon illustrating "snow and rain-drop" type of erosion. (Photo by Zion Picture Shop)

In widening their channels and in producing the remarkable architectural features of the park, the streams have played but a minor role. At no time were the streams as wide as the present canyons, probably never wider than at present. They have cut vertically downward and in so doing have retained the meanders and straight stretches that marked their original courses on higher ground. Each of the present alcoves and valleys was once a series of deep, narrow trenches like the "transepts" in the wall of the head of Willis Creek. Thus, in forming canyons, down-cutting precedes widening and also is much more rapid; many canyons hundreds of feet deep are less than 50 feet wide. (See Fig. 3, 10).

As the streams sink their runways deeper and deeper into solid rock, they expose larger and larger expanses of wall to the destructive work of atmospheric agencies (weathering). Frost in cracks. plant roots in the soil, rain that beats against the cliffs, wind that sweeps the surface, acids in the air, and water in the ground decompose and break up the solid rock, causing grains, fragments. and even large blocks to fall to the stream which carries them to the mouth of their canyon and on to the Paria and the Colorado and eventually to the sea. Within the alcoves and on the lands below them, weathering, like down-cutting, is rapid for the climate, and the composition of the rocks are favorable. Snow to the depth of a foot or more lies over the ground for several months, and the slow melting permits the water to seep into cracks and dissolve the cement that holds the rock grains together. The quarrying action of frost is particularly noticeable not only at the beginning and the end of the cold season but also at night and morning. Alternate freezing and thawing crowds wedges of ice into rock cracks for as many as 275 days a year.

Figure 5. Alternate hard and soft layers are responsible for this banded and ribbed design which is so characteristic of Bryce Canyon rocks. (Photo by Zion Picture Shop)

Though weathering is universal, its destructive activity is not everywhere the same. The rate and manner of weathering vary with the composition of the rocks, and therefore weathered rocks of different kinds assume different architectural forms. Because the pink limestone which forms the rim of Paunsaugunt Plateau is a compact mass in which the cement and the body of the rock are chiefly calcium carbonate, it is decomposed largely by acids carried in the air and in rain water. The result is a cliff, a steep slope, or a pinnacle without prominent benches. (See Fig. 5) The generally smooth surface of this rock is bare and hard or is coated with fine debris—the residue of chemical reaction which incidentally has produced much of the striking color. Where quartz sand cemented with lime is a considerable constituent, decomposition extends inward, forming niches, recesses, and caves. Some narrow walls are completely perforated and stand as the frames of windows and natural bridges. In the thick beds of sandstone, the weak cement that holds together the rounded grains of quartz is readily dissolved by rain that wets the walls and by water that seeps through the rock, loosening the tiny particles and permitting them to fall or to be swept from cliffs and ridges by passing showers or wind. The rock thus disintegrated bit by bit may stand in cliffs or broad flat surfaces with a series of broad benchlike steps whose height of 30 to 200 feet is a measure of the thickness of individual beds. The thin-bedded sandstones and the rocks composed chiefly of clay (the shales) weather more rapidly, particularly where they include coal (Cretaceous rocks of Paria Valley), and therefore form slopes on which each bed is represented by a step a few inches or a few feet high. The rocks most slowly and unevenly weathered are the sandstones and shales that are tightly held together by iron. They form the brown knobs seen here and there on the flat lands and as caps of buttes and pillars.

The topographic features that give the Bryce Canyon National Park region its eminence among areas of superb scenery have been produced by erosion, chiefly by streams—the same streams that now run over the flat lands and down the deep gorges. Ever since the lands in the park attained their present positions, the streams have been gnawing at the rocks. in some places their work is nearly finished, in other places, vigorously in progress, and still other places, barely begun. As a result, the landscape consists of three unlike parts: a vertical wall (or rim), a nearly flat highland above it and a rugged lowland. The wall is the Pink Cliffs, the beautifully carved and beautifully painted rim of the Paunsaugunt Plateau. The highland is the top of the plateau, and the lowland is the huge amphitheater cut at the base of the plateau at the head of Paria Valley and bordered by much-broken slopes that extend upward to the base of the pink wall. (See Fig. 6)

Figure 6. The alcove at the head of Bryce Canyon. This shows the sharp boundary line between the even surface of the Paunsaugunt Plateau (background) and the "under the rim" area in process of erosion by tributaries to the Paria River.